Development and Experimental Investigation of a Marine Vessel Utilizing the Energy Ship Concept for Far Offshore Wind Energy Conversion

The energy ship is a concept for offshore wind energy capture which has received very little attention until today. To this date, there had not been yet an experimental proof of concept. In order to tackle this issue, an experimental platform and data acquisition system has been developed. A 5.5m long sailing catamaran served as a platform equipped with a 240mm diameter water turbine. The energy ship platform has been tested several times in the actual river to investigate the workability of the platform and data acquisition system. Results show that energy ship platform can produced 500W electric power for a true wind speed of 10 knots.


INTRODUCTION
Most of the wind energy resource is far out at sea. Unfortunately, it cannot be exploited with current wind energy technologies (grid-connected bottom-fixed or floating wind turbines) due to grid-connection cost, moorings and installation cost, and maintenance cost would be prohibitive. Therefore, there is a need to develop new wind energy technologies in order to enable harvesting the far-offshore wind energy resource.
A solution for this problem is using the energy ship concept which was been proposed by (R. Abd Jamil et. Al.,2019) with greater capacity factor acceding 80% due to optimizing weather forecast in order to maximizing the capacity factor. An energy ship is a ship driven by the wind using sails and equipped with a water turbine in order to convert the kinetic energy of the ship into electricity. Energy ships include an onboard energy storage system to store the produced energy. This concept is not new since it was first patented by (Salomon,1982). Note that he suggested hydrogen for energy storage. Since then, a few researchers have proposed designs of energy ships (Meller, 2007;Gizara,2007;Kim & Park, 2010;Pelz et. al.,2016;Gilloteaux & Babarit, 2017;Ouchi & Henzi, 2017;Babarit et. al.,2019;Babarit et. al.,2020) as in Figure 1.   The hydro-generators are widely installed in sailing ship as the renewable energy mix provider to accommodate extra power demand for navigational lights, radio, and GPS. For fast sailing ship, the hydro-generator can produced high and stable electricity production as long as the ship is sailing.
The major difference between a sailing ship and an energy ship is the presence of a large water turbine underneath the ship. The main effect of the turbine is to generate an additional drag force. Energy production by the ship depends on the product of this force and the induced velocity in the disk swept by the rotor of the turbine. It has been shown in (Gilloteaux & Babarit, 2017;Babarit et. al., 2019) that optimizing the drag force and the induced velocity is key to maximize energy production. However, this hasn't been confirmed experimentally yet. Addressing this gap requires the development of an experimental platform, which is one of the aims of the present study. The second aim is to achieve an experimental proof-of-concept for the conversion of wind energy into electricity using the energy ship concept.

Hull and Sails
In this research, a used Hobie Cat Tiger sport catamaran as in Figure 2 is used as the basis for the test platform. Its length is 5.51 m. It is fitted with a Bermuda rig. The mainsail area is 17 m², the jib sail area is 3.45 m² and the spinnaker sail area is 19 m². This ship is normally operated by a crew of 2 people. Other main characteristics are given in Table 1

Selection of the water turbine
In order to select the water turbine, an estimation of the power that can be absorbed by the platform for typical wind conditions if true wind speed W is 10 knots (approximately 5 m/s), true wind angle β=90° and ship velocity U=10 knots. The apparent wind speed and the apparent wind angle derive from the true wind speed W and the true wind angle β in Figure 3 as follows:

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(1) V 2 = U 2 + W 2 +2UW cos β W sin β = V sin α In the aforementioned conditions, the apparent wind velocity V is 14 knots and the apparent wind angle α is 45°. The thrust force delivered by the sails can be estimated using equation derived by (Babarit et. al.,2019): where ρ a is the air density, A s is the total sail area and C L and C D are the lift and drag coefficients of the rig. For a Bermuda rig, C L is in the order of 1.5 and C D is in the order of 0.5 (Charrier, 1979). Thus, the thrust force is in the order of 470 N. At equilibrium and without the water turbine, the thrust force is equal to the water resistance of the hull of the ship R w . In a first approach, it is assumed that this water resistance is simply proportional to the square of the ship velocity: As the thrust force is estimated to be 470 N, r w is in the order of 18 N.s 2 /m 2 .
When the water turbine is added, it creates an additional source of drag R T . Therefore, the ship velocity will be smaller with the water turbine than without the water turbine. According to the momentum theory, the drag force can be written: Where a Є [0, ] is the axial induction factor, ρ w is the water density and A T is the turbine disk area. The produced power can be written: At equilibrium, the thrust force is equal to the water resistance plus the drag force from the water turbine: Using Equations 1, 2 and 6 it leads to an equation relating the ship velocity to the water turbine drag: Using equations 7 and 5 one can calculate the ship velocity and produced power as function of the axial induction factor. Results of such calculation are shown in Figure 4 for a true wind speed of 10 knots and a true wind angle of 90°. Note that in this calculation, it was assumed that the diameter of the water turbine is 240 mm. One can see that, as expected, the ship velocity reduces with increasing induction factor (because of increasing turbine drag force).
For the produced power, one can see that a maximum of approximately 500 W is obtained for an optimal induction factor of approximately 0.25. The corresponding turbine drag is approximately 200 N and the corresponding ship velocity is approximately 8 knots. Therefore, the cruising 600 hydrogenerator by Watt & Sea was selected as it matches well the requirements for the test platform. Indeed, this hydrogenerator has a nominal power of 600 W as shown in Figure 5. It can be fitted with a turbine of diameter 240 mm. For that turbine, it can deliver approximately 300 W electric power for 4 m/s ship velocity.

