Solar photovoltaic converters. The principle of the solar battery: how the solar panel is arranged and works
Solar energy- the 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. The production of energy from solar power plants fits well with the concept of distributed power generation.
Photovoltaics- a method of generating electrical energy by using photosensitive elements to convert solar energy into electricity.
Solar energy- one of the ways of practical use of a renewable energy source - solar energy, used to convert solar radiation into the heat of water or a low-boiling liquid heat carrier. Solar energy is used both for industrial production of electricity and for heating water for domestic use.
Solar battery- a household term used in colloquial speech or unscientific press. Usually, the term "solar battery" or "solar panel" refers to several combined photovoltaic converters (photovoltaic cells) - semiconductor devices that directly convert solar energy into direct current.
The term "photovoltaic" means the normal operating mode of a photodiode, in which the electric current is generated solely from the converted light energy. In fact, all photovoltaic devices are varieties of photodiodes.
Photovoltaic converters (FEP)
In photovoltaic systems, the conversion of solar energy into electrical energy is carried out in photovoltaic converters (PV converters). Depending on the material, design and production method, it is customary to distinguish between three generations of FEP:
PVC of the first generation based on crystalline silicon wafers;
PEC of the second generation based on thin films;
FEP of the third generation based on organic and inorganic materials.
To increase the efficiency of solar energy conversion, PV cells are being developed on the basis of cascade multilayer structures.
FEP of the first generation
PECs of the first generation based on crystal plates are most widely used today. Over the past two years, manufacturers have managed to reduce the production cost of such PVCs, which has strengthened their positions in the world market.
Types of FEP of the first generation:
monocrystalline silicon (mc-Si),
polycrystalline silicon (m-Si),
based on GaAs,
ribbon-technologies (EFG, S-web),
thin layer polysilicon (Apex).
FEP of the second generation
The technology for the production of second-generation thin-film PVCs implies the deposition of layers by a vacuum method. The vacuum technology, in comparison with the technology for the production of crystalline PVCs, is less energy consuming, and also characterized by a lower volume of capital investments. It makes it possible to produce flexible low-cost PVCs of a large area, however, the conversion coefficient of such elements is lower in comparison with the first generation PVCs.
Types of FEP of the second generation:
amorphous silicon (a-Si),
micro- and nanosilicon (μc-Si / nc-Si),
silicon on glass (CSG),
cadmium telluride (CdTe),
(di) copper- (indium-) gallium selenide (CI (G) S).
FEP of the third generation
The idea of creating a third-generation solar cell was to further reduce the cost of a solar cell, to abandon 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 by printing methods.
Currently, the bulk of projects in the field of third-generation PV cells are at the research stage.
Types of FEP of the third generation:
dye photosensitized (DSC),
organic (OPV),
inorganic (CTZSS).
Installation and use
FEPs are assembled into modules that have standardized mounting dimensions, electrical parameters and reliability indicators. For the installation and transmission of electricity, solar modules are equipped with current inverters, batteries and other elements of the electrical and mechanical subsystems.
Depending on the field of application, the following types of solar system installations are distinguished:
private small power plants located on the roofs of houses;
small and medium-sized commercial plants located both on the roofs and on the ground;
industrial solar stations that provide energy to many consumers.
The maximum values of the efficiency of photocells and modules, achieved in laboratory conditions
Factors affecting the efficiency of photocells
From the performance characteristics of the photovoltaic panel, it can be seen that the correct selection of load resistance is required to achieve the highest efficiency. To do this, the photovoltaic panels are not directly connected to the load, but use a photovoltaic system controller that ensures the optimal operation of the panels.
Production
Very often, single photocells do not produce enough power. Therefore, a certain number of PV cells are combined into so-called photovoltaic solar modules and a reinforcement is mounted between the glass plates. This build can be fully automated.
Dignity
The general availability and inexhaustibility of the source.
Safe for the environment - although there is a possibility that the widespread introduction of solar energy could change the albedo (a characteristic of the reflective (scattering) ability) of the earth's surface and lead to climate change (however, with the current level of energy consumption, this is extremely unlikely).
disadvantages
Dependence on weather and time of day.
The need for energy storage.
In industrial production - the need to duplicate solar power plants with maneuverable power plants of comparable power.
The high cost of construction associated with the use of rare elements (for example, indium and tellurium).
The need for periodic cleaning of the reflective surface from dust.
Heating the atmosphere over the power plant.
The conversion efficiency depends on the electrophysical characteristics of the inhomogeneous semiconductor structure, as well as the optical properties of the PVC, among which the most important role is played by photoconductivity. It is caused by the phenomena of the internal photoelectric effect in semiconductors when they are irradiated with sunlight.
The main irreversible energy losses in PVC are associated with:
reflection of solar radiation from the surface of the transducer,
the passage of part of the radiation through the PVC without absorption in it,
scattering of excess photon energy on thermal vibrations of the lattice,
by recombination of the formed photo-pairs on the surfaces and in the volume of the PVC,
internal resistance of the converter, etc.
