# Improving the Energy Conversion Efficiency for Hydrokinetic Turbines Using MPPT Controller
## 3 Findings and Interpretation of Results  
The algorithm used the specific parameters of the previously tested turbine system (rotor and electric generator) [21], as shown in Table 1. The simulation was performed for the same water velocities for which the turbine had been tested. Thus, the variation of the water velocity decreasing from 1.33 m/s to 1 m/s and further to 0.83 m/s was considered. This enabled the validation of the simulation with the experiments that had been carried out in [21]. The simulated water velocity variation over time is shown in Figure 7. Various water velocity profiles were considered in order to represent, as realistically as possible, the water flow by adding white noise and a gradual transition from one velocity value to another. \begin{table}
\begin{tabular}{c c c} \hline
**PMSG–Electric Generator** & **Value** & **Unit** \\ \hline Stator resistance Rs & 3.32 & \(\Omega\) \\ Number of pole pairs p & 4 & - \\ Direct inductance Ld & 3.529 & mH \\ Quadrature inductance Lq & 3.529 & mH \\ Synchronous inductance Ls & 3.529 & mH \\ PM peak flux \(\uppsi_{\text{PM}}\) & 0.149 & Wb \\ Moment of inertia J & 0.0008 & kgm\({}^{2}\) \\ \hline
**Hydro-Kinetic Turbine** & & \\ \hline Turbine radius R & 0.25 & m \\ Blade angle \(\beta\) & 0 & degrees \\ \hline
**Boost Converter** & & \\ \hline Boost capacitor C1 & 1000 & \(\upmu\)F \\ Boost inductance L1 & 1 & mH \\ Inductance equivalent resistance \(\tau_{\text{L}}\) & 0.02 & \(\Omega\) \\ On state switch resistance \(R_{\text{em}}\) & 0.086 & \(\Omega\) \\ Diode forward voltage drop \(U_{\text{fd}}\) & 0.6 & V \\ \hline
**Buck Converter** & & \\ \hline On state switch resistance \(R_{\text{em}}\) & 0.00002 & \(\Omega\) \\ Inductance equivalent resistance \(\tau_{\text{L}}\) & 0.00008 & \(\Omega\) \\ Buck inductance L2 & 10 & mH \\ Buck capacitor C2 & 1000 & \(\upmu\)F \\ Diode forward voltage drop \(U_{\text{fd}}\) & 0.6 & V \\ \hline
**Resistive Load** & 10 & \(\Omega\) \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of the components considered for the hydrokinetic turbine system. Figure 7: Water velocity variation over time. The correction \(\Delta D\) applied increases or decreases the duty cycle of the converter. Figure 8 shows the variation of rectified DC power in response to the MPPT correction as the water velocity changed. It can be seen that the power output closely followed the variation of the water velocity, which, in turn, determined the change of the rotational speed of the generator. Analyzing the graphs shown in Figures 1 and 8, we can compare the results of the performed simulation to the experimental results obtained during on-site testing. The results are summarized in Table 2.  
According to Table 2, the simulation predicted a power output of 105 W for a water velocity of 1.33 m/s, 60 W for 1 m/s, and 30 W for 0.83 m/s. The algorithm provided more accurate results for increased water velocities (\(\geq\)1 m/s) because at lower velocities the low voltage output determines system instabilities when trying to ensure a stable voltage output, as seen in Figure 9 (Vdc). This occurs due to the fact that the system has an increased voltage threshold that is necessary to ensure the specification of the load, especially if considering a battery charging system. Because the first DC converter was a boost-type converter, the correction (which was actually the duty cycle of the converter) decreased. Therefore, the output voltage of the converter increased, as shown in Figure 9, along with the increase in the rectifier DC power from Figure 8, as the algorithm tracked the maximum point for the particular water velocity. It was found that the response time to a change in velocity was fast. However, once the maximum point was reached, there were small oscillations in MPPT correction, as expected for this type of algorithm; these oscillations can be reduced by a more precise tuning of the MPPT increment factor and the sampling time of the algorithm.