Middle-East Journal of Scientific Research 12 (12): 1704-1709, 2012
ISSN 1990-9233
© IDOSI Publications, 2012
DOI: 10.5829/idosi.mejsr.2012.12.12.32
Thermo-Electrically Cooled Solar Still
S. Ravindran and M.E. Dean
Department of Mech. Engg., Bharath Inst. of Sci. and Tech.,
Bharat University, Selaiyur, Chennai-600 073, India
Abstract: Solar still is a thermal device, which is capable of trap solar light energy, evaporate water and
condense water vapour, to obtain distilled water. An attempt was made to augment the performance of a solar
still with the use of Thermo-electric cooling arrangement. Cooling and condensing capacity was augmented by
two single stage thermo-electric modules (TEM) of 80W each. This paper presents the initial data on the
performance of a hybrid solar still. The yield of water from the solar still, without TEM was 700 ml per day.
With TEM, the yield improved to 1200 ml per day, an improvement of 71 percent. As the initial data on the
performance of a hybrid solar still gets presented it needs further experimentation.
Key words: Solar Still
Performance
Augmentation
INTRODUCTION
A report by WHO /1/ has estimated that earth
consists of 97.5 percent of salty seawater and 2.5 percent
fresh water. Of this, fresh water 70 % is estimated to be at
the polar icecaps and 30 % as soil moisture or
underground aquifers. This leads to an estimate of less
than 1 % of the world’s fresh water i.e. only 0.7 percent of
the total amount of water on the earth is accessible for
direct human use. This observation indicates the need
for a new source of water. Potable/fresh water is
available from rivers, lakes, ponds and wells. In the form
of rain cycle, nature is carrying out the process of water
desalination since ages. Potable water is required for
domestic, agriculture and industries.
As per the different surveys done by Lorna F., et al
/2/ and MAW of KSA /3/, ocean covers 71 recent of the
earth's surface-140 million square miles with a volume of
330 million cubic miles and has an average salt content of
35,000 ppm. Underground saline/brackish water contains
dissolved salts of about 2,000-2,500 ppm. Brackish/saline
water is strictly defined as the water with less dissolved
salts than sea water but more than 500 ppm. For
agricultural purposes, water containing salt content of
1000 ppm is considered as the upper limit. Modern steam
power generation plant needs water with less than 10
ppm. Potable water (fresh water) suitable for human
Thermoelectric Cooling
consumption should not contain dissolved salts more
than 500 ppm. Demand of fresh water / potable water is
estimated to be in the range of 75-100 litres / person / day.
Solar still converts brackish water to produces
distilled water. Solar still uses a low temperature heat
energy source, solar thermal energy, for its working.
The solar still has a single chamber for three activities;
namely (1) energy collection, (2) evaporation and (3)
condensation /6/. The main aim of the project is to
produce potable water for the drinking purpose by using
the solar still and to improve its performance. One of the
small cooling systems is Thermoelectric module. It was
proposed to be used for additional condensation of the
water vapour by cooling and increases the output
amount of fresh water from Solar still. In this line of
thinking, a hybrid solar still was fabricated and tested
for its performance. Two thermoelectric coolers were
used in the new design of the solar still. The solar distiller
was fabricated and tested for 7 days. With the help of the
modified solar still, an improvement in its performance
was obtained, in the order of 58 percent.
Literature Review:
Solar Still: Solar stills are very simple in construction.
The specialty of these distillers is that they have single
chamber for heat collection, evaporation of salty water
and to condense the water vapor. Brackish water or
Corresponding Author: S. Ravindran, Department of Mech. Engg., Bharath Inst. of Sci. and Tech., Bharat University,
Selaiyur, Chennai-600 073, India. Cell: +9840864819.
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Middle-East J. Sci. Res., 12 (12): 1704-1709, 2012
seawater could be used for distillation. They have a small
area for heat collection, in the order of one to two square
meters.
Madani and Zaki /4/ developed a prototype of a
simple solar still, with double slope, at King Abdul Aziz
University, Kingdom of Saudi Arabia. The yield of the
still was in the order of 2.8 to 3.42 l/m2d. Thermal
efficiency of the system was as low as 20 to 30 percent.
James /5/ analyzed the different causes of low efficiency
of a solar still. Factors affecting the efficiency are (i)
feeding methods, (ii) geometrical configuration, (iii) type
and thickness of insulation, (iv) air circulation aspects (vi)
cover material and (vii) slope of the cover. Reduction of
heat conductance of glass cover due to condensing
steam and presence of air were found to be the two main
causes for the low yield of the solar still. Factors
invariably desired for better efficiency are (i) low thermal
capacity of the unit, (ii) leak proof cover and (iii) reliable
basin liner insulation.
