Real-Time Control of WECs Based on NAR, NARX and LSTM Artificial Neural Network

In this study, we aim to improve WECs’ performance for maximizing energy absorption through a sub-optimal method of phase control by latching is applied to the device.The forecasting of future wave force is required for the optimal control command deducted.An artificial neural network, namely LSTM (Long Short-Term Memory) is proposed to accurately predict the short-term wave force. The hydrodynamic properties of a point absorber is analyzed based on the 3D potential flow theory in frequency-domain. Cummin's equation and a 4^th-order state-space model are used to efficiently represent the hydrodynamic behavior of the WEC under irregular waves in time-domain. The Nonlinear Autoregressive artificial neural network(NAR-ANN) and NARx network are used to verify the method proposed in this paper. The simulation results show that the mean square error value, root mean square error value and R² value based on the LSTM prediction model are better than those of the NAR prediction model. The prediction performance of LSTM is more suitable for processing the time series.

Wave energy is a kind of marine renewable energy. It is regarded as a forward-looking solution for sustainable power generation because of its high power density, high stability, good economic benefits and energy flow without intermission.Furthermore,the total exploitable amount is the same as the power consumption. A mainstream Wave energy converters (WECs) is the Oscillating Buoy(OB).To lower the levelized cost of energy (LCOE), a real-time latching control methods is being investigated. The latching control was first introduced by Budal and Falnes (Budal, 1980). Power absorption is achieved by locking and releasing the buoy alternately to keep the phase of WEC align with the wave excitation force. Babarit and Clement (Babarit, 2006) assessed the power extraction of an oscillating-body WEC with latching control. Previous studies focus on the optimal control, which assumed that the coming wave force was already known. But real-time latching control is result in non-causal transfer functions, so it make sense only if future wave force is known. Meanwhile, the information of future force is difficult to measure directly in real sea state.