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close this bookComparative Study of Solar and Sun Drying of Fish in Ecuator (NRI)
close this folderSection 1 - Equipment and methods
View the document(introduction...)
View the documentSolar dryers
View the documentSun-drying methods
View the documentFish preparation
View the documentOperating procedure
View the documentFinal product analysis

Solar dryers

Solar tent dryer

The solar tent dryer was developed by Doe and others, (Doe et al. 1977; Doe, 1979) and initially tested in Bangladesh with fish. TPI has tested this dryer in Africa, SE Asia and Latin America for production of various dried fish products and it has shown considerable promise.

A sketch of the dryer as built at the CDRS is shown in Figure 1, Appendix 1. For this exercise, it differed in one important aspect from the original design of Doe et al., in that both sides of the tent were of clear plastic sheet, whereas for the dryers built in Bangladesh, only the side facing the sun was of clear plastic, the other being of black plastic sheet. The reason for this was that the working site at the CDRS being almost exactly on the equator, the path of the sun was virtually due east to due west for the period of the exercise (April/May). Therefore, it was considered that the slight increase in efficiency of solar collection effected by a side of black plastic would be more than offset by the reduction in the 'greenhouse effect'.

The principle of operation of the tent dryer is that insolation passes through the clear plastic sides and ends of the tent and is absorbed on the black plastic base. Air at the base is thereby heated and rises, thus inducing a draught within the tent. Openings at the base along both sides allow air to be drawn in, and vents in the apex at both ends allow air to exhaust. Some control of the internal temperature, and flow of air through the dryer, can be maintained by adjusting the height of the side openings.

Construction of the dryer was very simple, using a bamboo framework and plastic sheet. Black polyvinyl chloride (PVC) was used for the base of the tent and clear ultra-violet-resistant polyethylene for the sides and ends. Staples were used to attach the plastic sheet to the framework. The drying rack was built along one side of the tent using bamboo and black plastic mesh. Access to the rack was through a movable plastic flap forming half of one end of the tent. The flap could be closed and fastened when not in use. Construction time for the tent dryer was about 6 manhours.

SCD solar dryer

This dryer was developed by Exell and others (Exell and Kornsakoo, 1978; Exell et al., 1979; Exell, 1980) at the Asian Institute of Technology (AIT) in Thailand. It differs principally from the tent dryer in that the solar collector and the drying chamber are distinctly separate as can be seen from the sketch of the dryer in Figure 2, Appendix 1. The dryer was developed for use with paddy but recent work has been conducted at AIT using fish (Exell, private communication).

The solar collector consists of a black plastic base with an inclined transparent plastic cover with a narrow opening across the full width of one end of the collector. Air is heated during its flow through the collector and passes into the drying chamber before exhausting through the chimney. The function of the extended chimney is that the black sides absorb insolation and so heat the air within, thereby enhancing the natural convective flow of air through the dryer.

Identical materials to those used for the solar tent were employed; the base of the collector, the base and back of the drying chamber, and the sides of the chimney were of black PVC, and the collector cover and the top and sides of the drying chamber were of clear polyethylene. Access to the drying rack was provided by plastic flaps on either side of the drying chamber, which also provided a rudimentary means of control of the internal temperature. It should be noted however that for larger models it would be necessary to provide access flaps at the back of the dryer for ease of loading as described by Exell and Kornsakoo, 1978; Exell et al., 1979 and Exell 1980. Construction time for the dryer was about 15 man-hours.

Solar cabinet dryer

This design of dryer was pioneered by Lawand (1966) and the Brace Research Institute (1973) and is probably the most widely-used dryer developed to date, being utilised for a large number of commodities. As its name suggests, and as can be seen from Figure 3, Appendix 1, it is essentially a rectangular cabinet with an inclined transparent cover. The optimum angle of inclination of the cover is dependent upon the geographical latitude (Brace Research Institute, 1973); for the Galapagos this angle was 15°.

Air inlet ports in the base of the cabinet provide an entry for air which is then heated within the cabinet and rises to exhaust through outlet ports in the upper parts of the front and sides. The potential of the cabinet to absorb insolation is enhanced by blackening all interior surfaces. It is normally recommended that the front, sides and base be of double-wall construction with the cavity being filled with a material with good insulation properties, e.g. sawdust. However, the cavity was left unfilled during this exercise to prevent excessive internal temperatures being attained since there is the risk of 'case-hardening' and 'cooking'. Case-hardening is the formation of an impermeable surface layer of the fish, caused by too rapid drying initially, before the moisture in the deeper layers has had an opportunity to diffuse to the surface. The resulting product has a hard, well-dried surface, but the centre remains moist and will spoil.

For this exercise, the cabinet was constructed of plywood on a frame of 50 mm X 50 mm wood; 12-ply thickness sheet was used for the external walls of the sides and front, and top of the base, and 5-ply sheet for the internal walls of the sides and front and the bottom of the base. Two doors of 5-ply sheet formed the back of the cabinet, allowing loading of the trays. All interior surfaces were painted with black matt paint. Plastic hose was used to form ducts through the two walls for the inlet and outlet ports. The cover was a single sheet of clear polyethylene stapled to the frame. The dryer was mounted on legs to facilitate air entry through the base and to reduce the risk of entry of pests into the cabinet. Simple temperature control was achieved by leaving the doors partially open. Construction time for the cabinet dryer was about 25 man-hours.