Design and Fabrication of Multi-functional Photovoltaic-Membrane. Distillation-Evaporative Crystallizer for Water Desalination, Electricity

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Distillation-Evaporative Crystallizer for Water Desalination, Electricity

Generation, Salt Crystallization and Solar Cell Cooling

Thesis by

Sara Ali S Aleid

In Partial Fulfillment of the Requirements

For the Degree of

Master of Science

King Abdullah University of Science and Technology

Thuwal, Kingdom of Saudi Arabia

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EXAMINATION COMMITTEE PAGE

The thesis of Sara Ali S Aleid is approved by the examination committee.

Committee Chairperson: Prof. Peng Wang

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COPYRIGHT

© November, 2019

Sara Aleid

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ABSTRACT

Ensuring electricity availability and acquiring access of potable water during emergencies in remote areas are becoming a global challenge around the world. Utilizing solar energy electricity generation by photovoltaics and clean water production by solar distillation have shown its great potential to meet the world’s future energy and water demands. In this work, we fabricated a photovoltaic-membrane distillation-evaporative crystallizer device (PV-MD-EC), in which high electricity generation efficiency (~15%), clean water production rate (~2.66 kg/m2 h) and salt crystallization from seawater can be achieved in such an integrated system under one sun illumination. In addition, the solar cell operates in a much lower temperature at around 48 oC, which is much lower than previous work. The advanced performance is attributed to the utilization of a highly porous and thinner hydrophobic membrane. This design provides a new strategy to address the challenge of water-energy nexus.

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DEDICATION

This couldn’t have happened without my family. To my parents, my father Ali Aleid thank you for being my backbone and support throughout this whole journey. My mother, Fatimah Almohaisen thank you for your kind heart and constant encouragement. You two made me the person I am today. I am sincerely grateful for the unconditional love and support you have offered me throughout my entire life. To my brothers Salman, Salih and Abdulmohsin you are the joy of my life. To my sister Haya, Thank you for always being there for me. Last but not least, to my grandfather, thank you for pushing me forward and for your constant encouragements. I aspire to be the person I want to be because of you. Your strength has taught me well, and your patience has taught me that goals are achievable.

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ACKNOWLEDGEMENTS

This study is a part of a master research project at the environmental science and engineering department, King Abdullah University of Science and Technology. I gratefully acknowledge the continuous support from the university and office of research.

I would like to express my gratitude to Professor Peng Wang, my research supervisor, for his encouragement, valuable critiques, and positive energy during the processes of this research and internship. His continuous support and time is highly appreciated. I would also like to thank Wenbin Wang for his guidance and assistance during the process of this research. Also, I would like to extend my thanks and appreciation to the thesis committee members, Prof. Pascal Saikaly and Prof Zhiping Lai, for your time. I am also thankful to all lab members; environmental nanomaterials lab, for their assistance and positive energy.

Special thanks to my close friends, Yara Aldrees and Mohammad Hashem for always having my back no matter what. Hard times were made easier, and good times were made better thanks to you. This journey wouldn’t have been as wonderful without you being a huge inspiration and I am sincerely grateful for having to share this opportunity with you two.

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Table of Contents

EXAMINATION COMMITTEE PAGE ... 2

COPYRIGHT ... 3 DEDICATION... 5 ACKNOWLEDGEMENTS ... 6 LIST OF ABBREVIATIONS ... 10 LIST OF FIGURES ... 12 Chapter 1: Introduction ... 13 1.1 Solar cell ... 15 1.2 Solar still ... 17

1.3 Simultaneous production of clean water and electricity ... 18

Chapter 2: Materials and Methods ... 21

2.1 Chemicals and Materials ... 21

2.2 Device Fabrication ... 22

2.2.1 Multistage Membrane Distillation Component ... 22

2.2.2 Evaporative Crystallizer... 23

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2.3.1 Performance Evaluation Using Clean Water as Source Water ... 23

2.3.2 Seawater Desalination ... 24

2.4 Characterization ... 25

2.4.1 Membrane Porosity ... 25

2.4.2 Water quality analysis and UV-Vis-NIR ... 25

2.5 Clean water production rate calculation ... 25

2.6 Vapor pressure calculation ... 26

2.7 Thermal Conductivity Calculation ... 26

Chapter 3: Results and Discussion ... 28

3.1 Structure of the PV-MD-EC device ... 28

3.2 Theoretical Calculation ... 30

3.3 UV-VIS-IR Spectra of the Solar Cell ... 34

3.4 Evaluation of the PV-MD-EC Device Using Clean Water as Source Water ... 35

3.5 Seawater Desalination ... 38

Chapter 4. Conclusion ... 43

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LIST OF ABBREVIATIONS

Ca: Calcium Cd: Cadmium CO2 Carbon dioxide e.g.: example

EC: evaporative crystallizer i.e.: in essence

ICP-OES: Inductively coupled plasma optical emission spectrometry Isc: Short circuit current

K: Potassium

MED: Multi-effect distillation MFD: multi-flash desalination Mg: Magnesium Na: Sodium NOx : Nitrogen oxides PE: polyethylene PTFE: Polytetrafluoroethylene

PV-MSMD: photovoltaic-multistage membrane distillation device

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RO: Reverse osmosis SO2: Sulfur Dioxide

