Solar Electricity Basics: photovoltaics, solar cells, irradiation, orientation, tilt angle, and watt peak. |
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How Photovoltaics work?
For solar electricity you need a solar panel that consists of one or more solar cells. When sunlight falls onto a solar cell, the solar cell material absorbs some of the light particles (so called photons). Each photon contains a small amount of energy. When a photon is absorbed it starts a process of freeing an electron in the material of the solar cell. Because both sides of a solar cell are electrically connected with a wire, a current will flow when the photon is absorbed. The solar cell now produces electricity, which can be used instantly or stored in a battery.
1. light (photons)
2. front contact
3. negative layer
4. diversion layer
5. positive layer
6. back contact
As long as the solar cells are exposed to light this process of creating free electrons continues and electricity is produced. Materials that possess this photovoltaic effect are some semiconductors. In a special production process solar cells are made out of this semiconductor material.
A solar panel can produce clean electricity for 20 years or more. Wear is mainly due to exposure to the environment. A well-mounted solar panel will be a reliable, silent and clean source of energy for many years.
Solar Cells
The silicon solar cells can be of the type single crystalline, multi crystalline or amorphous. The difference between these cells is how the silicon atoms are ordered, the crystalline structure. There is also a difference in efficiency. By efficiency is meant the percentage of sunlight that is converted into electricity. Single and multi crystalline solar cells have almost the same and the highest efficiency with respect to amorphous silicon.
A standard solar cell is composed of a number of layers. First there is a back contact layer and then two layers of silicon. On top are the front metal contacts with an anti-reflection layer, which gives the solar cell its typical blue colour.
The last decade new types of solar cells of other materials than silicon are being developed. These are for instance thin-film solar cells and CIS (copper indium diselenide) and CdTe (cadmium telluride) solar cells. These cells are beginning to become commercially available.
| Cell efficiencies: | |
| Single crystalline: | 15-18 % |
| Multicrystalline: | 13-15 % |
| Amorphous: | 5-8 % |
| Cadmium telluride: | 6-9 % |
Solar Panels
Solar panels consist of solar cells. As one single solar cell does not produce sufficient energy for most purposes, solar cells are put together in solar panels so that they produce more electricity jointly.
Solar panels (also called Photo Voltaic or PV modules) are produced in many types and sizes. The most common ones are 50 Wp (Watt peak, producing a maximum of 50 Watts of solar electricity in bright sunlight) and consist of Silicon solar cells. Such panels are about 0,5 m2. However, a wide range of smaller and larger panels is commercially available. Solar panels can be interconnected to generate more solar electricity (two interconnected 50 Wp solar panels equal one 100 Wp panel).
The efficiency of the solar panels that are commercially available varies from 5-15%. This means that 5-15% of the energy of all sunlight that reaches the cell will actually be transformed into electricity. Research laboratories all over the world are developing new materials with higher efficiencies (upto 30%). Production costs are evenly important. Some new technologies (such as thin film) allow labour extensive large scale production which would decrease the cost significantly.
Irradiation
The sun is continuously emitting enormous amounts of energy. A fraction of that energy reaches the earth. The fraction of the energy from the sun that reaches the earth in just one day is still more than enough to cover the energy use of the world in a whole year. However, not all the energy of the sun that reaches the earth can be used effectively. Part of the sunlight is absorbed in the earth's atmosphere or reflected back into space.
The intensity of the sunlight that reaches the earth varies with time of the day and year, location, and the weather conditions. The total energy on a daily or annual basis is called irradiation and indicates the strength of the sunshine. Irradiation is expressed in Wh/m² per day or for instance kWh/m² per day.
To simplify calculations with irradiation data solar energy is expressed in equivalents of hour's bright sun light. Bright sun light corresponds with a power of about 1,000 W/m² so one hour of bright sunlight corresponds with an amount of energy of 1 kWh/m².
This is approximately the solar energy when the sun shines on a cloudless day in the summer on a surface of one square meter perpendicular to the sun.
Please visit the PVGIS: Geographical Assessment of Solar Energy Resource and Photovoltaic Technology for solar irrradiation levels in Greece and the rest of Europe.
The irradiation varies with time of day but it can also vary considerably from location to location, especially in mountainous areas. Irradiation various from an average of 1,000 kWh/m² per year for northern European countries such as Germany, to 2,000 to 2,500 kWh/m² per year for desert areas. Weather conditions and the difference in the relative position of the sun (solar elevation) in the sky, which depend on the latitude of each place on earth, cause these variations (see also orientation and tilt angle).
Orientation
The sunlight travels in a straight line from the sun to the earth. When it enters the earth's atmosphere part of the light is scattered and part of it reaches the surface in a straight line. Another part of the light is absorbed by the atmosphere. This scattered sunlight is what we call diffuse radiation or diffuse light. The sunlight that reaches the surface without being scattered or absorbed is of course direct radiation. Direct radiation is the most intense as everyone knows from sunbathing or working outdoors.
1. direct
2. absorption
3. reflection
4. indirect
Only a small fraction of all sunlight actually reaches the surface of the earth.
A solar panel produces electricity even when there is no direct sunlight. So even with cloudy skies a solar energy system will produce electricity (see How does it work). The best conditions, however, are bright sunlight and the solar panel facing towards the sun. To benefit most of the direct sunlight a solar panel has to be oriented as best as possible towards the sun. For places on the Northern Hemisphere this is south, for countries on the Southern Hemisphere this is north.
In practice, the solar panels should therefore be positioned at an angle to the horizontal plane (tilted). Near the equator the solar panel should be placed slightly tilted (almost horizontal) to allow rain to wash away the dust.
A small deviation of these orientations has not a significant influence on the electricity production because during the day the sun moves along the sky from east to west.
Tilt Angle
The sun moves across the sky from east to west. Solar panels are most effective when they are positioned facing the sun at a perpendicular angle at noon. Solar panels are usually placed on a roof or a frame and have a fixed position and cannot follow the movement of the sun along the sky. Therefore they will not face the sun with an optimal (90 degrees) angle all day. The angle between the horizontal plane and the solar panel is called the tilt angle.
Due to motion of the earth round the sun there are also seasonal variations. In the winter the sun will not reach the same angle as in summer. Ideally, in the summer solar panels should be placed somewhat more horizontal, to benefit most from the sun high in the sky. However these panels will then not be placed optimally for the winter sun. To achieve the best year round performance solar panels should be installed at a fixed angle, which lies somewhere between the optimum angle for summer and for winter. For each latitude there is an optimum tilt angle. Only near to the equator the solar panels should be placed horizontally.
1. winter sun
2. summer sun
Optimum tilt angle in winter and summer time.
Small deviations of about 5 degrees to this optimum tilt angle only have a minor effect on the electricity production. The differences due to weather conditions in the electricity production are more important. For stand alone systems, the optimum tilt angle depends on the monthly demand pattern.
Watt Peak
A solar cell produces electricity when it is exposed to light. Depending on the intensity of the light (the irradiance in W/m²) a solar cell produces more or less electricity: bright sunlight is preferable to shade and shade is better than electric light. To compare solar cells and panels it is necessary to know the so-called nominal power of such a cell or panel. The rated power, expressed in Watt peak or Wp, is a measure of how much energy such a solar panel can produce under optimal conditions.
To determine and compare the nominal power of solar panels, the output is measured under standard test conditions (STC). These conditions are:
- An irradiance of 1,000 W/m²
- Solar reference spectrum AM 1.5 (this defines the type and colour of the light)
- Cell temperature of 25 °C (Importantly, the efficiency of a solar panel drops when the cell temperature rises).