Photovoltaics

Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially utilized for electricity generation and as photosensors.

A photovoltaic system employs solar modules, each comprising a number of solar cells, which generate electrical power. PV installations may be ground-mounted, rooftop mounted, wall mounted or floating. The mount may be fixed or use a solar tracker to follow the sun across the sky.

PV has become the cheapest source of electrical power in regions with a high solar potential, with a bid for pricing as low as 0.01567 US$/kWh in Qatar in 2020.^[1] Panel prices have dropped by the factor of 4 between 2004 and 2011. This competitiveness opens the path to a global transition to sustainable energy, which would be required to help mitigate global warming. The emissions budget for CO
2 to meet the 1.5 degree target would be used up in 2028 if emissions remain on the current level. However, the use of PV as a main source requires energy storage systems or global distribution by high-voltage direct current power lines causing additional costs, as well as a number of other specific disadvantages such as unstable power generation and the requirement for power companies to compensate for too much solar power in the supply mix by having more reliable conventional power supplies in order to regulate demand peaks and potential undersupply.

Solar PV has specific advantages as an energy source: once installed, its operation generates no pollution and no greenhouse gas emissions, it shows simple scalability in respect of power needs and silicon has large availability in the Earth’s crust, but other materials required in PV system manufacture such as silver will eventually contrain further growth in the technology. Other major contraints identified are competition for land use and lack of labour in making funding applications.^[2] Production and installation does cause pollution and greenhouse gas emissions and there are no viable systems for recycling the panels once they are at the end of their lifespan after 10 to 30 years.

Photovoltaic systems have long been used in specialized applications as stand-alone installations and grid-connected PV systems have been in use since the 1990s.^[3] Photovoltaic modules were first mass-produced in 2000, when German environmentalists and the Eurosolar organization got government funding for a ten thousand roof program.^[4]

Advances in technology and increased manufacturing scale have in any case reduced the cost,^[5] increased the reliability, and increased the efficiency of photovoltaic installations.^[3][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] More than 100 countries now use solar PV.

In 2019, worldwide installed PV capacity increased to more than 635 gigawatts (GW) covering approximately two percent of global electricity demand.^[8] After hydro and wind powers, PV is the third renewable energy source in terms of global capacity. The International Energy Agency expects a growth by 700 – 880 GW from 2019 to 2024.^[9]

In 2020, a rooftop photovoltaic system recoups the energy needed to manufacture it in 1.28 years in Ottawa, Canada, 0.97 years in Catania, Italy, and 0.4 years in Jaipur, India.^[10]

PV has grown as an energy source primarily as a result of technological development delivering decreasing costs.^[11]

Etymology

The term “photovoltaic” comes from the Greek φῶς (phōs) meaning “light”, and from “volt”, the unit of electromotive 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 “photovoltaic” has been in use in English since 1849.^[12]

Solar cells

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 by the photovoltaic effect.^[13][14]

Solar cells produce direct current electricity from sunlight 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 systems for power generation. In this case an inverter is required to convert the DC to AC. There is still a smaller market for stand alone systems for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, and cathodic protection of pipelines.

Photovoltaic power generation employs solar modules composed of a number of solar cells containing a semiconductor material.^[15] 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.^[16][17][18]

Cells require protection from the environment and are usually packaged tightly in solar modules.

Photovoltaic module power is measured under standard test conditions (STC) in “W_p” (watts peak).^[19] The actual power output at a particular place may be less than or greater than this rated value, depending on geographical location, time of day, weather conditions, and other factors.^[20] Solar photovoltaic array capacity factors are typically under 25%, which is lower than many other industrial sources of electricity.^[21]

Current developments

For best performance, terrestrial PV systems aim to maximize the time they face the sun. Solar trackers achieve this by moving PV modules to follow the sun.^{[citation needed]}. Static mounted systems can be optimized by analysis of the sun path. PV modules 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 photovoltaic modules.^[53]

Drivers and Barriers to Growth

Generous Feed-in tariff (FIT) and supporting policies such as tax exemptions are found to be the key proximate drivers of Vietnam’s solar PV boom. Underlying drivers include the government’s desire to enhance energy self-sufficiency and the public’s demand for local environmental quality.^[78]

A key barrier is limited transmission grid capacity.^[78]

Top 10 PV countries in 2019 (MW)

Installed and Total Solar Power Capacity in 2019 (MW)^[79]

#	Nation	Total Capacity	Added Capacity
1	China	204,700	30,100
2	United States	75,900	13,300
3	Japan	63,000	7,000
4	Germany	49,200	3,900
5	India	42,800	9,900
6	Italy	20,800	600
7	Australia	15,928	3,700
8	United Kingdom	13,300	233
9	South Korea	11,200	3,100
10	France	9,900	900

Applications

Photovoltaic systems

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.^[108] 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.^[109] 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.^[110]

• 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.^[111][112]

• Power stations

Satellite image of the Topaz Solar Farm Many utility-scale solar farms have been constructed all over the world. In 2011 the 579-megawatt (MW_AC) Solar Star project was proposed, to be followed by the Desert Sunlight Solar Farm and the Topaz Solar Farm in the future, both with a capacity of 550 MW_AC, to be constructed by US-company First Solar, using CdTe modules, a thin-film PV technology. All three power stations will be located in the Californian desert.^[113] When the Solar Star project was completed in 2015, it was the world’s largest photovoltaic power station at the time.^[114]

