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Remember this fairy-like iced food in the restaurant? Or a beautiful stage...
The material used to create this scene is dry ice, which is the solid state of carbon dioxide. Carbon dioxide is a greenhouse gas, but if it can make ice, does it subvert your cognition! The most indispensable thing in the Winter Olympics is the various types of ice. How to make high-quality ice surfaces that meet the requirements of the competition is very important. In the Beijing competition area, 7 of the 15 ice surfaces in the Winter Olympics arena were made using carbon dioxide transcritical direct cooling ice-making technology, making the 2022 Beijing Winter Olympics the first Olympic Games in history to use carbon dioxide refrigerants on a large scale. The Capital Gymnasium completed ice making on the venue. Picture from the Beijing News (provided by the Beijing Major Projects Office)
A historic first? Is there some secret behind the CO2 transcritical direct cooling ice-making technology? Don’t worry, let’s learn about the special features of ice-making at the Beijing Winter Olympics. Why choose carbon dioxide? To explain clearly what carbon dioxide transcritical direct cooling ice making is, we must first break down the term: carbon dioxide, transcritical, and direct cooling ice making.
Let's talk about carbon dioxide first. Most people may know it as a gas in the air, and it is also a greenhouse gas. Those who know more about it will think of dry ice, but carbon dioxide has more roles than that. Yes, carbon dioxide is indeed a greenhouse gas, but in the field of refrigeration, it has become an environmentally friendly refrigerant. The refrigeration we are talking about here refers to artificial refrigeration, which is to use refrigeration equipment to reduce the temperature of the object to be cooled to less than the surrounding ambient temperature. Take carbon dioxide transcritical direct cooling ice making as an example, which is to lower the water temperature until it becomes ice. In the process of artificial refrigeration, the role of refrigerant is very important. It will circulate in the refrigeration equipment and play a role in heat transfer. Freon, which we often hear about, is a very common refrigerant. So why can carbon dioxide be a refrigerant? Generally speaking, refrigerants achieve heat exchange through the mutual transformation between liquid and gas, and carbon dioxide can also absorb or release heat in such a physical state transformation. Simply put, carbon dioxide is in gaseous state at normal temperature and pressure. When a certain pressure is applied, it can be liquefied into liquid or even condensed into solid (solid carbon dioxide is what we often call dry ice). When the pressure is reduced, liquid or solid carbon dioxide can quickly vaporize (or sublimate) into gas and absorb a large amount of heat to achieve the purpose of lowering the ambient temperature (letting water condense into ice). National Speed Skating Oval Ice Rink, picture source Beijing Daily In fact, the use of carbon dioxide as a refrigerant is nothing new. It is the "old man" among refrigerants and was put into use as early as the end of the 19th century. It was only because of the emergence of refrigerants such as chlorofluorocarbons that carbon dioxide gradually faded out of people's sight. It was not until recent years that the adverse effects of refrigerants such as chlorofluorocarbons on the environment gradually emerged, and carbon dioxide, a more environmentally friendly refrigerant, regained favor. Some people may ask, since it is a greenhouse gas, where is the environmental protection characteristic of carbon dioxide as a refrigerant? First of all, carbon dioxide itself does not destroy the ozone layer, but many chlorofluorocarbon refrigerants are very destructive to the ozone layer (such as Freon R12). Secondly, although carbon dioxide is a greenhouse gas, its greenhouse effect is much lower than that of refrigerants such as chlorofluorocarbons, so it is indeed environmentally friendly. In addition, carbon dioxide has the advantages of wide sources, safety and non-toxicity, so it naturally stands out from many refrigerants. Why choose direct cooling to make ice? Now that we have figured out the "status" of carbon dioxide in the field of refrigeration, let's talk about what direct cooling ice making is. In fact, direct cooling is relative to indirect cooling (i.e. refrigerant refrigeration). Indirect cooling refers to the process in which the cold obtained by the evaporation of the refrigerant is transferred to the ice surface through the refrigerant to complete the cooling process. Direct cooling does not require the use of a refrigerant. It directly uses the cold generated by the evaporation of the refrigerant to exchange heat with water, gradually freezing the water into ice cubes. Direct cooling ice making avoids the energy loss caused by the use of a refrigerant for heat exchange. The heat transfer coefficient and refrigeration efficiency are very high. Moreover, the temperature fluctuation range can be controlled within 0.5 degrees Celsius, and the temperature of the ice rink can be kept basically consistent, which is very important for competitions such as speed skating.
Another thing you may have noticed is that the ice on the ice rink is milky white, but the ice you freeze at home is transparent. What's going on? National Bobsleigh and Luge Center, image source: Yanqing District Media Integration Center Generally speaking, the white color of the ice rink used for competitions is post-processed. The ice surface made using traditional ice-making technology is actually transparent and needs to be sprayed white with special ice paint. However, the ice rink that uses direct cooling does not require the step of spraying white with ice paint. This is because during the direct cooling process, the speed at which water freezes into ice is very fast (the ice rink that takes 7 days to make using traditional ice-making technology can be made in 3 days with direct cooling), so that the bubbles in the water are frozen in the ice before they can escape, and the final ice surface naturally appears milky white. What is transcriticality? Finally, let’s look at how to understand “transcritical”.
As we have said before, carbon dioxide has been used as a refrigerant for a long time, but the early carbon dioxide refrigeration used a subcritical circulation system, which had low refrigeration efficiency, especially when the ambient temperature was high, the refrigeration capacity of carbon dioxide would be greatly reduced. What does the "critical" in "subcritical" and "transcritical" mean? We know that substances can be converted between gas, liquid, and solid, and the limiting thermodynamic state of gas and liquid coexisting in equilibrium is its critical state. The temperature and pressure at this time are called critical temperature and critical pressure, respectively. Taking carbon dioxide as an example, its critical temperature is 31.1 degrees Celsius, and the critical pressure reaches 7.38 MPa. Carbon dioxide as a refrigerant must be circulated in a refrigerator, which is mainly composed of four parts: a compressor, a condenser (cooler), an expansion valve, and an evaporator. In the subcritical cycle system, the pressure of carbon dioxide transcritical refrigeration cycle is lower than the critical pressure of carbon dioxide during condensation; in the transcritical cycle system, carbon dioxide is in a supercritical state in the cooler, releasing heat at a constant pressure without a complete phase change. The pressure in the evaporator is less than the critical pressure. At this time, carbon dioxide exchanges heat with the surrounding environment, absorbs heat, and reduces the temperature of the surrounding environment. The carbon dioxide transcritical refrigeration cycle system has high pressure and large pressure difference, and has high strength requirements for system components, so it has not been widely used for a long time. The Beijing Winter Olympics successfully used carbon dioxide transcritical direct cooling ice-making technology to complete the preparation of 7 ice surfaces. The technical support behind it is the iterative upgrade of core hardware such as compressors, heat exchangers and regenerators, which is a manifestation of scientific and technological hard power. It seems that the ice on the Winter Olympics field is not only the "fastest" ice, but also the ice with high technological content. |
