Time Domain Modeling of a Point Absorber

Abstract

The demand for renewable energies is rising due to climate goals and high oil prices. Not only the established renewable sources like wind and solar are interesting to exploit. There is a vast amount of energy stored in the world’s oceans. To harvest this energy, one needs a Wave Energy Converter (WEC). There
are already some prototype WECs tested around the world but there is no leading design that proves to be a cost-effective way to convert this energy to electricity.

A company from the Netherlands, Dutch Wave Power, also tries to build a cost-effective device to harvest the ocean’s energy. Their design consists of a floating tube or cylinder that converts the heave and sway motion into a pitch motion. This pitch motion drives a generator which generates electrical power. The generator is connected to the outer wall of the float and rotates as the float pitches. Inside the generator is a shaft that is kept in place by an inside pendulum. Dutch Wave Power validated their concept with experimental tests in a wave flume. The next step in the development is a numerical model. This model
gives insight into the effects that influence the dynamics and power production of the WEC. Secondly, a numerical model eliminates the need for new experimental tests each time a design parameter is changed.

The numerical model estimates the dynamics of the WEC. The BEM software NEMOH is used to estimate the diffraction and radiation forces coefficients. A state-space approximation of the Cummins equation is used to capture the memory effect of the radiation forces. The Froude-Krylov forces are fully non-linear and are evaluated for each time step. Lastly, some friction and drag forces are included. The PTO system is described as a linear damper. The numerical model is validated with experimental tests executed by Dutch Wave Power.

Next, the numerical model is adapted to also estimate the hydrodynamics in irregular waves. The irregular waves used in this thesis are based on a JONSWAP spectrum. Using the parameters of the JONSWAP spectrum, a sea-state with the desired significant wave height and peak period can be generated. The
Froude-Krylov and the diffraction forces are estimated based on the principle of superposition. With this irregular wave model, a power matrix is constructed for a WEC with twice the size that was used in the experimental tests. This power matrix gives an indication of the ideal operational wave conditions, where the efficiency of the WEC is the highest, and it makes it enables the possibility to compare the performance of the WEC at different sea-states.