Experimental Study of A Glazed Transpired Solar Air Heater in Natural Convection Mode

Abstract

The experimental study of a glazed transpired solar air heater (also called glazed transpired collector, GTC) in natural convection mode has been carried out. An experimental rig consisting of a perforated absorber plate, glazing, insulated casing, air duct and chimney was designed and set up in the field for the study. Data was collected at hourly interval covering a six months‟ period. Instrumentation included a solarimeter for radiation data, a hot wire anemometer for system air temperature and velocity data, a thermocouple meter and probe for absorber plate temperature measurement and a wind vane anemometer for wind speed and ambient temperature measurement. The study considered the following: the influence of absorber hole diameter and hole pitch to diameter ratio on the performance of the GTC; the influence of dryer chimney height and air suction on the performance of the GTC and a comparison of the performance of the glazed non-transpired collector (GNTC) with that of the GTC in natural convection mode. The scope of this study also covered the development of heat transfer correlation equations for the GTC in natural convection mode and the determination of the suitability of the GTC for crop drying in natural convection mode. The results of the study showed that Air outlet temperature increased with decrease in the absorber hole diameter and decreased with increase in air suction velocity. Heat exchange effectiveness decreased with increase in absorber hole diameter as well as with increase in Rayleigh number. Thermal efficiency increased with increase in absorber hole diameter. It also increased with increase in air suction velocity, Rayleigh number and pressure difference across the absorber. The pressure difference across the absorber, induced air suction velocity and Rayleigh number all increased with increase in chimney height. The thermal efficiency of the GTC for absorber hole diameter range from 2.5 mm to 3.0 mm was higher than that of the GNTC, while for diameter range from 1.0 mm to 2.0 mm, the thermal efficiency became lower than that of the GNTC with a transition between 2.0 mm and 2.5 mm. The average efficiencies of the GTC with absorber hole diameters of 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm were 3.4%, 11.9%, 47.9.0%, 56.9% and 74.3% respectively while the average efficiency of the GNTC was constant at 50.8%. The GTC heat transfer correlation based on influence of hole diameter was .

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and the correlation based on influence of pitch was .

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. The Nusselt number correlation for influence of hole diameter predicted the Nusselt number with less than 10% error for 2.0 mm to 3.0 mm absorber hole diameter but failed to predict the Nusselt number for 1.0 mm and 1.5 mm absorber hole diameters. Low suction velocities associated with these absorbers were difficult to measure. The low suction velocities for these absorbers were due to high resistance to air suction and this resulted in errors in the computed values of Rayleigh numbers. However, the Nusselt number correlation for predicted all measured Nusselt numbers well with maximum error less than 9%. The GTC with absorber plate having 3.0 mm diameter holes provided the best performance with thermal conversion efficiency of 74.3%. The chimney height of 8.04m provided the highest pressure difference and induced air suction velocity corresponding to the best performance. The study shows that the GTC has a promise for application in the areas of space heating and crop drying in locations where there is no electricity suppl