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How do solar cells work?? [With installation]

Solar cells working

In the last two decades the contribution of solar energy to the world's total energy supply has grown significantly. This post will show how solar cell or photovoltaic cell produce electricity. Energy from the Sun is the most abundant and absolutely freely available energy on planet earth. In order to utilize this energy we need help from the second most abundant element on earth, sand. 

solar power

  • The sand has to be converted to 99.999% pure silicon crystals to use in solar cells. To achieve this, the sand has to go through a complex purification process as shown. The raw silicon gets converted into a gaseous silicon compound form. This is then mixed with hydrogen to get highly purified polycrystal line silicon. 

  • These silicon ingots are reshaped, and converted into very thin slices called silicon wafers. The silicon wafer is the heart of a photovoltaic cell. When we analyze the structure of the silicon atoms you can see they are bonded together. 

When you are bonded with someone you lose your freedom. Similarly the electrons in the silicon structure also have no freedom of movement. To make the study easier let's consider a 2D structure of the silicon crystals. Assume that phosphorus atoms with five valence electrons are injected into it. Here one electron is free to move. In this structure when the electrons get sufficient energy they will move freely. Let's try to make a highly simplified solar cell only using this type of material. When light strikes them, the electrons will gain photon energy and will be free to move. 

However this movement of the electrons is random. It does not result in any current through the load. To make the electron flow unidirectional a driving force is needed. An easy and practical way to produce the driving force is a PN junction. Let's see how a PN Junction produces the driving force. Similar to n-type doping if you inject boron with three valence electrons into pure silicon there will be one hole for each atom. 

  • This is called p-type doping. If these two kinds of doped materials join together, some electrons from the N side will migrate to the P region and fill the holes available there. This way a depletion region is formed where there are no free electrons and holes. Due to the electron migration the N-side boundary becomes slightly positively charged. And the P side becomes negatively charged. An electric field will definitely be formed between these charges. 
  • This electric field produces the necessary driving force. Let's see it in detail. When the light strikes the PN Junction something very interesting happens. Light strikes the N region of the PV cell and it penetrates and reaches up to the depletion region. This photon energy is sufficient to generate electron hole pairs in the depletion region. 

  • The electric field in the depletion region drives the electrons and holes out of the depletion region. Here we observe that the concentration of electrons in the N region and holes in the P region become so high that a potential difference will develop between them. As soon as we connect any load between these regions, electrons will start flowing through the load. 

The electrons will recombine with the holes in the P region after completing their path. In this way a solar cell continuously gives direct current. In a practical solar cell you can see that the top N layer is very thin and heavily doped. Whereas the P layer is thick and lightly doped. This is to increase the performance of the cell. Just observe the depletion region formation here. You should note that the thickness of the depletion region is much higher here compared to the previous case. This means that due to the light striking the electron hole pairs are generated in a wider area compared to the previous case. This results in more current generation by the PV cell. 

The other advantage is that due to the thin top layer, more light energy can reach the depletion region. Now let's analyze the structure of a solar panel. You can see the solar panel has different layers. One of them is a layer of cells. You will be amazed to see how these PV cells are interconnected. After passing through the fingers the electrons get collected in busbars. 

The top negative side of this cell is connected to the back side of the next cell through copper strips. Here, it forms a series connection. When you connect these series connected cells parallel to another cell series you get the solar panel. A single PV cell produces only around 0.5 voltage. The combination of series and parallel connection of the cells increases the current and voltage values to a usable range. 

The layer of EVA sheeting on both sides of the cells is to protect them from shocks, vibrations, humidity and dirt. Why are there two different kinds of appearances for the solar panels? This is because of the difference in the internal crystalline lattice structure. In polycrystal line solar panels multi crystals are randomly oriented. If the chemical process of silicon crystals is taken one step further, the polycrystal line cells will become mono crystalline cells. Even though the principles of operation of both are the same, mono crystalline cells offer higher electrical conductivity. 

  • However mono crystalline cells are costlier and thus not widely used. Even though running costs of PV cells are negligible. The total global energy contribution of solar voltaic is only 1.3 percent. This is mainly because of the capital costs and the efficiency constraints of solar voltaic panels, which do not match conventional energy options. Solar panels on the roofs of homes have the option to store electricity with the help of batteries and solar charge controllers. 

However in the case of a solar power plant the massive amount of storage required is not possible. So generally they are connected to the electrical grid system in the same way that other conventional power plant outputs are connected. With the help of power inverters, DC is converted to AC and fed to the grid. Please support eengineering tech educational activities on eengineeringtech.in and also, don't forget to follow to our website. Thank you 

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