Some of the softer Science Fiction has its new technology powered by something rather...different to plain old electricity, nuclear power, Phlebotinum, Functional Magic, or petrol. Depending on the series, this can be either for something Powered by a Forsaken Child, or a source that gives power to no (or little) ill effect. Just to be clear, if the fuel source exists in Real Life, it isn't an example. Mysterious new substances and energy sources also don't count. Compare Magitek, which is usually fueled this way.