Carnot battery system integrated with low-grade waste heat recovery: Toward high energy storage efficiency

The low-grade waste heat is widely distributed in various scenarios and lacks suitable technologies for recovery. Carnot battery is a large-scale electrical energy storage technology, and pumped thermal energy storage (PTES) is one of the branches in which the waste heat can be efficiently utilized. The integration of the PTES system and waste heat promotes energy storage efficiency and tackles the problem of low-grade waste heat utilization. Aiming to improve the energy storage efficiency of the system, several PTES systems have been put forward, where the system configurations were modified and the waste heat is employed in both the heat pump cycle and the organic Rankine cycle. The novel concept of the PTES systems was examined, and the energy-conservation mechanism was further explored. Moreover, the effect of the heat storage temperature pair on the PTES system was evaluated, and the results indicate that the highest efficiency can attain when the low heat storage temperature is around 100.0 °C. The implementation of different working fluids in the charging and discharging process leads to the promotion of power-to-power efficiency, and the efficiency increases from 65.56 % to 66.52 % when the high heat storage temperature is chosen as 130 °C. In terms of various waste heat parameters, the power-to-power efficiency of the advanced PTES system can attain 99.3 % as the inlet temperature of the waste heat rises to 90.0 °C.