WO2023115164A1

WO2023115164A1 - Desalination apparatus and process

Info

Publication number: WO2023115164A1
Application number: PCT/AU2022/051590
Authority: WO
Inventors: Xuan WU; Haolan XU
Original assignee: University Of South Australia
Priority date: 2021-12-23
Filing date: 2022-12-23
Publication date: 2023-06-29

Abstract

The present disclosure relates to a water distillation apparatus comprising a transparent housing substantially encasing a photothermal substrate and a condensing surface; a photothermal substrate comprising a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the photothermal substrate having an upper end and a lower end; a condensing surface being spaced from the photothermal substrate; a water feeder in fluid contact with the upper end of the photothermal substrate, and configured to introduce a feed aqueous solution to the photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the photothermal substrate; wherein heat generated by the photothermal material evaporates water from the photothermal substrate to generate steam which condenses on the condensing surface to form condensate; a condensate collector positioned at a lower end of the transparent housing and configured to collect condensate that runs down the condensing surface.

Description

DESALINATION APPARATUS AND PROCESS

PRIORITY DOCUMENT

[0001] The present application claims priority from Australian Provisional Patent Application

No. 2021904245 titled “DESALINATION APPARATUS AND PROCESS” and fded on 23 December 2021, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure generally relates to photothermal evaporation. In particular, the present disclosure relates to photothermal evaporation devices that are particularly suitable for desalination or purification of other contaminated water.

BACKGROUND

[0003] Fresh water is in short supply in many parts of the world. As the population continues to grow, demands for supplies of fresh water will also continue to grow. One potential source of fresh water is water produced by desalination of sea water. Desalination is being used more and more around the world to provide people with needed fresh water.

[0004] Desalination by reverse osmosis is one procedure commonly used. Reverse osmosis is an effective means to desalinate saline water, but it must be done on a large scale to be useful to large populations, and the current processes are expensive, energy-intensive, and involve large-scale facilities.

[0005] Desalination by distillation is another process used throughout the world. Solar-driven interfacial evaporation (SIE) has emerged as a promising form of desalination by distillation. Interfacial evaporation systems place a light absorber at the water-air interface and enable just the air-liquid interface to be heated rather than the bulk water, resulting in a much higher thermal efficiency. In addition, SIE uses clean solar energy and produces no greenhouse gas emissions.

[0006] In a typical SIE system, a photothermal evaporation surface is placed on a thermal insulator support with only a fraction of the surface immersed in water to provide a water transportation path. Figure 1 depicts a typical double-layered solar-driven interfacial evaporation system which comprises a light absorber, a supporting substrate, a bulk water reservoir, incident light, and a vapour. Incident light is absorbed by the light absorber and converted into heat. In the meantime, water is adsorbed by the supporting substrate and transported up to the evaporation surface through interconnected water pathways with the aid of capillary forces. The heat generated through the light absorber raises the temperature of the water on the evaporation surface, which drives continuous evaporation from the bulk water. [0007] Typical 2D photothermal evaporators only have one effective solar evaporation surface and suffer from relatively poor solar to vapor energy conversion efficiency. 3D photothermal evaporators aim to improve on the efficiency of 2D evaporators. Compared to 2D photothermal evaporators, with the same occupied area, 3D photothermal evaporators have a larger evaporation surface area, receive more solar light, and have more energy input and, as a result, they have higher evaporation rates and vapour (i.e. clean water) outputs

[0008] However, 3D photothermal evaporators have limitations that have limited their scalability and, hence, their industrial applicability. As shown in Figure 2, 3D photothermal evaporators have a wicking limit (i.e. water supply limit) which can be about 20 cm depending on materials used. In principle, it should be possible to increase the height of a 3D evaporator to increase the evaporation surface area and thus the evaporation rate. However, due to the limitation of wicking effect, the maximum height of a 3D evaporator is about 20 cm. Therefore, the evaporation rate is limited. In addition, during seawater desalination, due to the wicking limit, the top part of the evaporator has little water supply and this leads to salt crystallisation and accumulation on the top part of the 3D evaporator. This is no good for practical seawater desalination. Furthermore, the long-term operation of 2D and 3D photothermal evaporators under varying weather conditions has also presented a significant challenge.

[0009] Accordingly, there remains a need for an SIE device and/or process that has satisfactory performance in one or more of the following aspects: (i) improved evaporation performance; (ii) reduced costs; (iii) long-term operability; (iv) industrial applicability; and/or (v) scalability.

