Patent Application
20190322543
The advent of the steam engine was one of the key developments that led to the first Industrial Revolution. Since then, the use of steam has influenced many aspects of modern life. For instance, thermal steam generation and condensation was one of the dominant technologies for seawater desalination before the introduction of reverse osmosis technologies. Although membrane-based technologies became the dominant solution to desalination, they are usually energetically demanding with serious environmental impacts arising from cleaning and maintenance. As a result, there is emerging global interest in developing alternative desalination technologies to address these issues. Solar vapor generation with no electrical input is proving to be a promising and environmentally benign solution, especially in resource limited areas. However, conventional techniques for generating solar vapor typically rely on costly and cumbersome optical concentration systems to enable bulk heating of a liquid, resulting in relatively low efficiencies (e.g., 30%-40%) due to heat absorption throughout the entire liquid volume that is not directly translated into vapor production. Recently, various advanced and expensive metallic plasmonic and carbon-based nanomaterials have been explored for use in solar vapor/steam generation. However, the vaporization efficiencies of these reported structures are still relatively low under 1 sun illumination (e.g., 48% (10)˜83%).
For practical outdoor solar still applications, stable and continuous solar illumination is not achievable in most areas of this planet due to varying weather conditions. Even with inexpensive moderate solar concentrators, a stable incident power higher than AM 1.5 solar light still cannot be guaranteed. Additionally, since most solar stills are covered by glass or other similar collection material, condensation can lead to optical scattering and a decrease in the incident solar power. Therefore, vapor generation under <1 solar illumination condition is an important, long-felt need, despite being neglected in most previously reported work.
The present disclosure provides an alternative approach to solar vapor generation using a supported substrate. In an extremely cost-efficient and effective embodiment, the substrate is a carbon black-dyed cellulose-polyester blend (CCP) and the support is expanded polystyrene foam (EPS). A system according to some embodiments of the disclosed technology achieved a record thermal conversion efficiency of ˜88% under non-concentrated solar illumination of 1 kW/m2. This corresponds to an optimized vapor generation rate that is ˜3 times greater than that of natural evaporation. Stable and repeated seawater desalination tests were performed in a portable prototype both in the laboratory and an outdoor environment, and achieved a water generation rate that was 2.4 times that of a commercial product. Also, desalination systems according to some embodiments of the present disclosure largely avoid the costs for seawater intake and pretreatment that are generally required for conventional reverse osmosis processes. Compared with previously reported advanced nanostructures, this CP-EPS system is extremely low-cost in terms of both materials and fabrication, environmentally benign, and safe to handle during production. These attributes enable such a system to be easily expanded to a large scale system. Furthermore, embodiments of the present system may be used for simultaneous fresh water generation and treatment from heavily contaminated source water. Membrane filters and photocatalysts may also be incorporated to purify contaminated source water. Considering the challenges in contaminated/waste water treatment and reuse, the development of low cost, electricity-free, and multi-functional technologies represents a significant advance in the field.
In some embodiments, the approach further utilizes cold vapor below room temperature, and provides a near unity conversion efficiency of absorbed solar energy. Due to the energy contribution from the surroundings, the measured total vapor generation is higher than the upper limit that can be produced by a given incident solar energy. Importantly, this breakthrough technique was realized using the extremely low cost CCP-foam system under 1 sun illumination, with no need for advanced and expensive nanomaterials. In addition, features for optically absorbing and evaporative materials for solar still systems are shown: i.e., under a given environment, a stronger natural evaporation capability will result in a lower surface temperature. This provides applications in solar still technology, evaporative cooling and solar evaporated mining applications, evaporation-driven generators and recently reported water-evaporation-induced electricity.
For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:
Unless defined otherwise herein, all technical and scientific terms used in this disclosure have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. The disclosure includes all combinations of all components and steps described herein. Throughout this application, the singular form includes the plural form and vice versa.
By utilizing extremely low-cost materials in this invention, economically viable large-area systems are now possible with no energy input required for operation. This prospect is particularly attractive for addressing global freshwater shortages, especially for individuals to purify water for personal needs (i.e., ˜2 liter/day) in developing regions. Because embodiments of the present disclosure do require special micro/nanofabrication processes and do not require solar concentrators, the disclosed technology is extremely low-cost and amenable to scaling up over large or huge areas for real applications.
Without being bound by any theory, due to the superior absorption, heat conversion, and insulating properties of the presently-disclosed CCP-foam structure, most of the absorbed energy can be used to evaporate surface water with significantly reduced thermal dissipation compared with previously reported architectures. Without being bound by any theory, due to the thermal insulation between the surface liquid and the bulk volume of the water and the suppressed radiative and convective losses from the absorber surface to the adjacent heated vapor, a record solar thermal conversion efficiency of >88% under illumination of 1 kW/m2 (corresponding to the evaporation rate of 1.28 kg/(m2·h)) was realized using an embodiment of the disclosure having no solar concentration. When scaled up to a 100 cm2 array in a portable solar water still system, the outdoor fresh water generation rate was 2.4 times of that of a leading commercial product. Furthermore, seawater desalination was also demonstrated with reusable stable performance.
