It has been proved to prevent bacteria, yeast, mold and enzymes spoiling a
food, its moisture content should be reduced to 10-20%. At the same time the
nutritional values and the flavour are concentrated and maintained (Scanlin,
1997). The typical traditional method to reduce the moisture content is
to use open sun drying. It involves spreading the agricultural products in a
thin layer such as crops, fruits, vegetables and tobacco on the ground and turned
regularly. Turning the products continuously until the food moisture content
reduced to the required moisture level (dried) for storage purpose. This method
is still widely used in the developing countries. However, there exist many
problems associated with open sun drying. It has been seen that open sun drying
has the following disadvantages. It requires long drying time and large surface
area. Since the products are open to the air they could easily get damaged by
the hostile weather conditions, degradation by overheating, infestation by animal
and contamination by foreign materials. Crops are also susceptible to re-absorption
of moisture if they are left on the ground during periods of no sun and the
drying process cannot be controlled. These could lead to slow drying rate, contamination
and poor quality of dried products and loss in production.
The aforementioned problems can be avoided if agricultural products are dried
in a sheltered area or chamber using a stream of heated air by solar energy
to reduce its relative humidity. Moreover, speed of operations from harvest
to storage forced the implementation and use of heated air for drying (Hall,
1980; Ergunes et al., 2005). Consequently,
various drying devices were implemented for drying crops such as woodfuel driers,
oil-burned driers, electric dryers and solar dryer. However, the high cost of
oil and electricity and their scarcity in the rural areas of most third world
countries have made some of these driers very unattractive. Therefore, interest
has been focused mainly on the development of solar driers (Veziroglu,
Based on the mode of air flow solar dryers are classified into two groups natural
convection and forced convection dryers. Forced convection dryer requires external
fan or blower to create the air current in the dryer which require power source.
Whereas in natural convection with the proper design of the dryer the air current
is generated due to density gradient of the air along the dryer, i.e., does
not require power source. However, low air flow rate and the long drying time
result in low drying capacity (Bala et al., 2005).
As a result natural convection solar dryer is limited for drying small amount
of agricultural products for family consumption. If large amount of products
are required to be processed, then forced convection dryers should be used.
Performance of forced convection maize and bean solar dryer was evaluated by
Gatea (2010, 2011). Stiling
et al. (2012) used concentrating solar panels to improve solar drying
for Roma tomatoes and showed a considerable increase in drying rate on sunny
days. This study focused on natural convection solar dryer so that it can be
used anywhere else as it does not need power input except the solar radiation.
The objective of this study was to analyze experimentally the drying characteristics of potato slices using free convection solar drying system. It is mainly concentrated on the practical field test performance of a locally manufactured, solar dryer. The experiment was conducted in the Faculty of Technology, Addis Ababa University in Ethiopia.
The solar collector is parallel piped shape with dimension of L = 2 mxW = 1
mx0.14 m having 80 mm channel depth, 40 mm gap between the absorber plate and
glass cover, 20 mm thick fiber glass insulation was used at the bottom of the
collector to reduce the back and edge losses. The solar collector which is inclined
at an angle of 12° from the horizontal is oriented along the N-S direction.
Blackened stainless steel flat sheet of thickness 1 mm is used as the absorber
plate. The solar collector is covered with 4 mm thick commercial glass. The
lower end face of the collector (1x0.08 m) is the air inlet whereas, its higher
face end is connected to the rectangular duct of the chamber. For the study
of the drying characteristics of potato slices in the solar dryer, measuring
instruments and thermocouples are used and are all connected to data logger.
Figure 1 shows the location and types of the sensors applied.
The drying chamber has 1.2x1.04x0.55 m outer dimensions. Out of the three shelves/trays only two trays TR1 and TR2 were placed inside the drying chamber. The depth of each tray is 19 mm with wire mesh at the bottom. The two drying trays TR1 and TR2 have areas of A1 = 0.39 m2 and A2 = 0.42 m2, respectively, with a total area of 0.81 m2. The relative positions of the three trays, TR1, TR2 and TR3 are 0.18, 0.32 and 0.46 m, respectively, above the drying chamber base (hot air entry point). Holes were drilled on the drying chamber for inserting the rod-shaped measuring probes.
The potato slices were characterized by analyzing their moisture content and
drying rate curves both on wet and dry basis. The initial moisture content on
wet and dry basis of the potato slice was determined by AMB (AMB 310) moisture
balance. The AMB balance test was set at Mode 1, with strobe time interval of
2 sec and drying temperature 160°C. Then, slices of potato samples were
placed on the AMB moisture balance tray. Samples of the potato of weight ωO
were dried in the moisture balance at 160°C until the dried sample weight,
ωd became stable. The moisture content on wet basis of the potato
used was 81.56% (42.3% moisture content on dry basis).
|| Schematic representation of the drying setup
The moisture content, dry basis, Mdo of the potato is expressed
For the determination of the moisture content, dry basis, Mdi of the potato at any time ti during the drying process, the following equation is used:
or moisture content, wet basis, of the potato at any time ti during the drying process where ωi is the weight of the potato at time ti is expressed as:
The moisture content on dry basis and wet basis is related by Eq.
