Cryogenics 2021 highlights: applications of liquid nitrogen
Cryopreservation of biological materials to favour biodiversity and cooling of superconducting cables to allow transmission of electrical energy with zero resistance are key applications of liquid nitrogen.
The applications of liquid nitrogen are numerous. These include the freezing and transport of food products, the cryopreservation of biological samples, the use as a coolant for superconductors, vacuum pumps and other materials and equipment. Liquid nitrogen is also used in cryotherapy to remove skin abnormalities, to shield materials from oxygen exposure, to cool materials for easier machining or fracturing, and even the molecular gastronomic preparation of unusual foods and beverages. (1)
During the IIR's Cryogenics 2021 conference last October, researchers presented two of the most notable applications of liquefied nitrogen.
A Ukrainian researcher (2) presented the history and prospects of liquid nitrogen for the cryopreservation of biological material. The development of human civilisation has significantly increased anthropogenic pressure on the natural environment in which it lives. Biologists were the first to notice the decrease in biological diversity of flora and fauna, and Nikolai Ivanovich Vavilov was one of the first to understand of the need to store plant resources ex situ in the form of seeds. He organised 180 expeditions (1924-1939) to all continents and collected "germplasm" from crop of cultivated plants and their wild species in the main centres of origin of the species . The collection consisted of several tons of seeds stored between +4°C and -10°C in the Kuban Genetic Seed Bank. The correlation between ultra-low temperature and viability of living objects has been studied by many researchers. Raoul Pictet, a Swiss physicist, was the first to liquefy nitrogen (-196°C) in 1877, paving the way for the long-term preservation of living material in fauna and flora species. The practical use of liquid nitrogen began in 20th century. The advantages of cryopreservation are simplicity and applicability to a wide range of objects. All parts of plants may be conserved by cryopreservation. The author concludes that the use of long-term conservation of living material at the temperature of liquid nitrogen is very important for the future of human civilisation. The issue of preserving seed viability ensures the maximum germination rate, which is the guarantee for a high and high-quality crop yield. The preservation of germplasm, primarily of the plant samples that makes human life possible, should be a top priority among the many cryopreservation facilities, especially in connection with climate change, which increases the risk of declining biological diversity.
German researchers (3) presented the latest advances in cooling of long superconducting cables with liquid nitrogen. High-temperature superconductors (HTS) for the transmission of electrical energy in distribution networks or industrial systems consist of ceramic materials that reach their critical temperature close to the boiling point of liquid nitrogen (-196°C / 77K) and then completely lose their electrical resistance . Thanks to this physical phenomenon, electrical current can be transported without loss. When comparing conventional cables with HTS cables in terms of energy, the savings in ohmic losses that heat up copper or aluminium transmission lines stands against the energy requirement for cooling the HTS systems. Superconducting wires are embedded in cable cryostats through which subcooled liquid nitrogen is circulated to eliminate heat input through the thermal insulation. The circulating nitrogen is re-cooled in a subcooler, in which liquid nitrogen evaporates under vacuum conditions to generate cold. The evaporation pressure can be reduced to 150 mbar, the evaporation temperature is then -209°C (64K). Lower temperatures are not admissible because nitrogen freezes at -210°C (63K). In addition to the heat input through the cryostat, hydraulic losses due to the flow of liquid nitrogen and the associated heat input from the circulation pumps are also of importance, especially with longer cables. The additional cooling capacity required can be provided by installing intermediate cooling stations.
In a new cooling concept proposed by the researchers, the cryostat is equipped with an actively cooled shield. As a result, the heat flow acting on the superconductor can be reduced by a factor of around 10 compared to a simple cryostat. The required liquid nitrogen mass flow is then also reduced by a factor of 10 and the flow pressure drop is reduced by a factor of 100. It is then possible to implement cooling concepts without circulation pumps and intercoolers, even with long cables. The costs of the cooling technology and the energy consumption decrease considerably.
(2) https://iifiir.org/en/fridoc/liquid-nitrogen-and-preservation-of-genetic-material-of-biological-144384 (free download for IIR members)
(3) https://iifiir.org/en/fridoc/cooling-of-long-superconducting-cables-with-liquid-nitrogen-144385 (free download for IIR members)