To enhance the vapor generation rate, typically the approach is to increase the operation temperature for a given solar illumination. However, this will inevitably increase the thermal loss to the surroundings mainly via conduction, convection and radiation losses. Therefore, high temperature solar vapor generation (e.g., with solar concentration) inherently suffers from limits in energy conversion efficiencies.
In some embodiments, present disclosure provides techniques which take an opposite approach, using solar energy to generate cold vapor below room temperature, to provide surprising results. This is a breakthrough pathway for efficient solar vapor generation since under illumination at low power densities, the absorbed-light-to-vapor energy conversion efficiency can reach ˜100% when the evaporation temperature is lower than the room temperature. Under this condition, the environment will provide additional energy for vapor generation, resulting in a total vaporization rate that is higher than the upper limit that can be produced using the input solar energy alone. This cold vapor generation technique was experimentally validated and demonstrated limit-breaking vaporization rates using an extremely low cost CCP-foam system.
With reference to
The system 10 may further comprise a housing 14. The substrate 20 and the support 22 may be located within the housing 14. In some embodiments, at least a portion of the vessel 12 may be located within the housing 14. The housing 14 may be configured so as to admit solar energy. For example, the housing 14 may have a transparent top. For example, the housing 14, or a portion thereof, may be made from a transparent plastic, a transparent glass, a transparent polymer membrane (e.g., microwave membrane), etc. In some embodiments, an interior surface of the cover is coated with a non-toxic, anti-mist super-hydrophobic surface treatment.
The system 10 may further comprise an air mover 30 configured to cause air (e.g., ambient air) to move adjacent to the substrate 20. The air mover 30 may be an electrically-powered fan 30, which may be powered by way of, for example, a solar cell 32.
In some embodiments, a temperature of the substrate 20 is maintained substantially at or below an ambient temperature. For example, in embodiments having a housing 14, the housing may be a temperature-controlled housing 14 for maintaining an ambient temperature above the temperature of the substrate 20. By maintaining a temperature substantially at an ambient temperature, it is intended that the temperature of the substrate be maintained to within 5° C. of the ambient temperature. In some embodiments, substantially at the ambient temperature means to maintain the temperature to within 1, 2, 3, or 4° C. or any other value therebetween to within a decimal position. In some embodiments, the substrate is maintained at a temperature below the ambient temperature.
In some embodiments, the system 10 is used as a solar still. For example, in such embodiments, the system 10 may be used to desalinate water for use as drinking water. In such embodiments, the system 10 may further comprise a condenser for condensing the generated vapor. For example, the housing 14 may be configured such that vapor condenses on the housing 14 (i.e., an inner surface of the housing) for recovery of the condensate. In other embodiments, a condenser, such as a condensation trap, may be located within the housing or outside of the housing.
As will be further described below under the heading “Further Discussion,” the substrate 20 may be configured as a planar sheet generally parallel to a top surface of the solution. In another embodiment, the substrate is tent-shaped, comprising two planar sheets connected to one another along an adjoining edge. The two planar sheets of a tent-shaped substrate may connect at any angle, for example, at an angle of between 1.0 and 180.0 degrees, all values and ranges therebetween to the first decimal place (tenths). In some embodiments, the two planar sheets connect at an angle of between 20.0 and 45.0 degrees, inclusive and all values and ranges therebetween to the first decimal place (tenths).
The substrate may be a porous material, such as, for example, a fabric. The substrate may comprise paper and/or plastic, for example, a porous fabric material comprising paper and/or plastic. In some embodiments, the substrate is a hydroentangled, non-woven 55% cellulose/45% polyester blend, such as TechniCloth™ Wiper TX609, available from Texwipe. The word “paper” does not signify, expressly or implicitly, any equivalence between the “paper” used in some embodiments of the subject disclosure and alternative paper material including any prior substrate which may have been called “paper,” but which may have a different or unknown composition or arrangement of fibers. The material may comprise material or material(s) suitable for the purposes of the present substrate as will be apparent in light of the present disclosure.
In some embodiments, the substrate comprises a cellulose/polyester blend. The blend may comprise about 35% to about 75% cellulose, including all integers and ranges therebetween, and about 45% to about 65% polyester, including all integers and ranges therebetween. In an embodiment, the blend may comprise about 55% cellulose and about 45% polyester. In another embodiment, the substrate may consist essentially of cellulose, while in a different embodiments, the substrate does not consist essentially of cellulose.
