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Turning Heat Into Sound Into Electricity
WrongSizeGlass writes "Science Daily is reporting on work by physicists at the University of Utah who have developed small devices that turn heat into sound and then into electricity. 'We are converting waste heat to electricity in an efficient, simple way by using sound [...] It is a new source of renewable energy from waste heat.' They report that technology holds promise for changing waste heat into electricity, harnessing solar energy and cooling computers and radars."
Why bother?
[1] Thermodynamics, not Robotics
Lacking <sarcasm> tags,
http://www.physics.utah.edu/~woolf/acoustics/
It would be interesting to hear all the questions there. I imagine yours will be handled pretty well.
Obviously the conversion to sound can't beat Carnot's Theorem, and it says in the article it doesn't start until there's a temperature gradient of at least 90 degrees F. In other words, it's not very efficient.
I was trying to figure out where I had seen something like this recently:
- stove-generator-refrigerator-combo-aimed-at-develo ping-nations.html
http://arstechnica.com/news.ars/post/20070527-new
this thing also uses thermoacoustic technology.
Why, yes I have been touched by His noodly appendage. And I plan to sue.
you need a significant heat differential as well as the fact that AC needs fast dissipation to work.
Ac coils need to shed that heat fast, even faster when the ambient temperature is up there like 100degF (I hope you mean 100F and not 100C) This process relies on a wider temperature differential and not shedding it fast.
so it will not work in most places where waste heat energy recovery would be a benefit.
Do not look at laser with remaining good eye.
This IS a stirling engine. Specifically, a modification of the thermoacoustic stirling engine originally developed at LANL.
What are the advantages to converting the heat to sound first, rather than directly to electricity via thermoelectric processes?
Current thermopiles are pretty inefficient. The main problem is that they unavoidably leak heat from the hot to the cold side. In peltier cells (the ones in those cheap "coolers" and CPU heatsinks) leak several times as much heat as they make use of when running as generators (and leak most of the heat they pump, so they have to pump it several times to get it dumped). There's a more efficient one in the labs, which doesn't have a lot of charge (and thus heat) carriers in the hot/cold bridge. But it's still far from perfect.
They also have to operate at temperatures that don't destroy their materials - typically semiconductors. That limits how hot the hot end can get, and thus how much energy you can get out of the heat (since they can't break the carnot cycle rules).
These devices are gas-working-fluid heat engines, with the gas (and the piezo power takeoff) as the only moving part(s). In principle the gas "prime mover" should be able to approach carnot cycle efficiency (which is as good as you CAN get) - and that's what this group is trying for. Being made of gas and metal, the "hot end" can get very hot, too, so you aren't as limited as with semiconductor heat converteres. Meanwhile, piezos are extremely efficient as well - and some (like quartz) can also handle very high temperatures.
As simple mechanical systems they should also be easier to fabricate than semiconductors, making them a garage-shop item that doesn't require your garage to be a clean-room in silicon valley with 100 megabux of specialized equipment.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
Just read about piezo efficiency the other day. 60% is pretty common for polycrystalline. 90+% for single crystal. I'll hunt down the link if you're dieing to read about it yourself.
The world is made by those who show up for the job.
Let's say you have a heat reservoir (e.g. a coal fire) and a cold reservoir (e.g. a cooling tower). You could just let the heat from the hot reservoir flow to the cold reservoir with nothing else happening. You could also set up a steam engine so that the flow of heat from the hot reservoir to the cold reservoir caused some of the heat to be "converted" to mechanical energy (or electrical energy or something equivalent). Now, ideally you would want as little heat as possible to flow between the reservoirs with as much heat as possible being converted to mechanical energy. Carnot's Theorem places an upper limit on how "efficient" this process can be. Basically, the smaller the difference in temperature between the two reservoirs the more heat will flow between the reservoirs and the less heat will be converted to mechanical energy.
Let's now consider a different scenario. Suppose you have some mechnical energy (e.g. some electricity) and you want to create a temperature difference between two heat reservoirs (e.g. you want to air condition your apartment). In this case, you want to do as little work as possible (keep the electric bill low) while moving as much heat from the cooler reservoir up to the hotter reservoir (moving the heat out of your apartment). Basically, you want to minimize the "conversion" of mechanical work to heat while maximizing the flow of heat between the reservoirs. Carnot's Theorem also applies here. You have to do less work to move heat between reservoirs that are at almost the same temperature and you have to do more work to move heat between reservoirs that are at very different temperatures.
For the second part of Carnot's Theorem, imagine that you found one (reversible) process where there was a lot of heat flow between the reservoirs for a given amount heat-work conversion and another (reversible) process where there was very little heat flow for a given amount of heat-work conversion - assuming the same temperature difference between heat reservoirs for both processes. You could hook these two processes together and have a perpetual motion machine of the second kind.
To put it another way, if you could find either an air conditioner or a power plant that was not limited by the Carnot Theorem then you could use your air conditioner to generate the temperature difference to run your power plant and you could use the electricity from your power plant to run your air conditioner all while having electricity left over to power your television (i.e. you'd get free energy from your power plant - no more having to burn coal).