WAVE ENERGY BREAKWATERS - A DEVICE COMPARISON

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1 CONFERENCE IN OCEAN ENGINEERING - COE' December 1996, Madras, INDIA WAVE ENERGY BREAKWATERS - A DEVICE COMPARISON PROF. DR.-ING. KAI-UWE GRAW Universitätsprofessor, University of Leipzig Marschnerstraße 31, Leipzig, Germany Tel , Fax ABSTRACT Efforts to construct a breakwater to harness the oceans' vast wave power resource, that means a breakwater also able to produce usable forms of energy from the wave power, have been made for years. According to this the Oscillating Water Column () is a known construction which meets the requirements and some promising prototypes have been built. Nevertheless the knowledge about this technique and its use is still limited and therefore new breakwater constructions are not likely to be built this way. INTRODUCTION The present tendency to overcome the environmental problems caused by thermal energy production is to use all possible kinds of renewable energy. This enhances the status of water energy in general, the renewable energy widely used for a very long time. Especially the use of the wave energy - a huge unexploited global energy reservoir - becomes a new centre of interest. At the time the first wave energy devices were developed, a commercial possibility of using these devices was not seen because the only facts of interest were the price of the energy produced - which was too high - and that the time dependent energy production was not considered to be acceptable. Nowadays the external costs of energy production are more and more taken into account and it is taken for granted that the price of solar energy will never be less than that of wave energy. Some inconveniences regarding the availability of energy will be accepted - at least in the near future - due to the large environmental problems connected with other production methods. Many different systems were developed to convert the wave energy into electrical energy, but none of them is without problems (especially in storm situations), as it was not yet possible to use the energy of the surface region without obstructing this region. The most interesting and durable device - the shore based - will never be built on normal beaches as its presence would do great damage to the natural environment. But if the is included in a breakwater (e.g. for harbour protection) a construction place is available with no further environmental impact. OSCILLATING WATER COLUMN DEVICE Introduction The most successful and most extensively studied device for extracting energy from sea waves is the oscillating water column device (). It is based on the exchange of energy between the water and the air that is trapped above the water surface in an air chamber which is open at the bottom. Approaching waves force the free surface in the chamber to move up and down, which causes an oscillation of the pressure of the air inside the chamber. This forces an air flux backwards and forwards through an air turbine installed in a duct which connects the chamber to the atmosphere. The first demonstration plants of this kind were completed in in 1983 and Norway in 1985, others are under construction or are already producing energy in several countries (table 1 and 2). My work in this field of research started with a number of tests performed during an Indo-German Joint Research Project [Graw, ]. The tests were carried out on behalf of the Ocean Engineering Centre (OEC) of the Indian Institute of Technology (IIT) in Madras (India) in the large wave flume of the Institut für Wasserbau und Wasserwirtschaft (IWAWI) of the Technical University in Berlin (Germany). They represent a small part of the research program undertaken at the IIT to develop a breakwater

2 Installed power Location, Country Type Width, Water depth Period 375 kw 1000 kw 560 kw Sea, floating B: 12 m, d: 40 m kw Toftestalen, Norway : 10 m, d: 70 m kw 75 kw Trivandrum, India B: 8 m, d: 10 m since kw Isle of Islay, B: 17 m, d: 3 m since kw Sakata, B: 20 m, d: 18 m since kw Sanze, B: 17 m, d: 3 m kw Niigata, B: 13 m, d: 6,5 m kw Kujukuri, with storage 10 : 2 m, d: 2 m since kw Dawanshan, China B: 4 m, d: 10 m since 1990 Table 1: -devices Installed power Location, Country Type EU - contribution 2 MW Near-shore, 2*250 kw Azores, Portugal 600 kw Isle of Islay, Table 2: -devices planned in the EU ECU ECU ECU device (figure 1) well suited to the sea state parameters of the Indian Ocean. Since India has a lot of potential for new fishing harbours in the near future, the development of new cost effective breakwater systems is useful. A wave energy breakwater becomes profitable because the costs are shared between the breakwater wall and the power plant. An energy absorbing breakwater is also a better engineering design as compared to a rubble wall breakwater which takes all the beating from the waves. The research carried out at the IIT to develop the [Ravindran et al., 1989; Raju et al., 1991; Koola et al., 1993] and the realization of the prototype [Raju et al., 1992] are described elsewhere. One aim of this presentation is to promote the idea of such a wave energy breakwater, as at the present time this idea is not very widely propagated in Europe. The European research into wave energy is focused on the development of a real wave power station (table 2). The double use of the construction is not popular here.

3 Figure 1: prototype developed at the IIT in Madras Figure 2: Look at the Sakata prototype Figure 3: The prototype developed at the IIT An outline of s in operation at present Completing the survey during a visit to in 1993 I was able to see all three larger devices presently operating. This may be a good opportunity to give a very personal outline of the projects - without repeating the technical data. These and other devices are presented in table 1. The very first glance sets off the Sakata device (figure 2, [Miyazaki, 1993; Takahashi et al., 1992]): compared to it, the other two almost look like a do it yourself structure. The reason is evident, since there is a very different approach to the problem: the Sakata breakwater was conceived as a test of an. A possible failure of the system and with it a failure of the whole breakwater had to be excluded. A second reason is the fact that it is the only construction built inside an industrial harbour, with all the technical equipment possible. But the same look at the s can also show us that a much simpler technology - as applied for the construction of the near Trivandrum (India) - can lead to similar results. It is a very good reminder for European (also American, ese,..) technicians that you can build a structure also without a floating dock and without a lot of cranes and other machines. The type of chosen and studied thoroughly at the IIT - and realized near Trivandrum (figure 3) - has a rectangular shape (thus it is also possible to integrate it into a breakwater) and two guide walls in front to concentrate the energy for the design wave period towards the chamber. Learning about the hydraulic performance of the devices makes clear that the Indian is the one with the best hydraulic efficiency. After a very

