Heliostat. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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In devices called equatorial tracking systems, the angle of inclination of the axis to the ground is kept constant. In this regard, with the change of seasons, there will be a constant decrease in the efficiency of photovoltaic conversion. To obtain maximum efficiency, it is necessary to introduce additional adjustment of the angle of inclination.

The convenience of introducing adjustment depends on the specific installation. It is not advisable to change the value of the polar angle, otherwise the very meaning of such a tracking device disappears. Therefore, it is necessary to adjust the angle at which the solar panel is attached to the axis.

It would be useful to have a solar tracking system capable of tracking the position of the sun in two planes, i.e. a two-axis tracking system. A tracking system with two degrees of freedom is often called a heliostat.

Heliostats

The term heliostat is often used to refer to solar panels, but this is somewhat incorrect. In fact, a heliostat is a motor-driven reflector (mirror) mounted on the upper surface of the support, which follows the sun and reflects its light constantly in the same place. Since it is the heliostat that monitors the sun, let's take a closer look at its work.

Due to the complexity of the movement process, the heliostat is usually placed on a vertical support and driven by an azimuthal tracking system. The azimuth servo system differs from the equatorial one in a number of significant ways.

First, the supports of almost all azimuthal systems are installed vertically (Fig. 1). The vertical support has many advantages over the inclined one used for polar tracking systems. First of all, there are no bending stresses in the support. When the support is tilted, tension appears at the point of contact with the ground.

Ris.1

The magnitude of the stress is directly proportional to the weight of the equipment placed on the support, and this always causes certain difficulties. On the other hand, a straight column transmits force vertically downwards. Therefore, if the column is not under lateral stress, it has a lightweight design. Think of the dandelion stem, which supports the vertically applied weight of the flower, but can easily break when bent.

Of course, there are inclined supports of azimuth tracking systems (located at an angle equal to the latitude of the installation location). But in this case, they can be attributed to the type of equatorial servo systems, even if they are controlled in two different planes.

This type of tracking system is mainly used by astronomers. And although the telescope rotates around two axes, only the polar drive is constantly used. The elevation angle of the telescope is often set only once.

Azimuthal tracking systems differ from equatorial tracking systems mainly in that they simultaneously track an object in two different planes. Therefore, two motors are required for the drive. One motor moves the receiver of solar radiation in a horizontal plane, the other - in a vertical one. There is no fixed position or orientation. Without any restrictions, the azimuth tracking system can point to any point in the sky at any time.

Obviously, to provide such a range of movements, a more complex device is required than a simple clockwork. Often such a complex movement is controlled by a computer. (Referring to clock mechanisms used to point telescopes at a certain point in the starry sky).

Of course, in our tracking system we do not need a computer, but we will use some properties of computer logic. With the help of a unique combination of normal shadows cast by objects and electronic logic, we will be able to get the necessary control commands to track the Sun.

Principle of operation

I consider the photosensitive head to be the "brain" of the tracking system due to its special properties and shape. Let's first look at the mechanical aspects of the solar sensor. On fig. 2 the head is shown disassembled, and in fig. 3 - assembled.

Ris.2

Ris.3

The sensitive head consists of an opaque base, in the center of which are four light-sensitive sensors. Our device uses infrared phototransistors for this purpose.

The phototransistors are separated by two thin metal semicircular partitions, in which grooves are sawn to the middle, which allows for connection, as shown in Fig. 2. This design is preferable to outdated cardboard. Note that each transistor is in its own section.

If you position the device as shown in Fig. 3, then all the phototransistors, except for the one closest to us, will disappear from view. This situation is equivalent to the most familiar working position of the device under lighting. In other words, one sensor captures the sun's rays while the others are in the shade. Let's take advantage of this phenomenon.

Let us position the sensitive head so that its partitions are oriented in the north-south and east-west directions, as shown in Fig. 4. Each section with a phototransistor is marked with the letters A, B, C, D. Now let's consider various options for the relative position of the sensitive head and the sun.

