How to design the solar PV system by yourself ?

29/12/2014

Till now, we have learnt the basics of the solar electricity, here in this post we will learn to design the solar system by ourselves.
As you know that the basic components of the system are:

Solar Panels
Charge controller
Inverter
Battery

The whole designing of the system is based on the energy consumption of the electrical appliances that need to be powered by the solar panels.

The step by step procedure of designing the system as follows:

(1) Determine the Power consumption demand

Let us take an example, in a house there are:

4 CFL of 18 watts which operates for 4 hours in day
Two 80 watt fans used 3 hours in day and
1 television of 150 watt operates 4 hours in day.

From the above given information we can easily calculate the total appliances usage in watt-hours:

CFL: 4 no’s x 18 watts x 5 hours = 360 watt-hours/day
Fans: 2 no’s x 80 watts x 4 hours = 640 watt-hours/day
TV: 1 no x 150 watts x 4 hours = 600 watt-hours/day

Adding all the above values,

(360 + 640 + 600) watt-hours/day

= 1600 watt-hours/day
This is the daily energy requirement that needs to be backed by the solar panels.

(2) Sizing the PV panels

Energy required from the solar panels = 1600 x 1.3 (as some of energy is lost in the form of heat)

= 2080 watt-hours/day

Now, divide this value by the solar insolation for that area where the system is to be installed to get the peak wattage of the solar panels (The average solar insolation Delhi is 5.5 KWhr per square metre daily.)

=1778.4/5.5
=378 watts
Therefore, number of 100 watt panels needed: 378/100= 3.78 or at least 4 numbers

(3) Inverter sizing

The total watt of the appliances is

CFL: 4 no’s x 18 watts = 72 watts
Fan: 2 no’s x 80 watts = 160 watts
TV: 1 no x 150 watts = 150 watts

Adding all the three values, (72 watts + 160 watts + 150 watts) we get 382 watts.
(For safety purposes the inverter should be at least 25%-30% bigger in size)

Therefore, the rating of the inverter should be 382 x 1.3 = 496.6 watts or greater.

(4) Sizing the batteries

Calculate the total wattage of the load (as calculated above) = 382 watts
Divide this value by 0.8 (battery loss) = 382/0.8 = 477.5 watts
Further divide 477.5 watts by 0.8 (conversion loss by inverter while converting current from dc to ac current) = 477.5/0.8 = 596.87 watts
Divide 596.87 watts by 0.8 for the depth of discharge (for long life don’t fully discharge the batteries, keep 20% charge remaining in the batteries) = 596.87/0.8 = 746 watts

The 746 watts is the amount of power which is required from the batteries to run the appliances.
Now let us suppose that backup required from the batteries is 5 hours.

Multiply 746 watts by 5 hours, we get 3730 watt-hours which are required from the batteries.
Now, divide the result by 12 volts, the nominal battery voltage, to get the capacity of the battery, we get 3730/12 = 310.8 ampere-hours.

So, the battery should be rated 12 volts 310.8 Ah for 5 hours back up.

(5) Sizing the solar charge controller

The rating of the charge controller = total short circuit current of the PV array x 1.3

The 100 watt PV module specification is:

Power in watts (Pm) = 100 watts
Open circuit voltage (Voc) = 21 volts
Short circuit current (Isc) = 6.35 amperes.
Voltage at maximum power (Vmp) = 17 volts
Current at maximum power (Imp) = 5.88 amperes

We assumed that the solar panels are arranged in the parallel combination. The current will get add up i.e. 6.35 x 4 = 25.4 amperes. Therefore, the rating of the solar charge controller is 25.4 x 1.3 = 33.02 amperes.
So, the solar charge controller should be rated 33.02 amperes at 12 volts or greater.
This is the basic designing that one can do oneself and evaluate the cost of the system based on the specification of the components.

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