Photoelectric Conversion of Solar Energy

Basic Principle of Photovoltaics

Molten Salt Power Tower

Photovoltaics is to do with the direct conversion of solar radiation into electricity. A photoelectric converter uses the photoelectric effect typical of certain materials. This effect is exhibited when photons of light fall onto a material and electrons are emitted as a result, generating photoelectric current.

To put it simply, when falling onto a solar cell made of two materials with different electric conductivity (N-type and P-type), the photon dislodges an electron, creating a free electron and a hole. As a result, the balance of the so-called PN junction is distorted, which causes electric current to flow. 

A photovoltaic cell’s spectral response depends on the wavelengths of the falling light and thinness of the cell top layer. In clear weather a silicon solar cell generates electric current of about 25 mA at a voltage of 0.5 V per 1 cm2 of the cell area, that is, 12-13 mW/cm2. The theoretical efficiency of silicon solar cells is about 28%, and practical efficiency – 14 to 20%.

Silicon solar cell diagram

When series-parallel connected, solar cells make up a solar (photovoltaic) array. The capacity of solar arrays produced in the industry is 50 to 200 W. In photovoltaic power plants solar modules are used to build photoelectric generators. The service life of such a power station is 20 to 30 years, at low operating costs.

The drawbacks to using flat photocells to produce electrical energy are their high cost (up to 5 USD/W) and large areas required for setting up of photo power plants.  

Solar power plant chart

Concentrator photovoltaics

One of the important developments in the field of photovoltaics is the advancement of concentrator photovoltaic installations. The system of solar energy concentration consists of a concentrator and a solar tracker because CPV units pick up only direct solar radiation.

Nowadays the basis for production of low concentration CPV modules is silicon. For instance, Australia produces crystalline silicon modules with a concentration of 11 suns and efficiency of 21.6%, while the USA manufactures silicon modules with a concentration of 40 suns and efficiency of 20%.

To improve the efficiency of solar energy conversion, gallium arsenide is applied, the high-temperature photovoltaic loss of which is much lower than that of silicon.

With the use of gallium arsenide there have been created two- and three-junction modules with a high efficiency at a concentration of 1000 suns  and more. There have been created laboratory samples of CPV cells with an area of 0.5 cm2, a concentration of 500 suns and efficiency of 40%.

Specialists in the field of photovoltaics predict that the most promising CPV modules are going to be the ones with a concentration of 1000 suns, operating with the next-generation multi-junction gallium arsenide solar elements. 

Applications of Solar Cells

One of the examples of using solar cells is photoelectric transport, with many companies creating vehicles powered by solar cells. The first solar plane was made in 1980; its flying distance was 160 km. In 2015 Solar Impulse 2 became the first solar plane to make a round-the-world trip, having spent 23 days without landing.

Some research has also been done in the development of solar powered car; the first automobile powered by a solar array was built in 1977. Currently, auto manufacturers are developing solar roof options to power their cars.  

In addition, photovoltaics plays an important role in powering satellites and telescopes, helping achieve a significant economy of fuel consumption.

Source (Russian language): http://energetika.in.ua

Alexei Tiatiushkin
Marketing manager
KharkovEnergoPribor Ltd.
marketing@keppowertesting.uk
http://www.kep.ua