solar energy

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Solar power from a parabolic reflector.
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The amount of solar energy available to the Earth in one minute exceeds global energy demand for a year.
Renewable energy
Solar energy is energy from the sun. It supports life on Earth and drives the Earth's weather. Solar energy predominantly arrives in the form of infrared, visible and ultraviolet light, and is either returned back to space or is absorbed. Nearly all of the absorbed energy is converted directly to heat, with a small but important fraction converted to chemical energy, such as in ozone production, photosynthesis or photovoltaic energy production.

Solar energy also broadly describes technologies that utilize sunlight. These technologies are diverse and date back millennia. The Greeks, Native Americans and Chinese warmed their buildings by orienting them toward the sun. In Europe, farmers used elaborate field orientation and thermal mass to increase crop yields during the Little Ice Age. Modern solar technologies continue to harness the sun to provide water heating, daylighting and even flight.[1][2]

Solar power generally describes technologies that convert sunlight into electricity and in some cases thermal or mechanical power. In 1866, the French engineer Auguste Mouchout successfully powered a steam engine with sunlight. This is the first known example of a solar powered mechanical device. Over the next 50 years inventors such as John Ericsson, Charles Tellier and Frank Shuman developed solar powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are concentrating solar power plants.[2]

The modern age of solar power arrived in 1954 when researchers at Bell Laboratories developed a photovoltaic cell capable of effectively converting light into electricity. This breakthrough marked a fundamental change in how power is generated. Since then solar cells efficiencies have improved from 6% to 15% with experimental cells reaching efficiencies over 40%. Prices on the other hand have fallen from $300 per watt to less than $3 per watt.[3]

The utilization of solar energy and solar power spans from traditional technologies that provide food, heat and light to electricity which is uniquely modern. Solar energy is used in a wide variety of applications, including:

Energy from the Sun

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Solar power as it is dispersed on the planet and radiated back to space. Values are in PW =1015 W

The Earth receives 174 petawatts (PW) of solar radiation at the upper atmosphere. While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. The absorption of solar energy by atmospheric convection (sensible heat transport) and by the evaporation and condensation of water vapor (latent heat transport) drive the winds and the water cycle.[4]

Atmospheric conditions not only reduce the quantity of light reaching the Earth's surface but also affect the qualities of light by altering its spectrum and diffusing approximately 20% of the incoming light.[5] After passing through the Earth's atmosphere approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared spectrum, and a small part of ultraviolet radiation.[6] Upon reaching the surface, sunlight is absorbed by the oceans, earth and plants. The energy captured in the oceans drives the thermohaline cycle. As such, solar energy is ultimately responsible for temperature driven ocean currents such as the thermohaline cycle and wind driven currents such as the Gulf Stream. The energy absorbed by the earth in conjunction with that recycled by the Greenhouse effect warms the surface to an average temperature of approximately 14°C.[7] The solar energy captured by plants and other phototrophs is converted to chemical energy via photosynthesis. All the food we eat, wood we build with, and fossil fuels we use are products of photosynthesis.[8]

The flows and stores of solar energy are vast in comparison to human energy needs.
  • The total solar energy available to the earth is approximately 3850 zettajoules (ZJ) per year.[9]
  • Oceans absorb approximately 285 ZJ of solar energy per year.
  • Winds can theoretically supply 6 ZJ of energy per year.[10]
  • Biomass captures approximately 1.8 ZJ of solar energy per year.[11][9]
  • Worldwide energy consumption was 0.471 ZJ in 2004.[13]
The map on the right (top) shows how solar radiation at the top of the earth's atmosphere varies with latitude. The bottom map shows annual average ground level insolation. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).[14] At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.[15]

Types of technologies

Many technologies use solar energy. Some classifications of solar technology are active, passive, direct and indirect.
  • Active solar systems use electrical and mechanical components such as tracking mechanisms, pumps and fans to process sunlight into usable outputs such as heating, lighting or electricity.
  • Passive solar systems use non-mechanical techniques of controlling, converting and distributing sunlight into usable outputs such as heating, lighting, cooling or ventilation. These techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the sun.
  • Direct solar generally refers to technologies or effects that involve a single conversion of sunlight which results in a usable form of energy.
  • Indirect solar generally refers to technologies or effects that involves multiple transformations of sunlight which result in a usable form of energy.

Architecture and Urban planning

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The Zion National Park Visitor's Center incorporates several aspects of solar design.
Solar architecture controls the use of the sun to provide comfortable temperatures, lighting and air quality. The basic elements of solar architecture are building orientation, proportion, thermal mass and window placement. The solar architecture and design process tailors these elements to the local climate and environment.

