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Research paper : Development of a small-size cogeneration system using thermoelectric power generation (R. Funahashi et al.)−87 Synthesiology - English edition Vol.1 No.2 (2008) module close to the burner. The thermoelectric module was installed in the space between the heat converter and the burner. Technologies necessary to manufacture the pipe-type module included technologies for: manufacturing thermoelectric element; joining of p- and n-type elements at low resistance and high strength; junction of element and pipe with high heat transfer, sturdiness, and electric insulation property; heat collection to transfer gas combustion heat into the module; and low cost manufacturing. To synthesize these technologies, “upstream” technologies were necessary for materials that not only possess high thermoelectric performance but also have high durability in natural gas combustion and high temperatures both chemically and mechanically, as well as materials to stably join electrode to element in high temperatures. Moreover, the material must not contain toxic and/or rare elements due to safety and cost concerns. Basic researches such as physics to design such material and chemistry for nanotechnology were also necessary. In this paper, basic research undertaken by the Authors to develop a small gas cogeneration system using thermoelectric power generation, intermediate integration technology that combines the technologies, and the power generation performance of pipe-type module installed in a water heater will be explained.5 Basic research: birth of new materialIn 1998, the Authors started a search for safe, inexpensive thermoelectric oxides that were stable in high temperatures and air. The design concept of the material was a low-dimensional material or a layer structure that was drawing attention at that time[3]. One of the Authors, Funahashi, has been working on superconducting oxides with layer structure, and has synthesized Co layered oxide derived from that research. However, this was removed from the list of development material since no significant property was found. Fortunately, upon assessing the thermoelectric property, it was discovered it had good p-type property in high temperatures and air. The composition of this oxide is Ca3Co4O9 (Co-349), and Figure 4 (a) shows the diagram of its crystal structure[4]. This oxide has a layer structure where CoO2 layer composed of octahedron with 6 Os arranged around Co and Ca2CoO3 layer with rock salt (NaCl) structure are stacked alternatively. The dimensionless thermoelectric figure-of-merit (ZT) [Equation(1)] at 973 K for oxide monocrystal was about 1.1. ZT = S2T/ρκ ... (1)Here, Z is called thermoelectric figure-of-merit, and when it is multiplied by absolute temperature T, it is called dimensionless thermoelectric figure-of-merit. S, ρ and κrepresent Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. Greater the ZT, better it is as thermoelectric material.ZT for Co-349 is normally at the same level as the highest figures for compound semiconductors in bulk, but these values are measured in vacuum, and only Co-349 shows high thermoelectric performance in high temperatures and air (Figure 4(b)).To construct an efficient thermoelectric generation system, development of n-type thermoelectric material was necessary. However, since it was extremely difficult to find excellent material as in the above case, the Authors developed high-efficiency search technology for thermoelectric materials using sol-gel synthesis to increase chances of discovery. Using this technology, LaNiO3 (Ni-113), an n-type material stable in high temperatures and air, was discovered, although its ZT was about 0.01 in 973 K and its performance was still insufficient[5]. We also succeeded in manufacturing thermoelectric generation module that brought out the performance of these oxides to a maximum, but the conversion efficiency was still about 1.5~2 %. Fig. 3 Technologies needed for gas/thermoelectric cogeneration system.We constructed “upstream” technology to respond to demand from “downstream”.・Pipe-type module・Heat transfer of element-water pipe・Electric insulation of element-water pipe・Adhesive strength of element-water pipe・Junction strength of element-electrode・Junction electric resistance of element-electrode・Thermoelectric performance of element・Mechanical property of element・High-temperature durability of element etc.Intermediate integration technologyIntermediate integration technology・Heat exchange・Heat collection, cooling・Mechanical strength・High-temperature durability・Generation performance・Mass production etc.Installed in water heaterInstalled in water heater・By natural gas combustion・Hot water supply・Superheated steam・Power generationGas-thermoelectric cogeneration systemGas-thermoelectric cogeneration system・Material design・Solid physics & chemistry・Nanotechnology・Crystal structure control・High throughput screening ・Departure from rare metals・Detoxification etc.Basic researchBasic researchFig. 4 Crystal structure of Ca3Co4O9 (Co-349) (a) and temperature dependence of dimensionless figure-of-merit ZT (b).Co-349 has a structure where electrically conductive CoO2 layer and insulating Ca2CoO3 layer are stacked alternatively. The ZT of monocrystal of this oxide was 1.1 at 973 K. This is equivalent to conversion efficiency of over 10 %. The performances of metal materials with high ZT are also shown in the graph. Excluding Co-349, ZT of all other materials were measured in vacuum.~~~~~~~~1.51.20.90.60.3040060080010001200 CoCa(b)(a)BiSbTeObca0.51.53ZnSbCoSbSiGeCo-34943CeFe3.50.512PbTeTemperature (K) Dimensionless figure-of-merit ZT(15)−

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