solar stills

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Review of researches and developments on solar stills

A.E. Kabeel , S.A. El-Agouz

Mechanical Power Engineering Department, Faculty of Engineering Tanta University, Egypt

a b s t r a c ta r t i c l e i n f o

Article history:

Received 22 November 2010

Received in revised form 15 March 2011

Accepted 16 March 2011

Available online 17 April 2011

Keywords:

Solar still

Desalination

Review

Single-effect solar still is a simple solar device used for converting the available brackish or waste water into

potable water. This device has many advantages like, easily fabricated from locally available materials and

cheap maintenance with low skilled labor. A lot of works were undertaken to improve the productivity of the

still. Throughout the review on solar still performance, the results indicated that, the basin water depth isconsidered the main parameter that affects the still performance. Also the review showed that; the solar still

cover with inclination equal to latitude angle receives sun rays close to normal sun rays throughout the year.

The still productivity also increases with decreasing the cover thickness and increasing its thermal

conductivity. The still basin material plays an important role in improving the productivity of the still, rubber

basin was considered the best used material in improving the absorption, the storage and the evaporation

effects. The previous studies also showed that, the daily production of still was greatly enhanced by using

sponge cubes, fins and stepped. The coupling of a solar collector, hot water tank, external re flector, internal

condenser and greenhouse with a still increased the productivity. Finally; from the previous efforts it was

clear that; using the sun tracking is more effective than the fixed system in enhancing the still productivity;

also the PCM is more effective for lower masses of basin water on winter season.

2011 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Single-effect solar still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Solar still modifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4. Solar still coupled with sponge cubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

5. Solar still coupled with condenser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6. Solar still coupled with sun tracking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

7. Solar still coupled with reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

8. Solar still coupled with sun tracking and reflector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

9. Solar still coupled with a flat plate solar collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

10. Solar still coupled with phase change material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

11. Solar still concave surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

11.1. Cost analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

11.2. Vibratory harmonic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

12. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

Distillation technologies have been used for about a century in

land-based plants and on ships to provide water for a crew. The

regular use of distillation technologies accelerated after World War II,

as the demand for fresh water in arid countries increased. The cost for

distillation has been decreasing rapidly, especially in recent years

with theintroduction of efficient and more cost effective technologies.

Distillation is one of the many processes available for water

purification, and sunlight is one of the several forms of heat energy

that can be used to power that process. Sunlight has the advantage of

zero fuel cost but it requires more space (for its collection) and

Desalination 276 (2011) 112

Corresponding author.

E-mail addresses: [email protected] (A.E. Kabeel), [email protected]

(S.A. El-Agouz).

0011-9164/$ see front matter 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.desal.2011.03.042

Contents lists available at ScienceDirect

Desalination

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http://dx.doi.org/10.1016/j.desal.2011.03.042http://dx.doi.org/10.1016/j.desal.2011.03.042http://dx.doi.org/10.1016/j.desal.2011.03.042mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.desal.2011.03.042http://www.sciencedirect.com/science/journal/00119164http://www.sciencedirect.com/science/journal/00119164http://dx.doi.org/10.1016/j.desal.2011.03.042mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.desal.2011.03.042


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generally more costly equipment. In principle, the water from a solar

still should be quite pure. The slow distillation process allows only

pure water to evaporate from the basin and collect on the cover,

leaving all particulate contaminants behind.

Solar stills have been thoroughly studied and tested for the

production of desalinated water using solar energy. The effect of

different factors such as; solar input, ambient temperature, water

depth, and wind velocity, on the performance of the still was

incorporated by many researchers. For most cases, even underoptimized operating conditions, the reported efficiency of the single

basin solar still was in the range of 3045%, with less than 5 L/m2/day

of fresh water production. This low efficiency mainly is due to the

complete loss of latent heat of condensation of water vapor on the

solar still glass cover. Multi-effect solar stills were used to improve

production of desalinated water but only in small capacities, this is

because the condenser is an integral part of the still. The low heat and

mass transfer coefficients in this type of still require an operation at

relatively high temperatures and thus the use of large, expensive,

metallic surfaces for evaporation and condensation. A solar still, with

its lower productivity, does not compete with other desalination

techniques. However, when the demand of fresh water does not

exceed a few cubic meters, the solar still is an available option. Since

the productivity of the solar still increases as the saturation pressure

of the water increases, this is determined by the temperature at the

brine surface.

Solar stills use exactly the same processes that occurred in nature

that generates rainfall, namely evaporation and condensation. The

solar still construction can be easily described as follows; a

transparent cover encloses a pan of saline water, this construction

heats up the water causing evaporation and condensation on the

inner face of the sloping transparent cover. The formed distilled water

is generally potable; the quality of the distillate is very high because

all the salts, inorganic and organic components and microbes are left

behind in the bath. During reasonable conditions of sunlight, the

temperature of the water will rise sufficiently to kill all pathogenic

bacteria anyway. A film or layer of sludge is likely to develop in the

bottom of the tank and this should be flushed out as often as

necessary. In order to evaporate 1 kg of water at a temperature of30 C about 2.4106J is required. For a solar radiation of 250 W/m2,

during an averaged time of 24 h, this energy could evaporate a

maximum of 9 L/m2/day. In practice, and due to the heat losses, the

expected productivity from the solar still is in therange 45 L/m2/day.

