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Source Knol: Solar Energy
by William Pentland, Senior Energy Systems Analyst at Pace Energy & Climate Center
The Mechanics of Energy
Energy is the capacity of matter to perform work. Energy exists in multiple forms - mechanical, thermal, chemical, electrical, radiant, and atomic. One form of energy can be converted into any other form of energy if exposed to the appropriate processes. For example, sunlight, a form of radiant energy, is converted into carbohydrates, a form of chemical energy, by plants through a process called photosynthesis. Animals transform chemical energy stored in plants into either kinetic energy (physical movement) or the chemical bonds - a second form of chemical energy - that hold together a living person's body. Otherwise, plants die and over eons of time morph into fossil fuels like oil and natural gas.
Synopsis of Solar Energy
The Sun is about 900,000 miles across and is at least 10 million degrees at its center. The surface of the sun is roughly 6,000°C and its hot gases emit light that has a spectrum ranging from the ultraviolet, through the visible, into the infrared. Photovoltaic or solar cells convert solar power directly into electrical power. Light consists of discrete particle-like packets of energy called photons. Sunlight contains photons with energies that reflect the sun’s surface temperature; in energy units of electron volts. The energy density packed into the photons vary, but the visible region of the light spectrum tends to contain among the highest concentrations of energy that hits the planet.
More energy from sunlight strikes the Earth in one hour than all the energy consumed on the planet in a year. At high noon on a cloudless day, the surface of the Earth receives 1,000 watts of solar power per square meter. Sunlight provides by far the largest of all carbon-neutral or clean-energy sources. Heat travels in all directions from the Sun and is the ultimate source of all energy on Earth. This energy is responsible for all sorts of weather events, not only scorching heat waves. For instance, wind occurs when sunlight heats the ground, which heats the air above it, which rises, so that cool air whisks in to take its place.
In the past decade, solar energy has attracted significant attention from investors, policymakers and the public generally because it is widely available, geopolitically secure and environmentally sustainable. Indeed, solar energy does not create greenhouse gases as a byproduct of generating electricity. Not surprisingly, it is widely considered among the most compelling solutions available for the world's need for clean, abundant sources of energy. Skeptics need only consider the $7.5 billion solar-energy industry - still growing at a rate of more than 30% every year — to appreciate the growing popularity of solar energy in mainstream electricity markets. Still, in 2001, solar electricity provided less than 0.1% of the world's electricity.
What "Efficiency" Means in the Solar-Energy Sector?
The efficiency of a solar cell is a measure of its ability to convert the energy that falls on it in the form of EM radiation into electrical energy, expressed as a percent. The power rating of a solar cell is expressed in watts, as either as peak watt (Wp), which is a measure of maximum possible performance under ideal conditions, or under more real-life conditions including normal operating cell temperature and AMPM (whole day rather than peak sunshine) standard ratings. The following chart shows solar-efficiencies for several of the leading-edge solar cell technologies.
Solar energy is the conversion of the sun’s energy into electricity. Light emitted by the sun is a form of electromagnetic (EM) radiation, and the visible spectrum comprises the majority of solar radiation. EM radiation that falls below the visible spectrum (the infrared region) contains less energy while radiation above the visible spectrum (the ultraviolet region) contains more energy. Solar cells respond to various forms of EM radiation in different ways, depending on the material used to construct the cells.
Crystalline silicon, for example, is able to use the entire visible spectrum, plus a portion of the infrared spectrum. Energy in EM radiation that is outside of the useable region of a solar cell is generally lost as heat. Insolation is the amount of energy present in sunlight falling on a specific geographical region, which is determined by a range of factors that include time of day, time of year, climate, air pollution and several other factors. As a result, the economics of solar energy depend heavily on appropriate geographic siting.
A Very Short History of Solar Energy
“I’d put my money on the sun & solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that. I wish I had more years left.”
-Thomas Edison, 1931
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.
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.
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 semiconductor that would release electrons when exposed to radiation within the visible spectrum. During this time, researchers at the Bell Telephone Laboratories were developing similar semiconductors to be used in communication systems. By accident, Bell scientists Calvin Fuller and Daryl Chapin found the perfect semiconductor: a hybridized crystal called a " doped" cell, which was made of phosphorous and boron. The first solar cells using these new crystals debuted in 1954 and yielded a conversion efficiency of nearly six percent. Later improvements in the design increased the efficiency to almost 15 percent.
