Hydro power plants capture the energy released by water falling through a turbine and converts this into mechanical power, which drives generators to produce electricity. Today’s hydropower turbines are capable of converting more than 90% of available energy into electricity.
Hydropower is currently the second most used renewable energy source in the world, just behind solid biomass. In terms of electricity production, hydro power is the most important renewable energy source, both in the EU and globally. About 20% of globally supplied electricity is generated by hydropower and in some countries it provides more than 50% of electricity supply.
Approximately 70% of the earth’s surface is covered with water, a resource that has been exploited for many centuries. The first recorded use of water power was a clock, built around 250 BC. Since that time, people have used falling water to provide power for grain and saw mills. The first use of moving water to produce electricity was a waterwheel on the Fox river  in Wisconsin, USA, in 1882, and shortly thereafter the first of many hydro electric power plants at Niagara Falls was completed. Hydro power has played a major role in the expansion of electrical services since then, both in North America and around the world (Oud, 2002). Hydro power plants capture the energy released by water falling through a turbine and converts this into mechanical power, which drives generators to produce electricity. Today’s hydropower turbines are capable of converting more than 90% of available energy into electricity, which is more efficient than any other form of generation (the best fossil fuel power plant is only about 50% efficient) (Bartle, 2002).
The use of hydropower has been characterised by continuous technical development and it is currently the second most used renewable energy source in the world, just behind solid biomass. In terms of electricity production, hydro power is the most important renewable energy source, both in the EU and in the World: in 2004, 90% of renewable electricity in the World was produced with hydro power (IEA, 2006). About 20% of globally supplied electricity is generated by hydropower and in some countries it provides more than 50% of the electricity supply. In Norway 99% of the electricity production is generated by hydropower and New Zealand uses hydropower for 75% of its electricity (WIVC, 2004).
Presently, at least 35,000 large dams exist in the World. The number and size of large dams have increased in recent decades, with most of them in developing countries. Most industrialised countries have either developed larger prospective sites or have excluded them from development due to environmental concerns (Bartle, 2002). An overview of different sizes of hydro power installations is shown in the table below.
|TYPE||Hydro power category description|
|Large hydro||More than 100 MW and usually feeding into a large electricity grid|
|Medium hydro||15 - 100 MW (usually feeding into a grid)|
|Small hydro||1 - 15 MW (usually feeding into a grid)|
|Mini hydro||Above 100 kW and below 1 MW (either stand alone schemes or more often feeding into the grid)|
|Micro hydro||From 5 kW up to 100 kW (usually providing power for a small community or rural industry in remote areas away from the grid)|
|Pico hydro||From a few hundred watts up to 5 kW|
|Source||Egre and Milewski (2002)|
The most common type of hydroelectric power plant is an impoundment facility, which typically uses a dam to store river water in a reservoir (see Figure 1 below). Electricity is produced when water is released from the reservoir and spins a turbine to activate a power generator. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level (US Department of Energy, 2005).
The amount of electricity which can be generated at a hydropower plant depends on two factors: the vertical distance through which the water falls (the ‘head’), and the flow rate, measured as volume of water per unit time. ‘High head’ power plants are the most common and generally utilise a dam to store water at an increased elevation. The dam is also used for providing the stored water during rainy periods and releasing it during dry periods. Sometimes the heads for this type of power plant may be larger than 1000 m. High-head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet electricity demand.
‘Low head’ hydro plants generally have heads of only a few meters or less. These may utilise a low dam or weir to channel water, or no dam and simply use the ‘run of the river’. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power (Oud, 2002).
Pumped hydro requires an upper and a lower storage reservoir with an initial water charge. After that, it only requires topping up. When there is a sudden power demand, water is released from the upper reservoir passed through the turbines to generate electricity, which commands a premium price At night when there is an abundance of cheap electricity the turbines are reversed and become pumps to pump the water from the lower to higher reservoir for use in the next peak in demand (University of Strathclyde Glasgow, 2004).
