Method and system for thermomechanical energy storage

Abstract

A method for the thermomechanical storage of thermal energy, wherein, in a storage stage, thermal energy is introduced via thermal energy input means into a thermal reservoir comprising at least one storage unit, said unit comprising a storage material capable of undergoing a phase transition from a lower internal energy phase to a higher internal energy phase, a substantial part of the storage material is transformed into its higher internal energy phase by absorption of the thermal energy introduced into the reservoir, and wherein, in a release stage, thermal energy is transferred from the at least one storage unit to a working fluid, a quantity of the working fluid, at least partly in gaseous form, is brought into contact with a thermal energy-to-mechanical energy converter, and mechanical energy is produced by the converter.Through volumetric expansion of the said working fluid, mechanical energy is transferred to mechanical energy output means for its use.

Description

Title of the invention: Method and system for thermomechanical energy storage

[0001] The invention relates to a method for the thermomechanical storage of thermal energy, in particular waste energy from industrial processes or energy from solar fields. It also relates to a system for implementing the method.

[0002] In the context of sustainable development, the challenges posed by energy storage are of paramount importance. The production of renewable energies, for example from solar or wind power, is inherently intermittent and cannot reach its full potential without a solution for storing these energies between the time of their production and the time of their consumption. When these renewable energies are in the form of electricity, the use of batteries is costly and raises the issue of their recycling, made more acute by their limited lifespan. Moreover, their manufacture itself uses minerals whose extraction still generates significant pollution today. In the specific case of domestic energy use, many households have installed panels to capture solar energy, in thermal or electrical form. On very sunny days, a surplus of energy is produced.When energy is in electrical form, surpluses are usually injected into the electrical grid, but the cost of this solution for distribution companies cannot be ignored, is passed on to all users, especially as the number of homes equipped with solar energy capture systems increases sharply and is encouraged at the environmental level.

[0003] There is therefore a need for simple, environmentally sustainable and efficient energy storage systems.

[0004] In US2016 / 0032783, an energy storage and recovery system is described in which, during the storage phase, a fluid is extracted as vapor from a first container, then compressed, cooled by passing through a regenerator, and finally condensed in a second container whose pressure and temperature increase during storage. During the recovery phase, the fluid in the second container is vaporized, reheated by passing back through the regenerator, and finally expanded, the energy of the expansion being used. In this known process, the compression and expansion means must operate at pressures that vary considerably depending on the degree of load on the device. Since the energy is mainly stored in the form of a compressed liquid, high pressures are necessary to obtain a satisfactory energy density. High temperatures are also required, typically several hundred degrees Celsius, which poses thermal insulation difficulties and greatly increases the cost of the process.

[0005] EP 2673586 describes a thermal energy absorption device comprising a plurality of cells containing a phase-change material. The dimensions of the different cells have a distribution enabling the device to respond to both fast and slow transients.

[0006] Various families of phase change materials are described on the PCMPRODUCTS website.

[0007] US4192144 describes a direct contact heat exchanger that uses a change The liquid-vapor phase of a working fluid within the exchanger, as well as an energy production system incorporating this exchanger, are all factors considered. Variations of the exchanger utilize phase-change materials.

[0008] WO 2014 / 183892 describes a thermal energy storage device comprising a phase-change storage material that is integrated into the circuit of a working fluid, such that condensation and evaporation of the working fluid can occur in contact with the phase-change material. The working fluid can be used in an organic Rankine cycle.

[0009] WO79 / 01004 describes a system for converting solar energy into electrical energy in which the working fluid, feeding a Rankine cycle, circulates in a heat exchanger thermally coupled to a composition of fusible salts acting as a thermal energy reservoir.

[0010] US2011 / 0083436 A1 describes a system and method for energy storage thermal energy and its transfer to a unit for conversion into mechanical energy. They include the use of two phase-change materials, the first of solid-liquid type for the actual storage and the second, of liquid-vapor type for the transfer to the conversion unit which may include a Stirling cycle.

[0011] WO2018 / 170533 also describes a storage and retrieval system thermal energy is initially produced by the dissipation of electrical energy. The stages of production, storage in a phase-change material, and recovery of thermal energy are organized in superimposed layers.

