Decentralized water purification using solar thermal energy

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Decentralized water purification

using solar thermal energy

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Decentralized water purification

using solar thermal energy

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op vrijdag 12 februari 201 om 15:00 uur

door

Rajat BHARDWAJ scheikundig ingenieur,

Techniche Universiteit Delft, Nederland geboren te Delhi, India.

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Composition of the doctoral committee:

Rector Magnificus Chairman

Prof. dr. R. F. Mudde Delft University of Technology, promotor Independent members:

Prof. dr. ir. M. van Sint Annaland Eindhoven University of Technology Prof. dr. G.N. Tiwari Indian Institute of Technology Delhi, India Prof. dr. M.D. Kennedy UNESCO-IHE, Delft

Prof. dr. ir. L.C. Rietveld Delft University of Technology Prof. dr. ir. H.E.A. van den Akker Delft University of Technology

Prof. dr. ir. C. R. Kleijn Delft University of Technology, reserve member Other member:

Dr. M.V. ten Kortenaar Dr Ten B.V.

This work was supported by the Dr TEN B.V.

Printed by: Proefschriftmaken.nl & Uitgeverij BOXPress Published by: Uitgeverij BOXPress, ’s-Hertogenbosh

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without the prior permission of the author.

ISBN: 978-94-6186-606-6

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Summary

Provision of clean drinking water to unprivileged societies can prevent a large of number of deaths and illnesses amongst children around the world. In 2010, about 0.75 million child deaths were caused due to diarrhea, and a further 22.5 million years of life were lost due to ill-health, disability or early death caused by unimproved water and inadequate sanitation around the world. Most of these people live in regions with limited or no electricity, and abundance of sunshine and salt water or rain water.

Solar thermal distillation using solar stills is the most potent way of cleaning the salt water in the absence of electricity. Historically, the use of solar still has been known to man for more than four centuries. Over the years, several hundreds of designs, patents and peer reviewed papers have been published. Several products such as the inflatable stills for life rafts, water-cone and water pyramid have been demonstrated for the purification of salt water. Yet, the use of solar stills as a widespread commercial product is not seen.

The mass based application of solar stills has faced several challenges. The use of plastic as a material of construction causes a reduction in the production of water from the solar still. The reduction in the water production is caused by the reflection of incoming sunlight and dripping of purified water from the condensation surface. However, plastic is relatively easier to process, manufacture, mass produce, transport and maintain than glass as a material of construction. Further, there are very few examples of commercially available plastic based solar stills which can yield at least 2.5 liters/day of drinking water, which is sufficient for at least two children in a family. Moreover, there is an unavailability of a solar still product which is easy to mass manufacture in different countries around the world. Finally, the conversion of distilled water coming out of the solar still to mineralized water still remains to be demonstrated.

This thesis gives a direction for the use of plastics for making solar stills which can produce at least 2.5 liters of drinking water/day. We have shown that the loss of water due to the reflection of sunlight and the dripping can be considerably reduced by using a hydrophilic condensation surface. We have shown that the length of the condensation surface beyond a particular value results in the dripping of purified water. Moreover, we have indicated the preferred contact angle and inclination angle of the condensation surface to avoid dripping.

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increase the yield of water. For this, we have demonstrated the use of an additional area of the condensation surface to increasing the yield from the solar still by more than a factor of five. The use of additional area further provides an opportunity to use evaporation cooling without blocking the incoming solar radiation. Another way of improving the yield of water from the condensation surface using salt was also demonstrated.

Furthermore, we have shown two possible ways for the production of plastic based solar stills. Firstly, we have demonstrated a plastic based inflatable solar still for the production of 1 liter/hour of drinking water. This solar still can be seen as an example to be used by a family for their drinking water needs. Secondly, we have shown the use of an ”air gap membrane distillation” unit which can be scaled up for the production of 2.5 – 3.5 l/day of drinking water. The membrane solar still showed consistent purification of high saline feed water containing NaCl (25% by weight) to a permeate with a concentration of < 500 mg/l for a period of 60 hours. Additionally, the still was also able to purify As (500 ppb), Fl (50 ppm) and NO3 (1000 ppm) to concentration levels below the guidelines values set by

the WHO. Finally, in the appendices, we have shown important aspects of solar distillation which contribute to the practical application of solar stills. These cover re-mineralization of distilled water to the levels of total dissolved solids recommended by the WHO, increase in the production of water from solar still by decreasing the temperature of the condensation surface and the manufacture of new membranes for air gap membrane distillation.

Overall, the work of this thesis gave a very strong view on the manufacture of plastic based solar stills capable of providing at least 2.5 liters/day of drinking water. These results are being used by Dr Ten B.V. for the development of a simple and replicable mass manufacturing process for plastic based stills. Dr Ten B.V. was the initiator and coordinator for the projects of the solar still product and the direct funding organization of this thesis with support for R&D funding too from the Dutch Ministry of Economic Affairs. Further much R&D work was done at and in close collaboration with the TU Delft. Future projects for the use of solar stills are under consideration with several governments and organizations like UNHCR. Global NGO organizations like World Vision and BRAC have shown their interest to implement the water distillation product in under-privileged regions of India and Bangladesh, respectively. Also, talks with UNICEF are

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in progress for the use of a family scale solar distillation product. However, arriving at full professional products with mass production is still a step to go for the company. The R&D team is grateful for all the help by the Dutch government and expresses hope to go on in order to fulfill these goals and so helping the lives of the needy ones.

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Samenvatting

Levering van schoon drinkwater aan kansarme samenlevingen kan een groot aantal sterfgevallen en ziektes onder kinderen over de hele wereld voorkomen. In 2010 werden ongeveer 0,75 miljoen sterfgevallen onder kinderen veroorzaakt als gevolg van diarree, en werden wereldwijd nog eens 22,5 miljoen levensjaren verloren (YLL) als gevolg van slechte gezondheid, handicap of vroege dood veroorzaakt door verontreinigd water en het gebrek aan sanitaire voorzieningen. De meesten van deze mensen wonen in regio’s met een beperkte of geen elektriciteit, en een overvloed aan zon en zout water of regenwater.

