Hydroelectric generation

Hydroelectric schemes which exploit height difference in the flow path of water are the oldest method of generation from water. It involves damming a watercourse to create the necessary pressure to drive high speed impulse turbines. The Boulder Dam scheme in the USA was the first large-scale project implemented in the 1930s as a means of driving the country out of recession.

One of the first major projects to be completed after the Second World War was the Aswan Dam scheme initiated by Colonel Nasser, the Egyptian President. Work started in 1960 to create the huge Lake Nasser as the storage facility and as a potential irrigation source for a major part of the country. It cost $1 billion ($10 billion at current prices) and began operations in 1968, delivering 2000 megawatts (MW) of power.

The project has served to illustrate some of the problems which accompany hydroelectric schemes of this massive scale. For example, evaporation from the lake has been much greater than anticipated, and the country is considering reactivating storage schemes beyond its borders. At the same time, the dam has so disrupted the flow of the Nile that it threatens the agriculture of the delta.

A further problem is that, historically, the Nile has conveyed millions of tonnes of silt per year, mostly soil, from the Ethiopian highlands. The silt, part of which used to be deposited in the Nile flood plain, is now trapped behind the dam, a fact which is calculated to have done irreparable damage to the fertility of the Nile valley and delta. To compensate for the loss Egypt is now one of world's heaviest users of agricultural chemicals.

One of the worst drawbacks concerns saline pollution. Salts are dissolved in river water and modern irrigation systems leave salts behind - about one tonne per hectare. Large areas of fertile land are being threatened by the salt which makes the ground toxic to plants and ultimately causes it to revert to desert. There is now a project to remove saline water from two million hectares of land at a cost which exceeds the original price of the dam (New Scientist, pp. 28-32, 7 May 1994).

In December 1994 work commenced on the Three Gorges scheme on the Yangtze River. The dam is two kilometres long and some 100 metres high. It has created a lake 600 kilometres long displacing over one million people. In return the country will receive 18 000 MW of power which is 50 per cent more than the world's existing largest dam, the Itaipu Dam in Paraguay. Even so, in the long term this dam will make a relatively small impact on China's dependency on fossil fuel. In addition, in November 1994, plans were revived to generate up to 37 000 MW along the course of Mekong River, again with drastic potential social consequences.

With the exception of projects on the River Danube, Europe gains most of its hydroelectricity from medium to small-scale plants. Most of Norway's supply is from hydro sources; in Sweden it is 50 per cent of the total and Scotland produces 60 per cent of its electricity from non-fossil sources, mostly hydro. According to the Department of Trade and Industry, 'The UK has a considerable untapped small-scale hydro resource' such as the discreet plant at Garnedd in Gwynedd, North Wales. Given the right buying-in rates from the National Grid, such ventures could become a highly commercial proposition.

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