Large-scale hydropower systems require a very large dam, or series of dams, to store the enormous quantities of water they need. For example, when full, the Kariba dam between Zimbabwe and Zambia holds 160 billion m3 of water. Such dams are often used as a resource for irrigation and fishing as well as supplying the water for power generation (Boyle, 2004).
Figure 1 above shows how the dam keeps the water upstream in a reservoir, or large artificial lake. By opening the gates, the water from the reservoir flows downwards through a penstock and eventually reaches a turbine. The vertical blades of the turbine are connected to a generator located above of it. Each turbine can weight as much as 172 tonnes and turn at a rate of 90 revolutions per minute. The turbine blades turn in unison with a series of magnets inside the generator (see Figure 2). The large magnets rotate past copper coils which produce alternating current and which is converted into a higher-voltage current to be delivered to the grid. The water used flows to the river downstream. There are various types of turbine and their applicability depends mainly on the pressure head available and the design flow for the proposed hydropower installation.
While hydropower turbine manufacturers have incrementally improved turbine technology to improve efficiencies, the basic design concepts have not changed. Improvement can be achieved with respect to:
- Performance and production;
- Environmental issues;
- Innovative operations and maintenance; and
- Advanced hydro turbine development and deployment.
When considering a site for hydropower generation it is essential to explore whether the landscape is suitable for dam building and whether the potential output of a scheme is attractive. No project can proceed unless there is a right to utilise all the land concerned and it is important to find out how contractors will access the different parts of the scheme with the necessary equipment. It is therefore wise to approach the relevant land-owners at an early stage to identify any objections to the proposed scheme and to negotiate access. Since water courses often form property boundaries, the ownership of the banks and existing structures may be complex. Failure to settle this issue at an early stage may result in delays and cost penalties later during a project.
Although large hydropower is considered to have large potential for further deployment, there are some barriers that must be taken into consideration:
- Economic barriers: Large hydropower projects require very high initial investment cost.
- Technical barriers: There is still need for R&D activities with respect to new and better materials, more efficient, cheaper and environmentally-friendly equipment, and better design. Hydropower is only suitable for sites with large volumes of flowing water (Boyle, 2004).
- Regulatory barriers: There is limited legislative support for hydro dams, especially in developing countries, which is mainly due to difficulties related to gaining permission to use water from rivers.
- Environmental barriers: Hydroelectric projects can be disruptive to surrounding aquatic ecosystems as it could degrade the habitat for fish and other aquatic organism, increasing the undesirable growth and spread of algae and aquatic weeds since the reservoir is stagnant compared to a free-flowing river and block upstream and downstream movements of migratory fish. Large-scale hydroelectric dams are more likely to be involved in these environmental issues, as in the case of the Aswan Dam  (Egypt) and the Three Gorges Dam  (China), which have created environmental problems both upstream and downstream (Sternberg, 2008).
Large hydropower remains one of the lowest-cost energy technologies, although environmental constraints, resettlement impacts, and the availability of sites have limited further growth in many countries. IEA (2008) reports a global, technically-feasible hydro-electricity potential of some 14,320 TWh per year. In a speech to the World Congress on Advancing Sustainable Hydropower in Antalya, Turkey, 2007 , International Hydropower Association (IHA) Executive Director Richard Taylor stated that about 6000 TWh per year are considered to be realistic and cost-effective. Presently, 'only' 808 GWe of hydropowercapacity in operation or under construction worldwide. In Africa, Asia and Latin America most large scale hydro power resources have been developed (WEC, 2006). In developing countries, the development of hydropower is now focused more on smaller sites, on refurbishing and upgrading existing hydropower plants, and on retrofitting dams constructed for other purposes. Large and intermediate-scale dams, however, will continue to be very important in developing countries, the former Soviet Union and in some other industrialised nations, such as Canada, which has a long tradition with hydropower (Taylor and Upadhyay, 2005).
