Renewable energy technologies are being increasingly exploited worldwide. Countries around the world are resorting to integrating renewable energy resources into their energy policy to reduce fossil fuel usage and carbon emissions [1
]. Electricity demand is growing rapidly as countries develop, with increased use of electricity to meet a range of needs. According to the projections of the International Energy Agency (IEA), the global energy need has increased by about
since 1990, and a
increase is expected by 2030 [5
]. This fast-rising energy demand will require some US
trillion in new infrastructure investment by 2030 [6
]. The renewable energy technologies can improve energy security and decrease dependence on fossil fuels. The International Energy Outlook (IEO2016) confirms that these systems are related to a growing renewable energy converters. Renewable energy consumption increases by an average of
per year between 2012 and 2040 [7
]. The technical potential for renewable energy is far greater than current human energy use, and studies suggest it could supply
of global energy demand by 2050, and double its current share by 2030, at a relatively low net cost [8
]. Marine renewable energy has become the focus of national research and development because of its abundant, renewable, and non-polluting characteristics [10
]. Tidal energy represents an important energy source as the tidal energy potential is estimated to be around 450 TWh/year, with about 24 TWh/year on the European coasts [13
Among marine renewable energy converters, Tidal Stream Generator (TSG) promises to be an environmentally friendly way to generate renewable electric energy with no emission of greenhouse gases during normal operation [14
]. The horizontal axis tidal stream turbine has apparent similarities with the wind turbine. Nevertheless, they have different operational behaviors. In normal conditions, the fluid is over eight hundred times
denser than the air [16
]. This is due to the huge kinetic energy density of the water. Therefore, at equal power, the tidal turbine will be more compact than a wind turbine. This will lead to a significant difference in the rotor size [17
]. Consequently, the advantages of these opposing views will appear in construction, transportation, and charge of installation. In addition, the differences are mainly in the load design, size, and the inertia of the rotor. These characteristics are figured in different operating conditions. In effect, studies demonstrate that variations of the rotational speed for a TST are higher than for a windmill system, despite the perturbation in the wind speed is much higher than that of marine current [16
]. Concerning the tidal stream converters, the swell effect is supposed to be the most disturbing phenomenon for the tidal current speed input [18
]. This fluctuation will affect the harnessed output power.
In this area of research, control strategies have a valuable role to enhance the dynamic behavior of the TSG plant. In this context, several control approaches have been used. The Maximum Power Point Tracking (MPPT) strategy is employed to search the maximum harvested power from tides and tracking the Optimal Regimes Characteristic (ORC) operation [19
]. Other research focused on the control of the active and reactive powers through the Doubly Fed Induction Generator (DFIG) by the use of the Rotor Side Converter (RSC) and the Grid Side Converter (GSC). The RSC control is used to keep the generator speed at its reference signal and the GSC control is dedicated to ensuring that the DC-link voltage remains constant [20
]. In the literature, the PI controller has been used to control the operation of the marine current turbine through the back-to-back power converter aiming to maximize the captured energy [22
]. One can envisage two designs: the torque and rotational speed control loops. However, the torque control loop is sensitive to the parametric variations and the turbulent tidal resource [23
]. In addition, advanced control approaches can be employed to provide better performance especially to ensure the robustness under the modeling uncertainties [24
]. In this framework, the sliding mode control approach is a suitable method for nonlinear systems [26
]. It has been used in the field of marine energy conversion [27
]. Its robustness to the disturbances and parametric uncertainties renders unnecessary a precise knowledge of the system. However, the main drawback of this method is the chattering phenomenon which is the high-frequency oscillations. This can negatively affect the generator because of discontinuous control. Many approaches were proposed to deal with this drawback as presented in [28
]. Furthermore, the artificial intelligence techniques are capable of handling nonlinear problems in various signal processing applications, from pattern recognition and extended to renewable energy converters [29
]. An artificial neural network is considered a technique which is well accepted for nonlinear statistical adjustment applications [30
]. As discussed in [31
], the Artificial Neural Networks (ANN) technique is used to more accurately determine the wind speed distribution law of a site, enabling the better assessment of wind energy potential and wind generator performances. The approach enables wind speed prediction with less errors. An application of using neural networks in wave energy systems is detailed in [32
], where the assessment of the wave energy potential in near shore coastal areas is investigated by means of ANNs. The ANN model developed forecast wave energy potential with great accuracy. The Fuzzy Gain Scheduling (FGS) technique has been used as well for renewable energy converters. The MPPT controller for photovoltaic systems using an FGS strategy has been studied in [33
]. This approach creates an adaptive MPPT controller and achieves better overall system performance. Furthermore, the proposed technique detailed in [34
] is applied to design FGS-PID (Proportional Integral Derivative) controllers of superconducting magnetic energy storage for power system stabilization. The study confirms that the controller provides high robustness under various operating conditions and large disturbances. From the benefits of both advanced approaches, which are the ANN and FGS, this study focuses on the power output improvement of the TSG system by implementing a hybrid neural fuzzy design.
