What Is Thermoelectric Cooling?

The so-called thermoelectric effect is a phenomenon in which currents or charges are accumulated when electrons (holes) in a heated object move from a high temperature region to a low temperature region with a temperature gradient. The magnitude of this effect is measured using a parameter called thermopower (Q), which is defined as Q = E / -dT (E is the electric field due to charge accumulation, and dT is the temperature gradient).

Chinese name: Thermoelectric effect
Foreign name: thermo electric effect

Solid: A phenomenon
the reason: Generate current or charge buildup
Object: Electrons in a heated object

Introduction to thermoelectric effects

Alumite Alunite hexagonal KAl3 (OH) 6 (SO4) 2 is a potassium, sodium, and aluminum sulfate mineral containing hydroxide. Its cleavage surface is pearly luster, and the rest is glassy luster. Hardness 3.5 ~ 4, white streaks,

Thermoelectric effect Specific gravity of 2.58 ~ 2.75, with gray, white, slightly yellow, slightly red, etc. It has a strong thermoelectric effect, is insoluble in water, hardly soluble in hydrochloric acid, nitric acid, hydrofluoric acid and ammonia, but can be dissolved in strong alkali and sulfuric acid. Or perchloric acid. Alum stones are irregular deposits and veins. The alum stones of the Datunshan volcanic group form fine-grained crystals and coexist with quartz, opal and clay minerals. Some are veined, and some explain the matrix and crystals in andesite. Stone alum stone is produced in granular or scaly form in ore deposits and metamorphic surrounding rocks. As a source of alum and potassium sulfate, it can also be used for refining aluminum and papermaking, food processing, water purification agents, dyes and other uses.

The rare mineral stone with obvious thermoelectric effect is selected as the raw material and added to the wall material. In contact with the air, it can polarize and discharge to the outside to play a role in purifying indoor air.

Thermoelectric effect

American scientists have discovered that a colloid in the shark's nose can convert changes in seawater temperature into electrical signals and send them to nerve cells so that sharks can sense subtle temperature changes and find food accurately. Scientists speculate that similar colloids may exist in other animals. The nature of this current due to temperature differences is similar to the thermoelectric effect of semiconductor materials. The artificial synthesis of this colloid is expected to be used in the microelectronics industry.

A scientist at the University of San Francisco

Thermoelectric effect Ran reported that he extracted a gel similar to ordinary gelatin from the skin pores of the shark's nose and found that it was very sensitive to temperature, and a temperature change of 0.1 degrees Celsius would cause it to produce a significant voltage change.

The small holes in the skin of the shark's nose are filled with nerve cells that are very sensitive to current. The temperature changes in the seawater cause currents in the colloid to stimulate the nerves and make the sharks sense the temperature difference. Scientists believe that with this colloid, sharks can sense a temperature change of 0.001 degrees Celsius, which is good for them to forage in seawater.

Mammalian temperature is sensed by ion channels on the surface of the cell: changes in external temperature cause charged ions to enter and exit the channel, generating electrical currents, stimulating nerves, and making animals feel cold and warm. Unlike mammals, sharks use colloids to sense temperature changes without the need for ion channels.

Thermoelectric effect life application

Thermoelectric refrigeration is also called temperature difference electric refrigeration, or semiconductor refrigeration. It is a refrigeration method that uses the thermoelectric effect (Peltier effect).

In 1834, the French physicist Peltier connected a bismuth wire to each end of the copper wire. After connecting the two bismuth wires to the positive and negative poles of the DC power supply, one connector became hot and the other connector was found. Get cold. This shows that when an electric circuit composed of two different materials passes a direct current, heat absorption and release occur at the two joints. This is the basis of thermoelectric cooling.

Semiconductor materials with high thermoelectric potential can be successfully used to make small thermoelectric refrigerators. Figure

Thermoelectric effect formula Reference numeral 1 shows a thermocouple cooling element composed of an N-type semiconductor and a P-type semiconductor. The copper plate and the copper wire are used to connect the N-type semiconductor and the P-type semiconductor into a loop, and the copper plate and the copper wire only play a conductive role. At this time, one contact becomes hot and one contact becomes cold. If the direction of the current is reversed, the cold and heat effects at the junction are reciprocal.

