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Solar drying is in practice since the time im-memorable for preservation of food and agriculture crops. This was done particularly by open sun drying under open the sky. This process has several disadvantages like spoilage of product due to adverse climatic condition like rain, wind, moist, and dust, loss of material due to birds and animals, deterioration of the material by decomposition, insects and fungus growth. Also the process is highly labor intensive, time consuming and requires large area. With cultural and industrial development artificial mechanical drying came in to practice. This process is highly energy intensive and expensive which ultimately increases product cost. Thus solar drying is the best alternative as a solution of all the drawbacks of natural drying and artificial mechanical drying.
Solar dryers used in agriculture for food and crop drying ,for industrial drying process, dryers can be proved to be most useful device from energy conservation point of view. It not only save energy but also save lot of time, occupying less area, improves quality of the product, make the process more efficient and protects environment also. Solar dryers circumvent some of the major disadvantages of classical drying. Solar drying can be used for the entire drying process or for supplementing artificial drying systems, thus reducing the total amount of fuel energy required.
Solar dryer is a very useful device for
Textile industries for drying of textile materials. Drying implies the partial removal of water from the material. But when fuel fired equipment is used in the process then the term dehydration is often applied. Condition essential to both drying and dehydration include a supply of heat energy to evaporate the water and a supply of air to carry away the water vapor produced.
In open air Solar drying the heat is supplied by direct absorption of solar radiation by material being dried. The vapor produced is carried away by air moving past the material, the air motion being due either to natural convection resulting from contact with the heated material or to winds. In dehydration using fuel heat, the material being dried is placed in an enclosure and heated air is blown past the surface of the surface of the material to remove moisture. Solar dryer make use of solar radiation, ambient temperature, relative humidity. Heated air is passed naturally or mechanically circulated to remove moisture from material placed in side the enclosure.
Many variation of solar dryers offer the processor a wide range of opportunity to select various type of solar dryers depending on degree of control over the drying process and handaling of the material. The principal types of solar dryers are described briefly below.
Solar Cabinet Dryer :
Solar Cabinet dryer mainly consist of a drying cabinet . One side of the cabinet is glazed to admit solar radiation, which is converted in to low grade thermal heat thus raising the temperature of the air, the drying chamber, and the produce. Usually the sun light shines directly on the material being dried. The moisture evaporated by solar heat is removed by air circulation. This is accomplished either by designing to encourage natural convective air flow or by forcing circulation with fans or blowers. The material to be dried is placed in shallow layers on trays inside the drying cabinet. Proper air vents are provided for displacement of hot air. Solar Cabinet drivers are ,
Solar green house dryers :
Solar green house dryers are characterized by having extensive glazing on their south facing side while the other sides are well insulated. Inside the enclosed area, some means are provided to store the daytime excess heat. Vents are strategically sized and positioned to control air flow. A well designed greenhouse dryer permits a greater degree of control over the drying process than the solar cabinet dryers and should be used where relatively large quantity of product is to be dried.
Indirect Solar Dryer :
In this type of dryer, the produce is placed on trays inside an opaque drying chamber to which is attached an air type solar collector. The sun does not shine directly on the material to be dried, instead the air heated in solar collector is ducted to the drying chamber for dehydration. Air circulation can be by natural convection, however it is often forced by blowers. These dryers result in higher temperature than the cabinet dryers or sun drying, and can produce higher quality product.
In many large scale commercial drying operations, air type solar collectors are being combined with fuel fired dehydrators in order to reduce the fuel consumption and yet maintained fine control of the drying conditions. The solar heated air is used directly if it is hot enough, otherwise, the fossil fuel system boost the air temperature to the required level Thus the effect of fluctuations in energy out put from the collector is less serious since the fuel fired system is automatically controlled to provide specific optimum temperature.
Design and specifications of different type of dryers vary from case to case and it should be worked out individually.
Performance:
Performance of a solar dryer changes for different type of dryer operated at different temperature. Basic requirement for installation of a solar dryer is the open space free from any obstacle to the solar radiation. For indoor drying/dehydration processes by solar heated air, air heating collectors are to be installed in open space on ground, on roof or on a terrace and ducted to the drying chamber.
Operation and maintenance :
Operation of solar dryers is very simple and it is considered to be maintenance free. However for efficient performance general maintenance of cleaning of the glazing,regular coating of absorbing surface and instruments and controls.
http://www.geda.org.in/solar/so_slr_dry.htm