In some embodiments, the substrate is made from non-woven fibers. In other embodiments, the substrate is made from woven fibers (e.g., yarns). In other embodiments, the substrate is a composite material. For example, the substrate may be made from one or more non-woven layers and/or one or more woven layers. In another example of a composite, the substrate may be made from more than one layer, each layer made from the same or different materials. Plastic or paper filter (virgin kraft paper) may also be used as the substrate. In a further embodiment, the substrate does not consist essentially of any one of the following: coral fleece fabric, cotton, wool, nylon, jute cloth, coir mate or polystyrene sponge.
In some embodiments, the substrate has a dark hue au naturale. In some embodiments, the substrate is coated, dyed, or otherwise colored to attain a dark hue. In some embodiments, the substrate is black or substantially black. For example, the substrate may be coated, dyed, or otherwise colored with carbon black. In some embodiments, the carbon black comprises nanoporous carbon black, microporous carbon black, or a mixture thereof. In another embodiment, the carbon black consists essentially of nanoporous carbon black. Selecting carbon black of a particular sized porosity may be helpful in cleaning contaminated water. However, it is not necessary for the distillation of water, in which general purpose black carbon may be used. Other black or dark pigments may also be used to dye or coat the substrate.
In some embodiments, the substrate may have a length of about 8 cm to about 14 cm and all integers and ranges therebetween. The length was determined by the water transportation capability of the substrate. The exemplary length of about 10 cm to about 14 cm was used in an exemplary embodiment for a hydroentangled (non-woven) substrate consisting of about 55% cellulose and about 45% polyester. The width may be greater for more substrates with greater liquid transport potential. The length may be less than 10 cm or greater than 14 cm according to the application at hand.
In some embodiments, the substrate may have a width of about 8 cm to about 14 cm and all integers and ranges therebetween. The width was determined by the water transportation capability of the substrate. The exemplary width of about 8 cm to about 14 cm was used with a hydroentangled (non-woven) substrate consisting of about 55% cellulose and about 45% polyester. The width may be greater for more substrates with greater liquid transport potential. The width may be less than 8 cm or greater than 14 cm according to the application at hand.
In some embodiments, the substrate has the shape of a cross. In some embodiments, the substrate has the shape of a square or rectangle. The substrate may be any shape suitable to the application.
In some embodiments, the substrate is corrugated, in whole or in part (see, e.g.,
In some embodiments, the substrate and its support float at the surface of the solution. For example, the solution may be source water to be distilled. In such embodiments, where the substrate and its support float on the source water, the dimensions of the support and of the substrate may be selected so that the ends of the substrate overlap the edges of the support and contact the source water as shown in
In some embodiments, the support has a length of about 8 to about 10 cm. In some embodiments, the support has a width of about 8 to about 10 cm. The support has a height of about 8 to about 14 cm. The height can be greater for more absorbent substrates or substrates with enhanced liquid transport (wicking) capability. As before, these dimensions were optimized for a hydroentangled (non-woven) substrate consisting of about 55% cellulose and about 45% polyester. The dimensions of the support and of the substrate may be selected so that the ends of the substrate overlap the edges of the support as shown in
A support 122 is disposed within the vessel 112, and a substrate 120 is disposed on the support 122. As described above, the support 122 may be made from any suitable material, such as, for example, EPS foam. Also as described above, the substrate 120 may be made from a suitable wicking material, such as, for example, CCP. Other materials may be used for the support 122 and/or the substrate 120. The some embodiments, the support 122 is configured to float on water contained within the vessel 112. The substrate 120 may be configured to wick water contained within the vessel 112. The system 100 may include a solar concentrator 130—such as, for example, a Fresnel lens—for increasing the solar energy directed towards the substrate 120.
The system 100 further includes a housing 140, which may be in the shape of a cone, a dome, a pyramid, or any other shape suitable to the purpose as is described herein. The housing 140 is arranged to contain the vessel 112 within. In this way, water vapor evaporating from the water in the vessel 112 will condense on an inner surface of the housing 140 and run down the inner surface for collection in a collection container 150. The collection container 150 may be constructed so as to encourage condensation. For example, the collection container 150 may be constructed using a single-layer of material, such as a plastic or metal material. The system 100 may further include an outlet 152 whereby condensate (distillate) may be accessed for further use/storage.
In another embodiment, a system 200 is configured to be used in a body of water 290 (see, e.g.,
In some embodiments, the support includes an air gap 323 between a portion of the substrate 320 and a portion of the support 322 (see, e.g.,
In another aspect, the present disclosure may be embodied as a method 400 for solar vapor generation including placing a solution, such as a water-based solution in an open-topped vessel (see, e.g.,
The substrate may be configured in any way described herein. The substrate may be disposed 403 on the solution using a support, such as a foam support, to float the substrate at or near a top surface of the solution. The substrate is exposed 406 to solar energy thereby causing evaporation of the solvent (e.g., water), or increasing the rate of evaporation of the solvent over the rate at which evaporation would occur without a substrate and/or exposure to solar energy. The method 400 includes maintaining 409 the substrate at a temperature which is below the ambient temperature. The method may include moving air adjacent to the substrate to further increase the rate of evaporation and/or cool the substrate.