Zonnewarmte destillatie met behulp van een zonne destilleer-apparaat (solar still) is de meest krachtige manier om zout water in de afwezigheid van stroom te zuiveren. Historisch gezien is de mens al meer dan vier eeuwen bekend met het gebruik van de solar still. In de loop der jaren zijn er honderden verschillende ontwerpen, octrooien en intercollegiaal getoetste wetenschappenlijke artikels gepubliceerd. Van diverse producten, zoals de opblaasbare stills voor reddingsvlotten, water-kegel en water piramide is de werking aangetoond voor het zuiveren van zout water. Toch wordt het gebruik van solar stills niet als wijdverspreid commercieel product gezien. Het op grote schaal toepassen van solar stills staat voor verschillende uitdagingen. Het gebruik van kunststof als constructiemateriaal veroorzaakt een vermindering van de waterproductie door de solar still. De vermindering van de waterproductie wordt veroorzaakt door de reflectie van invallend zonlicht en door het druipen van gezuiverd water van het condensatie oppervlak. Echter, kunststof is relatief makkelijker te verwerken, te vervaardigen, in massa te produceren, te transporteren en te onderhouden dan glas als constructiemateriaal. Verder is er slechts een beperkt aantal voorbeelden van commercieel verkrijgbare standalone solar stills op kunstofbasis, die minstens 2,5 liter drinkwater per dag leveren, voldoende voor ten minste twee kinderen uit een gezin. Bovendien bestaat er nog geen solar still product, dat geschikt is voor wereldwijde massa-productie. Tot slot moet de omzetting van het gedestilleerde water afkomstig uit de solar still naar gemineraliseerde water nog worden aangetoond.

Dit proefschrift geeft richting aan het gebruik van kunststof voor het maken van solar stills die ten minste 2,5 liter drinkwater / dag kunnen produceren. We hebben aangetoond dat het verlies van water als gevolg van de reflectie van het zonlicht en het druipen aanzienlijk kan worden

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beperkt moet worden tot zijn afdruiplengte. Bovendien hebben we een voorkeurscontacthoek en hellingshoek van het condensatie oppervlak aangegeven om druipen te voorkomen.

Verder hebben we manieren getoond om het condensatie oppervlak onafhankelijk te maken van het oppervlak dat de inkomende straling ontvangt met als gevolg een hogere opbrengst aan water. Hiervoor hebben we de werking laten zien van een extra gedeelte aan condensatie oppervlak waardoor de opbrengst van de solar still meer dan een factor vijf toeneemt. Het gebruik van dit extra oppervlak verschaft bovendien de mogelijkheid om verdampingskoeling te gebuiken zonder de invallende zonnestralen te blokkeren. Een andere manier om met zout de opbrengst van water afkomstig van het condensatie oppervlak te verbeteren werd eveneens aangetoond.

Daarnaast hebben we twee manieren laten zien voor de productie van solar stills op kunststofbasis. Ten eerste demonstreren we een een opblaasbare solar still op kunststofbasis met een drinkwaterproductie van 1 liter/uur. Deze solar still kan als een voorbeeld worden beschouwd om gebruikt te worden voor de drinkwaterbehoeften van een familie. Ten tweede hebben we het gebruik van een luchtspleet membraan destillatie-eenheid laten zien, die kan worden opgeschaald tot een productie van 2,5 - 3,5 l drinkwater per dag. De membraan solar still vertoonde gedurende 60 uur een consistente zuivering van zeer zout met toevoerwater 25% (gewichts) NaCl tot een permeaat met een concentratie van minder dan 500 mg/l. Bovendien was de still ook in staat om As (500 ppb), Fl (50 ppm) en NO3(1000 ppm) te zuiveren

tot concentratieniveaus beneden de waarden, die de WHO als richtlijn heeft opgesteld. Tot slot laten we in de bijlagen belangrijke aspecten van zonne destillatie zien, die een bijdrage leveren aan de praktische toepassing van solar stills. Deze behandelen de re-mineralisatie van gedestilleerd water tot de niveaus van volledig opgeloste vaste stoffen zoals aanbevolen door de WHO, toename van de productie van water door de solar still door een afname van de tempretuuur van het condensatie oppervlak en de fabricage van nieuwe membranen voor luchtspleet membraandestillatie.

Algemeen beschouwd, geeft het werk beschreven in dit proefschrift een zeer sterke visie op de productie van solar stills op kunststofbasis, die in staat zijn ten minste 2,5 liter drinkwater per dag te leveren. Deze resultaten worden gebruikt door Dr Ten bv voor de ontwikkeling van een eenvoudig en reproduceerbaar massproductie proces voor solar stills op kunststofbasis.

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Dr Ten bv was de initiatiefnemer en co ¨ordinator van de projecten voor het solar still product en de direct financierende organisatie van dit proefschrift met bijkomende ondersteunende O&O fondsgelden vanuit het Nederlandse Ministerie van Economische Zaken. Verder is veel van het O&O werk uitgevoerd aan en in nauwe samenwerking met de TU Delft. Toekomstige projecten voor het gebruik van solar stills zijn in beraad bij diverse overheden en organisaties als de UNHCR. Wereldwijde NGO organisaties als World Vision en BRAC hebben hun belangstelling getoond om het water destillatie product in kansarme regio’s van, respectievelijk, India en Bangladesh te verwezenlijken. Ook zijn er gesprekken met UNICEF aan de gang voor het op familie schaal gebruiken van het zonne destillatie product. Echter, om tot volledig professionele, massafabriceerbare producten te komen, is nog een stap te ver voor het bedrijf. Het O&O team is dankbaar voor alle hulp van de Nederlandse overheid en spreekt de hoop uit om door te kunnen gaan om met het bereiken van dit doel en zo het leven van de behoeftigen vooruit te helpen.

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Introduction

1.1 Background of the investigation

1.1.1 Present situation of drinking water

Global assessments by the WHO and UNICEF show that a large proportion of the worlds population does not have access to an adequate or microbiologically safe sources of water for drinking and other essential purposes. An estimated 748 million people lack access to an improved source of drinking water [WHO and UNICEF, 2014]. An improved source is one that, “by nature of its construction, adequately protects the source from outside contamination, particularly fecal matter” [WHO and UNICEF, 2013]. However, several authors have pointed out that this definition is unaccountable for the measurements of the quality of water [Bain et al., 2014; Pr ¨uss et al., 2002]. Many more people use sources that are classified as ‘improved’ but are still unsafe for consumption. Bain et al. [2014] estimated that more than 1.8 billion people consume drinking water from a contaminated source through a systematic review of 96,737 water samples.

Figure 1.1 shows the regions affected with physical and economic water scarcity in the world. It shows that, a significant portion of Africa and south-east Asia suffers from economic water scarcity. In these areas, the human, institutional and financial resources are limited to provide an access to drinking water, even though water is available locally to meet the human demands. Additionally, a major portion of Asia suffers from physical water scarcity arising due to, either limited availability of natural water sources or exploitation of water sources. By, 2025, an estimated 1.8 billion people will be

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living in countries or regions with water scarcity, and two-thirds of the world population could be living under water stress conditions [UNWATER, 2013].

