Experimental investigations of the operating behavior of a low-flow drainback solar heating system

Solar heating plants operated as drainback systems (DBS) have proven potential for cost reduction and protection against system overheating and freezing. By selecting a lower flow rate in the collector loop, investment and operation costs can be further reduced. However, the impact of low-flow operation on hydraulic and thermal performance and the associated aspects during planning needs better understanding. Therefore, the phenomena of water/air two-phase flow characteristics under different flow conditions, planning and installation issues, improvements for low-flow operation, and associated malfunctions of a DBS with heat pipe evacuated tube collector are investigated. The low-flow operation of DBS with a concentric manifold configuration creates unforeseen problems of flow break and reduced collector performance. The air that penetrates the system and accumulates in the collector manifold creates these problems. The study indicates that, after removing the air, the collector performance outweighs the reduction in performance induced by lowering the specific flow rate from 40.8 l/(h∙m 2 ) to 11.5 l/(h∙m 2 ) for reduced temperature differences above 0.04 K∙m 2 /W. Concerning the design of the collector manifold, an eccentric reducer with 0.2° inclination performs better than the concentric reducers do at low-flow operation. The air entering the collector loop is well pushed out of the manifold. • Air accumulation in a drainback system impacts its filling, operation and draining. • At low-flow air accumulates in collectors with concentric manifold endings. • Air accumulation and low-flow operation reduce the performance of the collector. • Manifold design with eccentric endings and 0.2° inclination avoids air accumulation.