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Battery energy storage for intermittent
renewable electricity production
A review and demonstration of energy storage
applications permitting higher penetration of
renewables
Steffen Görtz
EN1510
Examensarbete för civilingenjörsexamen i Energiteknik
Abstract
Driven by resource politics and climate change, the transition from conventional fossil fuel based and
centralized energy generation to distributed renewables is increasing rapidly. Wind and solar power
generation offer carbon dioxide neutral electricity but also present some integration difficulties for
energy system operators and planners due to intermittent power output. A promising way of dealing
with the intermittency from renewables is energy storage.
The method of storing energy in the electricity grid, especially by the means of electrochemical storage,
has gained a lot of attention over the last years in the energy sector. While most utilities and energy
market stakeholders have the basic understanding of energy storage, a more profound knowledge of grid
storage applications is often lacking. This thesis aims to highlight and explain possible energy storage
applications with focus on renewables integration.
Battery energy storage can allow higher amounts of renewable electricity generation to be integrated by
smoothening power output, time shifting generated energy to follow demand and increase hosting
capacities through peak shaving. Power quality related issues due to intermittency can be mitigated by
controlling the storage’s charging patterns to respond to grid variables. For optimal utilization and
maximum storage value, several applications should be within the operational repertoire of the storage
unit. Other applications including arbitrage, grid investment deferral and load following are discussed.
Several battery technologies which have been developed and tested for such applications including lead
acid, sodium sulfate and lithium-ion are presented. The most promising battery energy storage
technology is lithium-ion with exceptional storage characteristics and most importantly a favorable near
term price development.
Two case studies on two of Umeå Energy’s low voltage networks simulating high penetrations of solar
generation have been carried out to demonstrate mitigation of overvoltage and peak shaving with battery
energy storage systems. The simulations show that energy storage systems can successfully be used to
aid the integration of renewables in the electricity grid. Present capital costs are still too high for energy
storage to be feasible but falling pricing and a developing market is foreseen to lower the hurdles.
The main obstacle for energy storage at grid scale besides high capital costs are, in principle, nonexisting legal frameworks regulating the ownership of energy storage systems and system technology
standardization. Further discussions on the matter in combination with testing and pilot projects are
needed to gain national and international experience with battery energy storage for the successful high
share integration of renewables.
i
Sammanfattning
Sinande naturresurser och växthuseffekten driver på övergången från centraliserad kraftproduktion
baserad på fossila bränslen till distribuerad förnyelsebar energiproduktion i rask takt. Vind- och solkraft
levererar koldioxidneutral el men ställer samtidigt balansansvariga och elnätsplanerare inför en rad
problem på grund av periodiskt återkommande och tidvis ostabil effektgenerering. Energilager
presenteras som en lovande lösning på problemen orsakade av förnyelsebara energikällor
Att lagra energi i elnätet, i synnerhet med batterier, har fått en hel del uppmärksamhet de senaste åren i
energibranschen. De flesta elnätsbolag och intressenter på energimarknaden har en grundläggande
förståelse kring energilagring i elnätet men saknar ofta mer djupgående kunskap. Detta examensarbete
syftar att belysa och förklara användningsområden och potentialer för energilagring med fokus på
integreringen av förnyelsebara energikällor.
Teorin beskriver hur batterilager kan användas för tillåta integreringen av en hög andel förnyelsebar
elproduktion. Några tillämpningar är; effektutjämning, lagring av producerad energi för senare bruk
samt ökad nätkapacitet genom att kapa toppar. Problem relaterade till försämrad elkvalité orsakad av
varierande kraftproduktion visas kunna pareras med hjälp av programmerbara energilagringssystem som
läser av storheter på elnätet såsom spänning och frekvens. För att utnyttja energilagret optimalt och
komma åt dess maximala värde bör flera användningsområden kombineras. Därför diskuteras även
andra användningsområden såsom arbitrage, lagringskapacitet för att skjuta upp eller undvika
förstärkning av elnätet och lastföljning.
Ett flertal batteriteknologier aktuella för de diskuterade användningsområdena såsom bly-,
natriumsulfat- och litium-jonbatterier presenteras. Den mest lovande teknologin är litium-jon tack vare
dess utmärkta egenskaper och framförallt mycket gynnsamma förväntade prisutveckling.
Två fallstudier av två av Umeå Energi´s nätområden med hög simulerad andel solenergiproduktion har
utförts för att demonstrera utnyttjandet av energilager för reglering av överspänning och kapning av
toppar. Simuleringarna visar att energilagringssystem med framgång kan underlätta integreringen av
förnyelsebara energikällor. Dagens kapitalkostnader är fortfarande för höga för att energilagring ska
vara ekonomiskt försvarbart men fallande priser och en växande marknad väntas verka till teknikens
fördel.
Det visar sig att regelverk gällande ägandeskapet och standardiseringen av energilager är i det närmaste
obefintliga vilket utgör ytterligare hinder för tekniken. Fortsatta diskussioner gällande dessa punkter i
kombinationen med test- och pilotanläggningar för att införskaffa erfarenhet av energilagring i elnätet
krävs.
ii
Preface and acknowledgements
This master thesis of 30 ECTS credits completes my studies for the degree Master of Science in Energy
Technology at the Faculty for Applied Physics and Electronics at Umeå University. The work was
carried out on the behalf of Umeå Energi Elnät AB during the period of 2015.01.19 to 2015.06.01.
First I would like to thank my supervisors at Umeå Energi Elnät AB, Agneta Linder and Negar
Ghanavati for their contributions to my work in the form guidance, comments and numerous discussions
and clarifications in several areas concerning electrical networks. I would also thank my supervisors at
the faculty, Jan-Åke Olofsson and Johan Pålsson for your advice and support during the project and
writing of the report.
I would also thank all employees at Umeå Energi Elnät AB who have helped me during my work for
their openness and knowledge inputs. A special thanks goes to Mikael Antonsson for the discussions on
power quality as well as making the visit to the energy storage in Falköping possible.
I am also grateful for the opportunity given to me by Stefan Carlson from Falbygdens Energi AB, to
visit Sweden’s first grid-connected energy storage. It was very interesting to hear about the project and
experiences from it. I also want to thank Lars Olsson, former CEO of Falbygdens Energi AB, for your
knowledge and insights into recent processes in the field of grid energy storage.
I also want to thank the DIgSILENT support team, Franscesco Marra and Boštjan Blažič for their
comments on the simulations.
Umeå, May 2015
Steffen Görtz
iii
Table of Contents
Abstract .................................................................................................................................................... i
Sammanfattning....................................................................................................................................... ii
Preface and acknowledgements.............................................................................................................. iii
Abbreviations ....................................................................................................................................... viii
1.
2.
3.
4.
Introduction ................................................................................................................................. - 1 1.1
Purpose ................................................................................................................................ - 2 -
1.2
Research questions .............................................................................................................. - 2 -
1.3
Methods ............................................................................................................................... - 2 -
1.4
Constraints ........................................................................................................................... - 3 -
The electricity network ................................................................................................................ - 4 2.1
Traditional power production and the electricity grid ......................................................... - 4 -
2.2
Power quality and reliability ............................................................................................... - 5 -
2.2.1
Voltage dips ................................................................................................................. - 6 -
2.2.2
Voltage limits .............................................................................................................. - 6 -
2.2.3
Flicker.......................................................................................................................... - 6 -
2.2.4
Asymmetrical voltage .................................................................................................. - 7 -
2.2.5
Harmonics ................................................................................................................... - 7 -
Intermittent renewables ............................................................................................................... - 7 3.1
Micro generation ................................................................................................................. - 8 -
3.2
Photovoltaic power .............................................................................................................. - 8 -
3.3
Wind Power ......................................................................................................................... - 9 -
3.4
The effects of distributed intermittent generation ............................................................. - 11 -
3.5
Renewable market trends and integration forecast ............................................................ - 13 -
3.5.1
Photovoltaics ............................................................................................................. - 13 -
3.5.2
Wind .......................................................................................................................... - 14 -
Alternatives to energy storage ................................................................................................... - 15 4.1
Production curtailment ...................................................................................................... - 15 iv
5.
4.2
Transmission and distribution grid investments ................................................................ - 15 -
4.3
Dynamic line rating ........................................................................................................... - 16 -
4.4
Demand response .............................................................................................................. - 16 -
4.5
PV system integrated reactive power control .................................................................... - 17 -
Battery energy storage technology ............................................................................................ - 17 5.1
Terms and definitions of storage technology .................................................................... - 18 -
5.2
Battery technologies .......................................................................................................... - 19 -
5.2.1
History ....................................................................................................................... - 19 -
5.2.2
Lead acid ................................................................................................................... - 20 -
5.2.3
Nickel electrode batteries .......................................................................................... - 21 -
5.2.4
Lithium-ion batteries ................................................................................................. - 22 -
5.2.5
NaS batteries.............................................................................................................. - 23 -
5.2.6
Flow batteries ............................................................................................................ - 24 -
5.3
6.
Choice of storage technology ............................................................................................ - 26 -
Energy storage applications ....................................................................................................... - 31 6.1
Renewable energy integration with BES ........................................................................... - 31 -
6.1.1
Renewable energy time shift/spinning reserve .......................................................... - 31 -
6.1.2
Capacity firming/Ramp support/forecast hedging ..................................................... - 32 -
6.1.3
Voltage support/Voltage stability .............................................................................. - 33 -
6.1.4
Increasing hosting capacity ....................................................................................... - 34 -
6.2
Other energy storage applications ..................................................................................... - 35 -
6.2.1
Arbitrage/electric energy time shift ........................................................................... - 35 -
6.2.2
Demand charge reduction .......................................................................................... - 36 -
6.2.3
Customer power reliability and quality ..................................................................... - 36 -
6.2.4
Supply capacity/spinning reserve .............................................................................. - 37 -
6.2.5
Load following and area regulation ........................................................................... - 38 -
6.2.6
Voltage support ......................................................................................................... - 39 -
6.2.7
Transmission support and frequency regulation ........................................................ - 40 -
v
6.2.8
Transmission congestion relief and upgrade deferral ................................................ - 41 -
6.2.9
Peak shaving to minimize grid losses ........................................................................ - 42 -
6.3
Simultaneous energy storage applications......................................................................... - 43 -
6.4
Placement of storage units ................................................................................................. - 46 -
6.5
Auxiliary components ....................................................................................................... - 47 -
7.
6.5.1
Power inverter ........................................................................................................... - 47 -
6.5.2
Battery management system ...................................................................................... - 47 -
6.5.3
System supervisory controller ................................................................................... - 48 -
Regulations and frameworks ..................................................................................................... - 49 7.1
Ownerships of energy storage ........................................................................................... - 51 -
7.1.1
Grid operator ............................................................................................................. - 51 -
7.1.2
Electricity supplier..................................................................................................... - 51 -
7.1.3
Third part – the aggregator ........................................................................................ - 51 -
8.
Concluding remarks .................................................................................................................. - 52 -
9.
Case study I – High PV penetration in rural grid ...................................................................... - 53 9.1
Method............................................................................................................................... - 53 -
9.1.1
System description..................................................................................................... - 53 -
9.1.2
Construction of grid simulation ................................................................................. - 54 -
9.1.3
Assumptions .............................................................................................................. - 55 -
9.1.4
Simulation setup ........................................................................................................ - 55 -
9.2
Results and analysis........................................................................................................... - 56 Case study II – Load leveling with CES ............................................................................... - 64 -
10.
10.1
Method............................................................................................................................... - 64 -
10.1.1
System description..................................................................................................... - 64 -
10.1.2
Assumptions and simulation setup ............................................................................ - 65 -
10.2
Results and analysis........................................................................................................... - 65 -
10.2.1
Off-peak storage for 75% PV penetration ................................................................. - 66 -
10.2.2
Load leveling and arbitrage ....................................................................................... - 68 -
vi
11.
Discussion ............................................................................................................................. - 71 -
11.1
Uncertainties in case study I .............................................................................................. - 71 -
11.2
Uncertainties in case study II ............................................................................................ - 71 -
11.3
Battery energy storage as alternative to traditional solutions ............................................ - 72 -
11.4
Future prospects ................................................................................................................ - 73 -
11.5
Future work ....................................................................................................................... - 74 -
12.
Conclusions ........................................................................................................................... - 75 -
References ......................................................................................................................................... - 77 Appendix A – Studied networks......................................................................................................... A
Appendix B – Alternative solution overview for case study I ............................................................ B
Appendix C – Application synergies .................................................................................................. D
vii
Abbreviations
BES
‒ Battery Energy Storage
BESS ‒ Battery Energy Storage System
BMS
‒ Battery Management System
CES
‒ Community Energy Storage
CHP
‒ Combined Heat and Power
DER
‒ Distributed Energy Resources
DLR
‒ Dynamic Line Rating
DSO
‒ Distribution System Operator
ES
‒ Energy Storage
HV
‒ High Voltage
LV
‒ Low Voltage
MV
‒ Medium Voltage
OPF
‒ Optimal Power Flow
PV
‒ Photovoltaic
SSC
‒ System Supervisory Controller
viii
1.
Introduction
The electricity grid of today is undergoing a modernization by the implementation of power flow
measurement, gathering information on electricity use and controlling power production and
distribution. With an increasing interest for distributed renewables and micro generation (1) in Sweden
and globally (2) the energy production becomes more and more decentralized. While distributed solar
and wind power production are necessary for a sustainable energy system the intermittent nature of
energy supply from renewables does not follow electricity consumption.
Traditional power production such as nuclear, hydro and co-generation from combustion of fossil or bio
fuels can be controlled by an operator, they are dispatchable. The electricity production to the grid can
be increased or decreased in short notice to match demand. Renewable power production from solar and
wind power cannot be controlled in the same way as they depend on energy resources the operator has
no control over. The power production varies as the sun shines and wind blows. Thus the production
form renewables has little to no correlation with consumption. Without an intermediate storage the
produced energy is not dispatchable. A solution to make renewable electricity more dispatchable is to
store a portion of it in an energy storage which can be dispatched to the grid when the electricity is
needed.
In a study by the International Energy Agency (IEA) it was concluded that an annual production share
up to 45% from distributed renewables do not require significant increases in power system costs with
current system flexibility and grid capacity. If larger share of distributed renewable energy production
is to be integrated cost-effective a system wide transformation is needed however. (3)
Battery energy storage has mostly been associated with off-grid renewable electricity generation for
remote locations and was only used for grid applications as backup power. Grid connected energy
storage is a relative new phenomena following in the wake of increased installation of renewable energy
production in the 21st century. As different issues regarding the intermittency of solar and wind power
production become clearer, solutions are discussed to resolve them. Large investments are placed in
research in many countries where the installation of renewable energy production has progressed fast
over the last years such as Germany, the United States of America and China.
According to some market analytics the time has come for energy storage to be implemented in large
scale with an estimated 6 GW of annual installation of global storage capacity from 2017. Other analyses
estimate annual growth rates for grid scaled battery energy storage systems of 63%. The growing market
for battery energy storage is thereby projected to reach market potentials around 18 billion US dollar by
2023. (4)
-1-
As energy storage has become an emerging topic among grid planners and smart grid developers at both
state and local level, the need to broaden the understanding of its applications in order to harness its full
potential is becoming more and more important. This thesis is meant to clarify and show the benefit of
some of the possible applications of battery energy storages as a part of the grid.
1.1
Purpose
The aim of this thesis is to identify possible applications of battery energy storage in the distribution
grid in order to allow an increased integration of renewables in the energy system. Two case studies
serve to demonstrate the use of energy storage in distribution grids with high penetration of renewables.
1.2
Research questions
The central questions to be answered in this thesis are the following:

How are local grids affected by high penetrations of renewable micro generation?

What are the possible applications of energy storage for allowing a higher penetration of
renewable micro generation and who benefits from them?

Is energy storage an economical feasible alternative to traditional reinforcement in the grid?

Are regulatory frameworks beneficial for energy storage and if not what general changes are
needed?
1.3
Methods
The information and results has been collected through a literature study within the field including recent
publications, reports and interviews.
The simulations were carried out in DIgSILENTs PowerFactory with grid data from Umeå Energi Nät´s
DIgPRO. Hourly load data from multifamily households in the studied part of the local grid as well as
10 minute values from a grid connected photovoltaic system in Umeå´s distribution grid was provided
by Umeå Energi Nät AB.
-2-
1.4
Constraints
This thesis intends to explain the applications of energy storage rather than giving in depth an analytic
evaluation and assessment.
In depth aspects of economic validation and market assessments including payback periods are left or
future work as well as in depth discussion of regulatory aspects. The reason for this is that technology
prices and the market changes and develop rapidly making such assessments less significant.
Several energy storage technologies other than batteries exist, including; pumped hydro storage,
compressed air storage and super capacitors. However to avoid a too broad approach and because
batteries have a beneficial general technical maturity and price development the focus of this thesis lies
on battery energy storage system
When it comes to determine the uses and benefits of energy storage applications regulations and
frameworks will not be considered eternal. As an example, in Sweden a utility company cannot own an
energy storage with the intent of using it for arbitrage. The reason for overlooking policies is that they
are governing over which energy storage applications are and will be profitable which could keep the
technology and market from getting the most out of some applications. After all, regulations and
frameworks are not written in stone and can be altered in favor of sustainable development.
