Effectively converting free rays of the sun into energy that can be used to power homes and other facilities is the cherished dream of many green energy apologists.
But the principle of operation of the solar battery and its efficiency are such that there is no need to talk about the high efficiency of such systems yet. It would be nice to have your own additional source of electricity. Is not it? Moreover, even today in Russia, with the help of solar panels, a considerable number of private households are successfully supplied with “free” electricity. Still don't know where to start?
Below we will tell you about the design and operating principles of a solar panel; you will learn what the efficiency of a solar system depends on. And the videos posted in the article will help you assemble a solar panel from photocells with your own hands.
There are quite a lot of nuances and confusion in the topic of “solar energy”. It is often difficult for beginners to understand all the unfamiliar terms at first. But without this, it is unreasonable to engage in solar energy, purchasing equipment for generating “solar” current.
Unknowingly, you can not only choose the wrong panel, but also simply burn it when connecting it or extract too little energy from it.
Image gallery
The maximum return from a solar panel can only be obtained by knowing how it works, what components and assemblies it consists of, and how it is all connected correctly
The second nuance is the concept of the term “solar battery”. Typically, the word “battery” refers to some kind of electrical storage device. Or a banal heating radiator comes to mind. However, in the case of solar batteries the situation is radically different. They do not accumulate anything in themselves.
Details Published 12/27/2019Dear readers! The library team wishes you a Happy New Year and Merry Christmas! We sincerely wish you and your families happiness, love, health, success and joy!
May the coming year give you prosperity, mutual understanding, harmony and good mood.
Good luck, prosperity and fulfillment of your most cherished desires in the new year!
Test access to EBS Ibooks.ru
Details Published 12/03/2019Dear readers! Until December 31, 2019, our university has been provided with test access to the EBS Ibooks.ru, where you can familiarize yourself with any book in full-text reading mode. Access is possible from all computers in the university network. Registration is required to obtain remote access.
"Genrikh Osipovich Graftio - on the 150th anniversary of his birth"
Details Published 12/02/2019Dear readers! In the “Virtual Exhibitions” section there is a new virtual exhibition “Henrikh Osipovich Graftio”. 2019 marks the 150th anniversary of the birth of Genrikh Osipovich, one of the founders of the hydropower industry in our country. An encyclopedist scientist, a talented engineer and an outstanding organizer, Genrikh Osipovich made a huge contribution to the development of domestic energy.
The exhibition was prepared by employees of the library's scientific literature department. The exhibition presents the works of Genrikh Osipovich from the LETI history fund and publications about him.
You can view the exhibition
Test access to the IPRbooks Electronic Library System
Details Published 11/11/2019Dear readers! From November 8, 2019 to December 31, 2019, our university was provided with free test access to the largest Russian full-text database - the IPR BOOKS Electronic Library System. EBS IPR BOOKS contains more than 130,000 publications, of which more than 50,000 are unique educational and scientific publications. On the platform, you have access to current books that cannot be found in the public domain on the Internet.
Access is possible from all computers in the university network.
To obtain remote access, you must contact the electronic resources department (room 1247) VChZ administrator Polina Yurievna Skleymova or by email [email protected] with the topic "Registration in IPRbooks".
Types of photoelectric converters
The most energy-efficient devices for converting solar energy into electrical energy (since this is a direct, single-stage energy transition) are semiconductor photovoltaic converters (PVCs). At an equilibrium temperature characteristic of PV cells of the order of 300-350 Kelvin and solar T ~ 6000 K, their maximum theoretical efficiency is >90%. This means that, as a result of optimizing the structure and parameters of the converter, aimed at reducing irreversible energy losses, it will be quite possible to increase the practical efficiency to 50% or more (in laboratories, an efficiency of 40% has already been achieved).
Theoretical research and practical developments in the field of photovoltaic conversion of solar energy have confirmed the possibility of achieving such high efficiency values with solar cells and identified the main ways to achieve this goal.
Energy conversion in PV cells is based on the photovoltaic effect, which occurs in inhomogeneous semiconductor structures when exposed to solar radiation. The heterogeneity of the PV structure can be obtained by doping the same semiconductor with different impurities (creating p-n junctions) or by connecting different semiconductors with unequal band gaps - the energy of electron removal from the atom (creating heterojunctions), or by changing the chemical composition semiconductor, leading to the appearance of a gradient of the band gap (creation of graded-gap structures). Various combinations of the above methods are also possible. The conversion efficiency depends on the electrical characteristics of the inhomogeneous semiconductor structure, as well as the optical properties of the solar cell, among which the most important role is played by photoconductivity, caused by the internal photoelectric effect in semiconductors when irradiated with sunlight. The operating principle of PV cells can be explained using the example of converters with p-n junctions, which are widely used in modern solar and space energy. An electron-hole junction is created by doping a wafer of single-crystal semiconductor material with a certain type of conductivity (i.e., either p- or n-type) with an impurity, ensuring the creation of a surface layer with conductivity of the opposite type.
The dopant concentration in this layer must be significantly higher than the dopant concentration in the base (original single crystal) material in order to neutralize the main free charge carriers present there and create conductivity of the opposite sign. At the boundary of the n- and p-layers, as a result of charge flow, depleted zones are formed with an uncompensated volumetric positive charge in the n-layer and a volumetric negative charge in the p-layer. These zones together form a p-n junction. The potential barrier (contact potential difference) that appears at the transition prevents the passage of the main charge carriers, i.e. electrons from the p-layer side, but freely allow minority carriers to pass in opposite directions. This property of p-n junctions determines the possibility of obtaining photo-emf when irradiating a solar cell with sunlight. The nonequilibrium charge carriers (electron-hole pairs) created by light in both layers of the photovoltaic cell are separated at the p-n junction: minority carriers (i.e. electrons) freely pass through the junction, and majority carriers (holes) are retained. Thus, under the influence of solar radiation, a current of nonequilibrium minority charge carriers - photoelectrons and photoholes - will flow through the p-n junction in both directions, which is exactly what is needed for the operation of the solar cell. If we now close the external circuit, then the electrons from the n-layer, having done work on the load, will return to the p-layer and there recombine (unite) with holes moving inside the solar cell in the opposite direction. To collect and remove electrons into an external circuit, there is a contact system on the surface of the semiconductor structure of the solar cell. On the front, illuminated surface of the converter, the contacts are made in the form of a grid or comb, and on the rear they can be solid.
