windgen.org vawt discussion

Winglets will create a small increase in the lift to drag ratio, and that will create a small increase in the overall energy conversion. Most of the energy conversion for conventional VAWT occurs on the upwind pass of the blades (about 80%). So if the winglet extends inward from the blade, it will have the most effect. A winglet extending outward from the blade must be considered from the perspective of added drag when upwind versus less tip vortex losses when downwind.

A far larger increase in overall energy conversion can be achieved by using the Sicard/Bayly-Kentfield VAWT with passive pitch control based on the principle of a centrifugal-pendulum-spring. An even large increase in overall energy conversion can be achieved by using a Sharp VAWT with passive, dynamic pitch control based on the same principle of pitch control. The S/B-K VAWT and the Sharp VAWT also provide very relilable self-starting and a 50% increase in annual energy conversion.

That last bit of information is contained in a letter to me from Professor Kentfield         (2004) interpreting the calculated power and torque curves for the Bayly-Kentfield VAWT (1981) tested full scale (15 feet by 15 feet) at the University of Calgary. The Sharp VAWT works very similarly and should be a little more responsive to gusting winds. If a dynamically balanced Sharp VAWT is tipped 25 to 30 degrees away from the wind to expose the downwind blades to clean air, the "effective Cp" will "exceed" the Betz limit. This is based on using the original upright swept area to calculate the Cp; there is actually no violation of the Betz limit, only an increase in the swept area and power due to tipping the rotor to create a larger swept area.

3d simulation is starting to give me some clues on why my VAWT doesn't behave exactly like it does in 2d simulation.

The red bubble is the high pressure area on the leading edge of the blade.  This causes drag.  Notice it goes full length of the blade, and a 2d sim would represent that well.

The  green is the low pressure lift bubble that drives the turbine.  It doesn't go full length because some air flows over the top of the wing.  A 2d sim that assumes this lift goes full length of the wing would come up with too much force.

After I let this spin for a while, I'll add winglets and see if there is any difference.  If there is I'll try it in real life.

itsandbits1 wrote:

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I have found with the miniature testing that the aoa is needed even for the type of blade i am  playing with and I wonder how much power u are going to get even if u get up to speed if u are not producing lift in any reasonable amount. With your airfoil 1-2 degrees at the speed u are at would probably do it, if that. I find if I look for the lift outside in clean air I get higher rpm. translation, turn your tips out or up a degree or 2. I may slim down my design and clean up the airflow to chase some higher numbers myself. I have to keep remembering though that it has to be a speed with power that is sustainable when u load it up

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If I add AOA then the blade will stall sooner and the top speed will be more limited.

The amount of lift actually goes up with speed, until about 180 RPM in a 15 mph wind.

Power = speed * torque, so if we can keep the speed up we need less torque.  And speed makes the alts more efficient.  That's why I'm shooting for more speed.

 

May be a link to your youtube video would be helpfull...

Bernd