Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold

A study is reported of thermosiphon circulation in solar water heaters incorporating glass tubular evacuated collectors and a water-in-glass manifold of extremely simple design. The manifold is characterised by the absence of partitioning of the inner volumes of the absorber tubes into inlet/outlet channels and buoyancy effects are utilised to convey heat to a header pipe at the open end of the tubes. Solar energy input to the tubes has been simulated by electric heating. The thermosiphon system design is unusual in that there are no risers within the collector tubes, thus, the pressure head responsible for thermosiphon flow originates entirely from relatively short pipe runs between manifold and storage tank. Thermosiphon flow has been measured for a number of system designs and a wide range of operating conditions. The relative impedances of the system components has been evaluated allowing optimization of the system design. An investigation of the effect of withdrawal of hot water from the storage tank, with associated injection of cold water to the bottom of the tank, has illustrated that the self-regulating nature of the thermosiphoning system results in a large proportion of heat stored in the wate filled collector tubes being effieciently transferred to the storage tank, providing some water is drawn off intermittently.