![temperature regime of integrated pebble bed solar collector/storage]](https://www.researchgate.net/profile/Wilfred-Okonkwo-2/publication/278967158/figure/fig2/AS:294359766454273@1447192245962/temperature-regime-of-integrated-pebble-bed-solar-collector-storage_Q320.jpg)
Content uploaded by Wilfred Ifeanyi Okonkwo
Author content
All content in this area was uploaded by Wilfred Ifeanyi Okonkwo on Jun 22, 2015
Content may be subject to copyright.
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 67
PERFORMANCE EVALUATION OF A PEBBLE BED
SOLAR CROP DRYER
W. I. OKONKWO1 and OKOYE, E. C.2
1National Centre for Energy Research and Development
University of Nigeria, Nsukka, Nigeria
E-mail: ifeanyiwilfred@yahoo.com
2Department of Agricultural Engineering
University of Nigeria, Nsukka,
ABSTRACT
The design, development and test performance evaluation of an integrated of passive solar
energy crop drying system was undertaken. The solar crop dryer consists of an imbedded pebble
bed solar heat storage unit/solar collector and crop drying chamber measuring 67 cm x 110 cm
x 21cm and 50 cm x 90 cm respectively. The crop-drying chamber is made of drying trays of wire
gauze while the roof is made of transparent glazing. Test performance evaluation of the solar
crop dryer indicates that maximum absorber temperature of 72 0C, heat storage bed temperature
of 58 0C and chamber temperature of 57 0C were obtained using the dryer when the maximum
ambient temperature was 34 0C. Further test using cassava, showed a moisture reduction from
73% initial moisture content to 10.2% final moisture content in 3 days of drying process while
open air sun drying was 22.2% under the same period of drying.
Products dried under the passive solar crop dryer were of high quality while there were mould
build up on the open air sun dried products. This indicates that drying under solar crop dryer
offers high quality products and is time saving than the open-air sun drying.
Keywords: pebble bed, solar, storage, crop dryer.
1.0 Introduction
Food drying is a major processing activity,
which involves dehydration of moisture
from agricultural produce for easy handling
and storage. It is a preventive technique
against post-harvest losses. Post- harvest
losses, which amount to food insecurity,
could be tamed if adequate precautionary
measures are taken as soon as the farmer
harvests his produce. The issue of food
insecurity rested on a tripod of non-
availability, non-stability of price and non-
accessibility of food. According to Ogugua
(2004) the situation of post harvest losses
has caused a setback in the level of
nutritious food consumption. Food price
stability is influenced by the extent of post-
harvest losses in the agricultural produce.
The farmer produce so much food but along
the way he losses much of the produce as he
harvests the produce due mainly to lack of
preservative method for a prolonged storage.
The problem of food availability all
year round is not a matter of producing
enough but a function of preservative
methods employed by the farmer for a
sustained availability. One of such methods
is drying. Despite the availability of
artificial methods of food drying in Nigeria,
most farmers still employ the open-air sun
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 68
drying technique in order to dry their
agricultural produce. The open-air sun
drying is prevalent and very common in the
rural areas. Farmers spread their agricultural
produce such as maize, cassava, pepper,
tomatoes etc. along major roads in order to
get them dried. In Nigeria this is common
along such roads as Ejule - Abuja, Okene -
Lokoja, Obolo afor - Otukpo roads and so
on. This method of drying process has
inherent limitations, which include dust
contamination, decay, insect and rodent
attacks mould (fungal) build up due to
prolonged drying periods. In most cases,
what could take few hours to dry under
modern techniques take days to dry. Same
time, the available non-solar drying systems
such as electricity and fossil fuel based
systems are very expensive and therefore
unattainable to the poor local farmers. A
possible alternative cheaper crop drying
system then could be the use of solar energy
drying system.
The abundance of solar radiation in
Nigeria could make crop drying with solar
dryers very easy and simple. Economic
appraisal of solar drying in Nigeria showed
that the use of solar energy in food
processing is much more cost effective than
electricity and fossil fuel drying systems.