The main irreversible energy losses in solar cells are associated with:
- reflection of solar radiation from the surface of the converter,
- the passage of part of the radiation through the photovoltaic cell without absorption in it,
- scattering of excess photon energy on thermal vibrations of the lattice,
- recombination of the formed photopairs on the surfaces and in the volume of the photovoltaic cell,
- internal resistance of the converter,
- and some other physical processes.
To reduce all types of energy losses in solar power plants, various measures are being developed and successfully applied. These include:
- the use of semiconductors with optimal band gaps for solar radiation;
- targeted improvement of the properties of the semiconductor structure through its optimal doping and creation of built-in electric fields;
- transition from homogeneous to heterogeneous and graded-gap semiconductor structures;
- optimization of PV design parameters (pn-junction depth, base layer thickness, contact grid frequency, etc.);
- the use of multifunctional optical coatings that provide antireflection, thermal regulation and protection of solar cells from cosmic radiation;
- development of solar cells that are transparent in the long-wave region of the solar spectrum beyond the edge of the main absorption band;
- creation of cascade PV cells from semiconductors specially selected for their bandgap width, making it possible to convert in each cascade the radiation that has passed through the previous cascade, etc.;
Also, a significant increase in the efficiency of solar cells was achieved through the creation of converters with double-sided sensitivity (up to +80% of the existing efficiency of one side), the use of luminescent re-emitting structures, and the preliminary decomposition of the solar spectrum into two or more spectral regions using multilayer film beam splitters (dichroic mirrors). ) with subsequent transformation of each part of the spectrum by a separate photovoltaic cell, etc.
In energy conversion systems of solar power plants (solar power plants), in principle, any types of solar cells of various structures based on various semiconductor materials that have been created and are currently being developed can be used, but not all of them satisfy the set of requirements for these systems:
- high reliability with a long (tens of years!) service life;
- availability of source materials in quantities sufficient for the manufacture of elements of the conversion system and the possibility of organizing their mass production;
- energy costs for creating a conversion system that are acceptable in terms of payback periods;
- minimum energy and mass costs associated with managing the energy conversion and transmission system (space), including the orientation and stabilization of the station as a whole;
- ease of maintenance.
For example, some promising materials are difficult to obtain in the quantities required for the creation of solar power plants due to the limited natural reserves of raw materials and the complexity of their processing. Certain methods for improving the energy and operational characteristics of solar cells, for example, by creating complex structures, are poorly compatible with the possibilities of organizing their mass production at low cost, etc. High productivity can be achieved only by organizing fully automated PV production, for example, based on tape technology, and creating a developed network of specialized enterprises of the appropriate profile, i.e. in fact, an entire industry, comparable in scale to the modern radio-electronic industry. The production of solar cells and the assembly of solar panels on automated lines will reduce the cost of the battery module by 2-2.5 times.
Silicon and gallium arsenide (GaAs) are currently considered as the most likely materials for photovoltaic systems for converting solar energy into SES, and in the latter case we are talking about heterophotoconverters (HPCs) with an AlGaAs-GaAs structure.
FECs (photovoltaic converters) based on a compound of arsenic with gallium (GaAs), as is known, have a higher theoretical efficiency than silicon FECs, since their bandgap width practically coincides with the optimal bandgap width for semiconductor solar energy converters =1 .4 eV. For silicon, this indicator = 1.1 eV.
Due to the higher level of absorption of solar radiation, determined by direct optical transitions in GaAs, high efficiency PV cells based on them can be obtained with a significantly smaller PV cell thickness compared to silicon. In principle, it is enough to have a GFP thickness of 5-6 microns to obtain an efficiency of the order of at least 20%, while the thickness of silicon elements cannot be less than 50-100 microns without a noticeable decrease in their efficiency. This circumstance allows us to count on the creation of lightweight film HFPs, the production of which will require relatively little starting material, especially if it is possible to use not GaAs as a substrate, but another material, for example, synthetic sapphire (Al2 O3).
GFCs also have more favorable operational characteristics in terms of requirements for SES converters compared to silicon PV cells. Thus, in particular, the possibility of achieving small initial values of reverse saturation currents in p-n junctions due to the large band gap makes it possible to minimize the magnitude of negative temperature gradients of efficiency and optimal power of the HFP and, in addition, significantly expand the region of the linear dependence of the latter on the luminous flux density . Experimental dependences of the efficiency of HFPs on temperature indicate that increasing the equilibrium temperature of the latter to 150-180°C does not lead to a significant decrease in their efficiency and optimal specific power. At the same time, for silicon solar cells, an increase in temperature above 60-70°C is almost critical - the efficiency drops by half.
Due to their resistance to high temperatures, gallium arsenide solar cells can be used as solar radiation concentrators. The operating temperature of GaAs-based HFP reaches 180 °C, which is already quite operating temperatures for heat engines and steam turbines. Thus, to the 30% intrinsic efficiency of gallium arsenide HFPs (at 150°C), we can add the efficiency of a heat engine using the waste heat of the liquid cooling the photocells. Therefore, the overall efficiency of the installation, which also uses the third cycle of low-temperature heat extraction from the coolant after the turbine for space heating, can be even higher than 50-60%.
Also, GaAs-based HFCs are much less susceptible to destruction by high-energy proton and electron flows than silicon FECs due to the high level of light absorption in GaAs, as well as the small required lifetime and diffusion length of minority carriers. Moreover, experiments have shown that a significant part of radiation defects in GaAs-based HFPs disappears after their heat treatment (annealing) at a temperature of just about 150-180 °C. If GaAs HFCs constantly operate at a temperature of the order of 150°C, then the degree of radiation degradation of their efficiency will be relatively small throughout the entire period of active operation of the stations (this is especially true for space solar power plants, for which the low weight and size of the FEC and high efficiency are important) .
