The principle of operation of solar panels. Principles of operation of solar panels and how they work

In recent years, so-called “alternative energy” has become increasingly popular. Particular attention is paid to the use of solar radiation. This is quite natural, because if you create an element that is capable of converting light rays into electricity, you can get a free inexhaustible energy source. And such an element was created. It was called a “solar photocell” or “solar battery”, and how a solar battery works is quite simple to understand.

Operating principle

The main thing is not to confuse photovoltaic batteries with solar collectors (both are often called “solar panels”). If the principle of operation of collectors is based on heating the coolant, then photocells directly produce electricity. Their work is based on the photoelectric effect, which consists in generating current under the influence of sunlight in semiconductor materials.

Semiconductors are substances whose atoms either contain an excess number of electrons (n-type), or, conversely, lack them (p-type). And those areas of the structure of p-elements where electrons could potentially be located are called “holes”. Accordingly, a semiconductor-based photocell consists of two layers with different types conductivity.

How do solar cells with this structure work? As follows. The inner layer of the element is made of a p-semiconductor, the outer, much thinner one, is made of an n-semiconductor. At the boundary of the layers, the so-called “ p-n zone transition”, formed due to the formation of positive bulk charges in the n-layer and negative ones in the p-layer.

In this case, a certain energy barrier appears in the transition zone, caused by the difference in charge potentials. It prevents the penetration of major charge carriers, but freely allows minor ones to pass through, and in opposite directions. Under the influence of sunlight, some photons are absorbed by the surface of the element and generate additional “hole-electron” pairs. That is, electrons and holes move from one semiconductor to another, giving them an additional negative or positive charge. In this case, the initial potential difference between the n- and p-layers decreases, and an electric current is generated in the external circuit.

Features of the structure

Many modern photocells have only one p-n junction. In this case, freely transferring charge carriers are generated only by those photons whose energy is either greater than or equal to the width of the “gap” at the transition boundary. This means that photons with lower energy levels are simply not used, which in turn significantly reduces the efficiency of the cell. To overcome this limitation, multilayer (more often four-layer) photostructures were created.

They allow the use of a significantly larger part of the solar spectrum and have more high performance. Moreover, the photocells are positioned in such a way that the rays hit the junction with the widest bandgap first. In this case, more “energy-intensive” photons are absorbed, while photons with less energy travel deeper and stimulate other elements.

What types of solar panels are there?

Solar cells, the operating principle of which is based on the photoelectric effect, have been created for a long time. The main difficulty in their production is the selection of materials capable of generating a sufficiently powerful current. The first experiments were carried out with selenium cells, but their efficiency was extremely low (about 1%). Nowadays, photovoltaic cells mainly use silicon; the productivity of such devices is about 22%. In addition, new cell samples are constantly being developed (for example, using gallium or indium arsenide) with higher efficiency. The maximum efficiency of solar panels today is 44.7%.

But such elements are very expensive and are so far produced only in laboratory conditions. Cells based on monocrystalline or polycrystalline silicon, as well as thin-film elements, have become widespread. Photobatteries based on monocrystals are more expensive, but have greater performance, while polycrystals are cheaper, but due to their heterogeneous structure, they are less efficient. In the production of thin-film cells, it is not crystals that are used, but silicon layers deposited on a flexible substrate.

We often write about various types alternative energy, including solar. This article begins a series of articles about the principles of operation various devices running on renewable energy. And the first thing that will be considered is solar panels. Solar energy in lately used everywhere: in natural light rooms, heating water, drying and sometimes even in cooking. However, the most important use The energy of the sun is, perhaps, the generation of electricity. And the main element of such generation is a solar battery!

Structure of solar panels

A solar battery consists of photocells connected in series and parallel. All photocells are located on a frame made of non-conductive materials. This configuration allows you to assemble solar cells with the required characteristics (current and voltage). In addition, this makes it possible to replace failed photocells with a simple replacement.

