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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The use of solar cells. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources Welcome to the world of photovoltaics, to the world of electricity from the sun. If the reader has not yet been familiar with photoelectricity, then he will get real pleasure and will be rewarded for this acquaintance. We will talk about the use and application of silicon solar cells. Regardless of where the device will be used, solar cells are its integral part and are interesting in themselves. Thus, it is important to understand their nature and learn how to use them. There is nothing difficult in this chapter. We're just going to talk about "cogs and nuts". Basic principles of work The principle of operation of a solar cell is quite simple and is as follows. When illuminated, a silicon solar cell generates an electrical voltage of 0,5 V. Regardless of the type and connection scheme, all (large and small) silicon solar cells generate a voltage of 0,5 V. The situation is different with the output current of the element. It depends on the intensity of the light and the size of the element, which refers to the surface area. It is clear that a 10 x 10 cm2 element is 4 times larger than a 5 x 5 cm2 element, therefore, it produces 4 times more current. The strength of the current also depends on the wavelength of the light and its intensity, and it is directly proportional to the intensity of the radiation. The brighter the light, the more current is generated by the solar cell. Increasing the output characteristics of solar cells Solar cells would be used very rarely if operated within the parameters mentioned. Only in some cases is such a low voltage (0,5 V) required for arbitrary requirements for the amount of current consumed.
Fortunately, there are no restrictions here. Solar cells can be connected in series and in parallel to increase output characteristics. We will consider solar cells as ordinary batteries. It is known that several batteries are used to increase the brightness of the flashlight. In essence, when batteries are connected in series, the total voltage increases (Fig. 1). The same can be done with solar cells. By connecting the positive terminal of one cell to the negative terminal of the other, a voltage of 1 V can be obtained from two cells. Similarly, three cells will give 1,5 V, four cells 2 V, etc. Theoretically, the voltage developed by series-connected solar cells, provided that there is enough of them, can reach thousands of volts! Unfortunately, from the point of view of increasing the output current, the series connection has an inherent disadvantage. When the batteries are connected in series, the output current does not exceed the level characteristic of the worst element in the circuit. This is true for all power sources, whether they are batteries, power supplies, or solar cells. This means that for any number of 2A solar cells in a circuit, a 1A cell will determine the total output current, i.e. 1A. Therefore, if you are looking to achieve maximum performance, you must match the currents of all elements in the circuit. Okay, tension is clear. But how to increase the output current of a solar cell? After all, the sun shines with a certain brightness. The output current depends on the surface area of the element, and so the natural way to increase the current is to increase the area of the element (or elements). Elements? Exactly!
If we take four elements of 5x5 cm2 each and connect them in parallel, as shown in fig. 2, it is possible to achieve the same result as when replacing four elements with one size of 10x10 cm2 (in both cases, the surface area is the same and is 100 cm2). It must be learned that with a parallel connection, only the magnitude of the current increases, and not the voltage. Regardless of the number of elements connected in parallel (4 or 50), the generated voltage will be no more than 0,5 V. Photovoltaic batteries You can guess what will be discussed. Indeed, in order to take advantage of both switching methods, it is possible to combine series and parallel connection of elements. This combination is called a battery. Batteries can be made in any desired combination. The simplest battery is a chain of series-connected cells. You can also connect chains of elements in parallel, individual elements in chains, or combine them in any other combination. On fig. 3 shows only three examples of possible combinations.
Differences in the nature of the connections of elements in fig. 3, although they all have the same output characteristics, are dictated by different reliability requirements. On fig. 3, and three consecutive chains of elements are connected in parallel. This method is used when there is a high probability of a short circuit of individual elements. On fig. 3, b shows a diagram of a parallel-series connection of elements. With such a connection, the failure of one of the elements, for example, due to the appearance of a crack, does not lead to the loss of the entire chain due to a chain break. In the last example (Fig. 3, c), both cases with a minimum of connections are taken into account. Other types of connections are possible, and their choice should be determined by the specific operating conditions of your device. One important condition should be remembered. Regardless of the flight of your imagination, parallel connected chains of elements must necessarily match each other in voltage. You cannot connect a chain of 15 elements and a short chain of 5 elements in parallel. With this connection, the battery will not work. reverse bias When working with solar panels, as a rule, they encounter a phenomenon that does not occur when using conventional power supplies. This phenomenon is associated with the so-called reverse bias. To understand what this is, let's look at Fig. 4.
