Photovoltaics (PV) is a term which covers the conversion of light into
electricity using semiconducting materials that exhibit the
photovoltaic effect, a phenomenon studied in physics, photochemistry,
A typical photovoltaic system employs solar panels, each comprising a
number of solar cells, which generate electrical power. PV
installations may be ground-mounted, rooftop mounted or wall mounted.
The mount may be fixed, or use a solar tracker to follow the sun
across the sky.
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.
PV systems have the major disadvantage that the power output is
dependent on direct sunlight, so about 10-25% is lost if a tracking
system is not used, since the cell will not be directly facing the sun
at all times. Dust, clouds, and other things in the atmosphere also
diminish the power output. Another main issue is the
concentration of the production in the hours corresponding to main
insolation, which do not usually match the peaks in demand in human
activity cycles. Unless current societal patterns of consumption
and electrical networks mutually adjust to this scenario, electricity
still needs to be stored for later use or made up by other power
sources, usually hydrocarbon.
Photovoltaic systems have long been used in specialized applications,
and standalone and grid-connected PV systems have been in use since
the 1990s. They were first mass-produced in 2000, when German
environmentalists and the
Eurosolar organization got government
funding for a ten thousand roof program.
Advances in technology and increased manufacturing scale have in any
case reduced the cost, increased the reliability, and increased the
efficiency of photovoltaic installations.
Net metering and
financial incentives, such as preferential feed-in tariffs for
solar-generated electricity, have supported solar PV installations in
many countries. More than 100 countries now use solar PV.
After hydro and wind powers, PV is the third renewable energy source
in terms of global capacity. At the end of 2016, worldwide installed
PV capacity increased to more than 300 gigawatts (GW), covering
approximately two percent of global electricity demand. China,
Japan and the United States, is the fastest growing
Germany remains the world's largest producer, with solar
PV providing seven percent of annual domestic electricity
consumption. With current technology (as of 2013), photovoltaics
recoups the energy needed to manufacture them in 1.5 years in Southern
Europe and 2.5 years in Northern Europe.
2 Solar cells
3 Current developments
3.3 Environmental impacts of photovoltaic technologies
6.1 Photovoltaic systems
8 See also
10 Further reading
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.
Main article: Solar cell
Solar cells generate electricity directly from sunlight.
Average insolation. Note that this is for a horizontal surface. Solar
panels are normally propped up at an angle and receive more energy per
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.
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 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. 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.
Solar photovoltaic 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. Cells require
protection from the environment and are usually packaged tightly in
Photovoltaic power capacity is measured as maximum power output under
standardized test conditions (STC) in "Wp" (watts peak). 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. Solar photovoltaic array capacity factors are typically
under 25%, which is lower than many other industrial sources of
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.
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
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) or built into the roof or walls of a building
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." This is a significant claim, as 20%
efficiency is typical among solar panels which use more expensive
Solar cell efficiency
Best Research-Cell Efficiencies
Electrical efficiency (also called conversion efficiency) is a
contributing factor in the selection of a photovoltaic system.
However, the most efficient solar panels are typically the most
expensive, and may not be commercially available. Therefore, selection
is also driven by cost efficiency and other factors.
The electrical efficiency of a PV cell is a physical property which
represents how much electrical power a cell can produce for a given
insolation. The basic expression for maximum efficiency of a
photovoltaic cell is given by the ratio of output power to the
incident solar power (radiation flux times area)
displaystyle eta = frac P_ max Ecdot A_ cell .
The efficiency is measured under ideal laboratory conditions and
represents the maximum achievable efficiency of the PV material.
Actual efficiency is influenced by the output Voltage, current,
junction temperature, light intensity and spectrum.
The most efficient type of solar cell to date is a multi-junction
concentrator solar cell with an efficiency of 46.0% produced by
Fraunhofer ISE in December 2014. The highest efficiencies achieved
without concentration include a material by
Sharp Corporation at 35.8%
using a proprietary triple-junction manufacturing technology in
2009, and Boeing Spectrolab (40.7% also using a triple-layer
design). The US company
SunPower produces cells that have an
efficiency of 21.5%, well above the market average of 12–18%.