[0012] The invention aims to provide a method for thermomechanical energy storage that is simple, economical, and produces and uses little pollution. In particular, it aims to provide such methods with improved overall thermodynamic efficiency.

[0013] Consequently, the invention relates to a method for the thermomechanical storage of thermal energy according to which, in a storage step,

[0014] • thermal energy is introduced via thermal energy input means into a thermal reservoir comprising at least one unit of storage, said unit comprising a storage material capable of undergoing a phase transition from a lower internal energy phase to a higher internal energy phase, • a substantial portion of the storage material is transformed into its higher internal energy phase by absorbing the thermal energy introduced into the reservoir,

[0015] and according to which, in a restitution step,

[0016] • thermal energy is transferred from at least one storage unit to a working fluid, the working fluid being in contact with said storage unit, a substantial part of the storage material contained in the storage unit being thereby progressively transformed into its lower internal energy phase, • a quantity of the working fluid, at least partly in gaseous form, is brought into contact with a thermal energy-to-mechanical energy converter, • Mechanical energy is produced by the converter, through volumetric expansion of the said working fluid, • Mechanical energy is transferred to means of mechanical energy output for use.

[0017] According to the invention, the working fluid volume that is in contact with the converter is in fluidic communication with the working fluid volume that is in contact with at least one storage unit, during at least part of its volumetric expansion.

[0018] Thermomechanical storage is understood to mean that the process or system according to the invention can receive thermal energy and release energy both in thermal and mechanical form. In certain particular embodiments, it can also receive energy in electrical form.

[0019] The method according to the invention comprises at least one storage step and one release step. In practice, it comprises many. During operation, it frequently happens that a storage phase occurs, at least partially, during a release phase, depending on the ratio between the available thermal energy and the mechanical energy required. The thermal reservoir is capable of storing a certain maximum quantity of thermal energy. When all the storage material contained in the reservoir (where applicable, distributed in several storage units) is in its phase with the highest internal energy, the reservoir is fully charged. A release-discharge phase must then take place in order to at least partially discharge the reservoir before a new storage-charge phase can occur.

[0020] By thermal reservoir we mean a thermally insulated hollow body capable of containing among other things at least one storage unit and a sufficient quantity of working fluid, allowing their mutual contact.

[0021] A storage unit is defined as a solid, single-piece body comprising storage material and capable of being immersed in working fluid over at least 50% of its surface, preferably 65%, advantageously 80%, and preferably 90%. Where applicable, the storage unit must accommodate the volume change experienced by the storage material during its phase change.

[0022] In both the storage and release phases, a substantial portion of the storage material undergoes a phase change. This means that the amount of material that changes phase is macroscopic, related to the amount of energy either stored or released, by conservation of energy. Microscopic quantities that change phase due to thermodynamic fluctuations but do not affect the amounts of substances in their respective phases are not considered.

[0023] In the remainder of this description, the expression "storage material capable of undergoing a phase transition from a phase with lower internal energy to a phase with higher internal energy" will generally be abbreviated as "phase change material" or "PCM". Examples of such materials are well known and extensively described in the prior art. They have the advantage of being able to store a large amount of energy and release it while remaining at a theoretically constant temperature. In practice, the temperature of some PCMs (in particular organic PCMs) may vary slightly between the beginning and end of the phase transition (for example, between the fully charged and fully discharged states). It is recommended to use PCM materials whose temperature varies by less than 20°C, advantageously less than 15°C, and preferably less than 10°C between the beginning and end of the phase change.Materials for which this temperature difference is less than 5°C are particularly preferred. Many types of phase-change materials can be used in the invention. They can be organic (such as polyethylene, paraffins, fatty acids, polyols such as pentaerythritol) or inorganic (e.g., salt hydrates, eutectic salt mixtures).

[0024] It is recommended to use MCPs with a storage capacity exceeding 50 MJ / m3, advantageously 100 MJ / m3, preferably 200 MJ / m3.

[0025] It is also recommended that the MCP not exhibit too large a volume change during the phase change. This volume change should preferably be less than 100%, advantageously less than 20%, ideally less than 10%.

[0026] In one embodiment of the invention, the lower energy phase of the material The phase transition is a solid phase, and the higher energy phase is a liquid or solid phase. In a preferred variant, both the lower energy and higher energy phases are solid. Such phase transitions are therefore of the "solid-solid" type.

