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The Technology


What is Thermoacoustics ?

Thermoacoustics is a physical phenomenon describing the relationship between acoustic energy and heat transfer. Acoustics or sound waves consists of oscillations in pressure, temperature and displacement. Although the temperature oscillations are small, research has shown that this "thermoacoustic" effect can be harnessed to produce efficient heat engines. Thus, these engines have the potential to be both simple and reliable.

Thermoacoustics has been relatively obscure within the commercial engineering community, due to its initial specialized use in space for military applications. Recent interest in thermoacoustics as an alternative heat transfer process underpins the timing of investing and developing this technology to replace applications relying on environmentally hazardous refrigeration gases.

History of Thermoacoustics

The history of thermoacoustics is thought to have started 200 years ago with European glass blowers that noticed sound generated when a cold glass tube was placed next to a hot glass stem. 150 years later, in a series of research papers, the first accurate quantitative discussion on thermoacoustics was formally published. The theory of thermoacoustics refrigeration was identified more recently when heat transfer phenomena was observed in a gas resonating in a cylinder in 1975. Ten years later, the use of oscillating fluid particles to induce a cooling effect resulted in the development of a "cyrocooler". In the late '80s, thermoacoustic research was pursued at the Naval Postgraduate School where a 5W Space Thermoacoustic Refrigerator, dubbed STAR, was developed and launched aboard the Space Shuttle Discovery (STS-42) in 1992, picture below right. The Navy demonstrated the use of thermoacoustics and cooled off the electronics aboard the USS Deyo in 1995. More recently, interest in thermoacoustics has intensified, motivated by its relative mechanical simplicity, potential for broad integration into industrial and consumer applications and benign environmental impact. At present, only low temperature applications have been commercialized in niche markets.

Thermoacoustics vs. Competitive Technologies

The apparent benefits and commercial opportunity of developing thermoacoustic refrigeration and air conditioning applications becomes evident when evaluated against conventional technologies. The following table compares thermoacoustics to conventional vapor compression technologies, including the industry's recent advances in adopting carbon dioxide and hydrocarbons as alternative refrigerant gases.

A/C and Refrigeration Technologies Compared
Technology
Under Development
Current Technology
Recently Introduced
Technology
Recently Introduced
Technology
Relative Performance Measures
Thermoacoustics
Vapor Compression
Hydrocarbons
Carbon Dioxide
Ozone Depletion Potential
None
CFCs, HCFCs
None
None
Greenhouse Effects
None
Very High
Low
Low
Energy Efficiency - power grid
High
Moderate
Moderate High
Moderate
Energy Efficiency - solar power
100%
Not Practical
Not Practical
Not Practical
Operating Mode
Proportional
On/Off
On/Off
On/Off
Initial Equipment Costs
Low
High
High
Very High
Maintenance Costs
Low
Moderate
Moderate
Moderately High
Operating Life Cycle
Very Long
Moderate
Moderate
Moderate

Specific Benefits of Thermoacoustics for Air Conditioning Applications

Preliminary research has established thermoacoustics as suitable technology for most heating & cooling applications. Feasibility studies show thermoacoustics is better suited for the operating temperatures of A/C applications versus low temperature application (cryogenic level temperatures) that have been successfully commercialized. Thermoacoustic based A/C equipment should have substantially lower manufacturing costs compared to conventional compressors. The mechanical simplicity of these systems versus conventional compressors, which require gears, pistons, valves and timing chains along with no lubrication requirements or scheduled maintenance, should extend the service life - potentially 200 to 300 percent longer than present systems. Photo to the right is a thermoacoustic unit build by students at a University.

Thermoacoustic systems can maintain a constant temperature by matching input power to load conditions and produce higher efficiency than systems using an on/off cycle to control temperature as most vapor compression systems do today. Other technologies may be pursued, as compliance with the Montreal Protocol progressively reduces the use of banned gases, however many of these technologies have substantial drawbacks - including flammability, corrosiveness and extremely high operating pressures.

Cool Sound Industries and Thermoacoustics

Cool Sound Industries was established as a commercial platform to bring together the various technical disciplines required to successfully develop residential and commercial air conditioning applications. Frank Wighard, the company's founder, has over 35 years of experience in the design, installation and service of environmental control equipment (air conditioning systems). In the late '90s Mr. Wighard recognized the potential benefits of thermoacoustics for air conditioning systems and initiated his investigation by collaborating with the academic and research community studying this phenomena. In 1996, Mr. Wighard filed an initial patent on a double driver configuration that adapts thermoacoustic heat transfer process to air conditioning applications. He has since proposed numerous improvements to his basic design, although formal public disclosure has been tactically withheld. His early involvement and consulting activities have resulted in valuable business relationships that will benefit the launch and future success of the company.

Patent drawing of a Double-acting Acoustic Driver