David Brandt R&D Scientist EWC Weather Consult GmbH
Schönfeldstraße 8 76131 Karlsruhe
Germany
david.brandt@weather-consult.com www.weather-consult.com
Adapting Wind Resource Estimation for Airborne Wind Energy Converters
David Brandt1, Martin Busch1, Alexander Bormann2, Maximilian Ranneberg2 1EWC Weather Consult GmbH
2EnerKíte GmbH
Wind resource estimation refers to any technique that as-sesses the wind potential in order to evaluate whether wind energy is economically viable at the investigated lo-cation. Thereby usually the annual energy production (AEP) is calculated based on the wind speed frequency distribution and the power curve. Interruption of opera-tion, as necessary for maintenance and repairs, is usually considered as a capacity factor, e.g. 0.98. Regarding air-borne wind energy converters (AWEC) however, one may need to consider additional influences towards the ca-pacity 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 coun-tries, 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 op-erational 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 investi-gated 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 val-ues are available as timeseries with hourly resolution cov-ering 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 light-ning 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 evalua-tion of air temperature and humidity which can both be taken from 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 ca-pacity factor will be shown as well as a site specific sum-marized capacity factor. The findings are discussed espe-cially in the context of optimization strategies as well as with respect to operational management of AWECs.
