*2.2. Small Hydropower*

The expansion of these installations was due to the development of the Francis turbine (for medium heads) [31], contributing both to the reduction of greenhouse gases and to the establishment of electrical service in remote rural areas or consumption points located away from supply points. This "social contribution" should be taken into account in viability studies in developing countries. As in large hydropower, the leading countries in small hydropower energy are China, Brazil, India, Canada, and some European countries. In 2013, China had a total installed capacity above 80 GW, which supplied more than 650 rural areas [8], with a range of installed power between 0.5 and 10 MW in each plant [32]. Brazil had 397 power plants in operation in 2011, with an installed capacity of 3.5 GW. Currently, the potential capacity is equal to 25.9 GW [33]. The United States Department of Energy tabulated more than a half million sites, with an installed potential of 100 GW [34], representing 10% of the current generation in the United States.

Currently, Australia has 60 small hydropower plants with a total installed capacity of 0.15 GW, which is 10% of the potential capacity. Australian Administration has projected three new plants with an installed capacity of 20, 8.4, and 7 MW, which will be developed in the future [35]. In India, the potential capacity is 15 GW, of which 2.4 GW are currently installed in 674 plants, with an expected increase of 9.4 GW in 2017 [36]. Ushiyama [37] established an installed power capacity of 30 GW for Japan in 2010, where small hydropower was practically non-existent. However, Japan has already started to develop projects in remote areas with a rate of capacity installation greater than 300 MW per year, with an installed power and potential capacity in the near future of 3.52 and 6.82 GW, respectively [38]. Across the African continent, small hydropower is also being developed in rural zones, where these plants are generating significant social benefits with a lower installed capacity of 300 kW [39].

According to the European Small Hydropower Association (ESHA), the total installed capacity in 2005 was 12.4 GW, of which six European Union members (Italy, France, Spain, Germany, Austria, and Sweden) in addition to Switzerland and Norway possess more than 90% of the installed capacity [40]. Alonso-Tristán et al. [41] presented the distribution of the small hydropower installed in Spain using data from Red Eléctrica de España (REE). This country represented 23.1% of the hydropower generation in European small hydropower in 2008 [42]. The potential growth in installed power capacity is 10 GW, with an annual production above 38,000 GWh [30], and the installed power will reach 17.3 GW (an increment of 39.51%) [41] in the period 2005–2020, as depicted in Figure 3.

**Figure 3.** Planned installation of small hydropower capacity in 2005–2020 by European Union countries (adapted from [40]).

Small hydropower has several advantages: less negative impacts than large hydropower and available potential to increase renewable energy production. ESHA estimated an annual reduction of 29 × 10<sup>6</sup> tonnes of CO2 as a result of the 13 GW installed capacity in Europe [43]. Amponsah et al. [44]

analyzed different values of the carbon footprint of small hydropower and established a range between 2 and 74.9 gCO2-e/kWh based on the installation and the type of plant. In the particular case of micro-hydropower, Gallagher et al. [45] analyzed the carbon footprint of three plants with installed capacities of 15, 90 and 140 kW. The results of this analysis were 2.14, 4.39, and 2.78 gCO2-e/kWh, respectively. These values emphasize the positive environmental impact of hydropower solutions.

Regarding economic aspects, Kosnik [34] developed an economic analysis based on several small plants, obtaining a non-linear relationship between the cost of implementation and installed power (small, micro or pico). Ogayar and Vidal [46] also analyzed the distribution of costs for small hydropower, which are distributed among civil work (40%), turbine (30%), electro-mechanical and regulation equipment (22%), and construction managemen<sup>t</sup> (8%). This type of renewable energy project is viable when the required investment is below 2000 \$/kW [47], although special attention should be paid to the environmental and social benefits provided by these installations. At the European level, according to the General Direction for Environment, the average cost of investment for plants with an installed capacity below 10 MW is between 2941 and 4072 €/kW, depending on the characteristics of the system (e.g., flow, head, orography) [40]. Mishra et al. [48] proposed formulas that use the turbine, installed power capacity, and net head to estimate the required investment. These expressions can be used to determine the associated costs.

