Hydropower

Modern usage

There are several forms of water power currently in use or development. Some are purely mechanical but many primarily generate electricity. Broad categories include:

  • Waterwheels, used for hundreds of years to power mills and machinery
  • Hydroelectricity, usually referring to hydroelectric dams, or run-of-the-river setups (eg hydroelectric-powered watermills)
  • Damless hydro, which captures the kinetic energy in rivers, streams and oceans
  • Vortex power, which creates vortices which can then be tapped for energy
  • Tidal power, which captures energy from the tides in horizontal direction
  • Tidal stream power, which does the same vertically
  • Wave power, which uses the energy in waves
  • Osmotic power, which channels river water into a container separated from sea water by a semipermeable membrane.
  • Marine current power which captures the kinetic energy from marine currents.
  • Ocean thermal energy conversion which exploits the temperature difference between deep and shallow waters.

Hydroelectric power now supplies about 715,000 megawatts or 19% of world electricity[2]. Large dams are still being designed. The world’s largest is the Three Gorges Dam on the third longest river in the world, the Yangtze River. Apart from a few countries with an abundance of hydro power, this energy source is normally applied to peak load demand, because it is readily stopped and started. It also provides a high-capacity, low-cost means of energy storage, known as “pumped storage“.

Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2.

Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.

The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.

Not all hydroelectric power requires a dam; a run-of-river project only uses part of the stream flow and is a characteristic of small hydropower projects. A developing technology example is the Gorlov helical turbine.

Hydro-powered electricity, however is not without its drawbacks. Dam failures can be very hazardous, e.g. the Banqiao Dam, which killed 171,000. Also, rivers move silt, and therefore dams fill with silt, and eventually become unable to store enough water to provide water and power in dry weather. [6]

In addition to the significant threat that dams pose to fish populations and the ecosystems of rivers and streams, hydropower can negatively impact both the flow and quality of water. Lower levels of oxygen in the water can present a threat to animal and plant life [7]. However, these issues can be addressed if fish ladders are put in place to ensuThisre safe passage around the area, and the water is aerated on a regular basis to maintain adequate oxygen levels safe for animal and plant life [7]. The flow of water should be monitored closely to prevent the ecological dangers associated with over-stressing bodies of water. These dangers can easily be avoided by shutting down pumping operations temporarily to allow balance to return to damaged ecosystems

http://en.wikipedia.org/wiki/Hydropower

Hydroelectricity is electricity generated by hydropower, i.e., the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants. Worldwide, an installed capacity of 777 GWe supplied 2998 TWh of hydroelectricity in 2006.[1] This was approximately 20% of the world’s electricity, and accounted for about 88% of electricity from renewable sources.

Greenhouse gas emissions

Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide (a greenhouse gas). While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation. One measurement of greenhouse gas related and other externality comparison between energy sources can be found in the ExternE project by the Paul Scherrer Institut and the University of Stuttgart which was funded by the European Commission.[6] According to this project, hydroelectricity produces the least amount of greenhouse gases and externality of any energy source[7]. Coming in second place was wind, third was nuclear energy, and fourth was solar photovoltaic[7]. The extremely positive greenhouse gas impact of hydroelectricity is found especially in temperate climates. The above study was for local energy in Europe; presumably similar conditions prevail in North America and Northern Asia, which all see a regular, natural freeze/thaw cycle (with associated seasonal plant decay and regrowth).

Related activities

Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.

ailure Hazard

Dam failures have been some of the largest man-made disasters in history. Also, good design and construction are not an adequate guarantee of safety. Dams are tempting industrial targets for wartime attack, sabotage and terrorism.

For example, the Banqiao Dam failure in Southern China resulted in the deaths of 171,000 people and left millions homeless. Also, the creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963. [9]

Smaller dams and micro hydro facilities create less risk, but can form continuing hazards even after they have been decommissioned. For example, the small Kelly Barnes Dam failed in 1967, causing 39 deaths with the Toccoa Flood, ten years after its power plant was decommissioned in 1957. [10]

Large Power Outages caused by dam failures

Large dams, whilst generally reliable can suffer catastrophic failure to the dam itself, or the connections and substations, leading to extremely large and sudden loss of output, which can plunge an entire network off line, for hours or even months depending on the damage. Hence whilst these are regarded as “firm” or “despatchable” sources, in reality duplication or back up has to be provided. Examples are:

These are very large losses of power; for comparison, the average UK power demand is around 37 GW.

