Compact, efficient, and affordable absorption Carnot battery for long-term renewable energy storage

The growing penetration of renewable energy poses significant challenges to the stability of the power grid, necessitating the development of advanced energy storage systems to facilitate power grid decarbonization with enhanced flexibility. Nonetheless, current energy storage technologies face obstacles including geographical constraints, high expenses, and short lifespans. In this work, a novel Carnot battery (power-heat-power conversion) based on absorption-desorption processes of hygroscopic salt solutions, absorption Carnot battery (ACB), is proposed for large-scale renewable energy storage with remarkable energy storage density (ESD), competitive round-trip efficiency (RTE), and negligible self-discharging rate (SDR). Through the integration of heat-generation, heat-storage, and power-generation sub-cycles into a single compact system, the ACB can save space and cost compared to previous Carnot batteries. A dynamic model is established with high accuracies to explore the characteristics of the proposed system. The dynamic temperatures, pressures, concentrations, mass flow rates, powers, and efficiencies of the ACB are analyzed to elucidate its energy conversion/storage mechanism. Based on the multi-objective optimization, the optimum operating concentration range of [45%, 60%] is determined, demonstrating the best comprehensive performance with an RTE of 45.80% and an ESD of 16.26 kWh/m3. Compared to the existing energy storage systems, the ACB stands out due to the competitive RTEs (30.5%–48.4%) and higher ESDs (7.6–21.8 kWh/m3). Even during an 80-day standby period, the ACB exhibits a small SDR of only 0.74%, which is significantly lower than that of Rankine pumped thermal energy storage (RPTES) at 33.01%. Despite the ACB yields a higher initial cost, it demonstrates a markedly lower levelized cost of storage (0.342 $/kWh) compared to the RPTES (0.749 $/kWh) because of its higher ESD, thus confirming the economic feasibility of the proposed system.