Thermal performance of an active single basin solar still (ASBS) coupled to shallow solar pond (SSP)

Desalination 280 (2011) 183–190

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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

Thermal performance of an active single basin solar still (ASBS) coupled to shallow solar pond (SSP) A.A. El-Sebaii 1, S. Aboul-Enein, M.R.I. Ramadan, A.M. Khallaf ⁎ Department of Physics, Faculty of Science, Tanta University, Tanta, Egypt

a r t i c l e

i n f o

Article history: Received 11 April 2011 Received in revised form 11 June 2011 Accepted 1 July 2011 Available online 30 July 2011 Keywords: Active single basin still Shallow solar pond Productivity Computer simulation

a b s t r a c t In order to enhance the productivity of single basin solar stills especially during the night, a shallow solar pond (SSP) was coupled to the still. An analytical model for the various elements of the system (the pond and the still) was performed. Numerical calculations were carried out under Tanta prevailing weather conditions. The daily productivities of the active single basin solar still (ASBS) were found to be 5.740 and 1.830 (kg/m 2 day) with and without the SSP, respectively. The daily productivity Pd and efficiency ηd of the active still were found to decrease with increasing the thickness dw and mass flow rate m˙ of the water flowing over the basin liner of the still up to typical values of 0.030 m and 0.015 kg/s. Moreover, the monthly average of the daily productivity P d had minimum values of 3.0 and 1.570 (kg/m 2 day) in December with and without the SSP, respectively. The maximum values of P d were found to be 6.68 and 5.29 (kg/m2 day) in July with and without the SSP, respectively. To validate the proposed mathematical models, comparisons between experimental and theoretical results had been performed. Good agreement had been achieved. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Water is a nature's gift and it plays a key role in the development of an economy and in turn for the welfare of a nation [1]. Many countries in the world suffer from a shortage of natural fresh water. Increasing amounts of fresh water will be required in the future as a result of the rise in population rates and enhanced living standard, together with the expansion of industrial and agricultural activities [2]. Desalination is the oldest technology used by people for water purification in the world. In desalination, the brackish or saline water is evaporated using thermal energy, and the resulting steam is collected and condensed as the final product. Vapor compression is the process of distillation in which water vapor from boiling water is compressed adiabatically and vapor gets superheated [3]. Solar desalination systems were mainly classified as passive and active solar stills. In the passive solar still, the solar radiation is received directly by the basin water and is the only source of energy for raising the water temperature; consequently, the evaporation leading to a lower productivity. In order to overcome this problem, many active solar stills had been developed [4]. The factors affecting the productivity of solar stills were investigated in previous work [5–8]. Murugavel et al. [9] reviewed the progress in improving the effectiveness of the single

⁎ Corresponding author. Tel.: + 20 040 3070311. E-mail address: [email protected] (A.M. Khallaf). 1 Present Address: Physics Department, Faculty of Science, King Abdulaziz University, 80203 Jeddah 21589, Saudi Arabia. 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.07.004

basin passive solar still. An experimental evaluation of the behavior of a solar still was presented by Voropoulos et al. [10] where a thermal storage tank with hot water was integrated to the still. Vimal Dimri et al. [11] conducted theoretical and experimental analysis of a solar still integrated with flat plate collectors with various condensing cover materials. The performance of a solar still coupled to a flat plate solar collector operating under the forced circulation mode was studied by Rai and Tiwari [12]. The effect of wind speed and other operational parameters on the productivity of some kinds of active and passive solar stills had been presented [13,14]. Tiwari et al. [15] presented parametric study of passive and active solar stills integrated with a flat plate collector. Tiwari and Dhiman [16] performed a transient analysis of a solar still integrated with a panel of collectors through a heat exchanger. Transient study of a single basin solar still coupled to a flat plate solar collector under the thermosyphone mode of operation was studied by Yadav [17]. He concluded that the enhancement in the yield was 30–35% as compared to the uncoupled still. Badran et al. [18] studied the performance of a solar still augment with a flat plate solar collector. Analysis of solar stills coupled to solar collectors operated in the natural circulation mode has been studied by Zaki et al. [19]. They showed that the productivity of the coupled stills increases with increasing the solar collector area. Dwivedi and Tiwari [20] experimentally studied the double slope active solar still under natural circulation mode. Tiwari et. al. [21] developed thermal models for all types of solar collector integrated active solar stills based on energy balance equations in terms of inner and outer glass temperatures. Kumar et al. [22] studied the effect of several parameters on the annual performance of active solar stills.

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As far as the authors now, few papers had appeared concerning the integration of solar ponds to single basin solar stills where effect of using mini solar pond for preheating saline water of solar stills had been investigated [23–25]. In previous work [26,27], thermal performance of the shallow solar pond under open and closed cycle modes of heat extraction has been investigated theoretically and experimentally. The overnight productivity of the active solar stills equals zero due to the absence of the storage capability because the pump connected between the still and the solar collector is allowed to operate only during sunshine hours to avoid the heat losses caused by reverse flow of heat during off-sunshine hours. In this paper, the shallow solar pond was used under the closed cycle mode of heat extraction to improve the daily productivity of active solar stills. Another objective was the comparison between the effect of the open and closed cycle modes on the still performance which is not present in the literature. The thermal performance of the system had been studied by computer simulation. Comparisons between calculated and measured [26] results showed that good agreement was achieved.

