Solar-driven hygroscopic hydrogel device for atmospheric water harvesting

Abstract

A system-level design, optimization, fabrication, and implementation of a passive solar-driven sorbent-based atmospheric water harvesting (SAWH) device is provided. The SAWH device can use a hydrogel-salt composite as a sorbent material to passively harvest water from both rural and arid environments via a daily cycle of absorption during nighttime and desorption of the hydrogel-salt composite during daytime using solar energy.

Description

Atty. Dkt. No.: MIT 26185 PCT | 88212-427689SOLAR-DRIVEN HYGROSCOPIC HYDROGEL DEVICE FOR ATMOSPHERIC WATER HARVESTINGCROSS REFERENCE TO RELATED APPLICATION

[0001] The present disclosure claims priority to and the benefit of U.S. Provisional Application No. 63 / 713.567. entitled “Solar-Driven Hygroscopic Hydrogel Device for Atmospheric Water Harvesting,” filed on October 29, 2024, the content of which is incorporated by reference herein in its entirety7.FIELD

[0002] The present disclosure relates to compositions and methods for decentralized water production that is both fully renewable and low-cost, and more particularly relates to compositions and methods of a hydrogel that can be used for atmospheric water harvesting applications.BACKGROUND

[0003] Current assessments of global water supply predict that population and economic growth will lead to an additional 1.8 billion people living in moderate to severe water scarcity regions by 2050, with 80% of those residing in developing countries. Limited access to potable w ater leads not only to community health decline and transmission of waterborne diseases, but also profoundly impacts the economic uncertainty of a region, further reducing the quality of life for local inhabitants. While filtration and desalination technologies offer potential solutions to the potable water crisis in coastal regions, their high economic, energetic, and environmental costs motivate decentralized and passive technologies as alternatives, especially for inland regions. To this end, sorbent-based atmospheric water harvesting (SAWH) has emerged in recent years as a technology with the potential to help mitigate water scarcity7at the local level by conlrollably harvesting moisture from ambient air via solar driven processes. Additionally, by leveraging an ubiquitous source of energy, in the form of sunlight, and moisture, SAWH can be successfully deployed in inland, arid climate regions whose geography and low humidity limits access to other renewable water sources such as desalination and dewing.

[0004] Recently, significant effort has been made to develop sorbent materials for SAWH devices, including metal-organic frameworks (MOFs), zeolites, and hygroscopic hydrogels.Atty. Dkt. No.: MIT 26185 PCT | 88212-427689These efforts have led to rigorous characterization of the properties and performance of each material in controlled, laboratory conditions. In particular, hydrogel salt-composites have demonstrated desirable performance, including a large ability to capture moisture, tunable speed of capture and release, and low energy requirements for desorption, making them promising candidates for SAWH. Despite the promise of hydrogel-salt composites, their optimized modeling and demonstration in SAWH has thus far been material-focused, crucially limiting the performance of SAWH devices. For instance, previous works have developed first-principle thermodynamic and heat and mass transport models to describe material -lev el behavior of hydrogel-salt composites. Despite the success of these models in quantifying performance as a function of material variables such as pore size, thickness, and chemical composition, the system-level integration of these models into the design of SAWH devices has not been demonstrated. Due, at least in part, to the lack of system-level physicsbased models, decisions of component materials, insulation, and system length scales are typically empirical, leading to system nonidealities and reduced material performance. Moreover, despite significant efforts in material level performance, optimal system integration of hydrogels in sorption systems remains a significant limitation to realizing cost- effective, high-performance devices. Still further, SAWH device that function in one environment may exhibit exceptionally poor performance when placed in another environment, and / or when weather conditions change.

[0005] Accordingly, there is a need for improvement of water production in atmospheric water harvesting devices with regards to both material performance and following exposure to varying environmental conditions.SUMMARY

[0006] The present application is directed to compositions and methods for providing decentralized water production. The present embodiments can design, optimize, and demonstrate deployment of lithium chloride-loaded polyacrylamide (PAM-LiCl) hydrogels in a passive atmospheric w ater harvesting device to provide potable w ater with high thermal efficiency and water production. As provided for herein, a comprehensive heat and mass transport model can be developed to design an optimal device architecture for maximal thermal efficiency and water output. Performance of the hydrogel device of the present embodiments can be optimized using a system-level heat and mass transport model forAtty. Dkt. No.: MIT 26185 PCT | 88212-427689 hydrogel-salt composite-based atmospheric water harvesting, considering impacts of both material and system parameters on heat and mass transfer during device operation.

[0007] One embodiment of a sorbent-based atmospheric water harvesting (SAWH) device includes a hydrogel-salt composite, an absorber stage, a condenser, a cover, and a thermal insulation. The hydrogel-salt composite is formed from an acrylamide monomer polymerized in an aqueous solution of lithium chloride and de-ionized water. The absorber stage is configured to convert incident solar flux into heat. The condenser is configured to capture water vapor from the hydrogel and output it as liquid water during desorption. The device thermal insulation is disposed around the hydrogel-salt composite, the absorber stage, the condenser, and the cover.

