Solar Energy

Solar energy is the radiant energy (light or heat) that comes from the sun. Only a small amount of the sun’s energy strikes the Earth, one part per 2 million. But even that one part is an enormous amount of energy.

Solar energy is the most abundant energy source available. In fact, according to National Geographic, every hour, the sun beams enough energy to meet the entire world’s energy needs for a year! The tricky part is harnessing the energy effectively to meet those needs. Solar power depends on the sun being out—and when it’s not, solar panels generate no power. However, storage systems can collect the sun’s energy to be used during cloudy weather or at night.

Solar power systems, once they’re up and operational, last for 15-30 years. And while in 2008 solar only provided .02% of the world’s energy, that number is growing every year.

How Solar Energy is Formed and Used

Sunlight can be directly used to create electricity using solar, or photovoltaic (PV) cells. The solar cells are made using silicon, the same thing that makes up sand. Even though silicon is found almost everywhere, making a solar panel is a difficult and expensive process. According to a report in the New York Times, “generating power from photovoltaic panels costs more than four times as much as coal, and more than twice what wind power costs.” Polysilicon, a material made up of small silicon crystals that is used to make photovoltaic cells, is sometimes in short supply, which means prices for solar equipment can be volatile.

The silicon is heated and melted at super high temperatures and then molded into thin wafers. When sunlight strikes the solar cell, electrons are loosened and move toward a treated front surface, making an imbalance between the front and back. Electricity occurs when a connector, such as a wire, joins the negative and positive sides together.

The production of photovoltaic cells, which are used to change sunlight directly into electricity, causes far less pollution than fossil fuel-burning power plants. While manufacturing the cells does require harmful metals such as lead and mercury and also produces some greenhouse gases, the toxic emissions are up to 300 times lower than those created by coal power plants, according to scientists with the Brookhaven National Laboratory in New York.

PV cells by themselves are small, at most about 4 inches across, and do not generate enough power for most applications. That’s why, in most photovoltaic systems, PV cells are grouped into larger panels or modules that collect sunlight and convert it into electricity. These panels can be small (like those found on solar-powered calculators) or huge, covering acres of land and providing power for whole buildings.  Most photovoltaic cell panels need a lot of space. Because the sun doesn’t deliver that much energy to any one place at any one time, a large surface area is required to collect the energy at a useful rate.

Solar thermal plants also use sunlight to generate power, but in a different way from PV systems. Solar thermal plants use the sun’s rays to heat liquid, which is then used to heat water and produce steam. This steam then powers a turbine to generate electricity. There are three main types of solar thermal power systems: solar dish, solar power tower, and parabolic trough.

Solar dishes are designed to always point at the sun, collecting concentrated energy and, with the help of a turbine and electric generator, turning that energy into electricity. Solar power towers use mirrors to concentrate the sun’s energy into a central receiver tower, which contains fluid that collects heat. A parabolic trough system uses a long reflector to focus the sun’s rays onto a receiver pipe. Fluid moving through the pipe is then heated and used to heat water, creating steam. This steam is then used in a turbine to generate electricity.

Solar energy is used for heating water for domestic use, heating buildings, drying agricultural products, and generating electrical energy. Solar panels are used in some homes to convert the sun’s rays into electricity. Although not sold commercially, solar-powered vehicles can be found at demonstrations and educational sites globally. Solar cars use PV cells to convert the sun’s energy directly into electrical energy.

See the US Energy Information Administration’s Energy Kids Solar Energy pages for more information.


Photosynthesis is solar power at work in nature! It is the process of converting carbon dioxide into organic compounds using energy from the sun’s rays. Photosynthesis is needed to make plants, bacteria and algae grow. It is vital for life on Earth—photosynthesis gives us the fruits, vegetables and grains we eat and also maintains the normal level of oxygen in our atmosphere.

History of Solar Energy

Solar energy has been used since the 7th century B.C. The sun’s energy was used for heat, cooking, light, and to light fires. Ancient Greeks and Romans designed architecture to use the sun’s capacity to light and heat indoor spaces.

7th Century B.C. – Ancient peoples use a magnifying glass to concentrate the sun’s rays onto fuel and light a fire.

3rd Century B.C. – Greeks and Romans use mirrors to light torches.

2nd Century B.C. – Greek scientist Archimedes experiments with using the reflectivity of bronze shields to focus sunlight and set fire to enemies’ wooden ships.

A.D. 20 – Chinese people use mirrors to light torches.

1st-4th Century – Roman bath houses are designed to use the sun’s warmth to heat water.

13th Century – Pueblo ancestors in North America build cliff dwellings facing south to take advantage of the sun’s warmth during winter.

1767 – Swiss scientist Horace de Saussure builds the first solar collector.

1816 – Scottish minister Robert Stirling applies for a patent for his economiser heat engine, which will later be used in solar thermal electric technology that concentrates the sun’s thermal energy to produce electric power.

