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The solar-driven water evaporation technique to produce clean water has shown enormous potential towards an energy-efficient solution for global freshwater scarcity. Designing a cost-effective and efficient solar vapor generator (SVG)...
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Materials
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February 2020
Pages 001-200
ISSN 2633-5409
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and A. Bera, Mater. Adv., 2021, DOI: 10.1039/D1MA00361E.
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a.Department of Physics, Indian Institute of Technology Jammu, Jammu and
Kashmir, 181221 India *E-mail: ashok.bera@iitjammu.ac.in
Electronic Supplementary Information (ESI) available: experimental methods,
calculation of solar absorptance, calculation for evaporation efficiency, table for
comparison of CCH with other recently reported 3D SVGs, Photograph of
prototype CCH evaporator, table for pH and conductivity. See
DOI: 10.1039/x0xx00000x
Received 00th April 2021,
Accepted 00th January 2021
DOI: 10.1039/x0xx00000x
Environment Pollutant to Efficient Solar Vapor Generator; an Eco-
Friendly Way of Freshwater Production
Tawseef Ahmad Wani,a Parul Garg,a Ashok Bera*a
The solar-driven water evaporation technique to produce clean
water has shown enormous potential towards an energy-efficient
solution for global freshwater scarcity. Designing a cost-effective
and efficient solar vapor generator (SVG) using an underutilized
natural source causing environmental pollution will be an eco-
friendly way for freshwater generation. We developed an SVG
simply by carbonizing the surfaces of a piece of coconut husk using
a household liquefied petroleum gas stove. The naturally porous
structure of coconut husk enhances light absorption, and the 3D
assembly promotes heat energy harvesting from the environment,
leading to an effective energy input of 1.6 kW m-2 under 1sun
illumination (1.0 kW m-2). Our carbonized coconut husk
evaporator shows an evaporation rate of 3.6 kg m-2 h-1. The simple
design technique of carbonized coconut husk-based SVGs,
negligible material cost, and its large-scale availability combined
with the already existing coconut coir industries can lead to an
affordable way of freshwater generation.
Freshwater scarcity is now a global concern due to the
increase in water pollution and the human population.1–4
Finding a cost-effective solution for freshwater generation
using renewable energy sources can ultimately encounter
global freshwater scarcity.5 Recently, solar-driven water
purification technology is being considered an energy-efficient
way of producing clean water.6–12 The principle of this
technology is to convert solar energy into heat that generates
vapor/steam, and the main focus is to prepare efficient solar
vapor generators (SVG) using un-concentrated solar light.13,14 It
has been found that heating a small amount of water near the
evaporator surface while restricting the heat flow to the entire
water improves the evaporation efficiency significantly, known
as interfacial vapor generators.14–18 Such interfacial vapor
generators have been used both in 2D and 3D configurations.
In general, 2D SVGs consist of an absorber layer on a
hydrophilic porous substrate followed by a heat-insulating
layer and an arrangement of controlled water flow to the
absorber layer using capillary action.19–21 Heat loss from the
absorber layer due to diffuse reflectance and radiation limits
the evaporation rate of 2D SVGs that have been addressed
using 3D SVGs.22–27 Efforts have been made to fabricate highly
efficient solar vapor generators by investigating different types
of photothermal materials, including carbon materials,28,29
oxides,30,31 polymers.32,33 Efficiency of evaporators has been
increased by reducing the enthalpy of vaporization using
hydrogel as well.34 The cost of absorber materials and
complicated design technologies of such SVGs restricts the
application to their full potentials.
Energy shortage with increasing energy demands and
environmental crisis has forced researchers to find low-cost,
eco-friendly, novel functional materials that can be
commercialized shortly.35,36 Recently, the carbonization of
varieties of plants and related products has been utilized to
design SVGs.37–41 The unique structure of the plant stem works
as channels for conducting water in these SVGs. But, the
carbonization of plants required high-temperature annealing
under a controlled environment. High-temperature annealing
destroys the mechanical strength and surface morphology of
plants.42 Uniform deposition of the carbonized species on a
substrate or self-assembling them in the desired form creates
difficulties in the large-scale design of SVGs. Low efficiency of
the plant-based SVGs is one of the significant challenges; for
example, SVGs made from mushrooms,43 lotus seed,44
daikon,45 potato,46 rice husk,47 sugar cane,48 and banana
peels49 have an evaporation rate of less than 1.5 kg m-2 h-1
under 1 sun illumination (1 kW m-2). Industry scale availability
for some of these plants might be a challenge as well.
Although the evaporation rate of bamboo stem-based SVGs
reported having an evaporation rate of 3.1 kg m-2 h-1, it
required multistage annealing of the bamboo stem in different
temperatures as high as 1200 °C.50 Finding a plant-based
source material with large-scale availability, simple design
techniques, and efficient SVG performance can lead to an
affordable solution for a freshwater generation.
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The coconut (Cocos Nucifera) belongs to the Arecaceae
family. Almost every part of the coconut plants is used in
diverse applications due to their advantages like availability in
large quantities, low-costs, renewable nature, eco-friendly,
and biodegradable.51,52 Out of all the coconut plant products,
the coconut husks are still reported as the most underutilized
resource causing environmental pollution.53,54 Designing SVGs
using coconut husk will further improve ecological
management. Coconut husks have very low thermal
conductivity and have potential application in the thermal
isolation layer.55 Coconut plant-based materials are known to
have high porosity that helps water conduction by capillary
action. Hence, designing SVGs using coconut husk is expected
to have higher efficiency. The high mechanical strength of the
coconut coir's56 and the resiliency of coconut husk in water54
will prolong the durability of the evaporator as well.
