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Friday, Aug. 23, 2002

FOR KIDS

What water can do


By ELLEN WHYTE
Special to The Japan Times

If you put your hand under the kitchen tap or stick your toe into a fast-flowing river, you can feel the push of the water. Water has great power. This is something that the ancient Greek hero Hercules knew only too well. He used the strength of water to clean the stables of King Augeas. They were so dirty that no human being had the strength to scour them. But Hercules diverted a river toward the stables, so that its strong flow washed away all the dirt.

Through the centuries engineers have built machines capable of harnessing the power of water and converting it into mechanical energy. Like Hercules, their challenge has been to use water to perform laborious tasks and to make our lives easier.

Early projects

In ancient times, flour was made by hand. This was very tiring. Laborers placed the grain on a flat stone and then used another stone, or perhaps a stone pestle, to pound the grain into flour. However, in ancient China, Egypt and Persia, inventors found a way to make the work easier. They built machines called waterwheels to grind grain, such as wheat and corn, into flour.

Waterwheels were simple to make. First engineers made a large wheel with a series of paddles mounted along the edge. This wheel was connected to a shaft, which was connected, in turn, to a stone called a millstone.

When the wheel was lowered into a river, the flow of the water would push against the paddles, turning the wheel. The wheel would then move the shaft, and the shaft would move the millstone up and down. Engineers placed another stone directly underneath the moving millstone so that the two stones would bash together as the river pushed the waterwheel around. All the laborers had to do was pass a small trickle of grain on the stationary millstone and wait for it to be crushed into flour.

Over time waterwheels were used to power a variety of machines that performed sophisticated tasks like sawing up wooden logs.

Modern projects

Naturally, the more water that's available, the more power that can be generated. So engineers first look for a place where lakes and rivers come together.

Then they build a dam. This holds the water in a large lake called a reservoir. When the gates of the dam are opened a little, the water falls from a great height. This fast-flowing water is run through a building complex called a powerhouse. Modern powerhouses work along the same principles as old-fashioned waterwheel setups, but instead of the water power being used to make flour or cut logs, it is used to create electricity.

When water is funneled through the powerhouse, the stream is directed to hit a large disk made of curved blades called a turbine.

The turbine looks and acts like a waterwheel. This turbine is connected to a shaft, which is in turn connected to a generator. When the water moves the turbine, the shaft moves up and down. This works the magnets and copper coils inside a machine called a generator and creates electricity.

The electricity travels out of the powerhouse along a system of cables called a grid to places where it is needed, such as factories and cities. Otherwise it is stored for future use.

Environmental impact

Today's hydroelectric plants provide about one-fourth of all the electricity in the world. However there are many pros and cons associated with such projects.

On the plus side, hydropower doesn't cost as much as other forms of power. Also, while the dam is being built, there are hundreds of workers who need houses, shops and entertainment. They bring good business to local communities.

One of the minuses is that modern dams need lots of water, so dam builders often interfere with the natural flow of rivers. When the river course changes, whole communities are relocated and local ecosystems destroyed.

Plus, as the dam ages, it needs expensive repairs. These costs are passed on to consumers, who have to pay more for electricity.

A classic case study is the Aswan High Dam project, completed in 1970. This dam barricades the Nile, the longest river in the world.

Before the dam was built, the Nile flooded every year, depositing a rich mud called silt on the river banks. This made the land very fertile. But the floods would often destroy houses and inundate entire towns.

Supporters of the dam point out that the annual flooding of the Nile River can now be controlled. But project opponents complain that without the Nile's annual flooding, farmers can no longer have their fields fertilized for free.

There is a similar debate about the most ambitious dam project today, the Three Gorges Dam on the Yangtze in China, scheduled for completion in 2009.

But because of dams and the power they provide, our lives are much simpler. We use electric mixers to beat eggs and washing machines to clean clothes. We've come a long way from grinding our flour by hand.

For an illustrated essay about waterwheels and mills from ancient Egypt and China to the late Middle Ages, see www.geocities.com/Athens/Forum/3807/features/watermills.html To learn more about the ins and outs of a modern working dam, go to www.howstuffworks.com/hydropower-plant.htm What sort of shape should a dam be, can a broken dam be repaired and what happens if it becomes unstable? For answers, see www.pbs.org/wgbh/buildingbig/dam To learn more about the Aswan High Dam, go to www.bol.ucla.edu/~hw1110


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