Views:1 Author:Site Editor Publish Time: 2018-07-12 Origin:Site
Floor heating adopts environmentally friendly energy sources such as solar energy and low-valley electricity, but these two forms of energy are not very consistent with the heating period. For example, when the sun is strongest, the heating load is often the smallest, and when the electricity price is underestimated, it is often in the early hours. The period of time only accounts for a small part of the heating period. If you want to use these two forms of energy reasonably
and effectively, it will inevitably involve the problem of heat storage and heat preservation. The low-temperature hot water floor heating system generally uses a hot water storage tank for heat storage. In this way, the heat transfer medium is required to be water, and it occupies a space for use of the building. The heat storage form in which the floor heating does not occupy the building area is the buried heat storage material.
2.1 Phase change material definition and its characteristics
Phase Change Materials (PCM) refers to substances that change morphology with temperature and provide latent heat. The process of changing a phase change material from a solid to a liquid or from a liquid to a solid is called a phase change process, in which case the phase change material will absorb or release a large amount of latent heat. Phase change materials have the ability to change their physical state over a range of temperatures. It is this endothermic and exothermic phenomenon of phase change materials that makes phase change materials a hot spot in the world.
The study of PCM applied to building materials began in 1982 and was initiated by the US Department of Energy Solar. There are thousands of phase change materials found so far, but not all PCMs can be used for energy storage. The ideal PCM should have high melting latent heat, high thermal conductivity and heat capacity, small deformation, non-toxic, non-corrosive, too cold or too hot. Considering the requirements of people for the comfort of residential buildings, the phase change materials applied to the building also need to meet the following conditions: (1) The phase change temperature of the PCM must be in the vicinity of the indoor comfort temperature range, such as the room temperature in winter at 18 ° C to 22 ° C; The room temperature of the room temperature of 22 ° C ~ 26 ° C (2) in summer should be within this range. PCM can't leak from the wallboard, the long-term cycle does not deteriorate, and it is compatible with building materials. Table 1 lists the phase change materials commonly used in building energy storage.
2.2 Energy-saving principle of phase change energy storage building materials
The principle of phase change materials applied in building energy conservation is that the phase change material undergoes phase change accompanied by the release and absorption of phase change heat, that is, during the heat transfer process, the cold load in the phase change material is stored in the energy storage structure. The outdoor temperature is reduced, part of the stored heat is released to the outside, which reduces the building's cooling load; the other part is released into the room, increasing the cold load of the night building. According to the above theory, taking the phase change energy storage structure as an example, applying the phase change material to the existing building can greatly increase the heat storage capacity of the building structure, and a small amount of material can store a large amount of heat. Due to the heat storage effect of the phase change energy storage structure, the heat flow fluctuation between the indoor and outdoor buildings is weakened and the action time is prolonged, thereby reducing the design load of the building heating and air conditioning system and achieving the purpose of energy saving.
2.3 Shaped phase change materials
The shaped phase change material is a composite energy storage material composed of a phase change material and a polymer support and encapsulating material. Due to the micro-encapsulation and support of the polymer capsule, the phase change material as the core material does not undergo solid-liquid phase change. It flows out, and the entire composite material maintains its original shape and has a certain strength even after the core material is melted. Disadvantages of ordinary solid-liquid phase change materials: (1) phase separation; (2) packaging, expensive containers; (3) large thermal resistance of the container; (4) leakage of the container. Shaped phase change materials overcome these shortcomings, and the shape of the material does not change during the phase change. This type of material has the above advantages to reduce packaging cost and difficulty, and to reduce the thermal resistance between the phase change material and the heat transfer fluid. This kind of material has broad application prospects in the field of building HVAC and building materials.
2.4 Classification of phase change materials
Phase change materials can be divided into solid-liquid phase change materials, solid-solid phase change materials, liquid-gas phase change materials and solid-gas phase change materials according to the phase change mode, although liquid-gas and solid-gas The latent heat of phase change accompanying the conversion is much greater than the phase change heat during solid-solid and solid-liquid conversion, but it is difficult to use because of the gas generation during the liquid-gas and solid-gas phase changes. In actual engineering.
Organic solid-liquid phase energy storage materials mainly include aliphatic hydrocarbons, fatty acids, alcohols and polyenols. The advantages are that phase separation and supercooling are less likely to occur, corrosion is less, and latent heat of phase change is large. Easy to leak. At present, most of the applications are mainly aliphatic hydrocarbons and polyhydric alcohol compounds. The phase change material has large heat storage capacity and good thermal stability, but it is easy to leak when reaching the phase change temperature, and requires container packaging.
The organic solid-solid phase change energy storage material is energy storage and release energy through the conversion of the crystal form of the material, and has the advantages of small volume change, no leakage, no corrosion and long service life in the phase change process, and the material of the container And the production technology requirements are not high, and the phase change latent heat is in the same order of magnitude as the solid-liquid phase change material. There are three main types of solid-solid phase change materials that have been developed with economic potential: polyols, polymers, and layered perovskites.