Wind power is already one of the most economic ways of generating electricity from regenerative sources. Yet in order to successfully compete with photovoltaics and other technologies, the cost must still be reduced further. Fraunhofer IWES develops methods that enable an individual wind turbine to autonomously adjust itself to the environmental conditions at its site and to the damage it has already accumulated. These methods lead to better utilization of material, the use of less material, more efficient maintenance, and increased power output.
Wind conditions at a site are the main drivers for the design and the expected lifetime of a turbine. During turbine development, only insufficient knowledge about wind conditions is available, but has to be taken into account nevertheless. This is done by classifying them into one of the three wind turbine classes in accordance with IEC61400-1. Within one class, identical turbines are operated with minimal adaptations to very different sites. The turbine design is based on nominal site conditions. Since the turbines must always be able to withstand the most adverse conditions of a wind turbine class, most turbines are over-dimensioned.
Longer operation period or maximum power production in 20 years?
Wind turbine life-time is limited by fatigue loads. Usually, turbines are designed for a minimum service life of 20 years. In most cases, some usable service life remains after the end of the planned time of operation, which can sometimes be exploited by extending the service life period. However, structural integrity must be assessed for each individual turbine to show that it can continue to operate safely.
However, there are cases in which it is not possible or desirable to have a lifetime extension. Reasons can be due to e.g. economic, legal or technical constraints. A different operating strategy comes into play in these cases: maximum energy yield during the pre-specified useful life of the turbines is desired. Increased power of a wind turbine, for example by employing highly dynamic control or by accepting short-term overloading, can usually only be achieved at the expense of increased degradation.
At the same time, this must not lead to unexpected early failures so as not to diminish system reliability. Ideally, the power of the turbine is increased just enough to distribute the damage over its complete useful lifetime. This way, the physically limited service life is exploited completely and at the same time, maximum energy production is achieved.
Automated adaption to site and damage
Fraunhofer IWES is developing higher-level control strategies for wind turbines for this purpose. They detect the current degradation state of the turbine and feed this information back to the operational control. Closed-loop reliability control is comprised of the detection of the current degradation state, determination of the required turbine configuration and an adjustment of the operational controllers. Reliability control thus autonomously adjusts the behavior of a standard turbine to its individual site.
The site conditions are taken into account via the real operating history, the loads and the accumulated damage. If the turbine is located at a site where the loads are lower than assumed during the design phase, this is converted into a permanent increase in turbine performance. The inverse of this is also possible: a turbine designed for a low-load site can be operated at a site with slightly increased loads. In this case, the performance is reduced automatically in order to achieve the desired useful life.
Automated reliability control contributes to a reduction of the cost of energy in several ways:
• Lower investment costs through better utilization of material and use of less material.
• Increase of turbine performance such that the desired useful lifetime is achieved with a minimal safety margin. This reduces the variance of the time-to-failure, safety factors can be reduced, and the turbine can be designed less robust.
• Better maintenance planning and more efficient execution of maintenance work reduce operating costs. Critical components or the turbine as a whole are systematically subjected to lower loads to defer imminent faults to coincede with other planned maintenance actions.
• All measures increase the maximum power output, contributing to a further reduction in the cost of energy
In addition to these operational benefits, a direct increase in profit is possible: the balance between damage induced in the turbine and performance can be set to prioritize performance. This increases power output and allows to use temporarily increased feed-in tariffs fully to increase profit.