Experimental Study on the Performance of ModuleRaft
Wave Energy Converter
Watchara Tongphong *, Ali Alkhabbaz *, Ho-Seong Yang* and Young-Ho Lee ** Department of Mechanical Engineering, Korea Maritime and Ocean University, Busan, Korea
E-mail: lyh@kmou.ac.kr

Abstract
This article proposes a novel wave energy converter (WEC), referred to as the ModuleRaft WEC. The WEC consistsof a floating modular flap and four rafts hinged at the main floating structure. ModuleRaft WEC is unique due to its
ability to convert both wave potential energy and wave kinetic energy by utilizing the pitch motion of rafts and floating
modular flap. The performance of the ModuleRaft WEC model was evaluated by conducting the wave flume tests. The
experimental tests were performed for both unidirectional regular and irregular waves. The experimental results showthat the wave height, wave period, significant wave height, average wave period and power take-off (PTO) damping havea significant influence on the capture factor of the ModuleRaft wave energy converter. The capture factor of theModuleRaft WEC which is a combination of modular flap and rafts is higher than only a modular flap without rafts by
27.7 % in regular wave condition and 6.7 % in irregular wave condition. It indicates that rafts play a positive role byfloating and increasing the overall capture factor of the conventional floating modular flap-type wave energy converter.
Keywords: Wave energy converter, Raft, Floating modular flap, Wave flume, Experimental test

1. Introduction
Renewable energy technologies are being developed to meet rising electrical energy demand [1]. Ocean waves areboth clean and renewable sources of energy with tremendous global potential for generating electricity. Over the years,different types of wave energy converters have been invented, developed and tested [2]. Among all kinds of devices
proposed, raft-type wave energy converters (WECs) consisting of two or more floaters hinged at each floater end use
wave curvatures (potential energy) as a source of reaction and power absorption. It has been proven to have the highest
wave energy conversion efficiency and good survivability [3]. Andrew [4] presented the McCabe Wave Pump (MWP).
The device is constructed with three narrow rectangular steel pontoons (one forward and one aft pontoon hinged to asmaller center pontoon with a damping plate) at an approximate length. The forward and aft pontoons move relative tothe center pontoon by pitching around the hinges. Linear hydraulic rams extract the energy from the rotation of the hinges.
This energy can be used to generate electricity using a hydraulic motor attached to a generator.
Whereas, pendulum-type wave energy converters, consist of a pendulum which converts wave kinetic energy touseful energy. Some of its advantages include wide frequency response and high energy conversion efficiency undernormal sea conditions. T. Mäki presented “WaveRoller”. The WaveRoller WEC is a bottom-mounted, near-shore device
which operates at depths of between 8 and 20 meters, approximately 0.3-2 km from the shore. It comprises a single waveactivated body, a flap, attached via a pivot to a basement structure and a foundation template on the seabed. The WEC
converts incoming wave power to electrical power by utilizing the pitch motion of the flap.
In this study, we proposed the ModuleRaft wave energy converter, a new concept of wave energy converter inspiredby using the benefits of raft type and pendulum type wave energy converters. The wave energy converter is unique due
to its ability to convert both wave potential energy and wave kinetic energy by utilizing the pitch motion of the rafts andfloating modular flap. The objective of this study is to: 1) Investigate the motion characteristic, performance and optimal
condition of the ModuleRaft wave energy converter model based on wave flume testing and 2) Investigate the parameterswhich affect capture factor of the ModuleRaft wave energy converter.

2. Experimental Test
The experiments were performed in the Flow Informatics Laboratory, Korea Maritime and Ocean University,
Republic of Korea as shown in Fig. 1. The wave flume is 7.3 m long (wave direction), 1.0 m wide and has a mean waterlevel of 1 m. The ModuleRaft wave energy converter model was positioned on the central line of the wave flume at adistance of 1.7 m from the wave generator. To calibrate the wave generator and monitor the incident and transmittedwaves, two wave gauges (Ultralab ULS sensor, model USS20130) with a resolution of 0.18 mm and a measurement
range of 200 to 1300 mm were installed upstream 0.35 m and downstream 0.55 m away from the model.
0.35 m0.55 m

Wave Gauge No. 1

Wave Gauge No. 2

Wave Generator
Wave Absorber

1.8 m

1m

ModuleRaft WEC

1.7 mFig. 1 Schematic sketch of the experimental setup (scaled).
The ModuleRaft wave energy converter model is shown in Fig. 2. The ModuleRaft wave energy converter consistsof a floating platform, a modular flap, four hinged rafts and power take-off (PTO) system. The modular flap consists ofthree modules which are installed at the center of the platform. The raft wave energy converter consists of four rafts,
which are connected to the floating platform by hinged joints. The modular flap and rafts each have a PTO system
consisting of a compression and tension load cell, electromagnetic brake and angle sensor as shown in Fig. 3. Twoquantities were measured i.e. torque and angular velocity on the rotating shaft of the modular flap and raft no.1-4. The
electromagnetic brake (model FB19) is used to provide appropriate resisting torque (load) which is representative of atypical external PTO mechanism. Compression and tension load cell (model UMMC-5kgf) is used to measurecompression and tension forces. The force multiplies the length of the lever arm (r) then the torque on the shaft iscalculated. The angle sensor (model STS60V3-1N) is used to measure the pitch angle of the modular flap, raft no. 1-4and platform. The derivative of the pitch angle with time, then the pitch angular velocity is calculated. The digital signals
from all the sensors are stored at the same time using a data logger (model PT-1624). The experimental rig is shown inFig. 4.
Compression & Tension Load Cell