Thus, motivated by coconut husk's natural properties, we
prepared a 3D cylindrical SVG by flaming coconut husk's
surfaces in an environmental condition using a liquefied
petroleum gas (LPG) stove. Our carbonized coconut husk (CCH)
based evaporators show a maximum evaporation rate of 3.6
kg m-2 h-1 under 1 sun illumination (AM 1.5) and offering a
thermal efficiency of 144%. Although the evaporation rate is
comparable to other reported 3D SVGs, easy processability
combined with negligible material cost and high stability of
coconut husk in water establish the superiority of CCH for
practical application. The evaporation rate remains unaffected
up to a measured time of 10 hours during continuous steam
generation from seawater and has the capability of self-
cleaning salt under dark. The evaporation efficiency remains
unchanged for higher intensities and multiple cycles as well.
The efficient evaporation capacity of CCH evaporators can be
attributed to the increased surface area, effective three-
dimensional structure, and heat harvesting capability from the
environment through the larger cylindrical facet of the CCH
evaporators.
To prepare the CCH evaporators, raw coconut husk (RCH)
was cut into pieces, washed thoroughly using KOH solution
followed by DI water, compressed tightly into a cylindrical
shape, and dried at 70 °C in an oven (Fig. 1). The density of
coconut husk was approximately 0.33 g cm-3 after drying.
Finally, its cylindrical surfaces and top surface were carbonized
using a household LPG stove in an environmental condition.
The detailed carbonization process is given in the supporting
information. The cross-section image of the CCH (extreme
right) shows that only the surface has been carbonized, and
the bulk remains the same, which will help in maintaining the
mechanical strength of the coconut husk. As it does not
require any specific tool to fabricate, an ordinary person can
easily design the same CCH evaporator for domestic use. The
chemical composition and functional groups of RCH and CCH
were determined using Fourier Transform Infrared (FTIR)
Spectroscopy, and the results are shown in Fig. 2a. The peaks
in RCH at 3258 cm-1 (O-H stretching vibration) and 1027 cm-1
(O-H bending vibration) are absent in the CCH, which confirms
the degradation of the oxygen-containing groups coming from
the hydroxyl groups 57. The diminished peak at 2897 cm-1
indicates the formation of unsaturated carbon-carbon
bonding, which is one of the reasons to enhance light
absorption.57 XPS analysis was performed to determine the
type of chemical bonding present between the elements in
CCH. An increase in the carbon concentration was estimated
by the carbon to oxygen (C/O) ratio from the XPS spectra given
in Fig. 2b. The measured element content of C and O in raw
coconut husk is 73.27% and 21.11%, and in the carbonized
coconut husk, the content of C and O is 82.27% and 14.71%,
showing the C/O ratio was increased from 3.47 to 5.59 after
carbonization. Further, the fitted high-resolution C1s spectra
of raw coconut husk (Fig. S1a) and carbonized coconut husk
(Fig. 2c) show three standard peaks of C-C, C-O-C, and O-C=O
located around the binding energies 284.8eV, 286eV, and
288.7eV and the deconvoluted O1s spectra of coconut husk
(Fig. S1b) and carbonized coconut husk (Fig. 2d) consist of two
distinct peaks of C=O and C-O at binding energies 533eV and
531.5eV, respectively. The occurrence of the bonds between
carbon and oxygen in O1s and C1s spectra suggested the
hydrophilic nature of the CCH.58 Furthermore, the immediate
absorption of the water droplets, when added on top,
confirms the hydrophilicity of both RCH and CCH (shown in
video S1).
The coconut husk consists of fibres bonded by spongy
coconut pith. The same structure was maintained after
carbonization, as shown in the scanning electron microscope
images given in Fig. 3a-b. The encircled region of Fig. 3b
Fig. 1 Photographs showing stepwise fabrication of carbonized coconut husk
evaporator.
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displays a fibre surrounded by the coconut pith. Furthermore,
the tubular fibres with long channels (Fig. 3c) help in fast water
transport. The porous structure of the CCH will enhance solar
absorption by trapping the incident photons, increase the
effective surface area of evaporation, and act as the escape
path to the vapor from the evaporator, as shown in the model
in Fig. 3d. The porous nature of coconut husk was quantified
by measuring the surface area before and after carbonization
using the BET (Brunauer-Emmett-Teller) method (Fig. S2), and
the estimated surface area of RCH and CCH was 87.54 and
41.084 m2 g-1, respectively. The reduced surface area of CCH
may be caused by the shrinkage of coconut husk after
carbonization.43,50
The diffused reflectance of CCH was measured in both dry
and wet states in the wavelength range of 300-2500 nm (Fig.
4a) to estimate the light-absorbing capability within the entire
solar spectrum. Since the transmittance is zero, the CCH
exhibits an average absorption of about 96% and 97.5% (Note
S1) in dry and wet states, respectively, indicating the superior
light-absorbing ability of CCH after carbonization. On the
contrary, 52% of solar absorption of RCH in the dry state (Fig.
