Theoretical analysis of inclined solar still with baffle plates for improving the fresh water yield

Accepted Manuscript Title: Theoretical analysis of inclined solar still with baffle plates for improving the fresh water yield Author: Ravishankar Sathyamurthy D.G. Harris Samuel P.K. Nagarajan PII: DOI: Reference:

S0957-5820(15)00145-7 http://dx.doi.org/doi:10.1016/j.psep.2015.08.010 PSEP 612

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

12-3-2015 27-7-2015 23-8-2015

Please cite this article as: Sathyamurthy, R., Samuel, D.G.H., Nagarajan, P.K.,Theoretical analysis of inclined solar still with baffle plates for improving the fresh water yield, Process Safety and Environment Protection (2015), http://dx.doi.org/10.1016/j.psep.2015.08.010 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.

Highlights

1. A new model using baffles is proposed for improving the yield of fresh water with reduced area. 2. The present RHN model can be applied to stepped and weir cascade solar still.

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3. Characteristic equation for yield on effect of mass flow, internal heat transfer and basin

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temperature are determined.

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Theoretical analysis of inclined solar still with baffle plates for improving the fresh water yield Ravishankar Sathyamurthya,*, D.G. Harris Samuela, P.K. Nagarajanb a

an

Department of Mechanical Engineering, Hindustan Institute of Technology and Science, Kelambakkam, Chennai-603103, India

b

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Department of Mechanical Engineering, S.A.Engineering College, Poonamale, Chennai-600064, India

Highlights

Ac ce pt e

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*Corresponding author Research Assistant, Department of Mechanical Engineering, Hindustan Institute of Technology and Science, Chennai-603103, Tamil Nadu, India Phone number:-+91-909-436-7381 Email id: - [email protected]

•

A new model using baffles is proposed for improving the yield of fresh water with reduced area.

•

The present RHN model can be applied to stepped and weir cascade solar still.

•

Characteristic equation for yield on effect of mass flow, internal heat transfer and basin temperature are determined.

Abstract

This work presents the theoretical analysis on the effect of mass flow, feed water temperature, internal heat transfer coefficient and the absorber plate temperature of an improved inclined solar still for producing fresh water. The solar intensity and other environmental parameters are considered for simulating the solar still in order to examine the effect of mass flow; feed water temperature; internal heat and mass transfer parameters; basin temperature on yield and effect of air gap distance between plates. The results show that, at a minimum mass flow rate, yield was increased by 57.14% and the maximum average water temperature for the flow rates of 0.0833, 0.1666, 0.3222 and 0.4166 kg/min are found to be 62, 45, 40 and 38oC respectively. The average temperature of hot water collected in the lower storage was 47.9oC. The effect of inlet feed water temperature shows that, there is an increase in yield by 65% with a flow rate of 0.0833 kg/min and the inlet temperature 60oC respectively. Also, introducing baffle plates in the basin absorbs some of the intensities which will heat up the flowing water for achieving higher brine temperature. The present RHN (Ravi-Harris-Nagarajan) model can be applied to stepped and weir cascaded solar still for determining the average water temperature in the solar still. Keywords: - Inclined solar still; Baffle plates; flow rate; inlet temperature; fresh water yield

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1.

Introduction

Solar desalination is the traditional and cheapest method for converting saline water into drinking water thereby solving the problem in relation to the shortage of drinking water. There were two techniques involved in desalination using solar energy namely (i) direct and (ii) indirect desalination. Direct desalination utilizes solar radiation and their capacities are relatively low. Whereas in indirect approach it is well suitable for larger capacities (Tiwari, G. N et al. 2003), Nagarajan et al (2014), Sathyamurthy et al (2015, 2015a, b), Arunkumar et al (2015). Several low cost solar still have also been discussed (Ahsan et al (2013, 2014), Syuhada et al (2013)).

Direct solar radiation heats up the top surface layer of water

b.

Evaporation of water,

c.

Trapping of water vapor in the clouds and,

d.

Condensation of vapor.

1.1.

BASIN TYPE SOLAR STILL

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cr

a.

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The principle behind solar desalination is similar to a hydrological cycle and as follows:

1.1.1. Single basin

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The basin type solar still was commonly used type for the distillation process, and the yield of fresh water is relatively lower than other methods (Singh and Tiwari (1991)). Shadow from the side walls, falls on the water surface during the morning and evening hours in the still reduces the productivity of fresh water, which has been discussed by various researchers. The use of internal and external reflectors in the solar still increased the hourly yield of fresh water. But on the economic aspect this increases the cost of distilled water and the initial capital cost.

1.1.1.1.

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Tiwari (1987) studied theoretically and experimentally the effect of water depth and inlet feed water temperature on a solar still. It has been reported that the yield of a solar still is maximum for a brine temperature of more than 45oC. On the same transient analysis, the yield will increase with a decrease in water depth when the water is at a temperature of 40oC.

Inverted absorber

The radiation absorbed by the absorber convected to the absorber plate where the water is placed. The rest of radiations are lost to the atmosphere and glass covers. Dev and G.N.Tiwari (2011) concluded that there is an increase in temperature of water in the absorber due to the reduction of heat loss from absorber and increased value of absorptivity. The yield of the inverted absorber solar still is double the conventional solar still. As the water depths increases, there is no significant change in the convective and radiative heat transfer coefficient. Evaporative heat transfer coefficient depends on the depth of water due to the increase in water temperature as the depth decreases. 1.1.1.2.

Finned

Velmurugan et al (2008) examined the effect of introducing fins inside the solar still. Incorporating fins in the solar still increases the free surface area of the water. The maximum accumulated fresh water from the solar still is about 2.2 kg/m2day. 1.1.1.3.

Cascaded solar still

Ziabari et al (2013) experimentally investigated a cascaded solar still which reported that fresh water production increases by 26% with a maximum productivity of 6.2 kg/m2day. Also, it has been reported that the quality of drinking water is in the WHO range. 1.1.1.4.

Weir type solar still

Zoori et al (2013) experimentally carried out a study to analyze the exergy and energy efficiency of a weir type solar still. The experimental study shows that for minimum and maximum flow rates of brine water in the basin produces 3.8 and 6.11% of exergy efficiency. Maximum energy and exergy from the model is 83.4% and 10.5% respectively Page 2 of 29

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with a flow rate of 0.065 kg/min and their theoretical studies show that the dependence of input parameters such as intensity, the thickness of flowing water, ambient conditions are affiliated with the exergy and energy efficiency. M.Dashtban and F.F.Tabrizi (2011) theoretically examined the utilization of PCM as a storage medium. Studies were also made to see the effect of distance between two parallel plates (absorber to cover). The Result shows that the reduction in air gap improves the fresh water production by 35% with PCM in the solar still.