Alternatives to energy storage for the integration of renewable energy generation such as transmission
upgrades and curtailment are not studied in detail more than to be mentioned and for simplified price
comparisons.
-3-
2. The electricity network
In this section the Swedish electricity grid and energy system is briefly described to establish basic
understanding for forthcoming theories.
2.1Traditional power production and the electricity grid
The power system consists of mainly three components; generation, transmission and distribution of
electric power. The generation of electric power is mostly dispatchable meaning the amount of power
generated can be controlled by operators. The operators themselves are obliged to follow the regulation
requirements which the Transmission System Operator (TSO) has on them. In Sweden the TSO is
Svenska Kraftnät, owned by the state and is responsible for keeping the balance of generation and
consumption of electric power. Keeping the balance at all times is important as imbalance leads to
frequency variations. The base power production in Sweden comes from nuclear and hydro power with
each standing for near half of the electricity production complemented by mainly wind (8%) and
combined heat and power (CHP) plants (5%) (5) (6). Hydro power and combined heat and power plants
are dispatchable and can therefore be used for short and long term power regulation. As CHP plants are
regional they are sometimes used for regulation in the distribution grid indirectly* controlled by DSO’s
whereas hydro power comes into use for national regulation and are by indirectly controlled by TSO’s.
An important role for hydro power is the regulation of wind power production. Fluctuations are
compensated by adjusting the power output from hydro power plants to rebalance demand and
consumption.
The generated electricity is transported from the major power stations via the national grid (400 kV –
220 kV) to the regional network (130 kV – 40 kV) and local distribution grid (40 kV – 400 V) to the
consumers. See Figure 1. The transmission network is owned by the Swedish state (Svenska Kraftnät).
The regional networks are owned by the major electricity companies and the local grids are owned by
local utility companies. As the grid owner the companies have regional monopoly and are obliged take
care of the grid and allow any consumer or production such as wind parks to connect to its network if
technical requirements for are met.
Traditional grid planning of the distribution network is based on a radial power system structure where
electricity generation and consumption are separate parts of the grid and the power flow goes in the
direction of the consumer. Lines and transformers are dimensioned above the maximum expected power
flows in order to endure peak demand hours without greater losses. Long feeders in the distribution
*
By indirect it is meant that that the DSO and TSO ask for regulations services from the plant operator.
-4-
network are dimensioned to supply the outermost costumer with electricity of satisfactory quality which
can leave lines close to secondary substations over-dimensioned. This can be the case primarily for nonurban sections of the grid where the reliability is not prioritized as it would become too expensive to
build a ring fed network which is the case in urban areas with many costumers. Here the reliability is
prioritized higher as power failures affect a larger number of costumers which could results in substantial
repayments in case of power cut during longer periods.
Figure 1 Simplified schematic of the electricity grid structure. The national grid corresponds to the transmission grid
and the regional and local to the distribution grid. Wind parks are occasionally connected to the regional grid. (7)
Grid planning is based on two fundamental principles for establishing a fault free electric network that
provides customers with electric energy that satisfies their needs. Electronic equipment is standardized
to ensure that no components have a significant impact on the electrical environment they are connected
to.
2.2 Power quality and reliability
Power quality is a collective concept with different criteria to primarily assess the technical quality of
the transmission and delivery of electricity to customers. In a summary by Eurolectrics “Power Quality
in European Electricity Supply Networks” from 2002, the concept is divided into two main parts;
voltage quality, which is the degree of conformation of grid voltage to specified ranges and power
reliability, which is the degree of how much a costumer can rely on continued availability of electricity.
(8)
-5-
The grid owner is considered to be the responsible for the network they manage. Local costumers
however have a part of the responsibility as well by limiting their negative influence on power quality.
Most issues come from the grid operator, the costumer or the power equipment used in the grid.
Therefore all three parts carry the responsibility by avoiding equipment and practices that create or
amplify power quality issues.
The main quality issues relevant for this thesis are those which mainly are consequences of electricity
generation like voltage dips, exceeding of voltage limits, unsymmetrical voltages, flicker and transients.
2.2.1 Voltage dips
Voltage dips are occasions where the voltage level drops to under 90% of the nominal voltage for time
periods longer than 10 milliseconds (ms) and shorter than 90 seconds (s). The amplitude of the voltage
drop is defined as the difference of the actual voltage during the voltage dip and the nominal voltage.
Voltage dips are caused by a electric fault either at transmission level or at the customer. The reasons
range from lightning, birds on electrical lines causing short-circuits or damages to ground lying power
lines. Voltage dips where voltages drop to zero or near zero Volt and last for 10 ms to 90 s are defined
as short-while power cuts which in the worst case scenarios can knock out whole industry facilities. A
strong grid infrastructure with safety relays, overvoltage arresters and grounding in combination with
high requirements for power equipment can reduce the occurrence and severity of voltage dips.
2.2.2 Voltage limits
The voltage level of the grid varies depending on the loads and topology of the grid. If overhead lines
are dominating voltage differences can be relative large across a long feeder line, from secondary
substation to the furthest customer. As all electrical appliances are constructed for a specific voltage
range they might operate worse or not start at all if the limits are exceeded. The sensitivity varies from
appliance to appliances with electrical motors often as the most sensitive managing a voltage difference
of sometimes only ± 5% for optimal performance. The limits for voltage differences in LV grid is not
allowed to be more than 10% of the nominal phase-voltage (230V) for more than 10 minutes (9). This
translates to an accepted voltage range of 207 V-253 V. Large voltage differences across long feeders
are commonly resolved by set the voltage from the secondary substation a few percentage units above
the nominal voltage.
2.2.3 Flicker
Fast periodic reoccurring variations of the effective voltage can lead to flicker which can be observed
as flickering light in e.g. light bulbs. Common causes of flicker in LV grids are welding equipment,
copying machines and elevator generators and compressors but also older types of wind turbines. The
-6-
flicker is due to very fast load variations and can be explained by the ratio of the variations and the short
circuit power in the connection point. Flickering electric lights can be unpleasant for people but the
effects are subjective. The amount of flicker can be reduced by changing conventional light bulbs to
low-energy lamps or high frequency strip lightning which possess a lower sensitivity to voltage
fluctuations. (8)
2.2.4 Asymmetrical voltage
A symmetric three-phase voltage is characterized by all phases having the same amplitude and the
interrelated phases dislocations are equal. If one the properties are not true anymore the voltage is said
to be asymmetric. Asymmetry in LV grids is caused by asymmetric connected line loads as might the
case for single-phase connected loads or PV systems. The result can be overloading of AC-machines or
dysfunctional power inverters. (8)
2.2.5 Harmonics
Harmonics are voltage or current components with frequencies of integer multiples of the grid frequency
50 Hz. Overlaid they are a measurement of the periodic deformity of the sinusoidal current and voltage
curves. Harmonics cause increased losses in power equipment and lines as well as shortening life times
of some equipment like condensers for phase compensation. Depending on the integer of the harmonics
they may also trigger residual current breakers and fuses or cause pulsating torques in electrical motors.
The harmonics are caused by non-linear loads where the ratio between voltage and current is not constant
during periods. The non-linear load draws current that deviates from the sinusoidal shape of the grid
current curve and by this deforming it. The resulting current harmonics then cause corresponding voltage
harmonics. Non-linear loads causing harmonics include rectifiers, low-energy lamps and home
electronics with semi-conductors such as computers. (8) (10)
3. Intermittent renewables
In this subsection the technology of intermittent renewables† wind and photovoltaic (PV) power are
briefly described to give the reader an overview. This includes mechanisms, power production profiles
and how they can affect the grid.
†
Technically also wave power counts as intermittent renewable but as this technology is not present in the Swedish
energy system to date it will be kept left out.
-7-
3.1 Micro generation
Small scale renewable production sites with the intention to cover a costumers’ own electricity needs
go under the epithet micro generation. According to the Swedish Electricity Act (1997:857) (11), micro
generation is considered as sites connected to a fuse subscription of 63 Ampere (A) with a maximum
power of 43,5 kW. The most common type of micro generation in Sweden is photovoltaic on residential
roof area complemented by small scale wind power, hydro and renewable based combined power and
heating (12). In order to connect micro generation to the grid several technical requirements for safety
and operational causes have to be met.
On January 1 2015 a tax reduction system for micro generation of renewable electricity was introduced
in Sweden. The owner of a micro generation plant is thereby paid for feeding excess electricity into the
grid by reduced annual taxes. The reduction is 0,60 SEK/kWh electricity with a maximum of 18 000
SEK per year. (13) The tax reduction includes costumers with a fuse subscription of up to 100 A which
according to Ohm’s law corresponds to an upper limit of 69 kW peak power. Requirements for receiving
tax reduction include installation of equipment for hourly measurement and that the generation plant is
connected to the grid such that outage for the costumer from the grid and feeding to the grid occurs at
the same connection point.
3.2 Photovoltaic power
The vast majority of commercial photovoltaic solar panels sold today are based on semiconductor
technology using silicon as crystalline or amorphous cells. Silicon as variety of other semiconductors
release electrons upon being submitted to solar light. This is called the photovoltaic effect. The excited
electrons act as charge carriers and are collected by electrodes on the top of the cell. By connecting the
cell to an electric circuit, electrons are allowed to recombine with the holes in the semiconductor which
the excited electrons created. This creates a direct current which is converted to AC with an inverter
which allows the use for home power equipment and grid connection.
PV technologies based upon other materials than silicon exist, but are often still in a laboratory phase
with higher production cost or lower conversion efficiencies. These include Grätzel cells, Perovskite
based cells and organic cells (14). However some of these are promised a bright future as the
development continues and might become cheaper and more efficient than commercial silicon cells.
Solar panels consist of photovoltaic cells and are connected in series and parallel forming arrays. The
voltage is scaled by the number of solar panels connected in series. As the electrons are excited by solar
light, the power output of a cell is directly proportional to the amount of incident light. Laboratory silicon
cells have a conversion efficiency of around 25% (15) with theoretical limits of 30%. However, due to
the presence of impurities in the material and system losses, the conversion efficiency of a commercial
-8-
photovoltaic power system is around 13-15% (16). The solar radiation on to the atmosphere amounts to
1350 W/m2 and around 1000 W/m2 reach the earth. Taking the conversion efficiency of a PV system to
be 15%, a power production of 150W/m2 is achieved on a sunny day with optimal conditions.
The variability of power production from PV is directly proportional to the solar irradiation. Therefore,
the peak power output will be around noon when the sun is at its highest point in the sky. Due to
occasional clouds passing by in the sky the production of a PV system can vary between these values
creating large differences in power output to the grid if connected. Another factor becomes relevant
when a large number of PV system is connected to a LV distribution grid which could be a possible
scenario in newly established housing quarters or rural areas with environmental friendly owners seek
to provide a part of their own electricity through installing PV systems on their houses. When PV
systems are presented in the same part of the LV grid, a sunny day could lead to a substantial increase
in local production. Typical problems for areas with high PV penetration are local overvoltage and
overloading of distribution lines and secondary substation equipment. In Germany, which has a rather
high penetration of LV-connected PV, this is leading to expensive grid infrastructure reinforcement
investments. (17)
Figure 2 Left: Power output from a 5kWp photovoltaic system on a clear day. Right: Power output from a 5kWp
photovoltaic system on a day with varying cloudiness. (18)
3.3 Wind Power
A typical wind turbine consists of a tower, a machine house for the generator and gearbox and a
horizontal axis with 3 blades parallel to the tower. The blades are designed to catch the wind passing by
converting kinetic energy to mechanical energy. A generator is connected to the axis converting the
mechanical energy to electricity. The size of wind turbines generally correspond to the peak power
-9-
output which varies between a few kilowatts (kW) to several megawatts (MW). Two turbines can have
the same rated peak power and have different heights as the height of the tower is primarily chosen to
attain sufficient wind speed which increase by height.
The power generation from a wind turbine is directly proportional the wind speed cubed. Therefore a
relative small increase in wind speed leads to a rather large increase of power generated by a wind
turbine. As wind speeds are hard to predict this leads to a large uncertainty, as the power output from a
single wind turbine can vary over a relative short time period. Figure 3 illustrates this by comparing the
normalized power output from a single turbine with that of all wind turbines in Sweden at the same hour
of the year. The relative variation will decrease as a result from wind farm being distributed across large
geographical areas which results in the smoothened curved in Figure 3. (19)
Figure 3 Normalized wind power output for a single turbine and the entire country of Sweden. (19)
It is clear that the combined output of all turbines is very different from that of a single turbine or even
a wind farm, making distributed wind generation hard to predict. This mainly affects the TSO and
balance responsible parties, as wind farms generally are connected to the MV grid at transmission level.
However, the variations are the same for micro generation which are connected to the LV grid at
distribution level. The energy from forthcoming wind front can be predicted fairly well. However the
arrival can be an hour or two delayed or early giving large errors in prediction. This is a problem for
balancing operator that are responsible for keeping power generation in level with consumption at an
hourly basis.
Grid connected wind power generation has been associated with increased levels of harmonics due to
their power electronic converters which are known to cause distorted currents. The emissions of
harmonic frequencies have shown to be lower than for industrial or commercial installations but they
- 10 -
show larger emissions of interharmonics which are non-integer multiples of the grid frequency. (10)
(20). In general, the cause of harmonics is not the intermittency of the power output but rather the
characteristic of the wind turbines power converter, which can be improved by design or adding active
filters to mitigate the harmonics.
3.4 The effects of distributed intermittent generation
Distributed generation from renewable energy sources has the trait of not following consumption as well
as frequent uncontrolled alternations in power output due natural variations in wind speed and solar
irradiance. The variability of distributed electricity generation and its impact on the transmission system
varies for different renewables and can be measured in several ways.
Ramp rate is the change in production per time period in relation to its nominal power rating and is
given as a percentage. Fast ramp rates due to passing clouds in case of PV systems or wind gusts in case
of wind turbines lead to fast varying power outputs as described earlier and need to be compensated in
order to keep the balance between generation and consumption. The correlation coefficient is the
normalized covariance between the consumption of electricity and its consumption. A value close to
zero indicates a weak or almost non present statistical correlation whereas a value close to one indicates
a strong correlation. Negative correlation values indicate that production is highest when consumption
is low as often is the case for wind power which usually produces the most during night time. The
capacity factor for power plants is a measure of how well utilized a plant is. It is defined as the ratio of
the actual power output over a period of time and its output if it had operated at its rated name plate
power over the given time period. The capacity factor is higher if the power plant such as a wind park
or PV system is placed where it best can utilize the natural resources available during the day. Placing
a wind turbine where there is little wind yields a much lower capacity factor than if it was placed where
the average wind speed is high. Another factor that can decrease a renewable power plants capacity
factor is if its power output does not match with the momentarily consumption in the grid it is connected
to and needs to be curtailed to maintain the balance. A lowered capacity factor leads to lowered return
in produced energy and therefore the capital investment yields lower return of investment.
The capacity factor for production is generally an economic concern of the production owner. The
production unit however is connected to a grid which is dimensioned for peak capacities which might
change with the introduction of distributed generation. The efficient utilization of the grid is of
importance for the grid operator due to profitability of grid investments but also for the grid owners as
additional investments would be added to the tariff. Thus the capacity factor for the electrical network
infrastructure including lines and transformers is of interest. A definition could be the average used grid
- 11 -
capacity in comparison to the designed rating for handling the peak capacity. (19) Renewable generation
tends to require a fairly high grid capacity as it is dimensioned for power peaks. These peaks normally
only last for a couple of hours which leaves the grid under-utilized for long time periods when production
goes down. This results in significant costs per amount of produced energy and a lower grid capacity
factor. If the peaks would be taken care of, meaning they are reduced, it would be possible to build new
DER without having to upgrade the grid. (19)
When integrating large scale renewables the main challenge at distribution level is the grid capacity
rather than the variable nature of wind and sun. If the correlation coefficient is low or even negative,
large variations in peak power flow are the result. The term hosting capacity stands for the amount of
renewable electricity production that is allowed to be integrated into an electrical grid without causing
decreased reliability and power quality of the grid. (19) The hosting capacity is a property of the
individual grid and varies depending on layout and rating of lines and transformers. The concept of
hosting capacity is shown Figure 4 which shows the net power flow exceeding the hosting capacity
twice during overproduction and once during peak consumption.
Figure 4 Example of how low correlation of production and consumption can lead to scenarios where the grids hosting
capacity is exceeded. (19)
Typically the grid infrastructure is over dimensioned to tolerate occasional peak loads in a region.
Exceeding the hosting capacity therefore only becomes a problem when the rated power of the
renewable power installation exceeds the peak demand. In Sweden such large increases in potential
power flow might be hard to obtain as peak demand occurs most frequently during winter and the grid
- 12 -
is dimensioned thereafter. Possible situations where the hosting capacity is reached might be in part of
rural grids with a low number of end customers or summer residences.
