The technical principle of phase change materials is to use the phase change process of matter to store and release heat. Specifically, matter has three phases: solid, liquid, and gas. When matter changes from one state (phase) to another state (phase), it will absorb or release energy, and the temperature does not change during the process. The heat is academically defined as the latent heat of phase change.
There are many types of phase change materials, and their classification standards are also diverse. According to the transition mode between the phase states of matter, it can be divided into the following four types: the phase transition between solid and solid (solid-solid phase transition), the phase transition between solid and liquid (solid-liquid phase transition), Phase transitions between gas and solid (gas-solid phase transition) and phase transitions between gas and liquid (gas-liquid phase transition). The energy storage principle of solid-solid phase change materials is as follows: when a substance changes from one crystalline state to another, energy conversion occurs, and the purpose of energy storage can be achieved by using this process.
The characteristics of this type of phase change material are: 1. Very small latent heat storage density; 2. Compared with the volume change of solid-liquid phase transition, the volume change of solid-solid phase transition is small. The solid-solid phase transition has an obvious advantage: it brings flexibility in container design because the requirements for the container are not high. Compared with solid-solid phase change materials, the latent heat of phase change of these two types of materials, solid-gas phase change and liquid-gas phase change, is larger, but these two types of phase change materials have an obvious shortcoming: that is, in the process of phase change, the two Phase-change-like materials will be accompanied by the generation of a large amount of gas, which has high requirements on the airtightness of the container, thus making the easy design complex and impractical. Although the phase change enthalpy of the solid-liquid phase change material is slightly smaller than that of the gas-liquid phase change material, the volume change during the gas-liquid phase change process is 10 times the volume change during the heat storage process of the solid-liquid phase change material. Therefore, the similar phase change enthalpy and significantly small volume change during the heat storage process make solid-liquid phase change materials considered to be a latent heat storage material with great industrial application value.
There are various classifications of solid-liquid phase change materials, which can be divided into low temperature (<100C), medium temperature (100^200 °C) and high temperature phase change materials (200 1000 C) according to the phase change temperature during phase transition. In addition, solid-liquid phase change materials can also be divided into organic and inorganic phase change materials according to their chemical properties, or into economical and non-economical phase change materials according to the use price and cost. Usually, the main difference between economical and non-economical phase change heat storage materials is whether the price is suitable for large-scale application. Although some phase change materials have very good thermophysical parameters such as phase change enthalpy and thermal stability, they are expensive and not suitable for large-scale applications. Use at scale.