Improvements in Solar Heat Collection: In order to
increase the product water and thereby the efficiency of
a solar still, many researchers have attempted to improve
the effectiveness of the three heat transfer processes,
namely, the heat collection by configuration changes,
evaporation and condensation of water vapor. The hybrid
stills with additional technical gadgets attached to a
simple solar distiller were also tested. They also have
coverage of few square meters of land.
Exhaustive reviews have been presented by
Tiwari et al. /6/, Raghu Raman et al. /7/ and Aref /8/.
Performance of solar stills with different glazing methods
like single slope; double slope and asymmetric glazing
were reported. Different materials of construction like
aluminum, steel, concrete, different plastics, FRP and
galvanized iron were used. Different insulation materials
widely used are calcium silicate, PUF, fiber glass and
sawdust.
Joachim Koschikowski et al., /9/ made use of a
combination of solar / membrane techniques. Zaki et al
/10/ tested (a set of five) solar stills to simultaneously
study the effects of three different slopes of the
glass sheet, heat storing rocks at the basin and mass
circulation within a still with the help of a flat plate
collector. Fath et al /11/ tested a still with the
pyramidal configuration. Barrera /12/ developed a
spherical solar still. Mohamed /13/ made an attempt to
minimize the cost of a solar still with the help of
appropriate technology.
Improvement of Evaporation and Condensation in
Simple Solar Still: Black coloured absorber floating
in water was used by Riera et al /14/. Mousa et al /15/
experimented with a
double exposure solar still.
Cappelletti /16/ performed tests on double condensing
and double evaporating system. Bassam /17/ obtained six
percent improvement by cooling the glass surface with
water cooling. Solar heating with multi stage or multi
effect systems were tested by Ouahes et al /18/, Le et al
/19/ and Tanaka et al /20/ to boost up the yield of a solar
distiller. Bouchekima /21/ tested capillary film solar
distiller. Ohshiro /22/ tested the effect of wicks made of
polytetrafluoroethylene in a multi effect solar system.
Nabil /23/ had a separate condenser in a pool of water to
obtain 70 percent efficiency. Herbert /24/ developed a
new approach with three sections in a solar still,
namely, evaporator, condenser and absorber cum tripper.
Air circulation and heat recovery were possible with the
design. Badawi /25/ made use of a wiped film rotating disk
evaporator in solar powered distillation plant. Hilal /26/
compared a double effect solar still with a single effect
solar still. Tiwari et al /27/ conducted experiments with
three different conditions of the condensing surface,
which included air conditioned surroundings and by
keeping ice on the condensing surface. Ravindran et al
/28,29/ performed initial tests with Thermoelectrically
Cooled Solar Still.
Thermoelectric Cooling Module: Thermoelectric module
(TEM) is solid state heat pump that dissipates heat
utilizing the Peltier Effect /5/. During operation, DC current
flows through the TEM to create a temperature differential
across the ceramic surfaces, causing one side of the
TEM to be cold and the other side hot. Nowadays
commercial standard single stage TEMs are available to
achieve temperature differentials in the range of 75 to
80°C. Multistage TEMs can produce chillness well
below ambient temperatures, down to minus 100°C.
However, with the modern growth of processing and
methods of semiconductor materials, the limits may
exceed this range.
TEMs have several advantages over alternate
cooling technologies as listed below:
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They have no moving parts, so the solid state
construction results in high reliability.
Thermoelectric are capable of heating or cooling
by simply reversing the polarity, which changes the
direction of heat transfer.
Middle-East J. Sci. Res., 12 (12): 1704-1709, 2012
Precise temperature control, up to ±0.01°C can be
maintained under steady-state conditions.
In heating mode, TEMs are much more effective
than conventional resistant heaters because they
generate heat from the input power supplied plus
additional heat generated by the heat pumping action
that occurs.
TEMs are available in different shapes. Typical
shapes are squares, rectangles, circles and rings.
EMs are available in different sizes. They are as small
as 2 x 2 mm to 62 x 62 mm.
TEMs are light in weight. This makes TEMs ideal for
applications with tight geometric space constraints
or low weight requirements when compared too much
larger cooling technologies, such as conventional
compressor based systems.
TEMs can also be used as a power generator
converting waste heat into energy as a small DC
power sources in remote locations.
TEMs have some disadvantages over alternate
cooling technologies as listed below:
The COP is lower, compared to other heat pumps.
They are suitable for limited temperature differences
only.
In spite of the above mentioned disadvantages,
thermoelectric modules are greatly used, particularly in
developing countries because of their durability, low
maintenance, low cost and clean environment. There is a
lot of scope for developing materials specifically suited
for TE cooling purpose and these can greatly improve the
COP of these devices. Development of new methods to
improve efficiency catering to changes in the basic
design of the thermoelectric module set up like better
heat transfer, miniaturization etc., can give very effective
enhancement in the overall performance of thermoelectric
Module.
The junctions connecting the thermo elements
between the hot and cold plates are interconnected
using highly conducting metal (e.g. copper) strips.
