Dynamic Analysis of a Floating Vertical Axis Wind Turbine using the Actuator Cylinder Flow Theory

160 Pages·2016·6.06 MB·English

by  Boy Solo Koppenol

Dynamic Analysis of a Floating Vertical
Axis Wind Turbine using the Actuator
Cylinder Flow Theory
Comparative study on a land-based versus
spar vertical axis wind turbine concept and a
code-to-code comparison
Boy Solo Koppenol
Wind Energy
Submission date: September 2016
Supervisor: Zhen Gao, IMT
Norwegian University of Science and Technology
Department of Marine Technology

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An ever-increasing demand for electricity pushes the wind energy industry to deep waters that
favor floating concepts. Vertical axis wind turbines (VAWTs) failed for onshore applications
in the 1980s, but its advantages raise the potential for competitive floating designs and
ultimately lowering the cost of energy. This thesis aims to (1) better understand the dynamic
response characteristics of such floating concept and (2) critically evaluate numerical tools able
to perform fully coupled dynamic analysis on floating VAWTs.
The studied wind energy converter is a combination of DeepWind’s two-bladed Darrieus rotor
and the OC3 spar floater. A comparative study is made between this spar concept and the
equivalent land-based VAWT in both steady and unsteady wind-wave environments. The
dynamic responses are calculated by the aero-hydro-servo-elastic code SIMO-RIFLEX-AC,
designed to model physical phenomena as accurately and efficiently as possible. Efficiency-
driven design solutions imply simplifications in the field of aerodynamic, hydrodynamic and
structural modeling. The effect of modeling theories on the calculated dynamic response of the
spar VAWT is studied in a code-to-code comparison between HAWC2 and SIMO-RIFLEX-
AC. Special attention is devoted to the aerodynamic models, which are both based on the
Actuator Cylinder (AC) flow theory.
The model-comparison shows high energy content at the 2P frequency in most responses, this
characterizes the two-bladed Darrieus rotor. Aerodynamic loads excite the first tower modes
(close to 2P) of the land-based VAWT, whereas the spar VAWT hints to a tower mode around
the 4P frequency. Tower tilt of the spar concept – pitch offset extends up to – induces
gravitational loads at the tower base and blades, and also reduce the generated power up to 5%.
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Dynamic content from hydrodynamic loads is strongly felt at the tower base, but the effect of
wind turbulence is more significant in terms of platform motions and rotor speed. Introducing
wind turbulence reduces the aerodynamic and structural loads.
The code-to-code comparison shows that HAWC2’s aerodynamic model calculates lower
loads in steady wind, but higher aerodynamic torque in turbulent wind. Dynamic stall
significantly increases the lift force in the downwind half of the rotor, causing an overall
increase of aerodynamic load at low tip-speed ratios. The wave-induced platform response is
stronger in HAWC2, which is also felt at the tower base. The physical mooring system in
SIMO-RIFLEX-AC endures an increased mean tension, but its standard deviation is lower.
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This Master’s thesis is carried out in order to obtain the degrees Offshore Engineering from
TU Delft and Wind Energy from NTNU. I have gotten familiar with the topics through a
specialization project that took place in Fall 2015 in Trondheim. The major work that is
presented in this thesis has been carried out from February 2016 through September 2016 in
Delft. My daily supervisors Carlos, Zhen and Zhengshun have assisted me in this period
through regular (Skype) meetings and conversations via email. I owe my gratitude to them for
each in their own way. Carlos, thank you for your general guidance on writing and structuring
this thesis. Zhen, thank you for your input on designing (and adjusting) the approaches and
methods applied in this thesis. Zhengshun, thank you for your countless efforts on solving my
problems related to the numerical tools, I appreciate it.
With this thesis I close off a truly well-spend two years of my life. The European Wind Energy
Master has put me in a very international program that takes place at the technical universities
in Copenhagen, Delft and Trondheim. After a rough start, I was forced to work extra hard at
TU Delft and NTNU in order to remain with my fellow students and finish in time. I would
like to thank my fellow students – and everyone around it – for this unforgettable experience.
Last but not least, I would like to show my appreciation and love to my girlfriend and family.
They have not been able to help me out on any technical issues, but they did provide me with
moral and emotional support that shaped me to the person I have become today.

Delft, the Netherlands September 29th 2016

____________________
Boy S. Koppenol

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