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General: Tidal power

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  1.  
    posted by Fostertom, Tony, DJH and Ed Davies on another thread[the Moon causes tides by] maintaining a standing wave as Earth rotates under it.

    By increasing friction via tidal turbines, letting water in and out of barages out of sync etc we are leveraging the mass of the moon to slow down Earth's rotation.

    Now we think that wind tide and waves are harmlessly 'renewable' infinite resources that we can harness forever, but ultimately what we're depleting is the planet's rotational momentum.

    Not in any meaningful sense.
    It would be measurable, though

    tidal has adverse impact orbital taking energy out of the moon eventually it will get further away and take longer to go round the earth.
    tide is mostly due to the influence of the moon

    https://en.wikipedia.org/wiki/Tidal_acceleration#Angular_momentum_and_energy


    I find the 'standing wave' idea a bit theoretical, because it ignores all the continents, which would stop such a wave running constantly round the Earth at 1000mph. That could only work in a belt around the Antarctic, or under the Arctic Ocean.

    I think a better model is: a number of tide waves circulate around the edges of each ocean basin, like you get when you swirl a teacup. The waves are forced by the Earth's rotation and by the Moon and Sun's gravity pulse.

    Depending on the shape of each coast, the tide waves will resonate to a greater or lesser height, depending on frequency. EG less resonance in the Med, and more amplitude round Scotland and Wales.Turns out that the dominant resonance is at 2x the Moon's frequency, so about two of the waves pass us per day.

    Tidal generators would add (a little) to the turbulent damping of the resonance, so reduce the tide height and flow, so reduce (a little) the turbulent friction over the seabed and so form a new equilibrium of lower tides.

    Adding very very many tidal generators would end up with diminishing returns, plus mess up tidal ecosystems. In that sense, tidal energy is a finite resource.

    There's also a very finite number of good sites, and a Betz limit how much energy you can get from each site.

    The tide mechanism couples the Earth's rotation to the Moon's orbit. Tides draw energy from the Earth's rotation (not the Moon), slowing us down. They transfer a little of the energy to speed up the Moon, so it slingshots to a higher orbit above Earth. This is naturally lengthening the Earth's day (measurably, requiring leap seconds) and raising the Moon's orbit (measurably). Humans extracting tidal energy would delay this natural process (possibly not measurably).
    •  
      CommentAuthordjh
    • CommentTimeJul 5th 2020 edited
     
    Posted By: WillInAberdeenI find the 'standing wave' idea a bit theoretical, because it ignores all the continents, which would stop such a wave running constantly round the Earth at 1000mph. That could only work in a belt around the Antarctic, or under the Arctic Ocean.

    Indeed and that's all explained in the wikipedia articles.

    There's also a very finite number of good sites, and a Betz limit how much energy you can get from each site.

    Agreed as to the finite nature of the resource and would be surprised if it weren't (!!!) and not too surprised that there's a fair number around our islands.

    The Betz limit is flow-related (and was apparently actually discovered by Lanchester some years earlier!) so I suppose that some non-flow based method of recovering the potential energy 'stored' in tides may be possible and that would avoid the Betz limit; I don't know? But it seems sensible to assume it applies unless/until somebody shows it doesn't.

    They transfer energy to speed up the Moon, so it slingshots to a higher orbit above Earth.

    I think you'll find they increase its [potential] energy but decrease its kinetic energy (i.e. slow it down rather than speed it up).

    My understanding is that for every individual project, you need to look at its effect on the resonances to understand whether it speeds up or slows down the process. I don't pretend to understand the fundamentals of how it all fits together. Presumably extracting energy will change the orbital mechanics somehow, but I don't know how.
  3.  
    The [tides] transfer energy to speed up the Moon, so it slingshots to a higher orbit above Earth.
    Exactly as written, but maybe too condensed to be clear:
    1 The tidal coupling speeds up the Moon, it gains kinetic energy from the Earth.
    2 So the moon slingshots to a higher orbit, so converting kinetic to potential energy.
    3 The Moon is now at higher orbit, so to conserve angular momentum as well as energy, it must now have less speed and kinetic energy and more potential energy than it started out with. There's only one orbital height that conserves both energy and momentum.

