Solar energy absorption properties of some agricultural products

Abstract

The purpose of this study is to investigate the possibility of using agricultural products as direct solar energy collectors in solar energy crop drying systems by determining the solar absorptance and emittance properties, and collector efficiencies of the products. For a good grain solar energy absorber, such a system would result in considerable savings in the equipment and maintenance costs, and would be more suitable to developing countries. Solar absorptances and collector efficiencies of grain absorbers were determined by calorimetric technique and compared, under identical conditions, with those of a reference standard-flat-black paint-which is known to be a good solar energy absorber. Seven experiments were conducted under relatively clear sky conditions using previously dried wheat, brown rice, white rice, brown beans, yellow corn, Peanuts in-shell and cowpeas (black-eyed white beans) as grain absorbers. The relative absorptance (ratio of the total solar energy absorbed by the grain and the reference flat-black paint) and the relative efficiency (ratio of useful energy gain by the grain collector and the reference flat-black paint collector) were also obtained for the seven grain absorbers. The experimental results obtained showed that the solar absorptance and collector efficiency of the flat-black paint varied from 0.97 to 0.91 and 0.524 to 0.638, respectively, for solar angle of incidence ranging from 38.5 to 58.9°. A relative absorptance and relative efficiency of 80 percent with respect to the reference flat-black paint was used as a criterion for determining the performance of each grain as a good solar energy absorber. All the seven products tested, except white rice, had relative absorptances and relative efficiencies above the 80 percent rating. Peanuts in-shell had the highest relative solar absorptance and relative efficiency of 0.967 and 1.09, respectively, with the corresponding values of 0.726 and 0.718 for white rice. Colour, porosity, surface roughness, and the amount of exposed surface area seemed to affect the solar absorption characteristics and collector efficiencies of these agricultural products. As far as absorption of solar energy and heat exchange with the drying air are concerned, the six different grains tested and most other agricultural products could be effectively used as absorbers in solar energy crop drying systems. The major hindrance might be consideration for handling ease and possible reduction in the air drying effectiveness due to the level of and variations in outdoor air relative humidity, and initial grain moisture content. The material cost for a covered metal flat-plate solar energy collector is estimated at about $2.00/m² more than for a covered grain collector.