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Solar photovoltaic cell on a pocket calculator

Renewable energy

by Chris Woodford. Last updated: January 9, 2014.

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. Here, a very small solar cell is trickling a couple of volts into my pocket calculator.

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 versus renewables

Chart showing percentage of US energy supplied by different fossil fuels and 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. From the pie chart, you can see that about 84–85% percent of US energy comes from fossil fuels, while the remainder comes from renewables and nuclear. Looking at the renewables alone, in the bar chart on the right, you can see that hydroelectric and biomass provide the lion's share, while wind and solar provide very little at all. Solar provides just 1 percent of US renewable energy.

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

Chart: Percentage of total US energy supplied by different fossil fuels and renewables in 2008. Source: Office of Coal, Nuclear, Electric and Alternate Fuels, Energy Information Administration, US Department of Energy. Data published 2010.

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.

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.

225kW wind turbine in Staffordshire, England

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 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.

Photo: This 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.

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

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

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.

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

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 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.

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. So you'll need 2500–5000 wind turbines–and an awful lot of land to put them on. Is it doable? The world's biggest wind farm 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.

Artwork: It takes about 1000 wind turbines (1000 small blue dots) to make as much power as a single coal-fired power plant (one big black dot).

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

Futuristic house with blue solar panels on the roof

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 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. 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 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.

Photo: One day, we may all live in environmentally friendly eco-homes. In the meantime, 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).

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Text copyright © Chris Woodford 2009, 2011. All rights reserved. Full copyright notice and terms of use.

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

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