[0027] It is also advantageous for the phase change temperature to be between 100 and 200°C, preferably between 110 and 190°C, and more preferably between 120 and 180°C. Indeed, it is within these temperature ranges that the thermal energy is best utilized by the invention. In cases where the PCM temperature does not remain strictly constant during the phase change (which is, for example, the case with commercial polyethylenes that have a molecular weight distribution), this temperature is the DSC (Differential Scanning Calorimetry) peak temperature.

[0028] In general, the nature of the phase change material (PCM) does not allow direct contact between it and the working fluid. Therefore, one of the functions of the storage unit is to protect the PCM when necessary while ensuring the best possible heat exchange between the PCM and the working fluid. However, it is recommended that at least one storage unit (hereinafter referred to as the "storage unit") contain, per unit volume, as many PCMs as possible.

[0029] It is recommended that the storage unit comprise at least 50%, advantageously 60%, more advantageously 70%, preferably 80%, more preferably 90% by volume of phase change material.

[0030] In particular embodiments, the storage unit comprises at least 95%, advantageously 96%, more advantageously 97%, preferably 98%, more preferably 99% by volume of phase change material.

[0031] PCM is commonly in the form of particles. A particle is understood to be a single, continuous quantity of material. In the process according to the invention, the particles preferably have a geometric shape and dimensions that promote rapid heat transfer. Typically, the particle has an average characteristic dimension of less than 5 mm, advantageously less than 1 mm. If the particle is approximately spherical, this refers to the diameter of the smallest sphere that can contain it. In the case of fibrous particles, it refers to the diameter of the smallest cylinder that can contain the fiber. Finally, for example, in the case of lamellar particles, it refers to the thickness of the lamella, defined in a similar manner.

[0032] The thermal reservoir can comprise a highly variable number of storage units.

[0033] In a first extreme embodiment of the method according to the invention, in which the thermal reservoir comprises a very large number of storage units, the storage unit consists of only a single PCM particle. In this case, the unit of storage advantageously includes an average volume of MCP (the average being taken over the particle population in the reservoir) less than lcm3, generally less than 100 mm3, advantageously less than 10 mm3, preferably less than 1 mm3.

[0034] In this first embodiment, the PCM particle is often encapsulated. Encapsulation allows the particle to maintain its individuality during the phase change if necessary, as in the frequent case where the phase change is solid-liquid. It also protects the PCM from the working fluid. However, in a particular embodiment of this first embodiment, the PCM particle is not encapsulated. In this case, the storage unit preferably comprises 100% PCM, and the working fluid is in contact with the PCM. This case can, for example, arise when the PCM undergoes solid-solid phase transitions.In this first embodiment, in which the thermal reservoir comprises a very large number of storage units, these can advantageously be grouped in at least one container, for example perforated, allowing the storage units to move relative to each other and allowing the working fluid to circulate freely between the storage units.

[0035] In a second variant of the process, the storage unit comprises a larger quantity of MCP, typically a quantity having an average volume greater than 1 cm3, generally greater than 10 cm3, advantageously greater than 100 cm3.

[0036] In a first particular embodiment of this second variant, the storage unit has a more complex geometry that maintains the speed of heat exchange. For example, the storage unit comprises PCM lamellae fixed to a support that keeps them spaced apart to allow the working fluid to circulate. If not encapsulated, the PCM material may then contain a filler that gives it the necessary mechanical properties.

[0037] In another embodiment of this second variant, the storage unit comprises a hollow body having a shape conducive to heat exchange, the hollow body containing the desired quantity of PCM. In this embodiment, the PCM no longer needs to be encapsulated, the hollow body being able to protect the PCM from the working fluid and, if necessary, maintain the individuality of the storage units during phase changes. The hollow body may advantageously have the shape of thin cylinders, a circular cross-section, or advantageously a rosette shape, so as to increase the contact surface between the storage unit and the working fluid for a given quantity of PCM.

[0038] In the process according to the invention, a substantial portion of the storage material is transformed into its higher internal energy phase by absorption of the thermal energy introduced into the reservoir. Any suitable means can be used to to transfer thermal energy to the storage material. In practice, a heat exchanger connected to the thermal energy input can be used for this purpose. In this case, it is advantageous for it to be at least partially immersed in the working fluid, in which the storage unit is itself immersed, the working fluid then ensuring the thermal connection between the exchanger and the storage unit.

