Solar power

Solar power
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The PS10 concentrates sunlight from a field of heliostats on a central tower.

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Solar power is the generation of electricity from sunlight. This can be direct as with photovoltaics (PV), or indirect as with concentrating solar power (CSP), where the sun's energy is focused to boil water which is then used to provide power. Solar power provided 0.02% of the total world energy consumption in 2008. The largest solar power plants, like the 354 MW SEGS, are concentrating solar thermal plants, but recently[clarification needed] multi-megawatt photovoltaic plants have been built. Completed in 2008, the 46 MW Moura photovoltaic power station in Portugal and the 40 MW Waldpolenz Solar Park in Germany appear to be characteristic of the trend toward larger photovoltaic power stations. Larger ones are proposed, such as the 100 MW Fort Peck

Solar Farm[citation needed], the 550 MW Topaz Solar Farm, and the 600 MW Rancho Cielo Solar Farm. Terrestrial solar power is a predictably intermittent energy source, meaning that whilst solar power is not available at all times, we can predict with a very good degree of accuracy when it will and will not be available. Some technologies, such as solar thermal concentrators have an element of thermal storage, such as molten salts. These store spare solar energy in the form of heat which can be made available overnight or during periods that solar power is not available to produce electricity. Orbital solar power collection (as in solar power satellites) avoids this intermittent issue, but requires satellite launching and beaming of the collected power to receiving antennas on Earth. The increased intensity of sunlight above the atmosphere also increases generation efficiency.

Contents
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1 Applications 2 Concentrating solar power 3 Photovoltaics 4 Experimental solar power 5 Development o 5.1 1800-1900 o 5.2 1950-1970 o 5.3 1970-2000 o 5.4 2000-Present 6 Energy storage methods 7 Economics o 7.1 Energy payback time and energy returned on energy invested o 7.2 Power costs o 7.3 Grid parity o 7.4 Net metering o 7.5 Financial incentives o 7.6 Investment 8 Environmental impacts o 8.1 Location o 8.2 Greenhouse gases o 8.3 Cadmium 9 Solar power usage o 9.1 Germany o 9.2 India o 9.3 Iran o 9.4 Israel o 9.5 United States 10 See also

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11 Notes 12 References 13 External links

[edit] Applications
Solar power is the conversion of sunlight to electricity. Sunlight can be converted directly into electricity using photovoltaics (PV), or indirectly with concentrating solar power (CSP), which normally focuses the sun's energy to boil water which is then used to provide power, and technologies such as the Stirling engine dishes which use a Stirling cycle engine to power a generator. Photovoltaics were initially used to power small and medium-sized applications, from the calculator powered by a single solar cell to off-grid homes powered by a photovoltaic array. The three types of photovoltaic panels are Monocrystalline, Polycrystalline and Amorphous, each has its advantages and disadvantages. The fact with photovoltaics is that it is difficult to create a commercially viable system because of the overall cost of the system required to generate a useful amount of power.[1] Solar power plants can face high installation costs, although this has been decreasing due to the learning curve.[2][3] Developing countries have started to build solar power plants, replacing other sources of energy generation.[4][5][6] In 2008, Solar power supplied 0.02% of the world's total energy supply. Use has been doubling every two, or fewer, years. If it continued at that rate, solar power would become the dominant energy source within a few decades.[7] Since solar radiation is intermittent, solar power generation is combined either with storage or other energy sources to provide continuous power, although for small distributed producer/consumers, net metering makes this transparent to the consumer. On a larger scale, in Germany, a combined power plant has been demonstrated, using a mix of wind, biomass, hydro-, and solar power generation, resulting in 100% renewable energy.[8]

