The 2022 Beijing Winter Olympics is over!
With the blessing of various high-tech technologies, many records in the Winter Olympic Games have been broken. The ice of the "Ice Ribbon" of the National Speed Skating Hall was called "record-breaking ice" by netizens.
"Ice Ribbon" is the largest speed skating hall in Asia, with the largest ice surface in Asia. It is worth noting that its ice surface is made of carbon dioxide direct cooling, which is also the world's first Winter Olympic speed skating venue that uses carbon dioxide transcritical direct evaporative refrigeration.
Inside the National Speed Skating Hall "Ice Ribbon"
(Image source: Beijing News reporter Tao Ran photo)
Part.1
CO2 refrigeration: A refrigeration technology that kills two birds with one stone
Carbon dioxide is no stranger to it, it has a prominent physical feature: it can become a liquid under pressurized and cooling conditions, continue to reduce the temperature will become snowflake-like, and then after compression, it is easy to form dry ice. After the pressure is reduced, the dry ice will quickly evaporate or sublimate, and this process will take away a lot of heat, so that the ambient temperature will be reduced, which is the principle of carbon dioxide refrigeration.
The greenhouse effect caused by the large amount of carbon dioxide emissions has brought many adverse effects to the earth. If the atmospheric carbon dioxide is used as a safe, economical and environmentally friendly "natural" refrigerant to achieve efficient refrigeration, it can be said that killing two birds with one stone: it can effectively use the emitted carbon dioxide to alleviate the greenhouse effect, and can significantly improve the performance of artificial refrigeration.
Carbon dioxide cooling is expected to transform the greenhouse effect into a schematic of a comfortable environment
(Image source: the author made it himself)
The principle of carbon dioxide refrigeration looks simple, but the operation process is difficult. Specifically, the entire process of transcritical direct cooling of carbon dioxide to ice requires compressors, gas coolers, expansion valves, evaporators, lines, valves, etc.
First, the pressure of the carbon dioxide is raised to a supercritical state by means of a compressor.
The compressor then discharges high-temperature, high-pressure carbon dioxide through the gas cooler to cool its temperature, and the released heat is used for waste heat recovery.
The CO2 refrigerant then flows through the expansion valve, where its pressure drops rapidly.
Finally, through the rapid evaporation of carbon dioxide in the evaporator, the heat of the surrounding environment is absorbed, so as to achieve the effect of refrigeration or ice making.
Schematic of carbon dioxide refrigeration
Part.2
There are so many refrigerants, why use carbon dioxide for refrigeration?
In the choice of refrigerants for indoor ice rinks, carbon dioxide is not the first choice, and early people used ammonia or Freon as a refrigerant. However, considering that synthetic refrigerants damage the ozone layer and have a significant greenhouse effect, they have now been phased out and no longer used, and ammonia working fluids (working fluids refer to the medium used to achieve the conversion of thermal energy and mechanical energy) are dangerous to be used in indoor ice rinks.
In contrast, the use of carbon dioxide refrigeration is more environmentally friendly, carbon emissions tend to zero, and the four main ice surfaces in the "Ice Ribbon" stadium can reduce carbon dioxide emissions by about 900 tons during competition. Not only that, compared with the traditional refrigeration technology, carbon dioxide cooling efficiency is high, energy efficiency can be increased by more than 20%, and even refrigeration can be achieved, and the temperature difference between different positions on the ice surface is basically controlled within 0.5 ° C, and its hardness and flatness are almost the same and conducive to sliding.
Part.3
Which is the strongest refrigeration technology? Why did ice ribbon choose it?
After choosing a refrigerant, it is necessary to consider refrigeration technology. The current artificial refrigeration technology mainly includes indirect refrigeration technology and direct evaporative refrigeration technology.
Indirect refrigeration technology refers to the fact that the cooling volume generated by the system evaporator cannot be directly used by the user, but needs to rely on the heat transfer carrier to achieve the purpose of refrigeration through the conveying and heat exchange equipment.
The direct evaporative refrigeration technology is that the system evaporator directly exchanges heat with the cooled space to achieve the effect of refrigeration.
For the "ice ribbon" that has just finished the ice and snow event, how should the refrigeration technology be chosen?
(Source: Beijing Daily)
Whether it is intercooled or direct-cooled, the ice surface temperature uniformity is required, and the working fluid flow required for the two methods is different, so the power consumption becomes the main difference between the two. In contrast, the advantages of carbon dioxide transcritical direct cold ice making technology are revealed.
