Latent heat storage with integrated condition monitoring

Abstract

The invention relates to a latent heat storage device (1) for storing thermal energy comprising a heat storage body (2) comprising a phase change material (3), and a monitoring unit for monitoring a charging and / or material state of the heat storage body (2) comprising at least one piezoelectric element (4) arranged in the heat storage body (2). In this way, a latent heat storage (1) is provided, which can be used to monitor the charging and / or material condition in a particularly efficient manner and in different application areas.

Description

[0001] The invention relates to a latent heat storage device for storing thermal energy.

[0002] Physical heat storage is divided into sensible (or perceptible) and latent heat. In sensible heat storage, the storage material is heated and releases heat over time. Both liquids and solids can be used. Typical examples of sensible heat storage are water tanks, which are particularly cost-effective and suitable for both heating and cooling applications. The thermal energy Q sens This results from the product of the mass m and the specific heat capacity c p and the temperature change of the material ΔT. Since no phase transitions take place, c remains p constant and Q sens It behaves linearly. The following applies: Qsens=CΔT=mcpΔT.

[0003] Unlike sensible heat storage systems, latent heat storage systems, also known as phase change materials (PCMs), utilize the energy of phase transitions. Phase changes include both changes in the state of matter (solid-liquid, liquid-gas) and recrystallization between different crystal structures (solid-solid). The stored energy Q lat This varies considerably (ΔH = enthalpy change and Δh = specific enthalpy change of the phase transition), both between the transition forms and depending on the material. The following applies: Qlat=ΔH=mΔh.

[0004] Latent heat storage materials often possess a high melting point and are particularly interesting for heat transport in the fluid state as well as heat absorption and storage in the immobilized solid state. Typical applications for PCMs are found in solar technology, heat recovery, and industrial energy storage in buildings. Since the supplied thermal energy is used for the corresponding conversion process during a phase change, PCMs can also be used for temperature stabilization. This is useful, for example, for electronic systems such as battery stacks. Both organic and inorganic materials are suitable as latent heat storage materials. From a technical perspective, it is important to avoid material loss. Therefore, the material used should have the lowest possible vapor pressure. Furthermore, leakage can easily occur in the molten state. To prevent this, PCMs are often encapsulated or microencapsulated in a support material.Due to the very narrow temperature range in which the phase transition occurs, temperature sensors are only conditionally suitable for monitoring the degree of crystallization, hereinafter referred to as the state of charge. Since information about the state of charge can be time-critical depending on the application, due to temperature-sensitive processes or unexpectedly high energy input, PCM storage devices are currently unsuitable for some applications. Therefore, the object of the invention is to provide a latent heat storage device in which the state of charge can be monitored and which simultaneously overcomes the aforementioned disadvantages.

[0005] This problem is solved by the subject matter of claim 1. Preferred embodiments are found in the dependent claims.

[0006] According to the invention, a latent heat storage device for storing thermal energy is thus provided with a heat storage body, which includes a phase change material and a monitoring unit for monitoring a charging and / or material state of the heat storage body, comprising at least one piezoelectric element arranged in the heat storage body.

[0007] A key aspect of the invention is that the heat storage element has at least one integrated piezoelectric element. The piezoelectric element is permanently arranged within the heat storage element and can remain there. Furthermore, the use of a probe for inserting the piezoelectric elements is never necessary.

[0008] A piezoelectric element is a component that utilizes the piezoelectric effect to convert mechanical energy into electrical energy and vice versa. It consists of materials that generate an electrical voltage when mechanically deformed, whether by compression, tension, or bending. This effect occurs because the material's crystal structure causes a shift in the centers of charge when stressed, resulting in the generation of a measurable electrical voltage.

[0009] Piezoelectric elements can be used to employ a range of methods, such as electromechanical impedance spectrum, acoustic ultrasonics guided waves and acoustic emission, to integrate the charge state and / or the general material condition of the PCM memory and thereby simultaneously provide cost-effective monitoring of the general condition (structural health monitoring, SHM).

[0010] In addition to or as an alternative to electromechanical impedance measurements, measurements using ultrasound can be performed. Ultrasound measurements utilize high-frequency sound waves, particularly waves with a frequency greater than 20 kHz, generated by at least one piezoelectric element, to examine material properties and / or the material condition for defects. At defects in the interfaces, the waves are reflected, scattered, or attenuated. These modified waves are then measured again with the at least one piezoelectric element to draw conclusions about the material condition.

