Alternating operation of a supercritical CO2 heat pump / gas turbine for electrical energy storage
To ensure a reliable and efficient energy sector with a growing share of intermittent renewable energy, energy storage is essential. One of the promising solutions to store excess energy (charging mode) and deliver deficit energy (discharging mode) is Thermo-Electrical Energy Storage (TEES). During the charging mode, excess electrical energy is converted into heat at high temperature using a heat pump and during the discharging mode, the stored heat is converted into electrical energy using a heat engine. Hereby, an attractive working fluid for the heat pump and the heat engine cycles is CO2, due to its non-flammability, non-toxicity, high density, and near-ambient critical temperature. This paper proposes an innovative TEES based system consisting of a supercritical CO2 heat pump in the charging mode and a supercritical CO2 gas turbine in the discharging mode, with a heat storage in between. Hereby, an advantage of the proposed system is the ability to use the same components in the two alternating operational phases. Next to the energy conversion systems, a suitable heat storage medium is required, which allows heat transfer at low temperature differences and is compact. For the proposed system, schematic diagrams, pressure-enthalpy and temperature-entropy diagrams are given. Furthermore, thermodynamic models of the supercritical CO2 gas turbine and the supercritical CO2 heat pump are developed. To calculate the COP of the proposed heat pump and the thermal efficiency of the proposed gas turbine a thermodynamic analysis is performed. Hereby, results for the influence of the pressure and the temperature conditions on the round-trip efficiency are presented. The results show that the system can achieve high thermodynamic performance with a round-trip efficiency between 55% and 65%. This indicates that the supercritical CO2 heat pump / gas turbine system is a competitive energy storage technology for decarbonisation of the energy sector.