Performance of a hybrid battery energy storage system

The use of energy storage systems is inevitable in a power grid dominated by renewable generators. This paper presents a performance overview of a 100 kW/270 kWh, grid-connected, hybrid battery energy storage system. The hybrid system uses two types of battery chemistries, li-ion and lead–acid connected directly at the DC bus — without power electronic converters. After a brief introduction and a short technical description of the project, the paper presents a three year, 2019 to 2021, operational data set. The battery data is later split into individual charge/discharge cycles and analyzed in terms of power and strings current sharing, energy, round-trip efficiency and energy transfer between the strings. The analysis shows that the average round-trip energy efficiency of the system is 90% and depends on the depth of discharge. The energy transfer between the strings can happen during charge or discharge and the average values are 5.5% (during charge) and 2.47% (during discharge) of the total discharged energy. Minimum capacity loss was recorded for the lead–acid cells and practically no capacity degradation for the li-ion cells.