The principle of nuclear fusion is the conversion of matter into energy. Current safe methods of using nuclear energy involve converting nuclear energy into other forms of energy for reuse.
Other principles of energy production: chemical energy can be converted into electrical energy, which is also a commonly used principle of energy production, such as batteries, such as electric eels, and temperature ; the differences can be converted into electrical energy, as I have only heard of the principle, but I don't know if such a battery has been developed yet.
I hope this can be useful to you.