Abstract
In this study, an experimental outdoor investigation of the thermal efficiency and outlet air temperature was conducted on an unglazed, double-pass, solar air heater with a perforated absorber plate and packing wire mesh layers as a supplemental absorbent area. This was done to observe their effects on the thermal performance of the solar air heater. The double-pass collector was constructed with a bed height of 0.05 m, and a collection area of 1.5 m2. The height of the upper channel was fixed at 0.015 m to improve the thermal efficiency, and the outlet temperature at air flow rates between 0.003 and 0.018 kg/s. The collector was mounted with a slope of 42° facing south, to maximize the intensity of solar irradiance during winter. The effects of the air flow rate, ambient temperature, inlet temperature, outlet temperature, and solar intensity were experimentally investigated. The results showed that thermal efficiency could be improved by increasing the air flow rate, where the highest thermal efficiency achieved was 86% at 0.018 kg/s. However, the temperature difference was increased to a maximum value of 38.6 °C, when the air flow rate was decreased to 0.003 kg/s. Furthermore, the results demonstrated a significant improvement in the thermal efficiency and outlet temperature; and when compared with previous research, the experimental results and the predictions for the outlet temperature using the theoretical model agreed.
Highlights
Solar air collectors have several advantages compared to application of other, liquid-type solar energy systems, such as no freezing or corrosion problems, less maintenance required, and ability to use a perforated plate system
Some researchers investigated the thermal performance of a double-pass, counter-flow solar collector with porous material in the second air passage [19]; they studied the behavior of the solar collector with and without porous media, and compared them according to various governing parameters, such as the air mass flow rate, inlet air temperature, spacing between the top cover and absorber plate, and the intensity of the solar radiation [20]
the expression for (To) evaluate the performance of the solar heater system, the first law of thermodynamic was applied, and the thermal efficiency was defined as the ratio of the useful energy to receive energy, and the useful energy was defined as the thermal energy, and receives energy as heat flux or solar radiation energy
Summary
Several investigators have undertaken experiments regarding heat transfer analysis of solar energy systems, such as the unglazed transpired solar heaters without cover glasses that have been studied by Bhushan and Singh [8]. A double-pass counter-flow was conducted to improve the thermal performance of the transpired solar heater [15]. Some researchers investigated the thermal performance of a double-pass, counter-flow solar collector with porous material in the second air passage [19]; they studied the behavior of the solar collector with and without porous media, and compared them according to various governing parameters, such as the air mass flow rate, inlet air temperature, spacing between the top cover and absorber plate, and the intensity of the solar radiation [20]. The purposes of the Sustainability 2020, 12, 3619 used double-pass, counter-flow collector include: First, increasing the air path to gain more heat and minimize heat loss to the surroundings; and second, maximizing heat transfer to the airstream in the upper channel
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