Abstract
Wave energy converter (WEC) devices are proposed to provide wave sheltering for coastlines, offshore wind turbines, and offshore aquaculture. This study evaluates those claims by comparing the transmission and reflection coefficients of different WEC architectures to a standard floating breakwater design. The transmission coefficient (KT) describes the percent of wave height traveling past the body. The reflection coefficient (KR) describes the percentage of wave height the body sends back upstream. Far-field energy balance through the wave field is characterized by these coefficients. The WECs analyzed here are a heaving point absorber (PA), an oscillating surge WEC (OSWEC), and an attenuator. The coefficients are estimated in multiple fashions using linear potential flow in Capytaine. The first method calculates the coefficients directly from the wave height generated by the solver, averaging wave height over wavelength. The second calculates the disturbance coefficient, averaging peak disturbance over wavelength. The final method finds KT and KR from the body’s hydrodynamic coefficients, radiation potential, and exciting force and is convenient for evaluating the effect of controls. The effect of arrays must also be quantified. Array configurations may affect an individual body’s transmission and reflection capabilities. If the effect is not significant, each body’s coefficients can be approximated as isolated bodies. However, if array effects are significant, array configuration will become imperative when designing for wave sheltering. The bodies are first evaluated without controls to compare performance from a geometry-based hydrodynamic perspective. Controls are then added to analyze the magnitude of their effect.
These coefficients are analyzed in the period range of 13-6 seconds (0.5-1.047 rad/s). The bodies are fixed in all degrees of freedom sans the mode of motion they extract energy from. Less reflection is noted for all individual bodies at longer wavelengths (>150 m or >10 s period). Without controls, PAs do a poor job at wave damping, reflecting almost no waves (< 3% for all periods). The OSWEC sees maximum reflection of ~59% at T=6s but declining performance at T > 7.5 s. The attenuator’s transmission is middling, around 60-70% between T=6-8 s. However, its reflection is only marginally higher than the point absorber, and simulations show more energy is dissipated radially than reflected back. The breakwater performs poorly in these sea states. This is likely because breakwaters are designed for shallow areas with T < 5 s, and simulations show reflection does not beat transmission until T=4s. Preliminary array studies were conducted with 3-body, regular array configurations. They show that interactions affect PA’s and attenuators minimally. However, the OSWECs performance significantly increases in an array, with reflection surpassing transmission at T=8s (compared to T=6s), reaching reflection up to 75%. Breakwaters also perform slightly better in an array, with reflection surpassing 20% by T=6s (compared to max 15%). Different configurations will be analyzed as well for comprehensive interaction modeling. Controls are ongoing work.