Today's state-of-the-art single-effect solar stills have an efficiency of

about 3040%. The daily amount of drinking water needed by humans

varies between 2 L and 8 L per person and the typical requirement for

distilled water is 5 L per person per day, hence approximately 2 m2 of

still are needed for each person served.

2. Single-effect solar still

Single basin solar still is a popular solar device used for converting

available brackish or waste water into potable water. Because of itslower productivity, it is not popularly used. A number of works are

undertaken to improve the productivity of the still. The still

productivity and efficiency depended on parameters like location,

solar radiation intensity, atmospheric temperature, basin water

depth, glass cover material, thickness and its inclination, wind

velocity and the heat capacity of the still.

For a given cover material, the lower angle of incidence of sun rays

that causes the transmittance is higher and the reflectance is lower.

The inclination and the direction of inclination of the cover depend on

the latitude of the location [1,2]. The cover with inclination equal to a

latitude angle will receive the sun rays close to normal throughoutthe

year. Rate of evaporation depends on the intensity of solar radiation.

Hence the angle of inclination is optimized with early average

variation of solar azimuth angle and solar intensity of the place. The

performance of solar still under different inclination angles in the

range from 10 to 50 [3] were studied. The heat transfer through the

cover plate increases with decrease in thickness and increase in

thermal conductivity. Experimental results showed that a solar still

with glass cover plate with 3 mm thickness gives 16.5% more

production than the cover with 6 mm glass thickness [4]. Material

selection for solar stills is very important; the cover material may be

made of either glass or plastic. Glass is the preferred material for

cover, since it has higher solar transmittance for various angles ofincidence and long service life, whereas a plastic (such as polyeth-

ylene) can be used for short-term use.

Lowering the cover temperature helps in increasing the produc-

tivity. Increasing the temperature difference between the glass and

the basin water, increasesthe natural circulationof air mass inside the

still. It increases both convective and evaporative heat transfer

between basin water to cover. The cooler inner glass surface increases

the rate of condensation. The glass cover temperature is reduced by a

film of cooling water continuously flowing over the glass [5] or

intermittentflow of cooling water on the cover [6]. The cooling water

gains the latent heat of condensation and this heat is regenerated by

passing this water into a basin.

Thewind velocity is also affectingthe cover temperature. At higher

wind velocity the convective heat transfer from the cover to the

atmosphere increases due to the increase in convective heat transfer

coefficient between the cover and the atmosphere. This effect

increases the condensation and evaporation rates and productivity

of the still [7,8].

The basin water depth has a significant effect on the productivity

of the basin. Investigations show that the water depth is inversely

proportional to the productivity of still [912]. The variation of the

convective heat transfer coefficient and thermal modeling of solar

stills were studied by many authors [9,10], where the water depth

parameter is incorporated as a major parameter that affects the still

performance. The effect of water depth on heat and mass transfer in a

passive solar still in summer climatic conditions have been studied

also [11]. As water depth increases the volumetric heat capacity of the

basin is reduced, hence the water temperature is decreased for the

given solar radiation input, but the temperature and production rateare uniform and will not be affected by sudden solar intensity

variation due to cloud passing for a short period of time. The heat

stored in the water mass is released during the absence of sunshine

and production is continuous even during the night. In addition, the

performance analysis for six different water depths in a single slope

passive solar still (Fig. 1) of cover inclination of 30 were studied. The

lower depth has been found giving the highest annual yield.

Increasing the water depth decreases the yield of the still up to

depths of about 0.1 m but at greater depths than this the yield

becomes almost constant. The daily yield of the lower water depth

0.02 m has been found to be 32.57% and 32.39% more than the daily

yield of the higher water depth 0.18 m in summer and winter

respectively. The daily yield of summer, of the lowest water depth

(0.02 m) has been found to be 66.9% more than the correspondingvalue of winter for the same water depth [12].

A solar still of desalination plant is considered to have the lowest

thermal efficiency and productivity among others. This could be

improved by various passive and active methods [13], for example,

the effect of using black ink and dye on the productivity of a single-

basin solar still is studied, an enhancement of 45% and 60% for black

ink and dye was reported. Different studies [14,15] were carried to

determine the still efficiency; the efficiency was ranged from 15% to

25%. A schematic diagram of the designed solar still is shown in Fig. 2.

