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Active solar distillation—A 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, Indiab Department of Automobile Engineering, PSG College of Technology, Coimbatore 641004, Tamilnadu, Indiac Department of Mechanical Engineering, KSR College of Engineering, Tiruchengode 637215, Tamilnadu, India

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505

2. 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 hybrid PV/T system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516

4. 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) 1503–1526

A R T I C L E I N F O

Article history:

Received 6 November 2009

Received in revised form 15 December 2009

Accepted 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 different studies on active solar distillation system over the years. Thermal modelling was done

for various types of active single slope solar distillation system. This review would 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 at ScienceDirect

Renewable and Sustainable Energy Reviews

journa l homepage: www.e lsev ier .com/ locate / rser

1364-0321/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.rser.2010.01.023


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;b�a convective heat transfer coefficient from basin to

ambient (W/m2 8C)

hr;b�a radiative heat transfer coefficient from basin to

ambient (W/m2 8C)

ht;b�a total heat transfer coefficient from basin to

ambient (W/m2 8C)

hc;g�a convective heat transfer coefficient from glass

cover to ambient (W/m2 8C)

hr;g�a radiative heat transfer coefficient from glass cover

to ambient (W/m2 8C)

ht;g�a total (convective and radiative) heat transfer

coefficient from glass cover to ambient (W/m2 8C)

hc;w�g convective heat transfer coefficient from water to

glass cover (W/m2 8C)

he;w�g evaporative heat transfer coefficient from water to


hr;w�g radiative heat transfer coefficient from water to


ht;w�g 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 solar radiation over the inclined surface

of the solar collector (W/m2)

I(t)s intensity of solar radiation 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;w�g rate of convective heat transfer from water to glass

cover (W/m2)

qe;w�g rate of evaporative heat transfer from water to

glass cover (W/m2)

qr;w�g rate of radiative heat transfer from water to glass

cover (W/m2)

qt;w�g rate of total heat transfer from water to glass cover

(W/m2)

qr;g�a rate of radiative heat transfer t from glass cover to

ambient (W/m2)

qc;g�a rate of convective heat transfer from glass cover to

ambient (W/m2)

qt;g�a 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)

K. Sampathkumar et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1503–15261504


1. Introduction

Water is a nature’s gift and it plays a key role in thedevelopment of an economy and in turn for the welfare of anation. Non-availability of drinking water is one of the majorproblem faced by both the under developed and developingcountries all over the world. Around 97% of the water in the worldis in the ocean, approximately 2% of the water in the world is atpresent stored as ice in polar region, and 1% is fresh water availablefor the need of the plants, animals and human life [1]. Today,majority of the health issues are owing to the non-availability ofclean drinking water. In the recent decades, most parts of the worldreceive insufficient rainfall resulting in increase in the watersalinity. The pollution of water resources is increasing drasticallydue to a number of factors including growth in the population,industrialization, urbanization, etc. These activities adverselyaffected the water quality in rural areas and agriculture. Globally,200 million hours are spent each day, mostly by females, to collectwater from distant, often polluted sources. In the world, 3.575million people die each year from water related diseases. The basicmedical facilities never spotted numerous villages in the develop-ing and under developed countries. Majority of the rural people arestill unaware of the consequences of drinking untreated water.

Desalination is the oldest technology used by people for waterpurification in the world. Various technologies were invented fordesalination from time to time and it has been accepted by peoplewithout knowing future environmental consequences. Majordesalination 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 havebeen significantly affected by energy crisis because of heavydependency on conventional energy sources (coal power plants,fossil fuels, etc.), which has directly affected the environment andeconomic growth of these countries. The changing climate is one of

the major challenges the entire world is facing today. Gradual risein global average temperatures, increase in sea level and melting ofglaciers and ice sheets have underlined the immediate need toaddress the issue. All these problems could be solved only throughefficient and effective utilization of renewable energy resourcessuch as solar, wind, biomass, tidal, and geothermal energy, etc.

Solar energy is available in abundant in most of the rural areasand hence solar distillation is the best solution for rural areas and hasmany advantages of using freely available solar energy. It is a simpletechnology and more economical than the other available methods.A solar still operates similar to the natural hydrologic cycle ofevaporation and condensation. The basin of the solar still is filledwith impure water and the sun rays are passed through the glasscover to heat the water in the basin and the water gets evaporated.As the water inside the solar still evaporates, it leaves allcontaminates and microbes in the basin. The purified water vapourcondenses on the inner side of the glass, runs through the lower sideof the still and then gets collected in a closed container [2]. Manysolar distillation systems were developed over the years using theabove principle for water purification in many parts of the world.This paper reviews the technological developments of various activesolar distillation systems developed by various researchers in detail.The review also extends to thermal modelling of some active solardistillation 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 passivesolar still and active solar still. The numerous parameters areaffecting the performance of the still such as water depth in thebasin, material of the basin, wind velocity, solar radiation, ambienttemperature and inclination angle. The productivity of any type ofsolar still will be determined by the temperature differencebetween the water in the basin and inner surface glass cover. In apassive solar still, the solar radiation is received directly by thebasin water and is the only source of energy for raising the watertemperature 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 solarstills have been developed. Here, an extra thermal energy issupplied to the basin through an external mode to increase theevaporation rate and in turn improve its productivity. The activesolar distillation is mainly classified as follows [2]:

(i) High temperature distillation—Hot water will be fed into thebasin from a solar collector panel.

(ii) Pre-heated water application—Hot water will be fed into thebasin at a constant flow rate.

(iii) Nocturnal production—Hot water will be fed into the basinonce in a day.

3. Active solar distillation system

The performance of a solar still could neither be predicted norimproved by some of the uncontrollable parameters like intensityof solar radiation, ambient temperature and wind velocity. But,there are certain parameters such as depth of water, glass coverangle, fabrication materials, temperature of water in the basin andinsulation thickness, which affects the performance of the solarstill that could be modified for improving the performance. The stillperformance can be increased by reducing the water depth andthereby increasing the evaporation rate. The temperature differ-ence between water in the basin and condensing glass cover alsohas a direct effect in the performance of the still. The increasedtemperature 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 absorptivity

av thermal diffusivity of water vapour (m2/s)

a0 fraction of energy absorbed

(at) absorptance–transmittance 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 (5.67 � 10�8 W/m2 K4)

Subscripts

a ambient

b basin liner

c collector

eff effective

g glass cover

s solar still

w water

K. Sampathkumar et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1503–1526 1505


difference between the evaporating and condensing surfaces. Toachieve better evaporation and condensation rate, the temperatureof water in the basin could be raised by feeding thermal energyfrom some external sources.

3.1. High temperature active solar distillation

The water temperature of the conventional still is increased bysupplying additional thermal energy through solar collectors to thebasin. The temperature is increased from 20–50 8C to 70–80 8C inhigh temperature distillation for better evaporation.

3.1.1. Solar still coupled with flat plate collector

The solar still coupled with flat plate collector is working as hightemperature distillation method. The solar still coupled with flatplate collector (FPC) works either in forced circulation mode ornatural circulation mode. In forced circulation mode, a pump isused for supplying water. In natural circulation mode, water flowsdue to the difference in the density of water.

3.1.1.1. Forced circulation mode. The flat plate collector gives anadditional thermal energy to the basin of the solar still. A pump isused to circulate the water from the basin via flat plate collector tothe basin. Many researches have been carried out in this method andthe first being reported by Rai and Tiwari [3]. They found that, thedaily distillate production of a coupled single basin still is 24% higherthan that of an uncoupled one using forced circulation mode. Aschematic diagram of an active solar still integrated with a flat platecollector under forced circulation mode is shown in Fig. 1. Rai et al.[4] experimentally studied the various modes of operations in singlebasin solar still coupled with flat plate collector. From their studyshows that, the rate of daily distillate deceases with the saltconcentration. The addition of salt increases the surface tension andhence decreases the rate of evaporation. The best performance wasobserved in a single basin still coupled with a flat plate collectorhaving forced circulation and blackened jute cloth floating over thebasin water and a small quantity of black dye added to the water. Andalso found that, the rate of distillation increased by 30% when a smallquantity of black dye is added to the water. The bottom insulation isan important design parameter of the active solar still and fordrinking purposes, the conventional solar still will give betterperformance because, the efficiency of the system reduces with theincrease in the effective area as reported by Tiwari and Dhiman [5].Their experimental study showed that, there was only 12% rise inyield of the system if the length of the heat exchanger is varied from6.0 to 12.0 m and the overall efficiency of the system varied from 15to 19%.

Sanjeev Kumar and Tiwari [6] observed that, temperature ofwater and thermal efficiency decreased with an increase in basinarea due to the large storage capacity of the water mass in the basinand depth of water, respectively. Yield increased with increase inthe number of collectors, as expected, owing to increased heattransfer from the collector panel into the basin and the optimumnumber of collectors for maximum yield is 8 m2 since beyond thatthe increase in gain will be lower than the thermal loss. SanjeevKumar et al. [7] suggested that, for maximum annual yield, theoptimum collector inclination for a flat plate collector is 208 andthat of still glass cover is 158 for New Delhi climatic condition.

Tiwari et al. [8] inferred that, the internal heat transfercoefficients should be determined by using inner glass covertemperature for thermal modelling of passive and active solarstills. The heat transfer coefficients mainly depends on the shape ofthe condensing cover, material of the condensing cover andtemperature difference between water and inner glass cover. Onthe basis of the numerical computation, Singh and Tiwari [9] foundthat, the annual yield is at its maximum when the condensing glasscover inclination is equal to the latitude of the place and theoptimum collector inclination for a flat plate collector is 28.588, fora condensing glass cover inclination of 18.588 for New Delhiclimatic condition. Rajesh Tripathi and Tiwari [10] inferred that theconvective heat transfer coefficient between water and innercondensing cover depends significantly on the water depth of thebasin. It is also observed that more productivity was obtainedduring the off shine hours as compared to day time for higherwater depths in solar still (0.10 m and 0.15 m) due to storage effect.Vimal Dimri et al. [11] conducted theoretical and experimentalanalysis of a solar still integrated with flat plate collector withvarious condensing cover materials. The results indicated thatyield is directly related to thermal conductivity of condensingcover materials; copper gives a greater yield compared to glass andplastic due to higher thermal conductivity.

Tiwari et al. [12] presented the parametric study of passive andactive solar stills integrated with a flat plate collector. Computerbased thermal models were developed based on two assumptions:Tgi = Tgo and Tgi 6¼ Tgo. The results show that (i) there is an effect ofthe inner and outer glass temperature on the daily yield of bothactive and passive solar stills. (ii) The mean estimated errorinvolved in predicting the hourly yield of the passive solar still andactive solar still using the thermal model based on the assumptionthat Tgi = Tgo is 6% and 3%, respectively. Hence, the thermal model ofsolar stills should be developed based on the assumption thatTgi 6¼ Tgo. (iii) The results of the thermal model for the active solarstill for N = 1 show that the daily yield values are 3.08 l and 2.85 lfor Tgi = Tgo and Tgi 6¼ Tgo, respectively. Tiwari and Tiwari [2]reported the performance of single slope passive still coupled withmulti flat plate collectors. In their new design, rather than couplinga single collector, multiple collectors were integrated with thesolar 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 theoptimum number of collectors for single effect, double effect andtriple effect were 10, 9 and 6, respectively. In single effect, if thereare more than 8 collectors, the daily yield is higher than the doubleeffect but at the cost of additional collectors.