[edit] Concentrating solar power
Main article: Concentrating solar power

Solar troughs are the most widely deployed. A legend claims that Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse.[9] Augustin Mouchot used a parabolic trough to produce steam for the first solar steam engine in 1866.[10] Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the parabolic trough, the concentrating linear fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the Sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.[11] A parabolic trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The receiver is a tube positioned right above the middle of the parabolic mirror and is filled with a working fluid. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. Parabolic trough systems provide the best land-use factor of any solar technology.[12] The SEGS plants in California and Acciona's Nevada Solar One near Boulder City, Nevada are representatives of this technology.[13][14] The Suntrof-Mulk parabolic trough, developed by Melvin Prueitt, uses a technique inspired by Archimedes' principle to rotate the mirrors.[15] Concentrating Linear Fresnel Reflectors are CSP-plants which use many thin mirror strips instead of parabolic mirrors to concentrate sunlight onto two tubes with working fluid. This has the advantage that flat mirrors can be used which are much cheaper than parabolic mirrors, and that more reflectors can be placed in the same amount of space, allowing more of the available sunlight to be used. Concentrating linear fresnel reflectors can be used in either large or more compact plants.[16][17] A Stirling solar dish, or dish engine 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. Paraboloidal coordinates ("parabolic") dish systems give the highest efficiency among CSP technologies.[18] The 500 m2 ANU "Big Dish" in Canberra, Australia is an example of this technology.[19] The Stirling solar dish combines a parabolic concentrating dish with a Stirling heat engine which normally drives an electric generator. The advantages of Stirling solar over photovoltaic cells are

higher efficiency of converting sunlight into electricity and longer lifetime. A solar power tower uses an array of tracking reflectors (heliostats) to concentrate light on a central receiver atop a tower. Power towers are more cost effective, offer higher efficiency and better energy storage capability among CSP technologies.[13] The Solar Two in Barstow, California and the Planta Solar 10 in Sanlucar la Mayor, Spain are representatives of this technology.[13][20] A solar bowl is a spherical dish mirror that is fixed in place. The receiver follows the line focus created by the dish (as opposed to a point focus with tracking parabolic mirrors).

[edit] Photovoltaics
Main article: Photovoltaics

11 MW Serpa solar power plant in Portugal At the end of 2009, the cumulative global PV installations surpassed 21-gigawatts.[21] A solar cell, or photovoltaic cell (PV), is a device that converts light into electric current using the photoelectric effect.[22] This is based on the discovery by Alexandre-Edmond Becquerel who noticed that some materials release electrons when hit with rays of photons from light, which produces an electrical current.[23] The first solar cell was constructed by Charles Fritts in the 1880s.[24] 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.[25] Following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954.[26] These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%.[27] As of late 2009, the highest efficiency PV cells were produced commercially by Boeing/SpectroLab at about 41%. Other, similar, multilayer cells are close. These are very expensive however, and are used only for the most exacting applications. Thin film PV cells have been developed which are made in bulk and are far less expensive and much less fragile, but are at most around 20% efficient. The most recent development (from Caltech, March 2010) is the experimental demonstration of a new design which is 85% efficient in plain sunlight and 95% efficient at certain wavelengths. It has only been produced in experimental laboratory examples, but may have some possibility for low cost bulk production in the future.[28]

There are many competing technologies, including at least fourteen types of photovoltaic cells, such as thin film, monocrystalline silicon, polycrystalline silicon, and amorphous cells, as well as multiple types of concentrating solar power. It is too early to know which technology will become dominant.[29] The earliest significant application of solar cells was as a back-up power source to the Vanguard I satellite in 1958, which allowed it to continue transmitting for over a year after its chemical battery was exhausted.[30] 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 them.[31] After the successful application of solar panels on the Vanguard satellite it still was not until the energy crisis, in the 1970s, that photovoltaic solar panels gained use outside of back up power suppliers on spacecraft.[32] Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar, and they remain vital to the telecommunications infrastructure today.[33]

Building-integrated photovoltaics (BIPV), shown here on the roof of the "Friedenskirche" in Tübingen, Germany, cover the roofs of an increasing number of homes. 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, offshore oil rigs, navigational buoys and railroad crossings. [34] These off-grid applications accounted for over half of worldwide installed capacity until 2004.[35] The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s.[36] 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.[37] 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.[38] Since the mid-1990s, leadership in the PV sector has shifted from the US to Japan and Europe. 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.[39] As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999,[40] and worldwide production growth increased to 30% in the late 1990s.[41]

Concentrating photovoltaics in Catalonia, Spain. Germany became the leading PV market worldwide since revising its feed-in tariffs as part of the Renewable Energy Sources Act. Installed PV capacity in Germany has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007.[42][43] After 2007, Spain became the largest PV market after adopting a similar feed-in tariff structure in 2004, installing almost half of the photovoltaics (45%) in the world, in 2008, while France, Italy, South Korea and the U.S. have seen rapid growth recently due to various incentive programs and local market conditions.[44] The power output of domestic photovoltaic devices is usually described in kilowatt-peak (kWp) units, as most are from 1 to 10 kW.[45] Concentrating photovoltaics (CPV) are another new method of electricity generation from the Sun. CPV systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties may be used, which are often mounted on a solar tracker in order to keep the focal point upon the cell as the sun moves across the sky. Tracking can increase flat panel photovoltaic output by 20% in winter, and by 50% in summer.[46]

