Photovoltaics

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The Solar Settlement, a sustainable housing community project in Freiburg, Germany.
Photovoltaic SUDI shade is an autonomous and mobile station in France that provides energy for electric vehicles using solar energy.
Solar panels on the International Space Station

Photovoltaics (PV) is the name of a method of converting solar energy into direct current electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon commonly studied in physics, photochemistry and electrochemistry. A photovoltaic system employs solar panels composed of a number of solar cells to supply usable solar power. The process is both physical and chemical in nature, as the first step involves the photoelectric effect from which a second electrochemical process take place involving crystallized atoms being ionized in a series, generating an electric current.[1] Power generation from solar PV has long been seen as a clean sustainable[2] energy technology which draws upon the planet’s most plentiful and widely distributed renewable energy source – the sun. The direct conversion of sunlight to electricity occurs without any moving parts or environmental emissions during operation. It is well proven, as photovoltaic systems have now been used for fifty years in specialized applications, and grid-connected PV systems have been in use for over twenty years.[3] They were first mass-produced in the year 2000, when German environmentalists including Eurosolar succeeded in obtaining government support for the 100,000 roofs program.[4]

Driven by advances in technology and increases in manufacturing scale and sophistication, the cost of photovoltaics has declined steadily since the first solar cells were manufactured,[3][5] and the levelised cost of electricity from PV is competitive with conventional electricity sources in an expanding list of geographic regions.[6] Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries.[7] With current technology, photovoltaics recoups the energy needed to manufacture them in 1.5 to 2.5 years in Southern and Northern Europe, respectively.[8]

Solar PV is now, after hydro and wind power, the third most important renewable energy source in terms of globally installed capacity. More than 100 countries use solar PV. Installations may be ground-mounted (and sometimes integrated with farming and grazing) or built into the roof or walls of a building (either building-integrated photovoltaics or simply rooftop).

In 2014, worldwide installed PV capacity increased to at least 177 gigawatts (GW), sufficient to supply 1 percent of global electricity demands. Due to the exponential growth of photovoltaics, installations are rapidly approaching the 200 GW mark – about 40 times the installed capacity of 2006.[9] China, followed by Japan and the United States, is the fastest growing market, while Germany remains the world's largest producer, with solar contributing about 7 percent to its annual domestic electricity consumption.[10]

Etymology[edit]

The term "photovoltaic" comes from the Greek φῶς (phōs) meaning "light", and from "volt", the unit of electro-motive force, the volt, which in turn comes from the last name of the Italian physicist Alessandro Volta, inventor of the battery (electrochemical cell). The term "photo-voltaic" has been in use in English since 1849.[11]

Solar cells[edit]

Main article: Solar cell
Solar cells generate electricity directly from sunlight

Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. The photovoltaic effect was first observed by Alexandre-Edmond Becquerel in 1839.[12][13] The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode.

Solar cells produce direct current electricity from sun light which can be used to power equipment or to recharge a battery. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. There is a smaller market for off-grid power for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, and cathodic protection of pipelines.

Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide.[14] Copper solar cables connect modules (module cable), arrays (array cable), and sub-fields. Because of the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years.[15][16][17]

Solar photovoltaics power generation has long been seen as a clean energy technology which draws upon the planet’s most plentiful and widely distributed renewable energy source – the sun. The technology is “inherently elegant” in that the direct conversion of sunlight to electricity occurs without any moving parts or environmental emissions during operation. It is well proven, as photovoltaic systems have now been used for fifty years in specialised applications, and grid-connected systems have been in use for over twenty years.

Cells require protection from the environment and are usually packaged tightly behind a glass sheet. When more power is required than a single cell can deliver, cells are electrically connected together to form photovoltaic modules, or solar panels. A single module is enough to power an emergency telephone, but for a house or a power plant the modules must be arranged in multiples as arrays.

