WO2019113020A1

WO2019113020A1 - Membrane-based selective surface-integrated water desalination system

Info

Publication number: WO2019113020A1
Application number: PCT/US2018/063781
Authority: WO
Inventors: Shanker NARAYANAN
Original assignee: Rensselaer Polytechnic Institute
Priority date: 2017-12-04
Filing date: 2018-12-04
Publication date: 2019-06-13

Abstract

A system for desalinating a fluid including an evaporator and a condenser are disclosed. The evaporator includes a membrane and a selective fight absorption surface. The selective light absorption surface includes a plurality of nanostructures including a metallic substrate, a ceramic template on the metallic substrate, and metallic inserts within the ceramic template and on the metallic substrate, which preferentially reflect light having long wavelengths and preferentially absorb light having shorter wavelengths. The evaporator is positioned so that the nanostructured surface receives light transmitted through a light-permeable cover. Evaporator flow channels are positioned to bring a fluid into contact with the porous membrane, where heat from the nanostructured surface causes at least some of the fluid to evaporate and diffuse across the porous membrane. The vapor is then condensed as a product fluid, such as distilled water.

Description

MEMBRANE-BASED SELECTIVE SURFACE-INTEGRATED WATER

DESALINATION SYSTEM

CROSS REFERENCE TO RELATED APPLICATIONS) [0001] This application claims the benefit of U.S. Provisional Application Nos.

62/774,376, filed December 3, 2018, and 62/594,337, filed December 4, 2017, which is incorporated by reference as if disclosed herein in its entirety.

BACKGROUND

[0002] Global water supply is being increasingly stressed due to population growth, competing energy demands, and. disruption of natural hydrologie cycles. Natural disasters also have the power to produce localized water supply issues. For example, U.S. suffered significant loss during hurricanes Harvey, Irma and Jose in 2017, with people were stranded without potable water.

[0003] State-of-the-art reverse osmosis (RO) is considered unsuitable for

desalination of higher water recovery ratios and high salinity feed solutions, with solar thermal desalination being used instead. Solar thermal desalination involves absorption of light and heat transfer to a bulk fluid. However, these methods often lead to significant losses and low throughput. Other approaches include the use of volumetric receivers, nanofluid, floating solar absorbers. Since desalination relies on phase-change, the use of low intensity solar flux has typically been insufficient to reach the desired high

temperatures to compensate for losses and manage the large latent heat for evaporation. Consequently, optical concentrators have been used. However, these systems suffer from significant heat loss that lowers the overall desalination efficiencies, poor thermal and vapor transport characteristics resulting in low evaporation rates, mechanical failure due to salt stagnation, resulting in clogging or diffusion-limited evaporation flux, the requirement for expensive materials, and loss in absorption efficiency due to absorber sealing. SUMMARY

[0004] Some embodiments of the disclosed subject matter are directed to a system for desalinating a fluid including a light-permeable cover, an evaporator positioned to receive light transmitted through the light-permeable cover, and an effluent conduit positioned proximate the evaporator and configured to output a vapor phase transmitted through a porous membrane. In some embodiments, the evaporator includes a porous membrane including a selective light absorption surface positioned to receive light transmitted through the light-permeable cover, and having a pore size and chemistry to favor diffusion of a vapor phase over a liquid phase. In some embodiments, the evaporator includes one or more evaporator flow channels positioned to bring a fluid into contact with the porous membrane, an evaporator inlet channel configured to deliver the fluid to the evaporator flow channels, and an evaporator outlet channel downstream of the evaporator flow channels. In some embodiments, the selective light absorption surface includes a plurality of nanostructures including a metallic substrate, a ceramic template on the metallic substrate, and metallic inserts within the ceramic template and on the metallic substrate.

