Active Solar Distillation—a Detailed Review (Articulo)

8/21/2019 Active Solar Distillationa Detailed Review (Articulo)

1/24

Active solar distillationA detailed review

K. Sampathkumar a,*, T.V. Arjunan b, P. Pitchandi a, P. Senthilkumar c

a Department of Mechanical Engineering, Tamilnadu College of Engineering, Coimbatore 641659, Tamilnadu, IndiabDepartment of Automobile Engineering, PSG College of Technology, Coimbatore 641004, Tamilnadu, IndiacDepartment of Mechanical Engineering, KSR College of Engineering, Tiruchengode 637215, Tamilnadu, India

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15052. Classification of active solar distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505

3. Active solar distillation system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505

3.1. High temperature active solar distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506

3.1.1. Solar still coupled with flat plate collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506

3.1.2. Solar still coupled with parabolic concentrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509

3.1.3. Solar still coupled with evacuated tube collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1511

3.1.4. Solar still coupled with heat pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1511

3.1.5. Solar still coupled with solar pond. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512

3.1.6. Solar still coupled with hybridPV/Tsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512

3.1.7. Multistage active solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1513

3.1.8. Multi effect active solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1514

3.1.9. Air bubbled solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515

3.1.10. Hybrid solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515

3.2. Pre-heated water active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515

3.3. Nocturnal active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15164. Theoretical analysis of active solar distillation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517

4.1. Heat transfer in active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517

4.1.1. Internal heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517

4.1.2. External heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519

4.2. Thermal modelling of active solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519

4.2.1. Inner and outer surface of glass cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519

4.2.2. Inner surface of glass cover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520

4.2.3. Outer surface of glass cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520

Renewable and Sustainable Energy Reviews 14 (2010) 15031526

A R T I C L E I N F O

Article history:

Received 6 November 2009

Received in revised form 15 December 2009Accepted 25 January 2010

Keywords:

Active solar still

Desalination

Flat plate collector

Review

Solar pond

Thermal modelling

A B S T R A C T

All over the world, access to potable water to the people are narrowing down day by day. Most of the

human diseases are due to polluted or non-purified water resources. Even today, under developed

countries and developing countries face a huge water scarcity because of unplanned mechanism and

pollution created by manmade activities.Water purification without affecting the ecosystem is the need

of the hour. In this context, many conventional and non-conventional techniques have been developed

for purification of saline water. Among these, solar distillation proves to be both economical and eco-

friendly technique particularly in rural areas. Many active distillation systems have been developed to

overcome the problem of lower distillate output in passive solar stills. This article provides a detailed

review of differentstudies on activesolar distillation system over the years. Thermal modellingwas done

forvarioustypesof activesingle slope solar distillation system. This reviewwould also throw light on the

scope for further research and recommendations in active solar distillation system.

2010 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +91 421 2332544; fax: +91 421 2332244.

E-mail addresses: [email protected](K. Sampathkumar), [email protected](P. Senthilkumar).

Contents lists available atScienceDirect

Renewable and Sustainable Energy Reviews

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / r s e r

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

doi:10.1016/j.rser.2010.01.023
mailto:[email protected]:[email protected]://www.sciencedirect.com/science/journal/13640321http://dx.doi.org/10.1016/j.rser.2010.01.023http://dx.doi.org/10.1016/j.rser.2010.01.023http://www.sciencedirect.com/science/journal/13640321mailto:[email protected]:[email protected]


2/24

4.2.4. Basin liner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520

4.2.5. Water mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520

5. Discussion and scope for further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1524

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1524

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1525

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1525

Nomenclature

Aa aperture area of concentrating collector (m2)

Ac area of solar collector (m2)

AET absorber tube diameter times collector length in

ETC (m2)

Ar receiver area of concentrating collector (m2)

Ass area of sides in solar still (m2)

As area of basin in solar still (m2)

C constant in Nusselt number expression

Cp specific heat of vapour (J/kg 8C)

Cw specific heat of water in solar still (J/kg 8C)

FR heat removal factor

g acceleration due to gravity (m/s2)Gr Grashof number

hc;ba convective heat transfer coefficient from basin to

ambient (W/m2 8C)

hr;ba radiative heat transfer coefficient from basin to

ambient (W/m2 8C)

ht;ba total heat transfer coefficient from basin to

ambient (W/m2 8C)

hc;ga convective heat transfer coefficient from glass

cover to ambient (W/m2 8C)

hr;ga radiative heat transfer coefficient from glass cover

to ambient (W/m2 8C)

ht;ga total (convective and radiative) heat transfer

coefficient from glass cover to ambient (W/m2 8C)hc;wg convective heat transfer coefficient from water to

glass cover (W/m2 8C)

he;wg evaporative heat transfer coefficient from water to


hr;wg radiative heat transfer coefficient from water to


ht;wg total heat transfer coefficient from water to glass

cover (W/m2 8C)

hw convective heat transfer coefficient from basin

liner to water (W/m2 8C)

hb overall heat transfer coefficient from basin

liner to ambient through bottom insulation (W/

m2 8C)I(t)c intensity of solarradiation over the inclined surface

of the solar collector (W/m2)

I(t)s intensity of solarradiation over the inclined surface

of the solar still (W/m2)

Ki thermal conductivity of insulation material (W/

m 8C)

Kg thermal conductivity of glass cover (W/m 8C)

Kv thermal conductivity of humid air (W/m 8C)

Kw thermal conductivity of water (W/m 8C)

L latent heat of vaporization (J/kg)

Li thickness of insulation material (m)

Lg thickness of insulation glass cover (m)

Ma molecular weight of dry air (kg/mol)

mew hourly output from solar still (kg/m2 h)

Mew daily output from solar still (kg/m2 day)

Mw mass of water in the basin (kg)

Mwv molecular weight of water vapour (kg/mol)

n constant in Nusselt number expression

Pgi partial vapour pressure at inner surface

glass temperature (N/m2)

Pr Prandtl number

Pt total vapour pressure in the basin (N/m2)

Pw partial vapour pressure at water temperature (N/

m2)

qc;wg

rate of convective heat transfer from water to glass

cover (W/m2)

qe;wg rate of evaporative heat transfer from water to

glass cover (W/m2)

qr;wg rate of radiative heat transfer from water to glass

cover (W/m2)

qt;wg rate of total heat transfer from water to glass cover

(W/m2)

qr;ga rate of radiative heat transfertfrom glass cover to

ambient (W/m2)

qc;ga rate of convective heat transfer from glass cover to

ambient (W/m2)

qt;ga rate of total heat transfer from glass cover to

ambient (W/m2)

qw rate of convective heat transfer from basin liner to

water (W/m2)

qb rate of heat transfer from basin liner to ambient

(W/m2)

Qu useful thermal energy gain from the solar collector

(W/m2)

Ra Rayleigh number

Ra0 modified Rayleigh number

t time (s)

Ta ambient temperature (8C)

Tb basin temperature (8C)

Tgi inner surface glass cover temperature (8C)

Tgo outer surface glass cover temperature (8C)

Tsky temperature of sky (8C)

Tw water temperature (8C)

DT temperature difference between water and glass

surface (8C)

Ub overall bottom heat loss coefficient (W/m2

8C)

Us overall side heat loss coefficient (W/m2

8C)

ULC overall heat transfer coefficient for solar collector

(W/m2 8C)

ULS overall heat transfer coefficient for solar still (W/

m2 8C)

Ut overall top heat loss coefficient from water surface

to ambient air (W/m2 8C)

K. Sampathkumar et al./ Renewable and Sustainable Energy Reviews 14 (2010) 150315261504


3/24

1. Introduction

Water is a natures gift and it plays a key role in the

development of an economy and in turn for the welfare of a

nation. Non-availability of drinking water is one of the major

problem faced by both the under developed and developingcountries all over the world. Around 97% of the water in the world

is in the ocean, approximately 2% of the water in the world is at

present storedas ice in polar region, and 1% is fresh water available

for the need of the plants, animals and human life [1]. Today,

majority of the health issues are owing to the non-availability of

clean drinking water.In therecent decades,most partsof theworld

receive insufficient rainfall resulting in increase in the water

salinity. The pollution of water resources is increasing drastically

due to a number of factors including growth in the population,

industrialization, urbanization, etc. These activities adversely

affected the water quality in rural areas and agriculture. Globally,

200 million hours are spent each day, mostly by females, to collect

water from distant, often polluted sources. In the world, 3.575

million people die each year from water related diseases. The basicmedical facilities never spotted numerous villages in the develop-

ing andunder developedcountries. Majority of the rural people are

still unaware of the consequences of drinking untreated water.

