Experimental study of unglazed collector with perforated absorber plate and metal cover

Preheating air with solar power is much cleaner than heated space using fossil fuels. In this study, several tests were carried out in Baghdad, Iraq (33oN and 44oE), to investigate improvements in energy and exergy efficiencies of unglazed solar collectors. The experimental setup mainly involved a perforated plate and a covered metal plate. The unglazed solar collector is a suitable application for preheating a large building area, preheating ventilation air with solar energy, and it has potential for saving fuel and money. This paper presents energy and exergy analysis of a conventional unglazed collector perforated absorber plate and metal cover. The thermal analysis results were based on solar intensity, air velocity, temperature differences and heat parameter (Tout - Tin)/I. The results showed that the daily exergy output rate varied inversely with the air velocity with maximum exergy efficiency gains at lower air velocities: 0.03% the maximum exergy efficiency at 2.5 m/s and 37oC temperature differences. In contrast, the energy efficiency output rate varied progressively with the air velocity, with the highest energy efficiency of 83% at a greater air velocity of 5.5 m/s and 22oC temperature differences. The perforated plate with a metal cover implemented higher energetic efficiencies than exergetic efficiencies. Thermal efficiencies have different behaviours than exergy efficiencies since thermal efficiencies depend on beneficial energy gain and rate of heat transfer from the absorber perforated plate to flowing air; thermal efficiencies increase with the increase in air velocity. Temperature differences are affected by the variation of air velocity. For low air velocity, the passing air had more time to absorb heat than the flowing air, so the maximum temperature difference was 37oC at a low air velocity of 2.5 m/s. The solar intensity value increased from sunrise until midday; the highest solar intensity values were between 880 W/m2 and 988 W/m2.