INTRODUCTION
Clean water and Salination Issue + Desalination method
In the recent time, one of global problem is crisis of the clean water Water is everywhere, but clean water is lacking According to the World Economic Forum
2019, the clean water crisis is one of four global threats that have a great impact on the lives of human beings [21] Moreover, Water Center, University of Twente
(2016) showed us that over 65% of the world's population, have to face to the shortage of clean water for at least one month a year as a result of climate change and drought [14] Vietnam is one of the country’s most vulnerable to climate change and is currently facing a serious shortage of freshwater and irrigation due to drought and especially surface intrusion over the years According to Vietnam Disaster Management Authority, in the Mekong Delta, nearly 160,000 households are using polluted water, saline intrusion affects 40% of the fruit land area [19]
Figure 1.1 reveals drought and saline intrusion in several provinces in Vietnam The salinization (water with salinity of > 4‰) is alarming, and in some areas the saline instruction occurs 80-100 km from the coast Several predictions affirm that Vietnam's GDP will be reduced by 10% by 2030 because of drought and saline intrusion Therefore, constructing a system to product the clean water from the sea water is expected to be a valuable solution to face with the situation of clean water scarcity
Figure 1.1: Drought in Vietnam (left), warning of saline intrusion in Ben Tre province, and Vinh Long province on the television news
To find a solution with this situation, a lot of technologies for producing fresh water from the saline water have been developed and applied, such as distillation, ion exchange, membrane filtration, and so on [4], [7], [18] However, these methods have limitations, such as high cost, high consumption of materials and low performance because of the sea water’s corrosion and salt precipitation Nowadays, technology for producing the clean water from the saline water using solar energy is receiving much attention The potential of this technology is to create an eco- friendly, cheap, high-performance system.
Solar Steam Generation
The solar energy is a kind of green energy which available in the nature Moreover, it is an endless source of energy for human life However, we have not used the solar energy optimally According to California Institute of Technology, the amount of sunlight energy that reaches the earth in 1 hour is equal to the total amount of energy that humans use within 1 year [22] In the world, Vietnam is one of the countries with a lot of sunshine hours in a year (around 2000 to 2600 hours/year, equivalent to 6-7 hours/day), which is a huge source of energy coming from the sun
This is an extremely good condition, giving Vietnam many advantages when setting up devices that use solar energy Solar Steam Generation (SSG) is a system that uses solar energy to turn water into steam That steam is passed through a condensation system to obtain the clean water SSG system has many advantages, such as no electricity in use, no CO2 emissions, simplicity, and competitive price
With the sunshine hours of 6-7 h/d as in the South of Vietnam, a normal device can produce 15-30 L/h, equivalent to the minimum water demand of a household per day [25]
A complete SSG system is divided into 3 main components: the light absorber and converter, (2) water supply system, and (3) the clean water collector [23] The principle of the system is the process of converting light energy into thermal energy
Absorber material receives sunlight and converts that energy into heat energy The surface temperature of the material goes up extremely high The water supply transports the water from the bottom to the surface of the absorber material through a water transport system (usually a capillary system) A sufficient amount of water when reaching the surface, will exchange heat with the absorber and converter system, to receive energy to transform the state from liquid to vapor Steam is involved in the condensation process by a refrigeration device Finally, clean water is collected The above process is operated continuously Water is constantly being transferred up to a specified amount so that the absorber layer’s surface temperature is always at maximum The performance of the system depends on the photo- thermal energy conversion efficiency of the absorber layer In order to achieve the highest photo-thermal conversion efficiency, light absorber materials must have strong absorption in the sunlight range (from 300 nm to 3000 nm) Many categories of materials have been used for developing light absorbers, such as metal nanoparticles [2], [3], metal oxides [17], polymers [11], [20], semiconductors [6],
[26], bio-inspired materials [5], [10], [24], etc The photo-thermal materials have been designed with the porous and capillary structures or bio-inspired structures
Photo-thermal conversion (PTC) process has classified into 3 main types The first type is PTC based on the plasmonic localized heating of metals There are two main kind of materials that operate on this mechanism, metals nanoparticles and metal oxides Several metals with nanostructures such as gold, silver, and copper have been utilized for absorber layer [1], [8] They exhibit extremely strong absorption due to the resonant effect of free electrons on the surface excited by the electromagnetic of the incident light Yang et al showed SSG system with copper nanoparticles, which is a scalable and eco-friendly system [13] The copper nanoparticles exhibited strong absorption (around 97,7%) in the region of 200 nm to
