4. Comparison with field data for a large wind turbine - the Vestas V80 2megawatt (offshore).
There is surprisingly little published data that gives full details of the operating performance of a large wind turbine in an installed situation. However, there are two reports on offshore wind farms in the UK. One of them is produced by E-ON (UK) on the performance of their offshore Scroby Sands wind farm off the Norfolk coast and the other was produced by Npower Renewables Ltd on the offshore North Hoyle wind farm off the North Wales coast. In both cases, Vestas V80 2mw offshore turbines were used. Both reports cover an operating period of a year - 2005 for Scroby Sands and 2003-2004 for North Hoyle. The table below lists the important results.
The figure below shows the steady power curve and turbine efficiencies for the Vestas V80 (offshore design) obtained from Vestas information pamphlet. With peak efficiencies of 45%, this reflects how advanced turbine blade design has become for large commercial turbines.
The figure below shows a comparison between the predictions of the WindPower program and the field data. The mean power output has been calculated for the default value of the standard deviation of 62% and also for the Rayleigh distribution of 52%. The agreement between the field data and the WindPower predictions is very good. It is worth mentioning that this graph was produced from an Excel spreadsheet by exporting the data from the WindPower program - which is one of the menu options available. It should be stressed that the mean power output for the Vestas has been calculated for the period when they were fully operational. In practice, it seems that it is difficult to get the percentage time for the turbines to be operational much above 85% - data obtained since these reports continues to show about an 85% operational level. It was apparently hoped that this figure would have been closer to 95%.
Because the reports on the two offshore wind farms contained details of installation and running costs, it is interesting to look at the cost per kilowatt-hour of electricity produced by these offshore turbines. At 2004/05 prices, the up-front installation costs worked out at around £2,600,000 for a Vestas V80 2 megawatt turbine. In calculating the full installation costs, it is usual to include interest payments and if we take an annual interest rate of 10% with a repayment period of twenty years, this gives a final installation cost of £6,107,900. Additionally, the reports shows that annual maintenance costs were at an average of about £75,000 per turbine. The figure below shows the cost per kilowatt-hour with and without the annual costs. These figures would need to be increased by between 10-15% to allow for the fact that the turbines seem only able to operate for about 85-90% of the time. Gearbox failures seem a particularly prevalent cause of breakdown.
The costs per kilowatt-hour shown in this figure are within the range normally quoted for offshore wind farms but, by plotting them against mean wind speed, it gives greater emphasis than the usual average figure to the importance of choosing a site with care. The WindPower program makes it very easy to carry out these calculations.
Next, some comments on the performance of small wind turbines.