Evaluation of thermal characteristics of a flat plate solar collector with a back mounted air channel

Abstract

Flat plate solar collectors for domestic hot water systems are known to experience problems under stagnation conditions, when the heat transfer fluid is at rest in the absorber flow passages. The solar collector absorber plate may reach temperatures approximately 170°C or more especially when power supply fails or periods when there is minimal energy demand. This paper evaluates the thermal characteristics of an commercial Enerworks Residential Collector integrated with a back mounted air channel under stagnation conditions which is the continuation of previous investigations of current authors both experimentally and numerically to optimize the performance of the collector during both operation and stagnation. Air flow and thermal conditions in the inclined air channel of the solar collector were simulated using the validated numerical model adopting the Reynolds Averaged Navier-Stokes (RANS) modeling approach with SST-k-ω turbulence model. The radiation exchange between the surfaces of the air channel was modeled using the Discrete Ordinate radiation Model. The numerical results for velocity contours, turbulence intensity, temperature distribution, heat transfer coefficient and averaged Nusselt numbers were obtained. A correlation of averaged Nusselt numbers with modified Rayleigh numbers was developed. From the results obtained it was concluded that the incorporation of a passive air cooling channel at the back of flat plate solar collectors with a suitable control valve at the outlet opening could be helpful in reducing the maximum temperature arrived under stagnation conditions approximately from approximately 170°C to 120°C and will prolong the life of solar collectors.