Phase change material (PCM) thermal storage device with regeneration of the heat exchange surface

Abstract

The present invention relates to a device comprising at least one tank (1), at least one phase change material PCM (2) that is liquid (200) / solid (201) contained in this tank (1), and at least one compact heat exchange system (4) inside the tank, which heat exchange system uses a method for detaching the solid phase change material (201) from the surfaces of the heat exchange system (4) by active or passive heating means, or mechanical means, or pneumatic or hydropneumatic inflation means, or means for treating the surface of the heat exchange system (4) chosen specifically according to the properties of the phase change material (2) used, which device is characterised in that the maximum volume footprint dedicated to the heat exchange system (4) is designed so as to be much lower, in a ratio at least equal to 1:3, than the volume footprint dedicated to the storage capacity, and in that the phase change material (2) moves in its liquid (200) or solid (201) forms within the tank (1), whereby the suspended solid phase (201) must free up the space near the heat exchange surface (4) so that a new liquid volume (200) can be in contact for production, latent storage and thermal energy restitution in industrial and commercial applications.

Description

Thermal storage device using phase change materials (PCM) with regenerative heat exchange surface

[0001] The present invention relates to a thermal storage system for the production, storage, and release of heat and cold for industrial and residential applications. More specifically, it relates to a thermal exchange and storage system.

[0002] Thermal energy, historically stored in fossil form, gas and fuel oil, must find new storage solutions in order to enable the deployment of solutions that emit less carbon dioxide and are more competitive on an industrial scale.

[0003] Decarbonizing the energy mix also requires proposing effective solutions to address heating needs, which represent 51% of humanity's total energy requirements. The development of renewable heat production and thermal storage needs to expand just as much as renewable electricity production and storage. Similarly, it will be necessary to develop cold storage for industrial and residential needs.

[0004] The intermittency and uncontrollability of renewable energy production methods raise concerns about the marginal cost of energy consumed during periods of high demand (peak and off-peak hours), both economically and in terms of carbon dioxide emissions. Curtailing consumption during these periods by generating energy reserves during off-peak hours addresses both economic and environmental needs.

[0005] In addition to energy storage in electrical, chemical and mechanical form, thermal storage meets thermal and electrical needs, generally over storage periods exceeding 8 hours, thanks to low variable costs per unit of stored energy compared to other technologies.

[0006] Thermal energy storage technologies fall into three categories: sensible heat storage – achieved by increasing the temperature of a solid or liquid material; latent heat storage – achieved through the phase change of a material from solid to liquid or liquid to solid; and thermochemical storage – achieved through reversible endo- and exothermic reactions. These various solutions are still not widely implemented in industry due to the previous energy model, which focused on storing energy reserves in the form of fossil fuels consumed on demand.

[0007] Latent energy storage is a very attractive option for industrial thermal energy storage. Compared to sensible energy storage, latent energy storage is more compact and allows for energy release at a constant temperature. Compared to thermochemical energy storage, latent energy storage is significantly less expensive per unit of stored energy, more mature, and easier to operate. The ability to meet thermal energy needs at a constant temperature, with controlled thermal power, low heat losses, and a competitive cost makes latent energy storage highly advantageous from a techno-economic perspective, particularly for industrial applications involving significant capacity.

[0008] Latent heat storage uses phase-change materials that consume and release heat during liquefaction and solidification processes, respectively. The choice of phase-change material depends on the desired temperature level, the material cost, and its chemical nature, which may lead to considerations of compatibility with the rest of the installation and long-term stability.

[0009] When heat is extracted from a latent heat storage system, a solid layer of phase-change material appears at the exchange interface. The phase-change material is initially in a solid-liquid biphasic state. This solid layer thickens until the entire liquid phase has solidified, which, in the case of a final heat release requirement, corresponds to the maximum discharge point of the latent heat storage. However, during its thickening, this solid layer increases the thermal resistance of the transfer, which significantly reduces the performance and feasibility of certain heat exchanges.

[0010] When heat is supplied to a latent heat storage system, the solid phase of the phase-change material at the exchange interface is liquefied. The phase-change material then exists in a solid-liquid biphasic state. The proportion of solid phase decreases until all of it is liquefied, which, in the case of a final heat release requirement, corresponds to the maximum load point of the latent heat storage.

