SASEC2015 Third Southern African Solar Energy Conference 11 – 13 May 2015

SASEC2015 Third Southern African Solar Energy Conference 11 – 13 May 2015
SASEC2015
Third Southern African Solar Energy Conference
11 – 13 May 2015
Kruger National Park, South Africa
COMPARISON OF ENERGY STORAGE OPTIONS AND DETRMINATION OF
SUITABLE TECHNIQUE FOR SOLAR POWER SYSTEMS
Ghenai C.* and Janajreh I.
*Author for correspondence
Sustainable and Renewable Energy Engineering Department,
College of Engineering, University of Sharjah,
Sharjah, UAE
[email protected]
ABSTRACT
bioenergy, geothermal, and ocean (wave, current, temperature
gradients) as source of renewable energy [3]. Solar and wind energy
are among the most abundant and potentially readily available.
Solar and wind power technologies have grown quickly. Globally,
the total electricity from installed wind power reached 336 gigawatts
(GW) in June 2014, and wind energy production was around 4% of the
total worldwide electricity usage [4]. The World Energy Council
estimates that new wind capacity worldwide will total up to 474 GW
by 2020. The output from photovoltaic (PV) module installations is
currently growing at 40% per year worldwide [5]. By the end of 2013,
worldwide installed PV capacity reached 139 GW and an estimated
40-50 GW was added in 2014 [5]. Solar power is expected to become
the world’s largest source of electricity by 2050 [5]. However, solar
and wind are not constant and reliable sources of power. The variable
nature of these renewable sources causes significant challenges for the
electric grid operators because other power plants such as fossil fueled
power plants need to compensate for the variability. For example, wind
power profiles peak at night when demand is low and solar power is
generated only during the daytime and varies when clouds pass by.
The energy can be stored when the supply is high and demand is low.
The energy can be recovered and used when the supply is low and the
demand is high. Another concern is the fact that the renewable
resources are localized and are often away from load centers. For
example in the United States, wind sources are concentrated in the
Midwest regions, and solar sources in southwest regions.
To smooth out the intermittency of renewable energy production,
high performance and low-cost electrical energy storage will become
necessary. Energy storage has been considered as a key enabler of the
smart grid or future grid, which is expected to integrate a significant
amount of renewable energy resources [6-7]. Energy storage
technologies will (1) help electric utilities increase the grid efficiency,
capacity, and reliability (balance load – shift energy consumption;
bridge power – no break in service during the switch from one power
generation source to another; and power quality management – control
voltage and frequency), (2) meet the need of energy consumers and (3)
help the integration of renewable energy into to the grid by managing
the intermittency.
A comparison study between energy storage options is presented in
this paper. The principal objective of this comparison study is to
determine the most suitable energy storage techniques for power
systems with intermittent resources such as solar and wind.
The efficiency and cost of renewable solar and wind power
systems using intermittent resources could significantly be improved
by developing low cost, high efficiency and more sustainable energy
storage systems. A comparison study between energy storage options
is presented in this paper. The energy storage options include: (1)
electro chemical storage: lead acid, Li-ions, Nickel-Cadmium, Nickel
metal hydride, Sodium Sulfur, and vanadium flow batteries; (2)
electro-magnetic energy storage: super capacitors and super
conducting magnetic energy storage; (3) hydrogen storage: onboard
systems and utility scale; (4) mechanical storage: compressed air,
flywheel, pumped hydro, spring (composite and metal), and (5)
thermal energy storage. The resource intensities and operational
parameters of the energy storage options are compared in this study.
The main objective is to review the various types of storage techniques
and their characteristics and to determine the most appropriate
technique for solar and wind energy applications: energy storage
system with suitable discharge time, lowest resource intensities, best
operation performance and lowest cost. Based on the results obtained
in this study, super capacitors, super conducting magnetic, and
flywheel energy storage systems could be a good option for solar and
wind applications: they offer fast discharging/charging times, greater
performance (high specific power, high cycle efficiency, high cycle
life and they) and are very attractive with respect to the operating
costs.
