## 7 Conclusions  
This paper describes an experimental study of the implementation of three control systems for an Archimedes screw turbine. Several evaluation indicators (electricity production, micro-hydropower plant efficiency, and gearbox fatigue) were analysed for each control system implemented in an asynchronous electrical generator (a maximum power point tracking system, an optimal water level control system, and a water level control system) at the same micro hydropower plant. The Barreda micro-hydropower plant (Spain) was used for the study. This micro-hydropower plant uses an Archimedes screw turbine coupled to an asynchronous electric generator via a gearbox. The results show the advantages of using the optimal water level control and maximum power point tracking systems. In summary, the following conclusions can be drawn:  
1. The optimal water level control system and the maximum power point tracking system achieve similar levels of electrical production. The energy gain obtained is less than 0.5%. In contrast, the water level control system obtains the worst results; in fact, the energy gain is around \(-12\%\) for low flow rates and \(-3\%\) for high flow rates.
2. The efficiency of the micro-hydropower plant is similar with the implementation of the optimal water level control system and the maximum power point tracking system, regardless of the available flow. The average efficiency gain is less than 0.9%. On the contrary, the water level control system obtains the worst results; specifically, the average efficiency gain is around \(-15\%\) for low flows and \(-5\%\) for high flows.
3. The optimal water level control system reduces mechanical stress on the gearbox and therefore allows for a significant reduction in fatigue load rates compared to the maximum power point tracking system. The maximum power point tracking system produces 7.14 times more mechanical stress on the gearbox than the optimal water level control system for low flow rates, and 3.57 times more for high flow rates. When compared to the water level control system, these amounts are 10 times higher and 5 times higher for low flow rates and high flow rates, respectively.  
Figure 14: Time variation in the torque.  
It can be concluded that the optimal water level control system performs better as concerns the three assessment indicators used. The maximum power point tracking system shows similar behaviour to the optimal water level control system with respect to energy generation, but shows worse behaviour with respect to mechanical stress. Finally, the water level control system shows the worst performance of the three control systems analysed in relation to the energy produced.  
Conceptualization, A.B., F.G.-G. and L.B.; Methodology, A.B. and L.B.; Software, R.G.; Validation, A.B., F.G.-G. and L.B.; Investigation, F.G.-G. and R.G.; Data curation, F.G.-G.; Writing--original draft, R.G.; Writing--review and editing, R.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the data protection policy of the company SinFin Energy.  
We wish to thank SinFin Energy [28] for its contributions to this paper.  
The authors declare no conflicts of interest.