Text | "China Science News" reporter Ni Sijie
In September 2008, the Shenzhou-7 spacecraft was launched, and the mainland became the country that independently mastered the key technology of out-of-cabin activities after the United States and Russia. While the whole country is celebrating, scientists from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Physics and Chemistry) are thinking about an important issue related to the sustainable development of China's aerospace industry.
In order to develop the aerospace industry, it is necessary to rely on large carrier rockets with greater thrust, higher efficiency, and clean and environmentally friendly. As a rocket propellant with high specific thrust and environmental protection, liquid hydrogen and liquid oxygen can produce a larger velocity increment under the same conditions, and the rocket engine using liquid hydrogen and liquid oxygen is more efficient. Hydrogen can only be turned into a liquid state at -253°C, and in order to produce liquid hydrogen, a large cryogenic refrigeration unit is required. However, on the other side of the ocean at this time, the United States explicitly prohibited various institutions from exporting cryogenic refrigerators and core components below -250 °C to China.
"Large-scale cryogenic technology is one of the indispensable key technologies for the space security system." In January 2009, scientists from the Institute of Physics and Chemistry took the initiative to apply to the Chinese Academy of Sciences to overcome the "fortress" of large-scale hydrogen helium cryogenic refrigeration technology.
A battle for the localization of large-scale low-temperature refrigeration equipment has quietly begun.
Prelude: "You have to do it by smashing pots and selling iron"
The creation of a low-temperature environment relies on the cryogenic refrigerator to continuously pump the heat in the environment outward, and at the same time prevent the heat from the outside from entering. Large-scale low-temperature refrigeration equipment is known as the "super low-temperature factory". Like a super-large refrigerator, it can reduce the temperature to below -253°C and even to -271°C, and maintain a cooling capacity of 100 to 10,000 watts.
This kind of "super cryogenic factory" is an important strategic support equipment of the country, and plays an irreplaceable role in important fields such as aerospace cryogenic propellant support, special material extraction, hydrogen energy utilization and hydrogen energy storage, strategic helium resource exploitation, and major national scientific and technological infrastructure.
Because of this irreplaceability, some Western countries have tried to lock up the development of China's related technologies. Since the 50s of the last century, Academician Hong Chaosheng and Academician Zhou Yuan, the older generation of scientists of the Institute of Physics and Chemistry, have started the research of cryogenic technology and tried to break out of the encirclement.
However, until 2009, the state has not officially approved the R&D project of large-scale cryogenic refrigeration devices for new needs, and the piecemeal small topics cannot support the establishment of key problems. Relevant scientists were anxious and proposed to the Chinese Academy of Sciences.
After learning more about the situation, the leaders of the hospital immediately decided: "What are you waiting for, you have to do it by smashing the pot and selling iron!" "The Chinese Academy of Sciences has approved special funds to the Institute of Physics and Chemistry to urgently deploy projects in important directions.
At the same time, the relevant departments of the Chinese Academy of Sciences have also begun to study whether the large-scale cryogenic refrigeration device has the conditions to become a major national scientific research equipment development project.
At that time, it coincided with the pilot launch of the national major scientific research equipment development project. The Ministry of Finance has arranged special funds to support the independent innovation of major scientific research equipment, and has taken the Chinese Academy of Sciences as a pilot to explore the national financial support model for the independent innovation of major scientific research equipment.
During the project establishment and demonstration, Li Qing, a researcher at the Institute of Physics and Chemistry who has been engaged in the research and development of low-temperature refrigeration technology in the liquid hydrogen temperature zone for a long time, led the team to repeatedly demonstrate the plan. He often sweats through his clothes due to back pain, but he is still working hard for the mainland's large-scale low-temperature refrigeration technology to occupy a place in the world. He once lived in the office and was busy almost every day until the middle of the night.
