ةزغ ةيملاسلإا ةعماجلا · III ABSTRACT Solar still is a simple solar device used...

الجامعة اإلسالمية غزة

عمادة الدراسات العليا

كلية الهندسة

قسم الهندسة المدنية

البنية التحتية

Islamic University of Gaza

High Studies Deanship

Faculty of Engineering

Civil Engineering Department

Infrastructure

Modified stepped solarstill for brackish water

desalination

تعديالت مقترحة على المقطر الشمسي ذي العتبات لتحلية المياه المالحة

Prepared by

Ismail Amin Abu Hassaneen

Supervised by

Dr. FahidKh. Rabah

A Thesis Submitted In Partial Fulfillment of The Requirements for The Degree

of Master of Science In Civil-Infrastructure.

October 2015

II

"بسم هللا الرحمن الرحيم"

وَ مَحْيَايَ وَ مَمَاتِي لِلَّه رَبْ ونُسُكِيقُلْ إِنَّ صَالَتِي

الْعَالَمِنيَ

(261سورة األنعام )

III

ABSTRACT

Solar still is a simple solar device used for converting the available brackish or waste

water into potable water. This device has many advantages like, easily fabricated from

locally available materials and cheap maintenance with low skilled labor. A lot of works

were undertaken to improve the productivity of the still. Throughout the review on solar

still performance, the results indicated that, the basin water depth is considered one of

the main parameter that affects the still performance. Also the research showed that;

the solar still cover with inclination equal to latitude angle receives sun rays close to

normal sun rays throughout the year. The still productivity also increases with

decreasing the cover thickness and increasing its thermal conductivity. The still basin

material plays an important role in improving the productivity of the still. The research

also showed that, the daily production of still was greatly enhanced by using sponge

cubes, fins and stepped. The coupling of a solar collector, hot water tank, glass cover

cooling, external reflector, internal condenser and internal reflector increased the

productivity. Finally; from the previous efforts it was clear that; Maximum distillate

water achieved in the still was 6.670 L/(13 hrs (7:00 am to 20:00 pm))) by using stepped

solar still with pre-heating water and glass cover cooling.

In conclusion, this system proved to be promising and can be future developed to

achieve better results as well as Gaza Strip suffers from electrical energy shortage and

provide desalinated water with less price during catastrophes, wars and remote areas

with low population.

Key words: Solar still, desalination, stepped solar still, brackish water.

IV

الدراسةملخص

تعد الخلية الشمسية وسيلة بسيطة تستخدم لتحويل المياه الجوفية والعادمة الى مياه مقطرة، وهذه الطريقة

لها مزايا عديدة حيث انه يسهل تصنيعها من مواد متوفرة محليا، كما انه يسهل صيانتها مع قليل من الجهد.

هذه الخلية الشمسية، واظهرت أداءل متابعة من خالل هذا البحث قمت ببعض االعمال والتحسينات من خال

لية الخ إنتاجيةالنتائج ان عمق الماء يعد واحد من اهم المؤثرات التي تعمل على زيادة اإلنتاجية، كما أن

إلنتاجااظهر البحث أن أيضاتتزايد مع تقليل سمك الماء وزيادة التوصيلية الحرارية للغطاء في القاع،

د بشكل كبير باستخدام المقطر الشمسي ذي العتبات والمحاط باإلسفنج والمتصل بجهاز اليومي للخلية يتزاي

التكثيف، كما ان استخدام المقطر الشمسي ذي العتبات المتصل بمزود للمياه الساخنة والغطاء الزجاجي

.اإلنتاجيةأيضاالمبرد والعواكس الداخلية والخارجية تزيد من

صباحا .ساعة من 31لتر/) 676.6الحصول عليه من المياه المقطرة هي الذي تم األقصىوقد كان الحد

مساءا( عبر تسخين الماء المغذى وتبريد طبقة الزجاج الخارجي. 8وحتى

النائية قمناطفي ال هليتم استغالل في الختام أثبت هذا النظام انه واعد ويمكن تطويره لتحقيق نتائج أفضل

الكوارث والحروب. وإدارةطة وفترات انقطاع التيار الكهربائي ذات الكثافة السكانية البسي

V

DEDICATION

This research is dedicated to:

My Father and Mother for their love pray, and

continuous sacrifices…

My dear wifeHelen,myson Al Braa Ben Malek.

To all my brothers Said, Moath, Khaled, Osama

and my sister Enas.

To all my friends and colleagues…

VI

ACKNOWLEDGEMENTS

First, all praises and glory are due to ALLAH for all the bounty and support granted to

me. This work would not be done without God's endless guidance and support.

I would like to take this opportunity to sincerely thank all individuals who have helped

me in this effort. Primarily, I would like to thank my supervisor and mentor Dr.

FahidKh. Rabahfor his unlimited guidance, encouragement, and support. I am really

indebted to this man for his valuable advice and his vision which inspired this research.

Of course, this work could not be accomplished without the help of MEDRC which

was the donor for my Master.

Additionally, I would like to extend my acknowledgement to Mr. Said Nassar Mayer

of Municipality of Deir Al Balah. I forward my special thanks to Eng. Hisham Dirawi

from Municipality of Deir Al Balah, Eng. Alaa Abu Hassaneen from UNRWA and

also for Eng. Ahmad Baraka from Palestinian Water Authority for his support and

encouragement.

I would like to express my grateful appreciation and thanks to everyone who gave me

support to bring this research into reality especially to my friends.

VII

TABLE OF CONTENT

I ................................................................................................................................. إقرار

ABSTRACT ................................................................................................................. III

III .......................................................................................................................... الخالصة

DEDICATION .............................................................................................................. V

ACKNOWLEDGEMENTS ......................................................................................... VI

TABLE OF CONTENT ............................................................................................. VII

LIST OF TABLES .................................................................................................... VIII

LIST OF FIGURES ...................................................................................................... X

LIST OF ABBREVIATIONS ...................................................................................... XI

1INTRODUCTION ....................................................................................................... 1

1.1 Background .............................................................................................................. 1

1.2 Problem Statement ................................................................................................... 2

1.3Justification of the Study .......................................................................................... 3

1.4Aim and Objectives................................................................................................... 3

1.5Research Methodology ............................................................................................. 3

2LITERATURE REVIEW ............................................................................................ 5

2.1 Introduction .............................................................................................................. 5

2.2 The need for solar Desalination ............................................................................... 5

2.3 Direct desalination system ....................................................................................... 5

2.3.1 Solar humidification dehumidification desalination ............................................ 5

2.3.2 Solar chimney ....................................................................................................... 6

2.3.3 Solar still ............................................................................................................... 6

2.3.3.1 Solar still coupled with sponge cubes ................................................................ 7

2.3.3.2 Solar still coupled with sun tracking .................................................................. 7

2.3.3.3 Solar still coupled with flat plate collector ........................................................ 8

2.3.3.4 Solar still coupled with evacuated tube collector ............................................. 8

2.3.3.5 Solar still coupled with internal and external reflector ...................................... 9

2.3.3.6 Solar still coupled with condenser ................................................................... 10

2.4 Indirect desalination systems ................................................................................. 11

2.4.1 Non-membrane processes ................................................................................... 11

2.4.1.1 Solar multi stage flash desalination ................................................................. 11

VIII

2.4.1.2 Solar multi effect distillation ........................................................................... 11

2.4.1.3 Vapor compression desalination ...................................................................... 12

2.4.2 Membrane processes ........................................................................................... 12

2.4.2.1 Solar powered reverse osmosis desalination.................................................... 12

2.4.2.1.1 Solar PV powered RO desalination .............................................................. 12

2.4.2.1.2 Solar thermal powered RO desalination ....................................................... 12

2.4.2.2 Solar powered electro dialysis (ED) ................................................................ 13

2.5Techniques used to improve the performance of the solar still .............................. 13

2.5.1Basin construction materials ................................................................................ 13

2.5.2Insulation.............................................................................................................. 13

2.5.3 Vacuum technology ............................................................................................ 14

2.5.4 Wick .................................................................................................................... 14

2.5.5 Glass cover cooling ............................................................................................. 14

2.5.6 Inclination of cover ............................................................................................. 15

2.5.7 Different trays depth, width and shape ............................................................... 15

2.6 Heat transfer mechanisms in a solar still ........................................................... 16

2.7 Modes of heat transfer in a solar still ................................................................. 17

2.7.1 Internal heat transfer ................................................................................... 17

2.7.1.1 Convection heat transfer ..................................................................... .17

2.7.1.1.1 Radiation heat transfer .................................................................. 18

2.7.1.1.2 Evaporation heat transfer ............................................................. .19

2.7.1.1.3 External heat transfer .................................................................... 20

2.7.1.2 Top loss heat transfer. .......................................................................... 20

2.7.1.2.1 Bottom and side loss heat transfer ............................................... .21

2.7.1.2.2 Calculation of yield and thermal efficiency .................................. 23

2.7.1.3 Energy balance ..................................................................................... 23

2.8 Monthly optimum inclination of glass cover with internal and top external

reflector ........................................................................................................................ 24

2.9 Monthly optimum inclination of glass cover with bottom external reflector .... 26

2.10 Comparative study ............................................................................................ 27

3 MATERIALE AND METHOD ................................................................................ 29

3.1 Introduction ........................................................................................................ 29

3.2 Materials ............................................................................................................ 29

IX

3.2.1 Apparatus .................................................................................................... 29

3.2.1.1 Fabricated Conventional solar still ...................................................... 29

3.2.1.2 Fabricated Stepped solar still ............................................................... 30

3.2.3 Evacuated solar water heater ...................................................................... 31

3.2.4 Temperature device ..................................................................................... 32

3.2.5 Glass cover .................................................................................................. 33

3.2.6 The brackish water tanks ............................................................................ 33

3.3 Physical-chemical properties of inlet water sample ........................................... 33

3.4 Method ............................................................................................................... 34

3.4.1 Experimental Design ................................................................................... 34

3.4.2Analytical Method ....................................................................................... 40

3.4.3water quality................................................................................................. 41

4RESULTS AND DISCUSSION ................................................................................ 43

4.1 Effect of using conventional and steeped solar still without modification on the

performance of solar still ............................................................................................. 43

4.2 Effect of using different water depth on steeped solar still ............................... 44

4.3 Effect of using internal mirror on steeped solar still.......................................... 45

4.4 Effect of using external and internal mirror on steeped solar still ..................... 46

4.5 Effect of using glass cover cooling on steeped solar still .................................. 47

4.6 Effect of using pre heating water on steeped solar still ..................................... 48

4.7 Effect of using pre heating water with glass cover cooling on steeped solar still

...................................................................................................................................... 49

4.8 Effect of water glass temperature on productivity ............................................. 50

4.9 Accumulated productivity of the stepped solar modification ............................ 51

4.10Cost evaluation.................................................................................................. 52

5 CONCLUSION AND RECOMMENDATION ....................................................... 55

5.1 Conclusions ........................................................................................................ 55

5.2 Recommendations .............................................................................................. 56

References .................................................................................................................... 57

Appendix ...................................................................................................................... 62

X

LIST OF TABLES

Table2.1: Optimum reflector inclination for each glass covers inclination throughout

the year. ..................................................................................................................... 26

Table 2.2: Physico-chemical properties of inlet water sample .................................. 27

Table2.3: Illustrates the method used for the analysis of the required parameter ..... 28

Table 3.1: The results water samples before and after distillation with drinking water

standereds ………………………………………………………….…………..……34

Table 3.2: The results brine samples after one, two, three distillation dayes ............ 40

