Research On Offshore Wind Energy Resources Remote Sensing Technology And Applications Based On The Satellite-borne SAR

The development of offshore wind farm needs accurate assessment of offshore wind energy, for the wind field data of offshore high spatial coverage density can not be obtained using traditional methods. In addition, using the numerical simulation technology to estimate offshore wind energy resources, the accuracy and resolution of results are low. With the development of satellite remote sensing technology, synthetic aperture radar (SAR) provides new methods for offshore wind energy resources assessment. In this paper, offshore wind energy remote sensing technology based on space-borne SAR is researched. The long time sequence sea surface wind speed data along the Zhejiang coast at the height of10meters are retrieved from443scene ENVISAT ASAR imagery, which are obtained from2005～2011. The resolution and accuracy of the retrieved sea surface wind speed data is also high. Based on the wind speed data, the wind energy resources are assessed accurately in the Zhejiang coast. The macroscopic distribution and microcosmic characteristics of the wind energy resources distribution in the waters are studied and analyzed.In order to retrieve high-precision wind speed data of the Zhejiang coast waters, the main factors which have an influence on the ENVISATA ASAR wind filed retrieval are studied, and a system of working methods is established. To reduce the impact of prevalent upwelling along the coast in summer on the Zhejiang coastal ASAR wind field retrieval, in light of the numerical model high resolution wind field data, a coastal upwelling region SAR wind speed correction method is especially built and the underestimated wind speed is corrected. The result shows that the accuracy of SAR wind speed retrieval has been improved in coastal upwelling.The three common SAR wind field retrieval models-geophysical model functions (GMFs)(such as CMOD4,COMD5and COMD-IFR2) are researched, which are suitable for C wave band and VV polarization. For HH polarization SAR imagery, the polarization ratio model is also studied. Using the three models, the wind speeds are retrieved from SAR imagery and are compared respectively with buoy observations. According to the results of the comparative study, the CMOD4is chosen to retrieve the sea surface wind speed along the Zhejiang coast. Then those wind speed data are compared with the mast and buoy observed wind speed. The results show that the root mean square error is1.42m/s and0.89m/s. The results demonstrate that the accuracy of wind speed data retrieved from ENVISAT ASAR is high. So they can be used for wind energy resources assessment.Based on the high resolution and accuracy wind speed data, the wind energy resources are assessed along the Zhejiang coast and the characteristics of the Zhejiang coast wind resource distribution are researched. The main conclusions are as follows: (1) Spatial distribution characteristics:The wind energy resources along the Zhejiang coast are rich. The sea surface annual average wind speed at the height of10m is5m/s-10m/s, and the annual average wind power density can reach100W/m300W/m2within50km away from the coastline. The region richest in wind energy resources is located in south sea of the Zhejiang coast, and the largest wind power density can reach500W/m2. In general, wind energy density shows an overall gradient change trend from coastline to the waters away from the coastline which is decreasing from East to West. So the wind energy resources of the islands along the Zhejiang coast are also rich.(2) Time (seasonal) distribution characteristics:The seasonal feature of wind resource shows that the wind power is rich in the autumn and winter, while it is poor in the spring and summer along the Zhejiang coast. The sea surface average wind power density of spring and summer is100W/m2-200W/m2within50km away from coastline, and the average wind power density of autumn and winter is200W/m2～300W/m2.(3) Vertical distribution characteristics:The exponential wind profile near the sea surface is applied to transfer wind speed at an elevation of10meters to the wind speed at the height of70meters above the sea level. The statistics show wind resources at70m height are richer than at10m. In the coastal waters, the annual average wind speed is6m/s～11m/s, and the annual average wind power density is200W/m2～800W/m2.(4) Based on ENVISAT ASAR, the wind energy resources along the Zhejiang coast are assessed and the result is consistent with the result of buoy observation wind speed data. The relative error of weibull distributed parameter scaling factor A is1.59%. The relative error of shape parameter k is7.83%. The relative error of average wind speed is4.86%and the relative error of average wind power density is2.0%. The assessment accuracy is higher than that of NCEP-NCAR numerical simulation wind field data. It also agrees well with the result of wind energy resources assessment using traditional methods. The results show that the resolution and the accuracy of the wind energy resources assessment based on the ENVISAT ASAR imagery are high.