| 1. |
- 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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| 2. |
- Arnqvist, Johan, 1985-, et al.
(författare)
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Investigation of Turbulence Accuracy When Modeling Wind in Realistic Forests Using LES
- 2019
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Ingår i: Progress In Turbulence Viii. - Cham : SPRINGER INTERNATIONAL PUBLISHING AG. - 9783030221966 - 9783030221959 ; , s. 291-296
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Konferensbidrag (refereegranskat)abstract
- This study presents an evaluation of wind field simulations, in neutral atmospheric conditions, above a heterogeneous forest. The calculations were performed with Large-Eddy Simulation (LES) code OpenFOAM, with explicit modelling of the forest through drag coefficient and forest density. The findings indicate that a large modelling domain is needed in order to reproduce the measurements in different wind directions, since the effect of far upwind forest characteristics influence the wind and turbulence profiles. It is further shown that even though the low resolution of the LES simulations lead to slightly misrepresented single point turbulence characteristics, two point turbulence characteristics are well predicted due to spatial filtering of the small scales.
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| 3. |
- 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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| 4. |
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| 5. |
- Arnqvist, Johan, 1985-, et al.
(författare)
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Robust processing of airborne laser scans to plant area density profiles
- 2020
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 17:23, s. 5939-5952
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Tidskriftsartikel (refereegranskat)abstract
- We present a new algorithm for the estimation of the plant area density (PAD) profiles and plant area index (PAI) for forested areas based on data from airborne lidar. The new element in the algorithm is to scale and average returned lidar intensities for each lidar pulse, whereas other methods do not use the intensity information at all, use only average intensity values, or do not scale the intensity information, which can cause problems for heterogeneous vegetation. We compare the performance of the new algorithm to three previously published algorithms over two contrasting types of forest: a boreal coniferous forest with a relatively open structure and a dense beech forest. For the beech forest site, both summer (full-leaf) and winter (bare-tree) scans are analyzed, thereby testing the algorithm over a wide spectrum of PAIs. Whereas all tested algorithms give qualitatively similar results, absolute differences are large (up to 400 % for the average PAI at one site). A comparison with ground-based estimates shows that the new algorithm performs well for the tested sites. Specific weak points regarding the estimation of the PAD from airborne lidar data are addressed including the influence of ground reflections and the effect of small-scale heterogeneity, and we show how the effect of these points is reduced in the new algorithm, by combining benefits of earlier algorithms. We further show that low-resolution gridding of the PAD will lead to a negative bias in the resulting estimate according to Jensen's inequality for convex functions and that the severity of this bias is method dependent. As a result, the PAI magnitude as well as heterogeneity scales should be carefully considered when setting the resolution for the PAD gridding of airborne lidar scans.
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| 6. |
- Avila, M., et al.
(författare)
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Daily cycle simulations of thermally stratified flows over forests
- 2019
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Ingår i: Wake Conference 2019 22–24 May 2019, Visby, Sweden. - : IOP PUBLISHING LTD.
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Konferensbidrag (refereegranskat)abstract
- The aim of the present work is to obtain a better understanding of how to model the thermally stratified wind field over a forest during full diurnal cycles. The setup of the study assumes a horizontally homogeneous forest, with the objective of finding a simple and efficient way to model the canopy flow using time-dependent input data, obtained from measurements and mesoscale simulations. With this, new insights can be gained for future microscale modelling of complex forested terrains using mesoscale input data. In terrain without forest a diurnal cycle is commonly simulated by imposing time-dependent ground temperature. However, the presence of forests partially isolates the temperature at ground level from the flow above the canopy, making this common approach ineffective. This work proposes imposing the time-dependent net radiation at the forest canopy top to drive the thermal stratification changes along the diurnal cycle. To this end, several full days of simulation are driven by prescribing the net radiative heat flux balance measured on top of the canopy, together with a geostrophic pressure gradient. The advantage of the method is its simplicity and that the input data can be easily obtained from mesoscale modelling. When compared to the observations at the Swedish site Ryningsnas, the new method dramatically improves estimations of wind speed, wind direction and turbulent kinetic energy compared to simulations that only assume neutral stratification. Out of the variables studied, temperature and turbulent heat flux profiles were the ones that qualitatively followed the measurements the best, while wind speed and turbulent kinetic energy showed a larger disagreement.
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| 7. |
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| 8. |
- Floors, Rogier, et al.
