Solar power refers to the use of energy from the sun. This use occurs naturally as a part of life. For example, plants that are capable of photosynthesis use solar power as a source of the energy they need to survive and grow. Without the energy of the sun warming Earth, no life would exist; surface life would die off quickly and, as the atmosphere cooled, even the deepest parts of the ocean would freeze.
More practically, solar power has been harnessed for warmth for thousands of years, and, beginning in the nineteenth century, as a means of generating energy to provide heat, light, and electricity.
Solar power is an increasingly attractive alternative energy source as it is a stand-alone energy source; that is, it does not require a connection to a power grid, and the sun provides an unlimited and virtually endless supply of energy. As well, solar power can be used to supplement electricity generated conventionally using water (hydroelectric power) and burning fossil fuels.
In an era when the human-related generation of greenhouse gases such as carbon dioxide have been acknowledged by the Intergovernmental Panel on Climate Change (IPCC) to be a major reason for the increasing warming of Earth’s atmosphere that has been occurring for the past 150 years, and which has accelerated since the mid-twentieth century, the use of solar power reduces the greenhouse-gas emissions associated with conventional generation of electricity. This is one reason why solar power is moving more into the mainstream as an energy source.
Historical Background and Scientific Foundations
Solar power has existed ever since there was material that could be warmed by the rays of the sun. Roman bathhouses built in the first century operated essentially like greenhouses, letting sunlight in through south-facing rooms. The same design was used in the thirteenth century in dwellings made by the Anasazi in Arizona and New Mexico; some of these dwellings still exist. In the fifteenth century, Italian inventor and artist Leonardo da Vinci (1452–1519) made sketches of a device that would concentrate sunlight to weld copper. A device that concentrated solar energy to provide a temperature high enough to cook food was first built in 1767. In 1839, French physicist Edmond Becquerel (1820–1891) discovered the photovoltaic effect (which was experimentally proven in 1916) and in 1891, the first solar water heater was patented in the United States.
Solar power is still used today to generate heat in a more sophisticated form known as concentrated solar thermal systems. These consist of an arrangement of mirrors that track the motion of the sun and reflect the sunlight to a small central area. All this reflected sunlight creates temperatures of up to 932°F (500°C), which is used to heat water or oil, which is, in turn, transferred to a facility to generate power. These facilities can convert up to 40% of the incoming sunlight to usable power.
The first operational photovoltaic cell was developed at Bell Laboratories in New Jersey in 1954. It was capable of converting solar energy to provide enough electricity to run everyday electrical equipment. Within several years the cell’s efficiency had been increased from 4% to 11%, which is remarkable considering that the versions in common use in 2008 have only a marginally higher efficiency of about 15%.
The solar panels that power objects ranging from Earth-bound pocket calculators to orbiting satellites are photovoltaic cells. The word photovoltaic, which comes from photo (meaning light) and voltaic (electricity), describes the function of the process, in which sunlight is directly converted into electricity.
A photovoltaic cell is typically composed of silicon. When light contacts silicon, some of the energy is absorbed. This added energy dislodges electrons from the silicon; once freed, the electrons can move. The movement of electrons represents a current. The current can be tapped to power a device like a calculator, or can be stored.
A solar panel is made up of many photovoltaic cells. The collective power produced by an array of solar panels, such as are found on a house roof, can supply at least some of the building’s electricity needs.
Silicon is able to function in a solar panel because of its structure. In a silicon crystal, each atom has 14 electrons that are in orbit around it. The electrons tend to arrange to be most energetically stable. The result is a three-tiered arrangement of electrons (each tier is known as a shell). The two shells of electrons that are nearest to the central atom are full and thus are stable. However, the outermost shell has only four electrons, leaving it half-full. As this is not energetically stable, one atom will tend to share its four outermost electrons with a neighboring silicon so that each atom will have an outer shell containing eight electrons.
Because this arrangement produces outer shells that are stable, it becomes more difficult to get electrons to dislodge and flow, which is what is required for a solar photovoltaic cell to operate. So, in reality, the silicon is present along with other atoms such as phosphorus. Phosphorous has five electrons in its outer shell. It can share four electrons with silicon, leaving one unpaired electron. It is these unpaired electrons that can be dislodged to generate the electrical current.