Photovoltaic power capacity is measured as maximum power output under standardized test conditions (STC) in "Wp" (Watts peak).[18] The actual power output at a particular point in time may be less than or greater than this standardized, or "rated," value, depending on geographical location, time of day, weather conditions, and other factors.[19] Solar photovoltaic array capacity factors are typically under 25%, which is lower than many other industrial sources of electricity.[20]

Current developments[edit]

For best performance, terrestrial PV systems aim to maximize the time they face the sun. Solar trackers achieve this by moving PV panels to follow the sun. The increase can be by as much as 20% in winter and by as much as 50% in summer. Static mounted systems can be optimized by analysis of the sun path. Panels are often set to latitude tilt, an angle equal to the latitude, but performance can be improved by adjusting the angle for summer or winter. Generally, as with other semiconductor devices, temperatures above room temperature reduce the performance of photovoltaics.[21]

A number of solar panels may also be mounted vertically above each other in a tower, if the zenith distance of the Sun is greater than zero, and the tower can be turned horizontally as a whole and each panels additionally around a horizontal axis. In such a tower the panels can follow the Sun exactly. Such a device may be described as a ladder mounted on a turnable disk. Each step of that ladder is the middle axis of a rectangular solar panel. In case the zenith distance of the Sun reaches zero, the "ladder" may be rotated to the north or the south to avoid a solar panel producing a shadow on a lower solar panel. Instead of an exactly vertical tower one can choose a tower with an axis directed to the polar star, meaning that it is parallel to the rotation axis of the Earth. In this case the angle between the axis and the Sun is always larger than 66 degrees. During a day it is only necessary to turn the panels around this axis to follow the Sun. Installations may be ground-mounted (and sometimes integrated with farming and grazing)[22] or built into the roof or walls of a building (building-integrated photovoltaics).

Another recent development involves the makeup of solar cells. Perovskite is a very inexpensive material which is being used to replace the expensive crystalline silicon which is still part of a standard PV cell build to this day. Michael Graetzel, Director of the Laboratory of Photonics and Interfaces at EPFL says, “Today, efficiency has peaked at 18 percent, but it's expected to get even higher in the future.”[23] This is a significant claim, as 20% efficiency is typical among solar panels which use more expensive materials.

Efficiency[edit]

Main article: Solar cell efficiency
Best Research-Cell Efficiencies

Although it is important to have an efficient solar cell, it is not necessarily the efficient solar cell that consumers will use. It is important to have efficient solar cells that are the best value for the money. Efficiency of pv cells can be measured by calculating how much they can convert sunlight into usable energy for human consumption. Maximum efficiency of a solar photovoltaic cell is given by the following equation: η(maximum efficiency)= P(maximum power output)/(E(S,γ)(incident radiation flux)*A(c)(Area of collector)).[24] If the area provided is limited, efficiency of the PV cell is important to achieve the desired power output over a limited area.

The most efficient solar cell so far is a multi-junction concentrator solar cell with an efficiency of 43.5%[25] produced by Solar Junction in April 2011. The highest efficiencies achieved without concentration include Sharp Corporation at 35.8% using a proprietary triple-junction manufacturing technology in 2009,[26] and Boeing Spectrolab (40.7% also using a triple-layer design). The US company SunPower produces cells that have an energy conversion ratio of 19.5%, well above the market average of 12–18%.[27][citation needed]

There have been numerous attempts to cut down the costs of PV cells and modules to the point that will be both competitive and efficient. This can be achieved by significantly increasing the conversion efficiency of PV materials. In order to increase the efficiency of solar cells, it is necessary to choose the semiconductor material with appropriate energy gap that matches the solar spectrum. This will enhance their electrical, optical, and structural properties. Choosing a better approach to get more effective charge collection is also necessary to increase the efficiency. There are several groups of materials that fit into different efficiency regimes. Ultrahigh-efficiency devices (η>30%)[28]] are made by using GaAs and GaInP2 semiconductors with multijunction tandem cells. High-quality, single-crystal silicon materials are used to achieve high-efficiency cells (η>20%).

