(Click anywhere to close preview.)

Sustainable Homes for the 21st Century


Walmart Foundation

Solar Energy is not only future, but it is our past and present

Our world is changing and growing every day. Along with all of these changes come increased demands on our resources to provide such fundamentals as food and energy. Right now, we heavily rely on fossil fuels such as oil, natural gas and coal to provide us with energy to our homes and our various modes of transportation. However, fossil fuels don't last forever. We need to increase our use of renewable resources such as Solar Energy to help us meet our future energy demands. In order to do that, we must first look at the past: the progression of solar energy, the present: how we use solar energy today, and finally the future: what will be doing years down the road to harness more energy from the sun.

Most people today are familiar with solar panels, perhaps you even know someone who has solar panels on their home or where they work, perhaps on their car as well. You may be surprised to learn that you yourself may have them in your home right now! Try to find a calculator that does not require batteries. However, harnessing solar energy has been around for a very long time. How long do you think we have been harnessing energy from the sun?

Do you have an answer yet? Is it 50 years? 100 years? 1000 years. Other then the sun just warming us naturally, how long have we been focusing or managing its energy on purpose? The answer may be surprising. People have been harnessing energy from the sun as far back as the 7th century BC. That is more than 2700 years! So what did they do?

How Ancient Civilizations used Mirrors and Lenses

In the 7th century BC, people would use a magnifying glass to focus rays of light from the sun. People focused the rays of light for a variety of reasons. The light rays were focused for warmth, cooking or to light some fuel source. It wasn't that long before people realized they could focus these light rays for other uses. Some speculated that they too burned ants.

Mirrors came into play a short time later. Mirrors were also used to focus sunlight for various religious purposes with the Greeks and Romans starting around the 3rd century BC. In fact, some speculate that this is how the Olympic torch was lit (Figure 1). This idea is strongly rooted in Greek Mythology. This flame commemorates Prometheus, one of the Titans, stealing fire from Zeus so that he could give it to all humans. The sacred flame was lit from the sun's rays that originated at Olympia. Thus, the flames remained lit throughout the entire Olympic ceremonies.

greek-torch

Figure 1: Ancient Greeks showing how you could light a torch with a parabolic mirror.

It wasn't long after this that people figured out how turn solar power into a weapon. Legend has it that as early as 212 B.C., Archimedes used the reflective properties of bronze shields to focus the sunlight and then set fire to the Romans wooden ships. It is known that Syracuse was invaded at this time and when Archimedes himself lost his life. However, there is no evidence that these "death rays" were used to ward off the attacking ships (Figure 2). But could they? The debate continues on and has yet to offer a definitive answer. In 1973, The Greek navy recreated this experiment and they were able to burn a wooden ship about 50 m (160 ft) away from them. MIT students did this in 2006 as well and recently Mythbusters showed that they needed perfect conditions in order to get even cinders on the wooden shape. So did Archimedes actually create and use this mirror system? Nobody knows for sure! We do know that modern parabolic mirrors can focus the sun's rays and raise the temperature to an incredible 400 F (Figure 3)!

heat ray

Figure 2: A possible array of mirrors used to start a fire on enemy ships.

solar cooking 1

Figure 3: Examples of Parabolic mirrors.

"What was Archimedes' death ray?"

This is an extra article which provides additional insight into Archimedes' famed death ray.

How Solar Power was First Introduced into the Home

Shortly into the start of the 1st century through the 4th century AD, solar power was used in the homes. Nothing like how we use solar power today, but rather people would build their homes and other structures like Roman bath houses to maximize the suns energy. For example, Roman bath houses had large windows with southern exposure to allow a great deal of sunlight into the bath houses. In fact, over the next couple hundred of years, that sunrooms in houses and buildings are so common laws are put into place in the 6th century AD to ensure every building has access to the sun. This is what the Justinian code (legal code of Ancient Rome) called "Sun Rights."

It wasn't just the Romans who recognized the importance of building homes to maximize the sun's energy. In the 13th century, the Anasazi (ancestors of the Pueblo people) built their cliff dwellings facing the south (Figure 4). They did this again to maximize the amount of energy from the sun. So this beckons the question, why is a southern exposure so desirable?

housing

Figure 4: A typical cliff dwelling of the Anasazi people.