In general, we can conclude that the energy, mass and operational characteristics of GaAs-based HFCs are more consistent with the requirements of SES and SCES (space) than the characteristics of silicon FECs. However, silicon is a much more accessible and widely used material than gallium arsenide. Silicon is widespread in nature, and the supply of raw materials for creating solar cells based on it is almost unlimited. The technology for manufacturing silicon solar cells is well established and is constantly being improved. There is a real prospect of reducing the cost of silicon solar cells by one to two orders of magnitude with the introduction of new automated production methods, which make it possible, in particular, to produce silicon tapes, large-area solar cells, etc.
Prices for silicon photovoltaic batteries have decreased over 25 years by 20-30 times from 70-100 dollars/watt in the seventies down to 3.5 dollars/watt in 2000 and continue to decline further. In the West, a revolution in the energy sector is expected when prices cross the 3-dollar mark. According to some calculations, this could happen as early as 2002, and for Russia, with current energy tariffs, this moment will come at a price of 1 watt of solar energy of 0.3-0.5 dollars, that is, at an order of magnitude lower price. All factors taken together play a role here: tariffs, climate, geographic latitudes, and the state’s ability to set real prices and make long-term investments. In actual structures with heterojunctions, the efficiency today reaches more than 30%, and in homogeneous semiconductors such as monocrystalline silicon - up to 18%. The average efficiency in solar cells based on monocrystalline silicon today is about 12%, although it reaches 18%. It is mainly silicon SBs that can be seen today on the roofs of houses around the world.
Unlike silicon, gallium is a very scarce material, which limits the possibility of producing GaAs-based HFPs in the quantities required for widespread implementation.
Gallium is mined mainly from bauxite, but the possibility of obtaining it from coal ash and sea water is also being considered. The largest reserves of gallium are found in seawater, but the concentration there is very low, the recovery yield is estimated at only 1% and, therefore, production costs are likely to be prohibitive. The technology for the production of GaAs-based HFPs using liquid and gas epitaxy methods (oriented growth of one single crystal on the surface of another (on a substrate)) has not yet been developed to the same extent as the technology for the production of silicon PVS, and as a result, the cost of HFPs is now significantly higher (by orders) of the cost of silicon solar cells.
In spacecraft, where the main source of current is solar panels and where clear ratios of mass, size and efficiency are very important, the main material for the sun. The battery, of course, is gallium arsenide. The ability of this compound in solar cells not to lose efficiency when heated by 3-5 times concentrated solar radiation is very important for space solar power plants, which accordingly reduces the need for scarce gallium. An additional reserve for saving gallium is associated with the use of synthetic sapphire (Al2O3) rather than GaAs as the HFP substrate.
The cost of HFPs during their mass production based on improved technology will probably also be significantly reduced, and in general, the cost of the conversion system of an SES power conversion system based on GaAs HFPs may be quite comparable with the cost of a silicon-based system. Thus, at present, it is difficult to completely give clear preference to one of the two semiconductor materials considered - silicon or gallium arsenide, and only further development of their production technology will show which option will be more rational for ground-based and space-based solar energy. Insofar as the SBs produce direct current, the task arises of transforming it into industrial alternating current 50 Hz, 220 V. A special class of devices - inverters - copes with this task perfectly.
Calculation of photovoltaic system.
The energy of solar cells can be used in the same way as the energy of other power sources, with the difference that solar cells are not afraid of short circuits. Each of them is designed to maintain a certain amount of current at a given voltage. But unlike other current sources, the characteristics of a solar cell depend on the amount of light incident on its surface. For example, an incoming cloud can reduce power output by more than 50%. In addition, deviations in technological conditions entail a scatter in the output parameters of elements of one batch. Consequently, the desire to ensure maximum efficiency from photovoltaic converters leads to the need to sort cells by output current. As a clear example of “a lousy sheep spoiling the entire flock,” the following can be cited: insert a section of pipe with a much smaller diameter into a break in a large-diameter water pipe; as a result, the water flow will sharply decrease. Something similar happens in a chain of solar cells with heterogeneous output parameters.
Silicon solar cells are nonlinear devices and their behavior cannot be described by a simple formula such as Ohm's law. Instead, to explain the characteristics of an element, you can use a family of easy-to-understand curves - current-voltage characteristics (CVC)
The open circuit voltage generated by one element varies slightly from one element to another in the same batch and from one manufacturer to another and is about 0.6 V. This value does not depend on the size of the element. The situation is different with current. It depends on the intensity of the light and the size of the element, which refers to its surface area.
An element measuring 100-100 mm is 100 times larger than an element measuring 10-10 mm and, therefore, under the same illumination, it will produce a current 100 times greater.
By loading the element, you can plot the dependence of output power on voltage, obtaining something similar to that shown in Fig. 2
The peak power corresponds to a voltage of about 0.47 V. Thus, in order to correctly assess the quality of the solar cell, as well as for the sake of comparing elements with each other under the same conditions, it is necessary to load it so that the output voltage is equal to 0.47 V. After the solar the elements are selected for the job, they need to be soldered. Serial elements are equipped with current-collecting grids, which are designed for soldering conductors to them.
The batteries can be arranged in any desired combination. The simplest battery is a chain of elements connected in series. You can also connect chains in parallel, resulting in a so-called series-parallel connection.
An important point in the operation of solar cells is their temperature regime. When the element is heated by one degree above 25°C, it loses 0.002 V in voltage, i.e. 0.4%/degree. Figure 3 shows a family of current-voltage characteristic curves for temperatures of 25°C and 60°C.
On a bright sunny day, the elements heat up to 60-70°C, losing 0.07-0.09 V each. This is the main reason for the decrease in the efficiency of solar cells, leading to a drop in the voltage generated by the element. The efficiency of a conventional solar cell currently ranges from 10-16%. This means that an element measuring 100-100 mm under standard conditions can generate 1-1.6 W.
All photovoltaic systems can be divided into two types: autonomous and connected to the electrical network. Stations of the second type release excess energy into the network, which serves as a reserve in the event of an internal energy shortage.
An autonomous system generally consists of a set of solar modules placed on a supporting structure or on the roof, a battery, a battery charge/discharge controller, and connecting cables. Solar modules are the main component for building photovoltaic systems. They can be manufactured with any output voltage.
