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4 Vs 5 Terminal dual-rate meter confusion

Discussion in 'Electrics UK' started by d000hg, 19 Nov 2018.

  1. JohnW2

    JohnW2

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    As I said, I don't know how typical my location is - we're very 'central', and thereby tend to get spared some of the worst of the either.

    However, as for 'here', the wind today definitely was exceptional, the likes of which we definitely don't see for more than a small number of days per year. As I said, it brought down a large bit of (in fact, most of) one of my trees, and I see that a lot of my neighbour's fence has also been blown down. We definitely don't see that sort of thing very often.

    Kind Regards, John
     
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  3. plugwash

    plugwash

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    IIRC turbines have a maximum wind speed at which they can safely operate. So exceptionally high winds can be as bad as no wind.
     
  4. JohnW2

    JohnW2

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    Yes, I believe there is a safe maximum, and that certainly makes sense. However, they obviously cannot take the blades down when there's high wind, so I can but presume that, under such conditions, they partially feather the blades so that they perform at about their 'maximum safe' level.

    Kind Regards, John
     
  5. JohnW2

    JohnW2

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    It's not really "the last few years" that I was talking about. What surprised me that the day/night cost ratio had remained much the same for about 30 years, and then changed fairly dramatically at some point between April 2017 and April 2018. Can you explain that in terms of changes in nuclear generation?

    Kind Regards, John
     
  6. SimonH2

    SimonH2

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    It will obviously depend on the design - there are still many turbines with fixed blades since feathering blades add a considerable amount of complexity* to the hub.
    Any fixed-blade turbine has to shut down above it's max design speed. Some near here (where I've had a tour with the local engineering society) has the outer section of the blades that can "pop out" and rotate 90˚ to act as an aerodynamic brake to bring the rotor speed right down before they apply the mechanical brake.
    Given that the wind power is (IIRC) proportional the fourth power of the wind speed, it's clear that the power available drops off rapidly below the maximum speed so there's a tricky trade-off between losing generation due to over-speed shutdown, and losing generation due to setting the max design speed too high.

    On larger & newer turbines they do have moveable blades, but this isn't the panacea for all wind speeds since as you feather the blades, the blade roots can go into negative angle of attack which must make the stress calculations "interesting". At some point it is likely that the turbine will have to shut down since there will become some very turbulent airflow around parts of the blades which is generally "not a good thing" and, for land based ones at least, will significantly increased noise.

    * With fixed blades, there's just a ring of bolts. Once the blades are moveable then you have bearings and the actuating mechanism - and the bearings must add significant structural complication.
    I guess "marketing" has caught up !
     
  7. StephenOak

    StephenOak

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    It is all there on the site you linked to. There is a download option and you can select which items and which time period to include.

    Over the past year (00:05 30 Nov 2017 to 00:05 30 Nov 2018) there are 104,990 five minute reporting periods returned, so you can look as finely or as coarsely as you want.

    In that time
    6.41% came from coal
    21.93% came from nuclear
    41.94% came from CCGT
    14.03% came from wind

    Wind supplied less than 10% of demand 40% of the time and more than 30% of demand 7% of the time.

    I have never seen any consolidated date for the total amount of wind power installed, but I would be very surprised if the nameplate value was less than that for CCGT. Wind units delivers somewhere between 20 and 30 per cent of their nameplate value, whereas FF & nuclear plants deliver 85-95 per cent of theirs.


    I am not aware of anyone 'sneering' at so-called renewables because they don't (or can't) work but because they are not economic.

    Not only are they more expensive than reliable electricity supplies but their variable supply makes them worth less than reliable supplies. In a free market I don't think that any electricity company would buy wind derived electricity at any price.
     
    Last edited: 30 Nov 2018
  8. boringoldcodger

    boringoldcodger

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    deleted, as no longer relevant.
     
    Last edited: 30 Nov 2018
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  9. StephenOak

    StephenOak

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    Yes I did, now fixed, thank you.
     
