Abstract
Applying solar collectors is a popular tool for harnessing solar energy. In this work, a flat plate solar air collector was investigated under direct solar radiation in an endeavor to enhance the thermal efficiency of solar air collector with a slatted glass cover, perforated absorber aluminum sheets (porosity 0.0177, 0.0314 and absorber thickness of 1.25, 2.5 mm) which is the most suitable for a solar dryer. The effects of porosity and thickness on absorber performance of collector were evaluated. Six levels of air mass flow rates (0.0056 to 0.0385 kg m−2 s−1) were adopted. The tests were conducted in three replications on very clear sky days in September and October. The experimental results showed that thermal efficiency of collector was increased by an increase in the porosity of the absorber. The absorber with lower porosity showed a better thermal efficiency at lower air mass flux. In the minimum air flow rate, absorber efficiency with porosity 0.0177 and 0.0314 was 0.31 and 0.29, respectively, whereas at the maximum flow, efficiency showed an enormous change of 0.83 and 0.88, respectively. This solar air heater can be used for drying agricultural products, heating the space of greenhouse, and so forth.
Highlights
Solar air heaters are inherently low in thermal efficiency due to low heat capacity and low thermal conductivity of the air in comparison to the liquid-type solar collectors [1]
In porous type, absorbing the solar radiant heat and the heat convection between air and the absorber can effectively strengthen the air that passes through the collector absorber which improve the quality of heat transfer coefficient and thermal efficiency, it would be better in thermal characteristics in comparison with nonporous absorber collector [3]
Porous absorber plates have been studied in different studies such as absorber made of aluminum foil cutted [4], wire nets [5, 6], retail glass [7], pieces of coal [8], hollow spheres [9], pages made of black synthetic fiber [10], and thick black cotton fabric [11]
Summary
Solar air heaters are inherently low in thermal efficiency due to low heat capacity and low thermal conductivity of the air in comparison to the liquid-type solar collectors [1]. Various absorber plates and glazing systems have been used in solar collectors and reported in the literature, porous (transpired) absorber and nonporous one and different types of glazing. The main disadvantage of nonporous absorber is depletion of complete heat transfer between absorber and fluid that leads to low thermal efficiency because the coefficient of convective heat transfer between air and the absorber plate is quite low, the temperature of the absorber plate would be high and radiation loss is quite large. In porous type, absorbing the solar radiant heat and the heat convection between air and the absorber can effectively strengthen the air that passes through the collector absorber which improve the quality of heat transfer coefficient and thermal efficiency, it would be better in thermal characteristics in comparison with nonporous absorber collector [3]. The advantage of porous absorber is attributed to the lower thermal energy dissipation to environment due to absorption and penetration depth of solar radiation [1]
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