Hydroelectricity

From Environment & Energy Wiki

Contents 1 Description 2 Detailed description 2.1 Alpine types 2.1.1 Safety 2.1.2 Environmental impact 2.2 Major river dams 2.2.1 Safety 2.2.2 Environmental impact 2.3 Small-scale dams 2.3.1 Safety 2.3.2 Environmental impact 2.4 River flow type 2.4.1 Safety 2.4.2 Environmental impact 2.5 Pumped storage type 2.5.1 Safety 2.5.2 Environmental impact 3 Commentary 4 References

Description

Hydroelectricity (HE) is a source of renewable energy. It consists of using water turbines to drive generators or alternators, much as, in the past, water wheels were used to drive mills. There are several types of HE generation. In many countries, HE generation is non-existent or very limited.

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Detailed description

About 19% of global electricity generation is hydroelectric.^[1] It is not known how this is split by type. Note that many dams are used to provide also irrigation or potable water.

Alpine types

This type consists of a dam across a narrow defile at a high altitude. The water is carried to a low altitude generating station through tunnels or pipes, called penstocks. The energy is therefore at high pressure with a comparatively low volume. An outstanding example is the Grande Dixence^[2] in Switzerland, with an altitude difference of 1800 m. The dam itself is the highest in Europe.

Safety

Much concern has been expressed about Alpine type dams, in that if one burst, the results would be catastrophic. It is estimated that if the Grande Dixence burst, the whole of the Bas Valais region would be flooded, up to the defile of Saint Maurice, with a wave of tens of metres in height, destroying the towns of Sion and Martigny, as well as many other villages. Beyond St Maurice, a surge of a few metres would occur along the whole of Lac Léman, as far as Geneva. Such a catastrophe is unlikely to happen because the geological stability of the dam is constantly monitored by radar, laser beams and satellites.^[3] In the unlikely event of danger, the dam would be emptied rapidly but safely, as occurred with the Rawyl dam some 20 years ago. Nevertheless, the danger of the unforeseen does exist and the memory of the Vajont disaster in Italy, in 1963, survives, due to a mountainside falling into the lake. On a smaller scale, the accident at Mattmark, Switzerland, is an illustration of a disaster in that a large piece of glacier broke off and landed on a construction site of a dam, killing 88 workers; if it had happened a few years later, when the dam had filled, the results would have been much worse.

Other than this, the very high pressure of the water at the power station is a source of potential danger. An illustration is the accident that occurred below the Grande Dixence in 2000 when three persons were killed and considerable damage done to a village when a penstock burst.

As a further point, the disastrous earthquake in China in May 2008 has produced unsubstantiated media reports that 400 dams in the mountainous regions of Szechuan are suspected to have been damaged, some being emptied to avoid the risk of collapse.

Environmental impact

Strangely, despite the massive scale of larger Alpine type dams, the environmental impact is relatively small. Generally, the terrain is rocky and above the tree line, so the deep flooded valleys are small in area with relatively little vegetation. Much mammal life is nomadic (chamois, ibex etc.) and there is relatively little bird life that could not avoid rising waters. There may be small microclimate changes due to the mass of water. The construction of the dams themselves involve vast quantities of concrete but the greenhouse gas balance sheet is very favourable, compared with that of fossil-fuel power stations of equivalent capacity, over just a few years.

Major river dams

There are numerous dams of this type, such as the Kariba, Aswan and, above all, the Three Gorges dams, They mostly have a head of 80 to 180 m and rely on the combination of a moderate pressure and high flow to turn the turbines. Their capacity is high, from 1 000 to up to more than 10 000 MWe. They are characterised by having large lakes (reservoirs). They are usually controversial for a number of reasons.

