Test methods for the thermophysical properties of phase change materials in supercooled unstable hydrated salt systems

Abstract

This invention discloses a method for testing the thermophysical properties of a supercooled unstable hydrated salt phase change material. The method includes: performing a first DSC cycle test on the supercooled unstable hydrated salt phase change material: heating from an initial temperature to a first temperature, then cooling to a second temperature; performing at least one pretreatment on the material obtained from the first DSC cycle test: heating from a second temperature to a third temperature, then cooling to a second temperature; and performing a DSC cycle test on the pretreated material. This invention achieves separation of exothermic and endothermic peaks by performing at least one pretreatment—heating and then cooling again within a range below the material's melting temperature—after initial cooling, thus avoiding interference between the exothermic peak and the endothermic peak.

Description

Technical Field

[0001] This invention belongs to the technical field of performance testing of phase change materials in hydrated salt systems, specifically relating to a method for testing the thermophysical properties of phase change materials in supercooled unstable hydrated salt systems. Background Technology

[0002] Currently, the thermophysical performance testing of phase change energy storage materials in hydrated salt systems is usually carried out by differential scanning calorimetry (DSC). For supercooled and unstable hydrated salt systems, due to the influence of the material's supercooling, the material does not have time to undergo a crystallization phase transition during the cooling process. This is reflected in the DSC curve as the absence of an exothermic peak. However, during the heating process, the material begins to undergo a crystallization phase transition, resulting in the simultaneous appearance of exothermic peaks (crystallization process) and endothermic peaks (melting process) in the heating segment. In some cases, the exothermic and endothermic peaks may even be closely connected (partially overlapping), which can interfere with subsequent thermophysical performance analysis.

[0003] The thermophysical properties of hydrated salt phase change energy storage materials (e.g., melting phase transition enthalpy, melting temperature, crystallization phase transition enthalpy, crystallization temperature) are mainly tested using DSC and T-History methods. The main difference between the two methods lies in the size of the sample being studied. The sample size required for T-History (greater than tens or hundreds of grams) is about 10,000 times larger than that required for DSC (1-20 mg). However, DSC is faster and yields more accurate results. The vertical axis of the DSC curve represents the rate of heat absorption or release, and the horizontal axis represents temperature or time. Generally, a complete melt-solidification process (heating to complete melting and then cooling to complete crystallization) is considered a cycle. However, when the supercooling behavior of the material is unstable, there may be no exothermic peak during cooling, but a delayed exothermic peak during heating. Sometimes, the material begins to melt again before complete crystallization, which can lead to coupling of exothermic and endothermic peaks, easily causing deviations in analytical conclusions during actual data analysis.

[0004] To address this issue, existing technologies primarily involve crystallizing the sample during preparation and performing only one DSC test. This method can lead to inaccurate test results for samples requiring DSC cycling. Alternatively, for samples requiring cycling, the T-History method can be used for macroscopic cycling, followed by a single DSC test. However, this method is not only time-consuming, but also affects the uniformity of heating due to the large sample volume and the interfacial water exchange caused by equipment sealing, which in turn affects the material's cycling performance and thus has a certain impact on the test results. Summary of the Invention

[0005] The main objective of this invention is to provide a method for testing the thermophysical properties of phase change materials in supercooled unstable hydrated salt systems, in order to overcome the shortcomings of existing technologies. The thermophysical properties of existing phase change energy storage materials in supercooled unstable hydrated salt systems are mainly tested by the DSC method. The main drawback of this method is that the exothermic peak and the endothermic peak are mixed during the heating phase of DSC, which can easily lead to deviations in the data analysis results.

[0006] To achieve the aforementioned objectives, the technical solution adopted by this invention includes:

[0007] This invention provides a method for testing the thermophysical properties of phase change materials in a supercooled unstable hydrated salt system, comprising:

[0008] Provide phase change materials for supercooled unstable hydrated salt systems;

[0009] The supercooled unstable hydrated salt system phase change material was subjected to a first DSC cycle test: the temperature was increased from the initial temperature to a first temperature, and then decreased to a second temperature; wherein the initial temperature was lower than the phase change temperature of the supercooled unstable hydrated salt system phase change material, the first temperature was higher than the melting temperature of the supercooled unstable hydrated salt system phase change material, and the second temperature was lower than the melting temperature of the supercooled unstable hydrated salt system phase change material.

[0010] The material obtained from the first DSC cycle test is subjected to at least one pretreatment: the temperature is raised from a second temperature to a third temperature, and then cooled back to the second temperature; wherein the third temperature is lower than the melting temperature of the phase change material of the supercooled unstable hydrated salt system.

