Photovoltaic array

File:Bp-solarmodul.JPG

Solar Panel (photovoltaic array)

US annual average solar energy received by a latitude tilt photovoltaic cell

A laundromat in California with solar hot water panels on the roof.

The solar panels on this small yacht at sea can charge the 12 V batteries at up to 9 Amps in full, direct sunlight.

A solar heater

Larger solar arrays can provide electricity to habitations in isolated, well-lighted areas.

A solar panel on top of a parking meter

Solar panels are devices for capturing the energy in sunlight. The term solar panel can be applied to either solar hot water panels (usually used for providing domestic hot water) or solar photovoltaic panels (providing electricity).

Current Development

Right now, countless corporations and institutions are developing ways to increase the practicality of solar power. While private companies conduct much of the research and development in this area, colleges and universities also work on solar-powered devices, especially solar-powered vehicles. Solar-powered cars have commonly appeared at many car and technology shows, and now solar boats are an interesting application of the technology. Colleges and Universities compete against each other for superiority in this field of technology. They meet in competitions such as the Solar Splash^[1]competition in North America, or the Frisian Nuon Solar Challenge^[2] in Europe.

In 2005 the most important issue with Solar Panels is the cost, which has been coming down to about $3-4 (US) a watt. Also grid tied systems are the largest growth area. In the USA, with incentives from States, power companies and in 2006 and 7 from the Federal government growth will continue to climb. Net-metering lets you get credit for any extra power you send back into the grid. Most is true net-metering with even prices for you to equal what you get charged, a few only give avoided cost at about 1/3 what they charge you.

In Germany you get paid 8 times what the power company charges you. That large premium has made a huge demand in solar panels for that area. As manufacturers increase production the cost will continue to drop in the years to come.

The price of silicon used for most panels is now being pressed and the price has increased. This has caused developers to start using other materials and thinner silicon to keep cost down. Renewable energy like solar PV gets less costly as we use and buy more.

Solar hot water

A solar water heater uses the sun's energy to heat a fluid, which is used to transfer the heat to a heat storage vessel. In the home, for example, sanitary hot water would be heated and stored in a hot water cylinder. Panels on the roof have an absorber plate to which fluid circulation tubes are attached. The absorber (usually coated with a dark selective coating) assures the conversion of the sun's radiation into heat, while fluid circulating through the tubes carries the heat away where it can be used or stored. The heated fluid is pumped to a heat exchanger (a coil in the storage vessel or an external heat exchanger) where it gives off its heat and is then circulated back to the panel to be reheated. This provides a simple and effective way of harnessing the sun's energy.

Solar photovoltaics

Solar photovoltaic panels contain arrays of solar cells that convert light into electricity. They are called solar after the sun or "Sol" because the sun is the most powerful source of the light available for use. The solar cells are sometimes called photovoltaic cells, photovoltaic meaning literally "light-electricity". Solar cells or PV cells rely on the photovoltaic effect to absorb the energy of the sun and cause current to flow between two oppositely charged layers.

On a bright day, the sun delivers about 1 kW/m² to the Earth's surface. Today's solar panels are known to have an average efficiency of 12 %. This would result in 120 W/m². However, not all days have bright suns and are fortunate enough to be blessed by such energy.

At middle northern latitudes, taking the daylight cycle and weather conditions into account, on average 100 W/m² in winter and 250 W/m² in summer reach the ground. With a conversion efficiency of about 12%, one can expect to obtain between 12 and 30 watts per square meter of solar cell. Accordingly, at the current $0.08/kWh, a square meter will generate up to $0.06 per 24 hr day, and a square kilometer (250 acres) would generate up to 30 MW, or $50,000/km²/day. For reference, the unpopulated Sahara desert is over 9 million km², with less cloud cover and better solar angle, giving closer to 50MW/km², or 450TW (terrawatt) total. The Earth's current energy consumption is near 12-13 TW at any given moment (including oil, gas, coal, nuclear, hydro.)

The real issue with solar panels is the capital cost, as shown at the net energy gain article, requiring up to over 7 years recovery period before any profit is made, out of a 40+ year useful life. In contrast, nuclear or coal plant recovers its capital cost in under a mere month, not considering the limited fuel supplies and thus fuel cost.

Use of solar PVs

Together with a backup battery, they have become routine in certain low-power applications, such as powering buoys or devices in remote areas or simply where connection to the electricity mains would be impractical.

The relatively high cost of purchase and installation still prohibits their use in large-scale power generation. Solar PV panels currently make up a very small portion of the world's electricity production.

In experimental form they have even been used to power automobiles in races such as the World solar challenge across Australia. Many yachts and land-vehicles use them to charge on-board batteries away from grid power. Large-scale incentive programs, offering financial incentives like the ability to sell excess electricity back to the public grid, have greatly accelerated the pace of solar PV installations in Spain, Germany, Japan, the United States and other countries.

