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

Performance correlations for basin type solar stills Abdul Jabbar N. Khalifa ⁎, Ahmad M. Hamood Nahrain University, College of Engineering, Jadiriya, P.O. Box 64040, Baghdad, Iraq

a r t i c l e

i n f o

Article history: Accepted 27 June 2009 Available online 27 September 2009 Keywords: Desalination Correlations Performance Solar still

a b s t r a c t Many experimental and numerical studies have been done on different conﬁgurations of solar stills to reach the optimum design by examining the effect of climatic, operational and design parameters on its performance. Some of the most important parameters investigated were solar radiation, cover tilt angle, brine depth, and using dyes with the brine. The majority of the investigators presented their results in scatter diagrams rather than correlations. Four correlations are derived in this work to illustrate the effect of solar radiation, dyes, cover slope and brine depth on the productivity of the basin type solar still using the available data given by the different investigators. The correlations developed illustrate that the still productivity could be inﬂuenced by the brine depth alone by up to 33% and by the tilt angle alone by up to 63%. A cover tilt angle of about 30° gives the highest productivity. The still productivity could be enhanced by adding dark soluble dye to the brine by up to 20%. The still productivity increases with the increase of insulation thickness of the still and the solar radiation received. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Interest in the simple solar still has been mainly due to its simple design, construction, and low operation and maintenance costs. However, its low productivity stimulated the development of methods to increase efﬁciency. Many experimental and numerical studies have been done on different conﬁgurations of solar stills to reach the optimum design by examining the effect of climatic, operational and design parameters on its performance. Four of the most important parameters investigated were solar radiation, cover tilt angle, brine depth and using dyes with the brine. The majority of the above investigators presented their results in scatter diagrams rather than correlations; may be due to the small number of data points. Four correlations are derived in this work to illustrate the effect of solar radiation, dyes with water, cover slope and different brine depths on the productivity of the basin type solar still using the available data given by the different investigators. The following is a citation of the most relevant research work categorized in relation to the parameters investigated. 1.1. Brine depth Cooper [4] investigated by a digital simulation some of the more common variables such as water depth, wind velocity, still insulation and cover slope. The results of the simulation indicated that water depth had little effect on the daily productivity, insulation improved

⁎ Corresponding author. Tel.: +964 7902 545412. E-mail address: [email protected] (A.J.N. Khalifa). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.06.011

the productivity at shallow water depths and both double glass covers and high cover slopes were not justiﬁable. Garg and Mann [6] investigated a year round performance of the single and double-sloped solar stills. The productivity of still was found to increase with the decrease in water depth. The effect of water depth on daily yield of a still using a transient analysis method was conducted by Tiwari and Madhuri [11]. It was concluded that the daily yield increases with brine depth for an initial temperature of the brine ≥45 °C and decreased with depth for an initial temperature of the brine ≤40 °C. Kamal [12] used a theoretical model based on solving the time dependent simultaneous energy equations for the water, basin and glass cover to predict the effect of the water depth and insulation thickness on a still with its cover angle close to the annual optimum value. An experimental investigation veriﬁed the theoretical model. Sharma and Mullick [15] predicted analytically the hourly performance of a solar still for different water depths in the range of 5 to 15 cm. Al-Hinani et al. [19] used a mathematical model to predict the productivity of a simple solar still under different climatic, design and operational conditions. A shallow water basin with water depth of 0.02 m was found to be the optimum that produced an average annual yield of 4.15 kg/m2day. Al-Hayek and Badran [20] concluded that the productivity of the still could be increased by decreasing the water depth. The effect of wind speed on the daily productivity of solar stills by computer simulation was investigated by El-Sebaii [21]. A critical mass (depth) of basin water was indicated beyond which the productivity increases with the increase in wind speed up to a typical velocity. The critical mass (depth) was found to be 45 kg (4.5 cm). Singh and Tiwari [22] Developed analytical expressions for water and glass cover temperatures and yield as a function of water depth among other variables. On the basis of numerical computations, it was

