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This paper compared the thermal performance of two solar collectors. The first collector is coated by a dark black absorbent solar collector, and the second one coated by the chromium trioxide. The results of the second collector show a better selectivity in terms of greater radiation absorption, and less emission compared to the first collector. At the minimum solar irradiance time, the absorbed energy is increased from 908.28J to 1221.5J, and the thermal efficiency is improved from 37.3% to 50.1% when the chromium trioxide coating is used. Besides, at the maximum solar irradiance time, the absorbed energy is increased from 1340.5J to 1528.4J, and the thermal efficiency is improved from 63.9% to 78.9%. After three months exposing the collectors to outdoor conditions, the chromium trioxide coated collector show superior thermal performance over dark black coated collector. At the minimum solar irradiance time, the absorbed energy is increased by 877J and the thermal efficiency is improved by about 34% when the chromium trioxide coating is used. In addition, the absorbed energy and thermal efficiency are also increased from 501.1J and 20% to 1440.7J and 62.2% respectively when the solar irradiance is at its maximum value

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