Abstract
A multi-axis point absorber wave energy converter (MA-PAWEC) can be defined as a point absorber wave energy converter that absorbs energy from multiple modes of body motion using a power-take-off (PTO) system operating in multiple degrees of freedom. There is a lack of knowledge around whether MA-PAWECs could produce a lower cost of energy compared to the most common point absorber type, the heaving device. This research seeks to address this gap. A generic spherical MA-PAWEC with PTO on the heave and surge axes is assessed on an energy absorption and cost of energy basis relative to an equivalent heaving device. Linear theory was used to model the energy absorption under motion and power constraints. For heave+surge MA-PAWECs the results suggest that for low power constraints (relative to wave climate) and large available excursions then surge as the primary axis with heave as a secondary axis may be the most cost effective option. If the power constraint is large and excursions are tightly limited heave should be the primary axis with surge a secondary axis. By selecting axes that are best suited for different wave types a MA-PAWEC can absorb energy more consistently. This gives better utilisation of grid connection infrastructure: a MA-PAWEC with the same rated grid connection as a single axis equivalent can deliver significantly more energy. A MA-PAWEC should have its PTO system sized with the sum of the individual axes PTO limits higher than the rated device output. The relative cost-of-energy for the MA-PAWEC vs. the heave device under the modelling conditions considered here suggests that MA-PAWECs have the potential to be both significantly better and significantly worse than the incumbent heave devices. MA-PAWECs are therefore not a clear-cut advancement over heave devices, but the performance upside justifies further research in to this device type.