**1. Hydropower Generation**

Society's energy consumption worldwide has increased by up to 600% over the last century. This increase has been a direct result of population growth since the industrial revolution, in which energy has been provided mainly by fossil fuels. Nevertheless, today and in the near future, renewable energies are expected to be more widely implemented to help maintain sustainable growth and quality of life and, by 2040, to reduce energy consumption down to the 2010 levels [1].

Sustainability must be achieved by using strategies that do not increase the overall carbon footprint, considering all levels of production (macro- and microscale) of the different supplies. These strategies' development has to be univocally linked to new technologies [2]. Special attention must be paid to those new strategies that are related to energy recovery. These new techniques have raised interesting environmental and economic advantages. Therefore, a deep knowledge of the water-energy nexus is crucial for quantifying the potential for energy recovery in any water system [3], and defining performance indicators to evaluate the potential level of energy savings is a key issue for sustainability, environmental, or even managemen<sup>t</sup> solutions [4].

Energy recovery, with the aim of harnessing the power dissipated by valves (in pressurized flow) or hydraulic jumps (in open channels), is becoming of paramount importance in water distribution networks. Recovery will allow the energy footprint of water (i.e., the energy unit cost needed to satisfy each stage of the water cycle: catchment, pumping, treatment, and distribution) to be reduced, even considering that energy generation is not a priority for these systems [5–7], although this recovery contributes to the development of more sustainable systems. This production could also contribute to the exploitation costs reduction in these systems, increasing the feasibility of drinking and irrigation water exploitation.

Among all of the different types of renewable energy (e.g., photovoltaic, solar thermal, tidal, and wind), the hydropower plant stands out for its feasibility. Historically, large installations can be found in dams around the world to take advantage of the potential energy created by different water levels. The most important hydropower plants are located in countries such as China, the United States, Brazil, and Canada. Currently, China has the greatest installed capacity (exceeding 240 GW), with production greater than 800 TWh in 2012 and an average growth of capacity of 20 GW/year [8]. In Brazil and Canada, hydropower plants represent 84% and 56% of the total energy consumption, respectively. The production of this type of energy in these countries reaches 16% of the total consumed energy [9,10]. Figure 1 shows the technical, economic and exploited potential on each continent.

**Figure 1.** Worldwide hydraulic potential (adapted from [11]).

Going further, Spänhoff [12] performed a worldwide projection of the installed capacity of renewable energy for the United States Energy Information Administration. Hydropower has been the largest renewable source of energy in the period 2004–2010, and it will probably have the highest installed capacity in 2035. According to these forecasts, the installed capacity of hydropower will exceed 1400 GW. This installed capacity will be three times higher than that of wind energy and more than fifteen times greater than that of photovoltaic energy (Figure 2). The actual implementation of these renewable energy systems (solar and wind) in 2016 has been lower than the predicted values (Figure 2). The installed solar capacity is only 30% of the predicted capacity; the wind capacity is only 70% of the predicted value, and the hydropower capacity is approximately the value indicated in Figure 2 [13].

In Europe, renewable energy generation has increased by 96.17% in the period 2002–2013, with production equal to 2232.5 TWh in 2013 [14]. In this decade, the power produced by hydropower plants increased only 16.38%, but other sorts of energy (e.g., solar, wind, and biomass) have experienced greater increases. For instance, in Spain, the increase in renewable energy generation was 152% in this period, but the increase was 73% for Spanish hydropower production.

**Figure 2.** Trends for worldwide renewable energy (adapted from [13]).

Considering all of these renewable energy systems, wind energy has increased by 477%, with a total generation of 53.90 TWh/year. Photovoltaic energy has increased by 4300%, producing 22.85 TWh in 2013 [14]. Nevertheless, the growth of hydropower production has been uneven due to the irregularity of rainfall in the Iberian Peninsula, although the trend is upward [15]. In Spain, the Institute for Energy Diversification and Saving estimated that the untapped generation potential of small hydropower is 1000 MW [16]. Therefore, the development of renewable energy has a promising future, if the potential exploitation is considered. This promising development has positive aspects (e.g., lower environmental impact and generation of stable electrical supply) compared to other renewable energies (e.g., intermittent generation, such as solar or wind) [17]. In addition, this type of energy generation can be very important in the development of multipurpose water systems, where generation is another possible water use [18].

In the near future, part of the growth of hydropower production must come from the retrieval of potential energy embedded in water distribution networks. Considering the potential reduction of natural resources due to extensive agriculture or unsustainable water use on a global level, any investment in energy water recovery is crucial. Therefore, the whole water cycle must be included in the process of energy recovery, including both drinking and irrigation systems. This coupled water-energy nexus will allow the consideration of these systems as a new sustainable and efficient source of energy.

In this framework, the state of the art in the traditional field of hydropower (installed capacity) with a more advanced vision of the energy implications in drinking and irrigation systems is presented, considering the possibility of installing energy recovery in water distribution networks. The objective is to show the hydropower potential in water distribution networks. The installation of these systems will help increase energy efficiency. In the particular case of irrigation, improving the efficiency will allow the reduction of exploitation costs, decreasing pressure on the profit margin, as well as the environmental [19] and economic aspects [20].

### **2. Energy Recovery in Water Networks**

Although there is not a consensus at the European level, the accepted demarcation between large and small hydro by the European Commission is whether the installed power capacity exceeds 10 MW [21]. When the installed capacity is below 100 kW, the generation system is called micro hydropower and when the generated power is below 10 kW, it is called pico hydropower.