in contact with the pond water so that no evaporation of water occurs; hence the cooling effect due to water evaporation was prevented. A serpentine heat exchanger made of copper with length 5.0 m and diameter of 0.01 m was welded to the upper surface of the pond absorber plate. Another serpentine heat exchanger was included within the water in the storage tank. The outlet fluid of the storage tank was used as a fluid flowing through the pond heat exchanger's tube to extract the heat from the pond. The fluid at the outlet of the pond heat exchanger goes back as the inlet fluid for the storage tank under the closed cycle mode of heat extraction. The cold water from the cold water tank was used as a fluid flowing through the storage tank heat exchanger for extracting the heat from the storage tank. The heated fluid obtained at the outlet of the storage tank heat exchanger was fed at the inlet of the still to flow as a thin layer over the basin liner. The still was placed at the same level of the pond; however, the storage tank was put at a relatively high place to achieve the required pressure difference needed for water flowing. 3. Performance analysis

2. Construction of the system The system consists of a single basin solar still operating under the active mode by coupling the still to a shallow solar pond (SSP) as shown in Fig. 1. For enhancing the thermal performance of the SSP, a completely insulted storage water tank was integrated to the pond. A galvanized iron sheet was used for fabricating the still basin with an area of 1.0 m 2. A glass sheet with a thickness of 0.003 m had been used as a glass cover for the still with an optimum inclination angle of 10 o [13] with respect to the horizontal. The surface of the basin facing the sun was painted black for maximum absorption of solar radiation. The bottom and sides of the basin were insulated to minimize heat losses. A galvanized iron sheet was used for fabricating the SSP with a depth of 0.088 m and a bottom surface area of 1.0 m 2, which acts as the absorber surface for the incident solar radiation. The bottom and sides of the pond were insulated to minimize heat losses. Two glass cover sheets (0.003 m thick) with a gap of 0.027 m in between were used as the cover for the pond. The lower glass cover of the pond was always

In previous work [26], construction and the performance of a shallow solar pond under the closed cycle mode of heat extraction had been investigated. It was concluded that thermal performance of the SSP under the closed cycle mode was more efficient than that under the open cycle mode of heat extraction. Therefore in this paper, the SSP under the closed cycle mode of heat extraction was integrated with an active single basin solar still (ASBS) (as shown in Fig. 1) for enhancing the still productivity. In order to write the energy balance equations for the various elements of the system, the following assumptions are made. (i) The heat capacities of the pond absorber plate, basin liner of the still, glass covers and insulation materials are negligible compared to those of the pond and still water, (ii) The lower glass cover of the pond is in contact with the pond water surface to prevent the evaporative heat transfer between the water surface and the lower glass cover. This assumption is justified by achieving the required good contact using the filling and overflow pipes (see Fig. 1) (iii) There is no temperature gradient across the thickness of

Fig. 1. A schematic diagram of the active solar still coupled with a shallow solar pond and a storage water tank.

A.A. El-Sebaii et al. / Desalination 280 (2011) 183–190

the pond and still water. This assumption is justified by taking a small depth of water (0.088 m). The energy balance equations for different elements of the SSP under the closed cycle mode of heat extraction have been carried in a previous work [26]. The energy balance equations for the various components of the ASBS, i.e. basin liner, elemental water length and glass cover may be written as follows:

 Iτg τw αp Ap = h1 Ap Tp −Tw + Ub Ap Tp −Ta

ð1Þ

 

 ∂Tw ˙ w Iτg α w bdx + h1 Tp −Tw bdx = h2 Tw −Tg bdx + mC dx ∂x   ∂Tw + ρdw Cw bdx ∂t

ð2Þ




 Iαg Ag +h2 Aw Tw −Tg = hcga Ag Tg −Ta + hrgs Ag Tg −Ts

ð3Þ

where h2 = hcwg + hrwg + hewg is the total internal heat transfer coefficient, Ag = ApSecβ is the surface area of the glass cover, β is the inclination angle of the glass cover with horizontal. From Eqs. (1) and (3)Tp and Tg are obtained as
Tp =

Iτg τw αp + Ub Ta + h1 Tw

Tg =

Using the initial conditions Tw(x = 0, t = 0) = Twi, Twi is the initial temperature of the water flowing over the basin liner of the still, i.e. the outlet temperature of the SSP under the closed cycle mode of heat extraction. The solution of Eq. (8) is given as Tw ðεÞ = Twi exp½−εðb1 = 2Þ + ðf1 ðt Þ = b1 Þf1− exp½−εðb1 = 2Þg

ð9Þ

where f1 ðt Þ is the average value of f1(t) for a certain time period Δt, and it may be treated as a constant [28]. Hence, the temperature of the water flowing over the basin liner of the still as a function of x and t is given by Tw ðx; t Þ = Twi exp½−ðb1 = 2Þðx + t = a1 Þ
 + f1 ðt Þ = b1 f1− exp½−ðb1 = 2Þðx + t = a1 Þg

ð10Þ

The outlet and average temperatures of the water flowing over the basin liner are obtained as: Two = Tw ðx; t Þx = L

ð11Þ

= Twi exp½−ðb1 = 2ÞðL + t = a1 Þ
 + f1 ðt Þ = b1 f1− exp½−ðb1 = 2ÞðL + t = a1 Þg

 ð4Þ

ðh1 + Ub Þ


185

Iαg Ag + h2 Aw Tw + hcga Ag Ta + hrgs Ag Ts
 h2 Aw + h3 Ag

and L



Twav = ð1 = LÞ∫ Tw ðx; t Þdx 0

!

