Chapter 15: Solar Energy

We have always used the energy of the sun as far back as humans have existed on this planet. As far back as 5,000 years ago, people "worshipped" the sun. Ra, the sun-god, who was considered the first king of Egypt. In Mesopotamia, the sun-god Shamash was a major deity and was equated with justice. In Greece there were two sun deities, Apollo and Helios. The influence of the sun also appears in other religions - Zoroastrianism, Mithraism, Roman religion, Hinduism, Buddhism, the Druids of England, the Aztecs of Mexico, the Incas of Peru, and many Native American tribes.

We know today, that the sun is simply our nearest star. Without it, life would not exist on our planet. We use the sun's energy every day in many different ways.

When we hang laundry outside to dry in the sun, we are using the sun's heat to do work -- drying our clothes.

Plants use the sun's light to make food. Animals eat plants for food. And as we learned in Chapter 5, decaying plants hundreds of millions of years ago produced the coal, oil and natural gas that we use today. So, fossil fuels is actually sunlight stored millions and millions of years ago.

Indirectly, the sun or other stars are responsible for ALL our energy. Even nuclear energy comes from a star because the uranium atoms used in nuclear energy were created in the fury of a nova - a star exploding.

Let's look at ways in which we can use the sun's energy.

Solar Hot Water

In the 1890s solar water heaters were being used all over the United States. They proved to be a big improvement over wood and coal-burning stoves. Artificial gas made from coal was available too to heat water, but it cost 10 times the price we pay for natural gas today. And electricity was even more expensive if you even had any in your town!

Many homes used solar water heaters. In 1897, 30 percent of the homes in Pasadena, just east of Los Angeles, were equipped with solar water heaters. As mechanical improvements were made, solar systems were used in Arizona, Florida and many other sunny

parts of the United States. The picture shown here is a solar water heater installed on the front roof of a house in Pomona Valley, California, in 1911 (the panels are circled above the four windows).

By 1920, ten of thousands of solar water heaters had been sold. By then, however, large deposits of oil and natural gas were discovered in the western United States. As these low cost fuels became available, solar water systems began to be replaced with heaters burning fossil fuels.

Today, solar water heaters are making a comeback. There are more than half a million of them in California alone! They heat water for use inside homes and businesses. They also heat swimming pools like in the picture.

Panels on the roof of a building, like this one on the right, contain water pipes. When the sun hits the panels and the pipes, the sunlight warms them.

That warmed water can then be used in a swimming pool.

Solar Thermal Electricity

Solar energy can also be used to make electricity.

Some solar power plants, like the one in the picture to the right in California's Mojave Desert, use a highly curved mirror called a parabolic trough to focus the sunlight on a pipe running down a central point above the curve of the mirror. The mirror focuses the sunlight to strike the pipe, and it gets so hot that it can boil water into steam. That steam can then be used to turn a turbine to make electricity.

In California's Mojave desert, there are huge rows of solar mirrors arranged in what's called "solar thermal power plants" that use this idea to make electricity for more than 350,000 homes. The problem with solar energy is that it works only when the sun is shining. So, on cloudy days and at night, the power plants can't create energy. Some solar plants, are a "hybrid" technology. During the daytime they use the sun. At night and on cloudy days they burn natural gas to boil the water so they can continue to make electricity.

Another form of solar power plants to make electricity is called a Central Tower Power Plant, like the one to the right - the Solar Two Project.

Sunlight is reflected off 1,800 mirrors circling the tall tower. The mirrors are called heliostats and move and turn to face the sun all day long.

The light is reflected back to the top of the tower in the center of the circle where a fluid is turned very hot by the sun's rays. That fluid can be used to boil water to make steam to turn a turbine and a generator.

This experimental power plant is called Solar II. It was re-built in California's desert using newer technologies than when it was first built in the early 1980s. Solar II will use the sunlight to change heat into mechanical energy in the turbine.

The power plant will make enough electricity to power about 10,000 homes. Scientists say larger central tower power plants can make electricity for 100,000 to 200,000 homes.

Solar Cells or Photovoltaic Energy

We can also change the sunlight directly to electricity using solar cells.

Solar cells are also called photovoltaic cells - or PV cells for short - and can be found on many small appliances, like calculators, and even on spacecraft. They were first developed in the 1950s for use on U.S. space satellites. They are made of silicon, a special type of melted sand.

