Photovoltaics

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Nellis Solar Power Plant at Nellis Air Force Base in the USA. These panels track the sun in one
axis.

Photovoltaic system 'tree' in Styria, Austria

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into
direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic
power generation employs solar panels comprising a number of cells containing a photovoltaic
material. Materials presently used for photovoltaics include monocrystalline silicon,
polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium
selenide/sulfide.[1] Due to the growing demand for renewable energy sources, the manufacture of
solar cells and photovoltaic arrays has advanced considerably in recent years.[2][3][4]
As of 2010, solar photovoltaics generates electricity in more than 100 countries and, while yet
comprising a tiny fraction of the 4800 GW total global power-generating capacity from all
sources, is the fastest growing power-generation technology in the world. Between 2004 and
2009, grid-connected PV capacity increased at an annual average rate of 60 percent, to some 21
GW.[5] Such installations may be ground-mounted (and sometimes integrated with farming and
grazing)[6] or built into the roof or walls of a building, known as Building Integrated
Photovoltaics or BIPV for short.[7] Off-grid PV accounts for an additional 3–4 GW.[5]

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.[8] Net
metering and financial incentives, such as preferential feed-in tariffs for solar-generated
electricity, have supported solar PV installations in many countries.

Contents
[hide]

 1 Overview
 2 Current developments
 3 Applications
o 3.1 Power stations
o 3.2 In buildings
o 3.3 In transport
o 3.4 Standalone devices
o 3.5 Rural electrification
o 3.6 Solar roadways
o 3.7 Solar Power satellites
 4 Performance
o 4.1 Temperature
o 4.2 Optimum Orientation of Solar Panels
 5 Advantages
 6 Disadvantages
 7 See also
 8 Notes
 9 References

[edit] Overview
Solar cells produce 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 electricity. The photovoltaic effect refers to photons of light
knocking electrons into a higher state of energy to create electricity. 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.

Average solar irradiance, watts per square metre. Note that this is for a horizontal surface,
whereas solar panels are normally mounted at an angle and receive more energy per unit area.
The small black dots show the area of solar panels needed to generate all of the world's energy
using 8% efficient photovoltaics.

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. Although the selling price of modules is still too high to compete with grid
electricity in most places, significant financial incentives in Japan and then Germany, Italy and
France triggered a huge growth in demand, followed quickly by production. In 2008, Spain
installed 45% of all photovoltaics, but a change in law limiting the feed-in tariff is expected to
cause a precipitous drop in the rate of new installations there, from an extra 2500 MW in 2008 to
an expected additional 375 MW in 2009.[9]

A significant market has emerged in off-grid locations for solar-power-charged storage-battery

based solutions. These often provide the only electricity available.[10] The first commercial
installation of this kind was in 1966 on Ogami Island in Japan to transition Ogami Lighthouse
from gas torch to fully self-sufficient electrical power. Due to the growing demand for renewable
energy sources, the manufacture of solar cells and photovoltaic arrays has advanced dramatically
in recent years.[2][11][12]

Photovoltaic production has been increasing by an average of more than 20 percent each year
since 2002, making it the world’s fastest-growing energy technology.[13][14] At the end of 2009,
the cumulative global PV installations surpassed 21,000 megawatts.[14][15] Germany installed a
record 3,800 MW of solar PV in 2009.[16] Roughly 90% of this generating capacity consists of
grid-tied electrical systems. Such installations may be ground-mounted (and sometimes
integrated with farming and grazing) [17] or built into the roof or walls of a building, known as
Building Integrated Photovoltaics or BIPV for short.[18] Solar PV power stations today have
capacities ranging from 10-60 MW although proposed solar PV power stations will have a
capacity of 150 MW or more.[1]

World solar photovoltaic (PV) installations were 2.826 gigawatts peak (GWp) in 2007, and 5.95
gigawatts in 2008, a 110% increase.[19][20] The three leading countries (Germany, Japan and the
US) represent nearly 89% of the total worldwide PV installed capacity. According to Navigant
Consulting and Electronic Trend Publications, the estimated PV worldwide installations outlooks
of 2012 are 18.8GW and 12.3GW respectively. Notably, the manufacture of solar cells and
modules had expanded in recent years.

