Vol.1 No.2 2008

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Research paper : Development of a small-size cogeneration system using thermoelectric power generation (R. Funahashi et al.)−89 Synthesiology - English edition Vol.1 No.2 (2008) 6.2 Electric insulation technologyThermoelectric module to be installed in gas cogeneration system must convert water used for cooling into superheated steam at the same time as generating power. To accomplish this, both high heat transfer and electric insulation must be maintained between the water pipe (stainless steel) and the thermoelectric element. Water pipe and thermoelectric element were insulated by insulating paste and ZrO2 coating formed by thermal spraying on the surface of the water pipe. Here, the problem was breach in insulation due to of Ag from Ag paste that joined the element and electrode. The dispersed Ag pierced the ZrO2 layer and reached the water pipe. This dispersal was prevented by adding other elements to the Ag paste. Since technology for preventing Ag dispersal by adding Pd particles was already used in industry, we investigated the effect of compositing Pd paste into Ag paste composited with Co-349 powder.6.2.1 Sample manufacturing and assessment methodThe assessment method for breach of electric insulation due to dispersal of Ag to insulating paste is shown in Figure 6(a). Ag paste composited with 0~10 wt.% Pd paste was applied and solidified on an alumina plate. Insulating paste and silver sheet were layered on top, and the paste was solidified. After heat treatment of a layered sample at 1023 K for 30 h, conductivity was assessed with a tester. Five samples with varied Pd contents were assessed, and the insulation property was calculated.6.2.2 Electric insulation propertyFigure 6(b) shows the insulation property at different conditions. Insulation increased with increased amount of Pd paste, and in case of 10 wt.%, all samples maintained electric insulation property even after heat treatment at 1023 K for 30 h. Upon observation of fine structure under SEM, it was found that Pd composite prevented Ag from dispersing into the insulation paste.6.3 Structure and manufacture of pipe-type moduleThe structure of pipe-type thermoelectric module manufactured in this research is shown in Figure 7. The pipe-type module has a structure where 2 rows of elements composed of 27 pairs of elements sandwich the stainless steel water pipe. Ca2.7Bi0.3Co4O9 was used as sintered compact for p-type element, and CaMn0.98Mo0.02O3 for n-type element. In this research, we installed this module in a small water heater. The combustion room of the water heater was small, and the module had to be kept compact, so the number of thermoelectric elements was limited. However, numerous elements were necessary to obtain high voltage by thermoelectric generation. To solve this dilemma, we manufactured the module using CaMn0.98Mo0.02O3 which had higher S values than La0.9Bi0.1NiO3. ZrO2 coat of 60~70 µm was formed on the surface of the water pipe by thermal spraying to provide insulation between water pipe and thermoelectric element. Commercial insulating paste was applied over this to bond the element rows and the water pipe. The thickness of insulating paste after solidification was 150~300 µm. To create the element row, the thermoelectric element was formed into arch shape by grinding the sintered oxide, and using Ag paste composited with 6 wt.% Co-349 powder, it was joined at 1123 K under pressure of 50 kg/cm2 on the arch section. The element row was joined to the water pipe whose surface was insulated, to create a pipe-type module with a length of 30 cm (54 elements in pairs)[9].7 Construction of small gas-thermoelectric cogeneration system[10]Two of the above modules were bundled and installed in the main-stop type water heater (Figure 8). By gas combustion of water heater, exterior of the module was heated, and hot water (about 313 K) was supplied to the water pipe at flow Fig. 6 Assessment method for electric insulation of multilayer structure (a) and dependency on Pd paste content for insulation of multilayer sample after heat treatment at 1023 K for 30 h (b).Five samples were created with varying contents of Pd paste composites. The insulation property between Ag paste and Ag sheet mediated by insulating paste after heat treatment was assessed.00351020406080100(a)Silver sheetResistance measurementusing testerInsulating pasteSilver pasteAlumina substrate(b)Pd paste content (wt.%)Insulation (%)Fig. 7 Schematic diagram of Pipe-type module.The total length of the pipe is 30 cm. The space between thermoelectric element and stainless steel water pipe is multilayer of Ag paste, Ag sheet, insulating paste, and ZrO2. Although having thin structure is desirable to increase heat transfer, it is necessary to prevent breach of electric insulation by dispersal of Ag from the Ag paste.Thermoelectric elementAg electrodeInsulating pasteZirconium coatAg pasteThermoelectric elementCross section of module Stainless steel pipeStainless steel pipe30 cm4 mm(17)−

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