a review of desalination by solar still

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23-25 February 2006 NATO ARW Hammamet. Tunisia 1 A Review of Desalination by Solar Still Hikmet S. Aybar Department of Mechanical Engineering Eastern Mediterranean University G.Magosa, KKTC, Mersin 10 Turkey

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Page 1: A Review of Desalination by Solar Still

23-25 February 2006 NATO ARW Hammamet. Tunisia1

A Review of Desalinationby Solar Still

Hikmet S. AybarDepartment of Mechanical EngineeringEastern Mediterranean UniversityG.Magosa, KKTC, Mersin 10 Turkey

Page 2: A Review of Desalination by Solar Still

23-25 February 2006 NATO ARW Hammamet. Tunisia2

Introduction

Distillation is one of many processes available for obtaining fresh water from salty, brackish or contaminated water; and sunlight is one of several forms of heat energy that can be used to power that process. The aim of this study is to present basic principle of distillation by solar still, type of solar still systems, and the recent developments on the solar still systems.

Page 3: A Review of Desalination by Solar Still

23-25 February 2006 NATO ARW Hammamet. Tunisia3

Distillation by Solar Still

Solar still is a device to distillate impure water like brackish or saline water. It is a simple device to get potable/fresh distilled water from impure water, using solar energy as fuel.

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23-25 February 2006 NATO ARW Hammamet. Tunisia4

Principle of Solar Still

Page 5: A Review of Desalination by Solar Still

23-25 February 2006 NATO ARW Hammamet. Tunisia5

Water Quality

In principle, the water from a solar still should be quite pure.The slow distillation process allows only pure water to evaporate from the pan and collect on the cover, leaving all particulate contaminants behind.

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23-25 February 2006 NATO ARW Hammamet. Tunisia6

Types of Solar Still

The solar distillation systems are classified into two groups in term of energy supply:Passive Solar Stills: The passive solar still systems are conventional solar still systems which use solar energy as solely source of thermal energy.Active Solar Stills: In the active solar stills, an extra thermal energy is given to the passive solar still for faster evaporation.

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Types of Solar Still

In terms of structure of the conventional solar still, different types of the basin-type solar still systems can be found in the literature:Single-slope double-basin solar still (Al-Karaghouliand Alnaser, 2004),Single-slop triple-basin solar still (El-Sebaii, 2005),Pyramid-shaped solar still (Fath et al. 2003b),Double-slope single-basin solar still (Al-Hayek and Badran, 2004).

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Types of Solar Still

There are some other interesting solar still designs:Vertical Solar Still (Boukar and Harmin, 2005) which has vertical single-wick (holding the feed water) and vertical condensing surface.Vertical Solar Still with multiple vertical-wick and single vertical condensing surface with flat-plate reflector (Tanaka and Nakatake, 2005).

Page 9: A Review of Desalination by Solar Still

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Types of Solar Still

Inclined Solar Distillation System (Aybar et al. 2005) in which water flows down on the inclined absorber plate (bare plate or covered with black wick) and evaporated water condenses on the inclined glass cover.Capillary Film Solar Still system (Bouchekimaet al., 2001).Basin-type Solar Still (El-Bahi and Inan, 1999) which is coupled with a condenser to increase the condensation rate.

Page 10: A Review of Desalination by Solar Still

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Modeling of Solar Still

Any type of solar still systems can be modeled using energy balance and mass balance equations for the system.Since the main energy source is solar intensity which depends on the time during the day, the basic energy and mass balance equations must be time dependent.

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Modeling of Solar Still

In the modeling of the solar still, the most important parameters are:Convection heat transfer coefficient to estimate heat transfer from the glass,Convection heat transfer coefficient to estimate heat losses from the bottom and sides of the cavity,Evaporation rate (or evaporation coefficient), andCondensation rate (or condensation coefficient).

Page 12: A Review of Desalination by Solar Still

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Performance of Solar Still

The thermal instantaneous efficiency of a solar still is defined (Tiwari, 2002) as

The production rate performance (PRP) of absorber plate as (kg of distilled water per m2/ kJ solar energy per m2)

( ))()( tI

TThtI

q gwewewi

−==

∑∑

∆⋅

∆⋅=

tItm

PRP i&

Page 13: A Review of Desalination by Solar Still

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Conclusion

Solar still is most simple device to get potable/fresh distilled water from impure water using solar energy as fuel.Researchers have modified the conventional solar still system for better performance, such as multi-basin, multi-slop solar still systems, and coupled with solar collector to increase the water temperature.Especially, solar stills look like best choice to obtain fresh drinkable water in the remote areas for domestic usage.However, still the rate of distilled water production of the solar stills is low which is about 4-7 liters/day per meter square.

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23-25 February 2006 NATO ARW Hammamet. Tunisia14

An Application:Inclined Solar Water Distillation System

Experimental and mathematical modeling studies have been performed on Inclined Solar Water Distillation System.Inclined Solar Water Distillation System generates fresh water and hot water at the same time.

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System Description

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Experimental Study

In the experimental study, three cases of the absorber plate have been tested:Bare absorber plate,Black-cloth wick,Black-fleece wick.

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Results of Experimental Study

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Results of Experimental Study

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Description of Thermal Processes

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Simulation Parameters

Parameter Symbol Value

Mass of absorber plate Mp (kg/m2) 7.9

Specific heat of absorber plate Cp (J/kg K) 477

Absorptivity of absorber plate α 0.96

Emissivity of absorber plate εp 0.08

Mass of glass Mg (kg/m2) 2.7

Specific heat of glass Cg (J/kg K) 800

Transmissivity of glass τ 0.88

Emissivity of glass εg 0.98

Density of water ρw (kg/m3) 989

Specific heat of water Cw (J/kg K) 4184

Mass of air Ma (kg) 0.24

Latent heat of vaporization hfg (J/kg) 2400×103

Convection heat transfer coefficient hc,p-w (W/m2 °C) 40

Convection heat transfer coefficient hc,g-a (W/m2 °C) 40

Page 21: A Review of Desalination by Solar Still

23-25 February 2006 NATO ARW Hammamet. Tunisia21

Changing of plate, water exit, and glass temperatures within time with constant solar intensity and constant air temperature.

0 1000 2000 3000 4000

Time (s)

25

35

45

55

65

T (°

C)

T PlateT WaterT Glass

Page 22: A Review of Desalination by Solar Still

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Changing of evaporation and condensation rates, and relative humidity within time with constant solar intensity and constant air temperature.

0 1000 2000 3000 4000

Time (s)

0.00000

0.00002

0.00004

0.00006

0.00008

0.00010

0.00012

0.00014

0.00016

0.00018

Mas

s Fl

ow R

ate

(kg/

s)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Rel

ativ

e H

umid

ity

EvaporationCondensationRH

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Changing of plate, water exit, and glass temperatures within time with variable solar intensity and air temperature.

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Time (hours)

20

30

40

50

60

70

80

90Te

mpe

ratu

re (°

C)

0

100

200

300

400

500

600

700

800

900

Sol

ar In

tens

ity (W

/m2 )

Solar IntT AirT PlateT WaterT Glass

12:00 Noon 5:00 PM