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Bad Journalism Energy Vs Power

  1. Dec 15, 2008 #1


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    I like to point out bad journalism, and here's one I saw this weekend in Time magazine:

    kW is power and is already in units of energy per unit time: there is no such thing as a kW per hour. You can actually plug ten 100W light bulbs into it and run them for 1 hour, 2 hours, 15 hours, or however long the wind lasts.

    Ironically, I didn't see this article, I saw a response letter to the editor - the response talked about how little power (or energy, can't remember which) that was (so we should do nuclear power), but didn't catch the error.
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  3. Dec 15, 2008 #2


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    Staff: Mentor

    I sent them this email:
  4. Dec 15, 2008 #3
    As I'm always explaining to people, if you read it in a news article, it was probably run through a blender somewhere between reality and the printer.
  5. Dec 15, 2008 #4


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    You sure got them. :rolleyes:
  6. Dec 15, 2008 #5
    You sure got him.
  7. Dec 15, 2008 #6


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    I sure did.
  8. Dec 15, 2008 #7
    i think they probably mean 1.5 kW-hours/day, which makes sense given a 100W light bulb for their example. it's a good example of why energy policy opinions of people with liberal arts degrees should be taken with a grain of salt, but overall, it's not a huge screwup.
  9. Dec 15, 2008 #8


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    I don't understand this...
    ...or this...
    It takes 1.5 kW-hours of ENERGY to power a (100W) lightbulb for 15 hours.

    "1.5 kW-hours per hour" and "1.5 kW-hours/day" are both amounts of power, not energy.
  10. Dec 15, 2008 #9


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    [response to Proton Soup] No, it really is a 1.5 kW turbine. http://www.swiftwindturbine.com/specifications.php

    1.5 kWh/day would be less than 100 W and wouldn't be worth having.

    I probably wouldn't have sent them an email, but the fact that they printed the same error twice without correcting it implies they still don't know they made the error.
  11. Dec 15, 2008 #10


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    Yes, that's why I said it would be cumbersome - it is a non-standard power unit.

    But think of it this way (by my units): every hour it generates enough energy to light a 100 W bulb for 15 hours.
  12. Dec 16, 2008 #11
    that's 1.5kW with a 14m/s(31mph) wind, which isn't realistic for most locations if you're talking averages. if you're buying a windmill, you'd want to know what the average power production is at your location to determine if the unit is economically viable. if this is the max power output, it's valuable to me as an engineer, but for the public at large, it just seems intentionally confusing and makes it sound like it's more powerful than it is.

    now the next spec is valuable and should have been what the writer focused on:
    in my state, this works out to $217.80/year. in Doug Morrell's state of Michigan, $236/year. and all of this at a price tag of $16,000.00. for that, it'd better last a very long time with zero maintenance. if you could manage 5% a year on a $16000 investment, that'd be $800 a year and you get to keep not only the interest but reinvest most of the principle.

    federal subsidies aside, it's a really bad investment. and i think that's what makes the journalism bad.

    and who wants 5 windmills in their yard? :yuck:

    yes, i know that kW-hours are energy. and i honestly didn't realize you'd have problems interpreting that the 15 hours @ 100W is also on a per day basis.
  13. Dec 16, 2008 #12
    Wow. My computer takes about 500W, not counting the monitor, modem, or router. And how much does a kWh of power cost? Like 7 cents or something? I think I'll pass on this.
  14. Dec 16, 2008 #13
    If you want to kill some time. Google windmill failure.

    When the blades start flapping they can do some serious damage. The same thing happens on helicopters. If the helicopter blade flaps at excessive angles it cuts off the tail boom (just like this windmill cut off the tower) or cuts into the cockpit killing the pilots.

    A windmill is simply a helicopter blade oriented vertically.
    Last edited by a moderator: Sep 25, 2014
  15. Dec 16, 2008 #14
    The article didn't state kilowatts PER hour, it said kilowatts AN hour. I think what he was going for was kilowatts for an hour. I've seen that kind of unit of notation used before in some technical publications as kwh doesn't make much sense to people. Its kind of a stupid unit for energy anyway.

    This is completely and utterly wrong. A helicopter rotor and a wind turbine are both turbomachinery that operate in an open fluid but that's about all that they have in common. Blade flapping on helicopters is caused by a difference in relative velocity at the blades due to the helicopters forward motion. The blade flapping is always normal to the fuselage, not the parallel to it. If a helicopter has a boom strike or a cockpit strike (I dont think this has ever happened) its because of a poor rotor head or blade design, not because of flapping.

