RU2786119C1 - Thermoelectric generator and method for its manufacture - Google Patents

Abstract

FIELD: thermal generators.

SUBSTANCE: invention relates to thermal generators based on the Seebeck effect and can be used for power supply of unattended equipment complexes in hard-to-reach areas. The thermal power generator contains branches connected in series with metal conductors, separated by insulating inserts and placed between electrically insulated hot and cold heat conductors. The plates forming the thermocouple branches are oriented in a plane parallel to the heat flow, separated by thin layers of a high-temperature dielectric, and connected in a series circuit along the end surfaces by welding. The electrical insulation of heat conductors is represented by an oxide layer on their contact surfaces. As materials of thermocouples, the choice of a combination of constantan-chromel is preferable. In the manufacture of a thermoelectric generator, metal plates are assembled into a package, alternating dissimilar metals and laying dielectric layers between them, fixed with a coupler, leveling the common surface of the ends. Pairwise welding of metal plates is carried out using a deposited electrode based on a copper alloy. After that, the surfaces of the welds are ground to the 5th roughness class, covered with a heat-conducting paste and installed between the hot and cold heat pipes.

EFFECT: expansion of the operating temperature range, reduction of the occupied area, reduction of heat losses.

2 cl, 7 dwg

Description

The invention relates to thermoelectric generators based on the Seebeck effect and can be used for power supply of unattended equipment complexes in hard-to-reach areas.

The principle of operation of thermoelectric generators is based on the occurrence of thermoelectric power in the contact area of various conductive materials that form thermocouple branches. Historically, thermoelements were made from matched pairs of metals, later replaced by semiconductors. Thermoelectric generators are attractive in that they do not contain moving parts, and only a heat source is required for their operation. This quality allows them to be used where maintenance is rarely provided, for example, at points of electrochemical protection of main pipelines. In conditions of inaccessibility, the priority is the reliability of equipment, achieved at least at the expense of other indicators, including efficiency. For example, diesel, gasoline and gas generators have an efficiency of converting thermal energy into electricity at the level of 30-40%, while thermoelectric generators (TEG) - at the level of a few percent. However, TEGs are increasingly being used precisely because of their potentially higher reliability.

The consumer characteristics of TEG are evaluated by the quality factor, which relates the heat flux to the generated power per unit area. The quality factor is defined as Z=α ² /ρχ, where α is the Seebeck coefficient of the thermogenerator elements, µV/deg, ρ is the resistivity, µOhm⋅m, χ is the thermal conductivity, W/m⋅°K.

The Seebeck coefficient ranges from tens to hundreds of μV/deg for different materials; therefore, to obtain an acceptable voltage, hundreds of branches have to be connected in series.

A typical design of a semiconductor TEG, shown in Fig. 1 comprises a series of rectangular semiconducting posts soldered in a series circuit by metal conductors and placed between electrically insulated hot and cold heat sinks.

There are a number of TEGs produced by Kryotherm LLC specifically for remote unattended points [https://layothermtec.com/assets/dir2attz/solution_rus.pdf] and operating on the thermal energy of flared gas. TEG modifications include generators with power from 30 to 1000 W.

Technologies for improving the efficiency of TEGs are constantly evolving. However, serious achievements are associated with extreme labor intensity and high cost. The complexity of TEG manufacturing can be illustrated by patents [RU 2624615], [RU 2518353], which provide sequences of dozens of complex technological operations. In addition, despite the measures taken, semiconductor TEGs are not free from such shortcomings as degradation of characteristics due to diffusion of constituent elements, exposure to air oxygen and water vapor, and thermomechanical cycles. In this regard, the task of creating the most reliable TEG becomes urgent. Paradoxically, in the light of these requirements, a real alternative arises to return from semiconductor TEGs to metal ones that existed back in the 19th century. Despite the fact that typical metal thermocouples have a Seebeck coefficient that is several times lower than that of semiconductor thermocouples, their operating temperature range can be extended without sacrificing reliability to such an extent that this difference is leveled. In addition, the cost of metallic materials and technological operations performed is many times less than the cost of semiconductor materials and technologies.

