Street Light Free Essays

string(56) " and referencing the position of a building to the Sun\." INDEX |S. NO |TITLE |PAGE NO | |1 |Introduction |1 | |2 |Solar Energy |4 | |3 |Photovoltaics |24 | |4 |Solar Cell |28 | |5 |Solar Roadway |51 | |6 |Component description |55 | |7 |Working of Project |82 | |8 |Conclusion |86 | |9 |Images |91 | |10 |Bibliography |93 | INTRODUCTION INTRODUCTION: Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar energy technologies include solar heating, solar photovoltaics, solar thermal electricity and solar architecture, which can make considerable contributions to solving some of the most urgent energy problems the world now faces. We will write a custom essay sample on Street Light or any similar topic only for you Order Now Solar power is the conversion of sunlight into electricity, either directly using photovoltaic (PV), or indirectly using concentrated solar power (CSP). Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. Photovoltaics convert light into electric current using the photoelectric effect. A Street light, lamppost, street lamp, light standard, or lamp standard is a raised source of light on the edge of a road or walkway, which is turned on or lit at a certain time every night. Modern lamps may also have light-sensitive photocells to turn them on at dusk, off at dawn, or activate automatically in dark weather. In older lighting this function would have been performed with the aid of a solar dial. It is not uncommon for street lights to be on posts which have wires strung between them; such as on telephone poles or utility poles. New street lighting technologies, such as LED or induction lights, emit a white light that provides high levels of scotopic lumens allowing street lights with lower wattages and lower photopic lumens to replace existing street lights. Photovoltaic-powered LED luminaires are gaining wider acceptance. Preliminary field tests show that some LED luminaires are energy-efficient and perform well in testing environments. This project is a LED based Solar Lights is an automatic street lightening system using a LDR and 6V/5W solar panel. During day time, the internal rechargeable battery receives charging current from the connected solar panel. Here IC 555 is wired as a medium current inverting line driver, switched by an encapsulated light detector (LDR). When ambient light dims, the circuits drive the white LEDs. When the ambient light level restores, circuit returns to its idle state and light(s) switched off by the circuit. Block Diagram: SOLAR ENERGY SOLAR ENERGY Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar energy technologies include solar heating, solar photovoltaics, solar thermal electricity, solar architecture and artificial photosynthesis, which can make considerable contributions to solving some of the most urgent energy problems the world now faces. Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air. In 2011, the International Energy Agency said that â€Å"the development of affordable, inexhaustible and clean solar energy technologies will have huge longer-term benefits. It will increase countries’ energy security through reliance on an indigenous, inexhaustible and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating climate change, and keep fossil fuel prices lower than otherwise. These advantages are global. Hence the additional costs of the incentives for early deployment should be considered learning investments; they must be wisely spent and need to be widely shared†. The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth’s surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet. Earth’s land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth’s surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14  °C. By photosynthesis green plants convert solar energy into chemical energy, which produces food, wood and the biomass from which fossil fuels are derived. The total solar energy absorbed by Earth’s atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year. In 2002, this was more energy in one hour than the world used in one year. Photosynthesis captures approximately 3,000 EJ per year in biomass. The technical potential available from biomass is from 100–300 EJ/year. The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earth’s non-renewable resources of coal, oil, natural gas, and mined uranium combined. Solar energy can be harnessed at different levels around the world, mostly depending on distance from the equator. [pic] Average insolation showing land area (small black dots) required to replace the world primary energy supply with solar electricity. 18 TW is 568 Exajoule (EJ) per year. Insolation for most people is from 150 to 300 W/m2 or 3. 5 to 7. 0 kWh/m2/day. Solar energy refers primarily to the use of solar radiation for practical ends. However, all renewable energies, other than geothermal and tidal, derive their energy from the sun. Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. You read "Street Light" in category "Essay examples" Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies. APPLICATIONS OF SOLAR TECHNOLOGY Average  insolation  showing land area (small black dots) required to replace the world primary energy supply with solar electricity. 18 TW is 568 Exajoule (EJ) per year. Insolation for most people is from 150 to 300 W/m2  or 3. 5 to 7. 0 kWh/m2/day. Solar energy refers primarily to the use of  solar radiation  for practical ends. However, all renewable energies, other than  geothermal  and  tidal, derive their energy from the sun. Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered  supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies ARCHITECTURE AND URBAN PLANNING [pic] Darmstadt University of Technology  in Germany  won the 2007  Solar Decathlon  in Washington, D. C. with this  passive house designed specifically for the humid and hot subtropical climate. Sunlight has influenced building design since the beginning of architectural history. Advanced solar architecture and urban planning methods were first employed by the  Greeks  and  Chinese, who oriented their buildings toward the south to provide light and warmth. The common features of  passive solar  architecture are orientation relative to the Sun, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and  thermal mass. When these features are tailored to the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature range. Socrates’  Megaron House is a classic example of passive solar design. The most recent approaches to solar design use computer modeling tying together  solar lighting,  heating  and  ventilation  systems in an integrated  solar design  package. Active solar equipment such as pumps, fans and switchable windows can complement passive design and improve system performance. Urban heat islands (UHI) are metropolitan areas with higher temperatures than that of the surrounding environment. The higher temperatures are a result of increased absorption of the Solar light by urban materials such as asphalt and concrete, which have lower  albedos  and higher  heat capacities  than those in the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. Using these methods, a hypothetical â€Å"cool communities† program in  Los Angeles  has projected that urban temperatures could be reduced by approximately 3  Ã‚ °C at an estimated cost of US$1  billion, giving estimated total annual benefits of US$530  million from reduced air-conditioning costs and healthcare savings. [23] AGRICULTURE AND HORTICULTURE [pic] Greenhouses  like these in the Westland municipality of the  Netherlands  grow vegetables, fruits and flowers. Agriculture  and  horticulture  seek to optimize the capture of solar energy in order to optimize the productivity of plants. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields. [24][25]  While sunlight is generally considered a plentiful resource, the exceptions highlight the importance of solar energy to agriculture. During the short growing seasons of the  Little Ice Age, French and  English  farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground and facing south, but over time, sloping walls were developed to make better use of sunlight. In 1699,  Nicolas Fatio de Duillier  even suggested using a  tracking mechanism  which could pivot to follow the Sun. [26]  Applications of solar energy in agriculture aside from growing crops include pumping water, drying crops, brooding chicks and drying chicken manure. [27][28]  More recently the technology has been embraced by vinters, who use the energy generated by solar panels to power grape presses. [29] Greenhouses  convert solar light to heat, enabling year-round production and the growth (in enclosed environments) of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to produce  cucumbers  year-round for the Roman emperor  Tiberius. [30]  The first modern greenhouses were built in Europe in the 16th century to keep exotic plants brought back from explorations abroad. [31]  Greenhouses remain an important part of horticulture today, and plastic transparent materials have also been used to similar effect in  polytunnels  and  row covers. TRANSPORT AND RECONNAISSANCE [pic] Australia hosts the  World Solar Challengewhere solar cars like the Nuna3 race through a 3,021  km (1,877  mi) course from Darwin to Adelaide. Development of a solar powered car has been an engineering goal since the 1980s. The  World Solar Challenge  is a biannual solar-powered car race, where teams from universities and enterprises compete over 3,021 kilometres (1,877  mi) across central Australia from  Darwin  to  Adelaide. In 1987, when it was founded, the winner’s average speed was 67 kilometres per hour (42  mph) and by 2007 the winner’s average speed had improved to 90. 87 kilometres per hour (56. 46  mph). [32]  The  North American Solar Challenge  and the planned  South African Solar Challenge  are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles. [33][34] Some vehicles use solar panels for auxiliary power, such as for air conditioning, to keep the interior cool, thus reducing fuel consumption. [35][36] In 1975, the first practical solar boat was constructed in England. [37]  By 1995, passenger boats incorporating PV panels began appearing and are now used extensively. [38]  In 1996,  Kenichi Horie  made the first solar powered crossing of the Pacific Ocean, and the  sun21  catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006–2007. [39]  There are plans to circumnavigate the globe in 2010. [40] [pic] Helios UAV  in solar powered flight. In 1974, the unmanned  AstroFlight Sunrise  plane made the first solar flight. On 29 April 1979, the  Solar Riser  made the first flight in a solar powered, fully controlled, man carrying flying machine, reaching an altitude of 40 feet (12  m). In 1980, the  Gossamer Penguin  made the first piloted flights powered solely by photovoltaics. This was quickly followed by the  Solar Challenger  which crossed the English Channel in July 1981. In 1990  Eric Scott Raymond  in 21 hops flew from California to North Carolina using solar power. [41]  Developments then turned back to unmanned aerial vehicles (UAV) with the  Pathfinder  (1997) and subsequent designs, culminating in the  Helios  which set the altitude record for a non-rocket-propelled aircraft at 29,524 metres (96,864  ft) in 2001. 42]  The  Zephyr, developed by  BAE Systems, is the latest in a line of record-breaking solar aircraft, making a 54-hour flight in 2007, and month-long flights are envisioned by 2010. [43] A  solar balloon  is a black balloon that is filled w ith ordinary air. As sunlight shines on the balloon, the air inside is heated and expands causing an upward  buoyancy  force, much like an artificially heated  hot air balloon. Some solar balloons are large enough for human flight, but usage is generally limited to the toy market as the surface-area to payload-weight ratio is relatively high. [44] DAYLIGHTING [pic] Daylighting features such as this  oculusat the top of the  Pantheon, in  Rome, Italy have been in use since antiquity. The history of lighting is dominated by the use of natural light. The Romans recognized a  right to light  as early as the  6th century  and English law echoed these judgments with the Prescription Act of 1832. [45][46]  In the 20th century artificial  lighting  became the main source of interior illumination but daylighting techniques and hybrid solar lighting solutions are ways to reduce energy consumption. Daylighting  systems collect and distribute sunlight to provide interior illumination. This passive technology directly offsets energy use by replacing artificial lighting, and indirectly offsets non-solar energy use by reducing the need for  air-conditioning. 