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Modeling and experimental study of a corrugated wick type solar still: Comparative study with a simple basin type
Energy Conversion and Management 105 (2015) 1261–1268
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Modeling and experimental study of a corrugated wick type solar still: Comparative study with a simple basin type K.K. Matrawy a,b,⇑, A.S. Alosaimy a, A.-F. Mahrous a,c a
Mechanical Engineering Department, Taif University, Al-huwayah, P.O. Box: 888, 21974, Saudi Arabia Mechanical Engineering Department, Faculty of Engineering, Assiut University, Egypt c Mechanical Power Engineering Department, Menoufiya University, Shebin El-Kom 32511, Egypt b
a r t i c l e
i n f o
Article history: Received 20 April 2015 Accepted 1 September 2015
Keywords: Solar still Corrugated surface Porous material Inclined reflector Productivity
a b s t r a c t In the present work, the productivity of a solar still is modified by forming the evaporative surface as a corrugated shape as well as by decreasing the heat capacity with the use of a porous material. This target has been achieved by using black clothes in a corrugated shape that are immersed in water where the clothes absorbs water and get saturated by capillary effect. Along with the proposed corrugated wick type solar still, a simple basin still type was fabricated and tested to compare the enhancement accomplished by the developed solar still. Inclined reflectors were used to augment the solar radiation incident on the plane of the developed solar stills. The energy balance in the developed mathematical models takes into consideration the glass covers, the porous material, along with the portion of water exposed to the transmitted solar radiation as well as the portion of water shaded by the corrugated surface. The developed mathematical model was validated by fabricating and testing two models for the proposed and simple basin solar stills under the same conditions. Good agreement between the simulated and experimental results has been detected. It has been found that an improvement of about 34% in the productivity for the proposed wick type solar still is gained as compared to the simple basin case. Also, the best tilt angle for the inclined reflector has been found to be about 30° with respect to the vertical direction of the setup under consideration. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Supplying fresh and healthy water is still one of the major problems in different parts of the world, particularly in remote arid areas [1]. Solar still may provide a solution for those areas where it is cheap and having low maintenance cost, but it suffers from the lower productivity [2]. Accordingly, rigorous theoretical and experimental studies have been made to enhance the solar stills’ productivity. Developed work by Tiwari and Tripathi [3] has been carried out on both passive and active solar distillation systems. They recommended that only passive solar stills can be economical to provide potable water. Rai et al. [4] studied experimentally a single basin solar still coupled with a flat plate collector under various modes of operation. The modes of operation incorporated the effect of water salinity as well as the thermosyphon and forced circulations. The experimental results showed that the daily variation
⇑ Corresponding author at: Mechanical Engineering Department, Taif University, Al-huwayah, P.O. Box: 888, 21974, Saudi Arabia. Tel.: +966 5495 31562; fax: +966 (2) 724 0614. E-mail address: [email protected] (K.K. Matrawy). http://dx.doi.org/10.1016/j.enconman.2015.09.006 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
of the stills’ productivity ranged from 1.6 kg/m2 to 2.4 kg/m2 when adding a small amount of dye. Badran [5] found that the productivity was increased by about 52% in case of coupling the still with a flat plate collector. Akash et al. [6] showed that the productivity of basin solar still decreases with salinity and also decreases in a linear relationship while increasing the water depth. A maximum hourly yield with a value of 0.6 kg/m2 was attained in the study. Omara and Kabeel [7] studied the performance of different sand beds solar stills. The influences of sandy bed height, type of sand, and water height above the sandy bed level on the solar still performance have been investigated. Maximum accumulated distillate yield of 4 kg/m2 was achieved while the corresponding yield was 2 kg/m2 in the conventional type. Nafey et al. [8] enhanced the productivity of the single slope solar still using black rubber and black gravel material as storage medium. They found that an enhancement of 19–20% in the stills’ productivity can be achieved when using these materials. Other technique to enhance the stills’ productivity was developed by feeding a water film over the glass cover of a multi- wick solar still to increase the productivity and also performs self-cleaning for the glass cover [9]. Incropera and Dewitt [10] have investigated the convective heat transfer between
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Nomenclature A Cp h hfg I m Md L T W Q Qs
area (m2) specific heat (J/kg K) heat transfer coefficient (W/m2 K), height (m) latent heat of condensation (J/kg C) solar radiation (W/m2) mass (kg) mass of distillated (kg) length (m) temperature (°C) width (m) heat (W) absorbed solar radiation (W/m2)
Greek letters a absorptivity b slope angle (°) h incidence angle (°) d Boltzman constant (W/m4 K) s transmissivity
the cooling film and glass cover while Sherwood et al. [11] have studied the heat transfer due to water evaporation associated with the cooling film. A solar desalination plant consists of solar parabolic collectors, steam generators, and MED unit was simulated and optimized using multi-objective genetic algorithm by Mokhtari et al. [12]. Recently, various technologies have been developed to meet the increasing demand of potable water such as double slope solar still [13], providing low pressure inside the still basin [14], using nanofluids and integrating the still basin with external condenser [15], enhancing the stepped solar still using internal and external reflectors [16], using a flat and ripped absorber in ‘‘V” wick type solar still [17], floating cum tilted wick solar still [18], using a corrugated galvanized iron steel as an absorber in between the wick strips [19] and multiple porous blackened jute absorbers floated on the water basin [20]. Moreover, Huang et al. [21] studied the multi effect diffusion type solar still (MEDS) coupled with a vacuum tube solar collector. They showed that the 10-effect MEDS produces pure water ranged from 13.7 to 19.7 kg/day/m2 when the incident solar radiation ranges from 600 to 800 W/m2, respectively. For the 20-effect solar still, the productivity increases by 32% compared to the10-effect one. Thus, it may be concluded that the most recommended strategies to enhance the productivity are: 1. Decreasing the water depth in the basin. 2. Using a forced circulation to increase the rate of evaporation processes. 3. Feeding a water film over the cover to decrease the cover temperature. 4. Using a sand bed or black rubber as a storage medium. 5. Integrating an external condenser and using nanofluids. 6. Providing low pressure inside the still to decrease the evaporating temperature. 7. Floating blackened jute on the water basin. 8. Applying energy recovery for the condensate vapor (i.e. multi effect solar still). Accordingly, the main objective of the present study was to enhance the evaporation processes in a simple basin solar still. This was satisfied by increasing the evaporative area using corrugated shape and decreasing the heat capacity of the evaporating water
q
reflectivity emissivity
e
Subscripts 1 corrugated solar still 2 simple basin solar still a air b bottom, beam c cover, convention d diffused e evaporative G global p porous pt porous top pb porous bottom r radiative, reflector s still w water, wind
with the use of a porous material. Removing the salt and some impurities from the porous material was taken place through a periodic cleaning for the porous material. Description of the developed solar stills, mathematical modeling and experimental set up were presented in Sections 2–4, following by the results and discussions for the simulated and experimental results. 2. Description of the proposed corrugated wick type and simple basin type solar stills A schematic diagram for the proposed corrugated wick type solar still with an inclined flat reflector is shown in Fig. 1. From Fig. 1, it can be seen that the main components of the proposed solar still are: 1. Glass cover with a thickness of 6 mm to transmit the incident solar radiation and the reflected radiation from the reflector to both the porous material and the water in the basin. Also, the generated vapor is condensed along the lower surface of the cover and collected at the lower end. 2. Porous material that is black clothes with a thickness of 2 mm made in a corrugated shape. The porous material is partially immersed and wetted by the water in the basin by capillary effect to increase the rate of evaporation.
Inclined Relector
Glass Cover
Black clothes
Water
Insulation
Fig. 1. Corrugated wick type solar still with an inclined reflector.
