How do photovoltaics panels work?

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. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. 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[citation needed], significant financial incentives in Japan and then Germany, Italy, Greece 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 2.5 GW in 2008, to an expected additional 375 MW in 2009.

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.

Due to the growing demand for renewable energy sources, the manufacture of solar cells and photovoltaic arrays has advanced dramatically in recent years.

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. At the end of 2009, the cumulative global PV installations surpassed 21 GW.[15] 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) or built into the roof or walls of a building, known as Building Integrated Photovoltaics or BIPV for short, while BAPV is Building Applied PV -– it’s a retrofit added to the building long after construction. 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.

World solar photovoltaic (PV) installations were 2.826 GW peak (GWp) in 2007, and 5.95 GW in 2008, 7.5 GW in 2009, and 18.2 GW in 2010.[17][18][19][20] The three leading countries (Germany, Japan and the US) represent nearly 89% of the total worldwide PV installed capacity.

Germany installed a record 3.8 GW of solar PV in 2009;[21] in contrast, the US installed about 500 MW in 2009. The previous record, 2.6 GW, was set by Spain in 2008. Germany was also the fastest growing major PV market in the world from 2006 to 2007 industry observers speculate that Germany could install more than 4.5 GW in 2010.[21][22] In fact Germany installed 7,25 GW in 2010 [21][22] 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.

Photovoltaic power capacity is measured as maximum power output under standardized test conditions (STC) in "Wp" (Watts peak).[23] 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 electricity. Therefore the 2008 installed base peak output would have provided an average output of 3.04 GW (assuming 20% × 15.2 GWp). This represented 0.15 percent of global demand at the time.

The EPIA/Greenpeace Advanced Scenario shows that by the year 2030, PV systems could be generating approximately 1.8 TW 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.

Photovoltaic panels based on crystalline silicon modules are being partially replaced in the market by panels that employ thin-film solar cells (CdTe CIGS, amorphous Si, microcrystalline Si), which are rapidly growing and are expected to account for 31 percent of the global installed power by 2013. Other developments include casting wafers instead of sawing, 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.

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. It is expected that by 2009 over 90% of commercial photovoltaics installed in the United States will be installed using a power purchase agreement. An innovative financing arrangement in Berkeley, California, funded by grants from the United States Environmental Protection Agency and the Bay Area Air Quality Management District, lends money to a homeowner for solar system, to be repaid via an additional tax assessment on the property which remains in place for 20 years. This allows installation of the solar system at "relatively little up-front cost to the property owner."

The San Jose-based company Sunpower produces cells that have an energy conversion ratio of 19.5%, well above the market average of 12–18%. The most efficient solar cell so far is a multi-junction concentrator solar cell with an efficiency of 43.5% produced by the National Renewable Energy Laboratory in April 2011. The highest efficiencies achieved without concentration include 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). A March 2010 experimental demonstration of a design by a Caltech group led by Harry Atwater which has an absorption efficiency of 85% in sunlight and 95% at certain wavelengths is claimed to have near perfect quantum efficiency. However, absorption efficiency should not be confused with the sunlight-to-electricity conversion efficiency.

The new European Photovoltaic Industry Association (EPIA) report predicts a promising future for photovoltaics. "The future of the PV market remains bright in the EU and the rest of the world," the report said. "Uncertain times are causing governments everywhere to rethink the future of their energy mix, creating new opportunities for a competitive, safe and reliable electricity source such as PV." By 2015, between 131 and 196 gigawatts (GW) of photovoltaic systems could be installed around the globe. (Until now, only 40 GW have been installed.)