The HEATSEP method to maximize the efficiency of pumped thermal energy storage

Storing electricity requires the conversion of the available electrical energy into energy of different form, which can be more easily stored over time. Among the possible options, thermal energy can be generated, accumulated, reconverted using mature technologies that are easy to scale, not constrained by geographic area, and that use safe materials and working fluids. During the charging phase of Pumped Thermal Energy Storage (PTES) the available electricity drives a vapor compression heat pump system (reverse cycle) transferring (pumping) thermal energy from a cold thermal storage to a hot one. During the discharging phase, a heat engine (direct cycle) uses the thermal energy stored in the hot storage to generate electricity and discharges thermal energy into the cold storage. This paper analyses all possible configurations of PTES through a multiobjective optimization approach to find the best tradeoff between roundtrip efficiency and energy density, the latter depending on the specific work of the discharge cycle. The optimization is based on the HEATSEP method, which allows extracting the basic structures of the configurations, named Basic Configurations, understood as an assembly of compression and expansion stages and their interconnections. Results shows that the maximum roundtrip efficiency of 53 percent is obtained by a Rankine PTES versus a maximum value of 42 percent of the Brayton Joule PTES; energy density shows instead an inverse ranking between Rankine (30 kWh/m3) and Brayton Joule (70 kWh/m3) configuration due to the higher temperatures at which heat can be pumped in the latter configuration. Finally, it was found that the little margin for improvement makes it not convenient to further complicate the configurations by increasing the number of compression and expansion stages.