Wind Turbines: which turbine is right for the wind speeds in Devon and Cornwall?
The recent Super storm Sandy that devastated parts of the US east coast reminded me of a question Capture Energy often get asked, that is, 'do wind turbines survive high winds?' To attempt to put things in context, Superstorm Sandy had reported windspeeds in the region of 65mph.
There have been 150mph winds in the Shetlands, Scotland in recent years, with Kingspan KW6 turbines not just surviving, but continuing to generate! Not all turbines can survive this kind of extreme wind climate. Whilst most parts of the UK do not experience such extreme winds, it is important to look at a wind turbines suitability for the local wind climate.
Without getting too technical, one of the little discussed topics by wind turbine installers in the 5-500kW range is wind turbine classification. There are various elements to the IEC 61400 turbine classification but the one I want to discuss is the "Wind Speed Parameters for Wind Turbine Classes". It sounds like a boring, difficult topic – but it's actually essential to understand if you are considering installing a wind turbine. Hopefully my little introduction to the subject of wind turbine classification won't be too boring or difficult to get your head around.
There are four basic wind turbine classes, 1, 2, 3 & 4. The four 'classes' relate to the wind conditions a turbine has been tested (or designed) to survive and cover the following three areas which all relate to the wind climate at the turbine hub height:
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· Annual Average Wind Speed
· 50 year return gust
· 1 year return gust
The table above shows the values for the above three parameters for the four wind turbine classes, and the classification of our turbines.
The key element in the classification is the 'annual average wind speed'. Whilst the windier it is, the more a turbine will generate is true, not all turbines on offer are suitable for all UK wind climates. The most common turbine classification in the small-medium wind industry is class 3. A class 3 turbine is designed to survive an annual average wind speed of 7.5m/s at hub height. If the annual average wind speed at hub height is over 7.5m/s, then a class 2 or class 1 turbine is required.
The Northern Power 100 allows for a good example of how to choose the right turbine based upon site wind speed classification. It is available as a class 2 or a class 3 turbine, the difference being the rotor diameter. In lower windspeed sites, below 7.5m/s average, larger 24m blades are used to increase energy generation in class 3 site conditions. For higher wind speed sites, for example 8m/s at hub height, smaller, 21m diameter blades are used to allow the wind turbine to reliably survive the windier climate.
Following on from the 'annual average wind speed' are peak 50 year and 1 year gusts. A class 1 designed turbine like the KW6 can be installed in very remote windy locations (such as Alaska or the Shetland Islands) because it can withstand annual gusts of 115mph and 50 year gusts of 154mph. Whilst the right turbine can survive extreme conditions, the most problems happen with incorrect choice of turbine between classes 2 and 3 sites. We have seen many quite obvious class 2 windspeed sites, with class 3 turbines installed. This may provide excellent yields in early years, but more repairs and maintenance will be required over the 20 year life time. It is sometimes difficult to precisely define the classification of a site because wind speeds vary year on year and average wind speeds provided by desktop models will never be completely accurate.
Just to confuse matters a little, turbines will have a turbine classification based upon a test report; however they may be designed to withstand higher classifications of windspeed. For example, the Kingspan KW6 is a class 2 certified wind turbine, however it is designed for class 1 wind sites. The simple reason is that it has never been formally accredited with class 1 certification; however this should take place in the near future. Some manufacturers of class 3 turbines do occasionally grant concessions to allow their turbines to be installed in class 2 environments if the site has low turbulence. If any turbine is to be installed outside of its formal wind classification environment then approval will need to be sought from the manufacturer to confirm the warranty is not invalid.
In most sites in the UK a class 3 turbines will be suitable, however in parts of Scotland, Wales, Cornwall and other very windy sites, a class 2 turbine is likely to be required. Capture Energy offer a range of turbines to suit all wind climates.