Synergistic design of a combined floating wind turbine - wave energy converter
Author(s)Kluger, Jocelyn Maxine
Massachusetts Institute of Technology. Department of Mechanical Engineering.
Alexander H. Slocum and Themistoklis P. Sapsis.
MetadataShow full item record
Offshore energy machines have great potential: higher capacity factors, more available space, and lower visual impacts than onshore machines. This thesis investigates how combining a wave energy converter (WEC) with a floating wind turbine (FWT) may produce offshore renewable energy cost savings. Attaching the WEC to the FWT greatly reduces the WEC's steel frame, mooring lines, electric transmission lines, and siting/permitting costs, which may comprise 56% of a standalone WEC's cost. A 5 MW FWT currently requires up to 1700 tons of platform steel and 5700 tons of ballast concrete for stabilization in the ocean. This required material may be reduced if the WEC stabilizes the FWT. This thesis addresses several challenges to designing a combined FWT-WEC. First, parameter sweeps for optimizing ocean machine performance are limited by high dimensionalities and nonlinearities, including power takeoff control and wave viscous forcing, which normally require computationally expensive time-domain simulations. This thesis develops a statistical linearization approach to rapidly compute machine dynamics statistics while accounting for nonlinearities in the frequency domain. It is verified that the statistical linearization method may capture significant dynamics effects that are neglected by the traditional Taylor series linearization approach, while computing the results approximately 100 times faster than time domain simulations. Using Morison's equation for wave viscosity and quasi-steady blade-element/momentum theory for rotor aerodynamics, we find that viscous effects and nonlinear aerodynamics may increase the FWT motion and tower stress by up to 15% in some wind-sea states compared the the Taylor series linearized system. Second, the WEC must stabilize rather than destabilize the FWT. This thesis investigates the dynamics statistics of dierent FWT-WEC configurations using a long wavelength, structurally coupled model. It is shown that simultaneous targeted energy transfer from both the FWT and waves to the WEC when the WEC and FWT are linked by a tuned spring is unlikely. That being said, this thesis considers heave-mode oscillating water column WEC's that are linked to the FWT platform by 4-bar linkages, so that the FWT and WEC's are uncoupled for small heave motions and rigidly coupled in all other degrees of freedom. It is shown that this configuration allows the WEC to move with a large amplitude in its energy harvesting degree of freedom, and therefore harvest a significant amount of power without significantly increasing the FWT motion in the same direction. In the rigidly-connected modes, the WEC inertial resistance to motion must be greater than the wave forcing, as these properties are transmitted to the FWT. Third, the WEC requires power robustness in dierent sea states. Typical WEC's require control schemes to maintain good power performance when the ocean wave dominant frequency differs from the WEC resonant frequency. This thesis introduces a nonlinearity into the WEC design that passively increases power adaptability in dierent sea states. While the optimized nonlinear WEC requires 57% more steel than the optimized linear WEC, the nonlinear WEC produces 72% more power on average, resulting in a 3% lower levelized cost of energy. Further optimization of the nonlinear WEC may find improved performance. This thesis determines that attaching a single linear hinged floating spar oscillating water column to the FWT reduces the levelized cost of energy from $0.31/kWh for the standalone system to $0.27/kWh (13%) without changing stress on the FWT tower. Attaching a single nonlinear hinged floating spar oscillating water column to the FWT reduces the levelized cost of energy to $0.26/kWh (16%) and reduces the lifetime equivalent fatigue stress on the FWT tower from 32.4 MPa to 31 MPa (5%). A 6-unit array of the nonlinear WEC's encircling the FWT platform may generate an average of 400 kW while reducing the FWT tower stress by over 50%. In wave tank experiments, the response statistics of four dierent combined FWT-WEC configurations are measured, verifying the FWT-WEC dynamics model.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 241-251).
DepartmentMassachusetts Institute of Technology. Department of Mechanical Engineering.
Massachusetts Institute of Technology