Wave-to-Wire Efficiency Maximisation for Oscillating-Water-Column Systems

Wave energy is a significant source of renewable energy which is harnessed by wave energy converters (WECs). However, due to the relatively high levelised cost of energy, wave energy has not attained a commercial stage yet. One of the key pathways to achieve commercialisation of WECs is to design effective control strategies to optimise the overall wave-to-wire (W2W) energy conversion process. This paper particularly focuses on W2W efficiency maximisation for oscillating-water-column (OWC) WECs. In OWC systems, the displacement of a water column compresses/decompresses a volume of air, consequently generating a bidirectional air flow. The air flow is typically used to drive a self-rectifying air turbine, which is directly coupled with a suitable electric generator. Due to the demanding issue of turbine efficiency, current OWC control strategies aim to maximise turbine efficiency by controlling the turbine rotational speed, albeit ignoring hydrodynamic performance. However, for Wells turbines, variations in the rotational speed affect the hydrodynamic efficiency (i.e., the wave-to-pneumatic energy conversion process) of the OWC system. Furthermore, the generator performance also depends on rotational speed and, therefore, rotational speed should be ideally modulated to improve the overall W2W efficiency, rather than just turbine efficiency. To this end, this paper investigates the benefits of W2W efficiency maximisation through Wells turbine rotational speed modulation, for a fixed OWC system. Results from numerical simulation show that, for Wells turbines, appropriate rotational speed control can further improve the overall OWC W2W energy conversion process, especially due to the impact of rotational speed on the hydrodynamic performance. Copyright (c) 2023 The Authors.