Wave Energy Conversion

Abstract

Since the awareness that the conventional energy sources will run short, the use of various renewable energy sources has been investigated, these are solar, wind, ocean and geothennal energy. Quite some countries are interested in wave energy conversion. In several countries some full-scale wave energy converting pilot plants have been tested. Some of these are still operating. Several types of wave energy converting devices exist. There are some methods to classify these converters. According to their size and orientation three types can be distinguished: (I) point absorbers, devices which are small compared to a typical wave length, (2) tenninators, wide structures perpendicular to the incident waves and (3) attenuators, long structures parallel to the wave propagation. It is expected that in future, a number of point absorbers, installed some kilometres offshore will be used as large wave power plants. The advantage of these point absorbers is that they can capture wave energy from a larger width than the width of the structure. At present, an useful power plant is the in Norway developed tapered channel, TAPCHAN. The waves are converted by a rising channel into potential energy and subsequently by a turbine into electricity. Also the combination of a breakwater with wave energy conversion converting devices is expected to have good prospects. This study deals with the design of wave power converting breakwaters. Three types of wave energy converting devices have been investigated for the combination with a breakwater. Potential energy converting devices, flap type devices and oscillating water column devices. Oscillating water column devices have a good perfonnance, while they are able to convert large wave power values and they are not sensitive to damage. It is concluded that these devices are most suitable for combination with a breakwater. Two types of oscillating water column (= OWC) devices can be discerned: (1) devices with a single air chamber above a column and consequently one particular resonance frequency or (2) devices with in front of the chamber a 'harbour' such that the devices become multi-resonant. In Japan, Sakata Port, a wave power converting caisson with only an air chamber has been constructed. The British inventors expect that a breakwater with 'harbour' type devices has the best prospects. These devices are placed at intervals in the breakwater and operate as point absorbers. In this study the 'harbour' type devices have been investigated. Several theories (mainly numerical methods) exist to model the hydrodynamic characteristics of 'harbour' type devices. Most theories show roughly the same results. Comparison of the results of the different theories and several designs of 'harbour' type devices has been made possible by dimensionless presentation of design parameters. In that way, general applicable design rules have been derived. With these rules, the dimensions of a 'harbour' type device can be detennined without the help of complicated numerical methods. With the derived design rules, a wave power converting breakwater has been designed for the Port of Bilbao, North Spain. Two design conditions are investigated, namely (1) the ultimate limit state (U.L.S.) required for the stability and strength of the breakwater and (2) the serviceability limit state (S.L.S.), for functioning of the breakwater for sheltering Bilbao Harbour and for wave power conversion.