Why use Supercritical carbon dioxide instead of water and steam


We know that since Watt invented the Steam engine, fire and water have been leading the process of heat, machinery, electricity and energy conversion. Watt's Steam engine uses coal to burn water, converts it into steam, pushes the piston to move, and generates mechanical power. Now, this principle is also used for thermal power generation, by causing the piston to move back and forth to rotate the turbine, to drive the generator set or use a Stirling engine for power generation. So, why use Supercritical carbon dioxide instead of water and steam?

We know that since Watt invented the steam engine, fire and water have led the process of thermal, mechanical, electrical, and energy conversion. Watt's steam engine burns water with coal, converts it into steam, propels the piston movement, and generates mechanical power. Now this principle is also used for thermal power generation, by reciprocating the piston to rotate the steam turbine, to drive the generator set or use the Stirling engine to generate electricity. So, let's take a look at why supercritical carbon dioxide is used instead of water and steam?

Supercritical carbon dioxide

Water and steam simply act as engineers to transfer energy throughout the process. Water has always been a very good energy transfer project, cheap and reliable, and can be used anywhere, so both thermal power plants and nuclear power plants use water and steam to generate electricity.


So why replace water and steam with supercritical carbon dioxide? This depends on the advantages of supercritical CO2.


The difference between gases and liquids of a substance is the difference in density. When a gas-liquid equilibrium system continues to heat and pressurize, when the temperature and pressure rise to a certain extent, the density of gases and liquids is equal, and their boundary lines disappear. Thermal expansion causes the density of the liquid to decrease, and the density of the gas increases with the increase of pressure. This state of a substance is its critical state. At this time, the gas-liquid state is very close, and the supercritical liquid has better fluidity and thermal conductivity than the original material liquid state. When a substance reaches a supercritical state, the gaseous substance flows like a liquid, and the liquid is compressed like a gas. The use of supercritical state as the energy transmission working fluid of the power generation system can greatly reduce the energy loss in the transmission process.


When the pressure reaches 220 atmospheres and the temperature reaches 374, the density of water expanding at high temperature is exactly the same as the density of water vapor compressed at high pressure. At this time, the liquid of water is no different from the gas, completely dissolved, and becomes a gas with a new high-pressure and high-temperature state, and the water becomes "supercritical water" from the usual state. The pressure and temperature at which this moisture is dissolved is called the "tipping point". Water that exceeds the "tipping point" is supercritical water. To make water reach the supercritical state, it is necessary to meet the two conditions of high temperature and high pressure, and the supercritical state of water has certain oxidation and corrosion, which has high requirements for the strength and corrosion resistance of the system.


Similarly, by controlling the temperature and pressure of supercritical carbon dioxide, specifically, when the temperature is 31 and the air pressure reaches 72.8 atmospheres on the surface, so that these two parameters reach a certain threshold at the same time, supercritical carbon dioxide will also reach a special supercritical state. Supercritical carbon dioxide becomes a supercritical fluid. At this time, it can flow like a liquid, but it will be compressed like a gas, and it is easy to form a supercritical state. Supercritical carbon dioxide is a very dense fluid, with gas characteristics, low viscosity, strong fluidity, and will expand like gas when placed in the container, filling the entire container. Supercritical carbon dioxide cycle without phase change, compression power consumption in the compression process effectively reduced, only 30%, critical temperature and pressure far lower than the critical point of water, system cycle loss is small, with liquid characteristics, high density, high heat transfer efficiency, strong function. The conventional helium cycle accounts for about 45%, and the gas turbine accounts for 50~60%. The analysis of foreign research charts shows that when the supercritical carbon dioxide temperature reaches 550, the conversion efficiency of the supercritical carbon dioxide power generation system from heat energy to output power usually reaches more than 45%. As the temperature rises, the efficiency also increases further.


Small size and light weight. The circulating medium in the supercritical carbon dioxide power generation system is carbon dioxide, and the volume and weight of the supercritical carbon dioxide power generation system are about 50% of the traditional steam power generation system. During the whole cycle, supercritical carbon dioxide is in a supercritical state, no phase change occurs, high density, large kinetic energy, and small size of cooler and pipeline accessories.


Low noise. For rotating machinery and equipment, the speed is high, and the high-frequency vibration spectrum is the mainstay, which is conducive to vibration isolation and noise reduction, and its vibration characteristic frequency is mainly concentrated above the axial frequency. Supercritical carbon dioxide power generation systems generally use high-speed turbine generator sets.


The above is why supercritical carbon dioxide is used instead of water and steam? If you want to know more, feel free to contact us! Our company has many years of experience and is always looking forward to your joining.



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