Renewable energy

Last updated: March 4, 2009.

Running low on fuel? Just zip to the gas station and fill up your tank. The only trouble is, you won't be able to do that forever because Earth itself is running low on fuel. Most of the energy we use comes from fossil fuels like oil, gas, and coal, which are gradually running out. Not only that, using these fuels produces air pollution and carbon dioxide—the gas most responsible for global warming. If we want to carry on living our lives in much the same way, we need to switch to cleaner, greener fuel supplies—renewable energy, as it's known. Let's take a closer look at what it is and where it comes from.

Please note: If you want a general overview of the science of energy, you might prefer to look at our detailed introduction to energy.

Photo: Solar energy will come into its own as fossil fuel supplies dwindle and renewables become more economic. But at the moment, it supplies only a tiny fraction of world energy. Photo by Warren Gretz courtesy of US Department of Energy/NREL.

What is renewable energy?

Broadly speaking, the world's energy resources (all the energy we have available to use) fall into two types called fossil fuels and renewable energy:

• Fossil fuels are things like oil, gas, coal, and peat, formed over hundreds of millions of years when plants and sea creatures rot away, fossilize, and get buried under the ground, then squeezed and cooked by Earth's inner pressure and heat. Fossil fuels supply about 80-90 percent of the world's energy.
• Renewable energy means energy made from the wind, ocean waves, solar power, biomass (plants grown especially for energy), and so on. It's called renewable because, in theory, it will never run out. Renewable sources currently supply about 10-20 percent of the world's energy.

What's the difference between fossil fuels and renewable energy?

In theory, fossil fuels exist in limited quantities and renewable energy is limitless. That's not quite the whole story, however.

The good news is that fossil fuels are constantly being formed. New oil is being made from old plants and dead creatures every single day. But the bad news is that we're using fossil fuels much faster than they're being created. It took something like 400 million years to form a planet's worth of fossil fuels. But humankind will use something like 80 percent of Earth's entire fossil fuel supplies in only the 60 years spanning from 1960 to 2020. When we say fossil fuels such as oil will "run out," what we actually mean is that demand will outstrip supply to the point where oil will become much more expensive to use than alternative, renewable fuel sources.

Just as fossil fuel supplies aren't exactly finite, neither is renewable energy completely infinite. One way or another, virtually all forms of renewable energy ultimately come from the Sun and that massive energy source will, one day, burn itself out. Fortunately, that won't happen for a few billion years so it's reasonable enough to talk of renewable energy as being unlimited.

Photo: Oil won't "run out," but will get gradually get more expensive until other forms of energy become more economic. Photo by Warren Gretz courtesy of US Department of Energy/NREL.

What are the different types of renewable energy?

Almost every source of energy that isn't a fossil fuel is a form of renewable energy. Here are the main types of renewable energy:

Solar power

For as long as the Sun blazes (roughly another 4-5 billion years), we'll be able to tap the light and heat it shines in our direction. We can use solar power in two very different ways: electric and thermal. Solar electric power (sometimes called active solar power) means taking sunlight and converting it to electricity in solar cells (which work electronically). This technology is sometimes also referred to as photovoltaic (photo = light and voltaic = electric, so photovoltaic simply means making electricity from light) or PV. Solar thermal power (sometimes called passive solar power or passive solar gain) means absorbing the Sun's heat into solar hot water systems or using it to heat buildings with large glass windows.

Wind power

Depending on where you live, you've probably seen wind turbines appearing in the landscape in recent years. There are loads of them in the United States and Europe, for example. A turbine is any machine that removes kinetic energy from a moving fluid (liquid or gas) and converts it into another form. Windmills, based on this idea, have been widely used for many hundreds of years. In a modern wind turbine, a huge rotating blade (similar to an airplane propeller) spins around in the wind and turns an electricity generator mounted in the nacelle (metal casing) behind it. It takes roughly several thousand wind turbines to make as much power as one large fossil fuel power plant. Wind power is actually a kind of solar energy, because the winds that whistle round Earth are actually made when the Sun heats different parts of our planet at different times, causing huge air movements over its surface.