    Flapping on wind turbines is caused by shadowing from the mast or supporting structural column of the turbine. The failure in the video wasn't caused by blade flapping or a column strike. It was caused by a structural failure near the blade root. That wind turbine should have been shut down at such high wind speeds.

    Last edited: Dec 16, 2008
  16. Dec 16, 2008 #15


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    Does make some sense - if the generator is 1.5kW and on average runs for an hour a day then it makes more sense to say it generate 1.5KWh, a unit people recognise and which electricity is priced in, every day.
    Better than saying it is a 1.5Kw generator with a load capability factor of 6.25%
  17. Dec 16, 2008 #16


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    It should - the fault was caused by the gearbox/braking system failing to automatically stop the blades when it went overspeed.
    The montiors did warn the company that there was a fault and the evacuated an area - the surprise was how far the failed blades went.
  18. Dec 16, 2008 #17
    Unfortunately, it has happened. My professor who wrote a book on helicopter aerodynamics, told us so. I also wrote something about this in my paper for the class too. See:


    When the rotational RPM of the helicopter rotor becomes to low, excessive blade flapping occurs. You are correct that the blades flap in the sense of a teetering rotor (which is what your thinking of), but fully articulated blades are able to lead/lag and bend up/down. Down to the point that they have to put stops so they don't droop down and hit the tarmac when at rest.

    It is basic aerodynamic knowledge that blade flapping is a result of a balance between centrigual forces from rotation and lift from the airfoil section that defines the coning angle of the blade.

    A wind turbine is a helicopter in autorotation. Although a helicopter is probably more in the vortex ring state than a wind turbine is (though I could be wrong on that one).

    Also: I agree with your analysis as to the cause of failure on the windturbine though. I also don't argue about what you said in terms of the mast causing flapping in the case of a wind turbine.
    Last edited: Dec 16, 2008
  19. Dec 16, 2008 #18
    I know helicopters have had boom strikes and many have had fuel probe strikes. My comment in parentheses was regarding cockpit strikes. Is your professor Gordon Leishman?

    Do you have a source for this? What is causing the oscillation if the blade is in balance?

    I don't think so. Autorotation refers to a balance of induced inner and outer profile torque from an inflow through the rotor essentially creating a half turbine and half propeller rotor. I believe the autorotation you are referring to is not actually autorotation (the word is commonly misused) but "windmilling" where the rotor acts as a turbine in order to keep it spinning in order to prevent crashing from power loss. BTW, a rotor can not be in autorotation and in VRS at the same time. Also, wind turbines typically never enter a VRS and in the worst case operate in a turbulent wake state.

    Sorry to hijack your thread russ.
  20. Dec 16, 2008 #19

    When the blades rotate they are always subjected to perturbations from unsteady air loads, wind gusts, etc. Because of the low rotational RPM, the centrigual force can't keep the blades straight out, so any distrubance will make the tip path plane start to oscillate.


    Yes, that's correct.


    The point is autorotation. It's the point where the rotor is extract exactly enough energy to overcome the drag to slow it down. A helicopter never wants to act like a turbine (i.e extract power from the air), or it will overspeed the rotor and cause it to explode (exactly like in the video). I would agree with the word windmilling in the sense that you are using it.

    See the graph in the second link above.

    You know a good deal about wind turbines. Do you work with them?
  21. Dec 16, 2008 #20
    I wouldn't call blade coning flapping, but I understand your point about flapping at low rotor head speeds.

    I guess this is just formalities. If the rotor shaft has drag, it must be absorbing power to keep it spinning which would put the point below the power line on the graph which I used in so many power point presentations. If you neglect friction, then yes the rotor is not absorbing any power. I just like to be clear about this as I used to easily confuse people at my previous place of employment.

    Again, formalities. Notice it says "Point of IDEAL autorotation" and not practical autorotation. If you can design an autogyro that can operate in VRS at steady state, your a better man than me. Also, I think that text your referencing is a fantastic book and IMO the best modern and up to date reference for helicopter aerodynamic design, you can tell your professor I said this. It was one of the only text books that always stayed on my desk at work because I used all the time. Although I would have liked to see some mention of china weights in there.

    I've never actually worked with windmills or helicopters before but I have worked with something similar. Between undergrad and grad school I worked for a company on a venture capital project. Which if you watch this VIDEO you will quickly see why I had to know so much about these things.
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