Even with these incentives, the start-up costs associated with solar electric panels currently push their likely 'pay-back' period into decades rather than years in applications where conventional "grid" power is readily available. As fossil fuel energy costs climb, production experience and economies of scale reduce prices, and technological advances increase the efficiency of solar cells, this may not be true in the relatively near future. Many installations at this time are motivated by tax incentives and green sensibilities.

World solar power production

Total peak power of installed solar panels is around 2,600 MW as of the end of 2004^[3].

Installed PV Power as of the end of 2004 ^[4]

Country	PV Capacity
	Cumulative			Installed in 2004
	Off-grid PV [kW]	Grid-connected [kW]	Total [kW]	Total [kW]	Grid-tied [kW]
Australia	48,640	6,760	52,300	6,670	780
Austria	2,687	16,493	19,180	2,347	1,833
Canada	13,372	512	13,884	2,054	107
France	18,300	8,000	26,300	5,228	4,183
Germany	26,000	768,000	794,000	363,000	360,000
Italy	12,000	18,700	30,700	4,700	4,400
Japan	84,245	1,047,746	1,131,991	272,368	267,016
Korea	5,359	4,533	9,892	3,454	3,106
Mexico	18,172	10	18,182	1,041	0
Netherlands	4,769	44,310	49,079	3,162	3,071
Norway	6,813	75	6,888	273	0
Spain	14,000	23,000	37,000	10,000	8,460
Switzerland	3,100	20,000	23,100	2,100	2,000
United Kingdom	776	7,386	8,164	2,261	2,197
United States	189,600	175,600	365,200	90,000	62,000

Large PV power plants

World's largest PV power plants ^[5]

DC Peak Power	Location	Description	MWh/year
6.3 MW	Mühlhausen, Germany	57.600 solar modules	6,750 MWh
5 MW	Bürstadt, Germany	30,000 BP solar modules	4,200 MWh
5 MW	Espenhain, Germany	33,500 Shell solar modules	5,000 MWh
4.59 MW	Springerville, AZ, USA	34,980 BP solar modules	7,750 MWh
4 MW	Geiseltalsee, Merseburg, Germany	25,000 BP solar modules	3,400 MWh
4 MW	Gottelborn, Germany	50,000 solar modules (when completed)	8,200 MWh (when completed)
4 MW	Hemau, Germany	32,740 solar modules	3,900 MWh
3.9 MW	Rancho Seco, CA, USA	n.a.	n.a.
3.3 MW	Dingolfing, Germany	Solara, Sharp and Kyocera solar modules	3,050 MWh
3.3 MW	Serre, Italy	60,000 solar modules	n.a.

Cost of solar photovoltaic panels

Costs of photovoltaic panels seem, in 2005, to be about $1 to $2 per watt in ~400kW quantities. As production rates increase, costs are likely to continue to go down.

Installed, costs seem to be in the $3-$7 per watt range.

Theory and construction

See the solar cell article for a description of the conversion of light energy into electrical energy.

Crystalline silicon and gallium arsenide are typical choices of materials for solar cells. Gallium arsenide crystals are grown especially for photovoltaic use, but silicon crystals are available in less-expensive standard ingots, which are produced mainly for consumption in the microelectronics industry. Polycrystalline silicon has lower conversion efficiency but also lower cost.

When exposed to direct sunlight at 1 AU, a 6-centimeter diameter silicon cell can produce a current of about 0.5 ampere at 0.5 volt. Gallium arsenide is more efficient.

Crystalline ingots are sliced into wafer-thin disks, polished to remove slicing damage, dopants are introduced into the wafers, and metallic conductors are deposited onto each surface: a thin grid on the sun-facing side and usually a flat sheet on the other. Solar panels are constructed of these cells cut into appropriate shapes, protected from radiation and handling damage on the front surface by bonding on a cover glass, and cemented onto a substrate (either a rigid panel or a flexible blanket). Electrical connections are made in series-parallel to determine total output voltage. The cement and the substrate must be thermally conductive, because the cells heat up from absorbing infrared energy that is not converted to electricity. Since cell heating reduces the operating efficiency it is desirable to minimize the heating. The resulting assemblies are called solar panels or solar arrays.

A solar panel is a collection of solar cells. Although each solar cell provides a relatively small amount of power, many solar cells spread over a large area can provide enough power to be useful. To get the most power, solar panels have to be pointed directly at the sun.

It is claimed that if one fourth of the nation's pavement and buildings in cities alone were converted to incorporate solar panels, these could power the entire United States.

Solar panels on spacecraft

Probably the most successful use of solar panels is on spacecraft, including most spacecraft that orbit the Earth and Mars, and spacecraft going to other destinations in the inner solar system. In the outer solar system, the sunlight is too weak to produce sufficient power and radioisotope thermal generators are used.