INTRODUCTION
Today most of the global energy demand is derived from the
combustion of gas, oil and coal. The reliance on fossil fuels is
expected to diminish in the coming decades due to (1) the new
emissions regulations – reduction of carbon dioxide and other
greenhouse gases (Nitrogen oxides NOX and Sulfur Oxides SOX for
example), (2) the reduction of the coal, oil and natural gas reserves, (3)
the need to reduce the dependence on foreign imports (use local
resources), and (4) the projected triple increase in the energy demands
by 2050 [1]. How this energy will be generated when the oil, gas and
coal reserves become depleted. Nuclear and renewable energies are the
two alternatives to conventional power generation using fossil fuels
[2].
Renewable energy could supply most of the energy demands (80%)
by 2050 [1]. Renewable energy technologies that can be integrated in
the present and future energy systems includes solar, wind, hydro,
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DESCRIPTION OF THE ENERGY STORAGE SYSTEMS
storage accounts for 99% (~127,000 MW) of the worldwide capacity
[15]. The energy efficiency of the pumped storage Hydroelectricity
varies between 70% and 80% [15].
Flyweel Storage: The Energy can be stored as kinetic energy (W = ½
I , where I is the rotational moment of inertia and  is the angular
velocity of the flyweel) by spinning a flywheel. The stored energy is
then released by allowing the flywheel to run a generator and produce
electricity. The flywheel is designed to have as large moment of inertia
as possible in order to spin fast and maximize the energy stored.
Frictional losses (in the bearings and surroundings air) reduce the
efficiency of the flywheel storage system. The flywheel systems are
usually used for short term storage. To reduce the losses related to the
air resistance, the flywheel can be placed in a vacuum.
Superconducting magnetic bearings can be used to levitate the mass
and reduce the bearings losses.
Spring (composite and metal): It is the storage of energy as
mechanical potential energy by compressing or extending material
elastically [9]. The springs have very low specific energy (MJ/kg). The
specific energy is given by W/m = ½ (y2/ E), where  is the density,
E is the Young Modulus and y is the yield stress for the material).
There are no examples of springs being used as a utility scale device,
nor to store energy in automobiles. The spring storage method with
low energy density using today material was added in this paper just
for comparison purpose. The development of new materials in the
future will help to increase the energy density of the spring [9].
A description of the energy storage systems for power systems is
presented in this section. The energy storage options include: Electro
chemical storage (lead acid Li-ions, Nickel-Cadmium, Nickel metal
hydride, Sodium Sulfur, and vanadium flow batteries), electromagnetic energy storage (super capacitors and super conducting
magnetic energy storage), hydrogen storage (onboard systems and
utility scale), mechanical storage (compressed air, flywheel, pumped
hydro, spring), and thermal energy storage (See Fig. 1).
Figure 1 Energy storage systems
Thermal Storage
Mechanical Storage
It is the storage of energy by heating up a material [10]. The
energy can be used to (1) heat a solid or liquid and the heat capacity
Cp (J/kg K) of the material is used to retain the energy, or (2) heat a
solid to melt and the latent heat of fusion Lm (J/kg) is used to capture
the energy. When excess energy is available, this energy is used to
create a temperature gradient and the energy is stored as heat.
Alternatively, the energy can be used to melt or freeze material,
thereby storing energy as latent heat. The energy is released by using
the temperature gradient either to directly heat/cool space, or to run an
engine and generate electricity. Utility scale thermal energy storage is
increasingly being used as part of concentrated solar plants. The sun’s
heat is used to heat up molten salt from a cold tank (typically 40%
potassium nitrate, 60% sodium nitrate), which is then held in a hot
tanks until the energy is needed. The molten salt from the hot tank then
returns to the cold tank via a steam generator, which runs a turbine and
generates power.
