Designing a Battery-Based System Step-by-Step

Designing a Battery-Based System Step-by-Step
Designing a Battery-Based System
Step-by-Step
Brian Teitelbaum
Application Engineer
Tuesday, January 19, 2016
San Diego, CA
Introduction
• Disclaimer
AEE Solar is a distributor of goods and services used in the
deployment of PV and wind distributed power systems. We are not
accountants, attorneys or Code-making experts. The information
presented here represents the equipment, rules and best practices
that we are aware of. However, local and project-specific
requirements can vary widely.
Types of Battery-Based
Systems
PV/wind direct
• Loads are run directly from renewable energy source
• No energy storage (batteries)
• Loads typically motors (pumps, fans, etc.) that run directly from the
energy source
DC-Only
• All loads run on DC from a battery
• Batteries charged by PV, wind turbine, generator, etc.
AC-only
• All loads run on AC power from an inverter or AC generator
• Most common type of Off-Grid system used for homes
AC/DC
• Both AC and DC loads are powered by the system
Hybrid
• Derives energy from more than one source
• i.e. PV and wind, PV and utility grid, or PV and a generator
Designing a Battery-Based System
Step-by-Step
System Sizing
User Data You Will Need to
Collect
•
Daily consumption (Watt-hours)
‒ How much energy will the application consume each day? Is it seasonal?
‒ For each load, multiply the power draw by the hours it is used per day
‒ For appliances, divide the Energy Star annual consumption kWh by 365
‒
http://www.energystar.gov/index.cfm?c=products.pr_find_es_products
•
Peak load (Watts) and characteristics (VDC/VAC/Hz)
‒ Sum of all loads that may be run simultaneously
‒ Separate AC and DC Loads
‒ Voltage and frequency the loads require
•
Days of Autonomy
‒ How many days in a row will the loads need to run with little or no sun
‒ Don’t neglect heavy snows
•
Sun-Hours per day during darkest month (kWh/day)
‒ This is the available solar resource
‒ Use Winter Solstice time frame rather than annual average if loads will be run
during winter
Load Analysis
•
Load Worksheet –
‒ 2016 AEE Solar Design Guide and Catalog – Page 9
Don’t Forget “Stand-by” Loads
123.5W X 24hrs = 2964Wh = 3kWh/day
System Options You Will
Need to Choose
•
Battery Type: Flooded, AGM, Gel or Advanced
‒ Consider: Maintenance, shipping and storage requirements
•
Battery bank DC voltage : 12, 24, 48 VDC (or high-voltage)
‒ Consider: Voltage of DC loads, size of system, available inverters
‒ 48VDC is generally most efficient and cost-effective for AC systems over 2kW
•
Charge controller type: PWM or MPPT
‒ Pulse Width Modulated (PWM) controllers
Pros: Inexpensive and compact
Cons: Low capacity (up to 60A), input voltage limitations (12/24/48VDC array)
‒ Maximum Power Point Tracking (MPPT) controllers
Pros: Maximized energy harvest, wider input voltage range (48 to 600VDC),
higher capacity (up to 100A)
Cons: More Expensive , physically larger
•
Module Type: 36-cell, 60-cell or 72-cell
‒ PWM controllers require 36-cell (12 VDC nominal) or 72-cell (24 VDC nominal)
modules
‒ Consider transportation and mounting limitations as well as module cost per watt
PV Array Sizing for
Off-Grid Systems
• PV arrays for Off-Grid systems are based on the peak sun-hours
during the darkest month of the year, not the yearly average
• Most Off-Grid systems require some sort of back-up power,
usually a generator, for extended cloudy weather.
PV Array Sizing
Peak Sun-Hours
NREL Red Book
PV Array Sizing
Peak Sun-Hours
PV Array Sizing
Peak Sun-Hours
AEE Solar Catalog Maps
•
In Reference Section
in the back of the
Catalog
•
These maps show the
Peak Sun-Hours for
the darkest month of
the year
•
NOT the yearly
average
•
They are useful for
sizing off-grid
systems, not grid-tie
systems
PV Array Sizing
For PWM Controllers
•
Find the current the array must produce
𝐷𝑎𝑖𝑙𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝐴ℎ) × 1.2
= 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐴𝑟𝑟𝑎𝑦 𝐴𝑚𝑝𝑠
𝑘𝑊ℎ 𝑝𝑒𝑟 𝑚2 𝑝𝑒𝑟 𝑑𝑎𝑦
•
Select a PV module
‒ Find the peak current rating (Imp) on the module data sheet
•
Determine the number of parallel strings required
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐴𝑟𝑟𝑎𝑦 𝐴𝑚𝑝𝑠
= 𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑟𝑖𝑛𝑔𝑠
𝑀𝑜𝑑𝑢𝑙𝑒 𝐼𝑚𝑝
•
Determine the number of modules per string
12 VDC modules (36-cell) :
24 VDC modules (72-cell) :
•
12 VDC
10 A
12 VDC system = 1 per string
24 VDC system = 2 per sting
12 VDC system = NA
24 VDC system = 1 per string
Determine the total number of modules
𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑟𝑖𝑛𝑔𝑠 × 𝑆𝑡𝑟𝑖𝑛𝑔 𝑙𝑒𝑛𝑔𝑡ℎ = 𝑇𝑜𝑡𝑎𝑙 𝑚𝑜𝑑𝑢𝑙𝑒𝑠
24 VDC
5A
MPPT Charge Controllers
•
Charge controllers for modern 60 cell modules must be
MPPT type
‒ MPPT = Maximum Power Point Tracking
•
An MPPT controller will convert the input voltage to the correct
voltage for charging the battery
•
Array voltage must be higher than the battery voltage
•
MPPT Charge controllers are current limited
A 4kW array will need one 80 A change controller for a 48 VDC battery,
but will require two controllers for a 24 VDC battery
80 A x 48 VDC = 3,840 W
80 A x 24 VDC = 1,920 W
•
The PV array will rarely put out full rated power except at high
altitude sites
‒ Reasonable oversizing of array vs. charge controller can minimize cost
‒ Always have overcurrent protection between array and controller
Charge Controllers
String Sizing (MPPT)
•
String operating voltage must be between battery charging voltage
and controller limit (usually 150 VDC)
‒ Power will drop off dramatically if the charge point of the array falls below
the battery voltage
• A 48 VDC battery charges at 56 VDC or more

Requires at least three 60-cell modules to charge correctly
2 modules x 27 VDC = 54 VDC
2 modules in series will typically not charge a 48 VDC battery at full power in
hot weather
•
Most 60 cell modules will exceed 150 VDC in strings of 4 in most
locations
•
A 48 VDC battery requires 3 modules in series only
•
A 24 VDC battery can typically use either 2 or 3 modules in series
PV Array Combiner Boxes
•
Each parallel string of modules must have circuit protection
‒ Most modules have a 15 A circuit rating
‒ Under fault conditions, the array can be exposed to full short circuit current of the
battery bank
•
The breakers in these circuits must be rated for the maximum voltage
‒ Maximum voltage for many of these systems will be 150 VDC
•
High-voltage charge controllers require higher voltage breakers or fuses and a
proper combiner box for them.
•
All array circuits going to each charge controller must have a separate and
isolated feeder to that charge controller
•
Some combiner boxes have the capacity for two separate circuits
‒ Multiple combiners may be more convenient for wire management
Balance of System:
Charge Controller Circuit Protection
•
Disconnects and circuit protection are required between the PV array and the
charge controller, and between the charge controller and the battery
•
A circuit breaker is normally used for 150 VDC PV input circuits to controller
‒ This breaker must be sized for 156% (125% x 125%) of Isc of array (STC)
‒ Breaker not to exceed the maximum input amperage rating for the charge controller
‒ Wire between breaker and the combiner box must meet or exceed the current rating of
the breaker used
•
The charge controller breaker and disconnect serves as the battery breaker
‒ If it matches the charge controller output rating it must be rated for continuous duty
‒ If not rated for continuous duty at full amperage, size to 125% of max current
‒ For OutBack systems, this is typically the GFDI breaker
‒ If the charge controller will be operated near its limit, oversize the battery breaker
slightly to avoid nuisance tripping
Balance of System:
Ground and Arc Fault Protection
•
Ground fault detection and disconnect (GFDI) is required for most residential
systems
‒ DC-GFDI or “DC-GFP” assemblies
•
A DC-GFDI is simply a small breaker’s pole connected to larger breaker poles
‒ The smaller breaker pole (usually 0.5A) connects the negative array conductor to ground
‒ This connection is the DC negative-to-ground bond
‒ If the GFDI breaker trips from current flowing between negative and ground, it also
disconnects the PV array and/or charge controller
•
The GFDI is traditionally installed between the array breaker and charge
controller
‒ OutBack also uses it between the charge controller and battery, serving as the charge
controller disconnect
•
The 2011 NEC code also requires Arc Fault detection in the PV system
‒ Stand-alone arc fault detectors are not available at this time
Designing a Battery-Based System
Step-by-Step
Charging Batteries
Flooded Cell
(Deep Cycle Type)
Pros
Cons
•
Lower initial cost than VRLA in •
smaller sizes
•
Excellent cycle life
•
Electrolyte loss is replaceable
•
Less sensitive to overcharging
•
Not sealed
o Cannot be mounted on its side
o Cannot be shipped by air
o Requires isolation & ventilation
Maintenance required
o Water must be added ~monthly
o Regular equalization charge
o Reduced life at higher temperatures
•
Less efficient
Higher self discharge,
Much higher float current >1kWh
per day likely.
