Analysis of an inclined solar still with baffles for improving the yield of fresh water

Accepted Manuscript Title: Analysis of an inclined solar still with baffles for improving the yield of fresh water Author: Nagarajan P.K. S.A. El-Agouz Harris Samuel D.G. Edwin M. Madhu B. Magesh babu D. Ravishankar Sathyamurthy Bharathwaaj R. PII: DOI: Reference:

S0957-5820(16)30288-9 http://dx.doi.org/doi:10.1016/j.psep.2016.11.018 PSEP 921

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

29-8-2015 11-11-2016 17-11-2016

Please cite this article as: P.K., Nagarajan, El-Agouz, S.A., D.G., Harris Samuel, M., Edwin, B., Madhu, D., Magesh babu, Sathyamurthy, Ravishankar, R., Bharathwaaj, Analysis of an inclined solar still with baffles for improving the yield of fresh water.Process Safety and Environment Protection http://dx.doi.org/10.1016/j.psep.2016.11.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Analysis of an inclined solar still with baffles for improving the yield of fresh water

Nagarajan.P.K.a, S.A. El-Agouzb, D.G.Harris Samuelc, Edwin. M. d, Madhu.B.a, Magesh babu. Da, Ravishankar Sathyamurthya,*, Bharathwaaj. Ra

a

Department of Mechanical Engineering, S.A. Engineering College, Chennai, India.

b

Department of Mechanical and Power Engineering, Tanta University, Egypt.

c

Department of Mechanical Engineering, Hindustan Institute of Technology and Science, Chennai, India. d

Department of Mechanical Engineering, University College of Engineering, Nagercoil, Tamil Nadu, India.

*Corresponding author, Ravishankar Sathyamurthy, Professor, Department of Mechanical Engineering, S.A. Engineering College, Chennai-600077, Tamil Nadu, India. [email protected], [email protected]

Research Highlights 

Experiments are conducted for January, February, March and April 2014.

 Theoretical results are validated by experimental data. 

Total yield from solar still with and without baffles are found as 5.4 and 3.4 kg/m 2 respectively.



With an average solar intensity of 8.12 kWh/m2 and wind velocity of 1.88 m/s yield is maximum.

Abstract In the present study, the performance of an inclined solar still with and without baffles are studied. From the study, it is identified that the yield of fresh water and the evaporation of water inside the solar still are completely dependent on the retention time of water with solar radiance. Furthermore, a theoretical and experimental analysis is carried out in order to evaluate the performance of the solar still using RHN (Ravi- Harris- Nagarajan) Model. The yields are 5.4 kg/m2day ± 3.6% for solar still with baffles and 3.4 kg/m2day ± 3% for solar still without baffles during April month with a total daily intensity of 8.125 kWh/m2. The yield of the solar still is increased by 1.68 times the solar still without baffles. Results prove that the improvement of yield from inclined solar stills depends on the contact time of flowing water with solar radiance and temperature of the absorber. Keywords: -Inclined solar still; baffles; flow rate; improvement; yield

1. Introduction Water is one of the prime sources for the survival of human beings. As there is a larger shortage of drinking water, people living in the urban and rural community are suffering to get fresh drinking water. People living in the remote villages and urban cities are largely dependent on the use of ground water sources, and industries use these sources for their development. The use of Multi-Stage Flash (MSF), Membrane Distillation (MD) is largely used for mass production of fresh water. Even compact RO systems are now used for

