The amount of solar energy available in a given area can have a large impact on eventual energy production. Areas with low cloud cover and long days are optimal. The amount of solar energy available in southwestern states in the United States may be as high as 8 kilowatt hours per square meter, four times the solar energy available in northeastern states.

Modern, large-scale energy installations designed to provide power to urban grids have been and are being built in areas with abundant solar energy, such as Los Angeles, Phoenix, Las Vegas, southern Europe, India, Iran, and China. Not only do these areas have considerable solar resources; they also are near strong, already existing energy grids.

Passive solar energy is used for more than space heating. Glass and plastics made it possible to build solar water heaters, which have become widespread, particularly in Europe and Asia. Water purification is an important use of solar energy in developing areas without cheap sources of fuel to boil water. Evaporation devices and even the exposure of water to sunlight in clear plastic bottles are inexpensive ways to purify water.

The first solar electricity projects to be connected to commercial grids were concentrated solar power (CSP) systems. These projects use troughs, towers, or dishes to focus sunlight, which generates superheated steam for conventional turbogenerators to produce electricity. Peak solar-to-electric conversion efficiencies range from just over 20 percent for troughs, to 23 percent for power towers, and 29 percent for dishes. The efficiency in transforming the solar radiation to electricity over the course of a year, or net annual efficiency, is approximately 11-16 percent for troughs, 7-20 percent for towers, and 12-25 percent for dishes (NREL 2008).

In spite of their relatively poor efficiency ratings, parabolic troughs are the most mature commercial technique. Indeed, until the late 1990s the nine plants in the Mojave Desert near Barstow, California, which used parabolic troughs, accounted for more than 90 percent of the world's concentrated solar power capacity. These plants were built between 1984 and 1990, and their total installed capacity is 354 megawatts with unit sizes between 14 and 80 megawatts (NREL 2010). Other trough CSP projects include small plants in Egypt, Iran, Morocco, Spain, Greece, and India. Large projects have been completed in Iran, northern Morocco, Egypt, Mexico, and Rajasthan, India.

Larger installations such as these can minimize the energy fluctuations between day and night, and sunny and cloudy days. Trough, tower, and dish installations heat materials that retain heat well with solar arrays and then store them for later use for powering steam turbines. Common materials used for heat storage include water, oil, or molten salt. Storage materials significantly increase the efficiency of these technologies over photovoltaic panels because a plant can produce energy even at night. Many modern plants also include a secondary oil or natural gas furnace to create the steam used for the electricity turbines.

In 1839 French experimental physicist Edmund Becquerel found that the electricity generation of an electrolytic cell increased when exposed to light. Few studies were conducted on this phenomenon, known as the photovoltaic (PV) effect, until the 1870s, when the discovery of the increased photoconductivity in the light by selenium made it possible to make the first PV cells that increased electrical production with increased exposure to light. Although the efficiencies of early PV cells were too low for any practical application, they enabled scientists to study the phenomenon in more detail. In 1918 Polish chemist Jan Czochralski demonstrated how to grow large silicon (Si) crystals.

The decisive technical breakthrough in producing practical PV panels came in 1954, when a team at the Bell Laboratories in Murray Hill, New Jersey, produced silicon cells that had a 4.5 percent solar-to-electric conversion efficiency; they raised that performance to 6 percent just a few months later. By March 1958, when Vanguard I became the first PV-powered satellite, Hoffman Electronics had made cells that were 9 percent efficient. In 1962 Telstar, the first commercial telecommunications satellite, had 14 watts of PV power, and just two years later Nimbus rated 470 watts (Perlin 1999).

During the last four decades of the twentieth century, PV cells became an indispensable, and spectacularly successful, component of the rapidly expanding satellite industry. PV arrays have come to power scores of communication, meteorological, Earth-observation, and spy satellites. Land-based applications remained uncommon, however, even after David E. Carlson and Christopher R. Wronski at RCA Laboratories fabricated the first amorphous silicon PV cell in 1976. Cheaper alternate forms of energy kept the PV panels from wide use.

