Experimental Investigation of a Double Slope Solar Still with a Latent Heat Storage Medium

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, Jan - Feb (2013) © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4 Issue 1 January- February (2013), pp. 22-29 IJMET
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2012): 3.8071 (Calculated by GISI)
www.jifactor.com ©IAEME

EXPERIMENTAL INVESTIGATION OF A DOUBLE SLOPE SOLAR
STILL WITH A LATENT HEAT STORAGE MEDIUM

Ajeet Kumar Rai *, Vivek Sachan and Maheep Kumar

Department of Mechanical Engineering, SSET, SHIATS-DU Allahabad, U.P., India
*E-mail raiajeet@rediffmail.com

ABSTRACT

Single basin solar still is a very simple solar device used for converting available
brackish or water into fresh drinking water. This device can be fabricated easily with locally
available materials. The maintenance is also cheap and no skilled labor is required. The
device may be a suitable solution to solve drinking water problem but because of its low
productivity it is not popularly used. Number of works are undertaken to improve the
productivity of the still. The use of latent heat storage system using phase change material
(PCMs) is an effective way of storing thermal energy and has the advantage of high energy
density and the isothermal nature of the storage process. Double slope single basin solar still
is experimented by adding a heat reservoir in the basin using Zinc Nitrate Hexahydrate. It is
a material which changes its phase during addition and removal of heat. It is observe that an
increment of 33.5 % is observed in the collection of distillate when the still is used with PCM
as Zinc Nitrate Hexahydrate.

Key Words: double Slope Solar Still, Phase Change Materials

1.INTRODUCTION

Water is the primary source of life. Next to oxygen, fresh water is the most important
substance for sustaining human life. Water shortage is a worldwide problem, where 40% of
the world population is suffering from water scarcity [1]. Although Water is one of the most
abundant resources on Earth, covering approximately three-quarters of the planet's surface.
About 97% of the Earth's water is salt water in the oceans. 3% of all fresh water is in ground
water, lakes and rivers, which supply most of that needed by humans and animals.

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However, rapid industrial-growth and the population explosion world-wide have
resulted in a large escalation of the demand for fresh water. Added to this is the problem of
pollution of rivers and lakes by industrial wastes and the large amounts of sewage discharged.
On a global scale, man-made pollution of natural sources of water is becoming the single
largest cause for fresh-water shortages. Besides the only inexhaustible sources of water are
the oceans. Their main drawback, however, is the high salinity of such water. It would be
attractive to tackle the water-shortage problem with desalination of this water, which may be
mixed with brackish water increase the amount of fresh water and reduce the concentration of
salts to around500 ppm [2].
Solar distillation has been practiced for many generations. All desalination methods
require fossil fuel or electrical energy but solar distillation is one of many processes that can
be used to produce fresh water by using the heat of the sun directly in a simple equipment to
purify water. The equipment, commonly called a solar still [3,4]. Solar still is most simple
device to get potable/fresh distilled water from impure water. Among other available designs
of solar still, the Double Slope Solar Still is most popular. The construction and design of this
solar still is simple. The problem is poor productivity. A large number of attempts are made
to improve the productivity from solar still. Studies are performed to predict the performance
of solar still [5]. Effect of variation of parameters on the total output is also studied by
various researchers [6].They have analyzed the effect of water depth on the performance of
DSS. Due to intermittent nature of solar energy, distillate production is not continuous and
night time production is almost nil. By using energy storage mediums, distillate may be
produced during non-Sunshine hours. These energy storage systems may store heat energy in
two ways (i) Sensible Heat (ii) Latent Heat. Thermal energy can be stored as a change in
internal energy of a material as sensible heat, latent heat or combination of these two. In
sensible heat storage (SHS), thermal energy is stored by raising the temperature of a solid or
liquid. SHS utilizes the heat capacity and the change in temperature of the material during the
process of charging and discharging. The amount of heat stored depends on the specific heat
of the medium, the temperature change and the amount of storage material [7].

்௙
Q=‫்׬‬௜ ݉‫ܶ݀݌ܥ‬ (1)

=݉‫݌ܥ‬ሺ݂ܶ െ ܶ݅ሻ (2)

LHS is based on the heat absorption or release when a storage material undergoes a phase
Change from solid to liquid or liquid to gas or vice versa. The storage capacity of the LHS
system with a pcm medium is given by-

்௠ ்௙
Q=‫்׬‬௜ ݉‫ ݐ݀݌ܥ‬൅ ݉ܽ݉∆݄݉ ൅ ‫்׬‬௠ ݉‫ݐ݀݌ܥ‬ (3)

Q=݉ሾ‫݌ݏܥ‬ሺܶ݉ െ ܶ݅ሻ ൅ ܽ݉∆݄݉ ൅ ‫݌݈ܥ‬ሺ݂ܶ െ ܶ݉ሻሿ (4)

