Battery bank design for 1KW solar stand alone system at Barmer (Rajasthan)

Battery bank design for 1KW solar stand alone system at Barmer(Rajasthan)

The stand alone solar system works only in day, generally from 9.00 to 3.00 hours solar time. Rest of time it generates negligible energy to suit owner requirements. So for other times some storage device essential for running lights, fans, motors, freeze, etc.

Battery is a electric storage device. It sores electricity in the form of chemical energy. Generally there are two types of batteries, first is automobile battery or thin plate battery and other is deep cycle battery or thick plate battery. For our application in solar system deep cycle battery is used.  We can use 80%  of its capacity.

Example:  A 48Volt DC has an AC loads of 6 tube lights 40watts, 5 fans60watts, freeze, water pump etc.

(A)Average daily energy (Wh)

1.   (No. of tube lightsxwattsxNo. of hoursxno.of days /week)/7days/week

(6x40x5x7)/7-Wh

1200Wh

In this way calculate Wh for fans, freeze, motors etc. Suppose total  it came 4600Wh, than add extra 20% . Now it became (4600+20%x4600) 5520Wh.

2.   The  other method is electric bill, if you have electric connection. See how much unit you have consumed per month. Take average of all months and calculate per day electric consumption . Wh. Suppose it came 5500Wh.

(B) If there is cloudy days, the solar energy will not be available that day. So we keep the reserve energy for three days autonomy. Now the battery bank is designed for:

 (Average daily consumption)x3

=5520x3

=16560Wh

=16.560KWh

The battery capacity is denoted by AH, so divide 16560Wh by 48Volt

=16560/48 Ah

=345Ah

This 345Ah is usable capacity of battery, It is only 80% of battery bank capacity. So total capacity of battery is :

=345/0.8

=431.25Ah

Temperature correction:

The battery is designed to operate at room temperature 25°C. Below this temperature the effective capacity decreases as rate of chemical reaction slow down. So the storage capacity must be corrected upward for condition of use. In winter the room temperature go down to 10°C. The effective capacity reduced to 1%/°C below 25°C. Thus the differential is (25-10=15°C).Thus the 15% capacity reduced. Our battery capacity 431.25Ah is corrected to

=431.25/0.85

=507Ah

So we shall use 500Ah at a discharge rate C/20, placing it at room temperature 25°C.

Take 24 cells of two volts and 500Ah to connect in series to make battery bank 48Volt, 500Ah. The other combinations are eight six volt and 500Ah, Four 12Volts and 500Ah, connected in series. Donot use more than two parallel connections.

   Effect of discharge rate  

As the discharge rate increases the capacity decreases and vice versa. For example Surrette S460 has a capacity of 360Ah at C/24. Meaning the 15A load discharges in 24 hours. (100% discharge). If discharge rate C/100 than capacity increases 466Ah, as there is more time to active material for participating in chemical reaction. If the discharge rate increases to C/4, than less time is available to active material for participating chemical reaction. The battery capacity reduced