Composite phase change device and materials for thermal energy storage

Abstract

The invention relates to a device for the storage of thermal energy. The device comprises containers arranged between two plates, wherein the containers are filled with a phase change material that store cold thermal energy when the phase change material solidifies. The device comprises a fan that produces heat transfer via forced convection. The phase change material is based on an aqueous solution of inorganic salts comprising a mixture of graphite and copper as a nucleating agent and a thickening agent to keep the nucleating agent in suspension.

Description

[0001] COMPOSITE PHASE CHANGE DEVICE AND MATERIALS FOR THERMAL ENERGY STORAGE

[0002] TECHNICAL FIELD

[0003] This disclosure relates to energy storage devices. In particular, this disclosure relates to thermal energy storage devices that use phase-change materials to store thermal energy.

[0004] DESCRIPTION OF THE STATE OF THE ART

[0005] Thermal energy storage devices are used to overcome the intermittency of renewable energy sources and optimize energy utilization in various applications. In particular, thermal energy storage devices using phase-change materials offer high energy storage density and relatively constant temperature during the phase change.

[0006] Phase-change materials can absorb and release large amounts of thermal energy during phase changes (solid to liquid and vice versa) without a significant temperature change. This offers advantages such as high-density energy storage per unit volume, a wide operating temperature range allowing for diverse applications, and quasi-isothermal behavior, meaning that during phase changes, these materials maintain a nearly constant temperature, providing a stable source of heat or cold.

[0007] Therefore, there is a need for thermal energy storage devices that utilize phase-change materials. Prior art disclosures such as WO2021076741A1 and US6059016A, which relate to thermal energy storage devices, are identified.

[0008] WO2021076741A1 discloses a thermal energy management kit comprising one or more thermal storage cells. The individual thermal storage cells are arranged in a container. The container may be made of a thermally conductive material and has an internal volume. The thermal storage cell further includes a phase-change material arranged within the cell's internal volume.

[0009] Specifically, WO2021076741A1 discloses that thermal storage cell containers comprise an outer surface defining an inner volume. The container can have any shape or arrangement. For example, in some embodiments, the container is in the form of a plate, blade, grid, or panel.

[0010] WO2021076741A1 also discloses that thermal energy management kits comprise one or more thermal storage cells. The thermal storage cells may be configured in an array or group, or they may be mounted or arranged separately within an enclosure. A thermal storage battery may comprise a single thermal storage cell, although thermal energy management kits may contain any number of thermal storage cells. The number of thermal storage cells can be selected based on the desired heating capacity of the thermal storage battery formed by the thermal energy management kit. The thermal energy management kit may be modular, allowing for the addition or removal of thermal storage cells to meet the desired heating or cooling capacity of the thermal storage battery.Thermal storage cells comprise a container and a phase-change material disposed within an internal volume of the container. The cells may be in the form of an elongated structure such as a rod, tube, or other elongated prismatic structure having an internal volume that can be totally or partially filled with a phase-change material. WO2021076741A1 further discloses that the kit also comprises at least one fan that directs airflow from a first side of the thermal storage cells to a second side of the thermal storage cells. Such a fan can facilitate efficient transfer of thermal energy from the phase-change material disposed within the containers of the thermal storage cells. WO2021076741A1 further discloses a kit comprising a plurality of fans.For example, in some cases, a kit comprises a first fan that rotates clockwise and a second fan that rotates counterclockwise.

[0011] WO2021076741A1 states that the phase-change material may have any composition, provided it is not inconsistent with the disclosure objectives. In some embodiments, for example, a phase-change material comprises an inorganic composition. In other cases, a phase-change material comprises an organic composition. In some cases, a phase-change material comprises a salt hydrate. The phase-change material may also be water. In other embodiments, the phase-change material is not water. A phase-change material may store or release thermal energy in the process of undergoing a phase transition (such as between a solid and a liquid state, or between a solid and a gel state).

[0012] WO2021076741A1 further indicates that the phase-change material can be modified by the inclusion of one or more additives. The additive can be mixed with a phase-change material for use in the thermal management kit. In some embodiments, the additive comprises a thermal conductivity modulator. A thermal conductivity modulator increases the thermal conductivity of the phase-change material. A thermal conductivity modulator comprises carbon, including graphitic carbon.

[0013] US6059016A, for its part, refers to a thermal energy storage system comprising a casing that defines an interior region. The phase-change material is arranged in sealed, elongated containers placed in a stacked arrangement within the interior region. US6059016A specifically discloses that the geometry of the casing may vary. The casing may be constructed from a variety of commonly available, lightweight, and durable metals and plastics. The casing includes a first wall that is shaped to incorporate an inlet for receiving airflow or flow of other heat exchange fluids. The location of the inlet may vary depending on design constraints. The inlet may be formed in other walls of the casing or on a lower surface of the casing.

[0014] US6059016A also discloses that the housing includes a second wall (typically the top wall) shaped to include at least one air outlet. The second wall may be removable to allow access to the interior of the housing. The housing may include at least one fan adjacent to each outlet to induce airflow from the interior of the housing through the outlet. Elongated, sealed containers filled with phase-change material are arranged within the housing in a stacked configuration. The housing may also include upper and lower racks or shelves to ensure that the container assembly remains compact. The containers are arranged so that their long axes are perpendicular to the direction of airflow through the interior of the housing.

