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Active solar distillationA detailed review
K. Sampathkumar a,*, T.V. Arjunan b, P. Pitchandi a, P. Senthilkumar c
a Department of Mechanical Engineering, Tamilnadu College of Engineering, Coimbatore 641659, Tamilnadu, IndiabDepartment of Automobile Engineering, PSG College of Technology, Coimbatore 641004, Tamilnadu, IndiacDepartment of Mechanical Engineering, KSR College of Engineering, Tiruchengode 637215, Tamilnadu, India
Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15052. Classification of active solar distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505
3. Active solar distillation system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505
3.1. High temperature active solar distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506
3.1.1. Solar still coupled with flat plate collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506
3.1.2. Solar still coupled with parabolic concentrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509
3.1.3. Solar still coupled with evacuated tube collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1511
3.1.4. Solar still coupled with heat pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1511
3.1.5. Solar still coupled with solar pond. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512
3.1.6. Solar still coupled with hybridPV/Tsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512
3.1.7. Multistage active solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1513
3.1.8. Multi effect active solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1514
3.1.9. Air bubbled solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515
3.1.10. Hybrid solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515
3.2. Pre-heated water active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515
3.3. Nocturnal active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15164. Theoretical analysis of active solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517
4.1. Heat transfer in active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517
4.1.1. Internal heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517
4.1.2. External heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519
4.2. Thermal modelling of active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519
4.2.1. Inner and outer surface of glass cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519
4.2.2. Inner surface of glass cover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520
4.2.3. Outer surface of glass cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520
Renewable and Sustainable Energy Reviews 14 (2010) 15031526
A R T I C L E I N F O
Article history:
Received 6 November 2009
Received in revised form 15 December 2009Accepted 25 January 2010
Keywords:
Active solar still
Desalination
Flat plate collector
Review
Solar pond
Thermal modelling
A B S T R A C T
All over the world, access to potable water to the people are narrowing down day by day. Most of the
human diseases are due to polluted or non-purified water resources. Even today, under developed
countries and developing countries face a huge water scarcity because of unplanned mechanism and
pollution created by manmade activities.Water purification without affecting the ecosystem is the need
of the hour. In this context, many conventional and non-conventional techniques have been developed
for purification of saline water. Among these, solar distillation proves to be both economical and eco-
friendly technique particularly in rural areas. Many active distillation systems have been developed to
overcome the problem of lower distillate output in passive solar stills. This article provides a detailed
review of differentstudies on activesolar distillation system over the years. Thermal modellingwas done
forvarioustypesof activesingle slope solar distillation system. This reviewwould also throw light on the
scope for further research and recommendations in active solar distillation system.
2010 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +91 421 2332544; fax: +91 421 2332244.
E-mail addresses: [email protected](K. Sampathkumar), [email protected](P. Senthilkumar).
Contents lists available atScienceDirect
Renewable and Sustainable Energy Reviews
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / r s e r
1364-0321/$ see front matter 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.rser.2010.01.023
mailto:[email protected]:[email protected]://www.sciencedirect.com/science/journal/13640321http://dx.doi.org/10.1016/j.rser.2010.01.023http://dx.doi.org/10.1016/j.rser.2010.01.023http://www.sciencedirect.com/science/journal/13640321mailto:[email protected]:[email protected]8/21/2019 Active Solar Distillationa Detailed Review (Articulo)
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4.2.4. Basin liner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520
4.2.5. Water mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520
5. Discussion and scope for further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1524
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1524
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1525
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1525
Nomenclature
Aa aperture area of concentrating collector (m2)
Ac area of solar collector (m2)
AET absorber tube diameter times collector length in
ETC (m2)
Ar receiver area of concentrating collector (m2)
Ass area of sides in solar still (m2)
As area of basin in solar still (m2)
C constant in Nusselt number expression
Cp specific heat of vapour (J/kg 8C)
Cw specific heat of water in solar still (J/kg 8C)
FR heat removal factor
g acceleration due to gravity (m/s2)Gr Grashof number
hc;ba convective heat transfer coefficient from basin to
ambient (W/m2 8C)
hr;ba radiative heat transfer coefficient from basin to
ambient (W/m2 8C)
ht;ba total heat transfer coefficient from basin to
ambient (W/m2 8C)
hc;ga convective heat transfer coefficient from glass
cover to ambient (W/m2 8C)
hr;ga radiative heat transfer coefficient from glass cover
to ambient (W/m2 8C)
ht;ga total (convective and radiative) heat transfer
coefficient from glass cover to ambient (W/m2 8C)hc;wg convective heat transfer coefficient from water to
glass cover (W/m2 8C)
he;wg evaporative heat transfer coefficient from water to
glass cover (W/m2 8C)
hr;wg radiative heat transfer coefficient from water to
glass cover (W/m2 8C)
ht;wg total heat transfer coefficient from water to glass
cover (W/m2 8C)
hw convective heat transfer coefficient from basin
liner to water (W/m2 8C)
hb overall heat transfer coefficient from basin
liner to ambient through bottom insulation (W/
m2 8C)I(t)c intensity of solarradiation over the inclined surface
of the solar collector (W/m2)
I(t)s intensity of solarradiation over the inclined surface
of the solar still (W/m2)
Ki thermal conductivity of insulation material (W/
m 8C)
Kg thermal conductivity of glass cover (W/m 8C)
Kv thermal conductivity of humid air (W/m 8C)
Kw thermal conductivity of water (W/m 8C)
L latent heat of vaporization (J/kg)
Li thickness of insulation material (m)
Lg thickness of insulation glass cover (m)
Ma molecular weight of dry air (kg/mol)
mew hourly output from solar still (kg/m2 h)
Mew daily output from solar still (kg/m2 day)
Mw mass of water in the basin (kg)
Mwv molecular weight of water vapour (kg/mol)
n constant in Nusselt number expression
Pgi partial vapour pressure at inner surface
glass temperature (N/m2)
Pr Prandtl number
Pt total vapour pressure in the basin (N/m2)
Pw partial vapour pressure at water temperature (N/
m2)
qc;wg
rate of convective heat transfer from water to glass
cover (W/m2)
qe;wg rate of evaporative heat transfer from water to
glass cover (W/m2)
qr;wg rate of radiative heat transfer from water to glass
cover (W/m2)
qt;wg rate of total heat transfer from water to glass cover
(W/m2)
qr;ga rate of radiative heat transfertfrom glass cover to
ambient (W/m2)
qc;ga rate of convective heat transfer from glass cover to
ambient (W/m2)
qt;ga rate of total heat transfer from glass cover to
ambient (W/m2)
qw rate of convective heat transfer from basin liner to
water (W/m2)
qb rate of heat transfer from basin liner to ambient
(W/m2)
Qu useful thermal energy gain from the solar collector
(W/m2)
Ra Rayleigh number
Ra0 modified Rayleigh number
t time (s)
Ta ambient temperature (8C)
Tb basin temperature (8C)
Tgi inner surface glass cover temperature (8C)
Tgo outer surface glass cover temperature (8C)
Tsky temperature of sky (8C)
Tw water temperature (8C)
DT temperature difference between water and glass
surface (8C)
Ub overall bottom heat loss coefficient (W/m2
8C)
Us overall side heat loss coefficient (W/m2
8C)
ULC overall heat transfer coefficient for solar collector
(W/m2 8C)
ULS overall heat transfer coefficient for solar still (W/
m2 8C)
Ut overall top heat loss coefficient from water surface
to ambient air (W/m2 8C)
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1. Introduction
Water is a natures gift and it plays a key role in the
development of an economy and in turn for the welfare of a
nation. Non-availability of drinking water is one of the major
problem faced by both the under developed and developingcountries all over the world. Around 97% of the water in the world
is in the ocean, approximately 2% of the water in the world is at
present storedas ice in polar region, and 1% is fresh water available
for the need of the plants, animals and human life [1]. Today,
majority of the health issues are owing to the non-availability of
clean drinking water.In therecent decades,most partsof theworld
receive insufficient rainfall resulting in increase in the water
salinity. The pollution of water resources is increasing drastically
due to a number of factors including growth in the population,
industrialization, urbanization, etc. These activities adversely
affected the water quality in rural areas and agriculture. Globally,
200 million hours are spent each day, mostly by females, to collect
water from distant, often polluted sources. In the world, 3.575
million people die each year from water related diseases. The basicmedical facilities never spotted numerous villages in the develop-
ing andunder developedcountries. Majority of the rural people are
still unaware of the consequences of drinking untreated water.
