A solar phase change thermal storage device with enhanced heat storage through light passing through an optical waveguide

Abstract

The present invention discloses a solar phase-change thermal storage device that enhances heat storage by transmitting light within an optical waveguide. The device comprises a concentrating device, an optical fiber positioned below the concentrating device, and a thermal storage material surrounding the optical fiber. The optical fiber is divided into a non-side-emitting portion and a side-emitting portion. The non-side-emitting portion is positioned below the concentrating device and exposed to the air outside the thermal storage material, with the light-incident end face of the non-side-emitting portion located on the focal plane of the concentrating device. The side-emitting portion extends through the thermal storage material. This device optimizes the light distribution within the thermal storage material on a spatial scale, regulates the phase change interface, and increases the thermal storage material's charging phase change rate, enabling efficient and rapid storage of solar thermal energy over long distances within the thermal storage material.

Description

Technical Field

[0001] The present invention relates to the technical field of solar thermal storage, and in particular to a solar phase change thermal storage device capable of enhancing thermal storage by passing light through an optical waveguide. Background Art

[0002] Phase-change thermal storage is a key technology in the field of solar thermal utilization, attracting widespread attention from both academia and industry due to its low cost and high dispatchability. Phase-change thermal storage boasts large heat storage capacities and stable heat release temperatures, and is well-suited to the intermittent and temporally uneven nature of solar energy resources, making it a key area of ​​development in solar utilization technology. However, a drawback of solar phase-change thermal storage is that phase-change materials generally have low thermal conductivity, resulting in slow heat storage and low solar energy utilization efficiency.

[0003] In order to improve the efficiency of solar thermal utilization, it is necessary to enhance heat transfer. Currently, there are several methods to enhance heat transfer: (1) adding fin structures; (2) adding high thermal conductivity materials such as graphite and metal; (3) using a three-dimensional porous directional heat transfer skeleton to encapsulate the heat storage material; (4) encapsulating the heat storage material in an organic or inorganic shell to prepare a microcapsule composite phase change medium. Adding a fin structure limits the natural convection during the phase change process to a certain extent and occupies a certain space. There is a high interfacial thermal resistance between graphite, metal high thermal conductivity materials and porous directional heat transfer skeletons and the heat storage material, resulting in a slow heat transfer rate inside the medium. The preparation process of microcapsule composite phase change medium is relatively complicated. In addition, in the above methods, due to the absorption of incident photons by the filler or skeleton structure, the light transmission depth in the heat storage material is only a few millimeters. The phase change interface movement inside the medium is still mainly completed by heat diffusion, resulting in a very slow heat transfer rate inside the medium.

[0004] Currently, the biggest obstacle to achieving fast and efficient solar thermal storage in large-scale media is the distance photons travel within the phase-change medium. Optimizing the spatial distribution of sunlight within the thermal storage material can achieve rapid phase-change thermal storage and improve solar thermal utilization efficiency. Therefore, a new phase-change thermal storage light transmission technology and phase-change thermal storage device are needed to increase thermal storage speed.

[0005] Patent CN 207019162 U discloses an indoor radiant heating device based on optical fiber transmission of solar energy, including an outdoor concentrating device and an indoor radiant heat dissipation device. The outdoor concentrating device includes an outdoor fixed bracket, one end of which is equipped with a transmissive point-focusing Fresnel lens, the rear of which is provided with a biconcave curved lens, the rear of which is provided with an optical fiber, the optical fiber is located on the optical axis of the biconcave curved lens, and the optical fiber is connected to the indoor radiant heat dissipation device; the indoor radiant heat dissipation device includes an insulation layer, a heat storage layer and a radiant heat dissipation aluminum plate; in this device, light can only be emitted from the end face of the optical fiber, and only the heat storage material in a very small area near the end face of the optical fiber can quickly absorb light and then undergo phase change. The transfer of heat in the heat storage material in most other areas mainly relies on heat conduction, which may lead to problems such as local overheating and slow heat storage rate inside the heat storage material. Summary of the Invention