SUMMARY

[0010] According to a first aspect, there is provided a water distillation apparatus comprising: a transparent housing substantially encasing a photothermal substrate and a condensing surface; a photothermal substrate comprising a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the photothermal substrate having an upper end and a lower end; a condensing surface that is spaced from the photothermal substrate; a water feeder in fluid contact with the upper end of the photothermal substrate, and configured to introduce a feed aqueous solution to the photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the photothermal substrate; wherein heat generated by the photothermal material evaporates water from the photothermal substrate to generate steam which condenses on the condensing surface to form condensate; a condensate collector positioned at a lower end of the transparent housing and configured to collect condensate that runs down the condensing surface. [0011] In use, it has been found that the water distillation apparatus improves the evaporation rate by 5-20 times, compared to the traditional 3D photothermal evaporators.

[0012] In certain embodiments, the condensate collector is hydraulically isolated from the photothermal substrate in order to prevent mixing of collected condensate and feed aqueous solution that is present in or on the photothermal substrate.

[0013] In certain embodiments, the porous hydrophilic photothermal material is selected from, but not limited to, graphene oxide (GO), reduced graphene oxide (rGO), graphite, carbon nanotubes (CNT), polypyrrole (PPy), carbon black nanoparticles, biomass carbon, polydopamine, black nickel, CuO, Cu2-_xS (0<x<2), Fe3O4, CO3O4, TizO;. TiN, CuFeSz, and plasmonic metal (e.g. Au) and combinations thereof.

[0014] In certain embodiments, the water feeder comprises a water reservoir configured to hold a volume of feed aqueous solution and positioned at the upper end of the photothermal substrate and in fluid connection with at least the hydrophilic and porous photothermal material on the photothermal substrate so that feed aqueous solution can be fed from the water reservoir to the hydrophilic and porous photothermal material on the photothermal substrate and then transported by gravity and wicking from the upper end to the lower end of the photothermal substrate.

[0015] In certain embodiments, the transparent housing is a transparent condensing sleeve, the condensing surface is an inner surface of the transparent condensing sleeve, and the photothermal substrate is a columnar photothermal substrate and is substantially encased by the transparent condensing sleeve. Accordingly, there is provided a water distillation apparatus comprising a columnar photothermal substrate comprising a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the columnar photothermal substrate having an upper end and a lower end; a transparent condensing sleeve substantially encasing the columnar photothermal substrate, the condensing sleeve comprising an inner condensing surface that is spaced from the columnar photothermal substrate; a water feeder in fluid contact with the upper end of the columnar photothermal substrate, and configured to introduce a feed aqueous solution to the columnar photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the columnar photothermal substrate; wherein heat generated by the photothermal material evaporates water from the columnar photothermal substrate to generate steam which condenses on an inner condensing surface of the condensing sleeve to form condensate; a condensate collector positioned at a lower end of the transparent condensing sleeve and configured to collect condensate that runs down the inner condensing surface. [0016] In certain embodiments, the columnar photothermal substrate comprises an inner columnar support and an outer layer comprising the porous hydrophilic photothermal material. In certain other embodiments, the columnar photothermal substrate comprises an inner columnar support, an outer layer comprising the porous hydrophilic photothermal material and an intermediate layer between the inner columnar support and the outer layer comprising the porous hydrophilic photothermal material, the intermediate layer comprising a water absorbent material.

[0017] In certain embodiments, the photothermal substrate is applied to an inner surface of the transparent housing, for example in the form of a layer or sheet, and the water distillation apparatus further comprises a condensing component substantially encased by the transparent housing and configured to provide the condensing surface. Accordingly, there is provided a water distillation apparatus comprising a transparent housing substantially encasing a photothermal substrate and a condensing surface, a photothermal substrate applied onto an inner surface of the transparent housing, which comprises a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the photothermal substrate having an upper end and a lower end; a condensing component substantially encased by the transparent housing and comprising a condensing surface that is spaced from the photothermal substrate; a water feeder in fluid contact with the upper end of the photothermal substrate, and configured to introduce a feed aqueous solution to the photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the photothermal substrate; wherein heat generated by the photothermal material evaporates water from the photothermal substrate to generate steam which condenses on the condensing surface to form condensate; a condensate collector positioned at a lower end of the transparent housing and configured to collect condensate that runs down the condensing surface.

[0018] According to a second aspect, there is provided a water distillation system comprising the apparatus of the first aspect and a pump for transferring feed aqueous solution to be distilled from a source of feed aqueous solution to the water feeder.