47]  Although difficult to quantify, the use of  natural lighting  also offers physiological and psychological benefits compared to  artificial lighting. [47]  Daylighting design implies careful selection of window types, sizes and orientation; exterior shading devices may be considered as well. Deciduous trees at the east and west ends of buildings offer shade in the summer and do not block the sun in the winter. [48]  Individual features include sawtooth roofs,  clerestory windows, light shelves,  skylights  and  light tubes. They may be incorporated into existing structures, but are most effective when integrated into a  solar design  package that accounts for factors such as  glare, heat flux and  time-of-use. When daylighting features are properly implemented they can reduce lighting-related energy requirements by 25%. [49] Hybrid solar lighting  (HSL) is an  active solar  method of providing interior illumination. HSL systems collect sunlight using focusing mirrors that  track the Sun  and use  optical fibers  to transmit it inside the building to supplement conventional lighting. In single-story applications these systems are able to transmit 50% of the direct sunlight received. [50] Solar lights that charge during the day and light up at dusk are a common sight along walkways. [51]  Solar-charged lanterns have become popular in developing countries where they provide a safer and cheaper alternative to kerosene lamps. [52] Although  daylight saving time  is promoted as a way to use sunlight to save energy, recent research reports contradictory results: several studies report savings, but just as many suggest no effect or even a net loss, particularly when  gasoline  consumption is taken into account. Electricity use is greatly affected by geography, climate and economics, making it hard to generalize from single studies. [53] SOLAR THERMAL Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation. [54] WATER HEATING [pic] Solar water heaters facing the  Sun  to maximize gain. Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40  degrees) from 60 to 70% of the domestic hot water use with temperatures up to 60  Ã‚ °C can be provided by solar heating systems. [55]  The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools. [56] As of 2007, the total installed capacity of solar hot water systems is approximately 154  GW. [57]  China is the world leader in their deployment with 70  GW installed as of 2006 and a long term goal of 210  GW by 2020. [58]  Israel  and  Cyprus  are the per capita leaders in the use of solar hot water systems with over 90% of homes using them. 59]  In the United States, Canada and Australia heating swimming pools is the dominant application of solar hot water with an installed capacity of 18  GW as of 2005. [18] HEATING, COOLING AND VENTILATION [pic] Solar House #1 of  Massachusetts Institute of Technology  in the United States, built in 1939, used  Seasonal thermal energy storage (STES)  for year-round heating. In the United States,  heating, ventilation and air conditioning  (HVAC) systems account for 30% (4. 65  EJ) of the energy used in commercial buildings and nearly 50% (10. 1  EJ) of the energy used in residential buildings. [49][60]  Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy. Thermal mass is any material that can be used to store heat—heat from the Sun in the case of solar energy. Common thermal mass materials include stone, cement and water. Historically they have been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night. However they can be used in cold temperate areas to maintain warmth as well. The size and placement of thermal mass depend on several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment. [61] A solar chimney (or thermal chimney, in this context) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an  updraft  that pulls air through the building. Performance can be improved by using glazing and thermal mass materials[62]  in a way that mimics greenhouses. Deciduous  trees and plants have been promoted as a means of controlling solar heating and cooling. When planted on the southern side of a building, their leaves provide shade during the summer, while the bare limbs allow light to pass during the winter. [63]  Since bare, leafless trees shade 1/3 to 1/2 of incident solar radiation, there is a balance between the benefits of summer shading and the corresponding loss of winter heating. 64]  In climates with significant heating loads, deciduous trees should not be planted on the southern side of a building because they will interfere with wint er solar availability. They can, however, be used on the east and west sides to provide a degree of summer shading without appreciably affecting winter solar gain. [65] WATER TREATMENT [pic] Solar water disinfection  in  Indonesia [pic] Small scale solar powered sewerage treatment plant. Solar distillation can be used to make  saline  or  brackish water  potable. The first recorded instance of this was by 16th century Arab alchemists. [66]  A large-scale solar distillation project was first constructed in 1872 in the  Chilean  mining town of Las Salinas. 67]  The plant, which had solar collection area of 4,700  m2, could produce up to 22,700  L  per day and operated for 40  years. [67]  Individual  still  designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick, and multiple effect. [66]  These stills can operate in passive, active, or hybrid modes. Double-slope stills are the most economica l for decentralized domestic purposes, while active multiple effect units are more suitable for large-scale applications. [66] Solar water  disinfection  (SODIS) involves exposing water-filled plastic  polyethylene terephthalate  (PET) bottles to sunlight for several hours. 