[0011] In addition, the pretreated material was subjected to DSC cycle testing.

[0012] This invention also provides a method for testing the thermophysical properties of the aforementioned supercooled unstable hydrated salt phase change material, which is applied to testing the cycling performance of materials with supercooled instability.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention provides a method for separating exothermic peaks and endothermic peaks by performing a pretreatment of heating and cooling once or several times within a range below the melting temperature of the material after the initial cooling of the material, thereby avoiding the interference of exothermic peaks on endothermic peaks and realizing accurate analysis of melting characteristic peaks by DSC. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is the DSC curve of the sodium acetate trihydrate phase change energy storage material in Example 1 of this invention during its first cycle;

[0016] Figure 2 This is the DSC curve of the second cycle of the sodium acetate trihydrate phase change energy storage material in Example 1 of this invention;

[0017] Figure 3 This is the DSC curve of the sodium acetate trihydrate system after adding the heating and cooling program segment (-50℃→30℃→-50℃) in Example 1 of the present invention;

[0018] Figure 4 This is the DSC curve of the PCG-10% phase change system during its first cycle in Embodiment 2 of the present invention;

[0019] Figure 5 These are the DSC curves of the PCG-10% phase change system in Embodiment 2 of the present invention for the 2nd, 50th, and 100th cycles;

[0020] Figure 6 This is the DSC curve of the PCG-10% system after adding a heating and cooling program segment (-50℃→50℃→-50℃) in Embodiment 2 of the present invention.

[0021] Figure 7 This is the DSC curve of the PCG-8% phase change system during its first cycle in Embodiment 3 of the present invention;

[0022] Figure 8 These are the DSC curves of the PCG-8% phase change system in Embodiment 3 of the present invention for the 2nd, 50th, and 100th cycles;

[0023] Figure 9 This is the DSC curve of the PCG-8% system after adding a heating and cooling program segment (-50℃→50℃→-50℃) in Embodiment 3 of the present invention. Detailed Implementation

[0024] In view of the shortcomings of the existing technology, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. The main problem is that the thermophysical properties of existing supercooled unstable hydrated salt phase change energy storage materials are mainly tested by DSC method. The main drawback of this method is that the exothermic peak and the endothermic peak are mixed during the heating stage of DSC, which can easily lead to deviations in the data analysis results. A pretreatment stage is designed in which the exothermic peak and the endothermic peak are separated once or several times in a temperature range below the melting temperature of the material after the cooling stage is completed. This avoids the interference of the exothermic peak on the endothermic peak.

[0025] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] One aspect of this invention provides a method for testing the thermophysical properties of a phase change material in a supercooled unstable hydrated salt system, comprising:

[0027] Provide phase change materials for supercooled unstable hydrated salt systems;

[0028] The supercooled unstable hydrated salt system phase change material was subjected to a first DSC cycle test: the temperature was increased from the initial temperature to a first temperature, and then decreased to a second temperature; wherein the initial temperature was lower than the phase change temperature of the supercooled unstable hydrated salt system phase change material, the first temperature was higher than the melting temperature of the supercooled unstable hydrated salt system phase change material, and the second temperature was lower than the melting temperature of the supercooled unstable hydrated salt system phase change material.

[0029] The material obtained from the first DSC cycle test is subjected to at least one pretreatment: the temperature is raised from a second temperature to a third temperature, and then cooled back to the second temperature; wherein the third temperature is lower than the melting temperature of the phase change material of the supercooled unstable hydrated salt system.

[0030] In addition, the pretreated material was subjected to DSC cycle testing.

[0031] In some more specific implementations, the supercooled unstable hydrated salt system phase change material includes sodium acetate trihydrate hydrated salt system phase change energy storage material and / or hydrated salt system phase change gel composite material.

[0032] Furthermore, the hydrated salt system phase change gel composite material is made from hydrated salt phase change energy storage material and organic polymer.

[0033] Furthermore, the hydrated salt system phase change energy storage material includes sodium acetate trihydrate hydrated salt system phase change energy storage material and / or Mg(NO3)2·6H2O-MgCl2·6H2O magnesium-based eutectic salt phase change energy storage material, but is not limited thereto.

[0034] Furthermore, the hydrated salt system phase change gel composite material includes magnesium-based eutectic salt phase change gel composite material and / or sodium acetate trihydrate phase change gel composite material, but is not limited thereto.

[0035] In some more specific implementations, the supercooled unstable hydrated salt system phase change material exhibits supercooled instability.

