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Solar Panel

Solar panel

with solar hot water panels on the roof.]] Solar panels are devices for capturing the energy in sunlight. The term solar panel can be applied to either solar hot water panels (usually used for providing domestic hot water) or solar photovoltaic panels (providing electricity).

Current Development

Right now, countless corporations and institutions are developing ways to increase the practicality of solar power. While private companies conduct much of the research and development in this area, colleges and universities also work on solar-powered devices, especially solar-powered vehicles. Solar-powered cars have commonly appeared at many car and technology shows, and now solar boats are an interesting application of the technology. Colleges and Universities compete against each other for superiority in this field of technology. They meet in competitions such as the [http://www.solarsplash.com/ Solar Splash]competition in North America, or the [http://www.frisiannuonsolarchallenge.com/ Frisian Nuon Solar Challenge] in Europe. In 2005 the most important issue with Solar Panels is the cost, which has been coming down to about $3-4 (US) a watt. Also grid tied systems are the largest growth area. In the USA, with incentives from States, power companies and in 2006 and 7 from the Federal government growth will continue to climb. Net-metering lets you get credit for any extra power you send back into the grid. Most is true net-metering with even prices for you to equal what you get charged, a few only give avoided cost at about 1/3 what they charge you. In Germany you get paid 8 times what the power company charges you. That large premium has made a huge demand in solar panels for that area. As manufacturers increase production the cost will continue to drop in the years to come. The price of silicon used for most panels is now being pressed and the price has increased. This has caused developers to start using other materials and thinner silicon to keep cost down. Renewable energy like solar PV gets less costly as we use and buy more.

Solar hot water

A solar water heater uses the sun's energy to heat a fluid, which is used to transfer the heat to a heat storage vessel. In the home, for example, sanitary hot water would be heated and stored in a hot water cylinder. Panels on the roof have an absorber plate to which fluid circulation tubes are attached. The absorber (usually coated with a dark selective coating) assures the conversion of the sun's radiation into heat, while fluid circulating through the tubes carries the heat away where it can be used or stored. The heated fluid is pumped to a heat exchanger (a coil in the storage vessel or an external heat exchanger) where it gives off its heat and is then circulated back to the panel to be reheated. This provides a simple and effective way of harnessing the sun's energy.

Solar photovoltaics

Solar photovoltaic panels contain arrays of solar cells 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 available for use. The solar cells are sometimes called photovoltaic cells, photovoltaic meaning literally "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 charged layers. On a bright day, the sun delivers about 1 kW/m² to the Earth's surface. Today's solar panels are known to have an average efficiency of 12 %. This would result in 120 W/m². However, not all days have bright suns and are fortunate enough to be blessed by such energy. At middle northern latitudes, taking the daylight cycle and weather conditions into account, on average 100 W/m² in winter and 250 W/m² in summer reach the ground. With a conversion efficiency of about 12%, one can expect to obtain between 12 and 30 watts per square meter of solar cell. Accordingly, at the current $0.08/kWh, a square meter will generate up to $0.06 per 24 hr day, and a square kilometer (250 acres) would generate up to 30 MW, or $50,000/km²/day. For reference, the unpopulated Sahara desert is over 9 million km², with less cloud cover and better solar angle, giving closer to 50MW/km², or 450TW (terrawatt) total. The Earth's current energy consumption is near 12-13 TW at any given moment (including oil, gas, coal, nuclear, hydro.) The real issue with solar panels is the capital cost, as shown at the net energy gain article, requiring up to over 7 years recovery period before any profit is made, out of a 40+ year useful life. In contrast, nuclear or coal plant recovers its capital cost in under a mere month, not considering the limited fuel supplies and thus fuel cost. Solar energy on Earth is not bound by limited reserves.

Use of solar PVs

Together with a backup battery, they have become routine in certain low-power applications, such as powering buoys or devices in remote areas or simply where connection to the electricity mains would be impractical. The relatively high cost of purchase and installation still prohibits their use in large-scale power generation. Solar PV panels currently make up a very small portion of the world's electricity production. In experimental form they have even been used to power automobiles in races such as the World solar challenge across Australia. Many yachts and land-vehicles use them to charge on-board batteries away from grid power. Large-scale incentive programs, offering financial incentives like the ability to sell excess electricity back to the public grid, have greatly accelerated the pace of solar PV installations in Spain, Germany, Japan, the United States and other countries. Even with these incentives, the start-up costs associated with solar electric panels currently push their likely 'pay-back' period into decades rather than years in applications where conventional "grid" power is readily available. As fossil fuel energy costs climb, production experience and economies of scale reduce prices, and technological advances increase the efficiency of solar cells, this may not be true in the relatively near future. Many installations at this time are motivated by tax incentives and green sensibilities.

World solar power production

Total peak power of installed solar panels is around 2,600 MW as of the end of 2004.

Large PV power plants

Cost of solar photovoltaic panels

Costs of photovoltaic panels seem, in 2005, to be about $1 to $2 per watt in ~400kW quantities. As production rates increase, costs are likely to continue to go down. Installed, costs seem to be in the $3-$7 per watt range.

Theory and construction

See the solar cell article for a description of the conversion of light energy into electrical energy. Crystalline silicon and gallium arsenide are typical choices of materials for solar cells. 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. Polycrystalline silicon has lower conversion efficiency but also lower cost. 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. 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). Electrical connections are made in series-parallel to determine total output voltage. The cement and the substrate must be thermally conductive, because the cells 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 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. It is claimed that if one fourth of the nation's pavement and buildings in cities alone were converted to incorporate solar panels, these could power the entire United States.