The photoelectric method of converting solar energy into electrical energy is based on the phenomenon of the photoelectric effect - the release of conduction electrons in the radiation receiver under the influence of solar radiation quanta.
This effect is used in semiconductor materials, in which the energy of radiation quanta hn creates, for example, on p–n-transition photocurrent
I f=eN e,
where N e- the number of electrons creating a potential difference at the junction, as a result of which a leakage current will flow at the junction in the opposite direction I equal to the photocurrent, which is constant.
Energy losses during photoelectric conversion are due to the incomplete utilization of photons, as well as the scattering, resistance and recombination of the conduction electrons that have already arisen.
The most common commercial solar cell (solar cell) is the silicon wafer cell. There are also other types and designs that are being developed to improve the efficiency and reduce the cost of solar cells.
The thickness of a solar cell depends on its ability to absorb solar radiation. Semiconductor materials such as silicon, gallium arsenide, etc. are used because they begin to absorb solar radiation with a sufficiently long wavelength, and can convert a significant portion of it into electricity. The absorption of solar radiation by various semiconductor materials reaches the highest value when the thickness of the plates is from 100 to 1 micron or less.
Reducing the thickness of the solar cell can significantly reduce the consumption of materials and the cost of their manufacture.
Differences in the absorption capacity of semiconductor materials are explained by differences in their atomic structure.
The efficiency of converting solar energy into electricity is not high. For flint elements no more than 12 ... 14%.
To increase the efficiency of solar cells, antireflection coatings are used on the front side of the solar cell. As a result, the proportion of transmitted solar radiation increases. Uncoated elements have a return loss of up to 30%.
Recently, a number of new materials have been used for the manufacture of solar cells. One of them is amorphous silicon, which, unlike crystalline silicon, has no regular structure. For an amorphous structure, the probability of photon absorption and transition to the conduction band is higher. Hence, it has a great absorbency. Gallium arsenide (GaAs) is also used. The theoretical efficiency of GaAs-based elements can reach 25%, real elements have an efficiency of about 16%.
Thin-film solar cell technology is being developed. Despite the fact that the efficiency of these elements in laboratory conditions does not exceed 16%, they have a lower cost. This is especially valuable for reducing cost and material consumption in mass production. In the USA and Japan, thin-film elements are made on amorphous silicon with an area of 0.1 ... 0.4 m 2 with an efficiency of 8 ... 9%. The most common thin-film solar cell is cadmium sulfide (CdS) cells with an efficiency of 10%.
Another advancement in thin-film solar cell technology has been the production of multilayer cells. They allow you to cover most of the solar spectrum.
The active material of a solar cell is quite expensive. For more efficient use, solar radiation is collected on the surface of the solar cell using concentrating systems (Fig. 2.7).
With an increase in the radiation flux, the characteristics of the element do not deteriorate if its temperature is maintained at the same temperature as the ambient air using active or passive cooling.
There are a large number of concentrating systems based on lenses (usually flat Fresnel lenses), mirrors, total internal reflection prisms, etc. If there is a very uneven irradiation of solar cells or modules, this can lead to the destruction of the solar cell.
The use of concentrating systems can reduce the cost of solar power plants, since concentrating cells are cheaper than solar cells.
As the price of solar cells declined, it became possible to build large photovoltaic installations. By 1984, 14 relatively large solar power plants with a capacity of 200 kW to 7 MW had been built in the USA, Italy, Japan, Saudi Arabia and Germany.
A solar photovoltaic plant has a number of advantages. It uses a clean and inexhaustible source of energy, has no moving parts and therefore does not require constant monitoring by the maintenance personnel. Solar cells can be produced in mass series, which will lead to a decrease in their cost.
Solar panels are assembled from solar modules. At the same time, there is a large selection of types and sizes of these devices with the same energy conversion efficiency and the same production technology.
Since the supply of solar energy is periodic, it is most rational to include photovoltaic systems in hybrid power plants that use both solar energy and natural gas. A new generation of gas turbines can be used at these stations. Hybrid low power plants, consisting of photovoltaic panels and diesel generators, are already reliable energy suppliers.
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Energy conversion in PVC is based on the photovoltaic effect that occurs in inhomogeneous semiconductor structures when exposed to solar radiation. The inhomogeneity of the PVC structure can be obtained by doping the same semiconductor with different impurities (creating p - n junctions) or by combining different semiconductors with an unequal energy gap-energy of electron detachment from an atom (creating heterojunctions), or by changing the chemical composition semiconductor, leading to the appearance of a gradient of the bandgap (creation of graded-gap structures). Various combinations of the above methods are also possible. The conversion efficiency depends on the electrophysical characteristics of the inhomogeneous semiconductor structure, as well as the optical properties of the PVC, among which the most important role is played by photoconductivity due to the phenomena of the internal photoelectric effect in semiconductors when they are irradiated with sunlight. The principle of operation of PVC can be explained by the example of converters with p-n-junction, which are widely used in modern solar and space energy. An electron-hole junction is created by doping a plate of a monocrystalline semiconductor material with a certain type of conductivity (i.e., either p- or n-type) with an impurity, which ensures the creation of a surface layer with the opposite type of conductivity.
