To analyze the effect of un-load exposure on the heat loss of flat-plate collectors（FPCs）, the thermal performance of three different coatings（the blue-film coating, the anode oxidizing coating and the black chromium coating） of FPCs were tested by the quasi-steady state method. In addition, the variation trend of heat loss with the average temperature of the heat absorbing plate, inclination angle and wind speed were also analyzed. The results show that after un-load exposure aging, the total heat loss coefficient （the blue-film,the anode oxidizing and the black chromium） of the FPCs is 5.0724, 5.2457 and 5.9961 W/（m2·K） respectively. The emissivity of the coating and the average temperature of the heat absorbing plate are the decisive factors for the total heat loss coefficient. Wind speed and inclination angle are the important factors for the total heat loss coefficient, and the environment temperature has little effect on the total heat loss coefficient. The relative error of the total heat loss coefficient obtained by theoretical calculation and the fitting of experimental data is not more than 4%, which shows the better accuracy of the calculation results for the FPCs with different coatings. The observation and analysis by the SEM, it is shown that the black chromium and anode oxidizing coating are superior to the blue-film coating in corrosion resistance, oxidation resistance and thermal stability. Avoiding the long-term high un-load exposure of the FPCs and reducing the emissivity of the coatings can effectively reduce the total heat loss coefficient of flat solar collector.
Key words
flat-plate solar collectors /
stability /
heat loss coefficient /
numerical analysis /
heat absorber
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