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Hydropower Technologies
Hydropower Technologies Beatrice Wagner, Dipl.-Ing. Institute of Water Management, Hydrology and Hydraulic Engineering, Department of Water, Atmosphere and Environment, University of Natural Resources and Life Sciences, Vienna, Austria Christoph Hauer, Priv.-Doz. Dipl.-Ing. Dr.nat.techn., Institute of Water Management, Hydrology and Hydraulic Engineering, Department of Water, Atmosphere and Environment, University of Natural Resources and Life Sciences, Vienna, Austria Angelika Schoder BA, Dipl.-Ing., Institute of Social Ecology, Faculty of Interdisciplinary Studies, Alpen Adria University, Vienna, Austria Helmut Habersack, Univ.Prof. Dipl.-Ing. Institute of Water Management, Hydrology and Hydraulic Engineering, Department of Water, Atmosphere and Environment, University of Natural Resources and Life Sciences, Vienna, Austria Ó 2017 Elsevier Inc. All rights reserved.
Introduction The basic technology of hydropower follows quite a simple approach that emerged from the mechanical use of falling water and developed over time to a highly complex technology with many different technical and functional facets (IHA/IEA/CHA, 2000). The majority of hydropower plants are conventional facilities, that is, run-of-river plants, storage, and pumped-storage plants; however, in the last few years, new, innovative devices, such as hydrokinetic energy conversion systems (HKEC) transforming energy from the free-flowing water, have also been developed (Laws and Epps, 2016). Today, hydropower is used in 159 countries, and its share of worldwide electricity production amounts to approximately 16%, which corresponds to 85% of the global renewable electricity generation (OECD/IEA, 2014). With an installed capacity of 1212 GW (IHA, 2016) and an annual generation of about 3500 TWh (OECD/IEA, 2012), hydropower represents the largest single renewable energy source worldwide. The great advantage of hydropower is that it is a proven, safe, and reliable technology, mainly providing electricity but also a variety of other services, such as flood control, irrigation, and water supply. Although the energy output basically depends on precipitation and water runoff, the diversity of facility types enables a flexible operation of hydropower facilities. Dependent on the storage capacity, hydropower plants can cover both base load and peak load electricity demands (Crona, 2012). Moreover, hydropower technology is characterized by low operational and maintenance costs, usually ranging between 2 and 5 USD per MWh (IEA/NEA, 2010). However, as with most renewable energy technologies, capital costs are high, and construction periods, especially for large hydropower projects, are comparatively long (OECD/IEA, 2012). Generally, hydropower that is considered to be almost carbon neutral (IPCC, 2012) represents an important energy source in decarbonizing the energy mix in order to meet the political targets to combat climate change worldwide (European Commission, 2016; OECD/IEA, 2012). According to the latest figures, the remaining potential for additional hydropower amounts to 81% of the total global potential (IEA, 2010) and varies considerably between the countries. While hydropower is actively developed in Europe and North America, the most undeveloped technical potential can be located in Africa (92%), Asia (80%), and Latin America (74%) (IJHD, 2010; OECD/IEA, 2012). Corroborated by the political agreements on global greenhouse gas (GHG) reduction targets, ambitious hydropower exploitation plans at regional and national scales have been developed. According to the hydropower industry, a market potential of more than 8700 TWh/year could be reached in 2050 (IJHD, 2010). However, despite its positive effects on economy (e.g., growth, investments, job creation), the intended hydropower deployment is also accompanied by social and environmental impacts (Kaunda et al., 2012). The most relevant social implications are safety and health issues as well as impacts on local populations (e.g., livelihood adaptations and resettlements due to larger-scale hydropower projects) (IPCC, 2012). With respect to ecological impacts, hydropower plants may affect aquatic and riparian river ecology (e.g., fish migration barrier, residual flow, hydropeaking, reservoir flushing) (Fette et al., 2006). This book chapter (i) provides a brief insight into the chronological history of hydropower, starting from simple devices to a modern technology; (ii) gives an overview of the broad spectrum of hydropower types and approaches to classify them; (iii) describes the technology of the basic conventional types of hydropower plants; (iv) presents technological improvements and new hydropower approaches; and (v) summarizes ecological and environmental aspects due to hydropower use, including a lifecycle assessment (LCA) with respect to GHG emissions.
A Concise History of Hydropower The first hydroelectric plantsdturbines or water wheels coupled to an electric generatordcame up in the late 19th century, aided by technological developments of the Industrial Age. However, people had used the energy of flowing water for many centuries before to perform diverse mechanical tasks. Although aiming for a global outlook, the brief historical overview provided in this section is necessarily limited to certain geographic regions, not least because of available research and historical sources. How waterpower could be employed by humankind throughout history was closely linked both to material and biophysical factors (hydrology, topography, technical infrastructure) and sociocultural ones.
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From Mills to a Modern Technology The precise origin of the first primitive devices using waterpower (water lever, noria, horizontal water wheel) is unclear. The more sophisticated vertical undershot wheel probably appeared around 100 BC, possibly spreading from Asia to the Mediterranean regiondbut again, its first emergence and subsequent diffusion are open to speculation. Reliable references to the overshot wheel, which involves a dam and an elevated headrace and allows the energy of the water to be used more efficiently (energy conversion efficiency h [] ¼ 0.5–0.7 compared to h ¼ 0.15–0.3 for the undershot wheel), date back to the 3rd century AD (Reynolds, 1983). Initially mostly used for grinding grain and for pumping, the use of water mills increasingly diversified since Antiquity. In medieval Europe, as well as in parts of Asia and Northern Africa, they were adapted to various industrial processes, for example, sawing timber, hammering ore, fulling textiles, crushing seeds, operating bellows of forges, sharpening tools, and turning lathes. At least from the 16th century onward, theoretical analysis and experimental work in the field of hydraulics lead to an optimized design of water wheels (e.g., iron breast wheel) and weirs and to a more systematic exploitation of stream flows. Industrialization brought about an increasing number of mills in Europe and North America, leading to a considerable impact on the hydrology and sediment regime in some watersheds (cf. “Environmental Aspects section”). Competition for water resources caused that efficiency to become the paramount goal of technological development. Certain overshot wheels (h approaching 0.9) could easily compete with early turbines (cf. “Development of Specific Technologies Linked to Hydropower” section) in this regard, but due to some disadvantages (e.g., size and rotational speed), they were gradually replaced by the latter since the mid-19th century, albeit with regional differences (Reynolds, 1983; Lucas, 2005).
Development of Specific Technologies Linked to Hydropower Hydropower plants being complex technical structures (cf. “Current Hydropower Technologies section”), their emergence was based on certain inventions and technological developments of the 19th and 20th century. The discovery of technical uses for electricity (lighting, mechanical work) was of course an important prerequisite, and in turn, the decentralized location of many hydropower facilities favored the emergence and spread of power grids (Hughes, 1983). Progress in river engineering, tunneling, surveying, and hydrological monitoring were important for hydropower development, as were certain construction machinery and materials. Not least, turbine and dam technology were fundamental for modern hydroelectric plants. Like the water wheel, which can be regarded as its precursor, a water turbine is a prime mover converting the energy of the fluid into mechanical rotational energy that can be used for various ends (e.g., producing electricity). Initially, the major difference was the turbine’s specific configuration of axis, blades, and casing, allowing higher rotational speed and thus a smaller size to process a certain amount of water. A further advantage is the submerged installation (full exploitation of the available head and better protection from ice and debris). The first workable turbine is usually credited to French engineer Fourneyron, who designed it in the 1820s based on previous experimental designs and scientific research on mill technology (cf. “From Mills to a Modern Technology section”). Further important milestones are the turbines developed by Francis (1849), Pelton (patent 1880), and Kaplan (patent 1913), each of them adapted to specific head and flow conditions (cf. “Classification Approaches” section) (Giesecke et al., 2009; Reynolds, 1983). The type of hydropower dam is likewise closely connected with hydrological factors and with the plant type. Barriers to control the flow of waters have existed for millennia in different parts of the world. Before the advent of hydropower, they were mainly used for irrigation, water supply, flood protection, and fluvial transport (the first is globally still their most important purpose). The International Commission on Large Dams (ICOLD) lists a Roman gravity dam located in Spain, dating from 130 AD, as the oldest “large” dam still in existence. Preindustrial dams were built of earth, rock, wood, and masonry; since the late 19th century, new construction materials (concrete, steel) and optimized forms (arch dam, arch-gravity dam) allowed dams to increase tremendously in sizedcf. for example, Hoover Dam (USA), 221 m, completed 1936; Grand Dixence (Switzerland), 285 m, completed 1964; Jinping 1 (China), 305 m, completed 2013 (Blackbourn, 2006; ICOLD, no date).