Ris.4

Let's do something like a map reading exercise. When the sun is north of the sensing head, it illuminates sections A and B. Sunlight hitting the sensing head from the east will be detected by phototransistors B and C. If the sun is in the northeast, light will only fall on photosensor B.

Now the idea is clear. A similar consideration is valid for any direction of the incident rays. The reader is given the opportunity to analyze all these cases in detail.

The logic of the circuit

The information from these four sensors is used by the tracking system to track the movement of the sun across the sky. This is where computer logic is used. But for it, it is necessary to prepare the initial data.

This problem is solved by the circuit shown in Fig. 5. To simplify the reasoning, we reduce it to a block diagram.

Ris.5

Without going into details just yet, suffice it to say that when the phototransistor Q1 is not lit, the output of IC2A is high. The same is true for phototransistors Q2, Q3 and Q4: if they are not illuminated, the corresponding outputs of IC2 are high potential. It is these four outputs that will be used to control the two motors.

The logical control task is solved by the IC3 chip. It consists of four NAND elements combined in one body (all four elements work independently of each other). If a high potential is applied to both inputs of the AND-NOT element, a low level voltage will be set at the output.

To understand how IC3 converts this messy data into control commands, let's look at an example. Assume first that all outputs of inverters IC2 are at high potential (corresponding to the dark time of the day). Then suppose that the rays of the morning sun enter section A, illuminating the phototransistor Q1. As a result, the output of IC2 is driven low.

The output of IC3 will go high. Recall that there will be a high potential at the output of the NAND element as long as there is no high voltage at both inputs. Sounds strange, but it's negative logic.

The output voltage of the NAND element is controlled by a V-groove MOS field-effect transistor, in the drain circuit of which a relay is connected. The relay is activated when a high voltage appears at the output of the logic element. In total, there are four shapers and four relays in the circuit.

The relay contacts are connected in such a way that the relays RL1 and RL2 control one motor, and the relays RL3 and RL4 control the other. Then, on a signal from the phototransistor Q1, the IC3A chip will turn on the relay RL1.

When relay RL1 closes, the motor is energized and the azimuth servo turns north because if the light falls on Q1, the sun must be in the north. This is how the system searches for the sun.

However, lowering the output voltage of IC2A also has another effect. The output of the IC3C chip (whose input is connected to the output of IC2A) is set to a high potential, and the relay RL3 is activated. Logic IC3C quite rightly "determined" that the sun was west of sections B, C, and D, and began to rotate the system in a westerly direction.

As a result, both motors simultaneously move the device in the northwest direction, since that is where the sun is located.

The illumination of transistor Q4 will correspond to the average position of the sun between the north and south sensors of the sensing head. As soon as this happens, the output of IC2D will go low, and the output of IC3B will go high, and relay RL2 will operate. Both outputs of the motor are connected to the same pole of the power supply and the motor will stop. At the same time, the tracking system continues to search for the sun in the west direction. The direction to the sun is found when both transistors, Q2 and Q3, are illuminated by its rays. As a result, relay RL3 is activated and the east-west orientation motor of the system stops.

When all four sensors are lit, all four relays turn on and the motors do not work. The sensitive head has detected the sun and is now pointed precisely in its direction. Any shift of the sun from this position will cause at least two sensors to obscure and the logic to re-fire.

In the above example, the sun was rising in the northwest, which, of course, is impossible. Nevertheless, such an assumption was made to illustrate the wide possibilities of the heliostat tracking system. It doesn't matter where the sun rises. The tracking system will find this direction.

Signal conversion

When explaining the principle of operation of the logic circuit, the important features of signal conversion were not specifically considered. Let's do it now.

During the operation of the circuit, certain phenomena take place. Each of the four phototransistors operates independently of the others, so the signal conversion process occurs four times. Nevertheless, we will assume that all four channels work identically, and it is more expedient to consider the operation of only one of them.