Outer Frame
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3. Stills’ basin contains the warm water including an amount of water that directly absorbs the transmitted radiation besides a shaded amount that is heated by convective and radiative heat transfers from the back surface of the corrugated black clothes. 4. Outer frame that is a wooden box with dimensions of 1.35 0.9 m to protect the system from the outer ambient conditions. It includes an insulation material (k = 0.024 W/m K) with a thickness of 50 mm to decrease the heat losses from the hot water in the basin. 5. Flat inclined stainless steel reflector with dimensions of 0.5 m and 0.8 m for the height and width, respectively. The slope of the reflector with the vertical direction can be changed using a hinge as shown in Fig. 1. On the other hand, the simple basin solar still shown in Fig. 2 consists of the typical components with the same descriptions as in Fig. 1, except the absence of corrugated black clothes in the basin. The dimensions and specification of the component are the same as those mentioned in Fig. 1.
3. Mathematical modeling
Qs,c1
Tc1 Qs,w1 Qp1—c1 Qw1-c1
hw1
1. All the properties are independent on the temperature. 2. No longitudinal temperature distribution for the glass cover, water in the basin and the corrugated surface. 3. No temperature gradient through the thickness of glass cover, corrugated shape and water depth. 4. One dimensional heat flow through the thickness of: glass cover, porous material, water in the basin and back insulation. 5. The heat capacity of the glass cover and insulation material of the still, is negligible. 6. Performance is steady state.
Qb1
Fig. 3. Periodic segment of the corrugated wick type solar still.
Thus, according to a periodic segment of the corrugated wick type solar still shown in Fig. 3, the energy balances for the glass cover, porous material and water basin, respectively are:
ðmC p Þc1
dT c1 ¼ Q s;c1 þ Q p1c1 þ Q w1c1 Q c1a dt
ð1Þ
ðmC p Þp1
dT p1 ¼ Q s;p1 Q p1c1 Q p1w1 dt
ð2Þ
dT w1 ¼ Q s;w1 þ Q p1w1 Q w1c1 Q b1 dt
ð3Þ
mC p
w1
where Qs,c1, Qs,p1, and Qs,w1 are the absorbed solar radiations through cover, porous material, and water, respectively. Their values are given in detail in Appendix A while the other quantities are given as:
Q p1c1 ¼ Ap1 ðhr;p1c1 þ hc;p1c1 þ he;p1c1 ÞðT p1 T c1 Þ
ð4Þ
Q w1c1 ¼ Aw1 ðhr;w1c1 þ hc;w1c1 þ he;w1c1 ÞðT w1 T c1 Þ
ð5Þ
Q c1a ¼ Ac1 ðhr;c1a þ hw ÞðT c1 T a Þ
ð6Þ
Q p1w1 ¼ Ap1 ðhr;pt1w1 þ hr;pb1w1 þ hc;pt1w1 þ hc;pb1w1 ÞðT p1 T w1 Þ ð7Þ Q b1 ¼ ðUAÞ1 ðT w1 T a Þ
ð8Þ
where (UA)1 is the loss coefficient- area product. It is a function of the area of bottom and side walls of the still as well as the thickness and type of insulation material.The amount of distillated water Md1 in the corrugated solar still is:
Md1 ¼ Inclined Relector
Tp1 Qp1-w1
Tw1
3.1. Modeling of corrugated wick type solar still The developed simulation model for the proposed corrugated wick type solar still is based on energy balance for the main components shown in Fig. 1. The simulated components include the condensing surface (glass cover) and the evaporative one. The evaporative surface in this type consists of a part of the porous material above the water level beside the part of water surface exposed to the transmitted radiation (there is anther part of water surface shaded by the corrugated surface). The developed model is based on the following assumptions:
Qs,p1
Qc1-a
Ap1 he;p1c1 ðT p1 T c1 Þ þ Aw1 he;w1c1 ðT w1 T c1 Þ hfg
ð9Þ
3.2. Modeling of the simple basin solar still Glass Cover
Water
Insulation
Fig. 2. Simple basin solar still with an inclined reflector.
Outer Frame
Modeling of the simple basin solar still is derived based on the same assumptions given in Section 3.1, but without the porous material. The energy balances for the components are based on a periodic segment as shown in Fig. 4.
ðmC p Þc2
dT c2 ¼ Q s;c2 þ Q w2c2 Q c2a dt
ð10Þ
ðmC p Þw2
dT w2 ¼ Q s;w2 Q w2c2 Q b2 dt
ð11Þ
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hr;ij ¼
rðT i þ T j ÞðT 2i þ T 2j Þ 1ei A i ei
þ Ai F1ij þ
1ej A j ej
ð20Þ
where the view factor Fi j is determined as an enclosure problem depending on the configuration of surface i related to j as explained in Ref. [25]. This includes the corrugated and water surfaces compared to the cover and also the upper and lower corrugated surfaces compared to the water one. 4. Experimental test rig
Fig. 4. Periodic segment of the simple basin solar still.
Q w2c2 ¼ Aw2 ðhr;w2c2 þ hc;w2c2 þ he;w2c2 ÞðT w2 T c2 Þ
ð12Þ
Q c2a ¼ Ac2 ðhr;c2a þ hw ÞðT c2 T a Þ
ð13Þ
Q b2 ¼ ðUAÞ2 ðT w2 T a Þ
ð14Þ
The amount of distillated water Md2 in the simple basin still is:
M d2 ¼
Aw2 he;w2c2 ðT w2 T c2 Þ hfg
ð15Þ
Based on energy balances for the developed models, the temperatures variation and distillated amounts of water for the corrugated and simple basin solar stills can be calculated using Euler integration method cited in Ref. [2]. 3.3. Heat transfer coefficients The developed simulation model includes different heat transfer coefficients, namely convective, evaporative and radiative coefficients between different components of the corrugated wick type and simple basin solar stills. The convective and evaporative heat transfer coefficients between water surface and glass cover are given, as cited in [22] as:
hc;wc ¼ 0:884 T w T c þ
ðPw Pc ÞT w
1=3
268:9 103 Pw
he;wc ¼ 16:237 103 hc;wc
Pw Pc Tw Tc
ð16Þ
ð17Þ
where Pw and Pc are the saturation partial pressures at Tw and Tc, respectively. Values of Pw and Pc (for the range of temperature 10–90 °C) can be obtained from expression developed by Fernadez and Chargoy [23] as:
5144 PðTÞ ¼ exp 25:317 T þ 273
ð18Þ
Eqs. (16) and (17) can be used to calculate convective and evaporative heat transfer coefficients in Eqs. (5) and (12) based on Tw and Tc. Also, for the porous material, the value of Tw is replaced by Tp to calculate the same coefficients. For the convection heat transfer coefficient between the sides of the corrugated surface and water one, they have been treated as inclined and horizontal plates. Churchill and Chu in [24] recommended the following equation:
NuL ¼ 0:68 þ
0:67Ra0:25 L ½1 þ ð0:492=PrÞ9=16
4=9
RaL 6 109
ð19Þ
Radiative heat transfer coefficient hr,i j between two surfaces i and j is given as:
An experimental set up was prepared for both the corrugated wick type and the simple basin solar stills. The test rig is built on the roof of Solar Energy Laboratory in Taif University (latitude angle 22.23°) at Saudi Arabia with a photograph shown in Fig. 5. The dimensions and specifications are as mentioned in Section 3.1. The two examined solar stills are set beside each other and both face south to maximize the incident solar radiation. The setup is instrumented to measure the temperatures of glass cover, water in the basin, porous surface, and ambient air. The thermocouple sensors (type-K) were calibrated with an accuracy of ±0.5 °C and connected to thermocouple data logger which is programmed and connected to PC to record the temperatures with a time interval of 15 min. The global solar radiation on a horizontal surface is measured near the tested solar stills using a solar pyrometer (Type SR 11) with an accuracy of ±1 W. The condensate water from each still is collected and measured using a graduated vessel with a capacity of 500 ml and an accuracy of ±5 ml. Condensate water was recorded manually with a time interval of 30 min. The test procedure was started by putting the same amount of salty water in the corrugated and simple basin solar stills. The two stills were shaded for one day to ensure a complete saturation of the porous material by water observing a complete wetting for the material in the morning of the second day and to reach steady state conditions. Recording of the different temperatures, solar radiation, wind speed, condensate water were started from 9 a.m up to 6 p.m. in the month of May with the time interval mentioned above. The experiments were repeated for two days in clear sky conditions with slight variation in the recorded values of ±0.5%. The accumulated amount of distillated water was calculated by summing the hourly values through the day. 5. Results and discussions The results and discussions for the calculated and measured data for the proposed wick type and simple basin solar stills are presented in this section. The calculations have been carried out using the following specifications:
