The heat from the sun coupled with the wind has been used to dry food crops for preservation for several thousand years. This sun drying using direct sunlight and open air harnessing power of the sun as solar heat has been used for food preservation of agricultural commodities and products since early stages of mankind in best ways Other crops such as timber need to be dried before they can be used effectively, in building for instance.

The sun is the central energy producer of our solar system. It has the form of a ball and nuclear fusion take place continuously in its centre. A small fraction of the energy produced in the sun hits the earth and makes life possible on our planet. Solar radiation drives all natural cycles and processes such as rain, wind, photosynthesis, ocean currents and several other which are important for life. The whole world energy need has been based from the very beginning on solar energy. All fossil fuels (oil, gas, coal) are converted solar energy.

The solar radiation intensity outside the atmosphere is in average 1,360 W/m2 (solar constant). When the solar radiation penetrates through the atmosphere some of the radiation is lost so that on a clear sky sunny day in summer between 800 to 1000 W/m2 (global radiation) can be obtained on the ground.

This sun-drying has often developed into solar-drying, where the drying area is in an enclosed ventilated area – often with polythene, acrylic or glass covering as a more efficient harnessing of the elements of the drying operation. There are innumerable designs in use and each has its advantages and disadvantages. However, there are three basic designs upon which others are based: solar cabinet dryer, Solar Tent, tent-dryer, Solar Dehydrator and solar tunnel dryer.

These are discussed below after a brief description of the principles of drying/dehydrator.

Basic principles of drying and design consideration of drying depends upon:

• Temperature, humidity (moisture content) and quantity of air used

• Size of the pieces being dried

• Physical structure and composition

• Air flow patterns within the drying system

Heat is not the only factor which is necessary for drying. The condition, quality and amount of air being passed over and through the pieces to be dried determine the rate of drying. The amount of moisture contained in the air to be used for drying is important and is referred to as absolute humidity. The term relative humidity (RH) is more common and is the absolute humidity divided by the maximum amount of moisture that the air could hold when it is saturated. RH is expressed as a percentage and fully-saturated air would have an RH of 100%. This means that it cannot pick up any more moisture. Air containing a certain quantity of water.

at a low temperature will, when heated, have a greater capacity to hold more water. The table below gives an example of air at 29oC with an RH of 90%. Such air, when heated to 50oC will then have an RH of only 15%. This means that instead of only being able to hold only an extra 0.6 grams of water per kilogram (at 29oC), it is able to hold 24 grams per kilogram. Its capacity to pick up moisture has been increased because it has been heated.

When placed in a current of heated air, food initially loses moisture from the surface. This is the constant rate period. As drying proceeds, moisture is then removed from inside the food material, starting near the outside. Moisture removal becomes more and more difficult as the moisture has to move further from deep inside the food to the surface. This is the falling-rate period. Eventually no more moisture can be removed and the food is in equilibrium with the drying air.

During the falling-rate period, the rate of drying is largely controlled by the chemical composition and structure of the food. Design of a dryer depends upon the drying rate curve of the material to be dried but these curves are indicative only and depend upon the factors mentioned above.