Winds over the ocean typically have higher wind speeds, resulting in high efficiencies of wind turbines and making this an attractive environment for generating a lot of renewable energy. But how efficient are turbines going to be when offshore wind farms become larger and larger?
Figure. As wind farms expand in scale, they increasingly reduce regional wind speeds. This effect results in lower turbine efficiencies. The numbers represent typical climatological averages for the North Sea. Source: MPI-BGC/Agora Energiewende.
As it turns out, the answer is not to be found in technology, but in atmospheric science. It goes back to our work about ten years ago in which we looked at the limits to how much wind energy can at best be used (published in Earth System Dynamics here). We looked at this in a rather hypothetical way, plastering whole continents with wind turbines in a climate model. The key point is about what turbines do to the atmosphere: they remove kinetic energy, so they leave less behind, because the ability of the atmosphere to generate and transport kinetic energy is thermodynamically constrained to a rather low power and renewal rates. For a single turbine, this effect does not matter. But the more turbines are being considered, the more pronounced this effect is going to be, which we showed here. This effect goes well beyond the wakes found behind each turbine, which is well understood, modelled, and accounted for (see Figure). Wakes disappear by mixing with the surrounding flow, but kinetic energy has nevertheless been removed from the atmosphere by the turbines and some more has been dissipated by the mixing. To account for this effect, one needs to have a way to deal with the wind speed reductions in the surrounding flow that is caused by the turbines. This is what a climate model does well, but observations cannot do. The basic finding of our study have since been reconfirmed by follow-up studies (e.g., by Jacobsen and Archer, Marvel et al, from us (Miller et al.), or Volker et al.). What it implies is that as wind energy use expands in scale, wind turbines must become less efficient in generating electricity because regional wind speeds need to be lowered because of the turbines (which we showed here). This effect is irrespective of the technology. It results from a limitation set by the atmospheric environment.
The next question is, of course, whether this effect of lower winds is just hypothetical, or whether it is relevant? This is where the Agora Energiewende study comes into play. It started with informal exchanges in 2016 and subsequent workshops, where we met some of our critics (based on this comment by Badger and Volker and our reply). It led to a small project, called OffPot, in which we re-evaluated some of the scenarios for offshore wind energy in German energy scenarios for 2050 and their underlying assumptions regarding turbine efficiencies. We found that for those scenarios, the effects already become relevant, the expected yields can become quite a bit lower, and these effects need to be accounted for in the scenarios.
While this may seem like not such great news for offshore wind energy, I found this outcome a very rewarding scientific experience. Our initial, contrary views on wind energy have converged into a better, more differentiated view. Yes, efficiencies are likely to go down with the expansion of offshore wind farms because of reduced winds, but there is still a lot of renewable energy to be made. This is a science-informed perspective that is more realistic than some idealists like Jacobsen claim, but still a lot more optimistic than some of the critics of renewable energy. In my view, this is what progress is about.