JP5093520B2 - Thermoelectric conversion system - Google Patents

Description

  The present invention relates to a thermoelectric conversion system in which strontium is used as a heat source, and the heat source unit and a thermoelectric conversion unit are combined. This technique can effectively use strontium 90, which is a problem as a heat source for high-level waste, and is useful as a power source for ships or remote islands, for example.

  The power source usually requires natural energy (hydropower, wind power, sunlight, etc.) or fuel (fossil fuel, nuclear fuel). When using natural energy, it is unstable because the energy supply depends on the external natural environment, and it is necessary to develop a specific environment (dam construction, etc.) or select (air volume, sunshine duration, etc.). Therefore, for example, it is often difficult to use natural energy with a power source for a remote island. On the other hand, when fuel is used, periodic refueling is required, and the cost of refueling is high. In particular, a power source that is stable and does not require refueling for a long period of time is desired for ships or remote islands.

  By the way, a power source that combines a radioisotope such as strontium 90 and thermoelectric power generation is conventionally known (for example, see Non-Patent Document 1). However, in this technique, since the thermal conductivity of the strontium compound is low, there is a limit to the size of the heat source part, and the design of a small-scale power source of several kW or less is limited to a special application. In addition, an RI battery using a sintered solidified body containing strontium has been proposed (Patent Document 1), but in the case of a sintered solidified body, both the heat generation density and the thermal conductivity are low, and the output density of the thermoelectric conversion system is assumed to decrease. Therefore, it is not suitable for a compact power source installed on a ship or the like.

Japanese Patent Laid-Open No. 06-138298

Paul J. Dick and John W. McGrew, “Application of Strontium-90 to Thermo-electric Power Generation”, Transactions of American Nuclear Society, vol. 15, p. 91

  The problem to be solved by the present invention is to solve the problem of heat transport in a heat source so that it can be used for general purposes, and to achieve both a large output and a compact power supply.

  The present invention comprises a heat source part in which a heat source pin in which a strontium heat source is covered with a cladding tube is embedded in a copper heat medium, and a thermoelectric conversion part, and the heat source part is in a state in which the heat source pin is in a vertical direction. It is arrange | positioned so that it may contact the heating side of the said thermoelectric conversion part, It is a thermoelectric conversion system characterized by the above-mentioned. As the thermoelectric conversion unit, an alkali metal thermoelectric conversion unit is typically used. For example, the heat source pin has a structure in which a Hastelloy cladding tube is filled with a strontium fluoride heat source and closed with an upper end plug and a lower end plug, and a plurality of the heat source pins are arranged in parallel in a copper heat medium. The heat source part is formed so as to be embedded.

  Such a thermoelectric conversion system has a replaceable module configuration, and can be easily configured as a large output thermoelectric power generation device of a necessary scale by arranging as many as necessary in two and two dimensions vertically and horizontally. it can.

  As the heat source, strontium in the reactor spent fuel can be used. Strontium 90, an isotope of strontium, has a high calorific value and a long half-life of about 30 years. Since strontium in the nuclear reactor spent fuel contains a large amount of strontium 90, it is a problem as a heat source for high-level waste. However, if this strontium is separated from waste and used as a heat source for thermoelectric conversion, the problem of high-level waste disposal can be solved, and it will be promising as a stable heat source for a long period of time.

  The thermoelectric conversion system according to the present invention has a structure in which a heat source pin, in which a strontium heat source is covered with a cladding tube, is embedded in a copper heat medium as a heat source part, so that there is a problem of heat transport of the heat source part (that is, strontium Since the thermal conductivity of the compound is low, the problem that there is a limit to the size of the heat source part) can be solved, and it is possible to achieve both large output and compactness of the power source. In addition, the present invention can be composed of a heat source part using strontium as a heat source and an alkali metal thermoelectric conversion part, both of which have a simple structure and no moving parts, and are therefore excellent in maintainability.

  Since the thermoelectric generator according to the present invention can be designed as an assembly of small modules that can replace the thermoelectric conversion system, even if a failure occurs, the influence is limited to the inside of a specific module, and repair is performed by module replacement. It can be easily handled. Therefore, the present invention can supply stable power over a long period of time, and does not require a high level of expertise for maintenance. Therefore, it is particularly useful as a power source for ships or remote islands, and can also be used as a stationary power source.

The conceptual diagram of the thermoelectric conversion system which concerns on this invention. Explanatory drawing of the thermoelectric generator which assembled and modularized it.

  FIG. 1 is a conceptual diagram of a thermoelectric conversion system according to the present invention. This thermoelectric conversion system includes a combination of a heat source unit 10 using strontium as a heat source and an alkali metal thermoelectric conversion unit 12. Here, it is set as the structure which installs an alkali metal thermoelectric conversion part below, and installs a heat-source part above it.