The oldest principle of solar architecture is building orientation. The entire building can be positioned and angled to be oriented towards or away from the sun, overshadowing from other structures or natural features can be avoided or used, and the building can be set into the ground using earth sheltering techniques.
  • As a general rule, a solar building's axis should run lengthwise east to west and the structure should be twice as long as wide.
  • Windows facing the equator should be equal to 5-7% of the building's floor space.[16] If heating is a concern, window area facing away from the equator should be minimized.
  • The thermal mass in the building should be sized to smooth out temperature swings.
  • Spaces can be designed to naturally circulate air. Cooling elements such as a solar chimney can be incorporated to help with ventillation.
  • Lighting quality and energy use are strongly influenced by window design. In cold climates insulated glazing with low-emissivity coatings can maximize solar gain and reduce heat losses by 30-50%. In hot climates low-emissivity coatings on the outside of window panes can be used to reduce and control solar gain.[17]
  • The albedo of an object indicates the percentage of light it reflects. Asphalt has an albedo of around 10% while the average albedo of the Earth is 30%.[18] Urban heat islands (UHI) are metropolitan areas with significantly higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as concrete and asphalt which have lower albedos and higher heat capacities than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. A hypothetical "cool communities" program in Los Angeles, California called for the planting of ten million trees, the reroofing of almost 5 million homes and painting one-quarter of the roads. These measures are estimated to reduce urban temperatures by approximately 3°C. The projected costs of such a program are approximately $1 billion. The annual savings from reduced air-conditioning costs are estimated at $170 million with an additional yearly health benefit of $360 million in smog-reduction savings.[19][20]


Main articles: Daylighting and Light tube
The history of lighting is dominated by the use of natural light. The Romans recognized the Right to Light as early as the 6th century and English law echoed these judgements with the Prescription Act of 1832. It wasn't until the 1900s that artificial lighting took over as the main source of interior illumination. The 1973 oil and 1979 energy crises brought attention to conservation measures such as natural lighting but interest waned on both occasions with the restoration of energy supplies. Approximately 20% of the electricity used in the United States is for lighting. When daylighting techniques are appropriately applied natural light can supply interior lighting for many hours of the day.[5]

Daylighting is a passive solar method of using sunlight to provide illumination. Daylighting directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling load.[22] Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting. Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, skylights and light tubes.[23] These features may be incorporated in existing structures but are most effective when integrated in a solar design package which accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.

Hybrid solar lighting (HSL) is an active solar method of using sunlight to provide illumination. Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun. The collected light is transmitted via optical fibers into a building's interior to supplement conventional lighting. In single story applications, these systems are able to transmit 50% of received direct sunlight.[5]

Daylight saving time (DST) utilizes solar energy by matching available sunlight to the time of the day in which it is most useful.

Water heating

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Solar water heaters, on a rooftop in Jerusalem, Israel
Solar hot water systems use sunlight to heat water. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were thereafter gradually replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they lost in the 1920s. That being said, the recent price spikes and erratic availability of conventional fuels is renewing interest in solar heating technologies.[2][25]

As of 2005, the total installed capacity of solar hot water systems is 88 GWth and growth is 14% per year.[26] China is the world leader in the deployment of solar hot water systems with 80% of the market.[27] Israel is the per capita leader in the use of solar hot water with 90% of homes using this technology.[26] In the United States heating swimming pools is the most successful application of solar hot water.[25]

Solar water heating is highly efficient (up to 86%) and is particularly appropriate for low temperature (25-65 °C) applications such as domestic hot water, heating swimming pools and space heating. The oldest and simplest type of solar water heater is a black water tank which is exposed to the sun. These are called batch systems but there are many other configurations. Some configurations are designed to heat water to high temperatures while other are designed for economy. [25]

A solar pond is a pool of salt water that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after coming across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's waters which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem.[29] The pond consisted of layers of water that successively increased from a weak salt solution at the top to a high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar to electric efficiency of 2%. Current, representatives of this technology include a 150 KW pond in En Boqeq, Israel, and another used for industrial process heat at the University of Texas El Paso.[30]

Heating, cooling and ventilation

  • A thermal mass is a body that absorbs and holds heat. In the context of solar energy, it is a mass designed to store heat during sunny periods and release heat when sunlight levels are reduced or unavailable. A properly sized thermal mass will smooth out temperature swings and help keep rooms at a comfortable temperature throughout the day and night.
  • A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle sunlight stores heat in the thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored heat.[31]
  • A transpired collector is a perforated sun-facing wall. The wall absorbs sunlight and pre-heats air as much as 22°C as it is drawn into the ventilation system. These systems are highly efficient (up to 80%) and can pay for themselves within 3-12 years in offset heating costs.[32]
  • Solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles and solar mechanical processes. In 1878 Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device.[2]
  • A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a hollow thermal mass connecting the inside and outside of a building. As the chimney warms the air inside is heated causing an updraft which enhances the natural stack ventilation through a building. These systems have been in use since Roman times and are common in the Middle east.


