WO2019053638A1

WO2019053638A1 - Photothermal distillation apparatus

Info

Publication number: WO2019053638A1
Application number: PCT/IB2018/057033
Authority: WO
Inventors: Haolan XU; Dianxue Cao
Original assignee: Huasheng Graphite Stock Corporation Limited
Priority date: 2017-09-15
Filing date: 2018-09-14
Publication date: 2019-03-21

Abstract

A distillation apparatus comprises a distillation chamber comprising a water reservoir configured to hold a volume of water to be distilled. A porous photothermal substrate is configured to contact a surface of water in the water reservoir. The photothermal substrate is capable of generating heat and the generated heat evaporates water from the water reservoir to generate steam and/or condensate. A steam and condensate collector is configured to draw the generated steam or condensate to a condenser which is in thermal contact with the water in the reservoir and/or a source of water to be used in the reservoir, and a condensate collection chamber for collecting condensate from the condenser. A self-cleaning or self-washing porous photothermal material that is capable of converting light energy to vaporise water in contact therewith in a photothermal desalination apparatus is also disclosed. The photothermal material is in the form of a shape that is rotatable on the surface of the water to be vaporised.

Description

PHOTOTHERMAL DISTILLATION APPARATUS

PRIORITY DOCUMENTS

[0001] The present application claims priority from:

• Australian Provisional Patent Application No. 2017903764 titled "PHOTOTHERMAL

DISTILLATION APPARATUS" and filed on 15 September 2017; and

• Australian Provisional Patent Application No. 2017903767 titled "SELF-CLEANING OR

SELF-WASHING PHOTOTHERMAL MATERIALS FOR DESALINATION" and filed on 15 September 2017.

[0002] The content of each of these applications is hereby incorporated by reference in their entirety. TECHNICAL FIELD

[0003] The present disclosure relates to distillation apparatus for desalting seawater and/or purifying brackish and recycled water. The present disclosure also relates to self -cleaning or self -washing photothermal materials for desalination that have elongated operating life.

BACKGROUND

[0004] Potable water is scarce in semi-arid areas, desert areas and in countries with little land. The generation of potable water from sea water, brackish water, grey water and recycled water is a major interest.

[0005] Multi-stage flash distillation, reverse osmosis, membrane distillation, and electrodialysis have been used or suggested as processes for the desalination of sea water. However, many of these processes consume relatively high amounts of energy which is not only costly but can make access to the technologies in remote and rural areas where access to electricity may not be available, a point-of-use desalination system, driven by renewable energy, is not only more preferable, but also the only choice, which unfortunately is hardly achievable by the existing technologies.

[0006] There is thus a need to provide an apparatus and process for the sustainable generation of potable water from sea water or contaminated water and/or for a useful alternative to existing apparatus and processes for generating potable water from sea water or contaminated. [0007] Alternatively, there is a need for self -cleaning or self -washing photothermal materials for desalination that have elongated operating life and/or provide an alternative to existing photothermal materials for desalination.

SUMMARY

[0008] According to a first aspect of the present disclosure, there is provided a distillation chamber comprising a water reservoir configured to hold a volume of water to be distilled, a porous photothermal substrate configured to contact a surface of water in the water reservoir, the photothermal substrate capable of generating heat and wherein the generated heat evaporates water from the water reservoir to generate steam and/or condensate, a steam and condensate collector configured to draw the generated steam or condensate to a condenser, the condenser in thermal contact with the water in the reservoir and/or a source of water to be used in the reservoir, and a condensate collection chamber for collecting condensate from the condenser.

[0009] According to a second aspect, there is provided a desalination apparatus comprising a distillation chamber comprising a salt water reservoir configured to hold a volume of salt water to be distilled, a porous photothermal substrate configured to contact a surface of water in the salt water reservoir, the photothermal substrate capable of generating heat when it is exposed to light and wherein the generated heat evaporates water from the salt water reservoir to generate steam and/or condensate, a steam and condensate collector configured to draw the generated steam or condensate to a condenser, the condenser in thermal contact with the salt water in the reservoir and/or a source of salt water to be used in the reservoir, and a condensate collection chamber for collecting condensate from the condenser.

[0010] According to a third aspect, there is provided a process of distilling water, said process comprising:

contacting a porous photothermal substrate with a surface of a volume of water to be distilled in a water reservoir, the photothermal substrate capable of generating heat when it is exposed to light;

exposing the photothermal substrate to light under conditions to generate heat such that the generated heat evaporates water from the water reservoir to generate steam and/or condensate;

collecting steam and/or condensate that is generated by the photothermal substrate;

passing the collected steam and/or condensate through a condenser that is in thermal contact with the water in the water reservoir and/or a source of water to be used in the reservoir to transfer heat from the collected steam and/or condensate to the water in the water reservoir and/or to the source of water to be used in the reservoir; and

collecting condensate from the condenser.

[0011] According to a fourth aspect, there is provided a desalination process, said process comprising: contacting a porous photothermal substrate with a surface of a volume of sea water to be distilled in a water reservoir, the photothermal substrate capable of generating heat when it is exposed to light; exposing the photothermal substrate to light under conditions to generate heat such that the generated heat evaporates water from the water reservoir to generate steam and/or condensate;

passing the collected steam and/or condensate through a condenser that is configured to transfer heat from the steam to salt water in the water reservoir or to a source of salt water to be used in the reservoir; and

collecting condensate from the condenser.

[0012] According to a fifth aspect, there is provided a desalination system comprising the desalination apparatus of the second aspect and further comprising:

a sea water storage tank comprising a condenser configured to transfer heat from steam from the desalination apparatus to fresh sea water in the sea water storage tank;

a heat exchanger tank comprising a heat exchanger pipe that is located internally in heat exchanger tank and configured for sea water in the sea water storage tank to be fed to the heat exchanger pipe and for warm concentrated sea water from the desalination apparatus to be fed into the heat exchanger tank where it comes in to thermal contact with heat exchanger pipe so that heat from the warm concentrated sea water can be transferred to fresh sea water from the sea water storage tank as it passes through heat exchanger pipe; and

a photothermal heater through which sea water that has passed through the heat exchanger pipe passes through heater.