Hydropower Development in the 20th Century Hydropower potential depends on hydrology and topography and is therefore geographically highly variable. How much of this potential can actually be used in a certain region is closely linked to technological, socioeconomic, political, and cultural factors, for example, financial means, technical know-how, available alternative energy sources, and cultural preferences. Therefore, historic development paths of hydropower differ greatly among countries; see, for example, Fig. 1 for selected European countries. In spite of the importance of site-specific factors, two major global trends affecting hydropower development in the 20th centurydand up to the presentdcan be discerned: “high-modernist” (Scott, 1998) planning of large water projects and the emergence of ecological awareness. Hydropower plants at large rivers initially faced technical and also institutional constraints. From its outset, hydropower competed with other users (e.g., navigation, timber floating, irrigation) for limited water resources. Multipurpose schemesdfrequently planned by state institutions and with huge dimensionsdaimed at a reconciliation of those demands and at the economic development of certain regions. They were built under different political regimes, as part of the United States’ “New Deal” (see e.g., “Tennessee Valley Authority” or “Central Valley Project”) as well as in postwar Soviet Russia. Many large European rivers, like the Rhône and the Danube, were envisaged as “power-waterways,” with a chain of hydropower dams and locks to facilitate navigation. Even some highly utopian
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Fig. 1 Technically exploitable and developed hydropower potential 1938, 1965, and 2010 in selected European countries with high gross surface potential. Data from: Economic Council of Europe. (1968). The hydro-electric potential of Europe’s water resources, Vol. I. New York: United Nations, cited in Myllyntaus, T. (1991). Electrifying Finland. The transfer of a new technology into a late industrialising economy. pp. 298–301, Houndmills, Basingstoke, Hampshire, Helsinki: Macmillan); Eurelectric. (2011). Hydro in Europedpowering renewables. Synopsis report. p. 10, Brussels).
plans (e.g., damming the Mediterranean sea) emerged under this paradigm, while successful schemes became role models for projects in the developing world (Pritchard, 2011). In industrialized countries with highly used potential (i.e., few free-flowing river sections), however, the evident ecological impact of a large number of dams, combined with an increased sensitivity toward environmental issues in general, caused growing public concern over hydropower since the 1970s. Developers reacted with measures to better integrate plants into the landscape, to improve ecological conditions in impoundments and reservoirs, and to facilitate the migration of biota (by installing “fish ladders”). More recently, hydropower development has gained new impetus by the debate around climate change and the need to exploit renewable energies.
Current Hydropower Technologies Hydropower technology follows a functional principle transforming the potential energy resulting from gravitational forces into kinetic energy, utilizing the available height difference between headwater and tailwater, the upper and the lower reservoir, respectively. This energy of the flowing water is converted into mechanical energy by turning the blades of a turbine. Finally, the turbine shaft drives a generator converting the mechanical energy into electrical energy (Giesecke et al., 2009). An important parameter forming the basis of hydropower technology is the electrical power P (kW), which is determined by the volume of discharge available Q (m3/s) and the drop height (the vertical distance the water falls) hf (m) (see Eq. 1). The coefficient cP (kg/s2m2) represents, amongst others, the overall efficiency of the facility and is between approximately 8.0 for small and 8.8 for large hydropower plants. P ¼ cP $Q$hf
(1)
Generally, hydropower facilities are characterized by a high diversity in terms of size, type, and function (OECD/IEA, 2012). However, there are some components that all conventional hydropower plants have in common; (a) a dam or weir to maximize the available head, (b) a penstock/pipe transporting the water to the turbine, (c) a powerhouse that contains the turbine(s), (d) a generator and further technical equipment, and (e) transformers and transmission lines (Sommers, 2004).
Classification Approaches Classification schemes are based on different criteria and vary related to the level of detail. However, when categorizing hydropower schemes, it is evident that quite often, a clear distinction is impeded by close relationships and smooth transitions (Giesecke et al., 2009).
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A commonly used classification distinguishes according to the facility size (measured by the installed capacity in MW) between large and small hydropower plants. However, there is no globally standardized threshold value on the size categories (OECD/IEA, 2012). In China and Canada, for example, the group of small hydropower plants are defined by a capacity of up to 50 MW (Liu et al., 2013), whereas in Austria and Norway, the threshold is set at 10 MW (Gonzalez et al., 2011). In Germany, however, an additional capacity classdmedium hydropower plants (1–100 MW)dhas been established (Giesecke et al., 2009). Furthermore, in some countries, the terms “mini-hydro,” “micro-hydro,” and “pico-hydro” representing very low installed capacities have been implemented. Moreover, a categorization based on technical (structural and river engineering) aspects can be made, generally distinguishing between run-of-river power plants (cf. “Run-of-river plants” section), storage plants, (cf. “Storage plants” section), and pumpedstorage plants (cf. “Pumped-storage plants” section), each type with subcategories according to the structural composition of the facility (e.g., location of the powerhouse). In addition to these main technical hydropower types, special types, such as hydrokinetic energy conversion systems (cf. “Technical Improvements and New Technologies” section), tidal power plants, and wave power plants, can be distinguished (Giesecke et al., 2009; IPCC, 2012), and the last two mentioned types will be described in another chapter of this book. With regard to the hydraulic head, hydropower stations can be classified according to the difference between the headwater (upstream) and the tailwater (downstream) levels. In general, this classification scheme is often applied because a number of additional specific characteristics to a category can be assigned. However, similar to the installed capacity, there exist no common scales, and differences between the countries with respect to the threshold values defining the categories can be determined. The International Energy Agency, for example, differs between low head (drop height < 30 m) and high head technologies (drop height > 300 m) (OECD/IEA, 2012). Another approach described by Giesecke et al. (2009) divides low pressure (drop height < 15 m), medium pressure (drop height 15–50 m) and high pressure plants (drop height > 50 m). The common thread of these approaches is that head/water pressure and discharge define the type of hydraulic turbine to be installed (OECD/IEA, 2012). The most commonly used turbines for low heads and large flows are Kaplan runners, propeller turbines, and bulb turbines. Moreover, low head hydropower plants are mainly river power plants and diversion plants and are located in lowlands. Francis runners are characterized by a wide range of head (20–700 m), small to very large flow conditions, a high hydraulic efficiency (OECD/IEA, 2012), and are commonly used in medium head facilities (diversion, but also river power plants) often situated in the low mountains. Finally, Pelton turbines are dominating in hydropower plants (generally diversion plants) with high heads and small flows, located in the low and high mountain regions (Giesecke et al., 2009). In general, hydraulic turbines can be divided in two categories, impulse and reaction turbines. Impulse turbines (e.g., Pelton turbine, cross-flow turbine), mainly applied at high head technologies, use runners rotated by water jetted onto them at high velocities, whereas reaction turbines (e.g., Francis, Kaplan) use the water flow to generate an upward hydrodynamic force that rotates the runner blades (Loots et al., 2015). According to the operation, a differentiation between isolated and interconnected operation can be carried out. Another classification differs based on the operating mode (e.g., run-of-river operation or peaking operation at river power plants) (Habersack et al., 2011). A further possible distinction considers energy–economic aspects with respect to electricity supply and categorizes depending on the base load, medium load, and peak load capability of a hydropower plant, whereby the type medium load power plant is often omitted, and commonly, only a classification between base load (mostly run-of-river plants) and peak load hydropower power plants (mostly large (pumped-) storage plants) is made. Less frequently, a categorization related to topographical aspects is applied. Here, a differentiation can be made between (a) river power plants in the lower reaches of rivers, (b) hydropower facilities in the low mountains (run-of-river and storage plants), and (c) storage and pumped-storage plants in the high mountains. Finally, a classification can be made regarding the purpose of a hydropower facility. Thus, hydropower plants can be divided in (a) facilities only for electricity production, (b) multipurpose plants, and (c) plants where electricity production is subordinate to other usage. Only 17% of the single-purpose dams worldwide have been constructed solely for hydroelectricity (ICOLD, 2016). Many of the large storage facilities, for example, the Hoover Dam on the border between Nevada and Arizona (USA) or the Atatürk Dam in the Southeastern Anatolia Region of Turkey, are of major importance for the provision of irrigation water and the control of water flows (flood control) in addition to the hydropower function (Bureau of Reclamation, 2016). Other potential multipurpose benefits of hydropower plants may be (a) the supply of water for domestic, municipal, and industrial use; (b) the improvement of the conditions for fishing, navigation, tourism, and recreation (Egré and Milewski, 2002); (c) and the regulation of ground water levels and river sections (Giesecke et al., 2009).
