OPTIMIZED ASSEMBLY HEAT TRANSFER FLUID HEAT EXCHANGER AND A PHASE CHANGE MATERIAL THERMAL ENERGY STORAGE DEVICE INCLUDING SAID EXCHANGER

Description

TECHNICAL FIELD

[0001] The present invention relates to a thermal energy storage device using a phase change material according to claim 1.

[0002] The field of the invention relates to Thermal Storage Systems (TSS) using Phase Change Materials (PCM).

[0003] The invention relates more particularly to the integration of a thermal storage system in concentrated solar power plants, for example for direct steam generation plants, or to the recovery of waste heat from industries. STATE OF THE ART

[0004] Thermal storage systems offer numerous benefits for industrial management. For example, in concentrated solar power plants, there is a time lag between available solar energy and energy demand. This problem arises for all intermittent heat production units. The development of storage systems makes it possible to defer the use of heat produced by collecting heat during periods of high solar irradiance and releasing it later during periods of low sunlight or at night.

[0005] In the process industry, thermal storage systems are a way to utilize waste heat. This heat, generated from industrial waste heat, is available at temperatures between 30 and 90 °C in the food processing, paper and cardboard, and chemical industries, and even between 200 and 500 °C in the metal, glass, and cement industries. However, in most cases, the periods of availability differ from the periods of use, and without a storage solution, all the excess energy, unusable at the time of its production, is simply released into the environment (into the atmosphere as gaseous effluents or into natural water systems as liquid effluents). With a storage system, the excess energy produced during the day could be fed into an urban heating network during peak consumption periods, in the morning and evening.Heat storage would also make it possible to transport this energy to another industrial site that would need a thermal input.

[0006] There are different types of storage: sensitive, latent, and thermochemical.

[0007] Latent heat storage utilizes Phase Change Materials (PCMs) to store heat. Thermal storage is achieved through the storage of enthalpy of phase change. It is the enthalpy of phase change, most often during the solid / liquid phase transition, that is stored. This energy, absorbed during melting and released during solidification, results from the formation or breaking of interatomic or intermolecular bonds. Charging the storage system is accompanied by the melting of the storage material, while discharging is achieved through the solidification of said material. The material must be carefully selected based on the target temperature of the storage system, ensuring that its melting temperature falls within the operating temperature range.The liquid-to-gaseous phase change is much more interesting from an energy point of view, but the implementation difficulties related to the management of large volumes of gas under pressure and at high temperature limit their development.

[0008] The heat exchanged is called enthalpy of phase change or latent heat, and the amount of energy is on the order of 200 J / g. Therefore, by using such solid-liquid PCMs, it is possible to reduce storage volumes. Consequently, the quantities of materials are reduced, which lowers the cost of the system and limits heat losses, which are proportional to the external surface area of ​​the tank.

[0009] The energy density can thus be doubled compared to sensitive systems. It is, in fact, on the order of 15-60 kWh / m³ for sensitive storage systems and 60-120 kWh / m³ for latent storage systems. One of the major advantages of this technology is that the phase change can occur at constant pressure and temperature. Consequently, the discharge of stored energy takes place at a constant temperature.

[0010] However, the low thermal conductivity of PCMs, on the order of 0.2 to 0.5 W / (mK), limits heat transfer within these materials. PCMs are therefore inserted into tanks with increased surface area heat exchangers, such as tubes with circular or longitudinal fins, to maximize heat transfer.

[0011] One well-known technology is the shell-and-tube heat exchanger. A shell containing a fluid is traversed by a bundle of tubes through which another fluid flows. The two fluids exchange energy by conduction through the thickness of the tubes. In the case of thermal storage, this technology is adapted. Instead of an exchange between two moving fluids, the heat transfer fluid circulates in the tubes and a PCM (progressive capacitor) is fixed in the shell (apart from natural convection currents in the liquid phase). During charging, the heat transfer fluid reaches a temperature above the PCM's melting point and transfers energy to it, causing it to melt. During discharging, the heat transfer fluid enters at a temperature below the PCM's melting point and recovers the previously stored energy, causing the PCM to solidify.

