Multidisciplinary Modelling of Water Piston Oscillations in Wave Energy Converters : Assessment of Flow Resistance Through CFD Modelling with Fluid Structure Interactions

Abstract: As the world’s need for electricity increases, so does its demand for sustainable energy with low to no greenhouse gas emissions. One of these renewable sources of energy is the Ocean which is one of the world’s largest and most predictable energy source, where the extraction of energy can be from waves or tidal current, with zero greenhouse gas emissions during production. A company which works with wave energy is Waves4Power which has developed the wave energy buoy WaveEL.WaveEL is comprised of a buoy which is eight meters in diameter with a 36 meters long vertical cylinder which goes through the buoy. In the cylinder is a piston that oscillates in pace with the waves and generates electricity. Between the piston and the cylinder wall is a gap where the water can move from one side of the piston to the other in pace with the piston’s oscillations. The gap is called the leakage clearance. The leakage clearance effect on the flow resistance is the focus of this master thesis as something which has not been studied before in scientific articles for similar wave energy buoys or in other fields.The aim of the master thesis is to improve the understanding of how the water flow, because of the leakage clearance in the WaveEL buoy, affects the force which the piston is subjected to, and in turn how much electricity can be generated. As it is a complex system the focus will be to determine the dynamic flow resistance parameter because of the leakage clearance and the changes to the dynamic flow resistance parameter as the dimensions of the piston is varied.The leakage clearance effect on the flow resistance has been studied with the help of Computational Fluid Dynamics (CFD) in the software COMSOL Multiphysics 6.0 in two different models. In the first model, model A, the piston is locked in different positions in the cylinder and the pressure at the bottom of the cylinder varies to reflect the motion of the waves. For the second model, model B, the piston is allowed to move vertically in the cylinder due to a set force which reflects the motion of the waves.The dynamic flow resistance parameter for model A is lower at higher Reynolds number and within an interval between 0.4 and 1.6 within the working area. Outside of the working area the dynamic flow resistance parameter is lower at a higher Reynolds number and higher than in the working area at an interval between 0.7 and 45.For model B, the dynamic flow resistance parameter has only been calculated for the working area and is within an interval of 0.1 and 7, the value for the dynamic flow resistance parameter is low when the Reynolds number is high. Dissimilar to model A where the piston is locked into position, the piston oscillates in model B. There is a phase shift between the velocity of the piston and the velocity of the water, which leads to the piston being subjected to a larger force than in model A at lower water velocities. This is one of the reasons why the dynamic flow resistance parameter is higher in model B than model A at low Reynolds number. As model B calculates the dynamic flow resistance parameter based on the relative velocity between the piston and the water, the dynamic flow resistance parameter becomes lower than for model A at higher Reynolds number.For the performed sensitivity analysis, the results shows that a more advantageous value on the dynamic flow resistance parameter can be achieved by altering the dimensions of the piston. A more advantageous result was achieved for example when the rounded edge on the piston became sharper or when the leakage clearance width was increased by 10%. If this master thesis work is to be extended, the studies should focus on elaborating model B either more in depth or with values derived from experiments from the WaveEL buoy for a more realistic model and thus determine a more accurate dynamic flow resistance parameter.The results from the sensitivity analysis justifies a future study where the dynamic flow resistance parameter should be investigated with greater variations of the piston diameters, as this can increase the flow resistance and thus generate more electricity. However, it should be investigated in relation to the cost of manufacture, to obtain the ultimate design which generates maximum electricity for a reasonable manufacturing cost.