Photovoltaics:

Above - A large photovoltaic array on a commercial building roof, which adds to the power supply needs of that building.

Photovoltaics is one form of solar energy. The term “photo” comes from the Greek “phos” meaning light. “Voltaic” is named after Alessandro Volta (1745-1827), a pioneer in the study of electricity for whom the term “volt” was named. Photovoltaics, then, means “light electricity”. A solar cell or photovoltaic (PV) cell is a “semiconductor” device which is able to convert sunlight directly into electricity. This conversion - known as the photovoltaic effect – was discovered by a French experimental physicist Henri Becquerel in 1839. He observed that certain materials would produce a small electric current when exposed to light. This theory was further developed by Albert Einstein, who won the Nobel Prize in 1921 for his experiments with solar power and photovoltaics.

To see a flash movie on PV cells, go to:
http://www.energex.com.au

For more information about the operation of a photovoltaic cell, go to the section “how are solar cells made?” at :
http://www.rise.org.au/info/Tech/pv/index.html

The amount of power available from a PV device is determined by:

• The type and area of the material;
• The intensity of the sunlight;
• The wavelength of the sunlight;
• The temperature of the PV cell; and,
• The operating point.

A typical single PV cell of 100cm² will produce about 1.5 watts of power at 0.5 volts DC and 3 amps under full summer sunlight. The power output of the cell is almost directly proportional to the intensity of the sunlight. (For example, if the intensity of the sunlight is halved, the power will also be halved).

Please refer to the following diagram to measure the current and voltage output of a solar cell:


Source: Research Institute for Sustainable Energy.

An important feature of PV cells is that the voltage of the cell does not depend on its size, and remains fairly constant with changing light intensity. However, the electric current in a device is almost directly proportional to the light intensity and the size of the cell. Therefore, when people want to compare different sized PV cells, they record the current density, or the amps per square centimetre of the cell area. 

The power output of a solar cell can be increased quite effectively by using a tracking mechanism to keep the PV device directly facing the sun, or by concentrating the sunlight using lenses or mirrors. However, there are limits to this process due to the complexity of the mechanisms and the need to cool the cells.

Single PV cells have “a working voltage” of about 0.5 volts and they are usually connected together in a series (positive to negative) to provide larger voltages. PV panels are made in a wide range of sizes for different purposes. They generally fall into one of three basic categories:

1. Low voltage/lower power panels have a total area of a few square centimetres, voltages that are between 1.5 and six volts, and outputs of a few milliwatts. Although each of these panels is very small, the total production is large. They are used mainly in watches, clocks and calculators.
   
   
2. Small panels have an area from 100cm² and 1,000cm². They also have voltages that are between three and 12 volts, and outputs that are between one and 10 watts. Small panels are used for radios, toys, small pumps and electric fences.
   
   
3. Large panels have an area between 1,000cm² and 5,000cm² and they are usually made by connecting between 10 and 36 full-sized cells in a series. The large PV panels range from 10 watts to 60 watts and generally either six or 12 volts. They are used for small pumps and caravan power. These panels are also linked together to provide power for houses, communications pumping and remote area power supplies (known as RAPS).

Solar cells are not the only components of a working photovoltaic (PV) system. Many other parts are usually required for the PV system to provide a satisfactory electricity supply.


Above - Different configurations of PV-diesel hybrid energy systems.
Source: “Solar Electricity” from the UNESCO Energy Engineering Series, Energy Engineering Learning Package (Year 2000 edition).

Many PV systems contain a device - such as a battery bank – which will provide for energy storage. Through this energy storage device, the PV system will be able to supply electricity at night or during severe weather.

Solar cells provide direct current (DC). However, most electrical appliances only work with alternating current (AC). Therefore, PV systems also contain some form of power conditioning, such as an inverter, which will convert DC into AC.

Other power conditioning or control elements are also needed to connect the different parts of the PV system, and to allow for the variable nature of the converted solar energy. All these components have to be properly interconnected, sized and specified for effective PV operation.

The photovoltaic system consists of a number of parts or subsystems:

1. The photovoltaic generator with mechanical support and, possibly, a sun-tracking system.
2. Batteries (the storage subsystem).
3. Power conditioning and control equipment, including provision for measuring and monitoring.
4. The system may contain a supplementary or back-up generator (for example, a diesel generator), to form a “hybrid system”.

Many applications are best served by a combination of renewable energy (e.g. solar energy) and conventional energy sources (e.g. a diesel generator). These combine to form a hybrid system.

Photovoltaic-diesel hybrid energy systems generate AC electricity by combining a photovoltaic array with an inverter, which can operate alternately or in parallel with a conventional engine-driven generator. These systems can be classified according to their configuration as:

• Series hybrid energy systems.
• Switched hybrid energy systems.
• Parallel hybrid energy systems.

Some practical examples of photovoltaic-diesel hybrid energy systems are shown in the above diagrams.

To find out more about Australian projects in PV technology, go to:
http://www.greenhouse.gov.au/renewable/recp/pv/index.html

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