[0011] Phase change materials used in most applications have relatively low thermal conductivities, which severely penalizes the performance of heat exchange systems, particularly during thermal conduction phenomena through solid phases.

[0012] To address the problem of low thermal conductivity in phase change materials, a first state-of-the-art solution involves improving the intrinsic thermal conductivity of the material through the addition of additives. Patents CN111662688B and US9027633B2 propose solutions for modifying the manufacturing protocol of the phase change material and adding conductive nanoparticles which, when mixed with the material, improve its thermal conductivity. These are called Nano-enhanced Phase Change Materials (NeMCPs). This approach to improvement significantly increases the complexity of the chemical stability of the phase change material, as well as the tooling and expertise required for its fabrication, as mentioned in Punniakodi et al.

[0013] To address the problem of low thermal conductivity of phase change materials, a second state-of-the-art solution involves improving heat conduction through the reservoir by using preferential conductive pathways structurally integrated into the phase change material. This solution reduces thermal resistance along certain axes. Patent CN114656939B proposes a phase change material impregnated in expanded graphite to exhibit anisotropic and improved thermal conductivity along specific axes. Patent EP2825611B1 proposes a methodology for molding and manufacturing a graphite structure according to a specific arrangement. Similar to the previous point, this approach significantly complicates the manufacturing of the phase change material.

[0014] To address the issue of progressively increasing the thickness of the solid phase-change material layer, a state-of-the-art solution involves increasing the heat exchange surface area to storage volume ratio. This ensures that the half-distance between two heat-generating or heat-consuming surfaces inside the tank never exceeds the maximum thickness tolerated by the desired system efficiency. The heat exchange surface area can be increased using a finned tubular heat storage solution adapted for negative temperature production (patent EP3529549A1), or a multi-channel coil heat storage solution (patent WO2024005643A1).

[0015] The inventions proposed to address the problems of low thermal conductivity of phase change materials do not allow for the deployment of these solutions from a commercial point of view because the manufacturing costs associated with the addition of nanoparticles or the creation of preferential conductive paths are too high, which contradicts the key aspect of thermal storage with regard to its low variable cost per quantity of energy stored.

[0016] The inventions proposed to address the challenges of progressively increasing the thickness of the solid phase-change material layer can be grouped under the category of thermal storage devices. These solutions result in extremely large heat exchange surfaces, proportional to the maximum desired storage capacity, and oversized pipe lengths, leading to increased pressure losses, refrigerant quantities, and significantly higher costs, particularly for large storage capacities.

[0017] There is therefore an unmet need for heat exchange systems that can store latent heat in a compact, economical and efficient manner, while also being able to regenerate the heat exchange surface in order to be able to load or release heat at variable power.

[0018] The present invention aims, in particular, to overcome at least some of the drawbacks of the prior art. Specifically, one objective of the invention is to reduce the costs associated with implementing a compact, efficient thermal storage solution tailored to the specific needs of an industrial user or individual, enabling them to benefit from the advantages of latent energy storage by removing the technological barriers that currently prevent these technologies from being deployed on a large scale.

[0019] Another objective of the invention is to consider phase change material storage solutions for (very) high temperature applications on state-of-the-art decarbonization technologies, mainly heat pumps, which in the field of industrial process decarbonization require the use of refrigerants capable of withstanding high pressures.

[0020] Another objective of the invention is to consider a simplified manufacturing process as well as a more compact design for heat exchangers compared to current solutions.

[0021] Another objective of the invention is to allow operators to benefit from greater flexibility in the use of thermal storage by allowing them to vary the charging or discharging powers according to the need.

[0022] These objectives, along with others that will become clearer later, are achieved according to the invention of a device comprising a reservoir containing a storage medium—a phase-change material that can exist in either liquid or solid form, depending on the state of charge of the thermal storage. The latter is loaded and unloaded through a compact heat exchange system internal to the reservoir, which facilitates heat input and output. When the phase-change material changes from its liquid to its solid form, i.e., when heat is extracted from the reservoir, the solid phase appears on the heat exchange surface at the interface between the heat exchange system and the volume containing the phase-change material. This solid phase is detached from the exchange surface using a detachment method.The device is characterized by the fact that the maximum volumetric footprint dedicated to the heat exchange system is designed to be significantly smaller, by a ratio of at least 1:3, than the volumetric footprint dedicated to the storage capacity containing the phase-change material. This material appears and moves within the tank in either its solid or liquid form.

