Body Heat Could Charge Your Cellphone

An anonymous reader writes to mention Nature is reporting that scientists have discovered a much more efficient way to use silicon to convert heat into electricity. This offers the possibility of many different applications including possibly charging your portable electronics just by wearing them close to your skin. "The concept of converting waste heat into electricity isn't exactly new, but it never really materialized due to efficiency hurdles. Now, scientists from the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley think they may have found a key [to] increase the conversion efficiency by a factor of 100."

Why just cellphones?

[ivan1011001]

Think bigger. Put these and some batteries near your brakes or under the hood of your car. Line the Shuttle with them, attach them to the boosters that get jettisoned. Tape them to politicians so all the hot air charges them.

Needs a temperature gradient

[the_other_chewey]

Note that this of course does also need a temperature gradient to work. This is no magic electricity creating cooling device.

Basic physics: no.

[goodmanj]

Paging Dr. Carnot, Dr. Nicolas Carnot, call for you on line 2...

Human body thermal output is about 120 watts on average, skin surface area is 2 m^2, so a 50-cm^2 cell phone body can intercept 0.6 watts of body heat. BUT, the laws of thermodynamics place a limit on how much of that heat can be converted into useful work to charge the batteries in the phone. That limit depends on the temperature of the heat source and sink.

Suppose one side of the phone is in contact with your skin at 32 C (305 Kelvin), and the other side is in contact with room-temperature air at 27 C (300 K). (In practice, the temperature difference will be smaller, because the air near your body will be warmed above room temp.) The maximum efficiency one could get from these thermodynamic efficiency is (305-300)/300 = 1.7%.

And that's the theoretical maximum possible conversion efficiency. Real systems rarely come close to that.

SO, the most energy we could possibly get out of this generation system is 0.6 * 1.7% = 10 milliwatts. My iPhone's battery holds about 2400 mW-hours of juice, so if I installed this charging system and held it against my skin 24/7, it would take about 10 days to charge in the theoretical best case... and in practice, much longer than that.

This idea's dead in the water at the basic physics stage, before we even get to the engineering considerations.

Re:Needs a temperature gradient

[rmauger]

Right, I'm not a scientist, but TFA talks about all sorts of fantastic things (e.g. charging your iPod with body heat) that aren't mentioned in the Abstract of the cited piece in Nature. While the article does quote one of the authors for such claims, what the Nature article Abstract (I'm not paying $ to read stuff online) alludes to is improving the efficiency of potential co-generation from sources of waste heat from combustion--power plants, car engines, etc. Besides, if it's cold enough out that I'm wearing a winter coat, do I really want to shove a heat sucking iPod down my pants? Not really. But sticking it against my car's engine? Sure.

Also TFA mentions "...the discovery will depend on whether these rough nanowires will be efficient enough to make commercial sense." So yeah, I don't see it (soon) being any more energy efficient to manufacture heat converting nanowires for my iPod Nano than it would be to say, construct a battery. But maybe they have something that could better convert a massive heat gradient into electrical energy cheaper for the purposes of co-generation. Who knows, maybe even cut down on heat pollution.

Re:Basic physics: no.

[TheSHAD0W]

To defend GP, the Slashdot article DOES imply the electronics would only charge themselves.

Re:The inverse makes silent sub air-conditioning

[jc42]

The effect has been known for a century... don't need to worry about classification!

Actually, at least here in the US, you probably should still worry about it being classified information.

For example, various historians have mentioned that the Rosenbergs were executed for giving "secrets" to the Russians that apparently were available in a number of college-level physics textbooks at the time.

Decades later (but still a few decades before today), I did an end-of-chapter exercise in a physics text that was something like: Using equations E and F from this chapter, and table T in appendix A, calculate the critical masses of the following isotopes .... There was an asterisk at the end, and the footnote said that telling any of the answers to a non-citizen was a felony under US law and listing the possible penalties (which included execution).

The US government's security agencies don't consider previous publication in school textbooks to be a restriction on their right to classify information.

For another example, google for RSA encryption. I have one of those t-shirts that has the 4-line perl implementation of RSA, and on the back "Warning: This t-shirt is a munition" plus a reference to the appropriate regulation. I never had the nerve to wear it to the airport on an international flight, though I did wear it to a number of techie meeting where there were non-citizens. I kept wishing someone would get arrested for wearing one, since the trial could have been entertaining. But I suppose now they wouldn't bother with a trial; you'd just disappear to an undisclosed location in an unstated country for a few years and then dropped off on a hillside in Macedonia when they're done with you.