Thermodynamic analysis of a high temperature heat pump coupled with an organic Rankine cycle for energy storage

Abstract

[EN] Energy storage is one of the bottlenecks to increase the share of renewable energy in electricity production. This paper presents an interesting solution, which is to produce heat at high temperature during times of excess electricity production (e.g. from wind turbines), store the heat and recover it at a later stage in order to produce electricity by means of an organic Rankine cycle (ORC). The heat production is ensured by a high temperature heat pump (HTHP), and the heat is stored in latent and sensible heat storage systems. This work has been developed under the frame of the European project CHESTER (Compressed Heat Energy STorage for Energy from Renewable sources). A thermodynamic model has been developed using Engineering Equation Software (EES) to evaluate the system performance. This study analyzes the impact of three phase change material (PCM) with different melting temperatures (133, 149 and 183 °C), the selection of the refrigerant (dry, isentropic or wet fluid) and the choice of the thermodynamic cycle. The results indicate that isentropic fluids have the best overall system performance. Among the assessed refrigerants, R1233zd(E) is the best working fluid for the low and medium melting temperatures. For a melting temperature of 133 °C, a roundtrip efficiency of 1 can be reached if the source temperature is 75 °C. For the high temperature storage (183 °C), R141b is the best working fluid. A two-stage compression in the HTHP does not appear to be beneficial for the system, nor does the introduction of a recuperator or regenerator in the ORC cycle.