3.5 Renewable market trends and integration forecast
With the increased global political awareness of climate changes and the EU2020 energy goals including
the goal of 20% share of renewable energy production by 2020, investments in renewables lead to large
increases of PV and wind power installations. In 2013, added renewable electricity capacity surpassed
added capacity that burns fossil fuels for the first time in history. According to predictions the shift will
accelerate that by 2030 4 times more as much renewables will be added (2). The situation is not different
in Sweden and installation of renewables increases annually. From the nest two sections it can be
assumed that renewables will continue to strengthen their role in the Swedish electricity generation.
3.5.1 Photovoltaics
The global prices for photovoltaics have decreased steadily since their commercial introduction and are
expected to continue to decrease by 20-25% every year the next few years (2). The PV costs in Sweden
have also plummeted the last years with record low system prices to follow. Figure 5 shows typical
prices for PV systems installed by Swedish installation companies as turnkey systems.
Figure 5 Typical prices for turnkey PV systems excluding VAT reported by installation companies in Sweden. (1)
The prices for grid connected systems in Sweden are depending on the global market price and the size
of the Swedish market. Direct capital subsidy fueled the price reduction leading to a relative high
demand for grid connected PV systems in Sweden for 2013. The cumulative installed PV capacity in
Sweden until the end of 2013 is shown in Figure 6. As a result from decreasing system prices, grid
connected systems accounted for most of the installed capacity and the largest increase where installed
capacity more than doubled from 2012 to 2013 from 7,5 MWp to 17,9 MWp. (1)
- 13 -
Figure 6 Cumulative installed PV power capacity in Sweden in 4 sub markets and the yearly installed capacity. (1)
3.5.2 Wind
In 2014, the cumulative installed wind power capacity in the European Union, as shown in Figure 7,
reached 128,8 GW after a growth of 9,8% from the previous year. The installed capacity in Sweden
stood for 5,4 GW of the EU capacity which equates to 4,3 %. (21)
(GW)
Figure 7 Cumulative wind power installations in the EU (GW) (21)
The national ambition for wind power until 2020 set by the Swedish Energy Agency in 2007 is 30 TWh
of annual electricity production. With the help of electricity certificates supporting larger scaled
renewable energy production and enforcing actions to simplify the application processes for wind power
installations in Sweden, the cumulative installed capacity has steadily increased since 2007. This is
shown in Figure 8 which depicts the development over the last years and the projection of the base case
scenario. This scenario is based on confirmed future installations and an estimate that 5% of all
installation projects under investigation respectively 15% of all installation projects with permit are to
be implemented. (22)
- 14 -
Figure 8 Base case scenario by Svensk Vindenergi of cumulative wind power installation and energy production. (22)
4. Alternatives to energy storage
4.1 Production curtailment
In situation where electricity demand decreases and intermittent renewable generation is peaking the
only possible method to maintain the grids operational limits is to decrease the power output. For wind
turbines this is achieved by stall regulating the rotor blades such that the wind passes by rather than
drives the blades. In extreme cases calling for fast power regulation the whole turbine nacelle to turn out
of wind. Wind park operators as required by Svenska Kraftnät to have the ability to reduce the power
output to below 20% within 5 seconds if needed (23). There are currently no requirements on PV system
owners to regulate the power output as problematic penetration levels have not been achieved yet.
However there have been studies evaluating power regulation of PV systems to reduce overvoltage in
distribution grids (24). For both wind and PV power, the method requires the active power output to be
reduced to levels that are manageable for the grid capacity. The downside for the curtailment method is
the loss of expected income for the production owner even if the curtailed energy is only a fraction of
the total production as would be the case in a scenario similar to what can be seen in Figure 4. Economic
contracts for dividing the compensation for the loss of income and legal frameworks for regulation are
needed to ensure that high penetrations of renewables are both economically and technically possible
(19)
4.2 Transmission and distribution grid investments
The approach first considered by most grid operators is to invest in the grid by building new
infrastructure such as lines or transformers or upgrading existing infrastructure. Hereby the hosting
capacity of the grid infrastructure can be strengthened to better endure peak loads and load variations.
- 15 -
This might however lead to binding large capital investments to single areas when future load situations
can be uncertain. Payoff times for grid investments are often 30-40 years which can become costly for
grid owners and in extension for the costumers. Misplaced grid investments for taking care of infrequent
peak loads would thereby lead to decreased grid utilization and capacity factor.
4.3 Dynamic line rating
Dynamic line rating (DLR) is said to make up to 25% additional usable capacity available for system
operations (25). Instead of calculating static ratings for transmission lines with normal, long-term
emergency and short-term conditions DLR enables transmission overs to determine capacity and apply
line ratings in real time. Different to static ratings indicating maximum line currents based on fixed
assumptions, dynamic ratings are calculated in real time based on the actual operating conditions at
specific moments. This often results in higher line ratings which increase capacities and help avoiding
congestion. Additional grid investments due to the integration of renewables in large scale can be
avoided using this method. The requirements for implementing DLR are specialized instruments like
thermal rate systems, weather stations and sensors for currents and conductor temperatures as well as
using supervisory control and data acquisition (SCADA) systems (25). Implementing these
requirements might call for qualitative investment for grid operators such as training and changed
procedures which require incentives. However, this method remains a promising component in smart
grids and as it only has been around since 1970 it can become more widely used in the near future. (25)
4.4 Demand response
Shifting peak loads be altering daily electricity consumption through the participation of consumers is
called demand response and is considered as an increasingly valuable option for grid modernization (26)
. The methods engages consumers to take part in balancing supply and demand by measures including
time-based rates for peak, time-of-use and critical peak repayments that make it economical beneficial
to adjust household and commercial loads to the current electricity supply. Two examples of how critical
peak rebates and peak pricing are shown in Figure 9. Besides time-based tariffs the implementation of
demand response programs requires evaluation of supply conditions e.g. solar radiation and wind
forecasting. In addition there need to be means of communication to request action from involved users
and equipment.
- 16 -
Figure 9 Example of how critical peak rebates and peak pricing are implemented to achieve price elasticity.
Effectively, electricity prices are elevated or reduced during certain hours of the day to promote time of energy use in
order to level demand and reduce load peaks (27)
The approach with demand response to mitigate load peaks and reduce load imbalances requires a high
share of motivated customers to have an effect on load situations. The induced habit alterations have to
be consistent for the method to work which still remains to be shown practicable.
4.5 PV system integrated reactive power control
Residential distribution networks with high penetration of PV generation have been observed to
experience overvoltage due to the increased amount of active power fed into the grid during midday
when generation is high and consumption low (17) (24). Voltage rises can be reduced by using the PV
inverter to inject an amount of reactive power to the grid along with active power. This way the power
output from the PV system is not curtailed. A higher portion of controlled reactive power keeps the
voltage at constant levels reducing the risk of overvoltage. The drawback is that the higher active power
output rises the more reactive power is needed to keep the voltage inside the limits. This demands
inverters with higher power ratings resulting in increased losses from the PV system as well as higher
losses in the feeder due to a lowered power factor. (24)
5. Battery energy storage technology
In the following chapter the most important terms regarding energy storage and specifically battery
energy storage are introduced. A selection of established battery technologies is described to give the
reader an understanding of the technological variation. In the end of this chapter some additional factors
for the selection of storage technology are discussed.
- 17 -
5.1 Terms and definitions of storage technology
It is important to be able to tell apart the terms power and energy to understand the application attributes
of storage technology. Energy can be defined as a quantity or volume whereas power is the rate of which
the amount of energy changes. Energy in storage applications is measured as kilowatt-hours (kWh) or
megawatt-hours (MWh) and power is measured in kilowatts (kW) or megawatts (MW). Energy and
power are prioritized differently for different storage applications. Load shifting during longer periods
of time for example require a large energy volume of storage capacity whereas a power quality
application such as voltage stability requires power to be absorbed or injected fast rather than during
long periods. This creates a distinguishing between cost of power capacity (dollar/kW) and cost of
energy capacity (dollar/kWh) which becomes important when choosing storage technology.
It is also important to distinguish between real power and reactive power. The electric grid is almost
entirely based on an alternating current (AC) system which means that voltage and current follow a
sinusoidal wave pattern where the voltage is positive half the time and negative the other half. The
current meanwhile flows in one direction half the time and in the reverse the other half. In Scandinavia
and most parts of the world this cycle occurs 50 times per second (50 Hertz). Real power is transmitted
from an energy source to an electric component that consumes it to do some form of work such as
turning a motor. Reactive power, measured in volt-ampere reactive, is power that travels back and forth
between components as a result of the repetitive absorption and release of energy but it does no work.
Reactive power is, however, necessary to maintain the voltage to deliver active power through
transmission lines. It converts the flow of electrons into useful work required by motor load and other
loads (28). While reactive power is not consumed it can have a significant impact on power networks.
It causes the current flowing through the system to increase which in turn will increase heat losses which
leads to voltage stability complications.
Another term to be familiar with is pulse power which, opposed to constant discharging, is the ability
to discharge a volume of energy quickly. Pulse power is needed for mainly power quality applications
and is found in batteries and capacitors allowing energy to be stored and quickly discharged at high
power and voltage as a pulse. (29)
Response time is how quickly a storage technology can be ready for use and discharge energy. Different
applications require different response times. Power quality applications for example, require fast
response times as the energy is needed almost momentarily when quality issues occur, whereas load
shifting often is planned ahead and slower response times are sufficient. The discharge duration
indicates how long a storage device can maintain its required power output. The response time and
discharge duration is crucial when choosing the right storage technology and even among batteries these
attributes can vary widely.
- 18 -
Depth of discharge (DoD) denotes the percentage of energy discharged relative to the full storage
capacity before the storage is recharged. Some battery technologies are sensitive to how deep the
discharging goes and can reduce the life time expectancy. The optimal DoD varies between battery types
as some technologies come with a “memory effect” where the output voltage is depressed due to shallow
discharging when they operate best at full discharge cycles whereas some operate best under shallow
discharging conditions. The frequency of discharge refers to how often a storage unit will be discharged
during its operation. Also here the application dictates the frequency, where some storage units are
almost never completely discharged and some are cycled continuously.
The efficiency or also called round trip efficiency is defined by the input to output energy ratio of the
storage cycle. Energy is lost during charging and discharging processes in storage systems, often caused
by the converting AC to DC and back again to AC after storing. Other losses are standby losses defined
as the energy lost from the moment of charging to right before discharging e.g. storing electrical energy
for longer periods as chemical energy where the ambient temperature plays a role in the kinetics of cell
reactions perceived as decreased power output. Some technologies also require ancillary devices such
as inverters and filters required for grid connection and draw power. They are therefore considered as
parasitic losses in a similar way as standby losses that accumulate over time.
5.2 Battery technologies
In general a battery consists of two electrodes, one negative called the anode and one positive called
cathode, an electrolyte which is either a liquid or solid that transports the charges from anode to cathode.
There exists a large number of battery technologies ranging from proven lead acid batteries to more
recent but emerging like lithium ion batteries which all have different attributes and possible
applications. Key characteristics are summarized in Table 1. This section is intended to describe some
of the more promising candidates when it comes to choosing battery technology for energy storage
applications, starting with a short background on the history of battery development.
5.2.1 History
The by far most common energy storage capacity wise is pumped hydro storage and the first energy
storage facility was started up around 1909 at Schaffhausen, Switzerland, providing around 1 MW of
power (29) (30). Pumped hydro stores potential energy by pumping water in basins for later use and is
dispatched like a regular hydro power plant. Batteries are the dominating technology for storing
electrical energy (30). This comes from the various types of batteries that have found a place in everyday
life from button cells in watches providing a few watts to megawatt sized installations providing load
levelling services. Alessandro Volta, in the year of 1800 built the first battery composed of alternating
- 19 -
discs of copper and zinc separated by cardboard with a brine solution acting as electrolyte. From the
Voltas cell, the Daniel cell was developed using two different electrolytes (1836) and later the Leclanche
cell (1866) which used carbon for the cathode instead of copper.
Figure 10 Simple schematic of a lithium ion battery. (30)
The alkaline cells of today using alkaline electrolytes and manganese oxide for the cathode were not
invented until 1949. These cell types are known as primary batteries as they are not rechargeable. Fuel
cells are usually also not rechargeable and can be described as primary batteries. Secondary batteries on
the other hand are rechargeable and can be used to store energy repetitively. Rechargeable batteries
have progressed from the early lead acid batteries in 1859 through nickel-cadmium, NiCd, from 1899
and nickel-metal hydride, NiMH, around the mid of the 1980´s. The latest and considered to be the most
promising battery technology are Lithium-ion based batteries which first emerged in 1977. (30)
5.2.2 Lead acid
Lead acid batteries are made of two lead alloy electrode grids and sulfuric acid as electrolyte. The alloy
is typically made of a blend of antimony, calcium, tin or selenium and lead to improve the mechanical
strength of the cathodes.
Lead acid technology is made out of two main categories; vented (flooded) and valve-regulated (sealed).
While vented lead acid batteries’ electrodes are immersed in liquid, valve-regulated lead acid batteries
uses gel or an absorbent separator to immobilize the electrolyte. Vented lead acid batteries are used for
short power burst as used in power quality applications with short life time expectancies around 3-7
years or up to 1000 cycles with 10% discharging depending on use. A subcategory of vented lead acid
- 20 -
batteries used for stationary applications such as standby emergency power and telecommunications
system have longer life expectancy ranging up to 30 years. They are thus a common choice for energy
storage projects. The valve-regulated types are used for uninterruptible power supply (UPS) and possess
a low life expectancy ranging from 5 to 10 years due to their sensitivity to temperature and corrosion
etc. Their optimal operation temperature is around 25°C with deviations leading to possible explosion
(below -40°C) and overheating causing faster degradation. Common disadvantages include also; selfdischarge, sulfatation which reduces cell power and degradation (general deprivation of structures and
components) leading to battery failure. With more recent research so called advanced lead acid batteries
have been developed, reducing maintenance and increasing life expectancies.
Worldwide around 35 MW of installed power capacity for energy storage are based on lead-acid
technology. Starting as early as in the 1870’s lead acid batteries have been used for load leveling and
peaking in central electric plants of that time. Despite low energy and power density, short cycle life,
toxicity and high maintenance requirements they remain a popular choice for energy storage application
thanks to their low cost and technical maturity.
5.2.3 Nickel electrode batteries
Nickel electrode batteries are known as dry cells where each cell contains a pair of electrodes. One is a
positive nickel electrode and the other a negative electrode made of cadmium, zinc, iron, hydrogen or
metal halide. The porous electrodes are separated by a partition and liquid electrolyte is circulated into
them. Only chemistries using cadmium and iron electrodes have been developed and been installed for
storage demonstrations so far where the most popular is nickel cadmium is the most popular.
The round trip efficiency ranges from 65-85% for nickel electrode batteries in general and 60-70% for
nickel cadmium batteries. Nickel cadmium batteries are prone to irreversible degradation and efficiency
losses due to temperature sensitivity and depth and frequency of discharge. Life expectancies vary
between different constructions ranging from around 100 cycles to 3 500 cycles with 80% depth of
discharge. This translates roughly into 10 to 15 years for lightly cycled applications. The exception is
nickel iron batteries with long service lives up to 25 years.
Thanks to their relative low cost, high power and energy densities, reliability and life expectancies,
nickel cadmium batteries are a popular choice for storage applications despite being more costly than
lead acid technology. In addition to relative low round trip efficiencies of nickel electrode batteries the
some electrode metals give reason for environmental and safety concerns, especially the toxic nature of
cadmium. Careful monitoring and efficient recycling is thus important for the use of nickel electrode
batteries.
- 21 -
5.2.4 Lithium-ion batteries
A lithium ion battery is made out of negative graphite electrodes and positive metal-oxide separated by
a micro-porous polymer and an ether as organic electrolyte with dissolved lithium ions, as shown in
Figure 11. During charging the lithium ions flow from the positive metal oxide to the negative graphite
electrode. When the battery is discharge the ion flow is reversed. The oxide used for the positive
electrode is typically made of cobalt, manganese or iron and phosphate. The electrode material
determines the technical characteristics of the lithium ion battery but some general qualities can be
identified. In general they have a very high energy and power density giving them a much higher cell
voltage than other battery technologies which in turn requires less cells for the same power output. The
cells have a short response time in the order of 20 milliseconds and relatively high round trip efficiency
ranging from 85 to 90% (29). The life expectancy is estimated to around 2 000 - 3 000 cycles or 10 to
15 years with 80% depth of charge while some theoretical estimations of up to 5 000 cycles (31) have
been made.
Figure 11 Lithium ion battery schematic (29)
Lithium-ion batteries have been on the market since 1991 when they were commercially introduced by
Sony and are compared to lead acid batteries a much less mature technology. They have mostly been
used for consumer electronics such as cell phones and laptops for their high energy density offering
lower weight, their low standby losses and cycling tolerance. For the same reasons lithium ion batteries
are believed to become the first choice in electric vehicle manufacturing and are becoming increasingly
interesting for utility functions. (29) (31)
With their superior characteristics the biggest hurdle for the breakthrough of lithium ion batteries is the
high capital cost relative to other technologies like lead acid and NiCd. The driving force for the price
reduction of Li-ion battery technology is believed to be the automobile industry where this type of
battery has become the most commonly used. The most promising of the different types of lithium ion
battery is LiFePO4 which is likely to be the best choice for large energy storage applications thanks to
- 22 -
its greater lifespan and safety compared to other lithium ion batteries (31). Figure 12 shows the lithiumion battery packs of a test facility in Falköping, Sweden. In general it is assumed that production costs
will go down in the near future as production volumes and material costs will go down. (29) (31). This
is further discussed in section 5.3.