68]  Exposure times vary depending on weather and climate from a minimum of six hours to two days during fully overcast conditions. [69]  It is recommended by theWorld Health Organization  as a viable method for household water treatment and safe storage. [70]  Over two million people in developing countries use this method for their daily drinking water. [69] Solar energy may be used in a water stabilisation pond to treat  waste water  without chemicals or electricity. A further environmental advantage is thatalgae  grow in such ponds and consume  carbon dioxide  in photosynthesis, although algae may produce toxic chemicals that make the water unusable. [71][72] COOKING [pic] The Solar Bowl in  Auroville,  India, concentrates sunlight on a movable receiver to produce  steam  for  cooking. Solar cookers use sunlight for cooking, drying and  pasteurization. They can be grouped into three broad categories: box cookers, panel cookers and reflector cookers. [73]  The simplest solar cooker is the box cooker first built by  Horace de Saussure  in 1767. [74]  A basic box cooker consists of an insulated container with a transparent lid. It can be used effectively with partially overcast skies and will typically reach temperatures of 90–150  Ã‚ °C. [75]Panel cookers use a reflective panel to direct sunlight onto an insulated container and reach temperatures comparable to box cookers. Reflector cookers use various concentrating geometries (dish, trough, Fresnel mirrors) to focus light on a cooking container. These cookers reach temperatures of 315  Ã‚ °C and above but require direct light to function properly and must be repositioned to track the Sun. [76] The  solar bowl  is a concentrating technology employed by the Solar Kitchen at  Auroville, in  Tamil Nadu,  India, where a stationary spherical reflector focuses light along a line perpendicular to the sphere’s interior surface, and a computer control system moves the receiver to intersect this line. Steam is produced in the receiver at temperatures reaching 150  Ã‚ °C and then used for process heat in the kitchen. [77] A reflector developed by  Wolfgang Scheffler  in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. Polar tracking  is used to follow the Sun’s daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450–650  Ã‚ °C and have a fixed focal point, which simplifies cooking. [78]  The world’s largest Scheffler reflector system in Abu Road,  Rajasthan, India is capable of cooking up to 35,000 meals a day. [79]As of 2008, over 2,000 large Scheffler cookers had been built worldwide. [80] PROCESS HEAT Solar concentrating technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial system was the  Solar Total Energy Project  (STEP) in Shenandoah, Georgia, USA where a field of 114 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory. This grid-connected cogeneration system provided 400  kW of electricity plus thermal energy in the form of 401  kW steam and 468  kW chilled water, and had a one hour peak load thermal storage. [81] Evaporation ponds are shallow pools that concentrate dissolved solids through  evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. [82] Clothes lines,  clotheshorses, and clothes racks dry clothes through evaporation by wind and sunlight without consuming electricity or gas. In some states of the United States legislation protects the â€Å"right to dry† clothes. [83] Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22  Ã‚ °C and deliver outlet temperatures of 45–60  Ã‚ °C. [84]  The short payback period of transpired collectors (3 to 12  years) makes them a more cost-effective alternative than glazed collection systems. 84]  As of 2003, over 80 systems with a combined collector area of 35,000  m2  had been installed worldwide, including an 860  m2  collector in  Costa Rica  used for drying coffee beans and a 1,300  m2  collector in  Coimbatore, India used for drying marigolds. [28] ELECTRICITY PRODUCTION [pic] The  PS10  concentrates sunlight from a field of heliostats on a central tower. Solar power is the conversion of sunlight into  electricity, either directly using  photovoltaics  (PV), or indirectly using  concentrated solar power  (CSP). CSP systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. PV converts light into electric current using the  photoelectric effect. Commercial CSP plants were first developed in the 1980s. Since 1985 the eventually 354 MW  SEGS  CSP installation, in the Mojave Desert of California, is the largest solar power plant in the world. Other large CSP plants include the 150 MW  Solnova Solar Power Station  and the 100 MWAndasol solar power station, both in Spain. The 250 MW  Agua Caliente Solar Project, in the United States, and the 214 MW  Charanka Solar Park  inIndia, are the  world’s largest  photovoltaic plants. Solar projects exceeding 1 GW are being developed, but most of the deployed photovoltaics are in small rooftop arrays of less than 5 kW, which are grid connected using net metering and/or a feed-in tariff. [85] Concentrated solar power Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the parabolic trough, the concentrating linear fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the Sun and focus light. In all of these systems a  working fluid  is heated by the concentrated sunlight, and is then used for power generation or energy storage. [86] PHOTOVOLTAICS PHOTOVOLTAICS A solar cell, or photovoltaic cell (PV), is a device that converts light into electric current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in the 1880s. In 1931 a German engineer, Dr Bruno Lange, developed a photo cell using silver selenite in place of copper oxide. Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4. 5–6%. By 2012 available efficiencies exceed 20% and the maximum efficiency of research photovoltaics is over 40%. OTHERS Besides concentrated solar power and photovoltaics, there are some other techniques used to generated electricity using solar power. These include: †¢Dye-sensitized_solar_cells, Luminescent solar concentrators (a type of concentrated photovoltaics or CPV technology), †¢Biohybrid solar cells, †¢Photon Enhanced Thermionic Emission systems. Development, deployment and economics Beginning with the surge in coal use which accompanied the In dustrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce. However development of solar technologies stagnated in the early 20th century in the face of the increasing availability, economy, and utility of coal and petroleum. [109] The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999. [57] Although generally underestimated, solar water heating and cooling is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007. The International Energy Agency has said that solar energy can make considerable contributions to solving some of the most urgent problems the world now faces: The development of affordable, inexhaustible and clean solar energy technologies will have huge longer-term benefits. It will increase countries’ energy security through reliance on an indigenous, inexhaustible