A methodology for the structural design of LiftWEC: a wave-bladed cyclorotor

Novel wave energy convertors (WECs) need to be designed to ensure both longevity and hydrodynamic efficiency. Hence there is a need to develop methodologies that tackle both the hydrodynamic and structural requirements of WECs. Here we demonstrate an integral methodology for the design of LiftWEC: a two-foil wave bladed cyclorotor. The hydrofoils follow the orbital motion of the wave particles and rotate around a central axis. The span of the hydrofoils is aligned to the crest of the wave, making the device a wave terminator, i.e. a device that cancels the incoming wave. The phase of the rotation is different to that of the incoming wave. This phase difference generates lift and sustains the rotation of the hydrofoils. In this paper, a low-order two-dimensional hydrodynamic model and a structural model based on beam theory are weakly coupled to assess power production and structural stresses on the device. We estimate the forces on the hydrofoils due to regular waves under design conditions. By studying two rotor configurations and two typical loading cases on the hydrofoils, we demonstrate that LiftWEC is structurally resilient to design conditions. We show that, for the selected wave design conditions, the optimum radius to span ratio is about 0.8, which ensures maximum mean power output, but also, a reduced structural penalty. We therefore demonstrate a powerful design tool and pave the way for future frequency analysis studies for this type of devices.