A.J.N. Khalifa, A.M. Hamood / Desalination 249 (2009) 24–28

inferred that the annual yield signiﬁcantly depended on water depth, and that the annual yield for a given water depth increased linearly with the collector area. Tripathi and Tiwari [23] conducted experiments during winter months for different water depths namely 0.05, 0.1 and 0.15 m. It was observed that more yield was obtained during the off-sunshine hours compared to daytime hours for water depths of 0.10 and 0.15 m due to the storage effect. Abu-Arabi and Zurigat [24] outlined simulations of three different types of solar stills to compare their productivity using typical meteorological year data. These were the regenerative, conventional, and still with doubleglass-cover cooling. Increasing the water in the lower basin moderately reduced the productivity of the three stills. 1.2. Solar radiation Morse and Read [1] solved by means of characteristic charts heat and mass transfer relationships which govern the operation of a solar still in the unsteady state. The method was used to ﬁnd the effect on output when various parameters, such as solar radiation, wind speed, ambient temperature and heat loss change. It was shown that the effect of solar radiation was of great importance, the effect of wind was unimportant, the inﬂuence of ambient temperature and thermal losses through the base were of considerable importance. Cooper [5] identiﬁed the factors that determine the efﬁciency of a solar still. From knowledge of equations governing the internal and external heat transfer, it was possible to postulate an ideal solar still whose efﬁciency was the ultimate attainable. The output and the efﬁciency for the experimentally measured solar radiation levels were indicated by means of charts. Garg and Mann [6] investigated year round performance of the single and double-sloped solar stills. The long axis of the conventional double-sloped still was oriented in east–west direction. At high latitude stations, a single sloped solar still received more radiation than a double-sloped one at low and high latitudes. The productivity of solar still was found to increase with the increase in total solar radiation, ambient air temperature and wind speed. Tanaka et al. [8] conducted experimental study on the performance of solar stills, both under actual insolation and outdoor simulated conditions. The measured performance was then compared with the results obtained by theoretical analysis. Kamal [12] showed that the productivity of the still was very sensitive to variation in the solar energy input. Zaki et al. [14] investigated experimentally the performance of a simple basin type solar still coupled to an integrated plate type condensing surface. Experiments have been conducted under an average solar ﬂux of 400–600 W/m2. The condensation rate on the condenser reached 3.7 compared to 1.15 l/m2day on the glass surface under the same climatic conditions. Zein and Al-Dallah [16] investigated the performance of two different solar stills under variable meteorological conditions. A close correlation between the total solar radiation and the output was established. Singh and Tiwari [22] developed analytical expressions for water and glass cover temperatures and yield as a function of solar intensity among other factors. 1.3. Cover tilt angle The effect on still efﬁciency of various cover slopes of 5, 10, 15, 20, 25, 30, and 40° was investigated by Baibutaev and Achilov [2,3]. The optimum angle was found to be 30°. The mean condensate velocity and the ﬁlm thickness were determined as a function of the inclination of the condensation surface. Garg and Mann [6] investigated a year round performance of the single and double-sloped solar stills. A glass angle of 10° was found to be the optimum. Al-Jubouri and Khalifa [9] studied the combined effect of the inclination of the glass cover, hence, the air space between the water surface and glass cover, and the area of condensation surfaces on the productivity and efﬁciency of a solar still. Four identical single sloped

