We will introduce in this second part, the solar photovoltaic energy. That is to say among the various forms of solar power will focus on one of them which is the direct production of electricity. The principle of solar cell is described on this sphere. The first step is absorption of solar photons, yellow. In a material which is a semiconductor, silicon in general. The absorption of the solar photon will excite electrons initially bonded to atoms then becoming free. It will therefore remain in the material positive charges which are the so-called holes. So we created an electron-hole pair. This is what we see later in the course. In general, this pair will recombine. In fact, these electron-hole pairs will be separated by an internal electric field in the structure. The so-called diode. This internal electric field is created by the introduction of impurities into the semiconductor which will be called doping. This separation will create negative charges on an electron interface with the internal circuit. Also positive charges on the other electron. If these two electrodes are connected by an external circuit, we will obtain a voltage in the internal circuit on a DC electric current. These solar cells can be associated in photovoltaic modules for higher power production. What type of energy efficiency can be achieved with a solar cell? Do a simple thermodynamic estimate of the Carnot efficiency. The sun can be considered as a black body at 6,000 degrees K, yellow color. Its energy spectrum is described by Planck's law. When exposed to sunlight, the cell will eat up, say about 300 degree K. These corresponds to a blackbody in the infrared. Therefore, the boundary of the Carnot efficiency is close to 95 percent. This is a very high yield. This is because the sun is an excellent heat source at 6,000 degree K. If we consider the continuum of Planck's law, we obtain the maximal yield at about 85 percent. We will see later in the course that you never get this kind of performance. But we can get about half by combining several semiconductor materials which is already an impressive performance. We know evokes the different applications of photovoltaics. Consider first applications connected to the grid. Depending on the power, the first application is domestic such as solar panels on the roof of house. The corresponding power is of the order of 10 to 20 kilowatts. Larger application can be considered. School, hospital, parking, this corresponds to higher powers which can reach megawatt. And then, we can achieve the so-called utilities of solar farms on very large long areas. The installed capacity can the reach several hundreds of megawatts. Of course, the cost of energy will decrease from left to right. Other photovoltaic applications are autonomous without great connection. This can range from automotive roof to power a ventilation for example, to telecommunication units to name a few. A solar module, consisting in this example of about 30 solar cells, is a DC generator that depends on sunshine. We present the typical theoretical characteristic IV depending on the sunlight intensity. The first blue line correspond to the maximum sunlight. Noon sunny, about 1 kilowatt per meter square. The current decreases linearly with the sunshine. The maximum voltage also decreases with sunshine but in a slightly different way. We will see that is logarithmically. While keeping the data above, now consider the variation of the power P equal VI as function of the voltage. The green curve correspond to the characteristic IV, the most favorable in the previous figure. The curve P on V passes to a maximum as will be demonstrated later. The conversion efficiency is the maximum power delivered divided by the incident light power. This later amount is normalized at noon solar spectrum am 1.5. as we will define later in the course. The short-circuit current Isc which correspond to V equal zero, an open circuit voltage Voc, which correspond to I equal zero, are defined in the figure on the fill factor FF. FF is very affected by the slopes of the characteristic at the origins. Finally, we obtain the formula of the yield based on the product Isc, Voc. We will return in detail on this concept later. We now illustrate the different photovoltaic elements. We take the example of the crystalline silicon technology which represents more than 90 percent of the global market. We start from the cell typically, 15 centimeters by 15. Generally, obtain from a cylinder of 20 centimeters in diameter which gives the Carnot effects. The Voc of the cell is about 0.6 volts. A solar module is a set of solar cells connected in series in order to obtain sufficient voltage for charging a battery. At least 12 volts. Typically, 30-36 or 72 cells. The inverter used for alternating current transformation requires higher voltage cell batteries. Then the modules can be assembled to achieve high power solar power plants several hundred megawatts utility scale. What energy yields can be obtained with silicon technology? Here we show a commercial advertisement from Sunpower subsidiary of the French oil major Total. Sunpower therefore, sells solar modules with an efficiency of 21.5 percent. This is currently the world record. Note that a few years ago, this record was not more than 17 percent. The performances of the silicon technology are still improving as will be seen later. Let's return to the system aspects that play an important role in the total cost of electricity. Before connecting the solar modules to the grid, DC to AC conversion, it requires using an inverter. However, this transformation is not necessary in the case of standalone installation as in remote areas where there is no electricity grid. Particularly, in Africa. However, the use of batteries for storage becomes necessary in the later case. This figure shows an overview of photovoltaic applications. During the last decade, there has been a rapid gross in the share of the grid connected applications which now account for almost all the market. The share of production from utility scale is growing since it represents now approximately 60 percent of the production capacity especially in China,84 percent on the United States. So we're just describing the principle of solar photovoltaics on PV applications field. Later, we will look at the values photovoltaic technologies. Thank you.