UV-VIS-NIR: Ultraviolet-visible-near infrared spectroscopy Voc: Open circuit voltage

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LIST OF FIGURES

Figure 1. Strategies used for solar cell cooling: a photovoltaic cooler45, b air flow cooling46, c

using lake as cooler47 and d water flow cooling48 ... 16

Figure 2. The solar distillation system60 ... 18

Figure 4. The scheme of the hydrophilic membrane used in the evaporative crystallizer. ... 23

Figure 5. Scheme of the PV-MD-EC device ... 28

Figure 6. Relationship between the thickness of the membrane and membrane coefficient. ... 30

Figure 7. a Experimental setup b UV-Vis-IR spectra of the solar cell and sunlight c Mass change of the clean water production and evaporation d Temperature of the solar cell e J-V curve. ... 34

Figure 8. a Clean water production rate from seawater and evaporation rate b Efficiency of solar cell during seawater desalination ... 38

Figure 9. Crystallization of salt under the PV-MD-EC device via the evaporative crystallizer ... 40

Figure 10. ICP-OES measurements of ion concentrations in source (before) and desalinated (after) water with respect to WHO standards for drinking water quality ... 41

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Chapter 1: Introduction

Energy generation and clean water production are evidently intertwined. The water-energy nexus has become a hot topic due to its significant role in the achievement of global sustainability1-3. With the booming of world’s population, water scarcity is regarded as one of the most important challenges in many countries4, 5. Large-scale seawater desalination has been practiced for many years by reverse-osmosis (RO)6-8, multi-effect distillation (MED)9-11 and multi-flash desalination (MFD)12, 13. These approaches however entail massive amounts of electricity. For instance, in Saudi Arabia, 20% of the total national electricity is consumed for desalination alone14. On the other hand, in the United States and Western Europe, about 50% of water withdrawals are for energy production. The ramification of water-energy nexus is and will remain a global concern. At a global level, fresh water consumption rate has increased more than twice as fast as the population growth over the last century, pushing many countries and regions gradually into water scarce state. One-third of the global population live under conditions of severe water scarcity at least one month of the year, while half a billion people in the world face severe water scarcity all year round.4, 15 It has been reported by the World Health Organization (WHO) that over 1.7 billion kids suffer from diarrhea annually due to unhygienic water, leading to more than 500 thousand deaths every year and making it a leading cause of death of children globally.

On the other hand, more than 60% of the world’s electricity is currently generated by burning non-renewable fossil fuels16, which causes an increase in the emission of CO2 and other hazardous gases such as NOx and SO217-20. The emission of CO2 leads to the greenhouse effect, disrupting the global carbon cycle and leading to a planetary warming effect. Moreover, NOx and SO2 are

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considered harmful to human health 21-23 and are major contributors to acid rain which harms terrestrial ecosystems and degrade the water quality in lakes24-26.

Solar energy is rising as the most clean and renewable energy source, having a great potential to address the very strained water-energy nexus and the global warming concern. Solar energy can be directly converted into electricity by the photovoltaic (PV) effect.27 It is predicted that the CO2 emission level can be reduced in 2050 by 62% from the level in 2005 if PV solar cells were implemented along with supplementary renewable energy.28, 29

It was reported that, in the United States, a solar cell-based system yielded up to 98% less greenhouse gasses and heavy metals28 compared to a coal burning plant. For example, the CO

2 emissions in the life-cycle of CdTe thin film photovoltaics is about 20 g/kWh while the emission from fossil-fuel plants is about 500-1000 g/kWh. Furthermore, the life-cycle emissions from photovoltaics showed that CdTe solar cells emit about 0.02g Cd/kWh, in comparison to 2g Cd/kWh from a coal burning plant.

Over the past decades, many strategies have been proposed to balance the relations between the water scarcity and energy. For example, the utilization of the solar energy to generate electricity by solar cells30-32 or producing clean water by solar still33-35. These examples can reduce the dependence on the fossil fuels and minimize the emission of CO2 and other hazardous gases. However, only 33.7% of the solar energy can be theoretically converted into electricity in a single junction solar cell according to Shockley-Queisser limit36. Also, a conventional solar still shows a very low clean water production performance (i.e., 0.5-4 kg/m2 day)37-39. A photovoltaic-membrane distillation (PV-MD) device was designed in 2019 to effectively generate clean water and electricity simultaneously40. However, there still remain some problems in the PV-MD device

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such as high solar cell temperature, heat sink demands for dissipating residual heat and concentrated brine water production.

1.1 Solar cell

The PV efficiency is constrained by the bandgap energy of the materials used in the solar cell, resulting in a maximum electricity generation efficiency of 33.7%. In most of the commercial solar cell, the efficiency is generally below 22%41, and the rest of the solar energy is mainly converted into heat, leading to an increased solar cell temperature up to 30 oC above the atmosphere temperature42, 43. The rise in solar cell temperature results in the decline of open circuit voltage (Voc ), fill factor and power output of around 2-2.3 mV/oC, 0.1-0.2%/oC and 0.4-0.5%/oC respectively, with increase in short circuit current (Isc) of 0.06-0.1%/oC. Furthermore, every 10 oC temperature rise leads to a doubled aging rate of solar cell44.