[0005] Some embodiments of the disclosure subject matter include a method for desalinating a fluid by providing an evaporator as discussed above. In some

embodiments, the evaporator is positioned to receive light transmitted through a light- permeable cover. In some embodiments, light is transmitted through the light-permeable cover to the selective light absorption surface. A target fluid is delivered to the evaporator flow channels and heat is transferred from the evaporator to the target fluid. In some embodiments, the target fluid is salinated water. In some embodiments, a vapor phase is transmitted through the porous membrane and condensed into a fluid product. In some embodiments, the final product is distilled water.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: [0007] FIG. 1 A is a schematic representation of a desalinating fluid system according to some embodiments of the present disclosure;

[ 0008] FIG. 1 B is a schematic representation of a porous membrane of a desalination system according to some embodiments of the present disclosure; [0009] FIG. 1C is a schematic representation of a light absorbing porous membrane according to some embodiments of the present disclosure;

[0010] FIG. 2A is a schematic representation of a selective light absorption surface according to some embodiments of the present disclosure;

[0011] FIG. 2B is a schematic representation of a selective light absorption surface according to some embodiments of the present disclosure;

[0012] FIG. 2C is a graph portraying spectral intensity of solar radiation (blackbody, extraterrestrial and terrestrial) and spectral emissivity of a selective light absorption surface according to some embodiments of the present disclosure;

[0013] FIG. 3 is a graph portraying evaporation flux calculated for different operating conditions;

[0014] FIG. 4 is a graph portraying maximum and minimum temperatures

(continuous lines) and mass flux (dotted lines) resulting from thermal concentration of sunlight using different geometries of a selective light absorption surface according to some embodiments of the present disclosure; [0015] FIG. 5 A is a graph portraying increases in salt concentration with time on the surface of an evaporating pool of water capped with a microporous layer;

[0016] FIG. 5B is a graph portraying distribution of salt concentration at steady state for systems according to some embodiments of the present disclosure;

[0017] FIG. 5C is a graph portraying pumping power versus mass flow of fluid supplied at the inlet (feed solution) of desalination systems according to some

embodiments of the present disclosure; [0018] FIG. 6 is a schematic representation of a system according to some embodiments of the present disclosure; and

[0019] FIG, 7 is a flowchart of a method for desalinating a fluid according to some embodiments of the present disclosure. DETAILED DESCRIPTION

[0020] Referring now to FIG. 1A, aspects of the disclosed subject matter include a system 100 for desalinating a fluid. In some embodiments, system 100 is disposed in a monolithic body 102. In some embodiments, body 102 is composed of an insulating material, in some embodiments, body 102 is composed of acrylic. In some

embodiments, system 100 includes a plurality of bodies 102, e.g., to process high fluid volumes. In some embodiments, various components are distributed across two or more bodies 102 in communication with each other via a series of conduits (embodiment not shown). In some embodiments, body 102 includes a cover 104. Cover 104 is light- permeable, meaning that light 106 is transmitted from outside body 102 through cover 104 to an interior of body 102. In some embodiments, the material for cover 104 is chosen to provide thermal insulation, which traps heat inside body 102. in some embodiments, light 106 includes direct sunlight, indirect sunlight, diffused ambient radiation, or combinations thereof in some embodiments, cover 104 reflects less than about 50% of incident light. In some embodiments, cover 104 reflects less than about 25% of incident light. In some embodiments, cover 104 reflects less than about 10% of incident light. In some embodiments, cover 104 reflects less than about 5% of incident light. In some embodiments, cover 104 reflects less than about 1 % of incident light in some embodiments, cover 104 is composed of one or more suitable light-permeable natural or synthetic materials, such as natural glass, po!y(methyl methacrylate), polycarbonates, aerogels, and the like, or combinations thereof. In some embodiments, body 102 is enclosed or somewhat-sealed to avoid (minimize) the ingress of air into system 100.

[0021] In some embodiments, system 100 includes an evaporator 108 positioned to receive light 106T transmitted through cover 104. In some embodiments, evaporator 108 includes porous membrane 110. Porous membrane 110 is positioned to receive light 106T. In some embodiments, porous membrane 110 is positioned to receive light 106T directly, indirectly, or combinations thereof. In some embodiments, porous membrane 110 is light-absorbing. In some embodiments, porous membrane 110 is light- permeable and used in conjunction with other light absorbing components, as will be discussed In greater detail below. In some embodiments, porous membrane 110 reflects less than about 50% of incident light, in some embodiments, porous membrane 110 reflects less than about 25% of incident light, in some embodiments, porous

membrane 110 reflects less than about 10% of incident light. In some embodiments, porous membrane 110 reflects less than about 5% of Incident light. In some

