A key metric for energy storage systems is the amount of energy released versus the amount of input energy. This ratio is the Round Trip Efficiency.
Conover et al [2] definition: The useful energy output from an ESS divided by the energy input into the ESS over one duty cycle under normal operating conditions, expressed as a percentage. Taking into account auxiliary power when applicable.
It is always important to know the round-trip efficiency of your BESS system, but it is equally important to state what part of the system you are measuring.
Juho Heiska
Starting from the cell level, many factors affect efficiency: SOC, C-rate, and SOH, to mention a few. Naturally, DC block efficiency will be much higher than AC block efficiency, and it will decrease further when measured at the whole system level (step up transformers). In addition, it should be clarified whether all ancillary loads (lighting, HVAC, etc.) are included in the calculation and at which stage.
In the end, large-scale BESS should achieve over 80% round-trip efficiency if the system is well designed.
AC Round Trip Efficiency
- Rimac SineStack AC round trip efficiency >92%
This number includes inverter efficiency (it is important to check that this is included) and hence will be lower than the DC Round Trip Efficiency.
DC Round Trip Efficiency
Quoted values for DC Round Trip Efficiency fall in the range 92 to 98%.
(data from and included in the Battery Pack Database)
References
- Vedran Bobanac, Hrvoje Basic, Hrvoje Pandzic, Determining Lithium-ion Battery One-way Energy Efficiencies: Influence of C-rate and Coulombic Losses, IEEE EUROCON-2021
- David R. Conover, Alasdair J. Crawford, Summer R. Ferreira, Jason Fuller, Sri Nikhil Gourisetti, David M. Rosewater, David A. Schoenwald, Vilayanur Viswanathan. Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage Systems. Pacific Northwest National Labs and Sandia National Labs Report, 2016.