The sizes of conventional thermoelectric devices vary
from 3 mm2 by 4 mm thick to 75 mm2 by 5 mm thick.
Most of thermoelectric modules are not larger than 50 mm
in length due to mechanical consideration. The height of
single stage thermoelectric modules ranges from 1 to 5
mm. The modules contain 3 to 127 thermocouples.
There are
multistage
thermoelectric devices
designed to meet requirements for large temperature
differentials. Multi-stage thermoelectric modules can be
up to 20 mm in height, depending on the number of
stages. Commonly used semiconductor material is
Bismuth telluride.
Modified Solar Still: Solar still was purchased from
Lakshmi Solar Industries, Pune. The model number is CD
105L. TEC module was fixed on the rear of the solar still
near to the centre of the horizontal line of the wall.
Two TEMs of model HT8,F2,4040,TA,W6 were used in
the current test. They were purchased from M/s ISA
IMPEX Pvt. Ltd., Bangalore. The TEMs were fitted on the
backside wall of the solar still. Sealed TEMs were
preferred as the condensation is likely to occur inside the
system also. A pair of small copper plates was used to fix
the TEMs on the thin walls. Silica gel was used to make
the joints leak proof. On the outer side, TEMs were
placed, in such way that the cooler side of the TEMs will
be facing the interior of the solar still while the hot side
shall be pumping heat to the environment. Care was taken
to maintain a good surface contact free from air packets
between the copper and ceramic surfaces of the TEMs.
To achieve this condition a conducting paste was used.
On both the sides of the TEMs, fins / heat sinks were
provided. A small fan, with plastic mountings, was used
to improve the air circulation on the hot side. On the
condensing side, only a heat sink was provided. Both the
units were held tight with the help of bolts and nuts.
A small turf was provided below the heat sinks, in order
to collect the additional water condensed. In fig. 1 and 2
the different Components and the assembled view of the
TEM unit are shown. Figure 6 shows the solar still with
the TEMs mounted in location. The additional water turf
also can be seen.
Instrumenation: As the atmospheric temperature
varies in accordance to the intensity of light from
the sun, a Digital Lux meter was used to measure the
intensity of solar radiation, fig.7. shows the Lux
meter. A pair of alcohol in-glass thermometers was used
to measure the atmospheric air temperature and
temperature inside the solar still. Quantity of water was
measured with a measuring jar of 1.2 litre capacity.
Two 5A DC power sources were used to energize the
TEMs.
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Middle-East J. Sci. Res., 12 (12): 1704-1709, 2012
Date: 16-03-2013
Sl. No.
Fig. 1: Components of the TEM assembly
Intensity of light
Tatm
Time Interval
Amount of water
1200ml
1.
152
25
09:00-10:00
2.
169
26
10:00-11:00
3.
201
30
11:00-12:00
4.
232
31
12:00-01:00
5.
248
31.5
01:00-02:00
6.
241
32
02:00-03:00
7.
170
31
03:00-04:00
8.
154
28
04:00-05:00
Table 2: Amount of water collection at different sunlight intensity with
installed TEC device.
Date: 18-03-2013
Sl.No.
Fig. 2: The TEM assembly
Intensity of light
Tamb
Time Interval
Amount of water
700ml
1.
176
24
9-10
2.
194
26
10-11
3.
255
30
11-12
4.
260
31
12-1
5.
265
31.5
1-2
6.
232
32
2-3
7.
210
32
3-04
8.
181
33
4-5
CONCLUSION
Fig. 3: Fully assembled Solar Still Functioninng
Exprimental Results: Table 1 and 2 show the data of
water accumulated from the solar still before and after
fixing the thermoelectric cooling modules. The water
collection before and after fixing the TEC are 700 ml and
1200 ml respectively. This observation leads to an
estimate of 58 percent improvement in performance.
Table 1: Temperature and amount of water output at various intensity of
sun light
From the tables 1 and 2. It can be seen that
the data of water accumulated from the solar still before
and after fixing the thermoelectric cooling modules are
different. The water collection before and after fixing the
TEC are 700 ml and 1200 ml respectively. This
observation leads to an estimate of improvement or
augment in yield or performance of the Hybrid solar still
to be 58 percent.
The observation, which has been made is only a
starting point in the water production in small quantities.
It has to be explored further deeply.
There is a need for more studies that combine
several techniques, exploiting the best of each and using
these practically.
ACKLOWLEDGEMENT
The author thanks all his colleagues Mrs. B. Sharmila,
Mr. S. Brahmdeo, Mr. C. Mritunjay and Mr. M. Manish for
their support during the research work.
Nomenclature
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Middle-East J. Sci. Res., 12 (12): 1704-1709, 2012
COP
ppm
TEC
TEM
Tatm
WHO
MAW
Coeff. of Performance
parts per million
Thermoelectric Cooling
Thermoelectric Module
atmospheric temperature
World Health Organisation
Ministry of Agriculture and Water
12.
13.
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