## 4 Assessment Indicators
### Investment Costs  
The cost of installing a micro-hydropower plant varies from one location to another. It depends on several factors, such as the existing infrastructure [41] and the distance to the grid connection point, etc. Generally, for equal performance, the cost of a low head plant is higher than that of a higher head plant [41].  
[PERSON] et al. [42] proposed guidelines for determining the costs of a small hydropower plant. These costs are:  
1. Civil engineering costs. Civil engineering costs are related to the construction and hydro-structural costs of the project. In our study, these costs include the water conveyance system, the water pensstock structure, and the tailrace structure;
2. Electro-mechanical equipment costs. These costs include the turbine, the generator, the gates, the control system, the power substation, and the electrical and mechanical auxiliary equipment;
3. Power transmission line costs. These costs include the electricity transmission line to connect the power substation to the power transmission grid;
4. Indirect costs. These costs mainly include engineering and design (E&D), supervision and administration (S&A), as well as inflation during the construction period (I).  
This procedure has been used in several studies [43; 44].  
Due to the characteristics of our study, we were most interested in estimating the cost of electro-mechanical equipment as all other costs are considered constant. Graphs are available to determine the approximate cost of the electro-mechanical equipment [44]. However, the drawback is that these graphs have not been recently updated [44]. Moreover, as each installation is different, turbine and alternator manufacturers do not provide any cost information.  
To determine the electro-mechanical equipment costs (\(EMEC\)),many studies use equations based on the power output of the turbine (\(P_{l}\)) and the head of the hydraulic turbine (\(H\)) of a small hydropower plant [44; 45]. [PERSON] and [PERSON] propose the following cost function for a semi-Kaplan turbine [46]:  
\[EMEC=a\cdot P_{t}^{b-1}\cdot H^{c} \tag{6}\]  
where \(a\), \(b\), and \(c\) coefficients depend on the geographical, space or time field in which they are used. For the determination of these parameters, in [46] a best-fit analysis (applying logarithms) was carried out. The value of the constants for semi-Kaplan turbines obtained in [46] is:  
\[EMEC=19498\cdot P_{t}^{-0.58338}\cdot H^{-0.113901} \tag{7}\]  
where \(EMEC\) is the electro-mechanical equipment costs (\(\xi\)/kW), \(P_{t}\) is the power output of the turbine (kW), and \(H\) is the head of the hydraulic turbine (m). Equation (7) has been used in several studies [47; 48]. The use of Equation (7) incurs errors within a fluctuation band between 19.52% and \(-\)9.50% [46].  
The equations for the determination of costs refer to a distant period of time, as they are more than 10 years old. Moreover, these costs vary considerably from one location to another. Therefore, the actual costs of the Rebolluelo micro-hydropower plant will be used for this study [34].  
To assess this indicator, the ratio between the micro-hydropower plant cost (\(MHPC\)) of the two technologies was used. The \(MHPCR\) can be calculated as the difference between the costs of using both technologies:  
\[MHPCR=\frac{MHPC_{2}-MHPC_{1}}{MHPC_{1}}\cdot 100 \tag{8}\]  
where subscript 1 represents the Rebolluelo micro-hydropower plant with fixed-speed technology, and subscript 2 represents the Rebolluelo micro-hydropower plant with variable-speed technology.