[0012] Typically, fluid flows through the tube bundle from top to bottom during the charging phase and from bottom to top during the discharging phase. The tubes therefore have inlet and outlet ports in opposite positions, usually one at the top and one at the bottom. The tubes are connected at the top and bottom, for example, using a hydraulic manifold. The tube connections at their ends are bulky, creating dead space at the top and bottom of the tank. Furthermore, assembling the heat exchanger within the tank requires a fairly complex procedure, resulting in significant time loss and high manufacturing costs.

[0013] Document DE 40 06 965 describes a high-temperature heat exchanger comprising a eutectic salt capable of storing heat. Tubes are immersed in the chamber containing the eutectic salt. A liquid or gas, for example, high-pressure, high-temperature steam, flows through these tubes.

[0014] Therefore, there is a need to optimize heat exchangers in order to optimize thermal energy storage for a given device. DESCRIPTION OF THE INVENTION

[0015] The present invention proposes for this purpose a thermal energy storage device using Phase Change Material (PCM) according to claim 1.

[0016] According to the invention, the heat transfer fluid is a two-phase fluid in liquid and gaseous states intended to circulate in the heat exchanger. The distributor pipe and the manifold pipe are configured to open at the top of the heat exchanger.

[0017] Thus, the introduction and evacuation of the heat transfer fluid in the exchanger can be carried out through the upper part of the exchanger.

[0018] This feature of the invention significantly reduces the dead volumes generated by the heat exchanger tube connections. Installation of the heat exchanger in a tank filled with PCM is facilitated by being carried out from only one side of the exchanger.

[0019] Furthermore, the presence of a two-phase liquid / gas heat transfer fluid in the exchanger allows for more efficient latent heat storage than simple sensible heat storage. The combination of a two-phase heat transfer fluid with the introduction and exhaust of the fluid at the top is particularly striking. Specifically, gravity dictates a natural separation between the liquid and gaseous phases of the heat transfer fluid, and maintaining the liquid phase at the top and recovering it is not a straightforward process.

[0020] The exchanger according to the invention allows an arrangement in a tank filled with PCM of a storage device of which only the upper part and advantageously the upper wall is configured to allow the entry and exit of the heat transfer fluid of the exchanger.

[0021] The manufacturing of tanks and therefore storage devices is significantly improved.

[0022] According to another separable aspect, the device according to the invention comprises a plurality of exchangers.

[0023] Advantageously, the device includes expansion joints arranged between the heat exchangers and between the heat exchangers and the tank, so as to advantageously ensure the tank is watertight and advantageously compensate for the different expansions of the heat exchangers. The heat exchanger according to the invention allows it to be arranged resting on the bottom of the tank without requiring any connections in that area.

[0024] In another aspect, the invention relates to a thermal energy storage method comprising a charging stage and a discharging stage. For each stage, the heat transfer fluid is introduced into the upper part of the tank and is discharged from the upper part as well. BRIEF DESCRIPTION OF THE FIGURES

[0025] The aims, objects, features and advantages of the invention will become clearer from the detailed description of an embodiment thereof, which is illustrated by the following accompanying figures in which: Figure 1 : longitudinal cross-sectional view of an exchanger according to the invention filled with a two-phase liquid-gas heat transfer fluid. Figure 2 : top view of a heat exchanger according to the figure 1 . Figure 3 : longitudinal cross-sectional view of a storage device according to the invention comprising a plurality of exchangers filled with two-phase liquid-gas heat transfer fluid immersed in the tank containing a PCM. Figure 4 : perspective view of an embodiment of a distributor body and / or a collector body in the form of a hexagonal perforated plate. Figure 5: schematic top views of several examples of tank of device according to the invention comprising several exchangers respectively a) 19 exchangers, b) 31 exchangers, c) 29 exchangers. Figure 6 : schematic top view of a tank of a device according to the invention comprising 37 exchangers. DETAILED DESCRIPTION OF SPECIFIC IMPLEMENTATION METHODS

[0026] Before beginning a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are stated below.

[0027] First, it is recalled that the invention relates to a device according to claim 1.

