# Experimental proof-of-concept of an energy ship propelled by a Flettner rotor

B Elie, B Bognet, T Boileau, M Weber, Jean-Christophe Gilloteaux, Aurélien Babarit

► To cite this version:

B Elie, B Bognet, T Boileau, M Weber, Jean-Christophe Gilloteaux, et al.. Experimental proof-of-concept of an energy ship propelled by a Flettner rotor. Journal of Physics: Conference Series, 2022, 2265, 10.1088/1742-6596/2265/4/042057. [hal-05342952]

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# Experimental proof-of-concept of an energy ship propelled by a Flettner rotor

To cite this article: B Elie et al 2022 *J. Phys.: Conf. Ser.* **2265** 042057

View the [article online](#) for updates and enhancements.

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# Experimental proof-of-concept of an energy ship propelled by a Flettner rotor

B Elie<fnref n="1" />, B Bognet<fnref n="1" />, T Boileau<fnref n="1" />, M Weber<fnref n="2" />, J-C Gilloteaux<fnref n="2" /> and A Babarit<fnref n="1" />

**Abstract.** The energy ship is a new concept for offshore wind energy capture. It consists of a wind-propelled ship that generates electricity using water turbines attached underneath its hull. Since it is not grid-connected, the generated energy is stored aboard the ship (for instance, using batteries or through conversion to hydrogen using an electrolyser). This concept has received little attention until today. In this paper, a proof-of-concept of an energy ship propelled by a Flettner rotor is presented. The experimental results are compared to numerical simulations. A very good agreement is obtained, which validates previous numerical simulations that indicate that a full-scale energy ship equipped with Flettner rotors can lead to positive net energy production.

## Introduction

The energy ship concept consists in a vessel propelled by the wind, under which one or more water turbines are attached. These water turbines generate electricity. The generated electricity produced is stored on board, either in batteries or in the form of a fuel (hydrogen, ammonia, methanol, etc. [1]).

A key advantage of this concept is that it enables the harvesting of wind energy in the high seas, which is challenging with conventional floating wind turbines because of grid-connection cost, installation cost and operation costs which increase significantly with increasing distance to shore.

<figure><img src="image_7.png" /><figcaption>**Figure 1.** Pictures of concepts of energy ships.</figcaption></figure>

<figure><img src="image_8.png" /></figure>

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ Farwind Energy, 1 rue de la Noë, 44300 Nantes, France</footnote>

<footnote n="2">² LHEEA Lab., Ecole Centrale de Nantes, 1 rue de la Noë, 44300 Nantes, France</footnote>
The concept of the energy ship turbine was patented in 1982 [2]. However, it did not receive much attention until the end of the first decade of the 2000s. Thus, there has been only a limited number of energy ship proposals to date [2][9]. They are shown in figure 1.

Regarding academic works, the energy ship has also been the focus of only few studies, most of which are limited to theoretical and numerical studies [3]-[12]. The first experimental study was carried out in 2019 on the Erdre river in France by Ecole Centrale de Nantes [13]. That study allowed on the one hand to experimentally investigate the effect of the water turbine drag on energy production and on the other hand to verify that significant quantities of energy can be produced with the energy ship. A proof of concept (POC) was specially developed for that purpose using a Hobie Cat Tiger catamaran as the base for the experimental platform (figure 2). This type of catamaran was selected for its low water resistance (which is essential for maximizing energy production [8][11]), its low cost on the second-hand market, and because it fairly corresponds to a 1/14 scale version of the energy ship described in [8].

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Picture of the 1st experimental proof-of-concept.</figcaption></figure>

A second version of the prototype has been developed and tested by Ecole Centrale de Nantes and Farwind Energy (figure 3). The objectives of these second experiments were twofold. First, it was to achieve an experimental validation of an energy ship propelled a Flettner rotor, and remotely controlled. Second, it was to generate experimental data suitable for validation of numerical models which can be used to estimate the performance of energy ships at full-scale.

The remainder of the paper is organized as follows. First, the experimental platform is described in section 2. Then, experimental campaign and its conclusions are respectively presented in sections 3 and 4.

