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Battery energy storage efficiency calculation including auxiliary losses: Technology comparison and operating strategies

The overall efficiency of battery electrical storage systems (BESSs) strongly depends on auxiliary loads, usually disregarded in studies concerning BESS integration in power systems. In this paper, detailed electrical-thermal battery models have been developed and implemented in order to assess a realistic evaluation of the efficiency of NaS and Li-ion batteries. BESSs have been sized in order to operate on a real low voltage distribution network, based on load and photovoltaic generation measurements during an 8-month period. The performance of NaS and Li-ion batteries have been evaluated for two different operating strategies. Results show that, considering auxiliary losses, overall efficiencies of both technologies are very low with respect to the charge/discharge efficiency. Finally, two simplified formulas, able to evaluate the efficiency and the auxiliary losses of a NaS BESS, are presented.