Performance study of Different Direct Solar Dryers
written by .
Syed Mudabbar Hussain Shah, The Author is final year student of B.Sc (Hons.) in Food Engineering Department of Food Engineering University of Agriculture, Faisalabad.
In a direct solar dryer, the moisture of the product to be dried is taken away by the direct impingement of solar radiation on the product itself with or without the natural air circulation. A schematic view of direct solar dryer is shown in Fig (1).
Direct solar dryers have a drying chamber which is an insulated box covered by a transparent cover made of glass or plastic and having air holes to allow air to enter and exit the chamber. When the solar radiation impinges on the glass cover, the air heat up and circulates either naturally or by wind pressure using external source (e.g., fan, blower, etc.) or combination of both. A part of impinged solar radiation will get reflected back to the atmosphere whereas the other part will get transmitted inside the dryer cabinet. This transmitted part is again reflected back from the product surface and rest is absorbed by it which increases the product temperature and reduces its moisture content by evaporation.
Direct solar dryer has a simple and cheaper construction which protects the drying product from dust, rain, debris, dews, etc. But direct solar dryers also have some drawbacks in their functioning like product overheating, undesirable product quality, and limited drying capacity. Lutz et al. developed a multipurpose solar crop dryer comprised of a solar air heater and a tunnel dryer for drying various agricultural products (Fig. 2). As compared to the traditional sun drying methods, the drying time and mass losses were reduced significantly with the payback period of 1–3 years. Energy required for the drying of 1000 kg grapes was reported to vary from 11.2 to 23.0 kWh depending on the weather conditions and it cost to 1.2 to 2.0 US$ which was absolutely negligible as compared to the additional earning. However, dissemination of solar drying system to a particular area depends on the availability of electricity and its requirement.
Ahmad designed and fabricated a cylindrical collector (5 m length and 0.36 m diameter) having a black interior band covered with transparent insulation. The collector was combined with a greenhouse dryer and obtained a higher temperature of about 10 °C than ambient for farming product drying. The schematic view of solar dryer with cylindrical collector is shown in Fig (3).
From the literature it has been observed that direct solar dryers are the most commonly used devices for drying agricultural and food products. The average drying efficiency of direct solar dryers is observed to vary from 20 to 40% depending on the product types, air flow rate, and drying location. The product quality obtained by direct solar dryers is acceptable and can be improved by using certain types of pre-treatment chemical processes.
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