domestic purposes and these techniques require electricity for functioning and require more maintenance. Solar energy is widely used for power generation and some thermal applications such as water and living space heating, cooling and drying techniques. Solar desalination technique was invented during the Egyptian period as they were using distillation columns for the production of fresh water. During the 19th-century, basin type solar stills are designed and fabricated to produce fresh water. Due to their low yield, during the 20th and 21st century several studies were carried out to improve the yield of basin type solar stills. (Murugavel et al. 2013, Nagarajan et al. 2014, 2014a, Sathyamurthy et al. 2014, 2014a, b, 2015, 2015a-c and Arunkumar et al. 2015). Methods include pulsating heat pipe, double basin, parabolic trough collectors, waste heat recovery, parabolic concentrator, bubbling column type, solar water heater and the integration of PV/T collectors into basin type solar stills to improve the yield of fresh water. From an economic point of view, solar stills integrated with the techniques as mentioned above are not feasible. Sathyamurthy et al. (2015) investigated a inclined solar still with baffles on a semi-circular trough absorber at different mass flow rates. The results concluded that the use of (Poly Vinyl Chloride) PVC as absorber material is economically feasible and reduced the corrosion and maintenance of solar still. Also, it is identified that the semi-circular trough absorber solar still can be integrated as a water heating unit (Sathyamurthy et al. 2015b) to a conventional solar still for improving the yield of fresh water. From his study, it is identified that the air gap distance between the absorber and glass is not uniform. The heat and mass transfer correlations between two parallel plates have been previously determined. J.T. Mahdhi et al. (2011) theoretically and experimentally investigated an inclined solar still with jute with a charcoal coating as wick material. Results provided valuable information regarding the effect of mass flow inside the solar still and absorption tendency of water with jute cloth for evaporation. With a minimum mass flow of 1kg/hr inside the basin the yield was higher. Similarly, Anburaj et al. (2013) and Samuel et al. (2015) investigated different new wick materials and wire meshes on inclined flat and rectangular stepped solar still. Wick materials include polystyrene sponge, water coral fleece and wood pulp. El-Agouz et al. (2015) theoretically studied inclined solar still desalination system with a continuous water flow. They were studied for inclined solar still desalination system with and without water close loop. The results showed that the inclined solar still with a makeup water was superior in productivity (57.2% improvement) compared with a conventional basin-type solar still. The water film thickness, and velocity, as well as wind velocity, was played important roles in improving the still productivity and efficiency.

This work aims to improve the fresh water yield from an inclined solar still with baffles. The total area of the solar still fabricated is about 0.42m2. Use of baffles in the solar still improves the contact time of water with solar radiance. Also, the flow rate of water is reduced by a factor of 4, as water contacts the entire surface of the basin. For the theoretical analysis, the effect of longitudinal and transversal distance on mass flow is taken into consideration for these reduced flow rates, as described by Sathyamurthy et al. 2015c. 2. Theoretical approach Fig. 1 shows the schematic energy balance in the solar still. The different modes of heat transfer occurring inside the solar still are convection, evaporation and radiation. Evaporation of saline water inside the solar still depends on the energy received by the water flowing inside the solar still and inlet temperature of water. The previous numerical study Sathyamurthy et al. 2015c considered that the conductive heat transfer is negligible and in the present study it is not considered. The various modes of heat transfer inside and outside the still are;

2.1.

i.

Convective heat transfer from cover to ambient Qc, g-a,

ii.

Radiative heat transfer from cover to ambient Qr,g-a,

iii.

Convective heat transfer from water to cover Qc,w-g,

iv.

Evaporative heat transfer from water to cover Qe,w-g,

v.

Radiative heat transfer from water to cover Qr,w-g and,

vi.

Convective heat loss from water to basin Qc,w-b

Solar still without baffles

Energy balance of outlet water temperatureis given as,

I g Ag 1   w   m fwC pw Tout  Tin   Ub Ab (Tb  Ta )

(1)

And the heat lost from the basin to the surroundings was neglected as it is assumed in section 2. From equation (1) the outlet water temperature of solar still without baffles is determined as,  I g Ag (1   w )  U b Ab (Tb  Ta )  Tout     Tin m f C pw  

(2)

And the average water temperature in the solar still can be expressed as,

 I g Ag (1   w )  U b Ab (Tb  Ta )  TW  Tin    2  m f C pw  

(3)

2.2.