On the basis of organizing experts to conduct extensive research and strict demonstration, in 2010, the Ministry of Finance and the Chinese Academy of Sciences jointly launched the "large-scale low-temperature refrigeration equipment development" project, led by the Institute of Physics and Chemistry, with a project fund of 173 million yuan, and Li Qing served as the chief scientist.
This project came to be known to many as the "Phase 1 Project". With the support of this project, Chinese scientists independently developed a 10,000-watt liquid hydrogen temperature zone refrigeration equipment, which broke through five key technologies: high-speed helium gas bearing turboexpander stability technology, ultra-low leakage rate plate-fin cryogenic heat exchanger design and manufacturing technology, high-precision oil separation technology, pneumatic cryogenic control valve manufacturing technology and system integration control technology.
In the early stage of development, due to the lack of design and manufacture of large-scale low-temperature refrigeration-related equipment in China, the sealing performance of domestic compressors, low-temperature heat exchangers and other key equipment was two orders of magnitude worse than the demand. The project team built a simple small building next to Building 5 of the Institute of Physics and Chemistry. In the summer, when everyone bandages the insulation material, sweat flows down. In winter, everyone relied on a small heater and worked all night with cold hands and feet.
Hard work pays off. On April 29, 2015, with the efforts of Li Qing and his team, the first phase of the project passed the acceptance, and the mainland initially had the ability to independently design and manufacture large-scale low-temperature refrigeration equipment with liquid hydrogen temperature level, which solved the problem of low-temperature refrigeration equipment in large-scale liquid hydrogen temperature zone from scratch.
It is like a key that opens the door to the localization of China's large-scale low-temperature refrigeration equipment. And for scientists, their journey has only just begun.
Goal: "Keep going down"
Since the "Twelfth Five-Year Plan" period, the mainland has deployed a large number of large-scale scientific installation projects. Watching the devices "put on the ground" one by one, the scientists of the Institute of Physics and Chemistry thought further.
Many large scientific devices require the use of superconducting devices, including superconducting magnets and superconducting high-frequency cavities. In order to achieve good performance, most of these superconducting devices must operate in the liquid helium temperature region (around -269°C) to the superfluid helium temperature region (around -271°C). It has been said that "without the refrigeration equipment of large liquid helium or superfluid helium temperature zones, most large scientific installations are nothing more than a pile of scrap metal". Therefore, cryogenic chillers in lower temperature zones become necessary.
At that time, there was no domestic capacity to produce large-scale cryogenic refrigeration equipment in lower temperature zones. Some large-scale scientific equipment projects have fallen into passivity in the international scientific and technological competition in the race against time because of the uncontrollable supply time of imported products.
"We can't just be satisfied with the liquid hydrogen temperature zone, the temperature will continue to drop, covering the whole temperature zone, down to the liquid helium temperature zone or even the superfluid helium temperature zone." Zhang Liping, then director of the Institute of Physics and Chemistry, discussed with Zhan Wenshan, the former director and general consultant of the first phase of the project.
With the technical capabilities at that time, it was not easy for a large cryogenic refrigerator to achieve -253°C. If you want to drop to -269°C or even -271°C, you can imagine the difficulty. After discussion, it was decided to "squeeze" a small part of the funds from the first phase of the fund to demonstrate the feasibility of realizing large-scale low-temperature refrigeration technology in the whole temperature zone.
On this basis, the Institute of Physics and Chemistry submitted a report to the Chinese Academy of Sciences in 2014, requesting the continuation of the project.
The report has been revised no less than 25 times, with the most debated question being whether to follow "one step at a time" or "one step at a time".
"Step by step" refers to the establishment of a project to overcome the refrigeration equipment in the liquid helium temperature zone, and then a project to overcome the refrigeration equipment in the superfluid helium temperature zone. This approach is steady, but slow.
"One step across two steps" refers to the establishment of only one project, the equipment to conquer the liquid helium and superfluid helium temperature zone, and the development of a device that can provide both -269 °C and -271 °C temperatures. It's fast, but it's risky.
