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| 2. |
- Molinder, Jennie, et al.
(författare)
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Probabilistic Forecasting of Wind Turbine Icing Related Production Losses Using Quantile Regression Forests
- 2021
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Ingår i: Energies. - BASEL, SWITZERLAND : MDPI. - 1996-1073. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- A probabilistic machine learning method is applied to icing related production loss forecasts for wind energy in cold climates. The employed method, called quantile regression forests, is based on the random forest regression algorithm. Based on the performed tests on data from four Swedish wind parks available for two winter seasons, it has been shown to produce valuable probabilistic forecasts. Even with the limited amount of training and test data that were used in the study, the estimated forecast uncertainty adds more value to the forecast when compared to a deterministic forecast and a previously published probabilistic forecast method. It is also shown that the output from a physical icing model provides useful information to the machine learning method, as its usage results in an increased forecast skill when compared to only using Numerical Weather Prediction data. A potential additional benefit in machine learning for some stations was also found when using information in the training from other stations that are also affected by icing. This increases the amount of data, which is otherwise a challenge when developing forecasting methods for wind energy in cold climates.
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| 3. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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Using land-based stations for air–sea interaction studies
- 2020
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Informa UK Limited. - 0280-6495 .- 1600-0870. ; 72:1, s. 1-23
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Tidskriftsartikel (refereegranskat)abstract
- In situ measurements representing the marine atmosphere and air-sea interaction are taken at ships, buoys, stationary moorings and land-based towers, where each observation platform has structural restrictions. Air-sea fluxes are often small, and due to the limitations of the sensors, several corrections are applied. Land-based towers are convenient for long-term observations, but one critical aspect is the representativeness of marine conditions. Hence, a careful analysis of the sites and the data is necessary. Based on the concept of flux footprint, we suggest defining flux data from land-based marine micrometeorological sites in categories depending on the type of land influence:1. CAT1: Marine data representing open sea,2. CAT2: Disturbed wave field resulting in physical properties different from open sea conditions and heterogeneity of water properties in the footprint region, and3. CAT3: Mixed land-sea footprint, very heterogeneous conditions and possible active carbon production/consumption.Characterization of data would be beneficial for combined analyses using several sites in coastal and marginal seas and evaluation/comparison of properties and dynamics. Aerosol fluxes are a useful contribution to characterizing a marine micrometeorological field station; for most conditions, they change sign between land and sea sectors. Measured fluxes from the land-based marine station Ostergarnsholm are used as an example of a land-based marine site to evaluate the categories and to present an example of differences between open sea and coastal conditions. At the Ostergarnsholm site the surface drag is larger for CAT2 and CAT3 than for CAT1 when wind speed is below 10m/s. The heat and humidity fluxes show a distinctive distinguished seasonal cycle; latent heat flux is larger for CAT2 and CAT3 compared to CAT1. The flux of carbon dioxide is large from the coastal and land-sea sectors, showing a large seasonal cycle and significant variability (compared to the open sea sector). Aerosol fluxes are partly dominated by sea spray emissions comparable to those observed at other open sea conditions.
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| 4. |
- Mohr, Matthias, et al.