Conventional Hydropower Technologies Run-of-river plants As the name indicates, run-of-river plants make use of the runoff of rivers and take advantage of the height difference of the water level upstream and downstream of a weir. They convert both potential and kinetic energy of the water into hydroelectric energy. In the case of a river power plant, the powerhouse and the weir system are situated directly in the river (instream) and form a single unit. Accordingly, all essential components for power generation are located within the river flow (Fig. 2A) (Habersack et al., 2011). With respect to the constructional designs, a basic distinction between (a) connected structures (e.g., power plants in blocks where
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Fig. 2 Schematic overview of a run-of-river plant: (A) river power plant, (B) diversion plant. Modified from Habersack, H., Wagner, B., Hauer, C. et al. (2011). DSS_KLIM:ENdEntwicklung eines Decision Support Systems zur Beurteilung der Wechselwirkungen zwischen Klimawandel, Energie aus Wasserkraft und O¨kologie. Report, Vienna, Austria.
the essential plant components form one construction unit or bay power plants where the powerhouse is situated lateral to the original river course in an artificial bay), (b) separated structures (e.g., twin construction plants representing a special type of power plants in blocks where the powerhouse is located on both sides of the river or pier/pillar power plants where the machine units are situated in the piers of the weir), and (c) submerged structures where the machine units are located in the same plant component that provides also impoundment and flood relief can be made (Giesecke et al., 2009). Some impressions of different technical designs are given in Fig. 3. In the case of a diversion plant, water is abstracted from the main river channel to be utilized for electricity generation (offstream) and subsequently returned into the river concerned or into a different water body (Fig. 2B). A reduced discharge (residual flow) can be observed in the main channel. For a run-of-river operation, which is generally suitable for providing base load electricity, the storage of the water is limited to the impoundment space or permit. In most cases, a significant storage of water is not possible. Thus, electricity generation depends on the current discharge and can only be adapted to demand to a small extent. In the case of peaking operation (hydropeaking), water is temporarily stored in the upstream impoundment. This operation mode allows the adaptation of discharge rates and is therefore generally suited for daily peak load electricity production or for balancing short-term fluctuations. In addition, other purposes such as flood protection (retention of flood waves) can be fulfilled but only to a very limited extent (Habersack et al., 2011).
Storage plants In the case of storage plants, the height difference between one or more reservoirs with natural inflow in higher altitude and a lowerlying hydropower plant is used. Water flows from the reservoir through pressure tunnels and penstocks to the turbines located in the powerhouse. Storage plants are relatively independent from current dischargedthe usable amount of discharge is stored in the reservoir and can be released in times of high demand (storage operation). Therefore, they are suited for providing peak load electricity and ancillary services (e.g., energy to balance grid fluctuations). According to the storage volume related to inflow, a differentiation between daily (shifting capacity up to 0.5%), weekly (shifting capacity between 0.5% and 2%), and annual reservoirs (shifting capacity over 20%) can be made; seasonal storage would be a further option. In the case of a barrage power plant, the powerhouse is located directly at the downstream side of the dam (Fig. 4A), whereas at diversion plants, the powerhouse and the dam are spatially separated (Fig. 4B) (Habersack et al., 2011). In addition to the aforementioned storage technology, special types such as glacier power plants using the energy potential of glacier meltwater (e.g., Karahnjukar hydropower plant at the Vatnajökull glacier in Island) and hydropower plants with underground storage systems/caverns (e.g., Central Java) exist (Giesecke et al., 2009).
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Fig. 3 Different technical designs of run-of-river plants illustrated by the facilities (A) Iron Gate I/Djerdap I at the Danube River (Serbia), (B) Apalachia Dam at the Tennessee River (USA), (C) Kappelerhof at the Limmat River (Switzerland), and (D) McNary Dam at the Columbia River (USA).
Fig. 4 Schematic overview of a storage plant: (A) barrage power plant, (B) diversion plant. Modified from Habersack, H., Wagner, B., Hauer, C. et al. (2011). DSS_KLIM:ENdEntwicklung eines Decision Support Systems zur Beurteilung der Wechselwirkungen zwischen Klimawandel, Energie aus Wasserkraft und O¨kologie. Report, Vienna, Austria.
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Pumped-storage plants Pumped-storage plants operate with an upper and a lower reservoir. In general, the upper reservoir is (partially) filled with water that is pumped from the lower reservoir; however, pumped-storage plants with natural inflows also exist. Pumped-storage plants are characterized by their temporal flexibility. In times of high electricity demand, water flows from the upper reservoir through pressure tunnels and penstocks to the powerhouse and subsequently into the lower reservoir. In turn, during periods of low electricity demand (mostly at night), the water of the lower reservoir is again pumped into the upper reservoir (pumped-storage operation). Thus, in contrast to “normal” storage plants (cf. “Storage plants” section), pumped-storage plants can also use electricity from the grid to pump the water into the upper reservoir. This technology enables the storage of quickly available electricity generation for peak load periods or for providing ancillary services. Therefore, pumped-storage plants are considered to be an important hydropower technology for balancing electricity supply and demand and serve as “storage batteries” for more volatile renewable energy sources (e.g., wind and solar power). Analogous to storage plants, a classification according to the storage volume and the location of the powerhouse (barrage power plant; Fig. 5A resp. diversion plant; Fig. 5B) can be made (Habersack et al., 2011). In Fig. 6, some practical applications of storage and pumped-storage plants are illustrated.
