Experimental and modeling studies on thermosiphon domestic solar water heaters with flat-plate collectors at clear nights

To investigate effects of water temperature in the storage tank and height difference between collector loop connections at the tank on freeze protection of flat-plate collectors at clear nights in terms of outlet water temperature of the thermosiphonic reverse flow from the collector (referred to as Tout), two sets of thermosiphon domestic solar water heaters (DSWH, in short) were constructed and tested. Experimental measurements showed that, for given water temperature in tanks, Tout in the system with a vertical cylindrical tank was slightly higher than that in the one with a horizontal cylindrical tank; Tout increased with the increase of water temperature in the tank but was lower than the ambient air temperature all night. Meanwhile, a mathematical model was developed and experimentally validated for further investigating effects of structural and performance parameters of the system on Tout at clear nights. This model allows predicting Tout and the time at the moment ice formation inside absorber of the collector begins. Results by simulations showed that the collector-tank height difference and the thermal emissivity of absorbers had significant effects on the freeze protection of collectors in terms of Tout; for the system with a collector of non-selective absorber, the expected Tout was lower than the ambient air temperature, as observed in experiments; whereas for the system with a collector of solar selective absorber, the Tout, depending on water temperature in the storage tank, was higher than the ambient air temperature. This finding implied that, in thermosiphon DSWHs, flat-plate collectors with a non-solar-selective absorber might suffer from freezing-damage, but those with a solar selective absorber would not at clear nights with the ambient air temperature near or even lower than the freezing temperature.