Autonomous Kite-Sailing Power Generation

Shinji HIEJIMA
Professor, Graduate School of Environmental and Life Science, Okayama University, Okayama, Japan

1. Abundant Offshore Wind Energy around Japan
Japanese Prime Minister Yoshihide Suga announced that Japan will reduce overall greenhouse gas emissions tozero by 2050. In order to achieve this pledge, larger amount of renewable energy resources should be developed.
The most promising renewable energy seems to be offshore wind power, because Japanese islands are surroundedby vast oceans. Japanese Ministry of the Environment1) and New Energy and Industrial Technology Development
Organization2) (NEDO) have estimated that offshore wind energy potential around Japan is approximately 1200
~ 1500 GW. Moreover, this is the energy potential estimation confined to the nearshore sea areas within 30km
distance from the homeland. As shown in Fig.1, offshore wind energy potential becomes higher in the offshore
sea areas farther from the homeland. The wind energy in the offshore sea areas exceeding 30km far from thehomeland is still more abundant, and it could supply a demand for overall electricity consumption in Japan.

Fig. 1 Offshore wind energy around Japan3)
However, most of offshore wind turbines have been installed in nearshore sea areas within 30km distance fromlands. Because laying longer subsea power transmission cables to farther offshore wind turbines raises theinstallation and maintenance costs significantly in addition to anchoring costs at deep waters. Meanwhile, there
are many difficulties such as interference with fishing activities to install wind turbines in the nearshore areas.
If we accessed to the abundant wind energy in the offshore areas exceeding 30km far from lands, we could avoid
the interference with fishing activities and obtain sufficient energy to reduce overall greenhouse gas emissions

to zero. In order to realize that, breakthrough technologies are necessary to offshore wind power generation.

2. Proposal of the Autonomous Kite-Sailing Power Generation
We have proposed the Autonomous Kite-Sailing Power Generation as a next-generation offshore wind power
generating system shown in Fig. 2. The floating vessel sails exploiting the towing force induced by a kite
capturing offshore wind. This kite is flown at high speed by describing a figure-of-eight trajectory in the air
through manipulation of a pair of tethers connected to the kite. This high-speed flight amplifies the kite’s towing
force effectively. The longer the tethers are, the higher wind energy is obtained at higher altitude. Then the water
current turbines mounted under the floating vessel generate electric power by exploiting the water current
induced by the kite sailing. The generated electric power is stored in a battery inside the vessel instead ofconnecting to electric power transmission cables. The other way to store the generated power is converting tohydrogen gas by electrolysis of seawater. Without any anchoring or cable connection, the Autonomous KiteSailing Power Generation system is capable of moving around on the sea surface and generating electric power
regardless of water depth and distance of electricity transmission. Accordingly, it can access abundant windenergy in the offshore areas exceeding 30km far from lands. Moreover, this system does not occupy any fishingarea and hence not interfere with fishing activities.

wind

TetherKite
Floating vessel
Battery
water current

TurbineFig. 2 The Autonomous Kite-Sailing Power Generation
The Autonomous Kite-Sailing Power Generation system generates electric power not directly from the offshore
wind but from the water current induced by the offshore wind through the kite. In general, the electric powers
generated by propeller turbines are proportional to the fluid density, the square of rotor diameter and the cube of
fluid velocity. Although the fluid velocity of the water current is lower than the wind, the fluid density of thewater is about 800 times higher than the wind. Accordingly, a smaller size of water current turbine is expected
to generate electric power same as a larger size of wind turbine, and then the water current turbines are morecompact in size than wind turbines. This contributes to reduce the manufacturing cost of the Autonomous KiteSailing Power Generation system.
The Autonomous Kite-Sailing Power Generation works as an unmanned surface vehicle. It generates and stores
sufficient electric power in the offshore areas, and it autonomously return to its home ports to supply the electric
power to lands. Then it goes autonomously to the offshore areas again to obtain more wind energy. This repetitive
work of energy transport is conducted by the kite-sailing power without fuel to propel. Then the Autonomous
Kite-Sailing Power Generation system becomes a kind of energy carrier and could achieve a low-cost energy

transmission compared to laying subsea power transmission cables.
The advantages of the Autonomous Kite-Sailing Power Generation are listed as follows compared toconventional wind turbines.


