Friday, May 2, 2008
From Wikipedia, the free encyclopedia
Solar energy is energy from the Sun in the form of radiated heat and light. It drives the climate and weather and supports life on Earth. Solar energy technologies make controlled use of this energy resource.
Solar power is a synonym of solar energy or refers specifically to the conversion of sunlight into electricity by photovoltaics, concentrating solar thermal devices or various experimental technologies.
In building design, daylighting techniques use sunlight to provide interior lighting and to warm the air instead of using electric lighting and powered heating. Solar water heaters heat swimming pools and provide domestic hot water. In agriculture, greenhouses grow specialty crops and photovoltaic-powered pumps bring water to grazing animals. Evaporation ponds find applications in the commercial and industrial sectors where they are used to harvest salt and clean waste streams of contaminants.
Solar distillation and disinfection techniques produce potable water for millions of people worldwide. Family-scale solar cookers and larger solar kitchens concentrate sunlight for cooking, drying and pasteurization. More sophisticated concentrating technologies magnify the rays of the Sun for high temperature material testing, metal smelting and industrial chemical production. A range of prototype solar vehicles provide ground, air and sea transportation.
Applications of solar energy technology
Solar energy technologies use solar radiation for practical ends. Technologies that use secondary solar resources such as biomass, wind, waves and ocean thermal gradients can be included in a broader description of solar energy but only primary resource applications are discussed here. Because the performance of solar technologies varies widely between regions, solar technologies should be deployed in a way that carefully considers these variations.
Solar technologies such as photovoltaics and water heaters increase the supply of energy and may be characterized as supply side technologies. Technologies such as passive design and shading devices reduce the need for alternate resources and may be characterized as demand side. Optimizing the performance of solar technologies is often a matter of controlling the resource rather than simply maximizing its collection.
[edit] Architecture and urban planning
Sunlight has influenced building design since the beginning of architectural history.[15] Fully developed solar architecture and urban planning methods were first employed by the Greeks and Chinese who oriented their buildings toward the south to provide light and warmth.[16]
The elemental features of passive solar architecture are Sun orientation, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and thermal mass.[15] When these features are tailored to the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature range. Socrates' Megaron House is a classic example of passive solar design.[15] The most recent approaches to solar design use computer modeling to tie together solar lighting, heating and ventilation systems in an integrated solar design package. Active solar equipment such as pumps, fans and switchable windows can also complement passive design and improve system performance.
Urban heat islands (UHI) are metropolitan areas with higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as asphalt and concrete that 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. Using these methods, a hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 °C at an estimated cost of US$1 billion, giving estimated total annual benefits of US$530 million from reduced air-conditioning costs and healthcare savings.[17]
[edit] Agriculture and horticulture
Agriculture inherently seeks to optimize the capture of solar energy, and thereby plant productivity. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields.[18][19] While sunlight is generally considered a plentiful resource, there are exceptions which highlight the importance of solar energy to agriculture. During the short growing seasons of the Little Ice Age, French and English farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground with a south facing orientation but over time sloping walls were developed to make better use of sunlight. In 1699, Nicolas Fatio de Duillier even suggested using a tracking mechanism which could pivot to follow the Sun.[20] Solar energy is also used in many areas of agriculture aside from growing crops. Applications include pumping water, drying crops, brooding chicks and drying chicken manure.[21][22]
Greenhouses control the use of solar heat and light to grow plants in controlled environments, enabling year-round production and the growth of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to grow cucumbers year-round for the Roman emperor Tiberius.[23] The first modern greenhouses were built in Europe in the 16th century to conserve exotic plants brought back from explorations abroad.[24] Greenhouses remain an important part of horticulture today, while plastic transparent materials have also been used to similar effect in polytunnels and row covers.
[edit] Solar lighting
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 judgments with the Prescription Act of 1832.[25][26] In the 20th century artificial lighting became the main source of interior illumination.