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Photovoltaic (PV) modules are composed of multiple PV cells. Two or more interconnected PV modules create an array.
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Map of solar electricity potential in Europe
Main article: Photovoltaics

A solar cell or photovoltaic cell is a device that converts light into electricity using the photovoltaic effect. The first working solar cells were constructed by Charles Fritts in 1883. These prototype cells were made of selenium and achieved efficiencies of around 1%. Following the fundamental work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954.[33]

Until recently, their use has been limited because of high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and spacecraft.

To take advantage of the incoming electromagnetic radiation from the sun, solar panels can be attached to each house or building. The panels should be mounted perpendicular to the arc of the sun to maximize usefulness. The easiest way to use this electricity is by connecting the solar panels to a grid tie inverter. However, these solar panels may also be used to charge batteries or other energy storage device. Solar panels produce more power during summer months because they receive more sunlight.

Total peak power of installed PV is around 6,000 MW as of the end of 2006. Installed PV is projected to increase to over 9,000 MW in 2007.[34][35]

Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4 per watt between 1990 and 2005.[36] With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market.

In 2003, worldwide production of solar cells increased by 32%.[37] Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%.[38] 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.[39] Analysts have predicted similar supply problems for 2006 and 2007.[40]

Solar power plants

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Solar Two power tower surrounded by a field of heliostats.

Solar power plants use a variety of methods to collect sunlight and convert this energy into electricity, distill water or provide heat for industrial processes. Concentrating solar thermal power plants have traditionally been the most common type of solar power plant; however, multi-megawatt photovoltaic sites have seen recent rapid deployment.

Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. CST technologies require direct insolation to perform properly. This requirement makes them inappropriate for significantly overcast locations.[41]

The three basic CST technologies are the solar trough, solar power tower and parabolic dish. Each technology is capable of producing high temperatures and correspondingly high thermodynamic efficiencies but they vary in the way they track the sun and focus light.
  • Line focus/Single-axis
  • A solar trough consists of a linear parabolic reflector which concentrates light on a receiver positioned along the reflector's focal line. These systems use single-axis tracking to follow the sun. A working fluid (oil, water) flows through the receiver and is heated up to 400 °C before transferring its heat to a distillation or power generation system.[42].[43] Trough systems are the most developed CST technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.[42]
  • Point focus/Dual-axis
  • A power tower consists of an array of flat reflectors (heliostats) which concentrate light on a central receiver located on a tower. These systems use dual-axis tracking to follow the sun. A working fluid (air, water, molten salt) flows through the receiver where it is heated up to 1000 °C before transferring its heat to a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and energy storage capability.[44] The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.
  • A parabolic dish or dish/engine system consists of a stand-alone parabolic reflector which concentrates light on a receiver positioned at the reflector's focal point. These systems use dual-axis tracking to follow the sun. A working fluid (hydrogen, helium, air, water) flows through the receiver where it is heated up to 1500 °C before transferring its heat to a sterling engine for power generation.[43][44] Parabolic dish systems display the highest solar-to-electric efficiency among CST technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia are representatives of this technology.
A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse which funnels into a central tower. As sunlight shines on the greenhouse the air inside is heated and expands. The expanding air flows toward the central tower where a turbine converts the air flow into electricity. A 50 kW prototype was constructed in Ciudad Real, Spain and operated for eight years before decommissioning in 1989.[45]


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Solar Cookers use sunshine as a source of heat for cooking as an alternative to fire.
Main article: Solar cooker
Solar cookers (or solar ovens) use sunlight for cooking, drying and pasteurization. Solar cookers offset fuel costs and reduce demand for local firewood. Solar cookers also improve local air quality by removing a source of smoke. The most common designs are box cookers, concentrating cookers and panel cookers.
  • Solar box cookers consist of an insulated container with a transparent lid. Horace de Saussure developed this design in 1767 after observing: "It is a known fact, and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass." These cookers can be effectively used with partially overcast skies and can typically reach temperatures of 50-100°C. These are the cheapest and most widely used cooker design.[2][46]
  • Concentrating solar cookers use a parabolic reflector to concentrate light on a container positioned at the reflector's focal point. These designs cook faster and at higher temperatures (up to 315°C). As with other concentrating technologies these cookers require direct light and must be repositioned to track the sun.[46]
  • Solar Panel cookers (SPC) use flat reflectors to concentrate sunlight on a container within a transparent covering. Roger Bernard is credited with introducing panel cookers in 1994. This design uses partial concentration and will maintain effective operation with limited repositioning.

Solar chemical

Solar chemical processes convert solar energy into chemical energy. These processes use both light (photochemical) and heat (endothermic) to drive chemical, thermochemical or thermoelectric reactions. Solar chemical reactions can be used to store solar energy or replace energy that would otherwise be required from an alternate source.