Solar panels on spacecraft

Probably the most successful use of solar panels is on spacecraft, including most spacecraft that orbit the Earth and Mars, and spacecraft going to other destinations in the inner solar system. In the outer solar system, the sunlight is too weak to produce sufficient power and radioisotope thermal generators are used. Research is underway to develop solar power satellites: space-based solar plants — satellites with large arrays of photovoltaic cells that would beam the energy to Earth using microwaves or lasers. Japanese and European space agencies have announced plans to develop such power plants in the first quarter of the 21st century. As opposed to chemical rockets, which are powered by a chemical reaction of the propellant, and uses the exhaust gases as reaction mass, some spacecraft propulsion methods have a method of expelling reaction mass powered by electricity. Either solar energy or nuclear energy is used. These methods typically have a higher specific impulse. The amount of reaction mass needed always grows exponentially with the delta-v to be produced, but more mildly if the specific impulse is high (but it should not be too high because for large specific impulse the power needed is proportional to it). With solar power the acceleration that can be produced is very low (much too low for a launch), but enduring. Typical burn times are months instead of minutes. The power the solar panel produces per kg, as an upper limit of the power the spacecraft has at its disposal per kg spacecraft (including solar panels) is an important factor. See also energy needed for propulsion methods. 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. 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. To date, solar power, other than for propulsion, has been practical for spacecraft operating no farther from the sun than the orbit of Mars. For example, Magellan, Mars Global Surveyor, and Mars Observer used solar power as did the Earth-orbiting, Hubble Space Telescope. For future missions, it is desirable to reduce solar array mass, and to increase the power generated per unit area. This will reduce overall spacecraft mass, and may make the operation of solar-powered spacecraft feasible at larger distances from the sun. The Rosetta space probe, launched March 2, 2004, will use solar panels as far as the orbit of Jupiter (5.25 AU); previously the furthest use was the Stardust spacecraft at 2 AU. Solar power for propulsion is currently used on the European lunar mission SMART-1 with a Hall effect thruster. Solar array mass could be reduced with thin-film photovoltaic cells, flexible blanket substrates, and composite support structures. Solar array efficiency could be improved by using new photovoltaic cell materials and solar concentrators that intensify the incident sunlight. Photovoltaic concentrator solar arrays for primary spacecraft power are devices which intensify the sunlight on the photovoltaics. This design uses a flat lens, called a Fresnel lens, which takes a large area of sunlight and concentrates it onto a smaller spot. The same principle is used to start fires with a magnifying glass on a sunny day. 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.

References

# [http://www.oja-services.nl/iea-pvps/isr/index.htm Overview] # [http://www.oja-services.nl/iea-pvps/countries/ Country Information] # [http://www.pvresources.com/en/top50pv.php Solar Records]

External links

- http://www.enf.cn Solar industry directory, news and government contracts
- http://www.tectosol.staticip.de/index_en.htm Solar electricity yield of a photovoltaic system
- [http://www.talcoelectronics.com/ Solar Panels] Solar panels for marine, RV, & industrial use.
- http://www.solarbuzz.com Solarbuzz tracks the price of industrial solar panels
- http://www.solar-panels.cc More information on solar panels and how it works.
- http://www.californiasolarco.com/power-systems-photo-gallery.html Residential photovoltaic systems - photo gallery
- http://www.tucsonelectric.com/Company/News/PressReleases/ReleaseTemplate.asp?idRec=3 Tucson Electric Power describes the 2.4 MW array at Springerville
- http://news.nationalgeographic.com/news/2005/01/0114_050114_solarplastic.html nano breakthrough Category:Renewable energy Category:Spacecraft components

Electric boat

Electric Boat is sometimes used as a colloquial abbreviation for the US Electric Boat Corporation. This article is about electric boats themselves, rather than this corporation. While most boats on the water today are powered by diesel engines, and sail power and gasoline engines are also popular, it is perfectly feasible to power boats by electricity too. Electric boats were very popular from the 1880s until the 1920s, when the internal combustion engine took dominance. Since the energy crises of the 1970s, interest in this quiet and potentially renewable marine energy source has been increasing steadily again. With the present state of technology so developed, many believe that the time is right for electric boats to become popular again.

Components

The main components of the drive system of any electrically powered boat are similar in all cases, and similar to the options available for any electric vehicle.

Charger

Electric energy will have to be obtained for the battery bank from some source.
- Mains charger allows the boat to be charged from a shore-side power point when one is available. This calls into question claims that the boat is 'non-polluting' and uses 'renewable energy', but at least it does not directly pollute the water in which it sits as would the use of any petroleum-based motor. Shore-based power stations are subject to much stricter environmental controls than the average marine diesel or outboard motor.
- Solar panels can be built into the boat in reasonable areas in the deck, cabin roof or as awnings. Modern solar panels, or photovoltaic arrays, can be flexible enough to fit to slightly curved surfaces and can be ordered in unusual shapes and sizes. It is still true that the heavier, rigid mono-crystalline types are more efficient in terms of energy output per square meter. The efficiency of solar panels rapidly decreases when they are not pointed directly at the sun, so some way of tilting the arrays while under way is very advantageous.
- Towed generators are common on long-distance cruising yachts and can generate a lot of power when travelling under sail. If an electric boat is to have sails as well, and will be used in deep water (deeper than about 15 m or 50 ft), then a towed generator will help build up battery charge while sailing. There is no point in trailing such a generator while under electric propulsion - we are not trying to create a perpetual motion machine! The extra drag from the generator will waste more electricity than it generates. Some electric power systems use the free-wheeling drive propeller to generate charge through the drive motor when sailing, but this system, including the design of the propeller and any gearing, cannot be optimised for both functions. It may be better locked off or feathered while the towed generator's more efficient turbine gathers energy.
- Wind turbines are also common on cruising yachts and can be very well suited to electric boats. There are safety considerations regarding the spinning blades, especially in a strong wind. It is important that the boat is big enough that the turbine can be mounted out of the way of all passengers and crew under all circumstances, including when alongside and when coming alongside a dock, a bank or a pier. It is also important that the boat is big enough and stable enough that the top hamper created by the turbine on its pole or mast does not compromise its stability in a strong wind or gale.
- If the boat is to have an internal combustion engine anyway, then its alternator will of course provide significant charge when it is running. This does rather defeat the original purpose, however. The weight saving that we would expect by not having this engine and all its associated tanks, pipework and other fittings would help to add to the efficiency that electric propulsion needs. In all cases, a charge regulator will be needed. This is to ensure that the batteries are charged at the maximum rate that they safely can stand when the power is available. It must also ensure that they are not overcharged when nearing full charge and not overheated when they are discharged and a great deal of charge current becomes available.

Battery bank

There have been significant technical advantages in battery technology in recent years, and more is to be expected in the future.
- Lead-acid batteries are the most viable option at the moment. Deep-cycle, 'traction' batteries are the obvious choice. There is no denying that they are heavy and bulky, but not much more so than the diesel engine, tanks and fittings that they may be replacing. They need to be securely mounted, low down and centrally situated in the boat. It is essential that they cannot move around under any circumstances. Care must be taken that there is no risk of spilled, strong acid in the event of a capsize as this could be dangerous or even fatal. At the same time, venting of explosive hydrogen and oxygen gases is also necessary.
- Nickel metal hydride and other hi-tech, solid-state batteries are becoming available, but are still expensive. These are the kind of batteries currently common in rechargeable hand tools like drills and screwdrivers, but they are relatively new to this environment. They require specialised charge controllers.
- Fuel cells are going to provide significant advantages in the years to come, and one day heavy lead-acid batteries will seem 'pre-historic' by comparison. Today (2005) however they are very expensive and require specialist equipment and knowledge, making them all but impractical for any but their dedicated enthusiasts. The size of the battery bank will determine the range of the boat under electric power alone. The speed that the boat is motored at will also affect this - a lower speed can make a big difference to the energy required to move a hull. Other factors that affect range include sea-state, windage and any charge that can be reclaimed while under way, for example by solar panels in full sun. A wind tubine in a good following wind will help, and motor-sailing in any wind could do so even more.