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F.F.Tabrizi et al (2010a) theoretically studied the effect of internal mass flow of saline water. The increase in flow rate decreases the productivity of fresh water and the evaporative heat transfer co-efficient could not be able to satisfy the Dunkle’s co-relation due to the change in geometry. The system beyond 0.2 kg/min has a significant effect on system co-relation with Dunkle’s model. Due to the number of passes of water in the basin, water absorbs the maximum intensity for a particular amount of time step. The daily productivity for minimum and maximum flow rates is found to be 7.4 and 4.3 kg/m2day respectively.

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cr

F.F.Tabrizi et al (2010) experimentally investigated the effect of phase change material on a weir type solar still for improving fresh water production, and it is concluded that the production of fresh water is higher in case of sunny days compared to cloudy days. The heat from solar intensity during the sunshine periods is stored inside the PCM, so that the heat that is stored during sunshine hours are completely used during the off-shine period. The total productivity of solar still was found to be 5.1 kg/m2day and 4.8 kg/m2day.

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1.1.2. Double basin

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1.1.3. Triple basin

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Bapeshwararao et al (1983) theoretically investigated in a single slope-double basin solar still, by reducing the temperature of upper glass cover in the upper basin by flowing water. This technique reduces the temperature very quickly than cooling by air, due to higher heat capacity of water heat from the upper basin glass cover can be easily removed for enhanced productivity. The Results shows that the lower basin water temperature is lower than the upper basin, which the productivity of the lower basin is lower.

El-Sebaii (2005) studied theoretically the thermal performance of the triple basin solar still. The energy balance equations are solved analytically using the elimination method show that the productivity of water is higher in the lower basin than that of the middle and upper basin and it is maximum for a least water masses in the upper basin and middle basin. When the mass of water in the basin is higher, the productivity becomes less dependent on heat capacities. The daily productivity of water increases with an increase in the velocity of wind. For a triple basin the results indicate the daily productivity equals 12.635 kg/m2day with an average intensity of 651 W/m2.

1.1.4. Stepped solar still

Abdulla (2013) investigated the performance of a stepped solar still. The still productivity is about 3.5 kg/m2day, which is higher than that of conventional solar still which is about 2.2 kg/m2day. This is due to the increased number of steps resulting in increased surface area of water.

Kabeel et al (2013) experimentally studied a stepped solar still, which utilized a blower to heat up the water in the basin. The rise in water temperature is due to the blowing effect of air into the basin. Experimental results concluded that the productivity increased by 56% than a typical solar still.

1.2.

Inclined solar still

Aybar et al (2005) mathematically modelled an inclined solar still with jute cloth as wick material. It has been given that the inclined solar still can be used for water heating purpose. The average hot water temperature collected in the Page 3 of 29

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tank was about 40oC, which can be utilized for domestic purpose. The same has been experimentally studied and validated by Aybar et al (2006) Anburaj et al (2013) experimentally investigated an inclined solar still with ridges inside the solar still. The rectangular ridges reduce the flow rate inside the still. Using different wick’s, the system performance was found to be maximum for black cotton cloth of 4.2 L/day and at an optimum inclination of 30o. 1.2.1. Wick type solar still

Different geometry

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1.3.

cr

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Sodha et al [21] experimentally concluded that the effect of having wicks in a simple solar still increase the production rate by 25% by the capillary effect than the conventional solar still. Also, the shadow effect from the side walls are reduced by deepening the basin to keep the wicks on the surface for better evaporation of water.

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On the other aspect to reduce the initial cost a new geometry of the solar still was discussed by, Ravishankar et al (2013), Sathyamurthy et al (2014, 2014a, 2014b, 2015, 2015b), Nagarajan et al (2014), Arunkumar et al (2012a, 2013, 2013a), Syuhada (2013) and Ahsan et al (2008, 2009, 2010, 2011, 2012, 2013, 2013a).

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1.3.1. Triangular pyramid

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Sathyamurthy et al (2014),concluded in a triangular pyramid solar still that the depth of water and wind velocity over the solar still are the primary parameter which will be affecting the fresh water production. Also, it has the concluded that, at a minimum depth of water the productivity is higher and with higher wind velocity as it increased the temperature difference. Nagarajan et al (2014) investigated the performance of a solar still at minimum water depth. It has been concluded that the accumulated yield of 4.2 kg/m2day was achieved. Ravishankar et al (2013) and Sathyamurthy et al (2014a, 2014b), experimentally arrived at a conclusion that the effect of adding phase change material on a triangular pyramid solar still and PCM increased the efficiency by 35% than a typical solar still. Also, the fresh water production during summer and winter were found to be 4.5 and 3.4 kg/m2day respectively.

1.3.2. Pyramid type

Y. Taamneh and M. M. Taamneh (2012) investigated the utilization of forced convection over the glass surface. The cooling of the glass surface is done with a help of DC solar powered forced draught fan. The productivity of forced convection is increased by 10.81% than the free convection. The accumulated yield of the natural and forced convection pyramid solar still is found to be 3.3 and 3.7 kg/m2day respectively.

1.3.3. Tubular

Arunkumar et al (2012a, 2013) and Ahsan et al (2013a) experimentally investigated the effect of water-cover temperature difference on a tubular solar still. The conclusions show that the fresh water production is completely depending on the water-glass temperature difference, which were directly proportional. A linear proportionality between the total heat transfer co-efficient and mass transfer co-efficient are plotted. It is also reported that the convective heat transfer co-efficient is lower than that of evaporation/condensation.

1.3.4. Concentric tubular

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Arunkumar et al (2012, 2013) designed a new tubular solar still and by flowing air and water over the cover observed the enhanced condensation rate as water is having a higher heat capacity than that of air, which takes away the heat from the cover for better condensation of vapor inside the solar still. Also, the system performance is analysed with and without the effect of flowing air and water. The increase in productivity of fresh water using air and water was found to be 48.3% and 143.4% as a cooling medium respectively.

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1.3.5. Hemispherical

Enhancement methods

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1.4.