The output of thermoelectric cooler is generally small, so it is not suitable for large-scale and large-scale refrigeration. However, because of its flexibility, simple and convenient switching between hot and cold, it is very suitable for the field of micro refrigeration or cold places with special requirements.

The theoretical basis of thermoelectric refrigeration is the solid thermoelectric effect. In the absence of an external magnetic field, it includes five effects: heat conduction, Joule heat loss, Seebeck effect, Peltire effect, and Thomson. effect.

General cold air and refrigerators use fluorochloride as the refrigerant, causing the destruction of the ozone layer. Refrigerant-free refrigerators (air-conditioning) are therefore an important factor for environmental protection. Using the thermoelectric effect of semiconductors, a refrigerant-free refrigerator can be manufactured.

This method of power generation is to directly convert thermal energy into electrical energy, and its conversion efficiency is limited by the second law of thermodynamics, Carnotfficiency. As early as 1822, Sieber was discovered, so the thermoelectric effect is also called Seebeckeffect. .

It is not only related to the temperature of the two junctions, but also to the nature of the conductor used. The advantage of this power generation method is that there is no rotating mechanical part and there will be no wear and tear, so it can be used for a long time. Heat source, sometimes using several layers of thermoelectric material (cascade or staging) to achieve high efficiency.

Sources of thermoelectric effects found

Thomas John Seebeck (also translated as "West Burke") was born in Tallinn in 1770 (then part of East Prussia, now the capital of Estonia). Seebeck's father is a German of Swedish descent, and perhaps because of this, he encouraged his son to study medicine at the universities of Berlin and Göttingen where he had studied. In 1802, Seebeck received a medical degree. Because he chose the direction of physics in experimental medicine, and spent most of his life engaged in education and research in physics, he was generally considered a physicist.

After graduation, Seebeck entered the University of Jena, where he met Goethe. Seebeck was deeply affected by the German Romantic movement and Goethe s opposition to Newton s theory of light and color. Since then, he has been working with Goethe on theoretical research on light and color effects. Seebeck's research focuses on the solar spectrum, and in 1806 he revealed the effects of heat and chemistry on the different colors in the solar spectrum

Thomson effect A compound of ammonia and mercury oxide was first obtained in 1808. In 1812, while Seebeck was engaged in the phenomenon of light polarization in stress glass, he did not know that two other scientists, Brewster and Bio, had first discovered in this field.

Around 1818, Seebeck returned to the University of Berlin to carry out independent research activities. The main content was the magnetization of steel when a current passed through a conductor. At that time, Arago and Davy only discovered the magnetization effect of current on steel. Secbe carried out a lot of experiments on different metals and discovered the irregular reaction of magnetized hot iron, which is now The so-called hysteresis. During this period, Seebeck also studied photoluminescence, thermal effects in different wavelengths of the solar spectrum, chemical effects, polarization, and the magnetic properties of currents.

In the early 1820s, Seebeck experimentally studied the relationship between current and heat. In 1821 Seebeck connected two different metal wires together to form a current loop. He connected the two wires end-to-end to form two nodes. He suddenly discovered that if one of the junctions was heated to a very high temperature and the other junction was kept at a low temperature, there was a magnetic field around the circuit. He couldn't believe that a current would be generated when heat was applied to a junction made of two metals, which could only be explained by thermomagnetic currents or thermomagnetic phenomena. Over the next two years (1822-1823), Seebeck reported his continuing observations to the Prussian Scientific Society, describing the discovery as "metal magnetization caused by temperature differences."

Seebeck's experimental instrument, when one of the ends is heated, the pointer rotates, indicating that a magnetic field is generated by the wire. It is magnetized in a certain direction instead of forming a current. The scientific society believes that this phenomenon is caused by a temperature gradient that causes an electric current, which in turn creates a magnetic field around the wire. Seebeck was very annoyed by this explanation. He countered that the eyes of scientists were blinded by the experience of Oersted, the pioneer of electromagnetics, so they would only use the theory that "magnetic fields are generated by electric current" to explain , And there is no other explanation. but,

Thermoelectric effect Seebeck himself can hardly explain the fact that if the circuit is cut, the temperature gradient does not generate a magnetic field around the wire. Therefore, most people agreed with the view of the thermoelectric effect, and it was confirmed in this way.