Figure 1.1:Physical and economic surface water scarcity in the world. Physical water scarcity: Water resources development is approaching or has exceeded sustainable limits. 75% of river flows are withdrawn for use. Approaching physical water scarcity: More than 60% of river flows are withdrawn. These basins will experience physical water scarcity in the near future. Economic water scarcity: Human, institutional and financial resources limit an access to water even though water is available to meet local demands. Less than 25% of water is withdrawn from rivers for human purposes. Little or no water scarcity:Abundant water resources relative to use, with less than 25% of water from rivers available for human purposes. Source: Comprehensive assessment of water management in agriculture, 2007

1.1.2 Burden of contaminated water on health

Disease related to contamination of drinking water constitute a major burden on public health. Figure 1.2 shows the distribution of diseases leading to death amongst children under the age of 5. The burden of water related diseases is highest in low income rural areas where diarrhea remains a leading cause of child deaths. In 2010, almost 0.75 million child deaths were caused due to diarrhea [Liu et al., 2012]. Furthermore, a substantial amount of time was lost due to ill-health, disability or early death due to water related diseases. The disability-adjusted life year (DALY) is a measure of overall disease burden [Murray and Lopez, 1996]. In 2010, an approximate

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1.1 Background of the investigation 5

22.5 million years of life were lost due to ill-health, disability or early death caused by unimproved water and sanitation around the world [Liu et al., 2012]. Additionally, contamination of water with arsenic, fluoride, salts and heavy metal also lead to a significant impact on health in various regions of the world. Contamination of arsenic and fluoride in drinking water affects approximately a hundred million people in developing countries [Gadgil, 1998].

Figure 1.2: Distribution of child mortality in the world. Source: Institute of health metrics and evaluation, 2012.

A comparison of Figures 1.1 and 1.2 show that the regions with high water scarcity are most of the times also the regions suffering with high child mortality. Furthermore, Figure 1.3 shows the distribution of causes leading to child deaths in the year 2010. Diarrhea accounts for almost 10% of the total deaths amongst children under the age of 5. Additionally, it also shows that Pneumonia as a leading cause of deaths amongst children which is partly also caused by contaminated water.

1.1.3 Rural-urban divide in drinking water quality

Of the 2.1 million people who gained access to an improved drinking water source since 1990, almost two-thirds (1.3 million), lived in urban areas [WHO and UNICEF, 2014]. By the end of 2011, 83% of the population without an access to an improved drinking-water source lived in rural areas [WHO and UNICEF, 2014]. In urban and densely populated areas, the

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Figure 1.3:Distribution of leading causes of child mortality in the world. Source: [Liu et al., 2012]

principle of “economy of scale” generally favors central solutions for the supply, distribution and treatment of water [Peter-Varbanets et al., 2009]. In most of the rural areas of the developing and transitional countries, centralized drinking water treatment is generally prohibitively expensive, leading to the frequent use of untreated natural water sources (rivers, lakes, groundwater or rain). These sources are generally not well protected and may contain chemical or microbial pollutants, mostly derived from a lack of adequate sanitation [Gadgil, 1998]. In rural or informal urban or semi-urban communities of the developing countries, where a centralized water supply is lacking, decentralized systems are consequently often the only means to improve the quality of water obtained from contaminated sources.

1.1.4 Decentralized water purification

Development of decentralized water purification for rural communities is crucial for the provision of clean drinking water across the world [Peter-Varbanets et al., 2009]. The decentralized water purification systems can serve water to a household or a community either for its drinking purposes and/or its cooking purposes. Most common decentralized systems work on an individual method or a combination of several methods such as thermal treatment, ultra-violet methods, physical removal and chemical treatment [Peter-Varbanets et al., 2009]. Furthermore, the systems for

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1.2 Previous work on solar distillation 7

decentralized applications should preferably be independent of utilities such as electricity or tap pressure. Additionally, they should be easy to use, low in cost, easy to maintain and socially acceptable. The responsibility of securing food, water and health of children in rural areas largely falls on the women in the families. Most of these women have limited or no formal educational training. The use of a simple water purification device with low cost and maintenance has a greater possibility of acceptance in rural conditions. Furthermore, delivery of drinking water can have a big impact on the overall health of children living in rural conditions. In 2013, UNICEF has asked for desalination devices capable of producing at least 2.5 liters/day of drinking water for use in the developing countries (Technology in action conference, Geneva, 2013). The device is targeted to satisfy the demand of drinking water of at least the kids in the family. Finally, most of the regions facing a scarcity of water also have a high amount of annual incoming solar radiation. This makes these regions attractive for application of solar water purification.

1.2 Previous work on solar distillation

Solar distillation mimics nature’s hydrological water cycle by purifying water through evaporation and condensation (rain). It is also referred as solar desalination as it is capable of removal of contaminants such as salts or other non-volatile components from water. It is one of the most basic water purification system which can be applied to obtain high quality drinking water. It has been found to be preferable for application in villages and small islands [Eltawil et al., 2009; Kalogirou, 2005; Kumar and Tiwari, 2009; Xiao et al., 2013]. Kalogirou [2005] concluded that solar distillation technology present the best technical solution to supply remote villages or settlements with fresh water without depending on high technology and expertise. Xiao et al. [2013] has listed several advantages for the use of solar distillation.

1. It works on cheap and renewable solar energy.

2. It hardly any use of electricity and hence no carbon emissions.

3. There is often plenty of solar energy available in regions of scarce drinking water resource.

4. Solar stills are easy to build and operate.

Also, solar stills can be more economical than other desalination technologies for providing water to households and small communities. Kumar and

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Tiwari [2009] suggested that a solar distillation plant with a capacity less than 200 kg/day is more economical than other types of desalination plants of this scale.

The first solar still plant was built in 1872 by the Swedish engineer Charles Wilson in northern Chile. This glass covered basin still was 4700 square meters in size, produced more than 23,000 l of drinking water per day, and was in operation for not less than 40 years [Flendrig et al., 2009]. During world war II, Maria Telkes designed a small scale inflatable still for the use in the life rafts of the United states armed forces. With over 200,000 units, it is the most produced solar still ever [Flendrig et al., 2009]. Amongst the various types of solar still tested and manufactured till date, plastic based solar stills have been the preferred choice of solar still for commercial production [Abd; Hay, 1973; Stranicky, 1989; Ushakoff, 1948]. Plastic based solar stills present several advantages. Plastic materials are easy to machine and manufacture for large scale production. Plastics presents the advantage of being extremely light in weight and collapsible or foldable into very small space, making it easy to store and transport. Use of thermoplastic material makes the heat sealing and bonding of parts easier and quicker for mass manufacturing. Furthermore, plastics present an ideal solution for mass manufacturing by injection or extrusion molding [Wassouf et al., 2011]. Additionally, plastics are cheap and globally available. Thus, easier to locally mass manufacture and sell. All these properties make plastics attractive for large scale production of solar stills. However, most of the currently available solar stills like the water cone or the water pyramid are either too small or too large for a family [Flendrig et al., 2009; Wassouf et al., 2011]. Figure 1.4 show images of a plastic based solar still previously used.

