Active and passive technologies of solar energy

More than half a century, scientists have tried a huge number of different options and ways to extract and use solar energy. Expensive and inefficient technologies gave way to attractive and inexpensive developments that do not stop improving over the years. Let’s highlight the most common technology groups of the “solar” industry and try to identify the most attractive options for the consumer. First, it is necessary to determine the classification of “solar” technologies, separated by scientists into 4 groups: active, passive, direct (or “direct”) and indirect (indirect).

Active – together with the transducers mechanisms are involved, electric motors, pumps. Solar energy is used for heating water, lighting, ventilation.

Passive – differ from active by the absence in the contours of systems of any mechanisms, moving parts. A special feature of constructing passive solar structures for the organization of ventilation and heating systems is the selection of appropriate building materials for physical parameters, a specific layout of the room, and the placement of windows.

Direct or “direct” technologies include systems that convert solar energy in the course of one level or stage.
To the group of “indirect” technologies belong systems whose process of functioning includes multilevel transformations and transformations to obtain the required form of energy.

Based on the above classification of groups of solar energy technologies, it is possible to easily characterize the spheres of human activity, where the energy of the sun has become most widespread.


Advantages and disadvantages of solar energy

1. The general availability and inexhaustibility of the source.
2. Theoretically, complete safety for the environment, although there is a possibility that the widespread introduction of solar energy can change the albedo of the earth’s surface and lead to climate change (however, with the current level of energy consumption, this is highly unlikely).

1. Dependence on weather and time of day.
2. As a consequence, the need for energy storage.
3. High cost of construction.
4. The need for constant cleaning of the reflecting surface from dust.
5. Heating of the atmosphere above the power plant.

Is solar energy from the technical and economic point of view ideal? Unfortunately, not really. We will try to highlight the main advantages and disadvantages of this method of extracting energy.

Let’s start with the positive sides. First, “raw materials”, i.e. Sunlight, will never end. The second plus of solar energy is its general availability, as the sun shines in the south and west, in Africa and Europe.
The issue of absolute safety of these technologies for the environment is contradictory. Of course, this is not nuclear energy and not the extraction of oil and gas, but at this stage of development of “solar” technologies, harmful substances are used in the manufacture of batteries, which in one way or another can harm nature. Ready-made samples (photocells) contain poisonous substances, such as lead, cadmium, gallium, arsenic.

As for the service life of the converters (30-50 years), the problem arises here for the subsequent processing of obsolete modules, and the solution of the problem of their utilization has not yet been found. The obvious shortcoming of the process of extracting energy is the so-called impermanence. Solar systems are not able to work at night, and in the evening and in the early twilight the efficiency of stations falls several times.

Severe influence is exerted by weather factors. Many complain about the relative high cost of solar cells, inadequate efficiency in terms of material costs and payback (at the moment). The problem of technical support and maintenance is the “underwater stone” of the functioning of modern “solar farms”. The developers argue that the intense heating of photocells significantly reduces the efficiency of the system as a whole, so here it is necessary to provide a solution to the problem of cooling modules. Also, solar panels must be periodically cleaned from dust and dirt, and in the case of working with an installation area of ​​several square kilometers with cleaning, considerable difficulties can arise.

Ideal, at first glance, energy production technology even today there are a number of shortcomings, but you can be sure that this is just an indicator of improving solar energy. Every day of technological progress can eradicate one flaw after another, so it’s a matter of time.

What is a solar collector?

These devices today are the most common type of solar converters. The device operates at a temperature of from one hundred to two hundred degrees.

Talking about the application of these settings can be endless.

Already in our days, solar collectors perform a huge range of work. With the help of collectors, they heat food, save salt, extract water from wells.

Through concentrated solar energy, you can dry vegetables or fruits, and also freeze food.

It should be said that the main advantage of using a thermal solar converter is to provide a high efficiency.
Thus, recent developments allow us to speak about forty-five and even sixty percent. By the way, the level of efficiency of thermal solar receivers can be increased by supplementing them with special mirror surfaces.

The main function of such a surface is to concentrate the incoming radiation. If we consider these devices as a means of providing energy to an apartment house, then the most practical promises are so-called focons.

These are flat solar cells with linear concentrators. This device is represented in the form of a V-shape. By the way, the device can be not only flat, but also paraboloid.

Of course, such an improved design will cost the consumer much more expensive, but the effect will be appropriate.

For household needs, the collector, which performs the role of a water heater, is perfect. The structure includes a box with a coil, a tank with cold water, a tank-battery and pipes.

The main thing is to install the box correctly. It should be at an angle of 30-50 degrees and be directed to the south. Cold water is at the bottom of the box, it is heated and replaced by incoming cold water, enters the storage tank.