[0008] One or more of the following features can be included. The device can be configured to operate passively. The absorber stage can be positioned adjacent to the hydrogel-salt composite. The condenser can be positioned below the hydrogel-salt composite. The device can further include a plurality of air-vapor gaps, with at least a first air-vapor gap of the plurality of air-vapor gaps being positioned above the hydrogel-salt composite and at least a second air-vapor gap of the plurality of air-vapor gaps being positioned below the hydrogel-salt composite. A first of the plurality of air-vapor gaps can separate the cover from a surface of the absorber stage. A length of the first of the plurality of air-vapor gaps can be about 40 millimeters. A length of the first of the plurality of airvapor gaps can be approximately in a range of about 1 millimeter to about 10 millimeters. A second of the plurality of air- vapor gaps can separate the condenser from a surface of the hydrogel-salt composite. A length of the second of the plurality of air-vapor gaps can be about 40 millimeters. A length of the second of the plurality of air-vapor gaps can be approximately in a range of about 5 millimeters to about 100 millimeters. The atmospheric water harvesting device can be configured to achieve at least about 1.7 L / m2 / day water output with a thermal efficiency of greater than about 16% in arid climates.

[0009] In another aspect, embodiments relate to a method of atmospheric w ater harvesting includes absorbing moisture from a condenser surface using a hydrogel formed from an acrylamide monomer polymerized in an aqueous solution of lithium chloride. The method also includes desorbing the hydrogel to cause condensate to form on the condenser surface and harvesting the condensate.Atty. Dkt. No.: MIT 26185 PCT | 88212-427689

[0010] One or more of the following features can be included. Harvesting can occur on a daily cycle, in which absorption occurs when the hydrogel is exposed to nighttime ambient air, and desorption occurs during daytime. The hydrogel can be synthesized via radical polymerization of acrylamide monomers. Desorbing the hydrogel can further include heating the hydrogel. The hydrogel can be heated by solar energy'. The solar energy can include incident solar irradiation that is converted to thermal energy for hydrogel desorption. The moisture on the condenser surface can be formed by exposing the condenser surface to air flow. The hydrogel can be disposed in the device described above. Accordingly, the hydrogel can be disposed in a sorbent-based atmospheric harvesting (SAWH) device. The device can include a hydrogel-salt composite, an absorber stage, a condenser, a cover, and a thermal insulation. The hydrogel-salt composite can be formed from an acrylamide monomer polymerized in an aqueous solution of lithium chloride and de-ionized water. The absorber stage can be configured to convert incident solar flux into heat. The condenser can be configured to capture water vapor from the hydrogel and output it as liquid water during desorption. The device thermal insulation can be disposed around the hydrogel-salt composite, the absorber stage, the condenser, and the cover. Other features of the device described above can also be included in the SAWH device in which the hydrogel can be disposed.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] This disclosure will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:

[0012] FIG. 1 A is schematic illustration of operation of a device of the present embodiments during nighttime and daytime;

[0013] FIG. IB is a graph illustrating an isotherm for a hydrogel-salt composite used in the device of FIG. 1A and an inset (i) illustrating a microscale of the hydrogen-salt composite;

[0014] FIG. 1C is a perspective view of the device of FIG. 1A exposed to the ambient air;

[0015] FIG. 2 A is a schematic illustration of the device of FIG. 1 A showing energy and mass flows between the components of the device;

[0016] FIG. 2B is a graph illustrating impact of solar absorber emissivity and glass transmission of the device of FIG. 1A following thermofluidic optimization;Atty. Dkt. No.: MIT 26185 PCT | 88212-427689

[0017] FIG. 2C is a graph illustrating impact of the finned condenser structure and ambient heat transfer coefficient for both glass cover and no glass cover designs of the device of FIG. 1A following thermofluidic optimization;

[0018] FIG. 2D is a graph illustrating impact of environmental temperature and relative humidity of the device of FIG. 1A following thermofluidic optimization;

[0019] FIG. 2E is a graph illustrating impact of vapor gap length and hydrogel thickness on device productivity of the device of FIG. 1A following thermofluidic optimization;

[0020] FIG. 2F is a graph illustrating impact of vari abi 1 i ty of both productivity and thermal efficiency on incident solar flux for fixed hydrogel thicknesses of the device of FIG. 1A following thermofluidic optimization;

[0021] FIG. 3 A is a perspective view of an embodiment of an experimental setup of the device of the present embodiments exposed to ambient air in an urban area;

[0022] FIG. 3B is a graph illustrating measured temperatures on the absorber, glass, and condenser for the setup of FIG. 3 A;

[0023] FIG. 3C is a graph illustrating incident solar flux and water output data over an approximately nine (9)-hour testing period, showing time-dependent water output from the device;