# 1. Description of the experimental platform

## 1.1. General arrangement

The platform of the first prototype (a second-hand Hobie Cat Tiger catamaran, figure 2) was re-used for the second prototype. Its length is 5.51 m (18 feet). The general dimensions and characteristics of the platform can be found in [13]. Two main modifications were implemented. First, the rig was replaced by a Flettner rotor. Second, it was equipped with a control system which allows the rudder angle and rotational velocity of the Flettner rotor to be controlled remotely. Using a dedicated Graphic User Interface (GUI), the operator can monitor the sensors information (TWS, TWA, Boatspeed, Heading, Heel, ...) and adjust the Flettner rotor velocity and the rudder angle. The control system also includes
an autopilot. Two modes are available, one in which the control system maintains the ship heading (heading mode) and the other in which the control system maintains the true wind angle.

<figure><img src="image_10.png" /><figcaption>**Figure 3.** Picture of the second prototype</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 4.** Overview of the electrical and control architecture.</figcaption></figure>

## 1.2. Instrumentation and equipments

Figure 4 shows an overview of the instrumentation and equipments of the prototype. It was equipped with the following sensors and equipments (see also Figure 3):

* Actuators: a wind propulsion system consisting of a Flettner rotor built by CMP Composites and driven by a Dunkermotoren brushless motor. A Linak LA14 helm cylinder (with integrated linear feedback) which operates the port rudder. A Watt & Sea Cruise 600 water turbine
mounted on a force sensor system and its associated Watt & Sea 48V AC-DC conversion system. An actuator control system: Motion controller Esprit Austral Electronics which operates the rudder.

* A set of sensors: 2 LCJ CV7-C anemometers, 2 Ultrasonic Speedo NKE Locks, 1 GNSS Navilock NL-82004P MD6 (placed inside the on-board electronics box), 1 IMU (Inertia Measurement Unit): SBG IG-500A, the force sensor measuring the forces on the water turbine.

* The energy storage: A panel containing the electrical protection devices for all the components. A system for measuring the energy consumption of all the mains systems (Hall sensors + Arduino). 2 EasyLi Li-ion 20Ah 48 volts batteries to power the prototype. 2 Victron Energy DC-DC converters to supply the 24V and 12V stages. A Watt & Sea resistive dissipation bench to protect the system in the event of a power surge.

* The control system: The prototype control unit is a Quantum Lite Austral Electronics computer. This computer manages the data collected by the sensors and controls the boat actuators. The data is stored on-board in a database.

* The network: A wired local network connects most of the boat's sensors and actuators through serial to Ethernet converters as required. A NK2 (NMEA2000) bus is connecting the on-board computer to the actuator control device. The Flettner rotor and the water turbine force sensor are directly connected to the on-board control system. A wifi point to point local network is set between the prototype and the boat carrying the operator using two HP Ubiquity Bullet M2 and Wireless Instruments Dual Band Omnidirectional Antennas. The operator connects to the prototype with laptop which runs the GUI.

## 1.3. Actuators

1.3.1. *Flettner rotor.* The initial rig of the Hobie cat Tiger catamaran (a 17 m² mainsail and a 4.15 m² jib) was replaced by a Flettner rotor in the second prototype. The rotor height is approximately 2.8 m and the diameter is 0.45 m. Taking into account the high lift coefficient of Flettner rotors, it is approximately equivalent to half of the surface of the original sail.

Farwind Energy has selected the Fletter rotor technology for its energy ship concept because of its superior aerodynamic performance in comparison to other rigs on beam-reach, and because it is simple to operate (no halyards, no sheets)). Indeed, the only control parameter of the aerodynamic force is the rotational speed of the skin.

The Flettner rotor was installed on the port hull at the connection with the front transverse beam. It can be noted that this installation creates a static imbalance of the ship and shifts the center of thrust of the rig to the port side.

Figure 5 describes the Flettner rotor. The rotor is composed of 3 main parts: an envelope or “skin” (in black) which generates lift (and drag) when it rotates in an incident wind. This envelope is made of sandwich carbon composite. The skin is divided into two tubes to facilitate the assembly. The lower part incorporates a supporting structure made of aluminium. This structure includes a mast which supports an electrical motor (geared motor with brake and control electronics) connected to a driving disk which rotates the skin on the one hand, and on the other hand parts providing the pivot connection between the skin and the mast. The Flettner rotor is attached to the hull by means of a carbon fiber interface piece (Figure 6).