1300 nm The SSG based on Cu NPs had high efficiency up to 73% at 2 sun (equivalent with P= 2 kW.m -2 ) illumination Naomi et al developed the SSG using SiO2/Au nano-shells particles [15] This material strongly absorbs the light range from 500 nm to 1200 nm, leading to the system’s conversion efficiency reached up to 80% at 1 sun However, the SSG system using these materials has limitations, such as: complicated fabrication procedure, expensive, and so on Constructing this system on an industrial scale is an uneconomical option For the metal oxides, several groups published some initial results in the construction of the SSG system
By using WO 2.9 , SSG system manufactured by Wang et al can absorb more than 90% of sunlight [16] PTC efficiency of the system reached over 85%, higher than that of nanoparticles Deng et al constructed the absorber layer by generation Fe 3 O 4 nanoparticles on the surface of graphene [17] The temperature of the water contained this material rose to 1000 0 C when exposing to sunlight The photo- thermal materials based on metal oxides had higher light absorbability and greater conversion efficiency than metal nanoparticles Those materials get same limitations of metal nanoparticles
The second type of PTC is non-radiative relaxation of semiconductor
Semiconductor materials absorb light to transfer electrons from the valence band to the conduction band At the conduction band, electrons perform non-radiative relaxation before returning to the valence band These relaxation causes the temperature of the material to heat up, making the evaporation process faster Hu et al developed an SSG system using a membrane with CuFeSe 2 nanoparticles decorated wood [12] CuFeSe 2 had a narrow bandgap (0.45 eV) so it can absorb all photons with energy greater than 0.45 eV (equivalent to wavelength shorter than
2755 nm) This SSG system achieved a solar thermal efficiency of 86.2% under 5 sun illumination The evaporated water amount obtained with another semiconductor materials was from 0.85 to 1.3 kg.m -2 h SSG system using the semiconductor as a photo-thermal material will be difficult to fabricate with high price and be complicate to deal with large-scale system
The third type of PTC is thermal vibration of molecules Several materials have extremely high absorption, and all absorbed photon energy will be converted to thermal energy in the form of vibrations of the molecules of the material through the photo-thermal conversion Especially, carbon-based materials with zero band gap can absorb sunlight as a black body They are cheap, stable and easy to fabricate Carbonization process, which can turn any material with a specific structure into carbon-based materials with the same structure, plays an important role in making carbon-based materials Lin et al synthesized SSG system by annealing to make the carbonized melamine foams, realizing highly efficient SSG with a water evaporation rate of 1.270 kg.m -2 h -1 and an energy conversion efficiency of 87.3% under 1 kW.m -2 solar illumination [9] Zhu et al developed the SSG devices based on the natural materials, such as mushroom, wood, so on [24]
The mushroom was carbonized at high temperatures As a result, the surface of the mushroom turned into black and the absorbance of the carbonized mushroom increased to above 95% in the wavelength range of 30-3000 nm The SSG device performed a high photo-thermal conversion efficiency as above 85%, and an evaporation rate of 1.47 kg.m -2 h -1 The evaporated water quality met the standards of WHO for the drinking (or clean) water Carbonization material provides a good performance for SSG system, simple fabrication process, environmentally friendly, promising to be a key material for setting up SSG system In particular, carbonization materials of biological origin (naturally occurring) -named bio- metamaterials- are currently receiving much attention Constructing the SSG system using bio-metamaterial materials is a potential, and feasible direction
1.2.3 Purpose of this master thesis
The pomelo is the most common fruit in Vietnam The pomelo is not only a kind of human food, other components of the pomelo also have many application in our life
Pomelo's outer skin contains many essential oils, those essential oils are used as a remedy for hair loss, sinusitis, etc Pomelo peel is used as some traditional folk dishes and also have applications in medicinal healing In this master thesis, pomelo peel have been considered because of their porous structure With the porous structure of the pomelo peel, the carbonized pomelo peel (CPP) promises not only the good light-absorber material, but also the good ability to transport water onto the surface itself Carbonized pomelo peel promises to be a good bio-metamaterial for constructing the SSG system We hope that the SSG using the carbonized pomelo peel as PTC material will have high absorbance, high efficiency of photo- thermal conversion, good ability in desalination and purification
This master thesis aims at:
• Fabricating the photo-thermal conversion (PTC) material from natural sources by the carbonization method
• Studying on the PTC characteristics
• Evaluating the absorption of bio-metamaterial under the sunlight
• Developing a SSG system, using bio-metamaterial as a absorber layer (will mention in the chapter 2)
• Assessing the evaporation capacity of the system
• Demonstration in producing clean water from saline water using the Solar Steam Generation system, which is based on solar energy, bio-inspired materials with light absorbability and one-dimensional capillary sheets with high efficiency.