S3) shows its non-suitability towards solar steam generation.
The high absorption is accredited to the inherent light-
absorbing property and porous nature of CCH. The heat
harvesting capability of the CCH evaporator from the
surrounding was observed by the transient temperature
response in the dark. Fig. 4b shows the variation of
temperature of the top of the 2D CCH evaporator and its
surroundings after it was in contact with water. The
surrounding temperature away from the CCH was
approximately constant at 23 °C (neglecting the minimal
~
fluctuations) throughout the measurement time. In contrast,
the temperature of the evaporator surface slowly decreased to
19 °C after about 45 minutes, followed by saturation. The
~
inset of Fig. 4b shows the spatial distribution of temperature
after saturation. The gradual decrease in the temperature
towards the evaporator is due to the evaporative cooling,59
indicating constant heat energy capture by CCH from the
surrounding to vaporize water.
Further, the illuminated surface temperature of CCH based
SVG under 1 sun was recorded using an infrared camera to
investigate the photothermal effect. Under 1 sun, in the wet
condition, the average temperature increased from 19 °C to
~
31 °C (Fig. 4c), whereas, for dry CCH temperature reaches
~
from 23 °C to 50 °C, suggesting the efficient light to heat
~
~
conversion. Also, under light illumination, the temperature of
the middle part of the evaporator was always lower than the
ambient temperature (Fig. S4). These results confirmed the
low thermal conductivity and energy harvesting capability of
the CCH evaporator, which are the beneficial properties to get
a higher evaporation rate.
The heat exchange of the CCH evaporator with the
environment at 1 sun intensity was examined using
𝑆𝜀𝜎
for convective heat transfer and for
(
𝑇
4
𝑇
4
𝑎𝑚𝑏
)
𝑆ℎ
(
𝑇
𝑇
𝑎𝑚𝑏
)
radiative heat transfer,60 where is the area of illuminated
𝑆
surface, the emissivity, the Stefan Boltzmann constant,
𝜀
𝜎
𝑇
the average temperature (in °K) of the solar absorbing surface,
represents the ambient temperature (in °K), and h is the
𝑇
𝑎𝑚𝑏
convection heat transfer coefficient (10 W m-2 K-1).61 Under
light illumination, the top surface's average temperature (T2)
was 31 °C compared with the 23 °C ( ) of the
~
~
𝑇
𝑎𝑚𝑏
surrounding atmosphere; hence, the convective and the
radiative heat losses to the environment by CCH were 0.0266
W and 0.0149 W, respectively. The heat loss to the underlying
water was neglected as the height of the CCH is too high for
heat conduction. Therefore, the total energy loss to the
environment, including the reflection loss (estimated from Fig.
4a), was 0.0490 W. Similarly, the convective and radiative
Fig. 3 SEM images of the carbonized coconut husk evaporator (a) side view and (b) top
view. (c) SEM image of the top of a single fibre. (d) Schematic showing the advantage of
coconut coir in light absorption and steam escape.
Fig. 4 (a) Diffused reflectance spectra of dry and wet CCH. The solar irradiation
spectrum is shown in a light blue area. (b) Transient response of environment
temperature (black line) and top surface temperature (blue line) after keeping the 2D
evaporator in contact with water in the dark. The thermal image in the inset shows the
spatial distribution of temperature after saturation. (e) Time course of the surface
temperature of dry and wet 3D CCH under 1 sun and insets are IR thermal images of 3D
CCH in dry and wet states after 1 h of illumination. (d) Schematic of the coconut husk
showing the heat harvesting capacity. The maximum radius of the CCH was kept to 1
cm. Total effective energy input = (Solar flux + convective gain + Radiative gain) =
0.5073 W for 3.13 cm2 area 1.6 kW m-2 under 1 sun illumination (1 kW/m2).
~
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energy gain by cold cylindrical surfaces (with temperature T1)
of 3D CCH from the ambient environment was 0.1507 and
0.0426 W, respectively. Under the illumination of 1 sun, for a
CCH based evaporator with a radius of 1 cm and length of 6
cm, total effective energy input = solar flux + convective gain +
radiative gain = 0.5073 W ( 1.6 kW/m2), which is 1.6 times of
~
the solar radiation reaching to earth surface (Fig. 4d). The
energy efficiency (η) of CCH based SVG was calculated as
90.3% by using the following equation61–64
𝜂
=
𝑄
𝑠𝑜𝑙𝑎𝑟
+
𝑄
𝑔𝑎𝑖𝑛
𝑄
𝑙𝑜𝑠𝑠
𝑄
𝑠𝑜𝑙𝑎𝑟
+
𝑄
𝑔𝑎𝑖𝑛
Where, , , and are the energy of solar flux of 1
𝑄
𝑠𝑜𝑙𝑎𝑟
𝑄
𝑔𝑎𝑖𝑛
𝑄
𝑙𝑜𝑠𝑠
sun, energy gain from the environment by the evaporator, and
energy loss by the evaporator to the environment,
respectively. Since the input energy supplied to the CCH
evaporator was only , the effective energy efficiency
𝑄
𝑠𝑜𝑙𝑎𝑟
goes to 145.9% by considering only in the denominator
𝑄
𝑠𝑜𝑙𝑎𝑟
of the above equation. The detailed calculations are given in
Note S2 of the supporting information.
