Pareto front analysis for the design and the working fluid selection in ORC-based pumped thermal energy storage technology in both pure electric and cogenerative applications

Abstract

Carnot Batteries are a sub-technology of Pumped Thermal Energy Storage concept where closed cycles are used in both charging and discharging phase. In charging mode, cheap off-peak electricity from the grid is used to store heat at a temperature different than the ambient one (generally higher) while in discharging mode the stored heat is exploited for power production. Carnot Batteries can be designed with different thermodynamic cycles (Brayton cycle, Steam Rankine cycle, Organic Rankine cycle) and can adopt different types of thermal energy storage technologies. For low duration storage (daily or weekly cycling) sensible or latent energy storage with phase change materials can be adopted, while for long duration storage (months, seasonal) the use of thermochemical reactions could be a valid option, as proposed by H2020 RESTORE project. Performance of Carnot Batteries is highly affected by the thermal storage temperature, the condensation temperature in discharging mode and the availability of residual heat at a temperature higher than the ambient one for boosting the heat pump coefficient of performance in charging cycle. This paper focuses on the optimal design of a reversible thermodynamic system working as a heat pump (HP) cycle in charging mode and as an organic Rankine cycle (ORC) in discharging mode. A dedicated numerical model developed in Python is employed to compare the techno-economic performances of different working fluids and select the most promising candidates. Main difficulty is related to the use of reversible heat exchangers which design impacts both on the charging and discharging operation, strongly affecting the system round trip efficiency, and requiring a discretized off-design modelling. A Pareto front of optimal round trip efficiency against total area of heat exchangers is obtained by varying the main design parameters, such as heat transfer temperature differences and heat exchangers pressure drops. Results show that for a given overall heat transfer area low critical temperature fluid are mainly penalized by poor HP performance due to high compressor specific work, while high critical temperature fluids are mostly penalized by high pressure drops caused by the low fluid density. The most eaching a RTE close to 35% for the pure electric configuration at its maximum RTE over total heat transfer area parameter