Photovoltaic 'tree' in Styria, Austria

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Photovoltaics (PV) is the field of technology and research related to the application of solar cells for energy by converting sun energy (sunlight, including sun ultra violet radiation) directly into electricity. Due to the growing demand for clean sources of energy, the manufacture of solar cells and photovoltaic arrays has expanded dramatically in recent years.^[1][2][3]

Photovoltaic production has been doubling every 2 years, increasing by an average of 48 percent each year since 2002, making it the world’s fastest-growing energy technology.^[4] At the end of 2008, the cumulative global PV installations reached 15,200 megawatts.^[5][6] 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) ^[7] or built into the roof or walls of a building, known as Building Integrated Photovoltaics or BIPV for short.^[8]

Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries including Australia, Germany, Israel,^[9] Japan, and the United States.^[1]

Overview

Photovoltaic cells produce electricity directly from sunlight

Average solar irradiance, watts per square metre. Note that this is for a horizontal surface, whereas solar panels are normally propped up 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.

Map of solar electricity potential in Europe

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 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, roadside emergency telephones, remote sensing, and cathodic protection of pipelines.

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 installations there, from 2500 MW in 2008 to 375 MW in 2009.^[10]

Perhaps not unexpectedly, a significant market has emerged in off-grid locations for solar-power-charged storage-battery based solutions. These often provide the only electricity available.^[11]

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.^[12]

Current development

The most important issue with solar panels is capital cost (installation and materials). Newer alternatives to standard crystalline silicon modules including casting wafers instead of sawing,^[13] thin film (CdTe^[14] CIGS,^[15] amorphous Si,^[16] microcrystalline Si), 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.^[17]

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.^[18][19] It is expected that by 2009 over 90% of commercial photovoltaics installed in the United States will be installed using a power purchase agreement.^[20] 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.^[21]

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 23.4%, well above the market average of 12-18%. 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.^[22]

Worldwide installed totals

Wikinews has related news: PV Taiwan 2007 starts with photovoltaic solutions and applications PV Taiwan 2007: ITRI Taiwan awards winners of Jinyi Award and shows the solutions on photovoltaic industry

World solar photovoltaic (PV) installations were 2.826 gigawatts peak (GWp) in 2007, and 5.95 gigawatts in 2008, a 110% increase.^[23][24] The three leading countries (Germany, Japan and the US) represent nearly 89% of the total worldwide PV installed capacity.

Germany was the fastest growing major PV market in the world from 2006 to 2007. By 2008, 5.337 GWp of PV was installed, or 35% of the world total.^[6] 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.^[1] Photovoltaic power capacity is measured as maximum power output under standardized test conditions (STC) in "Wp" (Watts peak).^[25] 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.^[26] Solar photovoltaic array capacity factors are typically under 25%, which is lower than many other industrial sources of electricity.^[27] 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.^[28]

Produced, Installed & Total Photovoltaic Peak Power Capacity (MWp) as of the end of 2007