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Figure 1. Strategies used for solar cell cooling: a photovoltaic cooler45, b air flow cooling46, c using lake as cooler47 and d water flow cooling48

Some strategies were proposed for solar cell cooling. For example, since the heat can be dissipated into the environment by thermal radiation, the emissivity of the solar cell was designed to be as high as ~0.9549, 50. Fan’s group successfully developed radiative cooling, which was achieved by designing a one-dimensional photonic cooler that reflects undesired solar spectrum and releases heat via thermal emission. However, this approach cools down the solar cell by only 5.7 oC which can only increase the efficiency by roughly 2.25%45. In another work, thermal cooling was achieved by reducing 13 oC of temperature as a result of inserting a transparent thermal black-body over the solar cell51. In addition, increasing the thermal conduction or convection loss from the solar cell using flowing lake water as cooler and air flow can lead to a lower solar cell temperature46, 52-54. However, electricity consumption is required to power the water flow or air

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flow. These strategies, without any exception, regard the heat generated from the solar cell as “waste” and dump it into environment.

1.2 Solar still

Solar distillation has recently been investigated by many groups in the context of emerging interfacial heating scheme. In an interfacial heating-based solar distillation system, the solar energy is absorbed by the photothermal material which is floated on the interface of the water and converts solar energy into heat55-57. This interfacial heating process can increase the evaporation rate of the water by 2~5 times, typically in the range of 0.9~1.5 kg/m2/h, leading to high energy efficiency.

However, when a condensation system is integrated with the interfacial evaporation system (Figure 2), the whole system shows a decreased clean water production rate of 0.3-0.7 kg/m2/h58, 59. In the absence of a condensation chamber, the generated vapor is diffused into the atmosphere directly where the vapor has little effect on the humidity of its surroundings and a high evaporation can persist. When the evaporator is covered by a condensation chamber, the latent heat is released on the inner surface of the chamber, resulting in a temperature rise in the condensation chamber. In a whole evaporation-condensation system, the driving force is the vapor pressure difference between the evaporation surface and condensation surface which is caused by the temperature difference according to the Antoine equation. With a lower temperature difference, the driving force for water condensation is reduced. In this case, the chamber has a high temperature due to the condensation of the vapor, leading to a lower condensation rate. For the evaporator, since the generated vapor is not diffused nor condensed in time, the humidity of the surroundings will increase, resulting in a lower evaporation rate.

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Figure 2. The solar distillation system60

1.3 Simultaneous production of clean water and electricity

Recently, the concept of the simultaneous production of electricity and clean water via sunlight has attracted tremendous attention due to its potential to balance the relationship between the water and energy supply, but met with very limited success. For example, piezo-pyroelectric material was used to generate electricity from the temperature fluctuation of the water vapor in a solar-driven water evaporation process in an open space (i.e., no water condensation). Resulting in a solar-to-water-evaporation efficiency and electricity generation efficiency of 90% and ~0.01%61, respectively. In another work, Nafion membrane was applied to produce electricity from the

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salinity gradient generated in a solar seawater distillation process, resulting in a 75% water evaporation efficiency in an open space and 0.1% electricity generation efficiency under one-sun illumination62. Recently, the thermoelectric module was employed in a solar still to convert the latent heat released during water vapor condensation step to electricity via thermoelectric effect, producing a 72.2 % clean water production efficiency and 1.23% electricity generation efficiency under highly concentrated solar irradiation (i.e., 30 sun)63.

Figure 3. Schematic illustration of the integrated photovoltaic panel-membrane distillation (PV-MD) devices with three stages MSMD devices operated at a dead-end mode and b cross-flow mode

In 2019, Wang’s group developed a photovoltaic-membrane distillation (PV-MD) device which can achieve a simultaneous production of clean water and electricity via sunlight radiation with high energy efficiency on both (Figure 3)40. In this device, the PV-MD is mainly composed by two parts: top and bottom. The top is solar cell, which captures and converts solar energy into electricity and heat. The bottom side of the solar cell is a multistage membrane distillation device (MSMD)

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which can utilize the heat generated from the solar cell to power the water desalination process. For a single stage, it includes a thermal conduction layer, evaporation layer, porous hydrophobic membrane and condensation layer. The water in the evaporation layer is heated up and evaporated to generate the vapor, then the vapor will pass through the porous hydrophobic membrane that is only permeable to vapor, and condensation will subsequently take place on the condensation layer. The condensation process will release the latent heat, which is in turn used as the heat source for the next stage.

In order to avoid the crystallization of the salt in the evaporation layer, a cross-flow mode is designed. The source water flows into the device driven by a mechanical pump or gravity and then flows out of the device before reaching saturation. Although highly efficient clean water production (1.64 kg/m2 h) and electricity generation (11%) could be achieved in the PV-MD device, the solar cell exhibited a high temperature (58 oC in a three-stage PV-MD device). In addition, a heat sink immersed bulk water was used to dissipate the latent heat generated in the bottom condensation layer into the environment. These constrain the wider application of PV-MD technology.

In this work, an evaporative crystallizer (EC) is further designed for PV-MD and EC can recycle the latent heat in the bottom condensation layer to evaporate the concentrated brine water produced from the multistage membrane distillation. The new device, denoted as PV-MD-EC in short, produces salt solids as the only waste.

PV-MD-EC integrates four functions in one device: electricity generation, water desalination, solar cell cooling and zero-liquid-discharge (ZLD).

