Imagine the world’s most efficient engine design, Stirling Engine, running off the heat of your coffee mug.
When the heat expands the air, the air pushes up on the piston. When the air cools, it contracts, pulling the piston down. This heating and cooling moves the piston, making the propeller turn.
This engine needs only a 40 degree differential (either up or down) to run. So imagine an engine running off your coffee cup or your bowl of ice cream.
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