[0015] US6059016A further discloses that the containers arranged within the housing are elongated, flexible cylinders made of plastic that define an internal chamber for containing phase-change material. The type of plastic will vary depending on the temperature requirements. Metal containers may be necessary when applications require the use of phase-change material with a particularly high melting point. The dimensions of the containers will vary with the application.

[0016] US Patent 6059016A finally discloses that a wide variety of phase-change materials can be used. Phase-change materials are generally chosen based on their latent heat and melting point characteristics, but they can also be selected for additional qualities. Typical classes of usable phase-change materials include paraffin waxes, salt hydrate solutions, and water.

[0017] However, the cited prior art documents do not show any thermal energy storage devices that reduce undesirable effects, such as the degree of supercooling and limited thermal conductance, thereby improving the device's energy efficiency and effectiveness for thermal energy storage.

[0018] BRIEF DESCRIPTION

[0019] This disclosure relates to a thermal energy storage device. The device comprises a first plate and a second plate, the two plates being arranged opposite each other and forming a space between them. In particular, the second plate includes a central opening to which a fan is connected.

[0020] The device further comprises a plurality of containers arranged in the space between the first and second plates and arranged around the fan. Specifically, the containers are filled with a phase-change material. The phase-change material is based on an aqueous solution of inorganic salts and comprises a nucleating agent and a thickening agent.

[0021] The phase change material comprises between 0.5% and 5% of a graphite and copper mixture. The graphite and copper mixture has a proportion of between 90% and 95% graphite by weight and between 5% and 10% copper by weight. The phase change material also comprises between 1% and 5% of a thickening agent. Specifically, the composition of the phase change material is based on an aqueous solution of inorganic salts.

[0022] BRIEF DESCRIPTION OF THE FIGURES

[0023] Figure 1 shows an isometric view of one embodiment of a thermal energy storage device. Figure 2 shows an isometric view of one embodiment of the first plate of the thermal energy storage device.

[0024] FIG. 3 shows an isometric view of one modality of the second plate of the device for thermal energy storage.

[0025] FIG. 4 shows an isometric view of one type of container that stores the phase change material.

[0026] FIG. 5 shows one modality of the thermal energy storage device installed in a refrigerator in different positions.

[0027] DETAILED DESCRIPTION

[0028] Renewable energy sources produce energy intermittently. For example, wind turbines require wind to generate electricity, and photovoltaic panels can only produce electricity when there are minimum levels of solar radiation. Equipment that requires power around the clock, such as perishable food storage equipment like refrigerators or freezers, therefore needs devices that store cold thermal energy and can release it when solar energy is unavailable, allowing them to operate completely autonomously.

[0029] This disclosure relates to a device for storing thermal energy. The device comprises two plates between which containers filled with a phase-change material are arranged. The device may further comprise a fan located on one of the plates or between them, which circulates air around the containers to facilitate heat exchange.

[0030] To charge (i.e., store thermal energy) and discharge (i.e., release the stored thermal energy) in the thermal energy storage device of this disclosure, heat exchange must be promoted between the phase-change material and the air surrounding the containers holding the phase-change material. In a cold thermal energy storage application, during charging, the phase-change material solidifies by dissipating heat to the air, which must be at a temperature lower than the solidification temperature of the phase-change material. During discharge, the phase-change material melts and absorbs heat from the air, which is at a temperature higher than the melting temperature of the phase-change material.

[0031] On the other hand, and for the purposes of this disclosure, average thermal conductance is the factor that determines the effectiveness of the thermal energy storage device for heat transfer. Thermal conductance is a measure of the thermal energy storage device's ability to transfer heat between the phase-change material and the air flowing through the device. It is defined as the rate of heat transfer to or from the phase-change material per unit temperature difference; that is, how many watts (W) of heat are transferred to or from the phase-change material for each Kelvin (K) difference between the melting point and the fluid inlet temperature. Storage capacity refers to the amount of energy that the thermal energy storage device can store.

[0032] Referring to FIGs. 1, 2 and 3, in one modality of disclosure, the thermal energy storage device (20) comprises:

[0033] - a first plate (1) and a second plate (5) opposite each other and forming a space between them, the second plate (5) comprises a central opening (6);

[0034] - a fan (9) connected in the central opening (6) of the second plate (5);

[0035] - a plurality of containers (11) arranged in the space between the first plate (1) and the second plate (5) and arranged around the fan (9); wherein the plurality of containers (11) are filled with a phase-change material (15); wherein the phase-change material (15) is based on an aqueous solution of inorganic salts, and wherein said phase-change material comprises a nucleating agent and a thickening agent. At the start of loading in a cooling application, the phase-change material (15) is in a liquid state at a temperature above its melting point. Then, a fluid, for example, air at a temperature below the melting point of the phase-change material (15), is propelled by the fan (9) between the first plate (1) and the second plate (5), and said fluid comes into contact with the external surfaces of the plurality of containers (11).When the fluid comes into contact with the plurality of containers (11), it absorbs heat from the phase-change material, which cools to its melting point and then begins to solidify. Therefore, when the fluid flows out of the first plate (1) and the second plate (5), it exits at a temperature higher than the inlet temperature. Conversely, during discharge in a cooling application, the phase-change material is initially in a solid state and at a temperature lower than its melting point. A fluid, such as air, passing between the plates dissipates heat to the phase-change material, which heats up and then begins to melt. At discharge, this fluid exits at a temperature lower than the inlet temperature, thus achieving a cooling effect.