When sunlight strikes the solar cell, electrons (red circles) are knocked loose. They move toward the treated front surface (dark blue color). An electron imbalance is created between the front and back. When the two surfaces are joined by a connector, like a wire, a current of electricity occurs between the negative and positive sides.

These individual solar cells are arranged together in a PV module and the modules are grouped together in an array. Some of the arrays are set on

special tracking devices to follow sunlight all day long.

The electrical energy from solar cells can then be used directly. It can be used in a home for lights and appliances. It can be used in a business. Solar energy can be stored in batteries to light a roadside billboard at night. Or the energy can be stored in a battery for an emergency roadside cellular telephone when no telephone wires are around.

Some experimental cars also use PV cells. They convert sunlight directly into energy to power electric motors on the car.

But when most of us think of solar energy, we think of satellites in outer space. Here's a picture of solar panels extending out from a satellite.

Make a solar cell in your kitchen

A solar cell is a device for converting energy from the sun into electricity. The high-efficiency solar cells you can buy at Radio Shack and other stores are made from highly processed silicon, and require huge factories, high temperatures, vacuum equipment, and lots of money.

If we are willing to sacrifice efficiency for the ability to make our own solar cells in the kitchen out of materials from the neighborhood hardware store, we can demonstrate a working solar cell in about an hour.

Our solar cell is made from cuprous oxide instead of silicon. Cuprous oxide is one of the first materials known to display the photoelectric effect, in which light causes electricity to flow in a material.

Thinking about how to explain the photoelectric effect is what led Albert Einstein to the Nobel prize for physics, and to the theory of relativity.

Materials you will need

The solar cell is made from these materials:

1. A sheet of copper flashing from the hardware store. This normally costs about $5.00 per square foot. We will need about half a square foot.

2. Two alligator clip leads. 3. A sensitive micro-ammeter that can read currents between 10 and 50

microamperes. Radio Shack sells small LCD multimeters that will do, but I used a small surplus meter with a needle.

4. An electric stove. My kitchen stove is gas, so I bought a small one-burner electric hotplate for about $25. The little 700 watt burners probably won't work -- mine is 1100 watts, so the burner gets red hot.

5. A large clear plastic bottle off of which you can cut the top. I used a 2 liter spring water bottle. A large mouth glass jar will also work.

6. Table salt. We will want a couple tablespoons of salt. 7. Tap water. 8. Sand paper or a wire brush on an electric drill. 9. Sheet metal shears for cutting the copper sheet.

How to build the solar cell

My burner looks like this:

The first step is to cut a piece of the copper sheeting that is about the size of the burner on the stove. Wash your hands so they don't have any grease or oil on them. Then wash the copper sheet with soap or cleanser to get any oil or grease off of it. Use the sandpaper or wire brush to thoroughly clean the copper sheeting, so that any sulphide or other light corrosion is removed.

Next, place the cleaned and dried copper sheet on the burner and turn the burner to its highest setting.

As the copper starts to heat up, you will see beautiful oxidation patterns begin to form. Oranges, purples, and reds will cover the copper.

As the copper gets hotter, the colors are replaced with a black coating of cupric oxide. This is not the oxide we want, but it will flake off later, showing the reds, oranges, pinks, and purples of the cuprous oxide layer underneath.

The last bits of color disappear as the burner starts to glow red.

When the burner is glowing red-hot, the sheet of copper will be coated with a black cupric oxide coat. Let it cook for a half an hour, so the black coating will be thick. This is important, since a thick coating will flake off nicely, while a thin coat will stay stuck to the copper.

After the half hour of cooking, turn off the burner. Leave the hot copper on the burner to cool slowly. If you cool it too quickly, the black oxide will stay stuck to the copper.

As the copper cools, it shrinks. The black cupric oxide also shrinks. But they shrink at different rates, which makes the black cupric oxide flake off.

The little black flakes pop off the copper with enough force to make them fly a few inches. This means a little more cleaning effort around the stove, but it is fun to watch.

When the copper has cooled to room temperature (this takes about 20 minutes), most of the black oxide will be gone. A light scrubbing with your hands under running water will remove most of the small bits. Resist the temptation to remove all of the black spots by hard scrubbing or by flexing the soft copper. This might damage the delicate red cuprous oxide layer we need to make to solar cell work.