[0036] In some more specific implementations, when the supercooled unstable hydrated salt system phase change material is subjected to DSC cycle testing, no exothermic peak appears in the cooling section of its DSC curve, or the exothermic peak in the cooling section of its DSC curve is too small or incomplete, and an exothermic peak appears in the heating section.

[0037] In some more specific implementation schemes, the test method specifically includes:

[0038] The first DSC cycle test was performed on the supercooled unstable hydrated salt phase change material: the temperature was increased from the initial temperature to the first temperature at a heating rate of 5-15℃ / min, and then cooled to the second temperature at a cooling rate of 5-15℃ / min.

[0039] Furthermore, the initial temperature is at least 20°C lower than the phase change temperature of the supercooled unstable hydrated salt system phase change material.

[0040] Furthermore, the first temperature is more than 20°C higher than the melting temperature of the phase change material in the supercooled unstable hydrated salt system.

[0041] Furthermore, the second temperature is more than 20°C lower than the melting temperature of the phase change material in the supercooled unstable hydrated salt system.

[0042] In some more specific implementation schemes, the test method specifically includes:

[0043] The material obtained from the first DSC cycle test is subjected to at least one pretreatment: the temperature is increased from the second temperature to the third temperature at a heating rate of 5 to 15 °C / min, and then cooled back to the second temperature at a cooling rate of 5 to 15 °C / min.

[0044] Furthermore, the third temperature is 5–30°C lower than the melting temperature of the phase change material in the supercooled unstable hydrated salt system.

[0045] In some more specific embodiments, the testing methods for the thermophysical properties of the supercooled unstable hydrated salt system phase change material include:

[0046] (1) Samples of the newly prepared hydrated salt system phase change energy storage material were subjected to DSC cycle testing. The program was set to "reach a temperature below the phase change temperature (initial temperature point) → heat up to the highest temperature (first temperature) → cool down to the lowest temperature (second temperature)" as one complete cycle and multiple cycle tests were performed (greater than or equal to 2 times). The "lowest temperature" was tens of degrees below the melting temperature to ensure that the sample could achieve crystallization phase change, and the "highest temperature" was higher than the melting temperature to ensure that the sample could achieve sufficient melting phase change.

[0047] (2) Obtain the cyclic DSC curves of the thermophysical properties (enthalpy of melting, melting temperature, enthalpy of solidification, and solidification temperature) of the sample through step (1);

[0048] (3) If, in multiple cycles, the exothermic peak of the sample appears in the cooling section and only the endothermic peak appears in the heating section, then the hydrated salt system is determined to have stable supercooling behavior; if the sample does not show an exothermic peak in the cooling section of the DSC curve or the exothermic peak is obviously too small or not fully displayed, and an exothermic peak appears in the heating section, then the hydrated salt system is determined to have supercooling instability.

[0049] (4) For hydrated salt systems with supercooling instability, an additional program segment needs to be embedded in the original program: "Minimum temperature → heat up to a temperature below the melting temperature (third temperature) → cool down to minimum temperature" → heat up to maximum temperature → cool down to minimum temperature (the quotation marks are the embedded pretreatment heating and cooling cycle, which can also be repeated multiple times to ensure sufficient crystallization of the sample). Multiple cycle tests are performed for one complete cycle, which can ensure that the exothermic peak appears in the cooling section and the endothermic peak appears in the heating section in the measured DSC cycle curve, without mutual interference.

[0050] This invention addresses the problem that, during the cycling performance testing of existing supercooled unstable hydrated salt phase change energy storage materials, the cooling section of the DSC curve does not exhibit an exothermic peak even when the temperature drops far below the phase change temperature due to the material's supercooling instability. Instead, the material begins a delayed phase change during the heating process, resulting in the simultaneous appearance of exothermic peaks (solidification process) and endothermic peaks (melting process). In some cases, the exothermic and endothermic peaks may even be closely linked, interfering with subsequent thermophysical performance analysis. The invention provides a method to separate the exothermic and endothermic peaks by performing a pretreatment after the material has undergone initial cooling, heating and then cooling once or several times within a temperature range below the material's melting point. This avoids interference between the exothermic and endothermic peaks.

[0051] For hydrated salt systems with supercooling instability, this invention requires embedding an additional pretreatment program segment into the original program: "Minimum temperature → heat up to the highest temperature below the melting temperature → cool down to the minimum temperature". This pretreatment can also be repeated multiple times to ensure sufficient crystallization of the sample.