[0024] FIG. 4A is a perspective view of another embodiment of an experimental setup of the device of the present embodiments exposed to ambient air in desert conditions;

[0025] FIG. 4B is a graph illustrating humidity, temperature, and solar irradiance of the setup of FIG. 4A for absorption and desorption;

[0026] FIG. 4C is a graph illustrating measured and predicted system temperatures of the setup of FIG. 4A; and

[0027] FIG. 4D is a graph illustrating actual and predicted water output for the setup of FIG. 1A.Atty. Dkt. No.: MIT 26185 PCT | 88212-427689DETAILED DESCRIPTION

[0028] Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the systems compositions, designs, and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the systems, compositions, and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present disclosure is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present disclosure. Like-numbered components across embodiments generally have similar features unless otherwise stated or a person skilled in the art would appreciate differences based on the present disclosure and / or his / her knowledge.Accordingly, aspects and features of every embodiment may not be described with respect to each embodiment, but those aspects and features are applicable to the various embodiments unless statements or understandings are to the contrary.

[0029] Further, to the extent features, layers, sides, objects, steps, or the like are described as being “first,” “second,” third,” etc., and / or “lower,” “upper,” “middle,” etc., such numerical and / or location ordering / identification is generally arbitrary, and thus such numbering can be interchangeable unless indicated or otherwise understood by those skilled in the art to not be interchangeable. To the extent that the instant disclosure includes various terms for components and / or processes of the disclosed systems, compositions, designs, and methods, and the like, one skilled in the art, in view of the claims, present disclosure, and knowledge of the skilled person, will understand such terms are merely examples of such components and / or processes, and other components, designs, processes, and / or actions are possible.

[0030] The present disclosure generally provides systems and compositions for leveraging a hydrogel-salt composite for practical, cost effective water harvesting using solar energy to capture moisture from ambient air. In some embodiments, the hydrogel-salt composite can be used in an effective system-level design, optimization, fabrication, and / or implementation of a passive solar-driven sorbent-based atmospheric water harvesting (SAWH) device which, in conjunction with a heat and mass transport model, can be used to leverage and identify keyAtty. Dkt. No.: MIT 26185 PCT | 88212-427689 system parameters that can significantly impact the productivity and / or thermal efficiency of a hydrogel-salt composite. The device can then be optimized for maximum production and efficiency by selecting parameters that maximize water productivity and / or thermal efficiency in a variety of environments, including a semi-arid environment.

[0031] One example embodiment of a passive, solar-driven SAWH device 100 is shown in FIG. 1A. The device 100 can be designed to effectively cycle a polyacrylamide-lithium chloride (PAM-LiCl) hydrogel-salt composite or hydrogel 102 and demonstrate its record performance in both urban and rural environments. PAM-LiCl can be chosen as the sorbent material due, at least in part, to its favorable moisture uptake, even in low humidity environments (as shown in FIG. 1C), low desorption enthalpies, and proven mechanical stability when deployed as a cyclically loaded SAWH material. Moreover, the PAM-LiCl hydrogel-salt composite 102 can exhibit rigorously quantified heat and mass transport properties in all types of environments.

[0032] As shown in FIG. 1A, the device 100 can include a solar absorber stage 104 positioned adjacent to the hydrogel 102 and a cover 106. The cover 106 can be made of glass, and is discussed as such through the present disclosure, though use of other transparent and semi-transparent materials is possible in some embodiments. A condenser 108 can be positioned below the hydrogel 102 to permit air flow that is absorbed by the hydrogel 102. The device 100 can operate on a daily cycle, which includes an absorption phase 110, during which the hydrogel 102 is exposed to nighttime ambient air and captures moisture therefrom, and a desorption phase 112 during which the device 100 can be sealed from ambient air and daytime incident solar irradiation is converted to thermal energy for hydrogel desorption. Heating of the hydrogel 102 can cause condensate to form on the condenser 108, which can be harvested, e.g., in a vessel 109. A detailed discussion of the interaction of the various components of the device 100 is included below with respect to FIG. 2A.

[0033] Heat and mass transport optimization of the device 100 can include consideration for thermal efficiency of this day-night water production cycle but can focus primarily on daily productivity as the performance metric for maximization. For passive, solar-driven SAWH devices, daily productivity, often denoted LMD, can quantify the amount of water a device produces as liters of water per square meter of solar absorber per day, which is the metric that the device of the present embodiments optimizes and improves. Specifically, the device can achieve record device efficiency and water productivity in a scalable, cost-Atty. Dkt. No.: MIT 26185 PCT | 88212-427689 effective form factor, such as a device productivity of about 1.7 L / m2and a thermal efficiency of about 16%.