Figure 12 Lithium-ion battery packs of the test facility in Falköping operated by Falbygdens Energi AB. (32)
5.2.5 NaS batteries
Sodium-sulfur batteries (NaS) count to the group of high temperature or molten salt batteries where the
electrodes are molten. They are similar to conventional batteries but share some traits with thermal
storage technologies. NaS batteries cells use molten sodium at 300° to 360°C as anode and molten sulfur
as cathode and a solid ceramic as electrolyte, shown in Figure 13. The high tempered batteries need to
be properly handled as contact with water can cause explosions.
- 23 -
Figure 13 Construction scheme of a sodium sulfur cell (29)
NaS batteries have the beneficial property of being able to be used for power and energy applications
simultaneously. In combination with relative high round trip efficiencies of 70-90% this makes them
particularly useful as current installations have showed to be suitable for several energy storage
applications. Their primary function is energy storage for longer periods but their short response time
of around 1 millisecond and pulse power ability makes them suitable for power quality applications as
well (29). Depending on frequency and depth of discharge the life expectancy for NaS batteries lies
between 10 to 15 years with an estimated 2 500 lifecycles. They have a relative high power and energy
density demanding less space and leaving a smaller footprint. However, NaS batteries are still an
emerging grid scale storage technology with high cost estimates as they despite being officially
categorized as commercial still are in the early stages pf development.
An alternative to NaS as a molten salt battery is the sodium nickel chloride battery which has been
commercially available since 1995. The main focus of research has been around applications for vehicles
but other applications are possible. With response times in the millisecond range and round trip
efficiency of 85-90% in addition to higher tolerance to overcharge and discharge as well as higher cell
voltage the ZEBRA, as they are also called, is an alternative with potential.
5.2.6 Flow batteries
Unlike both molten salt and conventional batteries because of their construction, shown in Figure 14,
flow batteries are the subcategory of batteries best suited for energy applications requiring discharging
for longer than 5 hours.
- 24 -
Figure 14 Schematic of a flow battery. The electrolytes are stored in external tanks and are pumped to the electrodes
in the cell stack where they interact to generate current and voltage. The catholyte is the positive electrolyte and
anolyte the negative electrolyte. (29)
To the disadvantages of flow batteries count the initial cost and complexity of construction due to
plumbing, tanks and other non-electrochemical components which also increase probability of repair
cost and electrolyte leakage. The supporting equipment used to pump electrolytes from tanks to cells
decrease overall efficiency due to losses. Although their ability to increase energy capacity by simply
installing larger electrolyte tanks lower energy and power density than for conventional batteries are a
restriction. The greatest general advantages of flow batteries is their low energy capacity cost and ability
to independently vary energy and power capacity to efficiently suit intended application (29). This
makes flow batteries ideal for storing large amounts of energy from e.g. wind parks over longer time
periods.
Two of the most promising flow battery technologies are vanadium redox flow batteries (VRB) and
zinc bromine flow batteries (ZnBr). The development of VRB dates back to the 1970’s when NASA
began to work on redox battery technology. Since then they have been use for various energy storage
applications around the world. The electrolyte of VRB consists of vanadium ions dissolved in an acid
aqueous solution. The anolyte and catholyte being the same electrolyte but with negatively and
positively charged ions is separated by an ion exchange membrane in the cell where they react with a
carbon felt electrode generating current. The zinc bromine flow batteries have a zinc covered electrode
which during discharge is dissolved into the bromine electrolytes which are separated by microporous
membrane. Charging the battery deposited zinc back onto the electrodes. The electrolytes differ only in
the concentration of dissolved elemental bromine. (29)
The efficiency for both types of flow battery are good, ranging from 70 to 80% for ZnBr. The overall
efficiency for flow battery systems is however lower due to parasitic losses and heat dissipation
depending on system design which gives the systems efficiencies around 60-65%. Life time
expectancies depending on application are high, often over 10 000 cycles giving 10-15 years when
- 25 -
cycled 1 000 times a year. Response times for the cell on their own is very short, less than 1 millisecond.
Due to longer response times of pumps and power electronics the response time is prolonged to around
5 milliseconds which still places flow batteries in the same class as other electrochemical storage
technologies. Due to their low energy density flow batteries require more space than other battery types.
VRB having existed on the market longer than ZnBr are considered as more mature and more
extensively tested. Due to uneven buildup of zinc on the ZnBr electrode the cells need to be fully
discharged every 5-10 cycle whereas VRB do not require such maintenance procedures. Otherwise flow
batteries are almost maintenance free with pumps and plumbing being the major source of unreliability.
(29)
5.3 Choice of storage technology
From the previous section it is clear that when designing a BES it is not only important to know what
the required power and energy output are but also what battery technology suits the application best.
Here factors like present cost and efficiency seem most important as it is the capital and operational
costs that determines the feasibility of the storage concept. It is although also important to not to forget
that many battery technologies are still in development phase with possible future cost and operational
improvements. While there exist some storage technologies that have been proven to work well, i.e.
pumped hydro and compressed air energy storage, batteries are considered to have the broadest range
of application with the greatest advances in development (4). Many battery technologies are sufficiently
commercial with beneficial cost development on both the short and long term perspective. There are a
few factors to consider when choosing among the different battery technologies; technical maturity,
scalability and development of cost and market.
The technical maturity is a measurement of how much of the potential technical improvements have
been realized. New technologies generally have a greater potential for improved performance and costs
but they also come with a greater uncertainty for estimations of cost.
Figure 15 shows some storage technologies in comparison to their anticipated research and development
cost. While it is important to have technical maturity in mind such comparison can be biased as it refers
to the application for smart grids. While NaS batteries have been deployed already, other technologies
such as lithium ion are still in demonstration phase when it comes to grid energy storage applications
and therefore described as immature. However NaS have almost exclusively been manufactured for grid
storage demonstrations only and production volumes are low. Lithium ion batteries on the other hand
have large production volumes and a commercial market due to automobile and consumer electronic
applications
- 26 -
Figure 15 The level of technological maturity against the anticipated R&D investment cost for a variety of energy
storage technologies. The left end of arrows indicates current status. Right end of the arrow indicates the estimated
maturity level in 2020. (29)
The scalability of a storage technology indicates how the unit cost is affected by the unit size where
some technologies show a decreased cost for larger units. This is seen in wind power production where
a MW sized turbine can have up to 5-10 times lower costs than a kW sized turbine. For PV systems,
bigger installations have no cost advantage leading to a more small installations as the cost power square
meter is almost the same. A similar development is expected for some battery technologies as the BES
are composed of a large number small cells. This is especially true for lithium ion batteries as the small
cells often are the same as those used in present automobile and consumer applications there are low in
cost per kWh due to large production volumes (4). A study performed by EPRI in 2010 (33) it was found
that some storage technologies such as compressed air energy storage and pumped hydro only fit bulk
energy storage while other, especially battery technology, can be utilized in a broader range of systems
sizes.
In general batteries do not show good scalability as sizing power and energy up means to increase the
number of cells connected in series and parallel. Flow batteries being similar to fuel cells can be seen as
one large cell with exchangeable and refillable electrolyte whose energy capacity is limited by the
electrolyte volumes. By increasing the size of the electrolyte tanks the energy capacity can be scaled to
suit the application.
Because the cost is the determinant factor in the design of a storage system the cost development of
battery technologies is important to understand. Increased production volumes most times lead to lower
costs for the ready-to-use system solution. Annual investment trends analytics, Bloomberg New Energy
Finance (BNEF), estimate the cost of lithium ion batteries to decrease at a similar rate as PV panels have
had according to their latest studies. This trend predicts in annual price reductions of more than 20%, as
shown in Figure 16.
- 27 -
Figure 16 Lithium-ion EV battery experience curve compared with solar PV experience curve (2)
As the same battery cells used for EV applications are used for energy storage only in larger amounts
the cost of a BESS is assumed to decrease accordingly. A meta-analysis performed in a joint project
between the University of Melbourne and IBM Research Australia (34) confirms the trend and
prediction of the future decrease in battery pack costs as can be seen in Figure 17.
Figure 17 Cost predictions for full automotive lithium ion battery packs (34)
The forecasts of decreasing prices for battery packs are supported by the automobile industry with
several manufacturers including Nissan offering a change of battery packs for their EV at a cost of 5500
USD. This corresponds to a battery cost of 230 USD per kWh for their 24 kWh battery pack. The cited
prices are for EV battery packs only. For a complete BESS costs for measurement equipment, converters
and PLC’s are added to the total price. For a home energy storage unit in the kWh-range a simple rule
of thumb is to duplicate the price for the battery pack to get the total price. When installing a BESS in
an existing PV system some of the equipment can already present reducing the cost for the added storage
unit.
- 28 -
The actual and forecasted price development for a small scale home storage unit is shown in Figure 18.
From the curves it can be seen that there is a delay in cost compared to EV batteries which today cost
around 400 €/kWh giving the cost for a home energy storage system according to the rule of thumb of
800 €/kWh which here is the expected price in 2016. The delay can be explained by the relative small
production volumes of today but the price is expected to decrease by an annual 20% which would results
in a price near 400 €/kWh in 2020. (4)
Figure 18 Price forecast for a 6 kWh lithium-ion home storage system (35)
- 29 -
Table 1 Comparison between common solid state battery technologies. Data from (29) (31) (36) (37) (38)
Specifications
Lead acid
Nickel electrode
NiCd
Wh/kg
Cycle life (80% DoD)
NiMH
30-50
45-80
60-120
80-140
200-300
1000
300-500
5000-6000
Typical lifetime(years)
3-15
Recharge time (hours)
8-16
1
5%
2V
Self-discharge/month (room temp.)
Nominal cell voltage
Operating temperature (°C)
Round trip efficiency (%)
Maintenance
§
Cobalt
ZnBr
20-30‡
60008000
Manganese
Phosphate
30-45§
150-190
100-135
90-120
1500-2500
3500-8000
3500-8000
3500-8000
8-15
8-15
8-15
2-4
9
≤1
4
2-4
≤1
≤1
20%
30%
62-70%
58-68%
62-70%
< 10%
< 10%
< 10%
1,2 V
1,2 V
2V
1,41 V
1,85 V
3,6 V
3,8 V
3,3 V
70 to 80
70-90
Very high
VRB
5-10
70-80
Thermally stable
Li-ion
10-20
300-360
Topping charge
Flow batteries
12-20
-20 to 65
Safety requirements
‡
15-20
-20 to 50
every 3-6 months
Toxicity
NaS
Discharge
Discharge
every 30-60
every 60-
days
90 days
Thermally stable, fuse
protection
Very high
Not
required
Avoid
contact
with water
Low
Wh/L electrolyte
Wh/L electrolyte
- 30 -
High
-5 to 55
60-65
60-65
-20 to 60
85-92
Leakage control
Special care during
decommissioning,
High
85-92
85-92
Not required
Protection circuit mandatory
Low
Low
Low
6. Energy storage applications
This chapter presents and explains the principles of possible applications for energy storage systems.
First the applications for the explicit integration of renewable distributed generation are presented,
followed by additional applications for grid support and energy management. The concept of application
synergies is discussed as well as placement and auxiliary components conclude this chapter.
6.1Renewable energy integration with BES
This section presents the application explicitly targeting increased integration of distributed renewables.
6.1.1 Renewable energy time shift/spinning reserve
Bulk energy storage is the storing of large amounts intermittent electricity when it is produced in order
to utilize it during low production periods i.e. storing solar energy during the day to use it in the evening.
The stored energy can be discharged for different applications. One possible application is to charge and
discharge the storage unit to follow the system load, so called load following. The storage unit is charged
during net surplus of renewable energy production and discharged when demand is higher, as shown in
Figure 19. As the net surplus from wind production often peaks during night time the electricity is either
exported or the wind production is curtailed. Charging storage units with the surplus energy to fill the
demand gap during low production and high demand has two benefits. First, power production peaks
are reduced, thus leading to a better capacity factor and relieving of the grid infrastructure. Second, the
needs to curtail and regulate wind power are greatly reduced, releasing hydro power from its regulator
role in the energy system and strengthen its role as base power generation.
Figure 19 Wind energy time shifting with energy storage (39)
- 31 -
Another application of bulk energy storage in smaller scale is the combination of a residential PV system
and a storage unit for home storage. The energy storage is charged with the net generated solar energy
from the PV system during the whole day, or depending on the storage size with the energy produced
during mid-day, to reduce peak power. This is sometimes called energy time shift. The stored energy
can be used to completely or partly cover the residential electricity demand or reduce its peak demand.
Here the storage size is determined by the size of the PV system and the energy demand to be covered
alternatively the peak load.
6.1.2 Capacity firming/Ramp support/forecast hedging
The variable power output from renewable power plants such as solar or wind can be maintained at a
stable level for a period of time by smoothing the output and controlling the ramp rate (MW/min). This
is called capacity firming or ramp support. Capacity firming generally requires more storage capacity
than ramp support as its main intent is to allow intermittent electricity supply resources to be used as
near constant power source. Reducing power output fluctuations from wind power and keeping power
production at a stable level also decreases the need to regulate with hydro storage making it possible to
produce more renewable energy. Figure 20 illustrates how storage can be used to level the power output
from distributed renewables. In order to regulate intermittent wind power balance responsible parties
usually predict the power production through wind forecast which are based on weather conditions and
stochastic methods. Even though forecasts predict the bulk of the power generation from wind some
variations in peak production can still occur. By smoothing out the power output from the wind turbines
the gaps in actual and forecasted are reduced which sometimes is called forecast hedging.
Figure 20 Capacity firming of solar (left) and wind power (right) generation with energy storage (39)
Ramp support is the light version of capacity firming where the changes in power output over short
durations from intermittent renewables are smoothened to decrease frequency fluctuations and other
power quality issues. By damping the power output the variability of the production is greatly decreased
thus reducing the risk for grid frequency fluctuations and rapid voltage swings. An example of using
energy storage for ramp support of a 30 MW wind park near Oahu, Hawaii, is shown in Figure 21.
- 32 -
Figure 21 Ramp support of a 30 MW wind park during normal operation (40)
6.1.3 Voltage support/Voltage stability
As described earlier, active power injection to a feeder can cause voltage raises above the allowed limits.
The voltage rise can be expressed by (1) and (2)
∆𝑈 =
(𝑅∙𝑃+𝑋∙𝑄)
𝑉𝐺
𝑃 = 𝑃𝑃𝑉 − 𝑃𝐿
(1)
(2)
,where R and X are the cable resistance and reactance at a distance from the transformer, P and Q are
the active and reactive power which is exchanged with the grid at the generator point of connection, VG
is the voltage of the grid. PPV and PL are the generated power from a PV system and the load power
respectively. (41) When the active power output from a PV inverter without reactive power control the
voltage rises and high enough overvoltage forces the PV system to disconnect itself from the grid. This
leads to losses in production and hence lower efficiency and economical value for the owner. By
connecting a BESS to a point in the grid where overvoltage can become problem a voltage support
algorithm controlling the storage unit can mitigate the critical voltage rises or drops. By only absorbing
or injecting minimal amounts of real power from a storage system the voltage fluctuations can
effectively be damped. (42). As the voltage fluctuation is proportional to the cable resistance which
increases the further the connection point is located from the secondary substation, voltage support is
especially important in long radial feeders. A PV system in combination with a storage unit can thereby
reduce the effects of unacceptable voltage excursions like voltage dips and overvoltage on neighboring
customers (41). Voltage support can also be provided by feeding reactive power into the grid. A voltage
support algorithm can control the batteries reactive power output by measuring the output voltage and
- 33 -
feed reactive power into the grid. Several suppliers of PV inverters offer equipment with reactive power
control which lessens the need for curtailment (43) (44). However, a rethinking of solutions for voltage
support including storage technology is necessary as the amount of reactive power for voltage support
will increase as PV penetration increases. To avoid increased power losses due to higher amounts of
reactive power voltage support by reactive power compensation must be located only where it is needed
to avoid increased network losses.
6.1.4 Increasing hosting capacity
Residential scaled solar power generation can be managed by installing relative small storage units
behind the energy meter as a part of the customer’s private energy system. This solves possible problems
with local voltage stability before they develop. Large scale wind parks and solar power plants are
connected to the grid at MV level, challenging the grid infrastructure and operators when low
consumption creates a surplus of power in the grid causing overloading of lines and transformer etc. An
energy storage which absorbs occasional peaks in renewable power production could defer grid
infrastructure investments and reduce the need for curtailment. The question is how large such a storage
system would have to be in order to increase the hosting capacity without requiring immense energy
capacities. A study discussing the use of electrochemical energy storage (45) hypothesized that the
amount of storage capacity needed to store and adjust the power output from a 1MW solar power plant
to electricity demand at 100% efficiency, requires between 12,5 to 25 MWh of energy capacity. The
equivalent for wind power is double the size, from 47,5 to 57,5 MWh. Installing such large storage
capacity cannot be considered economical feasible.