3.1.1.2. Natural circulation mode. The working of solar thermaldevices under thermosyphon mode has been more advantageousthan the forced circulation mode in terms of simplicity, reliabilityand cost effectiveness. Theoretical study on single basin solar stillcoupled with flat plate collector through heat exchanger have beenreported by Lawrence and Tiwari [13]. The results show that, theefficiency of active solar still is less than that of a simple solar stilland 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].



increase in water depth, while for an active solar still, it is thereverse (Fig. 2).

Yadav [14] studied the performance of a solar still coupled witha flat plate collector using thermosyphon mode and forcedcirculation mode for New Delhi climatic condition. The authorfound that, the system using the forced circulation mode gives 5–10% higher yield than that of the thermosyphon mode and 30–35%enhancement in the yield was observed with simple solar still. Thesteady state condition of the system was achieved after 2–3 days.Yadav [15] studied the transient performance of a high tempera-ture solar distillation system. The study reveals that it isworthwhile to consider a temperature dependent evaporativeheat transfer coefficient when evaluating the performance of ahigh temperature distillation. Tiris et al. [16] conductedexperiments on two flat plate solar collectors integrated with abasin type solar still. From their study, the collector integratedsolar still gave an average increase of 100% in yield in comparisonwith 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 integratedsystem, while the corresponding solar radiation is 24.343 MJ/m2 day.

Ali A Badran et al. [17] performed the tests in solar stillaugmented with flat plate collector using tap water and salinewater. They found that the mass of distilled water production usingaugmentation increased by 231% in case of tap water as a feed andby 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 solarstill productivity. The results showed that, the output of the still ismaximum for the least water depth in the basin (2 cm). Also, theincrease in water depth has decreased the productivity, while thestill productivity is found to be proportional to the solar radiationintensity.

Dwidevi and Tiwari [19] experimentally studied the doubleslope active solar still under natural circulation mode. From thestudy, they observed that, the double slope active solar still under

natural circulation modes gives 51% higher yield in comparison tothe double slope passive solar still. The thermal efficiency ofdouble slope active solar still is lower than the thermal efficiency ofdouble slope passive solar still. However, the exergy efficiency ofdouble 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 anothermain parameter, which affects the performance of the solar still.The rate of evaporation increased with reduction of glasstemperature. The rate of evaporation of water from a watersurface will be higher than the rate of release of heat from the glasscover to ambient by convection and radiation processes. If the heatloss from glass cover to ambient can be increased and that heat lossis used for further distillation, then overall efficiency of thedistillation unit under active modes of operation can be increasedsignificantly, as in the case of double basin solar still. This can beobtained by flowing the water over the glass cover for fast heattransfer 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 experimentalstudy that, there is an increase of about 20% and 30% yield for inlettemperature equal to ambient temperature for a passive and activesolar still, respectively. If the inlet temperature is increased, theoutput from the upper basin is increased but the output from thelower basin is appreciably reduced due to a lower value of water-glass temperature difference in the lower basin. Bapeshwararao etal. [21] presented from transient analysis that the distillate outputincreases with increase in the initial water temperature in bothbasins, the dependence on lower basin water temperature showsmore effect than that of upper one comparatively and remarkableincrease in the efficiency of the present system over that of thesimple solar still in all the cases. Tiwari and Sharma [22] studiedthe double effect solar distillation under active mode of operationusing heat exchanger. The study shows that, there is an increase ofabout 30% in the active solar still due to water flow through theupper basin and there is a marginal increase in efficiency withincrease in the length of the heat exchanger.

Kumar Sanjeev and Tiwari [23] presented the performance ofdaily yield for an active double effect distillation system withwater flow. The results show that, a higher yield from the lowerbasin with a maximum yield of 3.34 kg/m2/h at noon is due to thehigh water temperature of 95 8C at that time (Fig. 4). With theincrease in water masses, the operating water temperature in thelower basin is lowered resulting in reduced yield and efficiency.The daily yield increases with an increase of collector area, becausethe 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].



increases. Sanjay Kumar and Tiwari [24] studied the performanceof single and double effect active solar distillation, with andwithout water flow over the glass cover. The study shows that, anactive solar still with water flow arrangements over the glass coverproduces maximum distillate output. The solar still operating inthe double effect mode does not enhance the daily outputsignificantly because of the difficulties in maintaining reasonablylow and uniform flow rates over the glass cover (10 ml/min).

Sanjay Kumar and Tiwari [25] conducted experiments toestimate the convective mass transfer in active solar still. Themodified values of ‘C’ and ‘n’ for Nu = C(GrPr)n, are proposed as‘C’ = 0.0538; ‘n’ = 0.383 for 5.498 � 106 < Gr < 9.128 � 106 in anactive solar still. The percentage of deviation between theexperimental and theoretical results was found be within anaccuracy of 12%. The authors also recommended that beforepredicting the performance theoretically, an experiment may becarried out on a particular model of still for a given climaticcondition to evaluate the values of ‘C’ and ‘n’ from a thermal model.Yadav [26] conducted the experiments on double basin solar stillcoupled to a collector in the thermosyphon mode and still coupledto a collector in the forced circulation mode (Fig. 5). The resultsshow that, the double basin solar still coupled to a flat plate

collector performs better in the forced circulation mode than in thethermosyphon mode; however, these performances are still betterthan those of the uncoupled double basin solar still. The efficiencyof the high temperature distillation system decreases withincreasing area of the collector panel.

3.1.1.4. Regenerative active solar still. The higher evaporation rate isachieved, when the solar still works in high temperature by means ofsupplying heat to the basin in active mode. Thus, the glass cover willreceive more latent heat of vaporization. In turn, the temperature ofthe glass cover increases, and temperature difference between theglass cover and basin water decreases. This causes low vaporizationand, thus, low yield. To decrease the glass temperature, cold water ismade to flow over the glass cover. Heat is transferred from the glassto the flowing water which, in turn, keeps the temperaturedifference large. Moreover, if the temperature of the flowing waterat the outlet becomes higher than the basin water temperature, thenit can be fed to the basin for higher yield. This system is known as aregenerative active solar distillation system and its cross-sectionalview is shown in Fig. 6. Tiwari and Sinha [27] observed based onexperimental study on active regenerative solar still that the passiveregenerative solar stills have better thermal efficiency than activeregenerative solar stills and the thermal efficiency increases withincrease in the flow of water.

Singh and Tiwari [28] studied the thermal performance of aregenerative active solar distillation system working under the

Fig. 4. Hourly variation of yield in lower and upper basin [23].

Fig. 5. (a) Schematic view of uncoupled double basin solar still. (b) Double basin still

coupled to a collector in the thermosyphon mode. (c) Double basin still coupled to a

collector in the forced circulation mode [26].

Fig. 6. Cross-sectional view of an active regenerative solar still [27].



thermosyphon mode of operation for New Delhi climatic condi-tion. The authors inferred that, (i) there is a significant improve-ment in overall performance due to water flow over the glass cover.(ii) The hot water available due to the regenerative effect does notenhance the output. (iii) The overall efficiency of the active stills(conventional and regenerative) is lower than that of the passivestills (conventional and regenerative) at any common depth ofwater because the active stills are operating at higher temperature.Tiwari et al. [29] observed that the instantaneous thermalefficiency of the system decreases with an increase of collectorarea, due to the higher operating temperature range of thedistillation system. Yousef H. Zurigat et al. [30] proved that, thethickness of water on top of the first glass cover and the mass flowrate of the water going into the second effect have marginal effecton the productivity of the regenerative solar still.

3.1.1.5. Solar still coupled with parallel flat plate collector. Yadav andPrasad [31] experimentally studied the solar still integrated withparallel flat plate collector. The schematic diagram of a solar stillintegrated with a parallel flat plate solar energy collector is shownin Fig. 7. The collector essentially consists of a parallel flat plateplaced over the insulation with an air gap through which the waterwill 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) isblackened by black board paint before the glass cover is placed overthe absorber. The collector outlet is connected to the still by a pipecovered with insulation. The circulation of water between thecollector and the still can be made either via a pump (forcedcirculation system) or by placing the collector over a supportingstructure at such a height as to provide adequate head for naturalcirculation of water (thermosyphon) in the system. The resultsshow that, a significant rise in the distillate output is observedwhen the still is coupled with the collector and this system can bepreferred 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 isshown in Fig. 8. The distillation unit consists of ‘n’ parallel verticalplates. The first plate is insulated on its front side and the last plateis exposed to ambient.

Each plate in the enclosure is covered with wetted cloth on oneside. The cloth is extended into a feed through along the upper edgeof each plate. Feed water in the through is then drawn onto theplate surface by capillary. Excess water moves down the plate andis 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 over 5, and it increased by about 34% and15% 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 parabolicconcentrator is shown in Fig. 9. The parabolic shaped concentratoror solar collector concentrates the incident solar radiation on largesurface and it focuses on to a small absorber or receiver area. Theperformance of concentrators is much affected by the sun trackingmechanism. The tracking mechanism should move the collectorsthroughout the day to keep them focused on the sun rays toachieve the higher efficiency. These types of solar collectors reachhigher temperature compared to flat plate collectors owing toreduced heat loss area.

The various types of concentrators were used over the yearsbased on the applications. To achieve higher yield, the contractor iscoupled with solar still by means of increasing water temperaturein the basin. The water or oil will be supplied to trough receiverpipe by natural circulation mode or forced circulation mode. Singhet al. [33] found an analytical expression for water temperature ofan active solar still with flat plate collectors and parabolicconcentrator through natural circulation mode.

The results show that, the efficiency of the system withconcentrator is higher than parabolic collector as the evaporativeheat transfer coefficient is higher in concentrator. Garcia Rodriguezand 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].



distillation system coupled to a parabolic through collector (PTC)for sea water desalination and suggested the following, (i) theannual energy production is about 23% grater for a north–southcollector than for an east west one. (ii) The optimum axis height fora single collector is 298 and it is 12% higher production than ahorizontal collector for an inlet/outlet thermal oil temperature of225 8C/300 8C. (iii) The maximum yearly average of the dailyoperation time is only about 12 h/day in coastal areas in southernSpain.

Scrivani et al. [35] presented the concept of utilizing throughtype solar concentration plants for water production, remediation,waste treatment and the system can be used for processing landfillpercolate in arid regions where conventional depuration systemsare expensive and impractical. Zeinab and Ashraf [36] conductedexperimental and theoretical study of a solar desalination systemcoupled with solar parabolic through with a focal pipe and simpleheat exchanger (Fig. 9). The results show that, as time goes on, allthe temperatures increase and begin to decrease after 4.00 pmwith respect to the solar radiation, although the temperaturevalues of the modified system are still higher than the conventionalone. In case of the modified design, the fresh water productivityincreased an average by 18%.