Speed controller

To make the boat usable and manoeuvrable, a simple-to-operate forward/stop/backwards speed controller is needed. This must be efficient - i.e. it must not get hot and waste energy at any speed - and it must be able to stand the full current that could conceivably flow under any full-load condition.

Electric motor

Electric motor technology is also complex and changing. Permanent magnet, brushless motors are considered very suitable by some specialists.

Drive chain

Depending on the size of the boat and the choice of electric motor, a standard propeller shaft, bearings and propeller may be available. In some cases some reduction gearing may be required, but from the point of view of efficiency, wear-and-tear and routine maintenance this should be avoided if at all possible, perhaps by choosing a different propeller.

Types

There are as many types of electric boat as there are boats with any other method of propulsion, but some types are significant for various reasons.
- Historical and restored electric boats exist and are often important projects for those involved. See the [http://www.marygordon.org.uk/marygordon.htm Mary Gordon Electric Boat] for example.
- River and lake boats. Electric boats, with their historically limited range and poor performance against a strong headwind or current, have tended to be used on inland waterways. In this environment, their complete lack of local pollution is also a significant advantage.
- Electric outboards and trolling motors have been available for some years at prices from about $100 (US) up to several thousand. These still require external batteries in the bottom of the boat, but by being a one-piece item apart from that, the manufacturers have been able to optimise the combination of the motor, the speed controller and the drive chain including the all-important propeller. Electric outboards have for some years provided an ideal drive system for inland waterway fishermen. They are quiet and they do not pollute the water, so they do not to scare away or harm the fish and other wildlife. As technologies improve, they should bring these benefits to many other dinghy users too, such as for yacht tenders and other inshore pleasure boats.
- Cruising yachts usually have an auxiliary engine, and there are two main uses for this engine. One is to power ahead or motor-sail at sea when the wind is light or from the wrong direction. The other is to provide the last 10 minutes or so of propulsion when the boat is in port and needs to be manoeuvred into a tight berth in a crowded and confined marina or harbour. Electric propulsion is ideally suited to the second case. The first case provides many - especially beginning yachtsmen and women - with a cause for anxiety, as a powerful diesel auxiliary has often been known to get a boat into harbour when sail power alone, or sail with limited help from an electric system, would miss the tide, the hours of daylight, or a rising gale. In fact with good seamanship, good passage-planning, sailing skills, the ability and willingness to keep to the deep sea or to anchor off in unfavourable conditions, none of this need be a problem, except perhaps for those in a hurry to get home, maybe for work in the morning or to feed the cats.
- Diesel-electric. There is a third potential use for the trusty diesel auxiliary and that is to charge the batteries, when they suddenly start to wane far from shore in the middle of the night, or at anchor after some days of living aboard. In this case, where this kind of use is to be expected, perhaps on a larger cruising yacht, then a combined diesel-electric solution may be designed from the start. The diesel engine is installed with the prime purpose of charging the battery banks, and the electric motor with that of propulsion. There is some reduction in efficiency if motoring for long distances as the diesel's power is converted first to electricity and then to motion, but there is a balancing saving every time the wind-, sail- and solar-charged batteries are used for manoeuvring and for short journeys without starting the diesel. There is the flexibility of being able to start the diesel as a pure generator whenever required. The main losses are in weight and installation cost, but on the bigger cruising boats that sit at anchor running large diesels for hours every day, these may not be too big an issue, compared to the savings that can be made at other times.

External links

- [http://www.westmarine.com/webapp/wcs/stores/servlet/SiteSearch?storeId=10001&catalogId=10001&keyword=electric+motor&x=0&y=0 A search for 'electric motor' at West Marine]
- [http://www.mindspring.com/~jimkerr1/sebebts.htm Solar Electric Boats]
- [http://www.econogics.com/ev/evboats.htm Electric Boats and Outboards]
- [http://www.findarticles.com/p/articles/mi_m0BQK/is_1_5/ai_61555347 Electric Boats Charging Back]
- [http://www.marygordon.org.uk/marygordon.htm Mary Gordon Electric Boat - history and restoration]
- [http://www.lvmshop.co.uk/item.asp?sid=&ls=&id=2664 Towed generator]
- [http://www.duffyboats.com/ Duffy Electric Boat Company] Category:Electric vehicles

Water heater

A water heater or hot water heater is an appliance for heating water above its ambient temperature. In industrial usage, as well when used to heat buildings through steam, large water heaters are called boilers.

Tank heaters

In household and commercial usage, most water heaters are of the tank type. These consist of tanks in which a given amount of water is kept continuously hot and ready for use. Typical sizes for household use are 20 to 50 US gallons (75 to 200 L). These may run on electricity, natural gas, propane, fuel oil, or other energy sources. The most popular in the United States is the natural gas type. Tank-type water heaters can be made more efficient by installation of additional insulation jackets around the tank, check valve devices at their inlet and outlet, cycle timers, electronic ignition (in the case of fuel-using models), sealed air intake systems (in the case of fuel-using models), and pipe insulation. The sealed air-intake system types are sometimes called "band-joist" intake units. "High efficiency" units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cool and are mechanically ventilated without the need of a chimney. At high efficiencies a drain must be supplied to handle the water condensed out of the combustion products. In British English, water heaters are known as boilers, or "Geysers" (pronounced "geezers"), feeding a separate hot water tank. Such tanks are often fitted with a backup electrical heater for a quick boost, known as an immersion heater. It is mandatory that these hot water storage vessels are 'indirect'. That means the water from the boiler circulates via a separate internal exchanger and does not come into contact with the stored hot water. (It is common for first time American users to burn themselves if not warned.)

