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.
This article is a brief, general introduction; we also have lots of detailed articles about
the different kinds of renewable energy you can explore when you're ready.
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. Most of the sun-facing roof of this new school in Swanage, England is covered with 50kW of photovoltaic solar panels (only half of which are shown here). They save around $4000 (£3000) each year in electricity bills—and inspire students to think about where their energy comes from.
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 in most countries (although, as the chart below shows, the amount varies widely from country to country).
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.
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. To keep things in perspective, it's worth noting that hardly any countries use more
than 20 percent renewable energy. Nations like Brazil and Norway score highly because they make great use of hydroelectricity.
Chart: Percentage of energy supplied by renewables and hydroelectricity in 2021 for 10 example countries. The world average is shown by the black bar in the center. Green-colored countries are better than average; red-colored countries are
worse. Source: BP Statistical Review of World Energy 2021.
In the United States, the breakdown looks like this:
Chart: Percentage of total US energy supplied by different fossil fuels and renewables in 2020. Source: Office of Coal, Nuclear, Electric and Alternate Fuels, Energy Information Administration, US Department of Energy. Data published April 2021. Note that figures are individually rounded and may not add to 100%.
From the pie chart, you can see that about 80% of US energy still comes from fossil fuels (down from 84% in 2008 and virtually unchanged since 2014), while the remainder comes from renewables and nuclear. Looking at the renewables alone, in the bar chart on the right, you can see that wind, hydroelectric, and biomass provide the lion's share. Wind and solar provide just 37 percent of US renewable energy and are steadily increasing in importance: solar currently provides 11 percent of total US renewable energy (up from 4 percent in 2014), while wind provides 26 percent (up from 18 percent in 2014).
That doesn't sound too bad, but remember that renewables make up just 12 percent of all our total energy use.
So solar provides about 1.3 percent of the total energy (11 percent of 12 percent) and wind provides about 3.1 percent
(26 percent of 12 percent).
Renewables have increased from 7% to 12% of the total since 2008, which is a much bigger increase than it might sound. But don't forget the bottom line: 80 percent of our energy—the vast majority—is still coming from fossil fuels.
Please note that these charts cover total energy and not just electricity.
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:
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 energy 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.
Photo: This relatively small wind turbine, in Staffordshire, England makes up to 225kW of electricity, which is about enough to power 100 electric kettles or toasters at the same time. The
world's most powerful wind turbines can make a maximum of about 13 megawatts (13,000 kilowatts), which is about 60 times as much as this one.
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 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 made when the Sun heats different parts of
our planet by different amounts, causing huge air movements over its
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 used was transmitted by an 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
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
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
Ocean thermal energy conversion (OTEC), warmer surface water flows into the top of a giant column
(perhaps 450–1200m or 1500–4000ft tall), mounted vertically
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.
Biomass is a fashionable, fancy word that really just means plants
(or other once-living things) used as fuel (especially ones grown specifically for that reason).
Wood fuel gathered by people in an African
country is biomass; biofuels such as ethanol, used to make diesel for car engines, is also biomass; and chicken manure used to fire power plants
is biomass too. The great thing about biomass is
that it's a kind of renewable energy: plants grow using sunlight,
which they convert into chemical energy and store in their roots,
shoots, and leaves. Burning biomass releases most of that energy as
heat, which can we use to warm our homes, generate electricity,
and fuel our vehicles.
Chart: Biomass (including wood) supplied about 39 percent of the renewable energy (18% + 21% in the chart higher up the page) used in the United States in 2020 and 5 percent of the total energy (39 percent of the 12 percent total energy that renewables contribute). Although you might expect most of it comes from wood, quite a lot comes from biofuels (mostly ethanol) as well. Biomass supplied 45 percent of the USA's total renewable energy in 2018, though the proportion has fallen significantly since then due to the huge growth in solar and wind. Data from US Office of Energy Administration, 2021.
Biomass is more environmentally friendly and sustainable than fuels such as coal for three main reasons.
Unlike coal (which takes many millions of years to form from plant remains), biomass can
be produced very quickly and we can easily grow new plants or trees
to replace the ones we cut down and burn (in other words, biomass can be genuinely
Plants absorb as much carbon dioxide
from the air when they grow as they release when they burn, so in
theory there is no net carbon dioxide released and burning
biomass does not add to the problem of global warming. (That's why
biomass is sometimes called a carbon neutral form of energy.)
I say "in theory" because in practice growing, harvesting, and
transporting biomass may use energy (tractors or trucks running on
oil might well be involved, for example) and that
reduces the overall environmental benefit.
Also, new young trees don't absorb as much carbon dioxide as the older trees that are cut down.
Biomass is often simply wasted or sent to landfill. Burning something like waste wood offcuts from a lumber
yard or chicken manure from a poultry factory not only gives us
energy, it also reduces the waste we'd otherwise need to dispose of.