Other effects

Thomson effect

William Thomson was born in Ireland in 1824. His father, James, was a professor of mathematics at the Royal College of Belfast. After teaching at the University of Glasgow, his family moved to Glasgow, Scotland, when William was 8 years old. Thomson entered the University of Glasgow at the age of ten (you don't need to be surprised, at that time, Irish universities would admit the most talented elementary school students), began studying college-level courses at about 14 years old, and at the age of 15 with an article entitled The "Shape of the Earth" article won the university's gold medal. Thomson later went to study at Cambridge University and graduated with a second place in the year. After graduating, he arrived in Paris where he conducted experimental research for a year under the guidance of Lenio. In 1846, Thomson returned to Glasgow University as a professor of natural philosophy (that is, physics), and officially retired in 1899.

Thomson established the first modern physics laboratory at the University of Glasgow; published a monograph on thermodynamics at the age of 24, establishing an "absolute thermodynamic temperature scale" for temperature; and published a book on "Theory of Thermodynamics" at the age of 27, establishing the second thermodynamics

Thermoelectric cooler Law, making it the basic law of physics; co-discovering the Joule-Thomson effect when gas is diffused with Joule; after 9 years to establish a permanent Atlantic submarine cable between Europe and the United States, thus obtaining the title of "lord Kelvin" aristocracy.

Thomson's life span was quite extensive. He has made significant contributions in mathematical physics, thermodynamics, electromagnetics, elastic mechanics, ether theory, and earth science. Leaving aside these, return to the theme of the "Thomson effect". Before introducing the Thomson effect, let me introduce the work done by the predecessors.

In 1821, the German physicist Seebeck discovered that in a closed circuit composed of two different metals, when the temperature at the two contact points is different, an electric potential is generated in the loop. This is called the Seebeck effect. Seebeck also later made measurements on some metal materials and arranged a sequence of 35 metals (ie Bi-Ni-Co-Pd-U-Cu-Mn-Ti-Hg-Pb-Sn-Cr-Mo- Rb-Ir-Au-Ag-Zn-W-Cd-Fe-As-Sb-Te -...), and pointed out that when any two metals in the sequence form a closed loop, the current will be from the earlier sorted metal Flow through the hot joint to the later sorted metal. In 1834, French experimental scientist Peltier discovered its opposite effect: the Peltier effect.

In 1856, Thomson used the thermodynamic principles he founded to comprehensively analyze the Seebeck effect and the Peltier effect, and established a relationship between the originally unrelated Seebeck coefficient and the Peltier coefficient. Thomson believes that at absolute zero, there is a simple multiple relationship between the Peltier coefficient and the Seebeck coefficient. On this basis, he theoretically predicted a new temperature difference electrical effect, that is, when an electric current flows through a conductor with a non-uniform temperature, in addition to generating irreversible Joule heat, the conductor must also absorb or emit a certain amount of heat (Called the Thomson fever). Or conversely, when the temperature of the two ends of a metal rod is different, a potential difference is formed between the two ends of the metal rod. This phenomenon was later called the Thomson effect, and became the third thermoelectric effect after the Seebeck effect and the Peltier effect.

The Thomson effect is a phenomenon that generates a potential when there is a temperature difference between the two ends of the conductor. The Peltier effect is a phenomenon that generates a temperature difference between two ends of a charged conductor (one of which generates heat and the other absorbs heat). Baker effect.

The physical explanation of the Thomson effect is that when the temperature is not uniform in the metal, the free electrons at higher temperatures have greater kinetic energy than the free electrons at lower temperatures. Like gas, thermal diffusion occurs when the temperature is not uniform, so free electrons diffuse from the high end of the temperature to the low end of the temperature, and accumulate at the low end, thereby forming an electric field in the conductor and forming a potential difference across the metal rod. This diffusion of free electrons continues until the effect of the electric field force on the electrons is balanced with the thermal diffusion of the electrons.