[edit] Experimental solar power
Main articles: Solar updraft tower and Thermogenerator 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.[47] Thermoelectric, or "thermovoltaic" 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,[48] 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.[49] 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%.[50] A new technology, developed by the Idaho National Laboratory, uses nanoantennas to harvest solar power. Nanoantennas use the infrared radiation of the sun to convert energy. During the day the Earth's atmosphere lets some of the infrared radiation to pass through it and absorbs the rest. At night the earth emits it.[51]

[edit] Development
Main article: Deployment of solar power to energy grids

Nellis Solar Power Plant, the largest photovoltaic power plant in North America

[edit] 1800-1900
Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce. However development of solar technologies stagnated in the early 20th century in the face of the increasing availability, economy, and utility of coal and petroleum.[52]

[edit] 1950-1970
In 1965 Ormat Industries established to commercialise the Organic Rankine Cycle turbine concept. The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies.[53][54] Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included

the formation of research facilities in the US (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE).[55]

[edit] 1970-2000
Between 1970 and 1983 photovoltaic installations grew rapidly, but falling oil prices in the early 1980s moderated the growth of PV from 1984 to 1996.[56]

[edit] 2000-Present
Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007,[35] and 14.73 GW in 2008.[57] Since 2006 it has been economical for investors to install photovoltaics for free in return for a long term power purchase agreement. 50% of commercial systems were installed in this manner in 2007 and it is expected that 90% will by 2009.[58] Nellis Air Force Base is receiving photoelectric power for about 2.2 ¢/kWh and grid power for 9 ¢/kWh.[59][60] Commercial concentrating solar thermal power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a levelized energy cost (LEC) of 12–14 ¢/kWh.[61] The 11 MW PS10 power tower in Spain, completed in late 2005, is Europe's first commercial CSP system, and a total capacity of 300 MW is expected to be installed in the same area by 2013.[62] In August 2009, First Solar announced plans to build a 2 GW photovoltaic system in Ordos City, Inner Mongolia, China in four phases consisting of 30 MW in 2010, 970 MW in 2014, and another 1000 MW by 2019. As of June 9, 2009, there is a new solar thermal power station being built in the Banaskantha district in North Gujarat. Once completed, it will be the world's largest.[63][64] World's largest concentrating solar thermal power stations Capacity Technology Name Country Location Notes (MW) type Solar Energy parabolic Mojave desert Collection of 9 354 Generating USA trough California units Systems Martin Next near parabolic Expected 75 Generation Solar USA Indiantown, trough[65] Late 2010 Energy Center [66] Florida parabolic Las Vegas, 64 Nevada Solar One USA trough Nevada parabolic Completed 50 Andasol 1 Spain Granada trough November 2008 solar power PS20 solar power Completed April 20 Spain Seville tower tower 2009

11

solar power tower

PS10 solar power tower

Spain Seville

Europe's first commercial solar tower

Solar installations in recent years have also begun to expand into residential areas, with governments offering incentive programs to make "green" energy a more economically viable option. In Ontario, Canada, the Green Energy Act passed in 2009 created a feedin-tariff program that pays up to 80.2¢/kWh to solar PV energy producers, guaranteed for 20 years.[67] The amount scales up based on the size of the project, with projects under 10KW receiving the highest rate. (People participating in a previous Ontario program called RESOP (Renewable Energy Standard Offer Program), introduced in 2006, and paying a maximum of 42¢/kWh, were allowed to transfer the balance of their contracts to the new FIT program.[68] The program is designed to help promote the government's green agenda and lower the strain often placed on the energy grid at peak hours. In March, 2009 the proposed FIT was increased to 80¢/kWh for small, roof-top systems (≤10 kW).[69] World's largest photovoltaic (PV) power plants[70] DC Name of PV power Peak GW·h Capacity Country Notes plant Power /year factor (MW) Olmedilla Spain 60 85 0.16 Completed September 2008 Photovoltaic Park Strasskirchen Solar Germany 54 57 Park Lieberose Photovoltaic Germany 53 53[71] 0.11 2009 Park[71] Puertollano Spain 50 2008 Photovoltaic Park Moura photovoltaic Portugal 46 93 0.16 Completed December 2008 power station[72] Kothen Solar Park Germany 45 2009 Finsterwalde Solar Germany 42 2009 Park 550,000 First Solar thinWaldpolenz Solar Germany 40 40 0.11 film CdTe modules. Park[73][74] Completed December 2008 Planta Solar La Magascona & La Spain 34.5 Magasquila Arnedo Solar Plant Spain 34 Completed October 2008 Planta Solar Dulcinea Spain 31.8 Completed 2009 Merida/Don Alvaro Spain 30 Completed September 2008 Solar Park