Organic photovoltaic cells (OPVs) are also viable alternative that relieves energy pressure and environmental problems from increasing combustion of fossil fuels. Recent development of OPVs made a huge advancement of power conversion efficiency from 3% to over 15%.[29] To date, the highest reported power conversion efficiency ranges from 6.7% to 8.94% for small molecule, 8.4%–10.6% for polymer OPVs, and 7% to 15% for perovskite OPVs.[30] Not only does recent development of OPVs make them more efficient and low-cost, they also make it environmentally-benign and renewable.

Several companies have begun embedding power optimizers into PV modules called smart modules. These modules perform maximum power point tracking (MPPT) for each module individually, measure performance data for monitoring, and provide additional safety. Such modules can also compensate for shading effects, wherein a shadow falling across a section of a module causes the electrical output of one or more strings of cells in the module to fall to zero, but not having the output of the entire module fall to zero.[31]

At the end of September 2013, IKEA announced that solar panel packages for houses will be sold at 17 United Kingdom IKEA stores by the end of July 2014. The decision followed a successful pilot project at the Lakeside IKEA store, whereby one photovoltaic (PV) system was sold almost every day. The panels are manufactured by the Chinese company Hanergy.[32]

One of the major causes for the inefficiency of cells is overheating. The efficiency of a solar cell declines by about 0.5% for every 1 degree Celcius increase in temperature. This would mean that a 100 degree increase in surface temperature could decrease the efficiency of a solar cell by about half. Self-cooling solar cells are a solution to this problem. Rather than using energy to cool the surface, pyramid and cone shapes can be formed from silica, and fastened to the surface of a solar panel. Doing so allows visible light to reach the solar cells, but causes a deflection of infrared rays (which carry heat).[33]

Growth[edit]

Worldwide growth of photovoltaics on a semi-log plot since 1992
Projected Global Growth (MW)
100,000
200,000
300,000
400,000
500,000
600,000
2009
2011
2013
2015
2017
2019
Projected global cumulative capacity (SPE)

      historical cumulative capacity
      average projection for 2015 (+55 GW, 233 GW)
      low scenario reaches 396 GW by 2019
      high scenario reaches 540 GW by 2019

Further: Growth of photovoltaics#Forecast

Solar photovoltaics is growing rapidly and worldwide installed capacity reached at least 177 gigawatts (GW) by the end of 2014. The total power output of the world’s PV capacity in a calendar year is now beyond 200 billion kWh of electricity. This represents 1% of worldwide electricity demand. More than 100 countries use solar PV.[10][34] China, followed by Japan and the United States is now the fastest growing market, while Germany remains the world's largest producer, contributing more than 7% to its national electricity demands.[10] Photovoltaics is now, after hydro and wind power, the third most important renewable energy source in terms of globally installed capacity.[35]

Several market research and financial companies foresee record-breaking global installation of more than 50 GW in 2015.[36][37][38][39] China is predicted to take the lead from Germany and to become the world's largest producer of PV power by installing another targeted 17.8 GW in 2015.[40] India is expected to install 1.8 GW, doubling its annual installations.[38] By 2018, worldwide photovoltaic capacity is projected to doubled or even triple to 430 GW. Solar Power Europe (formerly known as EPIA) also estimates that photovoltaics will meet 10% to 15% of Europe's energy demand in 2030.[41][42]

The EPIA/Greenpeace Solar Generation Paradigm Shift Scenario (formerly called Advanced Scenario) from 2010 shows that by the year 2030, 1,845 GW of PV systems could be generating approximately 2,646 TWh/year of electricity around the world. Combined with energy use efficiency improvements, this would represent the electricity needs of more than 9% of the world's population. By 2050, over 20% of all electricity could be provided by photovoltaics.[43]

