Peking University, February 6, 2022:  During the afternoon of February 5, 2022, Dutch speedskater Irene Schouten won gold in the women’s 3000 meters with an Olympic record of 3 min 56.93sec at the National Speed Skating Oval, breaking a long-standing Olympic record in 20 years. With her unparalleled achievements, comes the ice-making technology which granted her the stage where she was allowed to showcase her record-breaking performance. Professor Zhang Xinrong and his team at PKU is dedicated to the fundamental research and application of CO2 transcritical refrigeration system, the technology that is being used at the Beijing Winter Olympic Games today.


Zhang Xinrong is a professor at the College of Engineering, Peking University and also director of Beijing Engineering Research Center of City Heat. For decades, Zhang and his team have investigated the theories behind functional thermo-fluids preparations, flow dynamic, heat and mass transfer, and thermodynamic cycle from both the micro/nano scale and macroscopic scale. In preparation for the 2022 Beijing Winter Olympics, Zhang’s group took on the crucial task of snow- and ice-making based on their achievements in developing CO2 thermodynamic systems.


Snow- and ice-making in 2022 Beijing Olympics

Stepping into the old Shougang Industrial Park, you will see the famous Big Air Shougang – the only snow event venue in downtown Beijing – with its ribbon-like track fully covered by thick, spongy snow. Here, athletes of freestyle skiing and snowboarding will pursue their best performance in the upcoming 2022 Beijing Winter Olympics.

But for those who lived in Beijing for a long time, such a view would seem unreal and counter-intuitive. Considering the fact that the two Olympic events (Olympics and Paralympics, respectively) will be held respectively in February and March, the temperature during this time will fluctuate within the range of 2~3 oC, which makes it difficult to form ice or snow naturally, let alone hosting games of outdoor snow sports. The dry weather of Beijing aggravates the problem, since the city usually does not have precipitation during winter. In fact, the scarcity of snow has been a common issue for many host cities of winter games, making man-made snow an indispensable requirement for these areas.

The history of snow-making can be traced back to the 1980 Winter Olympics in Lake Placid. In recent times, the proportion of man-made snow was once raised to 90% in the 2018 Olympics in Pyeongchang. Typically, a snow-making machine would compact a mixture of water and air to form huge amounts of ice nuclei. When the mixture gets shot into the air, the nuclei of the mixture would quickly precipitate into the free water molecules, forming snow with structures similar to the natural ones. However, this equipment would not work when the atmosphere temperature surpasses 2 oC, which means that for the success of the Winter Olympics, Beijing must come up with a new method.

Over the past 5 years, Professor Zhang Xinrong and his colleagues at the PKU College of Engineering has taken over the key project of snow-making and -storage at the ambient temperature above 0 oC. They invented a new form of indoor snow-making guns that could tune the humidity and granularity of snow by altering the direction and jet speed of water. “Under the temperature of 0~15 oC, the traditional method would use crushed ice as a substitute of snow. However, such ‘snow’ greatly differs from its natural counterpart due to the fact that ice is much harder in density than snow, which may pose potential risks to athletes.” As Professor Zheng Qiuyun, member of Zhang’s research team, indicated. “But snow produced by our machine boasts superior properties similar to natural precipitates even at the ambient temperature of 25 oC, with a production speed of 6 m3 per hour.” Furthermore, the whole heat recovery can be made from the snow-making process, which can be used to create an outdoor low temperature ambient enough for thermal comfort.

Their innovations were also widely applied in the production of ice. The ice used at the National Speed Skating Oval which covers an area of 12,000 m3, making it the largest in the world – was also created and sustained by the natural fluid CO2 transcritical refrigeration system developed by Zhang’s group. According to Zhang’s innovation, the difference in temperature across a vast ice surface can be lower than 0.5 oC, thereby allowing the ice to have even hardness across the entire surface. This will act as a favorable element for athletes to break their records in the 2022 Olympics, as their speeds depend on not only their hard training, but also the evenness of the ice. “We aimed to make our ice even so that it can yield world records. During the Olympic international test trials, many athletes achieved their best records on our ice.” The transcritical CO2 refrigeration technologies innovated by Zhang’s groups are currently applied in 5 out of the 9 ice venues used for the 2022 Olympics and represent the highest level of sport-related technologies in the world. The whole waste heat is recovered for supplying heat for dehumidification, hot water, antifreeze and preheat in venues. He is the first whistler to put forward systems using CO2 for the ice rinks in 2022 Beijing Olympic Games.


CO2 thermal cycle underpins a green Olympics

The progress made by Zhang’s research team exceeds beyond the manufacture of ice and snow. The refrigeration fluid they have created out of carbon dioxide was also a significant accomplishment.

It would at first seem nonsensical that Zhang’s group would set their ice-production system dependent on the most widely-known greenhouse gas, CO2, especially when countries across the globe are working hard to reduce its emission and save our planet from global warming. China has prioritized a reduction of CO2 emission before 2030 and reaching carbon neutrality before 2060. Within the framework of these agendas, Beijing aims to host the 2022 Winter Olympics in a green and sustainable way. Therefore, many people may be perplexed when Zhang’s design was described to rely on CO2. In reality, however, Zhang’s refrigeration equipment features environment friendly and energy-efficient methods.

