Thermodynamic Analysis of High-Temperature Carnot Battery Concepts

Within the thermal energy storage initiative, National Demonstrator for IseNtropic Energy (NADINE) storage, three projects are carried out focusing on thermal energy storage at different temperature levels. Thermal storage units are key components of Carnot batteries, which are based on the intermediate conversion of electric energy into heat. Pumped thermal energy storage (PTES) is an emerging Carnot battery concept variant for the flexible management of supply and demand of electricity, heat, and cold. A counterclockwise thermodynamic cycle operated by surplus electricity is used to charge a thermal storage, which delivers heat to operate a power cycle during discharge. The absence of geographic constraints, a theoretical roundtrip efficiency of 100%, and a small environmental footprint are promising features of PTES. PTES is also able to provide low-cost backup capacity in case of shortages in available renewable energy. Various options are proposed for the technical implementation of PTES, using various combinations of engines, thermal storage units, and working fluids. The resulting systems differ in efficiency, costs, maturity, and complexity. Herein, a thermodynamic analysis of five different PTES variants is presented. The results should help to identify priorities in the further development of the PTES variants.