The authors of another study investigating energy storage as a mean to increase integration of wind
power (46) stated that combining energy storage with curtailment could drastically reduce the required
energy capacity of the storage. In the study, an energy storage alone storing all overproduction caused
by 11 MW added generation capacity would require 160 MWh of storage which would not be
commercially feasible. The combination of storage and curtailment required only a 1/40 of the required
size in order to prevent 100% overloading. They concluded that the storage should be dimensioned
according to the phenomena “diminishing returns per unit of energy storage capacity”. This means that
for energy storages exceeding this size it quickly becomes less attractive to install more energy capacity
in respect to the benefit. The same conclusion was drawn in a study investigating the sizing of ES in
micro grids (47). The benefit for the cited study was overloading of grid infrastructure due to increased
peak production from added wind power in a Swedish grid. The solution was to combine a storage
dimensioned to take care of the overloading up to a certain level with a low level of curtailment of the
- 34 -
added wind power production to prevent overloading. The result was in total 86 GWh additional energy
power per year with 4,7 GWh curtailed. (46)
6.2 Other energy storage applications
Energy storage is no limited to applications intended for the direct integration of renewables. Grid
operators and end customer can find several benefits from using energy storage to defer investments or
reduce cost in form of grid losses and energy costs.
6.2.1 Arbitrage/electric energy time shift
The concept of arbitrage takes advantage of price variations on the electricity spot market. The BESS
can be programmed to charge its batteries when the spot price is low and discharge when the price is
high. The total revenue is simply the difference for the cost of charging and value of the electricity that
is dispatched back to the grid. For example, if the spot price for electricity is 210 SEK/MWh between
02.00 and 04.00 AM the storage is charged during that time period. If the storage capacity is 2MWh and
power factor is 1 MW the price for the energy charge is 420 SEK. During peak demand hours in late
afternoon the spot price can be around 250 SEK/MWh and energy in the amount of 2MWh during that
time period costs 500 SEK. If the consumer instead uses the energy coming from the storage that was
charged during the night the worth of those 2MWh is 500 SEK instead of 420 SEK yielding savings of
80 SEK. This scenario assumes 100% AC-to-AC conversion. Real BES have about 80-90% AC-to-AC
conversion efficiency and thus the real revenue will be less. The values are from an actual day on the
Swedish electricity spot market and are taken as an example for this application, see Figure 22.
Figure 22 Spot price for the regions SE1, SE2, SE3 and SE4. a) Shows the price variations during a 24h time period
for the 19th of February 2015. b) Shows the price variations during a 7 days between 10th of December and 16 of
December 2015 (48)
- 35 -
Arbitrage can theoretically be an application for both utility customers with residential energy storage
and DSO’s with community or distribution support storages. The ownership is discussed in section 7.1.
6.2.2 Demand charge reduction
A battery energy storage system used to store solar energy form a residential PV system will be idle
during days when there is little or no solar irradiance. By using the storage unit to provide power during
the households peak demand it can save money and at the same time lower the overall load on the grid
during periods of peak demand. Even a home without micro generation can benefit from the use of an
energy storage system that lowers peak demand as the electricity bill often has a peak demand and
energy charge. The energy charge depends on the amount of energy the customer consumes over a
month and is expressed in a fee per kWh. The peak demand charge is calculated differently depending
on the utility’s tariffs** but it often takes one or the average of several peak power measurements during
one month and charges thereafter, expressed in a fee per kW. Subsequently, the occasion with the highest
monthly power draw determines the demand charge. A reduction of the maximum power draw thus in
combination with arbitrage reduce the electricity bill accordingly.
This method obviously requires the end customer to be charged by tariffs based on power charge rather
than or in combination with energy charges. Grid owners design the peak demand charges to cover their
expenses of the grid. Lower peak demand would mean less income possibly making it less attractive to
support for utilizes. However, as the grid infrastructure is designed for maximum peak demand a
reduction would mean lower requirements for future investments and possibly deferring infrastructure
investments in existing grids.
6.2.3 Customer power reliability and quality
Power cuts in distribution networks are relative common and can occur during storm season or be caused
by construction and restoration of grid infrastructure. While residential end-users are not as sensitive
many industrial utility customers rely on uninterruptible power supply as even short power cuts lasting
a few seconds can become very costly for production (49). Utility companies are responsible for paying
customers for in the incident of power cuts lasting longer than 12 hours (50). Energy storage can be used
to bridge power outages of extended duration ranging from a few second to a few hours depending on
storage energy capacity.
**
For Umeå Energi Elnät AB a common tariff for peak demand charge is based on the two highest monthly peak
draws of which the average determines the peak demand charge. The peak demand determines the fuse size by
which the customer is charged
- 36 -
Storages providing power reliability are well suited to also provide power quality services as they are
constantly online. The electric power quality applications uses stored energy to protect on-site loads
against short duration event that affect power quality. These events include:

Low power factor (voltage and current out of phase)

Variations in voltage magnitude

Frequency variations

Harmonics (the presence of voltages or currents at other frequencies than primary
frequency)

Power interruptions
Energy storage requirements for power reliability and quality are high quality power output with suited
power electronics capable of providing power for the needed duration. Needless to say, ES for customer
power reliability and quality are best placed near or within the customers’ facility. (39)
6.2.4 Supply capacity/spinning reserve
Whenever electric demand in a grid increases there need to be generation capacity to provide electricity
at the same moment. Utility companies, having the responsibility for electricity supply to the customers,
have stand by power plants ready to produce electricity when peak capacity is needed. This supply
capacity is either owned or rented by the utility and usually consists of a fossil fuel or biomass-fired
combined power plant. Energy storage units could be used to reduce the need for owning such facilities
or renting generation capacity from the wholesale market. Depending on annual hours, frequency and
duration of operation different operating profiles can be specified for a storage used as supply capacity.
Discharge durations depend on flexibility, if the capacity is priced per hour or fixed during certain hours
of the day. There are three types of supply capacity;
Spinning reserve – Power generation capacity that is unloaded but online so it can respond within 10
minutes to compensate for outages in the distribution or transmission network. They are the first reserves
that are used in case of shortfall and can also be used for frequency regulation by being synchronized
with the grid frequency.
Supplemental reserve - Power generation capacity which is offline but can respond within 10 minutes
and is used after all spinning reserves are online. Supplemental reserves can also include blocks of
curtailable interruptible loads.
- 37 -
Backup supply – Serves as backup for spinning and supplemental reserves and can pick up load within
one hour.
Storage for the use of energy supply has to be reliable and being able to respond to appropriate control
signals from DSO or TSO. Also the storage capacity must be enough to serve as a reserve for the required
time can usually last for at least one hour.
6.2.5 Load following and area regulation
Load following is generally done by ramping conventional power plants (generation supply) up and
down to follow the local and system energy demand (load) and is one of the ancillary services required
to operate an electrical grid. The power output from generation is varied every several minutes to
respond to changes in grid frequency and timeline loading to maintain the system frequency and
established interchanges with other areas within predetermined limits. During load following generation
supplies vary their output between base load or being offline (depending on if they are base generation
or spinning reserves) and their maximum design output. Operating at part load relative to design output
results in increased fuel consumption and air emissions (39) (51). Running base load generation at design
output complemented by storage could increase load following energy efficiencies if the storage
technology is sufficiently good. Storage units can be online and operational any time of the day and can
provide double capacity because it can stop charging and start discharging at the same time when
following the load up. Similarly it follows the load down with double capacity by stop discharging and
beginning to charge. A simple illustration of load following is shown in Figure 23 Load following with
regulation
- 38 -
Figure 23 Load following with regulation (39)
Area regulation also counts as ancillary service and works much like load following but at a smaller
scale by “interchanging flows with other control areas to match closely the scheduled interchange flows”
and managing momentary variations in demand within the control area (52). Energy storage is especially
well suited for area regulation for the same reason as it is well suited for load following. Storage has a
superior part load efficiency with only conversion efficiency as drawback. Thereby efficient storage
systems can provide two times its name-plate capacity for regulation. The response time is generally
fast†† and power output can be varied rapidly form none to full or vice versa in the matter of seconds
while conventional power plants might require several minutes to ramp down or up generation. (39)
Regulation services are sometimes referred as simply power regulation or fast regulation with a time of
discharge duration which varies from about 15 minutes to one hour depending on regulation
requirement.
6.2.6 Voltage support
An important ancillary service for grid operation is voltage support and management. Maintaining the
necessary voltage levels and voltage stability for efficient grid operation requires management of a
phenomena called reactance. Equipment that generates, transmits or uses electricity often has
characteristics of inductors and capacitors causing reactance. Voltage support offsets reactive power to
maintain or restore system voltage of the grid. As reactive power transmitted over long distances
††
Some developers of energy storage claim that storage systems are better at fast regulation than
conventional load balancing provided by combustion and hydro power plants. (76)
- 39 -
increases power losses, voltage support is a particularly attractive applications for distributed energy
storage as it is placed close to load centers where the most reactance occurs. Storage units equipped with
power electronics capable of reactive power (VAR) support can inject reactive power where and when
it is needed. Effective methods of VAR support provided by energy storage in combination high PV
penetration causing voltage rises have been investigated (41) (53). Placing energy storage with smart
PV inverters capable of VAR support close to the end of feeders or at load centers was found to be
effective while keeping the storage unit operational for other applications which increased their value.
6.2.7 Transmission support and frequency regulation
Electrical anomalies and perturbations such as voltage dips, poor voltage stability and sub-synchronous
resonance decrease transmission and distribution system performance which can be improved by
transmission support provided by energy storage. By compensating the anomalies and disturbances by
means such as voltage control, sub-synchronous resonance damping and load shedding reduction,
system performance can be improved. This requires high reliability, communication and control as well
as partial state-of-charge operation with many charge cycles. An energy storage for transmission support
cannot be used for other applications simultaneously as it needs to be available for operation whenever
the unstable situations arise.
One of the most promising applications for energy storage is frequency regulation (54) (55). Similar
to using hydro power or combustion power plants energy storage can contribute to balancing load and
consumption in any part of the grid. The energy storage can contribute in frequency regulation by
delivering active power when the grid frequency is dropping below a certain lower threshold and charge
when the frequency is above a certain upper threshold. The amount of energy charged and discharged
is proportional to the frequency deviation and can vary from nearly 0 to the battery’s maximum power
capacity. (55). Compared to conventional power plants which usually have slow response times,
providing frequency regulation can take several minutes while some methods using BESS can deliver
the same service in milliseconds. (56)
A study carried out by FEAB (57) tested the frequency regulation capability of their pilot battery storage
where the storage would charge or discharge upon measuring the frequency of the grid it was connected
to in order to correct it. The upper and lower limits for charging and discharging were 50.01 Hz and
49,97 Hz respectively. It was concluded that the active power output of the ES could be controlled by
the frequency of the grid and thus could be used for keeping the right frequency.
A predominant future role of energy storage systems in energy systems with high penetrations of
intermittent renewables is to provide means for grid stability. When more and more solar and wind
- 40 -
power replaces conventional power generation much of the inertia from large electrical generators is
lost while introducing more disturbances due to intermittency. In order to mitigate reduced power
stability energy storage systems can provide inertia support through frequency regulation. This can be
done by either installing large flywheels or battery energy storages with pulse power abilities. (58)
6.2.8 Transmission congestion relief and upgrade deferral
As electric network usually have a long payback time, areas which they serve can undergo considerable
changes in increased population and electricity demand causing transmission congestion during peak
demand periods. Many times the number of days when peak demand causes congestions is low as can
be seen in Figure 24. This poses the issue of whether upgrade of the congested transmission or
distribution infrastructure is profitable or not.
1,8
1,6
Power [MW]
1,4
Grid dimensioning point
1,2
1
0,8
0,6
0,4
0,2
0
0
1000
2000
3000
4000
5000
6000
7000
8000
Hours
Figure 24 Load duration curve for electricity consumption during one year. (59)
Often the amount of energy casing congestion is only a fraction of the total annual energy demand
making infrastructure a costly solution. With an energy storage unit providing the peak power during
high demand periods the need to upgrade infrastructure is deferred and increased overall cost to rate
payers avoid. The amount of the savings by this method or if storage even would be an option varies
from case to case and needs to be evaluated for each scenario.
- 41 -
6.2.9 Peak shaving to minimize grid losses
One of the more straight forward applications for energy storage is peak shaving to reduce losses in the
grid. In the grid losses come from shunt losses and series losses. Shunt losses, U2G where G us the shunt
conductance exists in e.g. transformers. The storage can reduce the shunt losses by reducing the voltage
with active or reactive power as mentioned in the previous section. However the potential gains will be
small as these losses can only vary over a small range. (19) Series losses, I2R where R is the series
resistance exist in e.g. transmission lines. The series losses, in average, can be expressed as:
𝐼2 𝑅 =
(𝑃2 +𝑄2 )
𝑅
2
2
𝑅 = 2 [( 𝑃 ) + 𝜎𝑝2 + 𝑄 ]
2
𝑈
𝑈
, where U is the grid voltage, I is the current, R is he series resistance, P and Q are the active and reactive
powers respectively and σP is the standard deviation of the actual power from the average power which
can be defined as:
2
𝜎𝑝2 = (𝑃 − 𝑃)
The losses due to series losses can be divided into three parts. The first term in the simplified equation
are losses due to the net energy transfer. These are proportional to the square of the average transferred
energy. As the active power is the useful energy transferred in the grid this term can only be altered by
adjusting electricity consumption and generation. The third term describes the losses due to reactive
power which can be compensated by electronic converters of the storage or other components like
capacitor banks and PV inverters with reactive power control.
The second term describes the losses due variations in the transferred energy and are proportional to the
square of the standard deviation of the active power. A large variability therefore leads to larger losses
and needs to be smoothened. The smoothening can be done by storing the highest portion of the loads
which exceeds the transformer capacity. This process is also referred to as “peak shaving” and increases
the hosting capacity of the grid by reducing the standard deviation of the power flow. The storage is
controlled such that the maximum current during overloading is minimized and the stored energy is
dispatched to fill the valleys, as shown in Figure 25. The storage is charged when the load reaches the
hosting capacity and discharges the energy during low power flows by increasing the lowest current.
- 42 -
Figure 25 Optimized charge and discharge of ES for peak shaving (19)
Some simulation studies, (60) (61) have shown that the use of storage systems with a peak shaving
algorithm can reduce line and transformer losses in both MV and LV radial feeders by up to 3 percent.
For wind power peak shaving the procedure stated above combined with reactive power compensation
and quick discharge can reduce the losses by up to almost 8 per cent. (62) As the losses are reduced
more intermittent power generation can potentially be integrated into the grid.
However, as found in (19) and subsequently in (62), storage conversion losses were several times higher
than the reduced losses. As mentioned in section 5, storage conversion efficiencies can vary between
70-90% with relatively large losses as a result when the storage is handling large amounts of energy
which can be the case for wind power peak shaving. In the case for the studied grid‡‡ in (62) an energy
storage with the sole purpose of reducing losses would not be possible due to its internal losses.
6.3 Simultaneous energy storage applications
Energy storage can have more applications than the integration of renewable energy production. The
commercial feasibility of a storage unit is in fact increased by utilizing it for several applications which
have synergies. Finding ways to benefit from a storage unit may it be in a residential energy system
behind the meter or at transmission network level can yield several system benefits.
Energy storage for the integration of renewables can be used for other applications while they are not
providing their intended service. Bulk energy storage that shifts generated renewable electricity can
‡‡
This study was part of a joint project between the STRI AB and Falbygdens Energi AB, the owner of the
energy storage mentioned in section 6.2.7 (57).
- 43 -
provide ancillary services like voltage stability and frequency regulation by utilizing only small amounts
of the store energy.
The combined applications may justify investment in a BESS by adding the financial benefits from each
application. Table 2 shows the estimated market values for energy storage applications based on
Electricity Energy Storage Technology Option: A White Paper Primer on Applications, Costs, and
Benefits published in 2010 by the Electric Power Research Center (EPRI) (33). The target values
represent an average value for stakeholders who might consider investing in energy storage while high
values represent the value for niche markets that place a particularly high value on the benefits from ES
applications and deem those as very important. Each benefit is modeled in isolation using a battery of 1
MW power capacity and 2 MWh energy capacity, with a 15 year life and 10% discount rate.
Table 2 Relative comparison of present values for the benefits from ES applications. Target values represent an
average value for commercialization for the broader market. High values represent the storages value on niche
markets where a high value is placed on that particular application. (33)
From this table it is clear that regulation services for the electricity market and deferral of grid
investments are the most profitable applications of grid energy storage. The driving incitements for this
kind of energy storage will most likely be the applications with the largest monetary benefit. The report
states that the tabled value are site specific and can vary depending on the actual site. The values can
- 44 -
also not simply be added together but must be weighted according to interoperability, meaning how one
applications requirements on operation affect others. Some application require the system to constantly
charge and discharge with varying SOC as result where other applications require full storage capacity.
