Does offshore wind turbine performance deteriorate with age?

18 December 2020

In an earlier article we looked at the performance of onshore wind turbines in Great Britain. In this article we perform a similar analysis on offshore turbines.

As a useful reference, take a look at this paper, which not only sets out the issues and arguments involved but also quantifies the deterioration or at least the scale of the deterioration.

Many of Great Britain's offshore wind turbines have now been operational for several years. What do the actual observed generation data for these wind turbines tell us?

Load Factor vs Age

The key metric we use is the load factor, which is the ratio of actual performance to rated capacity.

This chart shows the load factor for 2019 for larger offshore wind farms in Great Britain. Measured load factor is plotted against wind farm age in years. Hover over a plotted value to see which wind farm the point refers to. A full list of the wind farms appears at the end of this article.

As with onshore turbines, there seems to be a relationship between load factor and age. The older the wind farm, the lower the load factor appears to be. This aligns with the results of other research into this as referenced on our Wind page.

The red line (line of regression) show the "best fit" for the load factors. Regression Analysis indicates a deterioration of about 1% of rated capacity per annum. For a twenty five year old wind farm that initially achieved a load factor of about 45%, after 20-25 years this will have about halved.

In terms of statistical significance, the coefficient of correlation is -0.76, which would support the theory that there is an inverse linear relationship between load factor and age, with a very high level of confidence.

Have older turbines always been less efficient?

Could it be that older turbines have always been less efficient, and that the relationship doesn't reflect deterioration in turbine efficiency over time? Perhaps it shows that newer turbines are more efficient.

We can check that out using historical data on individual wind farms. The Renewable Obligation Certificate data from Ofgem provides monthly generation data, and we'll look here at one of the older wind farms and then one of intermediate age. (For very new wind farms natural variations in wind patterns make it difficult to detect trends with enough confidence.)

This shows deterioration, although at a lesser rate. Note that this is a small offshore wind farm in comparison with current capacities.

Again, the deterioration is noticeable and significant.

What does the National Grid say?

The load factor used by the National Grid for planning purposes is around 40% - 43% for offshore wind.

Their plans and scenarios show little or no appreciable deterioration in load factor for offshore wind over time, usually staying fairly flat or even increasing a little.

Given that they are modelling performance for 30 years into the future, all of the wind turbines currently in use in Great Britain will have been replaced or repowered at least once during that period. It follows that replacement or repowering will need to restore the load factor for each wind farm to a level that maintains the validity of the National Grid's idealised model.

At present the National Grid appears to be significantly overestimating the amount of energy that will be produced by offshore wind farms in the future, particularly as the older turbines will represent a greater proportion of those in operation over time.

The oldest large offshore wind farms are only generally half way through their planned lives. As they approach end of life their performance will continue to deteriorate. If they are not replaced or adequately repowered, the National Grid's projections for offshore wind generation will become even more over-optimistic.

Over the coming years we will be able to see whether offshore wind farms are achieving the performance that the National Grid is basing its plans on.

Offshore wind farms in our analysis

The following table lists the 31 offshore wind farms used in our analysis.

These were chosen for the following reasons:

• The wind farm was fully commissioned before 1 January 2019, and it has been operational since then
• The wind farm has a rated capacity of 60MW or more.

At the time of writing all offshore wind farms in Great Britain that satisfy all of these requirements are included in the list except Rampion.

Rampion Wind Farm was out of action for a significant proportion of 2019. The published cause was an issue with the power supply. Our purpose here is to look at deterioration due to "age", and so we have chosen to omit Rampion from our analysis as its presence would distort our analysis. Having said that, clearly breakdowns can affect load factor, and presumably the National Grid's modelling and planning includes an allowance for loss of energy due to such breakdowns.