The absence of national grid connections
and high cost of fossil fuel in rural areas
make agro-processing activities very
difficult in the Nigeria. Studios (Aarinze
1985 and Adesuyi 1991), showed that the
use of solar energy in crop drying is
possible, the studies are commendable ones.
Solar drying is nonetheless without some
major problems such as inability to under-
take drying process over the night or during
the off sunshine hours. A solar dryer that
could dry agricultural materials during the
off sunshine periods could be have an
advantage over the existing system and will
be of immense benefit to farmers. Such a
solar dryer would incorporate energy storage
device for drying purposes when needed or
for all the day round. Many methods of solar
energy storage materials are available
(Duffie and beckman, 1979). One such
method is storage of solar energy as sensible
heat using materials such as pebbles (rocks)
Pebble is an inexpensive material and
therefore locally available in Nigeria. Its
utilization in solar crop dryers could pose no
burden to famers.
In this report the design,
development and performance evaluation of
pebble bed passive solar energy storage crop
dryer is presented.
2.0 Design Consideration
The solar crop dryer is designed for drying
agricultural produce that needs low
temperature rise above the ambient. The
produce of study is cassava. Cassava is
chosen for the study because of its
importance in Nigeria economy. Nigeria is
the largest world cassava producer and is
being exported to other countries of the
world. Therefore, proper preservation of the
crop through drying will enhance the
storability and transportability, and hence
will further increase the economic gains
from the produce. The environmental impact
of solar drying is considered friendly, non
contaminating and non polluting. Heat
transfer medium is essentially by air current
and the drying method is passive. The
intermittent nature of solar radiation is an
important factor to consider in solar designs.
The design therefore incorporates a storage
system which stores and supplies heat to the
drying chamber during the off-sunshine
hours. Materials of construction are locally
available and friendly to end-users with low
maintenance cost.
2.1 Description of the Solar Dryer
The integrated solar energy pebble bed solar
dryer (fig.1) is made of two important
compartments
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 69
i. The solar energy collector/storage unit (A)
The crop drying chamber (B) ii.
2.1.1 The solar energy collector (heat
storage unit)
The solar collector is a dual function
apartment, which consists of a solar energy
collector and an imbedded pebble bed heat
storage unit. The solar energy collector /
heat storage unit (A) is a flat –plate solar
energy collector that collects and stores solar
energy. It has internal dimensions measuring
110cm x 65 cm x 20cm giving radiation
absorption surface area of 0.715m2 and
0.143m3 volume. The wall of the collector
(a) is constructed of 2 cm wooden materials,
which serves as an insulating material.
Three quarters (3/4) of the volume is filled
with uniformly sized pebbles, which
constitutes the pebble bed (b). The pebbles
on the top surface of the bed are painted
black to form the solar energy absorber. This
is covered with transparent glass-perspex
cover (c) at a 2 cm distance above the solar
absorbing surface. The pebbles store solar
energy during the sunny hours of the day,
which could be used during off-sunshine
hours such as night and cloudy periods of
the day. This provision ensures that drying
process takes place all the day round. The
solar collector is inclined at 220 to horizontal
and oriented towards south for all year
round solar energy collection. Two openings
(d) and (e) for fresh air entrance are located
at the lower end of the solar collector while
another opening (g) at the upper end allows
exit of hot air form the collector into the
drying chamber. The lower air openings are
provided with shutters either to close or
open for fresh air entrance into the collector.
The top opening is kept opened during the
sunny hours of the day while the lower one
is closed. During the night and cloudy
periods of the day the reverse is the case.
Also to prevent heat loss through the upper
surface of the collector during the night or
cloudy hours a hinged door (f) is fasten to
the collector. This is used to close or open
the solar collector when in operation.
2.1.2 The crop drying chamber
The crop drying chamber (B) is made of
wooden material measuring 50 cm x 50 cm
x 90 cm. It has an hot air inlet (h), which is
located at the bottom end of the chamber.