Desalination is the oldest technology used by people for water
purification in the world. Various technologies were invented for
desalination from time to time and it has been accepted by people
without knowing future environmental consequences. Major
desalination techniques like vapour compression distillation,
reverse osmosis and electrolysis used electricity as input energy.
But in the recent years, most of the countries in the world have
been significantly affected by energy crisis because of heavy
dependency on conventional energy sources (coal power plants,
fossil fuels, etc.), which has directly affected the environment and
economic growthof these countries. Thechanging climate is one of
the major challenges the entire world is facing today. Gradual rise
in globalaverage temperatures, increase in sea level and melting of
glaciers and ice sheets have underlined the immediate need to
address the issue. All these problems could be solved only through
efficient and effective utilization of renewable energy resources
such as solar, wind, biomass, tidal, and geothermal energy, etc.
Solar energy is available in abundant in most of the rural areas
andhence solar distillation is thebest solution forruralareas andhas
many advantages of using freelyavailable solar energy.It is a simple
technology and more economical than the other available methods.
A solar still operates similar to the natural hydrologic cycle of
evaporation and condensation. The basin of the solar still is filled
with impure water and the sun rays are passed through the glass
cover to heat the water in the basin and the water gets evaporated.
As the water inside the solar still evaporates, it leaves all
contaminates and microbes in the basin. The purified water vapour
condenses on the inner side of theglass, runs through thelower side
of the still and then gets collected in a closed container [2]. Many
solar distillation systems were developed over the years using the
above principle for water purification in many parts of the world.
Thispaperreviewsthe technological developments of various active
solar distillationsystemsdevelopedby various researchers in detail.
The review also extends to thermal modelling of some active solar
distillation systems, comparative studies of different active solarstills, scope for further research and recommendation.
2. Classification of active solar distillation
The solar distillation systems are mainly classified as passive
solar still and active solar still. The numerous parameters are
affecting the performance of the still such as water depth in the
basin, material of the basin, wind velocity, solar radiation, ambient
temperature and inclination angle. The productivity of any type of
solar still will be determined by the temperature difference
between the water in the basin and inner surface glass cover. In a
passive solar still, the solar radiation is received directly by the
basin water and is the only source of energy for raising the water
temperature and consequently, the evaporation leading to a lowerproductivity. This is the main drawback of a passive solar still.
Later, in order to overcome the above problem, many active solar
stills have been developed. Here, an extra thermal energy is
supplied to the basin through an external mode to increase the
evaporation rate and in turn improve its productivity. The active
solar distillation is mainly classified as follows[2]:
(i) High temperature distillationHot water will be fed into the
basin from a solar collector panel.
(ii) Pre-heated water applicationHot water will be fed into the
basin at a constant flow rate.
(iii) Nocturnal productionHot water will be fed into the basin
once in a day.
3. Active solar distillation system
The performance of a solar still could neither be predicted nor
improved by some of the uncontrollable parameters like intensity
of solar radiation, ambient temperature and wind velocity. But,
there are certain parameters such as depth of water, glass cover
angle, fabrication materials, temperature of water in the basin and
insulation thickness, which affects the performance of the solar
still that could be modified forimproving theperformance. Thestill
performance can be increased by reducing the water depth and
thereby increasing the evaporation rate. The temperature differ-
ence between water in the basin and condensing glass cover also
has a direct effect in the performance of the still. The increased
temperature of the water in basin can increase the temperature
v wind velocity (m/s)
Xv mean characteristic length of solar still between
evaporation and condensation surface (m)
Xw mean characteristic length of solar still between
basin and water surface (m)
Greek letters
a absorptivityav thermal diffusivity of water vapour (m
2/s)
a0 fraction of energy absorbed
(at) absorptancetransmittance product
b coefficient of volumetric thermal expansion factor
(1/K)
e emissivity
g relative humidity
mv viscosity of humid air (Pa s)
rv density of vapour (kg/m3)
s Stefan Boltzman constant (5.67108 W/m2 K4)
Subscripts
a ambientb basin liner
c collector
eff effective
g glass cover
s solar still
w water
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difference between the evaporating and condensing surfaces. To
achieve better evaporation and condensation rate,the temperature
of water in the basin could be raised by feeding thermal energy
from some external sources.
3.1. High temperature active solar distillation
The water temperature of the conventional still is increased by
supplying additionalthermal energy through solarcollectors to the
basin. The temperature is increased from 2050 8C to 7080 8C in
high temperature distillation for better evaporation.
3.1.1. Solar still coupled with flat plate collector
Thesolarstill coupled with flatplatecollector is working as high
temperature distillation method. The solar still coupled with flat
plate collector (FPC) works either in forced circulation mode or
natural circulation mode. In forced circulation mode, a pump is
used for supplying water. In natural circulation mode, water flows
due to the difference in the density of water.
3.1.1.1. Forced circulation mode. The flat plate collector gives an
additional thermal energy to the basin of the solar still. A pump is
used to circulate the water from the basin via flat plate collector to
the basin. Many researches havebeen carried out in this methodandthe first being reported by Rai and Tiwari[3]. They found that, the
daily distillateproduction of a coupledsinglebasin still is 24% higher
than that of an uncoupled one using forced circulation mode. A
schematic diagram of an active solarstill integrated with a flat plate
collector under forced circulation mode is shown in Fig. 1. Rai et al.
[4] experimentally studied the various modes of operations in single
basin solar still coupled with flat plate collector. From their study
shows that, the rate of daily distillate deceases with the salt
concentration. The addition of salt increases the surface tension and
hence decreases the rate of evaporation. The best performance was
observed in a single basin still coupled with a flat plate collector
having forced circulation and blackened jute cloth floating over the
basin water anda small quantity of black dyeadded to thewater. And
alsofoundthat, therate of distillation increased by 30%when a smallquantityof blackdye is added to the water. The bottom insulation is
an important design parameter of the active solar still and for
drinking purposes, the conventional solar still will give better
performance because, the efficiency of the system reduces with the
increase in the effective area as reported by Tiwari and Dhiman[5].
Their experimental study showed that, there was only 12% rise in
yieldof the system if the length of the heat exchanger is varied from
6.0 to 12.0 m and the overallefficiency of the system variedfrom 15
to 19%.
Sanjeev Kumar and Tiwari [6] observed that, temperature of
water and thermal efficiency decreased with an increase in basin
area dueto thelargestorage capacity of thewatermass in thebasin
and depth of water, respectively. Yield increased with increase in
the number of collectors, as expected, owing to increased heat
transfer from the collector panel into the basin and the optimum
number of collectors for maximum yield is 8 m2 since beyond that
the increase in gain will be lower than the thermal loss. Sanjeev
Kumar et al.[7] suggested that, for maximum annual yield, the
optimum collector inclination for a flat plate collector is 208 and
that of still glass cover is 158 for New Delhi climatic condition.
Tiwari et al. [8] inferred that, the internal heat transfer
coefficients should be determined by using inner glass cover
temperature for thermal modelling of passive and active solar
stills. Theheat transfer coefficients mainly depends on the shape of
the condensing cover, material of the condensing cover and
temperature difference between water and inner glass cover. On
the basis of the numerical computation,Singh andTiwari [9] found
that, the annual yield is at its maximum when the condensing glass
cover inclination is equal to the latitude of the place and the
optimum collector inclination for a flat plate collector is 28.588, for
a condensing glass cover inclination of 18.588 for New Delhi
climatic condition.Rajesh Tripathi and Tiwari [10] inferred that the
convective heat transfer coefficient between water and innercondensing cover depends significantly on the water depth of the
basin. It is also observed that more productivity was obtained
during the off shine hours as compared to day time for higher
water depths in solar still (0.10 mand 0.15 m)due to storage effect.
