Concentration difference power generation system, power generation method, and metamaterial absorber

Abstract

Provided is a concentration difference power generation system 100 which utilizes a concentration difference liquid including a first liquid 22 and a second liquid 42 having a concentration lower than that of the first liquid, in order to provide renewable energy power generation which utilizes sunlight and easily adapts power generation amount to demand. The first liquid is in contact with a metamaterial absorber 10. Upon irradiation with sunlight, the metamaterial absorber accelerates evaporation of a solvent from the first liquid in accordance with energy from the sunlight, and increases the concentration of the solute of the first liquid. The concentration difference power generation system additionally comprises, as needed: a first liquid storage unit 30 and an evaporation pool 20 for the first liquid 22; a second liquid storage unit 40 for supplying the second liquid 42; and a concentration difference power generation unit 50.

Description

Concentration difference power generation system, power generation method, and metamaterial absorber

[0001] The present disclosure relates to a concentration difference power generation system, a power generation method, and a metamaterial absorber.

[0002] Power generation using renewable energy (hereafter referred to as "renewable energy power generation") is becoming more widespread, utilizing sunlight, solar thermal, and wind power in addition to conventional hydroelectric power generation. Among these, solar power generation is seen as promising from the standpoints of carbon footprint and safety, and its adoption is progressing worldwide. However, renewable energy power generation faces a number of challenges. With solar power generation and solar thermal power generation, the conversion efficiency of obtaining electricity from the energy source is an issue. With wind power generation, there is the problem of limited locations suitable for power generation.

[0003] On the other hand, nanostructures with minute structures are also attracting attention due to their diverse functions. Nanostructures can exhibit properties that differ from the physical properties of the same material in its bulk state. In particular, their size and fine structure can alter (control) the physical properties of the bulk state, or they can exhibit properties that are not possible in the bulk state, and they are also called metamaterials. Patent Document 1 discloses that a fluid containing dispersed titanium nitride nanoparticles can promote the evaporation or distillation of a liquid.

[0004] Patent No. 6587170 Patent No. 7325122

[0005] Rajiv Gogna et al., "Is battery storage a good investment opportunity?" January 2021, URL: https: / / www.lcp.com / energy / publications / is-battery-storage-a-good-investment-opportunityThe U.S. Energy Information Administration, "A Case Study of Transmission Limits on Renewables Growth in Texas", July 2023, URL: https: / / www.eia.gov / electricity / markets / quarterly / archive / 2023 / transmission_limits_07_2023.pdfMeng Ye et al., " Charge-Free Mixing Entropy Battery Enabled by Low-Cost Electrode Materials", ACS Omega 2019 4 (7), 11785-11790, DOI: 10.1021 / acsomega.9b00863Mudachathi, R. and Tanaka, T. "Up Scalable Full Colour Plasmonic Pixels with Controllable Hue, Brightness and Saturation", Sci Rep 7, 1199 (2017), DOI: 10.1038 / s41598-017-01266-6Meng-Ju Yu et al., "Plasmon-Enhanced Solar-Driven Hydrogen Evolution Using Titanium Nitride Metasurface Broadband Absorbers", ACS Photonics 8 (11), 3125-3132, DOI: 10.1021 / acsphotonics.1c00927

[0006] The renewable energy sources mentioned above, with the exception of hydroelectric power generation, face the challenge of their hourly power generation (power output) being strongly affected by the natural environment. For example, solar power generation cannot generate electricity at night, resulting in large fluctuations in power output between day and night. Power generation, which is strongly affected by the natural environment, also faces the challenge of matching the temporal fluctuations in power output and energy demand. For these reasons, many renewable energy sources are difficult to use as baseload power sources or as peak power sources to meet demand fluctuations. Even as renewable energy generation is increasingly adopted, there are currently situations where capacity is limited or a portion of the electricity is wasted. According to a report by the UK company Lane Clark & ​​Peacock (Non-Patent Document 1), it is estimated that in 2020, curtailment of wind power, a type of renewable energy generation, in the UK resulted in the waste of more than 3.6 TWh, or more than the annual electricity consumption of one million households. Similar situations have also been reported in the United States (Non-Patent Document 2). The amount of curtailment is increasing year by year.

[0007] One potential solution is to store energy, using grid batteries and pumped storage hydroelectric power plants. However, both grid batteries and pumped storage hydroelectric power plants face challenges such as insufficient installed capacity, high investment requirements, and environmental impacts.

[0008] What is now strongly needed in renewable energy generation using solar power is technology that can absorb fluctuations in power generation and adapt it to electricity demand.

[0009] The present disclosure provides a novel method for solving at least one of the above problems, and contributes to the stabilization and diversification of power supply means, thereby further improving the practicality of renewable energy power generation.