Compressed air energy storage (CAES): When the energy demand
is low and the supply is high, the energy is used for compressing and
storing air. When the demand increases, the energy is released by
expanding the air through a pneumatic motor or turbine, connected to a
generator to generate electricity. The compressed air can be stored in
small tanks for automotive applications, or in underground chambers
for utility scale applications. During the compression process, the air
temperature increases and this result in losses of energy as heat. To
reduce or eliminate the energy loses during the compression processes,
the compression air chamber should be well insulated (adiabatic
compression) [14]. The other option is to have a compression at
constant temperature. This can be accomplished by compressing the
air slowly, and allowing the heat to escape and be stored separately so
that the temperature of the compressed air remains constant
(isothermal compression). During the expansion process, the air cools
rapidly and can freeze the turbine. To avoid this problem, the air can
be heated during the expansion process. If the heat generated during
compression can be stored and used during the expansion process, the
efficiency of the storage system will improve considerably. The
compressed air can also be uses in gas turbine engine to produce
electricity. This is a hybrid compressed air energy system and is
similar to a conventional gas power plant. In a conventional gas power
plant, up to 60% of the output from the plant is used to compress air.
The remaining 40% is directed to the generator [14]. In hybrid
compressed air system, the stored compressed air removes the need for
this (all the power from CAES turbine is directed to the generator),
increasing the efficiency of the gas plant up to 70% [14].
Pumped Hydro Storage: Energy is stored as potential energy. Low
cost-off peak power is used to pump water from low to high reservoirs.
The energy is then stored as gravitational potential energy in a mass m
(W = m g h, where g is the acceleration due to the gravity). During
periods of high electrical demand, the stored water is released through
turbines to produce electric power [8]. The pumped hydro energy
Electro Chemical Energy Storage - Batteries
The batteries store energy as chemical energy. The battery
consists of two half cells, each containing a metal and a salt solution of
that metal (e.g. metal sulfate) [13]. The half-cell with the more reactive
metal (metal A, in this example) is the anode. The metal is oxidized,
becoming a metal A ion (A+2x) and releasing some electrons (e-):
A → A+2x + 2xe-. The half- cell with the less reactive metal
(metal B) is the cathode. The metal ions (B+2x) from the solution are
reduced: B+2x + 2xe- → B. The metals from each half cell are
connected through a conducting circuit. The electrons are transported
from the anode to the cathode through this circuit, where they provide
electricity. In order to maintain balance of charge, anions (negatively
charged ions) are allowed to pass through a porous disk or salt bridge
from one solution to the other. These are some examples of batteries:
Lead-acid batteries: they have a lead anode and a lead dioxide
cathode [13]. On discharge, both of these become lead sulfate. At the
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anode lead (Pb) reacts with bisulfate ions (HSO4-) to form lead sulfate
(PbSO4), hydrogen ions (H+) and electrons:
Pb + HSO4- → PbSO4 + H+ + 2eAt the cathode, lead dioxide (PbO2) reacts with hydrogen ions and
bisulfate ions to form lead sulfate and water (H2O):
PbO2 + 3H+ + HSO4- + 2e- → PbSO4 + 2H2O
Li-ion batteries: Lithium-ion batteries have an anode of graphite
intercalated with lithium, and a cathode of lithium compounds [13].
During discharge, lithium ions Li+ move from the graphite/lithium
(LixC6) anode: LixC6 → xLi+ + xe- + 6C
and are inserted into the lithium compounds (typically a lithium metal
oxide compound (Li1-x MO)), to become lithium compounds with
more lithium (LiMO): Li1-x MO + xLi+ + xe- → LiMO
Nickel Cadmium batteries: Nickel-cadmium batteries have a
cadmium plated anode and a nickel oxide-hydroxide plated cathode
[13]. On discharge, the cadmium (Cd) at the anode is oxidized with
hydroxide ions (OH-) to form cadmium hydroxide (Cd(OH)2) and
electrons: Cd + 2OH- → Cd(OH)2 + 2eThe nickel oxide-hydroxide (NiO(OH)) is reduced with water (H2O)
and electrons to form nickel hydroxide (Ni(OH)2) and hydroxide ions:
NiO(OH) + H2O + e- → Ni(OH)2 + OHNickel-metal hybrid batteries: These batteries are similar to nickelcadmium batteries, but the anode is plated with a metal hydride rather
than cadmium [13]. The metal hydride can be one of a series of
different metals, common examples of which are lanthanum,
neodymium, praseodymium or cerium [13]. On discharge, the metal
hydride (MH) is oxidized with hydroxide ions (OH-) to form solid
metal (M), water (H2O) and electrons: MH + OH- → M + H2O + e-.
