Hydrodynamic design and build of a direct drive heaving buoy wave energy converter.

Abstract

To extract power from a wave energy converter, it is necessary to apply an electrical force which opposes some external force over a distance (Power = force x distance). This is done by a power take off system reacting a force against a moving capture element. The system needs an inertial reference or reaction element to apply this force. I.e. the power take off needs to be attached to something with a large inertia. This paper will describe the recent progress on the design build and demonstration of a direct drive wave energy converter based on a heaving buoy and integrated inertial reference – based on the IPS buoy concept.

The purpose of an inertial reference is to allow the power take off to apply a force to resist the motion of a capture element. Shore mounted and shallow devices have an inherent inertial reference, but in deep water it can be challenging. The inertial reference must be sufficiently stable to facilitate large reaction forces during low and moderate sea states. In storm seas, where there is risk of damage, it is beneficial to be able to decouple the power take off from the reference. This lead to the development of the IPS concept, where the inertial reference is a piston located within a cylinder of water. If the piston leaves the cylinder, the inertial reference is decoupled and the power take off is protected.

The influence of waves diminishes with distance below the surface. The relationship is exponential, and so after a certain depth (compared to the wave length) the wave excitation force is proportionally quite small. A simple model of an IPS buoy is one that assumes there is no wave force acting on the piston. Recent hydrodynamic analysis, however, has highlighted limitations of this simplified approach.

This paper will present a hydrodynamic investigation of a prototype being developed to showcase a direct drive electrical power take off system. Ansys AQWA has been used to investigate the operational characteristics of the concept when operating in the North Sea wave climate. The results imply that to be fully effective, the depth of the piston must be at least 10m.

To validate the work, a 10^th scale model is under test in a wave tank. The full paper will include the hydrodynamic investigations, results of 10^th scale wave tank work plus an overview of the electrical machine design and an update on the build of a full scale 2m diameter prototype being developed. As of December 2024 the full scale buoy and electrical machine have been built but not integrated together.