Photoelectric energy converters. How is the process of converting solar energy into electrical energy. Maximum efficiency values of photocells and modules achieved in laboratory conditions

Zagatin Sergey

The theme of my work "Photoelectric conversion of solar energy" is the most relevant at the present time.

In the abstract, I described methods for converting solar energy that can provide rapidly growing energy needs for many thousands of years. Electricity is the most convenient form of energy for use and transmission, since solar radiation is a practically inexhaustible source of energy.

In my opinion, the large-scale development of photoenergy will give a huge impetus to the development of areas of the Earth with a high average annual influx of solar radiation.

Download:

Preview:

Completed by: Zagatin S.V.

10 A student

Head: Luchina T.V.

Physics teacher

2008

	INTRODUCTION…………………………………………………………
	SOLAR ENERGY CONVERSION IS A PROMISING WAY FOR ENERGY DEVELOPMENT…...
	PHOTOELECTRIC CONVERSION OF SOLAR ENERGY……………………………………………………………..
	CONCLUSION……………………………………………………
	LITERATURE………………………………………………………

INTRODUCTION

The rapid growth of energy consumption is one of the most characteristic features of the technical activity of mankind in the second half of the 20th century. Until recently, the development of energy has not encountered fundamental difficulties. The increase in energy production was mainly due to an increase in the production of oil and gas, the most convenient in consumption. However, the energy industry turned out to be the first major sector of the world economy, which faced the situation of depletion of its traditional resource base. In the early 1970s, an energy crisis erupted in many countries. One of the reasons for this crisis was the limited availability of fossil energy resources. In addition, oil, gas and coal are also the most valuable raw materials for the intensively developing chemical industry. Therefore, it is now increasingly difficult to maintain a high rate of energy development by using only traditional fossil energy sources.

The nuclear power industry has recently also encountered significant difficulties associated, first of all, with the need for a sharp increase in the costs of ensuring the safety of nuclear power plants.

Pollution environment combustion products of fossil sources, primarily coal and nuclear fuel, is the cause of the deterioration of the ecological situation on Earth. The "thermal pollution" of the planet, which occurs when any type of fuel is burned, is also significant. The permissible upper limit of energy production on Earth, according to some scientists, is only two orders of magnitude higher than the current world average. Such an increase in energy consumption can lead to an increase in temperature on the Earth's surface by about one degree. Violation of the planet's energy balance on such a scale can lead to irreversible dangerous climate changes. These circumstances determine the growing role of renewable energy sources, the widespread use of which will not lead to disruption of the ecological balance of the Earth.

SOLAR ENERGY CONVERSION - A PROMISING PATH

Most renewable energy sources - hydropower, mechanical and thermal energy of the world's oceans, wind and geothermal energy - are characterized by either limited potential or significant difficulties in widespread use. The combined 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 middle star. Our luminary supplies the Earth with a power of about 10 17 W - such is the power of the "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/m 2 . With the development of highly efficient methods for converting solar energy, the Sun can supply explosively 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:

The specific power of solar radiation is small, and large-scale conversion of solar energy will require very large areas.
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 world energy budget? Obviously, this area depends on the efficiency of the converter systems used. To evaluate the efficiency of photovoltaic converters that directly convert solar energy into electrical energy using semiconductor photocells, we introduce the concept of the efficiency factor (COP) 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 an efficiency of solar converters equal to 10% (typical values of efficiency for silicon photocells, widely mastered in mass industrial production for the needs of ground-based energy), for the production of 10 12 W of electricity would be required to cover an area of 4 10 with photoconverters 10 m2 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 during 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. Possible ways to create economical solar energy converters will be considered in the following sections of this article.

The above considerations are a fairly strong argument: the problem of converting solar energy must be solved today in order to use this energy tomorrow. This problem can be considered at least jokingly within the framework of solving energy problems in 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 has been carried out in the field of solar energy in the world, which showed that in the near future this method of obtaining energy can become economically justified and be widely used.

Russia is rich natural resources. We have significant reserves of fossil fuels - coal, oil, gas. However, the use of solar energy has for our country great importance. Despite the fact that a significant part of the territory of Russia lies in high latitudes, some very large southern regions of our country, due to 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 areas close to this belt, characterized by a high level of solar energy, 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 kWh/m 2 .

The main areas of work in the field of solar energy conversion are currently:

direct thermal heating (receiving thermal energy) and thermodynamic conversion (receiving electrical energy with intermediate conversion of solar energy into thermal energy);
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 countries of the equatorial belt in solar heating, 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, heats up and is removed from it by means of a pump or natural circulation. Then the heated liquid enters the storage, from where it is consumed as needed. Such a device resembles domestic hot water systems.

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 (Fig. 1) 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 thermal receivers and low power converters.

PHOTOELECTRIC CONVERSION OF SOLAR ENERGY

Academician A.F. Ioffe. He dreamed of using semiconductor photovoltaic 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 for that time = 1%.

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 types. The large groundwork developed by scientists in the field of solar batteries for space purposes has also made it possible to launch work on ground-based photovoltaic energy.

The basis of photocells is a semiconductor structure with r-p transition(Fig. 2) arising at the interface of two semiconductors with different conduction mechanisms. Note that this terminology originates from English words positive (positive) and negative (negative). Different types of conductivity are obtained by changing the type of impurities introduced into the semiconductor. So, for example, atoms of the III group of the Periodic system of D.I. Mendeleev, introduced in crystal lattice silicon, give the latter hole (positive) conductivity, and impurities of the V group - 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 cause of the contact potential difference. When p- and n-type semiconductors are combined in one single crystal, a diffusion flow of electrons from an n-type semiconductor to a p-type semiconductor occurs and, conversely, a hole flow from p- to n-semiconductor. As a result of such a process, the part of the p-type semiconductor adjacent to the p-n junction will be negatively charged, and the part of the n-type semiconductor adjacent to the p-n junction, on the contrary, will acquire a positive charge. Thus, near the p-n junction, a double charged layer is formed, 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, tends to return electrons to the n-region. Similarly, the p-n junction field counteracts the diffusion of holes from the p- to the 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 the n-semiconductor, and holes from the 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 expend a certain energy. Without dwelling on the description of electrical characteristics p-n transition, which is widely used in rectifiers, transistors and other semiconductor devices, consider work p-n transition in photocells.

