Comparative analysis of different Rankine PTES system configurations

In this paper, four main configurations of a Rankine-based Pumped Thermal Energy Storage (PTES) system are proposed and compared in terms of achievable electrical and exergy roundtrip efficiency and energy density. The analysis considers a conventional setup employing commercial heat pumps and Organic Rankine Cycle (ORC) systems integrated with a Thermal Energy Storage (TES) unit as reference. The initial findings indicate that off-the-shelf systems result in roundtrip efficiencies lower than 40%, even under optimal high-temperature conditions at the heat pump evaporator inlet. This is primarily due to significant exogenous exergy destructions inherent in the commercial equipment. The study then explores upgraded alternatives to the reference PTES configuration, focusing on optimizing the heat pump layout and selecting non-conventional working fluids. This optimization process includes evaluating various working fluids, where n-hexane is identified as the optimal choice for achieving the highest electrical and exergy roundtrip efficiencies, particularly at evaporator inlet temperatures above 60°C. For lower temperature ranges, acetone emerges as a more suitable fluid due to its favorable thermodynamic properties. Further enhancements are made by optimizing the ORC layout, specifically through the introduction of an additional thermal storage tank and improved heat exchangers. These modifications are aimed at minimizing heat transfer losses and thereby boosting the overall system performance. With these changes, the PTES system's roundtrip efficiency reaches approximately 70%. The most advanced configuration integrates the heat pump and ORC systems into a single assembly, utilizing the working fluid not only for energy transfer but also as a storage medium. This integration reduces the number of required components and further increases efficiency. As a result, roundtrip efficiencies of about 80% are achieved, representing a significant advancement over current commercial systems.