Direct conversion of solar energy into electricity. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

 Comments on the article

From the shortcomings inherent in machine converters, to a certain extent, power plants with so-called machineless converters are free: thermoelectric, thermionic and photovoltaic (solar batteries), which directly convert the energy of solar radiation into electric current.

thermoelectric method

Thermoelectric generators (TEG) are based on the discovery in 1821 by the German physicist T.I. Seebeck thermoelectric effect, consisting in the occurrence of thermo-EMF at the ends of two dissimilar conductors, if the ends of these conductors are at different temperatures.

The open effect was originally used in thermometry to measure temperatures. The energy efficiency of such thermocouple devices, implying the ratio of the electrical power released at the load to the heat supplied, was fractions of a percent. Only after Academician A.F. Ioffe proposed the use of semiconductors instead of metals for the manufacture of thermoelements, the energy use of the thermoelectric effect became possible, and in 1940-1941 the world's first semiconductor thermoelectric generator was created at the Leningrad Institute of Physics and Technology. In the 40-50s, the theory of the thermoelectric effect in semiconductors was developed, and very effective (to this day) thermoelectric materials were synthesized. According to the developed theory, the TEG efficiency expression is given by the formula:

Where

z is the quality factor of the semiconductor material, 1/K; T_Г - temperature of the hot junction of the thermoelement, K; T_Х - cold junction temperature, K; T_cf. - average temperature of thermoelement leg, K,

M - Ioffe criterion, a - reduced differential thermo-EMF of thermoelement legs, µV/K; s, l - reduced electrical conductivity and thermal conductivity of thermoelement legs, respectively, in 1/(Ohm m) and W/(m•K).

It makes sense to dwell on the given formula for efficiency, since it characterizes the efficiency of not only thermoelectric generators, but also other devices for direct energy conversion. It is noteworthy that the TEG efficiency depends on the same factors as the efficiency of any heat engine: the thermal efficiency of the reversible Carnot cycle (the first factor in the formula) and the coefficient of irreversible energy losses (the second factor). In TEG, internal irreversible losses are mainly associated with heat transfer along the positive 3 and negative 4 branches from hot 1 (Fig. 3a) to cold 5 junctions (junctions, usually made of copper, are separated from the branches by antidiffusion layers 2 (Fig. 3 ,A)). As follows from the formula, the lower the irreversible losses, the higher the quality factor of the materials used. However, theory and many years of practice have shown that the value of the quality factor of the order of 3 • 10-3 1/deg is, apparently, its limiting value.

Fig.3. Scheme of thermoelectric converter: a - separate thermoelement, b - thermoelectric module on the concentrator

By interconnecting individual thermoelements, it is possible to create sufficiently powerful thermopiles, one of which is shown in Fig. 3b. The battery is located in the focal plane of the concentrator 3; its hot junctions 1 are directly heated by concentrated solar radiation, and heat is removed from cold junctions 2 by radiation. There are energy characteristics of a space power plant, similar to that shown in Fig. 3b, but without the concentrator. The expected specific gravity of the plant is up to 50 W/kg. This means that a 10 GW power plant can weigh up to 200 tons.

Reducing the weight of the power plant is directly related to increasing the efficiency of converting solar energy into electricity, which, as can be seen from the above formula, can be achieved in two ways: increasing the thermal efficiency of the converter (the efficiency of the Carnot cycle) and liquefying irreversible energy losses in all elements of the power plant. The first way is, in principle, possible, since concentrated radiation makes it possible to obtain very high temperatures. However, this greatly increases the requirements for the accuracy of solar tracking systems, which is hardly achievable for concentrating systems of enormous size. Therefore, the efforts of researchers have been invariably aimed at reducing irreversible losses, primarily at reducing the heat transfer from hot junctions to cold junctions by thermal conductivity. To solve this problem, it was necessary to achieve an increase in the quality factor of semiconductor materials.

However, as already mentioned, after many years of attempts to synthesize semiconductor materials with a high quality factor, it became clear that the achieved value (2,5-2,7) • 105 is the limiting value. Then, while continuing to search for new ways to reduce the heat flow, the idea arose to separate the hot and cold junctions with an air gap, as is the case in a two-electrode lamp - a diode. If one electrode, cathode 1, is heated in such a lamp (Fig. 4), and at the same time the other electrode, anode 2, is cooled, then a direct current arises in the external electrical circuit.

Fig.4. Schematic diagram of thermionic transducer

Thermionic transducer (TEC)

The phenomenon discovered by Edison was called thermionic emission. Like thermoelectricity, it has long been used in low current technology. Later, scientists drew attention to the possibility of using the method to convert heat into electricity. And although the nature of thermoelectricity and thermionic emission is different, they have the same expressions for efficiency:

where h_к - efficiency of the reversible Carnot cycle; h_unmod. - coefficient taking into account irreversible losses in the thermionic (thermoelectric) converter.

The main components of irreversible losses in the TEC are associated with the non-isothermal nature of the heat supply and removal at the cathode and anode, the heat transfer from the cathode to the anode through the structural elements of the TEC, as well as with ohmic losses in the elements of the series connection of individual modules.

