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|Evaluating my site for a Vertical axis wind generator|
NOTE! In researching roof mounted wind turbines I have come to conclude based on expert advice that they are considered impractical. It is easy to become seduced by the hype and the idea of having a sexy "green power" device on the roof. Please read the bottom of this page to see what I have learned.
March 28, 2010
One of the things I have noticed about the Maine weather since installing our solar power system is that when there's no sun, there is wind - sometimes a LOT of wind. So I figured I would do some research on wind generators. It would be great to offset the loss of solar power with a small 1 to 5 KW wind generator on the same roof top. The first thing I learned was that Maine is not a good place for residential wind systems. But it's all a matter of siting and selecting the right equipment, so I kept an open mind.
In my research I learned that the conventional wisdom is that you can't site a wind generator too close to trees or buildings. The rule of thumb being that the generator needs to be at least 20-30 feet above the nearest obstructions within 250 feet. These rules apply to the typical horizontal axis wind turbine (HAWT) that look like a big propeller. If these units are sited too low the upper part of the swept area will see more wind pressure on the blades than the lower half and this unbalanced load can quickly destroy the turbine. More importantly, the wind near any horizontal surface - including roofs - is too turbulent to yield good performance.
Our buildings, like most in Maine (known the Pine Tree State) are surrounded by trees. We have maybe a half acre of open land as you can see from the Google map - my workshop building is the lower right one. The strongest winds tend to blow from the north and the evergreen windrow at the north edge of our property is about 200 feet from the building. The road is lined by deciduous trees 50 feet to the east and some are as tall as the building. To the west is wide open for 100 feet and the tree line to the south are all less than 30 feet high. So my site is not optimal for a wind generator of any type - be it mounted on a tall mast or roof mounted.
|I read on the manufacturers web sites that vertical axis wind turbines (VAWT) can be sited in less than ideal locations like urban roof tops. They look like egg beaters and have the advantage that they can tolerate turbulent air and do not need to swivel into the wind. They turn slower, start at lower wind speeds, and tend to be quieter than their HAWT cousins. A number of companies have sprung up to make these turbines for residential roof top mounting and the list is growing. (This list on ecobusinesslinks is the most comprehensive). Many of them are quite attractive and could pass for modern art! The unit at right is made by Urban Green Energy and is designed to mount on flat urban roofs. The 30 ft tall 1KW Windspire at left can be roof or ground mounted with a relatively low ground clearance. Based on the availability - and relative affordability of these turbines I thought I would see if my site might be a viable candidate for a vertical turbine. Generally a 1 to 2 KW turbine with grid intertied electronics will cost in the range of $5-7000 from what I have seen to date. This means that the point at which they break even in ideal circumstanced is well over 10 years, but since ideal circumstances do not exist on a roof top I guess you would be looking at 40+ years. Not too exciting.|
Nonetheless I choose to be well informed. The best way to do a site survey for wind generators is to put up a wind gauge at the site and record data for a year. So I researched weather stations, and was originally put off by the high prices - most were well over $400 not including mounts etc. But then I found the Ambient Weather WS-1080 weather station that sells for under $100. This is a full professional grade weather station with the capability to log data and display it's data graphically on the web. So I ordered one on-line and went over to Radio Shack to get an antenna mount for the roof so I could locate the equipment right where the turbine would go about 10 feet above the middle of the roof.
NOTE: The original station (above) is no longer available, and I have upgraded it to the WS-1400-IP (left) includes a built in weather server that can sends data directly Weather Undergound for about $150. Other weather stations are also available here from Ambient Weather.
April 1, 2010
I got the WS-1080 and quickly set up the instruments to test it in the back yard for a week per the instructions. This is to make sure that it all works and the wireless system sends the information properly - it has a range of 100 feet to the base station. I'll move it to the roof when I'm sure it all works right. Both units are battery powered, and the touch screen base station connects to a PC via a USB port.
After several days of testing the system on the ground I climbed
up on the roof and installed it on a TV antenna mast. I used a tripod mount and 2 5 foot lengths of
It went up fairly quickly with my
Rebekah as ground crew and photographer. I was careful
to ground the mast to the solar panel grounding system - it's a
big lightning rod that may attract a strike.
April 3, 2010
I connected the web server to Weather Underground's web site, here's my official weather station page, and, and below is the current summary:
The weather instrument is visible above the middle of the roof as a white dot. I am really enjoying being a "weather geek" , looking at my weather station data has become an obsession for me. The more data I collect the less practical a wind turbine seems, but the process of learning about wind is very engaging.
May 1, 2010
Weather station update:
The issue now is how to extract meaningful information from the recorded wind data. If I were to take the average wind speed per day it would not represent the real performance for a wind turbine if half the day was calm and the other half was all 20+MPH wind. So I need to calculate average wind in smaller increments, I think that 10 minute averages make sense and hope to come up with a way to put those averages in a spreadsheet. Then I could input the performance characteristics of various wind turbines to see if they could yield a reasonable return on investment that would compete with solar panels.
Wind realities April 18,2010
I have had some email dialog with Mick Sagrillo a wind expert who writes about wind power systems for Home Power and other magazines. He has clarified that most small VAWT units do not live up to the hype in an excellent open letter on the AWEA site. They will not yield a return on investment that is viable if installed on a roof top. Here is a helpful FAQ. Unless the site has an average wind of over 15MPH of good clean wind these units will simply not perform well. Clearly this is not my site, nor is it likely that most roof tops would have the ideal siting for good performance. Ian Woofenden, an experienced wind installer has written an excellent 1 page article that debunks many of the VAWT myths for the June/July 2011 issue of Home Power (p.124) click here to download it.
But if I consider wind power as a augment to solar only on days when solar is low or at night then it may have value. It would still be cheaper to put up more solar panels, but I'm hoping a low cost unit may become available. I must admit that I just kinda like the idea of putting up a piece of "wind art" on my roof!
I will continue to monitor the wind and at the end of a year I will evaluate the number of hours where the wind exceeds an average of 15MPH and then see if there is a VAWT that has a proven track record that could produce useful power at my site. Mick thinks that this is unlikely and cites the formula Power = Velocity cubed. From this he suggested that if I wanted to generate 400 Watts in 15MPH wind I might need a turbine with a swept area of 300 square feet. That could look like a unit 10 feet wide and 30 feet tall! No so small, and definitely not cheap - nor would something that size be roof mountable or have a return on investment less than 100 years or more at my site.
The only other option would be for me to consider leasing a spot in my neighbors large (100+ acre) hay field and install a tower mounted turbine at 60 or more feet from the ground and 250 feet from the trees surrounding the field. Steve Wilke of Bergey Windpower has said , “You cannot change the site, but you can increase the height.” Well, given a choice of site and height - I could do both, but it would get VERY expensive.
Updated March, 2011
Well my average wind speed does not look good. Here's a graph of the data logged for the last year from my Weather Underground page:
click on the image to see complete weather data for the period shown
The average wind speed for this period was 4.9MPH. Note that the maximum wind gusts are significantly higher than the average wind. A small turbine is able to respond rapidly to sustained gusts and thus extract usable energy. So the daily average wind may not be the most realistic way to evaluate wind sites. Winter is definitely windier and I would hope that a small wind generator could offset the reduced solar hours - especially when it blows hard all day and night.
When you look at a fairly average windy day like April 30, 2011 below you see sustained wind of more than 10 MPH for several hours. This often happens on overcast days and at late into the night. But I would need a sustained wind of at least 8-10 MPH most days for a wind turbine to be a realistic investment.