ð5Þ =

where h3 = hrgs + hcga is the total external heat transfer coefficient from the glass cover to surroundings. On substituting Tp and Tg and using Eqs. (4) and (5), Eq. (2) may be simplified as: ∂Tw ∂T + a1 w + b1 Tw = f1 ðt Þ ∂x ∂t

ð6Þ

where ð6aÞ


 8 9 h1 Ub Ap = b < h2 Ag Aw hcga + hrgs + b1 = ; mC ˙ w :h2 Aw + hcga Ag + hrgs Ag h1 + Ub ;

ð6bÞ



ð12Þ The hourly productivity Ph of the active still is given by

Ph =

ρbdw ; a1 = m ˙

f1 ðt Þ 2 expðb1 t=2a1 Þ−2 exp½ð−b1 = 2ÞðL + t=a1 Þ 1− b1 Lb1   2Twi + ½ expð−b1 t=2a1 Þ− expð−b1 = 2ÞðL + t=a1 Þ Lb1


 hewg Tw −Tg × 3600

ð13Þ

Lw

where Lw is the latent heat of vaporization of water (J/kg), calculated using the following correlation proposed by Sharma and Mullick [29]: 2

Lw = 3044205:5−1670:1109 Tw −1:14258 Tw

ð14Þ

The daily productivity Pd is given as

and

+

(

24h

h1 Iτg τw αp h2 Iαg Ag Iτg αw + + h1 + Ub h2 Aw + hcga Ag + hrgs Ag ! h2 hcga Ag h1 Ub + T h1 + Ub h2 Aw + hcga Ag + hrgs Ag a

b f1 ðt Þ = mC ˙ w

h2 hrgs Ag Ts + h2 Aw + hcga Ag + hrgs Ag

Pd = ∑ Ph

ð6cÞ

The daily efficiency of the active still is given by the following formula P L ηd =
d ave × 100 Ap ∑I Δt

g

ð15Þ

ð16Þ

In order to solve Eq. (6), the following transformation relations are used [28]

where Lave is the daily average of the latent heat of vaporization of water (J/kg) and Δt is the time interval during which the solar radiation is measured.

ε = x + t = a1

4. Numerical calculations and experiments

ð7Þ

Using Eq. (7), Eq. (6) becomes   dTw + b1 Tw = f1 ðt Þ 2 dε

ð8Þ

Computer programs were prepared for the solution of the energy balance equations for different parts of the active solar still, shallow solar pond and the storage water tank. The input parameters to the programs include climatic, design and operational parameters. The

climatic parameters are the ambient temperature, wind speed and solar intensity. They were taken from their measured values for Tanta (Lat. 30 o 47 / N, Egypt) during the summer months of 2009. The design parameters are applicable values of the materials from which the active still, shallow solar pond and storage water were constructed. The relevant parameters, which had been used for numerical calculations for the still, shallow solar pond and storage tank, are given in Table 1. Numerical calculations were carried out assuming, the temperatures of the various components of the system are equal to the ambient temperature at t = 0. Using the initial temperatures, different internal and external heat transfer coefficients of the system: viz.; h1,hcwghrwg, hewg, hcga and hrgs were calculated using the standard correlations given in the literature [30–33]. The correlations which were used for calculating the various internal and external heat transfer coefficients are given in the Appendix. Using the obtained values of the various heat transfer coefficients along with the values of different climatic parameters, the temperatures of the different parts of the system were calculated for a short time interval Δt (10 min). Numerical calculations indicated that decreasing the time interval beyond 10 min does not significantly improve the accuracy of estimation of the still performance. The above procedure was repeated with the new values of the different temperatures for an additional time interval Δt and so on. The productivity was then calculated over the selected time interval. The hourly and daily productivities as well as the daily efficiency of the system were also calculated. Numerical calculations had also been performed for the still without the solar pond for the purpose of comparison. In order to study the effect of climatic and operational parameters on the performance of the SSP, experiments for the SSP under the open and closed cycle modes of heat extraction were conducted during the summer season of 2009 [26]. The system was oriented to face south to maximize the solar radiation received by the pond's cover. The global solar radiation incident on a horizontal surface was measured using an Eppley-Precession Spectral Pyranometer (EPSP) coupled to an Instantaneous Solar Radiation meter model no. 455. Calibrated NiCr– Ni thermocouples connected to a FLUKE 73 digital multimeter were used to measure the temperatures of different elements of the system (the pond and the still) at half-hour intervals. The ambient temperature was also recorded. The mass flow rates of the pond and storage tank heat exchangers (HE) fluids had been measured by collecting a certain volume of the flowing water over a certain time period. The mass of the collected water was divided by the time in seconds to give the mass flow rate in (kg/s). 5. Results and discussions Hourly variations of the measured horizontal solar radiation I, ambient air temperature Ta and measured temperatures of the various elements of the SSP when the pond operates under the open cycle mode of heat extraction on 15 August 2009 [26] are presented in Fig. 2. It is seen that the maximum value of the solar radiation is 951 W/m 2 at noon. The temperatures of different elements of the SSP;

120

1200 Tp Tw Tfo 1000 Tgl Tgu 800 Ta I 600

15/8/2009

100

L=b=1.0 m X w =0.088 m

80 60 40

400

20

200

0

Solar intensity (W/m2)