Germany installed a record 3,800 MW of solar PV in 2009; in contrast, the US installed about
500 MW in 2009. The previous record, 2,600 MW, was set by Spain in 2008. Germany was also
the fastest growing major PV market in the world from 2006 to 2007Industry observers speculate
that Germany could install more than 4,500 MW in 2009.[16][21] The German PV industry
generates over 10,000 jobs in production, distribution and installation. By the end of 2006, nearly
88% of all solar PV installations in the EU were in grid-tied applications in Germany.[2]
Photovoltaic power capacity is measured as maximum power output under standardized test
conditions (STC) in "Wp" (Watts peak).[22] 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.[23] Solar photovoltaic array capacity
factors are typically under 25%, which is lower than many other industrial sources of electricity.
[24]
Therefore the 2008 installed base peak output would have provided an average output of 3.04
GW (assuming 20% × 15,200 MWp). This represented 0.15 percent of global demand at the
time.[25]
The EPIA/Greenpeace Advanced Scenario shows that by the year 2030, PV systems could be
generating approximately 1,864 GW of electricity around the world. This means that, assuming a
serious commitment is made to energy efficiency, enough solar power would be produced
globally in twenty-five years’ time to satisfy the electricity needs of almost 14% of the world’s
population.[26]

Current developments

Map of solar electricity potential in Europe. Germany is the current leader in solar production.

Photovoltaic panels based on crystalline silicon modules are being partially replaced in the
market by panels that employ thin-film solar cells (CdTe[27] CIGS,[28] amorphous Si,[29]
microcrystalline Si), which are rapidly growing and are expected to account for 31 percent of the
global installed power by 2013[30]. Other developments include casting wafers instead of sawing,
[31]
, concentrator modules, 'Sliver' cells, and continuous printing processes. 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. By early
2006, the average cost per installed watt for a residential sized system was about USD 7.50 to
USD 9.50, including panels, inverters, mounts, and electrical items.[32]

In 2006 investors began offering free solar panel installation in return for a 25 year contract, or
Power Purchase Agreement, to purchase electricity at a fixed price, normally set at or below
current electric rates.[33][34] It is expected that by 2009 over 90% of commercial photovoltaics
installed in the United States will be installed using a power purchase agreement.[35] An
innovative financing arrangement is being tested in Berkeley, California, which adds an amount
to the property assessment to allow the city to pay for the installed panels up front, which the
homeowner pays for over a 20 year period at a rate equal to the annual electric bill savings, thus
allowing free installation for the homeowner at no cost to the city.[36]

The current market leader in solar panel efficiency (measured by energy conversion ratio) is
SunPower, a San Jose based company. Sunpower's cells have a conversion ratio of 24.2%, well
above the market average of 12-18%.[37] However, advances past this efficiency mark are being
pursued in academia and R&D labs with efficiencies of 42% achieved at the University of
Delaware in conjunction with DuPont by means of concentration of light[38] The highest
efficiencies achieved without concentration include Sharp Corporation at 35.8% using a
proprietary triple-junction manufacturing technology in 2009,[39] and Boeing Spectrolab (40.7%
also using a triple layer design). A March 2010 experimental demonstration of a design by a
Caltech group which has an absorption efficiency of 85% in sunlight and 95% at certain
wavelengths (it is claimed to have near perfect quantum efficiency).[40] However, absorption
efficiency should not be confused with the sunlight-to-electricity conversion efficiency.

Applications
Power stations

President Barack Obama speaks at the DeSoto Next Generation Solar Energy Center.
Main article: List of photovoltaic power stations

As of October 2009, the largest photovoltaic (PV) power plants in the world are the Olmedilla
Photovoltaic Park (Spain, 60 MW), the Strasskirchen Solar Park (Germany, 54 MW), the
Lieberose Photovoltaic Park (Germany, 53 MW), the Puertollano Photovoltaic Park (Spain, 50
MW), the Moura photovoltaic power station (Portugal, 46 MW), and the Waldpolenz Solar Park
(Germany, 40 MW).[41]

As of October 2009, the largest photovoltaic power plant in North America is the 25 MW
DeSoto Next Generation Solar Energy Center in Florida. The plant consists of over 90,000 solar
panels.[42]

World's largest photovoltaic (PV) power plants (40 MW or larger)[41]

Nominal
Name of PV power Country GW·h Capacity
Power Notes
plant /year factor
(MWp)
Olmedilla Siliken crystalline silicon modules.
Spain 55[43] 85[41] 0.16
Photovoltaic Park Completed September 2008
Strasskirchen Solar
Germany 54
Park
Lieberose Photovoltaic 700'000 First Solar CdTe modules,
[44][45] Germany 53 53[45] 0.11
Park opened 2009[46]
Puertollano 231'653 crystalline silicon modules,
Spain 47.6
Photovoltaic Park Suntech and Solaria, opened 2008
Moura photovoltaic
Portugal 46 93[47] 0.23 Completed December 2008
power station[47]
Kothen Solar Park Germany 45 2009
Finsterwalde Solar
Germany 41 2009
Park
550,000 First Solar thin-film CdTe
Waldpolenz Solar
Germany 40 40[49] 0.11 modules. Completed December
Park[48][49]
2008

Topaz Solar Farm is a proposed 550 MW solar photovoltaic power plant which is to be built
northwest of California Valley in the US at a cost of over $1 billion.[50] Built on 9.5 square miles
(25 km2) of ranchland,[51] the project would utilize thin-film PV panels designed and
manufactured by OptiSolar in Hayward and Sacramento. The project would deliver
approximately 1,100 gigawatt-hours (GW·h) annually of renewable energy. The project is
expected to begin construction in 2010,[51] begin power delivery in 2011, and be fully operational
by 2013.[52]

High Plains Ranch is a proposed 250 MW solar photovoltaic power plant which is to be built by
SunPower in the Carrizo Plain, northwest of California Valley.[52]

In buildings

Photovoltaic arrays are often associated with buildings: either integrated into them, mounted on
them or mounted nearby on the ground.