Some embodiments include chemically treating the substrate and/or the carbon to be more hydrophilic. In some embodiments, the substrate and/or the carbon is treated with sodium alginate.
As previously mentioned, in some embodiments, the subject invention provides methods and systems for solar distillation of water comprising a substrate on a support. The substrate may be referred to herein as a wick.
The sides, base, distillate channel, and collection container may each independently comprise metal, plastic or wood. The plastic may be acrylic. For the base, plastic or metal are preferred.
Optionally, foam or other material less dense than water may be added to ensure that the system floats (see, e.g.,
In an alternative embodiment, at least an interior surface of the base may angled so that the substrate and its support are angled to face the sun.
Some embodiments of the presently-disclosed techniques are particularly advantageous for use in mining applications, and more particularly, in salt mining applications. Solar salt mining is a common practice to obtain a plethora of different salts ranging from table salt, NaCl, to Lithium-based salts (e.g., Lithium Carbonate, Lithium Hydroxide, Lithium Chloride, etc.), and Sodium/Potassium/Iodine salts for battery, food, and medical applications. While salt processing plants have the ability to process large amounts of raw salt product every year, these plants rarely run at full capacity due to bottlenecks in the production of raw salts from solar evaporation of salt brine. Using embodiments of the present disclosure, the solar evaporation of salt brines can be increased by 3-5× times the natural rate. A low cost carbon nanomaterial based substrate was developed and shown to be >88% efficient at converting solar light into heat (see below under the heading “CCP Discussion and Experimental Details”). This carbon substrate can easily be applied using a roll-to-roll process for extremely feasible scalability and modular systems, allowing the continued use of the existing infrastructure for solar evaporation ponds while providing greatly improved solutions to enhance salt production. To further maintain current infrastructure, the material used may be mechanically stable, thereby allowing the continued use of current collection vehicles to drive over and scoop up the raw salts. In addition to being low cost and scalable, the present carbon-based substrate is chemically inert as to prevent contamination and preserve purity of salt products.
In another aspect suitable for use in mining applications, the present disclosure may be embodied as an apparatus for improved salt separation in an evaporation pond. The apparatus is similar to the above-described system where the open-topped vessel is a pre-existing evaporation pond. As such, the apparatus includes a substrate configured to wick solution from the evaporation pond. The apparatus may include a support, configured to support the substrate at a position near the surface of the solution. A temperature of the substrate is maintained below an ambient temperature. The substrate of such an apparatus may be of any type described herein and may be configured as a planar sheet or a tent-shaped configuration as described herein.
In some embodiments, the substrate is configured in a geometric shape—i.e., having a geometric circumferential shape. In a particular example (illustrated in
The substrate may configured for mechanical separation of the salt. For example, the substrate may be a durable material capable of withstanding mechanical separation (scraping, beating, etc.) As such, the substrate may be reusable, such that once the salts have been removed (substantially removed), the substrate may be used to obtain salts again. In some embodiments, the substrate is washable. Here again, such ability to be washed allows for re-use of the substrate.
While solar salt mining focuses on the evaporation of brine water to collect the salts left behind, embodiments of the present system will also enable reclamation of the evaporated water in a condenser unit. In this way, miners and staff may be provided with a fresh supply of drinking water. This means for no additional energy input, other than the natural solar radiation, raw salt production can be enhanced 3-5× while saving time, money, and other resources associated with providing these often remote mining locations with clean drinking water.