• Agrivoltaics

Main article: Agrivoltaic A number of experimental solar farms have been established around the world that attempt to integrate solar power generation with agriculture. An Italian manufacturer has promoted a design which track the sun’s daily path across the sky to generate more electricity than conventional fixed-mounted systems.^[115]

• Rural electrification

Developing countries where many villages are often more than five kilometres 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.^[116][117] Cuba is working to provide solar power for areas that are off grid.^[118] More complex applications of off-grid solar energy use include 3D printers.^[119]RepRap 3D printers have been solar powered with photovoltaic technology,^[120] 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, in 1995 solar rural electrification projects had been found to be difficult to sustain due to unfavorable economics, lack of technical support, and a legacy of ulterior motives of north-to-south technology transfer.^[121]

• Standalone systems

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,^[122]parking meters,^[123][124]emergency telephones, trash compactors,^[125] temporary traffic signs, charging stations,^[126][127] and remote guard posts and signals.

• Floating solar

Where land may be limited, PV can be deployed as floating solar. 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.^[128] The primary benefit of a floating 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.^[129] Another benefit of a floating solar 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.^[130]

• In transport

Solar Impulse 2, a solar aircraft 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.^[131] 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^[132] and airplanes^[133] have been demonstrated, with the most practical and likely of these being solar cars.^[134] The Swiss solar aircraft, Solar Impulse 2, achieved the longest non-stop solo flight in history and completed the first solar-powered aerial circumnavigation of the globe in 2016.

• 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.^[135]

• Spacecraft applications

Part of Juno‘s solar array Solar panels on spacecraft are usually the sole source of power to run the sensors, active heating and cooling, and communications. A battery stores this energy for use when the solar panels are in shadow. In some, the power is also used for spacecraft propulsion—electric propulsion.^[136] Spacecraft were one of the earliest applications of photovoltaics, starting with the silicon solar cells used on the Vanguard 1 satellite, launched by the US in 1958.^[137] Since then, solar power has been used on missions ranging from 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.^[136]

• Specialty Power Systems

Photovoltaics may also be incorporated as energy conversion devices for objects at elevated temperatures and with preferable radiative emissivities such as heterogeneous combustors.^[138]

• Indoor Photovoltaics (IPV)

Indoor photovoltaics have the potential to supply power to the Internet of Things, such as smart sensors and communication devices, providing a solution to the battery limitations such as power consumption, toxicity, and maintenance. Ambient indoor lighting, such as LEDs and fluorescent lights, emit enough radiation to power small electronic devices or devices with low-power demand.^[139] In these applications, indoor photovoltaics will be able to improve reliability and increase lifetimes of wireless networks, especially important with the significant number of wireless sensors that will be installed in the coming years.^[140] Due to the lack of access to solar radiation, the intensity of energy harvested by indoor photovoltaics is usually three orders of magnitude smaller than sunlight, which will affect the efficiencies of the photovoltaic cells. The optimal band gap for indoor light harvesting is around 1.9-2 eV, compared to the optimum of 1.4 eV for outdoor light harvesting. The increase in optimal band gap also results in a larger open-circuit voltage (VOC), which affects the efficiency as well.^[139]Silicon photovoltaics, the most common type of photovoltaic cell in the market, is only able to reach an efficiency of around 8% when harvesting ambient indoor light, compared to its 26% efficiency in sunlight. One possible alternative is to use amorphous silicon, a-Si, as it has a wider band gap of 1.6 eV compared to its crystalline counterpart, causing it to be more suitable to capture the indoor light spectra.^[141] Other promising materials and technologies for indoor photovoltaics include thin-film materials, III-V light harvesters, organic photovoltaics (OPV), and perovskite solar cells.

• Thin-film materials, specifically CdTe, have displayed good performance under low light and diffuse conditions, with a band gap of 1.5 eV.^[142]

• Some single junction III-V cells have band gaps in the range of 1.8 to 1.9 eV, which have been shown to maintain good performances under indoor lighting, with an efficiency of over 20%.^[143][144]

• There has been various organic photovoltaics that have demonstrated efficiencies of over 16% from indoor lighting, despite having low efficiencies in energy harvesting under sunlight.^[145] This is due to the fact that OPVs have a large absorption coefficient, adjustable absorptions ranges, as well as small leakage currents in dim light, allowing them to convert indoor lighting more efficiently compared to inorganic PVs.^[139]

• Perovskite solar cells have been tested to display efficiencies over 25% in low light levels.^[146] While perovskite solar cells often contain lead, raising the concern of toxicity, lead-free perovskite inspired materials also show promise as indoor photovoltaics.^[147] While plenty of research is being conducted on perovskite cells, further research is needed to explore its possibilities for IPVs and developing products that can be used to power the internet of things.

Source: Photovoltaics, https://en.wikipedia.org/w/index.php?title=Photovoltaics&oldid=1013046809 (last visited Mar. 21, 2021).