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Research paper : Development of a small-size cogeneration system using thermoelectric power generation (R. Funahashi et al.)−90 Synthesiology - English edition Vol.1 No.2 (2008) volume of 16 cm3/min from the water heater to produce temperature difference in thermoelectric element to generate power.During gas combustion, temperature around the thermoelectric module reached approximately 1473 K. When the water heater was burned at full force, open voltage (Vo) and maximum output (Pmax) reached 1.3~1.5 V and 0.28 W, respectively. After continuous generation for 1 h under heating condition where the Vo of the module reached 0.6 V or 1.0 V, combustion was stopped, allowed to cool to room temperature, and generation property was measured repeatedly. As a result, there was no deterioration of generation property by repeated heating and cooling for 1 h. Also, superheated steam of about 473 K was obtained from the end of the module water pipe. By installing pipe-type thermoelectric module that allowed direct heat conversion, an ordinary water heater became a multifunctional cogeneration system (Figure 9). Moreover, exhaust gas temperature was higher and CO partial pressure was reduced when gas combustion was done with pipe-type thermoelectric module installed in the water heater, compared to when water pipe without thermoelectric elements was installed. This was probably because by covering the water pipe with oxide thermoelectric element, incomplete combustion was prevented by halting the decrease in gas combustion temperature that occurred when water pipe with low surface temperature was installed.In general, waste heat recovery is considered to be the use of exhaust gas after completion of heat engine cycle (bottoming). Although natural gas burns at about 1473 K, the temperature of hot water that comes out of the water heater is 323 K at the most. This means that the heat energy produced by combustion is not efficiently used. Therefore, heat use with high total efficiency becomes possible if unused high-temperature heat energy (potential waste heat) can be used for thermoelectric generation while the water is heated using waste energy from thermoelectric conversion (topping). Heat recovery by topping is possible due to oxide material that can be used in high temperatures, and this is a new method for using thermoelectric generation.8 Future prospectsThe development of oxide thermoelectric generation system for efficient use of high-temperature waste heat was explained. It was necessary to start from thermoelectric material to construct this system. We were fortunate in this aspect, and were able to find Co layered oxide with excellent conversion efficiency and durability. This substance not only enabled practical application of thermoelectric generation in high temperatures and air, but also was highly acclaimed in the academic society as a demonstrative example of high thermoelectric performance by nano-block integration with different functions. However, for construction of a generation system, various technologies and know-hows for joining, electric insulation, and heat transfer described in this paper must be integrated and mass production technology must be developed along with the development of new high-performance materials. Also, reduction in use or alternatives to rare metals must be sought for Co for p-type material, La of n-type, and Pd used in paste to widely diffuse thermoelectric generation in the future.The market for thermoelectric generation is being developed at this moment. For practical application, we must create value for the users in thermoelectric generation using waste heat. To do so, it is necessary to give added function to the module in addition to thermoelectric conversion, or provide Fig. 8 Pipe-type module and main-stop water heater equipped with module.By gas combustion in main-stop water heater equipped with pipe-type module, the water heater produced hot water and the module produced superheated steam and electricity simultaneously.30 cmModuleFig. 9 Home water heater becomes cogeneration system. By installing thermoelectric module and conducting topping heat recovery, it was possible to produce electricity and superheated steam in addition to hot water. Also, the CO content in exhaust gas was reduced.Current gas water heaterPower generationMist saunagas water heaterPower outlet unnecessaryHeat exchangerHot waterHot waterGas cogeneration systemElectricity necessaryEquipped with thermoelectric moduleSuperheated steamCookerHeat exchangerthermoelec-tric moduleHarmless exhaust gas(18)−

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