III.4. Solar drying

It is possible to harness the sun’s energy to produce drying conditions superior to those prevailing under natural drying. A number of simple experimental designs have been tested with varying degrees of success. These designs include structures in the form of tents made with wooden or bamboo frames covered with clear and dark polythene, wooden black boxes, or some other simple designs made from wood or brick and glass.

The principle underlying solar drying is simple. Air inside the drier is heated as it flows over dark surfaces which absorb the sunlight, thus resulting in air temperatures higher than those of ambient air. A convection current or upward flow of air takes place as air flows from the vents located at floor level to those located at the top of the structure. The fish, which are placed on wire racks, are dried by this flow of air which gets progressively warmer as it rises upwards and leaves the structure by the top vents. Depend-in on the design of the solar dryer, temperatures of 70° C and over can be achieved if there is no ventilation (Szabo, 1970). The temperature can be lowered by opening the air vents thus allowing free movement of air.

A tent drier, made from a bamboo frame covered with clear and black polythene (e.g. of the type shown in Fig.II.6 and Plate II.16) was evaluated in Bangladesh (Doe et. al, 1972). This drier attained a maximum temperature of 48° C, which is suitable for drying fish, the ambient air temperature being equal to 27° C. Dried fish were produced within a marginally shorter period than in the case of natural drying and were superior in quality, mainly due to the lack of insect infestation. The temperature within the tent was high enough to kill adult flies which would have otherwise laid eggs on the drying fish. Sun dried fish already infested with fly larvae were disinfected to a considerable extent within three hours when placed inside a solar tent drier at about 45°C, twenty hours at this temperature being sufficient for a complete disinfection. In this case the solar drier was more useful for keeping away insects and for the de-infestation of dried fish, than for reducing the drying time. In general, further development work is required with various designs of solar driers, before the method can be widely recommended for commercial use.