Introduction to Wind speed monitoring for wind turbines

This the 1^st 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:

• Height
• Location
• Size of the wind turbine

 Height

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, 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:

• Average, maximum and standard deviation wind speeds: These are usually given in periods of 10 minutes.
  • Average: This is the mean average of wind speed during the 10 minutes period, usually given in meters per second [m/s]
  • Maximum: This is the maximum or gust wind speed measured during the 10 minutes intervals
  • Standard deviation: This is an indication of the wind turbulence intensity during the 10 minutes interval. We need to make sure the “wind turbine is going to work with the wind and not against it”
• Wind direction: you will need to know from where the wind comes most of the time and at which wind speeds.
• MET mast or Wind mast: This is used by professionals with anemometry or wind sensors mounted onto them. Masts need to be free of any vibration or “wobbling” as these will impact the readings taken by the wind logger. Usually a wind mast will have several sensors at different heights in order to get a better of the wind at the site. Things like roughness length or wind shear are very important but also temperature and pressure.
• Wind shear: This is calculated by measuring wind speed at different heights. By measuring at different heights we can get a better understanding of the turbulence intensity but also the impact on wind speeds for higher heights. This is also often used to “extrapolate” to higher heights when measuring at hub height of the wind turbine is not possible. Some other benefits of measuring at different heights is to keep options open when it comes to choosing different wind turbines.

(Example of average, maximum and standard deviation wind speed on LeSENSE, provided by Logic Energy Ltd)

Wind monitoring Q&A

• How long do I record or monitor wind data?
  • You will need several months to take an informed decision. Ideally no less than 6 months and better to have full year.
• When you have all your wind data ready what do you do with it?
• Ideally let a wind analyst to look at it, definitely if the wind speed seems reasonable and the site promising.
• Send it to your preferred wind turbine supplier, they will be able to tell you how much energy you should expect from your site.
• Calculate the average wind speed, this will give you a generic idea of how much wind speed the site has.
  • In general for small wind turbines that are going to be privately funded, things start to look a bit interesting over 4m/s.
  • If the wind turbine is bank funded, usually you will need wind speeds over 6m/s
• Calculate frequency distribution, this will give you a much clearer view of the wind profile of the site by distributing or dividing the different wind speeds into sections.
  • Once you know how many hours the wind has been blowing at different wind speeds (usually in bands or bins of 1m/s) you can multiply these by the power curve given by the wind turbine supplier. The sum of all these will give you the energy your site would have generated if you had that particular wind turbine on the site.
• Failing anything else… contact us and we will put you in contact with some specialist.

(Frequency distribution chart using LeSENSE, provided by Logic Energy Ltd)

Equipment

• Anemometers, these should be calibrated with a traceable certificate. With an accuracy better than 1% and a resolution of 0.1m/s. It is always recommended to use at least two anemometers.
• Wind vane, ideally 1* resolution
• Wind logger or monitor:
  • It should be able to store historical data. Minimum the following parameters for the wind speed:
    • Average, Maximum and Standard deviation of the wind speed
  • Wind direction
  • Advisable to measure temperature and/or pressure, humidity.
  • Advisable to have spare channels for extra anemometry equipment or other sensors
  • Power supply of the wind logger or monitor device.
    • For short periods of time it is ok to use Alkaline batteries when data can be retrieved quite frequently
    • For longer periods of time it is highly recommended to retrieve the data remotely (ie: via GSM or GPRS) and use PhotoVoltaic kit to power the wind monitor

 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.