Optimization method for phase change thermal storage material

Abstract

The application discloses an optimization selection method of phase change heat storage materials, and belongs to the technical field of material optimization selection methods, and mainly comprises the following steps: preselecting materials, establishing an energy balance equation, determining evaluation indexes and their weights, performing good and bad sorting, and comparison and verification. In the weight determination process, the influence degree of different evaluation indexes on the optimization target is considered, and the weight determination is more reasonable. In the optimization selection process of the phase change heat storage materials, different time scales of specific applications are considered, so that the optimization selection result is more targeted and reliable, and the application provides an effective way for the optimization selection of the phase change heat storage materials.

Description

Technical Field

[0001] This invention relates to the field of material selection methods, and in particular to an optimization method for phase change thermal storage materials. Background Technology

[0002] Against the backdrop of "dual carbon" (carbon dioxide, carbon emissions, and carbon sequestration), accelerating the development of renewable energy is an inevitable choice for my country's green and low-carbon energy transformation. Renewable energy, such as solar and wind power, has advantages such as cleanliness, efficiency, and sustainability, and its research and utilization are gradually increasing. However, its inherent volatility and intermittency pose serious challenges to the stable and reliable operation of energy systems. Energy storage technology can compensate for the mismatch between energy supply and demand in time, space, and form within energy systems. Therefore, promoting the coupling of energy storage technology and renewable energy, enabling their synergistic development in different application scenarios, can, to a certain extent, achieve flexible regulation of renewable energy systems.

[0003] According to the form of energy storage, energy storage technologies are mainly divided into thermal energy storage, electrochemical energy storage, electrical energy storage, chemical energy storage, and mechanical energy storage. Currently, thermal energy storage, electrochemical energy storage, and mechanical energy storage technologies are relatively mature. Among them, thermal energy storage technology has a longer service life and lower cost, and has a wider range of applications. (Reference: He Yaling, Yan Junjie, Yang Weiwei, et al. Efficient energy storage in distributed energy systems [J]. China Science Foundation, 2020, 34(3):9.)

[0004] The main methods of thermal energy storage include sensible heat storage, latent heat storage, and thermochemical heat storage. Among these, latent heat storage has advantages such as high heat storage density, near-isothermal heat storage and release processes, and easy integration with operating systems. It has seen rapid development and widespread application in fields such as power system peak shaving, solar energy utilization, waste heat recovery, and HVAC. In phase change thermal storage systems, the thermophysical properties of the phase change thermal storage material have a significant impact on the system's performance. Therefore, selecting the phase change thermal storage material with the best overall performance from the diverse range of materials available for specific applications and objectives is crucial to determining the performance of the phase change thermal storage system.

[0005] Currently, the main methods for material selection are the Ashby method and the multi-criteria decision making (MCDM) method. The Ashby method is often used in the initial selection stage of materials due to the limited number of indicators in material selection (Reference: Yang K, Zhu N, Chang C, et al. A methodological concept for phase change material selection based on multi-criteria decision making (MCDM): A case study [J]. Energy, 2018, 165 (PT.B(1085-1): 1085-1096.). The MCDM method is more widely used in the optimization selection of phase change thermal storage materials in different application fields (For example, Reference: [1] Anilkumar BC, Maniyari R, Anish S. Optimum selection of phase change material for solar boxcooker integrated with thermal energy storage unit using multi-criteria decision-making technique [J]. The Journal of Energy Storage, 2021, 40(1): 102807. [2] Rastogi M, Chauhan A, Vaish R, et al. Selection and performance assessment of Phase Change Materials for heating, ventilation and air-conditioning applications[J]. Energy Conversion & Management, 2015, 89: 260-269. (etc.) is considered the most promising method for optimizing the selection of phase change thermal energy storage materials.

[0006] By summarizing existing research on the optimization and selection of phase change thermal storage materials based on the MCDM method, it can be found that the current optimization methods mainly have the following two shortcomings:

[0007] (1) When optimizing the selection of phase change thermal storage materials based on the MCDM method, the methods for determining the weights of evaluation indicators mainly include subjective weight determination methods (analytic hierarchy process, AHP, etc.) and objective weight determination methods (entropy weight method, etc.). Among them, subjective weight determination methods mainly rely on the knowledge and experience of experts, while objective weight determination methods mainly rely on the relationship between the original data. Neither subjective nor objective weight determination methods consider the impact of changes in evaluation indicators on the optimization objective, resulting in poor pertinence and practicality of weight determination.

