An electrothermal electricity storage based on thermal energy storage combined with a heat pump and thermal engine utilizing organic rankine cycles is presented herein. During periods of excess electricity generation, a heat pump is used to transform heat to a higher temperature level, effectively storing the electricity in the thermal storage system. When required, the stored thermal energy is reconverted to electricity using an organic rankine cycle. The concept of reversible heat pump and heat engine operation for energy storage is not novel. The idea has however gained renewed interest in the context of load management issues introduced through the increasing share of fluctuating renewable electricity generation. Several implementations utilizing e.g. steam or supercritical CO2 cycles have been proposed as an alternative to pumped hydro-storage and CAES.
The particular realization investigated herein is based on an organic Rankine working fluid and uses water as the thermal storage medium. Charging and discharging operation were modelled in Aspen Plus for a kW scale configuration. The performance and the influence of important thermodynamic parameters on the latter is discussed in detail. Through the integration of low grade waste heat, efficiencies comparable to CAES solutions can be reached. Combined with possibly attractive capital costs due to the lower operating temperature and pressure, electrothermal electricity storage systems hold a potential to become a cost-effective electricity storage technology. Through heat extraction from the high temperature storage, the electrothermal electricity storage further has the potential to reduce the costs associated with high temperatue applications, such as heating and cooking.