Heat storage technology: improve the efficiency of comprehensive utilization of heat energy
At present, in many energy utilization systems, there is a contradiction between energy supply and demand mismatch, resulting in unreasonable energy utilization and a large amount of waste. Energy efficiency such as solar energy and industrial waste heat is low, which not only wastes resources, but also causes non-negligible thermal pollution to the atmospheric environment.
For this reason, improving energy conversion and utilization has become a major issue that countries must prioritize to implement sustainable development strategies, and the development of heat storage technology for comprehensive and effective use of heat energy is of paramount importance.
Abundant resources available
Solar energy is the most important basic energy source among renewable energy sources. It is "inexhaustible and inexhaustible" and is widely distributed and pollution-free. It is an economical clean energy. The sun can release energy of 391×1021 kW per second. Even if the energy radiated to the surface of the earth is only one-2.2 billionth of it, it is equivalent to 80,000 times the world's power generation. my country is a relatively rich country in solar energy. More than two-thirds of the country has an annual solar radiation of more than 6 GJ·m2 and annual sunshine hours of more than 2,200 h. The annual solar radiant energy received by the earth's surface in my country is about 50×1019 kJ, which is equivalent to 170 billion tons of standard coal. Such abundant solar energy resources also provide good conditions for my country's development and utilization of solar power generation. Industrial waste heat mainly comes from industries such as metallurgy, building materials, and chemicals. Statistics in 2010 showed that industrial waste heat resources accounted for up to 67% of the total fuel heat, of which the recovery rate reached 60%. However, the overall utilization rate of waste heat resources in my country is low, and the waste heat utilization rate of large iron and steel enterprises is about 30%. ~50%.
There is a lot of room for improvement in the utilization rate of industrial waste heat resources in my country. Take the metallurgical industry as an example. In 2010, my country’s crude steel output was 627 million tons. The energy contained in the flue gas produced was equivalent to 30 million tons of standard coal, and the amount of steel slag produced was approximately 280 million tons, and the contained thermal energy was equivalent to 10 million tons of standard coal. . At present, the utilization rate of flue gas waste heat in domestic iron and steel enterprises is about 30%, and the utilization rate of iron and steel slag waste heat is almost zero. If the waste heat utilization rate of flue gas can be increased to 90% and the utilization rate of steel slag waste heat can be increased to 60%, 21.6 million tons of standard coal can be saved every year, CO2 emission reduction of about 50 million tons, and 3.3 billion kWh of power generation can be generated.
It can be seen that waste heat recovery is a major demand of my country's energy strategy, with immeasurable economic benefits, and is of great significance to my country's economic development, social progress, and national energy security. However, whether it is solar energy or industrial waste heat resources, there are problems of intermittent and instability, which seriously hinder the promotion and application of related technologies.
Urgent need for medium and high temperature latent heat storage technology
The use of heat storage technology can alleviate the contradiction between thermal energy supply and demand in terms of time, intensity and space, and is an important means for optimized operation of thermal energy systems. Heat storage mainly includes three forms: sensible heat storage, latent heat storage and chemical reaction heat storage.
Chemical reaction heat storage is still in the experimental research stage due to its complex system, technical difficulty, and poor operability; although sensible heat storage technology has been widely used, heat storage is caused by the low heat storage density per unit volume of heat storage materials The large amount of materials makes the large-capacity heat storage system bulky, complicated in process, and high in cost.
Latent heat storage is to use the latent heat released or absorbed by the phase change process of the heat storage material to store and release heat. Compared with sensible heat storage technology, latent heat storage has the advantage of large heat storage density per unit volume, and has a larger energy absorption and release within the phase transition temperature range, and the storage and release temperature range is narrow, which is beneficial to charge and release The temperature of the thermal process is stable.
In order to improve energy conversion efficiency and reduce costs, solar thermal utilization technology is moving towards higher operating temperatures. The operating temperature of thermal power generation has exceeded 600°C, and the temperature of a large amount of industrial waste heat is also very high (for example, the converter flue gas temperature is 1600°C. about).
These all urgently need to research and develop medium and high temperature latent heat storage technologies. Although many scholars at home and abroad have carried out research from different levels such as materials and processes for a long time, so far, there is still no mature medium and high temperature latent heat storage system that operates stably.
