Thermoacoustics: Sounding Off on a Hot Idea Joe Geddes

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

How it works Standing wave device Stack Source QcQc QhQh

Thermo- acoustics 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 vs. Vapor Compression

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 isn’t the whole story –Must take into account quality of energy used –Account for efficiency of electricity production

Adjusted efficiency (on an envelope’s 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 compression’s efficiency is: 0.5X –Thermoacoustic efficiency is (0.7)(0.7)X = 0.49X Environmentally, thermoacoustics gain ground … but economics probably won’t work out

Increases in efficiency

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

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