A comprehensive thermal-hydraulic assessment of solar flat-plate air heaters

This paper evaluates the thermal-hydraulic performance of flat-plate solar air heaters having roughened ducts using different turbulence generators. About 220 roughened ducts have been selected from the literature based on the availability of experimental f-factor and Nusselt number data. In particular, roughness elements are rectangular/square ribs, arc/wavy ribs, V-shaped ribs, wedge-shaped ribs, protrusions, and metal mesh. To assess the performance of different solar air heaters, collector efficiency (as a function of useful energy and pumping power) and entropy generation are estimated. The optimal duct is chosen based on the maximum collector efficiency and the minimum entropy generation and hence, is suggested to be a benchmark. The results show that the Reynolds number range of 10000 –16000 is recommended to be used for operating solar air heaters to get the optimal performance. The solar duct GKS3 is found to have the optimal performance when Re < 11000, and the duct GK23 has the optimal performance for 11000 < Re < 16000. These two solar air heaters (GKS3 and GK23) are recommended to be benchmarks for comparing and assessing the performance of all flat-plate solar air heaters. For Re > 16000, the solar collector GB5 is found to have optimal performance.