The presence of other atoms in the crystalline arrangement of silicon atoms is termed an impurity. But, this does not imply that impurities are accidental. Rather, impurities are introduced deliberately; the process is referred to as doping. Silicon doped with phosphorus atoms is known as N-type silicon (N stands for negative, because the free electrons produce an overall negative charge). N-type silicon conducts electricity much better than pure silicon does.
A photovoltaic cell also has another component called P-silicon, which is generated by using boron atoms to create the impurity instead of phosphorus atoms. A boron atom has only three electrons in its outermost shell, in effect creating a hole into which one electron can be occupied.
The photovoltaic cell is designed so that not all the electrons become energetically stable. The result is the creation of an electrical field (known as voltage) between the N-silicon and P-silicon regions. When sunlight hits the cell, the flow of electrons provides the current. The combination of the voltage and current is electrical power.
A photovoltaic cell also has a coating that inhibits the reflection of sunlight, since reflection will reduce the
amount of light that contacts the panel and the resulting insolation. As well, the panel is covered by glass as a protective layer.
This process is not as efficient as it sounds in theory. Only about 15% of the sunlight striking a cell is converted to electrical energy, because energy is required to dislodge electrons and because doped silicon is not close to being 100% efficient.
Still, when produced on a daily basis (when enough sunlight strikes the panel), storage of the energy in batteries can be a longer-term source of electricity, similar to the powering of a laptop computer by its battery when it is not connected to a wall receptacle.
Some solar panels also have water-filled tubes runing though them. The circulating water that is heated during a back-and-forth passage through the array of solar panels can then be used as a source of hot water for laundry, bathing, or as heat in radiant flooring. Often the heated water is not used directly, but is routed to the building’s hot water heater to supplement the hot water and so reduce the amount of conventional (and purchased) electricity required to heat the tank of water.
Impacts and Issues
The popularity of solar power has long been based on its economy and as a renewable alternative to the nonrenewable use of fossil fuels to generate electricity. With global warming, the use of solar power has become even more attractive, as it does not generate carbon dioxide.
As well, solar power represents a form of energy that can be harnessed rapidly and with little controversy, in contrast to, for example, a nuclear power plant. Establishing a solar power facility requires money and space; once these are available, the solar collectors can be set up with minimal construction. Acquiring the means to rapidly generate electricity may become urgent, according to a 2008 study from the Scripps InstitutionPage 752 | Top of Article of Oceanography. The study examined Lake Mead in the Western United States, a key water source for millions of Americans in the Southwest and the main source of power (in the form of hydroelectricity) for Las Vegas, Nevada. Current climate-related change could dry the reservoir by 2021 if water use is not altered.
The amount of energy used globally in a year is contained in the sunlight that strikes Earth for only 40 minutes. The large amount of land in the United States that could be used to install solar arrays means that the country could become energy self-sufficient.
Already several U.S. companies have recognized the potential of large-scale solar power projects as becoming feasible as the need for energy becomes more difficult or environmentally unacceptable to be met by conventional technologies.
In March 2008, the U.S. Department of Energy announced an initiative to invest almost $14 million over the next three years in a number of projects that are aimed at increasing the efficiency of the conversion of sunlight to electricity by solar photovoltaic cells. Using different materials and solar cell designs, the goal is to manufacture a solar cell that could convert 45% of the sun’s energy into electricity.
Kemp, William. The Renewable Energy Handbook: A Guide to Rural Energy Independence, Off-Grid and Sustainable Living. Tamworth, Ontario, Canada: Aztext Press, 2006.
Thomas, Isabel. The Pros and Cons of Solar Power. New York: Rosen Central, 2008.
Natural Resources Canada. “Technologies and Applications: About Solar Energy.” April 26, 2005. www.canren.gc.ca/tech_appl/index.asp?CaId=5&PgId=121 (accessed March 19, 2008).
U.S. Department of Energy. “DOE to Invest up to $13.7 Million in 11 Solar Cell Projects.” March 12, 2008. www.eere.energy.gov/news/news_detail.cfm/news_id=11638 (accessed March 19, 2008).
Brian D. Hoyle