EXPERIMENTAL METHOD
Carbonized pomelo peel synthesis and characteristics
• Fresh pomelo used in this master thesis was purchased from Vin-mart Before fabrication process, fresh pomelo is stored in the refrigerator at 50 0 C
• It, then, was cut into slices to take pomelo peel slices (figure 2.1) Fresh Pomelo slices used in SSG system have the dimensions from 2 cm to 3.5 cm, with the thickness divided into 3 groups: thin groups with a thickness of 2 mm to 6 mm, the average group with a thickness of 7 mm to 12 mm , and the thick group is 12 mm to
Figure 2.1: Fresh pomelo (a) and (b), and Fresh pomelo peel (c)
• Pomelo peel slices were washed by the solution of domestic water and absolute ethanol 3 times with the stirrer at 200 rpm for 30 minutes at room temperature
• After that, they were dried at 400 0 C for 12 hours The moderate temperature helps the water evaporate while keeping the sample from bending after drying
• Dried-pomelo peel was taken part in a carbonization process at controllable temperature and time in the nitrogen atmosphere The carbonization process was performed as follows:
1 Put the pomelo peel sample into the annealing tube, the lock the valve
2 Vacuum for 5 minutes Then blow nitrogen into the tube until the pressure is equal to atmospheric pressure Do this step at least 3 times to make the annealing tube clean
3 Blow nitrogen so that the pressure is maintained at 1.5 atmospheres
4 Increase the temperature with the speed of 50 0 per minute
5 When the system reaches the desired temperature, keep the system at that temperature for a specified time
6 Stop heating the system, wait until the system temperature returns to room temperature, then take the sample Figure 2.2 shows a sample of carbonized pomelo peel
7 Store the sample in a dry condition
Figure 2.2: Pomelo peel and carbonized pomelo peel
• To study the structure and composition of materials, the following methods were used:
1 SEM to observe the material morphology
2 XRD, FTIR, Raman to study on the structure and composition of materials
• To investigate other properties of the material:
1 Absorption properties of carbonized pomelo peel
2 Solar heating behavior of carbonized pomelo peel
SSG System construction and evaluation
2.2.1 Construction of the SSG system
Figure 2.3 describes the structure of light absorber and converter part and water supply part of the SSG system Carbonized pomelo peel is placed at the top of the beaker containing water The area of the sample is designed to be equal to the surface area of the beaker Separating the water supply and carbonized pomelo peel has a Polystyrene Foam sheet This sheet helps prevent the heat exchange between materials and the water supply Surrounding the foam sheet is a gauze pad, the gauze pad acts as a water transport channel, transport water from the water supply to the carbonized pomelo peel Water is transported to the surface through the porous structure and tube structure of the material Finally, the baker is wrapped around with aluminum foil
Figure 2.3: Light absorber and converter and water supply
• To study the SSG system evaluation, the following methods were used:
1 Evaluate the evaporation index of the system
Figure 2.4 shows the experimental diagram for determining the evaporation rate of the SSG system The beaker (constructed in section 2.2.1) is placed on the electronic balance The mass of the system will be collected at different times, to determine the amount of evaporated water After that, the evaporation rate of the system is equal to the lost mass divided by the illuminated sample area
Figure 2.4: System to calculate the evaporation rate in the laboratory
2 Evaluate the desalination and purification ability
For the desalination and purification processes, all experiments are done in the real condition The baker containing the carbonized pomelo peel is placed into a sealed glass box designed as in Figure 2.5 The sun shines on the system, and makes the water evaporate That vapor will exist in a sealed glass box A cooling system, placed next to that glass case (dry ice), leading to the condensation of the steam inside the box Finally, the steam condenses into water and flows out through the hole at the bottom of the box For the desalination, sea water will be selected as a source of input water After taking enough amount of purified water, the ion concentration of both solutions is analyzed via SW-846 Test Method 6010D by using Skalar ++ CP-OES For the purification process, crystal violet and methyl orange solutions are pick as the wastewater The color and transparency of the purified solution will be assessed to evaluate the performance of the system
Figure 2.5: Mechanism of the system in the real condition
Table 2.1 shows some equipment used in measurement in the thesis
Table 2.1: Some equipment used in this master thesis
UV-Vis Lambda 950 for observing the absorption properties of materials in the range of 200 nm to 2500 nm
UV/Vis Jasco V670 (Jasco, Japan) for observing the absorption properties of materials in the range of
X-ray Diffraction (XRD Mini Flex
600, Rigaku, Japan) spectra of sample were obtained using a Bruker
4600, Jasco, Japan) spectra of samples were recorded on a Nicole