Country or Region Report Nat. Int.	off grid Δ	on grid Δ	Installed 2007	off grid Σ	on grid Σ	Total 2007	Wp/capita Total	Module Price €/Wp	kW·h/kWp·yr Insolation	Feed-in Tariff EU¢/kW·h
Germany[29][30]	35	1,100	1,135	35	3,827	3,862	46.8	04.0 4.0–5.3	1000 1,000–1,300[31]	51.8 51.8–56.8
Japan[30][32]	1.562	208.8	210.4	90.15	1,829	1,919	15	02.96 2.96	1200 1,200–1,600	Ended(2005)
United States[30][33]	55	151.5	206.5	325	505.5	830.5	2.8	02.98 2.98	0900 900–2,150[31]	01.2 1.2–31.04(CA)
Spain ?[30]	22	490	512	29.8	625.2	655	15.1	03.0 3.0–4.5	1600 1,600–2,200	18.38 18.38–44.04
Italy[30][34]	0.3	69.9	70.2	13.1	107.1	120.2	2.1	03.2 3.2–3.6	1400 1,400–2,200	36.0 36.0–49.0
Australia[30][35]	5.91	6.28	12.19	66.45	16.04	82.49	4.1	04.5 4.5–5.4	1450 1,450–2,902[36]	0 0–26.4(SA'08)
South Korea[30][37]	0	42.87	42.87	5.943	71.66	77.60	1.6	03.50 3.50–3.84	1500 1,500–1,600	56.5 56.5–59.3
France[30][38]	0.993	30.31	31.30	22.55	52.68	75.23	1.2	03.2 3.2–5.1	1100 1,100–2,000	30.0 30.0–55.0
Netherlands[30][39]	0.582	1.023	1.605	5.3	48	53.3	3.3	03.3 3.3–4.5	1000 1,000–1,200	01.21 1.21–9.7
Switzerland[30][40]	0.2	6.3	6.5	3.6	32.6	36.2	4.9	03.18 3.18–3.30	1200 1,200–2,000	09.53 9.53–50.8
Austria ?[30]	0.055	2.061	2.116	3.224	24.48	27.70	3.4	03.6 3.6–4.3	1200 1,200–2,000	>0
Canada[30][41]	3.888	1.403	5.291	22.86	2.911	25.78	0.8	03.76 3.76	0900 900–1,750	0 0–29.48(ON)
Mexico ?[30]	0.869	0.15	1.019	20.45	0.3	20.75	0.2	05.44 5.44–6.42	1700 1,700–2,600	None
United Kingdom[30][42]	0.16	3.65	3.81	1.47	16.62	18.09	0.3	03.67 3.67–5.72	0900 900–1,300	0 0–11.74(exprt)
Portugal ?[43]	0.2	14.25	14.45	2.841	15.03	17.87	1.7		1600 1,600–2,200
Norway[30][44]	0.32	0.004	0.324	7.86	0.132	7.992	1.7	11.2 11.2	0800 800–950	None
Sweden[30][45]	0.271	1.121	1.392	4.566	1.676	6.242	0.7	03.24 3.24–7.02	0900 900–1,050	None
Denmark[30][46]	0.05	0.125	0.175	0.385	2.69	3.075	0.6	05.36 5.36–8.04	0900 900–1,100	None
Israel[30][47]	0.5	0	0.5	1.794	0.025	1.819	0.3	04.3 4.3	2200 2,200–2,400	13.13 13.13–16.40
World (Total of countries listed)	127.9	2,130	2,258	662.3	7,178	7,841		02.5 2.5–11.2	0800 800–2,902	0 0–59.3
Country or Region Report Nat. Int.	off grid Δ	on grid Δ	Installed 2007	off grid Σ	on grid Σ	Total 2007	Wp/capita Total	Module Price €/Wp	kW·h/kWp·yr Insolation	Feed-in Tariff EU¢/kW·h

Notes: Off grid refers to photovoltaics which are not grid connected. On grid means connected to the local electricity grid. Δ means the amount installed during the previous year. Σ means the total amount installed. Wp/capita refers to the ratio of total installed capacity divided by total population, or total installed Wp per person. Module price is average installed price, in Euros. kW·h/kWp·yr indicates the range of insolation to be expected. While National Report(s) may be cited as source(s) within an International Report, any contradictions in data are resolved by using only the most recent report's data. Exchange rates represent the 2006 annual average of daily rates (OECD Main Economic Indicators June 2007).
Module Price: Lowest:2.5 EUR/Wp^[30] (2.83 USD/Wp^[48]) in Germany 2003. Uncited insolation data is from maps dating 1991-1995.
PV Power (2007-June)^[43][49] IEA PVPS website.

Applications

Power stations

Solar array at Nellis Air Force Base. These panels track the sun in one axis.

