Electrothermal energy storage with transcritical CO₂ cycles

A novel type of bulk electricity storage – electrothermal energy storage (ETES) – is presented. The concept is based on heat pump and heat engine technologies utilizing transcritical CO₂ cycles, storage of pumped heat in hot water, and ice generation and melting at the cold end of the cycles. The paper first describes the growing need for large scale electrical energy storage and the role of storage in the integration of renewable intermittent generation such as wind energy into the electricity network. The background and a short review on ETES is given and the main principles of (i) reversible¹ heat pumping using vapor compression, (ii) thermal energy storage, and finally (iii) back conversion of thermal energy into electricity via a thermal engine are explained. Following the introduction of ETES as a general concept, the transcritical CO₂ based system is presented by providing a description of the thermodynamic cycles and the corresponding operating conditions. Next the overview of an envisioned transcritical ETES plant is given with information on the main equipment including the turbomachines such as compressor and turbine, high pressure plate heat exchangers, and ice storage. Key properties of the proposed transcritical ETES system are then reviewed with an emphasis on energy storage efficiency, scalability, site-independence, and minimal environmental impact. Information about the operating characteristics such as start-up and standby times and storage duration of the proposed system is also given. The paper is concluded by discussing the future perspectives for the proposed system mainly by focusing on potential technology improvements for the CO₂ machines and the storage materials for both hot and cold ends of the system.