Photovoltaic Physics
Advantages and Disadvantages
Challenges Ahead
References
 

Photovoltaic Physics:

Photovoltaic cells use semiconductor technology to capture the energy in sunlight. A conventional solar cell consists of a wafer of silicon that is about 1/50th of an inch thick. Typical cells that are four inches in diameter produce about one watt of power, and are grouped into modules of dozens of cells. Modules are further grouped into panels and then arrays, which may produce several kilowatts of power.

When light shines on a crystal of pure silicon (A-B), particles called "electrons" are ejected from silicon atoms and move about
the crystal somewhat randomly (C). The place the electron came from is called a "hole". It takes energy from the light to eject
the electron from its normal resting place, and energy is released when the electron returns to an atom that is missing an
electron, and recombines with a hole (D).

To create a semiconductor, two halves of a crystal of pure silicon are contaminated, or "doped", with two different types of
material called "dopants", one that contains excess electrons, and one that is electron deficient. The junction between the halves
is critical to the operation of the cell.

Because of the presence of the dopants, an "electric field" exists across the junction of the two halves of the crystal that sweeps
free electrons across the junction in one direction only. It is this property of the junction that causes current flow in a solar cell.

If an electron is freed in the half of the cell that has excess electrons, the junction prevents the electron from drifting into the
other half, recombining with a hole, and losing its energy. If an electron is freed in the half of the cell with excess holes, the
electric field sweeps the electron into the other half. These effects induce electrons to flow in only one direction across the
junction.
 

Advantages:

• Mechanically simple, there are no moving parts in a PV cell.
• Production of DC current means battery storage is simple.
• PV cell make no noise and give off no exhaust.
• Allow the use of electrcity in remote areas where it would be expensive or impossible to run power lines.
• Trackers and concentrators increase the amount of sunlight reaching each PV cell.

• PV power is currently more expensive than power from utilities.
• Light is required to generate electricity; hence, during the night and on cloudy days PV cells are unable to produce power.
• To use AC appliances inverters must be used.
• Battery storage means additional maintenance and replacement.
• Some of the materials used in PV production are toxic

• Residential customer demand may exceed the power produced by a typical home sized photovoltaic system, necessitating storage or supplmental power supply system.
• Solar insolation varies from region to region as does the value of other energy options.
• Cost per kWh produced by a PV system are currently higher then the average price for electricity from most utilities.
• Lack of utility experience with PV and lack of knowledge of solar resources within service territories limit adoption.
• Without energy storage, PV system cannot provide continuous power.
• Some of the existing tax and other financial incentives to use photovoltaic generation are not available for utility property.
• There are insufficient high quality system integrators. Currently, utility suppliers of energy services have to provide custom-design systems every time they undertake a new installation.
• The manufacturing sector is undercapitalized for mass production.

• Joint government, industry, utility, and equipment manufacturer efforts are underway for demonstration and performance benefit analysis.
• Participation in the Utility PhotoVoltaic Group (UPVG) efforts to understand better potential PV applications in the electric utility sector is expanding; also participation in the broader collaborative effort,  PV-COMPACT.
• Use of existing tax and other financial incentives to overcome high inital capital costs.

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