As of April 2009, the largest photovoltaic (PV) power plants in the world are the (Spain, 60 MW), the (Spain, 50 MW), the Moura photovoltaic power station (Portugal, 46 MW), and the Waldpolenz Solar Park (Germany, 40 MW).^[50]

The 14 MW Nellis Solar Power Plant is the largest solar photovoltaic system in North America,^[51] and is located within Nellis Air Force Base in Clark County, Nevada, on the northeast side of Las Vegas. The Nellis solar energy system will generate in excess of 25 million kilowatt-hours of electricity annually and supply more than 25 percent of the power used at the base.^[52]

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.^[56] Built on 9.5 square miles (25 km²) of ranchland,^[57] 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,^[57] begin power delivery in 2011, and be fully operational by 2013.^[58]

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.^[58]

In buildings

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,^[59] and are one of the fastest growing segments of the photovoltaic industry.^[60] Typically, an array is incorporated into the roof or walls of a building, and roof tiles with integrated PV cells can now be purchased. Arrays can also be retrofitted into existing buildings; in this case they are usually fitted on top of the existing roof structure. Alternatively, an array can be located separately from the building but connected by cable to supply power for the building.

Where a building is at a considerable distance from the public electricity supply (or grid) - in remote or mountainous areas – PV may be the preferred possibility for generating electricity, or PV may be used together with wind, diesel generators and/or hydroelectric power. In such off-grid circumstances batteries are usually used to store the electric power.

In locations near the grid, however, feeding the grid using PV panels is more practical, and leads to optimum use of the investment in the photovoltaic system. This requires both regulatory and commercial preparation, including net-metering and feed-in agreements. To provide for possible power failure, some grid tied systems are set up to allow local use disconnected from the grid. Most photovoltaics are grid connected. In the event the grid fails, the local system must not feed the grid to prevent the possible creation of dangerous islanding.

The power output of photovoltaic systems for installation in buildings is usually described in kilowatt-peak units (kWp).

In transport

PV has traditionally been used for auxiliary 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. Recent advances in solar race cars, however, have produced cars that with little changes could be used for transportation.^[61]

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 parking meters,^[62] emergency telephones,^[63] 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.^[64][65] Cuba is working to provide solar power for areas that are off grid.^[66] 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

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.^[67]

Performance

Temperature

Generally, temperatures above room temperature reduce the performance of photovoltaics.^[68]

Optimum Orientation of Solar Panels

For best performance, 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 and winter.

Economics

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US average daily solar energy insolation received by a latitude tilt photovoltaic cell.

In photovoltaics, the solar value added chain is the set of steps from sand or raw silicon to the completed solar module and photovoltaic system completion and installation^[69].

Power costs

The PV industry is beginning to adopt levelized cost of energy (LCOE) as the unit of cost. For a 10 MW plant in Phoenix, AZ, the LCOE is estimated at $0.15 to 0.22/kWh in 2005.^[70]

The table below is a pure mathematical calculation. It illustrates the calculated total cost in US cents per kilowatt-hour of electricity generated by a photovoltaic system as function of the investment cost and the efficiency, assuming some accounting parameters such as cost of capital and depreciation period. The row headings on the left show the total cost, per peak kilowatt (kWp), of a photovoltaic installation. The column headings across the top refer to the annual energy output in kilowatt-hours expected from each installed peak kilowatt. This varies by geographic region because the average insolation depends on the average cloudiness and the thickness of atmosphere traversed by the sunlight. It also depends on the path of the sun relative to the panel and the horizon.

Panels can be mounted at an angle based on latitude,^[71] or solar tracking can be utilized to access even more perpendicular sunlight, thereby raising the total energy output. The calculated values in the table reflect the total cost in cents per kilowatt-hour produced. They assume a 10% total capital cost (for instance 4% interest rate, 1% operating and maintenance cost, and depreciation of the capital outlay over 20 years).

Physicists have claimed that recent technological developments bring the cost of solar energy more in parity with that of fossil fuels. In 2007, David Faiman, the director of the Ben-Gurion National Solar Energy Center of Israel, announced that the Center had entered into a project with Zenith Solar to create a home solar energy system that uses a 10 square meter reflector dish.^[72] In testing, the concentrated solar technology proved to be up to five times more cost effective than standard flat photovoltaic silicon panels, which would make it almost the same cost as oil and natural gas.^[73] A prototype ready for commercialization achieved a concentration of solar energy that was more than 1,000 times greater than standard flat panels.^[74]