The concentration of the dopant in this layer should be significantly higher than the concentration of the impurity 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. Depletion zones with uncompensated positive volume charge in the n-layer and negative volume charge in the p-layer are formed at the boundary of the n and p layers as a result of charge flow. Together, these zones form a p-n-junction. The potential barrier (contact potential difference) arising at the transition prevents the passage of the majority charge carriers, i.e. electrons from the side of the p-layer, but minority carriers freely pass in opposite directions. It is this property of p-n junctions that determines the possibility of obtaining a photo-emf by irradiating a PVC with sunlight. The nonequilibrium charge carriers (electron-hole pairs) created by light in both layers of the PVC are separated at the p-n-junction: minority carriers (i.e., electrons) freely pass through the junction, while the major carriers (holes) are delayed. Thus, under the action 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 PVC. 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 (combine) with holes moving inside the PVC in the opposite direction. There is a contact system on the surface of the semiconductor structure of the PVC for the collection and removal of electrons into the external circuit. On the front, illuminated surface of the converter, the contacts are made in the form of a grid or comb, and on the rear surface they can be solid.
The main irreversible energy losses in PVC are associated with:
- reflection of solar radiation from the surface of the transducer,
- the passage of part of the radiation through the PVC without absorption in it,
- scattering of excess photon energy by thermal vibrations of the lattice,
- by recombination of the formed photocouples on the surfaces and in the volume of the PVC,
- internal resistance of the converter,
- and some other physical processes.
To reduce all types of energy losses in the FEP, various measures are being developed and successfully applied. These include:
- the use of semiconductors with an optimal bandgap for solar radiation;
- directed improvement of the properties of a semiconductor structure by means of its optimal doping and creation of built-in electric fields;
- transition from homogeneous to heterogeneous and graded-gap semiconductor structures;
- optimization of the design parameters of the PVC (the depth of the p-n-junction, the thickness of the base layer, the frequency of the contact grid, etc.);
- the use of multifunctional optical coatings that provide antireflection, thermal control and protection of solar cells from cosmic radiation;
- development of PVCs that are transparent in the long-wavelength region of the solar spectrum beyond the edge of the main absorption band;
- creation of cascade PVCs from semiconductors specially selected for the width of the forbidden zone, which make it possible to convert radiation in each cascade that passed through the previous cascade, etc .;
Also, a significant increase in the efficiency of PVC was achieved by creating converters with two-sided sensitivity (up to + 80% to the already existing efficiency of one side), the use of luminescent re-emitting structures, preliminary decomposition of the solar spectrum into two or more spectral regions using multilayer film beam splitters (dichroic mirrors ) with the subsequent transformation of each part of the spectrum by a separate PVC, etc.
In energy conversion systems of SES (solar power plants), in principle, any types of PV cells of various structures created and being developed at present can be used on the basis of various semiconductor materials, but not all of them satisfy the set of requirements for these systems:
- high reliability with a long (tens of years!) resource of work;
- the availability of raw materials in sufficient quantity for the manufacture of elements of the transformation system and the possibility of organizing their mass production;
- energy costs acceptable in terms of payback periods for creating a conversion system;
- the minimum energy and mass consumption associated with the control of the power conversion and transmission system (space), including the orientation and stabilization of the station as a whole;
- ease of maintenance.
So, for example, some promising materials are difficult to obtain in the quantities necessary for the creation of SES 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 PVC, 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 with the organization of fully automated production of FEP, for example, based on tape technology, and the creation of a developed network of specialized enterprises of the corresponding profile, i.e. in fact, an entire industry, commensurate in scale with the modern radio-electronic industry. Manufacturing solar cells and assembling solar batteries on automated lines will reduce the cost of a 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 SES, and in the latter case we are talking about heterophotovoltaic converters (HFP) with the AlGaAs-GaAs structure.
PVCs (photovoltaic converters) based on a compound of arsenic with gallium (GaAs) are known to have a higher theoretical efficiency than silicon PVCs, since their band gap practically coincides with the optimal band gap for semiconductor solar energy converters = 1 , 4 eV. For silicon, this index = 1.1 eV.
Due to the higher absorption of solar radiation, determined by direct optical transitions in GaAs, high efficiency of PVCs based on them can be obtained at a much smaller thickness of the PVCs compared to silicon. In principle, it is sufficient to have a HFP 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 makes it possible to count on the creation of light film HFPs, for the production of which relatively little starting material is required, especially if it is possible to use not GaAs as a substrate, but another material, for example, synthetic sapphire (Al2O3).
