THE HYDRODYNAMICS OF A DISTENSIBLE WAVE ENERGY CONVERTER

The Anaconda is a new concept for wave energy conversion. It is just a rubber tube in the sea, full of water, closed at both ends, anchored head to waves. It is squeezed or enlarged locally by pressure variations that run along its length due to the waves. Squeezing a water-filled rubber tube starts a bulge wave running. The bulge wave travels at a speed that is determined by the geometry and material properties of the tube. The Anaconda is designed so that its bulge wave speed is close to the speed of the water waves above. In these conditions the bulges grow as they travel along the tube, gathering wave energy. Inside the tube, the bulge waves are accompanied by a periodically reversing flow. One way of extracting power from the Anaconda is to use a pair of duck-bill valves to convert this into a rectified flow past a turbine between high and low pressure reservoirs. We have proved the concept of the Anaconda at a scale of about 1:85 in a laboratory wave flume. At this scale a large part of the converted wave energy is lost in heating up the thin wall rubber from which the tube is made, and in the turbulent flow through the valves. Nevertheless, the model absorbed all of the incident wave power over a front equal in width to as much as 5 times its diameter. A power take-off system accounted for about 20% of this, corresponding to more than 250kW for a 7m diameter Anaconda, 150m long, in waves 2m high. At larger scale, energy losses would be much less significant and the proportion of useful power conversion much higher. A device rated at 1MW would contain about 100 tonnes of rubber, making the Anaconda an exceptionally light wave energy converter for its power.When it comes to predictions, the Anaconda is like no other marine vessel or structure. It has some features in common with Pelamis, but it is much more compliant, has many more degrees of freedom, and does not necessarily follow the motion of the water surface. Our mathematical models of it are rather basic, and in many respects are not in very good agreement with laboratory measurements. The aim of this research is to develop a better understanding of the hydrodynamics of the device, and formulate a comprehensive and validated numerical with which to make more reliable estimates of full scale performance. Experiments will be carried out at scales of 1:28 and 1:14, with tubes of diameters 0.25m and 0.50m, at which rubber hysteresis losses will be proportionately much lower than at smaller scale. Three types of experiments are planned, to provide measurements of internal pressures, tube displacements, radiated waves, mooring forces, and absorber power:(1) measurements in still water with bulge waves generated mechanically at one end of the tube, and absorbed at the other,(2) measurements in regular and irregular waves,(3) measurements in extreme waves.The results will provide insights into the mechanics of the device and support the development of a numerical model that will for the first time include the effects of wave radiation and other factors so far neglected. Some features of the work will be relevant to other examples of wave interactions with compliant surfaces.