(författare)
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From lidar scans to roughness maps for wind resource modelling in forested areas
- 2018
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Ingår i: Wind Energy Science. - : COPERNICUS GESELLSCHAFT MBH. - 2366-7443 .- 2366-7451. ; 3:1, s. 353-370
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Tidskriftsartikel (refereegranskat)abstract
- Applying erroneous roughness lengths can have a large impact on the estimated performance of wind turbines, particularly in forested areas. In this study, a new method called the objective roughness approach (ORA), which converts tree height maps created using airborne lidar scans to roughness maps suitable for wind modelling, is evaluated via cross predictions among different anemometers at a complex forested site with seven tall meteorological masts using the Wind Atlas Analysis and Application Program (WAsP). The cross predictions were made using ORA maps created at four spatial resolutions and from four freely available roughness maps based on land use classifications. The validation showed that the use of ORA maps resulted in a closer agreement with observational data for all investigated resolutions compared to the land use maps. Further, when using the ORA maps, the risk of making large errors (>25%) in predicted power density was reduced by 40-50% compared to satellite-based products with the same resolution. The results could be further improved for high-resolution ORA maps by adding the displacement height. The improvements when using the ORA maps were both due to a higher roughness length and due to the higher resolution.
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| 9. |
- Hallgren, Christoffer, et al.
(författare)
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Classification and properties of non-idealized coastal wind profiles - an observational study
- 2022
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Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
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Tidskriftsartikel (refereegranskat)abstract
- Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.
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| 10. |
- Hallgren, Christoffer, et al.
(författare)
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Looking for an Offshore Low-Level Jet Champion among Recent Reanalyses : A Tight Race over the Baltic Sea
- 2020
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 13:14
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Tidskriftsartikel (refereegranskat)abstract
- With an increasing interest in offshore wind energy, focus has been directed towards large semi-enclosed basins such as the Baltic Sea as potential sites to set up wind turbines. The meteorology of this inland sea in particular is strongly affected by the surrounding land, creating mesoscale conditions that are important to take into consideration when planning for new wind farms. This paper presents a comparison between data from four state-of-the-art reanalyses (MERRA2, ERA5, UERRA, NEWA) and observations from LiDAR. The comparison is made for four sites in the Baltic Sea with wind profiles up to 300 m. The findings provide insight into the accuracy of reanalyses for wind resource assessment. In general, the reanalyses underestimate the average wind speed. The average shear is too low in NEWA, while ERA5 and UERRA predominantly overestimate the shear. MERRA2 suffers from insufficient vertical resolution, which limits its usefulness in evaluating the wind profile. It is also shown that low-level jets, a very frequent mesoscale phenomenon in the Baltic Sea during late spring, can appear in a wide range of wind speeds. The observed frequency of low-level jets is best captured by UERRA. In terms of general wind characteristics, ERA5, UERRA, and NEWA are similar, and the best choice depends on the application.
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| 11. |
- Ivanell, Stefan, 1974-, et al.
(författare)
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Micro-scale model comparison (benchmark) at the moderately complex forested site Ryningsnäs
- 2018
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Ingår i: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 3:2, s. 929-946
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Tidskriftsartikel (refereegranskat)abstract
- This article describes a study in which modellers were challenged to compute the wind field at a forested site with moderately complex topography. The task was to model the wind field in stationary conditions with neutral stratification by using the wind velocity measured at 100 m at a metmast as the only reference. Detailed maps of terrain elevation and forest densities were provided as the only inputs, derived from airborne laser scans (ALSs) with a resolution of 10 m x 10 m covering an area of 50 km x 50 km, that closely match the actual forest and elevation of the site. The participants were free to apply their best practices for the simulation to decide the size of the domain, the value of the geostrophic wind, and every other modelling parameter. The comparison of the results with the measurements is shown for the vertical profiles of wind speed, shear, wind direction, and turbulent kinetic energy. The ALS-based data resulted in reasonable agreement of the wind profile and turbulence magnitude. The best performance was found to be that of large-eddy simulations using a very large domain. For the Reynolds-averaged Navier-Stokes type of models, the constants in the turbulence closure were shown to have a great influence on the yielded turbulence level, but were of much less importance for the wind speed profile. Of the variety of closure constants used by the participating modellers, the closure constants from Sogachev and Panferov (2006) proved to agree best with the measurements. Particularly the use of C-mu approximate to 0.03 in the k-epsilon model obtained better agreement with turbulence level measurements. All except two participating models used the full detailed ground and forest information to model the forest, which is considered significant progress compared to previous conventional approaches. Overall, the article gives an overview of how well different types of models are able to capture the flow physics at a moderately complex forested site.
