Appliance Design - April 2008 - (Page 18)

COOLING TECHNOLOGIES Stirling Strikes Fig. 1. FPSC layout. Old idea finds new life. by jesse edwards Jesse Edwards is mechanical engineer/business manager at Global Cooling Mfg. Co., Athens, Ohio. 18 applianceDESIGN April 2008 f the practical thermodynamic cycles such as Otto, Diesel, Brayton, and Rankine, the Stirling Cycle and its variants are the only fully reversible cycles that have the theoretical potential to match the ideal efficiency expressed by the Carnot cycle. Stirling machines, like the Stirling Cycle, are also fully reversible and can be operated in two directions. The first Stirling machines, called engines, were configured to utilize pumped heat to generate power. Later, machines were configured to run in the opposite direction and utilize power to pump heat. Any Stirling machine has the potential to run either way, but design optimizations limit the overall capability of the machine when operating in the “other” configuration. In reality, issues such as friction, manufacturing tolerances, real world materials, and cost pressures limit the efficiencies achievable by real Stirling cycle machines. Of all Stirling machines, freepiston designs, pioneered by Sunpower Inc. and Global Cooling, both of Athens, Ohio, have the most potential to achieve efficiencies approaching Carnot due to their simplicity and low internal friction. (See Fig. 1 and Graph 1.) This limit keeps moving upward as new machines are designed. Currently, the limit is around 56 percent Carnot for coolers (held by a 250 W machine, designed by a licensee of Global Cooling) and 60 percent Carnot for engines (held by Sunpower’s O Advanced 80 We Stirling converter). Over the years, proponents of Stirling technology have promised great achievements for it. Until fairly recently, however, these promises more often resulted in failure rather than success. Things began to change in 1992 when Sunpower began production of its first free-piston Stirling cryocooler, the M77. This was followed in 1995 when Global Cooling began commercial production of the M100, a 100 W capacity free-piston Stirling cooler (FPSC). (See Fig. 2.) This compact, high-efficiency machine was the first production model FPSC to be manufactured for “high” temperature applications above –120 DegC and broke many established records for power density and efficiency. Then, in 2003, Twinbird Inc., a licensee of Global Cooling, began production of the TB40, a low cost, 40 W capacity, free-piston Stirling cooler. The FPSC was designed to compete with thermoelectric (Peltier) cooling units by offering previously unobtainable performance and efficiencies in such a small form factor. (See Fig. 3.) Twinbird’s initial FPSC design was originally slated for integration into a portable refrigerator/ freezer (see Fig. 4) that was targeted at the camping and recreational market, but has since expanded its application portfolio to encompass a wide variety of other applications, including: 4The CoolCargo, a collapsible, temperature controlled transport and storage refrigerator/freezer. www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - April 2008 Contents Editorial Shipments/Forecasts News Watch A Variant of Thermoacoustic Refrigeration Known as Acoustic-Stirling is Expanding Its Range Outside Cryocooling Into More Conventional Applications Such As Refrigeration and Air Conditioning Free-Piston, Stirling Coolers Are Now Showing Up in Mainstream Applications As Costs Come Down and Heat Transport Solutions Are Devised Heat Exchanger Innovations Are Helping To Improve the Overall Efficiency of Vapor Compression Systems Environmental Concerns Have Once Again Ignited a Debate Over Refrigerants A New Glass Ceramic Material for Induction Cooktops Can Be Decorated In a Variety of Glimmering Metallic, 3-D Looks That Can Employ a Number of Shades, Patterns, Styles, Colors, Intensities, and Textures Colorful, Reflective Coatings for Glass Create a Variety of Design Options To Help Designers Establish a Unique Look for Their Products Appliance Designers Faced With Different Motor and Drive Options Need To Understand the Differences and Tradeoffs Before Choosing A New MEMS Valve Offers a Number of Advantages Over Solenoids In Certain Applications, Including the Opportunity To Cut Energy Use The AHR Expo Set Attendance Records and Showcased Thousands of Innovative New Products Classifieds Design Marts Advertiser’s Index Association Report: CEA

Appliance Design - April 2008

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