GLASS ROOF SOLAR DRYER (Brazil)
CASE STUDYÍ I
Status t operational Type : chamber dryer
Rooting mode*, hybrid
A solar dryer for small and middle scale drying operations of cacao has been optimized and its performance compared with the traditional sun drying platform or "barcaja" used in Brazil. The prototype is less expensive and uses solar radiation more efficiently, it is supplemented with gas heaters for rainy and night periods.
In general, the unit is similar to a regular greenhouse structure and has a special roof peak cap acting as a flue and protecting the inside of the dryer against rain. This cap, made of folded zinc sheet, allows the heated air charged with the moisture removed from the cocoa to escape, thereby permitting the entry of fresh air through the side shutters provided in the structure. The dryer is aligned lengthwise along a north-south axis. Basically it consists of two parallel rows of drying platforms with a central passage for an operator. A fixed glass roof above the drying platform allows the radiation of the sun to penetrate inside the dryer and also prevents the ingress of rain or dew at night. All surfaces inside the dryer are painted black to facilitate the absorption of solar radiation. See Figure 1 and 2.
The drying surface is made of galvanized iron wire mesh laid over wooden beams fixed aeross the platform.Strong metal wires stretched perpendicularly over the wooden beams and under the wire mesh provide additional support to the loaded wire mesh during the drying operation. This permits the solar or gas heated air to pass easily through the wire mesh on which the drying product is spread.
In the prototype dryer, the gas heaters are situated underneath one of the two rows of drying platforms and are intended to serve during rainy or night periods only for shortening the drying time.
A free vertical space between the lower edge of the glass roof and the outer edge of the platform allow for the proper ventilation of the dryer. Six wooden shutters on hinges are located in this space along the length of the dryer on each side, and they can be opened or closed independently to regulate the air flow inside the dryer during the solar drying hours, (see figure 3) .
Dimensions; The overall dimensions of the dryer are 12,80 m long x 5,20 m wide x 2 m high at the centre and 1 m high at the outer edges. The drying platforms are 12 m long x 1,76 m wide and are located at 0,8 m from the base of the dryer. The drying platforms are provided with a 10 cm ridge along their edges. The drying surface lays over wooden beams set at a spacing of 0,5 m. The metal wires reinforcing the wire mesh drying surface are set at a pitch of 0,4 m. The vertical space where the ventilation shutters are located is approximately 10 cm. See also the drawing Figure 5.
Materials of construction:
Transparent cover: glass panels, 3 mm thick
Frames: all frame work made with wood insulation: wood and waterproof composite panels (eucatex)
for walls, and windows. The dryer is designed so as not to be completely insulated.
Trays: drying platforms made of wire mesh supported over wooden frame and metal wire reinforcement.
All materials used are locally available.
Location: Centro de Pesquisas do cacau
Latitudei 14°47'S Longitude: 39°16*W
Summary of records over a period of 10 years.
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