Photovoltaics, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert energy from the sun into electricity. Photovoltaics is also the field of study relating to this technology.
Solar cells produce direct current electricity from the sun’s rays, which can be used to power equipment or to recharge a battery. Many pocket calculators incorporate a solar cell.
When more power is required than a single cell can deliver, cells are generally grouped together to form “PV modules”, or solar panels, that may in turn be arranged in arrays. Such solar arrays have been used to power orbiting satellites and other spacecraft and in remote areas as a source of power for applications such as roadside emergency telephones, remote sensing, and cathodic protection of pipelines. The continual decline of manufacturing costs (dropping at 3 to 5% a year in recent years) is expanding the range of cost-effective uses including roadsigns, home power generation and even grid-connected electricity generation.
Large-scale incentive programs, offering financial incentives like the ability to sell excess electricity back to the public grid ('feed-in'), have greatly accelerated the pace of solar PV installations in Spain, Germany, Japan, the United States, Australia, South Korea, Italy, Greece, France, China and other countries.
Many corporations and institutions are currently developing ways to increase the practicality of solar power. While private companies conduct much of the research and development on solar energy, colleges and universities also work on solar-powered devices.
The most important issue with solar panels is cost. Because of much increased demand, the price of silicon used for most panels is now experiencing upward pressure. This has caused developers to start using other materials and thinner silicon to keep cost down. Due to economies of scale solar panels get less costly as people use and buy more — as manufacturers increase production, the cost is expected to continue to drop in the years to come. As of early 2006, the average cost per installed watt for a residential sized system was about USD 6.50 to USD 7.50, including panels, inverters, mounts, and electrical items.
Grid-tied systems represented the largest growth area. In the USA, with incentives from state governments, power companies and (in 2006 and 2007) from the federal government, growth is expected to climb. Net metering programs are one type of incentive driving growth in solar panel use. Net metering allows electricity customers to get credit for any extra power they send back into the grid. This causes an interesting role reversal, as the utility company becomes the buyer, and the solar panel owner becomes the seller of electricity. To spur growth of their renewable energy market, Germany has adopted an extreme form of net metering, whereby customers get paid 8 times what the power company charges them for any surplus they supply back to the grid. That large premium has created huge demand for solar panels in that country.
PV in buildings
Solar arrays are increasingly incorporated into new domestic and industrial buildings as a principal or ancillary source of electrical power. Typically, an array is incorporated into the roof or walls of a building, roof tiles can now even be purchased with an integrated PV cell (B.I.P.V.- Building Integrated PhotoVoltaics) . 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 only possibility for generating electricity, or PV may be used together with wind and/or hydroelectric power. In such off-grid circumstances batteries are usually used to store the electric power. However, the largest installations are grid-connected systems (see table below). These systems are connected to the utility grid through a direct current to alternating current (DC-AC) inverter. When the load required in the building is more than that supplied by the PV array then electricity will be drawn from the grid; conversely when the PV array is generating more power than is needed in the building then electricity will be exported to the grid. Batteries are not required and standard AC electrical equipment may be used. The average lowest retail cost of a large PV module declined from USD 7.50 to USD 4 per watt between 1990 and 2004. However, prices have gone up 15-20% in 2005-2006 due to increased demand (mainly due to increased incentives and subsidies) and silicon shortages. The silicon shortage is expected to persist until at least 2008. With many jurisdictions now giving tax and rebate incentives, and/or net metering solar electric power can now pay for itself in ten to twenty years in a few places.
In August 2006 there was widespread news coverage in the United Kingdom of the major high street electrical retailers (Currys) decision to stock PV modules, manufactured by Sharp, at a cost of one thousand pounds sterling per module. The retailer also provides an installation service. The agency that administers UK government grants for domestic solar power systems estimates that an installation for an average-sized house would cost between £8,000 and £18,000, and yield annual savings between £75 and £125.
There is intensive research interest in solar-powered vehicles and the technology is developing rapidly. Solar-powered cars have commonly appeared at solar races such as the World Solar Challenge and at car and technology shows. Solar boats are a new application of the technology. Solar Boats from colleges and universities compete in the Solar Splash competition in North America, and the Frisian Nuon Solar Challenge in Europe.
PV power stations
Deployment of solar power depends largely upon local conditions and requirements. But as all industrialised nations share a need for electricity, it is clear that solar power will increasingly be used to supply a cheap, reliable electricity supply. In 2004 the worldwide production of solar cells increased by 60% but silicon shortages reduced growth afterwards.
The list below shows the largest photovoltaic plants in the world. For comparison, the largest non-photovoltaic solar plant, the solar trough-based SEGS in California produces 350 MW and the largest nuclear reactors generate more than 1,000 MW. A plant in Australia, which will not come into service until 2008, is expected to be 154 MW when it is completed by 2013.
The political purpose of incentive policies for PV is to grow the industry even while the cost of PV is significantly above grid parity, to allow it 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
Two incentive mechanisms are used:
investment subsidies:the authorities refund part of the cost of installation of the system,
feed in tariffs/net metering: the electricity utility buys PV electricity from the producer under a multiyear contract at a guaranteed rate.
With investment subsidies, the financial burden falls upon the taxpayer, while with feed-in tariffs the extra cost is distributed across the utilities' customer base. 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, so reward overstatement of power, and tolerate poor durability and maintenance. Feed in tariffs reward the number of kWh produced over a long period of time.
The price paid per kWh 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.
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.
In 2004, the German government introduced the first large scale feed in tariff system, under a law known as the 'EEG' (see below) which resulted in explosive growth of PV installations in Germany. 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.
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 for building integrated systems.
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. Incentives are scheduled to decrease in future depending as a function of the amount of PV capacity installed.
The price/kWh or kWp of the FIT or investment subsidies in stimulating the installation of PV is only one of three factors. The other two are insolation (the more sunshine, the less money is needed) and administrative ease of obtaining permits and contracts (Southern European countries are reputedly relatively complex)