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HOMEPAGE PARRY THOMAS HENRY SEAGRAVE JOHN COBB M CAMPBELL D CAMPBELL C BREEDLOVE K WARBY R NOBLE D VESCO INDEX |
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A SHORT HISTORY OF SOLAR CELLS
What a marvelous idea. Catch the Sun's radiant light energy and convert it into electrical energy. It's nothing new of course. Nature has been capturing the energy in light for millions of years. Each leaf is a form of solar cell, producing energy for plants and trees to grow in a chemical process known as Photosynthesis.
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SPACE AGE TECHNOLOGY - Solar cells, also known as Photo Voltaic Cells, were rapidly developed to provide electrical energy for space missions. The beauty of solar cells is that provided the Sun shines, they keep on producing free electricity. Well, sort of free. Solar panels are still expensive to manufacture. It is the high purchase price and installation cost that effectively limits their use.
There are many types of solar cell. Polycrystaline (more than one crystal), monocrystaline and thin film. Monocrystaline is presently the most efficient at converting light energy into electricity. Sometimes as high as 20% but more usually 15%. A monocrystaline cell is made from a thin slice cut from a single crystal of silicon. A grid of metal is then embedded over the wafer ending in the contacts and other layers added. Thin film cells are plated onto a plate of glass. They are much cheaper to produce, but only around 5% efficient and heavy. Vehicle designers will normally want to capture as much energy as possible for a given area and weight.
A single cell is not of much practical use, producing less than a volt. Several cells have to be connected in a series of cells to produce a useable voltage. The voltage increases proportionally. 10 cells connected in series will produce about 7.5 volts. 20 cells 15 volts and so on. A number of cells (a battery) linked and mounted together is known as a solar panel.
HOW MUCH POWER - The Sun's energy reaching the surface of our planet is roughly 1 kilowatt per square meter. Before entering our atmosphere it is about 20% more: 1.2 kilowatts. That's why astronauts always look so bright. At 15% efficiency 10 panels each measuring 1meter by 1 meter would power 1 1/2 bars on an electric heater. 20 panels would power an electric kettle. This of course assumes that the sun is shining. Just imagine how many panels Solar Navigator needs to cross an ocean? Why not try and guess the area of panels on Solar Navigator - there's a £100 prize and certificate for the winner.
Essentially, solar panels are devices that convert light into electricity. They are called solar after the sun or "Sol" because the sun is the most powerful source of the light to use. They are sometimes called photovoltaics which means "light-electricity". Solar cells or PV cells rely on the photovoltaic effect to absorb the energy of the sun and cause current to flow between two oppositely charge layers.
Photovoltaics:
Photovoltaic (or PV) systems convert light energy into electricity. The term "photo" is a stem from the Greek "phos," which means "light." "Volt" is named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. "Photo-voltaics," then, could literally mean "light-electricity." Most commonly known as "solar cells," PV systems are already an important part of our lives. The simplest systems power many of the small calculators and wrist watches we use every day. More complicated systems provide electricity for pumping water, powering communications equipment, and even lighting our homes and running our appliances. In a surprising number of cases, PV power is the cheapest form of electricity for performing these tasks.
Photovoltaic cells convert light energy into electricity at the atomic level. Although first discovered in 1839, the process of producing electric current in a solid material with the aid of sunlight wasn't truly understood for more than a hundred years. Throughout the second half of the 20th century, the science has been refined and the process has been more fully explained. As a result, the cost of these devices has put them into the mainstream of modern energy producers. This was caused in part by advances in the technology, where PV conversion efficiencies have improved considerably.
French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839, but it remained a curiosity of science for the next three quarters of a century. At only 19, Becquerel found that certain materials would produce small amounts of electric current when exposed to light. The effect was first studied in solids, such as selenium, by Heinrich Hertz in the 1870s. Soon afterward, selenium PV cells were converting light to electricity at 1% to 2% efficiency. As a result, selenium was quickly adopted in the emerging field of photography for use in light-measuring devices.
Major steps toward commercializing PV were taken in the 1940s and early 1950s, when the Czochralski process was developed for producing highly pure crystalline silicon. In 1954, scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell, which had an efficiency of 4%.
