Why is it not possible to put the tremendous amount of thermal energy in the ocean to work?

Question

The oceans contain a tremendous amount of thermal (internal) energy. Why, in general, is it not possible to put this energy to work?

This is a Physics concept question and I have no idea.

Answer 1

To transform thermal energy into work, it must flow from a hot reservoir (the ocean) to a cold one (perhaps deeper in the same ocean). The efficiency is small when the temperature difference is small. So, yes, there's a lot of thermal energy in the ocean but no practical way to extract it.