It is remarkable that one of the most basic water purification technologies around has still not been widely applied yet, despite hundreds of designs, patents and peer reviewed papers. Over the years it has become clear that several factors like low purification yield per meter square of solar still surface, ease of use and maintenance of solar stills, relatively high initial investment and the size of the solar still play an important role in the acceptance of solar distillation [Chaibi, 2000; Flendrig et al., 2009; Goosen et al., 2000].

1.2.1 Challenges in application of solar stills

Lower production of water from the solar stills has been a major limitation in its commercialization [Xiao et al., 2013]. A higher production of water can be achieved by including design features like a pump to create a thin

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1.2 Previous work on solar distillation 9

Figure 1.4: Solar still are currently available either on an individual scale or on a community scale. Source: www.landfallnavigation.com; www.watercone.com; www.aaws.nl

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water film that rapidly heats up in the sun [Abu-Abdou, 1994] or V-through solar concentrators that heat up water before entering the solar still [Mahdi and Smith, 1994]. Furthermore, cooling of the condensation surface can also be used for increasing the production of purified water in solar distillation devices or solar stills. However, most of these methods are costly, complex and involve regular monitoring. Simple, easily operable and cheap methods for increasing the output of water from the solar stills can make them attractive to be used by families at a large number of rural locations.

Another major challenge for using solar still in practice is to work with the undesirable properties of glass as a material for mass production for solar stills [Burch and Thomas, 1998; Phadatare and Verma, 2007; Tleimat and Howe, 1969]. Glass is heavy, brittle and has high replacement costs. On the other hand, plastic is light weight, relatively unbreakable, easy to transport and easy to process. Historically, plastic solar stills have been commercially more successful than glass solar stills [Hay, 1973]. However, due to higher amount of water production amongst other materials, glass has been the superior choice of material for its use as condensation surface inside a solar still [Tleimat and Howe, 1969].

Another challenge faced by many solar still products has been loss of condensed water due to the dripping of droplets from the cover, back into the still [Delano, 1947; D.S. Halacy, 1967]. This observation is found for solar stills with plastic as a condensing surface. Once the weight of the water droplet becomes larger than the holding force of the surface tension, dripping of water droplets is expected. A condensing cover made with a more wettable material and use of higher angles of inclination angles has been suggested by [Delano, 1947; D.S. Halacy, 1967]. However, it is not clear which plastic material and what inclination angle is most suitable for use in a solar still.

Distilled water is not fit for direct use in drinking and cooking applications because of the lack of minerals that are highly desirable for the human body. Furthermore, addition of zinc to drinking water in suitable concentrations can reduce the mortality rate of diarrhea diseases by 23% [Fischer Walker and Black, 2010]. A post treatment method for addition of necessary minerals into distilled water is essential and beneficial for the consumers. Although, re-mineralization of desalinated water has been applied for large reverse osmosis units, not much in known for re-mineralizaion of distilled water from solar stills. It is not shown on how to re-mineralize the distilled water from the solar stills to meet the desired mineral composition set by WHO in a simple, cheap and reliable method. Furthermore, there is no literature found

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1.3 Present work and research questions 11

which has shown the effect of different flow rates and different temperatures on the concentration of minerals in water from a solar still. Can a system of re-mineralization be applied for a flow rate of 2.5 liters/day of distilled water? These questions are essential to be discussed before making a product based on solar distillation.

Finally, development of robust membranes for solar distillation for decentralized water purification has also been a major challenge. Use of membrane offers several advantages such as lower operating temperatures and pressure, 100% (theoretical) rejection of non-volatile solute and microbial contamination, ability to purify highly saline water, modular design and a decentralized capacity with solar energy, and lastly, low sensitive to feed variations (e.g., pH, total dissolved solids, etc.). However, membranes are prone to fouling and can cause difficulty in operation and maintenance. A strong robust plastic membrane which has a capacity of handling highly saline feed water in sunny conditions can be useful in implementation of modular decentralized solar water purification systems.

1.3 Present work and research questions

The present work is motivated for overcoming challenges which limit the access to clean drinking water to single families living in remote locations. More specifically, the focus of the present work is in answering the questions related to making a water desalination device capable of providing at least 2.5 liters/day of clean drinking water: a simple and easily to use solar distillation device for providing clean drinking water to a family of five, especially the children in the family. All the challenges are presented below in the form of research questions. The answers to these questions can enable the designers and manufacturers to make solar stills for application in remote locations. Furthermore, the product is envisioned to work with limited or no requirement for infrastructure, maintenance and formal support or training.

Firstly, we address the choice of the material for the use of the condensing cover of the solar still. Glass has been the preferable choice of material as it gives higher water production than other materials. However, glass is brittle, heavy and costly to replace and transport to remote locations and villages. The research question is “which material property of the condensing cover significantly determines the production of water from a solar still?”

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Secondly, we study the effect of dripping of condensed water from the inclined surface to answer the following questions: “ What is the effect of the contact angle (material) on the dripping from the condensing cover of a solar still”? Furthermore, ”Is there a critical length and an inclination angle up to which the dripping is negligible?”

Thirdly, we study the effect of increase in the area of the condensation surface of the solar still on the production of water. The research question is “What is the optimum area up to which increasing of the condensation area gives a considerable (>50%) increase in the production of water”?

Furthermore, we also perform a demonstration study to find “if the effect of the increase of condensation area can also be applied for a bigger, 1.8 m2

distillation device.

Finally, we study the effect of using thick membranes in a solar air gap membrane distillation unit. We address the question “ What is the effect of using thick poly-ethylene membranes on the production of water from a solar distillation unit?” Further, “Can such membranes be useful for high saline (up to 20%) feed water?”

1.4 Outline

This thesis consists of articles published or submitted to journals and reports intended for future publications and patents. The details of the experimental setup and the applied experimental techniques vary from chapter to chapter, and are therefore described in each chapter separately.

Chapters 2 and 3 describe the effect of condensing cover material on the production of water from a solar still. Chapter 2 describes the significance of reflection and wiping on the condensing surface of the solar still. Chapter 3 specifically shows the effect of dripping from different materials of condensing cover at various lengths and inclinations. The effect of different materials such as polyethylene, Teflon, glass and aluminum on the production of water from the solar still is shown. Most of the solar experiments for this section were done in Delft, The Netherlands.

