# Main Energy Sources

## Main energy sources.

### 1) Definition of energy

Energy is an abstract physical quantity, which is linked to dynamic state of a closed system and that remains unchanged over time. You can also define the power of open systems, IE, parts not isolated from each other in a more closed system. A statement of the classical Newtonian physics states that energy is not created or destroyed, only transformed.

Energy is not a real physical state, nor an “intangible substance” but only a scalar number that is assigned to the state of the physical system, IE, energy is a mathematical abstraction or a property of physical systems. For example, one can say that a system with no kinetic energy is at rest.

The magnitude of energy use in practical terms is justified because it is much easier to work with scalar quantities, such as energy, vector magnitude, such as speed and position. So you can fully describe the dynamics of a system according to the kinetic energy, potential and other components. In isolated systems, in addition, the total energy has the property of “conservative”, IE be invariant over time. Mathematically, the conservation of energy for a system is a direct consequence of the evolution equations of this system is independent from the time in question, according to No ether’s theorem.

### 2) Major sources of energy in nature

#### Wind Energy

is the energy derived from wind, or the kinetic energy generated by the effect of air currents, and is transformed into other useful forms for human activities.

The wind comes from the Latin word Aeolus, belonging or related to Colo, the god of the winds in Greek mythology. Wind energy has been used since ancient times to move boats propelled by sail or operate machinery mills to move their blades.

At present, wind energy is mainly used for producing electricity through wind turbines. At the end of 2007, worldwide capacity of wind generators was 94.1 megawatts. While wind power generates about 1% of global electricity consumption, representing approximately 19% of electricity production in Denmark, 9% in Spain and Portugal, and 6% in Germany and Ireland (2007 data).

Wind energy is an abundant, renewable, clean and helps reduce emissions of greenhouse gases by replacing thermal fossil fuel, making it a kind of green energy. However, the main drawback is its intermittent.

#### Solar Energy

Is the energy gained by the abstraction of light and heat emitted by the Sun Solar radiation reaching the Earth could be harnessed through the heat it produces, but also through the absorption of radiation, egg optical devices or otherwise. It is a so-called renewable energy, particularly the non-pollutant, known as clean energy or green energy.

The power of radiation varies according to time of day, weather conditions that soften and latitude. It can be assumed that in a good position to influence the value is approximately 1000 W / m² in the earth’s surface. This power is known as irradiates.

Radiation is usable in direct and diffuse components, or the sum of both. Direct radiation is coming directly from the solar source, without intermediate reflections or refractions. The diffuse is issued by the daytime sky thanks to the many phenomena of reflection and refraction in the solar atmosphere, clouds and other atmospheric and terrestrial elements. Direct radiation can be reflected and concentrated on its use, while it is not possible to concentrate the diffused light coming from all directions.

The direct normal irradiates (or perpendicular to the sun) out of the atmosphere called the solar constant and has an average value of 1354 W / m² (which corresponds to a maximum value at the perihelion of 1395 W / m² and a minimum value at aphelion 1308 W / m²).

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