[0013] In certain embodiments, the desalination system further comprises a reflector positioned beneath the distillation chamber and configured to reflect sunlight or scattered light from the ambient environment to the bottom and/or sides of the desalination apparatus.

[0014] In addition to the first to fifth aspects, the present disclosure also provides a self-cleaning or self- washing photothermal material for desalination, a desalination system comprising the self-cleaning or self -washing photothermal material, and a process of desalination.

[0015] Thus, in a sixth aspect, there is provided a self-cleaning or self-washing porous photothermal material that is capable of converting light energy to vaporise water in contact therewith in a photothermal desalination apparatus, wherein the photothermal material is in the form of a shape that is rotatable on the surface of the water to be vaporised.

[0016] In a seventh aspect, there is provided a desalination process which includes (i) placing a plurality of the self -cleaning or self -washing photothermal materials of the first aspect on the surface of a body of water to be evaporated, (ii) exposing the plurality of the self-cleaning or self-washing photothermal materials to a light energy, wherein the light energy is converted to thermal energy by the photothermal materials, (iii) vaporising water in contact with the surface of photothermal materials, wherein the water passes through the pores and channels of the self-cleaning or self-washing photothermal materials, and (iv) condensing the water vapour in a condensation unit.

[0017] In an eighth aspect, there is provided a desalination system comprising a photothermal desalination apparatus and a plurality of the self -cleaning or self-washing photothermal materials of the first aspect.

BRIEF DESCRIPTION OF DRAWINGS

[0018] Embodiments according the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0019] Figure 1 is a part cross section side view of a distillation apparatus according to embodiments of the disclosure;

[0020] Figure 2 is a plan view of a distillation apparatus according to embodiments of the disclosure with the lid removed;

[0021] Figure 3 is a schematic representation of a distillation system according to embodiments of the disclosure that uses heliostats to concentrate sunlight and a mirror to direct concentrated sunlight onto a distillation apparatus;

[0022] Figure 4 is a part cross section side view of a distillation apparatus according to embodiments of the disclosure;

[0023] Figure 5 is a schematic representation of a distillation system according to embodiments of the disclosure that uses a distillation apparatus of embodiments of the disclosure;

[0024] Figure 6 shows some illustrative forms of a self-cleaning or self-washing photothermal materials according to embodiments of the disclosure; and

[0025] Figure 7 shows a side view of an embodiment of a plurality of the self -cleaning or self-washing photothermal materials according to embodiments of the disclosure staying at the water-air interface in a photothermal desalination apparatus. [0026] In the following description, like reference characters designate like or corresponding parts throughout the figures.

DESCRIPTION OF EMBODIMENTS

[0027] Before proceeding it is important to note that various terms that will be used throughout the specification have meanings that will be well understood by a skilled addressee. However, for ease of reference some of these terms will now be defined.

[0028] As used herein, the term "rotatable" means the shaped material can rotate on water surface driven by water evaporation, wind, water movement caused by stirring forces, factors that cause horizontal density gradients and/or surface/interface tension discrepancy etc.

[0029] As used herein, the term "light energy" includes light from artificial and natural light sources including, but not limited to, light energy from the sun.

[0030] As used herein, the term "photothermal" means the material is able to absorb light and convert it into heat through non-radiative decay. United States Patent Application No. 2015/0353385A1 discloses that some metallic nanostructures and certain carbon-based nanomaterials, such as carbon nanotubes and graphene nanosheets, possess a non-radiative photothermal effect, which is believed to be a highly energy-efficient way of generating dramatic localized heating as the non-radiative decay minimizes the energy loss via light radiation.

[0031] Disclosed herein is a distillation apparatus 10 comprising a distillation chamber 12 comprising a water reservoir 14. The water reservoir 14 is configured to hold a volume of water 16 to be distilled. A porous photothermal substrate 18 is configured to contact a surface of the water 16 in the water reservoir 14. The photothermal substrate 18 is capable of generating heat. The generated heat evaporates water from the water reservoir 14 to generate steam and/or condensate. A steam and condensate collector 20 is configured to draw the generated steam or condensate to a condenser 22. The condenser 22 is configured to transfer heat from the steam to water 16 in the reservoir 14 or to a source of water 17 to be used in the water reservoir 14. Condensate from the condenser 22 is collected in a condensate collection chamber 24.

[0032] The distillation apparatus 10 can be used for desalination. Therefore, also disclosed herein is a desalination apparatus 10a comprising a distillation chamber 12 comprising a salt water reservoir 14. The salt water reservoir 14 is configured to hold a volume of salt water 16 to be distilled. A porous photothermal substrate 18 is configured to contact a surface of the salt water 16 in the salt water reservoir 14. The photothermal substrate 18 is capable of generating heat. The generated heat evaporates water from the salt water reservoir 14 to generate steam and/or condensate. A steam and condensate collector 20 is configured to draw the generated steam or condensate to a condenser 22. The condenser is configured to transfer heat from the steam to salt water 16 in the reservoir 14 or to a source of salt water 17 to be used in the water reservoir 14. Condensate from the condenser 22 is collected in a condensate collection chamber 24.

[0033] The distillation apparatus 10 can be used to distil any aqueous solution including, but not limited to, seawater, brackish water, recycled water, waste water, and industrial waste water. For example, in one specific application the distillation apparatus 10 can be used to desalt seawater.