[0036] For example, in locations or situations where the availability of electrical power is variable, the thermal energy storage device (20) can be placed inside a refrigerated compartment. During periods of high electrical power availability, the refrigeration cycle can operate in conjunction with the fan (9) to solidify and cool the phase-change material within the plurality of containers (11) by exchanging heat with the air in the refrigerated compartment, which reaches a temperature slightly below the melting point of the phase-change material. The phase-change material solidifies at a temperature appropriate for the specific application.In the case of a preservation compartment, the phase-change material has a melting point between 0 and 4 degrees Celsius, and in the case of a freezing compartment, the melting point can be between -18 and -6 degrees Celsius, depending on the freezer's classification. At the end of the loading process, the fan (9) switches off to stop the heat exchange between the thermal energy storage device (20) and the refrigerated compartment. During periods of reduced electrical power availability, the refrigeration cycle stops, and the fan (9) switches on, circulating air from the refrigerated compartment between the first plate (1) and the second plate (5), where the air comes into contact with the external surfaces of the plurality of containers (11).When the fluid comes into contact with the plurality of containers (11), it dissipates heat to the phase-change material, which begins to melt. The air temperature drops to a value close to the melting point of the phase-change material (15). Therefore, the air is discharged back into the refrigerated compartment at a lower temperature than the inlet temperature, allowing the temperature inside the refrigerated compartment to be maintained at appropriate levels and ensuring the preservation of the products within it for a longer period.

[0037] Referring to FIG. 2, in particular, the thermal energy storage device (20) comprises a first plate (1) which can be square, rectangular, polygonal, or circular. The shape of the plate can also be adapted to the specific application conditions for which the thermal energy storage device (20) is to be used. In one embodiment of the disclosure, the first plate (1) is square.

[0038] On the other hand, the first plate (1) may comprise a central zone (2) where the base of a fan (9) can be connected. The first plate (1) may further comprise first cavities (3) arranged around the central zone (2) and on the surface of the first plate (1). A first end (12) of containers (11) may be arranged in the first cavities (3).

[0039] Referring to FIG. 3, the thermal energy storage device comprises a second plate (5), which may be square, rectangular, polygonal, or circular. The second plate (5) may also include a central opening (6) into which the fan (9) can be connected. The second plate (5) comprises second cavities (7) into which a second end (13) of the containers (4) can be connected. In one embodiment of the present disclosure (not illustrated), the first plate (1) comprises first cavities arranged across its entire surface. In this embodiment, the fan (9) is arranged in the central opening (6) of the second plate (5) and is connected to said second plate (5) such that it protrudes above this opening.In this configuration, the second plate (5) has a first face facing the first plate (1), and a second face opposite the first face, where the fan (9) protrudes above the second face. Accordingly, and because the fan (9) is positioned over and protrudes above the central opening (6), it does not touch the first plate (1). In this configuration, there is a space between the fan (9) and the first plate (1), allowing containers (4) to be placed across the entire surface of the first plate (1), as will be explained later.

[0040] Both the first plate (1) and the second plate (5) can be made of a metallic material selected from aluminum, copper, brass, carbon steel, cast iron, galvanized iron, chromium steels, chromium-nickel steels, chromium-nickel-titanium steels, nickel-chromium-molybdenum-tungsten alloy, ferrous chromium-molybdenum alloys, stainless steel 301, stainless steel 302, stainless steel 304, stainless steel 316, stainless steel 405, stainless steel 410, stainless steel 430, stainless steel 442, manganese alloy steel and combinations thereof.

[0041] Furthermore, the first plate (1) and the second plate (5) may also be made of a polymeric material selected from epoxy resin, polyamide (PA), polybutylene terephthalate (PBT), polycarbonate (PC), polyetheretherketone (PEEK), polyethylene (PE), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyoxymethylene (POM), styrene acrylonitrile (SAN), and combinations thereof, with or without filler material such as carbon fibers, glass fibers, or aramid fibers. In one embodiment of the disclosure, the first plate (1) and the second plate (5) are made of polymethyl methacrylate. In a particular example of the disclosure, the first plate and the second plate are made of polycarbonate.Polycarbonate allows the first plate (1) and the second plate (5) to have rigidity, mechanical durability and adequate chemical resistance against corrosive substances that can be used as a phase change material.

[0042] The first cavities (3) of the first plate (1) and the second cavities (7) of the second plate (5) allow the containers (11) filled with the phase change material (15) to be held fixed between the first plate (1) and the second plate (5).

[0043] The first cavities (3) of the first plate (1) and the second cavities (7) of the second plate (5) can be holes made on the surface of the first plate (1) and the second plate (5) respectively. A first end (12) of the plurality of containers (11) is inserted into the first cavities (3) of the first plate (1), and a second end (13) of the containers (11) is inserted into the second cavities (7) of the second plate (5).

