Dr Gordon Hughes
The Performance of Wind Farms in the United Kingdom and Denmark
1. Onshore wind turbines represent a relatively mature technology, which ought to have achieved a satisfactory level of reliability in operation as plants age. Unfortunately, detailed analysis of the relationship between age and performance gives a rather different picture for both the United Kingdom and Denmark with a significant decline in the average load factor of onshore wind farms adjusted for wind availability as they get older. An even more dramatic decline is observed for offshore wind farms in Denmark, but this may be a reflection of the immaturity of the technology.
2. The study has used data on the monthly output of wind farms in the UK and Denmark reported under regulatory arrangements and schemes for subsidizing renewable energy. Normalized age-performance curves have been estimated using standard statistical techniques which allow for differences between sites and over time in wind resources and other factors.
3. The normalized load factor for UK onshore wind farms declines from a peak of about 24% at age 1 to 15% at age 10 and 11% at age 15. The decline in the normalized load factor for Danish onshore wind farms is slower but still significant with a fall from a peak of 22% to 18% at age 15. On the other hand for offshore wind farms in Denmark the normalized load factor falls from 39% at age 0 to 15% at age 10. The reasons for the observed declines in normalized load factors cannot be fully assessed using the data available but outages due to mechanical breakdowns appear to be a contributory factor.
4. Analysis of site-specific performance reveals that the average normalized load factor of new UK onshore wind farms at age 1 (the peak year of operation) declined significantly from 2000 to 2011. In addition, larger wind farms have systematically worse performance than smaller wind farms. Adjusted for age and wind availability the overall performance of wind farms in the UK has deteriorated markedly since the beginning of the century.
5. These findings have important implications for policy towards wind generation in the UK. First, they suggest that the subsidy regime is extremely generous if investment in new wind farms is profitable despite the decline in performance due to age and over time. Second, meeting the UK Government’s targets for wind generation will require a much higher level of wind capacity – and, thus, capital investment – than current projections imply. Third, the structure of contracts offered to wind generators under the proposed reform of the electricity market should be modified since few wind farms will operate for more than 12–15 years.