•
Sensitive to shock and vibration
Gel-Cell VRLA
Pros
Cons
•
Low maintenance
•
Sensitive to overcharging
•
Sealed
•
Reduced life at higher temperatures
•
Lower charge & discharge rates
•
Slightly higher cost than AGM
•
Lower capacity
units available
than flooded cell
or AGM
o Can often be installed on its
side
o Can often be shipped by air
•
Very good cycle life
•
High efficiency
Low self discharge and float
current
•
Shock and vibration resistant
Absorbed Glass Mat
(AGM) VRLA
Pros
•
Low maintenance
•
Sealed
•
High efficiency
Low self discharge and float
current
o Can be installed on its side
o Can often be shipped by air
•
Shock and vibration resistant
•
Works in high-power
applications
•
Large capacity batteries
available
Cons
•
Sensitive to overcharging
•
Reduced life at higher temperatures
•
Electrolyte cannot be replaced
•
Higher initial cost in smaller sizes
than flooded
AGM with Carbon
•
Carbon is added to the negative plate of lead acid
batteries, either sealed or flooded, to improve partial
state of charge performance
•
Extended partial state of charge (PSoC)
Sulfation on the negative plate is minimized
Much less need for regular full charge
Excellent for self-consumption or off-grid systems
No advantage for standby backup systems
•
Longer cycle life in real conditions
‒ Can be cycled in mid state of charge
‒ Up to 44% improvement in cycle life
‒ Only about 15% higher price
•
Available in AGM or Flooded
OutBack Energy Cell Nano-Carbon AGM, in a wide range of
sizes
Trojan SmartCarbon flooded commercial and industrial
Aqueous Hybrid Ion
(Aquion)
Pros
•
High cycle life at high cycle depth
o
o
o
•
Durable and abuse tolerant
o
o
o
o
•
Completely tolerant of partial state of
charge
Self balancing in parallel strings
Can be completely depleted without harm
No active management necessary
Very safe
o
o
o
o
•
Up to 3,000 cycles @ 100% DoD
Up to 6,000 cycles @ 50% DoD
Operates from -5°C to 40°C with full life
Limited power output to shorts
Will not support flame
Will not explode
No thermal runaway issues
Cradle to Cradle certified
o
o
All common non-toxic materials
Sodium, and a little Lithium, salt water
electrolyte
Cons
•
Moderately expensive
o
o
•
2 to 3 X price of lead-acid batteries
Available in 48 V blocks only
Low power and energy density
o
o
o
o
Requires more space for the same capacity
Wider voltage range ~40 V to 57 V
Low rate of charge and discharge
Limited capacity at higher rates of charge and
discharge
Tesla Powerwall
Pros
•
High power and energy density
o
o
o
•
High cycle life at high cycle depth
o
o
o
o
•
Up to 3,000 cycles @ 100% DoD
High round trip efficiency up to 92.5%
Control of thermal issues
Use between -20°C to 50°C with internal
cooling and heating
Easy to install
o
o
o
o
o
•
6.4 kWh daily version or 9.1 kWh weekly
version
High voltage to substantially reduce
current and wire size
Takes up little space, wall hung
Integrates to solar inverters, SolarEdge to
start
Light weight
Touch safe
Outdoor rated
Multiple units for larger capacity up to 7
units
Pleasing design
o
Decorative cover
Cons
•
Moderately expensive
o
o
•
2 to 3 X price of lead-acid batteries
May have limited calendar life ~10 years
Availability
o
o
o
Installers must complete special training
and legal agreements before selling
Limited to two parallel units to start
Limited inverter compatibility
Lithium Ion
Pros
•
•
Widely used in laptops, EVs and
handheld devices
•
Expensive
o
o
2 to 5 X price of lead-acid batteries
May have limited calendar life ~10 years
High power and energy density
o
o
•
Cons
Discharge rates up to 40X nominal Ah
capacity
Up to ~40 Wh/lb
•
Thermal management challenges
o
o
o
High cycle life at high cycle depth
o
Up to 3,000 cycles @ 80 to 90% DoD
•
Requires sophisticated battery management
system
o
o
•
Operates between ~32 °F and 100 °F
Risk of thermal runaway  fire or explosion
LiFePO and related chemistries less prone to
thermal runaway risk, but more expensive and
less energy density
Adds to cost
Adds significant design complexity
Non-recyclable and hazardous
o
o
o
Disposal costs not typically included in price
Shipping restrictions
Will support combustion in a fire
Nickel Based Batteries
NiCd, NiFe, NiMH
Pros
•
Good energy and power density
o
•
o
Up
for
Up
for
to 11,000 cycles or more @ 80% DoD
flooded NiFe
to 5,000 cycles or more @ 80% DoD
flooded NiCd
Robust
o
o
o
o
•
About 60% of Lithium-ion
High cycle life at high cycle depth
o
•
Cons
Flooded cells only, not sealed
No sudden failures
Tolerant of under and overcharging
Very wide operating temperature range
•
Flat voltage vs. state of charge curve
•
Less hazardous materials
Very Expensive
o
o
•
Low cell voltage
o
o
•
•
3 - 5 X price of lead-acid batteries
Heavy metals, Nickel and/or Cadmium
Low cell voltage (~1.2 VDC)
More cells required per system
High self-discharge rate
Memory effect in sealed NiCd cells
o
o
Need to fully cycle about once per month
Limited reconditioning is possible
Battery Capacity
•
Capacity = Ampere-hours provided to
the load(s)
‒ Changes according to discharge rate,
temperature, age, etc.
‒ 20-hour rating = 100% capacity = C20
‒
8-hour rating = 88% of C20
‒
6-hour rating = 84% of C20
‒
3-hour rating = 74% of C20
‒
1-hour rating = 59% of C20
•
Batteries deliver less than 100% rated
capacity when new.
‒ Poor maintenance will affect capacity
•
Batteries self discharge if stored for
long periods of time without being
charged.