What is a biomass furnace?
Photo: Stoves have evolved quite a bit, but they haven't changed all that much over the years.
Modern woodburners produce less indoor pollution than old stoves, but they still throw significant amounts of pollution outside.
People tend to burn biomass in two ways. The simplest method is to
use a wood-burning stove, an enclosed metal box made from
something like cast iron, with opening doors at the front where the
fuel is loaded up and a small chimney called a flue to
carry away carbon dioxide, smoke, steam, and so on. This generally
provides heat in a single room, much like a traditional coal fire.
A biomass furnace is a more
sophisticated option that can heat an entire building. Unlike a wood-burning stove, a biomass furnace does the
same job as a central-heating furnace (boiler) powered by natural gas, oil, or
electricity: it can provide both your home heating and hot water and
it can even power modern underfloor central heating. It's not like a dirty and labor-intensive coal-fire and doesn't
require huge amounts of starting up, cleaning, or maintenance. All
you have to do is load in your biomass (generally, you'd use wood
pellets, wood chips, chopped logs, cereal plants, or a combination of them)
and periodically (typically every 2–8 weeks, depending on the
appliance) empty out the ash, which you can recycle on your compost.
Photo: This large biomass generator turns woodchips into electricity. Photo by Jim Yost courtesy of US Department of Energy/NREL.
While wood-burning stoves have to be manually filled up with logs,
biomass furnaces are often completely automated: they have a large
fuel hopper on the side that automatically tops up the furnace
whenever necessary. Unlike with a coal fire, you don't have to mess
around trying to get the fuel lit: biomass furnaces have simple,
electric ignition systems that do it all for you. It's perfectly
possible to run a system like this all year round, but in summertime
when you don't need home heating it might be excessive to have your
furnace running purely to make hot water. Many people switch off their
furnaces entirely for the summer months, relying on solar thermal hot
water systems (glass panels on the roof that warm up water using
the Sun's heat), electrical immersion heaters (a heating element
fitted inside a hot water tank), or an electric shower to tide them
through until fall or winter. It's perfectly possible to couple
together a biomass furnace with a solar hot-water panel so the furnace
switches on when the panel can't produce enough hot water for your
Photo: Biomass furnaces scale up very well: this is a 50megawatt power plant in Burlington, Vermont that produces electricity for local people using wood fuel.Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).
Biomass furnaces and wood-burning stoves are generally considered
to be far more environmentally friendly than home heating systems
powered by fossil fuels, but one drawback is worth bearing in mind:
burning biomass is cleaner than burning coal but still produces air
pollution. If you're considering buying a biomass stove or furnace,
ask about emissions (sales brochures usually mention how much
dust, carbon monoxide, and oxides of nitrogen appliances produce);
and be sure to find out whether there are pollution or other planning
restrictions in your area before you commit yourself to an expensive
purchase. And as with any form of home heating that involves burning
fuel, be absolutely sure to install a carbon monoxide detector for
your own safety: badly ventilated heating appliances can
kill, whether they're environmentally friendly or not!
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!
The amount of this we might actually recover is more like 70–80 gigawatts (70,000–80,000 megawatts),
which is about 50 of the same 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).
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.
If you want to use renewable power in a car, you have to swap the
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 cells are quiet, powerful, and make no pollution. Probably
the worst thing they do is puff steam from their exhausts!
How can New York City go renewable?
Talking about "renewable energy" can be very abstract. It sounds great in theory, and no-one would disagree with using
more environmentally friendly forms of power, but what would it actually mean in practice?
Suppose I make you Mayor of New York City (NYC) for a week and we agree that your top priority is to figure out
how to power the entire city with renewable energy.
How are you going to deliver eco-friendly electricity to one of the world's biggest cities?
How much energy do we need?
First off, you'll need to know how much energy the city uses. The amount is going to go up and down and you'll need
to be able to meet huge peaks in demand as well as day-to-day, average power. But let's just worry about the average
power for now. A quick bit of searching reveals that NYC's average power demand is of the order of 5 gigawatts
[Source: Accent Energy]. It may be more or less, but for this exercise it really doesn't matter.
What does 5 gigawatts actually mean? 5 gigawatts is the same as 5,000 megawatts, 5 million kilowatts, or 5 billion watts. A big old-fashioned
(incandescent) lamp uses about 100 watts, so NYC is consuming the same amount of energy as 50 million of those lamps glowing at the same time. If you prefer, think of an electric toaster, which uses about 2500 watts. NYC is like 2 million toasters burning away
all at once—a line of toasters stretching 500 km (roughly 300 miles) into the distance! It sounds like we're talking about an awful lot of energy!
How do we make that much energy right now?