Naturally, hydro power is highly dependent on a country’s geography, which implies that in Europe over 80% of hydro capacity is installed in Italy, France, Spain, Germany and Sweden. Asia, especially China, is set to become a leader in hydro-electricity generation. Present developments in Australia and New Zealand are focussing on small hydropower plants. Canada is developing small hydropower as a replacement for expensive diesel generation in remote off-grid communities (Union of Concerned Scientists, 2005).
Canada is the world's largest hydroelectric power producer. In 1999, it generated more than 340 billion kWh of power, or 60% of its electric power. In the USA over 2,000 hydropower plants are operational which produce around half of the country’s renewable energy; in 1999, these plants produced 8% of the total US electrical power (US Department of Energy, 2005). The largest US hydropower plant is the 6,800 MW Grand Coulee power statio n on the Columbia River in Washington State. Completed in 1942, the Grand Coulee today is one of the world’s largest hydropower plants, following the 13.3 GW Itaipu hydroelectric plant  on the Paraná River between Paraguay and Brazil.
In Uganda, currently, only about 15% of existing hydropower potential (300 MW) in Uganda is utilised, and power demand, which is growing at a rate of 8% per year, exceeds available supply. The shortage in generation capacity limits growth in many sectors of the Ugandan economy. The Government has formulated a Hydropower Development Master Plan  to guide the hydropower planning and development process in the country. The plan includes a comprehensive study of all the potential large- and small-scale hydropower schemes in the country and outlines the energy development strategy based on criteria such as power demand forecast, project generation potential, environmental effects, and cost criteria (UNESCO, no date).
By 2020, China aims to produce 200–240 GW of hydroelectricity, which means adding 7–9 GW of new hydropower capacity per year. To meet this goal, China will need to build the equivalent of roughly one Three Gorges dam  every 2 years. There is an enormous potential for hydroelectric power in the upper reaches of the Yangtze River, especially in the section of the mainstream above Yibin and below Yushu, namely the Jinsha River. Flowing for 2,360 km from the eastern Tibetan Plateau to the low-lying Sichuan Basin, with a drop of 3,280 m, the Jinsha River has an enormous potential to supply hydropower — as high as 112.4 GW, of which about 75.120 GW is exploitable (Institute of Geographic Sciences and Natural Resources Research, 2006).
The hydropower capacity in Africa and the Middle East have not yet been widely exploited. Most of the hydroelectric power projects in the Middle East are located in Turkey and Iran. In Africa, the largest installed hydroelectric capacities are in Egypt and Congo (Kinshasa). Although several African countries – including Ivory Coast, Kenya, and Zimbabwe – rely almost exclusively on hydropower for commercial electricity generation, this is mainly because of the absence of an electricity infrastructure in these and many other African countries rather than the presence of an extensive hydroelectric system (Water Institute of South Africa, 2006).
The technology is commercially and technically mature. Innovations in design, equipment and control/instrumentation could improve performance and increase access to export markets, as would systems to mitigate environmental impact. In general, large hydroelectric plants have little difficulty in competing in the market place with conventional generation technologies, because of its relatively low operational costs.
Hydropower has contributed to economic growth through regional development and expansion of industry (International Hydropower Association, 2002). Dams often have multiple tasks, including supplying water for irrigation, industrial production, and residential use, as well as flood prevention and habitat maintenance. In many developing countries, demand for water is increasing and competition for water resources is most likely to intensify (United Nations, 2004).
Hydro-dam large hydropower is a relatively cost-effective and reliable energy technology, which could result in the following socio-economic and environmental beneftis (Boyle, 2004):
- The power source is domestic and secure since it is not subject to foreign supply disruptions, cost fluctuations, and transportation issues.
- It is highly efficient with turbines capable of converting more than 90% of available energy into electricity, which is more efficient than any other form of generation.
- It is climate-friendly and does not produce air pollution or create any toxic by-products.
- It is a long-lasting and robust technology - most turbines have a lifetime more than 40 years, although this could be lengthened with proper maintenance (Astrom, 2006).
- Hydroelectric energy has relatively low operating and maintenance costs.
- Hydroelectric energy technology is a proven technology that offers reliable and flexible operation.
- Hydropower offers a means of responding within seconds to changes in load demand.