Table 3.3: The cost for the best fabricated Stepped still per m2 ............................... 53

Table A.1: Calculation Results Sheet No. 1 of Appendex A. .................................. 59









XI Page MS.c Thesis- I. Abu Hassaneen

LIST OF FIGURES

Figure 1.1: Cost of RO desalination Plant. .................................................................... 2

Figure 1.2:Annual variation in solar radiation in the Gaza strip.................................... 3

Figure 1.3: Research methodology steps ....................................................................... 4

Figure 2.1: Solar still with sponge. ................................................................................ 7

Figure 2.2: Schematic diagram of a simple sun tracking mechanism............................ 8

Figure 2.3:Solar still coupled with flat plate collector. .................................................. 8

Figure 2.4:Schematic diagram of evacuated tube collector. .......................................... 9

Figure 2.5:Reflected sunrays from vertical and inclined external reflectors on the

basin liner in winter. ...................................................................................................... 9

Figure 2.6: Reflected sunrays from vertical and inclined external reflectors onthe

basin liner in summer. .................................................................................................. 10

Figure 2.7: Solar still with external condenser. ........................................................... 10

Figure 2.8:Single wick still .......................................................................................... 14

Figure 2.9: The stepped solar still with film cooling ................................................... 14

Figure 2.10:(a) surface of flat type solar still, (b) absorber surface of convex type solar

still, (c) absorber surface of concave type solar still. ................................................... 15

Figure 2.11:Schematic of energy flow in a single basin single slope solar still. ......... 16

Figure 2.12:Daily amount of distillate of NRS varying with glass cover Inclination

throughout the year at 30_N latitude ........................................................................... 24

Figure 2.13:Daily amount of distillate of IS varying with glass cover Inclination

throughout the year at 30_N latitude ........................................................................... 24

Figure 2.14:Daily amount of distillate ......................................................................... 25

Figure 3.1:locally fabricated Conventional still........................................................... 29

Figure 3.2:locally fabricated Stepped solar still .......................................................... 30

Figure 3.3: tray of fabricated Stepped solar still .......................................................... 30

Figure 3.4:Evacuated solar water heater ...................................................................... 31

Figure 3.5: (LM35) temperature device ....................................................................... 31

Figure 3.6:water tanks .................................................................................................. 32

Figure 3.7:Sketch for first set which done on 20/07/2015 ........................................... 33

Figure 3.8:Sketch for second set which done on 20-23-25/07/2015 ........................... 34

Figure 3.9:Sketch for third set which done on 25/07/2015 and 05/08/2015 ................ 35

Figure 3.10:Sketch for fourth set which done on 25/07/2015 and 29/08/2015 ........... 36

Figure 3.11:Sketch for fifth set which done on 25/07/2015and 07/08/2015................37

XII Page MS.c Thesis- I. Abu Hassaneen

Figure 3.12:Sketch for sixth set which done on 25/07/2015 and 02/08/2015............38

Figure 3.13:Sketch for seventh set which done on 25/07/2015and 09/08/2015 .......... 38

Figure 4.1:Produced distillate water of conventional and stepped solar still .............. 43

Figure 4.2:Produced distillate water of different stepped solar still depth .................. 44

Figure 4.3:Produced distillate water with and without internal mirror on stepped solar

still................................................................................................................................ 45

Figure 4.4: Produced distillate water with and without internal and external mirror on

stepped solar still.......................................................................................................... 46

Figure 4.5:Produced distillate water with and without glass cover cooling on stepped

solar still ....................................................................................................................... 47

Figure 4.6: Produced distillate water with and without pre heating water on stepped

solar still ....................................................................................................................... 48

Figure 4.7: Produced distillate water with using pre heating water and glass cover

cooling on stepped solar still........................................................................................ 49

Figure 4.8: Effect of water–glass temperature difference on productivity .................. 50

Figure 4.9:Effect of water–glass temperature difference on productivity. .................. 51

Figure 4.10:Cumulative variation of fresh water productivity per unit area for all

experiments. ................................................................................................................. 52

Figure 4.11:The average cost of distillated water for different types of solar still and

my best distilled project. .............................................................................................. 54

XIII Page MS.c Thesis- I. Abu Hassaneen

Nomenclature: Symbols 𝐴 Area.

𝐴𝑎 Aperture area of solar collector.

Ab Area of basin of the solar still.

𝐴𝑐 Area of solar collector.

𝑏 Width of the still.

𝑑𝑡 Time interval.

εeff Effective emittance between water mass and glass cover.

ℎ Heat transfercoefficient.

hb The heat transfer coefficient between basin liner and the atmosphere

through the insulation.

hc,w−gi Convective heat transfercoefficient betweenwatermassandglasscover

innersurface.

he,w−giEvaporative heat transfer coefficient between water mass and glass cover

inner surface.

ℎ𝑡,𝑤−𝑔𝑖The total internal heat transfer coefficient between water mass and glass

cover inner surface.

ht,go−aThe total top heat loss coefficient between glass cover outer surface and

atmosphere.

hr,go−aThe radiative heat transfer coefficient between glass cover outer surface

and the surrounding .

hr,w−giRadiative heat transfer coefficient between water mass and glass cover inner

surface.

hw The convective heat transfer coefficient from basin liner to the water.

𝐼(𝑡) Intensity of solar radiation.

𝐼𝑒𝑓𝑓 Effective solar radiation.

𝑘 Thermal conductivity.

𝑘𝑓 Thermal conductivity of humid air.

𝑙𝑏 Length of basin.

𝑙𝑚 Height ofexternalreflector.

𝑙𝑠 The step length.

𝑚 Mass per unit basin area.

XIV Page MS.c Thesis- I. Abu Hassaneen

𝑚𝑒𝑤 Hourly yield from solar still.

𝑃 Saturated partial pressure.

𝑃𝑎 Atmospheric pressure.

𝑃𝑑 Partial pressure of vapor at dew point temperature.

𝑃𝑤 Partial vapor pressure at water surface temperature.

qb The rate of conduction heat transfer between basin liner and the atmosphere.

qc,w−g Convective heat transfer rate inside the solar still.

qcd,gi−goThe rate of conductive heat transfer from glass cover inner surface to the

glass cover outer surface.

qc,go−aThe convection heat loss from glass cover outer surface of the solar still to the

Atmosphere.

qe,w−giThe rate of evaporative heat transfer between water mass and glass cover

inner surface.

qt,go−aThe total top heat loss is the summation of convective and radiative heat

Losses.

qr,go−aThe radiation heat loss from glass cover outer surface of the solar still to the

Surroundings.

qr,w−gi Radiative heat transfer rate between water and glass cover inner surface.

qw The rate of convective heat transfer between basin liner and the water mass.

Qloss The heat losses by convection through the basin base and sides to the ground

and surrounding.

t Time.

𝑇 Temperature.

Ta Ambient temperature.

Tb Basin liner temperature.

Tw Water temperature.

Tgi Glass cover inner surface temperature.

Tg out Glass cover outer surface temperature.

Ub The overall bottom heat loss coefficient between water mass and atmosphere.

Ubs The total bottom and side heat loss coefficient from water mass to atmosphere.

Uss The overall side heat loss coefficient between water mass and atmosphere.

XV Page MS.c Thesis- I. Abu Hassaneen

𝑤 The tray width.

Greek

𝛼 Solar altitude angle.

𝛾 Solar azimuth angle.

𝜃𝑠 Tilt angle of the glass cover.

σ Stefan Boltzmann constant.

εEmissivity.

ΔTTemperature difference.

Abbreviations ED Electro dialysis.

ETCs Evacuated tube collectors.

FPCs Flate plate collectors.

SP Solar pond.

MED Multi effect distillation.

MVC Mechanical vapor compressor.

RO Reverse osmosis.

WHO World Health Organization.

NRS Internal nor the external reflectors.

ISOne with an internal reflector only.

3 Page MS.c Thesis- I. Abu Hassaneen

1INTRODUCTION

1.1 Background

Water is essential to sustain life, and a satisfactory (adequate, safe and accessible)

supply must be available to all. Improving access to safe drinking water can result in

tangible benefits to health. Every effort should be made to achieve a drinking water

quality as safe as practicable. Safe drinking water, as defined by the World Health

Organization (WHO) standard, does not represent any significant risk to health over a

lifetime of consumption, including different sensitivities that may occur between life

stages. Access to safe water represents one of the most important basic human needs of

the Palestinian people and is vital to a growing economy and a healthy population. The

quality of the pumped groundwater is the main concern ( WHO, 2008).

Desalination, Vapor compression, Reverse osmosis (RO) and Electro dialysis (ED) are

being used to provide freshwater from saline water. But the cost of energy consumption

of these methods is high. On the other hand availability of energy in remote areas and

most arid regions is low. Solar desalination is a solution for these problems ( Esfahani et

al., 2011).

Stepped solar still is an eco-friendly, small scale, cheap equipment which utilizes the

natural solar energy and is the best solution to purify water. Potable water can be

produced by the solar still at a reasonable cost. Solar desalination is the solution for

purifying the impure water in remote locations. It is the suitable method to purify water

where there is only saline water and ample amount of solar energy is available ( Ayoub

et al., 2014).

The various factors affecting the productivity of Stepped solar still are solar intensity,

wind velocity, ambient temperature, water glass temperature difference, free surface

area of water, absorber plate area, temperature of inlet water, glass angle and depth of

water. The solar intensity, wind velocity, ambient temperature cannot be controlled as

they are metrological parameters. Whereas the remaining parameters can be varied to

enhance the productivity of the solar stills ( Omara et al., 2014).


The high incident solar radiation in the Gaza strip encourages the local manufacture

and vendor to start deal with solar desalination technology. Therefore, this paper

presents the testing results of an attempt to design and manufacture a Stepped solar still

connect with Evacuated tube collectors (ETCs), internal and external reflectors with

optimal inclination to get high productivity of distilled water.

1.2 Problem Statement

Water scarcity in the Gaza Strip forms a real crisis for people in this area. For the Gaza

strip the only source of natural freshwater is the coastal aquifer, which suffer from rapid

decline in both quality and quantity.

To face this problem, the citizens and the authority in the Gaza Strips use many options;

the major of these options is water desalination where the Reverse osmosis (RO)

technology, which needs electricity, is applied.

Energy cost in desalination plants comprises about 30% to 50% of the total cost of the

produced water based on the type of energy used. Therefore, the total cost of

desalination can be reduced significantly by reducing the energy consumption. The

following figure 1.1 shows approximate vision about the cost details for any RO plant

(Ghali et al., 2010).

Figure 1.1 Cost of RO desalination Plant (Ghali et al., 2010).

This system proved to be promising and can be future developed to achieve better

results as well as Gaza Strip suffers from electrical energy shortage and provide

desalinated water with less price during catastrophes, wars and remote areas with low

population.


1.3Justification of the Study

The Gaza strip is semi-arid as well coastal region. Solar insulation in the Gaza strip is

relatively high. The daily average solar radiation on horizontal surface is about 222

W/𝑚2 (7014 MJ/𝑚2/yr). The following Figure 1.2 illustrates the variation in the daily

average, in the total insulation on horizontal surface for each month (Alaydi,2011).

Figure 1.2 Annual variation in solar radiation in the Gaza strip ( Alaydi, 2011).

Therefore we intended to utilize the natural sources of brackish water, sea water and

high solar radiation in development of appropriate, household-scale water purification

technology.

1.4Aim and Objectives

This study aims to make a development of small scale solar desalination technology in

the Gaza strip, by using local market materials to enhance the productivity of distilled

water to face electricity problems and shortage of good water quality.

