Analysis of hot-coffee powered iPhone charger

The Ephiphany One Puck looks like a hockey puck with a USB cable. It's got a Stirling Engine inside that operates on the temperature difference between a hot or cold drink placed on top of it and the ambient temperature. Can it really charge an iPhone?

Edward Vogel has looked into it and posted his analysis in the comments section of the Kickstarter campaign. It sounds like it's more of a fun curiosity than a practical device. UPDATE: Edward adds, "I do still think the device is 'practical' in the sense that it would conceivably 'capture' waste heat throughout the day and act as a charger when its internal battery is charged. If the price came down to $50 and did the same thing my $20 'extra' battery does along with the waste heat recovery thing. That would be practical."

The Carnot efficiency is about 8%.

The calculator I used includes working gas shuttle losses and other considerations. To really get 5 Watts out of the engine it would require a heat source of ~ 100 Watts. This is not what a single cup of coffee is going to provide however this still can work because (The Epiphany dude has some comments further down that "explain" some operating requirements):

1. You use as a coffee coaster it throughout the day and it charges a battery the puck's internal battery.

2. Use could use a tea candle if you really want some serious 5 Watts capability

3. My own thought - they will also charge the battery by motion activation from walking like a motion powered watch

My impression is (I am buying one) is that this product will not charge your phone directly from the energy of a single cup of coffee but it will largely be recharging from via the battery pack. It will be doing something cool whenever you plug it into a smart phone that you can view with an app. It is kind of like having a model LTD (low temperature difference) Stirling engine as a science demo/toy but this one you can take with you and demonstrate/experiment with thermodynamics.

Anaysis of Epiphany One Puck


  1. I don’t see how you’re going to get any useful amount of energy out of this thing unless you’re a 12 cup a day kind of coffee drinker.  The power available and efficiency are just too low.  It would probably work a lot better if you put it on top of the coffee cup instead of below it at least, especially since that way the cool side won’t be mashed up against your desk and unable to vent heat adequately.  If the batteries it uses are specifically designed to have very low power loss over time, and the circuits are also carefully designed to isolate the battery as much as possible, then it might be possible to charge your phone after several days and several cups of coffee, but overall this looks fundamentally flawed. 

    It’s like those solar phone chargers that try to charge an iPhone off of 8 square inches of solar panel. There is some power there, but it’s so weak that you need extremely efficient storage and a lot of time to make it work at all.

  2. ???

    A countertop is an insulator. This thing needs a big aluminum CPU cooler under it to work at all efficiently.

  3. So it will (but probably won’t) charge your phone while you sit there, at your desk, in front of a computer, which is 100 percent capable of charging your iPhone. Why would anyone pay for this stupid thing?

    1. Maybe it could be used for camping?  You point one side towards the fire, the other is facing the cool night air.  I’m not sure why you would want your phone when you’re out camping though.  Maybe for hikers that are out for many days at a time and want to recharge their GPS at night? 

      It makes absolutely no sense if you have wall power available though.  Cell phones use only pennies worth of power in their entire lifetime, so it will never pay for itself and it will always be worse than a tiny usb charging brick in just about every way. 

  4. I must agree that it’s pretty pointless and almost certainly won’t work in any usable way. 

    But I don’t fail to see that it’s a very cool device. It’d be fun. Android phones can tell you precisely what voltage etc. they’re receiving through the USB port (and iPhones probably can too), and if they provide a special app to use with it, all the better. You can experiment with different heat sources or whatever and learn about the science behind it.

    Of course, there are other ways to do that, as have been featured on BoingBoing and can be found for pretty cheap that are more viscerally exciting – but you don’t get easy-to-use data, you get to watch how fast or slow the wheel spins. With a digital device like this, you can do some much more interesting stuff. I bet you could even use it in Arduino projects and the like – mostly just for fun, of course.

  5. Heh, nice. Because the appeal is how it works (else you’d just use a peltier device) I think the case really wants to be transparent to display the cool workings inside.
    Most transparent materials would work against the functional need for the case to dissipate heat, but perhaps a heatsink could be used that looks like a tiny stand for it to sit on? Like a miniture mug-sized coffee table. Coffee table – geddit!? [sorry] :)

  6. Cmon, this is a joke.  Even allowing that this could work efficiently (which it doesn’t), in what situation do you have a brewed cup of coffee that you do not have a power outlet nearby? Save this for 4/1/13.

    1. Outdoor seats or tables.

      (Edit: assuming it goes on top, as was suggested. Outdoors you ‘re not likely to have a heat-conductive mug, so that wouldn’t be a thing either, as it stands.)

  7. Someone should calculate how long it’ll take for this thing to pay for itself… considering the iPhone costs under $.50 to keep charged for a year I’ll ballpark it and say roughly 500 years. 

    Sterling engines are pretty neat pieces of engineering though, I’ll give them that. I think a lot of people are mistaking this as a practical piece of technology (which the kickstarter page also seems to do).

  8. Let’s not forget an unpleasant side-effect: your coffee will become cold faster.  If the device drains heat from the bottom of your cup to power your toy phone, then the bottom of the cup will cool faster, which creates a heat delta between the cup bottom and the coffee, which will cause the coffee to cool as transfers some of its heat to the cup bottom.  That whole conservation of energy thing, you know.  Or, as my hero Robert Heinlein would point out, TANSTAFFL…

  9. I agree that it makes more sense to have it rest on top of the cup than on the bottom to be effective. But as for its overall efficiency, who’s under the illusion that it’s supposed to compete with an electrical charger? This is clearly meant for a “something is better than nothing” situation (besides that it’s also clearly meant to be a novelty).

    That said, the price point needs to be pretty low for it to satisfy a whim.

  10. Come on guys, use your brains.

    From the project page:

    “…..Say you are out for a night at the bar and your phone battery dies. Rather than awkwardly moving to a corner to plug in your charger, or leaving altogether, you can pull out the Epiphany onE Puck, power it up with that cold beer you’re holding, and charge away! ….”

    Think about it. If you suck energy from a cold beer, what happens to it?
    That´s right. It will get even colder ! Way!!!!

    I´m backing this, it´s the coolest invention since liquid nitrogen…

  11. I’m wary of moving my hot coffee around near an expensive electronic device. The only thing that could make me warier is the dumb idea of attaching the two with a cable.

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