After the solar cells are selected, they need to be soldered. Serial elements are equipped with current-collecting grids for soldering conductors to them. Batteries can be made in any combination.
The simplest battery is a chain of elements connected in series.
You can connect these chains in parallel, obtaining a so-called series-parallel connection. In parallel, only chains (lines) with identical voltage can be connected, and their currents are summed up according to Kirchhoff’s law.
For terrestrial use, they are usually used to charge batteries with a nominal voltage of 12 V. In this case, as a rule, 36 solar cells are connected in series and sealed by lamination on glass, PCB, or aluminum. The elements are located between two layers of sealing film, without an air gap. Vacuum lamination technology allows you to fulfill this requirement. In the case of an air gap between the protective glass and the element, reflection and absorption losses would reach 20-30% compared to 12% without an air gap.
The electrical parameters of a solar cell are presented as well as an individual solar cell in the form of a current-voltage curve under standard conditions (Standard Test Conditions), i.e., with solar radiation of 1000 W/m2, temperature - 25 ° C and solar spectrum at a latitude of 45 ° (AM1.5) .
The point of intersection of the curve with the voltage axis is called no-load voltage - Uxx, the point of intersection with the current axis is called short-circuit current Is.
The maximum power of the module is defined as the highest power under STC (Standard Test Conditions). The voltage corresponding to the maximum power is called the maximum power voltage (operating voltage - Up), and the corresponding current is called the maximum power current (operating current - Ip).
The operating voltage for a module consisting of 36 elements will thus be about 16...17 V (0.45...0.47 V per element) at 25o C.
This voltage margin compared to the voltage of a full battery charge (14.4 V) is necessary in order to compensate for losses in the battery charge-discharge controller (which will be discussed later), and mainly - a decrease in the operating voltage of the module when the module is heated by radiation : The temperature coefficient for silicon is about minus 0.4%/degree (0.002 V/degree for one element).
It should be noted that the open-circuit voltage of the module depends little on the illumination, while the short-circuit current, and accordingly the operating current, is directly proportional to the illumination.
Thus, when heated under real operating conditions, the modules heat up to a temperature of 60-70°C, which corresponds to a shift in the operating voltage point, for example, for a module with an operating voltage of 17 V - from 17 V to 13.7-14.4 V ( 0.38-0.4 V per element).
Based on all of the above, it is necessary to approach the calculation of the number of series-connected elements of the module. If the consumer needs to have alternating voltage, then an inverter-converter of direct voltage to alternating voltage is added to this kit.
The calculation of FES means the determination of the rated power of modules, their number, connection diagram; selection of type, operating conditions and battery capacity; power of the inverter and charge-discharge controller; determination of connecting cable parameters.
First of all, it is necessary to determine the total power of all consumers connected simultaneously. The power of each of them is measured in watts and is indicated in the product data sheets. At this stage, you can already select the power of the inverter, which should be no less than 1.25 times greater than the calculated one. It should be borne in mind that such a cunning device as a compressor refrigerator at the moment of startup consumes power 7 times more than the rated power.
The nominal range of inverters is 150, 300, 500, 800, 1500, 2500, 5000 W. For powerful stations (more than 1 kW), the station voltage is selected at least 48 V, because At higher powers, inverters operate better with higher initial voltages.
The next stage is determining the battery capacity. The battery capacity is selected from a standard range of capacities, rounded to the side larger than the calculated one. And the calculated capacity is obtained by simply dividing the total power of consumers by the product of the battery voltage and the depth of discharge of the battery in fractions.
For example, if the total power of consumers is 1000 Wh per day, and the permissible discharge depth of a 12 V battery is 50%, then the calculated capacity will be:
1000 / (12 x 0.5) = 167 Ah
When calculating the battery capacity in a fully autonomous mode, it is necessary to take into account the presence of cloudy days in nature during which the battery must ensure the operation of consumers.
The last stage is determining the total power and number of solar modules. The calculation will require the value of solar radiation, which is taken during the period of operation of the station, when solar radiation is minimal. In the case of year-round use, this is December.
The “meteorology” section provides monthly and total annual values of solar radiation for the main regions of Russia, as well as gradation according to different orientations of the light-receiving plane.
Taking from there the value of solar radiation for the period of interest to us and dividing it by 1000, we obtain the so-called number of pico-hours, i.e., the conditional time during which the sun shines with an intensity of 1000 W/m2.
For example, for the latitude of Moscow and the month of July, the value of solar radiation is 167 kWh/m2 when the site is oriented south at an angle of 40° to the horizon. This means that on average the sun shines in July for 167 hours (5.5 hours per day) with an intensity of 1000 W/m2, although the maximum illumination at noon on an area oriented perpendicular to the luminous flux does not exceed 700-750 W/m2.
The module with power Pw during the selected period will generate the following amount of energy: W = k Pw E / 1000, where E is the insolation value for the selected period, k-coefficient equal to 0.5 in summer and 0.7 in winter.
This coefficient corrects for the loss of power of solar cells when heated in the sun, and also takes into account the inclined incidence of rays on the surface of the modules during the day.
The difference in its value in winter and summer is due to less heating of the elements in winter.
Based on the total power of energy consumed and the above formula, it is easy to calculate the total power of the modules. And knowing it, simply dividing it by the power of one module, we get the number of modules.
When creating a solar power plant, it is strongly recommended to reduce the power of consumers as much as possible. For example, use (if possible) only fluorescent lamps as illuminators. Such lamps, with a consumption of 5 times less, provide a luminous flux equivalent to the luminous flux of an incandescent lamp.
For small PV systems, it is advisable to install its modules on a rotating bracket for optimal rotation relative to the incident rays. This will increase the station's capacity by 20-30%.
A little about inverters.
Inverters or converters of direct current to alternating current are designed to provide high-quality power supply to various equipment and devices in conditions of absence or poor quality of an alternating current power supply network with a frequency of 50 Hz and a voltage of 220 V, various emergency situations, etc.
The inverter is a pulse converter of direct current with a voltage of 12 (24, 48, 60) V into alternating current with a stabilized voltage of 220 V with a frequency of 50 Hz. Most inverters have a STABILIZED SINEUSOIDAL voltage at the output, which allows them to be used to power almost any equipment and devices.