The objectives of this research are to investigate the performance of a stepped solar still

by:

Adding wick on the vertical sides.

Supplying preheated water into the solar still.

Using trays with constant depth and width.

Using internal and external reflectors.

Using glass cover cooling.

1.5Research Methodology

Methodology consists of eleven stages. The first stage is the research concept, search

for optimum design of Stepped solar still, this comes from reading literature, paper and

several books. The second stage is to design a Stepped solar still prototype for enough


for household. The third stage is to search in the market for selected material. Forth

stage was to the fabricate the design Stepped solar still, after locally fabrication the

Stepped solar still. Fifth stage is to make primary testing of the fabrication. After

primary testing sixth stage is to add some extra modification were added on the design

and change some of the selected materials. After adding modification and changing

materials, The seven stage is to make primary testing on the locally modified solar still.

The eight stage is to start experiments on the modified Stepped solar still. The ninth

stage was to calculate on the collected data by Excel sheet figures. Tenth stage started

to have results and discussions from Excel sheet figure. Finally stage number Eleven is

to make conclusion and recommendations. Figure 1.3 shows research methodology

steps.

Figure 1.3 Research methodology steps


2 LITERATURE REVIEW

2.1 Introduction

Desalination technologies have been used for about a century in land-based plants and

on ships to provide water for a crew. The regular use of desalination technologies

accelerated after World War II, as the demand for fresh water in arid countries. The

cost for desalination has been decreasing rapidly, especially in recent years with the

introduction of efficient, more cost effective technologies. For solar distillation

systems, sunlight has the advantage of zero fuel cost but it requires more space (for its

collection) and generally more costly equipment. In principle, the water from a solar

still should be quite pure. The slow desalination process allows only pure water to

evaporate from the basin and collect on the cover, leaving all particulate contaminants

behind. A solar still is a simple device, which can be used to convert saline, brackish

water into drinking water. Solar stills use exactly the same processes, which in nature

generate rainfall, namely evaporation and condensation. Its function is very simple a

transparent cover encloses a pan of saline water ( kabeel et at., 2010).

2.2 The need for solar desalination

All the water treatment processes use a large amount of energy to remove a portion of

pure water from a salt water source. Salt water (feed water) is fed into the process, and

the result is one output stream of pure water and another of waste water with a high salt

concentration. Large commercial desalination plants using fossil fuel are in use in a

number of oil-rich countries to supplement the traditional sources of water supply.

Other countries in the world have neither the money nor oil resources to allow them to

develop in a similar manner and because of this energy demand and high cost of plants,

we prefer solar energy for the desalination process ( Murugavel, 2013).

2.3 Direct desalination system

2.3.1 Solar humidification dehumidification desalination

The main idea behind the solar humidification–dehumidification process is that the

moisture carrying capacity of the air increases with the increase in temperature. When

hot air heated by solar collector circulated in natural or forced mode comes in contact

with saline water which is sprayed in the evaporator, a certain quantity of vapor is


extracted by the air which could be recovered by condenser where saline feed water is

preheated. Four types of humidification–dehumidification desalination configurations

are closed air, open water cycle; closed air, closed water cycle; open air, open water

cycle and open air, closed water cycle. There always exists an optimum mass flow rate

ratio of water to dry air for maximum thermal energy recovery rate for a given spray

water temperature and condenser water temperature and the thermal energy recovery

rate could be increased by increasing the number of stages ( Sharon and Reddy, 2015).

2.3.2 Solar chimney

Solar chimney converts solar thermal energy into kinetic energy which in turn is

converted into electrical energy using turbo generator. The main components of solar

chimney are large diameter solar collectors, turbine, generator and long chimney.

Collectors used are mainly glass or plastic sheet which act as greenhouse, trapping heat

and causes the earth below the collector to get warmed up resulting in temperature

difference between the ambient air and the air inside the system causing heated air to

flow through the chimney. The kinetic energy of the moving air causes rotation of

turbine mounted below the chimney to produce power ( Sangi, 2012).

2.3.3 Solar still

The simple solar still is the oldest and most basic, low-tech desalination system

currently in use, and many improvements have been suggested over the years to

improve its efficiency. In essence, solar stills mimic the natural distillation process off

the hydrological cycle that generates rainfall: evaporation and condensation. In all solar

stills, a transparent cover (typically glass or plastic) encloses a basin of saline water. As

the sun shines through the glass, water heats up to a boil, causing evaporation and

condensation on the inner surface of the transparent cover. The distillate produced is of

very high quality, as all salts and other inorganic and organic components remain in the

basin, and pathogenic bacteria are killed in the boiling process. Modifications of the

solar still, including the basin still, wick still, and diffusion still, increase the thermal

efficiency of a simple still by at most 50%, and so were also regarded as impractical for

the purpose of this project ( Qiblawey and Banat , 2008).

2.3.3.1 Solar still coupled with sponge cubes

A solar pond (SP) is a thermal solar collector that includes its own storage system. A

solar pond collects solar energy by absorbing direct and diffuse sunlight. Therefore,

. Page MS.c Thesis- I. Abu Hassaneen

sponge cubes in the saline water was used to improve the evaporation rate as shown in

Figure 2.1 ( Velmurugana and Sritharb , 2007).

Figure 2.1 Solar still with sponge ( Velmurugana and Sritharb , 2007).

2.3.3.2 Solar still coupled with sun tracking

A Sun Tracking mechanism is a device incorporated into a solar still which follows the

movement of the sun across the sky with the aim of ensuring that maximum solar

irradiance is transmitted through the glass cover of the still into the basin and is

absorbed by the brine from sunrise to sunset, throughout the day as shown in figure 2.2

,The sun tracking mechanism is grouped into two types, the single axis and the double

axis models. The single axis is usually on a horizontal axle or vertical axle depending

on the region of use and application. The horizontal is used in the tropics where the sun

is very high at midday, but with shorter days while the vertical is used in high latitudes

where the sun is slightly high, but with very long summer days. The double axis sun

tracking mechanism has both a horizontal and vertical axle so, can be deployed

anywhere in the world

Figure 2.2A schematic diagram of a simple sun tracking mechanism


2.3.3.3 Solar still coupled with flat plate collector

Flat-plate collectors (FPCs) are used as heat transfer fluid, which circulates through

absorber pipes made of either metal or plastic. The absorber pipes are assembled on a

flat plate and they usually have a transparent protective surface in order to minimize

heat losses. FPCs is integrated with the solar still, to increase the temperature of the

basin water. The preheated water from the solar collector was circulated by a tube

through the basin water. The tube is acting as a heat exchanger and exchanging heat

from the preheated water to the basin water. Thus the basin water gets heated. In an

another arrangement, as shown in Figure 2.3, the basin water is directly circulated

through the flat plate collector ( Qiblawey and Banat, 2008).

Figure 2.3 Solar still coupled with flat plate collector ( Qiblawey and Banat, 2008).

2.3.3.4 Solar still coupled with evacuated tube collector

Heat losses are minimized in evacuated tube collectors (ETCs) by an evacuated cover

of the receiver. This cover is tubular and made of glass. In addition, a selective coating

of the receiver minimizes the losses due to infrared radiation. There are two different

technologies of evacuated tubes: (1) Dewar tubes two coaxial tubes made of glass,

which are sealed each other at both ends; and (2) ETC with a metallic receiver, which

requires a glass to metal seal ( Kalogirou, 2005).

The evacuated tube solar collector (ETC) has more advantageous than the flat plate

collectors for water heating purposes. The evacuated tubes greatly reduce the heat

losses as vacuum is present in the tubes as shown in Figure 2.4 , It consists of two

coaxial tubes with evacuated space between an outer surface of inner tube and inner

surface of outer tube. A selective coating is applied to the outer surface of the inner

tube. The heat transfer fluid enters through small diameter delivery glass tube and exits


from the same end of the tube through annular space between delivery tube and

selective coated absorber tube. The annular space between selectively coated tube and

borosilicate outermost glass tube is evacuated to minimize the convection loss from the

selective surface ( Sampathkumar et at., 2010).

Figure 2.4 Schematic diagram of evacuated tube collector ( Sampathkumar et at., 2010).

2.3.3.5 Solar still coupled with internal and external reflector

Internal and external reflectors can increase the distillate productivity of solar stills,

useful and inexpensive modification to increase the distillate productivity of solar stills.

Because of the usage of reflectors, more solar radiation is introduced into the still

compared to other solar stills and thus increment in the daily productivity ( Omara et al.,

2014).

In winter, the altitude angle of the sun decreases so a consider able amount of the

reflected radiation from a vertical external reflector would escape to the ground without

hitting the basin liner. Therefore, the reflector should be inclined slightly forward to

absorb the reflected sunrays on the basin liner effectively as shown in Figure 2.5 the

reflector in winter ( Tanaka , 2010).

Figure 2.5 Reflected sunrays from vertical and inclined external reflectors on

the basin liner in winter (Tanaka , 2010).


On the other hand, the altitude angle of the sun increases in summer and the vertical

external reflector cannot effectively reflect sunrays to the basin liner. So the external

reflector should be inclined slightly toward the back as shown in Figure 2.6 reflector in

summer( Tanaka , 2010).

Figure 2.6 Reflected sunrays from vertical and inclined external reflectors on

the basin liner in summer ( Tanaka , 2010).

2.3.3.6 Solar still coupled with condenser

Condenser is attached with a solar still as shown in Figure 2.7 to enhance the

productivity of the solar still. The condensation occurs due to the temperature

difference not only on the glass surface but also on the four sidewalls, which can be

cooled by water circulation through tubes attached on the wall surface for efficiency

enhancement ( Velmurugan and Srithar , 2011).

Figure 2.7 Solar still with external condenser ( Velmurugan and Srithar , 2011).

2.4 Indirect desalination systems

2.4.1 Non-membrane processes

In non-membrane desalination process, the heated feed water is allowed to evaporate

in distillation units to produce water vapors which are condensed using condenser to

produce distillate. The distillate produced is of high quality and the rate of distillation


can be enhanced by incorporating vacuum. In this process temperature of feed water,

condensing surface and pressure plays an important role in distillate yield.

2.4.1.1 Solar multi stage flash desalination

In multi stage flash desalination system, the feed saline water is heated above the

saturation temperature in brine heater and is made to flash in the vessel where low

pressure is maintained using vacuum pump. The brined is charged from the previous

stage is allowed to flash in successive stages and the vapors formed in each stage is

condensed using condenser where inlet saline water is preheated

2.4.1.2 Solar multi effect distillation

Multi effect distillation (MED) unit consists of vessels which are generally called

effects maintained successively at low pressure where saline water is sprayed. The heat

required to cause evaporation in first effect is supplied by solar energy or by combustion

of fossil fuel and the vapors thus formed are used to heat the feed in the next effect.

Thus, the latent heat of the produced vapors in the previous effects are successfully

utilized for the next effect in MED. MED systems are gaining more market share

because of its better compatibility with solar thermal desalination ( Mezher et. at, 2011).

2.4.1.3 Vapor compression desalination

In vapor compression desalination, the feed saline water heated by external heat source

is allowed to flash, the vapors thus produced are compressed using mechanical vapor

compressor (MVC) or thermo vapor compressor (TVC) to raise the condensation

pressure and temperature of the vapor and the compressed vapor is used to heat the

same stage or feed water of other stages (Sharon and Reddy, 2015).

2.4.2 Membrane processes

In membrane process, fresh water is produced from saline water by allowing passage

of water molecules (in case of reverse osmosis) or ions (in case of electro dialysis)

through membranes by applying high pressure (above osmotic pressure) or electrical

potential.