Motivated by these favourable characteristics, the CCH was
employed to check its potential for solar steam generation in
ambient conditions for 2D and 3D shapes. First, the CCH
evaporator's height was optimized by measuring the
evaporation rate with varying heights from 0.7cm (2D) to 7 cm
under 1 sun illumination, and their performances are given in
Fig. S5a. CCH provides the best performance at the height of
around 6 cm. This also shows the advantage of the porous
nature of the coconut husk that maintains controlled water
flow up to a vertical height over 6 cm with the help of capillary
action. The evaporation performances of the 2D and 3D CCH
evaporators with 6 cm height under dark and 1 sun
illumination conditions are plotted in Fig. 5a. Under dark, it
shows an evaporation rate of 0.4 kg m-2 h-1 and 1.3 kg m-2 h-1
for 2D and 3D configurations, respectively, which are much
faster than pure water (0.09 kg m-2 h-1). The impressive dark
evaporation rate mainly originated from the heat harvesting
capacity of the CCH evaporators, as discussed earlier. Under 1
sun illumination, the evaporation rate of the 2D evaporator
was 2.2 kg m-2 h-1, and the 3D evaporator shows an average
evaporation rate of 3.5 ± 0.15 kg m-2 h-1 (plotted in Fig. S5b)
with a maximum value of 3.6 kg m-2 h-1, which is comparable to
the recently reported other 3D SVGs (Table S1). The 3D
evaporation rate under 2 and 3 sun was 5.6 kg m-2 h-1 and 7.7
kg m-2 h-1 respectively, indicating the evaporation rate
increases linearly with intensity (Fig. S6). Under higher solar
irradiance power densities, the durability of CCH based SVG
was estimated. The 3D CCH based evaporator shows stable
performance for more than 15 cycles, each cycle of 2 h,
confirming its good recycling stability (Fig. 5b). The thermal
efficiency ( ) of our CCH evaporator was calculated using the
η
𝑡ℎ
equation 14 where is the net
η
𝑡ℎ
=
𝑚
𝐿𝑉
/
𝐶
𝑜𝑝𝑡
𝑃
𝑖𝑛
,
𝑚
evaporation rate, is the enthalpy of vaporization of water-
𝐿𝑉
vapor phase change, is the optical concentration, is
𝐶
𝑜𝑝𝑡
𝑃
𝑖𝑛
the solar irradiation at 1 sun. The estimated for 3D CCH
η
𝑡ℎ
was 144.4% under 1sun illumination, and 90.2% considering
effective as 1.6. These values are similar to the value
𝑃
𝑖𝑛
calculated earlier using energy efficiency.
To demonstrate the practical water purification, soap-
water, detergent-water, and methylene blue (MB) solution
were selected to simulate wastewater using a handmade
prototype purification setup (Fig. S7). The absorption spectrum
was used to examine the concentration of the pollutants (Fig.
5c). The CCH evaporators efficiently reject contaminants after
the purification, as confirmed by the zero-light absorption and
the color changes shown in the insets of Fig. 5c. To further
evaluate the desalination effect, artificial seawater was
prepared by dissolving 3.5 wt% NaCl in water. The stability of
seawater purification was measured by continuously
measuring the evaporation rate for 10 hours under 1 sun
illumination. A nearly uniform evaporation rate in Fig. 5d
shows the retention ability of the CCH evaporator without any
salt accumulation on the evaporator surface. The qualities of
the purified water from all these solutions were further tested
by measuring the pH and conductivity. The pH and
conductivity values of water after purification from different
contaminated solutions was found comparable with that of RO
and DI water (Table S2), indicating that CCH evaporators can
effectively decontaminate sewage. Noteworthy to mention
that during the high-intensity illumination (more than 3 sun),
the salt crystals slowly start to accumulate on the evaporator's
surface after 10 hours of continuous operation. As the height
of the evaporator is optimized for 1 sun illumination, lack of
sufficient water flow at 3 sun causes the salt accumulation.
However, under dark, these accumulated salt crystals dissolve
back into the bulk water (inset of Fig. 5d), ensuring the
endurance of the CCH evaporator in seawater desalination.
Fig. 5 (a) The mass change of water in 2D and 3D CCH evaporators in the dark and 1
sun. (b) Evaporation rate versus cycle number at a series of solar power intensities. (c)
UV-Vis absorption spectra of polluted and purified water. I: detergent water. J: purified
detergent water. K: MB solution. L: purified MB water. M: soap water. N: purified soap
water. The insets are the optical images of MB, detergent and soap solution before and
after purification. (d) Retention of 3D CCH evaporator for saltwater for a duration of 10
h. The insets are the optical image of accumulated salt on 3D CCH evaporator’s surface
after 10 h under 1 sun for higher salt concentration and self-cleaned under dark for
another 10 h.