Bechir Chaouchi et al. [37] designed and built a small solardesalination unit equipped with a parabolic concentrator (Fig. 10).The results show that, the maximum efficiency corresponds to themaximum solar lightning obtained towards 14:00. At that hour, theboiler was nearly in a horizontal position, which maximizes theoffered heat transfer surface. The experimental and theoretical study

concluded with an average relative error of 42% for the distillate flowrate. This is due to the imperfections in paraboloid geometry, the sunmanual follow up and especially to the system’s tilt variation duringthe day, which does not make it possible always to keep the absorbersurface covered with salted water. Lourdes Garcia Rodriguez et al.[38] proposed and evaluated the application of direct steamgeneration into a solar parabolic through collector to multi effectdistillation. The obtained results were useful in finding the mostsuitable conditions in which solar energy could compete withconventional 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 doubleeffect, solar distillation unit coupled compound parabolic concen-tration (CPC) collector under forced circulation mode (Fig. 11).Theauthors suggested that, (i) the temperature of the water in thelower basin is increased in comparison with single effectdistillation due to the reduced upward heat losses. (ii) The hourlyoutput in the lower basin is reduced due to the reducedtemperature difference between the water and glass temperatures.However, the overall output is increased due to reutilization of thelatent heat of evaporation in the second effect. (iii) The hourly yieldfrom the lower basin increases with increase of flow velocity due tothe decrease in the lower glass temperature. It is due to the factthat the lower glass cover temperature decreases due to the fastremoval of the latent heat of vaporization. (iv) The evaporativeheat transfer coefficient is a strong function of the operatingtemperature range. The convective and radiative heat transfercoefficients 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 glasstemperature of the solar still. Heat is transferred from the glass tothe flowing water which, in turn keeps the temperature differencelarge. This regenerative effect helps to achieve higher productivityof the solar still.

Sanjay Kumar and Sinha [40] conducted the experimentalanalysis of a double slope solar still coupled with a non-trackingcylindrical parabolic concentrator through an electric pump. Thesystem operates in a forced circulation mode to avoid the inherentproblems associated with a thermosyphon circulation mode. Theauthors observed that, the concentrator coupled still gives themaximum yield at all depths of the basin water (Fig. 12).

The concentrator assisted regenerative solar still has a muchhigher 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].



regenerative still at all water depths and they inferred that there isless thermal loss in the concentrator compared to the flat platecollector panel. From the analysis, an increase in the flow rate ofcold water over the glass cover also increases the overall thermalefficiency, followed by significant increase in its yield. LourdesGarcia Rodriquez et al. [41] studied the global analysis of the use ofsolar energy in seawater distillation under Spanish climaticcondition. They considered the following solar energy collectorsfor the analysis: salinity gradient solar ponds, flat plate collectors,evacuated tube collectors, compound parabolic collectors andparabolic through collectors for direct steam generation (DSG).Each of the collectors were compared for the parameters like, thefresh water production from a given desalination plant, attainablefresh water production if a heat pump is coupled to the solardesalination unit and area of solar collector required. Resultsshowed that direct steam generation parabolic through was apromising technology for solar assisted seawater desalination.

3.1.3. Solar still coupled with evacuated tube collector

The evacuated tube solar collector has more advantageous thanthe flat plate collectors for water heating purposes. Evacuated TubeCollectors (ETC) are well known for their higher efficiencies whencompared to flat plate solar collectors. In flat plate collectors, sunrays are perpendicular to the collector only at noon and thus aproportion of the sunlight striking the surface of the collector isalways likely to be reflected. But in evacuated tube collector, due toits cylindrical shape, the sun rays are perpendicular to the surfaceof the glass for most of the day. The evacuated tubes greatly reducethe heat losses as vacuum is present in the tubes. Owens-Illinois(OI) evacuated tube collector is shown in Fig. 13.

The OI collector consists of two coaxial tubes with evacuatedspace 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 diameterdelivery glass tube and exits from the same end of the tube throughannular space between delivery tube and selective coated absorbertube (which is sealed from one end). The annular space betweenselectively coated tube and borosilicate outermost glass tube isevacuated to minimize the convection loss from the selectivesurface.

Tiwari et al. [42] developed the thermal models for all types ofsolar collector integrated active solar stills based on energy balanceequations in terms of inner and outer glass temperature. The totaldaily yield of passive solar still, FPC, concentrating collector, ETCand ETC with heat pipe is shown in Fig. 14.

The authors have drawn the following points: (i) the maximumvalues of total heat transfer coefficient (htw) for active solar stillsintegrated with flat plate collector, concentrating collector,evacuated tube collector and ETC with heat pipe are 43, 86, 67and 76 W m�2 8C�1, respectively. (ii) The overall thermal efficiencyof 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 5.6–19.1 and0.25–0.85%, respectively. If the exergy out from FPC is considered,then average exergy efficiency of active solar still varies in therange 0.59–1.82%.

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 aheat pipe solar collector and a number of vertical and parallelpartitions in contact with saline soaked wicks. The system consistsof a heat pipe solar collector and a Vertical Multiple Effect Diffusiontype (VMED) still. The solar collector consists of a glass cover andcollector 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 withnarrow air gaps between them, and the partitions, with theexception of the outside one, are in contact with saline soakedwicks. Saline water is constantly fed to the wicks. The copper plateis in front of the first partition with a narrow gap. The gap becomesthe condensing path of the working fluid. The condensing path infront of first partition and the evaporating copper tubes attached tothe under surface of the collector plate are connected with twoconnecting pipes, so that a closed loop between the solar collectorand VMED still is formed. The constant mass of ethanol liquid ischarged into the closed loop and the closed loop is evacuated withan evacuating pump. The front surface of the VMED still and theunder 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].



The solar radiation transmits through the glass cover and isabsorbed on the collector plate and ethanol in the evaporatingcopper tubes attached to the under surface of the collector plate isheated up and evaporated. Ethanol vapour goes through the upperconnecting pipe to the top of the condensing path in front of firstpartition and flows downward the condensing path accompaniedwith condensation on the front surface of first partition.Condensate of ethanol returns to the evaporating copper tubesthrough the lower connecting pipe by gravity force. The latent heatof ethanol released by condensation on first partition enters theVMED 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 separatedwhen it is carried, so that the still would be easy to carry andshipping cost would be very cheap. (ii) The proposed still of 10partitions with 5 mm or 3 mm diffusion gap is theoreticallypredicted 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) The productivity of the proposed still is 13% larger thanthat of the VMED still coupled with a basin type still.

Hiroshi Tanaka et al. [44] found that, the optimum angle is 268when solar collector angle is fixed for the year if the proposed stillis used at 268N latitude. The overall daily productivity is 9% or 17%larger for the optimum solar collector angle stills than the fixed oneon the summer or winter solstices. The productivity increases witha decrease in the thickness of diffusion gaps between partitions,and the increase is considerable when the thickness of diffusiongaps is smaller than several millimetres. Hiroshi Tanaka et al. [45]conducted the indoor experiments on VMED solar still with a heatpipe solar collector, and the experimental results of the overallproduction rates of the multiple effect still were about 93%, whichindicates that the heat pipe of the proposed still can transportthermal energy well from the solar collector to the verticalmultiple effect diffusion type still.

3.1.5. Solar still coupled with solar pond

Solar pond is an artificially constructed pond in whichsignificant temperature rises are caused to occur in the lowerregions by preventing convection. Solar ponds are used forcollection and storage of solar energy and it is used for variousthermal applications like green house heating, process heat indairy plants, power production and desalination and this detailedreview of solar pond has been done by Velmurugan and Srithar[46]. Velmurgan and Srithar [47] theoretically and experimentallyanalysed the mini solar pond assisted solar still with sponge cube.

The results show that, average increase in productivity, when apond is integrated with a still is 27.6% and when pond and spongeare integrated with a still is 57.8%.

Velmurugan et al. [48] studied the augmentation of salinestreams in solar stills integrated with a mini solar pond. Industrialeffluent was used as feed for fin type single basin solar still andstepped solar still. A mini solar pond connected to the stills toenhance the productivity and tested individually. The schematicdiagram of experimental setup is shown in Fig. 16. The resultsshow that, maximum productivity of 100% was obtained when thefin type solar still was integrated with pebble and sponge. Theproductivity increases with increase in solar intensity and water-glass temperature difference and decreases with increase in windvelocity. Velmurugan et al. [49] experimentally investigated thepossibility of enhancing the productivity of the solar stills byconnecting a mini solar pond, stepped solar still and a single basinsolar still in series. Pebbles, baffle plates, fins and sponges are usedin the stepped solar still for productivity augmentation. Theirfinding shows that, maximum productivity of 78% occurred whenfins and sponges were used in the stepped solar still and also foundthat the productivity during night also improved when pebbleswere used in the solar stills.

Osamah and Darwish [50] studied a solar pond assisted multieffect desalination of sea water in an arid environment and it isrecommended that an optimum area ratio is used such that quasisteady operation is achieved. Huanmin Lu et al. [51] presented thedesalination coupled with salinity gradient solar ponds andobserved that, a multi effect–multi stage distillation unit producesthe high quality distillate. The total dissolved solid level of theproduct is about 2–3 mg/l. There is no significant influence ofoperating conditions on the quality of distillate. El.Sebai et al. [52]experimentally studied to improve the productivity of the singleeffect solar stills, a single-slope single basin solar still integratedwith a Shallow Solar Pond (SSP). They found that, the annualaverage values of daily productivity and efficiency of the still withSSP 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 ofthe solar radiation that is absorbed by a solar cell is not convertedinto electricity. The excess energy which goes unabsorbed by thesolar cell increases the temperature of the photovoltaic cell andreduces the efficiency. Natural or forced circulation of a fluid coolingmedium 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].



concentrating photovoltaic systems, in which the irradiance on thecell surface is high. An alternative to ordinary photovoltaic modulesis to use Photovoltaic-Thermal (PV/T) modules, which are photovol-taic modules coupled to heat extraction devices. Hence, thesesystems, in addition to converting sunlight into electricity, collectthe residual thermal energy and delivers both heat and electricity inusable forms. Shiv Kumar and Arvind Tiwari [53] conductedexperimental study of hybrid Photovoltaic/Thermal (PV/T) activesolar still and found that, the yield increased by more than 3.5 timesthan the passive solar still. The schematic diagram of hybrid (PV/T)active solar still is shown in Fig. 17.

