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.