Modeling of a novel concentrated solar still enhanced with a porous evaporator and an internal condenser

Available online at www.sciencedirect.com

ScienceDirect Solar Energy 114 (2015) 8–16 www.elsevier.com/locate/solener

Modeling of a novel concentrated solar still enhanced with a porous evaporator and an internal condenser Moh’d A. Al-Nimr a,1, Moh’d-Eslam Dahdolan b,⇑ a

Mechanical Engineering Department, Jordan University of Science and Technology, Jordan b Jordan University of Science and Technology, Jordan

Received 13 July 2014; received in revised form 18 January 2015; accepted 21 January 2015

Communicated by: Associate Editor G.N. Tiwari

Abstract This paper introduces a new design of solar stills utilizing concentration, porous evaporation, internal condensation, and thermosephonic circulation. A steady-state mathematical model of the still has been proposed and simulated. The results of the study are presented in figures showing both efficiency and distillate rate. The simulation has shown that the decrease of wind speed and condenser temperature increases the efficiency of the still and distillation rates. The increase of ambient temperature increases the efficiency and the distillation rates. On the other hand, the increase of solar intensity increases the distillation rate; but the efficiency does not increase for entire intensity range. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Solar distillation; Concentrated; Porous evaporator; Internal condenser

1. Introduction Water distillation is one of the most important processes in engineering and applied sciences in general. In fact, water distillation becomes more crucial because more than 97% of the earth’s water is salty and impure. Another issue to take under consideration is the continuous increase of the costs of fossil fuels. Inventing modern, cheap and easy ways of distillation is needed. Our need for energy makes us searching for new ideas for stills working on sustainable sources of energy. Many models of stills powered by sustainable sources of energy are designed continuously by ⇑ Corresponding author at: P.O. Box: 272, Irbid 21110, Jordan. Tel.: +962 79 703 7587. E-mail addresses: [email protected] (M.A. Al-Nimr), [email protected] (M.-E. Dahdolan). 1 P.O. Box: 3030, Mechanical Engineering, Jordan University of Science and Technology, Irbid 22110, Jordan. Tel.: +962 2 7201000x22546.

http://dx.doi.org/10.1016/j.solener.2015.01.021 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

many engineers around the world, and the best source of energy to be used for this purpose is the sunlight (Kabeel and El-Agouz, 2011). Solar stills are studied and designed around the world because of its simplicity and the availability of sunlight. Numerous designs and modifications have been proposed to enhance the performance of solar stills. Many review papers present excellent reviews about the most important designs and modifications of solar stills. A review on solar stills that have been classified into six sorts based on their design guidelines have been presented by Xiao et al. (2013). Different techniques that have been adopted to improve solar stills efficiency have been reviewed by Sivakumar and Sundaram (2013). About 19 types of active solar distillation systems have been discussed and reviewed by Sampathkumar et al. (2010). A detailed review for most known configurations and arrangements of wick type solar stills has been presented by Manikandan et al. (2013). Different Approaches used to enhance the performance of

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Nomenclature Ag As Ap;i D Egen Ein Eout g G hc;out hev hfg;w hin hr;in hr;in k k eff L Lc m_ Nu pc pp

surface area of the glass tube (m2) surface area of the still (m2) inner surface area of the porous layer (m2) the outer diameter of the glass tube (m) energy generation rate (W) rate of energy entering the system (W) rate of energy exiting the system (W) Gravitational Acceleration (m/s2) solar radiation (W/m2) convection heat transfer coefficient to the external ambient (W/m2 K) evaporation heat transfer coefficient (W/m2 K) latent heat of vaporization for water (kJ/kg) internal heat transfer coefficient inside the still (W/m2 K) radiation heat transfer coefficient inside the still (W/m2 K) radiation heat transfer coefficient outside the still (W/m2 K) thermal conductivity (W/m K) effective thermal conductivity (W/m K) length (m) length scale in Rac (m) mass flow rate (kg/s) Nusselt number partial pressure of the vapor near the condenser (Pa) partial pressure of the vapor near the porous evaporator (Pa)