Moreover, a parametric study was performed [16] on a conventional

double-sloped single-basin solar still under climatic conditions of the

Sultanate of Oman at the Gulf region (Fig. 2). This study showed that

under optimum design conditions, the still tends to give an average

annual solar yield of approximately 4 L/m2

/day. The performance of

2 A.E. Kabeel, S.A. El-Agouz / Desalination 276 (2011) 112


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single- and double-effect solar stills was investigated in Oman [17]. It

was found that the average annual yield was significantly higher for

thedouble-effect still andwith a potentialunit cost savingfor distilled

water of 15.7%.

In order to improve the performance of conventional solar stills,

several other designs have been developed, such as the double-basin

type [18,19], multi-basin type [19,20], a wick basin type [21] and a

multi wick singleslopesolarstilltype [22]. Integration of solar still in a

multi-source and the multi-use environmental type was also studied

[23]. Theeffect of several parameters on the annual performance of an

active solar still has been studied [24]. The effects of the heat

exchanger length, mass flow rate offluid in the heat exchanger loop

andwaterdepth in thebasinon theperformance of an activesolarstill

were investigated [25]. The effect of using black rubber and black

gravel for augmenting the productivity of the solar still is performed[2527]. These studies showed that black rubber, black gravel and

floating perforated black aluminum plate in the solar still increases

the solar still productivity by 20%, 19% and 15%, respectively. Some

authors worked on improving the performance of solar desalination

systems by using two modifications for solar desalination systems

[28]. The first modification wasusing a packedlayer that was installed

in the bottom of the basin to increase the efficiency of the still (Fig. 3).

The second modification, was using a rotating shaft installed close to

the basin water surface. The results showed that the two modifica-

tions enhanced the performance of the solar desalination system. The

efficiency of the first modification increased by 5% at May, 6% at June,

and 7.5% at July, while it increased by 2.5% at May, 5% at June, and 5.5%

at July for the second modification.

The performance of a stepped still is also reported to have higher

productivity [29]. An important issue with these designs is the

formation of scale on the absorber surface, which significantly affects

the absorptive of the surface and hence the productivity of the still. A

stepped still with two different depth of trays was also analyzed [30].

The basin plate contains twenty-five trays with 10 mm depth and

twenty-fi

ve trays with 5 mm depth. Theexperiments were carried outby integrating small fins in basin plate and adding sponges in the

trays. Theoretical and experimental analyses were made for fin type,

sponge type, and combination of fin and sponge type stepped solar

still. When the fin and sponge type stepped solar was used, the

average daily water production has been found to be 80% higher than

the ordinary single basin solar still. The productivity of the single

basin solar still was augmented by integrating fins at the basin

plate [31]. It was found that productivity increased with increase in

solar intensity and decreased with increase in wind velocity. From

experimental results, it was observed that the average evaporation

rate in the conventional solar still was 1.66 L per 8 h. The evaporation

rate increased by about 53% (2.54 L per 8 h) when fins were

integrated at the basin plate. A comparison between the performance

of ordinary single basin solar still and wick type still was performed

[32]. The enhanced evaporation of the still basin water, fins and

sponges was integrated at the basin of the still. It was found that

productivity increased 29.6%, when wick type solar still was used,

productivity increased 15.3% when sponges were used and produc-

tivity increased 45.5% when fins were used.

A theoretical and experimental investigation of a weir-type

inclined solar still (Fig. 4) was investigated [33]. A weir-type solar

still was proposed to recover rejected water from the water purifying

systems for solar hydrogen production. A weir-type solar still consists

of an inclined absorber plate formed to make weirs, as well as a top

basin and a bottom basin. Water is flowed from the top basin over the

weirs to the bottom collection basin. A small pump was used to return

the unevaporated water to the top tank. The results show that the

average distillate productivities for double and single pane glass

covers are approximately 2.2 and 5.5 L/m2/day in the months ofAugust and September in Las Vegas, respectively. Mathematical

models that can predict the hourly distillate productivity are

developed. The productivity of the weir-type still is approximately

20% higher.

3. Solar still modifications

Several improvements have been proposed, such as the use of

sponge cubes, a greenhouse, an external condenser, sun tracking,

reflectors, sun tracking and reflectors, flat plate solar collector and

phase change material integrated with solar stills. Each method has

some drawbacks, namely the effects of the dye on distillate quality.

4. Solar still coupled with sponge cubes

A solar pond (SP) is a thermal solar collector that includes its own

storage system. A solar pond collects solar energy by absorbing direct

and diffuse sunlight. Therefore, sponge cubes in the saline water was

used to improve the evaporation rate (Fig. 5) [34]. The effects of

sponge cube size, percent volume of sponge, water depth, water

salinity and the use of black coal and black steel cubes were also

investigated. They had proved that the distillate productivity

increased by 18273% compared to an identical still without sponge

cubes under the same conditions.

