Virtual Wave Energy Converter

A reference floating Oscillating Water Column (OWC) concept [1] has been adapted in order to match Bimep environment and site conditions.

The selected wave converter is a floating structure equipped with an Oscillating Water Column (OWC) inside, which allows to transform the wave energy into electrical energy. The relative movement between the waves and the platform causes the movement of the free surface inside the chamber, creating the circulation of the indoor air through the turbine, installed on the top of the device. The energy production in these kind of devices depends mainly on the interaction wave-structure. The designed converter has been adapted to be installed at depths of 90 meters.

The support structure that contains the Oscillating Water Column is a SPAR structure with 3 different sections:

  1. Oscillating Water Column (Chamber): hole located in the central part of the device along its entire length. The bottom of the chamber is open allowing the seawater to enter into the device. The turbine for the production of electrical energy, due to the flow of air inside the chamber, is located at the top of the structure .
  2. Floater: cylindrical body placed around the top part of the chamber, which produces the necessary bouyancy to ensure the structure does not sink.
  3. Ballast: cone-shaped body located at the bottom  of the chamber that provides to the structure the stability necessary to resist the sea states.

The turbine has been simulated with an aperture located in the top part of the chamber. Knowing the free surface inside the chamber and the pressure, the pneumatic output of the device can be calculated. The experimental tests have been done with two different configurations for the upper aperture. 1) Configuration 1 (Operational): Open aperture: Chamber partially open. 2) Configuration 2 (Survival): closed chamber.

The floating structure is anchored to the seabed with an anchoring system composed of 4 moorings lines. Each line is composed by 2 chains and an intermediate buoy.

Mooring system

The mooring system designed for the OWC is composed of 4 catenaries (link chains) placed every 90 degrees. In order to allow the vertical movements of the OWC, an intermediate buoy has been placed in each line. With the intermediate buoy, the vertical component of the loads of the mooring system on the platform is reduced to practically zero, so the vertical movement of the OWC is “free”.

The WEC (Wave Energy Converter) concept has been fully tested at laboratory scale. The tests have been carried out in the CCOB facilities as well.  The test program was focused on operational and survival conditions coupled wave and current action over the WEC to analyze the dynamic response of the platform under different metocean circumstances.  

The dynamics of a floating OWC is highly complex. Usually they are spar buoys where coupled motions may dominate the general dynamics of the WEC. Moreover, the internal dynamics of the OWC as well as the mooring influence have a strong impact over the performance of the WEC. Within the framework of the TRL+ consortium, an integrated numerical model able to predict the general performance of the floating structure, the OWC, as well as the dynamics of the mooring system.

The numerical model developed to analyze the behavior of the device consists of different submodels coupled together. These submodels analyze the main components: the floating platform, the mooring system, the oscillating water column and their interaction with the environment. In summary, the submodels used are:

  • Rigid body model: Newton´s second law is used to obtain the dynamic of the structure as a rigid solid with six degrees of freedom, three translations and three rotations. For this, it is necessary to know the inertia matrix of the platform and calculate all the forces and moments that act on it.
  • Hydrostatic and hydrodynamic models: with the hydrostatic model, we obtain the combination of buoyancy and gravity forces acting on the device due to the stiffness matrix. For the calculation of hydrodynamic forces, linear potential theory is used, characterizing the response of the platform with hydrodynamic coefficients obtained with the boundary element method (BEM)
  • Mooring system model: to model the lines used in the mooring system two options are considered. On the one hand, a quasi-static model that ignores the dynamic effects on the lines, based on the catenary equation,
    significantly faster, on the other hand a more elaborate dynamic model, based on the finite element method (FEM)
  • Oscillating water column model: this model is based on the hydrodynamic model mentioned above. In this case, the aperture on the top of the device is considered as a new degree of freedom: the free surface in the chamber. This affectation also affects the rest of the hydrodynamic coefficients of the platform. Finally, the OWC’s energy extraction system (PTO) is modeled, so its energy production can be obtained.

Once all the submodels are defined, they are coupled together in a single model. To do this, at each time step, the position and the speed of the platform are used in the others submodels to calculate the forces on it.

References

[1] R. P. F. Gomes, J. C. C. Henriques, L. M. C. Gato y A. F. O. Falcao, «Design of a floating oscillating water column for wave energy conversion,» Conference European Wave Tidal Energy (EWTEC), Proceeding 9th, Southamptom (UK) , 2011.