and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating climate change, and keep fossil fuel prices lower than otherwise. These advantages are global. Hence the additional costs of the incentives for early deployment should be considered learning investments; they must be wisely spent and need to be widely shared. In 2011, the International Energy Agency said that solar energy technologies such as photovoltaic panels, solar water heaters and power stations built with mirrors could provide a third of the world’s energy by 2060 if politicians commit to limiting climate change. The energy from the sun could play a key role in de-carbonizing the global economy alongside improvements in energy efficiency and imposing costs on greenhouse gas emitters. The strength of solar is the incredible variety and flexibility of applications , from small scale to big scale†. We have proved †¦ that after our stores of oil and coal are exhausted the human race can receive unlimited power from the rays of the sun. —Frank Shuman, New York Times, July 2, 1916 SOLAR CELL SOLAR CELL A solar cell made from amonocrystalline silicon wafer Solar cells can be used devices such as this portable monocrystalline solar charger. A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell (in that its electrical characteristics—e. g. urrent, voltage, or resistance—vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source. The term â€Å"photovoltaic† comes from the Greek (phos) meaning â€Å"light†, and from â€Å"Volt†, the unit of electro-motive force, the volt, which in turn comes from the last name of the Italian physicist Alessandro Volta, inventor of the battery (electrochemical cell). The term â€Å"photo-voltaic† has been in use in English since 1849. Photovoltaics is the field of technology and research related to the practical application of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight. Cells can be described as photovoltaic even when the light source is not necessarily sunlight (lamplight, artificial light, etc. ). In such cases the cell is sometimes used as a photodetector (for example infrared detectors), detecting light or other electromagnetic radiationnear the visible range, or measuring light intensity. The operation of a photovoltaic (PV) cell requires 3 basic attributes: 1. The absorption of light, generating either electron-hole pairs or excitons. 2. The separation of charge carriers of opposite types. 3. The separate extraction of those carriers to an external circuit. In contrast, a solar thermal collector collects heat by absorbing sunlight, for the purpose of either direct heating or indirect electrical power generation. Photoelectrolytic cell† (photoelectrochemical cell), on the other hand, refers either a type of photovoltaic cell (like that developed by A. E. Becquerel and modern dye-sensitized solar cells) or a device that splits water direct ly into hydrogen and oxygen using only solar illumination. FURTHER IMPROVEMENTS In the time since Berman’s work, improvements have brought production costs down under $1 a watt, with wholesale costs well under $2. â€Å"Balance of system† costs are now more than the panels themselves. Large commercial arrays can be built at below $3. 40 a watt,[12][13]  fully commissioned. As the semiconductor industry moved to ever-larger boules, older equipment became available at fire-sale prices. Cells have grown in size as older equipment became available on the surplus market; ARCO Solar’s original panels used cells with 2 to 4  inch (51 to 100  mm) diameter. Panels in the 1990s and early 2000s generally used 5  inch (125  mm) wafers, and since 2008 almost all new panels use 6  inch (150  mm) cells. This material has less efficiency, but is less expensive to produce in bulk. The widespread introduction of  flat screen televisions  in the late 1990s and early 2000s led to the wide availability of large sheets of high-quality glass, used on the front of the panels. In terms of the cells themselves, there has been only one major change. During the 1990s, polysilicon cells became increasingly popular. These cells offer less efficiency than their monosilicon counterparts, but they are grown in large vats that greatly reduce the cost of production. By the mid-2000s, poly was dominant in the low-cost panel market, but more recently a variety of factors has pushed the higher performance mono back into widespread use. CURRENT EVENTS Other technologies have tried to enter the market. First Solar  was briefly the largest panel manufacturer in 2009, in terms of yearly power produced, using a thin-film cell sandwiched between two layers of glass. Since then silicon panels reasserted their dominant position both in terms of lower prices and the rapid rise of Chinese manufacturing, resulting in the top producers being Chinese. By late 2011, efficient production in China, coupled with a drop in European demand due to budgetary turmoil had dropped prices for crystalline solar-based modules further, to about $1. 09[13]  per watt in October 2011, down sharply from the price per watt in 2010. A more modern process, mono-like-multi, aims to offer the performance of mono at the cost of poly, and is in the process of being introduced in 2012[citation needed]. APPLICATIONS [pic] Polycrystalline  photovoltaic cells laminated to backing material in a module [pic] [pic] Polycrystalline photovoltaic cells Solar cells are often electrically connected and encapsulated as a  module. Photovoltaic modules often have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the emiconductor  wafers  from abrasion and impact due to wind-driven debris,  rain,  hail, etc. Solar cells are also usually connected in  series  in modules, creating an additive  voltage. Connecting cells in parallel will yield a higher current; however, very significant problems exist with parallel connections. For example, shadow effects can shut down the weaker (less illuminated) parallel string (a number of series connected cells) causing substantial power loss and even damaging the weaker string because of the excessive  reverse bias  applied to the shadowed cells by their illuminated partners. Strings of series cells are usually handled independently and not connected in parallel, special paralleling circuits are the exceptions. Although modules can be interconnected to create an  array  with the desired peak DC voltage and loading current capacity, using independent MPPTs (maximum power point trackers) provides a better solution. In the absence of paralleling circuits, shunt diodes can be used to reduce the power loss due to shadowing in arrays with series/parallel connected cells. To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected  photovoltaic systems); in stand-alone systems, batteries are used to store the energy that is not needed immediately. Solar panels can be used to power or recharge portable devices. THEORY The solar cell works in three steps: 1. Photons  in  sunlight  hit the solar panel and are absorbed by semiconducting materials, such as silicon. 