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cr

Arunkumar et al (2012a) experimentally studied the effect of hemispherical solar still with and without cooling of the top cover. The driving force of solar desalination is the temperature difference between water and cover temperature. The production rates depend on water, ambient and cover temperatures. The flow rate of water is kept at a minimum over the cover (10ml/min), which improved the efficiency from 34 to 42%. Also, the output of solar still with the same amount of solar radiation was improved by 1.25 times due to the effect of cooling water over the cover. The average output of 3.5 l/m2/day without cooling and 4.2 l/m2/day with cooling of the cover.

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1.4.1. Internal and External reflectors

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Tanaka et al (2009, 2009a, 2009b, 2010, 2011, 2011a) has introduced the utilization of internal and external reflectors in a basin type solar still. The cost of adding internal and external reflectors increased the overall capital cost. There is an increase in productivity by 32% and 40% with internal and external reflectors respectively. Optimizing techniques were used to find the optimum inclination of reflectors inside and outside the still.

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Tanaka (2009) examined the effect of the bottom reflector on an inclined wick solar still. From theoretical analysis, the optimum reflector angle was found as 35o which increases the productivity by 30% and 13% over conventional solar still. Tanaka (2011a) analysed the effect of adding an external boosting mirror for an inclined flat plate collector attached to an inclined solar still. The results reported that the increase in productivity of fresh water with boosting mirror by 15%, when the mirror is tilted. The mirror held vertical almost to 90o, and the productivity increased by 25% although the mirror length is same as the solar still length. 1.4.2. Flat plate collectors

Voropoulus et al (2001) investigated the use of integrating a solar flat plate collector to a typical solar still. The results indicate that, the increase in brine temperature enhances the evaporation of saline water, which is done through a flat plate collector for enhancing fresh water production. The temperature of brine solution is raised by passing the hot fluid through a heat exchanger from the FPC. Also, it has been reported that the brine temperature can be increased by utilizing the waste heat from the thermal processes.

Badran et al (2005) identified the use of internal reflectors in a single slope solar still coupled to a flat place collector. The results show that the integration of FPC’s increased productivity by 36% and studies was made to see the effect of environmental parameters that will be affecting the performance of solar still. The environmental parameters also play an important role in fresh water production. Higher wind velocity over the glass cover decreases the glass temperature for a higher rejection of latent heat of condensation over the glass surface for improving fresh water production.

1.4.3. Solar pond

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Velmurugan and Srithar (2007, 2008) and Velmurugan et al (2008, 2009, 2009a) analytically and experimentally investigated the integration of a mini solar pond with a single slope solar still and stepped solar still. Results indicate that the use of a mini solar pond enhances the productivity by 27%, rather in the case of a stepped solar still it is 32%.

1.4.4. Phase change material

cr

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El-Sebaii et al (2009) investigated theoretically the use of latent heat thermal energy storage in a single basin solar still. The use of phase change material extracts the heat from intensity and stores it. During the off-shine period energy stored in the PCM’s are discharged for heating up of water in the basin. Theoretical results shows that the use of PCM during the daylight period, the productivity is low, which is due to the charging of higher mass PCM reducing the water temperature. During the off-shine period for the same case, the productivity is higher, which is due to the increase in the mass of PCM and a larger amount of energy stored in it. Total productivity of solar still with and without PCM was found as 9 and 5 kg/m2day respectively, and fresh water production increased by 80%.

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1.4.5. Sensible heat storage material

2.

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Murugavel et al (2008) and Rajaseenivasan et al (2013, 2013a) investigated the utilization of sensible heat storage material on a single and double basin double slope solar still. The results show that the use of sensible heat storage material inside the basin increases the fresh water production by 45% with 3/4”quartz rock. The materials used in the basin were mild steel scraps, 1/4” quartz rock, washed stones and red bricks. Productivity thus not only depends on the specific heat capacity and also on size of material. The size of material inside the basin increases the free surface area inside the basin for better evaporation of saline water.

Mathematical modelling

d

The following assumptions are considered during the mathematical modeling.

Ac ce pt e

a. There is no vapor leakage from the solar still.

b. The temperature of water and the basin are equal.

c. Latent heat rejected by the vapor is absorbed by the inner glass surface; hence all vapors are converted to distilled water. d. The flow of water throughout the basin is uniform.

e. The surface area of absorber is approximately equal to the surface area of the glass. f. Flow of wind velocity over the solar still is unidirectional and kept constant. g. There is no heat loss from the basin to the surroundings. h. The average water temperature in the solar still is almost equal to the absorber temperature.

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2.1.

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Fig. 1 Schematic diagram of the inclined solar still with baffles

Modes of heat transfer in the solar still

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The different modes of heat transfer occur 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 various modes of heat transfer inside and outside the still are; convective heat transfer from cover to ambient hc,g-a,

b.

radiative heat transfer from cover to ambient hr,g-a,

c.

convective heat transfer from water to cover hc,w-g,

d.

evaporative heat transfer from water to cover he,w-g,

e.

radiative heat transfer from water to cover hr,w-g and,

f.

convective heat loss from water to basin hc,w-b

Ac ce pt e

d

a.

2.1.1. Heat transfer inside the solar still

2.1.1.1.

Convective heat transfer between water and glass

This mode of heat transfer is taken place between evaporated water and the inner surface of the glass. This parameter is completely depends on the difference in temperature between water and the inner surface of the glass. The convective heat transfer co-efficient (hc) is a correlation between Nusselt number (Nu) and thermal conductivity (ka) of the vapor mixture. Where the Nusselt number for an inclined solar still is given by the expression as follows Hollands et al (1976), 1/3   1708   1708(sin1.8θ )1.6   RaH Cosθ  Nu = 1+1.44 1−  1−  +   −1 RaH Cosθ   5830    RaH Cosθ  

When

(1)

θ ≤ 60o

The flow will be in between two plates with an inclination angle of 0<θ<60o, at the height of “H”.

The convective heat transfer co-efficient between water and glass (hc,w-g) can be rewritten as Kamaraj et al (1980),

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Nu.k a H

(2)

And Rayleigh number is expressed as,

RaH =

g β∆TH 3

(3)

ν aα a

Where αa is the diffusivity of air and

β=

1 Tw + 273.15

∆T is expressed as Dunkle (1961) and Tiwari and Lawrence (1991),

∆T = (Tw − Tg ) +

( Pw − Pg )(Tw + 273.15)

(4) The convective heat transfer rate between water and glass (Qc,w-g) can be expressed as,

Evaporative heat transfer between water and glass

(5)

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2.1.1.2.