[0039] The function of the working fluid is to produce mechanical energy by volumetric expansion of its gaseous phase. This expansion occurs upon contact with a thermal energy-to-mechanical energy converter. The working fluid comes into contact with the converter, and its expansion transfers energy to it. In principle, any type of thermal energy-to-mechanical energy converter can be used in the invention, in which work is produced by the expansion of a gas whose pressure is increased by the addition of heat from the PCM. The use of conventional turbines is, for example, possible. The converter can be open, located in the thermal reservoir, and, for example, include a piston. Generally, however, the converter includes a housing into which the working fluid enters.In these common cases, the converter typically has working fluid inlet means, which may be equipped with a shut-off system to control the flow of the working fluid. The converter is generally equipped with outlet means for extracting the working fluid once its expansion has finished. In this case, the outlet means are advantageously equipped with a shut-off system to control the timing of this extraction when it is not continuous.

[0040] The volumetric expansion of the working fluid in contact with the converter occurs as a result of the pressure increase that the working fluid undergoes when it receives heat from the PCM. This heat may originate in part from the sensible heat of the PCM. However, the invention utilizes the large amount of latent energy contained in phase-change materials, and a substantial portion of the storage material contained in the storage unit is therefore progressively transformed into its lower internal energy phase due to the energy transfer to the working fluid.

[0041] The working fluid may be entirely in gaseous form. It may, for example, be air or an organic gas. However, in a preferred embodiment of the invention, the working fluid comprises a gaseous phase and a liquid phase, a portion of the liquid phase being transformed into a gaseous phase by contact with the storage unit. The latent energy required for the vaporization of the relevant portion of the working fluid is received from the storage material upon contact. In this embodiment, at least a portion of the working fluid that is in contact with at least one storage unit is in liquid form. The proportion of the surface area of ​​the storage unit in contact with the liquid phase of the working fluid is advantageously equal to minus 10%, preferably at least 20%, recommended at least 30%.

[0042] Examples of working fluids that may have a liquid phase and a gaseous phase in the invention are, for example, organic fluids such as propane or butane, or inorganic fluids. The choice of working fluid is dictated by the transition temperature of the PCM, and therefore by the temperature to which the available thermal energy, recovered by the process according to the invention, can heat it. It is recommended to use fluids having, all other characteristics being equal, the best possible thermal conductivity. The working fluid is advantageously water.

[0043] In the process according to the invention, the working fluid is kept in contact with at least one storage unit for a sufficient time for a substantial part of the storage material it contains to be transformed into its lower energy phase, this part depending on the amount of mechanical energy required by the process according to the invention to return.

[0044] The contact is physical in the material sense, with fluid molecules in contact with a storage unit, but it is also thermal, thereby facilitating heat exchange from the storage unit to the working fluid. The working fluid is considered as a whole, in the case where it is present in different phases.

[0045] To promote heat exchange during contact, it is recommended that the working fluid be in motion relative to the storage unit. This relative flow is preferably at least partially turbulent.

[0046] The working fluid volume that is in contact with the converter is in fluidic communication with the working fluid volume that is in contact with at least one storage unit, during at least part of its volumetric expansion.

[0047] Fluid communication between the two working fluid volumes is understood to mean that these volumes are connected and that the working fluid can therefore flow freely between them. This results in an additional supply of thermal energy from the PCM to the converter by conduction and convection, which increases the work done by the converter. It also results in the penetration of an additional quantity of working fluid into the converter during expansion, due to the concomitant pressure drop in the converter. The portion of the expansion during which fluid communication is established should ideally be as long as possible and must in any case be sufficient for this technical effect to manifest itself optimally. Those skilled in the art should determine this portion based on the technical details implemented.

[0048] It is generally recommended, more specifically, that fluid communication be maintained for the highest possible proportion of the time during which the working fluid expands, which promotes heat transfer, in particular due to The temperature and pressure of the working fluid tend to decrease while it is performing work. Indeed, even if in some cases the expanding working fluid region is some distance from a storage unit, heat transfer is enhanced by thermal conductivity and convection. It is often recommended that fluid communication be maintained with at least one storage unit for at least 25% of the time during which the working fluid is expanding and performing work. It is advantageous for this proportion to reach at least 50%, preferably 75%, and even more preferably 80% or 85%. Ideally, this proportion should exceed 90%. Ideally, connectivity is maintained throughout essentially the entire expansion.

