Off-Grid System Design - Solar Wholesale...

Off-Grid System Design

Brian Teitelbaum

Applications Engineer

January 27, 2017

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.

What is an “Off-Grid” system?

• No access to the electrical grid

• Batteries used to provide for loads when it’s dark or cloudy

Often referred to as Battery-Based systems

• Modules and racking are usually the same as used in grid-tie systems

• Most other components are specialized

Types of Off-Grid 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

• Load is run directly from renewable power source, without energy storage (batteries).

• Generally used only with motors, like pumps and fans, which can run directly from a DC energy source.

• Loads can only run when energy is available from the production source since there is no energy storage

• Very simple systems– minimal equipment needed

• Loads run at variable speed depending on energy available from source: PV-direct systems do not work at night

• Current limited: only the amount of current that is directly produced by the energy source is present. Often means that overcurrent devices are not needed as long as the conductors are sized to handle full current

For AEE Internal Use Only.

PV, Hydro, or Wind-direct Systems

Example of a PV-Direct system


Grundfos SQFlex water pumping system

1) Grundfos SQFlex pump

2) Submersible pump cable

3) Cable ties

4) Pump support rope or cable

5) Well seal

6) PV array

7) PV array mounting structure

12) IO-50 SQFlex on/off switch box

• Battery bank size & voltage drives design

DC load magnitude and run time determines minimum battery size

No AC loads or inverter

• PV, wind, etc. used to charge the battery bank

Constant power from intermittent source

• Common applications include lighting, communications, telemetry, signage and data logging

• Example: Weather station

PV array and mounting structure

12 VDC Battery

PWM Charge controller

Weather sensors and data logger/transmitter


DC-Only systems

AC –Only systems

• AC from an inverter powers all of the electrical loads

Any type of AC load can be powered

Inverter output limits total load draw

Appliances and wiring are standard and easy to find

• Inverter draws from battery bank

PV array/wind turbine/etc. charges batteries as in DC-only system

PV Arrays and other charging sources are connected to charge controller(s) rather than inverter

Most inverters can enable battery-charging from an AC generator


AC/DC systems

• Both AC and DC loads are powered by the off-grid power system

DC loads powered directly from battery output

AC loads powered by inverter or generator

• Requires separate AC & DC electrical circuits

• Most common in Boats and RV’s

Note that mobile inverters have special grounding requirements


Hybrid systems

• Energy is produced by more than one source, for instance PV and wind, or PV and hydro, or PV and a generator.


How does Off-Grid system design differ from Grid-Tie system design?

• Energy production based on daily or weekly consumption

• Some energy used directly as it’s produced, and some energy is stored in batteries

• PV array sized based on energy production during the darkest time of year

• Inverter size based on peak AC load, not on array size

• Complex – many components

• If the system fails, the lights go out

• Energy production based on yearly consumption

• Some energy used directly as it’s produced, and some energy is sent to the grid (stored as credit)

• PV array size based on yearly energy production

• Inverter size matches array size

• Simple – few components

• If the system fails, the lights stay on

Off-GridGrid-Tie (Net-Metered)

Off-Grid System Components

• Battery-based inverter(s)

• Charge controller(s)

• Energy production source(s):

PV, wind turbine, micro-hydro turbine, engine generator, etc

• PV array combiner box

• Battery bank

• DC switchgear and over-current protection

• AC switchgear and over-current protection

• Battery monitor and other system controllers


Off-Grid System Diagram

Off-Grid System Sizing

The purpose of the off-grid power system is to provide power to run loads

System design begins with an analysis of the loads that need to be powered

User Information That 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 (Energy storage)

How many days in a row will the loads need to run with little or no sun

Don’t neglect to account for 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

http://www.energystar.gov/index.cfm?c=products.pr_find_es_products

Load Analysis

Load Worksheet –

2016 AEE Solar Catalog and Design Guide – Page 9

Load Analysis Excel version of Loads Worksheet


Load Analysis for Refrigerators and Freezers


Use kWh per year figure for Watt-hour calculation

Enter in running Watts for peak load figure

Load AnalysisAdd the inverter tare loss as a load

(inverter self consumption)


Load Analysis Peak Load


• The Peak Load figure is used to size the minimum full power output

of the inverter required.