The nickel oxide hydroxide (NiO(OH)) is reduced with water and
electrons to form nickel hydroxide (Ni(OH)2) and hydroxide ions:
NiO(OH) + H2O + e- → Ni(OH)2 + OH-.
Sodium-sulfur batteries: Sodium-sulfur batteries have an anode of
molten sodium (Na), and a cathode of molten sulfur (S) [13]. On
discharge, the sodium is oxidized, and the ions pass through an
alumina electrolyte to reduce the sulfur and create sodium polysulfide
(Na2S4). The overall reaction is: 2Na + 4S → Na2S4
Vanadium flow batteries: Vanadium redox flow batteries operate in a
slightly different manner to other batteries [13]. The electrolytes are
stored in tanks, and are pumped through the battery cell, where they
are reduced or oxidized. The anode and cathode electrolytes are made
up of a solution of vanadium in different oxidation states. The anode
electrolyte is a solution of vanadium (II) ions (V+2), which are
oxidized to vanadium (III) ions (V+3) on discharge: V+2 → V+3 + eThe cathode electrolyte is a solution of vanadium (V) oxide ions
(VO2+), which are reduced with hydrogen ions (H+) and electrons, to
form vanadium (IV) oxide ions (VO+2) and hydroxide ions (OH-):
VO2+ + H+ + e- → VO+2 + OH-
electrolyte, rather than between two plates. Because of the large
surface area to volume ratio of activated carbon, and the vanishingly
thin distance over which the charge is stored, ultra capacitors have
much greater capacitance densities and consequently much greater
energy densities of 0.01-0.1MJ/kg.
Superconducting magnetic energy storage: It is the storage of
energy as magnetic energy by charging up a superconducting magnet
with current, which creates a magnetic field [11]. Because there is no
resistance, the current continues to flow. Energy is then released by
discharging the current through an external field. The difficulty with
this system is that the superconducting material needs refrigeration,
which reduces the efficiency, especially over large periods. For this
reason, it tends to be used for short term storage (e.g. for frequency
regulation). However, high temperature superconductors (HTS) (e.g.
YBCO – Yttrium Barium Copper Oxide), which become
superconducting below ~77K require less power for refrigeration than
low temperature superconductors (LTS) which only become
superconducting below ~4K.
Hydrogen Storage
Energy can be stored as the chemical energy of hydrogen [12].
Energy is used to electrolyze water into hydrogen and oxygen, and the
hydrogen stored. When energy is needed again, the stored hydrogen
can be passed through a fuel cell to generate electricity. The water
electrolyzer works by passing a current through water. The only way
this current can flow is if the water (H2O) is broken up into positive
hydrogen ions (H+) and oxygen (O2). At the anode: 2H2O →O2 + 4H+
+ 4eThe hydrogen ions then recombine at the cathode to produce hydrogen
(H2): 2H+ + 2e- → H2
The fuel cell works in a similar way to a battery. Hydrogen passes the
anode, where it is oxidized into hydrogen ions and electrons at the
anode: H2 → 2H+ + 2eAt the cathode, oxygen is reduced with hydrogen to form water:
O2 + 4e- + 4H+ → 2H2O
An electrolyte sits between the anode and cathode, and only allows
hydrogen ions to pass. The electrons are then forced through an
external circuit, where they deliver energy. Catalysts are required at
the cathode and anode of the hydrogen fuel cell to encourage these
reactions to occur. At the anode, platinum is used; at the cathode,
nickel is typically used. An overview of the hydrogen storage methods
available today with respect to their progress made recently and
problems are summarized in the study by Zhou [ ].
