Experimental Study on a Solar Air Heater with Various Perforated Covers and Bed Heights

In this study, the thermal performance of the single and double pass solar air heaters with normal glass cover, Plexiglas cover, quarter and half perforated covers were investigated experimentally. In this system, the conventional absorber plate was replaced with fourteen steel wire mesh layers. The solar air collector was tested with four different perforated covers. On two perforated covers the holes were made on the first quarter at the top side of the cover in an area of 100 × 36 cm2 . On the other two covers, half of the cover area (i.e. 100 × 72 cm2 ) on the top side was perforated. The holes made on one of the quarter and one of the half perforated covers had the center-to-center distance (dc) of 20D (6cm) and on the other two covers dc was 10D (3cm). D was the hole diameter that is fixed to be 0.3cm. The air mass flow rate was varied between 0.011 kg/s and 0.037 kg/s. The solar collector was also tested with three different bed heights (3, 5.5 and 8cm) in order to examine the effect of duct height on the efficiency of the air heater. The height of upper channel was 2cm in double pass collector. In this research the effects of different mass flow rates, various perforated covers and different bed heights on the outlet temperature and thermal efficiency were studied. The obtained results show that as the mass flow rate increases, the thermal efficiency also increases but the temperature difference between the outlet and inlet air, ΔT, decreases. The efficiency of the double pass collector with glass cover was always greater than the single pass collector with the same cover by 5 - 22.7% for the same air mass flow rates. In case of solar air heater with the quarter perforated cover, it was found that, the efficiency of the air heater with the 10D quarter perforated cover is slightly higher than the one with 20D quarter perforated cover for both single and counter flow collectors and that is because the 10D cover was cooler than 20D cover during the tests and this reduces the heat lost from 10D cover to the ambient. It was found that the solar air heater with quarter perforated cover reaches to higher ΔT than with the half perforated or normal Plexiglas cover. At mass flow rate (ṁ) of 0.032 kg/s, the maximum efficiency of the double pass solar collector with quarter perforated covers 20D and 10D were, 57.60% at 13:00h and 60.49% at 15:00h, respectively while at the same ṁ, for the same collector with half perforated covers 20D and 10D the maximum value of efficiency obtained were 52.66% at 12:00h and 57.93% at 15:00h, respectively. The results show that the thermal efficiency of the solar air heater decreases by increasing the bed height of the collector. The collector with duct height of 3cm was shown higher performance compared with the one with bed height of 5.5 or 8 cm. Keywords: Solar air heater, perforated cover, packed bed, thermal efficiency