A.A. El-Sebaii et al. / Desalination 280 (2011) 183–190

Temperature (oC)

186

0 8

10

12

14

16

18

20

Time of day Fig. 2. Hourly variations of the measured solar intensity I, ambient temperature Ta and temperatures of the different elements of the SSP under the open mode of heat extraction.

the absorber plate Tp, the pond water Tw, the outlet temperature of the pond HE's fluid Tfo and lower Tgl and upper Tgu glass covers of the pond are closely follow the solar radiation. The maximum temperature of the pond water is found to lag about 1–2 and 3–4 hours behind the maximum ambient air temperature and solar radiation, respectively. The latter results agree with those outlined by Ali et al. [34,35]. They have concluded that the maximum hourly temperature of the pond water lags behind the maximum hourly ambient temperature and solar radiation by around 1–2 and 3–5 h, respectively. To validate the proposed mathematical models, the pond has been investigated experimentally and theoretically under the same operational conditions for the open and closed cycle modes of heat extraction [26]. Fig. 3 shows comparisons between the measured and calculated temperatures of the pond water Tw when both the length L and width b of the SSP equal 1.0 m. From the results of Fig. 3, it is obvious that the agreement between the measured and calculated temperatures is fairly good. The relative percentage differences between the daily average values of the measured and calculated Tw are found to be −0.56% and 2.1% for the closed and open cycle modes, respectively. The difference between measured and calculated results may come from the correlations which are used for calculating the various heat transfer coefficients. Heat capacities of the pond elements e. g. absorber plate, glass cover, insulation, etc. are not considered in the mathematical analysis and they may represent another source of error. Hourly variations of the temperatures of the water flowing over the basin liner Tw of the ASBS when the still is used with and without SSP are presented in Fig. 4. It is clear from the results of Fig. 4 that Tw 55 calc. (closed) meas. (closed) calc. (open) meas. (open)

50

Table 1 Relevant parameters used in numerical calculations [33]. Relevant parameters

Numerical values

Relevant parameters

Numerical values

kb = ks kg Xg Xb = Xs τw Cw τg V

0.059 (W/m K) 0.78 (W/m K) 0.003 m 0.05 m 0.36–0.08 Lndw[41] 4190 (J/kg K) 0.90 2 m/s for summer 3 m/s for winter

αp εg αg σ

0.90 0.88 0.05 5.669 × 10−8(W/m2 K4)

kw Xw αw

0.6405 (W/m K) 0.088 m (1 − τw)

40

Tw

(oC)

45

35

under closed cycle mode

30

under open cycle mode

= 0.008 kg/s (13/8/2009)

= 0.007 kg/s (19/8/2009)

25 8

10

12

14

16

18

20

Time of day Fig. 3. Comparisons between measured and calculated temperatures of the pond water Tw under open and closed cycle modes of heat extraction.

A.A. El-Sebaii et al. / Desalination 280 (2011) 183–190

65

6 17/8/2009

17/8/2009

with SSP without SSP

60

= 0.005 kg/s

5

dw = 0.02 m

Pd (kg/m2 day)

55

Tw (oC)

187

50 45 40

= 0.005 kg/s

4 3 2 L varies, b= 1.0 m (with SSP) L varies, b = 1.0 m (without SSP) b varies, L = 1.0 m (with SSP) b varies, L = 1.0 m (without SSP)

dw = 0.02 m

35

L = b = 1.0 m

1

30 25

0 0

4

8

(8.0 AM)

12

16

Time (hr)

20

0

24

2

4

6

8

10

12

14

16

18

20

22

L or b (m)

(8.0 AM next day)

Fig. 4. Hourly variations of the calculated temperature of the flowing water Tw for the ASBS with and without SSP.

Fig. 6. Variations of the daily productivity for different values of length L and width b of the ASBS with and without SSP.

for the ASBS with the SSP is higher than that without the SSP because with the SSP, the water flowing over the basin liner of the still was preheated by the SSP under the closed cycle mode of heat extraction. It is clear also from Fig. 4 that the difference between Tw with and without SSP is more pronounced after 11 AM because of the increased heat stored within the flowing water when the still is used with the SSP. The daily average values of Tw equal 45.22 and 35.15 °C for the ASBS with and without the SSP, respectively. The thermal performance of the active solar still (ASBS) has been investigated by computer simulation. Variations of the hourly productivity Ph of the ASBS with and without the SSP on 17 August 2009 when dw = 0.02 m and m˙ = 0.005 kg/s are shown in Fig. 5. The still hourly productivities increase with time until they achieve their maximum values of 0.42 and 0.30 (kg/m 2 h) with and without the SSP, respectively. The daily productivities are found to be 5.74 and 1.83 (kg/m 2 day) with and without the SSP, respectively. The daily productivity of the active solar still with shallow solar pond is higher than that without the shallow solar pond because the water flowing over the basin liner of the still was preheated by the SSP under the closed cycle mode of heat extraction. It is also noticed that, Ph of the still without the SSP reaches zero at sunset. However with the SSP, the still continuous to produce fresh water until the early morning of the next day due to the heat gained from the SSP. Fig. 6 presents variation of the daily productivity Pd with the still length L or width b when dw = 0.02 m and m˙ = 0.005 kg/s when the ASBS is used with and without the SSP. It is obvious from Fig. 6 that Pd increases with the increase of L and/or b up to a typical value of both L and b of 6.0 m. These results are expected because; increasing the still length or

