Dam it

Looking over the Yangtze River, the third longest river in the world, Chinese developers (envisioning a huge source of hydropower) said, "Dam it." Under development when we wrote this, the Three Gorges Dam on the Yangtze River is a 15-year, 825 billion project that will be the world's biggest hydropower dam by the time it's complete in 2009.

The dam's impact on locals and on the environment has been widely criticized. The dam has already displaced 1 million people, the majority of whom are poor, and critics say the government hasn't properly resettled the displaced. When it's complete, the dam will bring water levels up to a height that would completely inundate old towns — including dumps, mines, and factories that will seriously pollute the waters. The Yangtze also affects fisheries — with much less freshwater and sediment getting to the ocean, scientists expect fish catches to decline.

This example shows that even renewable energy sources can have their social and environmental issues. Future hydropower developers need to closely consider the whole picture before they implement large structural changes.

Hydropower plants can play a huge part in reducing fossil fuel use. Eighty percent of Brazil's electricity comes from hydropower. Consequently, the country's power sector produces four times less carbon dioxide than comparable power sectors in other countries. On another positive note, hydropower is one of the cheapest renewable energy options available today because developers and power providers have already developed and built so much of its technology and infrastructure, which are in wide use.

Ocean power

If you've ever spent time on the sea coast, you know that the tides come in and flow out in a daily cycle, pulled by the moon's gravitational force. You may find yourself scooting your towel up the beach a few feet while the hours of the afternoon go by. Not only can the tides move you off the beach; they can move turbines, too.

Ocean power functions in basically the same way as hydropower, using the force created by the movement of water. But rather than coming from river flow, this water power comes from the movement of the currents, tides, and waves. Here's how each works:

I Currents: Turbines are placed in flow regions of naturally occurring strong currents.

I Tides: At full tide, water is held back with gates. When the ocean reaches low tide, the gates are lifted and the water flows out forcefully, spinning turbines to generate electricity.

I Waves: Turbines are put in the areas of strong wave action, and each wave that hits the turbines spins them.

As with almost all renewable energy, tidal power sources have a high start-up cost, but the environmental benefits could be huge; tidal and ocean energy give off no emissions. For countries that have long coastlines, including Canada, the U.S., and Australia, ocean power holds huge potential. France and China are already using tidal power.

Tidal technologies are still being perfected, however. Ocean power developers are currently working on new pilot projects that use more efficient technology that does not involve damming bays or estuaries. These new projects employ turbines on the floor of coastal zones (previous projects worked on the surface of the waves).

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