[0008] (2) The multi-timescale characteristics of the application have an important impact on the optimal selection of phase change thermal energy storage materials, while the traditional MCDM-based phase change thermal energy storage material optimization selection method cannot examine the impact of the multi-timescale of the application on the optimal selection of phase change thermal energy storage materials. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and propose an optimization selection method for phase change thermal storage materials that comprehensively considers the influence of evaluation indicators on optimization objectives and the multi-timescale characteristics of the application.

[0010] This invention provides a method for optimizing the selection of phase change thermal storage materials, comprising the following steps:

[0011] (1) Pre-selected materials:

[0012] Pre-selection of phase change thermal storage materials based on actual application scenarios;

[0013] (2) Establish the equation:

[0014] Based on the actual application scenarios and the actual form of the phase change thermal storage system, the energy balance equation of the phase change thermal storage system is established, and the thermophysical parameters of the phase change thermal storage material that affect the performance of the phase change thermal storage system are determined.

[0015] (3) Determine the weights:

[0016] Based on the optimization objectives of phase change thermal storage systems in practical application scenarios, the influence of thermal property parameters on the optimization objectives is analyzed, and the weights of thermal property parameters under multiple time scales are determined.

[0017] (4) Ranking by merit:

[0018] Based on the MCDM method and the weight determination method in step (3), the pre-selected phase change thermal storage materials in step (1) are ranked in order of merit to obtain the optimal phase change thermal storage materials under multiple time scales.

[0019] Preferably, in step (2), when the actual application scenario is a three-tube phase change heat storage unit, the three-tube phase change heat storage unit consists of an inner tube, a sleeve, and an outer tube from the inside out. Water, as the heat transfer fluid, exchanges heat with the phase change heat storage material filled in the sleeve through the inner and outer tubes. The energy balance equation is:

[0020]

[0021] Where ρ is density, c is specific heat, T is phase transition temperature, t is time, λ is thermal conductivity, and L is latent heat of phase transition. The value represents the liquid phase fraction of the phase change material, x is the axial coordinate, r is the radial coordinate, and the subscript p indicates the type of phase change thermal storage material.

[0022] Preferably, in step (3), the method for determining the weights includes the following steps:

[0023] S1: Design an orthogonal experiment based on the range of thermophysical properties of the pre-selected phase change thermal storage material;

[0024] S2: Numerical simulation studies were conducted on orthogonal experiments to obtain numerical simulation results at multiple time scales;

[0025] S3: Perform range analysis on the simulation results to obtain the weights of thermophysical parameters at various time scales under practical applications and optimization objectives.

[0026] Preferably, in step S1, the thermophysical parameters include phase change temperature, density, specific heat, thermal conductivity, and latent heat of phase change.

[0027] Preferably, in step S3, the range analysis is performed as shown in Equations 2 and 3:

[0028] k ij =K ij / 4 Formula 2

[0029] R j =max(k ij )-min(k ij Equation 3

[0030] Among them, K ij The sum of the optimization objectives corresponding to the i-th level of the j-th factor; k ij R represents the mean of the optimization objective corresponding to the i-th level of the j-th factor; j is the range of the j-th factor.

[0031] Preferably, in step S3, the weights are obtained using Equation 4:

[0032]

[0033] Where, ω jrepresents the weight of the factor in column j.

[0034] Preferably, the following steps are also included:

[0035] (5) Comparative verification:

[0036] The optimal phase change thermal storage material obtained in step (4) is compared with the optimal phase change thermal storage material obtained by combining the traditional weight determination method and the MCDM method to verify its rationality.

[0037] Compared with the prior art, the advantages of the present invention are as follows:

[0038] First, this invention considers the impact of changes in evaluation indicators on the optimization objective in the process of determining the weights of evaluation indicators: the impact of evaluation indicators on the optimization objective is quantitatively studied based on orthogonal experimental design, numerical simulation research and range analysis, and then the weights of evaluation indicators are determined based on the magnitude of the impact, which improves the pertinence and rationality of weight determination.

