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: Hydropower: A Low-Carbon Power Source

Anund Killingtveit

Introduction and Summary

Hydropower is a renewable energy source where electrical energy is derived from the potential energy of water moving from higher to lower elevations. Hydropower is probably the most efficient technology for production of renewable energy, with an efficiency of 90% or more, “water-to-wire”. Hydropower projects can be grouped into four main types: Run-of-River (ROR), Storage (Reservoir-based), Piunped- Storage (PSH) and In-stream (Hydrokinetic). Hydropower projects are always site-specific, tailored for an optimal use of available head and water resources. Hydropower stations can be found in the range of less than 1 kW up to more than 20 GW (20000 MW). In fact, 9 out of the 10 largest power plants in the world are hydropower plants!

Energy from hydropower today fulfils about 16% of the global electricity demand, and the potential is still not frilly utilized. It has been estimated that only around 25-30% of the global hydropower resources have yet been developed, and that most of the remaining economic potential probably could be fully developed by 2050. This could increase capacity and energy production to more than two times the current level. Hydropower is the largest source of renewable energy in the electricity sector, with a share of 62% of the renewables, nearly twice all other renewables combined.

Hydropower can be very cost competitive and has traditionally been the only renewable technology that could produce electricity at equal or lower cost, compared to thermal energy sources, like coal, oil or gas. typically in the range of 2-10 U.S. ceut/kWh.

Hydropower offers significant potential for carbon emission reductions due to generally low greenhouse gas (GHG) emissions compared both to other renewables and thermal power plants.

In addition to its role in energy generation, hydropower often has a vital role by supporting grid stability and security of supply, energy storage and frequency and voltage control. Hydropower reservoirs can store large quantities of energy and, in the future, is expected to have an increasingly important role in balancing other more variable renewable energy sources, like wind and solar power, which carmot store energy.

Hydropower systems can also provide important water management sen-ices, like flood control, navigation, water supply and irrigation. Many reservoirs are designed for multi-purpose use, where income from energy generation pays most of the investment and operation costs, while other uses may benefit from water storage and regulated release of water.

Hydropower projects can har e a substantial impact on flow regime in rivers, and may lead to negative and positive impact on ecosystem and humans. Therefore, both for existing and future projects, environmental and social impacts need to be carefully assessed, evaluated and mitigated as far as possible.

On a global basis, the potential for further hydropower development is significant. The remaining economic resource potential is large, > 5000 TWli, and in the near and medium future, resource depletion is not likely to restrict hydropower development. Since only 25% of the technical potential and less than 50% of the economic potential has been developed, the prospects for further development are good. If these TWh's are replacing electricity from coal fired plants, the total reduction of GHG emissions could amount to 5000 Million tons of CO, each year.

Hydropower History and Status Today

Historical development

The first hydroelectric power plant was installed in a country house in Cragside, Rothbury, England, in 1870. Industrial use of hydropower started a few years later, for example in Grand Rapids, Michigan (1880) and at Niagara Falls, New York (1881). Hie world’s first hydroelectric power station with a capacity of 12.5 kW was commissioned on 30 September 1882 on Fox River at the Vulcan Street Plant, Appleton, Wisconsin, USA. lighting two paper mills and a residence (IPCC, 2011). Early hydropower plants were much more reliable and efficient than the fossil fuel-fired plants and this resulted in a rapid deployment of small- to medium-scale hydropower stations distributed where there was an adequate supply of water and a need for electricity. By 1886, there were 45 hydroelectric power plants in operation in the USA. Many other countries followed suit, such as Norway, where the first hydropower plant began operation in 1885 in the town of Skien. Only a few years later, in 1891, the town of Hammerfest, far north of the Arctic circle, had electric streetlights supplied from a municipal hydropower system and in 1897 India saw its first hydroelectric power station of 130 kW, supplying a small town.

The share of hydropower in the global energy mix has been around 16-17% for many decades, and is expected to remain high even though strong growth in other renewables, in particular wind and solar PV, may lead to a somewhat lower share in the coming decades. The technical potential for increased hydropower generation is large enough to meet substantial further deployment both in the medium (2030) and long term (2050).

World hydropower generation has increased by more than a factor of 6 over the years from 1960 (690 TWh) to 2018 (4239 TWh), see Figure 1. The first 35 years, from 1960 to 1994, there was a steady increase of 50 TWh/year. Then, during the next 10 years, the growth slowed down to less than 20 TWh/year.

The slowdown in the decade around 2000 has been explained to be a result of an increasingly negative view on hydropower due to its environmental impacts, and to changes in funding policy for large infrastructure and energy projects. Then, from 2004 onwards, the growth increased again, now at around 100 TWh/year. mostly as a result of a very rapid expansion of hydr opower in China and some other developing countries.

