This the 1st of a series of articles that I hope to publish in the next coming months for anyone trying to find out a bit more about wind turbines and what makes them turn. I will start with small wind turbines and very basics points and little by little increase the technicality and complexity of wind site assessment which is often the bread and butter of Wind Energy Analysts.
These articles try to summarise the typical questions we get everyday from end users contacting us and by no means implies you should not look a wind analyst’s assistance.
When thinking of installing a small wind turbine at your home or farm, there are a few things to consider, but probably the most important can be summarised in three key points:
This is simple, the taller the wind turbine tower the better. The higher, the fewer obstacles and therefore the less turbulence on the wind and usually means higher wind speeds
(Real data provided by Logic Energy Ltd)
Above shows the comparison with two anemometer on the same wind mast, one at 40 meters and the other at 30 meters. The 40m anemometer is showing an average wind speed of 1m/s more than the 30m one.
Location, location, location, like the property market this point is probably one of the most important things to consider when thinking about installing a wind turbine. We may be in a windy area but the particular spot we may be looking at could be surrounded by obstacles like trees, houses or even towns a few miles away. These could have an unwanted effect not just on the increase of wind turbulences but also on the wind speed itself which could be severely reduced.
As you can see above, the higher the hub height of the wind turbine and the better located the wind turbine, the less turbulence on the wind and more steady flow of wind.
Size of the wind turbine
Think about the sails of a ship, the bigger they are the more wind they capture and therefore the more energy they can harvest from it. Wind turbines are not much different, the bigger the rotor the more wind they will capture.
Of course there are techniques to improve in efficiency output from the wind and all sort of technical advances but it is a rule of thumb worth keeping in mind.
Final point to make here which ties with Locations is that in certain areas where wind is too gusty or turbulent, or even too fast, in these places it may not be the best solution to install a wind turbine with a big rotor and other smaller wind turbines will be able to take full advantage of those high wind speeds.
Your wind analyst or consultant will be able to advice you on this when you have collected enough wind data measurements.
Planning your wind speed monitoring project
With these three rules in mind, we can look for the “perfect” site to install a wind turbine but before that, you will need to monitor your site’s wind speed for a period of time, the longer the better. This can be done in different ways but in general it is advisable to use a professional wind logger which will not just record wind data but also do real time calculations with the data in order to provide all the relevant information for the wind analyst.
A few things to get familiar with:
(Example of average, maximum and standard deviation wind speed on LeSENSE, provided by Logic Energy Ltd)
Wind monitoring Q&A
(Frequency distribution chart using LeSENSE, provided by Logic Energy Ltd)
More generic Q&A
Why do I need a wind logger or to record wind speeds at all?
By taking a decision on buying a wind turbine and not knowing the exact wind profile of your site, you may be taking a gamble. Small deviations on measured wind speeds as small as 0.4m/s could convert in several £10,000s over the 20-25 years of the feed in tariff. Wind analysts and Banks know all this too well, there’s no secret, just maths and good wind data.
I have checked on wind databases and my site comes with a good/bad average annual wind speed. Is this enough to make a decision?
No, it is not. Individual micro sitting of wind turbines are affected by the site’s landscape complexity. Sometimes a good average wind speed from a database can have not so good real wind data due to surroundings (trees, houses, valleys) and other times a not so good annual average wind speed from a database may result in a very good site given the individual characteristics of that micro site (ie: high elevation over the landscape)
“Weibull distribution" and other fancy words, what are they?
When you have an average wind speed and a site’s wind profile, you can then calculate a constant value to use on Weibull distribution algorithm. This allows to produce more long term energy forecasting. The key thing here is that you will need to know the constant parameter which with micro sites will come from measuring the wind speed.
Example with a small wind turbine
Let´s have a look at this chart, both of them belong to the same site. One is giving a wind probability based on Weibull distribution with k=2 (which is what the industry standards assume) and the other based on real data logged with the LeNETmobile wind data monitor. Both average wind speed of 3.5 m/s.
The chart below shows on the horizontal axis the wind speed in meters per second and on the vertical axis the wind probability per wind speed. In the case of the Real data there’s no probability, just real data!!
With the predicted data we found that we have a 12% probability of wind at 6m/s but with the real data, it shows only 6.5%. This seems bad news at first but look at the higher end of the wind speed: 10m/s, 12m/s, ... these are the sort of wind speeds that usually wind turbines work best, and have their higher efficiency!!
So far so good, but how does this relate to energy production?
We find the numbers of hours the wind has been blowing at different wind speeds for both sets of data (Real data and Weibull). With the total number of hours at different wind speeds, we put them together with the typical power curve supplied by any wind turbine manufacturer, and we find very different values:
Data collected (Real data): 1,492kWh in one month
Data estimated using Weibull: 1,086kWh in one month
You see the difference between knowning real data from your site and guessing it? And in this case it has been a positive approach but what about if it is the other way around? What about if we estimate more energy than really is available on the site? Definitely it is well worth to knowing the real potential of your site before investing in a wind turbine.
But... 1,500kWh per month is not a lot is it?
Well, all depends of the size of the wind turbine and the wind available at the site. This study was done with a very small wind turbine.
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