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| 12. |
- Janzon, Erik
(författare)
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Local Effects On Icing Forecasts for Wind Power In Cold Climate
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis will examine the local effects of land cover on icing forecasts. In Paper I, a single column model was used to test the sensitivity of icing forecasts to land cover fraction. Here, the ice accretion forecast was found to be highly sensitive to the wind magnitude response to the surface roughness. Diabatic effects related to the surface albedo played a secondary role, significant in cases with strong solar irradiance. Paper II examined the impact of 2-dimensional patterns of land cover heterogeneity on the effective surface roughness and blending height using large eddy simulation over a diurnal cycle of solar irradiance. The blending height--or the elevation at which the atmospheric response to the underlying land cover becomes horizontally homogeneous--has been proposed as a guide for coupling numerical weather models to surface parameterizations. In stable conditions, when the atmospheric boundary layer height was shallow, the blending height over surfaces with large heterogeneity length scale was found to be much lower than that of analytical models from previous studies. A new formula for a dynamic blending height was proposed taking this effect into account. The effective surface roughness was found to decrease with increasing land cover heterogeneity. The wind power response from an idealized wind turbine with 80-meter hub height to the effective surface roughness was shown, with a positive response in wind power with increasing land cover heterogeneity. The wind power response was smaller and less systematic with wind turbines extending above the blending height, further highlighting the utility of an accurate formulation for this variable.
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| 13. |
- Janzon, Erik, et al.
(författare)
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Modelling the effects of surface heterogeneity on the internal boundary layer during a diurnal cycle
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- To characterize the effects of subgrid surface heterogeneity, the blending height concept has been developed as a coupling strategy for surface parameterization schemes used in numerical weather prediction (NWP) models. Previous modelling studies have tested this concept using stationary conditions with one-dimensional strips of surface roughness. Here, Large Eddy Simulations (LES) are used to examine the response of the blending height and effective surface roughness to two-dimensional chessboard patterns of alternating high and low vegetation given a diurnal cycle of solar irradiance. In each experiment, the length scale of the roughness elements is increased while the total domain fraction of each vegetation type is kept constant. The effective surface roughness was found to decrease with increasing length scale of surface cover heterogeneity, which is shown to have a significant impact on estimated wind turbine power calculated from logarithmic wind profiles. In stable conditions, the blending height in cases with large heterogeneity length scales was found to exist well above the surface layer. As the behavior of the blending height has implications for coupled models, a simple model for the blending height as a function of heterogeneity length scale is introduced.
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| 14. |
- Janzon, Erik, et al.
(författare)
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Modelling the flow response to surface heterogeneity during a semi-idealized diurnal cycle
- 2023
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Ingår i: Journal of Applied Meteorology and Climatology. - : American Meteorological Society. - 1558-8424 .- 1558-8432. ; 62:4, s. 511-527
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Tidskriftsartikel (refereegranskat)abstract
- To characterize the effects of subgrid surface heterogeneity, the blending height concept has been developed as a coupling strategy for surface parameterization schemes used in numerical weather prediction (NWP) models. Previous modelling studies have tested this concept using stationary conditions with one-dimensional strips of surface roughness. Here, Large Eddy Simulations (LES) are used to examine the response of the blending height and effective surface roughness to \reva{tiled land cover heterogeneity, or a two-dimensional chessboard pattern }of alternating high and low vegetation given a diurnal cycle of solar irradiance \revg{in subarctic conditions}. In each experiment, the length scale of the roughness elements is increased while the total domain fraction of each vegetation type is kept constant. The effective surface roughness was found to decrease with increasing length scale of surface cover heterogeneity, which is shown to have a significant impact on estimated wind turbine power calculated from logarithmic wind profiles. In stable conditions, the blending height in cases with large heterogeneity length scales was found to exist well above the surface layer. As the behavior of the blending height has implications for coupled models, a simple model for the blending height as a function of heterogeneity length scale is introduced.
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| 15. |
- Janzon, Erik, et al.