Vimal Dimri et al.[11] conducted theoretical and experimental
analysis of a solar still integrated with flat plate collector with
various condensing cover materials. The results indicated that
yield is directly related to thermal conductivity of condensing
cover materials; coppergives a greater yield compared to glass and
plastic due to higher thermal conductivity.
Tiwariet al. [12] presented theparametric study of passive and
active solar stills integrated with a flat plate collector. Computer
based thermal models were developed based on two assumptions:
Tgi=Tgoand Tgi6Tgo. The results show that (i) there is an effect ofthe inner and outer glass temperature on the daily yield of both
active and passive solar stills. (ii) The mean estimated error
involved in predicting the hourly yield of the passive solar still and
active solar still using the thermal model based on the assumption
that Tgi=Tgo is 6% and3%, respectively. Hence,the thermal model of
solar stills should be developed based on the assumption that
Tgi6Tgo. (iii) The results of the thermal model for the active solar
still forN= 1 show that the daily yield values are 3.08 l and 2.85 l
for Tgi=Tgo and Tgi6Tgo, respectively. Tiwari and Tiwari [2]
reported the performance of single slope passive still coupled with
multi flat plate collectors. In their newdesign, rather than coupling
a single collector, multiple collectors were integrated with the
solar still. The results show that, for New Delhi climatic conditions,
the daily yield increases with number of collectors for basin area1 m2, collector area 2 m2, mass of saline water 150 kg and also the
optimum number of collectors for single effect, double effect and
triple effect were 10, 9 and 6, respectively. In single effect, if there
are more than 8 collectors, the daily yield is higher than the double
effect but at the cost of additional collectors.
3.1.1.2. Natural circulation mode. The working of solar thermal
devices under thermosyphon mode has been more advantageous
than the forced circulation mode in terms of simplicity, reliability
and cost effectiveness. Theoretical study on single basin solar still
coupled with flatplate collectorthrough heat exchanger have been
reported by Lawrence and Tiwari[13]. The results show that, the
efficiency of active solar still is less than that of a simple solar still
and the daily yield from the simple solar still decreases with theFig. 1.Schematic of an active solar still integrated with a flat plate collector [12].
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increase in water depth, while for an active solar still, it is the
reverse (Fig. 2).
Yadav [14] studied the performance of a solar still coupled with
a flat plate collector using thermosyphon mode and forced
circulation mode for New Delhi climatic condition. The author
found that, the system using the forced circulation mode gives 5
10% higher yield than that of the thermosyphon mode and 3035%enhancement in the yield was observed with simple solar still. The
steady state condition of the system was achieved after 23 days.
Yadav[15]studied the transient performance of a high tempera-
ture solar distillation system. The study reveals that it is
worthwhile to consider a temperature dependent evaporative
heat transfer coefficient when evaluating the performance of a
high temperature distillation. Tiris et al. [16] conducted
experiments on two flat plate solar collectors integrated with a
basin type solar still. From their study, the collector integrated
solar still gave an average increase of 100% in yield in comparison
with the simple basin solar still. Maximum yield was 2.575 l/
m2 day for the simple basin and 5.18 l/m2 day for the integrated
system, while the corresponding solar radiation is 24.343 MJ/
m2
day.Ali A Badran et al. [17] performed the tests in solar still
augmented with flat plate collector using tap water and saline
water.They found that themass of distilledwater production using
augmentation increased by 231% in case of tap water as a feed and
by 52% in case of salt water as a feed. Badran and Al-Tahainesh [18]
presented the effect of coupling a flat plate collector on the solar
still productivity. The results showed that, the output of the still is
maximum for the least water depth in the basin (2 cm). Also, the
increase in water depth has decreased the productivity, while the
still productivity is found to be proportional to the solar radiation
intensity.
Dwidevi and Tiwari [19] experimentally studied the double
slope active solar still under natural circulation mode. From the
study, they observed that, the double slope active solar still under
natural circulation modes gives 51% higher yield in comparison to
the double slope passive solar still. The thermal efficiency of
double slope activesolar still is lower than thethermal efficiencyof
double slope passive solar still. However, the exergy efficiency of
double slope active solar still is higher than the exergy efficiency ofdouble slope passive solar still.
3.1.1.3. Double effect active solar still. Glass temperature is another
main parameter, which affects the performance of the solar still.
The rate of evaporation increased with reduction of glass
temperature. The rate of evaporation of water from a water
surface will be higher than the rate of release of heat from the glass
cover to ambient by convection and radiation processes. If the heat
loss from glass cover to ambient can be increasedand that heat loss
is used for further distillation, then overall efficiency of the
distillation unit under active modes of operation can be increased
significantly, as in the case of double basin solar still. This can be
obtained by flowing the water over the glass cover for fast heat
transfer through the lower glass cover and then condensing theevaporated water from the upper glass cover as distillate ( Fig. 3).
Tiwari and Lawrence [20] observed from the experimental
study that, there is an increase of about 20% and 30% yield for inlet
temperature equal to ambient temperature fora passive and active
solar still, respectively. If the inlet temperature is increased, the
output from the upper basin is increased but the output from the
lower basin is appreciably reduced due to a lower value of water-
glass temperature difference in the lower basin. Bapeshwararao et
al.[21]presented from transient analysis that the distillate output
increases with increase in the initial water temperature in both
basins, the dependence on lower basin water temperature shows
more effect than that of upper one comparatively and remarkable
increase in the efficiency of the present system over that of the
simple solar still in all the cases. Tiwari and Sharma[22]studiedthe double effect solar distillation under active mode of operation
using heat exchanger. The study shows that, there is an increase of
about 30% in the active solar still due to water flow through the
upper basin and there is a marginal increase in efficiency with
increase in the length of the heat exchanger.
Kumar Sanjeev and Tiwari[23]presented the performance of
daily yield for an active double effect distillation system with
water flow. The results show that, a higher yield from the lower
basin with a maximum yield of 3.34 kg/m2/h at noon is due to the
high water temperature of 95 8C at that time (Fig. 4). With the
increase in water masses, the operating water temperature in the
lower basin is lowered resulting in reduced yield and efficiency.
Thedaily yield increases with an increase of collectorarea,because
the thermal energy in the basin increases as the collector area
Fig. 2.Schematic diagram of (a) active solar still working under natural circulation;
(b) design of heat exchanger[13].
Fig. 3.Schematic of double effect solar still coupled with flat plate collector [23].
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increases. Sanjay Kumar and Tiwari[24]studied the performance
of single and double effect active solar distillation, with and
without water flow over the glass cover. The study shows that, an
active solar still with water flow arrangements over the glass cover
produces maximum distillate output. The solar still operating inthe double effect mode does not enhance the daily output
significantly because of the difficulties in maintaining reasonably
low and uniform flow rates over the glass cover (10 ml/min).
Sanjay Kumar and Tiwari [25] conducted experiments to
estimate the convective mass transfer in active solar still. The
modified values of C and n for Nu=C(GrPr)n, are proposed as
C = 0.0538; n = 0.383 for 5.498106
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thermosyphon mode of operation for New Delhi climatic condi-
tion. The authors inferred that, (i) there is a significant improve-
ment in overall performance dueto water flow over theglass cover.
(ii) The hot water available due to the regenerative effect does not
enhance the output. (iii) The overall efficiency of the active stills
(conventional and regenerative) is lower than that of the passive
stills (conventional and regenerative) at any common depth of
water because the active stills areoperating at higher temperature.
Tiwari et al. [29] observed that the instantaneous thermal
efficiency of the system decreases with an increase of collectorarea, due to the higher operating temperature range of the
distillation system. Yousef H. Zurigat et al.[30]proved that, the
thickness of water on top of the first glass cover and the mass flow
rate of the water going into the second effect have marginal effect
on the productivity of the regenerative solar still.