268900 − Pw

cr

Qc , w − g = hc , w − g Ab (Tw − Tg )

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hc , w− g =

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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 co-efficient and temperature difference. This is given as,

m f C pw (Two − Twi ) = he , w− g Ab (Tw − Tg )

(6)

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The evaporative heat transfer co-efficient (he,w-g) can be estimated using the following heat and mass transfer analogy with heat flux as follows Sayigh, A. A. M., and E. M. A. El-Salam (1977), (7) Qe , w − g = 0.016hc , w− g Ab ( Pw − Pg )

Qe ,w − g = he , w− g Ab (Tw − Tg )

(8)

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and

Ac ce pt e

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 

(9)

The amount of water condensed in the inner surface of the glass can be written as Zurigat and Mousa (2004),

me, w− g = 2.1.1.3.

Qe, w− g .3600 h fg

(10)

Radiative heat transfer between water and glass

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

Qr , w− g = ε effectiveσ (Tw 4 − Tg 4 )

(11)

Where,

ε effective

1 1  =  + − 1  ε g ε w 

−1

(12)

Qr , w − g = hr , w − g Ag (Tw − Tg )

(13)

Substituting (13) in (11) gives,

hr , w− g = ε effectiveσ (Tw 2 + Tg 2 )(Tw + Tg )

(14)

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2.1.2. Heat transfer outside the solar still 2.1.2.1.

Convective heat transfer 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 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 of higher water to the glass temperature difference. The convective heat transfer from the still is a function of wind flowing around the still and convective heat transfer co-efficient, which is expressed in the form of,

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hc , g −a = 2.8 + 3u

(15)

Madhlopa, A., and C. Johnstone (2009) For wind velocity <= 5 m/s

cr

hc , g − a = 5.7 + 2.8u

For wind velocity > 5m/s (Tanaka, 2010) The convective heat transfer is expressed as,

Qc , g −a = hc , g − a (Tg − Ta )

Radiative heat transfer between glass and ambient

(17)

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2.1.2.2.

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(16)

Tsky = 0.05525Ta1.5

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The radiative heat transfer outside the solar still is a function of sky (Tsky) and glass temperature (Tg). The sky temperature is give as a function of ambient temperature, which is expressed as Duffie, John A., and William A. Beckman (1977),

(18)

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Qr , g − a = ε gσ (Tg 4 − Tsky 4 )

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

(19)

Qr , g −a = hr , g − a (Tg − Ta )

(20)

Substituting (20) in (19), which gives Shukla and Sorayan (2005),

 Tg 4 − Tsky 4  hr , w− g = ε gσ    Tg − Ta  2.2.

(21)

Energy balance

2.2.1. Energy balance of outlet water temperature

nxy * Iτ g (1 − α w ) = m fwC pw (Tout − Tin )( xdy − dxdy ) − nxyQbaffle

(22)

And the heat lost from the basin to the surroundings was neglected as it is assumed in the section 2. From the above equation,

 Iτ gα w + Qbaffle  Tout = nxy   + Tin  m f C pw ( xdy − dxdy ) 

(23)

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

 nxy  Iτ g (1 − α w ) + Qbaffle     TW = Tin +   2m fwC pw ( xdy − dxdy)  dTbaffle Iτ gαb = mbaffleC p,baffle dt

(24)

(25)

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Tbaffle,t = dTbaffle + Tbaffle ,i

(26)

2.2.2. Energy balance of the glass surface 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 α g Ag + h2 (Tw − Tg ) = h3 (Tg − Ta )

(27)

The above equation can be rewritten as (El-Sebaii et al, 2009),

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 I α g Ag + h3Ta + h2Tw  Tg =   h2 + h3  

(28)

Where,

cr

h2 = he, w − g + hc , w − g + hr ,w − g h3 = hc , g − a + hr , g − a

(29)

Energy balance on basin surface

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2.2.3.

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(30) Fig.1 shows the schematic energy balance in the solar still. Table. 1 describes the physical properties used for mathematical modeling.

The basin temperature is determined as,

I (t )τ gτ wα b = h1 (Tb − Tw ) + U b (Tb − Ta ) I (t )τ gτ wα b + h1Tw − U bTa

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Tb =

(31)

h1 + U b

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h1= 109 W/m2K and Ub=14 W/m2K

(32)

Table. 1 Physical properties of material for mathematical modeling Property αg Ag τg ρg εg Cp,g νa

Value

Property

Value

0.05

ρw

1000 kg/m3

0.42m2

hfg

2250 kJ/kg K

αw

0.05

2500 kg/m

εw

0.8

0.8

αb

0.9

dx

0.13m

dy

0.1m

0.95

3

750 J/kgK -5

2

1.87*10 m /s -5

µa

1.998*10 kg/ms

x

0.65m

ka

0.029 W/mK

y

0.65m

Cpa

1006.9 J/kgK

n

4

ρa

1.06 kg/m3

Cp,baffle

900 J/kg K

Cpw

4186 J/kg K

Ab

0.42m2

3.

Inclined solar still

Fig. 2 shows a 3D view of an inclined solar still. It consists of a square tray of 0.65x0.65 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 top of solar still, so that the evaporated water is being condensed inside the glass over. For easy removal of glass from the still, glass support is fixed so that it can be removed for maintenance purpose. Baffle plates are held in the solar still as the saline water can have an increased time of contact with solar radiation. This raises the temperature

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cr

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of saline water than that of the other solar still. At the end of glass cover, a distillate collector is placed to collect fresh water in a calibrated flask. Fig. 3 (a) and (b) shows the arrangement of baffle plates. The successive distance between each baffle plates is arranged to be at 0.13m. The hot water from the collector is taken from the end of solar still and stored in a separate tank for continuous circulation. The baffle plates not only deflect the path of water, but it stores some of the solar energy in it. This is the basic idea behind the inclined solar still. Insulation materials are filled up on the side walls and the bottom of the solar still to avoid heat losses.

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Fig. 2 3D View of an inclined solar still

Fig. 3 (a) Placement of baffle plates (b) Flow of water inside the basin 4.

