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1. Solar Energy
1.1 Introduction
A Star is formed by the constant agglomeration of hydrogen atoms until they reach the gravitational collapse, this leads to an enormously rising of the pressure and thus high temperatures. These extremely hot temperatures in the core of the sun allow the strong nuclear force to fuse deuterium and tritium nuclei together, releasing one atom of helium a neutron and energy (~17,6MeV).
The energy released is radiated into space in the form of electromagnetic radiation.
Despite only a tiny fraction of this energy reaches the earth, the sun offers more energy in four hours than the human race uses in a whole year. The sun is the biggest energy supplier for the earth.
The sun will be available as an energy source for another 5 billion years, therefore it is considered by our human perspective an unlimited energy source [1].
Fig.1 – Representation of a fusion reaction emitting radiation [3]
1.2 Electromagnetic Radiation
An electromagnetic wave is characterized by its wavelength () and frequency (f). Frequency is inversely proportional to wavelength according to where ν is the speed of the wave; in vacuum ν = c = 299,792,458 m/s
Fig.2 - Representation of an electromagnetic wave. [2]
Its energy (E) depends on the frequency (ƒ) of the electromagnetic radiation according to Planck’s equation, where h is the constant of Planck (h ≈ 6.626069 × 10–34 J-s). The high-frequency electromagnetic waves have a short wavelength and high energy; low-frequency waves have a long wavelength and low energy.
Table 1 - Limits in the spectrum of electromagnetic radiation. [2]
All objects at temperatures greater than 0 K emit energy as electromagnetic radiation due to the movement of the electrons; this radiation is also called thermal radiation.
Irradiance is the power of electromagnetic radiation per unit area at a surface and can be expressed as:
Where  is the emissivity of the surface (0<, σ is the constant of Stefan–Boltzmann (σ = 5.67 × 10–8 [W / (m^2 K^4)] and T the temperature of the surface.
1.3 Solar Spectral Distribution
The energy radiated by the sun comes from its extremely high surface temperatures. The Sun radiates throughout the entire electromagnetic spectrum from the shortest x-rays to long-wavelength radio waves. The solar spectrum is equivalent to the one of a blackbody ( spectrum for an effective temperature near 5,800 K, peaking near 480 nm. [2]