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wind generator design question

Discussion in 'Home Power and Microgeneration' started by brian mitchell, Mar 29, 2006.

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  1. Hi,

    from looking around at all the information on wind turbines I get the
    sense that there's a certain scientific macho at work. Everything has to
    be super-efficient, with rotor-blades as thin as knitting needles,
    perfect pitch and razored lift, etc.

    As I (unscientifically) understand it, this is because the power
    generation is all predicated on wind *velocity*. This may be fine if you
    live where Atlantic gales come screeching up the loch and you need heavy
    stones on the roof to keep it in place, but not many of us do. I
    actually live in Wales in the UK, which can be quite windy, but I am in
    sight of a trio of large wind turbines and they never seem to be turning
    --though they look impressive.

    My question is whether, for people who don't live in ideal (regular,
    strong, non-turbulent) wind conditions, it wouldn't be better to think
    in terms of wind *pressure*? With this, the rotor-blade would be more a
    sail than a wing and one would be wanting to maximise area rather than
    lift. Being slow-turning, the alternator part would have to be geared.
    Or, as I've been pondering, be arranged around the rim of the rotor so
    as to utilise the tip speed.

    Does anyone know if a) this makes any sense, b) anyone else is thinking
    in this general direction? Are there plans available? I know there is
    the Savonius rotor, which is a pressure device, but would seem to me to
    suffer from only being driven for about a third of its rotation, the
    rest being windage. But I could be very wrong, I know. Does anyone have
    experience of these rotors?

    Looking forward to hearing some thoughts on this...

    brian mitchell
  2. Yes. To be very specific, with an aerofoil blade, such as is used in
    most wind generators, the wind passing over the leading edge creates a
    pressure differential, known as 'lift', between the two surfaces of the
    blade. Increased velocity = increased lift. But this doesn't address my
    main point, which is that you need quite high velocities to get useful
    speeds of rotation. This may be the most *efficient* way of utilising
    the kinetic energy of the wind, but not necessarily the most effective
    way in all conditions.

    Thanks. Torque is really what's at issue. In light winds, a rotor which
    uses the kinetic energy to simply *displace* its blades by converting a
    horizontal force into a rotational one, like a windmill or a fan in
    reverse, may develop more torque than one dependent on lift alone.
    Sailing a boat downwind in light airs you throw up as much canvas as you
    can. Since electricity generation increases with rotational speed, it
    might be a better trade-off to use the torque of a displacement-type
    rotor with a geared-up generator than a lift-type rotor with blades
    several metres long. Especially if you want to mount the whole thing on
    the roof of a suburban semi!

    I'm sure both these are true, but it seems to be a fact that aerodynamic
    wind turbines give disappointing results --electricity-wise-- in areas
    where the windstream is broken, turbulent, confused, etc., and in light
    winds anywhere. Rather than saying :- these conditions are no good for a
    wind turbine so don't bother- perhaps it would be better to ask: how can
    we take best advantage of these conditions?

    brian mitchell
  3. daestrom

    daestrom Guest

    It isn't so much that a particular type of turbine 'give disappointint
    results...', it's just how much power is available. If a given diameter
    wind turbine can develop 500 watts in a 10 mph wind, then it can't do better
    than 500/8 = 62.5 watts in a 5 mph. Just because there is that much less
    energy in the wind, and that much less air moving past the turbine.

    Yes, it is possible to design a turbine that generates more power at lower
    wind speeds. Just make the diameter larger. But there comes a trade off of
    how much a large turbine costs versus how much power it can generate.
    Designing for very light winds pushes the $/watt up pretty quickly. And a
    turbine that generates max output at 7 mph ends up *throwing away* an awful
    lot of energy when the wind speed is up to 14 mph.

    Large, multiple blades as you're suggesting can develop a lot more torque.
    But the lower revolution speed ends up taking over and the horsepower output
    goes down. And larger blades means bigger/stronger tower to hold them up
    against that wind. Large bladed units are also somewhat self-limiting as
    wind speed increases because although the available power increases, the
    turbine can't speed up much because of all the drag associated with spinning
    those large blades through the air.

  4. Mic

    Mic Guest

    Wind generator Links
    Home built boat wind genny

    From a seller on Ebay
    Wind turbine generator PMA Permanent Magnet Alternator

    If these voltages seem high just remember that standard automotive
    alternators are usually putting out 40 to 50 volts into the 12 volt
    battery in your car. That's right! It won't harm the battery AS long
    as the battery is not FULL it can take high levels of charging voltage
    no problem. A lead acid battery will easily buffer high input

    (You will never be able to see voltages this high because the battery
    pulls all power inputs DOWN to its own voltage level AND that's why
    you NEVER pull off the battery cables when an engine is running
    because your fuses, dash board lights and regulator blow up !!!!!!!
    (Especially if you rev the engine! ,,,, NO battery---- NO buffer!!)

    Have you every seen a battery charger that had a BOOST/START mode???
    How could this work since you can't force more amperage into an
    electrical circuit. Amperage has to be "Demanded" by an electrical
    device so the BOOST/START switch on your battery charge is actually
    increasing the VOLTAGE to about 18 to 20 volts thereby allowing you to
    extract more amperage. Relax, it is normal to charge batteries with
    much more power than their output ratings. Only start worrying when
    the battery is fully charged and THEN any additional power must stop
    coming into it or must be dumped to keep the battery cool and from
    producing explosive hydrogen gas.