There is an ongoing effort to increase the conversion efficiency of PV
cells and modules, primarily for competitive advantage. In order to
increase the efficiency of solar cells, it is important to choose a
semiconductor material with an appropriate band gap that matches the
solar spectrum. This will enhance the electrical and optical
properties. Improving the method of charge collection is also useful
for increasing the efficiency. There are several groups of materials
that are being developed. Ultrahigh-efficiency devices (η>30%)
are made by using GaAs and GaInP2 semiconductors with multijunction
tandem cells. High-quality, single-crystal silicon materials are used
to achieve high-efficiency, low cost cells (η>20%).
Recent developments in Organic photovoltaic cells (OPVs) have made
significant advancements in power conversion efficiency from 3% to
over 15% since their introduction in the 1980s. 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 21% for
perovskite OPVs. OPVs are expected to play a major role in the
PV market. Recent improvements have increased the efficiency and
lowered cost, while remaining 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
features. 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
One of the major causes for the decreased performance of cells is
overheating. The efficiency of a solar cell declines by about 0.5% for
every 1 degree Celsius increase in temperature. This means that a 100
degree increase in surface temperature could decrease the efficiency
of a solar cell by about half. Self-cooling solar cells are one
solution to this problem. Rather than using energy to cool the
surface, pyramid and cone shapes can be formed from silica, and
attached to the surface of a solar panel. Doing so allows visible
light to reach the solar cells, but reflects infrared rays (which
Main article: Growth of photovoltaics
Worldwide growth of photovoltaics
Worldwide growth of photovoltaics on a semi-log plot since 1992
Projected Global Growth (MW)
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 about 300 gigawatts (GW) by the end of 2016. Since
2000, installed capacity has seen a growth factor of about 57. The
total power output of the world’s PV capacity in a calendar year in
2014 is now beyond 200 TWh of electricity. This represents 1% of
worldwide electricity demand. More than 100 countries use solar
PV. 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
Photovoltaics is now, after hydro and wind power, the
third most important renewable energy source in terms of globally
Several market research and financial companies foresee
record-breaking global installation of more than 50 GW in
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.
India is expected to
install 1.8 GW, doubling its annual installations. 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.
In 2017 a study in Science estimated that by 2030 global PV installed
capacities will be between 3,000 and 10,000 GW. The
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.
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.
Deutsche Bank sees a "second gold rush" for the photovoltaic industry
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
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.
Top 10 PV countries in 2015 (MW)
Installed and Total Solar Power Capacity in 2015 (MW)
Data: IEA-PVPS Snapshot of Global PV 1992–2015 report, March
Solar power by country
Solar power by country for a complete and continuously
Environmental impacts of photovoltaic technologies
Types of impacts
While solar photovoltaic (PV) cells are promising for clean energy
production, their deployment is hindered by production costs, material
availability, and toxicity. Data required to investigate their
impact are sometimes affected by a rather large amount of uncertainty.
The values of human labor and water consumption, for example, are not
precisely assessed due to the lack of systematic and accurate analyses
in the scientific literature.
Life cycle assessment (LCA) is one method of determining environmental
impacts from PV. Many studies have been done on the various types of
PV including first generation, second generation, and third
generation. Usually these PV LCA studies select a cradle to gate
system boundary because often at the time the studies are conducted,
it is a new technology not commercially available yet and their
required balance of system components and disposal methods are
A traditional LCA can look at many different impact categories ranging
from global warming potential, eco-toxicity, human toxicity, water
depletion, and many others.
Most LCAs of PV have focused on two categories: carbon dioxide
equivalents per kWh and energy pay-back time (EPBT). The EPBT is
defined as " the time needed to compensate for the total renewable-
and non-renewable- primary energy required during the life cycle of a
PV system". A 2015 review of EPBT from first and second generation
PV suggested that there was greater variation in embedded energy
than in efficiency of the cells implying that it was mainly the
embedded energy that needs to reduce to have a greater reduction in
EPBT. One difficulty in determining impacts due to PV is to determine
if the wastes are released to the air, water, or soil during the
manufacturing phase. Research is underway to try to understand
emissions and releases during the lifetime of PV systems.
Impacts from first-generation PV
Crystalline silicon modules are the most extensively studied PV type
in terms of LCA since they are the most commonly used.
Mono-crystalline silicon photovoltaic systems (mono-si) have an
average efficiency of 14.0%. The cells tend to follow a structure
of front electrode, anti-reflection film, n-layer, p-layer, and back
electrode, with the sun hitting the front electrode. EPBT ranges from
1.7 to 2.7 years. The cradle to gate of CO2-eq/kWh ranges from
37.3 to 72.2 grams.
