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Turbine Generator Question

  1. Mar 15, 2012 #1
    This is my first time posting in this forum so please have patience.

    I am working on a small wind turbine design that will output 6-22 kWh (30kWh would be ideal but is not very plausible) and will be used to charge a lithium ion battery. The turbine will be used with wind speeds of 35-80 mph. The current desired dimensions are 6-9"L X 3-5"D.
    I plan on using 2-4 generators to distribute the load. The best generator that I have found based on size alone can only produce a combined 1-2kWh, that output is well above the power curve, and wouldn't be very efficient.

    Is there a way to amplify the power to reach higher output or does anyone know of a good compact turbine/ generator design that meets the desired output?

    Once I know what generator to use and its specs, I should be able to calculate the rest.
  2. jcsd
  3. Mar 15, 2012 #2


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    Welcome to PF!

    I think you mean "kW", not "kWh". But no, there is no way to amplify power without generating more power first.
  4. Mar 15, 2012 #3


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    Do those dimensions refer to height and turbine diameter? 20kW seems an awful lot to be getting from something that size. The sort of turbine that I have seen of that size would produce, perhaps a couple of kW. Have you done your research on what's available commercially - rather than theoretical calculations? (Sorry if you already have done but it's worth while mentioning)
  5. Mar 15, 2012 #4
    I think you mean "kW", not "kWh". But no, there is no way to amplify power without generating more power first.

    I'm looking at Kilo Watt hours. I.E. In 1 hours time of running produces 6-22 kW.

    yeah I know all of the ones that I have found commercially around that size I want either don't produce enough power or they require 1500-1800 RPM. I need something that can reach a desired RPM/ power curve without a whole lot of torque and quickly.

    However I can change the dimensions of the generator and make it a little bigger. I think a foot in diameter would have to be max, but that would also limit it to only two turbine systems. (11kW each)

    Thank you, both of you, for your feedback, If you have any more ideas in regards to available mechanical generators please feel free....

    I looked into alternators, but the biggest problem, like I said, is the high RPM that is required.

    Thinking about gear box's with an alternator but there is a lose of power output due to friction, not sure if it is worth pursuing.
  6. Mar 15, 2012 #5


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    Generators are specified by their Power (kW). The Energy (kWh) depends on how long you run them.

    I don't know where you get your example figures but I have a wind turbine on my boat which is about 18" in diameter and delivers about 24W, flat out.

    Your desired performance seems well over optimistic. Why should you think you can be better than commercial designs?
  7. Mar 15, 2012 #6
    The basic horizontal axis wind turbine (HAWT) is only about 40% efficient. The theoretical (Betz) limit is about 59%, but other factors limit it to about 40%. The HAWT is typically most efficient when the turbine blade tip speed is about 6 times the wind speed, so small diameter blades have a much higher RPM than large ones.

    The incident kinetic power P in wind (in mks units) incident on a HAWT with frontal area A = pi R2 is
    [tex] P=\frac{1}{2}\rho A v^3 \space \space \space watts [/tex]
    where ρ is the air density (about 1.2 kilograms per cubic meter) and v is the wind velocity (meters per second). This gives about 188 watts for a 1-meter diameter 40% efficient turbine in a 10 meter per second wind. The optimum blade tip speed is about 60 meters per second, and the optimum RPM is about 1150 RPM.

    Bob S
  8. Mar 15, 2012 #7
    35 to 80 MPH is a very windy place, please don't think you can mount a wind turbine on your car and generate power to drive it by the car's motion.
  9. Mar 15, 2012 #8
    Jobrag: I am well aware of the laws of physics when referring to relativity. :-)
    However there are many applications that have been overlooked!

    sophiecentaur: I would love to go with a commercial design, I'm just not seeing one. If you have any suggestions...?

    Bob s: thank you for the formulas, I was looking at Betz limit but didn't completely understand it.

    I am in the Nuclear Field, and I plan on using the same design concept for the blades as a turbine... not the three fan blades that is a common windmill design. However they will not be very long 6 inches radius max (there is a concept, then there is reality).

    Thank you all for your feedback, it is all very helpful.

    If I'm looking at solving this wrong please advise.... Should I be looking at this from blades to generator instead of generator to blades?
  10. Mar 15, 2012 #9


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    Well, the first thing to look at is the power available in the airstream. Since the power available is much smaller than the power required, you'll have to go back to square one and rethink the project constraints. There is no use in trying to find the best way to extract energy that doesn't exist!
  11. Mar 15, 2012 #10


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    If this were an easy problem then everyone would be doing it - instead of just megalomaniac politicians who like to see their names up in lights / turbines.

    Where is this source of high speed air? Are you sure you are being 'real' about this?
    Why would you think you can do better than available commercial designs? Are you an 'expert'?
    Last edited: Mar 15, 2012
  12. Mar 15, 2012 #11
    Here is the Weibull (Rayleigh) histogram (distribution) of wind velocities measured at a weather station in UK. It depends on the elevation above the ground. Use 10 meters per second in your calculations.


    Bob S
  13. Mar 15, 2012 #12

    Please explain.
  14. Mar 15, 2012 #13


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    Which part of post #6 don't you understand?
  15. Mar 15, 2012 #14
    Numbers, data... why not?

    If your basing the statement off of Bob S formulas then... ehhh I shall see tomorrow (I have been up for a day and half, need a little rest before I start running the numbers again)
  16. Mar 15, 2012 #15


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    Bob did a sample calc that wasn't the same as your situation, but still shows you're off by an order of magnitude or more.
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