embodiments, porous membrane 110 reflects less than about 1% of incident light. In some embodiments, porous membrane is composed of any suitable light-permeable or wavelength-selective light absorbing material, as will be discussed in greater detailbelow. In some embodiments, porous membrane 110 includes one or more coatings for separation of liquid phase and vapor phase, such as poly(ether sulfone),

poly(tetrafiuoroefhylene), cellulose acetate, and the like, or copolymers thereof. [0022] Referring no w to FIG. 1 B, in some embodiments, porous membrane 110 includes one or more pores 110P. In some embodiments, pores 110P. are straight and cylindrical in shape. In some embodiments, the pores 110P. are irregularly shaped, hi some embodiments, pore 110P. are sized to favor diffusion of a vapor phase for a given substance over a liquid phase for that substance. In some embodiments, pores 110P. have suitably chosen pore geometry (pore radius, height and porosity) and wettability or surface chemistry lo facilitate wetting on a first side 110A by a given substance and favor diffusion of the vapor phase of the substance to a second side 110B via the pores. In some embodiments, pores 110P have suitably chosen pore geometry (pore radius, height and porosity) and wettability or surface chemistry to facilitate non-wetting on a first side 110A by a given substance and favor diffusion of the vapor phase of the substance to a second side 110B via the pores to delay saline deposition on the first side 110A and prolong the use of the system 100 without maintenance. In some embodiments, the radius of pores 110P ranges from about 1 μm to about 100 μm. In some embodiments, the height of pores 110P. ranges from about 50 μm to about 1000 μm. In some embodiments, membrane 110 has a porosity of from about 25% to about 75%, Without wishing to be bound by theory, in general, larger pore size arid porosity and smaller pore height supports diffusion of the vapor phase of the target fluid. Further, in general, smaller porosity and larger height provide structural strength for porous membrane 110. In some embodiments, a pore radius around 10 um, pore height around 100 μηι, and porosity around 75% arc used. However, this invention does is not restrictive in the selection of pore geometry, [0023] Referring again to FIG. 1 A, evaporator 108 includes a selective light absorption surface 112, e.g., a wavelength-selective light absorption surface. En some embodiments, surface 112 is configured to preferentially reflect light having long wavelengths and preferentially absorb light having shorter wavelengths. In some embodiment, surface 112 includes a nanostructured surface 112S configured to preferentially reflect light having long wavelengths and preferentially absorb light having shorter wavelengths. In some embodiments, the nanostructured surface 112S is configured to preferential!y reflect infrared light and preferentially absorb visible and near-infrared light. Referring now to FIGs. 2A-2B, in some embodiments, surface 112S includes a plurality of nanostructures 112N. In some embodiments, nanostructures 112N are included in pores 110P. In some embodiments, at least one nanostructure 112Ν is included in each pore 110P. In some embodiments, the nanostructures 112Ν include a metallic substrate 112B, a ceramic template or matrix 112T on metallic substrate 112B, and metallic inserts 112I within ceramic template 112T. In some embodiments, metallic inserts 112I are also on metallic substrate 112B. In some embodiments, the plurality of nanostructures 112N are generally hexagonally arranged, although the present disclosure is not limited in this regard. In some embodiments, metal inserts 112I include, but are not limited to, nickel, copper, tungsten, carbon, or combinations thereof. In some

embodiments, the ceramic template 112T includes alumina. In some embodiments, the metallic substrate 112B can include aluminum, copper, stainless steel, or combinations thereof.

[0024] Still referring to FIGs. 2A-2B, in some embodiments, the diameter of nanostructures 112N (d_p) is about 10^-8 m to about 10^-6 m. In some embodiments, d_p is about 10^-8 m to about 10^-7 m, In some embodiments, d_p is about 10^-7 m to about 10^-6 m. In some embodiments, d_int is about. 10^-7 m to about 10^-5 m. In some embodiments, d_int is about 10^-7 m to about 10^-6 m. In some embodiments, d_int is about 10^-6 m to about 10^-5 m.

