MEDIUM SCALE SOLAR CROP DRYERS FOR AGRICULTURAL PRODUCTS
Oliver St C Headleyand William Hinds
Centre for Resource Management and Environmental Studies, University of the West Indies, Cave Hill Campus,
P.O. Box 64, Bridgetown, Barbados. Telephone 246 417 4316, Fax 246 424 4204,
Solar crop dryers have been built by the Solar Energy Project at the University of the West Indies since 1973. During that time we have dried a wide variety of agricultural crops and timber species such as sorrel (Hibiscus sabdariffa), bananas (Musa sapientum) and mahogany (Swietenia macrophylla). Dryers have varied in size from the 2.2 m2 wire basket dryer to the 149 m2 roof collector. Most solar dryers use solar air heaters and the heated air is then passed through the crop. The smaller solar dryers use natural convection or chimneys for air circulation, but for solar collectors of more than 10 m2, forced convection is usually necessary. In this paper, we report on two medium scale dryers which were developed for drying onions (Allium cepa), hay and similar crops which require a relatively low drying temperature - less than 45°C. For a drying temperature of 40 - 45°C, an unglazed collector may be employed since the radiative heat losses to the sky are relatively small at these temperatures. The roof of an existing barn is converted into a solar collector after it is painted flat black and fitted with a ceiling which becomes the duct through which air, the heat transfer fluid, is pumped. The advantage of using the roof of an existing farm building as a solar collector is that the dryer costs less than if it employs a dedicated collector.
The first dryer which we describe is of this type and uses a corrugated galvanized steel roof of 149 m2 and a fan which supplies air at 5.6m3/s with a pressure drop of 747 Pa. Air is sucked into the ceiling space from one end of the roof and is ducted to the fan which then blows it into the plenum chamber at floor level. Bags of onions or bales of hay are placed on the top of the plenum whose roof is perforated to allow the air to flow through it and then through the crop. The crop is stacked so that heated air does not bypass it and escape to the ambient without removing moisture. This dryer does not employ an auxiliary heating system since the farmer for whom it was built did not think it was necessary. Given the airflow rate, psychrometric calculations show that drying will take place even when the temperature rise is low. This dryer’s maximum capacity is ten tonnes of hay.
The second dryer uses a glazed 40m2 solar collector which is connected to a fan. The collector was glazed since we needed to raise its efficiency. The drying chamber in this case consists of two standard 6 metre (20 ft) shipping containers which have been modified to enhance heat retention. By means of the ducting system, the fan sucks air through the solar collectors and forces it into the containers where the onions are placed on racks. One of the containers is insulated and lined with concrete blocks for increased thermal mass while the other is only insulated. This dryer is fitted with electrical resistance heaters to supply heat during rainy or cloudy periods and to allow the dryer to be used at night. Even though electricity is 26˘US/kWh in Antigua - which is the second highest electricity tariff in the anglophone Caribbean - the Antigua Marketing Agency preferred to use this heat source since they consider it to be more reliable and less dangerous than liquefied petroleum gas even though the latter is cheaper. The maximum capacity of this dryer is four tonnes of onions.
Keywords: Crop drying, solar air heaters, unglazed collector, thermal mass, forced convection.