When light is absorbed in a semiconductor, electron-hole pairs are excited. In a homogeneous semiconductor, photoexcitation only increases the energy of electrons and holes without separating them in space, that is, electrons and holes are separated in the "energy space", but remain side by side in the geometric space. For the separation of current carriers and the appearance of a photoelectromotive force (photoEMF), an additional force must exist. The most efficient separation of nonequilibrium carriers takes place precisely in the region of the p-n junction (Fig. 2). The "minor" 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 field p-n transition and are ejected into the semiconductor, in which they become the majority carriers: electrons will be localized in the n-type semiconductor, and holes - in the p-type semiconductor. As a result, the p-type semiconductor receives an excess positive charge, and the n-type semiconductor receives a negative charge. Between the n- and p-regions of the photocell, a potential difference arises - photoEMF. The polarity of the photoEMF corresponds to "direct" offset p-n transition, which lowers the barrier height and facilitates the injection of holes from the p region into the n region and electrons from the n region into 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 value of photoEMF is established. In this case, the photoEMF value in a wide range of illumination increases in proportion to the logarithm of the light intensity. At a very high light intensity, when the potential barrier turns out to be practically zero, the photoEMF value goes to "saturation" and becomes equal to the barrier height at the unilluminated p-n junction. When exposed to direct, as well as concentrated up to 100 - 1000 times solar radiation, the value of photo-EMF is 50 - 85% of the value of the contact difference potential p-n transition.

We have considered the process of occurrence of photo-emf that occurs at the contacts p- and n-regions p-n transition. In the event of a short circuit of the illuminated p-n junction, 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 included in electrical circuit payload, such as a solar-powered calculator, the amount of current in the circuit will decrease somewhat. Usually, 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 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 (Fig. 2). As methods for creating these regions, for example, the impurity diffusion method or the method of growing one semiconductor onto another is used. Then the lower and upper electrical contacts are made (the electrodes are shaded in the figure), the lower contact is solid, and the upper one is made in the form of a comb structure (thin strips connected by a relatively wide current collector bus).

Silicon is the main material for producing solar cells. The technology for producing semiconductor silicon and photocells 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, and the second most common 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 fiddle in photovoltaic solar energy programs. Single-crystal silicon photocells combine the advantages of using a relatively cheap semiconductor material with high parameters of devices obtained on its basis.

Until recently, solar batteries for terrestrial applications, as well as space ones, were made on the basis of relatively expensive single-crystal silicon. The reduction in the cost of initial silicon, the development of high-performance methods for manufacturing 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 tape, polycrystalline silicon; development of cheap thin-film elements based on amorphous silicon and other semiconductor materials; implementation of the conversion of concentrated solar radiation using highly efficient elements based on silicon and a relatively new semiconductor material aluminum-gallium-arsenic.

Figure 3 shows two schematic diagrams of photovoltaic installations with solar radiation concentrators in the form of mirrors (top) and Fresnel lenses (bottom). The Fresnel lens is a plate made of plexiglass 1–3 mm thick, one side of which is flat, and on the other a profile is formed in the form of concentric rings, repeating the profile of a convex lens. Fresnel lenses are significantly cheaper than conventional convex lenses and at the same time provide a degree of concentration of 2 - 3 thousand "suns".

In recent years, the world has made significant progress in the development of silicon solar cells operating under concentrated solar irradiation. Silicon cells with an efficiency factor of > 25% have been created under conditions of irradiation on the Earth's surface at a concentration degree of 20 - 50 "suns". Significantly higher degrees of concentration allow photocells based on a semiconductor material aluminum-gallium-arsenic, first created at the Physico-Technical Institute. A.F. Joffe in 1969. In such solar cells, efficiency values > 25% are achieved at a concentration degree 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 solar radiation concentration due to a significant (up to 1000 times) decrease 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 plant makes it justified to complicate and increase the cost of a photocell if this ensures an increase in efficiency. This explains the attention currently paid to the development of cascade 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 on the basis of 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%.

CONCLUSION

From the foregoing, a conclusion follows about the prospects of photovoltaic solar energy. Solar radiation is a practically inexhaustible source of energy, it comes to all corners of the Earth, is "at hand" for any consumer and is an environmentally friendly affordable source of energy.

The disadvantage of solar radiation as an energy source is the unevenness of its arrival on the earth's surface, determined by the daily and seasonal cycles, as well as weather conditions. Therefore, the problem of accumulating electricity generated by solar power plants is very important. Currently, this problem is solved mainly by using conventional chemical storage devices - batteries. One of the promising methods of storage is the use of electricity for the electrolysis of water into hydrogen and oxygen, followed by the storage and use of hydrogen as an environmentally friendly fuel, since only water vapor is formed during the combustion of hydrogen.

The large-scale development of photovoltaics will give a huge impetus to the development of areas of the Earth with high average annual solar radiation. This applies primarily to desert and arid regions, which, with the "arrival" of solar electricity, will become areas suitable for active farming - the breadbaskets of the Earth. Does this mean that the efforts of specialists should be focused only on the development of photoelectric converters and the solution of problems directly related to them? Of course not. It is impossible to develop any one direction at the expense of suppression of other directions. The same applies to the electric power industry: it cannot be built based on only one type of resource. It should be based on many sources: solar, wind, nuclear and, of course, traditional, fossil sources. This will make it possible to find the optimal ways of their interaction, gradually moving to a perfect, environmentally friendly and reliable energy of the future.