3.1.1.5. Solar still coupled with parallel flat plate collector. Yadav and
Prasad[31]experimentally studied the solar still integrated with
parallel flat plate collector. The schematic diagram of a solar still
integrated with a parallel flat plate solar energy collector is shown
in Fig. 7. The collector essentially consists of a parallel flat plate
placed over the insulation with an air gap through which the water
will flow below the absorber.
There is a glass sheet over the absorber and the whole assemblyis enclosed in a wooden box. The top of the plate (absorber) is
blackenedby black boardpaintbefore theglasscoveris placedover
the absorber. The collector outlet is connected to the still by a pipe
covered with insulation. The circulation of water between the
collector and the still can be made either via a pump (forced
circulation system) or by placing the collector over a supporting
structure at such a height as to provide adequate head for natural
circulation of water (thermosyphon) in the system. The results
show that, a significant rise in the distillate output is observed
when the still is coupled with the collector and this system can be
preferred as cost effective compared to the flat plate collector.
3.1.1.6. Vertical solar still coupled with flat plate collector. Kiatsir-
iroat et al. [32]analysed the multiple effect of vertical solar still
coupled with flat plate solar collector. The schematic sketch is
shown inFig. 8. The distillation unit consists of n parallel vertical
plates. The first plate is insulated on its front side and the last plate
is exposed to ambient.
Each plate in the enclosure is covered with wetted cloth on one
side. Theclothis extended into a feed through along theupperedge
of each plate. Feed water in the through is then drawn onto the
plate surface by capillary. Excess water moves down the plate and
is conducted out of the still. The last plate is cooled by air or water.
The authors found that, the distillation output increases slightlywhen the plate number is over5, and it increased by about 34% and
15% when the evaporating plate numbers are 1 and 6, respectively.
3.1.2. Solar still coupled with parabolic concentrator
The schematic diagram of the solar still coupled with parabolic
concentrator is shown in Fig. 9. The parabolic shaped concentrator
or solar collector concentrates the incident solar radiation on large
surface and it focuses on to a small absorber or receiver area. The
performance of concentrators is much affected by the sun tracking
mechanism. The tracking mechanism should move the collectors
throughout the day to keep them focused on the sun rays to
achieve the higher efficiency. These types of solar collectors reach
higher temperature compared to flat plate collectors owing to
reduced heat loss area.The various types of concentrators were used over the years
based on the applications. To achieve higheryield, the contractor is
coupled with solar still by means of increasing water temperature
in the basin. The water or oil will be supplied to trough receiver
pipe by natural circulation mode or forced circulation mode. Singh
et al.[33]found an analytical expression for water temperature of
an active solar still with flat plate collectors and parabolic
concentrator through natural circulation mode.
The results show that, the efficiency of the system with
concentrator is higher than parabolic collector as the evaporative
heat transfer coefficient is higher in concentrator. Garcia Rodriguez
and Gomez Camacho[34]experimentally studied the multi effect
Fig. 7. Schematic diagram of a solar still integrated with a parallel flat plate water collector[31].
Fig. 8.Schematic sketch of the multiple effects still with a flat plate collector [32].
Fig. 9.Solar still coupled with parabolic concentrator. (1) Parabolic through, (2) oil
pipeline, (3) valves, (4) solar still, (5) oil heat exchanger, (6) pump [36].
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distillation system coupled to a parabolic through collector (PTC)
for sea water desalination and suggested the following, (i) the
annual energy production is about 23% grater for a northsouth
collectorthan for an east west one. (ii) The optimum axis height for
a single collector is 298 and it is 12% higher production than a
horizontal collector for an inlet/outlet thermal oil temperature of
225 8C/300 8C. (iii) The maximum yearly average of the dailyoperation time is only about 12 h/day in coastal areas in southern
Spain.
Scrivani et al.[35]presented the concept of utilizing through
type solar concentration plants for water production, remediation,
waste treatment and the system can be used for processing landfill
percolate in arid regions where conventional depuration systems
are expensive and impractical. Zeinab and Ashraf[36]conducted
experimental and theoretical study of a solar desalination system
coupled with solar parabolic through with a focal pipe and simple
heat exchanger (Fig. 9). The results show that, as time goes on, all
the temperatures increase and begin to decrease after 4.00 pm
with respect to the solar radiation, although the temperature
values of themodifiedsystem arestillhigher than theconventional
one. In case of the modified design, the fresh water productivityincreased an average by 18%.
Bechir Chaouchi et al. [37] designed and built a small solar
desalination unit equipped with a parabolic concentrator (Fig. 10).
The results show that, the maximum efficiency corresponds to the
maximum solar lightning obtained towards 14:00. At that hour, the
boiler was nearly in a horizontal position, which maximizes the
offeredheat transfersurface. The experimentaland theoretical study
concluded withan averagerelative error of 42%for thedistillateflow
rate. Thisis dueto theimperfectionsin paraboloidgeometry,the sun
manual follow up and especially to the systems tiltvariation during
theday, which doesnot make it possible always to keepthe absorber
surface covered with salted water. Lourdes Garcia Rodriguez et al.
[38] proposed and evaluated the application of direct steam
generation into a solar parabolic through collector to multi effect
distillation. The obtained results were useful in finding the most
suitable conditions in which solar energy could compete with
conventional energies in solar desalination.
3.1.2.1. Double effect still coupled with parabolic concentrator. B-
hagwan Prasad and Tiwari[39]presented an analysis of a double
effect, solar distillation unit coupled compound parabolic concen-
tration (CPC) collector under forced circulation mode (Fig. 11).The
authors suggested that, (i) the temperature of the water in the
lower basin is increased in comparison with single effect
distillation due to the reduced upward heat losses. (ii) The hourly
output in the lower basin is reduced due to the reduced
temperature differencebetween the waterand glass temperatures.
However, the overall output is increased due to reutilization of the
latentheat of evaporation in thesecond effect. (iii) Thehourly yield
from thelowerbasin increases with increase of flowvelocitydue to
the decrease in the lower glass temperature. It is due to the factthat the lower glass cover temperature decreases due to the fast
removal of the latent heat of vaporization. (iv) The evaporative
heat transfer coefficient is a strong function of the operating
temperature range. The convective and radiative heat transfer
coefficients does not vary significantly.
3.1.2.2. Regenerative solar still coupled with parabolic concentra-
tor. Flowing water over the glass cover is made to reduce glass
temperature of the solar still. Heat is transferred from the glass to
the flowing water which, in turn keeps the temperature difference
large. This regenerative effect helps to achieve higher productivity
of the solar still.
Sanjay Kumar and Sinha [40] conducted the experimental
analysis of a double slope solar still coupled with a non-trackingcylindrical parabolic concentrator through an electric pump. The
system operates in a forced circulation mode to avoid the inherent
problems associated with a thermosyphon circulation mode. The
authors observed that, the concentrator coupled still gives the
maximum yield at all depths of the basin water (Fig. 12).
The concentrator assisted regenerative solar still has a much
higher thermal efficiency than the flat plate collector assisted
Fig. 10. Desalination by a parabolic solar concentrator [37].
Fig. 11. Cross-sectional view of double effect active distillation system[39].
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regenerative still at all water depths and they inferred that there is
less thermal loss in the concentrator compared to the flat plate
collector panel. From the analysis, an increase in the flow rate of
cold water over the glass cover also increases the overall thermalefficiency, followed by significant increase in its yield. Lourdes
Garcia Rodriquez et al. [41] studied the global analysis of the use of
solar energy in seawater distillation under Spanish climatic
condition. They considered the following solar energy collectors
for the analysis: salinity gradient solar ponds, flat plate collectors,
evacuated tube collectors, compound parabolic collectors and
parabolic through collectors for direct steam generation (DSG).
Each of the collectors were compared for the parameters like, the
fresh water production from a given desalination plant, attainable
fresh water production if a heat pump is coupled to the solar
desalination unit and area of solar collector required. Results
showed that direct steam generation parabolic through was a
promising technology for solar assisted seawater desalination.
