You can produce energy by disolving salt in fresh-water. That means there has to be a hard phyisical energy limit to desalinating water, otherwise you get a fre

You can produce energy by disolving salt in fresh-water. That means there has to be a hard phyisical energy limit to desalinating water, otherwise you get a free energy producing machine. The energy required happents to go into separating the bonds between the salt ions and the water molecules. Those bonds are quite strong, so it takes some energy to break them. The engineering problem is more one about capital investment. The price of water, vs the price of electricity are not what block desalination. It is the cost (and maintenance) of the machines that you need to earn back. That cost is what tends to make desalination un-economical. And that is an area where engineering has a lot of space to improve.