A study on a single-tether spherical point absorber with an asymmetric mass distribution

Abstract

The adoption of renewable energy has long been regarded as an effective solution to fulfil the growing demand for electricity and reduce greenhouse gas emissions. Ocean wave energy is a promising and reliable resource in the renewable energy mix, which exhibits higher power density and continuity than solar and wind energy. Furthermore, the overall potential of ocean wave energy is estimated as being much as 3.7 terawatts, which is double the current global demand for electricity. Although the first wave energy converter (WEC) design appeared as early as 1790, the technology of wave energy conversion is still at an early stage of commercialisation. Compared with solar and wind energy plants, the existing WEC systems have relatively small power generation capacity and exhibit greater costs associated with investment, infrastructure and maintenance. Consequently, wave energy conversion is currently at an economic disadvantage in the renewable energy mix. This thesis studies the efficiency improvement of a single-tether submerged spherical point absorbing wave energy converter by utilising an asymmetric mass distributed buoy. The spherical point absorber with asymmetric mass distribution is referred to as SPAMD in the thesis. The main contribution lies in frequency-domain modal analysis, parametric optimisation and high-fidelity modelling of the system. This Ph.D. research answers three questions: (i) What effect does mass distribution have on the dynamics of a submerged spherical buoy; (ii) how does the mass distribution of the buoy affect the power output of the SPAMD in irregular waves; and (iii) do the nonlinear hydrodynamic effects compromise the performance of the SPAMD? To understand the working principle and evaluate the efficiency improvement of the SPAMD, a frequency-domain modal analysis is conducted over typical wave frequencies. The influence of a power take-off device on the performance of the SPAMD is discussed on the basis of a modal analysis. The efficiency improvement over a generic point absorber for regular waves is assessed over different frequency regimes. Recommendations pertaining to the application of an asymmetric mass distributed buoy in wave energy harvesting are provided. The design considerations of the mass distribution of the buoy are also investigated under irregular waves characterised by the Pierson-Moskowitz spectrum. A spectral-domain model, including viscous drag effects, is developed to evaluate the performance of the SPAMD efficiently. Attention is given to the power absorption bandwidth, the mean power output and the dynamic mooring loading caused by the configuration of mass distribution. Suggestions regarding the configuration of the mass distribution of the buoy are provided according to the facility cost and the system performance. The final part of this thesis explores the trajectory and power analysis of the SPAMD in a high-fidelity simulation. A numerical wave tank has been developed from the Navier-Stokes equations, to simulate the fluid-structure interface during the operation of the SPAMD. It was found that the nonlinear hydrodynamics significantly modify the trajectory of the device as the wave height grows. The large change in the motion trajectory of the buoy decreases the efficiency of the converter in terms of wave energy harvesting. The efficiency improvement of the SPAMD in comparison with the generic point absorber is demonstrated in the numerical wave tank experiment.