Chapter 4 and 5 shows the effect of using additional condensation surface area on the production of water from a solar still on a lab scale and on a family scale unit, respectively. Tests in the lab are done by using a constant

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1.4 Outline 13

energy input of approximately 650 W/m2_{. The measurements of temperature}

were recorded and shown for different experiments. The temperature for most of the experiments lies below 800_{C. Most of the solar experiments for}

this section were done in Delft, The Netherlands.

Chapter 6 describes the use of solar membrane air gap distillation for water purification. The chapter shows the effect of using thick (1-20 mm) polyethylene membranes for air gap membrane distillation. It demonstrates the use of membranes in sunny conditions and at high salinity. It further tests the use of the system with further contaminants such as As, Fl, NO3, Fe and

bacteria. The solar experiments for this chapter were done in New Delhi, India.

Chapter 7 presents the main conclusions of this work; it presents a discussion of the main findings and their usability for making a plastic based solar still. This section puts the experimental results in a broader perspective and provides a clear linkage to the practical applications. This chapter also presents an outlook for further possibilities for future research and possibilities for application of decentralized water purification unit.

Additionally, the appendices includes chapters showing a potential method for re-mineralizing distilled water, by passing water over a mineral tablet. The tablet slowly dissolves into the passing water and thus re-mineralizes the water again. We show that it is possible to re-mineralize water in such a way that the mineral composition meets the WHO standards. Consequently a tablet with the right composition of salts can be made to be able to meet the WHO requirements.

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Chapter 2

Influence of condensation

surface on solar distillation

Glass has been the preferable choice of material for its use as a condensation surface in solar distillation as it gives higher water production than other materials. However, the reason for the higher production of water is uncertain. In this paper, we study the influence of different condensation surfaces on the production of water from solar water distillation. For various condensation surfaces, we study the effect of thermal conductivity, contact angle and angle of inclination and wiping of the condensation surface on the production of water from solar water distillation. From our results, we conclude that the contact angle is the most important parameter for choosing the material of condensation surface inside a solar water distiller. Subsequently, we also conclude that the reflection of solar irradiation from the surface is the most important phenomenon affecting the differences in water production from solar distillation.

2.1 Introduction

One of the major challenge for putting solar still in practice is to work with the undesirable properties of glass as a material for mass production for solar stills [Burch and Thomas, 1998; Phadatare and Verma, 2007; Tleimat and Howe, 1969]. Glass is heavy, brittle and has high replacement costs. On the other hand, plastic is light weight, relatively unbreakable, easy to transport and easy to process. Historically, plastic solar stills have been commercially more successful than glass solar stills. More than 400,000 units of plastic

R. Bhardwaj, M.V. ten Kortenaar, R.F. Mudde, Influence of condensation surface on solar distillation, Desalination, Volume 326, 1 October 2013, Pages 37-45

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solar still have been sold [Hay, 1973]. Still, due to higher amount of water production amongst other materials, glass has been the superior choice of material for its use as condensation surface inside a solar still [Tleimat and Howe, 1969].

Factors like the type of material, roughness, inclination, shape, transmittance, wiping and vibration of the condensing surface were found to have a significant impact on the production of water from the solar still [A-hinai, 2000; Artley et al., 1999; Aybar, 2006; Dhiman, 1988; Dimri et al., 2008; Eldalil, 2010; ElSherbiny and Fath, 1993; Ghoneyem and Ileri, 1997; Hay, 1973; Menguy et al., 1980; Pollet and Pieters, 2002a; Singh et al., 1995; Tiwari and Tiwari, 2007; Tiwari, 2003a; Tiwari et al., 1994; Tleimat and Howe, 1969]. The effect of condensing surface was studied in more detail by Ghoneyem and Ileri [1997]; Tiwari and Tiwari [2007]; Tleimat and Howe [1969] and Dimri et al. [2008]. The use of mechanically modified plastic against glass lowered the production of water by 18% [Tleimat and Howe, 1969]. The production of water from a solar still was found to be linearly related to the thickness and thermal conductivity of the condensing surface. The production of water decreased by 7% with an increase in glass thickness from 2 mm to 6 mm. The use of copper metal against plastic increased the production of water by 18% [Dimri et al., 2008]. Further, the production of water was found to be highest for an inclination angle of 150 _{in summers and 45}0 _{in winters [Tiwari and}

Tiwari, 2007]. Other studies for the optimisation of glass cover inclination were also performed [Artley et al., 1999; Aybar, 2006; Singh et al., 1995; Tiwari et al., 1994]. Furthermore, renewal of surface through vibration or wiping also increased the production of water [Dhiman, 1988; Eldalil, 2010; Menguy et al., 1980]. Menguy et al. [1980] reported a 25% increase in production of water by wiping the condensation surface of a spherical solar still. Eldalil [2010] reported an increase in water production by 72% with the combined effect of modified absorbing surface and vibratory condensation surface. Still, there are two underlying issues which need to be resolved completely. Firstly, amongst heat transfer, dripping, droplet growth and reflection from the condensation surface, which phenomenon relating to the condensation surface critically effects the production of water? And secondly, which material property from either thermal conductivity or contact angle significantly affects this phenomenon. An answer to these questions will explain why the use of glass in a solar still results in a higher production of water than other materials. Consequently, it will also suggest if the properties of plastic material can be tailored to match the properties of glass, and further use it as a condensing surface in a solar still.

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2.2 Theory 17

Figure 2.1: (a) A schematic of a typical solar still. The incoming solar radiation heats up water, resulting in subsequent evaporation and condensation of vapors. (b) Thermal resistance diagram for heat transfer at the surface. R and h represent the thermal resistance and heat transfer coefficient respectively. Subscripts I, S and E represent internal, surface and external properties. (c) Phenomena associated with the condensation surface in a solar still.

2.2 Theory

Figure 2.1 shows the phenomena associated with the condensation surface of a solar still. These phenomena were first explained in detail by Dunkle [1961]. On a broader level, heat transfer and vapor condensation has been extensively studied by Rose [1999] and Beysens [2006]. Furthermore, the effect of reflection of sunlight has been studied within greenhouses by Pollet and Pieters [2002a], Briscoe and Galvin [1991b], Pieters et al. [1997] and Cemek and Demir [2005]. Finally, the role of inclined surface has been studied in the solar stills by several authors [Artley et al., 1999; Aybar, 2006; Ghoneyem and Ileri, 1997; Singh et al., 1995; Tiwari and Tiwari, 2007; Tiwari et al., 1994]. The following sections summarize these effects and their relationship with the solar still.