FLIR infrared camera for calculating the surface temperature of material
Ion concentration of solutions were obtained via SW-846 Test Method 6010D by using Skalar ++ CP-OES
Annealing equipment for carbonization process
SEM for observing the material morphology
HOBO pyranometer for measuring the sunlight intensity
RESULTS AND DISCUSSION
Carbonized pomelo peel
3.1.1 Physical characteristics of carbonized pomelo peel
Figure 3.1: Porous Structure and Tube Structure of Fresh Pomelo Peel (left) and
Carbonized Pomelo Peel Figure 3.1 describes the morphology of fresh pomelo peel and the CPP The annealing condition of this CPP was: 4000C for 2 hours On a large scale, both materials exhibit porous and tube structures Tube structure is surrounded by porous structures and is a minority This proves that, after participating in the carbonization process, the structure of the material remains unchanged Moreover, the porous structure, and especially the tube structure, can perform well as a water channel
Water in the water supply will be brought to the surface of the absorption layer through capillary force The water can then absorb heat easily and evaporate
Figure 3.2: Changes in structural dimensions before and after carbonization process
(a), (b), (c): Fresh Pomelo Peel; (d), (e), (f): Carbonized Pomelo Peel
Figure 3.2 shows the dimensions of the tube structure and the porous structure of the material before and after the carbonization process Fresh Pomelo Peel has a porous structure with a hole diameter of about 300- 400 àm After carbonization, although the porous structure remains the same, the diameter of the holes is significantly reduced Most holes in the porous structure range in size from 75 to
150 àm Similarly, the diameter of the tube mouth in the tube structure is also reduced This is explained by the evaporation of water in the material structure during carbonization process At high temperatures, the material loses water, causing all structures to shrink from 2 to 4 times As the size of the internal structure of the material becomes smaller, it is beneficial to transport water The higher capillary force, the better water transport system, also the better water retention ability
Figure 3.3: Water Capacity Ability of Carbonized Pomelo Peel The water capacity of carbonized pomelo peel is shown in Figure 3.3 By assessing the mass ratio of the material before and after hydration, the water capacity ability of the material is shown as the equation above The variable y is mass of the hydrated material and x is mass of the original material It implies that the amount of water that the material can hold can be more than 6 times its own weight In addition, water transport speed is also assessed through the change of average weight over time of exposure to water of the material The average water transport speed reaches the speed of 0.1 kg.m -2 h -1 The above proves that the material is carbonized pomelo peel capable of performing tasks as well as a water channel system
3.1.1.2 XRD, FTIR, and Raman spectra
Figure 3.4: XRD spectrum of Carbonized Pomelo Peel Figure 3.4 shows X-ray diffraction (XRD) of the carbonized Pomelo Peel It exhibits a weak and broad signal at the 2-theta degrees of 23 0 , that describes the typical reflection of graphite-like structure [7] This is understandable, because after carbonization process, most of the material has been carbonized, and the main constituent material is carbon Moreover, that are also confirmed by the Raman spectrum of Carbonized Pomelo Peel (figure 3.5)
Figure 3.5: Raman Spectrum of Carbonized Pomelo Peel
The Raman spectrum exhibits two prominent peaks at 1350 cm -1 (D) and 1590 cm -1 (G), which represented the graphitic lattice vibration mode and disorder in the graphitic structure of the biochar [24] Specifically, peak D refers to disordered sp2- hybridized carbon atoms with vacancies and impurities, while peak G causes from the stretching of sp2 atomic pairs in the carbon atom ring or carbon chain It can be assumed that the carbon atom is bonded to a sp2 hybridized covalent bond, while electrons which are not involved in hybridization form a π bond However, the intensity of G-band is much higher than that of D-band It implies that the amorphous carbon plays an important role in the formation of carbonized pomelo peel
Figure 3.6: FTIR spectra of Fresh Pomelo Peel and Carbonized Pomelo Peel Another method to examine the physical characteristic of the material is FTIR
Figure 3.6 show FTIR spectra of Fresh Pomelo Peel and Carbonized Pomelo Peel in the region of wavenumber from 400 cm -1 to 4000 cm -1 Fresh Pomelo Peel FTIR spectrum reveals vibration peaks of some functional groups of the carbonized pomelo peel Distinguishing vibration peaks of C-O bond, C=C bond, C=O bond,
-1 -1 cm -1 , 2900 cm -1 , and 3300 cm -1 , respectively On the other hand, the FTIR of Carbonized Pomelo Peel saw the significantly reduce of the intensity of (N-H)/(O- H), (C-H), (C=O), (C-O) due to carbonization process From the results of XRD, FTIR, Raman, the elemental composition and structure of the carbonized pomelo peel are clearly shown