[0006] In response to the above-mentioned prior art, the present invention aims to provide a solar phase-change thermal storage device that enhances heat storage by transmitting light through an optical waveguide. This device accelerates interface movement and improves the efficiency of light and heat utilization. It also features a simple structure and easy processing, making it suitable for phase-change thermal storage of large-scale media.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a solar phase change thermal storage device with enhanced heat storage by light passing through an optical waveguide, comprising a light concentrating device, an optical fiber located below the light concentrating device, and a thermal storage material wrapping the optical fiber; the optical fiber is divided into a non-side emitting portion and a side emitting portion; the non-side emitting portion of the optical fiber is located below the light concentrating device and exposed to the air outside the thermal storage material, and the light incident end face of the non-side emitting portion of the optical fiber is located on the focal plane of the light concentrating device; the side emitting portion passes through the interior of the thermal storage material.

[0009] Preferably, the optical fiber is a PMMA optical fiber or a silica optical fiber.

[0010] Preferably, the side-emitting portion of the optical fiber is prepared by the following method: physically stripping the cladding of the optical fiber or removing the cladding by soaking in an organic solvent to obtain a bare optical fiber, and soaking the bare optical fiber in a reagent to obtain a side-emitting optical fiber.

[0011] Preferably, for a PMMA optical fiber, the cladding of the optical fiber region requiring side-emitting light is physically stripped to obtain a bare optical fiber region, and the bare optical fiber region is immersed in a mixture of acetone and n-hexane in a volume ratio of 2:3 for 3-15 seconds to obtain an optical fiber including a non-side-emitting portion and a side-emitting portion.

[0012] For quartz optical fiber, the optical fiber area that needs side-luminescence treatment is immersed in acetone for 24 hours, and the organic cladding is stripped off to obtain a bare optical fiber area. The bare optical fiber area is immersed in a mixture of H2O2 and H2SO4 with a volume ratio of 3:7 and pretreated at 90°C for 1 hour; 2mmol of potassium titanium oxalate is dissolved in 40mL of a 75% volume fraction diethylene glycol aqueous solution, and the pretreated bare optical fiber area is inserted into the diethylene glycol aqueous solution. After hydrothermal reaction at 180°C for 6 hours, an optical fiber including a non-side-luminescence part and a side-luminescence part is obtained.

[0013] Preferably, there is at least one optical fiber, and the focusing device corresponds to the optical fiber one by one.

[0014] Preferably, the focusing device is a compound parabolic concentrator or a Fresnel lens.

[0015] Preferably, the light-concentrating device is located outside the heat storage material, and the focal plane of the light-concentrating device is located at the incident end face of the non-side-light-emitting portion of the optical fiber.

[0016] More preferably, the light-emitting portion of the optical fiber penetrates the interior of the heat storage material.

[0017] Preferably, the heat storage material is one or more of potassium nitrate, sodium nitrate, potassium chloride, magnesium chloride, sodium chloride, paraffin, and polyurethane.

[0018] More preferably, graphene is evenly dispersed in the thermal storage material. The graphene has excellent light-to-heat conversion performance, so that light emitted from the side of the optical fiber can be quickly absorbed by the graphene and converted into heat energy, causing the thermal storage material to heat up and undergo phase change.

[0019] During the thermal storage process, sunlight is focused by a concentrator onto the incident end face of the optical fiber's non-side-emitting portion and coupled into the fiber. In PMMA optical fibers, the surface of the fiber is etched with reagents to form depressions. During transmission through the optical fiber waveguide, the sunlight is scattered by these depressions and output into the thermal storage medium. In silica optical fibers, the silica (refractive index 1.45) in the side-emitting portion is covered with a layer of titanium dioxide (refractive index 2.49). During transmission through the optical fiber core (silica) waveguide, the sunlight cannot meet the conditions for total internal reflection and is refracted into the titanium dioxide. From there, it is emitted into the thermal storage medium. The photothermal material, graphene, dispersed within the thermal storage medium absorbs this sunlight and converts it into heat. This heat is then released to the phase-change thermal storage material, causing it to gradually heat up and undergo a phase change, storing the heat within the phase-change thermal storage material. During this process, the optical fiber guides the sunlight and outputs it into the phase change medium for photothermal conversion, which optimizes the light distribution inside the thermal storage material on a spatial scale, regulates the phase change interface, and increases the thermal charging phase change rate of the thermal storage material, enabling longer-distance, efficient, and rapid storage of solar thermal energy in the thermal storage material.