After many years of in-depth research in this field by many domestic and foreign research units, combined with the current status and trends of domestic and foreign technology development, it is believed that the medium and high temperature latent heat storage technology mainly faces the following outstanding problems.
First, there is a lack of medium and high temperature latent heat storage materials with comprehensive properties such as high heat storage density and strong thermal conductivity. The foundation of latent heat storage technology is phase change materials. At present, research on low temperature heat storage materials (<100°C) based on paraffin wax and hydrated salt has been extensive, and it has also been applied in the fields of construction and clothing. However, medium and high temperature heat storage materials, especially high temperature phase change heat storage materials with a melting point >600°C, are still lacking.
Secondly, the medium and high temperature phase change heat storage materials are mainly inorganic salts and alloys. On the one hand, the selection of candidate materials requires an in-depth understanding of the thermodynamics and kinetic mechanisms of the phase transition process of the material. On the other hand, it is necessary to reveal the influence of microstructure on the thermal properties of materials from two aspects: enhanced heat transfer and efficient heat storage.
In addition, the encapsulation of liquid-solid phase change materials and the decay of thermal properties during the service process are also indispensable contents in the research of medium and high temperature phase change materials. This is often a bottleneck problem in the research and development of such materials. High-performance heat storage materials to be developed
Many scientists at home and abroad have studied metals as heat storage materials. In 1980, Birchenall et al. measured and analyzed the thermophysical properties of binary and ternary alloys composed of Al, Cu, Mg, Si and Zn, which are abundant on earth, and found that the phase transition temperature is in the range of 780～850 K and rich in Si. Or Al alloys have the highest heat storage density, and then aluminum and silicon-based alloy phase change heat storage materials have been extensively studied.
Inorganic salt materials have a wide range of sources, large phase change enthalpy values, and moderate prices, and are particularly suitable for use as medium and high temperature phase change heat storage materials. The researchers studied the thermophysical properties of molten salt with a temperature higher than 450 ℃, and extended the application of inorganic eutectic salt with a temperature range of 220 ℃ to 290 ℃ to the field of solar thermal power generation, and passed tests such as differential scanning calorimetry. Method, the thermophysical properties of molten salt were measured.
In addition, the volume change rate of many molten salt systems before and after the phase change exceeds 10%. The larger volume change rate increases the voids in the molten salt phase change material system, affects the heat storage/release rate, and increases the heat storage. The design difficulty of the system equipment reduces the heat storage efficiency. For this reason, researchers have studied the compatibility of molten salt phase change heat storage materials with stainless steel, and the results show that stainless steel has a good anti-corrosion effect on most molten salts.
At the same time, the cycle performance of ternary aluminum-based alloy phase change materials and compatibility with containers; the compatibility of fluoride molten salts with cobalt, nickel and refractory metal element alloy steels; the compatibility of lithium hydroxide with structural alloy materials In other aspects, scientists have also conducted research.
Although some results have been achieved in the research of medium and high temperature phase change heat storage materials, the cost of metal and alloy phase change materials is high, and the heat storage density per unit mass is limited. In addition, the chemical activity of metal alloy phase change materials is stronger after phase change. , Severe high temperature corrosion greatly limits its wide application in the field of medium and high temperature heat storage.
As a phase change heat storage material, molten salt has a large phase change enthalpy, high heat storage density, and moderate price. It has great development potential in the field of medium and high temperature heat storage applications. However, molten salt has poor thermal conductivity and has serious high temperature corrosion problems with metal alloy phase change materials, which is still a problem that restricts its scale application.
Therefore, the development of high-performance heat storage materials and their preparation methods is an inevitable trend in the research of medium and high temperature heat storage materials and an inevitable way for the development of heat storage technology.
The dispersion of solar energy, industrial waste heat, large energy span, and the intermittent nature of renewable energy all require medium and high temperature phase change heat storage technology.
The research of large-scale heat storage technology involves the intersection of materials science, chemical engineering, mechanical engineering, heat and mass transfer and multiphase flow.
The development of high-performance medium and high temperature phase change heat storage materials is of great significance to the field of medium and high temperature heat storage, especially solar thermal power generation, industrial waste heat recovery and other fields.
