AN UPRIGHT DETACHED BREAKWATER INSTALLING PENDULOR WAVE POWER CONVERTER

Hideo Kondo

Prof. Emeritus of Murrain Institute of Tech. and President, Coastsphere Systems Inst. Inc.

#1003 Sapporo TM Buildg., 15-1 N7W2, Kita-Ku, Sapporo, 060-0807 JAPAN

Phone +81-11-738-3320, Fax. +81-11-738-3321, E-mail cosinehk@smile.ocn.ne.jp

Isao Yamauchi

Graduate Student, Dept. of Civil Eng. and Architecture, Muroran Institute of Technology

Senji Osanai

President, North Japan Port Engineering Consulting Company Inc.

Abstract: A system had been proposed by the senior author which has capability of wave power extraction and coastal protection at erosive coasts in 1997. It consists of a high efficiency power extractor of pendulum type named Pendulor installed in a pile supporting structure and a solid back wall which will act like a detached breakwater. The structural system is planned so as to distribute the incident wave power to the reflected, absorbed and transmitted powers, respectively, 20, 40 and 10%. The back wall has low crest elevation to decrease wave force at storm waves, and a clearance between its bottom and the seabed to allow on-off shore movement of sediment.

In order to verify the above requirements and to get the other related data, a series of two dimensional hydraulic experiments was performed for wave deformation, power absorbing efficiency, wave force and seabed scouring. The three dimensional basin experiment was carried out subsequently to investigate wave deformation and power absorption of the system in a detached breakwater group.

A cost analyses has been performed to estimate the production cost of electricity together with the total cost which includes an environmental cost and a social cost.

It is confirmed the present system has a high feasibility for erosive coasts.

Keywords: beach erosion, detached breakwater, wave power utilization, pendulor system, cost effectiveness

1 INTRODUCTION

Wave power utilization has not essentially been commercialized yet except small extractors for navigational buoys. Nevertheless wave power will be expected to become one of the most reliable energy among the several renewable energies. Meanwhile beach erosion has going on rapidly all coastal countries because of shortage of river sediment, large shoreline facilities, mean sea level rise, etc. Several countermeasures have been carried out against the erosion problems. In 1997 the senior author had proposed a hybrid system of wave power extraction and shore protection applying Pendulor device developed by Muroran Institute of Technology. Pendulor system in caisson had proved that the primary efficiency of 55% in the sea test, which is much higher than those reported from OWC devices. In the present study feasibility of the hybrid system is conducted on the basis of the two and three-dimensional hydraulic experiments, and the analysis of cost effectiveness.

2 CONCEPT OF THE SYSTEM

2.1 Configuration of structure

The system consists of Pendulor power extractor, a supporting frame piling structure for it and a submerged solid back wall to reflect shoreward waves from the oscillating pendulum plate or directly incoming oblique waves. The main structure consists of vertical piles and horizontal frames made of steel and/or r.c. The back wall is the only solid one. No side walls and bottom plate contrary to the original type of which major structure is r.c. caisson. In order to decrease the heavy wave forces during storms, the crest elevation of the back wall must be as low as possible to keep the power absorption ratio above mentioned. The back wall has a clearance at bottom to allow water and also on-off shore sediment movements at return for lower power absorption.

2.2 Wave deformation

When incident waves act on the pendulum plate hung from atop of the supporting pile structure, a part of it is reflected while the major part is absorbed by the oscillating motion of it with a standing wave motion generated within the water area between it and the solid back wall. A part of the incident power transmitted shoreward of system by allowing overtopping from the crest of and penetration under the back wall. The ratio of transmission becomes larger for larger waves. The target ratio of reflected, absorbed and transmitted will be about 20, 40 and 10 %, respectively.

2.3 Power extracter

Muroran Institute of Technology had studied Pendulor power extractor of bottom standing type that can be installed in upright caisson breakwaters as seen in Fig.2 since 1980. It consists of pendulum plate in a concrete caisson with seaside open and a power transmission system. Two side and one bottom walls of caisson together with the pendulum plate forms a water chamber where a standing wave system is generated by oscillation of the plate.

The power extraction was performed employing a rotary vane pump which had replaced in 1994 the original cylinder pump to overcome durability of the cylinder at storm periods.

Field experiments at Port Muroran in 1998-99 had proved it to get a high average of 55% primary efficiency (TANIGUCHI et.al, 2000).

3 EXPERIMENTAL SETUP

The two dimensional (2D) experiments which dealt with wave deformation, power absorption, wave forces and the bottom scouring beneath the structure were performed in the wave channel of 0.6 wide, 1.0 deep and 24 m long with the regular and random waves (See Fig.3 for arrangement). Constant water depth of h = 0.5 m was used where waves of wave height H = 0.05 - 0.15m and wave period of 1.1 - 2.7 sec. were generated. Regular and random waves were employed, the frequency spectrum of which was of Bretschneider-Mitsuyasu. The model rotary vane pump employed in the experiment is a simplified one to the prototype pump used in the field experiment.

A movable bed model was set to examine the scouring characters of the system.

The three dimensional (3D) basin experiments were carried out to investigate plane wave characters of wave deformation and power absorption for the case of a group composed of three detached breakwaters. The experimental arrangement is sketched in Fig.4. Three kinds of wave direction were applied of which oblique angle β=0°, 15°and 30°, respectively. Case β=0°means wave advances perpendicularly to structure. The model pump was set on the middle one among the three breakwaters to observe the extracted power. In order to analyze the difference between the two and three dimensional test results the quasi-two dimensional (Q2D) model was tested in which two sidewalls attached along the structure parallel to the incident wave direction.

4 RESULT OF EXPERIMENT

4.1 Back wall dimensions

At the first stage 2D experiments were done to find suitable value of the distance between back wall and pendulum plate B, and the crest elevation hc and the bottom clearance Cr of the back wall which satisfy the target power ratios explained 2.2. The test had confirmed the case of hc =0.1m and Cr = 0.1m in model to be optimum, that means hc/h =Cr/h =0.2.

4.2 Wave deformation

Wave height distribution in front of the structure which composed of the incident and diffracted waves in the three dimensional test were decreased with the increase of β. The reflection coefficient of structure KR, however, almost constant of 0.5-0.6 which is nearly the same to common rubble breakwaters, as seen in Fig.5. On the other hand the transmission coefficient KT is much sensitive for wave directions, and decreases with increase of β as shown in Fig.6. Comparison of 2D, Q2D and 3D showed a considerable effect of diffracted wave penetration shoreward of structure.

4.3 Power absorption

The primary power conversion efficiency h is defined in the present study the ratio of hydraulic power absorbed by the vane pump to the incident wave power per one breakwater length. It is well known for the Pendulor system of caisson that ratio of B to the wave length in the chamber L, B/L, is a dominant factor for h, maximum of which had been found for B/L=0.25. This has been confirmed to the present system in 2D experiment too and was higher than 40% in the wide range of B/L. In 3D experiment the effect of βon h is negligible( Fig.7). It barely keeps the target value of 40% at B/L=0.25 because of leaking power from two sides of plate in 3D case( Fig.8).

4.4 Wave force

The horizontal and the vertical wave force on the system were measured to find that they are less than those on the back wall without pendulum which is 80% or less of calculated one with common wave force formulae. The vertical force is small and about one tenth of horizontal one.

4.5 Bottom scouring

The 2D movable study revealed that the scour depth of the sandy bottom beneath the back wall was much smaller compared with the case of without pendulum. The reason is decrease of the shear stress due to wave power absorption by pendulum motion.

5 COST EFFECTIVENESS

The senior author studied the cost of electricity from wave power to Japanese coasts ( KONDO, 1996). In the study, the so-called total cost or true cost Pt (KONDO, et al., 1993) was estimated besides the production or the market cost Pm. The total cost Pt (￥/ kWh) consists of the market cost Pm , the environmental cost Pe and the social cost Ps.

Pt = Pm + Pe + Ps

Pm is estimated with conventional procedures. It was found at that time Pm was about 4 times of oil-fired on the basis of interest ratio of 0.06 in Japanese coasts. Pe is the cost to repair the environmental damages occurred in power production process, such as removal of exhausted gases including CO₂. For the case of renewable energies it can be minus when they improve the environment by power extraction.

The environmental cost of wave power is to save the beach erosion to which the expense is required to construct revetments or to nourish sand. Pe is the minus of the cost by those expenses. In the course of cost calculation the total efficiency from wave power to electricity is assumed 25% on the basis of 3D result shown above. A comparative case study of the renewable energies is performed. The renewable ones studied are sea current, tide, wind and wave. Their locations are chosen as the most prospective ones for each case.

The result is shown in Table 1 from which wave is the lowest generation cost Pt among the renewable ones.

Table 1 Market and total costs of renewwable energy in Japan (By ￥)

Energy	Location	Pg	Pe	Ps	Pt	Pt / Pt,o
Wave	Kantou	46.0	-12.5	0.9	34.4	1.4
Current	Tsugaru St	33.3	3.3	1.0	37.0	1.5
Tide	Ariake Sea	73.0	3.7	2.2	78.9	3.2
Wind	Muroran	36.0	0	1.0	37.0	1.5
Oil	Kantou	12.0	12.0	1. 0	25.0	1.0

6 CONCLUSION

In the present study a unique detached breakwater system for wave power extraction as well as shore protection has been studied from the standpoint of feasibility with the aid of hydraulic experiment and the cost analyses. The study has revealed that the system is prospective in at the coasts where beach erosion is severe and the coastal areas are well developed, like Japan.

Further study including a field experiment is required to practical application of this hybrid system.

Acknowledgements

The authors would like to thank Dr. T.WATABE, T-Wave Consulting Volunteer for his kind guidance on hydraulic system of Pendulor. The present study had been performed at Muroran Institute of Technology during the period of 1997-1999 academic years.. Thanks must go to Professor S.Touma, Technical Officer N.Oota and the graduate and the senior undergraduate students of River & Coastal Engineering Laboratory, who worked for the study earnestly.

The study had been funded mostly with Scientific Research Fund by Ministry of Education.

References

Kondo, Hideo (1997): A hybrid system wave power extraction and shore protection,

Proc. of Iahr, Energy and Water: Sustainable Development, D, pp.561-565.

Kondo, Hideo (1996): Cost effectiveness of wave power extraction at erosive coasts, Proc. 25th International Conf. on Coastal Engineering, Vol.4, ASCE, pp.4583-4590.

Kondo, K., K.Taniguchi, T.Watabe and K.Hamada (2000): Field experiment of new Pendulor wave power extractor, Proc. Japanese Coastal Engineering Conf, 46-2, pp.1261-1265 (in Japanese).

Kondo, H., S.OSANAI and I. Sugioka (1993): The concept of true cost of energy and its application to ocean energies, Proc. of International Symposium on Ocean Energy Development, Muroran Inst. Tech. and Cold Region Port & Harbor Eng. Research Center, pp.101-106.

Osanai, S., HKONDO, Y.Mizuno and T.Watabe (1966): Feasibility test of new pendulum wave energy apparatus, 25th International Conf. on Coastal Engineering, Vol.4, ASCE, pp.4591-4600.

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