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Conclusions
In summary, we have demonstrated that coconut husk can be
used as an efficient 3D solar vapor generator only by
carbonizing its surfaces. 3D structures with increased effective
surface area promote energy harvesting from the environment
leading to an evaporation rate of 3.6 kg m-2h-1 with effective
efficiency of 90.2%. The CCH evaporator shows the
sustainability towards seawater desalination and sewage
purification. Large-scale availability, negligible materials cost,
combined with the simple design techniques, the coconut
husk-based solar vapor generator may be an attractive
alternative to conventional freshwater generators. Further
optimization may be needed for freshwater collection after
steaming for industrial-scale application. According to recent
reports, countries like Indonesia, Philippines, India, Srilanka,
and Mexico have increased stress in freshwater.65,66 Also, the
density of coconut farming is very high in these parts of the
world.67 Hence, designing efficient SVGs using coconut husk
can give an immediate eco-friendly solution in these coastal
parts of the world.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
We acknowledge Department of Science and Technology,
Govt. of India via Project DST/INSPIRE/04/2016/000269.
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... For freshwater production, Wani et al. [94] carbonized a biomass-based coconut husk (CCH) using a liquefied petroleum gas (LPG) stove. The method of fabricating the CCH is illustrated in Fig. 18. ...
... The evaporation efficiencies of Fig. 18. The processing steps for making carbonized coconut husk (CCH) [94]. Taylor et al. [111] investigated gold nanoparticles (Au-NPs) coated on 5 mm thick carbonized wood (Au-wood) for solar desalination. ...
... The LPG stove flame was used to carbonize the surface for solar vapor generation. It was observed that the carbonized husk evaporated 3.60 kg m − 2 h − 1 under one sun illumination [94]. The carbonized magnolia fruit exhibited at maximum Fig. 22. (a-b) A pictorial view of algae in the field and sample of Enteromorpha Prolifera (EP) and (c) carbonized EP-driven solar desalination setup [116]. ...
Article
Solar desalination is one of the green energy processes to treat saline water and wastewater. Solar evaporation systems, formally solar stills, have been widely used to evaporate water to purify it. However, the evaporation rate in solar stills is typically low due to incoming energy used to heat the entire bulk water. In order to minimize the bulk heating, researchers have developed capillary flow-based, self-floatable, broadband photothermal absorbers (250–2500 nm wavelength) for fast solar evaporation. Recently, interfacial solar steam generation (ISSG) has attracted attention due to significant advantages in desalination and water treatment. In general, ISSG materials are classified into plasmonic metals, semiconductors, black carbon and polymer-based materials. The basic requirements for these photothermal materials include being self-floatable and having high solar absorption, fast water transport (capillary action) and low thermal conductivity to confine the heat locally. Some natural plant species satisfy these prerequisites and have been used as photothermal materials in solar steam generation (SSG). The present review exclusively focuses on the carbonized botanical species, including bamboo, corncob, corn-stalk, coconut-husk, carrot, fruit residues (cherry, grape, orange and apple), green algae, loofah fruit, magnolia fruit, mushroom, lotus leaf and seedpods, sugarcane, sunflower head, tofu, wheat flour and wood pieces for improving the evaporation rate and efficiency. Carbonization technique improves the solar absorption by increasing the carbon concentration. In addition, these floatable solar absorbers evaporate the water with the aid of natural microchannels. These materials not only improve the efficiency, but also have economic and environmental benefits.
... Black TiO 2-x with narrow band-gap (J. Wang et al., 2017a;Yan et al., 2016;Ye et al., 2017), TiN with lossy plasmonic resonance (Ishii et al., 2016), and biochar materials (Guan et al., 2021;Mnoyan et al., 2020;Wang et al., 2020;Wani et al., 2021;Yang et al., 2019) showed a great potential in photothermal water evaporation. Furthermore, synchronous photocatalytic pollutants degradation and photothermal water evaporation by dual functional materials became a promising strategy for water cleaning and optimized photonic energy utilization Xue et al., 2019). ...
Article
Relatively large band-gap, fast charge carriers recombination, and mono-functionality of photocatalytic materials are still representing stumbling hurdles against their optimal usage for water cleaning. Herein, a novel black titanium oxide/plasmonic titanium [email protected] coconut biochar (TiO2-x/[email protected]) composite was designed to have both photocatalytic and photothermal functions. Intermediate states of black TiO2-x, plasmonic effect of TiN, and high electrons (e⁻) capacity of biochar enhanced band-gap narrowing, light absorbance extension, and charge carriers separation respectively. Black TiO2-x and plasmonic TiN sensitization via visible/infrared (Vis/IR) portion of photonic spectrum in addition to the confirmed close contact of composite constituents explained the demonstrated major role of e⁻ in photocatalytic mechanism through efficient excitation and facile transfer. Thanks to black photocatalytic semiconductor and carbonic materials for their ultimate photons harnessing and efficient photothermal conversion where the composite exhibited a remarkable photothermal water evaporation upon Vis/IR illumination as well. TiO2-x/[email protected] composite revealed 92.8 and 89.7% photocatalytic reduction of hexavalent chromium (Cr(VI)) and water evaporation efficiencies up to 92.9 and 51.1% upon IR and Vis light illumination respectively. This study proposes a new approach for efficient water cleaning by coupling of oxygen deficient and plasmonic semiconductors supported on naturally derived carbonic material as a broad spectrum harvester and bi-functional photocatalytic and photothermal material.
... The primary essence of solar steam generation technology is to absorb the entire solar light, and the same can be achieved through the development of efficient photothermal materials [8] with advanced structural designs [9,10]. Various materials such as metal oxides [11][12][13], polymers [14][15][16][17], carbon-based [18][19][20][21], and plasmonic materials [22][23][24][25] with broadband light absorption have been investigated for solar steam generation. Environmental stability, bandgap tunability, non-toxicity, and facile low-cost synthesis make metal oxides promising absorber materials in interfacial steam generators [13,26]. ...