A comparison between theoretical and experimental analysis of a

mini solar pond assisted solar is presented [35], in a mini solar pond,

experiments were conducted for different salinity (Fig. 6). Effect of

sponge cubes in the still, effect of integrating mini solar pond with the

Fig. 1. Schematic arrangement and various energy transfers associated with the solar

still [12].

Fig. 2. Energy balance for the solar still [16].

3A.E. Kabeel, S.A. El-Agouz / Desalination 276 (2011) 112
http://image%20of%20fig.xn--1rc/


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still and combination of both were discussed. It was found that the

optimum value of salinity in the mini solar pond is 80 g/kg of water.

The average daily production of solar still was found to be increased

considerably, when it is integrated with a mini solar pond.

In an attempt to improve the daily productivity of the single effect

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

shallow solar pond (SSP) [36] was studied to perform solar distillation

at a relatively high temperature (Fig. 7). Numerical calculations were

carried out on typical summer and winter days in Tanta (latitude 30

47 N) for different thicknesses and mass flow rates of the flowingwater to study theeffect of these parameters on thedaily productivity

and efficiency of the system. The results show that, the optimum

values of the flowing water thickness and the mass flow rate for this

typical configuration of the SSP-active solar still were obtained as

0.03 m and 0.0009 kg/s. The annual average values of the daily

productivity and efficiency of the still with the SSP were found to be

higher than those obtained without the SSP by 52.36% and 43.80%,

respectively.

The transient performance of an active single basin solar still

integrated with a thin layer of a sensible storage material, beneath the

basin liner of the still, for the purpose of fresh water production

during the night was presented by [37]. Sand is used as a storage

material because it is cheap and available. The results show that, avalue of daily productivity of 4.005 (L/m2/day) with a daily efficiency

of 37.8% has been obtained using 10 kg of sand compared to 2.852 L/

m2/day with a daily efficiency of 27% when the still is used without

Fig. 3. Schematic diagram of the modified solar still using rotating shaft [28].

Fig. 4. Weir-type inclined solar still [33].



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storage. The annual average of daily productivity of the still with

storage is found to be 23.8% higher than that when it is used without

storage.

5. Solar still coupled with condenser

Few authors [38] studied the effect of adding a passive condenser

on the performance of the single slope, basin type solar still. Two solar

thermalelectrical methods are described to purify water by distilla-

tion. In the first method, air saturated with water vapor is removed

from a basin type still by using a low power exhaust fan, and is passed

through a condenser. The thermal efficiency of the still is increased

more than twice the thermal efficiency of the conventional still. In the

second design, a concentratorcollector is used to boil water in the

absorber tube. A low power vacuum pump is employed to lower the

boiling temperature of water by about 10 C. The yield of distillate

from the still is nearly doubled. Some authors [39] suggested the

utilization of a forced condensing technique in a moving film inclined

solar desalination. Thesolar still consists of four main parts. The watervapor is extracted as soon as it is formed before it reaches the glass

cover and is allowed to condense in a separate unit kept at a much

lower temperature. In a study to find optimum performance of the

system, the thermal and overall efficiency were found to be about 70%

and 60%, respectively. An experimental study [40] was carried out to

evaluate the effect of using an internal condenser on the performance

of a single-effect solar still. The still was tested in two different ways:

first, it was used for water vapor condensation without condenser,

and second, it wasused for water vapor condensation with condenser.

The still was single-sloped with a double pass internal condenser. The

results showed that combining an internal condenser with basin type

solar still caused an improvement in the still performance. The still

daily productivity was increased from 5.5 kg/m2/day for the first test

to 5.9 kg/m2/day for the second test. The improving efficiency of solar

still was carried out [41], the effect of adding an outside passivecondenser to a single-basin-type solar still with minimum inclination

(4) was investigated experimentally (Fig. 8). The solar still yielded a

daily output of up to 7 L/m2 and efficiency of 75% during the summer

months. The solar still was operated without a condenser, it yield

decreased to 70% of that with a condenser.

The analysis of a parallel double glass solar still with separate

condenser was studied [42], a solar still designed and built utilizes

direct and reflected (from a reflector) solar radiation incident on a

parallel double glass cover to evaporate sea or brackish water. Water

vapor purges from the evaporator and diffuses to an integrated

condenser due to pressure difference that exists because of the

volume ratio and temperature difference between the evaporator and

condenser. The variations of solar radiation, ambient temperature,

basin water temperature, vapor temperature and other important

temperatures at different locations in the solar still is studied. The

efficiency was increased from 48% to more than 70% when the

condenser cover was cooled down.

A passive solar still with separate condenser hasbeen modeled and

its performance was evaluated [43], the schematic diagram of that

study is shown in Fig. 9. Theperformance of the systemwas evaluated

and was compared with that of a conventional solar still under the

same meteorological conditions. Results show that the distillate

productivity of the present still is 62% higher than that of the

conventional type. The first, second and third effects contribute 60, 22

and 18% of the total distillate yield respectively. It is also found that

the productivity of the solar still with separate condenser is sensitive

to the absorptance of the liner of basin 1, and the mass of water in

basins 1 and 2. The mass of water in basin 3 and wind speed have

marginal effect on distillate production.

