## 6 Conclusions  
In this paper, a comparative evaluation was carried out on several indicators (the phenomenon of cavitation, investment costs, electricity production and economic benefit) for the same micro-hydropower plant, located in the north of Spain, in an irrigation infrastructure (the Reboluluelo micro-hydropower plant) featuring two types of asynchronous electric generator control system (a fixed-speed control system (scenario 1) and a variable speed control system (scenario 2)). The assessment indicators were analysed for four different years: 2016, 2017, 2021, and 2022. In scenario 1, the year 2016 was chosen as it  
Figure 16: Economic benefits of each scenario.  
was the best year for electricity production, and the year 2017 was chosen because the operation of the micro-hydropower plant was halted due to the phenomenon of cavitation. The Rebolluelo micro-hydropower plant started operating at variable speed in July 2020. Therefore, in scenario 2 the years 2021 and 2022 were chosen as these are the only years for which all data are available. This micro-hydropower plant uses a semi-Kaplan turbine coupled to an asynchronous electric generator through a gearbox. Older micro-hydropower plants operate at fixed speed. When there ia a lack of rainfall, irrigation infrastructure prioritises the use of water for crops, which leads to these micro-hydropower plants operating outside their design flow and, thus, to various problems (such as cavitation, reduced turbine efficiency, and reduced operating hours). In summary, the following conclusions can be drawn:  
1. The use of variable-speed technology eliminated the possibility of cavitation. It allowed operation in a speed range of 584 rpm (when the flow rate is high) to 233 rpm (when the flow rate is low);
2. Evidently, the use of variable-speed technology increased the total cost of the Rebolluelo micro-hydropower plant. The micro-hydropower plant cost ratio is 11.96%;
3. The use of variable speed technology significantly increased the power output ratio. Using 2016 as a basis for comparison, the power output ratio increased by 93.03% and 35.87% in 2021 and 2022, respectively;
4. The use of variable-speed technology significantly increased the economic benefit. Using 2016 as a basis for comparison, the economic benefit ratio increased by 108.72% and 29.31% in 2021 and 2022, respectively.  
This work shows the superiority of variable-speed technology in micro-hydropower plants for three of the four indicators assessed. Although the cost is higher, the rest of the indicators recommend the implementation of this type of control system in micro-hydropower plants. This work could be used to help make decisions with regard to future micro-hydropower plant installations.  
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 & editing, R.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
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 [34] for its contributions to this paper.