The entropy of an isolated system always do what when natural processes occur?

Answer 1

increase

Answer 2

if you mix hot and cold water and get warm water the entropy is higher, even if the energy did not change. The availability of that energy is what changed. Between a glass of hot water and a glass of cold water you can run an engine. However there is not much you can do with a glass of warm water alone even though the amount of heat energy is the same.

Answer 3

This is the trouble with OTEC (Offshore Thermal Energy Conversion) where the difference in temperature between deep ocean water and water at the surface (in tropics of course) can be used to run a turbine system and generate electricity or whatever. However the efficiency is low because although the surface water seems warm and the deep water cold, the difference in degrees Kelvin is just not very much so the efficiency stinks no matter what you do.