Comparative Study between Modified Solar Dryer and Open Sun Drying for Drying of Onion Slices

Solar greenhouse drying is a method of removing moisture from the food material in which drying medium is solar energy which is easily available and low in cost for farmers. The solar energy is decreases in the monsoon months in comparison to non-monsoon months. The decrease in the solar energy results in lower temperatures in the solar greenhouse dryer. The aluminium foil which has 88% reflectivity, reflects the radiation inside solar greenhouse dryer and heats the air inside dryer and maintains heat for longer times.Aluminium foil was applied on eastern, western and southern walls of the dryer and the parameters viz; temperature and relative humidity were recorded in both modified solar greenhouse dryer, solar greenhouse dryer and ambient conditions. Assessment of temperature in ambient condition, solar greenhouse dryer with and without modification showed the maximum temperature difference between solar greenhouse dryer without modification and ambient condition was about 11.5°C whereas 17.5°C maximum temperature difference between modified solar greenhouse dryer and ambient condition. The dryer was able to reduce moisture content of onion from initial moisture content of 679 per cent dry basis to 5.7 per cent in 31 h of drying whereas the traditional sun drying was able to reduce moisture content of onion from initial moisture content of 679 per cent dry basis to 29 per cent in 31 h of drying time. The results showed a considerable advantage of solar dryer over the traditional open sun drying method in term of drying rate and less risk for spoilage.