width leads to increasing the absorber surface area; but increasing the surface area is also followed by increasing losses. At a typical value of the still length, the gain in energy compensates for the losses due to increased absorber area owing to increasing still length [36]. Aybar et al. [37] concluded that the longer the absorber plate is the greater the rate of evaporation, leading to the increase in the amount of distiller water. The effect of thickness of the flowing water dw over the basin liner of the still on the daily productivity Pd and efficiency ηd of the ASBS with and without the SSP when m= ˙ 0.005 kg/s is presented in Fig. 7. It is obvious from the results of the Fig. 7 that, Pd and ηd are found to decrease with increasing dw. It is clear also from the results of Fig. 7 that the decrease in Pd and ηd occurs up to a typical value of dw of 0.030 m due to the increased heat capacity which leads to an increase in the time required to warm up the flowing water. Beyond 0.030 m, the decrease in Pd and ηd is insignificant. These results are in agreement with those reported by El-Sebaii et al. [38]. The daily average values of Pd are found to be 5.29 and 2.25 (kg/m 2 day) for the ASBS with and without SSP, respectively. The daily average values of ηd equal 47.54 and 20.45% for the ASBS with and without SSP, respectively. The effect of mass flow rate of the water flowing over the basin liner of the still m˙ on the daily productivity Pd and efficiency ηd of the ASBS with and without the SSP when dw = 0.02 m is presented in Fig. 8. It is seen from Fig. 8 that Pd and ηd are found to decrease with increasing m. ˙ It is clear also from the results of Fig. 8 that the decrease in Pd and ηd occurs up to a typical value of m˙ of 0.015 kg/s due to the increased heat capacity which leads to an increase in the time required to warm up the flowing water [39]. Beyond 0.015 kg/s, the decrease in Pd and ηd is insignificant. The daily average values of Pd are

17/8//2009

0.4

8 = 0.005 kg/s

Pd (kg/m2 day)

= 0.005 kg/s

Ph (kg/m2 hr)

80

10 with SSP without SSP

17/8/2009

dw = 0.02 m

0.3

L = b = 1.0 m

0.2

Pd (with SSP) 70 Pd (without SSP) Eff. (with SSP) 60 Eff. (without SSP) 50

6

40 4

30

Efficiency (%)

0.5

20

0.1

2 10

0.0 0

(8.0 AM)

4

8

12

Time (hr)

16

20

24

(8.0 AM next day)

Fig. 5. Variations of the calculated hourly productivity Ph of the active solar still with and without SSP.

0 0.00

0.01

0.02

0.03

0.04

0.05

0 0.06

Water thickness (m) Fig. 7. Effect of the flowing water thickness dw on the daily productivity Pd and efficiency ηd of the ASBS with and without SSP.

188

A.A. El-Sebaii et al. / Desalination 280 (2011) 183–190

8

100

dw = 0.02 m

Pd (kg/m2 day)

6

90 17/8/2009

Curve m (kg/s)

0.001 0.002 0.003 0.004 0.006 0.010

80 80

60 4 40

70

Two (oC)

Pd (with SSP) Pd (without SSP) eff. (with SSP) eff. (without SSP)

Efficiency (%)

17/8/2009

60

hollow symbols without the SSP filled symbols with the SSP

50 40

2

dw = 0.02 m

20 30 0 0.000

0.005

0.010

0.015

0.020

0 0.025

Mass flow rate (kg/s)

20 0

4

8

(8.0 AM)

12

16

20

Time (hr)

24

(8.0 AM next day)

Fig. 8. Variations daily productivity (Pd) and efficiency (ηd) for different values of the mass flow rate of the flowing water (m) ˙ of the ASBS with and without SSP.

Fig. 10. Hourly variations of the calculated outlet temperature of the flowing water Two for different values of the mass flow rate m˙ with and without the SSP.

found to be 5.02 and 1.94 (kg/m 2 day) for the ASBS with and without the SSP, respectively. The daily average values of ηd equal 45.13 and 19.59% for the ASBS with and without the SSP, respectively. The effect of variations of both the thickness dw and mass flow rate m˙ of the flowing water on the outlet temperature of the water flowing over the basin liner Two when the still was operated with and without the SSP is summarized in Figs. 9 and 10, respectively. It is clear from the results of Figs. 9 and 10 that Two for the ASBS with the SSP is more than that without the SSP because the water flowing over the basin liner of the still was preheated by the SSP under the closed cycle mode of heat extraction. It is seen also from Figs. 9 and 10 that during sunshine hours, Two is found to decrease with increasing both dw and m˙ due to the increased heat capacity with increasing dw and m. ˙ On the other hand, during off-sunshine hours, Two slightly increase with increasing dw and m˙ due to the heat stored in the water flowing over the basin liner of the still. The daily average values of Two for the ASBS with the SSP are decreased from 47.47 to 45.02 °C with increasing dw from 0.005 to 0.06 m. However, for the ASBS without the SSP, Two decreased from 42.87 to 35.77 °C for the same values of dw. On the other hand, for the ASBS with the SSP, Two decreased from 48.03 to 45.14 °C with increasing m˙ from 0.001 to 0.010 kg/s. But for the ASBS without the SSP, Two decreased from 44.73 to 36.09 °C for the same values of m. ˙ Comparisons between the monthly average of daily P d , daylight P dl and overnight P on productivities of the ASBS with and without the SSP during the year 2000 [40] when m˙ = 0.005 kg/s and dw = 0.02 m are presented in Fig. 11. It is clear that P d and P dl have the minimum values of 3.0 and 2.5 (kg/m 2 day) for the ASBS with the SSP and 1.57 and 1.5 (kg/m 2 day) for the ASBS without the SSP in

