Below is a graph of hourly production from Denmark’s onshore versus onshore wind last year.
Notice anything interesting? Yes, a 1-1 relationship between hourly onshore and onshore wind production does not exist. Instead, offshore wind seems to saturate with increasing onshore wind production.
[A quick note: Denmark has just over 1200 MW of offshore wind capacity.]
To make this clear, below I have plotted onshore versus offshore wind in alternative months for 2014. The blue line is a simple smooth through the data points.
It’s clear that there is a saturating effect every month.
As a mathematician my first thought after plotting this is what can be causing it. One possibility is that there isn’t a 1-1 relationship between the kinetic energy in onshore and offshore winds. This is possible, but I’m going to have to plead ignorance on that one. Perhaps a more enlightened reader can point me towards data on the relationship between wind speeds on and offshore.
An alternative explanation relates to the way wind turbines function. Kinetic energy follows the 1/2 * mass * velocity ^ 2 formula most people are familiar with. Therefore, the kinetic energy of the wind turbines that capture that energy goes up with the cube of wind speed.
However, designing a wind turbine to be able to capture energy at all wind speeds, or to capture it with equal efficiency makes little sense. As Eliza Doolittle had problems telling us, hurricanes hardly ever happen. And there is little point designing a wind turbine to be able to turn the energy in hurricanes into electricity.
Put simply, above a certain wind speed the turbine will simply throw out electricity at its rate capacity. There are diminishing returns from greater wind speeds. This can be put alternatively in terms of efficiency, as shown below:
So, above a certain wind speed you get less relative return on the wind energy flowing through the turbine.
And this possibly explains the relationship between offshore wind and onshore wind shown above. Offshore wind farms typically see much stronger winds than onshore wind farms. This is shown by the average capacity factors of Britain’s onshore wind farms (27%) versus offshore wind farms (35%).
Hourly wind speeds in onshore and offshore wind farms are strongly correlated. However, because those in offshore locations are stronger, the output from offshore wind farms is more likely to be capped. This is most noticeable at the times when offshore wind output is around 1,200 MW. Here almost all of Denmark’s offshore wind turbines are running at full capacity. Increased wind speeds on these occasions will make little difference offshore, but will onshore.
This will result in the saturating effect seen above.
Hourly onshore and offshore wind data is taken from PF Bach’s website.
Note to regular readers
I’m on a 3 month, and possibly permanent, break from Twitter. I’ll be writing 1 or 2 data driven posts like this for the next while. There is an automated thing set up here to tweet new posts to my Twitter account, but if you are tempted to respond on Twitter don’t because I’m on a break.
I will be trying to avoid hot button issues. And I will not be writing about the attempts of a certain libertarian Silicon Valley superhero to convince rich people to dole out $5,000 to keep the lights on during blackouts.