Water heater safety

Water heaters potentially can explode and cause significant damage to a house if certain safety devices are not installed. When the water temperature exceeds 210 degrees Farenheit, the pressure and increasing heat of the water will cause a violent explosion. A safety device called a TPR valve, temperature pressure relief, is normally fitted on the top of the water heater. The TPR valve senses when the pressure is too great and will discharge water out of the valve. Often a discharge pipe is connected to the valve to direct the flow of water to a drain. Most home inspectors recommend the discharge pipe extend to outside the home so that it does not cause flooding damage. If a water heater is installed in a garage, it is recommended that it be elevated 18 inches off the ground so that the potential for catching fire due to low ground flowing exhaust is eliminated. Some building codes will allow for the discharge pipe to terminate in the garage. For older homes where the water heater was centrally located, some plumbers will install a Watts 210 device which is installed in place of the TPR valve. When the Watts 210 device senses that the temperature reaches 210 degrees, it will shut off the gas and eliminate the heat from water heater tank. In addition, an outside pressure relief valve is required to be installed on one of the cold water hose bibs on the exterior of the house. If the water pressure is too great in the water heater, it will discharge out through the cold water system to the exterior of the house. In California, state law mandates that all homes sold, whether new or old, have water heaters strapped and securely bolted to wall studs to prevent the tank from toppling over in a serious earthquake.

Tankless heaters

Tankless water heaters, also called instantaneous, inline or instant-on water heaters, are also available and gaining in popularity. These water heaters heat the water as the water flows through the device, and do not retain any water internally except for what is in the pipe. Tankless heaters may be installed in at faucet or bathroom that is far from the central water heater or larger models can be used to provide hot water for the entire house. Tankless heaters can be far more efficient than storage water heaters. In both kinds of installation, the absence of a tank saves energy as conventional water heaters have to reheat the water in the tank as it cools off. With a central water heater of any type, water is wasted waiting for water to heat up because of the cold water in the pipes between the faucet and the water heater. Point of use tankless water heaters are located right where the water is being used, so the water is almost instantly hot, which saves water. They also save even more energy than centrally installed tankless water heaters because there is not any hot water being left in the pipes after the water is shut off. However, point of use tankless water heaters are usually used in combination with a central water heater, as the expense of buying a heater for every kitchen, laundry room, bathroom or sink, often outweighs the money saved in water and energy bills. In addition, point of use water heaters are almost always electrical, and electricity is far more expensive than propane and natural gas. The most cost effective configuration is usually to use a central tankless water heater for the most of the house, preferably natural gas, and install a point of use tankless water heater at any distant faucets or bathrooms. However, this may vary according to how much electricity, gas and water costs in the area, the layout of the house and how much hot water is used. Only electric tankless water heaters were available at first and they are still used for almost all point of use heaters, but natural gas and propane heaters are now common. Since the water must be heated instantly, the tankless water heaters use a lot of electricity or fuel while they are on. If a storage water heater is being replaced with a tankless one, the size of the electrical wire or gas pipeline may have to be increased to handle the load.

Solar heaters

wire]] In some locales, solar water heaters are used. These are installed outside dwellings, typically on the roof or nearby, and consist of a tank and of a panel in which water circulates. The tank and the panel are painted a dark color in order to maximize the reception of solar heat.

External links

- [http://www.titanheater.com/tankless_water_heater_inside.php Tankless Water Heater Interior Components]
- [http://www.allhvacinfo.com/Contractors_Residential_Water_Heaters/Contractors_Residential_Water_Heaters.htm Directory of Water Heater Contractors]
- [http://home.howstuffworks.com/water-heater.htm Howstuffworks "How Water Heaters Work"]
- [http://www.industrialresource.biz/waterheater.html Water Heater Articles] Collection of articles about industrial water heaters
- [http://www.ruralenergy.co.nz/dairyaudit/content/main/4_waterheating.html Water Heating for Dairies] - Issues relating to farm dairy water heating Category:Plumbing Category:HVAC Category:Home appliances

Solar cell

A solar cell, or photovoltaic cell, is a semiconductor device consisting of a large-area p-n junction diode, which in the presence of sunlight is capable of generating usable electrical energy. This conversion is called the photovoltaic effect. The photovoltaic effect was discovered in 1839 by French experimental physicist Alexandre-Edmond Becquerel. He observed that certain materials would produce a small current when exposed to light. Light is comprised of packets of energy called photons. When light hits the p-n junction of a semi-conductor the absorbed photon energy releases a electron from the n-type region and moves it to the p-type filling a hole and creating a current. The field of research related to solar cells is known as photovoltaics. Solar cells have many applications. They are particularly well suited to, and historically used in, situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones and water pumping applications. Solar cells (in the form of modules or solar panels) on building roofs can be connected through an inverter to the electricity grid in a net metering arrangement. net metering

Introduction

Etymology

The term "photovoltaic" comes from the Greek photos meaning light, and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) are named. It means literally of light and electricity.

History

Main article: Timeline of solar cells The photovoltaic effect was first recognised in 1839 by French physicist Alexandre-Edmond Becquerel. However it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 ([http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662 US2402662], "Light sensitive device"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells.

Materials and efficiency

Various materials are being investigated for solar cells. Peformance in the two main criteria, efficiency and costs, varies greatly. Efficiency is the ratio of the electric power output to the light power input. Around noon on a clear day, the solar radiation at the equator is about 1000 W/m². So a 10% efficient module of 1 square meter has a power output of about 100 W. Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 30% or higher with multiple-junction research lab cells. The common method to express economic costs of electricity generating systems is to calculate a price per kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. To make actual use of the solar generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in autonomously operating systems, batteries are used to store the electricity that is not needed immediately. Using the present (2005) commercially available solar cells and system technology leads to system efficiencies between 5 and 15%. Electricity generation costs range from around 50 eurocents/kWh (middle of Europe) down to around 25 eurocents/kWh in regions of high solar irradiation. By far the most common material for solar cells (and all other semiconductor devices) is crystalline silicon. Crystalline silicon solar cells come in three primary categories:
- Single crystal or monocrystalline wafers made using the Czochralski process. Most commercial monocrystalline cells have efficiencies on the order of 14%; the SunPower cells have higher efficiencies, around 20%. Single-crystal cells tend to be expensive, and because they are cut from cylindrical ingots, they cannot completely cover a module without a substantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at the corners of four cells. [http://www.sunpowercorp.com/html/ Sunpower] and Shell Solar are among the main manufacturers of this type of cells.
- Poly or multi crystalline made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. These cells are cheaper than single crystal cells, but also somewhat less efficient. However, they can easily be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells, and this compensates for their lower efficiencies. See [http://www.gtsolar.com/products/hem.php GT Solar HEM Furnace], [http://www.bp.com/modularhome.do?categoryId=4320&contentId=7004540 BP Solar], [http://solar.sharpusa.com/solar/home/0,2462,,00.html Sharp Solar] and [http://www.kyocerasolar.com Kyocera Solar].
- Ribbon silicon, formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells are typically the least efficient, but there is a cost saving since there is very little silicon waste, as this approach does not require sawing from ingots. See [http://www.evergreensolar.com/ Evergreen Solar], and [http://www.rweschottsolar.com/ RWE Schott Solar]. These technologies are wafer-based manufacturing. In other words, in each of the above approaches, self-supporting wafers of about 300 micrometres thick are fabricated and then soldered together to form a module. Thin film approaches are module-based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Two main thin film approaches are amorphous silicon and CIS:
- Amorphous silicon films are fabricated using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). These cells have low efficiencies of around 8%.
- CIS stands for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2. While these films can achieve 11% efficiency, their costs are still too high. There are additional materials and approaches. For example, Sanyo has pioneered the HIT cell. In this technology, amorphous silicon films are deposited onto crystalline silicon wafers. The chart below illustrates the various commercial large-area module efficiencies and the best laboratory efficiencies obtained for various materials and technologies. 500px

