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Photovoltaic solar panels covering the roof of Swanage School in Dorset England

Renewable energy

by Chris Woodford. Last updated: December 15, 2020.

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.

Contents

  1. What is renewable energy?
  2. What's the difference between fossil fuels and renewable energy?
  3. What are the different types of renewable energy?
  4. How can New York City go renewable?
  5. How you can use more renewable energy
  6. Find out more

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:

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

Chart showing percentage of US energy supplied by different fossil fuels and renewables.

Chart: Percentage of total US energy supplied by different fossil fuels and renewables in 2019. Source: Office of Coal, Nuclear, Electric and Alternate Fuels, Energy Information Administration, US Department of Energy. Data published April 2020. Note that figures are individually rounded and may not add to 100%.

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. 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 over a third of US renewable energy and are steadily increasing in importance: solar currently provides 9 percent of total US renewable energy (up from 4 percent in 2014), while wind provides 24 percent (up from 18 percent in 2014). That doesn't sound too bad, but remember that renewables make up just 11 percent of all our total energy use. So solar provides about 1 percent of the total energy and wind provides about 2.6 percent. Renewables have increased from 7% to 11% 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 is still coming from fossil fuels.

Please note that these charts cover total energy and not just electricity.

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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 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.

Wind power

225kW wind turbine in Staffordshire, England

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 8–10 megawatts (8000–10,000 kilowatts), which is about 35–45 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 surface.

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 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 up above.

Ocean power

Model of an ocean thermal energy plant

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).

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

Biomass

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.

Pie chart showing sources of biomass energy for the United States.

Chart: Biomass (including wood) supplied about 5 percent of the total energy used in the United States in 2019 and 44 percent of the renewable energy (19% + 25% in the chart higher up the page). Although you might expect most of it comes from wood, quite a lot comes from biofuels (mostly ethanol) as well. Use of biofuels has been increasing for years, notching up 45 percent of the total in 2019, though it has fallen slightly from the 2016 figure of 48 percent in response to growing environmental concerns. Wood comes in at 46 percent (up from 41 percent in 2016). Figures don't add to 100 percent because of rounding. Data from US Office of Energy Administration, 2020.

Biomass is more environmentally friendly and sustainable than fuels such as coal for three main reasons.

  1. 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 sustainable).
  2. 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.
  3. 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?

Glowing coals in a typical wood-burning biomass stove.

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.

A compact combined heat and power (CHP) woodchip gasification engine.

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 needs.

A power plant running on biomass at Burlington, Vermont.

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 Dave Parsons courtesy of US Department of Energy/NREL.

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!

Geothermal energy

A geothermal electricity generator at a power plant in Imperial County, California.

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).

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!

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 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.

Diagram demonstrating that it takes about 1000 wind turbines to make as much power as one coal power plant.

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.

How you can use more renewable energy

Chart showing worldwide growth in renewable energy from 2007 onwards

Chart: World consumption of renewable energy is currently growing at about 16–17 percent per year. Chart drawn using data from BP Statistical Review of World Energy 2020 (and earlier editions for older data).

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:

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 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.

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Woodford, Chris. (2009/2020) Renewable energy. Retrieved from https://www.explainthatstuff.com/renewableenergy.html. [Accessed (Insert date here)]

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