The document summarizes an experiment performed on a double slope solar still to analyze its energy and exergy efficiency. Key findings include: - The daily energy efficiency was higher for the system with a south-facing glass cover (29%) compared to the north-facing cover. However, the exergy efficiency was higher for the north-facing cover system. - Equations were developed to calculate the instantaneous and overall thermal efficiency, exergy input and output, and exergy destruction within the system. - The average convective heat transfer coefficient was higher for the north-facing cover (1.85 W/m2K) than the south-facing cover (1.57 W/m2

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
47
ENERGY AND EXERGY ANALYSIS OF A DOUBLE SLOPE SOLAR STILL
Ankur Kumar Singh, Dr. Ajeet Kumar Rai, Vivek Sachan
MED, SSET, Sam Higginbottom Institute of Agriculture Technology and Sciences, Allahabad (U.P.)
India
ABSTRACT
In this work, an attempt has been made to perform energetic and exergetic analysis of a
double slope solar still. Experiments were performed on a single basin double slope solar still in the
premises of SHIATS-DU Allahabad. Energy and exergy balance equations were written for the
system. It is observed that the energy efficiency of the system with south glass cover is higher than
that of north side glass cover, whereas exergy efficiency of the system with north cover is higher
than that of the system exergy efficiency with south side glass cover.The daily energy efficiency of
the system is 29%.
INTRODUCTION
Supplying of fresh water is an urgent need for drinking, cleaning, agriculture and domestic
usages. Nowadays, remote areas and arid zones suffer from water scarcity. Surveys reveal that about
79% of available water is salty, only 1% is fresh and the rest 20% is brackish [1]. Desalination is a
process in which fresh water is produced from saline water. Solar stills are used for water
desalination in remote areas and rural places with low congestion and limited demand. Direct solar
stills use the solar energy to produce distillate directly in the solar collector and the system that
combine conventional desalination system with solar collector are called indirect systems [2]. The
various forms of energy are due to random thermal motion, kinetic energy, potential energy
associated with a restoring force, or the concentration of species relative to a reference state. Exergy
analysis provides a method to evaluate the maximum work extractable from a substance relative to a
reference state (i.e dead state).The reference state is arbitrary ,but for terrestrial energy conversion
the concept of exergy is most effective if it is chosen to reflect the environment on the surface of
earth. Solar radiation is free, abundant, easily available and need no transportation. Distillation is a
natural phenomenon. Solar energy heats water source, evaporates it and condenses by clouds and
back to the earth as rainfall. Solar stills are simulating this natural process in small scale. In order to
establish how much work potential a resource contains, it is necessary to compare it a against a state
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
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defined to have zero work potential. An equilibrium environment which cannot undergo an energy
conversion process to produce work is the technically correct candidate for a reference state
[3].Exergy is the expression for loss of available energy due to the creation of entropy in irreversible
systems or processes. The exergy loss in a system or component is determined by multiplying the
absolute temperature of the surroundings by the entropy increase. Entropy is the ratio of the heat
absorbed by a substance to the absolute temperature at which it was added. While energy is
conserved, exergy is accumulated [4].
EXPERIMENTAL SET-UP
Fig shows the photograph and schematic diagram of a double slope solar still. The
experimental setup consists of a passive solar distillation unit with a glazing glass cover inclined at
2605having an area of 0.048m x 0.096 m. This tilted glass cover of 3 mm thickness, served as solar
energy transmitter as well as a condensing surface for the vapor generated in the basin. Glass basin,
made up of Galvanized Iron, has an effective area of 0.72 m2. The basin of the distiller was
blackened to increase the solar energy absorption. 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. Rubber gasket
was fixed all along the edges of the still. All these 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 system has the capability to collect distillates from two sides of the still (i.e. East &
West sides and North & South sides). Thermocouples were located indifferent places of the still.
They record different temperature, such as inside glass cover & water temperature in the basin and
ambient temperature. In order to study the effect of salinity of the water locally available, table salt
was used at various salinities. All experimental data are used to obtain the internal heat and mass
transfer coefficient for double slope solar still.
Fig 1: Experimental Setup of a Double Slope Solar still