"What the ancients knew: Roman bath culture" (2:25)

This video not only talks about how the bath houses maximized the sunlight but it also talks about the social significance of the bath houses.

Why do we Care about Southern Exposure?

Ask any realtor and they will tell you the importance of southern exposures. However, this is only true in the northern hemisphere (Figure 5). If you were a realtor in South America, Northern exposure is more important. Here in the U.S. the sunlight comes from the south. This is for two reasons, one the Earth is roughly spherical and two, the Earth is tilted on the axis by about 230 (this is also gets into why we have seasons). Look at the following illustration:

earth-house-summer

The arrangement of the house and sun in the summer

earth-house-winter

The arrangement of the house and sun in the winter

Figure 5: Arrangements of the house and sun in different seasons.

In the summer, you can see a house sitting on the Earth in the picture above. The sunlight comes in on the left side, but from the perspective of the house, it is directed to the South. Even in the winter, when the Earth is on the opposite side of the sun, the sunlight still appears to come in from the South (from your perspective inside the house). Now this brings in another architectural design: that is the placement of eaves in your house (Figure 6). Eaves are those small overhangs on the side of your house. If you have a deep eave, that will allow the winter sunlight to come in, but during the summer time, the sunlight will be blocked which will prevent the house from getting warmer. This has to do with the position you see the sun in the sky. If you look at the figure (Figure 6), the summer sun is much higher in the sky then the winter sun. Thus, the eaves will help to let all of the warmth from the winter sun in but not let in too much warmth in during the summer.

earth-house3

Figure 6: Position of the sun during different seasons.

Cooking without Gas or Electricity just the Sun

Surprisingly, it took a little bit of time before people started to use solar energy beyond their home. One of the next big steps in the use of solar energy is for cooking food. A big part of the reason that solar energy was used for cooking was due to another material that was becoming widely used in the 18th century: glass. If you ever sat in a sunroom or a car on a hot day with the windows up or you notice that you get pretty warm pretty quickly. People in the late 1700s noticed this as well. Nobody did any scientific work with this idea until a gentleman by the name of Horace de Saussure came along. Though his colleagues were more interested in burning things with mirrors, he was interested in understanding how well glass could trap thermal energy.

De Saussure first built a miniature greenhouse five walls thick (Figure 7). He constructed it from five square boxes of glass, decreasing in size from 12 in. on a side by 6 in. high to 4 in. on a side by 2 in. high. The bases of the boxes were cut out so the five boxes could be stacked one inside the other atop a black wooden table. After exposing the apparatus to the sun for several hours, and rotating the model so that solar rays always struck the glass covers of the boxes perpendicularly, de Saussure measured the temperature inside. The outermost box was the coolest, and the temperature increased in each succeeding smaller box. The bottom of the innermost box registered the highest temperature—189.5º F. "Fruits. . . exposed to this heat were cooked and became juicy," he wrote.1

Saussure1

Figure 7: Artist's rendition of de Saussure's hot box.

So how do solar cookers work? (Figure 8) We first need to understand one thing about sunlight: sunlight is a form of electromagnetic radiation. Sunlight itself isn't "hot." It's the effect that the radiation has on objects that makes us feel warm from sunlight. When the light hits your skin, the electro-magnetic radiation causes the molecules in your skin to speed up. As the molecules speed up, the amount of energy they possess also increases, and your skin feels warmer because you have more thermal energy. Solar cookers work the same way. With the solar cooker, the glass top allows the radiation to penetrate through the glass. The sunlight hits the surface on the inside and causes the energy to increase which in turn makes objects warmer. Though the glass lets the sunlight it, it does not let the thermal radiation out, so energy keeps being added to the cooker, but does not easily leave it. As a result, the temperature gets very high inside the solar cooker. To increase the amount of energy going to the food being cooked, the inside of the cooker will be covered with mirrors or some other type of reflective surface. Temperatures can get upwards of 300 degrees Fahrenheit, or roughly 150 degrees Celsius. At this temperature you can safely cook many types of foods. You can go try cooking your own meal with your solar cooker. You can go searching on the Internet and buy a solar cooker to use in your own home or you can make your own.

solar-cooking-2

Figure 8: How a solar box cooker works.

