where nday is the number of autonomous days powered solely by
the battery storage bank; Eload is the daily energy consumption;
Ec = nday Eload is the summary of energy demand for the continuous
number of autonomous days; gBI is the overall battery and inverter
efficiency; VB is the battery rated voltage, DOD is the allowable
depth of discharge.
According to the evaluation results of battery performance in
Stage 1 [41], the battery bank roundtrip efficiency is approximately
74.3% in 2011. To simplify the calculation, a roundtrip efficiency of
75% was assumed in the whole lifespan of the battery. The other
important design criterion for sizing batteries is the depth of discharge
(DOD). The deeper the regular discharges the fewer the
number of charge/discharge cycles a battery can provider over its
service lifetime. In this study, the maximum DOD was taken as
75% depending on the suggestion from a design handbook [52].
With 5 days of autonomy allowed in this study, the actual DOD will
often be less than 30% during periods of good sunshine, according
to experience from onsite performance evaluation results [41]. The
deep-cycle battery (Sonnenschein Exide Dryfit A600 OPzV) used in
Stage 1 of the project is still chosen for the current study. According
to capacity curve (relationship between DOD and number of
cycles) based on IEC 896-2, if the state of charge (SOC) is strictly
kept above 25%, the corresponding number of cycles is about
2100, which means that the battery would be replaced approximately
every 5 years.
The same PV system configuration in Stage 1 was also adopted
in this option, which means it employs two types of inverters.
Based on the performance evaluation results of the PV inverter
(SMC10000TL) and the bidirectional inverter (SI5048) in Stage 1
[41], the overall efficiency of the inverting process can be estimated
as 79%. Thus the efficiency of inverter-battery efficiency
gBI is 0.75 0.79 = 0.59.
Therefore, the total number of batteries in this option is calculated
as 1474. However, the number of batteries should be rounded
into an integral multiple of 24 since there must be 24 batteries
connected in series to meet the voltage requirement of inverter
SI5048. Thus the total number is rounded into 1488, with 24 connected
in series as one string and with 62 strings in parallel.