Solar drying is another very important application of solar energy. Solar dryers use air collectors to collect solar energy. Solar dryers are used primarily by the agricultural industry. The purpose of drying an agricultural product is to reduce its moisture content to a level that prevents its deterioration. In drying, two processes take place: One is a heat transfer to the product using energy from the heating source, and the other is a mass transfer of moisture from the interior of the product to its surface and from the surface to the surrounding air.
Traditionally, farmers used the open-to-the-sun or natural drying technique, which achieves drying by using solar radiation, ambient temperature, relative humidity of ambient air, and natural wind. In this method, the crop is placed on the ground or concrete floors, which can reach higher temperatures in open sun, and left there for a number of days to dry. Capacity wise, and despite the very rudimentary nature of the process, natural drying remains the most common method of solar drying. This is because the energy requirements, which come from solar radiation and the air enthalpy, are readily available in the ambient environment and no capital investment in equipment is required. The process, however, has some serious limitations. The most obvious ones are that the crops suffer the undesirable effects of dust, dirt, atmospheric pollution, and insect and rodent attacks. Because of these limitations, the quality of the resulting product can be degraded, sometimes beyond edibility. All these disadvantages can be eliminated by using a solar dryer.
The purpose of a dryer is to supply more heat to the product than that available naturally under ambient conditions, thus increasing sufficiently the vapor pressure of the crop moisture. Therefore, moisture migration from the crop is improved. The dryer also significantly decreases the relative humidity of the drying air, and by doing so, its moisture-carrying capability increases, thus ensuring a sufficiently low equilibrium moisture content.
There are two types of solar dryers: the ones that use solar energy as the only source of heat and the ones that use solar energy as a supplemental source. The airflow can be either natural convection or forced, generated by a fan. In the dryer, the product is heated by the flow of the heated air through the product, by directly exposing the product to solar radiation or a combination of both.
The transfer of heat to the moist product is by convection from the flowing air, which is at a temperature above that of the product, by direct radiation from the sun, and by conduction from heated surfaces in contact with the product.
Absorption of heat by the product supplies the energy necessary for vaporization of water from the product. From the surface of the product, the moisture is removed by evaporation. Moisture starts to vaporize from the surface of the product when the absorbed energy increases its temperature sufficiently and the vapor pressure of the crop moisture exceeds the vapor pressure of the surrounding air. Moisture replacement to the surface is by diffusion from the interior, and it depends on the nature of the product and its moisture content. If the diffusion rate is slow, it becomes the limiting factor in the drying process, but if it is fast enough, the controlling factor is the rate of evaporation from the surface, which occurs at the initiation of the drying process.
In direct radiation drying, part of the solar radiation penetrates the material, and it is absorbed within the product, thus generating heat both in the interior of the product and on its surface. Therefore, the solar absorptance of the product is an important factor in direct solar drying. Because of their color and texture, most agricultural materials have relatively high absorptance.
By considering product quality, the heat transfer and evaporation rates must be closely controlled to guarantee both optimum drying rates and product quality. The maximum drying rate is required so that drying is economically viable.
Solar energy dryers are classified according to the heating mode employed, the way the solar heat is utilized, and their structural arrangement. With respect to the heating mode employed, the two main categories are active and passive dryers. In active systems, a fan is used to circulate air through the air collector to the product, whereas in passive or natural circulation solar energy dryers, solar-heated air is circulated through the crop by buoyancy forces as a result of wind pressure. Therefore, active systems require, in addition to solar energy, other non-renewable energy sources, usually electricity, for powering fans for forced air circulation or for auxiliary heating.
With respect to the mode of solar energy utilization and structural arrangements, the three major sub classes are distributed, integral, and mixed-mode-type dryers. These sub-classes belong to both active and passive solar energy dryers. In a distributed-type solar energy dryer, the solar energy collector and the drying chamber are separate units. In an integral-type solar energy dryer, the same piece of equipment is used for both solar energy collection and drying, i.e., the dryer is capable of collecting solar energy directly, and no solar collectors are required. In the mixed-mode-type, the two systems are combined, i.e., the dryer is able to absorb heat directly but the process is enhanced by the use of a solar collector. These types are explained in more detail in the following sections.
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