Working Fluid Selection and Thermodynamic Optimization of the Novel Renewable-Energy Based RESTORE Seasonal Storage Technology

Seasonal based energy storage aimed at increasing the dispatchability of renewable energies is expected to be one of the main options for the decarbonization of the space heating sector. Technologies available today can only partially fulfill the requirements of efficiency, energy density and affordability, being based on the use of hot water. This work focuses on the analysis of a non-conventional system based on the adoption of pumped thermal energy storage concept to maximize the usage of renewable energies and waste heat excess during summer season and make them available during winter months. Organic fluid-based cycles are adopted for both the heat upgrade during hot season (heat pump) and to produce electricity and hot water during cold season (power unit). Upgraded thermal energy is used for supporting an endothermic reaction producing dehydrated solid salts that can be stored for months using inexpensive, high efficiency and high energy density (up to 2 GJ/m³) solutions. This paper focuses on the design of the thermodynamic cycles, comparing the performance attainable with several different working fluids. Two different systems are investigated: coupled systems sharing the same heat exchangers and the same fluid in both operating modes and decoupled systems. Analysis is completed with a preliminary economic assessment of the system, including a sensitivity analysis on electricity and heat cost. Cyclopentane resulted to be one of the most promising working fluids for coupled systems, reaching a competitive round-trip efficiency of around 35%, maximizing the ratio between performance and total heat transfer surface, without showing excessive turbomachinery volume ratios and volumetric flow rates. Economic analysis also highlighted that in this cogenerative application the use of working fluids with lower efficiency, but also low system capital cost, can achieve competitive payback times. On the contrary, decoupled cycles systems result to be less attractive, being able to reach only slightly higher thermodynamic performance, but requiring a higher capital cost and thus possibly being of interest only in specific applications.