Photovoltaic power as an alternative to coal
Photovoltaics (PVs) are arrays of cells containing a solar photovoltaic material that converts solar radiation into direct current electricity. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, microcrystalline silicon, cadmium telluride, and copper indium selenide/sulfide.
Photovoltaic production has been doubling every 2 years, increasing by an average of 48 percent annually since 2002. At the end of 2008, the cumulative global PV installations reached 15,200 megawatts. Roughly 90% of this generating capacity consists of grid-tied electrical systems. Solar PV power stations today have capacities ranging from 10-60 MW, although proposed solar PV power stations would have a capacity of 150 MW or more.
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. Policy can also drive down costs: net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in countries such as Germany.
Google Clean Energy 2030 advocates that solar energy, both photovoltaic (PV) and concentrating solar power (CSP), could grow from about 1 GW today to 250 GW by 2030, generating 12% of demand. Concentrated solar power systems use lenses or mirrors to focus a large area of sunlight onto a small area. Electrical power is produced when the concentrated light is directed onto photovoltaic surfaces.
The solar cells used in calculators and satellites are called photovoltaic (PV) cells (photo meaning "light" and voltaic meaning "electricity"), and convert sunlight directly into electricity. Photovoltaic cells are made of special materials called semiconductors such as silicon, which is currently used most commonly. When light strikes the cell, a certain portion of it is absorbed within the semiconductor material, and the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. In short, the photovoltaic effect refers to photons of light knocking electrons into a higher state of energy to create electricity.
PV cells have one or more electric field that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, the current can be drawn off for external use. This current, together with the cell's voltage (its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.
Solar cells produce direct current (DC) 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 alternating current (AC). There is a smaller market for off-grid power for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, cathodic protection of pipelines, and microgeneration.
When more power is required than a single cell can deliver, cells are electrically connected together to form photovoltaic modules. A module is a group of cells connected electrically and packaged into a frame, more commonly known as a solar panel, which can then be grouped into larger solar arrays. 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.
A significant market has emerged in off-grid locations for solar-power-charged storage-battery based solutions. These often provide the only electricity available. 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.
How Solar Panels Work
Silicon has special chemical properties that make it useful for solar panels. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells - which hold two and eight electrons respectively - are completely full. The third shell has four electrons, but can hold eight, leaving room for four more electrons. If one silicon atom contacts another silicon atom, each receives the other atom's four electrons, creating a strong bond. There is no positive or negative charge because the eight electrons satisfy the atoms' needs. Silicon atoms can combine for years to result in a large piece of pure silicon. This material is used to form the plates of solar panels.
By itself pure crystalline silicon is a poor conductor of electricity because none of its electrons are free to move about, unlike the electrons in more optimum conductors like copper. To address this issue, other atoms are purposefully mixed in with the silicon atoms, such as phosphorous, which has five electrons in its outer shell rather than four.  When phosphorous bonds with silicon, there is an extra free electron; it can't leave, because it is bonded to the other phosphorus atoms even though it isn't needed by the silicon. This new silicon/phosphorus plate is therefore considered to be negatively charged.
In order for electricity to flow, a positive charge must also be created. This is achieved in solar panels by combining silicon with an element such as boron, which only has three electrons in its outer shell. A silicon/boron plate therefore still has one spot left for another electron, meaning the plate has a positive charge.
The positive and negative plates are sandwiched together in solar panels, with conductive wires running between them. When the negative plates of solar cells are pointed at a proper angle to the sun, photons bombard the silicon/phosphorus atoms, eventually knocking the extra 9th electron off of the outer ring. This electron doesn't remain free for long - the positive silicon/boron plate draws it into the open spot on its own outer band. As the sun's photons break off more electrons, electricity is generated.
Researchers and companies such as NanoSolar are looking into nanotechnology to increase the absorption rate of photovoltaics.
World solar photovoltaic (PV) installations were 2.826 gigawatts peak (GWp) in 2007, and 5.95 gigawatts in 2008, a 110% increase.
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. 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.
As of May 2010, 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). The largest photovoltaic power plant in North America is the 25 MW DeSoto Next Generation Solar Energy Center in Florida, which consists of over 90,000 solar panels.
