It's in A Closed and Common Orbit: "By then, your onboard generator will have harvested enough kinetic energy to keep you going. You’ll be able to power yourself by that point. Unless you spend several days completely motionless in bed, you’ll always have enough power."
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I'd joke that if as a kid I'd mentioned a self-powered machine like that to my father, he'd have beat me until my ears leaked brain matter but what he would have actually done was make me explain why it couldn't work. Violence was how he taught math, not thermo. (con't)
Something similar came up on a road trip: why can't you make an engine that works by turning liquid water to ice, and then using the heat liberated to power the engine?