Desalination is the oldest technology used by people for water

purification in the world. Various technologies were invented for

desalination from time to time and it has been accepted by people

without knowing future environmental consequences. Major

desalination techniques like vapour compression distillation,

reverse osmosis and electrolysis used electricity as input energy.

But in the recent years, most of the countries in the world have

been significantly affected by energy crisis because of heavy

dependency on conventional energy sources (coal power plants,

fossil fuels, etc.), which has directly affected the environment and

economic growthof these countries. Thechanging climate is one of

the major challenges the entire world is facing today. Gradual rise

in globalaverage temperatures, increase in sea level and melting of

glaciers and ice sheets have underlined the immediate need to

address the issue. All these problems could be solved only through

efficient and effective utilization of renewable energy resources

such as solar, wind, biomass, tidal, and geothermal energy, etc.

Solar energy is available in abundant in most of the rural areas

andhence solar distillation is thebest solution forruralareas andhas

many advantages of using freelyavailable solar energy.It is a simple

technology and more economical than the other available methods.

A solar still operates similar to the natural hydrologic cycle of

evaporation and condensation. The basin of the solar still is filled

with impure water and the sun rays are passed through the glass

cover to heat the water in the basin and the water gets evaporated.

As the water inside the solar still evaporates, it leaves all

contaminates and microbes in the basin. The purified water vapour

condenses on the inner side of theglass, runs through thelower side

of the still and then gets collected in a closed container [2]. Many

solar distillation systems were developed over the years using the

above principle for water purification in many parts of the world.

Thispaperreviewsthe technological developments of various active

solar distillationsystemsdevelopedby various researchers in detail.

The review also extends to thermal modelling of some active solar

distillation systems, comparative studies of different active solarstills, scope for further research and recommendation.

2. Classification of active solar distillation

The solar distillation systems are mainly classified as passive

solar still and active solar still. The numerous parameters are

affecting the performance of the still such as water depth in the

basin, material of the basin, wind velocity, solar radiation, ambient

temperature and inclination angle. The productivity of any type of

solar still will be determined by the temperature difference

between the water in the basin and inner surface glass cover. In a

passive solar still, the solar radiation is received directly by the

basin water and is the only source of energy for raising the water

temperature and consequently, the evaporation leading to a lowerproductivity. This is the main drawback of a passive solar still.

Later, in order to overcome the above problem, many active solar

stills have been developed. Here, an extra thermal energy is

supplied to the basin through an external mode to increase the

evaporation rate and in turn improve its productivity. The active

solar distillation is mainly classified as follows[2]:

(i) High temperature distillationHot water will be fed into the

basin from a solar collector panel.

(ii) Pre-heated water applicationHot water will be fed into the

basin at a constant flow rate.

(iii) Nocturnal productionHot water will be fed into the basin

once in a day.

3. Active solar distillation system

The performance of a solar still could neither be predicted nor

improved by some of the uncontrollable parameters like intensity

of solar radiation, ambient temperature and wind velocity. But,

there are certain parameters such as depth of water, glass cover

angle, fabrication materials, temperature of water in the basin and

insulation thickness, which affects the performance of the solar

still that could be modified forimproving theperformance. Thestill

performance can be increased by reducing the water depth and

thereby increasing the evaporation rate. The temperature differ-

ence between water in the basin and condensing glass cover also

has a direct effect in the performance of the still. The increased

temperature of the water in basin can increase the temperature

v wind velocity (m/s)

Xv mean characteristic length of solar still between

evaporation and condensation surface (m)

Xw mean characteristic length of solar still between

basin and water surface (m)

Greek letters

a absorptivityav thermal diffusivity of water vapour (m

2/s)

a0 fraction of energy absorbed

(at) absorptancetransmittance product

b coefficient of volumetric thermal expansion factor

(1/K)

e emissivity

g relative humidity

mv viscosity of humid air (Pa s)

rv density of vapour (kg/m3)

s Stefan Boltzman constant (5.67108 W/m2 K4)

Subscripts

a ambientb basin liner

c collector

eff effective

g glass cover

s solar still

w water

K. Sampathkumar et al./ Renewable and Sustainable Energy Reviews 14 (2010) 15031526 1505


4/24

difference between the evaporating and condensing surfaces. To

achieve better evaporation and condensation rate,the temperature

of water in the basin could be raised by feeding thermal energy

from some external sources.

3.1. High temperature active solar distillation

The water temperature of the conventional still is increased by

supplying additionalthermal energy through solarcollectors to the

basin. The temperature is increased from 2050 8C to 7080 8C in

high temperature distillation for better evaporation.

3.1.1. Solar still coupled with flat plate collector

Thesolarstill coupled with flatplatecollector is working as high

temperature distillation method. The solar still coupled with flat

plate collector (FPC) works either in forced circulation mode or

natural circulation mode. In forced circulation mode, a pump is

used for supplying water. In natural circulation mode, water flows

due to the difference in the density of water.

3.1.1.1. Forced circulation mode. The flat plate collector gives an

additional thermal energy to the basin of the solar still. A pump is

used to circulate the water from the basin via flat plate collector to

the basin. Many researches havebeen carried out in this methodandthe first being reported by Rai and Tiwari[3]. They found that, the

daily distillateproduction of a coupledsinglebasin still is 24% higher

than that of an uncoupled one using forced circulation mode. A

schematic diagram of an active solarstill integrated with a flat plate

collector under forced circulation mode is shown in Fig. 1. Rai et al.

[4] experimentally studied the various modes of operations in single

basin solar still coupled with flat plate collector. From their study

shows that, the rate of daily distillate deceases with the salt

concentration. The addition of salt increases the surface tension and

hence decreases the rate of evaporation. The best performance was

observed in a single basin still coupled with a flat plate collector

having forced circulation and blackened jute cloth floating over the

basin water anda small quantity of black dyeadded to thewater. And

alsofoundthat, therate of distillation increased by 30%when a smallquantityof blackdye is added to the water. The bottom insulation is

an important design parameter of the active solar still and for

drinking purposes, the conventional solar still will give better

performance because, the efficiency of the system reduces with the

increase in the effective area as reported by Tiwari and Dhiman[5].

Their experimental study showed that, there was only 12% rise in

yieldof the system if the length of the heat exchanger is varied from

6.0 to 12.0 m and the overallefficiency of the system variedfrom 15

to 19%.

Sanjeev Kumar and Tiwari [6] observed that, temperature of

water and thermal efficiency decreased with an increase in basin

area dueto thelargestorage capacity of thewatermass in thebasin

and depth of water, respectively. Yield increased with increase in

the number of collectors, as expected, owing to increased heat

transfer from the collector panel into the basin and the optimum

number of collectors for maximum yield is 8 m2 since beyond that

the increase in gain will be lower than the thermal loss. Sanjeev

Kumar et al.[7] suggested that, for maximum annual yield, the

optimum collector inclination for a flat plate collector is 208 and

that of still glass cover is 158 for New Delhi climatic condition.

Tiwari et al. [8] inferred that, the internal heat transfer

coefficients should be determined by using inner glass cover

temperature for thermal modelling of passive and active solar

stills. Theheat transfer coefficients mainly depends on the shape of

the condensing cover, material of the condensing cover and

temperature difference between water and inner glass cover. On

the basis of the numerical computation,Singh andTiwari [9] found

that, the annual yield is at its maximum when the condensing glass

cover inclination is equal to the latitude of the place and the

optimum collector inclination for a flat plate collector is 28.588, for

a condensing glass cover inclination of 18.588 for New Delhi

climatic condition.Rajesh Tripathi and Tiwari [10] inferred that the

convective heat transfer coefficient between water and innercondensing cover depends significantly on the water depth of the

basin. It is also observed that more productivity was obtained

during the off shine hours as compared to day time for higher

water depths in solar still (0.10 mand 0.15 m)due to storage effect.