Electrochemical cells, commonly known as batteries, convert electrical energy into chemical energy. Solar energy can indirectly be converted into chemical energy in a system involving a photovoltaic to electrochemical cell exchange. A more direct approach involves the use of photoelectrochemical cells which use light to produce hydrogen in a process similar to the electrolysis of water. A third approach involves the use of thermoelectic devices which convert a temperature difference between dissimilar metals into an electric current between those metals. This current can be use to produce hydrogen and oxygen through the electrolysis of water. The solar pioneer Mochout envisioned using the thermoelectric effect to store solar energy for later use during darkness; however, his experiments toward this end never progressed beyond primitive devices.[2]

Concentrating solar thermal technologies can be used to drive high temperature chemical processes.
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Renewable energy utilizes natural resources such as sunlight, wind, tides and geothermal heat, which are naturally replenished. Renewable energy technologies range from solar power, wind power, and hydroelectricity to biomass and biofuels for transportation.
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Renewable energy
  • Biofuels
  • Biomass
  • Geothermal power
  • Hydro power
  • Solar power
  • Tidal power
  • Wave power
  • Wind power
Biofuel (also called agrofuel[1]
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Biomass refers to living and recently dead biological material which can be used as fuel or for industrial production.
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Geothermal power is energy generated by heat stored beneath the Earth's surface. As of 2007, geothermal power supplies less than 10% of the world's energy.[1] Geothermal comes from the Greek words geo, meaning earth, and therme, meaning heat.
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Hydroelectricity is electricity produced by hydropower. Hydroelectricity now supplies about 715,000 MWe or 19% of world electricity (16% in 2003), accounting for over 63% of the total electricity from renewables in 2005.
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Solar energy is energy from the sun. It supports life on Earth and drives the Earth's weather. Solar energy predominantly arrives in the form of infrared, visible and ultraviolet light, and is either returned back to space or is absorbed.
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    Tidal energy, sometimes called tidal power, is a form of hydropower that exploits the movement of water caused by tidal currents or the rise and fall in sea levels due to the tides.
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      Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work - including electricity generation, desalination, and the pumping of water (into reservoirs). Wave power is a form of renewable energy.
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      Wind power is the conversion of wind energy into more useful forms, such as electricity, using wind turbines. At the end of 2006, worldwide capacity of wind-powered generators was 73.
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      Ancient Pueblo People or Ancestral Puebloans were a prehistoric Native American culture centered around the present-day Four Corners area of the Southwest United States, noted for their distinctive pottery and dwelling construction styles.
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      Little Ice Age (LIA) was a period of cooling occurring after a warmer era known as the Medieval climate optimum. Climatologists and historians find it difficult to agree on either the start or end dates of this period.
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      Auguste Mouchout was a 19th century French inventor of the earliest solar-powered engine, converting solar energy into mechanical steam power.

      Auguste Mouchout, a mathematics instructor at the Lyce de Tours, he supported the idea of finding new alternative energy sources, as
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      John Ericsson (July 31, 1803 – March 8, 1889) was a Swedish inventor and mechanical engineer, as was his brother, Nils Ericson. He was born at Långbanshyttan in Värmland, Sweden, but primarily came to be active in the United States.
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      Photovoltaics, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert light from the sun directly into electricity.
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      Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry both hot water and water heated to steam have many uses.
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      HVAC (pronounced either "H-V-A-C" or, occasionally, "aitch-vak") is an initialism/acronym that stands for "heating, ventilation, and air conditioning".
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      Cooking is the act of preparing food for eating by the application of heat. It encompasses a vast range of methods, tools and combinations of ingredients to alter the flavor or digestibility of food.
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      Daylighting is the practice of placing windows, or other transparent media, and reflective surfaces so that, during the day, natural light provides effective internal illumination.
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      Electricity (from New Latin ēlectricus, "amberlike") is a general term for a variety of phenomena resulting from the presence and flow of electric charge. This includes many well-known physical phenomena such as lightning, electromagnetic fields and electric currents,
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      Photovoltaics, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert light from the sun directly into electricity.
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      A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle.
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      There are many applications of photovoltaics in transport either for motive power or as auxiliary power units, particularly where fuel, maintenance, emissions or noise requirements preclude internal combustion engines or fuel cells.
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      A solar car is an electric vehicle powered by solar energy obtained from solar panels on the surface of the car. Photovoltaic (PV) cells convert the sun's energy directly into electrical energy.
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      Desalination, desalinization, or desalinisation refer to any of several processes that remove excess salt and other minerals from water. Desalination may also refer to the removal of salts and minerals more generally,[1] as in soil desalination,
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      Biomass refers to living and recently dead biological material which can be used as fuel or for industrial production.
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      peta- (symbol: P) is a prefix in the SI (system of units) denoting 1015, or 1 000 000 000 000 000. For example:

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