Interconnection and modules

Usually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc). Solar cells are usually connected in series in modules, so that their voltages add.

Theory

Background

In order to understand how a solar cell works, a little background theory in semiconductor physics is required. For simplicity, the description here will be limited to describing the workings of single crystalline silicon solar cells. Silicon is a group 14 (formerly, group IV) atom. This means that each Si atom has 4 valence electrons in its outer shell. Silicon atoms can covalently bond to other silicon atoms to form a solid. There are two basic types of solid silicon, amorphous (having no long range order) and crystalline (where the atoms are arranged in an ordered three dimensional array). There are various other terms for the crystalline structure of silicon; poly-crystalline, micro-crystalline, nano-crystalline etc, and these refer to the size of the crystal "grains" which make up the solid. Solar cells can be, and are made from each of these types of silicon, the most common being poly-crystalline. Silicon is a semiconductor. This means that in solid silicon, there are certain bands of energies which the electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the "band gap". The allowed and forbidden bands of energy are explained by the theory of quantum mechanics. At room temperature, pure silicon is a poor electrical conductor. In quantum mechanics, this is explained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a better conductor, it is "doped" with very small amounts of atoms from either group 13 (III) or group 15 (V) of the periodic table. These "dopant" atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as "holes") are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. Silicon which is doped with group 13 atoms (aluminium, gallium) is known as p-type silicon because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group 15 atoms (phosphorus, arsenic) is known as n-type silicon because the majority charge carriers (electrons) are negative. It should be noted that both n-type and p-type silicon are electrically neutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-type silicon, some of the negative charges are free to move around, while the converse is true for p-type silicon.

Light generation of carriers

n-type When a photon of light hits a piece of silicon, one of two things can happen. The first is that the photon can pass straight through the silicon. This (generally) happens when the energy of the photon is lower than the bandgap energy of the silicon semiconductor. The second thing that can happen is that the photon is absorbed by the silicon. This (generally) happens if the photon energy is greater than the bandgap energy of silicon. When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one less electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs. A photon only needs to have energy greater than the band gap energy to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy.

The p-n junction

A solar cell is a large-area semiconductor p-n junction. To understand the workings of a p-n junction it is convenient to imagine what happens when a piece of n-type silicon is brought into contact with a piece of p-type silicon. In practice, however, the p-n junctions of solar cells are not made in this way, but rather, usually, by diffusing an n-type dopant into one side of a p-type wafer. If we imagine what happens when a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then what occurs is a diffusion of electrons from the region of high electron concentration - the n-type side of the junction, into the region of low electron concentration - p-type side of the junction. When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. This diffusion of carriers does not happen indefinitely however, because of the electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. Electrons from donor atoms on the n-type side of the junction are crossing into the p-type side, leaving behind the (extra) positively charged nuclei of the group 15 donor atoms, leaving an excess of positive charge on the n-type side of the junction. At the same time, these electrons are filling in holes on the p-type side of the junction, becoming involved in covalent bonds around the group 13 acceptor atoms, making an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric field which opposes further diffusion of charge carriers across the junction. This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region". The electric field which is set up across the p-n junction creates a diode, allowing current to flow in only one direction across the junction. Electrons may pass from the n-type side into the p-type side, and holes may pass from the p-type side to the n-type side. But since the sign of the charge on electrons and holes is opposite, conventional current may only flow in one direction.

Separation of carriers by the p-n junction

Once the electron-hole pair has been created by the absorption of a photon, the electron and hole are both free to move off independently within the silicon lattice. If they are created within a minority carrier diffusion length of the junction, then, depending on which side of the junction the electron-hole pair is created, the electric field at the junction will either sweep the electron to the n-type side, or the hole to the p-type side.

Connection to an external load

Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load. Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there.

Equivalent circuit of a solar cell

metal metal To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode. In practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The result is the "equivalent circuit of a solar cell" shown on the left. Also shown on the right, is the schematic representation of a solar cell for use in circuit diagrams.

Manufacture and devices

Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and memory chips. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells. Single crystalline wafers which are used in the semiconductor industry can be made in to excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production. Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (250 to 350 micrometre) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, an n-type diffusion is performed on the front side of the wafer, forming a p-n junction a few hundred nanometres below the surface. Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically applied next. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). The wafer is then metallised, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminum. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The metal electrodes will then require some kind of heat treatment or "sintering" to make Ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed.

Energy conversion efficiency

Typical module efficiencies for commercially available screen printed multicrystalline solar cells are around 12%. A solar module's energy conversion efficiency, (or just efficiency) is the ratio of the maximum output electrical power divided by the input light power under "standard" test conditions. The "standard" solar radiation (known as the "air mass 1.5 spectrum") has a power density of 1000 watts per square metre. Thus, a typical 1 m² solar panel in direct sunlight will produce approximately 120 watts of peak power. A more technical description of efficiency is the maximum power, made up of the fill factor x the open circuit voltage x the short circuit current, divided by the input power. Note : A typical 4 square centimetre solar cell produces electrical energy of the order of 0.4 to 0.5 volts at 6 milliamperes.

Applications and implementations

See the article solar panel for information about applications and implementations of solar cells and panels.