[0028] Advantageously, according to preferred but non-limiting variants, the invention is such that: Pipe 8 of manifold 6 extends towards distributor 5 and is suitable for immersion in MCP 13. Tubes 2 extend longitudinally between distributor 5 and manifold 6. Tubes 2 are straight and parallel to each other. Tubes 2 extend vertically. Distributor 5 comprises a body 24 formed from a tube plate with a core, cooperating with the first end 3 of tubes 2. Manifold 6 comprises a body 25 formed from a tube plate with a core, cooperating with the second end 4 of tubes 2. The body 24 of distributor 5 and the body 25 of manifold 6 have a hexagonal cross-section. the tank 14 comprises a lower wall 15, side walls 16 and an upper wall defining an internal volume receiving at least one MCP 13. the pipe 7 of the distributor 5 and the pipe 8 of the collector 6 pass through the upper wall of the tank 14 to open outside the tank 14.said exchanger 1 rests in contact with the lower wall 15 of the tank 14. The device includes expansion joints 20 configured to ensure a seal between the distributor of the heat exchanger 1 and the tank 14. The device includes several exchangers 1 juxtaposed in the tank 14 and several expansion joints 20 arranged respectively between the distributors 5 of the exchangers 1. The PCM 13 is a solid-liquid PCM.

[0029] According to another aspect, the invention relates to a method for storing thermal energy in a device of the invention comprising a charging phase during which: The heat transfer fluid enters the exchanger 1 at the top through the pipe 7 of the distributor 5 opening at the top 17 of the tank 14, the distributor 5 at the top of the exchanger 1 distributes the heat transfer fluid in gaseous state to the plurality of tubes 2, the heat transfer fluid in gaseous state condenses in the tubes 2 causing the melting of the PCM 13, the heat transfer fluid in liquid form is collected by the collector 6 at the bottom of the exchanger 1 and is evacuated from the exchanger 1 through the pipe 8 of the collector 6 opening at the top 17 of the tank 14.

[0030] Advantageously, according to preferred, cumulative or alternative but non-limiting variants, the invention is such that: The process includes a discharge phase during which: the heat transfer fluid enters the heat exchanger 1 at the top 17 via the pipe 8 of the manifold 6 opening at the top 17 of the tank 14; the manifold 6 at the bottom of the heat exchanger 1 distributes the heat transfer fluid in liquid form to the plurality of tubes 2; the heat transfer fluid in liquid form vaporizes in the tubes 2, causing the PCM 13 to solidify; the heat transfer fluid in gaseous form is collected by the distributor 5 at the top and is discharged from the heat exchanger 1 via the pipe 7 of the distributor 5 opening at the top 17 of the tank 14; the heat transfer fluid is introduced into the heat exchanger 1 and recovered from the heat exchanger 1 via the top 17 of the tank 14; the heat transfer fluid is in liquid form throughout the pipe 8 of the manifold 6.

[0031] The invention relates to a heat exchanger 1 comprising a heat transfer fluid. The heat exchanger 1 according to the invention is intended to be arranged in a tank 14 to form a thermal energy storage device. The tank comprises at least one PCM 13, and the heat exchanger 1 is advantageously at least partially immersed in the PCM 13 contained in the tank 14.

[0032] The heat transfer fluid is preferably a two-phase fluid, meaning that it is configured to alternately take two phases or states under the operating conditions of the invention. According to the invention, the heat transfer fluid is in a liquid and gaseous state under the operating conditions, i.e., temperature and pressure, of the heat exchanger 1 and the thermal energy storage device of the invention.

[0033] The change of phase or state of the heat transfer fluid occurs during the charging and discharging of the thermal energy storage device within the exchanger 1 by transfer of thermal energy between the heat transfer fluid and the MCP 13.

[0034] Advantageously, during the pressurization phase, the heat transfer fluid enters heat exchanger 1 in gaseous form. Energy is transferred to the PCM 13 as the heat transfer fluid releases its thermal energy. The heat transfer fluid then cools and changes state to a liquid at the outlet of heat exchanger 1. During the discharge phase, the heat transfer fluid enters heat exchanger 1 in liquid form. Energy is transferred to the heat transfer fluid as the PCM 13 releases its thermal energy. The heat transfer fluid then cools and changes state to a gaseous at the outlet of heat exchanger 1.

[0035] Latent heat storage allows the absorption of the condensation energy of the heat transfer fluid vapor during charging, and the release of this latent heat to evaporate the liquid heat transfer fluid during discharging. This process is advantageously carried out without significant variations in temperature and, furthermore, without affecting the quality of the energy.

