A study of working fluids for transcritical pumped thermal energy storage cycles

Electricity storage is widely regarded as critical to a sustainable energy future, and currently deployed technologies such as pumped hydroelectric storage have drawbacks which limit the scale to which they can be implemented. Pumped thermal energy storage (PTES) has recently started to attract interest as an alternative. This article focuses on transcritical cycles and aims to identify the best working fluids, in a configuration with a single hot store and no cold store. Three different storage media were considered for the hot store: water, Therminol D12, and Therminol 66. For the transcritical cycle, 176 different working fluids were screened for thermodynamic, environmental and safety suitability, and the resulting list of 8 fluids was tested with cycles at a range of storage temperatures. The optimal round-trip efficiency is a trade-off between heat exchanger losses and turbomachinery losses. Pareto fronts were used to rank the fluids for efficiency, power density, and heat to work ratio. The most promising fluid was found to be trifluoroiodomethane (R13I1) with a peak round-trip efficiency of 57.6%.