Having been one of the original pioneers of wind energy in the UK, Windcluster is now amongst the first to explore wind turbine life extension. Our Vestas V52 turbines were installed in 2005 and were originally designed for a 20 year lifespan. As detailed below, the turbines are still in such good shape after 15 years that the directors have successfully applied for the consents for a life extension to the turbines to enable the site to run until 2040.
The Turbines
A schematic illustrating the modelling of remaining life. The projected failure rates of key components are predicted based on the life history of the turbines. Each will lie within a bell shaped distribution, indicating which components will need to be replaced, refurbished or can be left in place.
The turbines at the Windcluster site were classified as IEC Class 1A turbines. This means that they were designed to withstand an annual average windspeed of 10m/s and turbulent wind conditions (18% turbulence rate). Our site at Haverigg has an annual mean windspeed of around 8.2m/s and a turbulence intensity of 12%, considers to be “exceptionally low” by Garrad Hassan, the authors of our original wind speed measurement report. The reason for this is that the site is very exposed to winds from the sea, which are much smoother than those from the land.
Structural fatigue
All engineering structures are designed to resist a combination of extreme loads and long-term cyclical loading. For wind turbines, the long-term cyclical loading cases tend to be the main design drivers. This is because of a phenomenon known as structural fatigue. If a component is subjected to repeated load, even at levels much below its extreme failure load level, it will eventually fail, and different materials have different fatigue characteristics. In general, most engineering materials (and certainly the main materials in wind turbines) follow a power law relationship between the applied stress and the number of cycles. This means that even a relatively small difference in stress levels can result in large differences in the number of cycles that a structure can tolerate before failure.
Because the severity of the cyclical loading is strongly dependent upon the turbulence of the wind regime, we expect that our turbines still have substantial reserves against fatigue failure.
One careful owner
In addition to operating in conditions that are relatively benign when compared to the design specification, our turbines have been very well maintained. We have had the comprehensive Vestas AOM4000 service contract throughout their lifetime and also the site has been managed by WPO.
Re-powering?
As an alternative to extending the life of our existing turbines, we did consider re-powering the site, but rejected it for a number of reasons. Firstly, the electrical connection capacity at the site is very limited, so we would have had to more or less replace like with like. However, since 2004 when we ordered our turbines, the industry has changed dramatically and the smallest machines that most manufacturers now make are at least twice the capacity of the V52s. This greatly reduced our options for new turbines.
While the planning regime for re-powering is less onerous than for new projects, we were advised that it was still a potentially expensive and risky path to follow, with life extension likely to be a much smoother process.
The failure stress of a component follows a power law, which appears as a more or less straight line on a logarithmic plot. Towards the extreme end of the curve, even a small difference between the operational and design stress levels can result in large increases in the number of cycles before failure, and thus the lifetime of the component.
Lastly, even though the embedded energy in our turbines has been repaid many, many time already, it was appealing to follow the re-use, repair route rather than replace.
Life extension permissions
We needed two consents in order to extend the length of time that the turbines could remain on the site - the agreement of the landowner and planning consent from the local authority. The Craghill family, our landlords, readily agreed to extend our existing lease, so the next step was to secure planning consent. We engaged Arcus Consulting to help us and undertook a range of environmental studies to meet the requirements of Natural England. An application to extend the consent by 15 years (until 2040) was submitted in April 2020 and despite Covid-19, Copeland Borough Council were able to process the application and it was unanimously approved in October.
What to do?
We have several years until the turbines reach the end of their original life in 2025, so we have ample time to plan how we are going to manage the life extension. In addition to reaching 20 years of operation, 2025 will see the end of our Renewables Obligation support, so the income will drop and with it the optimum balance between costs and availability will change.
Our first steps are to commission an engineering study of the remaining life, so that we can identify which, if any, components may need to be either replaced or refurbished at that point. At the same time we will install a condition monitoring system so that we have better knowledge of the condition of the turbine components. We will also establish a baseline dataset from which we can monitor future performance of key components, and develop a preventative maintenance strategy.