4. Comparison with field data for the Vestas V80 2mW and V90 3mW large wind turbines.
The Vestas V80 2mW offshore turbine - Scroby Sands and North Hoyle UK wind farms.
There are 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 that use the Vestas V80 2mW offhsore turbine. 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.Both reports cover an operating period of a year - 2005 for Scroby Sands and 2003-2004 for North Hoyle - and they are both available from our reference library page. The table below lists the important results.
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 offshore turbine. Additionally, the reports shows that annual maintenance costs were at an average of about £75,000 per turbine. If the capital cost were financed by a loan repayable over 20 years at an annual interest rate of 10%, the total cost would be £6,107,900. The figure below shows the cost per kilowatt-hour for capital cost only, capital cost plus annual maintenance charges and, finally, capital cost repayable over 20 years with 10% annual interest rate and the annual maintenance charges. 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.
It should be noted that even larger wind turbines are now the norm for off-shore wind farms but the cost of wind turbines and their foundations have risen sharply over the last five years to nearly double those at 2005. However, there is some expectation that they may fall again over the next decade by possibly around 20% to 30%.
The Vestas V90 3mW offshore turbine - Kentish Flats UK wind farm.
The size of offshore wind turbines is set to rise as time goes on and turbines with rated outputs nearing 10 megawatts are now in prospect. One of the more recent UK wind farms - Kentish flats in the Thames estuary - uses thirty Vestas V90 3 megawatt units. Details of the first full year's operation in 2006 is described in a report by Vattenhall - see the reference library. Figures are given for both the monthly output of individual turbines along with the monthly averaged wind speeds. The figure below shows the power curve and efficiency curve of the Vestas V90 offshore wind turbine . The peak efficiency is again close to 45% and the design achieves this efficiency in the range from 8 to 10 metres per second. The expected mean wind speed at the site was 8.7 metres/second at the hub height of 70 metres and it is clear that the blade design has been optimised to achieve the highest efficiencies at speeds around the expected mean wind speed.
The figure below shows a comparison between the mean power calculated from the WindPower program and the field data obtained over monthly periods for those turbines that had no downtime. The result for the annual mean power output is also shown. There is some scatter in the results which almost certainly reflects the fact that there is a significant variation in the probability density distribution of the wind speed from month to month. Generally speaking, there is better agreement with the mean power results calculated using a standard deviation of speed compared to the mean speed of 52% - the Rayleigh distribution.
Next, some comments on the performance of small wind turbines.