HFPs also have more favorable operating characteristics from the point of view of requirements for SES converters in comparison with silicon PVCs. Thus, in particular, the possibility of achieving small initial values of the reverse saturation currents in pn junctions due to the large bandgap allows minimizing the value of negative temperature gradients of the efficiency and optimal HPP power and, in addition, significantly expanding the region of the linear dependence of the latter on the luminous flux density ... The experimental dependences of the efficiency of HFP on temperature indicate that an increase in 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 PVCs, 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 make it possible to apply solar radiation concentrators to them. The working temperature of HFP on GaAs reaches 180 ° C, which is already quite working temperatures for heat engines and steam turbines. Thus, to the 30% intrinsic efficiency of gallium arsenide HFP (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, may even be higher than 50-60%.
Also, GaAs-based HFPs are, to a much lesser extent than silicon PVCs, subject to destruction by high-energy proton and electron fluxes due to the high level of light absorption in GaAs, as well as the small required values of the lifetime and diffusion length of minority carriers. Moreover, experiments have shown that a significant part of radiation defects in HFP based on GaAs disappears after their heat treatment (annealing) at a temperature of just about 150-180 ° C. If GaAs HPPs will constantly operate at a temperature of about 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 small weight and size of PVCs and high efficiency are important) ...
In general, we can conclude that the energy, mass, and operational characteristics of HFP based on GaAs are more consistent with the requirements of SES and SCES (space) than the characteristics of silicon PVCs. However, silicon is a much more readily available and widely used material in production than gallium arsenide. Silicon is widespread in nature, and the supply of raw materials for the creation of PVC on its basis is practically unlimited. The technology for manufacturing silicon solar cells is well developed and is constantly being improved. There is a real prospect of reducing the cost of silicon solar cells by one or two orders of magnitude when introducing new automated production methods, which, in particular, make it possible to obtain silicon ribbons, solar cells of a large area, etc.
Prices for silicon photovoltaic batteries have decreased 20-30 times over 25 years from $ 70-100 / watt in the seventies to $ 3.5 / watt in 2000 and continue to decline further. In the West, a revolution in the energy sector is expected at the time the price crosses the $ 3 milestone. According to some calculations, this may happen already in 2002, but for Russia with the current energy tariffs, this moment will come when the price of 1 watt SB $ 0.3-0.5, that is, at an order of magnitude lower price. Taken together, they play a role: tariffs, climate, geographic latitudes, the state's ability to make real pricing and long-term investments. In actually operating structures with heterojunctions, the efficiency reaches today more than 30%, and in homogeneous semiconductors such as single-crystal silicon - up to 18%. The average efficiency in solar cells based on monocrystalline silicon today is about 12%, although it reaches 18%. It is, basically, silicon SBs that can be seen today on the roofs of houses around the world.
In contrast to silicon, gallium is a very scarce material, which limits the possibilities of producing HFPs based on GaAs in quantities required for widespread adoption.
Gallium is mainly mined 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 its concentration there is very low, the recovery is estimated at only 1% and, therefore, the production costs will probably be prohibitive. The technology for the production of HFP based on GaAs using liquid and gas epitaxy methods (oriented growth of one single crystal on the surface of another (on a substrate)) is not yet developed to such an extent as the technology for the production of silicon PVC, and as a result, the cost of HFP is now significantly higher (by orders) of the cost of PVC from silicon.
In spacecraft, where solar batteries are the main source of current 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. Very important for space SES is the ability of this compound in PVCs not to lose efficiency when heated by 3-5 times concentrated solar radiation, 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 a HFP substrate.
The cost of HFPs in their mass production based on the improved technology will probably also be significantly reduced, and in general, the cost of the conversion system of the power conversion system of the SES based on HFP from GaAs may be quite comparable to the cost of the system based on silicon. Thus, at the present time it is difficult to completely give a clear preference to one of the two considered semiconductor materials - silicon or gallium arsenide, and only further development of their production technology will show which option will be more rational for terrestrial and space solar power engineers. Insofar as SBs give out direct current, the task arises of transforming it into an industrial alternating current of 50 Hz, 220 V. A special class of devices - inverters - perfectly copes with this task.
Calculation of the 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 current strength at a given voltage. But unlike other current sources, the characteristics of a solar cell depend on the amount of light falling on its surface. For example, an oncoming cloud can reduce the output power by more than 50%. In addition, deviations in technological modes entail a scatter of the output parameters of the elements of one batch. Consequently, the desire to maximize the efficiency of photovoltaic converters leads to the need to sort the cells according to the output current. As an illustrative example of “a lousy sheep spoiling the whole herd,” we can cite the following: cut a section of a pipe with a much smaller diameter into a rupture of a large-diameter water pipe, as a result, the watercourse will be sharply reduced. Something similar occurs in a chain of solar cells that are non-uniform in terms of their output parameters.