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  11. JohnW2

    JohnW2

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    Fair enough. It's far from any of my areas of expertise, but I would have feared that if one simply 'applied a brake' to the blades (without altering the angle of attack), whilst one would obviously eliminate any over-speed problems, the stresses on the structure could be pretty large - the blades are, after all, trying hard to rotate rapidly.

    Kind Regards, John
     
  12. SimonH2

    SimonH2

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    As well as the Gridwatch site, there is the BM Reports site as part of NETA (New Electricity Trading Arrangement) - I believe it's where Gridwatch gets it's data from. it used to have some nice pretty graphs, but they went when they did the "new improved" site.
    Also, one of the tables they had included the metered capacity which made it easy to work out % of rating plate capacity actually being produced. For renewables, there are a lot of smaller sites that aren't metered (their output only appears as a reduction in total demand) - and I don't think there are any figures for how much capacity they add up to.
    It's the latter that I have an issue with - and the costs to the rest of the industry (not to mention the £11,500,000,000 project to foist smart meters on us).
    Well they certainly stop the turbine when the wind is too strong - otherwise something would break, either overloading the mechanics and generator trying to control the speed, or simply flying apart when the speed got too high.
    I don't know how other turbines do it - I suspect smaller ones just use the brake, but on larger ones that could be a lot of power to dissipate.
    So I did a search, and came across a few links.
    First, a video of a turbine being stopped by fully feathering the blades. Once fully feathered, there's a lot of sail to drag through the air - so highly effective. Also, there's less twist in the blade profile than I thought.
    Fluid dynamic model of a turbine doing such a stop.

    What happens when it goes wrong :eek:
    What they do with aircraft engines to stop the bits going through the passengers - A380 blade off test, or how to destroy a £9m engine in five minutes :whistle:

    A summary of the methods in use according to the Danish Wind Industry Association.
    Seems I'm a little out of date, but then I've not been following the area that closely.

    And for a bit of fun, a compilation of Dyno run fails :whistle:
     
  13. JohnW2

    JohnW2

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    I wasn't really thinking about the act (and method) of 'stopping it' (which, as you say, could require dissipation of a lot of energy) but, rather, the situation after it had been stopped. In that situation, there would be no energy to dissipate but, if not feathered, there could be enormous forces fighting against the brake (and whatever it was attached to) or, ultimately, trying to twist/bend the blades or even the entire tower.

    Kind Regards, John
     
  14. SimonH2

    SimonH2

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    Actually, the forces could be lower than when running. The AoA (Angle of Attack) of the airflow over the blades would be considerably higher and it's likely that the blade will be stalled (aerodynamic stall, the airflow has "broken off" the blade and there's turbulence behind the blade). Obviously the desired direction of lift is different with a turbine blade than an aircraft wing, but the aerodynamics are much the same - ye cannae defy the laws of physics cap'n.
    https://en.wikipedia.org/wiki/Angle_of_attack
     
  15. ban-all-sheds

    ban-all-sheds

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    I think they should be designed so that at maximum speed the blade tips have gone supersonic.
     
  16. JohnW2

    JohnW2

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    I thinkthat's the case with aircraft propellers and helicoptor rotor bladed, isn't it? However, that obviously unrelated to my thoughts about the forces that would exist when the turbine was braked to a halt in the face of very high wind.
     
  17. SimonH2

    SimonH2

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    So that they make lots more noise :whistle:
    It is indeed an issue with some propellors or rotor blades - but not for normal flight. Some fixed-pitch, large diameter props can have the tips go supersonic at take-off power and low airspeed (especially when starting the take-off run). But the blade tips going supersonic reduced efficiency - the supersonic bits are basically turning power into noise :eek:
    Similarly with rotor blades. At high speed, the forward travelling blade increases airspeed - but also the rearwards travelling blade has lower airspeed. While the blade tip going supersonic is bad, retreating blade stall is worse : https://www.skybrary.aero/index.php/Retreating_Blade_Stall
     
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