Maybe the thought is that if you drop a 4000m-deep-to-surface pipe into Monterey Canyon, there will be a pressure differential at the base of the pipe (since se

Maybe the thought is that if you drop a 4000m-deep-to-surface pipe into Monterey Canyon, there will be a pressure differential at the base of the pipe (since sea water is denser than fresh water). I don't think the math works out all that spectacularly though. The pressure difference would be a about 1.1 MPa, and desalination of seawater takes ~6MPa, so by running a pipe down to the bottom of Monterey Canyon you could... save almost 20% of the energy costs. Which makes sense. You can't build a perpetual motion machine by sticking a pipe with a membrane at the bottom and a turbine at the top into the ocean. Even if it's a very long pipe.