"Weighting the determined rate of wear may include dividing the determined rate of wear by the cost of electricity value. Implementation of this embodiment of the invention is simple and leads to good results because a higher cost of electricity value will automatically lessen the influence of the actual wear on the TLC system while a low cost of electricity value will throttle the power output of the wind turbine even more if little profit is to be expected.
"The cost of electricity value may be a relative cost of electricity value. The relative cost of electricity value may be a function of a current cost of electricity value and of an expected future cost of electricity value. Considering expected future cost of electricity values in addition to the current cost of electricity is advantageous because any extension or reduction of lifetime of the wind turbine due to the TLC will result in a profit or loss proportional to the cost of electricity towards the end of the lifetime of the wind turbine.
"Alternatively the relative cost of electricity value may be a predetermined value. Such a predetermined value can be either set manually or provided as part of the control routine. Using predetermined values results in a cost effective implementation which is especially suitable for single wind turbines or relatively small wind farms.
"In some embodiments of the invention the method may further comprise determining a current time. In such a case the predetermined value is selected based on the determined current time and from a data set comprising typical cost of electricity values as a function of time.
"In an embodiment of the method the data set comprises typical cost of electricity values as a function of daytime. Typically the consumption of electric energy is high during midday and in the evenings while it is very low in the early morning hours. On week-ends the consumption may remain lower throughout the morning. Even though changes in this scheme may occur, the inventive method yields good results even when based on such a relatively simple model of the cost of electricity.
"Alternatively or in addition the data set may comprise typical cost of electricity values as a function of season information. For some regions the consumption of electricity will be higher in the winter season than in the summer due to the increased use of electric light. In other regions there may be a higher consumption in summer times when air-conditioning is used broadly. Accordingly, in some embodiments of the invention the predetermined value may be selected based on or taking geographic data of the site of the wind turbine into account.
"The inventive method may comprise receiving stock information online and computing the relative cost of electricity value based on the received stock information. In such a case, the relative cost of electricity value will reflect unforeseeable events allowing the method to be receptive to unexpected market developments. Embodiments of the invention using received stock information are especially suitable for larger wind farms.
"The stock information may comprise at least one of an electricity price, a gas price and a coal price as these prices have a direct influence on the profitability of the wind turbine.
"The relative cost of electricity value may be computed based on a mathematical model which takes variations in the received stock information into account. A suitable mathematical model could be based on predictors as known from control theory.
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