[0049] In certain embodiments, particularly those in which the converter includes piston-type means, it is recommended that fluidic communication be maintained with at least one storage unit for more than 0.1 seconds, advantageously more than 0.25 seconds, preferably more than 0.5 seconds, ideally more than 1 second. In these embodiments, the converter's movements are therefore slow, and it must be sized accordingly to obtain the desired power.

[0050] Thanks to the fluidic communication according to the invention, the average temperature (the average being taken over the connected volume) of the working fluid remains close to the phase change temperature of the phase-change material. It is recommended that the difference between these two temperatures, during the period of time in which connectivity and therefore fluidic communication is established, remain below 20°C, preferably below 15°C, and if possible below 10°C. In preferred embodiments of the invention, it is possible to limit this difference to 5°C. Because connectivity is established during at least part of the expansion of the working fluid in contact with the converter, this expansion can indeed approach isothermal expansion, which increases the efficiency of the converter and thus further improves the overall efficiency of the process.

[0051] In embodiments of the invention in which the working fluid comprises a liquid phase and a gaseous phase, a portion of the liquid phase being transformed into a gaseous phase by contact with the storage unit, the working fluid is in phase equilibrium and in contact with at least one storage unit, which remains at its phase change temperature. Two phase equilibria are thus present. The fluid communication according to the invention allows the pressure in the expanding gaseous phase volume to decrease less, by transferring additional working fluid, this additional fluid being produced by vaporization upon contact with at least one storage unit. This further increases the work produced by the converter. In practice, for example, in embodiments in which the The converter comprises a housing and working fluid inlet means equipped with a shut-off system, the latter remaining open during at least part of the working fluid expansion. The combination of these variants facilitates the entry of additional working fluid into the converter during expansion. For example, converters according to this embodiment may include piston-type means whose compression / expansion chamber is in fluidic communication with at least one storage unit during the expansion phase.

[0052] When the working fluid has completed its expansion, it exits the converter in essentially gaseous form, although the presence of condensed liquid (mist) formed during expansion is possible. This fluid can, if necessary, be used as is in certain applications. For example, it can be introduced into a conventional machine that produces work from a compressed gas. When the gas is steam, this can be any type of known steam engine.

[0053] The extraction of the working fluid from the converter can be continuous or discontinuous. The first situation (continuous extraction) occurs when the converter is of the turbine type. The second situation corresponds, for example, to piston-type converters. In this second situation, the converter generally includes working fluid outlet means equipped with shutters to perform the working fluid transfers appropriately. The piston is advantageously associated with return means, for example a spring, which tends to return it to its starting position before the working fluid expands. These return means also assist in the extraction of the expanded fluid.

[0054] In general, as described above, particular care is taken in the design of the converters to make the best use of the possibility of heat transfer from the PCM to the working fluid during its expansion and the possibility for additional working fluid to enter the converter during expansion.

[0055] In a recommended embodiment of the invention, the expanded working fluid extracted from the converter circulates through a secondary heat exchanger, whereby it transfers some of its thermal energy to a utility fluid. This utility fluid can be any industrial fluid that needs to be heated. Advantageously, the utility fluid is water. For example, in the domestic applications of the invention, it is particularly recommended that the utility fluid be water circulating in the heating system or domestic hot water. The overall energy efficiency of the process according to the invention is then exceptional. The utility fluid is generally contained in a secondary tank, with the secondary heat exchanger immersed in the utility fluid. The heat exchanger is connected to the working fluid outlet means of the converter. In practice, the secondary tank is ge Generally equipped with utility fluid inlet and outlet means. The inlet and outlet flow rates can be conveniently adjusted to maintain a setpoint value for the utility fluid.

[0056] In a variant of this embodiment, the working fluid is at least partially condensed in the heat exchanger. It is then advantageous for the condensed liquid to be reinjected into the thermal reservoir, the use of a pump generally being necessary to compensate for the pressure drop of the working fluid after it leaves the converter.