[0034] The distillation apparatus 10 comprises a distillation chamber 12. The distillation chamber 12 can take any form but generally consists of a water reservoir 14 that is configured to hold a volume of water 16 to be distilled. In the embodiments illustrated in Figures 1, 2 and 4 the distillation chamber 12 comprises a vessel that is rectangular or square in plan view and comprises a base 26 and four walls 28a, 28b, 28c, 28d that extend up from the base to form a vessel. The base 26 and walls 28 can be formed from any suitable material including, but not limited to, plastic, glass, concrete and metal. In some embodiments, the base 26 and walls 28a, 28b, 28c, 28d can be encapsulated by thermal insulator materials, which can assist in effectively retaining heat in the distillation chamber 12. In some embodiments, the walls 28, and optionally the base 26, of the chamber are made of a transparent material with low thermal conductivity, such as acrylic, or double layer materials with a vacuum interlayer. An inner surface of the transparent materials can be coated with a photothermal layer to absorb light and to heat the chamber 12.

[0035] The distillation chamber 12 can be any size and, for the purposes of generating reasonable volumes of distilled water, may have dimensions (i.e. height, width and depth) in metres, such as about 1 metre to about 5 metres.

[0036] The distillation apparatus 10 also comprises a lid 30 that, in use, rests on top of the walls 28a, 28b, 28c, 28d. The lid 30 is transparent so that light emanating from a light source from above the distillation apparatus 10 is able to pass through the lid and reach the porous photothermal substrate 18. The lid 30 can be made from any suitable transparent material such as glass, quartz, plastic, etc. The light source may be an artificial light source or a natural light source. In the embodiment shown in Figure 3, the light source is sunlight that is concentrated by a series of heliostats 32. Light from the heliostats is directed to a reflection mirror 34 which directs the concentrated sunlight at the distillation apparatus 10. This system can significantly enhance the productivity by providing concentrated solar light. The heliostats can make the system work in weak light environment such as a cloudy day. [0037] The lid 30 forms a substantially fluid and air tight seal with walls 28a, 28b, 28c, 28d. In this way, steam generated is not able to leave the distillation apparatus and is forced into the steam and condensate collector 20.

[0038] In the embodiment shown in Figure 1, the lid 30 is arched in cross section with the highest part of the lid being in the centre and the lid then sloping downwardly from the centre to sides 36a and 36b. The sloping sections of the lid 30 allow for any condensate that forms on the lid 30 to run down to the sides 36a and 36b which, in turn are positioned above the steam and condensate collector 20. It will be appreciated that, under normal operating conditions, a substantial amount of heat is generated in the distillation apparatus 10 and, as such, it is unlikely that steam will condense in the distillation chamber 12 (i.e. on internal surfaces of walls 28a, 28b, 28c, 28d or lid 30). Nevertheless, if the lid 30 is cooled in some manner then it may be possible for condensate to form.

[0039] In the embodiment shown in Figure 4, the lid 30 is arched or curved in cross section and may form a lens to collect more light and concentrate the collected light.

[0040] The porous photothermal substrate 18 comprises a photothermal material supported on a substrate. The photothermal material can include for example gold nanomaterials such as nanospheres, nanowires, nanostars or nanorods; carbon based materials including graphene nanosheets, carbon nanotubes, carbon particles, carbon black, and carbon nanofibers; silver nanomaterials such as nanospheres, nanowires or nanorods; copper materials such as CuO, CuS, Cu_xS_y, CuSe, Cu_xSe_y, nanospheres, nanoplates, nanowires, nanoleaves; CuO hollow spheres, for example those described in J Xu, X Li, X Wu, W. Wang, R Fan, X Liu, H Xu, Journal of Physical Chemistry C 120 (23), 12666- 12672, and G. Chen et al, Scientific Reports 2017, 7, 41895; iron oxide materials such as nanospheres, nanowires, or nanorods; polydopamine films and particles, for example those described in H Xu et al., Chemistry of Materials 2011, 23 (23) 5105-5110, W Xuan et al., Advanced Sustainable System, 2017, 1, 1700046; polypyrrole; nickel, nickel oxide; cobalt, cobalt oxide; manganese, manganese oxide; and mixtures of any of the aforementioned photothermally active substances.

[0041] In an embodiment, the photothermal material is selected from: polydopamine (for example as described in H Xu et al., Chemistry of Materials 2011, 23 (23) 5105-5110; and W Xuan et al., Advanced Sustainable System, 2017, 1, 1700046); CuO (for example as described in J Xu, X Li, X Wu, W. Wang, R Fan, X Liu, H Xu, Journal of Physical Chemistry C 120 (23), 12666-12672); and CuS (for example as described in H Xu, et al. Nanotechnology 2006, 17, 3649-3654).

[0042] In some embodiments, the diameter (or longest dimension for non-spherical particles) of the photothermal material can be nanometres to centimetres, such as 20 nm -100 cm. [0043] The photothermal material can be loaded onto a substrate by self-polymerization, spray coating, dip coating, layer-by-layer (LBL) assembly deposition, chemical vapour deposition, physical vapour deposition, or combinations thereof. The substrate can be pre -formed into a desirable shape. Alternatively, the photothermal materials may work without a substrate.

[0044] In some embodiments, the substrate is selected from porous polymer materials, which can be made from, for example, cellulose based foams, hydrogels, polyethylene, polypropylene, polyvinyl alcohol, polydimethylsiloxane, polyvinylidene fluoride, polyurethane, polyester, polylactide, polysulfone, polyvinyl chloride, polycarbonate, polyacrylonitrile, polybutylene, polybutylene terephthalate, polyimide, polymethyl methacrylate, polyethylene terephthalate. In certain specific embodiments, the substrate is made from polydimethylsiloxane, polyvinyl alcohol, and cellulose based foams.

[0045] In certain embodiments, the porous substrate contains open-cells throughout, which is beneficial for the water to reach the photothermal material loaded on the surface of substrate.