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solar stills with transparent lateral walls were constructed. The inclination angle of the glass cover was varied from 5 to 25°. The results indicated that the increase in the inclination angle of the glass cover increases the productivity and improves efﬁciency. However, the rate of increase in output decreases as the angle of inclination increases. Maximum output was found to be for an inclination of around 25°. Kamal [12] predicted the effect of water depth and insulation thickness on a still with its cover angle close to the annual optimum value. An experimental investigation veriﬁed the theoretical model and examined the inﬂuence of the cover slope and the still orientation. The optimum tilt angle was given as 12.5°. Ahmed [13] studied experimentally the effect of using an internal condenser on the performance of single-effect solar still. Its effective base area was 0.4 m2 and its cover slope was 30°. Akash et al. [18] investigated experimentally solar stills with various cover tilt angles of 15, 25, 35, 45 and 55°. An optimum tilt angle of 35° was found in May. The results showed that water production decreases in a linear relationship with increasing water depth. A mathematical model to predict the productivity of a simple solar still under different climatic, design and operational conditions was used by Al-Hinani et al. [19]. A 23° cover tilt angle was found to be the optimum design that produced an average annual yield of 4.15 kg/m2day. Singh and Tiwari [22] developed analytical expressions for water and glass cover temperatures and yield as a function of cover inclinations among other factors. On the basis of numerical computations, it was inferred that the annual yield signiﬁcantly depended on inclination of condensing cover. 1.4. Effect of adding dyes Garg and Mann [6] investigated a year round performance of the single and double-sloped solar stills. The productivity of still was found to increase with increasing the absorptivity of water by using dyes. Rajvanshi [7] studied analytically and experimentally the effect of adding dyes to the brine using two identical stills. The dyes used were black napthylamine, red carmoisine and dark green with various concentrations. The results showed that the dye increased the output by as much as 29%. The agreement between the experimental and analytical model was found to be excellent. Rai and Tiwari [10] studied theoretically the performance of a single-basin solar still coupled with a ﬂat plate collector. The use of dye in the water increased the daily distilled output per unit area of water surface, but the rate of increase with absorptance was more in uncoupled than coupled one. Akash et al. [17] studied the effect of different absorbing materials on the productivity of single-basin solar still. The experimental results showed that using absorbing black rubber mat and black ink increased the daily water productivity by 38% and 45% respectively. Black dye was the best absorber; its enhancement was about 60%. Al-Hinani et al. [19] used a mathematical model to predict the productivity of a simple solar still under different climatic, design and operational conditions. Asphalt coating base was found to be the optimum design that produced an average annual yield of 4.15 kg/m2day. Al-Hayek and Badran [20] showed that the productivity of the still could be increased by the addition of dye. An experimental investigation on the effect of various absorbing materials on the absorptivity of water for solar radiation was conducted by Nijmeh et al. [25]. The materials included dissolved salts, violet dye, and charcoal. Signiﬁcant increase in still efﬁciency and productivity was obtained. The addition of potassium permanganate resulted in 26% improvement in efﬁciency. 2. The developed performance correlations The simplest structure of a solar still, as shown in Fig. 1, is a basin having a certain quantity/depth of saline water and a cover transparent

26

A.J.N. Khalifa, A.M. Hamood / Desalination 249 (2009) 24–28 Table 1 Brine depth correlations. Author(s)

Correlation

R2

Garg and Mann [6] Tiwari and Madhuri [11] Kamal [12] Akash et al. [18] Al-Hayek and Badran [20] Concluding correlation

y = 3.462e− 0.0235(d) y = 4.196e− 0.0575(d) y = 3.516e− 0.0374(d) y = 3.961e− 0.0344(d) y = 3.746e− 0.0369(d) y = 3.884e− 0.0458(d)

0.971 0.991 0.985 0.970 0.986 0.832

water depth while Fig. 2 shows their data and the exponential regressions curve ﬁtted to the scattered points. −0:0458ðdÞ

y = 3:884e

Fig. 1. A schematic diagram of a basin type solar still.

to solar radiation, yet blocks the long wavelengths radiation emitted by the interior surfaces of the still. A sloped cover, which provides a cool surface for condensation of water vapor, facilitates an easy ﬂow of the water droplets into the condensate trough. The base of the still is blackened on the interior surface to maximize absorption of solar radiation, and insulated on the exterior surface to minimize heat losses. The still could be a single or double-sloped. Regression lines were ﬁtted to the data collected from the different investigators cited in the literature for both types as the monthly performance of both conﬁgurations show comparable performance around the year, Yadav [29]. A least square method was used; the type of the regression was chosen to give the best ﬁt to the data, i.e. the one with the best root mean square value R2. The correlations were developed for the following parameters.