Ls ¼ 1:25 m; Lr ¼ 0:5 m; W r ¼ W s ¼ 0:8 m; hs ¼ 22:23 ; hr ¼ 30 ;
ac ¼ 0:07; aw ¼ 0:9 and ap ¼ 0:95:
Fig. 6 shows the beam, diffused and global solar radiations incident on a horizontal surface in the month of May and located in the site under consideration. The presented values in the figure have been calculated according to the ASHREA model cited in Refs. [26,27]. The calculations have been carried out for beam, diffused and global radiations, while the measurements have been recorded only for the global radiation. It is clear from the figure that there is an agreement between the calculated and measured results with a maximum deviation of about 2–3%. Also, there is a slight variation in the diffused solar radiation along the day hours. The maximum percentages of diffused radiation are about 13% and 10.8% of the beam and global radiations, respectively. The best inclination angle for the flat reflector with respect to the vertical direction and different reflector angles is presented
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Relector Simple type Solar Still
Computer set
Corrugated type solar still
300 Calculated IG
1000
Measured IG Calculated Id
250
800 200 600 150 400 100
200
1000
250
800
200
600
150
Water or Porous Absorbed Radiation, W/m2
Calculated Ib
Diffused Radiation, W/m2
Beam or Global Radiation, W/m2
1200
400
100 Calculated Qs,w
200
50
Calculated Qs,p Calculated Qs,c
0
0 8
9
10
11
12
13
14
15
16
17
18
Cover Absorbed Radiation, W/m2
Fig. 5. Experimental setup.
Time, hr 0 8
10
12
14
16
50 18
Fig. 8. Absorbed solar radiations for the developed solar stills.
Time, hr Fig. 6. Calculated and measured solar radiations versus day hours.
85 120
Temperature, o C
Absorbed Radiation by Reflector, W/m2
95
100 80 60
Reflector angle,deg
Calculated Tw1
55
Measured Tw1 Calculated Tp1
45
Measured Tp1 Calculated Tw2 Measured Tw2
20
25 8
30
20
65
35
10
40
75
10
12
14
16
18
Time, hr
40
0 9
10
11
12
Time, hr
Fig. 9. Calculated and measured temperatures for the developed solar stills.
Fig. 7. Absorbed radiation by reflector with different reflector angles.
in Fig. 7. It is clear from that figure that the maximum reflected radiation for the whole day hours occurs at an angle of about 30o. This angle may be considered as the best angle for the reflector in the present case. It is important to note that the derived angle is considered only in May with a reflector length of 0.5 m. This value may differ in other cases. The derived best inclination angle for the flat reflector has been assigned as a constant and used as input data to the developed simulation model and also through the measurements of the designed test rig. The amount of absorbed radiations on the porous material, water in the basin, and glass cover are shown in Fig. 8. The presented values are calculated from Eqs. (A15) and (A16) in Appendix A based on the absorptivity for the components beside the incident radiations on planes of the solar still as well as the flat reflector. The same results are presented for the conventional type solar still shown in Fig. 2, but without including the absorbed radiation
through the porous material. From Fig. 8, it can be seen that the maximum absorbed radiation occurs with the porous material while the lower value occurs with the glass cover. The absorbed radiation on the water in the basin lied between the porous material and the glass cover. The trend of the absorbed radiation is similar to the incident solar radiations as shown in Fig. 6. Maximum absorbed radiation has a value of 900 W/m2 in the porous material while it is about 85 W/m2 for the glass cover. The lower absorbed radiation through the glass cover is mainly attributed to the lower absorptivity of glass cover used in calculations. The temperatures of the porous material and water in the basin of the corrugated wick type solar still are shown in Fig. 9. Temperature of water in the simple basin solar still is also presented in the same figure to simplify the comparison between the developed solar stills. It can be seen from the figure that the temperatures decrease from a maximum value of 87 °C in the porous material to a minimum one of 82.5 °C for the water of the simple type solar
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still. Temperature of the water basin of the corrugated solar still lies between the above values as seen in the same figure. It is important to understand that the water in the basin of corrugated solar still is heated by direct transmitted radiation beside the heat transfer by convection and radiation from the sides of porous surface. Thus, the behaviors of the presented temperatures are mainly attributed to the exposed area of the transmitted solar radiation and also to the heat capacity of the components. The measured temperatures are presented also in the same figure, which shows a quite agreement between the calculated and measured temperatures. The calculated and measured temperatures of the glass covers for the corrugated and simple solar stills are shown in Fig. 10. Ambient air temperature is also presented in this figure. It can be seen that the temperature of the glass cover in case of the corrugated solar still is higher than that of the simple type. This may be attributed to the higher temperature of the evaporating surface (porous material in corrugated solar still) than that of the simple type (water in the basin). Maximum values take place at 3 p.m. with values of 78 °C and 74 °C for the corrugated and simple type solar stills, respectively. The corresponding ambient temperature is 35 °C under these conditions. Moreover, it is observed that the measured temperatures are agreed well with the calculated values with a difference of about 2–3 °C. Based on the previous illustrations of the calculated and measured temperatures, the calculated and measured productivities are displayed in Fig. 11. Due to the higher temperature of the corrugated wick type surface, its productivity is higher than that for the conventional type at the same operating conditions. Maximum productivities of 0.93 kg/h and of 0.76 kg/h were
achieved for the corrugated and simple solar stills, respectively, as seen in Fig. 11. It is important to explain and discuss the contribution of both corrugate and water surfaces through the evaporation processes in corrugated solar still. Fig. 12 presents the productivity due to corrugated surface and water surface with respect to the total productivity produced from the corrugated solar still. From the shown figure, it is clear that the productivity of the corrugated surface is the dominant source of evaporation compared to the contribution of the water surface. At 2 p.m., the productivity of the corrugated surface is about 0.7 kg/h, while the corresponding value is 0.23 kg/h for the water surface in the basin. The higher contribution of the corrugated porous material is mainly attributed to the higher temperature compared to that of the water and also to the higher area of corrugated surface exposed to the transmitted radiation. Based on the geometry and configuration of the corrugated solar still, the surface area of corrugated shape was about 70% of evaporation area while the remaining percentage was for the water surface. This explains the higher contribution of the corrugated surface in evaporation process. The accumulated amounts of distillated water for wick and simple basin solar stills versus day hours are shown in Fig. 13. It is clear from this figure that the accumulated amount of distillated water from corrugated wick solar still is higher than that for the simple basin. The daily productivity of corrugated solar still at 6 p.m was about 5.9 kg/m2 while the corresponding for the simple basin is nearly 4.4 kg/m2. This means that the productivity of corrugated solar still with inclined flat reflector is increased by about 34% more than that for the simple basin with the same reflector.