[0011] Since information about the state of charge of PCM storage devices can be time-critical depending on the application, such as temperature-sensitive processes or unexpectedly high energy input, PCM storage devices have not been suitable for some applications to date. However, the latent heat storage device according to the invention makes this possible, as it allows the state of charge to be determined in near real time. Furthermore, the latent heat storage device according to the invention can provide important information for an energy management system, since the energy stored or available in the PCM becomes measurable, thus enabling the control of, for example, (nighttime) and mobile latent heat storage devices in industrial environments. This allows for the optimization and increased efficiency of the use of available energy, for example, from sustainable, non-constantly producing sources, as well as preventing damage to the PCM, for example, through overcharging.

[0012] Preferably, the phase change material comprises organic, inorganic, or eutectic materials. For example, the phase change material consists of paraffins, fatty acids, or other organic compounds such as alcohols, glycols, or polymers. Furthermore, the phase change material can also consist of salt hydrates, metals and alloys, or other inorganic salts such as sodium sulfate or calcium sulfate. "Eutectic materials" are understood to mean, in particular, combinations of two or more components that melt or solidify together at a specific temperature without losing their chemical identity. Such mixtures can consist of both organic and inorganic components. They offer precise melting points and can be specifically designed for particular temperature ranges. It is especially preferred that the phase change material consists at least partially of a polymer.Polymers as phase change materials (PCMs) offer specific advantages, particularly when flexibility, malleability, and adaptability are required. The latent heat storage medium can assume virtually any desired shape. Polymers are inherently flexible and can be processed into various forms, making them ideal for applications where the PCM needs to be integrated into specific structures or geometries, such as in textiles or flexible electronics. Another advantage of polymers is that their phase change temperatures can be precisely tailored to meet specific requirements through polymer selection or modification. Furthermore, polymers are often lighter than other materials like metals or salts, making them particularly attractive for applications where weight is a critical factor.

[0013] According to a preferred embodiment of the invention, the piezoelectric element is arranged to be completely enclosed by the heat storage body. The piezoelectric element is therefore fully integrated into the heat storage body and can measure changes in the heat storage body in all directions.

[0014] According to a preferred embodiment of the invention, the piezoelectric element is arranged in a fixed location within the phase-change material. "Fixed location" means, in particular, that the piezoelectric element is fixed or set to a specific location. This is particularly advantageous for specific data evaluation methods and physical measurement principles when using piezoelectric elements and is sometimes necessary to achieve comparability of measurements.

[0015] According to a preferred embodiment of the invention, the latent heat storage device comprises a holding device designed to position the piezoelectric element in a fixed location within the phase-change material. In this way, the stationary state can be achieved.

[0016] Alternatively or additionally, according to a preferred embodiment of the invention, the phase change material is designed to be dimensionally stable. The phase change material remains stationary even in the molten state, and no convective material transport can occur. This can be achieved, for example, by gelators or aerogels that provide a porous structure in which the PCM remains in the molten state due to capillary forces. If the mechanical stability of some gels is still insufficient, polymers can be added to improve it.

[0017] According to a preferred embodiment of the invention, the phase-change material comprises a gelator with a mass fraction of preferably less than 15%, particularly preferably less than 10%. In this way, dimensional stability can still be achieved. Due to this dimensional stability, a piezoelectric element arranged in this phase-change material is automatically also fixed in position, since it can no longer be moved by the phase-change material.

[0018] According to a preferred embodiment of the invention, the phase-change material comprises at least a portion of an electrically insulating material. In this way, the at least one piezoelectric element can be electrically isolated, thus preventing short circuits of the electrical contacts of the piezoelectric elements. For this purpose, the phase-change material can comprise at least a portion of paraffin, fatty acid esters, or other insulating materials.

[0019] According to the invention, a method for monitoring the charging and / or material state of a latent heat storage device described above is further provided. The method comprises the following process steps: S1) Performing electromechanical impedance measurements and / or ultrasonic measurements using the piezoelectric element and obtaining a frequency spectrum; and S2) Determining a state of charge and / or a material state of the heat storage body depending on the frequency spectrum.

[0020] Electromechanical impedance measurements measure the interaction between electrical and mechanical vibrations within a system. More precisely, electromechanical impedance measures how a system reacts to mechanical loads or vibrations. It captures both the system's resistance to mechanical movements and the electrical system's resistance to current flow. This vibration can be measured using the piezoelectric element, eliminating the need to transmit waves to another element or allow reflections back to the piezoelectric element. The electromechanical impedance measurements generate an impedance spectrum from which the state of charge of the thermal storage element can be derived. Reference measurements can be used for this purpose.

