I am listening again to a PDC taught by Bill Mollison in 1983, and at some point he talks about Solar Ponds
The concept is relatively simple, normal water circulates top to bottom (hot water rises, gives off heat to enviroment by evap, sinks, etc). Salt water has another property though, the higher the salt content, the higher the density of the water. The more dense the water, the more hot it can get. In normal cases this water would rise, but because of the increased salinity, it cannot mix with the less salty water on top, effectively locking it to cycle in the bottom. Using this method you can reach water temperatures between 70 and 90 degrees C (158 to 194 F)!!! This can then be used to heat homes, refrigirate, heat showers, desalinate water and last but not least: run generators. Can you say Sterling engine?
It seems to me that this is another one of those hugely applicable things that never got around to being developed, or perhaps even suppressed, because of the oil boom. Something like the Trombe airpressure system, but without the need for falling water