Silicon solar cells are non-linear devices and their behavior cannot be described by a simple formula like Ohm's law. Instead, to explain the characteristics of an element, you can use a family of easy-to-understand curves - current-voltage characteristics (VAC)
The open circuit voltage generated by one cell varies slightly from one cell to another in one batch and from one manufacturer to another and is about 0.6 V. This value does not depend on the size of the cell. The situation is different with the current. It depends on the intensity of light and the size of the element, which means its surface area.
An element with a size of 100 100 mm is 100 times larger than an element with a size of 10 10 mm and, therefore, under the same illumination, it will give out a current 100 times greater.
By loading the element, you can plot the dependence of the output power on the voltage, getting 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 a solar cell, as well as for the sake of comparing the elements with each other in the same conditions, it is necessary to load it so that the output voltage is 0.47 V. After the solar the elements are selected for work, you need to solder them. Serial elements are equipped with collector grids, which are designed for soldering conductors to them.
The batteries can be combined in any desired combination. The simplest battery is a string of cells connected in series. You can also connect strings in parallel to form 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 I - V characteristics for temperatures of 25 ° С and 60 ° С.
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 voltage drop generated by the cell. The efficiency of a conventional solar cell currently ranges from 10-16%. This means that a cell with a size of 100 to 100 mm under standard conditions can generate 1-1.6 watts.
All photovoltaic systems can be divided into two types: stand-alone and grid-connected. Stations of the second type give surplus energy to the grid, which serves as a reserve in the event of an internal energy deficit.
An autonomous system generally consists of a set of solar modules located on a supporting structure or on the roof, a storage battery (accumulator), a discharge controller - battery charge, 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 must be soldered. Serial elements are equipped with collector grids for soldering conductors to them. The batteries can be combined in any combination.
The simplest battery is a string of cells connected in series.
You can connect these strings in parallel to form a so-called series-parallel connection. In parallel, only chains (rulers) with identical voltages can be connected, while their currents are summed up according to Kirchhoff's law.
For ground use, they are usually used to charge storage batteries (accumulators) 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, aluminum. In this case, the elements are located between two layers of a sealing film, without an air gap. Vacuum lamination technology meets this requirement. In the case of an air gap between the protective glass and the element, the reflection and absorption losses would reach 20-30% compared to 12% without the air gap.
The electrical parameters of a solar cell are presented as a separate solar cell in the form of a volt-ampere curve under Standard Test Conditions, i.e., at a solar radiation of 1000 W / m2, a temperature of 25 ° C and a solar spectrum at a latitude of 45 ° (AM1.5) ...
The point of intersection of the curve with the axis of voltages is called the open-circuit voltage - Uxx, the point of intersection with the axis of currents - the short-circuit current Isc.
The maximum power of a module is defined as the highest power at 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 36-cell module will therefore be about 16 ... 17 V (0.45 ... 0.47 V per cell) at 25 ° C.
Such a voltage margin in comparison with the full charge voltage of the battery (14.4 V) is necessary in order to compensate for the losses in the battery charge-discharge controller (we will talk about it later), and basically - 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 cell).
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 in real operating conditions, the modules are heated to a temperature of 60-70 ° C, which corresponds to the offset of 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 cell).
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 an alternating voltage, then an inverter-converter of direct voltage to alternating voltage is added to this set.
The calculation of FES is understood as the determination of the rated power of the modules, their number, connection schemes; selection of the type, operating conditions and capacity of the battery; the capacities of the inverter and the charge-discharge controller; determination of the parameters of the connecting cables.
First of all, it is necessary to determine the total power of all consumers connected at the same time. The power of each of them is measured in watts and is indicated in the product passports. At this stage, it is already possible to select the power of the inverter, which should be at least 1.25 times the calculated one. It should be borne in mind that such a tricky device as a compressor refrigerator at the time of start-up consumes 7 times more power than the passport.
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 chosen at least 48 V, because at higher powers, inverters perform better with higher input voltages.
The next step is to determine the capacity of the battery. The battery capacity is selected from a standard range of containers 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 by the value of the battery discharge depth 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 A * h
When calculating the capacity of the battery in a completely autonomous mode, it is necessary to take into account the presence of cloudy days in nature during which the battery must ensure the work of consumers.
The last step is to determine the total power and the 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 section “meteorology” gives monthly and total annual values of solar radiation for the main regions of Russia, as well as with 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 get the so-called number of pic-hours, i.e., the conditional time during which the sun shines, as it were, 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 to the south at an angle of 40o to the horizon. This means that the average 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 a site 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 is the coefficient equal to 0.5 in summer and 0.7 in winter.
This factor makes a correction for the loss of power of solar cells when heated in the sun, and also takes into account the oblique 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 the consumed energy and the above formula, it is easy to calculate the total power of the modules. And knowing it, by simply dividing it by the capacity of one module, we get the number of modules.
When creating a PV 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 luminaires, with 5 times less consumption, provide a luminous flux equivalent to the luminous flux of an incandescent lamp.
For small photovoltaic power plants, it is advisable to install its modules on a swivel bracket for optimal rotation relative to the incident beams. This will increase the capacity of the station by 20-30%.
