Wave Energy Conversion with Floating Objects for the Coast of East

Java

Heri Saptono Warpindyasmoro and Hanny Hosiana Tumbelaka

Electrical Engineering Department, Petra Christian University, Jl. Siwalankerto 121 - 131 Surabaya, Indonesia

Keywords: Wave Energy Conversion, Cylindrical Absorber, Cone Absorber.

Abstract: The coast of East Java has ocean waves with varying significant wave heights and wave periods. To convert

wave energy into electrical energy, equipment is needed, which is a floating object. This floating object

serves to convert wave energy into mechanical energy which is then converted into electrical energy.

Energy conversion will be maximum if the ocean wave frequency same with natural frequency of floating

objects. The natural frequency of floating objects is determined by the shape of the floating object. This

study compares two floating objects, namely cylinder and cone shaped. From the results of simulations, the

cylinder shape is more suitable to be applied on the south coast of East Java, while the cone shape is more

suitable to be applied on the north coast of East Java.

1 INTRODUCTION

Energy produced by ocean waves is a very potential

energy in the world and the most efficient when

converted to electrical energy (Drew, et.al, 2009).

But the implementation as a real electricity generator

is still very minimum. Most are still on a laboratory

scale. The energy produced by ocean waves is

depended on the parameters of ocean waves, namely

the significant wave height and wave period. At each

location has a varying significant wave height and

wave periods (Faizal, et.al, 2014). Therefore we

need a wave energy conversion mechanism (Wave

Energy Converter, WEC) that is suitable for wave

conditions in each of these locations. Furthermore,

the mechanical energy is converted into electrical

energy. In general, WEC can be categorized as

oscillating water columns (OWC), overtopping

devices, attenuators and point absorbers (Aggidis

and Taylor, 2017). The OWC structure is a column

with two holes. The first hole faces the sea where a

wave comes. The wave then press the water that

pushes the air in the column. In the second hole, it

relates to an atmosphere where an air turbine is

placed to convert it into electrical energy (Ravinesh,

et.al, 2016). OWC is usually installed on the

shoreline. The advantage of this system is the ease

of installation. Whereas the weakness is the power

of the wave is not as big as offshore. The structure

of overtopping devices is a water reservoir that

higher position than sea level. If there is a wave,

water will collect in that place. Then the water flows

downward to move the turbine to convert it into

electrical energy (Frigaard, 2008). The WEC

attenuator is a long absorber that the incoming wave

will move it perpendicular to the direction of the

wave. So that each part of the attenuator moves

vertically (Lopez, et.al, 2013). Point absorber is a

floating object with a certain shape that is partially

or completely submerged at sea level. When a wave

comes, this floating object will move vertically

(heave). This floating object movement will drive

the generator with a certain mechanism to produce

electrical energy (Faizal, et.al, 2014).

Point absorber is one of the WECs that will be

developed in shape and size. By optimizing the

shape and size of floating objects, floating objects

will be suitable for that location, so the maximum

wave energy conversion will be obtained. When the

waves come, floating objects will move vertically

(heave). This floating object is called a point

absorber, because in this part the wave energy is

absorbed by the system. This movement will excite

the movement to produce mechanical energy to be

converted into electrical energy, called Power Take

Off (PTO). PTO will be very efficient if the

movement is limited to one dimension only (Pecher,

2017). To maximize energy, a condition is needed

where the natural frequency of floating objects must

Warpindyasmoro, H. and Tumbelaka, H.

Wave Energy Conversion with Floating Objects for the Coast of East Java.

DOI: 10.5220/0008375100870090

In Proceedings of the 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2018), pages 87-90

ISBN: 978-989-758-455-8

be equal to the frequency of ocean waves. Whereas

the natural frequency of floating objects depends on

the shape of the floating object. Therefore, to

optimize the energy conversion, the structure and

shape of the floating object becomes very important.

2 WAVE ENERGY ON THE

COAST OF EAST JAVA

Sea wave characteristics on the southern coast of

East Java (the Indonesian Ocean) and the northern

coast of North Java (Java Sea) have several

differences. The difference in characteristics

between the two locations is related to the

significant wave height (Hs) and wave period (T).

Figure 1: Wave characteristics in East Java.

From figure 1, the south coast has a significant

wave height Hs = 1-2 m with a wave period T = 10-

16 seconds. Whereas on the north coast has a

significant wave height Hs = 0 - 1 m with a wave

period T = 2 - 8 seconds (Warpindyasmoro, 2018).

The energy produced by ocean waves (Goncalves,

2014; Atan, et.al, 2016) is

𝑃

 





𝐻





𝑇 (1)

Where ρ is the density of seawater (kg/m3), g is

the gravitational acceleration (m/dt2), Hs is the

significant wave height (meter) and T is the wave

period (seconds). By using the equation (1), the

potential of electrical energy on the coast of East

Java can reach 232 MWh/m/year (Warpindyasmoro,

2018). Energy potential is large enough to be

developed further.

Characteristic differences between waves on the

north coast (Java Sea) and the south coast

(Indonesian Ocean), especially related to wave

periods, to optimize the energy produced, floating

objects are needed as Wave Energy Converter

(WEC) which has the maximum Response

Amplitude Operator (RAO) according to the wave

period at each wave location at the sea.

3 POINT ABSORBER

MODELLING

One WEC that can be directly connected with a

generator is a point absorber. Parameters that need to

be considered when designing a point absorber are

shape, length, volume, mass, draft, center of weight,

buoyant force and moment of inertia (Nielsen, et.al,

2014). Modelling of absorber points can be seen in

Figure 2.

Figure 2: Point absorber modelling.