Vimal Dimri et al.[11] conducted theoretical and experimental

analysis of a solar still integrated with flat plate collector with

various condensing cover materials. The results indicated that

yield is directly related to thermal conductivity of condensing

cover materials; coppergives a greater yield compared to glass and

plastic due to higher thermal conductivity.

Tiwariet al. [12] presented theparametric study of passive and

active solar stills integrated with a flat plate collector. Computer

based thermal models were developed based on two assumptions:

Tgi=Tgoand Tgi6Tgo. The results show that (i) there is an effect ofthe inner and outer glass temperature on the daily yield of both

active and passive solar stills. (ii) The mean estimated error

involved in predicting the hourly yield of the passive solar still and

active solar still using the thermal model based on the assumption

that Tgi=Tgo is 6% and3%, respectively. Hence,the thermal model of

solar stills should be developed based on the assumption that

Tgi6Tgo. (iii) The results of the thermal model for the active solar

still forN= 1 show that the daily yield values are 3.08 l and 2.85 l

for Tgi=Tgo and Tgi6Tgo, respectively. Tiwari and Tiwari [2]

reported the performance of single slope passive still coupled with

multi flat plate collectors. In their newdesign, rather than coupling

a single collector, multiple collectors were integrated with the

solar still. The results show that, for New Delhi climatic conditions,

the daily yield increases with number of collectors for basin area1 m2, collector area 2 m2, mass of saline water 150 kg and also the

optimum number of collectors for single effect, double effect and

triple effect were 10, 9 and 6, respectively. In single effect, if there

are more than 8 collectors, the daily yield is higher than the double

effect but at the cost of additional collectors.

3.1.1.2. Natural circulation mode. The working of solar thermal

devices under thermosyphon mode has been more advantageous

than the forced circulation mode in terms of simplicity, reliability

and cost effectiveness. Theoretical study on single basin solar still

coupled with flatplate collectorthrough heat exchanger have been

reported by Lawrence and Tiwari[13]. The results show that, the

efficiency of active solar still is less than that of a simple solar still

and the daily yield from the simple solar still decreases with theFig. 1.Schematic of an active solar still integrated with a flat plate collector [12].



5/24

increase in water depth, while for an active solar still, it is the

reverse (Fig. 2).

Yadav [14] studied the performance of a solar still coupled with

a flat plate collector using thermosyphon mode and forced

circulation mode for New Delhi climatic condition. The author

found that, the system using the forced circulation mode gives 5

10% higher yield than that of the thermosyphon mode and 3035%enhancement in the yield was observed with simple solar still. The

steady state condition of the system was achieved after 23 days.

Yadav[15]studied the transient performance of a high tempera-

ture solar distillation system. The study reveals that it is

worthwhile to consider a temperature dependent evaporative

heat transfer coefficient when evaluating the performance of a

high temperature distillation. Tiris et al. [16] conducted

experiments on two flat plate solar collectors integrated with a

basin type solar still. From their study, the collector integrated

solar still gave an average increase of 100% in yield in comparison

with the simple basin solar still. Maximum yield was 2.575 l/

m2 day for the simple basin and 5.18 l/m2 day for the integrated

system, while the corresponding solar radiation is 24.343 MJ/

m2

day.Ali A Badran et al. [17] performed the tests in solar still

augmented with flat plate collector using tap water and saline

water.They found that themass of distilledwater production using

augmentation increased by 231% in case of tap water as a feed and

by 52% in case of salt water as a feed. Badran and Al-Tahainesh [18]

presented the effect of coupling a flat plate collector on the solar

still productivity. The results showed that, the output of the still is

maximum for the least water depth in the basin (2 cm). Also, the

increase in water depth has decreased the productivity, while the

still productivity is found to be proportional to the solar radiation

intensity.

Dwidevi and Tiwari [19] experimentally studied the double

slope active solar still under natural circulation mode. From the

study, they observed that, the double slope active solar still under

natural circulation modes gives 51% higher yield in comparison to

the double slope passive solar still. The thermal efficiency of

double slope activesolar still is lower than thethermal efficiencyof

double slope passive solar still. However, the exergy efficiency of

double slope active solar still is higher than the exergy efficiency ofdouble slope passive solar still.

3.1.1.3. Double effect active solar still. Glass temperature is another

main parameter, which affects the performance of the solar still.

The rate of evaporation increased with reduction of glass

temperature. The rate of evaporation of water from a water

surface will be higher than the rate of release of heat from the glass

cover to ambient by convection and radiation processes. If the heat

loss from glass cover to ambient can be increasedand that heat loss

is used for further distillation, then overall efficiency of the

distillation unit under active modes of operation can be increased

significantly, as in the case of double basin solar still. This can be

obtained by flowing the water over the glass cover for fast heat

transfer through the lower glass cover and then condensing theevaporated water from the upper glass cover as distillate ( Fig. 3).

Tiwari and Lawrence [20] observed from the experimental

study that, there is an increase of about 20% and 30% yield for inlet

temperature equal to ambient temperature fora passive and active

solar still, respectively. If the inlet temperature is increased, the

output from the upper basin is increased but the output from the

lower basin is appreciably reduced due to a lower value of water-

glass temperature difference in the lower basin. Bapeshwararao et

al.[21]presented from transient analysis that the distillate output

increases with increase in the initial water temperature in both

basins, the dependence on lower basin water temperature shows

more effect than that of upper one comparatively and remarkable

increase in the efficiency of the present system over that of the

simple solar still in all the cases. Tiwari and Sharma[22]studiedthe double effect solar distillation under active mode of operation

using heat exchanger. The study shows that, there is an increase of

about 30% in the active solar still due to water flow through the

upper basin and there is a marginal increase in efficiency with

increase in the length of the heat exchanger.

Kumar Sanjeev and Tiwari[23]presented the performance of

daily yield for an active double effect distillation system with

water flow. The results show that, a higher yield from the lower

basin with a maximum yield of 3.34 kg/m2/h at noon is due to the

high water temperature of 95 8C at that time (Fig. 4). With the

increase in water masses, the operating water temperature in the

lower basin is lowered resulting in reduced yield and efficiency.

Thedaily yield increases with an increase of collectorarea,because

the thermal energy in the basin increases as the collector area

Fig. 2.Schematic diagram of (a) active solar still working under natural circulation;

(b) design of heat exchanger[13].

Fig. 3.Schematic of double effect solar still coupled with flat plate collector [23].



6/24

increases. Sanjay Kumar and Tiwari[24]studied the performance

of single and double effect active solar distillation, with and

without water flow over the glass cover. The study shows that, an

active solar still with water flow arrangements over the glass cover

produces maximum distillate output. The solar still operating inthe double effect mode does not enhance the daily output

significantly because of the difficulties in maintaining reasonably

low and uniform flow rates over the glass cover (10 ml/min).

Sanjay Kumar and Tiwari [25] conducted experiments to

estimate the convective mass transfer in active solar still. The

modified values of C and n for Nu=C(GrPr)n, are proposed as

C = 0.0538; n = 0.383 for 5.498106


7/24

thermosyphon mode of operation for New Delhi climatic condi-

tion. The authors inferred that, (i) there is a significant improve-

ment in overall performance dueto water flow over theglass cover.

(ii) The hot water available due to the regenerative effect does not

enhance the output. (iii) The overall efficiency of the active stills

(conventional and regenerative) is lower than that of the passive

stills (conventional and regenerative) at any common depth of

water because the active stills areoperating at higher temperature.

Tiwari et al. [29] observed that the instantaneous thermal

efficiency of the system decreases with an increase of collectorarea, due to the higher operating temperature range of the

distillation system. Yousef H. Zurigat et al.[30]proved that, the

thickness of water on top of the first glass cover and the mass flow

rate of the water going into the second effect have marginal effect

on the productivity of the regenerative solar still.