Solar still with baffles

Energy balance of outlet water temperature is given as,

nxy * I g 1   w   m fwC pw Tout  Tin  xdy  dxdy   nxyQbaffle

(4)

From equation (4) the outlet water temperature is given as (Sathyamurthy et al. 2015c),  I g (1   w )  Qbaffle  Tout  nxy    Tin  m f C pw  xdy  dxdy  

(5)

And the average water temperature in the solar still can be expressed as,

 nxy  I g 1   w   Qbaffle   TW  Tin     2m fwC pw ( xdy  dxdy ) 

(6)

Heat energy absorbed by the glass + Heat liberated by vapor inside the still = Heat lost due to convection and radiation by the outside glass surface. I g Ag  h2 (Tw  Tg )  h3 (Tg  Ta )

(7)

The above equation can be rewritten as (El-Sebaii et al, 2009),  I g Ag  h3Ta  h2Tw  Tg    h2  h3  

(8)

Where, h2  he, w g  hc,w g  hr ,w g

h3  hc, g a  hr , g a

(9) (10)

The convective heat transfer rate between water and glass (Qc,w-g) can be expressed as, Qc,w g  hc,w g Ab (Tw  Tg )

(11)

The evaporated water inside the basin will be in the form of water vapor that later rejects its heat to the cover by forming a film condensation on the surface. Mass flow rate is directly proportional to the heat transfer coefficient and temperature difference. Evaporative heat transfer between water and glass is given as, m f C pw (Two  Twi )  hc Ab (Tw  Tg )

(12)

The evaporative heat transfer coefficient (he,w-g) can be estimated using the following heat and mass transfer analogy with heat flux as follows Sayigh and El-Salam (1977),

Qe,w g  0.016hc ,w g Ab ( Pw  Pg )

(13)

and Qe, w g  he, w g Ab (Tw  Tg )

(14)

Substituting Qe,w-g in the above equation which gives Tiwari et al. (1989),  Pw  Pg  he, w g  0.016hc ,w g    Tw  Tg 

(15)

The amount of water condensed on the inner surface of the glass can be written as Zurigat and Mousa (2004), me, w g 

Qe, w g  3600

(16)

h fg

The assumption made for radiative heat transfer is that it is considered as heat transfer between two parallel infinite plates. The radiative heat transfer between water and cover glass (hr,w-g) per unit area is expressed as, Charters WW (1977)

Qr ,w g   effective (Tw4  Tg 4 )

(17)

Where,

 effective

1  1     1   g  w 

1

Qr ,w g  hr ,w g Ag (Tw  Tg )

(18) (19)

Substituting (25) in (23) gives,

hr ,w g   effective (Tw2  Tg 2 )(Tw  Tg )

(20)

2.3.Convective heat transfers between glass and the ambient Heat transfer from the surroundings is majorly in the form of convection and radiation. It is mainly due to the heat is taken away by the wind from the glass surface, side walls and bottom of the solar still. The heat loss from the side walls and the bottom can be reduced by insulation layers. The heat in the form of convection over the glass surface reduces the glass temperature, which increases the driving force. The convective heat transfer from the still is a function of wind flowing around the still and convective heat transfer coefficient, which is expressed in the form as, For wind velocity <= 5 m/s Madhlopa and Johnstone (2009) hc, g a  2.8  3u

(21)

For wind velocity > 5m/s (Tanaka, 2010) hc, g a  5.7  2.8u

(22)

The convective heat transfer is expressed as, Qc, g a  hc , g a (Tg  Ta )

(23)

2.4.Radiative heat transfers between glass and ambient air The radiative heat transfer outside the solar still is a function of the sky (Tsky) and glass temperature (Tg). The sky temperature is given as a function of ambient temperature, which is expressed as Duffie and William (1977),

Tsky  0.05525Ta1.5

(24)

Radiative heat transfer rate is given by Shukla and Sorayan (2005),

Qr , g a   g (Tg 4  Tsky 4 ) Qr , g a  hr , g a (Tg  Ta )

(25) (26)

Substituting (20) in (19), which gives Shukla and Sorayan (2005),  Tg 4  Tsky 4  hr , w g   g    Tg  Ta 

(27)

3. Experimental methodology Fig. 2 shows the schematic diagram of inclined solar still with baffles. It consists of a square tray of 0.65 x 0.65 m2 area and the height of the solar still is 0.15m. The saline water is fed into the basin using a flow control valve. An inclined glass is mounted on the solar still so that the evaporated water is being condensed inside the glass cover. For easy removal of glass from the still, glass support is fixed so that it can be removed for maintenance purposes. Baffle plates are held in the solar still for increased time of contact for water with solar radiation for increasing the temperature of water for enhanced evaporation. At the end of glass cover, a distillate collector is placed so that fresh drinking water can be collected from it. The consecutive distance between each baffle is maintained at 0.1m. The hot water from the solar still is taken from the other end and stored in a separate tank for other purposes. The baffle plates not only deflect the path of water also store some of the solar energy in it.