‒ The hotter the temperature the more
self discharge
Temperature vs. Capacity
Battery Wiring
•
Series: Voltage is additive; capacity remains same
‒ Positive of Battery 1 is connected to negative of
Battery 2 and so on
Example: Two 12 VDC, 220 Ah batteries in series will
yield a 24 VDC, 220 Ah battery
Series connection
•
Parallel: Capacity is additive; voltage remains same
‒ The positives are connected to each other, same for
negatives
‒ Output leads must be from first and last battery for
electrical balance
‒ Manufacturers typically limit maximum number of
parallel strings to three
Example: Two 12 VDC, 220 Ah batteries in parallel will a
12 VDC, 440 Ah battery
Parallel connection
Battery Charging
•
Lead Acid Batteries should be charged after every use to ensure they
are never stored in a discharged condition (May not be as necessary
for added Carbon batteries)
‒ If batteries are stored for extended periods of time they should be charged
approximately every 6 weeks
‒ Between 105-120% of previously discharged capacity must be returned for
full charge
‒ No need to fully discharge prior to charging
‒ Charging should be temperature corrected
‒
•
Always use charge controller’s temperature sensor when available
Flooded Batteries need to be overcharged to ensure proper mixing of
the electrolyte and avoid stratification
‒ “Equalization” - deliberate, periodic overcharge to prevent electrolyte
stratification
‒ Most charge controllers have an equalize charge setting
Battery Voltages
Condition (@77F)
•
Nominal Battery Bank Voltage
12 VDC
24 VDC
48 VDC
Fully charged – no load
12.7 VDC
25.4 VDC
50.8 VDC
20% charged – no load
11.6 VDC
23.2 VDC
46.4 VDC
90% charged – charging
15 VDC
30 VDC
60 VDC
Fully charged - equalizing
>15 VDC
>30 VDC
>60 VDC
Fully charged – under heavy load
11.5 VDC
23 VDC
46 VDC
20% charged – under heavy load
10.2 VDC
20.4 VDC
40.8 VDC
Charging voltage must increase as temperature falls
‒ Charging voltage must be higher in cold weather and lower in hot weather
•
A battery monitor keeps track of amp-hours, voltage and temperature to
determine state of charge
‒ OutBack FlexNet DC, Magnum BMK, Schneider Battery Monitor, Bogart Trimetric
•
Check battery voltages or specific gravity during any maintenance call
‒ This is often the first indication that something could be wrong
Depth of Discharge
•
Deep Discharging will shorten
battery life
•
Most lead-acid batteries
designed for 50% DoD
•
Deep-Cycle Batteries
designed for up to 80% DoD
‒Shallower cycles will enable
longer life
•
Never leave batteries
discharged for more than 1-2
days!
‒Sulfating of electrodes will
permanently decrease capacity
(much less so for Nano-Carbon)
Watering Flooded Batteries
•
Add distilled water to cells AFTER charging
‒ Never add acid/electrolyte to cells
‒ Fill to 1/8” below the bottom of the fill well or
to maximum level indicator
•
Do not overfill the batteries
•
If the plates are exposed, add water to discharged
batteries to just above the plates
‒ Do not fill all the way!
•
Never add water to discharged batteries if the
electrolyte is visible above the plates!
‒ This will cause the batteries to spill over when
they are charged
•
Single-point watering systems can make it much
easier and safer
Be sure system is compatible with batteries used
Battery Safety
•
Always wear personal protective
equipment when handling batteries
‒ Chemical-resistant gloves, goggles or
face shield, acid-resistant apron and
boots
•
Keep flames, sparks or metal objects
away from batteries
‒ Use insulated tools
‒ Do not smoke near batteries
‒ No metal jewelry
•
Neutralize acid spills with baking soda
immediately
•
Ensure that vent caps are securely in
place before charging
•
Provide proper ventilation to prevent
gas build up
Periodic Inspection and Cleaning
•
Keep batteries clean and dry
•
Check that all vent caps are tight
•
Check that all connections are tight
•
Terminal protector should be
applied to terminals to reduce
corrosion
•
Use a solution of baking soda and
water to clean any acid residue on
batteries or corrosion on the
terminals
Battery Enclosures
•
Batteries must be properly housed and sited
•
Keep batteries in temperature-controlled space
‒ Basements or garages are typical
•
Battery enclosure must be ventilated
‒ Active ventilation may be required for flooded
batteries in hot or tightly enclosed spaces
•
Protect terminals from people, pets and falling
objects
•
Ensure that battery rack/enclosure meets any
applicable seismic requirements for your region
•
Breakers/Switches help improve safety when
using and servicing batteries
Battery Bank Sizing
•
Daily energy consumption
𝑨𝒉 =
𝑾𝒉
𝑽𝑫𝑪𝒃𝒂𝒕𝒕𝒆𝒓𝒚 𝒃𝒂𝒏𝒌
‒ Sum the DC loads and convert to Ah
‒ Sum the AC Loads and convert to Ah, then divide by inverter efficiency
• 𝑹𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑩𝒂𝒏𝒌 𝑪𝒂𝒑𝒂𝒄𝒕𝒊𝒕𝒚 =
𝑨𝒗𝒆 𝑫𝒂𝒊𝒍𝒚 𝑪𝒐𝒏𝒔𝒖𝒎𝒑𝒕𝒊𝒐𝒏 × (𝑫𝒂𝒚𝒔 𝒐𝒇 𝑨𝒖𝒕𝒐𝒏𝒐𝒎𝒚)
× 𝑻𝒆𝒎𝒑 𝒎𝒖𝒍𝒕𝒊𝒑𝒍𝒊𝒆𝒓
(𝑹𝒆𝒄𝒐𝒎𝒎𝒆𝒏𝒅𝒆𝒅 𝑫𝒆𝒑𝒕𝒉 𝒐𝒇 𝑫𝒊𝒔𝒄𝒉𝒂𝒓𝒈𝒆)
•
𝑹𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑩𝒂𝒏𝒌 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚
