Chapter 24Development of a Three-Line Ground-ActuatedAirborne Wind Energy Converter

Alexander Bormann, Maximilian Ranneberg, Peter Kovesdi, Christian Gebhardt,Stefan Skutnik

Abstract EnerKite GmbH designed, built and demonstrated a three line AWE sys-tem. This article presents history of the enterprise and the decisions involved. Thebuilt ground station is described in detail, and fl ight data obtained during the courseof a year in development is presented.

24.1 Introduction

This article portrays the company EnerKı te which set out in 2010 to build a AWECprototype using a three-line actuated system, the EK30. This includes companygoals and design decisions with focus on the built ground station and the acquiredexperience with the prototype. Section 24.2 outlines the history of the company un-til now, and Sect. 24.3 presents the company goals and the design decisions that ledto the construction of the prototype, which is detailed in Sect. 24.4. Finally, Sect.24.5 shows results of the fi rst year of operation, including progress in controllerdevelopment as well as fl ight data plots of trajectories and power quantities.

24.2 Brief History

The necessity and opportunity to invent alternatives to conventional wind turbinesbecame apparent in the mid nineties. The towers for multi-megawatt turbines re-quire cost extensive solutions due to restrictions of industrial manufacturing andtransportation [13]. At the same time Germany’s heavy industry was seeking for in-novative transport solutions by means of giant airships, which should have remedied

Alexander Bormann (�) · Maximilian Ranneberg · Peter Kovesdi · Christian Gebhardt · StefanSkutnikEnerKite GmbH, Ackerstr. 76, 13355 Berlin, Germany, e-mail: a.bormann@enerkite.com

427427U. Ahrens et al. (eds.), Airborne Wind Energy, Green Energy and Technology,DOI: 10.1007/978-3-642-39965-7_24, � Springer-Verlag Berlin Heidelberg 2013

428 Alexander Bormann et al.

these issues in parts. After years of research and development in that field, it becameobvious that the planned lighter-than-air solutions are working only on quite a smallscale and at cost levels rather suited for the military or very special purposes. Eveninnovative developments in lighter-than-air technology did not change that picture[1, 2, 7, 8, 11]. Without this option for bulky and heavy cargo transport and fac-ing the huge amount of material needed in order to build higher support structureswe came to the conclusion, that manufacturing and erection of large horizontal axiswind turbines (HAWT) are becoming cost intensive and inefficient, especially forlower wind onshore sites.

The core team of today’s EnerKıte GmbH has been advancing the idea of AWESystems since 2002. Reliable autonomous flight control and automation was iden-tified as the major challenge for efficient energy generation. Thus, after first towingtests with soft wings in 2003 the concept for an autonomous ground actuated ad-vertising kite system with bionically shaped and helium supported wings of up to24 m2 was developed together with Festo. The so called CyberKite system was per-forming its first autonomous flights in Berlin in May 2008. After comprehensivetests flights of several hundred hours the development ended with the presentationat the Hannover industry fair in April 2010 [14].

In late 2010, the EnerKıte GmbH was founded in Brandenburg state near Berlin.After 3 months the funding was secured by equity and federal grants and the de-velopment began. Within only one year the EK30, a 30 kW offgrid AWE prototypeand research platform (see Fig. 24.1), was designed and brought to operation by

Fig. 24.1 EK30, the mobile 30kW AWEC for research and development during operation at thetest site in Brandenburg, Germany.

24 Development of a Three-Line Ground-Actuated Airborne Wind Energy Converter 429

the team of six engineers. After its virgin flight in March 2012, the field test andcontroller development began. Part of the development process were test flights atvarious sites across Europe. The EK30 performed a joint flight program and firstautonomous flights together with the kitepower team of Delft University of Tech-nology on 22 May 2012 [3] and was presented at the Highwind Workshop in Leuvenon 23 May 2012 [10]. In September 2012, the system was presented in operation togeneral public at the famous Husum wind energy fair [5]. Autonomous operationduring extreme weather conditions and rain strongly confirmed the feasibility ofthis AWE technology.

Fig. 24.2 Flight at HUSUM Wind 2012 [5]. Wing equipped with onboard camera.

24.3 Design, Philosophy and Roadmap

The goals of sustainable renewable energy technologies are cost-efficiency, mate-rial and resource efficiency, flexible application and smart integration [9]. Compre-hensive internal studies on the feasibility and optimization of AWE systems werecarried out in order to approach these goals. A capacity factor of 50% or 4400 fullload hours per year represents a default EnerKıte design criterion for ideal-typicalonshore sites, almost doubling conventional WEC full load hours [4]. For today’sHAWT low nominal wind speeds result in prohibitive costs due to structural reasonsand lower wind speeds on-shore. EnerKıte, using durable high-lift wing configura-tions and operating at a capacity factor of 50% or more, aims significantly reducedlevelized costs and about 95% savings in material per energy generated in compari-son to conventional HAWT.