Michael Liebreich, from Bloomberg New Energy Finance, anticipates a tipping point for solar energy. The costs of power from wind and solar are already below those of conventional electricity generation in some parts of the world, as they have fallen sharply and will continue to do so. He also asserts, that the electrical grid has been greatly expanded worldwide, and is ready to receive and distribute electricity from renewable sources. In addition, worldwide electricity prices came under strong pressure from renewable energy sources, that are, in part, enthusiastically embraced by consumers.[44]

Deutsche Bank sees a "second gold rush" for the photovoltaic industry to come. Grid parity has already been reached in at least 19 markets by January 2014. Photovoltaics will prevail beyond feed-in tariffs, becoming more competitive as deployment increases and prices continue to fall.[45]

In June 2014 Barclays downgraded bonds of U.S. utility companies. Barclays expects more competition by a growing self-consumption due to a combination of decentralized PV-systems and residential electricity storage. This could fundamentally change the utility's business model and transform the system over the next ten years, as prices for these systems are predicted to fall.[46]

Environmental impacts of photovoltaic technologies[edit]

PV technologies have shown significant progress lately in terms of annual production capacity and life cycle environmental performances, which necessitates the assessment of environmental impacts of such technologies. The different PV technologies show slight variations in the emissions when compared the emissions from conventional energy technologies that replaced by the latest PV technologies.[47] With the up scaling of thin film module production for meeting future energy needs, there is a growing need for conducting the life-cycle assessment (LCA) of such technologies to analyze the future environmental impacts resulting from such technologies.[48] The manufacturing processes of solar cell involve the emissions of several toxic, flammable and explosive chemicals. Lately, in the field of photovoltaic research, there has been a continual rise in research and development efforts focused on reducing mass during cell manufacture. Such efforts have resulted in reducing the thickness of solar cells and thus the next generation solar cells are becoming thinner and eventually risks of exposure are reduced nevertheless, all chemicals must be carefully handled to ensure minimal human and environmental contact. The large scale deployment of such renewable energy technologies could result in potential negative environmental implications. These potential problems can pose serious challenges in promulgating such technologies to a broad segment of consumers.[49][50][51]

There are studies which have shown that the PV environmental impacts come mainly from the production of the cells; operation and maintenance requirements and associated impacts are relatively small. There has been a significant progress in the published literature on LCA of thin film PV technologies. Research groups are applying life-cycle assessment approach to emerging PV technologies in order to facilitate a robust comparison of emerging next generation thin film photovoltaic technologies competing with each other in the photovoltaic market. In a 2014 study,[47] Collier et al. conducted the LCA for CZTS and Zn3P2 PV technologies for the first time. In this study, the cradle to gate environmental impacts from CZTS and Zn3P2 are assessed and compared with those from current commercial PV technologies such as CdTe and CIGS. The four impacts including Primary energy demand, global-warming potential, freshwater use and eco-toxicity were primarily studied. For all four impacts studied, CdTe and Zn3P2 performed better than CIGS and CZTS. In general, the contribution of raw (absorber) material extraction and processing to the total impacts was low compared with impacts coming from electricity consumption during manufacturing. Therefore, to reduce environmental impact, future PV technology development should focus more on the process improvement.[47] Apart from conducting the LCA of emerging PV technologies, there is a vital need to assess the energy payback period of next generation PV technologies. Bhandari, Collier et al. (2015),[52] conducted a systematic review and a meta-analysis of the embedded energy, energy payback time (EPBT), and energy return on energy invested metrics for the crystalline Si and thin film PV technologies published in 2000–2013. Across different types of PV, the variation in embedded energy was greater than the variation in efficiency and performance ratio suggesting that the relative ranking of the EPBT of different PV technology today and in the future depends primarily on their embedded energy and not their efficiency.[52]

Economics[edit]

Source: Apricus[53]

