Symphony: The Ultimate Wave Energy Controller

Abstract

It is well established that control strategies are essential to enhance the economics associated with wave energy converter (WEC) technologies. Specifically, it is proven that active control strategies may improve WEC performance (in terms of power capture) in a factor varying from two to four. Briefly, this is because energy-maximising control can adapt the WEC characteristics to different waves and wave climates, while preserving the device integrity by guaranteeing the operation within the system physical constraints. Ultimately, however, the degree of performance enhancement depends on the WEC prototype, power take-off, and specific control algorithm employed. 

Control algorithms for WECs may be broadly divided into optimisation-based (OB) and non-optimisation-based (!OB). Constrained energy-maximising control can only be achieved with OB strategies. However, OB control relies on numerical methods to solve a constrained optimal control problem, which makes OB strategies heavily dependent on the WEC model precision, and computationally demanding. Alternatively, !OB control strategies represent computationally tractable solutions. However, !OB strategies are approximate solutions to the optimal control problem, generally incur model dependency and, to date, are incapable of efficiently handling the WEC physical constraints. 

In this paper, \textit{Symphony}, a simple phase wave energy controller, is presented. The proposed controller bridges the gap between OB and !OB strategies, preserving the implementation simplicity of !OB control algorithms, but retaining the features and performance of OB strategies. Specifically, Symphony is the first !OB controller capable of dynamically handling hard position, velocity, and force/torque constraints, while exhibiting close-to-optimal performance. Furthermore, Symphony is relatively independent of the WEC model, which represents a seminal advancement; this feature is of pivotal importance, particularly considering the difficulties in accurate hydrodynamic modelling for WECs. Additionally, key features of Symphony include simplicity of design, implementation, tuning, and use, combined with robustness and versatility. 

To illustrate the potential of Symphony, a comparison with the spectral and model predictive WEC controllers is performed. The evaluated metrics are power absorption in constrained scenarios and computational efficiency. The results show above 98% of power absorption in constrained scenarios with respect to the constrained OB solutions, but with a significantly reduced computational burden, demonstrating that Symphony represents the ultimate wave energy controller.