With respect to the design criteria “safety first” and “complexity matters” thefollowing pragmatic configuration came up: The system needs to be able to operatesafely even without airborne sensors and actuators. Every vitally important systemthat has to fly and reliably communicate with the ground station increases the riskof failure and the cost of development, production and maintenance significantly.

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Thus, a ground actuated three line tether system was chosen as the most promisingconcept.

Other lessons learned from experience both in wind engineering and in thelighter-than-air field are scaling and the necessity of an industrialized manufac-turing. Rather small units may become more cost competitive if standardized andmanufactured in high quantities. Industrialized manufacturing, low erection costsand simple integration are implying to achieve the desired capacity by installingmultiple units rather than upscaling much above the MW scale. Just two types ofEnerKıte AWEC are planned. The EK200 will generate about 500 MWh annuallyat levelized cost with grid parity for the commercial consumers requirements. Forthe local demand of small enterprises and consumers multiple EK200 can be placedand operated on grid or in combination with other sources and storage just likeone facility. The EK1M is scaled up in order to gain grid parity at the utility level.Multi-Megawatt facilities result from an according array of these units. EnerKıtewill confirm the reliability and operability in the small wind sector and plan to runa continuous pilot operation from 2014 on with the EK200.

24.4 EK30 - The mobile development platform

The initial step towards realization was to built a fully functional and operational au-tonomous AWE system with minimal efforts in time and costs, which ultimately re-sults in shortening the time-to-markets. The EK30 is a full-functioning wind-energysystem with advanced drive and inverter technology, battery storage, load resistorand the three-line winch built into a mobile platform. The ground station was de-signed to operate with a peak power output of 54 kW and 30 kW nominal continuouspower output. Nevertheless, no compromises were made in terms of actuator powerand response. The reinforced kites were optimized to better withstand the high loadsand the dynamic load characteristics of an AWE system. Figure 24.3 shows the wind

Fig. 24.3 Wind tunnel test setup (left) and ground station module lab assembly (right).

24 Development of a Three-Line Ground-Actuated Airborne Wind Energy Converter 431

tunnel setup for load analysis of ram-air wing segments and the ground station mod-ule before mounting on the carrier vehicle. The mechanical and electrical propertiesare given in Table 24.1.

Mechanical (Ground Station)

tether force (main line), RMS 8.2 kNtether force (main line), peak 16 kNmax. collective line speed 26 ms−1

max. relative control line speed 3.2 ms−1

max. collective line acceleration 26 ms−2

max. control line acceleration 20 ms−2

max. rotational speed winch platform 20 min−1

max. acceleration winch platform 4 rads−2

max. tether storage 1200 m

Electrical (Ground Station)

max. regenerative power 57 kWnominal regenerative power 30 kWcapacity storage (600 V - lead acid battery) 50 kWhgrid connection 230 V, 3 × 400 V

Geometrical (Ground Station)

diameter (w/o vehicle and w/o storage) 2.10 mheight (w/o vehicle and w/o storage) 0.75 mweight (incl. lead acid storage, w/o vehicle) 4100 kg

Ram-Air Kites

area (flat) 15 m2 - 21 m2

area (projected) 12.8 m2 - 17.9 m2

wingspan (flat) 9.3 m - 12.1 mweight (kite only) 4.2 kg - 5.8 kg

Table 24.1 Mechanical, electrical and geometrical data of the EK30

24.5 Research and Operation

Since March 2012 the EK30 is operated for AWE research and development. Theemphasis lies in the development of suitable kites for AWE systems, model vali-dation and controller development. Additional research areas include the acousticprofile of AWE systems [6] and innovation in electrical storage devices [12].

The on-site controller development began with flight testing in March 2012. Theamount of operational experience and the progress in terms of flight automation isshown in Fig. 24.4. The first tests focused on load-control and kite adaption. Conse-quently, the emphasis shifted to trajectory control, which necessitated the automatic

432 Alexander Bormann et al.

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Fig. 24.4 Progress of automatic flight with the EK30. The occasional numbers above the barsindicate the percentage of automated flight (occasional photoshoots included). Days 25 to 27 shownin this graph are corresponding to the demonstration flights at the Husum WindEnergy fair, whenthe system had to withstand heavy rain showers and gusty winds [5].

adaption to changing wind directions at the height of the kite and the developmentof suitable backtracking maneuvers.