2.4.2.1 Solar powered reverse osmosis desalination

Reverse osmosis (RO) is a pressure driven desalination process in which pressurized

feed water is allowed to pass through the cross flow membrane module. If the applied

pressure is higher than the osmotic pressure, fresh water permeates across the


membrane and it is collected through the permeate tube and the brine is drained out (Greenlee, 2009).

2.4.2.1.1 Solar PV powered RO desalination

In solar PV powered RO unit, the power required for the desalination process is

supplied by photovoltaic panels and the system can be operated with (or) without

batteries, Solar PV operated reverse osmosis unit has better socio-economic and

environment benefits compared to diesel generator operated reverse osmosis unit (

Eltawil .et al , 2008).

2.4.2.1.2 Solar thermal powered RO desalination

In solar thermal powered RO desalination, the mechanical energy produced by the solar

organic cycles is directly used to run the high pressure pumps of RO unit(Penate, 2012).

The solar thermal driven RO desalination unit is a more promising technology, any

development in RO technology would be useful for developing RO technology based

on solar thermal systems.. The unit cost of water produced by RO plant could be

reduced by using hybrid solar assisted steam cycle for supplying required shaft power

for RO high pressure pump ( Eltawil .et al , 2008).

2.4.2.2 Solar powered electro dialysis (ED)

Electro dialysis (ED) is the process of removal of salts from saline water and the ED

unit consists of large number of compartments filled with saline water and separated by

cation and anion exchange membranes. When DC polarity is applied across the cathode

and anode, the negative ions passes through the anion exchange membrane and positive

ions passes through the cation exchange membranes and these ions gets accumulated

in a particular compartment and is discharged out as brine. Reversal of polarity is

usually followed every 20 min to prevent deposition of salts in the membranes. The

solar powered ED desalination system is suitable for (a) areas having less or no electric

power, (b) areas with no access to low cost fuel supply and

(c) areas with abundant sunshine ( Charcosset, 2009).

2.5Techniques used to improve the performance of the solar still

Researchers have taken efforts to make different designs of solar still for higher

distillate yield and inferred that solar stills are effective and efficient.


2.5.1Basin construction materials

Solar radiation that passes through the transparent cover is absorbed by saline water

and the basin liner of a solar still. So, the basin liner acts as an absorber of solar radiation

and it is important for the liner to have a relatively high absorbance for solar radiation,

basin liners can be made of plastic or metal-sheet. Some plastics are relatively cheap

while others are expensive. Common metal sheets applied in solar collection are copper,

aluminum and steel . The important property of a metal for application in solar

engineering is thermal conductivity. Copper and aluminum have relatively high thermal

conductivities ( Murugavel .et al, 2014).

2.5.2Insulation

Thermal insulation is the simplest way to prevent heat losses and to achieve economy

in energy usage especially in solar thermal systems. Thermal insulation serves many

significant functions such as, to conserve energy, to reduce heat loss or heat gain, to

maintain an efficient operation of the system (or chemical reaction), to assist in

sustaining a product at a constant temperature, to prevent condensation, to create a

comfortable environmental, to protect personnel. Conventional insulation materials are

often opaque and suitably classified into, fibrous, cellular, granular and reflecting types

materials. Some commonly used thermal insulation materials are as; glass, fiber,

alumina silicate, mineral wool and calcium silicate ( Saxena et al , 2015).

2.5.3 Vacuum technology

The effect of vacuum inside the still is to avoid any heat transfer due to convection in

the still. The heat loss from the water in an insulated still is due to evaporation and

radiation only. In the presence of vacuum, the effect of the non-condensable gas, which

reduces the rate of condensation, was also avoided ( Velmurugan and Srithar , 2011).

2.5.4 Wick

Wick still mostly come under inclined type still. In a wick still, the feed water flows

slowly through a porous, radiation-absorbing pad (the wick). Two advantages are

claimed over basin stills. First, the wick still can be tilted so that the feed water presents

a better angle to the sun. It reduces reflection and presents a large effective area. Second,

less feed water is in the still at any time and so the water is heated more quickly and to

a higher temperature. The main disadvantage in this still is while cloud passing or after

sunset, it does not produce distillate. However, in the case of basin still the productivity


continuous for some time due to heat stored in the basin water, the figure 2.8 shows

Single wick still ( Kumar.et al, 2015).

Figure 2.8 Single wick still ( Kumar.et al, 2015).

2.5.5 Glass cover cooling

To increase the performance of the stepped solar still outlet water film cooling is

recycled as a makeup water, as shown in figure 2.9 .It was found that film cooling

thickness, volumetric flow rate, and water film inlet temperature have a significant

effect on the daily distillate productivity. The presence of the glass cover water film

cooling may increase the stepped still daily productivity by about 8.2% but the value of

this percentage mainly depends on the combinations of film cooling parameters. On the

other hand, the presence of the film cooling neutralized the effect of air wind speed on

the still distillate productivity ( Kabeel et at., 2015).

Figure 2.9 The stepped solar still with film cooling ( Kabeel et at., 2015).

2.5.6 Inclination of cover

The yield from a solar still heavily relies on the tilt angle of the solar glass. This angle

in turn depends on inclination and the direction the cover is facing, and also its latitude.

It is expected that covers that has an inclination that is aligned with the angle of the


latitude will be the recipient of a normal solar radiation annually. This is deemed as

important due to the fact that evaporation is reliant on intensity of solar radiation. This

leads to the adjustment of the angle of inclination with respect to the solar azimuth

angle and solar intensity (Muftah et al, 2014).

2.5.7 Different trays depth, width and shape

Depth of water in the solar still inversely affects the productivity of the solar still.

Investigations indicated that a reduction of the brine depth in the still improves the

productivity, mainly due to the higher basin temperature. For maintaining minimum

depth, wicks, plastic water purifier and stepped solar still were used. So that stepped

solar stills can increase the distillate productivity about conventional solar stills, many

reports studied the performance of stepped solar still ( Abdullah , 2013).

The figure 2.9 shown different absorber surface of stepped solar still such absorber

surface of flat type solar still, absorber surface of convex type solar still, absorber

surface of concave type solar still .

Figure 2.10 (a) surface of flat type solar still, (b) absorber surface of convex type solar still,

(c) absorber surface of concave type solar still ( Kabeel et al., 2015).

2.6 Heat transfer mechanisms in a solar still

Generally, a heat transfer process is broadly classified as being either steady or

transient. During steady heat transfer process, the temperature or heat flux remains

unchanged with time, but in transient process these properties are time dependent. Most

of the heat transfer processes encountered in practice are transient in nature. The


transient heat transfer processes are difficult to analyze, but they could be analyzed

based on some presumed steady conditions. The heat transfer in a solar still is

considered as the transient heat transfer process due to the variation in temperature or

heat flux with respect to time. Figure 2.10 shows the energy flow that take place inside

as well as outside of the single basin single slope solar still during the desalination

process ( Elango .et al, 2015).

Figure 2.11 Schematic of energy flow in a single basin single slope solar still.

2.7 Modes of heat transfer in a solar still

The heat transfer process in a solar still can be broadly classified into internal and

external heat transfer processes based on energy flow in and out of the enclosed space.

The internal heat transfer is responsible for the transportation of pure water in the vapor

form leaving behind impurities in the basin itself, whereas the external heat transfer

through the condensing cover is responsible for the condensation of pure vapor as

distillate ( Elango .et al, 2015).

2.7.1 Internal heat transfer

The heat exchange between water surface and glass cover inner surface of the solar

still is known as internal heat transfer. There are three modes, namely convection,

radiation and evaporation processes, by which the internal heat transfer process within

the solar still is governed. These three modes of internal heat transfer process are

described as follows

3. Page MS.c Thesis- I. Abu Hassaneen

2.7.1.1 Convection heat transfer.

Convection heat transfer process is complicated in nature by the fact that it involves

fluid motion as well as heat conduction. The convection heat transfer strongly depends

on fluid properties and geometry and roughness of solid surface involved. In a solar

still, the convection heat transfer takes place between basin water and glass cover inner

surface across humid air due to temperature difference between them ( Elango .et al,

2015).

The convective heat transfer rate inside the solar still can be expressed in terms of water

temperature (Tw) and glass cover inner surface temperature (Tgi) by the following

relation:

qc,w−g = hc,w−gAw(Tw − Tgi) (2. 1)

In the above expression is the convective heat transfer coefficient between water mass

and glass cover inner surface and can be calculated as follows:-

hc,w−gi = 0.884 {(Tw − Tgi) +(Pw−Pgi)(Tw+273.17)

268900−Pw}

1/3

(2. 2)

The saturation vapor pressures at water temperature and glass cover inner surface

temperature are evaluated by the following expressions:-

Pw = exp (25.317 −5144

Tw+273) (2. 3)

Pg = exp (25.317 −5144

Tg+273) (2. 4)

2.7.1.1.1 Radiation heat transfer

The radiation heat transfer occurs through a mechanism that involves the emission of

internal energy of the object. The energy transfer by radiation is the fastest and it suffers

no attenuation in a vacuum. Also, the radiation heat transfer occurs in solids as well as

in liquids and gases. Even it can occur between two bodies separated by a medium

which is colder than both the bodies. The radiative heat transfer occurs at inside of the

solar still between water mass and glass cover inner surface ( Elango .et al, 2015).


The view factor plays a major role in determining the rate of radiative heat transfer. In

solar still, the view factor is assumed as unity since the inclination of glass cover with

horizontal is small.

The radiative heat transfer rate between water and glass cover inner surface can be

obtained by the following relation:-

qr,w−gi = hr,w−gi(TW − Tgi) (2. 5)

where hr,w−gi is the radiative heat transfer coefficient between water mass and glass

cover inner surface and evaluated by:-

hr,w−gi = σεeff{(Tw + 273.15)2 − (Tg + 273.15)2}(Tw + Tg + 546) (2. 6)

The effective emittance between water mass and glass cover is given as:-

εeff = (1

εw+

1

εg− 1)−1 (2. 7)

2.7.1.1.2 Evaporation heat transfer.

Evaporation occurs at the liquid- vapor interface when the vapor pressure is less than

the saturation pressure of the liquid at a given temperature. The evaporation heat

transfer occurs in the solar still between water and water–vapor interface ( Elango .et al,

2015).

The rate of evaporative heat transfer between water mass and glass cover inner surface

is given by:-

qe,w−gi = he,w−gi(Tw − Tgi) (2. 8)

In the above expression, he,w−gi is called as evaporative heat transfer coefficient

between water mass and glass cover inner surface and determined by:-

he,w−gi = (16.237 ∗ 10−3)hc,w−gi(Pw − Pgi)/(Tw − Tgi) (2.9)

The total internal heat transfer rate is the summation of convective, radiative, and

evaporative heat transfer rates between water mass and glass cover inner surface which

is given as:-

qt,w−gi = qc,w−gi + qr,w−gi + qe,w−gi (2.10)

Also,


qt,w−gi = ht,w−gi(Tw − Tgi) (2.11)

The total internal heat transfer coefficient between water mass and glass cover inner

surface ℎ𝑡,𝑤−𝑔𝑖 is obtained by the following expression:-

ht,w−gi = hc,w−gi + hr,w−gi + he,w−gi (2.12)

The rate of conductive heat transfer from glass cover inner surface to the glass cover

outer surface is given by:-

qcd,gi−go =Kg

Lg(Tgi − Tgo) (2.13)

2.7.1.1.3 External heat transfer

The external heat transfer consists of conduction, convection, and radiation processes

which are independent of each other. It is considered as the loss of heat energy from

the solar still to the atmosphere. The heat lose in the solar still from glass cover outer

surface to the atmosphere is called as top loss heat transfer process and from water mass

to the atmosphere through insulation is called as bottom and side loss heat transfer

process. The higher the former the higher will be the yield from the solar still and lower

the latter better will be the yield. They are described briefly in the following section

2.7.1.2 Top loss heat transfer. The heat energy from the glass cover outer surface is lost to the atmosphere by

convection and radiation heat transfer processes.

