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Thursday, 13 March 2014
09:00 - 10:30 The model chain: First steps towards tomorrow's technology
Resource Assessment  


Room: Tramuntana
Session description

Recent advances in the software and computational resources available to the wind industry have opened a new frontier; the ‘model chain’. A single approach to such a concept has yet to emerge as the industry standard, although a general idea of a dynamic process across progressively smaller scales is emerging. This session intends to give delegates a broad view of how research institutes and private companies are dealing with this challenge, what the most promising approaches are and which range of applications is foreseen for the coming years.

Learning objectives

  • Understand some of the challenges of multi-scale modelling
  • Get a first glimpse of current approaches in this topic
  • Talk directly to the main players in this research field
Lead Session Chair:
Pep Moreno, Vortex, Spain

Co-chair(s):
Hans E. Joergensen, DTU Wind Energy, Denmark
Xiaoli Guo Larsén Technical University of Denmark, Denmark
Co-authors:
Xiaoli Guo Larsn (1) F P
(1) Technical University of Denmark, Roskilde, Denmark (2) 2Climate Service, South African Weather Service, Pretoria, South Africa (3) Climate Service, South African Weather Service, Pretoria, South Africa

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Xiaoli Guo Larsén is senior scientist at Wind Energy Department, Technical University of Denmark. She received her Ph.D. on the topic of Air-Sea interaction in Uppsala University and has been working on Wind Energy for 10 years. Her major activity topics includes basic atmospheric researches, wind and wave interactions, wind resources and particularly, extreme wind modeling and estimation. For the last topic, she developed a series of most up-to-date methodologies.

Abstract

Extreme gust wind estimation using mesoscale modeling

Introduction

The estimation of the extreme gust wind, e.g. the 50-year winds of 3 seconds values, currently in the IEC standard, is based on a statistical model that assumes a Gaussian process to convert the 1:50-year wind values from the 10 min resolution. The current study follows a theory that gusts at the surface are produced by the deflection of air parcels flowing at the top of the surface layer, and uses the mesoscale Weather Forecasting and Research (WRF) model to simulate the gust values. These values are then used to estimate deign values which can be used for turbine design.

Approach

This study follows the approach proposed by Brasseur (2001) in which he argued that an air parcel will be able to reach the surface if the vertical component of the turbulence kinetic energy (TKE) is strong enough to counteract the buoyancy force. For a targeted area, we identified all storms that contributed to the extreme wind climate from the CFSR data 1998 – 2010, which served as input to the WRF model, run at 4 km resolution. The profiles of TKE and buoyancy were calculated to track the winds that satisfy the above described conditions, considered to be the gust values.

Main body of abstract

The experiments have been done to regions in Denmark and South Africa. A number of individual cases have been validated with available gust and mean surface wind speed measurements. The method captures the gust wind satisfactorily for the mid-latitude cyclones (existing both in Denmark and South Africa), but shows discrepancies where wind gusts occur due to thunderstorms. This is one of the main sources of extreme winds in South Africa. To assist with the optimal mapping of design wind values for South Africa, the model was run for the south-western Cape region, where the occurrence of strong winds, complex topography and high population density makes the accurate assessment of extreme wind statistics imperative. The maximum wind gust value for each storm event at each grid point was estimated and used to calculate the 1:50-year value. The map from the mesoscale modeling will be compared with a map developed from a limited number of measurement stations. The estimation will also be compared with the IEC recommended procedure.

Conclusion

Our study supports the assumption that the gust observed at the surface is brought down by large eddies, under the condition that TKE counteracts the buoyancy force. The mesoscale modeling using WRF provides the samples of gust values through physics, in contrast to the statistical approaches in the existing methods, and the 50-year gust value is calculated through the downscaling of the CFSR reanalysis data. We expect to have completed the validation of the results before the EWEA 2014 conference.


Learning objectives
This is the first time that the estimation of extreme gust value, e.g. the 50-year value, will be based on physics modeling. The inclusion of large scale eddies through the modeling provides an innovative angle to estimate gust values. The results also emphasize the different physical processes involved in the development of strong gusts, which should be considered in the estimation of design wind speed values