2

; R = 0:832

ð1Þ

where (y) is the productivity in (l/m2 day) and (d) is the brine depth in (cm). It is evident that the shallower the basin layer, the higher the productivity. A thin layer of water attains higher temperatures as compared to a deep layer because of its lower capacity. Table 1 shows the correlations created using the results of each of the individual investigators and the concluding correlation of brine depth. 2.2. Solar radiation Solar radiation is the parameter with the highest effect on the productivity as it provides the energy required for water evaporation. Many researchers investigated the effect of solar radiation on productivity including Garg and Mann [6], Tanaka et al. [8], Ahmed [13], Zaki et al. [14], Cooper [5], Zein and Al-Dallah [16], Zaki et al. [27] and Tahir [28]. Fig. 3 shows the diagram of their data ﬁtted by a polynomial regression of the second order. The curve and Eq. (2) illustrate the productivity of the still as a function of the solar radiation. 2

2

y = 0:0036ðIÞ + 0:0701ðIÞ + 0:2475; R = 0:762

ð2Þ

Garg and Mann [6], Tiwari and Madhuri [11], Kamal [12], Akash et al. [18], Al-Hayek and Badran [20] and Abu-Arabi et al. [26] had studied experimentally the effect of brine depth on the productivity. Eq. (1) gives the correlation between the productivity and the basin

where (y) is the productivity in (l/m2 day) and (I) is the solar radiation in (MJ/m2 day). An obvious behavior of increase in the productivity (output) with the increase in the solar radiation (input) can be noticed. Table 2 shows the correlations created using the results of each individual investigator and the concluding correlation of solar radiation effect.

Fig. 2. Variation of still productivity with brine depth.

Fig. 3. Variation of still productivity with solar radiation.

2.1. Brine depth

A.J.N. Khalifa, A.M. Hamood / Desalination 249 (2009) 24–28 Table 2 Solar radiation correlations.

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Table 3 Tilt angle correlations.

Author(s)

Correlation

R2

Author(s)

Correlation

R2

Cooper [5] Garg and Mann [6] Tanaka et al. [8] Ahmed [13] Zeki et al. [14] Zien and Al-Dallah [16] Zaki et al. [27] Tahir [28] Concluding correlation

y = 0.0013(I)2 + 0.1708(I) − 0.3333 y = 0.0079(I)2 − 0.0110(I) − 0.2908 y = 0.0038(I)2 + 0.0901(I) + 0.1991 y = 0.0057(I)2 − 0.0236(I) + 0.5971 y = 0.0039(I)2 + 0.0292(I) + 0.9581 y = 0.0161(I)2 − 0.4482 (I) + 4.9415 y = 0.0708(I)2 − 2.8675 (I) + 31.935 y = 0.0093(I)2 − 0.1878 (I) + 1.9620 y = 0.0036 (I)2 − 0.0701(I) − 0.2475

0.948 0.912 0.991 0.982 0.931 0.982 0.958 0.941 0.762

Baibutaev and Achilov [2] Baibutaev and Achilov [3] Al-Jobouri and Khalifa [9] Kamal [12] Akash et al. [18] Concluding correlation

y = − 0.0450(a) + 4.900 (two points only) y = − 0.0017(a)2 + 0.1296(a) + 0.7007 y = 0.0018(a)2 + 0.0250(a) + 2.1340 y = 0.0042(a)2 − 0.0710(a) + 3.0700 y = − 0.0031(a)2 + 0.1944(a) + 0.1824 y = − 0.0025(a)2 + 0.1562(a) + 0.8430

