Humans have always made use of solar energy. When man’s earliest ancestor realised that a patch of sunlight was warmer than the shade, or that rocks heated all day by the sun remained warm long into the night, solar energy was being used.
Ancient Roman structures were routinely built with south-facing windows to gather in the warmth of the sun. So many Roman residences and public buildings made use of passive solar heating that the imperial enactments of the Justinian Codex, a part of the Corpus Juris Civilis issued between 529 and 534 by order of the Roman Emperor Justinian I, addressed individual citizens’ sun rights.
Passive solar design is still popular today. Whether it is as simple as determining on which wall to place a barn entrance or as complex as the glass material selection and window placement for a multimillion dollar design, architects and craftsmen continue to design and build structures that take full advantage of the sun’s warmth and light. The only thing new about solar daylighting or passive solar heating is the name.
All early attempts to harness solar energy focused on heat. This is understandable because the warmth of sunlight is an obvious, tangible property. The basis for the Industrial Revolution was the steam engine, and so early works with solar energy attempted to concentrate the sun’s heat to produce steam. In 1767, a Swiss scientist named Horace-Benedict de Saussure constructed an insulated box with an opening covered by three layers of glass. This device is generally considered to have been the world’s first solar collector, and it could reach internal temperatures of 230 degrees Fahrenheit. In the 1830s, Sir John Herschel took one of these devices, commonly known as a Saussure’s oven, on his South Africa expedition to cook food.
Solar economisers or concentrators are still widely used today. Systems of mirrors and lenses focus sunlight onto reservoirs of thermal transfer fluids to produce steam, control building temperatures, or heat saline storage ponds. Unlike Saussure’s ovens, modern solar concentrators reach temperatures in the thousands of degrees. To the average consumer, however, heat isn’t energy. Electricity is energy. Solar power is considered energy only when it is converted to useable electricity.
The first step in converting sunlight to electricity occurred in 1839 when a French scientist named Edmond Becquerel exposed two electrodes in an electrolyte to sunlight. He observed an increase in electrical current that he could not explain. This was a defining moment in the history of solar power. Although he did not understand the physics behind his observation, Becquerel is credited with first observing what is now known as the photovoltaic effect. It was a scientific curiosity, but it was not put to little practical use for many years.
Investigation of solar energy has long been tied to the price structure and supply of other fuels. In the 19th century, France purchased coal to fuel their industrial growth from England. A French inventor named Augustin Mouchot believed that the supply was exhaustible and, in the hands of the English, unreliable. In 1860, he began building upon Saussure’s oven and created a water-filled container that was enclosed in a glass envelope. Exposure to sunlight concentrated heat inside the glass envelope and caused the water to boil in the container. By connecting a small steam engine to this device, Mouchot created the first solar-powered steam engine.
His work was inventive, practical, and financially supported by the French government. Unfortunately for the history of solar power, France soon negotiated a new deal with England for cheaper coal and more reliable deliveries. Mouchot’s work was no longer viewed by the French monarchy as a priority for that country, and his funding was discontinued. Without financial backing, his work fell to the wayside.
Other scientists continued to toy with the curious property discovered by Becquerel , and in 1873 another important advance in solar energy occurred when Willoughby Smith discovered that selenium was a photoconductive material. William Grylls Adams and Richard Evans Day experimented further with selenium. They were not able to produce sufficient quantities of electricity to do any useful work, but by 1876 they were able to demonstrate, for the first time, that a solid material with no moving parts could be used to convert sunlight directly into electrical energy.
An American inventor, Charles Fritts, used their discovery to create solar cells from selenium wafers ten years later. These primitive cells converted less than two percent of the available sunlight into electricity, but this was still a tremendous achievement at the time.
Most researchers at that time were also looking for a way to store electricity for later use. It was inconvenient to operate a conventional generating facility in the middle of the night, and it was impossible to collect sunlight at night. In 1904, Henry Willsie constructed two large solar generators and storage facilities in California. His facilities were the first to use power at night that had been generated through solar photovoltaics during the daylight hours. His facilities were expensive to operate, however, and his company went bankrupt without inspiring any additional innovations.
Albert Einstein contributed a tremendous advancement in solar energy when he published the explanation of the photoelectric effect in 1905. With his explanation of the phenomenon, physicists began to experiment with photovoltaics and to design materials that they predicted would demonstrate a photovoltaic effect.
