Today, most of the energy we use comes from fossil fuels. Coal, oil, and natural gas are all fossil fuels. Over millions of years, the decay of plants, dinosaurs, and other animals was formed into fossil fuels. These fuels lie buried between layers of earth and rock. The only way to get them out is to drill or mine for them. While fossil fuels are still being created today by underground heat and pressure, they are being consumed more rapidly than they are created. For that reason, fossil fuels are considered nonrenewable; that is, they are not replaced as soon as we use them. So, we could run out of them sometime in the future. Or, we might someday use so much fossil fuel that we won't be able to drill or mine fast enough to keep up with the demand.

Because our world depends so much on energy, we need to find sources of energy that will last a long time. What if there was a type of energy that never ran out? There is. It is called renewable energy.

In addition, because there are so many people on the earth using fossil fuels, we create a lot of pollution. So, we should also use energy sources that produce as little pollution as possible. While all energy sources cause some pollution in their creation or their consumption, renewable energy systems generally are less polluting than fossil fuel systems.

What is renewable energy?

Renewable energy systems use resources that are constantly replaced and are usually less polluting. Examples of renewable energy systems include solar, wind, and geothermal energy (getting energy from the heat in the earth). We also get renewable energy from trees and plants, rivers, and even garbage.

We can use the energy in sunshine to warm and light our homes, heat our water, and provide electricity to power our lights, stoves, refrigerators, and other appliances. This energy comes from processes called solar heating, solar water heating, photovoltaic energy (converting sunlight directly into electricity), and solar thermal electric power (when the sun's energy is concentrated to heat water and produce steam, which is used to produce electricity).

Solar heating

Have you ever sat in a car that was closed up on a sunny day? Did you notice how hot it was in the car? This warmth is just one example of solar heating. We can use the sun to heat other things, including our homes. Today, more than 200,000 houses in the United States have been designed to use features that take advantage of the sun's energy. These homes often use passive solar designs, which do not normally require pumps, fans, or other mechanical equipment to store and distribute the sun's energy. In contrast, active solar designs need additional mechanical components.

A passive solar home or building naturally collects the sun's heat through large, south-facing windows, which are just one aspect of passive design. Once the heat is inside, we need to capture and absorb it. Think about a sunny spot on the floor of your house on a cold day. That "sun spot" is nice and warm, right? It is warm because it holds the sun's heat, and we call such things absorbers.

In solar buildings, sunspaces are built onto the south side of the structure and act as large absorbers. The floors of sunspaces are usually made of tiles or bricks that absorb heat throughout the day, then release heat. When the air is colder than the floor, the tiles or bricks release the heat to the air.

A challenge with solar heating is keeping the heat inside the house. One way to do this is to use special windows that reflect the heat back into the house. Another aspect of solar heating is that the house absorbs heat even during hot weather, when the last thing you need is more heat! So, passive solar homes need to be designed to let the heat in during cold months and block the sun in the hot months. How can you do this?

You can use deciduous trees or bushes in front of the south-facing windows. These plants lose their leaves in the winter and allow most of the sun in, while in the summer, the leaves will block out a lot of the sunshine and heat. Or, you can design your house to have overhangs above the south-facing windows. This will block out the summer sunshine when the sun is high in the sky but let it in when the sun is lower in the winter.

Solar water heating

The sun also can heat water for bathing and laundry. Most solar water-heating systems have two main parts: the solar collector and the storage tank. The collector heats the water, which then flows to the storage tank. The storage tank can be just a modified water heater, but ideally it should be a larger, well-insulated tank. The water stays in the storage tank until it is needed for something, say a shower or to run the dishwasher.

A common collector is called a flat-plate collector, and is usually mounted on the roof. This collector is a rectangular box with a transparent cover that faces the sun. Small tubes run through the box, carrying the water or other fluid such as antifreeze to be heated. The tubes are mounted on a metal absorber plate, which is painted black to absorb the sun's heat. The back and sides of the box are insulated to hold in the heat. Heat builds up in the collector, and as the fluid passes through the tubes, it heats up.