[0052] Another aspect of the present invention provides the application of the method for testing the thermophysical properties of the aforementioned supercooled unstable hydrated salt phase change material in testing the cycling performance of materials with supercooled instability.

[0053] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. This embodiment is implemented on the premise of the technical solution of the invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0054] Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0055] Example 1: Cyclic Testing of Thermophysical Properties of Phase Change Energy Storage Material in Sodium Acetate Trihydrate (SAT) Hydrated Salt System

[0056] (1) Samples of the newly prepared sodium acetate trihydrate hydrate phase change energy storage material were subjected to DSC testing. Figure 1 The temperature range was 0℃→100℃→-50℃. The thermophysical properties of the sample during the first cycle were obtained: enthalpy of melting (287.5 J / g), melting temperature (58.14℃, from...). Figure 1 No exothermic peak was observed in the sample; both the heating and cooling rates were 10℃ / min.

[0057] (2) Continue to perform DSC testing on the above samples. Figure 2 The temperature range was -50℃→100℃→-50℃. The DSC curve of the sample in the second cycle was obtained. At this time, the sample showed two upward exothermic peaks during the heating process (-50℃→-20℃). The exothermic peak was adjacent to the endothermic peak at -18℃, indicating that the material system has supercooling instability.

[0058] (3) The sodium acetate trihydrate sample was first cooled from 100℃ to -50℃. Figure 3 (Light gray dashed line), then heat to 30°C below the melting temperature of 58°C, then cool to -50°C. Figure 3 (black dashed line), and finally raise the temperature to 100℃ (…). Figure 3 (Solid line) The DSC curve of the sample is obtained, and it can be found that there is only an endothermic peak during the heating process, and it is no longer affected by the exothermic peak.

[0059] Example 2: Thermophysical property testing of hydrated salt system phase change gel composite material (PCG-10%)

[0060] (1). A phase change gel composite material (PCG-10%) was prepared from Mg(NO3)2·6H2O-MgCl2·6H2O magnesium-based eutectic salt phase change energy storage material and acrylamide (AAM). The mass ratio of magnesium-based eutectic salt phase change energy storage material to monomer, crosslinking agent, and initiator was 100:10. The crosslinking agent was polyethylene glycol diacrylate (PEGDA), and the initiator was 1-hydroxycyclohexylphenyl ketone. The composite ratio was m(monomer):m(crosslinking agent):m(initiator) = 100:2:2. Samples were taken for DSC testing. Figure 4 The temperature range was -30℃→90℃→-30℃. The DSC curve of the sample during the first cycle was obtained. It was found from the figure that the sample did not show an exothermic peak. The heating rate and cooling rate were both 10℃ / min.

[0061] (2) Continue to perform DSC cycle testing on the above samples. Figure 5 The temperature range was -50℃→90℃→-50℃. The DSC curves of the sample for the 2nd, 50th and 100th cycles were obtained. It can be clearly seen from the figure that the exothermic peak and the endothermic peak both appeared in the heating stage of the DSC curve and were closely connected. The sample melted again as the temperature rose before it was fully crystallized, which made it difficult to analyze the thermophysical properties of the sample at this time.

[0062] (3) To completely separate the exothermic peak and the absorption peak, the sample was first cooled from 90℃ to -50℃. Figure 6 (Light gray dashed line), then heat to 50°C below the melting temperature, then cool to -50°C ( Figure 6 (black dashed line), and finally raise the temperature to 90℃ (…). Figure 6 (Solid line) The DSC heating curve of the sample is obtained. It can be clearly seen from the figure that the exothermic peak and the endothermic peak are separated in the heating stage, achieving the goal of "only endothermic peak (melting phase change) in the heating stage", avoiding the interference of the exothermic peak on the endothermic peak.

[0063] Example 3: Thermophysical property testing of hydrated salt system phase change gel material (Mg-PCG-8%)

[0064] (1). A phase change gel material (PCG-8%) was prepared from magnesium-based eutectic salt phase change energy storage material Mg(NO3)2·6H2O-MgCl2·6H2O and acrylamide (AAM) (mass ratio 100:8). The mass ratio of the magnesium-based eutectic salt phase change energy storage material to the monomer, crosslinking agent, and initiator was 100:8. The crosslinking agent was polyethylene glycol diacrylate (PEGDA), and the initiator was 1-hydroxycyclohexylphenyl ketone. The composite ratio was m(monomer):m(crosslinking agent):m(initiator) = 100:2:2. Samples were taken for DSC testing. Figure 7 The temperature range was -30℃→90℃→-50℃. The DSC curve of the sample during the first cycle was obtained. The figure showed that the sample had only one weak exothermic peak, and it was impossible to determine whether the sample was completely crystallized. The heating rate and cooling rate were both 10℃ / min.