1This paragraph of information was taken from http://solarcooking.org/saussure.htm. The authors did such a wonderful job in describing De Saussure's work that we could not make any modifications or improvements.

"Simply Science: Solar Cooking" (2:02)

This video shows how one family is able to cook a meal with a home-made solar cooker.

The First Steps: Sunlight to Electricity

When we think about solar energy, we may not think about solar cookers and southern exposures, we think about those solar panels on top of our houses. It was only about 70 years after DaSaussure's work with solar ovens that Becquerel discovered something very fascinating about light and electricity. In 1869 Edmund Becquerel discovered the photoelectric effect (also called the photovoltaic effect). Amazingly, he was only 19 years old when he discovered this. His setup involved placing two metal electrodes into an electricity-conducing solution. A setup of his apparatus is shown (Figure 9).

BECQ

Figure 9: An example of Becquerel's setup.

Becquerel found that different colors and types of light had different effects. The best results were those from lights of lower wavelengths, mostly blue lights and ultra-violet colored light. Even coating different materials on the electrodes produced different results, the best of which appeared to be Silver Chloride AgCl or Silver Bromide AgBr. Though Becquerel is credited as the first to discover this effect, he was not the one to explain it. We will need to wait about another 70 years for that explanation. In the meantime, solar energy grew in its applications! For example, in the 1860s, August Mouchet, a French mathematician proposed ways that solar energy could be used to power engines to do tasks such as making ice (when he hooked up his steam engine powered by the sun to his refrigeration device). Some argue that August Mouchet is the father of modern solar energy.

Other substances exhibit the photoelectric effect - such as Selenium (Se). What makes Selenium so special? Well, it was first discovered in 1873 that it has photoconductive properties by Willoughby Smith. Then, in 1876 William Adams and Richard Day were able to use it to produce electricity. Though the amount of electricity it produced was so small it couldn't power electrical motors, it showed for the first time that a solid material, as opposed to a liquid, could create electricity without the need for moving parts or thermal energy (Figure 10).

VSELEN

Figure 10: Adams and Days' Selenium glass tube.

Adams and Day credited the current generated by the light to crystallization on the Selenium bar. Again, it was several years before scientists truly understood what was going on. But, it was only another seven years until the next big step for solar energy was discovered, something that very closely resembles to what we think of today when we think of solar energy.

In 1883 Charles Fritts, an American Inventor, made a solar cell of very thin Selenium wafers (Figure 11). Fritts was able to create very thin Selenium films by compressing the molten selenium and then compress them to two different metals such as gold and brass.

 

THINSEL

Figure 11: Fritts' thin-film Selenium wafer from 1883.

Fritts realized the great potential of these devices. For one, the devices were relatively inexpensive to make, thus they could make a lot of them. Furthermore, the current created by the devices could be used immediately, or stored in batteries for a time when it was needed. Lastly, the current could also be transmitted through wires to distances far away from where it was created. Later, in 1902 it was discovered by Philip Eduard Anton von Lenard that when the frequency of the light on the materials increased, so did current production.

The Magic Behind the Solar Cell

It wasn't until 1905 that the photoelectric effect was explained. The man credited with the explanation was Albert Einstein. This was also the year that he published his influential paper on relativity. However it wouldn't be until 1921 when he receives a Nobel Prize for his work on the photoelectric effect. In order to understand the photoelectric effect, we need to understand a little bit about the debate of what light actually is, or was thought to be at that time.

Anton von Lenard's work in 1902 challenged the current ideas of how light behaved. The prevailing theory of the time of how light behaved was proposed by James Clerk Maxwell. Maxwell believed that light was a wave based on his diffraction experiments. If this were true then you would only need to increase the amount of the light to increase the energy in the substance. Einstein was able to solve this issue (as described in the next paragraph). He believed that light consisted of photons, or rather discrete quanta's of energy. Another way to say this is that a quanta of light is a small bundle of a specific amount of energy.