90
70
Calculated Ta
1
Measured Ta
0.9
Calculated Tc1
0.8
Productivity, kg/hr
Temperature, o C
80
Measured Tc1 Calculated Tc2
60
Measured Tc2
50 40 30
0.7 0.6
Md1 (total)
0.5
Md (from porous)
0.4
Md (from water)
0.3 0.2 0.1
20
0 9
10 8
9
10
11
12
13
14
15
16
17
18
10
11
12
19
13
14
15
16
17
18
Time, hr
Time, hr Fig. 10. Calculated and measured temperatures for the glass covers of corrugated and simple basin solar stills.
Fig. 12. Calculated productivity for corrugated and water, surfaces of the corrugated wick type solar still.
6
1
Accumulated Md, kg
Productivity, kg/hr
0.9 0.8 0.7 0.6 0.5 Calculated Md1
0.4
Measured Md1
0.3
Calculated Md2
0.2
Measured Md2
0.1
5 4 3 2 Measured Md1
1
Measured Md2
0
0 9
10
11
12
13
14
15
16
17
18
19
Time, hr Fig. 11. Calculated and measured productivity for corrugated and simple basin solar stills.
9
10
11
12
13
14
15
16
17
18
Time, hr Fig. 13. Measured accumulated amounts of distillated water for corrugated and simple basin solar stills.
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6. Conclusions
The value of sc(hr) is calculated according to Eq. (A3) by replacing hs by hr. Also, the values of L1, L2, L3, and L4, are given as:
The present work aimed to enhance the productivity of the conventional type solar still by introducing a corrugated wick type solar still. The performance of the developed solar still is compared with that for the simple type. Theoretical analysis for the absorbed solar radiation on the glass cover, corrugated porous material and water was developed. The analyses are based on the calculated beam, diffused solar radiation using ASHREA model. The results obtained by mathematical model were compared to the experimental results with a quite agreement between the calculated and experimental results. The diffused radiation was about 13% and about 10.8% of the beam and global solar radiation, respectively. The corrugated porous surface in the corrugated solar still was shown to contribute by about 75% of the total productivity. At the same time, the daily productivity of corrugated solar still was increased by about 34% more than that for the simple basin type.
L1 ¼ Lr fcos br tanðx3 þ 2br Þ sin br g
ðA11Þ
L2 ¼ L1 tan x1
ðA12Þ
L3 ¼ L1 tan x2
ðA13Þ
cos w L4 ¼ Lr sin br þ cos br tan /
ðA14Þ
where the angles x1, x2, x3 are given in detail in Ref. [30] as a function of other lengths L5 to L12 beside the given lengths. Finally, the total absorbed solar radiations on the water basin and cover are given in [31] as:
Q s;w ¼ Q b;w þ Q r;w þ Q d;w
ac Q b;w þ Q r;w Q d;w þ aw sc ðhs Þ sc ðbs Þ
ðA15Þ ðA16Þ
Appendix A
Q s;c ¼
The use of an external reflector can be useful to increase the productivity of the solar still as reported in both [28,29]. The developed analyses by Tanaka [30] and by Tanaka and Nakatake [31] for the inclined reflector are used in the present study to augment the incident solar radiation on the planes of corrugated and simple basin solar stills. As explained in details in both [30,31], the absorbed beam and diffused solar radiation in the water basin of a south facing solar still can be expressed as:
Similarly, total absorbed radiation on the corrugated surface due to the reflector is obtained using Eq. (A15) through substitution of ap instead of aw. The area of the water and porous corrugated surfaces related to the total area should be considered in the calculations.
sin bs cos w Q b;w ¼ Ib sc ðhs Þaw W s Ls cos bs þ tan /
ðA1Þ
Q d;w ¼ Id sc ðbs Þaw W s Ls
ðA2Þ
sc ðhs Þ ¼ 2:642 cos hs 2:163 cos2 hs 0:32 cos3 hs þ 0:719 cos4 hs ðA3Þ
sc ðbs Þ ¼ 2:03 105 b2s 2:05 103 bs þ 0:667 bs in ½ ðA4Þ where the Ib and Id are beam and diffused radiation on horizontal surface, respectively. Values of Ib and Id are obtained using ASHREA model [27], while the incidence angle hs is obtained from Eq. (A5) as given in Ref. [30]. Other symbols are given in nomenclatures list.
cos hs ¼ sin / cos bs þ cos / sin bs cos w
ðA5Þ
The angles / and w are altitude and azimuth angles of the sun, respectively. Values of / and w are given in [27]. Similarly, the absorbed radiation on the water due to the south facing inclined reflector is given as [30]:
Q r;w ¼ Ib
L4 sc ðhr Þqr aw L1 fW s 0:5ðL2 þ L3 g L1
ðA6Þ
where hr is the incidence angle of radiation on the plane of the inclined reflector and is defined by:
cos hr ¼ sin /0 cos bs þ cos /0 sin bs cos w0
ðA7Þ
w0 ¼ 180 w
ðA8Þ
w ¼ tan1
L2 L1 þ Lr sin br
ðA9Þ
/0 ¼ tan1
Lr cos br cos w L1 þ Lr sin br
ðA10Þ
References [1] Tiwari GN. Solar energy: fundamentals, design, modeling and application. New York/New Delhi: CRS Press/Narosa Publishing House; 2003. [2] Duffie JA, Beckman WA. Solar engineering of thermal process. John Wiley & Sons, Inc; 1991. [3] Tiwari SGN, Tripathi RHN. Present status of solar distillation. Sol Energy 2003;75(5):367–73. [4] Rai SN, Dutt DK, Tiwari GN. Some experimental studies of a single basin solar still. Energy Convers Manage 1990;30(2):149–53. [5] Badran AA, Al-Hallag AA, Solman IF, Odat MZA. Solar still augmented with a flat plate collector. Desalination 2005;172:227–34. [6] Akash BA, Mohsen MS, Nayfeh W. Experimental study of the basin still under local climate conditions. Energy Convers Manage 2000;41:883–90. [7] Omera ZM, Kabel AE. The performance of different sand beds solar stills. Int J Green Energy 2014;11:240–54. [8] Nafey AS, Abdelkader M, Abdelmotalip A, Mabrouk AA. Solar still productivity enhancement. Energy Convers Manage 2001;42:1401–8. [9] Dhiman NK, Tiwari GN. Effect of water flowing over the glass cover of a multiwick solar still. Energy Convers Manage 1990;30:245–50. [10] Incropera F, Dewitt D. Fundamentals of heat transfer. John Wiley & Sons; 1981. [11] Sherwood TK, Pigford RL, Wilke CH. Mass transfer. New York: McGraw Hill; 1975. [12] Mokhtari H, Bidi M, Gholinejad M. Thermoeconomic analysis and multiobjective optimization of a solar desalination plant. J Solar Energy 2014 [Article ID 892348]. [13] Babalola TA, Boyo AO, Kesinro RO. Effect of water depth and temperature on the productivity of a double slope solar still. J Energy Nat Res 2015;4(1):1–4. [14] Gnanadason MK, Kumar PS, Sivaraman G, Daniel JES. Design and performance analysis of a modified vacuum single basin solar still. Smart Grid Renew Energy 2011;2:388–95. [15] Kabel AE, Omera ZM, Essa FA. Enhancement of modified solar still integrated with external condenser using nanofluids: an experimental approach. Energy Convers Manage 2014;78:493–8. [16] Omera ZM, Kabel AE, Younes MM. Enhancing the stepped solar still performance using internal and external reflectors. Energy Convers Manage 2014;78:876–81. [17] Suneesh PU, Tayaprakash R, Namshad T, Kumar S. Performance of corrugated wick in ‘‘V” type solar still. Smart Grid Renew Energy 2013;4:483–7. [18] Aruna RK, Janarthanan B. Simulation modeling of floating cum tilted wick type solar still. Int J Innovative Res Sci Eng Technol 2014;3(6). [19] Senger SH, Mohad AG, Khandeted YP, Modak SP, Gupta DK. Design and development of wick type solar distillation system. J Soil Sci Environ Manage 2011;2:125–33. [20] Srivastava PK, Agrawal SK, Agrawal A. Effect of absorbed material on the performance of basin type solar still with multiple floating porous absorbers. Int J Chem Tech Res 2013;5:1046–53. [21] Huang B, Chong T, Chang H, Wu P, Kao Y. Solar Distillation system based on multi-effect diffusion type still. J Sust Dev Energy, Water Environ Syst 2014;2:41–50.