[0020] Beneficial effects of the present invention:

[0021] When the device of the present invention is in the heat storage process, sunlight is concentrated by the focusing device to the incident end face of the optical fiber, transmitted by the optical fiber to the side-emitting portion, and output by this side-emitting portion to the interior of the heat storage material. This optimizes the light distribution inside the heat storage material on a spatial scale, regulates the phase change interface, effectively improves the heat storage rate of the phase change medium, and enables long-distance, efficient, and rapid storage of solar heat in the heat storage material.

[0022] This invention uses optical waveguides to transport light and output it laterally into the thermal storage material. By optimizing the spatial distribution of light within the medium, this overcomes the drawback of short light transmission distances within the medium, effectively increasing the phase change thermal storage rate and, in turn, the efficiency of solar thermal utilization. Furthermore, the long-distance transmission of light within optical fibers makes the technology of enhancing the phase change thermal storage rate through optical waveguides suitable for phase change thermal storage in large-scale media. Furthermore, the present invention's internally transmitted phase change thermal storage device boasts a simple structure and low cost, making it easy to manufacture and apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 : A schematic diagram of a solar phase change thermal storage device according to the present invention having only one optical fiber;

[0024] Figure 2 : A schematic diagram of a solar phase change thermal storage device according to the present invention having multiple optical fibers;

[0025] Figure 3: Longitudinal section of the compound parabolic concentrator of the present invention. DETAILED DESCRIPTION

[0026] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0027] As described in the background art, light travels only a few millimeters deep within thermal storage materials, and phase transition interface movement within the medium still primarily relies on thermal diffusion, resulting in a very slow heat transfer rate within the medium. Based on this, the present invention provides a solar phase-change thermal storage device that utilizes light-enhanced thermal storage within an optical waveguide, comprising a concentrating device, an optical fiber positioned below the concentrating device, and a thermal storage material encapsulating the optical fiber. The optical fiber is divided into a non-side-emitting portion and a side-emitting portion. The non-side-emitting portion of the optical fiber is positioned below the concentrating device and is exposed to the air outside the thermal storage material, with the light-incident end face of the non-side-emitting portion of the optical fiber positioned on the focal plane of the concentrating device. The side-emitting portion extends through the interior of the thermal storage material.

[0028] The optical fiber is a PMMA optical fiber or a silica optical fiber; the side-emitting portion of the optical fiber is prepared by the following method: for PMMA optical fiber, the cladding of the optical fiber region requiring side-emitting treatment is physically stripped off to obtain a bare optical fiber region, and the bare optical fiber region is immersed in a mixture of acetone and n-hexane (volume ratio of 2:3) for 3 to 15 seconds to obtain an optical fiber including a non-side-emitting portion and a side-emitting portion; for quartz optical fiber, the optical fiber region requiring side-emitting treatment is immersed in acetone for 24 hours, and the organic cladding is stripped off to obtain a bare optical fiber. The bare optical fiber region is immersed in a mixture of H2O2 and H2SO4 (the H2O2 and H2SO4 used are purchased pure solutions) with a volume ratio of 3:7, and pretreated at 90°C for 1 hour; 2mmol of potassium titanium oxalate is dissolved in 40mL of a 75% by volume aqueous diethylene glycol solution, and the pretreated bare optical fiber region is inserted into the solution. After a hydrothermal reaction at 180°C for 6 hours, a layer of titanium dioxide will grow on the surface of the bare optical fiber region, and this region has the property of side-emitting light. Therefore, the optical fiber includes a non-side-emitting part and a side-emitting part.

