Focusing Wave Energy for Wave Energy Converter Applications

Wave tank tests at Stevens Institute of Technology quantified the ability of near-surface platforms to concentrate wave energy over the platform.

Due to the instantaneous change in water depth, mass, energy, and power are conserved in this process. The energy and power concentration factors ranged from 1 to 4 times the incident wave power as a function of incident wave period, wave height, and platform depth. Platform slope was set to zero for all 300 plus wave runs at platform top surface depths varying from 0.15 m to 1.10 m.

This data set is extremely valuable to the MHK industry as water particle velocities over the platform were recorded at velocities on the order of 4x incident maximum orbital velocities based on Airy/Navier-Stokes theory. This term has been used "A change in effective water depth over which waves propagate". The only way I have been able to get the data to align with Airy wave theory is to use the top of tension leg platform (TLP) depth and a wave height corresponding to the change in the free surface elevation over the platform.

The discrete change in effective water depth over which waves propagate is a topic of interest for fundamental hydrodynamic research as this implies there is an instantaneous convergence of group and phase velocities of waves at the TLP edge which shears the incident waves. This high shear rate makes the inviscid and irrotational assumptions and potential flow analysis invalid. This data set can be used as part of benchmarking any CFD which may be used to analyze this flow field.

Using the top of the TLP as the "h" and full free-surface elevation change over the platform for "H", the maximum orbital velocities measured align with Airy/Navier-Stokes equations. If the tank depth is used for "h", or incident wave height is used for "H", the equations do not align with the data.

Note that the SurfWEC system involves a non-inertial reference frame as the fully-submerged TLP is continuously experiencing positive and negative accelerations in most wave conditions; therefore, when a spring-mass (regenerative AHC winch - float) system is used for PTO, the "pseudo" centrifugal force must be accounted for in the loading to the system.