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Friday, 22 February 2013

The Stirling Engine and the iPhone

The Epiphany One Puck
Credit: Epiphany Labs
A couple of weeks ago I read about a Kickstarter project to develop a phone charger powered by a hot cup of coffee.  The device is nicknamed The Puck, or the Epiphany One Puck in long form.

Two things interest me.

Will it work?  I'm interested not just from an engineering perspective but also curious as to whether Kickstarter and other crowd funding platforms will ultimately end up pushing developments in engineering and technology or simply fund lifestyle projects with no realistic use, value or chance of success.

And the resurgence of the Stirling engine - invented nearly 200 years ago - used not just in The Puck but undergoing something of a renaissance of late, featuring in a variety of other micro-generation and transportation projects that have come to my attention.

How can a technology conceived of in 1816 suddenly come of age like this? A lesson perhaps for any inventor or entrepreneur hoping to make a fortune from a Killer App or other gadget.


If The Puck works - as in is capable of charging a mobile phone from a common domestic heat source - it could spawn a revolution in micro-generation which until recently has focussed on fuel cell technology.

The physics says it could work - but it's on the margins and a lot will depend on how efficient The Puck is at turning heat into electricity.

A typical smartphone battery stores around 18,000 joules (5 Watt-hours) or energy and is surprisingly efficient to charge - manufacturers claim 80-90%.

So we'll need around 5.5-6 Watt-hours of energy to fully-charge a phone.

A 350ml cup of boiling water has theoretical stored energy of 117,600 joules (32.7 Watt-hours) in a room at a temperature of 20 degrees C (weight of water in grammes multiplied by the specific heat capacity of water (4.2) multiplied by the temperature difference in Celsius/Kelvin).

So in theory the Stirling engine in The Puck need only achieve 17% efficiency to fully-charge a modern smartphone, and the laws of thermodynamics says the Stirling engine is - in theory at least (and yes, I do hold a Physics degree) - extremely efficient at turning heat energy into mechanical motion.


But that is sadly just theory.  In practice larger Stirling generators on the market have an efficiency of between 10-28% (example) without heat regenerators.

Running at a lower temperature difference means the theoretical thermodynamic efficiency of The Puck will be far higher (Carnot cycle), however the lower power output at lower temperature differences means mechanical friction will play a much greater role in lowering the overall efficiency.

A low temperature difference could also add to the engineering challenge in building a usable generator into a small space.

And then, in the efficiency equation, we have the dynamo required to turn the mechanical energy into electricity, and the voltage stabilisation circuitry to provide a clean 5v of power to your phone.

If I had to guess I'd say the manufacturers will struggle to top 10% efficiency in turning heat into 5-Volts of electrical power but this is just my opinion.  I've been unable to find any published claims by the manufacturers.


Still, that would charge even the most power-hungry phone to two thirds from one cup of coffee, which ain't that bad... Or would it?

When talking about the theoretical energy in a cup of boiling water I didn't mention leakage - heat lost into the local environment through a surface other than the Epiphany One Puck.

Heat rises - or rather the heat energy in the cup of hot water concentrates at the top due to convection currents in the water, and consequently far more heat is lost through the top of the cup than the sides or bottom, where The Puck sits.

And then there's evaporation.  A large percentage - it can be as much as 70% from a well-insulated cup - of heat energy lost from a hot liquid is via evaporation.

Without taking extra steps to insulate your heat source (coffee), using a lid to minimise evaporation and perhaps placing The Puck above rather than below your well-insulated cup you could end up losing 90% of the energy stored in the boiling coffee.

And now we're only talking about charging 6% of your phone's battery and being left with stone-cold coffee in the process.

And I haven't even mentioned a potential mismatch between the rate of cooling of your coffee and the power consumed by your phone.  My phone takes a couple of hours to charge from mains electricity - making insulation of the heat source used by The Puck essential over this time frame.


Yet I'm still quite excited by the project.  Coffee is only one heat source - there's always the sun (in some countries) and in cold countries it's easier to find a heater on e.g. a train in the UK than it is an electrical socket.

I like the idea of having a handy, portable, well-packaged Stirling generator even though I might need a dozen cups of coffee to charge my phone.  If Epiphany Labs can pull this off they'll have solved a number of engineering challenges that make projects like this very worthwhile.


1 comment:

  1. "How can a technology conceived of in 1816 suddenly come of age like this?"

    Easy.. too many science teachers make the mistake of focusing on the math and failing to engage the imagination of their pupils by showing them things like the Sterling Engine. Even today I still think they're amazing but a lot of modern classes are all about charts, statistics, reactions (sleeping material for the brain) and not enough practical science anymore.

    As for the Puck, well I agree with more or less everything you said. I view it as more of a novelty because realistically you'd need to drink a lot to recharge modern smartphones. On the other hand it's rare that solar panels actually come out as a better solution than something :).



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