In addition, the CCP structure can also be applied to evaporation enhancement for water having only a low concentration of salt. In such applications, accumulated salt can re-dissolve into the water solution, providing a “self-cleaning” feature and reducing the maintenance required for operation. Additionally,
Additionally, the presently-disclosed process includes the geometric assembly of the substrate. Based on geometry, the carbon substrate can be arranged to induce higher airflow speed which increases evaporation rates, prevents adsorption of salts onto the surface of the substrate and easily transfers salts to different collection containers, which aids in overall collection and ease of use/maintenance. As such, the apparatus for salt separation may include one or more air movers (for example, as shown in
In contrast to water purification applications, solar mining may utilize extra components/devices to accelerate the vapor generation rate. For instance, electricity driven or solar driven fans can be employed in the solar vapor generation for salt mining. According to preliminary experiment results, an air flow from 0.4 to 2 m/s can enhance the vapor generation rate by 1000% (dark environment) ˜15% (under 3× sun illumination). In particular, solar driven fans can be included in each solar evaporator model (
As illustrated in
P=αC
opt
q
i−εσ(T24−T14)−h(T2−T1)−qwater (1)
Here, α is the optical absorption coefficient, Copt is the optical concentration, qi the normal direct solar irradiation (i.e., 1 kW/m2 for 1 sun at AM 1.5), ε the optical emission, σ the Stefan-Boltzmann constant (i.e., 5.67×10−8 W/(m2·K4)), T2 the temperature at the surface of the evaporative material, T1 the temperature of the adjacent environment, h the convection heat transfer coefficient, and qwater the heat flux to the bulk water. This equation describes most major processes (if not all) involved in the evaporation process, i.e., the absorption of light, αCoptqi, the net radiative loss to the surroundings, εσ(T24−Ti4), the convective loss to the ambient, h(T2−T1), and the radiative and conductive loss to the bulk water, qwater. By manipulating the energy distribution among these channels, unique solar vapor generation mechanisms can be realized. For instance, a selective absorber and a bubble wrap cover can be introduced to decrease the infrared thermal radiation (ε) and the convective loss (h) to the surroundings, respectively, to produce 100° C. steam under one sun illumination. However, for high temperature solar vapor generation systems, these losses can only be reduced but not eliminated completely. An important question is what happens when T2≤T1? In this steady case (with a stable surface temperature), the system will actually take energy from the environment and the absorbed solar energy can only be consumed in the liquid-to-vapor phase transition, corresponding to near perfect solar energy conversion. Next, a thermally isolated CCP on foam was employed as a low-cost test bed to analyze the energy balance and heat transfers under both dark and illuminated conditions.
In an exemplary embodiment, a substrate of carbon-coated cellulose and polyester blend (CCP) was fabricated using commercially available materials: paper (Texwipe™ TX609) and carbon powder (Sid Richardson Carbon & Energy Company). In some embodiments, evaporation performance can be further manipulated by engineering features of carbon nanomaterials. For example, the light-absorbing substrate can be enhanced with hydrophilic features. In particular, it may be advantageous to provide a substrate that comprises a black material able to absorb water and sunlight simultaneously and evaporate moisture at a higher rate. To improve these characteristics, the porosity of a carbon nanomaterial may be manipulated in some embodiments. In some embodiments, the substrate and/or the carbon may be chemically treated to increase hydrophilicity. In some embodiments, the substrate and/or the carbon may be treated with sodium alginate.
In an experiment to demonstrate such features, water diffusion height was employed as the figure of merit to evaluate the absorptivity of materials under test (
2 g carbon powder was dispersed into 400 mL water. 8 mL acetic acid was added to make carbon powder easier to attach to fibers. The solution was mixed in a 1000 ml beaker and blended well using an ultrasonic cleaner (Branson Ultrasonics Bransonic® B200) for 5 minutes. Subsequently, the prepared white substrate was put into the mixed solution to vibrate and stir for 3 minutes so that carbon powders can dye the substrate uniformly. After that, the CCP was dried at 80° C. on a heating stage. This procedure was repeated three to four times to realize a desired dark color.
The absorption spectrum using an integration sphere spectroscopy (Thorlabs IS200-4 integrated with Ocean Optics USB2000+, Ocean Optics Jaz, and Avantes AvaSpec-NIR256-1.7TEC for ultraviolet, visible and infrared wavelength range, respectively). By weighting optical absorption spectrum of CCP (the topmost curve in
Solar Vapor Generation
To measure the water evaporation rate, a 150 mL beaker with an inner diameter of 5 cm filled with ˜140 g water was placed under an intensity-tunable solar simulator (Newport 69920), as shown in
Dark Evaporation
Water evaporation is a natural process which occurs under any conditions regardless of solar illumination. As shown in
One can see that the surface temperature of the CCP is ˜14.3±0.2° C. (T2), which is lower than that of the room temperature (i.e., T1=22.3-23.3° C.). This was characterized in a laboratory environment (with the humidity of 16˜25% in winter time at Buffalo, N.Y.) showing that the average evaporation rate in the dark environment was 0.275 kg/(m2·h). Due to natural evaporation, this process will consume 6.78×105 J/(m2·h) energy from the environment (considering the enthalpy of vaporization at 14.3° C.). Therefore, the energy balance and heat transfer diagram under dark environment (or low intensity illumination condition) is different from that in a previously reported solar heating situation. As shown in
In this experiment, a solar simulator (Newport) was employed to illuminate the CCP samples (
To minimize these loss channels, the incident power was reduced to ˜0.2 kW/m2. As shown by the upper panel in
In previously reported solar vapor generation literature, the dark evaporation was usually considered as a background which was subtracted from the total vapor generation to obtain the net solar-induced vapor generation. However, by simply comparing
To interpret this intriguing problem, here the energy balance was analyzed using a “water container” model, as illustrated in
Surpassing the Solar Upper Limit: Reducing the Power Density using Larger Surface Areas
As illustrated in
When the same light was employed to illuminate the triangle samples with larger surface areas (
In describing the present techniques for limit-breaking solar vapor generation rate beyond the input solar energy limit, the theoretical upper limit was estimated as described below.
