205 problems carry the numbers don't work

The solution is real or clearly buildable; the economics kill it. Desalination works but costs too much for the people who need the water. A special case worth knowing: a design choice made upstream for good product reasons — stretchy fibers in clothing, glued-in batteries — creates a heavy cost downstream for whoever has to recycle or handle the product, and that downstream party has no way to send the bill back to the designer, who therefore has no reason to change. The fix here is usually a new business model, price signal, or policy channel, not a new machine.