A
solar cell is any device that directly converts the energy in light into
electrical energy through the process of photovoltaics. The development of
solar cell technology begins with the 1839 research of French physicist
Antoine-César Becquerel. Becquerel observed the photovoltaic effect while
experimenting with a solid electrode in an electrolyte solution when he saw
a voltage develope when light fell upon the electrode. "What is often considered the first genuine solar cell was built around 1883 by Charles Fritts, who used junctions formed by coating selenium (a semiconductor) with an extremely thin layer of gold... These early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This impasse was finally overcome with the development of the silicon solar cell by Russell Ohl in 1941. Thirteen years later three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight." - Encyclopedia Britannica
Solar Panels
A solar panel or battery converts the sun's energy to electricity. Gerald Pearson, Calvin Fuller and Daryl Chapin invented the first sun energy battery in 1954. The inventors created an array of several strips of silicon (each about the size of a razorblade), placed them in sunlight, captured the free electrons and turned them into electrical current. Bell Laboratories in New York announced the prototype manufacture of a new solar battery. Bell had funded the research. The first public service trial of the Bell Solar Battery began with a telephone carrier system (Americus, Georgia) on October 4 1955. In 1954, G.L. Pearson, C.S. Fuller, and D.M. Chapin invented the first solar panel battery.
Solar panel array on Solar Navigator development model.
Solar Panels in Space:
Extract: Nasa Jet Propulsion Laboratory ReporCrystalline silicon and gallium arsenide are typical choices of materials for solar panels for deep-space missions. Gallium arsenide crystals are grown especially for photovoltaic use, but silicon crystals are available in less-expensive standard ingots, which are produced mainly for consumption in the microelectronics industry.
When exposed to direct sunlight at 1 AU, a 6-centimeter diameter silicon cell can produce a current of about 0.5 ampere at 0.5 volt. Gallium arsenide is more efficient. Crystalline ingots are sliced into wafer-thin disks, polished to remove slicing damage, dopants are introduced into the wafers, and metallic conductors are deposited onto each surface: a thin grid on the sun-facing side and usually a flat sheet on the other. Spacecraft solar panels are constructed of these cells cut into appropriate shapes, protected from radiation and handling damage on the front surface by bonding on a cover glass, and cemented onto a substrate (either a rigid panel or a flexible blanket), and electrical connections are made in series-parallel to determine total output voltage. The cement and the substrate must be thermally conductive, because in flight the cells tend to heat up from absorbing infrared energy that is not converted to electricity. Since cell heating reduces the operating efficiency it is desirable to minimize the heating. The substrate is supported on a deployable structural framework. The resulting assemblies are called solar panels or solar arrays.
A solar panel is a collection of solar cells. Although each solar cell provides a relatively small amount of power, many solar cells spread over a large area can provide enough power to be useful. To get the most power, solar panels have to be pointed directly at the Sun. Spacecraft are built so that the solar panels can be pivoted as the spacecraft moves. Thus, they can always stay in the direct path of the light rays no matter how the spacecraft is pointed. Spacecraft are usually designed with solar panels that can always be pointed at the Sun, even as the rest of the body of the spacecraft moves around, much as a tank turret can be aimed independently of where the tank is going. A tracking mechanism is often incorporated into the solar arrays to keep the array pointed towards the sun.
Solar panels need to have a lot of surface area that can be pointed towards the Sun as the spacecraft moves. More exposed surface area means more electricity can be converted from light energy from the Sun. Sometimes, satellite scientists purposefully orient the solar panels to "off point," or out of direct alignment from the Sun. This happens if the batteries are completely charged and the amount of electricity needed is lower than the amount of electricity made. The extra power will just be vented by a shunt into space as heat.
Solar panels power satellites
Solar panels are very hardy. Compared to alternative power sources, they wear out very slowly. The principal factor affecting the loss in power with time is the Space radiation environment. For low radiation environments, such as low Earth orbiting, their effectiveness decreases around 1 to 2 percent a year. This means after a five year mission the solar panels will still be making more than 90% of what they made at the beginning of the mission (as long as they haven't gotten farther away from the Sun). In contrast, for missions in higher radiation environments, such as mid altitude Earth orbit (2000 to 10000 kilometers), arrays can lose half their power within 1 year. That is one reason few missions fly in this orbital range.
Photovoltaic concentrator solar arrays for primary spacecraft power are devices, which intensify the sunlight on the photovoltaics. This design uses lenses, called Fresnel* lenses, which take a large area of sunlight and direct it towards a specific spot by bending the rays of light and focusing them. Some people use the same principle when they use a magnifying lens to focus the Sun's rays on a pile of kindling or paper to start fires.
Solar concentrators put one of these lenses over every solar cell. This focuses light from the large concentrator area down to the smaller cell area. This allows the quantity of expensive solar cells to be reduced by the amount of concentration. Concentrators work best when there is a single source of light and the concentrator can be pointed right at it. This is ideal in space, where the Sun is a single light source. Solar cells are the most expensive part of solar arrays, and arrays are often a very expensive part of the spacecraft. This technology allows costs to be cut significantly due to the utilization of less material.
*Fresnel lenses have been around since Augustin Jean Fresnel invented them in 1822. Theaters use them for spotlights and lighthouses use them to make their lights visible at greater distances.
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