Structurally, the inverter is made in the form of a desktop unit. On the front panel of the inverter there is a product operation switch and a converter operation indicator. On the rear panel of the product there are pins (terminals) for connecting a DC source, for example, a battery, a grounding pin for the inverter housing, a hole with a fan mount (cooling), and a three-pole Euro socket for connecting the load.
Stabilized voltage at the inverter output allows you to provide high-quality power supply to the load when the input voltage changes/fluctuates, for example, when the battery is discharged, or fluctuations in the current consumed by the load. Guaranteed galvanic isolation of the DC source at the input and the AC circuit with the load at the inverter output allows you to not take additional measures to ensure operational safety when using various DC sources or any electrical equipment. Forced cooling of the power part and low noise level during operation of the inverter make it possible, on the one hand, to ensure good weight and size characteristics of the product, on the other hand, with this type of cooling they do not create inconvenience during operation in the form of noise.
- Built-in control panel with electronic display
- Capacitance potentiometer that allows precise adjustments
- Normalized strip with pin connection: WE WY STEROW
- Built-in reverse braking
- Radiator with fan
- Aesthetic fastening
- Power supply 230 V - 400 V
- Overload 150% - 60s
- Ramp-up time 0.01...1000 seconds
- Built-in electric filter, class A
- Operating temperature: -5°C - to +45°C
- RS 485 port
- Frequency step adjustment: 0.01 Hz - 1 kHz
- Protection class IP 20
Functionally provides: increase, decrease in frequency, control of overload, overheating.
Most renewable types of energy - hydropower, mechanical and thermal energy from the world's oceans, wind and geothermal energy - are characterized by either limited potential or significant difficulties in widespread use. The total potential of most renewable energy sources will increase energy consumption from current levels by only an order of magnitude. But there is another source of energy - the Sun. The Sun, a star of spectral class 2, a yellow dwarf, is a very average star in all its main parameters: mass, radius, temperature and absolute magnitude. But this star has one unique feature - it is “our star”, and humanity owes its entire existence to this average star. Our star supplies the Earth with a power of about 10 17 W - such is the power of the “sun bunny” with a diameter of 12.7 thousand km, which constantly illuminates the side of our planet facing the Sun. The intensity of sunlight at sea level in southern latitudes, when the Sun is at its zenith, is 1 kW/m2. By developing highly efficient methods for converting solar energy, the Sun can supply rapidly growing energy needs for many hundreds of years.
The arguments of opponents of large-scale use of solar energy boil down mainly to the following arguments:
1. The specific power of solar radiation is small, and large-scale conversion of solar energy will require very large areas.
2. Converting solar energy is very expensive and requires almost unrealistic material and labor costs.
Indeed, how large will the area of the Earth covered by converter systems be to produce a significant share of electricity in the global energy budget? Obviously, this area depends on the efficiency of the converter systems used. To assess the efficiency of photovoltaic converters that directly convert solar energy into electrical energy using semiconductor photocells, we introduce the concept of coefficient of performance (efficiency) of a photocell, defined as the ratio of the power of electricity generated by a given element to the power of a sunbeam incident on the surface of the photocell. Thus, with an efficiency of solar converters equal to 10% (typical efficiency values for silicon photocells, widely used in serial industrial production for the needs of ground-based energy), to produce 10 12 W of electricity it would be necessary to cover an area of 4 * 10 10 m 2 with photoconverters equal to the square with a side of 200 km. In this case, the intensity of solar radiation is taken to be 250 W/m 2, which corresponds to the typical average value throughout the year for southern latitudes. That is, the “low density” of solar radiation is not an obstacle to the development of large-scale solar energy.
The above considerations are a fairly compelling argument: the problem of converting solar energy must be solved today in order to use this energy tomorrow. One can at least jokingly consider this problem within the framework of solving energy problems of controlled thermonuclear fusion, when an effective reactor (the Sun) is created by nature itself and provides a resource for reliable and safe operation for many millions of years, and our task is only to develop a ground-based converter substation. Recently, extensive research has been carried out in the world in the field of solar energy, which has shown that in the near future this method of generating energy can become economically justified and find wide application.
Russia is rich in natural resources. We have significant reserves of fossil fuels - coal, oil, gas. However, the use of solar energy is also of great importance for our country. Despite the fact that a significant part of Russia's territory lies at high latitudes, some very large southern regions of our country have a very favorable climate for the widespread use of solar energy.
The use of solar energy has even greater prospects in the countries of the Earth’s equatorial belt and areas close to this belt, characterized by a high level of solar energy. Thus, in a number of regions of Central Asia, the duration of direct solar irradiation reaches 3000 hours per year, and the annual arrival of solar energy on a horizontal surface is 1500 - 1850 kW o hour / m 2.
The main directions of work in the field of solar energy conversion are currently:
— direct thermal heating (receipt of thermal energy) and thermodynamic conversion (receipt of electrical energy with intermediate conversion of solar energy into heat);
— photoelectric conversion of solar energy.
Direct thermal heating is the simplest method of converting solar energy and is widely used in the southern regions of Russia and in the equatorial countries in solar heating installations, hot water supply, building cooling, water desalination, etc. The basis of solar heat-using installations are flat solar collectors - absorbers of solar radiation. Water or other liquid, being in contact with the absorber, is heated and removed from it using a pump or natural circulation. The heated liquid then enters storage, from where it is consumed as needed. This device is reminiscent of domestic hot water supply systems.
Electricity is the most convenient type of energy to use and transmit. Therefore, the interest of researchers in the development and creation of solar power plants that use the intermediate conversion of solar energy into heat with its subsequent conversion into electricity is understandable.
In the world now, the most common solar thermal power plants are of two types: 1) tower type with the concentration of solar energy on one solar receiver, carried out using a large number of flat mirrors; 2) dispersed systems of paraboloids and parabolic cylinders, at the focus of which thermal receivers and low-power converters are located.
