detlev heinemann

Carl von Ossietzky Universität OldenburgInstitute of Physics • Energy Meteorology Group

Detlev Heinemann

Conditions for Offshore Wind Energy Use

Detlev Heinemann

ForWind Carl von Ossietzky Universität Oldenburg

Institute of Physics/Energy Meteorology Group

Wind Power R&D Seminar – Deep Sea Offshore Wind – Trondheim, 20 January 2011

Donnerstag, 20. Januar 2011

Carl von Ossietzky Universität OldenburgInstitute of Physics • Energy Meteorology Group

Detlev Heinemann

Conditions for Offshore Wind Energy Use

Detlev Heinemann

ForWind Carl von Ossietzky Universität Oldenburg

Institute of Physics/Energy Meteorology Group

Visualization of flow conditions in Horns Rev wind farm through mixing of almost saturated air of different temperature [Vattenfall]


OFFSHORE WIND ENERGY RESEARCH

CONTENTS

‣ Marine boundary layer conditions

‣ Vertical wind profile over sea

‣ Wind flow in and behind large offshore wind farms

‣ Outlook & future research



A GENERAL REMARK

Basic Physics

Measurements

ParameterizationsAssumptions on

atmospheric flow ModelsDescription of

MBL flow



A GENERAL REMARK

Basic Physics

Measurements



MBL flow

mostly proven for non-complex onshore wind flow



A GENERAL REMARK

Basic Physics

Measurements



MBL flow

Finite knowledge of offshore wind conditions limits our modeling success!

mostly proven for non-complex onshore wind flow



ONSHORE (INLAND) vs. OFFSHORE (MARINE) WINDS

• Isallobaric Winds‣ local wind effect due to time-varying pressure fields

‣ more significant over sea due to decreased friction

‣ potential source of large wind forecast errors

• Still vs. Moving Surface‣ surface moves under the influence of wind forcing

‣ momentum from tides, ocean currents, and wind-driven currents

‣ wave generation driven by momentum transfer from wind

• Atmospheric Stability‣ poor PBL parameterizations in case of non-stable

thermal stratifications

‣ highly variable frictional turning over water linked to wave height and stability

Marine winds are fundamentally different from inland winds in four principal ways:

• Land-Water Interface‣ varying coastal wind effects

‣ regionally important



MARINE BOUNDARY LAYER CONDITIONS

‣ MBL is rather shallow compared to continental air masses (higher moisture content >lower lifting condensation level)

‣ Low variability of sea surface temperature (SST), nearly unlimited energy source and sink> smaller diurnal oscillation in air temperature

‣ MBL flow is more geostrophic in both speed and direction as over land given the same atmospheric conditions

‣ NWP issues:‣ Errors in model BL profiles of wind, temperature, and dew point due to turbulence and

convective parameterizations‣ Simple algorithms for wind-wave coupling‣ Lack of real-time data for model initialization (> data assimilation, > remote sensing)

‣ Poor resolution of near-surface variablesforecast winds (and waves) are often erroneous during both stability extremes in the MBL




Turbulence (I)

Mechanical Turbulence

‣ through interaction of the wind and surface air mass with sea surface waves

‣ resulting eddies formed by the rising and falling sea surface can extend vertically for tens of meters

‣ Extent of eddies is based on wave height and vertical near-surface wind shear




Turbulence (II)

Convective Turbulence

‣ Due to rising plumes of warm air and compensating downdrafts

‣ Range from 100 to more than 1000 m height

‣ Stability is the main factor for the depth of frictional coupling in the MBL due to convective turbulence

‣ Stratified lower atmosphere: Mixing is minimal and the surface air mass will be decoupled from the winds aloft

‣ Temperature difference between rather constant SST and temperature of overlying air mass is primary factor impacting stability over water



DIFFERENCES BETWEEN MARINE AND CONTINENTAL ATMOSPHERIC BOUNDARY LAYERS (I)

‣ Near-surface air is always moist, relative humidity typically ~ 75–100%

‣ Weak diurnal cycle, since surface energy fluxes distribute over a large depth (10–100+ m) of water (large heat capacity!)

‣ Small air-sea temperature differences, except near coasts. Air is typically 0–2 K cooler than the water due to radiative cooling and advection, except for strong winds or large sea-surface temperature (SST) gradients.