It can access abundant wind energy in the offshore areas exceeding 30km far from lands by using kitesailing system without anchoring and connecting to power transmission cables.

Its kite system can access higher wind energy at higher-altitude area than wind turbines.



It does not interfere with fishing activities because it is not anchored and does not occupy any fishing area.



It is more compact in size than wind turbines and generates electric power same as lager-size wind turbines.



It can transport offshore wind energy to lands by its sailing system instead of high-cost subsea powertransmission cables.

3. The DeepSky Concept
As mentioned above, the Autonomous Kite-Sailing Power Generation system is capable of carrying out longterm missions autonomously by sailing without fuel over a wide range of offshore areas. Therefore, it can beutilized for multiple purposes by equipping various devices and sensors as shown in Fig. 3. Moreover, bydeploying a large number of the multipurpose Autonomous Kite-Sailing Power Generation systems andcollaborating each other as a network, they can work as a common platform of various offshore infrastructures
as follows.


Offshore early-warning system equipped with weather observation devices for typhoon and tsunamiAutonomous maritime security system equipped with surveillance devices for marine salvage, illegal fishingor maritime self-defense of isolated islands

Autonomous submarine prospecting system equipped with sonars to explore scarce resources such as rare
metal



Offshore internet facility utilizing the high-altitude kite as a communication antennaEnergy-saving offshore datacenter equipped with data server cooling by seawaterAutonomous clean-up system for marine plastic debrisOffshore charging station utilized for electric vessels

Fig. 3 The Autonomous Kite-Sailing Power Generation equipped with devices and sensors

The area of the exclusive economic zone (EEZ) in Japan is the sixth largest in the world, but it has not beenutilized actively. The Autonomous Kite-Sailing platform can accelerate the high-degree application of this vast
ocean space. If the above-mentioned offshore infrastructures were constructed on the ocean, various businesses
exploiting oceanic big data obtained from them could be created. Moreover, the above multiplex businesses
based on the Autonomous Kite-Sailing platform are expected to establish a more profitable and sustainablebusiness model of offshore wind power generation.
We have propounded the “DeepSky” concept which aims to achieve energy self-sufficiency ratio of 100% by
exploiting the sky-high wind energy over the deep ocean spaces based on the Autonomous Kite-Sailing platform.
This also aims to change the unutilized vast ocean spaces to a virtual homeland through the common platform of
the multipurpose infrastructures established by the Autonomous Kite-Sailing system. This platform contributes
not only to achieve the reduction of overall greenhouse gas emissions to zero by 2050, but also to realize theMaritime Domain Awareness promoted by the Japanese government, which aims to gather and share various
oceanic big data about forthcoming large earthquakes, incursion of foreign vessels, climate change and fisheryresources.

4. Research and Development of the Autonomous Kite-Sailing Power Generation(1) Similar energy conversion method based on kite flight
Airborne wind energy4), which is a kite-based wind energy conversion method similar to the Autonomous KiteSailing Power Generation, has been developed in many European countries. The long tether of the kite is wound
on a reel placed on the ground, and it is reeled out with the kite flown away. At the same time, a power generator
attached on the reel rotates with the reel rotation and generates electric power. After the tether is reeled out tothe end, reversely the tether is reeled in with the kite flown down. Although the energy is consumed to reel thetether in, the energy consumption can be minimized by reeling in with the kite flown down slowly. The othermethod of the air borne wind energy is to fly the kite with wind turbines installed on the kite. In this case, theinstalled wind turbine is restricted to lightweight, but it can generate electric power exploiting high-altitude
strong wind power.

Fig. 4 Ship propulsion systems using towing kites5)6)Some ship propulsion systems using towing kites have been developed as shown in Fig. 4. These kites arecontrolled to describe the figure-of-eight trajectory and amplify the towing forces similarly to the Autonomous
Kite-Sailing Power Generation. These kite-sailing ships have already been commercialized and succeeded toreduce fuel consumption of the ship propulsion. If water current turbines were installed to these kite-sailing ships,
they could be equivalent systems to the Autonomous Kite-Sailing Power Generation.