2.2.1 Reflection from the condensing surface

The phenomenon of reflection of light off a surface with condensing droplets has been studied previously [Briscoe and Galvin, 1991b; Cemek and Demir, 2005; Pieters et al., 1997; Pollet and Pieters, 2000, 2002a]. Of the total solar radiation incident on the top of the solar still, a considerable part might get reflected. The reflection happens at two surfaces. First, at the condensation surface and then at the condensing droplet attached to the bottom of the condensation surface. The amount of reflection depends on the refractive indices of materials and the shape or mode of condensing droplet [Briscoe and Galvin, 1991b]. The mode of condensation depends on the contact angle

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of the condensation surface [Beysens, 2006]. Filmwise condensation mode occurs at surfaces with low contact angles resulting in flat droplet formation. Dropwise mode occurs on surfaces with high contact angles resulting in hemispherical droplets. For materials having contact angles greater than 480_{, the transmittance from the condensation surface is given by Briscoe and}

Galvin [1991b]. Pollet and Pieters [2000] and Pollet and Pieters [2002b] applied this study for greenhouse materials and reported a decrease of transmittance up to 25% when using plastic instead of glass.

2.2.2 Heat transfer from the condensing surface

The heat and mass transfer within the solar still have been previously investigated by Dunkle [1961], Tiwari [2003a] and Lof et al. [1961]. After reflection from the surface, the majority of the transmitted solar radiation is absorbed at the bottom of the solar still. The bottom of the still is usually a black material with a layer of water above its surface. The heat from the black bottom is then transferred to the water above it. The heat from the water is brought to the condensation surface in the form of vapor via internal convection. The vapor condenses and transfers its heat to the condensation surface. The difference between the water temperature and the condensation surface is the driving force for the heat transfer. Finally, the condensation surface conducts it to the outside environment via external convection. The overall heat transfer flux ϕq, from vapor inside the solar still

to the environment outside is given as ϕq = U ∆T = ∆T /R. Where ∆T is

the overall temperature difference from the vapor side to the environment. It is the driving force for the heat transfer. U is the overall heat transfer coefficient and represents the ability to allow transfer of heat. R = 1/U is the overall thermal resistance and represents the ability to resist heat transfer. Furthermore, the value of the overall thermal resistance for the solar still is represented as [Dunkle, 1961] : R = ( 1 hI + 1 hS + 1 hE ) ; hS = ( xm Km + xf Kf )−1 (2.1) where h , K and x represent the heat transfer coefficient and thermal conductivity and thickness of the condensation surface respectively. Subscripts I, S, E represent internal, surface and external phenomena. Subscripts m and f represent the thickness of the materials and the water film. Additionally, the overall heat transfer coefficient and efficiency η of the processes are calculated as [Dunkle, 1961] :

U = ( (mCp(Tv− Ts) + m∆Hvap A∆T t ) ; η = m∆Hvap ϕqinAt (2.2)

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2.2 Theory 19

where, m represents the mass of water condensed for a particular time duration t, A is the area of surface, Cp and ∆Hvap are the heat capacity

and the latent heat of condensation of water respectively. T represents temperature and subscripts v and s represent vapor and surface respectively.

ϕqinrepresents the incoming solar radiation heat flux.

2.2.3 Filmwise and dropwise condensation

The contribution of condensation towards overall heat transfer is extensively studied [Beysens, 2006; Rose, 1999, 2002; Yamali and Merte Jr, 2002]. The comparison of hSversus U identifies the importance of the thermal resistance

of the surface in limiting the overall heat transfer. Consequently, it explains the importance of the thermal resistance of the condensation surface on the production of water. The thermal resistance of the surface is a combination of the thermal resistance offered by the condensation surface and the water film or the droplets attached to it. Thus, such a comparison will clarify two things. First, the role of conductivity of the surface material on the overall production of water. Second, the importance of filmwise versus dropwise condensation in the improvement of the overall water production.

2.2.4 Droplet growth and dripping

Depending on the contact angle of the surface the condensed water will form tiny hemispherical droplets or large flat droplets [Beysens, 2006; Cemek and Demir, 2005; Rose, 1999] . The growth of the condensed water layer takes place in three stages [Beysens, 2006]. It starts with formation of droplets by heterogeneous nucleation, followed by growth of droplets in isolation and finally growth of droplet by coalescence. The first two stages are marked by low surface coverage of the material and the last phase happens when the surface coverage typically exceeds 30%. The final growth stage leads to a self-similar or self-repeating growth pattern in time. Further, out of all the regimes, the condensation rate in the third regime reaches a maximum value and remains constant.

As the droplet increases its size, it becomes heavier and flowable on the inclined surface [Briscoe and Galvin, 1991a; Chen et al., 2005]. For a pendant drop, there is a critical volume at which the droplet starts to slide and further detach from the surface. The critical volume depends on the contact angle of the surface and the angle of inclination which the surface makes with the horizontal [Briscoe and Galvin, 1991a; Chen et al., 2005]. Further, Chen et al. [2005] reported that the volume of a droplet hanging from a surface decreases,

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Figure 2.2: (a) Thermal still setup for replicating the effect of solar still inside the laboratory. (b) Parameters and material properties which were varied in the thermal still to measure their effects on phenomena described in Figure 2.1. All these parameters are tested in the absence of sunlight. Only the effect of contact angle on reflection is studied in the sunlight.

with an increase in the contact angle of the material and an increase in the angle of inclination of the condensing surface. Inside a solar still, falling of hanging droplets back to the impure water is undesirable and should be prevented [Aybar, 2006; ElSherbiny and Fath, 1993; Ghoneyem and Ileri, 1997].

2.3 Materials and methods

The experiments were designed to observe and measure the effect of parameters and material properties on phenomena described previously in Figure 2.1. The parameters and material properties which affect the condensation phenomenon are shown in Figure 2.2(b). These are inclination angle α, contact angle Θ, thermal conductivity K, time and wiping frequency. To test the parameters, experiments were conducted in a controlled lab environment and in the sunlight. The experimental setup for test in the sunlight is referred to as solar still and the experimental setup for the tests inside the laboratory testing is referred to as thermal still. Both these stills are explained in the following section.

2.3.1 Solar still setup

The experimental setup consisted of two hollow cuboid shaped boxes with an open ceiling. The base of the boxes had an area of 0.27x0.27 m2 _{and the}

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2.3 Materials and methods 21

four walls of the boxes were 0.12 m in height. Polymethyl methacrylate (PMMA or perspex) with a thickness of 8 mm was used for construction of the boxes. A Styrofoam material of 20 mm thickness was used for insulation of the PMMA boxes. Aluminum foil was pasted to the inside surfaces of the insulation. Each of the PMMA boxes contained a black sponge with an area of 0.25x0.25 m2 _{and a thickness of 0.02 m. The ceiling of the first box had a}

polyethylene terephthalate (PET) surface and the ceiling of the second box had a glass surface. Both the ceilings had a dimension of 0.28x0.28 m2 _and

a thickness of 2 mm. At the start of the experiment, 350 ml of water was absorbed in pieces of black sponge and kept in each box. The boxes were then sealed with a transparent ceiling and kept in the sunlight for a duration of 7.5 hours i.e from 11:00 to 18:30. After 18:30 the experimental setup was in shade and the experiment was stopped. During the time of experiment, the amount of water condensed on the ceiling was collected after the end of each hour. Additionally, the thermocouple probes were placed to measure the temperature of the sponge inside individual boxes. Furthermore, for the ceiling, transparent surfaces of polymethyl methacrylate (PMMA) or perspex and polycarbonate (PC) were also used. Table 2.1 gives a summary of materials and their dimensions used for construction of the solar still. The solar still setup is solely used to test the effect of reflection while using different transparent materials. The remaining effects described in Figure 2.1 are tested on the thermal still detailed in the following section.