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
49
Performance of double slope solar still
Energy efficiency
Instantaneous efficiency
The expression for instantaneous efficiency (ηi)
ηi =
୫ୣ౭‫୐כ‬
୍ሺ୲ሻ‫כ‬୅౭
Overall thermal efficiency
The expression for overall thermal efficiency (ηpassive)
ηpassive =
∑ ୫‫୐כ‬
୅౭୍ሺ୲ሻୢ୲
Exergy analysis
The general exergy balance for solar still can be written, Hepbalsi (2006)
Exsun – (Exevap +Exwork ) = Exdest
The exergy input to the solar still is radiation and can be written as
Exsun = Exin =Aw * I ( t)*[ 1 - ర
య
( ౐౗
౐౩
) + భ
య
ሺ ౐౗
౐౩
ሻ4
]
The exergy output of a solar still can be written as
Exevap =Aw* hew*( Tw – Tc ) )*[ 1 - ሺ ౐౗
౐౭
ሻ ]
The exergy of work rate for solar still
Exwork = 0
The exergy destructed in solar still can be written as
Exdest = Mw *Cw * ( Tw – Ta )* [ 1 - ሺ ౐౗
౐౭
ሻ ]
The exergy efficiency of solar still us defined, Hapbalsi (2006)
ηEx =
୉୶ୣ୰୥୷ ୭୳୲୮୳୲ ୭୤ ୱ୭୪ୟ୰ ୱ୲୧୪୪
୉୶ୣ୰୥୷ ୧୬୮୳୲ ୭୤ ୱ୭୪ୟ୰ ୱ୲୧୪୪
ሺ୉୶ୣ୴ୟ୮ሻ
ሺ୉୶୧୬ሻ
ηEx = 1 -
୉୶ୢୣୱ୲
୉୶୧୬
hcw=.884[(Tw-Tg)+(Pw-Pg)(Tw+273)/268.9x103
-Pw]1/3
hew=.01623.hcw.(Pw-Pg)/(Tw-Tg)
RESULTS AND DISCUSSION
Fig.2: Variation of Solar Intensity with time of a day
0
200
400
600
800
1000
1200
1400
8:30
9:00
9:30
10:00
10:30
11:00
11:30
12:00
12:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
SolarIntensity(W/m2)
Time of a day(hr)
Solar Intensity(N)
Solar Intensity(S)

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
50
Figure 2 shows the variation of solar intensity on the north side glass cover and south side
glass cover with time of a day. Solar intensity continuously increased and reached its maximum
value in the noon on the both side of the glass cover since south side glass cover is facing sun so
these two covers distant except evening and morning.
Fig.3: Variation of Wind Speed with time of a day
Figure 3 shows variation of wind velocity on a particular day of experiments in the month of may.
Fig.4: Variation of Temperature with time of a day
Figure 4 shows variation of temperature of south side glass, north side glass, water, ambient.
Water temperature is always higher than glass temperature. A max value of water temperature of
530
c is reached at around 2 o’clock.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
WindSpeed(m/s)
Time of a day(hr)
WIND SPEED
0
10
20
30
40
50
60
Temperature(0C)
Time of a day(hr)
Temperatue(S)
Temperatue(N)
Temperatue(W)
Temperatue(A)

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
51
Fig.5: Variation of Distillate with time of a day
Figure 5 shows the variation of distillate outfut for north and side glass cover. Distillate for
north side glass cover is higher than south side glass cover because of temperature difference
between water and glass is higher than temperature difference between water and south side glass.
Fig.6: Variation of Convective Heat Transfer Coefficient with time of a day
Figure 6 shows variation of convective heat transfer coefficient for north side is higher than
convective heat transfer of south side. Average value of convective heat transfer coefficient for north
and south side are 1.85W/m2
K and 1.57W/m2
K.
0
10
20
30
40
50
60
70
80
90
100Distillate(ml)
Time of a day(hr)
Distillate(N)
Distillate(S)
0
0.5
1
1.5
2
2.5
3
ConvectiveHeatTransferCoefficient
(W/m2K)
Time of a day(hr)
Convective Heat Transfer
Coefficient(N)
Convective Heat Transfer
Coefficient(S)

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
52
Fig.7: Variation of Convective Heat Transfer Coefficient with time of a day
Figure 7 shows variation of evaporative heat transfer coefficient for north side is higher than
evaporative heat transfer of south side. Average value of evaporative heat transfer coefficient for
north and south side are 12.30W/m2
K and 12.00m2
K.
Fig.8: Variation of Energy Efficiency with time of a day
Figure 8 shows variation of daily efficiency of north side is 51.11% and south side is 19.62%.
0
5
10
15
20
25
9:00
9:30
10:00
10:30
11:00
11:30
12:00
12:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
EvaporativeHeatTransferCoefficient
(W/m2K)
Time of a day(hr)
Evaporative Heat Transfer
Coefficient(N)
Evaporative Heat Transfer
Coefficient(S)
0
10
20
30
40
50
60
70
80
9:00
9:30
10:00
10:30
11:00
11:30
12:00
12:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
EnergyEffiicency
Time of a day(hr)
Energy Efficency(N)
Energy Efficency(S)

7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 6, June (2014), pp. 47-54 © IAEME
53
Fig.9: Variation of Exergy Efficiency with time of a day
Figure 9 shows variation of daily exergy efficiency of north side is 0.64% and south side is 0.81 %.
CONCLUSION
The following points can be concluded from the present work.
• The energy efficiency for solar still with glass as a condensing cover for north and south are
51.11% and 19.63%.
• The exergy efficiency for solar still with glass as a condensing cover for north and south are
0.64% and 0.81%.
• The exergy efficiency of solar still is lower than energy efficiency due to lower evaporative
heat transfer rate.
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