[0019] In certain embodiments, the water distillation system further comprises a controller for regulating the flow rate of feed aqueous solution into the water reservoir. Advantageously, the controller can be used to control the flow rate of feed aqueous solution so as to dilute as required any excess feed aqueous solution that passes down the columnar photothermal substrate. For example, in the case of seawater desalination, brine solution containing salt and excess water exits the columnar photothermal substrate at the lower end. The concentration of the exiting brine solution can be controlled by increasing or decreasing the flow rate of seawater into the water reservoir. This can be used to dilute the brine solution so that it has a brine concentration that allows it to be disposed of in municipal sources, such as into the sea.

[0020] According to a third aspect, there is provided a water distillation process comprising: providing a water distillation apparatus of the first aspect; continuously introducing feed aqueous solution to an upper end of the photothermal substrate; and collecting condensate that runs down the condensing surface.

[0021] In certain embodiments, the length and/or the diameter of the columnar photothermal substrate comprised by the water distillation apparatus are/is chosen to achieve desirable water distillation.

[0022] In certain embodiments of the first, second and third aspects, the feed aqueous solution is an aqueous fluid that is selected from, but not limited to, tailings, lithium brine, industry wastewater, bore water, river water, seawater, brackish water, and combinations thereof.

BRIEF DESCRIPTION OF THE FIGURES

[0023] Embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0024] Figure 1 shows a schematic illustration of a typical prior art 2D photothermal evaporator (left) and a schematic illustration of a typical prior art 3D photothermal evaporator (right).

[0025] Figure 2 shows a schematic illustration of a component of a typical prior art 3D photothermal evaporator using bottom water supply by capillary effect. The distance D is the wicking distance.

[0026] Figure 3 shows a schematic illustration of a water distillation apparatus according to embodiments of the present disclosure.

[0027] Figure 4 shows a schematic illustration of a water distillation apparatus according to embodiments of the present disclosure.

[0028] Figure 5 shows a photograph of a water distillation apparatus according to embodiments of the present disclosure.

[0029] Figure 6 shows a photograph of water distillation apparatuses according to embodiments of the present disclosure, which have different lengths (E) and diameters (D) of the columnar photothermal substrate. [0030] In the following description, like reference characters designate like or corresponding parts throughout the figures.

DESCRIPTION OF EMBODIMENTS

[0031] Aspects of the present disclosure arise from the inventors’ research into 3D photothermal evaporation systems.

[0032] Figure 3 shows a water distillation apparatus 10 according to embodiments of the present disclosure. The water distillation apparatus 10 comprises a columnar photothermal substrate 12. The columnar photothermal substrate 12 comprises a hydrophilic and porous photothermal material 13. The hydrophilic and porous photothermal material 13 is capable of generating heat upon exposure to sunlight. The columnar photothermal substrate 12 has an upper end 14 and a lower end 16.

[0033] A transparent condensing sleeve 18 substantially encases the columnar photothermal substrate 12. The condensing sleeve 18 comprises an inner condensing surface 20 that is spaced from the columnar photothermal substrate 12.

[0034] A water feeder 22 is in fluid contact with the upper end 14 of the columnar photothermal substrate 12 and is configured to introduce feed aqueous solution 24 to the upper end 14 of the columnar photothermal substrate 12 such that feed aqueous solution 24 is continuously fed to the upper end 14 and moves to the lower end 16 of the columnar photothermal substrate 12. In use, heat generated by the hydrophilic and porous photothermal material 13 evaporates water from the columnar photothermal substrate 12 to generate steam which condenses on the inner condensing surface 20 of the condensing sleeve 18 to form condensate. A condensate collector 26 is positioned at a lower end 28 of the transparent condensing sleeve 18 and configured to collect condensate that runs down the inner condensing surface 20.

[0035] Figure 4 also shows a water distillation apparatus 10 according to embodiments of the present disclosure. The water distillation apparatus 10 comprises a photothermal substrate 12 applied onto an inner surface of the transparent housing 18. The photothermal substrate 12 comprises a hydrophilic and porous photothermal material. The hydrophilic and porous photothermal material is capable of generating heat upon exposure to sunlight. The photothermal substrate 12 has an upper end 14 and a lower end 16.

[0036] A transparent housing 18 substantially encases the photothermal substrate 12 and a condensing surface 20 provided by a condensing component 15. The condensing surface 20 is spaced from the photothermal substrate 12. [0037] A water feeder 22 is in fluid contact with the upper end 14 of the photothermal substrate 12 and is configured to introduce feed aqueous solution 24 to the upper end 14 of the photothermal substrate 12 such that feed aqueous solution 24 is continuously fed to the upper end 14 and moves to the lower end 16 of the photothermal substrate 12. In use, heat generated by the hydrophilic and porous photothermal material comprised by the photothermal substrate 12 evaporates water from the photothermal substrate 12 to generate steam which condenses on the condensing surface 20 of the condensing component 15 to form condensate. A condensate collector 26 is positioned at a lower end 28 of the transparent housing 18 and configured to collect condensate that runs down the condensing surface 20. The exiting solution 40 may be discharged from the apparatus 10 at a lower end of the transparent housing 18, for example, to a container.

