Experimental investigation of subcritical CO2 forced convective heat transfer in high-temperature cast-iron thermal storage channels under regulated variable-flow conditions

Solid sensible heat thermal energy storage can improve power system flexibility and support renewable energy integration. This study experimentally investigates heat extraction by subcritical carbon dioxide flowing through channels in a high temperature cast-iron storage body. A closed CO 2 loop was operated at 0.2 MPa under high Reynolds number conditions. During discharge, the distributed control system adjusted the CO 2 mass flow rate according to the measured main steam flow, compensating for the decreasing thermal driving force and maintaining the heat extraction rate within a limited range. The tests therefore represent variable flow operation with an approximately constant average heat flux over the test section, while the cast iron remained in a transient and spatially nonuniform thermal state. Effective convective heat transfer coefficients and Nusselt numbers were determined from the measured flow rate, temperature variation, and temperature dependent CO 2 properties. Several empirical correlations were developed using polynomial regression and logarithmic multiple linear regression and compared with classical correlations. The results show that heat transfer performance is strongly associated with Reynolds number. The recommended correlation achieves a coefficient of determination of 0.988, a mean absolute percentage error of 0.804%, and a maximum relative deviation of 2.313%, providing improved predictive accuracy for the present system. These results provide experimental data and a practical correlation for evaluating subcritical CO 2 heat extraction under regulated variable flow discharge conditions. The correlation can support the thermal design and performance prediction of high temperature solid sensible heat storage systems using CO 2 as the heat transfer fluid.