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What is Sun energy

The sun is the largest energy source ever. The sun is the star at the center of our solar system. Inside of this huge nuclear reactor energy hydrogen is melted down to helium at temperatures of 15 Million degrees. This process produces energy. At the surface of the sun, temperatures are extremely high with approx. 5700°C. From its surface light and heat is distributed into space. Parts of the radiation from the sun reach the earth surface and can be converted into energy by solar power plants. According to estimates, the sun daily provides the global energy demand of eight years.

There are different kinds of solar power plants:

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CSP (Concentrated Solar Power)
CSP systems use lenses or mirrors and tracking systems to bundle a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant.

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PV (Photovoltaic)
A solar cell, or PV cell, is a device that converts light into electric current using the photoelectric effect.

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CPV (Concentrated Photovoltaics)
Contrary to conventional PV systems, CPV uses lenses and curved mirrors to bundle sunlight onto small, but highly efficient solar cells.


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History of Solar Energy

Since ancient times the sun has been energy source for different cultures. However, the active use of solar energy is an achievement of modern times.

In 1839 the French physician Alexandre Edmund Bequerel discovered the photoelectric effect, which set the start for today’s solar cells. While experimenting with batteries with galvanic elements, he discovered that the electrical voltage increased when light shines on it. However, he could not explain this phenomenon at this time. It has been Albert Einstein, who discovered the physical background of photovoltaic in 1905.

The predecessor of today’s PV modules has been developed by Charles Fritts in 1883. It was him, who set the basis for further research on the photoelectric effect.

Solar cells based on Silicon are relatively new. In 1954 a research team of the US Corporation Bell Laboratories among the specialists Calvin Fuller, Daryl Chapin and Gerald Pearson developed the first solar cell with a coefficient of performance of up to 6%. Further developments of Photovoltaic are linked especially with space technology.

Also, solar heat has been used since ancient times. In ancient Egypt and Greece people used burning and concave mirrors to focus sun beams. Since ancient times the Olympic torch has been ignited traditionally with burning mirrors.

The Swiss natural scientist Horace-Bénédict de Saussure invented the predecessor of today’s solar collectors in the 18th century. However, it took a long time until the seventies of the last century to develop concepts for using solar heat.

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Photovoltaic: How Does it work and characteristics of a solar cell

The direct conversion of sun light into electric energy by means of solar cells is called Photovoltaic. The conversion is based on the photoelectric effect, which describes the release of positive and negative loads within a solid body by means of light beams.

Most of the worldwide produced solar cells consist of the semiconductor silicon. Two silicon layers with different loads are the essential element of a solar cell. A boundary layer is established between the two layers. Released loads can pass through the boundary layer by means of sun beams. The created electric power can be used via pins, connected on the front and back side of the solar cell. A full contact surface is usually on the backside of the solar cell. A thin grid is usually on the front to lead light direct on the surface.

In general solar cells are 10x10cm, in recent times the dimension of 15x15cm are more and more attractive. On the cell surface is covered with a transparent anti-reflection layer to protect the solar cell and to reduce reflection.

The voltage provided by the solar cell depends on the semiconductor, e.g., silicon produces 0.5V. While the voltage is only slightly dependent on sun irradiation, the current increases with higher luminosity. The performance of a solar cell is temperature-dependent. Higher cell temperatures lead to lower performance and hence to a lower coefficient of efficiency. The coefficient of efficiency indicates how much of the sun light can be converted into usable electrical energy.

Depending on the type of crystal, there are different cell types: monocrystalline, polycrystalline and amorphous.

Material Coefficient in %
Coefficient in %
Monocrystalline silicon approx. 24 14 ... 17
Polycrystalline silicon approx. 18 13 ... 15
Amorphous silicon approx. 13 5 ... 7

 Source: Wikipedia

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Solar Thermal energy: How Does it work

In contrary to photovoltaics, solar thermal power plants indirectly generate power from sun light. The conversion of solar irradiation into thermal energy follows the Principle of Absorption. It can be performed by solar collectors, which directly absorb the sun irradiation. Large thermal solar power plants work with concentrators following the principle of reflection. The concentrators bundle the sun beams to increase the intensity of the incident light on the absorber. Thus the temperature in the heat carrier is increased.

Thermal solar power plants use different methods for power generation, three of them work with focusing mirrors.

Parabolic trough
Parabolic trough focus the sun beams on an absorber tube filled with a fluid. The fluid is transported to a heat engine, where the heat is converted to electricity, which boosts as gas a turbine.

Parabolic solar dish
A parabolic dish system uses large, reflective, parabolic dishes. It focuses all the sunlight that strikes the dish up onto a single point above the dish, where a receiver captures the heat and transforms it into a useful form. Typically the dish is coupled with a Stirling engine, but also sometimes a steam engine is used. Parabolic solar dish systems are known to have the highest efficiency of all solar technologies.

Power towers or heliostat power plants
Power towers capture and focus the sun's thermal energy with thousands of tracking mirrors (called heliostats). A tower is placed in the center of the heliostat field. The heliostats focus concentrated sunlight on a central absorber, which is on top of the tower. Within the absorber the concentrated sunlight heats up to over 538 C. The steam drives a standard turbine to generate electricity.

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