The main application of the proposed control is affected by the change in velocities that are not predictable by astronomical tide—in particular, the swell effect phenomenon. For that reason, the robustness of the investigated control strategies has been compared by acquiring data from realistic tidal site in order to show how much energy will be saved. In particular, the novel hybrid Neural Fuzzy design is investigated to reach the power output improvement extraction by varying the rotational speed. The Artificial Neural Networks (ANN) based-MPPT approach has the advantage to approximate and interpolate multi-variate data that require huge databases. Furthermore, the fuzzy gain scheduling based control eliminates the fixed gains during operation. Therefore, the proposed fuzzy block will provide the adaptive change of controller gains which adequately vary according the variable tidal speed. In the operation in variable speed, the FGS-PI-based control is applied to the RSC. This enables the TSG to track the MPPT strategy. The MPPT approach uses a multilayer feed-forward ANN that enhances a fuzzy rotational speed controller. The aim of this command is to control the TSG plant, which, at each tidal velocity, must follow the optimal rotational speed where the maximum generated power is satisfied. The analysis of the investigated control approaches has been tested in the case of an irregular tidal resource and the occurrence of a disturbance during normal operation.
The rest of this article is structured as follows. In Section 2
, the TSG system is described and modeled. Section 3
is devoted to the control design of the MPPT-based ANN and the FGS-based rotational speed control. In Section 4
, the control robustness and disturbance rejection are investigated. Two study cases are presented and discussed using numerical input and real measured input. Finally, concluding discussions are drawn in Section 5
In this article, a tidal stream generator system has been designed and controlled. A hybrid neural fuzzy design has been developed to deal with the power disturbances due to the swell effect.
The hybrid design consists of an ANN-based MPPT approach which adequately generates the reference rotational speed in order to drive coupled with a fuzzy gain schedule that drives the system in the maximum power. The ANN design adaptively changes its weights to provide the suitable trajectory for each marine velocity. The block design is the fuzzy gain scheduling which controls the rotational speed control loop. The fuzzy controller adaptively changes its gains using the designed fuzzy supervisors.
To test the effectiveness of the novel hybrid FGS PI-controller, two realistic tidal sites were investigated. The first scenario is proposed with a variable spring and neap marine velocity provoking swell effect disturbances on the Western coast of Europe. The results found prove that approaches successfully deal with these perturbations that enable the TSG plant to harness the maximum output power. A second scenario based on the realistic data from the Cook Inlet, USA was considered. Comparing with the uncontrolled case, the hybrid neural fuzzy controller shows the power generation improvement offered by the developed control schemes.
Another case of study was considered to assess the robustness of the implemented control strategies under disturbances with an excellent reference tracking. The proposed hybrid FGS-PI control that has been enhanced with an ANN provides very good output power performance improvement from the tidal stream generator system.
This study consists of enhancing the operation of the tidal stream generator system for several study cases by varying the input profile. In effect, the plant is controlled in a way to maximize the harnessed power. The work proves that regulating the tidal turbine in variable speed functioning lead to a high energy yield by operating with a maximum power coefficient.