PCM - Phase Change Material refers to changing the state of substances and providinglatent heatSubstances.The process of changing physical properties is called phase change process. At this time, the phase change material will absorb orreleaseA lot of latent heat.
Once widely used in human life, this material will become the best for energy conservation and environmental protectionGreen and environmental protectionThe carrier has been listed as a national R&D and utilization sequence in China.
Phase change materials can be divided intoOrganic(Organic) andinorganic(Inorganic) phase change material.It can also be divided into hydrated salts phase change materials and paraffin wax phase change materials.
Our most common phase change materials arewaterWhen the temperature drops to 0 ° C, water changes from liquid to solid (freezing).When the temperature is higher than 0 ° C, the water changes from solid to liquid (dissolved).A large amount of cold energy is absorbed and stored in the process of icing, while a large amount of heat energy is absorbed in the process of dissolution.The greater the amount (volume) of ice, the longer the dissolution process takes.This is a typical example of phase change materials.
The application of phase change materials in the electric heating industry is a revolutionary transformation from traditional electric heating to energy-saving electric heating. Phase change thermoelectric heaters are representative products. Compared with traditional electric heaters, there is no need to expand the capacity of electricity meters, and they can enjoy 24-hour heating while saving electricity costs.
It can be seen from the above examples that phase change materials can actually be used as energy storage.This characteristic has great significance in energy saving, temperature control and other fields.Therefore, phase change materials and their applications have become a wide range of research topics.
The biggest difference between organic phase change materials and inorganic phase change materials is the difference in durability and fire resistance when applied to building materials, and the latter is more superior to the former.
Mechanism and characteristics of heat storage
Phase change materials have the ability to change their physical state within a certain temperature range.Take the solid-liquid phase transition as an example. When heated to the melting temperature, the phase transition from solid to liquid occurs. During the melting process, the phase change material absorbs and stores a large amount of latent heat;When the phase change material is cooled, the stored heat will be distributed to the environment within a certain temperature range, and the reverse phase change from liquid to solid will be carried out.The energy stored or released during these two phase transitions is called latent heat of phase transition.When the physical state changes, the temperature of the material itself almost remains unchanged before the completion of phase transformation, forming a wide temperature platform. Although the temperature remains unchanged, the latent heat absorbed or released is quite large.
Classification of phase change materials Phase change materials mainly include inorganic PCM, organic PCM and composite PCM.Among them, inorganic PCM mainly includes crystalline hydrated salts, molten salts, metals or alloys, etc;Organic PCM mainly includes paraffin, acetic acid and other organic substances;Composite phase-change heat storage materials emerged at the historic moment, which can not only effectively overcome the shortcomings of single inorganic or organic phase-change heat storage materials, but also improve the application effect of phase-change materials and expand their application range.Therefore, the development of composite phase change thermal storage materials has become a hot research topic in the field of thermal storage materials.However, the mixed phase change materials may also bring about the decline of latent heat of phase change, or easy to change in the long-term phase change process.
What is phase transition
The process of changing a substance from one phase to another.The uniform part in the material system with identical physical and chemical properties and obvious interface with other parts is called phase.Corresponding to the three states of solid, liquid and gas, substances include solid phase, liquid phase and gas phase.
First order phase transition
When a phase change occurs, there is volume change and heat absorption or release. This type of phase change is called "first order phase change".For example, under the condition of 1 atmospheric pressure of 0 ℃, 1 kg of ice will be transformed into water at the same temperature, which will absorb 79.6 kcal of heat, while the volume will also shrink.Therefore, the transformation between ice and water belongs to the first order phase transition.
Second-order phase transition
When phase change occurs, the volume does not change, nor does it accompany the absorption and release of heat, butHeat capacity、Coefficient of thermal expansionAnd isothermal compressibility, etcphysical quantityChanges, which are calledSecond-order phase transition。Normal liquid helium (helium I) andSuperfluidHelium（Helium II）Transition between, normalconductorAndSuperconductorThe transition between paramagnets and ferromagnets, and the transition between ordered and disordered states of alloys are all typical examples of second-order phase transformation.[1]
extensive use
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space flight
becausethe outer spaceThe temperature belongs to extremely cold or hot environment, and the protection requirements for astronauts and spacecraft are very strict. Ordinary materials cannot adapt to harsh conditions, so special materials are required for protection.Scientists from the United States and the former Soviet Union first developed phase change materials, which enabled the application of astronauts' clothing, return capsule shells, etc.This technology has always been in a monopoly position.Since China entered the 21st century, with the continuous efforts of scientists, it has overcome the key technology and started to apply it in practice.
Architecture
The introduction of phase change materials to architecture is a revolutionary development in the field of architecture.The main result is energy saving.
clothing
In the field of clothing, using phase change materials and embedding them into fibers can greatly change people's quality of life. Without using any energy, ordinary clothes can be turned into micro air conditioners.