This adds another dimension to the planning of energy storage systems in order to gain their full
potential. In Table 3 some characteristics for storage applications are shown.
Table 3 Standard assumptions for energy storage applications. (39) (33)
Application
Desired
Size*
Duration
Cycles*
Renewable energy time shift
1 kW- 500 MW
3-5 hours
> 4000
15-20 years
Renewable capacity firming
1 kW- 500 MW
2-4 hours
5000 (10 000 full
15-20 years
lifetime
energy cycles)
0,2 kW – 500MW
10 sec. – 15 min
> 4000
15-20 years
1 kW- 500 MW
1-6 hours
< 50/year**
15-20 years
Load following
1 MW – 500 MW
2- 4 hours
N/A
20 years
Area regulation
1 MW – 40 MW
15 – 30 min.
N/A
20 years
Spinning reserve
1 MW – 500 MW
1-4 hours
N/A
20 years
Voltage support
1 MW – 10 MW
15 min. -1 hour
> 4000
20 years
Transmission support
10 MW – 100 MW
2-5 sec.
300-500/year
15-20 years
Transmission congestion
relief
T&D upgrade deferral
1 MW – 100 MW
3-6 hours
300-500/year
15-20 years
250 kW – 5 MW
3-6 hours
300-500/year
15-20 years
Renewable voltage stability
Increase hosting capacity
Time of use energy cost
management
Demand charge management
1 kW – 1 MW
4-6 hours
150-400/year
15 years
50 kW – 10 MW
5-11 hours
400-1500/year
15 years
Power reliability
0,2 kW – 10 MW
5 min. – 1 hour
<50/year
10 years
Power quality
0,2 kW – 10 MW
10 sec- 1 min.
<50/year
10 years
* Depends on type and scale of renewable energy source. Residential PV requires much less storage capacity than
wind power parks
** Based on the assumption that overloading due to surplus production occurs only a few times per year, from (46)
It has to be added that some applications are relying more on pulse power abilities than long discharge
durations and the opposite can be the case for other applications. In general it can be stated that energy
storage applications with short discharge durations are power applications requiring pulse power ability
while applications with long discharge duration are energy application with energy capacity as the most
important quality.
Due to application synergies which lets several applications be implemented for one storage unit
without impeding on each other’s benefits it would be possible to maximize the use of energy storage
- 45 -
and include integration of renewables. While considering varying technical and operational needs for
different storage applications, with some shown in Table 3, there are several possible combinations that
can become attractive in the near future. Notable synergies are renewables energy time shifting and
capacity firming which may reduce grid effect caused by volatile wind power output or cloud induced
voltage dips from PV generation. Those applications also pair well with several others including voltage
support, demand charge management and grid investment deferral. They are less compatible with load
following and regulation services as the SOC of the system needs to be sufficient for the intended use.
A more comprehensive visualization of all application synergies is shown in Appendix C – Application
synergies.
6.4 Placement of storage units
ES intended for ramp support and capacity firming are by intuition best located at the connection point
for renewable generation in order to mitigate intermittency and provide synergy applications. As the
feasibility of an energy storage is highly depending on investment costs, the optimal placement giving
the most control with minimal storage capacity is pursued. It can seem plausible that energy storage
units intended for renewables integration should be placed near the perturbations i.e. the connection
points with volatile power output. However some studies investigating the matter have found that this
might not always be the case.
In a computational study of a grid with high renewables penetration (63) the results pointed out that
nodes with the highest degree of control over congestion not necessarily have to be the nodes with
renewable generation connected to them. In one case only one of the ten nodes giving the highest degree
of control was a renewable site. Another study investigating optimal storage placement or different
storage budgets (64) concludes that optimal placement of the storage is more dependent on the network
structure than where renewables are connected the grid. This holds for situations where no line-flow
limits are present while such limits (causing congestion) have a significant impact on storage placement.
When planning deployment of an energy storage it is therefore reasonable to invest time in
computational assessments to find out the optimal location as it could reduce capital costs.
- 46 -
6.5 Auxiliary components
6.5.1 Power inverter
For the appliance of a BES to power AC-driven electronic equipment or grid connection the DC power
from the battery need to be converted to AC by a power inverter. The inverter transforms electricity so
that it has the necessary voltage and power factor. Together with the battery´s control system the storage
output is synchronized with the oscillations of AC power on the grid. For the opposite application,
charging the battery with grid electricity a inverter converts the grids AC power to DC power needed
by the storage. Power generated by a PV systems, which is DC output, also need to be conditioned to
meet the storage´s voltage level. (39)
6.5.2 Battery management system
A major concern about batteries is their cycle life and how reduced cell states affect their performance.
All batteries have a maximum intrinsic cycle life which is based on their composition and chemistry
which varies wildly among different types. What all batteries have in common is that their cycle life is
heavily depending on which conditions they are operating in. Too high or too low ambient temperatures
or inhomogeneous cell voltages across the battery pack but also deep charging can degrade the batteries
state of health. Exceeding the batteries operating conditions can also lead to cell failure and ultimately
battery system failure. Figure 26 shows six failure regions caused by cell voltage and temperature in a
graphite anode lithium-ion battery system.
Figure 26 Failure regions for graphite anode Li-ion batteries. Failures are a combination of cell voltage and
temperature deviating from optimal values. (65)
In order to counteract battery degradation and increase operation safety the BESS requires a battery
management system (BMS). The BMS measures cell voltage, current, temperature and in case for flow
- 47 -
batteries the electrolyte ion concentrations. The BMS ensures safe operations of the BESS by controlling
thermal management, current and voltage limits and fault detection and shutdown. It also determines
the SOC and time left for the applied load profile. In enclosed battery systems such as lithium-ion the
BMs also balances cell voltage across all cells in the system to avoid failure and uneven discharging.
6.5.3 System supervisory controller
The BESS is controlled by system supervisory controller (SSC) which basically is a programmable
logic controller, a computer. The SSC controls charging and discharging patterns according to a perdetermined load profile or any desired application which may require additional input e.g. grid frequency
or PV power output. The SOC, battery temperature and cell voltage and current are given by the BMS.
The SSC analyses those inputs and if needed alters the charging/discharging current and ambient
temperature to protect the cells from internal cell degradation and fading capacity.
- 48 -
7. Regulations and frameworks
With a large number of studies investigating and confirming the values of electrical energy storage for
grid applications and renewable integration the number of actual commitments is low (29) (39) (41) (66)
(19) (61). In some cases legal frameworks are preventing the use of storage for some grid applications
by limiting TSO’s and DSO’s to own energy storage. Utility companies in Sweden are allowed
ownership of ES for applications with the exclusive intent of reducing grid losses, or as temporary back
up in case of power outage (10). Regulations of this kind this prohibits the use of energy storage unit for
other applications by utilities such as area regulations and voltage support. The reason for this case is
primarily the absence of a definition for energy storage in the Swedish Electricity Act making it unclear
whether it is a production unit or not.
Utilities are only allowed to buy and sell electricity on the market under limited conditions which are
by covering grid losses. An energy storage both consume and injects electricity to the grid which
requires electricity trading making the ownership illegal if the profits are greater than the cost to be
covered from grid losses. It is however pointed out that there are no restrictions for utilities to purchase
energy storage application based services by another company as long as the stated rules are followed
(10).
Despite regulatory obstacles for commercial use, various demonstration and test objects exist and are
being deployed introduced energy storage technology in a number of countries around the world. With
different national regulations and frameworks the integration approach of energy storage in the grid
varies. In some countries including Italy and USA (California) the responsibility and incentives for
energy storage integration is placed upon TSO’s and DSO’s. Systems as such provide development
stimulus for central energy storage for load following pointed out by EPRI (33) to most interesting.
Germany and Japan with California in some extent are providing subsidies to reduce the investment
costs for home energy storage units with Germany focusing on combining PV with storage and Japan
exclusively subsidizing Li-ion storage. (4) A brief overview of implemented ES integration policies is
shown in Table 4.
- 49 -
Table 4 BES integration policies for several countries. Reproduced from (66)
California (USA)
Puerto Rico
Japan
China
South Korea
Germany
Italy
Action
Timescale
State mandate for utilities to procure 1325 MW of storage by
Announced in October 2013
2020. 1,62 USD/W “produced” electricity from ES (67)
New renewable generation must include 30% of capacity as
Announced in December 2013
storage
Government subsidies for homeowners and companies to
Announced in March 2014
install batteries and PV paying for up to 2/3 of the price
Current 5-year plan forecasts 25 GW of storage for wind
Announced in October 2011
power
Revision of electricity rates to encourage storage
Announced in September 2013
Subsidy for 30% (up to 3000€) cost of storage associated
Announced in May 2013
with PV
75 MW of batteries for use in transmission and distribution
TSO and DSO are authorized to use
by 2015
batteries under certain conditions
While financial incentives such as subsidies lower the capital cost of ESS making the use of energy
storage implementable the procedure of integrating ES into the grid needs conform to consensus-based
standards. Technical standards provide guidelines for the planning, testing and operation of devices to
ensure safety, reliability and interoperability. Without standards for ES quality and safety of the systems
can be impoverished. As the use of energy storage for grid applications is fairly new there exist no
applicable standards for BESS or the integration of such in Sweden or elsewhere at the moment.
However the discussion of such standards is initiated by International Electrochemical Commission
(IEC) among others (68). The standards discussed include appropriate terminology, testing methods to
ensure the performance of the ES or guidelines for the planning and installation of ES to be used by
power system planners and system integrators. Other important parts to be covered by standards are
environmental issues and safety considerations. Altogether to standards should cover every stage of the
system ranging from already well standardized power converters systems to new innovative types of
batteries.
In order to ensure suitable standards for the Swedish market the future energy storage stakeholders the
participation in the discussion and development of said standards should be of interest for Swedish
industry and government. However some experience with storage systems and integration is needed to
be able to contribute and benefit from such standardization. The best way of gaining that experience is
believed to be by allowing and supporting test and demonstration projects around Sweden. (4) (58)
- 50 -
7.1 Ownerships of energy storage
The utilization of ES is limited by the legal frameworks in force which disallows certain uses for some
owners. Possible owner of an ES can be divided into the grid operator, electricity supplier and third part.
7.1.1 Grid operator
The basic working principle for a grid operator is to provide the grid infrastructure for the transmission
and distribution of electricity much like the Swedish Road Administration (Vägverket) provides
transportation infrastructure. The Swedish Electricity Act, chapter 4, 4§ states that the grid operators
activities may include projection, construction and service of establishments, measurements and
calculation of transmitted power and energy as well as other activities. The term “other activities” could
include energy storage if it is need to maintain reliable electricity supply of good quality. An energy
storage owned by the grid operator for the compensation of power quality and load volatility due to
increased integration of renewables can therefore be justified. However, because a grid operator is not
allowed to own an ES for other uses than reducing grid losses or as backup in case of power outage the
range of applications described in previous sections is limited. As the potential of the ES is limited,
ownership might become commercially infeasible for Swedish grid operators with current legal
frameworks. (4)
7.1.2 Electricity supplier
The electricity supplier is not bound in ownership of ES by the regulatory framework like the grid
operator and is permitted to buy and sell electricity on the regulation market where the greatest profits
are projected. The electricity supplier is permitted to provide power quality and load following services
for the integration of DR which the grid operator might be interested in. The cost of purchased ES
services can be added to the grid operators operating cost and be included in the revenue regulation. (10)
7.1.3 Third part – the aggregator
The business model of selling ancillary services provided by ES in the same way as an electricity
supplier can also be applied to a third part other than the supplier. A third part that owns the ES, buys
and sells electricity and makes profit via arbitrage i.e. utilizes the diurnal price variations on the spot
and regulation market. The role of a so called “aggregator” is defined as an entity which acts on the
electricity markets by buying and selling electricity from and to small customers with small scale or
micro production from (possibly) renewables. The aggregator pays the customer for increasing or
decreasing their electricity consumption by fixed price, a reduction of the energy price or a combination
of both. Both the third part and the aggregator can by not being bound in ES ownership by regulatory
frameworks utilize several if not all ES applications described in previous sections. (10) The aggregator
- 51 -
could make it possible for the end customer to sell grid services without directly having to invest in a
home energy storage unit.
It needs mentioning that agreements concerning the regulation responsibility of electricity to the storage
delivery and from the storage to customers have to be made if the ES owner is someone other than a
grid operator or regulator. It is also concluded that there is not enough experience of the aggregator role
in order to determine requirements on balance responsibility. In case of large scale deployment of energy
storage and aggregators on the electricity market a clarification of and possibly added legal frameworks
concerning the purchase of grid services by TSO’s and DSO’s. (10)
8. Concluding remarks
An increased share of distributed renewables can be expected to increase the need for investments in
power stability and quality methods and technology. While a number of traditional and upcoming
solutions exist, additional methods might be needed.
Battery energy storage can allow higher amounts of renewable electricity generation to be integrated by
smoothening power output, time shifting generated energy to follow demand and increase hosting
capacities through peak shaving. Power quality related issues due to intermittency can be mitigated by
controlling the storage’s charging patterns to respond to grid variables.
For optimal utilization and maximum storage value, several applications should be within the
operational repertoire of the storage unit. Other applications including arbitrage, grid investment deferral
and load following can assist grid planners and operators.
The most promising battery energy storage technology is lithium-ion with exceptional storage
characteristics and most importantly a favorable near term price development. It remains to be seen if
the predictions come true.
Current regulatory frameworks are more prohibiting than enabling the implementation of energy
storage. Discussion regarding incitements and standardization are presumably needed for utilizing the
potentials of energy storage systems in the grid.
- 52 -
9.
Case study I – High PV penetration in rural grid
The scope of this case study is to demonstrate the use of a BESS to prevent voltage rises due to high
penetration of PV systems above the permitted limit of 10% in distribution feeders. For this objective a
rural distribution feeder was chosen and where voltage issues have been reported previously. (69) (70)
9.1
Method
In this section the system used in the case study is described and assumptions presented.
9.1.1 System description
The studied grid is the rural distribution network Haddingen 2 located 19 km northwest of Umeå. It
22 costumers connected to the secondary substation with 3 customers directly connected and the
remaining 19 connected to four feeders as shown schematically in Figure 27 A visual overview is
given in
- 53 -
Appendix A – Studied networks. The secondary substation’s transformer is rated to 200 kVA and all
customer are three-phase connected. The lengths of the feeders are presented in Table 5.
Figure 27 Grid structure of Haddingen 2. The grid is divided into 4 feeders and 3 costumers directly connected to the
LV bus of the secondary substation.
Table 5 Number of customers and maximum lengths of feeders in Haddingen 2
Feeder
1
2
3
4
Number of customers
4
8
3
8
Length
162 m 384 m 195 m 894 m
9.1.2 Construction of grid simulation
The network for this case study was created in DIgSILENT PowerFactory using data provided by Umeå
Energi Elnät AB from DIgPro where all line and transformer data and costumer energy consumption
were available. Every line section and connection points is dimensioned as to make a representation of
the actual grid of Haddingen 2.
- 54 -
9.1.3 Assumptions
As the voltage drop is proportional to the line impedance and the lines in the studied grid are of equal
size the largest voltage rises are to be expected in the longest feeder. Therefore voltage rises are to be
simulated for feeder 4 only. The connection point located the furthest from the secondary substation and
thereby expected to show the highest voltage rise is terminal 40.
The electricity demand for the customers connected to Haddingen 2 is assumed to be the same for all
and is shown in Figure 28. In reality the electricity demands as well as annual energy consumptions
differ largely among the costumers leading to different dimensioning of a BESS to counteract voltage
rise or off-peak storage. This case study however is intended to simulate the principle of BESS and is
not a detailed analysis of possible variations and perturbations.
Load [kW]
4,0
Individual demand profile for customers
of Haddingen 2
Demand
3,0
2,0
1,0
0,0
0
4
8
12
16
20
24
Time [h]
Figure 28 General electricity demand profile for customers of Haddingen 2.
It needs mentioning that the two outermost costumers have a logged annual energy consumption of less
than 600 kWh meaning they would not count as regular customers in the sense of electricity demand
and PV generation. This also applies to a third customer halfway along the feeder. A lower load at the
time of high PV generation would most likely increase the voltage rise as less active power is consumed
in that feeder. A customer with such low annual energy consumption would be less likely to install a PV
system of significant size that could contribute to the overvoltage. To avoid adding too much complexity
to the study these customers are therefore assumed to follow the same demand profile and utilizing the
same size of PV installation as the other costumers.
9.1.4 Simulation setup
To simulate how a BESS can prevent overvoltage in distribution feeders caused by the active power
output from PV installations during times of low demand and high generation two scenarios are set up.
For both scenarios the data for demand profiles and PV power generation are from the same day, 21st of
- 55 -
July 2014 which was a particularly sunny day. As a consequence a situation of minimal demand and
maximum power generations is achieved, which is a premise for overvoltage due to active power
generation. Hereby a worst case scenario is created which determines the maximum requirements of a
BESS for voltage regulation. The measurement values were provided by Umeå Energi Elnät AB. The
customer values are from the internal hourly based measurement system and the PV generation values
are taken from a solar energy producing customer in Holmsund, 10 km south of Umeå.
