Experiment and simulation study on convective heat transfer of all-glass evacuated tube solar collector

Solar collector with horizontal double-row all-glass evacuated tubes has been extensively implemented in the solar water heating system engineering. The temperature distribution and stratification of horizontal double-row all-glass evacuated tube collector with 24 evacuated tubes have been studied. Validation of instantaneous efficiency under different declination angle θm were studied by means of experiments. Moreover, numerical simulations are carried out for four different declination angle θm (0°/2°/4°/6°). The results showed that the declination angle θm has significant effect on energy conversion efficiency, flow patterns and stratification inside evacuated tubes. When θm > 0, with the declination angle increased, the instantaneous efficiency also increased, nevertheless, the temperature stratification weakened and the heat loss coefficient demonstrated no significant change. Declination angle 6° allowed to achieve significant higher temperatures, nevertheless, along with inactive area at the bottom of evacuated tubes appears. If declination angle θm < 0, inversion phenomenon appears. With the increase of |θm|, more heat is trapped in the sealed end of the evacuated tube, which is not conducive to the flow heat transfer in evacuated tube solar collector.