In December 2012 the Renewable Energy Foundation published a study by Dr. Gordon Hughes [backup link] that detailed the degradation of capacity factors in Denmark and the UK over time.  As it happens, I’ve been posting on this issue ever since John Harrison first brought it to my attention over a year ago.  John and I and others have run our own numbers, obtaining results that are fairly consistent with Hughes’ results.  First I’ll summarize his report and then discuss the similarities.

Hughes’ report runs a total of 52 pages, of which the first 22 are the body and the remainder are appendices.  The body’s format is 45 points, each of which provides a paragraph on his various findings.  I’m not (thankfully!) going to go through all 45 as the body is fairly well written and is accessible to a general audience.  In summary he went through the same sort of process as John and I did in a more rigorous manner.  He took the published output of wind projects in the UK and Denmark, normalized them with wind speeds and graphed the declines over time.  His sources of data look to be more complete than what John and I had to work with, so his numbers are likely more accurate.

He calculated the capacity factor declines for 3 different groups of turbines:

  1. UK Onshore: the CF went from 24% to 15% over 10 years and further decreased to 11% over the next 5 years.  This averages 0.9% decline per year.
  2. Denmark Onshore: the CF went from 22% to 18% over 15 years, a decline of 0.26% per year.
  3. Denmark Offshore: the CF went from 39% to 15% over 10 years, a decline of 2.4% per year.

These declines were statistically significant; there is little chance the slopes were the wind industry’s hoped-for zero.

To me the more important of Hughes’ 45 points are:

  • Large wind projects in the UK perform less well (and degrade faster) than small wind projects.
  • More recent projects in the UK perform less well (start at a lower point) than older projects.
  • Given the declines of the projects in the UK, a repowering would seem to be necessary at about the 15-year mark.
  • Given the declines of the projects in the UK, getting to their renewable energy goals will be much more difficult and expensive than current plans.
  • Denmark’s declines of onshore turbines may be due to better maintenance, or smaller projects, or better citing.  Or some combination.
  • Denmark’s large declines of their offshore turbines may be due to a harsher environment or that they are earlier in the development cycle.
  • The UK’s plans for offshore turbines might be in trouble, given the large declines of Denmark’s offshore turbines.

While of course one can quibble with his numbers and conclusions, Hughes’ work appears to be fundamentally sound.  The fact that his work is consistent with what I’ve posted here previously certainly helps.

The issuance of this study was duly reported in the press.  It got enough notice that EWEA felt obliged to respond.  I’ll have to note that EWEA’s response is pretty typical for wind proponents – lots of ad hominems but no numbers of their own and no refutations of his methods.  Science at its finest.

How do his numbers compare with what I’ve posted?  Here’s a listing (all in annual CF percentage declines):

UPDATE – Bach has posted a response to the Hughes study, in which he discusses how his results can differ from Hughes.  It gives you a good sense of the issues all of us encounter when trying to dig out the truth.  In addition, if you want a non-glossy view of Denmark’s electrical system his entire site is worthwhile.