Operating principle

In general, this method of generating electricity should be the most effective, because it is single-stage. Compared to other technology of converting solar energy through thermodynamic transition (Rays -> Water Heating -> Steam -> Turbine Rotation -> Electricity), less energy is lost in transitions.

Structure of a photocell

A semiconductor-based photocell consists of two layers with different conductivities. To layers with different sides The contacts that are used to connect to the external circuit are soldered. The role of the cathode is played by a layer with n-conductivity (electronic conductivity), the role of the anode is played by the p-layer (hole conductivity).

The current in the n-layer is created by the movement of electrons, which are “knocked out” when light hits them due to the photoelectric effect. The current in the p-layer is created by the “movement of holes”. A “hole” is an atom that has lost an electron; accordingly, the jumping of electrons from “hole” to “hole” creates the “movement” of holes, although the “holes” themselves, of course, do not move in space.

At the junction of layers with n- and p-conductivity, a p-n junction is created. It turns out to be a kind of diode that can create a potential difference due to the ingress of light rays.

Physical mechanism of action

The potential difference (and, accordingly, the emf) that a photocell can create depends on many factors: the intensity of solar radiation, the area of ​​the photocell, the efficiency of the structure, and temperature (when heated, the conductivity decreases).

What are photocells made of?

The world's first photocell appeared in 1883 in the laboratory of Charles Fritts. It was made of selenium plated with gold. Alas, this set of materials showed poor results - about 1% efficiency.

A revolution in the use of photocells occurred when the first silicon cell was created in the bowels of the Bell Telephone laboratory. The company needed a source of electricity for telephone exchanges, and could be said to be the first company to use alternative source on solar energy.

Silicon is still the main material for the production of solar cells. In general, silicon (Silicium, Silicon) is the second most common element on Earth, its reserves are huge. However, in its industrial use there is one big problem- its cleaning. This process is very labor-intensive and expensive, so pure silicon is expensive. A search is now underway for analogues that would be as efficient as silicon. Compounds of copper, indium, selenium, gallium and cadmium, as well as organic solar cells, are considered promising.

Solar batteries (Assemblies)

However, the potential difference created by a single solar cell is small for industrial applications. To be able to use solar cells to power devices, they are connected together. This produces solar cells ( solar assemblies, solar modules). In addition, photocells are covered with various protective layers of glass, plastic, and various films. This is done in order to protect the fragile element.

Main performance characteristic solar battery is the peak power, which is expressed in Watts (W, W). This characteristic shows output power batteries in optimal conditions: solar radiation 1 kW/m 2, ambient temperature 25 o C, solar spectrum width 45 o (AM1.5). Under normal conditions, it is extremely rare to achieve such indicators, the illumination is lower, and the module heats up higher (up to 60-70 degrees).

By connecting photocells in series we increase the potential difference, while connecting in parallel we increase the current. Thus, by combining connections, you can achieve the required parameters for current and voltage, and therefore power. In addition, not only photocells within one solar battery, but also solar batteries as a whole can be connected in series or in parallel.

Solar battery: design and principle of operation

Quite recently, when we were still in school, a solar battery for generating electricity seemed like something fantastic. It seemed to us that they could only be used on spaceships. But 20-25 years have passed and solar batteries have not only appeared in watches and calculators, but are also already capable of providing electricity to private homes and cottages. And modern solar power plants can provide electricity to small towns. Solar batteries have become widespread in European countries, the USA, Israel and other regions with high solar insolation. And their use already provides significant savings in electricity and hot water supply.

Solar energy can be converted into heat and electricity. Man took the very first steps in using solar energy in the direction of generating heat. We can say that in this case there is no transformation. The operating principle is simple. It consists of collecting solar heat. That’s why devices for this are called solar collectors. The operating principle of such installations is to collect heat using an absorber and transfer it to the coolant. The latter is water or air. Such installations are often used for heating and hot water supply of private houses. The second use case is to convert it into electricity.

Plants on our planet have been converting solar energy from chemical bonds for millions of years. This process, called photosynthesis, produces glucose. The principle of photosynthesis has been known to humans for a long time. Read more about this at the given link.

