A hydroelectric dam can be built across a tidal inlet on the coast. Gates in the dam open to allow water in as the tide rises every day. At high tide the gates are closed, and as the tide falls again, the water behind the barrage is allowed out through turbines linked to electricity generators. Some systems like this tidal plant in France exploit both the incoming and outgoing tide. Tidal barrages produce clean electricity, but they can be extremely damaging to coastal wildlife habitats because they disrupt or even prevent the natural ebb and flow of the tides.
Huge rotors drive waterproofed electricity generators
Wind is already being harnessed on a large scale by"wind farms" of hi-tech wind turbines. Many are sited on land, often in hilly, scenic regions where people resent their impact on the landscape. Offshore wind farms such as this one near Copenhagen, Denmark, experience stronger, more consistent winds and they can be built well away from land, although there are concerns about seabirds being killed by the huge rotors. The main practical problem is that wind turbines work only when the wind blows. This means that there must be back-up power generation systems, which often have to use fossil fuels.
Most solar power systems are small-scale, but a few big power plants have been built in reliably sunny regions such as California and Spain. They use mirrors to focus the Sun's rays on a tower that collects the heat and uses it to run a turbine and generator. More efficient power lines may enable such plants to distribute power to cities in less sunny areas.
In volcanic regions, energy can be tapped from hot rock and water beneath the ground. It can be used directly to heat water and houses, as in Iceland where it heats 90 percent of the buildings. The heat can also run the turbines of power stations like this one at Wairakei, New Zealand. Geothermal power in California, Mexico, Indonesia, Italy, and Iceland is producing as much electricity as about ten large coal-fired power plants, but it could be used far more widely, wherever there is volcanic activity.
Crop of sugar cane
Burning any plant product as a fuel releases carbon dioxide. However, if more plants are grown to replace them when they are harvested, the new crop should absorb the carbon dioxide that is released. Some of these crops can be used as fuel in their raw state, but plants such as switchgrass and sugarcane are converted into fuel oils and ethanol. However, growing enough biofuels to replace all the fossil fuels that we rely on would use up all the farmland that we now use to grow food, or require massive deforestation. It would also generate greenhouse gases through the use of fertilizers and machinery.
Solar energy is free, abundant, and non-polluting. The most efficient way to use it on a small scale is for heating the water we use for bathing and washing. Many systems use roof panels made of copper pipes in glass tubes. Sunlight passing through the glass heats fluid pumped through the pipes, and the hot fluid heats the water. A conventional boiler can act as back-up in winter, but in summer the Sun can supply enough energy to raise the water temperature to more than 140°F (60°C).
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