Advances on wind energy resource mapping from SAR
Charlotte Hasager(1) , Morten Nielsen(1) , Merete Christiansen(1) , Rebecca Barthelmie(1) , and Poul Astrup(1)
Risoe National Laboratory,
In Europe the wind energy potential offshore is considerably greater than onshore and the offshore installed wind energy capacity is growing fast (1). One of the most important external conditions for the siting of wind farms is the wind resource. It is challenging to assess the offshore wind resource rapidly and accurately. Satellite SAR provides an option in parallel to meteorological mast data and modelling. Advances on using series of satellite SAR wind map for the estimation of offshore wind climatology and statistics relevant for wind engineering is presented. In this process the following steps are performed. Satellite SAR scenes are calibrated (e.g. using BEST from ESA). Then wind direction is found from streak directional analysis, meteorological model data, scatterometer wind direction or mast observations dependent upon availability. Finally wind speed is calculated from inversion of the geophysical model functions (CMOD4/5). The following step is the treatment of the series of wind maps using footprint- weighting function upwind of each point of interest in each wind map (i.e. averaging the grid cells upwind for each location as a function of wind direction with a probability density function). The time-series of wind speed and wind direction for each location is finally used to fit Weibull distribution function shape and scale parameter and the uncertainties are calculated. The last step is calculation of the hub-height winds, typically 100 m above sea level, from the 10m satellite-based wind statistics. Satellite-based wind resource results presented are based on ERS-2 SAR and Envisat ASAR covering the Danish Seas in which more than 400 MW of the total installed 600 MW offshore capacity globally is located. Comparison to meteorological observations and to wind resource model results underlines the complementarities of satellite-based wind resource statistics to the traditional sources. Major advantages of SAR for wind resource mapping are: 1) coverage of the coastal zone in which most wind farming projects are in progress; 2) sufficient number of available data for much of the globe in the ESA archives; 3) fast and well established methodology for applied use for wind engineers are available (see e.g. RWT, WASP and EO-windfarm). The major limitation is that the accuracy is sufficient in pre-feasibility phase but not bankable. This means satellite-based wind resource mapping in the early phase of a wind farm project is relevant. At a later stage, when the financing is decided, other sources of wind resource statistics are needed.
1. Executive Committee for IEA ( International Energy Agency). "IEA Wind Energy. Annual Report 2004." 2005.