Grid parity

Grid parity, the point at which photovoltaic electricity is equal to or cheaper than grid power, is achieved first in areas with abundant sun and high costs for electricity such as in California and Japan.^[75]

Grid parity has been reached in Hawaii and other islands that otherwise use fossil fuel (diesel fuel) to produce electricity, and most of the US is expected to reach grid parity by 2015.^[76][77]

General Electric's Chief Engineer predicts grid parity without subsidies in sunny parts of the United States by around 2015. Other companies predict an earlier date:^[78] the cost of solar power will be below grid parity for more than half of residential customers and 10% of commercial customers in the OECD, as long as grid electricity prices do not decrease through 2010.^[79]

The fully-loaded cost (cost not price) of solar electricity is $0.25/kWh or less in most of the OECD countries. By late 2011, the fully-loaded cost is likely to fall below $0.15/kWh for most of the OECD and reach $0.10/kWh in sunnier regions. These cost levels are driving three emerging trends:^[79]

1. vertical integration of the supply chain;
2. origination of power purchase agreements (PPAs) by solar power companies;
3. unexpected risk for traditional power generation companies, grid operators and wind turbine manufacturers.

Abengoa Solar has announced the award of two R&D projects in the field of Concentrating Solar Power (CSP) by the US Department of Energy that total over $14 million. The goal of the DOE R&D program, working in collaboration with partners such as Abengoa Solar, is to develop CSP technologies that are competitive with conventional energy sources (grid parity) by 2015.^[80] Concentrating photovoltaics (CPV) could reach grid parity in 2011.

Financial incentives

The political purpose of incentive policies for PV is to facilitate an initial small-scale deployment to begin to grow the industry, even where the cost of PV is significantly above grid parity, to allow the industry to achieve the economies of scale necessary to reach grid parity. The policies are implemented to promote national energy independence, high tech job creation and reduction of CO₂ emissions.

Three incentive mechanisms are used (often in combination):

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

With investment subsidies, the financial burden falls upon the taxpayer, while with feed-in tariffs the extra cost is distributed across the utilities' customer bases. While the investment subsidy may be simpler to administer, the main argument in favour of feed-in tariffs is the encouragement of quality. Investment subsidies are paid out as a function of the nameplate capacity of the installed system and are independent of its actual power yield over time, thus rewarding the overstatement of power and tolerating poor durability and maintenance. Some electric companies offer rebates to their customers, such as Austin Energy in Texas, which offers $4.50/watt installed up to $13,500.^[81]

With feed-in tariffs, the financial burden falls upon the consumer. They reward the number of kilowatt-hours produced over a long period of time, but because the rate is set by the authorities, it may result in perceived overpayment. The price paid per kilowatt-hour under a feed-in tariff exceeds the price of grid electricity. Net metering refers to the case where the price paid by the utility is the same as the price charged. Net metering is particularly important because it can be done with no changes to standard electricity meters, which accurately measure power in both directions and automatically report the difference, and because it allows homeowners and businesses to generate electricity at a different time from consumption, effectively using the grid as a giant storage battery. As more photovoltaics are used ultimately storage will need to be provided, normally in the form of pumped hydro-storage. Normally with net metering deficits are billed each month, while surpluses are rolled over to the following month and paid annually.

Where price setting by supply and demand is preferred, RECs can be used. In this mechanism, a renewable energy production or consumption target is set, and the consumer or producer is obliged to purchase renewable energy from whoever provides it the most competitively. The producer is paid via an REC. In principle this system delivers the cheapest renewable energy, since the lowest bidder will win. However, uncertainties about the future value of energy produced are a brake on investment in capacity, and the higher risk increases the cost of capital borrowed.

The Japanese government through its Ministry of International Trade and Industry ran a successful programme of subsidies from 1994 to 2003. By the end of 2004, Japan led the world in installed PV capacity with over 1.1 GW.^[82]

In 2004, the German government introduced the first large-scale feed-in tariff system, under a law known as the 'EEG' (Erneuerbare Energien Gesetz) which resulted in explosive growth of PV installations in Germany. At the outset the FIT was over 3x the retail price or 8x the industrial price. The principle behind the German system is a 20 year flat rate contract. The value of new contracts is programmed to decrease each year, in order to encourage the industry to pass on lower costs to the end users. The programme has been more successful than expected with over 1GW installed in 2006, and political pressure is mounting to decrease the tariff to lessen the future burden on consumers.