In same embodiments, the height of metallic inclusion (h_m) is about 10^-8 m to about 10^-6 m. In some embodiments, h_m is about 10^-8 m to about 10^-7 m. In some embodiments, h_m is about 10^-7 m to about 10^-6 m. In some embodiments, the height of the ceramic templates (h_p) is about 10^-7 m to about 10^-4 m. In some embodiments, h_p is about 10^-6 m to about 10^-5 m.

|0025| In some embodiments, evaporator 108 includes a plurality of surfaces 112. In some embodiments, surface 112 is positioned to receive light 106T transmitted through cover 104. In some embodiments, surface 112 is positioned to receive light 106T directly, indirectly, or combinations thereof. In some embodiments, surface 112 is positioned to receive light 106M transmitted through porous membrane 110. In some embodiments, surface 112 is positioned to receive light 106M directly, indirectly, or combinations thereof. In some embodiments, surface 112 absorbs more than about 50% of visible and near-infrared light from light 106T/106M. In some embodiments, surface 112 absorbs more than about 25% of visible and near-infrared light from light 106Τ/106Μ. In some embodiments, surface 112 absorbs more man about 10% of visible and near-infrared light from light 106T/106M. In some embodiments, surface 112 absorbs more than about 5% of visible and near-infrared light from light 106T/106M In some embodiments, surface 112 absorbs more than about 1% of visible and near-infrared light from light 106T/106M.

[0026] The structure of surface 112 minimizes heat loss via radiation, thus increasing the heat available for use in evaporator 108. Referring now to FIG. 2C, a computational simulation of radiative thermal transport indicates that a metallic foil coated with a selective surface will significantly outperform a simple black-colored surface. When exposed to normally incident solar radiation in the absence of convective losses, the b!ack surface resulted in a temperature of 84°€, while the selective surface reached 431 °C. As more of light 106T/106M is absorbed by surface ! 12, the temperature of that surface rises. Heat from surface 112 is then transferred to a target fluid to be evaporated by evaporator 108, as will be discussed in greater detail below.

[0027] Referring again to FIG, 1 A, in some embodiments, evaporator 108 includes one or more evaporator flow channels 114. The evaporator flow channels are positioned and configured to bring a target fluid into contact with the membrane 110 and thermal communication with surface 112. In some embodiments, flow channels 114 extend generally longitudinally along evaporator 108 in the general direction of How by the target fluid. In some embodiments, flow channels 114 are disposed above, below, between, or in surface 112, or combinations thereof. In some embodiments, the flow channels 114 are disposed in the selective light absorption surface 112. In some embodiments, evaporator 108 includes a plurality of flow channels 114. In some embodiments, the plurality of flow channels 114 ate distributed laterally across surface 112. in some embodiments, the fluid flow channels are narrow and shallow to promote localized heating of the liquid and minimize loss of the energy and salt accumulation from the target fluid. In some embodiments, flow channels 114 have a depth of about 1 mm to about 10 mm. In some embodiments, flow channels 114 have a width of 1 mm to 100 mm. The optimal selection of the geometry is application specific. For example, for desalination of sea water, channel depth of 5 mm and channel width of 10 mm is found optimal under certain conditions. In some embodiments, one or more ridges are positioned in flow channels 114 to delay salt deposition and clogging membrane 110. Without wishing to be bound by theory, the one or more ridges mitigate clogging with minimum disruption to fluid flow through evaporator 108,

[0028] Referring again to FIG. 1B, and discussed above, flow channels 114 bring the target fluid into contact with membrane 110 and thermal communication with

surface 112. In some embodiments, pores 110P. are disposed only over flow

channels 114, which has the benefit of maximizing absorption of light via surfaces without pores 110F. . The liquid phase of the target fluid is substantially confined to one side of membrane 110, however any vapor phase produced via heating by surface 112 is allowed to diffuse across membrane 110. In some embodiments, the liquid phase Is confined via capillary pressure, e.g., by membrane 110, flow channels 114, or combinations thereof with the use of suitable non-wetting coating, as mentioned above.

In some embodiments, the flow rate of the fluid through flow channels 114 is faster than the evaporation rate of the fluid, to delay or avoid clogging of pores in membrane 110.