3.1.1.5. Solar still coupled with parallel flat plate collector. Yadav and

Prasad[31]experimentally studied the solar still integrated with

parallel flat plate collector. The schematic diagram of a solar still

integrated with a parallel flat plate solar energy collector is shown

in Fig. 7. The collector essentially consists of a parallel flat plate

placed over the insulation with an air gap through which the water

will flow below the absorber.

There is a glass sheet over the absorber and the whole assemblyis enclosed in a wooden box. The top of the plate (absorber) is

blackenedby black boardpaintbefore theglasscoveris placedover

the absorber. The collector outlet is connected to the still by a pipe

covered with insulation. The circulation of water between the

collector and the still can be made either via a pump (forced

circulation system) or by placing the collector over a supporting

structure at such a height as to provide adequate head for natural

circulation of water (thermosyphon) in the system. The results

show that, a significant rise in the distillate output is observed

when the still is coupled with the collector and this system can be

preferred as cost effective compared to the flat plate collector.

3.1.1.6. Vertical solar still coupled with flat plate collector. Kiatsir-

iroat et al. [32]analysed the multiple effect of vertical solar still

coupled with flat plate solar collector. The schematic sketch is

shown inFig. 8. The distillation unit consists of n parallel vertical

plates. The first plate is insulated on its front side and the last plate

is exposed to ambient.

Each plate in the enclosure is covered with wetted cloth on one

side. Theclothis extended into a feed through along theupperedge

of each plate. Feed water in the through is then drawn onto the

plate surface by capillary. Excess water moves down the plate and

is conducted out of the still. The last plate is cooled by air or water.

The authors found that, the distillation output increases slightlywhen the plate number is over5, and it increased by about 34% and

15% when the evaporating plate numbers are 1 and 6, respectively.

3.1.2. Solar still coupled with parabolic concentrator

The schematic diagram of the solar still coupled with parabolic

concentrator is shown in Fig. 9. The parabolic shaped concentrator

or solar collector concentrates the incident solar radiation on large

surface and it focuses on to a small absorber or receiver area. The

performance of concentrators is much affected by the sun tracking

mechanism. The tracking mechanism should move the collectors

throughout the day to keep them focused on the sun rays to

achieve the higher efficiency. These types of solar collectors reach

higher temperature compared to flat plate collectors owing to

reduced heat loss area.The various types of concentrators were used over the years

based on the applications. To achieve higheryield, the contractor is

coupled with solar still by means of increasing water temperature

in the basin. The water or oil will be supplied to trough receiver

pipe by natural circulation mode or forced circulation mode. Singh

et al.[33]found an analytical expression for water temperature of

an active solar still with flat plate collectors and parabolic

concentrator through natural circulation mode.

The results show that, the efficiency of the system with

concentrator is higher than parabolic collector as the evaporative

heat transfer coefficient is higher in concentrator. Garcia Rodriguez

and Gomez Camacho[34]experimentally studied the multi effect

Fig. 7. Schematic diagram of a solar still integrated with a parallel flat plate water collector[31].

Fig. 8.Schematic sketch of the multiple effects still with a flat plate collector [32].

Fig. 9.Solar still coupled with parabolic concentrator. (1) Parabolic through, (2) oil

pipeline, (3) valves, (4) solar still, (5) oil heat exchanger, (6) pump [36].



8/24

distillation system coupled to a parabolic through collector (PTC)

for sea water desalination and suggested the following, (i) the

annual energy production is about 23% grater for a northsouth

collectorthan for an east west one. (ii) The optimum axis height for

a single collector is 298 and it is 12% higher production than a

horizontal collector for an inlet/outlet thermal oil temperature of

225 8C/300 8C. (iii) The maximum yearly average of the dailyoperation time is only about 12 h/day in coastal areas in southern

Spain.

Scrivani et al.[35]presented the concept of utilizing through

type solar concentration plants for water production, remediation,

waste treatment and the system can be used for processing landfill

percolate in arid regions where conventional depuration systems

are expensive and impractical. Zeinab and Ashraf[36]conducted

experimental and theoretical study of a solar desalination system

coupled with solar parabolic through with a focal pipe and simple

heat exchanger (Fig. 9). The results show that, as time goes on, all

the temperatures increase and begin to decrease after 4.00 pm

with respect to the solar radiation, although the temperature

values of themodifiedsystem arestillhigher than theconventional

one. In case of the modified design, the fresh water productivityincreased an average by 18%.

Bechir Chaouchi et al. [37] designed and built a small solar

desalination unit equipped with a parabolic concentrator (Fig. 10).

The results show that, the maximum efficiency corresponds to the

maximum solar lightning obtained towards 14:00. At that hour, the

boiler was nearly in a horizontal position, which maximizes the

offeredheat transfersurface. The experimentaland theoretical study

concluded withan averagerelative error of 42%for thedistillateflow

rate. Thisis dueto theimperfectionsin paraboloidgeometry,the sun

manual follow up and especially to the systems tiltvariation during

theday, which doesnot make it possible always to keepthe absorber

surface covered with salted water. Lourdes Garcia Rodriguez et al.

[38] proposed and evaluated the application of direct steam

generation into a solar parabolic through collector to multi effect

distillation. The obtained results were useful in finding the most

suitable conditions in which solar energy could compete with

conventional energies in solar desalination.

3.1.2.1. Double effect still coupled with parabolic concentrator. B-

hagwan Prasad and Tiwari[39]presented an analysis of a double

effect, solar distillation unit coupled compound parabolic concen-

tration (CPC) collector under forced circulation mode (Fig. 11).The

authors suggested that, (i) the temperature of the water in the

lower basin is increased in comparison with single effect

distillation due to the reduced upward heat losses. (ii) The hourly

output in the lower basin is reduced due to the reduced

temperature differencebetween the waterand glass temperatures.

However, the overall output is increased due to reutilization of the

latentheat of evaporation in thesecond effect. (iii) Thehourly yield

from thelowerbasin increases with increase of flowvelocitydue to

the decrease in the lower glass temperature. It is due to the factthat the lower glass cover temperature decreases due to the fast

removal of the latent heat of vaporization. (iv) The evaporative

heat transfer coefficient is a strong function of the operating

temperature range. The convective and radiative heat transfer

coefficients does not vary significantly.

3.1.2.2. Regenerative solar still coupled with parabolic concentra-

tor. Flowing water over the glass cover is made to reduce glass

temperature of the solar still. Heat is transferred from the glass to

the flowing water which, in turn keeps the temperature difference

large. This regenerative effect helps to achieve higher productivity

of the solar still.

Sanjay Kumar and Sinha [40] conducted the experimental

analysis of a double slope solar still coupled with a non-trackingcylindrical parabolic concentrator through an electric pump. The

system operates in a forced circulation mode to avoid the inherent

problems associated with a thermosyphon circulation mode. The

authors observed that, the concentrator coupled still gives the

maximum yield at all depths of the basin water (Fig. 12).

The concentrator assisted regenerative solar still has a much

higher thermal efficiency than the flat plate collector assisted

Fig. 10. Desalination by a parabolic solar concentrator [37].

Fig. 11. Cross-sectional view of double effect active distillation system[39].



9/24

regenerative still at all water depths and they inferred that there is

less thermal loss in the concentrator compared to the flat plate

collector panel. From the analysis, an increase in the flow rate of

cold water over the glass cover also increases the overall thermalefficiency, followed by significant increase in its yield. Lourdes

Garcia Rodriquez et al. [41] studied the global analysis of the use of

solar energy in seawater distillation under Spanish climatic

condition. They considered the following solar energy collectors

for the analysis: salinity gradient solar ponds, flat plate collectors,

evacuated tube collectors, compound parabolic collectors and

parabolic through collectors for direct steam generation (DSG).

Each of the collectors were compared for the parameters like, the

fresh water production from a given desalination plant, attainable

fresh water production if a heat pump is coupled to the solar

desalination unit and area of solar collector required. Results

showed that direct steam generation parabolic through was a

promising technology for solar assisted seawater desalination.