LITERATURE

Vasiliev A.M., Landsman A.P. Semiconductor photoconverters. M.: Sov. radio, 1971.
Alferov Zh.I. Photovoltaic solar energy / In the collection: The future of science. M.: Knowledge, 1978. S. 92-101.
Koltun M.M. Optics and metrology of solar cells. Moscow: Nauka, 1985.
Andreev V.M., Grilikhes V.A., Rumyantsev V.D. Photoelectric conversion of concentrated solar radiation. L.: Nauka, 1989.
Koltun M.M. Solar cells. Moscow: Nauka, 1987.
Grilikhes V.A., Orlov P.P., Popov L.B. Solar energy and space flights. Moscow: Nauka, 1984.

Rapid growth in energy consumption leads to limited fossil energy resources. It is becoming increasingly difficult to maintain a high pace of energy development through the use of traditional energy sources. The theme of my work "Photoelectric conversion of solar energy" is the most relevant at the present time.

In my opinion, the large-scale development of photoenergy will give a huge impetus to the development of areas of the Earth with a high average annual influx of solar radiation.

Review

In the abstract "Photoelectric conversions of solar energy" Sergey fully revealed the chosen topic. In this paper, topical issues of solar energy conversion are considered: direct thermal heating and photoelectric conversion.

Revealing the topic Zagatin S. relies on the works of A.F. Ioffe. In his work, he considers the use of semiconductor photovoltaic cells in solar energy, the history of the use of solar batteries, as well as the process of the emergence of photo-EMF.

Sergey's work has logical integrity, the volume of parts of the abstract is sustained. The presentation of the material is scientific and interesting, illustrated by drawings. There is a personal assessment of the issue under study.

In preparing for the work on the abstract, a sufficient amount of literature was used.

I consider it possible to evaluate the work done by Zagatin S.

to "5".

Supervisor

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, p–n-transition photocurrent

I f=eN e,

where N e- the number of electrons that create a potential difference at the junction, as a result of which a leakage current will flow in the opposite direction at the junction I, equal to the photocurrent, which is constant.

Energy losses during photoelectric conversion are due to the incomplete use of photons, as well as scattering, resistance, and recombination of conduction electrons that have already arisen.

The most common of the commercially available solar cells (photocells) is lamellar silicon cells. There are also other types and designs that are being developed to increase 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 proportion of it into electricity. The absorption of solar radiation by various semiconductor materials reaches its greatest value when the plate thickness is from 100 to 1 µm 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 absorptive capacity of semiconductor materials are explained by differences in their atomic structure.

The efficiency of converting solar energy into electrical energy is not high. For silicon elements no more than 12...14%.

To increase the efficiency of solar cells, antireflection coatings are applied to the front side of the solar cell. As a result, the proportion of transmitted solar radiation increases. Uncoated elements have reflection losses 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, does not have a regular structure. For an amorphous structure, the probability of photon absorption and transition to the conduction band is higher. Therefore, it has a large absorption capacity. Gallium arsenide (GaAs) also finds use. The theoretical efficiency of GaAs-based cells can reach 25%, real cells have an efficiency of about 16%.

The technology of thin-film solar cells 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 cells are cadmium sulfide (CdS) cells with an efficiency of 10%.

Another advance in thin-film solar cell technology has been the production of multilayer cells. They allow you to cover a large part of the spectrum of solar radiation.

The active material of a solar cell is quite expensive. For more efficient use, solar radiation is collected on the surface of a 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 level of the ambient air temperature 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 highly uneven irradiance of photocells or modules, this can lead to the destruction of the solar cell.

The use of concentrating systems reduces the cost of solar power plants, since concentrating cells are cheaper than solar cells.

As the price of solar cells declined, the possibility of building large-scale photovoltaic installations arose. By 1984, 14 relatively large solar power plants with a capacity from 200 kW to 7 MW were built in the USA, Italy, Japan, Saudi Arabia and Germany.

Solar photovoltaic installation 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 maintenance personnel. Solar cells can be mass-produced, which will reduce their cost.

Solar panels are assembled from solar modules. However, there is a large selection of types and sizes of these devices with the same energy conversion efficiency and the same manufacturing 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. At these stations, a new generation of gas turbines can be used. Hybrid small power plants, consisting of photovoltaic panels and diesel generators, are already reliable energy providers.

End of work -

This topic belongs to:

Department of industrial heat power engineering.. lecture notes on the course of nivie gribanov a i .. the text was printed ..

If you need additional material on this topic, or you did not find what you were looking for, we recommend using the search in our database of works:

What will we do with the received material:

If this material turned out to be useful for you, you can save it to your page on social networks:

All topics in this section:

Energy resources of the planet
Energy resources are material objects in which energy is concentrated. Energy can be conditionally divided into types: chemical, mechanical, thermal, electrical, etc. To the main energy resources from

Possibilities for the use of energy resources
Fusion energy Fusion energy is the energy of fusion of helium from deuterium. Deuterium is a hydrogen atom whose nucleus consists of one proton and one neutron.

Energy resources of Russia
Russia has huge reserves of energy resources and, especially, coal. Theoretical potential is fuel reserves that are not specifically verified. Technical Potential

Getting energy at thermal power plants
As in most countries of the world, most of the electricity in Russia is generated at TPPs that burn fossil fuels. Thermal power plants use solid, liquid and gaseous fuels as fuel.

Variable power consumption schedule
During the day, electricity consumption is not the same. During peak hours, it increases sharply, and at night it decreases significantly. Therefore, the power system must have base capacities operating in

Electricity transmission problems
The transmission of electrical energy over long distances is associated with losses in power lines. Loss of electrical energy is equal to the product of the current strength and el. wire resistance. wired mo

Gas turbine and combined cycle plants (GTU and CCGT)
Currently, gas turbine and combined cycle plants are the most promising of all plants for the production of thermal and electrical energy. The use of these installations in many countries around the world

Magnetohydrodynamic installations (MGDU)
The use of power plants based on a magnetohydrodynamic generator is also promising. The MGDU cycle is the same as the GTP, i.e. adiabatic compression and expansion of the working fluid, isobaric supply

fuel cells
At present, fuel cells are used to generate electrical energy to generate electricity. These elements transform energy chemical reactions into electrical energy. Chemical

Heat pumps
TN are called devices operating on the reverse thermodynamic cycle and are designed to transfer heat from a low-potential energy source to a high-potential one. Second law

