Technical and Economic Parameters

for the Adoption of Solar Drying Systems

 on Small Farms

 Clement K. Sankat
 Department of Mechanical Engineering
 University of the West Indies
 St. Augustine, Trinidad


 Solar cabinet dryers appear suitable for use by small Carib-
bean farmers, many with farms less than 1 ha in size.
Perishable products like sorrel, hot peppers, sweet potatoes
and mango slices may be dried for safe storage. In a solar
cabinet dryer of the multi-rack design, an overall drying effi-
ciency of 25% can be expected. Crops can be dried in thin
layers with loading densities of less than 5 kg/m2, in two days


of fair weather. For the small farmers, with holdings ranging
from 0.125 to 1 ha, and with 50% of their annual crop yield
dried, dryers with cover areas ranging approximately from 3.8
m2 to 30.8 m2 respectively, will be adequate for a crop like sor-
rel. The minimum estimated cost of such dryers would cor-
respondingly increase from US$75 to US$600.
Keywords: Drying, solar, efficiency, cost.


 A major problem faced by farmers producing crops which are
seasonal and perishable, is the adverse effect on farm income as a
result of seasonal gluts. Drying of such commodities so as to
facilitate storage, processing, marketing and utilisation provides
an opportunity for stabilising and even increasing farm income.
Traditional open sun drying of perishables such as ginger, sorrel,
hot pepper, coconut and shrimp is extensively practiced in the
Caribbean, though on a limited scale, and usually by small
farmers with surpluses from the fresh market trade. Sun drying,
from an economic viewpoint, is attractive to the small farmer, as
it requires a negligible investment in capital. However, open sun
drying methods have a number of disadvantages including:
 1. A dependency upon fair weather, with minimal rainfall re-
 quiring extra handling.
 2. Excessive handling of the crop during the normal drying
 cycle.
 3. A reduction in crop quality due to contamination with
 dust, dirt, etc.
 4. Crop losses due to handling and open drying.
 For small Caribbean farmers, many with farm sizes of less than
1 ha, an alternative approach to open sun drying is the use of a
simple solar dryer, described as the cabinet dryer (Lawand, 1966).
In such a direct, passive solar dryer, the crop to be dried is placed
in a thin layer in the cabinet which has a transparent cover. Heat
is generated by the absorption of solar radiation by the crop, as
well as by the internal surfaces of the dryer chamber. This heat
evaporates the moisture in the crop and expands the air in the
cabinet, thus creating a natural air flow through the crop bed and
removing evaporated moisture.
 There are other types of passive solar dryers (Szulmayer, 1971;
Headley and Singh, 1979), but direct dryers of the cabinet type
permit the shortest and fastest operation, although crop quality
may not be the best (Lof, 1962). The cabinet dryer, with its
simplicity in design and construction therefore appears to be an
attractive option for the small farmer. However, the initial cost of
such a dryer must be an important consideration for the farmer,
with a negligible investment, when compared to the traditional
open sun drying.


264


SOLAR CABINET DRYER PERFORMANCE
Description
 A solar cabinet dryer was designed, built and tested at the
University of the West Indies, St. Augustine, Trinidad. It was
built as a multi-rack unit to facilitate easy handling of the pro-
duce to be dried, and to facilitate improved dryer performance as
discussed by Sandhu et al. (1979).
 In Figure 1, a pictorial drawing of the assembled dryer shows
the essential functional features and major dimensions. The
transparent glass cover was 1.67 m2, inclined at 10 to the
horizontal, and faced south. The side and end panels which sup-
ported the glass cover were made from 2 cm thick wooden planks.
There were 5 cm x 86 cm openings (with protective wire mesh
screens) for air movement on the front and rear end panels. The
base of the dryer was comprised of a flat galvanised sheet screwed
onto the wooden sides, with 4 cm thick "Styrotex" insulation
below the metal sheet. Four trays, each 34 cm x 85 cm, made
with a wooden frame and a wire mesh base, could be inserted in-
to the dryer through slits in the side panels. The trays were sup-
ported by wooden runners in the dryer. All internal surfaces of
the dryer were painted with a flat, black paint.

Testing
 Sorrel, sweet potatoes, green mangoes and hot peppers were
dried in the solar dryer. In the case of sorrel, seeds were manually
removed prior to drying. The sweet potatoes were washed, diced
into 1 cm cubes, and treated with a potassium metabisulphite
solution. The green mangoes were cut into 0.5 1.5 cm thick
slices, and blanched in boiling water before drying. Hot peppers
were cut into two sections before drying.
 All crops were spread in a thin layer on the drying trays,
averaging 1.07 kg, 1.70 kg., 1.35 kg and 1.3 kg in weight per tray
of sorrel, hot pepper, mango slices and sweet potatoes respective-
ly. The initial moisture contents of all the crops were measured by
the oven method, and during drying the weight changes in the
crops were measured by removing and weighing the trays at
2 3 h intervals. Air temperatures within the cabinet dryer were
measured initially by thermometers and later by thermocouples.

 PROCEEDINGS of the CARIBBEAN FOOD CROPS SOCIETY-VOL. XX