*Permanent address: Dept. of Mech. Engineering, Faculty of Engineering, Mansoura University, Egypt ** The General Organization for Potable Water, Mansoura, Egypt

Thermal energy of a pours-basin solar still

M. Mosaad * and M. Ragab**

Faculty of Technological Studies, Public Authority of Applied Education & Training of Kuwait

Abstract

In this experimental study, two different single-basin solar stills are investigated under

the climatic conditions of Cairo city (latitude: 30°6'N and longitude: 31°24'E) One of two

stills is a conventional black-basin solar still, while the other is a pours-basin solar still.

Both stills are of equal basin area 0.85 m2and covered by a single glass sheet with an

inclination angle of 20o to the horizontal basin base. Ten daily tests is performed. The

daily test starts at 9 a.m. and ends at 6 p.m. During the test, measurements are made in

terms of solar flux, distillate productivity, and temperatures of ambient, glass-cover and

basin-water. The effect of basin water depth on the performance of two solar stills is

investigated.

1. Introduction

The shortage of drinking water in most countries rises from day to day due to population

growth and contamination of fresh water resources. In last few decades, various water

distillation techniques have been proposed to convert brackish water into distillate water

in order to cover the increasing demand for drinking water. Among those techniques is

that one of solar distillation, where the clean regenerative solar energy is used for

supplying the required heating energy. The solar still is one of the popular devices of

solar distillation. As a brief description of the solar still concept, the solar energy flowing

inside evaporates the basin water, the generated vapors condenses on the inner surface of

a glass sheet covering the still basin, the condensate flows down by the effect of gravity

to be collected as distillate water. The concept is similar to that of natural rain formation

[1].

Conventional solar stills are economic and simple in construction, operation and

maintenance. Therefore, it can be used to supply desalinated water for small

communities in remote coastal and dessert areas, where no electricity is available.

Therefore, the topic of solar still has received an attention in recent research work.

Single-basin or double-basin solar stills with different glass-cover shapes (e.g., single-

slope [2], pyramid [3], hemispherical [4] and tubular [5] glass cover) have been reported

in literatures.

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The first practical work was conducted in 1954 by Wilson [6], who used a series of

single-basin solar stills with total basin area of 4700 m2 to supply drink water. Since then,

various studies have been conducted to improve the solar still productivity.

The use of a wick material in the basin of a solar still to increase its volumetric

heat capacity and, consequently, improve its performance has been investigated by some

researchers [7-10]. The performance of such a wick-basin solar still depends on the

capillary action of the wick material, which enables high water evaporation rate at

minimum heating [7]. Frik and Sommerfeld [8] proposed a single-wick solar still, and

concluded that the system is economical but suffers from the problem that a part of the

wet wick becomes sometimes dry. Soda et al. [9] investigated experimentally the

performance of a multiple-wick solar still, by using a number of separated jute pieces of

different length fixed on an incline to be wet at all times. The results showed that on a

cold sunny day in Delhi, the distillate production was 2.5 l/m2 per day, which

corresponds to overall efficiency of 34%.

Other attempts have been made for enhancing the productivity of a solar still by

using a solid porous material in the basin. Hence, the volume of water in the pours basin

is less than that in a conventional still basin having the same area and water depth.

Consequently, this pours-basin structure results in a high distillate productivity compared

to a conventional basin solar still. Madian and Zaki [11] used a carbon powder to

construct a pours basin. An average productivity of 2.5 - 4 l/m2 per day could be

achieved. They noted also that removing the basin insulation results in a reduction of still

productivity by about 13-17%.

From the above review, it is evident that the available data of pours solar stills are

scares and obtained for certain places of different climatic conditions. Thus, it cannot be

used as a reference for the design of pours solar stills in other places of different climatic

conditions. Therefore, in the present work, an experimental investigation is conducted

on a conventional single-basin solar still under the climatic condition of Cairo city, and

the effect of using a porous layer in the still basin is examined for different basin-water

depth.