(författare)
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Single Column Model Simulations of Icing Conditions in Northern Sweden : Sensitivity to Surface Model Land Use Representation
- 2020
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 13:16
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Tidskriftsartikel (refereegranskat)abstract
- In-cloud ice mass accretion on wind turbines is a common challenge that is faced by energy companies operating in cold climates. On-shore wind farms in Scandinavia are often located in regions near patches of forest, the heterogeneity length scales of which are often less than the resolution of many numerical weather prediction (NWP) models. The representation of these forests-including the cloud water response to surface roughness and albedo effects that are related to them-must therefore be parameterized in NWP models used as meteorological input in ice prediction systems, resulting in an uncertainty that is poorly understood and, to the present date, not quantified. The sensitivity of ice accretion forecasts to the subgrid representation of forests is examined in this study. A single column version of the HARMONIE-AROME three-dimensional (3D) NWP model is used to determine the sensitivity of the forecast of ice accretion on wind turbines to the subgrid forest fraction. Single column simulations of a variety of icing cases at a location in northern Sweden were examined in order to investigate the impact of vegetation cover on ice accretion in varying levels of solar insolation and wind magnitudes. In mid-winter cases, the wind speed response to surface roughness was the primary driver of the vegetation effect on ice accretion. In autumn cases, the cloud water response to surface albedo effects plays a secondary role in the impact of in-cloud ice accretion, with the wind response to surface roughness remaining the primary driver for the surface vegetation impact on icing. Two different surface boundary layer (SBL) forest canopy subgrid parameterizations were tested in this study that feature different methods for calculating near-surface profiles of wind, temperature, and moisture, with the ice mass accretion again following the wind response to surface vegetation between both of these schemes.
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| 16. |
- Lindberg, Oskar
(författare)
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Analysis and Forecasting of Utility-Scale Hybrid Wind and PV Power Parks
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increasing share of wind and photovoltaic (PV) power in the electricity generation mix pose challenges in power system management due to their non-dispatchable and intermittent nature. Co-locating wind and PV parks, forming utility-scale hybrid power parks (HPPs), means that the sources can share grid connection, land, permitting procedures as well as operation and maintenance work. On top of this, the combined power output is generally smoothened due to the anti-correlated characteristics of the respective resources.This licentiate thesis contributes to the state-of-the-art and progress the knowledge of co-located wind and PV parks, where the total number of studies on HPPs are limited compared to the respective fields. According to the results, the power output of co-located wind and PV parks are generally anti-correlated for all studied time scales (seasonal, mid-term, synoptic, diurnal and hourly resolution). The useful anti-correlation is found on the seasonal and diurnal time scale where wind turbine sites are likely to be more anti-correlated than any randomly chosen site. On the synoptic time scale, the useful anti-correlation is consistent, although to a lesser extent.The smoothing effect as a result of co-location is also studied in terms of probabilistic forecasting, which corresponds to estimating the uncertainty of power production predictions by means of a probabilistic distribution. By forecasting aggregated time series of co-located wind and PV, the probabilistic forecasts can be improved which is explained by the aggregated time series being smoother and therefore more straightforward to predict. The value of improved forecasts could also be realized in the day-ahead market, where sharper and more reliable forecasts lower the regulation costs.
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| 17. |
- Lindberg, Oskar, et al.
(författare)
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Analyzing the mechanisms behind temporal correlation between power sources using frequency separated time scales : A Swedish case study on PV and wind
- 2022
-
Ingår i: Energy. - : Elsevier. - 0360-5442 .- 1873-6785. ; 259
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Tidskriftsartikel (refereegranskat)abstract
- The temporal correlation between different power generation sources is important for quantifying the reduction in variability when constructing co-located hybrid power parks (HPPs) that combine multiple power sources. This study investigates the physical mechanisms behind correlation on time scales relevant for the power system using frequency separated time scales. The methodology is universally applicable to any data set consisting of at least two power sources and could be adjusted accordingly. The methodology is demonstrated and validated in a case study across Sweden for wind and PV power generation, using the meteorological reanalysis dataset CosmoREA-6. All studied time-scales (seasonal, mid-term, synoptic and diurnal) showed anti-correlated characteristics, although the magnitude of temporal correlation is highly dependent on the time-scale considered. The highest potential for useful anti-correlation is found on the seasonal cycle, followed by the diurnal cycle where existing wind turbine sites, on average, have stronger anti-correlation than the average site. The validation showed good correspondence with measurements for all time-scales. However, an underestimations of the results were found for the diurnal and seasonal cycle while this was shown to have a minor effect when analyzing the correlation on different time scales. The methodology of the case study should be generally valid for all similar climates.