[0044] In one embodiment of this disclosure, the first plate (1) and the second plate (5) are squares of equal area and are positioned opposite each other, creating a space between them. This allows a fluid, which may be in a gaseous state such as air, to circulate between the containers (11) located between the first plate (1) and the second plate (5). The particular shape of this embodiment allows the thermal energy storage device (20) to be placed on the shelves of a household refrigerator or freezer, and the airflow between the first plate (1) and the second plate (5) to be more uniform.

[0045] Referring again to FIG. 1, the fan (9) can be placed in the space between the first plate (1) and the second plate (5).

[0046] In one embodiment of the disclosure, the fan (9) is a centrifugal fan that draws air in through its front opening. In this embodiment, the fan (9) is located in the central opening (6) of the second plate (5) and circulates air between the containers (11) positioned between the first plate (1) and the second plate (5). Furthermore, in this embodiment, the fan (9) has a base, which is located in a central area (2) of the first plate (1). For the purposes of this disclosure, the base of the fan (9) is understood to be the frame or support that attaches the fan (9) to a surface. In the embodiment where the fan (9) is a centrifugal fan, the base attaches the fan (9) to the first plate (1).

[0047] For the purposes of this disclosure, a centrifugal fan is a fan that generates an airflow perpendicular to the fan's axis. The technical effect of using a centrifugal fan (9) is that it allows the fan to be positioned between the plates, enabling a more compact design of the energy storage device (20). Furthermore, the centrifugal fan (9) can generate a pressure differential that allows air to circulate between the containers (11) even if the containers (11) are arranged more densely, i.e., with less space between them.

[0048] In one embodiment of the disclosure, the fan (9) is an axial fan. The axial fan can be connected to the central opening (6) of the second plate (5) so that it protrudes above the second plate (5). In this embodiment, the fan (9) is not positioned between the first plate (1) and the second plate (5). In this embodiment, the axial fan receives air from its rear and discharges it from the front, circulating air between the containers (11) and exiting through the central opening (6) of the second plate (5). The axial fan (9) is implemented in applications of the thermal energy storage device (20) where high flow rates are required and a high pressure differential is not necessary. Furthermore, since the fan does not occupy space between the plates, this space is filled by containers (11).For the purposes of this disclosure, an axial fan is a fan in which the airflow occurs parallel to the axis of the fan.

[0049] The fan (9) of the thermal energy storage device (20) enables forced convection heat exchange, as it activates air movement between the first plate (1) and the second plate (5) and around the plurality of containers (11). In the forced convection heat transfer mechanism, significantly higher convection coefficients can be achieved compared to natural convection. Therefore, the surface area required for the containers (11) filled with phase-change material for heat transfer is reduced, and the full required thermal energy load can be achieved with a more compact design.

[0050] For the purposes of this disclosure, forced convection is a heat transfer process between a fluid and a solid surface—in this case, the air flowing between the plates and the containers with the phase-change material—that occurs due to the induced movement of the fluid by an external force. In the present invention, the external force is a fan. Natural convection, or free convection, is a heat transfer process that occurs in a fluid due to differences in density and temperature within the fluid itself, without the intervention of external devices to induce movement.

[0051] The thermal energy storage device (20) may further comprise fastening means (14) that join the first plate (1) and the second plate (5). The fastening means (14) are selected from the group comprising: rivets, screws and bolts, pins, staples, clamps, clips, spacers, welding or fusion with external filler material or of the same material as the plates, or other fastening means known to a person reasonably skilled in the art. In one embodiment of the disclosure, the fastening means (14) are spacers and screws that are attached to the first plate (1) and the second plate (5). The spacers maintain the distance between the first plate (1) and the second plate (5), while the screws ensure the mechanical connection of the parts. The fastening means (14) provide structural rigidity to the thermal energy storage device (20).In one form of disclosure when the first plate (1) and the second plate (5) are square, the joining means (14) are arranged at the corners of the plates.

[0052] The thermal energy storage device (20) further comprises a plurality of containers (11) arranged in the space between the first plate (1) and the second plate (5). In the embodiment of the disclosure in which the thermal energy storage device (20) comprises a centrifugal fan (9), the plurality of containers (11) can be arranged around the fan (9). In the embodiment of the disclosure in which the thermal energy storage device (20) comprises an axial fan (3), the plurality of containers (11) can occupy the entire surface between the first plate (1) and the second plate (5).

[0053] The containers (11) can have a square, rectangular, polygonal, or circular cross-section. In one embodiment of the disclosure, the containers (11) have a circular cross-section. Each container (11) can also have a volume that allows it to store between 1 ml and 5 ml of phase-change material (15), between 5 ml and 10 ml of phase-change material, or between 10 ml and 100 ml of phase-change material. These ranges allow for different types of containers to store larger or smaller quantities of phase-change material (15) and to use the thermal energy storage device (20) in different applications.

[0054] In thermal energy storage applications in appliances such as household refrigerators, these containers can have volumes on the order of 1 to 5 ml. A volume in this range allows for good heat transfer due to the low thermal conductivity of the phase-change material, compared to containers with larger volumes where heat transfer is lower. In other applications requiring containers with volumes greater than 5 ml, containers with a high aspect ratio are preferred; that is, tall cylinders with small diameters, which allows for adequate heat transfer.

