Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 141

Integral type natural-circulation solar-energy dryers are both simple and cheaper
to construct than those of the distributed type for the same loading capacity. However
the potential drawbacks of the former are (i) a liability to localised over-heating and
(ii) relatively slow overall drying rates. To overcome these limitations, a “solar
chimney'' is employed to increase the buoyant force on the air stream and thus provide
an increased rate of moist-air removal. Two generic dryer types can thus be identified,
namely: the cabinet dryer and the ventilated greenhouse dryer.
A natural-circulation solar-energy cabinet dryer is simply a single or double-glazed
insulated container referred to as a cabinet at small scales or a silo at large scales. Solar
radiation is transmitted through the cover and is absorbed on the blackened interior
surfaces as well as on the product itself, thus raising the internal temperature. Vents at
both the base and lower parts of the cabinet or silos enable air ventilation, with warm
air leaving via upper apertures under the action of buoyant forces, drawing in replenish-
ing fresh air at the base. Shallow layers of the product are placed on perforated trays
inside the enclosure. Solar cabinet dryers, constructed from cheap locally-available
materials, are usually relatively small units used to preserve “household'' quantities of
fruits, vegetables, fish and meat.
The major drawback of small-scale cabinet dryers is the poor air circulation often
that reduces the drying rate, and incurs very high internal temperatures that can
overheat crops. Drying air temperatures above 70 ◦ C are excessive, particularly for
perishables fruits and vegetables. Relatively large air inlet ducts with appropriately-
designed solar chimneys are essential for effective circulation within the cabinet to
minimise temperature elevations.
Natural-circulation solar cabinet dryers are probably the most widely used type
of solar dryer. A cabinet dryer equipped with a solar chimney shows higher efficiency
than that of a natural-circulation distributed type which the incoming air was heated as
it passed through a solar-energy collector and for more efficient than open sun drying.
Natural circulation solar-energy greenhouse dryers are larger than most cabinet
dryers and are characterised by extensive glazing on their sides. Usually the glazing
is on the front side (i.e. sun facing side) of the dryer while the rear side is insulated.
Insulant panels may be drawn over the glazing at night to reduce heat losses and heat
storage may also be provided. Designed properly, a solar greenhouse dryer allows a
greater degree of control over the drying process than the solar cabinet dryer and are
more appropriate for large-scale drying.
Typical later designs of natural-circulation solar greenhouse dryers include the
widely-reported polyethylene-tent fish dryer built (Doe et al., 1977) that consists of
a ridged tent-like bamboo framework clad with clear polyethylene sheet both on the
side orientated towards the sun and on the ends. The rear side was clad with black
polyethylene sheet, which was also spread on the floor. The cladding at one end was
arranged to allow access into the drying chamber for loading and unloading. The clear
plastic cladding at the bottom edge of the front side was rolled around a bamboo pole
which could be adjusted to control airflow into the chamber, while the vents at the top
of the ends served as the exit for the moist exhaust air. The maximum temperature for
the drying of fish is 50 ◦ C: above which the fish will cook. Dryer temperatures of about
45 ◦ C are however desirable as flies and larvae infestations within the fish are boiled.
The high 5 ◦ C temperature merger is difficult to maintain with thermometry connected
to vent aperature controls.