[0023] The heat exchange system comprises internal and external components of the tanks, dedicated to supplying or extracting heat for thermal production or consumption. The internal part of this system is compact and consists of flat or profiled plates. The most common profile features corrugations along the various axes of the plate plane. These plates can supply or extract heat to / from the outside either by conduction, in which case they are solid, or by convection through the passage of refrigerants or heat transfer fluids, in which case they have internal channels. The profiled nature of each plate is determined based on the heat transfer fluid / refrigerant combination and the phase-change material. Its purpose is to improve heat exchange performance by increasing the contact surface area.

[0024] The heat transfer fluids or refrigerants that supply or remove heat from the heat exchange system comprise a plurality of independent fluids. Thus, a plate may have several distinct and independent channels.

[0025] The internal channels within the plates, which ensure the circulation of the refrigerant or heat transfer fluid, have cross-sections with diameters varying from 1 to 15. This diameter variation compensates for the density variations of these fluids during heat input or output. If the fluid evaporates or condenses within the channels, the diameter variation helps to keep the flow velocities within ranges that maintain optimal operating conditions.

[0026] These internal channels in the plates can be manufactured using metalworking technologies such as friction stir welding, laser welding, tungsten inert gas welding or rolling welding.

[0027] To extract the solid phase of the phase-change material from the surface of the heat exchange system and transfer it into the volume of the tank dedicated to thermal storage, the device uses various detachment methods. These include active or passive heating methods, mechanical methods, pneumatic or hydropneumatic inflation methods, or surface treatment methods for the heat exchange system. The choice of the latter depends on the nature of the phase-change material used in the tank.

[0028] To facilitate this separation, the heat exchange surface between the phase change material and the internal heat exchange system of the tank can be subjected to different types of surface treatments allowing an increase or reduction in the adhesion of the phase change material.

[0029] Once detached, the suspended solid phase must clear the space near the heat exchange surface so that a new volume of liquid can come into contact – this is when the exchange surface is completely regenerated. Means of moving the solid or liquid phase within the tank include gravity, thanks to the difference in density between the liquid and solid phases of the phase-change material, or forced displacement mechanisms, such as liquid circulators or other mechanical extraction devices, which allow the solid phase to be removed.

[0030] These natural or forced movements can be supplemented by the use of screens permeable to liquid and impermeable to solid in order to contain the latter in certain specific places within the reservoir.

[0031] The storage medium, or phase-change material selected, depends on the desired storage temperature in the tank. Among other options, trimethylpenthane (C6H2) is chosen. 14 ), methanol (CH3OH), H2O-NaCl mixtures, water and propylene glycol mixtures, water and ethylene glycol mixtures, fatty acids or paraffins, organometallic compounds (C x Zn, C x Mn), polyol sugars (such as erythritol), and nitrates. These different materials currently offer the best technical and economic optimizations for latent thermal storage over temperature ranges from -163°C to above 150°C.

[0032] The mechanical lifting system allows the immersion height of the internal heat exchange system within the tank to be adjusted from outside the tank. This enables the addition or removal of liquid phase-change material to be adapted to variations in the tank's height.

[0033] The pressure adjustment system allows for the mechanical adjustment of the pressure applied within the tank, for example, via a hydraulic diaphragm system or expansion vessel. For technical and economic reasons related to the simplicity of tank manufacturing, atmospheric pressure is preferable. This allows for the adjustment of the thermodynamic properties of the phase-change material, as well as compensating for the density variation between the solid and liquid phases of the material.

[0034] The chemical composition of the phase-change medium or material contained within the reservoir can be altered to modify the thermodynamic properties of the latent storage. This alteration can be achieved by adding particles of a different nature, or, if the phase-change material is a mixture of several pure substances, by modifying the concentration of these substances within the mixture.

