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Study for the Hull shape of a wave energy converter-point absorber; design optimization & modeling improvement

Kalofotias, F. (2016) Study for the Hull shape of a wave energy converter-point absorber; design optimization & modeling improvement.

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Abstract:Global warming and the subsequent climate change have led governments to the pursuit of clean sources of energy. This kind of energy sources are the so-called renewable energy sources which are naturally replenished in human timescale. Wave energy, i.e. energy transported through sea masses via wind waves, can be a sustainable source of energy in the future. Wave energy can be harnessed by devices called Wave Energy Converters. Many types of Wave Energy Converters exist. Point Absorbers are one of these types and according to the author's opinion the most promising one. Nevertheless, wave energy conversion is still in R&D phase and far from being commercially applied as many challenges have to be overcome. In this context, the report studies the Design Optimization and Modeling Improvement of a Point Absorber. In terms of Design Optimization, the shape and dimensions of the hull of the Point Absorber are considered. Three different shapes are evaluated, namely the Cylinder, the Bullet and the Cone. Each of the three shapes is dimensioned for deriving the highest Efficiency in power extraction. Speciffic restrictions regarding maximum dimensions of the hull were applied. The dimensioning of the three shapes is conducted by building a model deriving the average power extraction of each design in the Frequency Domain. Hydrodynamic input to the model is provided by a Boundary Element Method (BEM) model using 3D- Diffraction Theory, namely NEMOH. Once dimensions for the three different shapes are derived, the three final designs are compared in terms of Efficiency. For the comparison, a model is built which derives the average power extraction of the Point Absorber in the Time Domain. NEMOH is used again for hydrodynamic input. Additionally, the Computational Fluid Dynamics (CFD) code, ComFLOW3 is employed for assessing the nonlinear effect of viscous damping. A CFD model and a methodology are produced for deriving drag coefficients for any studied design. The methodology makes use of the raw data derived by ComFLOW3 without adopting linear, non-viscous assumptions. Furthermore, an alternative approach for estimating the resulting, from viscous damping, drag force is presented and implemented in the Time Domain model. The Bullet design was selected as the most efficient. Results showed that the most efficient Bullet shape design proved to produce less viscous damping for fast oscillations where viscous damping is more important. Additionally, the Cylinder shape of the hull was found to produce much larger viscous damping than the other shapes and reasonably it should be avoided. In any case, it can be argued that the design optimization of the hull of the Point Absorber is a coupled problem in terms of shape and dimensions. Additional size restrictions can have signifficant influence and they can be a determinant factor. Regarding Modeling Improvement, two additions are made to the Time Domain model as this was derived by Wellens (2004) and Kao (2014). Final Model 1 is derived for including viscous (drag) force more accurately. Drag force is estimated by adjusting the drag coefficient in a time-step manner. The adjustment is made by deriving the ow conditions at every time step using the dimensionless Reynolds and Keulegan-Carpenter numbers. A set of Forced Oscillation Tests is conducted for parameterizing the behavior of the drag coefficient in different ow regimes around the body. It was found that the inclusion of drag force in the Time Domain model can decrease the predicted extracted power significantly. Additionally, the inclusion of viscous damping shifts the position of the optimum configuration of the Power Take Off (PTO) device as this was derived by Wellens (2004). In general, it can be argued that viscous damping should be included both in the design optimization phase and in any control strategy applied such the one produced by Kao (2014). Then, Final Model 2 was produced so as to assess the influence of the changing position in the force exrted on the hull by waves, i.e. the excitation force. In both studies of Wellens (2004) and Kao (2014), the excitation force was estimated always at equilibrium position, i.e. the position at which the hull rests in calm water. The excitation force was divided to two components, namely the Froude-Krylov force and the Diffraction. The Froude-Krylov force is estimated by integrating the pressure around the hull without taking into account the wave/hull interaction. The Diffraction force is estimated by NEMOH runs in various positions and interpolation at every time step. The influence of the varying positions of the hull due was proved to be computationally expensive without adding significant information to the model. It was concluded that the assumption of estimating wave forces with the hull always at equilibrium position is valid at least for relatively large bodies. Finally, validation of the used models for estimation of forces provided good agreement with forces derived by CFD simulations.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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