1839 – French scientist Edmond Becquerel discovers the photovoltaic effect. He finds that while experimenting with an electrolytic cell made up of two metal electrodes in an electricity-conducting solution, the electricity generation increases when exposed to light.

1860s – French mathematician August Mouchet starts work on a solar-powered steam engine; over the next 20 years, he and his assistant, Abel Pifre, build these engines, which are early versions of the modern parabolic dish solar collectors.

1873 – Willoughby Smith discovers the photoconductivity of selenium.

1876 – William Grylls Adams and Richard Evans Day discover that selenium produces electricity when exposed to light.

1880 – Samuel P. Langley invents the bolometer, which measures light from distant stars as well as the sun’s heat rays.

1883 – American Charles Fritts conceives of the first solar cells made of selenium wafers.

1887 – Heinrich Hertz discovers that ultraviolet light changes the lowest voltage capable of causing a spark to jump between two metal electrodes.

1891 – US inventor Clarence Kemp patents the first commercial solar water heater.

1904 – Wilhelm Hallwachs discovers that copper and cuprous oxide put together are photosensitive.

1905 – Albert Einstein publishes a paper on the photoelectric effect (along with his famed theory of relativity!).

1908 – William J. Bailey invents a solar collector that uses copper coils and an insulated box. This is roughly the same design solar collectors use today!

1914 – Scientists note a barrier layer in photovoltaic devices.

1916 – Robert Millikan logs experimental proof of the photoelectric effect.

1918 – Polish scientist Jan Czochralski discovers how to grow single-crystal silicon.

1921 – Albert Einstein wins the Nobel prize for this theories explaining the photoelectric effect.

1932 – The photovoltaic effect in cadmium sulfide is discovered.

1947 – Solar buildings are in demand as energy becomes scarce during World War II.

1954 – US scientists Daryl Chapin, Calvin Fuller and Gerald Pearson develop the first silicon photovoltaic cell—the first solar cell capable of generating enough power from the sun to run everyday electrical equipment.

Mid-1950s – Architect Frank Bridgers designs the world’s first commercial office building, complete with solar water heating.

1958 – New PV cells that are more resistant to radiation are developed; this feature will be important for solar cells used in space.

1963 – Japan installs a 242-watt PV array on a lighthouse.

1964 – NASA launches the first Nimbus spacecraft, a satellite powered by a 470-watt photovoltaic array.

1969 – A “solar furnace” using an eight-stories-tall parabolic mirror is built in Odeillo, France.

1970s – Dr. Elliot Berman and the Exxon Corp. design a less expensive solar cell. As a result, solar cells enjoy wider use.

1972 – The University of Delaware establishes the Institute of Energy Conversion, the world’s first lab dedicated to PV research and development. The following year, the university builds “Solar One,” a PV-thermal hybrid system.

1976 – The NASA Lewis Research Center starts installing the first of 83 PV power systems around the world, providing power for medical clinic lighting, water pumping, classroom television and more.

1977 – The US government launches the Solar Energy Research Institute.

1981 – Paul MacCready builds the first solar-powered aircraft, which he flies from France to England.

1982 – Australian Hans Tholstrup drives the first solar-powered car almost 2,800 miles between Sydney and Pert in 20 days.

1986 – The world’s largest (at the time) solar thermal power plant is commissioned in California.

1994 – The first solar dish generator to use a free-piston Stirling engine is hooked up to a utility grid.

1998 – Subhendu Guha leads the development of flexible solar shingles, which can be used for converting sunlight to electricity in buildings.

2001 – British Petroleum and BP Solar open a service station in the US with a solar-electric canopy that contains PV modules made of thin silicon films.

2002 – PowerLight Corp. in California installs the largest rooftop solar power system in the US.

2008 – The world’s largest thin-film PV power system, Waldpolenz Solar Park, goes online in Germany.

Source: US Department of Energy


Solar Energy in Developing Countries

Solar power—and other clean and renewable energy sources—has the potential to make a huge impact on the 1.6 billion people around the world who live without access to electricity. Several organizations are working to promote sustainable energy programs in the world’s developing countries. Solar Energy International is one such organization; it works to promote sustainability, educate children about solar energy, engineer solar projects and more around the world.

Another organization, Solar Electric Light Fund (SELF), provides technical and financial assistance for solar power and wireless communications in developing countries and rural areas, including Nigeria, Nepal and Vietnam.

In India, which is densely populated but has an underdeveloped electricity infrastructure, a $9 billion plan was announced in mid-2009 to produce 20 GW of solar power by 2020. The plan, dubbed the National Solar Mission, would provide solar-powered lighting for 3 million homes by 2012 and prevent 42 million tons of carbon dioxide emissions. The plan could have a great positive effect on the country, where 56% of the population does not have electricity.

Kenya is home to more than more solar power systems per capita than any other country. The people of Kenya buy more than 30,000 small solar panels every year, each panel producing 12-30 watts. Many Kenyans opt for this power source rather than trying to tap into the country’s unreliable power grid.