Kim and Park7) have proposed a kite-sailing power generation system similar to the Autonomous Kite-SailingPower Generation. In their system, the kite is flown statically at fixed point in the air over the lateral directionof the ship. However, the lift force induced by such static kite is lower than the kite flown by describing thefigure-of-eight trajectory. Furthermore, most of the towing force induced by the kite flown in the lateral directionof the ship does not contribute to propel the ship. Accordingly, the energy harvesting efficiency of their systemseems to be lower than the Autonomous Kite-Sailing Power Generation.
(2) Performance simulation of the Autonomous Kite-Sailing Power Generation
We have developed the kite flight simulator8) to examine the energy harvesting performances of the Autonomous
Kite-Sailing Power Generation. This simulator is based on the coupling equations between the kite flying along
a figure-of-eight trajectory and the vessel sailing by the towing kite. As shown in Fig. 5, the kite with a wingspanof 20m and a chord length of 5m is employed in the simulation. NACA4415 airfoil is used for the wing crosssection of the kite, and the wing has a span-wise curvature with a central angle of 90°. We assumed that twowater current turbines with a diameter of 5m are mounted under the vessel with a weight of 100ton and they are
towed by the kite. These specifications of the vessel and the turbines are determined by considering the SR250
floating tidal turbine of Orbital Marine Power Ltd9). The specifications used in the simulation are listed in Table
1. Wind speed is assumed to be 15m/s at 10m altitude and increase with altitude according to logarithmic law.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

cross-section of kite
NACA4415

Fig. 5 Kite and its cross-section geometry used in the simulation
Table 1 Kite, tether, vessel and turbine specifications used in the simulation

kite

tethervesselturbinemass
wing area
wingspanchord length
lengthdiameterdrag coefficientmassunderwater cross-section area
drag coefficientnumber of turbines
total swept area of rotor
power coefficient

77 kg
100 m2
20 m
5m
200 m
0.03 m
1.2
100 ton
3.14 m2
1.0
239.27 m2
0.35

Fig. 6 Schematic of the simulation for the Autonomous Kite-Sailing Power Generation

5vs (m/s)c = 15deg, c = 0deg

430

c = 15deg
c = 0deg

 (deg)

3
2

25

1
20

t (s)

0
0

15

500
10

targeted
simulated
-15

-10

20

30

40

50

60

c = 15deg, c = 0deg

P (kW)

400
300

5

(deg) 200

0
-5

10

0

5

10

15 100

t (s)

0
0

10

20

30

40

50

60

Fig. 7 Example of the simulation results in flight case of 𝜃 = 15°, 𝜙 = 0°
In the simulation, the kite flies on the surface of a quarter sphere in the air, and its position is denoted by elevation
angle  and azimuth angle  as shown in Fig. 6. Then the center position of the figure-of-eight trajectory of thekite is denoted by c and c. The directions of the natural wind and the sailing path of the floating vessel are
assumed to coincide with the direction of  = 0°. Fig. 7 is an example of the simulation results in the trajectory
case of 𝜃 = 15° and 𝜙 = 0°. The flight direction of the kite changes with the kite’s roll angle adjusted bymanipulating a pair of tethers connected to the kite. Fig. 7 shows that the kite can fly close to the targetedtrajectory. At the same time, the floating vessel velocity 𝑣 and the power 𝑃 generated by the turbines are 𝑣 =3.6m/s and 𝑃 = 324kW on average, respectively. This is just an example to show the energy harvesting
performance of the Autonomous Kite-Sailing Power Generation, and the performances could be higher if larger
sizes of kites and turbines were employed.