[0034] FIG. IB illustrates an isotherm for the hydrogel-salt composite 102 at about 25 °C, which can be quantified using direct vapor sorption (DV S) testing, and used to model sorbent equilibrium uptake based, for example, on ambient nighttime humidity. The inset (i) of FIG. IB illustrates the hydrogel-salt composite 102 on the microscale, detailing the crosslinked architecture of the salt-loaded hydrogel composite material. As shown, the hydrogel 102 can include a plurality of polymer chains 113 attached via crosslinks 115 with salt ions 117 interspersed therebetween. The composition of the hydrogel 102 of the present embodiments can be synthesized as follows, with the appreciation that this composition is merely exemplary:

[0035] Synthesis ofPAM-LiCl Hydrogels: The hydrogel-salt composite 102 can be synthesized via radical polymerization of acrylamide monomers (AM, Sigma-Aldrich) in an aqueous solution of lithium chloride (LiCl, Sigma- Aldrich). The hydrogel 102 was fabricated to achieve approximately a 4 g salt per g polymer hydrogel at about 20% RH. First, approximately 173.36 g of LiCl was dissolved in approximately 477.8 ml of deionized water (DI H2O. Sigma- Aldrich) and stirred the solution until cool, approximately 1 hour. Then, approximately 44.68 g AM was added and stirred for an additional approximately 15 minutes. Next, approximately 0.1518 g ammonium persulfate (APS, Sigma- Aldrich), approximately 0.267 g N,N’ - methylenebisacrylamide (MBA, Sigma- Aldrich), and approximately 128.3 pL tetramethylethylenediamine (TEMED. Sigma- Aldrich) was added to the solution, stirred for approximately one (1) minute, and the solution was poured directly into the hydrogel stage of the device. The hydrogel-salt composite can be polymerized rapidly but was allowed approximately six (6) hours to set prior to use to ensure equilibrium. The table below summarizes the ingredients and their approximate quantities:Table 1 . Ingredients and their approximate quantitiesIngredient Quantity [g]De-ionized water 477.800Lithium chloride (LiCl) 173.360Acrylamide (AM) 44.680Ammonium Persulfate (APS) 0. 152N,N’-methylenebisacrylamide (MBA) 0.267Tetramethylethvlenediamine (TEMED) 0.128Atty. Dkt. No.: MIT 26185 PCT | 88212-427689

[0036] Dynamic Vapor Sorption (DVS) Experiments: The sorption isotherm of our PAM- LiCl hydrogel-salt composite was characterized using the dynamic vapor sorption vacuum instrument (Surface Measurement Systems Ltd.). To do so, an approximately 30 g sample was dried at approximately 80 °C for approximately six (6) hours, followed by thermal equilibration at approximately 25 °C for approximately four (4) hours. Then, the mass rate of change of the sample was recorded as a function of time for a series of approximately 5 % RH steps, approximately ranging from about 0% to about 90%. by controlling the vapor pressure of water vapor at approximately 25 °C. The sample was allowed to equilibrate at each humidity step, where the continuation criterion was set to a sample mass change less than approximately 0.002 wt % / min to ensure isotherm precision. Finally, the measurements halted at approximately 90 % RH for a set approximately 90 minutes, with a maximum testing duration of approximately 10,000 minutes.

[0037] Fabrication of Device: Commercial black spray paint was applied directly to an aluminum stage and used as the solar absorber for our final device. Aluminum was used as the structural interfacial material between paint and hydrogel rather than a more conductive material, such as copper, due, at least in part, to the low cost of bulk aluminum. Acrylic was used as the vapor barrier for desorption, preventing leakages while confining vapor transport to between the hydrogel and condenser surface only, and commercial polyisocyanurate board foam (Lowes) was used as a thermal barrier to minimize heat leakage from absorber to ambient. For successful vapor confinement, dynamic sealing surfaces were designed perpendicular, or substantially perpendicular, to the direction of device clamping. This can incite a maximal sealing pressure and thus can inhibit leakage from vapor gap to ambient. Food-grade EPDM rubber was used as the gasket material at the dynamic seal to prevent vapor leakage while minimizing potential contamination. Additionally, all material interfaces can be permanently sealed with, for example, either a silicone-based sealant (GE Advanced Silicone Caulk) or water-resistant adhesive (J-B Weld MarineWeld), depending, at least in part, on a free or load bearing interfacial surface, respectively. An aluminum heat sink with an area ratio Ar= 5 was machined to fit into the aluminum housing of the device (Tyenaza, Amazon.com) and interfaced with the backside of the condenser surface using Super Lube 98003 Silicone Heat Sink Compound. To minimize the potential impact of variable ambient air flow speeds on device performance, five (5) de fans were arranged in a checkerboard pattern on the back of the device, providing ~ 0.5 m / s airflow to the condenser backing regardless of ambient wind. Each fan was coupled to a 5 Vdc PV cell to maintain the solar-Atty. Dkt. No.: MIT 26185 PCT | 88212-427689 driven nature of the device. It will be appreciated that wind speeds during the testing presented in this work exceeded flow speeds supplied by the fans (» 0.5 m / s) and so the inclusion of active cooling is negligible for environmental testing presented in this work. During absorption all mounting bolts are removed and the top hydrogel stage is disconnected from the body of the device. The hydrogel stage is then manually placed in an open air environment, leveraging ambient airflow for absorption during the night. When transitioning to desorption, the hydrogel stage can be re-installed onto the top of the device and can be mounted to the body using, for example, M3 screws and threaded bolts.