2. DEVELOPMENT OF SOLAR ENERGY
In the late 70s and early 80s, seven pilot solar power plants (SPPs) of the so-called tower type with a power level of 0.5 to 10 MW were built in different countries of the world. The largest solar power plant with a capacity of 10 MW (Solar One) was built in California. All of these solar power plants are built on the same principle: a field of heliostat mirrors placed at ground level that track the sun reflects the sun's rays onto a receiver mounted on the top of a fairly high tower. The receiver is, in essence, a solar boiler in which water steam of average parameters is generated, which is then sent to a standard steam turbine.
The second project, the PHOEBUS tower solar power plant, is being implemented by a German consortium. The project involves the creation of a demonstration hybrid (solar-fuel) solar power plant with a capacity of 30 MW with a volumetric receiver in which atmospheric air will be heated, which is then sent to a steam boiler, where water steam is generated, which operates in the Rankine cycle. On the air path from the receiver to the boiler, a burner is supposed to burn natural gas, the amount of which is regulated so as to maintain the specified power throughout the daylight hours. Calculations show that, for example, for an annual solar radiation of 6.5 GJ/m2 (similar to that typical for the southern regions of Ukraine), this solar power plant, which has a total heliostat surface of 160 thousand m2, will receive 290.2 GW *h/year of solar energy, and the amount of energy contributed with fuel will be 176.0 GWh/year. At the same time, the solar power plant will generate 87.9 GWh of electricity per year with an average annual efficiency of 18.8%. With such indicators, the cost of electricity generated at a solar power plant can be expected to be at the level of thermal power plants using fossil fuels.
Since the mid-80s, in Southern California, the LUZ company has created and put into commercial operation nine solar power plants with parabolic cylindrical concentrators (PCC) with unit capacities that increased from the first solar power plant to the next from 13.8 to 80 MW. The total capacity of these solar power plants reached 350 MW. In these SESs, PCCs with an aperture were used, which increased during the transition from the first SES to the next ones. By tracking the sun on a single axis, the concentrators focus solar radiation onto tubular receivers enclosed in evacuated tubes. A high-temperature coolant fluid flows inside the receiver, which heats up to 380°C and then transfers the heat of the water vapor to the steam generator. The design of these solar power plants also provides for the combustion of a certain amount of natural gas in a steam generator to produce additional peak electricity, as well as to compensate for the reduced insolation.
These solar power plants were created and operated at a time when there were laws in the United States that allowed solar power plants to operate break-even. The expiration of these laws at the end of the 80s led to the fact that the LUZ company went bankrupt, and the construction of new solar power plants of this type was stopped.
The KJC (Kramer Junction Company), which operated five of the nine solar power plants built (from 3 to 7), set itself the task of increasing the efficiency of these solar power plants, reducing the costs of their operation and making them economically attractive in the new conditions. This program is currently being successfully implemented.
Switzerland has become one of the leaders in the use of solar energy. According to data from 1997, approximately 2,600 solar installations based on photoelectric converters with a capacity of 1 to 1,000 kW were built here. The program, called “Solar-91” and carried out under the slogan “For an energy-independent Switzerland,” makes a significant contribution to solving environmental problems and energy independence of a country that today imports more than 70% of its energy. A solar power plant with a capacity of 2-3 kW is most often installed on the roofs and facades of buildings. This installation produces an average of 2,000 kWh of electricity per year, which is enough for the domestic needs of an average Swiss home. Large companies install solar installations with a capacity of up to 300 kW on the roofs of production buildings. Such a station covers the enterprise's electricity needs by 50-60%.
In the Alpine highlands, where it is unprofitable to lay power lines, high-power solar power plants are also being built. Operating experience shows that the Sun is already able to meet the needs of all residential buildings in the country. Solar installations, located on the roofs and walls of houses, on noise barriers of highways, on transport and industrial structures, do not require expensive agricultural territory for their placement. An autonomous solar installation near the village of Grimsel provides electricity for round-the-clock lighting of the road tunnel. Near the town of Shur, solar panels installed on a 700-meter section of noise barrier provide 100 kW of electricity annually.
The modern concept of using solar energy was most fully expressed during the construction of the buildings of the window glass plant in Arisdorf, where solar panels with a total power of 50 kW were assigned an additional role during the design as floor and façade elements. The efficiency of solar converters noticeably decreases with strong heating, so ventilation pipelines are laid under the panels to pump outside air. Dark blue photoconverters sparkling in the sun on the southern and western facades of the administrative building, supplying electricity to the network, act as decorative cladding.
In developing countries, relatively small installations are used to supply electricity to individual houses, in remote villages to equip cultural centers, where, thanks to PMTs, you can use televisions, etc. In this case, it is not the cost of electricity that comes to the fore, but the social effect. Programs for the introduction of photovoltaics in these countries are actively supported by international organizations; the World Bank takes part in their financing on the basis of the “Solar Initiative” put forward by it. For example, in Kenya over the past 5 years, 20,000 rural houses have been electrified with the help of photovoltaics. A large program for the introduction of photomultipliers is being implemented in India, where in 1986 - 1992. Rs 690 million were spent on installing PMTs in rural areas.
In industrialized countries, the active implementation of photomultipliers is explained by several factors. Firstly, PMTs are considered as environmentally friendly sources that can reduce harmful impacts on the environment. Secondly, the use of PMTs in private homes increases energy autonomy and protects the owner in the event of possible interruptions in the centralized power supply.
3. PHOTOVOLTAIC CONVERSION OF SOLAR ENERGY
An important contribution to understanding the mechanism of action of the photoelectric effect in semiconductors was made by the founder of the Physico-Technical Institute (PTI) of the Russian Academy of Sciences, Academician A.F. Ioffe. He dreamed of using semiconductor photocells in solar energy already in the thirties, when B.T. Kolomiets and Yu.P. Maslakovets created sulfur-thallium solar cells at the Physicotechnical Institute with a record efficiency of 1% for that time.
Wide practical use of solar panels for energy purposes began with the launch in 1958 of artificial Earth satellites - the Soviet Sputnik-3 and the American Avangard-1. Since that time, for more than 35 years, semiconductor solar batteries have been the main and almost the only source of energy supply for spacecraft and large orbital stations such as Salyut and Mir. The extensive groundwork accumulated by scientists in the field of solar batteries for space applications has also made it possible to develop work on ground-based photovoltaic energy.