Ship Velocity, Heading and Motions
For measuring the ship velocity, ultrasonic speedo sensors from NKE Marine Electronics were selected. Model 90-60-479 was selected to provide speed measurement range in between 0.3 to 50 knots and provide resolution of two decimal points. Speed sensors were installed vertically under the port and starboard hull which were closed to the axis of the ship and installed in front of dagger board to achieve optimal performance. The installation can be visualized as described in Figure 6. The redundancy is needed for the case of failure or one of the hulls is flying on the air.
A BU-353S4-USB GPS was selected for measuring the geographic position of the ship with the horizontal position accuracy below than 2.5 meter. This sensor also allows the ship velocity over ground (speed over ground -SOG) and heading (course over ground -COG) to be measured through differentiation of the catamaran's positions. Sampling rate for the GPS is 1 Hz. Note that there can be differences between the velocity measurement from the speed sensors and the SOG ground depending on current. Motions of the ship were captured by BNO055IMU sensor at 100 Hz sampling rate. This sensor measured the heading, pitch and roll of the ship. IMU is calibrated for the best performance before conducting any test.

Wind Velocity and Direction Measurement
For measuring wind speed and direction, an anemometer model CV7-LCJ Capteurs was mounted at the top of the mast with additional position sensor from Honeywell. This position sensor is placed at the bottom of the mast for wind direction correction. The current designed allows the mast to be rotated at 90 degrees to the port and starboard respectively. Due to the angle limitation of current position sensor, an additional method is used to limit the rotation of the mast approximately 15 degrees to the port and starboard respectively by tying the mast to the sail rig adjuster on the port and starboard side of the ship.

Drag Force Sensor
One of the objective in the experiment is to investigate the effect of the drag force in order to maximise the energy production. So it requires that force to be measured. For that purpose, a dedicated balance has been designed. It is based on a set of three strain gauges which enables the drag force in the x direction as well as the moments in the y and z directions to be measured as shown in Figure 7. Load cell from HBM Z6 was selected for this task with triangular shape because of robustness and withstands extreme mechanical stress and aggressive media. For this purposes, 2 unit of load cell with maximum load 100 kg and 1 unit of load cell with maximum load 50N were selected. The balance is attached to the Hobie Cat Tiger at the center back of the platform. The load cells presented a linearity and hysteresis ideal for the test with error below 0.1% of the vertical and 0.4% horizontal for a full scale (200 N).