[0035] During the heat transfer process to the reservoir—that is, when the solid phase of the phase-change material transitions from solid to liquid—the liquid phase can be extracted from the reservoir, used to collect heat in a closed loop via a secondary heat exchanger (usually external to the reservoir), and then returned to the reservoir. Its mixing with the solid phase in the reservoir consequently induces the melting of the solid phase. This alternative operating mode, which utilizes a slight superheating of the liquid phase of the phase-change material, can be particularly advantageous in applications requiring high thermal power.

[0036] The use of the aforementioned device is for the purpose of producing, distributing and storing heat and cold.

[0037] The attached drawings illustrate the invention.

[0038] The diagram shows a cross-section of a thermal storage tank (1) filled with phase-change material (2) in liquid (200) and solid (201) form, during the loading of a cold storage tank or the unloading of a hot storage tank. The tank has an external heat exchange loop consisting of a heat exchanger (7) external to the tank (1). The diagram illustrates the concepts of thermal loading and unloading of the tank (1) via a heat exchange system (4) composed of flat plates (400), the height of which is adjusted by a mechanical lifting system (402), and heat-generating and heat-consuming units (3). The circulation of the liquid phase (200) is ensured by a forced displacement drive (8) through the external heat exchanger (7) to the tank (1). The solid phase (201) is contained by screens (5). Pressure and volume variation are controlled by the system (6).

[0039] Figure 1 shows a cross-section of the thermal storage tank (1) during the loading of a cold storage unit or the discharge of a hot storage unit, without an external heat exchange loop. This illustrates a design alternative to the previous one, eliminating the need for an external loop. The phase-change material (2) also serves as the heat transfer fluid for the downstream application.

[0040] Figure 400 represents a top view of two types of submerged flat plates. The plate on the left comprises a single channel and can contain a single refrigerant or heat transfer fluid (401), while the plate on the right comprises several channels and can contain a plurality of refrigerants or heat transfer fluids (401). These plates (400) include internal channels (403) through which heat transfer fluids or refrigerants (401) circulate.

[0041] represents a top view of a single-channel, flat, submerged plate (400) having internal channels with variable diameter passage cross-section (404) in which heat transfer fluids or refrigerants (401) circulate.

[0042] With reference to the first two drawings, the device comprises a thermal reservoir 1, in which a storage medium – a phase-change material 2 – is in liquid form 200 and solid form 201. The cooling unit 3, i.e., the heat extraction unit for the reservoir 1, is external to the reservoir 1 and allows the latter to be loaded via flat, solid plates 400 positioned vertically with the mechanical lifting system, shown in the drawing, by a screw system, 402. In this case, the heat transfer between the cooling unit 3 and the plates 400 occurs by conduction; therefore, there is no need for plates containing channels 403 and 404, internal to the plate 400. The heat is extracted from the liquid phase 200, which solidifies 201 and is progressively detached from the plate 400 via a detachment system, not shown in the drawing.The screens 5 are used to prevent the entire solid phase 201 from being directed to the bottom of the tank 1. The entire tank 1 is maintained at a constant pressure, despite changes in volume of the phase change material 2, between the liquid phase 200 and solid phase 201, thanks to the pressurization system 6.

[0043] When cooling is required for a specific purpose, in this case, it is necessary to supply heat to reservoir 1, meaning that the reservoir transfers its cooling capacity to meet the downstream process requirements. Thus, the first embodiment described in the diagram includes a recirculation loop external to reservoir 1, comprising an external heat exchanger 7. The liquid phase 200 is drawn from the top of reservoir 1, via forced displacement induced by a drive unit 8, to ensure that no solid particles 201 enter the external loop. The liquid phase is superheated by the secondary fluid located in the secondary circuit of the external heat exchanger 7. When the liquid phase of the phase-change material 200, superheated by a few degrees, is reintroduced into reservoir 1, it comes into contact with solid particles 201, which melt, while maintaining a uniform temperature within reservoir 1.Here, it is important, via the mechanical adjustment system 402, to ensure that the immersion height of the plates 400 is low enough to avoid any routing of solid phase 201 through the external recirculation loop and more particularly through the exchanger 7. In other cases, where no external recirculation is required, it is preferable to position the plates 400 as high as possible in the tank 1 in order to be able to use the entire useful volume.