[0057] In another particular embodiment of the invention, the thermal reservoir includes means for converting electrical energy into heat. The electrical energy then advantageously comes from renewable energy sources, for which storage is necessary, preferably photovoltaic panels. This embodiment makes it possible to reach higher temperatures, and therefore to use PCMs with higher transition temperatures, which have significant storage capacities. It also makes it possible to increase the thermodynamic efficiency of the process. This embodiment can be recommended, for example, in the case of a surplus of available electrical energy.

[0058] The process according to the invention makes it possible to store and release in thermomechanical form thermal energy from many sources.

[0059] For example, in a first practical embodiment of the invention, the thermal energy introduced into the thermal reservoir is waste energy produced by a primary process. The invention thus makes it possible to recover this energy, which would otherwise be lost, or at least to make better use of it. Waste energy is understood to be energy whose production is not sought as such but which inevitably accompanies the (so-called primary) process that gives rise to it. For example, it may be the heat contained in cooling fluids from various processes, industrial or otherwise, such as metallurgy, conventional combustion, or combustion in fuel cells. The process according to the invention makes it possible to extract mechanical energy from these cooling fluids before releasing them.

[0060] In a second practical embodiment of the invention, the thermal energy introduced into the thermal reservoir comes at least in part from the conversion of excess electrical energy into heat. This energy can, for example, be produced by photovoltaic solar panels during periods of high sunlight.

[0061] In a third practical embodiment of the invention, which is particularly advantageous, the thermal energy introduced into the thermal reservoir comes from solar thermal collectors. Solar collectors are understood to be Thermal systems that cause a heat transfer fluid to rise in temperature by passing through pipes exposed to the sun. This embodiment is advantageously combined with those in which the utility fluid is water, preferably domestic hot water. Indeed, the free energy contained in the heat transfer fluid is then used much more efficiently than in commercial systems where significant entropy generation and free energy loss occur in the contact between the heat transfer fluid and the hot water tank, the temperature of which is maintained below 100°C for safety reasons.

[0062] It is understood that the variants and modes of execution of the invention described in this memorandum can be combined with each other, when they are not manifestly mutually exclusive.

[0063] The invention also relates to a system for the thermomechanical storage of thermal energy comprising:

[0064] • a thermal energy reservoir comprising at least one thermal energy storage unit, said unit comprising a storage material capable of undergoing a phase transition from a lower internal energy state to a higher internal energy state, • means of inputting the thermal energy to be stored, intended to transmit this thermal energy to at least one storage unit and to allow a substantial part of the storage material to be transformed into its higher internal energy phase by absorption of this energy,

[0065] • means for containing a working fluid, at least partly in gaseous form, intended to produce work by volumetric expansion, said means allowing the fluid to be in contact with at least one storage unit, • a thermal energy-to-mechanical energy converter, designed to come into contact with the working fluid and to receive energy through its volumetric expansion, • means for outputting the mechanical energy produced by the converter.

[0066] The system according to the invention includes means for maintaining the volume of working fluid, which is in contact with the converter, in fluidic communication with the volume of working fluid, which is in contact with at least one storage unit, and this during at least part of its volumetric expansion.

[0067] In the system according to the invention, the converter is arranged so as to allow the working fluid to be in contact with at least one storage unit so as to receive energy from it. This energy transfer results from the phase change of at least a portion of the material undergoing a phase change from its higher energy phase. to its lower energy phase.

[0068] In a variant of the system according to the invention, the means for containing the working fluid are at least partly contained in the thermal reservoir.

[0069] According to an advantageous embodiment of this variant, the converter is also at least partially contained within the thermal reservoir. This variant greatly simplifies the system construction.

[0070] In yet another variant of the system according to the invention, the thermal energy-to-mechanical energy converter comprises piston-type means.

[0071] The system according to the invention is intended to implement the method according to the invention. The embodiments and variants described for the method according to the invention therefore apply mutatis mutandis to the system according to the invention. For example, the concept of contact must be understood in the same way. Where applicable, it is understood that the system includes means for implementing the variants of the method, the implementation of these means not posing any particular problems for a person skilled in the art.

[0072] The accompanying figure illustrates a particular embodiment of the system according to the invention.

