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B Getting energy from deep ocean pressure

  1. May 21, 2017 #1
    Let's say we would take a pipe made of some very strong material, and connect it to the ocean at the deepest level possible. For example 500 meters.

    Then we would dig a huge area on land, again, as deep as possible. Lead the pipe to the lowest level of this huge basin we created and then use the pressurized water coming out to generate energy.

    Part of the energy will be used to turn the water into steam (so our hole never fills up completely), among other methods like possibly having arrays of mirrors pointing towards the basin, or choosing an area with volcanic activity where the used water is lead into to generate even more steam.

    We would be getting distilled water, salt and energy out of this process as I imagine it.

    So why hasn't it be done? Where did my thinking go wrong? It would seem like an almost infinite source of energy one could get out of this, without polluting the environment all while generating distilled water and possibly clouds which could be used to irrigate desert areas.
  2. jcsd
  3. May 21, 2017 #2
    Unfortunately, It takes just as much energy to get water out of your hole as you can get from letting it in.
    Evaporating water costs 2.2MJ/kg, while you only get 5KJ/kg from letting it flow into the hole. If you used the sun to evaporate the water, this would only make a really inefficient solar collector.
  4. May 21, 2017 #3
    I was thinking of an array of mirrors inside the large basin attached to the walls, all pointing towards a glass tower the water would be led through.

    Also, why would it be inefficient to have a really large basin and let the sun in the sahara desert for example do the work of turning the water into steam while you control how much new water is pumped in, keeping the balance.

    What would be your estimate of the area required per KW generated, if we were to rely on just the sun evaporating the water pumped in, without the hole filling up?

    How about the volcanic area idea, where we would send the water into a hole with some volcanic activity? Also, how much water would the ground itself be capable of soaking up without the hole being filled?
    In other words, can you see any possible method where this could actually turn out to become efficient?
  5. May 21, 2017 #4
    Adding to the above,

    It is quite difficult for me to understand why this idea is seemingly dismissed so easily, when getting rid of the water does not seem such a huge fundamental problem.

    You could dig tunnels to move around the water to other areas for easier dissipation at no cost at all, including areas with volcanic activity. Gravity itself would do the moving, with no pumping needed at all. Your tunnels would just have to be slightly below the level of the hole.
  6. May 21, 2017 #5


    Staff: Mentor

    Test your idea yourself. Stick a hose into a bucket of water and see how fast the water flows out. I think you will find that the water level in the hose will not raise higher than the water level in the bucket. Why?

    If the bucket was as deep as the ocean, the result would be the same.

    A second experiment: Put the first full bucket on a table and put a second empty bucket on the ground. Fill the hose with water. Put one end in the first bucket and the other in the second bucket. That is called a siphon. Repeat the experiment with the second bucket on the table and the first bucket on the ground What can you learn from that?
  7. May 21, 2017 #6
    I'm disimissive because getting rid of the water is a huge fundamental problem.
    Because It takes 440x as much energy to evaporate the water as you get from it, any method to get rid of the water is better used to produce energy directly. If you use volcanic heat to get rid of the water, it will be much better to produce energy with it directly. It's not hard to beat an efficiency of 0.22%
    The only thing that would work is just to let the water flow in a huge basin and let the sun evaporate it. This will get you about 2W/m^2. You'll need 500 km^2 for a 1GW installation, and you'll have to get rid of much more salt then you'll ever want. There have been plans to flood the Qatarra depression in Egypt in this way.

    Attached Files:

  8. May 21, 2017 #7
    I am not sure you understood my proposal,

    The hole in the ground i am proposing would be below water level. As deep as possible technically.

    Ideally it would have the shape of a dish, with as many mirrors inside covering it, pointing all towards one point. You would firstly generate energy from the water exiting the pipe and then again gain energy by turning water into steam with your array of mirrors all pointing to a tower in the middle the water moves through.

    You would also get distilled water and salt out of this among other things.