Shiv Kumar and Tiwari [54] have made an attempt to estimate theinternal heat transfer coefficients of a deep basin hybrid (PV/T) activesolar still for composite climate of New Delhi. The authors observed

that, Kumar and Tiwari model better validate the results than theother model and the average annual values of convective heattransfer coefficient for the passive and hybrid (PV/T) active solar stillare 0.78 and 2.41 W m�2 K�1, 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 andsuggested the following, (i) the lowest cost per kg of distilledwater obtained from the passive and hybrid (PV/T) active solarstills is estimated as Rs. 0.70 and Rs. 1.93, respectively. It is mucheconomic in comparison to the bottled water available, which costsaround Rs. 10 per kg in Indian market for consumers. (ii) Thepayback periods of the passive and hybrid (PV/T) active solar stillsare obtained in the range of 1.1–6.2 years and 3.3–23.9 years,respectively, for the selling price of distilled water in the range ofRs. 10 to Rs. 2 per kg. Therefore, passive solar stills are acceptablefor potable use. (iii) The energy payback times (EPBT) of passiveand hybrid (PV/T) active solar stills are estimated as 2.9 years and4.7 years, respectively.

3.1.7. Multistage active solar distillation system

Nishikawa et al. [56] developed and tested the triple effectevacuated solar still. The authors reported that, the highestdistillation performance of 73.6 kg/day was obtained that corre-sponds to the 9.44 kg m�2 day�1 fresh water distilled at a conditionof the solar radiation of 13.85 MJ m�2 day�1 (108.3 MJ day�1). Thetotal latent heat of the distillation (178.8 MJ day�1) was about 1.7times the solar radiation. The power consumption of the vacuumpump was only 326 W day�1 (1.17 MJ day�1) when the solar cellsgenerated 952.5 Wh day�1 (3.43 MJ day�1) at 12.25 MJ m�2 day�1

(45.33 MJ day�1) solar radiation.Ahmed et al. [57] designed, fabricated and tested the multistage

evacuated solar still system that consists of three stages stacked onthe 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].



environment using rock-wood and aluminium foil layers toprevent any losses to the ambient environment. The three stagesare mounted on top of each other and a good sealing is maintainedbetween the stages to prevent any vapour leakage through thecontact surfaces. A thick insulation is also used to reduce heatlosses of the still to the ambient. A solar collector is used to supplyheat to the system through the lower stage, which is maintained ata pressure lower than atmospheric by means of a heat exchanger. Asolar operated vacuum pump is used to evacuate the non-condensable gases from the stages. Fig. 18 shows a schematicdiagram of the multistage evacuated solar still. Saline water is fedinto each stage from the tank located at the top of the third stage.Vapour generated in the lower stage condenses on the bottomsurface of the intermediate stage, giving its heat to the saline waterin the intermediate stage.

Vapour generated in the intermediate stage condenses at thebottom surface of the upper stage giving its heat to the saline waterin the upper stage. The fed water is preheated by the heat given toit by condensation of the vapour generated at the upper stage,which condenses at the bottom of the feed water tank. Pressureinside each one of the three stages is kept lower than the previousstage. Vacuum is generated using a solar operated vacuum pump. Aset of valves is used to control the vacuum inside the differentstages. The results show that, the maximum production of the solarstill was found in the first stage and is 6 kg/m2/day, 4.3 kg/m2/dayin second stage and 2 kg/m2/day in first stage at a vacuum pressureof 0.5 bar. Indeed, the total productivity of the solar still is affectedvery much by changing the internal pressure. The productivitydecreased as the pressure increased due to the lower evaporationrates at the higher pressure values.

Mahmoud et al. [58] experimentally investigated the perfor-mance of a multi stage water desalination still connected to a heatpipe evacuated tube solar collector. The results of tests demon-strate that the system produces about 9 kg/day of fresh water andhas a solar collector efficiency of about 68%. Schwarzer et al. [59]developed the multistage solar desalination system with heatrecovery. The results show that, the system produces about 15–18 l/m2/day, which is 5–6 times higher than simple still.

3.1.8. Multi effect active solar distillation system

The multi effect solar distillation system is working based onthe multiple condensation–evaporation cycle. Multi effect solar

still is an efficient method for the production of desalinated waterat relatively low temperature up to 70 8C. Adel M Abdel Dayem [60]demonstrated experimentally and numerically the performance ofa simple solar distillation unit. The basic distillation unit consists ofair humidifiers (evaporators) and dehumidifiers (condensers).There is no wall separating the two enclosures. The brine is passedthrough the hot storage tank-2 where its temperature rises. It thenpasses through evaporators where water vapour and heat are givenup to the counter-current air stream, reducing the brinetemperature. The air is heated and humidified simultaneouslysince the humidity of saturated air is decreased in the condenserside. On the other side, the evaporator consists of two horizontalpipes with small holes provided on the lower side of the pipe. Theholes work as injectors that inject the hot salt water to increase theevaporation rate. Fig. 19 gives a schematic diagram of the system.

The results show that, the system can work continuously andthe productivity of the distilled water is high for the collector meantemperature of 50 8C and the estimated optimum collector areabased 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. ZhengHongfei and Ge Xinshi [61] conducted the experimental study of asteady state closed recycle solar still with enhanced falling filmevaporation and regeneration. Based on the experimental results,the authors found that, the performance ratio of the unit is abouttwo to three times greater than that of a conventional basin typesolar still (single effect). Shaobo Hou and Hefei Zhang [62] studiedthe hybrid solar desalination process of the multi effecthumidification–dehumidification and the basin type unit. Thegain output ratio of this system was raised by 2–3 at least throughreusing 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].



Frieder Grater et al. [63] experimentally investigated the multieffect still for hybrid solar/fossil desalination of sea and brackishwater. The results show that, heat recovery from the outlet massflows of concentrate and distillate has only little effect on distillateoutput but the Gained Output Ratio (GOR) increases considerably.With blowers and intermediate screens, installed inside thedistillation effects, the distillate yield can be increased by morethan 50% and the GOR by 60% related to results of a configurationwithout heat recovery and blowers. Garg et al. [64] presented anexperimental design and computer simulation of multi effecthumidification–dehumidification solar desalination and the de-veloped model which is useful in the estimation of the distillationplant output and optimized various components of the system like,solar water heater, humidification chamber, and condensationchamber.

Ali M. El-Nashar [65] studied the multiple effect solardesalination plant and found that dust deposition and its effectson performance depend strongly on the season of the year and thefrequency of jet cleaning should be adjusted accordingly. LianyingZhang et al. [66] developed a specifically designed solar desalina-tion system with a solar collector and tested under practicalweather conditions. The results show that, the yield is about two tothree times more than that of a conventional single basin solar stillunder the same conditions. Ben Bacha et al. [67] conductedexperimental validation of the distillation module of a desalinationstation using the solar multiple condensation–evaporation cycleprinciple. The results show that, a correct choice of a packed bedmaterial, which permits higher exchange coefficients and the solarcollector should be selected with high efficiency performance.

3.1.9. Air bubbled solar still

Pandey [68] reported the effect of dried, forced air bubbling andcooling of glass cover in solar still. The results show that, thesimultaneous bubbling of dry air and glass cooling gives thehighest 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 showthat, about a threefold increase in yield was achieved comparedwith that of a basin type solar still of the same area and with thesame irradiation. Mink et al. [70] presented the performance teston air blown, multiple effect solar still with thermal energy recycleconsisting of an upper evaporation chamber and lower condensa-tion chamber. The experimental result indicated that the stillperformance can be enhanced further by increasing the liner airstream 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 desalinated and hot waterfrom the same system. These types of designs have moreadvantages over the other type of systems. Voropoulos et al.

[71] experimentally investigated the hybrid solar still coupled withsolar collectors (Fig. 21). The results show that, (i) the productivityof the coupled system is about double that of the still only. (ii)Significant raises in distilled water productivity have beenobtained not only during the day but mainly during nightoperation of the system, reaching triples the solar only systemproductivity. (iii) The continuous heating of basin water from tankwater result in higher production rates in all operation periods as aresult of significantly higher differences between water and covertemperatures, mainly at night. Voropoulos et al. [72] studied theenergy behaviour of hybrid solar still and concluded that, thedeveloped method can be a valuable tool for the systemoptimization, used during its design and also for evaluation ofan existing solar distillation installation through short termtesting.

Mathioulakis and Belessiotis [73] investigated the possibilitiesof using optimization of a simple solar still through its incorpo-ration in a multi-source and multi-use environment and observedthat, the design of such systems depending on the available heatsources and/or expected consumption of hot water usage.

Voropoulos et al. [74] conducted experimental study of a hybridsolar desalination and water heating system. The results show that,the output of a conventional solar still can be significantlyincreased if it is coupled with a solar collector field and hot waterstorage tank. The distilled water production was graduallyreduced, when the increase delivered energy through hot waterdraw-off. Ben Bacha et al. [75] developed a mathematical model togive the ability to estimate the expected performance of the systemunder given climatic conditions, allowing the choice of the properdesign 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 watertemperature in the basin. The waste hot water is available fromvarious sources like paper industries, chemical industries, thermalpower plants and food processing industries and the same may beutilized for solar distillation plant to increase the productivity. Thehot 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].



exchangers. Proctor [76] proposed the technique of using wasteheat in a solar still and predicted that productivity increased 3.2times compared with ordinary still.

Sodha et al. [77] presented the experimental results onutilization of waste hot water for distillation. In that test, twomodes were studied: (i) flowing waste hot water from thermalpower plants at constant rate through the solar still. (ii) Feedingwaste hot water obtained from thermal power plants once a day.Their results showed that, length of solar still, depth of water inbasin, inlet water temperature and solar radiation are theparameters which affects the performance of the still and thestill fed with hot water at constant rates gives higher yield incomparison to a still with hot water filled only once in a day.

Tiwari et al. [78] studied the performance on effect of waterflow over the glass cover of a single basin solar still with anintermittent flow of waste hot water in the basin (Fig. 22). Based onthe experimental study, the authors made following points, (i) thetemperature of the water flowing over the glass cover alwaysremains of the same order as the ambient temperature and theglass cover temperature is slightly higher than this. (ii) With theflow of waste hot water during off sunshine hours, one can have ahigher yield than that of stationary water. (iii) The still productivityincreases with the increase in mass flow rate for higher inlet watertemperatures and decreases for inlet water temperatures less thatthe average ambient temperature. (iv) The still productivity isbetter for the waste hot water flows during off sunshine hours thanthe continuous flow of hot water for lower inlet temperatures. Butfor higher inlet water temperatures, a continuous flow of water isbetter. Ashok Kumar and Tiwari [79] investigated the use of hotwater in double slope solar still through heat exchanger (Fig. 23).

The authors observed that, the evaporative heat transfercoefficient depends strongly on temperature and advised to usethe waste hot water with either higher temperature or during offsunshine hours. Also found that, the efficiency of the system wasimproved with the inlet temperature of the working fluid.

Yadav [80] analysed the performance of double basin solar stillcoupled to a heat exchanger. Based on the analysis, the authorobserved the following points, (i) the efficiency of a double basinsolar still coupled to a heat exchanger is significantly less, ascompared to that without heat exchanger. (ii) The efficiency of adouble basin solar still coupled to a heat exchanger is a strongfunction of the heat exchanger length and the mass flow rate of theworking fluid. Yadav and Yadav [81] proposed the solar stillintegrated with a tubular solar energy collector for productivityenhancement.