[0046] The photothermal material functions to convert light energy into thermal energy, which can be used to heat the water that is in contact with the porous photothermal substrate 18 for evaporation. A certain amount of pores and channels are required to be present among the loaded photothermal material, and thus allow the water to pass through (driven by capillary force) and to be heated by the photothermal materials for evaporation when exposed to light energy.

[0047] In embodiments, the photothermal material comprises photothermal nanomaterials loaded onto a porous substrate. The substrate is selected from porous polymer materials, which can be made from, for example, cellulose based foams, hydrogel, polyethylene, polyvinyl alcohol, polypropylene, polydimethylsiloxane, polyvinylidene fluoride, polyurethane, polyester, polylactide, polysulfone, polyvinyl chloride, polycarbonate, polyacrylonitrile, polybutylene, polybutylene terephthalate, polyimide, polymethyl methacrylate, polyethylene terephthalate.

[0048] The porous photothermal substrate 18 can take any form and, as shown in the illustrated embodiments, may be in the form of a block or raft. Alternatively, the porous photothermal substrate 18 may be in the form of spheres, semi-spheres, cones, cylinders, etc. some examples of which are shown in Figure 6 and described in detail later.

[0049] In some embodiments, the density of the photothermal material allows the porous photothermal substrate 18 to spontaneously float on the water surface. In other embodiments, the density of the photothermal material does not allow the porous photothermal substrate 18 to freely float on the water surface but it is kept in place at the water surface with the aid of a supporting device, for instance, a supporting mesh whose height is adjustable. The mesh described here can be made of polymer materials or metals.

[0050] A layer of hydrophilic polymer/surfactant coating can be applied to the photothermal material. Such coating may be helpful for dragging water through the pores and channels, and form a thin water film on the surface of photothermal materials, and thereby enhancing water evaporation rate when exposed to light energy. For the present purpose, examples of the hydrophilic polymer include polydopamine, polyvinyl alcohol, polyethylene glycol, poly(acrylic acid), poly(allylamine hydrochloride), poly(diallyldimethylammonium chloride), poly(stryenesulfonate), etc.

[0051] Alternatively, a layer of non-fouling coating can be applied to the photothermal material. Such coating may be helpful in preventing adhesion of contaminants including salt and organisms to the photothermal material. For the present purpose, examples of suitable coatings include zwitterionic coatings, and nanoparticle coatings such as Ag, etc.

[0052] The porous photothermal substrate 18 is configured to contact a surface of the volume of water 16 in the water reservoir 14. Specifically, at least part of a bottom surface 38 is in fluid contact with the volume of water 16. Furthermore, the porous nature of the porous photothermal substrate 18 means that some water enters the pores by capillary action and is transferred to the upper surface of the photothermal substrate to form a thin water film. As discussed, the photothermal substrate 18 is capable of generating heat when the substrate 18 is exposed to light. The generated heat then evaporates the thin water film which will be continuously refreshed by the water from the water reservoir 14 to generate steam and/or condensate. Steam may rise from the surface and through the pores of the porous photothermal substrate 18 and/or through any gaps between the porous photothermal substrate 18 and the walls 28a, 28b, 28c, 28d. The steam rises into the space in the distillation chamber 12 that is above the porous photothermal substrate 18 and the volume of water 16. In some cases, the steam may condense at this stage. However, under normal operating conditions, the temperature inside the distillation chamber 12 will be too high for condensation to occur.

[0053] The generated steam or condensate (if any) is then drawn into steam and condensate collector 20. The steam and condensate collector 20 can take any form and may, for example, be any outlet in the distillation chamber 12 that is in fluid connection with the steam in the space in the distillation chamber 12 that is above the porous photothermal substrate 18 and the volume of water 16. In the embodiments illustrated in Figures 1 and 2, the steam and condensate collector 20 is in the form of rectangular ducts 40. The ducts 40 are formed by duct walls 42 that are fixed to an inside surface of the walls 28a and 28c, with the latter forming part of the wall of the ducts 40. The ducts 40 can take any form and may, for example, be tubular ducts. [0054] The ducts 40 comprise an opening 44 that is positioned above the level of the volume of water 16. The duct walls 42 extend substantially vertically down from the opening 44 and a distal end of each duct 40 is in fluid connection with an inlet 46 of the condenser 22. The ducts 40 are therefore configured to draw steam and/or condensate to the condenser 22.

[0055] In the illustrated embodiments, steam and/or condensate is drawn into the steam and condensate collector 20 and the condenser 22 by a pump 48, the details of which will be discussed later. The pump 48 draws a vacuum downstream of the condenser 22. In other embodiments, the steam and/or condensate may be drawn into the steam and condensate collector 20 and the condenser 22 using a temperature differential. Thus, a cooler (not shown) may be positioned downstream of the condenser 22. The cooler may draw heat from the steam to cause condensation. The heat loss may then result in steam being drawn from the distillation chamber 12 via the steam and condensate collector 20 and the condenser 22. The cooler may be cooled by a refrigerant, fan, or any other cooling means known in the art. If desired, heated air/fluid from the cooler may be transferred to the volume of water 16 in the distillation chamber 12 in order to heat the water and assist in the evaporation process. The heated air can be transferred using insulated pipes, as is known in the art.

[0056] The distillation apparatus includes a preheating system configured to preheat the water that is to be distilled. In the embodiments illustrated in Figures 1 and 4, the preheating system is in the form of condenser 22 that is configured to transfer heat from the steam to the volume of water 16 in the water reservoir 14. The condenser 22 is in the form of a pipe 50 that is positioned in the volume of water 16. Thus, the pipe 50 is in thermal contact with the volume of water 16. The pipe 50 forms a serpentine or spiral fluid path in order to maximise the surface area between the pipe 50 and the volume of water 16. Steam and condensate (if present) enters the pipe 50 via inlets 46. Heat is then transferred from the steam through the pipe 50 to the volume of water 16. In this way, the condenser 22 acts as a heat exchanger. The pipe 50 can be made from any suitable material. Materials having a high thermal conductivity, such as copper or aluminium, may be particularly suitable.