winter (December). The maximum values of P d and P dl are found to be 6.68 and 5.9 (kg/m 2 day) for the ASBS with the SSP during July and 5.29 and 5.2 (kg/m 2 day) for the ASBS without the SSP during June. It is also seen from the results of Fig. 11 that, the overnight productivity of the ASBS without the SSP P on equals zero; but, P on for the ASBS with the SSP has the minimum value of 1.34 (kg/m 2 day) in summer (June) and the maximum value of 2.7 (kg/m 2 day) in winter (February). 6. Conclusions To enhance the thermal performance of the active single basin solar still (ASBS), the still has been integrated with a shallow solar pond (SSP). From the obtained results, it is concluded that he daily productivity Pd of the ASBS with the SSP is found to be higher than that of the still alone. The daily efficiency of the ASBS with the SSP is higher than that obtained without the SSP by 54.98%. The monthly average values of daily productivity P d of the ASBS have the minimum values of 3.0 and 1.57 (kg/m 2 day) in December with and without the SSP, respectively. However, the maximum values of P d are found to be 6.68 and 5.29 (kg/m 2 day) in July for the ASBS with and without the SSP, respectively. The present results clearly proved that coupling of the shallow solar pond, under the closed mode of operation, to a single basin solar still significantly enhances the productivity and efficiency of the still all year round. In addition to using the system for fresh water production, the present ASBS may be used as a source of hot water required for most domestic applications. The present system as a source of potable water is relatively simple to construct, operate and test for several years without major attention of maintenance and

85

10 dw = 0.005 m

75

dw = 0.020 m

70

dw = 0.030 m

Two (oC)

2

dw = 0.050 m

60

dw = 0.060 m

55

hollow symbols without the SSP filled symbols with the SSP

50 45 40 m = 0.005 kg/s

35

8

Pd (without SSP)

V = 2.0 m/s (summer) V = 3.0 m/s (winter)

dw = 0.02 m

dw = 0.040 m

65

Pd (with SSP)

m= 0.005kg/s

9

Productivity (kg/m day)

17/8/2009

80

L = b = 1.0 m

Pdl (with SSP)

Pdl (without SSP)

7

Pon (with SSP)

6 5 4 3 2 1

30 25 0

(8.0 AM)

4

8

12

Time (hr)

16

20

24

(8.0 AM next day)

Fig. 9. Hourly variations of the calculated outlet temperature of the flowing water Two for different values of the basin water thickness dw with and without the SSP.

0 1

Jan.

2

3

4

5

6

7

Month

8

9

10

11

12

Dec.

Fig. 11. Comparisons between the monthly average values of daily P d , daylight P dl and overnight P on productivities of the ASBS with and without SSP during the year 2000.

A.A. El-Sebaii et al. / Desalination 280 (2011) 183–190

repair. It consists of wood, iron and glass sheets and serpentine heat exchanger tubes made of copper. These materials are cheap and available. On the other hand, the costs of this device are relatively low compared to the conventional sources of energy that are usually used for fresh water production. Therefore, the considered ASBS offers a suitable solution to the fresh water problems faced by the people living in remote and rural areas in the world especially in developing countries. Nomenclature Ag surface area of the glass cover (m 2) Ap surface area of the basin liner (m 2) Aw surface area of the flowing water (m 2) b width of the still (m) Cw specific heat of water (J/kg K) Dhe diameter of the pond heat exchanger tube (m) dw thickness of the flowing water (m) Gr Grasshof number (dimensionless) hcga convective heat transfer coefficient between the glass cover and the ambient air (W/m 2 K) hcwg convective heat transfer coefficient between the flowing water in the solar still and the glass cover (W/m 2 K) hewg evaporative heat transfer coefficient between the flowing water in the solar still and the glass cover (W/m 2 K) hrgs radiative heat transfer coefficient between the glass cover of the pond and the sky (W/m 2 K) hrwg radiative heat transfer coefficient between the flowing water in the solar still and the glass cover (W/m 2 K) h1 convective heat transfer coefficient from the basin liner to the flowing water (W/m 2 K) h2 total internal heat transfer coefficient (W/m 2 K) h3 total external heat transfer coefficient from the glass cover to surroundings (W/m 2 K) kb thermal conductivity of the back insulating material (W/m K) kg thermal conductivity of the glass cover of the still (W/m K) ks thermal conductivity of the sides insulation material (W/m K) kw thermal conductivity of the pond water (W/m K) I solar radiation on a horizontal surface (W/m 2) L length of the solar still (m) Lave daily average of the latent heat of vaporization for the flowing water (J/kg) Lhe heat exchanger length (m) Lw latent heat of vaporization for the flowing water (J/kg) m˙ mass flow rate of the flowing water in the active solar basin still (kg/s) m˙ f mass flow rate of the pond heat exchanger's fluid (kg/s) Nu Nusselt number (dimensionless) Pd daily productivity of the still (kg/m 2 day) Pdl daily daylight productivity of the still (kg/m 2 day) Pg partial pressures of saturated vapour at the glass cover temperature (N/m 2) Ph hourly productivity of the still (kg/m 2 h) Pon daily overnight productivity of the still (kg/m 2 day) Pr Prandtl number (dimensionless) Pw partial pressures of saturated vapour at the basin water temperatures (N/m 2) t desired time period (s) Ta ambient air temperature (°C) Tg glass cover temperature (°C) Tgl temperature of the lower glass cover of the pond (°C) Tgu temperature of the upper glass cover of the pond(°C) Tfo outlet temperature of the fluid in the pond heat exchanger tube (°C) Tp absorber plate temperature (°C)

