Hey there!  You know how, each morning, you have a cup of hot coffee and then mess around on your phone, reading Facebook updates or whatever?

And you know how, every time you do that, you wish that your phone was simultaneously being charged and that your coffee was getting cold faster?

No?  What’s that?  You’re saying that your phone is already charged because it’s morning, and that you’d like your coffee to stay warm longer?  WELL THEN DO WE HAVE THE PRODUCT (not) FOR YOU!  This is called the onEPuck, made by Epiphany, because names are hard.

It’s a tiny little Stirling engine in that puck there, which turns a piston that charges the phone.  A Stirling engine is a form of heat engine, which works by exploiting the flow of heat from a hot area to a cold one.  A Stirling engine specifically works when heat expands gases in one chamber, which then cool by way of a heat sink into a cooler area.  The cooler gases contract, powering the other half of the rotation, and then the heat expands them again and the cycle continues.  It is impossible to find an animated gif that actually helps you to understand this process, so I picked a pretty one instead.

So the physics is sound.  The problem lies, as is often the case, in the quantitative.  Is there in fact enough heat in a cup of coffee to power a Stirling engine that will charge a phone?

As it turns out, there is!  Isn’t that nice?  Yes, there’s roughly 31 watt-hours worth of “extra” energy in a 12-ounce cup of coffee.  By “extra” I mean energy responsible for it being hotter than room temperature, since once it’s cooled to room temperature, there will no longer be any transference of heat and the engine will stop running.  And the charger claims to be able to generate about five watts of power, which would give you several hours of charge time.  Not bad.

But there’s a problem.  Stirling engines aren’t 100% efficient, because nothing is.  The best efficiency you can hope for (and in this case, efficiency is measured in terms of heat energy lost over electrical energy generated) is around 31%.  So that means that you’re actually only getting an hour of charge.  And that’s what you get if you suck ALL the heat out of your coffee.  If it’s only running while you’re drinking the coffee, then you’re getting a charge for the amount of time it takes to drink your coffee.  Maybe 20-30 minutes.  Not super useful.

That’s not the issue.  The issue is from their Kickstarter page.  On it, they go through the usual FAQ like device compatibility, power output, how long it takes to charge, etc.  But then they assert that this simple device will change the world by adding it to their solar water heaters.

This is a problem.

You see, the engine will have to extract heat from the hot water in the solar heaters, which will make them cooler, which will defeat the point of heating it in the first place.  And the more electricity you’re getting, the less hot the water is.  And of course, the energy transfer, as I have repeatedly pointed out, is less efficient than just using the sun to power a heat engine in the first place.  This is, like many ideas, adding an unhelpful middleman to the whole “generate power” problem.

So, is this a cute toy?  Yes.  Is it functional?  Yes.  Is it going to change the world?

One thought

  1. On camping trips you could use it to charge your phone by placing it over a candle or something. Or run off the hot water from the campfire.

    I dont think it will be cooling your coffee quicker, it will mainly just be getting some work out of the heat that is already escaping unless you are comparing it to an insulated coffee mug..

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