There have been major changes in the underlying costs, industry structure and market prices of solar photovoltaics technology, over the years, and gaining a coherent picture of the shifts occurring across the industry value chain globally is a challenge. This is due to: “the rapidity of cost and price changes, the complexity of the PV supply chain, which involves a large number of manufacturing processes, the balance of system (BOS) and installation costs associated with complete PV systems, the choice of different distribution channels, and differences between regional markets within which PV is being deployed”. Further complexities result from the many different policy support initiatives that have been put in place to facilitate photovoltaics commercialisation in various countries.[3]

The PV industry has seen dramatic drops in module prices since 2008. In late 2011, factory-gate prices for crystalline-silicon photovoltaic modules dropped below the $1.00/W mark. The $1.00/W installed cost, is often regarded in the PV industry as marking the achievement of grid parity for PV. Technological advancements, manufacturing process improvements, and industry re-structuring, mean that further price reductions are likely in coming years.[3]

Financial incentives for photovoltaics, such as feed-in tariffs, have often been offered to electricity consumers to install and operate solar-electric generating systems. Government has sometimes also offered incentives in order to encourage the PV industry to achieve the economies of scale needed to compete where the cost of PV-generated electricity is above the cost from the existing grid. Such policies are implemented to promote national or territorial energy independence, high tech job creation and reduction of carbon dioxide emissions which cause global warming. Due to economies of scale solar panels get less costly as people use and buy more—as manufacturers increase production to meet demand, the cost and price is expected to drop in the years to come.

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 44.0% with multiple-junction concentrated photovoltaics.[54] Solar cell energy conversion efficiencies for commercially available photovoltaics are around 14–22%.[55][56] Concentrated photovoltaics (CPV) may reduce cost by concentrating up to 1,000 suns (through magnifying lens) onto a smaller sized photovoltaic cell. However, such concentrated solar power requires sophisticated heat sink designs, otherwise the photovoltaic cell overheats, which reduces its efficiency and life. To further exacerbate the concentrated cooling design, the heat sink must be passive, otherwise the power required for active cooling would reduce the overall efficiency and economy.

Crystalline silicon solar cell prices have fallen from $76.67/Watt in 1977 to an estimated $0.74/Watt in 2013.[57] This is seen as evidence supporting Swanson's law, an observation similar to the famous Moore's Law that states that solar cell prices fall 20% for every doubling of industry capacity.[57]

As of 2011, the price of PV modules has fallen by 60% since the summer of 2008, according to Bloomberg New Energy Finance estimates, putting solar power for the first time on a competitive footing with the retail price of electricity in a number of sunny countries; an alternative and consistent price decline figure of 75% from 2007 to 2012 has also been published,[58] though it is unclear whether these figures are specific to the United States or generally global. The levelised cost of electricity (LCOE) from PV is competitive with conventional electricity sources in an expanding list of geographic regions,[6] particularly when the time of generation is included, as electricity is worth more during the day than at night.[59] There has been fierce competition in the supply chain, and further improvements in the levelised cost of energy for solar lie ahead, posing a growing threat to the dominance of fossil fuel generation sources in the next few years.[60] As time progresses, renewable energy technologies generally get cheaper,[61][62] while fossil fuels generally get more expensive:

The less solar power costs, the more favorably it compares to conventional power, and the more attractive it becomes to utilities and energy users around the globe. Utility-scale solar power can now be delivered in California at prices well below $100/MWh ($0.10/kWh) less than most other peak generators, even those running on low-cost natural gas. Lower solar module costs also stimulate demand from consumer markets where the cost of solar compares very favorably to retail electric rates.[63]

As of 2011, the cost of PV has fallen well below that of nuclear power and is set to fall further. The average retail price of solar cells as monitored by the Solarbuzz group fell from $3.50/watt to $2.43/watt over the course of 2011.[64]

For large-scale installations, prices below $1.00/watt were achieved. A module price of 0.60 Euro/watt ($0.78/watt) was published for a large scale 5-year deal in April 2012.[65]

