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The present study was conducted in order to design and test a dual source solar dryer (D1) in the shape of a truss for drying some agricultural crops, including: figs, strawberry commodities, grapes and tomato commodities by using solar cells, and a source of illumination – in Egypt. Furthermore, in comparison, the dryer was tested without a solar cell, and the results were compared with those of another (half-inclined) solar dryer (D2), as well as an electrical dehydrator (D3), and circle electric and solar dehydrator (D4) – in Jeddah, Saudi Arabia.
5.1 Effects of dryer design:
The maximum temperature was 70.1° when drying figs in dual source solar dryer (D1) in summer and the minimum temperature was 24.2° when drying strawberry in dual source solar dryer D1 that was in spring.
5-1-2 Relative humidity.
The minimum moisture humidity was 10% when drying figs in (D1), (D2), whole strawberries, and other products it was 12% when drying tomato slices in (D1) when running fan and heat resource without sun.
5.1.3 Air velocity:
The greatest air velocity inside the dryer was 11.3 m/s when dried strawberry juice and tomato slices inside (D1) with running heat source and fan under the sun in Cairo (Egypt) in a summer day, the latest air velocity was 0.9 m/s from the fan inside the electric dryer (D4) when running with fan with solar cells in a winter day in KSA.
5.1.4 Solar Radiatio:
The greatest solar radiation in Cairo, Egypt was 900 W/m2 at 1 pm, and the minimum solar radiation was 368 W/m2 at 6 pm in summer before sunset; whereas the greatest solar radiation in Jeddah, KSA was 949 w/m2 at 1 pm, and the latest solar radiation was 341 W/m2 at 5 pm in winter before sunset.
5.1.5 Moisture content:
The moisture content of all fruits decreased by drying duration for all dryers. The latest moisture content (dry basis) for all products for all dryers was 1.8% when drying tomato juice in 1000 Watt electric dehydrator and the greatest moisture content (dry basis) was 1362.5% when drying tomato slices in dual source solar dryer with running heat source and fan under the sun. The greatest coefficient k when drying figs was 0.1 h-1 resulting from dual source solar dryer with running fan, the greatest coefficient k was when drying whole strawberry at 0.124 h-1 resulting from the 1000 Watt electric dehydrator. The greatest k was when drying strawberry slices at 0.431 h-1 resulting from the 1000 Watt electric dehydrator. The greatest was k when drying strawberry juice at 0.205 h-1 resulting from dual source solar dryer with running heat source and fan under the sun. The greatest coefficient was when drying red grapes at 0.283 h-1 resulting from without running fan. The greatest k was when drying green grapes at 0.129 h-1 resulting from 1000 Watt electric dehydrator. The greatest k was when drying tomato slices at 0.219 h-1 resulting from 1000 Watt electric dehydrator. The greatest k was when drying tomato juice at 0.35 h-1 resulting from 1000 Watt electric dehydrator.
5.1.6 Solar System Drying Efficiency:
The greatest solar system drying efficiency reached 95.72% For circuit electric and solar dryer when use it with solar fan without electricity when drying strawberry slices in Jeddah. The efficiencies reached 92.7% For dual source solar dryer with running fan when drying figs in Cairo. The latest solar system drying efficiency was 0.36% when drying red grapes in solar half inclined dryer in Cairo. “It was noticed that solar system drying efficiency tended to decrease as time passes during the early period of drying. This due to reduced moisture potential in product and also due to excessive losses near noon by convection and radiation” ((Khalil, 2002).
5.2 Cost Analysis
The drying costs ranged from 624 to 2806 LE/ton. The lowest dryer cost (624 LE/ton) was obtained with the dual source solar dryer, while the highest cost (2806 LE/ton) was for the circuit electric and solar dryer. Drying costs per ton of fruits ranged between 624 LE / ton for the dryer with dual source solar dryer this 77.7% less than 350 Watt circuit electric and solar dryer and 4% less than 1000 Watt electric dehydrator. The cost per ton for the dryer with dual source solar dryer is relatively high, it should be kept in mind that, cost of a prototype is always high. The dryer size is experimental (small). So, producing the dryer with large number and large scale will cut down the cost to the lowest.
It is recommended to use a dryer with dual source solar dryer because it gave high cabinet temperature 70.1°, it also gave an economical benefit of 624 LE/ton, 0.624 LE/Kg 4% less than 1000 Watt electric dehydrator, 77.7% less than 350 watts circle electric and solar dehydrator.
5-3 Chemical Analysis,
The maximum humidity percentages were 95.6 % when drying tomato slices in 1000 Watt electric dehydrator, and the minimum humidity percentages were 63.3% when drying whole strawberries in dual source solar dryer, the maximum rehydration ratios were 36 and 35.2 for tomato juice when dried in 1000 Watt electric dehydrator, and strawberry juice when dried in dual source solar dryer, respectively; whereas the minimum rehydration ratio was 1.5 for figs when dried in 1000 Watt.
5-4 Statistical Analysis:
The ambient temperature and the intensity of solar radiation had an impact on the temperature inside the dryer as shown in the following multiple regression equation as inferred from SPSS:
Tdryer= 25.326 - 0.394 Tout +0.42 IT
The value of the correlation coefficient is 1, and the coefficient of determination R2 is 1, which is very high. Hence, this shows that the regression equation is good, and that the correlation is significant. The hot air temperature inside the dryer has a direct impact on the decreased relative humidity inside the solar dryer as revealed in the multiple regression between them as shown in the following equation:
Hin = 118.669 – 2.262 Tout + 0.22 It – 0.902 Tin
The value of the correlation coefficient R is 0.830 and R2 is 0.689.