After this general presentation of the Photovoltaic Solar Energy, we will now briefly present in this chapter, the operation of solar cell on PV modules. Photons are first absorbed by your solid , this energy is transferred to an electron that was initially bonded. It will then become free. In a further step, it will be possible to separate the positive or negative charges using an electrical field. Finally, the charges will be transferred to the metallic electrodes. So, a potential difference will appear. If the two electrodes are connected to a load circuit, power generation will take place, on the end of the cycle, the electron recombines. In reality, the process is more complex. As the first lost of energy, the path of the recedant photon flux is reflected. Then, all the photon energy is now transported to the electron. Some part is lost, this is called Thermalization. This phenomenon will be addressed later in the course. There after, the electron separation from positive charges is not perfect and some recombination takes place in the device. And finally, the contact between the solid semiconductor on the external circuit can depart from perfect behavior and you see new losses. The electron recombines at the end. Before discussing the physics of solar cell, first, let's look at solar radiation, which is a reliable source of energy. Then, it will be seen as a principle of operation of the solar cell as the ideal case. The real case will first focus on the limits of the conduction efficiency and then, to the solar cell optics, and finally, introduce the concept of photovoltaic module. Solar Radiation is defined from two quantities, which are, first, Irradiance, in watts per square meter, which is the power density delivered by the solar flux. This is a instantaneous quantity of power. The other is a irradiation, which is therefore, the time integral of irradiance energy. These quantities depend on the time of day, day of the season, on latitude of the location considered. The sun is refer from two different angle, zenith and azimuth, as shown in the figure. First, the zenith, we check on for the height of the sun, which is in fact the complimentary angle of elevation. When the sun is vertical, the zenith is zero. And then, the azimuth varies from east to west during the day. We will now focus on seasonal variations in solar irradiance, which is illustrated in this animation, where we see the rotation of the earth around the sun during the 12 months of the year. The earth is tilted with respect to its plane of rotation. This is the origin of the various seasons. So, for six months of the year, the Northern Hemisphere is oriented towards the sun, and during the other six months, it is the case of the Southern Hemisphere. We will look now at a few particular days, which are the equinox and solstice, as shown in the animation. Let's start with the autumn equinox. The autumn equinox as the particularity that the Sun/Earth's axes is located in the equatorial plane. So that the length of the day is 12 hours. The Northern and Southern Hemispheres being equivalent. This equinox is that, at noon the zenith is equal to the latitude. For the winter solstice, Norther Hemisphere, the sun elevation is minimum. It corresponds to the shutters there of the year. We take here as an example of the latitude of the Ecole polytechnique. Then, we pass through the spring equinox, which is exactly equivalent to the autumnal equinox, the length's 12 hours. Then, we arrive at the summer solstice, the longest day of the year. Saint John, in the Christian world, which is illustrated in this animation. I surmise the previous data here. Data is zenith, complimentary of elevation on five latitude. But again, taking the example of the location of polytechnique, that is to say, latitude 48.7 degree, at polytechnique at summer solstice, the zenith is 23 degrees. That is to say, the sun is high, still 67 degree at noon. In contrast to the winter solstice, the sun elevation is only 18 degree at noon, which is very low. An important element to describe solar radiation is called Air-Mass, that measures the amount of atmosphere caused by the sunlight. More specifically, as illustrated in the figure, the Air-Mass is the inverse of the cosine of the zenith. It is zero outside the atmosphere, and then increases as a function of the inclination of the sun. If we consider Ecole polytechnique, at summer solstice at noon, which is the most favorable case, the Air-Mass value is little more than one. As the winter solstice soars end of December, it is more than three. At the equinox, the Air-Mass is 1.5. This Air-Mass ratio varies with the latitude, obviously. This figure shows the variations of the Air-Mass in the Brussel region in Belgium on northern , of course. Depending on the time of the day for all months of the year, the winter Air-Mass is very large, especially, early or late in the day. It is larger than three even at noon. However, the closer you are to the summer solstice, the more Air-Mass is small. In June, July, even August, the Air-Mass is smaller than two for most of the day. Let's consider now changes in the azimuth of the sun from east to west. Here, we present the polar coordinates. The red arrow indicates the zenith year. We always consider the case of the Paris region. Contrary to what we often think, the sun does not rise in the east and sets in the west. This behavior is strictly to two days a year that does the equinox. However, in winter, the sun rises in the south-east and sets in the south-west. Summer, on the contrary, the sun rises at the north-east and sets in the north-west. Such data are affected by the latitude. We investigated the earth's trajectory around the sun. We will look thereafter to the solar spectrum on each variation, depending on the wavelength of the light. Thank you.