Planta Solar Ose de la Spain Vega Planta Fotovoltaico Spain Casas de Los Pinos Planta Solar Fuente Spain Alamo DeSoto Next Generation Solar Energy Center[75][76] USA

30 28 26 25 44 40 SunPower. President Obama visited October 27, 2009. Completed October 2009

See also: List of photovoltaic power stations Financial incentives supporting installation of solar power generation are aimed at increasing demand for solar photovoltaics such that they can become competitive with conventional methods of energy production.[citation needed] Another innovative way to increase demand is to harness the green purchasing power of academic institutions (universities and colleges). This has been shown to be potentially influential in catalyzing a positive spiral-effect in renewables globally.[77]

[edit] Energy storage methods
Main articles: Grid energy storage and V2G

This energy park in Geesthacht, Germany, includes solar panels and pumped-storage hydroelectricity.

Seasonal variation of the output of the solar panels at AT&T Park in San Francisco. Solar energy is not available at night, making energy storage an important issue in order to provide the continuous availability of energy.[78] Both wind power and solar power are

intermittent energy sources, meaning that all available output must be taken when it is available and either stored for when it can be used, or transported, over transmission lines, to where it can be used. Wind power and solar power can be complementary, in locations that experience more wind in the winter and more sun in the summer, but on days with no sun and no wind the difference needs to be made up in some manner.[79] The Solar Two used this method[clarification needed] of energy storage, allowing it to store enough heat in its 68 m³ storage tank to provide full output of 10 MWe for about 40 minutes, with an efficiency of about 99%.[80] Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems, have the potential to eliminate the intermittency of solar power, by storing spare solar power in the form of heat; and using this heat overnight or during periods that solar power is not available to produce electricity. This technology has the potential to make solar power dispatchable, as the heat source can be used to generate electricity at will. Solar power installations are normally supplemented by storage or another energy source, for example with wind power and hydropower. Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity. With grid-tied systems, excess electricity can be sent to the transmission grid. Net metering programs give these systems a credit for the electricity they deliver to the grid. This credit offsets electricity provided from the grid when the system cannot meet demand, effectively using the grid as a storage mechanism. Credits are normally rolled over month to month and any remaining surplus settled annually.[81] Pumped-storage hydroelectricity stores energy in the form of water pumped when surplus electricity is available, from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water: the pump becomes a turbine, and the motor a hydroelectric power generator.[82] Combining power sources in a power plant may also address storage issues. The Institute for Solar Energy Supply Technology of the University of Kassel pilot-tested a combined power plant linking solar, wind, biogas and hydrostorage to provide load-following power around the clock, entirely from renewable sources.[83]

[edit] Economics
This section may stray from the topic of the article into the topic of another article, Renewable energy commercialization. Please help improve this section or discuss this issue on the talk page. (May 2010) This section may contain original research. Please improve it by verifying the claims made and adding references. Statements consisting only of original research may be removed. More details may be available on the talk page. (September 2007) See also: Renewable energy commercialization

US average daily solar energy insolation received by a latitude tilt photovoltaic cell. In photovoltaics, the solar value added chain is the set of steps from sand or raw silicon to the completed solar module and photovoltaic system completion and installation.[84]

[edit] Energy payback time and energy returned on energy invested
The energy payback time of a power generating system is the time required to generate as much energy as was consumed during production of the system. In 2000 the energy payback time of PV systems was estimated as 8 to 11 years[85] and in 2006 this was estimated to be 1.5 to 3.5 years for crystalline silicon PV systems[86] and 1-1.5 years for thin film technologies (S. Europe).[86] Another economic measure, closely related to the energy payback time, is the energy returned on energy invested (EROEI) or energy return on investment (EROI)[87], which is the ratio of electricity generated divided by the energy required to build and maintain the equipment. (This is not the same as the economic return on investment (ROI), which varies according to local energy prices, subsidies available and metering techniques.) With lifetimes of at least 30 years[citation needed], the EROEI of PV systems are in the range of 10 to 30, thus generating enough energy over their lifetimes to reproduce themselves many times (6-31 reproductions) depending on what type of material, balance of system (BOS), and the geographic location of the system.[88]