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[22] Medugu DW, Ndatuwong LG. Theoretical analysis of water distillation using solar still. Int J Phys Sci 2009;4(11):705–12. [23] Fernadez J, Chargoy N. Multistage indirectly heated solar still. Sol Energy 1990;44:212–5. [24] Incropera FP, Dewitt DP, Lergman TL, Lavine A. In: Fundamental of heat and mass transfer. John Wiley & Sons, Inc; 2007. [25] Kreith F, Boehm RF. Heat and mass transfer. Boca Raton: CRC Press LLC; 1999. [26] Matrawy KK, Mahrous A-F, Youssef MS. Energy management and parameteric optimization of an integrated PV solar house. Energy Convers Manage 2015;96:377–83.
[27] Elsayed MM, Taha IS, Sabbagh JA. Design of solar thermal systems. 1st ed. Scientific publishing centre, King Abdulaziz University; 1994. [28] Malik MAS, Tiwari GN, Kumar A, Sodha MS. Solar distillation. UK: Pergamon Press; 1982. [29] Tsumura K, Yamashita M, Watanabe H, Inaba H. Bull Jpn Soc Sea Water Sci 1985;39:129. [30] Tanaka H. Increase in distillate productivity by inclining the flat plate reflector of a tilted –wick solar still in summer. Energy Sci Technol 2011;1:11–20. [31] Tanaka H, Nakatake Y. Improvement of the tilted wick solar still by using a flat plate reflector. Desalination 2007;216:139–46.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Modeling and experimental study of a corrugated wick type solar still: Comparative study with a simple basin type K.K. Matrawy a,b,⇑, A.S. Alosaimy a, A.-F. Mahrous a,c a
Mechanical Engineering Department, Taif University, Al-huwayah, P.O. Box: 888, 21974, Saudi Arabia Mechanical Engineering Department, Faculty of Engineering, Assiut University, Egypt c Mechanical Power Engineering Department, Menoufiya University, Shebin El-Kom 32511, Egypt b
a r t i c l e
i n f o
Article history: Received 20 April 2015 Accepted 1 September 2015
Keywords: Solar still Corrugated surface Porous material Inclined reflector Productivity
a b s t r a c t In the present work, the productivity of a solar still is modified by forming the evaporative surface as a corrugated shape as well as by decreasing the heat capacity with the use of a porous material. This target has been achieved by using black clothes in a corrugated shape that are immersed in water where the clothes absorbs water and get saturated by capillary effect. Along with the proposed corrugated wick type solar still, a simple basin still type was fabricated and tested to compare the enhancement accomplished by the developed solar still. Inclined reflectors were used to augment the solar radiation incident on the plane of the developed solar stills. The energy balance in the developed mathematical models takes into consideration the glass covers, the porous material, along with the portion of water exposed to the transmitted solar radiation as well as the portion of water shaded by the corrugated surface. The developed mathematical model was validated by fabricating and testing two models for the proposed and simple basin solar stills under the same conditions. Good agreement between the simulated and experimental results has been detected. It has been found that an improvement of about 34% in the productivity for the proposed wick type solar still is gained as compared to the simple basin case. Also, the best tilt angle for the inclined reflector has been found to be about 30° with respect to the vertical direction of the setup under consideration. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Supplying fresh and healthy water is still one of the major problems in different parts of the world, particularly in remote arid areas [1]. Solar still may provide a solution for those areas where it is cheap and having low maintenance cost, but it suffers from the lower productivity [2]. Accordingly, rigorous theoretical and experimental studies have been made to enhance the solar stills’ productivity. Developed work by Tiwari and Tripathi [3] has been carried out on both passive and active solar distillation systems. They recommended that only passive solar stills can be economical to provide potable water. Rai et al. [4] studied experimentally a single basin solar still coupled with a flat plate collector under various modes of operation. The modes of operation incorporated the effect of water salinity as well as the thermosyphon and forced circulations. The experimental results showed that the daily variation
⇑ Corresponding author at: Mechanical Engineering Department, Taif University, Al-huwayah, P.O. Box: 888, 21974, Saudi Arabia. Tel.: +966 5495 31562; fax: +966 (2) 724 0614. E-mail address: [email protected] (K.K. Matrawy). http://dx.doi.org/10.1016/j.enconman.2015.09.006 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
of the stills’ productivity ranged from 1.6 kg/m2 to 2.4 kg/m2 when adding a small amount of dye. Badran [5] found that the productivity was increased by about 52% in case of coupling the still with a flat plate collector. Akash et al. [6] showed that the productivity of basin solar still decreases with salinity and also decreases in a linear relationship while increasing the water depth. A maximum hourly yield with a value of 0.6 kg/m2 was attained in the study. Omara and Kabeel [7] studied the performance of different sand beds solar stills. The influences of sandy bed height, type of sand, and water height above the sandy bed level on the solar still performance have been investigated. Maximum accumulated distillate yield of 4 kg/m2 was achieved while the corresponding yield was 2 kg/m2 in the conventional type. Nafey et al. [8] enhanced the productivity of the single slope solar still using black rubber and black gravel material as storage medium. They found that an enhancement of 19–20% in the stills’ productivity can be achieved when using these materials. Other technique to enhance the stills’ productivity was developed by feeding a water film over the glass cover of a multi- wick solar still to increase the productivity and also performs self-cleaning for the glass cover [9]. Incropera and Dewitt [10] have investigated the convective heat transfer between
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Nomenclature A Cp h hfg I m Md L T W Q Qs
area (m2) specific heat (J/kg K) heat transfer coefficient (W/m2 K), height (m) latent heat of condensation (J/kg C) solar radiation (W/m2) mass (kg) mass of distillated (kg) length (m) temperature (°C) width (m) heat (W) absorbed solar radiation (W/m2)
Greek letters a absorptivity b slope angle (°) h incidence angle (°) d Boltzman constant (W/m4 K) s transmissivity
the cooling film and glass cover while Sherwood et al. [11] have studied the heat transfer due to water evaporation associated with the cooling film. A solar desalination plant consists of solar parabolic collectors, steam generators, and MED unit was simulated and optimized using multi-objective genetic algorithm by Mokhtari et al. [12]. Recently, various technologies have been developed to meet the increasing demand of potable water such as double slope solar still [13], providing low pressure inside the still basin [14], using nanofluids and integrating the still basin with external condenser [15], enhancing the stepped solar still using internal and external reflectors [16], using a flat and ripped absorber in ‘‘V” wick type solar still [17], floating cum tilted wick solar still [18], using a corrugated galvanized iron steel as an absorber in between the wick strips [19] and multiple porous blackened jute absorbers floated on the water basin [20]. Moreover, Huang et al. [21] studied the multi effect diffusion type solar still (MEDS) coupled with a vacuum tube solar collector. They showed that the 10-effect MEDS produces pure water ranged from 13.7 to 19.7 kg/day/m2 when the incident solar radiation ranges from 600 to 800 W/m2, respectively. For the 20-effect solar still, the productivity increases by 32% compared to the10-effect one. Thus, it may be concluded that the most recommended strategies to enhance the productivity are: 1. Decreasing the water depth in the basin. 2. Using a forced circulation to increase the rate of evaporation processes. 3. Feeding a water film over the cover to decrease the cover temperature. 4. Using a sand bed or black rubber as a storage medium. 5. Integrating an external condenser and using nanofluids. 6. Providing low pressure inside the still to decrease the evaporating temperature. 7. Floating blackened jute on the water basin. 8. Applying energy recovery for the condensate vapor (i.e. multi effect solar still). Accordingly, the main objective of the present study was to enhance the evaporation processes in a simple basin solar still. This was satisfied by increasing the evaporative area using corrugated shape and decreasing the heat capacity of the evaporating water
q
reflectivity emissivity
e
Subscripts 1 corrugated solar still 2 simple basin solar still a air b bottom, beam c cover, convention d diffused e evaporative G global p porous pt porous top pb porous bottom r radiative, reflector s still w water, wind
with the use of a porous material. Removing the salt and some impurities from the porous material was taken place through a periodic cleaning for the porous material. Description of the developed solar stills, mathematical modeling and experimental set up were presented in Sections 2–4, following by the results and discussions for the simulated and experimental results. 2. Description of the proposed corrugated wick type and simple basin type solar stills A schematic diagram for the proposed corrugated wick type solar still with an inclined flat reflector is shown in Fig. 1. From Fig. 1, it can be seen that the main components of the proposed solar still are: 1. Glass cover with a thickness of 6 mm to transmit the incident solar radiation and the reflected radiation from the reflector to both the porous material and the water in the basin. Also, the generated vapor is condensed along the lower surface of the cover and collected at the lower end. 2. Porous material that is black clothes with a thickness of 2 mm made in a corrugated shape. The porous material is partially immersed and wetted by the water in the basin by capillary effect to increase the rate of evaporation.