Table 2.1:Design values for materials used in solar still and the dimensions of different components of the solar still

Component Material Thickness(mm)

Insulation Styrofoam 20

Solar still PMMA (Perspex) 8

Top condensation surface Glass, PET, PC, PMMA 2

Dimensional component Value

Area of condensation surface 0.27x0.27 m2

Depth of water* 20 mm

*Dimension are measured from the bottom of the solar still

2.3.2 Thermal still setup

General description

The experimental setup consists of three equipment; 1. Thermal still, 2. Temperature controller and 3. Hot water bath as shown in Figure 2.2(a). The

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thermal still forms the heart of the process in which all the measurements were taken. The temperature controller and hot water bath were used for regulating and maintaining the temperature of water, TW. Additionally,

thermocouples TV, TS and TA were positioned to measure vapor, surface

and ambient temperatures respectively. During operation, hot water from the water bath flows through a coil placed inside the thermal still. The heat input increases the temperature of the water resulting in the formation of vapor. The vapor rises and subsequently condenses on the under-side of condensation surface. The condensate from the condensation surface was collected batch wise in the bottom section of the thermal still. Figure 2.2(b) shows the parameters and the material properties that were tested on the thermal still for different phenomena described in Figure 2.1(b).

Table 2.2: Design values for materials used in thermal still and the dimensions of different components of the thermal still

Component Material Design parameter Value

Heating coil Copper Heat transfer area 0.01 m2

Top condensation surface Teflon, PE, PVC, Thickness 2 mm PET, Glass, Al plate

Thermal still PMMA (Perspex) Thickness 8 mm

Dimensional component Value

Area of condensation surface 0.28x0.28 m2

Depth of water* 25 mm

Height of bottom of condensation surface* 0.16 m

Height of top of condensation surface* α = 900

0.44 m *Dimension are measured from the bottom of the thermal still α = 300 0.33 m

Test section and materials

Table 2.2 describes the materials used for constructing the thermal still along with their dimensions. The thermal still is made of PMMA of 8 mm thickness. The heating coil is made of a copper tube with internal diameter of 12.7 mm and length of 0.25 m. The heat transfer area of the heating coil is 0.01 m2_{. The ceiling or the condensation surface at the top has a thickness of 2}

mm and an area of 0.28*0.28 m2_{. The area of the condensation surface was}

kept constant even when the inclination angle α, was changed from 300 _to

900. This modification was achieved by adding another support surface as shown with a dotted line in Figure 2.2(b). Consequently, the height of the thermal still was 0.44 m and 0.33 m when the inclination angle, α was 900

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2.3 Materials and methods 23

polyethylene (PE), polyvinyl chloride (PVC), PET, glass and aluminium (Al) as test materials. Out of these materials, only glass, PMMA and PET were used in experiments in the sunlight. The thickness of all surface materials was 2 mm. Finally, a NESLAB GP 600 hot water circulating bath and an in-house temperature controller are used as auxiliary equipment. The water in the thermal still was maintained at a depth of 0.025 m or 1.96 liters at the start of each experiment. The thermocouples were placed at locations as shown in Figure 2.2(a). The real time image of the setup is included in the supplemental info.

Table 2.3: Contact angle measurement for materials using contact angle microscope. The droplet volume used was 5 µl. Standard deviation from five measurements was± 2.500

Material Teflon PE PVC PC PET Glass Aluminum

Contact angle (0

) 105 90 83 81 71 30 ∼ 20

Table 2.3 lists the measured contact angles of materials used as condensation surfaces in the experiments. A KRUSS , Easy Drop Contact Angle Microscope was used for measuring the contact angle. The sessile drop method was used to estimate the contact angle by the microscope. Note that contact angle for Al plate is represented as∼ 200_{as the sessile method was}

unable to identify the contact shape accurately below this angle.

Measurement Procedure

The measurement procedure is described on the basis of the setup shown in Figure 2.2.

1. The water inside the hot water bath (equipment 3) was heated to a definite temperature before connecting to the thermal still (equipment 1). The value of the definite temperature was set to 5 0_{C above the}

temperature desired to be achieved in the thermal still. This method was adopted to limit the time it takes for water in equipment to reach desirable levels.

2. The temperature in the controller (equipment 2) was set to a desired value (usually 60 0_{C) and then connected to the thermal still. The hot}

water bath follows the controller and circulates hot water through the coil placed inside the thermal still. This procedure raises and maintains the temperature of the still to the desired value.

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3. The condensed water from the underside of the condensation surface was collected after a decided batch time (usually two hours). A typical example for measurement of temperatures for a two hour experiment is included in the last section of the chapter.

2.4 Results and discussion

2.4.1 Reflection of sunlight from glass and PET surfaces

Figure 2.3:(a) Reflection of sunlight from condensation surface of a solar still with glass and PET material. (b)Measurement of water temperature in solar stills for a duration of 7.5 hours of sunshine. (c) Measured volume of water condensed in the two stills at the end of each hour. (d) Normalized volume in ml/m2

at different times. Secondary axis represents the solar irradiation received at the location in MJ/m2. (e) Instantaneous efficiency calculated at the end of every hour of the measurement.

Figure 2.3(a) shows solar stills with a PET (right) ceiling and glass (left) ceiling. The PET is visually shinier than glass. The small condensed droplets formed on the surface of PET make it optically more reflective than the surface of glass. The condensation droplets on the glass surface are puddled shaped. The puddle shaped condensation surface allows much more light to pass through. A larger amount of light leads to larger input of solar energy. At equilibrium, energy coming in should match the energy going out. A larger amount of energy loss from the ceiling indicates a greater condensation

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2.4 Results and discussion 25

volume and hence greater efficiency of the solar still.