[0036] The tank 14 contains at least one PCM 13. Mixtures of PCM 13 may be used. In the following description, references to a single PCM 13 are not limiting. Various PCMs may be used, including solid-to-solid transition PCMs or, preferably, solid-to-liquid transition PCMs. The invention is suitable for a wide variety of PCMs and therefore for a broad range of storage temperatures. The two main categories of PCMs that can be used are organic (paraffin, fatty acids, alcohols, etc.) and inorganic (salts, salt hydrates, metal alloys, etc.). For superheated water heating networks (-180°C), the PCM used will have a melting point of approximately 100-120°C, for example: erythritol, sebacic acid, paraffin, etc.It is preferable to target a material that does not oxidize and that does not oxidize the metallic structure of the exchanger 1 or the tank 14, and that has a good specific enthalpy of phase change and is non-toxic. Preferably, the PCM 13 will have good heat capacity and the highest possible thermal conductivity.

[0037] PCM 13 is a two-phase material, preferably solid and liquid, in which the transition between these two phases stores or releases energy. Preferably, the transition from the first phase to the second phase requires heat, which is therefore stored in the PCM in its second phase. Conversely, the transition from the second phase to the first phase is exothermic and releases the stored heat.

[0038] When the energy storage system is operating to store thermal energy, i.e., during charging, heat exchanger 1 supplies heat to tank 14. Heat is exchanged from the heat transfer fluid to the PCM 13 via the heat exchanger. This heat enables the PCM 13 to transition from the first phase to the second phase, which then stores the heat from the heat transfer fluid. When the system is operating to release thermal energy, i.e., during discharging, heat exchanger 1 cools the PCM 13. Heat is exchanged from the PCM 13 to the heat transfer fluid via heat exchanger 2, allowing the transition from the second phase to the first phase. This transformation is exothermic. The released heat is recovered by the heat transfer fluid.

[0039] The exchanger 1 comprises a plurality of tubes 2. Advantageously, the heat transfer fluid circulates in the exchanger 1 and more particularly in the tubes 2. It is within the tubes 2 of the exchanger 1 that the phase change of the heat transfer fluid takes place.

[0040] The exchanger 1 according to the invention is intended to be at least partially immersed in a tank 14 containing at least one MCP 13.

[0041] The tubes 2 comprise a first end 3 and a second end 4. Advantageously, the first end 3 is suitable for being arranged in the upper part of the exchanger 1 while the second end 4 is suitable for being arranged in the lower part of the exchanger 1. This arrangement is thus found when the exchanger 1 is arranged in the tank 14 of the device, that is to say that the first end 3 of the tubes 2 is arranged in the upper part 17 of the tank 14 and the second end 4 of the tubes 2 is arranged in the lower part 18 of the tank 14.

[0042] The upper part is understood to be the area located above, along a vertical axis, a horizontal median plane. The upper part is preferably the upper third of tank 14 or exchanger 1, respectively.

[0043] The lower part is understood to be the area located below, along a vertical axis, a horizontal median plane. The lower part is preferably the lower third of tank 14 or exchanger 1, respectively.

[0044] The heat exchanger 1 includes a distributor 5 that connects the first ends 3 of the tubes 2 with fluid flow, and a manifold 6 that connects the second ends 4 of the tubes 2 with fluid flow. All the first ends 3 of the tubes 2 open into the distributor 5. All the second ends 3 of the tubes 2 open into the manifold 6. The tubes 2 open at the top of the distributor and at the bottom of the manifold. That is to say, the first end of a tube is open and located at the top of the heat exchanger. This first end is connected with fluid flow to the distributor 5. That is to say, the second end of a tube is open and located at the bottom of the heat exchanger. This second end is connected with fluid flow to the manifold 6.The distributor 5 and the manifold 6 have a functional role in forming the fluidic network for the circulation of the heat transfer fluid in the exchanger 1. Advantageously, the distributor 5 and the manifold 6 have a mechanical role in maintaining and mechanically connecting the plurality of tubes 2 to each of the two ends 3, 4 of the tubes 2. Preferably, the distributor 5 and the manifold 6 each comprise a body 24, 25 formed for example by a plate as illustrated in the figure. figure 4 This plate is drilled through to allow the distribution and collection of the heat transfer fluid at the ends of the various tubes. The plate includes fluid circulation channels formed longitudinally within its thickness. The use of this plate helps to reduce dead volumes and facilitates tube connections.

[0045] The distributor 5 is arranged in the upper part of the exchanger and the collector 6 is arranged in the lower part of the exchanger.