The strong upswing from 2004 can be explained by the increasing concern about climate change and the political determination to move from carbon-based technologies to renewable sources. Another important driver may have been the efforts led by the International Hydropower Association (IHA) and a multi-stakeholder range of partners in promoting greater sustainability thr ough the development and use of “sustainability guidelines” (IHA, 2019).

The fast growth in hydropower seen since 2004 also continued during the last years, up to 4239 TWh in 2018 (IEA, 2018). Last year, from 2017 to 2018, the growth in hydropower (129 TWh) was nearly the

Increase in world hydropower generation from I960 to 201S (Ritchie and Roser, 2019; ША, 2018)

Figure 1. Increase in world hydropower generation from I960 to 201S (Ritchie and Roser, 2019; ША, 2018).

Growth in electricity generation 2017 to 2018 by technology (ША, 2018)

Figure 2. Growth in electricity generation 2017 to 2018 by technology (ША, 2018).

same as that of wind (132 TWh) and solar PV (136 TWh), see Figure 2. Unfortunately, the strong growth in renewables was still not enough to meet total demand increase (900 TWh) and generation from fossile fuel powered plants increased even more than for renewables, by 258 TWh for coal and 236 TWh for gas, according to IEA (IEA, 2018).

Status today

Hydropower is today a major source of electricity, with an annual generation of 4239 TWh (2018), representing almost 16% of the global electricity generation. This means that more than 1 billion people covered their electricity consumption from hydropower. Hydropower generation in six different world regions in 2016 can be seen in Table 1. Most recent regional data are still from 2016. Hydropower is the third largest source of electricity generation, behind coal (37.9%) and natural gas (22.8%) but well ahead of nuclear (10.2%), wind (4.6%) and solar (2.1%), see Figure 3. It is worth noticing that hydropower generation is still more than twice that of wind and solar power combined.

Today, hydropower is produced in 159 countries, the top 10 hydropower producing countries in the world in 2016 are listed in Table 2. Four leading countries: China, Canada, Brazil and the USA, together produced 2253 TWh, more than half of the world total. China alone produced ahnost twice as much

Table 1. Hydropower generation and capacity in six different regions (IHA, 2017; IRENA, 201SA).


Hydropower generation 2016 (TWli/year)

Hydropower capacity *) 2016 (GW)

Capacity factor (Average)

North America




South America
















Australasia Oceania








*) Not including Pumped Storage.

Share of fuel source for world electricity generation (IEA, 201S; Enerdata, 2018)

Figure 3. Share of fuel source for world electricity generation (IEA, 201S; Enerdata, 2018).

Table 2. Generation in the 10 top hydropower countries in 2016 (Enerdata, 2018; IRENA, 201SA).


Total generation (2016)

Renewable (RES)


Hydro % share of








































































Share of total electricity generation fi'om hydropower in 2016 (Enerdata, 2018; IRENA, 201SA)

Figure 4. Share of total electricity generation fi'om hydropower in 2016 (Enerdata, 2018; IRENA, 201SA).

hydropower as the whole of Europe, and is still increasing its hydropower generation capacity rapidly. Nearly 20% of all electricity generation in China now comes from hydropower. The table illustrates the high share of hydropower among renewables in these countries, from 46% to nearly 100% and 75% on average. The hydropower share of total generation varies from only 6.8% (USA) to 96% (Norway), on average 18.6%, see Figure 4.

It may come as a suiprise, but almost all the largest power plants in the world are based on hydropower. Figure 5 shows the capacity and typical annual generation in the 10 world’s largest power plants (by capacity). Nine out of these are hydropower plants. The only non-hydro is the nuclear plant Kashiwazaki-Kariwa in Japan. This station was, however, taken out of operation after the Fukushima disaster and has not been operational since, so generation is not shown.

Capacity and annual generation in the 10 largest power plants in the world (Wikipedia, 2019)

Figure 5. Capacity and annual generation in the 10 largest power plants in the world (Wikipedia, 2019).

Main Components of Hydropower System

Structure of a hydropower plant

A hydropower plant includes civil structures to store, divert and transport water, and mechanical and electrical components in order to convert energy to mechanical and electrical energy. A hydropower system typically consists of the following main components:

  • - Storage reservoir(s) (Not for Run-of-River plants)
  • - Intake in the reservoir or in the river
  • - Wateiways to collect and transport water to the powerhouse
  • - Powerhouse with mechanical and electrical equipment
  • - Wateiways to transport water back from powerhouse to outlet in downstream water body.

The main technical components are illustrated on Figure 6:

  • - Dam creating a reservoir or intake pond
  • - Intake with gates for control of water release
  • - Wateiway (“head race”) to transport water to the power plant (tunnel, canal, pipe...)
  • - Surge tank for control of pressure variations and to eliminate water hammer
  • - Penstock (Except instream and ultra-low head projects)
  • - Powerhouse housing electro mechanical equipment
  • - Turbine (“Prime mover”)
  • - Generator
  • - Control equipment for monitoring and operation
  • - Transformer
  • - Power lines
  • - Wateiway (“tail race”) to transport water from the powerplant to downstream water body.
Structure and mam components of a high-head storage hydropower plant

Figure 6. Structure and mam components of a high-head storage hydropower plant.