Results and Discussions

4.1.1. Effect of mass flow rate Fig. 4(a) shows diurnal variation of average water temperature and wind velocity at different water flow, while Fig. 4(b) shows the hourly variation of absorber temperature and solar intensity. Due to the increased flow of water the absorber temperature is reducing as it reduces the water temperature. The complete absorption of energy from the absorber plate to water is not utilized by flowing water. Also, due to the increased flow the solar radiance is not completely utilized by flowing water. The minimum mass flow of saline water inside the basin absorbs the maximum intensity at a constant inlet water temperature of 32oC. It has been observed that, the due to excessive movement inside the basin, saline water absorbs minimum energy from solar radiation. The current model average water temperature has a greater match over the Dunkle’s previous model. Also, the rise in water temperature is not only Page 11 of 29

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from solar radiation, but also from the baffle plates. Baffle plates absorb some of the energy, which is transferred to the flowing water passing through it. The maximum average temperature of water at 0.0833, 0.166, 0.33, 0.4166 kg/min were found to be 65, 45, 40, and 38oC respectively. Fig. 5 shows the hourly variation of evaporative heat transfer co-efficient. It has been found that, the evaporation of saline water is quicker in case of minimum mass flow. The phenomenon behind is that, the saline water is having a greater contact over intensity during the flow. Also, due to natural convection and larger contact of water inside the basin, the vapor raises and condenses them back quickly. The evaporative heat transfer co-efficient acts as a driving force in fresh water production in any solar still. Thus, it depends not only on flow rates, but also the consecutive distance between two plates, as Nusselt number is a dimensionless parameter.

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Fig. 4(a) Diurnal variation of average water temperature at different mass flow rates at constant inlet feed water temperature (Tw,in=32oC) and wind velocity

Fig. 4(b) Diurnal variation of average absorber temperature at different mass flow rates at constant inlet feed water temperature (Tw,in=32oC) and solar intensity

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Fig. 4(c) Diurnal variation of ambient temperature used for theoretical analysis

Fig. 5 Diurnal variation of hourly evaporative heat transfer co-efficient at different mass flow rates The hourly variation of yield from solar still is shown in Fig. 6. It can be seen that, the decrease in flow rate increases the yield of fresh water. The maximum yield was found to be 0.65 kg/m2hr at noon with a minimum mass flow. There is a gain in energy from the side walls and baffles in the solar still for a small raise in productivity at 1500 hrs at minimum mass flow in the still. Also, it is found that there is no change in yield, when it is increased by 0.33 and 0.4166 kg/min respectively. The simulated result is then compared with the previous wick type inclined solar still developed by Jassim Talib Mahdi (1992). The results compared are well on agreement with the previous model Jassim Talib Mahdi (1992). Maximum hourly yield with Charcoal cloth as wick material with a 5 kg/hr mass flow over it was 0.55 kg/m2 hr.

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Fig. 6 Hourly variation of fresh water production at different mass flow rate

Fig. 7 Diurnal variation of accumulated yield for different mass flow rate

Fig. 7 shows the accumulated yield of fresh water production. The yield of fresh water decreased by approximately ½ times with an increase in mass flow rate. The accumulated yield of fresh water for mass flow rates of 0.0833, 0.166, 0.33 and 0.4166 kg/min were found to be 3.5, 1.75, 0.75 and 0.5 kg/m2day respectively at a constant inlet water temperature and constant wind velocity of 32oC and 3 m/s respectively. For further study, the effect of water temperature inside the still was also studied. 4.2. Effect of inlet water temperature

A process heater is used in the storage tank and water is heated at regular interval of time. Using a stirrer the water bath is maintained at constant temperature. The variation in evaporative heat transfer co-efficient at different inlet water temperature is shown in Fig.8. Fig. 8 shows the hourly variation of evaporative heat transfer co-efficient at a minimum mass flow rate of 0.0833 kg/min. Results show that, the inlet water temperature has a great impact over the yield of fresh water. The evaporation rate of saline water inside the solar still increases due to the partial pressure developed in it. Also, due to the minimum flow inside the still and larger rejection of heat from the cover and vapor increases the hourly yield of fresh water.

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Fig. 8 Effect of inlet feed water temperature at mass flow rate of 0.0833kg/min on evaporative heat transfer coefficient

Fig. 9 Effect of inlet feed water temperatures and mass flow rates on daily yield

Fig. 9 and 10 shows the variation of the accumulated yield of solar still at different flow rates of water. It is observed that the yield of water decreases with the increase in flow rate for different inlet water temperature. Also, the yield of fresh water is higher in case of the highest water temperature and the lowest mass flow rate. The inlet water temperature with 60oC produced about 10 kg/m2day of fresh water, which the evaporative heat transfer co-efficient of 100 W/m2K. The sudden exposure of heated water into the basin develops the partial pressure between water and glass, rejects its latent heat of vaporization from the water vapor through the glass to the ambient for attaining thermal equilibrium and condensing of water. Vapor inside the still releases its latent heat through the glass due to the poor thermal conductivity of material. The vapor inside the still releases its heat not only due to thermal conductivity, also due to the surrounding temperature and wind velocity.

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Fig. 10 Variation of yield on inlet feed water temperatures

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Fig. 11 shows the hourly accumulated yield of fresh water at different inlet water temperatures. A maximum yield of 10 kg/m2day with an error of ±5% standard deviation is achieved. Similarly, the accumulated yield for different water temperature of 32, 40, 50 are found to be 3.5, 4.2, 7 kg/m2day respectively. The maximum yield is achievable with a fact that, the water will be completely flowing down very slowly and completely receiving all the solar intensity. The basin receives the intensity and store the energy in it. The temperature of absorber is also an important parameter on fresh water yield. While comparing it to the J.T.Mahdhi et al (2011), the daily efficiency is improved by 40% with 464.875W/m2, 3 m/s, 32oC and 30o of solar intensity, wind velocity, ambient temperature and inclination respectively. While the same system compared to a conventional solar still, the yield is increased by 113%.

Fig. 11 Accumulated yield variation on different inlet feed water temperatures at 0.0833 kg/min

Table. 2 Polynomial equations for different operating temperatures

S.no

Inlet feed water temperature Tin, (oC)

Correlation co-efficient values R2

Constants

Non linear (Polynomial) Equation a

b

c

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16

1

32

0.169Tin-2.663 2

0

0.169

-2.663

0.999

2

40

0.001Tin +0.129Tin-3.690

0.001

0.129

-3.690

1

3

50

0.121Tin-4.224

0

0.121

-4.224

0.999

0.001

0.095

-3.841

0.999

4

2

60

0.001Tin +0.095Tin-3.841

y = ax 2 + bx + c

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Fig. 12 shows the variation of productivity at different inlet feed water temperature. Yield of fresh water is linearly increasing with an increase in feed water temperature. The polynomial equation is chosen for the curve fitting, and it seems to be it is linearly increasing. The generalized form of 3rd order polynomial is given by,

cr

(31)

us

The values of a, b, c were given in the Table. 2.

