In the last 20 years, wind energy has developed very successfully with an important increase of installations every year all over the world. Most of the wind turbines are installed onshore but offshore wind turbine installations constitute an increasing part of the total wind market. However, the harvested energy from offshore wind turbines are nearly double as expensive as energy harvested from turbines erected on land, and it will become even more expensive to go to deep water. Therefore reducing the cost of energy for offshore wind energy is highly urgent. The development of new technologies for reducing the cost of energy will be the focus of further research in offshore wind energy.
Offshore sites have the advantage of being relatively far from populated areas and wind resources at offshore sites are very rich. This gives the possibility of designing wind turbines at a higher tip-speed, and thus increasing the power efficiency. Offshore conditions in China are different than in other countries. Typhoon is a typical wind in China and there are many times of typhoon periods every year. Therefore wind turbines should be specially designed to run under such wind conditions.
The “state of the art” in the development of offshore wind energy is that wind turbines at offshore sites were erected with rotors designed for onshore use, and efforts were made on reducing the cost of foundation and floating structure. The present project focuses on designing optimal wind turbine rotors under local offshore wind conditions in China to reduce the cost of energy. To obtain high performance, wind turbines are proposed to run at a tip-speed higher than that wind turbines are performing at today. The reason for performing wind turbines at a lower tip speed was mainly due to the noise problem. As offshore sites are relative far from populated areas, the noise regulation is less strict. Thus designing optimal and faster rotors is a feasible way to reduce the cost of energy. Moreover, noise generated from wind turbines can be optimized in the design stage through the optimal design of low noise airfoils and rotors. In the few past years, DTU developed some advanced low noise techniques [1-2]. As a consequence, these techniques will be incorporated in the design of optimal wind turbines under offshore wind conditions in China.
Today, tools for designing wind turbine rotors [3]-[6] are based on incompressible flow assumptions and the designed wind turbines are supposed to optimally run at a tip-speed of less than 90 m/s. To design high speed rotors (>100m/s), the compressibility effects become more important. Thus the existing design tools should be extended to take into account the compressibility effects. The related tasks are: (1) the further development of the Q3uic viscous-inviscid code [7] in the design stage of wind turbine airfoils; (2) the further development of the tip loss correction model and the yaw model in the design stage of rotors; (3) the further development of the CFD code “EllipSys” in the stage of design verification. The EllipSys code developed at DTU and Risø National Laboratory [8]-[11] is based on solving incompressible Navier-Stokes equations with both Reynolds-Averaged Navier-Stokes models (RANS) and Large Eddy Simulation (LES) techniques. The code has been successfully employed by Danish wind turbine manufacturers (Vestas and LM) for verifying their products.
With the loading obtained from the aerodynamic design step, the structure requirements for both airfoils and rotors have to be considered and structural optimization should be performed using e.g. the sandwich structure methodology developed in [12].
To design modern wind turbines, different control techniques must be adapted to the local wind conditions. Furthermore, these techniques are rarely studied in the typhoon case which could give the extreme loads that the turbine has to be designed for. The control systems comprise the control of generator torque, yaw, and blade pitch, etc. The latter can be the collective pitch type for power control and supplemented with cyclic pitch or individual pitch for load alleviation [13]–[15]. The sensor inputs to the blade pitch control or the flap control are crucial for the ability of the system to reduce the loads. Local flow measurement in the vicinity of the rotating blade e.g. with a five whole pitot tube has shown very promising results in aero-elastic simulations [16]. Those simulations require an accurate modelling of the turbulent inflow to the turbine including the wakes from upstream turbines in order to obtain a correct design of the control system.
The aim of the project is to develop computational methodologies/tools capable of designing optimal wind turbine rotors under offshore wind conditions in China including extreme conditions such as typhoons.
References
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