Can a “retained heat cycle” enhance the antifreeze performance of large-scale solar collector systems?

The freezing of solar collector systems poses a significant challenge to maintaining safe and stable solar heating during winter. Conventional antifreeze measures for large-scale systems—most notably the use of antifreeze fluids—incur high costs, increase hydraulic resistance during daytime operation, and reduce collection efficiency. This underscores the need for more economical and efficient antifreeze strategies. Exploiting the intrinsic spatial and temporal heterogeneity of heat loss in such systems, we propose an antifreeze approach based on a “retained-heat” circulation concept, designed to maximise the utilization of retained heat. We investigated fluid heat-loss under both static and dynamic operating modes, developed performance evaluation indices, and characterised the cooling behaviour of large-scale systems. Our results show that the dynamic mode has a heat-loss ratio about 30 % higher than the static mode, yet increases the minimum fluid temperature by more than 2.1 °C. This demonstrates the feasibility of using a system's retained heat to prevent freezing. Increasing the initial fluid temperature, flow rate, main pipeline diameter, and insulation thickness further enhances antifreeze performance. These findings offer a novel, theoretically grounded framework for developing cost-effective antifreeze strategies in large-scale solar collector systems.