[0057] In other embodiments illustrated in Figure 5, the preheating system is in the form of condenser 22 that is configured to transfer heat from the steam to a source of water to be used in the reservoir. The source of water may be external to the water reservoir 14. More specifically, the condenser 22 is in the form of a pipe 70 that is positioned in a volume of water 72 that is held in a water storage tank 74. The volume of water 72 in tank 74 is water that will be introduced into the distillation apparatus 12. The pipe 70 forms a serpentine or spiral fluid path in order to maximise the surface area between the pipe 70 and the volume of water 72. Steam and condensate (if present) enters the pipe 70 via inlet 76. Heat is then transferred from the steam through the pipe 70 to the volume of water 72. In this way, the condenser 22 acts as a heat exchanger. The pipe 70 can be made from any suitable material. Materials having a high thermal conductivity, such as copper or aluminium, may be particularly suitable. [0058] The preheating system may include heating elements in addition to or instead of the condenser 22, such as

[0059] In the embodiment illustrated in Figure 4, a lower portion of the distillation chamber 12 is filled with a low specific heat material 75 (lower than that of water), such as sand, and/or a photothermal material. This can reduce the volume of water 16 in the chamber 12, enhance light absorption from the environment, and increase the temperature in the distillation chamber 12.

[0060] As will be seen from the embodiments illustrated in Figures 1 , 4 and 5 the condenser 22 may located internally in the distillation apparatus 12, such as in the embodiments illustrated in Figures 1 and 4, and/or the condenser may be located externally of the distillation apparatus 12, such as in Figure 5. It is contemplated that the condenser 22 may be formed from two separate condenser units, with one located internally in the distillation apparatus 12 and one located externally of the distillation apparatus 12. For example, the distillation apparatus 12 shown in the system illustrated in Figure 5 may not have a condenser 22 located internally in the distillation apparatus 12.

[0061] Condensate formed in the condenser 22 (and any steam that has not condensed) then exits the condenser 22 via outlet 52 after which it is collected in condensate collection chamber 24. The condensate collection chamber 24 is a vessel that can take any suitable form. An inlet 54 of the condensate collection chamber 24 is in fluid connection with outlet 52. In the embodiments illustrated in Figures 1 and 4 the outlet 52 is positioned at a middle section of the condensate collection chamber 24. In the embodiments illustrated in Figure 5 the outlet 52 is positioned at an upper section of the condensate collection chamber 24. A pump connection 56 is positioned at a top surface of the condensate collection chamber 24 and is connected to a pump 48. The pump 48 can be any suitable vacuum pump and draws a vacuum through the condensate collection chamber 24, the condenser 22 and the steam and condensate collector 20. The vacuum can be adjusted to ensure the residence time for steam in the condenser 22 is sufficient for condensation to occur. In the embodiment illustrated in Figures 1 and 3, the condensate collection chamber 24 includes a set of condenser baffles 58 that are positioned in the chamber 24 between inlet 54 and pump connection 56. The condenser baffles 58 provide an additional surface area on which steam can condense and prevent water evaporation in the condensate collection chamber 24.

[0062] The condensate collected in the condensate collection chamber 24 is distilled water that can be used for any suitable purpose. For example, when the volume of water 16 is sea water, the condensate will be substantially pure and/or desalinated water that may be suitable for drinking.

[0063] Figure 5 shows a distillation system 80 that includes a distillation chamber 12. The photothermal distillation system 80 is designed to maximise the utilization of solar energy for distillation or desalination. Specifically, the system 80 is designed to fully use the light including sunlight, concentrated sunlight and scattered/reflective sunlight from the environment to generate heat to heat up the porous photothermal substrate 18 at the water-air interface, a thin water film on the surface of the porous photothermal substrate 18, as well as to heat up the distillation chamber 12, so as to maximise water evaporation for distillation or desalination.

[0064] The distillation system 80 shown in Figure 5 comprises a distillation chamber 12, which may be as described earlier. The distillation system 80 will now be described with reference to use as a desalination system 80 for the desalination of sea water. The system 80 comprises a sea water storage tank 74. Fresh sea water enters the sea water storage tank 74 via fresh sea water inlet 82 which, in turn, is connected to a source of fresh sea water via sea water inlet pipe 84. Fresh sea water passes through a sea water filter 86 prior to entering the sea water storage tank 74. The sea water filter 86 can be any suitable filter and may be a filter to remove coarse or gross contaminants from the sea water. Flow of sea water into the sea water storage tank 74 is controlled by inlet pump 88.

[0065] Sea water storage tank 74 can be any suitable capacity and can be made from any suitable material, such as metal (e.g. stainless steel), fiberglass, plastic, etc. The volume of fresh sea water stored in sea water storage tank 74 can be from about 10 litres to about 100,000 litres.

[0066] Sea water storage tank 74 contains a condenser 22. As described previously, the condenser 22 is configured to transfer heat from steam from the distillation chamber 12 to the fresh sea water in the sea water storage tank 74. The condenser 22 is in the form of a pipe 70 that is positioned in a volume of sea water 72 that is held in sea water storage tank 74. The pipe 70 forms a serpentine or spiral fluid path in order to maximise the surface area between the pipe 70 and the volume of sea water 72. Steam and condensate (if present) enters the pipe 70 via inlet 76. Heat is then transferred from the steam through the pipe 70 to the volume of sea water 72. In this way, the condenser 22 acts as a heat exchanger and heat is transferred to the volume of sea water 72 that will eventually be fed into the distillation chamber 12. The pipe 70 can be made from any suitable material. Materials having a high thermal conductivity, such as copper or aluminium, may be particularly suitable. Condensate formed in the pipe 70 exits the pipe via outlet 90 and is transferred to the condensate collection chamber 24 via condensate transfer pipe 92.

