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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Christian Kress ETH Zürich, Switzerland
Co-authors:
Christian Kress (1) F P Ndaona Chokani (1) Reza S. Abhari (1) Takashi Hashimoto (2) Masatoshi Watanabe (2) Takahiko Sano (2) Mitsuru Saeki (2)
(1) ETH Zürich, Zürich, Switzerland (2) Hitachi Ltd., Tokyo, Japan
Poster Award Winner

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Mr. Kress is a PhD Assistant in the Laboratory for Energy Conversion (LEC), ETH Zürich. LEC’s broad-based wind energy program includes experiments on dynamically scaled wind turbines, measurements on full-scale wind turbines, and development of simulation tools of atmospheric flows and wind farm micrositing. Mr. Kress’ research, in the ETH Wind Turbine Test Facility, is focused on the design of novel offshore wind turbine rotors; his recent work assessed the performance, unsteady structural loading and operational stability of upwind and downwind wind turbines. This experimental work is complemented by expensive computational studies.

Abstract

Economical and operational merits of offshore multi-megawatt downwind turbines

Introduction

Downwind turbines are well suited for use on offshore floating platforms, and offer substantial potential to lower the cost of wind-generated electricity. As blade-tower collisions are impossible on downwind turbines, both lighter and more flexible blades can be used and the overhang distance between the tower centre and the rotor hub can be made smaller, compared to an upwind turbine that is operated in the same environment. The advantageous operational stability characteristics of downwind wind turbines furthermore lower the cost of wind-generated electricity. The restoring moment of a downwind rotor tends to beneficially align the turbine with the incoming flow.

Approach

The Hitachi 5MW offshore downwind wind turbine is designed to maximise the potential opportunity space in the offshore wind market. In this regard, a comprehensive experimental and computational assessment of the benefits and drawbacks of multi-megawatt downwind turbines has been undertaken. In the first part of the paper, experiments on models of multi-megawatt turbines performed in the ETH Wind Turbine Test Facility in well-controlled flow conditions are described. In the second part of the paper, full-rotor Navier-Stokes simulations of the Hitachi 5MW offshore downwind wind turbine are discussed.

Main body of abstract

The models tested in the ETH Wind Turbine Test Facility are configured with different rotor designs, and can be operated either downwind or upwind. The impact of the rotor design and turbine operating conditions on the stability, performance and unsteady loads are measured. The measurements show all downwind configurations have yaw stability, whereas upwind configurations are either unstable or have significantly reduced yaw stability compared to the corresponding downwind rotor configurations. Furthermore, downwind configurations yield up to 5% more power, but with 3% higher thrust than upwind configurations. Downwind rotors must operate in the presence of the tower’s wake, with which high fatigue loads are associated. The measurements in the ETH Wind Turbine Test Facility show that the fatigue loads are substantially larger on downwind configurations compared to upwind configurations. Although the fatigue loads are smaller on upwind configurations, the fatigue loads increase at non-optimum tip speed ratios; whereas on an optimum downwind configuration, the fatigue loads are insensitive to tip speed ratio and minimum compared to non-optimum downwind configurations.
The simulations of the Hitachi 5MW offshore downwind wind turbine show that the nacelle has a favourable blockage effect, which results in higher axial velocities ahead of the rotor and higher flow incidences on the blade, both of which yield a higher power of downwind configurations compared to upwind configurations. Furthermore, the overall aerodynamics, including efficient passive cooling of the compact nacelle, of the 5MW offshore downwind wind turbine are detailed.

Conclusion

On a downwind turbine, the nacelle can be made more compact and lighter, and due to the lower towerhead mass the overall turbine can be made lighter. The resultant lighter and cheaper turbine contributes to the aforementioned lower cost of energy. The resultant lighter turbine contributes to lighter foundation and provides a high economical wind turbine system.
The tendency of downwind turbines to align with the incoming flow is advantageous in stormy weather, as well as when access to the turbine is inhibited, since in the event of an on-board electrical failure, a free-yawing downwind turbine always remains safely oriented.


Learning objectives
Control approaches, such as individual blade pitch control, may be designed for multi-megawatt downwind turbines to reduce the fatigue loads resulting from the blade’s passage through the tower’s wake.
Lastly, as the blades on downwind configurations are more highly loaded inboard, a critical design perspective of 10-20MW floating offshore downwind wind turbines that can provide a competitive cost of wind-generated electricity are discussed.