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I have a passing interest in water desalination projects. One of the challenges of desalination is simply moving the water from the ocean to the desalination plant. Typically the plant is built above the height of sea level, possibly by tens to hundreds of meters.

Is there a highly efficient mechanical method already devised, to use the weight of the waste water returning to the ocean, to lift more supply water to the desalination plant?

Since purified water is removed by the desalination plant, the brackish return water will be slightly lower in volume than the supply, and there will be losses from viscous drag in the pipes, so there's not a huge amount of energy recovery available.

But it seems like some useful percentage of energy saving / energy recovery should be possible, using the stored potential energy of the lifted return water, to offset the energy requirements of lifting the supply water.

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    $\begingroup$ Is the energy required to transport the water really at all significant relative to the energy required to do the actual desalination? $\endgroup$
    – Dave Tweed
    Commented Jun 5, 2016 at 17:51
  • $\begingroup$ Have you read any articles on the design and implementation of existing systems? $\endgroup$ Commented Jun 6, 2016 at 11:55
  • $\begingroup$ I have very little knowledge of water desalination plants, but it sounds rather inefficient to build one hundreds of meters above sea level! That notwithstanding, there are plenty of hydroelectric plants in the world, which use the potential energy of water at height to produce electricity. Whether one would be efficient at the scale of water volumes for a desalination plant - I have no clue. $\endgroup$
    – AndyT
    Commented Jul 6, 2016 at 9:42
  • $\begingroup$ @Dale: Are you confusing "as efficiently as possible" with "at lowest cost"? A simple reliable system may be more "efficient" in terms of running and maintenance costs. $\endgroup$
    – Transistor
    Commented Jul 16, 2016 at 15:53
  • $\begingroup$ Some variation of a hydraulic ram might be of use. $\endgroup$ Commented Jul 19, 2016 at 15:11

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Hydroelectric dams approximate your description.

Look at the below schematic diagram of a hydroelectric power plant. As you can see, the kinetic energy of the water flow spins a turbine which powers a generator to produce electric power. That electric power can be used to (partially) power a pump to pump the water to whatever elevation is required.

enter image description here

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You can construct two elevator shafts with the cables connected via a pulley. If you fill one elevator with with processed water to go down and fill the other one with fresh sea water, then you just need a small electric engine to offset the weight difference. This will be as efficient as you can get it in terms of wasted energy. But the capital costs and the low throughput make it rather unpractical.

You may find this a contrived example, but the losses of viscous drag that you mention yourself can be reduced to an arbitrarily low value by choosing a sufficiently large pipe diameter. It will cost a lot, though.

So, you need to specify what the capital cost is allowed to be for a given throughput.

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