𝑼𝒏𝒊𝒕 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚
= 𝑻𝒐𝒕𝒂𝒍 𝑷𝒂𝒓𝒂𝒍𝒍𝒆𝒍 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑺𝒕𝒓𝒊𝒏𝒈𝒔
‒ Limit total strings to 3 or less (for common lead-acid batteries)
•
𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑩𝒂𝒏𝒌 𝑽𝒐𝒍𝒕𝒂𝒈𝒆
𝑼𝒏𝒊𝒕 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑽𝒐𝒍𝒕𝒂𝒈𝒆
•
𝑩𝒂𝒕𝒕𝒆𝒓𝒊𝒆𝒔 𝒑𝒆𝒓 𝑺𝒕𝒓𝒊𝒏𝒈 × 𝑻𝒐𝒕𝒂𝒍 𝑷𝒂𝒓𝒂𝒍𝒍𝒆𝒍 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑺𝒕𝒓𝒊𝒏𝒈𝒔 = 𝑻𝒐𝒕𝒂𝒍 𝑩𝒂𝒕𝒕𝒆𝒓𝒊𝒆𝒔
•
May need to try multiple different battery sizes
= 𝑩𝒂𝒕𝒕𝒆𝒓𝒊𝒆𝒔 𝒑𝒆𝒓 𝑺𝒕𝒓𝒊𝒏𝒈
Designing a Battery-Based System
Step-by-Step
Inverters:
Making AC from
Batteries
Battery-Based vs. Grid-tie
Inverters
Battery-Based Inverters
•
•
Inverter fed from steady
12/24/48VDC from batteries
PV array MPPT provided by charge
controller
o
•
Inverter builds its own waveform
Fixed low-voltage input
o
•
•
Voltage sourced
o
Charge controller dictates string
size
Modest efficiency: 75-90%
•
Grid-tie Inverters
Low-voltage
single-phase 120
or 240VAC output
Current sourced
o
•
Actively controlled voltage input
o
•
Inverter uses grid waveform
Inverter uses MPPT and accepts wide
range of voltages
Inverter provides PV array MPPT
o
String sizing dictated by inverter
•
High efficiency: 95-98%
•
Range of outputs
o
240VAC or 208/277/480VAC 3-phase
Inverter Sizing
•
Find maximum AC load
‒ Identify and sum all loads that may run
simultaneously
‒ Sum up total Watts – this is the minimum inverter
continuous power rating
•
Identify any loads with high start-up or surge
current draws
‒ Motors in compressors, fans and appliances
‒ Gauss rifles/rail guns, electromagnets and other large
inductive loads
‒ Largest surge load determines minimum inverter
surge rating
•
Identify any loads that may require 240 VAC
‒ 240 VAC often requires transformer or dual inverters
•
Be sure to consider inverter’s no-load-draw when
sizing battery system and array
Inverter Selection Considerations
•
Will there be an AC generator in the system?
‒ Be sure inverter has generator input for battery
charging
‒ Be sure that inverter and generator are compatible
•
Will the system connect to the utility grid?
‒ Inverter must be “grid interactive” – these inverters
have internal transfer switches to comply with UL
1741/IEEE 1547 anti-islanding requirements
•
Will the inverter be mounted outdoors?
‒ Most battery-based inverters are only rated for indoor
use (NEMA 1)
•
Do the loads include sensitive electronics or audio
equipment?
‒ Modified sine wave inverters may damage certain
electronics and will interfere with most speakers
Integration Hardware
•
Over-current devices – breakers
and fuses
•
DC Ground-Fault Protection (GFP)
•
Bus Bars
•
Combiner boxes
•
Grounding
•
Generator Start Controls
•
Amp-Hour Meters
•
System Monitoring
Balance of System (BOS):
Power Panels
Balance of System (BOS):
Power Panels
•
A central location is desired to connect wiring and install breakers
•
•
A battery-based inverter can have a very large current draw
•
•
Especially when battery voltage is lower
The main DC breaker for these inverters is 125 A to 250 A
•
•
•
There needs to be a DC load center and an AC load center, or one load
center for both AC and DC
The inverter manufacturer or supplier will generally specify breaker sizes
Beaker size is generally max power output in watts divided by battery
voltage x 1.5, but sometimes larger
Wire size for battery and inverter circuits will commonly be 2/0 or 4/0
AWG
•
•
Keep connection as short as possible to minimize voltage drop
Under 10 ft is best
Balance of System (BOS):
Power Systems
•
Factory pre-wired power systems are
available to simplify design and installation
•
•
Several common sizes & configurations
Pre-wired power systems include:
•
Inverter(s)
•
Controller and networking devices
•
Battery monitor
•
Integration hardware and BOS
•
Enclosures, Breakers, GFDI, Bypass, etc.
•
Charge controllers
Balance of System (BOS):
System Controllers
• Automate battery management
•
Float and equalization charge timing
• Turn on/shut off inverters and/or
generators according to time of day or
battery state of charge
• Remote monitoring/control via Internet
Example: AC-Only System
•
Off-Grid house outside of Moab, UT
•
•
•
•
Involuntary loads
•
•
•
•
•
Full-time residence
Typical 2-3 bedroom house
Propane available for water and space
heating
Refrigerator & Freezer
HVAC Air handler
Bathroom/kitchen vent fans
Water pump
Voluntary loads
•
•
•
•
•
•
•
Lights
Computer/entertainment system
Washer/gas dryer
Microwave oven
Toaster
Hair dryer
Battery chargers
•
Other considerations
•
•
Snow may reduce output for up to
2 days at a time
Well is separate PV-direct system,
but house pressure is from AC
pump
Descriptions of AC loads run by inverter
furnace blower or pumps
well pump
septic pump
lights
computer & monitor
router and network
printer & peripherals
tv & dvd player or similar
satellite dish
stereo
refrigerator
400
(annual kWh from energy guide above)
freezer
500
(annual kWh from energy guide above)