Cost analysis

The US retail module costs are in the $3.50 to $5.00/Wp range ([http://www.solarbuzz.com/ see SolarBuzz]). Additional installation costs for a residential rooftop retrofit in California (CA) is around $3.50/Wp or more. So on the low side, installed system costs are about $7.00/Wp in CA, and probably higher in places with less experience. Federal, state, utility, and other subsidies combined pay about half the cost. So CA rule of thumb is that the installed system PV will cost you at the low end, $3.50/Wp. Under net metering, one offsets regular retail utility rate which for CA is about 11 cents/kWh. Knowing installed system costs, amount of sunshine, and the utility rates, one can figure out the years till payback with or without financing costs. Assuming no financing costs and a $6/Wp installed system cost (lower than current $7), one can take sunshine and utility rate information from around the globe and come up with a payback graph such as shown below. The addition of subsidies brings down the years to payback proportionately. For example, if the years to payback were 24 years at $6/Wp, and subsidies brought that down to $3/Wp, the years to payback would be 12. Image:PVYear2Payback.gif

Current research

There are currently many research groups active in the field of photovoltaics at universities and research institutions around the world. Much of the research is focused on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica sand. The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% in purity) is produced with the emission of about one and half tonnes of carbon dioxide. It is recently reported that solid silica can be directed converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius). [refs. T. Nohira et al, ‘Pinpoint and bulk electrochemical reduction of insulating silicon dioxide to silicon’, Nat. Mater., 2 (2003) 397. X. B. Jin et al, Electrochemical preparation of silicon and its alloys from solid oxides in molten calcium chloride’, Angew. Chem. Int. Ed., 43 (2004) 733.] While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such produced electrolytic silicon is in the form of porous silicon which turns readily into a fine powder (particle size: a few micrometers), and may therefore offer new opportunities for development of solar cell technologies. Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells. The invention of conductive polymers, (for which Alan Heeger was awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. Because photovoltaic panels convert a small fraction of the received light energy to electricity, there has been continued interest in laminating photovoltaic cells onto solar thermal panels to make PVT panels. Research in this area has found many difficulties and not much success (see References), for four reasons:
- Photovoltaic panels are usually designed to reflect light not used, rather than absorb it, and the difference requires quite a bit of development.
- Solar thermal panels for domestic hot water or space heating are usually glazed, and the glass tends to reflect some of the light that the PV cell might absorb.
- Because the panels convert less light to heat, they require more area for the same heat and suffer larger radiative and convective losses, all of which reduces the overall cost effectiveness of the solar thermal system.
- Because the manufacturing process is less developed, costs are higher, which is crippling in the extremely cost-sensitive solar market.

Thin-film solar cells

The next step in reducing the cost of solar cells and panels seems certain to come from thin-film technology. Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials. Thin Film solar cells are mainly deposited by PECVD from silane gas and hydrogen. This process produces a material without crystalline orientation : amorphous silicon. Depending on the deposition's parameters both protocrystalline silicon, which has been shown to exhibit the most stability, and nanocrystalline silicon can also be obtained. These types of silicon present dandling and twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) as well as in the deformation of the valence and conduction bands (band tails). This contributes to reduce the efficiency of Thin-Film solar cells by reducing the number of collected electron-hole pair by incident photon. Amorphous silicon (a-Si) has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgap as c-Si, the two material can be combined by depositing two diodes on top of each other : the tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. One particularly promising technology is crystalline silicon thin-films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach. From the [http://www.pacificsolar.com.au/ Pacific Solar] website: :"Crystalline Silicon on Glass (CSG) [is] the photovoltaic technology developed by Pacific Solar that is now being commercialised by [http://www.csgsolar.com.au/ CSG Solar]. A very thin layer of silicon, less than two micrometres thick, is deposited directly onto a glass sheet whose surface has been roughened by applying a layer of tiny glass beads. The silicon is not crystalline when first deposited, but becomes so after heat treatment in an oven. The resulting layer is processed using lasers and ink-jet printing techniques to form the electrical contacts needed to get the solar-produced electricity out of the thin silicon film." In 2005, a full-scale production factory is being built in Thalheim, Germany to commercialise this technology (project management by IB Vogt GmbH). CSG Solar expects to release its first product for sale in 2006. Each solar module will have a rated power exceeding 100 watts and will be cheaper than competing solar panels. Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications.

Emerging Materials

For special applications, such as Deep Space 1, high-efficiency cells can be made from gallium arsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different band gap energy so that it absorbs its share of the electromagnetic spectrum with very high efficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of each other, each absorbing a different spectrum of light, efficiency as high as 35.2% have been achieved. The multiple junction solar cells may be very efficient, but are prohibitively expensive to make. Cost-effective use of these cells could be achieved with concentrating optics so that less of the array consists of actual semiconductor devices. Experimental non-silicon solar panels can be made of quantum heterostructures, eg. carbon nanotubes or quantum dots, embedded in a special plastic. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost. While conventional solar cells only generate electricity from the visible light spectrum, experimental cells have been made that use the infrared spectrum. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. If panels that absorb both visible and infrared spectrums are able to be manufactured, the panels may be able to achieve up to 30 percent efficiency. (McDonald, et al., 2005) Some of the most efficient solar cell materials are cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. Polymer or organic solar cells are built from ultra thin layers (typically 100 nm) of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only a crude description of the functioning of such cells, as electron hopping and other processes also play a crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. In the reverse mode, the technology has however already successfully been commercialised in organic LEDs and organic displays, also called polymer displays. Graetzel cells (sometimes called photoelectrochemical cellsor Dye Solar Cells) were first announced in Nature in October 1991. The cell depends on a layer of nanoparticulate titanium dioxide, sensitised by a dye. In contrast to the classical solar cell the dye absorbs the radiation, mimicking the process of photosynthesis. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. As a result, this type of cell allows a more flexible use of materials, and typically are manufactured by screen printing, with cost advantages over the more expensive manufacturing techniques and equipments used for traditional and thin film solar cells, and significantly less embodied energy. This is an emerging technology with commercial impact forecast within this decade.

Solar cells and energy payback

There is a common but mistaken notion that solar cells never produce more energy than it takes to make them. While the expected working lifetime is around 40 years, the energy payback time of a solar panel is anywhere from 1 to 30 years (usually under five) depending on the type and where it is used (see net energy gain). This means solar cells are net energy producers and can "reproduce" themselves (from 6 to more than 30 times) over their lifetime. For details see [http://jupiter.clarion.edu/~jpearce/Papers/netenergy.pdf Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells.]