Inclined Relector
Glass Cover
Black clothes
Water
Insulation
Fig. 1. Corrugated wick type solar still with an inclined reflector.
Outer Frame
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3. Stills’ basin contains the warm water including an amount of water that directly absorbs the transmitted radiation besides a shaded amount that is heated by convective and radiative heat transfers from the back surface of the corrugated black clothes. 4. Outer frame that is a wooden box with dimensions of 1.35 0.9 m to protect the system from the outer ambient conditions. It includes an insulation material (k = 0.024 W/m K) with a thickness of 50 mm to decrease the heat losses from the hot water in the basin. 5. Flat inclined stainless steel reflector with dimensions of 0.5 m and 0.8 m for the height and width, respectively. The slope of the reflector with the vertical direction can be changed using a hinge as shown in Fig. 1. On the other hand, the simple basin solar still shown in Fig. 2 consists of the typical components with the same descriptions as in Fig. 1, except the absence of corrugated black clothes in the basin. The dimensions and specification of the component are the same as those mentioned in Fig. 1.
3. Mathematical modeling
Qs,c1
Tc1 Qs,w1 Qp1—c1 Qw1-c1
hw1
1. All the properties are independent on the temperature. 2. No longitudinal temperature distribution for the glass cover, water in the basin and the corrugated surface. 3. No temperature gradient through the thickness of glass cover, corrugated shape and water depth. 4. One dimensional heat flow through the thickness of: glass cover, porous material, water in the basin and back insulation. 5. The heat capacity of the glass cover and insulation material of the still, is negligible. 6. Performance is steady state.
Qb1
Fig. 3. Periodic segment of the corrugated wick type solar still.
Thus, according to a periodic segment of the corrugated wick type solar still shown in Fig. 3, the energy balances for the glass cover, porous material and water basin, respectively are:
ðmC p Þc1
dT c1 ¼ Q s;c1 þ Q p1c1 þ Q w1c1 Q c1a dt
ð1Þ
ðmC p Þp1
dT p1 ¼ Q s;p1 Q p1c1 Q p1w1 dt
ð2Þ
dT w1 ¼ Q s;w1 þ Q p1w1 Q w1c1 Q b1 dt
ð3Þ
mC p
w1
where Qs,c1, Qs,p1, and Qs,w1 are the absorbed solar radiations through cover, porous material, and water, respectively. Their values are given in detail in Appendix A while the other quantities are given as:
Q p1c1 ¼ Ap1 ðhr;p1c1 þ hc;p1c1 þ he;p1c1 ÞðT p1 T c1 Þ
ð4Þ
Q w1c1 ¼ Aw1 ðhr;w1c1 þ hc;w1c1 þ he;w1c1 ÞðT w1 T c1 Þ
ð5Þ
Q c1a ¼ Ac1 ðhr;c1a þ hw ÞðT c1 T a Þ
ð6Þ
Q p1w1 ¼ Ap1 ðhr;pt1w1 þ hr;pb1w1 þ hc;pt1w1 þ hc;pb1w1 ÞðT p1 T w1 Þ ð7Þ Q b1 ¼ ðUAÞ1 ðT w1 T a Þ
ð8Þ
where (UA)1 is the loss coefficient- area product. It is a function of the area of bottom and side walls of the still as well as the thickness and type of insulation material.The amount of distillated water Md1 in the corrugated solar still is:
Md1 ¼ Inclined Relector
Tp1 Qp1-w1
Tw1
3.1. Modeling of corrugated wick type solar still The developed simulation model for the proposed corrugated wick type solar still is based on energy balance for the main components shown in Fig. 1. The simulated components include the condensing surface (glass cover) and the evaporative one. The evaporative surface in this type consists of a part of the porous material above the water level beside the part of water surface exposed to the transmitted radiation (there is anther part of water surface shaded by the corrugated surface). The developed model is based on the following assumptions:
Qs,p1
Qc1-a
Ap1 he;p1c1 ðT p1 T c1 Þ þ Aw1 he;w1c1 ðT w1 T c1 Þ hfg
ð9Þ
3.2. Modeling of the simple basin solar still Glass Cover
Water
Insulation
Fig. 2. Simple basin solar still with an inclined reflector.
Outer Frame
Modeling of the simple basin solar still is derived based on the same assumptions given in Section 3.1, but without the porous material. The energy balances for the components are based on a periodic segment as shown in Fig. 4.
ðmC p Þc2
dT c2 ¼ Q s;c2 þ Q w2c2 Q c2a dt
ð10Þ
ðmC p Þw2
dT w2 ¼ Q s;w2 Q w2c2 Q b2 dt
ð11Þ
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hr;ij ¼
rðT i þ T j ÞðT 2i þ T 2j Þ 1ei A i ei
þ Ai F1ij þ
1ej A j ej
ð20Þ
where the view factor Fi j is determined as an enclosure problem depending on the configuration of surface i related to j as explained in Ref. [25]. This includes the corrugated and water surfaces compared to the cover and also the upper and lower corrugated surfaces compared to the water one. 4. Experimental test rig
Fig. 4. Periodic segment of the simple basin solar still.
