# Improving the Energy Conversion Efficiency for Hydrokinetic Turbines Using MPPT Controller
## 1 Introduction  
River-based hydrokinetic turbines represent a clean and ecological way to harness the water energy of streams and could have a significant contribution in the future to the renewable energy sector due to their potential in a variety of river crossing areas around the world. However, hydrokinetic turbines are still in the early stage of development and various optimized solutions can be identified in the literature and in specific projects. The main drawback of such devices is their sensitivity to water flow rates and rotational speed variations because the rotor is placed directly into the water current. In the field of hydrokinetic turbines, some improvements were made recently on the rotor design, including blade optimization and the use of various shroud shapes that aimed to increase the power coefficients. Different studies and research have focused on identifying the most suitable technical solutions and operating conditions that lead to an increased power extraction. The authors in [1] numerically investigated a patented new concept of 1 kW ducted isokinetic turbine with improved axial velocity given by two joined concentric runners. Some studies [2] have assessed the blade chord length influence on the efficiency of a shrouded horizontal axis hydrokinetic turbine, while others [3] have focused on identifying a power prediction method for a given velocity and a range of hydrokinetic turbines by using experimental data for different rotors with similar geometry. A power prediction curve is obtained by adapting the power coefficient variation with tip speed ratio. Other studies have analyzed experimentally [4] or both numerically and experimentally [5] the development of ducted/shroudedturbines, also called diffuser-augmented turbines, aiming to assess the increased power output. In [6], the operation range of a kinetic turbine (marine currents turbine) was experimentally investigated in order to avoid cavitation. By performing tests under different hydrodynamic conditions in a cavitation tunnel and a towing tank, the authors determined the power and thrust characteristics for a range of rpm, flow speed, pitch angles, and immersion depth for the operation of single and twin rotors. The best performance of the turbine was obtained for tip speed ratios between 5 and 7 and hub pitch angles of approximately 20\({}^{\circ}\). Besides assessing the interference between rotors and areas of cavitation inception, the research also identified that a reduced immersion depth leads to a low extracted power. Since harnessing the kinetic energy of water is the subject of numerous studies that analyze different scientific, technological, and economic aspects related to kinetic turbines use, reviews focusing on the current development and challenges of this technology have been published [7], as well as study cases for certain countries [8; 9] or areas [10], characterized by specific available flow velocities. For example, the steps for achieving a kinetic turbine suitable for ensuring the power of remote communities are presented in [10], giving details regarding turbine sizing, blade design, and construction. An electronic power converter and control system can also be used to increase the power output. The majority of solutions aiming maximum power point tracking (MPPT) optimization are focused on wind turbines and photovoltaic (PV) power plants due to their spread and accessibility. Micro-hydropower plants can also benefit from the optimization systems mentioned in the literature [11; 12; 13; 14; 15; 16; 17; 18; 19]. The controllers used for this purpose can be classified into three main control methods, namely, tip speed ratio (TSR) control, power signal feedback (PSF) control, and hill-climb search (HCS) control. The proposed model for MPPT algorithm uses a hill-climb search (HCS) method based on perturbation and observation (P&O) of the self-regulating system, implemented in the simulation.