References

- PMID 15640806
- PVNET European Roadmap for PV R&D Ed Arnulf Jager-Waldan Office for Publications of the European Union 2004

External links

- [http://home.att.net/~africantech/solar/amorphous/amorphous1.htm Use of solar cells in ] Kenya and Uganda, in Africa
- Pennicott, Katie, "[http://physicsweb.org/article/news/5/12/2 Solar cell edges towards endless energy]". 7 December 2001. PhysicsWeb.
- [http://dcwww.epfl.ch/lpi/solarcellE.html Dye Sensitized Solar Cells] (DYSC) based on Nanocrystalline Oxide Semiconductor Films
- News searching: [http://news.google.com/news?hl=da&q=%22Solar+Cell%22 Solar Cell], [http://news.google.com/news?hl=da&q=Photovoltaic Photovoltaic]
- [http://www.atse.org.au/index.php?sectionid=391 Historical: Photovoltaic Solar Energy Conversion: An Update]
- [http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html Wladek Walukiewicz, Materials Sciences Division, Berkeley Lab.: Full Solar Spectrum Photovoltaic Materials Identified.] Quote: "... Maximum, theoretically predicted efficiencies increase to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps, respectively...."
- [http://news.cnet.com/investor/news/newsitem/0-9900-1028-21199489-0.html CNET: SunPower Announces World's Most Efficient, Low-Cost Silicon Solar Cell] (12 May 2003) Quote: "...[http://www.nrel.gov/ The National Renewable Energy Laboratory (NREL)] has verified 20.4 percent conversion efficiency for the A-300...."
- [http://www.sunpowercorp.com/html/Products/Datasheets/A-300/A-300.pdf SunPower A-300 (pdf)], [http://www.sunpowercorp.com/ SunPower]
- [http://www.sciam.com/article.cfm?chanID=sa003&articleID=0004C094-02CC-1CD0-B4A8809EC588EEDF Scientists Create New Solar Cell] (29 March 2002) Quote: "...semiconducting plastic material known as P3HT... 1.7 percent for sunlight..."
- [http://www.newscientist.com/news/news.jsp?id=ns99993380 'Denim' solar panels to clothe future buildings] (15 February 2003) Quote: "... Unlike conventional solar cells, the new, cheap material has no rigid silicon base..."
- [http://www.californiasolarco.com/power-systems-photo-gallery.html Residential Solar Power Systems - Photo Gallery]
- [http://www.sma-america.com/installations.html Examples of Photovoltaic Systems ]
- [http://science.howstuffworks.com/solar-cell.htm howstuffworks.com: How Solar Cells Work]
- [http://www.azonano.com/news.asp?newsID=548 azonano.com: Carbon Nanotube Structures Could Provide More Efficient Solar Power(28 February 2005)
- [http://www.newton.mec.edu/Brown/TE/HOT/TIMELINES/SOLAR/solar_timeline.html Solar energy timeline]
- [http://www.ecn.nl/docs/library/report/2004/rx04056.pdf PV-Thermal collector development -- an overview of the lessons learnt]] Zondag et al, 2004
- http://news.nationalgeographic.com/news/2005/01/0114_050114_solarplastic.html>Spray-On Solar-Power Cells Are True Breakthrough

Yield data

- http://www.tectosol.staticip.de/index_en.htm electricity yield of a solar power system
- http://www.sunny-portal.de Yield Portal for solar power system users

Theory

- [http://www.nrel.gov/buildings/pv/factsheets.html National Renewable Energy Laboratory (NREL): Photovoltaics for Buildings: PV Technology for the Home Factsheets]
- [http://www.nrel.gov/research/pv/docs/pvpaper.html 1993, National Renewable Energy Laboratory (NREL): Photovoltaics: Unlimited Electrical Energy From the Sun] BrokenLink
- [http://www.cefetba.br/fisica/NFL/PBCN/solar/solardeu.html#ideal Electrical models of solar cells]

Dye solar Cells

- [http://www.dyesol.com] commercialising Dye Solar Cell technology

Cost Benefit

- [http://rredc.nrel.gov/solar/codes_algs/PVWATTS/pvwatts_index.html PVWATTS - A Performance Calculator for Grid-Connected PV Systems]

Do-it-yourself

;PEC (Photo Electro Chromic)
- [http://www.chemistry.ucsc.edu/teaching/Winter98/Chem1B/photo/Solar_Kit_Word_6.html How to Build Your Own Solar Cell]
- [http://www.solideas.com/solrcell/cellkit.html DIY (Do It Yourself): Nanocrystalline Dye-Sensitized Solar Cell Kit] Quote: "... sunlight-to-electrical energy conversion efficiency is between 1 and 0.5 %..." ;Cuprous oxide solar cells
- [http://www.scitoys.com/scitoys/scitoys/echem/echem2.html#solarcell Make a solar cell in your kitchen], [http://www.scitoys.com/scitoys/scitoys/echem/echem3.html#sflatpanel A flat panel solar battery]
- [http://www.zetatalk.com/energy/tengy17f.htm From: How to Build a Solar Cell That Really Works by Walt Noon]

Indexes

- Open Directory Project: [http://www.dmoz.org/Business/Energy_and_Environment/Renewable/Solar/ Solar]

Newsgroups

- [http://groups.google.com/groups?q=alt.solar.photovoltaic alt.solar.photovoltaic]

Patents

- [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662 US2402662] -- Light sensitive device -- R. S. Ohl
- [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=1289369.WKU.&OS=PN/1289369&RS=PN/1289369 US1289369] -- Method of increasing the capacity of photosensitive electrical cells Category:Electrical components Category:Energy conversion Category:Renewable energy ko:태양 전지 ja:太陽電池

Electricity

Electricity is a general term applied to phenomena involving a fundamental property of matter called an electric charge. This article will introduce and explain some of the basic principles of electricity.

Related concepts

being radiated as light as the air of Earth's atmosphere is shifted from gas to plasma and back. ]] In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms.
- Electric charge: a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
- Electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity.
- Electric potential the potential energy per unit charge associated with a static (time-invariant) electric field.
- Electric current: a movement or flow of electrically charged particles.
- Electrical energy: energy made available by the flow of electric charge through a conductor or from the forces between charged particles.
- Electric power: The rate at which electric energy is converted into another form, such as light, heat, or mechanical energy (or converted from another form into electric energy).

History

Ancient

According to Thales of Miletus, writing circa 600 BCE, a form of electricity was known to the Ancient Greeks who found that rubbing fur on various substances, such as amber, would cause a particular attraction between the two. The Greeks noted that the amber buttons could attract light objects such as hair and that if they rubbed the amber for long enough they could even get a spark to jump. The origin of the word "electricity" is from the Greek word ēlektron, a word the ancient Greeks used for both "amber" and "electrum," and derives from an old root, ēlek- = "shine." The same word was used for both amber and electrum, probably because of the pale yellow color of some varieties of electrum (see electrum). An object found in Iraq in 1938, dated to about 250 BCE and called the Baghdad Battery, resembles a galvanic cell and is believed by some to have been used for electroplating. Additionally, some egyptologists associate the ancient goddess Hathor with artificial light (see Hathor temple). But, remaining unproven are the conjectures that these and other similar ancient artifacts had electrical function and that their associated ancient technology contributed to the development of modern electrical knowledge.