Insulation materials are filled up on the side walls and the bottom of the solar still to avoid heat losses. Experiments are conducted on the inclined solar still with baffles at two different mass flow rates. To increase the contact time between water and solar radiance, baffles are placed inside the solar still. The increase in temperature of water depends on the flow rate and transfer of energy absorbed by the absorber to water. In an ordinary solar still the entire absorber temperature is not completely used by flowing water. The fresh saline water is entered into the parallel plate and hence it completely depends on solar intensity and not on the absorber temperature. Table.1 shows the physical properties used for mathematical calculations. Experimental results of temperature of water, yield are validated against the equation (6) and Equation (16) respectively. For different months (January-April 2014) experiments are carried out in a hot and humid climate of Chennai, India and experiments are started from 7 AM to 6 PM. Different elements such as inlet water, outlet water, glass, absorber temperatures are measured using a PT-100 (RTD sensor) with an error of ±0.1%. Solar intensity and wind velocity are measured using TES1333R solar power meter and AM4836C wind anemometer with data logging facility. The solar still is inclined at an angle of 30o while the glass is facing south during the months of January-April 2014.

4. Results and Discussion The average hourly variation of solar intensity for months of January - April 2014 is shown in Fig. 3. It is observed that the intensity during the April month is higher than the other months. And the experiments are conducted from winter till summer. The hourly variation of wind velocity and ambient temperature is plotted in Fig. 4 (a) and (b). During the morning hours it is observed the wind velocity is lower and during the evening hours the wind velocity is increasing. Fig. 5 and Fig. 6 show the theoretical and experimental hourly variation of the basin, water and glass temperature of inclined solar still without and with baffles. From both the cases, the theoretical and experimental values are almost equal with a deviation of 3.56%. While analysing the yield from a solar still the maximum temperature of the water is during the month of April 2014 as the solar intensity is higher. The maximum temperature of water for solar still with and without baffles is found to be 72 and 68°C respectively during the month of April 2014. For an average solar intensity (I=500 W/m2) it was found to be 68 and 58°C from solar still with and without baffles respectively. The complete utilization of

absorber heat and solar intensity tends to increase the water temperature. In an inclined solar still the absorber temperature is completely utilized but the flowing of water has to have a longer retention time to reach higher temperature set point. The theoretical and experimental results show that the longitudinal and transversal direction of water flow increases the water temperature. Fig. 7 and 8 shows the variation of predicted and experimental variation between yields, temperature of basin, water and glass temperature of inclined solar still with and without baffles. It is clearly observed that the deviation of between experimental and theoretical is in the range of ± 2%. Similarly, the error including uncertainty the deviation is observed as ± 3.56%. Fig. 9-11(a-b) shows the margin of error in temperature of basin, water and glass temperature of solar still with and without baffles. It is observed that the margin of error in basin temperature with and without baffles is found as ±1.3 and 1.1% respectively. While the margin of error in water and glass temperature with and without baffles is found as ±1.1 and 1% respectively. Fig. 12 shows the variation of theoretical and experimental hourly yield from inclined solar still without baffles at a feed mass flow rate of 0.0833 kg/min. It can be observed that the maximum achievable hourly yield of solar still without baffles is found to be 0.22 kg/hr during the month of April 2014. Also, it is observed that the yield during the month of March 2014 is similar to that of April 2014, as the high solar intensity during this month provides more energy. The yield of the solar still without baffles is lower than that of the solar still with baffles, as it is used to control the flow of water inside the absorber. Due to the noncontact of water and non-continuous heat extraction from the basin, the average water temperature has no raised with respect to its inlet temperature.