[0010] Metamaterials are optical materials that utilize artificially controlled nanostructures to achieve optical properties that cannot be achieved with materials directly available in nature. These materials can be considered artificial optical materials. Furthermore, the inventors focused on the technique of concentration-difference power generation. Concentration-difference power generation generates electricity by combining two or more liquids with different concentrations (hereinafter referred to as "concentration-difference liquids"). As long as the concentration-difference liquids are available, electricity can be generated at any time. This allows for both converting solar energy during the day into a storable form and generating electricity as needed at a later time. In particular, the inventors conceived the idea that using artificial optical materials to generate concentration-difference liquids would fully utilize the flexibility of concentration-difference power generation, making it an ideal configuration. In other words, if concentration-difference liquids could be efficiently generated using a nanoscale light absorber (hereinafter referred to as a "metamaterial absorber") and sunlight, electricity could be supplied on demand by storing at least one of the concentration-difference liquids. Storing concentration-difference liquids would thus store solar energy.

[0011] That is, in one embodiment of the present disclosure, there is provided a concentration difference power generation system that utilizes a concentration difference liquid including a first liquid that is a solution in which a certain solute is dissolved in a certain solvent, and a second liquid that contains the solvent, and in which the concentration of the solute in the solvent is lower than that of the first liquid, wherein the first liquid is in contact with a metamaterial absorber, and when irradiated with sunlight, the metamaterial absorber promotes evaporation of the solvent from the first liquid in response to energy from the sunlight, thereby increasing the concentration of the solute in the first liquid.

[0012] Furthermore, in one embodiment of the present disclosure, there is provided a concentration difference power generation method that utilizes a concentration difference liquid containing a first liquid that is a solution in which a certain solute is dissolved in a certain solvent, and a second liquid that contains the solvent and has a lower concentration of the solute in the solvent than the first liquid, the power generation method including the steps of bringing a metamaterial absorber into contact with the first liquid, irradiating the metamaterial absorber in contact with the first liquid with sunlight, thereby promoting evaporation of the solvent from the first liquid in accordance with energy from the sunlight and increasing the concentration of the solute in the first liquid, storing the first liquid with the increased solute concentration, and performing concentration difference power generation using the stored first liquid with the increased solute concentration and the second liquid as the concentration difference liquid.

[0013] Although it is known that evaporation or distillation of liquids can be promoted by using fine particles such as titanium nitride nanoparticles (Patent Document 1), it is unclear what kind of metamaterial absorber is suitable for evaporation or distillation of liquids. Therefore, the inventors have created a metamaterial absorber suitable for generating liquids with a concentration difference.

[0014] In addition, in one embodiment of the present disclosure, there is provided a metamaterial absorber comprising a pair of conductor portions and a dielectric portion sandwiched between the pair of conductor portions, which is adapted to resonate with light of at least one wavelength within the range of the sunlight spectrum in an environment filled with a solution having a solute dissolved in seawater and a solvent of water, and which exhibits the property of absorbing light of the at least one wavelength from sunlight irradiated in the environment and converting it into heat.

[0015] In this disclosure, "liquids with different concentrations" refers to two or more liquids with different concentrations. For example, highly concentrated saltwater and fresh water can be examples of such liquids. This highly concentrated saltwater can also be concentrated seawater. In this disclosure, the two or more liquids that constitute the liquids with different concentrations are distinguished as a first liquid and a second liquid as necessary. When referring to liquids with different concentrations, or as the first or second liquid, the solvents and solutes of these liquids are not particularly limited. However, the second liquid is intended to be a liquid in which the concentration of the same solute as the first liquid in the same solvent as the first liquid is lower than that of the first liquid. Further details are described in the detailed description of the invention below.

[0016] According to any of the embodiments of the present disclosure, the amount of power generated in renewable energy power generation using sunlight can be matched to demand.

[0017] FIG. 1 is a block diagram illustrating an exemplary configuration during operation of an active differential power generation system according to an embodiment of the present disclosure. FIG. 2 is a perspective view illustrating a metal-insulator-metal structure (hereinafter, MIM structure), which is a typical example of a metamaterial absorber according to an embodiment of the present disclosure. FIG. 3 is a graph of measured absorption and transmission spectra of an MIM structure in which a dielectric is sandwiched between two aluminum thin films. FIGS. 4A to 4C are perspective views illustrating core-shell metamaterial absorbers according to an embodiment of the present disclosure. These metamaterial absorbers include a metamaterial absorber in which a conductor portion is provided in the form of a shell on the surface of a spherical dielectric portion, which serves as a core, and an annular slit is provided in the conductor portion so as to trace a great circle ( FIG. 4A ), a metamaterial absorber in which the slit traces part of the great circle of the spherical dielectric portion ( FIG. 4B ), and a metamaterial absorber in which the slit traces two great circles formed by mutually orthogonal planes and the center of the dielectric portion ( FIG. 4C ).