Figure 2.3(b) shows that the temperature of water in the solar still with a glass ceiling is higher than that measured for a PET ceiling. A larger glass temperature results in a larger temperature gradient for condensation heat transfer. The experiment was performed for a duration of 7.5 hours between 11:00 and 18:30 on June 2, 2013. The measurements of condensed water was taken at the end of each hour starting at 12:00. The data for incoming radiation was obtained from the Royal Dutch Meteorological Institute (KNMI). Location co-ordinates: longitude(east): 4.444, latitude (North):51.955 and elevation:-4.80 m. Figure 2.3(c) shows the measured amount of water condensed after each hour of experiment. The overall water condensed in the solar still having a glass ceiling is∼ 27% higher than the water condensed in the solar still with PET ceiling. Similar trends are observed for normalized volume and efficiency shown in figure 2.3(d)and (e).

The amount of condensate volume collected during the first hour of experimentation for the solar still with glass surface is considerably higher than PET. This is because of large amount of water still hanging at the PET surface at the end of first hour. The water stays attached to the PET surface due to its higher contact angle. The water has condensed on the PET surface but has not slid down towards the collection area. Additionally in Figure 2.3 (e), the efficiency of the solar still remains considerably high even during the end of the sunshine hours. This is because the water temperature in the still is high enough to drive condensation even at less solar irradiation.

The increase in the volume of water condensed for glass against PET is similar to the increase in the energy transmitted through glass against plastic reported earlier by various authors [Briscoe and Galvin, 1991b; Cemek and Demir, 2005; Pieters et al., 1997; Pollet and Pieters, 2000, 2002a]. These studies concluded that, for materials covered with a layer of condensate, the transmittance of solar radiation decreases with an increase in contact angle of the material. Additionally, for the same material, the transmittance can decrease to a maximum of 25% for plastics and 20% for glass. The percentage decrease was taken relative to the transmitted solar radiation in the dry state of these materials. It was also concluded that the inclination angle does not have a significant impact on the transmittance of incoming solar radiation. The latter study was carried out keeping the angle of incidence of incoming solar radiation normal to the inclined surface.

Furthermore, separate experiments were performed in a solar simulator to test the efficiency of a solar still in controlled conditions. The efficiency of the still was calculated as 28.5% based on an incoming solar radiation of 800 W/m2 _{for a duration of three hours. An Atlas Suntest XXL solar}

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Figure 2.4: (a) Reflection from transparent materials on a winter day. (b) Volume of water evaporated as measured from the black sponge place at the bottom of the solar stills.

Moreover, a solar still with glass and plastic ceiling shows similar trend for the temperature profiles inside the controlled conditions of a solar simulator. The amount of condensate achieved in several tests show variation of less than 10% for different transparent materials. A possible reason for this limited variation can be due to the angle of irradiation of artificial light inside the solar stills, which is much different from the angle of irradiation in sunlight. The irradiation angle in solar simulator is normal to the inclined surface, where as the irradiation angle for sunlight is dependent on the location of experimentation and the time of the day. Furthermore, the equipment could only operate at a minimum irradiation of 800 W/m2_{. It could not replicate the}

variation in intensity of irradiation as expected in the sunlight. The results of the experiments are not a good representation of experiments in sunlight and hence are not included here.

Furthermore, the effect of reflection is extended to four transparent surfaces viz. PET, PC, PMMA and glass. Figure 2.4(b) shows the effect of contact angle on the water production inside a solar still. The experiment was performed simultaneously with four different transparent materials and the temperature profiles are included in the supplemental information.

This section concludes the tests performed on the solar still. The following sections would cover the tests in laboratory in the absence of sunlight. From our findings in this section, we can conclude that the transmittance of light through a surface with condensed droplets plays an important role in determining the production of water from a solar water distiller. A more detailed study on the effect of inclination angle and incidence angle on the transmittance of light through different condensing surfaces for greenhouse ceilings has been performed by [Pollet and Pieters, 2000] and [Pollet and Pieters, 2002a]. Additionally, the effect of inclination angle in solar stills has

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2.4 Results and discussion 27 Table 2.4: Calculated values of heat transfer coefficient of the condensation surface, hS

Material Teflon PE PVC PC PET Glass Aluminum Water

K (W/m.K) 0.25 0.4 0.19 0.2 0.24 1.05 250 0.58

hS(W/m2.K) 119.83 187.10 91.99 96.67 115.23 444.53 2834.25

*Thickness of all the materials was 2 mm.

been studied and optimized for maximizing the incoming solar irradiation by several authors [Artley et al., 1999; Aybar, 2006; Singh et al., 1995; Tiwari and Tiwari, 2007; Tiwari et al., 1994]. A combination of these studies along with the effect of different materials at different inclination angles can be taken forward for future research.

2.4.2 Effect of heat transfer on the production of water

Heat transfer through the condensing surface has two components viz. heat transfer through the condensing surface material and heat transfer through the water film attached to the surface. These are discussed in the following sections.

2.4.3 Effect of thermal conductivity of material on the heat

transfer

Table 2.4 shows that for the given geometry, the calculated heat transfer coefficient of the condensation surface, hS is much larger than the calculated

values of the overall heat transfer coefficient, U. The value of U is ∼ 6 (J.s−1.m−2.K−1), based on the experimental measurements discussed later in the following section. A comparison of the values suggests that the contribution of condensation surface towards the overall thermal resistance is much smaller than the thermal resistance offered by internal and external convection. The thickness of the water film is taken as 0.2 mm, a reasonable assumption based on similar cases found in literature [Memory and Rose, 1991]. However, even at higher film thicknesses, the rate of overall heat transfer will remain independent of the heat resistance offered by the water film. A practical description of the effect of growing water film is shown below.

Effect of growing water film on the heat transfer

To check whether there is a change in production of water inside a thermal still for dropwise and filmwise condensation with time, experiments were

Decentralized water purification using solar thermal energy

Decentralized water purification

using solar thermal energy

Decentralized water purification

using solar thermal energy

PROEFSCHRIFT

Summary

Samenvatting

Contents

Chapter 1

Introduction

1.1

Background of the investigation

1.1.1

Present situation of drinking water

1.1.2

Burden of contaminated water on health

1.1.3

Rural-urban divide in drinking water quality

1.1.4

Decentralized water purification

1.2

Previous work on solar distillation

1.2.1

Challenges in application of solar stills

1.3

Present work and research questions

1.4

Outline

Chapter 2

Influence of condensation

surface on solar distillation

2.1

Introduction

2.2

Theory

2.2.1

Reflection from the condensing surface

2.2.2

Heat transfer from the condensing surface

2.2.3

Filmwise and dropwise condensation

2.2.4

Droplet growth and dripping

2.3

Materials and methods

2.3.1

Solar still setup

2.3.2

Thermal still setup

2.4

Results and discussion

2.4.1

Reflection of sunlight from glass and PET surfaces

2.4.2

Effect of heat transfer on the production of water

2.4.3

Effect of thermal conductivity of material on the heat

transfer