Like solar-designed buildings, solar water-heating systems can be either active or passive. The most common systems are active, which means they use pumps to move the heated fluid from the collector and into the storage tank.

While a solar water-heating system can work well, it can't heat water when the sun isn't shining—and we all know it can be cloudy for days at a time! For that reason, homes also have a conventional backup system that uses fossil fuels.

Photovoltaic energy

The sun's energy can also be made directly into electricity using photovoltaic (PV) cells, sometimes called solar cells. PV cells make electricity without moving, making noise, or polluting. They are used in calculators and watches. They also provide power to satellites, electric lights, and small electrical appliances such as radios. PV cells are even being used to provide electricity for homes, villages, and businesses. Some electric utility companies are building PV systems into their power supply networks.

Although the PV cells used in calculators and watches are tiny—less than a half inch (1.2 centimeters) in diameter—PV cells for larger power systems are about 4 inches (10 centimeters) in diameter. When more power is needed, PV cells can be wired together to form a module. A module of about 40 cells is often enough to power a small light bulb. For more power, PV modules are wired together into an array. PV arrays can produce enough power to meet the electrical needs of your house—or for even larger uses.

Today, PV systems are mostly used for water pumping, highway lighting, weather stations, and other electrical systems located away from power lines. For example, if you had a cabin on a mountain top, a PV system would allow you to read some of your favorite books before you went to sleep!

Because PV systems can be expensive, they are not used in areas that have electricity nearby. But if someone needs electricity in a remote place, PV can be quite economical. Another aspect of PV power is "intermittency," which means that if the sun isn't shining, the system can't make electricity. Because PV systems only produce electricity when the sun is shining, these remote systems need batteries to store the electricity.

Solar thermal electric power

Solar thermal systems can also change sunlight into electricity, but not in the same way as PV cells. In most cases, solar thermal systems concentrate (focus) sunlight to produce heat. This heat boils water to make steam. The steam rotates a turbine, which is made of several rows of blades mounted on a large shaft. The steam's pressure flows through the turbine, pushes against the blades, and causes the shaft to turn, much like you can make a pinwheel spin by blowing on it. The turbine is attached to a generator that makes electricity.

Like electricity from PV systems, solar thermal power can be intermittent. To avoid this problem, many systems use a backup system that relies on natural gas to heat the water. Because solar thermal systems concentrate the sun's energy, they need to be located in areas of the world that receive a lot of intense sunshine.

Did you know that wind is considered an indirect form of solar energy? This is because the wind is driven mainly by temperature differences on the surface of the earth that are caused by sunshine.

For centuries, the wind has been used to sail ships, grind grain, and pump water. Now, people use the wind to generate electricity. The windmills built long ago had many blades, but today's wind turbines usually have just two or three blades that turn when the wind blows. But the blades on wind turbines are much longer than those you might see on a windmill. In fact, wind turbine blades can be up to 82 feet (25 meters) long!

The blades drive a generator that produces electricity, much like steam turbines. The longer the blades and the faster the wind speed, the more electricity the turbine generates. Wind turbines are placed on towers because the wind blows harder and more steadily above the ground.

To produce the most electricity, wind turbines need to be located in areas where the wind blows at a constant speed, which it does not do in all parts of the world. Wind speed is described by seven "classes." For example, Class 7 winds are extremely strong, while Class 2 winds are mild breezes. Generally, Class 4 winds and above are considered adequate for a wind turbine to produce electricity.

Large groups of wind turbines, called wind farms or wind plants, are connected to electric utility power lines and provide electricity to many people. New turbine designs now take advantage of less windy areas by using better blades, more electronic controls, and other improvements. Some new turbines can also operate efficiently over a wide range of wind speeds.

An advantage of wind turbines over some forms of renewable energy is that they can produce electricity whenever the wind blows (at night and also during the day). In theory, wind systems can produce electricity 24 hours every day, unlike PV systems that can't make power at night. However, even in the windiest places, the wind does not blow all the time. So, while wind farms don't need batteries for backup storage of electricity, small wind systems do need backup batteries. And, we're still learning about local wind patterns and how they affect wind turbines and blades.