‣ The MBL air is usually radiatively cooling at 1–2 K/day, and some of this heat is supplied by sensible heat fluxes off the ocean surface. If the air is much colder than the SST, vigorous convection will quickly reduce the temperature difference.



DIFFERENCES BETWEEN MARINE AND CONTINENTAL ATMOSPHERIC BOUNDARY LAYERS (II)

‣ Small ‘Bowen ratio’ of sensible to latent heat flux due to the small air-sea temperature difference:latent heat fluxes: ~ 50–200 Wm-2, sensible heat fluxes: ~ 0–30 Wm -2

‣ Most of marine boundary layers include clouds.

Excepting near coasts, when warm, dry continental air advects over a colder ocean, tending to produce a more stable shear-driven BL which does not deepen to the LCL of surface air.

Clouds can greatly affect MBL dynamics. It also affects the surface and top-of-atmosphere energy balance and the SST.



VERTICAL WIND PROFILE OVER SEA

‣ Generally, Monin-Obukhov theory has been found to be applicable over open sea(although developed over land...)

‣ We need: homogeneous and stationary flow conditions

‣ Coastal areas show strong inhomogeneities due to – roughness change at coastline – heat flux change through different surfaces

‣ Common example: Warm air advection over cold water (> well-mixed layer below an inversion)

‣ Systematic deviations from Monin-Obukhov theory at offshore sites!

u(z) =u!!

[ln(z

z0)!!m(

z

L)]




Example: Ratio of wind speeds at 50m and 10m as a function of stability parameter 10/L for different estimation methods for L

Determination of L by different methods: sonic, gradient & bulk methodData: Rødsand, Baltic sea, 50m, 1998-1999; solid line: MO theory

‣ Evidence of larger deviations from MO for stable stratification

‣ Results depend on „measurement“ of L Lange (2002)




Example: Ratio of wind speeds at 50m and 10m as a function of stability parameter 10/L for different estimation methods for L

Results show:

‣ Established theories may fail when basic assumptions are no longer valid

‣ Availability of (more) high quality measurement data is essential

‣ Results may depend on specific techniques and data for analysis (usually indicator for non-optimal solution...)



WIND FLOW IN AND BEHIND WIND FARMS: WAKE EFFECTS

Significant reduction of wind speed downstream of a wind farm



Vertical wind profile

measured and modeled wake effect in 3D distance behind the rotor

in comparison with an undisturbed logarithmic profile





Operation results of Horns Rev offshore wind farm showed 20% less power output than calculated (Barthelmie et al., 2009)

‣ Fundamental different flow conditions in large wind farms compared to small ones

‣ Suboptimal consideration of interaction of wind flow in a wind farm and atmospheric boundary layer

‣ Lack of adequate measurement data

‣ Larger effects expected for spatially „coupled“ wind farms



LES approach

‣ High-resolution modeling of idealized development of „far wakes“ (10 m)• Periodic boundary conditions (non-periodic in development)• Development of wakes in the boundary layer • Validation with on-site measurements

Coupling of mesoscale model & LES

‣ Analysis of wake development under real meteorological conditions • „nested“ areas • WEC model (MYJ-TKE scheme): - sink of kinetic energy - source of turbulent kinetic energy

• Analysis of wind farm effects




‣ Measurements for optimization of micro- and meso-scale meteorological models

incl. satellite remote sensing (vertical structure?!)

‣ Improved wake modeling (multiple wakes, wind farm wakes, > LES)

‣ Offshore Wind Resource Assessment:‣ Wake Effects and Climate Impacts of Offshore Wind Farms ‣ Wakes from large wind farms ‣ Impact of wakes on local/regional climate: boundary layer height, boundary layer clouds ‣ Future climates and wind resources ‣ Validate new mesoscale parameterization for offshore conditions

OUTLOOK & RESEARCH NEEDS (I)



OUTLOOK & RESEARCH NEEDS (II)

‣ Surface waves and turbulent boundary layers and their mutual relationships:

‣ complex wave surfaces in ABL and OBL LES

‣ coupled wind-wave and wave-current models

‣ OBL and ABL mixing parameterizations with wave effects;

‣ wave and turbulence mechanics in high winds (e.g., hurricanes)

‣ wave-breaking structure and statistical distributions

‣ disequilibrium, mis-aligned wind-wave conditions


detlev heinemann

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