Solar Energy

SOLAR ENERGY

All electromagnetic waves are constituted from photons (the radio waves, gamma rays and visible light). The photons are the fundamental components of light.
Solar energy is the source of all forms of energy production used today on Earth, with the exception of nuclear energy, geothermal and tidal energy.

Solar energy can be transformed into:

• Chemical energy (photosynthesis),
• Kinetic energy (thermodynamic),
• Thermal energy,
• Electrical energy (solar panel),
• Biomass.

Currently, there are two main ways to exploit solar energy (and another undergoing expansion):

• Solar photovoltaic which directly transforms the radiation into electricity;
• Solar thermal which directly transforms the radiation into heat;
• The solar says "thermodynamic" is a variant form of solar thermal. This technique differs in that it uses solar thermal energy to transform it, in a second time, in electricity.

It is relatively complex to recover solar energy to convert it into electricity or heat, and, even more, to store it. The exploitation of this energy source is recent and rapidly developing in sunny regions. Nevertheless, it is still expensive. Many research programs are under way to improve performance of new technologies of solar energy exploitation.

Characteristics of solar radiation on Earth:         (click on image to enlarge)                                      

 

Modeling the uneven distribution of solar energy in the world (source: ez2c.)

Solar power received by planets

The solar energy received in a point on the globe depend of:

• Solar energy supplied by the sun, which fluctuates with its fusion activity;
• The nebulosity (clouds, fog, etc...), which is, for example, important near equator and lowest in inter-tropical areas;
• The latitude, season and time, which act on the height of the sun in the sky, and also impact on the energy received per ground surface unit.

Three families of processes transforming solar energy exist today:

Photovoltaic solar energy:

Movable solar panel

Photovoltaic solar energy is to convert the sun's energy directly into electricity through a cell.The photovoltaic cells are made from semiconductor materials such as silicon, produced from a raw material of high purity. 
This form of energy stays today quantitatively limited, it is nevertheless promised to a great future thanks to the progress expected, the decreasing cost, its simplicity and versatility.

Durable and Maintenance-free, it can be operated with or without connection to a power grid; it can meet the electrical energy needs of a home (panels on the roof) or industry.
The first application appeared in the spatial domain for satellites. Other areas then used photovoltaic technology, including telecommunications, maritime buoyage and air markup, home lighting and water pumping.
But as the sun is not visible at a point of the Earth round the clock, this application requires the use of batteries or other systems that ensure the energy storage with a view to consume it off period of sunshine.
(See “Photovoltaic Solar Energy”)
 
Thermal solar energy:
Thermal solar energy is to use the heat from solar radiation:

Solar water heater

• In direct use of heat: solar water heaters and solar heating, solar cookers and dryers;
• In indirect use: heat serving as another purpose: solar cooling, desalination, etc...

In fact, it is used mainly for heating water or accommodation spaces. This is done using thermal collectors. There are several categories of collectors, but the principle is always the same: thermal collector absorbs solar photons and converts it into heat. The heat is then passed to a liquid or a gas that transports it (the gas is called "heat carrier") to a heat storage tank.  
(See “Solar Thermal Energy”)

Thermodynamic solar energy:

Solar power plant

The thermal energy from the sun can also produce electricity by thermodynamic path. The principle is identical to that of a conventional power station:
The production of steam or gas under high pressure is turbined to then be transformed into electricity. This process requires high temperatures (250°C to over 1000°C), which are obtained by concentrating light with the solar mirrors toward a heat carrier fluid.

Among the major concentration technologies we can list:

• Solar power plants with parabolic trough (linear concentration);
• Solar power plants with Fresnel mirrors (linear concentration) equipped with blades slightly curved mirrors;
• Tower power plants with heliostats which return radiation on the concentrator (punctual concentration);
• Parabolic concentrators Dish-Stirling (punctual concentration) equipped with a Stirling engine (hot air engine).

It allows the storage of a portion of the energy as heat. This storage can reduce the effects of intermittency of the solar resource; for example, continue to produce electricity after sunset. 
(See “Thermodynamic Solar Power”)

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