Despite the fact that they were not widely used at that time, PV cells have obvious advantages for electricity generation. They have no moving parts (and hence low maintenance requirements), operate silently at atmospheric pressure and ambient temperature with a minimal environmental impact, and are inherently modular. Photovoltaics could be translated into commercial terrestrial applications only after (1) the cells reached conversion efficiencies close to, and in excess of, 10 percent, and (2) as the average price of PV modules (typically made up of forty PV cells) and arrays (made up of about ten modules) was reduced by nearly an order of magnitude, which took place between 1975 and 1995. The price of PV installations has continued to decline.

The three most widely produced types of solar panel are high-purity single crystals, polycrystalline cells, and thin films. Single crystals are large aligned sheets of atoms, while polycrystalline cells are created from sandwiched slices of smaller crystals, which are easier to produce. Thin film panels are created by depositing materials onto a substrate, usually glass. Single crystals are theoretically the most efficient, but are also the most difficult and expensive to produce, while polycrystalline cells are more common and cheaper to produce but less efficient. Single crystal and polycrystalline cells are most commonly made with silicon, while thin PV films are made of amorphous silicon or compounds such as gallium arsenide or cadmium telluride. Thin films have promise to be very cheap, but have not yet been widely adopted, in part due to lower efficiencies (NREL 2010).

Efficiency gaps remain between laboratory and field performances, as well as between high-purity single crystals, polycrystalline cells, and thin films. Theoretical single-crystal efficiencies are 25 percent, with more than 23 percent achieved in laboratories (NREL 2010). Lenses and reflectors can be used to focus direct sunlight onto a small area of cells and boost conversion efficiencies to more than 30 percent. Stacking cells sensitive to different parts of the spectrum could push the theoretical peak to 50 percent. Thin-film cells convert as little as 3 percent after several months, and multijunction amorphous silicon cells, with multiple thin films layers of different materials, convert at least 8 percent. Cells under development include photoelectrochemical devices based on nanocrystalline materials and cheap conducting polymer films that allow cells to capture light from all angles and can be fashioned into electricity-producing windows. These advances promise even greater efficiency.

As efficiencies have risen, so too have annual worldwide shipments of cells and modules. During the 1990s such shipments, measured in peak megawatts, increased from about 43 peak megawatts to 288 peak megawatts as the typical price fell by 250 percent. The largest PV cell and module producers in 2008 were Q-Cells in Germany, First Solar in the United States, and China's Suntech. Installed capacity in the year 2008 amounted to about 1.1 peak gigawatts in the United States, a twofold increase since 2000. Globally, installed capacity in 2008 was 13.9 gigawatts, increasing from almost 1 peak gigawatt in 2000. In 2008, grid-connected modules accounted for 94 percent of global installed solar capacity; the United States accounts for 71 percent of the total (NREL 2010).

Non-grid-connected PV installations are less likely to be included in surveys and may be under-reported. Recreational uses (mainly camping and boating), home solar systems (remote-generation preferable to expensive hookups to a distant grid), village generation in poor countries, and water pumping are common uses. New photovoltaic power applications are continuously developing, including highway signs and sensors, weather-sensing equipment, and lighting. Focused solar ovens and water purification are among the more simple applications.

Recent growth of PV capacity at 155 percent a year has led to many optimistic forecasts for its continued contribution to electricity production. At the beginning of the twenty-first century, the US Department of Energy set a goal of 1 million rooftop PV systems with about 3 peak gigawatts by 2010, and Japan planned to install annually ten times as many rooftop units by 2010 as it did in 1999. In 2008, installed rooftop capacity in the United States reached only about 218 megawatts, however, missing the target significantly (NREL 2008).

Whatever their eventual progress may be, one key fact remains: conversions of solar radiation harness the largest potential renewable source of energy. Its affordable mastery would revolutionize the terrestrial supply of electricity no less than it has already done in space applications. Solar radiation conversion is in the early stages of development. Continuing improvements in the net energy gain, cost, durability, and efficiency of PV cells and improvements in the efficiency of thermal power plants will make them leading choices for household use and energy grid supply in a not-too-distant future.

Vaclav SMIL, University of Manitoba

This article was adapted by the editors from Vaclav Smil's 2003 article “Solar Energy” in Shepard Krech III, J. R. McNeill, & Carolyn Merchant (Eds.), the Encyclopedia of World Environmental History (pp. 1138-1140). Great Barrington, MA: Berkshire Publishing.

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