[0065] (2) Continue to perform DSC testing on the above samples. Figure 8 The temperature range was -50℃→90℃→-30℃. The DSC curves of the sample for the 2nd, 50th and 100th cycles were obtained. It can be clearly seen from the figure that both the exothermic peak and the endothermic peak appeared in the heating stage of the DSC curve, which interfered with the analysis of the heat absorption and exothermic process of the sample.

[0066] (3) Cool the sample from 90℃ to -50℃ first. Figure 9(Light gray dashed line), then heat to 50°C below the melting temperature, then cool to -50°C ( Figure 9 (black dashed line), and finally raise the temperature to 90℃ (…). Figure 9 (Solid line) The DSC heating curve of the sample is obtained. It can be clearly seen from the figure that the exothermic peak and the endothermic peak are separated in the heating stage, achieving the goal of "only endothermic peak (melting phase change) in the heating stage", avoiding the interference of the exothermic peak on the endothermic peak.

[0067] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0068] It should be understood that the technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made to the technical solutions of the present invention without departing from the spirit and scope of the claims are within the scope of protection of the present invention.

Claims

1. A method for testing the thermophysical properties of a phase change material in a supercooled unstable hydrated salt system, characterized in that, include: Provide a supercooled unstable hydrated salt system phase change material; when the supercooled unstable hydrated salt system phase change material is subjected to DSC cycle testing, no exothermic peak appears in the cooling section of its DSC curve, or the exothermic peak in the cooling section of its DSC curve is too small or incomplete and an exothermic peak appears in the heating section. The supercooled unstable hydrated salt system phase change material was subjected to a first DSC cycle test: the temperature was increased from the initial temperature to a first temperature, and then decreased to a second temperature; wherein the initial temperature was lower than the phase change temperature of the supercooled unstable hydrated salt system phase change material, the first temperature was higher than the melting temperature of the supercooled unstable hydrated salt system phase change material, and the second temperature was lower than the melting temperature of the supercooled unstable hydrated salt system phase change material. The material obtained from the first DSC cycle test is subjected to at least one pretreatment: the temperature is increased from the second temperature to the third temperature at a heating rate of 5~15℃ / min, and then cooled back to the second temperature at a cooling rate of 5~15℃ / min; wherein the third temperature is 5~30℃ lower than the melting temperature of the supercooled unstable hydrated salt phase change material. In addition, the pretreated material was subjected to DSC cycle testing.

2. The test method according to claim 1, characterized in that: The supercooled unstable hydrated salt system phase change material includes hydrated salt system phase change energy storage material and / or hydrated salt system phase change gel composite material.

3. The test method according to claim 2, characterized in that: The hydrated salt system phase change gel composite material is made from hydrated salt phase change energy storage material and organic polymer.

4. The test method according to claim 2, characterized in that: The hydrated salt system phase change energy storage material includes sodium acetate trihydrate hydrated salt system phase change energy storage material and / or Mg(NO3)2·6H2O-MgCl2·6H2O magnesium-based eutectic salt phase change energy storage material.

5. The test method according to claim 2, characterized in that: The hydrated salt system phase change gel composite material includes magnesium-based eutectic salt phase change gel composite material and / or sodium acetate trihydrate phase change gel composite material.

6. The test method according to claim 1, characterized in that: The supercooled unstable hydrated salt system phase change material exhibits supercooled instability.

7. The test method according to claim 1, characterized in that, Specifically, it includes: The first DSC cycle test was performed on the supercooled unstable hydrated salt phase change material: the temperature was increased from the initial temperature to the first temperature at a heating rate of 5~15℃ / min, and then cooled to the second temperature at a cooling rate of 5~15℃ / min.

8. The test method according to claim 7, characterized in that: The initial temperature is more than 20°C lower than the phase change temperature of the supercooled unstable hydrated salt system phase change material.

9. The test method according to claim 7, characterized in that: The first temperature is more than 20°C higher than the melting temperature of the phase change material in the supercooled unstable hydrated salt system.

10. The test method according to claim 7, characterized in that: The second temperature is more than 20°C lower than the melting temperature of the phase change material in the supercooled unstable hydrated salt system.

11. The method for testing the thermophysical properties of the supercooled unstable hydrated salt phase change material according to any one of claims 1-10 is used in testing the cycling performance of materials with supercooled instability.

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