Daylighting systems collect and distribute sunlight to provide interior illumination. These systems directly offset energy use by replacing artificial lighting, and indirectly offset non-solar energy use by reducing the need for air-conditioning.[27] Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting.[27] Daylighting design carefully selects window type, size and orientation and may also consider exterior shading devices. Individual features include sawtooth roofs, clerestory windows, light shelves, skylights and light tubes.[28] These features may be incorporated into existing structures but are most effective when integrated in a solar design package that accounts for factors such as glare, heat flux and time-of-use. When daylighting features are properly implemented they can reduce commercial lighting-related energy requirements by 25%.[29]
Hybrid solar lighting (HSL) is an active solar method of using sunlight to provide illumination. HSL systems collect sunlight using focusing mirrors that track the Sun and use optical fibers to transmit the light into a building's interior to supplement conventional lighting. In single-story applications, these systems are able to transmit 50% of the direct sunlight received.[30]
Although daylight saving time is promoted as a way to use sunlight to save energy, recent research is limited and reports contradictory results: several studies report savings, but just as many suggest no effect or even a net loss, particularly when gasoline consumption is taken into account. Electricity use is greatly affected by geography, climate and economics, making it hard to generalize from single studies.[31]
[edit] Solar thermal
Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.[32]
[edit] Water heating
Solar hot water systems use sunlight to heat water. When sited in low latitudes (below 40 degrees), solar heating system can provide around 60 to 70% of domestic hot water use with temperatures up to 60 °C.[33] The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools.[34]
As of 2007, the total installed capacity of solar hot water systems is approximately 154 GW.[35] China is the world leader in the deployment of solar hot water with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020.[36] Israel is the per capita leader in the use of solar hot water with 90% of homes using this technology.[37] In the United States, Canada and Australia, heating swimming pools is the dominant application of solar hot water, with an installed capacity of 18 GW as of 2005.[38]
[edit] Heating, cooling and ventilation
In the United States, heating, ventilation and air conditioning (HVAC) systems account for over 25% (4.75 EJ) of the energy used in commercial buildings and nearly 50% (10.1 EJ) of the energy used in residential buildings.[39][29] Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy.
Thermal mass, in the most general sense, is any material that has the capacity to store heat. In the context of solar energy, thermal mass materials are used to store heat from the Sun. Common thermal mass materials include stone, cement and water. These materials have historically been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night, but they can also be used in cold temperate areas to maintain warmth. The size and placement of thermal mass should consider several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures without consuming energy.
A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. Performance can be improved by using glazing and thermal mass materials in a way that mimics greenhouses. These systems have been in use since Roman times and remain common in the Middle East.
Deciduous trees and plants can be used to provide heating and cooling. When planted on the southern elevation of the building, the leaves can provide shade during the summer while the bare limbs allow light and warmth to pass during the winter.
[edit] Process heat
Concentrating solar technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial process heating project was the Solar Total Energy Project (STEP) in Shenandoah, Georgia where a field of 120 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory.[40] This system generated 400 kW of electricity and 3 MW of thermal energy in the form of steam, and had a thermal storage system which allowed for peak-load shaving. A prototype Scheffler reflector is currently being constructed in India for use in a solar crematorium.[41]
Evaporation ponds are shallow pools that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams.
Clothes lines, clotheshorses, and clothes racks dry clothes through evaporation. These devices use wind and sunlight instead of electricity or natural gas. Florida legislation specifically protects the 'right to dry' and similar solar rights legislation has been passed in Utah and Hawaii.[42]
Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22 °C and deliver outlet temperatures of 45-60 °C.[43] The short payback period of transpired collectors (3 to 12 years) makes them a more cost-effective alternative than glazed collection systems.[43] As of 2003, over 80 systems with a combined collector area of 35,000 m² had been installed worldwide,including an 860 m² collector in Costa Rica used for drying coffee beans and a 1,300 m² collector in Coimbatore, India used for drying marigolds.[22]
[edit] Cooking
Solar cookers use sunlight for cooking, drying and pasteurization. Solar cooking offsets fuel costs, reduces demand for fuel or firewood, and improves air quality by reducing the generation of smoke. The simplest type of solar cooker is the box cooker first built by Horace de Saussure in 1767. A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 50-100 °C.[44][45] Concentrating solar cookers use reflectors to focus light on a cooking container. The most common reflector geometries are flat plate, disc and parabolic trough type. These designs reach temperatures up to 315 °C but require direct light to function properly and must be repositioned to track the Sun.[45]
The solar bowl is a unique concentrating technology employed by the Solar Kitchen in Auroville, India. The solar bowl is a stationary spherical reflector that focuses light along a line perpendicular to the sphere's surface and a computer control system moves the receiver to intersect this line. Steam is produced in the receiver at temperatures reaching 150 °C and then used for process heat in the kitchen.[46]
A reflector developed by Wolfgang Scheffler in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. Polar tracking is used to follow the Sun's daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450-650 °C and have a fixed focal point which improves the ease of cooking.[47] The world's largest Scheffler reflector system in Abu Road, Rajasthan, India is capable of cooking up to 35,000 meals a day.[48] By early 2008, over 2,000 large Scheffler cookers had been built worldwide.[49]