fans
microwave
Washer
Dryer
Phone & device chargers
Toaster
Hair dryer
Inverter no-load draw
Loads
Peak watts
Inverter efficiency
87%
X Hours/Week X Watt-Hrs/Week
Watts
250
250
5
5
130
50
25
100
135
110
15
250
42
15
168
1
35
28
35
250
134
1000
240
240
25
850
850
30
4934
1250
1250
0
5460
750
4200
100
4725
3080
525
7692
9615
28
1.9
10
10
32.5
0.5
1
28
Total AC Wh/Week
Total AC Wh/Day
5671 Adjusted for inv. eff.
Divide by 7 days to get average daily DC watt-hours
3752
1900
2400
2400
813
425
850
840
0
52027
7432
59801
8543
System Data
•
Daily consumption (Watt-hours from worksheet )
•
Peak load (Watts) and characteristics (VDC/VAC/Hz)
•
Days of Autonomy
8543 Wh
5671 W @ 120VAC
2 Days
Finding Solar Resource
Example system location:
Moab, UT
Site Latitude: 38.6°N
Be sure to set the correct
Tilt Angle and Orientation
that the PV array will
actually have
Peak Sun-Hours during
darkest month
(December) is 4.7
System Options
•
Battery Type: Flooded, AGM or Gel
‒ Considerations: Large system, user on-site
‒ Flooded battery likely to be most cost effective, but AGM may be better for
the user
•
Battery bank DC voltage : 12, 24, or 48 VDC
‒ Considerations: large system, all loads go through inverter
‒ 48 VDC battery bank likely to be most cost-effective
•
Charge controller type: PWM or MPPT
‒ Considerations: Large system, 48 VDC battery bank
‒ MPPT controller only type large enough
‒
•
Module Type: 36-cell, 60-cell or 72-cell
‒ Considerations: MPPT charge controller, multi-kW sized array
‒ 60-cell modules likely most cost effective
Battery Bank Sizing
• Daily energy consumption
A𝒉 =
𝑾𝒉
𝑽𝑫𝑪𝒃𝒂𝒕𝒕𝒆𝒓𝒚 𝒃𝒂𝒏𝒌
=
𝟖𝟓𝟒𝟑 𝑾𝒉
𝟒𝟖 𝑽𝑫𝑪
= 𝟏𝟕𝟖 𝑨𝒉
• 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝐵𝑎𝑛𝑘 𝐶𝑎𝑝𝑎𝑐𝑡𝑖𝑡𝑦 =
𝑻𝒐𝒕𝒂𝒍 𝑫𝒂𝒊𝒍𝒚 𝑪𝒐𝒏𝒔𝒖𝒎𝒑𝒕𝒊𝒐𝒏 × (𝑫𝒂𝒚𝒔 𝒐𝒇 𝑨𝒖𝒕𝒐𝒏𝒐𝒎𝒚)
× 𝑻𝒆𝒎𝒑 𝒎𝒖𝒍𝒕𝒊𝒑𝒍𝒊𝒆𝒓
(𝑹𝒆𝒄𝒐𝒎𝒎𝒆𝒏𝒅𝒆𝒅 𝑫𝒆𝒑𝒕𝒉 𝒐𝒇 𝑫𝒊𝒔𝒄𝒉𝒂𝒓𝒈𝒆)
𝟏𝟕𝟖 × (𝟐)
𝑹𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝑩𝒂𝒏𝒌 𝑪𝒂𝒑𝒂𝒄𝒕𝒊𝒕𝒚 =
× 𝟏. 𝟎 = 𝟕𝟏𝟐 𝑨𝒉
(𝟓𝟎%)
•
What battery to use?
3-string configuration:
𝑴𝒊𝒏𝒊𝒎𝒖𝒎 𝒃𝒂𝒕𝒕𝒆𝒓𝒚 𝒓𝒂𝒕𝒊𝒏𝒈 =
𝑹𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚 𝟕𝟏𝟐 𝑨𝒉
=
= 𝟐𝟑𝟕 𝑨𝒉
𝟑 𝒔𝒕𝒓𝒊𝒏𝒈𝒔
𝟑
2-string configuration:
𝑴𝒊𝒏𝒊𝒎𝒖𝒎 𝒃𝒂𝒕𝒕𝒆𝒓𝒚 𝒓𝒂𝒕𝒊𝒏𝒈 =
𝑹𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚 𝟕𝟏𝟐 𝑨𝒉
=
= 𝟑𝟓𝟔 𝑨𝒉
𝟐 𝒔𝒕𝒓𝒊𝒏𝒈𝒔
𝟐
1-string configuration:
𝑴𝒊𝒏𝒊𝒎𝒖𝒎 𝒃𝒂𝒕𝒕𝒆𝒓𝒚 𝒓𝒂𝒕𝒊𝒏𝒈 =
𝑹𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚 𝟕𝟏𝟐 𝑨𝒉
=
= 𝟕𝟏𝟐 𝑨𝒉
𝟏 𝒔𝒕𝒓𝒊𝒏𝒈𝒔
𝟏
Battery Bank Sizing
Battery
Volts
Ah
#/string
Strings
QTY
Unit $
Total $
IND9-6V
6 VDC
445
8
2
16
$1,250
$20,000
IND13-6V
6 VDC
673
8
1
8
$1,705
$13,640
EnergyCell
800RE
2 VDC
672
24
1
24
NA
$14,161
EnergyCell
1100RE
2 VDC
960
24
1
24
NA
$18,345
𝑺𝒕𝒓𝒊𝒏𝒈 𝒍𝒆𝒏𝒈𝒕𝒉 =
𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝒃𝒂𝒏𝒌 𝒗𝒐𝒍𝒕𝒂𝒈𝒆
𝑩𝒂𝒕𝒕𝒆𝒓𝒚 𝒖𝒏𝒊𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆
PV Array Sizing
•
Loads consume 8543 Wh/day and site receives ~ 4.7 kWh/m2/day in winter
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐴𝑟𝑟𝑎𝑦 𝑊𝑎𝑡𝑡𝑠 =
•
𝐷𝑎𝑖𝑙𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝑊ℎ) × 1.2
8543 𝑊ℎ × 1.2 10252
=
=
= 𝟐𝟏𝟖𝟐 𝑊
𝑘𝑊ℎ 𝑝𝑒𝑟 𝑚2 𝑝𝑒𝑟 𝑑𝑎𝑦
4.7 𝑘𝑊ℎ
4.7
Select a PV Module
Considerations: 60-cell, MPPT charge controller, standard roof-mount racking
PV Array Sizing
•
Minimum required PV array wattage = 2182 W
•
Determine the number of modules required
𝑃𝑉 𝑎𝑟𝑟𝑎𝑦 𝑠𝑖𝑧𝑒
‒ 𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑑𝑢𝑙𝑒𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 𝑀𝑜𝑑𝑢𝑙𝑒 𝑁𝑎𝑚𝑒𝑝𝑙𝑎𝑡𝑒 =
2182 𝑊
255𝑊
= 8.6
‒ Most MPPT charge controllers only accept 3 modules per string for a 48 VDC
system
3 𝑚𝑜𝑑𝑢𝑙𝑒𝑠 × 4 𝑠𝑡𝑟𝑖𝑛𝑔𝑠 = 9 𝑚𝑜𝑑𝑢𝑙𝑒𝑠 
Always round up when determining number of modules!
 Recommend oversizing to 12 modules
Charge Controller Selection
•
System Requirements
‒ Compatible with sealed AGM batteries, array size is 3,060 W
•
Note input voltage limit is 150 VDC
•
Maximum charge current for the EnergyCell
1100RE is 212 A
•
4 strings of 3 modules each
Inverter Selection
•
System requirements
‒ 5,671 W Peak AC Load, All loads 120 VAC/60 Hz, 48 VDC battery bank
‒ Maximum surge load (Washer motor startup): 12 A (1,440 W)
‒ AC generator?
System Components
X4
Designing a Battery-Based System
Step-by-Step
Grid-tie Battery
Backup: Backing
up the Grid
Types of Grid-tie Battery Systems
•
Backup system separate from the solar grid-tied system
•
Grid-tied solar systems with battery backup and protected loads panel
‒
‒
‒
‒
•
Basic DC coupled grid-tie w/ Battery Backup (GTBB)