inclined solar stills have been reviewed by Kalidasa Murugavel et al. (2013). Different adopted techniques that aim to increase the productivity of the multi-effect solar still have been reviewed by Rajaseenivasan et al. (2013). Another review that highlights the various factors that affect the sill productivity has been presented by Velmurugan and Srithar, 2011. Other articles present general review about the subject (Kabeel and El-Agouz, 2011) (Kaushal and Varun, 2010). Some of the ideas in the mentioned reviews are: installing reflectors, like a basin type solar still with internal and external reflectors (Tanaka, 2009a,b) (Tanaka, 2011), using solar collectors, like a solar still augmented with a flat-plate collector (Badran et al., 2005), enhancing condensation, like a passive solar still with a separate condenser (Madhlopa and Johnstone, 2009), increasing free surface area, like a fin type solar still (Velmurugan et al., 2008), recovering vapor latent heat, like a vertical multiple effect diffusion solar still (Tanaka, 2009a,b), using heat storage, like an integrated basin solar still with a sandy heat reservoir (Tabrizi and Sharak, 2010), improving performance of the absorber basin, like baffle suspended absorber plates (Qiblawey and Banat, 2008) and charcoal particles

Pr Qc;in Qc;out Qev Ql;in Ql;out Qr;in Qr;out rc;o rp;i Rac ReD Req Tc Tp T sky T1 V a b eg g t r s k

Prandtl number convection heat transfer rate inside the still (W) convection heat transfer rate outside the still (W) evaporation heat rate (W) heat losses rate inside the still (W) heat losses rate outside the still (W) radiation heat transfer rate inside the still (W) radiation heat transfer rate outside the still (W) outer radius of the condenser (m) inner radius of the porous evaporator (m) Rayleigh number Reynolds number equivalent thermal resistance (K/W) condenser temperature (K) porous evaporator temperature (K) sky temperature (K) ambient temperature (K) wind speed (m/s) thermal diffusivity (m2/s) volumetric thermal expansion coefficient (K1) emissivity still efficiency (%) Kinematic viscosity (m2/s) Stephan–Boltzman constant (W/m2 K4) transmissivity Mirror Utilization Factor

absorbers (Naim and Abd El-Kawi, 2003), using glass cover inclination in solar stills (Tiwari et al., 1994), enhancing evaporation by vacuum techniques, like an innovated water desalination system using low-grade solar heat (AlKharabsheh and Yogi-Goswami, 2003) and using sun tracking systems, like the system studied by Garcı´aRodrı´guez et al., 2002. Also, many other models like active double effect solar stills (Kumar and Tiwari, 1996), active regenerative solar stills (Tiwari et al., 1993), double effect stills with a parabolic concentrator (Bhagwan and Tiwari, 1996) and air bubbled solar stills (Pandey, 1984). Tubular designs have been proposed and studied in addition to the mentioned designs. A three effect tubular solar still have been analyzed by Chen et al. (2013). A heat and mass transfer model of a tubular solar still has been provided by Ahsan et al. (2010). Another experimental study of a tubular solar still has been done by Ahsan et al. (2010). A tubular solar still with a rectangular basin has been suggested by Arunkumar et al. (2013). A multi-sleeve tubular still filled with different gas media has been analyzed by Zheng et al. (2013). A solar still integrated with evacuated tube collector has been studied by Singh et al. (2013). A tube-type solar still equipped

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M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 114 (2015) 8–16

with heat accumulation has been developed by Murase et al. (2008). Effective sky cool temperature has been utilized as a heat sink to cool the external condenser of solar stills in order to enhance the productivity of these stills under steady and transient conditions (Al-Nimr and Haddad, 1998; AlNimr et al., 2000; Haddad and Al-Nimr, 2002). A theoretical and experimental investigation has been conducted (Al-Nimr et al., 1998) to examine the possibility of harvesting pure water by condensing the water vapor in the ambient humid air on a cool surface that has been cooled by the effective sky temperature. This system does not require the presence of solar radiation to evaporate water because it works on condensing the natural humid air. The objective of this paper is to present a mathematical model of a proposed novel tubular solar still enhanced with a porous evaporator and an internal condenser. This parametric study is carried for steady-state conditions, and it shows the effect of solar intensity, ambient temperature, condenser temperature and wind speed on the efficiency of the still as well as the daily distillation rates. 2. System overview 2.1. Anatomy and mechanism Referring to Fig. 1, the proposed novel still consists of a parabolic mirror that concentrates solar radiation on a tubular still located at the focus of the parabola. A cold condenser pipe passes through the still. The condenser is attached to the same finned tank containing the impure water; it is also inclined by a small degree to collect the distillate water. Referring to Fig. 2 the tubular part consists of a glass layer (A), a porous evaporator (B), a condenser pipe (D) filled with water (E) supplied by the finned tank. The novelty of the proposed solar still is presented by the following: the existence of a porous evaporator which can be made of dark painted sponge or hay, an internal condenser and a parabolic concentrator all together, and the fact that it is a passive stand-alone solar still, the water in the condenser comes from the impure water tank and it needs no electricity to function.