[0039] Secondly, the proposed method takes into account different time scales of application, and can obtain the optimal phase change thermal storage material for application at different time scales, thereby improving the flexibility and reliability of phase change thermal storage material selection. Attached Figure Description

[0040] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention, wherein:

[0041] Figure 1 A flowchart illustrating the optimized selection of phase change thermal storage materials provided by this invention;

[0042] Figure 2 This is a schematic diagram of the three-tube phase change thermal storage heat exchange unit provided by the present invention. Detailed Implementation

[0043] The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed.

[0044] An optimization method for phase change thermal storage materials, the specific process of which is as follows: Figure 1 As shown, the main steps include:

[0045] Step one: Pre-select phase change thermal storage materials based on the specific application scenario. Specifically, select phase change thermal storage materials that meet the phase change temperature requirements of the actual application.

[0046] For example, solar thermal utilization can be divided into high temperature (120~270℃), medium temperature (80~120℃) and low temperature (40~80℃) applications (Reference: [1] Bhupendra G, Jyoti B, Shlok S, et al. Phase change materials in solar energy applications: A review [J]. Materials Today: Proceedings 46 (2021) 5550-5554 2020.). Taking medium temperature solar thermal utilization as an example, phase change thermal storage materials with a phase change temperature range of 80~120℃ can be selected as pre-selected materials.

[0047] Step 2: Based on the specific application scenario and the specific form of the phase change thermal storage system, establish the energy balance equation of the phase change thermal storage system and determine the thermal property parameters of the phase change material that affect the performance of the phase change thermal storage system.

[0048] For example, taking a three-tube phase change thermal storage heat exchange unit as a specific application, its physical model is as follows: Figure 2 As shown, it consists of three concentric circular tubes, from the inside out: an inner tube, a casing, and an outer tube. During the heat storage and release process, water, as the heat transfer fluid, exchanges heat with the phase change thermal storage material filled in the casing through the inner and outer tubes. The phase change thermal storage material stores and releases heat through melting and solidification.

[0049] The corresponding energy balance equation is as follows:

[0050]

[0051] In Equation 1, ρ represents density, kg / m³ 3 ; c represents specific heat, J / (kg·K); T represents phase transition temperature, K; t represents time, s; λ represents thermal conductivity, W / (m·K); L represents latent heat of phase transition, J / kg; The liquid phase fraction of the phase change material; x represents the axial coordinate; r represents the radial coordinate; the subscript p represents the phase change heat storage material.

[0052] According to the energy balance equation (Equation 1) of phase change thermal storage materials, the thermophysical parameters that affect the performance of the thermal storage system include: phase change temperature T, density ρ, specific heat c, thermal conductivity λ, and latent heat of phase change L.

[0053] Step 3: For specific optimization objectives (optimization objectives refer to the goals that the phase change thermal storage system needs to achieve in practical applications, such as good economic efficiency and energy saving), analyze the impact of changes in the thermophysical parameters of the phase change thermal storage material on the optimization objectives, and determine the weights of the thermophysical parameters of the phase change thermal storage material at different time scales. The specific steps are as follows:

[0054] ① Design an orthogonal experimental scheme based on the range of thermophysical property parameters of the pre-selected phase change thermal storage material;

[0055] ② Numerical simulation studies were conducted on different orthogonal experimental schemes to obtain numerical simulation results at different time scales;

[0056] ③ Range analysis was performed on the experimental results at different time scales to obtain the weights of different thermophysical parameters of the phase change thermal storage material at different time scales under specific applications and optimization objectives, as shown in Equations 2 and 3 below:

[0057] k ij =K ij / 4 (2)

[0058] R j =max(k ij )-min(k ij (3)

[0059] In the formula, K ij The sum of the observed variables (optimization objective) corresponding to the i-th level of the j-th factor; k ij R represents the mean of the observed variable corresponding to the i-th level of the j-th factor; j This represents the range of the j-th factor.

[0060] According to range analysis, the weights of different thermophysical parameters of phase change thermal storage materials can be expressed as Equation 4:

[0061]

[0062] In the formula, ω j This represents the weight of the factor in column j.