RESOURCE
INTENSITIES
PARAMETERS
AND
OPERATIONAL
A deep understanding of the resources intensities and operational
parameters is needed for the development of sustainable, efficient and
cost effective energy storage systems. A review and comparison of
resource intensities is very important for the economic and
sustainability accounting. The operational parameters review and
comparison are needed for the design and performance evaluation
studies and the development of more efficient storage system.
Electro-magnetic Energy Storage
Super-capacitors: Capacitors store electrical energy directly.
Conventional capacitors consist of two plates placed in close
proximity, with a dielectric insulator between them [13]. Energy is
used to remove charge from one and place it on the other, creating a
potential difference between them. Energy is extracted by allowing the
charge to return, after passing through an external circuit where it
delivers energy. Although these can be charged and discharged very
quickly, their specific energy is very small at 0.00005-0.0001MJ/kg.
However, electric double-layer capacitors (EDLCs), or "supercapacitors" offer scope for further improvements. EDLCs store the
charges at the interface between activated carbon and a liquid
Resource Intensities
The resource intensity is a measure of the resources needed for the
energy storage systems [13]. This is a measure of the efficiency of the
resource used and the carbon released to the atmosphere per unit of
storage capacity (MJ). The resource intensities are the key concepts
used for sustainability measurements. The resource intensities for the
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RESULTS
energy storage systems include the type and amount of material the
energy storage system requires; the total area of land the system
occupies; the total energy required (material, fabrication, and
transport) of the energy storage system; the amount of CO2 released to
the atmosphere during the construction of the system, and the cost for
building or purchasing the storage system [13].
 Capital intensity ($/MJ) is the ratio of the total money value of
capital equipment (cost of building or purchasing the storage system
- $) to the total potential output (storage capacity - MJ).
A comparison between energy storage options (electro chemical
and electromagnetic, mechanical, hydrogen, and thermal) for power
systems is presented in this paper. The comparison includes the
resource intensity (capital, material, area, energy and carbon foot print
intensities), and operational parameters (cycle efficiency, cycle life,
specific energy, specific power, and discharging/charging times).
Figure 2 shows the material intensity in kg/MJ for the energy
storage systems. The results show that some of the energy storage
systems such as spring, pumped hydro, flywheel, super capacitor and
super conducting magnetic use more materials per energy storage
capacity. The electro chemical batteries, compressed air, hydrogen and
thermal energy storage systems have the lowest material intensity. The
material intensity is one of the metrics used for sustainability
accounting. Using less material for the construction of the energy
storage system will help not only to reduce the material consumption
but also reduce the energy consumption and CO2 emissions during the
extraction and processing of the material and reduce the cost of the
energy storage system depending on the type of material used. It is
noted that solar power systems have the lowest material intensities
compared to the other renewables, fossil and nuclear power systems
[3]. It is important to select the energy storage system with the best
material intensity for the solar power systems with respect to
sustainability and economic criteria.
 Material intensity (kg/MJ) is the ratio of the total mass of material
(kg) that the storage system typically requires to the total potential
output (storage capacity – MJ).
 Area Intensity (m2/MJ) is the ratio of the total area of land (m2) that
the storage system typically occupies to the total potential output
(storage capacity – MJ).
 Energy Intensity (MJ/MJ) is the ratio of the total energy required to
create the storage system (energy needed to extract and process raw
materials, fabricate components, and construct the storage system,
with transport at different stages of the construction process taken
into account - MJ) to the total potential output (storage capacity –
MJ).
 Carbon intensity (kg/MJ): The ratio of the CO2 (equivalent) released
to the atmosphere (kg of CO2) during the construction of the storage
system to the total potential output (storage capacity – MJ).
Operational Parameters
The development of energy storage devices will dependent on the
cost and efficiency of these systems. It is necessary to know the
operational parameters during the design, analysis and performance
evaluation of the energy storage systems. The operational parameters
are usually selected as the design basis, performance evaluation and
the cost analysis of a given storage system. These parameters include:
the specific energy, energy density, specific power, economic storage
capacity, cycle efficiency, cycle life and the operational cost [13].