The place of small-scale power generation in the energy sector of Russia
Non-traditional energy sources include small hydroelectric power plants, diesel power plants, gas-piston power plants, and small nuclear power plants. The guarantor of reliable power supply, heat

Gas turbine and steam-gas small power plants
Low power gas turbine power plants are compact units manufactured according to the block-container principle. The components of the GTPP make it possible to generate not only electricity, but also

Mini CHP
At present, there is increased interest in combined heat and power generation with small installations using small installations with a power of several tens of kW to several

Diesel power plants
In some hard-to-reach regions of Russia, where it is unprofitable to build power lines, gasoline and diesel power plants are used to supply the population of these regions with energy. In the regions of the far north, the number

Gas piston power plants
Because diesel fuel prices are constantly rising, the use of diesel power plants on diesel fuel becomes expensive, so there is now a lot of interest in the world

Small hybrid power plants
To improve the reliability and efficiency of power supply systems, the creation of multifunctional energy complexes (IEC) is required. Also, complexes can be created on the basis of small hybrid power plants.

Small NPPs
Recently, considerable interest has been shown in small-capacity nuclear power plants. These are modular stations, they allow you to unify the equipment and work autonomously. Such stations can be reliable

Small hydropower
China is the leader in the development of small hydropower. The capacity of small hydropower plants (SHPPs) in China exceeds 20,000 MW. In India, the installed capacity of SHPPs exceeds 200 MW. Widespread SHPP

The main non-renewable energy resources will sooner or later be exhausted. Now about 80% of the energy consumption on the planet is provided by fossil fuels. With this use, organic

hydropower
The hydroelectric power station uses the energy of the water flow as an energy source. Hydroelectric power stations are built on rivers, constructing dams and reservoirs. 2 main factors are needed for efficient energy production at hydroelectric power plants

solar energy
Solar energy is the result of a nuclear fusion reaction of the light elements deuterium, tritium and helium, which are accompanied by a huge amount of energy. Source of all energy except t

Converting solar energy into thermal energy
Solar energy can be converted into thermal energy using a collector. All solar collectors have a surface or bulk heat sink. Heat can be removed from the collector or stored

Thermodynamic conversion of solar energy into electrical energy
Methods for the thermodynamic conversion of solar energy into electrical energy are based on the cycles of heat engines. Solar energy is converted into electricity at solar power plants (

Prospects for the development of solar energy in Russia
In 1985, in the village of Shchelkino, Crimean Region, the first in the USSR solar tower-type power plant SES-5 with an electric power of 5 MW was put into operation. 1600 heliostats (flat grains

Features of the use of wind energy
The main reason for the occurrence of wind is the uneven heating of the earth's surface by the sun. Wind energy is very high. According to the World Meteorological Organization, the energy reserves of wind

Electricity generation with wind turbines
Using wind turbines to generate electricity is the most efficient way to convert wind energy. When designing wind turbines, it is necessary to take into account their following features

Wind power in Russia
The energy wind potential of Russia is estimated at 40 billion kW. hours of electricity per year, that is, about 20,000 MW. WPP with a capacity of 1 MW at an average annual wind speed of 6 m/s saves 1

Origin of geothermal energy
At the core of the Earth, the temperature reaches 4000 °C. The release of heat through the solid rocks of the land and the ocean floor occurs mainly due to thermal conductivity and less often - in the form of convective flows of molten

Geothermal Heat Extraction Technique
Geothermal energy sources can be divided into five types. 1. Sources of geothermal dry steam. They are quite rare, but the most convenient for the construction of GeoTPP. 2. Source

Electricity
The transformation of geothermal energy into electrical energy is carried out on the basis of the use of a machine method using a thermodynamic cycle at GeoTPP. For the construction of GeoTPP, the most used

The scale of the use of geothermal heat for heating and hot water supply is more significant. Depending on the quality and temperature of the thermal water, there are various geothermal schemes.

Impact of geothermal energy on the environment
The main environmental impact of GeoTPP is associated with the development of the field, the construction of buildings and steam pipelines. To provide the GeoTPP with the necessary amount of steam or hot water,

Geothermal energy in Russia
In Russia, 47 geothermal deposits have been explored with reserves of thermal waters, which make it possible to obtain more than 240 × 103 m3 / day. thermal waters, and steam hydrothermal

Causes of hot flashes
Tides are the result of the gravitational interaction of the Earth with the Moon and the Sun. The tidal force of the Moon at a given point on the earth's surface is defined as the difference between the local value of the attractive force

Tidal power plants (TPPs)
The water raised at high tide to the maximum height can be separated from the sea by a dam. The result is a tidal basin. The maximum power that can be obtained by passing in

Impact of PES on the environment
The possible impact of tidal power plants on the environment can be associated with an increase in the amplitude of the tides on the ocean side of the dam. This can lead to flooding of land and built

Tidal energy in Russia
In Russia, the use of tidal energy in the coastal zones of the seas of the Arctic and Pacific Oceans is associated with large capital investments. The first in our country Kislogubskaya TPP power

Wave energy
From sea waves you can get a huge amount of energy. The power carried by waves in deep water is proportional to the square of their amplitude and period. Of greatest interest are the long

The energy of ocean currents
The entire water area of the World Ocean is crossed by surface and deep currents. The stock of kinetic energy of these currents is about 7.2∙1012 kW∙h/year. This energy with

Ocean thermal energy resources
The oceans are a natural accumulator of solar energy. In tropical seas, the upper layer of water several meters thick has a temperature of 25 ... 30 ° C. At a depth of 1000 m, the water temperature is

Ocean thermal power plants
Several types of devices are proposed for converting the energy of the temperature difference in the ocean. Of greatest interest is the conversion of thermal energy into electrical energy using thermodynes

Biomass resources
The term "biomass" refers to organic matter of plant or animal origin, which can be used to produce energy or technically convenient fuels by

Thermochemical conversion of biomass (combustion, pyrolysis, gasification)
One of the main areas of wood waste recycling is their use for heat and electricity generation. The main technologies for obtaining energy from wood waste are