[0029] In some embodiments, surface 112 interfaces with membrane 110. In some embodiments, flow channels 114 separate surface 112 from membrane 110. In some embodiments, surface 112 is disposed on membrane 110 itself. In some embodiments, a first portion of surface 112 interfaces with membrane 110, while a second portion of surface 112 is separated from membrane 110 by a flow channel 114. Referring now to FIG, 1C, in some embodiments, surface 112 includes pores 110P, and thus membrane 110 and surface 112 are one and the same. In some embodiments, pores 110P are coated with nanostructures 112N. Incorporating the nanostruclures 112N into pores 110P closely interfaces the light absorption and evaporation mechanisms which makes the evaporation of the target fluid more efficient. [0030] Referring again to FIG. 1 A, in some embodiments, evaporator 108 includes an evaporator inlet channel 116. Inlet channel 116 is configured to deliver the target fluid from outside of body 102 and/or evaporator 108 to flow channels 114. In some embodiments, evaporator 108 includes an evaporator outlet channel 118. Outlet channel 118 is downstream of flow channels 114 and configured to evacuate fluid from the evaporator and/or body. In some embodiments, the target fluid is a salinated liquid. "Salinated liquid" as used herein means any brine, saline, sea water, river water, brackish water, petrochemical waste, or other salt-containing solution upon which a desalination treatment produces a purer liquid. Heat absorbed by surface 112 is held in evaporator 108 for transfer to the target liquid, e.g., via conduction of the heat to pores 110P. As the salinated liquid enters evaporator 108, it is brought into contact with

membrane 110/surface 112, where it is heated in the flow channels 114 by the

surface 112, producing a vapor phase to diffuse across membrane 110. In embodiments where the salinated liquid is salinated water, this vapor phase is substantially distilled water vapor. The heated liquid still in flow channels 114, now a brine having increased concentration of salt, continues to outlet channel 118. In some embodiments, at least some of the brine in outlet channel 118 is recycled back into inlet channel 116. As shown in FIG. 1 A, in some embodiments, inlet channel 116 and outlet channel 118 are in thermal communication with each other, so that at least some heat in the fluid exiting from outlet channel 118 is recovered in the fluid entering at inlet 116, e.g., through body 102, evaporator 108, or combinations thereof. In some embodiments, flow through inlet channel 116 is countercurrent the flow through outlet channel 118.

[0031] Still referring to FIG. 1 A, an effluent conduit 120 is positioned proximate the evaporator 108. Effluent conduit 120 is configured to output the vapor phase transmitted through membrane 110. Effluent conduit 120 can be any suitable shape and size so long as it transports the vapor phase away from membrane 110. In some embodiments, effluent conduit 120 is in fluid communication with a condenser 122. Condenser 122 is operated at a lower temperature than evaporator 108 and is configured to condense the vapor phase transmitted through membrane 110 into a liquid product, e.g., distilled water. In some embodiments, condenser 122 includes a heat sink 124. In some embodiments, heat sink 124 is a passive heat sink. In some embodiments, condenser 122 is in thermal communication with inlet channel 116, so that at least some heat in the fluid in

condenser 122 is recovered in the fluid entering at inlet 116. In some embodiments, condenser 122 includes a product fluid outlet channel 126 to remove the liquid product from system 100.

[0032] In some embodiments, system 100 includes an energy storage system 128. In some embodiments, energy storage system is a thermal energy storage system, solar- thermal energy storage system, geothermal energy storage system, or combinations thereof, e.g., existing storage systems. Thermal energy storage system 128 is in thermal communication with evaporator 108 to deliver thermal energy to the evaporator on an as- needed basis, e.g., at night. In some embodiments, evaporator includes one or more heating channels 128C to provide heat to fluid in evaporator 108 from thermal energy storage system 128. In some embodiments, heating channels 128C are positioned between the one or more flow channels 114. In some embodiments, heating

channels 128C are positioned in flow channels 114. In some embodiments, heating channels 128C are positioned on surface 112.

