Why Might an Ocean-Based Thermal Energy Scheme Fail to Be Sustainable?

[dsfranca]

Hi guys, I saw this problem today and the solutions was not obvious to me, especially because I believe it is not clearly stated.

Homework Statement

Consider a scheme in which thermal energy is extracted from the ocean. Some of the extracted energy is used to perform mechanical work(run the ship) and the rest is discarded back into the ocean. Why will this not work?

Homework Equations

The Attempt at a Solution

Thank you!

 

[dsfranca]

Just got a solution. It needs two energy sources, one hot and one cold. Should have thought more before posting it here...
Sorry