 Specific energy (MJ/kg): is the energy the system can store (MJ) per
unit of its mass.
 Energy density (MJ/m3) is the energy the system can store per unit
of volume.
 Specific power (W/kg): represent the rate at which the energy can
be drawn from the system per unit of its mass.
 Economic energy storage capacity (MW) is the range of energy
capacity for which a particular storage system is economically
viable.
 Cyle efficiency (%): is the percentage of the energy put into a
storage system that can be recovered when the energy is retrieved.
The cycle efficiency = (Energy output / Energy input) x 100. The
cycle efficiency is measured for a typical cycle time, and will
usually decrease if the cycle is usually long.
 Cycle life (cycles) is the the number of times an energy storage
system can be charged and discharged before the capacity of the
system drops below 80% of its initial capacity. The cycle life is
limited by factors such as fatigue in mechanical systems and
electrolyte degradation in electrochemical systems.
 Operating cost ($/MJ/cycle) is the approximate cost of one
charge/discharge cycle per unit of energy, and includes the cost of
maintenance, heating, and labor.
Figure 2 Material intensity
Figure 3 shows the energy intensity or the total energy used for the
construction of the energy storage systems per unit of energy storage
capacity. Spring, Super capacitor, and super conducting magnetic use
more energy for the construction of the energy storage systems per
energy output. Compressed air is the energy storage system with the
lowest energy intensity. It is also noted that renewable power systems
such as hydro power (steel reinforced concrete), tidal power (barrage),
deep geothermal power, and solar power plants have high energy
intensities ( MJ/kW: energy used for the construction of the power
plants per power output) compared to conventional power plant such
as natural gas power plants. Solar power systems use more energy
during the construction of the power plant (high energy intensity).
Compressed air energy storage system is the best option to reduce the
total energy used for the construction of the integrated system (solar
power plant and energy storage system).
The CO2 (kg/MJ) intensities during the construction of the energy
storage system show a linear variation of the CO2 intensity with the
energy intensity. The high energy used during the construction of the
energy storage systems for spring storage system is associated with
high carbon footprint or CO2 emissions released in the atmosphere.
The compressed air storage system with the lowest energy intensity
has the lowest CO2 intensity. With respect to electricity generation,
renewable power systems are not carbon free if we look for the life
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cycle of the system. Emission of CO2 is released in the atmosphere
during their construction and their maintenance. The energy and CO2
intensities are two metrics for sustainability accounting. The goal is to
develop an integrated system (renewable power systems such as solar
and energy storage system) with the lowest energy and CO2 intensities.
storage systems show high specific energy. The energy storage
systems with high specific energy (MJ/kg) and high energy density
(MJ/m3) are suitable for small and light storage systems.
Figure 5 Capital Intensity
Figure 3 Energy intensity
The area intensities (m2/MJ) of the energy storage systems are
presented in Figure 4. Renewable power system such as wind and solar
require land area far in excess to conventional power generation
technologies. Most of the renewable energy systems use more area: 50
to 150 times than conventional (coal and natural gas) and nuclear
power systems except geothermal power based plant [3]. It is
important to select more efficient energy storage system with the low
total area of land the storage system typically occupies. Thermal,
compressed air, hydrogen, and electrochemical batteries are examples
of energy storage systems with low area intensities.
Figure 6 Specific energy
Figure 7 shows the results of the specific power of the energy
storage systems. The specific power defines how fast the energy stored
in the system can be discharged or charged. Energy storage system
with high specific power provides peak power requirements: fast
response frequency regulation and short bursts of power that stabilize
the grid. Super capacitors, super conducting magnetic, flywheel and
Lithium ions batteries are examples of energy storage systems that can
be used for energy storage and provide peak power requirements.
