THERMODYNAMIC FEASIBILITY OF A PUMPED THERMAL ENERGY STORAGE DRIVEN BY OCEAN TEMPERATURE GRADIENT

Abstract

The significant future penetration of variable renewable energy, necessary to decarbonize power production, will require energy storage to stabilize the electric grids. Electrochemical batteries may not be the best solution for all energy storage needs due to their limited lifetimes, performance degradation and use of critical raw materials. Pumped thermal energy storage (PTES) is an attractive solution since it does not suffer from the abovementioned limitations and might be preferable for long-duration applications. Particularly Thermally Integrated-PTES (TI-PTES), exploiting an external heat source during the charging or discharging phase, represents a significant improvement of this technology regarding the round-trip efficiency . Several TI-PTES systems have been proposed using solar and geothermal energy or industrial waste heat. This paper introduces a novel TI-PTES concept exploiting the temperature difference between the surface and deep oceanic Water in tropical areas. The system comprises a vapour compression chiller, a phase change material (PCM) cold storage, and an organic Rankine cycle. During the charging phase, the chiller operates between the cold seawater and the thermal storage, converting electric energy into a cooling effect. In these conditions, the chiller Energy efficiency ratio (EER) is in the range of 4.6 - 9.0 due to the deep seawater's low temperature. During the discharge phase, the organic Rankine cycle operates between the hot surface and the storage to produce electricity. Results show that the round-trip efficiency ranges from 0.30 to 0.52 depending on the fluids adopted in chiller and ORC and PCM storage temperature, making the investigated technology a promising alternative to other long-duration storage technologies.