[0063] Since the time scale required for phase change thermal storage processes varies in different practical applications, the different time scales for different application needs will cause inconsistent degrees of influence of the thermal property parameters of phase change thermal storage materials on the optimization objective, thus affecting the weight of the thermal property parameters and consequently the optimization selection results. Therefore, applying multi-factor, multi-level orthogonal experiments to analyze the influence of the thermal property parameters of phase change thermal storage materials on the optimization objective can reduce the actual workload.

[0064] Step four: Based on the MCDM method combined with the weight determination method proposed above (Step three), the pre-selected phase change thermal storage materials (Step one) are ranked in order of merit to obtain the optimal phase change thermal storage material at different time scales. For details, please refer to the article "A methodological concept for phase change material selection based on multi-criteria decision making (MCDM): A case study", which uses the TOPSIS method combined with the optimal combined weights to optimize the selection of phase change materials.

[0065] Step 5: Compare the optimized selection results of phase change thermal storage materials with the optimal phase change thermal storage materials obtained by combining the traditional weight determination method with the MCDM method to verify the rationality of the proposed method.

[0066] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the claims.

[0067] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for optimizing selection of a phase change thermal storage material, characterized by, Comprising the following steps: (1) Pre-selecting materials: According to the actual application scenario, pre-selecting the phase change heat storage material; (2) Establishing equations: According to the actual application scenario and the actual form of the phase change heat storage system, the energy balance equation of the phase change heat storage system is established, and the thermal physical parameters of the phase change heat storage material affecting the performance of the phase change heat storage system are determined; (3) Determine the weight: According to the optimization target of the phase change heat storage system in the actual application scenario, analyze the influence of the thermal physical parameters on the optimization target, and determine the weight of the thermal physical parameters under multiple time scales; (4) Superiority and inferiority sorting: Based on the MCDM method combined with the weight determination method in step (3), the phase change heat storage materials pre-selected in step (1) are sorted in terms of superiority and inferiority, and the optimal phase change heat storage material under multiple time scales is obtained; In step (3), the weight determination method comprises the following steps: S1: According to the thermal physical parameter range of the pre-selected phase change heat storage material, design orthogonal experiment; S2: Numerical simulation research is carried out on the orthogonal experiment, and numerical simulation results under multiple time scales are obtained; S3: Range analysis is performed on the simulation results to obtain the weight of the thermal physical parameters under actual application and the optimization target under multiple time scales; In step S3, the range analysis is as shown in formula 2 and formula 3: Formula 2 Formula 3 in, K ij For the first j The first of the listed factors i The sum of optimization objectives corresponding to the level; k ij For the first j The first of the listed factors i The mean of the optimization objective corresponding to the level; R j For the first j The range of the listed factors; In step S3, the weight is obtained by formula 4: Formula 4 wherein, ω j is the weight of the i-th j column factor.

2. The method of optimizing selection of a phase change thermal storage material according to claim 1, wherein In step (2), when the actual application scenario is a three-tube phase change heat storage heat exchange unit, the three-tube phase change heat storage heat exchange unit is composed of an inner tube, a sleeve tube and an outer tube from inside to outside, water as a heat transfer fluid passes through the inner tube and the outer tube to exchange heat with the phase change heat storage material filled in the sleeve tube, and the energy balance equation is: Formula 1 wherein, ρ is the density, c is the specific heat, T is the phase change temperature, t is the time, λ is the thermal conductivity, L is the latent heat of phase change, is the liquid fraction of the phase change material, x is the axial coordinate, r is the radial coordinate, subscript p is the type of phase change material.

3. The method for optimizing selection of a phase change thermal storage material according to claim 1, wherein In step S1, the thermal physical parameters include phase change temperature, density, specific heat, thermal conductivity and phase change latent heat.

4. The method for optimizing selection of a phase change thermal storage material according to claim 1, wherein Further comprising the following steps: (5) Comparison and verification: The optimal phase change heat storage material obtained in step (4) is compared with the optimal phase change heat storage material obtained by combining the traditional weight determination method with the MCDM method, and the rationality is verified.

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