[0029] The focusing device is a compound parabolic concentrator or a Fresnel lens. The focusing device is located outside the heat storage material, and the non-side-emitting portion of the optical fiber is also located outside the heat storage material. The focal plane of the focusing device and the light incident end face of the non-side-emitting portion of the optical fiber are at the same position, so that the light is focused onto the focal plane by the focusing device and introduced into the optical fiber.

[0030] The heat storage material is one or more of potassium nitrate, sodium nitrate, potassium chloride, magnesium chloride, sodium chloride, paraffin, and polyurethane, and graphene is evenly distributed in the heat storage material.

[0031] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below with reference to specific embodiments.

[0032] The test materials used in the examples of the present invention are all conventional test materials in the art and can be purchased through commercial channels.

[0033] Example 1

[0034] A solar phase change thermal storage device with enhanced heat storage through light passing through an optical waveguide, such as Figure 1 As shown, it includes a focusing device, a PMMA optical fiber and heat storage material.

[0035] The PMMA optical fiber is divided into a non-side-emitting portion and a side-emitting portion. The non-side-emitting portion of the optical fiber is located outside the thermal storage material, and the focal plane of the focusing device is co-located with the light incident end face of the non-side-emitting portion of the optical fiber. The side-emitting portion of the optical fiber extends through the thermal storage material. The side-emitting portion of the optical fiber is prepared by physically stripping the cladding of the optical fiber region to be treated for side-emitting light to obtain a bare optical fiber region. This bare optical fiber region is then immersed in a mixture of acetone and n-hexane in a volume ratio of 2:3 for 10 seconds to obtain an optical fiber comprising both the non-side-emitting portion and the side-emitting portion.

[0036] The concentrator is a compound parabolic concentrator located outside the thermal storage material. The non-laterally emitting portion of the optical fiber is also located outside the thermal storage material. The focal plane of the concentrator is located at the light-incident end face of the non-laterally emitting portion of the optical fiber. The thermal storage material is paraffin wax, which is evenly distributed with graphene.

[0037] The steps to determine the parameters of the compound parabolic concentrator are as follows: the longitudinal section of the compound parabolic concentrator is as follows: Figure 3 As shown. Concentrator geometric concentration ratio Sincident and Sexcout represent the incident and exit end surface areas, respectively. According to the properties of the parabolic concentrator, under the premise of determining the exit end surface radius a′ and the geometric concentration ratio C, the geometric parameters of the concentrator can be uniquely determined by formulas (1)-(4).

[0038]

[0039]

[0040] f=a'(1+sinθ)(3)

[0041]

[0042] In the above formula, θ is the maximum acceptance angle, a is the incident end face radius, a′ is the exit end face radius, C is the concentrator geometric concentration ratio, f is the focal length, and L is the concentrator length.

[0043] Example 2

[0044] A solar phase change thermal storage device with enhanced heat storage through light passing through an optical waveguide, such as Figure 2 As shown, the device comprises nine light-concentrating devices, nine PMMA optical fibers, and a thermal storage material. The PMMA optical fibers are divided into a non-side-emitting portion and a side-emitting portion. The non-side-emitting portion of the optical fibers is located outside the thermal storage material, and the focal plane of the light-concentrating devices is co-located with the light-incident end face of the non-side-emitting portion of the optical fibers. The side-emitting portion of the optical fibers extends through the thermal storage material. The side-emitting portion of the optical fibers is prepared by physically stripping the cladding from the region of the optical fiber to be treated for side-emitting light, obtaining a bare optical fiber region. This bare optical fiber region is then immersed in a mixture of acetone and n-hexane in a volume ratio of 2:3 for 10 seconds, yielding an optical fiber comprising both a non-side-emitting portion and a side-emitting portion.

