Thailand is an agricultural country. Consequently, a large amount of dried fruits and vegetables are produced across a country. A traditional natural sun drying method has been widely used for producing dried agricultural products in Thailand. Despite a low drying cost, products dried with this method are subjected to contaminations by dirt and dust, insect infestation, and loss by birds and animals. As Thailand is situated in the tropics where solar radiation is abundant, a utilization of solar drying technology is considered as a promising solution of the problem of drying fruits and vegetables in this country. As a result, we have developed a polycarbonate sheet-covered greenhouse solar dryer. It consists of a parabolic roof made from polycarbonate sheet on a concrete floor. The parabolic cross-sectional shape help to reduce wind load in case of a tropical storm. The structure of the dryer is made of galvanized iron bars. The products to be dried are placed in a thin layer on arrays of trays. These arrays of trays are placed on single-level raised platforms with passages between the platforms for loading and unloading the products inside the dryers. Polycarbonate sheet is used as a cover of the dryer, because it has high transmittance (about 0.5), thus creating a good greenhouse effect in the dryer. In addition, the polycarbonate sheet has a light weight and easy to bend and cut, thus reducing the construction cost. DC fans operated by pv-modules are installed in the wall opposite to air inlet to ventilate the dryer. Three sizes of this type of dryer are available: small size with the base area of 6x8 m2 and the loading capacity for fruits or vegetables of 200 kg, medium size with the base area of 9x12 m2 and the loading capacity of 500 kg and large size with the base area of 9x20 m2 and the loading capacity of 1000 kg. A version of this type of dryer with LPG burner as a supplementary heater for continuous drying in case of rain or cloudy skies has been also developed. All sizes of the prototype of the dryers have been tested at Silpakorn University and high quality of dried products such as chillies, longans and bananas were obtained. As a result of this success, the Department of Alternative Energy Development and Efficiency of Thailand had set up a dissemination program to promote a wide spread use of this type of dryer for small-scale dried food industries in Thailand and more that 200 units of this type of dryer are being used in many locations across the country.
Development and Dissemination of Polycarbonate Sheet-covered Greenhouse Solardryer for Small-scaled Dried Food Industries in Thailand