In this calculation, the solar energy was assumed to transfer solely to the liquid-vapor transition without any other losses. Therefore, the obtained solar vapor generation rate was equal to the solar intensity (J/(m2·h)) divided by the enthalpy of evaporation (J/kg).
The solar intensity was measured by placing the aforementioned S305C thermal sensor perpendicular to the light beam. For triangle structures shown in
The enthalpy of evaporation is temperature dependent. Therefore, an analysis was performed of the temperature distribution on the CCP surface, which was non-uniform (
A hydrophilic porous material, a fiber-rich nonwoven 55% cellulose/45% polyester blend (TechniCloth™ Wiper TX609, available from Texwipe™) was selected for use in a test embodiment. This substrate was chosen for its extremely low cost (i.e., retail price of ˜$1.05/m2), chemical-binder-free make up, and has excellent water transport properties. Its microstructure is shown in
Sample preparation: 0.8 g carbon powder (Sid Richardson Carbon & Energy Co.) was dispersed into a 160 mL water. 3 mL acetic acid was added to make carbon powder easier to attach to fibers. The mixed solution was blended well using an ultrasonic cleaner (Branson Ultrasonics Bransonic™ B200) for 5 minutes. Subsequently, the 2 cm×2 cm white paper (TechniCloth™ Wiper TX609, available from Texwipe™) was put into the mixed solution to vibrate for 3 minutes so that carbon powders can dye the paper uniformly. After that, the CCP was dried at 80° C. on a heating stage. This procedure was repeated three to four times to realize a dark shade (see
As a result of the dying process, the fibers were coated with carbon nanoparticles, as shown in
Stability/durability test: To demonstrate the stability/durability of carbon powder attached on the paper fibers, a CCP sample was cleaned ultrasonically in clean water. The water solution was changed every 30 minutes to visualize the effect of the ultrasonic cleaning. As shown in
To demonstrate the baseline for solar vapor generation performance, a direct comparison was performed under several different conditions as shown in
To measure the water evaporation rate, a 250 mL beaker (open area of the beaker was 35.3 cm2) filled with ˜165 g water was placed under a solar simulator (Newport 69920). The CCP floated on the water surface with or without the EPS foam. The residual water surface was covered by EPS foam to eliminate natural evaporation. Two pieces of Fresnel lens (26 cm×17.8 cm, focal length: 300 mm, OpticLens) were used to concentrate solar light. 1-10 times concentrated solar light was calibrated using a powermeter (PM100D, Thorlabs Inc.) equipped with a thermal sensor (S305C, Thorlabs Inc.) The evaporation weight change was measured by an electronic scale every 10 minutes.
In a dark environment (i.e., at room temperature of 21° C. and humidity of 10%), the water weight loss was 0.44 g/h. Therefore, the average evaporation rate in the dark environment was 0.125 kg/(m2·h), which was subtracted from all subsequent measured evaporation rates to eliminate the effect of natural water evaporation. Under solar illumination using a solar simulator (Newport 69920 with the solar intensity of 1 kW/m2, i.e., AM1.5), the weight loss increased to 1.11 g/h. After that, a 4×4 cm2 white paper and a 4×4 cm2 CCP were placed on top of the water surface, and the weight change increased to 1.16 g/h and 1.48 g/h, respectively. To interpret the weight change difference, a portable thermal imager (FLIR ONE®) was used to characterize the temperature of these samples. The thermal imaging characterization was confirmed by a direct measurement using a thermocouple sensor probe, indicating a reasonable accuracy (i.e., ≤0.4° C. in the 33-35° C. range).
To demonstrate the accuracy of the thermal imaging used in the experiment, two samples (i.e., black Al foil and CCP sample) were placed on a heat plate (Super-Nuova™, HP131725).
To interpret the evaporation rate difference, the IR thermal imager (FLIR ONE, FLIR system) was used to measure the surface temperature of different samples. The vapor and liquid temperatures were also measured by a thermometer equipped with two K-Type thermocouple sensor probes (Signstek 6802 II). One of the probes was placed above the CCP sample and covered by a small piece of white cardboard to eliminate the heating effect of direct illumination (
As shown in
However, this heating effect was not well isolated from the bulk water (i.e., the bulk water was heated to 31.7° C.), resulting in less efficient vapor generation effect. One can see that the water evaporation speed with the CCP was 1.33 times higher than that of pure water under the 1 kW/m2 solar illumination.
