Exergy-based performance comparison of the Rankine-based Carnot battery with different heat pump systems and multi-stage thermal storage modes

The Rankine-based Carnot battery is an effective solution for integrating renewable energy and enabling cogeneration in regional energy systems, owing to its long lifespan, low specific investment, and geographical adaptability. This study presents a performance analysis and comparison of the standalone Rankine-based Carnot batteries with different heat pump systems and multi-stage thermal storage modes based on the exergy method. The mathematical models of these Rankine-based Carnot batteries were developed and validated, the performance indicators were introduced, and then the highest roundtrip efficiencies of different system configurations were sought through parameter optimization. The results reveal that after introducing latent thermal storage in the hot storage side, the heat pump cycle achieves better match with the ORC, leading to a significant improvement in performance. The COP of the heat pumps are between 2.01 and 2.56, and the efficiencies of the ORC are between 15.54 % and 16.31 %. The roundtrip efficiencies of the cascade heat pump configurations without and with latent thermal storage are 31.22 % and 38.21 %, respectively. For the flash heat pump configurations without and with latent thermal storage, the roundtrip efficiencies are 34.80 % and 41.06 %, respectively. Additionally, replacing cold storage medium with latent thermal materials can further enhance system performance, increasing the roundtrip efficiency up to 46.67 %. Exergy losses are concentrated in heat exchange processes, compressors, and turbines, and the introduction of latent thermal storage in the hot and cold storage sides can effectively lower exergy losses during charge and discharge processes.