[0055] The containers (11) can be made of a metallic material selected from aluminum, bronze, copper, brass, carbon steel, cast iron, galvanized iron, chromium steels, chromium-nickel steels, chromium-nickel-titanium steels, nickel-chromium-molybdenum-tungsten alloy, ferrous chromium-molybdenum alloys, stainless steel 301, stainless steel 302, stainless steel 304, stainless steel 316, stainless steel 405, stainless steel 410, stainless steel 430, stainless steel 442, manganese alloy steel and combinations thereof.

[0056] The containers (11) may also be made of a polymeric material selected from epoxy resin, polyamide (PA), polybutylene terephthalate (PBT), polycarbonate (PC), polyetheretherketone (PEEK), polyethylene (PE), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyoxymethylene (POM), styrene acrylonitrile (SAN), and combinations thereof, with or without filler material such as carbon fibers, glass fibers, or aramid fibers. In one embodiment of the disclosure, the containers (11) are made of polypropylene.

[0057] Referring to FIG. 4, the containers (11) have a first end (12) and a second end (13) where the first end (12) of the containers (11) can be rounded, so that the first end (12) is inserted into the first cavities (3) of the first plate (1).

[0058] The second end (13) of the containers (11) may comprise an opening that is closed by a lid, wherein said opening allows the containers to be filled with a phase-change material (15). The second end (13) of the containers (11) is inserted into the second cavities (7) of the second plate (5). One of the technical effects of having a lid at the second end (13) of the containers (11) is that it allows for changing the phase-change material, thereby enabling the thermal energy storage device to be used in different applications requiring different types of phase-change materials. For example, in applications such as transporting medicines, vaccines, or biological materials, a phase-change material with a high latent heat of fusion may be required because it allows for storing more energy and thus provides longer autonomy in order to maintain these products at the required temperature for a longer period.The phase-change material used in this case may differ from the phase-change material used in a household refrigeration application. In this application, phase-change materials other than eutectic solutions could be used, particularly if melting temperatures above zero degrees Celsius are required. In such cases, other phase-change materials such as paraffins, fatty acids, or salt hydrates may be used. In one embodiment of this disclosure, the containers (11) may also be capsules sealed with a bonding medium, such as adhesives, heat-sealing, or similar bonding means. The container (11) is filled with the phase-change material and sealed. This facilitates the mass production of the containers (11) filled with phase-change material.

[0059] On the other hand, in one embodiment of the disclosure, the plurality of containers (11) may be interconnected by a distribution or collection conduit that allows the circulation of the phase-change material between the containers (11). In this embodiment, with all the containers (11) interconnected, a filling port allows all the containers to be filled simultaneously, such that each container (11) does not have an individual filling port with a lid. The technical effect is to facilitate the filling of the containers (11), as it is not necessary to fill each container (11) individually with the phase-change material.

[0060] Furthermore, in one embodiment of the disclosure, the containers with the phase-change material may not include a lid. In this embodiment, the phase-change material is added to the containers (11) during the manufacture of the thermal energy storage device (20). In this embodiment of the disclosure, the phase-change material in the thermal energy storage device (20) can be modified by replacing the containers (11) with other containers (11) that have been filled with a different phase-change material.

[0061] Also, the cavities (3, 7) of the plates (1, 5) can have a shape that is integral with both the first end (12) and the second end (13) of the containers (11). For example, one of the technical effects of the rounded shape of the first end (12) of the containers (11) is that it allows them to be coupled to the second cavities (7) of the second plate (5) without the use of adhesives or mechanical joining means.

[0062] The containers (11) of the thermal energy storage device (20) are filled with a phase-change material (15). For the purposes of this disclosure, a phase-change material (15) is a substance that has the ability to store and release large amounts of thermal energy when it undergoes a phase transition between solid and liquid. During the solidification or melting process, the material releases or absorbs thermal energy without experiencing a significant change in its temperature.

[0063] The phase change material (15) may be based on an aqueous solution of inorganic salts. For the purposes of this disclosure, an aqueous solution is a homogeneous mixture of a solute and water, which acts as the solvent.

[0064] Furthermore, the aqueous solution of inorganic salts on which the phase-change material (15) of this disclosure is based may be a eutectic solution. For the purposes of this disclosure, a eutectic solution is a mixture of two or more substances that combine to form a homogeneous liquid solution. Such a homogeneous liquid solution has a lower melting point than any of the individual components of the eutectic solution separately. This lower melting point is known as the eutectic melting point, since at this temperature all components of the solution change phase simultaneously. A eutectic salt solution, or eutectic salt, is a solution comprising salts and water that can change phase at relatively low temperatures and has a high capacity for storing latent thermal energy.

[0065] The phase change material (15) is based on an aqueous solution of inorganic salts selected from the group consisting of: ammonium chloride, potassium chloride, barium chloride, zinc sulfate, sodium chloride, ammonium sulfate, sodium bisphosphate, sodium sulfate, potassium nitrate, sodium hydroxide, and sodium carbonate.

[0066] For the purposes of this disclosure, inorganic salts are chemical compounds resulting from the combination of a metal with a nonmetal through ionic bonds. Inorganic salts form when the atoms of the elements involved transfer electrons to each other, creating positive ions (cations) and negative ions (anions). Cations are generally metals, while anions can be nonmetals or polyatomic. The phase-change material (15) further comprises a nucleating agent and a thickening agent.

