1. E. Dellwik, A. Papettaa, J. Arnqvist, M. Nielsena and T. J. Larsena
Inflow conditions and wake effects for wind turbines in forested terrain
E. Dellwik, A. Papettaa, J. Arnqvist, M. Nielsena and T. J. Larsena

2. Motivation 1: There are many wind farms in forested areas.
2: We have previously focused on the wind climate and now want to study its interaction with wind turbines.

3. Motivation continued
3: We have observed shallow/high-gradient boundary layers in the typical height range of a wind turbine rotor and want to study how this gradient influence loads and wakes.

4. Sites Hornamossen, 2015- Skogaryd, 2010 Ryningsnäs, 2008-2011 U
We are here

5. Occasional very strong mean wind gradients
Skogaryd September 2010:
Occasional very strong mean wind gradients
Prototype
ZephIR lidar,
40 km from the coast
Message: curious about
How a turbine would react

6. Hornamossen: 180m tower, time series also showing “split” message it is not just one site.

7. Hornamossen: 180m tower, time series also showing “split” message it is not just one site.

8. Ryningsnäs CONFIDENTIAL Vattenfall (2008-2009): Turbine loads,
Controller signals
Well-instrumented
tower
DTU/UU ( ):
Six sonic anemometers
Surface energy balance
Campaigns with remote
Sensing instruments
Vattenfall’s test site for turbines in forested
terrain.
100m
CONFIDENTIAL
80m
Nordex turbines, 2.5MW

9. Outline Inflow conditions Ryningsnäs: quantification
Inflow conditions Ryningsnäs: generation with HAWC2
Examples of HAWC2 loads
Wakes (results ready Monday at 12 )
Conclusions

10. Inflow conditions: hc = 20m hc = 20m
Bergström et al. 2013: Wind power in forests: Winds and effects on loads, Elforsk report 13:09
hc = 20m
hc = 20m

11. Inflow conditions: pdf-s of turbulence gradients and selected cases
“HOM”
“HET”

12. Simulation of wind field for load estimation: Mann parameters from spectra
Chougule et al. 2014: Spectral tensor parameters for wind turbine load
modeling from forested and agricultural landscapes, Wind Energy

13. HAWC2 wind field output with power law gradient: case hom

14. HAWC2 wind field output with power law gradient: case het

15. HAWC2: tweaking of wind fields

16. HAWC2: tower loads on 2.5MW turbine

17. HAWC2: blade loads

18. Consequences for wake affected loads
We use the Dynamic Wake Meandering model
The wake deficit is transported downstream where the transport process is governed by the crosswind flow of the large scale ambient turbulence – same principles as smoke from a chimney.
The model has previously validated with LIDARs, model and full scale experiments of on and offshore wind turbines.
The ambient turbulent level is crucial for the meandering motion and the turbulent mixing process.

19. Onshore Forrest Conditions
Wake study
Two turbines are studied at wind speed at 8m/s for two spacings: 3D and 5D
Relative wind speed direction -45,-44,…,45˚
Offshore Conditions
Turbulence intensity 6%
Shear α
0.12
Onshore Forrest Conditions
Turbulence intensity
22%
Shear α
0.45

20. Influence on production
At 8m/s power production is higher in forrest that offshore due to the higher shear level
The wake losses are smaller in forrest that offshore due to the higher turbulence level

21. Influence on blade flap fatigue loads
Significantly higher blade loads are seen in forrested area than offshore. The relative impact from wakes is smaller in forrests than offshore

22. Influence on tower bottom fatigue loads
Significantly higher tower loads are seen in forrested area than offshore. The relative impact from wakes is smaller in forrests than offshore

23. Influence on yaw loads
The yaw bearings are significant higher loaded in forrested areas. In forrested ares the wake effects only contribute to increased loading for very close spacings <5D

24. Conclusions
Tall turbines are necessary in forested area to move away from high-turbulence near-surface conditions.
Tall turbines in a forested landscape experiences - as a rule – inhomogeneous turbulence fields.
A way to use observed gradients in load simulations for optimization of wind turbine was demonstrated.
In case of large turbulence gradients, some loads exceed those casued by more homogeneous (and higher) turbulence if the hub height wind speed is above 8 m/s.
Wake effects has a relative less though not ignorable impact on turbines in forrested areas when comparing to offshore conditions. It is, however, still an important load contributor with respect to tower load level.

25. References
Arnqvist et al Wind Statistics from a forested landscape, Boundary-Layer Meteorol, DOI /s x
Chougule et al 2014, Spectral tensor parameters for wind turbine loadmodeling from forested and agricultural landscapes, DOI /we.1709 Wind Energy.
Larsen et al. 2008, Wake meandering - a pragmatic approach. Wind Energy, 11, pp. 377–395.
Larsen, T.J. et al Validation of the Dynamic Wake Meander Model for Loads and Power Production in the Egmond aan Zee Wind Farm. Wind Energy, Volume 16, Issue 4, pp. 605–624.

27. Simulation of wind field for load estimation: Mann parameters from spectra
Chougule et al. 2014: Spectral tensor parameters for wind turbine load
modeling from forested and agricultural landscapes, Wind Energy