Multicriteria optimization of Brayton-like pumped thermal electricity storage with liquid media

Abstract

A multi-objective and multi-parametric optimization of a Pumped Thermal Electricity Storage system based on Brayton cycles is presented by the calculation of different Pareto fronts and the associated Pareto optimal sets for energetic and design analysis, respectively. A large range of internal and external irreversibilities and the thermodynamic properties of the storage media are taken into account. The analysis shows that the heat capacity of the working fluid and the heat capacity of the storage media should be the same in the contact with the hot reservoirs and in the contact with the cold reservoir in the heat pump, but in the contact with the cold reservoir for the heat engine the ratio should be 0.33, this offers information regarding the mass flow increasing significantly the achievable values for the round-trip efficiency, power output and the heat engine efficiency in the discharge process. Optimal values are given in terms of the degree of irreversibilities in the system and a comparison is made with extreme cases of infinite and minimum sizes for the storage system. Round-trip efficiencies in the so-called optimum scale/mass-flow-ratio design point exhibits noticeably larger values compared to previously reported results including the so-called endoreversible limit, where no internal irreversibilities are considered and where the improvement can achieve 49% over the endoreversible case in the most ideal scenario. Explicit numerical values of the maximum round trip efficiency, power output, and efficiency are given for a broad range of both internal and external irreversibilities.