[0045] The concentrator is a compound parabolic concentrator located outside the thermal storage material. The non-light-emitting portion of the optical fiber is also located outside the thermal storage material. The focal plane of the concentrator is located at the incident end face of the non-light-emitting portion of the optical fiber. The thermal storage material is paraffin wax, which is evenly distributed with graphene.

[0046] The steps to determine the parameters of the compound parabolic concentrator are as follows: the longitudinal section of the compound parabolic concentrator is as follows: Figure 3 As shown. Concentrator geometric concentration ratio Sincident and Sexcout represent the incident and exit end surface areas, respectively. According to the properties of the parabolic concentrator, under the premise of determining the exit end surface radius a′ and the geometric concentration ratio C, the geometric parameters of the concentrator can be uniquely determined by formulas (1)-(4).

[0047]

[0048]

[0049] f=a'(1+sinθ)(3)

[0050]

[0051] In the above formula, θ is the maximum acceptance angle, a is the incident end face radius, a′ is the exit end face radius, C is the concentrator geometric concentration ratio, f is the focal length, and L is the concentrator length.

[0052] Comparative Example 1

[0053] This comparative example does not contain PMMA optical fiber, and the rest is the same as Example 1.

[0054] Comparative Example 2

[0055] In this comparative example, the PMMA optical fiber is a non-side-light-emitting portion inside the heat storage material, and the rest is the same as in Example 2.

[0056] Experimental example

[0057] The devices of Examples 1-2 and Comparative Example 1-2 were used to measure the thermal storage material charging phase change rate. Specifically, the devices of Examples 1-2 and Comparative Example 1-2 were simultaneously placed under sunlight, and an infrared camera was used to record the evolution of the temperature distribution on the side of the device over the illumination time. In Examples 1-2 and Comparative Example 1, the phase change interface (temperature corresponding to the phase change temperature of the thermal storage material) shifted from the top to the bottom of the thermal storage material with increasing illumination time. Therefore, the relationship between the position of the phase change interface (distance from the phase change interface to the top surface of the thermal storage material) and illumination time in Examples 1-2 and Comparative Example 1 was analyzed. The optical fiber in Comparative Example 2 is not capable of side-emitting light. Light transmitted through the optical fiber can only be output from its bottom end face, where it is absorbed by the surrounding photothermal material and converted into heat energy. This heat energy causes the thermal storage material in the area near the optical fiber's light-emitting end face to rapidly heat up and undergo a phase change. Simultaneously, heat is transferred to the thermal storage material in other areas via heat conduction. In Comparative Example 2, the light-emitting end of the optical fiber is located at the bottom of the thermal storage material. Therefore, the thermal storage material at the bottom of the thermal storage material in Comparative Example 2 heats up first, reaching the phase transition temperature. This heat energy is then transferred to the thermal storage material in the upper region, causing it to heat up and undergo a phase transition. Similarly, the phase transition interface in Comparative Example 2 moves from the bottom to the top of the thermal storage material as the illumination time increases. Therefore, the phase transition interface position recorded in Comparative Example 2 is the distance from the phase transition interface to the bottom surface of the thermal storage material. The experimental results for Examples 1-2 and Comparative Examples 1-2 are shown in Table 1.

[0058] Table 1

[0059]

[0060] The greater the distance between the phase change interface and the surface of the thermal storage material, the more thermal storage material reaches above the phase change temperature; for the same illumination time, the more phase change material reaches above the phase change temperature, the faster the thermal storage rate.

[0061] Compared with Comparative Example 1, the distance between the phase change interface and the surface of the thermal storage material in Example 1 is smaller at 15 minutes, and gradually increases from 20 to 65 minutes compared with Comparative Example 1. The thermal storage rate of Example 1 is the fastest.