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. Built on 9.5 square miles (25 km2) of ranchland, 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, begin power delivery in 2011, and be fully operational by 2013.
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.
Photovoltaic versus coal
Currently, coal power is seen as cheaper than renewable sources of power like solar. The Standard and Poors chart cited in Khosla Venture (KV)'s "The War on Coal: Think Outside the (Coal) Pits 2007 Report" lists the average baseline cost for pulverized coal at 5.8 cents per kilowatt hour (c/kwh), or the more realistic cost of coal power generated from Powder River Basin (PRB) coal, a coal which usually has fewer contaminants and therefore somewhat bypasses the sulfur caps on coal-fired generation, is priced at 6.8 c/kwh. By comparison, the same chart lists wind at 7.1 c/kwh (while noting shortages and energy transportation factors are not included) and concentrated solar (CSP) at between 7 and 11 c/kwh.
The KV report goes on to suggest that the price of coal-fired power is going up due to a variety of reasons and is poised to do so even further, due to pending legislation to reduce carbon emissions from power generation. The report calculates that this alone is likely to drive the cost of coal generation to be within the same range as renewables: 7.9 c/kwh for pulverized coal, 8.4 c/kwh with Powder River Basin coal.
Other countries show that energy policy has a significant effect on cost. Although the selling price of solar panel 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 due to feed-in tariff policies (selling electricity back to the grid).
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.
In 2006, U.S. 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. An innovative financing arrangement is being tested in Berkeley, California, called Berkeley First, 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.
Photovoltaic versus nuclear
A 2010 report by NC Warn, Solar and Nuclear Costs — The Historic Crossover: Solar Energy is Now the Better Buy found that solar photovoltaic system costs have fallen steadily for decades and are projected to fall further over the next 10 years, while projected costs for construction of new nuclear plants have risen steadily over the last decade, and continue to rise. Looking at North Carolina, the report finds that electricity from new solar installations is cheaper than electricity from proposed new nuclear plants, making planned nuclear projects for N.C. an unwise decision out of line with state policy, as state law requires that the development of the electricity system follow a “least-cost” path with available resources added as necessary. Less expensive resources are to be added first, followed by more expensive ones, provided that system reliability is maintained. Energy efficiency, wind power, solar hot water, and cogeneration (combined heat and power), were already cheaper sources than new nuclear plants, and yet "the state’s largest utilities are holding on tenaciously to plans dominated by massive investments in new, risky and ever-more-costly nuclear plants, while they limit or reject offers of more solar electricity. Those utilities seem oblivious to the real trends in energy economics and technology that are occurring in competitive markets."
According to the report, commercial-scale solar developers in North Carolina are already offering utilities electricity at 14 cents or less per kWh. Yet power companies like Duke Energy and Progress Energy are limiting or rejecting these offers and pushing ahead with plans for nuclear plants that, if ever completed, would generate electricity at much higher costs — 14–18 cents per kilowatt-hour according to present estimates, with the delivered price to customers somewhat higher for both sources. While solar electricity enjoys tax benefits that help lower costs to customers, "since the late 1990s the trend of cost decline in solar technology has been so great that solar electricity is fully expected to be cost-competitive without subsidies within the decade. Nuclear plants likewise benefit from various subsidies — and have so benefitted throughout their history. Now the nuclear industry is pressing for more subsidies. This is inappropriate." The report therefore advocates state investment in solar power over nuclear: "Commercial nuclear power has been with us for more than forty years. If it is not a mature industry by now, consumers of electricity should ask whether it ever will be competitive without public subsidies. There are no projections that nuclear electricity costs will decline. Very few other states are still seriously considering new nuclear plants. Some have cancelled projects, citing continually rising costs with little sign of progress toward commencing construction. Many states with competitive electricity markets are developing their clean energy systems as rapidly as possible. North Carolina should be leading, not lagging, in the clean energy transition."
The key drawback in solar energy generation is that when the sun goes down, there is no new energy created. As such, its use is dependent on storage of energy for use during the night. A number of storage options have been implemented or suggested, ranging from high capacity electric batteries, to compressed air storage to use to turn turbines, to hydrogen generation for use in fuel cells. Solar power can also be alternated with other power sources, such as wind.
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