Please turn on javascript in your browser to play chess.
Science Forum

Science Forum

  1. 10 Mar '15 21:00 / 1 edit
    http://phys.org/news/2015-03-benefits-adding-smaller-rotor-turbines.html

    they say it can increase their efficiency by 18%.
    Hopefully the extra energy generated from this 18% improvement will more than compensate for the extra costs for having the engineering complexity of adding those extra blades on the turbines.
  2. Standard member DeepThought
    Losing the Thread
    10 Mar '15 22:14
    Originally posted by humy
    http://phys.org/news/2015-03-benefits-adding-smaller-rotor-turbines.html

    they say it can increase their efficiency by 18%.
    Hopefully the extra energy generated from this 18% improvement will more than compensate for the extra costs for having the engineering complexity of adding those extra blades on the turbines.
    Are they connected to the same crank shaft (or whatever they call them on windmills)?

    A major limiter for wind power is that the generated power has to be transmitted to the ground and the top of the windmill has to be able to rotate with arbitrary winding number as the wind doesn't always blow from the same direction. This means that a wire could end up twisted round arbitrary numbers of times. There needs to be a slip ring to cope with this and the brushes suffer from wear. I wonder if a better (as in needs replacing less often) solution would either be a universal joint with the generator mounted in the shaft below the windmill head or some sort of inductive coupling where the power is transmitted in the way it is in a transformer - but arranged so that rotation doesn't matter. That would do away with the need for a slip ring and the associated down time and maintenance costs.

    Having two rotating cranks would render the universal joint set-up impossible as the two drive shafts would have to rotate around each other. The inductive coupling idea could still work.
  3. 10 Mar '15 22:31
    Originally posted by DeepThought
    Are they connected to the same crank shaft (or whatever they call them on windmills)?

    A major limiter for wind power is that the generated power has to be transmitted to the ground and the top of the windmill has to be able to rotate with arbitrary winding number as the wind doesn't always blow from the same direction. This means that a wire could ...[text shortened]... ve shafts would have to rotate around each other. The inductive coupling idea could still work.
    Two thoughts...

    Could they not mount the Generator in the pillar and use a bevel gear to
    translate the power through 90'?
    I'm assuming that the problem would be the size and weight of the required gearing?


    Could they transfer power from the generator to the ground via contact-less induction
    transfer? Induction 'chargers' are getting much more efficient. >95%
  4. Standard member DeepThought
    Losing the Thread
    10 Mar '15 22:43
    Originally posted by googlefudge
    Two thoughts...

    Could they not mount the Generator in the pillar and use a bevel gear to
    translate the power through 90'?
    I'm assuming that the problem would be the size and weight of the required gearing?


    Could they transfer power from the generator to the ground via contact-less induction
    transfer? Induction 'chargers' are getting much more efficient. >95%
    Yeah - but if there's a torque the windmill will tend to rotate counter to the rotation we're trying to transmit - although that's a problem with the universal joint set up too. It's the approach with traditional windmills.

    The inductance transmission you're describing is something like what I had in mind. 5% losses would be more than compensated by humy's extra turbine. As long as there's no actual contact they could be as close to touching as you like so there's plenty of scope to make something specialised.
  5. 10 Mar '15 23:11 / 1 edit
    Originally posted by DeepThought
    Yeah - but if there's a torque the windmill will tend to rotate counter to the rotation we're trying to transmit - although that's a problem with the universal joint set up too. It's the approach with traditional windmills.

    The inductance transmission you're describing is something like what I had in mind. 5% losses would be more than compensated by ...[text shortened]... ld be as close to touching as you like so there's plenty of scope to make something specialised.
    Apparently people have got there before us...

    http://powerbyproxi.com/2011/02/5-reasons-to-use-contactless-slip-rings/
  6. 11 Mar '15 08:11 / 4 edits
    Originally posted by DeepThought
    Are they connected to the same crank shaft
    Yes. But not sure if there is a 'joint' (or some sort of mechanical arrangement ) in that shaft to allow the two sets of blades to rotate at different spreads independently. If there is, I assume there would have to be two separate electric generators, one for each of the sets of blades -unless there is some sort of ingenious gear mechanism locking the two together but so that the smaller set of blades always rotate, say, twice the speed of the larger blades?
  7. 11 Mar '15 10:43
    Originally posted by humy
    Yes. But not sure if there is a 'joint' (or some sort of mechanical arrangement ) in that shaft to allow the two sets of blades to rotate at different spreads independently. If there is, I assume there would have to be two separate electric generators, one for each of the sets of blades -unless there is some sort of ingenious gear mechanism locking the two toge ...[text shortened]... but so that the smaller set of blades always rotate, say, twice the speed of the larger blades?
    No, they are fixed together.

    The larger blades generate little torque near the centre due to the blades
    being almost round at that point for structural support.

    Adding smaller blades in that region allows torque to be generated over the
    entire facing area.

    Thus the smaller blades are and must be fully attatched to the same shaft.
  8. 11 Mar '15 12:02
    Originally posted by googlefudge
    No, they are fixed together.
    Are you sure? I couldn't find any mention of it in the article, and it doesn't make sense for them to be fixed together - especially in the model in the photo where they are in line with each other.
  9. Subscriber joe shmo
    Strange Egg
    11 Mar '15 14:55
    Originally posted by twhitehead
    Are you sure? I couldn't find any mention of it in the article, and it doesn't make sense for them to be fixed together - especially in the model in the photo where they are in line with each other.
    I seem to think it especially makes sense for them to be fixed to the same shaft in the model where they are aligned, or offset.