Efficient Vapor Generation using Thermally Isolated CCP
A thermal-isolating strategy was employed to confine the heating effect at the top surface for more efficient vapor generation. The finite thickness, large contact area and fluid transport of previously studied porous substrates led to relatively poor thermal insulation performance (e.g., in two previous studies, the thermal conductivities were 0.49 W/(m·K) and 0.426 W/(m·K)). Without being bound by any theory, a strategy was utilized for the test embodiment to make full use of the capillary force of the porous paper to draw fluid up around the support rather than through it, thus minimizing the thermal loss to the bulk fluid below. As shown by the upper panel in
To eliminate water evaporation from other open areas, the surrounding exposed water surface was covered with EPS foam with a square hole for the CCP (
A potential concern for reduced liquid flow cross section would decrease the liquid flow rate to the CCP surface. To characterize this practical upper limit, the liquid transportation capability of the CCP was characterized. The original weight of a CCP sample was measured, and then an edge of the sample was placed into water and the IR imager was used to monitor water flow as the function of time. The 4-cm-long sample was saturated by water in ˜25 seconds after which the weight of the wet-CCP was measured. It was noted that the flow rate was not a constant when the paper was saturated. By considering the small cross-sectional area of the CCP-layer (i.e., ˜0.2 mm×2 cm), the practical upper limit of the CCP sample was well over 1,500 kg/m2/h, which is higher than the theoretical upper limit under 1,000× solar concentration. Therefore, the reduced liquid flow rate was not a limitation in the test system under small to moderate solar concentration.
In most previously reported work, the sample surface was always wet, indicating that the performance was limited by surface temperature only. Therefore, the ultimate performance can be improved by introducing concentrated solar illumination. Thus, the vapor generation performance was analyzed under moderate solar concentration conditions to better compare with previously reported nanostructures. In this experiment, an inexpensive planar PVC Fresnel lens (e.g., OpticLens®, $2.39/piece with the area of 26 cm×17.8 cm) was employed to focus the incident light from the solar simulator. As shown in
To evaluate the solar-vapor generation performance quantitatively, the solar conversion thermal efficiency, ηth, was calculated, using Equation (2):
where {dot over (m)} is the mass flux, hLV is the total enthalpy of liquid-vapor phase change, Copt is the optical concentration, and qi is the normal direct solar irradiation (i.e., 1 kW/m2). Particularly, the calculation of the total enthalpy of liquid-vapor phase change, hLV, should consider both the sensible heat and the temperature-dependent enthalpy of vaporization.
The thermal conversion efficiency, ηth, is widely employed in the literature as an important figure of merit in evaluating the performance of solar vapor generation. However, the detailed values for parameters employed in those literature are slightly different. Therefore, it is necessary to explain the calculation in detail to demonstrate that the presently-obtained ηth was unambiguously higher than previously reported results.
The most frequently used equation for thermal conversion efficiency is
The variable parameter employed in different calculation was the total enthalpy of liquid-vapor phase change, hLV, containing two parts: i.e., the sensible heat and the enthalpy of vaporization (i.e., hLV=C×(T−T0)+Δhvap). In the present experiments, T0 was the initial temperature of water, i.e., 21° C. T was the vapor temperature measured by the thermometer, which was in the range of 40° C. to 90° C. (see data listed in Table 3 below). In this temperature range, the specific heat capacity of water, C, was a constant, i.e., 4.18 J/g·K. However, the enthalpy of vaporization, Δhcap, was highly dependent on the temperature, which was larger at lower temperature. Recent literature employed different values of hLV in their calculation, resulting in certain inaccuracies in the resulting calculated ηth.
For instance, a first paper directly employed a constant Δhvap at 100° C. (2260 kJ/kg) as hLV to calculate ηth. Another paper employed a temperature-dependent enthalpy of vaporization Δhvap as hLV to calculate ηth. These sources did not consider the sensible heat (i.e., C×(T−T0)). In contrast, another paper considered the sensible heat but employed a constant Δhvap at 100° C. (2260 kJ/kg). By considering these two terms more accurately, the solar thermal conversion efficiencies of the presently-disclosed structure under 1, 3, 5, 7, 10 times concentrated solar illumination were calculated in Table 3. Fortunately, the sensible heat (i.e., C×(T−T0)) was much smaller than Δhvap, especially under small solar concentration conditions, as shown by the data listed in Table 3. Therefore, previously reported values under 1 sun illumination are still reliable but may contain up to >10% difference under 10× solar concentration.
Thus, for energy conversion efficiency estimation, the sensible heat should be considered since this energy is actually consumed by the vapor. But if one focuses on vapor generation performance, this term can be neglected since it just results in higher temperature vapor rather than generates more vapor.
