Drying is defined as a phenomena of removal of liquid by evaporation from a solid and comprises of two fundamental and simultaneous processes:
(i) Heat is transferred to evaporate liquid.
(ii) Mass is transferred as a liquid or vapor within the solid and as a vapor from the surface.
Open Sun Drying (OSD)
Open sun drying is whereby the grains are placed on a matt or well prepared pavements specifically constructed for drying. Its a traditional method for reducing the moisture content of food stuffs and is attained by spreading the grains under the sun. The solar radiation heats up the grains as well as the surrounding air and thus increases the rate of water evaporating from the grains. The grains are turned several times and days before they are fully dried and then packed in sacks for storage.
It requires little investment and is environmentally friendly since it uses the sun as the heat source and therefore produces no carbon dioxide. However, sun drying tends to be labour intensive and has limited capacity. Temperature control is also difficult in this method and grains can easily be overheat causing cracked grains which leads to low milling quality. It is also not possible to sun dry at night or during rain seasons.
Solar drying has been used since time immemorial to dry plants, seeds, fruits, meat, fish, wood, and other agricultural, forest products. In order to benefit from the free and renewable energy source provided by the sun several attempts have been made in recent years to develop solar drying mainly for preserving agricultural products. However, for large?scale production the limitations of open?air drying are well known. Among these are high labour costs, large area requirement and lack of ability to control the drying process, possible degradation due to biochemical or microbiological reactions, insect infestation, and so on. The drying time required for a given commodity can be quite long and result in post?harvest losses (more than 30%). Solar drying of agricultural products in enclosed structures by forced convection is an attractive way of reducing post?harvest losses and low quality of dried food crops.
Principle of open solar drying (OSD)
Drying of the crops is achieved by spreading it on the ground, mat or cement floor where they receive short wavelength solar energy during a major part of the day and also natural air circulation. Figure 2.1 below shows the working principle of open sun drying by using the open sun energy. A part of the energy is reflected back and the remaining is absorbed by the surface depending upon the colour of the crops. The absorbed radiation is converted into thermal energy and the temperature of the material starts to increase. However there are losses like the long wavelength radiation loss from the surface of crop to ambient air through moist air and also convective heat loss due to the blowing wind through moist air over the crop surface. Generally, the open sun drying method does not fulfill the required quality standards and sometimes the products cannot be sold in the international market. With the awareness of inadequacies involved in open sun drying, a more scientific method of solar energy utilization for crop drying has been developed termed as solar drying.
Figure 2.1 Working principle of an open sun dryer (Sharma et al. 2009)