(författare)
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Wind power in forests II : Forest wind
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Sweden has good conditions for wind power. However, most of Sweden (ca. 70%) is covered by forest. Forests decrease wind speeds and create turbulence, something which is not favourable for wind power. Several Swedish wind maps have shown that forests in Nordic countries can be well suited for wind power (e.g. Bergström and Söderberg 2011, Byrkjedal and Åkervik 2009).At the same time, there is uncertainty over wind conditions over forests at very high altitudes (ca. 150 m above ground). How good do wind resource assessment models agree with measurements? How much energy is a wind turbine in forest going to produce and which loads will a wind turbine in forest experience?This project has investigated all these issues. Work was concentrated in the following work packages:Wind resource at very high heightsTurbulence- and wind measurements at very high heights above forestAnalysis of turbulence data from forestsModel simulations with wind flow modelsModel simulations with very-high-resolution weather forecast modelsModel simulations with Large Eddy Simulation (LES) modelsImproved specification of so-called “synthetic turbulence” over forestAnalysis of airborne laser altimeter measurements over forestForest’s effects on wind turbine energy productionLoad simulations for wind turbines over forestWP1 studies how wind speed and direction varies with height over forest (up to ca 150 m above ground and higher up). Several profile relations are studied here. Frequency distributions of wind shear and veer are presented. WP2 describes turbulence and wind measurements that have been carried out within the project at Hornamossen. Moreover, the measurement campaign that was carried out in a line over the Hornamossen-hill within the New European Wind Atlas project is described. WP3 analyses turbulence data from Hornamossen together with turbulence data from Ryningsnäs. Of special interest is how turbulence intensity decreases with height as well as if the IEC-standard class A, B or C for wind turbines is complied with at different heights. WP4 describes the newly developed linearised wind flow model ORFEUS with a dedicated forest module. WP5 describes model simulations with WRF and the MIUU model, their sensitivity for surface roughness and turbulence parameterisations. Mean wind profiles from the models are compared to Hornamossen. WP6 describes LES simulations with Chalmers LES model and WRF-LES. LES-resultats depend to a large degree on how the turbulent vortices are initialised at the inflow boundaries of the LES model. Several different methods for that are described. WP7 describes a new turbulence model (the Segalini & Arnqvist model) that includes atmospheric stability. This is a further development of the IEC turbulence model (=Mann model) for neutral stability. Coherence of turbulent winds as well as phase profiles are other improvements of the IEC model. WP8 describes a new method to compute leaf/needle/plant area density from laser scans of the Swedish forest and how one estimates surface roughness and zero plane displacement from that. The new method is compared with two other methods. Results are also compared with official forest data (“skoglig grunddata”). The effect on the wind profile is also shown. WP9 describes the new methods for estimating AEP from the Power Curve Working Group and the IEC standard for Power Performance Testing. Effects on estimated AEP are shown. A new simple model for calculating turbulence effects on energy production is developed and compared with data from a wind farm. Within WP10 a new generic open-source wind turbine is developed and used for load simulations with aero-elastic simulations. Results show that the new coherence model for turbulence gives much smaller loads than the turbulence model of the IEC standard.For more information on the different parts of the project the reader is referred to the report’s introduction, the ”Summary and Conclusions” of each chapter as well as the overall summary (”Executive Summary”) at the end of the report.
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| 5. |
- Arnqvist, Johan, et al.
(författare)
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Canopy waves, observations and predictions from lineartheory
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper the existence of canopy waves is examined using measurements from a 138 m high tower placed in a forest. Characteristics of the waves are examined in relation to wind energy. Using wavelet analysis it is shown that when the wave signal is clear, the phase lag between horizontal and vertical velocity is close to 90 degrees, which limits the contribution of the waves to themomentum flux. Results from numerical solution of linear wave equations is shown to agree with measurements in terms of wave period and the vertical shape of the wave amplitude. Linear analysis and measurements suggests that Kelvin-Helmholtz instability causes unstable wave growth and that the most unstable wave number normally has a period of 10-100 s. In addition to the Kelvin-Helmholtz instability, the linear analysis predicts that instabilities of the Holmboe kind, with higher frequency, can develop over forests in certain conditions.
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| 6. |
- Arnqvist, Johan, 1985-, et al.
(författare)
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Flux-profile relation with roughness sublayer correction
- 2015
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 141:689, s. 1191-1197
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Tidskriftsartikel (refereegranskat)abstract
- Calculation of momentum flux using Monin–Obukhov similarity theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this article is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux-profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for mesoscale modelling, scalar transport models, or sound propagation models.
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| 7. |
- Arnqvist, Johan, 1985-
(författare)
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Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine.This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile.Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower.The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space.Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.
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| 8. |
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| 9. |
- Arnqvist, Johan, et al.
(författare)
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Wind Statistics from a Forested Landscape
- 2015
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 156:1, s. 53-71
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Tidskriftsartikel (refereegranskat)abstract
- An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40 wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.
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