Known thermoelectric generator [RU 2131156], taken as a prototype, containing series-connected metal conductors branches that create emf of different polarity, separated by insulating inserts and placed between electrically insulated hot and cold heat conductors. In the prototype, these branches are made in the form of thin plates (layers) oriented in a plane perpendicular to the direction of heat flow, as shown in Fig. 2. This overlapping plate configuration implies a way of joining them by soldering, which is also evidenced by the name of metal conductors by soldering. This circumstance limits the operating temperature of the generator to the melting temperature of the solder. The plenary arrangement of branches implies an irrational use of the area in terms of the need to place hundreds of branches. The small thickness of the planar structure implies its high thermal conductivity, against which the heat losses of heat pipes with electrical insulation layers begin to play a significant role, reducing the overall efficiency.

The technical result of the invention is the expansion of the operating temperature range, the reduction of the occupied area, the reduction of heat losses, as well as the reduction in the cost of materials and technological operations.

This result is achieved by the fact that the heat generator containing branches connected in series with metal conductors, creating an emf of different polarity, separated by insulating inserts and placed between electrically insulated hot and cold heat conductors, is distinguished by the fact that in it the plates forming the branches of thermocouples are oriented in a plane parallel to the thermal flow, separated by thin layers of a high-temperature dielectric and connected in a series circuit along the end surfaces, and the choice of a combination of constantan-chromel is preferable as thermocouple materials, the end surfaces are connected by welding, and the electrical insulation of heat conductors is represented by an oxide layer on their contact surfaces.

This result is also achieved by the fact that in the manufacture of TEG, metal plates are assembled into a package, alternating dissimilar metals and laying dielectric layers between them, fixed with a coupler, leveling the common surface of the ends and carrying out their pairwise welding using a deposited electrode based on a copper alloy, after which they are ground surfaces of welded seams up to the 5th class of roughness, are covered with a heat-conducting paste and installed between hot and cold heat pipes.

The achievability of the technical result is determined as follows.

The choice of the constantan-chromel combination as the thermocouple material corresponds to the highest quality factor for widely used alloys. This excludes materials with a higher thermoelectric power, such as antimony (high brittleness and toxicity) and bismuth (low melting point). Despite the fact that the Seebeck coefficient is maximum for a pair of chromel-kopel, the difference in thermal conductivity and electrical conductivity levels out the quality factors for two pairs of chromel-constantan and chromel-kopel, however, kopel is much more expensive than constantan, so its use can be considered unreasonable.

Welding the ends of the plates compared to soldering provides the maximum operating temperature and mechanical strength, which is important in conditions of thermal deformation. The copper-based alloy fuses well with the plates during the welding process, without leading to the formation of craters. The welding seam rises above the surface of the ends, creating conditions for their grinding without understating relative to the original surface. The orientation of the planes of the plates forming the branches of thermocouples, parallel to the direction of the heat flow, reduces the area occupied by a single element, which significantly reduces the dimensions of the device. This arrangement also makes it easy to separate the plates with a high-temperature dielectric pad, such as mica or glass cloth 0.1 mm thick. Since, for reasons of low ohmic resistance and ease of welding, the thickness of the metal plates should be 0.7-1 mm, the insulating layers slightly increase the thickness of the package.

The oxidation of heat conductors, which are usually made of aluminum, provides thermal contact with a minimum thickness of the electrically insulating layer. With a typical oxide thickness of 20 μm, its thermal resistance is an order of magnitude smaller than the thermal resistance of thermocouple legs. Preferably the so-called cold oxidation, characterized by a greater density and strength of the coating. It is also preferable to make a heat conductor from an AMg-2 alloy, which has a relatively high thermal conductivity and temperature resistance. The application of thermally conductive paste eliminates air gaps that increase thermal resistance. It has been experimentally established that the domestic paste KTP-8 retains its performance at least up to 480°C, and the performance of the domestic lubricant MS-1600 is announced up to 1000°C. The use of imported pastes with a thermal conductivity of more than 1 W/m⋅°K (in particular, pastes with a thermal conductivity of up to 10 W/⋅°K are advertised) is not possible, since their declared operating temperatures do not exceed 200°C. It is desirable to have a minimum thickness of the gap filled with paste, on the order of 10 μm, so that the thermal resistance of the gap is a small part of the thermal resistance of the plates. In this regard, it is preferable to grind the ends of welded thermocouples to roughness class 5, characterized by a height of irregularities of the order of 5 μm. More careful polishing is redundant. In addition to improving the quality of the thermal contact area, the paste allows the contact surfaces to slide, reducing mechanical stress during temperature changes.