By the end of 2012, the "best in class" module price had dropped to $0.50/watt, and was expected to drop to $0.36/watt by 2017.[66]

In many locations, PV has reached grid parity, which is usually defined as PV production costs at or below retail electricity prices (though often still above the power station prices for coal or gas-fired generation without their distribution and other costs). However, in many countries there is still a need for more access to capital to develop PV projects. To solve this problem securitization has been proposed and used to accelerate development of solar photovoltaic projects.[67][68] For example, SolarCity offered, the first U.S. asset-backed security in the solar industry in 2013.[69]

Photovoltaic power is also generated during a time of day that is close to peak demand (precedes it) in electricity systems with high use of air conditioning. More generally, it is now evident that, given a carbon price of $50/ton, which would raise the price of coal-fired power by 5c/kWh, solar PV will be cost-competitive in most locations. The declining price of PV has been reflected in rapidly growing installations, totaling about 23 GW in 2011. Although some consolidation is likely in 2012, due to support cuts in the large markets of Germany and Italy, strong growth seems likely to continue for the rest of the decade. Already, by one estimate, total investment in renewables for 2011 exceeded investment in carbon-based electricity generation.[64]

In the case of self consumption payback time is calculated based on how much electricity is not brought from the grid. Additionally, using PV solar power to charge DC batteries, as used in Plug-in Hybrid Electric Vehicles and Electric Vehicles, leads to greater efficiencies. Traditionally, DC generated electricity from solar PV must be converted to AC for buildings, at an average 10% loss during the conversion. An additional efficiency loss occurs in the transition back to DC for battery driven devices and vehicles, and using various interest rates and energy price changes were calculated to find present values that range from $2,057.13 to $8,213.64 (analysis from 2009).[70]

For example in Germany with electricity prices of 0.25 euro/kWh and Insolation of 900 kWh/kW one kWp will save 225 euro per year and with installation cost of 1700 euro/kWp means that the system will pay back in less than 7 years.[71]

Applications[edit]

Photovoltaic systems[edit]

Main article: Photovoltaic system

A photovoltaic system, or solar PV system is a power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and directly convert sunlight into electricity, a solar inverter to change the electric current from DC to AC, as well as mounting, cabling and other electrical accessories. PV systems range from small, roof-top mounted or building-integrated systems with capacities from a few to several tens of kilowatts, to large utility-scale power stations of hundreds of megawatts. Nowadays, most PV systems are grid-connected, while stand-alone systems only account for a small portion of the market.

