Making the Simplest DIY Wind Energy Generator - How Hard Could it Be? -- Episode 608

Hang on, those blades don't seem right. It is important to decide whether you are aiming for a rotor that uses lift, or one that uses drag.

The latter, known as the Savonius, has an assymetric appearance - typically using "scoops" whereby the wind sees the backs of the scoops on one side of the rotor and the open fronts on the other side.  The drag force on the open fronts is greater than the force on the backs, so a nett rotational force is produced. The spiral rotor that the presenter first tried (and which fell apart) was an example of a Savonius rotor. Because Savonius rotors rely on drag, the blades cannot move faster than the air that is pushing them, so they tend to be high-torque, low speed rotors. 

The second attempt, using two polyethylene sheets with a 90 degree twist, presents a symmetrical profile to the wind, so it doesn't seem like it can operate as a Savonius (drag) rotor.

The other type of rotor used in vertical axis turbines is the Darrieus. The blades have an aerofoil cross-section and experience lift as the air passes over them. They appear symmetrical at first glance, but of course the back of the aerofoil is presented to the wind on one side, and the front on the other. Importantly, the rotational force relies on the blades experiencing their own "headwind" as they rotate.  The resultant lift force from the headwind combines with the lift generated by the wind to produce a rotational force.  This means that vertical axis Darrieus rotors *are not self-starting*.  They do, however, spin considerably faster than the Savonius, albeit with less force. 

Those strips of polyethylene don't seem to have any aerofoil shape to them, so I don't see how they can generate lift.  But even if they do somehow take on an aerofoil shape when the wind blows on them, the rotor still wouldn't be self-starting.  And bearing in mind that they present a symmetrical cross-section to the wind, I don't see how they can act as a Savonius rotor either.

I presume that design - with the polyethylene strips twisted through 90 degrees - was found on the Internet somewhere, so perhaps they do somehow work to a limited extent, but they certainly aren't going to be "good" in any sense apart from simplicity of manufacture.

The Savonius is self-starting, quite easy to make, and generates higher torque than the Darrieus, albeit at considerably lower rotor speeds (because it uses drag rather than lift). I think the presenter gave up too easily on his first attempt at a rotor. That "air scoop" design - twisted so the torque is constant as it rotates - has a lot going for it. It just needs to be made more robust. I think he should get his generator working with the Savonius rotor and see how effective it is. Upgrading to a Darrieus - after having done a bit more research - can be the subject of another project and video. 

thanks for explaining the different kinds of wind generators designs and how they work, I'm sure alot of people find this interesting.

thanks for explaining the different kinds of wind generators designs and how they work, I'm sure alot of people find this interesting.