The Thomson effect is extremely weak due to the voltage generated, and has not yet been found in practical applications. (The flameout protection method in gas stoves --- Thermoelectric type: This device also uses the thermal energy generated during the combustion of gas. The thermoelectric flameout safety protection device consists of two parts, a thermocouple and a solenoid valve. Thermocouples are composed of two different types. Combination of alloy materials. Different alloy materials produce different thermoelectric potentials under the action of temperature. Thermocouples are manufactured using different thermoelectric potentials generated by different alloy materials under the action of temperature. It uses different alloy materials. Difference in electric heating.)

When searching the information, it was found that in addition to William Thomson, there was another English physicist by the same name, Joseph John Thomson (1856-1940), who proved that the cathode ray was actually an electron beam.

Peltier effect

Two different metals form a closed loop. When a DC current is present in the loop, a temperature difference will occur between the two joints. This is the Peltier Effect.

Maybe you still remember the Seebeck effect (also called thermoelectric effect, the temperature difference causes the potential at the junction of two metals) introduced earlier. The Peltier effect can be regarded as the opposite effect of the Seebeck effect. The Seebeck effect is usually called the first thermoelectric effect, the Peltier effect is called the second thermoelectric effect, and the Thomson effect to be described later is called the third thermoelectric effect.

The Peltier effect was discovered by the French scientist Peltier in 1834, so when it comes to Peltier's name, it is easy to associate him with the Peltier effect and mistakenly think that he is a physicist. In fact he was at most an amateur physicist.

JeanCharlesAthanasePeltier (1785 1845)

Born in Somme, France, Peltier was originally a watchmaker. At the age of 30, he gave up this profession and turned to experimental and scientific observation. In most of his many papers, most of them are about the observation of natural phenomena, such as sky electricity, tornadoes, sky blueness measurement and light polarization, sphere water temperature, polar boiling point, etc. There are also a few natural sciences.

In 1837, Russian physicists (Lenz, 1804 to 1865) discovered that the direction of the current determines whether heat is absorbed or generated. The amount of heat (cooling) is proportional to the magnitude of the current. The proportionality factor is called "Peltier" coefficient".

Q = · I = a · Tc · I, where = a · Tc

In the formula: Qexothermic or endothermic power

proportional coefficient, called Peltier coefficient

Iworking current

aTemperature difference electromotive force rate

Tccold junction temperature

The Peltier effect was found to have not been applied for more than 100 years because the Peltier effect of metal semiconductors is very weak. Until the 1990s, research by the former Soviet Union scientist Yu Fei showed that compounds based on bismuth telluride were the best thermoelectric semiconductor materials, which led to the emergence of a practical semiconductor electronic cooling elementThermoelectric Cooling (ThermoElectriccooling, (Referred to as TEC).

TEC kit (illustrated) (TEC + DC power supply), can be used as CPU and GPU heat sink

Compared with air cooling and water cooling, semiconductor cooling fins have the following advantages: (1) can reduce the temperature below room temperature; (2) accurate temperature control (using closed-loop temperature control circuit, accuracy can reach ± 0.1 ); (3 ) High reliability (refrigeration components are solid devices, no moving parts, life expectancy exceeds 200,000 hours, and low failure rate); (4) No working noise.

TEC basic working process: When an N-type semiconductor and a P-type semiconductor are formed into a galvanic couple, as long as a DC power supply is connected to the galvanic couple circuit, a current will flow through the galvanic couple, and energy transfer will occur at a contact. Exothermic (or endothermic) on the other side, and vice versa on the other contact.

The physical explanation of the Peltier effect is that a charge carrier moves in a conductor to form a current. Because the charge carrier is at different energy levels in different materials, when it moves from a high energy level to a low energy level, it releases excess energy; on the contrary, when it moves from a low energy level to a high energy level, it absorbs energy from the outside. Energy is absorbed or released as heat at the interface between the two materials.

