Techno-economic comparison of high-temperature and sub-ambient temperature pumped-thermal electricity storage systems integrated with external heat sources

Pumped-thermal energy storage (PTES) is a promising grid-scale energy storage technology that stores electrical energy as thermal exergy, and whose roundtrip efficiency can be significantly improved by incorporating external heat sources. In this study, two thermally integrated pumped-thermal energy storage (TI-PTES) variants, namely “hot TI-PTES” employing a high-temperature vapor compression heat pump (HT-VCHP) and the “cold TI-PTES” employing vapor compression refrigeration (VCR), along with a thermal energy storage (TES) unit and an organic Rankine cycle (ORC) system, are investigated and compared from a thermo-economic perspective. The effects of different design parameters such as the heat source and ambient temperatures, the annual charging and discharging durations, nominal power rating, and the temperature glide (i.e., the temperature difference between two tanks for the sensible storage medium) are discussed. Results show that the roundtrip efficiency, exergy efficiency, total investment cost (C_invest), and levelized cost of storage (LCOS) decrease while the energy density increases as the temperature glide increases. Roundtrip efficiency values of 0.89 and 0.74 and the exergy efficiency values of 0.4 and 0.38 are achieved by the “hot TI-PTES” and “cold TI-PTES” systems, while providing a heat source temperature of 80 °C, temperature glide of 14 °C, and nominal power rating of 0.5 MW. The energy density, C_invest, and LCOS reach values of 0.80 kWh/m³, 4.31 M$, and 0.33 $/kWh for the former storage system, and 0.99 kWh/m³, 3.21 M$, and 0.31 $/kWh for the latter. Moreover, the LCOS for a grid-scale storage system (5 MW nominal power rating, 8 h storage duration) can be as low as 0.25 $/kWh for the “hot TI-PTES”, and 0.23 $/kWh for the “cold TI-PTES”. The proposed energy storage systems can offer a range of integration options with district heating and cooling networks, which can improve overall energy system operational flexibility.