Development And Evaluation Of An Integral Passive Solar Crop Dryer

ABSTRACT 

A direct-heated solar crop dryer suitable for rural farmers and households was developed and evaluated. The simple structured dryer was fabricated using square and round pipes, aluminium sheet, Perspex, wire gauze, bolts and nuts. Its performance was evaluated using cassava chips and three cylindrical aluminium chimneys of heights 1200mm, 800mm and 400mm; all of equal diameter. Mature cassava roots were peeled, washed, chopped into flat circular chips of 10mm thickness and spread on racks in the drying chamber of the solar dryer. The dryer with its content was fully exposed in the sun for ten hours daily (8.00 hours to 18.00 hours) for three consecutive days. For comparison, control tests were set up alongside that of the dryer, by drying in open air. The following parameters were monitored on hourly basis (i) in-let air (ambient) temperature, i.e temperature of the air flowing into the solar dryer; (ii) drying chamber temperatures, i.e. temperature of the air inside the drying chamber;   (iii) exhaust (exit) air temperatures and (iv) weights of the drying samples of the cassava chips. The observations and results were recorded and systematically analysed. Through a drying period of thirty hours cumulatively, the cassava chips spread in the open air showed moisture loss of up to 396g, while the chips spread inside the solar dryer lost 550g water. Moisture content of the cassava chips spread inside the dryer reduced from 62%(wb) to 9.7%(wb ), as against 23.8%(wb) for the chips dried in open air. An average drying efficiency of 95.5% was attained using the aluminium chimney of height 1200mm. Further analyses showed that the variation in chimney height from 0mm to 1200mm had negligible effect on the temperature gain inside the solar dryer; temperatures inside the dryer rose with the daily ambient temperature. Chimney height, however, appeared to have  very slight influence on the overall moisture reduction effect on the solar dryer, thus the cassava chips dried using chimney height 1200mm attained a moisture content as low as 9.7%(wb) in 30hours; whereas the chips dried without chimney attained a moisture content of 13.0%(wb).   On the other hand, the analyses of variance showed that temperatures inside the solar dryer, drying rate and the overall performance of the integral solar dryer were more significantly affected by the daily ambient temperatures rather than the chimney height.