Introduction to Photovoltaics
What are photovoltaics?
Photovoltaic cells are usually known as 'solar cells'. We already use them in our everyday lives in small calculators, wristwatches and parking payment machines and in larger systems on the roofs of buildings.
How do they work?
- Photovoltaic cells work by transforming the photon energy in solar radiation directly into electrical energy without an intermediate mechanical or thermal process.
How are they made?
- A photovoltaic cell consists of layers of semiconductor materials in contact with each other and fitted with metallic contacts to transfer the released electrons to the external load. Most commercial photovoltaic cells now available are manufactured from crystalline silicon, which is doped to provide the required semiconductor qualities. This is then fitted with the metallic contacts and encapsulated for protection.
What is the principle on which PV cells operate?
- PV cells operate on the principle that electricity will flow between two different semiconductors when they are put in contact with each other and exposed to light. By linking a number of these cells together a flow of electricity can be achieved.
What are the current uses of PV technology?
- It is an energy source for the 'post-fossil era'.
- Since 1988 world-wide production of photovoltaic modules has increased at a rate of up to 40% per year. Today, it is used for a wide range of applications, including stand-alone systems, grid-linked systems and building integrated systems.
- Major national and international investments in research, development, demonstration and dissemination have resulted in important technical improvements and a drop in the price of PV cells by a factor of more than 20 over the last two decades. This has opened up opportunities for cost-effective uses and both commercial and donor-supported applications are resulting in continued major growth in the global markets.
- It is used for a variety of off-grid applications in developing countries where PV technology is considered as a promising alternative to the slow arrival of grid electricity, particularly as its cost is expected to decrease. Its reliability, very low operational and maintenance costs, and its modularity which provides for easy expansion, make it very advantageous in many rural settings.
Overview of the technology
How do PV cells work?
- Sunlight is composed of photons containing energy which correspond to the different wavelengths of the solar spectrum. When photons strike a PV cell, their energy is transferred to an electron in the semiconductor material of the cell. With this extra energy, the electron is then able to escape from its normal position in the atom creating a "hole", which will become part of a current in an electrical circuit.
- A diode is formed when two layers of semiconductor materials are doped so that one will conduct negative carriers and the other positive carriers. When photons fall on these layers they transfer energy and momentum to charge carriers, which increase their potential energy by an amount depending on the diode's material properties. Because of their electrical properties, PV modules produce direct current (DC) rather than alternating current (AC). In the simplest PV systems, DC current is used immediately in applications but where AC is required, an inverter is added to the system to convert DC into AC.
What are the various aspects of the technology and its applications?
- The efficiency of the photovoltaic conversion process would be about 85% if each photon could transfer all its energy into that of charge carriers. However, this is normally not the case as any transfer of energy from photon to charge carrier can only be of the amount given by the band-gap of the semiconductor material. Photons with energies below the energy band-gap of the material are lost from the photovoltaic effect and converted into heat. In addition, photons with energies above the band-gap transfer no more than the band-gap energy, and any excess energy is lost. In today's cells, both of these effects individually limit the theoretical efficiency to 50%.
- Currently, practical maximum efficiencies are in the range of 15-20%.
Ideally, PV cells would consist of material layers with different band-gaps, for each photon to be absorbed exactly where its energy matches the band-gap energy.
- The output from a PV module depends on the amount of incident light and other factors such as temperature and the cleanliness of the cell surface.
- Modules are rated in terms of their peak output (Peak Watts, or Wp), which is the maximum power that they will produce given calibrated solar input and operating conditions. However, PV cells can produce useful quantities of power in less than ideal solar conditions.