Refrigeration plays an indispensable, enabling role in the emerging industry of high temperature superconductivity. New advances are being made in technologies and business models to enable a broad range of superconducting applications. On-site cryogenic refrigeration systems, designed for economical and reliable long-term operation, are under development that can greatly expand the market opportunity for HTS technology. The industrial gas industry, with its cryogenic expertise and infrastructure, is poised to play a critical role in accelerating HTS technology development and deployment.
The emergence of high-temperature superconductivity (HTS) has tremendous potential for consumers and the entire U.S. economy to benefit because of its expected high-impact in applications, ranging from electric power to transportation. Many HTS applications operate in the temperature range of liquid nitrogen (77 K or -196 °C). This fact necessitates extremely reliable and cost effective onsite cryogenic refrigeration systems, as well as an overall system approach that takes into account both the equipment requirements and the need for an infrastructure to provide long-term support.
For over a hundred years, the industrial gas industry has been supplying the cryogenic refrigeration needs for U.S. industry in such diverse areas as chemicals, low temperature superconductivity (LTS) and food products. Refrigeration is a core technology for the industrial gas industry. Some salient facts about this industry include the following: 
A core element of the cryogenic refrigeration system for HTS is expected to be mechanical refrigeration units (cryocoolers), which will typically be supported by on-site liquid nitrogen for back-up. The industrial gas industry has a broad range of such on-site systems in industries, ranging from electronics to pharmaceuticals. These types of onsite systems are routinely monitored and controlled from remote operations centers.
The industrial gas industry stands ready to meet the needs for HTS commercialization by providing cryogenic cooling equipment and services on a highly reliable “utility” basis suitable for many applications. The industry is already based on several key principles that support such a business model, including the following:
Projects currently underway are demonstrating how the capabilities of the industrial gas industry can be matched to the commercial requirements for cryogenic refrigeration in HTS. For example, three HTS cable demonstration projects have an industrial gas company partner providing both the refrigeration system and ongoing service and support.
Commercially viable cryogenic systems for HTS must meet both reliability and cost requirements. There is no single cooling solution for all HTS technologies. Rather, there is a broad range of potential HTS applications that will require flexibility in how technology is applied. The business model of cryogenic cooling being provided as a “utility” is being demonstrated and will further evolve. Methods of implementation will vary depending on circumstances, for example, in remote locations such as on board ships.
When produced on an industrial scale, liquid nitrogen can be the most energy efficient, simple and reliable means of producing refrigeration for larger HTS applications such as cable. Electrical power is required by all industries, many of whom also require industrial gasses. With regular pump boost and temperature regulation stations along the cable, it is envisaged that the cable could actually act as a delivery pipeline to the other industrial users of liquid nitrogen and make a substantial reduction in truck delivery miles.
Generally, the process of determining the lowest life cycle cost for any system, regardless of reliability requirements, requires a combined assessment of equipment, system engineering, monitoring and maintenance. There may be numerous sources for particular pieces of cryogenic equipment. The key objective is to engineer the overall system to achieve the lowest life cycle cost that can be achieved within reliability constraints.
A key element of HTS cryogenic refrigeration systems is the mechanical refrigeration unit or cryocooler. The basic technology for very large and small-scale applications is in many respects well developed, based on considerable industrial experience in cryogenic distillation, as well as the years of experience of many manufacturers of small-scale cryocoolers. However, there is a need for ongoing research and development to meet the large-scale refrigeration needs, cost targets and rigorous reliability requirements for the full range of HTS application opportunities.
One of the most promising technology developments for large cryocoolers is in the area of pulse tubes. Orifice pulse tube refrigerators operate in a closed cycle, using helium as a working fluid. The cold is generated by the use of acoustic (sound) waves that substitute for the typical pistons or rotating equipment found in other cryocoolers. This technology promises major advantages for HTS applications. These include the absence of cold moving parts, leading to extremely high reliability, and a theoretically high cycle efficiency, which is expected to translate into low operating costs. The development challenge is to fully achieve the high reliability and efficiency, while simultaneously reducing manufacturing costs. The focus of current development efforts is to achieve these goals, as well as to produce increasingly larger units with cooling capacities in excess of 1,500 watts at 77 K. Currently available units are capable of producing up to 1,100 watts at 77 K.
Free piston Stirling cycle cryocoolers have been available in commercial volumes for HTS electrical devices since 2000. The Stirling cryocooler employs gas bearings, a single piston and displacer, a combination of gas and mechanical springs, efficient heat exchangers and a passive balancer used to minimize casing vibration. The compact, high performance and extremely reliable cryocooler is field-proven with Mean Time Between Failure (MTBF) of well over one million hours, with over 6,000 units deployed, logging over 400 million cumulative run-time hours. The utilization of this proven technology will be critical in scaling cryocooler size to meet the needs of new HTS applications. 
In conclusion, the basic equipment and infrastructure already exists to support HTS cryogenic refrigeration systems. To optimize the HTS opportunity, however, there is a continuing need to improve overall system designs with an eye toward commercial operation. The industrial gas industry is well positioned to provide refrigeration in the form of a cooling service.
As HTS applications begin to move towards commercial reality, it becomes increasingly necessary to demonstrate cryogenic refrigeration systems that are cost effective and reliable, and that can be serviced and supported by a proven infrastructure. The following areas require specific focus: