2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 156 -
transparent materials. First, they have high transmittance
in the short wavelength solar radiation. For thermal long
wavelength radiation, the polycarbonate sheets have a
transmittance of about 0.2 [9]. The transmittance for the
short wavelength and the long wavelength indicate that
polycarbonate sheets have good optical properties for the
creation of the greenhouse effect. Second, polycarbonate
sheets have low thermal conductivity due to the air
channels in the sheets, which help reduce heat losses.
Third, their low density and high flexibility facilitate the
construction. Finally, they have reasonable price with a
long life span of over ten years. The greenhouse dryer is
3
u
4.2 m
2
effective floor covering area having the
central height and height of walls as 3.06 m and 1.85 m,
respectively. All side walls of the dryer are covered with
polycarbonate sheets with the thickness of 6 mm. The
front side wall of the dryer has two air inlets. The
dimension of inlet air is 300
u
400 cm
2
. One door of 1
u
1.85 m
2
made of glass was situated on the middle of the
front side wall. To ventilate the dryer, four DC axial flow
fans were installed in the wall opposite to the air inlets to
suck out moist air from the inside dryer to the
surrounding environment. Two channels are made at the
bottom near the door to allow the outside air intake into
the dryer as shown in Fig. 1. A 30 W solar cell module
was installed to power the fans directly during the day.
Metallic shelves with two levels of drying trays were
constructed and installed inside the greenhouse dryer for
placing products to be dried. The dryer is oriented in the
east-west direction.
The dryer was never shaded by trees or buildings.
Drying experiments were carried out in the month of
October 2010. The global solar radiation incident on a
horizontal surface was measured using a pyranometer
(Kipp and Zonen B.V. model CM 11, accuracy
r
10 Wm-
2). The greenhouse dryer was instrumented with K-type
thermocouples for measuring the temperatures of the air
inside the dryer, and the cover plate as shown in Fig. 1.
Ambient humidity and temperature were measured using
a Testo model 175-2 (accuracy
r
0.5
q
C,
r
3% RH). The air
flow rate was calculated from the air velocity, measured
by a hot wire anemometer (Testo model 445, accuracy
r
0.03 ms-1) at the fans outlet (Fig. 2) and the equal area
method (Macferran 1999) was used for air velocity
measurement. Weighed loss of the products during the
drying period was measured with an electronic balance
(Sartorius model CP3202S accuracy
r
0.01 g). A digital
clamp tester (Kewtech model KT200 accuracy
r
0.05 V,
r
0.01 A) was used to measure the power consumption of
the fan. The sun dryer control samples were weighted as
well. All data were recorded at 10 min intervals. Pork
was slice to dimension of 5 cm
u
12 cm and 0.2 cm thick.
The sliced pork was cooked before drying. The moisture
contents of the pork were measured at the starting and
end of each run of experiments by an air oven method
using about 8 g of pork at 103
q
C for 72 hours [10]. The
sample was found to have an initial moisture content of
about 210% dry basis. Pork was then spread on stainless
steel screen trays in a thin layer. For each of the
experimental runs the dryer was loaded to the full
capacity about 40 kg of pork. The greenhouse dryer is
capable of drying 40kg of pork two times a day. Testing
started at 8:30 a.m. and ended at 4:30 p.m. each day.
Fig. 1. The structure and dimension of the greenhouse dryer and the
positions of the thermocouples (T) and relative humidity sensors (RH).
Fig. 2. The dimension and the positions of the thermocouple (T),
relative humidity (RH).
B. Analysis
The drying efficiency of the greenhouse dryer is
defined as the ratio of energy output of the dryer to
energy input in the dryer. Solar radiation input on the
dryer is:
³
t
d
s
dt tGA Q
0
)(
(1)
Where Q
s
is solar energy input on the dryer, J
G(t) is solar radiation at time t, W/m
2
