Temperature Gravity Wheel
The temperature gravity wheel is a heat engine. It is ultra low maintenance, and it requires low temperature difference.
More explanation about it in wikipedia: https://en.wikipedia.org/wiki/Minto_wheel
Some people build is as a hobby project. It works. Could be used for OTEC.
Working examples: A temperature gravity wheel, a high school project in Alaska. https://youtu.be/Z6AChkM3TkU
The temperature gravity wheel is also called the Minto Wheel. Other examples:
This is the one I (Spark2) built: https://youtu.be/gdmU6EEAnXI