3.1.3. Solar still coupled with evacuated tube collector
The evacuated tube solar collector has more advantageous than
theflat plate collectors forwaterheating purposes.Evacuated Tube
Collectors (ETC) are well known for their higher efficiencies when
compared to flat plate solar collectors. In flat plate collectors, sun
rays are perpendicular to the collector only at noon and thus a
proportion of the sunlight striking the surface of the collector is
always likely to be reflected. Butin evacuatedtube collector, dueto
its cylindrical shape, the sun rays are perpendicular to the surface
of the glass for most of the day. The evacuated tubes greatly reduce
the heat losses as vacuum is present in the tubes. Owens-Illinois
(OI) evacuated tube collector is shown inFig. 13.
The OI collector consists of two coaxial tubes with evacuated
space between an outer surface of inner tube and inner surface ofouter tube. A selective coating is applied to the outer surface of the
inner tube. The heat transfer fluid enters through small diameter
delivery glass tube andexits from thesame endof thetube through
annular space between delivery tube and selective coated absorber
tube (which is sealed from one end). The annular space between
selectively coated tube and borosilicate outermost glass tube is
evacuated to minimize the convection loss from the selectivesurface.
Tiwari et al.[42]developed the thermal models for all types of
solarcollectorintegrated active solarstills basedon energy balance
equations in terms of inner and outer glass temperature. The total
daily yield of passive solar still, FPC, concentrating collector, ETC
and ETC with heat pipe is shown inFig. 14.
The authors have drawn the following points: (i) the maximum
values of total heat transfer coefficient (htw) for active solar stills
integrated with flat plate collector, concentrating collector,
evacuated tube collector and ETC with heat pipe are 43, 86, 67
and76 W m2 8C1, respectively. (ii) The overall thermal efficiency
of active solar stills integrated with FPC, concentrating collector,
ETC and ETC with heat pipe is 13.14, 17.57, 17.22 and 18.26%,
respectively. (iii) The overall average thermal and exergy efficiencyof FPC integrated active solar still are in the range of 5.619.1 and
0.250.85%, respectively. If the exergy out from FPC is considered,
then average exergy efficiency of active solar still varies in the
range 0.591.82%.
3.1.4. Solar still coupled with heat pipe
Hiroshi and Yasuhito [43] proposed the newly designed,
compact multiple effect diffusion type solar still consisting of a
heat pipe solar collector and a number of vertical and parallel
partitions in contact with saline soaked wicks. The system consists
of a heat pipe solar collectorand a Vertical MultipleEffect Diffusion
type (VMED) still. The solar collector consists of a glass cover and
collector plate, on which the selective absorbing film is attached,
with an air gap between them. Copper tubes, are attached to theunder surface of the collector plate with a fixed pitch.
VMED still consists of vertical and parallel partitions with
narrow air gaps between them, and the partitions, with the
exception of the outside one, are in contact with saline soaked
wicks. Saline water is constantly fed to the wicks. The copper plate
is in front of the first partition with a narrow gap. The gap becomes
the condensing path of the working fluid. The condensing path in
front of first partitionand theevaporating coppertubes attached to
the under surface of the collector plate are connected with two
connecting pipes, so that a closed loop between the solar collector
and VMED still is formed. The constant mass of ethanol liquid is
charged into the closed loop and the closed loop is evacuated with
an evacuating pump. The front surface of the VMED still and the
under surface of collector plate are insulated.
Fig. 12.Variation of daily yield with water depth of still [40].
Fig. 13. Schematic diagram of Owens-Illinois evacuated tube collector[42].
Fig. 14. Total daily yield for active solar stills [42].
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The solar radiation transmits through the glass cover and is
absorbed on the collector plate and ethanol in the evaporating
copper tubes attached to the under surface of the collector plate isheated up and evaporated. Ethanol vapour goes through the upper
connecting pipe to the top of the condensing path in front of first
partition and flows downward the condensing path accompanied
with condensation on the front surface of first partition.
Condensate of ethanol returns to the evaporating copper tubes
through the lower connecting pipe by gravity force. The latent heat
of ethanol released by condensation on first partition enters the
VMED still and is recycled to increase the production of distillate
(Fig. 15). The authors observed from the experimental studies that,
(i) the solar collector and the VMED still can be folded or separated
when it is carried, so that the still would be easy to carry and
shipping cost would be very cheap. (ii) The proposed still of 10
partitions with 5 mm or 3 mm diffusion gap is theoretically
predicted to produce 19.2 or 21.8 kg/m2 day, respectively, on asunny autumn equinox day of daily solar radiation of 24.4 MJ/
m2 day. (iii) Theproductivity of theproposed still is 13%larger than
that of the VMED still coupled with a basin type still.
Hiroshi Tanaka et al.[44]found that, the optimum angle is 268
when solar collector angle is fixed for the year if the proposed still
is used at 268N latitude. The overall daily productivity is 9% or 17%
largerfor theoptimum solar collectoranglestills than thefixed one
on the summer or winter solstices. The productivity increases with
a decrease in the thickness of diffusion gaps between partitions,
and the increase is considerable when the thickness of diffusion
gaps is smaller than several millimetres. Hiroshi Tanaka et al. [45]
conducted the indoor experiments on VMED solar still with a heat
pipe solar collector, and the experimental results of the overall
production rates of the multiple effect still were about 93%, whichindicates that the heat pipe of the proposed still can transport
thermal energy well from the solar collector to the vertical
multiple effect diffusion type still.
3.1.5. Solar still coupled with solar pond
Solar pond is an artificially constructed pond in which
significant temperature rises are caused to occur in the lower
regions by preventing convection. Solar ponds are used for
collection and storage of solar energy and it is used for various
thermal applications like green house heating, process heat in
dairy plants, power production and desalination and this detailed
review of solar pond has been done by Velmurugan and Srithar
[46]. Velmurgan and Srithar[47]theoretically and experimentally
analysed the mini solar pond assisted solar still with sponge cube.
The results show that, average increase in productivity, when a
pond is integrated with a still is 27.6% and when pond and sponge
are integrated with a still is 57.8%.Velmurugan et al. [48] studied the augmentation of saline
streams in solar stills integrated with a mini solar pond. Industrial
effluent was used as feed for fin type single basin solar still and
stepped solar still. A mini solar pond connected to the stills to
enhance the productivity and tested individually. The schematic
diagram of experimental setup is shown in Fig. 16. The results
show that, maximum productivity of 100% was obtained when the
fin type solar still was integrated with pebble and sponge. The
productivity increases with increase in solar intensity and water-
glass temperature difference and decreases with increase in wind
velocity. Velmurugan et al. [49] experimentally investigated the
possibility of enhancing the productivity of the solar stills by
connecting a mini solar pond, stepped solar still and a single basin
solar still in series. Pebbles, baffle plates, fins and sponges are usedin the stepped solar still for productivity augmentation. Their
finding shows that, maximum productivity of 78% occurred when
fins and sponges were used in the stepped solar still and also found
that the productivity during night also improved when pebbles
were used in the solar stills.
Osamah and Darwish[50]studied a solar pond assisted multi
effect desalination of sea water in an arid environment and it is
recommended that an optimum area ratio is used such that quasi
steady operation is achieved. Huanmin Lu et al. [51]presented the
desalination coupled with salinity gradient solar ponds and
observed that, a multi effectmulti stage distillation unit produces
the high quality distillate. The total dissolved solid level of the
product is about 23 mg/l. There is no significant influence of
operating conditions on the quality of distillate. El.Sebai et al. [52]experimentally studied to improve the productivity of the single
effect solar stills, a single-slope single basin solar still integrated
with a Shallow Solar Pond (SSP). They found that, the annual
average values of daily productivity and efficiency of the still with
SSP were higher than those obtained without the SSP by 52.36%
and 43.80%, respectively.
