Michael Borg

A comparison between the dynamics of horizontal and vertical axis offshore floating wind turbines

The need to further exploit offshore wind resources in deeper waters has led to a re-emerging interest in vertical axis wind turbines (VAWTs) for floating foundation applications. However, there has been little effort to systematically compare VAWTs to the more conventional horizontal axis wind turbine (HAWT). This article initiates this comparison based on prime principles, focusing on the turbine aerodynamic forces and their impact on the floating wind turbine static and dynamic responses. VAWTs generate substantially different aerodynamic forces on the support structure, in particular, a potentially lower inclining moment and a substantially higher torque than HAWTs. Considering the static stability requirements, the advantages of a lower inclining moment, a lower wind turbine mass and a lower centre of gravity are illustrated, all of which are exploitable to have a less costly support structure. Floating VAWTs experience increased motion in the frequency range surrounding the turbine [number of blades]×[rotational speed] frequency. For very large VAWTs with slower rotational speeds, this frequency range may significantly overlap with the range of wave excitation forces. Quantitative considerations are undertaken comparing the reference NREL 5u2009MW HAWT with the NOVA 5u2009MW VAWT.

FloVAWT: Further Progresses on the Development of a Coupled Model of Dynamics for Floating Offshore VAWTS

Interest in potential wind farm sites in deeper waters and further offshore has substantially increased recently, and in parallel an increased interest towards floating, rather than bottom-fixed, offshore wind turbines: the Energy Technologies Institute (UK) recently announced a plan to invest £25m in offshore floating wind turbine projects. Furthermore, a recent document by the UK LCICG (Low Carbon Innovation Coordination Group), demonstrated that the “Development and demonstration of new concepts such as floating foundations for water depths >60m”, has a value in meeting emissions targets at low cost of up to £13bn. The present article is a follow on with the previous article presented at OMAE 2013 [1], in which the progresses on the development of an aero-hydro-servo-elastic coupled model of dynamics for VAWT are illustrated, called FloVAWT. The further progresses presented consist in: a) the model, in particular the hydrodynamic module, has been now validated against experimental data provided by the DeepCwind project (see OC4) for the semi-submersible support structure configuration, b) the additional velocity component due to the 6 degree-of-freedom motion of the supporting floating structure are now taken into account within the aerodynamic module, while previously only the displacement imposed by the support structure was considered, c) a new module dedicated to the mooring system has been developed and validated, capable of modelling catenary mooring systems with a quasi-static, energy-based approach. Some of the new capabilities of the program are illustrated through a case study of a Darrieus-type VAWT rotor coupled with the OC4 semi-submersible support structure. Comparisons with the previous version of the program are presented, giving an insight on the relative importance of the additional aspects taken into account.Copyright