[0044] In this case, the secondary loop is no longer required because the phase-change material 2 is identical to the heat transfer fluid present in the downstream thermal storage system. Therefore, the use of an intermediate heat exchange loop is no longer necessary.

[0045] In one embodiment of the invention, when heat is required, the arrangement necessitates extracting heat from the reservoir 1 via the internal plates 400. In this case, the internal heat exchange system 4 within the reservoir is mandatory, because as soon as heat is extracted from the phase-change material 2, the phase change occurs, solidifying the liquid phase 200 into a solid phase 201 at the heat exchange interface. Recirculation in an external loop is therefore not possible, and the phase-change material cannot be identical to the heat transfer fluid. Once a certain layer of solid phase-change material 201 has accumulated on the exchange surface of the plates 400, a detachment system allows the solid phase 201 to be removed from the exchange surface.Then, by the phenomenon of gravity or a phenomenon of forced circulation induced by a driving element 8, the solid phase 201 is moved away from the exchange surface in favor of a new liquid volume 200. This is how the exchange surface of the heat exchange system 4 is regenerated.

[0046] Referring to the third drawing, the two plates 400 shown depict a cross-section of two flat plates 400 comprising constant-section channels 403. The plate on the left has a single channel, while the one on the right has several channels. The use of the latter can be motivated for several reasons: to multiply the circuits within the plate 400 with the same refrigerant or heat transfer fluid 401 in order to cover a larger surface area while maintaining simple circuits to limit point pressure losses associated with a complex circulation design, and to reduce the diameter of each circuit, thus limiting the thickness of the plate 400 and ensuring the mechanical strength of the system. This last point not only simplifies the manufacturing process of the plates 400 but also reduces the thermal resistance of the exchange between the refrigerant or heat transfer fluid 401 and the phase-change material 2.By dedicating a channel 403 to each fluid 401, it becomes possible to have a refrigerant 401 from the generating unit 3 and a heat transfer fluid 401 from the consuming unit. This allows for direct heat exchange without passing through the phase change material 2, and, depending on production and consumption profiles, reduces the size of the reservoir 1 and the quantity of phase change material 2 required.

[0047] Referring to the fourth drawing, plate 400 shows a cross-section of a flat plate comprising a single channel with a variable cross-section 404. In the case of a refrigerant 401 that undergoes evaporation during its passage through plate 400, the inlet of plate 400 is shown in the lower left. The diameter of the initial cross-section is determined to achieve optimal heat exchange performance. As the fluid evaporates, it produces cold and solidifies a layer of phase-change material 201 at the exchange interface. Once evaporated, the overall density of the refrigerant decreases. However, due to conservation of mass, if the diameter remains constant, the fluid accelerates to technically impractical speeds, severely impacting the system's performance. Therefore, the diameter of the cross-section is progressively increased until a limit is reached.The latter is generally fixed for reasons of mechanical resistance to pressure and plate thickness of 400. The second option shown in the drawing is to divide the channel into a plurality of arms circulating in parallel, which allows to increase the passage cross-section while maintaining reasonable flow diameters.

[0048] The device that is the subject of the invention is capable of industrial applications, particularly in the field of thermal storage of heat and cold.

[0049] Its first industrial application is to provide an economically viable solution to thermal storage and the ability to buffer between production and consumption, both of which are deferred and fluctuating, for any type of application with a hot or cold thermal energy generating unit and a hot or cold thermal energy consumption unit.

[0050] Its second industrial application provides a solution for the latent storage of thermal energy for industrial processes by addressing the issues of performance, cost, feasibility, and independence between the charging and discharging power of the thermal storage. In particular, it makes it possible to consider the possibility of producing steam at 120-150°C from a decarbonized electrical source (non-dispatchable sources) using heat storage during off-peak hours, with a given power output over an extended period. This accumulated heat can then be released over a significantly shorter period, corresponding to the industrial thermal demand. Patent documents

[0051] CN111662688B “Boron nitride / graphene double-heat-conduction-base aerogel composite phase-change-material and preparation method thereof”

[0052] US9027633B2 “Nanoparticle-enhanced phase change material (NePCM) with improved thermal energy storage”

[0053] CN114656939B « Expanded graphite-based composite phase change material with anisotropic thermal conductivity and preparation method thereof »