2. Test system and measurements

Figure 1 shows a sketch of the test system. It involves two stills having an equal

basin area of 0.85 m2. The two stills are separated by a vertical sheet of 6-mm thickness

from glass to avoid shadow effect. The heat exchange between two rooms may be

considered negligible due to the low glass thermal conductivity and low temperature

difference between two rooms. The two flat horizontal basins are made from 1.25-cm

polished steel sheets, and covered by a 6-mm transparent glass sheet with a slop angle of

20o to the horizontal. The two basin surface is black painted to enhance the absorption of

incident solar energy. The outer bottom and sides of two solar stills are thermally

insulated by a foam layer of 5-cm thickness. One basin contains a 20-mm layer from black

solid grains of pazalt. The average volume of basalt grains is 0.22 m3, and the ratio of

porosity volume to grains volume is 0.514. Thus, the system comprises two different

types of solar stills. One is a conventional black-basin still, while the other is a pours-

basin still. Each still is provided with a V-drain aluminum channel to drainage the

condensate into a collection bottle. Copper-Constantine thermocouples, connected to a

voltmeter through an on-off switch, are used to measure the temperatures of basin water,

glass cover and ambient air. The temperature measurement accuracy is about ± 0.3 oC.

The solar radiation is measured by using a Swiss-made pyrometer (brand name:

HAENNI) with an accuracy of ± 4 W/m2.

The solar energy flows into the still heats up the basin water. Some water

evaporates leaving impurities behind. The generated vapour condensates on the inner

surface of the glass cover. The slope of glass sheet directs the condensate to a V-channel,

which in turn delivers it to a collection bottle. Over a period of fifteen days from 30th of

August to 18th of September, ten tests have been performed simultaneously on the two

solar stills, for different basin-water depth = 1, 1.6, 2.7, 5 and 8 cm. These water depth

values are the initial values at the test start. However, because of the decrease of basin

water depth during the test, a compensate amount of water is added during the expected

time of maximum productivity to keep it nearly constant. The maximum deviation in the

tested water depth was within ±1 mm. The compensate water is supplied manually to the

still basin at a temperature of about 35 oC from an official water supply line exposed to

sun on the building roof

During each daily test, solar radiation, temperatures of ambient air, glass cover and

basin water, and distillate production are measured in equal time intervals of 15 minutes.

The hourly mean values of these measurements are calculated by taking the average of

each four subsequent readings. The test time period extends from 9 a.m. to 6 p.m.

3. Results and discussion

A typical sample of measured data is plotted in Fig. 2; in terms of the variation in

solar radiation, distillate productivity, ambient air temperature, water temperature, and

glass cover temperature versus time intervals. It is observed that measured productivity and

temperatures follow nearly the trend of measured solar radiation.

Figure 3 shows the time variation in the measured temperatures of water and glass

cover at different basin-water depth; for conventional “non-porous basin” still (left) and

4

pours-basin still (right). The results of both stills indicate that for a higher basin water

depth, the maximum temperature of basin water and that of glass cover become lower,

and the time period required to reach this maximum values becomes longer. It is also noted

that the time, at which both basin water and glass cover take on the same temperature,

delays as the basin water depth increases.

Figure 4 displays the hourly distillate productivity of convectional basin solar still; for

different basin water depth. It is clear that for a certain water depth, the productivity

increases with time to reach its maximum value nearly afternoon, then, decreases again.

It is noted also that increasing the basin water depth decreases the maximum value of

distillate productivity as well as delays the time at which this value occurs. However, after

a few hours from this time of maximum productivity, the effect of water depth is reversed.

The higher the water depth the lower the still productivity will be. Figure 5 shows the

corresponding hourly distillate productivity of the pours-basin still. The effect of water

depth on the pours solar still productivity seems to be similar to that in the case of the

conventional solar still (cf., Fig. 4), except in the case of the low water depth of 1 cm. An

explanation to this exception case will be presented next.

A comparison between the daily productivity of two solar stills is presented in Fig. 6;

for the two different water depths of 1 and 2.7 cm. In the graph, the curves of the pours-

basin solar still are represented by dashed lines, while those of the conventional basin still

are plotted by solid lines. The displayed results indicate that in the case of 2.7-cm water

depth, the pours still productivity is higher than the conventional still productivity.

However, after a few hours from the maximum productivity time, this trend is reversed.

This is attributed to the low thermal capacity of a porous basin compared to that of a

convectional basin at the same water depth. However, in the case of 1-cm water depth,

the pours still productivity is found much lower than the conventional still productivity.