The rest of the assembly is very simple and quick.

Cut another sheet of copper about the same size as the first one. Bend both pieces gently, so they will fit into the plastic bottle or jar without touching one another. The cuprous oxide coating that was facing up on the burner is usually the best side to face outwards in the jar, because it has the smoothest, cleanest surface.

Attach the two alligator clip leads, one to the new copper plate, and one to the cuprous oxide coated plate. Connect the lead from the clean copper plate to the positive terminal of the meter. Connect the lead from the cuprous oxide plate to the negative terminal of the meter.

Now mix a couple tablespoons of salt into some hot tap water. Stir the saltwater until all the salt is dissolved. Then carefully pour the saltwater into the jar, being careful not to get the clip leads wet. The saltwater should not completely cover the plates -- you should leave about an inch of plate above the water, so you can move the solar cell around without getting the clip leads wet.

The photo above shows the solar cell in my shadow as I took the picture. Notice that the meter is reading about 6 microamps of current.

The solar cell is a battery, even in the dark, and will usually show a few microamps of current.

The above photo shows the solar cell in the sunshine. Notice that the meter has jumped up to about 33 microamps of current. Sometimes it will go over 50 microamps, swinging the needle all the way over to the right.

How does it do that?

Cuprous oxide is a type of material called a semiconductor. A semiconductor is in between a conductor, where electricity can flow freely, and an insulator, where electrons are bound tightly to their atoms and do not flow freely.

In a semiconductor, there is a gap, called a bandgap between the electrons that are bound tightly to the atom, and the electrons that are farther from the atom, which can move freely and conduct electricity.

Electrons cannot stay inside the bandgap. An electron cannot gain just a little bit of energy and move away from the atom's nucleus into the bandgap. An electron must gain enough energy to move farther away from the nucleus, outside of the bandgap.

Similarly, an electron outside the bandgap cannot lose a little bit of energy and fall just a little bit closer to the nucleus. It must lose enough energy to fall past the bandgap into the area where electrons are allowed.

When sunlight hits the electrons in the cuprous oxide, some of the electrons gain enough energy from the sunlight to jump past the bandgap and become free to conduct electricity.

The free electrons move into the saltwater, then into the clean copper plate, into the wire, through the meter, and back to the cuprous oxide plate.

As the electrons move through the meter, they perform the work needed to move the needle. When a shadow falls on the solar cell, fewer electrons move through the meter, and the needle dips back down.

A note about power

The cell produces 50 microamps at 0.25 volts. This is 0.0000125 watts (12.5 microwatts). Don't expect to light light bulbs or charge batteries with this device. It can be used as a light detector or light meter, but it would take acres of them to power your house.

The 0.0000125 watts (12.5 microwatts) is for a 0.01 square meter cell, or 1.25 milliwatts per square meter. To light a 100 watt light bulb, it would take 80 square meters of cuprous oxide for the sunlit side, and 80 square meters of copper for the dark electrode. To run a 1,000 watt stove, you would need 800 square meters of cuprous oxide, and another 800 square meters of plain copper, or 1,600 square meters all together. If this were to form the roof of a home, each home would be 30 meters long and 30 meters wide, assuming all they needed electricity for was one stove.

There are 17,222 square feet in 1,600 square meters. If copper sheeting costs $5 per square foot, the copper alone would cost $86,110.00 USD. Making it one tenth the thickness can bring this down to $8,611.00. Since you are buying in bulk, you might get it for half that, or about $4,300.00.

If you used silicon solar panels costing $4 per watt, you could run the same stove for $4,000.00. But the panels would only be about 10 square meters.

Or, for about a dollar, you can build a solar stove out of aluminum foil and cardboard. For about $20, you can build a very nice polished aluminum parabolic solar cooker.

A flat panel solar cell

I made a more portable version of the solar cell in a flat panel form. I used the clear plastic top from a plastic CD jewel case as the window, and lots of silicone rubber glue to both attach the pieces together and to insulate them from each other.

The first step is to make a cuprous oxide plate like we did in the first solar cell. This time I sanded one corner clean all the way down to the shiny copper, and soldered an insulated copper wire to it for the negative lead.

