3.3.1. Pumped Water Systems

Pumped water systems have been analyzed by different authors [99–103] whose main objective has been to minimize the energy costs. Rodriguez-Diaz et al. [99] proposed a new methodology with energy savings between 10% and 30% in real case studies, considering the most critical consumption points, which depend on needs and location. Moreno et al. [100] developed a methodology in which characteristic and efficiency curves are optimized depending on the recorded flows, obtaining a 32.33% average reduction of installed power in the studied networks.

In other research, energy reduction has been carried out using strategies to minimize energy consumption through optimal operating schedules, reducing energy footprints by 36.4% [101,102]. Costa et al. [103] presented a general optimization routine integrated with EPANET [104]. This routine allows the determination of strategic optimal rules of operation for any type of water distribution system. Cabrera et al. [105] developed a methodology to carry out an energy audit, which detects weaknesses in pressurized water networks. This methodology is applied in a real case, obtaining energy savings above 40%. In all of the cited cases, energy savings correspond to an economic reduction between 35% and 50% of the energy costs. Ferracota et al. [60] integrated a new technical solution with economic and system flexibility benefits, which replaces pressure reduction valves by pumps as turbines. In the majority of methods, when energy optimization is carried out in pumped water systems, the objective is easily defined as minimizing the energy consumption, with the solution being the establishment of optimized irrigation schedules according to the minimum necessary pressure and irrigation needs at each consumption point.

### 3.3.2. Gravity Water Systems

If an irrigation network is a gravity system, the best solution is not to convert an on-demand water irrigation into a scheduled network because this decision can irritate farmers, and reduce their operational freedom. Therefore, if a water manager wants to increase the system sustainability and energy efficiency of a network by the installation of energy recovery systems, the manager should know the flow distribution over time. The analysis of water distribution systems allows the establishment of some crucial aspects of the recovery system, such as the type of hydraulic machine, the best efficiency point (BEP) and the range of operating conditions for the energy converters [64].

Preliminary values of recoverable energy have been obtained using average circulating flows for both irrigation [26] and water supply networks [106] in some studies. The authors have used daily patterns in these studies. To analyze the variation of flows over time, Pérez-Sánchez et al. [107] developed a new methodology to estimate the hourly circulating flows in any line based on the opening probability of the irrigation points, which depends on farmers' habits (i.e., irrigation duration, maximum days between irrigation, weekly irrigation trend, and irrigation start). This methodology can also be used in water supply networks when behavioral patterns are known. These demand patterns allow the water network to be simulated and the energy balance to be calculated to determine the percentage of energy dissipated by friction losses, the energy necessary for irrigation, the non-recoverable energy, and the theoretically available energy. In the case of Vallada (Spain) [107], the energy dissipated by friction is approximately 4.10% of the provided energy (with a maximum energy footprint of 2.85 kWh/m3), with the theoretical recoverable energy in the network equal to 68.70%, when all of the irrigation points are considered elective places of recovery.

The feasibility of an energy project is not guaranteed when a high number of machines is installed in the network; thus, an analysis of the water network is needed to maximize the energy recovery. Samora et al. [108] developed a methodology that uses simulated annealing to maximize recovered energy in a water supply network [109]. This methodology selects the lines depending on the recovered energy, and considering the feasibility of the facilities according to an economic criterion. For these preliminary studies of feasibility, Castro [16] proposed a simple economic balance where the payback period is only determined through the investment cost, incomes and maintenance cost, which depend on the installed power. In an advanced or existing project, other more complex and detailed methods can be used, which consider the annual interest and the inflation rate [110,111].