[0038] While a PAM-LiCl hydrogel sorbent is discussed herein, some additional nonlimiting examples can include polyacrylamide (PAM), polyvinyl alcohol (PVA). calcium chloride (CaCh), lithium bromide (LiBr), metal organic frameworks, and / or zeolites. For example, during w ater absorption, the desiccant or sorbent can absorb w ater vapor, or adsorb in the event that LiCl is used, from the surrounding air, either passively or actively.

[0039] FIG. 2A illustrates the device 100 in greater detail while also highlighting the energy and mass flows through the device. As discussed above, the device 100 can include (1) a glass cover 106, which can transmit solar irradiation while eliminating convective cooling of the solar absorber stage 104; (2) the absorber stage 104, e.g., black adsorber stage, which can convert incident solar flux into heat and conducts thermal energy to the sorbent; (3) the hydrogel-salt composite 102, which can act as a sorbent material; (4) the condenser stage 108, which can capture w ater vapor from the hydrogel and output it as liquid water during the desorption phase 112 of operation; and (5) thermal insulation 111. which can reduce heat loss to ambient air. Sufficient insulation on all sides of the device 100 can be assumed for purposes of this model such that approximate heat transfer as a ID timedependent energy7flow from glass 106 to the condenser 108 parallel to the direction of vapor flow, as shown. In some embodiments, the device 100 can include two air-vapor gaps 114, 116, which can be considered in the optimization scheme. The top air gap 114 can separate the glass cover 106 from the solar absorber surface and can eliminate natural convection, providing an insulating layer of stagnant air. Additionally, the bottom air gap 116 between the hydrogel 102 and the condenser surface 108 can be a critical length scale for optimization and be designed to maximize device productivity.

[0040] Using this heat and mass transport model for the SAWH device 100, the relationship between design variables and system performance can be quantified. Moreover,Atty. Dkt. No.: MIT 26185 PCT | 88212-427689 the model of FIG. 2A can elucidate the significance of several design parameters which can significantly impact heat and mass transfer in the device. FIGS. 2B-2F can illustrate the results of the thermofluidic optimization of the present embodiments, including impact of solar absorber emmisivity and glass transmission, impact of condenser structure and ambient heat transfer coefficient for both glass cover 106 and no glass cover designs, impact of environmental temperature and relative humidity, impact of the vapor gap length and hydrogel thickness on device productivity, and variability of both productivity and thermal efficiency on incident solar flux for fixed hydrogel thicknesses, respectively.

[0041] For example, due to the radiative energy input for desorption from the sun, both absorber emissivity and glass transmission can be maximized to produce the highest energy flux and thus daily water production, as shown in FIG. 2B. With respect to FIG. 2C, a glass cover and finned condenser structure can both significantly improve device productivity compared to a design without a cover or one with a flat plate condenser, regardless of heat transfer rates to ambient. Specifically, FIG. 2C illustrates the impact of finned condenser structure and ambient heat transfer coefficient for both glass cover (dashed) and no glass cover (dotted) designs. As shown in FIG. 2B, the productivity of the device 100 can increase as the transmissivity of the glass increases due to higher solar heating. Similarly, as the emissivity of the absorber increases from 0.2, which corresponds to a poor solar absorber, to 0.99. which corresponds to a close-to-ideal absorber, the productivity of the device 100 can increase. It will be appreciated that, as shown in FIG. 2C, heat rejection from the condenser can be increased through a higher ambient convection coefficient and condenser surface area.

[0042] FIG. 2D illustrates the impact of environmental temperature and relative humidity on device productivity, with higher humidity leading to more moisture captured during absorption relative to drier environments. Interestingly, despite the slightly negative impact of elevated temperature on equilibrium absorption of sorbent materials and specifically of the PAM-LiCl composite 102. an increased ambient temperature can improve device productivity at higher relative humidities.

[0043] The system can be optimized for material properties of each of items ( 1 )-(5) discussed above. For example, one or more of such factors as ambient temperature, ambient air flow, assumptions on solar input, breeze data, and so forth can be accounted for during optimization. With these values, one or more parameters of the system can be adjusted. In some embodiments, a material of the condenser stage 108 and / or the absorber stage 104 canAtty. Dkt. No.: MIT 26185 PCT | 88212-427689 be changed to one or more of aluminum, copper, or the like to account for increased needs of heat retention of the system, for example. Additionally, or alternatively, a composition of the hydrogel 102 can be adjusted, e.g., increased or decreased thickness, altered consistency, and so forth, to account for the needs of the system.