In the TEC refrigeration chip, the semiconductor is connected by a metal deflector to form a loop. When the current passes from N to P, the electric field causes the electrons in N and the holes in P to flow in reverse. The energy they generate comes from the thermal energy of the crystal lattice. Therefore, heat is absorbed on the deflector, and heat is released on the other end, resulting in a temperature difference.

Peltier modules are also called heatpumps, and they can be used for both heating and cooling. A semiconductor cooling fin is a heat transfer tool. As long as the temperature of the hot end (the object to be cooled) is higher than a certain temperature, the semiconductor refrigerator starts to function, so that the temperature at the cold and hot ends is gradually balanced, thereby playing a cooling role.

History and development of thermoelectric effects

"Temperature difference power generation directly converts thermal energy into electrical energy. It can also be applied in the presence of only small temperature differences. It is a wide range of green and environmentally-friendly energy sources-it can even use human body heat to power various portable devices. Do turn waste into treasure. "Professor Tu Shandong and Associate Professor Luan Weiling of the School of Mechanical Engineering of East China University of Science and Technology believe that thermoelectric technology is becoming a hot spot in global research again and deserves the attention of China's science and technology research department ^[1] .

Regarding the mechanism of thermoelectric technology, the latest research progress in this field, the urgency of popularization and application, and the current breakthrough points that may be achieved, two experts engaged in energy material and equipment technology research accepted an exclusive interview with this reporter. Temperature difference power generation is realized through thermoelectric conversion materials, and the mark of the verification thermoelectric conversion material lies in its three basic effects: Peltier effect, Seebeck effect, and Thomson effect. "Associate Professor Luan Weiling said that it is these three effects that lay the foundation of thermoelectric theory in thermodynamics and also show broad prospects for the practical application of thermoelectric conversion materials. Among them, the Seebeck effect is the basis of thermoelectric power generation ^[1] .

In 1821, the German Seebeck discovered that in a circuit composed of two different metals (antimony and copper), if there is a temperature difference between the two joints, a magnetic field will appear around it, and further experiments revealed that there is an electromotive force in the circuit. The discovery of this effect laid the foundation for the manufacture of temperature measuring thermocouples, temperature difference power generation and temperature difference electrical sensors. Luan Weiling introduced that the thermoelectric conversion material directly converts thermal energy into electrical energy. It is an all-solid-state energy conversion method that does not require chemical reactions or fluid media. Therefore, it has no noise, no wear, no medium leakage, small volume, It has the advantages of light weight, convenient movement, and long service life. It has an "irreplaceable" status in special applications such as military batteries, remote space detectors, long-range communication and navigation, and microelectronics. With the deterioration of global environmental and energy conditions in the 21st century and the difficulty of fuel cells to enter practical applications, thermoelectric technology has become an attractive research direction ^[1] .

Luan Weiling described the working principle of temperature difference power generation. When two ends of two different types of thermoelectric conversion materials N and P are combined and placed in a high temperature state, the other end is left open and given a low temperature. The hole and electron concentrations are also higher than those at the low temperature end. Driven by this kind of carrier concentration gradient, holes and electrons diffuse to the low temperature end, thereby forming a potential difference at the low temperature open circuit end; if many pairs of P-type and N-type The thermoelectric conversion materials are connected to form a module, and a sufficiently high voltage can be obtained to form a temperature difference generator ^[1] .

According to reports, research on thermoelectric technology started in the 1940s, peaked in the 1960s, and successfully achieved long-term power generation on spacecraft. At that time, the Office of Space and Defense Power Systems of the US Department of Energy gave an appraisal that "thermoelectric power has been proven to be a reliable technology with low maintenance and long-term work in extreme harsh environments." In recent years, temperature difference generators have shown good application prospects not only in the military and high-tech areas, but also in the civilian area ^[1] .