Technical Improvements and New Technologies Compared to other renewable energy technologies, hydropower is technically well advanced and has a high efficiency and energy payback ratio. However, hydropower technology is also being continuously developed and improved in order to enhance reliability, efficiency, safety, and environmental aspects. With respect to improvements in turbines, enhancements are addressed to (a) an increased power output and hydraulic efficiency (however, peak efficiencies are already between 90% and 95%), (b) improved availability, (c) lower maintenance costs (e.g., costs due abrasion of technical components by sediments), (d) higher flexibility on the electricity market, and (e) enhanced environmental performance (e.g., fish friendly turbines; screw turbines enabling fish migration). Moreover, continuous improvements in material properties with respect to the weight of the runners; the strength of the technical components; and also the resistance to corrosion, cavitation, and abrasion are achieved (OECD/IEA, 2012). In addition to the technical improvements of conventional hydropower plants, there is a broad range of new, innovative technologies. Gravity water vortex power plants, for example, have a round basin with a central drain where the water forms a vortex that
Fig. 5 Schematic overview of a pumped-storage plant: (A) barrage power plant, (B) diversion plant. Modified from Habersack, H., Wagner, B., Hauer, C. et al. (2011). DSS_KLIM:ENdEntwicklung eines Decision Support Systems zur Beurteilung der Wechselwirkungen zwischen Klimawandel, Energie aus Wasserkraft und O¨kologie. Report, Vienna, Austria.
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Fig. 6 Different technical designs of (pumped-) storage plants illustrated by the facilities (A) Hoover Dam (USA), (B) Listertal Dam (Germany), (C) Zillergru¨ndl Dam (Austria), and (D) Glen Canyon Dam (USA).
drives a water turbine (Yaakob et al., 2014). In matrix power plants, the machine unit consists of several bulb turbines, which are situated in a shut-off device with the advantage that existing weir structures can be used to produce energy (e.g., applied at the Jebel Aulia dam on the White Nile in Sudan) (Andritz Hydro, 2013). Furthermore, a group of hydrokinetic energy conversion systems (HKEC) has been developed. The main difference of these systems from traditional hydropower devices is that the facility works without a dam/impoundment at river sections with small water elevation changes under 1.5 or 2 m (IPCC, 2012) and converts energy from the free-flowing water. Hence, commonly used names for these systems include instream flow turbines, free-flow turbines, and current turbines (Rourke et al., 2010). A general classification of HKEC technologies may divide turbine systems and nonturbine systems (e.g., flutter vane, piezoelectric, vortex induced vibration, oscillating hydrofoil and sails), whereby turbine systems can be further categorized into (i) axial flow turbines with horizontal or inclined axes and (ii) cross-flow turbines with vertical or in-plane axes (Rourke et al., 2010). Generally, the majority of HKECs are in research and deployment stages, and only a few devices are at a (pre-) commercial development stage; however, this technology is currently gaining significant attention since it is not only designed to be used in rivers but also in tidal estuaries, ocean currents, waves, and artificial waterways (Lago et al., 2010). The hydrokinetic technologies of marine wave and tidal power technologies will not be discussed in further detail here; this sector is treated in another chapter.
Environmental Aspects Due to highly diverse site conditions and different types of hydropower plants, the environmental impact of hydropower is variable. However, ecological and social issues related to hydropower are recognized as important topics worldwide.
Ecological and River Morphological Impacts In terms of ecological and river morphological aspects, the main impacts of hydropower are a consequence of the fragmentation of the river systems by dams (Siciliano et al., 2015) affecting hydrological and sediment regimes with regard to quality, quantity, and time and leading to an alteration of the aquatic and riparian ecology and geomorphology (Alldredge and Moore, 2014; Habersack, 2009). In fact, nearly 60% of all major river systems worldwide are fragmented by large dams (Revenga et al., 2000). In general, environmental impacts vary depending on (a) the type of hydropower plant; (b) its size and sequence of dams, reservoirs, and diversions; (c) the operation mode; and (d) the physiographic setting (e.g., climate region, geology) (Burke et al., 2009). The most frequent environmental impacts on rivers by hydropower plants are caused by the barrier effect of dams affecting upstream and downstream fish migration and sediment transport (trapping of sediment in reservoirs) (Habersack, 2009). Moreover, flow regulations, that is, water diversion (residual flow), reservoir flushing, and hydropeaking, may lead to a deterioration
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of the aquatic ecology (Hauer et al., 2013). Fish in diversion reaches (see Fig. 2B) may be reduced in terms of quantity and variety due to altered flow conditions. Potential consequences of reservoir flushing are caused by an increase in suspended sediment concentration (siltation of the tailwater) and a rapid dissolved oxygen decrease with negative effects on water quality, fish, and benthic invertebrates (e.g., substrate clogging) (Crosa et al., 2010). Finally, hydropeaking may cause stranding of aquatic organisms along the changing channel margins, downstream displacement of fish, a disturbance of spawning habitats, and a reduction of biodiversity (Young et al., 2011).
Lifecycle Assessment Hydropower is considered to be an important player in the global transition toward a low-carbon energy system. However, environmental aspects in all stages of the lifecycle of hydropower plants have to be considered. On the basis of an LCA, the environmental performance of hydropower with respect to the carbon footprint can be compared with other technologies (Wall and Passer, 2013). With respect to GHGs, emissions can occur (a) in the construction phase from the production and transportation of construction materials (e.g., concrete) and the operation of work equipment, (b) during operation and maintenance work, and (c) from dismantling of dams (IPCC, 2012). In general, the lifecycle GHG emissions from hydropower generation are estimated to be between 4 and 14 g CO2eq/kWh, and thus are 1–2 orders of magnitude lower than those from nonrenewable energy sources (OECD/IEA, 2012). A comparison of the different hydropower technologies shows that facilities with reservoirs cause significantly more GHG emissions than run-of-river plants (IPCC, 2012; Li and Zhang, 2014). This is mainly caused by decomposition of organic matter from flooded vegetation and soil when the reservoir is filled for the first time. The amount of GHG emissions varies greatly depending on the climate, the type of ecosystem flooded, the shape and depth of the reservoir, and the operation mode (International Rivers, 2016). In numbers, all large dams worldwide emit 104 million metric tonnes of CH4 from reservoir surfaces, including turbines, spillways, and downstream river reaches. That corresponds to at least 4% of human-caused climate change (Lima et al., 2008).
Conclusions The main advantages of hydropower can be summarized as follows. It is a well-developed and cost-competitive renewable energy technology. Due to its broad range of facility types and sizes, flexible use from base to peak load power is possible. In line with the current developments on the electricity market (e.g., growing proportion of solar and wind power), temporally flexible facilities with the possibility to store energy in large reservoirs, such as pumped-storage plants, are especially assumed to become increasingly important. Hence, in 2015, an additional 2.5 GW of pumped storage capacity could be realized worldwide. Moreover, particular focus is on multipurpose hydropower schemes enabling potential synergies between different uses (e.g., electricity production, irrigation, flood retention). In addition, new hydropower technologies (e.g., HKEC, matrix power plants) that have previously received much less attention are gaining ground, but existing facilities are also being upgraded with respect to technical, economic, and environmental improvements. Hydropower plays a significant role in achieving global energy and climate change targets and can contribute to a low-carbon future. In particular, the latest agreements at the climate conference in Paris (2015) on limiting global warming to well below 2 C by reducing emissions of greenhouse gases have a major impact on the hydropower sector and may drive governments and investors toward hydropower expansion, resulting in ambitious deployment plans at the regional and national level. Thus, sustainability of hydropower projects over the whole lifecycle (construction, operation, and dismantling phase) in terms of economic, environmental, and social aspects is becoming increasingly important. In this context, it should be noted that many of the ecological impacts connected with this technology (cf. “Ecological and River Morphological Impacts” section) are not captured in an LCA approach based solely on GHG emissions (cf. “Lifecycle Assessment” section) and nor are social aspects that are associated especially with large hydropower projects (cf. “Introduction” section).