Modern

In 1600 the English scientist William Gilbert returned to the subject in De Magnete, and coined the modern Latin word electricus from ηλεκτρον (elektron), the Greek word for "amber", which soon gave rise to the English words electric and electricity. He was followed in 1660 by Otto von Guericke, who is regarded as having invented an early electrostatic generator. Other European pioneers were Robert Boyle, who in 1675 stated that electric attraction and repulsion can act across a vacuum; Stephen Gray, who in 1729 classified materials as conductors and insulators; and C. F. Du Fay, who first identified the two types of electricity that would later be called positive and negative. The Leyden jar, a type of capacitor for electrical energy in large quantities, was invented at Leiden University by Pieter van Musschenbroek in 1745. William Watson, experimenting with the Leyden jar, discovered in 1747 that a discharge of static electricity was equivalent to an electric current. In June, 1752, Benjamin Franklin promoted his investigations of electricity and theories through the famous, though extremely dangerous, experiment of flying a kite during a thunderstorm. Following these experiments he invented a lightning rod and established the link between lightning and electricity. If Franklin did fly a kite in a storm, he did not do it the way it is often described (as it would have been dramatic but fatal). It was either Franklin (more frequently) or Ebenezer Kinnersley of Philadelphia (less frequently) who created the convention of positive and negative electricity. Franklin's observations aided later scientists such as Michael Faraday, Luigi Galvani, Alessandro Volta, André-Marie Ampère, and Georg Simon Ohm whose work provided the basis for modern electrical technology. The work of Faraday, Volta, Ampere, and Ohm is honored by society, in that fundamental units of electrical measurement are named after them. Volta worked with chemicals and discovered that chemical reactions could be used to create positively charged anodes and negatively charged cathodes. When a conductor was attached between these, the difference in the electrical potential (also known as voltage) drives a current between them through the conductor. The potential difference between two points is measured in units of volts in recognition of Volta's work. The invention of the electric telegraph showed that commercial and practical use could be made of electrical phenomena. By the end of the 19th century electrical engineering became a distinct profession, separate from the physicist or inventor. The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, inventor of the polyphase induction motor; Samuel Morse, inventor of the telegraph; Antonio Meucci, an inventor of the telephone; Thomas Edison inventor of the phonograph and a practical incandescent light bulb; George Westinghouse, inventor of the electric locomotive; Charles Steinmetz, theoretician of alternating current; Alexander Graham Bell, another inventor of the telephone and founder of a sucessful telephone business. The rapid advance of electrical technology in the latter 19th and early 20th centuries lead to commercial rivalry such as the so-called War of the Currents), between Edison's direct-current system or Westinghouse's alternating-current method. Often concurrent research in widely scattered locations lead to multiple claims to the invention of a device or system.

Electric charge

Electric charge is a property of certain subatomic particles (e.g., electrons and protons) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity" is used interchangeably with the phrases "charge of electricity" and "quantity of charge." There are two types of charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law.

Electric field

The concept of electric field was introduced by Michael Faraday. The electrical field force acts between two charges, in the same way that the gravitational field force acts between two masses. However, electric field is a little bit different. Gravitational force depends on mass, whereas electric force depends on the electric charge on both objects. A positive charge exerts away from the object and a negative charge pulls towards the object equally in all directions; thus it is symetric. The most common experience with electric charge in everyday life is that of static cling - when two particular types of materials are rubbed together, they tend to stick together, at least for a while.

Electric potential

The electric potential difference between two points is defined as the work done per unit charge (against electrical forces) in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradients indicates the direction of heat flows. Similarly, there is an electric potential at every point in space, and its gradient in the the electric field indicates where charges move.

Electric current

The electric charge which occurs naturally within conductors can be forced to flow, while the charges within insulators are locked in place and cannot be moved. Devices that use charge flow principles in materials are called electronic devices. A flow of electric charge is called an electric current. A direct current (DC) is a unidirectional flow; alternating current (AC) is a flow whose time average is zero, but whose energy capability (RMS level) is not zero. With AC the electric current repeatedly changes direction. Electric current is measured in Amperes Ohm's Law is an important relationship describing the behaviour of electric currents: See also: electrical conduction For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.

Electrical energy

Electrical energy, is the flow of electrons or ions. When electrons are flowing through a wire or through hundreds of feet of air in the case of lightning it is because they are being forced to do so by an electrical field. A force is exerted on the electrons and they move. Work is done on the charged particles. A force is pushing them through a distance. More properly, they are moving from outer orbitals of one atom to another, being pushed by the electromotive force. While the electrons are in motion they contain kinetic energy. Consquently, atomic level electricity is a form of kinetic energy.

Electric power

Electric power is the capacity of the circuit for performing work in a particular amount of time. When a charge moves in a conductor, work is done by that charge. Devices can be made which convert this work into heat (Electric arc furnaces), light (light bulbs and Fluorescent lamps), or motion, i.e. kinetic energy (electric motors). The unit for all forms of power is the watt (symbol: W). In practice, however, this is generally reserved for the real power component. Apparent power is conventionally expressed in volt-amperes (VA) since it is the simple multiple of rms voltage and current. The unit for reactive power is given the special name "VAR", which stands for volt-amperes-reactive.

SI electricity units

External links

- [http://amasci.com/miscon/whatis.html What is electricity?]
- [http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=electricity Merriam-Webster: Electricity]
- [http://www.bibliomania.com/2/9/72/119/21387/1.html Tyndall: Faraday as Discovery: Identity of Electricities]
- [http://www.eia.doe.gov/fuelelectric.html US Energy Department Statistics]
- [http://www.mouthshut.com/readreview/38842-1.html How to save on your electricity bills]
- [http://users.pandora.be/worldstandards/electricity.htm Electricity around the world]
- [http://www.tufts.edu/as/wright_center/fellows/bob_morse_04/ A Comprehensive Collection of Franklin’s Electrical Works: The Electrical Writings of Benjamin Franklin], Created and Collected by Robert A. Morse (2004)
- [http://www.telesensoryview.com/steverosecom/Articles/UnderstandingBasicElectri.html Understanding Electricity and some Electronics in 10 minutes](Steve Rose, Maui)
- [http://amasci.com/miscon/eleca.html Electricity Misconceptions]
- ko:전기 ja:電気 simple:Electricity

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