Experimental investigation and optimisation study of a direct thermosyphon heat-pipe evacuated tube solar water heater subjected to a northern maritime climate

A proprietary heat-pipe Evacuated Tube Solar Water Heater (ETSWH) originally designed for collection of solar energy using forced fluid circulation was operated using thermosyphon fluid circulation. The thermal performance of this solar water heating system was monitored from October 2006 to June 2007. The ETSWH array was inclined with the expected collector outlet at a higher datum level than the expected collector inlet to provide a hydrostatic pressure differential across the manifold. During the monitoring period it was observed that thermosyphon flow was not always in the expected direction, with flow reversal occurring when the collector inlet temperature was greater than that of the outlet. When the evacuated tube solar water heater manifold was inclined at 1 degrees to the horizontal, reverse fluid flow was observed to occur for 69% of the monitored diurnal periods. Diurnal reverse circulation lowers system efficiency by reducing thermal stratification in the hot water storage tank via convective entrainment and mixing. The thermal performance of the ETSWH was monitored continuously from January 2007 to June 2007 with the manifold inclined at 5 degrees to the horizontal. Over this time period it was found that fluid flow reversal arose for 22% of the diurnal periods considered, resulting in a 47% improvement in diurnal storage efficiency compared to when the system had its manifold inclined at 1 degrees. The long term diurnal storage efficiency of the optimised system inclined at 5 degrees was measured as 66%.