The most common refrigerants are Freon and ammonia, which are also listed as greenhouse gases. Freon, once diffused into the atmosphere, will destroy the ozone layer through unstoppable chain reactions, and ammonia is toxic by itself. Compared to its two counterparts, the carbon dioxide has unique advantages. “In fact, carbon dioxide is an environment-friendly refrigerant, since we can abstract it from the air without generating new emissions during the refrigeration cycle,” noted by Wang Wei, deputy chief engineer of Beijing Institute of Architectural Design. In thousands of steel tubes beneath the ice of the National Speed Skating Oval, liquid carbon dioxide gets evaporated to absorb heat from water and transformed into ice. The CO2 gas is then transported to air compressors which compacts it into a supercritical state containing huge amount of energy. While unstable CO2 gets condensed to release energy, a process of swelling into liquid form presumes and eventually CO2 returns back to tubes under the ice.


“The CO2 direct cooling system developed by our team could achieve snow and ice-making efficiently in limited space with low temperature,” Professor Zhang added. “This was a bold trial unprecedented in history.” Zhang’s team integrated the equipment of both cooling and heating into one system, where ice would be formed in the hypobaric chamber and energy would be recycled in the hyperbaric chamber, ultimately allowing “the energy released in the thermodynamic cycles to be utilized for the heating of venues and the provision of hot water supply for athletes.” This thermodynamic system would save 2 million kW·h of electricity and more than 30% of total energy, reducing the carbon dioxide emission of more than 900 tons every year.

The snowmaking guns developed by Zhang’s group also exhibited good performance in the pilot experiments carried out in the Shijinglong Sky Resort at Yanqing, Beijing. These new guns reduces the energy cost by half compared to those that used Freon as a refrigerant, and it utilizes only 1/3 of liquid nitrogen typically required in a traditional snow-making machine.


Listed in the basic principles of 2022 Beijing Winter Olympics and Paralympics, environmental protection policy was listed as the top priority to show China’s determination to reach its emission reduction goals. Zhang’s snow and ice-making system, together with other energy-efficient practices, will provide crucial reference for further carbon-reduction trials in China’s applied research toward carbon neutrality.

CO2 as a refrigerant has broader stage for green development

The application of CO2 refrigeration system encompasses more than cooling procedures in Professor Zhang’s blueprint. Compared to traditional methods, the eco-friendly CO2 system boasts high safety and low running cost that others cannot be on par with. Considering the sustainability element that the CO2 system embodies, the CO2 thermodynamic system poses as a promising choice in future sports events. Zhang has been devoted to the wider application of CO2 circulation and the development of supporting technologies over the past several decades. The strong compressibility of CO2 in its transcritical region grants its potential application in enhancing the efficiency of heat transfer and electricity generation. Based on this characteristic of CO2, Zhang and his collaborators invented a supercritical CO2 solar collector that enhanced the utilization of solar power. He is the first to propose CO2 as the working fluid for power generation cycles, which are very promising in applications for various future power plants.


The CO2 cooling-heating system is also used in coal mines. The intolerable environment in large-depth mines has long been a long-term problem, as the deep galleries are characterized by high temperature and humidity. Recent policy on eco-protection aggravates the problem as it banned the use of boilers that would result in serious pollution. However, Zhang saw the “heat hazard” as an opportunity to make full use of the huge amounts of energy otherwise released wastefully. They designed a CO2 unit to extract the thermal energy in the mine galleries and transferred it to supply heating above ground. Zhang hopes that in the future, the CO2 unit can be manufactured to replace the expensive old equipment.

Professor Zhang is also director of the Beijing Engineering Research Center of City Heat (BERCCH), which was co-founded by Peking University, Beijing Municipal Science & Technology Commission and Beijing Energy Conservation and Environmental Protection Center in 2014. This center aims to act as a bridge between industrial needs and applied research, as well as prompting the development of urban heat management through technological support. The BERCCH provides a broader platform for scientists whose interests lie in the conversion of thermal energy. Here, Zhang and his counterparts from other top academic institutes in China work together in projects aiming to upgrade technologies used in heat pump, heat storage, solar energy and thermal management integration. Their achievements include fluorine-free CO2 heat pump of the 100 kW level, hot sludge fuelization and heat extracting technology of sewage. In urban areas, however, there are still huge amounts of heat energy being wasted, as Professor Zhang mentioned in his interview, “There are many computing centers in Beijing, which cool down the room temperature with air conditioners that constantly transfer heat from indoors to outdoors without recycling available energy. However, we could store the energy and apply them to heating supplies for surrounding communities in winter through CO2 thermodynamic cycles.” BERCCH still has a long way to go and huge tasks to shoulder in Beijing’s plan to forge as an eco-friendly city.


In the history of refrigerants, CO2 is already a well-known substance with its roots traced back to 1850, when an American called Alexander Twining introduced it into the refrigeration of the steam compression system. The system was a revelation at the time, but nowadays, its cooling efficiency is severely limited due to its low critical temperature and high critical pressure, which explains its decline and the reality of being replaced by systems that utilize Freon and Ammonium. In the new era of sustainable scientific research, CO2 can now act as a natural refrigerant which might win the final victory in the battlefield of refrigeration against other fluoric counterparts. When that happens, humanity would be able to embrace a greener and more self-sufficient world with lower or even zero carbon emission. In other words, we are all working together for a shared green future – which is exactly what Professor Zhang and his team believes in.

Written by: Bai Qingwen
Edited by: Rose Li & Zhang Jiang