[0046] The distributor 5 and the collector 6 ensure an advantageously uniform distribution of the heat transfer fluid to the tubes 2.

[0047] The body 24, 25 includes a plurality of passages 22 formed transversely in the thickness of the body 24, 25 into which the ends 3, 4 of the tubes 2 are inserted and welded. The body 24, 25 also includes openings 23 corresponding to the mouths of fluid circulation conduits formed longitudinally in the thickness of the body.

[0048] Advantageously, the tubes 2 are joined to the body 24 of the distributor 5 and to the body 25 of the collector 6 respectively by their first ends 3 and by their second ends 4 which are for example welded to the bodies 24, 25.

[0049] The distributor 5 includes a pipe 7 intended for the introduction or exit of the heat transfer fluid from the exchanger 1, more precisely from the tubes 2.

[0050] The manifold 6 includes a pipe 8 intended for the introduction or exit of the heat transfer fluid from the exchanger 1, more precisely from the tubes 2.

[0051] As illustrated in figures 1 to 3 The distributor 5, more precisely the distributor body 24, has two faces: an upper face and a lower face. The tubes 2 are fixed to the distributor 5 at its lower face. Advantageously, the channel 7 is fixed to the distributor 5 at its upper face. The channel 7 of the distributor 5 is arranged oppositely to the tubes 2 relative to the distributor body 24.

[0052] According to the invention, the pipe 8 of the collector 6 extends towards the distributor 5. As illustrated in figures 1 to 3The collector 6, more precisely the collector body 25, has two faces: an upper face and a lower face. The tubes 2 are fixed to the collector 6 at its upper face. The pipe 8 is fixed to the collector 6 at its upper face. The pipe 8 of the collector 6 is arranged on the same side as the tubes 2 relative to the collector body 25.

[0053] The tubes 2 can be of any cross-section, with a marked preference for circular tubes 3, which are the simplest to design and offer the best pressure resistance. The arrangement of the tubes 2 can also vary; they can be in a square, a triangle, etc.

[0054] The two tubes can be made of steel; the classic grades for a pressure wall are P235GH, P265GH, and P355GH. For non-pressure walls, 304 and 316 stainless steels can be used, as well as aluminum, copper, or other metals or alloys. Steel is preferred for its low cost, strength, and pressure resistance, while aluminum is easy to work with and a good thermal conductor.

[0055] Typically, the tubes 2 are surrounded by circular fins increasing the heat exchange surface or by larger inserts which also help to keep the tubes together.

[0056] Pipes 7 and 8 can be formed like tubes 2 except for the presence of fins or inserts.

[0057] Advantageously, the pipe 7 of the distributor 5 and the pipe 8 of the collector 6 are configured to open into the upper part of the exchanger 1. The pipe 8 opens beyond the upper face of the distributor 5. The pipe 8 passes through the distributor 5 without being in fluidic communication with it.

[0058] The tubes 2 extend advantageously vertically. The tubes 2 extend longitudinally between the collector 6 and the distributor 5; preferably, the tubes 2 are straight and parallel to each other.

[0059] Pipe 8 of collector 6 is advantageously parallel to tubes 2. Pipe 8 extends over the entire height of the exchanger 1.

[0060] Preferably, the pipes 7, 8 open onto the upper face of the exchanger 1. The upper face is understood to be the surface of the exchanger 1 located above the first end of the tubes 2. The upper face of the exchanger 1 corresponds preferably to the upper face of the body 24 of the distributor 5.

[0061] Since the exchanger 1 is arranged in a tank 14, the pipes 7, 8 open into the upper part of the tank 14. Preferably at the level of the upper face of the tank 14.

[0062] The connections of the exchanger 1 are therefore made from the top, which facilitates the assembly of the exchanger 1 in the tank 14 as well as its extraction for repair or replacement.

[0063] The tank 14 advantageously comprises a lower wall 15, also called the bottom, side walls 16, and a top face. The top face is opposite the bottom 15.

[0064] According to this advantageous arrangement, the heat exchanger 1 is mounted directly on the bottom 15 of the tank 14. Advantageously, the heat exchanger 1 rests on the bottom 15 of the tank 14, for example, by feet 21, as illustrated in figure 3 .

[0065] As indicated above, the heat exchanger 1 is at least partially immersed in the PCM 13. The tubes 2 advantageously extend into contact with the PCM 13 to allow for heat exchange. The pipe 8 is also advantageously immersed at least partially in the PCM 13.