Finally, the classification of these installations is referenced in European legislation [47] according to the installed power. However, other classifications have been proposed that depend on the type of plant from an operational point of view [16,49,50].


### *2.3. Type of Hydraulic Machines*

Hydraulic machines are classified according to the system (pressurized or open channels) in which they are installed (Figure 4). In open channels, all types of hydraulic wheels have been traditionally used to take advantage of waterfalls. According to the type of energy used (potential, pressure, or kinetic), the machines are classified as gravitational, hydrostatic, or kinetic. Gravitational machines take advantage of different water levels to extract energy from the flow (e.g., Archimedes screw or waterwheel). Hydrostatic machines operate by the difference of hydrostatic pressures on both faces of a blade (e.g., hydrostatic pressure wheel). Finally, if a wheel uses the velocity of the flow to move the axis of the machine, this type of machine is called a kinetic machine. There are many different types of kinetic machines (e.g., helical turbine with vertical or horizontal axis, overshot wheel, and ducted turbine) [51].

In pressurized water systems, the most frequently used machines can be grouped into traditional machines (which are categorized as action and reaction machines) and adapted machines. The last group includes hydraulic machines that normally work not as turbines but as pumps. In reaction machines, the hydraulic power is transmitted to the axis of the machine by varying the pressure flow between the inlet and outlet of the impeller, which depends on the specific speed of the machine (e.g., Francis and Kaplan). In action turbines, the energy exchange (hydraulic to mechanical) is carried out at atmospheric pressure, and the hydraulic power is due to kinetic energy of the flow (e.g., Pelton and Turgo).

**Figure 4.** Classification of hydraulic machines.

These types of machines are used in large and small hydropower, depending on the nominal flow and the available head (Figure 5).

**Figure 5.** Selection of turbine depending on head and flow in: large (**left**); and small (**right**) hydropower [16,31].

Currently, most of the turbines installed for large hydropower are Pelton, Francis, Kaplan, or Deriaz turbines [31]. These turbines present different performance curves, which depend on their specific speed and discharge number [21,52]. Gordon [53] analyzed both the efficiency of 107 turbines that had been installed since 1908 and the increase in efficiency obtained with replacement impellers in 22 power plants, evaluating the improvement in the performance of propellers over time. Increases in the performance of the machines were obtained, rising from efficiencies lower than 50% in 1920 to above 96% in some current cases.

Regarding hydraulic machines installed for small hydropower, Paish [47] established the efficiency of these machines according to the type and head ratio. The efficiency has values of approximately 90% for Pelton turbines over a wide head ratio (0.2–0.8). In crossflow turbines, the values of efficiency are close to 80% for a head ratio (0.2–1). Francis and propeller turbines present efficiencies of approximately 85% for a head ratio (0.9–1).

The development and improvement of large and small hydropower systems have allowed the adaptation of the machines to water distribution networks, establishing the group called "adapted machines" (Figure 4). This group of machines is used in micro and pico hydropower plants. Pump as turbine (PAT) [54], tubular propeller [55], and positive displacement machine [56] are included in this group (Figure 6).

**Figure 6.** Hydraulic machines at IST-Universidade de Lisboa: PAT (**left**); and tubular propeller (**right**).

This group of machines can be installed in places where energy is currently dissipated for specific flow and pressure operating conditions. These conditions mainly depend on user demand and the minimum pressure required (when the machine is installed in a pressurized water network). The existing demand establishes the circulating flow over time in the line, whereas a required pressure establishes the maximum recovered head. In pressurized water networks, the excess of energy is dissipated with pressure reduction valves. In open channel flows, this dissipation of energy is carried out by means of hydraulic jumps. In micro and pico systems, conventional machines can be installed according to installed power and head characteristics (e.g., micro Francis, Pelton, Turgo, and Cross-Flow), but the high investment cost makes the installation not viable.

