بررسي تاثير دو پوشش شيشه اي پله اي و تخت بر بازده حرارتي يك هواگرمكن خورشيدي با جاذب متخلخل

The Effect of Two Different Glass Covers (Flat and Slatted) on Thermal Efficiency of a Porous Absorber Solar Air Heater

A distinguished drawback of using flat plate absorber in solar air heater is low value of convective heat transfer coefficient between absorber plate and cooling air which increases top heat loss and decreases thermal efficiency. In this study, a porous absorber solar air heater with slatted sheets and flat glass covers were designed and fabricated. The thermal efficiencies of these two solar air heaters were compared at different cooling air mass flow rates (0.0065 to 0.030 Kg/s.m2) at outdoor conditions. Air mass flow rates were measured by orifice plates, temperatures were measured by T-type thermocouples and solar radiation was recorded by a pyranometer. In this research, the use of porous absorber resulted in increasing convective heat transfer coefficient between flowing air and absorber and decreasing convective and radiative heat losses between absorber and cover systems leading to higher thermal efficiency of the solar air heaters. Results indicate that by increasing the air mass flow rate, the temperature of all points of the two solar air heaters were reduced, thus yielding a higher thermal efficiency. Furthermore, in the air heater with slatted glass sheets cover, the vertically downward air flow reduces convective heat loss from absorber to cover. Sucking air through the slots recovers part of the short wave radiation absorbed by the glass sheets as well as the long wave emission from the absorber. This process resulted in minimizing the convective and radiative heat losses through the cover. It was shown that, for the range of air mass flow rate studied, the thermal performance of the air heater with slatted glass sheets cover was always higher than the flat glass cover and this superiority improved at higher air mass flow rates. At the highest air mass flow rate, (0.030 Kg/s.m2 ); the thermal efficiencies of the air heaters with slatted and flat glass covers were 63%, and .51% respectively at outdoor conditions.