Electricity is a key form of energy. It powers all the lights visible in this view of Johannesburg

Energy comes in different forms.
Light is a form of energy. So is heat. So is electricity.
Often, one form of energy can be turned into another.
This fact is very important because it explains how we get electricity, which we use in so many ways.
Electricity is used to light streets, houses and buildings, to run computers and TVs, and to run many other machines and appliances at home, at school, and at work.
One way to get electricity is to burn a fuel like oil or coal. This makes heat.
The heat then makes water boil and turn into steam.

The steam runs a machine called a turbine that produces electricity.
Often, this electricity then goes into a public power system that sends it out, through wires, to homes, schools, and businesses over a wide area in South Africa.
This method for making Electricity is popular. But it has some problems.
Our planet has only a limited supply of oil and coal. They are not renewable fuels.
Once they are used, they are gone forever. Also, they give off gases when they are burned.
These gases may make the air dirty, or polluted, and some of them may change Earth’s climate.

Another way to make electricity uses sunlight. Sunshine is free and never gets used up. Also, there is a lot of it.
The sunlight that hits the Earth in an hour has more energy than the people of the world use in a year.
A little device called a solar cell can make electricity right from sunlight
(“solar” means having to do with the Sun).
A solar cell doesn’t give off any gases. It doesn’t even make any noise.
A solar panel is a group of solar cells that work together.
The use of solar cells is growing fast in South Africa and many other countries.

Solar cells and solar panels have lots of uses. They are in everyday things like calculators, watches, and flashlights.
There are solar-powered toys, radios, and MP3 players.
Using solar power with devices like these means you never have to worry about batteries.
Solar panels are sometimes used to make the electricity to light up houses.

Power for houses
The ceiling lights and all kinds of machines and appliances used at home, school, and work get their electricity from the wires running through the building.
Usually, this electricity comes to the building from the public power system, or grid. But solar panels can also be used along with power from the grid.
People sometimes put solar panels on their homes.
Large buildings may have them as well. They make it possible to use less of the grid’s costly electricity.
In addition, they are a backup in case of a power failure, or blackout.
In some areas the grid itself gets some power from solar panels.

The Sun constantly gives off energy. The energy is carried through space as electromagnetic radiation. There are several types of electromagnetic radiation.
Light is one type. Radio waves are another.Electromagnetic radiation travels like waves in water. Like water waves, it is a series of ups and downs.

One way various types of electromagnetic waves differ is in their wavelength. This is the distance between two ups (or two downs) in a row. The wavelengths of radio waves are longer than those of light.
Among types of light, red has a longer wavelength than blue.

How Solar Cells Use Light
Only part of the energy sent toward Earth by the Sun actually makes it to Earth’s surface. Some solar energy gets bounced back into space. Some gets absorbed by the air. Most of the solar energy that does make it to Earth’s surface is in the form of visible light.
Solar cells can use the energy of this light to make electricity. But they don’t work equally well with all forms of light.
Different types of solar cells use different wavelengths. This means a cell can use only some of the solar energy that it receives.

People often think of electricity as something that flows. In fact, that is pretty much right. Electricity is a stream of tiny particles called electrons.
The stream is called an electric current.

There are two kinds of current.
One is DC. This is direct current. It always flows in the same direction.
Batteries and solar cells make DC.
The other kind of current is AC. This is alternating current. It reverses direction many times a second. The grid has AC. Most home electrical devices use AC.

Measuring Electricity
Electric power is the rate at which electric energy is used. It is measured in watts.

A 100-watt light bulb is more powerful than a 60-watt bulb. It uses more electricity. (When people talk about large numbers of watts, they use larger units: a kilowatt is 1,000 watts; a megawatt is 1 million watts.)

Another important unit is the watt-hour.
It measures the electric energy produced or used during a period of time. It equals 1 watt of power over a period of one hour.
A similar but larger unit is the kilowatt-hour. This is 1,000 watts over an hour.
A 100-watt light bulb that stays on for 10 hours uses 1,000 watt-hours of electric energy.
This amount is the same as 1 kilowatt-hour.