    The Betz limit says that as much fluid must flow out of the generator as flows in, so you can't recover all of its energy, or you'd have a load of stationary fluid. Can't see how any generator could get round that.
    •  
      CommentAuthordjh
    • CommentTimeJul 5th 2020
     
    I don't think the universe understands our distinction between kinetic energy and potential energy. Indeed I think I dimly remember that Einstein and friends tell us that kinetic energy is actually a bit of extra mass. Just as information is energy, but that was somebody different. So I doubt the moon speeds up and then slows down again; I suspect it just slows down as it moves outward, regardless of how your accounting books measure it.
  5.  
    If only! Then a rocket wouldn't need to speed up to lift off from Earth and move toward orbit, it would just get heavier and more informative :-)
    •  
      CommentAuthordjh
    • CommentTimeJul 6th 2020
     
    Posted By: WillInAberdeenIf only!

    If only what, sorry? If you can be more explicit about what you're challenging, I can be more explicit in explaining my point.

    PS Clearly a rocket doesn't need to speed up to leave the earth, that's purely an artefact of the propulsion technology we currently use. With an orbital tether in place, it could go as slowly as it liked, for example. And the orbital speed depends on what frame you look at it from.
  7.  
    I'd have thought the much more relevant effects of harnassing tidal power would be in the way the alterations to the local water flow affected water levels, channel gouging or silting up, effect on marshes, wild life, etc.
    A bit like the unforeseen consequences of building dams.
    • CommentAuthorJeff B
    • CommentTimeJul 6th 2020 edited
     
    I admit I am completely out of my depth* here (and especially on the other thread where discussions about slowing down the Earth's rotation are going on) but it seems to me that the last contributor (C.Pope) has the most pertinent observation! Surely we need to get on and try out some tidal power schemes in order to ascertain how significant these bio/eco effects would be? Aren't there data from other parts of the world that can provide that sort of information already?

    A quick skirmish on the Internet found this article: https://www.sciencedirect.com/science/article/pii/S1364032115016676

    It seems there is a lot of work to be done yet!

    *No pun intended!
    • CommentAuthortony
    • CommentTimeJul 6th 2020
     
    Bring back Einstein

    I like tidal power especially in the form of underwater turbines in tidal races.

    I do not see any effect on rotation of the earth, if tides slowed this down then the planet would have already stopped.

    The standing wave is just the high tide on the opposite side of the planet to the moon.

    The effects of taking energy in the form of tidal power are immeasurably small in anything but the very long term. Energy removed will result in the moon’s orbital energy decreasing and the moon will get further away infinitesimally slowly.

    The sun does also influence the tides a little,

    I don’t agree with the idea of energy being transferred to or from the moon.
    • CommentAuthorJeff B
    • CommentTimeJul 6th 2020
     
    Re moon getting further away etc... what time frame are we talking here chaps before this has some dramatic influence on life as we know it?

    We know (believe?) that the sun will become a red dwarf and engulf the Earth in about 5 billion years, an unimaginably long time. In the meantime absolutely anything could have happened to humanity.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 6th 2020 edited
     
    The moon used to spin but that was gradually slowed by gravitational attraction until now it's locked to always show one face only to Earth. As no 'planet's' ball is completely homogeneous, it has one slightly densest hemisphere, which has the strongest attraction to Earth.

    Strange to think there must have been a time when it first failed to complete a whole revolution, but stopped and returned, tren a series of clockwise/anticlockwise oscillations which eventually petered out.

    One day, by same process, Earth will also stop rotating and become locked to the moon, or possibly to the sun.
    • CommentAuthorEd Davies
    • CommentTimeJul 6th 2020
     
    I don’t agree with the idea of energy being transferred to or from the moon.
    Since this effect has been directly measured in the form of both the slowing of the Earth's rotation and the increased distance of the Moon your agreement on this, as with other subjects, is rather moot.
    • CommentAuthorEd Davies
    • CommentTimeJul 6th 2020
     
    Posted By: Jeff BRe moon getting further away etc... what time frame are we talking here chaps before this has some dramatic influence on life as we know it?
    A long time. We'll have other problems before that. First the increased heat of the Sun increasing weathering so removing COâ‚‚ from the atmosphere resulting in the death of all the plants then the expansion of the Sun to a red giant which will likely engulf the Earth. So beyond the term of this parliament.
    • CommentAuthorphiledge
    • CommentTimeJul 6th 2020
     
    Posted By: Jeff B

    A quick skirmish on the Internet found this article: https://www.sciencedirect.com/science/article/pii/S1364032115016676

    It seems there is a lot of work to be done yet!


