Wind Power - A Rising Capacity Factor. Really?

The Capacity Factor (or annual fraction of maximum output achieved) has come to dominate the credibility of wind power schemes. The wind industry vigorously promotes the idea that the Capacity Factor (or CF) is rising. They eulogize wind power as an improving and developing technology. 

CF  has become the de facto metric by which energy generation is measured by. So a wind turbine system displaying a rising CF would undermine the growing view that wind is actually a moribund and subsidy addicted dead end. 

So there is a lot to play for by the wind turbine aficionados. Especially as today if you look at UK offshore wind turbine data it does look like the CF for later offshore wind farms in the UK is going up.

So is this due to an improving dynamic and forward looking technology?

Or is there something else going on here? Is this simply a “fix” - a manipulated figure. More smoke and mirrors to defend a stagnating technology?

While many factors ultimately determine the output of a wind turbine, the maximum output from a wind turbine is mostly determined by the the diameter of its rotor, the hub height and its location.

Yet the published Maximum Capacity (and so the calculated CF) of a wind turbine is determined from the size of the generator NOT from size of the rotor. Yet in reality the size of the attached generator is really a secondary limiter. It is rarely (if ever) run at maximum output and so makes little or no difference to the actual generation capability of the turbine.

The generator spends almost all of its life being driven at one fifth to one third of its maximum output. With this level of headroom, the CF is wide open to manipulation. It can easily be increased by reducing the relative size of the generator to the turbine swept area so that the smaller generator is driven harder and so shows a higher CF without actually increasing annual output. (In fact if you decrease the generator size too far you may push up the CF yet reduce the total energy output over the year.)

So is this happening to UK offshore wind? Are newer turbines being de-rated to increase the CF which will create the illusion that the technology is advancing? Has the Wind Industry any other potential motives as well?

It appears so.

Take the Walney Offshore Wind Park run by Dong Energy in the UK.

Walney consists of two phases. Walney One was commissioned in 2011 and Walney Two was finally commissioned in 2012. Both are now fully operational.

Walney One and Walney Two have 51 turbines each. All the turbines are rated at 3.6MW Maximum Capacity. But the turbine models are different.

Walney One uses Siemens SWT-3.6-107 turbines. These are 137m high, with a swept area of 9000m2 which gives a area/power density of 2.4 square meters per KW

The second tranche Walney Two uses Siemens SWT-3.6-120 turbines. These are 150m high, have a swept area of 11,300 square meters and a area/power density of 3.14 square meters per KW.

Essentially, while they both have the same size generators, Walney Two has bigger turbines.

Unsurprisingly, Walney Two has declared a higher capacity factor than Walney One. But given the quite large difference on swept area, the difference in CF is strangely small.

While the area power density differs by 30% the CF in the last year differs by less than 5%

If you normalize the turbine generator size on the area/power density of the Walney One turbines, (i.e so Walney Two would have an area/power density of 2.4 sqm/KW) then the Walney Two turbines should be rated and fitted with at least a  4.7MW generator.

If this was the size of generator attached to the Walney Two turbines then the capacity factor for last year (based on the 4.7MW generator size) for Walney Two actually decreases to a lowly 34%.

So then you have to ask: Why are these bigger turbines at Walney Two (in reality) being worked significantly less hard than their Walney One cousins? Why are they trading down the magnitude of the increased Capacity Factor?

Here I believe we have the second hidden agenda item associated with de-rating these turbines.

There have been long term and apparently intractable generic reliability problems with offshore wind turbines especially when under significant load. (see earlier post Here) So the trick to making your turbines avoid (example) catastrophic and immensely expensive gear box failure is to de-rate them and run them as far below their capability as is economically and practically possible. Even though the operator is paid around £150 per MWh, losing a gearbox will make a big dent in their profitability.

So for the wind industry, quietly fitting smaller generators to your turbines is a win-win. It falsely promotes the impression that turbine capacity factor has magically increased, while at the same time allowing them to de-rate these larger turbines and run them less hard so reducing costly repair and maintenance.

What this highlights is that the “maximum capacity” (based on generator size) as promoted by the wind industry is actually a fictitious value and bears little relationship to turbine capability or size. Calculating the effectiveness of wind turbines on this false flag is disingenuous.

So next time you hear some pro-wind zealot breathlessly announce that capacity factors are going up to 50% (and beyond) just ask them what the area/power value for this wondrous advance in turbine design is. I suspect they will look at you blankly.

Tell them that if they want to prove wind turbine capacity factor is significantly improving they need to compare LIKE with LIKE. But warn them, that if they do actually compare like with like, their magical improvements will most likely completely disappear. If not go backwards.