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Theoretical analysis indicates that when the thickness of the porous hydrophobic membrane used in the PV-MD-EC is reduced, it is possible to achieve a lower solar cell temperature and higher clean water production performance. Based on this, we fabricated a five-stage PV-MD-EC device by using a new hydrophobic membrane (thickness of 0.1 mm and porosity of 0.84). EC was placed right underneath the device as a cooler to replace the heat sink in previous PV-MD. In this case, the heat is consumed by the evaporation process. Compared to the PV-MD device, the solar cell in the new PV-MD-EC device shows a lower operating temperature (45 oC for PV-MD-EC, 63 oC for PV-MD) and higher clean water production performance (3.8 kg/m2 h for PV-MD-EC, 2.5 kg/m2 h for PV-MD).

Chapter 2: Materials and Methods

2.1 Chemicals and Materials

The stainless-steel mesh used for evaporation layer and condensation layer was supplied by Furun and its porosity is 82% and the pore size is 15μm. The hydrophobic membrane (PTFE) membrane was supplied by Zhejiang Kertice Hi-tech Fluor-material Co., LTD, with the porosity of 84% and the pore size of 0.45μm. The epoxy glue used to seal the device was supplied by ALTECO CHEMICAL PTE LTD. The type of stainless steel used for thermal conduction layer was 316L and the tube with the inner diameter of 0.7 mm and outer diameter of 0.9 mm used for water transport was made by 316L stainless steel. The solar cell was provided by KuHong and the hydrophilic membrane for evaporative crystallizer was provided by Kimberly-Clark. Seawater was obtained from Red Sea in the King Abdullah Monument inside KAUST campus.

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2.2 Device Fabrication

2.2.1 Multistage Membrane Distillation Component

The multistage membrane distillation component was assembled from the top to the bottom stages. The top stage was assembled as follows: the thermal conduction layer (316L stainless steel sheet) was cut into 5 cm2 square sheets. Then, a piece of stainless-steel mesh with the size of 3.9x3.9 cm was placed on the stainless-steel sheet (thermal conduction layer) serving as the evaporation layer. A piece of polytetrafluoroethylene (PTFE) membrane with the size of 5x5 cm was placed on the evaporation layer (stainless steel mesh). The stainless steel tubes were inserted into the two opposite sides of the stainless steel mesh, serving as the inlet and outlet of the water transport. The side areas were sealed by epoxy glue, which was made by mixing the A resin and B hardener with the ratio of 1:1. After the glue was solidified (two hours later), another piece of stainless steel mesh (3.9x3.9 cm) was placed between the hydrophobic layer and another piece of stainless steel sheet with the size of 5x5 cm serving as condensation layer. Then a stainless steel tube was inserted into one side of the stainless steel mesh, working as the outlet of the clean water. Finally, the side areas were sealed by the epoxy glue. The tube used to transport water was inserted into the evaporation layer and condensation layer before sealing process. These layers made up a single stage, and the processes were repeated four more times to assemble a 5-stage membrane distillation device. A piece of polycrystalline silicon solar cell (KuHong) was used and put on the device as the photovoltaic and photothermal components. The UV-VIS-NIR spectra of the solar cell was obtained from UV-VIS-Lambda 950. The range of the wavelength was from 250 nm to 2500 nm and the data interval was 1 nm.

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2.2.2 Evaporative Crystallizer

For the evaporative crystallizer, a hydrophilic membrane with the size of 3.9*3.9 cm was cut into the shape as shown in Figure 4 and then stuck to a piece of stainless-steel sheet by a melt method. A piece of polyethylene (PE) membrane with the thickness of 0.01 mm was put between the hydrophilic membrane and stainless-steel sheet, and then the stainless-steel sheet was put on a heater plate with the temperature of 180 oC and kept there for 30 seconds.

Figure 3. The scheme of the hydrophilic membrane used in the evaporative crystallizer.

2.3 Experimental setup

2.3.1 Performance Evaluation Using Clean Water as Source Water

The evaporative crystallizer was put on a holder and the wick of the EC was immersed in a container (container-1) which was full of clean water. The mass change of the container-1 was monitored by an electrical balance. Then, the multistage membrane component was put over the EC and a mechanical pump (ISMATEC tubing pump) was connected to the inlet of the MSMD

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component and the outlet of the concentrated source water in the MSMD component was connected to a second container (container-2) to store the water. The outlet of the condensation layer was connected to a third container (container-3) to collect the clean water. In this case, an electrical balance was used to monitor the mass change of the container-3. After that, a piece of solar cell was placed on the MSMD component and connected to a Keithley 2400 series source meter. A thermal couple was used to monitor the temperature of the solar cell. A solar simulator (Newport 94043A) with a standard AM 1.5 G spectrum optical filter and the intensity of 1000 W/m2 was used. The solar simulator was calibrated by a solar power meter (AMPROBE SOLAR-100).

2.3.2 Seawater Desalination

In this experiment, the setup was similar to the production of water using clean water source. However, the flow of water was controlled by gravity rather than using a mechanical pump. A fourth container (container-4) which contained seawater, was placed higher than the MSDM component to drive the flow of seawater using gravity. Before the experiment, the mass change of container-2 was firstly monitored by an electrical balance to determine the source water flow rate and to keep the flow rate constant. A mechanical pump was used to supply the seawater to container-4 constantly. In the evaporative crystallizer, the concentrated seawater obtained in the last day was used as source water. 0.05% of anti-scaling agent was added to it to prevent the blockage of the hydrophilic membrane in the EC.