[0021] Ultrasonic measurements utilize high-frequency sound waves, particularly waves with a frequency greater than 20 kHz, generated by at least one piezoelectric element, to examine material properties and / or the material condition for defects. At surface defects, the waves are reflected, scattered, or attenuated. These altered waves are then measured again by the at least one piezoelectric element to draw conclusions about the material condition. The most important parameters of the measured waves are transit time, amplitude, and frequency variations. The emitted wave is preferably a harmonic wave with a specific frequency, which, when represented as a spectrum, shows a single maximum, while all other frequencies only indicate the noise level. However, if the wave is altered by a defect, there can be multiple maxima or a shifted maximum.This method enables the precise, non-destructive testing of materials for cracks or damage.

[0022] The aim of this method is to determine the state of charge of a latent heat storage device, to make a statement about the stored energy content, and / or to check the material condition for defects and / or changes, for example, due to foreign substances. When using suitable reference materials, knowledge of the degree of crystallinity, the proportion of the solid phase, is not necessary for determining the state of charge at any point.

[0023] It has been shown that the state of charge can be measured with a deviation of less than 20%, preferably less than 10%, using the method according to the invention.

[0024] According to a preferred embodiment of the invention, the method comprises the following further process step: S1a) Evaluating predetermined frequency ranges of the frequency spectrum.

[0025] Specific frequency ranges are used to evaluate impedance measurements. These frequency ranges depend primarily on the application. Changes in frequency can provide information about the crystallization or discharge of the latent heat storage. Particularly large frequency changes can be observed in the region of the piezoelectric element's natural frequency. In these regions, the amplitude of the spectrum decreases, which can be explained by the higher stiffness of the crystalline phase of the heat storage material, which inhibits the piezoelectric element's oscillation.

[0026] To evaluate the ultrasonic measurements, the emitted wave, particularly the harmonic wave, is modified in the case of defects or other changes, so that multiple maxima or a shifted maximum become visible in the frequency spectrum. This method enables the precise, non-destructive testing of materials for cracks or damage.

[0027] The invention will now be explained in more detail with reference to the drawings and a preferred embodiment.

[0028] The drawings show Fig. 1a schematically a shape-stabilized latent heat storage device according to a preferred embodiment of the invention, Fig. 1b schematically a latent heat storage device according to a further preferred embodiment of the invention, Fig. 2 a flowchart of a method for monitoring a charging and / or material state of a latent heat storage device according to a preferred embodiment of the invention.

[0029] The Fig. 1a and Fig. Figures 1b each show a latent heat storage device according to a preferred embodiment of the invention. The latent heat storage device 1 comprises a heat storage body 2, which can assume almost any shape. The heat storage body 2 consists of a phase-change material 3 that fills the heat storage body 2. At least one piezoelectric element 4 is arranged in the phase-change material 3. Fig. 1a Two piezoelectric elements 4 are arranged axially symmetrically on opposite sides. Fig. 1b A piezoelectric element 4 is arranged in the center of the heat storage body 2. To ensure that the piezoelectric element 4 is positioned within the heat storage body 2, either the phase change material 3 – as in Fig. 1a shown - designed to be dimensionally stable, so that the piezoelectric element 4 is not movable at all in the phase-change material 3, or the piezoelectric element 4 - as in Fig. 1b shown - mechanically fixed in the heat storage body 2 with a holding device 5.

[0030] Fig.Figure 2 shows a method for monitoring the charge and / or material state of a latent heat storage device according to a preferred embodiment of the invention. In a first step S1, an electromechanical impedance measurement is performed. The vibrations in the phase-change material 3 can be detected by means of the piezoelectric element 4. An impedance spectrum can be acquired through the impedance measurement. In a subsequent step S1a, specific frequency ranges are evaluated within this impedance spectrum. It has been shown that particularly large changes can be measured in the ranges of the natural frequencies of the piezoelectric element. In these ranges, the amplitude of the spectrum decreases, which can be explained by the higher stiffness of the crystalline phase of the PCM, which inhibits the piezoelectric element during vibration.Based on the impedance spectrum and especially on amplitude and frequency changes, a statement about the state of charge of the latent heat storage can be made in a next step S2. Reference symbol list 1. Latent heat storage 2 heat storage elements 3 Phase change material 4 Piezoelectric elements 5 Holding device S1) Performing electromechanical impedance measurements using the piezoelectric element S1a) Evaluating predetermined frequency ranges of the impedance spectrum S2) Determining a charging and / or material state of the heat storage body

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