# Guesstimating in VAWT Design?

### A VAWT question arrived in my email.

Hello.  I enjoyed your various video’s showing your testing of the different generators and the build of your vawt. I was wondering if you had a way to determine the shape, number and dimensions of the blades for your vawt, or if you made it by “guesstimating”?

The FREEDOM pmg’s look to be a good choice after looking at all their info on their website.

Do you have figures or graphs showing the rpm of your vawt coupled to the FREEDOM pmg compared to wind speed? I am new to this windgen business and live in Belgium where the average yearly wind speed in my area is only 10-15 mph. I presume that I would need several pmgs or probably a (much) larger vawt connected to the FREEDOM to get enough rpm to generate acceptable power. I would use it to supplement solar power.

If you have some useful to guide my further decisions, I will gladly contribute to help you also.

Best regards,

Jon

Hello Jon,

I have a hard time explaining how I came up with my vertical axis wind turbine design, but I’ll try to share some of the thought process behind it. The shape, angle and number of blades were determined by experimenting with a small model in front of a very weak fan. But I did start the experiment with a few laws of physics in mind: The smaller the diameter of the VAWT, the faster it can spin. The greater the diameter of the VAWT the more torque it can generate as long as it can overcome the weight of its combined components. The more surface area you have for the wind to hit, the less wind it takes to move the turbine. On a horizontal wind turbine, the fewer the blades you use, the faster they can spin. On a vertical axis wind turbine, the fewer blades you use, the greater the wind resistance is on the blades cutting back into the wind. Too many blades and the wind will not penetrate into the turbine which means the wind will move around the turbine instead…

Another fact you should know, a vertical axis turbine cannot generate as high an RPM as a horizontal axis turbine. So to increase the RPM of a PMA being driven by a VAWT you would need to run it through belt and pulley, gears, or some type of transmission. Or to increase the power output you can use a higher output PMA than you would with a horizontal axis turbine. This is why I recommend the 48/96 volt Freedom II PMA from Missouri Wind and Solar. It’s like two 48 volt Freedom PMG’s in one housing. The two cores of the Freedom II can be connected in series which will about double the voltage output of a VAWT turning at a low RPM. I don’t worry about the VAWT making too much power because a VAWT is limited on how fast it can spin. A limit on RPM means a limit on power output. That may sound like a bad idea, but I like the power output to be more predictable which should make matching the rest of the power storage/distribution system easier to engineer.

Sorry I don’t have any power output graphs. No numbers I can Share. That would require a controlled environment like a wind tunnel to develop accurate information. That is something I just don’t have access to. So yes, I’ve used a lot of guesstimations in the design of my Stealth VAWT. However the length of the blades is determined by the sheet metal brake I have in my shop. If I had a sheet metal brake large enough, I’d make the blades 2 meters tall.

I hope this information is helpful. If you have other questions, or if I missed something please let me know…. The same goes for anyone reading this post. Use the comment section below to get the conversation started. Or private message me through the contact us page.

### 2 thoughts on “Guesstimating in VAWT Design?”

1. I really liked your tests but I doubt if Blue will like it. I like your humor. But I’m an old man to. I was wondering if you have thought about putting a stationary ring around it to direct the wind and get a steadier power out put on it. I was be buying from you shortly. Right now I’m working on a ceiling fan generator to use out in my shop. My power bills are really high here in Cincinnati. I’m changing all my light bulbs to LED’s and so on. Just to get my usage down a little. Good luck. Gene.

• I’ve thought of several different modifications that can be made to my design. Eventually I hope to have the finances and tools to explore all of those options. Good luck on that ceiling fan generator. Please let me know how well it works. I’m always looking for less expensive alternative generators for my projects. Thanks for the comment! Andrew

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