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A survey of the evidence for an industrial revolution in medieval Europe. Technology and Culture 46, 1–30. OECD/IEA (2012) Technology Roadmap. Hydropower. Report of the International Energy Agency, Paris, France, 68. OECD/IEA, 2014. Electricity production from hydropower sources (% of total). Available online: http://data.worldbank.org/indicator/eg.elc.hyro.zs. accessed 19.08.16. Pritchard, S.B., 2011. Confluence. The nature of technology and the remaking of the Rhône. Harvard University Press, Cambridge, MA. Revenga, C., Brunner, J., Henninger, N., Payne, R., Kassem, K., 2000. Pilot analysis of global ecosystems: Freshwater systems. World Resources Institute, Washington, DC. Reynolds, T.S., 1983. Stronger than a hundred men. A history of the vertical water wheel. John Hopkins University Press, Baltimore, London. Rourke, F.O., Boyle, F., Reynolds, A., 2010. Marine current energy devices: Current status and possible future applications in Ireland. 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Jurnal Teknologi 69, 1–7. Young, P.S., Cech, J.J., Thomposon, L.C., 2011. Hydropower-related pulsed-flow impacts on stream fishes: A brief review, conceptual model, knowledge gaps, and research needs. Reviews in Fish Biology and Fisheries 21, 713–731.
Introduction The basic technology of hydropower follows quite a simple approach that emerged from the mechanical use of falling water and developed over time to a highly complex technology with many different technical and functional facets (IHA/IEA/CHA, 2000). The majority of hydropower plants are conventional facilities, that is, run-of-river plants, storage, and pumped-storage plants; however, in the last few years, new, innovative devices, such as hydrokinetic energy conversion systems (HKEC) transforming energy from the free-flowing water, have also been developed (Laws and Epps, 2016). Today, hydropower is used in 159 countries, and its share of worldwide electricity production amounts to approximately 16%, which corresponds to 85% of the global renewable electricity generation (OECD/IEA, 2014). With an installed capacity of 1212 GW (IHA, 2016) and an annual generation of about 3500 TWh (OECD/IEA, 2012), hydropower represents the largest single renewable energy source worldwide. The great advantage of hydropower is that it is a proven, safe, and reliable technology, mainly providing electricity but also a variety of other services, such as flood control, irrigation, and water supply. Although the energy output basically depends on precipitation and water runoff, the diversity of facility types enables a flexible operation of hydropower facilities. Dependent on the storage capacity, hydropower plants can cover both base load and peak load electricity demands (Crona, 2012). Moreover, hydropower technology is characterized by low operational and maintenance costs, usually ranging between 2 and 5 USD per MWh (IEA/NEA, 2010). However, as with most renewable energy technologies, capital costs are high, and construction periods, especially for large hydropower projects, are comparatively long (OECD/IEA, 2012). Generally, hydropower that is considered to be almost carbon neutral (IPCC, 2012) represents an important energy source in decarbonizing the energy mix in order to meet the political targets to combat climate change worldwide (European Commission, 2016; OECD/IEA, 2012). According to the latest figures, the remaining potential for additional hydropower amounts to 81% of the total global potential (IEA, 2010) and varies considerably between the countries. While hydropower is actively developed in Europe and North America, the most undeveloped technical potential can be located in Africa (92%), Asia (80%), and Latin America (74%) (IJHD, 2010; OECD/IEA, 2012). Corroborated by the political agreements on global greenhouse gas (GHG) reduction targets, ambitious hydropower exploitation plans at regional and national scales have been developed. According to the hydropower industry, a market potential of more than 8700 TWh/year could be reached in 2050 (IJHD, 2010). However, despite its positive effects on economy (e.g., growth, investments, job creation), the intended hydropower deployment is also accompanied by social and environmental impacts (Kaunda et al., 2012). The most relevant social implications are safety and health issues as well as impacts on local populations (e.g., livelihood adaptations and resettlements due to larger-scale hydropower projects) (IPCC, 2012). With respect to ecological impacts, hydropower plants may affect aquatic and riparian river ecology (e.g., fish migration barrier, residual flow, hydropeaking, reservoir flushing) (Fette et al., 2006). This book chapter (i) provides a brief insight into the chronological history of hydropower, starting from simple devices to a modern technology; (ii) gives an overview of the broad spectrum of hydropower types and approaches to classify them; (iii) describes the technology of the basic conventional types of hydropower plants; (iv) presents technological improvements and new hydropower approaches; and (v) summarizes ecological and environmental aspects due to hydropower use, including a lifecycle assessment (LCA) with respect to GHG emissions.
A Concise History of Hydropower The first hydroelectric plantsdturbines or water wheels coupled to an electric generatordcame up in the late 19th century, aided by technological developments of the Industrial Age. However, people had used the energy of flowing water for many centuries before to perform diverse mechanical tasks. Although aiming for a global outlook, the brief historical overview provided in this section is necessarily limited to certain geographic regions, not least because of available research and historical sources. How waterpower could be employed by humankind throughout history was closely linked both to material and biophysical factors (hydrology, topography, technical infrastructure) and sociocultural ones.
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From Mills to a Modern Technology The precise origin of the first primitive devices using waterpower (water lever, noria, horizontal water wheel) is unclear. The more sophisticated vertical undershot wheel probably appeared around 100 BC, possibly spreading from Asia to the Mediterranean regiondbut again, its first emergence and subsequent diffusion are open to speculation. Reliable references to the overshot wheel, which involves a dam and an elevated headrace and allows the energy of the water to be used more efficiently (energy conversion efficiency h [] ¼ 0.5–0.7 compared to h ¼ 0.15–0.3 for the undershot wheel), date back to the 3rd century AD (Reynolds, 1983). Initially mostly used for grinding grain and for pumping, the use of water mills increasingly diversified since Antiquity. In medieval Europe, as well as in parts of Asia and Northern Africa, they were adapted to various industrial processes, for example, sawing timber, hammering ore, fulling textiles, crushing seeds, operating bellows of forges, sharpening tools, and turning lathes. At least from the 16th century onward, theoretical analysis and experimental work in the field of hydraulics lead to an optimized design of water wheels (e.g., iron breast wheel) and weirs and to a more systematic exploitation of stream flows. Industrialization brought about an increasing number of mills in Europe and North America, leading to a considerable impact on the hydrology and sediment regime in some watersheds (cf. “Environmental Aspects section”). Competition for water resources caused that efficiency to become the paramount goal of technological development. Certain overshot wheels (h approaching 0.9) could easily compete with early turbines (cf. “Development of Specific Technologies Linked to Hydropower” section) in this regard, but due to some disadvantages (e.g., size and rotational speed), they were gradually replaced by the latter since the mid-19th century, albeit with regional differences (Reynolds, 1983; Lucas, 2005).
