Savonius vertical wind turbines, no brake needed?

YostFMXYostFMX Solar Expert Posts: 94 ✭✭✭
I'm going to make a savonius vertical wind turbine with a DC treadmill motor for a back up power (very, very, very small back up power). I've made one already but it doesn't spin unless its really blowing (as in the picture). And on that one I left a rim to put a bike brake on it. So the question is do I even need a brake on savonius style vertical wind turbines? As what I have read they can't go faster than the wind. I will be wiring the turbine the same as my PV WITH OUT A DUMP LOAD, so thats why I had planed on a brake. But for my new savonius turbine I want to make I don't need one do I? Cause its not going to "freewheel" off like a normal wind generator can, right?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Savonius vertical wind turbines, no brake needed?

    You are correct... This type cannot spin faster than the wind. So--all you need to do is define your maximum wind speed (xxx MPH, KPH, M/S, etc.) and work out the maximum RPM and decide if that will be damaging to your system or not.

    If Max RPM> Safe RPM--then you still need the brake. If Max RPM < Safe RPM--then you don't need to brake.

    Lastly, you may want/need the brake anyway--Will you need to stop the turbine on a windy day for maintenance?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • YostFMXYostFMX Solar Expert Posts: 94 ✭✭✭
    Re: Savonius vertical wind turbines, no brake needed?

    Thanks, I can just get on my roof and stop it with something.

    So how would you figure out the wind speed to RPM unless you did a test? Or is that the only way? Mine will be about 2' in diameter and probably 16'' tall, a classic savonius 2 fin.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Savonius vertical wind turbines, no brake needed?

    Lets see if I can get this right (otherwise, I will have to give it to one of my school age kids as a homework assignment ;) ). Assuming that maximum tip speed is the wind speed (unloaded max RPM), using 100 MPH max wind and 2' diameter and pi=3.14:
    • Ft/Min wind/rim speed * 1/circumference in ft = RPM
    • 100 MPH * 5,280 ft/M * 1/60 hr per min * 1 / (2' dia * 3.14) * = 1,400 RPM max at 100 MPH
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Savonius vertical wind turbines, no brake needed?

    i don't think your design is quite inline with it having any real efficiency just by looking at it. too little of the wind is being caught so to speak. in the simplest design it would be a circle cut in half and slid so as to go about halfway. this will not start at all in some instances so another above it facing 90 degrees off of the first is used. from what i see most of the wind can pass right through your design and hence the poor startup. savonius designs are normally easy start, but low rpm compared to hawt
    as bill said, there will be a point when it will be considered as too fast and some experimentation will be needed as it depends on many factors as to what is the breaking point.
  • YostFMXYostFMX Solar Expert Posts: 94 ✭✭✭
    Re: Savonius vertical wind turbines, no brake needed?

    Yea, that one is the picture is the first one I made, it sucks haha. The one I'm making now is a classic savonius vertical wind turbines with two half "C" shapes, and than another one stacked on it at a 90o angle. It will be 2' wide by 20'' tall and is looking very cool right now...
  • loteklotek Solar Expert Posts: 65 ✭✭✭✭
    Re: Savonius vertical wind turbines, no brake needed?

    A question on this subject. Doesnt the generator create more resistance, the more power it produces? wouldnt this impact the need for a break?
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Savonius vertical wind turbines, no brake needed?

    lotek,

    Several issues here...

    A Savonius design just "catches" the wind and therefore cannot turn any faster than the wind. So, normally the design limit of RPM would be just the maximum RPM for maximum designed wind speed (faster wind, and the unit may self destruct if no brake).

    Other wind turbines use "lift" from an airfoil shaped blade section. They can rotate much faster than wind speed--so when unloaded can spin very quickly (3-4x operating RPM?) and self destruct. Remember that the centripetal force is F=mrw^2 and so 4x the RPM, you have 16x the force.

    On to resistance by the generator... Yes, in "normal" operation the generator/alternator produces a resistance to rotation and limits the top speed...

    However, there are limits... For example if your turbine produces maximum energy at 30 MPH, and the energy in the wind goes with the cube of the wind's velocity--at 100 MPH:
    • E= Y * (100^3 - 30^3) = 9,100x the amount of energy at 100 MPH vs 30 MPH
    So, there is no way a generator/alternator that is designed for 30 MPH wind could control the amount of wind energy at 100 MPH without either being massively over designed or some other method for limiting RPM.

    So--you are left with.
    1. Short the generator/alternator output so the blades turn slowly (airfoils are stalled). Done quite often. In very high winds, the alternator windings can overheat and fail from high short circuit current).
    2. External brake to stop spinning. A stopped turbine has no motion, so no heat is generated.
    3. Feather blades
    4. Turn blade face away from wind (can't be done with VAWT)
    5. Self-limiting designs (many VAWT do limit maximum RPM by physical design limitations).
    And, remember that anything electromechanical can fail. Generators can fail, slip rings can fail, electronics can fail, dump loads can fail, circuit breakers/fuses can fail, mechanical linkages can fail, brakes can fail, etc...

    Also, the loads themselves are variable... Grid Power can fail (no grid load). Batteries become fully charged--so they need a "dump load" to keep the wind turbine loaded, but limit charging current to the battery bank.

    So, generally, you want several independent methods to limit maximum RPM so that if one fails, there are backups that will work.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • peterakopeterako Solar Expert Posts: 144 ✭✭
    Re: Savonius vertical wind turbines, no brake needed?

    A remote stopping is always good. ( see above )

    But keep in mind that if your system is not balanced very good. It will start making noise on your roof and if you want to stop that and need to go on the roof whit high winds and at night rain etc.

    Or the bearing is getting dammaged and you want to stop it.

    Think about safety.

    P.s. any wind generator on a roof is a bad solution.

    Greetings from Greece
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