• Choose an inverter with a power rating above the peak wattage thatthe loads can draw

Load Analysis Average Daily Consumption


Divide weekly watt-hour consumption by 7 to get the average daily consumption:

• 57767 ÷ 7 = 8252 Wh/day average

• This figure is used as the basis for both battery sizing and PV array sizing

Don’t Forget “Phantom” Loads

123.5W x 24 hrs = 2964 Wh/day

Load Analysis Example

Daily consumption (Watt-hours )

7432 Wh – energy consumed by loads

8543 Wh – energy drawn from battery

Peak load (Watts) and characteristics (VDC/VAC/Hz)

4934 W @ 120VAC- use a 5000W inverter

Site Analysis

• What is the solar energy potential at the site?

• Peak sun-hours, seasonally?

• Shade or orientation issues?

• How much mounting space?

PV arrays for Off-Grid systems are based on the peak sun-hours during

the darkest month of the year, not the yearly average

• Are there any other possible energy sources other than PV?

• Wind potential?

• Hydro potential?

Wind or Hydro turbines can produce power when sunlight is not available

and can add greatly to the energy production reliability of off-grid systems

• How many cloudy days in a row should the design be based on?

• How much generator run time?

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


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


Yearly Average – 4.66

December average – 3.22

PV Array Sizingfor MPPT Controllers

• 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

2 modules x 26.1 VDC = 52.2 VDC

2 60-cell modules in series will typically not charge a 48 VDC battery

• Most 60 cell modules will exceed 150 VDC in strings of 4

• A 48 VDC battery requires 3 modules in series only

• A 24 VDC battery can use either 2 or 3 modules in series

• A 12 VDC battery can use 1, 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

• 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

If the charge controller will be operated near its limit, oversize the battery breaker slightly to avoid nuisance tripping

• Charge controllers must have DC Ground Fault Protection


Choosing System DC Voltage

Selecting the best DC system voltage depends on a variety of factors

• Peak load - 12VDC is fine for peak loads up to about 1000W.

- 24VDC is fine for peak loads up to 2000W

- 48VDC is best for peak loads over 2000W

• Size of PV array - A single MPPT 80A charge controller will handle up to 1000W of

PV in a 12V system, but will handle up to 2000W in a 24V system

and up to 4000W in a 48V system

• Battery bank - Higher system voltage means more 2V cells in series

so fewer parallel strings will be needed to gain the same amount of

energy storage

• Voltage of any DC loads - If there are DC loads, that may define what the system

voltage should be.


Energy Production Sources for Charging Batteries

• PV modules

● Very reliable and predictable energy source

● Usable almost everywhere

• Wind Turbines

● Excellent source of supplemental energy in windy areas

● Generally intermittent and unpredictable

● Site specific

• Micro-Hydroelectric Turbines

● Excellent primary or secondary energy source

● May be seasonal or year-round

● Site specific

• Engine Generators

● Diesel, Gasoline, Propane

● Noisy, dirty, but predictable (if maintained properly)

● Use non-renewable energy (unless bio-diesel)


PV Charge Controller Types

• Pulse Width Modulated (PWM) controllers

Examples: Morningstar SunSaver, ProStar, TriStar PWMSchneider C-Series

Pros: Inexpensive ($150-$300) and compact

Cons: Input voltage limitations (12/24/48VDC)

• Maximum Power Point Tracking (MPPT) controllers

Examples: OutBack FM-60 and FM-80

MidNite Solar Classic

Morningstar TriStar MPPT

Schneider XW MPPT-150 or XW MPPT-600

Pros: Maximized energy harvest

Wider input voltage range (up to 150VDC, 200VDC, 250VDC, or even

600VDC)

Cons: More Expensive; Physically larger


Off-Grid PV Array Sizing Worksheets – pages 12-13 (2013 edition)


PV Array Sizingfor PWM controllers

• Find the current the array must produce

𝐷𝑎𝑖𝑙𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝐴ℎ) × 1.2

𝑘𝑊ℎ 𝑝𝑒𝑟 𝑚2𝑝𝑒𝑟 𝑑𝑎𝑦= 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐴𝑟𝑟𝑎𝑦 𝐴𝑚𝑝𝑠

