# Hydroenergy Harvesting Assessment: The Case Study of Alviela River

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## Abstract

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## 1. Introduction

## 2. Methodology

**Step****1**- consists of identifying the primary purpose of the water supply or collectors system, its main characteristics and the potential locations for energy harvesting, which must combine excessive head, large discharges and sufficient available space to install the powerhouse and the turbine. For the identified locations, available head and discharge data must be collected and should include, at least, three consecutive years representative of the system operation. If necessary, it should be identified, the derived discharge that cannot be turbined. Above all, the construction of MHP cannot compromise the primary purpose of the system, which can be the water supply or the wastewater drainage or treatment.
**Step****2**- focuses on the selection of possible technological solutions based on the available head and discharges range and the technical features of each solution. For this purpose, Figure 2a), which presents performance characteristics of possible solutions, can be used. Usually, the best solutions are: the AST, Kaplan, PAT, Cross-flow and waterwheel turbines for low-heads (1–20 m) [29]; PAT, Francis and Cross-flow turbines for medium heads (20–100 m); and Francis and Pelton turbines for high heads (>100 m). For each solution, the maximum and minimum operating ranges of heads and discharges as well as the efficiency curve versus discharge should be defined. Figure 2b) presents the variation of the turbine efficiency with the maximum discharge percentage for several turbines.
**Step****3**- consists of the simulation of the energy harvesting over one year for each energy recovery solution, considering the head and discharge data and different values of design discharge. The proposed method requires the following input data: the pressure head and discharge data over at least three consecutive years with a time-stamp that can vary between 15 min, in systems highly demand dependent (e.g., networks), and 1–5 days, in systems with high seasonal variation (e.g., transmission systems, storage tanks); the interval of acceptable design discharges; and the discharge range of operation of the turbine $[{Q}_{\mathrm{min}},{Q}_{\mathrm{max}}]$ (Figure 2a).Initially, the discharge probability of occurrence curve is calculated over the assessment period (e.g., three years) based on discharge time history, $Q\left(t\right)$ and used to define the range of design discharges, ${Q}_{d}$. The annual energy harvesting is calculated for each design discharge, ${Q}_{d}$, taking into account the probability of occurrence curve.The turbined discharge at simulated time step is determined considering the following rules: (i) if the available discharge, $Q\left(t\right)$, is higher than the maximum for the considered design discharged, ${Q}_{\mathrm{max}}\left({Q}_{d}\right)$ than the turbine will only operate with ${Q}_{\mathrm{max}}\left({Q}_{d}\right)$ and the remaining discharge will be derived through a bypass; (ii) if the available discharge, $Q\left(t\right)$, is within the operating range of the turbine, $[{Q}_{\mathrm{min}},{Q}_{\mathrm{max}}]$, the turbine will use $Q\left(t\right)$ for energy production; and (iii) if the available discharge is lower than the minimum, the turbine will not operate.The annual harvested energy, E (kWh), for each design discharge, ${Q}_{d}$, is calculated by the time integration of the product of the available power, $\gamma QH\left(Q\right)$, by the respective efficiency for each discharge over one year:$$E\left({Q}_{d}\right)=\frac{\int \gamma QH\left(Q\right)\eta \left({\textstyle \frac{Q}{{Q}_{d}}}\right)\mathrm{dt}}{3600\times 1000}\phantom{\rule{-6.0pt}{0ex}}$$$$P=\frac{\gamma {Q}_{d}{H}_{d}{\eta}_{\mathrm{max}}}{1000}$$The annual volume used by the turbine, the annual harvested energy and the total power are calculated for each design discharge.
**Step****4**- consists of the economic analysis of the project based on the annual harvested energy for each design discharge. Capital costs, operation and maintenance (O&M) costs and gross and net revenues are calculated. Three economic indicators are typically used to evaluate the feasibility of the project: the net present value (NPV), the payback period (PBP) and the internal rate of return (IRR). These are calculated for each design discharge and for each selected turbine. The additional input data to calculate these indicators are: the discount rate, ${t}_{a}$; the project lifetime, n (years); the energy cost unit, $Cu$ (€/kWh); the capital cost (CC); and the annual O&M costs described as a percentage of CC. The CC includes the equipment control, management, civil works and turbine generator setup. The civil works and equipment costs can be estimated by using empirical equations for the initial costs [31] or cost estimations based on similar projects [32]. When only the turbine cost is known, the civil works’ cost can be estimated based on its weight on the total cost of the project and, then, estimated the CC [33]. The CC, net revenue, accumulated revenues over the period of analysis and the referred economic indicators are calculated for each design discharge and for each technological solution. These indicators will contribute to finding the best solution which corresponds to the maximum NPV of the project. Finally, a sensitivity analysis to the main uncertain parameters, such as the capital cost, the O&M costs and the discount rate, should be carried to assess their impact on the final decision.
**Step****5**- consists of the comparison of analysed technological solutions to select the best technical and economical option for energy harvesting in the analysed water system. The main parameters to be compared for each design discharge scenario are harvested energy, CC, PBP, NPV and IRR. The final solution is the one that leads to the highest NPV with an acceptable IRR (>10%) and an adequate payback period ideally lower than 10 years [34]. The project is only feasible, if the NPV is higher than zero, which means that the investment is recovered within the project lifetime. If the NPV is equal to zero, the investment cost is retrieved and the minimum rate of return of capital is achieved, so the profitability of the project is doubtful. If the NPV is negative, the project is financially impractical.It should be highlighted that the project design discharge depends not only on the energy harvested but also on the economic analysis, since higher discharges not only lead to higher revenues but also to higher capital and O&M costs.A computational tool has been implemented in MATLAB for carrying out the described simulations.