6. Solar still coupled with sun tracking

Sun tracking systems were used by some researchers to enhance

the distillation production [44]; a sun tracking system for use with

various collectors and platforms was studied. An experimental

Fig. 5. Solar still with sponge [34].

Fig. 6. A mini solar pond and a solar still [35].



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investigation on a collector consisting of six parabolic troughs with

trackers was conducted [45]; whereas a tracking system which can be

used with single-axis solar concentrating systems as an enhancer was

described by others [46]. An experimental study to investigate the

effect of using two axes sun tracking system on the thermal

performance of compound parabolic concentrators CPC was per-formed [47]; the tracking of CPC collector showed a better

performance with an increase in the collected energy of up to 75%

compared with an identical fixed collector. Two axes sun tracking

system with PLC control to evaluate the performance of photovoltaic

panels (PV) was erected [48]; this study showed that, the suntracking

systems have a better performance with an increase in the collected

energy up to 41.34% compared with the fixed surface.

The improvement in the performance of a traditional single slope

solar still through three design modifications was an important

matter [49]. An addition of internal reflecting mirrors on all interiorsides of still, using step-wise water basin instead of flat basin, and

coupling the solar still with a sun tracking system was studied. The

inclusion of internal mirrors improved the system thermal perfor-

mance up to 30%, while step-wisebasinenhanced theperformanceup

Fig. 7. Schematic diagram of a single-basin solar still coupled with the shallow solar pond [36].

Fig. 8. Solar still with inbuilt condenser [42].

http://image%20of%20fig.xn--60c/


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to 180% and finally the coupling of the step-wise basin with sun

tracking system gave the highest thermal performance with an

average of 380%.

A sun-tracking system [50] is developed for enhancing the solar

still productivity; a computerized sun-tracking device was used for

rotating the solar still with the movement of the sun. A comparison

between fixed and sun tracked solar stills showed that the use of sun

tracking increased the productivity by around 22%, due to theincrease

of overall efficiency by 2%. It can be concluded that the sun tracking is

more effective than thefixedsystem andit is capable of enhancingthe

productivity.

7. Solar still coupled with reflectors

An external reflector could be a useful and inexpensive modifica-

tion to increase the distillate productivity of single-effect stills. A solar

still was constructed and operated with and without reflectors and

black dye under different weather conditions [51]; the study showed

that the addition of a black dye to the water basin and the installation

of reflectors (mirrors) on the inside walls of the still considerablyenhance the productivity. An analysis of an inverted absorber solar

still has been presented [52]; the condensed water trickles down the

condensing surface under gravity and is finally collected through

drainage provided at the lower end.

The vertical multiple-effect diffusion still coupled with a basintype

has a simple structure, but the volume and weight of the still become

large and it is difficult to carry or store, since a basin type still is bulky

and heavy. The vertical multiple-effect diffusion still coupled with a

heat pipe solar collector has a great advantage in its compact size

compared with one coupled with a basin type, but it may be difficult

for local technicians to make and maintain a heat-pipe solar collector,

which should be tightly sealed and thereforerequiresa relatively high

technique for construction. A vertical multiple-effect diffusion-type

still coupled with a basin type still has been proposed [5355], inthese studies, a number of vertical partitions were arranged to the

rear vertical wall of a single-slope basin type still. Thefirst partition of

the vertical multiple-effect diffusion-type still could absorb inclined

direct solar radiation as well as the latent heat of condensation

released from the water vapor which evaporates from the basin liner

where vertical solar radiation could be absorbed. A theoretical

analysis in detail was performed [53,54] and long-term outdoor

experiments [55] were conducted, theresults showedthat thevertical

multiple-effect diffusion still coupled with a basin type still has

greater productivity than the conventional inclined still, since the

diffusion gaps between partitions can be smaller for the vertical still

than for the inclined still. A vertical multiple effect diffusion still

coupled with a heat-pipe solar collector has been proposed [56]; in

the still, solar energy was transported from a solar collector to the

vertical multiple-effect diffusion still as latent heat of working fluid.

The theoretically analyzed characteristics of the still were performed

[57] and also indoor experiments were executed [58], and found that

the productivity of the still is larger than that of the vertical multiple

effect diffusion-type still coupled with a basin type still, which has a

higher productivity than conventional multiple-effect diffusion-type

solar stills.

In addition, numerical and experimental work was studied

[59,60]; the vertical multiple-effect diffusion solar still coupled with

a flat plate reflector was considered. The flat plate reflector would be

set at horizontal or slightly tilted upward from horizontal, and the still

is designed to be rotated (or the orientation of the still would be

changed) for azimuth tracking (Fig. 10). They found that the distillate

productivity of the still could be drastically increased by rotating the

still just once a day at southing of the sun.

