When people think of clean energy, they usually think first of solar energy -- and for good reason. It's the ultimate in renewable power, it's almost totally non-polluting, it's available almost everywhere, and it's free. In a sense, most energy sources -- wind, hydropower, even fossil fuels -- originate with power from the sun.
The three main technologies associated with solar energy are photovoltaic cells (PV cells), the large panels on rooftops that convert sunlight directly into electrical energy; solar heat collectors like solar water heaters that use sunlight to heat water or a water-alcohol mixture; and solar concentrators that use mirrors or lenses to focus sunlight on a liquid container, heating the liquid enough to drive an electricity-generating steam turbine. Of course, people have been using passive solar -- like the warmth from a sunny window -- to heat their homes since the dawn of civilization.
Deep beneath the surface of the Earth is an enormous furnace of hot molten rock called magma. The heat within 30,000 feet of the surface contains roughly 50,000 times more energy than all the oil and natural gas reserves in the world. As underground water comes into contact with this heat, it boils up to the surface and is tapped as a constant source of green energy for powering electric steam turbines.
Another kind of geothermal energy is used in geothermal heat pumps, which take advantage of the fact that the ground immediately beneath the Earth's surface is at a near constant temperature of about 55 degrees Fahrenheit. In geothermal heat pumps (sometimes called heat exchange pumps or ground-source heat pumps), air or liquid is pumped through pipes that are buried underground, then re-circulated into the building. In summer, the pipes absorb heat from the building and move it into the ground, thus cooling the building. In winter, the pipes move warmer air or liquid from underground to the building's heating system, so the system has less work to do -- it's easier to warm air that's 55 degrees than air that's 25 degrees -- thus saving energy..
Wind has been a source of clean energy since the first sailor propped a sheet of canvas onto his boat to move it along faster, and it's been used from the windmills of Holland (used to grind grains into flour) to the old iron wind-powered pumps -- erroneously called windmills -- that pumped groundwater for pioneers on America's Great Plains. According to the Union of Concerned Scientists, wind energy is world's the fastest-growing source of electricity. In 2008, more than 27,000 megawatts of new electrical capacity were installed worldwide, marking a 36 percent increase over 2007 and representing $51.5 billion in new wind farm investments.
Modern wind turbines today dot the landscape across the world, though they're more popular in Europe than in North America. Spain, according to the Union of Concerned Scientists, now meets about 13 percent of its electricity demand from wind power -- in Denmark, that figure is even higher, at 20 percent.