[0018] Hereinafter, a concentration gradient power generation system according to an embodiment of the present disclosure will be described. In the description, common parts or elements are designated by common reference numerals unless otherwise specified. Furthermore, in the drawings, elements of each embodiment are not necessarily shown to scale.

[0019] 1. Concentration Gradient Power Generation The inventors have been working on technological development in the field of metamaterials, and have focused on concentration gradient power generation technology, which generates electricity by utilizing the difference in concentration of a solution, such as the difference in salt concentration between saltwater and freshwater. The advantages of concentration gradient power generation are its low environmental impact, as no carbon dioxide is emitted during the power generation process and no radioactive waste is produced, as well as its flexibility, as there are no particular restrictions on when power can be generated using stored liquid.

[0020] Concentration gradient power generation is a technology that generates electricity by utilizing the concentration difference between two or more liquids with different concentrations, i.e., a concentration gradient liquid. Here, a concentration gradient liquid refers to two or more liquids with different concentrations, i.e., multiple solutions with different concentrations of a solute. If the solvent is water and the solute is salt, a combination of saltwater with different concentrations or a combination of saltwater and fresh water could be a concentration gradient liquid. In this context of two or more liquids with different concentrations or multiple solutions with different solute concentrations, a solution containing only a solvent without a solute could also be considered a concentration gradient liquid. A combination of saltwater and freshwater, such as seawater and freshwater, is a typical concentration gradient liquid. The concentration gradient power generation method employed in this disclosure is optional. In this disclosure, a metamaterial absorber is used as a pre-processing step for such concentration gradient power generation to efficiently generate a concentration gradient liquid. A highly concentrated solution is generated by evaporating the solvent from the solution, and a concentration gradient liquid is generated that uses that solution as the first liquid. Generating a liquid with a different concentration means generating at least the first liquid among the liquids with a different concentration that can be combined with the first liquid to generate electricity due to concentration difference. In other words, if the second liquid is fresh water or freshwater, for example, and the environment allows for easy preparation of an amount of fresh water or freshwater sufficient to perform active differential power generation, generating a liquid with a different concentration can be achieved by generating only the first liquid.

[0021] A concentration gradient power generation system according to an embodiment of the present disclosure will be specifically described with reference to Fig. 1, taking as an example a configuration in which the concentration gradient liquid is a combination of saltwaters with different concentrations (including a combination of saltwater and freshwater). Fig. 1 is a block diagram showing an example configuration of a concentration gradient power generation system 100 according to this embodiment during operation.

[0022] The concentration difference power generation system 100 includes an evaporation pool 20 in which a first liquid 22 containing a metamaterial absorber 10 is placed, a first liquid storage unit 30 that stores the first liquid 22, a second liquid storage unit 40 for supplying a second liquid 42, and a concentration difference power generation unit 50. The first liquid 22 is, for example, saltwater such as seawater, and the metamaterial absorber 10 is mixed therein. The evaporation pool 20 is disposed in a position that receives sunlight, for example, a position that is open to the atmosphere. When sunlight is irradiated onto the metamaterial absorber 10 in the first liquid 22, the solvent evaporates from the first liquid 22 in contact with the metamaterial absorber 10, increasing the concentration of the solute (e.g., table salt) and concentrating it. The concentrated first liquid 22 is appropriately stored until the time to generate electricity, for example, by remaining contained in the evaporation pool 20 or by being transferred to a separate container, the first liquid storage unit 30. This storage does not require any particular energy, and there is no particular limit to the storage period. In other words, the generated high-concentration first liquid 22 (e.g., high-concentration saltwater) serves as a raw material for concentration difference power generation, i.e., one of a pair of liquids that constitute the concentration difference liquid, and can be stored for any period of time by appropriate means. If the high-concentration first liquid 22 produced using only sunlight as an energy source is stored, it can be combined with a second liquid 42, which has a lower solute concentration than the first liquid 22, at a timing corresponding to the power demand, thereby operating the concentration difference power generation unit 50 at any timing to perform concentration difference power generation. If the first liquid 22 is saltwater, the second liquid 42 can be water (including fresh water, such as desalination water) with a lower salt concentration than the first liquid 22. The second liquid storage unit 40 is not necessarily required in the concentration difference power generation system 100; the concentration difference power generation system 100 can function as long as the second liquid 42 is supplied. Although the evaporation pool 20 and the first liquid storage unit 30 are shown separately in FIG. 1, the concentration difference power generation system 100 can also be implemented by using a storage unit that has the functions of both the evaporation pool 20 and the first liquid storage unit 30.