At first, most people supported a more secure route. "We are sure of the device in the liquid helium temperature zone, and many people have never seen a superfluid helium refrigerator, let alone manufactured it." Gong Lingling, a researcher at the Institute of Physics and Chemistry and the executive deputy commander of the second phase of the project, said.
However, the actual domestic demand for refrigeration and the stalemate of international competition do not wait for anyone. The Chinese Academy of Sciences and the Institute of Physics and Chemistry finally decided to "step into two steps in one step".
In 2015, the project application was recognized by the state. With the support of major national scientific research and equipment research projects, the second phase of the project "Development of Large-scale Cryogenic Refrigeration System from Liquid Helium to Superfluid Helium Temperature Zone" is seamlessly connected, with a funding of 187 million yuan.
Gong Linghui felt extremely stressed. The development of the second phase of the project includes three kinds of refrigerators, one is a 100-watt liquid helium refrigerator with a cooling capacity of 250 watts at -269 °C, the other is a kilowatt liquid helium refrigerator with a cooling capacity of 2500 watts at -269 °C, and the third is a 100-watt superfluid helium refrigerator with a cooling capacity of 500 watts at -271 °C. Not only that, but the latter two chillers also need to be integrated into a single unit.
"There are a lot of projects and time is tight, so I'm afraid I can't get it." Gong Lingling said frankly.
Journey: Towards -269°C
Unsurprisingly, the dual pressures of technology and time made the project doubly difficult.
According to the plan, the research team will first develop a liquid helium system with relatively small cooling capacity. In September 2016, the 250 watt liquid helium system was integrated and commissioning began. "I didn't think about it, it wasn't like that. As soon as I turned on the machine, there was a problem, and the temperature could not be lowered. Liu Xinjian, then deputy director of the Institute of Physics and Chemistry and commander-in-chief of the second phase of the project, said.
The discussion was held countless times, and the reasons were checked countless times, but the problem has not been solved, and everyone's confidence has fallen to the bottom little by little. "I can't do it halfway, what should I do?" Liu Xinjian beat a drum in his heart.
By the summer of 2017, everyone felt that the tinkering "conservative treatment" was no longer helpful, so they decided to break the boat: "Demolish! Reassemble! ”
The equipment to be dismantled is mainly a cold box. From the outside, the cold box looks like a large jar with many parts inside, where helium becomes liquid helium or superfluid helium.
The disassembly and assembly task was carried out in the Langfang Park of the Institute of Physics and Chemistry. At that time, the park had not yet been completed, and the factory had just been connected to water and electricity. In order to save time, Gong Ling led a team of young people to move mats, sheets, kettles, "hot fast" and boxes of instant noodles into the factory and live on the spot.
After working day and night for more than a month, they finally found the problem. The commissioning process then went smoothly. By October 2017, the 250-watt liquid helium refrigerator passed the expert acceptance, and the localization rate of key components reached 100%.
The confidence of the research team has doubled. Based on the successful experience, they further developed and perfected the key core equipment, the high-speed helium turboexpander, and integrated a 2500 watt liquid helium refrigerator. In September 2019, the performance of the large-scale liquid helium refrigeration system reached the design index.
Test and acceptance of helium screw compressors for 2500 watt cryogenic systems.
However, the development of 100-watt and kilowatt-level liquid helium refrigeration equipment has only completed half of the project tasks, and greater challenges await them.
Breakthrough: Sprint to -271°C
The other half of the project was the development of a 100-watt superfluid helium refrigerator. This part of the task was started almost simultaneously with the development of the liquid helium refrigerator.
The key equipment of the superfluid helium refrigerator is the centrifugal cold compressor, which is also the most feared equipment in the whole project.
