A significant increase of renewable energy generation such as photovoltaic and wind power causes fluctuations in frequency and voltage in power grid. Battery energy storage systems (BESSs) are expected as effective measures to mitigate these fluctuation problems. Among many of technical parameters which specify or characterize BESS performance, the paper focuses on “efficiency” as a key performance indicator for BESSs. The authors propose a comprehensive calculation model to evaluate efficiency of a BESS, where losses in its interconnecting transformer, power conditioning system (PCS), auxiliary equipment and energy storage device are taken into account. The paper demonstrates seasonal field results of the efficiency measured in an actual 100kW-300kWh BESS. The measured results are compared with the calculation results based on the designed values of the BESS, so that the proposed evaluation model for BESS efficiency is verified to be a useful tool for the comparison of BESSs.
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