Fine-scale modeling of wind farm impact on ambient … -2015-met-ocean... · 2015. 5. 29. · Fine-scale modeling of wind farm impact on ambient meteorological conditions Nils Gunnar

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U N I V E R S I T Y O F B E R G E N

Fine-scale modeling of wind farm impact on ambient meteorological conditionsNils Gunnar Kvamstø, Joachim Reuder and Valerie Kumer

Met-ocean measurements and modelling for offshore wind energy, May 28, 2015Japan – Norway Energy Science Week 2015

Geophysical Institute, Faculty of mathematics and natural sciences

University of Bergen

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Outline

• The atmospheric boundary layer – what are wemodelling?

• Turbulence in numerical weather prediction models – a parameterisation problem

• Validation of NWP • Idealised modelling of windfarm atmosphere interaction• Closing remarks





The planetary boundary layer

Important parameters:

• Boundary layer height

• Strength of capping inversion

• Surface layer height



MABL

• Far less diurnal variation ∆θ ≈1K

• more moisture (clouds)

• closer to neutral stratification

(near surface)

The marine boundary layer (MABL)



Synoptic scale phenomena in troposphere matters!!

Iceland



To model windpark-atmosphere interaction we need to work on the weather scaleas there can be non-local causes and effects

Grid-box ∆x

– smallest resolved scale.

Sub-grid processes L< ∆x,

- must be parametrised

& ′

′

Mixing length theory classvs non-local approach

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Some examples and results



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A validation experiment

WRF experiments

• Integration: 1 year – 2005• BC: ERA-Interim• 5 simulations:

• YSU• ACM2• MYJ• MYNN2• QNSE

Observations• FINO1 (research platform)

• V, T measured every 10m• TKE, 40, 60, 80m• p, q, P, UV,….

(For more details see; Krogsæter and Reuder (2015), Wind Energy)



FINO1

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Example - validation of wind shear



Krogsæter, O. and Reuder, J. Wind Energy, DOI: 10.1002/we.1727, 2014.

• Relatively similar performance

• Most schemes overestimate wind

• MYJ gives best score

Important to quantify systematic errors.(For accuracy and bias correction)

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Modeling windfarm atmosphere interaction

• Large offshore wind farms are planned to increase energy production from renewable sources.

• Until now wind farms have been relatively small.• Small farms influence mainly lower part of atmospheric

boundary layer (BL).• As farms increase in size, begin to influence whole BL –

disturbance becomes regional in scale.• Most research has concentrated on local effects of

turbines and wakes – few have considered regional pressure field from large farms affecting efficiency.

• What to expect from simple models?



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Slide 3

Typical θ profile over sea

Side view

top view

- Generation of pressure gradients by wind farm:- θ in troposphere increases with height under typical stable conditions- As air lifted over farm, lower θ air brought up from below- This creates cold anomaly aloft and thus high pressure anomaly below (from hydrostatic

law) - – pressure gradients deflect wind.

MABL



Model wind farm as increased drag at model levels intersected by turbine blades

CT: velocity dependent drag coefficient – depends on turbineA: cross-sectional area of turbine

Add sink term to horizontal momentum equations in PBL scheme

Energy which does not go into electricity or mechanical losses goes back into the atmosphere in the form of TKE:

AUCF T2

21

FUuFx F

UvFy

3

21 UACP TKETKE

Parameterization of wind farm in NWPs

Fitch et al., 2012

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Idealised simulation of windfarm atmosphereinteraction• 2-way nested domains

– 600x600km, 3km resolution– 200x200km, 1km resolution

• Wind farm 10kmx10km– 1 turbine in each grid point– 100 turbines

• 81 vertical levels (30 below 1 km)

• ∆t=9s, 3s, T=360h• Only PBL! All other

parametrisations shut off(dry, adiabatic)

• Background: Uniform horizontal wind in x-direction



600km

200km

10km

10 m/s

Fitch et al., 2012



Some results – resolved wind

• Response in (resolved) horizontal windat hub height.

• Up to 18% reduction (relative to basicstate)

• Upstream deceleration

Vertical cross section of response in horizontalwind• Up to 18% reduction• Wake of 60-km e-folding distance• Affects capping inversion (gravity waves)

Fitch et al., 2012



• TKE same extent on response as on resolved wind• Up to 7 times increase of TKE within wind warm• Analysis of different terms in momentum equation

shows that both gravity waves and vertical transport is active

Fitch et al., 2012

TKE

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Geometry of power output



Fitch et al., 2012

Fractional power output for each turbine.(Power for each turbine divided by the maximum power output of a turbine in the wind farm)

100 turbines in a regular 1km x1 km grid

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Closing remarks

• Models as a numerical laboratory is a powerful tool– Response, interactions between farms– Sensitivity to design, layout etc– Sensitivity to individual processes– Estimation of extremes

• Important to validate models with observarions– Accuracy– Bias correction– Detect weak and strong abilities– Further development



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Marine Atmospheric Boundary Layer (MABL)



Source: http://www.ieawind.org/GWEC_PDF/GWEC%20Annex23.pdf

Challenge:

Include coupling to ocean

Documents

Fine-scale modeling of wind farm impact on ambient … -2015-met-ocean... · 2015. 5. 29. · Fine-scale modeling of wind farm impact on ambient meteorological conditions Nils Gunnar