A BLDC (Brushless DC) 48V Dunkermotoren motor was chosen to drive the Flettner rotor. This motor gives access to multiple control parameters (slopes, maximum speeds, ...) through its control interface as well as information on motor's behaviour (currents, temperature, Torque, ...).
<figure><img src="image_12.png" /><figcaption>**Figure 5.** Flettner rotor description.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 6.** Carbon fiber interface between the Flettner rotor and the boat's hull</figcaption></figure>

### 1.3.2. Water turbine.
The water turbine system is the same as the one used for the first prototype (a Watt & Sea Cruise 600 water turbine [14]). It was installed at the middle of the front transverse beam. Figure 7 describes the water turbine (left) and the force sensor (center and right) which was used to measure the generated drag.

The water turbine is composed of 3 parts: the propeller which converts the kinetic energy from the incoming flow into a mechanical torque applied on the generator shaft, the generator which converts the mechanical torque into electrical energy. This generator is embedded into the nacelle which is connected to a streamlined rotating mast. This mast is connected to the force sensor.

<figure><img src="image_14.png" /><figcaption>**Figure 7.** Water turbine (left), and its force sensor system (center, right).</figcaption></figure>

The propeller diameter is 240 mm. The propeller's shape is not adapted to the intended operating regimes of the final boat, so the global performance of the boat is not optimal. However, it is recalled
that the main objective of the present study was to generate experimental data suitable for validation of numerical models.

The force sensor system is composed of 3 HBM Z6 C3 load cells allowing the measurement of the drag force induced by the water turbine as well as the Y and Z associated moments. The drag force and the My moment give access to the drag hydrodynamic coefficient while the observation of Mz tells how well the water turbine is aligned with the flow. The Watt & Sea converter which ensures the conversion of the three-phase AC current into a 2 phase 48V DC current also collects data to characterize the generator's behaviour. That data gives access to the mechanical torque available on the propeller shaft.

We refer the reader to [14] for the description of the water turbine calibration which has been previously conducted in the Ecole Centrale Nantes (France) towing tank.

### 1.3.3. Rudder
The rudder of the boat is controlled by means of an electric cylinder Linak LA14. The cylinder stroke is regulated from the onboard computer through its analog voltage linear feedback. A calibration was carried out during the prototype preparation phase in order to obtain the relation between the rudder angle and this voltage. End stops are also included as well as a zero bar adjustment. These setting items are accessible from the GUI's settings tabs. This allows to fine-tune the boat before trials.

## 1.4. Control system

The proof of concept presented in this paper aimed, among other things, to demonstrate remote operation of an energy ship. An Austral Electronics Quantum computer has therefore been installed on board in order to control the boat's actuators based on the information received from the sensors (see section 0). The rudder is controlled using an autopilot which has three modes: one in which the control system maintains the ship's heading, the other in which the control system maintains the true wind angle, and the last one to manually control the rudder angle. The water turbine embeds two modes: a manual mode in which a given generator output voltage value can be defined through the GUI, or a MPPT (Maximal Power Point Tracking) mode which seek to maximize energy production. The Flettner rotor's rotational speed is controlled through the analog voltage inputs of the BLDC motor's logic bus. A calibration was carried out during the prototype preparation phase in order to obtain the relation between this voltage and the rotational speed of the Flettner rotor, allowing the rotational speed to be directly set in the GUI.

Figure 8 shows the Farwinder's operational GUI tabs. On the left side, the Wheelhouse tab allows a pilot to operate the boat. This page gives access to the settings of the actuators and allows the monitoring of the wind (for instance the true wind angle (TWA) and speed (TWS)), and of the boat's behaviour (for instance the Boat speed (BS) and the heading (HDG)). On the right hand side, the Engine room tab displays information on the electrical consumption of all the actuators and the forces acting on the water turbine. This page also informs the user about the state of charge of the batteries.