Safety

Most of the major dam disasters have been attributed to major river dams. Fears have been expressed as to the safety of the Three Gorges Dam, not so much because the massive concrete structure is likely to give, but because of the speculative risk of a geological fracture causing the dam to overflow. If this were to happen, the city of Yichang (2 million inhabitants), 20 km downstream would be seriously flooded within minutes. However, many catastrophic failures of this type of dam have occurred. In August 1975, a number of dams failed in China ^[4], with estimates of direct and indirect mortality varying between 150 000 and 250 000. This was due to immense precipitation around an unusual weather pattern. The Malpasset dam at Fréjus in France failed in 1959, with mortality approaching 500, probably because of a geologic fault.^[5]

Environmental impact

Major river dams are, by far, the least "environmentally-friendly" of all the renewable energy sources, on several counts. This is because of the vast size of the reservoirs. As an example, the retention lake of the Three Gorges Dam will be 600 km long and up to 180 m deep (plus the natural river depth, when it is filled. The sheer volume of water is expected to change the local climate. Upstream from the dam, hundreds of km² of rich flood-plain farmland will be lost and many biotopes destroyed. Two million farmers have been relocated to new purpose-built villages at higher altitudes, where the soil is poorer and the climate less suitable for the type of agriculture they know. Downstream from the dam, the flood plains will no longer receive sufficient water to bring down silt to fertilise the soil. Farmers will require to use chemical fertilisers (requiring fossil fuels to manufacture) to make up for this lack. In the gorges themselves, some of the most beautiful scenery will be lost for ever with much cultural material submerged, as well. However, the worst environmental effect will be due to silt. China's largest city, Chongqing, with a population exceeding 30 million, is at the head of the reservoir. Its sewage is largely untreated. A further estimated population of 15 million lives upstream from the dam, in the Yangtze catchment basin. When the basin is filled, all the sewage and natural silt from vegetable matter (including all the submerged vegetation) will no longer be aerobically decomposed in the fast-flowing and oxygenated river but will fall to the bottom of the comparatively stagnant reservoir, where it will undergo anaerobic decomposition. This will produce large volumes of methane, a powerful greenhouse gas. Although not substantiated, some have estimated that the climate change effect resulting from this methane may exceed that if the same quantity of electricity were generated from fossil fuels. Over a number of years, the accumulated silt will reduce the dam capacity and may even block the water flow. A further problem is that many species of edible fish in the Yangtze migrate upstream to spawn. It is expected that the fish population will diminish, possibly with extinction of some species. The famous Yangtze freshwater dolphin or Baiji ^[6] is already quasi-extinct (maybe 2 or 3 specimens still alive), but this dam will give the species its definitive coup-de-grâce.

These problems have already been seen with other large dams. The Aswan dam has silted up dramatically and the downstream agriculture has radically changed with the desert advancing towards the Nile where artificial irrigation is not carried out. No one old enough to remember will forget the extreme measures to save the wildlife behind the Kariba dam. It is almost certain that such dams have already caused extinctions of species but no one knows the extent of this, often because extensive biotope studies are not completed before the flooding.

Small-scale dams

This is an arbitrary classification, but they are those dams with a limited height (say, <80 m) and width (say, <250 m) and holding back a reservoir of smaller dimensions (say, <50 km²). Because of their small size, numerous construction techniques may be employed. Generally, their energy capacity is between 1 and 500 MWe.

Safety

Research has not revealed the frequency of small dam failures, but it would seem likely that several fail per year throughout the world. These do not always cause loss of life, although they often do but not necessarily on a large scale, so that they pass unnoticed by the media. The USA is particularly concerned by this problem and the Federal Emergency Management Agency (FEMA) state that of the 79,500 dams in the country, about one-third of them 'pose a "high" or "significant" hazard to life and property if failure occurs.'^[7] Note: most of these dams are used for purposes other than generating electricity, but the problems are similar.

The risk of dam failure is a combination of the construction type and materials, the risk of abnormal weather (e.g., flash floods), landslides into the reservoir, seismic activity etc. As a general rule, properly designed concrete dams present a minimal hazard. However, many dams are constructed from compressed earth, possibly with a core of boulders and poorly designed concrete dams may not be firmly implanted into bedrock. Failure may be due to long term seepage weakening the structure over years or decades. In the case of earthen dams, even the vegetable and animal life on the dam may have an impact.^[8] Vibration due to seismic activity or nearby civil engineering works can literally liquefy soil in earthen dams due to thixotropy.

Environmental impact

The environmental impact of small dams is exactly the same as that of large ones, scaled down proportionally.