Fig. 1. General overview of the main setting showing the parabolic concentrator, the finned tank, and the highlighted main tubular part.

Fig. 2. The main tubular part: (A) glass layer, (B) porous evaporator, (C) space filled with water vapor, (D) condenser, (E) cooling water.

The mechanism of this still is the following: water is supplied to the porous evaporator from the finned tank shown in Figs. 1 and 2 to the porous evaporator, water is spread within the thin porous layer by capillary effect. The concentrated solar radiation reflected by the parabolic mirror then enters the tubular still passing through the glass layer causing the porous layer to be heated and impure water to be evaporated. Water is condensed by the cold condenser. Cooling water is circulated inside the system by thermosephonic effect. As a result, the condensed pure water will drip downwards to be collected as distillate water. Fig. 3(a and b) shows a schematic diagram and a block diagram of the system. The cooling mechanism depends on circulating the impure water from the finned tank to the condenser pipe and back to the tank by the thermo-sephonic effect. Cold water enters the condenser from the away side (bottom) to absorb the latent heat of condensation from vapor, and while it is being heated up, it raises up returning to the tank. (Remark: the condenser is slightly inclined to collect distillate water and to enhance thermo-sephonic circulation). 2.2. Advantages The following advantages distinguish the proposed still:  The still provides good water distillation rates.  Since cooling water circulation within the still is caused by thermal sephonic effect, it can be considered as a stand-alone still or a passive system.  The cooling water inside the condenser comes from the same water resource of the salty water.  The porous evaporator has low total thermal capacity (less mass and lower specific heat capacity) with two advantages: (a) very large effective surface to volume ratio of the porous material enhances the evaporation rate from the evaporator. (b) most of the absorbed thermal radiation will be utilized to evaporate water due to the small thermal capacity of the porous evaporator. This also implies that the thermal time constant of the evaporator is relatively small, which gives a faster transient response.

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Fig. 3. (a,b). Description of the still’s mechanism.

 The absorbed latent heat of condensation will not be lost, but collected by the condenser to be utilized in other domestic applications that needs warm water like flushing toilets or warming small chicken coops. 2.3. Other configurations  The use of double and triple glazed glass instead of single glaze glass layer.  Same still with only the lower half of the porous evaporator to prevent salty water from dripping on the condenser and mixing with distilled water.  Same still with an arc above the condenser (umbrella) to prevent the dripping salty water -if existed- from mixing with distilled water.

3. Modeling and simulation 3.1. Mathematical model To describe the thermal behavior of the proposed still the following assumptions have been made:  The model is running under steady-state conditions.  The glass layer has zero thermal conductive resistivity.  The porous layer temperature equals the adjacent glass layer temperature because both are in perfect thermal contact. The purpose of the assumption is to relate the

glass temperature and the porous evaporator’s temperature. However, it leads to lowers outputs of the proposed model.  Properties of the air-vapor mixture in are taken as that of air at average temperature.  Solar irradiation used in the model is assumed to be the perpendicular component on the still.  The condenser collected all the evaporated water. Under the above assumptions, applying the energy balance equation on the evaporator is: Ein  Eout þ Egen ¼ 0

ð1Þ

Where: Ein ¼ kasAs G

ð2Þ

Eout ¼ Ql;in þ Ql;out

ð3Þ

where (Ql;in ) represents the heat losses inside the system, while (Ql;out ) represents the heat losses outside the system “the ambient”. Other parameters are defined in the Nomenclature. The term (Ql;in ) is also divided into radiation internal losses and convection internal losses as follows: Ql;in ¼ Qr;in þ Qc;in ¼

ðT p  T c Þ Req

Qr;in ¼ hr;in Ap;in ðT p  T c Þ

ð4Þ ð5Þ

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M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 114 (2015) 8–16

hr;in ¼ eg rðT p þ T c ÞðT 2p þ T 2c Þ

ð6Þ

The internal convection losses (Qc;in ) is given as: (Incropera, et al. n.d.) 2pLk eff ðT p  T c Þ   rp;i ln rc;o