In this material we will talk about generating electricity using solar panels. Photovoltaic cells are used for this. These are silicon-based semiconductors that produce direct electric current when exposed to light. Silicon compounds with cadmium, copper, and indium are used as materials for photocells. In addition, they may differ in manufacturing technology.

• Monocrystalline;
• Polycrystalline;
• Amorphous.

Photovoltaic panels made from silicon monocrystals are considered the most efficient and have high efficiency. Polycrystalline silicon solar cells are cheaper and have the lowest cost per watt of electricity. There are also photovoltaic cells based on amorphous silicon. They are made from them. They are produced from amorphous silicon. The production of such elements is simpler than mono and polycrystals. As a result, the price is lower, but the efficiency leaves much to be desired (5─6%). In addition, amorphous silicon panels have a shorter service life than the previous two types. To increase the efficiency of elements, copper, selenium, gallium, and indium are added to silicon.

Photovoltaic cells are combined to form a solar cell. Typically, the number of photocells in a battery is a multiple of 36, but there are other options. In addition to the solar battery, solar systems also include other devices for storing and distributing electricity. In particular, these are:

• Battery (one or more);
• Inverter (converts voltage from 12 or 24 to 220 volts);
• A controller for controlling the charge and discharge of the battery and supplying power to the network.

According to purpose, two can be distinguished large groups devices. Low power solar batteries (up to ten watts) are used in mobile gadgets or power bank for charging. Systems with more power are used to electrify private houses and cottages. They are usually located on the roofs and facades of houses, less often in areas near the house. There are devices that allow you to track the sun and change the angle depending on its position. Now let's see how a solar battery works and what determines the efficiency of its operation.

How does a solar battery work?

Solar energy is converted in series-connected photocells. Let's consider the principle of operation of a solar battery at the level of photovoltaic cells. The basis of a photocell is a silicon crystal. Silicon compounds are very common in nature. The most famous is silicon oxide or sand. A silicon crystal can be simply called a large grain of sand. The crystals are grown artificially in laboratory conditions. They are usually produced into cubic shapes and then into plates. The thickness of these plates is only 200 microns. This is 3-4 times thicker than a human hair.

The resulting silicon wafers are coated with a layer of boron on one side and phosphorus on the other. There is an excess of electrons at the points where the silicon wafer contacts the boron. On the other side, along the border of the silicon wafer with phosphorus, there are missing electrons. “Holes,” as they are commonly called, form there. This joining of boundaries with an excess number of electrons and their deficiency is called a p-n junction.

When sunlight hits the solar cells of a battery, their surface is bombarded with photons. They knock out excess electrons at the phosphorus interface, and they begin to move towards the “holes” at the boron interface. Thus, an electric current arises, which is the ordered movement of electrons. Metal tracks are connected to the photocell, through which the current is collected. This expresses the operating principle of a silicon photocell.

Today, solar panels are installed in their homes and cottages to save electricity. Such miniature solar systems operate all year round. The main thing is that the surface of the panels is clean and the sun is shining. In some cases, their effectiveness is higher on a frosty sunny day than on a summer day. This is explained by the fact that heating somewhat reduces the efficiency of their work.

It is immediately worth noting that it is not possible to completely abandon electricity from centralized networks. But by installing a solar battery, you will be able to save significantly on utility costs. The option, of course, is not suitable for an apartment. It is possible to operate such a system normally only in country house or in the country where there is enough space for installation solar panels.

In the central regions of Russia, a solar system pays for itself in about 5 years. In the southern regions, the payback period is significantly reduced. Often, collectors for heating a house are installed together with solar panels. Now there are factory-made solar collectors that can heat water all year round.

Regarding the installation of solar panels, the following points should be noted:

• The panels need to be installed on the south side of the roof, facade or on the site with the side facing south;
• The tilt angle corresponds to the latitude of your region;
• There should be no objects nearby that cast a shadow on the solar panels;
• The surface of the panels must be regularly cleaned of dirt and dust;
• It is advisable to use systems that track the position of the sun.