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| 18. |
- Lindberg, Oskar, et al.
(författare)
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Day-ahead probabilistic forecasting at a co-located wind and solar power park in Sweden : Trading and forecast verification
- 2023
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Ingår i: Advances in Applied Energy. - : Elsevier. - 2666-7924. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a first step in the field of probabilistic forecasting of co-located wind and photovoltaic (PV) parks. The effect of aggregation is analyzed with respect to forecast accuracy and value at a co-located park in Sweden using roughly three years of data. We use a fixed modelling framework where we post-process numerical weather predictions to calibrated probabilistic production forecasts, which is a prerequisite when placing optimal bids in the day-ahead market. The results show that aggregation improves forecast accuracy in terms of continuous ranked probability score, interval score and quantile score when compared to wind or PV power forecasts alone. The optimal aggregation ratio is found to be 50%–60% wind power and the remainder PV power. This is explained by the aggregated time series being smoother, which improves the calibration and produces sharper predictive distributions, especially during periods of high variability in both resources, i.e., most prominently in the summer, spring and fall. Furthermore, the daily variability of wind and PV power generation was found to be anti-correlated which proved to be beneficial when forecasting the aggregated time series. Finally, we show that probabilistic forecasts of co-located production improve trading in the day-ahead market, where the more accurate and sharper forecasts reduce balancing costs. In conclusion, the study indicates that co-locating wind and PV power parks can improve probabilistic forecasts which, furthermore, carry over to electricity market trading. The results from the study should be generally applicable to other co-located parks in similar climates.
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| 19. |
- Lindberg, Oskar, et al.
(författare)
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Review on power-production modeling of hybrid wind and PV power parks
- 2021
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Ingår i: Journal of Renewable and Sustainable Energy. - : American Institute of Physics (AIP). - 1941-7012. ; 13:4
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Forskningsöversikt (refereegranskat)abstract
- The interest for co-located wind and solar photovoltaic (PV) parks, also known as hybrid power parks (HPPs), is increasing both in industry and in the scientific community. Co-locating wind and PV can lead to synergies in power production, infrastructure, and land usage, which may lower the overall plant cost compared to single technology systems. This review paper summarizes the existing research on power output modeling related to utility-scale HPPs and identifies knowledge-gaps. The main literature shows that there is a need for improved modeling methodologies accounting for the variability of the combined power production. There is potential for immediate improvement by combining state-of-the-art models that have been developed in separate fields and harmonizing the vocabulary across the different research fields. The study also shows that the total number of peer reviewed studies on utility-scale HPPs is limited and further research, in particular comparative studies, is needed to give a comprehensive view of the benefits and challenges of combining technologies. Other areas such as physical design, control strategies, market participation, and quantification of the possible synergies for physical implementation of HPPs also need to be studied further.
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| 20. |
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| 21. |
- Mann, J., et al.
(författare)
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Complex terrain experiments in the New European Wind Atlas
- 2017
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 375:2091, s. 1-23
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Tidskriftsartikel (refereegranskat)abstract
- The New European Wind Atlas project will create a freely accessible wind atlas covering Europe and Turkey, develop the model chain to create the atlas and perform a series of experiments on flow in many different kinds of complex terrain to validate the models. This paper describes the experiments of which some are nearly completed while others are in the planning stage. All experiments focus on the flow properties that are relevant for wind turbines, so the main focus is the mean flow and the turbulence at heights between 40 and 300 m. Also extreme winds, wind shear and veer, and diurnal and seasonal variations of the wind are of interest. Common to all the experiments is the use of Doppler lidar systems to supplement and in some cases replace completely meteorological towers. Many of the lidars will be equipped with scan heads that will allow for arbitrary scan patterns by several synchronized systems. Two pilot experiments, one in Portugal and one in Germany, show the value of using multiple synchronized, scanning lidar, both in terms of the accuracy of the measurements and the atmospheric physical processes that can be studied. The experimental data will be used for validation of atmospheric flow models and will by the end of the project be freely available. This article is part of the themed issue 'Wind energy in complex terrains'.
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| 22. |
- 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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| 23. |
- Navarro Diaz, Gonzalo, et al.