[0067] The volume of sea water 72 in sea water storage tank 74 is water that will be introduced into the distillation apparatus 12. In some embodiments, sea water in the sea water storage tank 74 can be fed directly to the distillation chamber 12. However, in the embodiment illustrated in Figure 5 sea water in sea water storage tank 74 is fed to the distillation chamber 12 via a heat exchanger tank 94. The heat exchanger tank 94 comprises a heat exchanger pipe 96 that is located internally in heat exchanger tank 94. Sea water in sea water storage tank 74 is fed to the heat exchanger pipe 96 using a pump 98 via outlet 100 on sea water storage tank 74 and inlet 102 to heat exchanger pipe 96. Warm concentrated sea water from the distillation chamber 12 is fed into the heat exchanger tank 94 where it is in thermal contact with heat exchanger pipe 96 so that heat from the warm concentrated sea water can be transferred to fresh sea water from the sea water storage tank 74 as it passes through heat exchanger pipe 96. This allows for heat from the concentrated sea water from the distillation chamber 12 to be transferred to the fresh sea water. The concentrated sea water is transferred from the distillation chamber 12 to the heat exchanger tank 94 via concentrated sea water transfer pipe 104 and it may be transferred by a gravity feed or by using a pump (not shown). After heat exchange has taken place, concentrated sea water in heat exchanger tank 94 can be drained from the tank via drain pipe and valve 105.

[0068] Sea water that has passed through heat exchanger pipe 96 exits at outlet 106 and passes through heater 108. Heater 108 comprises one or more pipes 110 that are coated with a photothermal material (as described earlier) such as a solar vacuum tube. In use, the photothermal coated pipes 110 are exposed to natural or concentrated sunlight which causes the photothermal coating to heat up and this heat is then transferred to pipes 110 and, consequently, to sea water flowing through pipes 110. In this way, sea water that is to be transferred to the distillation chamber is heated by the condenser 22, the heat exchange tank 96 and the heater 108. This then accelerates water evaporation in the distillation chamber 12. In the illustrated embodiment, the heater 108 comprises four coated pipes 110, although it will be appreciated that any number of coated pipes 110 can be used. It is also contemplated that heater 108 could take the form of other heaters, such as electric heaters, gas fired heaters, etc.

[0069] Heated sea water exits the heater 108 via outlet 112 and is transferred to distillation chamber 12 via heated sea water pipe 114. The distillation chamber 12 functions as previously described.

[0070] Advantageously, in the embodiment illustrated in Figure 5 the base 26, walls 28 and lid 30 of distillation chamber 12 are formed from transparent material and have a photothermal coating on the inside surfaces. A reflector 116 is positioned beneath the distillation chamber 12. The reflector 116 comprises an arched reflective surface to reflect sunlight or scattered light from the ambient environment (shown schematically with arrows in Figure 5) to the bottom and/or sides of the distillation chamber 12. The photothermal coating on the inside surfaces of the base 26 and walls 28 of the distillation chamber 12 absorbs light reflected from the reflector 116 and heats up the distillation chamber 12, thereby increasing the rate of water evaporation in the chamber 12. The reflector 116 has an outlet drain 118 at a lowermost surface to drain rain water, etc from the reflector surface.

[0071] Steam that is generated in distillation chamber 12 is transferred via steam pipe 120 to condenser pipe 70, as described previously.

[0072] Fresh water formed from condensed steam from condenser 22 is transferred to condensate collection chamber 24. The condensate collection chamber 24 is a vessel that can take any suitable form. An inlet 54 of the condensate collection chamber 24 is in fluid connection with outlet 90. A fresh water outlet 52 is positioned at lower section of the condensate collection chamber 24 and is controlled by fresh water valve 122. The fresh water outlet 52 is connected to a fresh water filter 124 to filter particulate material from the fresh water. A pump connection 56 is positioned at a top surface of the condensate collection chamber 24 and is connected to a pump 48. The pump 48 can be any suitable vacuum pump and draws a vacuum through the condensate collection chamber 24, the condenser 22 and the steam and condensate collector 20. The vacuum can be adjusted to ensure the residence time for steam in the condenser 22 is sufficient for condensation to occur.

[0073] The distillation system 80 maximises the utilisation of heat by efficient heat exchange and recycle, such as heat exchange between the steam and the volume of water 16 in the distillation chamber 12, between the steam and the volume of sea water 72 that is held in a sea water storage tank 74, and between the concentrated seawater (ready for drain) and fresh seawater (ready to flow into the distillation chamber 12) via heat exchange pipe 96.

[0074] The distillation system 80 is driven by incident natural sunlight and/or concentrated sunlight. Electrical power for vacuum pump 48 and water pumps 88 and 98 can be provided by one or more solar panels, so that the overall system can be run without any power supply.

[0075] Taking seawater as an example, a problem that may be encountered with photothermal substrates, such as photothermal substrate 18 and others, is that after evaporation, salt will deposit on the material surfaces and in the pores/channels. As a result, the pores/channels can become blocked and the light absorption on the surfaces of the photothermal materials can be reduced. In addition, organisms in the seawater can also induce fouling in the pores/channels and on the surfaces of the photothermal materials, which can then lead to degradation of the desalination performance.

[0076] Also disclosed herein and shown in Figures 6 and 7 is a self-cleaning or self-washing porous photothermal material that is capable of converting light energy to vaporise water in contact therewith in a photothermal desalination apparatus. The photothermal material is in the form of a shape that is rotatable on the surface of the water to be vaporised.

[0077] The self -cleaning or self-washing porous photothermal material may form the porous

photothermal substrate 18 of the earlier described embodiments, or it may be used in other photothermal apparatus and processes.

