Science Buddies: "Ask an Expert"

Hi

I was wondering:

(1) Is it most efficient to build a windmill as light as possible? Or does it make much difference?

(2) I know windmills are usually built in areas with consistent wind speed, but the wind must gust occasionally. When the gusts (especially when it speeds up fairly slowly, but slows down very rapidly) occur, does the rotor speed of the windmill change as quickly as the wind speed? (in other words if the wind stops suddenly, does the rotor stop quickly too, or does its momentum keep it spinning for a while?)

Thank you for taking the time to read this. I hope I was clear enough what I was asking.

Hi,

Here are some general resources on windmills that may be of use:

http://earthsci.org/mineral/energy/wind/wind.html

http://209.85.173.132/search?q=cache:s8 ... cd=1&gl=us

http://en.wikipedia.org/wiki/Wind_turbine

http://www.aacog.com/air/curriculum/Tes ... erator.pdf

I regret I was not able to find definitive answers to your questions in the time I had available. I did see statements that commercial windmills are designed to adapt to variable wind speed, including to shutdown at high wind speeds, and to either operate at constant rotation rates or have compensating circuitry to produce constant current frequency with variable rotation rates. As to whether the windmill will respond instantly to wind speed change, I believe the answer is that it will respond rapidly but not instantly to wind velocity changes. You can test this experimentally yourself fairly easily(see for example the bottom link above).

I do not think that low inertial mass in the rotor helps the efficiency much - I have seen evidence that the primary efficiency driver is the airfoil design of the windmill rotor. You may want to explore the Betz limit:
http://www.reuk.co.uk/Betz-Limit.htm

And its derivation:
http://www.windpower.org/en/stat/betzpro.htm

The above link is to a Danish Wind Power Association website that deserves careful exploration.

You obviously have picked a topic of great current national importance. I wish you great success and fun with it!

Best regards,

Barrett Tomlinson