Techniques to produce multi-crystalline silicon (multi-si)
photovoltaic cells are simpler and cheaper than mono-si, however tend
to make less efficient cells, an average of 13.2%. EPBT ranges
from 1.5 to 2.6 years. The cradle to gate of CO2-eq/kWh ranges
from 28.5 to 69 grams. Some studies have looked beyond EPBT and
GWP to other environmental impacts. In one such study, conventional
energy mix in Greece was compared to multi-si PV and found a 95%
overall reduction in impacts including carcinogens, eco-toxicity,
acidification, eutrophication, and eleven others.
Impacts from second generation
Cadmium telluride (CdTe) is one of the fastest-growing thin film based
solar cells which are collectively known as second generation devices.
This new thin film device also shares similar performance restrictions
(Shockley-Queisser efficiency limit) as conventional Si devices but
promises to lower the cost of each device by both reducing material
and energy consumption during manufacturing. Today the global market
share of CdTe is 5.4%, up from 4.7% in 2008. This technology’s
highest power conversion efficiency is 21%. The cell structure
includes glass substrate (around 2 mm), transparent conductor
layer, CdS buffer layer (50–150 nm), CdTe absorber and a metal
CdTe PV systems require less energy input in their production than
other commercial PV systems per unit electricity production. The
average CO2-eq/kWh is around 18 grams (cradle to gate). CdTe has the
fastest EPBT of all commercial PV technologies, which varies between
0.3 and 1.2 years.
Copper Indium Gallium Diselenide (CIGS) is a thin film solar cell
based on the copper indium diselenide (CIS) family of chalcopyrite
semiconductors. CIS and CIGS are often used interchangeably within the
CIS/CIGS community. The cell structure includes soda lime glass as the
substrate, Mo layer as the back contact, CIS/CIGS as the absorber
layer, cadmium sulfide (CdS) or Zn (S,OH)x as the buffer layer, and
ZnO:Al as the front contact. CIGS is approximately 1/100th the
thickness of conventional silicon solar cell technologies. Materials
necessary for assembly are readily available, and are less costly per
watt of solar cell. CIGS based solar devices resist performance
degradation over time and are highly stable in the field.
Reported global warming potential impacts of CIGS range from 20.5 –
58.8 grams CO2-eq/kWh of electricity generated for different solar
irradiation (1,700 to 2,200 kWh/m2/y) and power conversion efficiency
(7.8 – 9.12%). EPBT ranges from 0.2 to 1.4 years, while
harmonized value of EPBT was found 1.393 years. Toxicity is an
issue within the buffer layer of CIGS modules because it contains
cadmium and gallium. CIS modules do not contain any heavy
Impacts from third generation
Third-generation PVs are designed to combine the advantages of both
the first and second generation devices and they do not have
Shockley-Queisser limit, a theoretical limit for first and second
generation PV cells. The thickness of a third generation device is
less than 1 µm.
One emerging alternative and promising technology is based on an
organic-inorganic hybrid solar cell made of methylammonium lead halide
Perovskite PV cells have progressed rapidly over the past
few years and have become one of the most attractive areas for PV
research. The cell structure includes a metal back contact (which
can be made of Al, Au or Ag), a hole transfer layer (spiro-MeOTAD,
P3HT, PTAA, CuSCN, CuI, or NiO), and absorber layer (CH3NH3PbIxBr3-x,
CH3NH3PbIxCl3-x or CH3NH3PbI3), an electron transport layer (TiO, ZnO,
Al2O3 or SnO2) and a top contact layer (fluorine doped tin oxide or
tin doped indium oxide).
There are a limited number of published studies to address the
environmental impacts of perovskite solar cells. The major
environmental concern is the lead used in the absorber layer. Due to
the instability of perovskite cells lead may eventually be exposed to
fresh water during the use phase. These LCA studies looked at human
and ecotoxicity of perovskite solar cells and found they were
surprisingly low and may not be an environmental issue. Global
warming potential of perovskite PVs were found to be in the range of
24–1500 grams CO2-eq/kWh electricity production. Similarly, reported
EPBT of the published paper range from 0.2 to 15 years. The large
range of reported values highlight the uncertainties associated with
these studies. Celik et al. (2016) critically discussed the
assumptions made in perovskite PV LCA studies.