This may be explained as follows. In this case, the water level is below the pours layer, and

consequently, the evaporation water surface area is limited by the gaps area between

basalt grains. Moreover, under this condition of low water depth, the capillary effect of

the pours layer becomes weak. This results in a low distillate predicatively compared to

that in the case of a conventional basin solar still.

The distillate productivity per day is displayed as a function of basin water depth in

Fig. 7 for both solar stills. It is clear that the increase in basin water depth has a

considerable effect on solar still productivity. The conventional basin still achieves a best

daily productivity of about 2.1 liter/m2 at basin water depth of 5 cm. On other side, the best

performance of the pours solar still could be achieved at water depth of nearly 1.6 cm. The

daily distillate productivity is about 2.3 liter/m2. Here, it is important to point out that the

night productivity after 9 p.m. was found to be about 5% of the daily productivity of the

pours solar still, while it was about 8% for the conventional solar still. This may be

attributed to the higher thermal capacity of the conventional still compared to the pours

solar still.

The solar still efficiency, defined by the ratio of the daily evaporative heat energy to the

daily incident solar energy on the still, is plotted as a function of basin water depth in Fig. 8

for both stills. It is clear from the graph that the pours-basin still has a maximum efficiency

of 41.3 % at basin water depth = 1.6 cm, while the conventional still has a maximum

efficiency of 37.5 % at water depth = 5 cm. These results are consistent with the data

plotted in Fig. 7.

Comparison of present productivity data with corresponding data obtained by Farid

and Hamad [10] is shown in Fig. 9. The comparison shows that the present data are in a

respectable agreement range with that of Farid and Hamad. This may be attributed to the

fact that the data of Farid and Hamad were obtained under climatic conditions similar to

that of the present test.

4. Conclusions

The performance of two types of single-basin solar stills has been investigated; namely:

a conventional black-basin solar still and a pours-basin solar still. The effect of the basin-

water depth on the distillate productivity of each sill has been investigated for water depth =

1, 1.6, 2.7, 5 and 8 cm. The results indicated that the use of a pours layer of black basalt

grains in a solar still basin improves the still productivity. The results showed also that the

pours-basin still with a pours layer of 2-cm height yields a maximum daily distillate

productivity of about 2.3 liter/m2 at water depth of 1.6 cm. However, in the case of

conventional black-basin solar still, a maximum daily distillate productivity of 2.1 liter/ m2

could be obtained at water depth of 5 cm. Present data have been compared with relevant data

from literature, and a reasonable agreement has been found.

For future studies, the following recommendations can be made:

1. Perform more testes for several weeks during the four year seasons to achieve a better

knowledge of the effect of using pours basin on the solar still productivity.

2. Investigate the effect of creating vacuum inside the pours solar still on its productivity.

3. Examine the effect of pours substance permeability on the solar still productivity.

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References

[1] 10] J.T. Mahdi, B.E. Smith, A. O. Sharif, An experimental wick-type solar still system:

design and construction, Desalination 267 (2011) 233–238.

[2] A.A. Al-Karaghouli, W. E. Alnaser, Experimental comparative study of the

performances of single and double basin solar-stills, Applied Energy 77 (2004) 317–325.

[3] H. E. S. Fath, M. El-Samanoudy, K. Fahmy, A. Hass- abou, Thermal-economic analysis

and comparison between pyramid-shaped and single-slope solar still configurations,

Desalination 159 (2003) 69–79.

[ 4 ] T . Arunkumar, R. Jayaprakash, D. Denkenberger, An experimental study on a

hemispherical solar still, Desalination 159 (2012) 342–348.

[5] A. Ahsan, T. Fukuhara, Evaporative mass transfer in tubular solar still, Journal of

Hydroscience and Hydraulic Engineering, Japan Society of Civil Engineers 26 (2008) 15–25.

[6] B. W. Wilson, Desalting of bore water by solar desalination, CSIRO, Report 71 (1954).

[7] K.K. Murugavel, K. Srithar, Performance study on basin type double slope solar still with

different wick materials and minimum mass of water, Renewable Energy 36 (2011) 612–620.

[8] G. Frik, J.V. Sommerfeld, Solar still of inclined evaporating clothes, Solar Energy 14 (1993) 427-

432.

