Solar power stations

1. The solar energy

 The Sun is a gas sphere mainly made up by helium, hydrogen and carbon. Its mass is 330,000 times the mass of the Earth and it is estimated that it is 6,000 million years old.

The Sun behaves as a nuclear reactor which transforms nuclear energy into radiation energy, energy that reaches the Earth. But not all the energy produced in the Sun reaches the earth surface. When going through the atmosphere, the radiation looses intensity due to the absorption, diffusion and reflection by the action of: gases, steam and suspended particles in the atmosphere.

Thus, the radiation the Earth receives from the Sun can be divided into the following:

• Direct radiation: that coming through the atmosphere without suffering any change in its direction.
• Disperse or diffuse radiation: that which is received after the reflection and diffusion.

We could say that the Earth receives a large amount of solar energy as radiations. This is why solar energy is a very important energy resource to be taken into account.

2.Explotation of solar energy

There are currently two main ways of exploiting the solar energy:

Thermal solar energy

The exploitation of the thermal solar energy consists in using the radiation of the Sun to heat up a fluid which, depending on its temperature, is used to produce hot water, steam or electric energy.

The systems to take advantage of the solar energy thermally can be divided into three groups:

• Low temperature systems. The heating of water is produced below its boiling water, that is to say, 100ºC. Most of the equipment based on this technology is applied to the production of domestic hot water and acclimatisation.
• Medium temperature systems. They are used in those applications requiring temperatures between 100 and 300ºC for heating, providing heat in industrial processes, supply of steam, etc.
• High temperature systems. They are used in applications requiring temperatures of more than 250 or 300ºC such as, for example, to generate steam or for the generation of electric energy in thermo solar power stations.

Photovoltaic solar energy

The photovoltaic solar energy is exploited by transforming it directly into electricity by means of the photovoltaic effect. This transformation is carried out by means of the use of photovoltaic cells.

3. What is a solar power station?

The solar power stations are installations designed to take advantage of the radiation of the Sun to generate electric energy. There are two types of installations with which the energy of the Sun can be used to generate electricity:

• In the thermo solar power station the generation of electricity is achieved by means of the heating up of a fluid with which using a conventional thermodynamic cycle, an alternator is moved thanks to the steam generated in the same.
• In the photovoltaic installation the obtaining of electric energy is carried out by means of photovoltaic panels that capture the luminal energy from the Sun to transform it into electricity. In order to achieve the transformation photovoltaic cells are used, which are manufactured with semi-conducting material.

4. Thermo solar power stations

A thermo solar power station is an installation that allows using the energy of the Sun to generate electricity using a thermal cycle similar to that of the conventional thermal power stations. There are different types of thermo solar power stations, although the most important are the following:

• Central tower power stations. They have a series of large directional mirrors that concentrate the solar radiation on a point. The heat is transferred to a fluid circulating along the inside of the boiler and transforms it into steam, thus starting a conventional water-steam cycle.
• Distributed collector power stations. They use the concentration collectors, which concentrate solar radiation received at the surface, which allows obtaining high performance temperatures of up to 300ºC, enough to generate steam at high temperature, which is used to generate electricity or for other industrial processes.

Operation of a thermo solar power station

A central tower thermo solar power station is made up by a field of large directional mirrors that reflect the light of the Sun and concentrate the rays on a boiler located in a tower at a great height.

In the boiler the calorific contribution of the solar radiation is absorbed by a thermal fluid which is taken to a steam generator, in which its heat is transferred to a second fluid (generally water) to transform it into steam.

This steam is taken to a turbine to transform its energy into mechanical energy which will be transformed into electricity in the alternator.

The steam is taken to a condenser where it is turned into water again in order to repeat a new cycle of steam production.

The production in a solar power station depends on the hours of sunlight. That is why in order to increase production thermal insulation systems inserted in the heating circuit are usually installed.

You can learn more about the thermo solar power stations in the following interactive game.

Limitations of the thermo solar power stations

The development of this type of power stations faces various limitations:

• Economic: their exploitation costs are still very high, that is why they are not competitive with other types of power stations .
• Technological: there are still many improvements in order to increase the efficiency of the concentration and storage systems.
• Seasonality: the viability of the solar radiation and the meteorological uncertainties have to be taken into account.

5. Photovoltaic farms

The photovoltaic effect is a physical phenomenon which consists of the conversion of luminous energy into electric energy. The generation of radiation (photons) on a heterogeneous structure (photovoltaic cell) is absorbed by the electrons of the most external layers of the atoms making up this material; this creates an electric current inside a determined voltage.

The cells are connected in series in order to make up a photovoltaic module.

Operation of the photovoltaic power stations

The basic element of a photovoltaic farm is the group of photovoltaic cells which capture the solar energy, transforming it into direct electric current. The photovoltaic cells are integrated in modules that, when joined together, will form photovoltaic panels.

The direct current generated is sent, initially, to a direct current cabinet where the transformation will take place with the assistance of a current inverter and, it is finally taken to a transformation centre where the current is adapted to the intensity and voltage conditions of the transport lines of the electric network.

You can learn more about the operation of the photovoltaic farms in the following game.

Limitations of the photovoltaic farms

The technology available has to be optimised for the efficiency of the photovoltaic cells to improve reaching figures of about 18-20%.

Spain in a pioneering country in the development of this technology and financial subsidies are provided to this type of electricity production.

6. Environmental impacts of the solar power stations

From the environmental point of view, the production of electricity from this type of system has great advantages:

• It does not generate any type of atmospheric emissions.
• Does not produce liquid effluents.
• Avoids the use of fossil fuel.

In spite of this, the large thermo solar power stations can cause considerable impact on the landscape and need large surfaces where to place the directional mirrors.

We should also mention that once their useful life is over, photovoltaic panels generate waste, which has to be treated specifically.