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| Photovoltaic Energy |
Photovoltaic solar energy is a clean and renewable energy, easy to install and maintain, which basic principle is the conversion of solar energy into electricity.
Although photovoltaic solar energy only represents 0.001 % of the world’s electric power supply, a fast and significant growth of its implementation is forecast, based on the current technology development and the environmental commitment of the most developed countries.
Its conversion principle takes place as follows: when sunlight falls on certain materials called semi-conductors, the photons that make up the light can convey their energy to the semi-conductor’s valence electrons, freeing the latter from the bond that ties them to their respective atoms.
For each broken bond there is an electron that is free to circulate inside the solid, in our case silicon. The space left by each freed electron in the broken bond is called a hole and can also move freely inside the solid from one atom to the next as a result of the movement of the rest of the electrons of the bonds. Holes behave in many respects like particles with a positive charge of the same value as the electrons.
The physical structure of these materials creates an electric field that establishes a path for the released electrons, thereby generating an electric current.
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| That effect is the fundamental base for the operation of a photovoltaic cell. The union of these cells forms a photovoltaic module or panel. |
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| From then on the conventional electricity generation starts: the direct current produced by the modules is conveyed to the inverters where it is converted into alternating current and after that to MT, heading later for a connection point of the general distribution network. |
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Solar Thermal  |
In general, a thermoelectric plant is a system that can generate electric power from thermal power by means of what it is called power cycle, and in order to be able to develop this cycle it is necessary to have a source of primary energy from which to obtain the necessary thermal power. If the primary energy source is the solar radiation, the thermoelectric plant is called Solar Thermal Power Plant [in Spanish, Central Energética Termosolar (CET)]
Unlike photovoltaic installations, a CET does not generate electricity directly from solar radiation but transforms this radiation into thermal power which is brought to a conventional power cycle and the latter transforms that thermal power into mechanical power. Subsequently, by means of an electric generator the mechanical power is transformed into electric power, the latter being injected into the electric network and reaching the consumption points.
CETS can incorporate a power storage system which allows continuing to supply power in the absence of solar radiation. The daily interval during which electric power can continue being supplied shall vary depending on the storage system capacity.
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| Types of plants: |
| There are three types of solar thermal power plants given the characteristics of the solar field, that is, the system used to convert solar radiation into thermal power. |
| 1. - Parabolic Cylinder Collector Plants (PCC) |
In this type of plants the solar field is made up of parallel rows of parabolic cylinder collectors (PCC), and each row can accommodate several collectors connected in series. Each collector is basically composed of a parabolic-cylinder mirror that reflects direct solar radiation concentrating it over a receiver tube disposed along the focal line of the parabola.
The concentration of solar radiation results in a warming of the fluid that circulates inside the receiver tube.
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| 2. - Central Receiver Plants or Tower |
| They consist of a field of heliostats (mirrors) that follow the sun’s position at all times (elevation and azimuth) and position the reflected ray to face the reflector placed at the top of a tower. |
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| 3. - Parabolic Dish Plants |
| They are small independent units made up of a reflector in the form of paraboloids of revolution that concentrates the rays onto the receiver positioned at the focal point of the paraboloid and which, in turn, comprises the power generation system based on a Stirling engine. A variant of this type of plants are the plants that instead of a reflector have several reflectors so that the group forms a structure that resembles a paraboloid of revolution. |
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| Frequently asked questions: |
| Can the energy produced be stored? |
The energy obtained in large photovoltaic plants is not usually stored, since it makes more sense financially to connect it directly to the distribution network.
The energy obtained in solar thermal installations is usually stored in part, thanks to the quality of the fluids and mineral salts used in these systems, as they can retain during a long period of time the high temperatures obtained after exposure to solar radiation.
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| What can be done with the electricity generated? |
| It can be used in a direct way (to draw water from a well, to generate electricity, to irrigate, etc.) or it can be stored in accumulators for later use (to generate electricity during the night). If the general electricity network reaches the installation, the best alternative is to sell all the electricity generated to the electricity company. In this way, there is an economic benefit for the private user or company owing to the fact that the selling price of the electricity generated is higher than the purchase price. |
| The sunnier it is the better performance of the installation? |
In the case of solar thermal energy, the sunnier the weather, the more hot water we obtain.
In solar photovoltaic installations, higher temperatures do not always result in a better performance. Photovoltaic modules suffer a reduction in power at high temperatures.
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| Can photovoltaic panels operate on cloudy days? |
Photovoltaic panels produce electricity even on cloudy days, nevertheless their efficiency decreases.
The production of electricity varies linearly with the incident light and so on a totally overcast day when the incident light is equivalent to approximately 10% of the total intensity of the sun, the efficiency of the panel decreases in proportion to this value.
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| What is the amortization period of the photovoltaic installations? |
There are different factors that determine the amortization period of an installation: the correct calculation of the needs, the system’s optimization, an adequate installation and the quality of the materials, public subventions and, mainly, the use of such installation.
Nevertheless, to provide a general idea, we can say that photovoltaic installations are amortized after 10 years.
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| Is there any kind of subvention for these installations? |
| There are subventions for both photovoltaic and solar thermal installations. The method of subvention consists of applying a bonus system to the selling price of electric power. |
| What is a solar park? |
| Solar parks are large photovoltaic installations located outside the city centre that pour all their electricity into the general distribution network. |
| What is the peak power of a panel? |
| It is the output power, in Watts, that a photovoltaic panel produces in conditions of maximum solar lighting, with a radiation of approximately 1 kW/m2 and a room temperature of 25ºC. |
| Are all panels the same? |
| No. In fact, broadly speaking, panels are divided into two technologies: |
| 1. - Crystalline Silicon: |
| These ones are also divided into two large groups of panels: monocrystalline silicon and polycrystalline silicon. |
| 2. - Thin Film: |
| Thin-film modules are based on different technologies: CdTe, amorphous silicon, etc. |
| What should be the inclination angle of the solar panels? |
| The optimum inclination of the solar panels depends on the latitude of the installation. |
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