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https://www.thedrive.com/the-war-zone/30256/scientist-behind-the-navys-ufo-patents-has-now-filed-one-for-a-compact-fusion-reactor
Stuff like this already exists, but yeah I guess its inaccessible.. for now
Crystal batteries can run seemingly forever
Waterpowered things have been suppressed for decades and I've seen devices using magnets work for generating power
At one point I read about some alien craft supposedly using vaccum generators (they create a suction and the suction makes a potential difference which powers stuff) I don't know how that works but its an interesting idea.
For a bunch of interesting but not too difficult to make power sources, see; https://www.youtube.com/user/lasersaber/videos