    The article seems to be from 2016 and I think things have moved on a bit. I think theres already operational turbines installed in the channel and in scotland with more coming into operation soon. Google Orbital Marine Power and Atlantis Ocean Energy.
    • CommentAuthorJeff B
    • CommentTimeJul 6th 2020
     
    Posted By: philedge
    Posted By: Jeff B

    A quick skirmish on the Internet found this article: https://www.sciencedirect.com/science/article/pii/S1364032115016676

    It seems there is a lot of work to be done yet!


    The article seems to be from 2016 and I think things have moved on a bit. I think theres already operational turbines installed in the channel and in scotland with more coming into operation soon. Google Orbital Marine Power and Atlantis Ocean Energy.


    I did wonder. West Wales seems to be doing its bit:

    https://www.swanseabaycitydeal.wales/news/regional-approval-for-homes-as-power-stations-project/
    https://www.swanseabaycitydeal.wales/news/green-light-for-60-million-pembrokeshire-marine-energy-project/

    Pity about the actual Swansea Bay barrage scheme though.
    • CommentAuthorJeff B
    • CommentTimeJul 6th 2020
     
    Posted By: Ed Davies
    Posted By: Jeff BRe moon getting further away etc... what time frame are we talking here chaps before this has some dramatic influence on life as we know it?
    A long time. We'll have other problems before that. First the increased heat of the Sun increasing weathering so removing COâ‚‚ from the atmosphere resulting in the death of all the plants then the expansion of the Sun to a red giant which will likely engulf the Earth. So beyond the term of this parliament.


    "So beyond the term of this parliament" ....that's a relief then. Not sure I could put up with Boris for the next several billion years.
    •  
      CommentAuthordjh
    • CommentTimeJul 6th 2020
     
    FWIW, there's a list of existing tidal power stations at https://en.wikipedia.org/wiki/List_of_tidal_power_stations

    The Rance Barrage is pretty nearby and has a lot of experience.
    • CommentAuthorJeff B
    • CommentTimeJul 7th 2020
     
    <blockquote><cite>Posted By: djh</cite>FWIW, there's a list of existing tidal power stations at<a href="https://en.wikipedia.org/wiki/List_of_tidal_power_stations" rel="nofollow">https://en.wikipedia.org/wiki/List_of_tidal_power_stations</a>

    The Rance Barrage is pretty nearby and has a lot of experience.</blockquote>

    Thanks. The Swansea Bay scheme is dead of course and the SeaGen operation has been decommissioned.
    • CommentAuthorEd Davies
    • CommentTimeJul 7th 2020
     
    Interesting lists on the Wikipedia page. Near me is MeyGen [¹] which currently has 4 1.5 MW turbines though it's planned to eventually be the largest of installed or under-construction tidal stations in the world (though some planned ones are much bigger) but only got a capacity factor of 25% in 2019.

    Meanwhile, round the corner at the Beatrice Offshore Wind farm (BOWL) [²] there are 84 7MW turbines which have been constructed, installed and commissioned since work started on MeyGen. I don't know their capacity factor but would expect it to be in the high 30s to low 40s percent, maybe even over 45%.

    Of course, tidal has the advantage that it's a lot more predicable and the lulls in production are shorter (and distributed round the country to some extent) but the wider use of the technology seems to me to make wind (and PV) the likely backbone of generation with tidal only providing a niche filler in certain places.

    That's probably why they're well on the way to starting another similar-sized wind farm in the Moray Firth while MeyGen is piddling around with a few more 1.5 MW turbines (not sure of their status) with a total capacity less than one of the offshore-wind turbines and not much more than one of the dozen or so wind turbines (5 MW each) in planning to go on shore from the BOWL farm and just up the road from my house site.