Table. 3 Polynomial equations for different mass flow rates

Mass flow rates (kg/min)

Constants

Non linear (Polynomial) Equation

an

S.no

2

Correlation co-efficient values R2

a

b

c

44.93

-30.54

5.542

0.966

45.14

-31.20

7.203

0.972

0.0833

44.93mf -30.54mf+5.542

2

0.166

45.14mf2-31.20mf+7.203

3

0.33

51.68mf2-35.30mf+9.924

51.68

-35.30

9.924

0.968

4

0.4166

52.96mf2-36.18mf+12.31

52.96

-36.18

12.31

0.978

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1

Fig. 12 Variation of fresh water yield at different inlet feed water temperature Fig. 13 observes that, the effect of flow rates on fresh water yield with total yield as a function of mass flow rate. The non-linear curves for the productivity against yield shows that, the internal mass flow rate also plays an important role in fresh water yield. Page 17 of 29

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The non-linear (Polynomial) 2nd order equation for 0.0833, 0.166, 0.33 and 0.4166 kg/min given in the Table. 3.

Effect of internal heat and mass transfer parameters

d

4.3.

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Fig.13 Variation of yield of fresh water with different flow rates of saline water

Ac ce pt e

Fig. 14, 15, 16 and 17 shows the internal parameter with a continuous flow of water inside the solar still. Here the parameters (Nusselt Number and convective heat transfer co-efficient) were plotted against the Rayleigh number whereas, heat transfer coefficient is a function of Nusselt number and distance between the plates. The distance between the bottom plate and cover is constant and assumed as 0.15m. The vapor in between the space transfer heat in the form of convection, radiation and evaporation. Convection is always higher than the radiation, whereas evaporation is always higher than convection and radiation. The Rayleigh number is associated with free flow convection of fluid that is given to be the buoyancy effect. The flow of vapor inside the basin area depends on Rayleigh number and consecutive distance between the basin and cover. Fig. 14 shows the variation of Nusselt number and heat transfer co-efficient with a Rayleigh number between the ranges of 4.64x105 and 1.46x106 at a flow rate of 0.0833 kg/min. The convective heat transfer co-efficient and the Nusselt number was found to be in the range of 10-12 W/m2K and 4.3-4.5 respectively. The linear equation is used for curve fitting, which is given as,

y = m1 x + c1

(32)

The heat transfer co-efficient and Nusselt number with Rayleigh number as a function and given as,

hc = 3E − 06 Rah + 7.627

(33)

Nu = 1.0 E − 06 Rah + 3.945

(34)

Fig. 15 shows the effect of variation in Nusselt number and heat transfer co-efficient at a flow rate of 0.133 kg/min. It is seen that the Rayleigh number is found to be in the range of 105 to 106. Due to the increase in water flow inside the basin, convection between water and glass found to be reduced, which is in the range of 5.7-8.5 W/m2K, where the Nusselt number decreased and in the range of 2.7-3.8. It is obvious that the increase in internal flow disturbs the evaporation of water inside the basin. Also, the Nusselt number is directly proportional to the heat transfer co-efficient

Page 18 of 29

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and inversely proportional to the inner distance between plates. The co-relation between heat transfer co-efficient and the Nusselt number with Rayleigh number as a parameter is given as follows, (35)

Nu = 2.0 E − 06 Rah + 2.637

(36)

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hc = 3E − 06 Rah + 5.098

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Fig. 14 Effect of Rayleigh number over Nusselt number and heat transfer co-efficient at flow rate of 0.0833 kg/min and 30o inclination

Fig. 15 Effect of Rayleigh number over Nusselt number and heat transfer co-efficient at flow rate of 0.166 kg/min and 30o inclination

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Fig. 16 Effect of Rayleigh number over Nusselt number and heat transfer co-efficient at flow rate of 0.33 kg/min and 30o inclination

(37)

Nu = 6 E − 06 Rah + 1.915

(38)

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hc = 1E − 05Rah + 3.702

M

an

The variation of Nusselt number and heat transfer for a flow rate of 0.33 kg/min is shown in Fig. 16. The range of Rayleigh number is found to be 5x104-1.72x105. The internal fluid flow has to take the complete heat for vaporization. The convective heat transfer between the water and glass is lower for which is in the range of 5-6W/m2K. The linear equation for heat transfer co-efficient and Nusselt number is given as follows,

Fig. 17 Effect of Rayleigh number over Nusselt number and heat transfer co-efficient at flow rate of 0.4166 kg/min and 30o inclination The co-relation between heat transfer co-efficient and Nusselt number against Rayleigh number is shown in Fig. 17. It is clear that with a further increase in mass flow rates have no significant changes in the internal heat transfer. The co-relation between Fig. 16 and 17 are similarly equal, and there are no further changes with increasing the flow rate. The co-relation for heat transfer co-efficient and Nusselt number with Rayleigh number as a parameter as follows,

hc = 1E − 05Rah + 3.428

(39)

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(40)

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Nu = 8E − 06 Rah + 1.773

d

Fig.18 Variation of heat transfer co-efficient over Rayleigh number

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Fig. 18 and 19 shows the variation of heat transfer co-efficient and Nusselt number over Rayleigh number. It can be observed that the convective heat transfer and evaporative heat transfer co-efficient are directly proportional. The equation for curve fitting is given by,

 c  y = ax + b ln( x) +   ln x 

(41)

Coefficient of Multiple Determination (R2) = 0.9177830986 (Fig.18)

Fig.19 Variation of Variation of Nusselt number over Rayleigh number

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Coefficient of Multiple Determination (R2) = 0.9176042454 (Fig. 19) From Fig. 18 and 19, it can be seen that the co-efficient of multiple determination was similar. The regression nonlinear co-efficient values a, b and c for heat transfer and Nusselt number are discussed in Table.4 and Table.5 respectively.

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The characteristic equation for convective heat transfer co-efficient and Nusselt number are as follows,

cr

 1.64  hcw− g = 19.17 Rah + 1.91ln( Rah ) −   (42)  ln Rah 

us

 0.63  Nu = −4.7813Rah + 8.24 ×10−8 ln( Rah ) +   (43)  ln Rah 

M

an

The equation is applicable when the flow rate is significantly greater than or equal to 0.0833 kg/min and Rayleigh number within the range of 103 and 2x106. There is no significant increase in heat transfer co-efficient and Nusselt number, when there is an increase in the flow rate. It is also observed that the convective heat transfer co-efficient matches with the Dunkle’s model.