The plant’s capacity during the day is about two kilowatt-hours per square meter. Water can be heated to sixty or seventy degrees, which allows it to be used for a variety of purposes (heating, shower, etc.).

Also, the device boasts a high efficiency. Usually it reaches forty percent. The principle of solar collectors in many ways resembles the principle of greenhouses. Such collectors can be made of different materials – wood, metal, plastic.

On the one hand they are closed with a single or double glass. To ensure complete absorption of sunlight, a sheet of metal is inserted into the box. As a rule, this sheet is painted in black.

The box contains air or water that is heated and then fed into the tank by the action of a fan or pump.

Types of solar cells

Monocrystalline silicon

The most effective and widespread for general consumption are monocrystalline silicon elements. To manufacture such elements, silicon is purified, melted and crystallized in ingots, from which thin layers are cut off. Externally, single-crystal elements look like a monochromatic surface of dark blue or almost black color. Through the silicon is a grid of metal electrodes. The efficiency of this element is from 16 to 19% under standard testing conditions (direct sunlight, + 250 ° C).
The service life of such panels from good manufacturers is usually 40-50 years. The productivity for every 20-25 years of service is gradually reduced by about 20%.

Polycrystalline silicon

Technology is not fundamentally different from monocrystalline elements, but the difference is that less pure and cheaper silicon is used for manufacturing. Outwardly, this is not a monotonous surface, but a pattern from the boundaries of many crystals. The effectiveness of this element is from 14 to 15%. Nevertheless, these panels are about the same popularity on the market as monocrystalline ones, since the price of production decreases proportionally to efficiency.

Silicon band

Fundamentally the same as the previous types, differs only in that silicon is not cut from the crystal, but is built up by a thin layer in the form of a tape. Anti-reflective coating gives an iridescent color to such panels. This technology could not conquer the market, occupying only about 2% on it.

Amorphous silicon

In this type, not crystals are used, but the thinnest layers of silicon sputtered in vacuum onto plastic, glass or metal. This type is the cheapest in production, but has a serious drawback. The layers of silicon burn out in the light much faster than in the previous types. Decrease in productivity by 20% can happen in two months. Very often in Russia people attracted by low price get such panels and then are disappointed, since in a year or two such an element ceases to give energy.
You can recognize such a panel in a more pale gray or dark color of incomprehensible shades.

Cadmium tellurium

This type of thin-layer solar cells has a potentially greater efficiency and uses tin oxide as a conductive component. The efficiency is 8-11%. At cost, these elements are not much cheaper than mono- and polycrystalline silicon and have the problem of using toxic cadmium. Now this type of elements takes less than 5% of the total market. The admission of such panels to undeveloped countries is undesirable primarily due to domestic inability to handle potentially toxic products.

Other elements

In addition to the above, there are many more solar cells that are not widely used. Potentially promising are copper-gallium, concentrating, composite and some other elements.

Raw material. What do solar panels make of?

We will touch on the problem of raw materials. Scientists say that silicon (the main resource for the production of most types of solar cells) is the second most prevalent element on our planet. Silicon accounts for more than a quarter of the total mass of the earth’s crust, but to what silicon?

The fact is that in most cases this substance occurs in the form of oxide – SiO2, but it is difficult, even problematic, to obtain a pure silicium (Silicium – as chemists call silicon) from this compound.

Here there are cost factors, features of technologies. It is interesting to note that the cost of pure “solar” silicon is equal to the cost of uranium for nuclear power plants, but only silicon reserves on our planet are 100,000 times larger.

Due to the high cost of silicon, reflected in the retail price of solar cells, research centers have been working for many years to find a worthy alternative. For example, German scientists at the Institute of Physical Electronics in Stuttgart proposed using synthetic fibers instead of silicon, capable of generating electric current under the influence of light.

New developments though can not boast of high efficiency, but they are cheap and suitable for powering low-power digital devices. A shirt made of “synthetic” fabric can provide power to a pocket PC, mobile phone or MP3 player. And if the seafarers try to sew a sail from such a canvas? To provide energy on-board electronics certainly enough.

Today, in the era of nanotechnology, when a person easily conquers the microcosm, the scientific contributions of engineers can speed up the development of the “solar” industry several times. A striking example of this can serve as a statement of employees of the Norwegian company Scatec AS. Scientists believe that panels made using nanotechnology will reduce the cost of solar energy in comparison with the now widely used photovoltaic cells in 2 times.