    [¹] https://en.wikipedia.org/wiki/MeyGen

    [²] https://en.wikipedia.org/wiki/Beatrice_Wind_Farm
    •  
      CommentAuthordjh
    • CommentTimeJul 7th 2020 edited
     
  21.  
    Also Hornsea 3 and 4 : 2.4GW and 1.8GW

    The Hywind one near Aberdeen is interesting, floating turbines.

    Apparently the moon and sun are involved in 'tidal' pressure waves and flows in the atmosphere, the same as in the oceans. But without the effect of shorelines to help with the resonance.

    So the tidal effect in the atmosphere is only a tiny% of the total wind at surface, but nonetheless I wonder if more tidal electricity is gathered today from the air than from the sea?
    • CommentAuthorJonti
    • CommentTimeJul 8th 2020
     
    Posted By: Jeff B
    Posted By: djhFWIW, there's a list of existing tidal power stations athttps://en.wikipedia.org/wiki/List_of_tidal_power_stations" rel="nofollow" >http:////en.wikipedia.org/wiki/List_of_tidal_power_stations" rel="nofollow">https://en.wikipedia.org/wiki/List_of_tidal_power_stations

    The Rance Barrage is pretty nearby and has a lot of experience.


    Thanks. The Swansea Bay scheme is dead of course and the SeaGen operation has been decommissioned.


    This was on the BBC website the other day don't know if it is all part of the same/similar scheme

    https://www.bbc.co.uk/news/uk-wales-53224180
    • CommentAuthorMike1
    • CommentTimeJul 9th 2020
     
    Posted By: Jeff BThe Swansea Bay scheme is dead of course

    Fortunately not - they've done some site clearance work, which is enough to have secured the planning permission in perpetuity - so it's effectively 'shovel ready'
    http://www.tidallagoonpower.com/news/2020/07/08/project-update-8th-july-2020/

    The main blocks seem to be funding (peanuts in post-Covid terms at only £1.5bn projected) & operational subsidy arrangements.
    •  
      CommentAuthordjh
    • CommentTimeJul 9th 2020
     
    I wonder if tidal lagoon schemes can operate as balancing services? That would give them an additional revenue scheme.
    • CommentAuthortony
    • CommentTimeJul 9th 2020
     
    Tidal lagoons would be better employed generating energy than balancing the grid
    • CommentAuthorEd Davies
    • CommentTimeJul 9th 2020 edited
     
    I think I agree with tony here (!); AIUI the efficiency of pumped-storage schemes increases with height though I'm not really clear on why. Anybody?
    •  
      CommentAuthordjh
    • CommentTimeJul 9th 2020
     
    Posted By: Ed DaviesAIUI the efficiency of pumped-storage schemes increases with height though I'm not really clear on why. Anybody?

    Isn't that easy? Greater height = greater energy and that implies smaller proportional losses?
    • CommentAuthordb8000
    • CommentTimeJul 10th 2020 edited
     
    Posted By: Mike1Fortunately not - they've done some site clearance work, which is enough to have secured the planning permission in perpetuity - so it's effectively 'shovel ready'

    i think that's what they'd like the world to believe but the DCO had a number of pre-conditions to commencement of development. I think they'll have an uphill battle proving lawful commencement and without that, the DCO lapses.

    It's interesting why another scheme came forward immediately - though a dragon shaped island sounds gimmicky!
  29.  
    Of course tidal power stations slow down the rotation of the earth - and the earth's day length is now considerably longer than it was billions of years ago. It's not difficult to calculate the angular momentum and how much of that must be removed by tidal power ... but the good news it that it will take millions of years to do much. That tidal power station in France has already measurably slowed the rotation of the earth down by a couple of milliseconds ... and the constant tidal drag of both water and continents shifting does slow the earth's rotation.

    As for pumped storage, the effectiveness increases with height more than the efficiency since the energy stored is proportional to height, but the frictional losses are pretty much the same once you get above a certain height as the speed of the water in the pipes is the same.

    Paul in Montreal.
  30.  
    Posted By: djhAh, so the distinction is between 'efficiency' and 'effectiveness'? You'd need to define both those terms for me; I'm just thinking in terms of energy out over energy in.


    Efficiency is how much of the potential energy stored you can get back, whereas effectiveness is how much energy you can store in a lake of a given size. You could have a very small efficient lake, but that might not be useful as the same sized lake much higher up with twice or three times the energy stored etc.

    Paul.

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