Due to the compactness of PCMs the latent heat is much higher than the sensible heat. These
materials are still a point of interest for researchers. Lorsch et. al. [8], Lane et. al. [9] and

23


Humphries and Griggs [10] have suggested a wide range of PCMs that can be selected as a
storage media. Latent Heat storage systems are having advantage of their isothermal nature of
storing heat energy. Kuznik et. al [11] has given a good explanation of how PCM stores and
releases latent heat. The external heat supplied to a PCM is spent in breaking the internal
bonds of lattice and thereby it absorbs a huge amount of latent heat at phase temperature.
Abhat et.al.[12] has given a detailed classification of PCMs along with their properties.
Dinser and Rosen [13] have also exercised the same. A large number of phase change
materials (organic, inorganic and eutectic) are available in any required temperature range.
El-Sebaii et al [14] and Shukla et al [15] have used phase change material as a energy storage
medium to study the performance of a single slope solar still. In the present study,
performance of a double slope solar still with Zinc Nitrate Hexahydrate as PCM has been
investigated in outdoor conditions in the month of October. Thermophysical properties of
Zinc Nitrate Hexahydrate are given. Melting temperature 36.10C specific heat (solid) 1.34
kJ/kg0C, specific heat (liquid) 2.26kJ/kg0C, Latent Heat of fusion 147.0 kJ/kg, Thermal
conductivity 0.464W/mK at 39.90C.

2. EXPERIMENTAL SET-UP AND PROCEDURE

2.1 Set-up

Figure 1 shows the photograph of two Double Slope Solar Still of same size and
shape. One DSS is without PCM and another DSS is having PCM in the basin. The DSS
consist of a passive solar distillation unit with a glazing glass cover inclined at 260 having an
area of 0.048m x 0.096 m. The tilted glass covers are of 3 mm thickness, Transmit solar
energy and work as an insulator of heat. It works as a condensing surface for the vapor
generated in the basin. Still basin, made up of Galvanized Iron, has an effective area of 0.72
m2. The basin of the distiller was blackened to increase the absorptive of the basin liner. A
distillate channel was provided at each end of the basin for the collection of distillate output,
a hole was drilled in each of the channels and plastic pipes were fixed through them with an
adhesive (Araldite). An inlet pipe and outlet pipe was provided at the top of the side wall of
the still and at the bottom of the basin tray for feeding saline water into the basin and draining
water from still for cleaning purpose, respectively. All arrangements are made to make the
still air tight. Water gets evaporated and condensed on the inner surface of glass cover. It runs
down the lower edge of the glass cover. The distillate was collected in a bottle and then
measured by a graduated cylinder. The distillate is collected from two sides of the still. Phase
Change Material is filled in the tubes and placed in the basin of the still. Tubes are made of
aluminum to offer little resistance to the heat transfer between water and PCM. Tubes are of
dia 10 mm and length as that of the basin inner side. Thermocouples were attached in
different locations of the still to record the temperatures of inside glass cover, water
temperature in the basin and ambient temperature. All experimental data are used to obtain
the internal heat and mass transfer coefficient for double slope solar still. The effect of use of
phase change material is also studied by comparative analysis.

24


Fig.1 Photograph showing experimental set-up (Double Slope Solar Still with and without PCM)

2.2 Procedure

The experiments were conducted in the campus of Sam Higginbottom Institute of
Agriculture, Technology and Sciences- Deemed University, Allahabad, India. All
experiments were started at 08:30 AM at local time and lasted for 08.30 hours. Water, glass,
water vapor and PCM temperatures were recorded with the help of calibrated Copper –
Constantan thermocouples having a least count of 10C. The ambient temperature is measured
by a calibrated mercury (ZEAL) thermometer having a least count 10C. The distillate output
was recorded with the help of a measuring cylinder of least count 1 ml. The solar intensity
was measured with the help of calibrated solarimeter of a least count of 2 mW/cm2. The
hourly variation of all above mentioned parameters were used to evaluate average values of
each for further numerical computation.

25


3.PRODUCTIVITY AND EFFICIENCY OF THE SOLAR STILL

Energy balance equations are written for the different components of a double slope
solar still without PCM in the basin[5], [6] and with PCM in the basin [14].

The hourly and daily productivity of solar still is

Mewh = hewh (Tw-Tgin)/L (5)

Mewd = ∑24 hrsmewh (6)

The instantaneous efficiency of the solar still is:

ηi = [ hewh (Tw-Tgin)/I (t)]x 100% (7)

ηd = [Mewd hew/(Ap∑I)(∆t)]x 100% (8)

4. RESULTS AND DISCUSSION

1000
900 Solar intensity East
Solar Intensity (W/m2)

800 Solar intensity West
700
600
500
400
300
200
100
0

Time Of the Day (hr)

Fig. 2. Variation of solar intensity with time of the day

Variation of solar intensity falling on the east and west side glass covers of the double slope
solar still for a particular day (17-10-12) is shown in fig.2. It is observed that the solar
intensity falling through east side glass cover is higher till 01:30 hrs. Maximum value is
observed around11:00 hrs on east glass cover.