Two new promising thin film technologies are copper zinc tin sulfide
(Cu2ZnSnS4 or CZTS), zinc phosphide (Zn3P2) and single-walled
carbon nano-tubes (SWCNT). These thin films are currently only
produced in the lab but may be commercialized in the future. The
manufacturing of CZTS and (Zn3P2) processes are expected to be similar
to those of current thin film technologies of CIGS and CdTe,
respectively. While the absorber layer of SWCNT PV is expected to be
synthesized with CoMoCAT method. by Contrary to established thin
films such as CIGS and CdTe, CZTS, Zn3P2, and SWCNT PVs are made from
earth abundant, nontoxic materials and have the potential to produce
more electricity annually than the current worldwide
consumption. While CZTS and Zn3P2 offer good promise for these
reasons, the specific environmental implications of their commercial
production are not yet known.
Global warming potential of CZTS and
Zn3P2 were found 38 and 30 grams CO2-eq/kWh while their corresponding
EPBT were found 1.85 and 0.78 years, respectively. Overall, CdTe
and Zn3P2 have similar environmental impacts but can slightly
outperform CIGS and CZTS. Celik et al. performed the first LCA
study on environmental impacts of SWCNT PVs, including a
laboratory-made 1% efficient device and an aspirational 28% efficient
four-cell tandem device and interpreted the results by using mono-Si
as a reference point. the results show that compared to
monocrystalline Si (mono-Si), the environmental impacts from 1% SWCNT
was ∼18 times higher due mainly to the short lifetime of three
years. However, even with the same short lifetime, the 28% cell had
lower environmental impacts than mono-Si.
Organic and polymer photovoltaic (OPV) are a relatively new area of
research. The tradition OPV cell structure layers consist of a
semi-transparent electrode, electron blocking layer, tunnel junction,
holes blocking layer, electrode, with the sun hitting the transparent
electrode. OPV replaces silver with carbon as an electrode material
lowering manufacturing cost and making them more environmentally
friendly. OPV are flexible, low weight, and work well with roll-to
roll manufacturing for mass production. OPV uses "only abundant
elements coupled to an extremely low embodied energy through very low
processing temperatures using only ambient processing conditions on
simple printing equipment enabling energy pay-back times". Current
efficiencies range from 1–6.5%, however theoretical analyses
show promise beyond 10% efficiency.
Many different configurations of OPV exist using different materials
for each layer. OPV technology rivals existing PV technologies in
terms of EPBT even if they currently present a shorter operational
lifetime. A 2013 study analyzed 12 different configurations all with
2% efficiency, the EPBT ranged from 0.29–0.52 years for 1 m² of
PV. The average CO2-eq/kWh for OPV is 54.922 grams.
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.
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. As of 2017 power-purchase
agreement prices for solar farms below $0.05/kWh are common in the
United States and the lowest bids in several international countries
were about $0.03/kWh.
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.
Solar cell energy conversion efficiencies for commercially available
photovoltaics are around 14–22%. 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. 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
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, 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, particularly when the time of generation is included, as
electricity is worth more during the day than at night. 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. As time progresses, renewable energy technologies
generally get cheaper, while fossil fuels generally get more
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.
Price per watt
Price per watt history for conventional (c-Si) solar cells since 1977.
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.
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.
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.
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. For example,
SolarCity offered, the
first U.S. asset-backed security in the solar industry in 2013.
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
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
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 to $8,213 (analysis from
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.
See also: List of photovoltaics companies
Overall the manufacturing process of creating solar photovoltaics is
simple in that it does not require the culmination of many complex or
moving parts. Because of the solid state nature of PV systems they
often have relatively long lifetimes, anywhere from 10 to 30 years. In
order to increase electrical output of a PV system the manufacturer
must simply add more photovoltaic components and because of this
economies of scale are important for manufacturers as costs decrease
with increasing output.
While there are many types of PV systems known to be effective,
crystalline silicon PV accounted for around 90% of the worldwide
production of PV in 2013. Manufacturing silicon PV systems has several
steps. First, polysilicon is processed from mined quartz until it is
very pure (semi-conductor grade). This is melted down when small
amounts of Boron, a group III element, are added to make a p-type
semiconductor rich in electron holes. Typically using a seed crystal,
an ingot of this solution is grown from the liquid polycrystalline.