Professor Tu Shandong introduced that in the field of remote space exploration, people have been continuously aiming to distant planets and even remote spaces outside the solar system since the middle of the last century. In these environments, solar cells are difficult to function, and the heat source is stable and structured. A compact, reliable, long-lived radioisotope temperature difference power generation system becomes an ideal choice. Because a coin-sized radioisotope heat source can provide continuous power for more than 20 years, which greatly reduces the load on the spacecraft, this technology has been used in the Apollo Lunar Module, Pioneer, Pirate, Used by Voyager, Galileo, and Ulysses ^[1] .

The application of radioisotope generators in the military field should not be underestimated. As early as the early 1980s, the United States completed the development of 500W to 1000W military temperature difference generators, and was officially included in military equipment in the late 1980s and placed in the deep sea as a component of the US missile positioning system network-radio signals. Forwarding system power. In 1999, the US Department of Energy launched the "Energy Harvesting Science and Technology Project" to study the use of thermoelectric modules to collect soldiers' body heat for battery charging ^[1] .

In addition, the advantages of small size, light weight, no vibration and no noise make the temperature difference generator very suitable for use as a small power source less than 5W, for various unmanned monitoring sensors, tiny short-range communication devices, and medical and physiological research Instrument-At present, related products have entered the practical stage. Recently, based on the Seebeck effect of thermoelectric conversion materials, scientists have also successfully developed many new temperature difference electrical sensors for low-temperature temperature measurement, single-pixel infrared and X-ray detection, hydrogen and other combustible gas leak detection ^[1] .

In the most attractive aspect of "turning waste into treasure", due to the nearly zero raw material cost and low operating cost, temperature difference power generation can completely achieve commercial competition with existing power generation methods. Seeing this prospect, Japan and the United States have launched a series of low-grade heat and waste heat and waste heat resource utilization projects in recent years. Utilizing heat sources throughout the industrial waste heat generated by chemical plants, steel industry, cement industry, paper industry, petroleum smelting industry, etc., rich in organic combustibles and extremely resource-efficient waste incineration heat, in automobile exhaust, cooling water, lubrication Waste heat from automobiles and solar radiation, natural heat such as solar radiant heat, ocean temperature difference heat, geothermal heat, and other dispersive heat sources such as waste heat from the remaining water in baths, heat dissipation from household heating stoves, etc. ^[1] .

Although there are many applications for temperature difference power generation, it has been restricted by thermoelectric conversion efficiency and large costs for a long time, and the popularization of temperature difference power technology to industry and civilian industry has been greatly restricted. Although the energy and environmental crisis has become increasingly prominent in recent years, and the development of a number of high-performance thermoelectric conversion materials has been successful, the research on thermoelectric technology has become a hot spot again, but the hope for breakthrough lies in the stable improvement of conversion efficiency ^[1] .

Luan Weiling introduced that the earliest temperature difference generator successfully developed in the former Soviet Union in 1942 had a power generation efficiency of only 1.5% to 2%. The temperature difference generators currently developed generally have a efficiency of 6% to 11%, which greatly limits its use. range. In this case, through in-depth research on thermoelectric conversion materials and development of new materials, continuously improving thermoelectric performance, and striving to increase electrical output power without changing the heat source have become the core content of thermoelectric technology research ^[1] .

Tu Shandong said that currently the developed countries of science and technology have included the development of temperature difference electricity technology in the medium and long-term energy development plan. Among them, the United States tends to applications in the military, aerospace and high-tech fields, Japan leads the world in waste heat utilization, and the European Union focuses on low-power power supplies, sensors, and product development using nanotechnology. China has a certain strength in the theoretical and application research of semiconductor thermoelectric refrigeration, but the research on thermoelectricity is still in its infancy. We must quickly increase the development efforts to realize the industrialization of thermoelectricity technology as soon as possible. The specific breakthrough point can be set in small thermoelectricity. Sensors and industrial and waste incineration power generation. Tu Shandong said that with the continuous research in the field of temperature difference electricity, many new concepts and application examples have appeared recently, including high energy density temperature difference power generation modules, cogeneration systems, heating cycle thermoelectric combustion systems, etc. With the further improvement of thermoelectric performance and the gradual maturity of manufacturing technology, mankind gradually resolves the energy crisis and eliminates environmental pollution caused by energy use. It will no longer be a dream ^[1] .