[0062] The distance between the upper and lower surfaces of the thermal storage materials in Example 1 and Comparative Example 1 was 5 cm, and the distance between the upper and lower surfaces of the thermal storage materials in Example 2 and Comparative Example 2 was 12 cm. The phase change interface of the thermal storage materials in Example 1 and Example 2 almost reached the lower surface of the thermal storage material, indicating that heat was conducted from the top to the bottom of the thermal storage material, the thermal storage material had a large heat storage capacity, and a fast heat storage rate. The distance from the phase change interface to the upper surface of the thermal storage material in Comparative Example 1 was only 1.9 cm, and heat was only conducted for 1.9 cm in the thermal storage material, resulting in low heat storage capacity and a slow heat storage rate. The distance from the phase change interface to the lower surface of the thermal storage material in Comparative Example 2 was only 4.9 cm, and the portion of the thermal storage material that stored heat accounted for 40.8%, resulting in low heat storage capacity and a slow heat storage rate.

[0063] When the device of the present invention is in the heat storage process, sunlight is concentrated by the focusing device to the incident end face of the optical fiber, transmitted by the optical fiber to the side-emitting portion, and output by this side-emitting portion to the interior of the heat storage material. This optimizes the light distribution inside the heat storage material on a spatial scale, regulates the phase change interface, effectively improves the heat storage rate of the phase change medium, and enables long-distance, efficient, and rapid storage of solar heat in the heat storage material.

[0064] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A solar phase change thermal storage device with enhanced heat storage by light passing through an optical waveguide, characterized in that: It includes a light-collecting device, an optical fiber located below the light-collecting device, and a heat storage material wrapping the optical fiber; the optical fiber is divided into a non-side-light-emitting part and a side-light-emitting part, the non-side-light-emitting part of the optical fiber is located below the light-collecting device and exposed to the air outside the heat storage material, and the light incident end face of the non-side-light-emitting part of the optical fiber is located on the focal plane of the light-collecting device; the side-light-emitting part runs through the interior of the heat storage material.

2. The solar phase change thermal storage device with enhanced heat storage by light passing through the optical waveguide according to claim 1, characterized in that: The optical fiber is PMMA optical fiber or quartz optical fiber.

3. The solar phase change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 2, characterized in that: The side-emitting portion of the optical fiber is prepared by the following method: physically stripping the optical fiber of its cladding or removing the cladding by soaking it in an organic solvent to obtain a bare optical fiber, and soaking the bare optical fiber in a reagent to obtain a side-emitting optical fiber.

4. The solar phase change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 3, characterized in that: For PMMA optical fibers, the cladding of the optical fiber region requiring side-emitting light is physically stripped to obtain a bare optical fiber region. The bare optical fiber region is then immersed in a mixture of acetone and n-hexane at a volume ratio of 2:3 for 3-15 seconds to obtain an optical fiber comprising a non-side-emitting portion and a side-emitting portion. For silica optical fiber, the optical fiber area that needs side emission treatment is immersed in acetone for 24 hours, and the bare optical fiber area is obtained after stripping the organic cladding. and The mixed solution with a volume ratio of 3:7 was pretreated at 90°C for 1 hour; 2 mmol potassium titanium oxalate was dissolved in 40 mL of a 75% diethylene glycol aqueous solution, and the pretreated bare optical fiber area was inserted into the diethylene glycol aqueous solution. After hydrothermal reaction at 180°C for 6 hours, an optical fiber including a non-side-emitting part and a side-emitting part was obtained.

5. The solar phase change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 1, characterized in that: The number of the optical fiber is at least one, and the focusing device corresponds to the optical fiber one by one.

6. The solar phase change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 1, characterized in that: The light concentrating device is a compound parabolic concentrator or a Fresnel lens.

7. The solar phase change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 1, characterized in that: The light-collecting device is located outside the heat storage material, and the focal plane of the light-collecting device is located at the incident end face of the non-side-light-emitting portion of the optical fiber.

8. The solar phase change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 1, characterized in that: The heat storage material is one or more of potassium nitrate, sodium nitrate, potassium chloride, magnesium chloride, sodium chloride and paraffin.

9. The solar phase-change thermal storage device with enhanced thermal storage by light passing through an optical waveguide according to claim 8, characterized in that: Graphene is also evenly distributed in the thermal storage material.

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