Abstract
The effects of suppression of thermal radiation, conduction and/or free convection heat loss from the absorber of a flat-plate solar collector are analytically studied. A two-dimensional, steady-state, heat-transfer analysis is developed for a collector for this purpose. The analysis accounts for the temperature gradients in the fluid flow and vertical directions in the collector, the collimated and diffuse nature of solar irradiation, the physical and thermodynamic properties of the materials in the collector, the number of cover plates, any selective surfaces, and the collector dimensions. The spectral nature of radiation heat transfer in the collector is modeled by two spectral bands, solar and thermal. The results illustrate that a single selective surface on the absorber or on the cover plate closest to the absorber to suppress thermal radiation heat loss is the most effective means of improving flat-plate collector performance when only suppressing one mode of heat transfer. Honeycomb structures placed in the air gap between the absorber and cover plate to suppress free convection or evacuation of the air gap to eliminate conduction and free convection are not as effective a method of improving collector performance as selective surfaces at higher (> 100°C) collector temperatures.
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