Multi-objective optimization of Pumped Thermal Energy Storage for large scale applications

Pumped Thermal Energy Storage (PTES) systems are ideal candidates for large scale applications due to high energy densities, no geographical constraints, and the use of safe materials and working fluids. This paper shows how considering the charge and discharge phases of a PTES system as if they were simultaneous allows to easily find the internal heat transfer that optimizes the design of the total system in each step of the optimization procedure. The application of this approach makes a homogeneous comparison of all PTES configurations and performance possible, easy to be carried out and interpreted. Round-trip efficiency and energy density are considered as performance metrics, and their optimum trade-off is obtained by means of a multi-objective optimization approach. Results show that the maximum round-trip efficiency of 53% is obtained by a Rankine PTES versus a maximum value of 42% of the Brayton-Joule PTES. Conversely, energy density shows an inverse ranking between Rankine (30 kWh/m3) and Brayton-Joule (70 kWh/m3) configurations due to the higher temperatures at which heat can be “pumped" in the latter. Finally, it was found that additional configuration changes, such as an increased number of compression and expansion stages, are not convenient for the slight performance improvement