While the solar still with baffles improved the yield as water having a greater contact with the solar radiation and the heat from the absorber is utilized by flowing water. Due to the lower heat capacity of absorber material, it is possible to store the energy by flowing water with a thin film for increasing the temperature of water. Fig. 13 shows the variation of theoretical and hourly yield from inclined solar still with baffles. It is observed that the yield of solar still is higher during the mid noon and thereafter falling in the yield. This is due to maximum intensity makes the water evaporate quickly at minimum flow. Also, it is due to the inconsistency of wind velocity over the surface of glass, as the temperature of glass increases in the solar still the temperature difference (driving force) inside the solar still reduces for

decreased yield. The maximum peak yield from the solar still with baffles is found as 0.3 kg/hr which is 26.66% higher than solar still without baffles. Similarly, the observed yield of solar still with baffles is higher with respect to time and month of experiment. Due to the decreasing solar intensity during afternoon most of the absorbed in the basin is used by flowing water and this tends to the increase in yield from both the solar still. Fig. 14 shows the margin of error in yield from solar still with and without baffles. It is observed that the margin of error in yield from both the cases is in between the range of ±0.02%, while the uncertainty of the distillate collecting jar is in the range of ± 10 ml (0.1 %) and the error is almost within the limit. Fig. 15 shows the regression between the average water temperature inside the solar still at two different mass flow rates and solar intensity. It can be observed that the increase in solar intensity increased the water temperature up to 73°C and the temperature increases linearly. While the mass flow rate is doubled, the water temperature decreases only by 28.5% (Imax=1025 W/m2) as the retention time of water with solar radiance is decreasing. A linear relation between solar intensity and water temperature is plotted. Fig. 16 shows the variation of yield of solar still with and without baffles as a function of solar intensity. It is clear evident that the yield of solar still with baffles has better performance as compared to solar still without baffles. Also, it can be seen that the yield of solar still without baffles have similar yield during the sunshine hours as compared to the yield of baffled system during the off shine hours. Fig. 17 shows the variation of the evaporative heat transfer coefficient of inclined solar still with and without baffles. The peak evaporative heat transfer coefficient was found as 114 and 160 W/m2K respectively. The increase in evaporative heat transfer coefficient of the present model with Dunkle’s model is found as 56.14% (increased by 2.28 times) and 68.75% (increased by 3.20 times) respectively. Fig. 18 (a) and (b) shows the cumulative yield from solar still with and without baffles while the flow rate in the solar still is 0.0833 kg/min. Due to the longitudinal and transverse flow movement inside the basin the yield is higher for solar still with baffles. With an area of

0.42 m2 of the basinthe maximum yield collected is about 2.1 kg/day from solar

still with baffles. Whereas the yield for a similar solar still without baffles the collected yield is 1.4 kg/day during the hottest condition of Chennai (April). Also, it is observed that the yield is majorly affected by wind velocity over the surface. Due to the lower wind velocity during the hot summer condition, the temperature of the glass is peaking to a maximum and thus, the equilibrium for the rate of condensation from the solar still is affected.

Fig. 19 shows the variation of yield from solar still with and without baffles with respect to average solar intensity. It can be clearly seen that the average yield from the solar still with baffles is found to be linearly increasing with respect to increase in average solar intensity. The maximum yield of 0.38 kg/m2 is achievable with an average solar intensity of 678 W/m2. The percentage increase in yield of fresh water from the baffled system was found as 30.23% as compared to the solar still without baffle. Similarly, the other environmental parameter that affects the solar still performance is the wind velocity and it is shown in Fig. 20. It can be clearly seen that the increase in the average wind velocity from 1.8 to 2.7 decreases the yield of fresh water linearly. The maximum wind velocity occurs during winter on the coastal area of Chennai. The percentage decrease in yield of fresh water from the baffle system by average wind velocity from 2.1 m/s, 2.5 m/s and 2.7 m/s is found as 10.2 %, 9.1 %, and 1.4% respectively. While analyzing the percentage decrease in yield from solar still without baffles is found as 13.2%, 6.7 % and 6.3 % for average wind velocity of 2.1 m/s, 2.5 m/s and 2.7 m/s respectively. It can be concluded that the increase in wind velocity during winter decreases the yield.

5. Conclusions From the theoretical and experimental analysis the following conclusions are derived:

The yields are 5.4 kg/m2day for solar still with baffles and 3.4 kg/m2day for solar still without baffles with an average solar intensity of (I=675 W/m2) during April month.



The yield of solar still is increased by 1.68 times the solar still without baffles.