Research is underway to develop solar power satellites: space-based solar plants — satellites with large arrays of photovoltaic cells that would beam the energy to Earth using microwaves or lasers. Japanese and European space agencies have announced plans to develop such power plants in the first quarter of the 21st century.

As opposed to chemical rockets, which are powered by a chemical reaction of the propellant, and uses the exhaust gases as reaction mass, some spacecraft propulsion methods have a method of expelling reaction mass powered by electricity. Either solar energy or nuclear energy is used. These methods typically have a higher specific impulse. The amount of reaction mass needed always grows exponentially with the delta-v to be produced, but more mildly if the specific impulse is high (but it should not be too high because for large specific impulse the power needed is proportional to it). With solar power the acceleration that can be produced is very low (much too low for a launch), but enduring. Typical burn times are months instead of minutes. The power the solar panel produces per kg, as an upper limit of the power the spacecraft has at its disposal per kg spacecraft (including solar panels) is an important factor. See also energy needed for propulsion methods.

Solar panels need to have a lot of surface area that can be pointed towards the Sun as the spacecraft moves. More exposed surface area means more electricity can be converted from light energy from the Sun. Sometimes, satellite scientists purposefully orient the solar panels to "off point," or out of direct alignment from the Sun. This happens if the batteries are completely charged and the amount of electricity needed is lower than the amount of electricity made. The extra power will just be vented by a shunt into space as heat.

Spacecraft are built so that the solar panels can be pivoted as the spacecraft moves. Thus, they can always stay in the direct path of the light rays no matter how the spacecraft is pointed. Spacecraft are usually designed with solar panels that can always be pointed at the Sun, even as the rest of the body of the spacecraft moves around, much as a tank turret can be aimed independently of where the tank is going. A tracking mechanism is often incorporated into the solar arrays to keep the array pointed towards the sun.

To date, solar power, other than for propulsion, has been practical for spacecraft operating no farther from the sun than the orbit of Mars. For example, Magellan, Mars Global Surveyor, and Mars Observer used solar power as did the Earth-orbiting, Hubble Space Telescope. For future missions, it is desirable to reduce solar array mass, and to increase the power generated per unit area. This will reduce overall spacecraft mass, and may make the operation of solar-powered spacecraft feasible at larger distances from the sun. The Rosetta space probe, launched March 2, 2004, will use solar panels as far as the orbit of Jupiter (5.25 AU); previously the furthest use was the Stardust spacecraft at 2 AU.

Solar power for propulsion is currently used on the European lunar mission SMART-1 with a Hall effect thruster.

Solar array mass could be reduced with thin-film photovoltaic cells, flexible blanket substrates, and composite support structures. Solar array efficiency could be improved by using new photovoltaic cell materials and solar concentrators that intensify the incident sunlight.

Photovoltaic concentrator solar arrays for primary spacecraft power are devices which intensify the sunlight on the photovoltaics. This design uses a flat lens, called a Fresnel lens, which takes a large area of sunlight and concentrates it onto a smaller spot. The same principle is used to start fires with a magnifying glass on a sunny day.

Solar concentrators put one of these lenses over every solar cell. This focuses light from the large concentrator area down to the smaller cell area. This allows the quantity of expensive solar cells to be reduced by the amount of concentration. Concentrators work best when there is a single source of light and the concentrator can be pointed right at it. This is ideal in space, where the Sun is a single light source. Solar cells are the most expensive part of solar arrays, and arrays are often a very expensive part of the spacecraft. This technology allows costs to be cut significantly due to the utilization of less material.

See also

• Electric vehicle
• Electric boat
• Solar fan

References

1. ^ "Welcome to SOLAR SPLASH". 2005-12-22.
2. ^ "Frisian Nuon Solar Challenge". 2005-12-22.
3. ^ Overview
4. ^ Country Information
5. ^ Solar Records

External links

• Leading photovoltaic panel manufacturer
• http://www.enf.cn Solar industry directory, news and government contracts
• http://www.tectosol.staticip.de/index_en.htm Solar electricity yield of a photovoltaic system
• Solar Panels Solar panels for marine, RV, & industrial use.
• http://www.solarbuzz.com Solarbuzz tracks the price of industrial solar panels
• http://www.solar-panels.cc More information on solar panels and how it works.
• http://www.californiasolarco.com/power-systems-photo-gallery.html Residential photovoltaic systems - photo gallery
• http://www.tucsonelectric.com/Company/News/PressReleases/ReleaseTemplate.asp?idRec=3 Tucson Electric Power describes the 2.4 MW array at Springerville
• http://news.nationalgeographic.com/news/2005/01/0114_050114_solarplastic.html nano breakthrough