This provision allows hot air from the
integrated solar collector/hear storage unit
into the chamber while an exhaust air outlet
(i) of 30 cm x 5 cm was provided at the
upper end of the drying chamber. The roof
(j) of the drying chamber is covered with a
transparent glass cover. This allows direct
solar radiation into the chamber thereby
enhancing crop drying operation. The
chamber also consists of three drying trays
(k) all separated at a distance of 20 cm from
the other. The trays are framed with wood
while the floors are of wire mesh. The trays
measure 48cm x 48cm x 5cm.The chamber
is constructed of wood and this insulates the
whole system against heat loss from the
chamber. For access into (l) is provided. The
chamber is placed on an iron stand (m).
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 70
Fig. 1: Isometric view of the pebble bed
solar crop dryer (a) collector casting (b)
pebbled bed (c) transparent glass cover (d)
lower air opening (c) upper air opening (f)
collector hinged door (g) hot air exit (h)
chamber hot air inlet (i) exhaust hot air exit
hot air (j)transparent chamber roof (k)
2.0 Design Equations
The collector was inclined at an optimum
angle equal to 220. The optimum tilt angle
was determined by local latitude plus 150
Nsukka has a latitude L of 6.80.
(1)
The collector area was determined by the
expression
(2)
Where
Ac = collector area, m2
Qu = useful energy gained by
collector, W
Ic = incident radiation on the
collector surface.
n = average collector efficiency
The useful energy gained by the collector
was determined by
(3)
Where
Va = Volumetric flow rate of air
m3/s
la = density of air kg/m3
Cpp = specific heat capacity of air
KJ/kg0C
The sensible heat Qst stored by the pebble
was established using
(4)
Where
Ou = amount of heat stored by
pebbles, W
Vst = volume of pebble, m3
p = density of pebble, m3
T = temperature change in
storage, 0C
Cpp = specific heat capacity of
storage pebble, KJ/kg0C
Assuming a 10 hours of non-sunshine hours,
the amount of heat needed to keep on drying
at T rise in a storage temperature above the
ambient is given by
While the total rate of heat transfer to the
drying agricultural product is a combination
of convective, conductive and radiative heat
transfer, which could be expressed as
(6)
Where
qc = convective heat transfer, W
qr = radiative heat transfer, W
qk = conductive heat transfer, W
and these are expressed as
(7)
(8)
(9)
hc = convective heat transfer coefficient
W/m2K
T = temperature of hot air coming
from the solar collector, 0C
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 71
Tch = temperature of chamber
(temperature of material), 0C
Tr = radiative heat transfer
temperature, 0C
A = drying area of material, m2
hr = radiative heat transfer
coefficient, W/m2.k
Uk = thermal conductivity of
material w/m2. k
Moisture loss from the drying material is
expressed with
(10)
Where
Xt = rate of drying %
wm = weight of material before
drying, kg
WS = weight of material after
drying
3.0 performance evaluation
The performance evaluation of the solar
crop dryer was undertaken by conducting a
physical test-run of the dryer and the dryer
loaded with agricultural product. The study
covers a prolonged period. The study covers
a prolonged period of six months (December
2003 to May 2004) of evaluation. The result
presented here reflects an average
performance of the period. The prevailing
physical conditions – temperature and
relative humidity of the dryer and ambient
conditions were monitored with
thermometers and relative humidity sensors
and thermocouple wire located at strategic
points within the solar collector/heart storage
unit and the drying chamber. The dryer was
filled with cassava chips, which are cut into
uniform size measuring 1.3 cm x 1.3 cm. a
similar quantity and size of cassava were also
spread outside under open-air sun drying as
control. Solar radiation data within the period of
experiment was collected from the
meteorological unit of the national centre for
Energy Research and Development,
University of Nigeria, Nsukka. The rates of
moisture loss of the drying materials were
determined using oven method. Temperature
and relative humidity measurements were
determined on hourly basis.
