Thermo-electric energy storage using co2 transcritical cycles and ground heat storage

Abstract

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 massive electricity storage concept based on CO2 transcritical cycles and ground heat exchangers, 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 MWe as the discharging process. A steady-state thermodynamic model is realized and several options, including regenerative or multi-stage cycles, are investigated. In addition, a one-dimensional design model of the geothermal heat exchanger network is used to optimize the number of wells for the ground heat storage. The results show the strong dependency between the charging and discharging cycles, and how the use of regenerative heat exchangers and a two-phase expander (in the charging cycle) could increase the system efficiency and lower the investment cost.