The movement of the point absorber is limited to

moving only in the z axis, so that the equation of

motion from the point absorber according to

Newton's Law (Pecher, 2017) is:

𝑚











𝐹



𝐹



𝐹



𝐹



𝐹



(2)

Where m is the mass of floating objects, (d^2

z)/(dt^2 ) is the acceleration of floating objects in the

direction of the z axis, Fex is exciting force, Frad is

radiation force, Fres is a hydrostatic restoring force,

Fdamp is the external damping force , Ftun the buoy

tuning force to phase-control. Because the point

absorber model uses the mass-spring-damper model,

the natural frequency of the floating object is

𝜔























(3)

where k is the hydrostatic restoring coefficient,

m is mass of the buoy, ma (ω) is added mass and

msup is supplementary mass. Because the context

ISOCEEN 2018 - 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

for waves is a period, natural period parameter will

be used which is the opposite of the natural

frequency. The development of point absorber has

been done to obtain the largest PTO (Falnes and

Hals, 2012; Shadman, 2018).

4 RESULTS AND DISCUSSION

Figure 3: Model of point absorber: (a) cylinder, (b) cone.

In this study two point absorber models were made,

namely cylindrical and cone shaped like Figure 3.

The size of each model is L = 1.5 D, where D is the

diameter. The diameter of the model varied, D = 6,

9, 12, 15 and 18 m. Figure 4 shows the comparison

of RAO between cylinders and cones for D = 18 m

with a 4 m draft. It appears that the maximum RAO

on the cylinder occurs at a greater period than the

maximum RAO at the cone.

Figure 4: Comparison of maximum RAO in cylinders and

cones.

To find out the relationship between the diameter

of the floating object and the wave period that

produces the maximum RAO, the diameter of the

floating object is simulated for diameter 6, 9, 12, 15

and 18 m. For cylindrical floating objects can be

seen in Figure 5.

Figure 5: Relationship between diameter and period to

produce maximum RAO for cylinder shape.

From Figure 5, it appears that the greater the

maximum floating diameter of the RAO object will

occur at the higher wave period. For maximum

diameter (D) 6-18 m, RAO occurs in the wave

period (T) 6-10 seconds. To produce maximum

RAO in a wave period of more than 10 seconds can

be done by increasing the diameter of the floating

object. For locations that produce waves with a wave

period of 10-16 seconds, such as on the southern

coast of the Java Sea (Indonesian Ocean) can use

cylindrical floating objects as point absorber for

WEC to produce maximum mechanical energy.

Figure 6: Relationship between diameter and period to

produce maximum RAO for cone shape.

Meanwhile for conical floating objects, as shown

in Figure 6, the maximum RAO with a diameter (D)

of 6-18 m occurs in the period of wave (T) 4-8

seconds. By reducing the diameter, the maximum

RAO will be obtained in a period smaller than 4

seconds. Thus the conical floating object can be used

as a point absorber in the WEC to be applied in

Wave Energy Conversion with Floating Objects for the Coast of East Java

locations with small wave periods (2-8 seconds)

such as on the north coast of East Java (Java Sea).

5 CONCLUSION

An overview of the cylindrical and cone shaped

point absorber has been carried out. Point absorber

with cylindrical floating objects produces maximum

RAO over a larger period than cone-shaped floating

objects. To produce the maximum RAO, the greater

the maximum RAO floating diameter will occur in

the larger period. Point absorber with cylindrical

floating objects 6 - 18 m in diameter can produce

maximum RAO in the wave period of 6 - 10

seconds. By enlarging the diameter, this shape is

more suitable for locations with higher wave periods

such as on the southern coast of East Java

(Indonesian Ocean). While the cone-shaped point

absorber can produce maximum RAO in the 4 - 8

second wave period. By reducing the diameter of

floating objects, this shape is more suitable to be

applied on the north coast of East Java (Java Sea).

ACKNOWLEDGEMENTS

This research was funded by the Directorate of

Research and Community Service of the Directorate

General of Strengthening Research and

Development of the Ministry of Research,

Technology and Higher Education in accordance

with the Agreement on the Implementation of

Research Program Implementation Number:

002/SP2H/LT/K7/KM/2018, 26 February 2018.

REFERENCES

A. Pecher, 2017. Handbook of Ocean Wave Energy, 7.

B. Drew, A. R. Plummer, and M. N. Sahinkaya, 2009.

JPE, 223, 887.

G. A. Aggidis and C. J. Taylor, 2017. IFAC-

PapersOnLine, 50, 1.

H. Saptono Warpindyasmoro, 2018. MATEC Web Conf.,

177, 01018.

I. López, J. Andreu, S. Ceballos, I. M. De Alegría, and I.

Kortabarria, 2013. Renew. Sustain. Energy Rev., 27,

413.

J. Falnes and J. Hals, Philos., 2012. Trans. R. Soc. A Math.

Phys. Eng. Sci., 370, 1959.

K. Nielsen, M. M. Kramer, F. Ferri, A. S. Zurkinden, and

M. Alves, 2014. Overview of wave to wire models,

Aalborg University.

K. Ravinesh, M. Rafiuddin, M. Asid, and Y. Lee, 2016.

JOES, 1, 77.

M. Faizal, M. R. Ahmed, and Y. H. Lee, 2014. Adv. Mech.

Eng.

M. Gonçalves, P. Martinho, and C. Guedes Soares, 2014.

Renew. Energy, 68, 774.

M. Shadman, S. F. Estefen, C. A. Rodriguez, and I. C. M.

Nogueira, 2018. Renew. Energy, 115, 533.

P. Frigaard, T. L. Andersen, and L. Margheritini, 2008.

8th International Congress on Advances in Civil

Engineering, 15.

R. Atan, J. Goggins, and S. Nash, 2016. Energies, 9, 11.

ISOCEEN 2018 - 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management