Article
Photothermal water evaporation provides a pathway toward a promising solution to global freshwater scarcity. Synergistic integration of functions in a material in diverse directions is a key strategy for designing multifunctional materials. Lanthanum-based perovskite complex oxides LaMO3 (M = Ni and Co) have narrow band gaps with a high absorption coefficient. These functionalities have not been appropriately explored for photothermal energy conversion. Here, we synthesized nanostructured metallic LaNiO3 and semiconducting LaCoO3 and used them to design interfacial solar steam generators. Effective light absorption capability over the entire solar spectrum of these materials leads to a photothermal efficiency of the order of 83% for both materials. Using a cone-shaped 3D interfacial steam generator with a LaNiO3 absorber, we achieved an evaporation rate of 2.3 kg m⁻² h⁻¹, corresponding to solar vapor generation efficiency of over 95%. To the best of our knowledge, this evaporation rate is higher than any oxide-based interfacial solar steam generator reported so far. Furthermore, we have also shown an effective way of using such evaporators for long-term seawater desalination.
... 5 Recently, solar-driven water purification technology is being considered an energy-efficient way of producing clean water. [6][7][8][9][10][11][12] The principle of this technology is to convert solar energy into heat that generates vapor/steam, and the main focus is to prepare efficient solar vapor generators (SVG) using un-concentrated solar light. 13,14 It has been found that heating a small amount of water near the evaporator surface while restricting the heat flow to the entire water improves the evaporation efficiency significantly, known as interfacial vapor generators. ...
Research
The solar-driven water evaporation technique to produce clean water has shown enormous potential towards an energy-efficient solution for global freshwater scarcity. Designing a cost-effective and efficient solar vapor generator (SVG) using an underutilized natural source causing environmental pollution will be an eco-friendly way for freshwater generation. We developed an SVG simply by carbonizing the surfaces of a piece of coconut husk using a household liquefied petroleum gas stove. The naturally porous structure of coconut husk enhances light absorption, and the 3D assembly promotes heat energy harvesting from the environment, leading to an effective energy input of 1.6 kW m-2 under 1sun illumination (1.0 kW m-2). Our carbonized coconut husk evaporator shows an evaporation rate of 3.6 kg m-2 h-1. The simple design technique of carbonized coconut husk-based SVGs, negligible material cost, and its large-scale availability combined with the already existing coconut coir industries can lead to an affordable way of freshwater generation.
Article
The progression of photothermal materials with broad solar absorption and improved photothermal conversion efficiency is critical for developing interfacial solar steam generation (ISSG)-based water desalination and purification systems. This green solar-driven water vaporization technology has regained popularity as a sustainable solution to water shortages. Among many other photothermal materials, natural porous material-based photothermal evaporators have piqued the interest of researchers owing to their biodegradability, abundance, low thermal conductivity, low cost, natural capillary mechanism, and hydrophilicity. In this review, we report on recent advances in photothermal material design based on various natural porous materials, categorize systems considering contact mode and water transportation route, assess optical absorbance, thermal management issues, and discuss water purification as well as desalination applications. This review will stimulate further investigation and research interest in the utilization of natural porous materials for large-scale ISSG-based application implementation.
Article
Full-text available
The main challenge of interfacial solar steam generation (ISSG) for desalination is salt accumulation on solar absorber surface, thus significantly decreasing the evaporation efficiency. The most common method is design of a hydrophilic/hydrophobic multilayer composites system, where the upper hydrophobic layer is used for light absorption and the lower hydrophilic layer is used for pumping water. Obviously, such a complex multilayer system results in unsatisfactory efficiency and high cost of solar desalination. Here, we propose a novel strategy to address this issue using self-floating superhydrophilicity porous carbon foam (SPCF) used as integrative solar absorber for desalination, resulting from the powerful water pumping capability of SPCF. Salt from bulk water can be quickly re-dissolution in the 3D porous structure of SPCF, no salt accumulation was observed on the surface of SPCF in simulated seawater during 8 h desalination. Together with superior light absorptance (96.19%), ultrafast solar-thermal response (a temperature increases of 92.7 °C within 10 s under 2 sun), low thermal conductivity and outstanding mechanical robust, a high energy efficiency (86% at 1 sun) and simultaneous salt resistant for vapor generation are achieved. The findings provide a new perspective to design self-desalting monolithic ISSG to satisfy the demand for eco-friendly, low cost, highly efficient, and enduring solar desalination.
Article
One way to harvest solar energy is to produce steam from liquid water. Steam can be used to provide freshwater even in a harsh environment, with extreme temperature and low groundwater supplies. It can also be extracted from polluted soil or water, purifying the water in the process at the same time. Water evaporation systems are energy effective, with efficiencies ranging from 60% to over 90%. Furthermore, they can be cost-effective and have a low environmental impact, using the right materials. In this article, we will review natural materials, mainly of biological origin, proposed for solar evaporation, from wood and plants to algae and other atypical biomass such as fungi or wastes such as pomelo peels, through gels and foams, raw or charred. Their evaporation effectiveness will be presented and discussed, as well as their energy efficiency. This timely review suggests the suitability of natural materials for this application and reports on the progress that has already been achieved, as well as on the advances that remain to be made to improve the performance of these low cost, low environmental impact but high performance systems.