And yet... five gigawatts is actually not as much as it sounds. A big, coal-fired power plant could make about two gigawatts, so you'd need
about 3 coal stations to power the city (4 to be on the safe side). Nuclear plants typically produce less (maybe 1–1.5 gigawatts), but a big nuclear station like Indian Point (just outside
NYC) can make two gigawatts. So going nuclear, you could manage with perhaps 3–6 good-sized plants.
See how easy it is to power a city the old way? You only need a handful of big old power plants.
Artwork: It takes about 1000 wind turbines (1000 small blue dots), working at
full capacity, to make as much power as a single coal-fired power plant (one big black dot).
How could we make that much energy with renewables?
This is where it starts to get tricky. Let's say you're keen on wind turbines. Great! How are you going to power NYC with wind?
We need 5 gigawatts of power and a modern turbine will deliver about 1–2 megawatts when it's working
at full capacity. So you'll need a minimum of 2500–5000 wind turbines–and
an awful lot of land to put them on. Is it doable? One of the world's biggest wind farms, at Altamont Pass in California, has almost 5000 small turbines and produces only 576 megawatts, which is about 11 percent of what we need for NYC. Now these are mostly old turbines, they're really quite puny by modern standards, and we could certainly build much bigger and more powerful ones—but, even so, powering NYC with wind alone seems to be a fairly tall order.
What about solar power? For simplicity, let's assume NYC is full of ordinary houses (and not huge skyscrapers). Cover the roof of a typical house with photovoltaic (solar-electric) panels and you might generate 5 kilowatts (5,000 watts) of power; stick those panels on a larger, municipal building and you might get three or four times as much. Let's assume every building could make 10 kilowatts for us. To generate 5 gigawatts, we'd need 500,000 buildings generating electricity all the time. That sounds like another tall order.
What other options do we have? How about harnessing the tidal power of the East River? That's been done already: six turbines installed
between 2006 and 2008 produce, altogether, about 200 kilowatts of the power used in Manhattan. [Source: Tidal Turbines Help Light Up Manhattan, MIT Technology Review, April 23, 2007.] That's a good start, but we'd need something like 140,000 of these turbines to generate our 5 gigawatts! There simply isn't enough power in the river.
Gulp. None of this is meant to put you off renewable energy; as far as I'm concerned, the world can't get away from fossil fuels fast enough. But looking at the science and the numbers, it's clear that if we're going to use renewables, and only renewables, we need an awful lot of them. Switching to renewables means building many thousands (and maybe hundreds of thousands) of separate power-generating units.
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? Given that you spend quite a lot of the money you earn on energy, try to direct that money where it will have the biggest effect. Here are some simple tips:
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 photovoltaic solar panels,
a solar hot water system,
or a ground-source 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. But, do your homework, and you have the reassurance of knowing
that the money you're spending is helping to reduce your own use of fossil fuels.
Using more by using less
The easiest way to save the planet is to use resources more wisely.
If you can't find a way to use more renewable energy, you can still try to 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 (LEDs are more efficient than CFLs and now just as cheap) and use energy monitors to help you measure and cut the cost of your other appliances. You can save money in your car too by giving some thought to fuel efficiency.
Renewable energy by Bruce Usher. Columbia University Press, 2019. A concise, balanced, up-to-date primer for non-technical readers.
The Switch by Chris Goodall. Profile, 2016. An optimistic look at why it makes economic sense to switch from fossil fuels to solar power.
Homeowners' Guide to Renewable Energy by Dan Chiras. New Society, 2011. A more practical guide that reviews and compares different renewable energy approaches, including solar, wind, and biomass. Chapter 6 (Wood Heat) covers retrofitting fireplaces, wood-burning stoves, pellet stoves, and masonry heaters.
Sustainable Energy—Without the Hot Air: An excellent online (and printed) book by Cambridge physicist David MacKay, who considers how we can meet our energy needs in the future without fossil fuels. Can we really supply all the energy we're going to need just from renewables? Prof MacKay's examples all come from the UK, where he lives, but his basic arguments apply worldwide.
Energy by Chris Woodford. New York/London, England: DK, 2007. My own, more general introductory book about energy compares all the different forms of renewable and fossil energy. Suitable for ages 8–12.
A Renewable Energy Boom by The Editorial Board. The New York Times, April 4, 2016. The falling cost of wind and solar offers hope for tackling climate change, but many obstacles remain.
What It Would Really Take to Reverse Climate Change by Ross Koningstein and David Fork. IEEE Spectrum. November 18, 2014. Google's RE<C project questions whether steady growth of renewables will be enough to make a dent in humankind's carbon dioxide emissions.
↑ This is based on 1500 megawatt power plants and an estimated figure for Earth's total surface flow
of 42 million megawatts, which comes from "Chapter 1: A Global Assessment of Geothermal Resources" by Marek Hajto et al in
Geothermal Water Management by J. Bundschuh et al, CRC Press, 2018, p.3.
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