Before initiating the first scenario the size of PV system and the level of PV penetration causing
overvoltage had to be determined. In four simulations different PV systems with peak power outputs of
5kWp, 7.5 kWp, 10kWp and 12,5 kWp were connected to all customers on feeder 4 and the resulting
voltage rise at the outermost connection point, terminal 40, were analyzed. In addition a single oversized
PV system with varying peak power output was connected to terminal 40 to analyze the effect a single
large PV system on the voltage magnitude.
In scenario 1 a BESS is used to absorb the active power causing the voltage rise. The energy is stored
and utilized during morning and evening hours to smoothen the customer’s active power demand profile.
The BESS would thereby act on the same intent as active power curtailment and PV inverters with active
power control. The benefit of using a BESS is that no PV panels have to be disconnected causing loss
of income due to lower generation or feeding reactive power into the grid causing higher losses. The
BESS will be designed for overvoltage absorption only by absorbing active power from the PV system
above the power level that causes overvoltage.
In scenario 2 a BESS with the capacity to store the net power generated by the PV system is applied.
The net power is defined as the customer’s active load subtracted from the power output of the PV
system. The BESS capacity is thereby greater than in the first scenario as it is intended to maximize the
utilization of the decentralized solar energy production on site and therefore needs to absorb larger
amounts of power.
All calculations where performed in Microsoft Office Excel 2013 while all simulations were performed
in DIgSILENT PowerFactory version 15.2.
9.2
Results and analysis
In this section the results of scenario 1 and 2 are presented and analyzed in term of effect, required
storage size and investment costs.
- 56 -
9.2.1.1
Scenario 1
To begin with the effect of a single oversized PV system connected at the outermost connection point
only was analyzed. The result of connecting PV systems with peak power output 15 kW, 30kW and 40
kW is shown in Figure 29. The unit p.u. which stands for per-unit indicates the voltage magnitude at
the measured point.
Voltage magnitude at Terminal 40
[p.u.]
PV for outermost customer
1,14
1,12
1,10
1,08
1,06
1,04
1,02
1,00
0,98
0,96
0,94
15 kW
30 kW
40 kW
0
20000
40000
60000
80000
Time [s]
Figure 29 Comparison of different sized PV systems in respect to voltage magnitude at terminal 40. PV system
connected only at the outermost customer.
The results for the voltage magnitude at terminal 40 show that very large PV systems are required to
cause a voltage rise greater than the permitted 10%. The installation of such large PV systems can be
considered unlikely for private customers due to high capital costs. The result however shows that a
single large PV system connected far from a secondary substation can result in significant a voltage rise
which would lead to temporal disconnections due to violation of voltage limits.
It follows to determine what size of PV system causes unpermitted voltage rises in the case of high PV
penetration. Figure 30 shows the voltage magnitude over a day when all customers on feeder 4 have
connected PV systems with 5kWp, 7,5 kWp, 10 kWp and 12,5 kWp.
- 57 -
Voltage magnitude at Terminal 40
[p.u.]
PV for all customers on feeder 4
1,14
1,12
1,10
1,08
1,06
1,04
1,02
1,00
0,98
0,96
0,94
5 kW
7,5 kW
10 kW
12,5 kW
0
20000
40000
60000
80000
Time [s]
Figure 30 Comparison of different sized PV systems and their effect on the voltage magnitude at terminal 40. PV
systems at all customers along feeder 4.
As can be seen in Figure 30 overvoltage occurs at the outermost connection point when PV systems
with a peak power output greater than 10 kW are connected at all customer connection points along
feeder 4. As the 12,5 kWp PV system was the only one to cause overvoltage this size will be applied in
the simulation with a BESS.
For the simulation of the BESS for voltage regulation the charging profile was designed to absorb the
active power that causes the voltage rise shown in Figure 30 and discharge during evening hours. The
charging profile is shown in Figure 31.
- 58 -
Charging profile of BESS for voltage regulation
3
Load [kW]
2
1
0
0
2
4
6
8
10
12
14
16
18
20
22
24
-1
-2
-3
Time [h]
Figure 31 Charging profile for the battery energy storage system when used for voltage regulation. Charging occurs
just before as the permitted voltage rise of 10% is reached. The storage is discharged during low PV power
generation.
The energy capacity required for charging the BESS according to the applied charging profile is 17,3
kWh with a charge depth of 80%. This equals to about 40% of the customers daily electricity demand.
The stored energy is discharged during evening hours for self- consumption in order to absorb the same
amount of solar energy the next day if the circumstances are right i.e. if it is sunny. The stored energy
could also be utilized for peak shaving of the demand during following days if solar radiation forecasts
predict low PV generation.
By absorbing the active power from the PV system according to the chosen charging profile voltage
rises in the distribution grid above the permitted limit of 10% are prevented, as shown in Figure 32.
- 59 -
Voltage magnitude at Terminal 40
[p.u.]
BESS for voltage regulation
1,14
1,12
1,10
1,08
1,06
1,04
1,02
1,00
0,98
0,96
0,94
12,5 kW PV only
12,5 kW PV with
BESS
0
20000
40000
60000
80000
Time [s]
Figure 32 Voltage magnitude at terminal 40 before and after appliance of BESS for regulation of overvoltage at all
customers with PV systems along feeder 4.
9.2.1.2
Scenario 2
The BESS in scenario 1 is designed for taking care of the active power that causes the overvoltage and
has the capacity for absorbing only a small amount of solar energy. For the use of storing the energy
that is produced during a sunny days for later use, off-peak storage, that BESS would be too small. If
the intent of installing solar cells is to achieve maximum self-sufficiency a greater storage capacity that
can absorb all the generated energy during one day is needed. In this scenario such a BESS is tested to
observe its effect on the voltage magnitude in long feeders.
During continuous days of sun and low consumption e.g. during summer, the difference between
electricity demand and DR production will be the largest and thus the voltage rise is expected to reach
its maximum value. The energy capacity of the battery is defined by the amount of net energy that is
generated i.e. the energy that would get sold and fed into the grid. The BESS for off-peak storage charges
as soon as the PV system generates more active power than consumed and discharges when the demand
is higher than production. The charging profile for the BESS connected to a 12,5 kWp PV system is
shown in Figure 33.
- 60 -
Charging profile of BESS for off-peak storage application
4
Load [kW]
3
2
1
0
-1
0
2
4
6
8
10
12
14
16
18
20
22
24
-2
-3
Time [h]
Figure 33 Charging profile for battery energy storage when used for off peak storage of PV generated energy during
one day in July. The storage is charged during a net power generation and discharged during hours when demand is
higher than PV generation.
The BESS charges during mid-day when solar energy generation is high and discharges during morning
and evening hours to cover the customer’s electricity demand. The energy capacity required for the
BESS is considerably higher than for the first scenario, 92 kWh with a charge depth of 80%. The
resulting load profile in comparison to the initial demand profile is shown in Figure 34.
Load profile for costumer 22
6
4
Load [kwh/h]
2
0
-2 0
2
4
6
8
10
12
14
16
18
20
22
24
Demand
-4
PV
-6
With BESS
-8
BESS
-10
-12
-14
Time [h]
Figure 34 Load profile for customer 22 with and without BESS for off peak storage. By charging the BESS when PV
generation is higher than demand and discharging when lower, the net load for the customers is zero during
one day in July
The PV generated energy stored by the BESS is discharged during low generation hours and thus making
the owner of the PV system and BESS self-sufficient during the day that is simulated in scenario 2. Here
it is assumed that the BESS provides the electricity needed during the first hours of the day by utilizing
- 61 -
stored energy from the day before. The total amount of solar energy stored during the simulated day is
74 kWh while the total electricity demand is 41 kWh. As the net energy production is greater than the
electricity demand for one day there will be surplus energy stored in the battery that can be used for
days when solar radiation is low.
By utilizing a BESS designed to absorb all the net generated solar energy no active power is fed into the
grid and thus a voltage raise is completely prevented in feeder 4 as shown in Figure 35. This shows that
overvoltage are not a problem for feeders with high penetration of PV generation if the system is
Voltage magnitude at terminal 40 [p.u.]
equipped with a storage device.
BESS for off-peak storage
1,14
1,12
1,10
1,08
1,06
1,04
1,02
1,00
0,98
0,96
0,94
With BESS
Without
BESS
0
20000
40000
60000
80000
Time [s]
Figure 35 Voltage magnitude at the terminal 40, the outermost customer 22. After applying a BESS with a capacity of
that absorbs the net generated solar energy the voltage rise is compensated completely.
The results form scenario 1 showed that the line voltage can be regulated to comply with existing voltage
regulations for distribution grids. The voltage was kept below the limit of 10% overvoltage by charging
a BESS without curtailment of the PV systems that would have been the alternative solution for a voltage
violation. The required energy capacity for the BESS used is 17,3 kWh with a charge depth of 80%.
Scenario 2 showed that voltage rises are completely prevented when a BESS capable of storing all net
generated solar energy is used. The required energy capacity for the BESS during one day in July is 92
kWh. As the useful energy capacity is nearly twice that of the energy consumption the storage could be
used for supplying the customer with energy during the following 2 days or peak shaving during even
longer periods.
- 62 -
Table 6 Comparison of investment cost with present and projected capital cost for the BESSs.
BESS
Grid investment
Larger PV inverter with
(71) (72)
reactive power control§§
(17,3 kWh for scenario 1)
1 – Voltage
Solution 1:
1890 SEK
2015: 358 684 SEK
regulation
341.000 SEK
Scenario
2 – Off-peak
storage
(92 kWh for Scenario 2)
2020: 67 482 SEK
Solution 2:
N/A
2015: 1 095 168 SEK
2020: 205 939 SEK
250.000 SEK
In the cost comparison, shown in Table 6, it is assumed that the owner of the equipment is responsible
for the cost e.g. the utility company pays for the grid enforcement and the owner of the PV system pays
for the upgraded PV inverter or BESS. However, for the PV inverter there has been a discussion on if
and how the utility company might compensate the owner of the PV facility for reactive power
compensation as a power quality service shifting the cost to from the customer to the utility company
(73). The same method could be used for a BESS for voltage regulation. Especially if other applications
come in question such as peak shaving and power regulation. This would lower the annual cost for a
BESS and make it more economical feasible.
For both scenarios the solution of using a BESS to mitigate violation of voltage limits is successful but
more expensive than the presented alternatives. Solution 1 constitutes of reducing the connection
impedance at terminal 40 by placing a second transformer 300 meters closer and laying thicker cable
for the remainder of the distance. Solution 2 consists of only laying thicker cable from a projected
secondary substation nearby. The two solutions are outlined in Appendix B – Alternative solution
overview for case study I. The PV inverter with reactive power control is significantly cheaper provided
that the BESS has no other values accredited to it. In the case for scenario 1 the BESS can become a
feasible alternative to grid investment. However, it has to be remembered that the capital cost for the
BESS represents one unit serving one customer whereas the grid investment serves the whole feeder.
§§
Upgrading the PV inverter by 2 kW to enable reactive power control (73)
- 63 -
10.
Case study II – Load leveling with CES
The scope of this case study is to demonstrate the use of a BESS for storing solar energy produced by
customer owned PV systems for a scenario with high PV penetration. The BESS is intended for the
application of storing solar energy to maximize self-use of the local micro-generation. This type of
storage can be called a community energy storage as it provides with the local community with storage
services.
10.1
Method
This section gives an overview of the studied network and assumptions made for the case study.
10.1.1 System description
The studied area is part of a newly built residential area in Umeå, Tavleliden. The area includes 84
residential customers connected to the secondary substation Stjärnbilden with a transformer of a 500
kVA power rating. The electricity demand profile for the transformer of the secondary substation
Stjärnbilden for one day in July is shown in Figure 36. A visual overview of the area is given in
- 64 -
Appendix A – Studied networks.
Load [kW]
Demand profile for customers of Stjärnbilden
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
22
24
Time [h]
Figure 36 Demand profile for customers of Stjärnbilden secondary substation during one day in July.
10.1.2 Assumptions and simulation setup
For this case study 75% of all customers connected to the secondary substation Stjärnbilden are equipped
with a rooftop PV system with a peak power output of 10 kW. A PV system of this size has an average
annual energy production of 9500 kWh based on an assumed electricity production of 950 kWh per
kWp. The median of annual electricity demand for customers connected to Stjärnbilden is 13 300 kWh.
A PV system with the given size would thus provide roughly 70% of the customers annual electricity
demand.
To simulate the worst case scenario e.g. when electricity demand is low and solar energy production is
highest a sunny day in July is simulated by taking the demand profile for customers of Stjärnbilden and
add the power output form the PV.
All simulations and calculations were carried out in Microsoft Office Excel 2013
10.2
Results and analysis
When no storage is applied most of the solar power generated by the residential micro-generation is fed
“backwards” into the secondary substation, reversing the power flow, as the demand is much lower than
production during most of the day. This is shown in Figure 37 for the area of Stjärnbilden when 75 5 of
all connected customers have installed a 10 kWp PV system. Here the rated power for the transformer,
- 65 -
500 kVA, is surpassed causing strain on the equipment. Overloading of transformers can cause shortened
life time leading to higher capital cost of the grid infrastructure.
Load [kW]
Power generation from PV
200
100
0
-100 0
-200
-300
-400
-500
-600
2
4
6
8
10
12
14
16
18
20
22
24
75%
No PV
Time [h]
Figure 37 The load on the transformer in secondary substation Stjärnbilden with 75% PV penetration during one day
in July.
A possible future problem for DSO´s emerges besides overloading secondary substations is when the
above described scenario occurs for several areas in a residential area. In that case the back feeding of
power to the transmission grid can result in penalties and higher losses.
10.2.1 Off-peak storage for 75% PV penetration
For the scenario of 75% PV penetration two applications of a BESS are considered. The first application
is a peak shaving BESS, denoted as BESS1, that charges surplus generated solar energy that would
cause overloading of the secondary substations transformer during times of low consumption and high
production. The stored energy is later discharged to supply the customers with electricity when needed.
The second application of a BESS could be that of storing a part of all generated solar energy by the
connected PV systems, denoted as BESS2. The energy can be sold to the electricity market during high
demand hours for larger profit or stored for later use by the community in order to reach a higher degree
of self-sufficiency. The application of BESS1 and BESS2 is shown in Figure 38
- 66 -
Net load for transformer Stjärnbilden with
75% PV penetration
200
100
Load [kW]
0
-100
0
2
4
6
8
10
12
14
16
18
20
22
24
75% PV
-200
PV + BESS1
-300
PV + BESS2
-400
-500
-600
Time[h]
Figure 38 Net load profiles for the load in Stjärnbilden with 75% PV penetration during one day in July. Two
applications of BESS are shown. BESS1 stores the generated energy that would cause temporary overloading of the
transformer. BESS2 stores half of the generated energy for later use in the community.
The application of BESS1 prevents overloading of the transformers by charging when the transformer
load is close to the rated power. A maximum 60 kW are “shaved” off the PV generated peak during one
occasion. The stored energy is discharged during high demand hours in the evening. The energy capacity
needed for the studied case is 120 kWh requiring a 150 kWh BESS when considering a charge depth of
80%.
BESS2 charges half of the surplus energy generated by the connected PV systems, lowering the overall
loading of the transformer to about half of what it would have been without the storage applied. The
storage provides the community with electricity during hours when the PV systems are producing less
than is demanded. During the studied day about 970 kWh of solar energy is stored by the BESS requiring
an energy capacity of roughly 1,2 MWh when considering a charge depth of 80%.
By applying a BESS as BESS1 or BESS2 an overloading of the secondary substation could be prevented.
In the case of BESS2, the stored energy is more than enough to cover the electricity demand of the
customers connected to Stjärnbilden. For periods with less sunny days the stored energy can be used
locally to make the community self-sufficient on electricity during times of high solar radiation e.g.
summer. When investing in a BESS solution it is important to find as many uses as technically possible
to maximize the value of the storage system and decrease payback time. A BESS intended for solar
energy storage only will not be economical feasible if left unused during times of low solar radiation
e.g. winter time in Scandinavia. Therefore other applications should be incorporated in the use of the
BESS. Such applications can range from power quality services to fast power regulation and arbitrage
- 67 -
of electricity. A possible use that does not require much regulation is load levelling of the secondary
substation load during times of high demand e.g. during winter. To prevent fluctuation in power demand
the BESS could charge itself during night when the electricity price is generally low and discharge
during high demand hours. The customers connected to the storage can purchase the electricity from the
storage during high demand for the price of when the electricity was charged, night time. Thus the
customers save money while decreasing load peaks in the energy system.
10.2.2 Load leveling and arbitrage
As a continuation of this case study the two BES systems, BESS1 and BESS2, are simulated to level the
load during 3 days in February. During the chosen days there is no significant solar irradiance and thus
no production from the PV systems .Because the storage is sited behind the secondary substation meter
charging the BESS leads to an increased load while discharging the stored energy decreases the load on
the secondary substation. The application of BESS1 with a total energy capacity of 150 kWh with charge
depth of 80% and AC-to AC- efficiency of 90% is shown in Figure 39 where is the maximum load has
been decreased from 247 kW to 202 kW during peak demand in the afternoon. The storage is charged
during night 00.00 to 04.00 with 40 kW keeping the energy stored until high demand hours during 16.00
to 21.00 where the BESS is discharged.