The basis of photocells is a semiconductor structure with a p-n junction that appears at the interface of two semiconductors with different conduction mechanisms. Note that this terminology originates from the English words positive (positive) and negative (negative). Various types of conductivity are obtained by changing the type of impurities introduced into the semiconductor. For example, atoms of group III of the Periodic Table D.I. Mendeleev, introduced into the crystal lattice of silicon, gives the latter hole (positive) conductivity, and group V impurities - electronic (negative). The contact of p or n semiconductors leads to the formation of a contact electric field between them, which plays an extremely important role in the operation of a solar photocell. Let us explain the reason for the occurrence of contact potential difference. When p- and n-type semiconductors are combined in one single crystal, a diffusion flow of electrons arises from the n-type semiconductor to the p-type semiconductor and, conversely, a flow of holes from the p- to n-semiconductor. As a result of this process, the part of the p-type semiconductor adjacent to the p-n junction will be charged negatively, and the part of the n-type semiconductor adjacent to the p-n junction, on the contrary, will acquire a positive charge. Thus, a double charged layer is formed near the p-n junction, which counteracts the process of diffusion of electrons and holes. Indeed, diffusion tends to create a flow of electrons from the n-region to the p-region, and the field of the charged layer, on the contrary, returns electrons to the n-region. Similarly, the field in the pn junction counteracts the diffusion of holes from the p- to n-region. As a result of two processes acting in opposite directions (diffusion and movement of current carriers in an electric field), a stationary, equilibrium state is established: a charged layer appears at the boundary, preventing the penetration of electrons from the n-semiconductor and holes from the p-semiconductor. In other words, in the region of the p-n junction an energy (potential) barrier arises, to overcome which electrons from the n-semiconductor and holes from the p-semiconductor must expend a certain energy. Without stopping to describe the electrical characteristics of the pn junction, which is widely used in rectifiers, transistors and other semiconductor devices, let us consider the operation of the pn junction in photocells.
When light is absorbed in a semiconductor, electron-hole pairs are excited. In a homogeneous semiconductor, photoexcitation increases only the energy of electrons and holes without separating them in space, that is, electrons and holes are separated in “energy space” but remain close together in geometric space. For the separation of current carriers and the appearance of photoelectromotive force (photoEMF), an additional force must exist. The most effective separation of nonequilibrium carriers occurs precisely in the region of the pn junction. “Minority” carriers generated near the p-n junction (holes in the n-semiconductor and electrons in the p-semiconductor) diffuse to the p-n junction, are picked up by the field of the p-n junction and thrown into the semiconductor, in which they become majority carriers: electrons will be localized in an n-type semiconductor, and holes in a p-type semiconductor. As a result, the p-type semiconductor receives an excess positive charge, and the n-type semiconductor receives a negative charge. A potential difference—photoEMF—occurs between the n- and p-regions of the photocell. The polarity of the photoEMF corresponds to the “forward” bias of the p-n junction, which lowers the barrier height and promotes the injection of holes from the p-region to the n-region and electrons from the n-region to the p-region. As a result of the action of these two opposite mechanisms—the accumulation of current carriers under the influence of light and their outflow due to a decrease in the height of the potential barrier—at different light intensities, different photovoltage values are established. In this case, the value of photovoltage in a wide range of illumination increases in proportion to the logarithm of the light intensity. At very high light intensity, when the potential barrier turns out to be practically zero, the photoEMF value reaches “saturation” and becomes equal to the height of the barrier at the unilluminated p-n junction. When exposed to direct, as well as solar radiation concentrated up to 100-1000 times, the photoEMF value is 50-85% of the contact potential difference of the p-n junction.
Thus, the process of occurrence of photovoltage that occurs at the contacts of the p- and n-regions of the p-n junction is considered. When an illuminated pn junction is short-circuited, a current will flow in the electrical circuit that is proportional to the illumination intensity and the number of electron-hole pairs generated by the light. When a payload, such as a calculator powered by a solar battery, is connected to the electrical circuit, the current in the circuit will decrease slightly. Typically, the electrical resistance of the payload in the solar cell circuit is chosen so as to obtain the maximum electrical power delivered to this load.
A solar photocell is made from a wafer made of a semiconductor material, such as silicon. Regions with p- and n-types of conductivity are created in the plate. Methods for creating these areas include, for example, the method of impurity diffusion or the method of growing one semiconductor onto another. Then the lower and upper electrical contacts are made, with the lower contact being solid, and the upper contact being made in the form of a comb structure (thin strips connected by a relatively wide current collection bus).
The main material for producing solar cells is silicon. The technology for producing semiconductor silicon and photocells based on it is based on methods developed in microelectronics - the most developed industrial technology. Silicon, apparently, is generally one of the most studied materials in nature, and also the second most abundant after oxygen. Considering that the first solar cells were made from silicon about forty years ago, it is natural that this material plays first fiddle in solar photovoltaic energy programs. Photocells made from monocrystalline silicon combine the advantages of using a relatively cheap semiconductor material with the high parameters of devices obtained from it.
Until recently, solar cells for terrestrial use, as well as for space applications, were made on the basis of relatively expensive monocrystalline silicon. Reducing the cost of initial silicon, the development of high-performance methods for manufacturing wafers from ingots and advanced technologies for manufacturing solar cells have made it possible to reduce the cost of ground-based solar cells based on them several times. The main areas of work to further reduce the cost of solar electricity are: obtaining elements based on cheap, including strip, polycrystalline silicon; development of cheap thin-film elements based on amorphous silicon and other semiconductor materials; Converting concentrated solar radiation using highly efficient silicon-based elements and a relatively new aluminum-gallium-arsenic semiconductor material.
A Fresnel lens is a plate made of plexiglass 1–3 mm thick, one side of which is flat, and on the other there is a profile in the form of concentric rings, repeating the profile of a convex lens. Fresnel lenses are significantly cheaper than conventional convex lenses and provide a degree of concentration of 2 - 3 thousand “suns”.