Examples [0033] Without wishing to be bound by theory, the local evaporation rate depends on both heat and mass transfer. However, for the chosen evaporator design, which closely interfaces solar absorption and evaporation mechanisms, mass transfer is expected to limit the evaporation flux. With pore diameters (~10 μm) much larger than the mean free path (~70 nm) of molecules, vapor transport across pores 110P is dominated by molecular diffusion. In this case, the local mass flux is estimated using Fick's Law as

where D_v is the vapor diffusivity, L_m is the pore depth, p_v(T_e) and p_v(T_c) are saturated vapor densities at evaporator (T_e) and condenser temperatures (T_c), respectively (see FIG. 1 B). Hence, mass flux can be improved with higher evaporator temperatures, T_e or lower vapor condenser temperatures, T_c. Assuming saturated vapor conditions on both sides of the pores, the evaporation flux as a function of T_e from 333 to 373 K, and T_c from 293 to 313 K, is given by FIG. 3. The calculation assumes a porosity of 0.785, which means 78,5% of area capping flow channel 114 is open due to pores 110P. A larger gap between T_e and T_c corresponds to higher evaporation flux. As a result,

T_s - 373 K and T_c - 293 K gives a mass flux of 0.0045 kg/m²s. Some underlying assumptions in the above calculations include that vapor condensation is not being a rate limiting mechanism, which can be ensured by using sufficiently large condensing surface. The calculated mass flux of 0.0045 kg/m² s is equivalent to a heat flux of 10,000 W/m², which indicates the benefits of energy concentration, either by optical or thermal means, [0034] Again, without wishing to be bound by theory, the geometry of

membrane 110 and surface 112 aid with thermal concentration. Referring again to FIG. 1C, varying the aspect ratio W_u/W_c can control the temperature distribution across the cross-section. Assuming saturated vapor density at the pore outlet, p_v(T_c— 293K), different aspect ratios result in different temperature distributions and local evaporation rates. FIG. 4 shows the evaporation ilux, maximum, and minimum cross-section temperatures as a function of the aspect ratio. By way of example, with normally incident radiation of 1 sun (1000 W/m²) and W_u/W_c = 10, a maximum temperature of

about 376 K can be obtained, while evaporation takes place at about 371 K, resulting in an average evaporation flux of 0.0043 kg/m²s. As W_u/W_e increases, the average temperature of surface 112 increases, which also improves the rate of evaporation.

[0035] Referring now to FIGs. 5A-5C, a lab-based comparison of system 100 to a floating absorber based desalination system was performed. Within six hours of continuous operation by the floating absorber, the concentration increased more than 400- times (see FIG. 5 A). This calculation assumes evaporation through porous layer, with diffusion limited flux given by the Fick's law (mentioned above), with L_m = 1 mm,

D_v = 10^-5 m²/s, T_e = 333 K and T_c = 293 K. Indeed, pore clogging in this lab arrangement using a floating absorber is unavoidable, and will result in a gradual decrease in evaporation flux followed by clogging, which will require frequent replacement or cleaning. [0036] Without wishing to be bound by theory, as the water flows through flow channels 114, the flow rate decreases and salinity increases due to evaporation, risking clogging. However, salt concentration in pores 110.P can be controlled by balancing advecfion of sail towards the pores (by physical movement of fluid, like saline wafer) and diffusion of salt (from high to low concentration) back into the fluid, FIG, 5B shows the concentration distribution for two channel geometries that are each 1 m long and 5 mm in height. In both cases, the mass flow rate at the channel, inlet is set 50% higher than the overall evaporation rate from the channel. However, one of the channel also incorporates periodic ridges as shown in FIG. 5B’s inset. The maximum concentration at steady state, even in the case of a simple channel, was lower than the concentration in the floating absorber after 6 hours. With a flow through setup, the concentration can be lowered further by an order of magnitude by disrupting the flow pattern using the ridges. The presence of ridges do not result in excessive pressure drop. In this case, for mass flow rates per channel of about 5 x 10^-5 kg/s and about 8 x 10^-5 kg/s, pumping powers of about 4 x 10^-7 and about 11 x 10^-7 W was sufficient. These flow rates were comparable to the total evaporation rate ( 1.2x to 1.8x, respectively). This indicates that for every about 10^-8 m³ of distilled water produced about 10^-7 J of pumping energy is necessary. This corresponds to 26, 2 kJ of energy expended per day to produce 10,000 m³ of distilled water. The pumping energy is low since flow rates are comparable to net evaporation rates, and unlike RO, feed water in this case is pumped through relatively wide channels rather than pores 110P, Even considering other pumping losses, and accounting for inefficiencies, the electrical power required to operate the pumps will be significantly low and derivable from a small array of photovoltaic panels.

