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  

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.

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.

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

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

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

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

Ryningsnäs CONFIDENTIAL Vattenfall (2008-2009): Turbine loads, Controller signals Well-instrumented tower DTU/UU (2010-2011): 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

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

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

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

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

HAWC2 wind field output with power law gradient: case hom

HAWC2 wind field output with power law gradient: case het

HAWC2: tweaking of wind fields

HAWC2: tower loads on 2.5MW turbine

HAWC2: blade loads

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.

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

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

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

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

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

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.

References Arnqvist et al. 2015. Wind Statistics from a forested landscape, Boundary-Layer Meteorol, DOI 10.1007/s10546-015-0016-x Chougule et al 2014, Spectral tensor parameters for wind turbine loadmodeling from forested and agricultural landscapes, DOI 10.1002/we.1709 Wind Energy. Larsen et al. 2008, Wake meandering - a pragmatic approach. Wind Energy, 11, pp. 377–395. Larsen, T.J. et al 2013. 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.

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