[0078] For the purpose of the present disclosure, the photothermal material comprises a photothermally active substance. The photothermally active substance may be supported on a substrate. [0079] The photothermally active substance can include for example gold nanomaterials such as nanospheres, nanowires, nanostars or nanorods; carbon based materials including graphene nanosheets, carbon nanotubes, carbon particles, carbon black, and carbon nanofibers; silver nanomaterials such as nanospheres, nanowires or nanorods; copper nanomaterials such as CuO, CuS, Cu_xS_y, CuSe or Cu_xSe_y nanospheres, nanowires or nanorods; CuO hollow spheres, for example those described in J Xu, X Li, X Wu, W. Wang, R Fan, X Liu, H Xu, Journal of Physical Chemistry C 120 (23), 12666-12672, and G. Chen et al, Scientific Reports 2017, 7, 41895; iron oxide nanomaterials such as nanospheres, nanowires, or nanorods; polydopamine films and particles, for example those described in H Xu et al., Chemistry of Materials 2011, 23 (23) 5105-5110, W Xuan et al., Advanced Sustainable System, 2017, 1, 1700046; polypyrrole; nickel, nickel oxide; cobalt, cobalt oxide; manganese, manganese oxide; and mixtures of any of the aforementioned photothermally active substances. In some embodiments, the photothermally active substance is selected from CuO nanomaterials, CuS nanomaterials, nickel nanomaterials, polydopamine particles, polydopamine coatings, and carbon based materials.

[0080] The diameter (or longest dimension for non-spherical particles) of the photothermally active substance can be nanometres to centimetres, such as from about 20 nm to about 100 cm.

[0081] The photothermally active substance can be loaded onto a substrate by self -polymerization, spray coating, dip coating, layer-by-layer (LBL) assembly deposition, chemical vapour deposition, physical vapour deposition, or combinations thereof. The substrate can be pre-formed into a desirable shape, which will be discussed in detail below.

[0082] In some embodiments, the substrate is selected from porous polymer materials, which can be made from, for example, cellulose based foams, polyethylene, polypropylene, polyvinyl alcohol, polydimethylsiloxane, polyvinylidene fluoride, polyurethane, polyester, polylactide, polysulfone, polyvinyl chloride, polycarbonate, polyacrylonitrile, polybutylene, polybutylene terephthalate, polyimide, polymethyl methacrylate, polyethylene terephthalate. In certain specific embodiments, the substrate is made from polydimethylsiloxane, polyvinyl alcohol or cellulose based foams.

[0083] The ratio by weight of the photothermally active substance to the substrate may vary according to practical need. In principle, the photothermally active substance needs to densely cover the surface of the substrate, so that the light can be effectively absorbed. In certain embodiments, the porous substrate contains open-cells throughout, which is beneficial for the water to reach the photothermally active substance loaded on the surface of substrate.

[0084] As described herein, the photothermal material functions to convert light energy into thermal energy, which can be used to heat the water that is in contact with the photothermal material for evaporation. A certain amount of pores and channels are required to be present in the photothermal material. The pores and channels allow water to pass through and reach the upper surface of the photothermal materials, and then to be heated by the photothermal materials for evaporation when exposed to light energy.

[0085] For the purpose of the present disclosure, the photothermal material is in the form of a shape that can rotate on water surface driven by water evaporation, wind, water movement caused by stirring forces, factors that cause horizontal density gradients and/or surface/interface tension discrepancy etc. The surface of the photothermal materials can be rough with nano- and/or microstructure to increase the surface area (i.e. water evaporation area). Examples of suitable shapes include spheres, semi-spheres, ellipsoids, cones, cylinders, tubes, etc. In certain embodiments, the photothermal material is in the form of a sphere. In certain other embodiments, the photothermal material is in the form of a cylinder. There is no special limitation on the size of a shape as long as it is suitable for rotating on a water surface to efficiently clean off the contaminants including salt and organisms deposited on the surface of the photothermal material.

[0086] In some embodiments, the density of the shaped photothermal material allows it to spontaneously float on the water surface. In other embodiments, the density of the shaped photothermal materials does not allow it to freely float on the water surface but the photothermal materials can be kept in place at the water surface with the aid of a supporting device, for instance, a supporting mesh whose height is adjustable.

[0087] Additionally, a layer of hydrophilic polymer or surfactant coating can be applied onto the shaped photothermal material. Such a coating may be helpful for dragging water through the pores and channels, and forming a thin water film on the surface of photothermal materials, and thereby enhancing water evaporation rates when exposed to light energy. For the present purpose, examples of suitable hydrophilic polymers include polydopamine, polyvinyl alcohol, polyethylene glycol, and poly(acrylic acid).

[0088] Compared to bulk water heating systems where a homogeneous high temperature is required for the entire bulk water, a locally high temperature at the water surface can be achieved using the present shaped photothermal material, which effectively drives water vapour, once generated, to leave the water surface and move into the air for collection. Contaminants including salts and organisms may deposit and accumulate on the surface of the photothermal materials, but can be easily cleaned off as a result of the photothermal materials rotating on the water surface when there is water evaporation, wind, water movement caused by stirring force, factors that cause horizontal density gradients and/or surface/interface tension discrepancy, etc. In this way, the present apparatus and system can be highly energy efficient for desalination and the photothermal materials can have elongated operating life without frequent replacement or extra cleaning. [0089] Also disclosed herein is a process of desalination, which includes (i) placing a plurality of the self -cleaning or self-washing photothermal materials of the present disclosure on the surface of a body of water to be treated, (ii) exposing the plurality of the self -cleaning or self -washing photothermal materials to light energy, wherein the light energy is converted to thermal energy by the photothermal materials, (iii) vaporising the water in contact with the surface of photothermal materials, wherein the water passes through the pores and channels of the self-cleaning or self-washing photothermal materials, and (iv) condensing the water vapour in a condensation unit.