  The heat source unit 10 has a structure in which a heat source pin 20 in which a strontium heat source is covered with a cladding tube is embedded in a copper heat medium 22. For example, the heat source pin 20 has a structure in which a Hastelloy cladding tube 30 is filled with a strontium fluoride heat source 32 and closed with an upper end plug 34 and a lower end plug 36. The copper heat medium 22 is a quadrangular prism body, and has a structure in which a plurality of holes are formed in parallel at equal intervals so as to extend in the longitudinal direction. The plurality of heat source pins 20 are embedded in each hole of the copper heat medium 22. Here, the heat source pin 20 and the copper heat medium 22 need to be in close contact with each other in consideration of heat transfer performance. Therefore, specifically, for example, it is desirable to integrally couple the copper heat medium 22 and the Hastelloy cladding tube 30 by hot isostatic pressing (HIP processing). In that case, the copper heat medium and the Hastelloy cladding tube are integrated, and then the lower end plug is attached to the Hastelloy cladding tube, the strontium fluoride heat source is filled therein, and the upper end plug is attached. In addition, a method may be employed in which a copper heat medium is accurately manufactured and a heat source pin filled with strontium fluoride is mechanically inserted into the formed hole. In this case, since thermal expansion of the copper heat medium occurs during operation, good contact can be ensured between the heat source pin and the copper heat medium.

  As is well known, the alkali metal thermoelectric conversion (AMTEC: Alkali Metal Thermo-Electric Conversion) unit 12 is a direct thermoelectric conversion device combining an alkali metal and a solid electrolyte having alkali metal ion conductivity. Electricity is generated by the movement of the alkali metal in the solid electrolyte using the vapor pressure difference between the high temperature alkali metal and the low temperature alkali metal as the driving source. Since such an alkali metal thermoelectric converter itself has already been proposed in various structures, it can be used. In the example shown in FIG. 1, it is configured by a β-alumina power generation element 40 and a capillary tube 42, and the container 44 is filled with an alkali metal 46. The alkali metal 46 in the lower part of the container 44 is kept at a low temperature (about 250 ° C.) and is transported to the β-alumina power generation element 40 in the upper part of the container 44 by the capillary 42. Since the upper portion of the container 44 is in contact with the heat source section 10, the alkali metal 46 is evaporated at this portion and transported again to the low temperature portion. Electricity is generated through transport of the alkali metal from the low temperature part to the high temperature part. However, the alkali metal thermoelectric converter usable in the present invention is not limited to such a structure.

  Furthermore, in this invention, it is also possible to utilize thermoelectric conversion systems other than alkali metal thermoelectric conversion. For example, it is possible to adopt a method in which a known combination of Co—Sb and Bi—Te is used as a thermoelectric conversion element, a method in which a combination of a silicon compound and Bi—Te is used as a thermoelectric conversion element, and the like. Since these are equivalent to alkali metal thermoelectric conversion in use temperature, efficiency, etc., it is possible to adopt an optimum one for the thermoelectric conversion part according to the development situation.

  The heat source unit 10 as described above is placed on the upper surface of the alkali metal thermoelectric conversion unit 12 with each heat source pin 20 in a vertical direction. Since it is sufficient for the alkali metal thermoelectric converter 12 and the heat source 10 to transfer heat, it is only necessary to place the heat source on the alkali metal thermoelectric converter with the joint surface being sufficiently smooth. With such a structure, since the alkali metal thermoelectric conversion part and the heat source part can be independently replaced, maintenance and repairability are also improved.

  FIG. 2 shows an example of a thermoelectric generator that uses a thermoelectric conversion system as a module and aggregates the modules to increase output. The thermoelectric conversion module 50 includes a combination of the heat source unit 10 and the alkali metal thermoelectric conversion unit 12. As described above, the heat source unit 10 has a structure in which a heat source pin in which a strontium heat source is covered with a cladding tube is embedded in a copper heat medium. Here, if the vertical and horizontal dimensions of each thermoelectric conversion module 50 are about 10 × 10 cm (for 36 heat source pins), manufacture and handling are easy from the viewpoint of HIP processing. In addition, when the heat source part is replaced after the end of its useful life, or when a problem occurs and the heat source part is replaced, the module can be replaced for each other, so that maintenance and repairability are improved. Furthermore, since the heat source unit 10 is independent of the alkali metal thermoelectric conversion unit 12 as described above, the heat source unit 10 and the alkali metal thermoelectric conversion unit 12 of each module can be individually replaced and repaired. Such thermoelectric conversion modules 50 are juxtaposed two-dimensionally in the number and the number according to the required output to form a large-capacity thermoelectric generator 52. By appropriately selecting the number of modules installed according to the application, the power generation output can be freely adjusted. Further, if a thermoelectric conversion module is added as necessary, it is possible to easily cope with an increase in output.

As the operating conditions of the alkali metal thermoelectric converter, when the high temperature side is 500 ° C. and the low temperature side is 250 ° C., efficiency is 19.7%, heat flux is 10.8 W / cm ² , heat flux is important In this case, an efficiency of 10.1% and a heat flux of 33.1 W / cm ² are possible. Strontium heat source decay heat generation decreases as one proceeds, is 0.289W / m ³ at 0.448W / m ^3, 20 years cooled from the reactor is taken out at 5 years cooling. For this reason, it is possible to operate with a stable output by performing high heat generation and low efficiency operation at the beginning of operation and low heat generation and high efficiency operation at the end of operation.

