Supercritical CO2 vs Transcritical CO2
Supercritical CO2 is a state of carbon dioxide (R744) and Transcritical CO2 is a cycle of a CO2 system. The key difference lies in their respective behaviors.
Supercritical CO2: Supercritical CO2 refers to carbon dioxide that is above its critical temperature 87.8 F (31.1°C) and critical pressure 1055 psig (72.8 barg). At these conditions, CO2 exists as a single phase that exhibits properties of both a gas and a liquid. It has a high density like a liquid but fills its container like a gas. There is no temperature pressure relationship when you are above the critical point of CO2 (R744). Supercritical CO2 is commonly used as a solvent in various industrial applications, such as extraction processes and cleaning technologies. It offers advantages such as high diffusivity, low viscosity, and easy separation from solutes. We are starting to see more and more in refrigeration and heating applications.
Transcritical CO2: Transcritical CO2 refers to a cycle in a CO2 refrigeration system that can operate in both the supercritical and subcritical states during the running cycle. This cycle occurs when CO2 refrigerant is above both its critical temperature and critical pressure (supercritical state) and when the pressure is reduced through the high pressure valve below the critical temperature and critical pressure (subcritical state). Transcritical CO2 cycles are often used in refrigeration, a/c and heat pump systems. In these applications, CO2 undergoes a compression process and experiences a pressure above the critical pressure before being expanded to lower pressures for cooling or heating purposes.
Check out this video where we dive into CO2 Extraction vs CO2 Refrigeration with James Seabrook from Vitalis where we explain the differences in terminology - Watch Here
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To summarize, the main difference between Supercritical CO2 state and Transcritical CO2 cycle lies in their behavior. Supercritical CO2 exists as a single phase with properties of both a gas and a liquid, while transcritical cycles involves a mixture of subcritical and supercritical states. The specific applications and uses of these two of CO2 depend on their unique properties and characteristics.
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Sustainable Applications Expert at Danfoss
1 年Thanks for sharing all the good educating material Trevor. I have a doubt about the definition of Transcritical CO2 here, like to check with you. I think when CO2 temperature is higher than critical temperature but the pressure is lower than critical pressure, it should be called superheated CO2 (for example, at compressor discharge). It is the same for any other refrigerant. and that is why the heat exchanger to recover the heat before the condenser/gas cooler is sometimes called de-superheater. I think subcritical and supercritical CO2 cannot co-exist. The term "Transcritical" is more about a CO2 system (not CO2 as a refrigerant, not a phase definition), which can operate and reject heat in both subcritical and supercritical phases. not at the same time. This depends on the heat rejection/heat reclaim control and the thermodynamics boundary conditions. All said, I am aware that the term "transcritical" is sometimes used to mean the "supercritical phase". an example helping me to remember these terms is: "This transcritical CO2 system operates in subcritical phase 70% of the year and in supercritical phase 30%. [on the high pressure side]"
In refrigeration we generally refer to transcritical operation when the compressor discharge is pushed above the critical point (both P and T), which means the compressor is discharging supercritical R744. The reasons we want supercritical CO2 could be for heat reclaim or high ambient conditions, but there is always only one phase of supercritical. Loss of either supercritical threshold (P/T), means the fluid is then back to a 2 phase flow of liquid and vapor. It is possible to have supercritical CO2 entering the gas cooler / heat reclaim and liquid or supercritical fluid leaving - but not both.
Masters of Philosophy Student (Mechanical Engineering] @ UoW
1 年Thanks for sharing Trevor. Compared with the other three Natural Refrigerants, the amount of effort required to make CO2 work, the high pressures involved, the complicated valves and associated piping it all seems like witchcraft, smoke & mirrors to me.