<figure><img src="image_15.png" /><figcaption>**Figure 8.** Pictures of the GUI. Wheelhouse (left), Engine room (right).</figcaption></figure>
## 2. Experiments

### 2.1. Methodology

Experiments were carried out on June 24th and 25th, 2021 on the Vioreau lake, France. Wind conditions were light (3 to 11 knots). The experimental method consisted in a series of roundtrips on beam reach (see Figure 9). This wind direction was chosen because it corresponds to the point of sail for which the performance of energy ship is maximum. On each round trip, the control settings (Flettner rotor's rotational velocity and water turbine output voltage) were changed to study their effect on energy production. 87 datapoints have been collected and analysed. Some of these results are presented below.

<figure><img src="image_16.png" /><figcaption>**Figure 9.** Example of GPS track during the experimental campaign.</figcaption></figure>

### 2.2. Data processing

Experimental data was first processed in order to remove cases for which the true wind speed was too low or for which there was sudden changes in the true wind direction. This was necessary to get data corresponding to steady conditions. Only the beam reach and constant SR time ranges have been retained. The signals were then averaged over each of these time intervals. Standard deviations were also calculated in order to keep trace of the data quality. Figure 10 shows an example of time recording of data collected during the experiments. The oscillations are due to inherent variations in environmental conditions over the test area, combined with the prototype's boatspeed and heading variation. Table 1 shows the curated data. The power area is defined as the ratio of the mechanical power on the shaft of the propeller to the available wind power density.
<figure><img src="image_17.png" /><figcaption>**Figure 10.** Example of raw data measured during the experimental campaign. Left: Angle values. Bottom right: Speed values</figcaption></figure>

<table><caption>**Table 1.** Curated data</caption><thead><tr><th>Case number</th><th>Mean TWA (°)</th><th>Std dev (°)</th><th>Mean TWS (m/s)</th><th>Std dev (m/s)</th><th>Ω Flettner rotor (rpm)</th><th>SR</th><th>Water Turbine Target Tension (v)</th><th>Water Turbine Tension (v)</th><th>BS (m/s)</th><th>Std dev (m/s)</th><th>Mean TWS/BS</th><th>Power area (m²)</th><th>Std dev (m²)</th><th>CP.meca</th><th>Std dev</th><th>Estim. Leeway (°)</th><th>Rudder Angle (°)</th></tr></thead><tbody><tr><td>11</td><td>-93.8</td><td>3.27</td><td>4.71</td><td>0.55</td><td>700</td><td>3.30</td><td>40</td><td>42.2</td><td>1.97</td><td>0.04</td><td>0.42</td><td>0.91737</td><td>0.09797</td><td>0.917</td><td>0.098</td><td>-2.60</td><td>-4.97</td></tr><tr><td>13</td><td>98.3</td><td>4.77</td><td>4.85</td><td>0.85</td><td>-700</td><td>3.33</td><td>40</td><td>41.3</td><td>1.85</td><td>0.12</td><td>0.38</td><td>0.72744</td><td>0.09162</td><td>0.727</td><td>0.092</td><td>2.00</td><td>1.52</td></tr><tr><td>16</td><td>83.3</td><td>2.60</td><td>4.49</td><td>0.60</td><td>-686</td><td>3.31</td><td>30</td><td>30.9</td><td>1.67</td><td>0.04</td><td>0.37</td><td>0.79461</td><td>0.09583</td><td>0.795</td><td>0.096</td><td>1.00</td><td>5.28</td></tr><tr><td>17</td><td>-100.0</td><td>2.74</td><td>4.11</td><td>0.51</td><td>700</td><td>4.03</td><td>30</td><td>32.5</td><td>1.79</td><td>0.07</td><td>0.44</td><td>1.38537</td><td>0.14452</td><td>1.385</td><td>0.145</td><td>-3.38</td><td>-5.91</td></tr><tr><td>20</td><td>-96.0</td><td>4.25</td><td>3.79</td><td>0.52</td><td>489</td><td>2.84</td><td>25</td><td>26.6</td><td>1.52</td><td>0.05</td><td>0.40</td><td>1.08719</td><td>0.17106</td><td>1.087</td><td>0.171</td><td>-3.77</td><td>-5.10</td></tr><tr><td>22</td><td>90.0</td><td>3.22</td><td>4.09</td><td>0.58</td><td>-698</td><td>3.67</td><td>20</td><td>22.1</td><td>1.62</td><td>0.07</td><td>0.40</td><td>1.11033</td><td>0.17106</td><td>1.110</td><td>0.171</td><td>1.91</td><td>4.85</td></tr><tr><td>23</td><td>-91.8</td><td>2.95</td><td>4.36</td><td>0.79</td><td>700</td><td>3.58</td><td>20</td><td>22.4</td><td>1.75</td><td>0.05</td><td>0.40</td><td>1.21469</td><td>0.14355</td><td>1.215</td><td>0.144</td><td>-3.76</td><td>-5.94</td></tr><tr><td>27</td><td>90.7</td><td>3.21</td><td>5.04</td><td>0.49</td><td>-692</td><td>3.14</td><td>10</td><td>12.2</td><td>1.72</td><td>0.14</td><td>0.34</td><td>0.75842</td><td>0.13315</td><td>0.758</td><td>0.133</td><td>3.74</td><td>4.12</td></tr></tbody></table>

