Recently, TEES (thermo-electric energy storage) systems have been proposed as a new method for large-scale energy storage wherein electric power is stored as thermal energy by using a heat pump and retrieved from the TES (thermal energy storage) by using a heat engine. The advantages of TEES systems are their higher energy density and independence from geological formations in comparison with pumped hydro storage and CAES (compressed air energy storage). In particular, a TEES system based on hot water, ice storage and transcritical CO2 cycles is considered to be a promising method for large-scale energy storage. This paper reviews current TEES systems and proposes a novel isothermal TEES system with transcritical CO2 cycles. For the given efficiencies of the compressor and expander, the maximum round-trip efficiency decreases rapidly with an increase in the back work ratio. It is shown that the round-trip efficiency of the isothermal TEES system can be increased because of a lower back work ratio than in Isentropic case. For the isothermal compression/expansion, the water from hot storage tank is used by pump/motor to compress/expand the supercritical CO2 as the liquid piston, and a portion of the water is sprayed to cool/heat the supercritical CO2 by a circulation pump. (C) 2012 Elsevier Ltd. All rights reserved.