The photoelectric effect. Solar panels

The photoelectric effect is the phenomenon of the emission of electrons by matter under the action of light. It was discovered in 1887 by Hertz, who discovered that a spark discharge in the air gap is easier to create if there is another spark discharge nearby. Hertz experimentally showed that this is due to ultraviolet radiation of the second order. In 1889, J. Thomson and F. Lenard found that when the metal surface is illuminated in an evacuated vessel, it emits electrons. Continuing these studies, Lenard demonstrated in 1902 that the number of electrons emitted per second from the surface of the metal is proportional to the intensity of light, whereas their energy depends only on the light wavelength, i.e. colors. Both these facts contradicted the conclusions of Maxwell’s theory of the mechanism of emission and absorption of light. According to this theory, the intensity of light serves as a measure of its energy and, of course, it must influence the energy of the emitted electrons.

Under certain conditions, the photoelectric effect is possible in gases and atomic nuclei, from which photons with a sufficiently high energy can knock out protons and give birth to mesons. The photoelectric properties of the metal surface are widely used to control the electric current through a light beam, when playing sound from the soundtrack of a film, as well as in numerous monitoring, counting and sorting devices. Photocells are also used in lighting.

Solar panels

In our time, the topic of developing alternative ways of obtaining energy is as relevant as possible. Traditional sources are rapidly drying up and after only some fifty years can be exhausted. And even now, energy resources are quite expensive and significantly affect the economy of many states.

All this forces the inhabitants of our planet to seek new ways of obtaining energy. And one of the most promising areas is obtaining solar energy. And this is quite natural. After all, it is the Sun that gives life to our planet and provides us with warmth and light. The sun heats all the corners of the Earth, controls the rivers and wind. Its rays grow at least one quadrillion tons of various plants, which, in turn, are food for animals.

Thus, we already use solar energy for our needs and all the traditional energy sources (oil, coal, peat) appeared on the globe thanks to the Sun.

A man from the earliest times learned to use the gifts of the Sun. Even the simple bonfire that warmed our ancestors thousands of years ago and continues to do this now is, in fact, the use of solar energy, which has accumulated wood. But the Sun is able to satisfy even more large-scale human needs. According to scientists, humanity needs ten billion tons of fuel.

If we calculate the number of such conditional tons that are provided by the Sun within a year, we will get a fantastic amount – about a hundred trillion tons. Thus, people get the amount of energy that exceeds the necessary resources tenfold. You just need to take this energy wealth. This question is extremely urgent for science.

The result of many years of work has become a device like a solar battery.

The physical principle of the photocell

The energy conversion in photoelectric converters is based on the photoelectric effect that occurs in inhomogeneous semiconductor structures when solar radiation is applied to them.
The heterogeneity of the structure in photoelectric converters can be obtained by doping the same semiconductor with various impurities (creating pn junctions) or by joining different semiconductors with an unequal width of the forbidden band-the energy of electron detachment from the atom (creation of heterojunctions), or by changing the chemical composition of the semiconductor , Resulting in a gradient of the width of the forbidden band (creation of the valine structures). Various combinations of these methods are also possible.

The conversion efficiency depends on the electrophysical characteristics of the inhomogeneous semiconductor structure, as well as the optical properties of photoelectric converters, among which photoconductivity plays the most important role. It is caused by the phenomena of the internal photoelectric effect in semiconductors when they are irradiated with sunlight.
The main irreversible energy losses in photoelectric converters are related to:

• reflection of solar radiation from the transducer surface,
• Passing part of the radiation through photoelectric converters without absorption in it,
• scattering by thermal vibrations of the grid of excess photon energy,
• recombination of the resulting photo pairs on surfaces and in the volume of photoelectric converters,
• the internal resistance of the converter,
• and some other physical processes.

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

• the use of semiconductors with the maximum width of the forbidden zone for solar radiation;
• directed improvement of the properties of the semiconductor structure by its optimal alloying and creation of built-in electric fields;
• the transition from homogeneous to heterogeneous and heterogeneous semiconductor structures;
• Optimization of design parameters of photoelectric converters (depths of p-n junction, base layer thickness, contact grid frequency, etc.);
• application of multifunctional optical coatings, providing clarification, thermal regulation and protection of photoelectric converters from cosmic radiation;
• development of photoelectric converters transparent in the long-wave region of the solar spectrum behind the edge of the main absorption band;
• Creation of cascade photoelectric converters from semiconductors specially selected for the width of the forbidden band, which allow one to convert radiation passing through the previous cascade in each cascade, etc .;

Also, a significant increase in the efficiency of photoelectric converters was achieved due to the creation of transducers with a two-sided sensitivity (up to +80% to the already existing efficiency of one side), the use of luminescence re-emitting structures, preliminary decomposition of the solar spectrum into two or more spectral regions by means of multilayer film beam splitters (dichroic Mirrors) with the subsequent transformation of each part of the spectrum by a separate photoelectric converter, etc.