In summary, after heating the sample at high temperature for a defined time, the CPP sample shrinks but still retains its structure After carbonization process, the main component of the CPP is carbon, the carbon-carbon bonding is predominant
With carbon as the main ingredient, the CPP material promises to exhibit a strong sunlight absorption Section 3.1.2 will study the light absorption properties of CPP material in the ultra violet, visible and infrared regions
Figure 3.7: Absorption properties of carbonized pomelo peel with 1 hour of annealing time Studying the absorption properties of materials is one of the good methods for selecting optimal material fabrication conditions The two most important factors in the carbonization process are temperature and annealing time The light absorption of carbonized pomelo peel with different temperature conditions, during one hour of annealing, is shown in Figure 3.7 There are three temperatures measured here, which are 200 0 C, 300 0 C and 400 0 C In the wavelength range from 300 nm to 550 nm, the absorbance of all three samples is very impressive (reaching over 90%)
However, at the range with wavelength larger than 550 nm, while the absorbance of the sample at 300 0 C and 400 0 C is almost unchanged, the absorbance of the 200 0 C sample decreases with increasing of the wavelength It implies that the temperature of 200 0 C is not optimal
Figure 3.8: Absorption properties of carbonized pomelo peel with 2 and 3 hours of annealing time When increasing both of annealing time and annealing temperature, the absorption of the material has a significant improvement Figure 3.8 shows the absorption spectrum of carbonized pomelo peel samples with annealing temperatures above
300 0 C, and annealing times of over 2 hours The absorbance of the samples reaches over 90% and increases gradually in the short wavelength range (from 300 nm to
400 nm), then remains at 95% in the region of the longer wavelength Not only well absorbed in the UV-Vis region, the sample also absorbs light very well in the infrared region Figure 3.9 describes the absorption properties of fresh pomelo peel and carbonized pomelo peel in the wavelength range from 300 nm to 2500 nm
While fresh pomelo peel has a poor absorbance, carbonized samples exhibit an excellent performance in absorbing light in that range With the annealing time of 3 hours, the absorbance of 4000C sample and 500 0 C sample is almost the same and remains at over 95% in the UV-Vis region, maintained above 90% in the infrared
This promises that the new material will do well as a photo-thermal conversion material in Solar Steam Generation system
Figure 3.9: Absorption properties of carbonized pomelo peel and fresh pomelo peel in the UV-Vis-IR region
Figure 3.10: Absorption spectrum of carbonized pomelo peel samples before being hydrated and after being hydrated
Light absorption of samples before and after being hydrated is shown in Figure 3.10
Solar steam generation system ability
3.2.1 Vapor steam creation capacity 3.2.1.1 Vapor steam creation capacity under an artificial sun
Figure 3.14: Vapor steam creation ability under an artificial sun Figure 3.14 describes the vapor steam creation ability of different samples with the same thickness of 4mm under an artificial sun with P=1 kW.m -2 (1 sun illumination)
The blue line exhibits the vapor evaporation in the dark condition while the black line shows that indicator in an artificial sun condition The amount of evaporated water will be calculated based on the change in mass of the system (as the steam evaporates away, the total mass of the system decreases) Within one hour, the amount of water evaporated in the dark is 0.05 kg.m -2 , while that amount in 1 sun illumination is around 0.4 kg.m -2 Thus, the sunlight promotes evaporation process about 10 times compared to original condition Water molecules receive energy from incident light and raise the temperature After that, the water molecules continue receiving more photon energy to convert the phase from liquid to vapor
With the attendance of carbonized pomelo peel, more photons are absorbed, leading to the higher in total energy input Thus, the evaporation index in these cases has a significant growth Within 1 hour, the amount of evaporated water of the 300 0 C- within 3 hours of annealing sample is 1.47 kg.m -2 That number of samples with condition 400 0 C-3h and 500 0 C-3h are almost the same and reach around 1.8 kg.m -2 This phenomenon is explained by the higher absorbances of the 400 0 C and 500 0 C samples compared to the 300 0 C sample, resulting in a greater total energy input
Figure 3.15: Vapor steam creation ability with different conditions of thickness under an artificial sun (a): Mass change within 1 hour; (b): Evaporation Rate within
1 hour; (c) Carbonized pomelo peel’s infrared photos when exposing to the sunlight Figure 3.15a describes the evaporation capacity of the SSG system when using carbonized materials with different thicknesses On the other hand, figure 3.15b gives information about the evaporation rate of the respective conditions When using a thicker carbonized pomelo peel as an absorber layer, the amount of the amount of evaporated water of SSG systems is 1.3 kg m -2 , 1.6 kg m -2 , 2.1 kg m -2 , and 2.4 kg m -2 with the carbonized pomelo peel thickness of 1 mm, 3 mm, 6 mm, and 10 mm respectively In figure 3.15b, the evaporation rate of all samples increases gradually during the first period, and the change is not significant after that As discussed before, thicker materials will have a higher saturation temperature Thus, the amount of evaporated water within a unit time is also greater