The geometrical model that was used to predict the effect of

inclining an external flat plate reflector on a basin type still described

in reference [61] differs significantly from the one with a vertical

reflector indicated in reference [62], and it is more complicated

(Fig. 11). In addition, it was found that the inclined external reflectorcan increase the distillate productivity of the still at any inclination of

the glass cover [62]. The external reflector inclination should be set at

about 15 from vertical. This would produce approximately a 16%

increase in distillate over a basin type still with a vertical reflector

Fig. 9. Solar still with inbuilt condenser [43].

Fig. 10. Schematic diagram of the basin type still with reflectors [59].

http://image%20of%20fig.xn--u4c/


8/12

when the reflector was half as long as the basin liner. For the tilted-

wick still, the geometrical model for the inclined external reflector

would also be more complicated than the one with a vertical reflector

[63] but the geometrical model for the inclined reflector of basin type

still can be easily applied to the tilted-wick still with some

modifications. It was found that the external reflector can increase

the distillate productivity in all but the summer seasons, and the

increase in the daily amount of distillate averaged over the four days

was predicted to be about 9%.

A theoretical analysis of a tilted-wick solar still with an inclinedflat

plate external reflector (Fig. 12) on a winter solstice day at 30 N

latitude was studied [64]. The daily amount of distillate of a still with

an inclined reflector would be about 15% or 27% greater than that with

a vertical reflector when the reflector's length is half of or the same as

the still's length.

An experimental investigation on the effect of internal and

external reflectors inclined at angles 0 (vertical), 10, 20 and 30on the output of simple-basin solar stills in summer, autumn and

winter was presented [65]. A simple still, which has a 20 cover tilt

angle and equipped with internal and external reflectors is investi-

gated at a latitude angle of 33.3N. The results show that, the average

daily yield is increasedby theuse of internal and/or externalreflectors

except for summer where the effect of the reflectors is found to be

negative. The increase in the productivity of the still with reflector(s)

compared to the still with no reflectors (increase ratio) is averaged at

19.9% and 34.5%, 34.4%, 34.8% and 24.7% for the still with internal

refl

ector only, still with internal and an inclined external refl

ectortilted at 0, 10, 20, and 30 respectively.

8. Solar still coupled with sun tracking and reflector

Numerical analysis of the vertical multiple-effect diffusion solar

still coupled with a flat plate reflector: optimum reflector angle and

optimum orientation of the still at various seasons and locations. A

parametric study on a vertical multiple-effect diffusion-type solar still

consisting of a flat plate reflector was presented [66], a number of

vertical parallel partitions in contact with saline-soaked wicks with

narrow air gaps between partitions, and casters for manual azimuth

tracking. The results show that the actual productivity under practical

conditions would still be significantly greater than that of the

conventional single-effect stills.

The optimum angle of a flat plate reflector and the optimum

orientation of a vertical multiple-effect diffusion solar still coupled

with a flat plate reflector throughout the year are numerically

determined with the assumption that the still is located at the

equator and at 10, 20, 30 and 40 northern latitude direction was

studied [67]; The optimum orientation of the still which maximizes

the distillate productivity of the proposed still was calculated

assuming that the orientation of the still is changed once during

daytime at southing of the sun. The results show that the angle of the

flat plate reflector should be basically fixed at 10 from horizontal and

changed to be 0 during the winter season (around December) at

higher latitudes, and the orientation of the still should be adjusted

according to month at any latitude. The daily productivity of the

proposed still was predicted to be more than 30 kg/m2/day at any

latitude throughout the year except for the winter season (fromNovember to January) at 40N latitude.

A new idea of one axis three position tracking PV module with low

concentration ratio reflector to provide a simpler PV tracking system,

which can reduce the power generation cost was presented [68]. The

analysis also shows that the effect of installation misalignment away

from the true south directionis negligible (b2%) if the alignment error

is less than 15. An experiment performed in the present study

indicates that the PV power generation can increase by about 23%

using low concentration (2) reflectors. Hence, combining with the

power output increase of 24.5%, by using one axis three position

tracking, the total increase in power generation is about 56%.

A theoretical analysis of a one step azimuth tracking tilted-wick

solar still with a vertical flat plate reflector is studied [69]. The still is

assumed to be rotated manually just once a day at southing of the sun.The theoretically determined the optimum tilt angle of the still and

the optimum orientation of the still on these 4 days were presented.

The results show that the increase in the daily amount of distillate of a

tilted-wick still would average about 41% for 4 days, and can be

achieved by the simple modification of using a flat plate reflector,

setting the still at a proper tilt angle according to seasons and rotating

the still just once a day.

9. Solar still coupled with a flat plate solar collector

Flat-plate collectors (FPCs) are used as heat transfer fluid, which

circulates through absorber pipes made of either metal or plastic. The

absorber pipes are assembled on a flat plate and they usually have a

transparent protective surface in order to minimize heat losses. They

Fig. 11. Schematic diagram of experimental apparatus [62].

