On the configuration of arrays of floating wave energy converters

Abstract

In this thesis, certain issues relating to a number of wave energy absorbers operating in the same vicinity are investigated. Specifically, arrangements of the devices within such an array are sought, such that beneficial hydrodynamic interference between members is exploited and unwanted effects mitigated. Arrays of `point absorber' devices as well as converters with multiple closely spaced floats are modelled and a frequency domain hydrodynamic solution derived. This is implemented as efficient computer code, capable of producing the full linear wave theory solution to any desired degree of accuracy. Furthermore, the results are verified against output from the boundary element code WAMIT. Initially, detailed analysis of an isolated absorber is conducted, with motion responses, forces, power output and velocity potentials at the free surface computed for a range of different device specifications. Elementary examples of arrays are then used to demonstrate the influence of factors such as device separation, wave heading angle, number of devices and array configuration upon collective performance. Subsequently, the power output from an array of five devices is optimised with respect to its layout, using two different routines. The first is a new heuristic approach, named the Parabolic Intersection (PI) method, that efficiently creates array con figurations using only basic computations. The second is a Genetic Algorithm (GA) with a novel `crossover' operator. Each method is applied to maximise the output at a given regular wave frequency and direction under two different power take-off regimes and also to minimise power in a third, cautionary example. The resulting arrays are then analysed and the optimisation procedures themselves evaluated. Finally, the effects of irregular seas on array interactions are investigated. The configurations that were optimised for regular wave climates are assessed in a range of irregular sea-states. The GA is then used once more to create optimal array layouts for each of these seas. The characteristics of the arrays are subsequently examined and the influence of certain spectral parameters on the optimal solutions considered. The optimisation procedures were both found to be effective, with the GA marginally outperforming the PI method in all cases. Significant positive and negative modifications to the power output were observed in the arrays optimised in regular waves, although the effects weakened when the same arrays were subjected to irregular sea-states. However, arrays optimised specifically in irregular seas exhibited differences in net power output equivalent to over half that produced from the same number of devices in isolation.