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 T _out), 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, T_out in the system with a vertical cylindrical tank was slightly higher than that in the one with a horizontal cylindrical tank; T_out 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 T_out at clear nights. This model allows predicting T_out 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 T _out; for the system with a collector of non-selective absorber, the expected T_out was lower than the ambient air temperature, as observed in experiments; whereas for the system with a collector of solar selective absorber, the T_out, 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.