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2.4 Characterization

2.4.1 Membrane Porosity

The membrane porosity was measured by a gravimetric method. The hydrophobic membranes were cut into the size of 5x5 cm first. Then, the membrane was measured in their dry state for their dry weight (W1). After that, the membrane was dipped into ethanol and the weight of the membrane was measured after five minutes to get wet weight (W2). The porosity was calculated according to the following equation64:

𝜀 = (𝑊2−𝑊1)/𝐷𝑒

[(𝑊2−𝑊1)/𝐷𝑒]+𝑊2/𝐷𝑚 (Equation 1)

Where W1 (g) is the dry weight of the membrane, and W2 (g) is the wet weight of the membrane. De and Dm are the density of the ethanol (0.789 g/cm3) and the membrane materials (PTFE, 2.20 g/cm3), correspondingly.

2.4.2 Water quality analysis and UV-Vis-NIR

The element concentration of the seawater and collected clean water was measured by ICP-OES and the detected elements included Na, Ca, K and Mg.

The UV-Vis-NIR diffuse reflectance spectra of the samples were recorded with an Agilent Cary 5000 spectrometer, with BaSO4 powder as reference.

2.5 Clean water production rate calculation

According to membrane distillation theory, the vapor flux across the porous hydrophobic membrane, which is also known as the clean water production rate, is a function of the membrane

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coefficient and vapor pressure difference between the evaporation layer and condensation layer and can be calculated as follows65:

𝐽 = 𝐶𝑚× ∆𝑝 (Equation 2) Where J (kg/m2 h) is the vapor flux across the hydrophobic membrane, Cm (kg/m2 h Pa) is the membrane coefficient, ∆𝑝 (Pa) is the vapor pressure difference between the evaporation layer and condensation layer.

2.6 Vapor pressure calculation

Vapor pressure in the evaporation layer and condensation layer is a function of the temperature and can be calculated by Antoine equation66.

𝑙𝑛𝑝 = 𝐴 + 𝐵

𝐶+𝑇 (Equation 3)

Where P (mmHg) refers to vapor pressure, A, B, and C are constant (18.4057 for A, 3903.66 for B and 231.6 for C) and T (K) is temperature.

2.7 Thermal Conductivity Calculation

Since the hydrophobic membrane is composed of fibers, the equation67 below is used to calculate the thermal conductivity of the hydrophobic membrane:

k = ∅ 1 𝑘𝑎+

1−∅ 𝑘𝑚

(Equation 4)

Where ∅ refers to the porosity of the membrane, ka and km (W/m k) refer to the thermal conductivity of the air and the material of the membrane correspondingly. The transferred heat through thermal conduction can be calculated from the following equation below:

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𝑄𝑐 = k × ∆𝑇

𝛿 (Equation 5)

Where the intensity is referred to 𝑄𝑐 (W/m), the thermal conductivity is k, ∆𝑇 (K) is the temperature difference between the evaporation layer and condensation layer and 𝛿 (m) stands for the thickness of the membrane.

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Chapter 3: Results and Discussion

Figure 4. Scheme of the PV-MD-EC device 3.1 Structure of the PV-MD-EC device

The structure of the device is shown in Figure 5. As can be seen, it is mainly composed of three parts, a solar cell on the top, a multistage membrane distillation (MSMD) in the middle and an evaporative crystallizer at the bottom. The main role of solar cell is to harvest the solar energy, converting it to electricity and heat under the illumination of sunlight. The generated heat is utilized by MSMD device to power the process of water desalination. The MSMD consists of several stages and each stage has four main layers composed of a first top thermal conduction layer following with the second evaporation layer right beneath. The third layer is the hydrophobic membrane followed by the fourth layer which is the condensation layer. These four layers make up one

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complete stage of the MSMD device. The water flows from the water source by using either gravity or mechanical pump and flows out of the device before reaching saturation. An evaporative crystallizer is designed and placed underneath the MSMD device to recycle the latent heat of the condensation layer in the bottom stage of the MSMD. The evaporative crystallizer serves as cooler to provide a temperature difference between the solar cell and the bottom. The source water of the evaporative crystallizer can be the concentrated water of the MSMD. For the concentrated water production rate, it can be calculated by the following equation:

𝐽𝑠 = 𝐽𝑤 − 𝐽𝑐 (Equation 6) Where Js (kg/m2 h) refers to the concentrated water production rate, Jw and Jc (kg/m2 h) are source water flow rate and clean water production rate of the MSMD.

