I have recently measured wind speed in my back garden for 1 year from April 23rd 2008 to April 23rd 2009 at 4 m. We live in Upperchurch in Co. Tipperary and the area would be considerd by locals and visitors a windy place all year round, and bleak spot in the winter.
I then applied a scaling factor of 0.61 (i.e. divide the reading by 0.61) to bring the wind speed up to 10 m height this figure was based on data provided by http://www.sustainability.ie/microwind.html and had a higher shear factor than RETscreen recommended i.e. I was generous in my estimation.
Accorrding to the wind maps the wind speeds for Upperchurch are 6 – 6.25 m/s at 50 meters. At 10 meters my average measured wind speed was 3.83 ms (this is upscaled) the measurement at 4 meters was 2.3363 m/s .
Number of hours for each wind speed were as follows.
| April 23 2008 – April 23 2009 extimated wind speed at 10 meters height |
|||||
| m/s | Hours | ||||
| 0 | 1261.833 | ||||
| 1 | 747.4167 | ||||
| 2 | 890.8333 | ||||
| 3 | 1443.333 | ||||
| 4 | 1395 | ||||
| 5 | 1040.583 | ||||
| 6 | 393.4167 | ||||
| 7 | 597.5833 | ||||
| 8 | 390.75 | ||||
| 9 | 243.9167 | ||||
| 10 | 84.91667 | ||||
| 11 | 110.9167 | ||||
| 12 | 75.16667 | ||||
| 13 | 40.91667 | ||||
| 14 | 23.25 | ||||
| 15 | 20.16667 | ||||
Using Mike Sagrillos Wind Swept Area Calculations I generated a payback table based on wind turbine diameter only I calculated the income that would be generated from selling back to the grid. The first 3,000 KW hrs are priced at 19 cents and the remainder at 9 cents.
The results are as follows:
| Diameter (meters) |
Turbine Output at 10m/s | Kw Hrs | Income |
| 2 | 0.77 | 1159 | 220 |
| 2.2 | 0.93 | 1402 | 266 |
| 2.4 | 1.1 | 1669 | 317 |
| 2.6 | 1.3 | 1958 | 372 |
| 2.8 | 1.5 | 2271 | 431 |
| 3 | 1.7 | 2608 | 495 |
| 3.2 | 1.96 | 2967 | 563 |
| 3.4 | 2.2 | 3349 | 601 |
| 3.6 | 2.48 | 3755 | 638 |
| 3.8 | 2.78 | 4184 | 676 |
| 4 | 3 | 4636 | 717 |
| 4.2 | 3.4 | 5111 | 760 |
| 4.4 | 3.38 | 5111 | 804 |
| 4.6 | 4 | 6131 | 851 |
| 4.8 | 4.4 | 6676 | 900 |
| 5 | 4.8 | 7244 | 952 |
| 5.2 | 5.18 | 7835 | 1005 |
| 5.4 | 5.59 | 8450 | 1060 |
| 5.6 | 6.02 | 9087 | 1117 |
| 5.8 | 6.45 | 9784 | 1177 |
| 6 | 6.9 | 10432 | 1238 |
These figures should help you to estimate the payback time. Look up the turbine’s diameter and look across to the income.
A breakdown for a 3 meter radius turbine is provided below
| 3 meter radius turbine | ||||||
| Rotor M | Rotor feet | m/s | mph | Power (kW) | Hrs | Energy (kWh) |
| 3 | 10 | 3 | 6.75 | 0.046624 | 1443.333 | 67.29332 |
| 3 | 10 | 4 | 9 | 0.110515 | 1395 | 154.1685 |
| 3 | 10 | 5 | 11.25 | 0.21585 | 1040.583 | 224.6096 |
| 3 | 10 | 6 | 13.5 | 0.372988 | 393.4167 | 146.7398 |
| 3 | 10 | 7 | 15.75 | 0.592292 | 597.5833 | 353.9436 |
| 3 | 10 | 8 | 18 | 0.884121 | 390.75 | 345.4701 |
| 3 | 10 | 9 | 20.25 | 1.258836 | 243.9167 | 307.051 |
| 3 | 10 | 10 | 22.5 | 1.726798 | 84.91667 | 146.6339 |
| 3 | 10 | 11 | 24.75 | 2.298368 | 110.9167 | 254.9273 |
| 3 | 10 | 12 | 27 | 2.983907 | 75.16667 | 224.2903 |
| 3 | 10 | 13 | 29.25 | 3.793775 | 40.91667 | 155.2286 |
| 3 | 10 | 14 | 31.5 | 4.738334 | 23.25 | 110.1663 |
| 3 | 10 | 15 | 33.75 | 5.827943 | 20.16667 | 117.5302 |
| Total Kw | 2608.053 | |||||
| Income | 495.53 | |||||
very interesting i had heard that windmills don’t pay for themselves i was looking into buying one but with this kinda information i don’t think i will bother
p.s.
well done it must have taken alot of time and effort to produce this information
Hi, Well done on the research – we need more of this.
I wonder if you tried plotting the shear using the calculator at http://www.windpower.org/en/tour/wres/calculat.htm – I can’t do it without having some idea of the roughness class of your land, but it would be interesting to see how the plot using SEI’s wind map figure of 6m/s at 50m and your roughness class and using the plot there to see how close that comes to your actual readings at 4m and your shear estimate at 10m?
I think most turbines with a 3m blade would have some control to prevent them giving over 5kw. This is always a design issue as the output really ramps up when you get above 11 or 12 m/sec, and most have an inverter that would blow a gasket if you sent 5kw down the line.
However, unless you live at a very windy site, the lost power from cutting out at 14 m/sec isn’t that much, and your figures bear that out. Although the power output at 15m/sec at 5kw is very tempting, 117kwhr per year probably doesn’t justify the cost of a larger inverter etc., especially if this additional power only brings in revenue at 9c / kwhr
Q
Liam,
I have just read some of your earlier posts and have tried to follow through on your later ones.
The measurement of the wind speeds is something that i have wondered about.
One of the questions raised by Quentin in his post is the roughness class and i would also be interested in knowing what the local factors were. As you measured the wind at a height of only 4m it is likely that this would be significantly influenced by any obstructions. Where was your anemometer mounted?
As you say the local wind speed from the wind atlas is 6m/s @ 50m. Using the shear formula and a roughness class of 0.1 (Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 500 metres) the wind speed at 10m is 4.446m/s and at 4m is 3.5615m/s.
Using a roughness of 0.4 the speeds further drop to 4m/s and 2.864m/s much closer to your measured 2.33m/s.
So it would be interesting to see what your opinion on the roughness of the location is and an actual description or sketch of the measurement set up.
A quick backward calc seems to say that your measured speed would mean the roughness class would be around 1.24 which would put it above an city centre figure!!
So, is it the maps, the measurment, the location? Or are we missing something…
Regardless well done on putting in the effort. Would be good to hear your thoughts on the above.
Hi Cormac,
Thanks for your comments.
You are right about the shear factor and this is something I am painfully aware off.
The issue I think is that the weather station was 30 meters from the house.
The idea behind the measuring was to assess if a windy back garden could support a wind turbine, local knowledge would say yes but measurement says no as the exact spot was not open to the winds from the south east to the north west.
Unfortunately our house blocks wind from the west and wind from the south west has to contend with a 8 metre house 80 meters away. There is a constant breeze/wind where we live but buildings do wreck havoc.
So even though its not the city centre the rule of thumb of being 20 times the height of your house away from a turbine does apply and make sense in this instance.
Regards
Liam