In recent years, significant progress has been made in the world in the development of silicon solar cells that operate under concentrated solar irradiation. Silicon elements with efficiency > 25% have been created under irradiation conditions on the Earth's surface at a concentration degree of 20 - 50 “suns”. Significantly greater degrees of concentration are allowed by photocells based on the semiconductor material aluminum-gallium-arsenic, first created at the Physico-Technical Institute. A.F. Ioffe in 1969. In such solar cells, efficiency values > 25% are achieved at concentration levels of up to 1000 times. Despite the high cost of such elements, their contribution to the cost of generated electricity does not turn out to be decisive at high degrees of concentration of solar radiation due to a significant (up to 1000 times) reduction in their area. The situation in which the cost of photocells does not make a significant contribution to the total cost of a solar power installation makes it justified to complicate and increase the cost of a photocell if this ensures an increase in efficiency. This explains the current attention paid to the development of cascaded solar cells, which can achieve a significant increase in efficiency. In a cascade solar cell, the solar spectrum is split into two (or more) parts, for example, visible and infrared, each of which is converted using photocells made from different materials. In this case, the energy losses of solar radiation quanta are reduced. For example, in two-element cascades the theoretical efficiency value exceeds 40%.
Solar energy- a direction of non-traditional energy based on the direct use of solar radiation to obtain energy in any form. Solar energy uses an inexhaustible source of energy and is environmentally friendly, that is, it does not produce harmful waste. Energy production using solar power plants fits well with the concept of distributed energy production.
Photovoltaics- a method of generating electrical energy by using photosensitive elements to convert solar energy into electricity.
Solar thermal energy- one of the methods of practical use of a renewable energy source - solar energy, used to convert solar radiation into heat of water or low-boiling liquid coolant. Solar thermal energy is used both for industrial production of electricity and for heating water for domestic use.
Solar battery- an everyday term used in colloquial speech or the non-scientific press. Typically, the term “solar battery” or “solar panel” refers to several combined photovoltaic converters (photocells) - semiconductor devices that directly convert solar energy into direct electric current.
The term "photovoltaics" refers to the normal operating mode of a photodiode in which electric current is generated solely by converted light energy. In fact, all photovoltaic devices are varieties of photodiodes.
Photoelectric converters (PVCs)
In photovoltaic systems, the conversion of solar energy into electrical energy is carried out in photovoltaic converters (PVCs). Depending on the material, design and production method, it is customary to distinguish three generations of PV cells:
First generation solar cells based on crystalline silicon wafers;
Second generation FEC based on thin films;
Third generation FEC based on organic and inorganic materials.
To increase the efficiency of solar energy conversion, solar cells based on cascade multilayer structures are being developed.
FEP first generation
First-generation solar cells based on crystalline wafers are currently the most widely used. Over the past two years, manufacturers have managed to reduce the cost of production of such PV cells, which has ensured the strengthening of their position in the global market.
Types of first generation solar cells:
monocrystalline silicon (mc-Si),
polycrystalline silicon (m-Si),
based on GaAs,
ribbon technologies (EFG, S-web),
thin layer polysilicon (Apex).
FEP second generation
The technology for producing second-generation thin-film solar cells involves applying layers using the vacuum method. Vacuum technology, compared to the production technology of crystalline solar cells, is less energy-consuming and is also characterized by a lower volume of capital investments. It allows the production of flexible, cheap solar cells with a large area, but the conversion coefficient of such elements is lower compared to the first generation solar cells.
Types of second generation solar cells:
amorphous silicon (a-Si),
micro- and nanosilicon (μc-Si/nc-Si),
silicon on glass (CSG),
cadmium telluride (CdTe),
Copper-(indium-)gallium (di)selenide (CI(G)S).
Third generation FEP
The idea of creating third-generation solar cells was to further reduce the cost of solar cells, abandoning the use of expensive and toxic materials in favor of cheap and recyclable polymers and electrolytes. An important difference is also the possibility of applying layers using printing methods.
Currently, the bulk of projects in the field of third-generation solar cells are at the research stage.
Types of third generation solar cells:
photosensitized dye (DSC),
organic (OPV),
inorganic (CTZSS).
Installation and use
PV cells are assembled into modules that have standardized installation dimensions, electrical parameters and reliability indicators. To install and transmit electricity, solar modules are equipped with current inverters, batteries and other elements of electrical and mechanical subsystems.
Depending on the area of application, the following types of solar system installations are distinguished:
private low-power stations located on the roofs of houses;
commercial stations of low and medium power, located both on roofs and on the ground;
industrial solar stations providing energy supply to many consumers.
Maximum efficiency values of photocells and modules achieved in laboratory conditions
Factors affecting the efficiency of photocells
From the performance characteristics of the photovoltaic panel it is clear that to achieve the greatest efficiency, the correct selection of load resistance is required. To do this, photovoltaic panels are not connected directly to the load, but use a photovoltaic systems control controller, which ensures optimal operation of the panels.
Production
Very often single photocells do not produce enough power. Therefore, a certain number of PV elements are connected into so-called photovoltaic solar modules and a reinforcement is mounted between the glass plates. This assembly can be fully automated.
Advantages
Public accessibility and inexhaustibility of the source.
Safety for the environment - although there is a possibility that the widespread introduction of solar energy could change the albedo (characteristic of reflectivity (scattering) ability) of the earth's surface and lead to climate change (however, given the current level of energy consumption, this is extremely unlikely).
Flaws
Dependence on weather and time of day.
The need for energy storage.
In industrial production, there is a need to duplicate solar ES with maneuverable ES of comparable power.
High cost of construction associated with the use of rare elements (for example, indium and tellurium).
The need to periodically clean the reflective surface from dust.
Heating the atmosphere above the power plant.
The conversion efficiency depends on the electrical characteristics of the inhomogeneous semiconductor structure, as well as the optical properties of the solar cell, among which photoconductivity plays the most important role. It is caused by the phenomena of internal photoelectric effect in semiconductors when irradiated with sunlight.
The main irreversible energy losses in solar cells are associated with:
reflection of solar radiation from the surface of the converter,
the passage of part of the radiation through the photovoltaic cell without absorption in it,
scattering of excess photon energy on thermal vibrations of the lattice,
recombination of the formed photo-pairs on the surfaces and in the volume of the photovoltaic cell,
internal resistance of the converter, etc.
