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
