Grid Integration and Power Smoothing of an Oscillating Water Column Wave Energy Converter

Abstract

This paper applies model predictive control (MPC) for the power processing of an oscillating water column (OWC) wave energy conversion (WEC) system to achieve smooth power delivery to the grid. The particular air turbine design adopted in this study produces large power pulses ranging from 0 to 1 MW in magnitude, and thus, direct connection to the grid is practically impossible, especially in weak grid conditions. Therefore, energy storage is an essential element that should be integrated into this particular WEC system in order to absorb power pulses and thereby ensure smooth delivery of power to the grid. Taking into account the repetitive nature, duration, and magnitude of the power pulses, this study has chosen "supercapacitor" as the suitable energy storage technology. The supercapacitor energy storage (SCES) is integrated into the dc-link of the back-to-back power converter of the WEC system through a bidirectional dc-dc converter. In order to achieve the desired operation of this complex power converter arrangement, a finite control set MPC strategy is proposed in this paper. Performance of the proposed energy storage system (ESS) and control strategy are evaluated through computer simulations. Simulation results show that the proposed SCES system and the control strategy are able to achieve smooth power delivery to the grid amidst power pulses coming from the generator. exhale and inhale air streams through the open end of the chamber. Energy of the air stream is then converted into electricity with the aid of a turbine coupled generator. The commonly used turbines are bidirectional which extract energy from both the inhale and exhale streams.The Australian maritime college, in collaboration with the Wave Swell Energy Ltd., has developed a new OWC air turbine technology, which has been recognized as an efficient and simple design compared with many other OWC technologies [6,7]. This particular configuration has passive, non-return air flow valves built into its chamber, which activate during the exhale stage and equalize the pressure inside the chamber to the atmosphere, which allows the rising water column to reach its maximum height. This creates a higher differential pressure during the inhale stage resulting in high velocity air stream through the turbine. Since this particular air turbine extracts energy only during the inhale stage, the output power inherently becomes discrete pulses. These large and discrete power pulses create major operational issues, such as frequency deviations, voltage sags/swells, and instabilities if delivered to the grid without smoothing. Therefore, energy storage is an essential feature that should be incorporated into the power converter of the WEC system. Nevertheless, in contrast to wind energy or tidal energy turbines, where the rotational speed should be changed according to the wind speed or tidal flow speed, the unidirectional air-turbine used in this study does not necessarily require variable speed operation. Therefore, according to [8], the turbine speed can be ...

Power Smoothing and Energy Storage Sizing of Vented Oscillating Water Column Wave Energy Converter Arrays

“…The parameters given in Table 1 were used to model each case study in MATLAB/Simulink to gain the simulation results discussed in the next section. The details of modelling the VOWC air turbine, turbine-generator, grid, energy storage systems (ESSs), and DC-to-DC bidirectional converters can be found in [17,18]. The PMS is modelled using the rules presented in [9].…”

Dimensioning Methodology of an Energy Storage System Based on Supercapacitors for Grid Code Compliance of a Wave Power Plant

“…Grid code compliance with SCESS has been studied in [28,29], validating control strategies for compensating WEC power oscillations. A finite predictive control is applied to the study proposed in [28], where the SCESS is located in the DC link of the back-toback power converter.…”

“…Grid code compliance with SCESS has been studied in [28,29], validating control strategies for compensating WEC power oscillations. A finite predictive control is applied to the study proposed in [28], where the SCESS is located in the DC link of the back-toback power converter. The analysis performed in [30] include an experimental validation in a laboratory test bench of a control that includes a state of charge compensator for the SCESS.…”

Improved Operation Strategy with Alternative Control Targets for Voltage Source Converter under Harmonically Distorted Grid Considering Inter-Harmonics

“…However, the performance of RC is undesirable, since its decreased gain in high frequency, and the response of RC is comparatively slow for its fractional denominator. Also, the approaches based on model predictive control (MPC) [14,15] and sliding mode control (SMC) [16] are employed to improve the performance of VSC when grid voltage are distorted.…”