Conceptual research of a multi megawatt downwind turbine

Gesine Wanke*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesisResearch

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Abstract

This thesis investigates the advantages and disadvantages of a downwind wind
turbine rotor concept compared to an upwind rotor concept. A commercial Suzlon 2.1 MW upwind turbine is used as the baseline and converted into a downwind configuration by moving the rotor downwind from the tower. The effect of the conversion on the loads is investigated. Dynamic stability investigations are made regarding the difference in edgewise damping and a free-yawing downwind option. Finally, new rotors are designed for the upwind and the downwind configuration to evaluate differences in turbine mass and cost. The overall objective of the thesis is to evaluate the possible economic benefits of the downwind configuration compared to the upwind configuration for the chosen example turbine. A comparison of a full design load basis simulated with HAWC2 according to IECstandard shows that the minimum blade tip to tower clearance can be increased by the turbine conversion. The downwind configuration shows a 10% lower extreme flapwise blade root moment due to the coning direction. The tradeoff is a 0.75% lower annual energy production and a 14% higher extreme tower bending moment as moments from the thrust and gravity on the rotor nacelle assembly are aligned. The tower shadow effect increases the blade fatigue loads, and for the edgewise direction a decrease in damping leads to further load increase. Consequently, the difference in edgewise damping of the downwind configuration in comparison to the upwind configuration is studied. This shows that the turbine conversion changes the interaction of the aerodynamic forces, the rotor, and the tower torsional motion. This interaction influences the out-of-plane component of the edgewise modes which is the main contributor to the change in damping. Turbine design parameter such as cone angle and tower torsional stiffness could be used to increase the edgewise damping. Furthermore, a free yawing downwind configuration is investigated as it could reduce the complexity of the yaw system. The equilibrium free yaw angle of the example turbine can be larger than 19° misalignment with the wind direction for high wind speeds. The tilt angle causes external moments from wind and torque projection onto the yaw bearing leading to the misalignment. A larger cone angle can be used to decrease the misalignment with the wind direction and also to increase the dynamic stability of the equilibrium yaw position. Flapwise blade flexibility can destabilize the equilibrium yaw position, as the steady-state blade deflection at high wind speeds counteracts the cone angle. Conclusively, the rotor for the upwind configuration and the downwind configuration are redesigned with a combination of a low fidelity optimization tool and the HAWTOpt2 framework. Turbine costs are scaled based on blade and tower masses as well as loads resulting from full load basis simulations with HAWC2. As a consequence of the load difference observed previously, the resulting rotor mass for the downwind configuration is 4.4% lower while the tower mass is 8.6% higher than for the downwind configuration. Both configurations show very similar capital expenditures, but due to the lower annual energy production of the downwind configuration, the cost of energy for the upwind configuration is 1% lower than for the downwind configuration.
Original languageEnglish
PublisherDTU Wind Energy
Number of pages180
DOIs
Publication statusPublished - 2019
SeriesDTU Wind Energy PhD
Number0094

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