[edit] Desalination and disinfection
Solar distillation is the production of potable water from saline or brackish water using solar energy. The first recorded use was by 16th century Arab alchemists.[50] The first large-scale solar distillation project was constructed in 1872 in the Chilean mining town of Las Salinas.[51] This 4,700 m² still could produce up to 22,700 L per day and operated for 40 years.[51] Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick and multiple effect.[50] These stills can operate in passive, active or hybrid modes. Double slope stills are the most economical for decentralized domestic purposes while active multiple effect units are more suitable to large-scale applications.[50]
Solar water disinfection (SODIS) is a method of disinfecting water by exposing water-filled plastic PET bottles to several hours of sunlight.[52] Exposure times vary according weather and climate from a minimum of six hours to two days during fully overcast conditions.[53] SODIS is recommended by the World Health Organization as a viable method for household water treatment and safe storage.[54] Over two million people in developing countries use SODIS for their daily drinking water needs.[53]
[edit] Solar electricity
Solar power technologies convert sunlight into electricity using photovoltaics, concentrating solar thermal devices, or various experimental technologies. PV has mainly been used to generate power for small and medium-sized applications, from the calculator powered by a single solar cell to off-grid homes powered by a photovoltaic array. For large-scale generation, concentrating solar thermal power plants like SEGS have been the norm but multi-megawatt PV plants are becoming more common. Completed in 2007, the 14 MW power station in Clark County, Nevada and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger photovoltaic power stations in the US and Europe.
[edit] Photovoltaics
A solar cell (or photovoltaic cell) is a device that converts light into direct current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in 1883.[55] Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery.[56] 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.[57] These early solar cells cost 286 USD/watt and reached efficiencies of 4.5-6%.[58]
The earliest significant application of solar cells was as a back-up power source to the Vanguard I satellite, which allowed the satellite to continue transmitting for over a year after its chemical battery was exhausted.[59] The successful operation of solar cells on this mission was duplicated in many other Soviet and American satellites, and by the late 1960s PV had become the established source of power for satellites.[60] Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar and continue to remain vital to the telecommunications infrastructure today.[61]
The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without grid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, navigational buoys and railroad crossings.[62] These and other off-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004.[36]
The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s.[63] Economies of scale which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 USD/watt in 1971 to 7 USD/watt in 1985.[64] Steadily falling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax credits associated with the Energy Tax Act of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996.[65]
Since the mid-1990s, leadership in the PV sector has shifted from the US to Japan and Germany. Between 1992 and 1994 Japan increased R&D funding, established net metering guidelines, and introduced a subsidy program to encourage the installation of residential PV systems.[66] As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999,[67] and worldwide production growth increased to 30% in the late 1990s.[68]
Germany has become the leading PV market worldwide since revising its Feed-in tariff system as part of the Renewable Energy Sources Act. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007.[69][70] Spain has become the third largest PV market after adopting a similar feed-in tariff structure in 2004, while France, Italy, South Korea and the US have also seen rapid growth recently due to various incentive programs and local market conditions.[71]
By 2006 more polysilicon was used in solar panels than in computer chips,[72] and the limited supply caused prices to flatten.[73][74] In 2008 new factories were built to meet the demand,[75] and thin film technologies were developed which use a printing process to make solar panels, and do not use polysilicon.[76] The global solar market is expected to grow from $13 billion to $32 billion by 2012, with thin film expanding 45% a year. [77]
[edit] Concentrating solar power
Concentrated sunlight has been used to perform useful tasks since the time of ancient China. A legend claims Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse. In 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine, and subsequent developments led to the use of concentrating solar-powered devices for irrigation, refrigeration and locomotion.[78]
Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exist; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.[79]
A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. Trough systems are the most developed CSP technology. The SEGS plants in California and Acciona's Nevada Solar One near Boulder City, Nevada are representatives of this technology.
A parabolic dish system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. Parabolic dish systems give the highest efficiency among CSP technologies. The Big Dish in Canberra, Australia is an example of this technology.
A solar power tower uses an array of tracking reflectors (heliostats) to concentrate light on a central receiver atop a tower. Power towers are less advanced than trough systems but offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 in Sanlucar la Mayor, Spain are representatives of this technology.
[edit] Experimental solar power
A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse that 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.[80]
A solar pond is a pool of salt water (usually 1-2 m deep) that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after he came across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's water, which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem.[81] 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 two percent.
