DESIGN NEW PROTOTYPE OF AN INDIRECT SOLAR- ELECTRICAL DRYER TO DRY WHITE ONIONS SLICES

Abstract

In the present research, a new specific model of an indirect solar–electrical dryer for onions slices was designed and constructed at a  Zagazig City , Sharkia Governorate, Egypt. Solar dryer consists of solar flat plate air collector with V- corriugated absorption plate. The total area of the collector is 1.75 m2. The size of the drying cabinet is (1.5 × 0.50 × 1) m (height, width  and depth). An experiment was conducted to study drying of onions slices 5 and 10 mm. The qualitative analysis for drying of  onions slices of 5 mm under three air velocities  (1.5,2.2 and 3) m/s, showed that moisture content was reduced from (669.23db to 6.95, 8.69 and 14.41db), (4.27,5.26 and 9.89db) and (2.04, 3.09 and7.52db) for( Tray1, Tray2, Tray3) while it were (19.33,26.90 and 33.51db), (14.41, 21.35 and 27.71db) and (9.89, 16.27 and 21.95db) for onions slices 10 mm respectiviely.The average moisture content of open sun dry were 20.60 and 50.95 db for onions slices of 5and10 mm. The average percentage thermal efficiency for the collector was found to be 33.03, 49.96 and 65.45 % while it were 21.37,15.43 and 10.29% of the dryer for air speed 1.5,2.2 and 3 m/s,, respectiviely.The average percentage value of the collector 34.07,37.59 and 42.69% while it were be24.82, 32.55and 41.85% for the dryer at air velocities were1.5,2.2 and 3 m/s, respectiviely. In order to estimate and select the suitable form of solar drying curves, five different mathematical models, were compared according to their coefficient of determination R2, MBE, RMSE, %E and chi square X2 to estimate experimental drying curves. The Modified page model in this condition proved to be the best for predicting drying behavior of onions  slices of 5 and10 mm with (R2 = 0.9961and0.9899) and (X2 = 0.000148 and0.000591). The effective moisture diffusivity (Deff) was obtained using Fick’s diffusion equation and its value varied from 1.72958× 10-9 to 2.37261× 10-9 m2/s and 4.31037× 10-9 to 5.61732× 10-9 m2/s while the value of activation energy ranged from 23.49 to 29.82 kJ/mol k and 16.09 to 18.01 kJ/mol k  for onions slices of 5and10 mm. Finally an economic evaluation was calculated using the criterion of payback period which is found very small 1.05 years compared to the life of the dryer 25 years.