(författare)
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Wind turbine wake inflow over a heterogeneous forest : comparison between measurement and LES simulation
- 2021
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 1934
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Tidskriftsartikel (refereegranskat)abstract
- In this work a new step in understanding the wind turbine (WT) wake behavior on forested areas is made. For this analysis, a pair of real scale WTs located on a site with moderately complex terrain and heterogeneous forest is simulated using Large Eddy Simulation (LES). This simulation is compared with met mast and power output measurements of two WTs in Ryningsnas, Sweden, considering near neutral stratification in the atmospheric boundary layer (ABL). Three validation steps are followed; first, the undisturbed wind profile is compared with met mast data and another similar LES code. Then, the wake for each WT wake impacting on the met mast at different directions is addressed. A feature of this pair of WTs is that these have different hub heights, but the same rated power and rotor diameter, which helps provide insight into how the tip clearance over the forest affects the operation and wake characteristics. Finally, power output deficits when the WTs are operating in each others wakes are compared to observed power deficits. For these simulations SOWFA, the OpenFOAM project for wind farms simulation in ABL, is used. In this code, three new additions are made; the forest model, the mesh modification for complex terrain and the representation of the WT using an actuator disc model with local force adaptation for wind farm flows. The simulation results show a good performance on quantitatively and qualitatively capturing the velocity in the wake, but for TKE the simulation underestimates the magnitude, and fails to match the measured structure of the wake for one of two WTs. The power deficit on the impacted WTs is well captured, despite the complexity related to turbines with different hub heights. This study makes one of the first steps on validating LES simulations for wind farms in forest.
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| 24. |
- Olivares-Espinosa, Hugo, et al.
(författare)
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Assessment of wind fields over forested sites with LES and a nacelle lidar
- 2019
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Ingår i: Wake conference. - : IOP Publishing.
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Konferensbidrag (refereegranskat)abstract
- The capability of a model based on Large-Eddy Simulations (LES) to reproduce turbulence measurements over a forest of heterogeneous density is assessed. Measurements are obtained from an experimental campaign with a continuous wave lidar mounted on the nacelle of a wind turbine (not considered in this work) that scans over a cone in the upstream direction. The measurements are then compared with the results of the LES of the atmospheric boundary layer in neutral stability conditions. The model comprises a full description of the forest over a large area upstream of the lidar by using plant area density data obtained with airborne laser scans, which also provides the terrain elevation. Although the relatively restricted mesh refinement of the LES leads to a limited representation of turbulence towards higher frequencies, comparisons with the measurements show that the model is capable of reproducing the turbulence levels and spatial coherence in the hypothetical rotor plane. Results permit to conclude that the LES-based model is a suitable tool to identify and predict the microscale effects that terrain features have in the wind resource for sites of high complexity. This work exemplifies the challenges associated to the process and interpretation of data from the employed lidar and its setup, for which a filtering technique potentially useful in future studies is presented.
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| 25. |
- Olivares-Espinosa, Hugo, et al.
(författare)
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Modelling of wind turbine wakes over forests along the diurnal cycle
- 2023
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Ingår i: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
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Konferensbidrag (refereegranskat)abstract
- This work presents a methodology for the Large-Eddy Simulation (LES) of the continuous transition of atmospheric stability over forests along the diurnal cycle and its effect on the turbulence characteristics of wind turbine wakes. The forest is modelled as a porous surface where temperature changes, transferred to the air via sensible transport, are caused by the variation of net radiation and in proportion to the tree height and leaf density. The flow is driven by a pressure gradient including Coriolis forcing to allow for the development of nocturnal inertial oscillations. An actuator disk is employed to model the wake of a wind turbine located in Ryningsnas, Sweden, for which metmast measurements are available to carry out a comparison. Results show a good prediction of the inflow and wake characteristics during daytime whereas turbulence fluctuations seem to be overestimated during night periods, attributed to a combination of an excess in geostrophic velocity and coarse mesh resolution. Observations of velocity, heat flux, potential temperature, velocity spectra and other higher order statistics are used to characterize the diurnal variations both in the inflow and across the wake. The results show that the model is capable of representing the turbulence flow dynamics during the diurnal stability transition, hence laying the ground to future studies to assess the performance of wind parks over forested areas.
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| 26. |
- Rodrigo, Javier Sanz, et al.
(författare)
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The New European Wind Atlas Model Chain
- 2020
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596.