First, the light is converted into an electronic signal. The phototransistor is responsible for converting light into electricity. The more light falls on the phototransistor, the more current flows through it.

A resistor is included in the emitter circuit of the transistor, on which a voltage drop is created when current flows. The voltage drop across a resistor is directly proportional to the current flowing, which in turn is proportional to the light intensity. Therefore, a large illumination causes an increase in voltage.

From the emitter resistor, voltage is applied to the non-inverting input of the voltage comparator. The reference voltage is applied to the inverting input. When the voltage coming from the emitter resistor exceeds the reference voltage, a high level voltage appears at the output of the comparator. If the emitter voltage is below the reference voltage, a low level voltage appears at the output of the comparator.

The operation of the circuit is determined by the magnitude of the reference voltage. As is known, a necessary property of a tracking system is the ability to determine the level of solar radiation intensity that is appropriate for practical use. This can be done with a reference voltage.

Since the voltage across the emitter resistor is a function of the intensity of sunlight, the value of this voltage can be used to judge that the radiation intensity reaches a practically acceptable level. This level is determined by the comparator: the input voltage exceeds the reference voltage, the required light level has been reached. Thus, the relay cannot operate until the voltage at the emitter exceeds the value corresponding to the minimum level of solar radiation intensity.

Moreover, all comparators are supplied with the reference voltage from the same source, and, therefore, one voltage setting affects all comparators. With an increase in the threshold for one channel, the threshold for all others increases.

In the output stage of the comparator there is an open collector transistor, to which a load resistance must be connected to remove the output signal. To match the input of the NAND elements and according to the logic of operation, the output signal of the comparator is passed through the inverter.

Sensing head design

If you immediately use the above recommendations, making a sensitive head is not difficult.

Shading sections are made of thin metal, such as aluminum sheet. Cut out a circle about 10 cm in diameter from it. Then cut it into two semicircles of the same size and shape.

Determine the midpoint of the straight edge of the semicircle and restore the perpendicular from this point to the intersection with the semicircle. Mark the middle of the perpendicular, it should be at a distance of 2,5 cm from the edge. Do these operations with both semicircles.

Set aside one of the details so as not to confuse. Make a notch in one of the parts from the base (straight edge) to the mark of the middle of the perpendicular. In another of the same part, make a similar notch, but this time from the outer (rounded) edge in the direction of the center to the mark of the middle of the perpendicular. See how it's done in Fig. 2.

Connect the parts together as shown in fig. 3. The tightest connection can be obtained if you use a hacksaw with a cutting edge thickness of the blade equal to the thickness of the metal. A cloth with fine teeth gives a finer cut.

The base of the head can be made of wood, plastic or metal. While metal is best, it is more difficult to machine.

A round disk with a diameter of about 10 cm is taken as the base, corresponding to the size of the disk used to make the shading sections. Draw the base into four equal sectors, as when cutting a cake.

Using a hacksaw, cut small grooves along these lines at least 0,8 mm deep or more (as the material allows), but no deeper than half the thickness.

When finished, you should get a cross-shaped lattice with an intersection in the center of the round base. The appearance of the grooves should resemble the crosshairs of a telescopic rifle, just as thin and accurate.

Drill a 6 mm hole in each quadrant as close as possible to the crosshairs of the grooves (fig. 4). However, some clearance must be left between the grooves and holes.

Now everything is ready to attach the sections to the base. The aluminum parts can be glued with epoxy glue. Parts made of other metal can be soldered. Remember that the design is not designed to carry any kind of load, and therefore the most important thing is that the individual parts of the head are firmly connected to each other.

However, it should be remembered that as a result of the heating of the structure by the sun's rays, stresses will appear. In this regard, it is undesirable to use materials with different coefficients of thermal expansion and cover the already finished assembled product with paint.

Insert the phototransistors into the corresponding holes and glue them. Collector terminals are connected to a common power supply, so they can be connected together. When using a metal base, common leads can be connected to it, since the base serves as a "ground" and shields the head from external noise.