Therefore, if the feasibility is studied, knowledge of the performance and head curves as functions of the flow in the selected machine is necessary to determine the real recovered energy. The proposed PAT curves by Rawal and Kshirsagar [112] and Singh [113] (Figure 8) can be used in the analysis to help select a PAT. These curves allow the impellers' diameter to be selected as a function of the specific rotational speed (*ns*), the discharge number (φ), and the head number (ψ). These parameters are defined by Equations (1)–(3) [24]:

$$ms = N \frac{\sqrt{P\_R}}{H\_R^{1.25}} \tag{1}$$

$$\Phi = \frac{Q}{ND^3} \text{ (discharge number)} \tag{2}$$

$$
\Psi = \frac{H}{N^2 D^2} \text{ (head number)}\tag{3}
$$

where *N* is the rotational speed (rpm); *PR* is the rated power (kW); *R* is the pump design point or the best efficiency condition; *HR* is the rated head (m w.c.); *Q* is the circulating flow (m3/s); *H* is the recovered head of the machine (m w.c.); and *D* is the impeller diameter (m).

If the previous premises are used, different studies have been developed in water systems (irrigation and drinking networks), considering strategies to maximize the energy efficiency. In the particular case study of Vallada (Spain) [107], where the annual water consumption is 930,000 m3/year, the actual recovered energy is 26.51 MWh/year. This recovery represents 9.55% of the energy provided to the network, with a simple payback period of 5.28. In a preliminary study of Alqueva (Portugal) [26], the theoretical energy recovery is at least 2.12 MWh/year in 68 ha of pressurized irrigation, for a water consumption of 179,000 m3/year.

**Figure 8.** Head number depending on the discharge number (adapted from [112,113]).

In water supply systems, a case study of Lausanne (Switzerland) [106] finds that the real recovered energy represents up to 5% of the available energy. In a case study of Fribourg in the same country, the recovered energy reaches 10% of the available energy [114]. Another energy study developed in collaboration with the Consortium of Commons for the Monferrato Aqueduct (Italy) [74] determines an energy recovery equal to 9585 kWh/year when the pressure reducing valve is replaced by a PAT with a constant flow of 7 L/s and head of 75 m w.c. These examples show the importance of this type of solution for economic and environmental sustainability in water systems, if similar solutions are implemented.

From the economic point of view, the benefits of selling energy and generating income can be quite significant in some cases (although this generation is irregular over time because it depends on the flow, which varies as a function of consumption in water networks). Some particular analyses of these systems (and, more specifically, of PATs) present payback periods less than five years, with an installed capacity between 5 and 500 kW [8]. However, the importance of these solutions consists in the generation of energy for self-consumption by the local communities, i.e., for extracting water from their own water wells, electric supply in irrigation communities, or individual use at the irrigation points, avoiding investment in the electric grid.

From the environmental point of view, the use of these renewable energy sources reduces the emission of greenhouse gases when they are compared with non-renewable energy (e.g., fuel, usually used in electric generators installed in irrigation communities or irrigation points). Therefore, these recovery systems can supply the users' demand for low energy consumption in their facilities. Regarding the environmental added value, the theoretical reduction of CO2 emission is 216.2 t/year in Vallada's network.

The development of studies to install energy recovery in pressurized water networks (mainly in supply systems) has been important, as previously discussed. The development of these studies has focused on the use of non-conventional hydraulic machines to generate energy and on energy analyses using different installation schemes for these machines. Table 1 summarizes the current development of small energy recovery in pressurized water supply networks.


**Table 1.** Analyzed topics related to recovery systems in water supply networks.

Table 1 shows different topics related to PATs that have been studied by different authors. The description, operating mode, characteristic curves (theoretical and experimental), and simulations (for hourly uniform patterns in all consumption joints) of PATs have been developed, enumerating the advantages (such as good efficiency values and low price) and limitations. The main limitations

are lower efficiency when the system operates with oscillating flows and the irregular generation of energy due to variable flow, which hinders both sale and self-consumption.

The development of different research is necessary to solve the previously cited limitations. Future work should focus on obtaining better knowledge of recorded flows over time in any line; improving the variable operating strategies to adapt the rotational speed of machines in each time step; and developing sustainable and feasible electric systems (grid connection or stand-alone operation). These electric systems will increase the viability of selling the energy to the national grid or using it for self-consumption.