The positive plate is a U shaped piece cut from the copper sheeting, a little bit larger than the cuprous oxide plate, with the cutout portion of the U a little bit smaller than the cuprous oxide plate. Another insulated copper wire is soldered to one corner of the U.

The first step in construction is to glue the U shaped copper plate to the plastic window. Use plenty of silicone glue, so the saltwater won't leak out. Make sure that the solder connection is either completely covered with glue, or is outside of the glue U, as shown in the photo (completely covered in glue is best).

The photo below shows the back side of the solar cell (the side not facing the sun) at this point in the construction.

The photo below shows the front side of the solar cell (the side that will face the sun) at this point in the construction. Notice that the silicone glue does not completely cover the copper, since some of the copper must eventually be in contact with the saltwater.

The next step is to lay a good size bead of glue onto the U shaped clean copper plate. This layer will act as an insulator between the clean copper

plate and the cuprous oxide plate, and must be thick enough to leave some room for the saltwater. Again, not all of the copper is covered, so there will be plenty of copper in contact with the saltwater.

Gently press the cuprous oxide plate onto this layer of glue. You should press hard enough to make sure the glue seals off any gaps, but not so hard that the two plates touch.

The photo below shows the back side of the solar cell (the side not facing the sun) at this point in the construction.

The photo below shows the front side of the solar cell (the side that will face the sun) at this point in the construction. Note that I added extra glue to form a funnel at the top to allow the saltwater to be added.

You can click on the photo above to get a bigger picture.

Not shown in the photo is a generous extra bead of glue all around the outside of the plates, to ensure that no saltwater will leak out. Allow the glue to cure before going on to the next step.

Next, use a large eyedropper to add the saltwater. Fill the cell up almost to the top of the copper plate, so it almost spills out. Then seal the funnel with another generous bead of glue, and allow the glue to cure at least a half hour.

In the photo above you can see the flat panel solar cell in action in the bright sun. It is delivering about 36 microamperes of current. You can also see the extra bead of glue around the edges of the plates, and filling the top of the funnel.

Finally, another shot of the author's shadow. Note that the meter now reads about 4 microamperes, since no sunlight is falling on it.

A note about power

Many readers were jumping straight to this page without reading the previous project, so I am repeating this note here:

The cell produces at most about 50 microamps at 0.25 volts. This is 0.0000125 watts (12.5 microwatts). Don't expect to light light bulbs or charge batteries with this device. It can be used as a light detector or light meter, but it would take acres of them to power your house.

The 0.0000125 watts (12.5 microwatts) is for a 0.01 square meter cell, or 1.25 milliwatts per square meter. To light a 100 watt light bulb, it would take 80 square meters of cuprous oxide for the sunlit side, and 80 square meters of copper for the dark electrode. To run a 1,000 watt stove, you would need 800 square meters of cuprous oxide, and another 800 square meters of plain

copper, or 1,600 square meters all together. If this were to form the roof of a home, each home would be 30 meters long and 30 meters wide, assuming all they needed electricity for was one stove.

There are 17,222 square feet in 1,600 square meters. If copper sheeting costs $5 per square foot, the copper alone would cost $86,110.00 USD. Making it one tenth the thickness can bring this down to $8,611.00. Since you are buying in bulk, you might get it for half that, or about $4,300.00.

If you used silicon solar panels costing $4 per watt, you could run the same stove for $4,000.00. But the panels would only be about 10 square meters.

Or, for about a dollar, you can build a solar stove out of aluminum foil and cardboard. For about $20, you can build a very nice polished aluminum parabolic solar cooker.

Build a hydrogen fuel cell.

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on image for a larger picture

A fuel cell is a device that converts a fuel such as hydrogen, alcohol, gasoline, or methane into electricity directly. A hydrogen fuel cell produces electricity without any pollution, since pure water is the only byproduct.

Hydrogen fuel cells are used in spacecraft and other high-tech applications where a clean, efficient power source is needed.

You can make a hydrogen fuel cell in your kitchen in about 10 minutes, and demonstrate how hydrogen and oxygen can combine to produce clean electrical power.

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To make the fuel cell, we need the following:

One foot of platinum coated nickel wire, or pure platinum wire. Since this is not a common household item, we carry platinum coated nickel wire in our catalog.