1 0.983 0.998 1 0.919 0.734

2.3. Cover tilt angle Conﬂicting conclusions were found in the literature regarding the effect of cover tilt angle on productivity. The main reasons for that are the difference in the season of these tests and the difference in the geographical locations (latitude angles). Baibutaev and Achilov [2,3], Al-Jubouri and Khalifa [9], Kamal [12] and Akash et al. [18] studied experimentally the effect of tilt angle on the productivity. Fig. 4 shows the diagram of their results ﬁtted by means of a second order polynomial regression. The polynomial curve and Eq. (2) illustrate the productivity of the still as a function of the tilt angle. The curve speciﬁcs some optimum cover angle around 30o. The effects of the cover tilt angle on the productivity can be the resultant of the effect of more than one parameter as the variation in the tilt angle makes a number of changes in the still. These may include the following: the volume available for water evaporation above the water surface; the heat transfer area of the cover; the speed at which the droplets travel along the interior surface of the cover towards the collecting tray; some drops will fall in the still if the angle is too low; the amount of radiation reﬂected by the cover may vary with seasons which might be explained by the fact that the sun's declination angle has negative values in the winter and positive values in the summer. 2

2

y = −0:0025ðaÞ + 0:1562ðaÞ + 0:843; R = 0:734

ð3Þ

where (y) is the productivity in (l/m2 day) and (a) is the cover tilt angle (degree). Table 3 shows the correlations created using the results of each investigator and the concluding correlation. 2.4. Effect of adding dyes Garg and Mann [6], Rajvanshi [7], Rai and Tiwari [10], Akash et al. [17] and Nijmeh et al. [25] examined experimentally the effect of

Fig. 4. Variation of still productivity with the tilt angle.

adding dyes to the brine on the productivity of the still. Eq. (4) shows the relation between the productivity with and without dyes. The dyes used were black or dark dyes that are soluble in the brine. Fig. 5 shows the diagram of the results of their experiments ﬁtted by a power regression which indicates that using dyes with the brine increases the productivity of the still due to the higher temperatures attained because of the higher brine absorptivity to solar radiation. 1:0467

yD = 1:2122ðyÞ

2

; R = 0:833

ð4Þ

where (yD) is the productivity with dye and (y) is that without dye, both in (l/m2 day). Table 4 shows the percentage increase in productivity caused by adding dye to the brine as reported by the different investigators and the correlation from their data. Eqs. (1)–(4) are applicable for the following conditions: • Passive basin type solar still under solar radiation between 8 and 30 MJ/m2 day. • Galvanized iron body. • Insulation thickness between 5 and 10 cm of polystyrene or any other insulation with equivalent conductivity. • Glass cover with tilt angle between 5° and 45°. • Brine depth ranging from 1 to 10 cm. • Latitude angles between 20° and 35° N. • Dye concentration ranging from 50 to 100 ppm [Eq. (4)]. 3. Conclusions A survey of literature was made to collect as much data as possible to show the effect of four of the most important parameters which affect the still productivity namely; water depth, solar radiation, tilt angle, adding dye. The data were correlated to obtain relations between these parameters and the productivity. The correlation could be used to predict the productivity under the speciﬁed conditions.

Fig. 5. The relation between still productivity with and without dye.

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A.J.N. Khalifa, A.M. Hamood / Desalination 249 (2009) 24–28

Table 4 Dye effect on productivity. Authors

Increase in output

Garg and Mann [6] Rajvanshi [7] Akash et al. [17] Nijmeh et al. [25] Rai and Tiwari [10] Concluding correlation

14.6% 20.3% 53.7% 23.3% 19.8% yD = 1.2122(y)1.0467, R2 = 0.833

From the correlations developed using the data of literature the following conclusions may be given: 1. Increasing the brine depth decreases the productivity of the basin type solar still. The correlation developed illustrates that the still productivity could be inﬂuenced by the brine depth alone by up to 33% for depth ranging from 1 to 10 cm. 2. The productivity of the still is directly related to the intensity of the solar radiation received. 3. A cover tilt angle of about 30° gives the highest productivity. The correlation derived illustrates that the still productivity could be inﬂuenced by the tilt angle alone by up to 63%. 4. The still productivity is enhanced by adding dark soluble dyes to the brine by up to 20%.

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

[27] [28]

References [1] R.N. Morse, W.R. Read, Solar Energy 11 (1968) 5–17. [2] K.B. Baibutaev, B.M. Achilov, Geliotekhnika 4 (1968) 69–72.

[29]

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