Solar Technology

1. PHOTOVOLTAIC SYSTEMS

French physicist, Edmund Bequerel, was the one to note the photoelectric effect back in 1839. He found that certain materials have property to produce small amounts of electric current when exposed to sunlight. In 1905, Albert Einstein described the nature of light and the photoelectric effect which has become the basic principle for photovoltaic technology. The first photovoltaic module was built by Bell Laboratories in 1954.

1.1. Photovoltaic cell

PV cell are basically semiconductor diode. This semiconductor diode has got a p-n junction which is exposed to light. When illuminated by sunlight it generates electric power [1], [2]. PV cell are made up of various semiconductor materials. But mono-crystalline silicon and poly-crystalline silicon are mainly used for commercial use

1.2 Photovoltaic module

The power produced by a single PV cell is not enough for general use. So by connecting many single PV cell in series (for high voltage requirement) and in parallel(for high current requirement) can get us the desired power. Generally a series connection is chosen this set of arrangement is known as a module. Generally commercial modules consist of 36 or 72 cells. The modules consist of transparent front side, encapsulated PV cell and back side. The front side material is usually made up of low-iron and tempered glass. The efficiency of a PV module is less than a PV cell. This is due to the fact that some radiation is reflected by the glass cover and frame shadowing etc

1.3 Photovoltaic Array

A photovoltaic array (PV system) is a interconnection of modules which in turn is made up of many PV cells in series or parallel. The power produced by a single module is seldom enough for commercial use, so modules are connected to form array to supply the load. The connection of the modules in an array is same as that of cells in a module. Modules can also be connected in series to get an increased voltage or in parallel to get an increased current. In urban uses, generally the arrays are mounted on a rooftop. In agricultural use, the output of an array can directly feed a DC motor

1.4  Working of PV cell

Working of a PV cell is based on the basic principle of photoelectric effect. Photoelectric effect can be defined as a phenomenon in which an electron gets ejected from the conduction band as a consequence of the absorption of sunlight of a certain wavelength by the matter(metallic or non-metallic solids, liquids or gases).So, in a photovoltaic cell, when sunlight strikes its surface, some portion of the solar energy is absorbed in the semiconductor material. If absorbed energy is greater than the band gap energy of the semiconductor, the electron from valence band jumps to the conduction band. By this, pairs of hole-electrons are created in the illuminated region of the semiconductor. The electrons thus created in the conduction band are now free to move. These free electrons are forced to move in a particular direction by the action of electric field present in the PV cells. These flowing electrons constitutes current and can voltage (created because of its built-in electric fields) produces required power. be drawn for external use byconnecting a metal plate on top and bottom of PV cell.

1.5 Characteristics of a PV cell

In a PV characteristic there are basically three important points viz. open circuit voltage (), short circuit current () and maximum power point (). The maximum power that can be extracted from a PV cell are at the maximum power points. Usually manufacturers provide these parameters in their datasheets for a particular PV cell or module. By using these parameters we can build a simple model but for more information is required for designing an accurate model.

 

1.6 Materials used in PV cell

As mentioned above the materials used in PV cells are described below.

1.6.1 Mono-crystalline Silicon

Earlier mono-crystalline silicon has been extensively used in the making of PV cells but nowadays polycrystalline silicon has replaced it. Mono-crystalline Silicon was in use just because of its stability and desirable chemical and physical properties of silicon.

1.6.2 Polycrystalline Silicon:

This material has surpassed mono-crystalline silicon because it is cheaper. The cost of silicon comprises of a major portion of the total cost of a cell. The manufacturing cost of poly-crystalline silicon is less. This results in cheaper cells.

1.6.3 Micro-crystalline Silicon:

Micro-crystalline also known as nano-crystalline is a form of porous silicon. It is an allotropic form of silicon with para-crystalline structure. Nano-crystalline differs from poly-crystalline in a way that the former contains small grains of crystalline silicon within the amorphous phase whereas poly-crystalline consists solely of silicon crystalline grains. The factor that differentiates poly-crystalline and micro-crystalline silicon is the grain size. It has got several advantages over mono-crystalline and poly-crystalline. It has got increased stability and also it is easier to fabricate.

1.6.4 Cadmium Telluride (CdTe):

It is a crystalline compound formed from cadmium and tellurium. In the formation of p-n junction photovoltaic solar cell cadmium telluride is usually sandwiched with cadmium sulfide.

1.6.5 Copper Indium Selenide

It is a compound semi conductor material. It is composed of copper, indium and selenium. The material is a solid solution of copper indium selenide. Unlike amorphous silicon, light-induced degradation does not take place in CIS solar cells.

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