A popsickle stick or similar small piece of wood or plastic. A 9 volt battery clip. A 9 volt battery. Some transparent sticky tape. A glass of water. A volt meter.

The first step is to cut the platinum coated wire into two six inch long pieces, and wind each piece into a little coiled spring that will be the electrodes in our fuel cell. I wound mine on the end of the test lead of my volt meter, but a nail, an ice pick, or a coat hanger will do nicely as a coil form.

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Next, we cut the leads of the battery clip in half and strip the insulation off of the cut ends. Then we twist the bare wires onto the ends of the platinum coated electrodes, as shown in the photo. The battery clip will be attached to the electrodes, and two wires will also be attached to the electrodes, and will later be used to connect to the volt meter.

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The electrodes are then taped securely to the popsickle stick. Lastly, the popsickle stick is taped securely to the glass of water, so that the electrodes dangle in the water for nearly their entire length. The twisted wire connections must stay out of the water, so only the platinum coated electrodes are in the water.

Now connect the red wire to the positive terminal of the volt meter, and the black wire to the negative (or "common") terminal of the volt meter. The volt meter should read 0 volts at this point, although a tiny amount of voltage may show up, such as 0.01 volts.

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Your fuel cell is now complete.

To operate the fuel cell, we need to cause bubbles of hydrogen to cling to one electrode, and bubbles of oxygen to cling to the other. There is a very simple way to do this.

We touch the 9 volt battery to the battery clip (we don't need to actually clip it on, since it will only be needed for a second or two).

Touching the battery to the clip causes the water at the electrodes to split into hydrogen and oxygen, a process called electrolysis. You can see the bubbles form at the electrodes while the battery is attached.

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Now we remove the battery. If we were not using platinum coated wire, we would expect to see the volt meter read zero volts again, since there is no battery connected.

The platinum acts as a catalyst, allowing the hydrogen and oxygen to recombine.

The hydrolysis reaction reverses. Instead of putting electricity into the cell to split the water, hydrogen and oxygen combine to make water again, and produce electricity.

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We initially get a little over two volts from the fuel cell. As the bubbles pop, dissolve in the water, or get used up by the reaction, the voltage drops, quickly at first, then more slowly.

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After a minute or so, the voltage declines much more slowly, as most of the decline is now due only to the gasses being used up in the reaction that produces the electricity.

Notice that we are storing the energy from the 9 volt battery as hydrogen and oxygen bubbles.

We could instead bubble hydrogen and oxygen from some other source over the electrodes, and still get electricity. Or we could produce hydrogen and oxygen during the day from solar power, and store the gasses, then use them in the fuel cell at night. We could also store the gasses in high pressure tanks in an electric car, and generate the electricity the car needs from a fuel cell.

How does it do that?

We have two things going on in this project — the electrolysis of water into hydrogen and oxygen gasses, and the recombining of the gasses to produce electricity. We will look into each step separately.

The electrode connected to the negative side of the battery has electrons that are being pushed by the battery. Four of the electrons in that electrode combine with four water molecules. The four water molecules each give up a hydrogen atom, to form two molecules of hydrogen (H2), leaving four negatively charged ions of OH-.

The hydrogen gas bubbles up from the electrode, and the negatively charged migrate away from the negatively charged electrode.

At the other electrode, the positive side of the battery pulls electrons from the water molecules. The water molecules split into positively charged hydrogen atoms (single protons), and oxygen molecules. The oxygen molecules bubble up, and the protons migrate away from the positively charged electrode.

The protons eventually combine with the OH- ions from the negative electrode, and form water molecules again.

The Fuel Cell

When we remove the battery, the hydrogen molecules that are clinging as bubbles to the electrode, break up due to the catalytic action of the platinum, forming positively charged hydrogen ions (H+, or protons), and electrons.

At the other electrode, the oxygen molecules stuck in bubbles on the platinum surface draw electrons from the metal, and then combine with the hydrogen ions in the water (from the reaction at the other electrode) to form water.

The oxygen electrode has lost two electrons to each oxygen molecule. The hydrogen electrode has gained two electrons from each hydrogen molecule. The electrons at the hydrogen electrode are attracted to the positively charged oxygen electrode. Electrons travel more easily in metal than in water, so the current flows in the wire, instead of the water. In the wire, the current can do work, such as lighting a bulb, or moving a meter.