For harnessing ocean thermal energy, the temperature difference between water at the surface and water at depths up to 2 km should be more than

Harnessing ocean thermal energy needs the surface-to-deep temperature difference to exceed about 20 degrees Celsius.

A gap of only 5 degrees is far too small to run the conversion cycle.

A 10 degree difference still cannot drive an efficient plant.

Even 15 degrees falls short of the practical threshold.

This large gradient occurs mainly in tropical seas and powers the working-fluid cycle.

Warm surface water vaporises a fluid whose vapour spins a turbine, then cold deep water condenses it.