### 3. Comparisons between experimental and numerical results

Farwind Energy has developed a numerical model which allows the velocity and power production of an energy ship to be estimated. It is based on that presented in [8]. It has been refined in order to include the drift velocity, the trim and heel angles, and the rudder angle. The modelling approach follows that of [15].

The model requires as inputs the characteristics of the rig (dimensions, aerodynamic coefficients), of the hull (dimensions, water resistance coefficients, windage coefficients), of the water turbines, and of the appendages.

The rotor lift and drag coefficients are based on the experiments of [16] (see Figure 11). The drag coefficient was increased by 2.5 in order to take into account windage. The hull water resistance coefficients were obtained using the REVA software [17]. In the experiments, only one daggerboard (0.42 m²) and one rudder (0.27 m²) were used. For the water turbine coefficients, the results obtained in the towing tank experiments were used [13]. The appendages were modelled as lifting surfaces with NACA0009 profile and aspect ratio of 6.
<figure><img src="image_18.png" /><figcaption>**Figure 11.** Experimental data of Charrier [16] for the lift and drag coefficients of a Flettner rotor as function of the spin ratio. The aspect ratio is 5. The rotor is equipped with two end plates of diameter 2D. The Reynolds number is 13 200.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 12.** Comparison of experimental data and of numerical results.</figcaption></figure>

Figure 12 shows the comparison of the experimental data and of results of the numerical model. One can see that there is a very good agreement between the numerical model and the experiments for the power area, the boat speed and the rudder angle. For the leeway angle, the agreement is very good for the negative angles. For the positive angles, two data points show good agreement and two data points show significant discrepancies. During this campaign, the prototype did not embed sensors to measure the leeway angle of the boat. Consequently, the experimental leeway results are based on the heading and COG (Course Over Ground) measurements assuming zero current. Therefore, the discrepancies may correspond to measurement problems. This is a point of improvement that will be considered in
future experiments. Nevertheless, those results indicate that the numerical model is able to predict the ship's performance and behaviour.

One can note that the Flettner rotor is an active wind propulsion system which requires a motor to spin. Therefore, one can wonder whether the power generated by the water turbine is greater than that consumed by the Flettner rotor. This was not the case at prototype scale because (i) the motor which was selected to spin the rotor was significantly oversized (which led to its electrical consumption being an order of magnitude greater than the mechanical power required to spin the rotor) and (ii) because the power consumption of the sensors and control systems is very high at small scale in comparison to the generated power. However, in [18], it was shown that a full-scale energy ship propelled by four 35 m tall Flettner rotors can produce a positive net power of approximately 1300 kW when sailing beam wind in a true wind speed of 10 m/s. As that prediction was obtained using the numerical model which we showed in this paper that it is able to predict the ship's performance and behaviour, it can be concluded that positive net power production can be achieved at full-scale with energy ships powered by Flettner rotors.

## 4. Conclusions

This paper presents experiments conducted with an energy ship prototype propelled by a Flettner rotor and remotely controlled. Comparisons of the experimental results with numerical simulations were carried out. A good agreement was found, which validates the numerical model. This validates numerical results which indicates that positive net power can be achieved using this concept.

## 5. Acknowledgement

This work was partially supported by the Pays de la Loire Region, France; and Nantes Métropole, Nantes, France [WEAMEC projects AUTOFLEET_Y1 and AUTOFLEET_PLUS].

## 6. References

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