[9] M.S. Sodha, A. Kumar, G.N. Tiwari, R.C. Tyagi, Simple multiple wick solar still, analysis

and performance, Solar Energy 26 (1981) 127-131.

[10] M. M. Farid, F. Hamad, Performance of a single-basin solar still, Renewable Energy 3

(1999) 73-83.

[11] A.A. Madian, G.M. Zaki, Yield of solar stills with porous basins, Applied Energy 52

(1995) 273-281.

[12] A. Kr. Tiwari, G.N. Tiwari, Effect of water depth on heat and mass transfer in a passive solar still

in summer, Desalination 195 (2006) 78-94.

Fig. 1 Test system layout 1. Water supply and drain line 2. distillate bottle 3. Pours-basin solar still 4. conventional -basin solar still

8

Fig. 2 Hourly variation of water temperature, glass cover temperature, ambient air temperature, solar radiation, and still productivity.

8 10 12 14 16 18 20Time, hr

0

100

200

300

400

500

600

700

Sola

r fl

ux,

W/m

10

30

50

70

20

40

60

Tem

pera

ture

, C

distillate

ambient

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Dis

till

ate

pro

du

cti

vit

y,

/ m

2

Solar

glass

water

2

o

l

8 10 12 14 16 18 20Time, hr

10

20

30

40

50

60

70

80

Tem

per

atu

re, C

10

30

50

70

0

20

40

60

80

Tem

per

atu

re,

C

glass

glass

water

(b) h = 1.6 cm

10

20

30

40

50

60

70

80

Tem

per

atu

re,

Co

o

10

20

30

40

50

60

70

80

Tem

per

atu

re,

C

water

(a) h = 1 cm

(c) h = 2.7 cm

(d) h = 8 cm

water

glass

water

glass

8 10 12 14 16 18 20Time, hr

10

20

30

40

50

60

70

80

Tem

per

atu

re,

C

10

30

50

70

0

20

40

60

80

Tem

per

atu

re,

C

glass

glass

water

(b) h = 1.6 cm

10

20

30

40

50

60

70

80

Tem

per

atu

re,

Co

o

10

20

30

40

50

60

70

80

Tem

per

atu

re,

C

water

(a) h = 1 cm

(c) h = 2.7 cm

(d) h = 8 cm

water

glass

water

glass

10

Fig. 3 Hourly variation of water and glass-cover temperatures for different water depth:

Conventional basin still (left) and pours-basin still (right)

Fig. 4 Hourly distillate productivity of conventional basin solar still; for different water depth.

8 10 12 14 16 18 20 Time, hr

0

100

200

300

400

Ho

url

y

dis

till

ate

pro

du

ctio

n,

m

/m

Black painted basinP = 0.0 gaugeh = 1.0 cm 2.7 " 5.0 " 8.0 "

l

2

8 10 12 14 16 18 20

Time, hr

0

100

200

300

400

Ho

url

y d

isti

llat

e p

rod

uct

ion

, m

/

m

Pours basin stil lPg = 0.0 h = 1.0 cm 2.7 " 5.0 " 8.0 "

2l

Fig. 5 Hourly distillate productivity of pours-basin solar still; for different water depth.

Fig. 6 Comparison of hourly productivity of two solar stills.

8 10 12 14 16 18 20

Time, hr

0

100

200

300

400

Hourl

y

dis

tillate

pro

ducti

on,

m

/

m2

l

P = 0.0 gaugeh = 1.0 cm 2.7 "Pours still : dashed lines

(a)

0 2 4 6 8 10Water depth, cm

1.3

1.8

2.3

1.0

1.5

2.0

Sti

ll p

rod

uct

ivit

y,

/ m

1.8

2.2

1.6

2.0

2.4

pours basin still

conventional basin still

2 l

12

Fig. 7 Daily still productivity as a function of basin water depth

Fig. 8 Solar still efficiency as a function of basin water depth

0 2 4 6 8Water depth, cm

20

24

28

32

36

40

44

Eff

icie

ncy

%

black basin

pours b

asin

8 10 12 14 16 18 20Time, hr

0.00

0.10

0.20

0.30

0.40

Pro

du

ctiv

ity,

/m

Farid & Hamad [10]present balck-basin still

present pours-basin still

l 2

Fig. 9 Comparison of present data with other data from ref. [1].