    To make a long story short a 12 volt battery that is being charged
    with a wind turbine putting out 50 volts will still only read "12
    volts" to a volt meter proving that the high voltage power is easily
    being swallowed up by the big lead plates of your battery.
    The Chispito Wind Generator is a 100W machine. It outputs 14volts at
    275 rpm, so it starts putting power into your batteries in low wind
    speeds (5-7mph).
    Assembly instructions
    "When modified, auto alternators can provide variable direct current
    at 0 to 120 volts for battery charging, etc...."
    A 20 amp charge controller that is installed between a solar panel and
    a battery. Use for general purpose solar regulation. For 12 Volt
    CirKits Links to Alternative Energy Web Sites Large
    Wind Turbine Power Output Calculator
    The Basic Chemistry of Gas Recombination in Lead-Acid Batteries
    Lots of good projects in including a CNC router???
    Building your own wind turbine can be both a very fulfilling
    experience or a disappointing exercise.
    Wireless cycle computer as an RPM monitor
    This is the WindGenZen SAIL model

    # The motor should produce 12 volts at 200-300 RPM.
    # To find out if a motor will do this, take the rated voltage and
    divide it by the rated RPM. I.E. a 50 volt motor rated for 1000 RPM
    will make 12 volts at about 250 RPM. Use a linear scale to calculate
    the voltage of any given RPM.
    # If the RPM required to make 12 volts is higher than 300 it probably
    will not work unless you have a big blade and screaming winds.

    # If the RPM required to make 12 volts is higher than 400 it will not
    work and you should consider a different generator.

    # For example, a motor that makes 72 volts DC at 1850 RPM will make 12
    volts DC at about 300 RPM. That will work.

    # A 50 volt motor rated for 1000 RPM would make 12.5 volts at 250 RPM.
    That will work.

    # A 36 volt motor rated for 800 RPM would make 12 volts at about 266
    RPM. That will work.

    # A 24 volt motor rated for 700 RPM would make 12 volts at about 350
    RPM. That will work if you have good wind.

    # A 36 volt motor rated for 1500 RPM would make 12 volts at about 500
    RPM. That will NOT work and you should reconsider the generator.

    # Do you see the relationship and ratio of RPM and voltage you need?
    Avoid motors that require too many RPM for making 12 volts unless you
    are charging 6 volt batteries: they won't work.

    # Large AC motors can be rectified for less than $30.00 and often
    produce tons of good DC power at low RPMs...lots, and often do it for
    only a little more than ameteks.
  5. Yes. The most adaptive machine possible seems the best idea.
    How would that be controlled?
    I don't know what pwm field control is...? But I agree that the
    electrical side should also be adaptive. Hugh Piggott's well-known diy
    generator can have its 6 coils wired in two configurations, star for low
    speeds and delta for higher speeds. He speaks of having a switch to
    change modes and it could be centrifugally operated inside the

    And I, in my Mad Victorian Inventor mode, wonder if it would be feasible
    to have a centrifugally operated auto-transformer, such that with
    increasing speeds the permanent magnet generator fed into a load of
    increasing impedance and the output would be a nearly constant voltage.
    Perhaps there's a wholly electronic way of achieving the same end, I
    don't know.

    brian mitchell
  6. Can't argue with that! Although a turbine which could develop that much
    power in 10mph winds and still be considered 'domestic' would be a
    desirable object.
    I can see that, but any turbine system is likely to end up throwing away
    energy. What happens when you're batteries are fully charged and it
    decides to blow a gale? And don't most small-scale systems use shunt
    voltage regulation to protect batteries?

    I'm trying to see the whole system as a unity, and that has to take
    actual conditions as the base point. I've learnt that the annual mean
    wind speed for my area is 8mph. That's at 10 metres above ground level,
    so probably less at roof level. This suggests to me that for *most* of
    the year one would be getting 8mph winds as a maximum, and one has to
    consider viability, cost/benefit, etc, over a whole year.
    It doesn't seem beyond possibility to me that wind as a wide-application
    alternative energy source is a bit of a chimera. The fastest I've ever
    seen those large wind turbines on the nearby hill turning is about one
    revolution in five seconds. Maybe that's enough to generate usefully
    large amounts of power, I don't know. But I do wonder about the economic
    viability of them.

    I'm working on the basis that a greater number of smaller systems which
    can utilise battery storage might be a better bet. Although in the UK
    you're allowed to connect really quite small systems into the national
    grid and sell power back, that isn't storage in any sense, and they pay
    you nearly nothing for it.

    At the moment, for a system that might be called 'domestic' (ie.,
    rooftop, easily made and installed) my mind is yawing towards vertical
    axis, low windspeed jobs. Quite a lot of people seem to be looking in
    that direction I now discover.

    brian mitchell
  7. Taking the Savonius-type rotor as an example, once one frees one's mind
    of the "55 gallon drum" imprint there are many modifications possible to
    address the problems and raise the overall efficiency. One instance of
    this I've seen is a shroud or cowl which leaves only the working side
    exposed and dramatically cuts down the drag of the returning 'cups'.
    This design sacrifices the omni-directionality but increases the
    efficiency, and it's only a lightweight cowl seeking the wind so there's
    no gyroscopic forces involved.

    Other designs have two tiers with staggered cups, or make them wider
    than they are tall to take advantage of increased leverage to turn a
    bigger load. And there's no reason why this kind of rotor shouldn't have
    a top bearing as well.

    Many developments, too, on the aerofoil, Darrieus-type vertical axis
    rotors. Different configurations of blades, variable pitch as they
    revolve, and so on. I don't know how tunable these are to low-wind
    conditions, though.

    brian mitchell
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