River flow type

This consists of a turbine which is directly powered by the water flow, such as in a natural defile. Lacking a defile, a weir and flume may be built to direct the water flow into an artificial one. A variation of this is where water may be diverted from a natural waterfall. This type of hydroelectricity may be considered as a "hi-tech" variation of the old water-wheel. Most such installations are small, of less than 50 MWe capacity, although the total maximum power capacity at Niagara exceeds 2 GWe (including pumped storage), with a drop of 52 m. This compares, for example, to another major waterfall HE scheme in the USA, at Snoqualmie, WA, with a capacity of 44 MWe, with a drop of 82 m.

Safety

As there are no dams (other than possibly small weirs), the risks involved are generally small. Waterfall types with heads of up to 100 m work at a comparatively low pressure and there is small risk of a penstock ceding, short of an external influence. In cases with a flume, to return of the water to the river bed should ideally be as close to the weir as possible. One point that often goes hand-in-glove with river-flow projects is "course correction". This usually consists of canalising the water away from flood plains, either by dredging the bed or by levées. In both cases, the result is that the water cannot escape from the canal and, instead of natural upstream flooding with high water and the water velocity is increased. In the event of exceptionally high water volumes, the canal will breach its banks, often with more catastrophic results than if nature had been allowed to take its course, because of the higher flowrate. In recent years, there have been moves in many countries to rehabilitate water courses with restoration of flood plains.^[9]

Environmental impact

The environmental impact of river flow installations is small, even with weirs. The water retained is usually small, as is the reservoir. Provided that the biotope is carefully examined before a project is started, there are usually few problems. "Salmon ladders", such as in the NW USA and in Scotland^[10], can cater for migratory fish. In other cases, fish can be periodically restocked to maintain the natural food chain.

Pumped storage type

A pumped storage hydroelectric scheme is the only practical way to store electrical energy on a large scale and with a high efficiency. It requires two reservoirs, either or both of which may be a natural watercourse. These must be at different altitudes. When excess electricity is available, such as at night, water is pumped up from the lower reservoir into the upper one. During peak consumption periods, the water in the upper reservoir is used, by gravity, to turn a turbine and then back to the lower one. The overall electricity-to-electricity efficiency is typically about 80 per cent. There are numerous installations of this type in the world. One example in Switzerland is the Hongrin-Veytaux system, with a 240 MW capacity. The bottom reservoir is Lac Léman (the Lake of Geneva). The top reservoir is an Alpine type dam, 878 m higher in the Vaudois préalpes.^[11] The Dinorwig station in Wales is the largest in Europe.^[12] In the USA, the Raccoon Mountain system, run by the Tennessee Valley Authority, is amongst the biggest in the world, with a peak capacity of 1600 MW.^[13]

Safety

As pumped storage types are usually of an Alpine character, the safety issues are the same (see above)

Environmental impact

As pumped storage types are usually of an Alpine character, the environmental impact is the same (see above), except when an additional reservoir is required at the lower level, which would normally be similar to a medium-scale river type.

Commentary

Much of the exploitable hydroelectricity capacity throughout the world is already in service. All energy generation involves a danger and HE systems are no exception. The numbers of persons killed because of accidents since 1945 (before that, bombing dams also caused a more deliberate death toll) has exceeded 100 000 by the most conservative estimates, certainly many times more than those due to all the civil nuclear accidents, including the Chernobyl disaster. Modern monitoring of major dams has greatly reduced the risks of catastrophic accidents, but they have not been eliminated, nor can they be when nature has been the cause more often than faulty engineering design.

References

1. ↑ Hydroelectricity [1]
2. ↑ Grande Dixence Dam[2]
3. ↑ Abstracts of a conference on dam security (in German and French)[3]
4. ↑ The catastophic dam failures in China [4]
5. ↑ La catastrophe de Malpasset (in French)[5]
6. ↑ Wikipedia entry: Baiji.[6]
7. ↑ FEMA: Dam Failure[7]
8. ↑ Dam owner's guide to animal impacts on earthen dams [8]
9. ↑ Flood Protection and Rehabilitation – New Directions for Our Rivers[9]
10. ↑ Salmon ladder[10]
11. ↑ Hongrin-Veytaux power station[11]
12. ↑ Dinorwig (Detailed PowerPoint presentation)[12]
13. ↑ Raccoon Mountain Pumped Storage Plant[13]