Fig. 12. Schematic diagram of a tilted-wick still with an inclined flat plate external

reflector [64].



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may have different selective coatings to reduce heat losses and to

increase radiation absorption. Thus the thermal efficiency increases

although the collector cost also increase. A typical flat-plate collector

is an insulated metal box with a glass or plastic cover and a dark

colored absorber plate.

The performance of a solar still coupled to a flat-plate solar energy

collector operating under the forced circulation mode has been

studied [70]. The study indicated that, the daily distillate production

of a coupled single-basin solar still is 24% higher than that ofuncoupled still. A transient analysis of a solar still integrated with a

panel of collectors through a heat exchanger was performed [71]; the

study was indicated that the internal convective and radiative heat

transfer coefficients may be considered as constants during operation

of the solar still; however, the internal evaporative heat transfer

coefficient is a very sensitive parameter of water and glass

temperatures. Transient study of a single-basin solar still coupled to

a flat-plate solar collector under the thermos phone mode of

operation has been studied [72]; from the study it is concluded that,

the enhancement in the yield was 3035% as compared to uncoupled

still.The effects of the heat exchangerlength, mass flow rate offluid in

the heat exchanger loop and water depth in the basin on the

performance of an active solar still were investigated [73], while the

effect of several parameters on the annual performance of an active

solar still was studied [74]. The influence of coupling the solar still

with a hot water tank, an asymmetric, single-effect solar still of

greenhouse type, integrated storage and a flat-plate collectors field

has been studied [75]; these studies have shown that coupling a solar

still with a hot water tank generallydoubles thedistilled water output

in the 24-h period, as a result of continuous heating of basin water

from tank water. Increases are higher at night than in the day, since at

night differences in water and cover temperatures are generally

higher, resulting in higher production rates. The hybrid design of this

system, apart from being able to supply desalted water together with

hot water, leads to significantly higher water productivity in the day

and night. Moreover, it can use available heat such as waste heat,

coming from thermal processes nearby, optimizing exploitability of

any available heat sources. A parametric investigation that was

theoretically performed for the vertical multiple-effect diffusion-typesolar still, which consists of a number of vertical partitions in contact

with saline-soaked wicks with narrow gaps between the partitions,

coupled with a heat-pipe solar collector has been presented [76]; the

proposed still has some advantages: the still's size is compact, the still

canproduce distilled water without electricity, and the productivity is

greater than that of conventional multiple-effect diffusion-type solar

stills. The variation of the convective heat transfer coefficient with

water depth in the basin of an active solar distillation system (Fig. 13)

was studied [77]; the performance of a solar still augmented with a

flat-plate collector was studied [78], the last study showed that, the

mass of distilled water production was increased by 231% in the case

oftapwateras a feedandby 52% inthecaseof saltwateras a feed. The

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

productivity of solar stills was incorporated in [79]; differentparameters like, water depth, direction of still and solar radiation

were studied to show the enhancement of the productivity. Single

slope solar still with mirrors fixed to its interior sides was coupled

with a flat plate collector. It was found that coupling of a solar

collector with a still has increased the productivity by 36%. Also the

increase of water depth has decreased the productivity, while the still

productivity is found to be proportional to the solar radiation

intensity.

10. Solar still coupled with phase change material

Another method that may be used for improving the productivity

of solar stills is by using storage systems. These systems could be

sensible or latent heat systems. This adopted method utilizes the heat

dissipated from the bottom of the still. The latent heat thermal energy

storage systems have many advantages over sensible heat storage

systems includinga large energystorage capacity perunit volumeand

almost constant temperature for charging and discharging [80].

Recently, many papers have appeared concerning the use of phase

change material (PCM) as storage media integrated with some solar

thermal energy systems; such as that considered the use of phase

change materials as storage media in solar stills [81]. This study of a

transient performance of a steeped solar still with built-in latent heat

thermal energy storage for heating and humidification of agricultural

green house was presented. He investigated the effect of thickness of

paraffin wax as a PCM and mass flow rate of air on the system

performance. His results indicated that decreasing the airflow rate has

a significant influence on the still yield, while the green house heat

load experiences a decrease. A total productivity is yield of about

4.6 L/m2 with an efficiency of 57%.

A transient mathematical model for a single slope-single basin

solar still with and without phase change material (PCM) under thebasin liner of the still (Fig. 14) was presented [82]; numerical

calculations were carried out using stearic acid as a PCM, on typical

summer and winter days. Effect of mass of the PCM (mpcm) on the

productivity daylight (Pdl), productivity overnight (Pon) and daily

productivity (Pd) and efficiency (d) of the still for different masses of

basin water mw was studied. Theresults of that study showed that, Pdldecreased as mpcm increased; but Pon and Pd were increased

significantly with an increase of mpcm due to the increased amount

of the stored heat within the PCM. During discharging of the PCM, the

convective heat transfer coefficient from the basin liner to basin water

is doubled; thus, the evaporative heat transfer coefficient is increased

by 27% on using 3.3 cm of stearic acid beneath the basin liner.