[0073] This system comprises the following elements. A thermal energy reservoir (5) includes a set of storage units (10) schematically represented by circles drawn at a distance from one another and supported by a grid (19). The system includes thermal energy input means (9) and (12) external to the reservoir and a heat exchanger (11) located inside the reservoir and surrounded by the storage units (10). The reservoir is further filled with a working fluid consisting of water. This working fluid comprises a portion (18) in liquid form below the grid (19), a portion (8) also in liquid form in which the units (10) are immersed and in which the heat exchanger (11) is immersed, and finally a portion (2) in vapor form. A thermal energy-to-mechanical energy converter (3) is also disposed in the reservoir (5).It is immersed in the portion (2) of the working fluid, which is in vapor form. This converter (3) includes mechanical energy output means (1) and working fluid inlet (6) and outlet (4) means, equipped with shutters. The particular system illustrated also includes a secondary reservoir (20) containing a utility fluid (15). A secondary heat exchanger, connected to the converter's outlet means (4) by a conduit (7), is immersed in the liquid (15). The secondary heat exchanger is intended to carry the expanded working fluid extracted from the converter. The outlet of the heat exchanger is connected to a conduit (16) entering the thermal reservoir, which includes a pump (not shown), allowing the working fluid to be reintroduced into the thermal reservoir. The end of the conduit (16) includes means (not shown) for promoting turbulence within the working fluid, improving heat transfer with the... storage units. The secondary tank is also provided with means for the inlet (13) and outlet (14) of utility fluid.

[0074] Details and features of the invention will become apparent from the following description of a particular embodiment of the invention, with reference to the attached figure.

[0075] Example

[0076] The system illustrated in [Fig.1] operates in the following manner, in accordance with an advantageous embodiment of the process according to the invention.

[0077] Water (18) is introduced into a tank (5) via water inlet means (not shown), until the tank is filled to approximately 75% of its free volume (excluding the converter). The tank, which includes a support grid (19), also contains a number of spherical storage units (10) filling approximately 50% of the tank's volume, resting on the grid (19) so as to leave a space without a storage unit in the lower part of the tank. The units (10), immersed in the water (18), comprise 95% by weight of PCMs having a solid-solid phase transition around 130°C. The PCM is finely encapsulated in a polymer that protects it from the water. The tank also includes a heat exchanger (11) through which a heat transfer fluid flows, equipped with fluid inlet / outlet means (9, 12). heat transfer fluid. The heat exchanger (11) is completely surrounded by the units (10) and the water (18).The heat transfer fluid is heated by a thermal field, comprising solar thermal panels, not shown in the figure. The thermal energy-to-mechanical energy converter (3), located in the thermal reservoir, comprises a piston (not shown in the figure) with vertical movement, to which return means tend to place it in the lowered position. The working fluid consists of water in liquid (18) and vapor (2) form. The vapor can enter the converter through the inlet means (6), which include a shutter. The utility fluid contained in the secondary reservoir (20) consists of water. The outlet means (4) also include a shutter.

[0078] During operation, in a storage phase, the solar panels are exposed to solar radiation and the flow rate of the heat transfer fluid is adjusted to maintain its temperature slightly above the phase transition temperature of the PCM material, here around 135-140°C. The inlet means (6) shutter is closed and the piston is in the lowered position. Contact between the heat transfer fluid and the PCM, ensured by the heat exchanger, causes the PCM to transition to its high-energy phase. The storage phase continues until 90% of the PCM material by volume has undergone a phase transition. The average temperature of the PCM in the thermal storage tank therefore remains close to its transition temperature, i.e., close to 130°C. It should be noted that it would be possible to continue the storage phase. Slightly beyond 100% transition, causing the PCM temperature to rise above the phase transition temperature, but this possibility is not used. At the end of the storage stage, the temperature of the working fluid in contact with the storage unit is also, after equilibrium, close to 130°C. The pressure in the thermal reservoir approaches 2.5 bar. During a release phase, the shutter (6) is open. Water vapor at 2.5 bar enters the converter, exerting a pressure on the piston significantly greater than the action of the return means. The piston rises and sets the mechanical energy output means (1) in motion. According to a particularly advantageous feature of this embodiment of the invention, during essentially the entire upward movement of the piston and the expansion of the working fluid that causes it, the shutter of the means (6) remains open.The pressure in the thermal reservoir initially tends to decrease following the entry of steam into the converter and its expansion, which causes further vaporization of the liquid phase in contact with the PCM. Part of the PCM returns to its lower energy phase, but, as its temperature remains constant, the pressure in the thermal reservoir remains close to 2.5 bar. Consequently, the expansion of the working fluid is essentially isobaric. When the piston, which has a cross-section of 0.25 m², reaches its upper position after approximately 2 seconds of travel, the inlet valve closes and the outlet valve opens. The piston stroke is 50 cm. The piston descends under the action of the return means, causing the expanded steam to circulate through the conduit (7) and the heat exchanger (17), which heats the utility fluid (15).The steam condenses in the heat exchanger, and the water is reintroduced into the thermal storage tank via conduit (16) and its associated pump. The flow of the reintroduced water is designed to improve the movement of the working fluid around the storage units and thus enhance heat transfer. After a sufficient number of heat recovery stages, the temperature of this fluid approaches 90°C and is then maintained around this value by drawing off hot water and introducing cold water via means (13) and (14).

