B2 - Adaptive rotor concepts for demand-responsive feed-in

In the past decades, the energy production costs of wind energy could be reduced drastically. For the decarbonization of the energy supply, further cost reductions are necessary. Here, the SFB will make a methodological contribution for the realization of wind turbines (WT) with a nominal capacity > 20 MW. At the same time, it is also necessary to improve the ratio of the "value" of wind power compared to its energy production costs. Here, the "value" is understood to include avoided system costs (e.g., lower costs for grid expansion, balancing power, and storage) in addition to the revenues that can be generated. Due to the increase in installed wind power capacity, the exchange prices for wind power now tend toward zero on windy days. Therefore, it would be economically attractive if future offshore wind farms were to feed in significantly more and more consistent wind energy than conventional wind farms at low to medium wind speeds. In return, at least for a larger part of these offshore wind farms, the power fed into the grid could be lower during periods of strong wind, since more than sufficient wind power is then offered by onshore wind energy, which is to be further expanded in Europe. In other words, there is a question of a paradigm shift in the design goal of WTs away from minimized costs at the wind farm level and toward optimization of performance and costs at the power system level.

A power characteristic with relatively more power in low wind and less feed in high wind contradicts both elementary physical and proven technical principles. To achieve this, the size of a WTs swept rotor area would have to be adjustable: a larger rotor area at low wind speeds and an effectively smaller area at higher wind speeds to manage the enormous loads. However, length-adjustable rotor blades do not appear feasible even in the foreseeable future. Thus, the scientific challenges arise to explore aerodynamic design methods and control system operating strategies that effectively limit the loads on extremely large rotor blades of up to 200 m in length at medium and high wind speeds and under extreme conditions. Against this background, the two working hypotheses of the subproject are:

1. Future very large offshore wind turbines (OWTs) can be designed and operated with aerodynamic performance characteristics that enable a more demand-driven and less fluctuating feed-in from wind energy to the interconnected grid and lower system costs.
2. The extreme and fatigue loads resulting from inhomogeneous wind fields on the extremely large rotors, which can only be influenced relatively slowly, can be limited by model-predictive operation management and control concepts. Such approaches can be validated experimentally in the wind tunnel by reproducible turbulent inflows.

The subproject pursues the innovative approach that at constant outer rotor diameter the WT can switch between different operating modes within about one minute. As a result, the outer area of the rotor becomes more "permeable" at higher wind speeds and is strongly relieved. In terms of fluid mechanics, this corresponds to a variable, radially strongly varying induction distribution along the rotor blades. The three-dimensional rotoraerodynamic effects of such a design with stationary or dynamic inflow have not yet been scientifically clarified. Methodically, the mentioned hypotheses shall be tested in four steps. A design procedure for rotors of future offshore wind turbines shall be developed, which allows a more demand-oriented inflow. Methodologically of central importance is the scaling of the problems to the size of the Oldenburg wind tunnel with its reproducible turbulent inflow conditions. Experiments with a model wind turbine will be carried out with two objectives: Exploring the effects of radially variable induction on rotor aerodynamics, and exploring and validating new model predictive approaches in operation control for switching between operating modes and load reduction.

"Figure 1 shows the experimental setup in the wind tunnel at Carl von Ossietzky University from a previous measurement campaign. Shown is the wind tunnel model of a wind turbine (MoWiTO 1.8) and the measurement of the inflow by laser Doppler anemometer under turbulent inflow. The unsteady wind fields are generated with the imaged active grid, which is operated in collaboration with subproject A01 (Wind and Turbulence Models for Wind Turbines at High Altitudes)."

Within the Design Rotor cluster, there is to be strong interaction with the two other subprojects on structural rotor blade design (SP B03) and dynamic flow separation on rotor blades (SP A02). An intensive exchange is planned with four of the five other clusters. There, SP B02 supports the characterization of turbulent inflow conditions and joint wind tunnel experiments (SP A01), the integrated design methodology for support structures (SP B01) and the central project (SP Z01).