The convection heat loss from glass cover outer surface of the solar still to the

atmosphere is given by:-

qc,go−a = hc,go−a(Tgo − Ta) (2.14)

The convective heat transfer coefficient hc,go−a is expressed in terms of wind

velocity(v) as follows:-

hc,go−a = 2.8 + (3.0 ∗ V) (2.15)

The radiation heat loss from glass cover outer surface of the solar still to the

surroundings is given by:-

qr,go−a = hc,go−a(Tgo − Ta) (2.16)


The radiative heat transfer coefficient between glass cover outer surface and the

surrounding is given as:-

hr,go−a = σεg{(Tgo + 273)4 − (Tsky + 273)4}/(Tgo − Ta) (2.17)

where

Tsky = (Ta − 6) (2.18)

The total top heat loss is the summation of convective and radiative heat losses

which is given as:-

qt,go−a = qc,go−a + qr,go−a − a (2.19)

Also

qt,go−a = ht,go−a(Tgo − Ta) (2.20)

The total top heat loss coefficient between glass cover outer surface and atmosphere

can be obtained by the following relation:-

ht,go−a = hc,go−a + hr,go−a (2.21)

The total top heat loss coefficient can also be determined directly in terms of wind

velocity (v) by considering the effect of both radiation and free convection from the

condensing cover by the following expression:-

ht,go−a = 5.7 + (3.8 ∗ V) (2.22)

2.7.1.2.1 Bottom and side loss heat transfer. The heat energy is lost from water to the atmosphere through basin liner and insulation

by conduction, convection and radiation processes. In case of solar still mounted on

stand, the bottom and side heat losses occur in sequence—first convection, then

conduction and, finally, convection and radiation losses to the ambient. But, in case of

grounded solar still, the bottom heat loss is first in the form of convection and then

conduction only (Elango .et al, 2015).

The rate of convective heat transfer between basin liner and the water mass is given

by:-


qw = hw(Tb − Tw) (2.23)

where hw is the convective heat transfer coefficient from basin liner to the water.

The rate of conduction heat transfer between basin liner and the atmosphere is given

by:-

qb = hb(Tb − Ta) (2.24)

The heat transfer coefficient between basin liner and the atmosphere through the

insulation is:-

hb = √Lins

Kins+

1

ht,b−a (2.25)

Where

ht,b−a = 5.7 + (3.8 ∗ V) (2.26)

The overall bottom heat loss coefficient between water mass and atmosphere is given

by:-

Ub =hw+hb

hw+hb (2.27)

The overall side heat loss coefficient between water mass and atmosphere is expressed

as:-

Uss =Ass

Ab+ Ub (2.28)

The total bottom and side heat loss coefficient from water mass to atmosphere can be

given by:-

Ubs = Ub + Uss (2.29)

For lower water depth, the overall side heat loss coefficient (Uss) can be neglected since

the area of side walls losing heat (Ass) is very small compared with area of basin

(Ab) of the solar still.

The overall external heat loss coefficient from water mass to the atmosphere through

top, bottom and sides of the solar still is expressed as:-

ULS = Ut + Ubs (2.30)


The heat losses by convection through the basin base and sides to the ground and

surrounding:-

Qloss = UbAb(Ab − As) ∗ (Tb − Ta ) (2.31)

Where Ub = Kl − Li , and Kl and Li are thermal conductivity and the thickness of the

insulation.

2.7.1.2.2 Calculation of yield and thermal efficiency

The hourly yield is given by the following equation:-

mew = he,w−g(Tb − Tg) ∗ 3600/(hfg) (2.32)

The daily efficiency,ηd, is obtained by the summation of the hourly condensate

production m, multiplied by the latent heat hfg , hence the result is divided by the daily

average solar radiation I(t) over the whole area A of the device:-

ηd =∑ m∗hfg

∑ A∗I(t) (2.33)

2.7.1.3 Energy balance

The analytical results are obtained by solving of the energy balance equations for the

absorber plate, saline water and glass cover of the solar still. The saline water

temperature, basin plate temperature and glass cover temperature can be evaluated at

every instant.

Energy balance for the basin plate:-

I(t)Abαb = mbcpb (dTb

dt) + Qc,b−w + Qloss (2.34)

Energy balance for the saline water:-

I(t)Awαw + Qc,b−w = mwcpw (dTw

dt) + Qc,w−g + Qr,w−g + Qe,w−g + Qfw (2.35)

Energy balance for the glass cover:-

I(t)Agαg + Qc,w−g + Qr,w−g + Qe,w−g = mwcpg (dTg

dt) + Qr,g−sky + Qc,g−sky (2.36)

For the stepped still with mirrors the solar radiation reflected by the mirrors per step

and absorbed by the basin (trays) and saline water can be determined as the product of


the direct solar irradiance, the shadow area of the vertical mirror, transmittance of the

glass cover, reflectance of the mirror, 𝜌𝑖𝑛𝑡, and absorptance of the basin liner or of saline

water, and this may be expressed as:

Qint,b = I(t)τgρintαb ∗ wls tan θs cos γ/ tan α (2.37)

Qint,w = I(t)τgρintαw ∗ wls tan θs cos γ/ tan α (2.38)

Where 𝑤 is the tray width and 𝑙𝑠 is the step length, 𝛼 solar altitude angle, 𝛾 solar azimuth

angle and 𝜃𝑠 tilt angle of the glass cover.

Energy balance for the basin plate (stepped still with mirrors):-

I(t)τgAbαb + (I(t)τgρintαb ∗ wls tan θs cos γ / tan α) ∗ 9 = mbcpb (dTb

dt) +

Qc,b−w + Qloss (2.38)

Energy balance for the saline water of stepped still with mirrors:-

I(t)τgAwαw + (I(t)τgρintαw ∗ wls tan θs cos γ / tan α) ∗ 9 + Qc,b−w =

mwcpw (dTw

dt) + Qc,w−g + Qr,w−g + Qe,w−g + Qfw (2.39)

2.8 Monthly optimum inclination of glass cover with internal and top

external reflector

The daily amounts of distillate produced by a still with neither the internal nor the

external reflectors (called NRS) and one with an internal reflector only (called IS)

varying with glass cover inclination 𝜃𝑠 throughout the year at 30_N are shown in Figure

3.2 and 3.3. The daily amount of distillate produced by NRS remains almost constant

to the glass cover inclination 𝜃𝑠 in any month, while one of IS increases with an increase

in glass cover inclination 𝜃𝑠 except for in summer (May–July). This is because the effect

of the inclination 𝜃𝑠 on the amount of the absorption of direct solar radiation is

negligible, but the absorption of solar radiation reflected by the internal reflector

increases with an increase in inclination 𝜃𝑠 since the area of the internal reflector

increases with an increase in inclination 𝜃𝑠 (Tanaka, 2010).


Figure 2.12 Daily amount of distillate of NRS varying with glass cover

Inclination throughout the year at 30_N latitude (Tanaka, 2010).

Figure 2.13 Daily amount of distillate of IS varying with glass cover

Inclination throughout the year at 30_N latitude ( Tanaka, 2010).

For a basin type still, it is difficult to change the glass cover inclination 𝜃𝑠 according to

seasons, so the actual operation for a basin type still would be done with the glass cover

angle fixed throughout the year. In this situation, the reflector inclination 𝜃𝑚 should be

adjusted to the optimum values for each month listed in Table 3.1 for a still with glass

cover inclination 𝜃𝑠 of 10–50_. Here, to facilitate ease of adjustment, inclinations 𝜃𝑠 and

𝜃𝑚 are assumed to be set at 5_ steps. The optimum combination of 𝜗𝑠 and 𝜗𝑚 to

maximize the daily amount of distillate for each month are underlined in Table 2. The


optimum glass cover inclination 𝜗𝑠would be 10_ in summer (May, June and July) and

50_ in other seasons, and the optimum reflector inclination 𝜗𝑚 is smallest in spring

and autumn at 0_, and greatest in summer and winter at 25_ with 𝜗𝑠= 10_ in June and

𝜗𝑠= 50_ in December ( Tanaka, 2010).

Table 2.1 Optimum reflector inclination for each glass covers inclination throughout

the year ( Tanaka, 2010).

𝜽𝒔

Optimum reflector inclination 𝝑𝒎for each glass cover inclination (∘)

Jan

ua

ry

Fe

bru

ary

Ma

rch

Ap

ril

Ma

y

Ju

ne

Ju

ly

Au

gu

st

Sep

tem

be

r

Octo

be

r

No

vem

be

r

Decem

be

r

10 5 0 0 15 20 25 20 15 0 0 5 10 15 10 0 0 15 20 20 20 15 0 0 10 10 20 10 5 0 10 15 20 15 10 0 5 10 10 25 10 5 0 10 15 15 15 10 0 5 10 15 30 15 10 0 5 15 15 15 10 0 10 15 15 35 15 10 0 5 10 15 10 5 0 10 15 20 40 20 10 5 5 10 10 10 5 5 10 20 20 45 20 20 5 0 10 10 10 0 5 15 20 20 50 20 20 10 0 5 10 5 0 5 15 20 25 10 5 0 0 15 20 25 20 15 0 0 5 10

2.9 Monthly optimum inclination of glass cover with bottom external

reflector

Bottom reflector can reflect the sunrays to the evaporating wick and increase the

distillate productivity of a tilted wick still as shown in figure 3.3 when the reflector

inclination 𝜃𝑚 is larger than about 15° on spring and autumn equinox and winter

solstice, and 25° on the summer solstice ( Tanaka, 2010).

Figure 2.14 Daily amount of distillate ( Tanaka, 2010).


2.10 Comparative study

The comparative study of performance improvement techniques used in solar stills and

their results are represented in Table 2.2

Table 2.2 The different review types and area of solar stills, climatic condition and

daylight hours ( Kabeel et al., 2010).

No. Ref. Type of solar still Climatic condition Area,m2 Daylight

hours

Fath et al. Single-slope Egypt 1.52 6-18

Samee et al. Single-slope Pakistan 0.54 10-19

Kumar and

Tiwari

Single-slope India 1 9-17

Kumar and

Tiwari

With solar collector India 1 9-17

Badran and

Tahaineh

With solar collector Jordan 1 8-18

Abdel-Rehim

and Lasheen

With solar

concentrator

Egypt 1 9-19

Abdallah and

Badran

With sun tracking Jordan 1 7-18

Fath et al. Pyramid-shaped Egypt 1.52 6-18

Al-Hinai et al. Pyramid-shaped Oman 1 8-20

Badran et al. Pyramid with collector Jordan 0.92 8-17

Velmurugan

et al.

With fin type India 1 8-17


Velmurugan

et al.

With wick and fin type India 1 9-17

Velmurugan

et al.

Stepped with fins and

sponges

India 0.5 9-17

Table 2.3 The cost comparison of different solar still ( Kabeel et al., 2010).