    "First, said the Iowa State University professor of aerospace engineering, check out the base of each blade. They're big, round structural pieces. They're not shaped like an airfoil. And so they don't harvest any wind, reducing a turbine's energy harvest by about 5 percent."

    Read more at: http://phys.org/news/2015-03-benefits-adding-smaller-rotor-turbines.html#jCp

    Why do you think the opposite?
  10. Standard member DeepThought
    Losing the Thread
    11 Mar '15 16:46 / 2 edits
    Originally posted by humy
    Yes. But not sure if there is a 'joint' (or some sort of mechanical arrangement ) in that shaft to allow the two sets of blades to rotate at different spreads independently. If there is, I assume there would have to be two separate electric generators, one for each of the sets of blades -unless there is some sort of ingenious gear mechanism locking the two toge ...[text shortened]... but so that the smaller set of blades always rotate, say, twice the speed of the larger blades?
    There is such a thing as continuously variable gearing. Imagine two cones with their vertical axes parallel, but pointed in opposite directions with a belt transferring the rotation. As the belt moves up and down the cones the gearing changes. I went to a talk about 25 years ago where the speaker, who was from the industry, was talking about the potential of using them in cars as an alternative to conventional gearboxes. It's fairly easy to think of problems with the set up though, so I can imagine that it never got past the feasibility testing stage.

    For a windmill the simplest arrangement is to have the blades connected to the same drive-shaft so I'd have thought they'd do that just to reduce the number of things that can go wrong. The reason I asked the question in the first place was because I was wondering about ways around the slip-ring problem.
  11. Subscriber joe shmo
    Strange Egg
    11 Mar '15 16:57
    Originally posted by DeepThought
    There is such a thing as continuously variable gearing. Imagine two cones with their vertical axes parallel, but pointed in opposite directions with a belt transferring the rotation. As the belt moves up and down the cones the gearing changes. I went to a talk about 25 years ago where the speaker, who was from the industry, was talking about the poten ...[text shortened]... question in the first place was because I was wondering about ways around the slip-ring problem.
    Subaru uses a CVT (continuously variable transmission), so they did make it into production vehicles.
  12. 12 Mar '15 05:29 / 1 edit
    Originally posted by joe shmo
    Why do you think the opposite?
    Because that big round structural piece gets in the way of the airflow. In fact, if they are fixed to the same shaft, why aren't they welded together for support? Even better, why aren't only the tips of the blades there attached to the front of the bigger blades. After all, it is the tips of the blades that provide most of the power.

    Also of importance is the fact that the smaller blade probably has to rotate faster to be efficient.
  13. Standard member DeepThought
    Losing the Thread
    12 Mar '15 16:17
    Originally posted by twhitehead
    Because that big round structural piece gets in the way of the airflow. In fact, if they are fixed to the same shaft, why aren't they welded together for support? Even better, why aren't only the tips of the blades there attached to the front of the bigger blades. After all, it is the tips of the blades that provide most of the power.

    Also of importance is the fact that the smaller blade probably has to rotate faster to be efficient.
    By that logic the blades would be the same length. But if the tips of the blades remove most of the power from the wind then the airflow at the same radius as the larger blades will be slower than the airflow nearer the centre. So the smaller blades are taking energy that the larger blades were unable to capture. Putting connecting rods in between the turbines would have aerodynamic consequences. Since the larger blades, when mounted alone, do not require it, there's no necessity for them on smaller blades.

    I suspect that the main engineering reason to have them connected to the same shaft is to keep it simple. The gain from having the rotors able to rotate independently may well be wiped out by increased mechanical failure rate and maintenance required by a more complicated arrangement.
  14. Subscriber joe shmo
    Strange Egg
    12 Mar '15 17:16 / 2 edits
    Originally posted by twhitehead
    Because that big round structural piece gets in the way of the airflow. In fact, if they are fixed to the same shaft, why aren't they welded together for support? Even better, why aren't only the tips of the blades there attached to the front of the bigger blades. After all, it is the tips of the blades that provide most of the power.

    Also of importance is the fact that the smaller blade probably has to rotate faster to be efficient.
    In fact, if they are fixed to the same shaft, why aren't they welded together for support?


    How would you change the pitch if they are welded?

    Even better, why aren't only the tips of the blades there attached to the front of the bigger blades. After all, it is the tips of the blades that provide most of the power


    That seems like an oversimplification. The angle of attack for any cross section of the blade at radius "r" from the center of rotation is optimized such that the force of drag is minimized and the force of lift is maximized for these types of wind turbines. Also, as well as for structural reasons, the blade is wider at the base. The force of lift on the blade results from integrating dF = p*dA, so "r & A" are inversely porportional radially approaching the tip. The linear velocity is increased with increasing "r", but the force is decreased, and power is porportional to the product F·V, so most certainly most of the power does not come from the tip.

    Also of importance is the fact that the smaller blade probably has to rotate faster to be efficient.


    The coefficient of performance for this type of turbine is a non-linear function of the advance ratio, and it is not necessarily the case that an increase in the advance ratio results in an increase in the COP.
  15. 13 Mar '15 06:40
    Originally posted by DeepThought
    But if the tips of the blades remove most of the power from the wind .....
    They don't.