As an example, a 1000 kW thermoelectric conversion system with a lifetime of 15 years was designed. The necessary heat source part dimensions were determined under these conditions. The required volume of the heat source is evaluated based on strontium attenuated at the end of the life of the thermoelectric conversion system (20 years after reactor removal) as a standard, and the electric capacity is 19.7% for efficiency and 10.8 W / cm ² for heat flux. The power density was 2.1 W / cm ² , and the height of the heat source part was 37 cm with strontium fluoride alone. In a state where the decay is not advanced at the beginning of the life (reactor removal 5 years), a heat flux of 16.7 W / cm ² can be provided, and the thermal efficiency on the alkali metal thermoelectric conversion side at this time is about 17%, and the electrical output Was estimated to be 2.8 W / cm ² . For this reason, even if a small evaluation is made, an output of ² W / cm ² is obtained constantly for 15 years. Assuming 1 MW output for the entire power generation facility, the total area of the thermoelectric conversion system is estimated to be 47 m ² . In this case, the total required amount of strontium for a 1 MW power generation facility was estimated to be 53 kg.

By the way, in a strontium heat source, heat removal property becomes a problem. In fact, in the case of a heat source height of 37 cm and a heat generation density of 0.448 W / cm ³ (reactor removal 5 years), if the thermal conductivity of strontium fluoride is 1.42 W / mK, the maximum temperature will be 20,000 ° C or higher. Therefore, in the present invention, a heat transport structure using a copper heat medium as shown in FIG. 1 is employed. In terms of dimensions, the Hastelloy cladding tube has an outer diameter of 13 mm, the strontium fluoride heat source has an outer diameter of 12 mm, an arrangement pitch of 16 mm, and the copper heating medium has a height of 100 cm. The alkali metal thermoelectric conversion part was 10 cm high. In this case, since the volume density of strontium fluoride is 44.2%, the required height of the heat source is estimated to be 85 cm. The heat generated from the strontium fluoride heat source is transported to the copper heat medium through the Hastelloy cladding tube, and is transported to the lower alkali metal thermoelectric conversion section by the copper heat medium.

  In order to confirm the effectiveness of the present invention, the temperature distribution was evaluated by numerical analysis. Since the heat source has a basically symmetric structure, the calculation focusing on the four heat source pins was performed. From the analysis result, the maximum temperature of the heat source part was evaluated as 929 ° C. (1202 K). The limit of the heat source part is 1000 ° C., which is the temperature limit of the contact part between the cladding tube and strontium fluoride, but it is evaluated that the temperature of the heat source part is lower than this, and the heat generated by the copper heat medium of the present invention The effectiveness of the transport structure was confirmed.

In the present invention, since both the alkali metal thermoelectric conversion part and the heat source part have a simple structure, the output can be freely selected according to the number of thermoelectric conversion modules. If the alkali metal thermoelectric conversion part in one thermoelectric conversion module is 100 cm ² (10 × 10 cm), the output per module is 0.2 kW. For 1000 kW, 5000 modules will be installed 100 vertically and 50 horizontally. In this case, the installation area is 10 × 5 m and is estimated to be 50 m ² . There are no movable parts in the alkali metal thermoelectric conversion part and the heat source part, and the cause of failure and maintenance items during operation are limited. The replacement required when the service life ends or a failure occurs can be dealt with by replacing the module. Also, as described above, the alkali metal thermoelectric converter 12 and the heat source 10 can be replaced independently, so that the maintenance is repaired. Also excellent in properties.

DESCRIPTION OF SYMBOLS 10 Heat source part 12 Alkali metal thermoelectric conversion part 20 Heat source pin 22 Copper heat carrier 30 Hastelloy cladding tube 32 Strontium fluoride heat source 34 Upper end plug 36 Lower end plug

Claims (4)

  A heat source pin in which a strontium heat source is covered with a cladding tube is embedded in a copper heat medium, and a thermoelectric conversion unit, and the heat source unit is in a state where the heat source pin is in a vertical direction. It arrange | positions so that the heating side of a part may be contacted, The thermoelectric conversion system characterized by the above-mentioned.   A heat source pin in which a strontium heat source is covered with a cladding tube is embedded in a copper heat medium, and an alkali metal thermoelectric conversion unit, and the heat source unit is in a state where the heat source pin is in a vertical direction. A thermoelectric conversion system, wherein the thermoelectric conversion system is disposed so as to contact the heating side of the alkali metal thermoelectric conversion section.   The heat source pin has a structure in which a Hastelloy clad tube is filled with a strontium fluoride heat source and closed with an upper end plug and a lower end plug, and the plurality of heat source pins are arranged in parallel in a copper heat medium. The thermoelectric conversion system according to claim 1, wherein a heat source part is formed by being embedded in the thermoelectric conversion system.   A thermoelectric power generator as a replaceable module in which the thermoelectric conversion system according to any one of claims 1 to 3 is replaced, and a large number of the modules are juxtaposed two-dimensionally in the vertical and horizontal directions.

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