No. Ref. CPL (cost of distilled

water per liter) ( $)

Fath et al. 0.035

Samee et al. 0.063

Kumar and Tiwari 0.14

Kumar and Tiwari 0.18

Badran and Tahaineh 0.115

Abdel-Rehim and Lasheen 0.058

Abdallah and Badran 0.23

Fath et al. 0.031

Al-Hinai et al. 0.0135

Badran et al. 0.103

Velmurugan et al. 0.054




3MATERIALES AND METHODS

3.1 Introduction

The experiments were carried out in Deir El Balah, Gaza, Palestine. The location lies

at 31.68° N latitude and 34.42° E longitude. The experiments were carried out during

the period of July to August 2015 from 7:00 am to 8:00 pm, it is installed South

direction to receive maximum solar radiation throughout the year. In this work, two

solar stills were designed and fabricated to compare and evaluate the performance of

proposed solar desalination systems, and to get most efficiency and highest production

distilled water. The first one is a conventional still, the second is a stepped solar still.

3.2 Materials

3.2.1 Apparatus

3.2.1.1 Fabricated conventional solar still

The conventional still (a single basin) shown as a photo in figure 4.1 has a basin area

of 1 𝑚2 (100 cm×100 cm). High-side wall depth is 50 cm and the low-side wall height

is 20 cm. The still is made of aluminum sheets (1.5 mm thick)that consist of four

sidewalls and a bottom, it is fixed inside external body made of wood (5mm thickness).

The whole basin surfaces are coated with black paint from inside to increase the

absorptivity. Also, the still is insulated from the bottom to the sidewalls with sawdust

4 cm thick to reduce the heat loss from the still to ambient. The insulation layer is

supported by a wooden frame. The basin is covered with a glass sheet (110 cm×120cm),

3 mm thick inclined at nearly 30° horizontally, to collect the distillate output, a trough

was fixed at the end of the low-side of basin. Plastic pipe was connected to the trough

to drain the fresh water (distillate) to external calibrated flask.


Figure 3.1 locally fabricated Conventional still.

3.2.1.2 Fabricated Stepped solar still

The Stepped solar still shown as a photo in figure 4.2 and figure 4.3 has a basin area of

1 𝑚2 (100 cm×100 cm). High-side wall depth is 110 cm and the low-side wall height

is 15 cm. The still is made of aluminum sheets (1.5 mm thick) that consist of four

sidewalls and a bottom, it fixed is inside external body made of wood (5mm thickness).

The whole basin surfaces are coated with black paint from inside to increase the

absorptivity. Also, the still is insulated from the bottom to the sidewalls with sawdust

4 cm thick to reduce the heat loss from the still to ambient. The insulation layer is

supported by a wooden frame. The basin is covered with a glass sheet (120 cm×140

cm), 3 mm thick inclined at nearly 30° horizontally. The absorber plate is made of 10

steps (each of size 10cm× 100cm) with tray depth 2mm,5mm, and 10mm and width

100cm. Integrating fins in the basin of the absorber plate on tray, the fins are made of

aluminum sheet with a height, length and breadth of 20,100 and 1 mm, respectively.

The pitch between two successive fins is taken as 100 mm and kept constant. The

mirrors added on the vertical sides of the steps as wicks as internal reflectors of stepped

still. The top external reflector inclined backward. The width and length of the external

( top and bottom ) reflector are 100cm×120cm. Two sprinklers are constructed and

installed on the top part of the glass of stepped solar still in order to ease splashing

method for cooling the glass cover with tap water with external Fan, To collect the

distillate output, a trough was fixed at the end of the low-side of basin. Plastic pipe was

connected to the trough to drain the fresh water (distillate) to external calibrated flask.


Figure 3.2 locally fabricated Stepped solar still.

Figure 3.3 Tray of fabricated Stepped solar still.

3.2.3 Evacuated solar water heater.

The evacuated solar water heater as shown as in figure 3.4 consisted of collector, water

tank, expansion vessel and frame. The solar collector consists of 12 vacuum tubes; each

tube has 5.8 cm outer diameter, 4.8 cm inner diameter and 1.8 m length. Inside each

evacuated glass tube there is a sealed copper heat pipe running through the inner tube.

The hollow copper heat pipe within the tube is evacuated of air but contains a small

quantity of a low pressure water–ethylene glycol plus some additional additives to


prevent corrosion or oxidation. A cylindrical stainless steel water tank having 120 l

capacity was used to feed the basin with brackish hot water through insulated tube.

Figure 3.4 Evacuated solar water heater.

3.2.4 Temperature measurement device

The temperature at various locations in the still were measured by (LM 35) as shown

in figure 3.5 coupled to digital microcontroller [its range from 10 to 130°C] the

accuracy of this device is in the range of 1.0°C for the temperature measurements

between 10 and 130°C. Five sensors were used to measure the following temperatures:

Basin, glass (in), ambient, water, and glass (out).

Figure 3.5 Temperature measurement device (LM35).

3.2.5 Glass cover

In this work window glass of 4 mm thickness was used and its average transmissivity

(t) of 0.88, it was fixed at an angel 30° with the horizontal. Glass cover has been sealed

with silicon rubber, which is the most successful because it will make strongly contact


between the glass and many other materials. The sealant is important for efficient

operation. It is used to secure the cover to the frame, take any up difference in expansion

and contraction between dissimilar materials.

3.2.6 The brackish water tanks

Two water tanks were made of 2mm thickness of plastic, it has a diameter of 100 cm,

a height of 150 cm and its volume is 1500 l. The first water tank is located at a suitable

level from the still unit to allow saline water to flow regularly from its outlet hole (at

its bottom) through control valve to Evacuated solar water heater, the second tank is

located after solar still which storage water exterior from the still then pump it to

Evacuated solar water heater.

Figure 3.6 Water tanks.

3.3 Physical-chemical properties of inlet water sample

Brackish water samples were used in this experiment, was obtained from one of the

wells in Deir El Balah (Al Turazy well), Initial pH of the water was 7.5, which is the

original pH of water samples, each condition was tested 3 times and an average value

was reported together with its corresponding standard deviation as shown in table 3.1


Table 3.1 Physical-chemical properties of inlet water sample from Al Turazy well.

Temperature 25 EC µS /cm 4120

pH 7.5 TDS mg/L 2616

Nitrate mg/L 128 Chloride mg/L 969

Sodium mg/L 706 Magnesium mg/L 85

Potassium mg/L 4 Alkalinity mg/L 307

3.4 Methods

3.4.1 Experimental Designs

In this research, seven sets of experiments were performed as follows:

First set: Distilled water from Conventional solar still and stepped solar still without

pre heating:-

It consists of a brackish water tank, a conventional still (single basin solar still) and a

stepped solar still as shown in figure 3.7

Figure 3.7 Sketch for first set which done on 20/07/2015 .


Second set: Distilled water from Stepped solar still with different tray depth without

pre heating:-

It consists of a saline water tank and stepped solar stills with different tray depth 2, 5,

and 10 cm. The modified stepped solar still has the same specification and dimensions

of the first set except the stepped solar still with 5 and 10 cm tray depth did not have

fins in the basin as shown in figure 3.8

Figure 3.8 Sketch for second set which done on 20-23-25/07/2015

Third set: Distilled water from Stepped solar still with and without heating water:-

It consists of a saline water tank, a vacuum tube solar collector, two stepped solar still,

storage tank, Amorphous silicon solar photovoltaic (PV), charge controller, battery and

inverter. Two modified stepped solar still have the same specification and dimensions

of the first set with tray depth 2 cm, the first stepped solar still experimental procedure

is the same in first set, the second stepped solar still feed water from vacuum tube solar

collector which used to preheat the feed water of the saline water, the water which exist

from the still storage in tank and pumped every two hour to saline water, the pump

which rise hot water from storage tank to saline water tank connect with solar


photovoltaic (PV), charge controller, battery and inverter which work with solar energy

as shown in figure 3.9

Figure 3.8 Sketch for third set which done on25/07/2015 and 05/08/2015

Fourth set: Distilled water from Stepped solar still with and without internal

reflectors:-

It consists of a saline water tank and two stepped solar stills. The first stepped solar still

experimental procedure is the same in first set; the second stepped solar still has an

internal reflector 10 mirrors (10 *100 cm) wicks on vertical side of steps to increase the

evaporation surface area of the water as shown in figure 3.9

Figure 3.9 Sketch for fourth set which done on25/07/2015 and 29/07/2015


Fifth set: Distilled water from Stepped solar still with and without external reflectors:-


experimental procedure is the same in first set; the second stepped solar still has the

same specification and dimensions of the first set except an external reflector 2 mirror

(top and bottom reflectors 100 * 120 cm) as shown in figure 3.10

Figure 3.10 Sketch for fifth set which done on25/07/2015 and 07/08/2015

Sixth set: Distilled water from Stepped solar still with and without internal and external

reflectors:-


experimental procedure is the same in first set; the second stepped solar still has the

same specification and dimensions of the first set except an external reflector 2 mirrors

(top and bottom reflectors 100 * 120 cm) and an internal reflector 10 mirrors (10 *100

cm) wicks on vertical side of steps to increase the evaporation surface area of the water

as shown in figure 3.11


Figure 3.11 Sketch for sixth set which done on25/07/2015 and 02/08/2015

Seventh set: Distilled water from Stepped solar still with and without external cooling

system:-

It consists of a saline water tank and two stepped solar stills and external fan. The first

stepped solar still experimental procedure is the same in first set; the second stepped

solar still has the same specification and dimensions of the first set except the cold water

flows over the glass cover was kept uniform and constant with the help of a regulator

and constant head tank . The evaporated water contacts the glass cover and condenses

then run down, and collected. Two sprinklers are constructed and installed on the top

part of the glass of stepped solar still in order to ease splashing method for cooling the

glass cover with tap water with external Fan as shown in figure 3.12 and 3.13

Figure 3.12 Sketch for seventh set which done on25/07/2015 and 09/08/2015


Figure 3.12 Sketch for seventh set which done on25/07/2015 and 02/08/2015

3.4.2 Analytical Method

The water analysis was performed according to standard methods for the examination

of water and wastewater as shown in Table 3.2

Table 3.2 illustrates the method used for the analysis of the required parameter.

No. Parameter Method

1 EC EC-Hach ECO20 cond

2 CL- 4500 CL-B. Argentometric method

In order to evaluate the distillate water chemistry, the distillate water samples were

collected during the month of August in 2015. In all the experiments, for statistical

purposes, each condition was tested 3 times and an average value was reported together

with its corresponding standard deviation. The results water samples before and after

distillation is shown in Table 3.3 with standards given World Health Organization

(WHO). The results revealed that, all the samples were well agreed with standard values

after distillation.


Table 3.3 The results water samples before and after distillation with drinking water

standards

No.

Paramete

r

Before

Distillation

After

Distillation

For set

no 1

After

Distillation

For set

no 2

After

Distillation

For set

no 3

After

Distillation

For set

no 4

After

Distillation

For set

no 5

After

Distillation

For set

no 6

WHO

Standard**

1 ECµS/c

m

4120 33 31 29 35 32 44 <250

2 TDSmg

/L

2616.2 16.5 15.5 14.5 17.5 16 22 <600

3 CL-

mg/L

969.7 8.5 8.5 8 9 8.5 9.5 <250

**WHO. Guidelines for drinking-water quality 4th ed. Geneva, Switzerland: World

Health Organization; 2011 [chapter 10].

