By Mathy Mpassy Isinki

Off grid solar PV systemsdesign 101

Energy One Ltd

By Mathy Mpassy IsinkiOff Grid Energy Solutions Business and

Technical Sales Professional

Off grid PV systems are sized ,based onthe energy demand of the appliances tobe connected.

Design process:Load assesmentAdding lossesEstimating PV array required energyoutputDetermining Equivalent Sun HoursEstimating PV array power outputPV array configurationDetermine operating VoltageController selectionDesigning battery storage bankInverter selection

Load assesment:

Identifying DC and AC loads;Determinining DC appliances contributionto load demand (W) and (Wh);Determining AC appliances contributionto load demand (W) and (Wh);Determining load demand (Wh).

Example, solar powering a houseequipped with:

12 lamps/ 5W DC each / used 6 hours daily;1 TV /150W / used 6 hours daily;1 desktop computer / 150W/ used 4 hoursdaily;1 fridge/ 80W/ used 12 hours a day.

Load assesment:

Total energy needs: 2820WhAc power demand: 380W

Energy output:To estimate PV array required energyoutput, you should take into accountlosses:

Inverter efficiency : 90%Combined controller, battery storage andcables efficiency : 85%

From our example:Total energy needs are 2820Wh

Losses:Inverter efficiency : 90%Combined controller, battery storage andcables efficiency : 85%

Required Daily PV array energy output:3639Wh

How much energy a PV module deliversdepends on several factors, such as:

local weather patterns, seasonal changes and; installation of modules.

From energy to power:Equivalent Sun Hours is the link between PV array energy output and PV arrayinstalled power.

When solar irradiation data are available fora particular location than the equivalent sunhours can be determined.

Coming back to our example:Required Daily PV array energy output:3639Wh;Assuming that the system is to beinstalled in India where ESH=4.5;Required PV array power output will be:809W

Module selection criteria:Market availabilityPerformance

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Back to ourexample; due tomarketavailability, you asthe systemdesigner decide touse 120W solarpanels.

Required PV array power output was809W; applied to 120W Pmax, minimumnumber of modules is 7.

Array configuration:By connecting modules in series, thevoltage of each module is added whilethe current remains the same.If the panels are connected in parallel,the voltage will remain the same whilethe current outputs will be added.

Array configuration influencing factors:System voltage;Aesthetic.

Recommended system voltage: 12Volts for energy demand bellow1kWh ;24Volts for energy demand above 1kWhand bellow 3 to 4kWh;48Volts for energy demand above 3 to4kWh.

As a general rule, system voltage increasesas the total load increases;

A higher voltage may be required tominimise power loss in the cables.

Configuration:Module voltage: 12VStrings: 2 module in serieArray: 4 parallel Strings

Coming back to our example, energydemand is 2820Wh, which is above 1kWhbut bellow 3kWh;

Recommended system voltage: 24Volts

Due to system voltage constraint, we haveadjusted modules number from 7 to 8.

Standard switched controller selectionparameters:

Voltage: refers to array VocCurrent: refers to array Isc

MPPT solar charge controllers allow touse high wattage (up to 3kW) solaroff-grid systems with 12V, 24V or 48Vbatteries, depending on the combinationof wattage and battery voltages.

Battery storage charging current shouldalso be considered when selecting aMPPT.

Referring to array configuration, ifselecting a standard switched controllerfor our design example, it should be capable to hold an input of:

2 Voc which is 43VDC and; 4 Isc which is 29.8A.

The controller must be oversize accordingto electrical standards. Mostly 125% forcurrent input and 120% for voltage.

If using a MPPT, the array output voltageand power should be within the range ofthe input range of the MPPT.

Battery storage sizing input:Total energy demandBattery, controller and cablescombined efficiencyDepth of dischargeSystem voltageDays of autonomyAvailable batteries capacity

Battery storage sizing output:Storage capacityNumber of batteries

Example, from the same house, 3 days ofautonomy are required.

Available bateries are of 200Ah and,Manufacturer depth of discharge(DOD) is 60%

Knowing that operating voltage is 24 Volts,the required battery storage is 758Ah,Required number of batteries is 8.

Inverter sizing input:AC Power demandEfficiencySystem voltage

Inverters selection criteria:input voltage refers to system voltage;output voltage refers to load nominalvoltage ;wave form;output power;surge capacitystackability.

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From the AC power demand, afteradjustment due to inverter efficiency, theminimum inverter size to cover the needsdescribed in our example is 422W. However,due to the presence of a fridge, before finalselection of an inverter, you should refer toinverters specifications to check for itssurge capability.

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