4 and 2:
The determination of the potato weight was done by weighing the drying tray with its load of potato at any time in the drying process. For the determination of the instantaneous drying rate RDdi, dry basis, Eq. 5 is applied:
where, ti-1 and ti are successive times corresponding to when two successive measurements of a drying material are made.
RESULTS AND DISCUSSION
Figure 2 shows the moisture content of potato as a function
of the drying time. Initially the moisture content of the potato slices is high.
The moisture content reduced with time and similar potato moisture content profile
during drying was shown by Srikiatden and Roberts (2008)
and Putranto et al. (2011). As may be expected,
TR1 the one placed nearer to the hot air, exhibits the most rapid drying. At
6 PM of the first day, the potato moisture content of TR1 and TR2 which are
located in the dryer and TR3 which is located in the open sun dropped to about
4.38, 7.88 and 11.49%, wet basis, respectively. During the second day, the moisture
content decreased gradually. By 10 AM of this day the moisture content dropped
to about 2.62, 3.05 and 5.6%. The final moisture contents at 1 PM were 2.62,
2.93 and 3.58% on wet basis and which are considered as the equilibrium moisture
contents of potato.
||Moisture content curves for potato slices in solar dryer and
open air sun dryer
||Drying rate curves plotted for potato slices on a dry basis
These moisture contents indicate that the first tray reached the equilibrium
moisture content after 2 h of the second day, whereas the potato in the open
sun tray reached equilibrium moisture content after seven hours of the second
drying day. This means a reduction of the drying period of 4-5 h was obtained
using the solar dryer compared to the traditional open sun drying which also
depends on the prevailing weather conditions. Other advantages are the protection
against direct sunshine, dust and insects. During the night times the inlet
and the exit of the dryer were closed and the control sample was placed in a
room to prevent the potato from moisture regain.
Figure 3 shows the drying rate of potato as a function of
the drying time. As can be seen from the curves in the figure, the drying rate
for the first tray, placed at the bottom of the drying chamber, expectedly has
the highest drying rate during the first 3 h.
|| Drying rate curves
||Temperature variations with respect to the vertical distance
from the drying chamber bottom
However, once a large quantity of moisture has been removed, its drying rate
decreases. The drying time of the potato on the second tray is higher than the
first tray because the drying air moisture-absorbing capacity is reduced after
it has absorbed moisture from the first tray.
During the initial stage of drying, the rate of moisture migration is sufficient
to maintain the surface in a completely wet condition, as shown in Fig.
4. During this period, the rate of drying of the material is controlled
by the rate of evaporation from the surface. This is controlled by the condition
of air adjacent to the surface. Thus, during this period, the rate of drying
is relatively constant as shown in Fig. 4, which is known
as the constant rate period. The point where the drying rate starts to decrease
is known as the critical moisture content. Thereafter, the period of drying
is known as the falling rate period. This is the period when the surface of
the material is not wetted completely, by migration of moisture. The drying
rate tends to go to zero when the rate of evaporation from the surface equals
the rate of absorption of moisture by the material and is known as the equilibrium
||Weather data for the test period: measured total solar radiation
and ambient temperature obtained from the pyranometer and temperature sensor
Since the drying rate decreases to zero with a certain bound moisture, potato
is a hygroscopic material. Srikiatden and Roberts (2007)
said potato is one of the hygroscopic foods. Furthermore, the drying rate trends
showed a typical hygroscopic material curve as reported by Mujumdar
and Devahastin (2000).
Figure 5 displays the variation of air temperature with vertical distance from the bottom of the drying chamber. Ambient air temperature is included in the graph for comparison. A major drawback of the shelf-type dryer is the uneven drying. As a result of the migration of the drying front, the materials at the entrance are dried while at the exhaust are under dried. This problem can be alleviated by rotating the drying shelves. In such a rotating operation, the hot air from the collector is used to heat the product already in the latter stages of drying, falling rate period while the unsaturated air is used to remove moisture from product in the upper shelves.
METRO LOGICAL DATA DURING THE TEST
The weather throughout the experiments was clear at day time. The maximum ambient temperature reached 22.65°C at 2:41 PM and radiation reached 1.0357 kW m-2 around noon. These are shown in Fig. 6. The average relative humidity of the ambient air was 30.41% compared to the relative humidity of the air at the collector exit which has an average of 11.89% in the morning, 8.7-8.9% between 11:00-15:00 h, with a minimum of 6.6% at 13:30 h and an average of 18.97% in late afternoon.
Solar energy was utilized to dry potato slices in the shelf or batch type solar dryer with 2m2 flat plate collector. The collector produced air temperatures of 14-29°C, from 8:30 AM- 4:00 PM, higher than the ambient air temperature in a clear day. The hot air was used to dry 6.3 kg m-2 potato slices on trays inside a dryer. The final moisture content of the potato is 2.62-2.93% which was observed after 10 sunny hours. The drying time required by traditional open sun drying is reduced by 4-5 h, about 30%, in natural convection dryer under the existing environmental conditions. The drying rate curves of potato slices were produced. Furthermore, the drying material is protected from direct solar radiation, infestation by insects and contamination by dust, producing a product with improved quality.
Special thanks go to the Solar Energy Research and Development Group of Faculty of Technology, Addis Ababa University, for providing us with equipment and facilities and Universiti Teknologi PETRONAS for providing finical support to publish the paper.