[0066] Characteristically, the pipe 8 of the manifold 6 is thermally insulated so as to limit heat exchange between the pipe 8 of the manifold 6 and the MCP 13. The thermal conductivity of the pipe 8 is advantageously from 0.02 to 0.1 W / m / K. By way of example, the pipe 8 is surrounded by at least one layer of thermally insulating material 9 such as calcium silicate, for example.

[0067] This arrangement ensures that the heat transfer fluid circulating in this pipe 8 of collector 6 does not undergo a phase change.

[0068] The heat transfer fluid is in a liquid state in the collector 6 and especially along the entire length of the pipe 8.

[0069] The heat exchanger 1 is, for example, a hexagonal unit. The shape of the heat exchanger 1 is defined in particular by the shape of the distributor 5 and the manifold 6, more precisely by the bodies 24, 25 of distributor 5 and manifold 6. As illustrated in figure 4 The body 24 of the distributor 5 has a hexagonal cross-section that lies in a plane perpendicular to the longitudinal axis of the tubes 2. This shape allows for an optimized arrangement of the tubes 2 by minimizing inactive areas. The heat exchanger 1 is advantageously configured to withstand high pressures, for example, from 20 to 150 bar.

[0070] The type of plate illustrated in the figure 4Its advantages include minimizing the dead volume of heat transfer fluid, easy access to tube welds for regulatory inspections and possible sealing or plugging of tubes, good fatigue resistance to temperature and pressure cycling, and manufacturing automation possibilities allowing economies of scale.

[0071] In a preferred embodiment, the tank 14 comprises conventionally metallic inner walls. In one option, the walls are preferably coated with a material designed to prevent contact between at least one PCM 13 and metallic parts. For example, the coating material is a polymer or resin, preferably a fluoropolymer such as PTFE, FEP, or PFA. This arrangement improves the PCM's storage capacity by limiting PCM oxidation during storage cycles in contact with oxygen and / or metal. Advantageously, this arrangement can also be useful in preventing corrosion of the tank by the PCM if the latter is corrosive.

[0072] Several heat exchangers 1 can be placed in a tank 14, advantageously a low-pressure tank containing at least one PCM 13. Advantageously, expansion joints 20 are arranged between the various distributors 5. In one option, the device includes expansion joints 20 advantageously placed between the distributors 6 of the heat exchangers 1 and the tank 14, if a seal is required at this point. The expansion joints 20 advantageously allow for differential thermal expansion between the distributors 5 and the tank 14. In another advantageous option, the expansion joints 20 also ensure the sealing of the gas head of the tank 14.

[0073] Expansion joints 20 are advantageously watertight seals and preferably have an elasticity that allows them to absorb thermal expansions between the exchangers 1.

[0074] Following this arrangement illustrated in figure 3 , the distributors 5 of the exchangers 1 form the upper face of the tank 14.

[0075] The upper part 17 of the tank 14, advantageously above the MCP 13, can advantageously be occupied by a gaseous space 19 comprising an inert gas to limit corrosion processes.

[0076] In figure 3 The figure illustrates a device according to the invention with a tank 14 in which heat exchangers 1 are placed, three of which are fully shown, and a PCM 13. The arrows indicate the direction of flow of the heat transfer fluid during the charging phase (condensation of the heat transfer fluid, melting of the PCM). Solid arrows represent the heat transfer fluid in the vapor state, while dashed arrows represent the heat transfer fluid in the liquid state. The flow direction is reversed during the discharge phase (evaporation of the heat transfer fluid, solidification of the PCM).

[0077] During the charging stage, the heat transfer fluid in gaseous form enters the heat exchanger 1 through the distributor 5 located in the upper part 17 of the tank 14. The heat transfer fluid enters through the upper face of the body 24 of the distributor 5 via the pipe 7. The distributor distributes the heat transfer fluid homogeneously into the tubes 2, where it condenses, causing the PCM 13 to melt. The heat transfer fluid in its liquid state is collected in the lower part 18 of the tank 14 by the manifold 6. The heat transfer fluid exits the tank through the manifold 6 and rises in the pipe 8 of the manifold 6 before being extracted from the tank 14.