To explain the change of evaporation rate with different carbonized pomelo peel thicknesses, energy exchange processes are considered At the beginning, heat absorbed through the thermal vibration of the molecules increases Besides, water is transported through the water channel to the surface and conducts heat exchange with absorbing material The energy mainly raises the temperature of the water and a small part is involved in the water phase transition, leading the increment of evaporation rate from 0 After that, when the heat exchange process becomes stable, the energy absorbed by the photon is balanced with the total of heat loss and the energy transferred to water; the water evaporation rate will increase slowly and arrive at the saturation state Figure 3.15c shows the temperature of the absorption layer (carbonized pomelo peel) from the 15th to the 75th minute during the evaporation process In the beginning, the CPP temperature gradually increased from the room temperature After 15 minutes, the surface temperature of the material has a relatively constant temperature, which shows the stability of the SSG system when performing evaporation process
3.2.1.2 Vapor steam creation capacity under the real condition
Figure 3.16 exhibits the vapor steam creation capacity under the real condition The power intensity is calculated by using the HOBO pyranometer (MCCD- VJU, Vietnam) For 160 minutes, with power intensity conditions of 0.7 to 0.85 kW m -2 , the maximum amount of evaporation water that the system can generate reaches around 7 kg m -2 for the 500 0 C-3h CPP sample Annealing conditions of 400 0 C for CPP exhibit a lower amount in the change of water’s mass, but still good (around 6 kg m -2 per 160 minutes) For the 400 0 C, the annealing time of 2 hours and 3 hours gives nearly the same results The evaporation rate of the SSG system in actual conditions is higher than that in laboratory conditions due to several factors Firstly, the evaporation rate depends on the humidity index of the environment In the real condition when the experiment is taken, the humidity index is small, so the evaporation of water is accelerated Secondly, the average temperature in the real condition and the temperature in the laboratory condition is 37 0 C and 29 0 C, respectively With higher temperature, the SSG system in the real condition requires less energy to change the state of water from liquid to vapor Moreover, the process of heat exchange with the environment of SSG system also occurs less frequently, because the difference in temperature is less For long enough, the water in the water supply system also exchanges heat with its surroundings and receiving the sunlight energy and raises the temperature, so the amount of evaporation of water increases with time If the evaporation process is only considered within the first 60 minutes, when the water supply water temperature has not increased much, the water evaporation rate in the real condition and the laboratory condition is similar
This proves that the SSG system not only works well in the laboratory condition, but also shows a great performance in the real condition
Figure 3.16: Vapor steam creation capacity under the real condition
The effect of power intensity of sunlight in the real condition should also be considered Figure 3.17 shows the evaporation of the SSG system when using
500 0 C-3h CPP as a PTC material under different conditions of solar energy intensity At the sunlight intensity of 0.7 to 0.85 sun (equivalent to 0.7 to 0.85 kW m -2 ), the evaporated water within 150 minutes is approximately 6.7 kg m -2 As the intensity of the sunlight decreases, the amount of the evaporated water during the same time also decreases At 0.4 sun and 0.2 sun, the evaporated water is 4.15 and 2.48 kg m -2 h -1 , respectively When the sunlight intensity decreases by 1.75 ~ 2 times, the amount of evaporated water decreases by 1.6 times When the sunlight intensity decreases by 3.5 to 4 times, the evaporated water decreases by 2.7 times
Thus, the amount of evaporated water is not linearly proportional to the intensity of sunlight When the intensity of sunlight is low, the temperature of PTC material is not high, leading to the reduction of heat loss due to exchange with the environment, leading to better system performance than that SSG system at the time of high sunlight intensity This proves that the system also works well under the low power intensity and responds to actual sunlight conditions in Vietnam
Figure 3.17: Vapor steam creation capacity under different power intensity
3.2.1.3 Photo-thermal conversion materials stability
The stability of PTC material is assessed based on the performance of the SSG system using that material after 30 days and compared with their first result Figure 3.18 shows the evaporation capacity of the SSG system using CPP 400 0 C-2h on day
