Optimal heat storage temperature and performance of ORC-based Carnot battery at various application scenarios

Long-term electricity storage technology is essential to achieve a high proportion utilization of fluctuating renewable energy. Carnot battery is an emerging long-term electricity storage technology with lower cost, larger capacity, and no geography restrictions. Using organic Rankine cycle (ORC) as the power unit is beneficial to integrate the low-grade waste heat, achieving a higher energy efficiency for Carnot battery. Heat storage temperature is a key parameter influencing the optimization and performance of ORC-based Carnot battery, but its optimal selection is affected by working fluid type and heat source temperature. For various application scenarios, the optimal heat storage temperatures and the highest power-to-power efficiencies of ORC-based Carnot battery are still unclear. This paper focuses on the ORC-based Carnot battery with various heat source temperatures and working fluid types. Influences of heat storage temperature on the optimization and performance of system are analyzed. Optimal heat storage temperature and the highest power-to-power efficiency at various application scenarios are given. Exergy performance characteristics of ORC-based Carnot battery are revealed, and its energy efficiency superiority is evaluated. Results indicate that the effects of heat storage temperature on system performance differ remarkably for using various working fluid types. The heat source temperature greatly affects the optimal heat storage temperature when it exceeds 60 °C. Exergy loss of system is mainly distributed in heat exchange processes, accounting for 69.4 % of the total exergy loss. The highest power-to-power efficiency of ORC-based Carnot battery can reach 1.09, exceeding the energy efficiency of combining battery and single ORC system.