[0078] Conversely, during the discharge stage, the heat transfer fluid in liquid form enters the heat exchanger 1 through the manifold 6 located in the lower part 18 of the tank 14. The heat transfer fluid enters through the upper face of the manifold body 25 via the pipe 8 opening into the upper part 17 of the tank 14. The manifold 6 distributes the heat transfer fluid homogeneously into the tubes 2, where the solidification of the PCM 13 allows the heat transfer fluid to vaporize. The heat transfer fluid in its gaseous state is collected in the upper part 17 of the tank 14 by the distributor 5. To exit the tank, the heat transfer fluid passes through the distributor 5 and rises in the pipe 7 of the distributor 5 before being extracted from the tank 14.

[0079] Advantageously, as illustrated for example in figure 3The exchanger 1 includes a liquid level 12 in the tubes 2 which varies according to the charging or discharging stage but is always above the manifold 6. The liquid level 12 illustrates the separation of the two phases of the heat transfer fluid, the gaseous phase, the vapor 11 being above the liquid phase 10. Thus, the heat transfer fluid is always in a liquid state in the manifold 6 and therefore in the pipe 8 and always in a gaseous state in the distributor 5.

[0080] In a tank 14 comprising several heat exchangers 1, the arrangement of the heat exchangers 1 can be in a hexagonal pattern to minimize the surface area of ​​the tank 14 and to optimize the number of heat exchangers 1 according to the required storage capacity. The heat exchangers 1 are placed side-by-side in the tank 14. The external shape of the tank 14 can take on several different forms, such as those shown in the diagram. Figure 5illustrating a) a device with a tank 14 of regular hexagonal shape with 19 exchangers 1, b) a device with a tank 14 of rectangular shape with 31 exchangers 1, c) a device with a tank 14 of elongated hexagonal shape with 29 exchangers 1.

[0081] In figures 5 and 6 , tank 14 is seen from above with the exchangers 1 each comprising a pipe 7 and a pipe 8, each exchanger being joined to the juxtaposed exchanger by at least one expansion compensator 20.

[0082] The advantage of placing several heat exchangers 1 in a single tank 14 is, in particular, to reduce the cost of the tank 14. Furthermore, having only one volume of PCM 13 is beneficial. Indeed, for high-temperature applications, filling the tank 14 with PCM 13 is a costly and complex operation, since the PCM 13 must be introduced in liquid, i.e., molten, form. Using a single tank 14 allows this operation to be carried out in one go. The overall thermal insulation of the system is improved due to the reduction in the surface area to be insulated.

[0083] The device will allow the storage of thermal energy from a heat transfer fluid over temperature ranges of between 100 and 1000°C, during a daily period, and the release of this energy at the same temperature level directly into the same heat transfer fluid.

[0084] The process of storing and releasing thermal energy in a device as described above, also called the thermal energy charging and discharging process, comprises a charging stage and a discharging stage. Advantageously, in both the charging and discharging stages, the inlet and outlet of the heat transfer fluid in the exchanger occur at the upper part of said exchanger 1. Example :

[0085] A concentrated solar power (CSP) plant operates at a nominal pressure of 110 bar. To deliver the amount of steam corresponding to 6 hours of operation at full load, the storage capacity must be 900 MWh. At the temperature and pressure levels targeted by this application, PCM storage represents nearly 50% of the total storage capacity, and NaNO3 is a suitable PCM. Under these conditions, the total volume of NaNO3 required for the plant would be approximately 4400 m³.

[0086] However, an optimized storage system, comprising for example 217 tubes 9 m long, would require a NaNO3 volume of 60 m3 per exchanger. Therefore, 74 exchangers are needed to achieve the storage capacity required by the solar power plant. Painting Characteristics of the MCP storage system for a 50 MWe direct steam generation power plant (without the use of an intermediate heat transfer fluid) Rated electrical power MWe 50 Rated thermal output MWt 150 Storage capacity h 6 Total storage capacity MWht 900 Storage capacity per MCP MWht 444 Latent heat of PCM (NaNO3) kJ / kg 172 Total mass of MCP t 9300 Total volume of MCP m3 4407 MCP volume per heat exchanger m3 60 Number of heat exchangers - 73,5

[0087] These 74 heat exchangers could ideally be distributed across 2 tanks of 37 exchangers each, as shown in the figure 6 , which would allow for optimization of the hexagonal shape of tank 14. REFERENCES

