Report KD 738 about the VIRYA-6 available

[Adriaan Kragten]

Report KD 738 can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports. The title of this report is: "Calculations executed for the 3-bladed rotor of the VIRYA-6 windmill (lambda design = 6) with the pendulum safety system with a torsion spring connected to the generator type TGET620-5KW-200R for grid connection or heat generation".

I think that this wind turbine is large enough to heat the main room of a house in the winter using three resistors in a water reservoir. A water reservoir is needed for the absorption of the fluctuation of the power coming out of the wind turbine. However, as wind power is much more in phase with the wanted power for heating in the winter than solar power, a much smaller reservoir is needed than for storage of the generated heat of solar panels. Storage of solar power has been investigated in the Dutch report KD 713 for solar panels combined with the VIRYA-10 windmill and a reservoir for four houses. It appeared that storage of solar energy is very difficult because it has to be stored for about half a year and during this long period a lot of energy is lost, even if the reservoir has a thick layer of isolation at the outside. Generation of heat with a wind turbine is explained in the Dutch report KD 709. These two reports are in Dutch because I have joined several meetings about the energy transition in my town Meierijstad.

[bigrockcandymountain]

I had a quick look at this report.  I like the simplicity of manufacture and the size.

Could you tell me approximately what the generator price would be?

How close to a failure stress are the blade roots when it is at rated speed? It seems like at some size, a constant chord wood blade would suffer failure at the root without the benefit of taper.  The largest load must be the axial bending of the blades due to lift, so maybe tapered blades don't have a very large advantage over constant chord.

As always, your reports get me thinking, but I'm not sure what to do with my thoughts.  Thanks for posting anyway. 

[Adriaan Kragten]

I don't know the price of the generator. If you go to the website of Hefei Top Grand following the path given in the report, you get all technical information but no price. To get a price, you have to ask the supplier. I have bought a small generator of this manufacture and the final price is much higher than the price off factory because of costs of transport and import taxes (see report KD 595).

I have good experiences with constant chord blades. It is true that you get the highest bending stress at the blade root and therefore you can't use a very slender blade. You also need a safety system which sharply limits the thrust but the pendulum safety system limits the thrust very well and a rated wind speed of 10 m/s isn't very high. The used airfoil Gö 711 is rather thick and has a thickness of 51.58 mm for a chord of 280 mm. So the moment of resistance is rather high for the chosen chord. The bending moment is reduced because the blade bends backwards and the centrifugal force in the blade partly compensates this bending. This roughly results in 75 % of the bending stress which occurs if the blade would not bend backwards. I have made such calculations for orther blades but not for the blades of the VIRYA-6 as I wouldn't make KD 738 too complicated. Such calculations are given in chapter 2 of report KD 579 for the blades of the VIRYA-6.5 rotor.

KD 738 only shows what a windturbine with a rotor diameter of 6 m can do. For Pel-V curve, see figure 7. But in the end of chapter 6, I show dat further development is a lot of work and using it for heating is also a new field which has still to be develloped.

In the attachment I have added figure 1 out of the report which gives a drawing of the rotor. This drawing shows the simplicity of a rotor with a constant chord and a constant blade angle.

[SparWeb]

The swoopy curves of the blades I build are as much about looks as performance, and I have read other performance test reports from the NREL that show good performance from constant-chord blades as Adriaan shows them here.  One drawback could be noise, although much of this can be mitigated by ensuring the blades don't turn too fast, and chamfer/radius details at the outer tip of the blade.  Noise control is a bit of a dark art, but the energy producing noise is reduced by the square of speed, so the most effective way to make a WT quiet is to slow the blades down.

[Adriaan Kragten]

Assume we compare two rotors with the same rotor diameter D, the same number of blades B and the same design tip speed ratio but one designed with the optimal lift coefficient resulting in tapered blades with an increasing chord and blade angle for decreasing r and one with blades with a constant chord c and a constant blade angle. This has been done in chapter 5.4 of report KD 35. The aerodynamic theory prescribes that for a certain radius r, you need a certain product of Cl * c (see formula 5.4 KD 35). For the constant chord blade, the chord at the blade tip is much larger than for the tapered blade and the lift coefficient must therefore be much smaller.

If you look at table 1 of KD 738, you see for the blade tip (station A) that the linearised lift coefficient is only 0.52 corresponding to an angle of attack of -1.7°. The pressure difference in between the front side and the back side of the airfoil causes a tip vortex which is larger as the pressure difference is larger. The main sound produced by a wind turbine blade is caused by this tip vortex. I expect that the tip vortex is smaller if the lift coefficient at the tip is smaller. So constant chord blades will make less noise than tapered blades if all other parameters are the same. My 2-bladed VIRYA-4.2 with constant chord blades and a design tip speed ratio of 8 has run for several years and this rotor made only a little noise if the generator is loaded such that the rotor runs about with the design tip speed ratio. An unloaded rotor is much noisier but that is the case for both types of blades.

If you reduce the design tip speed ratio, the tip speed at a certain wind speed will be lower and so the noise production will be lower too but this is valid for both types of blades. So a tapered blade with a design tip speed ratio of 4 can be more silent than a constant chord blade with a design tip speed ratio of 6. However, generally it is the opposite. Tapered blades of very big 3-bladed wind turbines have a design tip speed ratio of about 8. Constant chord blades of 3-bladed wind turbines with a design tip speed ratio of 8 would be too slender and so rather sensible for flutter because of the low torsion stiffness or not strong enough because of a too high bending stress at the blade root.

[Adriaan Kragten]

A new chapter 7 has been added to report KD 738 about the VIRYA-6. The title of this chapter is: "Using of a resistance as load for heating". It appears that switching from star to delta is a good way to make the rotor start at low wind speeds and to generate power at low wind speeds.