Refrigeration equipment
Traditional refrigeration equipment, such as air conditioners, refrigerated vehicles and cold storage, all use compressor refrigeration technology for refrigeration, which not only consumes electricity, but also is not environmentally friendly.The phase change technology can replace the compressor for refrigeration, saving more than 60% energy.
military
Once equipped, it will be a major contribution of phase change materials.Military vehicles, military clothing, ships, aircraft, tanks, submarines and other military aspects are all important areas of phase change materials application, which can greatly improve combat effectiveness and protection durability.
Communication and power
Phase change materials can save more than 75% of equipment cost in terms of cooling of equipment boxes (rooms) such as communication and power.In the field of communication, it has been widely used in the computer room and battery pack room of the communication base station, so that the traditional one-year equipment can be extended to 4 years or more.[2]
building material
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sketch
Phase change energy storage building materialsIt has the advantages of both ordinary building materials and phase change materials, and can absorb andreleaseAppropriate heat energy;It can be used together with other traditional building materials;No special knowledge and skills are required to install and use heat storage building materials;Able to produce with standard production equipment;It is competitive in economic benefits.
Phase change energy storage building materialsResearch on building materials began in 1982US Department of EnergySolar energy companylaunch.In the 1990s, PCM technology was developed to deal with building materials (such as gypsum board, wallboard and concrete components).Then, PCM is used in the concrete test blockGypsum wallboardThe research and application of building materials such as.In 1999, another kind ofNew building materials- Solid liquid eutectic phase change material, which can be poured in the wallboard or lightweight concrete precast slab to keep the indoor temperature appropriate.In addition, many companies in Europe and the United States use PCM to produce and sell outdoor communication wiring equipment and power transformation equipment in special cabins, which can be kept at a suitable working temperature in winter and summer.In addition, asphalt ground or cement pavement containing PCM can prevent roads, bridges, runways, etc. from freezing late in winter.
Composite process
The bonding process of PCM and building material matrix mainly includes the following methods: (1) PCM is sealed in a suitable container.(2) Seal the PCM and place it in the building materials.(3) PCM is infiltrated into porous building material matrix (such as gypsum wallboard, cement concrete test block, etc.) by soaking.(4) Mix PCM directly with building materials.(5) The organic PCM is emulsified and added to the building materials.A well-known domestic building energy saving enterprise successfully emulsified paraffin with different grades, and then mixed it with special phase change glue powder, waterPolyphenylene granuleThe light aggregate is mixed to prepare a phase change thermal storage slurry which can be used for the inner and outer layers of the building wall and has both thermal storage and insulation.The test work of the test building is in progress.At the same time, we are also developing phase change mortar, phase change putty and other products.
Application in architecture
The development of modern buildings towards high-rise buildings requires that the enclosure structure used beLightweight materials。However, the heat capacity of ordinary lightweight materials is small, resulting in large fluctuations in indoor temperature.This not only makes the indoor thermal environment uncomfortable, but also increases the air conditioning load, leading to an increase in building energy consumption.The latent heat of the phase change materials used can reach 170J/g or even higher, while ordinary building materials will need 190 times the mass of phase change materials to store the same heat when the temperature changes 1 ℃.Therefore, the composite phase-change building materials have a heat capacity unmatched by ordinary building materialsair conditioning systemThe stability of the working condition is very favorable.
The disadvantage of this composite product is that the strength is not enough.[3]
Material selection
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Features
be used forbuilding envelope It is very important to select appropriate phase change materials for the development of phase change building materials, which should have the following characteristics: (1) high melting latent heat, so that it can store or release more heat during phase change;(2) The phase change process has good reversibility, small expansion and contraction, and few supercooling or overheating phenomena;(3) There is an appropriate phase change temperature, which can meet the specific temperature that needs to be controlled;(4) High thermal conductivity, high density and high specific heat capacity;(5) Phase change materials are non-toxic, non corrosive, low cost and easy to manufacture.
In the actual development process, it is very difficult to find phase change materials that meet these ideal conditions.Therefore, phase change materials with appropriate phase change temperature and large latent heat are often considered first, and then comprehensive factors affecting research and application are considered.
Degradation of thermophysical properties of phase change materials during cycling;Phase change materials are easy to leak from the matrix;The effect of phase change materials on matrix materials.
Economic performance:
If we want to maximize the solution to the above problems, it will lead to an increase in the cost of unit heat energy storage, and will lose the competitive advantage with other heat storage methods or ordinary building materials.Phase change energy storage building materialsAfter more than 20 years of development, there is no doubt about its intelligent functionality.With people's increasing attention to building energy conservation and the gradual enhancement of environmental protection awareness, phase-change energy storage building materials will certainly be of great use in the field of building materials in the future, and will also be gradually recognized by people, with very broad application prospects.
representative figure
Xu ZuyaoAcademicianMartensitic transformation、Bainite transformation, shape memory materials and material thermodynamics.The diffusion of interstitial atoms in non diffusion martensitic transformation is revealed, which redefines martensitic transformation and corrects the classical dynamic equation;Iron base was successfully calculated from thermodynamics.
