Solar Dryer

Solar dryers are special structures that enhance the drying power of the sun and protect the agriculture products from dust, dirt and insects or basically are used to preserve them for a longer duration of time. The basic principle of a solar dryer is that the air is heated by the Sun in a collector and then passed over the material (which is to be dried). Many fruits can be dried in Solar Dryers that uses
the Sun's heat to evaporate the moisture content present in them. When the moisture content is low we can preserve the food and use it for the longer time.

Figure below shows the basic block diagram of working of a solar dryer.

Its working is very simple. As shown in the block diagram, we pass normal air inside a chamber which is known as solar air collector and this air is allowed to receive the solar radiations and get super heated. Then this super heated air is allowed to pass into another chamber which is known as the handling unit. This unit is responsible for the controlled release of super heated air at the time of its requirement. In the third and the final stage we have our drying unit in which we keep our material (which we need to get dried off) and then open the valves of the handling unit. The super heated air is allowed to pass over that material for a specific interval of time.

Solar dryers usually intake high airflow rates at low temperature over long period of time. Slow drying of fruits and vegetables preserve the flavour and quality of products which makes solar drying more attractive.

There are different types of solar dryers; most of them were developed for specific products or class of products. Selection of a solar dryer represents a compromise between dryer cost, product quality and installation convenience. According to their operating mode these can be classified as:

·         Direct solar dryers :

These are the dryers in which the product receives the direct heat from the sun .

The best example for a direct solar dryer is ‘The Box Type Solar Dryer’

They are used in the areas which receives the direct sun rays for longer duration of time. They are usually rectangular in shape and painted black from inside to create green house effect. It is covered by a plastic membrane or we can even use a transparent glass sheet. There is a layer made of mesh wire placed few centimetres above the base on which we keep our produce ( product to be dried). Holes are made at the base and at the top of the rare vertical walls, so that evaporated moisture will be ventilated out through these holes.

Images below showing the box type solar dryer.

Box Type Solar Dryer

                                                         

                      

·         Indirect solar dryers :

These are the dryers which are mentioned above in the block diagram, the sun's heat is first collected in the solar collectors and then ducted in the drying chamber.

The best example of this type of dryer is ‘Bin-type Grain Dryers’.

 In these type of dryers, the bin is fitted at the top and the super heated air is ducted from the bottom of the chamber. 

Images below showing the Bin-type Grain Dryers.

Multi Layer Grain Solar Dryer

Single Layer Grain Solar Dryer