(3) Development of the Hydro-VENUS turbine based on flow-induced oscillation
We have been developing the Hydro-VENUS (Hydrokinetic Vortex ENergy Utilization System) in Fig. 8 as awater current turbine appropriate for the Autonomous Kite-Sailing Power Generation. The Hydro-VENUS is apendulum-based energy converter exploiting flow-induced oscillations. When a circular cylinder pendulum10)11)
is employed for the Hydro-VENUS, the vortex-induced oscillation occurs due to the Karman vortices shedding
periodically from the pendulum. However, the amplitude of the vortex-induced oscillation is relatively small,and then the harvested energy is also small. On the other hand, the galloping oscillation, which is still stronger
than the vortex-induced oscillation, occurs and can harvest higher energy when a semi-circular cylinder
pendulum12) is employed for the Hydro-VENUS.
The galloping is a self-excited oscillation induced by the shear flows separated from a solid body. As shown in
Fig. 9, the shear flows are separated from the sharp edges on the both sides of the semi-circular cylinder crosssection. These separated shear flows incline toward the opposite direction of the pendulum velocity. These
asymmetric flows cause pressure difference between the both sides of the pendulum surface. This pressure
difference involves hydrodynamic forces perpendicular to the flow direction, and accelerates the pendulum
oscillation. This acceleration promotes the flow asymmetricity and increases the pressure difference further.
Accordingly, the hydrodynamic forces acting on the pendulum and the pendulum velocity continue to rise eachother. This feedback amplification between the hydrodynamic forces and the pendulum velocity causes selfexcited large-amplitude oscillations of galloping.

Fig. 8 Schematic of the Hydro-VENUS

flow
edge

edge

Force : small

Force : large

Separatedshear flow

Pendulum velocity : low

Pendulum velocity : highFig. 9 Mechanism of galloping oscillation

flow

Semi-elliptical cross-section

semi-minor
axis

major axis
Fig. 10 Semi-elliptical cross-section with aspect ratio of 6
According to the previous studies13)14) about the cross-section geometry of the Hydro-VENUS pendulum, it has
turned out that semi-elliptical cross-section with an aspect ratio of 6 in Fig. 10 is the most effective to induce
large-amplitude galloping oscillations and achieves 10 times higher energy harvesting performance than thecircular cylinder pendulum. The amplitudes of the semi-elliptical cross-section pendulum in galloping oscillation
exceed 360° and reach several rotations when the spring constant in Fig. 8 is small. It is found that one-way
rotation ultimately occurs in the pendulum without the spring. This rotational pendulum works as if it was aconventional propeller rotor. But its mechanism of the rotation is absolutely based on the galloping caused by
the feedback amplification between the pendulum and the separated shear flows around the pendulum.
It is revealed that this Hydro-VENUS rotor with semi-elliptical cross-section shows the high energy harvesting
performances equivalent to the conventional propeller turbines with airfoil blades14). In addition, this HydroVENUS rotor is capable of rotating in both the forward and the reverse direction quite differently from theconventional airfoil propellers. Thus, it can keep rotating and generating powers by changing the rotational
direction in accordance with the flow conditions even in the strongly turbulent flows. This direction-changeablerotation of the Hydro-VENUS rotor is also effective to avoid winding of driftage around the rotor. In addition,
the start-up performance of the Hydro-VENUS rotor from fully stalled static state is expected to be high becauseof the feedback amplification mechanism same as the galloping. The Hydro-VENUS pendulum is a columnar
body with uniform cross-section and without twisting angles. Accordingly, its three-dimensional shape is much
simpler than airfoil blades of conventional propellers. Then it is easy to manufacture, and its manufacturing cost
is expected to be low. It is also easy to reinforce the inside space of the pendulum with a small number ofstructural elements, and hence it is useful as a light-weight and high-strength turbine blade.