Limited Service Life

Almost all rivers convey silt. Dams on those rivers will retain silt in their catchments, because by slowing the water, and reducing turbulence, the silt will fall to the bottom. Siltation reduces a dam’s water storage so that water from a wet season cannot be stored for use in a dry season. Often at or slightly after that point, the dam becomes uneconomic. Near the end of the siltation, the basins of dams fill to the top of the lowest spillway, and may cause the dam to fail during any season. Some especially poor dams can fail from siltation in as little as 20 years. [11] Larger dams are not immune. For example, the Three Gorges Dam in China has an estimated life that may be as short as 70 years. [12]

Dams’ useful lives can be extended with sediment bypassing, special weirs, and forestation projects to reduce a watershed’s silt production, but at some point most dams become uneconomic to operate. [13]

Environmental damage

Large reservoirs required for the operation of hydroelectric powerplants result in submersion of extensive areas upstream of the dams, destroying biologically rich and productive lowland and riverine valley forests, marshland and grasslands. The loss of land is often exacerbated by the fact that reservoirs cause habitat fragmentation of surrounding areas.

Hydroelectric projects can be disruptive to surrounding aquatic ecosystems both upstream and downstream of the plant site. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed. Salmon spawn are also harmed on their migration to sea when they must pass through turbines. This has led to some areas transporting smolt downstream by barge during parts of the year. In some cases dams have been demolished (for example the Marmot Dam demolished in 2007)[14] because of impact on fish. Turbine and power-plant designs that are easier on aquatic life are an active area of research. Mitigation measures such as fish ladders may be required at new projects or as a condition of re-licensing of existing projects.

Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks.[15] Since turbine gates are often opened intermittently, rapid or even daily fluctuations in river flow are observed. For example, in the Grand Canyon, the daily cyclic flow variation caused by Glen Canyon Dam was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from pre-construction conditions. Depending on the location, water exiting from turbines is typically much warmer than the pre-dam water, which can change aquatic faunal populations, including endangered species, and prevent natural freezing processes from occurring. Some hydroelectric projects also use canals to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. Examples include the Tekapo and Pukaki Rivers in New Zealand.

Greenhouse gas emissions

Bonnington hydroelectric power station, River Clyde, Scotland.

Lower positive impacts are found in the tropical regions, as it has been noted that the reservoirs of power plants in tropical regions may produce substantial amounts of methane and carbon dioxide. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. According to the World Commission on Dams report[16], where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant.[17] Although these emissions represent carbon already in the biosphere, not fossil deposits that had been sequestered from the carbon cycle, there is a greater amount of methane due to anaerobic decay, causing greater damage than would otherwise have occurred had the forest decayed naturally.

The pipes supplying water from the River Clyde to Bonnington hydroelectric power station, Scotland.

In boreal reservoirs of Canada and Northern Europe, however, greenhouse gas emissions are typically only 2% to 8% of any kind of conventional fossil-fuel thermal generation. A new class of underwater logging operation that targets drowned forests can mitigate the effect of forest decay.[18]

In 2007, International Rivers accused hydropower firms for cheating with fake carbon credits under the Clean Development Mechanism (CDM), for hydropower projects already finished or under construction at the moment they applied to join the CDM. These carbon credits – of hydropower projects under the CDM in developing countries – can be sold to companies and governments in rich countries, in order to comply with the Kyoto protocol.[19]

Population relocation

Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction.[20] In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost. Such problems have arisen at the Three Gorges Dam project in China, the Clyde Dam in New Zealand and the Ilısu Dam in Southeastern Turkey.

Affected by flow shortage

Changes in the amount of river flow will correlate with the amount of energy produced by a dam. Lower river flows because of drought, climate change or upstream dams and diversions will reduce the amount of live storage in a reservoir therefore reducing the amount of water that can be used for hydroelectricity. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power.