Hydrogenerator and Power Management System
For reasons explained above, a Watt & Sea Cruise 600 hydrogenerator was selected for the test platform. This type of hydrogenerator is designed to produce high performance output during cruising with low drag profile. Cruising hydrogenerator is light, robust, compact and easy to install. Delivered with 240 mm propeller, power converter regulator and lifting fastening bracket. There are three different sizes of propeller available which are 200mm, 240mm and 280mm. Increase the propeller size can generate more power at lower speed. As an example, a 240mm diameter propeller provides maximum using speed at 14 knots meanwhile propeller with larger diameter 280mm provides maximum using speed at 11 knots. Lower pitch will provide better acceleration and higher pitch will provide higher top speed. The hydrogenerator also comes with variable rotational speed control function (pitch control). For getting maximum performance the rotor speed must vary with wind speed.
The hydrogenerator is operated with a power management system consist of a power converter, a 24V Lithium battery, a dump load and an Arduino NANO microcontroller as shown in Figure 8. The 24V15Ah battery is selected due several reasons such as light weight (3kg), longest life, produced constant power, fast charging and safe. The dimension of the battery is 330mm in length, 96mm in width and 104mm in height. The alternating current electricity produced by the turbine is converted into direct current by the converter and stored into the 24 V Lithium battery. The converter also enables to control the water turbine (rotational velocity, drag force, power absorption) The system includes sensors for measuring the produced energy (current, voltage, power) and the turbine rotational velocity. When the battery is fully charged, the excess energy can be dissipated by the dump load.
Note that the battery included in the hydrogenerator system is also used for powering the anemometer, the data acquisition system and the wifi antenna using power over ethernet (PoE). The voltage is stepped down to 5V by using buck regulators.
The hydroelectric generator component specification and function are summarized in Table 2. Power management system is located at the starboard side of the hull. Some alteration and modification need to be made at the starboard hull of the catamaran to allow compartment with waterproof hatch to be installed. Waterproof hatch on the starboard hull of the catamaran is designed to locate a few items such as Lithium battery and also dump load resistor.   Note that the battery included in the hydrogenerator system is also used for powering the anemometer, the data acquisition system and the wifi antenna using power over ethernet (PoE). The voltage is stepped down to 5V by using buck regulators.
The hydroelectric generator component specification and function are summarized in Table 2 As an interface to change the drag of the hydroelectric generator Battery Store the renewable energy and also act as a buffer Dump load resistor If the battery is in fully charged condition then the heat sink will be operated to discharge the energy is located at the starboard side of the hull. Some alteration and modification need to be made at the starboard hull of the catamaran to allow compartment with waterproof hatch to be installed. Waterproof hatch on the starboard hull of the catamaran is designed to locate a few items such as Lithium battery and also dump load resistor. Table 3.
Data variable recorded.

Data acquisition system and control
The data acquisition system is based on a Raspberry Pi 3. It is installed onboard in a Pelicase as in Figure 9. It is connected to the various system, sensors and transducers as illustrated in Figure 10. The sensors were selected to be wired connection. Waterproof pin connectors with IP68 standard were used for the connection. Several communication protocols such as NMEA 183, I²C, ethernet and USB have been used in developing data acquisition system. As an example, GPS, anemometer and hydroelectric generator were connected using USB protocol meanwhile sensor such as IMU was connected using I²C protocol. The data acquisition system has been designed to record the measured data and store it in a CSV file. The overall recorded data is simplified as listed in Table 3. Data acquisition is start and stop by pressing the corresponding buttons located on the top of the pelicase box.
Moreover, the data acquisition system has been designed to enable the live data streaming through wifi. Thus, the data can be visualized live on a remote computer by an operator located on-land or on another boat. This feature has been developed to Hydrogenerator drag Strain gauges reduce the workload of the catamaran crew, as it is expected that they will be fully busy with the navigation and operation of the boat.

Figure 10.
Illustration of sensors location.

RESULT AND DISCUSSION
Four test was conducted to check the workability of the energy ship system and the platform as well as the data acquisition system and labelled as Run02, Run03 and Run04. A few results of trajactory of energy ship and speed can be visualised in Figure 11 and Figure 12. The trials were carried out in light wind conditions (1 to 7 m / s), without current. The control setting of the hydro generator was modified in order to study its effect on production energy. The hydro generator was controlled remotely, allowing the crew to focus on navigating the sailing ship.  Sample of parameters recorded during the test by the data acquisition system can be visualized in Figure 13. Figure 13 (a) shows the ship trajectory with different hydrogenator control setting meanwhile Figure 13 (b) represents the ship speed during the testing which depended on the speed of the wind. Higher wind speed will produce higher ship speed. More number of satellites will produce more accurate data and this can be viewed in Figure 13 (c). Overall, seven satellite signals were captured during the testing. Figure 13 (d) shows the direction of the sailing ship which were in line the position of the ship that was mentioned earlier in 13 (a). Figure 13 (e) and (f ) show the reading of hydrogenator power and battery voltage of the energy ship respectively that correlated with the wind speed. Other than that, Figure 13 (g) represents the corrected wind direction which was adjusted by mast angle sensor throughout the testing. The wind speed influences the speed of the ship and the electrical power production of the energy ship as shown in Figure 13

g) Wind direction
h) Wind speed Figure 14 represents the average speed of the energy ship in function of the average true wind speed and the voltage of the hydro generator. The average wind speed over the data ranges selected ranged between 2.5 and 5.0 m / s. Note that a good approximation of the ship's speed is half of the real wind when the hydro generator is in operation. The formula given in Figure  14 can be used to estimate the speed of a ship and subsequently estimate the electrical power produced by an energy ship as shown in Figure 5.

CONCLUSION
This trial campaign successfully demonstrates the principle of energy ship or wind powered vessel. The amount of power that can be produced by this prototype is about 600W for the true wind speed, 11 m/s. Results obtained prove that an energy ship concept is able to produce significant amount of energy. For future studies, the usage of control pitch propeller and bigger diameter of propeller are proposed to be investigated.