[edit] Power costs
The PV industry is beginning to adopt levelized cost of energy (LCOE) as the unit of cost. For a 10 MW plant in Phoenix, AZ, the LCOE is estimated at $0.15 to 0.22/kWh in 2005.[89] The calculated total cost per kilowatt-hour of electricity generated by a photovoltaic system is a function of the investment cost, cost of capital and depreciation period. The annual energy output in kilowatt-hours expected from each installed peak kilowatt varies by geographic region because the average insolation depends on the average cloudiness and the thickness of atmosphere traversed by the sunlight. It also depends on the path of the sun relative to the panel and the horizon.

Panels can be mounted at an angle based on latitude,[90] or solar tracking can be utilized to access even more perpendicular sunlight, thereby raising the total energy output. The calculated values in the table reflect the total cost in cents per kilowatt-hour produced. They assume a 10% total capital cost (for instance 4% interest rate, 1% operating and maintenance cost, and depreciation of the capital outlay over 20 years). Physicists[who?] have claimed that recent technological developments bring the cost of solar energy more in parity with that of fossil fuels. In 2007, David Faiman, the director of the Ben-Gurion National Solar Energy Center of Israel, announced that the Center had entered into a project with Zenith Solar to create a home solar energy system that uses a 10 square meter reflector dish.[91] In testing, the concentrated solar technology proved to be up to five times more cost effective than standard flat photovoltaic silicon panels, which would make it almost the same cost as oil and natural gas.[92] A prototype ready for commercialization achieved a concentration of solar energy that was more than 1,000 times greater than standard flat panels.[93]

[edit] Grid parity
Further information: Low-cost solar cell and Solar America Initiative Grid parity, the point at which photovoltaic electricity is equal to or cheaper than grid power, is achieved first in areas with abundant sun and high costs for electricity such as in California and Japan.[94] Grid parity has been reached in Hawaii and other islands that otherwise use fossil fuel (diesel fuel) to produce electricity, and most of the US is expected to reach grid parity by 2015.[95][96] General Electric's Chief Engineer predicts grid parity without subsidies in sunny parts of the United States by around 2015. Other companies predict an earlier date:[97] the cost of solar power will be below grid parity for more than half of residential customers and 10% of commercial customers in the OECD, as long as grid electricity prices do not decrease through 2010.[98] The fully loaded cost (cost not price) of solar electricity is $0.25/kWh or less in most of the OECD countries. By late 2011, the fully loaded cost is likely to fall below $0.15/kWh for most of the OECD and reach $0.10/kWh in sunnier regions. These cost levels are driving three emerging trends:[98] 1. vertical integration of the supply chain; 2. origination of power purchase agreements (PPAs) by solar power companies; 3. unexpected risk for traditional power generation companies, grid operators and wind turbine manufacturers. Abengoa Solar has announced the award of two R&D projects in the field of Concentrating Solar Power (CSP) by the US Department of Energy that total over $14

million. The goal of the DOE R&D program, working in collaboration with partners such as Abengoa Solar, is to develop CSP technologies that are competitive with conventional energy sources (grid parity) by 2015.[99] Concentrating photovoltaics (CPV) could reach grid parity in 2011.[citation needed] Due to the growing demand for photovoltaic electricity, more companies enter into this market and lower cost of the photovoltaic electricity would be expected.

[edit] Net metering
Main article: Net metering Net metering is particularly important because it can be done with no changes to standard electricity meters , which accurately measure power in both directions and automatically report the difference, and because it allows homeowners and businesses to generate electricity at a different time from consumption, effectively using the grid as a giant storage battery. As more photovoltaics are used ultimately additional transmission and storage will need to be provided, normally in the form of pumped hydro-storage. Normally with net metering, deficits are billed each month while surpluses are rolled over to the following month and paid annually.