3.1.3. Solar still coupled with evacuated tube collector

The evacuated tube solar collector has more advantageous than

theflat plate collectors forwaterheating purposes.Evacuated Tube

Collectors (ETC) are well known for their higher efficiencies when

compared to flat plate solar collectors. In flat plate collectors, sun

rays are perpendicular to the collector only at noon and thus a

proportion of the sunlight striking the surface of the collector is

always likely to be reflected. Butin evacuatedtube collector, dueto

its cylindrical shape, the sun rays are perpendicular to the surface

of the glass for most of the day. The evacuated tubes greatly reduce

the heat losses as vacuum is present in the tubes. Owens-Illinois

(OI) evacuated tube collector is shown inFig. 13.

The OI collector consists of two coaxial tubes with evacuated

space between an outer surface of inner tube and inner surface ofouter tube. A selective coating is applied to the outer surface of the

inner tube. The heat transfer fluid enters through small diameter

delivery glass tube andexits from thesame endof thetube through

annular space between delivery tube and selective coated absorber

tube (which is sealed from one end). The annular space between

selectively coated tube and borosilicate outermost glass tube is

evacuated to minimize the convection loss from the selectivesurface.

Tiwari et al.[42]developed the thermal models for all types of

solarcollectorintegrated active solarstills basedon energy balance

equations in terms of inner and outer glass temperature. The total

daily yield of passive solar still, FPC, concentrating collector, ETC

and ETC with heat pipe is shown inFig. 14.

The authors have drawn the following points: (i) the maximum

values of total heat transfer coefficient (htw) for active solar stills

integrated with flat plate collector, concentrating collector,

evacuated tube collector and ETC with heat pipe are 43, 86, 67

and76 W m2 8C1, respectively. (ii) The overall thermal efficiency

of active solar stills integrated with FPC, concentrating collector,

ETC and ETC with heat pipe is 13.14, 17.57, 17.22 and 18.26%,

respectively. (iii) The overall average thermal and exergy efficiencyof FPC integrated active solar still are in the range of 5.619.1 and

0.250.85%, respectively. If the exergy out from FPC is considered,

then average exergy efficiency of active solar still varies in the

range 0.591.82%.

3.1.4. Solar still coupled with heat pipe

Hiroshi and Yasuhito [43] proposed the newly designed,

compact multiple effect diffusion type solar still consisting of a

heat pipe solar collector and a number of vertical and parallel

partitions in contact with saline soaked wicks. The system consists

of a heat pipe solar collectorand a Vertical MultipleEffect Diffusion

type (VMED) still. The solar collector consists of a glass cover and

collector plate, on which the selective absorbing film is attached,

with an air gap between them. Copper tubes, are attached to theunder surface of the collector plate with a fixed pitch.

VMED still consists of vertical and parallel partitions with

narrow air gaps between them, and the partitions, with the

exception of the outside one, are in contact with saline soaked

wicks. Saline water is constantly fed to the wicks. The copper plate

is in front of the first partition with a narrow gap. The gap becomes

the condensing path of the working fluid. The condensing path in

front of first partitionand theevaporating coppertubes attached to

the under surface of the collector plate are connected with two

connecting pipes, so that a closed loop between the solar collector

and VMED still is formed. The constant mass of ethanol liquid is

charged into the closed loop and the closed loop is evacuated with

an evacuating pump. The front surface of the VMED still and the

under surface of collector plate are insulated.

Fig. 12.Variation of daily yield with water depth of still [40].

Fig. 13. Schematic diagram of Owens-Illinois evacuated tube collector[42].

Fig. 14. Total daily yield for active solar stills [42].



10/24

The solar radiation transmits through the glass cover and is

absorbed on the collector plate and ethanol in the evaporating

copper tubes attached to the under surface of the collector plate isheated up and evaporated. Ethanol vapour goes through the upper

connecting pipe to the top of the condensing path in front of first

partition and flows downward the condensing path accompanied

with condensation on the front surface of first partition.

Condensate of ethanol returns to the evaporating copper tubes

through the lower connecting pipe by gravity force. The latent heat

of ethanol released by condensation on first partition enters the

VMED still and is recycled to increase the production of distillate

(Fig. 15). The authors observed from the experimental studies that,

(i) the solar collector and the VMED still can be folded or separated

when it is carried, so that the still would be easy to carry and

shipping cost would be very cheap. (ii) The proposed still of 10

partitions with 5 mm or 3 mm diffusion gap is theoretically

predicted to produce 19.2 or 21.8 kg/m2 day, respectively, on asunny autumn equinox day of daily solar radiation of 24.4 MJ/

m2 day. (iii) Theproductivity of theproposed still is 13%larger than

that of the VMED still coupled with a basin type still.

Hiroshi Tanaka et al.[44]found that, the optimum angle is 268

when solar collector angle is fixed for the year if the proposed still

is used at 268N latitude. The overall daily productivity is 9% or 17%

largerfor theoptimum solar collectoranglestills than thefixed one

on the summer or winter solstices. The productivity increases with

a decrease in the thickness of diffusion gaps between partitions,

and the increase is considerable when the thickness of diffusion

gaps is smaller than several millimetres. Hiroshi Tanaka et al. [45]

conducted the indoor experiments on VMED solar still with a heat

pipe solar collector, and the experimental results of the overall

production rates of the multiple effect still were about 93%, whichindicates that the heat pipe of the proposed still can transport

thermal energy well from the solar collector to the vertical

multiple effect diffusion type still.

3.1.5. Solar still coupled with solar pond

Solar pond is an artificially constructed pond in which

significant temperature rises are caused to occur in the lower

regions by preventing convection. Solar ponds are used for

collection and storage of solar energy and it is used for various

thermal applications like green house heating, process heat in

dairy plants, power production and desalination and this detailed

review of solar pond has been done by Velmurugan and Srithar

[46]. Velmurgan and Srithar[47]theoretically and experimentally

analysed the mini solar pond assisted solar still with sponge cube.

The results show that, average increase in productivity, when a

pond is integrated with a still is 27.6% and when pond and sponge

are integrated with a still is 57.8%.Velmurugan et al. [48] studied the augmentation of saline

streams in solar stills integrated with a mini solar pond. Industrial

effluent was used as feed for fin type single basin solar still and

stepped solar still. A mini solar pond connected to the stills to

enhance the productivity and tested individually. The schematic

diagram of experimental setup is shown in Fig. 16. The results

show that, maximum productivity of 100% was obtained when the

fin type solar still was integrated with pebble and sponge. The

productivity increases with increase in solar intensity and water-

glass temperature difference and decreases with increase in wind

velocity. Velmurugan et al. [49] experimentally investigated the

possibility of enhancing the productivity of the solar stills by

connecting a mini solar pond, stepped solar still and a single basin

solar still in series. Pebbles, baffle plates, fins and sponges are usedin the stepped solar still for productivity augmentation. Their

finding shows that, maximum productivity of 78% occurred when

fins and sponges were used in the stepped solar still and also found

that the productivity during night also improved when pebbles

were used in the solar stills.

Osamah and Darwish[50]studied a solar pond assisted multi

effect desalination of sea water in an arid environment and it is

recommended that an optimum area ratio is used such that quasi

steady operation is achieved. Huanmin Lu et al. [51]presented the

desalination coupled with salinity gradient solar ponds and

observed that, a multi effectmulti stage distillation unit produces

the high quality distillate. The total dissolved solid level of the

product is about 23 mg/l. There is no significant influence of

operating conditions on the quality of distillate. El.Sebai et al. [52]experimentally studied to improve the productivity of the single

effect solar stills, a single-slope single basin solar still integrated

with a Shallow Solar Pond (SSP). They found that, the annual

average values of daily productivity and efficiency of the still with

SSP were higher than those obtained without the SSP by 52.36%

and 43.80%, respectively.