Subsequently Spain, Italy, Greece and France introduced feed-in tariffs. None have replicated the programmed decrease of FIT in new contracts though, making the German incentive relatively less and less attractive compared to other countries. The French FIT offers a uniquely high premium (EUR 0.55/kWh) for building integrated systems. California, Greece, France and Italy have 30-50% more insolation than Germany making them financially more attractive.

In 2006 California approved the 'California Solar Initiative', offering a choice of investment subsidies or FIT for small and medium systems and a FIT for large systems. The small-system FIT of $0.39 per kWh (far less than EU countries) expires in just 5 years, and the alternate "EPBB" residential investment incentive is modest, averaging perhaps 20% of cost. All California incentives are scheduled to decrease in the future depending as a function of the amount of PV capacity installed.

At the end of 2006, the Ontario Power Authority (Canada) began its Standard Offer Program, the first in North America for small renewable projects (10MW or less). This guarantees a fixed price of $0.42 CDN per kWh over a period of twenty years. Unlike net metering, all the electricity produced is sold to the OPA at the SOP rate. The generator then purchases any needed electricity at the current prevailing rate (e.g., $0.055 per kWh). The difference should cover all the costs of installation and operation over the life of the contract.

The price per kilowatt hour or per peak kilowatt of the FIT or investment subsidies is only one of three factors that stimulate the installation of PV. The other two factors are insolation (the more sunshine, the less capital is needed for a given power output) and administrative ease of obtaining permits and contracts.

Unfortunately the complexity of approvals in California, Spain and Italy has prevented comparable growth to Germany even though the return on investment is better.

In some countries, additional incentives are offered for BIPV compared to stand alone PV.

• France + EUR 0.25/kWh (EUR 0.30 + 0.25 = 0.55/kWh total)
• Italy + EUR 0.04-0.09 kWh
• Germany + EUR 0.05/kWh (facades only)

Investment

There is an organized by EPIA.^[83]

Environmental impacts

Unlike fossil fuel based technologies, solar power does not lead to any harmful emissions during operation, but the production of the panels leads to some amount of pollution. This is often referred to as the energy input to output ratio. In some analysis, if the energy input to produce it is higher than the output it produces it can be considered environmentally more harmful than beneficial. Also, placement of photovoltaics affects the environment. If they are located where photosynthesizing plants would normally grow, they simply substitute one potentially renewable resource (biomass) for another. It should be noted, however, that the biomass cycle converts solar radiation energy to chemical energy ( with significantly less efficiency than photovoltaic cells alone). And if they are placed on the sides of buildings (such as in Manchester) or fences, or rooftops (as long as plants would not normally be placed there), or in the desert they are purely additive to the renewable power base.^{[citations needed]}

Greenhouse gases

Life cycle greenhouse gas emissions are now in the range of 25-32 g/kWh and this could decrease to 15 g/kWh in the future.^[84] For comparison, a combined cycle gas-fired power plant emits some 400 g/kWh and a coal-fired power plant 915 g/kWh and with carbon capture and storage some 200 g/kWh. Only nuclear power and wind are better, emitting 6-25 g/kWh and 11 g/kWh on average. Using renewable energy sources in manufacturing and transportation would further drop carbon emissions.

Cadmium

One issue that has often raised concerns is the use of cadmium in cadmium telluride solar cells (CdTe is only used in a few types of PV panels). Cadmium in its metallic form is a toxic substance that has the tendency to accumulate in ecological food chains. The amount of cadmium used in thin-film PV modules is relatively small (5-10 g/m²) and with proper emission control techniques in place the cadmium emissions from module production can be almost zero. Current PV technologies lead to cadmium emissions of 0.3-0.9 microgram/kWh over the whole life-cycle.^[84] Most of these emissions actually arise through the use of coal power for the manufacturing of the modules, and coal and lignite combustion leads to much higher emissions of cadmium. Life-cycle cadmium emissions from coal is 3.1 microgram/kWh, lignite 6.2, and natural gas 0.2 microgram/kWh.

