Heat transfer optimization of a Carnot battery using a partial cascade organic-steam Rankine cycle

Carnot battery is an emerging technology for low-cost, site-independent, environmentally friendly electricity storage and is also suitable for use in the heat sector. State-of-the-art Carnot battery uses subcritical steam Rankine cycles (SRC) for heat-to-power conversion because of the significant technical challenges of supercritical cycles, including increased leakage losses at high-pressure turbines and wetness at low-pressure turbines. The maximum operating temperature of about 565⁰C for molten salts limits the live steam pressure, water evaporation temperature (310-340⁰C) and power cycle efficiency. To increase the average temperature in the heating process and the cycle power efficiency, this paper proposes an innovative Carnot battery using a partial cascade organic-steam Rankine cycle integrated with molten salt storage. The organic fluid, a mixture of biphenyl and diphenyl oxide, absorbs heat from the molten salts and evaporates at a high temperature. It drives a turbine and then releases heat to generate steam for the bottom cycle. The steam is superheated and reheated by the molten salts. Fundamentals of the proposed system are illustrated, and mathematical models are built. Optimization is conducted. The results show the proposed Carnot battery has an efficiency of 8% higher than conventional ones.