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Figure 5. Relationship between the thickness of the membrane and membrane coefficient. 3.2 Theoretical Calculation

Since the vapor flux across the membrane is a function of the vapor pressure and membrane coefficient (Equation 2), it is of significance to calculate the membrane coefficient. This is a thermally driven transport of water through microscope hydrophobic membranes, which is exactly the same as the traditional direct contact membrane distillation (DCMD) and thus the mass transport model used in DCMD can be applied in this case. Generally, the calculation of the membrane coefficient is based on the assumption that gas permeates through the porous hydrophobic membrane and there are three processes and their associated models, including Knudsen diffusion, continuum diffusion and the transition between them. The relation between the

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mass transport with collisions (molecule-molecule collisions or molecule-pore wall collisions) is taken into account by these three process models. Therefore, the division of these three models depends on the relation between the pore size (r) of the membrane and mean free path (λ) of the vapor. For Knudsen region (r < 0.5 λ), molecule-pore wall collisions are predominant as the pore size of the membrane is very low. For continuum diffusion (r > 50 λ), molecule-molecule collisions are predominant as the pore size of the membrane is large enough. If 0.5λ < r < 50λ, both of the continuum diffusion and Knudsen diffusion need to be considered and thus this diffusion is known as transition diffusion. For water desalination, the mean free path of the vapor is estimated to be 137.7 nm at 40 oC and 1 atm.68 Since the molecule-pore wall diffusion can slow down the movement of the vapor molecule, membrane with smaller pore size results in a lower membrane coefficient. Generally, the pore size of the membrane used in this case is higher than Knudsen region. When the pore size of the membrane is very large (e.g. r > 6μm in continuum region), liquid water can pass through the membrane directly. Considering these reasons, the transition diffusion is more suitable in this case and thus the membrane coefficient can be calculated by the following equation:68: 𝐶𝑚= [ 3 2 𝜏𝛿 ∅𝑟( 𝜋𝑅𝑇 8𝑀) 1/2 +𝜏𝛿 ∅ 𝑃𝑎 𝑃𝐷 𝑅𝑇 𝑀] −1 (Equation 7)

Where τ, δ (m), and r (m) stand for pore tortuosity, thickness and pore radius of the membrane, accordingly. M (kg/mol) is the molecular weight of water, R is the gas constant and T is the temperature of the membrane. Pa and P (Pa) are the air pressure and the total pressure inside the pore, respectively. D is the water diffusion coefficient. The tortuosity of the membrane can be calculated according to Macki–Meares’s model69:

τ =(2−∅)2

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The value of water-air PD can be calculated by the following equation69:

PD = 1.895 × 10−5𝑇2.072 (Equation 9)

The main purpose of this project is to achieve a lower solar cell temperature, while maintaining high clean water production performance of the MSMD. For each stage, the heat is transferred via two pathways: thermal conduction and thermal convection (evaporation and condensation process). Therefore, it can be understood that decreasing the thermal conduction and increasing thermal convection can result in a higher clean water production performance. In the previous design, a thick porous hydrophobic membrane (with the thickness of 3 mm) was used in each stage to prevent the water flow from evaporation layer to condensation layer and provide the pathway for vapor only. According to equation 7 and equation 8, with the increased thickness of the hydrophobic membrane, the heat conducted from the evaporation layer to the condensation layer can be decreased. On the other hand, the vapor flux is a function of membrane coefficient and vapor pressure difference between the evaporation layer and condensation layer (equation 5). Therefore, the membrane coefficient plays an important role in the clean water production performance of the device. According to equations 2~4, the membrane coefficient in thicker membrane is lower due to the elongated pathway of the vapor given by the thicker hydrophobic membrane. A lower membrane coefficient requires a higher driving force (i.e., vapor pressure difference) to keep high clean water production performance. Hence, a higher temperature difference is demanded to provide higher driving force, and this can inversely increase the thermal conduction energy loss. All of this results in a higher solar cell temperature. Therefore, the key to decrease the solar cell temperature is to reduce the thermal resistance of the device. This can be

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achieved by using a thinner hydrophobic membrane, but its exact effect on the water production performance is still not clear.

In this project, the relation between the thickness of the membrane and membrane coefficient was first calculated and the results are shown in figure 6. It shows an inverse relationship between the membrane coefficient and the thickness of membrane where increasing the thickness decreases the membrane coefficient drastically. The major decrease is observable when the thickness is low, e.g. from 0.1 mm to 1 mm, the membrane coefficient decreased dramatically from 4.24 to 0.42 kg/m2 h kpa. While beyond 1 mm the change is insignificant and it only decreases by 0.32 kg/m2/h kPa from 1 mm to 4 mm. Thus, it can be predicted that when the thickness of the membrane is decreased to 0.1 mm, the solar cell temperature can be decreased a lot owing to its lower thermal resistance. High clean water production performance can also be possibly realized even in a lower temperature difference due to the significantly higher membrane coefficient. To test this hypothesis, we fabricated a five stage PV-MD-EC device and the thickness of the hydrophobic membrane used in this device was 0.1 mm as compared to the membrane of 3 mm in the previous one. 70

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Figure 6. a Experimental setup b UV-Vis-IR spectra of the solar cell and sunlight c Mass change of the clean water production and evaporation d Temperature of the solar cell e J-V curve.