[0023] Several methods of concentration difference power generation are known. Non-limiting examples of concentration difference power generation techniques that can be employed in the present disclosure include osmotic pressure power generation and reverse electrodialysis power generation. Osmotic pressure power generation uses a semipermeable membrane to convert a concentration difference into pressure, which is then used to generate electricity, for example, by converting it into potential energy and using a water wheel. Reverse electrodialysis power generation uses a semipermeable membrane to extract electricity through an electrochemical reaction. In addition to these, a mixing entropy battery, for example, can also be employed for the concentration difference power generation of the present disclosure (Non-Patent Document 3). Note that the method using a semipermeable membrane also has the advantage of facilitating recovery of the metamaterial absorber because the semipermeable membrane suppresses diffusion of the metamaterial absorber.

[0024] 2. Metamaterial Absorber 2-1. Metamaterial Absorber in Generating Concentration-Difference Liquid In an embodiment of the present disclosure, a concentration-difference liquid is generated with high efficiency by utilizing energy from solar radiation through the action of a metamaterial absorber. In the present disclosure, high efficiency in generating concentration-difference liquid may mean achieving some or all of the following simultaneously: increasing the concentration of the same amount of brine more quickly from the same amount of solar radiation; producing a larger amount of brine with the same concentration from the same amount of solar radiation; and producing the same amount of brine from the same amount of solar radiation with fewer metamaterial absorbers. Using brine as an example, the action of a metamaterial absorber increases the efficiency of concentrating brine using thermal energy, as in traditional solar salt production technology. In other words, a metamaterial absorber can produce brine with a higher concentration or a larger amount of brine, even with the same amount of solar radiation.

[0025] FIG. 2 is a perspective view of a metal-insulator-metal structure (hereinafter, MIM structure) 12, which is a typical example of a metamaterial absorber according to an embodiment of the present disclosure. The MIM structure 12 in the present disclosure sandwiches a dielectric 16 having a nanometer-scale thickness T between two conductive portions 14, 16. As shown in the figure, the MIM structure 12 may have a circular extension with a cylindrical side surface as its outer edge. When light is incident on the MIM structure 12, a non-radiative plasmon mode is excited within the MIM structure 12 due to the resonant interaction between the structure and the light wave, resulting in suppression of reflection and transmission of light. This means that the entire amount of light energy at the resonant wavelength is absorbed by the MIM structure 12. In particular, a metamaterial absorber that completely absorbs light of a specific wavelength with 100% efficiency can be realized. When sunlight is used to increase the concentration of the first liquid 22, the MIM structure 12 is designed to correspond to the spectrum of sunlight. The MIM structure 12 designed in this manner preferably has an outer diameter D of 10 μm or less. Other examples of shapes that the MIM structure of the present disclosure may have include a shape having a polygonal extension, a concentric circular shape, and a core-shell shape.

[0026] A polygonal shape is similar to the MIM structure 12 shown in Figure 2, with the outer edge formed like a polygonal prism. A concentric circular shape refers to a conductor pattern with multiple circular rings with a single center, and may also include split rings where the rings are partially interrupted. In a concentric circular MIM structure, at least one conductive layer has a concentric circular pattern. A core / shell shape is a shape in which the material of adjacent regions is changed between a core region defined by a concentric sphere with a smaller diameter inside a sphere and a shell region (which may have multiple concentric shell regions) excluding the core region. In a core / shell MIM, the material of one core region and one or more shell regions can be changed, and a structure can be formed in which a conductor, a dielectric, a conductor, etc. are arranged radially from the center to the outside. The outermost layer can be either a conductor or a dielectric. A spiral structure, such as that described in Patent Document 2, can also be used for the MIM structure of the present disclosure. The dielectric 16 preferably exhibits the property of transmitting at least a portion of sunlight when the material is a bulk material. Note that the M (metal) portion of the MIM structure in this disclosure is not necessarily limited to metal, and can be made of a conductive material.

[0027] A typical example of a metamaterial absorber is the microparticle disclosed in Patent Document 1. However, although the size of these microparticles is adjusted, the structure itself is not particularly ingenious, making them unsuitable for concentration gradient power generation systems. In the metamaterial absorber employed in this disclosure, each constituent element has an absorption cross-section extremely large compared to its physical cross-section at the resonant absorption wavelength. In other words, each individual element of the metamaterial absorber has super-Planckian properties, meaning that it can collect light energy over a range larger than the cross-section defined by its actual physical dimensions. A metamaterial absorber exhibiting such properties may potentially have the ability to absorb or radiate (emit) greater amounts of thermal radiation energy than a blackbody. Furthermore, such metamaterial absorbers can absorb sunlight without loss even when the number of elements is small or there are gaps between them. In other words, by making the most of this property, sufficient light absorption can be achieved even when the individual elements are arranged more loosely than in an ideal blackbody. As a result, by alternating elements with different resonant wavelengths, broadband and large light absorption characteristics can be achieved. However, with simple fine particles such as those disclosed in Patent Document 1, there is little freedom in adjusting the wavelength absorbed by the particles, i.e., the shape of the absorption spectrum, making it difficult to efficiently absorb light of a target wavelength.