In addition, there will usually be other important components, such as gates and valves, trash racks, ventilation system, drainage system, governor, power cables, switchyards, etc.

A hydropower plant is normally tailored to optimize the utilization of available water and head. All the components shown in Figure 6 may not be needed for all types of hydro projects, for example reservoir, surge tank, tunnel or penstock.

There are two basic types of turbine, reaction and impulse, that can be used in order to maximize efficiency and reliability, depending on head and water flow. The most common types are Peltou (impulse) and Francis turbine (reaction) for high and medium head situations, and Kaplan and Propeller turbines (reaction) for lower head systems. The efficiency of a turbine varies very much depending on relative discharge (% of frill load), therefore it is very important to be able to run at or as close as possible to the “best-point” for most of the time. For a mn-of-river plant with large variation in inflow, this may be difficult if only one turbine is installed. Two or more turbines will allow operation at better efficiency over a wider range of flow, but also increase the cost of construction.

Classification of hydropower projects

Flydropower projects can be found over a continuum in scale, from very small units < 1 kW up to megaprojects like China's Three Gorges with 22.5 GW installed capacity (22500000 kW) and an annual generation of close to 100 TWh year. Hydropower projects are usually classified into four main types, depending on their purpose and technical solutions:

  • - Storage hydro—with a reservoir
  • - Ruu-of-river (RoR) hydro—without a reservoir
  • - Pumped storage hydro (PSH)
  • - Instream/hydrokinetic hydro

Hydropower projects are also often classified according to size (pico, micro, mini, small, medium, large) or by head (low, medium, high). Classification according to size is administratively simple, but, to some degree, is arbitrary and used for simplifying the procedure of fiscal support and taxation. Concepts like ‘small hydro’ or Targe hydro' are not very useful as indicators of impacts, economics or other characteristics. There is yet no consensus on how to classify by size, different definitions are used in different countries (IPCC, 2011).

Run-of-river (RoR) hydropower plants

An RoR hydropower plant is a plant where little or no water storage is provided, it generates electricity from the available flow of the river at any given time. Such plants may sometimes include a short-term storage or “pondage”, giving from a few horns up to daily flexibility in adapting generation to the demand profile. The generation profile will, to a large degree, be determined by the natural river flow conditions, or by the release profile from upstream storage if it is part of a cascade. In the absence of any pondage or upstream reservoirs, generation depends entirely on flow and typically may have substantial daily, seasonal and year-to-year variations.

Storage hydropower plants

Storage hydropower plants include resenoir(s) to impound water, which can be stored and released later when needed. Water stored in reservoirs provides flexibility to generate electricity on demand, reducing dependency on the variability of inflow. Large reservoirs can store inflow for months or even years, but are usually designed for seasonal storage, to store water duiing wet seasons and supply water during dry seasons. With the ability to control water flow's, storage reservoirs are often also used in multipurpose projects, providing additional benefits like flood control, water supply, irrigation, navigation and recreation.

Pumped-Storage Hydropower plants (PSH)

In a PSH. water can be pumped from a lower reservoir into an upper reservoir when energy demand is low and released back from the upper reservoir through turbines to generate electricity later, when needed. This cycle can happen several times a day. The round-trip efficiency (pumping generation) is high, from 75% to 80%. In this way, excess electricity, for example from wind power, can be stored in a batteiy. Pumped storage currently represents 99% of all on-grid electricity storage in the world. PSH projects will usually not be a net producer, but may have some natural inflow to the upper reservoir, which will increase the generation. Energy stored in a PSH is directly proportional to the water volume stored in the upper reservoir and the elevation difference between reservoirs. Amajor advantage of PSHs is their ability to interact with other variable renewables, such as wind and solar power. PSH installations can store excess energy during periods of high wind or high insolation, and provide backup reserve which is immediately dispatchable duiing periods when the other variable power sources are unavailable.

In-stream (hydrokinetic) hydropower plants

In-stream energy can be derived from the movement (kinetic energy) of water flowing in rivers and canals, or from tidal flow and ocean currents. This technology differs from traditional hydropower plants, which rely on the elevation and pressure difference (head) between the intake and outlet. Hydrokinetic devices are placed directly in the stream of flowing water and energy is extracted from the kinetic energy in the water, like wind turbines in air. Kinetic energy in the flowing water is converted to mechanical energy by a propeller that drives a generator which produces electricity.

Because it is pow'ered by kinetic energy instead of potential energy, it is also known as a ‘zero- head’ turbine. As such, no dams and/or head differential are necessaiy for the operation of this device; the course of a river remains almost in its natural state. However, the efficiency and amount of energy that can be extracted is low', and the cost high. So far, very few' of these powrer plants liai e been put in operational use in rivers and contribution to electricity generation is still insignificant.

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