3.3. Nocturnal active solar still

Nocturnal production is the working of a solar still in theabsence of sunlight. This may be achieved by either the solarenergy 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 andmost of the thermal energy acquired by the water mass is storedwithin it. This stored energy is mostly utilized during off sunshinehours for the distillation, in the absence of solar radiation, and isknown as nocturnal distillation and this can also be achieved byfeeding 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 stillwith intermittent flow of waste hot water in the basin during offsunshine hours. The authors observed that, the yield increases inproportion to the increase in inlet water temperature during theflow of water and remain the same for stationary water. With theflow of waste hot water during off sunshine hours, one can havehigher yield than that of the continuous flow of hot water andstationary water. Gupta et al. [83] presented the analysis report oneffect of intermittent flow of waste hot water into the lower basinat a constant rate during off sunshine hours (Fig. 24).

The results show that, (i) initially, the temperature of glasscovers is greater than the temperature of the water in thecorresponding basin. Soon, after 2 days, the situation is reversed.Quasi-steady state is reached in about 5 days and evaporationbecomes significant. (ii) The yield of the still increases withincreasing inlet waste hot water temperature, while the otherparameters are kept constant. (iii) The daily productivity of the stillincreases with the rate of flow of waste hot water, provided thetemperature of the inlet waste hot water is greater than its criticalvalue. If temperatures of the inlet waste hot water is less than itscritical value, the productivity of the still decreases as the rate offlow of water increases. So, they suggested to use a higher flow 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].



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]. Resultsindicated that, on average, nocturnal distillation accounted for 78%of the total daily output of the 0.178 m deep still while accountingfor about 50% of the total daily output of the 0.076 m deep still.

Tiwari and Ashok Kumar [85] experimentally studied thetubular solar still design suggested by Tleimat and Howe. The stillconsists of a rectangular (0.1 m � 1.1 m � 0.0127 m) black metallictray placed at the diametric plane of a cylindrical glass tube(Fig. 25).

The length and diameter of the glass tube are slightly greaterthan the length and width of the tray, respectively. Duringoperation, the ends of the glass tube are sealed with gasketedwooden heads. The tray and glass tube are fixed slightly tilted fromthe horizontal plane but in opposite direction. Brine fed from oneend is partly evaporated, and the remainder discharged throughthe other end of the tube. The evaporated water condensed on theinside walls of the glass cover flows down and it is removed fromone end at the bottom of the glass tube.

Based on the study, the authors found that, (i) the average brinetemperature is independent of still length for higher flow ratewhile the output temperature of brine strongly depends on stilllength. (ii) The daily yield of distillate in the tubular solar still ishigher than that of the conventional solar still for the same set ofstill and climatic parameters. (iii) The internal heat transfercoefficient remains constant for constant inlet brine temperaturein contrast with the conventional solar still for higher flow rates.(iv) The purity of the product in the tubular solar still is greaterthan in a conventional one, and could be used for chemicallaboratories, 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 internaland external heat transfer. The details of various heat transfers thattake place in active solar still are shown in Fig. 26.

4.1.1. Internal heat transfer

The internal heat transfer occurs within the solar still fromwater surface to inner surface of the glass cover, which mainlyconsists of evaporation, convection and radiation. The convectiveand evaporative heat transfers takes place simultaneously and areindependent of radiative heat transfer.

4.1.1.1. Radiative heat transfer. The view factor is considered asunity 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;w�g ¼ hr;w�gðTw � TgiÞ (1)

The radiative heat transfer coefficient between water to glass isgiven as,

hr;w�g ¼ eeff sðTw þ 273Þ2 þ ðTgi þ 273Þ2

Tw þ Tgi þ 546

" #(2)

The effective emittance between water to glass cover ispresented as,

eeff ¼1

ð1=egÞ þ ð1=ewÞ � 1: (3)

4.1.1.2. Convective heat transfer. Natural convection takes placeacross the humid air inside the basin due to the temperaturedifference between the water surface to inner surface of the glasscover. The rate of convective heat transfer between water to glassis given by [47],

qc;w�g ¼ hc;w�gðTw � TgiÞ (4)

The convective heat transfer coefficient depends on thetemperature difference between evaporating and condensingsurface, physical properties of fluid, flow characteristic andcondensing cover geometry. The various models were developedto find the convective heat transfer coefficient. One of the oldestmethod was developed by Dunkle’s [86] and his expressions havecertain 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.



a. Valid only for normal operating temperature ffi50 �C in a solarstill and equivalent temperature difference of DT ¼ 17 �C.

b. This is independent of cavity volume, i.e., the average spacingbetween the condensing and evaporating surfaces.

c. This is valid only for upward heat flow in horizontal enclosed airspace, i.e., for parallel evaporative and condensing surfaces.

The convective heat transfer coefficient is expressed as [86],

hc;w�g ¼ 0:884½DT 0�1=3(5)

where

DT 0 ¼ ðTw � TgiÞ þðPw � PgiÞðTw þ 273Þð268:9� 10�3 � PwÞ

Pw ¼ exp 25:317� 5144

273þ Tw

� �� (6)

Pgi ¼ exp 25:317� 5144

273þ Tgi

� �� (7)

The value proposed in the above equation for ‘C’ and ‘n’ are0.075 and 0.33, respectively, for Gr > 3.2 � 105. The above equationis not used widely because of its limitations. Kumar and Tiwari [87]have proposed a thermal model for predicting the convective heattransfer coefficient using linear regression analysis and it is freefrom Dunkle’s shortcoming. Nusselt number for convective heattransfer coefficient is represented as,

Nu ¼ hc;w�g � Xv

Kv¼ CðGr � PrÞn (8)

or

hc;w�g ¼Kv

Xv� CðGr � PrÞn (9)

where, Grashof number (Gr) and Prandtl number (Pr) are expressedas follows,

Gr ¼ bgX3v rv

2 DT 0

mv2

(10)

Pr ¼ mvC p

Kv(11)

The unknown constants ‘C’ and ‘n’ will be calculated by linearregression analysis using experimental data. From the experimen-tal study, they proposed that value of ‘C’ and ‘n’ was 0.0278 and0.3513, respectively, for active single slope solar still. Chen et al.[88] developed the model of free convection heat transfercoefficient of the solar still for wide range of Rayleigh numberð3:5� 103 <Ra<106Þ and as follows,

hc;w�g ¼ 0:2Ra0:26 Kv

Xv(12)

Zheng et al. [89] have developed a modified Rayleigh numberusing Chen et al. [88] model for evaluating the convective heattransfer coefficient,

hc;w�g ¼ 0:2ðRa0Þ0:26 Kv

Xv(13)

where,

Ra0 ¼ X3v rvgbmvav

DT 00 (14)

where,

DT 00 ¼ ðTw � TgiÞ þPw � Pgi

ðMaPt=ðMa �MwvÞÞ � PwðTw þ 273:15Þ

� �(15)

The convective heat transfer between basin to water is given by[42]

qw ¼ hwðTb � TwÞ (16)

The convective heat transfer coefficient between basin to wateris given as,

hw ¼Kw

XwCðGrPrÞn where; C ¼ 0:54 and N ¼ 1=4: (17)

4.1.1.3. Evaporative heat transfer. The performance of solar stilldepends on the evaporative and convective heat transfercoefficients. Various scientists developed mathematical relationsto evaluate the evaporative and convective heat transfer coeffi-cients. The general equation for the rate of evaporative heattransfer between water to glass is given by [47],

qe;w�g ¼ he;w�gðTw � TgiÞ (18)

The evaporation takes place inside the solar still by addition ofheat in the water by means of solar radiation. Dunkle’s [86]developed a model to evaluate the evaporative heat transfercoefficient as follows,

he;w�g ¼ 16:273� 10�3 � hc;w�gPw � Pgi

Tw � Tgi

� �(19)

Malik et al. [90] developed a correlation based on Lewis relationfor low operating temperature range and it is expressed as,

he;w�g ¼ 0:013hc;w�g (20)

Kumar and Tiwari [87] developed a new model by consideringoperating temperature range, orientation of glass cover and solarstill cavity. They used regression analysis after conductingexperiments in actual field to evaluate the ‘C’ and ‘n’.

he;w�g ¼ 16:273� 10�3 � Kv

Xv� CðGr � PrÞn �

Pw � Pgi

Tw � Tgi

� �(21)

Clark et al. [91] developed model by using the rate ofevaporative mass flux in an air–water humidification situation as,

qe;w�g ¼K 0

2

� �� hc;w�gðPw � PgiÞ (22)

where K 0 ¼ 0:016273 and it is valid only when the rate ofevaporation and condensation are equal, which is only possible fora high operating temperature range (>80 8C) and spacing betweenevaporating and condensing surfaces is large.

The total heat transfer coefficient of water to glass is defined as,

ht;w�g ¼ hc;w�g þ he;w�g þ hr;w�g (23)

The rate of total heat transfer of water to glass is defined as,

qt;w�g ¼ qc;w�g þ qe;w�g þ qr;w�g

qt;w�g ¼ ht;w�gðTw � TgiÞ (24)

Shiv Kumar and Tiwari [54] compared the various internal heattransfer coefficients of different models in active solar still. On thebasis of results, the following points have been made. (i) On thebasis of hourly yield Kumar and Tiwari model is superior to theothers model under consideration with least percentage deviationexcept in extreme cases. The better fitting of the curves with highervalue of correlation coefficient is obtained for wide range of water



temperature. (ii) The values of ‘C’ and ‘n’ differ for each design ofthe 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 givenclimatic conditions to evaluate the values of ‘C’ and ‘n’ for aparticular design of solar still. Dwivedi and Tiwari [19] observedfrom their studies in passive solar still that, Dunkle’s model givesbetter agreement between theoretical and experimental results forlower depth (0.01–0.03 m).

4.1.2. External heat transfer

The external heat transfer in solar still is mainly governed byconduction, convection and radiation processes, which areindependent each other.

4.1.2.1. Top loss heat transfer coefficient. The heat is lost from outersurface of the glass to atmosphere through convection andradiation modes. The glass and atmospheric temperatures aredirectly related to the performance of the solar still. So, top loss isto be considered for the performance analysis. The temperature ofthe glass cover is assumed to be uniform because of smallthickness. The total top loss heat transfer coefficient is defined as[92],

qt;g�a ¼ qr;g�a þ qc;g�a (25)

qt;g�a ¼ ht;g�aðTgo � TaÞ (26)

where,

ht;g�a ¼ hr;g�a þ hc;g�a (27)

The radiative heat transfer between glass to atmosphere isgiven by [92],

qr;g�a ¼ hr;g�aðTgo � TaÞ (28)

The radiative heat transfer co efficient between glass toatmosphere is given as,

hr;g�a ¼ egsðTgo þ 273Þ4 � ðTsky þ 273Þ4

Tgo � Ta

" #(29)

where,

Tsky ¼ Ta � 6

The convective heat transfer between glass to atmosphere isgiven by [2],

qc;g�a ¼ hc;g�aðTgo � TaÞ (30)

The convective heat transfer coefficient between glass toatmosphere is given as,

hc;g�a ¼ 2:8þ ð3:0� vÞ (31)

Another direct expression for total top loss heat transfercoefficient in terms of function of wind speed is given by [2],

ht;g�a ¼ 5:7þ ð3:8� vÞ (32)

But, there is no significant variation in the performance of thedistillation system by considering Eq. (27) or Eq. (32).