[0067] For the purposes of this disclosure, a nucleating agent is a substance used to induce or accelerate a nucleation process. Nucleation, in turn, is the process by which the nuclei of a new phase form within a medium that is in another phase. The nucleating agent can be a solid particle, an impurity, or other defects that act as starting points where crystalline nuclei form. These starting points promote the rate of crystallization and the formation of more homogeneous and larger crystals.

[0068] The phase change material (15) of this disclosure may comprise a mixture of graphite and copper as the nucleating agent. The nucleating agent of the phase change material (15) of this disclosure may be a mixture of graphite and copper with a concentration of between 0.5% and 5% by weight of the total phase change material (15). In particular, the graphite and copper mixture of the phase change material (15) of this disclosure may have a graphite concentration of between 90% and 95% and a copper concentration of between 5% and 10% by weight.

[0069] The nucleating agent range is the minimum range found in experimental tests to achieve the nucleation effect, ensuring that components added to the phase-change material that do not solidify / melt do not reduce its thermal energy storage capacity. Accordingly, nucleating agent concentrations below 5% produced a reduction in supercooling. The concentration to be used depends on the salt used in the phase-change material.

[0070] On the other hand, the copper and graphite concentrations in the nucleating agent allowed for a reduction in supercooling, improved energy efficiency of the device, and enhanced effectiveness for thermal energy storage in experimental tests. For the purposes of this disclosure, energy efficiency will be understood as the ability to store and transfer a greater amount of thermal energy.

[0071] A thickening agent is a substance added to a liquid or mixture to increase its viscosity, making the mixture denser or thicker. This is achieved by altering the liquid's molecular structure, causing the particles to clump together or disperse, thus increasing resistance to flow. The thickening agent may be a naturally occurring thickener selected from guar gum, xanthan gum, pectin, starch, or other naturally occurring thickeners known to a person with a basic understanding of the subject, or combinations thereof. Alternatively, the thickening agent may be a synthetic thickener selected from acrylic polymers, carboxymethylcellulose, hydroxyethylcellulose, cellulose gum, polyacrylamide, or other synthetic thickeners known to a person with a basic understanding of the subject, or combinations thereof. In one version of the information, the thickening agent is xanthan gum.

[0072] The phase change material (15) of this disclosure comprises thickening agent in a concentration of between 1% and 5% by weight.

[0073] The technical effect of the thickening agent is to keep the nucleating agent in suspension within the phase-change material. Keeping the nucleating agent in suspension allows it to reduce supercooling by having its nucleation points homogeneously dispersed.

[0074] The phase-change material (15) described herein, comprising a mixture of graphite and copper as a nucleating agent, inorganic salts, and a thickening agent in the concentration ranges disclosed above, reduces undesirable effects such as supercooling that occur in phase-change materials. Furthermore, the phase-change material (15) described herein has beneficial effects such as increased thermal conductivity. For the purposes of this disclosure, supercooling is a phenomenon that occurs when a substance is cooled below its freezing point without actually solidifying; that is, the liquid substance is supercooled without initiating the transition to the solid state and remains in a metastable liquid state.

[0075] Supercooling is an undesirable phenomenon in phase-change materials because it reduces the energy efficiency of the phase-change material (15). Cooling the phase-change material below its solidification point to initiate the phase change requires the refrigeration system to operate at a lower temperature and with less efficiency, resulting in higher electricity consumption. Furthermore, a high degree of supercooling implies that the temperature range within the refrigerated compartment must be expanded, which is not feasible in some applications where low temperatures could negatively affect the stored products.

[0076] For the purposes of this disclosure, thermal conductivity is a physical property of materials that measures a material's ability to conduct heat. For example, a material with high thermal conductivity, such as copper or aluminum, can transfer heat at high power levels requiring a low temperature difference, while a material with low thermal conductivity, such as polystyrene or wood, transfers heat at low power levels even with a high temperature difference compared to materials like copper.

[0077] The size of the first plate (1), the second plate (5), the containers (11), as well as the size and power of the fan (9), are selected according to the application of the thermal energy storage device (20). Referring to Figure 5, in one embodiment of the disclosure, the thermal energy storage device (20) is used to maintain the temperature in a refrigerator. In this embodiment, the device must have a size, weight, and volume that allow it to be placed on one of the shelves of, for example, a refrigerator.

[0078] In one embodiment of the disclosure, the thermal energy storage device (20) may further comprise temperature sensors (not illustrated) configured to detect the temperature around the thermal energy storage device (20). The temperature sensors are connected to a computing unit and transmit temperature data to the computing unit. Additionally, the computing unit is connected to the fan (9), enabling control of the fan's rotational speed. The fan's rotational speed will increase when the temperature inside the cooler rises in order to increase airflow around the containers (11).

[0079] The computing unit can be selected from the group comprising: microcontrollers (e.g., PSoC 4BLE), microprocessors, DSCs (Digital Signal Controllers), FPGAs (Field Programmable Gate Arrays), CPLDs (Complex Programmable Logic Devices), ASICs (Application Specific Integrated Circuits), SoCs (System on Chips), PSoCs (Programmable System on Chips), computers, servers, tablets, cell phones, smartphones, signal generators, and computing units, processing units, or processing modules known to a person with a moderate knowledge of the subject, and combinations thereof. In one form of popularization, the computing unit is a microprocessor.