3.1.6. Solar still coupled with hybrid PV/T system

The problem encountered with normal PV cells is that, most of

the solar radiation that is absorbed by a solar cell is not converted

into electricity. The excess energy which goes unabsorbed by the

solar cell increases the temperature of the photovoltaic cell and

reduces the efficiency. Natural or forced circulation of a fluidcooling

medium reduces the cell temperature. Cooling is often applied for

Fig. 15. Schematic diagram of multiple effect diffusion type still coupled with heat pipe solar collector[45].



11/24

concentrating photovoltaic systems, in which the irradiance on the

cell surface is high. An alternative to ordinary photovoltaic modules

is to use Photovoltaic-Thermal (PV/T) modules, which are photovol-

taic modules coupled to heat extraction devices. Hence, these

systems, in addition to converting sunlight into electricity, collectthe residual thermal energy and delivers both heat and electricity in

usable forms. Shiv Kumar and Arvind Tiwari [53] conducted

experimental study of hybrid Photovoltaic/Thermal (PV/T) active

solarstill and found that,the yield increased by more than 3.5 times

than the passive solar still. The schematic diagram of hybrid (PV/T)

active solar still is shown in Fig. 17.

Shiv Kumar andTiwari [54] havemadean attemptto estimate the

internalheat transfer coefficientsof a deep basin hybrid(PV/T) active

solar still for composite climate of New Delhi. The authors observed

that, Kumar and Tiwari model better validate the results than the

other model and the average annual values of convective heat

transfer coefficient for the passive and hybrid (PV/T) active solar still

are 0.78 and 2.41 W m2 K1, respectively at 0.05 m water depth.

Shiv Kumar and Tiwari [55] presented the life cycle costanalysis of single slope hybrid (PV/T) active solar still and

suggested the following, (i) the lowest cost per kg of distilled

water obtained from the passive and hybrid (PV/T) active solar

stills is estimated as Rs. 0.70 and Rs. 1.93, respectively. It is much

economic in comparison to thebottled water available, which costs

around Rs. 10 per kg in Indian market for consumers. (ii) The

payback periods of the passive and hybrid ( PV/T) active solar stills

are obtained in the range of 1.16.2 years and 3.323.9 years,

respectively, for the selling price of distilled water in the range of

Rs. 10 to Rs. 2 per kg. Therefore, passive solar stills are acceptable

for potable use. (iii) The energy payback times (EPBT) of passive

and hybrid (PV/T) active solar stills are estimated as 2.9 years and

4.7 years, respectively.

3.1.7. Multistage active solar distillation system

Nishikawa et al. [56] developed and tested the triple effect

evacuated solar still. The authors reported that, the highest

distillation performance of 73.6 kg/day was obtained that corre-

spondsto the9.44 kg m2 day1 fresh water distilledat a condition

of the solar radiation of 13.85 MJ m2 day1 (108.3 MJ day1). The

total latent heat of the distillation (178.8 MJ day1) was about 1.7

times the solar radiation. The power consumption of the vacuum

pump was only 326 W day1 (1.17 MJ day1) when the solar cells

generated 952.5 Wh day1 (3.43 MJ day1) at 12.25 MJ m2 day1

(45.33 MJ day1) solar radiation.

Ahmed et al. [57] designed, fabricated and tested the multistage

evacuatedsolar still systemthat consists of three stages stacked on

the top of each other, and are carefully insulated from the outside

Fig. 16.Schematic diagram of the mini solar pond integrated with single basin and stepped solar still [48].

Fig. 17. Schematic of a hybrid (PV/T) active solar still[54].



12/24

environment using rock-wood and aluminium foil layers to

prevent any losses to the ambient environment. The three stages

are mounted on top of each other and a good sealing is maintained

between the stages to prevent any vapour leakage through the

contact surfaces. A thick insulation is also used to reduce heat

losses of the still to the ambient. A solar collector is used to supply

heat to the system through the lower stage, which is maintained at

a pressure lower than atmospheric by means of a heat exchanger. A

solar operated vacuum pump is used to evacuate the non-

condensable gases from the stages. Fig. 18 shows a schematic

diagram of the multistage evacuated solar still. Saline water is fed

into each stage from the tank located at the top of the third stage.

Vapour generated in the lower stage condenses on the bottom

surface of the intermediatestage, giving its heat to the saline waterin the intermediate stage.

Vapour generated in the intermediate stage condenses at the

bottomsurface of the upper stage giving its heat to thesaline water

in the upper stage. The fed water is preheated by the heat given to

it by condensation of the vapour generated at the upper stage,

which condenses at the bottom of the feed water tank. Pressure

inside each one of the three stages is kept lower than the previous

stage.Vacuumis generatedusing a solar operated vacuumpump. A

set of valves is used to control the vacuum inside the different

stages. Theresults show that, themaximum production of thesolar

still was found in the first stage and is 6 kg/m2/day, 4.3 kg/m2/day

in secondstage and2 kg/m2/day in first stage ata vacuumpressure

of 0.5 bar. Indeed, the total productivity of the solar still is affected

very much by changing the internal pressure. The productivitydecreased as the pressure increased due to the lower evaporation

rates at the higher pressure values.

Mahmoud et al. [58] experimentally investigated the perfor-

mance of a multi stage water desalination still connected to a heat

pipe evacuated tube solar collector. The results of tests demon-

strate that the system produces about 9 kg/day of fresh water and

has a solar collector efficiency of about 68%. Schwarzer et al. [59]

developed the multistage solar desalination system with heat

recovery. The results show that, the system produces about 15

18 l/m2/day, which is 56 times higher than simple still.

3.1.8. Multi effect active solar distillation system

The multi effect solar distillation system is working based on

the multiple condensationevaporation cycle. Multi effect solar

still is an efficient method for the production of desalinated water

at relatively lowtemperature up to 70 8C. Adel M AbdelDayem[60]

demonstrated experimentally and numerically the performance of

a simplesolardistillation unit. Thebasic distillationunit consists of

air humidifiers (evaporators) and dehumidifiers (condensers).

There is no wall separating the two enclosures. The brine is passed

through the hot storage tank-2 where its temperature rises. It then

passesthrough evaporators where water vapourand heat aregiven

up to the counter-current air stream, reducing the brine

temperature. The air is heated and humidified simultaneously

since the humidity of saturated air is decreased in the condenser

side. On the other side, the evaporator consists of two horizontal

pipes with small holes provided on the lower side of the pipe. The

holes work as injectors that inject the hotsalt water to increase theevaporation rate.Fig. 19gives a schematic diagram of the system.

The results show that, the system can work continuously and

theproductivity of thedistilled water is high forthe collectormean

temperature of 50 8C and the estimated optimum collector area

based on the system life cycle solar savings was obtained as 6 m2

rather than that used in the present system, i.e., 3.1 m2. Zheng

Hongfei and Ge Xinshi[61]conducted the experimental study of a

steady state closed recycle solar still with enhanced falling film

evaporation and regeneration. Based on the experimental results,

the authors found that, the performance ratio of the unit is about

two to three times greater than that of a conventional basin type

solar still (single effect). Shaobo Hou and Hefei Zhang[62]studied

the hybrid solar desalination process of the multi effect

humidificationdehumidification and the basin type unit. Thegain output ratio of this system was raised by 23 at least through

reusing the rejected water.

Fig. 18.Schematic diagram of the evacuated multistage solar still [57].

Fig. 19. Schematic diagram of the present solar distillation system [60].



13/24

Frieder Grater et al.[63]experimentally investigated the multi

effect still for hybrid solar/fossil desalination of sea and brackish

water. The results show that, heat recovery from the outlet mass

flows of concentrate and distillate has only little effect on distillate

output but the Gained Output Ratio (GOR) increases considerably.

With blowers and intermediate screens, installed inside the

distillation effects, the distillate yield can be increased by more

than 50% and the GOR by 60% related to results of a configuration

without heat recovery and blowers. Garg et al. [64]presented an

experimental design and computer simulation of multi effect

humidificationdehumidification solar desalination and the de-

veloped model which is useful in the estimation of the distillation

plant outputand optimized various components of the systemlike,

solar water heater, humidification chamber, and condensation

chamber.

