Energetica India Magazine nº88 March-April 2020

Battery bank capacity vs. life: Batteries should be capable of meeting both the power and en- ergy requirements of the system. Major design parameters that should be considered in the sizing of batteries are as follows: • Number of days of autonomy or reserve • Daily depth of discharge (DOD) • Daily energy demand • Maximum power demand • Surge demand • Maximum charging current The number of days of autonomy should be determined on the basis of acceptable level of loss of load probability (LOLP), equivalent no sunshine days, presence of backup generator and acceptable average daily depth of discharge. The opti- mum sizing of the battery and operating temperature ultimately determine the life of the battery. For example, if a deep cycle flooded lead acid battery operates at 30% average daily depth of discharge (DOD), at an operating temperature of 30oC, the battery will have a life of 4000 cycles i.e. approximately 11 years as shown in the figure below. If the same battery operates at 50% average daily DOD at the same operating temperature, the battery will have a life of 2000 cycles of approximately 5.5 years. Figure 3: Battery daily average depth of discharge vs. life cycle The batteries selected should be rated for the required d.c. nominal system voltage and capacity. It is not recommended to connect batteries in parallel as it may lead to a significant decrease in the battery life. If parallel batteries are unavoidable, the manufacturer’s recommendation for the maximum number of parallel strings should be followed. Determining PV array capacity: Broadly there are two different methods for determining PV array capacity based on whether the system will operate with PWM based charge controller or MPPT based charge controller. If siz- ing approach of MPPT based charge controller is applied to de- SOLAR POWER 37 energetica INDIA- March-April_2020