PhD Candidate: Maarten Holtslag Department: AWEP Section: Wind Energy Supervisor: W.A.A.M. Bierbooms Promoter: G.J.W. van Bussel Start date: 01-10-2011 Funding: FLOW Cooperations: ECN-Ecofys-Eneco
Far Shore Wind Climate Modelling
As shown in previous figures, shear is highest for stable conditions while turbulence is (generally) strongest for unstable conditions. Despite the fact that the guidelines do not overestimate either of these processes, the combined level of moderately strong shear and turbulence never occurs in reality. Wind shear and turbulence are thus not independent, but are both coupled and dependent on atmospheric stability. This fundamental coupling of processes in the atmosphere is lacking in guidelines and, as shown, results in an overestimation of wind turbine fatigue loads of 10%. As a final analyses, we assess if there is a significant difference in loads caused by shear and loads caused by turbulence by the guidelines. In this scope similar load simulations where carried out for an atmosphere with only turbulence present and for an atmosphere with only shear present. The relative ratio of these simulations is plotted in figure 4. It can be seen here that in fact for all wind speeds the balance between shear and turbulence is incorrect in the guidelines. This can be adjusted by lowering wind shear, which similarly also reduced simulated lifetime equivalent loads.
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Very Unstable 2,5
Equivalent turbulence [m/s]
Far shore wind conditions are expected to be favorable for wind energy power production due to increased mean wind speeds and reduced turbulence levels. Exact far shore wind conditions are not known however due to absence of intensive off shore measurement campaigns. This research aims not only to define far shore wind conditions, but also assess how general wind conditions should be transferred to wind turbine design parameters. Besides, it is aimed to define a methodology how one can determine wind conditions for a far shore site, either with or without local observation data. It is expected that this research also contributes to fundamental insight in turbulence, wind shear and atmospheric stability for offshore conditions. It is recognized that atmospheric stability is a fundamental parameter in meteorology, used to describe the general state of the atmosphere. Due to its importance, it is assessed if for a far shore site atmospheric stability does influence key wind turbine design parameters, and what the impact of atmospheric stability is on wind turbine fatigue loads compared to using reference guidelines.
Coupling of atmospheric conditions
σEQ as a function of U and L
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Very Stable Guidelines
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Figure 2: Equivalent turbulence as a function of atmospheric stability
Wind shear according to observations and shear models 2 Observations Bin-ave Observations Power Law Free Convection Holtslag
Stability and Fatigue Loads
u90/u27 [-]
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FEQ = C ∑
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Figure 1: Wind shear as a function of stability Stability and Wind Shear In meteorology Monin-Obukhov theory has been used extensively in the past decades to describe the lowest parts of the atmosphere. MO-Theory states that any nondimensional parameter is a function of the non-dimensional stability parameter ζ (Obukhov, 1971). Assuming validity of MO-Theory, wind profiles can be described by the stability corrected logarithmic wind profile (Businger et al., 1971)
U (z) =
u* z ln −ψ ( ζ ) κ z0
Analyses of observation data taken from the far shore meteomast IJmuiden shows this wind profile performs well offshore. Results are shown in figure 1, where it can be seen that for stable conditions wind shear increases.
Stability and Turbulence Since turbulence (in terms of the standard deviation of the horizontal wind speed) is a stochastic process, and one cannot simulate every possible TI, one has to consider a characteristic TI in wind turbine design. It can be shown that if TI is log-linear distributed, the characteristic turbulence level depends on wind turbine characteristics as well as on distribution parameters of the log-linear distribution of turbulence (Veldkamp, 2006). The impact of atmospheric stability on the equivalent turbulence can be seen in figure 2, where it is clear that turbulence increases for unstable conditions and there is a non-linear dependence of turbulence on mean wind speed. This nonlinear dependence is not included in the guidelines.
The last two terms in the summation are taken from the same observation dataset and represent respectively the chance that for a hub height wind speed U the stability class L occurs, and the chance that the hub height wind speed U occurs. The first term in the summation represents the equivalent damage that occurs for given (hub height) atmospheric conditions, and is calculated based on simulations carried out with the design software Bladed. For these simulations the 5MW NREL reference turbine is used as reference wind turbine. The cumulative loads before summation over all wind speeds are shown in figure 3. The difference in lifetime cumulative equivalent loads when using either guidelines or the stability dependant results shown in figure 1 and 2 is 10%. These difference are mainly found for moderate wind speeds, ranging from 8 to 14 m/s. Missing values prevented calculations for wind speeds below 6 m/s and above 21 m/s for stability dependant results. Since neither wind shear (figure 1), nor turbulence (figure 2) is by definition overestimated, it is questioned what causes the difference in calculated lifetime equivalent loads.
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IEC-guidelines Stability-Obs.
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Figure 4: Relative ratio of wind turbine loads for idealized atmospheres (shear only / turbulence only) Outlook The research presented so far is based on only 6 months of observation data, which is questionable based on the seasonal pattern of temperature (and thus stability). Besides, so far fatigue loads of only one component of the wind turbine are investigated. After proper extension of this research it is expected that two journal papers will be written, one dealing with far shore atmospheric conditions, and one dealing with the impact of atmospheric stability on wind turbine fatigue loads. Besides, results obtained so far will be presented at the 2nd International conference on Energy & Meteorology in Toulouse, June 2013.
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Figure 3: Cumulative loads as a function of wind speed
Publications -
30
Hub height wind speed [m/s]
FEQ per wind speed
120
Equivalent load [kNm]
Aerospace Engineering
Shear-loads / turbulence-loads
The validated wind profile and turbulence characteristics can be used to assess the overall impact of stability on wind turbine fatigue loads. This is done for the blade root bending moment. The cumulative lifetime equivalent loads are calculated as (Sathe et al., 2012)
Shear-loads / turbulence-loads [%]
1.8
Obukhov, A. M. (1971), “Turbulence in an atmosphere with non-uniform temperature”, Boundary Layer Meteorology 2, pp 7-29 Businger, J. A., Wyngaard, J. C., Izumi, Y. & Bradley, E. F. (1971), “Flux-Profile relationships in the atmospheric boundary layer”, Journal of the Atmospheric Sciences 28 pp181-189 Veldkamp, H. F. (2006), “Chances in Wind Energy: A probabilistic approach to wind turbine fatigue design”, PhD-Thesis, Technical University Delft” Sathe, A., Mann, J., Barlas, T., Bierbooms, W. A. A. M. & van Bussel, G. J. W. (2012), “Influence of atmospheric stability on wind turbine loads”, Wind Energy pp 49-61
