Temperature Gravity Wheel
The temperature gravity wheel is a heat engine. It is ultra low maintenance, and it does not require big temperature differences. The temperature gravity wheel is also called the Minto Wheel.
More explanation about it in wikipedia: https://en.wikipedia.org/wiki/Minto_wheel
Some people build it 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
This is the one I (Spark2) built: https://youtu.be/gdmU6EEAnXI