The cold compressor operates at low temperatures and uses electromagnetic bearings, requiring the control system to react accurately to possible rotor deviations within 0.5 milliseconds. Since 1959, the Cryogenic Technology Experimental Center (one of the predecessors of the Institute of Physics and Chemistry) has been developing various contact and non-contact bearing compressors, but has never developed electromagnetic bearing compressors.
For the sake of prudence, the project team has prepared two routes, one is to purchase the whole machine or parts from abroad, and the other is to develop all independently. Two routes, go hand in hand.
As a result, the first route was in full condition, and everyone's hearts went up and down like a roller coaster.
In 2015, the research team found a foreign company with experience in the production of cold compressors, and they had already negotiated the purchase, but before the bidding, the company sent an email saying: "We discussed and negotiated with the government departments last week to obtain an export license, but they had a more negative view of the issuance of the export license, on the grounds that your firm was on the government's ban list." Later, the scientific research team found another foreign company, but the cold compressor produced by this company was shipped to Langfang, and the first test run failed, and the repair progress was delayed again and again.
The second route, though equally difficult, is much more realistic.
When the first route was completely unworkable, the scientific research team had accumulated many years of technical experience and independently overcame the technical problems of electromagnetic bearings. Since then, the research team has focused all their efforts on the second route. By 2019, the cold compressor independently developed by the team entered the technical testing process.
Since the test required a large amount of liquid helium, they moved the test site to the High Magnetic Field Science Center of the Hefei Institute of Physical Sciences, Chinese Academy of Sciences, which had the test conditions. Here, the cold compressor test went very smoothly overall. In November 2019, the cold compressor was moved back to Langfang and integrated into the superfluid helium system test bench.
The next goal for the scientists is to achieve a 500-watt superfluid helium system combined with a 2,500-watt liquid helium system.
Joint debugging: Realize the dual-purpose of one machine
In November 2019, the prototype of the cold compressor was equipped with a superfluid helium system, which was jointly debugged with the liquid helium system for the first time. In December, the superfluid helium system reached -271°C for the first time in the second joint commissioning...... The experiment seemed to be going well. However, just as they were approaching the higher target of 500 watts, -271°C, the problem arose.
In February 2020, the third joint debugging obtained a good result and a bad result, the good result was that the superfluid helium system obtained an instantaneous cooling capacity of 502.9 watts at -271°C, and the bad result was a failure of the instrumentation system.
At that time, the test was often stopped while it was being done, the motor overheated, and the instrument alarmed. "With the equipment developed by ourselves, we can infer where the problem is, and we can continue to iterate on technology." Liu Liqiang, chief scientist of the second phase of the project and researcher of the Institute of Physics and Chemistry, is anxious but confident.
While continuing to improve the performance of the cold compressor, they worked hard to solve various problems including motor overheating, and the stable operation time of the superfluid helium system was gradually extended: at the beginning of July 2020, the ninth joint commissioning was carried out, and the stable operation was 1 hour; At the end of the month, the tenth joint debugging was carried out stably for 5 hours; In August, the 11th joint commissioning was carried out for 7 hours. However, the problem of motor overheating has not been cured.
With a ruthless heart, everyone decided to do a "major operation" on the cold compressor, replace all the coils in the motor, and optimize the cooling structure.
Everyone in the team felt that even if the progress was slow, the equipment handed over to the state would not allow for a trace of hidden dangers. "The 'conservative therapy' may be able to achieve a 72-hour stability target, but the appearance of any chance factor will make it unstable." Liu Liqiang said.
In October 2020, in Langfang Park, the motor coil was replaced and the twelfth joint debugging was ready for the twelfth joint commissioning. This time, they were going to challenge for 72 hours.
1 hour, 5 hours, 7 hours, 10 hours...... Liu Liqiang saw signs of success. In the past, there was always the problem of high or low values, but this time it was very smooth, and the system has been running smoothly.
12 hours, 24 hours, 36 hours, 48 hours...... The system remains stable. Until October 20, the equipment ran smoothly for 72 hours.
