coachgeo,

You idea about drawing a vacuum to lower the boiling point is correct. We used to use it on the supply boats in the Middle East to de-sal salt water. The thing is that it does not save you any energy. You have to pay for the energy to draw the vacuum and raise the temp. The amount of energy used is the same whether you raise the water to 212 deg. or to 180 deg. and draw a vacuum. We had free vacuum and heat off the Diesel engiines on the boats. Even so cost was not a problem.