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Tidskriftsartikel (refereegranskat)abstract
- The New European Wind Atlas (NEWA), the largest European project on wind resource assessment technology, has developed mesoscale-to-microscale wind atlas and site assessment methodologies alongside a validation strategy that leverages data from large field experiments as well as wind resource campaigns from industry through a formal verification and validation process. A probabilistic wind atlas approach, based on a multi-physics ensemble, provides means to quantify the uncertainty associated to the mesoscale configuration. Offline meso-micro coupling has been adopted to provide a modular approach for microscale models of different fidelities to share common mesoscale input data. An opensource model chain based on WRF and OpenFOAM codes has been released as reference for future model development and validation activities in connection to wind assessment best practices and standards.
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| 27. |
- Segalini, Antonio, et al.
(författare)
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A spectral model for stably stratied turbulence
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Ingår i: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645.
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Tidskriftsartikel (refereegranskat)abstract
- A solution of the inviscid rapid distortion equations of a stratied flow with homogeneous shear is proposed, extending the work of Hanazaki and Hunt (J. Fluid Mech., 2004,vol. 507, pp. 1-42) to the two horizontal velocity components. The analytical solution allowed the determination of the spectral tensor evolution at any given time starting from a known initial condition. By following the same approach adopted by Mann (J.Fluid Mech., 1994, vol. 273, pp. 141-168), a model for the velocity spectral tensor in the atmospheric boundary layer is obtained where the spectral tensor, assumed to be isotropic at the initial time, evolves until the break-up time where the spectral tensor is supposed to achieve its final state observed in the boundary layer. The model predictions are compared with atmospheric measurements obtained over a forested area, giving the opportunity to calibrate the model parameters and further validation is provided by lowroughness data. Characteristic values of the model coffecients and their dependence on the Richardson number are proposed and discussed.
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| 28. |
- Svensson, Nina, 1988-, et al.
(författare)
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Measurements and Modelling of Offshore Wind Profiles in a Semi-Enclosed Sea
- 2019
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Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 10:4
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Tidskriftsartikel (refereegranskat)abstract
- A conically scanning, continuous-wave LIDAR is placed on an island in the central Baltic Sea with large open-water fetch, providing wind and turbulence profiles up to 300 m height. LIDAR and Weather Research and Forecasting (WRF) profiles from one year are used to characterize the marine boundary layer, at the same time performing an evaluation of the WRF model against LIDAR measurements with a focus on low-level jet representation. A good agreement is found between the average wind speed profile in WRF and LIDAR, with the largest bias occurring during stable conditions. The LLJ frequency is highest in May with frequency of occurrence ranging between 18% and 27% depending on the method of detection. Most of the LLJs occur during nighttime, indicating that most of them do not have local origin. For cases with simultaneous LLJs in both data sets the WRF agrees well with the LIDAR. In many cases, however, the LLJ is misplaced in time or space in the WRF simulations compared to the LIDAR. This shows that models still must be improved to capture mesoscale effects in the coastal zone.
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| 29. |
- Zhang, Shuping, et al.
(författare)
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Mapping coastal upwelling in the Baltic Sea from 2002 to 2020 using remote sensing data
- 2022
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Ingår i: International Journal of Applied Earth Observation and Geoinformation. - : Elsevier. - 1569-8432 .- 1872-826X. ; 114
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Tidskriftsartikel (refereegranskat)abstract
- Coastal upwelling (CU) is an elementary phenomenon in coastal waters. CU brings up deep, often cold, saline water rich of nutrients and carbon, and plays a vital role in local air-sea exchange of gases and heat, marine ecosystem maintenance, and ocean physical dynamics. In this study, regional CU in the Baltic Sea was mapped on the daily MODIS SST from 2002 to 2020, using a method modified developed by Lehmann et al. (2012). Based on the individual CU event detected, the spatiotemporal distribution of CU in the Baltic Sea was depicted, the CU-wind relationship and potential CU drivers in the Baltic Sea on different temporal scale were analyzed. The results found that: 1) The modified approach can effectively delineate the CUs formed by upwelled cold water; 2) The 19 zones delineated with frequent CU occurrences aligned well with previous study and the overall CU occurrence spatial heterogeneity was casted by the different directional relationship between the local coastline and wind; 3) The occurrences of the CU detected in this study showed strong seasonality and primarily driven by SST seasonality and then intensified by local wind in fall; 4) The interannual difference of CU occurrences were affected by heatwaves and its monthly timing. The CUs detected in this study have a high potential for facilitating investigations with respect to oceanic modeling, air-sea exchange of heat and greenhouse gases, and physical dynamics in the Baltic Sea.
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