ð7Þ

 14 k eff Pr 1 ¼ 0:386 Ra4c 0:861 þ Pr k

ð8Þ

Qc;in ¼

gbðT p  T c ÞL3c at   43 rp;i 2 ln rc;o Lc ¼  5 3 3 3 rp;i5 þ rc;o5 Rac ¼

ð9Þ

ð10Þ

Eqs. (4), (5), (7)–(11) will be used to estimate the internal heat as a function of (T p ). After neglecting the conduction thermal resistance of the glass layer, and assuming that the glass temperature is equal to the porous media temperature, the external heat losses are given as: (Incropera et al. n.d.) Ql;out ¼ Qr;out þ Qc;out

ð11Þ

Qr;out

¼ hr;out Ag ðT p  T sky Þ

ð12Þ

hr;out ¼ eg rðT p þ T sky ÞðT 2p þ T 2sky Þ

ð13Þ

Qc;out ¼ hc;out Ag ðT p  T 1 Þ

ð14Þ

hc;out ¼

NuD k

ð15Þ

Fig. 4. (a, b). Solar Still Simulation Inputs.

P p ¼ ð0:14862ÞT p  ð0:36526  102 ÞT 2p þ ð0:11242

Sky temperature is given by Akhtar and Mullick (2007): T sky ¼ 0:0552 

T 1:5 1

ð16Þ

where Nu is given as (Incropera et al. n.d.): 1  5 !45 1 0:62Re2D Pr3 ReD 8 Nu ¼ 0:3 þ 1þ 0:423 14 282000 ð1 þ Pr Þ ReD ¼

VD t

ð17Þ

By substituting back through Eqs. (12)–(18) we get the external thermal losses as a function of (T p ). To find the evaporation heat transfer, which represents the generated heat term (Egen ), in terms of (T p ): ð19Þ

where hev is given as in Dunkle’s model which is valid for parallel condensation and evaporation surfaces (Ahsan et al., 2013): hev ¼ ð16:27  103 Þhin hin ¼

Ql;in Ap;i ðT p  T c Þ

Pp  Pc Tp  Tc

P c ¼ ð0:14862ÞT c  ð0:36526  10  103 ÞT 3c

ð18Þ

Qev ¼ hev Ap;i ðT p  T c Þ

 103 ÞT 3p

ð20Þ ð21Þ

ð22Þ 2

ÞT 2c

þ ð0:11242 ð23Þ

(P p ) and (P c ) in Eqs. (22), (23) are the vapor pressures of water at the evaporator and condenser temperatures (in °C) respectively. Substituting all energy terms (Ein ; Eout ; Egen ) into Eq. (1), a non-linear equation with one variable (T p ) will be obtained. Finally, to find the efficiency and the distillation rate: Qev G Q m_ ¼ ev hfg;w

g¼

ð24Þ ð25Þ

As a validation of the proposed model, sample runs have been made with still parameters changed to act like a conventional solar stills (no concentration, condenser temperature slightly less than ambient). The proposed mathematical model predicted efficiencies and productivities within or very close to practical ranges of (30–40%) and (4–5)-litre/day (Kabeel and El-Agouz, 2011).

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35

Parameter

Value

Outer diameter of the glass layer (D) Inner diameter of the glass layer (Dg,i) Porous evaporator layer thickness (t) Outer diameter of the pipe condenser (Dc,o) Perpendicular area of the concentrator (A) Length of the tubular part (L) Wind speed range (V) Solar intensity range (G) Ambient temperature range (T1) Condenser temperature (Tc) less than ambient temperature by: Absorptivity (a) Transitivity (s) Emissivity (e) Mirror Utilization Factor (k)

48.3-mm 40.9-mm 2.5-mm 21.3-mm 1-m2 1-m (2–20)-m/s (200–1000)-m/s (20–30)-°C (0–4)-°C 0.9 0.79 0.9 0.95

Sll Efficiency (%)

Table 1 Simulation inputs for the proposed still.

30 25

20-C 19-C

20

18-C

15

17-C

10 5

16-C

0

5

10

15

20

25

Wind Speed (m/s) Fig. 5. Relationship of efficiency with wind speed for ambient temperature of 20-°C, solar intensity of 500-W/m2 and different condenser temperatures.