A little about inverters.
Inverters or converters of direct current into alternating current are designed to provide high-quality power supply to various equipment and devices in the absence or poor quality of an alternating current mains with a frequency of 50 Hz and a voltage of 220 V, various emergency situations, etc.
The inverter is a 12 (24, 48, 60) V DC pulse converter into alternating current with a stabilized voltage of 220 V and a frequency of 50 Hz. Most inverters have a STABILIZED SINUSOID voltage at the output, which allows them to be used to supply power to almost any equipment and appliances.
Structurally, the inverter is made in the form of a tabletop unit. On the front panel of the inverter are the product operation switch and the inverter operation indicator. On the rear panel of the product there are leads (terminals) for connecting a DC source, for example, a battery, a grounding terminal for the inverter case, a hole with a fan (cooling), a three-pole euro socket for connecting the load.
The stabilized voltage at the output of the inverter allows you to provide high-quality power supply to the load with changes / fluctuations in the voltage at the input, for example, when the battery is discharged, or fluctuations in the current consumed by the load. The guaranteed galvanic isolation of the DC source at the input and the AC circuit with the load at the output of the inverter allows you not to take additional measures to ensure operational safety when using various DC sources or any electrical equipment. Forced cooling of the power section and low noise level during the operation of the inverter allow, on the one hand, to ensure good weight and dimensions of the product, on the other hand, with this type of cooling, it does not create inconveniences in operation in the form of noise.
- Built-in control panel with electronic display
- Capacitance potentiometer that allows precise adjustments to be made
- Normalized pin-to-pin strip: WE WY STEROW
- Built-in brake turn
- Radiator with fan
- Aesthetic fastening
- Power supply 230 V - 400 V
- Overload 150% - 60s
- Take-off time 0.01 ... 1000 seconds
- Built-in electric filter, class A
- Working temperature: -5 ° C - + 45 ° C
- RS 485 port
- Frequency step regulation: 0.01 Hz - 1 kHz
- Protection class IP 20
Functionally provides: increase, decrease in frequency, control of overload, overheating.
Most renewable energies - hydropower, mechanical and thermal energy of the 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 the current level by only an order of magnitude. But there is another source of energy - the sun. The Sun, a spectral class 2 star, 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 middle star. Our luminary supplies the Earth with a power of about 10 17 W - such is the power of a "sunbeam" 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 burgeoning 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 conversion systems be for the production of a noticeable share of electricity in the world energy budget? Obviously, this area depends on the efficiency of the used conversion systems. To assess the efficiency of photovoltaic converters that directly convert solar energy into electrical energy using semiconductor photocells, we introduce the concept of efficiency (efficiency) of a photocell, defined as the ratio of the power of electricity generated by this element to the power of a sunbeam falling on the surface of the photocell. So, with the efficiency of solar converters equal to 10% (typical values of the efficiency for silicon photovoltaic cells, widely mastered 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 equal to a square with a side of 200 km. In this case, the intensity of solar radiation is taken equal to 250 W / m 2, which corresponds to a 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 quite a strong argument: the problem of converting solar energy must be solved today in order to use this energy tomorrow. You can at least jokingly consider this problem within the framework of solving energy problems for controlled thermonuclear fusion, when an efficient reactor (the Sun) was created by nature itself and provides a resource of reliable and safe operation for many millions of years, and our task is only to develop a ground-based converter substation. Recently, extensive research in the field of solar energy has been carried out in the world, which has shown that in the near future this method of obtaining 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 the territory of Russia lies in high latitudes, some very large southern regions of our country, in terms of their climate, are very favorable for the widespread use of solar energy.
The use of solar energy in the countries of the equatorial belt of the Earth and regions close to this belt, characterized by a high level of solar energy supply, has even greater prospects. 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 areas of work in the field of solar energy conversion are currently:
- direct thermal heating (obtaining thermal energy) and thermodynamic transformation (obtaining electrical energy with intermediate conversion of solar energy into thermal energy);
- photovoltaic conversion of solar energy.
Direct thermal heating is the simplest method for converting solar energy and is widely used in southern regions of Russia and in countries of the equatorial belt in solar heating installations, hot water supply, cooling buildings, desalination, etc. The basis of solar heat-using installations is flat solar collectors - absorbers of solar radiation. Water or other liquid, being in contact with the absorber, is heated and by means of a pump or natural circulation is discharged from it. Then the heated liquid enters the storage, from where it is consumed as needed. Such a device resembles a domestic hot water supply system.
Electricity is the most convenient form of energy for use and transmission. Therefore, the interest of researchers in the development and creation of solar power plants using the intermediate conversion of solar energy into heat with its subsequent conversion into electricity is understandable.
In the world, solar thermal power plants of two types are now most common: 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, in the focus of which are placed heat receivers and low-power converters.