Load [kW]
Load leveling using 150 kWh BESS 1
280
260
240
220
200
180
160
140
120
100
Demand
Load leveling +
Arbitrage
0
4
8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76
Time [h]
Figure 39 Net load profile of Stjärnbilden during 3 days in February with and without load leveling by a 150 kWh
BESS.
A similar storage cycling profile is applied to BESS2 where the charging occurs during 00.00 to 05.00
and discharging from 16.00 to 21.00. As BESS2 has about nine time higher energy capacity than BESS1
more electric energy can therefore be stored. However to utilize the lower electricity price during the
- 68 -
early hours of the day the charging still occurs during that time. The difference to BESS1 is the increased
charging power of 100 kW instead of 40 kW to maximize the arbitrage. Charging more of the “cheaper”
electricity during night and decreasing the use of more “expensive” electricity during high demand hours
would yield a larger profit form arbitrage. For comparison the cycling profile similar to that of BESS1
with a maximum charging power of 50 kW is shown in Figure 40.
Energy cost management and arbitrage with BESS 2
260
240
Load [kW]
220
Demand
200
Load leveling
+ Arbitrage
Maximize
arbitrage
180
160
140
120
100
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76
Time [h]
Figure 40 Load leveling using a 250 kWh respectively 500 kW of a 1,2 MWh BESS during 3 days in February.
The approach for maximizing arbitrage could also be used to store local wind energy production which
is not a part of this case study. A concern of the DSO is that wind energy production tends to occur
during low demand hours resulting in feedback penalties as the large solar energy production in the case
of 75% PV penetration would cause. The community storage could in periods of surplus wind energy
production act as local power regulation and serve as an off-peak storage for wind energy on distribution
grid level. This application is however not treated in detail by the case studies of this thesis.
A benefit of installing more storage capacity than needed as it is the case for BESS2 in the performed
simulation, the charge depth during in the load leveling application is reduced to 21% compared to 80%
for BESS1 with the same result. As the application of storing solar energy only is possible for days when
the sun is shining and load leveling can be done every day in the year the main application of the BESS
will be load leveling and arbitrage. By keeping the charging cycle shallow the lifetime of the battery can
be increased and thereby the BESS services used for longer time yielding a lower payback time.
- 69 -
The transformer should not be exposed to loads over 80% to often as this causes the lifetime to decrease
due to increased wear (71). The dimensioning of BESS 1 was based on the objective to avoid loads
above the rated power for the transformer. For a deeper analysis of required storage capacity for optimal
equipment lifetime the number of days with occurrence of overloading has to be assessed. In addition
to reduced lifetime, the transformer losses increase by the square of the current leading to higher losses
when run with high load. Therefore a reduced load is beneficial for the transformers life time expectance
and transformer losses. If the load cannot be reduced a change of transformer is necessary in order to
cope with the high peak loads.
A simple comparison of capital costs for the two presented storage solutions and a traditional solution
is shown in Table 7. For both BESS 1 and BESS 2 prices for 2015 and 2020 are given based on the
average cost from Figure 18 and an assumed annual price reduction of 20%. The cost for upgrading the
secondary substation with a transformer of higher rating includes costs installation and power outage
compensation.
Table 7 Cost comparison of solutions to the overloading of the 500kVA rated transformer on Stjärnbilden with 75%
PV penetration.
Solution
Cost
Change of transformer from
500 kVA to 800 kVA (71) (72)
BESS1 – 150 kWh
BESS2 – 1,2 MWh
2015 : 1 785 000 SEK
2015 : 9 000 000 SEK
2020 : 585 000 SEK
2020 : 4 681 000 SEK
120 000 SEK
From the cost analysis in Table 7 both energy storage solutions come out as significantly more expensive
in terms of capital costs. This is not surprising regarding that the storage systems costs are not expected
to be at commercial level by 2020. Changing the transformer for increased rated power turns out to cost
less and therefore is preferred. However, this cost analysis stretches only until 2020 while process are
expected to fall beyond 2020. With continuous annual price reductions battery energy storage can
become feasible within the next 10 to 15 years.
What also has not been taken into account in this cost analysis are potential values from the storage by
the implementation of storage applications. The potential earnings from load leveling and arbitrage are
not assessed and is left for future work.
- 70 -
11.
Discussion
In this section the uncertainty in the case studies are assessed. The findings from the case studies and of
the literature studies are summarized and discussed. In addition future prospect and question for future
work are discussed.
11.1
Uncertainties in case study I
The case study of rural distribution grid Haddingen 2 with a long distribution feeder in combination with
high PV penetration intended to demonstrate how a BESS could be used for mitigation of overvoltage
due to solar generation. For this cause several assumptions were made to simulate the studied grid that
altered it from the original.
First, the high penetration of relative large residential PV systems in feeder 4 is hardly expected to
become reality within the next 5 or even 10 years making the occurrence of overvoltage of the same
magnitude unlikely. While the voltage magnitudes might not emerge in rural feeders anytime soon, the
voltage results still give a hint of what issues might become real if and when the prices of residential
grid connected PV systems decrease as projected.
The assumption of homogenous electricity demand for all customers does not reflect actual variations
in electricity use. Different demands would also require different sized PV systems if the demand is to
be covered by solar energy. The assumption also eliminated the effect of the three summer residences
connected to feeder 4. With the assumption of high PV penetration those residences are more likely to
install an off-grid PV system for private use when owners are present. An off grid system would not
affect feeder voltages. As two of the outermost customer in fact were most likely summer residences
line feeder voltages can be expected to have been lower due to shortened feeder length and reduced
active power flow. Changes in ownership of residences is also not taken into account. In those aspects
the simulated grid is not a real life future representation but more of a worst case scenario for gird
planners.
11.2
Uncertainties in case study II
Concerning the micro generation there are similar uncertainties in the case study of the Stjärnbilden area
as in case study 1. As the demand profile used for the calculations was an aggregation of all customers,
individual differences from the time and amount of electricity demand are not taken into account for the
choice of PV systems size. Rather than taking the average of all customer and covering 2/3 of annual
demand with solar energy the PV system size should be determined after each individual customer’s
- 71 -
demand. Hence a scenario closer to reality for the Stjärnbilden area should include PV systems of
different sizes.
Since the area of Stjärnbilden consists of many newly built residences it might be the case that the data
for electricity consumption only includes the residences which already were occupied at the time of data
collection. The actual number of households with considerable electricity demand can thereby be higher
in the future scenario which is studied. As all residences were assumed to have electricity generating
PV systems installed on their roof the actual net load would be slightly lower than obtained in the
simulations.
The day for which the simulations were performed represented a worst case scenario for the grid operator
during a period of high solar irradiation and low consumption. The energy storage capacity was designed
thereafter. The simulations although covered only one day (24h) disregarding weather conditions for the
preceding and following days. An energy storage for the applications described would, for optimal use,
be charged and discharged more than once during consecutive days. The given energy capacity for BESS
2 would therefore have to be higher or the energy storage has to be discharged during nighttime to serve
its purpose during the next coming day. A re-design of the storage capacity and control algorithm based
on simulations covering the diurnal variations of demand and solar generation would therefore be
recommended for future practical use.
11.3
Battery energy storage as alternative to traditional solutions
Battery energy storage can allow higher amounts of renewable electricity generation to be integrated by
smoothening power output, time shift generated energy to follow demand and increase hosting capacities
through peak shaving. Power quality related issues due to intermittency can be mitigated by controlling
the storage’s charging patterns to respond to grid variables. For optimal utilization and maximum storage
value several applications should be within the operational repertoire of the storage unit.
Solutions for grid operators in addition to renewable integration include grid investment deferral, power
quality and reliability in case of power outages. The traditional method of upgrading grid infrastructure
is costly and binds investments for decades whereas battery storage can be used to solve the problems
before they emerge and can be mobile. Energy storage in combination with minimal curtailment can
increase hosting capacities substantially and grid investments deferred. A future concern of increased
electric vehicle charging can be solved by letting energy storage supply a part of the required charging
power.
While dynamic line rating might become a possible approach to deal with overloading this method will
most likely require substantial reorganization of working procedures for most utilities with not
- 72 -
insignificant costs to follow. In addition energy storage for increased hosting capacity has more
exploitable applications increasing its value.
Solutions for end customers with micro generation from renewables include energy cost reduction by
arbitrage and peak shaving as well as increased self-sufficiency by storing their generated electricity for
private use. The issue of voltage stability in residential areas has been a concern for grid operators in
countries with rapidly increasing PV penetration, such as Germany. The general solution of curtailment
when voltage limits are violated reduces the financial benefit from micro generation but is a cheap and
effective method nonetheless. An up and coming solution is provided by power electronics
manufacturers that offer PV inverters with reactive power control. This is also a possible solution which
in comparison to energy storage, as shown in case study 1, is less expensive. Battery energy storage
systems are however not limited to voltage control and have has mentioned several applications that add
to its value.
11.4
Future prospects
Due to high capital costs the near time deployment of energy storage facilities is not likely unless price
projections turn out to be correct. In that case the prices per kWh of storage capacity can drop below
4000 SEK/kWh by 2020 making energy storage an attractive alternative to traditional solutions to issues
caused by the intermittency of renewables. The most promising battery technology is the Lithium-ion
type due to two major causes; excellent energy characteristics and promising price development. The
price development is mostly due to increased production volumes of EV batteries which are of Lithiumion type. A possible way, with need for evaluation, of decreasing capital costs for battery storage is to
use second hand batteries from EV’s that have lost up to 30% of their initial capacity. The breakthrough
for energy storage is however not only limited by price developments. Permitting regulations and market
development are important if the concept of energy storage is to become a part of the energy system.
Today Swedish grid operators are not allowed participate on the electricity market and are only allowed
make use of energy storage in order to reduce grid losses or cover power outages. A discussion and
update on the term “energy storage” in the Swedish Electricity Act allowing grid operators to utilize
more potential applications of energy storage systems is needed.
As prices for PV systems are steadily decreasing more and more end customer can be expected to
become both consumers and producers. Distributed generation can lower grid losses but also poses
issues for scenarios with large penetration of micro generation. In order to mitigate power quality
impoverishment and power flow imbalances, increased self-consumption of electricity with the use of
energy storage can be a viable solution in the near future. To motivate the use of home storage systems
similar subsidies as those existing in Japan and Germany can be of help. However as electricity prices
- 73 -
are considerably lower in Sweden other actions need to be implemented to make customers participate
in the balance market. Such an action can be the introduction of the aggregator role who utilizes the
flexibility an energy storage system gives to the end customer’s electricity use and participates in the
electricity market. The aggregator’s customer can be both utility end customers with energy storages
and producers of wind or solar power. A prerequisite for utility customers to be willing to practice time
of energy use shifting with energy storage are peak demand based tariffs and hourly measurement to
capture the benefits of energy management and flexibility. Policies for the promotion of low peak power
withdrawals and increased self-consumption of electricity benefit both end customers with micro
generation and grid operators. The grid operator is relieved of dealing with the issues posed by high
penetrations of intermittent renewable micro generation while grid-scale energy storage systems in
distribution feeders can provide load regulation and power quality services purchased from an
aggregator or other third part.
Peak withdrawal based tariffs in combination with critical pricing to promote demand response in
customers to shape load curves. These measures would also motivate energy time shifting with storage
systems.
11.5
Future work
This thesis work was intended to provide an outline of the applications of battery energy storage for the
integration of renewables. The topic of energy storage is very broad and includes technical aspects
ranging from optimal placement in the grid to the choice of storage technology and optimization of
cycling as well as economic evaluations of different applications and regulatory aspects of energy
storage. Below some of the topics that came up during the progression of the work are listed;




Development and economic evaluation of possible business models for BESS with
mapping of market potentials in Sweden.
Explicit development and testing of storage algorithms for the integration of DR at
residential and grid scale.
Environmental aspects and life cycle analysis (LCA) of a BESS – What happens to the
batteries after decommissioning of the storage system?
Investigate how DER in combination with BES can provide voltage stability, VAr support
etc
- 74 -
12.
Conclusions
To conclude this thesis and summarize the key findings, the research questions with the answers are
presented.

How are local grids affected by high penetrations of renewable micro generation?
The intermittency and volatility of solar and wind generation is expected to become an issue for grid
operators when production and consumption is unbalanced. If the correlation is too low, worst case
scenarios in which production exceeds consumption can cause overloading of transmission
infrastructure, local overvoltage in distribution grid and possibly frequency fluctuations.

What are the possible applications of energy storage for allowing a higher penetration of
renewable micro generation and who benefits from them?
Battery energy storage can help to smoothen out distributed power generation and store generated energy
for periods when it is lower than demand. This allows the owner of the production unit to manage and
utilize the energy to its fullest. Private customers with a combined system of a PV system and battery
storage can achieve a higher degree of self-sufficiency while using the battery to manage energy costs
and demand charges.
Besides direct control of the power output from the production site, battery energy storage can also defer
grid investments by providing power during peak demand or storing power peaks during overproduction.
For areas with high shares of distributed intermittent generation affecting power and voltage stability, a
battery energy storage on grid scale can provide frequency regulation and voltage support. For grid
operators these applications are likely to be the most beneficial.

Is energy storage an economical feasible alternative to traditional reinforcement in the grid?
The two case studies demonstrate that battery energy storage can be used to allow high shares of
renewable distributed generation in the distribution grid. However, the investment costs of today for
such applications are too high due to the high battery prices. Fortunately, according to various forecasts,
the price development is likely to enable the use of battery storage within the next 5-10 years. An
investment in battery energy storage might then be a feasible alternative solution regarding the
integration of distributed intermittent generation.
- 75 -

Are regulatory frameworks beneficial for energy storage and if not what general changes are
needed?
There exist no clear definition of what an energy storage is and it how should be regulated, making it
difficult for grid operators to draw benefit from possible storage applications. Discussions regarding
regulations for grid operators and possibly new business models including a clear definition of the
storage system are called for. There are also no subsidiaries for the financial support of storage systems
as for renewable production. If energy storage is to assist in the integration of distributed generation, a
subsidiary might be beneficial for the initial stimulation of the market.
Battery energy storage has just recently become an engaging topic in the energy industry after highly
anticipated price reductions. With the technology seemingly ready to support the integration of
distributed renewables, regulatory changes and commercial efforts are needed in order to fully integrate
energy storage as a part in our energy systems. With the combination of distributed renewable generation
and energy storage, a 100 % carbon free and sustainable energy system might not be that far away.
- 76 -
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Appendix A – Studied networks
Overview of the studied networks Haddingen 2 and Stjärnbilden in the Umeå region.
Figure 41 Overview of the Haddingen 2 LV network. Blue and green lines indicate underground cables and overhead
lines.
Figure 42 Overview of the Stjärnbilden network in green
A
Appendix B – Alternative solution overview for case study I
Visualization of the two alternative solutions presented in case study I. Figure 43 shows how lines are
drawn from the secondary substation of Haddingen 2 to the outermost costumer (terminal 40). The
additional secondary substation is placed in the bend. Figure 44 shows the second alternative where a
ground cable is drawn from a secondary substation under construction (Haddingsavan) to the outermost
customer (terminal 40).
Figure 43 Overview of the high cost alternative solution for case study 1.
B
Figure 44 Overview of the layout for the low cost alternative solution in case study 1.
C
Appendix C – Application synergies
This table shows how well different energy storage applications can be combined for the same storage systems. The table is taken from the report Energy Storage
for the Electricity Grid: Benefits and Market Potential Assessment Guide publicized by Sandia National Laboratories in 2010 which gives very good insight in
most topics regarding energy storage and is recommended for interested readers.
Table 8 Application synergies. Table including notes and annotations were taken from the Sandia report. (39)
D
Notes
a. For Area Regulation: Assume that storage cannot be connected at the distribution level.
b. For Voltage support: Assume that a) storage is distributed and b) the storage system includes reactive
power capability.
c. For Reserve Capacity: Must have stored energy for at least one hour of discharge (i.e., so can offer
useof the storage as reserve capacity on "hour-ahead"
d. For T&D Load Following: For load following up (mornings) or down (evenings) involving charging;
must pay prevailing energy price.
e. For T&D Deferral: Annual hours of discharge range from somewhat limited to none. So storage is
available for other applications during most of the year.
f. For Time-of-use Energy Cost Management and Demand Charge Management: Assume discharge for
5 hrs./day (noon to 5:00 pm), weekdays, May to Octo
g. Transmission Support (not shown) is assumed to be mostly or entirely incompatible with other
applications.
Annotations
¹Requires distributed storage that is located where needed.
x Somewhat to very circumstance-specific, especially regarding timing of operation and/or location.
* Most storage cannot provide power for both applications simultaneously.
† Presumably discharge is somewhat to very coincident for the two applications.
# For distributed storage: charging energy a) from onsite renewable generation and/or or b) purchased
from offsite renewable generation via the grid.
‡ Requires utility dispatch of onsite storage.
E
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