This figure shows 8 elements connected in series. The total output voltage of the circuit is 4 V, and the resistor RL is connected as a load. So far so good. But let's darken photocell D with an opaque object, like a hand, and see what happens. You probably think the voltage will drop to 3,5V, right? Nothing like this! A solar cell that does not produce electrical energy is a link with a high internal resistance, not a short circuit. The same happens as when the switch is opened, but this switch is not fully open - a small current flows through it. In most cases, the effective resistance of a darkened solar cell is many times greater than the value of the load resistor RL. Therefore, in practice, you can consider RL as a piece of wire connecting the negative and positive terminals. This means that element D now performs the load function. What do the other elements do? Supply energy to this load! As a result, element D heats up and, if heated sufficiently, can fail (explode). As a result, we are left with a battery from a serial chain with one inactive element - an unenviable situation.
An effective way to solve this problem is to connect shunt diodes to all elements in parallel, as shown in fig. 5. The diodes are connected so that when the solar cell is operating, they are reverse-biased by the voltage of the cell itself. Therefore, no current flows through the diode, and the battery functions normally. Let's now assume that one of the elements is shaded. In this case, the diode turns out to be forward biased and current flows through it into the load, bypassing the faulty element. Of course, the output voltage of the entire circuit will decrease by 0,5 V, but the source of the self-destructive force will be eliminated. An additional benefit is that the battery continues to function normally. Without shunt diodes, it would completely fail. In practice, it is impractical to shunt each battery cell. Consideration should be given to economic considerations and the use of shunt diodes based on a reasonable trade-off between reliability and cost. As a rule, one diode is used to protect 1/4 of the battery. Thus, only 4 diodes are required for the entire battery. In this case, the shading effect will result in a 25% (tolerable) reduction in output power. Cutting elements into pieces Not always serial elements exactly match your plan. Although they try to offer you as much choice as possible, there is no way to satisfy all requests. Fortunately, this is not required. Monocrystalline solar cells can be molded into any desired shape.
You should know that this is the case, because monocrystalline solar cells are made from a large single crystal. The silicon atom has four valence electrons and forms a cubic crystal lattice. On fig. 6 shows a typical round solar cell with a prominent granular structure. If a force is applied to this structure of strongly bound electrons, then a crack will appear along the defect line. This is very similar to a crack that occurs as a result of an earthquake. The structure of the crystal is known and hence the direction of the crack can be predicted. If force is applied to the edge shown in Fig. 6 of the plate at point A, then the mechanical forces acting inside the crystal will split it into two halves. Now instead of one element there are two. Let's say it is necessary to split such an element into four identical parts. This can be achieved by applying force first along the vertical defective line and then along the horizontal one. Fortunately, this can be done at the same time. Most single-crystal round elements are marked with a cross in the center. If you press at this point with a cross-tipped knife, the element will split into four neat pieces. Don't worry if you don't hit the exact center. The element will split, but not into equal pieces. The size of the fragments will be determined by the point of application of the force, but they will all be split along the same planes. The cleavage lines are always parallel to each other, and all intersections occur at right angles. Guided by these rules, you can get elements of any required size. When trying to split an element for the first time, you must be extremely careful: you cannot work on a hard surface. Applying a lot of force to an element lying on a hard flat surface, you can only make a hole in it. To create mechanical stress, it is necessary for the element to bend. I found that a couple of sheets of paper (maybe newsprint) is enough when splitting an element. Only single-crystal elements can be split in this way. Recently appeared polycrystalline elements (wacker cells) cannot be split symmetrically. If you try to do this, the solar cell will shatter into a million pieces. A polycrystalline element is easy to distinguish from a single crystal. As a result of processing, the single crystal has an even, smooth surface structure. Polycrystal looks like galvanized steel with its characteristic surface appearance. Soldering solar cells After the solar cells are selected for work, it is necessary to solder them. Usually, we have at our disposal serial solar cells equipped with current-collecting grids and back