[0028] 2-2. Properties of metamaterial absorbers with MIM structures By designing the structure and conducting actual prototyping, metamaterial absorbers can be adapted to sunlight, which contains radiation over a wide wavelength range, from visible light to infrared.

[0029] The inventors have succeeded in controlling optical absorption in the visible light range using a nanostructure in which a dielectric is sandwiched between two aluminum thin films, as an example of the MIM structure 12 shown in FIG. 2 , and have actually fabricated a metamaterial absorber that functions as a color emitter by absorbing light of a specific wavelength. In this example of a metamaterial absorber, it has been confirmed that a color emitter covering the visible range from red to purple can be fabricated. Specifically, by adjusting the size of the MIM structure, it is possible to prototype a metamaterial color emitter that exhibits various colors in a wavelength range covering all visible light. Figure 3 shows a graph of the measured absorption and transmission spectra of an MIM structure in which a dielectric is sandwiched between two aluminum thin films. It has been confirmed that the optical absorption rate at the resonant wavelength can be increased to 99.5%, almost to a state of complete absorption.

[0030] In addition, the inventors have confirmed that by integrating multiple types of MIM structures that absorb different wavelengths in one location, it is possible to realize an absorption band that is the sum of the absorption bands exhibited by each type of structure. For example, Non-Patent Document 4 shows an example of fabricating a metamaterial absorber that can realize an absorber that absorbs light over a wide wavelength band of visible light. The same document also discloses that when three structures that actually exhibit red, blue, and green colors are integrated, they appear black.

[0031] In the metamaterial absorber of this embodiment, the resonant wavelength is preferably determined taking into account the properties of the medium in which it is mixed. As described above, resonant interaction with light waves is useful for enabling a metamaterial absorber having an MIM structure to exhibit high absorbance. This is because the wavelength (resonant wavelength) at which a metamaterial absorber exhibits a resonance phenomenon that is strongly dependent on wavelength (or frequency) is affected by the refractive index and absorption coefficient of the medium surrounding the metamaterial absorber, i.e., the complex refractive index or dielectric function. This property is not limited to MIM structures and is generally observed in metamaterial absorbers. While much research on conventional metamaterial absorbers has already been published, most metamaterial absorbers are designed to operate in air, with little consideration given to the state in which they are mixed into a material with a surrounding refractive index different from that of air.

[0032] The light energy absorbed by the metamaterial absorber is ultimately converted into heat. When that heat is transferred to the medium surrounding the metamaterial absorber, the temperature rises and evaporation of the solvent is promoted. This helps to efficiently produce the first liquid that constitutes the concentration difference liquid. If the concentration of the first liquid is increased or a larger amount of the first liquid is produced with the help of the metamaterial absorber, the amount of power generated by concentration difference power generation between the first liquid and the second liquid can be increased. In other words, the metamaterial absorber of this embodiment also exhibits the effect of converting sunlight into thermal energy with high efficiency, and as a result, the efficiency of the concentration difference power generation system is increased.

[0033] The metamaterial absorber can be composed of a metal nanostructure, as in the case of the MIM structure. Metal nanostructures can be synthesized in large quantities through chemical processes and are not consumed themselves, allowing for repeated use. Therefore, the environmental impact (e.g., carbon footprint) of producing the first liquid using the metamaterial absorber is small. Furthermore, if the first liquid is saltwater, its toxicity is not an issue, and no explosive by-products such as hydrogen are produced. Rare elements such as Li are not particularly required for the first liquid. In other words, producing a metamaterial absorber and using it to operate a concentration gradient power generation system does not impose a significant environmental impact.

[0034] Generally, the performance of nanostructures considered metamaterials is often confirmed when they are periodically arranged on a substrate surface. However, the properties utilized in the metamaterial absorber of this embodiment are exhibited by the individual nanostructures, and it is not essential that the nanostructures be arranged to form a metamaterial absorber. In other words, the periodicity due to the arrangement is not required for the metamaterial absorber employed in this embodiment, and the MIM structures function as a light absorber even when randomly dispersed in a liquid or solid material. Therefore, the metamaterial absorber of this embodiment is the nanostructure itself as individual particles, and their aggregates can be prepared as having the properties of a powder. The metamaterial absorber thus prepared can be mixed with saltwater, which is a first liquid, and irradiated with sunlight to concentrate the saltwater. When the first liquid is seawater, the solute is mainly dissolved in seawater, which is table salt, and the solvent is water. A typical second liquid to be combined with the first liquid is a liquid with a higher solvent content and a lower solute concentration than the first liquid. If the first liquid is derived from seawater, the second liquid is typically freshwater or fresh water.