In 1957, Bell Telephone used a silicon solar cell to power a telephone repeater station in Georgia. The process was considered a success although it was still too inefficient to penetrate the general mmarketplace . The first real application of silicon solar cells came in 1958 when a solar array was used to provide electricity for the radio transmitter of Vanguard 1 , the second American satellite to orbit Earth. Solar cells have been used on almost every satellite launched since.
In 2004, global solar cell production had increased from less than 10 MW annually in 1980 to about 1,200 MW annually. The current total global PV installed capacity is more than 3 Gigawatts per year.
By the 1960s, photovoltaic cells were used to power U.S. space satellites. By the 1980s, the simplest photovoltaic systems were being used commercially to power small calculators and wrist watches. Today, advanced solar-energy systems provide electricity to pump water, power communications equipment and increasingly generate electricity on a commercial scale. Two solar-energy technologies currently dominate the market for solar-based electricity production. Concentrating solar power systems direct sunlight through a magnifying lens, which increases the heat energy and drives a generator that produces electricity. Photovoltaics systems (PV) convert solar energy into electricity with semiconductors. A third technology, solar heating, absorbs the sun's energy with solar collectors and provides low-grade heat used directly for solar water heating, solar space heating in buildings, and solar pool heaters.
From the mid 1950s to the early 1970s, PV research and development (R&D) was directed primarily toward space applications and satellite power. Large-scale development of solar collectors began in the United States in the mid-1970s under the Energy Research and Development Administration and continued under the auspices of the U.S. Department of Energy after 1976. In 1973, a greatly increased level of R&D on solar cells was initiated following the oil embargo in that year, which caused widespread concern regarding energy supply.
In 1976, the U.S. Department of Energy, along with its Photovoltaics Program, was created. DOE, as well as many other international organizations, began funding PV R&D at appreciable levels, and a terrestrial solar cell industry quickly evolved. 
By the late twentieth century, solar energy had become practical and affordable enough to warrant its broad-scale marketing as one of the primary energy sources of the future. During the 1990s, the price of solar energy plunged 50 percent as technology improved. Meanwhile, PV applications went from a niche source of electricity to bringing solar technology into the margins of the mainstream. More than 10,000 homes in the United States were powered exclusively by solar energy in the late 1990s while an additional 200,000 homes supplemented electricity consumption with some form of photovoltaic system, according to the Solar Energy Industries Association. Although the solar power industry was valued globally at $5 billion in 2003, solar power still represented only about one percent of all electric power in the United States that year, primarily due to its persistently high costs and the continuing availability of cheap energy via traditional sources. As you will discover in the following sections, these barriers have fallen dramatically in recent years and unleashed a small revolution in the role the sun plays in humanity's daily life.
Principal Solar-Energy Technologies
In 2004, solar energy accounted for only 0.039 percent of the world's total primary energy supply of 11,059 million metric tons of oil equivalent, according to the International Energy Agency. In other words, solar energy provided about 4 terawatt-hours of electricity generation, out of an estimated overall total production of some 17,450 terawatt-hours (1 terawatt = 1 trillion watts). The strength of the solar energy available at any point on the earth depends on the day of the year, the time of day, and the latitude of at which it hits the Earth.
Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight is absorbed by the material, electrons are dislodged from the material's atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface.
When the electrons leave their position, holes are formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. When the two surfaces are connected through an external load, electricity flows. To increase power output, cells are electrically connected into a packaged weather-tight module. Modules can be further connected to form an array. The term array refers to the entire generating plant, which can consists of as few as one solar module or several thousand modules. The number of modules connected together in an array depends on the amount of power output needed.
Several technologies have been developed to harness that energy, including concentrated solar-power systems; passive solar heating and daylighting, photovoltaic systems, solar hot water, and solar process heat and space heating and cooling. To understand the mechanics of these technologies, the best place to begin is the beginning of solar-energy technologies - photovoltaics.