Q w2c2 ¼ Aw2 ðhr;w2c2 þ hc;w2c2 þ he;w2c2 ÞðT w2 T c2 Þ
ð12Þ
Q c2a ¼ Ac2 ðhr;c2a þ hw ÞðT c2 T a Þ
ð13Þ
Q b2 ¼ ðUAÞ2 ðT w2 T a Þ
ð14Þ
The amount of distillated water Md2 in the simple basin still is:
M d2 ¼
Aw2 he;w2c2 ðT w2 T c2 Þ hfg
ð15Þ
Based on energy balances for the developed models, the temperatures variation and distillated amounts of water for the corrugated and simple basin solar stills can be calculated using Euler integration method cited in Ref. [2]. 3.3. Heat transfer coefficients The developed simulation model includes different heat transfer coefficients, namely convective, evaporative and radiative coefficients between different components of the corrugated wick type and simple basin solar stills. The convective and evaporative heat transfer coefficients between water surface and glass cover are given, as cited in [22] as:
hc;wc ¼ 0:884 T w T c þ
ðPw Pc ÞT w
1=3
268:9 103 Pw
he;wc ¼ 16:237 103 hc;wc
Pw Pc Tw Tc
ð16Þ
ð17Þ
where Pw and Pc are the saturation partial pressures at Tw and Tc, respectively. Values of Pw and Pc (for the range of temperature 10–90 °C) can be obtained from expression developed by Fernadez and Chargoy [23] as:
5144 PðTÞ ¼ exp 25:317 T þ 273
ð18Þ
Eqs. (16) and (17) can be used to calculate convective and evaporative heat transfer coefficients in Eqs. (5) and (12) based on Tw and Tc. Also, for the porous material, the value of Tw is replaced by Tp to calculate the same coefficients. For the convection heat transfer coefficient between the sides of the corrugated surface and water one, they have been treated as inclined and horizontal plates. Churchill and Chu in [24] recommended the following equation:
NuL ¼ 0:68 þ
0:67Ra0:25 L ½1 þ ð0:492=PrÞ9=16
4=9
RaL 6 109
ð19Þ
Radiative heat transfer coefficient hr,i j between two surfaces i and j is given as:
An experimental set up was prepared for both the corrugated wick type and the simple basin solar stills. The test rig is built on the roof of Solar Energy Laboratory in Taif University (latitude angle 22.23°) at Saudi Arabia with a photograph shown in Fig. 5. The dimensions and specifications are as mentioned in Section 3.1. The two examined solar stills are set beside each other and both face south to maximize the incident solar radiation. The setup is instrumented to measure the temperatures of glass cover, water in the basin, porous surface, and ambient air. The thermocouple sensors (type-K) were calibrated with an accuracy of ±0.5 °C and connected to thermocouple data logger which is programmed and connected to PC to record the temperatures with a time interval of 15 min. The global solar radiation on a horizontal surface is measured near the tested solar stills using a solar pyrometer (Type SR 11) with an accuracy of ±1 W. The condensate water from each still is collected and measured using a graduated vessel with a capacity of 500 ml and an accuracy of ±5 ml. Condensate water was recorded manually with a time interval of 30 min. The test procedure was started by putting the same amount of salty water in the corrugated and simple basin solar stills. The two stills were shaded for one day to ensure a complete saturation of the porous material by water observing a complete wetting for the material in the morning of the second day and to reach steady state conditions. Recording of the different temperatures, solar radiation, wind speed, condensate water were started from 9 a.m up to 6 p.m. in the month of May with the time interval mentioned above. The experiments were repeated for two days in clear sky conditions with slight variation in the recorded values of ±0.5%. The accumulated amount of distillated water was calculated by summing the hourly values through the day. 5. Results and discussions The results and discussions for the calculated and measured data for the proposed wick type and simple basin solar stills are presented in this section. The calculations have been carried out using the following specifications:
Ls ¼ 1:25 m; Lr ¼ 0:5 m; W r ¼ W s ¼ 0:8 m; hs ¼ 22:23 ; hr ¼ 30 ;
ac ¼ 0:07; aw ¼ 0:9 and ap ¼ 0:95:
Fig. 6 shows the beam, diffused and global solar radiations incident on a horizontal surface in the month of May and located in the site under consideration. The presented values in the figure have been calculated according to the ASHREA model cited in Refs. [26,27]. The calculations have been carried out for beam, diffused and global radiations, while the measurements have been recorded only for the global radiation. It is clear from the figure that there is an agreement between the calculated and measured results with a maximum deviation of about 2–3%. Also, there is a slight variation in the diffused solar radiation along the day hours. The maximum percentages of diffused radiation are about 13% and 10.8% of the beam and global radiations, respectively. The best inclination angle for the flat reflector with respect to the vertical direction and different reflector angles is presented
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Relector Simple type Solar Still
Computer set
Corrugated type solar still
300 Calculated IG
1000
Measured IG Calculated Id
250
800 200 600 150 400 100
200
1000
250
800
200
600
150
Water or Porous Absorbed Radiation, W/m2
Calculated Ib
Diffused Radiation, W/m2
Beam or Global Radiation, W/m2
1200
400
100 Calculated Qs,w
200
50
Calculated Qs,p Calculated Qs,c
0
0 8
9
10
11
12
13
14
15
16
17
18
Cover Absorbed Radiation, W/m2
Fig. 5. Experimental setup.
Time, hr 0 8
10
12
14
16
50 18
Fig. 8. Absorbed solar radiations for the developed solar stills.
Time, hr Fig. 6. Calculated and measured solar radiations versus day hours.
85 120
Temperature, o C
Absorbed Radiation by Reflector, W/m2
95
100 80 60
Reflector angle,deg
Calculated Tw1
55
Measured Tw1 Calculated Tp1
45
Measured Tp1 Calculated Tw2 Measured Tw2
20
25 8
30
20
65
35
10
40
75
10
12
14
16
18
Time, hr
40
0 9
10
11
12
Time, hr
Fig. 9. Calculated and measured temperatures for the developed solar stills.
Fig. 7. Absorbed radiation by reflector with different reflector angles.