GTBB using 120VAC only inverters with 120VAC loads
GTBB using 120VAC only inverters and transformer for 120/240VAC loads
GTBB using 120/240VAC inverters
Grid-tied solar systems with battery backup for whole house backup
‒ GTBB for whole house using a 4 pole manual transfer switch
‒ GTBB for whole house using line side tap and manual transfer switch
‒ GTBB for whole house using an automatic transfer switch
•
Grid-tied solar systems using AC coupling
‒ GTBB using AC coupling and protected loads panel
Direct Grid-tied System without
Backup
•
•
This is a simple grid-tie system without any backup power
When the grid power goes out, the PV system is useless
Backup System Separate from
Solar Grid-tied System
•
•
The backup system can be separate from the solar system
This could also be a generator only or have both inverter and generator
DC-Coupled Grid-Tie w/ Battery
Backup (GTBB)
•
•
•
The original and most common arrangement for grid-tie with backup
DC coupled with low voltage array, charge controller, and battery-based grid-tie
inverter
Protected loads panel for loads to be backed up
GTBB Using 120 VAC Inverters
with 120 VAC Loads
•
Some grid-tie battery inverters just provide a single 120 VAC leg
•
These inverters also have only one AC input
‒
Transfer switch needed for running with a generator
GTBB Using 120 VAC Inverters
with 120/240 VAC Loads
•
GTBB 120 VAC system w/protected load panel and transformer for 240 VAC loads
•
Use standard US house wiring at 120/240 VAC
•
Grid connection and generator input is still only 120 VAC
GTBB Using 120/240 VAC Inverters
with Protected Load Panel
•
Using a 120/240 VAC split phase inverter with separate generator input is the
most versatile and easy to use system
•
120/240 VAC protected loads sub-panel is used
•
Grid connection and generator input are both 120/240 VAC
Whole House Backup Issues
•
NEC 702 5(B) requires a system that can power all loads if an automatic
transfer switch is used,
‒ Exception for loads that are automatically disconnected
‒ A 200A service would require six Radian 8kW inverters
•
A Manual transfer switch only requires the system to power chosen loads
‒ User must manually shut off non-critical circuits before switching
‒ Manual transfer will leave the house in the dark until someone does the transfer
•
If the inverter cannot power all the loads it will be overloaded and shut off,
requiring a manual restart
•
The input/grid-feed and output of the inverter must never be connected
together
‒ This will irreparably damage the inverter and may cause a fire
‒ Connecting the input to the utility side of the meter is most common approach
Using a Line Tie Connection for a
Whole House Backup System
•
•
•
Line tie connection with fused disconnect and two pole transfer switch
Transfer switch must be manually activated to meet code
‒ Non-critical circuits must be disconnected first
Power from the utility must be disconnected to install the transfer switch
Whole House Backup with
Automatic Transfer Switch
•
•
•
Whole house backup with an automatic transfer switch requires a backup
system capable of powering all loads, or having loads automatically
disconnected NEC 702 5(B)
200A service requires six Radian 8kW inverters unless automatic circuit
disconnects are used
Requires a line tie connection to the utility
AC coupled GTBB system
•
•
•
•
•
Normally uses a protected loads panel
Uses both a direct grid-tie inverter and a battery-based inverter
Is useful when retrofitting an existing system
A kWh production meter can still be used
More expensive and complex, not all equipment is compatible
Battery Based Grid-Tie Inverters
Battery Based Inverters for
Grid-tie
•
All battery-based inverters must have a battery bank to function
•
OutBack FXR and SMA Sunny Island inverters are 120 VAC input and output
only
•
•
•
OutBack Radian and Schneider XW+ inverters are 120/240 VAC split phase
input and output
•
•
•
Up to 10 Radian inverters can be stacked for an 80 kW system
Up to 4 Schneider XW+ inverters can be stacked for 27.2 kW system
FXR and Sunny Island inverters have a single AC input
•
•
An autotransformer can be used with a single inverter to produce 120/240 VAC split
phase output, or 240 VAC input, but not both with a single transformer
Two inverters can be stacked to produce 120/240 VAC split phase output
External transfer switch required for use with a generator