Article
Interfacial solar steam generation offers a sustainable and affordable technology for seawater desalination and water treatment. During solar steam generation the temperature of the solar evaporation surface is generally higher than the bulk water, which results in energy loss to the bulk water by heat conduction. While many strategies have been developed to minimize and/or eliminate the conductive heat loss, this study focuses on completely reversing conductive heat loss and turning it into an energy extraction from the bulk water to enhance the evaporation during solar steam generation. This was achieved by introducing a certain area of cold evaporation surface between the solar evaporation surface and the bulk water, which led to the conductive heat loss from the solar evaporation surface being completely absorbed and consumed by the cold evaporation surface before reaching the bulk water. Meanwhile, due to its lower surface temperature, the cold evaporation was also able to extract energy from the bulk water, turning the heat conduction loss from the evaporator to the bulk water into the energy harvest from the bulk water. When the surface area of the cold evaporation surface was increased to a certain point (50.3 cm² in this work), heat flow was reversed, and energy was extracted from the bulk water by the evaporator to enhance solar evaporation. Theoretical simulations agreed well with the experimental results. In addition, as parasitic effects, the cold evaporation surface was also able to gain energy from the ambient air and lower the temperature of the solar evaporation surface, reducing both radiation and convection energy loss. As a result, the evaporation rate and the light-to-vapor energy efficiency of the evaporator were far beyond the theoretical limits, confirming that this strategy has great potential for further practical applications.
Article
Solar vapor generation has become a promising water purification technology owing to its eco-friendly and energy-saving features. However, it remains as a big challenge to further improve the solar-driven evaporation performance, especially to further increase the efficiency of solar energy utilization. Here, a novel plasmonic wooden flower is fabricated, in which Ag-polydopamine (PDA) core-shell structured nanoparticles ([email protected] NPs) are loaded on the wooden flower. The superior structural characteristics of the porous wooden flower and the [email protected] NPs provide a synergetic effect. Thus, it exhibits a high light-harvesting absorption of 98.65%. Moreover, the hydrophilic [email protected] NPs promote the formation of water films on both sides of the flower petal. Meanwhile, the abundant capillary channels and pinholes in the wood further enhance the heat convection and solar-driven evaporation of the water films on both sides of the petal. With the excellent thermal management performance, the plasmonic wooden flower device has a high vapor generation rate of 2.08 kg m⁻² h⁻¹ and an ultrahigh solar-to-vapor efficiency of 97.0% under 1 sun illumination, among the best values reported in the literature. It is promising for future water treatment applications in large scale including recovery of metal ions, brine desalination and sewage purification.
Article
Solar-steam generation is recognized as a promising pathway to mitigate the global issue of clean water shortage. Preparation of high-performance photothermal materials and efficient design of advanced evaporators are two key factors which need to be enhanced to facilitate the practical application of solar steam generation for clean water production. In this work, polydopamine coated nickel-cobalt bimetal (Ni1Co3@PDA) nanosheets were synthesized and employed as photothermal materials for solar steam generation. Ni1Co3@PDA nanosheets were coated on the surface of a commercial sponge using sodium alginate as a binding agent. The obtained photothermal sponges exhibited excellent light absorption (>99%) which benefited the light-to-heat conversion and solar evaporation. A kerosene lamp-like evaporator, which spatially separated the bulk water and the evaporation surface, was adopted to evaluate the performance of the Ni1Co3@PDA sponge for solar steam generation. It was found that the photothermal sponges could be stacked to form 3D evaporators of adjustable heights to achieve superior evaporation rates while maintaining the same footprint. The stacked photothermal sponges with a height of 6.0 cm showed the highest water evaporation rate of 2.42 kg m-2 h-1 under 1.0 sun with corresponding energy efficiency (109%) beyond the theoretical limit. This was due to the eliminated heat conduction loss, reduced radiation and convection loss, as well as net energy gain from the environment. The salinity of the collected clean water is only 2.26 ppm. The overall system is cost effective and highly efficient, thus shows great potential for future real-world applications.
Article
In this work, a new kind of material, tannic [email protected]@Fe³⁺ ([email protected]@Fe³⁺) composite coating, with multifunctional performance has been developed for solar steam generation. The [email protected]@Fe³⁺ has many advantages: (a) Scalability. The fabrication process is facile and mild; it can be realized in aqueous solution at room temperature, without high pressure, toxic organic solvents, or complex equipment. (b) Cost-effectiveness. The reagents used for preparation of [email protected]@Fe³⁺ are low-cost and readily available. (c) Universality and good stability. [email protected]@Fe³⁺ firmly adheres on surfaces of various substrates with diverse shapes (cotton, filter paper, wood, polyurethane sponge, and even chemically inert and highly hydrophobic polyvinylidene fluoride membranes), and can withstand rinsing treatment (3000 r/min for 96 h), cyclic frost-thaw test (−18 °C ⇄ 30 °C, 90 times), and both high and low pH environments. Many other reported coatings, such as carbon black, fail under similar process strain. (d) Anti-crude oil-fouling property. The [email protected]@Fe³⁺ possesses stable superhydrophilicity and underwater superoleophobicity, which collectively endow substrates with the ability to resist fouling by oils. (e) Broad and strong light absorption. [email protected]@Fe³⁺ can transform various substrates with diverse shapes into black materials with broadband light absorption due to its d-d transitions and rough surface. Together, these properties of [email protected]@Fe³⁺ enable substrates with nearly any structural design to be easily transformed into photothermal materials for efficient solar steam generation. As a proof of concept, poplar wood is treated with [email protected]@Fe³⁺, achieving a water evaporation rate of ∼1.8 kg m⁻² h⁻¹ (one sun), which is a record among wood-based photothermal materials.