Ts Tw Twav Twi Two Ub V Xb Xg Xs

189

sky temperature (°C). temperature of the flowing water (°C) average temperature of the flowing water (°C) initial temperature of the flowing water (°C) final temperature of the flowing water (°C) bottom loss coefficient (W/m 2 K) wind speed (m/s) thickness of the back insulation material (m) thickness of the glass cover (m) thickness of the side insulating material (m)

Greek symbols αg absorptivity of the glass cover αp absorptivity of the basin liner of the still αw absorptivity of the flowing water ηd daily collection efficiency of the solar still (%) εgl emissivity of the lower glass cover εg emissivity of the glass cover ρ density of the flowing water (kg/m 3) σ Stefan-Boltzmann's constant (W/m 2 K 4). τg transmissivity of the glass cover τw transmissivity of the flowing water β inclination angle of the glass cover of the still with respect to horizontal ( o)

Appendix The convective heat transfer coefficient from the basin liner to the basin water h1is given by the following correlation [30] Nu =

h1 z kw

ðA:1Þ 1=4

= 0:54ðGr:Pr Þ

Where z is the characteristic length taken as the width of the basin liner (m), kw is the thermal conductivity of the basin water (W/m K). h2 is the total internal heat transfer coefficient between the water flowing over the basin liner of the still and the glass cover. h2 is calculated using the following Eq. h2 = hcwg + hrwg + hewg

ðA:2Þ

where hcwg, hrwg and hewg are the convective, radiative and evaporative heat transfer coefficients (W/m 2 K) from the water surface to the glass cover, respectively. These coefficients are calculated using the following Dunkle's correlations [31] 
  1 = 3  Pw −Pg T hcwg = 0:884 Tw −Tg + 2016−Pw w

ðA:3Þ



 2 2 hrwg = 0:9σ Tw + Tg Tw + Tg

ðA:4Þ

hewg = 9:15 × 10

−7


 9 8
ðA:5Þ

where Pw and Pg are the partial pressures of saturated vapour (N/m 2) at the basin water and glass cover temperatures, respectively. The partial pressure of saturated vapour at any temperature from 10 to 150 °C is given by the following Keenan and Keyes correlation [42] − x a + bx + cx3 Þ = ½T ð1 + dxÞ P = 165960:72 × 10 ½ ð

ðA:6Þ

190

A.A. El-Sebaii et al. / Desalination 280 (2011) 183–190

where x = 647.27 − T, a = 3.2437814, b = 5.86826 × 10 −3 , c = 1.1702379× 10−8 and d = 2.1878462× 10 −3. T is the temperature in K. The total internal heat transfer coefficient h2 is the sum of hcga and hrgs. hcga is the convective heat transfer coefficient from the glass cover of the still to the ambient air. hcga is calculated by using the following correlation [32] hcga = 2:8 + 3:0V

ðA:7Þ

where V is the wind speed (m/s). hrgs is the radiative heat transfer coefficient from the glass cover of the still to the sky. It is calculated by using the following Eq. [33]:

 2 2 hrgs = εg σ Tg + Ts Tg + Ts

ðA:8Þ

References [1] K. Sampathkumar, T.V. Arjunan, P. Pitchandi, P. Senthilkumar, Active solar distillation — a detailed review, Renewable Sustainable Energy Rev. 14 (2010) 1503–1526. [2] Akili D. Khawaji, Ibrahim K. Kutubkhanah, Wie Jong-Mihn, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69. [3] Varun Aayush Kaushal, Solar stills: a review, Renewable Sustainable Energy Rev. 14 (2010) 446–453. [4] G.N. Tiwari, A.K. Tiwari, Solar Distillation Practice for Water Desalination Systems, Anamaya Puplisher, New Delhi, 2008. [5] V. Velmurugan, K. Srithar, Performance analysis of solar stills based on various factors affecting the productivity — a review, Renewable Sustainable Energy Rev. 15 (2011) 1294–1304. [6] A. Safwat Nafey, M. Abdelkader, A. Abdelmotalip, A.A. Mabrouk, Parametric affecting solar still productivity, Energy Convers. Manage. 42 (2000) 1797–1809. [7] O.O. Badran, Experimental study of the enhancement parameters on a single slope solar still productivity, Desalination 209 (2007) 136–143. [8] Samee Muhammad Ali, Umar K. Mirza, Majeed Tariq, Ahmad Nasir, Design and performance of a simple and single basin solar still, Renewable Sustainable Energy Rev. 11 (2007) 543–549. [9] K. Kalidasa Murugavel, Knksk Chockalingam, K. Srithar, Progress in improving the effectiveness of the single basin passive solar still, Desalination 220 (2008) 677–686. [10] K. Voropoulos, E. Mathioulakis, V. Belessiotis, Experimental investigation of the behavior of a solar still coupled with hot water storage tank, Desalination 156 (2003) 315–322. [11] Vimal Dimri, Biokash Sarkar, Usha Singh, G.N. Tiwari, Effect of condensing cover material on yield of an active still: an experimental validation, Desalination 227 (2008) 178–189. [12] S.N. Rai, G.N. Tiwari, Single basin solar still coupled with flat plate collector, Energy Convers. Manage. 23 (1983) 145–149. [13] S. Aboul-Enein, A.A. El-Sebaii, E. El-Bialy, Investigation of a single-basin solar still with deep basins, Renewable Energy 14 (1998) 299–305. [14] A.A. El-Sebaii, Effect of wind speed on active and passive solar stills, Energy Convers. Manage. 45 (2004) 1187–1204. [15] G.N. Tiwari, Dimri Vimal, Chel Arvind, Parametric study of an active and passive solar distillation system, Desalination 242 (2009) 1–18. [16] G.N. Tiwari, N.K. Dhiman, Performance study of a high temperature distillation system, Energy Convers. Manage. 32 (1991) 283–291.

