thermoacoustics is a science which involves converting thermal energy to acoustic energy... purpose is to extract energy from low grade heat (like car exhaust)... and convert it to electricity...
Heat Engine
according to the second law of thermodynamics... if there is a heat source and a heat sink then its possible to generate work... and the amount of work that you can generate off an engine connected between the heat source and the heat sink depends on how much heat is given by the source and how much heat is given to the sink... there must be a heat sink or else the engine wont work... so heating something wont do... you have to figure out a way to reject some part of that input heat...
and the maximum theoretical efficiency, &neta;max modeled based on the theoretical carnot cycle suggest that it is proportional to the ratio of the absolute temperature difference... and this is the maximum possible efficiency... there is no other thermodynamic cycle in this world which can generate a higher efficiency... there might be other solid state devices which may archive higher efficiencies but they dont fall under "classical thermodynamics"....
anyhoo...
Diesel engines operate at around 40% thermal efficiency... and thats dependent on the compression ratio... higher the compression ratio... higher the efficiency...
the acoustic heat engine works on the priciple of the stirling cycle... and achieving high efficiencies is a challenge esp since the maximum working temperature and temperature difference between the source and the sink is much lower than that of diesel engines...
one of the main disadvantage is that the stirling engine is not an internal combustion engine unlike diesel or petrol engines... with stirling engines there must be an external heat source and a heat sink...
the acoustic engine does not have any moving parts but the thermodynamic cycle is similar to the mechanical stirling engine cycle...
The ideal stirling cycle consists of four thermodynamic processes acting on the working fluid:
* Isothermal Compression
* Constant-Volume (isometric) heat-addition
* Isothermal Expansion
* Constant-Volume (isometric) heat-removal
the acoustic heat engine generates sound which is essentially a pressure vibration... which results in a change in thermo-fluid properties such as density, temperature, internal energy, entropy etc.
"A sound wave in a gas is usually regarded as consisting of coupled pressure and motion oscillations, but temperature oscillations are always present, too. When the sound travels in small channels, oscillating heat also flows to and from the channel walls. The combination of all such oscillations produces a rich variety of “thermoacoustic” effects. Research in thermoacoustics began with simple curiosity about the oscillating heat transfer between gas sound waves and solid boundaries. These interactions are too small to be obvious in the sound in air with which we communicate every day. However, in intense sound waves in pressurized gases, thermoacoustics can be harnessed to produce powerful engines, pulsating combustion, heat pumps, refrigerators, and mixture separators.
To me, the word ‚Äúthermoacoustics‚Äù represents one unifying analytical and conceptual approach to all of these devices and phenomena. The thermoacoustic approach begins with the assumptions that the oscillations of pressure p, temperature T, density ρ, velocity u, and entropy s can be thought of as ‚Äúsmall‚Äù and that they are adequately represented as sinusoidal functions of time. Results of engineering interest are obtained as time-averaged products of the oscillating variables: heat fluxes are proportional to the product of T and u, work to the product of p and u, mass fluxes to the product of ρ and u, etc." - Greg Swift, Los Alamos National Laboratory
Read this story from New Scientist...
New ways of turning heat into sound waves - and then into electricity - may be the next step toward a practical new source of alternative energy.
Scientists have known for decades that they can turn heat into sound using simple devices called acoustic heat engines. But this week a team of University of Utah researchers plan to show they’ve succeeded in miniaturising and optimising the devices, which then turn the sound into usable electricity.
If true, the advance could open the door to super-efficient power plants, cars, and computers, as well as a new generation of solar cells.
Acoustic heat engines usually use a copper plate to conduct heat to a high-surface-area material like glass wool, which then heats the surrounding air. The movement of the hot air generates a single frequency sound wave, rather like a flute. And this in turns vibrates a piezoelectric electrode, producing voltage.
High efficiency
Most engines are large or inefficient, though, making them undesirable for interfacing with computers or other small applications.
To improve their prospects, Orest Symko and his team built smaller engines ranging from 11 to 18 centimeters long. At 40% efficient, the engines rival gasoline and diesel engines at energy conversion.
The team’s discoveries have also raised some eyebrows, however. "I realise anything to do with energy is really important these days," says Scott Backhaus, who studies thermoacoustics at the Los Alamos National Laboratory. "But we’re working on some applications for diesel engines, and I can tell you we’re not getting anywhere near 40% efficiency. I’m sceptical."
Tiny engines
The Utah researchers have also built the smallest known acoustic heat engines, which at 1.8 millimeters long could produce 1 Watt of electricity per cubic centimeter when clustered together. Symko speculates that the clusters could be used as the 'cells' in a new type of solar panel.
He plans to test the devices within a year to produce electricity from waste heat at a military radar facility.
“It looks very promising, but at this point there is still much work to be done. We’re still working on an array,” he says, adding that he hopes to begin mass-production of miniature engines within the next year.
If all goes well, they could be installed on natural gas and coal-fired power plants shortly thereafter. The team will present their research on Friday at the annual meeting of the Acoustical Society of America in Salt Lake City, Utah.