3.2. Simulation parameters Simulation inputs are listed in Table 1 and Fig. 4: Wind speed is chosen as a parameter because of its unfavorable effect to the still. The main part is a tubular part which is exposed to ambient conditions. Moving wind is a fluid that has ambient temperature, flowing on the outside of the relatively hotter still causing it to cool down and lose efficiency. (See Table 1)

Disllate Rate (kg/day)

3 2.5 20-C

2

19-C

1.5

18-C 17-C

1 0.5

16-C

0

5

10

15

20

25

Wind Speed (m/s)

4. Results and discussion Simulation has been carried out by using MS Excel. Roots of the energy equation (Tp) has been found using the “Goal-Seek” data tool, with relative error smaller than (108). 4.1. Wind effect Fig. 5 shows the variation of solar still efficiency with wind speed. It is noticed that efficiency decreases as wind speed increases; this can be explained by the increase of external thermal losses due to external air flow. It is also noticed that efficiency of the still increases as temperature of the internal condenser decreases; this can be explained by the increase of potential to transfer energy from the porous evaporator to the colder condenser. The dependence of the still efficiency on wind speed decreases as wind becomes faster. On the other hand, dependence on condenser temperature becomes less for low wind speed. Fig. 6 shows the variation of distillate rates with wind speed. It is noticed that distillate rates decrease as wind speed increases; this can be explained by the increase of external thermal losses due to external air flow. It is also noticed that distillate rates of the still increase as temperature of the internal condenser decreases; this can be explained by the increase of potential to transfer energy from the porous evaporator to the colder condenser. The dependence of the still distillate rates on wind speed

Fig. 6. Relationship of distillate rates with wind speed for ambient temperature of 20-°C, solar intensity of 500-W/m2 and different condenser temperatures.

decrease as wind becomes faster. On the other hand, dependence on condenser temperature becomes less for low wind speed. Maximum and minimum values of the efficiency and distillate rates for 25-°C ambient temperature simulation are tabulated in Table 2. Compared to the previous two figures, it is noticed that the efficiency and distillate rates increases as ambient temperature increases, this can be explained by the decrease of the convective losses to the external ambient. 4.2. Solar intensity effect Fig. 7 shows the variation of solar still efficiency with solar intensity for different condenser temperatures. It is noticed that the efficiency of the still increases as solar intensity increases for condenser temperature equals to the ambient temperature, but for temperature difference of (1–4)-°C, a decrease is noticed for the first part of the intensity range, then an increase for the rest of the range. Generally, the increase of the efficiency can be explained by the increase of internal convective heat transfer associated with water evaporation due to solar power. However, the mentioned decrease can be explained by the decrease in

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M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 114 (2015) 8–16 Table 2 Maximum and minimum values for ambient temperature of 25-°C.

25 24 23 22 21

Efficiency and distillate rate at wind speed of 2m/s

Efficiency and distillate rate at wind speed of 20-m/s

Efficiency (%)

Distillate rate (kg/day)

Efficiency (%)

Distillate rate (kg/day)

31.3 31.9 32.6 33.2 33.8

2.77 2.83 2.89 2.94 3.00

11.9 13.1 14.3 15.5 16.7

1.05 1.16 1.27 1.38 1.48

ambient heating role acting on the still and enhancing its efficiency compared with the role played by the solar intensity. Another note to be observed is the increase in efficiency with the decrease of condenser temperature; this can be explained by the increase of potential to transfer energy from the porous evaporator to the colder condenser. Fig. 8 shows the variation of distillate rate with solar intensity for different condenser temperatures. It is noticed that distillate rate increases as solar intensity increases; this increase can be explained by the increase of internal convective heat transfer associated with water evaporation due to solar power. It is also noticed that distillate rate increases as the temperature of the condenser decreases similarly. Maximum and minimum values of the efficiency and distillate rates for 25-°C ambient temperature simulation are tabulated in Table 3. Compared to the previous two figures, it is noticed that the efficiency and distillate rates increases as ambient temperature increases, this can be explained by the decrease of the convective losses to the external ambient. Fig. 9 shows the relationship between the porous evaporator temperature (Tp) with solar intensity at constant wind speed, ambient temperature, and condenser temperature. It is noticed that a linear increase of the evaporator temperature occurs with increasing solar intensity. This can be explained by the solar radiation absorbed by the evaporator. 22

Sll Efficiency (%)

20 18

20-C

16

19-C 18-C

14

17-C 16-C

12 10

0

200

400

600

800

1000

1200

Solar Radiaon (W/m2) Fig. 7. Relationship of efficiency with solar intensity for ambient temperature of 20-°C, wind speed of 10-m/s and different condenser temperatures.