Ali M. El-Nashar [65] studied the multiple effect solar

desalination plant and found that dust deposition and its effects

on performance depend strongly on the season of the year and the

frequency of jet cleaning should be adjusted accordingly. Lianying

Zhang et al.[66]developed a specifically designed solar desalina-

tion system with a solar collector and tested under practical

weather conditions. The results show that, theyield is about twoto

three times more than that of a conventional single basin solar still

under the same conditions. Ben Bacha et al. [67] conductedexperimental validation of the distillation module of a desalination

station using the solar multiple condensationevaporation cycle

principle. The results show that, a correct choice of a packed bed

material, which permits higher exchange coefficients and the solar

collector should be selected with high efficiency performance.

3.1.9. Air bubbled solar still

Pandey [68] reported the effect of dried,forced air bubbling and

cooling of glass cover in solar still. The results show that, the

simultaneous bubbling of dry air and glass cooling gives the

highest increase followed by bubbling of dry air alone (Fig. 20).

Gyorgy Mink et al. [69] designed and conducted the experi-

ments on air blown solar still with heat recycling. The results show

that, about a threefold increase in yield was achieved comparedwith that of a basin type solar still of the same area and with the

same irradiation. Mink et al. [70]presented the performance test

on air blown, multiple effect solar still with thermal energy recycle

consisting of an upper evaporation chamber and lower condensa-

tion chamber. The experimental result indicated that the still

performance can be enhanced further by increasing the liner air

stream velocity in the lower chamber by decreasing its cross-

sectional area.

3.1.10. Hybrid solar distillation system

The hybrid solar still can produce the desalinatedand hot water

from the same system. These types of designs have more

advantages over the other type of systems. Voropoulos et al.

[71] experimentally investigated the hybrid solarstill coupled with

solar collectors (Fig. 21). The results show that, (i) the productivity

of the coupled system is about double that of the still only. (ii)

Significant raises in distilled water productivity have been

obtained not only during the day but mainly during night

operation of the system, reaching triples the solar only system

productivity. (iii) The continuous heating of basin water from tank

water result in higher production rates in all operation periods as a

result of significantly higher differences between water and cover

temperatures, mainly at night. Voropoulos et al. [72]studied the

energy behaviour of hybrid solar still and concluded that, the

developed method can be a valuable tool for the systemoptimization, used during its design and also for evaluation of

an existing solar distillation installation through short term

testing.

Mathioulakis and Belessiotis[73]investigated the possibilities

of using optimization of a simple solar still through its incorpo-

ration in a multi-source and multi-use environment and observed

that, the design of such systems depending on the available heat

sources and/or expected consumption of hot water usage.

Voropoulos et al. [74] conducted experimental study of a hybrid

solar desalination andwater heating system. Theresults show that,

the output of a conventional solar still can be significantly

increased if it is coupled with a solar collector field and hot water

storage tank. The distilled water production was gradually

reduced, when the increase delivered energy through hot waterdraw-off. Ben Bacha et al.[75]developed a mathematical model to

give theability to estimate theexpectedperformance of thesystem

under given climatic conditions, allowing the choice of the proper

design solutions in relation to the desired usage.

3.2. Pre-heated water active solar still

In this method pre heated water is used to increase the water

temperature in the basin. The waste hot water is available from

various sources like paper industries, chemical industries, thermal

power plants and food processing industries and the same may be

utilized for solar distillation plant to increase the productivity. The

hot water will be supplied directly to the basin or through heat

Fig. 20.Schematic diagram of air bubbled solar still [68].

Fig. 21.Schematic diagram of hybrid solar distillation system [71].



14/24

exchangers. Proctor[76]proposed the technique of using waste

heat in a solar still and predicted that productivity increased 3.2

times compared with ordinary still.

Sodha et al. [77] presented the experimental results on

utilization of waste hot water for distillation. In that test, two

modes were studied: (i) flowing waste hot water from thermal

power plants at constant rate through the solar still. (ii) Feeding

waste hot water obtained from thermal power plants once a day.

Their results showed that, length of solar still, depth of water in

basin, inlet water temperature and solar radiation are the

parameters which affects the performance of the still and the

still fed with hot water at constant rates gives higher yield in

comparison to a still with hot water filled only once in a day.

Tiwari et al. [78]studied the performance on effect of water

flow over the glass cover of a single basin solar still with an

intermittentflow of waste hotwaterin thebasin (Fig. 22). Based on

the experimental study, the authors made following points, (i) the

temperature of the water flowing over the glass cover always

remains of the same order as the ambient temperature and the

glass cover temperature is slightly higher than this. (ii) With the

flow of waste hot water during off sunshine hours, one can have a

higheryieldthan that of stationary water.(iii) Thestillproductivity

increases with the increase in mass flow rate for higher inlet water

temperatures and decreases for inlet water temperatures less thatthe average ambient temperature. (iv) The still productivity is

better forthe waste hot water flows duringoff sunshine hours than

the continuous flow of hot water for lower inlet temperatures. But

for higher inlet water temperatures, a continuous flow of water is

better. Ashok Kumar and Tiwari [79]investigated the use of hot

water in double slope solar still through heat exchanger (Fig. 23).

The authors observed that, the evaporative heat transfer

coefficient depends strongly on temperature and advised to use

the waste hot water with either higher temperature or during off

sunshine hours. Also found that, the efficiency of the system was

improved with the inlet temperature of the working fluid.

Yadav[80]analysed the performance of double basin solar still

coupled to a heat exchanger. Based on the analysis, the author

observed the following points, (i) the efficiency of a double basinsolar still coupled to a heat exchanger is significantly less, as

compared to that without heat exchanger. (ii) The efficiency of a

double basin solar still coupled to a heat exchanger is a strong

function of the heat exchanger length and the mass flow rate of the

working fluid. Yadav and Yadav [81] proposed the solar still

integrated with a tubular solar energy collector for productivity

enhancement.

3.3. Nocturnal active solar still

Nocturnal production is the working of a solar still in the

absence of sunlight. This may be achieved by either the solar

energy stored during day time is used during night or the supply of

waste heat available from various sources. The large water depths,

in a conventional solar still are heated during sunshine hours and

most of the thermal energy acquired by the water mass is stored

within it. This stored energy is mostly utilized during off sunshine

hours for the distillation, in the absence of solar radiation, and is

known as nocturnal distillation and this can also be achieved by

feeding the hot water available through any source (other thansolar energy) in the morning or evening for higher production[2].

Madhuri and Tiwari[82]conducted experiments on solar still

with intermittent flow of waste hot water in the basin during off

sunshine hours. The authors observed that, the yield increases in

proportion to the increase in inlet water temperature during the

flow of water and remain the same for stationary water. With the

flow of waste hot water during off sunshine hours, one can have

higher yield than that of the continuous flow of hot water and

stationary water. Gupta et al.[83]presented the analysis report on

effect of intermittent flow of waste hot water into the lower basin

at a constant rate during off sunshine hours (Fig. 24).

The results show that, (i) initially, the temperature of glass

covers is greater than the temperature of the water in the

corresponding basin. Soon, after 2 days, the situation is reversed.Quasi-steady state is reached in about 5 days and evaporation

becomes significant. (ii) The yield of the still increases with

increasing inlet waste hot water temperature, while the other

parameters arekept constant. (iii) The daily productivity of thestill

increases with the rate of flow of waste hot water, provided the

temperature of the inlet waste hot water is greater than its critical

value. If temperatures of the inlet waste hot water is less than its

critical value, the productivity of the still decreases as the rate of

flowof water increases. So,they suggestedto use a higherflow rate

Fig. 22.Schematic representation of the single basin solar still with water flowing

over the glass cover and inside the basin [78].

Fig. 23. Schematic diagram of double slope single basin solar still with heat

exchanger [79].

Fig. 24.Double basin solar still with constant flow rate [83].



15/24

of inlet water only when its temperature is above the critical value.

Nocturnal outputs from basin type stills were studied experimen-

tally for 0.178 m and 0.76 m depth by Onyegegbu [84]. Results

indicated that, on average, nocturnal distillation accounted for 78%

of the total daily output of the 0.178 m deep still while accounting

for about 50% of the total daily output of the 0.076 m deep still.