Variable

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Table.4 Regression Variable results for convective heat transfer co-efficient

Value

t-ratio

Confidence intervals

Standard Error

Prob(t)

68% (+/-)

90% (+/-)

95% (+/-)

99% (+/-)

a

19.17

7.12

2.69

2.70

4.5220

5.423

7.2422

0.0

b

1.91

5.09

3.75E-08

3.77E-08

6.308E-08

7.565E-08

1.010E-07

0.00001

c

-1.64

-5.33

30.84

31.02

51.80

62.134

82.97

0.0

Table.5 Regression Variable results for Nusselt number

Confidence intervals Variable

a

b

c

Value

t-ratio

Standard Error

Prob(t) 68% (+/-)

90% (+/-)

95% (+/-)

99% (+/-)

-4.7813

-3.49

1.369

1.3774

2.30038

2.7588

3.68412

0.00109

8.24E-08

3.7

2.228E-08

2.2411E-08

3.74282E-08

4.488E-08

5.99422E-08

0.00058

0.63

5.321

0.119349

0.120017

0.20043

0.24038

0.3210

0.0

Page 22 of 29

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4.4. Effect of basin temperature on yield

us

Fig. 20 Variation of yield over basin temperature at different mass flow rate of water

(41)

Jassim Talib Mahdi (1992)

Ac ce pt e

This is higher than the model predicted by

d

Pd = 0.017Tb − 0.571

M

an

The flow of saline water in the basin is majorly affecting the basin temperature. The fact is that the temperature of saline water inside the basin is a parameter which is normally considered to be equal to the ambient condition. In a practical case, the saline water in the storage tank receives the intensity, and there will be temperature disturbance. To avoid these conditions the storage tank is insulated that there is no heat gain or heat loss. The condition assumed for this study is that, the inlet feed water temperature and wind velocity is constant at 32oC and 3m/s respectively. Fig. 20 shows the variation of yield under different mass flow rates of saline water. From this study, the yield of solar still is higher in the case that the basin temperature is more than 50oC. The yield of fresh water under the mass flow rate of 0.0833 kg/min was found to be,

Fig. 21 Variation of daily efficiency under different mass flow rate Fig. 21 shows the variation of daily efficiency of solar still under different mass flow rates. It is observed that the efficiency of the solar still is higher in the case of the least mass flow rate and further decreases with an in the flow of saline water. The present model is also compared with the model investigated by J.T. Mahdhi et al. (2011) with the charcoal wick inside the basin of inclined solar still. 4.5. Effect of air gap distance between the plates on convective heat transfer co-efficient

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Fig. 22 Variation of average convective heat transfer co-efficient at different flow rates for different water glass distance

Conclusion

M

5.

an

Fig. 22 shows the variation of average convective heat transfer co-efficient at different distance between water and glass. It can be observed that the convective heat trasnfer co-efficient is higher with the least water flow. Due to the effect of geometry of the solar still, the flow inside the directly exposes to the incoming solar radiation for a longer period of time. A thin film of water will be touching the major portion of the absorber, to absorb maximum energy. Due to this effect water flowing through the basin water instantly evaporate. With an increase in the distance of water and glass, the vapor formed inside the still radiply rejects the latent heat to the surrounding.

In this system, the effect of different mass flow rates and inlet feed water temperatures were analysed and the following conclusions were arrived The accumulated yield of fresh water is maximum at a least water mass flow of 0.0833 kg/min of about 3.5 kg/m2 day at a constant wind velocity and the inlet feed water temperature of 3m/s and 32oC respectively.

•

The yield of fresh water is increases linearly with an increase in inlet feed water temperature.

•

From the regression analysis the characteristic equation for maximum yield at an inlet water temperature of Tin=60oC with R2=0.978, it was found as, Pd = 52.96m f 2 − 32.18m f + 12.31

Ac ce pt e

•

d

•

And from the regression analysis on the effect of inlet water temperature on yield is seen closely to linearity, and given as, Pd = 0.169Tin − 2.663

•

•

The characteristic equation for convective heat transfer co-efficient and the Nusselt number are found to be,  1.64   0.63  hcw− g = 19.17 Rah + 1.91ln( Rah ) −  Nu = −4.7813Rah + 8.24 × 10−8 ln( Rah ) +     ln Rah  and  ln Rah  respectively for the mass flow rate greater than equal to 0.0833 kg/min. The characteristic equation for total yield as basin absorber temperature is found to be,

Pd = 0.017Tb − 0.571 and the yield is completely depends on basin temperature while it is noticed that, the yield is higher when the absorber temperature is greater than 50oC. • •

24

The increase in air gap has to reduce the convective heat transfer co-efficient and due to the presence of baffle plates, excessive heat energy stored in is utilized by the flowing water for evaporation. At minimum mass flow, the convective heat transfer co-efficient linearly increases, and there is no further increase in the average value of convection when the flow is more than 0.33 kg/min. Page 24 of 29

Acknowledgements

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The corresponding author gratefully thank his dad late. Mr .M. Sathyamurthy for his constant encouragement and moral support in bringing out this work as modelling and fabrication. The corresponding authors sincerely thank Mrs. Shantharani Sathyamurthy for spending her valuable time in preparing the manuscript. The authors gratefully extend their sincere thanks to Prof .D. Vijayakumar, Veltech Multitech Dr.Rangarajan Dr.Sakunthala Engineering College, Chennai, Mr.A.P.Arun pravin and Mrs.M.Jeyashree, Assistant professors, Department of Mechanical Engineering, St.Peters College of Engineering and Technology, Chennai, for their moral support and friendly advice. The corresponding author extend his sincere thanks to Dr.G. Illavazhagan, Director (Research), Hindustan Institute of Technology and Science, Chennai, for his kind advice in preparing this manuscript. Also, the author would like acknowledge Hindustan Institute of Technology and Science for grant of fellowship Ref: - HITS/Regr/Ph.D./13 and Ref: - HITS/D(R)/IOC/VIII/2013 dated 7/08/2013 of Director (Research)/HITS.