  • Rooftop and building integrated systems
Rooftop PV on half-timbered house
Photovoltaic arrays are often associated with buildings: either integrated into them, mounted on them or mounted nearby on the ground. Rooftop PV systems are most often retrofitted into existing buildings, usually mounted on top of the existing roof structure or on the existing walls. Alternatively, an array can be located separately from the building but connected by cable to supply power for the building. Building-integrated photovoltaics (BIPV) are increasingly incorporated into the roof or walls of new domestic and industrial buildings as a principal or ancillary source of electrical power.[72] Roof tiles with integrated PV cells are sometimes used as well. Provided there is an open gap in which air can circulate, rooftop mounted solar panels can provide a passive cooling effect on buildings during the day and also keep accumulated heat in at night.[73] Typically, residential rooftop systems have small capacities of around 5–10 kW, while commercial rooftop systems often amount to several hundreds of kilowatts. Although rooftop systems are much smaller than ground-mounted utility-scale power plants, they account for most of the worldwide installed capacity.[74]
  • Concentrator photovoltaics
Concentrator photovoltaics (CPV) is a photovoltaic technology that contrary to conventional flat-plate PV systems uses lenses and curved mirrors to focus sunlight onto small, but highly efficient, multi-junction (MJ) solar cells. In addition, CPV systems often use solar trackers and sometimes a cooling system to further increase their efficiency. Ongoing research and development is rapidly improving their competitiveness in the utility-scale segment and in areas of high solar insolation.
  • Photovoltaic thermal hybrid solar collector
Photovoltaic thermal hybrid solar collector (PVT) are systems that convert solar radiation into thermal and electrical energy. These systems combine a solar PV cell, which converts sunlight into electricity, with a solar thermal collector, which captures the remaining energy and removes waste heat from the PV module. The capture of both electricity and heat allow these devices to have higher exergy and thus be more overall energy efficient than solar PV or solar thermal alone.[75][76]
  • Power stations
Satellite image of the Topaz Solar Farm
Many utility-scale solar farms have been constructed all over the world. As of 2015, the 579-megawatt (MWAC) Solar Star is the world's largest photovoltaic power station, followed by the Desert Sunlight Solar Farm and the Topaz Solar Farm, both with a capacity of 550 MWAC, constructed by US-company First Solar, using CdTe modules, a thin-film PV technology.[77] All three power stations are located in the Californian desert. Many solar farms around the world are integrated with agriculture and some use innovative solar tracking systems that follow the sun's daily path across the sky to generate more electricity than conventional fixed-mounted systems. There are no fuel costs or emissions during operation of the power stations.
  • Rural electrification
Developing countries where many villages are often more than five kilometers away from grid power are increasingly using photovoltaics. In remote locations in India a rural lighting program has been providing solar powered LED lighting to replace kerosene lamps. The solar powered lamps were sold at about the cost of a few months' supply of kerosene.[78][79] Cuba is working to provide solar power for areas that are off grid.[80] More complex applications of off-grid solar energy use include 3D printers.[81] RepRap 3D printers have been solar powered with photovoltaic technology,[82] which enables distributed manufacturing for sustainable development. These are areas where the social costs and benefits offer an excellent case for going solar, though the lack of profitability has relegated such endeavors to humanitarian efforts. However, solar rural electrification projects have been difficult to sustain due to unfavorable economics, lack of technical support, and a legacy of ulterior motives of north-to-south technology transfer.[83]
  • Standalone systems
A solar traffic light in Beijing, China
Until a decade or so ago, PV was used frequently to power calculators and novelty devices. Improvements in integrated circuits and low power liquid crystal displays make it possible to power such devices for several years between battery changes, making PV use less common. In contrast, solar powered remote fixed devices have seen increasing use recently in locations where significant connection cost makes grid power prohibitively expensive. Such applications include solar lamps, water pumps,[84] parking meters,[85][86] emergency telephones, trash compactors,[87] temporary traffic signs, charging stations,[88][89] and remote guard posts and signals.
  • Floatovoltaics
In May 2008, the Far Niente Winery in Oakville, CA pioneered the world's first "floatovoltaic" system by installing 994 photovoltaic solar panels onto 130 pontoons and floating them on the winery's irrigation pond. The floating system generates about 477 kW of peak output and when combined with an array of cells located adjacent to the pond is able to fully offset the winery's electricity consumption.[90] The primary benefit of a floatovoltaic system is that it avoids the need to sacrifice valuable land area that could be used for another purpose. In the case of the Far Niente Winery, the floating system saved three-quarters of an acre that would have been required for a land-based system. That land area can instead be used for agriculture.[91] Another benefit of a floatovoltaic system is that the panels are kept at a lower temperature than they would be on land, leading to a higher efficiency of solar energy conversion. The floating panels also reduce the amount of water lost through evaporation and inhibit the growth of algae.[92]
  • In transport
PV has traditionally been used for electric power in space. PV is rarely used to provide motive power in transport applications, but is being used increasingly to provide auxiliary power in boats and cars. Some automobiles are fitted with solar-powered air conditioning to limit interior temperatures on hot days.[93] A self-contained solar vehicle would have limited power and utility, but a solar-charged electric vehicle allows use of solar power for transportation. Solar-powered cars, boats[94] and airplanes[95] have been demonstrated, with the most practical and likely of these being solar cars.[96] The Swiss solar aircraft, Solar Impulse 2, achieved the longest non-stop solo flight in history and plan to make the first solar-powered aerial circumnavigation of the globe in 2015.
  • Telecommunication and signaling
Solar PV power is ideally suited for telecommunication applications such as local telephone exchange, radio and TV broadcasting, microwave and other forms of electronic communication links. This is because, in most telecommunication application, storage batteries are already in use and the electrical system is basically DC. In hilly and mountainous terrain, radio and TV signals may not reach as they get blocked or reflected back due to undulating terrain. At these locations, low power transmitters (LPT) are installed to receive and retransmit the signal for local population.[97]
  • Spacecraft applications
Part of one of Juno's solar array
Solar panels on spacecraft supply power to run the sensors, active heating and cooling, and telemetry, or they power is used for spacecraft propulsionelectric propulsion, sometimes called solar-electric propulsion.[98] Spacecraft operating in the inner solar system usually rely on the use of solar panels to derive electricity from sunlight. Solar PV cells on spacecraft was one of the earliest applications of photovoltaics. One of the first application of photovoltaic panels was on Earth-orbiting satellites, starting with the silicon solar cells used on the Vanguard 1 satellite, launched by the US in 1958.[99] Since then, solar power systems have been used on a wide variety of missions. Solar power has been used on missions ranging from the power system for the MESSENGER probe to Mercury, to as far out in the solar system as the Juno probe to Jupiter. The largest solar power system flown in space is the electrical system of the International Space Station. To increase the power generated per kilogram, typical spacecraft solar panels use high-cost, high-efficiency, and close-packed rectangular multi-junction solar cells made of gallium arsenide (GaAs) and other semiconductor materials.[98]