2. DEVELOPMENT OF SOLAR ENERGY
In the late 70s - early 80s, seven pilot solar power plants (SPP) of the so-called tower type with a power level from 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 Оne) was built in California. All these SES are built on the same principle: a field located at ground level of heliostat mirrors that follow the sun reflects the sun's rays onto a receiver-receiver mounted on top of a rather high tower. The receiver is, in essence, a solar boiler, which produces medium-sized steam, which is then sent to a standard steam turbine.
The second project - a tower solar power plant PHOEBUS 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 vapor is generated, which operates in the Rankine cycle. On the air path from the receiver to the boiler, a burner for burning natural gas is supposed, the amount of which is regulated so as to maintain the specified power throughout the day. Calculations show that, for example, for an annual solar radiation of 6.5 GJ / m2 (similar to that which is typical for the southern regions of Ukraine), this SPP, 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 introduced with fuel will be 176.0 GW * h / year. At the same time, the SPP produces 87.9 GW * h of electricity per year with an average annual efficiency of 18.8%. With such indicators, the cost of electricity generated at SES can be expected at the level of TPPs using fossil fuel.
Since the mid-80s, in Southern California, the LUZ company has created and put into commercial operation nine SPPs with parabolic cylindrical concentrators (PCC) with unit capacities, which increased from the first SPP to the next from 13.8 to 80 MW. The total capacity of these SPPs has reached 350 MW. In these SES, we used a PCC with an aperture that increased during the transition from the first SES to the next. By tracking the sun on a single axis, the concentrators focus the solar radiation on tubular receivers enclosed in evacuated tubes. A high-temperature liquid heat carrier flows inside the receiver, which heats up to 380 ° C and then gives off the heat of the water vapor to the steam generator. The scheme of these SPPs 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 reduced insolation.
These SESs were created and operated at a time when there were laws in the United States that allowed SES to operate without loss. 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 SPPs of this type was stopped.
The KJC company (Kramеr Junction Company), which operated five of the nine constructed SPPs (from 3 to 7), set itself the task of increasing the efficiency of these SPPs, reducing the cost of their operation and making them economically attractive in the new conditions. At this time, this program is being successfully implemented.
Switzerland has become one of the leaders in the use of solar energy. As of 1997, about 2,600 solar plants based on photovoltaic converters with a capacity of 1 to 1000 kW were built here. The program, called "Solar-91" and carried out under the slogan "For a non-volatile Switzerland", makes a significant contribution to solving environmental problems and energy independence of the country, which now imports more than 70% of energy. A solar power plant with a capacity of 2-3 kW is most often mounted on roofs and facades of buildings. Such an installation generates an average of 2,000 kWh of electricity per year, which is enough for the domestic needs of an average Swiss home. Large firms install solar installations on the roofs of industrial buildings with a capacity of up to 300 kW. Such a station covers the needs of the enterprise in electricity by 50-60%.
In the conditions of the Alpine highlands, where it is unprofitable to lay power lines, solar power plants of high power 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 protection fences of highways, on transport and industrial structures, do not require expensive agricultural territory for their own placement. An autonomous solar installation near the village of Grimsel provides electricity for round-the-clock lighting of a road tunnel. Near the town of Shur, solar panels mounted on a 700-meter section of a noise barrier generate 100 kW of electricity annually.
The modern concept of using solar energy is most fully expressed during the construction of the buildings of the window glass factory in Arisdorf, where solar panels with a total capacity of 50 kW were assigned an additional role of ceiling elements and facade decoration even during the design process. The efficiency of solar converters with strong heating is noticeably reduced, therefore, ventilation pipelines are laid under the panels for pumping outside air. Dark blue, sparkling in the sun, photoconverters on the southern and western facades of the administrative building, giving electricity to the grid, act as a decorative cladding.
In developing countries, relatively small installations are used to supply power to individual houses, in remote villages for - equipping cultural centers, where, thanks to PMTs, you can use televisions, etc. In this case, not the cost of electricity, but the social effect, comes to the fore. PMT implementation programs in these countries are actively supported by international organizations, and the World Bank participates 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 using PMTs. A large program for the introduction of PMT is being implemented in India, where in 1986 - 1992. 690 million rupees were spent on installing a PMT in rural areas.
In industrialized countries, the active introduction of PMT is explained by several factors. First, PMTs are considered as environmentally friendly sources that can reduce the harmful effect on the environment. Secondly, the use of PMTs in private houses increases energy autonomy and protects the owner in case of possible interruptions in centralized power supply.
3. PHOTOELECTRIC 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 (FTI) of the Russian Academy of Sciences, Academician A.F. Ioffe. He dreamed of using semiconductor solar cells in solar energy already in the thirties, when B.T. Kolomiets and Yu.P. Maslakovets created thallium sulfide photocells at the Physicotechnical Institute with a record efficiency of 1% for that time.
The widespread practical use of solar batteries 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 power supply for spacecraft and large orbital stations of the Salyut and Mir type. A large backlog, accumulated by scientists in the field of solar batteries for space purposes, also made it possible to launch work on ground-based photovoltaic energy.