4

Disllate Rate (kg/day)

Condenser temperature (°C)

3.5 3 2.5

20-C

2

19-C

1.5

18-C

1

17-C 16-C

0.5 0

0

200

400

600

800

1000

1200

Solar Radiaon (W/m2) Fig. 8. Relationship of distillate rates with solar intensity for ambient temperature of 20-°C, wind speed of 10-m/s and different condenser temperatures.

4.3. Comparison with other solar stills Different solar stills have been proposed and studied. Conventional solar stills have efficiencies within the range of (30–40)-% and a yield of (4–5)-kg/day (Kabeel and ElAgouz, 2011). Using sand as a storage material in a solar still yielded an efficiency of 37.8% and 4.005-L/m2/day (Kabeel and El-Agouz, 2011). A single effect solar still with internal condenser was found to have 5.9-kg/m2/day (Ahmad, 1988). Compared to the mentioned solar stills, the proposed mathematical model has shown efficiency range of (17.1– 21.3%) and distillate rate range of (1.51–1.89)-kg/day for ambient temperature of 25-°C, wind speed of 10-m/s and solar intensity of 500-W/m2, and it these ranges can be higher for higher ambient temperature, higher solar intensity and lower wind speed to reach more than 40% efficiency and more than 5-kg/day productivity. Also, the concentration makes the size of the proposed still less than other stills, so results are shown for a smaller still. 5. Summary In this paper, a steady-state mathematical model of a novel solar still with a porous evaporator and an internal condenser has been proposed. Combining concentration, porous evaporation, internal condensation, and thermossephonic fluid circulation is the main feature of this system.

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Table 3 Maximum and minimum values for ambient temperature of 25-°C. Efficiency and distillate rate at solar intensity of 200-m/s

Efficiency and distillate rate at solar intensity of 1000-m/s

Efficiency (%)

Distillate rate (kg/day)

Efficiency (%)

Distillate rate (kg/day)

25 24 23 22 21

14.1 17.0 19.7 22.4 24.9

0.500 0.602 0.699 0.794 0.884

20.4 20.9 21.4 21.9 22.3

3.62 3.71 3.79 3.88 3.96

Porous Evaporator Temperature (K)

Condenser temperature (°C)

316 314 312 310 308 306 304 302 300

0

200

400

600

800

1000

1200

Solar Intensity (W/m2) Fig. 9. Relationship between the evaporator temperature and solar intensity for wind speed of (10-m/s), condenser temperature of (25-°C), ambient temperature of (25-°C).

The simulation showed the behavior of the still discussed in Section 4 for parameters: ambient temperature, condenser temperature, wind speed and solar intensity. The still showed efficiency range of (17.1–21.3%) and distillate rate range of (1.51–1.89)-kg/day for ambient temperature of 25-°C, wind speed of 10-m/s and solar intensity of 500-W/m2. The simulation also showed the decrease of wind speed and condenser temperature increases the efficiency of the still and distillation rates. Also, the increase of ambient temperature increases the efficiency and the distillation rates. As for the increase of solar intensity, it increases the distillation rate, but not necessarily the efficiency. The variation of the still efficiency with solar intensity has been showed. Also, it was shown that the temperature of the evaporator increases linearly with solar intensity. References Ahmad, S.T., 1988. Study of single-effect solar still with an internal condenser. Sol. Energy 5 (6), 637–643. Ahsan, Amimul., Fukuhara, Teruyuki., 2010. Mass and heat transfer model of Tubular Solar Still. Sol. Energy 84 (7), 1147–1156. Ahsan, Amimul, Shafiul Islam, Kh.M., Fukuhara, Teruyuki, Ghazali, AbdulHalim, 2010. Experimental study on evaporation, condensation and production of a new Tubular Solar Still. Desalination 260 (1–3), 172–179. Ahsan, A., Imteaz, M., Dev, R., Arafat, H., 2013. Numerical models of solar distillation device: present and previous. Desalination, 173–181. Akhtar, N., Mullick, S.C., 2007. Computation of glass-cover temperatures and top heat loss coefficient of flat-plate solar collectors with double glazing. Energy 32, 1067–1074.

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