[0088] 1. Heat exchanger 2. Tube 3. First end 4. Second end 5. Distributor 6. Manifold 7. Pipe 8. Pipe 9. Thermal insulation 10. Liquid 11. Vapor 12. Liquid level 13. PCM 14. Tank 15. Bottom wall 16. Side walls 17. Top section 18. Bottom section 19. Gas head 20. Expansion joint 21. Foot 22. Passage 23. Opening 24. Distributor body 25. Manifold body

Claims

1. Thermal energy storage device using a phase change material (PCM) comprising at least one PCM (13) and a tank (14) intended to contain the at least one PCM (13), and at least one heat exchanger (1) comprising a heat transfer fluid, and at least partially immersed in the tank (14) containing the phase change material (PCM) (13), comprising: o a plurality of tubes (2), each tube (2) comprising a first end (3) arranged in the upper part of the heat exchanger (1) and a second end (4) arranged in the lower part of the heat exchanger (1), o a distributor (5) arranged in the upper part of the heat exchanger (1) and establishing fluid communication between the first ends (3), o a manifold (6) arranged in the lower part of the heat exchanger (1) and establishing fluid communication between the second ends (4), o the distributor (5) comprising an open pipe (7) for supplying or discharging the heat transfer fluid to / from the heat exchanger (1), ∘ the manifold (6) comprising an open pipe (8) for supplying or discharging the heat transfer fluid to / from the heat exchanger (1) and being capable of being immersed in the PCM (13), the heat transfer fluid being a two-phase fluid with a liquid state and a gaseous state and intended to flow within the heat exchanger (1), and in that the pipe (8) of the manifold (6) and the pipe (7) of the distributor (5) open out into the upper part of the heat exchanger (1), characterised in that the pipe (8) of the manifold (6) is thermally insulated and extends towards the distributor (5).

2. Device according to the preceding claim, wherein the distributor (5) comprises a body (24) formed by a tube plate drilled through the core and cooperating with the first end (3) of the tubes (2).

3. Device according to any one of the preceding claims, wherein the manifold (6) comprises a body (25) formed by a tube plate drilled through the core and cooperating with the second end (4) of the tubes (2).

4. Device according to any one of the preceding two claims, wherein the body (24) of the distributor (5) and the body (25) of the manifold (6) have a hexagonal cross-section.

5. Device according to any one of the preceding claims, wherein the tank (14) comprises a bottom wall (15), side walls (16) and a top wall defining an interior volume accommodating the at least one PCM (13).

6. Device according to any one of the preceding claims, wherein the PCM (13) is a solid-liquid PCM.

7. Method for storing thermal energy in a device according to any one of the preceding claims comprising a charging phase during which: - the heat transfer fluid enters the heat exchanger (1) at the upper part via the pipe (7) of the distributor (5) which opens out into the upper part (17) of the tank (14), - the distributor (5) in the upper part of the heat exchanger (1) distributes the heat transfer fluid in the gaseous state to the plurality of tubes (2), - the heat transfer fluid in the gaseous state condenses inside the tubes (2) causing the PCM (13) to melt, - the heat transfer fluid in liquid form is collected by the manifold (6) in the lower part of the heat exchanger (1) and is discharged from the heat exchanger (1) via the pipe (8) of the manifold (6) opening out into the upper part (17) of the tank (14).

8. Method according to the preceding claim comprising a discharging phase during which: - the heat transfer fluid enters the heat exchanger (1) at the upper part (17) via the pipe (8) of the manifold (6) which opens out into the upper part (17) of the tank (14), - the manifold (6) in the lower part of the heat exchanger (1) distributes the heat transfer fluid in the liquid state to the plurality of tubes (2), - the heat transfer fluid in the liquid state evaporates inside the tubes (2) causing the PCM (13) to solidify, - the heat transfer fluid in the gaseous state is collected by the distributor (5) in the upper part and is discharged from the heat exchanger (1) via the pipe (7) of the distributor (5) opening out into the upper part (17) of the tank (14).

9. Method according to any one of the preceding two claims, wherein the heat transfer fluid is supplied to the heat exchanger (1) and is recovered from the heat exchanger (1) via the upper part (17) of the tank (14).

10. Method according to any one of the preceding three claims, wherein the heat transfer fluid is in the liquid state throughout the pipe (8) of the manifold (6).