Fully agree, the only small remark being that it's
η = 1 - Tcold/Thot
and to put in perspective: If your metal rod on one side of the peltier element (that's inside the device) is at 150°C = 302° = 423K and the cold side (water reservoir) is at 20°C = 68°F = 293K the maximum achievable efficiency is 30%.
In practice you'll waste a lot of energy (probably the flame outputs about a kilowatt of power, normally you'd comfortably use it to heat a kettle of water or canned spaghetty or...) for a meager 2W or 3W of power to your cellphone. That's because the energy transfer from a flame to a simple rod is very, very inefficient.
η = 1 - Tcold/Thot
and to put in perspective: If your metal rod on one side of the peltier element (that's inside the device) is at 150°C = 302° = 423K and the cold side (water reservoir) is at 20°C = 68°F = 293K the maximum achievable efficiency is 30%.
In practice you'll waste a lot of energy (probably the flame outputs about a kilowatt of power, normally you'd comfortably use it to heat a kettle of water or canned spaghetty or...) for a meager 2W or 3W of power to your cellphone. That's because the energy transfer from a flame to a simple rod is very, very inefficient.