[0037] Referring now to FIG, 6, a 3 -dimensional study of fluid flow and radiative heat transfer was carried out to confirm that system 100 recovers heat effectively. The test system included a single evaporator with an acrylic cover. Unit cell dimensions are indicated in FIG, 6. Accounting for imperfections in the fabrication process and optical properties, the selective light absorption surface was assumed to have selective emittance given by and a

porosity of 0,70 for the regions capping the evaporator flow channel. In addition, vapor at the pore outlet was assigned a saturated vapor density at 293 K (ambient temperature). The optical and thermophysieal properties of all other components were assigned based on their respective bulk material properties. Saline water was fed into the evaporator channel at 2 x 10^-5 kg/s and 293.1 K. [0038] With normally incident radiation of 1 sun, the highest temperature on the evaporator was found to be ~490 K. A net evaporation rate of 1.76 X 10^-5 kg/s was found, indicating that >85% of feed water underwent evaporation at an average flux of 1.76 x 10^-3 kg/m²s over the evaporator flow channel. Further_* thermally interfacing the incoming and outgoing fluid streams was demonstrated as useful in minimizing energy loss. For the chosen conditions, the outgoing fluid left at 295 K and 0.3 x 10^-5 kg/s, allowed an overall energy efficiency of 80%. Even higher efficiencies can be achieved by including the energy recovered from a condenser, as discussed above.

[0039] Referring now to FIG. 7, some embodiments of the present disclosure are directed to a method 700 for desalinating a fluid. At 702, an evaporator is provided. As discussed above, in some embodiments, evaporator includes a porous membrane with a selective light absorption surface (or combined membrane/surface) positioned to receive light and having a pore size to favor diffusion of a vapor phase over a liquid phase, and one or more evaporator flow channels positioned to bring a fluid into contact with the porous membrane. At 704, the evaporator is positioned to receive light incident on the selective light absorption surface. In some embodiments, the light transmitted at 704 is first transmitted through a light-permeable cover. At 706, the light is transmitted to the selective light absorption surface, e.g., through the light-permeable cover, the porous membrane, etc. As discussed above, the selective light absorption surface reflects infrared light and preferentially absorbs visible and neai-inffared light. At 708, a target fluid is delivered to the evaporator flow channels. At 710, heat is transferred from the evaporator to the target fluid. At 712, a vapor phase is transmitted through the porous membrane. At 714, the vapor phase is condensed into a fluid product. In some embodiments, at 716, heated target fluid is brought into thermal communication with incoming target fluid to recover heat from the heated fluid.

[0040] Methods and systems of the present disclosure advantageously provide high- efficiency fluid desalination by greatly minimizing energy loss during treatment. Energy loss is minimized using the selective light absorption surface, which are far more absorbent than black surfaces and more efficiently transfer heat energy from a light source, e.g., the sun, to the fluid. Higher temperatures can be maintained to support larger evaporation rates, with desalination efficiencies >80% being achievable. The design of the evaporator also allows the inlet and outlet streams to be in close proximity to recover and recycle even more heat in the system. Evaporation rate, and thus throughput, is maximized with the positioning of the absorptive surfaces, fluid flow channels, and diffusion membrane, as well as the geometry and wettability of the membrane itself. System geometry and components also delay fouling, reducing the overall cost of maintenance for the system. Finally, the system can be fashioned from relatively inexpensive materials and using a scalable fabrication process, lowering the overall cost of the system significantly.

[0041] Although the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments can be combined, rearranged, etc., to produce additional embodiments within the scope of the invention, and that various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.

Claims

CLAIMS What is claimed is:

1. A system for desalinating a fluid comprising: a light-permeable cover; an evaporator positioned to receive light transmitted through the light- permeable cover, the evaporator including: a porous membrane including a selective light absorption surface positioned to receive light transmitted through the light-permeable cover, and having a pore size to favor diffusion of a vapor phase over a liquid phase; one or more evaporator flow channels positioned to bring a fluid into contact with the porous membrane; an evaporator inlet channel configured to deliver the fluid to the evaporator flow channels; and an evaporator outlet channel downstream of the evaporator flow channels; and an effluent conduit positioned proximate the evaporator and configured to output the vapor phase transmitted through the porous membrane.

