Thermoacoustics: Sounding Off on a Hot Idea

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Thermoacoustics Sounding Off on a Hot Idea

• Joe Geddes

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Thermoacoustic Engines and Refrigerators

• What are they?
• Acoustic energy conversion devices
• Convert thermal energy to acoustic energy (heat
  engine)
• Convert acoustic energy to heat energy
  (refrigerator)
• Multiple types
• Standing wave
• Traveling wave
• Why do we care?
• Refrigerators accounted for roughly 20 of
  Northeast residential electricity consumption in
  97

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How it works

• Standing wave device

Qh
Source
Stack
Qc
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Thermo-acoustics
Vapor Compression
vs.

• Pros
• Simple designs
• No moving parts
• No CFCs
• Adjustability
• Cons
• Low efficiency, especially for standing wave
  devices
• Not in wide use

• Pros
• Established technology
• Works well
• Safe for consumers
• Cons
• Gobble electricity
• Refrigerants

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Efficiency - a sometimes slippery term

• Measure of work done to move certain quantity of
  heat
• Often expressed as coefficient of performance
• Carnot efficiency is limit to our technology
• But in terms of environmental impact
• The operating efficiency isnt the whole story
• Must take into account quality of energy used
• Account for efficiency of electricity production

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Adjusted efficiency (on an envelopes back)

• Thermoacoustics are 20-30 less efficient than
  vapor compression
• Say vapor compression has an efficiency X
• If we weight the efficiency of the vapor
  compression device by the efficiency of the power
  plant (50 for natural gas turbine)
• Vapor compressions efficiency is 0.5X
• Thermoacoustic efficiency is (0.7)(0.7)X 0.49X
• Environmentally, thermoacoustics gain ground
  but economics probably wont work out

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Increases in efficiency
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Adjustability

• Vapor compression is binary -- it either
  refrigerates at full capacity or not at all
• Thermoacoustic devices are adjustible --deliver
  just enough cooling, and no more
• Efficiency of thermoacoustic devices increases
  with decreasing power
• Can find optimal set point

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Niches for Thermoacoustics (near term
applications)

• Places where there are large amounts of waste
  heat
• Near power plants
• Ships
• Automobiles (maybe)
• Remote Locations
• Natural gas recovery
• Vaccine coolers
• Applications where high degree of reliability is
  required
• Space missions
• Military operations