Photo: A wind turbine, in Wyoming, United States. Photo by Tom Hall courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Hydroelectric power

Hydro means water, so hydroelectricity means making electricity using water—not from the water itself, but from the kinetic energy in a moving river or stream. Rivers start their lives in high ground and gradually flow downhill to the sea. By damming them, we can make huge lakes that drain slowly past water turbines, generating energy as they go. Water wheels used in medieval times to power mills were an early example of hydro power. You could describe them as hydromechanical, since the water power the milling machines using elaborate systems of wheels and gears. Like wind power, hydroelectric power is (indirectly) another kind of solar energy, because it's the Sun's energy that drives the water cycle, endlessly exchanging water between the oceans and rivers on Earth's surface and the atmosphere up above.

Ocean power

The oceans have vast, untapped potential that we can use in three main ways: wave power, tidal barrages, and thermal power.

• Wave power uses mechanical devices that rock back and forth or bob up and down to extract the kinetic energy from moving waves and turn it into electricity. Surfers have known all about wave power for many decades!
• Tidal barrages are small dams built across estuaries (the points on the coast where rivers flow into the sea and vice versa). As tides move back and forth, they push huge amounts of water in and out of estuaries at least twice a day. A barrage with turbines built into it can capture the energy of tidal water as it flows back and forth. The world's best-known tidal barrage is at La Rance in France; numerous plans to build a much bigger barrage across the Severn Estuary in England have been outlined, on and off, for almost a century.



• Thermal power involves harnessing the temperature difference between warm water at the surface of the oceans and cold water deeper down. In a type of thermal power called OTEC (ocean thermal energy conversion), warmer surface water flows into the top of a giant column (perhaps 450m or 1500ft tall) mounted vertically in the distance some miles out to sea, while cooler water flows into the bottom. The hot water drives a turbine and makes electricity, before being cooled down and recycled. It's estimated that there is enough thermal energy in the oceans to supply humankind's entire needs, though little of it is recovered at the moment.

Photo: A model of an OTEC (ocean thermal energy conversion) plant that makes energy using temperature differences between different layers of ocean water. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Biomass

Biomass is the name given to any crop grown for the purpose of making energy. Biofuels are one example. Other examples include burning animal waste in a furnace to generate electricity. Biofuels are controversial because they often take up land that could be used to grow food, but they are generally a cleaner and more efficient way of making power than using fossil fuels. Because plants absorb carbon dioxide while they're growing and give it out when they're burned, biomass can provide energy without adding to the problem of global warming.

Geothermal energy

Earth may feel like a pretty cold place at times but, inside, it's a bubbling soup of molten rock. Earth's lower mantle, for example, is at temperatures of around 4500°C (8000°F). It's relatively easy to tap this geothermal (geo = Earth, thermal = heat) energy using heat pumps, which drive cold water deep down into Earth and pipe hot water back up again. In fact, Earth's entire geothermal supplies are equivalent to the energy you could get from about 25,000 large power plants!

Photo: A geothermal electricity generator in Imperial County, California. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Nuclear fusion

Conventional nuclear energy is not renewable: it's made by splitting up large, unstable atoms of a naturally occurring chemical element called uranium. Since you have to feed uranium into most nuclear power plants, and dig it out of the ground before you can do so, traditional forms of nuclear fission (the scientific term for splitting big atoms) can't be described as renewable energy. In the future, scientists hope to develop an alternative form of nuclear energy called nuclear fusion (making energy by joining small atoms), which will be cleaner, safer, and genuinely renewable.

Fuel cells

If you want to use renewable power in a car, you have to swap the gasoline engines or diesel engine for an electric motor. Driving an electric car doesn't necessarily make you environmentally friendly. What if you charge the batteries at home and the electricity you're using comes from a coal-fired power plant? A better alternative is to swap the batteries for a fuel cell, which is a bit like a battery that never runs flat, making electricity continuously using a tank of hydrogen gas. Hydrogen is cheap and easy to make from water. Fuel cell are quiet, powerful, and make no pollution. Probably the worst thing they do is puff steam from their exhausts!