5. Future Tasks of Research and Development
In the Autonomous Kite-Sailing Power Generation, the kite is controlled to describe the figure-of-eight trajectoryin the air by manipulating a pair of tethers automatically. In order to control the kite flying at high speed, it isnecessary to control the tether actuator in real time by measuring the kite position and wind speed with sensors
constantly. In order to achieve optimum control, it is important to reduce the effects of errors and time delays in
the controls and measurements. Various automatic kite control systems have been developed in the field ofAirborne Wind Energy4), and some of them have already been commercialized5)6)15). Although some autonomous
navigation systems16)17) of vessels have been developed and commercialized, autonomous navigation using the
kite-sailing system is necessary in the future.
The floating vessel with water current turbines of the Autonomous Kite-Sailing Power Generation has to maintainstable attitude under the various conditions of sea surface waves, thrust forces acting on the turbines and towing
forces induced by the kite. Submersible, semi-submersible and wave-piercing floating vessels seem to be usefulfor these purposes. Also, various floating type tidal turbines18)19)20)21) have been developed and could be diverted
to the Autonomous Kite-Sailing Power Generation. We have been developing the floating type turbine with theHydro-VENUS rotor as shown in Fig. 11. In addition, the battery technologies stably workable in the harsh
offshore environments are also necessary.

Fig. 11 Small-scale model of floating type turbine with the Hydro-VENUS rotor

References
1) Ministry of the Environment: Study of potential for the introduction of renewable energy (FY2010), 2011.
2) New Energy and Industrial Technology Development Organization (NEDO): Guidebook for the installationof bottom-mounted offshore wind turbines, 2018. (in Japanese)3) New Energy and Industrial Technology Development Organization (NEDO): NeoWins, http://app10.infoc.
nedo.go.jp/Nedo_Webgis/index.html
4) Schmehl, R.: Airborne Wind Energy, Springer, 2018
5) SkySails：https://skysails-group.com/index.html
6) Airseas：https://www.airseas.com
7) Kim, J. and Park, C.: Wind power generation with a parawing on ships, a proposal, Energy, Vol.35, pp.14251432, 20108) Hiejima, S., Endo, M. and Yamamoto, A.: Aerodynamic characteristics tests and flight simulator developmentof parafoil kites used for the Autonomous Kite-Sailing Power Generation, Wind Engineering Research,
Vol.26, pp.86-95, 2020. (in Japanese)9 ) Scotrenewables Tidal Power: Scotrenewables Tidal Power Ltd SR250 Deployment Fall of Warness:
Environmental Statement Volume II - Appendices, 201010) Hiejima, S., Oka, K., Hayashi, K. and Inoue, H.: Fundamental experiment for energy harvesting from flowinduced vibration of inverted pendulum, Journal of JSCE, Ser B3 (Ocean Engineering), Vol.69, No.1, pp.1221, 2013. (in Japanese)
11) Hiejima, S. and Oguma, H.: Characteristics of hydrodynamic force acting on circular cylinder pendulum inpendulum-based tidal energy converter, Journal of JSCE, Ser A1 (Structural Engineering & Earthquake
Engineering), Vol.72, No.2, pp.315-321, 2016. (in Japanese)
12) Hiejima, S., Takamatsu, H., Oguma, H. and Ueda, T.: Pendulum-based water current energy converter using
galloping oscillation, Journal of JSCE, Ser B3 (Ocean Engineering), Vol.73, No.1, pp.24-34, 2017. (inJapanese)
13) Hiejima, S., Takamatsu, H., Oguma, H. and Ueda, T.: An innovative turbine blade for water current power
generation utilizing excitation mechanism of flow-induced oscillation, Journal of JSCE, Ser B3 (Ocean
Engineering), Vol.74, No.1, pp.1-12, 2018. (in Japanese)
14 ) Hiejima, S., Ito, J., Kurioka, D., Sakahira, S., Hanafusa, Y. and Ueda, T.: An improvement in energy
harvesting performance of VENUS blade by suppressing blade tip vortices, Proc. 25th National Symposium

on Wind Engineering, pp.91-96, 2018. (in Japanese)
15) Kitepower: https://kitepower.nl16) ASV: https://www.asvglobal.com17) Maritime Robotics: https://www.maritimerobotics.com
18) Orbital Marin Power: https://orbitalmarine.com
19) Bluewater: https://www.bluewater.com
20) Magallanes Renovables: https://www.magallanesrenovables.com
21) Sustainable Marine Energy: https://www.sustainablemarine.com