contacts, which are designed for soldering conductors to them. During manufacture, the contacts are most often coated with solder containing a small amount of silver. Silver protects the tip of the soldering iron from destruction and possible adhesion of thin metal contacts during soldering. Remember that current collector grids are as fragile as the metal conductors of printed circuit boards. Solar cell manufacturers typically use special solder, flux, and conductors for the connections. Solder containing 2% silver can always be purchased at the store. Instead of rosin, a regular water-based flux should be used so that it can be easily washed off the surface of the element after soldering. The hardest thing to find is a flat, ribbon conductor, as it is rarely available for sale. However, you can make something similar if you take a piece of copper wire and flatten its end with a hammer. Instead, you can use copper foil or just thin copper wire. The soldering process itself is not difficult, but it must be done quickly. The silicon plate is a very good heat sink, and if you touch the element with a soldering iron for a long time, the soldering iron tip will cool down below the melting temperature of the solder. First you need to tin the wire using a little more solder than usual, but not too much. The solar cell is already tinned during manufacture. For work, it is recommended to use a soldering iron with a power of 30 or 40 watts. The tip of the soldering iron must be clean and warm. While the soldering iron is heating, flux is applied to the element and the tinned wire is pressed against the contact base of the element. Now touch the hot soldering iron to the surface of the wire. It is necessary that the joint be "enveloped" with molten solder and reliable contact of the wire with the element is ensured. Soldering is done in one touch: you need to work quickly, but carefully. The rear contact is soldered in the same way. To obtain a sequential chain of elements, the front contact of the first element is connected by a wire to the rear contact of the second. Then, with another piece of wire, the front contact of the second is connected to the back of the third, etc. The front contact is the negative electrode, while the back contact is the positive electrode. Another widely used method is the connection of elements in the form of a tiled roof. If you've ever seen a tiled roof, you've already got the idea. The front contact of one element is covered from above by the back contact of the other. The touch point is heated with a soldering iron, and thus the two elements are connected to each other. Such a connection is shown in Fig. 7.
It is necessary to collect some excess solder on the tip in order to reliably solder the elements. Be careful not to overheat the element, otherwise there will be no contact at all. In this way, it is better to solder small elements in which you can simultaneously heat the entire contact area. It is best to use a special rectangular soldering iron tip designed for desoldering integrated circuits from printed circuit boards. Uniform heating and pressure will be the key to success. Battery protection Now that the battery is assembled, it is necessary to protect it from mechanical damage and weather conditions. It is best to place the elements face down on a clean sheet of glass or Plexiglas. Safety glass is preferred, followed by toughened window glass, acrylic plastic, and normal window glass in descending order of safety. A transparent coating protects the battery from mechanical damage during shock and twisting, bending. But it does not protect well from moisture. As you know, silicon is slightly hygroscopic; this means that it absorbs very little water. However, after a long period of time, there is a gradual decrease in the output characteristics of the element due to the influence of humidity. Thus, battery life directly depends on the quality of moisture insulation. Moisture insulation can be provided in many ways. In accordance with one of them, the back side can be filled with liquid rubber. To do this, it is necessary to make a frame around the perimeter of the protective glass so that the liquid polymer does not overflow. In addition, a strong frame well protects the protective glass from side impact. Another method involves covering the back of the battery with a thick sheet of Mylar plastic and heating the entire battery, such as with an incandescent lamp, until the Mylar melts and adheres to the front protective cover. This operation requires some skill, especially in the case of large batteries. The back mylar cover can be simply glued on. This operation is often simpler than heating, but the insulating properties deteriorate. Finally, the back side of the battery cells can be covered with several layers of latex. It does not look as aesthetically pleasing, but provides fairly good moisture-proofing properties. Last but not least is the production of a moisture-proof hermetically sealed box for the elements. It is expensive, but provides the necessary moisture insulation. Author: Byers T.
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