Although early solar cells used selenium, the material most widely used in solar cells today is silicon. Jan Czochralski, a polish scientist, is credited with the discovery of the technique for growing single-crystal silicon in 1916. Many other materials also exhibit photovoltaic effects. Cadmium sulphide, for example, was shown 1932 to produce a current upon exposure to sunlight, and cadmium sulphide solar cells were used by the French government to power remote schools in Algeria.
Photovoltaic materials produce electricity in differing amounts according to the band gaps of the materials and the wavelengths of light to which they are exposed. In 1953, an American Chemist by the name of Dan Trivich published a series of theoretical calculations that predicted the efficiencies of various materials as solar cells based on their band gap widths and the spectrum of the sun.
The first real solar photovoltaic cell was created at Bell Laboratories a year later when Daryl Chapin, Calvin Fuller, and Gerald Pearson created a silicon crystal cell that could convert ordinary sunlight into a sufficient quantity of usable electricity to power equipment. The original cell had an efficiency of only four percent, but subsequent models reached 11 percent efficiencies.
In 1956, the United States began to develop solar cells for satellites. N-p junction silicon cells were invented by U.S. Signal Corps Laboratories in 1958. This design was much more resistant to radiation damage and was critically important for use in space, and the Vanguard I space satellite used a p-n junction silicon solar cell that produced less than one watt to power its communication radios. Explorer III, Vanguard II, and Sputnik-3 were all launched in 1958, and all featured p-n junction photovoltaic solar cell technology.
Silicon solar cells first became commercially available in 1959. By 1960, efficiency had been increased to 14 percent, and Bell Telephone Laboratories launched the Telstar telecommunications satellite, which featured a 14-watt solar-array panel, in 1962. By 1966, NASA satellites featured 1-kilowatt photovoltaic arrays.
Although solar cells were available commercially at this time, they were by no means cost-effective. Electricity produced by conversion of solar energy typically cost $100 per watt at this point. The production price dropped to $20 per watt in the 1970s due to advancements made by Dr. Elliot Berman. His inventions were used at off-grid locations where power was required for emergency or safety applications.
In 1972 the world’s first laboratory dedicated to the advancement of photovoltaic energy was founded at the University of Delaware. Thin-film PV systems were studied here. Silicon crystals are expensive to produce. One line of PV research has been the development of amorphous silicon photovoltaic cells. These cells lack crystalline structure and are much less efficient than silicon crystal solar cells, but they are also much cheaper to produce. Because they are cheaper, more cells can be produced and used. Economy of scale overcomes inefficiency, and solar electricity can be produced by thin film amorphous cells at lower costs per watt than through the use of crystal silicon PV cells.
History shows that advances in solar energy have been sporadic. In most cases, advances are seen to occur when conventional energy costs soar or when supplies are questioned. The advances made in the 1970s, for example, can be seen to correspond to the oil crisis of that same period. Mouchot’s solar steam engine was financed when France feared England would limit technological expansion by limiting the availability of coal. When costs drop, public and governmental interests in solar power generally wane. That paradigm, however, may be shifting.
Governments have made enormous investment in utility-scale solar plants during the past few years. Uncertainty about the Middle East, where much of the world’s petroleum supply is concentrated, has increased governmental interest in alternative fuel sources. Unlike previous flirtations with solar power, however, cost may not be the ultimate factor. Global warming phenomena and concerns over greenhouse gas emissions associated with energy production have also driven solar investments in recent years.
Current solar energy research focuses on cheaper methods of producing silicon solar cells, more effective means of storing solar energy, new super-thin copper-indium-gallium-selenide solar films, and the use of dye-laden glass or plastic plates to focus photons onto solar panels. These ideas may ultimately result in transparent materials that will turn every window into a solar panel, new construction materials that will allow all surfaces of a building to function as a giant solar panel, and batteries that store solar-produced electricity overnight or even for days.
As production prices dip lower and lower, arguments for solar energy production shift from economics to eco-friendliness. Many experts believe that the global economy will soon reach a point where solar energy is the preferred source of electricity even if the costs are slightly higher than conventional generation methods.
Andy Biggs,Solar Panels UK, http://www.solarpowerportal.co.uk/about/author_profile/59