Biotechnological conversion of biomass
Biotechnological conversion uses various organic wastes with a moisture content of at least 75%. Biological conversion of biomass is developing in two main directions: 1) farm

Environmental problems of bioenergy
Bioenergy installations contribute to the reduction of environmental pollution with all kinds of waste. Anaerobic fermentation is not only an efficient means of using animal waste

Characteristics of municipal solid waste (MSW)
Hundreds of thousands of tons of household waste accumulate annually in city dumps. Specific annual output of MSW per inhabitant modern city is 250 ... 700 kg. In developed countries, this value e

Waste recycling at landfills
At present, municipal solid waste is usually taken to landfills for disposal with the expectation of their subsequent mineralization. It is desirable that MSW be pressed before burial. This not only reduces

MSW composting
The second direction of MSW disposal is processing into organic fertilizer (compost). Can be composted up to 60% total mass household waste. The composting process is carried out in rotation.

Incineration of MSW in special waste incinerators
In economically developed countries, all more quantity MSW is recycled industrially. The most effective of them is thermal. It allows you to reduce the volume of waste by almost 10 times

This effect is used in semiconductor materials, in which the energy of radiation quanta h creates, for example, p–n-transition photocurrent

I f =en e ,

where N e- the number of electrons that create a potential difference at the junction, as a result of which a leakage current will flow in the opposite direction at the junction I, equal to the photocurrent, which is constant.

Energy losses during photoelectric conversion are due to the incomplete use of photons, as well as scattering, resistance, and recombination of conduction electrons that have already arisen.

Reducing the thickness of the solar cell can significantly reduce the consumption of materials and the cost of their manufacture.

Differences in the absorptive capacity of semiconductor materials are explained by differences in their atomic structure.

The efficiency of converting solar energy into electrical energy is not high. For silicon elements no more than 12...14%.

Another advance in thin-film solar cell technology has been the production of multilayer cells. They allow you to cover a large part of the spectrum of solar radiation.

The active material of a solar cell is quite expensive. For more efficient use, solar radiation is collected on the surface of a solar cell using concentrating systems (Fig. 2.7).

The use of concentrating systems reduces the cost of solar power plants, since concentrating cells are cheaper than solar cells.

As the price of solar cells declined, the possibility of building large-scale photovoltaic installations arose. 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.

From an energy point of view, the most energy-efficient devices for converting solar energy into electrical energy (since this is a direct, single-stage energy transition) are semiconductor photoelectric converters (PVCs). At an equilibrium temperature characteristic of solar cells of the order of 300-350 Kelvin and T of the sun ~ 6000 K, their limiting 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 is quite possible to raise 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 photoelectric conversion of solar energy have confirmed the possibility of realizing such high efficiency values with solar cells and have identified the main ways to achieve this goal.

The conversion of energy in a solar cell is based on the photovoltaic effect that occurs in inhomogeneous semiconductor structures when exposed to solar radiation. The heterogeneity of the solar cell structure can be obtained by doping the same semiconductor with various impurities (creating p - n junctions) or by combining different semiconductors with unequal band gap - the energy of detachment of an electron from an atom (creation of heterojunctions), or due to a change chemical composition semiconductor, leading to the appearance of a bandgap gradient (creation of graded-gap structures). Various combinations of these methods are also possible. The conversion efficiency depends on the electrical characteristics of the inhomogeneous semiconductor structure, as well as optical properties FEP, 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 the solar cell can be explained by the example of converters with pn junction, which are widely used in modern solar and space energy. An electron-hole transition is created by doping a plate of a single-crystal semiconductor material with a certain type of conductivity (ie, either p- or n-type) with an impurity that provides the creation of a surface layer with the opposite type of conductivity. 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 a conductivity of the opposite sign. At the boundary of the n- and p-layers, as a result of charge leakage, depleted zones are formed with an uncompensated positive volume charge in the n-layer and a negative volume charge in the p-layer. These zones together form a p-n junction. The potential barrier (contact potential difference) that has arisen at the junction prevents the passage of the main charge carriers, i.e. electrons from the side of the p-layer, but freely pass minor carriers in opposite directions. This property of p-n junctions determines the possibility of obtaining photo-emf when irradiating solar cells with sunlight. The non-equilibrium charge carriers (electron-hole pairs) created by light in both layers of the PVC are separated at the p-n junction: minor carriers (i.e. electrons) freely pass through the junction, and the main ones (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 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 (combine) with holes moving inside the FEP in the opposite direction. To collect and remove electrons to an external circuit, there is a contact system on the surface of the FEP semiconductor structure. On the front, illuminated surface of the converter, the contacts are made in the form of a grid or comb, and on the back they can be solid. The main irreversible energy losses in solar cells are associated with:

Ш reflection of solar radiation from the surface of the transducer,
Ø the passage of a part of the radiation through the solar cell without absorption in it,
Scattering on thermal vibrations of the lattice of excess photon energy,
Ш recombination of the resulting photopairs on the surfaces and in the volume of the solar cell,
W internal resistance of the converter,
Ш and some other physical processes.

To reduce all types of energy losses in solar cells, various measures are being developed and successfully applied. These include:

ь use of semiconductors with an optimal band gap for solar radiation;

ь targeted improvement of the properties of the semiconductor structure by its optimal doping and the creation of built-in electric fields;

b transition from homogeneous to heterogeneous and graded-gap semiconductor structures;

ь optimization of the design parameters of the solar cell (p-n-junction depth, base layer thickness, contact grid frequency, etc.);

ь application of multifunctional optical coatings that provide antireflection, thermal control and protection of solar cells from cosmic radiation;

l development of solar cells that are transparent in the long-wave region of the solar spectrum beyond the edge of the main absorption band;

- the creation of cascade solar cells from semiconductors specially selected according to the width of the band gap, which make 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 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 solar cell, etc.5

In SES energy conversion systems (solar power plants), in principle, any types of solar cells of various structures created and currently being developed based on various semiconductor materials can be used, but not all of them satisfy the set of requirements for these systems:

· high reliability with a long service life (tens of years!)
availability of raw materials in sufficient quantities for the manufacture of elements of the conversion system and the possibility of organizing their mass production;
· Acceptable from the point of view of the payback period, energy costs for the creation of a transformation system;
· minimal energy and mass costs associated with the control of the energy 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 to create a solar power plant due to the limited natural resources of the feedstock and the complexity of its processing. Separate 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 only be achieved by organizing a fully automated production of solar cells, for example, based on tape technology, and creating 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. The manufacture of solar cells and the assembly of 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 being considered as the most likely materials for photovoltaic solar energy conversion systems. In this case, we are talking about heterophotoconverters (HFP) with the AlGaAs-GaAs structure.

