Estimation, Optimisation and Validation of Power Curves for Airborne Wind Energy

EnerKíte team in front of the EK30 ground station (17 July 2014). 50

Maximilian Ranneberg Mathematician EnerKíte GmbH Fichtenhof 5 14532 Kleinmachnow Germany m.ranneberg@enerkite.com www.enerkite.com

Estimation, Optimisation and Validation of Power

Curves for Airborne Wind Energy

Maximilian Ranneberg, Alexander Bormann EnerKíte GmbH

Together with the financial aspects such as capital cost and operational cost, the power curve of a wind energy power-plant is the defining characteristic to deduce the economic viability for site specific wind and weather con-ditions.

Power curves for horizontal axis wind turbines have been studied, validated and optimized for decades.

This study is concerned with the detailed power output of two different airborne wind energy (AWE) systems that are using the YoYo principle.

Until now, no detailed and fully-dynamic model has been used to optimize, validate and publish a complete power curve for a YoYo converter.

Here, a detailed model of the EnerKíte EK30 AWE proto-type [1] is described and used to validate, obtain and com-pare power curves for two different wing technologies: Ram-Air Kites need a certain angle of attack to keep their shape. They are stiff with regard to the angle of attack due to their bridle. In comparison semi-rigid wings, which are not in need for a stiff bridle in all conditions, can be di-rectly retracted.

The power curve of currently employed ram-air wings is compared to experimentally obtained power measure-ments from test flights with the EK30 and a LIDAR wind measurement system. The validated simulation model allows to estimate and optimize both the machine design and operation and the power curve for the EK30

semi-rigid wings. With appropriate tether forces and torques more than 4 kW/m2_{wing-surface at wind speeds of 7.5} m/s can be achieved.

In the second part, a fast but detailed system model based on a point-mass model described in [2] is pre-sented. The model is used for rapid AWE design and power curve evaluation and compared to the detailed dy-namic simulations. The approach is similar to the meth-ods used in [3]. This model is used to gain insight into the effects of realistic altitude-dependent wind conditions, site constraints and different ground station and wing de-signs.

With regard to future codes for certification, the authors are proposing a standardized altitude depending repre-sentation of power curves which allows for the estimation of yield with respect to the time and altitude dependent nature of the winds.

References:

[1] Bormann A., Ranneberg M., Kövesdi P., Gebhardt C., Skutnik S.: Development of a three-line ground-actuated airborne wind en-ergy converter. In: Airborne Wind Enen-ergy. Springer (2013) [2] Ranneberg M.: Sensor setups for state and wind estimation for airborne wind energy converters. arXiv preprint arXiv:1309.1029 (2013)

[3] Fechner U., Schmehl R.: Model-based efficiency analysis of wind power conversion by a pumping kite power system. In: Airborne Wind Energy. Springer (2013)