> (Boiling water is very energy intensive ...

Only if you do it using a low technology approach:-).

For systems that need only 100C water and not steam you simply 
counterflow the output water against the input water. Energy is 
required only to make up thermal losses. Counterflow heat exchangers 
are one of the great under-developed marvels of our age :-)

If you want steam (distillation it's only slightly more complex. If 
you use pressure or vacuum to slightly alter the boiling point on the 
inlet and  outlet sides you can transfer almost all the latent heat of 
vaporisation back from the output to the input. You need then only 
make up losses as before.


        Russell



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