Claims

Claims

1. A method for the thermomechanical storage of thermal energy, wherein, in a storage step, • thermal energy is introduced via thermal energy input means (9, 11, 12) into a thermal reservoir (5) comprising at least one storage unit (10), said unit comprising a storage material capable of undergoing a phase transition from a lower internal energy phase to a higher internal energy phase, • a substantial part of the storage material is transformed into its higher internal energy phase by absorption of the thermal energy introduced into the reservoir (5), and according to which, in a restitution step, • thermal energy is transferred from the at least one storage unit (10) to a working fluid (2, 8, 18), the working fluid (2, 8, 18) being in contact with said storage unit (10), a substantial part of the storage material contained in the storage unit (10) being thereby progressively transformed into its lower internal energy phase, • a quantity of the working fluid (2, 8, 18), at least partly in gaseous form, is brought into contact with a converter of thermal energy (3) into mechanical energy, • mechanical energy is produced by the converter (3), by volumetric expansion of said working fluid (2, 8, 18), • mechanical energy is transferred to mechanical energy output means (1) for use, characterized in that the volume of working fluid (2, 8, 18) which is in contact with the converter (3) is in fluid communication with the volume of working fluid (2, 8, 18) which is in contact with at least one storage unit (10), during at least part of its volumetric expansion.

2. Method according to the preceding claim, in which the fluid of working unit (2, 8, 18) comprises a liquid phase and a gaseous phase, a part of the liquid phase being transformed into the gaseous phase by contact with the storage unit (10).

3. A method according to any preceding claim wherein the lower energy phase of the storage material is a solid phase and the higher energy phase is a liquid or solid phase.

4. A method according to any preceding claim, wherein the expanded working fluid extracted from the converter (3) circulates through a secondary heat exchanger (17) through which it transfers part of its thermal energy to a utility fluid (15).

5. Method according to the preceding claim, in which the utility fluid (15) is domestic hot water and the thermal energy which is introduced into the thermal reservoir (5) comes from thermal solar collectors.

6. System for the thermomechanical storage of thermal energy comprising: • a thermal energy reservoir (5) comprising at least one thermal energy storage unit (10), said unit comprising a storage material capable of undergoing a phase transition from a lower internal energy state to a higher internal energy state, • input means (9, 11, 12) for the thermal energy to be stored, intended to transmit this thermal energy to at least one storage unit (10) and to allow a substantial part of the storage material to be transformed into its higher internal energy phase by absorption of this energy, • means (5) for containing a working fluid (2, 8, 18), at least partly in gaseous form, intended to produce work by volumetric expansion, said means allowing the fluid (2, 8, 18) to be in contact with at least one storage unit (10),• a converter of thermal energy into mechanical energy (3), intended to come into contact with the working fluid (2, 8, 18) and to receive energy by volumetric expansion of the latter, • means for outputting (1) the mechanical energy produced by,

7. the converter, characterized in that it comprises means (6) for maintaining the volume of working fluid (2, 8, 18), which is in contact with the converter (3), in fluid communication with the volume of working fluid (2, 8, 18), which is in contact with at least one storage unit (10), and this during at least part of its volumetric expansion. System according to the preceding claim in which the converter of thermal energy into mechanical energy (3) comprises piston-type means.

Images