[0038] It will be evident from the following description that the water distillation apparatus 10 may have one or more of the following advantages:

• Easy setup/build

• Structurally stable

• Small occupied area

• No height limits

• Efficient condensation and water collection

• Maximum light absorption

• Controllable brine concentration for discharge

[0039] The water distillation apparatus 10 is suitable for producing distilled water from a range of aqueous solutions which can be a natural source such as seawater, river water, or an industrial source such as tailings, lithium brine, bore water, and brackish water, or combinations thereof. In certain embodiments, the water distillation apparatus 10 is used for desalination of seawater or other salt water.

[0040] Although the following description is generally directed to the water distillation apparatus represented by Figure 3, it is appreciated that similar considerations and simple adaptations may be made to fabricate the water distillation apparatus represented by Figure 4.

[0041] The water distillation apparatus 10 comprises a columnar photothermal substrate 12. As used herein, the term “columnar” is intended to mean that the object is shaped like a column which, in turn, is something that has a tall narrow shape. The term columnar when used herein does not necessarily mean that the object is cylindrical. [0042] In the illustrated embodiments, the columnar photothermal substrate 12 is a relatively tall and thin cylinder (i.e. it has a circular cross section). It is contemplated that the columnar photothermal substrate 12 could have a cross section that is non-circular, including triangular, square, rectangular, elliptical, pentagonal, hexagonal, heptagonal, octagonal, star shaped, irregularly shaped and combinations thereof.

[0043] The columnar photothermal substrate 12 comprises any suitable hydrophilic and porous photothermal material 13 that is capable of generating heat upon exposure to sunlight or other suitable light sources. Primary functions of the photothermal material 13 are light absorption, light-to-heat conversion, water transportation, and water evaporation. Ideally, the photothermal material 13 should be proficient in harvesting as much of the solar spectrum (i.e. 200-2500 nm) as possible. For example, the photothermal material 13 may be selected from, but not limited to, graphene, graphene oxide (GO), reduced graphene oxide (rGO), graphite, carbon nanotubes (CNT), polypyrrole (PPy), poly(l,3,5- hexahydro-l,3,5-triazines), carbon black nanoparticles, biomass carbon, polydopamine, black nickel, CuO, Cu2-_xS (0<x<2), Fe3O4, CO3O4, TizO;. TiN, CuFcS . and plasmonic metal (e.g. Au) and combinations thereof. In certain specific embodiments, the photothermal material 13 is rGO. Suitable rGO materials are available commercially, for example the rGO from Huasheng Graphite Co., Ltd.

[0044] The columnar photothermal substrate 12 may simply be a column of photothermal material 13 such as rGO. However, in order to minimise the amount of photothermal material 13 used, the columnar photothermal substrate 12 may be formed from an inner non-porous columnar support and an outer layer comprising the hydrophilic and porous photothermal material 13. The inner columnar support can be formed from a non-porous and suitably rigid material that is able to withstand the relatively high temperatures generated within the apparatus 10. Suitable materials include plastics, metals, ceramics, etc. In certain embodiments, the inner columnar support is formed from a length of PVC tube. The hydrophilic and porous photothermal material 13 in sheet form, is then wrapped onto an outer surface of the inner columnar support. In alternative embodiments, the columnar photothermal substrate 12 comprises an inner columnar support, an outer layer comprising the hydrophilic and porous photothermal material 13 (both as just described) and an intermediate layer between the inner columnar support and the outer layer comprising the hydrophilic and porous photothermal material 13. The intermediate layer comprises a water absorbent material. Suitable water absorbent materials include hydrophilic woven or non-woven fabric materials such as paper, bamboo paper, cotton sheet or synthetic material such as a polymer or natural fabric materials. In certain embodiments, the water absorbent material is bamboo paper. The water absorbent material assists in transporting water along the length of the columnar photothermal substrate 12.

[0045] As discussed earlier, many 3D photothermal evaporators have limitations that have limited their height and scalability and, hence, their industrial applicability because they have a wicking limit (i.e. water supply limit) which can be about 20 cm depending on materials used. The apparatus 10 disclosed herein has no such limitation because the feed aqueous solution 24 is fed from the upper end 14 of the columnar photothermal substrate 12 and travels along the entire length of the columnar photothermal substrate 12 as a result of wicking by the hydrophilic and porous photothermal material 13 and the water absorbent material (if present) as well as gravity. This means that the entire surface area of the columnar porous photothermal substrate 12 is wetted and evaporation can occur from this entire surface area.

