Two-bladed wind turbines are recently being discussed more often as the question arises for the most suitable offshore turbine concept.Regarding this turbine concept, a solution is required for the more challenging dynamics. A teetered hub has often been a load reduction concept of two-bladed turbines. During normal operation, a teetered hub eliminates the hub bending moment coming from unequal blade loading. But looking at extreme load cases, the teeter end impact is a major problem.The teeter end impact is quite often described as the occasion that destroys the load-reducing advantage of the teeter mechanism. The turbine must be designed to withstand the loads from the teeter impacts leading to additional weight, which is actually supposed to be reduced by using the teeter hinge.Although the teeter end impact is often described as a kind of 'killer aspect', a more detailed analysis and quantification of its nature is not given in open literature. This paper will do an analysis and quantification of the loads coming from teeter end impacts using an existing teetered turbine, the Controls Advanced Research Turbine 2 (CART2).First, there will be a look at analytical teeter equations to get an overview of the basic parameters leading to teeter movement.Then, teeter end impact behaviour will be analysed using aeroelastic load simulations of the CART2 according to International Electrotechnical Commission (IEC) 61400-1 edition 3.For each design load case, the most significant teeter response will be examined. A classification of teeter end impacts will be extracted from the simulation data. Results will be compared with a rigid turbine in order to get an evaluation of how severe teeter end impacts are, compared with extreme loads of a rigid turbine.Additionally, these results will be compared with modified teeter parameters of the CART2. These are the introduction of pitch-teeter coupling, a reduced free teeter angle and a different Lock number. It will be shown to what extent these parameters may reduce the intensity of teeter end impacts.Results show that it is worth discussing teetered turbines as an alternative to today's three-bladed turbines. According to this study, teeter end impacts need not be regarded as completely intolerable, and there are several turbine parameters that have a significant influence on them.
Two bladed wind turbines are discussed as a possible turbine alternative for offshore use as they show a potential to save cost of energy. But compared to three-bladed turbines, their dynamic behavior is much more challenging. A possible solution to handle these larger dynamic loads is the use of a teeter hinge, which can significantly reduce fatigue loads. In contrast to that, extreme loads, coming from teeter end impacts, are often described as a problem for teetered turbines.There are different design parameters of the teeter system of a turbine, which have an influence on extreme loads during teeter end impacts. Despite numerous studies on teeter movement and load reduction potentials of operational loads, scientific literature does not give information about suitable load-reducing combinations of teeter design parameters and their influence on extreme loads. This paper, which is a summary of a PhD thesis, 1 analyses which combination of teeter parameters has the largest load-reducing influence on extreme loads. Aeroelastic load simulations of the teetered turbine CART2 from the NREL test site and one of today's commercial two-bladed turbines, the SCD3MW from aerodyn (both pitch controlled upwind turbines), will be used. KEYWORDSCART2, extreme loads, SCD3MW, teeter, two bladed INTRODUCTIONIn today's literature, teeter end impacts are often described as situations that lead to critical loads. 2-8 Some of these studies even come to the conclusion that teeter end impacts make the load-reducing advantage of the teeter hinge obsolete. However, an overview of turbine design parameters influencing these loads is not given. A summary of research on teeter behaviour and a first approach of influences on teeter extreme loads has been done in Schorbach et al. 9 Closing this research gap is the aim of this study.
Large two-bladed offshore wind turbines possess cost-saving potentials in their complete life-cycle, together with general structural blade advantages, compared to similar three-bladed machines. However, the general dynamics and tower-eigenfrequency interactions are challenging. Therefore, a speed exclusion zone for the partial load region and two straightforward pitch- and generator-driven tower dampers for the whole power production region are introduced. They enable the incorporation of a reasonable controller for a two- and a three-bladed baseline reference to facilitate a fair and objective turbine comparison. The outlined control features significantly reduced tower fatigue loads by 18% and 70% in the case of a three-bladed 20 MW and a two-bladed 20 MW turbine, respectively.
Wind energy plants are subject to a vast optimization process with a multitude of design parameters. While one specific three-bladed turbine type dominates the on- and offshore market, two-bladed turbines offer promising, but not yet quantified, potentials for cost savings during manufacturing, erection, and maintenance, offshore. Nevertheless, the comparability of two- and three-bladed turbines is challenging, causing an ongoing discussion within research and industry about which alternative to prefer. A new approach could be to reduce the important changes made by maintaining the exact same energy yield. This results in an increase of design changes of the two-bladed rotor, due to slightly (about 2%) longer blades to counterbalance the inevitable losses of aerodynamic efficiency. Nevertheless, it has the great advantage that subsequent comparisons of loads, masses and costs do not have to be associated with a loss of power. This could serve as a solid basis to allow drawing conclusions and considering differences in dynamical loads, masses and costs more directly and hopefully more expediently, when comparing two- and three-bladed wind turbines.
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