[0035] 2-3. Materials of the MIM Structure The metamaterial absorber 10 shown in Figure 1 is typically an MIM structure 12. The conductor portions 14, 18 are typically made of metals, such as gold and silver. Gold and silver have low ohmic loss, i.e., low electrical resistance, and therefore strongly resonate with light waves. These precious metals are expensive, and the thermal properties of nanometer-sized gold or silver structures differ from those of bulk materials, resulting in low melting points and physical strength issues, such as deformation due to slight heating. When used as a metamaterial absorber, there is a risk of deformation due to heat associated with light absorption when increasing the amount of light absorption or when the irradiated light intensity increases. In such cases, this disclosure also allows for the development of a technology for forming metamaterial absorber elements as powders that actively utilize metal compounds such as titanium nitride (TiN), which have high melting points and physical strength (Non-Patent Document 5). Titanium nitride (TiN) has spectral characteristics similar to those of gold in the visible to near-infrared range, but is a material with advantages in terms of heat resistance, physical strength, and cost. The metamaterial absorber of this embodiment can incorporate the knowledge in the field of metamaterials described above, thereby realizing an absorber that efficiently absorbs the solar spectrum. The manufactured metamaterial absorber has a microparticle structure with a diameter of several hundred nanometers, composed of metal and a dielectric such as silica, as shown in Figure 2. Therefore, unlike organic molecules, it does not decompose under ultraviolet light and can exist relatively stably.

[0036] 2-4. Design and Evaluation In this embodiment, the amounts and concentrations of the first liquid (e.g., saltwater or seawater) and the second liquid (e.g., freshwater or freshwater) that operate the concentration gradient power generation system are determined based on the power generation conditions. The metamaterial absorber is used to increase the concentration in the first liquid, i.e., the concentration of the solute in a solution of a solvent and a solute. In a typical combination where the first liquid is saltwater and the second liquid is freshwater, the metamaterial absorber is used to increase the salinity of the saltwater. A metamaterial absorber that increases the efficiency of such a concentration gradient power generation system is designed to match its absorption wavelength to the solar spectrum, taking into account the environment of the first liquid. For example, in a case where saltwater is used as the first liquid, the metamaterial absorber is designed to match its absorption wavelength to the solar spectrum, assuming a saltwater environment. The specific method is as follows.

[0037] As a first step, an electromagnetic field simulation is performed that can faithfully reproduce the properties of the electromagnetic field. Examples of such electromagnetic field simulation methods include the finite element method (FEM), the finite-difference time-domain method (FDTD), and the rigorous coupled-wave analysis (RCWA). Among the conditions reflected in the calculation, a particularly useful feature for the concentration gradient power generation system of this embodiment is the ability to accurately predict the performance of the metamaterial absorber while incorporating the medium in which the metamaterial absorber is placed. That is, the surrounding medium can be set as a first liquid, and more specifically, the refractive index (dielectric properties) can be reproduced depending on its concentration. A typical design procedure for increasing light absorption efficiency involves designing the structure of a metamaterial absorber to absorb sunlight with as close to 100% efficiency as possible in that environment. While such efficiency is not necessarily achieved across a wide wavelength range, the structure of each metamaterial absorber is initially determined with the priority of achieving 100% light absorption.

[0038] In the second step, the resonant wavelength is controlled by changing the size, shape, or material of each structural component. The resonant wavelength can be adjusted by adjusting the size of each component or changing the material, and this degree of freedom in artificial adjustment is one of the characteristics of metamaterial technology. The resonant wavelength is determined taking into account the spectrum of light irradiated onto the first liquid. In this embodiment, in order to achieve the resonant wavelength characteristics over a wide wavelength range, multiple types of metamaterial absorbers are combined.

[0039] In the third step, metamaterial absorbers with different structures designed for different resonant wavelengths are integrated (mixed), ultimately determining the individual structures and mixing ratios of broadband metamaterial absorbers that can cover the entire solar spectrum.

[0040] For the purpose of explanation, the design procedure has been divided into three steps. In reality, however, the design is carried out by repeating each step or rearranging them in any order so that the final metamaterial absorber meets the purpose of operating the concentration gradient power generation system. Furthermore, by appropriately combining experimental confirmations corresponding to individual or multiple steps, the accuracy of the design can be controlled according to the purpose while investigating the correspondence between theory, numerical calculations, and reality. For example, by creating a prototype structure with an enlarged or reduced size taking into account the refractive index of the first liquid (e.g., saltwater) and evaluating its optical absorption characteristics using a visible / infrared spectrometer, the deviation between the designed and actual metamaterial absorber characteristics can be evaluated, and the design parameters of size, shape, and material can be adjusted. In this way, it is possible to fabricate a broadband metamaterial absorber (including a composite) that absorbs solar light in saltwater, or a broadband metamaterial absorber that absorbs the entire solar spectrum. The metamaterial absorber of this embodiment can achieve nearly 100% optical absorption across a wide absorption band that covers the entire solar spectrum.

