This thesis presents two wave energy converters (WECs), an oscillating surge wave energy converter and a heaving point absorber wave energy converter and a reverse osmosis desalination system which are numerically modeled and compared. The primary objective was to assess and contrast their performance in terms of Freshwater Production and Specific Energy Consumption (SEC) over a variety of realistic sea states, which are specified by significant wave heights (1–5 meters) and energy periods (5–9 seconds) incorporating a particular wave spectrum for irregular waves.

An open source MATLAB based solver was used to integrate hydrodynamic data from frequency-domain Boundary Element Method (BEM) solver into time-domain simulations. Both devices harnessed mechanical energy, which was then sent to the reverse osmosis unit via a hydraulic piston configuration. Power matrices displaying SEC, water production and permeate concentration were used to present the results, offering a direct comparison.

Results showed that the two devices differed significantly. Only in high-energy wave circumstances did the heaving device operate effectively, exhibiting low SEC of 2-4 kWh/m³ and significant freshwater output in certain sea states. However, in moderate or lower-energy seas, its efficiency significantly decreased. With SEC normally ranging from 2 to 4.5 kWh/m³ and consistently higher freshwater output, even in milder sea conditions, the oscillating surge device, on the other hand, showed more stable performance. According to the study, the oscillating surge device can provide desalinated water with salt concentrations below the threshold of 600 ppm suggested by the World Health Organization in the majority of sea states, whereas the heaving device often crosses this limit, with the exception of energetic wave conditions. These findings are in good alignment with previous studies on wave-powered desalination.

Limitations include the exclusion of operational issues such as membrane fouling or long-term system degradation, the assumption of linear interpolation for calculating intermediate sea states, and possible errors resulting from simplified models.

The use of complex interpolation techniques to improve accuracy, more simulations over multiple sea states, and experimental validation of results are some suggestions for future research. Additionally, combining environmental and economic evaluations will help in determining if larger-scale deployment of these wave-powered desalination systems is feasible. This study highlights oscillating surge devices as a strong option for sustainable freshwater generation and offers insightful information for enhancing wave energy desalination.

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.