INTRODUCTION
Drying is one of the ancient methods for preserving agricultural products. It means elimination of available moisture which is present in produce to above surface and later to removing the moisture which will left on drying surface material. It has been estimated that more than 20-30 percent food grains and 30-50 percent vegetables, fruits/fish etc. are lost while in post-harvest before reaching consumers. So, to avoid these losses drying is essential for most of grains and other agriculture products drying process ideally requires approximately 2.4 MJ energy for every one litre of water to evaporate. Based on moisture content of the product, for each product relative moisture content matches to a certain value if moisture is > 95% all organisms can grow in the product and up to 75% all yeasts, molds and most bacteria can survive. If moisture is removed from agriculture commodities or products, then produce will be safe and doesn’t get contaminated by microorganisms. Solar energy is diffuse in nature and thus suitable for crop drying, locally available and thus saves transportation, solar dryers can be made locally of any size and capacity and solar dryers are economical if cash crops are dried.
Classification of Solar Dryers
1. Direct type dryers
a. Direct type dryers or Natural convection:
The agricultural produce or grains etc. can be placed in shallow layers in a blackened enclosure with a transparent cover, then solar radiations are directly absorbed by the product itself, later the food product inside gets heated up and the moisture from the product evaporates and goes out by the natural convection or circulation. These dryers appear to be more attractive for use in developing countries since these do not use fan or blower to be operated by electrical energy. These dryers are low in cost and easy to operate.
Some of the problems with these dryers are low drying, no control on temperature and humidity, small quantity can be dried and some products change colour and flavour due to direct exposure to sun.
b. Solar cabinet dryer:
It consists of a wooden (or of any other material) box of certain width and length (length is generally kept as three times its width), insulated at its base and the sides should be covered with a transparent roof. Inside surfaces of the box should be coated with black paint and the agriculture product which is to be dried is kept in the trays made of wire mesh bottom. These trays loaded with product are kept through an open able door provided on the rear side of the drier. Ventilation holes can be made at the bottom from which fresh outside air is sucked automatically. Holes are also provided on the upper sides of the dryer through which moist warm air escapes. This dryer has given promising results and reduced the drying period by one third compared to open sun drying.
2. Indirect type dryers:
In these dryers the food product is placed in a drying chamber and the solar air heater with or without any electric fan along with a drying bin is used. It consists of a solar air heater made of a frame of bamboo poles and wire covered with 0.15 mm thick transparent PVC sheet. The ground is covered with burnt rice husk which absorbs the solar radiation and heats the air in contact. The hot air in this air heater rises to the drying chamber which either consists of transparent PVC sheets on bamboo frame absorbing directly the solar radiation or a bamboo frame should be covered with some opaque material all the four side. The drying material (rice etc.) is kept on the nylon net tray in thin layer through which hot air heated from air heaters enters its bottom and goes up into the chimney. The chimney is made from bamboo which looks exactly like cylinder and frame is covered with black PVC to keep the inside air warm. There is a cap at the top of the chimney, leaving some space in between chimney top and cap to allow warm humid air to go out and protecting the product from rain and other foreign materials.
The height of the chimney and the hot air inside it creates a pressure difference between its top and bottom thereby creating forced movement of air through the rice bed to the top of the chimney. The ventilation rate will depend on the depth of the bed, initial moisture present in the material, solar insolation, ambient temperature, and the design of the dryer.
3. Forced Circulation type Dryers:
In these dryers, hot air is continuously blown over the food product and the food product itself is loaded or unload continuously or intermittently.As the name indicates, such dryers use some kind of one or several electric operated blower/exhaust fan to circulate air between air heater/storage bin/drying chamber.Such dryers are more efficient, faster, and reliable preferred and can be used for drying large quantities of agricultural products.These dryers can be used at low as well as at high temperatures and used for drying large quantities of product.These dryers can be made like bin type, tunnel type, belt type, column type or rotary type.
Some forced circulation type solar dryers use some kind of thermal storage unit, heat recovery wheel and auxiliary heating arrangement. Supplementary energy can be supplied through electric heating or oil or by gas burners and used only when solar air heaters or the heat from the thermal storage device is not sufficient to supply required energy for drying the product. Several storage systems are proposed but the most preferred one is the rock bed storage system which stores the heat in the form of sensible heat and performs the dual function of storing the heat and acts as heat exchanger.
Advantages of Crop Drying or Agricultural Produce
It permits early harvesting and reduces the field losses of the products,
Reduces the risk of field losses caused by wild animals,
It permits better planning of harvesting season,
It reduces spoilage in storage drastically,
It permits the farmer to sell his product at better price during early period of harvesting season,
Quality of the product gets enhanced significantly and hence farmer gets more money for his product, and
Transportation is easy with dried product.
CONCLUSION
A solar drying method, particularly for agricultural produce is possible in developing countries because the cost of fuel energy is very high. In upcoming future larger systems can be designed utilizing solar panels, thermal or combined with wind power.
Solar and wind energy is necessarily intermittent, improvements in thermal and electrical energy storage is required to make use of renewable energy feasible in drying. Farmers have to use the locally available material for construction of solar dryers.