report KD 703 about a 4-buckets Savonious rotor available

[Adriaan Kragten]

Report KD 703 can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports. The tittle of this report is: "Ideas about a 4-buckets Savonious rotor called the VIRYA-1.45 for driving a positive displacement pump". A photo of a scale model of the rotor scale 1 : 5 is given on the front page of this report.

[Adriaan Kragten]

A new chapter 4 has been added to report KD 703 about the 4-buckets Savonious rotor. The title of this chapter is: "Alternative 4-buckets Savonious rotor with smaller blades and a larger rotor diameter".

The original rotor is rather heavy and so rather expensive. This alternative rotor uses blades which have half the width of the original rotor. This smaller blades can be positioned closer to the corners of the square plywood sheet which connects the blades to the hub and the rotor diameter is therefore larger (1.6 m in stead of 1.45 m). A picture of this alternative rotor is given in figure 2 of KD 703. I don't advise use of this rotor for generation of electricity but it might be a realistic option for driving a positive displacement pump. Figure 2 out of KD 703 is added as an attachment.

[Pierre159]

THank you for your marvellous work Adriaan,
not a little idea of the CP,
1 do you think the power of one unit is better than a savonius like this:
https://www.motherearthnews.com/sustainable-living/renewable-energy/savonius-super-rotor-zmaz74zhun/

2 do you think it could be enough torque to drive a car air contioning crompressor?

[Adriaan Kragten]

The point is that to keep it simple, I used a central plywood sheet half way the height. But aerodynamically it is much better to use a sheet on the top and on the bottom of the rotor. If there are two sheets, there is a channel in between the left and the right bucket and this makes that there will be a substantial flow through the rotor. This flow makes that the rotor works partially as a lift machine. The rotor will therefore have a much higher maximum Cp than a pure drag machine. But using two sheets makes that the tower ends at the bottom of the rotor and therefore you get a much higher bending moment in the shaft. The rotor will also be heavier with two sheets.

With only one central sheet, the flow can expand upwards and downwards and the flow through the rotor may therefore be much smaller than for two sheets. So I expect that the maximum Cp is much lower than for a normal 2-phase Savonious rotor (which has a maximum Cp of about 0.2) of for a 4-buckets Savonious rotor with two sheets but the starting torque coefficient might be large enough to drive a small positive displacement pump. The only way to be sure is to measure a scale model in the wind tunnel and to do that for several options, one with one central sheet and one with a sheet at the top and the bottom, one for the original rotor with square sheets and one for the alternative rotor with halved sheets.

But whatever option you chose, the rotor will be rather heavy and so rather expensive if compared to a HAWT, even if this HAWT has a low tip speed ratio and a large starting torque coefficient. So this extra investment is only justified if the costs of the transmission to the pump are much lower than for a slow running HAWT with a rectangular gear box at the head. Never use this rotor to generate electricity as a HAWT will run much faster and it will be much lighter and a high starting torque coefficient isn't required for a well designed PM-generator.

A compressor with pistons in it will have a much higher starting torque than the average torque. So to drive a compressor with a wind turbine, one needs a turbine rotor with an extremely high starting torque coefficient. I have done some test with a wind turbine driving a compressor and the air was used to drive an air bubble pump. This only works if the compressor is disconnected from the wind turbine rotor at low rotational speeds. The total efficiency of the system was very low.