[0044] The device 100 of the present embodiments may not operate in equilibrium sorption conditions, as there is a fixed absorption timeframe dictated by the sun. Additionally, an elevated temperature with a fixed relative humidity means that the environment has an increased amount of moisture per unit volume of air (absolute humidity ). Therefore, both chemical activity aw sand saturation pressure psatcan increase with temperature and result in faster kinetics for absorption relative to a lower temperature case. For a fixed timeframe and high relative humidity conditions, where equilibrium sorption is not achieved, this can equate to more moisture capture in the hydrogel-salt composite 102 during absorption. However, the difference in temperature between the hydrogel 102 and condenser surface 108 may be reduced due to. for example, ambient heating of the condenser at elevated temperatures. Therefore, the hydrogel-salt composite 102 can perform notably worse at higher ambient temperatures for low RH conditions due to, at least in part, the hot condenser, coupled with the minimal increase of moisture absorption, leading to low mass transfer rates in the system.

[0045] FIG. 2E illustrates impact of the vapor gap 114, 116 length and thickness of the hydrogel 102 on productivity' while FIG. 2F illustrates variability7of both productivity' and thermal efficiency on incident solar flux for fixed hydrogel thicknesses. For example, for gap lengths less than about 40 millimeters, heat transfer from the surface of the hydrogel 102 to the condenser 108 through the vapor gap 116 can become a significant heat loss pathway in the device 100. This may also reduce the density gradient driving vapor transport, yielding less water output overall, as shown in FIG. 2E. For gap lengths less than about 7 millimeters, e.g., the top air gap 114, the device 100 may not reach the critical Rayleigh number needed for thermobuoyancy, and so heat may be conducted rather than convected through the vapor gap. Conduction may further increase the rate of heat loss from the hydrogel surface compared to that of natural convection, decreasing device performance. However, at vapor gap lengths at or above about 40 millimeters, hydrogel heat loss due to internal natural convection can become less significant relative to mass transfer in the gap due to, at least in part, an inverse dependency of / tconv gon vapor gap length. Balancing compactness with performance, the device 100 of the present embodiments may include a vapor gap length ofAtty. Dkt. No.: MIT 26185 PCT | 88212-427689 about 40 millimeters, e.g., the bottom air gap 116, which may minimize hydrogel heat loss while maintaining a small device form factor. Other parameters, such as the thickness of the housing, absorber, glass, condenser, and silicone coating, may be minimized to reduce the impact of material choice on conductive heat transfer and energy storage in the device. To avoid heat loss due to natural convection in the insulating air gap, a gap thickness of the top air gap 114 can be chosen to be Lc= about 5 millimeters, though in some embodiments the gap thickness can be approximately in a range of about 1 millimeter to about 20 millimeters, or approximately in a range of about 5 millimeters to about 10 millimeters. For the bottom air gap 116, a gap thickness can be chosen to be Lg= about 40 millimeters, though in some embodiments the gap thickness can be approximately in a range of about 5 millimeters to about 100 millimeters, or approximately in a range of about 30 millimeters to about 60 millimeters.

[0046] Performance of the device 100 can be highly sensitive to initial hydrogel thickness Ho. Thick hydrogels have the potential to store more water from ambient during absorption, for example. However, these hydrogels may correspondingly store more energy as sensible heating and increase the resistance to thermal conduction, reducing the rate of desorption from the hydrogel surface. Therefore, there exists a balance between hydrogel thickness and solar irradiance, where the large water uptake of a thicker hydrogel can make it more advantageous than a thinner counterpart at high solar fluxes, as there is sufficient energy to desorb the larger amount of water contained within the hy drogel per m2. For example, for the average incident solar flux experienced in the summertime in Cambridge, MA, i.e. about 600 W / m2, a hydrogel thickness Ho= about 4 mm may be optimal with respect to both device productivity and thermal efficiency.

[0047] Validity of the design can be shown by fabricating and testing an atmospheric water harvesting device in a variety of extreme environmental conditions. For example, FIGS. 3A- 3C illustrate testing conditions and results in an urban area (Cambridge, MA), and FIGS. 4A- 4D illustrate testing conditions and results in a rural, arid area (Atacama Desert outside Antofagasta, Chile, for example). FIG. 3 A illustrates the device 100 setup and FIG. 3B illustrates incident solar flux and water output data over an approximately nine (9)-hour testing period, showing time-dependent water output from the device 100. Measured temperature data on the absorber 104 (a, A), glass cover 106 (b, B), and condenser 108 (c, C)Atty. Dkt. No.: MIT 26185 PCT | 88212-427689 show good agreement between the thermofluidic model and experimental results, respectively, and ambient experimental results (D) which are graphed in FIG. 3C.

