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 analyzes all possible configurations of PTES through a multi-objective optimization approach to find the best trade-off between round-trip 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% is obtained by a Rankine PTES versus a maximum value of 42% of the BraytonJoule 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 not convenient to further complicate the configurations by increasing the number of compression and expansion stages.