Angle Sensor

Raft No.2 & PTO System
Lever Arm

Modular Flap - PTO System(See Fig. 2 (b))

Mounting Studr
Electromagnetic Brake

Raft No.1 & PTO System
Raft No. 3

Raft No.4 & PTO System
Modular Flap

Shaft

Platform
Raft No.3 & PTO System(See Fig. 2 (c))

Fig. 2 The ModuleRaft WEC modelFig. 3 The PTO system

Wave Sensor No.1

Wave Sensor No.2

Wave Absorber

Wave Energy Converter

Data Logger(b)(a)Fig. 4 Experimental rig: (a) The 1:32-scale ModuleRaft WEC model in the wave flume, (b) Data recording tools

3. Test Conditions
In this study, experimental tests were performed in regular and irregular waves to understand the hydrodynamic
behavior of the ModuleRaft WEC. The first set of experimental tests were carried out for regular waves with water depth= 1 m. The main objective is to the effect of wave height, wave period and PTO damping on capture factor of theModuleRaft WEC. The second set of experimental tests were carried out for irregular waves with water depth = 1 m. The
regular and irregular wave conditions (REG) are summarized in Table 1.
Table 1 Regular (REG) and irregular (IRR) tested wave state
REG

T (s)

H (m)

REG

T (s)

H (m)

REG

T (s)

H (m)

1

0.8

0.015

7

0.8

0.030

13

0.8

0.045

2

0.9

0.015

8

0.9

0.030

14

0.9

0.045

3

1.0

0.015

9

1.0

0.030

15

1.0

0.045

4

1.1

0.015

10

1.1

0.030

16

1.1

0.045

5

1.2

0.015

11

1.2

0.030

17

1.2

0.045

6

1.3

0.015

12

1.3

0.030

181.30.045

IRRTe (s)Hs (m)

IRRTe (s)Hs (m)IRR

Te (s)Hs (m)

10.8

0.015

60.80.030

11

0.8

0.045

20.90.015

7

0.9

0.030

12

0.9

0.045

3

10.0158

10.03013

10.045

4

1.1

0.015

9

1.1

0.030

14

1.1

0.045

5

1.2

0.015

10

1.2

0.030

15

1.20.045

4. Results and Discussion
Fig. 5a shows the variation in capture factor of the ModuleRaft wave energy converter with wave period and wave
height under regular wave conditions. It can be seen that the capture factor for the raft wave energy converter is larger
than the modular flap wave energy converter for all wave period and height. Furthermore, the capture factors of theModuleRaft wave energy converter which is a combination of modular flap and rafts are larger than that of modular flap
wave energy converter without raft. The maximum capture factor of the ModuleRaft wave energy converter = 29.2 % atwave period = 1.2 s and wave height = 0.015 m. It implies that rafts play a positive role as a multifunctional device byfloating and increasing the overall capture factor of the conventional floating modular flap-type wave energy converter.

Fig. 5b shows variations in capture factor with a range of average wave energy periods = 0.8 to 1.2 s and significant
wave heights = 0.015, 0.03 and 0.045 m of modular flap, rafts and the combination of modular flap and rafts WEC underirregular wave conditions. It can be seen that the capture factor for the raft WEC and ModuleRaft WEC is higher thanthe modular flap WEC for all average wave energy periods and significant wave heights. In case of modular flap WEC,
the maximum capture factor = 2.3 % obtained at average wave energy period = 0.8 s and significant wave height = 0.045
m. For raft WEC and the combination of modular flap and rafts WEC, the maximum capture factor = 19.2 % and 9 %respectively, obtained at average wave energy period = 0.8 s and significant wave height = 0.045 m. Irregular waves
consist of a mixture of waves with different length, period and height. Therefore, it is quite difficult to operate the WEC
and meet the resonance condition under irregular wave condition. Thus, the capture factor of the modular flap, rafts andthe ModuleRaft WEC tends to be lower than the WEC which is tested under regular wave conditions.

(b)

(a)

Fig. 5 (a) Variations in capture factor of modular flap, rafts and the combination of modular flap and rafts with wave
period and wave height under regular wave conditions. (b) Variations in capture factor of modular flap, rafts and thecombination of modular flap and rafts with average wave energy period and significant wave height under irregular wave
conditions

5. Conclusions
In this paper, a hybrid concept of raft and modular flap-type WEC, named ModuleRaft WEC is presented. The resultsfrom regular and irregular waves test indicated that the wave height, wave period, significant wave height, average wave
energy period and PTO damping have a significant influence on capture factor of the ModuleRaft WEC. In case of regularwave conditions, the capture factor of the ModuleRaft WEC was larger by 27.7 % when a combination of modular flap
and rafts was used rather than only a modular flap without rafts at wave period = 1.2 s and wave height = 0.015 m. Incase of irregular wave conditions, the capture factor of the ModuleRaft WEC was larger by 6.7 % than only a modular
flap without rafts at average wave energy period = 0.8 s and significant wave height = 0.045 m. It indicates that rafts playa positive role by floating and increasing the overall capture factor of the conventional floating modular flap-type wave
energy converter.

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