Experimental study on heat transfer characteristics between high-pressure air and molten salt used in solar-aided compressed air energy storage systems

To establish a zero-emission, efficient, and reliable compressed air energy storage (CAES) system to support the large-scale integration of renewable energy into the grid, many studies are integrating concentrated solar power with the CAES, forming solar-aided compressed air energy storage (SA-CAES) systems. However, current integration schemes remain at the theoretical research stage. Furthermore, the heat transfer performance of the high-pressure air when exchanging heat with solar heat transfer fluids involved in the discharging process of the SA-CAES system has not been revealed by existing related experimental studies. Therefore, this paper designs a shell-and-tube heat exchanger and establishes a high-pressure air production system and a molten salt circulation system, to investigate the heat transfer performance of the high-pressure air inside the tubes. The novelty of this experiment lies in increasing the air-side pressure up to 5 MPa and proposing new air-side heat transfer correlations at high pressures, which provides important support for SA-CAES systems. The results indicate that the Reynolds number of air is the primary factor affecting heat transfer capability, higher Reynolds numbers result in better heat transfer rate and Nusselt number. An increase in air pressure has a minor negative impact on the heat transfer. In the turbulent region, the Gnielinski correlation still provides good predictive results, with a maximum deviation of 23 % within the experimental range. Whereas the Hansen correlation underpredicts the Nusselt number on the air side. New heat transfer correlations for high-pressure air have been established for both laminar and turbulent regions, showing prediction errors within ±10 % compared to experimental data, demonstrating high predictive accuracy.