The basis of photocells is a semiconductor structure with a p-n junction arising at the interface of two semiconductors with different conduction mechanisms. Note that this terminology originates from the English words positive and negative. Various types of conductivity are obtained by changing the type of impurities introduced into the semiconductor. So, for example, the atoms of the III group of the periodic table of D.I. Mendeleev, introduced into the crystal lattice of silicon, give the latter hole (positive) conductivity, and group V impurities - electronic (negative). 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 cell. Let us explain the reason for the occurrence of the contact potential difference. When p- and n-type semiconductors are connected in one single crystal, a diffusion flow of electrons from the n-type semiconductor to the p-type semiconductor arises and, conversely, a flow of holes from 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, while the field of the charged layer, on the contrary, aims to return electrons to the n-region. Similarly, the field in the p-n 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, which prevents the penetration of electrons from an n-semiconductor, and holes from a p-semiconductor. In other words, an energy (potential) barrier arises in the region of the p-n junction, to overcome which electrons from the n-semiconductor and holes from the p-semiconductor must spend a certain energy. Without dwelling on the description of the electrical characteristics of the p-n junction, which is widely used in rectifiers, transistors and other semiconductor devices, we will consider the work of the p-n 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 dividing them in space, that is, electrons and holes are separated in the "energy space", but remain close to each other in geometric space. For the separation of current carriers and the appearance of a photoelectromotive force (photoelectromotive force), an additional force must exist. The most effective separation of nonequilibrium carriers takes place precisely in the region of the p-n junction. Generated near the p-n junction "minority" carriers (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 pn junction and are ejected into the semiconductor, in which they become the main carriers: electrons will be localized in an n-type semiconductor, and holes in a p-type semiconductor. As a result, the p-type semiconductor is positively charged and the n-type semiconductor negative. Between the n- and p-regions of the photocell, a potential difference arises - the photo-emf. The polarity of the photo-emf corresponds to the "forward" displacement 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 action of light and their outflow due to a decrease in the height of the potential barrier - at different light intensities, a different photo-emf value is established. In this case, the magnitude of the photo-emf 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 photo-emf value reaches “saturation” and becomes equal to the barrier height at the unlit pn junction. When illuminated by direct, as well as concentrated up to 100 - 1000 times solar radiation, the magnitude of the photo-emf is 50 - 85% of the contact potential difference of the p-n junction.
Thus, the process of the appearance of the photo-emf arising at the contacts of the p- and n-regions of the p-n junction is considered. When the illuminated pn junction is short-circuited, a current will flow in the electrical circuit proportional to the magnitude of the illumination intensity and the number of electron-hole pairs generated by the light. When a payload, such as a solar-powered calculator, is connected to an 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 the load.
A solar cell is made on the basis of a plate 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 regions are, for example, the method of diffusion of impurities 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 one in the form of a comb structure (thin strips connected by a relatively wide current-collecting bus).
The main material for producing solar cells is silicon. The technology for producing semiconductor silicon and solar cells based on it is based on methods developed in microelectronics, the most advanced industrial technology. Silicon, apparently, is generally one of the most studied materials in nature, moreover, the second most abundant after oxygen. Considering that the first solar cells were made of silicon about forty years ago, it is natural that this material plays the first violin in solar photovoltaic energy programs. Monocrystalline silicon photocells combine the advantages of using a relatively cheap semiconductor material with high parameters of devices based on it.
Until recently, solar cells for terrestrial use, as well as for space, were made on the basis of relatively expensive monocrystalline silicon. Reducing the cost of initial silicon, the development of high-performance methods for the manufacture of wafers from ingots and advanced technologies for the manufacture of solar cells have made it possible to reduce the cost of ground-based solar cells based on them by 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; conversion of concentrated solar radiation using highly efficient silicon-based cells and a relatively new semiconductor material aluminum-gallium-arsenic.
The Fresnel lens is a 1–3 mm thick plexiglass plate, one side of which is flat, and the other has 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 concentration level of 2 - 3 thousand "suns".
In recent years, significant progress has been made in the world in the development of silicon solar cells operating under concentrated solar irradiation. Silicon cells with an efficiency> 25% have been created under irradiation conditions on the Earth's surface with a concentration of 20-50 "suns". Photocells based on the semiconductor material aluminum-gallium-arsenic, first created at the Physico-Technical Institute named after V.I. A.F. Ioffe in 1969. In such solar cells, efficiency values> 25% are achieved at a concentration level of up to 1000 times. Despite the high cost of such elements, their contribution to the cost of the generated electricity is not 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 photovoltaic cells does not make a significant contribution to the total cost of a solar power plant makes it justified to complicate and increase the cost of a photocell, if this provides an increase in efficiency. This explains the attention currently being paid to the development of cascade solar cells, which allow 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 on the basis of different materials. In this case, the energy losses of solar radiation quanta are reduced. For example, in two-element stages, the theoretical value of the efficiency exceeds 40%.
