There are many uses for the energy of moving water, some of which Man has known for great many years. Reference was made earlier to water-wheels, which use sun power indirectly - solar heat evaporates water from the Earth's surface, froming clouds which later release that water as rain elsewhere. If "elsewhere" is high ground, gravity allowss us to convert the kinetic energy of the flowing water into mechanical energy, and thence into work - whether that work is grinding corn or making electrical power. This principle was certainly used by the Greeks in 85 BC. Today hydro-electric stations produce more electricity in the world than nuclear stations - though this may soon change.

Cut-away section of the Dinowic pumped storage scheme, near Llanberis in North Wales. The water tunnel from the upper reservoir to the lower, via the power station, can be seen; the vertical seperation is about 500m. Six pump turbines will each generate 250 MW

The "Pelton wheel", developed by L. A. Pelton in California in 1880, is the best known impulse turbine, uses where is loe volume but a high "head" of water - as from mountians - which jets from a nozzle into buckets around the rim of a wheel. These can deliver up to 60 MW. Reaction turbines are best used in fairly fast-moving rivers, which drive vaned wheels by flowing through an outer casting. These produce 500 MW, abd could go up to 1,500 MW. A third type is the Axial flow turbine, resembles a ship's propellor encased in a wide tube. The efficiency of any of these turbines can exceed 90%. And to produce electricity and sue it to power machines is more efficient then the old method of using the energy of water directly through cogs and gears - attractive though an old water-mill looks in paintings.

Today, water is held behind huge dams and confined in reservoirs in order to even out seasonal floods and droughts (which can still sometimesx catch us out, as in 1976), to be used as required. Dams often have to be sited in areas of great scenic beauty, as for example in the US - where it is the fourth largest source of energy, Norway, Scotland and Wales, and ecological damage has to be avoided.

In order to be able to supply electricity at the time when the demand for it arrises, pumped storage schemes were develoved. The idea is simple: surplus electricity from off-peak periods (e.g. at night) is used to pump water to a reservoir at a high level, then the stored potential energy in the water is released to generate electricity at peak demand times, flowing into a lower reservoir. Electricity can also be produced at very short notice in the event of breakdown elsewhere. The classic example in Brittain is the 360 MW station at Ffestiniog in North Wales (which has actually received a citation from the Welsh Tourist Board).

Although the Electricity Boards take pains to minimise the impact on the environment, any new scheme to flood a valley is likely to meet as much opposition as did plans to mine the biggest coal deposits found this century, in the Vale of Belvoir, Rutland; or to recover the 4,000 mtce of uranium ore recently discovered in a 6.5 x 1.5 km area of one of the Orkney isles.

As there are not enough natural lakes around with convenient hills, an intriguing alternative use of pumped storage could be to make power stations drive compressors and pump compressed air into underground caves - which could be excavated with explosives - at off-peak times of storage. By supplying compressed air to the turbines instead of a large part of their own power being diverted for the purpose, their output could be more than doubled at peak times.

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