Using ram-air kites, the reel-in phase was carried out by flying upwind behindthe ground station and reeling in, with the wind, with high speed. This leads to acharacteristic pretzel-like trajectory if the flight path is seen from above. Trajecto-ries of such autonomous harvest operations are shown in Fig. 24.5. A Yo-Yo-AWE

Fig. 24.5 Trajectory of a ram-air kite shown from above.The ground station is locatedat the origin. During reel-out the typical figure-eighttrajectory is flown between100 m and 300 m. Then, thekite flies windwards to reel-in. The controller adapts tochanging wind directions.

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24 Development of a Three-Line Ground-Actuated Airborne Wind Energy Converter 433

13200

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Fig. 24.6 A typical work-over-time plot. The mechanical work is calculated as the sum of theproducts of measured forces and corresponding velocities of all three lines.

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Fig. 24.7 A typical force profile on the main line during reel-out and reel-in.

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An ultrasonic anemometer for wind assessment provides typical records for ourtest-site at a height of 6 m shown in Fig. 24.9 and Fig. 24.10. In addition, a Lidar

Fig. 24.9 Wind speed measurements obtained using an ultrasonic anemometer and Lidar at alti-tudes from 6 m to 200 m during a joint measurement program with Fraunhofer IWES at the testfield. 19 June 2013 was a hot summer day with high thermal activity and a thunderstorm withheavy gusts in the night. Peaks of about 30 m/s for 260 m are not shown for more clarity in thepicture.

Fig. 24.10 Turbulence intensity, that is the standard deviation assuming normal distribution with amean value of the running measured mean over 10 min divided by this mean wind speed. 19 June2013. During the day, turbulence intensities ranged from 20% to 40% across all altitudes. Duringthe night, the turbulence intensities dropped at all heights, with values of about 5% for altitudesabove 100 m. The spike after midnight occurred during a thunderstorm.

wind measurement system was used to capture wind speeds at higher altitudes up to200m and above during a recent joint measurement campaign. The 24 hour recordcovers a range of wind speeds from almost zero up to nearly 30m/s. In turbulent

03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:000

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24 Development of a Three-Line Ground-Actuated Airborne Wind Energy Converter 435

thermal conditions the average wind speed remains quite the same across the verticalprofile, while turbulence intensities are ranging from 20% to 40% at 100 m to 200 m.Even at higher altitudes wind drops occasionally down to almost zero. Nevertheless,autonomous power production was performed between noon and midnight. Duringthe night turbulence intensity drops down to approximately 5% above 100 m. At thesame time, wind speed increased significantly at higher altitudes with about 30%gain from 100 m to 200 m. Around midnight, flight operation was halted due to anoncoming thunderstorm.

The force and wind measurements are showing exemplary the high dynamic im-pact challenging the AWEC. Due to these loads we expect the full-load lifetime ofthe textile wings, even with significant strengthening of the high load areas, not tosurpass much more then 100 hours. Efficient and durable kite designs is one mainfocus of the ongoing research and development.

24.6 Summary and Outlook

The EnerKite EK30 has proven its reliability and performance in more than hundredhours of flights across Europe. As a mobile test rig it is perfectly suited to performresearch and development and to validate the prospective performance of airbornewind energy conversion.

No doubt - there is still a lot of research and development to be done before air-borne wind energy works reliable and economically 24 hours around the year. TheEnerKite team and their partners are therefore focusing on the implementation of adurable and efficient wing technology, the integration of an autonomous launch andlanding facility and the topics of permission and certification within the frame of afederal research grant program. As one of the leading enterprises in this field, En-erKıte strives to place a first and convincing product in the rapidly growing marketof smaller wind turbines with competitive levelized costs of electricity until 2015.

Acknowledgements The EnerKıte EK30 research and development was supported by federalfunds and regional funds of Brandenburg state. We thank the Ministry of Economy for the cur-rent support for the technology and product development. Lidar measurements were provided byFraunhofer IWES with friendly permission of the Ministry of Environment financing the “OnKite”study. Thank you all supporters, partners and sponsors! Special credits go to Franz Mitsch and alsoto Udo Zillmann and other investors of the early stage who both with finances, particular knowl-edge and their shared vision fundamentally contributed to the success of this company. You haveenabled EnerKıte to walk in the frontline of the reinvention of wind energy conversion.

436 Alexander Bormann et al.

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