The total internal heat loss coefficient ðht;w�gÞ and conductiveheat transfer coefficient of the glass ðKg=LgÞ is expressed as Uwo ¼½ð1=ht;w�gÞ þ ðLg=KgÞ� and the above equation could be rewrittenas,

Uwo ¼ht;w�gðKg=LgÞ

ht;w�g þ ðKg=LgÞ(33)

The overall top loss coefficient (Ut) from the water surface to theambient through glass cover,

Ut ¼ht;w�ght;g�a

ðht;g�a þ UwoÞ: (34)

4.1.2.2. Side and bottom loss heat transfer coefficient. The heat istransferred from water in the basin to the atmosphere throughinsulation and subsequently by convection and radiation from theside and bottom surface of the basin.

The rate of conduction heat transfer between basin liner toatmosphere is given by [93],

qb ¼ hbðTb � TaÞ (35)

The heat transfer coefficient between basin liner to atmosphereis given by [93],

hb ¼Li

Kiþ 1

ht;b�a

� ��1

(36)

where, ht;b�a ¼ hc;b�a þ hr;b�a and it is similar to Eq. (32). There is novelocity in bottom of the solar still. By substituting v ¼ 0, to obtainthe heat transfer coefficient. The bottom loss heat transfercoefficient from the water mass to the ambient through thebottom is expressed as,

Ub ¼1

hwþ 1

hb

� ��1

(37)

The above equation could be rewritten as,

Ub ¼hwhb

hw þ hb(38)

The conduction heat is lost through the vertical walls andthrough the insulation of the still and it is expressed as,

Us ¼Ass

As

� �Ub (39)

The total side loss heat transfer coefficient (Us) will be neglectedbecause of side still area (Ass) is very small compared with stillbasin area (As).

The overall heat transfer coefficient from water to ambientthrough top, bottom and sides of the still is expressed as [93],

ULS ¼ Ut þ Ub: (40)

4.2. Thermal modelling of active solar still

The thermal models of the single slope-single basin active solarstills are developed based on the energy balance equations. Thefollowing assumptions have been considered for writing theenergy balance equation in terms of w/m2

i. The solar still is vapour leakage proof.ii. The level of water in the basin is maintained at a constant level.

iii. Inclination of glass cover is small.iv. No stratification of water occurs in the basin of the solar still.v. The heat capacity of the glass cover, absorbing and insulation

materials (bottom and sides) is negligible.vi. The condensation that occurs through the glass is film type.

The energy balance equation of three main components ofactive solar still is as follows [42].

4.2.1. Inner and outer surface of glass cover

Tiwari et al. [12] conducted the parametric study of an activesolar stills for development of thermal models based on the two



assumptions that Tgi = Tgo and Tgi 6¼ Tgo and found that, the thermalmodel of solar stills should be developed based on the assumptionthat Tgi 6¼ Tgo.

The above conclusion is taken in to consideration for writing theenergy balance equation of glass cover. The rate of energy gainedby glass and rate of energy gained from water surface to glass byradiation, convection and evaporation is equal to the rate of energylost to air.

4.2.2. Inner surface of glass cover

a0gIðtÞs þ qr;w�g þ qc;w�g þ qe;w�g ¼Kg

LgðTgi � TgoÞ (41)

By substituting Eq. (24) in Eq. (41), the energy balance equationof inner surface of glass cover becomes,

a0gIðtÞs þ ht;w�gðTw � TgiÞ ¼Kg

LgðTgi � TgoÞ (42)

After simplifying the above Eq. (42), inner surface of glasstemperature is written as follows,

Tgi ¼a0gIðtÞs þ ht;w�gTw þ ðKg=LgÞTgo

ht;w�g þ ðKg=LgÞ: (43)

4.2.3. Outer surface of glass cover

Kg

LgðTgi � TgoÞ ¼ qr;g�a þ qc;g�a (44)

By substituting Eqs. (25) and (26) in Eq. (44), the energy balanceequation of outer surface glass cover becomes,

Kg

LgTgi � Tgo

� �¼ ht;g�a Tgo � Ta

� �(45)

By substituting Eq. (43) in L.H.S. of Eq. (45), it becomes,

Kg

LgðTgi � TgoÞ ¼

ht;w�gðKg=LgÞht;w�g þ ðKg=LgÞ

ðTw � TgoÞ þa0gIðtÞsðKg=LgÞ

ht;w�g þ ðKg=LgÞ(46)

By substituting Eq. (33) in Eq. (46) it becomes,

Kg

LgðTgi � TgoÞ ¼ UwoðTw � TgoÞ þ hka

0gIðtÞs

Kg

Lg(47)

where,

hk ¼Kg=Lg

ht;w�g þ ðKg=LgÞ

By substituting Eq. (47) in Eq. (45).it becomes,

a0gIðtÞshk þ UwoðTw � TgoÞ ¼ ht;g�aðTgo � TaÞ (48)

By simplifying above Eq. (48), outer glass temperature (Tgo) iswritten as follows,

Tgo ¼a0gIðtÞshk þ UwoTw þ ht;g�aTa

ht;g�a þ Uwo: (49)

4.2.4. Basin liner

The rate of energy absorbed by the basin plate is equal to therate of energy transferred to water and the rate of energy lost byconduction through bottom and sides.

a0bð1� a0gÞð1� a0wÞIðtÞs ¼ qw þ qb (50)

By substituting Eqs. (16) and (35) in Eq. (50), the above equationbecomes,

a0bð1� a0gÞð1� a0wÞIðtÞs ¼ hwðTb � TwÞ þ hbðTb � TaÞ (51)

After simplifying Eq. (51), the basin liner temperature of solarstill is written as follows,

Tb ¼�a0bIðtÞs þ hwTw þ hbTa

hw þ hb(52)

where,

�a0b ¼ a0bð1� a0gÞð1� a0wÞ: (53)

4.2.5. Water mass

The rate of energy absorbed and the rate of energy convectedfrom the basin liner is equal to the rate of energy stored and rate ofenergy transferred to the glass cover.

a0wð1� a0gÞIðtÞs þ qw þ Qu ¼ ðMCÞwdTw

dtþ ½qr;w�g þ qc;w�g þ qe;w�g �

(54)

Substituting Eqs. (16) and (24) in Eq. (54), the energy balanceequation of water mass in the solar still is as follows,

a0wð1� a0gÞIðtÞs þ hwðTb � TwÞ þ Qu

¼ ðMCÞwdTw

dtþ ½ht;w�gðTw � TgoÞ� (55)

By substituting values of Tgo and Tb from Eqs. (49) and (52) in Eq.(55), it becomes,

Qu þ IðtÞs a0bhw

ðhw þ hbÞþ a0w þ a0g

ht;w�g

ðht;g�a þ UwoÞ

� �

¼ ðMCÞwdTw

dtþ ht;w�ght;g�a

ðht;g�a þ UwoÞðTw � TgoÞ

� �

þ hwhb

ðhw þ hbÞðTw � TaÞ

� �(56)

By substituting Eqs. (34) and (38) in the above Eq. (56) and it isrewritten as,

Qu þ ðatÞeff IðtÞs ¼ ðMCÞwdTw

dtþ ðUt þ UbÞðTw � TaÞ (57)

where,

ðatÞeff ¼ a0bhw

ðhw þ hbÞþ a0w þ a0g

ht;w�g

ðht;g�a þ UwoÞ

� �

In an active solar still, additional thermal energy (Qu) is suppliedto the solar still with the help of solar collectors to increase thetemperature in the basin. For solving the above equation, flat platesolar collector (Qu) is considered. The useful energy per unit areafrom the flat plate solar collector is given as follows:

Qu ¼ NFR½ðatÞcIðtÞc � ULCðTw � TaÞ� (58)

Assuming number collectors N = 1, by substituting Eq. (56) inEq. (55) and it becomes,

FR½ðatÞcIðtÞc � ULCðTw � TaÞ� þ ðatÞeff IðtÞs

¼ ðMCÞwdTw

dtþ ðUt þ UbÞðTw � TaÞ (59)

By substituting Eq. (40) in the above Eq. (59), it becomes,

FRðatÞcIðtÞc þ ðatÞeff IðtÞs ¼ ðMCÞwdTw

dtþ ½ðULSÞ

þ ðFRULCÞ�ðTw � TaÞ (60)

The above equation is represented in the following form,

Ieff ¼ ðMCÞwdTw

dtþ Ueff ðTw � TaÞ (61)



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tio

n.

Co

lle

cto

ra

ng

le=

458

Fro

mth

ee

con

om

icp

oin

to

fv

iew

,th

eci

rcu

lati

ng

pu

mp

sho

uld

use

din

the

mo

rnin

ga

nd

ev

en

ing

.

Flo

wra

te=

1.1

5k

g/m

in

So

lar

stil

lco

up

led

wit

h

fla

tp

late

coll

ect

or

(Na

tura

lci

rcu

lati

on

mo

de

)

Ba

dra

na

nd

Al-

Ta

ha

ine

h[1

8]

Ty

pe

=si

ng

leb

asi

nso

lar

stil

l3

6%

mo

reth

an

the

sim

ple

sin

gle

ba

sin

sola

rst

ill.

Pro

du

ctiv

ity

isle

ssco

mp

are

dto

forc

ed

circ

ula

tio

n

mo

de

.

Am

ma

n,

Jord

an

Are

a=

1m�

1m

Ma

xim

um

dis

till

ate

of

3.5

l/m

2E

asy

for

op

era

tio

n.

Co

lle

cto

ra

ng

le=

358

Op

tim

um

an

gle

is1

08

for

win

ter

sea

son

inJo

rda

nD

ou

ble

slo

pe

sola

rst

ill

pro

du

ces

low

er

yie

ldth

an

sim

ple

sola

rst

ill.

Insu

lati

on

ma

teri

al=

Ro

ckw

oo

l

an

dth

ick

ne

ss=

6cm

Act

ive

do

ub

lee

ffe

ctso

lar

stil

lS

an

jay

Ku

ma

ra

nd

Tiw

ari

[24

]

Ty

pe

=si

ng

leb

asi

nso

lar

stil

lA

na

ve

rag

eo

f7

.5l/

da

yo

fd

isti

lle

dw

ate

rw

as

ob

tain

ed

inth

ea

ctiv

em

od

ew

ith

wa

ter

flo

wa

rra

ng

em

en

t.

Op

era

tio

na

nd

ma

inte

na

nce

isd

iffi

cult

.