In addition, this ηth actually describes the energy consumption in the vapor and has two major components: the energy used for water-to-vapor phase change and the energy used to heat the water/vapor. A larger ηth does not necessarily correspond to a higher vapor generation rate. For a given value of ηth, a higher temperature of the generated vapor will actually result in a lower generation rate since more energy is used to heat the water. Therefore, in terms of solar vapor generation rate, it was beneficial to analyze the theoretical upper limit and thermal loss channels in order to estimate the opportunity available for improvement.
Loss Channels
Recently, a strategy was reported to demonstrate the close to 100° C. steam generation under one sun enabled by a floating structure with “thermal concentration.” A detailed thermal loss analysis was performed, revealing that radiative loss and convective loss were two major thermal loss channels in the solar vapor generation systems. The radiative and the convective losses per area are expressed by Equations (3) and (4), respectively:
P
rad=εσ(T24−T14) (3)
P
con
=h(T2−T1) (4)
where ε is the emissivity of the CCP (i.e., 0.98), σ is the Stefan-Boltzmann constant (i.e., 5.67×10−8 W/(m2·K4)), T2 is the temperature at the surface of the CCP, T1 is the temperature of the adjacent environment, and h is the convection heat transfer coefficient (assumed to be 10 W/(m2·K)). Using these two equations, it was estimated that the radiative loss from the 100° C. blackbody absorber surface to the ambient environment (20° C.) was ˜680 W/m2 and the convective loss was ˜800 W/m2. Following this theoretical estimation, when the absorber surface was 44.2° C. (via experimental observation), the radiative loss to ambient was ˜147 W/m2 and the convective loss was ˜232 W/m2, corresponding to a total of 37.9% energy loss (i.e., 14.7%+23.2%). In this case, it seems that an efficiency ˜90% is impossible. An immediate question is why one can observe a record high vapor generation rate under 1 sun.
To interpret the unique features and physics of the proposed CCP-foam architecture, the thermal environment and heat transfer diagram was analyzed (
In further analysis of the microscopic thermal environment (
More importantly, in a real enclosed solar steam system, the vapor cannot be released immediately and the environment inside the system is thermally isolated from the cooler surrounding environment. Furthermore, typical acrylic or glass slabs are opaque to mid-infrared radiation. Consequently, thermal radiation cannot be emitted to the environment. Additionally, convective energy transfers are also largely suppressed when the internal environment is heated under near-thermal equilibrium conditions. In this case, the radiative and convective losses in a real system should be even more negligible. Intriguingly, in a recent report, the highest temperature of the generated steam was observed in a vapor chamber, demonstrating the accuracy of our physical picture.
Conventional desalination technologies are usually energy demanding (e.g., reverse osmosis membrane technology consumes ˜2 kW·h/m3) with serious environment costs. It was estimated that a minimum energy consumption for active seawater desalination is ˜1 kW·h/m3, excluding prefiltering and intake/outfall pumping. Passive solar desalination technologies, such as that of the present disclosure, are particularly attractive due to the electricity-free operation with minimum negative impacts on the environment.
To characterize the evaporation performance and reusability of our CCP-foam for desalination, salt water was prepared with 3.5 wt % NaCl and the solar water evaporation experiment was performed repeatedly. For each cycle, two CCP-foam samples were put on the surfaces of salt water and pure water, respectively, and illuminated under 1 kW/m2 for one hour. After that, the CCP samples were dried completely and reused for the next cycle. As shown in
After the 1-hour recycling test, a millimeter sized salt crystal was observed on the sample surface (see the first panel in
To investigate this issue, an 8-hour continuous experiment was performed in pure water and salt water in a beaker, respectively. Intriguingly, one can see that the evaporation speeds increased continuously and saturated at the 4th˜5th hour at ˜1.32 kg/(m2·h) and ˜1.42 kg/(m2·h) for salt water and pure water, respectively, as shown by the dots connected by the solid lines in
According to the experimental data shown in
An exemplary desalination solar still system is illustrated in
To overcome this weakness, a 5×5 CCP array (
As shown in
Considering the key components for solar-to-heat conversion employed in previously-reported literature (e.g., metal nanoparticles or nanorods dispersed in water, metal nanoparticles on nanoporous anodic alumina, exfoliated graphite on porous carbon foam, a selective absorber inserted between a polystyrene foam disk and a bubble wrap), the cost of embodiments of the presently-disclosed structure is the low. In
Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the spirit and scope of the present disclosure. Hence, the present disclosure is deemed limited only by the appended claims and the reasonable interpretation thereof.
This application claims priority to U.S. Provisional Application No. 62/428,138, filed on Nov. 30, 2016, now pending, and U.S. Provisional Application No. 62/517,604, filed on Jun. 9, 2017, now pending, the disclosures of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/063993 | 11/30/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62517604 | Jun 2017 | US | |
| 62428138 | Nov 2016 | US |