Arrays 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. In 2010, more than four-fifths
of the 9,000 MW of solar PV operating in Germany was installed on rooftops.[16]

Photovoltaic solar panels on a house roof.

Building-integrated photovoltaics (BIPV) are increasingly incorporated into new domestic and
industrial buildings as a principal or ancillary source of electrical power.[53] Typically, an array is
incorporated into the roof or walls of a building. Roof tiles with integrated PV cells are also
common.
The power output of photovoltaic systems for installation in buildings is usually described in
kilowatt-peak units (kWp).

In transport

Main article: Photovoltaics 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. A self-contained solar vehicle would have limited power and low utility, but a
solar-charged vehicle would allow use of solar power for transportation. Solar-powered cars
have been demonstrated.[54]

[edit] Standalone devices

Solar parking meter.

Until a decade or so ago, PV was used frequently to power calculators and novelty devices.
Improvements in integrated circuits and low power LCD 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
water pumps,[55] parking meters,[56] emergency telephones,[57] trash compactors,[58] temporary
traffic signs, and remote guard posts & signals.

Rural electrification

Developing countries where many villages are often more than five kilometers away from grid
power have begun 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 month's supply of kerosene.[59][60] Cuba is working to provide
solar power for areas that are off grid.[61] These are areas where the social costs and benefits offer
an excellent case for going solar though the lack of profitability could relegate such endeavors to
humanitarian goals.
Solar roadways

Main article: Solar roadway

A 45 mi (72 km) section of roadway in Idaho is being used to test the possibility of installing
solar panels into the road surface, as roads are generally unobstructed to the sun and represent
about the percentage of land area needed to replace other energy sources with solar power.[62]

Solar Power satellites

Main article: Solar power satellite

Design studies of large solar power collection satellites have been conducted for decades. The
idea was first proposed by Peter Glaser, then of Arthur D. Little Inc; NASA conducted a long
series of engineering and economic feasibility studies in the 1970s, and interest has revived in
first years of the 21st century.

From a practical economic viewpoint, the key issue for such satellites appears to be the launch
cost. Additional considerations will include developing space based assembly techniques, but
they seem to be less a hurdle than the capital cost. These will be reduced as photovoltaic cell
costs are reduced or alternatively efficiency increased.

Performance
Temperature

Generally, temperatures above room temperature reduce the performance of photovoltaics.[63]

Optimum Orientation of Solar Panels

For best performance, terrestrial PV systems aim to maximize the time they face the sun. Solar
trackers aim to 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.

Advantages
The 89 petawatts of sunlight reaching the Earth's surface is plentiful - almost 6,000 times more
than the 15 terawatts equivalent of average power consumed by humans.[64] Additionally, solar
electric generation has the highest power density (global mean of 170 W/m²) among renewable
energies.[64]

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.[65]
PV installations can operate for many years 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.

Solar electric generation is economically superior where grid connection or fuel transport is
difficult, costly or impossible. Long-standing examples include satellites, island communities,
remote locations and ocean vessels.

When grid-connected, solar electric generation replaces some or all of the highest-cost electricity
used during times of peak demand (in most climatic regions). This can reduce grid loading, and
can eliminate the need for local battery power to provide for use in times of darkness. These
features are enabled by net metering. Time-of-use net metering can be highly favorable, but
requires newer electronic metering, which may still be impractical for some users.

Grid-connected solar electricity can be used locally thus reducing transmission/distribution

losses (transmission losses in the US were approximately 7.2% in 1995).[66]

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 [67] and efficiencies are rapidly rising while mass-production
costs are rapidly falling.[68]

Disadvantages
Photovoltaics are costly to install. While the modules are often warranteed for upwards of 20
years, 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.[notes 1]

Solar electricity is seen to be expensive.[citation needed][notes 2]

Solar electricity is not produced at night and is much reduced in cloudy conditions. Therefore, a
storage or complementary power system is required.[notes 3]

Solar electricity production depends on the limited power density of the location's insolation.
Average daily output of a flat plate collector at latitude tilt in the contiguous US is 3-7
kilowatt·h/m²[notes 4][69][70][71] and on average lower in Europe.

Solar cells produce DC which must be converted to AC (using a grid tie inverter) when used in
existing distribution grids. This incurs an energy loss of 4-12%.[72]