Can Desalination Proceed without Electricity?

05Mar13

It always seems like desalination (i.e., removal of salt from sea water to make fresh water) is treated as a power problem.  It takes a lot of electricity to do that.  But I was wondering, can this be done without electricity?

I know this has surely been considered by many engineers.  But my search didn’t point toward an obvious writeup; so for now, here’s the scenario.

In brief, the concept is this:  Pacific Ocean tides are used to drive a pump, just as the old farm windmills drove pumps to bring water to the surface.  The pump lifts seawater to a pipe that flows inland from the coastline north of San Diego, toward Salton Sea.

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As its name suggests, that sea is no stranger to salt.  Moreover, it is below sea level.  Hence, the pipe would not have to be very high, except to reduce the amount of tunneling necessary to get the water through the mountains.  Some additional power would probably be needed to facilitate the water flow.  Some of that power might come from turbines utilizing the fall of the water toward the desert, on the east side of the mountains.

An alternate approach would be to bring the water up from the Gulf of California, in northwestern Mexico.  It is not much further away, and there would not be mountains in the way.  I do not know whether tides in the Gulf would offer pumping power comparable to that of the Pacific.  Presumably there would be both advantages and disadvantages of making this an international project.

The purpose of sending the water toward Salton Sea would be to take advantage of the inland heat to aid in evaporation.  This would be the other mechanical element of the scheme:  it seems possible to use sunlight rather than electricity to separate water from salt on a large scale.

Seawater, poured directly into Salton Sea, would likely damage its ecosystem.  Evaporation would also leave salt behind.  It would presumably be better to develop a separate system to process the evaporation.  That system could perhaps begin the evaporation process long before the water reached the desert floor, using heat directed toward relatively shallow flow channels along the way, like the coils in an air conditioner.

There may incidentally be some mix of salt and fresh water that could be shunted on to Salton Sea, to reverse its long-term process of drying out, and the environmental ills that may follow from that.  Overall, this evaporation scheme would have a seasonal element; perhaps the system could be constructed so as to accommodate an entire hot season’s salt buildup, and the cooler months could then be used for maintenance.

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