Therefore, on a summer day, a value of Pd of 9.005 L/m2/day with a

daily efficiency of 85.3% has been obtained compared to 4.998 L/m2/

Fig.13. Schematicdiagramof anactive solar still coupled with a flat-plate collector [77].

Fig. 14. Schematic diagram of the single slope-single basin solar still.



10/12

day when the still is used without the PCM. The PCM is more effective

for lower masses of basin water on winter season.

11. Solar still concave surface

Wick concave type solar still is designed and constructed [83]; a

concave shaped wick surface increases an evaporation rate because

the water surface level is lower than the upper limit of the wick

surface (Fig. 15). Results show that average distillate productivity in

day time was 4.1 L/m2 and the maximum instantaneous system

efficiency was found to be 45% and the daily efficiency of the still was

30%. The maximum hourly yield was 0.5 L/h per m2 after solar noon.

11.1. Cost analysis

Typically, in designinga solar still the main objectiveis to maintain

the cost minimal. The main part of the cost is for Plexiglas container.

However this is unavoidable because of portability of the solar still.

Economical analysis of water desalination unit is given by [8486].

The main parameters in cost analysis of solar stills are CRF (capitalrecovery factor), FAC (fixed annual cost), and SSF (sinking fund

factor), ASV (annual salvage value), average annual productivity (M)

and AC (annual cost). Also there are other parameters like AMC

(annual maintenance operational cost) and finally CPL (cost per

litter). AMC is used for calculation of maintenance cost for removing

salt deposits, maintenance of DC pump, fan and thermoelectric

module and regular filling of brackish water. Generally 15% of the

present cost has been considered as maintenance cost [82]. Table 1

provides a comparison between different types of solar still reported

in literature [84].

11.2. Vibratory harmonic effect

A new concept of active vibratory solar still was presented [87]. Aflexible packed stretched media installed in the bottom of the basin to

increase the efficiency of the still is applied. Theflexible packed media

is formed from stretched helical coiled copper wires, which are

considered as a good media for heat absorbing and transferring and as

a simple thermal storage system. The performance is compared with

the conventional solar stills (CSS). The governing equations including

the flexible packed media, the harmonic vibratory excitation, and

the energy storage were presented. The vibratory excitation effect is

accounted by two newparameters the vibratory performance gain and

the vibratory power effect. The study indicated that, the productivity

due to added backed helical wires is found to be 3.4 L/m2/day, with an

efficiency of about 35%, and the productivity with vibration is increased

to be 5.8 L/m2/day and the average daily efficiency is about 60%. The

nocturnal production ranges from 38% to 57%.

12. Conclusion

As a result of the above revision of a single basin passive type solar

still, the different methods and modifications used to improve the

productivity were listed as follows;

The still productivity and efficiency depended on parameters like

location, solar radiation intensity, atmospheric temperature, basinwater depth, glasscover material,thickness and its inclination, wind

velocity and the heat capacity of the still. When compared with

other parameters, the basin water depth is the main parameter that

affects the performance of the still.

The cover with inclination equal to latitude angle will receive the

sun rays close to normal throughout the year.

The productivity of the still decreases with an increase in depth of

water during daylight.

The still productivity increases with a decrease in thickness and

increase in thermal conductivity of cover plate.

Rubber is the best basin material to improve absorption, storage and

evaporation effects.

When the fin and sponge type stepped solar was used, the average

daily water production has been found to be 80% higher thanordinary single basin solar still.

The productivity of the weir-type still is approximately 20% higher.

The daily production of still can be greatly enhanced using sponge

cubes.

The distilled water output of the solar still integrated with

greenhouse type was higher than that of ordinary single basin

solar still type.

The results showed that combining an internal condenser with basin

type solar still caused an improvement in the still performance.

Thesun tracking is more effective than fixedsystem andit is capable

of enhancing the productivity.

Theexternalreflector can increase the distillate productivity and the

daily amount of distillate of a still with an inclined reflector greater

than that with a vertical reflector.

Fig. 15. Schematic diagram of concave wick solar still [83].

Table 1

Comparison between different types of solar still.

Type M L/m2 CPL $/L/m2

Pyramid shape 1533 0.031

Sun tracking 250 0.23

Single slope 1511 0.035

Transportable hemispherical 1026 0.18

A weir type 1001 0.054



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The coupling of a solar still with a hot water tank generally doubles

the distilled water.

The coupling of a solar collector with a still has increased the

productivity by 2436%.

The PCMis more effective for lower massesof basin water on winter

season.

The concave solar still efficiency reached about 45%.

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