Thermo-Electric Energy Storage involving CO2 transcritical ground heat storage

Multi-megawatt Thermo-Electric Energy Storage based on thermodynamic cycles is a promising alternative to PSH (Pumped-Storage Hydroelectricity) and CAES (Compressed Air Energy Storage) systems. The size and cost of the heat storage are the main drawbacks of this technology but using the ground as a heat reservoir could be an interesting and cheap solution. In that context, the aim of this work is (i) to assess the performance of a geothermal electricity storage concept based on CO2 transcritical cycles and ground heat exchanger, and (ii) to carry out the preliminary design of the whole system. This later includes a heat pump transcritical cycle as the charging process and a transcritical Rankine cycle of 1-10 MWel as the discharging process. A steady-state thermodynamic model is performed and several options, including heat regeneration, two-phase turbine and multi-stage design, are investigated. In addition, a one-dimensional model of the ground exchanger is performed and coupled to the thermodynamic model to optimize the number of wells for the ground heat storage. The results show a strong dependency between the charging and discharging processes and indicate how the use of heat regeneration in both processes could be advantageous. The results also measure the difference in performance between the basic and the advanced designs. (C) 2016 Elsevier Ltd. All rights reserved.