Photovoltaic solar cells convert solar radiation, or sunlight, directly into electrical power. Solar cells are the basic building blocks of photovoltaic systems. PV-based solar energy has become one of the most successful energy technologies the world has ever seen, achieving cost-reductions similar to those achieved by Ford during the era of the Model-T.
There are two types of photovoltaic solar-cells: crystalline silicon cells and thin-film solar cells. Crystalline silicon solar cells typically use silicon or polysilicon substrates. Individual cells vary in size from about 1/2 inch to about 4 inches across and include additional layers placed on top of the silicon to enhance light capture. In "thin-film" solar cells, the substrate is made of glass, metal or polymer substrates and has small deposits of gallium or semiconductor materials placed on top. The substrate may be just a few micrometers thick. Thin film solar cells are typically less efficient than crystalline silicon solar cells.
When sunlight strikes a solar panel, electricity is produced because sunlight releases electrons. Solar cells are frequently combined to produce a large amount of electrical energy in solar-modules and ultimately solar arrays. Solar cells with conversion efficiencies in the neighborhood of 20% were readily available at the beginning of the 21st century, with efficiencies twice as high or more achieved with experimental cells.
Energy conversion efficiency is an expression of the amount of energy produced in proportion to the amount of energy available to a device. The sun produces a lot of energy in a wide light spectrum, but we have so far learned to capture only small portions of that spectrum and convert them to electricity using photovoltaics. So, today's commercial PV systems are about 20% efficient. And many PV systems degrade a little bit (lose efficiency) each year upon prolonged exposure to sunlight. For comparison, a typical fossil fuel generator has an efficiency of about 28%.
Solar cells are typically combined into modules that hold about 40 cells; about 10 of these modules are mounted in PV arrays that can measure up to several meters on a side. These flat-plate PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.
The performance of a photovoltaic array is dependent upon sunlight. Climate conditions and environmental factors have a huge impact on the amount of solar energy received by a photovoltaic array. The current record for solar-cell conversion efficiency, established in August 2008 by the National Renewable Energy Laboratory, is 40.8 percent.
Photovoltaic cells, like batteries, produce direct electric current (DC) which is generally used to power fairly small loads like those usually required by electronic equipment. When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC.
Concentrating Solar Power (CSP) or Solar Thermal
Solar cells are often placed under a lens that focuses or concentrates the sunlight before it hits the cells. This approach has both advantages and disadvantages compared with flat-plate PV arrays. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. But because the lenses must be pointed at the sun, the use of concentrating collectors is limited to the sunniest parts of the country. Some concentrating collectors are designed to be mounted on simple tracking devices, but most require sophisticated tracking devices, which further limit their use to electric utilities, industries, and large buildings.
Concentrating solar power (CSP) systems channel sunlight through an optical lens that amplifies the strength and heat of the sun. There are currently three principal types of concentrating solar energy systems: trough systems, dish/engine systems and power towers. CSP plants deploy these systems in large numbers of mirror configurations that convert the sun's energy into high-temperature heat. The heat energy turns water into steam, which then powers a turbine and generates electricity.
On a large scale, and as a means of generating power, CSP has several advantages over photovoltaic cells. Power from concentrating solar heat is less variable than from photovoltaic solar (or from wind), an important consideration for a full-scale utility. Solar thermal facilities can be designed to store energy for several hours after sundown, helping a utility meet evening spikes in demand. And since solar thermal plants use the same steam turbines to generate power that other generating stations use, the plants can be hybridized to burn natural gas or other fuels during nighttime hours, to keep output constant and maximize use of the turbines.
Concentrated solar power is currently the fastest-growing, utility-scale renewable energy alternative after wind power, according to a December 2007 report by Emerging Energy Research, a Cambridge, Mass.-based consulting firm. The study describes the technique as "well-positioned to compete against other electricity generation technologies" and estimates that $20 billion will be spent on solar thermal power projects around the world from 2008 to 2013.
Concentrating collectors reflect solar energy falling on a large area and focus it onto a small receiving area, which amplifies the intensity of the solar energy. The temperatures that can be achieved at the receiver can reach over 1,000 degrees Celsius. The concentrators must move to track the sun if they are to perform effectively; the devices used to achieve this are called heliostats. There are three main types of concentrating solar power systems: parabolic-trough, dish/engine, and power tower.
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Source Knol: Solar Energy