3.1.6. Solar still coupled with hybrid PV/T system
The problem encountered with normal PV cells is that, most of
the solar radiation that is absorbed by a solar cell is not converted
into electricity. The excess energy which goes unabsorbed by the
solar cell increases the temperature of the photovoltaic cell and
reduces the efficiency. Natural or forced circulation of a fluidcooling
medium reduces the cell temperature. Cooling is often applied for
Fig. 15. Schematic diagram of multiple effect diffusion type still coupled with heat pipe solar collector[45].
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concentrating photovoltaic systems, in which the irradiance on the
cell surface is high. An alternative to ordinary photovoltaic modules
is to use Photovoltaic-Thermal (PV/T) modules, which are photovol-
taic modules coupled to heat extraction devices. Hence, these
systems, in addition to converting sunlight into electricity, collectthe residual thermal energy and delivers both heat and electricity in
usable forms. Shiv Kumar and Arvind Tiwari [53] conducted
experimental study of hybrid Photovoltaic/Thermal (PV/T) active
solarstill and found that,the yield increased by more than 3.5 times
than the passive solar still. The schematic diagram of hybrid (PV/T)
active solar still is shown in Fig. 17.
Shiv Kumar andTiwari [54] havemadean attemptto estimate the
internalheat transfer coefficientsof a deep basin hybrid(PV/T) active
solar still for composite climate of New Delhi. The authors observed
that, Kumar and Tiwari model better validate the results than the
other model and the average annual values of convective heat
transfer coefficient for the passive and hybrid (PV/T) active solar still
are 0.78 and 2.41 W m2 K1, respectively at 0.05 m water depth.
Shiv Kumar and Tiwari [55] presented the life cycle costanalysis of single slope hybrid (PV/T) active solar still and
suggested the following, (i) the lowest cost per kg of distilled
water obtained from the passive and hybrid (PV/T) active solar
stills is estimated as Rs. 0.70 and Rs. 1.93, respectively. It is much
economic in comparison to thebottled water available, which costs
around Rs. 10 per kg in Indian market for consumers. (ii) The
payback periods of the passive and hybrid ( PV/T) active solar stills
are obtained in the range of 1.16.2 years and 3.323.9 years,
respectively, for the selling price of distilled water in the range of
Rs. 10 to Rs. 2 per kg. Therefore, passive solar stills are acceptable
for potable use. (iii) The energy payback times (EPBT) of passive
and hybrid (PV/T) active solar stills are estimated as 2.9 years and
4.7 years, respectively.
3.1.7. Multistage active solar distillation system
Nishikawa et al. [56] developed and tested the triple effect
evacuated solar still. The authors reported that, the highest
distillation performance of 73.6 kg/day was obtained that corre-
spondsto the9.44 kg m2 day1 fresh water distilledat a condition
of the solar radiation of 13.85 MJ m2 day1 (108.3 MJ day1). The
total latent heat of the distillation (178.8 MJ day1) was about 1.7
times the solar radiation. The power consumption of the vacuum
pump was only 326 W day1 (1.17 MJ day1) when the solar cells
generated 952.5 Wh day1 (3.43 MJ day1) at 12.25 MJ m2 day1
(45.33 MJ day1) solar radiation.
Ahmed et al. [57] designed, fabricated and tested the multistage
evacuatedsolar still systemthat consists of three stages stacked on
the top of each other, and are carefully insulated from the outside
Fig. 16.Schematic diagram of the mini solar pond integrated with single basin and stepped solar still [48].
Fig. 17. Schematic of a hybrid (PV/T) active solar still[54].
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environment using rock-wood and aluminium foil layers to
prevent any losses to the ambient environment. The three stages
are mounted on top of each other and a good sealing is maintained
between the stages to prevent any vapour leakage through the
contact surfaces. A thick insulation is also used to reduce heat
losses of the still to the ambient. A solar collector is used to supply
heat to the system through the lower stage, which is maintained at
a pressure lower than atmospheric by means of a heat exchanger. A
solar operated vacuum pump is used to evacuate the non-
condensable gases from the stages. Fig. 18 shows a schematic
diagram of the multistage evacuated solar still. Saline water is fed
into each stage from the tank located at the top of the third stage.
Vapour generated in the lower stage condenses on the bottom
surface of the intermediatestage, giving its heat to the saline waterin the intermediate stage.
Vapour generated in the intermediate stage condenses at the
bottomsurface of the upper stage giving its heat to thesaline water
in the upper stage. The fed water is preheated by the heat given to
it by condensation of the vapour generated at the upper stage,
which condenses at the bottom of the feed water tank. Pressure
inside each one of the three stages is kept lower than the previous
stage.Vacuumis generatedusing a solar operated vacuumpump. A
set of valves is used to control the vacuum inside the different
stages. Theresults show that, themaximum production of thesolar
still was found in the first stage and is 6 kg/m2/day, 4.3 kg/m2/day
in secondstage and2 kg/m2/day in first stage ata vacuumpressure
of 0.5 bar. Indeed, the total productivity of the solar still is affected
very much by changing the internal pressure. The productivitydecreased as the pressure increased due to the lower evaporation
rates at the higher pressure values.
Mahmoud et al. [58] experimentally investigated the perfor-
mance of a multi stage water desalination still connected to a heat
pipe evacuated tube solar collector. The results of tests demon-
strate that the system produces about 9 kg/day of fresh water and
has a solar collector efficiency of about 68%. Schwarzer et al. [59]
developed the multistage solar desalination system with heat
recovery. The results show that, the system produces about 15
18 l/m2/day, which is 56 times higher than simple still.
3.1.8. Multi effect active solar distillation system
The multi effect solar distillation system is working based on
the multiple condensationevaporation cycle. Multi effect solar
still is an efficient method for the production of desalinated water
at relatively lowtemperature up to 70 8C. Adel M AbdelDayem[60]
demonstrated experimentally and numerically the performance of
a simplesolardistillation unit. Thebasic distillationunit consists of
air humidifiers (evaporators) and dehumidifiers (condensers).
There is no wall separating the two enclosures. The brine is passed
through the hot storage tank-2 where its temperature rises. It then
passesthrough evaporators where water vapourand heat aregiven
up to the counter-current air stream, reducing the brine
temperature. The air is heated and humidified simultaneously
since the humidity of saturated air is decreased in the condenser
side. On the other side, the evaporator consists of two horizontal
pipes with small holes provided on the lower side of the pipe. The
holes work as injectors that inject the hotsalt water to increase theevaporation rate.Fig. 19gives a schematic diagram of the system.
The results show that, the system can work continuously and
theproductivity of thedistilled water is high forthe collectormean
temperature of 50 8C and the estimated optimum collector area
based on the system life cycle solar savings was obtained as 6 m2
rather than that used in the present system, i.e., 3.1 m2. Zheng
Hongfei and Ge Xinshi[61]conducted the experimental study of a
steady state closed recycle solar still with enhanced falling film
evaporation and regeneration. Based on the experimental results,
the authors found that, the performance ratio of the unit is about
two to three times greater than that of a conventional basin type
solar still (single effect). Shaobo Hou and Hefei Zhang[62]studied
the hybrid solar desalination process of the multi effect
humidificationdehumidification and the basin type unit. Thegain output ratio of this system was raised by 23 at least through
reusing the rejected water.
Fig. 18.Schematic diagram of the evacuated multistage solar still [57].
Fig. 19. Schematic diagram of the present solar distillation system [60].
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Frieder Grater et al.[63]experimentally investigated the multi
effect still for hybrid solar/fossil desalination of sea and brackish
water. The results show that, heat recovery from the outlet mass
flows of concentrate and distillate has only little effect on distillate
output but the Gained Output Ratio (GOR) increases considerably.
With blowers and intermediate screens, installed inside the
distillation effects, the distillate yield can be increased by more
than 50% and the GOR by 60% related to results of a configuration
without heat recovery and blowers. Garg et al. [64]presented an
experimental design and computer simulation of multi effect
humidificationdehumidification solar desalination and the de-
veloped model which is useful in the estimation of the distillation
plant outputand optimized various components of the systemlike,
solar water heater, humidification chamber, and condensation
chamber.
