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.