in Fig. 7. It is clear from that figure that the maximum reflected radiation for the whole day hours occurs at an angle of about 30o. This angle may be considered as the best angle for the reflector in the present case. It is important to note that the derived angle is considered only in May with a reflector length of 0.5 m. This value may differ in other cases. The derived best inclination angle for the flat reflector has been assigned as a constant and used as input data to the developed simulation model and also through the measurements of the designed test rig. The amount of absorbed radiations on the porous material, water in the basin, and glass cover are shown in Fig. 8. The presented values are calculated from Eqs. (A15) and (A16) in Appendix A based on the absorptivity for the components beside the incident radiations on planes of the solar still as well as the flat reflector. The same results are presented for the conventional type solar still shown in Fig. 2, but without including the absorbed radiation
through the porous material. From Fig. 8, it can be seen that the maximum absorbed radiation occurs with the porous material while the lower value occurs with the glass cover. The absorbed radiation on the water in the basin lied between the porous material and the glass cover. The trend of the absorbed radiation is similar to the incident solar radiations as shown in Fig. 6. Maximum absorbed radiation has a value of 900 W/m2 in the porous material while it is about 85 W/m2 for the glass cover. The lower absorbed radiation through the glass cover is mainly attributed to the lower absorptivity of glass cover used in calculations. The temperatures of the porous material and water in the basin of the corrugated wick type solar still are shown in Fig. 9. Temperature of water in the simple basin solar still is also presented in the same figure to simplify the comparison between the developed solar stills. It can be seen from the figure that the temperatures decrease from a maximum value of 87 °C in the porous material to a minimum one of 82.5 °C for the water of the simple type solar
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still. Temperature of the water basin of the corrugated solar still lies between the above values as seen in the same figure. It is important to understand that the water in the basin of corrugated solar still is heated by direct transmitted radiation beside the heat transfer by convection and radiation from the sides of porous surface. Thus, the behaviors of the presented temperatures are mainly attributed to the exposed area of the transmitted solar radiation and also to the heat capacity of the components. The measured temperatures are presented also in the same figure, which shows a quite agreement between the calculated and measured temperatures. The calculated and measured temperatures of the glass covers for the corrugated and simple solar stills are shown in Fig. 10. Ambient air temperature is also presented in this figure. It can be seen that the temperature of the glass cover in case of the corrugated solar still is higher than that of the simple type. This may be attributed to the higher temperature of the evaporating surface (porous material in corrugated solar still) than that of the simple type (water in the basin). Maximum values take place at 3 p.m. with values of 78 °C and 74 °C for the corrugated and simple type solar stills, respectively. The corresponding ambient temperature is 35 °C under these conditions. Moreover, it is observed that the measured temperatures are agreed well with the calculated values with a difference of about 2–3 °C. Based on the previous illustrations of the calculated and measured temperatures, the calculated and measured productivities are displayed in Fig. 11. Due to the higher temperature of the corrugated wick type surface, its productivity is higher than that for the conventional type at the same operating conditions. Maximum productivities of 0.93 kg/h and of 0.76 kg/h were
achieved for the corrugated and simple solar stills, respectively, as seen in Fig. 11. It is important to explain and discuss the contribution of both corrugate and water surfaces through the evaporation processes in corrugated solar still. Fig. 12 presents the productivity due to corrugated surface and water surface with respect to the total productivity produced from the corrugated solar still. From the shown figure, it is clear that the productivity of the corrugated surface is the dominant source of evaporation compared to the contribution of the water surface. At 2 p.m., the productivity of the corrugated surface is about 0.7 kg/h, while the corresponding value is 0.23 kg/h for the water surface in the basin. The higher contribution of the corrugated porous material is mainly attributed to the higher temperature compared to that of the water and also to the higher area of corrugated surface exposed to the transmitted radiation. Based on the geometry and configuration of the corrugated solar still, the surface area of corrugated shape was about 70% of evaporation area while the remaining percentage was for the water surface. This explains the higher contribution of the corrugated surface in evaporation process. The accumulated amounts of distillated water for wick and simple basin solar stills versus day hours are shown in Fig. 13. It is clear from this figure that the accumulated amount of distillated water from corrugated wick solar still is higher than that for the simple basin. The daily productivity of corrugated solar still at 6 p.m was about 5.9 kg/m2 while the corresponding for the simple basin is nearly 4.4 kg/m2. This means that the productivity of corrugated solar still with inclined flat reflector is increased by about 34% more than that for the simple basin with the same reflector.
90
70
Calculated Ta
1
Measured Ta
0.9
Calculated Tc1
0.8
Productivity, kg/hr
Temperature, o C
80
Measured Tc1 Calculated Tc2
60
Measured Tc2
50 40 30
0.7 0.6
Md1 (total)
0.5
Md (from porous)
0.4
Md (from water)
0.3 0.2 0.1
20
0 9
10 8
9
10
11
12
13
14
15
16
17
18
10
11
12
19
13
14
15
16
17
18
Time, hr
Time, hr Fig. 10. Calculated and measured temperatures for the glass covers of corrugated and simple basin solar stills.
Fig. 12. Calculated productivity for corrugated and water, surfaces of the corrugated wick type solar still.
6
1
Accumulated Md, kg
Productivity, kg/hr
0.9 0.8 0.7 0.6 0.5 Calculated Md1
0.4
Measured Md1
0.3
Calculated Md2
0.2
Measured Md2
0.1
5 4 3 2 Measured Md1
1
Measured Md2
0
0 9
10
11
12
13
14
15
16
17
18
19
Time, hr Fig. 11. Calculated and measured productivity for corrugated and simple basin solar stills.
9
10
11
12
13
14
15
16
17
18
Time, hr Fig. 13. Measured accumulated amounts of distillated water for corrugated and simple basin solar stills.
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6. Conclusions
The value of sc(hr) is calculated according to Eq. (A3) by replacing hs by hr. Also, the values of L1, L2, L3, and L4, are given as:
The present work aimed to enhance the productivity of the conventional type solar still by introducing a corrugated wick type solar still. The performance of the developed solar still is compared with that for the simple type. Theoretical analysis for the absorbed solar radiation on the glass cover, corrugated porous material and water was developed. The analyses are based on the calculated beam, diffused solar radiation using ASHREA model. The results obtained by mathematical model were compared to the experimental results with a quite agreement between the calculated and experimental results. The diffused radiation was about 13% and about 10.8% of the beam and global solar radiation, respectively. The corrugated porous surface in the corrugated solar still was shown to contribute by about 75% of the total productivity. At the same time, the daily productivity of corrugated solar still was increased by about 34% more than that for the simple basin type.
L1 ¼ Lr fcos br tanðx3 þ 2br Þ sin br g
ðA11Þ
L2 ¼ L1 tan x1
ðA12Þ
L3 ¼ L1 tan x2
ðA13Þ
cos w L4 ¼ Lr sin br þ cos br tan /
ðA14Þ
where the angles x1, x2, x3 are given in detail in Ref. [30] as a function of other lengths L5 to L12 beside the given lengths. Finally, the total absorbed solar radiations on the water basin and cover are given in [31] as:
Q s;w ¼ Q b;w þ Q r;w þ Q d;w
ac Q b;w þ Q r;w Q d;w þ aw sc ðhs Þ sc ðbs Þ
ðA15Þ ðA16Þ
Appendix A
Q s;c ¼
The use of an external reflector can be useful to increase the productivity of the solar still as reported in both [28,29]. The developed analyses by Tanaka [30] and by Tanaka and Nakatake [31] for the inclined reflector are used in the present study to augment the incident solar radiation on the planes of corrugated and simple basin solar stills. As explained in details in both [30,31], the absorbed beam and diffused solar radiation in the water basin of a south facing solar still can be expressed as:
Similarly, total absorbed radiation on the corrugated surface due to the reflector is obtained using Eq. (A15) through substitution of ap instead of aw. The area of the water and porous corrugated surfaces related to the total area should be considered in the calculations.
sin bs cos w Q b;w ¼ Ib sc ðhs Þaw W s Ls cos bs þ tan /
ðA1Þ
Q d;w ¼ Id sc ðbs Þaw W s Ls
ðA2Þ
sc ðhs Þ ¼ 2:642 cos hs 2:163 cos2 hs 0:32 cos3 hs þ 0:719 cos4 hs ðA3Þ
sc ðbs Þ ¼ 2:03 105 b2s 2:05 103 bs þ 0:667 bs in ½ ðA4Þ where the Ib and Id are beam and diffused radiation on horizontal surface, respectively. Values of Ib and Id are obtained using ASHREA model [27], while the incidence angle hs is obtained from Eq. (A5) as given in Ref. [30]. Other symbols are given in nomenclatures list.
cos hs ¼ sin / cos bs þ cos / sin bs cos w
ðA5Þ
The angles / and w are altitude and azimuth angles of the sun, respectively. Values of / and w are given in [27]. Similarly, the absorbed radiation on the water due to the south facing inclined reflector is given as [30]:
Q r;w ¼ Ib
L4 sc ðhr Þqr aw L1 fW s 0:5ðL2 þ L3 g L1
ðA6Þ
where hr is the incidence angle of radiation on the plane of the inclined reflector and is defined by:
cos hr ¼ sin /0 cos bs þ cos /0 sin bs cos w0
ðA7Þ
w0 ¼ 180 w
ðA8Þ
w ¼ tan1
L2 L1 þ Lr sin br
ðA9Þ
/0 ¼ tan1
Lr cos br cos w L1 þ Lr sin br
ðA10Þ
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