[0046] This means that the columnar photothermal substrate 12 can be any length. In trials conducted to date, the columnar photothermal substrates 12 were 1 and 2 metre in length. However, it is envisaged that the columnar photothermal substrate 12 can be anywhere from 20 centimetres to 20 metres in length, such as for example 1 metre, 2 metres, 3 metres, 4 metres, 5 metres, 6metres, 7 metres, 8 metres, 9 metres or 10 metres in length. Clearly, the longer the columnar photothermal substrate 12, the higher the evaporation rate and the more condensate collected over a given time period.

[0047] As discussed, the columnar photothermal substrate 12 is in the form of a column. Thus, it is elongate and is used in a substantially vertical orientation in which it has an upper end 14 and a lower end 16. As used herein, the term “substantially vertical” should be taken to mean that the substrate is close to vertical but is not necessarily strictly vertical. For example, substantially vertical could mean ±10° from vertical.

[0048] In some embodiments, the photothermal substrate 12 is applied onto an inner surface of the transparent housing 18. The photothermal substrate 12 may be applied onto all or part of the inner surface of the transparent housing 18. For example, an adhesive can be used to stick the photothermal substrate 12 onto the inner surface of the transparent housing 18. It is appreciated that the adhesive should be transparent and should not have a substantially adverse impact on the photothermal effect of the photothermal substrate 12.

[0049] The transparent condensing sleeve 18 substantially encases the columnar photothermal substrate 12. The transparent condensing sleeve 18 forms a sealed enclosure around a majority of the length of the columnar photothermal substrate 12. The transparent condensing sleeve 18 can be formed from any material that is highly transparent to incoming light, such as sunlight, so that as much light as possible reaches the hydrophilic and porous photothermal material 13 on the columnar porous photothermal substrate 12. Suitable transparent materials include, but are not limited to, glass, polyvinyl chloride (PVC), polypropylene (PP), vinyl, acrylic, polycarbonate, etc. Similar considerations may be given to a transparent housing 18 shown in Figure 4.

[0050] The condensing sleeve 18 allows for light penetration, vapor condensation and collection of clean water. It comprises an inner condensing surface 20 that is spaced from the columnar photothermal substrate 12. The space between the inner condensing surface 20 and the columnar photothermal substrate 12 allows steam formed on the surface of the columnar photothermal substrate 12 to form in the space and the inner condensing surface 20 acts to condense steam that is in contact with it to form condensate which is distilled water. The space between the inner condensing surface 20 and the columnar photothermal substrate 12 also allows for separation of the “contaminated” aqueous solution present on or in the porous photothermal substrate 12 and the “clean” distilled water formed as condensate on the inner condensing surface 20.

[0051] The condensing component 15, for example the one shown in Figure 4, is substantially encased by the transparent housing 18 and configured to provide the condensing surface 20. For an embodiment represented by Figure 4, the condensing surface 20 is spaced from the photothermal substrate 12. The condensing surface 20 acts to condense steam that is in contact with it to form condensate which is distilled water. The condensate moves along the condensing surface 20 to reach the condensate collector 26. For the purpose of illustration, the condensing component 15 may be made from glass, polyvinyl chloride (PVC), polypropylene (PP), vinyl, acrylic, polycarbonate, etc.

[0052] In use, heat generated by the hydrophilic and porous photothermal material 13 on the photothermal substrate 12 evaporates water from the columnar photothermal substrate 12 to generate steam which condenses on the inner condensing surface 20 of the condensing sleeve 18 to form condensate.

[0053] The water feeder 22 is in fluid contact with the upper end 14 of the columnar porous photothermal substrate 12 and is configured to introduce feed aqueous solution 24 to the upper end 14 of the columnar photothermal substrate 12 such that feed aqueous solution 24 is continuously fed to the upper end 14 and moves to the lower end 16 of the columnar photothermal substrate 12. In the illustrated embodiments, the water feeder 22 comprises a water reservoir 30 configured to hold a volume of feed aqueous solution 24. The water reservoir 30 is positioned at the upper end 14 of the columnar photothermal substrate 12 and is in fluid connection with the hydrophilic and porous photothermal material 13 so that feed aqueous solution 24 can be fed from the water reservoir 30 to the hydrophilic and porous photothermal material 13 and then transported by wicking, capillary action and/or gravity from the upper end 14 to the lower end 16 of the columnar photothermal substrate 12. In the illustrated embodiments, the water reservoir 30 and the columnar photothermal substrate 12 are in fluid connection as a result of the hydrophilic and porous photothermal material 13 extending past the end of the columnar photothermal substrate 12 and folded back into the water reservoir 30 so that it contacts the feed aqueous solution 24 in the reservoir 30.