[0041] The evaluation of metamaterial absorbers is ultimately carried out from the perspective of photothermal conversion efficiency and evaporation rate. The evaporation rate can also be quantified as the concentration ability, which is the change in the concentration of the solute in the first liquid, or as the distillation ability. The photothermal conversion efficiency and evaporation rate are evaluated by comparing them with appropriate standards. In the example where the first liquid is saltwater, a mixture of carbon black and saltwater irradiated with sunlight is used as the reference sample, and the carbon black is replaced with a metamaterial absorber, and saltwater under the same conditions and sunlight irradiated under the same conditions are used to test and evaluate the photothermal conversion efficiency and evaporation rate of the sample.

[0042] The specific evaluation method is as follows, using an example in which the first liquid is saltwater and the second liquid is fresh water. First, an evaporation experiment of the saltwater is conducted by irradiating it with light from a solar simulator while allowing the prototype metamaterial absorber to interact with the saltwater. The volume and weight changes of the saltwater are measured, and by precisely measuring these, the efficiency of solar energy utilization can be evaluated. During this process, the structural changes and deterioration of the metamaterial absorber, as well as the decline in its light absorption properties, are investigated, as these will affect its practicality.

[0043] 2-6. Manufacturing of Metamaterial Absorber The designed metamaterial absorber can also be mass-produced as an absorber structure. There are no particular limitations on the processing techniques that can be employed for this purpose. A typical example of such processing techniques is nanofabrication technology. Various techniques included in nanofabrication technology can be applied to the manufacturing of the metamaterial absorber of this embodiment without particular limitations. Among them, it is preferable to employ thin film formation techniques such as sputtering technology, electron beam lithography technology, and dry etching technology. Furthermore, chemical self-assembly techniques are also promising for mass production of metamaterial absorber structures.

[0044] 2-7. Recovery of Metamaterial Absorber From a practical standpoint, a method for separating and recovering the metamaterial absorber from the first liquid is also useful. In the concentration gradient power generation system of this embodiment, the metamaterial absorber can be separated and recovered from the first liquid (e.g., saltwater). Specific methods include centrifugal separation, various membrane filters, and column filters. It is also preferable that a portion of the metamaterial absorber contains a magnetic material such as Fe, Ni, or Co. This is because metamaterial absorbers containing magnetic materials can be easily concentrated by utilizing the magnetic attraction effect when mixed with the first liquid, and then separated and recovered from the first liquid. Figure 1 shows a configuration including a recovery unit 60. Ni and Co can also be used as adhesive layers between the metal layer and the dielectric portion in MIM structures.

[0045] 2-8. Modified Examples of Metamaterial Absorber Modified examples of the metamaterial absorber of this embodiment include derivatives of a core-shell shape. Figures 4A to 4C are perspective views illustrating examples of core-shell metamaterial absorbers of this embodiment, and are, respectively, metamaterial absorber 10A (Figure 4A) in which a conductor portion is provided in the shape of a shell on the surface of a spherical dielectric portion 16S that serves as the core, and annular slits 15 are provided in the conductor portion so as to trace a great circle; metamaterial absorber 10B (Figure 4B) in which the slits 15 trace part of the great circle of the spherical dielectric portion; and metamaterial absorber 10C (Figure 4C) in which the slits 15 trace two great circles formed by mutually orthogonal planes and the center of dielectric portion 16S.

[0046] In metamaterial absorbers 10A-C shown in Figures 4A-C, dielectric portion 16S is a solid dielectric, and conductor portions are formed in layers on its surface. In metamaterial absorber 10A in Figure 4A, conductor portion 14A1 and conductor portion 14A2 are arranged in an MIM structure, with conductor portion 14A1 and conductor portion 14A2 forming a pair of conductor portions. In metamaterial absorber 10B in Figure 4B, slit 15 is not connected in an annular shape. In this case, conductor portion 14B1 and conductor portion 14B2, which are separated by slit 15 and partially connected to each other, form a pair of conductor portions, sandwiching dielectric portion 16S. In the metamaterial absorber 10C shown in FIG. 4C, the slits 15 are two annular, dividing the conductor portion 14 into four sections, with the conductor portion 14C1 and the conductor portion 14C3 arranged diagonally to form a pair of conductor portions, sandwiching the dielectric portion 16S. The same is true for the conductor portion 14C2 and the conductor portion 14C4. Furthermore, in the metamaterial absorber 10C, the conductor portion 14C1 and the conductor portion 14C2 are arranged adjacent to each other on both sides of the slit 15, forming a pair of conductor portions, sandwiching a portion of the dielectric portion 16S. This is true for all pairs of adjacent conductor portions 14C1 to 14C4. The metamaterial absorbers 10A to 10C shown in FIGS. 4A to 4C all function as optical absorbers, partially inheriting the properties of the MIM structure.