The calculation of the performance of a thermoelectric generator is based on the known physical characteristics of the materials used.

The thermoelectric power of the constantan-chromel pair is 2.95+3.4=6.35 mV/100°K.

The resistivity of the series constantan-chromel pair is 0.5+0.68=1.18 μOhm⋅m.

The thermal conductivity of a parallel pair of constantan-chromel is on average 25+25=50 W/m⋅°K.

Let us consider an assembly of plates, which has dimensions 30 × 30 mm, typical for semiconductor analogs. Let's take the assembly height as 10 mm. With a plate thickness of about 0.8 mm, there are 15 pairs in the assembly, creating a total EMF equal to 6.35×15≈95 mV/100°K. If you set the temperature difference between the heat sinks of the order of 400°K, the EMF will be 95×4=380 mV. The internal resistance of the thermoelectric generator, equal to the sum of the resistances of the plates, will be 0.007 Ohm. The power released in the load resistance, equal to the internal resistance, will be 4.92 W at a current of 25.8 A. To interface with a real consumer, a series of individual assemblies must be connected in series, as is done in all analogues. The thermal power passing through the thermoelectric generator under the above conditions will be 720 W.

Let's compare these characteristics with the characteristics of the serially produced Indigirka furnace" [https://teplo.guru/pechi/indigirka.html]. It combines 24 thermogenerator assemblies with a total output power of 50 W at a thermal power of 4000 W, or 2 W of electrical power and 167 W of heat per assembly. The comparison shows that the proposed generator is capable of delivering 4.9/2=2.45 times more electricity to the load, consuming 720/167=4.3 times more heat. At the same time, it has a significantly lower cost and incomparable reliability.

The invention is illustrated by the illustrations of Fig. 1-7. In FIG. 1 shows a typical semiconductor generator containing a plurality of columns connected in series. In FIG. 2 shows the structure of a thermoelectric generator according to the patent [RU 2131156], taken as a prototype, with plenary branches oriented perpendicular to the direction of the heat flow. In FIG. 3-7 shows the stages of manufacturing the inventive thermoelectric generator with branches oriented along the direction of the heat flow: FIG. 3 - the formation of a package of alternating plates of constantan and chromel, separated by insulating gaskets, fig. 4 - alignment of the end surfaces, Fig. 5 - welding of ends with filling of seams with a consumable electrode, fig. 6 - grinding of seams, fig. 7 - installation of heat pipes.

The thermoelectric generator, as shown in Fig. 7, contains branches 2, 3 connected in series by metal conductors 1, represented by welds, creating an emf of different polarity, separated by insulating inserts 4 and placed between electrically insulated hot 5 and cold 6 heat conductors. When operating as part of a generator set, the cold heat pipe communicates with the external air or liquid cooler, the hot heat pipe communicates with a heat source, such as a gas burner. The temperature difference at the ends of the branches causes a thermoelectric power, which creates a current in the external circuit, determined by the sum of the internal resistance of the branches and the load resistance. The equality of the indicated resistances ensures the transfer of maximum electric power to the external circuit.

In the manufacture of a thermoelectric generator, simple operations are carried out that do not require scarce materials and high technologies.

Claims (2)

1. A heat generator containing branches connected in series by metal conductors that create an emf of different polarity, separated by insulating inserts and placed between electrically insulated hot and cold heat conductors, characterized in that the plates forming the thermocouple branches are oriented in a plane parallel to the heat flow, separated by thin layers of a high-temperature dielectric and connected in a series circuit along the end surfaces, and the choice of a combination of constantan-chromel is preferable as thermocouple materials, the end surfaces are connected by welding, and the electrical insulation of heat conductors is represented by an oxide layer on their contact surfaces. 2. A method for manufacturing a thermoelectric generator according to claim 1, characterized in that the metal plates are assembled into a package, alternating dissimilar metals and laying dielectric layers between them, fixed with a coupler, leveling the common surface of the ends and carrying out their pairwise welding using a deposited electrode based on a copper alloy , after which the surfaces of the welds are ground to roughness class 5, covered with a heat-conducting paste and installed between the hot and cold heat pipes.

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