Adapting Wind Resource Estimation for Airborne Wind Energy Converters

David Wölfle, Martin Busch, Alexander Bormann, Max Ranneberg
Airborne Wind Energy Conference, Delft, The Netherlands
Wind resource estimation refers to any technique that assesses the wind potential in order to evaluate whether wind energy is economically viable at the investigated location. Thereby usually the annual energy production (AEP) is calculated based on the wind speed frequency distribution and the power curve. Interruption of operation, as necessary for maintenance and repairs, is usually considered as a capacity factor, e.g. 0.98. Regarding airborne wind energy converters (AWEC) however, one may need to consider additional influences towards the capacity factor, as AWEC may need to land in order to: • Prevent lightning impact • Allow de-icing • Avoid aircraft collision at low visibility In this work the influence of the before named situations on the AEP is examined for an EnerKíte AWEC, for an area covering Germany and parts of the surrounding countries, and for the years from 2012 till 2013. The system operates on altitudes from 50 to 300 m and may also be forced to land if the wind speed falls below the lower operational limit of 2 m/s. After conditions have improved a take-off is required to continue operation. For which it is also necessary that the lower operational wind speed at the lower operational altitude is reached, regardless stronger winds at higher altitudes. The power production of the Enerkíte AWEC depends on altitude and wind speed. Therefore, for every investigated location a wind profile must be used to consider optimal height of operation with respect to power. The wind speeds are taken from the COSMO-DE dataset produced by the German weather service DWD. The values are available as timeseries with hourly resolution covering the above mentioned area with a mesh size of 2.8 km. In vertical direction the wind speeds are interpolated to 12 height levels that are uniformly distributed between 25 and 300 m above ground level. Regarding the prevention of lightning impact, it is first necessary to evaluate the actual likelihood of a lightning striking into the AWEC during the investigated time span. Based on professional lightning records by Nowcast it is assumed that every lightning that has occurred within a certain radius around a potential site would have hit the airborne system. The actual radius depends thereby on the operational height. Furthermore it is presumed, that lightning threat is only given for situations where the line between kite and ground station is wet, as a dry line is supposed to be non-conducting. Finally an indicator for lightning risk is derived from correlating the virtual lightning impacts with radar reflectivity data provided by the DWD which is a sensitive diagnostic for thunderstorms. Based on this indicator it is possible to remove power that would be produced under lightning threat from the AEP. Potential icing of the AWEC can be identified by evaluation of air temperature and humidity which can both be takenfrom COSMO-DE. These two measures are also used for the spotting of low visibility situations. As a result, the impact of the individual threats on the capacity factor will be shown as well as a site specific summarized capacity factor. The findings are discussed especially in the context of optimization strategies as well as with respect to operational management of AWECs.
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David Woelfle