Development of Specific Technologies Linked to Hydropower Hydropower plants being complex technical structures (cf. “Current Hydropower Technologies section”), their emergence was based on certain inventions and technological developments of the 19th and 20th century. The discovery of technical uses for electricity (lighting, mechanical work) was of course an important prerequisite, and in turn, the decentralized location of many hydropower facilities favored the emergence and spread of power grids (Hughes, 1983). Progress in river engineering, tunneling, surveying, and hydrological monitoring were important for hydropower development, as were certain construction machinery and materials. Not least, turbine and dam technology were fundamental for modern hydroelectric plants. Like the water wheel, which can be regarded as its precursor, a water turbine is a prime mover converting the energy of the fluid into mechanical rotational energy that can be used for various ends (e.g., producing electricity). Initially, the major difference was the turbine’s specific configuration of axis, blades, and casing, allowing higher rotational speed and thus a smaller size to process a certain amount of water. A further advantage is the submerged installation (full exploitation of the available head and better protection from ice and debris). The first workable turbine is usually credited to French engineer Fourneyron, who designed it in the 1820s based on previous experimental designs and scientific research on mill technology (cf. “From Mills to a Modern Technology section”). Further important milestones are the turbines developed by Francis (1849), Pelton (patent 1880), and Kaplan (patent 1913), each of them adapted to specific head and flow conditions (cf. “Classification Approaches” section) (Giesecke et al., 2009; Reynolds, 1983). The type of hydropower dam is likewise closely connected with hydrological factors and with the plant type. Barriers to control the flow of waters have existed for millennia in different parts of the world. Before the advent of hydropower, they were mainly used for irrigation, water supply, flood protection, and fluvial transport (the first is globally still their most important purpose). The International Commission on Large Dams (ICOLD) lists a Roman gravity dam located in Spain, dating from 130 AD, as the oldest “large” dam still in existence. Preindustrial dams were built of earth, rock, wood, and masonry; since the late 19th century, new construction materials (concrete, steel) and optimized forms (arch dam, arch-gravity dam) allowed dams to increase tremendously in sizedcf. for example, Hoover Dam (USA), 221 m, completed 1936; Grand Dixence (Switzerland), 285 m, completed 1964; Jinping 1 (China), 305 m, completed 2013 (Blackbourn, 2006; ICOLD, no date).
Hydropower Development in the 20th Century Hydropower potential depends on hydrology and topography and is therefore geographically highly variable. How much of this potential can actually be used in a certain region is closely linked to technological, socioeconomic, political, and cultural factors, for example, financial means, technical know-how, available alternative energy sources, and cultural preferences. Therefore, historic development paths of hydropower differ greatly among countries; see, for example, Fig. 1 for selected European countries. In spite of the importance of site-specific factors, two major global trends affecting hydropower development in the 20th centurydand up to the presentdcan be discerned: “high-modernist” (Scott, 1998) planning of large water projects and the emergence of ecological awareness. Hydropower plants at large rivers initially faced technical and also institutional constraints. From its outset, hydropower competed with other users (e.g., navigation, timber floating, irrigation) for limited water resources. Multipurpose schemesdfrequently planned by state institutions and with huge dimensionsdaimed at a reconciliation of those demands and at the economic development of certain regions. They were built under different political regimes, as part of the United States’ “New Deal” (see e.g., “Tennessee Valley Authority” or “Central Valley Project”) as well as in postwar Soviet Russia. Many large European rivers, like the Rhône and the Danube, were envisaged as “power-waterways,” with a chain of hydropower dams and locks to facilitate navigation. Even some highly utopian
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Fig. 1 Technically exploitable and developed hydropower potential 1938, 1965, and 2010 in selected European countries with high gross surface potential. Data from: Economic Council of Europe. (1968). The hydro-electric potential of Europe’s water resources, Vol. I. New York: United Nations, cited in Myllyntaus, T. (1991). Electrifying Finland. The transfer of a new technology into a late industrialising economy. pp. 298–301, Houndmills, Basingstoke, Hampshire, Helsinki: Macmillan); Eurelectric. (2011). Hydro in Europedpowering renewables. Synopsis report. p. 10, Brussels).
plans (e.g., damming the Mediterranean sea) emerged under this paradigm, while successful schemes became role models for projects in the developing world (Pritchard, 2011). In industrialized countries with highly used potential (i.e., few free-flowing river sections), however, the evident ecological impact of a large number of dams, combined with an increased sensitivity toward environmental issues in general, caused growing public concern over hydropower since the 1970s. Developers reacted with measures to better integrate plants into the landscape, to improve ecological conditions in impoundments and reservoirs, and to facilitate the migration of biota (by installing “fish ladders”). More recently, hydropower development has gained new impetus by the debate around climate change and the need to exploit renewable energies.
Current Hydropower Technologies Hydropower technology follows a functional principle transforming the potential energy resulting from gravitational forces into kinetic energy, utilizing the available height difference between headwater and tailwater, the upper and the lower reservoir, respectively. This energy of the flowing water is converted into mechanical energy by turning the blades of a turbine. Finally, the turbine shaft drives a generator converting the mechanical energy into electrical energy (Giesecke et al., 2009). An important parameter forming the basis of hydropower technology is the electrical power P (kW), which is determined by the volume of discharge available Q (m3/s) and the drop height (the vertical distance the water falls) hf (m) (see Eq. 1). The coefficient cP (kg/s2m2) represents, amongst others, the overall efficiency of the facility and is between approximately 8.0 for small and 8.8 for large hydropower plants. P ¼ cP $Q$hf
(1)
Generally, hydropower facilities are characterized by a high diversity in terms of size, type, and function (OECD/IEA, 2012). However, there are some components that all conventional hydropower plants have in common; (a) a dam or weir to maximize the available head, (b) a penstock/pipe transporting the water to the turbine, (c) a powerhouse that contains the turbine(s), (d) a generator and further technical equipment, and (e) transformers and transmission lines (Sommers, 2004).
Classification Approaches Classification schemes are based on different criteria and vary related to the level of detail. However, when categorizing hydropower schemes, it is evident that quite often, a clear distinction is impeded by close relationships and smooth transitions (Giesecke et al., 2009).
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A commonly used classification distinguishes according to the facility size (measured by the installed capacity in MW) between large and small hydropower plants. However, there is no globally standardized threshold value on the size categories (OECD/IEA, 2012). In China and Canada, for example, the group of small hydropower plants are defined by a capacity of up to 50 MW (Liu et al., 2013), whereas in Austria and Norway, the threshold is set at 10 MW (Gonzalez et al., 2011). In Germany, however, an additional capacity classdmedium hydropower plants (1–100 MW)dhas been established (Giesecke et al., 2009). Furthermore, in some countries, the terms “mini-hydro,” “micro-hydro,” and “pico-hydro” representing very low installed capacities have been implemented. Moreover, a categorization based on technical (structural and river engineering) aspects can be made, generally distinguishing between run-of-river power plants (cf. “Run-of-river plants” section), storage plants, (cf. “Storage plants” section), and pumpedstorage plants (cf. “Pumped-storage plants” section), each type with subcategories according to the structural composition of the facility (e.g., location of the powerhouse). In addition to these main technical hydropower types, special types, such as hydrokinetic energy conversion systems (cf. “Technical Improvements and New Technologies” section), tidal power plants, and wave power plants, can be distinguished (Giesecke et al., 2009; IPCC, 2012), and the last two mentioned types will be described in another chapter of this book. With regard to the hydraulic head, hydropower stations can be classified according to the difference between the headwater (upstream) and the tailwater (downstream) levels. In general, this classification scheme is often applied because a number of additional specific characteristics to a category can be assigned. However, similar to the installed capacity, there exist no common scales, and differences between the countries with respect to the threshold values defining the categories can be determined. The International Energy Agency, for example, differs between low head (drop height < 30 m) and high head technologies (drop height > 300 m) (OECD/IEA, 2012). Another approach described by Giesecke et al. (2009) divides low pressure (drop height < 15 m), medium pressure (drop height 15–50 m) and high pressure plants (drop height > 50 m). The common thread of these approaches is that head/water pressure and discharge define the type of hydraulic turbine to be installed (OECD/IEA, 2012). The most commonly used turbines for low heads and large flows are Kaplan runners, propeller turbines, and bulb turbines. Moreover, low head hydropower plants are mainly river power plants and diversion plants and are located in lowlands. Francis runners are characterized by a wide range of head (20–700 m), small to very large flow conditions, a high hydraulic efficiency (OECD/IEA, 2012), and are commonly used in medium head facilities (diversion, but also river power plants) often situated in the low mountains. Finally, Pelton turbines are dominating in hydropower plants (generally diversion plants) with high heads and small flows, located in the low and high mountain regions (Giesecke et al., 2009). In general, hydraulic turbines can be divided in two categories, impulse and reaction turbines. Impulse turbines (e.g., Pelton turbine, cross-flow turbine), mainly applied at high head technologies, use runners rotated by water jetted onto them at high velocities, whereas reaction turbines (e.g., Francis, Kaplan) use the water flow to generate an upward hydrodynamic force that rotates the runner blades (Loots et al., 2015). According to the operation, a differentiation between isolated and interconnected operation can be carried out. Another classification differs based on the operating mode (e.g., run-of-river operation or peaking operation at river power plants) (Habersack et al., 2011). A further possible distinction considers energy–economic aspects with respect to electricity supply and categorizes depending on the base load, medium load, and peak load capability of a hydropower plant, whereby the type medium load power plant is often omitted, and commonly, only a classification between base load (mostly run-of-river plants) and peak load hydropower power plants (mostly large (pumped-) storage plants) is made. Less frequently, a categorization related to topographical aspects is applied. Here, a differentiation can be made between (a) river power plants in the lower reaches of rivers, (b) hydropower facilities in the low mountains (run-of-river and storage plants), and (c) storage and pumped-storage plants in the high mountains. Finally, a classification can be made regarding the purpose of a hydropower facility. Thus, hydropower plants can be divided in (a) facilities only for electricity production, (b) multipurpose plants, and (c) plants where electricity production is subordinate to other usage. Only 17% of the single-purpose dams worldwide have been constructed solely for hydroelectricity (ICOLD, 2016). Many of the large storage facilities, for example, the Hoover Dam on the border between Nevada and Arizona (USA) or the Atatürk Dam in the Southeastern Anatolia Region of Turkey, are of major importance for the provision of irrigation water and the control of water flows (flood control) in addition to the hydropower function (Bureau of Reclamation, 2016). Other potential multipurpose benefits of hydropower plants may be (a) the supply of water for domestic, municipal, and industrial use; (b) the improvement of the conditions for fishing, navigation, tourism, and recreation (Egré and Milewski, 2002); (c) and the regulation of ground water levels and river sections (Giesecke et al., 2009).