3.4.3 Water quality

Little research has been done regarding the water quality of the water produced by

solar-stills based on polluted or muddy water. However it is proven that nitrates,

chlorides, iron, heavy metals and dissolved solids are completely removed by the solar

still (Al-Hayek and Badran, 2004 and Zein and Al-Dallal, 1984). The process also

proved to be effective in the destruction of microbiological organisms present in the

feed water (Al-Hayek and Badran, 2004). The distillate is thus high purity water, which

also lacks essential dissolved minerals. Drinking demineralized water can have serious

health consequences, and it is thus of crucial importance that the essential minerals are

added to the water before consumption (WHO, 2004b). The advised quantities of

minerals where minimum or no adverse health effects are observed are shown are

shown in Table 3.4


Table 3.4 : Advised mineralogical quantities (from WHO, 2004b)

Total

Dissolve

d Solids

(mg/l)

Bicarbonat

e ion (mg/l)

Calciu

m

(mg//l)

Magnesiu

m (mg/l)

Hardnes

s

(mmol/l)

Alkalinit

y (meq/l)

Minimum 100 30 20 10

Optimum 250-500

40-80 20-30 2-4

Maximu

m

6.5

In order to evaluate the brine water chemistry. Table (3.5) shows the brine water

samples were collected after one, two and three day of experiments.

Table 3.5 The results brine samples after one, two, three distillation days.

No.

Parameter

Before

Distillation

After

On day

After

two day

After

three day

1 EC µS/cm 4120 4800 5630 6950

2 TDS mg/L 2616.2 3120 3605 4449

3 CL- mg/L 969.7 1105 1310 1590

** Not:- The results in table 3.5 for the brine sample which done on 25/07/2015 which

I must take more samples from all sets to be more scientific and to now when the brine

in the cell must pore and back wash the cell.


4 RESULTS AND DISCUSSION

In this research the desalination characteristics of brackish water (TDS 2616 mg/l) by

conventional and modified stepped solar still were studied. The main investigated were

depth of water, internal reflector, external reflector, feed water temperature, glass cover

cooling, collector efficiency, distillate water chemistry. At the end of the experiments

and data collection, results and calculations were discussed as follows:-

4.1 Effect of using conventional and stepped solar still without modification

on the performance of solar still

Figure 4.1 illustrates comparison between the hourly variation of fresh water

productivity per unit area for stepped solar still and conventional still, respectively for

both case the feed water TDS was 2616mg/l. Data are given in Calculation sheets No.

1and 4 of Appendix A

Figure 4.1 Produced distillate water (ml/hr) of conventional and stepped solar still

It is observed from Figure 4.1 that the produced distilled water increases as the time

increase till a maximum value at noon time i.e. 14:00 pm, the produced distillate water

was equal to 520 ml/hr in stepped solar still while the produced distillate water on

conventional still was equal to 500 ml/hr , and then start to decrease after that, and the

total produced distillate water for stepped solar still was (3.060 L/(13 hrs (7:00 am to

20:00 pm))) while the total produced distillate water for conventional still was (2.835

L/(13 hrs (7:00 am to 20:00 pm))), in this case the increase in distillate production for

stepped solar still is 7.35% higher than that for conventional still. Therefore the


produced distillate water increases with the increases of the ambient temperature and

decreases with decreases of the ambient temperature.

This phenomenon can be attributed to the fact that a smaller air volume trapped inside

the stepped solar still chamber than in the conventional still and therefore heating up

the trapped air will be much faster.

4.2 Effect of using different water depth on stepped solar still


productivity per unit area for different water depth on stepped solar still, respectively

for three cases the feed water TDS was 2616mg/l. Data are given in Calculation sheets

No. 1,2, and 3 of Appendix A

Figure 4.2 Produced distillate water (ml/hr) of different stepped solar still depth



was equal to 595 ml/hr in stepped solar still with water depth 2 cm while the produced

distillate water was equal to 520 ml/hr in stepped solar still with water depth 5 cm while

the produced distillate water was equal to 500 ml/hr in stepped solar still with water

depth 10 cm, and the total produced distillate water for stepped solar still with water

depth 2 cm was (3.355 L/(13 hrs (7:00 am to 20:00 pm))), and the total produced

distillate water for stepped solar still with water depth 5 cm was (3.060 L/(13 hrs (7:00

am to 20:00 pm))), and the total produced distillate water for stepped solar still with

water depth 10 cm was (2.785 L/(13 hrs (7:00 am to 20:00 pm))). In this case the


increase in distillate production for stepped solar still with 2 cm and 5 cm is 16.98% ,

8.98% higher than stepped solar still with 10 cm .

This phenomenon can be attributed to the fact that a reduction of water depth in the

still improves the productivity.

4.3 Effect of using internal mirror on stepped solar still


productivity per unit area for stepped solar still with and without internal mirror,

respectively for both case the feed water TDS was 2616mg/l. Data are given in

Calculation sheets No. 3and 5 of Appendix A

Figure 4.3 Produced distillate water (ml/hr) with and without internal mirror on stepped

solar still



was equal to 620 ml/hr when used internal mirror while the produced distillate water

was equal to 595 ml/hr without mirror, and then start to decrease after that, and the total

produced distillate water for stepped solar still with internal mirror was (3.535 L/(13

hrs (7:00 am to 20:00 pm))) while the total produced distillate water without mirror was

(3.355 L/(13 hrs (7:00 am to 20:00 pm))), also we found wide different change between

11:00 am to 14:00 pm and narrow different change between 15:00 pm to 20:00 pm. In

this case the increase in distillate production for stepped solar still with internal mirror

was 5.09 % more than without mirror with same water depth.


This phenomenon can be attributed to the fact that using wick increases the evaporating

surface area of the water by reflecting more solar radiation to the surface of water before

mid-day which heat water and more water evaporation and more productivity.

4.4 Effect of using external and internal mirror on stepped solar still


productivity per unit area for stepped solar still with and without internal and external

mirror, respectably for both case the feed water TDS was 2616mg/l. Data are given in


Figure 4.4 Produced distillate water (ml/hr) with and without internal and external

mirror on stepped solar still



was equal to 640 ml/hr when used internal and external mirror while the produced

distillate water was equal to 595 ml/hr without mirror, and then start to decrease after

that, and the total produced distillate water for stepped solar still with internal mirror

was (3.890 L/(13 hrs (7:00 am to 20:00 pm))) while the total produced distillate water

without mirror was (3.355 L/(13 hrs (7:00 am to 20:00 pm))). In this case the increase

in distillate production for stepped solar still with internal and external mirror was 13.75

% more than without mirror with same water depth.

This phenomenon can be attributed to the fact that using top and bottom reflector

increases the evaporating surface area of the water by reflecting more solar radiation to


the surface of water before and after mid-day which heat water and more water

evaporation, more glass temperature, and more productivity.

4.5 Effect of using glass cover cooling on stepped solar still


productivity per unit area for stepped solar still with and without glass cover cooling,



Figure 4.5 Produced distillate water (ml/hr) with and without glass cover cooling on

stepped solar still



was equal to 280 ml/hr when used glass cover cooling while the produced distillate

water was equal to 595 ml/hr without modification, and then start to decrease after that,

and the total produced distillate water for stepped solar still with glass cover cooling


without modification was (3.355 L/(13 hrs (7:00 am to 20:00 pm))), and with used glass

cover cooling the productivity decreased more than without glass cover cooling after

12:00 pm to 18:00 pm. In this case the decrease in distillate production for stepped solar

still with glass cover cooling was 42.02 % more than without modification with same

water depth.

This phenomenon can be attributed to the fact that using glass cover cooling decrease

water–glass temperature difference, which decrease the evaporative heat transfer rate.


4.6 Effect of using pre heated water on stepped solar still


productivity per unit area for stepped solar still with and without pre heating water,



Figure 4.6 Produced distillate water (ml/hr) with and without pre heating water on

stepped solar still



was equal to 740 ml/hr when used internal and external mirror while the produced

distillate water was equal to 595 ml/hr without mirror, and then start to decrease after

that, and the total produced distillate water for stepped solar still with internal mirror


without mirror was (3.355 L/(13 hrs (7:00 am to 20:00 pm))). In this case the increase

in distillate production for stepped solar still with pre heating water was 61.69 % more

than without pre heating water with same water depth.

This phenomenon can be attributed to the fact that using hot feed water to the still

reduces the time required to raise the water temperature and increase the temperature

difference between the boiler and stepped solar still.


4.7 Effect of using pre heated water with glass cover cooling on stepped

solar still


productivity per unit area for stepped solar still using pre heating water with glass cover

cooling and without modification, respectively for both case the feed water TDS was

2616mg/l. Data are given in Calculation sheets No. 3and 9 of Appendix A

Figure 4.7 Produced distillate water (ml/hr) with using pre heating water and glass cover

cooling on stepped solar still



was equal to 710 ml/hr when used pre heating water with glass cover cooling while the

produced distillate water was equal to 595 ml/hr without modification, and then start to

decrease after that, and the total produced distillate water for stepped solar still with pre

heating water and glass cover cooling was (6.670 L/(13 hrs (7:00 am to 20:00 pm)))

while the total produced distillate water without modification was (3.355 L/(13 hrs

(7:00 am to 20:00 pm))). In this case the increase in distillate production for stepped

solar still with pre heating water and glass cover cooling was 98.80 % more than

without modification with same water depth.

This phenomenon can be attributed to the fact that when the solar still works in high

temperature by means of supplying heat to the basin, the higher evaporation rate is

achieved. Thus, the glass cover will receive more latent heat of vaporization. In turn,

the temperature of the glass cover increases, and temperature difference between the

glass cover and basin water decreases. This causes low vaporization and, thus, low


yield. To overcome the glass overheating problem, a water cooling system was applied

to the glass acting.

4.8 Effect of water glass temperature on productivity

Figure 4.8 illustrates comparison between the hourly variations of fresh water

productivity for stepped solar still without modification against water glass temperature

difference. Data are given in Calculation sheet No.3 of Appendix A

Figure 4.8 Effect of water–glass temperature difference on productivity.

It is observed from Figure 4.8 that the for the maximum water-glass temperature

difference Tw-Tg of 21°C, productivity is 595 ml/hr and the average water-glass

temperature difference Tw-Tg of 13 °C, productivity is 400 ml/hr.

But in Figure 4.9 illustrates comparison between the hourly variations of fresh water

productivity for stepped solar still with pre-heating and glass cover cooling against

water glass temperature difference. Data are given in Calculation sheet No.9 of

Appendix A


Figure 4.9 Effect of water–glass temperature difference on productivity.

It is observed from Figure 4.9 that the for the maximum water-glass temperature

difference Tw-Tg of 42°C, productivity is 690 ml/hr and the average water-glass

temperature difference Tw-Tg of 25 °C, productivity is 500 ml/hr.

This phenomenon can be attributed to the fact that increases in water–glass temperature

difference increases the evaporative heat transfer rate. Hence the evaporation rate

increases with higher water–glass temperature differences.

4.9 Accumulated productivity of the stepped solar modification

Figure 4.10 illustrates comparison between the cumulative variations of fresh water

productivity per unit area for all experiments. Data are given in Calculation sheet No.

1 to 9 of Appendix A


Figure 4.10Cumulative variation of fresh water productivity per unit area for all

experiments

It is observed from Figure 4.10 that for the effect set that give the highest productivity

(6.670 L/(13 hrs (7:00 am to 20:00 pm))) by using pre-heating water with glass cover

cooling and the others sets give productivity less than 6.670 L/(13 hrs (7:00 am to 20:00

pm))).

This phenomenon can be attributed to the fact that increases in water–glass temperature

difference increases the evaporative heat transfer rate, and more consideration on

surface glass with is the same surface on all sets.

4.10 Cost evaluation

To make the solar desalination system perform well, it requires some maintenance.

