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In this paper, the performance of a double pass solar air collector with triangular integrated fins was investigated experimentally at Hungarian University of Agriculture and Life Sciences in Gödöllő, Hungary. The focus of this research is on energy-based performance evaluation. The thermal efficiency of the collector has been compared by testing two collectors that had the same design, with and without fins. The effect of the collector's air mass flow rate on thermal performance was investigated under various environmental situations. The results revealed that the temperature difference is always higher through the finned collector and the higher variation temperature between the inlet and outlet temperature leads to higher useful heat. The daily thermal efficiency of the finned collector was 56.57%, 59.41%, and 61.42%, while for the un-finned collector was 51.04%, 53.28%, and 57.08% for the mass flow rate 0.0081, 0.0101, and 0.0121 kg/s. The finned double pass solar air collector improved the thermal efficiency by 4.3–6.1% over the un-finned one. The efficiency of the finned collector is always higher than the un-finned one regardless of the mass flow rate. The presence of the fins to the top air channels significantly increases collector efficiency, owing to the increased absorbing surface area, which is responsible for increasing the internal thermal convective exchanges. Moreover, it creates a turbulence airflow, meaning that the air will be in good contact with the absorber plate and penetrate all regions, reducing the dead zones contributing to increased heat transfer.

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An attempt is established to investigate the heat transfer from a heated chimney of an indirect solar dryer and predicted the chimney outlet temperature using energy balance equation. Tests were carried out with no-load condition under natural convection mode. Data of air velocity, temperatures, ambient relative humidity and solar radiation of the drying process are presented in order to solve the heat transfer coefficients. The result shows that temperature difference between mean air inside a chimney and ambient was 23.8 °C at maximum solar radiation of 812 W/m 2 , where maximum airflow is obseved. The predicted chimney outlet temperature was found in good agreement with that of the experimental value. Temperature variation, correlation coefficient, and average absolute error are less than 10%, 0.977, and 5%, respectively. Replacing the normal chimney by heated chimney can significantly reduce the heat loss and increase the chimney efficiency.

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This paper presents the results of an experimental investigation on an indirect active type of solar dryer, using two distinct solar air collector and their impacts on drying agricultural products. The thermal performance of the proposed collectors has been evaluated using the first and second laws of thermodynamics. Experimental observations were done in climatic conditions Gödöllő, Hungary on the 2nd and 9th of October 2017. The experiments were also carried out to dry 2 kg of sliced apples spread over the drying trays. The mas flow rate of air was maintained as 0.038 kg/s and the dryer was operated from 10:00 a.m. to 3:00 p.m. When a double-pass solar air collector's results are compared to a single-pass solar air collector's, it's evident that the double-pass solar air collector produces much more energy and efficiency. The experimental results showed that single-pass and double-pass collectors have daily efficiencies of 42.77% and 56.10%, respectively, with average exergy efficiency values ranging from 31% to 49% for single-pass and 51% to 67% for double-pass. The most significant aspect determining the collectors' thermal efficiency was the temperature rise between the collector outlet and inlet. The average drying efficiencies of the solar dryer for the single and double-pass collectors were evaluated as 12.16% and 16.45%, respectively. The maximum temperature rise for double-pass was determined to be 20 °C, whereas single-pass was found to be 6.5 °C. Furthermore, the highest drying rate was achieved when sliced apples were dried with a double-pass collector mode. It reduced 52% of the water content in the apple in the same amount of time as single-pass drying, compared to 35% in the case of single-pass drying.