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Solar Energy | Photovoltaic

Friday, December 10, 2010 | | 6 comments

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Solar energy is the most promising clean energy source for new generations. Sun has been shinning for about 5 billion years (without malfunctions) and it will continue shinning for another 4 to 5 billion years.

Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used.

Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). CSP systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. PV converts light into electric current using the photoelectric effect.

Solar Cells

Solar cell used in Skylab
Solar Cell used in Skylab
Though it sounds unusual at first, it is a fact that solar cells represent a very important part of information technology revolution in the last few decades. Without solar cells there wouldn't be so many communication satellites, which found the information technology revolution. The consequences are fast development in the field of information technologies and computer sciences, Internet etc. Data and voice transfer possibilities, which are offered to you by modern communications techniques, were made possible by a small piece of silicon in the form of a solar cell.

First solar cells

A physical phenomenon allowing light-electricity conversion - photovoltaic effect, was discovered in 1839 by the French physicist Alexandre Edmond Becquerel. Experimenting with metal electrodes and electrolyte he discovered that conductance rises with illumination.

Willoughby Smith discovered photovoltaic effect in selenium in 1873. In 1876, with his student R. E. Day, William G. Adams discovered that illuminating a junction between selenium and platinum also has a photovoltaic effect. These two discoveries were a foundation for the first selenium solar cell construction, which was built in 1877. Charles Fritts first described them in detail in 1883.

In 1918, a Polish scientist Jan Czochralski discovered a method for monocrystalline silicon production, which enabled monocrystalline solar cells production. The first silicon monocrystalline solar cell was constructed in 1941.

The photovoltaic effect in other materials was observed in 1932 in cadmium-selenide (CdS). Nowadays, CdS belongs among important materials for solar cells production.

In 1963, Sharp Corporation developed the first usable photovoltaic module from silicon solar cells. The biggest photovoltaic system at the time, the 242 W module field was set up in Japan. A year later, in 1964, Americans applied a 470 W photovoltaic field in the Nimbus space project.

Photovoltaics (PV)

The term photovoltaics derives from the Greek word "phos" meaning light and the word "volt" (named by Alessandro Volta). Photovoltaics is a science, which examines light-electricity conversion, respectively, photon energy-electric current conversion. In other words it stands for light-current conversion. Both direct and diffuse solar radiation take part of the process. The light to current conversion takes place within solar cells, which can be amorphous, poly- crystalline or single-crystal, according to their structure.
  • Single-crystal cells are made in long cylinders and sliced into round or hexagonal wafers. While this process is energy-intensive and wasteful of materials, it produces the highest-efficiency solar cells—as high as 25 percent in some laboratory tests. Because these high-efficiency solar cells are more expensive, they are sometimes used in combination with concentrators such as mirrors or lenses. Concentrating systems can boost efficiency to almost 30 percent. Single-crystal accounts for 29 percent of the global market for PV.
  • Polycrystalline cells are made of molten silicon cast into ingots or drawn into sheets, then sliced into squares. While production costs are lower, the efficiency of the solar cells is lower too—around 15 percent. Because the solar cells are square, they can be packed more closely together. Polycrystalline solar cells make up 62 percent of the global PV market.
  • Amorphous silicon (a-Si) is a radically different approach. Silicon is essentially sprayed onto a glass or metal surface in thin films, making the whole module in one step. This approach is by far the least expensive, but it results in very low efficiencies—only about five percent.

Photovoltaic Cells

The basic PV or solar cell typically produces only a small amount of power. To produce more power, solar cells (about 40) can be interconnected to form panels or modules. PV modules range in output from 10 to 300 watts. If more power is needed, several modules can be installed on a building or at ground-level in a rack to form a PV array. About 10–20 PV arrays can provide enough power for a household.

Application Example

Most common application of solar cells applies to pocket calculators power supply, parking meters power supply and similar appliances. Solar-module consists of many solar cells, which are electrically connected and placed between glass and tedlar plate, and framed by an (usualy) aluminium frame. A number of solar-modules and other components (batteries, charge regulators, and inverters) can form large photovoltaic systems.

The first PV system applications developed were applied as an energy source for satellites and later for orbital stations in space. Nowadays, photovoltaic systems are applied as an energy source in many cases. Most commonly applied photovoltaic systems can be found in remote and rural areas where no public grid is available. However, quite often grid-connected systems are constructed in urban areas.

Photovoltaic systems are an excellent solution to electricity production regardless of your whereabouts - even at high latitudes of Himalayas or in Antarctica photovoltaic systems have been build. According to loads connected to the system and to the basic design principles, the following photovoltaic systems are used - direct coupled photovoltaic systems (systems without batteries - water pumping systems for example), standalone photovoltaic systems, grid-connected photovoltaic systems, hybrid systems (e.g. PV - wind or PV - diesel systems), concentrator photovoltaic systems. The applications below depict use of photovoltaic systems as an energy source in many interesting ways. 

Photovoltaic Power Plants
Photovoltaic power plants - Solar modules are nowadays parts of large standalone or grid-connected systems. Large photovoltaic power plants (MW range) have beeing constructed in Germany, Spain, USA, Italy, Netherlands etc. Worldwide more than 250 large PV power plants with peak power 1 MWp or more (each plant) are connected to the public grid(s).

Building Integrated Photovoltaics

Building integrated photovoltaics - Acronym of BIPV (Building Integrated Photovoltaics) refers to photovoltaic systems integrated with an object's building phase. It means that they are built/constructed along with an object. They are also planned together with the object. Yet, they could be built later on. Due to specific task cooperation of many different experts, such as architects, civil engineers and PV system designers, is necessary.

Photovoltaic Noise Barrier
Noise barriers - An efficient way of noise prevention by application of photovoltaic modules was first demonstrated in Switzerland in 1989. Later, the solution was applied also in some other European countries. Noise prevention is also a research subject of several projects conducted in European Union. Different photovoltaic noise barriers can be built considering motorway features, barrier construction, the height of the barrier, and other factor influence (environment etc.).

Photovoltaics: Design and Installation Manual
Photovoltaics: Design and Installation Manual

Practical Handbook of Photovoltaics: Fundamentals and Applications
Practical Handbook of Photovoltaics