Solar cells (photoelectric converters) based on arsenic-gallium (GaAs) compounds are known to have a higher theoretical efficiency than silicon solar cells, since their band gap practically coincides with the optimal band gap for semiconductor solar energy converters =1 .4 eV. For silicon, this indicator \u003d 1.1 eV.

Owing to more high level absorption of solar radiation, which is determined by direct optical transitions in GaAs, high efficiency of solar cells based on them can be obtained at a much smaller thickness of solar cells compared to silicon. In principle, it is sufficient to have an HFP thickness of 5–6 µm to obtain an efficiency of at least 20%, while the thickness of silicon elements cannot be less than 50–100 µm without a noticeable decrease in their efficiency. This circumstance makes it possible to count on the creation of light film HFPs, the production of which will require a relatively small amount of starting material, especially if it is possible to use not GaAs as a substrate, but another material, for example, synthetic sapphire (Al 2 O 3).

HFPs also have more favorable performance characteristics in terms of requirements for SES converters compared to silicon FEPs. Thus, in particular, the possibility of achieving low initial values of reverse saturation currents in pn junctions due to the large band gap makes it possible to minimize the magnitude of negative temperature gradients of the efficiency and the optimal power of the HFP and, in addition, significantly expand the region of the linear dependence of the latter on the light flux density . The experimental temperature dependences of HFP efficiency 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 solar cells, the temperature increase 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 operating temperature of HFP on GaAs reaches 180 °C, which is already quite operating temperatures for heat engines and steam turbines. Thus, to the 30% inherent efficiency of gallium arsenide HFPs (at 150°C), one 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 removal from the coolant after the turbine for space heating, can be even higher than 50-60%.

Also, GaAs-based HFPs, to a much lesser extent than silicon PVCs, are susceptible to destruction by high-energy proton and electron flows due to the high level of light absorption in GaAs, as well as the low required lifetime and diffusion length of minority carriers. Moreover, experiments have shown that a significant part of radiation defects in GaAs-based HFPs disappear after their heat treatment (annealing) at a temperature of just about 150–180°C. If GaAs HFPs 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 stations (this is especially true for space solar power plants, for which light weight and size of solar cells and high efficiency are important) .

On the whole, it can be concluded that the energy, mass, and operational characteristics of GaAs-based HFPs are more in line with the requirements of SES and SCES (cosmic) than the characteristics of silicon PVCs. However, silicon is a much more accessible and mastered material than gallium arsenide. Silicon is widely distributed in nature, and the stocks of raw materials for the creation of solar cells based on it are practically unlimited. The manufacturing technology of silicon solar cells is well established and is being continuously improved. There is a real prospect of reducing the cost of silicon solar cells by one or two orders of magnitude with the introduction of new automated production methods, which make it possible, in particular, to obtain silicon tapes, large-area solar cells, etc.

Prices for silicon photovoltaic batteries have decreased in 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 is expected in the energy sector at the moment the price passes the 3-dollar milestone. According to some calculations, this may happen as early as 2002, and for Russia with current energy tariffs, this moment will come at a price of 1 watt of SB 0.3-0.5 dollars, that is, at an order of magnitude lower price. Taken together, tariffs, climate, geographical latitudes, the ability of the state to real pricing and long-term investments. In actually operating structures with heterojunctions, the efficiency today reaches more than 30%, and in homogeneous semiconductors such as single-crystal silicon - up to 18%. The average efficiency in solar cells based on single-crystal silicon today is about 12%, although it reaches 18%. It is, basically, silicon SBs that can be seen today on the roofs of houses in different countries of the world.

In contrast to silicon, gallium is a very scarce material, which limits the possibility of producing HFPs based on GaAs in the amounts required for widespread use.

Gallium is extracted mainly from bauxites, but the possibility of obtaining it from coal ash and sea water is also being considered. The largest reserves of gallium are found in sea water, but its concentration there is very low, the extraction yield is estimated at only 1% and, therefore, production costs are likely to 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)) has not yet been developed to the same extent as the technology for the production of silicon PV cells, and as a result, the cost of HFP is now significantly higher (by orders) of the cost of a solar cell made of silicon.

IN spacecraft, where the main source of current is solar panels and where understandable ratios of mass, size and efficiency are very important, the main material for solar. 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 deficient gallium. An additional reserve for saving gallium is associated with the use of synthetic sapphire (Al 2 O 3) rather than GaAs as the HFP substrate. energy SES based on GaAs HFP can be quite commensurate with the cost of a system based on silicon. Thus, at present, 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 ground-based and space solar power engineering. Insofar as SBs give out direct current, the task of transforming it into an industrial variable 50 Hz, 220 V arises. A special class of devices, inverters, does an excellent job with this task.

Types of photoelectric converters

From an energy point of view, the most energy-efficient devices for converting solar energy into electrical energy (since this is a direct, single-stage energy transition) are semiconductor photoelectric converters (PVCs). At an equilibrium temperature characteristic of solar cells of the order of 300–350 Kelvin and T of the sun ~ 6000 K, their limiting 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 is quite possible to raise the practical efficiency to 50% or more (in laboratories, an efficiency of 40% has already been achieved).

