These estimates, in turn, depend on assessments of the wind speeds at the site and the performance of wind turbines.
However, the performance of wind turbines — in common with most mechanical plants — may deteriorate over time, leading to reduced output, increased costs, or both.
In the case of wind turbines, power output may deteriorate if the surface of the blade becomes soiled, and energy will be lost if the turbines need to be stopped in order to mend or replace defective components.
Repairs are particularly important in the case of offshore wind turbines, as weather conditions may prevent access to them, and so availability falls.
The Backpage slot in our November issue carried a brief critique of an assessment by the Global Warming Policy Foundation that argued that the performance of the world’s first offshore wind farm — Vindeby in Denmark — had fallen well short (by 45%) of expectations.
The Global Warming Policy Foundation was incorrect; the output over 20 years was about 15% lower than anticipated. (This is a more accurate estimate than the 20% figure quoted in November).
In order to establish whether this is part of a general trend the performance of other offshore wind farms can be examined, using data published by the Danish Energy Agency.
The second offshore wind farm to be commissioned was Tuno Knob, also in Denmark. This comprises ten Vestas 500kW turbines with 39-metre diameter rotors and was commissioned in 1995. It is still operating and has thus exceeded its projected lifetime of 20 years.
The output data from Tuno shows that, rather than a decline in performance, the output actually increased slightly over the 20-year period it has been operating.
The better-than-expected performance is probably the result of the availability being higher than assumed at the planning stage. The average capacity factor over the period 1996 to 2016 was a fraction over 31% compared with the projected value of 29.7%.
This result suggests that any decline in performance due to aerodynamic or mechanical factors is negligible, although the below-target capacity factors in some years may be due to reduced availability.
However, the lowest capacity factor at Tuno — in 2006 — was probably due to this being a year of below-average winds, according to the Danish Wind Index. Similarly, the high capacity factor recorded in 2015 corresponded to a year with high winds.
It is possible to correct the measured capacity factors to allow for the variations in the wind. This reduces the scatter on the plot and suggests that the average capacity factor at Tuno is around 32%, some two percentage points higher than the original projection.
In terms of age, the next offshore wind farm to be commissioned was sited off Copenhagen and has been operating for just over 16 years.
Applying the same method of analysis as for Tuno and Vindeby suggests that the capacity factor has declined by about one percentage point over the period. Energy production exceeds the estimated value, however.
Four other wind farms were commissioned between 2004 and 2008, and all these appear to exhibit an upward trend in capacity factor with time, but it is too early to draw definitive conclusions regarding the 20-year performance.
The slight increase in capacity factors for the Tuno wind farm from 1996 to 2016 could be due to the winds becoming stronger over time. However, there is no evidence that this is the case.
Whether or not the effects of climate change will lead to increases or decreases in wind speeds is uncertain. Some studies suggest wind speeds will increase, others that they will decrease, but there is a consensus that the changes will be small.
At a glance – This month’s data sources
Danish Energy Agency, Master data register for wind turbines
EMD International, Danish Wind Index vindstat.dk