Nocturnal reverse flow in water-in-glass evacuated tube solar water heaters

In this work, the thermal performance of water-in-glass evacuated tube solar water heaters (SWH) at nights was experimentally investigated. Measurements at nights showed that the water temperature in solar tubes was always lower than that in the water tank but higher than the ambient air temperature and Texp, the temperature of water inside tubes predicted in the case of the water in tubes being naturally cooled without reverse flow. This signified that the reverse flow in the system occurred at nights, making the water in solar tubes higher than Texp. It is found that the reverse flow rate in the SWH, estimated based on temperature measurements of water in solar tubes, seemed not sensitive to the atmospheric clearness but sensitive to the collector tilt-angle, the larger the tilt-angle of the collector, the higher the reverse flow rate. Experimental results also showed that, the reverse flow in the SWH was much higher as compared to that in a thermosyphonic domestic solar water heater with flat-plate collectors, but the heat loss from collectors to the air due to reverse flow in SWHs was very small and only took about 8-10% of total heat loss of systems.