3.3 UV-VIS-IR Spectra of the Solar Cell

The experimental setup for the MSMD device is shown in figure 7a. In this case the driving force for the water flow was provided by mechanical pump and the clean water was used as source water for both of the MSMD and EC. The UV-Vis-NIR spectrum of the solar cell was firstly measured to determine its absorptance and the result is shown in figure 7b. The solar cell shows high absorptance (>90 %) in the short wavelength range (300-800 nm). In this region, the solar energy is mainly converted to electricity through the photovoltaic effect, while for the longer wavelength range (>800 nm) it shows a lower absorptance (<80 %). The intensity of the solar radiation is not equivalent among different wavelengths which is shown in the gray line in figure 8b. Therefore, the solar absorptance was used to determine the ability of the solar cell to harvest solar radiation

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and it is defined as a weighted fraction between absorbed radiation energy and solar radiation energy. It can be calculated as follows71:

𝛼 =∫ 𝐼𝜆𝐴𝜆𝑑𝜆

2500 300

3002500𝐼𝜆𝑑𝜆

(Equation 10)

Where 𝛼 refers to solar absorptance, 𝜆 is the wavelength of the light, I and A are the intensity of the light and absorption, respectively. The solar absorptance of the solar cell used in our experiment is calculated to be 87% which means that around 87% of solar energy can be obtained by the solar cell.

3.4 Evaluation of the PV-MD-EC Device Using Clean Water as Source Water

The water production performance of the MSMD and the water evaporation of the EC was measured using the solar cell as heat source and the solar cell was connected to the source meter to measure its J-V curve. The results are shown in Figure 7c-e. In previous work, the PV-MD device was made by thicker hydrophobic membrane with the thickness of 3 mm, denoted as PV-MD-thicker. In this work, we fabricated a similar one with thinner hydrophobic membrane. The new device is compared with regards to the mass change of the clean water production and evaporation. The main difference of these two devices is the thickness of the hydrophobic membrane which was 3 mm and 0.1 mm for the old and new device respectively. The source water was wicked into the evaporation layer in the PV-MD-thicker while for the new device a mechanical pump was used to drive the flow of water into the evaporation layer. According to the mass change of the collected clean water (Figure 7c), it can be calculated (by the slope of the line). The PV-MD-thicker exhibited a clean water production rate of 2.20 kg/m2 h, and at the same time, the

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evaporation rate of the EC was 0.73 kg/m2 h. In the new device, the flow rate of the source water was controlled to be 7, 10 and 13 kg/m2 h. The corresponding clean water production rates were 3.67, 3.49 and 3.11 kg/m2 h, and the evaporation rate in the EC were 0.83, 0.78 and 0.63 kg/m2 h. This indicates that there is a trend that, with increasing the flow rate, both the water production performance of MSMD and evaporation performance of EC decrease. This is mainly because that part of the sensible heat is lost due to the water flow.

Compared to the PV-MD-thicker which had a thicker hydrophobic membrane, the new device exhibited a higher clean water production and evaporation rate. For example, when the flow rate of the source water was set to 7 kg/m2 h the average production rate was 3.67 kg/m2 h and the average evaporation rate was 0.83 kg/m2 h. However, for the PV-MD-thicker the average production and evaporation rate were 2.20 kg/m2 h and 0.73 kg/m2 h, respectively. It is worth noting that in the PV-MD-thicker, the source water was transported by capillary effect and no source water flew out from the device, so in this case it can be regarded as the dead-end mode and no sensible heat is taken away. The higher clean water production performance of the new device is mainly resulted from the use of thinner hydrophobic membrane, as calculated in figure 6. With the decrease in the thickness, the hydrophobic membrane shows a dramatic compromise in the membrane coefficient. For example, the membrane coefficient in the new device was calculated to be 4.24 kg /m2 h kpa which is nearly 30 times higher than it is in the PV-MD-thicker (0.14 kg /m2 h kpa). In addition, according to equation 5, the clean water production rate is a function of the membrane coefficient and vapor pressure difference between the evaporation layer and condensation layer, which is driven by the temperature gradient. Therefore, it can be inferred that the higher clean water production performance in the new device is mainly contributed to the utilization of the thinner hydrophobic membrane. For the evaporation rate in the EC, the decrease

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of the evaporation rate in the PV-MD-thicker may be caused by its higher solar cell temperature and higher thermal radiation energy loss which can be proved by the following temperature measurement.

The temperature of the solar cell was monitored and is shown in figure 7d. In the PV-MD-thicker the solar cell showed a temperature of 63 oC, while for the new device the temperature of the solar cell exhibited minor difference among the three different flow rates (44 oC ~ 45oC). Compared to the PV-MD-thicker, a lower temperature was achieved at 19 oC for the new device which is attributed to the utilization of the thinner hydrophobic membrane and its higher membrane coefficient. When the membrane coefficient is higher, the required driving force (vapor pressure difference) for same level of water evaporation and condensation is lower. Therefore, the temperature gradient required in each stage is reduced, resulting in a cooler solar cell. On the other hand, the lower temperature of the solar cell can also result in a reduced thermal radiation energy loss, leading to a higher clean water production performance as more heat can be transferred downward and a higher evaporation rate can be achieved as well.

Regarding the J-V curve of both devices in figure 7e, the electricity generation efficiency of the solar cell in the PV-MD-thicker was 13.62%. For the new device, the solar cell having different flow rate exhibited minor differences in the electricity generation efficiency and was measured to be among 15.15~15.26%. The open circuit voltage of the PV-MD-thicker (0.53 V) was lower than the new device (0.58 V). The higher electricity generation efficiency and open circuit voltage in the new device are mainly resulted from its lower operational temperature. The lower temperature can also result in a longer lifespan of the solar cell, which has been proven according to previous report72.

a

b

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3.5 Seawater Desalination

The main application of the PV-MD-EC device is for seawater desalination. Therefore, seawater was used as source water to determine its desalination performance, which was obtained from The Red Sea and pretreated by filtration. The seawater container was placed higher than the device and the height was controlled by a lift table. In this case, the flow of the seawater was driven by gravity and the flow rate can be controlled by adjusting the height of the seawater container. With the flow of the seawater from the container, the height of the water level would descend, leading to a declining water head. To keep the seawater flow in a stable rate, the seawater was supplied to the seawater container by a mechanical pump and the flow rate of the mechanical pump was kept to be the same with the water flow rate driven by gravity.