Conventional Hydropower Technologies Run-of-river plants As the name indicates, run-of-river plants make use of the runoff of rivers and take advantage of the height difference of the water level upstream and downstream of a weir. They convert both potential and kinetic energy of the water into hydroelectric energy. In the case of a river power plant, the powerhouse and the weir system are situated directly in the river (instream) and form a single unit. Accordingly, all essential components for power generation are located within the river flow (Fig. 2A) (Habersack et al., 2011). With respect to the constructional designs, a basic distinction between (a) connected structures (e.g., power plants in blocks where
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Fig. 2 Schematic overview of a run-of-river plant: (A) river power plant, (B) diversion plant. Modified from Habersack, H., Wagner, B., Hauer, C. et al. (2011). DSS_KLIM:ENdEntwicklung eines Decision Support Systems zur Beurteilung der Wechselwirkungen zwischen Klimawandel, Energie aus Wasserkraft und O¨kologie. Report, Vienna, Austria.
the essential plant components form one construction unit or bay power plants where the powerhouse is situated lateral to the original river course in an artificial bay), (b) separated structures (e.g., twin construction plants representing a special type of power plants in blocks where the powerhouse is located on both sides of the river or pier/pillar power plants where the machine units are situated in the piers of the weir), and (c) submerged structures where the machine units are located in the same plant component that provides also impoundment and flood relief can be made (Giesecke et al., 2009). Some impressions of different technical designs are given in Fig. 3. In the case of a diversion plant, water is abstracted from the main river channel to be utilized for electricity generation (offstream) and subsequently returned into the river concerned or into a different water body (Fig. 2B). A reduced discharge (residual flow) can be observed in the main channel. For a run-of-river operation, which is generally suitable for providing base load electricity, the storage of the water is limited to the impoundment space or permit. In most cases, a significant storage of water is not possible. Thus, electricity generation depends on the current discharge and can only be adapted to demand to a small extent. In the case of peaking operation (hydropeaking), water is temporarily stored in the upstream impoundment. This operation mode allows the adaptation of discharge rates and is therefore generally suited for daily peak load electricity production or for balancing short-term fluctuations. In addition, other purposes such as flood protection (retention of flood waves) can be fulfilled but only to a very limited extent (Habersack et al., 2011).
Storage plants In the case of storage plants, the height difference between one or more reservoirs with natural inflow in higher altitude and a lowerlying hydropower plant is used. Water flows from the reservoir through pressure tunnels and penstocks to the turbines located in the powerhouse. Storage plants are relatively independent from current dischargedthe usable amount of discharge is stored in the reservoir and can be released in times of high demand (storage operation). Therefore, they are suited for providing peak load electricity and ancillary services (e.g., energy to balance grid fluctuations). According to the storage volume related to inflow, a differentiation between daily (shifting capacity up to 0.5%), weekly (shifting capacity between 0.5% and 2%), and annual reservoirs (shifting capacity over 20%) can be made; seasonal storage would be a further option. In the case of a barrage power plant, the powerhouse is located directly at the downstream side of the dam (Fig. 4A), whereas at diversion plants, the powerhouse and the dam are spatially separated (Fig. 4B) (Habersack et al., 2011). In addition to the aforementioned storage technology, special types such as glacier power plants using the energy potential of glacier meltwater (e.g., Karahnjukar hydropower plant at the Vatnajökull glacier in Island) and hydropower plants with underground storage systems/caverns (e.g., Central Java) exist (Giesecke et al., 2009).
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Fig. 3 Different technical designs of run-of-river plants illustrated by the facilities (A) Iron Gate I/Djerdap I at the Danube River (Serbia), (B) Apalachia Dam at the Tennessee River (USA), (C) Kappelerhof at the Limmat River (Switzerland), and (D) McNary Dam at the Columbia River (USA).
Fig. 4 Schematic overview of a storage plant: (A) barrage power plant, (B) diversion plant. Modified from Habersack, H., Wagner, B., Hauer, C. et al. (2011). DSS_KLIM:ENdEntwicklung eines Decision Support Systems zur Beurteilung der Wechselwirkungen zwischen Klimawandel, Energie aus Wasserkraft und O¨kologie. Report, Vienna, Austria.
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Pumped-storage plants Pumped-storage plants operate with an upper and a lower reservoir. In general, the upper reservoir is (partially) filled with water that is pumped from the lower reservoir; however, pumped-storage plants with natural inflows also exist. Pumped-storage plants are characterized by their temporal flexibility. In times of high electricity demand, water flows from the upper reservoir through pressure tunnels and penstocks to the powerhouse and subsequently into the lower reservoir. In turn, during periods of low electricity demand (mostly at night), the water of the lower reservoir is again pumped into the upper reservoir (pumped-storage operation). Thus, in contrast to “normal” storage plants (cf. “Storage plants” section), pumped-storage plants can also use electricity from the grid to pump the water into the upper reservoir. This technology enables the storage of quickly available electricity generation for peak load periods or for providing ancillary services. Therefore, pumped-storage plants are considered to be an important hydropower technology for balancing electricity supply and demand and serve as “storage batteries” for more volatile renewable energy sources (e.g., wind and solar power). Analogous to storage plants, a classification according to the storage volume and the location of the powerhouse (barrage power plant; Fig. 5A resp. diversion plant; Fig. 5B) can be made (Habersack et al., 2011). In Fig. 6, some practical applications of storage and pumped-storage plants are illustrated.
