Design Development and Performance Evaluation of Solar Dryer for Drying Onion used as powder in Food

Abstract

Solar dryer was designed and manufactured at Fadis Agricultural Research Center workshop of Oromia Agricultural Research Institute. The framework of all the parts of the dryer were built by joining perforated angle irons of 40 mm 40 mm 4 mm and 20 mm  20 mm 4 mm by means of bolts and nuts. The dryer covers 3.0 m  3.0 m area of the ground of which the 1m² was used for drying chamber while the rest was saved for  collecting  solar radiation. The drying chamber surrounded by the collector from three sides, had five shelves positioned one on the top of another with 10 cm clearance in between. The roofs and walls of the dryer were covered with the flexible transparent plastic leaving the three sides of the solar collector open to allow air in. Preliminary tests with no load to the dryer showed that the solar collector raised the ambient air temperature of 20°C to 41°C to a warm air of 28°C to 64°C between the morning and midday. This lowered the relative humidity of air from average 26% in the morning to 5% at midday. The onion slices of 3 mm thickness, was loaded on the dryer,   at a rate of 4 kg/m², and dried from 87.10% (w.b) initial moisture content to 9.1% (w.b) final moisture content in 10 hours. The open air-sun drying tests conducted side by side with solar drying needed an average of 20 hours to reach the same final moisture contents. The maximum drying rate of onion slices attained was 2.6 kg of water per kg of dry matter-hr. while in the open-air sun drying, the maximum drying rate was 0.82 kg of water per kg of dry matter-hr.

Drying onion slices to its final moisture contents took two and three days in Natural convection solar drying (NCSD) and Open-air sun drying (OASD) of onion respectively. Drying rate coefficients ‘k’ (^-1hr) of Lewis model were statistically significantly different and could be used to describe solar and open-air sun drying characteristics of solar and open-sun drying of onion slices. From economic feasibility and payback analysis of the solar dryer, the payback period was determined and was very small (1.20 months) compared to the life of the dryer, so the dryer will dry product free of cost for almost its life period of 15 years.