1 and day 30 in an artificial sun with 1 sun illumination Both cases show the amount of evaporated water of about 1.56 to 1.71 kg m -2 within 80 minutes The amount of evaporated water on day 30 is about 10% smaller than that in the first day This implies that PTC material can work well after many times with stability up to several months After one month, the material has no change in shape and structure, the image is shown in the figure 3.18
Figure 3.18: Vapor steam creation capacity after 30 days
3.2.2 Desalination and purification capacity of the SSG system
Figure 3.19: Cation concentration (top), and Anion concentration (bottom) of Sea water and Purified water compared to the Standard of drinking water The carbonized pomelo peel exhibits an impressive performance of water evaporation from seawater and contaminated water To evaluate the water desalination application of the SSG system, the concentration of ions in the solution was determined The original water source was sea water, taken from Quynh Phuong beach, Hoang Mai, Nghe An province The SSG system was utilized to filter the original water source, then purified water was created The concentration of ions in the solution of Seawater and filtered water was analyzed by SW-846 Test Method 6010D by using Skalar ++ CP-OES Figure 3.19 shows the concentration of cations (top) and anions (bottom) in the seawater and the purified water The concentration of the ions in the seawater is much higher than the concentration of the same ion in the purified water, implies the particularly good desalination process of the SSG system Especially, the concentration of these ions is also much smaller than the maximum allowed for the standard of drinking water (black-dot line) Once again, this confirms the good application in desalination of the SSG system
Not only demonstrating good desalination capacity, SSG system works extremely well in purifying the wastewater Crystal Violet and methyl orange solutions are used to simulate wastewater After participating in the evaporation and condensation of SSG system, the solution obtained under both conditions becomes clean water with colorless and transparent Pictures of the original solution and the purified water placed side by side, are shown in Figure 3.20
CONCLUSION
The SSG system using carbonized pomelo peel as the photo-thermal conversion material is constructed and evaluated for the specific characteristics The carbonization process helps the pomelo peel retain its porous and tube structure, while increasing its absorption properties Carbonized pomelo peel’s absorbance is above 95% in the ultraviolet region and visible, more than 90% in the infrared The surface temperature of carbonized pomelo peel can reach 930C at 1 sun illumination (P=1 kW.m-2) The SSG system utilizing carbonized pomelo peel as the photo- thermal conversion material has an efficient performance, with a water evaporation coefficient of up to 2.4 kg.m-2.h at 1 sun illumination The purified water of the SSG system passes the drinking water standard provided by WHO and other prestigious organizations in the world The system also works well in wastewater treatment, shown by purifying the precursor color solutions into clear, colorless water
Table 4.1: The comparison of evaporation rate for each material and its disadvantages Material Evaporation rate
PGPU +Ag/Au 1 1 Low efficiency
Table 4.1 gives the information on the evaporation rate of materials when illuminated under the artificial sun with a specified capacity [23] When the power intensity of artificial sunlight is 1 (equivalent to P = 1kW m -2 -1 sun illumination), previously published materials have the evaporation rate between 1 kg m -2 h -1 and 1.5 kg.m -2 h -1 The SSG using MG/PNIPAm as a photo-thermal conversion material has a high evaporation rate when the intensity is less than 1, but its disadvantage is complexity Similarly, SSG systems using those materials often face some limitations, such as high cost, low efficiency, cumbersome systems, environment pollution, energy consumption, and so on Utilizing the carbonized pomelo peel material as a photo-thermal conversion material in the SSG system can be a great development in improving the evaporation rate of the system With a maximum evaporation rate of up to 2.4 kg.m -2 h -1 , the SSG system using carbonized pomelo peel will be a usefulness candidate to become a solution to handle water’s issues in the future
In the future, it is necessary to establish the process of manufacturing an SSG system using CPP as a PTC material For the system evaluation, methods of evaluating the source of the purified water will be considered, such as: assessing the purity of the water, measuring its ability to kill bacteria, the stability of the PTC materials during purification process of wastewater, etc The evaporation efficiency of the system is extremely high, but the condensation efficiency of the system still needs to be improved, so it is also one of the main tasks in the future Finally, in order to bring the SSG system to the market as a device, developing a larger area of the system is the most important In addition to CPP, we will continue to study other materials with good properties, which can be applied to the system with the criteria for cheap, easy to produce, high performance, and environmentally friendly
