Another application of solar energy is solar drying. Solar dryers have been used primarily by the agricultural industry. The objective in drying an agricultural product is to reduce its moisture contents to a level that prevents deterioration within a period of time regarded as the safe storage period. Drying is a dual process of heat transfer to the product from a heating source and mass transfer of moisture from the interior of the product to its surface and from the surface to the surrounding air. For many centuries farmers were using open-sun drying. Recently however, solar dryers have been used which are more effective and efficient.
The objective of a dryer is to supply the product with more heat than is available under ambient conditions, increasing sufficiently the vapor pressure of the moisture held within the crop, thus enhancing moisture migration from within the crop and decreasing significantly the relative humidity of the drying air, hence increasing its moisture-carrying capability and ensuring a sufficiently low equilibrium moisture content.
In solar drying, solar energy is used as either the sole heating source or a supplemental source, and the air flow can be generated by either forced or natural convection. The heating procedure could involve the passage of the preheated air through the product or by directly exposing the product to solar radiation, or a combination of both. The major requirement is the transfer of heat to the moist product by convection and conduction from surrounding air mass at temperatures above that of the product, by radiation mainly from the sun and to a little extent from surrounding hot surfaces, or by conduction from heated surfaces in conduct with the product. More information on solar dryers can be found in Chapter 7.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.