A point absorbing wave energy converter (WEC) is a complicated dynamical system. A semi-submerged buoy drives a power takeoff device (PTO), which acts as a linear or non-linear damper of the WEC system. The buoy motion depends on the buoy geometry and dimensions, the mass of the moving parts of the system and on the damping force from the generator. The electromagnetic damping in the generator depends on both the generator specifications, the connected load and the buoy velocity. In this paper a velocity ratio has been used to study how the geometric parameters buoy draft and radius, assuming constant generator damping coefficient, affects the motion and the energy absorption of a WEC. It have been concluded that an optimal buoy geometry can be identified for a specific generator damping. The simulated WEC performance have been compared with experimental values from two WECs with similar generators but different buoys. Conclusions have been drawn about their behaviour.

Reliable simulation tools are necessary to study performance and survivability of wave energy devices, since experiments are both expensive and difficult to implement. In particular, survivability in nonlinear, high waves is one of the largest challenges for wave energy, and since the wave loads and dynamics are largely model dependent, each device must be studied separately with validated

A full scale linear generator for wave power has been experimentally evaluated by measuring the line force and translator position throughout the full translator stroke. The measured line force, in relation to translator speed, generator damping and stator overlap, has been studied by comparing the line force and the damping coefficient, γ, for multiple load cases along the translator stroke length. The study also compares the generator's behavior during upward and downward motion, studies oscillations and determines the no load losses at two different speeds. The generator damping factor, γ, was determined for five different load cases during both upward and downward motion. The γ value was found to be constant for full stator overlap and to decrease linearly with a decreasing overlap, as the translator moved towards the endstops. The decline varied with the external load case, as previously suggested but not shown. In addition, during partial stator overlap, a higher γ value was noted as the translator was leaving the stator, compared to when it was entering the stator. Finally, new insights were gained regarding how translator weight and generator damping will affect the translator downward motion during offshore operation. This is important for power production and for avoiding damaging forces acting on the wave energy converter during operation.

This paper presents a study on how the power absorption and damping in a linear generator for wave energy conversion are affected by partial overlap between stator and translator. The theoretical study shows that the electrical power as well as the damping coefficient change quadratically with partial stator overlap, if inductance, friction and iron losses are assumed independent of partial stator overlap or can be neglected. Results from onshore experiments on a linear generator for wave energy conversion cannot reject the quadratic relationship. Measurements were done on the inductance of the linear generator and no dependence on partial stator overlap could be found. Simulations of the wave energy converter's operation in high waves show that entirely neglecting partial stator overlap will overestimate the energy yield and underestimate the peak forces in the line between the buoy and the generator. The difference between assuming a linear relationship instead of a quadratic relationship is visible but small in the energy yield in the simulation. Since the theoretical deduction suggests a quadratic relationship, this is advisable to use during modeling. However, a linear assumption could be seen as an acceptable simplification when modeling since other relationships can be computationally costly.

No abstract