[1] Chen, J., Feng, J., Li, Z., Xu, P., Wang, X., Yin, W., … Yin, Y (2018)
Space-Confined Seeded Growth of Black Silver Nanostructures for Solar Steam Generation Nano Letters doi:10.1021/acs.nanolett.8b04157
[2] Deng, Z., Zhou, J., Miao, L., Liu, C., Peng, Y., Sun, L., & Tanemura, S
(2017) The emergence of solar thermal utilization: solar-driven steam generation
Journal of Materials Chemistry A, 5(17), 7691–7709 doi:10.1039/c7ta01361b
[3] Liu, G., Cao, H., & Xu, J (2018) Solar evaporation of a hanging plasmonic droplet Solar Energy, 170, 184–191 doi:10.1016/j.solener.2018.05.069
[4] El-Nashar, A M (2001) Cogeneration for power and desalination — state of the art review Desalination, 134(1-3), 7–28 doi:10.1016/s0011-9164(01)00111-
[5] Fang, J., Liu, J., Gu, J., Liu, Q., Zhang, W., Su, H., & Zhang, D (2018)
Hierarchical porous carbonized lotus seedpods for highly efficient solar steam generation Chemistry of Materials doi:10.1021/acs.chemmater.8b01702
[6] Ghim, D., Jiang, Q., Cao, S., Singamaneni, S., & Jun, Y.-S (2018)
Mechanically Interlocked 1T/2H Phases of MoS2 Nanosheets for Solar Thermal Water Purification Nano Energy doi:10.1016/j.nanoen.2018.09.038
[7] Khawaji, A D., Kutubkhanah, I K., & Wie, J.-M (2008) Advances in seawater desalination technologies Desalination, 221(1-3), 47–69 doi:10.1016/j.desal.2007.01.067
[8] Kiriarachchi, H D., Awad, F S., Hassan, A A., Bobb, J., Lin, A., & El-Shall,
S (2018) Plasmonic Chemically Modified Cotton Nanocomposite Fibers for Efficient Solar Water Desalination and Wastewater Treatment Nanoscale doi:10.1039/c8nr05916k
[9] Lin, X., Chen, J., Yuan, Z., Yang, M., Chen, G., Yu, D., … Chen, X (2018)
Integrative solar absorbers for highly efficient solar steam generation Journal of Materials Chemistry A, 6(11), 4642–4648 doi:10.1039/c7ta08256h
[10] Lin, Y., Zhou, W., Di, Y., Zhang, X., Yang, L., & Gan, Z (2019) Low-cost carbonized kelp for highly efficient solar steam generation AIP Advances, 9(5),
[11] Liu, F., Zhao, B., Wu, W., Yang, H., Ning, Y., Lai, Y., & Bradley, R (2018)
Low Cost, Robust, Environmentally Friendly Geopolymer-Mesoporous Carbon Composites for Efficient Solar Powered Steam Generation Advanced Functional Materials, 1803266 doi:10.1002/adfm.201803266
[12] Liu, Hanwen , Chen, C., Wen, H., Guo, R., Williams, N., Wang, B., … Hu, L
(2018) Narrow Bandgap Semiconductor Decorated Wood Membrane for High- Efficiency Solar-Assisted Water Purification Journal of Materials Chemistry A doi:10.1039/c8ta05924a
[13] Mandal, J., Wang, D., Overvig, A C., Shi, N N., Paley, D., Zangiabadi, A.,
… Yang, Y (2017) Scalable, ―Dip-and-Dry‖ Fabrication of a Wide-Angle Plasmonic Selective Absorber for High-Efficiency Solar-Thermal Energy Conversion Advanced Materials, 29(41), 1702156 doi:10.1002/adma.201702156
[14] Mekonnen, M M., & Hoekstra, A Y (2016) Four billion people facing severe water scarcity, Sci Adv., 2, e1500323 doi: 10.1126/sciadv.1500323
[15] Neumann, O., Urban, A S., Day, J., Lal, S., Nordlander, P., & Halas, N J
(2012) Solar Vapor Generation Enabled by Nanoparticles ACS Nano, 7(1), 42–49 doi:10.1021/nn304948h
[16] Sun, L., Li, Z., Su, R., Wang, Y., Li, Z., Du, B., … Yu, M (2018) Phase- Transition Induced Conversion into a Photothermal Material: Quasi-Metallic WO2.9 Nanorods for Solar Water Evaporation and Anticancer Photothermal
Therapy Angewandte Chemie, 130(33), 10826–10831 doi:10.1002/ange.201806611
[17] Tao, P., Shu, L., Zhang, J., Lee, C., Ye, Q., Guo, H., & Deng, T (2018)
Silicone oil-based solar-thermal fluids dispersed with PDMS-modified Fe3O4@graphene hybrid nanoparticles Progress in Natural Science: Materials International doi:10.1016/j.pnsc.2018.09.003
[18] Van der Bruggen, B (2003) Desalination by distillation and by reverse osmosis — trends towards the future Membrane Technology, 2003(2), 6–9 doi:10.1016/s0958-2118(03)02018-4
[19] Vietnam Disaster Management Authority, 2020 Report of drought and salinization 2019-2020 http://phongchongthientai.mard.gov.vn/Pages/bao-cao- tong-hop-tinh-hinh-han-han-xam-nhap-man-khu-vuc-mien-nam-2019 2020.aspx
[20] Wang, X., Liu, Q., Wu, S., Xu, B., & Xu, H (2019) Multilayer Polypyrrole Nanosheets with Self‐Organized Surface Structures for Flexible and Efficient Solar – Thermal Energy Conversion Advanced Materials, 31(19), 1807716 doi:10.1002/adma.201807716
[21] World Economic Forum, Annual Report 2018–2019 https://www.weforum.org/reports/annual-report-2018-2019
[22] Working Draft Version 2006 Apr 20 Solar FAQs https://www.sandia.gov/~jytsao/Solar%20FAQs.pdf
[23] Xu, H., Lin, Y., Shan, X., Di, Y., Zhao, A., Hu, Y., & Gan, Z (2019) Solar steam generation based on photothermal effect: From designs to applications, and beyond Journal of Materials Chemistry A doi:10.1039/c9ta05935k
[24] Xu, N., Hu, X., Xu, W., Li, X., Zhou, L., Zhu, S., & Zhu, J (2017)
Mushrooms as Efficient Solar Steam-Generation Devices Advanced Materials, 29(28), 1606762 doi:10.1002/adma.201606762