[0054] Other water feeder configurations and arrangements are also contemplated, such as having a source of feed aqueous solution 24 separate from the apparatus 10 and in fluid connection with the upper end 14 of the columnar photothermal substrate 12. [0055] During operation, feed aqueous solution 24 (e.g. seawater) is injected to the water reservoir 30 at the upper end 14 of the columnar photothermal substrate 12 with a controlled injection rate. The feed aqueous solution 24 in the water reservoir 30 wicks in the hydrophilic and porous photothermal material 13 and is transferred to the side evaporation surfaces of the columnar photothermal substrate 12. In this scenario, the side surfaces are only wetted (e.g. by regulating surplus flow of feed aqueous solution 24) which is good for highly efficient evaporation. It will be appreciated that, if too much feed aqueous solution 24 is supplied, more thermal energy will be wasted to heat up the water. In the present apparatus 10, there is surplus feed aqueous solution 24 supply, however, by regulating the feed aqueous solution 24 injection, it is possible to minimise the surplus feed aqueous solution 24 supplies while balancing the concentration of the exiting solution (e.g. brine) at the lower end 16.

[0056] Due to this regulated supply mode, the height of the columnar photothermal substrate 12 (and hence the apparatus 10) can be very high. This overcomes one of the problems with known 3D photothermal evaporators. Advantageously, with this controlled injection of feed aqueous solution 24 one can ensure the evaporation surface near the lower end 16 of the porous photothermal substrate 12 is not dry to avoid salt crystallisation. Furthermore, it is also possible to control the concentration of the brine exiting the apparatus 10 to make sure it can be directly discharged.

[0057] Condensate formed on the inner condensing surface 20 runs down the surface and into condensate collector 26 that is positioned at a lower end 28 of the transparent condensing sleeve 18 where it is collected. The condensate collector 26 is hydraulically isolated from the columnar photothermal substrate 12 in order to prevent mixing of collected condensate and feed aqueous solution 24 that is present in or on the columnar photothermal substrate 12.

[0058] The condensate collector 26 is in the form of a vessel formed by the lower surfaces of the condensing sleeve 18. The condensate collector 26 is also formed by an annular wall 32 that is sealed to a base 34 of the condensing sleeve 18 and surrounds the columnar photothermal substrate 12. Thus, “clean” condensate water collects in the vessel formed by the annular wall 32, base 34 and the lower end 28 of the side wall of the condensing sleeve 18, whilst “contaminated” water from the porous photothermal substrate 12 is contained on the other side of the annular wall 32 and is not able to mix with the clean condensate water.

[0059] The condensate collector 26 has a valve 36 that can be used to remove condensate from the apparatus 10.

[0060] Brine or other feed aqueous solution 24 concentrate exits the columnar photothermal substrate 12 at the lower end 16 and below the base 34 of the condensing sleeve 18. [0061] The term “evaporation rate” used herein refers to the amount of water evaporated from a water source during a certain period. In an embodiment, it can be measured by the water weight loss or collected clean water during a certain period and expressed as, for example, kg per hour per m² or kg per day per m².

[0062] A plurality of water distillation apparatus 10 may be set up in an array. It will be appreciated that such an array will have a relatively small occupied area compared to a prior art 3D photothermal evaporator having a similar efficiency, evaporation rate or output.

[0063] Also disclosed herein is a water distillation system comprising the apparatus 10 disclosed herein and a pump for transferring feed aqueous solution 24 from a water source to the water feeder 22.

[0064] The water distillation system may further comprise a controller for regulating the flow rate of feed aqueous solution 24 into the water feeder 22 (e.g. the water reservoir 30). Advantageously, the controller can be used to control the flow rate of feed aqueous solution 24 so as to dilute as required any excess feed aqueous solution 24 that passes down the columnar photothermal substrate 12. For example, in the case of seawater desalination, brine solution containing salt and excess water exits the columnar photothermal substrate 12 at the lower end 16. The concentration of the brine solution can be controlled by increasing or decreasing the flow rate of feed aqueous solution 24 into the water reservoir. This can be used to dilute the brine solution so that it has a brine concentration that allows it to be disposed of in municipal sources, such as into the sea.

[0065] Also disclosed herein is a water distillation process comprising: providing a water distillation apparatus of the first aspect; continuously introducing feed aqueous solution 24 to be distilled to an upper end 14 of the columnar photothermal substrate 12; and collecting condensate that runs down the inner condensing surface 20.