Ne

wD

elh

i,In

dia

Are

a=

1m�

1m

Inth

ep

ass

ive

an

da

ctiv

em

od

es

wit

ho

ut

arr

an

ge

me

nts

for

wa

ter

flo

wa

ve

rag

eo

utp

ut

wa

s2

.2a

nd

3.9

l/d

ay

.

Do

ub

lee

ffe

ctm

od

ed

oe

sn

ot

en

ha

nce

the

da

ily

ou

tpu

tsi

gn

ifica

ntl

yb

eca

use

of

dif

ficu

ltie

sin

ma

inta

inin

gre

aso

na

bly

low

an

du

nif

orm

flo

wra

tes

ov

er

the

gla

ssco

ve

r.

Sti

lla

ng

le=

158

coll

ect

or

len

gth

=1

m

Co

lle

cto

ra

ng

le=

458

Flo

wra

te=

40

ml/

min

Ga

pb

etw

ee

ntw

og

lass

es

=2

0cm

Pu

mp

=0

.2H

P

Act

ive

reg

en

era

tiv

eso

lar

stil

lT

iwa

ria

nd

Sin

ha

[27

]

Ne

wD

elh

i,In

dia

Ty

pe

=S

ing

leb

asi

nso

lar

stil

l

wit

hre

ge

ne

rati

ve

eff

ect

Th

em

ax

imu

my

ield

of

1,0

.7,0

.3,a

nd

0.0

2k

g/m

2h

we

re

ob

tain

ed

at

13

hfo

ra

ctiv

ere

ge

ne

rati

ve

,a

ctiv

en

on

-

reg

en

era

tiv

e,

pa

ssiv

ere

ge

ne

rati

ve

an

dp

ass

ive

no

n-

reg

en

era

tiv

e,

resp

ect

ive

ly.

Hig

he

ry

ield

as

com

pa

reto

fla

tp

late

coll

ect

ors

.

Sti

lla

rea

=1

m2

Th

eth

erm

ale

ffici

en

cyo

fth

ea

ctiv

em

od

eo

fo

pe

rati

on

is

low

er

tha

nth

at

of

pa

ssiv

eso

lar

stil

l

Th

ein

itia

lco

stis

hig

h

Co

lle

cto

ra

rea

=2

m2

Co

mp

lex

ind

esi

gn

an

do

pe

rati

on

.

Flo

wra

te=

0.2

0k

g/s

Te

chn

olo

gy

kn

ow

np

ers

on

isre

qu

ire

dfo

ro

pe

rati

on

.

Insu

lati

on

thic

kn

ess

=0

.00

4m

So

me

acc

ou

nta

ble

loss

es

inh

ea

te

xch

an

ge

r.

He

at

ex

cha

ng

er

isu

sed

So

lar

stil

lco

up

led

wit

hp

ara

lle

l

fla

tp

late

coll

ect

or

Ya

da

va

nd

Pra

sad

[31

]T

yp

e=

sin

gle

ba

sin

sola

rst

ill

Ma

xim

um

pro

du

ctiv

ity

of

0.2

50

kg

/m2

ha

t1

.00

pm

is

ach

iev

ed

wh

ere

as

0.1

50

kg

/m2

hfo

rsi

mp

leso

lar

stil

l.

Sim

ple

de

sig

na

sco

mp

are

dto

the

fla

tp

late

coll

ect

or.

Ne

wD

elh

i,In

dia

Are

a=

1m�

1m

Ma

xim

um

wa

ter

tem

pe

ratu

reo

f6

88C

wa

sa

chie

ve

dC

ost

eff

ect

ive

as

com

pa

red

toth

efl

at

pla

teco

lle

cto

r.

Pa

rall

el

pla

teco

lle

cto

rle

ng

th=

1m

Pro

du

ctiv

ity

islo

wa

sco

mp

are

dto

the

fla

tp

late

coll

ect

or

Insu

lati

on

ma

teri

al=

Ro

ckw

oo

l

Insu

lati

on

thic

kn

ess

=0

.05

m

Ve

rtic

al

sola

rst

ill

cou

ple

d

wit

hfl

at

pla

teco

lle

cto

r

Kia

tsir

iro

at

et

al.

[32

]T

yp

e=

Ve

rtic

al

sola

rst

ill

Th

en

um

be

rso

fe

va

po

rati

ve

pla

tes

are

op

tim

ize

da

s5

for

the

wa

ter

flo

wra

teis

50

kg

/h.

On

lyfe

wre

sea

rch

es

ha

ve

be

en

rep

ort

ed

.

Ba

ng

ko

k,

Th

ail

an

dA

rea

=1

.52

m�

0.9

mT

he

av

era

ge

dis

till

ate

wa

ter

pro

du

ctio

no

f5

kg

/m2

da

y

wa

so

bta

ine

db

yu

sin

g5

nu

mb

ers

of

ev

ap

ora

tiv

ep

late

s.

Init

iala

nd

op

era

tio

na

lco

stis

hig

he

ra

sco

mp

are

dto

oth

er

fla

tp

late

coll

ect

ors

.

Co

lle

cto

ra

rea

=1

.4m

2P

um

pis

req

uir

ed

toru

nth

esy

ste

m.

Co

lle

cto

ra

ng

le=

158


Author's personal copyT

ab

le1

(Co

nti

nu

ed)

Ty

pe

of

act

ive

sola

rst

ill

Au

tho

r(s)

an

d

test

ing

pla

ce

Sp

eci

fica

tio

ns

Ex

pe

rim

en

tal

resu

lts

Re

ma

rks

Co

pp

er

tub

ed

iam

ete

r=

0.0

09

25

m

Co

pp

er

tub

ele

ng

th=

14

.4m

So

lar

stil

lco

up

led

wit

h

pa

rab

oli

cco

nce

ntr

ato

r

Ze

ina

bS

.Ab

de

l-R

eh

ime

ta

l.[3

6]

Ty

pe

=S

ing

leb

asi

nso

lar

stil

lT

he

pro

du

ctiv

ity

of

2.7

5l/

da

yw

as

pro

du

ced

by

mo

difi

ed

stil

l,w

he

rea

s2

l/d

ay

for

con

ve

nti

on

al

stil

l.

Th

issy

ste

mis

mo

ree

con

om

ica

lth

an

the

con

ve

nti

on

al

stil

l.

Giz

a,

Eg

yp

tS

till

are

a=

1m

2T

he

ma

xim

um

of

35

%w

as

ach

iev

ed

by

the

mo

difi

ed

stil

l

at

15

h.

Pu

mp

an

dm

oto

ra

reu

sed

inth

isst

ud

y.

Co

lle

cto

ra

rea

=8

0cm

lon

g

an

d0

.04

cmth

ick

ne

ss

Mo

ren

um

be

ro

fp

art

sth

an

the

oth

er

syst

em

.

Co

pp

er

pip

ele

ng

th=

2m

Do

ub

lee

ffe

ctst

ill

cou

ple

dw

ith

pa

rab

oli

cco

nce

ntr

ato

r

Bh

ag

wa

nP

rasa

da

nd

Tiw

ari

[39

]

Ty

pe

=si

ng

leb

asi

nso

lar

stil

lM

ax

imu

md

ail

yy

ield

of

14

.68

4k

g/d

ay

wa

so

bta

ine

d

thro

ug

hd

ou

ble

eff

ect

.

Hig

he

ry

ield

as

com

pa

red

tofl

at

pla

teco

lle

cto

rd

ue

tom

ore

con

cen

tra

tio

no

fso

lar

rad

iati

on

.

Ne

wD

elh

i,In

dia

Are

a=

1m�

1m

Th

eto

tal

ho

url

yy

ield

de

cre

ase

sw

ith

flo

wra

ted

ue

to

the

wa

ste

of

ho

tw

ate

rfr

om

the

up

pe

rb

asi

n.

Ma

inte

na

nce

eff

ort

sa

reh

igh

er.

CP

Cco

lle

cto

ra

rea

=1

m�

1m

Th

etr

ack

ing

can

be

ad

just

ed

ma

nu

all

yto

rece

ive

the

ma

xim

um

rad

iati

on

.

Ve

rysu

sce

pti

ble

ino

pe

rati

on

.

Co

lle

cto

rle

ng

th=

1m

Ma

inta

inin

gg

lass

cov

er

tem

pe

ratu

reis

acr

uci

al

fact

or.

Forc

ed

circ

ula

tio

nm

od

e

Flo

wra

te=

0.0

02

7k

g/s

Re

ge

ne

rati

ve

stil

lco

up

led

wit

hp

ara

bo

lic

con

cen

tra

tor

Sa

nja

yK

um

ar

an

d

Sin

ha

[40

]

Ty

pe

=D

ou

ble

slo

pe

sola

rst

ill

Co

nce

ntr

ato

rco

up

led

reg

en

era

tiv

eso

lar

wa

sp

rod

uce

d

8.2

l/m

2d

ay

,wh

ere

as

7.7

l/m

2d

ay

for

fla

tp

late

coll

ect

or

an

d4

.1l/

m2

da

yfo

rp

ass

ive

stil

l.

Re

ge

ne

rati

on

ad

de

dth

eh

igh

er

yie

ld.

Ne

wD

elh

i,In

dia

Sti

lla

rea

=1

m2

Th

eo

ve

rall

the

rma

le

ffici

en

cyo

fC

PC

cou

ple

d

reg

en

era

tiv

eso

lar

stil

lw

as

hig

he

rth

an

the

fla

tp

late

coll

ect

or

cou

ple

dre

ge

ne

rati

ve

sola

rst

ill

Mo

reco

mp

lex

inco

nst

an

tfl

ow

of

wa

ter.

Insu

lati

on

thic

kn

ess

=0

.00

4m

Pu

mp

isre

qu

ire

dfo

rci

rcu

lati

on

of

wa

ter.

CP

Ca

rea

=0

.08

6m

2N

ot

suit

ab

lefo

rru

ral

ap

pli

cati

on

s.

Flo

wra

te=

0.0

5k

g/s

Ca

pit

al

cost

ish

igh

com

pa

red

too

the

ra

ctiv

eso

lar

stil

ls.

Win

dsp

ee

d=

5m

/s

So

lar

stil

lco

up

led

wit

h

ev

acu

ate

dtu

be

coll

ect

or

Tiw

ari

et

al.

[42

]T

yp

e=

Sin

gle

ba

sin

sola

rst

ill

Th

eto

tald

ail

yp

rod

uct

ion

of

4k

g/m

2d

ay

wa

sca

lcu

late

d

usi

ng

the

ore

tica

la

na

lysi

s.

No

wa

da

ys

ev

acu

ate

dtu

be

coll

ect

or

be

com

ech

ee

p

tha

nth

efl

at

pla

teco

lle

cto

r.

Ne

wD

elh

i,In

dia

Sti

lla

rea

=1

m2

Th

eo

ve

rall

the

rma

leffi

cie

ncy

is1

7.2

2%

,wh

ich

ish

igh

er

tha

nth

efl

at

pla

teco

lle

cto

r.