[17] Y.P. Yadav, Analytical performance of a solar still integrated with a flat plate solar collector: thermosyphon mode, Energy Convers. Manage. 31 (1991) 225–263. [18] A.A. Badran, A.A. Al-Hallaq, I.A. Eyal Salman, M.Z. Odat, A solar still augmented with a flat-plate collector, Desalination 172 (2005) 227–234. [19] G.M. Zaki, A.M. Radhwan, A.O. Balbeid, Analysis of assisted coupled solar stills, Sol. Energy 51 (1993) 277–288. [20] V.K. Dwivedi, G.N. Tiwari, Experimental validation of thermal model of double slope active solar still under natural circulation mode, Desalination 250 (1) (2010) 49–55. [21] G.N. Tiwari, Vimal Dimri, Usha Singh, Arvind Chel, Bikash Sarkar, Comparative thermal performance evaluation of an active solar distillation system, Int. J. Energy Res. 31 (2007) 1465–1482. [22] S. Kumar, G.N. Tiwari, H.N. Singh, Annual performance of an active solar distillation system, Desalination 127 (2000) 79–88. [23] V. Velmurugan, S. Pandiarajan, P. Guruparan, L. Harihara Subramanian, C. David Prabaharan, K. Srithar, Integrated performance of stepped and single basin solar stills with mini solar pond, Desalination 249 (2009) 902–909. [24] V. Velmurugan, K. Srithar, Solar stills integrated with a mini solar pond-analytical simulation and experimental validation, Desalination 216 (2007) 232–241. [25] V. Velmurugan, J. Mandlin, B. Stalin, K. Srithar, Augmentation of saline streams in solar stills integrating with a mini solar pond, Desalination 249 (2009) 143–149. [26] A.A. El-Sebaii, M.R.I. Ramadan, S. Aboul-Enein, A.M. Khallaf, Thermal performance of shallow solar pond under open and closed cycle modes of heat extraction, Energy submitted for publication. [27] A.A. El-Sebaii, S. Aboul-Enein, M.R.I. Ramadan, A.M. Khallaf, Thermal performance of shallow solar pond under open cycle continuous flow heating mode of heat extraction, Energy Convers. Manage. 47 (2006) 1014–1031. [28] A.K. Singh, G.N. Tiwari, Thermal evaluation of regeneration active solar distillation under thermosyphone mode, Energy Convers. Manage. 34 (1993) 697–709. [29] V.B. Sharma, S.C. Mullick, Estimation of heat transfer coefficients, the upward heat flow, and evaporation in solar still, Trans. ASME, Sol. Energy Eng. 113 (1991) 36–41. [30] N.M. Ozisik, Heat transfer, McGraw-Hill, New York, 1988. [31] R.V. Dunkle, Solar water distillation: the roof type still and a multiple effect diffusion still, International development in heat transfer, University of Colorado, 1961, pp. 895–902, (Part 5). [32] J.H. Wattmuf, W.W.S. Carters, D. Prostor, Solar and wind induced external coefficients for solar collectors, COMPLES 2 (1977) 56. [33] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, 2th Edition, John Willey & Sons, New York, 1991. [34] H.M. Ali, Study on shallow solar pond applications at Tehran, Energy Convers. Manage. 27 (1987) 33–38. [35] H.M. Ali, M. Akhlaghi, Experimental study on a shallow solar pond, Sol. Wind Technol. 4 (1987) 425–430. [36] A.A. El-Sebaii, Parametric study of a vertical solar still, Energy Convers. Manage. 39 (1998) 1303–1315. [37] H.S. Aybar, F. Egelioglu, U. Atikol, An experimental study on an inclined solar water distillation system, Desalination 180 (2005) 285–289. [38] A.A. El-Sebaii, M.R.I. Ramadan, S. Aboul-Enein, N. Salem, Thermal performance of a single-basin solar still integrated with a shallow solar pond, Energy Convers. Manage. 49 (2008) 2839–2848. [39] H.S. Aybar, Mathematical modeling of an inclined solar water distillation system, Desalination 190 (2006) 63–70. [40] Khallaf AM, Investigation of thermal performance of shallow solar ponds (SSP), M.Sc. thesis, Tanta University, Egypt; 2003. [41] H.M. Yeh, N.T. Ma, Energy analysis for upward type, double-effect solar stills, Energy 15 (1990) 1161–1196. [42] J.H. Keenan, F.G. Keyes, In Thermodynamic Properties of Steam, Wiley and Sons, New York, 1936.