The conversion of energy in solar cells is based on the photovoltaic effect that occurs in inhomogeneous semiconductor structures when exposed to solar radiation. The heterogeneity of the FEP structure can be obtained by doping the same semiconductor with different impurities (creating p-n junctions) or by combining different semiconductors with a different band gap - the energy of detachment of an electron from an atom (creating heterojunctions), or by changing the chemical composition semiconductor, leading to the appearance of a bandgap gradient (creation of graded-gap structures). Various combinations of these methods are also possible. The conversion efficiency depends on the electrical characteristics of the inhomogeneous semiconductor structure, as well as the optical properties of solar cells, 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 the solar cell can be explained by the example of converters with a p-n-junction, which are widely used in modern solar and space energy. An electron-hole transition is created by doping a plate of a single-crystal semiconductor material with a certain type of conductivity (ie, either p- or n-type) with an impurity that provides the creation of a surface layer with the opposite type of conductivity.

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 a conductivity of the opposite sign. At the boundary of the n- and p-layers, as a result of charge leakage, depleted zones are formed with an uncompensated positive volume charge in the n-layer and a negative volume charge in the p-layer. These zones together form a p-n junction. The potential barrier (contact potential difference) that has arisen at the junction prevents the passage of the main charge carriers, i.e. electrons from the side of the p-layer, but freely pass minor carriers in opposite directions. This property of p-n junctions determines the possibility of obtaining photo-emf when irradiating solar cells with sunlight. The non-equilibrium charge carriers (electron-hole pairs) created by light in both layers of the PVC are separated at the p-n junction: minor carriers (i.e. electrons) freely pass through the junction, and the main ones (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 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 (combine) with holes moving inside the FEP in the opposite direction. To collect and remove electrons to an external circuit, there is a contact system on the surface of the FEP semiconductor structure. On the front, illuminated surface of the converter, the contacts are made in the form of a grid or comb, and on the back they can be solid.

The main irreversible energy losses in solar cells are associated with:

reflection of solar radiation from the surface of the transducer,
the passage of a part of the radiation through the solar cell without absorption in it,
scattering on thermal vibrations of the lattice of excess photon energy,
recombination of the formed photopairs on the surfaces and in the volume of the solar cell,
internal resistance of the converter,
and some other physical processes.

To reduce all types of energy losses in solar cells, various measures are being developed and successfully applied. These include:

the use of semiconductors with an optimal band gap for solar radiation;
targeted improvement of the properties of the semiconductor structure by its optimal doping and the creation of built-in electric fields;
transition from homogeneous to heterogeneous and graded-gap semiconductor structures;
optimization of the design parameters of the solar cell (p-n-junction depth, base layer thickness, contact grid frequency, etc.);
the use of multifunctional optical coatings that provide antireflection, thermal control 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;
the creation of cascade solar cells from semiconductors specially selected according to the width of the band gap, which make 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 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 section of the spectrum by a separate solar cell, etc.

high reliability with a long service life (tens of years!)
the availability of raw materials in sufficient quantities for the manufacture of elements of the conversion system and the possibility of organizing their mass production;
energy costs acceptable in terms of payback periods for the creation of a transformation system;
minimum energy and mass costs associated with the control of the energy 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 to create a solar power plant due to the limited natural resources of the feedstock and the complexity of its processing. Separate 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 only be achieved by organizing a fully automated production of solar cells, for example, based on tape technology, and creating 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. The production of solar cells and the assembly of 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 in SES, and in the latter case we are talking about heterophotoconverters (HFP) with the AlGaAs-GaAs structure.

Due to the higher level of absorption of solar radiation, which is determined by direct optical transitions in GaAs, high efficiency of solar cells based on them can be obtained at a much smaller thickness of solar cells compared to silicon. In principle, it is sufficient to have an HFP thickness of 5–6 µm to obtain an efficiency of at least 20%, while the thickness of silicon elements cannot be less than 50–100 µm without a noticeable decrease in their efficiency. This circumstance makes it possible to count on the creation of light film HFPs, the production of which requires a relatively small amount of starting material, 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 performance characteristics in terms of requirements for SES converters compared to silicon FEPs. Thus, in particular, the possibility of achieving low initial values of reverse saturation currents in pn junctions due to the large band gap makes it possible to minimize the magnitude of negative temperature gradients of the efficiency and the optimal power of the HFP and, in addition, significantly expand the region of the linear dependence of the latter on the light flux density . The experimental temperature dependences of HFP efficiency 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 solar cells, the temperature increase above 60-70°C is almost critical - the efficiency drops by half.

Also, GaAs-based HFPs, to a much lesser extent than silicon PVCs, are susceptible to destruction by high-energy proton and electron flows due to the high level of light absorption in GaAs, as well as the low required lifetime and diffusion length of minority carriers. Moreover, experiments have shown that a significant part of radiation defects in GaAs-based HFPs disappear after their heat treatment (annealing) at a temperature of just about 150–180°C. If GaAs HFPs 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 stations (this is especially true for space solar power plants, for which light weight and size of solar cells and high efficiency are important) .

Prices for silicon photovoltaic batteries have decreased in 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 is expected in the energy sector at the moment the price passes the 3-dollar milestone. According to some calculations, this may happen as early as 2002, and for Russia with current energy tariffs, this moment will come at a price of 1 watt of SB 0.3-0.5 dollars, that is, at an order of magnitude lower price. All together play a role here: tariffs, climate, geographic latitudes, the ability of the state to real pricing and long-term investments. In actually operating structures with heterojunctions, the efficiency today reaches more than 30%, and in homogeneous semiconductors such as single-crystal silicon - up to 18%. The average efficiency in solar cells based on single-crystal silicon today is about 12%, although it reaches 18%. It is, basically, silicon SBs that can be seen today on the roofs of houses in different countries of the world.

In contrast to silicon, gallium is a very scarce material, which limits the possibility of producing HFPs based on GaAs in the amounts required for widespread use.