[0047] 3. Conclusion The above provides a detailed description of the embodiments of the present disclosure. The above-described embodiments, modifications, and experimental confirmations are provided for the purpose of explaining the invention, and the scope of the present disclosure should be determined based on the claims. Furthermore, modifications within the scope of the present disclosure, including other combinations of the embodiments, are also encompassed within the scope of the claims. In other words, those skilled in the art may make various modifications, combinations, subcombinations, and substitutions of the components of the above-described embodiments within the technical scope of the present disclosure or its equivalents.

[0048] 100 Concentration difference power generation system 10, 10A, 10B, 10C Metamaterial absorber 12 MIM structure 14, 18, 14A1 to 14C4 Conductor section 15 Slit 16 Dielectric section 16S Dielectric section (sphere) 20 Evaporation pool 22 First liquid 30 First liquid storage section 40 Second liquid storage section 42 Second liquid 50 Concentration difference power generation section 60 Recovery section

Claims

1. A concentration difference power generation system that utilizes a concentration difference liquid containing a first liquid that is a solution in which a certain solute is dissolved in a certain solvent, and a second liquid that contains the solvent and has a lower concentration of the solute in the solvent than the first liquid, wherein the first liquid is in contact with a metamaterial absorber, and when irradiated with sunlight, the metamaterial absorber promotes evaporation of the solvent from the first liquid in response to energy from the sunlight, thereby increasing the concentration of the solute in the first liquid.

2. The concentration difference power generation system according to claim 1, wherein the metamaterial absorber exhibits the property of absorbing irradiated sunlight and converting it into heat.

3. The concentration difference power generation system according to claim 1, wherein the metamaterial absorber includes: a first metamaterial absorber that has an increased absorption amount of light of a first wavelength within the spectral range of the irradiated sunlight; and a second metamaterial absorber that has an increased absorption amount of light of a second wavelength different from the first wavelength within the spectral range of the irradiated sunlight.

4. The concentration difference power generation system according to claim 1, further comprising: a storage section for the first liquid in which the concentration of the solute has been increased.

5. The concentration difference power generation system according to claim 1, wherein the metamaterial absorber is dispersed in the first liquid, the metamaterial absorber is provided with a magnetic material, and a recovery unit is provided that recovers the metamaterial absorber from the first liquid by the action of magnetic force.

6. A concentration difference power generation system according to claim 1 or claim 4, wherein the solute is a substance dissolved in seawater, the solvent is water, and the first liquid is concentrated seawater.

7. The concentration difference power generation system according to claim 6, wherein the second liquid is fresh water.

8. A concentration difference power generation method that utilizes a concentration difference liquid containing a first liquid that is a solution in which a certain solute is dissolved in a certain solvent, and a second liquid that contains the solvent and has a lower concentration of the solute in the solvent than the first liquid, the power generation method comprising: bringing a metamaterial absorber into contact with the first liquid; irradiating the metamaterial absorber in contact with the first liquid with sunlight, thereby promoting evaporation of the solvent from the first liquid in accordance with the energy of the sunlight and increasing the concentration of the solute in the first liquid; storing the first liquid with the increased solute concentration; and performing concentration difference power generation using the stored first liquid with the increased solute concentration and the second liquid as the concentration difference liquid.

9. The method of generating electricity according to claim 8, wherein the step of contacting the metamaterial absorber with the first liquid comprises dispersing the metamaterial absorber in the first liquid.

10. The power generation method according to claim 9, wherein the solute is a seawater solution, the solvent is water, the first liquid is concentrated seawater, and the second liquid is freshwater.

11. A metamaterial absorber comprising a pair of conductor portions and a dielectric portion sandwiched between the pair of conductor portions, which is adapted to resonate with light of at least one wavelength within the range of the sunlight spectrum in an environment filled with a solution having seawater as the solute and water as the solvent, and which exhibits the property of absorbing light of at least one wavelength from sunlight irradiated in the environment and converting it into heat.

12. The metamaterial absorber according to claim 11, wherein the pair of conductor portions and the dielectric portion have a circular extension with a cylindrical side surface as the outer edge, and the dielectric portion is made of a bulk material that is transparent to sunlight.

13. The metamaterial absorber according to claim 11, wherein the pair of conductor portions and the dielectric portion have a polygonal extension with a side surface of a polygonal prism as an outer edge, and the material of the dielectric portion is a bulk material that is transparent to sunlight.

14. The metamaterial absorber according to claim 11, wherein the dielectric portion has a generally spherical outer shape, and the pair of conductor portions comprises a first conductor portion and a second conductor portion disposed on the surface of the dielectric portion and at least a portion of which is separated from each other by at least one slit.

15. The metamaterial absorber according to any one of claims 12 to 14, wherein the outer diameter of the spread is 10 μm or less.

16. The metamaterial absorber according to any one of claims 11 to 14, wherein at least one of the pair of conductor portions contains at least one material selected from the group of materials consisting of gold, silver, and titanium nitride.

17. The metamaterial absorber according to any one of claims 11 to 14, further comprising a magnetic material.

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