technology overview
Stirling coolers

What is a Stirling cooler?
A Free Piston Stirling Cooler (FPSC) is a single phase cooling device that moves heat from a cool source to a warm sink with the help of external heat exchangers. Applications of FPSCs range from exotic deep temperature units to machines that perform well in domestic applications, such as home refrigerators.

A FPSC differs from a compressor system in that the refrigerant is not pumped into the space that is to be cooled. Rather, a Stirling cooler has a cold head, where heat is transferred into the machine, and a warm head, where heat is transferred out of the machine. External heat exchangers are used to transfer heat from the refrigerated space to the Stirling and are simply clamped onto the appropriate head. External heat exchangers are customized for the specific needs of each application, but typically come in the form of a thermosyphon, pumped fluid loop, or forced air and fin system.

What are the advantages of FPSCs?

1. Extremely long life is reached through the use of non-contact running surfaces. Gas bearings allow the two moving parts to “ride on a cushion of air” keeping the running surfaces from making contact while the machine is operating.
   
2. High efficiency over a wide refrigerated and ambient temperature range. Unlike vapor compression systems outfitted with capillary tubes that have optimized performance at only one temperature condition, Stirling coolers maintain their high performance over a wide range of temperature conditions.
   
3. No CFC or other environmentally dubious chemicals. Global Cooling uses the environmentally safe, inert gas helium as the working fluid in their Stirling coolers.
   
4. Light weight machines. Light weight makes portable cooling a reality.
   
5. Infinitely variable lift leads to no “on/off” losses. The Stirling cooler can be modulated to provide cooling between 0% and 100% of the capacity. Modulating the cooler allows for precision cooling of the refrigerated space and removes the losses associated with “on/off” controlling, an energy saving or extension of battery life of approximately 20%. The coolers also have very low start current, being able to start with just a minimum input. No need for starting capacitors or expensive electronics.
   
6. Operation at high ambient temperatures. Stirling coolers perform well in harsh temperature environments, high ambient temperatures are limited only by the materials of the cooler.

Gas Bearing Technology
The Stirling coolers designed at Global Cooling utilize oil-free lubrication by the way of gas bearings. Gas bearings work by charging an internal volume in the piston during the compression stroke and then leaking the trapped gas out into the space between the piston and the cylinder wall. This produces a layer of high pressure gas between the two running surfaces that levitates the piston allowing the piston and cylinder to operate in a non-contact way. As long as the gas bearings are working properly and the machine is running, the moving parts will never make contact and last for an extremely long time.

Light Weight Coolers
Stirling coolers excel in small capacity cooling and have light weight making it the perfect solution for portable cooling. A 40 Watt Stirling cooler weighs 1.6 kg where a similarly sized compressor weights 4.3 kg. A plot of the mass vs. capacity for both compressors and Stirling machines is shown bellow. There are two Stirling trend lines, the solid line indicates the mass and capacity of prototype or limited production machines today, where the dashed line represents the projected performance and mass after becoming fully commercialized.



Performance Map of Stirling Cooler
Stirling coolers have high performance over a wide temperature range, making them ideally suited for applications with varying ambient or refrigerated space temperatures, such as outdoor vending machines or portable cool boxes. A performance map is shown below for the M100A optimized for near room temperature use.


Figure 1: Performance map of M100A with a fixed reject temperature of 30°C and a cold head temperature range between 5°C and -50°C.


Environmentally Responsible Cooling
Currently, the majority of the worlds cooling needs are being met by a vapor compression cycle that uses a compressor to circulate an artificial (typically) working fluid. These artificial working fluids, called refrigerants, typically hydro-fluorocarbons (HFCs) or hydro-chlorofluorocarbons (HCFCs such as R22), have been proven to have excellent thermal properties, remain stable when properly bottled, and are inexpensive to manufacture. Unfortunately, they are also harmful to the global environment by being major contributors to the depletion of the ozone layer and contributors to the global warming phenomena.

In 1987, negotiators ratified the Montreal Protocol to create binding commitments to reduce the manufacture and use of environmentally dubious chemicals harmful to the ozone layer. As of 2002, 183 countries have pledged to phase out the use of ozone depleting substances, including chlorofluorocarbons (CFCs), and hydro-chlorofluorocarbons (HCFCs). Developed countries that signed the Montreal Protocol fully phased out CFCs in 1996, and instituted a production freeze on HCFCs in 1996. By 2010, all CFC’s and the majority of HCFCs will no longer be in use.

Replacements for the phased out refrigerants have been developed; hydro-fluorocarbons (HFCs such as R-134a), and hydro-carbons (HCs such as butane) are the current options for refrigerants, however, both have their downfalls. HFCs do not have an ozone depleting nature, but do contribute to the global warming problem. Researchers have shown that once released into the environment, the most common HFC, R-134a, will contribute to the global warming problem for up to 100 years before it is finally broken down. HFs such as butane have the down fall of being explosive, causing a serious risk when used in indoor applications.