2. Electrons  (negatively charged) are knocked loose from their atoms, causing an electric potential difference. Current starts flowing through the material to cancel the potential and this electricity is captured. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. 3. An array of solar cells converts solar energy into a usable amount of  direct current  (DC) electricity. EFFICIENCY Solar panels on the International Space Station absorb light from both sides. These Bifacial cells are more efficient and operate at lower temperature than single sided equivalents. The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of each of these individual efficiencies. A solar cell usually has a voltage dependent efficiency curve, temperature coefficients, and shadow angles. Due to the difficulty in measuring these parameters directly, other parameters are measured instead: thermodynamic efficiency, quantum efficiency,integrated quantum efficiency, VOC ratio, and fill factor. Reflectance losses are a portion of the quantum efficiency under â€Å"external quantum efficiency†. Recombination losses make up a portion of the quantum efficiency, VOC ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of the quantum efficiency, VOC ratio. The fill factor is defined as the ratio of the actual maximum obtainable power to the product of the open circuit voltage and short circuit current. This is a key parameter in evaluating the performance of solar cells. Typical commercial solar cells have a fill factor ; 0. 70. Grade B cells have a fill factor usually between 0. 4 to 0. 7. 14] Cells with a high fill factor have a low equivalent series resistance and a high equivalent shunt resistance, so less of the current produced by the cell is dissipated in internal losses. Single p–n junction crystalline silicon devices are now approaching the theoretical limiting power efficiency of 33. 7%, noted as the Shockley–Queisser limit in 1961. In the extreme, with an infinite number of layers, the corresponding limit is 86% using concentrated sunlight. [pic] Reported timeline of solar cell energy conversion efficiencies (from National Renewable Energy Laboratory (USA)) MATERIALS [pic] [pic] The  Shockley-Queisser limit  for the theoretical maximum efficiency of a solar cell. Semiconductors with  band gapbetween 1 and 1. eV, or near-infrared light, have the greatest potential to form an efficient cell. (The efficiency â€Å"limit† shown here can be exceeded by  multijunction solar cells. ) Various materials display varying efficiencies and have varying costs. Materials for efficient solar cells must have characteristics matched to the spectrum of available light. Some cells are designed to efficiently convert wavelengths of solar light that reach the Earth surface. However, some solar cells are optimized for light absorption beyond Earth’s atmosphe re as well. Light absorbing materials can often be used in  multiple physical configurations  to take advantage of different light absorption and charge separation mechanisms. Materials presently used for photovoltaic solar cells include  monocrystalline silicon,  polycrystalline silicon,  amorphous silicon,  cadmium telluride, andcopper indium selenide/sulfide. [25][26] Many currently available solar cells are made from bulk materials that are cut into  wafers  between 180 to 240  micrometers thick that are then processed like other semiconductors. Other materials are made as  thin-films  layers, organic  dyes, and organic  polymers  that are deposited on  supporting substrates. A third group are made from  nanocrystals  and used as  quantum dots  (electron-confined  nanoparticles). Silicon remains the only material that is well-researched in both  bulkand  thin-film  forms. CRYSTALLINE SILICON [pic] Basic structure of a silicon based solar cell and its working mechanism. By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as â€Å"solar grade silicon†. Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, orwafer. 1. monocrystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells. 2. olycrystalline silicon, or multicrystalline silicon, (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less effi cient. United States Department of Energy data show that there were a higher number of polycrystalline sales than monocrystalline silicon sales. 3. ribbon silicon is a type of polycrystalline silicon: it is formed by drawing flat thin films from molten silicon and results in a polycrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots. 4. ono-like-multi silicon: Developed in the 2000s and introduced commercially around 2009, mono-like-multi, or cast-mono, uses existing polycrystalline casting chambers with small â€Å"seeds† of mono material. The result is a bulk mono-like material with poly around the outsides. When sawn apart for processing, the inner sections are high-efficiency mono-like cells (but square instead of â€Å"clipped†), while the outer edges are sold off as conventional poly. The result is line that produces mon o-like cells at poly-like prices. Analysts have predicted that prices of polycrystalline silicon will drop as companies build additional polysilicon capacity quicker than the industry’s projected demand. On the other hand, the cost of producing upgraded metallurgical-grade silicon, also known as UMG Si, can potentially be one-sixth that of makingpolysilicon. Manufacturers of wafer-based cells have responded to high silicon prices in 2004–2008 prices with rapid reductions in silicon consumption. According to Jef Poortmans, director of IMEC’s organic and solar department, current cells use between eight and nine grams of silicon per watt of power generation, with wafer thicknesses in the neighborhood of 0. 