• 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

𝑀𝑜𝑑𝑢𝑙𝑒 𝐼𝑚𝑝= 𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑟𝑖𝑛𝑔𝑠

• Determine the number of modules per string

12V modules (36-cell) : 12V system = 1 per string

24V system = 2 per sting

24V modules (72-cell) : 12V system = NA

24V system = 1 per string

• Determine the total number of modules 𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑟𝑖𝑛𝑔𝑠 × 𝑆𝑡𝑟𝑖𝑛𝑔 𝑙𝑒𝑛𝑔𝑡ℎ = 𝑇𝑜𝑡𝑎𝑙 𝑚𝑜𝑑𝑢𝑙𝑒𝑠


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 control 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


PV Array Sizing

8533 Wh/day x 1.25 = 10667 Wh - needs to be produced by

the PV array per day, on

average

The 1.25 factor accounts for the battery charge/discharge efficiency losses over the life of the battery

10667 ÷ 3.22 sun-hours (Dec) = 3313W minimum PV array watts

3313W ÷ 280W module rating = 11.83 modules

Round up to 12 modules minimum

15 modules would make the system more reliable and further reduce generator time

Battery SizingDesign Considerations

• Number of expected cloudy days in a row

● The battery needs to store enough energy to power the loads

during periods of low energy production

● This is called “days of autonomy”

● Use 3 days to 50% DOD in most of the US

● Use of a back-up generator can reduce the days of autonomy

● A larger PV array can reduce the days of autonomy


Battery Sizing Depth of Discharge

• Desired Daily Depth of Discharge (DOD) in percentage

● Batteries last longest with shallow daily DOD, and are worn out

faster with deeper daily discharges.

● Aim for 15-20% daily DOD, with only rare discharges of more

than 50% DOD for best battery life


Depth of Discharge

• Deep Discharging will shorten battery life

• Deep-Cycle Batteries are designed for up to 80% DoD

Shallower cycles will enable longer life

• Never leave batteries discharged for more than a few days!

Sulfation of the battery plates will permanently decrease capacity



● Peak load

● Peak Watts ÷ Battery Voltage = Peak Amps

● Batteries need to be large enough to deliver

the amperage drawn by the peak load without

excessive voltage drop.

● Multiply peak DC amps by 10 for ideal battery sizing



• Peak Charging Amperage

● Batteries can only absorb a charge at a certain rate

● Excessive charging current can cause violent gassing,

overheating, plate warping, and plate sloughing

● Sealed batteries should not be charged faster than a rate

equal to about 1/10th of their Ah capacity rating (C/10 rate)

● Flooded batteries should not be charged faster than a rate

equal to about 1/5th of their Ah capacity rating (C/5 rate)


Battery SizingTemperature Correction

• Correction for Battery Temperature -

● Batteries lose some of their apparent capacity at low

temperatures

● At 32°F a lead-acid battery will only deliver about 72%

of its rated capacity

● See temperature correction chart in the

Battery Sizing Worksheet on page 167

of the 2012 AEE Catalog or use

manufacturer’s recommendations


Battery Bank Wiring

• Series: Voltage is additive; rated Ah capacity remains same

Positive of Battery 1 is connected to negative of Battery 2 and so on

Example: Two 12V, 220Ah batteries in series will yield a 24V, 220Ah battery

• Parallel: Capacity is additive; DC 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 12V, 220Ah batteries in parallel will yield a 12V, 440Ah battery


Battery Charging

• Lead Acid Batteries should be charged after every use to ensure they are never stored in a discharged condition

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 lead-acid batteries prior to charging

Charging should be temperature corrected

Always use charge controller’s temperature sensor when available

• Flooded Batteries need to be overcharged occasionally 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


Maintaining 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


Maintaining Flooded Batteries

• 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 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

• 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


Battery Bank Sizing

Loads consume 8533 Wh/day

8533 Wh ÷ 48 VDC = 178 Amp-Hours/day (178 Ah)

Sunny winter areas should have about 2 days of energy storage to a 50% Depth of Discharge (50% DoD)

Multiply daily Ah by 4 - 178 Ah x 4 = 712 Ah battery bank

Dark winter areas should have about 3 days of energy storage to a 50% DoD

Multiply daily Ah by 6 - 178 Ah x 6 = 1068 Ah battery bank

Sizing Results

The Load Analysis told us that the inverter needed to be rated for 5000W of output power.