Light is a form of energy. When it hits a substance, if the energy is high enough, then it will excite the electrons in the material and it will cause them to leave, thus creating a moving charge which, by definition is a current (Figure 12). Einstein realized that the energy had to be above a certain level in order to excite the electrons. According to Maxwell, if we simply increased the number of the waves then the energy on the surface would increase. This increase in energy would then be enough to excite the electrons. Sadly, Maxwell was mistaken. Anton van Lenard found that the frequency needed to be increased to increase the energy. Einstein believed that the energy of each photon was equal to its frequency multiplied by some constant (which later was called Planck's constant). If a photon had a frequency high enough, then it had enough energy to kick out an electron from the substance, thus creating the photo-electric effect.

photoelectric effect

Figure 12: Electrons being excited from light which depicts the photoelectric effect.

You might wonder why, if Einstein was able to theorize this, why did it take so long for him to get the Nobel Prize for his work? Typically, a Nobel Prize is not awarded until someone else can also demonstrate that groundbreaking research is correct. Ironically, the man who helped verify Einstein ideas set out to initially disprove Einstein's theory- Robert Millikan. Though Millikan was more commonly known for his oil drop experiment, he spent almost a decade trying to prove Einstein wrong. The reason he spent so much time to prove him wrong was that there was so much evidence that light was a wave. If Einstein was correct, then what modern scientists believed at the time was wrong, or at the very least, had to be modified. However, more than a decade after Einstein theorized his work in 1905, Millikan's experiments confirmed Einstein's theories in every detail in 1916.

"Physics: Applications of the Photoelectric Effect" (9:43)

This video describes how solar cell works. From that, they are able to describe the photoelectric effect resulting applications of the effect.

Solar Cells: The Greatest Invention that Almost Never Was

While Einstein and Millikan worked on explaining the photoelectric effect, other scientists were discovering new ways to collector solar energies, such as, discovering the photoelectric effect in other materials, and finding ways to produce more silicon crystals. All of these advancements helped develop the solar panels that we use today. It wasn't until about 1954 that solar panels started to have any real type of efficiency that made them practical for use in generating electricity.

Some people say that 1954 was the birth year of modern photovoltaic technology. Scientists Daryl Chapin, Calvin Fuller and Gerald Pearson worked at Bell Laboratories and in 1954 they created the silicon photovoltaic cell or also known as a "PV cell". This was the first time that solar cells created enough energy that they could actually run equipment. The scientists at the labs later made solar cells that have efficiencies at 6% and then in 1960 Hoffman Electronics got efficiency up to 11%. What is so fascinating about this discovery is that Pearson did not actually set out to create a better solar collector. Pearson was looking at silicon for its applications with electronics, not with solar cells. However, like many things in science, he accidently discovered that it made a far better efficient solar cell then those cells made with selenium. The New York Times went on record to say that this is "the beginning of a new era, leading eventually to the realization of harnessing the almost limitless energy of the sun for the uses of civilization."

This new discovery opened up many possibilities and allowed for the creation of new products, including toys, solar powered radios, dollar bill changers, and devices that decoded punch cards in early computers. However it was not without its shortfalls. For one, it was very expensive to use with household items. For example, for a power plant to supply a house with one watt would only cost about 50 cents. If a solar cell were used to supply that same one watt, it was estimated that the cost would range from $300 to $1500. With such a huge difference in cost, it seemed like the ability to commercialize the solar cells was doomed. So what saved solar cells?

The answer was the U.S. Military with some help from the Russians. It was around this time that the great Space Race began. Sputnik was launched on October 4th, 1957. This single event spurred the space age and eventually helped lead to the founding of NASA. The U.S. Army and the Air Force were initially joined in a top secret military project but it was later turned over to the Navy. This project involved the creation of earth orbiting satellites. The Navy thought that solar cells would not be a viable power source because they were untried and not yet proven for satellites. They decided to pursue more traditional chemical batteries. This idea met with strong opposition from Dr. Hans Ziegler the leading expert in satellite instrumentation at the time. Ziegler understood that solar cells could power satellites for an extended length of time unlike a chemical battery that would quickly run out, rendering a million dollar satellite useless. After much debate, a compromise was made. The new Vanguard satellites would have a dual power system, a chemical battery and a solar cell. Ironically, just as Ziegler predicted, the chemical batteries died within a week.