To achieve high efficiency of the Carnot cycle, modern TECs are designed for cathode operating temperatures of 1700–1900 K, which, at temperatures of cooled anodes of about 700 K, makes it possible to obtain an efficiency of about 10%. Thus, due to the reduction of irreversible losses in the converter itself and with a simultaneous increase in the heat supply temperature, the TEC efficiency turns out to be twice as high as that of the TEG described above, but at significantly higher heat supply temperatures. To obtain such temperatures of the cathode surfaces in a geosynchronous orbit, the accuracy of orientation to the Sun of the TEC concentrator must be within 6°–8°, which, with the thermal power of the SCES of 10–20 GW and the corresponding areas of the concentrators, can become, as noted above, a serious technical problem.

It is quite possible that the noted circumstances played an important role in the choice of the photoelectric method for converting solar energy in the onboard power supply systems of the first and subsequent generations of spacecraft.

Photoelectric energy conversion method

The solar battery (Fig. 5) is based on the phenomenon of an external photoelectric effect, which manifests itself at the p-n junction in a semiconductor when it is illuminated with light. A p-n (or np) transition is created by introducing impurities with the opposite sign of conductivity into a single-crystal semiconductor base material. For example, aluminum or lithium is introduced into silicon. As a result, when solar radiation hits the p-n junction, the electrons of the valence band are excited and an electric current is generated in the external circuit. The efficiency of modern solar panels reaches 13-15%.

Fig.5. Solar battery diagram: 1 - solar cell, 2 - protective glass, 3 - switching bus, 4 - substrate

The most promising for the creation of SCES converters are ultrathin solar cells with an efficiency of about 15% with specific characteristics of 1 kW/m2 and 200 W/kg. When using these solar batteries as a converter of SCES with a capacity of 10 GW, their area would be 50 km2 with a weight of 10 thousand tons.

See other articles Section Alternative energy sources.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Alcohol content of warm beer 07.05.2024

Beer, as one of the most common alcoholic drinks, has its own unique taste, which can change depending on the temperature of consumption. A new study by an international team of scientists has found that beer temperature has a significant impact on the perception of alcoholic taste. The study, led by materials scientist Lei Jiang, found that at different temperatures, ethanol and water molecules form different types of clusters, which affects the perception of alcoholic taste. At low temperatures, more pyramid-like clusters form, which reduces the pungency of the "ethanol" taste and makes the drink taste less alcoholic. On the contrary, as the temperature increases, the clusters become more chain-like, resulting in a more pronounced alcoholic taste. This explains why the taste of some alcoholic drinks, such as baijiu, can change depending on temperature. The data obtained opens up new prospects for beverage manufacturers, ... >>

Major risk factor for gambling addiction 07.05.2024

Computer games are becoming an increasingly popular form of entertainment among teenagers, but the associated risk of gaming addiction remains a significant problem. American scientists conducted a study to determine the main factors contributing to this addiction and offer recommendations for its prevention. Over the course of six years, 385 teenagers were followed to find out what factors may predispose them to gambling addiction. The results showed that 90% of study participants were not at risk of addiction, while 10% became gambling addicts. It turned out that the key factor in the onset of gambling addiction is a low level of prosocial behavior. Teenagers with a low level of prosocial behavior do not show interest in the help and support of others, which can lead to a loss of contact with the real world and a deepening dependence on virtual reality offered by computer games. Based on these results, scientists ... >>

Traffic noise delays the growth of chicks 06.05.2024

The sounds that surround us in modern cities are becoming increasingly piercing. However, few people think about how this noise affects the animal world, especially such delicate creatures as chicks that have not yet hatched from their eggs. Recent research is shedding light on this issue, indicating serious consequences for their development and survival. Scientists have found that exposure of zebra diamondback chicks to traffic noise can cause serious disruption to their development. Experiments have shown that noise pollution can significantly delay their hatching, and those chicks that do emerge face a number of health-promoting problems. The researchers also found that the negative effects of noise pollution extend into the adult birds. Reduced chances of reproduction and decreased fertility indicate the long-term effects that traffic noise has on wildlife. The study results highlight the need ... >>

Random news from the Archive Dolphin paradox resolved 16.10.2009 In 1936, English biologist John Gray calculated that a dolphin could not swim at 35 kilometers per hour because its muscle mass was insufficient to overcome water resistance at that speed. Since dolphins actually achieve even faster speeds, biologists have started talking about the "dolphin paradox". Gray himself suggested that the matter was in the structural features of the dolphin skin, and since then the problem has been studied from this side. But recently, Professor Timothy Wei of Rensselaer Polytechnic Institute (USA) has shown that in fact the muscles of the dolphin are 10 times stronger than it was thought in Gray's time.

Other interesting news:

▪ Meike 85mm F/1.8 full-frame autofocus lens

▪ Garmin Smart Bike Headlight

▪ Navigator Garmin Speak

▪ The impact of hugs on stress levels

▪ Portable drive Samsung T5 EVO 8 TB

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the website Residual current devices. Selection of articles

▪ article Water, its pollution and consequences. Basics of safe life

▪ article What is the connection between a great playwright and a pirate? Detailed answer

▪ article Soldering aluminum and its alloys. HAM Tips

▪ article Engine sound and car horn simulator. Encyclopedia of radio electronics and electrical engineering

▪ article New KB receiver. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024