The ingot may also be cast in a mold. Wafers of this semiconductor
material are cut from the bulk material with wire saws, and then go
through surface etching before being cleaned. Next, the wafers are
placed into a phosphorus vapor deposition furnace which lays a very
thin layer of phosphorus, a group V element, which creates an N-type
semiconducting surface. To reduce energy losses an anti-reflective
coating is added to the surface, along with electrical contacts. After
finishing the cell, cells are connected via electrical circuit
according to the specific application and prepared for shipping and
Crystalline silicon photovoltaics are only one type of PV, and while
they represent the majority of solar cells produced currently there
are many new and promising technologies that have the potential to be
scaled up to meet future energy needs.
Another newer technology, thin-film PV, are manufactured by depositing
semiconducting layers on substrate in vacuum. The substrate is often
glass or stainless-steel, and these semiconducting layers are made of
many types of materials including cadmium telluride (CdTe), copper
indium diselenide (CIS), copper indium gallium diselenide (CIGS), and
amorphous silicon (a-Si). After being deposited onto the substrate the
semiconducting layers are separated and connected by electrical
circuit by laser-scribing. Thin-film photovoltaics now make up around
20% of the overall production of PV because of the reduced materials
requirements and cost to manufacture modules consisting of thin-films
as compared to silicon-based wafers.
Other emerging PV technologies include organic, dye-sensitized,
Perovskite photovoltaics. OPVs fall into the
thin-film category of manufacturing, and typically operate around the
12% efficiency range which is lower than the 12–21% typically seen
by silicon based PVs. Because organic photovoltaics require very high
purity and are relatively reactive they must be encapsulated which
vastly increases cost of manufacturing and meaning that they are not
feasible for large scale up. Dye-sensitized PVs are similar in
efficiency to OPVs but are significantly easier to manufacture.
However these dye-sensitized photovoltaics present storage problems
because the liquid electrolyte is toxic and can potentially permeate
the plastics used in the cell. Quantum dot solar cells are quantum dot
sensitized DSSCs and are solution processed meaning they are
potentially scalable, but currently they peak at 12% efficiency.
Perovskite solar cells are a very efficient solar energy converter and
have excellent optoelectric properties for photovoltaic purposes, but
they are expensive and difficult to manufacture.
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. 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. 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.
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
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.
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. 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.
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. Cuba is working to provide solar power
for areas that are off grid. More complex applications of
off-grid solar energy use include 3D printers.
RepRap 3D printers
have been solar powered with photovoltaic technology, 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.
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, parking meters, emergency telephones,
trash compactors, temporary traffic signs, charging
stations, and remote guard posts and signals.
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. 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. 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.
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. 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 and airplanes have been
demonstrated, with the most practical and likely of these being solar
cars. 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.
Part of Juno's solar array
Solar panels on spacecraft
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
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. Since then,
solar power has been used on missions ranging from the
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
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.
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. This abundance leads to
the suggestion that it will not be long before solar energy will
become the world's primary energy source. Additionally, solar
electric generation has the highest power density (global mean of
170 W/m2) among renewable energies.
Solar power is pollution-free during use, which enables it to cut down
on pollution when it is substituted for other energy sources. For
MIT estimated that 52,000 people per year die prematurely in
the U.S. from coal-fired power plant pollution and all but one of
these deaths could be prevented from using PV to replace
coal. Production end-wastes and emissions are manageable
using existing pollution controls. End-of-use recycling technologies
are under development and policies are being produced that
encourage recycling from producers.
PV installations can operate for 100 years or even more 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
Grid-connected solar electricity can be used locally thus reducing
transmission/distribution losses (transmission losses in the US were
approximately 7.2% in 1995).
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 and efficiencies are rapidly
rising while mass-production costs are rapidly falling.
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. Now known as PACE, Property Assessed Clean
Energy, 30 U.S. states have duplicated this solution.
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).
Wikimedia Commons has media related to Photovoltaics.
American Solar Energy Society
Anomalous photovoltaic effect
Copper in photovoltaic power generation
Cost of electricity by source
Electromotive force#Solar cell
List of photovoltaics companies
List of solar cell manufacturers
Quantum efficiency#Quantum efficiency of solar cells
Renewable energy commercialization
Solar module quality assurance
Solar photovoltaic monitoring
Solar thermal energy
Theory of solar cell
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SolarBankbility EU Project 09/2016. "Minimizing Technical Risks in
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