The solar cells that were created for these satellites took years to design. While these cells were being created, a very important discovery was made. In fact, that application is still used today. Mandelkorn created a solar cell that has a lower resistance to radiation. This is called an n-on-p cell and is described in the next section.

Modern Solar Cells: aka N-on-P Cells

What exactly is an "n-on-p" cell? The n portion of the cell is a special type of silicon cell that is a combination of silicon that has bonded with compounds that contain one more valence electron then silicon has, such as Phosphorus. This has a negative charge. Silicon only requires four electrons to bond with it, thus this extra electron is available to be conducted through the material. The p portion is the opposite, it is silicon combined with compounds containing one less valence electron, such as Boron, and has a positive charge. This means that only three electrons are available to bond with the silicon leaving one spot opened for a free electron. A picture of the junction is shown (Figure 13).

pn_junction

Figure 13: An n-on-p cell showing the different zones.

The depletion zone is the area where you have electrons moving from the n type to the p type. Moving electrons is current, thus you have the ability to create a current which can be used to power different objects.

The Vanguard I satellite was able to have a very small PV array. Though this array was less than a single watt, it was powerful enough to run the radios. The Vanguard satellite was the first of many that year to use PV power supplies for onboard electronics. Explorer III, Vanguard II and Sputnik -3 were also launched in 1958 and each had PV power supplies. In the following year, Explorers VI and VII were also launched, both containing PV power supplies. In fact, Explorer VI contained 9600 cells each of which were only 1 cm by 2 cm. These silicon solar cells become a cornerstone for all space missions and applications.

Throughout the sixties, the amount of power each solar cell could produce increased. Most of the cells were used in space missions, but others started appearing on buildings. For example, some lighthouses became powered by solar cells as well as the Astronomical Observatory installed a 1000 W cell in 1966 to supply power.

Solar technologies became much cheaper in the 1970s. Dr. Elliot Berman created new solar cells that lowered the cost from $100 per watt to $20 per watt. This drastic reduction in price led to a much larger market for solar cells. For example, at this point became practical to place solar panels as a source of power in remote areas where people cannot hook up to any kind of electrical grid. For example, at the end of the decade, NASA installed a 3.5 kilowatt photovoltaic system in southern Arizona on the Papago Indian reservation. This solar power system provided power for 15 homes which gave them electricity to use for, among other things, pumping groundwater. Other examples include the coast guard who uses solar power for its buoys and lighthouses. Also at this time, the first solar powered railroad crossing sign was created. Other warning lights, like those used on oil rigs started to become solar powered.

Solar Cells in the Sky and on the Road

The 1970's saw a large growth in applications of solar energy but the 80's brought about another change with solar energy -vehicles. Believe it or not, it wasn't an automobile that was the first solar powered vehicle, but an airplane! In 1981 Paul MacCready built the Solar Challenger (Figure 14): the first solar powered aircraft. This aircraft was flown from France to England across the English Channel a distance of 163 miles. The plane soared at 11,000 ft at a top speed of only 40 miles per hour and stayed in the air a total of 5 hours and 23 minutes. The wings contained more than 16,000 solar cells which powered a pair of motors. This airplane was an advancement of MacCready's prototype plane, the Gossamer Penguin (Figure 15). This prototype only had 3,920 solar cells and its public demonstration with NASA only flew for about 2 miles.

solarchallenger

Figure 14: An ariel view of the Solar Challenger.

Gossamer_penquin

Figure 15: A side view of the Gossamer Penguin.