## 3. Case Study: Alviela River Source

#### 3.1. Data Collection and Analysis

#### 3.2. Technology Selection

#### 3.3. Energy Harvesting Assessment

#### 3.4. Economic Analysis

#### 3.5. Final Recommendation

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 3.**Alviela river source (

**a**) catchments scheme; (

**b**) view A and (

**c**) view B (1—Weir 1; 2—Weir 2; 3—Pipe; 4—Alviela aqueduct inlet; R1-Upstream reservoir; R2-Downstream reservoir.

**Figure 5.**Results for the AST: (

**a**) operating discharge; and (

**b**) annual harvested and operating time.

**Figure 6.**AST economic analysis results as a function of the design discharge with and without the pipe cost: (

**a**) CC, O&M costs, revenues and NPV; (

**b**) NPV; (

**c**) PBP; (

**d**) IRR.

**Figure 7.**Economic analysis results as a function of the design discharge with and without the pipe cost: (

**a**) Kaplan, (

**b**) Propeller.

Turbine | Q_{d} (m^{3}/s) | P (kW) | E (MWh/year) | Operation (h/year) | V (hm^{3}) | Pipe (k€) | CC (k€) | O&M (k€/year) | Net Revenue (k€/year) | NPV (k€) | PBP (years) | IRR (%) |
---|---|---|---|---|---|---|---|---|---|---|---|---|

AST | 3 | 55 | 282 | 5183 | 55.98 | 50.4 | 160.7 | 0.8 | 23.9 | 137.3 | 8.3 | 13.7 |

Kaplan | 1 | 22 | 160 | 7326 | 26.37 | 0 | 88.3 | 3.5 | 10.5 | 42.6 | 11.1 | 10.2 |

Propeller | 1.5 | 33 | 179 | 5479 | 29.58 | 0 | 99.3 | 2.0 | 13.7 | 71.7 | 9.13 | 12.5 |

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**MDPI and ACS Style**

Oliveira, P.F.G.; Martins, N.M.C.; Fontes, P.; Covas, D.
Hydroenergy Harvesting Assessment: The Case Study of Alviela River. *Water* **2021**, *13*, 1764.
https://doi.org/10.3390/w13131764

**AMA Style**

Oliveira PFG, Martins NMC, Fontes P, Covas D.
Hydroenergy Harvesting Assessment: The Case Study of Alviela River. *Water*. 2021; 13(13):1764.
https://doi.org/10.3390/w13131764

**Chicago/Turabian Style**

Oliveira, Pedro F. G., Nuno M. C. Martins, Pedro Fontes, and Dídia Covas.
2021. "Hydroenergy Harvesting Assessment: The Case Study of Alviela River" *Water* 13, no. 13: 1764.
https://doi.org/10.3390/w13131764