200 mm. At 2008 spring’s IEEEPhotovoltaic Specialists’ Conference (PVS’08), John Wohlgemuth, staff scientist at BP Solar, reported that his company has qualified modules based on 0. 180 mm thick wafers and is testing processes for 0. 16 mm wafers cut with 0. 1 mm wire. IMEC’s road map, presented at the organization’s recent annual research review meeting, envisions use of 0. 08 mm wafers by 2015. Gallium arsenide multijunction: High-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W. [35] These multijunction cells consist of multiple thin films produced using metalorganic vapour phase epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2. [36] Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. Combinations of semiconductors are carefully chosen to absorb nearly the entire solar spectrum, thus generating electricity from as much of the solar energy as possible. GaAs based multijunction devices are the most efficient solar cells to date. In October 15, 2012, triple junction metamorphic cell reached a record high of 44%. [37] Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge p–n junctions, are seeing demand rapidly rise. Between December 2006 and December 2007, the cost of 4N gallium metal rose from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000–1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry. Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2003, 2005 and 2007, and also by the Dutch solar carsSolutra (2005), Twente One (2007) and 21Revolution (2009). The Dutch Radboud University Nijmegen set the record for thin film solar cell efficiency using a single junction GaAs to 25. 8% in August 2008 using only 4  µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film. THIN FILMS [pic] Market share of the different PV technologies  In 2010 the market share of thin film declined by 30% as thin film technology was displaced by more efficient crystalline silicon solar panels (the light and dark blue bars). Thin-film technologies reduce the amount of material required in creating the active material of solar cell. Most thin film solar cells are sandwiched between two panes of glass to make a module. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels. The majority of film panels have significantly lower conversion efficiencies, lagging silicon by two to three percentage points. 31]  Thin-film solar technologies have enjoyed large investment due to the success of First Solar and the largely unfulfilled promise of lower cost and flexibility compared to wafer silicon cells, but they have not become mainstream solar products due to their lower efficiency and corresponding larger area con sumption per watt production. Cadmium telluride  (CdTe),  copper indium gallium selenide  (CIGS) and  amorphous silicon  (A-Si) are three thin-film technologies often used as outdoor photovoltaic solar power production. CdTe technology is most cost competitive among them. [32]  CdTe technology costs about 30% less than CIGS technology and 40% less than A-Si technology in 2011. CADMIUM TELLURIDE SOLAR CELL A cadmium telluride solar cell uses a cadmium telluride (CdTe) thin film, a  semiconductor  layer to absorb and convert sunlight into electricity. Solarbuzzhas reported that the lowest quoted thin-film module price stands at US$0. 84 per  watt-peak, with the lowest crystalline silicon (c-Si) module at $1. 06 per watt-peak. [33] The  cadmium  present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during ? res in residential roofs. [34]  A square meter of CdTe contains approximately the same amount of Cd as a single C cell  Nickel-cadmium battery, in a more stable and less soluble form. [34] COPPER INDIUM GALLIUM SELENIDE Copper indium gallium selenide (CIGS) is a  direct band gap  material. It has the highest efficiency (~20%) among thin film materials (see  CIGS solar cell). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at  IBM  and  Nanosolar  attempt to lower the cost by using non-vacuum solution processes. GALLIUM ARSENIDE MULTIJUNCTION High-efficiency multijunction cells were originally developed for special applications such as  satellites  and  space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W. 35]  These multijunction cells consist of multiple thin films produced using  metalorganic vapour phase epitaxy. A triple-junction cell, for example, may consist of the semiconductors:  GaAs,  Ge, and  GaInP2. [36]  Each type of semiconductor will have a characteristic  band gap  energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb  electromagnetic radiation  over a portion of the spectrum. Combinations of semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible. GaAs based multijunction devices are the most efficient solar cells to date. In October 15, 2012, triple junction metamorphic cell reached a record high of 44%. [37] Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge p–n junctions, are seeing demand rapidly rise. Between December 2006 and December 2007, the cost of 4N gallium metal rose from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000–1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry. Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time  World Solar Challenge  winners  Nuna  in 2003, 2005 and 2007, and also by the Dutch solar carsSolutra (2005),  Twente One (2007)  and 21Revolution (2009). The Dutch  Radboud University Nijmegen  set the record for thin film solar cell efficiency using a single junction GaAs to 25. 8% in August 2008 using only 4  Ã‚ µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film. Light-absorbing dyes (DSSC) Dye-sensitized solar cells  (DSSCs) are made of low-cost materials and do not need elaborate equipment to manufacture, so they can be made in a  DIY  fashion, possibly allowing players to produce more of this type of solar cell than others. In bulk it should be significantly less expensive than older  solid-state  cell designs. DSSC’s can be engineered into flexible sheets, and although its  conversion efficiency  is less than the best  thin film cells, its  price/performance ratio  should be high enough to allow them to compete with  fossil fuel electrical generation. Typically a  ruthenium  metalorganic  dye  (Ru-centered) is used as a  monolayer  of light-absorbing material. The dye-sensitized solar cell depends on a  mesoporous  layer of  nanoparticulate  titanium dioxide  to greatly amplify the surface area (200–300 m2/g TiO2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the  light absorbing