Thermodynamic investigation of a Joule-Brayton cycle Carnot battery multi-energy system integrated with external thermal (heat and cold) sources

The electro-thermal conversion working mode implies that Carnot batteries have the potential to transform into multi-energy management systems by scheduling and converting different energy vectors according to energy demands. In this paper, a thermodynamic model of Joule-Brayton cycle Carnot battery multi-energy systems is established, based on which two methods of conversion and utilisation of external multi-grade heat and cold are proposed to respond to changes in energy demand. The effects of key parameters such as heat and cold source temperatures, the amount of absorbed heat and cold energy, working fluid mass flow rate and discharge duration on the performance of electricity efficiency, exergy efficiency and coefficient of performance are discussed. The results show that the electricity efficiency of the Carnot battery multi-energy system can be increased to 68.8%–78.0% when the system integrates heat sources only, and to 113.9%–115.2% when the system integrates both heat and cold sources. Additionally, the methods of increasing the working fluid mass flow rate and extending discharge duration allow such systems to integrate with multiple heat and cold sources in combined cooling, heating and power mode. Furthermore, the technical feasibility of such systems is also evaluated for a large energy hub in China, with the coefficient of performance, exergy efficiency and reduced CO2 emissions rate reaching 119.3%, 85.1% and 94.3 t/h, respectively. The Carnot battery multi-energy system possesses sufficient flexibility in the domain of multi-energy utilisation, demonstrating its potential to evolve into smart energy hubs for cities or districts.