First, one of the most significant advantages of CO2 transcritical direct cold ice making technology is its excellent temperature uniformity. Although the flow rate may be different at different locations when flowing over long distances, the temperature of the ice rink is extremely uniform due to the small viscosity of carbon dioxide and the fact that its evaporation temperature is guaranteed to be basically unchanged.
In the intercooled ice rink, the exothermic temperature zone of the carrier refrigerant is large, and there is a certain temperature difference between the inlet and outlet of the pipeline, which can generally reach 1.5 °C-2 °C. In order to ensure that the temperature of the ice rink is uniform enough, it is necessary to increase the flow rate of the refrigerant, when the working mass circulation is about ten times that of the direct evaporation type, it is also possible to cool with a very small temperature difference (basically within 0.5 ° C), but the power consumption of the circulation pump will increase significantly.
In addition, carbon dioxide as a low-temperature refrigeration cycle working fluid can have advantages in equipment miniaturization, heat exchange efficiency and maintenance.
Due to the small molecular weight and large refrigeration capacity of carbon dioxide compared to synthetic refrigerants, the compressor size can be reduced, which is convenient for efficient and compact design of the system.
Carbon dioxide has high density and low viscosity, its flow loss is small, and the heat transfer effect is good.
In addition, carbon dioxide is easy to meet the lubrication conditions of the equipment, it is difficult to corrode the refrigeration equipment, and can significantly improve the sealing of the compressor.
In addition, the transcritical cycle features carbon dioxide 'condensing' processes that can be cooled in a gas cooler, reducing the system's heat transfer losses.
With a CO2 heat pump system that efficiently recovers waste heat, more than 60% of the heat can be recovered and 70°C hot water is provided for the athlete's domestic hot water, ice melting pool melting and ice surface maintenance and ice pouring.
Combining the above advantages, the carbon dioxide transcritical direct cold ice making technology has become the best choice for ice making in the "ice ribbon" of the Winter Olympics.
Part.4
This technology is finally perfectly applied in the "ice ribbon"
Don't look at the surface of the ice ribbon only has a crystal clear ice surface, in fact, it is like a "sandwich", and there are several layers below, including reinforced concrete layer, insulation layer, rammed sand layer and so on. This design is conducive to thermal insulation at the lower end of the ice surface, and can also adjust the temperature throughout the ground in real time to ensure the consistency of the thickness and temperature of the ice layer.
Ground structure diagram of a typical artificial ice rink
(Image source: Reference 2)
In the process of adopting carbon dioxide transcritical direct cold ice making technology, "ice ribbon" uses an intelligent control system to allow carbon dioxide refrigerant to evaporate in the alloy steel pipe under the ice surface day and night.
Multiple carbon dioxide compressors work at the same time, and the low temperature carbon dioxide in the refrigeration pipe of the ice plate layer is heat exchanged with the reinforced concrete layer, so that the temperature gradually drops to minus ten degrees.
Then continue to sprinkle water on the ice board, you can freeze into a few millimeters per layer of ice surface, and finally after multiple rounds of ice making process, frozen into an ice surface with a thickness of 30mm, to ensure the good stability of the ice surface and temperature uniformity.
In addition, the ice surface of the ice ribbon adopts a sub-module control unit, like a "nine-square grid" with different temperatures, which divides the ice surface into several areas. Ice making is carried out according to different projects in different regions and sub-standards, and mass ice sports such as ice hockey, speed skating, figure skating, curling can be carried out at the same time, and the ice surface can be continuously used to meet the various needs of people's fitness.
Another question arises: Doesn't the refrigeration process generate a lot of heat? Where did it all go?
As mentioned earlier, most of the waste heat generated during the refrigeration process will be recovered and can be used for athletes' domestic hot water, ice melting in the ice rink, dehumidification of the stadium, air conditioning and heating, and antifreeze on the bottom of the ice rink, and even for the heat needs of surrounding residents.
In this way, about 18×10^6 kW·h can be saved a year, which is equivalent to 550 tons of standard coal or the electricity consumption of 360 households in Shanghai in a year.
(Source above: Sports Life News; Source below: Qingdao Danone Environmental Protection Equipment Co., Ltd.)
Advanced carbon dioxide transcritical direct cold ice technology not only shines in the Winter Olympics, but also can be applied to food freezing/refrigeration, automotive air conditioning, heat pump systems and other fields, and is expected to become a forward-looking technology to accelerate the realization of the "double carbon" goal and contribute more to the creation of a better future for mankind.
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Producer: Wu Shaofei (Institute of Refrigeration and Cryogenic Engineering, Shanghai Jiao Tong University),
Peng Gao (Institute of Refrigeration and Cryogenic Engineering, University of Shanghai for Science and Technology)
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