Abstract
Wave energy is a significant untapped renewable energy source which can be harnessed by wave energy converters (WECs). The oscillating water column (OWC) is one of the most promising WECs, due to its relative simplicity of operation and relatively small number of moving parts, all located above the water level. OWC power take-off systems also have lower levels of mechanical stress and more easily dissipate excess wave power, compared to other types of WECs, helping to increase the capacity factor. To improve the economic viability of OWC WECs, the performance of the associated energy-maximising control system is a major determining factor. However, energy maximisation alone does not necessarily imply that the economic return is maximised, since: (i) capital and operational costs of the OWC system should be considered and (ii) the additional goals of power quality and device integrity are also important. Indeed, to maximise return on investment, the optimisation pathway should ideally minimise the levelised cost of energy (LCoE). This critical review aims to: (i) provide a comprehensive analysis of the OWC control problem, (ii) offer an exhaustive review of available control strategies for OWCs, (iii) identify unexplored control and optimisation possibilities, and (iv) suggest future directions for OWC control. Ultimately, this review highlights that, to date, OWC control mainly focuses on turbine control, especially due to the importance of operating the turbine around its maximum efficiency point. However, comprehensive control strategies should maximise the overall, or wave-to-wire (W2W), efficiency of the device. To this end, control-oriented (complete and computationally simple) W2W OWC models should be considered in model-based control strategies. Finally, control co-design techniques should be adopted to guarantee optimal control-informed WEC design and to take into account cost-related aspects.