[0066] The apparatus 10, systems and processes disclosed herein may also include components such as a water feeding unit(s), water discharging unit(s), water level monitor(s), and measuring unit(s) that can be used to monitor surface temperature, brine concentration, etc.

[0067] Without wishing to be bound by any theory, any existing photothermal evaporators or any photothermal evaporators developed in the future that may be adapted to the apparatus 10, systems and processes disclosed in the present disclosure, factors such as absorption ability, photothermal conversion efficiency, material costs, fabrication costs, scalability, ease of transportation and being environmentally friendly are likely to play a role in designing a photothermal evaporator. [0068] In some embodiments, the apparatus 10 comprises a support frame for holding the photothermal substrate 12 and the condensing sleeve 18 in a substantially vertical position.

EXAMPLES

[0069] Fabrication of columnar photothermal substrate 12

[0070] Bamboo paper was purchased from a commercial source. rGO was supplied by Huasheng Graphite Co., Ltd. Sodium alginate (SA) was purchased from Sigma- Aldrich. Calcium chloride dihydrate and ethanol were purchased from Chem-Supply. For preparation of the columnar photothermal substrate 12, rGO nanosheet (1 mg mL ¹) and SA (5 mg mL ') were dispersed into a mixture solution of water and ethanol (Vwate_r:VEthanoi = 7: 1) via ultrasonication to form a homogenous rGO-SA suspension. Then, the black suspension was spray-coated onto bamboo paper, pre-frozen and freeze dried to form rGO-SA- bamboo paper aerogel sheet. The as obtained aerogel sheet was then immersed into an aqueous CaCL solution (5%, w/w) overnight for SA crosslinking, and was rinsed with tap water several times. The obtained photothermal aerogel sheet was then wrapped on the surface of a PVC tube.

[0071] Evaporation testing system setup

[0072] Test equipment was set up as shown in Figure 5 and Figure 6.

[0073] Results of evaporation testing

[0074] Water distillation apparatus comprising only a bamboo paper columnar material as a control (referred to in Table 1 as “Bamboo paper”) and a water distillation apparatus as described herein comprising columnar photothermal substrate comprising a hydrophilic and porous photothermal substrate were tested under various conditions. The results are shown in Table 1.

[0075] Table 1 - Photothermal evaporation data

[0076] The water distillation apparatus as described herein comprising a columnar photothermal substrate comprising a hydrophilic and porous photothermal substrate were set up by changing the length (L) and diameter (D) of the columnar photothermal substrate (i.e. in the form of a tube) (Figure 5). Specifically, columnar photothermal substrates with the following lengths and diameters were adopted: (1) L= 1 m, D=2.7 cm, (2) L= 2 m, D=2.7 cm, (3) L=1 m, D=4.5 cm, and (4) L=2 m, D=4.5 cm.

[0077] The above water distillation apparatuses were tested under various conditions. The results are shown in Table 2.

[0078] Table 2- Photothermal evaporation data

[0079] It has been advantageously found that structure parameters (such as length and diameter for the columnar photothermal substrate) of a water distillation apparatus may be adjusted to achieve a better clean water production. As can be seen from Table 2, when the length (L) of the photothermal tube increases from 1 m to 2 m, the clean water production more than doubled in most cases; when the diameter (D) of the photothermal tube increases from 2.7 cm to 4.5 cm, the clean water production significantly increased; and when the length (L) increases to 2 m and the diameter (D) increases to 4.5 cm at the same time, the clean water production more than tripled. This suggests that it is possible to get >740 mL clean water per day from a single unit with a diameter for the columnar photothermal substrate being only 4.5 cm.

[0080] Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “include” and variations such as “comprising” and “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0081] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

[0082] It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

Claims

1. A water distillation apparatus comprising: a transparent housing substantially encasing a photothermal substrate and a condensing surface; a photothermal substrate comprising a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the photothermal substrate having an upper end and a lower end; a condensing surface being spaced from the photothermal substrate; a water feeder in fluid contact with the upper end of the photothermal substrate, and configured to introduce a feed aqueous solution to the photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the photothermal substrate; wherein heat generated by the photothermal material evaporates water from the photothermal substrate to generate steam which condenses on the condensing surface to form condensate; a condensate collector positioned at a lower end of the transparent housing and configured to collect condensate that runs down the condensing surface.

2. The water distillation apparatus of claim 1, wherein the condensate collector is hydraulically isolated from the photothermal substrate in order to prevent mixing of collected condensate and feed aqueous solution that is present in or on the photothermal substrate.

