FIGURE 2. Drying efficiency with drying time for various crops.


Performance
 Tables 1 and 2 show the changes in moisture cc
crops with the time of day, drying time sunshine (
average temperature in the dryer, and the drying e
 The drying efficiency (qr) was calculated from the

 YI = Q,ffx 100
 Qi,
 = 27.777 x W, x L
 AxIxt

 where:


Qeff
Qi,
W,
L
A
I
t


= quantity of heat used in the drying pr
= quantity of heat entering the dryer
= crop weight loss (kg)
= latent heat of vaporisation (KJ/kg)
= dryer cover area (m2)
= solar radiation level (W/m2)
= drying time (h).


 Solar radiation intensity, I, was estimated from the mean daily
solar radiation data of Smith (1967), for Trinidad. An average
daily value for the dry months ofJanuary to May was estimated at
18.5 MJ/m2 day or 428 W/m2 for 12 hours of the day.
 The changes in drying efficiency during the drying cycle and
the drying times for the four crops are shown in Figure 2. As dry-
ing progresses, and as the rate of drying of the crop decreases in
the dryer, drying efficiency also falls. Lawand (1966) and Umarov
and Tairov (1982) have reported similar changes in drying effi-
ciency with drying time. Based upon the results given in Tables 1
and 2, average, weighted drying efficiencies for the entire drying
cycles can be calculated. These are 25 %, 44%, 28% and 37% for
mango slices, diced sweet potatoes, de-seeded sorrel and cut hot
peppers respectively. It should be noted that the hot peppers
were dried to a moisture content of 31%, and with further drying
necessary for safe storage, a lower drying efficiency can be ex-
pected. The sweet potatoes dried rapidly, principally due to the
size reduction, and hence showed the highest drying efficiency.
Pande et al. (1981) have reported average drying efficiencies in a
solar cabinet dryer for cluster bean pods, chilies and dates, of
19%, 20% and 18% respectively.


VOL. XX-PROCEEDINGS of the CARIBBEAN FOOD CROPS SOCIETY


 TECHNICAL AND ECONOMIC CONSIDERATIONS
)ntent of the Based on Equation [1], a moisture removal rate M (kg/m2 h),
e hours), the for a solar cabinet dryer may be described and is given by,
efficiency M = We/At
relationship, = 36 rI x 10-3/L [EQUATION 2]
 For many perishable crops, drying in a solar cabinet dryer can
 be completed in two full days, as seen in Tables 1 and 2. The
 dryer installed capacity Co (kg/batch), can therefore be mini-
 mised for the small farmer, and the dryer used throughout the
 [EQUATION 1 crop duration, if
 Co = 2N
 D [EQUATION 3]
 where
ocess N = quantity of crop to be dried per year (kg)
 D = duration of crop harvest (days).
 Under these conditions and assuming negligible sensible
 heating, the transparent cover area S (m2) required for the
 cabinet dryer can be evaluated,
 S = Co x (M,- M)
 M x t x (1- My) [EQUATION 4]


 = (2N) (M, -My)
 x
 (DMt) (1 My) [EQUATION 5]
 where
 M, = initial moisture content of the crop, decimal wet
 basis
 My = final moisture content of the crop, decimal wet basis
 t = drying time of 2 days, (24 h).
 The initial cost of the dryer X ($) can be given through a linear
relationship and is,
 X = KS [EQUATION 6]
 where
 K = cost of dryer per m2 of cover area ($/m2).

DISCUSSION
 Sorrel, with a yield of 12,000 kg/ha and a harvesting period of
12 weeks, may be used as an example of a crop with potential for
solar drying by small farmers. The crop could be dried from an in-
itial moisture content of 90%, to a final moisture content of 15 %
in 2 days with a cabinet dryer. It can be assumed that 50% of the
annual crop will be sold on the fresh market, with the remainder


265


 g/ass




 metal
 base




 92
screened
 opening


 100


 I 8o sweet potato
 \. -----* pepper
. ..........o sorrel
60 60
 ."*. ---. mango

 40 -
 40 ^S'
...





 0 4 8 12 16 20 24
 Drying time- hours


FIGURE 1. The solar cabinet dryer.


I