How it works?
When light shines on a PV cell, it may be reflected, absorbed, or pass right through. Only the absorbed light generates electricity.
Two layers of different semiconductor are placed in contact with one another. One layer is an "n-type" semiconductor with an abundance of electrons (negative). The other layer is a "p-type" semiconductor with an abundance of "holes" (positive)
Although both materials are neutral, n-type (-) silicon has excess electrons and p-type (+) silicon has excess holes. Sandwiching these together creates a p/n junction at their interface.
When n- and p-type silicon come into contact, excess electrons move from the n-type side (-) to the p-type side (+). The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.
The electrical field causes the electrons to move from the semiconductor toward the negative surface, where they become available to the electrical circuit. The holes move in the opposite direction, toward the positive surface, where they await incoming electrons from the sun.
Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.
Although the quantity of solar radiation striking the Earth varies by region, season, time of day, climate, and air pollution, the yearly amount of energy striking almost any part of the Earth is vast. Shown is the average radiation received on a horizontal surface across the continental United States in the month of June. Units are in kWh/m2.Power Stations
Largest PV plant in the world uses the Heliostat Concentrator Photovoltaic Technology to produces 154 MW. For a 10 MW plant in Phoenix, AZ, the levelized cost of energy (LCOE) is estimated at $0.15 to 0.22/kWh.
Buildings
Building-integrated photovoltaics (BIPV) are being incorporated in new structures (domestic and industrial) as an auxillary source of power. Usually fitted on the top of an existing roof structure, the PV arrays can be retrofitted into existing buildings.
Transport
Standalone Systems
Incentives?
The political purpose of incentive policies for PV is to grow the industry even where the cost of PV is significantly above grid parity, to allow it to achieve the economies of scale necessary to reach grid parity.
- Investment subsidies: the authorities refund part of the cost of installation of the system
- Feed-in Tariffs (FIT) also called Net Metering: the electricity utility buys PV electricity from the producer under a multiyear contract at a guaranteed rate.
- Renewable Energy Certificates (RECs)
In 2006 California approved the "California Solar Initiative". This offers a choice of investment subsidies or FIT up to medium sized 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.
Energy Payback Time (EPT) and Energy Returned on Energy Invested (EROEI)
The energy payback time is the time required to produce an amount of energy as great as what was consumed during production. The EROEI is the ratio of electricity generated divided by the energy required to build and maintain the equipment. In Europe silicon PV systems have a EPT of 2 years and EROEI around 15.
Pros:
- Using renewable energy sources in manufacturing and transportation would drop photovoltaic emissions to zero.
- Facilities can operate with little maintenance or intervention after initial setup.
- Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses
- 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 encouraged by net metering. Time-of-use net metering can be highly favorable to small photovoltaic systems.
Cons:
- Not availabe at night
- Solar electricity is in the form of DC power which is converted to AC (loss of 4-12%).
- More expensive than other resources.
- 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.
- Cadium which is used in a few of PV panels is a toxic substance that accumulates in the ecosystem. The amount of cadmium used in PV modules is small and with proper emission control can be almost zero.
UCLA Research
Major Solar companies
BP Solar, SunPower, Suntech, Yingli Green Energy, Kyocera, Q-cells, Sanyo, Sharp Solar, SolarWorld
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