What are the specific contents of the research progress of phase change materials? Zhongda Consulting will answer them below.
Energy is the blood of the survival and development of human society. With the energy and environmental crises caused by electric power supply attracting more and more attention, how to develop new green energy and improve energy utilization becomes increasingly important. increasingly important. At this stage, the new energy that people are more concerned about is solar energy, but there is a mismatch in time and space between solar energy utilization and waste heat recovery. Phase change energy storage materials can absorb energy from the environment and release energy to the environment, better solving the contradiction between energy supply and demand in time and space, and effectively improving energy utilization. At the same time, the temperature of phase change energy storage materials remains basically constant during the phase change process, and can be used to regulate the temperature of the surrounding environment and can be used repeatedly. These characteristics of phase change energy storage materials make it have broad application prospects in the fields of power peak shifting, energy saving in industrial and civil buildings and air conditioning, textiles, and military. 1 Research Progress of Phase Change Materials 1-1 Classification of Phase Change Materials Phase change materials can store a certain form of energy when it is higher than its phase change temperature, and release it for use when it is lower than its phase change temperature. Energy storage materials. It is mainly composed of main heat storage agent, phase change point adjuster, anti-overheating agent, anti-phase separation agent, phase change accelerator and other components. There are many types of phase change materials. Judging from the characteristics of the energy stored, they are divided into two categories: heat storage materials and cold storage materials. From the perspective of energy storage methods of energy storage materials, it can be divided into three categories: sensible heat energy storage, latent heat energy storage and chemical reaction energy storage. Among them, latent heat energy storage uses the phase change latent heat of phase change materials to store heat. The energy storage density is high, the heat storage device is simple and small, and the heat storage material is approximately constant temperature during the heat storage process, so it can easily achieve constant temperature of room temperature. Control, especially suitable for the field of building insulation and energy saving. Judging from the temperature range of heat storage, it can be divided into three categories: high temperature, medium and low temperature. High-temperature phase change materials are mainly molten salts and metal alloys; medium-temperature phase change materials are mainly hydrated salts, organic matter and polymer materials; low-temperature phase change materials are mainly ice and hydrogels. From the chemical composition of materials, they can be divided into three categories: inorganic phase change materials, organic phase change materials and mixed phase change materials. Inorganic phase change materials mainly include inorganic substances such as crystalline hydrated salts, molten salts, and metal alloys; organic phase change materials mainly include organic substances such as paraffin, carboxylic acids, esters, and polyols; mixed phase change materials mainly include organic and inorganic fusion materials. A mixture of phase change materials. From the perspective of the phase state of materials during the heat storage process, they can be divided into solid-liquid phase change materials, solid-solid phase change materials, solid-gas phase change materials and liquid-gas phase change materials. Since the latter two phase change methods are accompanied by the presence of a large amount of gas during the phase change process, the material volume changes greatly. Therefore, although they have large phase change enthalpy, they are rarely used in engineering applications. Solid-liquid phase change materials mainly include hydrated salts and paraffin waxes. During the phase change of solid-to-solid phase change materials, the phase state does not change. Instead, the crystal form of the phase change material changes. During the change of the crystal form, heat is absorbed and released. Solid-solid phase change materials mainly include high-density polyethylene, polyols and metal-organic compounds with layered perovskite crystal structures. 1-2 Screening and improvement of phase change materials In the 1980s, the American Dow Chemical Company tested nearly 20,000 phase change materials. The results showed that only 1% of the phase change materials were of use value and had appropriate melting points. Hydrated salts and some organic phase change materials. Since civil construction has strict restrictions on material properties and economic factors, there are even fewer phase change materials suitable for energy storage building materials. Ideal phase change materials for low energy consumption buildings should meet the following requirements: (1) The indoor design temperature of the phase change material or the temperature range required by the heating and air conditioning system; (2) Have a sufficiently large latent heat of phase change; ( 3) The expansion or contraction during phase change should be small; (4) The reversibility of the phase change should be good; (5) Non-toxic and non-corrosive; (6) The raw materials for production should be cheap and easy to obtain. But in fact, there is no ideal phase change material that can meet the above conditions. Therefore, improvements in phase change materials are needed. There are two main ways to improve phase change materials: (1) Combining several organic substances into binary or multi-component phase change materials, or combining organic substances with inorganic substances to obtain the appropriate phase change temperature and phase change latent heat phase change materials. (2) Prepare solid-liquid phase change materials that always maintain a solid shape. There are two main components of this type of phase change material: working substance and carrier matrix.