3. The water distillation apparatus of any one of claims 1 to 2, wherein the hydrophilic and porous photothermal material is selected from the group consisting of graphene oxide (GO), reduced graphene oxide (rGO), graphite, carbon nanotubes (CNT), polypyrrole (PPy), carbon black nanoparticles, biomass carbon, polydopamine, black nickel, CuO, Cu2-_xS (0<x<2), Fc^CL. CO3O4, TizO;. TiN, CuFeSz, plasmonic metal (e.g. Au) and combinations thereof.

4. The water distillation apparatus of any one of claims 1 to 3, wherein the water feeder comprises a water reservoir configured to hold a volume of feed aqueous solution and positioned at the upper end of the photothermal substrate and in fluid connection with at least the hydrophilic and porous photothermal material on the photothermal substrate so that feed aqueous solution can be fed from the water reservoir to the hydrophilic and porous photothermal material on the photothermal substrate and then transported by gravity and wicking from the upper end to the lower end of the photothermal substrate.

5. The water distillation apparatus of any one of claims 1 to 4, wherein the transparent housing is a transparent condensing sleeve, the condensing surface is an inner surface of the transparent condensing sleeve, and the photothermal substrate is a columnar photothermal substrate and is substantially encased by the transparent condensing sleeve.

6. The water distillation apparatus of claim 5, wherein the apparatus comprises a columnar photothermal substrate comprising a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the columnar photothermal substrate having an upper end and a lower end; a transparent condensing sleeve substantially encasing the columnar photothermal substrate, the condensing sleeve comprising an inner condensing surface that is spaced from the columnar photothermal substrate; a water feeder in fluid contact with the upper end of the columnar photothermal substrate, and configured to introduce a feed aqueous solution to the columnar photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the columnar photothermal substrate; wherein heat generated by the photothermal material evaporates water from the columnar photothermal substrate to generate steam which condenses on an inner condensing surface of the condensing sleeve to form condensate; a condensate collector positioned at a lower end of the transparent condensing sleeve and configured to collect condensate that runs down the inner condensing surface.

7. The water distillation apparatus of claim 6, wherein the columnar porous photothermal substrate comprises an inner columnar support and an outer layer comprising the hydrophilic and porous photothermal material.

8. The water distillation apparatus of either claim 6 or claim 7, wherein the columnar porous photothermal substrate comprises an inner columnar support, an outer layer comprising the hydrophilic and porous photothermal material, and an intermediate layer between the inner columnar support and the outer layer, the intermediate layer comprising a water absorbent material.

9. The water distillation apparatus of any one of claims 1 to 4, wherein the photothermal substrate is applied to an inner surface of the transparent housing, and the water distillation apparatus further comprises a condensing component substantially encased by the transparent housing and configured to provide the condensing surface.

10. The water distillation apparatus of any one of claims 1 to 4, wherein the apparatus comprises a transparent housing substantially encasing a photothermal substrate and a condensing surface, a photothermal substrate applied onto an inner surface of the transparent housing, which comprises a hydrophilic and porous photothermal material capable of generating heat upon exposure to sunlight, the photothermal substrate having an upper end and a lower end; a condensing component substantially encased by the transparent housing and comprising a condensing surface that is spaced from the photothermal substrate; a water feeder in fluid contact with the upper end of the photothermal substrate, and configured to introduce a feed aqueous solution to the photothermal substrate such that the feed aqueous solution is continuously fed to the upper end and moves to the lower end of the photothermal substrate; wherein heat generated by the photothermal material evaporates water from the photothermal substrate to generate steam which condenses on the condensing surface to form condensate; a condensate collector positioned at a lower end of the transparent housing and configured to collect condensate that runs down the condensing surface.

11. A water distillation system comprising the apparatus of any one of claims 1 to 10 and a pump for transferring feed aqueous solution to be distilled from a source of feed aqueous solution to the water feeder.

12. The water distillation system of claim 11, further comprising a controller for regulating the flow rate of feed aqueous solution into the water reservoir.

13. A water distillation process comprising: providing a water distillation apparatus of any one of claims 1 to 10; continuously introducing feed aqueous solution to an upper end of the photothermal substrate; and collecting condensate that runs down the condensing surface.

14. The water distillation process of claim 13, wherein the feed aqueous solution is an aqueous fluid that is selected from, but not limited to, tailings, lithium brine, industry wastewater, bore water, river water, seawater, brackish water, and combinations thereof.

15. The water distillation process of either claim 13 or claim 14, wherein the length and/or the diameter of the columnar photothermal substrate comprised by the water distillation apparatus are/is chosen to achieve desirable water distillation.