Article
Converting waste into versatile materials has been considered as an effective strategy to tackle environmental issues such as shortages in food, clean water and energy. Conventional carbonization technology has provided an effective approach to constructing photothermal materials for interfacial evaporation or crude oil removal. However, high energy consumption, poor shape retention and weak mechanical strength have significantly hindered the practical application of conventional carbonization technology. This paper discusses how a biomass-derived sponge from discarded pomelo peel (PP) may be used to collect photothermal polypyrrole in a cost-effective way with mild reaction. The photothermic sponge which is derived from this process can then be effectively employed to realize solar enhanced water evaporation and heavy crude oil removal. The polypyrrole-functionalized pomelo peel (FPyPP) can be attained by a mild, one-pot wet oxypolymerization method, resulting in over 95% absorption of sunlight and evaporation rates of about 1.22 kg/m2 under the sun. Furthermore, the concentrated heat on the sponge surface can efficiently reduce the viscosity of heavy crude oil and thereby lead to an effective clean-up. This study aims to provide a low-cost, low-energy-consumption and eco-friendly approach to the acquisition of the multifunctional photothermic sponge by recycling the waste and demonstrating potential applications in tackling clean water generation and polluted water treatment.
Article
Solar-powered water evaporation — the extraction of vapour from liquid water using solar energy — provides the basis for the development of eco-friendly and cost-effective freshwater production. Liquid water consumes and carries energy, and, thus, plays an essential role in this process. As such, extensive experimental and theoretical studies have been focused on water management to achieve efficient solar vapour generation. Many innovative materials have been proposed to enable highly controllable and efficient solar-to-thermal energy conversion to address the challenges in the energy–water nexus from the microscale to the molecular level. In this Review, we summarize the fundamental principles of materials design for efficient solar-to-thermal energy conversion and vapour generation. We discuss how to integrate photothermal materials, nanostructures/microstructures and water–material interactions to improve the performance of the evaporation system via in situ utilization of solar energy. Focusing on materials science and engineering, we overview the key challenges and opportunities for nanostructured and microstructured materials in both fundamental research and practical water-purification applications.
Article
Solar steam generation is one of the most promising technologies to mitigate the issue of clean water shortage using sustainable solar energy. Photothermal aerogels, especially the 3D graphene-based aerogels have shown unique merits for solar steam generation, such as light weight, high flexibility, and superior evaporation rate and energy efficiency. However, 3D aerogels require much more raw materials of graphene which limits their large-scale applications. In this study, 3D photothermal aerogels composed of reduced graphene oxide (RGO) nanosheets, rice straw derived cellulose fibres, and sodium alginate (SA) are prepared for solar-steam generation. The use of rice straw fibres as skeletal support significantly reduces the need for the more expensive RGO by 43.5%, turning the rice straw biomass waste into value-added materials. The integration of rice straw fibres and RGO significantly enhances the flexibility and mechanical stability of the obtained photothermal RGO-SA-cellulose aerogel. The photothermal aerogel shows a strong broadband light absorption of 96-97%. During solar-steam generation, the 3D photothermal aerogel effectively decreases radiation and convection energy loss while enhancing energy harvesting from the environment, leading to an extremely high evaporation rate of 2.25 kg m-2 h-1, corresponding to an energy conversion efficiency of 88.9% under 1.0 sun irradiation. The salinity of clean water collected during evaporation of real seawater is only 0.37 ppm. The materials are environmentally friendly and cost effective, showing great potential for real-world desalination applications.
Article
Solar steam generation (SSG) based on the photothermal effect has been considered to be a promising avenue for freshwater production. However, the fabrication of highly‐efficient photothermal layers, at large‐scale and low‐cost is still a challenge, hindering practical applications. Herein, it is demonstrated that carbonized towel‐gourd sponges (CTGS) are excellent photothermal materials. And a capillarity‐driven interfacial self‐coating method is developed to prepare the super‐hydrophilic CTGS/paper photothermal layer. The SSG device based on the CTGS/paper exhibits a high evaporation rate of 1.53 kg m⁻² h⁻¹ with an efficiency of 95.9% under 1 sun irradiation. The high evaporation rate only slightly reduces when confronting the diverse and complex practical SSG conditions, such as seawater and waste water. Moreover, CTGS/paper has the advantages of simple preparation, recyclablability, low‐cost (≈4 $ per m²), high‐efficiency, flexibility, and scalability, which are the important prerequisites for promoting SSG techniques for industrialization and practical applications. In addition, the versatile energy conversion ability of the CTGS/paper is also demonstrated. Assisted by the photothermal effect of CTGS/paper, solar energy is converted to mechanical energy and electricity.