I think the energy needed to vaporize water goes up slightly as you reduce the pressure. To condense the vapor you need to remove energy and that represents a l

I think the energy needed to vaporize water goes up slightly as you reduce the pressure. To condense the vapor you need to remove energy and that represents a loss in a single stage system. You can use multiple stages each at a lower and lower pressure to increase the efficiency. Using the heat of condensation to vaporize the water in the next stage.