Therefore the annual maintenance cost should be considered. For this system,

maintenance is required frequently due to the following reasons: (i) Continuous water

supply into the stills, (ii) replacement of broken or damaged parts, and (iii) cleaning of

solar stills, solar water heater and condenser. Cost estimation for various components

used in the solar desalination system for the best distillate system that I recommended

to use is given in Table 4.4


Table 4.4 The cost for the best fabricated Stepped still per 𝑚2

Item

Cost ($)

Aluminum sheet 130

Support legs 30

Paint 10

Inlet sawdust's 20

Glass cover 60

Fan 30

Photovoltaic system 250

Pump 70

Tanks 100

Evacuated solar water heater 500

Plastic pipe 30

Mirrors 50

Fittings 10

Temperature measurement device 100

Welding 60

Lab test 50

Total 1500$

The total fixed cost of Stepped still per 1𝑚2 was about F=1500 $. To obtain the average

value of the cost of distillate output, it is important to assume that V is the variable cost,

C is the total cost, where, C = F + V. In Egypt the variable cost ranged is from 25 to

30% from the fixed cost. Assume variable cost V equals to 0.3 F per year, as reported

in Omara and Eltawil , and the annual variable cost includes the maintenance cost, the

expected still life time is 10 years, then: C = 1500 + 0.3 × 1500 × 10 = 6000 $ where

the average daily productivity can be estimated from the analysis of different

experimental data, and it is taken as 6.670 l/day. To determine the annual cost of 1 l


assuming that the still operates 340 days in a year. The total productivity during the life

of Stepped still = 6.670 × 10 × 340 = 22678 l. Then the cost of 1 l from Stepped still =

6000/22678 = 0.26 $.

If we shows the average cost of distillated water for different types of solar still in

chapter 2 and my project as shown in figure 4.11

Figure 4.11 The average cost of distillated water for different types of solar still and my

best distilled project

The results obtained that my project give the maximum water production cost with 0.26

$.


6 CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

In this research the desalination characteristics of brackish water by Stepped solar still

was studied. The efficiency of the system was tested by exceeding some modification

to enhance the productivity such as adding wick on the vertical sides, supplying pre

heat water into the solar still, using trays with constant width, using internal and

external reflectors, and glass cover cooling. At the end of the experiments the following

important conclusions were drawn it was concluded that:-

1. The stepped solar still (the tray width equal the step width 10cm and step

depth 5cm ) achieved about 7.35% higher productivity than the conventional

solar still with water depth 5cm.

2. The stepped solar still incorporated with solar air heater and glass cover

cooling technique productivity was increased by 98.80 % more than stepped

solar still without modification with same water depth.

3. Glass cover cooling the external surface of the stepped solar still has a bad

effect on enhancing the productivity and the efficiency of the system

decreases approximately to 42%.

4. The daily productivity of the stepped still by using wick on the vertical sides

(internal reflector) and external reflector was increased by 13.75% more

than stepped solar still without modification with same water depth.

5. Daily productivity of the stepped still by preheating the feed water of the

stepped solar still was increased by 61.69 % more than without preheating

water with same water depth.

6. The daily productivity of stepped solar still by using wick on the vertical

sides (internal reflector) was increased by 5.09% more than stepped solar

still without modification with same water depth.

7. The daily productivity of stepped solar still Increasing by decreasing water

depth from 10, 5, to 2cm with efficiency 8.98%, 16.98% higher than

stepped solar still with 10 cm .

8. Increasing in water–glass temperature difference increases the evaporative

heat transfer rate.

9. Increasing the inclination angle of the bottom external reflector from

vertical to 30° has an adverse effect on the productivity in the summer

months.

10. Increasing the inclination angle of the top external reflector from vertical to

10° has an adverse effect on the productivity in the summer months.


11. The brine increasing in water with time of experimental which must release

after some days.

12. The results obtained that my project give the maximum water production

cost with 0.26 $.

6.2 Recommendations

1. It is recommended for future research to increase area of condensation

surface for solar distillation.

2. It is recommended for future research to find cheap components to reduce

the cost of this system in the market.

3. It is recommended for future research to mix distillated water with brackish

water without exceeding quality criteria of WHO with low cost to produce

sufficient amount of water for the residence in the same house .


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Appendix Table A:1 Experimental results of Stepped solar still with water depth 5 cm without

any modification on 20/07/2015 Sheet No. 1 of Appendix A

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

T gout

(°C)

T gi

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

0 0 0 26 27 26 27 26 7

1 10 10 28 32 29 30 28 8

7 40 30 30 44 37 38 29 9

11 115 75 33 53 44 45 31 10

14 235 120 38 57 52 53 33 11

15 540 305 42 63 57 58 34 12

19 925 385 44 67 63 64 35 13

20 1445 520 46 67 66 67 35 14

18 1935 490 46 65 64 65 34 15

15 2335 400 42 60 57 58 33 16

13 2655 320 39 52 52 53 32 17

9 2855 200 38 48 47 48 31 18

9 2975 120 36 43 45 46 30 19

9 3060 85 34 38 43 44 30 20


Table A:2 Experimental results of Stepped solar still with water depth 10 cm without

any modification on 23/07/2015 Sheet No. 2 of Appendix A

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

0 0 0 27 28 27 28 27 7

0 10 10 29 31 29 30 28 8

6 30 20 30 34 36 37 29 9

9 95 65 33 39 42 43 31 10

9 195 100 39 43 48 49 32 11

13 475 280 40 48 53 54 33 12

16 775 300 42 53 58 59 34 13

17 1275 500 43 57 60 61 34 14

15 1725 450 43 52 58 59 33 15

12 2110 385 42 49 54 52 32 16

10 2415 305 39 48 49 50 31 17

9 2610 195 37 46 46 47 30 18

9 2710 100 35 43 44 45 30 19

8 2785 75 32 42 40 41 29 20


Table A:3 Experimental results of Stepped solar still with water depth 2 cm

on 25/07/2015 Sheet No.3 of Appendix A

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

0 0 0 27 28 27 28 26 7

1 15 15 29 32 30 31 28 8

8 55 40 31 37 39 40 29 9

12 135 80 35 44 47 48 31 10

13 275 140 41 51 54 55 33 11

15 605 330 45 55 60 61 34 12

19 1045 440 47 57 66 67 35 13

21 1640 595 48 59 69 70 35 14

19 2165 525 46 56 65 65 34 15

15 2575 410 43 52 58 58 32 16

11 2905 330 40 48 51 52 31 17

11 3125 220 38 46 49 50 31 18

9 3270 145 37 44 46 46 30 19

9 3355 85 34 41 43 44 30 20


Table A:4 Experimental results of Conventional solar still with water depth 5 cm

on 20/07/2015 Sheet No.4 of Appendix A

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

0 0 0 27 27 27 28 27 7

1 10 10 28 31 29 30 28 8

6 40 30 30 35 36 37 30 9

9 110 70 33 39 42 43 31 10

10 220 110 36 44 46 47 32 11

11 500 280 40 47 51 52 33 12

13 890 390 43 51 56 57 33 13

14 1390 500 45 53 59 60 34 14

14 1830 440 43 51 57 58 34 15

13 2200 370 40 47 53 54 33 16

12 2475 275 38 46 50 51 32 17

13 2660 185 35 45 48 49 31 18

12 2770 110 34 44 46 47 29 19

12 2835 65 32 43 44 45 29 20



With internal mirror on 29/07/2015 Sheet No.5 of Appendix

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

1 0 0 27 28 28 29 27 7

3 20 20 28 33 31 32 28 8

9 60 40 30 39 39 40 29 9

15 145 85 34 46 49 50 31 10

17 315 170 39 53 56 57 32 11

19 690 375 43 56 62 63 33 12

22 1180 490 45 58 67 68 34 13

23 1800 620 47 59 70 71 35 14

23 2335 535 45 57 68 69 34 15

17 2755 420 43 53 60 61 32 16

12 3075 320 41 48 53 54 31 17

11 3300 225 38 47 49 50 31 18

10 3440 140 36 45 46 47 30 19

8 3535 95 34 43 42 43 29 20



With glass cover cooling on 02/08/2015 Sheet No.6 of Appendix

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

1 0 0 25 27 26 27 26 7

3 10 10 26 28 29 30 27 8

7 30 20 27 30 34 35 28 9

11 85 55 27 32 38 39 30 10

13 195 110 27 32 40 41 31 11

15 405 210 28 32 43 44 32 12

18 645 240 28 33 46 47 34 13

19 925 280 29 33 48 49 34 14

19 1195 270 28 33 47 48 33 15

15 1415 220 28 33 43 44 33 16

14 1605 190 28 33 42 43 32 17

13 1745 140 27 32 40 41 32 18

12 1865 120 27 31 39 40 31 19

10 1945 80 26 30 36 37 30 20



With pre heating water on 05/08/2015 Sheet No.7 of Appendix

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

14 0 0 29 38 43 44 26 7

11 120 120 38 45 49 50 28 8

9 325 205 45 51 54 55 29 9

9 635 310 50 56 59 60 29 10

11 1075 440 54 63 65 66 32 11

15 1570 495 58 68 73 74 33 12

17 2240 670 60 73 77 78 34 13

19 2980 740 62 77 81 82 35 14

24 3660 680 58 79 82 83 34 15

23 4220 560 55 75 78 79 32 16

22 4655 435 53 74 75 76 32 17

20 5000 345 51 70 71 72 31 18

18 5245 245 50 64 68 70 29 19

17 5425 180 49 62 66 67 29 20


Table A:8 Experimental results of Stepped solar still with water depth 2 cm With

internal and external mirrors on 07/08/2015 Sheet No.8 of Appendix

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

1 0 0 27 28 28 29 27 7

3 40 40 29 34 32 33 28 8

8 100 60 31 39 39 40 30 9

14 195 95 35 47 49 50 31 10

19 390 195 39 54 58 59 32 11

20 780 390 44 55 64 65 34 12

24 1300 520 46 56 70 71 35 13

24 1940 640 48 58 72 73 35 14

22 2520 580 46 57 68 69 34 15

19 3000 480 44 54 63 62 33 16

15 3370 370 43 51 58 59 32 17

10 3630 260 41 48 51 50 31 18

8 3785 155 39 47 47 48 31 19

8 3890 105 37 46 45 46 30 20


Table A:9 Experimental results of Stepped solar still with water depth 2 cm With

Pre heating water and glass cover cooling on 09/08/2015 Sheet No.9 of Appendix

Tw-Tg

out

(°C)

P com.

(ml/h) P (ml/h)

Tg out

(°C)

Tg in

(°C)

Tw

(°C)

Tb

(°C)

Ta

(°C)

Time

(h)

13 0 0 28 38 41 42 27 7

17 180 180 31 45 48 49 29 8

21 465 285 35 51 56 57 29 9

23 900 435 37 56 60 61 31 10

29 1410 510 38 63 67 68 33 11

34 2000 590 40 68 74 75 34 12

35 2640 640 42 73 77 78 35 13

40 3350 710 42 77 82 83 35 14

42 4040 690 40 79 82 83 34 15

42 4690 650 38 75 80 81 33 16

40 5280 590 37 74 77 78 32 17

38 5790 510 36 70 74 75 31 18

36 6250 460 34 64 70 71 30 19

35 6670 420 32 62 67 68 30 20

ةزغ ةيملاسلإا ةعماجلا · III ABSTRACT Solar still is a simple solar device used...

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Transcript of ةزغ ةيملاسلإا ةعماجلا · III ABSTRACT Solar still is a simple solar device used...