The average discharging/charging time for the energy storage
system is presented in Table 1. The average charging time is the ratio
of the average specific energy and the specific power. The results
show average charging times between few seconds to days. Super
capacitors, super conducting magnetic, and flywheel energy storage
systems have charging time of few seconds. Energy storage systems
with fast discharging/charging times or short bursts of power are
suitable for fast response frequency regulation and to stabilize the grid.
Lead acid, Nickel metals, compressed air, and vanadium flow energy
storage systems have charging time of hours. Pumped hydro and
thermal energy storage systems have charging times of day. Pumped
hydro and thermal energy storage systems are more suited for long
duration power systems.
Figure 8 shows also an important operational parameter for the
energy storage systems – the cycle efficiency. It is the ratio of the
energy provided to the user to the energy needed to charge the storage
system. It accounts for the energy loss during the storage period and
the charging and discharging cycle. The cycle efficiency is measured
for a typical cycle time, and will usually decrease if the cycle is long.
Hydrogen storage systems show the lowest cycle efficiency. The low
Figure 4 Area Intensity
The results of the capital intensity or the ratio of the total money
value of capital equipment (cost of building or purchasing the storage
system) to the total potential output or storage capacity are presented
in Figure 5. Solar power systems not only use more energy during the
construction of the power plant (high energy intensity) but also present
the highest capital intensity (US$/kW) compared to all power systems.
Based on the results presented in Figure 5, compressed air is the best
energy storage system to be integrated with solar power with respect to
the capital intensity. The energy storage system with the highest
capital intensity is the super conducting magnetic.
Figure 6-8 show the results of the operational parameters (specific
energy, specific power, average charging time, and cycle efficiency) of
the energy storage systems. The results presented in Figure 6 show the
specific energy or the energy the storage system can store per unit
mass. Hydrogen, electrochemical batteries, compressed air and thermal
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CONCLUSIONS
A comparison study between energy storage options for power
systems is presented in this paper. The comparison study will help to
select the best energy storage option for power systems with
intermittent resources. The results show that:
Discharging/charging time: Super capacitors, super conducting
magnetic, and flywheel energy storage systems have fast
discharging/charging times. They can be used for short bursts of power
and are suitable for fast response frequency regulation and to stabilize
the grid. Batteries and compressed air can be used to store energy for
hours. Pumped hydro and thermal energy storage systems have
charging times of day.
Operation performance: Super-capacitors, superconducting
magnetic and flywheels offer greater performance: high specific
power, high cycle efficiency and high cycle life. These energy storage
systems have high charge/discharge cycles and are more attractive
with respect to the operating costs.
Resource intensities: Super-capacitors, superconducting magnetic
and flywheels do not offer the best resource intensities. The material,
energy, storage area, CO2 and capital cost are higher during the
construction of these energy storage systems. Hydrogen, compressed
air and thermal storage offer the lowest resource intensities.
Based on these results super capacitors, super conducting
magnetic, and flywheel energy storage systems offer the best option
for solar power systems based on the fast discharging/charging times
and greater performance even the resource intensities are not the best.
efficiency of the energy storage systems is due to the losses during the
recovery of the stored energy. Super-capacitors, superconducting
magnetic and flywheels energy storage systems show high cycle
efficiency with the lowest energy losses. Super-capacitors,
superconducting magnetic and flywheels not only have high cycle
efficiency but also high cycle life. The cycle life is the number of
times an energy storage system can be charged and discharged before
the capacity of the system drops below 80% of its initial capacity.
Energy storage systems with low cycle life such as Lead acid, Lithium
ions, Nickel Cadium and Nickel Metal will make the system less
attractive due to the inconvenience of replacement and cost.
Based on the results obtained in this study and the data [13]
summarized in Table 1, super capacitors, super conducting magnetic,
and flywheel energy storage systems offer the best option for solar
power systems based on their fast discharging/charging times and
greater performance even they don’t offer the best resource intensities.
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Figure 7 Specific power
Figure 8 Cycle efficiency
Table 1 Resource intensities and operating parameters of energy
storage systems
210
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