    It would look similar to this


    except that you would also use every other reasonable method available to get more water out of the hole at a faster pace, after you extracted the energy via the turbine, as those I mentioned above or others I did not think of yet.
    Using volcanic ground areas for example, you pipe the extra water towards (using gravity) etc
  9. May 21, 2017 #8


    Staff: Mentor

    OK, the picture helps. You plan a hole deeper than the ocean, and a tunnel connecting it to the ocean floor. That sounds like a lot of digging. :nb)

    Actually, there was a real life demonstration of your idea. (I am unclear on whether the project was just proposed, or actually implemented.) Sorry, but I can't find a link to it. It was on the horn of Africa or on the Saudi peninsula if I remember correctly.

    There was a big bay connected to the ocean. They put a dam across the mouth of the bay. The sun evaporated the water in the bay until there was a significant height difference between the bay and the ocean. Then they let water come in from the ocean through a turbine to make power. If you adjusted the flow rate to match the evaporation rate, the water level would remain constant. The bay would eventually fill with salt, ending the project.

    But instead of a narrow deep hole like you are thinking of, the bay was very big in area and shallow in depth. I guess, perhaps 10000 ##km^2## in area and 10 meters below sea level.

    Again, you can do your own experiments. Using mirrors, how fast can you boil away a bucket of water?

    But you also propose using some of the energy from the turbines to boil the water. Are you able to do some simple calculations?
    Each of the Itaipu dam's turbines produces an electric capacity of 700MW, and a water flow rate 700 ##m^3/second##. How much power is needed to boil and evaporate 700 ##m^3/second## of water?
  10. May 21, 2017 #9
    On rethinking it, it might be a stupid idea to use the energy you got from the water pressure to evaporate the water out. Instead the mirrors inside the dish should do the job well enough. Another advantage of such a system would be as I see it, that at night when there is no sun, you could fill close by basins with the incoming water, while producing energy with the water pressure turbine still.
    Daylight you would run that water through the boiler and get rid of it via steam or bottled distilled water you tap at the top of the boiler.

    MAYBE it is even possible to use tides to get rid some of the water. Hence open a pipe back into the ocean from the top of the extra basins, whenever the ocean level is lowest. More thought needs to be put into this about how exactly it would work.
  11. May 21, 2017 #10
    For the record, I DID notice that my initial deep ocean water pressure power plant turned into a water pressure/solar/tidal power plant.
  12. May 21, 2017 #11
    Another question related to the above.

    What would happen if we dug the deepest hole possible technically, and supposed that we would not hit on any oil or water etc pockets, we would just pipe all incoming water after extracting the energy into said deep hole.. preferably one that reaches middle earth(joking).

    Would water turn into steam? If yes, perfect or would if the hole was deep enough and wide enough the water simply be absorbed by the ground(if yes, how much m^3 per day could one expect taking various scenarios into account)? Would it eventually fill with water to the brink? What would be possible realistic scenarios on this one, given a wide hole which would be as deep as technically possible?
  13. May 21, 2017 #12


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    I'm not convinced that you actually appreciate that, once the pipe has filled up the hole, there is no energy left. It's a one-off exercise and you might as well have used the energy used to dig that massive hole to feed a city full of people for a few years / months. You make a sort of reference to geothermal heating. There are some legs in that idea but you don't dig a big hole - you just send a pipe down a narrow bore hole to a hot region of rocks near the surface. You pump cold water down and it comes up warmer through a nearby pipe. Such systems have worked but I believe they tend to silt up and you have to use a fresh hole. You have to pick your location t make it pay.
  14. May 21, 2017 #13


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    That is called geothermal power. It has been used for decades, sometimes with no digging at all.
  15. May 21, 2017 #14


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    The deep hole doesn't really add anything to the system. The energy gained from the water flooding into the hole is less than the energy needed to remove the dirt from the hole or the water once it fills up. The only long term source of energy in this system is the sunlight hitting the mirrors. It would be a lot easier to just build the solar array on the surface. There is also an advantage to using liquid salt instead of water...

