PV-Off Grid Design

8/3/2019 PV-Off Grid Design

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Designing a PV System


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Input Data Required For PV SystemInput Data Required For PV System

SizingSizing

The daily or hourly load requirements during aThe daily or hourly load requirements during atypical yeartypical year

The required security of supply, taking intoThe required security of supply, taking intoaccount the back-up source, if any.account the back-up source, if any.

The mean daily irradiation in the plane of theThe mean daily irradiation in the plane of thearray at the chosen site for every month of aarray at the chosen site for every month of atypical year.typical year.


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Data Required cont`dData Required cont`d

The maximum number of consecutive sunless days likelyThe maximum number of consecutive sunless days likelyto be experienced.to be experienced.

The mean daily ambient temperature for every month ofThe mean daily ambient temperature for every month ofa typical year.a typical year.

The estimated cell temperature rise above ambient of theThe estimated cell temperature rise above ambient of the

modules in the array.modules in the array.

Typical current-voltage characteristics of the module atTypical current-voltage characteristics of the module atvarious irradiances.various irradiances.


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Data Required cont`dData Required cont`d

The selected DC bus voltage.The selected DC bus voltage. The maximum allowable depth of discharge ofThe maximum allowable depth of discharge of

the battery.the battery.

The estimated percentage energy losses in theThe estimated percentage energy losses in thebattery, power conditioning equipment andbattery, power conditioning equipment andcontrol system.control system.

The estimated losses in the array from moduleThe estimated losses in the array from module

mismatch, cables and voltage drop acrossmismatch, cables and voltage drop acrossblocking diodes.blocking diodes.

The estimated losses from dust and shading.The estimated losses from dust and shading.


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Types o f Ma te r i a ls

tSingle CrystaltPolycrystalline

t

Thin-Film


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Off-Grid Design


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Off-Grid Design Example

Step 1: Determine the DC Load.

DC Device Device X Hours of = DC Watt-HrsWatts Daily Use per Day

Refrigerator 60 24 1,440

Lighting fixtures 150 4 600

Device A 12 8 96

Total DC Watt-hrs/Day [A] 2,136


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Step 2: Determine the AC Load, Convert to DC

AC Device Device X Hours of = AC Watt-HrsWatts Daily Use per Day

Device B 175 6 1,050

Pump 80 0.5 40

Television 175 2 350

Total AC Watt-hrs/Day 1,440Divided by 0.85 (Inverter, losses)

Total DC Whrs/Day [B] 1,694


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Step 3: Determine the Total System LoadTotal DC Loads [A] 2,136Total DC Loads [B] 1,694

Total System Load 3,830Whrs/Day

Step 4: Determine Total DC Amp-hours/DayTotal System Load / System Nominal Voltage =(3,830 Whrs/Day) / 12 Volts = 319Amp-hrs/Day

Step 5: Determine Total Amp-hr/Day with Batteries

Total Amp-hrs/Day X 1.2(Losses and safety factor)319 Amp-hrs/Day X 1.2 = 382.8 or 383 Amp-hrs/Day


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Step 6: Determine Total PV Array Current

Total Daily Amp-hr requirement / Design Insolation*383 Amp-hrs / 5.0 peak solar hrs = 76.6 Amps

* Insolation Based on Optimum Tilt for Season

Step 7: Select PV Module Type

Choose BP Solar-Solarex MSX-60 module:Max Power = 60 W (STP)

Max Current = 3.56 AmpsMax Voltage = 16.8 VoltsNominal Output Voltage 12 Volts


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Total PV Array Current / (Module Operating Current) X(Module Derate Factor)76.6 Amps / (3.56 Amps/Module)(0.90) = 23.90 modules

Use 24 Modules

Step 8: Determine Number of Modules in Parallel

Step 9: Determine Number of Modules in Series

System Nominal Voltage / Module Nominal Voltage12 Volts / (12 Volts/module) = 1Module

Step 10: Determine Total Number of ModulesNumber of modules in parallel X Number of modules

in Series24 X 1 = 24 modules


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Step 11: Determine Minimum Battery Capacity

[Total Daily Amp-hr/Day with Batteries (Step 5)X Desired Reserve Time (Days)] / Percent ofUsable Battery Capacity(383 Amp-hrs/Day X 3 Days) / 0.80 = 1,436 Amp-hrs

Step 12: Choose a Battery

Use an Interstate U2S 100 Flooded Lead Acid BatteryNominal Voltage = 6 VoltsRated Capacity = 220 Amp-hrs


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Step 13: Determine Number of Batteries in Parallel

Required Battery Capacity (Step 11) / Capacity of

Selected Battery1,436 Amp-hrs / (220 Amp-hrs/Battery) = 6.5Use 6 Batteries

Step 14: Determine Number of Batteries in Series

Nominal System Voltage / Nominal Battery Voltage12 Volts / (6 Volts/Battery) = 2 Batteries

Step 15: Determine Total Number of BatteriesNumber of Batteries in Parallel X Number of Batteries

in Series6 X 2 = 12 Batteries


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+

-

+

- -+

3 A12 V

3 A12 V 3 A

24 V

3 A12 V

3 A12 V

+ +

- -

+

-6 A12 V

Series:Voltage is additive

Parallel:Current is additive


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Step 17: Complete Balance of System

a. Complete the design by specifying the:Charge ControllerInverterWire Sizes (Battery will have larger gage

due to higher currents)Fuses and DisconnectsStandby Generator, if neededBattery Charger, if neededManual Transfer Switch, if needed.

Step 16: Determine the need for a StandbyGenerator to reduce other Components

(number of Modules and Batteries). Severaliterations may be necessary to optimize costs.


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d. Obtain permits as required.

b. Determine mounting method:

Roof mountGround mount with racksGround mount with pole.

c. Assure proper grounding for safety.

Step 17 (Cont.):


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The Elegance of Simplicity

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PV-Off Grid Design