Ali M. El-Nashar [65] studied the multiple effect solar
desalination plant and found that dust deposition and its effects
on performance depend strongly on the season of the year and the
frequency of jet cleaning should be adjusted accordingly. Lianying
Zhang et al.[66]developed a specifically designed solar desalina-
tion system with a solar collector and tested under practical
weather conditions. The results show that, theyield is about twoto
three times more than that of a conventional single basin solar still
under the same conditions. Ben Bacha et al. [67] conductedexperimental validation of the distillation module of a desalination
station using the solar multiple condensationevaporation cycle
principle. The results show that, a correct choice of a packed bed
material, which permits higher exchange coefficients and the solar
collector should be selected with high efficiency performance.
3.1.9. Air bubbled solar still
Pandey [68] reported the effect of dried,forced air bubbling and
cooling of glass cover in solar still. The results show that, the
simultaneous bubbling of dry air and glass cooling gives the
highest increase followed by bubbling of dry air alone (Fig. 20).
Gyorgy Mink et al. [69] designed and conducted the experi-
ments on air blown solar still with heat recycling. The results show
that, about a threefold increase in yield was achieved comparedwith that of a basin type solar still of the same area and with the
same irradiation. Mink et al. [70]presented the performance test
on air blown, multiple effect solar still with thermal energy recycle
consisting of an upper evaporation chamber and lower condensa-
tion chamber. The experimental result indicated that the still
performance can be enhanced further by increasing the liner air
stream velocity in the lower chamber by decreasing its cross-
sectional area.
3.1.10. Hybrid solar distillation system
The hybrid solar still can produce the desalinatedand hot water
from the same system. These types of designs have more
advantages over the other type of systems. Voropoulos et al.
[71] experimentally investigated the hybrid solarstill coupled with
solar collectors (Fig. 21). The results show that, (i) the productivity
of the coupled system is about double that of the still only. (ii)
Significant raises in distilled water productivity have been
obtained not only during the day but mainly during night
operation of the system, reaching triples the solar only system
productivity. (iii) The continuous heating of basin water from tank
water result in higher production rates in all operation periods as a
result of significantly higher differences between water and cover
temperatures, mainly at night. Voropoulos et al. [72]studied the
energy behaviour of hybrid solar still and concluded that, the
developed method can be a valuable tool for the systemoptimization, used during its design and also for evaluation of
an existing solar distillation installation through short term
testing.
Mathioulakis and Belessiotis[73]investigated the possibilities
of using optimization of a simple solar still through its incorpo-
ration in a multi-source and multi-use environment and observed
that, the design of such systems depending on the available heat
sources and/or expected consumption of hot water usage.
Voropoulos et al. [74] conducted experimental study of a hybrid
solar desalination andwater heating system. Theresults show that,
the output of a conventional solar still can be significantly
increased if it is coupled with a solar collector field and hot water
storage tank. The distilled water production was gradually
reduced, when the increase delivered energy through hot waterdraw-off. Ben Bacha et al.[75]developed a mathematical model to
give theability to estimate theexpectedperformance of thesystem
under given climatic conditions, allowing the choice of the proper
design solutions in relation to the desired usage.
3.2. Pre-heated water active solar still
In this method pre heated water is used to increase the water
temperature in the basin. The waste hot water is available from
various sources like paper industries, chemical industries, thermal
power plants and food processing industries and the same may be
utilized for solar distillation plant to increase the productivity. The
hot water will be supplied directly to the basin or through heat
Fig. 20.Schematic diagram of air bubbled solar still [68].
Fig. 21.Schematic diagram of hybrid solar distillation system [71].
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exchangers. Proctor[76]proposed the technique of using waste
heat in a solar still and predicted that productivity increased 3.2
times compared with ordinary still.
Sodha et al. [77] presented the experimental results on
utilization of waste hot water for distillation. In that test, two
modes were studied: (i) flowing waste hot water from thermal
power plants at constant rate through the solar still. (ii) Feeding
waste hot water obtained from thermal power plants once a day.
Their results showed that, length of solar still, depth of water in
basin, inlet water temperature and solar radiation are the
parameters which affects the performance of the still and the
still fed with hot water at constant rates gives higher yield in
comparison to a still with hot water filled only once in a day.
Tiwari et al. [78]studied the performance on effect of water
flow over the glass cover of a single basin solar still with an
intermittentflow of waste hotwaterin thebasin (Fig. 22). Based on
the experimental study, the authors made following points, (i) the
temperature of the water flowing over the glass cover always
remains of the same order as the ambient temperature and the
glass cover temperature is slightly higher than this. (ii) With the
flow of waste hot water during off sunshine hours, one can have a
higheryieldthan that of stationary water.(iii) Thestillproductivity
increases with the increase in mass flow rate for higher inlet water
temperatures and decreases for inlet water temperatures less thatthe average ambient temperature. (iv) The still productivity is
better forthe waste hot water flows duringoff sunshine hours than
the continuous flow of hot water for lower inlet temperatures. But
for higher inlet water temperatures, a continuous flow of water is
better. Ashok Kumar and Tiwari [79]investigated the use of hot
water in double slope solar still through heat exchanger (Fig. 23).
The authors observed that, the evaporative heat transfer
coefficient depends strongly on temperature and advised to use
the waste hot water with either higher temperature or during off
sunshine hours. Also found that, the efficiency of the system was
improved with the inlet temperature of the working fluid.
Yadav[80]analysed the performance of double basin solar still
coupled to a heat exchanger. Based on the analysis, the author
observed the following points, (i) the efficiency of a double basinsolar still coupled to a heat exchanger is significantly less, as
compared to that without heat exchanger. (ii) The efficiency of a
double basin solar still coupled to a heat exchanger is a strong
function of the heat exchanger length and the mass flow rate of the
working fluid. Yadav and Yadav [81] proposed the solar still
integrated with a tubular solar energy collector for productivity
enhancement.
3.3. Nocturnal active solar still
Nocturnal production is the working of a solar still in the
absence of sunlight. This may be achieved by either the solar
energy stored during day time is used during night or the supply of
waste heat available from various sources. The large water depths,
in a conventional solar still are heated during sunshine hours and
most of the thermal energy acquired by the water mass is stored
within it. This stored energy is mostly utilized during off sunshine
hours for the distillation, in the absence of solar radiation, and is
known as nocturnal distillation and this can also be achieved by
feeding the hot water available through any source (other thansolar energy) in the morning or evening for higher production[2].
Madhuri and Tiwari[82]conducted experiments on solar still
with intermittent flow of waste hot water in the basin during off
sunshine hours. The authors observed that, the yield increases in
proportion to the increase in inlet water temperature during the
flow of water and remain the same for stationary water. With the
flow of waste hot water during off sunshine hours, one can have
higher yield than that of the continuous flow of hot water and
stationary water. Gupta et al.[83]presented the analysis report on
effect of intermittent flow of waste hot water into the lower basin
at a constant rate during off sunshine hours (Fig. 24).
The results show that, (i) initially, the temperature of glass
covers is greater than the temperature of the water in the
corresponding basin. Soon, after 2 days, the situation is reversed.Quasi-steady state is reached in about 5 days and evaporation
becomes significant. (ii) The yield of the still increases with
increasing inlet waste hot water temperature, while the other
parameters arekept constant. (iii) The daily productivity of thestill
increases with the rate of flow of waste hot water, provided the
temperature of the inlet waste hot water is greater than its critical
value. If temperatures of the inlet waste hot water is less than its
critical value, the productivity of the still decreases as the rate of
flowof water increases. So,they suggestedto use a higherflow rate
Fig. 22.Schematic representation of the single basin solar still with water flowing
over the glass cover and inside the basin [78].
Fig. 23. Schematic diagram of double slope single basin solar still with heat
exchanger [79].
Fig. 24.Double basin solar still with constant flow rate [83].
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of inlet water only when its temperature is above the critical value.
Nocturnal outputs from basin type stills were studied experimen-
tally for 0.178 m and 0.76 m depth by Onyegegbu [84]. Results
indicated that, on average, nocturnal distillation accounted for 78%
of the total daily output of the 0.178 m deep still while accounting
for about 50% of the total daily output of the 0.076 m deep still.
