Batteries are the key components of an off-grid renewable energy system. Selecting the right number and type of batteries that match your needs is essential for your PV plant to be succcessful.
Here are a few key points that you need to know before you start sizing a battery bank.
The battery sizing should be done after the inverter ( DC to AC ) sizing for AC loads and DC loads after the selection of DC to DC converter.
AS we know the design of solar PV systems proceeds in the opposite direction of energy flow. The inverter and the DC to DC converters will have some losses because they will have than 100% efficiency.
Therefore, we have to choose batteries in such a way that they should not only supply the power and energyrequired by the load, but also it should be able to supply the loss of energy in the inverter and the DC to DC converters. That means the loss incurred by the inverter/converter hsa to be compensated by the battery.
In order to size the battery, we have to consider several other parameters of batteries as well. These parameters are as follows :-
- Energy Consumption (Watt hours -W)
- System voltage ampere-hour (Ah) capacity of the battery
- Depth of Discharge of the battery
- Number of days of autonomy
- Battery Connections ( series or parallel)
For better understanding lets take this example.
We have to design a Solar PV system for an energy requirement of 2500Wh.
Now we have an inverter with 95% of efficiency. So the energy requirement at the inverter input side is 2500/95% = 2631.5 Wh.
Case 1 : Considering the System voltage
In this case the system voltage is choosen to be 12 volt. Therfore, inorder to findout the Ah capacity of the battery , we need to devide energy by system voltage
So, Ah = Wh/V = 2631.5/12 = 219.3 = 220Ah
Case 2 : Considering the Depth of Discharge (DoD) of battery
We, know that the DoD of batteries indicates how much of the total charge of the battery can be used. If the DoD is 50%, meanas only 50% of the total charge stored in the battery can be used.
In Solar PV systems, the deep discharge batteries are used with the DoD value in the range of 50 – 60 %. Due to this fact, that is DoD can not be 100%, the actual battery Ah capacity required to supply the required energy to the inverterwill be higher as compared to estimated value of 220Ah above.
So now the correct Ah of the battery is = 220Ah/50% = 440Ah
Case 3 : Considering the number of days of autonomy
We know that in standalone PV systems, batteries store energy to supply power to the system during non-sunshine hours. In situations like rainy seasons or cloudy days, it is a possibility that sun does not appear for several days. In such cases we need more energy to be stored in the batteries to be able to supply power during these days.
The number of cloudy days or no-sunshine days is referred to as Number of Days of autonomy. So this means whatever battery capacity we calculated on the basis of DoD, we have to increase the capacity of the battery to store extra energy as per the number of days of autonomy.
So if we need power for two days then we have to multiply the Ah capacity after DoD with 2 and so on.
So now taking number of days of autonomy to be 3 (three days of power), the battery capacity becomes = 440Ah x 3 = 1320 Ah.
the Ah capacity of the battery = (Power Input at Inverter x Number of days of Autonomy) / (DoD x System Voltage)
Now, from the above estimation, for the example of standalone PV system considered in this case, we need to provide 1320 Ah at a terminal voltage of 12 volt. Now, we have to choose appropriate batteries available in the market.
Assume, in the market only 120Ah batteries are available.
Now, out total Ah capacity of the battery system to be provided is 1320Ah and the battery that is available in the market is 120Ah.
So, the total number of batteries = Total Ah capacity required for the system/ Ah capacity of a single battery
= 1320Ah/120Ah = 11 Nos.
So we need 11 batteries of 120Ah capacity each to achieve our net power reuirement of 2500 Wh.
Series or Parallel connection between batteries is decide taking input voltage to inverter into consideration.