The former is used for energy storage, mainly solid-liquid phase change materials; the latter can maintain the immobility and processability of the material. The phase change temperature of the carrier matrix is ??generally higher. The carrier matrix must not only have the general characteristics of structural materials, It also needs to be compatible with phase change materials, non-corrosive, non-chemically reactive and low cost. 1-3 Preparation methods of phase change materials There are currently three main methods for preparing phase change materials: (1) Base material encapsulation phase change material method. The base material encapsulation phase change material method is to prepare the base material according to a certain forming process. Microcapsules, porous or three-dimensional network structures, and then the phase change material is poured into it or the carrier matrix is ??immersed in the molten phase change material. Among them, microencapsulation technology includes interfacial polymerization and in-situ polymerization: (1) Interfacial polymerization is to place two reaction monomers in the dispersed phase and continuous phase of the emulsion, and the polymerization reaction is in the phase. happens on the interface. The advantage of this process for preparing microcapsules is that it can be operated at normal temperature, is convenient, simple, and has good effects. The disadvantages are that the requirements for wall materials are high, and the coated monomers must have high reactivity; the prepared microcapsules contain a small amount of unreacted monomers; the permeability of the wall film formed by interfacial polymerization is generally high , not suitable for covering core materials that require strict sealing. (2) The technical characteristics of the in-situ polymerization method are: the monomers and initiators are all placed outside the capsule core and the monomers are required to be soluble, while the generated polymer is insoluble. The polymer is deposited on the surface of the capsule core and coated to form micro capsule. (2) Melting and mixing method of matrix and phase change material. This method uses the compatibility of phase change material and matrix to melt and mix them together to form an energy storage material with uniform composition. This method is more suitable for preparing low-temperature shaped phase change materials for industry and construction. (3) Mixing sintering method: In this method, the prepared micron-sized matrix material and phase change material are uniformly mixed, and then a certain amount of additives are added, ball milled, mixed, pressed and formed, and then sintered to obtain the energy storage material. 1-4 Characterization of phase change materials There is currently no unified standard for the characterization of phase change materials. Li Dong [1] and others have given 4 relatively comprehensive characterization methods, including differential scanning calorimetry (DSC) and thermal analysis. method (TA), TG analysis method, time-temperature curve method and scanning electron microscopy (SEM). Scanning calorimetry (DSC) and thermal analysis (TA) are mainly used to characterize the energy storage temperature range and energy storage density of phase change materials. The TG analysis method is mainly used to study the stability and heat storage capacity of phase change materials. The time-temperature curve method is mainly used to measure the time for complete phase change of phase change materials, thereby calculating its thermal conductivity. Scanning electron microscopy (SEM) is mainly used to observe the cross-section of phase change materials to determine the uniformity and stability of their structure. 2 Application of phase change energy storage materials in the construction field 2-1 History and current situation of application of phase change energy storage materials in construction The research on the application of phase change materials in construction began in 1982 and was initiated by the Solar Energy Corporation of the US Department of Energy. This research has been promoted by the U.S. Energy Storage and Distribution Office since 1988. In his book "Solar Energy Storage - Latent Heat Materials", Lane summarized the development of phase change materials and containers before the early 1980s. In the 1990s, the technology of using phase change materials to treat building materials such as gypsum board, wallboard and concrete components was developed. Subsequently, the research and application of phase change materials in gypsum board, wallboard and concrete components have been developed. The main purpose is to enhance the heat capacity of lightweight structures. Neeper in the United States estimates that phase change wall panels can transfer 90% of the sensible heat load in residents' air conditioning loads to the low periods of electricity consumption, and can reduce equipment capacity by 30%. Oakbridge National Laboratory concluded in 1990 that in solar rooms, phase change wall panels can significantly reduce additional energy consumption, with a payback period of approximately 5 years. Researchers from Kanagawa University and Tokyo Denki University in Japan studied the heat storage performance of phase change wall panels. They concluded that the use of phase change wall panels makes the heat load gentler, the radiation area more comfortable, the electricity consumption reduced, and it is possible to reduce the peak load. Domestic research on phase change building materials started late. Zhang Yinping studied a mixture of anhydrous sodium acetate and urea, with a phase change temperature of 28~31°C. Tongji University mainly uses industrial-grade butyl stearate as phase change material to conduct research on energy-saving concrete materials for buildings. 2-2 Encapsulation technology of phase change energy storage materials for construction There are three main ways to combine phase change materials with base materials: direct addition, immersion and encapsulation. The direct addition method is convenient for controlling the amount of addition, and the soaking method can process the finished building materials.
However, the phase change energy storage building materials prepared by these two methods have poor durability, which is mainly manifested by leakage of the phase change working fluid and corrosion of the base material. The encapsulation method effectively solves the above problems. Packaging includes large volume packaging and micro volume packaging. Large-volume packaging is to pack phase change materials into pipes, bags, plate containers or other containers. This containerized phase change material has been applied in the solar field by the market. However, due to the large area in contact with the environment during phase change, Small, making energy transfer not very efficient. Therefore, micro-volume packaging is attracting more and more attention. Micro-encapsulation refers to making the carrier matrix into microcapsules, porous foams or using substances that easily form films. At this stage, there are many studies on microencapsulation of phase change materials. Microcapsule phase change materials can be directly combined with traditional building materials. The process is simple, the chemical properties are stable, the heat storage is high, and the thermal conductivity is high. 2-3 Phase change mechanism of phase change energy storage materials.
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