3.3 Other Devices

In addition to the cooking, mechanical and electrical devices there are other mixed type of devices that tap the renewable energy and serve different purposes of the farmers and rural populations.

3.3.1 Drying

Open sun drying of various agricultural produce is the most common application of solar energy. With the objective of increasing drying rate and improving quality of the produce natural convection and forced convection type solar dryers have been developed for various commodities. The movement of air in the forced convection solar dryer is through a power blower, whereas in natural convection solar dryer air moves through the produce due to natural thermal gradient.

3.3.1.1 Domestic Solar Dryer


This is a small dryer meant for domestic uses of drying small quantities of products such as vegetables, fruits, condiments and spices. In this dryer, the convection of heated air is natural due to difference in temperatures. Solar energy is intercepted on the inclined aperture, which is glazed for trapping infrared radiation, and prevents unnecessary circulation of ambient air thereby maintaining the requisite temperature inside. The drying trays have been arranged one over the other on an inclined plane so that there is free circulation of heated air through the mass kept for drying. The products can be dried under shade or exposed to sunrays as desired. The dryer has provision for changing inclination of the aperture by 15° to capture more solar energy depending upon the season. Castor wheels make orientation easy in order to capture maximum solar radiation. The drier may be left unattended even during rains, as the products kept inside are not affected.

3.3.1.2 Step Type Solar Cocoon Stifler

Silk cocoon stifling is generally carried out using an electric oven or by using firewood and boiling water. The heat needed to kill the pupa is obtained from solar radiation in a solar cocoon stifler. If the pupas are not killed at the right time they grow out of the cocoon, damaging the silk fibres. Hence stifling kills the pupa and 12-15 days time is available for reeling the silk fibres. The solar cocoon stifler is a box type unit that is provided with insulation and double glazed cover for trapping solar heat. Wire mesh trays are arranged horizontally inside the stifler and can be loaded easily by opening the side panels. It has a small fan for re-circulating air for uniform heat distribution. An electric heater of 2 KW and thermostatic control is also provided to supply adequate heat during adverse weather. The quality of cocoon stifled in the solar stifler is similar to the cocoon stifled in the conventional process using the electric oven.

3.3.1.3 Simple Pyramid Shaped Solar Dryer

This dryer is conduction-convection type, pyramid shaped solar dryer with a base to height ratio of 1:1.5. The frame of the dryer is fabricated from wood and the trays (which also act like racks) are also of wooden frame and wire mesh. UV stabilized black polythene sheets cover the dryer. Since the wooden-wire-mesh trays are stored vertically, one over the other, the dryer allows an effective drying area equal to twice the size of the base area.
In summers the dryer attains temperatures of up to 15º to 20ºC more than ambient and in winters about 5º to 10ºC more than the ambient. Sunlight falling on the black UV stabilized polythene sheet heats up the sheet.

The heat absorbed by the sheet heats the air inside the pyramid shaped dryer. As the air gets heated inside, the convectional currents are initiated due to variations in the air densities.
Hot air moving up to the top of the dryer carries moisture as vapour from vegetables or fruits kept inside for drying in the wooden-wire-mesh trays/ racks. The holes provided at the top of the dryer allow the hot vapour to escape. Comparative colder air is drawn from outside through holes provided at the base of the dryer.

At the present cost of materials are incorporated in making this dryer, the standard model is prized at Rs. 2500. This dryer is being sold in North India around Delhi. Life of this solar drier is 5 years.

3.3.1.4 Data on Solar Dehydration of Selected Foods

Process data for fruits, vegetables, green leafy vegetables, spices, food items is given in Table.3.4.