Renewable energy
Last updated: December 9, 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.
Fossil fuels versus renewables
Different countries get their energy from different fuels. In the Middle East, there's more reliance on oil, as you'd expect, while in Asia, coal is more important.
In the United States, the breakdown looks like this. You can see that 92-93 percent of US energy comes from fossil fuels, while the remainder comes from renewables. Looking at the renewables alone, hydroelectric and biomass provide the lion's share, while wind and solar provide very little at all.
Chart: Percentage of total US energy supplied by different fossil fuels and renewables in 2006. Source: Office of Coal, Nuclear, Electric and Alternate Fuels,
Energy Information Administration, US Department of Energy. Data published July 2008.
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
technologies such as heat pumps, which drive cold water deep down into Earth and pipe hot water back up again. 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? One 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 with an electrolyzer. Fuel cell are quiet, powerful, and make no pollution. Probably
the worst thing they do is puff steam from their exhausts!
How to use more renewable energy
Photo: One day, we may all live in environmentally friendly eco-homes. But there's still lots we can do today to use energy more wisely. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
If you want to make a difference to the planet by making more use of renewable energy,
what's the best way to do it? You need to think and act carefully if you want to do your bit to ease Earth's environmental crisis, rather than simply making yourself feel a bit better.
Switching supplier
If you get most of your energy from electricity, you can switch supplier (or tariff) to
one that uses more renewable power. Sometimes this is less effective
than it sounds. If your supplier mainly operates hydroelectric power
plants and you switch from its ordinary power tariff to a green
tariff, will you actually be increasing the amount of green power in
the world or simply paying the company more money for doing exactly
the same as it was doing before? A better option is to switch to a
smaller supplier building new wind turbines or solar plants. That
way, you'll be helping the company to invest in more renewable energy and helping to
switch the world away from fossil fuels.
Making your own power
If you have more money to spend, you could investigate making some of your own
energy by installing something like a solar hot water system, a micro-wind turbine,
or a heat pump. Since you'll be using less energy
from utilities, making power this way saves money and helps the
environment too. Although making your own power pays for itself
eventually, the initial investment in turning your house into an eco
home can be costly.
Using more by using less
The easiest way to save the planet is to use resources more wisely. You can use more
renewable energy or you can use less conventional energy (from fossil
fuels). Being more efficient is surprisingly quick and easy and often costs nothing at
all. It costs nothing, for example, to share your car with a friend
and getting a bus or a train often saves you money, as well as saving
energy. Heat insulating your home is another good way of saving energy
(and money) at little or no cost, while turning down your thermostat
(and putting on an extra layer of clothing) is something anyone can do without spending so
much as a cent. Try switching to energy-efficient
light bulbs and use energy monitors to help you cut the cost
of your other appliances. You can save money in your car too by giving some
thought to fuel efficiency.