2. The system according to claim 1 , further comprising a source of salinated liquid in fluid communication with the inlet channel.

3. The system according to claim 1 , wherein the vapor phase is distilled water vapor.

4. The system according to claim 1 , wherein the one or more evaporator flow channels are disposed in the selective light absorption surface.

5. The system according to claim 4, wherein the one or more evaporator flow channels include a plurality of evaporator flow channels distributed laterally across the selective light absorption surface.

6. The system according to claim 1, wherein the selective light absorption surface

includes a nanostructured surface configured to preferentially reflect infrared light and preferentially absorb visible and near-infrared light.

7. The system according to claim 6, wherein the nanostructured surface includes a

plurality of nanostructures including a metallic substrate, a ceramic template on the metallic substrate, and metallic inserts within the ceramic template and on the metallic substrate.

8. The system according to claim 7, wherein porous membrane includes a plurality of pores and at least one of the plurality of nanostructures is included in the pores.

9. The system according to claim 1 , further comprising a thermal energy storage system in thermal communication with the evaporator,

10. The system according to claim 4, further comprising one or more heating channels positioned between the one or more evaporator flow channels, wherein a thermal energy storage system is in thermal communication with the heating channels.

11. The system according to claim 1, wherein the evaporator inlet channel is in thermal communication with the evaporator outlet channel to recover heat from fluid in the evaporator outlet channel via fluid in the evaporator inlet channel.

12. The system according to claim 1, further comprising a condenser in fluid

communication with the effluent conduit, the condenser including: a heat sink; and a product fluid outlet channel.

13. The system according to claim 12, wherein the heat sink is a passive heat sink.

14. The system according to claim 1, wherein the light transmitted through the light- permeable cover is direct sunlight, indirect sunlight, diffused ambient radiation, or combinations thereof.

15. A method for desalinating a fluid comprising: providing an evaporator, the evaporator including: a porous membrane including a selective light absorption surface positioned to receive light, and having a pore size to favor diffusion of a vapor phase over a liquid phase; and one or more evaporator flow channels positioned to bring a fluid into contact with the porous membrane; positioning the evaporator to receive light transmitted through a light- permeable cover, transmitting light through the light-permeable cover to the selective light absorption surface; delivering a target fluid to the evaporator flow channels; transferring heat from the evaporator to the target fluid; transmitting a vapor phase through the porous membrane; and condensing the vapor phase into a fluid product.

16. The method according claim 15, further comprising bringing heated fluid from the evaporator into thermal communication with the target fluid to recover heat from the heated fluid.

17. The method according to claim 15, wherein transmitting light through the light- permeable cover to the selective light absorption surface further comprises: preferentially, by the selective light absorption surface, reflecting infrared light and preferentially absorbing visible and near-infrared light.

18. The method according to claim 15, wherein the target fluid is salinated water and the fluid product is distilled water.

19. A system for desalinating a fluid comprising: a body having a light-permeable cover; an evaporator positioned to receive light transmitted through the light- permeable cover, the evaporator including: a light-permeable porous membrane positioned to receive light transmitted through the light-permeable cover, and having a pore size to favor diffusion of a vapor phase over a liquid phase; a selective light absorption surface positioned to receive light transmitted through the light-permeable porous membrane; one or more evaporator flow channels positioned to bring a fluid into contact with the light-permeable porous membrane; an evaporator inlet channel configured to deliver the fluid from outside of the body to the evaporator flow channels; and an evaporator outlet channel downstream of the evaporator flow channels and configured to evacuate fluid from the body; wherein the evaporator inlet channel is in thermal communication with the evaporator outlet channel to recover heat in the evaporator inlet channel from the evaporator outlet channel through the body; and an effluent conduit positioned proximate the evaporator and configured to output the vapor phase transmitted through the light-permeable porous membrane.

20. The system according to claim 19, wherein the body further comprises a condenser in fluid communication with the effluent conduit, the condenser including: a heat sink; and a product fluid outlet channel.