| Summary: | <p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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/m<sup>2</sup> more than for a covered grain collector.</p>
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