Compared with sensible heat storage, we will use the characteristics of materials that need to absorb (or release) heat during phase change to achieve heat storage, which is called latent heat storage, also called phase change heat storage or melting heat storage. The value of the heat absorbed (or released) when a unit phase change material undergoes a phase change is only related to the type of material, and has little influence on the outside world. Assuming that an object with a mass of m, the heat absorbed (or released) during a phase change is (Q), then:
Q=β • m (1)
In the formula: β is the latent heat of phase change of the substance.
Generally speaking, we call the heat absorbed by the substance from the solid state to the liquid state, which promotes its phase change, as the “latent heat of fusion”, and the heat released when the substance is condensed from the liquid state to the solid state is referred to as the “latent heat of solidification”. . In the same way, we also have “latent heat of evaporation, latent heat of condensation, latent heat of sublimation, and latent heat of sublimation”. The latent heat storage material should have a melting point temperature compatible with the energy storage environment, or equivalent to the melting point required by the energy storage technology; and should have a larger latent heat of fusion; a larger thermal conductivity, thermal diffusivity, and heat capacity; High chemical stability, that is, after multiple cycles of endothermic and exothermic heat, the characteristics of its latent heat storage material are still reversible, that is, the quality is stable; the volume change during phase change is small, and it has a very low vapor pressure; it can quickly release heat Crystal; non-flammable, non-toxic, non-corrosive; low price, abundant sources. In the choice of latent heat storage materials, inorganic salt hydrates are currently more commonly used.
For example: sodium sulfate (Na2SO4 • 10H2O), phase transition temperature 31~32℃, latent heat 57.9kCal/kg;
Calcium chloride (CaCl2 • 6H2O), phase transition temperature 29~39℃, latent heat 41.6kCal/kg;
Calcium carbonate (CaCO3 • 10H20), phase transition temperature 31~36℃, latent heat 64.5kCal/kg;
Calcium nitrate [Ca(NO3)2 • 4H2O], phase transition temperature 40~42℃, latent heat 50.1kCal/kg.
The latent heat material is too cold without crystallization, has a large latent heat, a large phase transition rate and a small volume change, is non-toxic, and has good chemical stability. It is generally widely used in the field of dryers or building energy saving, using solar air collectors to heat the air, directly using air as a working fluid; supplying heat to the latent heat storage system; or directly extracting heat from the latent heat storage system to dry objects; or building Keep warm.
In solar thermal power generation, latent heat storage is an important part of improving the stability and reliability of thermal power generation and reducing power generation costs. Since solar thermal power generation generally uses high-temperature superheated steam turbines to generate electricity, the system needs to obtain high steam temperature and pressure; at this time, if different working fluids are still used for heat transfer and heat storage, the loss of heat exchange efficiency will inevitably increase. In order to solve the problem of heat transfer loss, in 1984, the United States developed the use of molten nitrate as a heat transfer and storage medium at the same time, thus verifying the technical feasibility and flexibility of single working fluid molten salt for heat transfer and heat storage. On this basis, in 1996 the United States developed a composite molten salt composed of 60% NaNO3 + 40% KNO3 as a heat transfer and heat storage medium; this molten salt began to melt at 220°C, and its thermal performance was stable below 600°C; Practice has proved that molten salt heat transfer and heat storage technologies are of great significance for improving the power generation efficiency of the system, reducing the cost of solar thermal power generation, and improving the stability and reliability of the power generation system.