Edmond Becquerel of France first noticed that light can cause materials to make electricity. This was in 1839. Other scientists later studied the ties between light, matter, and electricity.
One of them was Albert Einstein.
In 1905 he explained how atoms take in electromagnetic radiation (such as light) and then give off electrons. This process is called the photoelectric effect.
Einstein won the Nobel Prize in 1921 for his work on it.

The First Solar Cells Russell Ohl was the first person to come up with a solar cell like the ones used today. He worked at Bell Laboratories in New Jersey.
His cell was made of silicon (silicon is found in sand and in many types of rock). He called the cell a “light-sensitive electric device.”
He filed for a patent on it in 1941. Five years later, he got the patent. In 1954, Bell Labs made the first practical solar cell. It was the first one to make enough electricity to run ordinary electrical devices.

Still, early cells didn’t make much electricity. Also, they were very costly. Their first important use was in space satellites, starting in 1958.
As cells became cheaper, they were used in other ways. The first power station able to make 1 megawatt of electricity with solar panels opened in Hesperia, California, in 1982.

Solar cells come in various sizes.
Some are tinier than a stamp. Some are 5 inches (12 centimeters) across.
The cells are made of a type of material known as a semiconductor.
Often, they are made of silicon Semiconductors can conduct, or carry, electricity.
They don’t do this as well as metals, however. That is why they are called “semi.” Because they only “semi” conduct electricity, they can be used to control electric current.

On their top and bottom they typically have metal contacts through which current can flow.
A typical simple cell has two layers of silicon. One is known as n-type. The other is p-type. The layers are different from each other.

How Solar Cells Make Electricity
The process of making electricity begins when the silicon atoms absorb some light. The light’s energy knocks some electrons out of the atoms. The electrons flow between the two layers.
The flow makes an electric current. The current can leave the cell through the metal contacts and be used.
When light hits a solar cell, much of its energy is wasted. Some light bounces off or passes through the cell. Some is turned into heat.
Only light with the right wavelengths, or colors, is absorbed and then turned into electricity.

A single simple solar cell makes only a little electricity. For most purposes more is needed. For this reason, cells are often linked together in groups known as solar modules.
A solar module has a frame that holds the cells. Some modules are several feet long and wide.
They usually can produce up to a few hundred watts of electricity. If more power is needed, modules can be
joined together to form a large solar array.

Modules are sometimes called solar panels. Arrays are also sometimes called solar panels.
Whatever you call a group of solar cells, the fact remains: the more cells you link together, the more electricity you make.
With enough modules, huge amounts of power are possible.

More Power
Many experts think even bigger power plants using solar panels will be built in the coming years. Someday there may be solar plants able to make as much as 500 megawatts of power.
That is about what a typical coal power plant produces today. Solar panels work best when they directly face the Sun. For this reason, the panels are often put on “trackers.” The trackers turn the panels so that they follow the Sun as it moves across the sky.

Types of Solar Panel cells
* Monocrystalline solar panels - The most efficient and expensive solar panels are made with Monocrystalline solar cells.
* Polycrystalline solar panels - less expensive and slightly less efficient than Monocrystalline solar cells.
* Amorphous solar panels - are much cheaper, but their energy efficiency is also much less so more square footage is required to produce the same amount of power as the Monocrystalline or Polycrystalline type of solar panel

Solar panels for buildings are no different from other panels. They must be able to receive enough sunlight to be useful.
Often, they are put on a roof that faces the Sun and is not shaded.
Sometimes they are simply built on the ground.
Solar panels come in various colors and designs. They may be put on a wall or roof and blend right in, so you don’t even notice them. Roof shingles and tiles can be made using thin-fi lm panels.
Usually solar panels and a few wires cannot by themselves supply electricity to a building. More equipment is needed.
Solar cells make DC electricity. This is fine for some electronic devices. But home appliances and lights usually run on AC. Houses are generally wired for AC.
To change the DC to AC, a device called an inverter is needed.