Finally, it is necessary to protect the device from adverse weather conditions with a transparent cap. It is preferable to use glass as it is more durable. A similar cap can be found in the gift department or pet store. It is better to first purchase a transparent cap, and then adjust the size of the base and sections to fit it. Glue the protective cap to the base with liquid glass.

PCB design

The electronic part of the circuit is made using printed wiring. The placement of parts is shown in fig. 6, drawing of the printed circuit board - in fig. 7 and 8. Note that the PCB is double sided.

Ris.6

Fig.7; Rice. eight

Due to the presence of the relay, the printed circuit board is quite large. Standard double-pole switch type relays in a transparent case are used. The contacts are rated for 10 A at 125 V AC.

However, the limiting factor is not the continuous current that the relay contacts can handle, but the current they can interrupt. Therefore, to increase the limiting switching currents, two pairs of contacts are connected in series.

It is known that when the contacts open, an electric arc occurs. It is called e. d.s. self-induction that occurs when the power supply circuit of the electric motor is broken. In an alternating current circuit, the arc quickly disappears when the direction of the electric field is reversed. However, in a DC circuit, the arc can sustain itself for quite a long time. Arc formation can be prevented by increasing the distance between the contacts and the speed of their separation.

When the relay contacts are connected in series, the total distance between the open contacts doubles and the speed of their separation increases. Therefore, the relay can switch a load that exceeds the passport value.

The relay is usually supplied with a connector, which is very useful for matching with servo motors, since the relays are available in various standard supply voltages ranging from 6V DC or AC to 120V.

I advise you not to solder the relay directly to the board, but to connect it through the connectors, then you can pick up a relay with any supply voltage.

For convenience, the relay power bus is isolated from the positive power wire. To connect the relay to the "plus" of the power supply, simply solder the jumper, as noted in the diagram.

If relays with a supply voltage of more than 60 V DC are used, it is necessary to select field-effect transistors that can withstand high voltages (they are produced for voltages above 400 V). Remember to also replace diodes D1 - D4 with higher voltage diodes, and never use diodes with AC powered relays.

Another part of the device that needs special attention is the emitter resistors R1, R2, R3 and R4. It is unlikely that you will be able to find four phototransistors with characteristics so close that their emitter voltages will match under the same illumination. To compensate for the spread of parameters, it is necessary to select the values ​​of the emitter resistors.

The nominal value of 1 kOhm is only an approximate value of the resistors during commissioning, and it must be selected more accurately. Keep in mind that the resistance value may vary with temperature.

The easiest way to choose the resistance value is to replace the constant resistor with a variable one. Start with a resistance value of 1 kΩ. By illuminating the sensing head with light at different intensity levels, a specific table of voltage values ​​can be obtained.

Don't try to replace sunlight with incandescent light. Phototransistors are sensitive to infrared radiation and react differently to these light sources.

If the measurement reveals that one phototransistor reacts too quickly to a change in illumination, reduce the value of the resistor. However, in this case, it is necessary to reduce the resistance of all resistors in order to maintain the normal operation of the circuit. Eventually you will find the values ​​at which the comparators from the signals coming from the corresponding phototransistors will work at the same light level.

Fig.9 (click to enlarge)

Measure the resulting value of the resistance of the variable resistor and replace it with a constant of the same value.

helpful hints

Adjustment changes the level of operation. In many cases it is not necessary to set this threshold too low or the tracking system will waste power. Given certain elements, you may want to adjust the trigger level of the circuit.

Although this tracking system has the widest viewing angle of any homemade product described in this book, it can still stop in an uncomfortable position at nightfall. In this case, several morning hours may be lost until the system starts to respond to the increased light level.

If you don't like this, have the servo system return to neutral after all relays have de-energized. This problem can be solved by a simple logic circuit. The best starting position is the middle one, pointing to the midday sky.

Author: Byers T.

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