[0048] For testing performed in the arid environment of Chile, the hydrogel stage was placed outside overnight, exposed to desert conditions, with an average temperature of about 11 °C and a relative humidity of about 38 % during the approximately 12-hour absorption phase. FIG. 4A illustrates the device 100 setup and FIG. 4B illustrates humidity, temperature, and solar irradiance in the absorption and desorption phases of testing. As shown in FIG. 4B, the solar flux averaged 517 W / m2during the 8-h desorption test, while during desorption, the ambient humidity decreased from ~38% RH to ~I5% RH, while the temperature increased from <5 °C to ~28 °C, reflecting the extreme conditions and temperature changes of the desert. The solar absorber reached a temperature of ~70 °C, reflecting the effective conversion of sunlight to heat and the thermal isolation due to the air gap between the glass and absorber. This role of the air gap can be further observed by the considerably lower temperature of the glass cover relative to the absorber. The condenser operated at slightly higher-than-ambient temperatures, as expected by the heat released by condensation and the heat transfer from the hot hydrogel-salt composite. A temperature difference of ~35 °C was achieved between condenser and absorber, which drove desorption even in an extremely dry environment due to a reduced ratio of vapor concentration at the hydrogel temperature to the vapor concentration of the surrounding environment, which is lower than the absorption RH, as shown in FIG. 4C. In particular, FIG. 4C is a graph illustrating predictions of the observed temperatures in all components of the device 100, taking into account the time- varying weather conditions, showing strong agreement between measured (experimental) and predicted (model) system temperatures for each component in field testing. Moreover, the w ater output comparison of the experimental and model results of the present embodiments, as shown in FIG. 4D, further reinforces the validity of the model values of the present embodiments as an optimization framework and as a performance prediction tool. For example, after testing, a water productivity of about 0.62 L / m2 / day with athermal efficiency of 9.3% can be achieved during testing.

[0049] While the system can efficiently use the thermal energy conducted through the absorber plate to harvest water from the hygroscopic hydrogel, it does not sequester a substantial amount of the original incident solar flux. Motivated in part by maintaining a low device cost, some energy loss can be intrinsic to the choice of materials, such as theAtty. Dkt. No.: MIT 26185 PCT | 88212-427689 borosilicate glass (rglass= 0.9) and black paint absorber (eads= 0.95). In some embodiments, the atmospheric water harvesting device can achieve about 1.7 L / m2 / day water output with a thermal efficiency of over about 16% in arid climates.

[0050] Examples of the above-described embodiments can include the following:1. A sorbent-based atmospheric water harvesting (SAWH) device, comprising: a hydrogel-salt composite formed from an acry lamide monomer polymerized in an aqueous solution of lithium chloride and de-ionized water; an absorber stage configured to convert incident solar flux into heat; a condenser configured to capture water vapor from the hydrogel and output it as liquid water during desorption; a cover; and a thermal insulation disposed around the hydrogel-salt composite, the absorber stage, the condenser, and the cover.2. The device of example 1, wherein the device is configured to operate passively.3. The device of example 1 or example 2, wherein the absorber stage is positioned adjacent to the hydrogel-salt composite.4. The device of any of examples 1 to 3, wherein the condenser is positioned below the hydrogel-salt composite.5. The device of any of examples 1 to 4, further comprising a plurality7of air-vapor gaps, with at least a first air-vapor gap of the plurality of air-vapor gaps being positioned above the hydrogel-salt composite and at least a second air-vapor gap of the plurality7of air-vapor gaps being positioned below the hydrogel-salt composite.6. The device of example 5, wherein a first of the plurality of air-vapor gaps separates the cover from a surface of the absorber stage.7. The device of example 6, wherein a length of the first of the plurality of air-vapor gaps is about 40 millimeters.Atty. Dkt. No.: MIT 26185 PCT | 88212-4276898. The device of example 6 or example 7, wherein a length of the first of the plurality of air-vapor gaps is approximately in a range of about 1 millimeter to about 10 millimeters.9. The device of any of examples 5 to 8, wherein a second of the plurality of air-vapor gaps separates the condenser from a surface of the hydrogel-salt composite.10. The device of example 9, wherein a length of the second of the plurality of air-vapor gaps is about 40 millimeters.11. The device of example 9 or example 10, wherein a length of the second of the plurality of air- vapor gaps is approximately in a range of about 5 millimeters to about 100 millimeters.12. The device of any of examples 1 to 11, wherein the atmospheric water harvesting device is configured to achieve at least about 1.7 L / m2 / day water output with a thermal efficiency of greater than about 16% in arid climates.13. A method of atmospheric water harvesting, comprising: absorbing moisture from a condenser surface using a hydrogel formed from an acrylamide monomer polymerized in an aqueous solution of lithium chloride; desorbing the hydrogel to cause condensate to form on the condenser surface; and harvesting the condensate.14. The method of example 13, wherein harvesting occurs on a daily cycle, in which absorption occurs when the hydrogel is exposed to nighttime ambient air, and desorption occurs during daytime.15. The method of example 13 or example 14. wherein the hydrogel is synthesized via radical polymerization of acrylamide monomers.16. The method of any of examples 13 to 15, wherein desorbing the hydrogel further comprises heating the hydrogel.17. The method of example 16, wherein the hydrogel is heated by solar energy.Atty. Dkt. No.: MIT 26185 PCT | 88212-42768918. The method of example 17, wherein the solar energy' comprises incident solar irradiation that is converted to thermal energy for hydrogel desorption.19. The method of any of examples 13 to 18, wherein the moisture on the condenser surface is formed by exposing the condenser surface to air flow.20. The method of any of examples 13 to 19, wherein the hydrogel is disposed in the device of any of examples 1 to 12.

