A Stirling cooler is a mechanical system that approximates the ideal gas cycle. Often associated with airborne or portable cooling applications that require compact design and low input power, Stirling coolers are suitable for such applications because of their high efficiency. Compared to Gifford-McMahon and Joule-Thomson cycles, Stirling offers more than twice the cooling performance in the capacity range from 1-100 W. The coolers have a variety of military and commercial applications in infrared receivers and thermal imaging equipment.
Although Stirling coolers have been widely adopted for these applications, the cooler's life, typically less than 7500 hours, has prevented broad commercial use. Stirling Technology Company (STC) has developed a family of long-life coolers based on flexure bearings, clearance seals, and low-cost linear drive motors. Two NASA SBIR contracts, with Marshall Space Flight Center (solicitation year 1994) and Goddard Space Flight Center (solicitation year 1994) provided funding to advance the core technologies used in these coolers. A low-cost linear drive compressor and flexure bearings serve as the technology platform for STC's BeCOOL™ cryogenic coolers, capable of more than 50,000 hours of continuous operation with no maintenance.
Through the Goddard SBIR Phase II contract, STC was able to develop a low-cost, highly reliable linear-drive gas compressor. The compressor provides a 60-Hz pressure wave that drives the Stirling gas cycle. Dr. Syed Nasar, chairman of the electrical engineering department at the University of Kentucky, provided the theoretical design, while STC converted his design into hardware. The compressor is seen on the left side of Figure 1.
The special features that provide low-cost manufacturing and maintenance-free operation that were realized in this program are the following:
- Moving iron motor: The linear motor mover is iron and the magnets are integrated into the stationary motor stator. In this configuration, the magnets are not exposed to cyclic motion that can induce cyclic stresses and cause demagnetization over time.
- Flexure bearings and clearance seals: The linear motor mover is supported by flexure bearings (see Figure 2). The flexures align the compressor piston along the longitudinal axis while preventing radial misalignment. This technique allows for the use of a clearance seal between the piston and the compressor housing. By using a very small clearance seal instead of a rubbing seal, there is no contact between the power piston and the compressor housing.
- Low-cost manufacturing: The linear drive compressor uses standard electric motor materials and fabrication methods. This technology is mature and very low in cost.
The use of flexure bearings in Stirling machines is a core technology that the company has been developing since its founding in 1984. Flexure bearings allow for axially free displacement while preventing radial movement (see Figure 2). This technique provides highly accurate piston tracking with no contacting parts. Since the flexures are essentially springs, they have simple mechanical configurations, and they can be accurately modelled. Flexure bearings are used in both the linear-drive compressor to support the motor mover and in the cold head to support the gas displacer. STC's bearing design is shown in Figure 2.
Overcoming price barriers and operating-life limitations allow BeCOOL to be used in a wide range of commercial applications. Since their introduction in 1996, the coolers have been sold for mainframe microprocessor cooling, ultralow-temperature biomedical freezers, and laser cooling. STC is actively pursuing new cooling applications in microwave communications and sensor devices. The combination of new cooling technologies and the emergence of new applications for ultralow-temperature cooling has enabled STC to establish a commercial production base for these coolers. Figure 3 shows the standalone laboratory model of the cooler. It provides 15 W of cooling at liquid nitrogen temperatures. The user can define a temperature setting or program a cooling cycle. All that is required to operate the machine is a 110-V electrical supply.