4 intensive design period an excellent hydraulic structure was obtained which - for comparable values of incoming wave energy and with a much smaller width - has much more output than the ese one (even if you take into account that the ese turbine should have been twice the size, which was not possible due to funding limitations [Takahashi, 1988]). The design of the turbine arrangement itself is by far more reliable in Sakata, and the much earlier developed Trivandrum design will hopefully be the last with a vertical chamber-turbine-structure (but the first drawings of the Osprey device (table 2, no.1) look very much like a vertical design...). The design of the constructed near Trivandrum was changed during Figure 4: The prototype on Islay lot about the possibilities and problems from both of them. If you go to Sakata, do not miss the touristic highlight you will find in the Sakata information leaflet among the museums, a fun bath and a hot water pool the and its show-room and you will learn a lot about how to promote a new kind of renewable energy, too. Advantages of the breakwater From the viewpoint of the electricity producer the breakwater has the advantage that the installation costs are reduced by sharing them with the harbour authorities (or similar) that want to shelter an area or a structure from the waves. From the viewpoint of someone who wants to protect something against wave attack the breakwater has the advantage that the installation costs are reduced by sharing them with an electricity producer. From the coastal engineering point of view the system has two large advantages over "normal" breakwaters: the wave height in front of the device is reduced, as the wave energy is absorbed and not reflected; the wave forces do not (only) act against the structure, they act to move the turbine, so the loads are reduced. Finally, what can be said about the European, the Islay device (figure 4) that I have not talked about yet? On the one hand it does not belong here, as it is no breakwater device. On the other hand it is presently the only European competitor. The comparison with the other two - in my opinion - shows that this is just a basic (turbine) testing facility, which was the best solution given the not so spectacular research funding for such projects in the past. Taking the much larger amount of energy ingress into account, a comparable construction may be the Sanze in. My opinion is supported by the fact that designs competitive to the two others (but still no breakwater designs) are now going to be funded by the EU research program JOULE. Nevertheless, I am curious about the results gained at the existing Islay device, especially concerning the turbine and electricity conversion performance. Both are still very important research topics, as the turbine design has still to be improved. Concluding my personal review, I would like to add two statements: If you have the possibility to go and see the two breakwater s: take the chance, you will learn a CONCLUSIONS AND OUTLOOK The advantages of the breakwater are well known. The design of the structure can still be improved but the basic layout of the construction can be deduced from good examples that are already in operation. The turbine layout will still be a problem, but it can be improved only with real installations in nature. Also in Europe consideration should be focused on combined breakwater s. As the ese and Indian devices show, this type of device does not depend on a vast wave power resource at the site. (A realization in the Mediterranean or Baltic Sea is a good possibility!) Costs are shared and therefore also limited incoming wave energy can lead to a sufficient output of useful power. LITERATURE [1] Graw, K.-U.; Wellenenergie - eine hydromechanische Analyse; 1995; Wuppertal: Mitteilung Nr.9 des Lehr- und Forschungsgebietes Wasserbau und Wasser-

5 wirtschaft, Bergische Universität - GH Wuppertal, Wuppertal, Deutschland [2] Graw, K.-U.; 1993; Scale 1:10 wave flume experiments on IIT oscillating water column wave energy device; Proc.: Int. Symposium on Ocean Energy Development (ODEC), Muroran,, pp [3] Kaldenhoff, H.; Graw, K.-U.; 1994; Unkonventionelle Küstenschutzbauwerke; Output 2, Zeitschrift der Bergischen Universität - Gesamthochschule Wuppertal, pp [4] Koola, P.M.; Ravindran, M.; Raju, V.S.; 1993; Design options for a multipurpose wave energy breakwater; Proc.: International Symposium on Ocean Energy Development (ODEC), Muroran [5] Miyazaki, T.; ese Wave Energy Devices; 1993; European Wave Energy Symposium, Edinburgh, pp [6] Raju, V.S.; Neelamani, S.; Concrete caisson for a 150 KW wave energy pilot plant: design, construction and installation aspects; 1992; Proc. of the 2nd Int. Offshore and Polar Eng. Conf., San Francisco, USA, June 14-19, pp [7] Raju, V.S.; Jayakumar,.; Neelamani, S.; 1992; Concrete caisson for a 150 kw wave energy pilot plant: design, construction and installation aspects; Proc.: 2nd International Offshore and Polar Engineering Conference, San Francisco [8] Raju, V.S.; Ravindran, M.; Koola, P.M.; 1991; Energy from sea waves - the Indian wave energy program; Proc.: 3rd Symposium on Ocean Wave Energy Utilization, JAMSTEC [9] Ravindran, M.; Swaminathan, G.; Koola, P.M.; 1989; Model studies for the sea trial of a 150 kw wave energy system; Proc.: 8th OMAE (ASME) Conference, The Hague [10] Sarmento, A. J.; 1992; Wave flume experiments on two-dimensional oscillating water column wave energy devices; Experiments in Fluids 12, [11] Shiraishi, N.; Numata, A.; Hase, N.; The effect and damage of the submerged breakwater in Niigata coast; 1960; Coastal Engineering in, JSCE, Vol.3, pp [12] Takahashi, S.; Nakada, H.; Ohneda, H.; Shikamori, M.; Wave power conversion by a prototype wave power extracting caisson in Sakata port; 1992; 23 ICCE, Venice, pp [13] Takahashi, S.; Hydrodynamic characteristics of wave-power-extracting caisson breakwater; 1988; 21 ICCE, Spain, pp