The yield from solar still increases with increase in solar intensity and decreases wind velocity during summer and vice versa for winter. The average yield from solar still with baffles is found as 0.38 and 0.386 kg/m2 for average solar intensity of 675 W/m2 and average wind velocity, U=1.88 m/s respectively during summer.



The temperature of the water and yield of the solar still is increased by increasing the contact time of water with solar radiance. Results prove that there is a significant improvement in the temperature and yield of fresh water.



The maximum hourly yield from solar still with and without baffles are found as 0.32 and 0.23 kg with a minimum mass flow of 0.083 kg/min.

Acknowledgement The corresponding author gratefully thank his dad late. Mr .M. Sathyamurthy for his constant encouragement and moral support in bringing out this work as modeling and fabrication. The corresponding author would like to thank the management of S.A. Engineering College (Chennai, Tamil Nadu, India) and Director Mr. Venkatesh Raja for providing infrastructural facility for preparing the manuscript and experimental facility. The corresponding author extends his sincere thanks to Dr.G. Illavazhagan, Director (Research), Hindustan Institute of Technology and Science, Chennai, for his kind advice in preparing this manuscript.

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Fig. 1.Mode of heat transfer in inclined solar still with baffles

Fig. 2.Schematic diagram of an inclined solar still with baffles for improving the yield

Fig. 3. Average hourly variation of solar intensity during the months of January-April 2014

Fig. 4 Variation of (a) wind velocity and (b) Ambient temperature during the months of January-April 2014

Fig. 5. Theoretical and experimental hourly variation of basin, water and glass temperature of inclined solar still without baffles (January-April ‘2014)

Fig. 6.Theoretical and experimental hourly variation of basin, water and glass temperature of inclined solar still with baffles (January-April ‘2014)

Fig. 7 Variation of experimental and predicted water, glass, basin temperature and yield from solar still with baffles

Fig. 8 Variation of experimental and predicted water, glass, basin temperature and yield from solar still without baffles

Fig. 9 Margin of error in basin temperature of solar still (a) with and (b) without baffles

Fig. 10 Margin of error in water temperature of solar still (a) with and (b) without baffles

Fig. 11 Margin of error in glass temperature of solar still (a) with and (b) without baffles

Fig. 12. Theoretical and Experimental variation of hourly yield from inclined solar still without baffles

Fig. 13. Theoretical and experimental variation of hourly yield from inclined solar still with baffles

Fig. 14 Margin of error in yield from solar still (a) with and (b) without baffles

Fig. 15.Variation of water temperature from inclined solar still with baffles with respect to solar intensity

Fig. 16 Variation of yield from solar still with and without baffles with respect to solar intensity

Fig. 17.Variation of evaporative heat transfer coefficient of inclined solar still (a) without baffles (b) with baffles

Fig. 18 Variation of cumulative yield from inclined solar still (a) without and (b) with baffles (mf=0.0833 kg/min)

Fig. 19 Variation of yield with respect to average solar intensity

Fig. 20 Variation of yield with respect to average wind velocity

Table. 1 Physical properties of material for theoretical analysis Property

Value

Property

Value

Absorptivity (αg)

0.05

Density (ρw)

1000 kg/m3

Area, (Ag)

0.42 m2

Latent heat of Vaporization (hfg)

2250 kJ/kg K

Transmissivity, (τg)

0.95

Absorptivity (αw)

0.05

Density (ρg)

2500 kg/m3

Emissivity (εw)

0.8

Emissivity (εg)

0.8

Absorptivity (αb)

0.9

Specific heat capacity (Cp,g)

750 J/kg K

Transverse distance, (dx)

0.13 m

Kniematic viscosity (νa)

1.87*10-5m2/s

Longitudinal distance, (dy)

0.1 m

Absolute viscosity (μa)

1.998*10 kg/m s

Breadth of absorber, (x)

0.65 m

Thermal Conductivity (ka)

0.029 W/m K

Lenth of absorber, (y)

0.65 m

Specific heat capacity (Cpa)

1006.9 J/kg K

Number of baffles (n)

4

Density (ρa)

1.06 kg/m3

Specific heat capacity (Cp,baffle)

900 J/kg K

Specific heat capacity (Cpw)

4186 J/kg K

Area, (Ab)

0.42m2

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