Figure 7. a Clean water production rate from seawater and evaporation rate b Efficiency of solar cell during seawater desalination

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To avoid the crystallization of the salt inside the evaporation layer, the flow rate of the source water should be set to be over two times higher than the clean water production rate and thus, the concentration of the residual seawater which flows out of the device is less than 7%. In this case, the flow rate of the source water was measured by an electrical scale and controlled to be 7 kg/m2 h. The collected concentrated seawater was used as source water for the EC for evaporation and the salt from the seawater was obtained. The experiment was kept for 12 hours under one sun illumination and then remained in dark for another 12 hours to simulate daily cycle. This cycle was repeated for five times and the results are shown in Figure 8 a.

In the five cycles, the device showed an uncompromised clean water production rate at the range of 2.54~2.66 kg/m2 h. Compared to the case when clean water was used as source water, the clean water production rate in this condition was a bit lower. It is mainly caused by the higher latent heat of the seawater. With higher salt concentration, the latent heat of the seawater is higher71. Therefore, more heat is required for the evaporation of the seawater. In addition, according to Clausius-Claperon relation73, the vapor pressure is a function of latent heat:

lnP = −𝐿 𝑅(

1

𝑇) + 𝑐 (Equation 10)

Where P refers to the vapor pressure, T is the temperature, L is the specific latent heat of vaporization and R is the specific gas constant and c is a constant. It can be deduced from the equation that with the rise in the latent heat, the vapor pressure will be decreased. Thus, the driving force for the water desalination in MSMD would be decreased, resulting in a lower clean water production performance.

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The evaporation rate of the EC in the five cycles changed a little at the range of 0.64~0.72 kg/m2 h, which is a little lower than the case where the clean water was used as source water. The compromised evaporation rate is mainly contributed to the higher latent heat of the concentrated source water. Also, it was observed that the salt fell off from the EC driven by gravity (Figure 9) and then was collected in a container beneath the device.

Figure 8. Crystallization of salt under the PV-MD-EC device via the evaporative crystallizer

In addition, the solar cell used in the PV-MD-EC device showed a stable temperature at 47.1~48.5 oC, 3 oC higher than having clean water as source water (figure 8 a). This is caused by the lower

vapor pressure of the seawater, because to keep sufficient driving force for the evaporation and condensation, higher temperature gradient is required in this case, resulting in a higher solar cell temperature. Furthermore, the efficiency of the solar cell in the five cycles kept unchanged at

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around 14.74~14.99%, which is a little lower due to its higher temperature compared to the situation when clean water was used as seen in figure 8 b.

Figure 9. ICP-OES measurements of ion concentrations in source (before) and desalinated (after) water with respect to WHO standards for drinking water quality

The ion concentration of the collected clean water was measured by ICP-OES in figure 10. The source water (seawater) was first diluted 1600 times before running this measurement. Na, Ca, K and Mg ion concentrations were detected to be 13440, 448, 640 and 1600 ppm for the source water, respectively. For the desalinated water, the ions of Na, Ca, K and Mg were measured to be 5.2, 0.12, 0.87 and 0.5 ppm respectively. It can be observed that all the ions concentration in the

Na

Ca

K

Mg

10

-2

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

Conc

en

tra

tio

n (

pp

m)

Before

After

WHO

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collected clean water was much lower than WHO (World Health Organization) drinking water standards.

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Chapter 4. Conclusion

In summary, a new design of the multistage membrane distillation device was presented. Using lower hydrophobic membrane with thickness of 0.1mm during the process of seawater desalination. Also, an evaporative crystallizer was used for the evaporation of concentrated brine water. Through this design, the latent heat generated from the solar cell was utilized and recycled through each stage giving high water production rate performance of 3.67 kg/m2 h and relatively low temperature of solar cell 44 oC ~ 45oC which is lower than the ones reported. However, for the seawater desalination processes, the clean water production rate of the newly designed PV-MD-EC device was 2.54~2.66 kg/m2h without exceeding the guidelines of the world health organization for safe water drinking limits. Furthermore, the temperature of the solar cell was 47.1~48.5 oC due to the lower vapor pressure of seawater when compared with non-saline water. Moreover, high electricity generation efficiency up to 14.5~15% was accomplished under the radiation of one sun during the seawater desalination process. This high performance can be explained from the consumption of the wasted heat generated from the solar cell and the presence of an evaporative crystallizer at the bottom, making the solar cell execute better efficiency results. A large scale of 1x1 m2 that can produce up to 26 Kg of clean water daily,is under study and evaluation and is to be implemented in the near future. The newly designed PV-MD-EC device can remarkably compliment the performance of electricity generation, clean water production rate and crystallization of salt and thus offers possible opportunities for the implementation of such systems in remote locations and emergency areas.

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