You'll never win by piping cold, denser water up a very long pipe. There's a reason it's way down there, and the potential energy difference is lost when one attempts to bring them together.

Instead, pump a much lighter (ammonia-based) liquid down there to be cooled, returning it to do work in the energy cycle.

The theory is that by using a substance with the approximate same ability to transfer heat/cold, but with less mass, you've beating the reason the water's there in the first place.

Only then will one extract energy from this scheme.

The alternitive line of thought would have us all install energy-generating electronics in the walls of our houses to generate electricity from the difference between the warm inside and cold outside so that we could use that to...

...heat our house.

Uh... Insulation is far more efficient.