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."

Comments

But....

[a.phoenicis]

But does it change waste heat into electricity? I'm not quite sure based on that write-up...

Sound to electricity.

[snowraver1]

Now they need to refine this to 100% effiecency and attach one to my wife.

Efficiency as opposed to thermoelectric?

[Anonymous Coward]

I just skimmed the article, but I didn't see mention of the efficiency of this process. What are the advantages to converting the heat to sound first, rather than directly to electricity via thermoelectric processes?

Re:Efficiency as opposed to thermoelectric?

[cosinezero]

And from the looks of that giant glass pipe lit by a blowtorch, my money's on the researchers being 100% Wasted while thinking this one up, too.

Re:Efficiency as opposed to thermoelectric?

[wsherman]

This is demonstrably false. Beating Carnot's theorem does not imply 100 percent (or greater) efficiency. The 2nd law would still be preserved.

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).

Massive /. potential

[u-bend]

There's so much waste heat here (Star Wars, Linux, browser, KDE/Gnome debates), that we could power a city and rock out at the same time.

No efficiency ratings

[denis-The-menace]

How efficient is it?

With double conversions it couldn't be much.

Why not convert heat into electricity DIRECTLY using a peltier device?
(aka Seebeck effect http://en.wikipedia.org/wiki/Thermoelectric_effect )

Re:No efficiency ratings

[drinkypoo]

Why not convert heat into electricity DIRECTLY using a peltier device?

Because peltier junctions are themselves horribly inefficient?

Maxwell's Daemon Rides Again?

[overshoot]

Unless they're claiming to have found a way around the Second Law, the efficiency of any such conversion is going to utterly suck. My CPUs run less than 10C above ambient, so the absolute Carnot limit on any converter recovering that heat is going to be about 3%.

Why bother?

[1] Thermodynamics, not Robotics

Dog Whistle

[Apocalypse111]

Symko says the devices won't create noise pollution. First, as smaller devices are developed, they will convert heat to ultrasonic frequencies people cannot hear.

So now we've turned my car into a mobile dog-whistle, causing even the well-behaved dogs to bark at me.

Ooh, on the other hand, maybe we could get the sound into the frequency range at which various crystal wine glasses shatter... I've got some asshole neighbors who could do without those particular bits of glasswear.

Diaphragms?

[bigattichouse]

would it be possible to do something with a speaker? (as an experiment). I understand TFA about the piezeo devices being compressed/released by the plates vibrating like a flute, but I started wondering about the image that immediately popped into my head, of tuned diaphragms responding to air pressure differences to vibrate a coil... I guess if you did the flute thing, you could just put a piezo crystal between a tuning fork and a solid surface... every note at that frequency, especially if sustained, would then make power.... So, how about making great huge "moaning towers" out in the middle of nowhere that do the same thing? I'll call it "BULLROAR"(tm) technology. Hell.. I wonder if the forces involved on a bullroar spinning aroud your head might generate power (say, with a couterweight like thos rechargable watches). This idea is kinda fun.

Heat to Sound to electricity.

[malvidin]

Well, luckily my wife doesn't need to be loud. She's that hot.

Re:Heat to Sound to electricity.

[jbeaupre]

This is Slashdot, which means we have to double check whether you are actually married or what you mean by hot. Let's hope you mean online "wife" and not that you set your real wife on fire.

Re:Heat to Sound to electricity.

[pan_piper]

Well, luckily my wife doesn't need to be hot. She's shocking.

Re:Heat to Sound to electricity.

[StikyPad]

Well, luckily my wife doesn't need to be shocking. She's battery powered.

Just a little prob with the numbers....

[Ancient_Hacker]

Nothing to see here. It's just a Prof that's spent $2 million on a wild goose chase. Now with the great smell of fish! The rub is multi-fold:

• Good old Carnot's law. The efficiency is limited by the temperature drop across the device compared to the absolute temperature. Now take two thermometers, stick one up your butt and fart. compute the temperature difference. Divide by 483. That's your efficiency in converting heated gas into sound. Prolly about 0.005% as a rough approx.
• For a less gross example, pucker your lips and blow. Do this for five minutes or until you pass out. You probably feel warm-- that's the heat. How much acoustic power did you generate? Well a loud whistle is about 100dbA, about a hundredth of a watt. Efficiency, 0.004% at best.
• Piezoelectic efficiency. Well, it's really high-- for an acoustic transducer. The Interwebs seem to reveal no figures for this, and in general a high level of coyness is a way of hiding embarrasing numbers. Let's assume a best-case number of say 40%.
• The impedances. Crystals are very high impedance devices, putting out LOTS of volts at vanishingly small amps, which is bad news for us, as most of our power sinks are low impedance. Getting a few milliamps at 40KV is not very compatible with powering your laptop, which is about a million times lower in impedance. It's particularly inconcvenient converting tens of kilovolts downwards with economy and efficiency.

So sorry, probably much less than nothing to see here, just another bundle of our taxpayer's money spent on a totally pointless technical exercise.