dye  are passed on to the  n-type  TiO2, and the holes are absorbed by an  electrolyte  on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by  screen printing  or use of  Ultrasonic Nozzles, with the potential for lower processing costs than those used for  bulk  solar cells. However, the dyes in these cells also suffer from  degradation  under heat and  UV  light, and the cell casing is difficult to  seal  due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade. The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations. [38] Quantum Dot Solar Cells (QDSCs) Quantum dot solar cells  (QDSCs) are based on the Gratzel cell, or  dye-sensitized solar cell, architecture but employ low  band gap  semiconductor  nanoparticles, fabricated with such small crystallite sizes that they form  quantum dots  (such as  CdS,  CdSe,  Sb2S3,  PbS, etc. ), instead of organic or organometallic dyes as light absorbers. Quantum dots (QDs) have attracted much interest because of their unique properties. Their size quantization allows for the  band gap  to be tuned by simply changing particle size. They also have high  extinction coefficients, and have shown the possibility of  multiple exciton generation. [39] In a QDSC, a  mesoporous  layer of  titanium dioxide  nanoparticles forms the backbone of the cell, much like in a DSSC. This TiO2  layer can then be made photoactive by coating with semiconductor quantum dots using  chemical bath deposition,  electrophoretic deposition, or successive ionic layer adsorption and reaction. The electrical circuit is then completed through the use of a liquid or solid  redox couple. During the last 3–4 years, the efficiency of QDSCs has increased rapidly[40]  with efficiencies over 5% shown for both liquid-junction[41]  and solid state cells. [42]  In an effort to decrease production costs of these devices, the  Prashant Kamat  research group[43]  recently demonstrated a solar paint made with TiO2  and CdSe that can be applied using a one-step method to any conductive surface and have shown efficiencies over 1%. [44] Organic/polymer solar cells Organic solar cells  are a relatively novel technology, yet hold the promise of a substantial price reduction (over thin-film silicon) and a faster return on investment. These cells can be processed from solution, hence the possibility of a simple roll-to-roll printing process, leading to inexpensive, large scale production. Organic solar cells and  polymer solar cells  are built from thin films (typically 100  nm) of  organic semiconductors  including polymers, such as  polyphenylene vinylene  and small-molecule compounds like copper phthalocyanine (a blue or green organic pigment) and  carbon fullerenes  and fullerene derivatives such as  PCBM. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials. However, it has improved quickly in the last few years and the highest  NREL  (National Renewable Energy Laboratory) certified efficiency has reached 8. 3% for the  Konarka  Power Plastic. [45]  In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an  exciton, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance. [46] In 2011, researchers at the Massachusetts Institute of Technology and Michigan State University developed the first highly efficient transparent solar cells that had a power efficiency close to 2% with a transparency to the human eye greater than 65%, achieved by selectively absorbing the ultraviolet and near-infrared parts of the spectrum with small-molecule compounds. 47]  [48]Researchers at UCLA more recently developed an analogous polymer solar cell, following the same approach, that is 70% transparent and has a 4% power conversion efficiency. [49]   The efficiency limits of both opaque and transparent organic solar cells were recently outlined. [50]  [51]  These lightweight, flexible cells can be produced in bulk at a low cost, and could be used to create power generating windows. Silicon thin films Silicon thin-film cells  are mainly deposited by  chemical vapor deposition  (typically plasma-enhanced, PE-CVD) from  silane  gas and  hydrogen  gas. Depending on the deposition parameters, this can yield:[52] 1. Amorphous silicon  (a-Si or a-Si:H) 2. Protocrystalline  silicon or 3. Nanocrystalline silicon  (nc-Si or nc-Si:H), also called microcrystalline silicon. It has been found that protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open circuit voltage. [53]  These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower  energy conversion efficiency  than  bulk  silicon, but are also less expensive to produce. The  quantum efficiency  of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon. An amorphous silicon (a-Si) solar cell is made of amorphous or microcrystalline silicon and its basic electronic structure is the  p-i-n  junction. -Si is attractive as a solar cell material because it is abundant and non-toxic (unlike its CdTe counterpart) and requires a low processing temperature, enabling production of devices to occur on flex ible and low-cost substrates. As the amorphous structure has a higher absorption rate of light than crystalline cells, the complete light spectrum can be absorbed with a very thin layer of photo-electrically active material. A film only 1 micron thick can absorb 90% of the usable solar energy. [54]  This reduced material requirement along with current technologies being capable of large-area deposition of a-Si, the scalability of this type of cell is high. However, because it is amorphous, it has high inherent disorder and dangling bonds, making it a bad conductor for charge carriers. These dangling bonds act as recombination centers that severely reduce the carrier lifetime and pin the Fermi energy level so that doping the material to n- or p- type is not possible. Amorphous Silicon also suffers from the Staebler-Wronski effect, which results in the efficiency of devices utilizing amorphous silicon dropping as the cell is exposed to light. The production of a-Si thin film solar cells uses glass as a substrate and deposits a very thin layer of silicon by  plasma-enhanced chemical vapor deposition  (PECVD). A-Si manufacturers are working towards lower costs per watt and higher conversion efficiency with continuous research and development on  Multijunction solar cells  for solar panels. Anwell Technologies Limited  recently announced its target for mul How to cite Street Light, Essay examples