[edit] Financial incentives
Main article: PV financial incentives The political purpose of incentive policies for PV is to facilitate an initial small-scale deployment to begin to grow the industry, even where the cost of PV is significantly above grid parity, to allow the industry to achieve the economies of scale necessary to reach grid parity. The policies are implemented to promote national energy independence, high tech job creation and reduction of CO2 emissions. Three incentive mechanisms are used (often in combination):
• • •

investment subsidies: the authorities refund part of the cost of installation of the system, Feed-in Tariffs (FIT): the electricity utility buys PV electricity from the producer under a multiyear contract at a guaranteed rate. Solar Renewable Energy Certificates ("SRECs")

Rebates With investment subsidies, the financial burden falls upon the taxpayer, while with feedin tariffs the extra cost is distributed across the utilities' customer bases. While the investment subsidy may be simpler to administer, the main argument in favour of feed-in tariffs is the encouragement of quality. Investment subsidies are paid out as a function of the nameplate capacity of the installed system and are independent of its actual power

yield over time, thus rewarding the overstatement of power and tolerating poor durability and maintenance. Some electric companies offer rebates to their customers, such as Austin Energy in Texas, which offers $2.50/watt installed up to $15,000.[100] Feed-in Tariffs (FiT) With feed-in tariffs, the financial burden falls upon the consumer. They reward the number of kilowatt-hours produced over a long period of time, but because the rate is set by the authorities, it may result in perceived overpayment. The price paid per kilowatthour under a feed-in tariff exceeds the price of grid electricity. Net metering refers to the case where the price paid by the utility is the same as the price charged. Solar Renewable Energy Credits (SRECs) Alternatively, SRECs allow for a market mechanism to set the price of the solar generated electricity subsity. In this mechanism, a renewable energy production or consumption target is set, and the utility (more technically the Load Serving Entity) is obliged to purchase renewable energy or face a fine (Alternative Compliance Payment or ACP). The producer is credited for an SREC for every 1,000 kWh of electricity produced. If the utility buys this SREC and retires it, they avoid paying the ACP. In principle this system delivers the cheapest renewable energy, since the all solar facilities are eligible and can be installed in the most economic locations. Uncertainties about the future value of SRECs have led to long-term SREC contract markets to give clarity to their prices and allow solar developers to pre-sell/hedge their SRECs. Financial incentives for photovoltaics differ across countries and even across states within the US, including Australia, China,[101] Germany,[102] Israel,[103] Japan, and the United States The Japanese government through its Ministry of International Trade and Industry ran a successful programme of subsidies from 1994 to 2003. By the end of 2004, Japan led the world in installed PV capacity with over 1.1 GW.[104] In 2004, the German government introduced the first large-scale feed-in tariff system, under a law known as the 'EEG' (Erneuerbare Energien Gesetz) which resulted in explosive growth of PV installations in Germany. At the outset the FIT was over 3x the retail price or 8x the industrial price. The principle behind the German system is a 20 year flat rate contract. The value of new contracts is programmed to decrease each year, in order to encourage the industry to pass on lower costs to the end users. The programme has been more successful than expected with over 1GW installed in 2006, and political pressure is mounting to decrease the tariff to lessen the future burden on consumers. Subsequently Spain, Italy, Greece (who enjoyed an early success with domestic solarthermal installations for hot water needs) and France introduced feed-in tariffs. None have replicated the programmed decrease of FIT in new contracts though, making the German incentive relatively less and less attractive compared to other countries. The

French and Greek FIT offer a high premium (EUR 0.55/kWh) for building integrated systems. California, Greece, France and Italy have 30-50% more insolation than Germany making them financially more attractive. The Greek domestic "solar roof" programme (adopted in June 2009 for installations up to 10 kW) has internal rates of return of 10-15% at current commercial installation costs, which, furthermore, is tax free. In 2006 California approved the 'California Solar Initiative', offering a choice of investment subsidies or FIT for small and medium systems and a FIT for large systems. The small-system FIT of $0.39 per kWh (far less than EU countries) expires in just 5 years, and the alternate "EPBB" residential investment incentive is modest, averaging perhaps 20% of cost. All California incentives are scheduled to decrease in the future depending as a function of the amount of PV capacity installed. At the end of 2006, the Ontario Power Authority (OPA, Canada) began its Standard Offer Program (SOP), the first in North America for small renewable projects (10MW or less). This guarantees a fixed price of $0.42 CDN per kWh over a period of twenty years. Unlike net metering, all the electricity produced is sold to the OPA at the SOP rate. The generator then purchases any needed electricity at the current prevailing rate (e.g., $0.055 per kWh). The difference should cover all the costs of installation and operation over the life of the contract. On October 1, 2009, OPA issued a Feed-in Tariff (FIT) program, increasing this fixed price to $0.802 per kWh.[105] The price per kilowatt hour or per peak kilowatt of the FIT or investment subsidies is only one of three factors that stimulate the installation of PV. The other two factors are insolation (the more sunshine, the less capital is needed for a given power output) and administrative ease of obtaining permits and contracts. Unfortunately the complexity of approvals in California, Spain and Italy has prevented comparable growth to Germany even though the return on investment is better. In some countries, additional incentives are offered for BIPV compared to stand alone PV.
• • •

