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The use of solar energy as a heat source (solar drying, solar assisted drying) has been evaluated intensively by many projects and institutions. While some solutions were proven to be technically feasible none was successfully commercialized for paddy drying because of the following reasons:
1. In natural convection dryers that were considered promising because they don’t need a fan and thus no additional energy is needed. The maximum layer depth of the grain is only 7 cm. Thicker layers provide too much resistance to the air that cannot be overcome by the small forces created by the thermal solar energy.
2. The capacity of those dryers referred to the floor area they is used for the solar collector is far too small for any serious application;
3.In solar tunnel dryers the drying air does not flow through the grain bulk but over the grain. The layer depth is therefore also limited to a few centimetres.
4. Generally in solar dryers more heat is generated when it is not needed, at midday when the air RH is low anyway. Heating the air is needed at night when it rains or in early morning, when the RH of the ambient air is too high for drying.
5. Some projects introduced additional devices to store the heat of the air during the day and release it when needed. This increases the capital cost beyond acceptable levels. The capacity is also limited;
6. Solar collectors need to be made from durable materials (UV stabilized plastic), which makes them expensive. They consume a lot of floor area for a very limited heat accumulation. Feedback from the field indicated that cheap structures were often quickly destroyed by animals (e.g. village dogs), wind or solar radiation; and
7. Generally capital cost per ton capacity is very high, which makes solar dryers suitable for high-value commodities that are dried in relatively small amounts (vanilla, spices, fruits, mushrooms, raisins etc.). Paddy is a high volume, low price commodity.