  16. May 21, 2017 #15

    Compare your system which requires water pumped towards it, in order to create steam to the system I proposed.

    The molten salt is just required if you want to store energy, to use at night. In my case I proposed two extra basins which have a similar functionality but also can be used for tidal energy generation partly.

    In my system, you won't have to pump water, as you will be getting pressurized water, which you can extract the energy of partly, before sending it into the solar power plant to generate steam.
    One more improvement to the design I proposed, would be to extend the pipe with a pipe which is thermally shielded and which siphons in hotter water from the top of the ocean, hence requiring less heating with the mirrors before it turns into steam.

    Being underground in a dish shaped hole, you can place more mirrors as well, with less effort.

    As for the geothermal part, one could imagine leading some pipes with a heat exchanger deep down underground where there is a lot of heat, to generate extra power through steam or other methods.
    Why would one want to use water on ground level for this, when you can have highly pressurized water at almost infinite quantities you can extract some energy of before sending it down the geothermal pipe? Unfortunately I don't have much of a clue of how exactly geothermic solutions work, let alone how deep one would have to go for water to turn into steam as well as which geographical areas would be best suited for this.

    Especially in the case of the solar power plant, the steam escaping would go up anyway. In my case, at about ground level, while in your case, above ground level. Since we are not extracting any other energy out of the escaping steam, my design would give you whatever your design gives in output PLUS the energy from the pressurized water.

    So now we are looking at hydro/solar/tidal/geothermal all at the same place, producing a lot of steam for distilled water. Cleaning/extracting the salt(and other) build up might be an issue however. Not sure how this is handled.
  17. May 21, 2017 #16


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    Since the stuff you dug out of the hole probably weighs more than the water that can fill it up, it won't provide a net output.
    It takes a lot of energy to turn water into steam.
    Perpetual motion machines are impossible. They violate the laws of physics (in this case, the law of conservation of energy), which is also why we don't allow discussion of them here.
    Now it is just a really complicated, expensive and inefficient way to use solar energy.
    Now you are describing every hydroelectric dam.
    Because closed thermodynamic cycles are more efficient than open ones.
    Well, the basic problem is that it is meandering idle speculation. It is far too incoherent to even be called an "idea". It is more like half a dozen fifths of an idea.
  18. May 21, 2017 #17


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    Your very first post describes an attempted perpetual motion machine. You've moved away from it by adding solar power, but with no other input besides the water's potential energy, that would be a perpetual motion machine.
  19. May 21, 2017 #18
    No it doesn't.

    At all points one of my main goals was to think about how to get the water out of the hole. Be it just by the sun shinning passively etc.

    If I can build a solar power plant on ground level, which requires water as well, that has to be pumped there,

    How is my design worse, if you could place that solar power plant below ground level, and have pressurized water at almost infinite quantities (no, that is NOT a perpetuum mobile i describe there) which you can extract energy of before sending it through the solar power plant.

    In both cases, those solar power plant would exhaust steam on top which you wouldn't be using for further energy generation. My design would exhaust the steam at about ground level while the standard design at above ground level which does not really give any additional benefit.

    So tell us.... which of the two designs would generate more energy? Let's assume that both are molten salt solar power plants to keep it simple for you.
  20. May 21, 2017 #19


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    Your first post doesn't mention the sun. It suggests boiling the water using the energy generated by bringing it into the hole.
    I already told you: closed thermodynamic systems are better than open ones because you don't waste the water or the residual heat in it.

    Anyway, this is largely moot because as you are going on with the discussion you are getting closer and closer to what is actually being done now with geothermal plants....it's just you are meandering your way there.
  21. May 21, 2017 #20
    Sure, if you ignore the "among other methods" part, like having an array of mirrors heat up the water and turn it into steam etc. But we are past the first post anyway. I already agreed with other posters here based on the input that it wouldn't be efficient enough.
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