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

Energy payback time and energy returned on energy invested

The energy payback time is the time required to produce an amount of energy as great as what was consumed during production. The energy payback time is determined from a life cycle analysis of energy.

Another key indicator of environmental performance, tightly related to the energy payback time, is the ratio of electricity generated divided by the energy required to build and maintain the equipment. This ratio is called the energy returned on energy invested (EROEI). Of course, little is gained if it takes as much energy to produce the modules as they produce in their lifetimes. This should not be confused with the economic return on investment, which varies according to local energy prices, subsidies available and metering techniques.

Life-cycle analyses show that the energy intensity of typical solar photovoltaic technologies is rapidly evolving. In 2000 the energy payback time was estimated as 8 to 11 years^[85], but more recent studies suggest that technological progress has reduced this to 1.5 to 3.5 years for crystalline silicon PV systems ^[84].

Thin film technologies now have energy pay-back times in the range of 1-1.5 years (S.Europe).^[84] With lifetimes of such systems of at least 30 years, the EROEI is in the range of 10 to 30. They thus generate enough energy over their lifetimes to reproduce themselves many times (6-31 reproductions, the EROEI is a bit lower) depending on what type of material, balance of system (or BOS), and the geographic location of the system.^[86]

Advantages

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• The 89 petawatts of sunlight reaching the Earth's surface is plentiful - almost 6,000 times more than the 15 terawatts of average power consumed by humans.^[87] Additionally, solar electric generation has the highest power density (global mean of 170 W/m²) among renewable energies.^[87]
• 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.^[88]
• Facilities can operate with little maintenance or intervention after initial setup.
• Solar electric generation is economically superior where grid connection or fuel transport is difficult, costly or impossible. Examples include satellites, island communities, remote locations and ocean vessels.
• When grid-connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is enabled by net metering. Time-of-use net metering can be highly favorable, but requires newer electronic metering, and tends to be impractical for small users.
• Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses (transmission losses were approximately 7.2% in 1995).^[89]
• Once the initial capital cost of building a solar power plant has been spent, operating costs are extremely low compared to existing power technologies.
• Compared to fossil and nuclear energy sources, very little research-money has been invested in the development of solar cells, so there is much room for improvement. Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40% and efficiencies are rapidly rising while mass production costs are rapidly falling.^[90]

Disadvantages

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• A major drawback is the sheer volume of land required to house a solar power generation plant. The 550MW California plant that is planned requires 9.5 square miles. Many areas of the country could not find this amount of unused land or assemble the large number of parcels required for this type of project. At the same time, photovoltaics take up no land at all when installed on existing rooftops or on land not otherwise used, such as decommissioned coal pits or in deserts.
• Depending on the cost of the installation and local electric rates the payback can be 14–20 years. While the modules are warranted for 20 years, the investment is mostly lost if you move. The city of Berkeley has come up with 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.^[91]
• Solar electricity is expensive. Once a PV system is installed it will produce electricity for the same cost until the inverter needs replacing (about 12 years). Current utility rates have increased every year for the past 20 years and with the increasing pressure on carbon reduction the rate will increase more aggressively. This increase will (in the long run) easily offset the increased cost at installation but the timetable for payback is too long for most.
• Solar electricity is not available at night and is less available in cloudy weather conditions from conventional silicon based technologies. Therefore, a storage or complementary power system is required. However, the use of germanium in amorphous silicon-germanium thin film solar cells provides residual power generating capacity at night due to background infrared radiation. Fortunately, most power consumption is during the day, so solar does not need to be stored at all as long as it offsets peak and "shoulder" consumption.
• Limited power density: Average daily insolation (output of a flat plate collector at latitude tilt) in the contiguous US is 3-7 kilowatt·h/m²^[92][93][94] and on average lower in Europe.
• Solar cells produce DC which must be converted to AC (using a grid tie inverter) when used in currently existing distribution grids. This incurs an energy loss of 4-12%.^[95]

Photovoltaics companies

See also