Also, if the building is not connected to the public power grid, there has to be some way of storing electricity for use when it is too dark for the solar panels to work. Usually, batteries are used to store the electricity.
Batteries can be helpful even in buildings that are connected to the grid. They can serve as a backup if the grid suffers a power loss.

Solar panels have a lot of strong points. The silicon used in most of them is a very common material.
Sand is made up mostly of silicon. Solar panels are reliable. The “fuel” they use—sunshine—is free. It is a renewable resource that will last nearly forever.

Scientists expect the Sun to keep shining for billions of years. Also, solar panels can make electricity right at the place where it is used. This removes the need for
wires or cables to carry electricity from a distant power plant.
When they are used, solar panels have almost no effect on the environment.
They are quiet.
They don’t release dirty or harmful gases into the air.
They don’t cause water pollution.
They don’t create hazardous waste.

Power plants using solar panels have several advantages. They can usually be built more quickly than oil, coal, or nuclear power plants. If more power is needed, they are easier to expand than other types of power

Along with strong points, solar panels have some weak points.
For one thing, they work best when the Sun shines brightly.
When the sky is cloudy, they make less electricity.
One problem is the same everywhere.
When it’s dark, solar panels don’t work at all.
If a solar-powered home isn’t linked to the grid, it needs to have batteries or some other way to store electricity for use at night.

While sunshine is free, solar panels are not. Getting a solar system for a building costs money. In many cases today,
the total cost may turn out to be so high that it’s cheaper to get power from the grid. This may change in the future, though, as the cost of electricity made by power plants using oil goes up.

Also, as more solar panels are used, the price of the panels may come down.
Solar power plants that are able to make large amounts of electricity need large amounts of land—and also lots of solar panels that are costly today.

Most electricity used in South Africa comes from the public power grid. Sometimes the demand for power is almost more than the grid can handle. People can help out by making some of their own electricity. This eases the load on the grid.
But that’s not all it can do. Using a renewable “fuel” like sunlight to make electricity saves on nonrenewable fuels, such as oil or coal. It also avoids the pollution that comes from burning oil or coal.

When experts compare solar cells, one thing they look at is how costly a cell is. If it costs too much, nobody will buy it. One problem with the first solar cells was that they usually cost more than other power sources.

That is why the first important use of solar cells was in space satellites. There were no cheaper ways to make electricity in space in the 1950s.
Another thing experts look at is a cell’s “efficiency.” This tells how good a cell is at using sunlight.
A high-efficiency cell turns more of the sunlight’s energy into electric energy than a low-efficiency cell.
Ever since solar cells were invented, scientists have worked to make them cheaper and more efficient. There has been a lot of progress.
The first solar cells had an efficiency of less than 4 percent. Today cells cost a lot less, and many have an efficiency of 15 percent or more. Some experimental cells do even better.

Making Better Solar Cells
Scientists continue to hunt for ways to make better cells. They are trying new materials, such as plastics. They are also looking for ways solar cells and panels can be more useful.
For instance, they have come up with a “photo capacitor.” This is a solar device that both makes electricity and stores it for later use. Experimental models were not good for practical use.
But if the device can be improved, it might someday eliminate the need to have batteries to store solar electricity, at least in some cases.

People keep coming up with new ideas for solar energy. They dream up new designs for solar cells.
They think of new materials for cells. The new kinds of solar cells that result make possible new ways of using solar energy. Lots of ideas for solar products have been talked about in recent years.
Some of them have begun to appear on the market.
Others are still experimental.
Ideas for the Future Thin-film solar cells can be put into cloth. The larger the area of the solar cells, the more current they can make.
If they were used in curtains, thin-film solar cells could provide a very useful amount of electricity.

Many people think devices called hydrogen fuel cells might someday be very good sources of power for many purposes.
They use hydrogen to make electricity. Hydrogen is very common.
It is in water, for instance. Getting the hydrogen out of water, however, takes energy. If solar panels get cheap enough and efficient enough, they might become a practical way of providing this energy.
Ways of getting lots of sunlight to solar cells are being studied.
One proposed concentrator could be used on windows. It involves putting special dyes into glass or plastic. Solar cells are put at the edges of the sheet of glass or plastic.
The dyes let some light through the window. They also capture some light energy, which flows to the cells.