Technical Improvements and New Technologies Compared to other renewable energy technologies, hydropower is technically well advanced and has a high efficiency and energy payback ratio. However, hydropower technology is also being continuously developed and improved in order to enhance reliability, efficiency, safety, and environmental aspects. With respect to improvements in turbines, enhancements are addressed to (a) an increased power output and hydraulic efficiency (however, peak efficiencies are already between 90% and 95%), (b) improved availability, (c) lower maintenance costs (e.g., costs due abrasion of technical components by sediments), (d) higher flexibility on the electricity market, and (e) enhanced environmental performance (e.g., fish friendly turbines; screw turbines enabling fish migration). Moreover, continuous improvements in material properties with respect to the weight of the runners; the strength of the technical components; and also the resistance to corrosion, cavitation, and abrasion are achieved (OECD/IEA, 2012). In addition to the technical improvements of conventional hydropower plants, there is a broad range of new, innovative technologies. Gravity water vortex power plants, for example, have a round basin with a central drain where the water forms a vortex that
Fig. 5 Schematic overview of a pumped-storage plant: (A) barrage power plant, (B) diversion plant. Modified from Habersack, H., Wagner, B., Hauer, C. et al. (2011). DSS_KLIM:ENdEntwicklung eines Decision Support Systems zur Beurteilung der Wechselwirkungen zwischen Klimawandel, Energie aus Wasserkraft und O¨kologie. Report, Vienna, Austria.
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Fig. 6 Different technical designs of (pumped-) storage plants illustrated by the facilities (A) Hoover Dam (USA), (B) Listertal Dam (Germany), (C) Zillergru¨ndl Dam (Austria), and (D) Glen Canyon Dam (USA).
drives a water turbine (Yaakob et al., 2014). In matrix power plants, the machine unit consists of several bulb turbines, which are situated in a shut-off device with the advantage that existing weir structures can be used to produce energy (e.g., applied at the Jebel Aulia dam on the White Nile in Sudan) (Andritz Hydro, 2013). Furthermore, a group of hydrokinetic energy conversion systems (HKEC) has been developed. The main difference of these systems from traditional hydropower devices is that the facility works without a dam/impoundment at river sections with small water elevation changes under 1.5 or 2 m (IPCC, 2012) and converts energy from the free-flowing water. Hence, commonly used names for these systems include instream flow turbines, free-flow turbines, and current turbines (Rourke et al., 2010). A general classification of HKEC technologies may divide turbine systems and nonturbine systems (e.g., flutter vane, piezoelectric, vortex induced vibration, oscillating hydrofoil and sails), whereby turbine systems can be further categorized into (i) axial flow turbines with horizontal or inclined axes and (ii) cross-flow turbines with vertical or in-plane axes (Rourke et al., 2010). Generally, the majority of HKECs are in research and deployment stages, and only a few devices are at a (pre-) commercial development stage; however, this technology is currently gaining significant attention since it is not only designed to be used in rivers but also in tidal estuaries, ocean currents, waves, and artificial waterways (Lago et al., 2010). The hydrokinetic technologies of marine wave and tidal power technologies will not be discussed in further detail here; this sector is treated in another chapter.
Environmental Aspects Due to highly diverse site conditions and different types of hydropower plants, the environmental impact of hydropower is variable. However, ecological and social issues related to hydropower are recognized as important topics worldwide.
Ecological and River Morphological Impacts In terms of ecological and river morphological aspects, the main impacts of hydropower are a consequence of the fragmentation of the river systems by dams (Siciliano et al., 2015) affecting hydrological and sediment regimes with regard to quality, quantity, and time and leading to an alteration of the aquatic and riparian ecology and geomorphology (Alldredge and Moore, 2014; Habersack, 2009). In fact, nearly 60% of all major river systems worldwide are fragmented by large dams (Revenga et al., 2000). In general, environmental impacts vary depending on (a) the type of hydropower plant; (b) its size and sequence of dams, reservoirs, and diversions; (c) the operation mode; and (d) the physiographic setting (e.g., climate region, geology) (Burke et al., 2009). The most frequent environmental impacts on rivers by hydropower plants are caused by the barrier effect of dams affecting upstream and downstream fish migration and sediment transport (trapping of sediment in reservoirs) (Habersack, 2009). Moreover, flow regulations, that is, water diversion (residual flow), reservoir flushing, and hydropeaking, may lead to a deterioration
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of the aquatic ecology (Hauer et al., 2013). Fish in diversion reaches (see Fig. 2B) may be reduced in terms of quantity and variety due to altered flow conditions. Potential consequences of reservoir flushing are caused by an increase in suspended sediment concentration (siltation of the tailwater) and a rapid dissolved oxygen decrease with negative effects on water quality, fish, and benthic invertebrates (e.g., substrate clogging) (Crosa et al., 2010). Finally, hydropeaking may cause stranding of aquatic organisms along the changing channel margins, downstream displacement of fish, a disturbance of spawning habitats, and a reduction of biodiversity (Young et al., 2011).
Lifecycle Assessment Hydropower is considered to be an important player in the global transition toward a low-carbon energy system. However, environmental aspects in all stages of the lifecycle of hydropower plants have to be considered. On the basis of an LCA, the environmental performance of hydropower with respect to the carbon footprint can be compared with other technologies (Wall and Passer, 2013). With respect to GHGs, emissions can occur (a) in the construction phase from the production and transportation of construction materials (e.g., concrete) and the operation of work equipment, (b) during operation and maintenance work, and (c) from dismantling of dams (IPCC, 2012). In general, the lifecycle GHG emissions from hydropower generation are estimated to be between 4 and 14 g CO2eq/kWh, and thus are 1–2 orders of magnitude lower than those from nonrenewable energy sources (OECD/IEA, 2012). A comparison of the different hydropower technologies shows that facilities with reservoirs cause significantly more GHG emissions than run-of-river plants (IPCC, 2012; Li and Zhang, 2014). This is mainly caused by decomposition of organic matter from flooded vegetation and soil when the reservoir is filled for the first time. The amount of GHG emissions varies greatly depending on the climate, the type of ecosystem flooded, the shape and depth of the reservoir, and the operation mode (International Rivers, 2016). In numbers, all large dams worldwide emit 104 million metric tonnes of CH4 from reservoir surfaces, including turbines, spillways, and downstream river reaches. That corresponds to at least 4% of human-caused climate change (Lima et al., 2008).
Conclusions The main advantages of hydropower can be summarized as follows. It is a well-developed and cost-competitive renewable energy technology. Due to its broad range of facility types and sizes, flexible use from base to peak load power is possible. In line with the current developments on the electricity market (e.g., growing proportion of solar and wind power), temporally flexible facilities with the possibility to store energy in large reservoirs, such as pumped-storage plants, are especially assumed to become increasingly important. Hence, in 2015, an additional 2.5 GW of pumped storage capacity could be realized worldwide. Moreover, particular focus is on multipurpose hydropower schemes enabling potential synergies between different uses (e.g., electricity production, irrigation, flood retention). In addition, new hydropower technologies (e.g., HKEC, matrix power plants) that have previously received much less attention are gaining ground, but existing facilities are also being upgraded with respect to technical, economic, and environmental improvements. Hydropower plays a significant role in achieving global energy and climate change targets and can contribute to a low-carbon future. In particular, the latest agreements at the climate conference in Paris (2015) on limiting global warming to well below 2 C by reducing emissions of greenhouse gases have a major impact on the hydropower sector and may drive governments and investors toward hydropower expansion, resulting in ambitious deployment plans at the regional and national level. Thus, sustainability of hydropower projects over the whole lifecycle (construction, operation, and dismantling phase) in terms of economic, environmental, and social aspects is becoming increasingly important. In this context, it should be noted that many of the ecological impacts connected with this technology (cf. “Ecological and River Morphological Impacts” section) are not captured in an LCA approach based solely on GHG emissions (cf. “Lifecycle Assessment” section) and nor are social aspects that are associated especially with large hydropower projects (cf. “Introduction” section).
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