Essentially, desal turns energy into water. You’re right that mass media has this tendency to write stories about breakthroughs, but they’re either tiny incremental improvements, or baloney.

In places that have almost no other water, like Israel, it makes sense. But if you have sources of water that cost half as much, it’s often cheaper to save a gallon of cheap water than it is to generate a gallon of desal water. So in places like Santa Barbara or northern San Diego County, it’s more like a last resort, the last water source they draws upon.

Desal is, at its heart, forcing water through a very fine filter, a filter so fine that it lets little more than water molecules though. The sort of pressure you need to force water through the filter is like pumping the water to a tank on a 1500’ tower. Which is doable, but at the scale of a city of 100,000 people it would be crazy expensive. At least compared to other cheaper sources of water.

There are consumable costs to an RO desal plant too, such as the filters themselves. They have to be replaced after a while and they’re not cheap. It’s great if those costs come down but they’re marginal compared just to the energy required to filter every single gallon of water. And that cost is really set by the global energy market.

Distillation and filtering are both energy expensive. So is electrolysis. If it weren’t we could produce hydrogen gas cheaply, run hydrogen fuel cells to generate electricity, and generate water as a waste product. There’s cheaper sources of hydrogen though, and a fuel cell bus emits just a tiny trickle of water from its exhaust. So if you try to use electrolysis to generate hydrogen to run a hydrogen fuel cell to power your electrolysis, it’s not going to give you free energy. It takes as much energy to break those bonds as is released when you form them.