Abstract:
Drying crops using solar energy is of countless economic significance all over the world,
particularly in Ethiopia, where most of the crops and grain harvests are lost to fungal and
microbial attacks. Due to those problems, an indirect, active type, eco-friendly, low-cost solar
dryer with a Parabolic dish solar concentrator with rock-bed thermal storage material was
designed to dry various agricultural products. Before the solar drying test, a preliminary
investigation was done using an oven drying test in order to determine the optimum drying
temperature for the mango slices and to investigate the effect of temperature on the physico-chemical value of the mango slices.Solar drying experiments were conducted to dry mango slices
and to investigate drying characteristics like drying rate, quality of dried mango in terms of
color,taste, rehydration ratio, and shrinkage ratio. Mango slices dried at 1 m/s air flow rate
were of the best quality in terms of color, taste, and shape when compared to those dried at 0.5
m/s and 2 m/s air flow rates in the drying chamber.
The dryer was designed based on the climatic conditions of Bahir Dar, Ethiopia, with an
average ambient air temperature of 22 and yearly average global solar radiation incident on
the horizontal surface of about 535w/m
2
and with average relative humidity of 48 . The
experimental tests were carried out with load and no-load conditions. With the no load tests, the
maximum amount of temperature was recorded relative to the load condition. The thickness of
mango slices was selected to be 3mm compared to 6 mm and 9mm interms of drying rate and
moisture removal rate, which is an appropriate size as thinner slices would lead to case
hardening and reduce the drying rate, whereas thicker slices would also reduce the drying rate
due to the diffusion process.During testing of mango drying its moisture content was lowered
from 81.4 to 15.3 . From the test results, the average collectore thermal and drying
efficiency with thermal storage was 65.5 and 16.5 and the maximum drying temperature for
mango slices is 80 , from the preliminary oven investigation. Different thin-layer drying models
were evaluated during the experimentation, and the page model was the best fitting model for
mango slice drying using criteria of high coeficient of determination (R
2
) close to unity and
lower value of mean bias error, root mean square error, and ch-square value closed to zero.