Indices already exist for Denmark, Germany, Netherlands and Sweden, calculated from the energy production reports of strategically selected wind plant. But this approach has not been possible in the UK because of the commercial sensitivity around wind farm production data.
Garrad Hassan has produced an index based on meteorological data. The firm's Colin Morgan explains that the problem, historically, with using met data has been ensuring that the Meteorological Office weather stations used are highly consistent and give good national coverage. "For our analysis, the met stations were selected by the same process we go through when selecting those to be used in a bespoke wind farm energy assessment," he says. This involves checking the long term consistency of station wind speed records and verifying the consistency of the records with Met Office staff at the stations and against stations at other locations.
"These all add up to a pretty onerous and time consuming set of criteria to apply," says Morgan. "Starting off with around 200 possible UK stations we find that for our ten year index, we end up with just nine stations." One of the problems has been that met stations have been through a series of equipment modernisation programs over the past 20 years. "This is good news for accuracy but bad news for consistency," he says. "However, we are confident that the nine remaining stations give satisfactory geographic coverage and we have a good degree of confidence in them."
Morgan adds that basing the index on met data gets around the difficulties experienced by other indices based on production data. "Production data is a function of the windiness, site exposure, hub heights and turbine performance," he explains. "To get a consistent index you need to find a way of maintaining consistency in the last three of these. This is difficult as through time projects are developed on lower wind sites with larger turbines and higher hub heights. Issues around this have led to a wholesale revision of the German index recently."
Nonetheless, one of the most interesting findings from the British index is its high degree of correlation with other Northern European indices, Morgan points out. In other words, a windy year in Britain is likely to correspond with a windy year in other Northern European countries. This news represents a blow to wind farm investors who believed they could mitigate wind risk by building a portfolio of projects across Northern Europe, he says.