Radian and XW+ inverters have two AC input circuits
•
Compatible with most generators
Battery Based Inverters
• Size inverter for the larger of maximum load or for array size
•
•
During an outage, the inverter must be large enough to run all of the loads
that will run at the same time
The inverter must be large enough to process the full solar array into the
grid
• Per 2011 NEC 705.12(D)(2), the grid connection circuit is rated at the
maximum inverter grid feed current x 1.25.
•
•
The grid intertie breaker can be larger than the intertie circuit rating so that
the intertie breaker can handle the maximum pass-through current
50 A or 60 A is a common breaker size for pass-through
• When a single 120 VAC inverter is used, the current for the grid intertie
will be double that for a given system size compared to a direct grid-tie
inverter running at 240 VAC
•
This may be a problem to meet 2008 NEC 690 64(B)(2), or 2011 NEC 705
12(D)(2)
Use Sealed Batteries
•
The battery must be large enough to
support the backup loads for the time
desired
• The battery bank must be large
enough to power the inverter
adequately
• Also consider: depth of discharge;
temperature; inverter efficiency;
degradation over lifetime
•
The battery bank may be the single largest
cost in the system
•
Sealed batteries require an enclosure, but
not special containment.
• A garage or basement is ideal
Load Analysis: Talk to your
Customer
•
Protected vs. non-protected loads: what do they really need
•
•
Heating systems vary widely and may or may not be practical to back up
•
•
Refrigerator/freezer, lights, ventilation, TV/computer, cell phone charger, alarms
Air handlers can be a very large energy load, even if the heat source is gas
How long do they really need the backup to last?
•
•
•
Batteries drive the total system cost more than the PV or inverter
The larger the loads and the longer they run, the more expensive the system will be
A 100 kWh battery bank (50 kWh/day for 2 days), is likely to cost about $40,000
•
Can they add a generator if the backup needs to be larger or last longer?
•
Manage the customer’s expectations!
•
•
•
“A protected loads panel is far more convenient and cost-effective than whole-house
backup”
Accurate load analysis is worth the time and will result in more satisfied customers
Ensure your customer understands how much energy they can expect from their
backup system and the risks of overloading it
Load Analysis
•
Sum the protected loads as you would if they were in a separate off-grid system
PV Array Sizing
• Sizing the PV array for GTBB systems is similar
to a grid-tie system
• Limits to array size:
•
•
•
100% offset of electrical use
Available space
Budget
• At minimum the array should power the
backup loads for an extended outage
• Sizing for an extended outage needs to
account for winter solar production unless the
system will also have a generator
Inverter Sizing Considerations
•
Inverter must meet the larger of array size or load size
•
•
•
•
The inverter needs to be large enough to power all of the loads that will be
put on it during a power outage
A grid-tied inverter also must handle entire PV array power
This connection does not exist for off-grid systems
Will any loads require 240 VAC power?
•
Same as for off-grid systems
•
Wire the protected loads panel for normal North American distribution
of 120/240 VAC split phase or just 120 VAC?
•
Will the installation use a backup generator?
•
will it be 120 VAC only or 240 VAC?
Battery Sizing
•
Battery sizing minimum for inverter power
•
From OutBack Radian manual:
•
•
•
•
•
“To prevent the inverter’s charger from overcharging, the minimum
recommended battery bank size is 350 amp-hours for every Radian
inverter/charger installed on the system”
”Systems intended to bridge short-term outages can use smaller battery
banks. In these cases, the bank can be as low as 200 amp-hours per
inverter However, the charge rate must be decreased to half the inverter’s
maximum using the MATE3”
One of the following conditions must also be true:
“The system is equipped with a backup generator that is programmed for
automatic start, or typical grid loss is 30 minutes or less, or the loads are
less than 2 kW”
Other inverters have similar requirements
Thank you!
support@aeesolar.com
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