Now you understand the principle of operation of solar panels and their capabilities. It is clear that we should not abandon the centralized supply of electricity. Modern solar systems are not yet able to fully provide a home with energy in cloudy weather. But as part combined system They are very appropriate for home energy supply.

It would seem that just recently the solar battery was strongly associated with spaceships, orbital stations and lunar rovers. And now, a device capable of extracting electricity from light can be found in any calculator. Moreover, in sun-rich countries with hot summers and mild winters (scientists call them “high insolation countries”), such as Italy, Spain, Portugal, the southern states of the USA, etc. Solar energy is a significant way to save money on electricity and heat supply. Moreover, this saving occurs both on the private initiative of citizens and in the form of mandatory government regulations, such as in Spain.

Attempts to make the energy of the sun work for themselves have been made by mankind for a long time, so according to legend, Archimedes burned the Roman fleet, ordering to focus many mirrors (in another version - shields polished to a shine) sunlight on the sails of Roman galleys. But the attempts to harness the energy of the sun gave noticeable results only in the last century. What are the ways to use solar energy?

How to get electricity

The most obvious way is to convert the light energy of the sun into heat. Strictly speaking, this cannot even be called a transformation, because light and heat have the same nature and differ only in frequency; it would be more correct to talk about heat collection. To collect solar heat, devices are called - (“collector” literally means collector). The principle of their operation is extremely simple - the coolant (water, less often air) is heated in a radiator made of heat-absorbing material. Such devices are widely used for hot water supply to private homes.

Another interesting way Nature tells us how to use the energy of the nearest star. Over millions of years of evolution, plants have learned to convert the energy of the sun into the energy of chemical bonds, synthesizing a complex compound - glucose - from simple substances. Anyone who didn’t skip botany at school, of course, guessed that we were talking about photosynthesis. But not everyone thought about the energy essence of this process, which consists precisely in the accumulation of solar energy and its further use (including in winter) for “personal” purposes. That is, we are talking about bioenergy. The real one, and not the one that home-grown magicians talk about. The method of using solar energy according to this operating principle is still awaiting its application in man-made technology.

As mentioned above, the easiest way to use solar energy for personal purposes is to collect thermal energy. However, “easiest” does not always mean “best.” The fact is that thermal energy is, one might say, a “perishable product.” Try to “conserve” the heat or transfer it to long distances. Most likely, the costs will cover all possible benefits. The most convenient type of energy for accumulation and transportation is electricity. It can be done without special problems collected in batteries or transmitted by wire to the place where it will work, with minimal losses. This leads to the third, most common way of using sunlight - converting it into electrical energy.

How does this work

The transformation of sunlight occurs in batteries (that is, series-connected groups) of photocells, which have acquired the name “solar batteries”. On what principle do solar panels work?

The heart of a photocell is a silicon crystal. We encounter silicon (more precisely, its oxides) every day - this is the familiar sand. Thus, we can say that a silicon crystal is a giant grain of sand grown in a laboratory. The crystals are cube-shaped and cut into platinum two hundred microns thick (about three to four times the thickness of a human hair).

A thin layer of phosphorus is applied to a silicon wafer on one side, and a thin layer of boron on the other side. Where silicon is in contact with boron, an excess of free electrons appears, and where silicon is in contact with phosphorus, on the contrary, electrons are in short supply, so-called “holes” appear. The junction of media with an excess and deficiency of electrons is called physics p-n transition. Photons of light bombard the surface of the plate and knock out excess phosphorus electrons to missing boron electrons. The ordered movement of electrons is electric current. All that remains is to “assemble” it by running metal tracks through the plate. This is how a silicon photocell works in principle.

The power of one photocell plate is quite modest; it is only enough to operate a flashlight bulb. That's why individual elements collected into battery systems. Theoretically, it is possible to assemble a battery of any power from the elements. The battery is placed on a metal substrate, reinforced to increase strength and covered with glass. It is important that the solar battery converts not only the visible part of the solar spectrum into electricity, but also the ultraviolet part of the solar spectrum, so the glass covering the battery must transmit ultraviolet radiation.