In spacecraft, where the main source of current is solar panels and where understandable ratios of mass, size and efficiency are very important, the main material for solar cells. 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 deficient 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 when mass-produced based on advanced technology is also likely to be significantly reduced, and in general, the cost of the conversion system of the energy conversion system of GaAs HFP solar power plants can be quite commensurate with the cost of a silicon-based system. Thus, at present, 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 ground-based and space solar power engineering. Insofar as SBs give out direct current, the task of transforming it into an industrial variable 50 Hz, 220 V arises. A special class of devices, inverters, does an excellent job with this task.

Calculation of a photovoltaic system.

You can use the energy of solar cells 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 incoming cloud can reduce output power by more than 50%. In addition, deviations in technological regimes entail a spread in the output parameters of the elements of one batch. Therefore, the desire to get the most out of photovoltaic converters leads to the need to sort cells by output current. As an illustrative example of “a lousy sheep spoiling the whole flock”, the following can be cited: cut a pipe section with a much smaller diameter into a break in a large-diameter water pipe, as a result, the water flow will drastically decrease. Something similar happens in a chain of non-uniform output parameters of solar cells.

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 the element, you can use a family of easy-to-understand curves - current-voltage characteristics (CVC)

The open-circuit voltage generated by one element changes slightly when moving from one element 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 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 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 the solar cell, as well as to compare the cells with each other under the same conditions, it is necessary to load it so that the output voltage is 0.47 V. After the solar elements are selected for work, it is necessary to solder them. Serial elements are equipped with current-collecting grids, which are designed for soldering conductors to them.

Batteries can be made in any desired combination. The simplest battery is a chain of series-connected cells. You can also connect chains in parallel, getting the 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 CVC 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 cell. The efficiency of a conventional solar cell currently ranges from 10-16%. This means that an element with a size of 100-100 mm under standard conditions can generate 1-1.6 watts.

All photovoltaic systems can be divided into two types: autonomous and connected to the electrical network. Stations of the second type transfer excess energy to 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 a roof, a battery, a discharge controller - battery charge, and connecting cables. Solar modules are the main component for building photovoltaic systems. They can be made with any output voltage.

After the solar cells are selected, they must 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 series-connected cells.

You can connect these chains in parallel, getting the so-called series-parallel connection. In parallel, only chains (lines) with identical voltage can be connected, while their currents, according to Kirchhoff's law, are summed up.

When used on the ground, they are usually used to charge rechargeable batteries (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, textolite, aluminum. In this case, the elements are located between two layers of sealing film, without an air gap. Vacuum lamination technology fulfills 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 the solar cell are presented as a separate solar cell in the form of a current-voltage curve under standard conditions (Standart 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 the open-circuit voltage - Uxx, the point of intersection with the current axis is called the short-circuit current Ikz.

The maximum power of the module is defined as the highest power under STC (Standart 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 value of the operating voltage for a module consisting of 36 elements, therefore, will be about 16 ... 17 V (0.45 .... 0.47 V per element) at 25 ° C.

Such a 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 (it will be discussed later), and mainly to reduce 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 does not depend much 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 ( 0.38-0.4V 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 kit.

The calculation of FES means the determination of the nominal power of modules, their number, connection scheme; choice of type, operating conditions and battery capacity; inverter and charge-discharge controller capacities; determination of parameters of 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 data sheets. At this stage, it is already possible to select the inverter power, which should be at least 1.25 times the calculated one. It should be borne in mind that such a cunning device as a compressor refrigerator at the time of launch consumes power 7 times more than the nameplate.

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 work better with higher input voltages.

The next step is to determine the capacity of the battery. The battery capacity is selected from the standard range of capacities rounded to the side greater 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 value of 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 depth of discharge 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 the determination of the total power and the number of solar modules. The calculation requires the value of solar radiation, which is taken during the operation of the station, when solar radiation is minimal. In the case of year-round use, this is December.

In the meteorology section, monthly and total annual values of solar radiation for the main regions of Russia are given, 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 pico-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, 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 a site oriented perpendicular to the light 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 value of insolation for the selected period, k is a coefficient equal to 0.5 in summer and 0.7 in winter.

This factor corrects for the power loss of solar cells when heated by 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 the lower 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 power of one module, we get the number of modules.

When creating a FES, 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, while consuming 5 times less, provide a luminous flux equivalent to that of an incandescent lamp.

For small FES, it is advisable to install its modules on a swivel bracket for optimal rotation relative to the incident rays. This will increase the station's capacity by 20-30%.

A little about inverters.

DC-to-AC inverters or converters are designed to provide high-quality power supply to various equipment and devices in the absence or low quality of an AC power supply with a frequency of 50 Hz and a voltage of 220 V, various emergency situations, etc.

The inverter is a pulsed DC converter with a voltage of 12 (24, 48, 60) V to AC with a stabilized voltage of 220 V at a frequency of 50 Hz. Most inverters have a STABILIZED SINUSOIDAL voltage 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 switch for the operation of the product and an indicator for the operation of the inverter. On the rear panel of the product there are leads (terminals) for connecting a DC source, for example, a battery, a grounding lead for the inverter case, a hole with a fan mount (cooling), a three-pole euro socket for connecting the load.

The stabilized voltage at the output of the inverter makes it possible to provide high-quality power supply to the load during changes / fluctuations in the input voltage, 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 output of the inverter make it possible not to take additional measures to ensure the safety of operation when using various DC sources or any electrical equipment. Forced cooling of the power section and low noise level during inverter operation allow, 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 inconveniences in operation in the form of noise.

Built-in control panel with electronic scoreboard
Capacitance potentiometer that allows fine adjustments to be made
Normalized pinned bar: WE WY STEROW
Built-in brake rotation
Radiator with fan
Aesthetic fastening
Power supply 230 V - 400 V
Overload 150% - 60s
Run-up time 0.01...1000 seconds
Built-in electric filter, class A
Operating temperature: -5°C - +45°C
RS485 port
Frequency step control: 0.01 Hz - 1 kHz
Protection class IP 20

Functionally provides: increase, decrease in frequency, control of an overload, an overheat.

Photoelectric energy converters. How is the process of converting solar energy into electrical energy. Maximum efficiency values ​​of photocells and modules achieved in laboratory conditions