Tiwari and Ashok Kumar [85] experimentally studied the
tubular solar still design suggested by Tleimat and Howe. The stillconsists of a rectangular (0.1 m1.1 m0.0127 m) black metallic
tray placed at the diametric plane of a cylindrical glass tube
(Fig. 25).
The length and diameter of the glass tube are slightly greater
than the length and width of the tray, respectively. During
operation, the ends of the glass tube are sealed with gasketed
woodenheads. Thetray and glass tube are fixed slightly tiltedfrom
the horizontal plane but in opposite direction. Brine fed from one
end is partly evaporated, and the remainder discharged through
the other end of the tube. The evaporated water condensed on the
inside walls of the glass cover flows down and it is removed from
one end at the bottom of the glass tube.
Based on the study, the authors found that, (i) the average brine
temperature is independent of still length for higher flow ratewhile the output temperature of brine strongly depends on still
length. (ii) The daily yield of distillate in the tubular solar still is
higher than that of the conventional solar still for the same set of
still and climatic parameters. (iii) The internal heat transfer
coefficient remains constant for constant inlet brine temperature
in contrast with the conventional solar still for higher flow rates.
(iv) The purity of the product in the tubular solar still is greater
than in a conventional one, and could be used for chemical
laboratories, etc.
4. Theoretical analysis of active solar distillation system
4.1. Heat transfer in active solar still
The heat transfer in solar still is mainly classified into internal
andexternal heat transfer.The details of various heat transfersthat
take place in active solar still are shown inFig. 26.
4.1.1. Internal heat transfer
The internal heat transfer occurs within the solar still from
water surface to inner surface of the glass cover, which mainly
consists of evaporation, convection and radiation. The convective
and evaporative heat transfers takes place simultaneously and are
independent of radiative heat transfer.
4.1.1.1. Radiative heat transfer. The view factor is considered as
unity because of glass cover inclination is small in the solar still.
The rate of radiative heat transfer between water to glass is given
by[2],
qr;wghr;wgTwTgi (1)
The radiative heat transfer coefficient between water to glass is
given as,
hr;wg eeffs Tw 273
2 Tgi 2732
TwTgi 546
" # (2)
The effective emittance between water to glass cover is
presented as,
eeff 1
1=eg 1=ew 1: (3)
4.1.1.2. Convective heat transfer. Natural convection takes place
across the humid air inside the basin due to the temperature
difference between the water surface to inner surface of the glass
cover. The rate of convective heat transfer between water to glass
is given by[47],
qc;wghc;wgTwTgi (4)
The convective heat transfer coefficient depends on the
temperature difference between evaporating and condensing
surface, physical properties of fluid, flow characteristic and
condensing cover geometry. The various models were developed
to find the convective heat transfer coefficient. One of the oldest
method was developed by Dunkles[86]and his expressions have
certain limitations, which are listed below.
Fig. 25.Schematic representation of a tubular solar still [85].
Fig. 26. Energy flow diagram of single slope active solar still.
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a. Valid only for normal operating temperature ffi 50 C in a solar
still and equivalent temperature difference ofDT 17 C.
b. This is independent of cavity volume, i.e., the average spacing
between the condensing and evaporating surfaces.
c. This is valid only for upward heat flow in horizontal enclosed air
space, i.e., for parallel evaporative and condensing surfaces.
The convective heat transfer coefficient is expressed as[86],
hc;wg 0:884DT01=3 (5)
where
DT0 Tw Tgi PwPgiTw 273
268:9 103 Pw
Pw exp 25:317 5144
273 Tw
(6)
Pgi exp 25:317 5144
273 Tgi
(7)
The value proposed in the above equation for C and n are
0.075and 0.33,respectively,for Gr>3.2105. Theabove equation
is notused widelybecause of itslimitations. Kumar andTiwari [87]
have proposed a thermal model for predicting the convective heat
transfer coefficient using linear regression analysis and it is free
from Dunkles shortcoming. Nusselt number for convective heat
transfer coefficient is represented as,
Nuhc;wg Xv
KvCGrPrn (8)
or
hc;wgKvXv
CGrPrn (9)
where,Grashof number(Gr) andPrandtl number(Pr) areexpressed
as follows,
GrbgX3vrv
2 DT0
mv2
(10)
PrmvCp
Kv(11)
The unknown constants C and n will be calculated by linear
regression analysis using experimental data. From the experimen-
tal study, they proposed that value of C and n was 0.0278 and
0.3513, respectively, for active single slope solar still. Chen et al.
[88] developed the model of free convection heat transfer
coefficient of the solar still for wide range of Rayleigh number
3:5 103
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temperature. (ii) The values of C and n differ for each design of
the solar still and for the operating water temperature range.
Therefore, it is recommended that before predicting the perfor-
mance theoretically, experiments must be carried out for given
climatic conditions to evaluate the values of C and n for a
particular design of solar still. Dwivedi and Tiwari[19]observed
from their studies in passive solar still that, Dunkles model gives
better agreement between theoretical and experimental results for
lower depth (0.010.03 m).
4.1.2. External heat transfer
The external heat transfer in solar still is mainly governed by
conduction, convection and radiation processes, which are
independent each other.
4.1.2.1. Top loss heat transfer coefficient. The heat is lost from outer
surface of the glass to atmosphere through convection and
radiation modes. The glass and atmospheric temperatures are
directly related to the performance of the solar still. So, top loss is
to be considered for the performance analysis. The temperature of
the glass cover is assumed to be uniform because of small
thickness. The total top loss heat transfer coefficient is defined as
[92],
qt;ga qr;gaqc;ga (25)
qt;ga ht;gaTgoTa (26)
where,
ht;ga hr;gahc;ga (27)
The radiative heat transfer between glass to atmosphere is
given by[92],
qr;ga hr;gaTgoTa (28)
The radiative heat transfer co efficient between glass to
atmosphere is given as,
hr;ga egs Tgo 273
4 Tsky 2734
TgoTa
" # (29)
where,
Tsky Ta 6
The convective heat transfer between glass to atmosphere is
given by[2],
qc;ga hc;gaTgoTa (30)
The convective heat transfer coefficient between glass to
atmosphere is given as,
hc;ga 2:8 3:0 v (31)
Another direct expression for total top loss heat transfer
coefficient in terms of function of wind speed is given by[2],
ht;ga 5:7 3:8 v (32)
But, there is no significant variation in the performance of the
distillation system by considering Eq. (27)or Eq.(32).
The total internal heat loss coefficient ht;wgand conductive
heat transfer coefficient of the glassKg=Lg is expressed asUwo
1=ht;wg Lg=Kg and the above equation could be rewritten
as,
Uwo ht;wgKg=Lg
ht;wg Kg=Lg (33)
Theoverall toploss coefficient (Ut) fromthe water surface tothe
ambient through glass cover,
Ut ht;wght;gaht;gaUwo
: (34)
4.1.2.2. Side and bottom loss heat transfer coefficient. The heat is
transferred from water in the basin to the atmosphere through
insulation and subsequently by convection and radiation from theside and bottom surface of the basin.
The rate of conduction heat transfer between basin liner to
atmosphere is given by[93],
qb hbTbTa (35)
The heat transfer coefficient between basin liner to atmosphere
is given by[93],
hb Li
Ki
1
ht;ba
1(36)
where, ht;ba hc;bahr;baand it is similar toEq. (32). There isno
velocity in bottom of the solar still. By substituting v 0, to obtain
the heat transfer coefficient. The bottom loss heat transfer
coefficient from the water mass to the ambient through thebottom is expressed as,
Ub 1
hw
1
hb
1(37)
The above equation could be rewritten as,
Ub hwhbhwhb
(38)
The conduction heat is lost through the vertical walls and
through the insulation of the still and it is expressed as,
Us Ass
As
Ub (39)
Thetotal side loss heat transfer coefficient (Us) will be neglectedbecause of side