France + EUR 0.16 /kWh (compared to semi-integrated) or + EUR 0.27/kWh (compared to stand alone) Italy + EUR 0.04-0.09 kWh Germany + EUR 0.05/kWh (facades only)

[edit] Investment
There is an International Conference on Solar Photovoltaic Investments organized by EPIA.[106]

[edit] Environmental impacts

The siting of solar power plants is an issue. Placement in environmentally sensitive locations can be an issue, as can the noise they produce. Unlike fossil fuel based technologies, solar power does not lead to any harmful emissions during operation, but the production of the panels leads to some amount of pollution.

[edit] Location
The location of solar power plants is an issue as more plants were built or planned. Locating a solar power plant in a pristine location such as the Mohave Desert raised objections. More acceptable to environmentalists is use of farmland taken out of production due to salinization or lack of water, or other contaminated locations such as reclaimed landfills or mines.[107] Noise, such as that caused by hundreds of sterling engines, is another issue.[108]

[edit] Greenhouse gases
Life cycle greenhouse gas emissions are now in the range of 25-32 g/kWh and this could decrease to 15 g/kWh in the future.[86] For comparison (of weighted averages), a combined cycle gas-fired power plant emits some 400-599 g/kWh,[109] an oil-fired power plant 893 g/kWh,[109] a coal-fired power plant 915-994 g/kWh[110] or with carbon capture and storage some 200 g/kWh, and a geothermal high-temp. power plant 91-122 g/kWh. [109] Only wind and geothermal low-temp. are better, emitting 11 g/kWh and 01 g/kWh[109] on average. Including the energy needed to mine uranium and the energyintensity of power plant construction and decommissioning, some place nuclear power plants' life-cycle greenhouse gas emissions below 40 g/kWh, but others give much higher figures.[111] Using renewable energy sources in manufacturing and transportation would further drop carbon emissions. BP Solar owns two factories built by Solarex (one in Maryland, the other in Virginia) in which all of the energy used to manufacture solar panels is produced by solar panels. A 1-kilowatt system eliminates the burning of approximately 170 pounds of coal, 300 pounds of carbon dioxide from being released into the atmosphere, and saves up to 105 gallons of water consumption monthly.[112]

[edit] Cadmium
One issue that has often raised concerns is the use of cadmium in cadmium telluride solar cells (CdTe is only used in a few types of PV panels). Cadmium in its metallic form is a toxic substance that has the tendency to accumulate in ecological food chains. The amount of cadmium used in thin-film PV modules is relatively small (5-10 g/m²) and with proper emission control techniques in place the cadmium emissions from module production can be almost zero. Current PV technologies lead to cadmium emissions of 0.3-0.9 microgram/kWh over the whole life-cycle.[86] Most of these emissions actually arise through the use of coal power for the manufacturing of the modules, and coal and lignite combustion leads to much higher emissions of cadmium. Life-cycle cadmium emissions from coal is 3.1 microgram/kWh, lignite 6.2, and natural gas 0.2 microgram/kWh.

Note that if electricity produced by photovoltaic panels were used to manufacture the modules instead of electricity from burning coal, cadmium emissions from coal power usage in the manufacturing process could be entirely eliminated.[citation needed]

