Direct solar drying (i.e. laying products out in the sun to dry) has traditionally been important for processing and preserving food, crops and other products. Drying involves the removal of moisture from produce so as to provide a product that can be safely stored for longer periods.
Solar dryers are specialized devices that control the drying process and protect agricultural produce from damage by insects, dusts and rain. In comparison to drying product in the open, solar dryers generate higher temperatures and lower relative humidity, and increase flow of air across the produce, resulting in shorter drying periods, lower product moisture content and reduced spoilage during the drying process.
Solar dryers come in various forms and sophistication. The basic principle is that air is heated in a collector by the green house effect. The hot air then dries the produce in a drying chamber.
Depending on the construction, both collector and drying chamber may be combined or separated.
In many places, use of solar drying devices is becoming an established part of the agricultural or food processing business, especially where the product can be sold at a higher price or transported more easily when dried. Products that are commonly dried include fruits, tea, coffee, lumber, pyrethrum, maize, fish, meat and others. Tobacco curing is also possible.
A number of commercially successful solar drying initiatives have been carried out in East Africa. Fruits of the Nile in Uganda is working with hundreds of women to dry pineapples, mangos, chillis and banana for export.
In Kenya, GTZ played a key role in introducing the technology. Most of the work was with simple direct, lowest cost type solar dryers. Such "simple" designs use frames made of wood, inside which screen trays are laid. A UV resistant plastic film is used as a cover.
KIRDI is actively working with institutions and women's groups in developing improved dryers for processing of fruits, vegetables and cereals on a commercial basis.
Solar drying is not simply a method for substituting fossil fuels by solar energy, but it is a technological process for producing dried materials of the required quality to replace fuel use. For example, there is potential to use it to cure tobacco (Brazil and Zimbabwe) dry wood (Brazil), fish and bricks.
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