[0051] One skilled in the art will appreciate further features and advantages of the disclosures based on the provided for descriptions and embodiments. Accordingly, the inventions are not to be limited by what has been particularly shown and described. To the extent the present disclosure includes illustrations and descriptions that include prototypes, bench models, or schematic illustrations of set-ups, a person skilled in the art will recognize how to rely upon the present disclosure to integrate the techniques, systems, composition, designs, and methods provided for into a product and / or production method. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0052] Some non-limiting claims that are supported by the contents of the present disclosure are provided below.

Claims

Atty. Dkt. No.: MIT 26185 PCT | 88212-427689We claim:

1. A sorbent-based atmospheric water harvesting (SAWH) device, comprising: a hydrogel-salt composite formed from an acrylamide monomer polymerized in an aqueous solution of lithium chlonde and de-ionized water; an absorber stage configured to convert incident solar flux into heat; a condenser configured to capture water vapor from the hydrogel and output it as liquid water during desorption; a cover; and a thermal insulation disposed around the hydrogel-salt composite, the absorber stage, the condenser, and the cover.

2. The device of claim 1, wherein the device is configured to operate passively.

3. The device of claim 1, wherein the absorber stage is positioned adjacent to the hydrogel-salt composite.

4. The device of claim 1, wherein the condenser is positioned below the hydrogel-salt composite.

5. The device of claim 1, further comprising a plurality of air-vapor gaps, with at least a first air-vapor gap of the plurality of air-vapor gaps being positioned above the hydrogel-salt composite and at least a second air-vapor gap of the plurality of air-vapor gaps being positioned below the hydrogel-salt composite.

6. The device of claim 5, wherein a first of the plurality of air-vapor gaps separates the cover from a surface of the absorber stage.

7. The device of claim 6, wherein a length of the first of the plurality' of air-vapor gaps is about 40 millimeters.

8. The device of claim 6, wherein a length of the first of the plurality of air-vapor gaps is approximately in a range of about 1 millimeter to about 10 millimeters.

9. The device of claim 5, wherein a second of the plurality of air-vapor gaps separates the condenser from a surface of the hydrogel-salt composite.Atty. Dkt. No.: MIT 26185 PCT | 88212-42768910. The device of claim 9, wherein a length of the second of the plurality of air-vapor gaps is about 40 millimeters.

11. The device of claim 9, wherein a length of the second of the plurality of air-vapor gaps is approximately in a range of about 5 millimeters to about 100 millimeters.

12. The device of claim 1, wherein the atmospheric water harvesting device is configured to achieve at least about 1.7 L / m2 / day water output with a thermal efficiency of greater than about 16% in arid climates.

13. A method of atmospheric water harvesting, comprising: absorbing moisture from a condenser surface using a hydrogel formed from an acrylamide monomer polymerized in an aqueous solution of lithium chloride; desorbing the hydrogel to cause condensate to form on the condenser surface; and harvesting the condensate.

14. The method of claim 13, wherein harvesting occurs on a daily cycle, in which absorption occurs when the hydrogel is exposed to nighttime ambient air, and desorption occurs during daytime.

15. The method of claim 13, wherein the hydrogel is synthesized via radical polymerization of acry lamide monomers.

16. The method of claim 13, wherein desorbing the hydrogel further comprises heating the hydrogel.

17. The method of claim 16, wherein the hydrogel is heated by solar energy'.

18. The method of claim 17, wherein the solar energy comprises incident solar irradiation that is converted to thermal energy for hydrogel desorption.

19. The method of claim 13, wherein the moisture on the condenser surface is formed by exposing the condenser surface to air flow.Atty. Dkt. No.: MIT 26185 PCT | 88212-42768920. The method of claim 13, wherein the hydrogel is disposed in a sorbent-based atmospheric water harvesting (SAWH) device, the device comprising: a hydrogel-salt composite formed from an acrylamide monomer polymerized in an aqueous solution of lithium chloride and de-ionized water; an absorber stage configured to convert incident solar flux into heat; a condenser configured to capture water vapor from the hydrogel and output it as liquid water during desorption; a cover; and a thermal insulation disposed around the hydrogel-salt composite, the absorber stage, the condenser, and the cover..

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