[0080] On the other hand, the computing unit may include a communications module or circuit with one or more connection ports configured to access a communications network, such as a wired network or combinations thereof. Examples of communications networks include VPNs, LANs, WANs, and other equivalent and similar communications networks familiar to anyone with a basic understanding of the subject, as well as combinations thereof.

[0081] Examples of wireless communication modules that use a wireless communication technology are selected from the group consisting of Bluetooth, WiFi, Radio Frequency RFID (Radio Frequency Identification), UWB (Ultra Wide Bandwidth), GPRS, Konnex or KNX, DMX (Digital Multiplex), WiMax, IR (infrared) and equivalent wireless communication technologies that are known to a moderately knowledgeable person and combinations of the above.

[0082] The wireless communications module allows remote control of the thermal energy storage device (20).

[0083] In one disclosure modality, the thermal energy storage device may comprise batteries that provide electrical power to the fan (9), the computing unit, and the temperature sensors.

[0084] Furthermore, this disclosure refers to a phase-change material composition comprising between 0.5% and 5% of a mixture of graphite and copper, wherein said mixture has a proportion of between 90% and 95% by weight of graphite and between 5% and 10% by weight of copper; and between 1% and 5% of a thickening agent, wherein said composition is based on an aqueous solution of inorganic salts.

[0085] The composition of the phase-change material may be based on an aqueous solution of inorganic salts. Furthermore, the aqueous solution of inorganic salts on which the composition of the phase-change material in this disclosure is based may be a eutectic solution.

[0086] The composition of the phase change material is based on an aqueous solution of inorganic salts selected from the group consisting of: ammonium chloride, potassium chloride, barium chloride, zinc sulfate, sodium chloride, ammonium sulfate, sodium bisphosphate, sodium sulfate, potassium nitrate, sodium hydroxide, and sodium carbonate.

[0087] The phase-change material composition described herein may include a mixture of graphite and copper as the nucleating agent. The nucleating agent in the phase-change material composition described herein may be a mixture of graphite and copper with a concentration of between 0.5% and 5% by weight of the total phase-change material (15). In particular, the graphite and copper mixture in the phase-change material composition described herein may have a graphite concentration of between 90% and 95% and a copper concentration of between 5% and 10% by weight. The range of nucleating agent in the phase-change material composition described herein is the minimum range found in experimental tests to achieve the nucleation effect, so that the addition of components to the phase-change material that do not solidify / melt does not diminish the thermal energy storage capacity.Based on the above, nucleating agent concentrations below 5% resulted in a reduction of supercooling. The concentration to be used depends on the salt used in the phase-change material composition.

[0088] On the other hand, the concentrations of copper and graphite in the nucleating agent of the phase change material composition of the present disclosure allowed in experimental tests to obtain a reduction in supercooling, improvement in the energy efficiency of the thermal energy storage device and effectiveness for thermal energy storage.

[0089] A thickening agent is a substance added to a liquid or mixture to increase its viscosity, making the mixture denser or thicker. This is achieved by altering the liquid's molecular structure, causing the particles to clump together or disperse, thus increasing resistance to flow. The thickening agent may be a naturally occurring thickener selected from guar gum, xanthan gum, pectin, starch, or other naturally occurring thickeners known to a person with a basic understanding of the subject, or combinations thereof. Alternatively, the thickening agent may be a synthetic thickener selected from acrylic polymers, carboxymethylcellulose, hydroxyethylcellulose, cellulose gum, polyacrylamide, or other synthetic thickeners known to a person with a basic understanding of the subject, or combinations thereof.In one embodiment of the disclosure, the thickening agent in the phase-change material composition of this disclosure is xanthan gum. The phase-change material composition of this disclosure comprises a thickening agent at a concentration of between 1% and 5% by weight. The technical effect of the thickening agent in the phase-change material composition of this disclosure is to keep the nucleating agent in suspension within the phase-change material. Keeping the nucleating agent in suspension allows it to reduce supercooling by having different nucleation points dispersed homogeneously.

[0090] The composition of the phase-change material described herein, comprising a mixture of graphite and copper as a nucleating agent, inorganic salts, and a thickening agent at the concentration ranges disclosed above, reduces undesirable effects such as supercooling that can occur in phase-change materials. Furthermore, the composition of the phase-change material (15) described herein has beneficial effects, such as increased thermal conductivity.

[0091] Supercooling is an undesirable phenomenon in phase change materials because it reduces the energy efficiency of the phase change material composition. Cooling the phase change material composition below its solidification point to initiate the phase change requires the cooling system to operate at a lower temperature and with less efficiency, resulting in higher electricity consumption.

[0092] Furthermore, a high degree of supercooling implies that the temperature range within the refrigerated compartment must be expanded, which is not feasible in some applications where low temperatures may affect the preserved products.