26


70 Glass cover temp. East with PCM Glass cover temp. East without PCM

60

50
Temperature (C0)

40

30

20

10

0

Time of the day (hr)

Fig.3 Variation of East glass covers temperatures of DSS with PCM and without PCM

Variation of east glass cover temperatures of DSS with PCM and without PCM is shown in
fig 3. It is observed that the DSS with PCM tubes will have higher glass cover temperature in
comparison to that of without PCM. This difference is higher in afternoon session. This is
due to thermal inertial effect produced by PCM tubes. This is also true for the west side glass
cover. A slight rise in temperature of the west side glass cover with PCM is shown in fig.4.
This little difference is due to low solar intensity on west side glass cover.

70 Glass cover temp. West with PCM Glass cover temp. West without PCM
60
Temperature (C0)

50

40

30

20

10

0

Time of a day (hr)

Fig. 4. Variation of West glass covers temperatures of DSS with PCM and without PCM

27


250 total output. with PCM
Total output. without PCM
200
Distillate output (ml)

150

100

50

0

Time of the day (hr)

Fig.5.Variation of total distillate output from DSS with PCM and without PCM

Hourly distillate output is measured in the daytime only from 08:30 AM till 5:00 PM. Last
reading shows total distillate collected during 5:00 PM to 8:00 AM. From fig. 5 it is clear that
output will increase due to use of PCM as heat reservoir. Total gain of 33.5% is observed in
output. Where daytime gain is 34.7% and nocturnal gain is of 31.7%.

5. CONCLUSION

This study explores the possibility of using latent heat energy storage mediums in
conventional solar still to ensure the continuous production of fresh water even after sunset.
Result shows that productivity increases by 30 to 35% with Zinc Nitrate Hexahydrate as
PCM. This can be further improved by a PCM with high latent heat of fusion and increasing
the mass of the PCM in the basin.

REFERENCES

[1]Abdullah S., Badranb O. & Abu-Khaderc M. M. (2008), Performance evaluation of a
modified design of a single slope solar still. Desalination 219pp. 222–230
[2]MalikMAS,Tiwari GN, Kumar A, Sodha MS.Solar distillation.Oxford: Pergamon
Press,1985.
[3]Tiwari, G.N., Tiwari, A.(2007), Solar Distillation Practice for Water Desalination
Systems, Anamaya, New Delhi,.
[4]Argaw N. ( 2001),"Renewable Energy in Water and Wastewater Treatment Applications".
National Renewable Energy Laboratory
[5] Shukla S.K. and Rai A. K. (2008), “Analytical thermal modeling of double slope solar
still using inner glass cover temperature” Thermal Science,vol 12,(3), 139-152.
[6] Rai Ajeet Kumar, Kumar Ashish and Verma Vinod Kumar (2012), “Effect of water depth
and still orientation on productivity of passive solar still”IJMET, vol 3, (2), 740-753.
[7] Rai Ajeet Kumar, Kumar Ashish (2012),” A Review on phase change materials and their
applications”IJARET, vol. 3,(2), 214-225.

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[8] Lorsch HG, Kauffman KW, Denton JC, “Thermal Energy Storage for Heating and Air
Conditioning, Future energy production system”. Heat Mass Transfer Processes; 1: pp 69-85
(1976).
[9] Lane GA, Glew DN, Clark EC, Rossow HE, Quigley SW, Drake SS, et al. “Heat of fusion
system for solar energy storage subsystems for the heating and cooling of building”.
Chalottesville, Virginia, USA, 1975.
[10] Humphries WR, Griggs EI. “ A designing handbook for phase change thermal control
and energy storage devices.” NASA Technical Paper, p. 1074, (1977).
[11] Kuznik F., Virgone, J. and Noel, J.: Optimization of a phase change material wallboard
for building use. Applied Thermal Engineering 28 (2008), 1291-1298.
[12] A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials,
Solar Energy30, pp 313-332 (1983).
[13] Dincer I., Rosen M.A. , Thermal energy storage, Systems and Applications John Willey
and Sons Chichester (England), 2002.
[14] El-Sebaii A. A., Al-Ghamdi A.A., Al-Hazmi F.S. and Faidah A.S. (2009), Thermal
performance of a single basin solar still with PCM as a storage medium, Applied Energy,
86,1187-1195.
[15] Al-Hamadani A.A.F. and Shukla S.K. (2011), Modeling of solar distillation system with
phase change material(PCM) storage medium, thermal science,

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Experimental Investigation of a Double Slope Solar Still with a Latent Heat Storage Medium

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