Energy is the capacity of matter to perform work as the result of its motion or its position in relation to forces acting on it. Sunlight is the primary source of energy on Earth. The sun’s energy warms the planet’s surface, powering titanic transfers of heat and pressure in weather patterns and ocean currents. The resulting air currents drive wind turbines. Solar energy also evaporates water that falls as rain and builds up behind dams, where its motion is used to generate electricity via hydropower.
Energy exists in various forms, including mechanical, thermal, chemical, electrical, radiant, and atomic.  Sunlight is a form of radiant energy, which like all forms of energy, can be converted from into any other form of energy by appropriate processes. In the process of transformation either kinetic or potential energy may be lost or gained, but the sum total of the two remains always the same.
Photosynthesis is the process living cells use to convert the sun's radiant energy into chemical energy. In other words, plant cells turn sunlight into carbohydrates - sugars and starches. If these plant carbohydrates are eaten by an animal, they will be broken down and their chemical energy turned into movement (kinetic energy), body heat (radiant energy), or new chemical bonds (stored chemical energy in the form of the animal's body). Through photosynthesis, solar energy drives plant life, or biomass, which can be subsequently converted into liquid fuels in the form of alcohol (ethanol) or methane. In fact, the fossil fuels are derived from geologically ancient plant life.
A Very Short History of Solar Energy
In 1767, Swiss scientist Horace de Saussure built the world's first solar collector, which was used years later by Sir John Herschel to cook food during his South African expedition in the 1830s. Meanwhile, on September 27, 1816, Robert Stirling applies for a patent for his economiser at the Chancery in Edinburgh, Scotland. This engine is later used in the dish/Stirling system, a solar thermal electric technology that concentrates the sun's thermal energy to produce electric power.
In 1839, Alexandre-Edmond Becquerel, a French physicist, discovered the so-called photovoltaic effect, when he built a device that could measure the intensity of light by observing the strength of an electric current between two metal plates. When sunlight is absorbed by a solar cell, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic (PV) effect.
Becquerel's conversion process transformed only 1% of the sunlight that fell on the submerged electrode into electricity. In other words, the conversion process was only 1% efficient. Following the initial discovery of the PV effect, scientists experimented with different materials in an attempt to find a practical use for PV systems. In the late nineteenth century, scientists discovered that the metal selenium was particularly sensitive to sunlight, and during the 1880 s Charles Fritts constructed the first selenium solar cell. His device, however, was inefficient, converting less than one percent of the received light into usable electricity.
Design Print of John Ericsson's Sun Motor, New York, 1872
John Ericsson, a Swedish inventor who lived and worked for most of his adult life in the United States, designed and built the world’s first solar-energy engine/dish in Pasadena, Calif. Ericsson presented the concept design for the solar machine (featured above) in 1876 at the centennial celebration in Philadelphia.
World's First Solar Energy Dish, Pasadena, Calif. 1901
The Fritts selenium solar cell was mostly forgotten until the 1950s, when the drive to produce an efficient solar cell was renewed. It was known that the key to the photovoltaic cell lay in creating a...
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