It was the very next year that the solar cells found their way onto a vehicle. Hans Tholstrup drove the first solar powered car in Australia for 2800 miles! Tholstrup drove his car, "The Quiet Achiever" (Figure 16) or as some called it ‘The Bathtub on Wheels", from Sydney to Perth in 20 days. Though this was only an average of 140 miles per day, it was actually 10 days faster then what it took the first gasoline automobile to drive this distance. Tholstrup is also credited with the creation of the first international solar race in 1987 called the "Solar Challenge" which still runs today.

Quiet Achiever

Figure 16: The Quiet Achiever en route to Perth.

Also in this year, Volkswagon started experimenting with solar panels on one of their vehicles -the Dasher station wagon. Solar arrays on the vehicle provided 160 Watts to help with the ignition system and reduce fuel usage

"Paul MacCready on Solar Planes" (21:32)

Paul MacCready is giving a talk at a TED conference where he discusses solar planes: specifically his work with them and where the future may go.

"How Solar Cars Work" (3:35)

This video investigates whether or not a car can effectively run on nothing but solar cells.

The World's Biggest Cooker?

Solar Furnace

Figure 17: Eight-story solar furnace.

This solar furnace (Figure 17) is eight stories tall and contains 10,000 mirrors which form one very large concave mirror. It is not the type of furnace like the one you may have in your home to help keep you warm, at least not in that same way. This concave mirror focuses the sunlight onto an area that is only the size of about a cooking pot. This "cooking pot" gets upwards to a temperature of about 5400o Fahrenheit. This can be used to boil away water into steam which is used in a process called electromagnetic induction (described later).

Supplying Electricity to the World

The 1980's brought not only solar powered vehicles, but different types of solar power plants. For example, in Barstow California, Solar One was built in 1982 (Figure 18). This was a 10 megawatt power tower system designed to show the feasibility of solar power generation on a large scale. The plant works in a similar fashion to the solar furnace described earlier. Solar One operated until 1986 and was then redesigned and renamed Solar Two. Solar One used 1,818 mirrors to concentrate the sun's rays into a focal point which was a central tower. A high temperature transfer fluid was used to bring the thermal energy to the boiler on the ground which was used to create steam which then turned the turbines which created electricity. Most of our modern day power plants use this same technology.

Solar One

Figure 18: An ariel view of Solar One.

The largest facility built during the 1980s was built in 1986 and remains the world's largest facility to date. This solar thermal facility was built in Kramer Junction California (which is part of a larger group of facilities) and is still in use today (Figure 19). Currently, there are 936,384 mirrors which if lined up would be 229 miles long. The facility covers a total of 1600 acres. This system is different by similar to Solar One. Each mirror is a parabolic mirror that focuses the sun's rays onto a tube filled with synthetic oil that can be heated up to 400o C (750o F). This oil transfers its thermal energy to water, which causes it to boil and evaporate, thus spinning the turbines which creates electricity. NextEra, the group running this plant claims that they provide power to 232,500 homes every year. This much electricity, in turn reduces 3800 tons of pollution that would have been created each year had these homes been powered by other fossil fuels such as oil.

Kramer Junction

Figure 19: A close up view of a solar collector at Kramer Junction .

"Light, Lenses and Lasers: Curved Mirrors" (0:47)

This video describes the uses of concave mirrors, focusing on the ability of these mirrors to focus light into powerful beams.

"Really Big Things: Solar Power Plant" (1:43)

Discovery Channel's "Really Big Things" visits a solar power plant in Albuquerque, New Mexico to see how the plant works.


"Powering the Future: Solar" (2:24)

This video visits a 600 mirror solar array that focuses sunlight onto a 400 foot water tower in Southern Spain.

Electromagnetic Induction – How we get electricity

To better understand how these plants work, we need to take a step back and understand an important principle in Physics: electromagnetic induction. An interesting phenomenon is that if you have an electrical current, this will create a magnetic field. If you hook up a wire to a battery and then place a compass above and below that wire, you will see that the compass will switch directions. The reverse is also true. If you have a magnetic field in a coil of wires, and you change that field in any way, that changing field will create a current. This is how a turbine (sometimes referred to as an electrical generator) works. To further help with this, look at (Figure 20):

PHET

Figure 20: A picture taken from www.phet.colorado.edu highlighting electromagnetic induction.