Tiwari and Ashok Kumar [85] experimentally studied the

tubular solar still design suggested by Tleimat and Howe. The stillconsists of a rectangular (0.1 m1.1 m0.0127 m) black metallic

tray placed at the diametric plane of a cylindrical glass tube

(Fig. 25).

The length and diameter of the glass tube are slightly greater

than the length and width of the tray, respectively. During

operation, the ends of the glass tube are sealed with gasketed

woodenheads. Thetray and glass tube are fixed slightly tiltedfrom

the horizontal plane but in opposite direction. Brine fed from one

end is partly evaporated, and the remainder discharged through

the other end of the tube. The evaporated water condensed on the

inside walls of the glass cover flows down and it is removed from

one end at the bottom of the glass tube.

Based on the study, the authors found that, (i) the average brine

temperature is independent of still length for higher flow ratewhile the output temperature of brine strongly depends on still

length. (ii) The daily yield of distillate in the tubular solar still is

higher than that of the conventional solar still for the same set of

still and climatic parameters. (iii) The internal heat transfer

coefficient remains constant for constant inlet brine temperature

in contrast with the conventional solar still for higher flow rates.

(iv) The purity of the product in the tubular solar still is greater

than in a conventional one, and could be used for chemical

laboratories, etc.

4. Theoretical analysis of active solar distillation system

4.1. Heat transfer in active solar still

The heat transfer in solar still is mainly classified into internal

andexternal heat transfer.The details of various heat transfersthat

take place in active solar still are shown inFig. 26.

4.1.1. Internal heat transfer

The internal heat transfer occurs within the solar still from

water surface to inner surface of the glass cover, which mainly

consists of evaporation, convection and radiation. The convective

and evaporative heat transfers takes place simultaneously and are

independent of radiative heat transfer.

4.1.1.1. Radiative heat transfer. The view factor is considered as

unity because of glass cover inclination is small in the solar still.

The rate of radiative heat transfer between water to glass is given

by[2],

qr;wghr;wgTwTgi (1)

The radiative heat transfer coefficient between water to glass is

given as,

hr;wg eeffs Tw 273

2 Tgi 2732

TwTgi 546

" # (2)

The effective emittance between water to glass cover is

presented as,

eeff 1

1=eg 1=ew 1: (3)

4.1.1.2. Convective heat transfer. Natural convection takes place

across the humid air inside the basin due to the temperature

difference between the water surface to inner surface of the glass

cover. The rate of convective heat transfer between water to glass

is given by[47],

qc;wghc;wgTwTgi (4)

The convective heat transfer coefficient depends on the

temperature difference between evaporating and condensing

surface, physical properties of fluid, flow characteristic and

condensing cover geometry. The various models were developed

to find the convective heat transfer coefficient. One of the oldest

method was developed by Dunkles[86]and his expressions have

certain limitations, which are listed below.

Fig. 25.Schematic representation of a tubular solar still [85].

Fig. 26. Energy flow diagram of single slope active solar still.



16/24

a. Valid only for normal operating temperature ffi 50 C in a solar

still and equivalent temperature difference ofDT 17 C.

b. This is independent of cavity volume, i.e., the average spacing

between the condensing and evaporating surfaces.

c. This is valid only for upward heat flow in horizontal enclosed air

space, i.e., for parallel evaporative and condensing surfaces.

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

hc;wg 0:884DT01=3 (5)

where

DT0 Tw Tgi PwPgiTw 273

268:9 103 Pw

Pw exp 25:317 5144

273 Tw

(6)

Pgi exp 25:317 5144

273 Tgi

(7)

The value proposed in the above equation for C and n are

0.075and 0.33,respectively,for Gr>3.2105. Theabove equation

is notused widelybecause of itslimitations. Kumar andTiwari [87]

have proposed a thermal model for predicting the convective heat

transfer coefficient using linear regression analysis and it is free

from Dunkles shortcoming. Nusselt number for convective heat

transfer coefficient is represented as,

Nuhc;wg Xv

KvCGrPrn (8)

or

hc;wgKvXv

CGrPrn (9)

where,Grashof number(Gr) andPrandtl number(Pr) areexpressed

as follows,

GrbgX3vrv

2 DT0

mv2

(10)

PrmvCp

Kv(11)

The unknown constants C and n will be calculated by linear

regression analysis using experimental data. From the experimen-

tal study, they proposed that value of C and n was 0.0278 and

0.3513, respectively, for active single slope solar still. Chen et al.

[88] developed the model of free convection heat transfer

coefficient of the solar still for wide range of Rayleigh number

3:5 103


17/24

temperature. (ii) The values of C and n differ for each design of

the solar still and for the operating water temperature range.

Therefore, it is recommended that before predicting the perfor-

mance theoretically, experiments must be carried out for given

climatic conditions to evaluate the values of C and n for a

particular design of solar still. Dwivedi and Tiwari[19]observed

from their studies in passive solar still that, Dunkles model gives

better agreement between theoretical and experimental results for

lower depth (0.010.03 m).

4.1.2. External heat transfer

The external heat transfer in solar still is mainly governed by

conduction, convection and radiation processes, which are

independent each other.

4.1.2.1. Top loss heat transfer coefficient. The heat is lost from outer

surface of the glass to atmosphere through convection and

radiation modes. The glass and atmospheric temperatures are

directly related to the performance of the solar still. So, top loss is

to be considered for the performance analysis. The temperature of

the glass cover is assumed to be uniform because of small

thickness. The total top loss heat transfer coefficient is defined as

[92],

qt;ga qr;gaqc;ga (25)

qt;ga ht;gaTgoTa (26)

where,

ht;ga hr;gahc;ga (27)

The radiative heat transfer between glass to atmosphere is

given by[92],

qr;ga hr;gaTgoTa (28)

The radiative heat transfer co efficient between glass to

atmosphere is given as,

hr;ga egs Tgo 273

4 Tsky 2734

TgoTa

" # (29)

where,

Tsky Ta 6

The convective heat transfer between glass to atmosphere is

given by[2],

qc;ga hc;gaTgoTa (30)

The convective heat transfer coefficient between glass to

atmosphere is given as,

hc;ga 2:8 3:0 v (31)

Another direct expression for total top loss heat transfer

coefficient in terms of function of wind speed is given by[2],

ht;ga 5:7 3:8 v (32)

But, there is no significant variation in the performance of the

distillation system by considering Eq. (27)or Eq.(32).

The total internal heat loss coefficient ht;wgand conductive

heat transfer coefficient of the glassKg=Lg is expressed asUwo

1=ht;wg Lg=Kg and the above equation could be rewritten

as,

Uwo ht;wgKg=Lg

ht;wg Kg=Lg (33)

Theoverall toploss coefficient (Ut) fromthe water surface tothe

ambient through glass cover,

Ut ht;wght;gaht;gaUwo

: (34)

4.1.2.2. Side and bottom loss heat transfer coefficient. The heat is

transferred from water in the basin to the atmosphere through

insulation and subsequently by convection and radiation from theside and bottom surface of the basin.

The rate of conduction heat transfer between basin liner to

atmosphere is given by[93],

qb hbTbTa (35)

The heat transfer coefficient between basin liner to atmosphere

is given by[93],

hb Li

Ki

1

ht;ba

1(36)

where, ht;ba hc;bahr;baand it is similar toEq. (32). There isno

velocity in bottom of the solar still. By substituting v 0, to obtain

the heat transfer coefficient. The bottom loss heat transfer

coefficient from the water mass to the ambient through thebottom is expressed as,

Ub 1

hw

1

hb

1(37)

The above equation could be rewritten as,

Ub hwhbhwhb

(38)

The conduction heat is lost through the vertical walls and

through the insulation of the still and it is expressed as,

Us Ass

As

Ub (39)

Thetotal side loss heat transfer coefficient (Us) will be neglectedbecause of side

Documents

Active Solar Distillation—a Detailed Review (Articulo)