h a l f b a k e r yThe embarrassing drunkard uncle of invention.
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A project in Boston nicknamed 'The Big Dig' has been in the news lately because of the numerous leaks that have sprung up in the huge tunnel they built beneath the city.
Down here in Tampa there's a de-salinization plant that's costing a lot more than was projected due to unforseen design problems.
If
we put the two troubled projects together there's an idea that England and France might be able to use if water shortages are ever a problem - enhance the Chunnel so it can double as a de-salinization plant. It would surely be more cost-effective than the plant they've built in Tampa. For one thing, saltwater wouldn't have to be pumped into the plant. The water pressure above the Chunnel would force it down into pipes added to the Chunnel. The pipes would have filters on them, using reverse osmosis to convert the saltwater into freshwater. Other than the pipes, all we'd need to do is build a pumping station on each end of the Chunnel to pump the freshwater out (via new pipes that were built into the Chunnel, not the Chunnel itself of course).
If the intake pipes were put near the center of this system then we'd also have a more economical way to get rid of the salt that's been extracted. Periodically the intake valves could be reversed and the pumping stations re-directed so water flowed backwards to flush the system out of the system. Then ocean currents would carry the salt away to keep piles of it from building up near the pipes.
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I am thinking that pumping the fresh water up from that depth would take as much energy as pumping seawater through a filter on the coast. |
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FJ - It might. I'm a generalist and not a specialist though so when it comes to details I have to rely on outside help to do the calculations. One thing to keep in mind - pumping it from the coast requires more than just movement, it takes force to get the water through the membranes. That part of the system would be taken care of by gravity in this case, as would moving the water through the pipeline along the Chunnel. The only time we'd need to add energy would be when the water reached the terminal and had to be lifted back up above ground level. |
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Could not the pressure force the freshwater upwards or am I misunderstanding how desalinisation works? |
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half - In this case the pressure would all be exerted in a downward direction. If the total diameter of the intake pipes was half that of the horizontal pipes, and the pumping stations drew water out at the same rate that it entered the system, then there'd always be more room in the horizontal pipes for water to fill in (preventing any backwards pressure from building up). |
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There was another idea posted here a while back, dealing with free desalinization powered by the difference in density between fresh water and salt water. It had some interesting calculations already done. |
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Basically, if the water is deep enough, you'll have enough pressure head to drive the process without a pump. As the water is forced from the bottom by pressure, the pressure head across the membrane decreases due to the pressure in the fresh water draw-off pipe. Eventually, there is not enough pressure across the membrane to sustain the process. |
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In a situation like this, you WILL need just as much energy to pump the fresh water up out of the chunnel as you would to pump salt water through the filters at ground level. There is no net benefit. |
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Interesting idea, but sorry, won't work. |
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freefall - Where would the pressure in the fresh water draw-off pipe come from if the water in that pipe was pumped out by the lift stations as fast as it came in through the intake pipes (and the horizontal pipe had a much larger diameter than the intake pipe)? No water could back up in the horizontal pipe so where would the pressure against the underside of the filters come from? |
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//It had some interesting calculations already done// and they were wrong. |
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This would work, if the head pressure above the membranes was high enough. |
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The freshwater reservoir below would fill back to sea level at worst, nullifying the head differential across the membrane. This can be easily pumped away, to maintain the differential. |
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Whether it takes energy to pump the fresh product away is immaterial. Most fresh water reservoirs need to be pumped, to attain working pressures. |
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Have the fresh water filter into something similar to the custard speedbumps, and have the gravitational force of cars running over them force the water through a unidirectional valve to the surface. |
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I thought about that, but it increases energy consumption in the tunnel, increasing pollution. |
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Perhaps a scaled down version of a speedbump? In a long enough tunnel and with an average throughput, I think small acquarium-type piping could be conjoined in manner that would provide a positive net effect at a lower cost (than an actual foot high speed bump every hundred feet.) Conservation of energy can't be ignored, but hey, there's probably an optimal solution waiting to be calculated. |
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You're probably right. Is there any known material that could be used for this purpose? Perhaps a layer of durable geotextile, sandwiched beneath a rubber underlay and the rail bed? As each train passes over the layer it pushes the water up the gradient. I'm unsure of the gradient in the Chunnel. |
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Given that the deepest point of submersion of the Chunnel is about 60 metres, it would be possible to attain an operating pressure only 6 atm. or 84psi. This would be a low pressure operating system, so would be less productive than a high pressure system. |
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The other factor that would affect performance is the turbidity level at that depth of the channel. This will be affected by shipping movements and any particulates stirred up would have to be prefiltered to about 5 micron size, I think. |
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freefall is right: there is no savings in pumping energy using this system. The plan essentially suggests putting a filter at the bottom of a hole to save pumping the water up to a pressure-head sufficient to push through said filter. The key word there is "up". All the effort expended to pump the fresh water up out of the hole will equal the effort that could have been expended in pumping saltwater up to pressure.
That said, there are some slight advantages to handling freshwater rather than salt, but absolutely no advantages to putting any water into the Chunnel. Try this system somewhere else, if you feel you must. |
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Wasn't there a recent idea about
a commuter- whitewater-rafting-
using-
a-water-filled-subway- system kind of
a
thing? |
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Could the two be combined here? |
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