Advantages[edit]

The 122 PW of sunlight reaching the Earth's surface is plentiful—almost 10,000 times more than the 13 TW equivalent of average power consumed in 2005 by humans.[100] This abundance leads to the suggestion that it will not be long before solar energy will become the world's primary energy source.[101] Additionally, solar electric generation has the highest power density (global mean of 170 W/m2) among renewable energies.[100]

Solar power is pollution-free during use. Production end-wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development[102] and policies are being produced that encourage recycling from producers.[103]

PV installations can operate for 100 years or even more[104] with little maintenance or intervention after their initial set-up, so after the initial capital cost of building any solar power plant, operating costs are extremely low compared to existing power technologies.

Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses (transmission losses in the US were approximately 7.2% in 1995).[105]

Compared to fossil and nuclear energy sources, very little research money has been invested in the development of solar cells, so there is considerable room for improvement. Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40% in case of concentrating photovoltaic cells[106] and efficiencies are rapidly rising while mass-production costs are rapidly falling.[107]

In some states of the United States, much of the investment in a home-mounted system may be lost if the home-owner moves and the buyer puts less value on the system than the seller. The city of Berkeley developed an innovative financing method to remove this limitation, by adding a tax assessment that is transferred with the home to pay for the solar panels.[108] Now known as PACE, Property Assessed Clean Energy, 30 U.S. states have duplicated this solution.[109]

There is evidence, at least in California, that the presence of a home-mounted solar system can actually increase the value of a home. According to a paper published in April 2011 by the Ernest Orlando Lawrence Berkeley National Laboratory titled An Analysis of the Effects of Residential Photovoltaic Energy Systems on Home Sales Prices in California:

The research finds strong evidence that homes with PV systems in California have sold for a premium over comparable homes without PV systems. More specifically, estimates for average PV premiums range from approximately $3.9 to $6.4 per installed watt (DC) among a large number of different model specifications, with most models coalescing near $5.5/watt. That value corresponds to a premium of approximately $17,000 for a relatively new 3,100 watt PV system (the average size of PV systems in the study).[110]

See also[edit]

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Further reading[edit]