[0093] Examples

[0094] Example 1

[0095] Referring to Figures 1 and 5, a thermal energy storage device (20) was designed for use in a refrigerator. The thermal energy storage device (20) is configured to weigh less than 4 kilograms and have a total volume of less than 6 liters. This allows the thermal energy storage device (20) to be installed on the shelves of a household refrigerator. To achieve this size and weight, the thermal energy storage device (20) is configured as follows:

[0096] The thermal energy storage device comprises:

[0097] - a first plate (1) and a second plate (5), the first plate (1) and the second plate (5) are square in shape with sides of 360 millimeters.

[0098] - The material from which the first plate (1) and the second plate (5) are manufactured must be easy to clean and prevent the risk of food contamination. In this example, the first plate (1) and the second plate (5) are made of polycarbonate.

[0099] - The first plate (1) has 24 first cavities (3) on each side so that in total it has 484 first cavities (3).

[0100] - The second plate (5) has 24 second cavities (7) so that in total it has 484 second cavities (7).

[0101] - The thermal energy storage device (20) comprises 484 containers (11), each container (11) being a circular cross-section container that stores two milliliters of phase change material (15).

[0102] - In total, the thermal energy storage device (20) comprises about one liter of phase change material (15). The first end (12) of each container (11) is arranged in a first cavity (3) of the first plate (1).

[0103] - The second end (13) of each container (11) is arranged in a second cavity (7) of the second plate (5).

[0104] Each container (11) is filled with a phase-change material. The material of the containers (11) in which the phase-change material (15) is to be encapsulated must be compatible with both inorganic and organic substances. Furthermore, to facilitate heat transfer, small containers are preferred, as they allow for an increased surface area per unit volume. The containers (11) are made of polypropylene. Polypropylene is selected for its good chemical stability and its compatibility with the phase-change material (15).

[0105] The thermal energy storage device further comprises a fan (9). The fan (9) is a Delta brand axial fan with a 120 mm side diameter, reference AFC1212D-SP19. The phase change material (15) is an aqueous solution comprising an inorganic salt, potassium chloride, at a concentration of 19.5%. The phase change material (15) also comprises a thickening agent, which is a mixture of 95% graphite by weight and 5% copper by weight. Additionally, the phase change material (15) includes a thickening agent, xanthan gum, at a concentration of 1.5% by weight.

[0106] The fan (9) is connected to a rechargeable battery and is manually activated. The fan (9) has an electrical power consumption of less than 3.84 Watts.

[0107] When the refrigerator is powered by electrical energy from a photovoltaic solar source, the thermal energy storage device (20) cools and solidifies the phase change material (15) in the containers (11), storing thermal energy.

[0108] When the refrigerator is no longer powered by electricity, the thermal energy storage device (20) releases the thermal energy stored in the containers (4) by melting the phase-change material, thus maintaining the temperature in the refrigerator. The forced airflow generated by the fan (9) helps transfer the thermal energy to / from the containers and / or the air in the refrigerated compartment with a small temperature difference.

[0109] With this configuration the thermal energy storage device (20) achieves an average thermal conductance of 15 W / K and a total storage capacity of approximately 80 Wh.

[0110] It should be understood that the present disclosure is not limited to the modalities described and illustrated, since, as will be evident to a person versed in the art, there are possible variations and modifications that do not depart from the spirit of the invention, defined by the following claims.

Claims

CLAIMS 1. A device for thermal energy storage (20), comprising: - a first plate (1) and a second plate (5) opposite each other and forming a space between them, the second plate (5) comprises a central opening (6); - a fan (9) connected in the central opening (6) of the second plate (5); - a plurality of containers (11) arranged in the space between the first plate (1) and the second plate (5); wherein the plurality of containers (10) are filled with a phase change material (15); wherein the phase change material (15) is based on an aqueous solution of inorganic salts, and wherein said phase change material comprises a nucleating agent and a thickening agent.

2. The device of Claim 1, wherein the fan (9) is arranged between the first plate (1) and the second plate (5) and the containers (11) are arranged around the fan (9).

3. The device of Claim 1, wherein the inorganic salts are selected from the group consisting of: ammonium chloride, potassium chloride, barium chloride, zinc sulfate, and sodium chloride.

4. The device of Claim 1, wherein the nucleating agent is a mixture of graphite and copper, and wherein said mixture has a concentration of between 0.5% and 5% by weight of the total phase change material (15).

5. The device of Claim 4, wherein the graphite and copper mixture has a graphite concentration of between 90% and 95% by weight and a copper concentration of between 5% and 10% by weight.

6. The device of Claim 1, wherein the containers (11) are connected to each other by means of a conduit that allows circulation of the phase change material (15) between the containers (11).

7. The device of Claim 4, wherein the thickening agent has a concentration of between 1% and 5% by weight.

8. The device of Claim 1, wherein the aqueous solution of inorganic salts is a eutectic solution.

9. A composition of a phase-change material, comprising: - 0.5% and 5% of a mixture of graphite and copper, wherein said mixture has a proportion of between 90% and 95% by weight of graphite and 5% and 10% by weight of copper; and - 1% and 5% of a thickening agent; wherein said composition is based on an aqueous solution of inorganic salts.

10. The composition of Claim 8, wherein the inorganic salts are selected from: ammonium chloride, potassium chloride, barium chloride, zinc sulfate, and sodium chloride.

11. The composition of Claim 9, wherein the aqueous solution of inorganic salts is a eutectic solution.

Images