4.0 Results and Discussion
4.1 Solar collector/ heat storage unit
Fig, 2 shows the temperatures regime of the
integrated pebble bed passive solar energy
storage crop dryer. The maximum air
temperature reached by the solar collector
was 630oC, while the storage temperature
was 58oC, the solar absorber temperature
was 72oC, and the ambient environmental
temperature was 340C. The average drying
chamber temperature was 57 0C. These
showed that the collector hot air storage,
solar absorber and the chamber temperatures
rose by 29 0C, 24 0C 38 0C 23 0C well above
the ambient temperature respectively. The
figure showed that the solar absorber had the
highest rise day time. This was recorded at
12 noon (point 7 on the time scale) starting
from 6 am (point 1); Ambient temperature,
storage temperature and the average drying
chamber temperature in that order followed
this. While the temperature of storage had the
highest temperature values during the night
period starting from 5 pm (point 12 on the time
scale), The trend continues till about 3 am (21)
when it dropped below the chamber and
collector air temperatures respectively. The
indication showed that by this period, the energy
stored the previous day by the storage unit has
been exhausted. The higher temperatures above
the ambient temperature exhibited by the storage
and the drying chamber during the night
periods indicated that the storage contributes
significant heat to the dryer during off sunshine
hours and therefore provides the dryer the ability
to undertake drying process into the hours of
night. The dryer and ambient relative humidity
were ranged between 43 to 74% and 37 to 83%,
respectively.
4.2.1 Drying material
Table 1 shows the result of the cassava chips
drying process. The cassava was at initial
moisture content of 73%. At the end of an
average period of 3 days a mean moisture
content of 10.5% was attained by the cassava
chips under the solar crop dryer while the open
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 72
air sun drying reduced to 22.2% at the same
period. It took another two additional days for
the open air sun drying (control) to attain about
11.2% moisture content (fig.3.). This showed a
significant reduction in time using the solar crop
dryer. Moreover, the samples dried in the solar
dryer were clean and of high quality with no
contamination through dust or insect and did
not change colour while those under open air
sun drying showed change in colour indicating
signs of deterioration in quality. It was also
observed that the samples at different levels in
the chamber trays were not drying at the same
rate. The tray at the topmost of the drying
chamber has the highest drying rate. However,
this was not the same during the night periods as
reverse was the case. This could be probably due
to the direct solar radiation, which comes
directly from the transparent roof cover of the
dryer to the topmost ray during the day hours in
addition to hot air coming from the solar
collector.
Fig. 2: temperature regime of integrated pebble bed solar collector/storage]
Nigerian Journal of Technology, Vol. 24, No. 2, September 2005 Okonkwo and Okoye 73
CONCLUSION
Pebble bed passive solar energy storage crop
dryer consisting of solar collector/ heat
storage unit and a drying chamber was
tested with cassava chips. The result of the a
prolonged performance evaluation showed
that a drying chamber temperature of up to
57 0C was attained with a solar collector
absorber temperature of 72 0C when the
pebble bed storage temperature was 58 0C,
The cassava was dried from initial moisture
content of 73% to an average storage
moisture content of 10.5% in 3 days of
drying process while it took about 5 days for
the open-air sun drying sample (control) to
attain a moisture content of 11.2% from the
same initial moisture- content. Materials
dried under the solar dryer were of high
quality and showed no sign to contamination
by change of colour while the open air sun
dried samples indicated some colour change
showing product deterioration and decay.
This showed that drying gives faster drying
process, yields high quality products and is
time saving than open air sun drying.
REFERENCES:
E.A Arinze (1985) Design and Performance
Evaluation of a Commercial Size Natural
Convection Solar Crop dryer Nigeria .J
Solar Energy, Vol 4 pp 106-115.
S.A Adesyi (1991) Assessment of a Solar
dryer for Drying Cassava and Plantain
Chips in Humid Tropics. Nigeria Solar
Energy, pp.8-18. Vol 10.
A.C Oguguo (2004). POST Harvest Losses
Major Cause of Food Insecurity. The
Guardian Newspaper, 025 April, 2004, pp
3.
Duffie J.A And Beckman W.A (1979). Solar
Energy Engineering Processes, John
Welley, New Yoke.