The first step is that you need to have something turn the magnet. This can be steam created from boiling water at solar power plants, falling water from a dam, or a windmill. The spinning magnet changes the direction of the magnetic field. Surrounding the magnet you have coils of copper wire where the current gets created. The magnetic field is changing inside the coils which is why the current gets created. This current can then light up the light bulb.

"Physics: What is Electromagnetic Induction" (14:28)

This video describes explains how electromagnetic induction works.


"Physics: Applications of Electromagnetic Induction" (11:51)

This video discusses applications of electromagnetic induction, such as speakers and microphones.


"Electricity and Magnetism: Generating Electricity" (8:58)

This is another video that explains how different power plants work including a description of electromagnetic induction. The video also discusses pros and cons of different types of power plants.

So where are Solar Cells Now?

Residential Use

In the past ten years, the number of solar cells used in private residences has been constantly increasing. Many homes, like the one shown in Figure x in Roswell, Georgia, use solar cells to power anything electrical in the house as well as heating water. Part of the reason for the huge growth in the number of homes with solar cells is government incentives. The U.S. Department of Energy awarded about $17.6 million to photo-voltaic projects in 2008. States such as California, New Jersey and Florida also passed incentives to help spur the growth of PV cells in residences. Finally, like most technologies, as time goes on solar cells become less expensive as the technology improves and mass production increases.

Solar Home Roswell GA

Figure XXXXXXX: A modern net-zero energy home featuring solar panels on the roof.

The growth of solar cells has been almost exponential in both residential and commercial Figure x)! New units (solar panels) installed in homes (which you can think of as mini power plants) between 2008 and 2009 nearly doubled (Figure x)!

PV Capacity Chart

Figure XXXXXXX: Annual Installed Grid-Connected PV Capacity by Sector (2000-2009).

Installation Chart

Figure XXXXXXX: Number of Annual US-Grid Connected PV Installations (2000-2009).

Commercial Use

This growth has been not only in residential units, but also in commercial power plants as four new concentrating solar plants were added to the solar grid in 2009. Though these plants were small, they provide initial data so that the plants can be replicated on a larger scale in the future. In fact there are eleven solar plants that are either under design or are in the planning stages, some of which will be completed between 2011 and 2016. If all of these plants become operational, this will almost provide an additional (at peak operations) 2300 MW.

Every state contributes in some way by adding energy to the grid from photovoltaic cells. However, some states contribute a lot more than others. Part of this reason is simply location. California and Nevada, for example, receive much more sunlight than states like Alaska. The state incentives discussed earlier also play a big role. In 2007, California enacted a 10 year, $3 billion "Go Solar" Campaign. As a result, California is the number one state in PV capacity. To give you an idea, in 2009 the total capacity for the US was 1256 MW, of which California supplied 768 MW. That is about 61% of the total production in the entire U.S.! Georgia brings in only about 0.2 MW, despite favorable sunlight conditions. Also surprising is the growth in PV in the United States in the last few years. The total U.S. capacity increased from 476 MW in 2007 to 1256 MW in 2009. California increased from 328 MW to 768 MW in that same time frame. New Jersey (127.5 MW), Colorado (59.1 MW), Arizona (46.2 MW) and Florida (38.7 MW) round out the other top 5 states. However, these four states combined are only about a third of what California produces.

Solar Vehicles

Photovoltaic cells are becoming more and more common in houses but that is not the only place you find them. We previously described how solar cells are being used on planes and automobiles (now you can find them on Toyota’s 3rd generation Prius), but now you can find them on bicycles, motorcycles, boats (figure x) and even spacecraft!

Sun21

Figure XXXXXXX: Sun21: An experimental catamaran built in 2007.

Hopefully with continued incentives from the government, reduced costs of photovoltaic cells and increased education about PV power, more and more homes will invest in photovoltaic cells to help make American houses more environmentally sustainable.

This is a playlist of 22 videos highlighting green homes throughout the US and the world.

Playlist: World’s Greenest Homes Videos