"It's finally done!" Liu Liqiang felt that the pressure on his shoulders was finally released.
On December 29, 2020, the liquid helium and superfluid helium cryogenic refrigeration device passed the acceptance of the expert group.
A kilowatt liquid helium refrigerator with a cooling capacity of 2500 watts at -269°C (left) and a 100-watt superfluid helium refrigerator with a cooling capacity of 500 watts at -271°C (right).
Future: To build the world's cryogenic technology industry highland
On April 15, 2021, the second phase of the project passed the project acceptance and scientific and technological achievement appraisal.
According to the acceptance opinion, the project "has made a comprehensive breakthrough in the core technology of large-scale helium cryogenic refrigeration equipment"; According to the appraisal opinion, the project "has formed the research and development capacity of a kilowatt-level large-scale helium cryogenic device, broken the technical monopoly of developed countries, and the overall technology of the project has reached the international level".
This is the first large-scale cryogenic refrigeration device in mainland China that can reach the superfluid helium temperature zone. From this day on, China has its own "super cryogenic factory".
After the second phase of the project, the Chinese Academy of Sciences has set up a strategic leading science and technology project to continue to support the Institute of Physics and Chemistry to develop a 5 ton/day large-scale hydrogen liquefaction system. On March 8, 2024, the system passed the test and acceptance, and the system operated stably at full load for 8.5 hours, with a hydrogen liquefaction rate of about 5.17 tons/day.
In the eyes of many witnesses, the success of the "super cryogenic factory" is due to a set of effective major project management systems and mechanisms.
Wang Xuesong, director of the Institute of Physics and Chemistry, introduced that during the implementation of the cryogenic equipment project, the Institute of Physics and Chemistry has explored a unique and effective management mechanism, and created an innovative R&D model and a complete development chain of "research, application, and transformation".
In terms of management, "we have broken the barriers between the original 'PI (project leader) system' research groups, integrated three research groups, established a research center oriented to major strategic goals, formed a scientific and technological framework to work together to do great things, and laid a solid foundation for the overall management of the project." Wang Xuesong said.
At present, 18 sets (sets) of large-scale cryogenic refrigerators have been applied in the Institute of Physics and Chemistry. Among them, the 100-watt liquid helium refrigerator is not only used in the superconducting magnet test device of the Institute of High Energy Physics of the Chinese Academy of Sciences, the linear accelerator of the China Institute of Atomic Energy, but also goes abroad and is used in the Korean Fusion Large Scientific Device (KSTAR); Liquid helium and superfluid helium refrigerators are used in the test line of accelerator-driven transmutation research equipment of the Institute of Modern Physics of the Chinese Academy of Sciences.
The first set of 200 watt liquid helium temperature zone refrigerator exported to the world.
At the same time, the Institute of Physics and Chemistry cooperates with enterprises in the development of screw compressors, helium valves, high-speed motors, electromagnetic bearings and other components. More than 20 enterprises in Shandong, Fujian and other places have achieved technological breakthroughs in a short period of time, and some of them have reached the international advanced level.
"We undertake major national projects, not only to complete the projects, but also to lay the industrial foundation for the country." Gong Lingdao said.
From the first phase of the project in 2010, to the second phase of acceptance in 2021, and then to the continuous breakthrough of the pilot project, Chinese scientists have completed the road of Western countries for decades in just over 10 years, and the mainland's large-scale low-temperature refrigeration technology has entered a new stage of development.
Now, these scientists have once again seen the country's new demand for cryogenic refrigeration devices, and have laid out research on extremely low temperature and large refrigeration capacity refrigerators, helping China become the world's highland of cryogenic technology and cryogenic industry, and fully supporting the development of national strategic resources, aerospace, and scientific and technological innovation.
Large cryogenic refrigeration team. Courtesy of the Institute of Physics and Chemistry
(Intern Zhao Yutong also contributed to this article)