In 1931, Thoma [54] implemented the first pump working as a turbine (PAT). Later, other authors presented more research that presented the description, operation, performance, and theoretical model of these machines [57–64]. PATs are normally used in pressurized water networks but they can also be used in open channel flows when complementary civil works are carried out to adapt to PAT operating conditions. These machines present a high range of flow and head for installation according to the typology of the machine (Figure 7). The best efficiencies vary between 40% and 70% as a function of the specific speed [58].

**Figure 7.** Range of application of PATs (adapted from [54]).

PATs become the technological solution to efficiently recover energy in water distribution networks. The main advantage of these machines is their immediate availability for installation and lower cost compared with conventional machines. Nourbakhsh and Jahangiri [65] established that the payback period of these machines is less than two years for installed capacities between 5 and 500 kW. These payback values make micro generation in water distribution networks feasible.

Elbatran et al. [64] listed the advantages of these machines in micro hydropower plants, such as a 50% reduction in the cost of the machine compared to a conventional turbine; the existence of a large availability of operating ranges depending on the hydraulic head and flow; simple managemen<sup>t</sup> and operation; and a lifespan of twenty-five years. Furthermore, they have lower installation costs, which can improve the viability of small projects [60].

### **3. Micro and Pico Hydropower Solutions**

### *3.1. Energy Recovery in Open Channel Networks*

Micro hydropower can use different hydraulic heads or diversion schemes in small dams in rivers and ravines, in open irrigation channels and in drainage systems. Some examples of these systems can be found in several regions, such as the western part of the Himalayas [66], Bangladesh [67], Nigeria [68], Laos [69], Europe [41], or Lithuania [70]. Although it might seem that the development of these hydropower plants has been recent, this is not true because watermills were present in all continents many years ago, were fundamental to moving other machines in the Industrial Revolution, and can still be found operating in some countries (e.g., United Kingdom, France, Spain, USA, Africa, and part of Asia) [71,72]. This technology is currently essential to generate electrical energy in rural areas and to support the social and economic development of these isolated regions.

Water wheels can also be used in other open channel flows. For example, these solutions can be located in water treatment plants. These elements can also be installed in urban infrastructure, where energy recovery systems are established to reduce the energy footprint of urban water systems [73]. In these facilities, Ramos et al. [28] proposed the use of an urban storm-water drainage system to take advantage of storm retention ponds and to develop energy recovery systems using the rain storage volume. This solution contributes a new source of clean energy, which is involved with the water drainage system. An example of energy recovery is analyzed by Novara et al. [74] in a wastewater treatment plant in the city of Asti (Italy). The flow in this channel oscillates between 0.07 and 0.83 m3/s, while the available head changes between 0.062 and 0.744 m. With these values, a hydrostatic pressure machine (HPM) is proposed to be installed. This wheel is an experimental waterwheel specifically designed for application in open channels with reduced head, developed and improved by Senior et al. [75], patented by Austrian inventor Adolf Brinnich, and tested under the HYLOW project as part of EU's Seventh Framework Program between 2008 and 2013 [76]. If the energy balance is carried out, the maximum electrical power is approximately 650 W with a flow equal to 0.29 m3/s, for a daily power of 10.9 kWh. In these conditions, 48% of the hydrostatic energy is converted into electrical power, 40% is mechanical loss, and 12% is electrical loss in the generator and transmission.

Similarly, in open channel irrigation systems, energy recovery can also be implemented by installed turbines in small dams or irrigation reservoirs [77,78]. An example of these installations is the analysis made by Butera and Balestra [79], who determined the potential generation by hydropower plants for the Piedmont region (Italy). This region has an installed capacity of 46 MW, of which 45% is pico hydropower, 49% is micro and 6% is small, with an average hydraulic potential of 1.5–2 kW/ha. Tarragó et al. [26] developed a preliminary study in the Alqueva's irrigation system, where twenty-two hydrostatic pressure machines were studied in different locations with hydraulic heads below three meters. Using this assumption, the theoretical energy recovery reached 406.64 MWh/year in 67,932 ha of this region.