Another Kind of Solar Power
Solar panels are not the only way to make electricity from sunshine.
Another method—called solar thermal power—is also popular. In fact, it is used in the largest solar power plants.
Solar thermal power plants collect sunlight with the help of concentrators. Often, the light heats a liquid to a high temperature, and this hot liquid then turns water into
steam. Or the light may directly heat water and turn it into steam. Either way, the steam is then used to drive a turbine that makes electricity.
Small thermal systems are sometimes used in buildings to provide heating or make hot water.

Did you know?
Environmental Questions
Solar panels use a renewable resource. Compared to other ways of
making electricity, they are very clean.
But they are not perfect. It takes energy to get silicon ready for use in a solar cell. It takes more energy to make the cells and panels.
This energy usually comes from burning nonrenewable fuels like coal or oil.
Burning them puts gases that cause pollution into the air.
Factories that make solar cells produce very little hazardous waste.
But still there is some.
Solar panels that are used for many years wear out.
Experts are working on finding ways to recycle them.

Did you know?
Sun Plus Wind
When the Sun doesn’t shine, solar panels can’t make electricity. But there is a renewable energy source that can: the wind. Some people have electric power systems that combine solar panels with a wind turbine. Such a system is called a hybrid. Of course, the wind doesn’t always blow at night. So if the system has no link with the grid, it will still need batteries for electricity on windless nights.

Did you know?
Power from Space
In space there are no clouds. Solar panels there can get lots of sunshine. Scientists have proposed using huge solar arrays on satellites to make power. The power would then be beamed down to Earth for use. A big question is whether long-distance beams are practical. An experiment in 2008 managed to send a small amount of energy about 90 miles (150 kilometers). The experiment, which took place in Hawaii, used radio waves to carry the energy.

Did you know?
Another name for solar cells is photovoltaic cells. This name is fitting,because photo means “light” and voltaic refers to electricity.

Did you know?
Solar Planes
Solar panels have even been used on airplanes—but only on a few. The problem is they are hard to use with planes. They need to cover a big area in order to make enough electricity. When it gets dark, they don’t work. The fi rst solar-powered plane to fl y a long distance was the Solar Challenger. It crossed the English Channel in Europe in 1981. Its wings had more than 16,000 solar cells. In 2008 the Zephyr-6 spent more than three days in the air. It was a plane without any pilot. It carried batteries that stored electricity from its panels for use at night.

Did you know?
Power in Remote Places
Solar panels are a handy way of getting electricity in very remote places
where there is no public power system. They power runway lights at
airstrips in Antarctica. They are used in spacecraft and in “rovers” on the
planet Mars.

Our earth gets most of its energy from the sun. We call this energy solar energy.
Sol means sun.
Solar energy travels from the sun to the earth in rays. Some are light rays that we can see.
Some are rays we can’t see, like x-rays. Energy in rays is called radiant energy.
The sun is a giant ball of gas. It sends out huge rays of energy every day.
Most of the rays go off into space. Only a small part reaches the earth.

When the rays reach the earth, some bounce off clouds back into space—the rays are reflected.
The earth absorbs most of the solar energy and turns it into heat. This heat warms the earth and the air around it—the atmosphere. Without the sun, we couldn’t live on the earth—it would be too cold.

Photovoltaic (PV) cells turn the sun’s energy into electricity. Photo means light and volt is a measure of electricity. PV cells are made of a piece of silicon, the main ingredient in sand.
Each side of the silicon has a different chemical added. When radiant energy from the sun hits the
PV cell, the sides of the silicon work together to change the energy into electricity.
Some toys and calculators use small PV cells instead of batteries. Big PV cells can make enough electricity for a house. They are expensive, but good for houses far away from power lines.
Some schools are adding PV cells to their roofs. The electricity helps lower the amount of money schools must pay for energy.
Today, solar energy provides only a tiny bit of the electricity we use. In the future, it could be
a major source of energy. Scientists are looking for new ways to capture and use solar energy












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