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The use of solar energy.

24 September 2018

Solar radiation is the main source of the diverse types of energy, both natural and artificial, that we know so far.

Focusing strictly on the use of solar energy, it must be said that not all energy coming from the Sun to the Earth is usable. Only 47% of solar energy reaches the earth's surface, being a smaller proportion of all the available energy outside the atmosphere as anyone can deduct. And why does not 100% of the energy arrive? What obstacles make us lose that valuable energy? First, we must consider it as a natural process and secondly that with the produced quantities, energy "would be more than enough" so the point is not to be focused on "the energy loss". Solar energy is subjected to three effects and / or processes before, during, and when passing through the atmosphere.

  1. Diffusion
  2. Absorption
  3. Reflection

Initially paying attention to the energy that manages to reach physical ground, it must be said, that this 47% is absorbed by the earth's surface and then its warming takes place. This warming effect is responsible for example of the water evaporation of oceans, lakes, rivers, as well as from the rest of the hydrological cycle. There is a tiny percentage of this energy, only 0.2%, which causes atmospheric movements and marine currents.

In comparison, the kinetic energy of wind is converted into electricity in wind farms. The same figure, 0.2%, is used in the production of living matter. It is the main natural process able to take advantage of solar energy directly. We then have a distribution between the direct energy that affects the earth's surface (31%), and a diffuse energy that also affects the earth's surface (16%).

Up to this point we analyzed the use of solar energy. The remaining, 53%, does not reach the earth's surface because 30% is reflected into outer space, or 23% is absorbed by the atmosphere itself, causing its warming. There is a change of direction that occurs, when it reflect with clouds, with the ground or by a diffuse transformation that redirects energy back to outer space.

The solar energy that reaches the earth's surface needs to be transformed into another type of useful energy that, in most cases, is electrical or thermal energy. According to this, the artificial processes of use of solar energy can be direct or indirect processes.

a) The first performs a single transformation process of energy to make it usable, the case of obtaining thermal energy, same as obtaining photovoltaic and photoelectric energy.

b) However, in indirect processes, solar energy is successively transformed into diverse types of energy until a necessary mode is achieved to make use of it. The most representative example of this indirect process is wind energy. In the first place for this use, solar energy must produce thermal differences in the atmosphere that originate wind and this kinetic energy moves the generators, where mechanical energy is transformed into electricity.

Finally, we have two conversion processes: thermal and electrical energy.

In the first case, solar energy is absorbed and converted into heat, due to the reflection capacity of the object's surface, (the less reflective the surface is, the greater is the absorption of the energy and the greater is the production of the heating effect. As for the absorption capacity, it depends primarily on the surface color (black objects absorb all visible radiation and for this reason they heat up faster). And finally, there is the incident radiation, that can be increased, for example, using solar tracking systems or parabolic reflectors.

The technology currently employed, requires the use of artificial devices called solar collectors. These are devices designed to collect the energy received from the sun and raises the temperature (at thermal level) of a fluid seeking to take use of it.

Collectors are divided into two large groups: low temperature collectors, used mainly in domestic heating systems, domestic hot water and pool heating and high temperature collectors, formed by mirrors and generally used to produce steam that moves a turbine which will generate electric power. In relation to these data you can see the article: Estado actual de la energía Termosolar (CSP) a nivel global 24 de julio de 2018.

Concerning the electrical energy conversion it is necessary to say, that there are two technological processes that allow us to obtain electrical energy directly from the electromagnetic energy of the photons. On the one hand, we have the photoelectric effect, which consists in the generation of an electric current by releasing electrons from the surface of metals, when photons of sufficient energy collide with it, and on the other hand we have the photovoltaic effect, which is based on the property of certain materials that are capable of generating a small potential difference (voltage).

As a glossary we leave some terms of relevance: More information in this link: 

Solar radiation (Solar irradiance) is the sum of two irradiances (direct radiation + diffuse radiation).

Direct radiation is the irradiance that reaches the surface in a horizontal plane coming only from the solar disk, without having suffered any atmospheric dispersion process.

Diffuse radiation (diffuse solar irradiance) Also reaches the surface in a horizontal plane but coming from - in this case - the rest of the sky due to the dispersion process that take place in the atmosphere. This means that the radiation that reaches the surface after being dispersed in the atmosphere by clouds, particles in suspension and molecules which constitute the atmospheric gases, at the same time, constitute on average - under clear sky conditions- between 15 and 20% of the total incident surface radiation or global radiation.

Irradiance (power of the radiation that hits a surface). It is used to define the amount of energy that falls on an area. Applied to solar energy, it would be the amount of solar radiation (electromagnetic waves) that falls on the earth's surface. It is expressed in unit of energy / unit of area. For example, KW / m2

Do not confuse with Radiance, which is a unit of power (used in Radiometry) which defines the radiant flux emitted per unit of area and per a solid angle.

Irradiation (energy per unit of area over a time). It is expressed in unit of energy / unit of time / unit of area. For example: KW / h / m2. Irradiation is an irradiance related to time. Solar irradiation is the density of solar energy that falls on a unit of surface area (land) for a certain time. It is usually expressed in KW / m2 / day.

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