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."

Comments

So...

[Icarus1919]

Between the cell phones being charged by shaking them, by solar power, and by body heat - at what point will they be blowing up in our pockets?

Re:So...

[thegnu]

If we've got shaking and body heat down as energy sources, I think we've just discovered vibrators that run forever.

And with solar power, we have at least one argument to get girls to masturbate outside. :-)

Downsides

[Tablizer]

Slap Slap Slap - But m'aam, I just wanted to charge my cellphone and your breasts were handy.

Re:Downsides

[Joebert]

This will sound funny, but, that's perfectly natural.

I'm sure plenty of people here have at one point thought that way about your mom.

Re:Downsides

[taiwanjohn]

Hehe... "Hey baby, if I said you had a hot body, would you hold it against my phone?" ;-)

Snap Crackle & Pop

[Tablizer]

Between the cell phones being charged by shaking them, by solar power, and by body heat - at what point will they be blowing up in our pockets?

I claim dibs on Rice Crispies-powered phones

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.

I want to see the users manual

[Brett Buck]

I just hope the word "insert" doesn't appear in the instructions.

        Brett

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.

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.

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:Basic physics: no.

[sgartner]

You put a lot of thought into that message, but it is negated by just RTFA which says that the potential applications include "...personal power-jackets that could use heat from the human body to recharge cell-phones...". They aren't talking about the cell phone itself drawing the power, but potentially the entire inner surface of a piece of clothing (which would also have the entire outer surface of the piece of clothing as the cooling surface for differential).

However, this does sound like we will only be recharging cell phones this way in the winter...

Re:Basic physics: no.

[TheSHAD0W]

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

Re:Basic physics: no.

[Anonymous Coward]

You are using the wrong denominator, (T_cold), in your calculation, although it hardly matters in this case because the temperature difference is so small. The formula for maximum theoretical efficiency is:

(T_hot-T_cold)/T_hot

Using your numbers: (305-300)/305 = 1.64%

Re:Basic physics: no.

[david.given]

SO, the most energy we could possibly get out of this generation system is 0.6 * 1.7% = 10 milliwatts... This idea's dead in the water at the basic physics stage, before we even get to the engineering considerations.

Actually, 10mW is loads. You're not going to charge batteries off that or run a GSM phone, but it's more than ample for powering things like wristwatches, calculators or medical sensors; and with appropriate design, there's no reason why you couldn't build a PDA that worked at that kind of power level. Microchip make a 16-bit PIC that runs at about 1.3mW per MIP. Combine this with an eink screen which only uses power when updating, burst radio powered by a capacitor for low bandwidth data transfer, and a lot of static RAM (which has standby loads in the microwatt range), and you could easily come up with a basic but useful device.

Re:Basic physics: no.

[Ewasx]

You did not read the article, did you? Of course you didn't, this is slashdot!
The article is talking about using the technology in the clothes and using the energy from that to power a cellphone. This would give you a lot more surface. And I don't know where you live, but "room temperature" here is not so tropical. So the theoretical maximum according to your calculation would be more like ((305-295)/295) * 120 = about 4 watts. Not a lot, but my telephone charger can provide about 2.2 watts and my battery is charged within 2 hours with that...

Re:Basic physics: no.

[Anonymous Coward]

the most energy we could possibly get out of this generation system is 0.6 * 1.7% = 10 milliwatts ... but combined with a form of fusion, the devices will have all the energy they would ever need.

Oh wait!

Re:Basic physics: no.

[ColdWetDog]

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

However you are forgetting the first two, pre-physics stages: Marketing and Investment.

The hot air (and subsequent thermal gradient) from these stages should allow thermodynamics to power these devices for at least as long as it takes the stock to tank.

The inverse makes silent sub air-conditioning

[MichaelCrawford]

My dad was an engineer for a Naval submarine shipyard. He told me that thermoelectric panels attached to the inside of the hull cooled the submarine silently when a voltage was applied.

Hope that wasn't classified or anything - but then Dad passed away a while back.

Re:The inverse makes silent sub air-conditioning

[jamesshuang]

Thermoelectric cooling - it's how peltiers work. The effect has been known for a century... don't need to worry about classification! :-p

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.

reversible?

[TheSHAD0W]

Is this effect reversible? Could it be turned into a super-efficient Peltier module?

There is a better way.

[headbulb]

You are always moving. Most likely the phone is in your pocket moving with you.

Have you seen those ever lasting flashlights. Which use a magnet, a coil and a capacitor. The same concept could be used here. It might not keep the phone charged for heavy users but it could be a nice supplement to the charger.

Maybe if they really got creative they could reuse the coil and magnet as the vibrator.
OH and I didn't read the article. These are both interesting idea's but how much power can we really extract.

Personal Thermonuclear Generators

[sgartner]

How about using these in a water-heater sized device in your home. With a isotope heat source at the bottom, a coil lined with these strings, filled with some kind of heat transferring liquid (say, water). You could put one of these in every home and without any moving parts (as in a traditional thermonuclear generator with giant turbines) it would be very reliable.

Re:Personal Thermonuclear Generators

[jc42]

How about using these in a water-heater sized device in your home.

I've seen this suggestion any number of times, often with snide comments about how engineers have missed something obvious. This usually causes some engineer to simply mention that there's a better way. No matter how your water heater is powered, it's always more efficient to add insulation to the water heater, and use the fuel that you save to directly power an electrical generator. This skips the stage of extracting power from the water heater's heat loss, and can thus extract more electricity from the fuel (or use less fuel to generate the same electricity).

Unfortunately for such schemes, it's only practical to extract energy from a heat gradient if the heat gradient is going to be there anyway. Then, if the extra weight isn't a problem, you might be able to use some of the heat you're losing to produce a small amount of electricity "for free" (i.e., at no additional fuel cost).

Bring 'em on !

[Joebert]

I'm always up for a heated discussion.

Next up: Lap-powered laptops

[Jeff Jungblut]

So how soon until I can recharge my MacBook Pro with crotch heat? I'm thinking, maybe, perpetual porn machine.

Seebeck effect

[piotrr]

I thought the Seebeck effect already had a conversion efficiency of 2-3%. How can you possibly increase that a hundredfold without breaking the laws of thermodynamics?

Copper Top

[Layth]

"[Scientists] may have found a key increase the conversion efficiency by a factor of 100." Let me guess, this is involves some form of fusion.. and the machines have found all the energy they will ever need. This happened in a dream once, that I was so sure was real..