Thermoelectric devices convert a temperature difference between dissimilar materials into an electric current. First proposed as a method to store solar energy by solar pioneer Mouchout in the 1800s,[82] thermoelectrics reemerged in the Soviet Union during the 1930s. Under the direction of Soviet scientist Abram Ioffe a concentrating system was used to thermoelectrically generate power for a 1 hp engine.[83] Thermogenerators were later used in the US space program as an energy conversion technology for powering deep space missions such as Cassini, Galileo and Viking. Research in this area is focused on raising the efficiency of these devices from 7–8% to 15–20%.[84]
[edit] Solar chemical
Solar chemical processes use solar energy to drive chemical changes. These processes offset energy that would otherwise be required from an alternate source and can convert solar energy into a storable and transportable fuel. Solar chemical reactions are diverse but can generically be described as either thermochemical or photochemical.
Hydrogen production technologies have been a significant area of solar chemical research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. The seemingly most direct of these routes uses concentrators to split water at high temperatures (2300-2600 °C), but this process has been limited by complexity and low solar-to-hydrogen efficiency (1-2%).[85] A more conventional approach uses process heat from solar concentrators to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield. Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue of hydrogen production. The Solzinc process under development at the Weitzman Institute is one such method. This process uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above 1200 °C. This initial reaction produces pure zinc which can subsequently be reacted with water to produce hydrogen.[86]
Sandia's Sunshine to Petrol (S2P) technology uses the high temperatures generated by concentrating sunlight along with a zirconia/ferrite catalyst to break down atmospheric carbon dioxide into oxygen and carbon monoxide (CO). The CO may then be used to synthesize fuels such as methanol, gasoline and jet fuel.[87][88]
Photoelectrochemical cells or PECs consist of a semiconductor, typically titanium dioxide or related titanates, immersed in an electrolyte. When the semiconductor is illuminated an electrical potential develops. There are two types of photoelectrochemical cells: photoelectric cells that convert light into electricity and photochemical cells that use light to drive chemical reactions such as electrolysis.[89]
A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy-rich chemical intermediates when illuminated. These chemical intermediates then react at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology.[90]
[edit] Solar mechanical
Passive solar tracking devices use imbalances caused by the movement of a low-boiling-point fluid to respond to the movement of the Sun. Tracking PV systems can generally produce 25% more electricity than fixed tilt PV systems.[91] Shading systems that respond to the movement of the Sun can also be used in buildings to maximize natural lighting during winter, lessen summer glare, and reduce cooling loads associated with unwanted solar gain.[92]
A light mill or Crookes radiometer consists of a glass bulb containing a set of vanes mounted on a spindle. Each vane has a dark side and a white side. When illuminated, the dark side becomes warm due to absorbing light but the white side reflects light and stays cool. The vanes rotate due to the motion of gases from the hot to the cool side of each vane.
[edit] Solar vehicles
Development of a solar powered car has been an engineering goal since the 1980s. The center of this development is the World Solar Challenge, a biannual solar-powered car race in which teams from universities and enterprises compete over 3,021 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph).[93] The 2007 race included a new challenge class using cars which could be a practical proposition for sustainable transport with little modification. The winning car averaged 90.87 kilometres per hour (56.46 mph). The North American Solar Challenge and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.
In 1975, the first practical solar boat was constructed in England.[94] By 1995, passenger boats incorporating PV panels began appearing and are now used extensively.[95] In 1996, Kenichi Horie made the first solar powered crossing of the Pacific Ocean, and the sun21 catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006–2007.[96] Plans to circumnavigate the globe in 2009 are indicative of the progress solar boats have made.
In 1974, the unmanned Sunrise II inaugurated the era of solar flight. In 1980, the Gossamer Penguin made the first piloted flights powered solely by photovoltaics. This was quickly followed by the Solar Challenger which demonstrated a more airworthy design with its crossing of the English Channel in July 1981. Developments then turned back to unmanned aerial vehicles with the Pathfinder (1997) and subsequent designs,[97] culminating in the Helios which set the altitude record for a non-rocket-propelled aircraft at 29,524 metres (96,860 ft) in 2001. The Zephyr, developed by BAE Systems, is the latest in a line of record-breaking solar aircraft, making a 54-hour flight in 2007, and month-long flights are envisioned by 2010.[98]
A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward buoyancy force, much like an artificially-heated hot air balloon. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is relatively high.
Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors to exploit radiation pressure from the sun. Unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the Sun shines onto the deployed sail and in the frictionless vacuum of space significant speeds can eventually be achieved.[99]
anyway, in short, it can be used for alot of stuff, so i think we'll mainly be concentrating on stuff for houses, cos we're trying to create a green house right?
so the main ones will probably be solar electricity, cooking, water heating/cooling, solar lighting!
and please research on solar energy for the above uses!
-chunhui+D