# Improving the Energy Conversion Efficiency for Hydrokinetic Turbines Using MPPT Controller
## 1 Introduction
Considering the benefits reported in literature when using MPPT control, it became obvious that it is possible to successfully apply this technique to hydrokinetic turbines. The algorithm applied to the hydrokinetic turbine considered in the present study is not different to the ones applied to wind turbines. Basically, an algorithm that has proven its efficiency in other wind turbine research can be also applied to kinetic turbines. The operating principle is similar for variable speed wind turbines and hydrokinetic turbines. Thus, an MPPT algorithm applied to wind turbines can be considered as the primary reference for efficiency improvement in this field. The challenge is to instantaneously adapt the control system to specific parameters of the hydrokinetic turbine so that the maximum available power could be extracted. Moreover, the power control system has to track the turbine behavior, permanently adjusting the load according to water velocity variation. Another important function of the controller is to ensure optimum conditions for turning the rotor again after a complete stop. This can be achieved by load disconnection until proper voltage is reached. For permanent magnet synchronous generators, the only method applicable for achieving power regulation is the load adjustment. The main objective of this paper is to design a power converter of the hydrokinetic power system in order to extract the maximum power from the flowing water. When the water velocity is high enough, a net positive power is produced by the hydrokinetic system. For certain characteristics of the hydrokinetic turbine, there is just one optimal operation point producing the maximum power for each tip speed ratio; the optimal operation point is determined by the water velocity and shaft rotational speed. Since the water velocity cannot be controlled, only the rotor speed can be adjusted to achieve the maximum power by changing the load. Therefore, the appropriate power conversion algorithm and rotor speed controller are the key components with significant impact on system efficiency [20]. For wind and water turbines, similar MPPT algorithms can be used. Different types of control systems can be implemented, depending on the required application, aiming to follow as closely as possible the ideal curve of the electric generator for extracting the maximum power. The variation of the power curves at different water velocities for a shrouded hydrokinetic turbine tested in real operating conditions during previous research [21] is shown in Figure 1. The investigated rotor had four Gottingen 450 blades with a diameter of 0.5 m.  
The parameters of the rotor and electric generator of this hydrokinetic turbine were used as input data for the MPPT algorithm. The novelty of the research is the implementation within the MPPT system of a DC/DC buck converter that controls the voltage applied to a certain load. This improvement is necessary for ensuring an adequate voltage in case the system is provided with a battery that requires a narrow range of voltage applied to the battery management system (BMS). For battery charging, the constant current-constant voltage (CC-CV) method was used. This method involves charging the battery at a constant current until the voltage reaches a certain threshold (depending on the battery type);then, the voltage is maintained constant and the current is adjusted until the battery is fully charged. Furthermore, the DC/DC buck converter allows for precise control of the voltage applied to the load; this aspect is very important when charging a battery because an increased voltage can damage it. The paper is structured in three main sections. The first one describes the design of the MPPT algorithm and also the design of the two converters. To test the algorithm, we developed mathematical models for the converters, which were used in a simulation to test the validity of the control system. In the second section, the results of the simulation under different operating conditions are presented and discussed.