[0090] In order to place a plurality of the self-cleaning or self-washing photothermal materials on the surface of a body of water to be treated, the shaped photothermal material can be made to spontaneously float on the water surface or can be maintained at the water surface with the aid of a supporting device, for instance, a supporting mesh whose height is adjustable.

[0091] Under natural solar illumination, the photothermal material can convert light energy to thermal energy, which will then heat up the water on the photothermal material and evaporate the water. The water vapour can be condensed and collected in a condensation unit for later use. A condensation unit is known in the art and, for example, the condensation unit can include a chamber with cold surfaces, which can be made of glass, silicon, or metal for the condensation of water vapour. In some embodiments, the condensation unit can include other components to collect and store the water.

[0092] Also provided herein is a desalination system comprising a photothermal desalination apparatus and a plurality of the self-cleaning or self-washing photothermal materials of the present disclosure.

[0093] In some embodiments shown in Figure 3, the desalination system further comprises a heliostat, which is used to keep reflecting sunlight to a predetermined target, for example, a second reflection mirror. In some embodiments, the desalination system also comprises a second reflection mirror, which gathers the concentrated sunlight and turns it towards the water surface where the self-cleaning or self- washing photothermal materials are located. The photothermal material will then convert light energy into thermal energy, which enables the water to evaporate from the surface, through the pores and channels into water vapour. In further embodiments, the desalination system comprises a condensation unit for condensing water vapour and collecting the condensed water.

[0094] In some embodiments, a plurality of the self-cleaning or self-washing photothermal materials will spontaneously float on the water surface upon desalination. In other embodiments, the desalination system comprises a supporting device, for example, a supporting mesh whose height is adjustable, for keeping a plurality of the self-cleaning or self-washing photothermal materials in place at the water surface upon desalination. [0095] 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 claim.

[0096] 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.

[0097] 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.

Claims

1. A distillation apparatus comprising a distillation chamber comprising a water reservoir configured to hold a volume of water to be distilled, a porous photothermal substrate configured to contact a surface of water in the water reservoir, the photothermal substrate capable of generating heat and wherein the generated heat evaporates water from the water reservoir to generate steam and/or condensate, a steam and condensate collector configured to draw the generated steam or condensate to a condenser, the condenser in thermal contact with the water in the reservoir and/or a source of water to be used in the reservoir, and a condensate collection chamber for collecting condensate from the condenser.

2. The distillation apparatus of claim 1 , wherein the porous photothermal substrate comprises a photothermal material supported on a substrate.

3. The distillation apparatus of claim 2, wherein the photothermal material is selected from the group consisting of polydopamine, CuO, CuS and carbon based materials.

4. The distillation apparatus of any one of claims 1 to 3, further comprising a heat exchanger configured to heat a source of water to be used in the reservoir.

5. The distillation apparatus of any one of claims 1 to 4, further comprising one or more heliostats configured to reflect sunlight to a predetermined target.

6. A desalination apparatus comprising a distillation chamber comprising a salt water reservoir configured to hold a volume of salt water to be distilled, a porous photothermal substrate configured to contact a surface of water in the salt water reservoir, the photothermal substrate capable of generating heat when it is exposed to light and wherein the generated heat evaporates water from the salt water reservoir to generate steam and/or condensate, a steam and condensate collector configured to draw the generated steam or condensate to a condenser, the condenser in thermal contact with the salt water in the reservoir and/or a source of salt water to be used in the reservoir, and a condensate collection chamber for collecting condensate from the condenser.

7. The desalination apparatus of claim 6, wherein the porous photothermal substrate comprises a photothermal material supported on a substrate.

8. The desalination apparatus of claim 7, wherein the photothermal material is selected from the group consisting of polydopamine, CuO, CuS and carbon based materials.

9. The desalination apparatus of any one of claims 6 to 8, further comprising a heat exchanger configured to heat a source of water to be used in the reservoir.

10. The desalination apparatus of any one of claims 6 to 9, further comprising one or more heliostats configured to reflect sunlight to a predetermined target.

11. A process of distilling water, said process comprising:

collecting condensate from the condenser.

12. A desalination process, said process comprising:

contacting a porous photothermal substrate with a surface of a volume of sea water to be distilled in a water reservoir, the photothermal substrate capable of generating heat when it is exposed to light; exposing the photothermal substrate to light under conditions to generate heat such that the generated heat evaporates water from the water reservoir to generate steam and/or condensate;

collecting condensate from the condenser.

13. A desalination system comprising the desalination apparatus of any one of claims 6 to 10 and further comprising:

14. The desalination system of claim 13, further comprising a reflector positioned beneath the distillation chamber and configured to reflect sunlight or scattered light from the ambient environment to the bottom and/or sides of the desalination apparatus.

15. The desalination system of any one of claims 13 to 14, further comprising a heat exchanger configured to heat a source of water to be used in the reservoir.

16. The desalination system of any one of claims 13 to 15, further comprising one or more heliostats configured to reflect sunlight to a predetermined target.

17. A self-cleaning or self-washing porous photothermal material that is capable of converting light energy to vaporise water in contact therewith in a photothermal desalination apparatus, wherein the photothermal material is in the form of a shape that is rotatable on the surface of the water to be vaporised.

18. A desalination process which includes (i) placing a plurality of the self -cleaning or self-washing photothermal materials of claim 17 on the surface of a body of water to be evaporated, (ii) exposing the plurality of the self -cleaning or self-washing photothermal materials to a light energy, wherein the light energy is converted to thermal energy by the photothermal materials, (iii) vaporising the water in contact with the surface of photothermal materials, wherein the water passes through the pores and channels of the self-cleaning or self-washing photothermal materials, and (iv) condensing the water vapour in a condensation unit.

19. A desalination system comprising a photothermal desalination apparatus and a plurality of the self -cleaning or self-washing photothermal materials of claim 17.