An important advantage of a solar battery is that it uses light, not heat, therefore, unlike a collector, a solar battery can work in winter, as long as clouds do not block the sunlight. There are projects to build huge fields of solar panels in the Arctic and Antarctic that will store energy during the six-month polar day, which occurs in the north in summer and in the south in winter, meaning two giant solar power plants will never be idle at the same time.

This is all in the long term, but you can benefit from the properties of a solar battery today by equipping your home with a miniature solar power plant. Such a station, of course, is unlikely to be able to fully satisfy the household’s electricity needs, but, without a doubt, it will become a sensitive factor in saving the family budget.

All life on earth arose thanks to the energy of the sun. Every second, a huge amount of energy enters the surface of the planet in the form of solar radiation. While we burn thousands of tons of coal and petroleum products to heat our homes, countries located closer to the equator are sweltering in the heat. Using the energy of the sun for human needs is a task worthy of inquiring minds. In this article we will look at the design of a direct converter of sunlight into electrical energy - a solar cell.

The simplest design of a solar cell (SC) based on monocrystalline silicon is shown in the figure.

A thin wafer consists of two layers of silicon with different physical properties. The inner layer is pure monocrystalline silicon with “hole conductivity” (p-type). On the outside, it is coated with a very thin layer of “contaminated” silicon, for example with an admixture of phosphorus (n-type). (About p-, n- and p-n types see article on diodes). A continuous metal contact is applied to the back side of the plate. U n-and boundaries p-layers, as a result of charge flow, depleted zones are formed with an uncompensated volumetric positive charge in the n-layer and a volumetric negative charge in the p-layer. These zones together form a p-n junction.

The potential barrier (contact potential difference) that appears at the transition prevents the passage of the main charge carriers, i.e. electrons from the p-layer side, but freely allow minority carriers to pass in opposite directions. This property of p-n junctions determines the possibility of obtaining photo-emf when irradiating a solar cell with sunlight. When the SC is illuminated, the absorbed photons generate nonequilibrium electron-hole pairs. Electrons generated in the p-layer near the p-n junction approach the p-n junction and those existing in it electric field are transferred to the n-region.

Similarly, excess holes created in the n-layer are partially transferred to the p-layer (Fig. a). As a result, the n-layer acquires an additional negative charge, and the p-layer acquires a positive charge. The initial contact potential difference between the p- and n-layers of the semiconductor decreases, and voltage appears in the external circuit (Fig. b). The negative pole of the current source corresponds to the n-layer, and the p-layer to the positive one.

Most modern solar cells have a single pn junction. In such an element, free charge carriers are created only by those photons whose energy is greater than or equal to the band gap. In other words, the photovoltaic response of a unijunction cell is limited to the part of the solar spectrum whose energy is above the bandgap, and lower energy photons are not used. Multilayer structures of two or more solar cells with different band gaps can overcome this limitation. Such elements are called multi-junction, cascade or tandem. Because they work with a much larger portion of the solar spectrum, their photovoltaic conversion efficiency is higher. In a typical multijunction solar cell, single solar cells are arranged one behind the other in such a way that sunlight hits the cell with the largest bandgap first, and the highest energy photons are absorbed.

Photons transmitted by the top layer penetrate into the next element with a smaller bandgap, etc. The main direction of research in the field of cascade cells involves the use of gallium arsenide as one or more components. The conversion efficiency of such solar cells reaches 35%! For technological reasons, separate solar cell It is possible to produce only a small size, therefore, for greater efficiency, several elements are combined into batteries.

Solar batteries have proven themselves in space as a fairly reliable and stable source of energy, capable of very long time. The main danger to solar cells in space is cosmic radiation and meteor dust, which cause erosion of the surface of silicon cells and limit the life of the batteries. For small inhabited stations, this current source will apparently remain the only acceptable and sufficiently effective one.