[0054] EP2825611B1 « Moldable mass comprising graphite and phase change material, and process for producing a molding from the mass »

[0055] EP3529549A1 « Dispositif pour accumulateur thermique a pris en glace »

[0056] WO2024005643A1 « Système de stockage d’énergie thermique » Littérature non-brevets

[0057] 10.1016 / j.applthermaleng.2023.119974 « Recent development in nano-enhanced phase change materials for solar thermal storage »

Claims

Device comprising at least one reservoir (1), at least one liquid (200) / solid (201) phase change material (PCM) (2) contained in this reservoir (1), at least one compact heat exchange system (4) internal to the reservoir, using a method of detaching the solid phase change material (201) from the surfaces of the heat exchange system (4) by active or passive heating means, or mechanical means, or pneumatic or hydropneumatic inflation means, or surface treatment means for the heat exchange system (4) specifically chosen according to the characteristics of the phase change material (2) used, characterized in that the maximum volumetric footprint dedicated to the heat exchange system (4) is intended to be significantly smaller, in a ratio of at least 1:3, than the volumetric footprint dedicated to the storage capacity,and because the phase-change material (2) moves in its liquid (200) or solid (201) forms within the reservoir (1), the suspended solid phase (201) must free up space near the heat exchange surface (4) so ​​that a new liquid volume (200) can come into contact. Device according to claim 1 characterized in that the heat exchange system (4) comprises flat or profiled plates (400), immersed in the phase change material (2) and ensuring the circulation of the refrigerant or heat transfer fluid (401) ensuring the supply or extraction of heat. Device according to any one of the preceding claims characterized in that the refrigerant or heat transfer fluid (401) ensuring the supply or extraction of heat from the heat exchange system (4) consists of a plurality of independent fluids. Device according to any one of the preceding claims characterized in that the internal channels (403) and (404) to the plates (400) and ensuring the circulation of the refrigerant or heat transfer fluid (401) benefit from sections with constant diameters (403) or sections with variable diameters (404) in a ratio of 1 to 15 allowing to compensate for the variations in flow velocities linked to the differences in density of the fluids. Device according to any one of the preceding claims characterized in that the heat supply and extraction channels (403) and (404) are manufactured according to metalworking technologies such as friction stir welding, laser welding, tungsten inert gas welding or rolling welding. Device according to any one of the preceding claims characterized in that the surface of the heat exchange system (4) in contact with the phase change material (2) inside the tank (1) is subject to different types of surface treatments allowing an increase or a reduction of the adhesion of the phase change material (2) on the surface of the heat exchange system (4). Device according to any one of the preceding claims characterized in that the device comprises means for moving the solid phase or the liquid phase within the reservoir exploiting the effect of gravity or exploiting forced displacement drive elements (8), such as liquid circulators or other mechanical extraction means allowing the regeneration of the exchange surfaces of the heat exchange system (4). Device according to any one of the preceding claims characterized in that a screen (5) permeable to the liquid (200) and impermeable to the solid phase (201), makes it possible to limit the movement of this solid phase (201) in any direction inside the reservoir (1). A device according to any one of the preceding claims, characterized in that the selected phase-change material (2) is a function of the desired temperature in the tank and is chosen, among others, from trimethylpenthane (C6H 14 ), methanol (CH3OH), H2O-NaCl mixtures, water and propylene glycol mixtures, water and ethylene glycol mixtures, fatty acids or paraffins, organometallic compounds (C x Zn, C x Mn), polyol sugars (such as erythritol), or nitrates. Device according to any one of the preceding claims characterized in that a mechanical lifting system (402) adjustable from outside the tank allows adjustment of the immersion height of the heat exchange system (4) internal to the tank (1). Device according to any one of the preceding claims characterized in that the chemical composition of the phase change material (2) can be altered, via the addition of particles of another nature, or in the case of a mixture where the phase change material is composed of several pure substances, via the modification of the concentration of one of these pure substances. Device according to any one of the preceding claims characterized in that the production of external cold is in particular achieved by circulation of the liquid phase in a closed loop through the existing exchange system (4) internal to the tank or via a secondary heat exchanger internal or external to the tank (7). Use of the device according to any of the preceding claims for the purpose of producing, restoring and storing heat and cold.

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