- A dam can be a useful resource for leisure, fishing, irrigation or flood control.
However, before selecting an appropriate site, several geologic, social, and environmental factors need to be evaluated. The process of damming a river and creating a reservoir can pose its own environmental, economic, health and social problems, among which are the displacement of floodplain residents and the loss of the most fertile and useful land in a given area.
Hydropower can achieve significant GHG emission reductions as it, depending on the energy mix of the country concerned, could replace fossil based technologies for electricity production. However, in the case of large scale dams emissions of methane from biomass decay in the water reservoirs need to be considered. In addition, as the water in the reservoir is stagnant compared to a free-flowing river, water-borne sediments and nutrients can be trapped, resulting in the undesirable growth and spread of algae and aquatic weeds. In some cases, water spilled from high dams may become supersaturated with nitrogen gas and cause gas-bubble disease in aquatic organisms inhabiting the tailwaters below the hydropower plant.
For calculation of GHG emission reduction of a large hydro power project, it is recommended to apply the Approved Consolidated Methodology ACM0002, which has been developed for grid connected renewable energy production projects under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html 
Hydropower projects involve large up-front investments costs, most of which are related to financing the dam and plant construction. The capital required for large dam hydro plants depends on the effective head, flow rate, geological and geographical features, the equipment (turbines, generators, etc.) and civil engineering works, and whether water flow is constant throughout the year. Moreover, there is a common lead time of approximately four to six years before a plant becomes operational, depending on the geological and geographical features. The capital investment cost varies from minus € 1400/kWh to € 1900/kWh, so that the payback period largely depends on the project case (Idaho National Laboratory, 2005). Apart from the investment and production costs, another principal cost element is operation and maintenance, including repairs and insurance. About € 0.033/kWh of electricity produced by a hydropower plant is needed to finance its operation and about € 0.025/kWh for its maintenance (EUSUSTEL, no date).
Because of the very high initial investment costs, large hydro power production is mainly feasible for big utilities with a large credit capacity . The main project risk for hydro power plants lies in varying electricity prices. Therefore, in countries with stable price agreements, projects are easier to be financed than in countries with volatile energy prices.
Some examples of large hydropower projects in developing countries, including information about costs, are described below:
In 1994, the Asian Development Bank (ADB) approved a loan of USD 60 million to the Lao Government for the 210 MW, USD 280 million Theun-Hinboun hydropower project . The project was co-financed by NORAD, the Norwegian Government’s aid agency. Despite the problems that have arisen during the implementation, ADB considers the Theun-Hinboun a ‘good example’ of how aid and private sector can co-operate in hydropower development in the region (International Rivers Network, 2000).
In Africa, several hydropower projects are either planned or under construction. In 2002, the construction of the 300 MW Tekeze hydroelectric project  in Ethiopia began, which, with an investment of USD 224 million, is the largest African joint venture with China. The China National Water Resources and Hydropower Engineering Corporation built the dam, which will be higher than China’s own Three Gorges Dam. The project will supply both electricity and water for irrigation to large parts of northern Ethiopia.
Mozambique plans to construct the Mepanda Uncua hydroelectric project , which will be located about 45 miles downstream from the existing 3.75 GW Cahora Bassa dam on the Zambezi River (construction started in 2005). The cost of the project has been estimated at USD 1.8 billion and its construction is to be completed in two phases, adding 1.3 GW in the first phase and another 1.1 GW in the second phase (Water Institute of Southern Africa, 2006).
For many nations in Africa, future hydroelectric projects depend on the ability to attract investment dollars. Due to their possible negative environmental and social impacts and the risks of defaulting on agreements, it has become increasingly difficult to find investors for such projects and for the instituations such as the World Bank to provide loans.
[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group ]
Project developers of hydro projects in the CDM pipeline mainly apply the following CDM methdologies:
CDM projects based on hydro represent 27.4% of all CDM projects in the pipeline and, as such, are the most common project type in the pipeline. The geographical distribution of hydro projects is concentrated in Asia, particularly in China.
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