Nomenclature

an

us

cr

Area (m2) Specific heat capacity (J/kgK) Heat transfer Co-efficient (W/m2K) latent heat of vaporization (J/kg) Total Radiation (W/m2) Partial pressure (N/m2) Wind velocity (m/s) Heat transfer (W) Rayleigh number Nusselt number Acceleration due to gravity (9.81 m/s2)

Ac ce pt e

d

PCM Phase change material Greek symbols α absorptivity σ Stefen Boltzman Constant (5.67 x 10-8 W/m2K4) ε emissivity θ Inclination angle (degrees) Subscripts a air atm atmosphere b basin c convection e evaporation f flow rate w water g glass r radiation in Inlet

M

A C h hfg I p U Q Rah Nuh g

References Abdullah, A. S., 2013. Improving the performance of stepped solar still. Desalination. 319, 60-65. Ahsan, A., M. Imteaz, U. A. Thomas, M. Azmi, A. Rahman, and NN Nik Daud., 2014. Parameters affecting the performance of a low cost solar still. Applied energy. 114, 924-930. Ahsan, Amimul, A. Rahman, A. Shanableh, N. N. Nik Daud, T. A. Mohammed, and A. N. A. Mabrouk., 2013. Life cycle cost analysis of a sustainable solar water distillation technique. Desalination and Water Treatment 51, no. 40-42: 7412-7419.

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Ahsan, Amimul, ABM Sharif Hossain, Abdul Halim Ghazali, Monzur A. Imteaz, and Zahangir Alam., 2011. Evaporation Phenomenon Inside a Solar Still: From Water Surface to Humid Air. INTECH Open Access Publisher. Ahsan, Amimul, and Teruyuki Fukuhara., 2010.Condensation mass transfer in unsaturated humid air inside tubular solar still. J Hydrosci Hydraul Eng. 28 : 1. AHSAN, Amimul, and Teruyuki FUKUHARA., 2008. Evaporative mass transfer in tubular solar still. Journal of hydroscience and hydraulic engineering. 26(2): 15-25.

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Ahsan, Amimul, and Teruyuki Fukuhara., 2010a. Mass and heat transfer model of tubular solar still. Solar energy. 84(7): 1147-1156.

cr

Ahsan, Amimul, Kh M. Shafiul Islam, Teruyuki Fukuhara, and Abdul Halim Ghazali., 2010b. Experimental study on evaporation, condensation and production of a new tubular solar still. Desalination. 260(1): 172-179.

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Ahsan, Amimul, Monzur Imteaz, Ataur Rahman, Badronnisa Yusuf, and T. Fukuhara., 2012. Design, fabrication and performance analysis of an improved solar still. Desalination. 292: 105112.

an

Ahsan, Amimul, Monzur Imteaz, Rahul Dev, and Hassan A. Arafat., 2013a. Numerical models of solar distillation device: Present and previous. Desalination. 311: 173-181.

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Amimul, Ahsan., 2009. Production Model of New Tubular Solar Still and Its Productivity Characteristics. Anburaj, P., R. Samuel Hansen, and K. Kalidasa Murugavel., 2013. Performance of an inclined solar still with rectangular grooves and ridges. Applied Solar Energy. 49(1): 22-26.

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Arunkumar, T., K. Vinothkumar, Amimul Ahsan, R. Jayaprakash, and Sanjay Kumar., 2012. Experimental study on various solar still designs. ISRN Renewable Energy. Arunkumar, T., R. Jayaprakash, Amimul Ahsan, D. Denkenberger, and M. S. Okundamiya., 2013. Effect of water and air flow on concentric tubular solar water desalting system. Applied Energy. 103: 109-115. Arunkumar, T., R. Jayaprakash, D. Denkenberger, Amimul Ahsan, M. S. Okundamiya, Hiroshi Tanaka, and H. Ş. Aybar., 2012a. An experimental study on a hemispherical solar still. Desalination. 286: 342-348. Arunkumar, Thirugnanasambantham, D. Denkenberger, Amimul Ahsan, and R. Jayaprakash., 2013a. The augmentation of distillate yield by using concentrator coupled solar still with phase change material. Desalination. 314: 189-192. Arunkumar, T., David Denkenberger, R. Velraj, Ravishankar Sathyamurthy, Hiroshi Tanaka, and K. Vinothkumar., 2015. Experimental study on a parabolic concentrator assisted solar desalting system. Energy Conversion and Management. 105: 665-674. Aybar, Hikmet Ş., 2006. Mathematical modeling of an inclined solar water distillation system. Desalination. 190(1): 63-70. Aybar, Hikmet Ş., Fuat Egelioğlu, and U. Atikol., 2005. An experimental study on an inclined solar water distillation system. Desalination. 180(1): 285-289. Badran, Ali A., Ihmad A. Al-Hallaq, Imad A. Eyal Salman, and Mohammad Z. Odat., 2005. A solar still augmented with a flat-plate collector. Desalination. 172(3): 227-234. Bapeshwararao, V. S. V., U. Singh, and G. N. Tiwari., 1983. Transient analysis of double basin solar still. Energy Conversion and Management. 23(2): 83-90. Page 26 of 29

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Charters WW., 1997. Solar energy engineering, in solar energy engineering. New York: Academic Press; 105–135 Dashtban, Mohammad, and Farshad Farshchi Tabrizi., 2011. Thermal analysis of a weir-type cascade solar still integrated with PCM storage. Desalination. 279(1): 415-422. Dev, Rahul, and G. N. Tiwari., 2011. Characteristic equation of the inverted absorber solar still. Desalination. 269(1): 67-77.

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Duffie, John A., and William A. Beckman., 1980. Solar engineering of thermal processes. Vol. 3. New York etc.: Wiley, 65

cr

Dunkle RV., 1961. Solar water distillation: the roof type still and a multiple effect diffusion still. In: International Developments in Heat Transfer, Int. Heat Transfer Conference, University of Colorado; p. 895-902 Part 5.

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El-Sebaii, A. A., 2005. Thermal performance of a triple-basin solar still. Desalination. 174(1): 2337.

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El-Sebaii, A. A., A. A. Al-Ghamdi, F. S. Al-Hazmi, and Adel S. Faidah., 2009. Thermal performance of a single basin solar still with PCM as a storage medium. Applied Energy. 86(7): 1187-1195.

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Fukuhara, Takeshi, Hiroaki Terasaki, Takafumi Yamaji, and Amimul Ahsan., 2013. Cost and production performance of a Tubular Solar Still. In Applications of Information Technology to Renewable Energy Processes and Systems (IT-DREPS), 2013 1st International Conference & Exhibition , pp. 9-14. IEEE. Hollands, K. G. T., T. E. Unny, G. D. Raithby, and L. Konicek., 1976. Free convective heat transfer across inclined air layers. Journal of Heat Transfer. 98(2): 189-193.

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