The Load Analysis told us that the loads, including inverter losses, consumed 8533 Wh/day

The Site Analysis told us that we needed to size the PV array for 3.22 peak sun-hours

The Load and Site Analyses results of 8533 Wh and 3.22 peak sun-hours , times 1.25, told us the minimum array PV Watts needed – 3313W

The array PV Watts divided by the module wattage rating told us the number of modules needed – 12 280W modules

The daily consumption of 8533 Wh ÷ battery voltage told us the daily AH consumption – 178 Ah

The daily consumption times the number of days of autonomy told us the battery bank rating in Ah – 178 Ah x 4 = 712 Ah for sunny-winter areas

- 178 AH x 6 = 1068 Ah for dark-winter areas

System Options You Will Need to Choose

Battery Type: Flooded, AGM, Gel or Advanced

Consider: Maintenance, shipping, handling, 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) charge controllers Inexpensive, compact

Usable with PV arrays having a “nominal” voltage the same as the battery

12/24/48 VDC nominal PV array

12V nominal modules - 36-cell 24V nominal modules - 72-cell

Maximum Power Point Tracking (MPPT) charge controllersMaximized energy production – operates array at maximum power point wattageArray voltages up to 150, 200, 250, or 600 VDC

Module Type: 36-cell, 60-cell or 72-cell

Consider transportation and mounting limitations as well as module cost per watt

Types of Off-Grid Inverters

• Modified Sine-Wave (Modified Square-Wave) inverters are inexpensive, but have a non-sinusoidal wave form. These are very efficient, but some loads cannot be run on them. They typically have a total distortion of up to 30%

• Sine-Wave inverters are the most commonly used in residential and commercial systems. They have very clean power output that will run almost any AC load. They typically have a total distortion of less than 5%

• Residential inverters are designed for stationary installations, such as homes and businesses. They should be Listed to UL 1741

• Mobile inverters are designed for RV and marine use. They have “ground switching” which allows for the system’s neutral/ground bond to either be inside the inverter, or outside in the “shore power” connection. They should be Listed to UL 458

• Inverters may or may not have built-in battery chargers for charging batteries from an AC source, such as a a generator.


Off-Grid 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 required

• Identify any loads with high start-up or surge current draws

Motors in pumps, compressors, and other appliances with large inductive loads

Largest surge load determines minimum inverter surge rating

• Identify any loads that may require 240VAC

To get 240VAC from a 120VAC inverter will require a transformer or dual inverters

• Be sure to consider inverter’s no-load-draw when sizing battery system and array


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


System Monitors & Controllers

• Automate battery management

Allows you to adjust the bulk, absorption, float and equalization charge timing and voltage set-points

• Turn on/shut off generators according to time of day or battery state of charge

• May allow for remote monitoring/control via Internet

• May track battery state-of-charge through amp-hour metering


Every Off-Grid system

should have an amp-

hour meter installed

Pre-Assembled Power Systems

• Factory pre-wired power systems simplify design and installation

Several common sizes & configurations

• Most Power Systems include:

Inverter(s)

Controller and networking devices

Battery monitor

Integration hardware and BOS

Enclosures, Breakers, GFDI, Bypass, etc.

Charge controllers

• AEE can provide custom configurations that are ETL marked to streamline inspections

Note that multi-inverter systems are shipped in crates


Inverter SystemsPoints to Consider

• A central location is desired to connect wiring and install breakers

There needs to be a DC load center and an AC load center, or one load center for both AC and DC. Most of these systems are for indoor mounting only

• 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 125A to 250A

The inverter manufacturer or supplier will generally specify breaker sizes

Breaker size is generally maximum power output in watts divided by battery voltage x 1.5, but sometimes larger


• Wire size for battery and inverter circuits will commonly be AWG 2/0 or AWG 4/0 cable

Keep connection as short as possible to minimize voltage drop

Under 10ft is best

Off-Grid System Design

Questions?

Contact: Brian Teitelbaum at [email protected] ext 7856

mailto:[email protected]

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