### *3.2. Energy Recovery Water Pipe and Irrigation Systems*

Currently, energy recovery in pressurized water distribution networks (both urban or irrigation water supply) has grea<sup>t</sup> significance. Relative to urban supply systems, the energy consumption in water supply networks represents 7% of the world's consumption of energy [80]. Water distribution involves an energy footprint between 0.18 and 0.32 kWh/m3, according to the California Energy Commission [81]. In addition to energy consumption, energy analysis of these networks has shown that an increase of pressure is correlated with increased leakage [82]. This problem justifies the installation of pressure reduction valves (PRVs) in many water distribution networks. These valves reduce pressure and, therefore, leakage volume. This directly proportional correlation between leakage and pressure caused the pioneering study of alternatives to leverage the dissipated energy by PRVs in water supply systems [57]. An unconventional solution was considered: replacing PRVs by PATs [57,59]. Ferracota et al. [60] studied leakage reduction. They presented and integrated a new technical solution with economic and system flexibility benefits, replacing pressure reduction valves by pumps used as turbines (PATs). The optimal operating point of the PATs was selected by using a variable operating strategy. Carraveta et al. [63] established a PAT operating scheme with a PRV in parallel. This operating scheme and the variability of flows over time in network pipelines due to user demand have fostered leading studies to develop variable operating strategies in these machines. These strategies allow the variation of the rotational speed of the hydraulic machine [83,84]. Ferracota et al. [85] have begun studies to improve efficiency prediction in the machine through experimental tests in semi-axial machines when the rotational speed varies. Preliminary studies in drinking water systems have been developed through computational simulations [59,60,86]. These studies considered average flows or hourly uniform patterns in all consumption joints for the development of simulations of water supply networks [87,88]. These energy recovery studies have promoted the use of water supply networks to generate clean energy, using the dissipated energy in PRVs [89]. These studies have resulted in some pilot installations emerging for evaluation (e.g., Murcia (Spain) [90], Portland (Oregon) [91], Hong Kong [89], and Kildare (Ireland) [92]).

In addition to water supply systems, water irrigation networks are very important for the improvement of energy efficiency in the water cycle. Worldwide water consumption is 3925 km3/year [93], which is distributed such that 69.53% of water is used for irrigation, 18.70% is used for industry, and 11.77% is used for drinking water systems. In Spain, water consumption is distributed as follows: 80% for irrigation, 15% for drinking, and 5% for industry. The annual volume used for agriculture equals 16,344 hm<sup>3</sup> [94].

Hence, because the volume of water consumed for irrigation is higher than in urban systems, the modernization of irrigation should not only be associated with high technology and automation but also with water managemen<sup>t</sup> that accounts for the sustainability of this infrastructure. The study of the installation of micro and pico hydropower is necessary because the irrigated surface area is huge (approximately 324 million hectares in the world are provided with irrigation installations, of which 86% are gravity irrigation, 11% sprinkler irrigation and 3% drip irrigation [95]). In Spanish economic terms, the irrigation water distribution cost was €1285 million in 2012. This value represents 20% of the total cost of the water supply service in Spain [96], considering that the irrigated surface area in Spain is 3.54 million hectares (1.09% of the worldwide irrigated surface area) [97].

Therefore, if the annual volume of water consumed in irrigation networks worldwide is measured, the development of systems to reduce the energy consumption is of the utmost importance. These new solutions should also try to improve, as much as possible, the environmental and economic sustainability of irrigation, considering that the modernization of irrigation water systems introduces an average increase of installed power equal to 2 kW/ha [98].

#### *3.3. Strategies for Sustainability and Energy Efficiency in Pressurized Water Networks*