Be

sto

pti

on

for

the

pro

du

ctio

no

fh

ot

an

dd

isti

lle

d

wa

ter

sim

ult

an

eo

usl

y.

Ma

sso

fw

ate

rin

the

ba

sin

=5

0k

g

Ev

acu

ate

dtu

be

coll

ect

or

are

a=

2m

2

Flo

wra

te=

0.0

35

kg

/s

So

lar

stil

lco

up

led

wit

h

he

at

pip

e

Hir

osh

iT

an

ak

ae

ta

l.[4

5]

Ty

pe

:V

ert

ica

lm

ult

iple

eff

ect

sola

rst

ill

Th

ed

isti

lla

tep

rod

uct

ion

rate

is0

.1g

/m2

sa

fte

r3

00

min

of

the

sta

rtin

gd

ay

.

Re

lati

ve

lyh

igh

er

pro

du

ctiv

ity

as

com

pa

red

too

the

r

sola

rco

lle

cto

rs.

Fuk

uo

ka

,Ja

pa

nC

oll

ect

or

are

a=

28

0m

m�

57

0m

mT

he

ma

xim

um

tem

pe

ratu

reo

f7

08C

iso

bta

ine

dd

uri

ng

the

test

.

Init

ial

cost

isa

lso

hig

he

r.

Sti

lla

ng

le=

268

Th

eo

ve

rall

pro

du

ctio

nra

tes

of

the

mu

ltip

lee

ffe

cts

stil

l

we

rea

bo

ut

93

%.

Be

sto

pti

on

for

hig

he

rp

rod

uct

ion

.

Insu

lati

on

ma

teri

al=

Gla

ssw

oo

l

Insu

lati

on

thic

kn

ess

=1

0m

m

Air

ga

p=

24

mm

So

lar

stil

lco

up

led

wit

h

sola

rp

on

d

Ve

lmu

rug

an

et

al.

[49

]T

yp

e=

Ste

pp

ed

sola

rst

ill

Ma

xim

um

pro

du

ctiv

ity

of

80

%in

sin

gle

ba

sin

sola

rst

ill

cou

ple

dw

ith

min

iso

lar

po

nd

wa

sa

chie

ve

du

sin

gfi

ns

an

dsp

on

ge

s.

Pa

yb

ack

pe

rio

dis

36

7d

ay

s.

Ma

du

rai,

Ind

iaA

rea

=1

m�

1m

Pe

bb

les

sto

rem

ore

the

rma

le

ne

rgy

an

dre

lea

ses

aft

er

sun

set.

Op

era

tio

na

ld

iffi

cult

yco

uld

by

occ

ur

ina

dd

ing

of

en

erg

yst

ori

ng

ma

teri

als

.

Sti

lla

ng

le=

985

50

Ind

ust

ria

le

fflu

en

tis

use

da

sfe

ed

.C

on

stru

ctio

nis

intr

ica

te.

Insu

lati

on

ma

teri

al=

saw

du

stB

est

op

tio

nfo

rin

du

stri

al

ap

pli

cati

on

s.

Dim

en

sio

ns

of

sola

rp

on

d:

To

pla

ye

r=

0.9

m

Bo

tto

mla

ye

r=

0.3

m

To

tal

he

igh

t=

0.3

m

So

lar

stil

lco

up

led

wit

h

hy

bri

dP

V/T

syst

em

Sh

ivK

um

ar

an

d

Tiw

ari

[54

]

Ty

pe

=S

ing

leb

asi

nso

lar

stil

lH

igh

er

yie

ldw

as

ach

iev

ed

com

pa

rew

ith

pa

ssiv

eso

lar

stil

l.

Hig

hca

pit

al

cost

(PV

mo

du

le)



Ta

ble

1(C

on

tin

ued

)

Ty

pe

of

act

ive

sola

rst

ill

Au

tho

r(s)

an

d

test

ing

pla

ce

Sp

eci

fica

tio

ns

Ex

pe

rim

en

tal

resu

lts

Re

ma

rks

Ne

wD

elh

i,In

dia

.A

rea

=1

m�

1m

Th

ea

ve

rag

ev

alu

es

of

con

ve

ctiv

ea

nd

ev

ap

ora

tiv

eh

ea

t

tra

nsf

er

coe

ffici

en

tso

fth

eP

V/T

act

ive

sola

rst

ill

are

3–

5

tim

es

hig

he

rth

an

the

pa

ssiv

eso

lar

stil

l

Th

ep

ay

ba

ckp

eri

od

of

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where

Ieff ¼ FRðatÞcIðtÞc þ ðatÞeff IðtÞs

Ueff ¼ ðULS þ FRULCÞ

Dividing Eq. (61) by (MC)w, it becomes,

dTw

dtþ

Ueff

ðMCÞw

� �Tw ¼

Ueff

ðMCÞw

� �Ta þ

Ieff

ðMCÞw

� �(62)

The above Eq. (62) can be written in first order differentialequation form as,

dTw

dtþ aTw ¼ f ðtÞ (63)

a ¼Ueff

ðMCÞw

� �and f ðtÞ ¼

Ieff þ Ueff Ta

ðMCÞw

� �

The following assumption have been made to find approximateanalytical solution,

1. The time interval Dt 0< t<Dt� �

is small.2. a is constant during the time interval Dt.3. The function f(t) is constant, i.e., f ðtÞ ¼ f ðtÞ for the time interval

between 0 and t.

By using the following boundary condition at t = 0,Twðt¼0Þ ¼ Tw0, the solution of Eq. (63) is derived as follows,

Tw ¼f ðtÞa

1� e�at�

þ Two e�at (64)

The hourly yield is given by the following equation

mew ¼he;w�gðTw � TgiÞ

L� 3600� As (65)

The total daily yield is given as follows

Mew ¼X24

i¼1

mew (66)

Similarly, yield expression will be obtained for other type ofsolar collectors using the following equations. The rate of heatenergy delivered by concentrating collector is given as,

Qu ¼ FR ðatÞcIðtÞc � ULCAr

Aa

� �ðTw � TaÞ

� �(67)

The rate of heat energy delivered by evacuated tube collector isgiven as,

Qu ¼ FR ðatÞcIðtÞc � ULCAL

AET

� �ðTw � TaÞ

� �(68)

where, AL ¼ pAET

The rate of heat energy delivered by evacuated tube heat pipecollector is given as,

Qu ¼ AcFR½ðatÞcIðtÞc � ULCðTw � TaÞ� (69)

Overall thermal efficiency of active solar still is,

hActi ¼P

mewLPðIðtÞc � Ac � 3600Þ þ

PðIðtÞs � As � 3600Þ : (70)

5. Discussion and scope for further research

The selection of solar still is a crucial factor, which directlyaffects successful implementation of solar energy systems in ruralareas. The following factors are to be considered for the selection ofsolar still: the availability of solar radiation, total water require-ment, salt/saline water available, cost of the still, operatingeasiness, maintenance cost, better utilization of available wastehot water and life of solar still. The comparative study on differenttypes of active solar stills with their productivity and their remarksare depicted in Table 1. The further research of active solar still maybe focussed on the following factors:

(a) The composite material may be used instead of FRP, GRP, GIsheet, aluminium sheet, etc., as a basin liner to increase thethermal conductivity and in turn the yield.

(b) The side and bottom heat losses may be minimized by goodinsulation materials like PUF, glass wool, etc.

(c) The natural circulation mode is to be used in active solar still toavoid electricity consumption by pump in forced circulationmode.

(d) The effect of energy storing materials in high temperaturedistillation may be taken to find productivity enhancement.

(e) More research may be carried out in active solar stills withother developed technologies like ETC, ETC with heat pipes andmultistage solar distillation.

6. Conclusion

Energy is a basic necessity for all of us to lead a normal life inthis wonderful world. Solar energy technologies and its usage isvery important and useful for the developing and under developedcountries to sustain their energy needs. It is very consistent and isnot significantly vulnerable to changes in climatic condition. Theuse of solar energy in desalination process is one of the bestapplications of renewable energy. Solar still has become morepopular particularly in rural areas. The solar stills are friendly tonature and eco-system. Various types and developments in activesolar distillation systems, theoretical analysis and future scope forresearch were reviewed in detail. Based on the review anddiscussions, the following could be concluded.

� The annual yield is at its maximum when the condensing glasscover inclination is equal to the latitude of the place.� The yield is directly related to thermal conductivity of

condensing cover materials; copper gives a greater yieldcompared to glass and plastic due to higher thermal conductivi-ty.� Solar still coupled with FPC with forced circulation mode gives

higher yield than that of the thermosyphon mode.� Double slope active solar still under natural circulation mode

gives higher yield in comparison with the double slope passivesolar still. The thermal efficiency of double slope active solar stillis lower than the thermal efficiency of double slope passive solarstill.� The exergy efficiency of double slope active solar still is higher

than the exergy efficiency of double slope passive solar still.� In active double effect solar still, a higher yield from the lower

basin at noon is due to the high water temperature at that time.� The hourly yield is only possible in the active mode of operation

and hence commercially viable.� The concentrator assisted regenerative solar still has much

higher thermal efficiency than the flat plate collector assistedregenerative still at all water depths and they inferred that thereis less thermal loss in the concentrator compared to the flat platecollector panel.



� The direct stream generation parabolic through is a promisingtechnology for solar assisted seawater desalination.� The maximum productivity is achieved, when energy storing

materials were used in the stepped solar still coupled with minisolar pond.� Higher productivity during night time is achieved by using

energy storing materials in the active solar stills.� The yield was high in hybrid photovoltaic/thermal (PV/T) active

solar still compared to the passive solar still.� The multistage solar desalination system with heat recovery

system produces higher yield than the simple solar still.� The length of solar still, depth of water in basin, inlet water

temperature and solar radiation are the major parameters whichaffects the performance of the still.� The solar still fed with hot water at constant rate gives higher

yield in comparison to a still with hot water filled only once in aday.� The evaporative heat transfer coefficient depends strongly on the

temperature and it is advisable to use the waste hot water eitherin higher temperature or during off sunshine hours.� The purity of the desalinated water in the tubular solar still is

greater than that of a conventional one.� Local climatic condition and application is to be considered while

selection of solar still.� The thermal model of solar stills should be developed based on

the assumption that Tgi 6¼ Tgo.� Kumar and Tiwari model is most suitable for evaluating the

internal heat transfer coefficients and hourly yield accuratelyexcept in extreme cases.� The values of ‘C’ and ‘n’ differ for each design of the solar still and

for the operating water temperature range. Therefore, it isrecommended that before predicting the performance theoreti-cally, experiments must be carried out for given climaticconditions to evaluate the values of ‘C’ and ‘n’ for a particulardesign of solar still.

Acknowledgements

The work was motivated by Prof. G.N. Tiwari, Centre for EnergyStudies, Indian Institute of Technology Delhi, New Delhi throughQIP short term course sponsored by MHRD, Government of India.The useful parley with Prof. M. Eswaramoorthy and Dr. S. Shankarfor the preparation of this article is gratefully acknowledged.

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