[edit] Solar power usage
Grid-Connected Solar PV Capacity (MW)[113][114] No Country 2006 2007 2008 2009 1 Germany 3,063 3,846 6,019 9,830 2 Spain 118 733 3,421 3,520 3 1,500 1,700 2,000 2,600 Japan 4 United States 300 500 700 1,200 5 Italy 58 120 458 1,032 6 1 4 55 466 Czech Republic 7 South Korea <100 100 400 400 — Others 100 500 700 2,000 — World 5,100 7,600 13,500 21,000 Solar heating (kWth)[115][116][117][113] Country 2009 2008 China 105,000,000 Germany 8,896,300 7,765,800 Turkey 7,500,000 Japan 4,100,000 Greece 2,851,940 2,707,740 Turkey 2,600,000 Brazil 2,400,000 Austria 2,517,812 2,268,231 United States 2,000,000 Total World 149,000,000 Grid-connected solar photovoltaics had an installed capacity of nearly 21,000 MW at the end of 2009, compared to 600 MW of concentrating solar thermal[113] and 149,000 MW of Solar heating. Germany and Japan have been leading in terms of cumulative PV capacity, China in solar heating. In recent years many countries (notably in Western Europe) have begun to enact financial incentives encouraging solar power.

[edit] Germany
Main article: Solar power in Germany Germany is one of the world's top photovoltaics (PV) installers, with a solar PV capacity in 2009 of 8,877 megawatts (MW), and 6,200 GWh of electricity generated in 2009.[118]

Solar power now meets about 1.1 percent of Germany's electricity demand, a share that some market analysts expect could reach 25 percent by 2050.[119]

[edit] India
Main article: Solar power in India India is both densely populated and has high solar insolation, providing an ideal combination for solar power. In solar energy sector, some large projects have been proposed, and a 35,000 km² area of the Thar Desert has been set aside for solar power projects, sufficient to generate 700 to 2,100 gigawatts. In July 2009, India unveiled a $19 billion plan, to produce 20 GW of solar power by 2020.[120] Under the plan, solar-powered equipment and applications would be mandatory in all government buildings including hospitals and hotels.[121] On November 18, 2009, it was reported that India was ready to launch its National Solar Mission under the National Action Plan on Climate Change, with plans to generate 1,000 MW of power by 2013.[122]

[edit] Iran
The average solar radiation for the whole of Iran is about 19.23 Mega joules per square meter, and it is even higher in the central part of Iran. The variation of radiation varies in the south-east part to 5.4 kWh/m in central region from 2.8 kWh/m. The calculations show that the amount of useful solar radiation hours in Iran exceeds 2800 hours per year. For this reason, the first Photovoltaic (PV) site, with capacity of 5 kW DC was established in the central region of Iran in Doorbid village Yazd in 1993. Following this, in 1998, the second photovoltaic site with 27 kW AC capacity was installed in Hosseinian and Moalleman villages in Semnan, 450 Km inland from Tehran. The capacity of these power plants has recently increased to 10 kW AC and 92 kW AC respectively. The power plant installed at Doorbid, works independently from the grid system, while the one installed at Hosseinian and Moalleman, is connected to grid. It is worth mentioning that all equipment of these sites is made in Iran.[123] Iran took its first step toward the large scale realization of that potential recently(2009) with the inauguration of its first solar energy plant. The plant was constructed with domestic materials and labour in Shiraz, the Fars province. This solar thermal plant joins some 4,075 small scale solar thermal installations throughout Iran–3,781 residential solar water heaters and 294 public baths heated with solar thermal energy. Iran makes less use of photovoltaic energy, but the Ministry of Energy News Agency mentions a 40 house solar village supplied with photovoltaic energy.[124]

[edit] Israel

Solar water heaters on a rooftop in Jerusalem As of the early 1990s, all new residential buildings were required by the government to install solar water-heating systems, and Israel's National Infrastructure Ministry estimates that solar panels for water-heating satisfy 4% of the country's total energy demand.[93] Israel and Cyprus are the per-capita leaders in the use of solar hot water systems with over 90% of homes using them.[125] Israeli research has advanced solar technology to a degree that it is almost costcompetitive with fossil fuels.[93] Its abundant sun made the country a natural location for the promising technology. The high annual incident solar irradiance in the Negev Desert has spurred an advanced solar research and development industry, with Harry Tabor and David Faiman of the National Solar Energy Center two of its more prominent members. At the end of 2008 a feed-in tariff scheme was approved, which resulted in the building of residential and commercial solar energy power station projects.

[edit] United States
Main article: Solar power in the United States Solar power in the United States accounted for less than 0.1% of the county's electricity generation in 2006.[not in citation given (See discussion.)] Renewable resources (solar, wind, geothermal, hydroelectric, biomass, and waste) provided nearly 12 percent of the nation's electricity supply in 2003.[126][not in citation given (See discussion.)] The DoE has established the goal of generating 10-15% of the nation's energy from solar sources by 2030.[citation needed]