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.)−86 Synthesiology - English edition Vol.1 No.2 (2008) 2 Strategy for realization of thermoelectric power generationAs the energy issue is becoming serious, the expectation for effective use of waste heat is increasing. Waste heat includes a wide range of temperatures from cool waste heat of about 80 K to high temperatures of over 873 K, as well as various forms of heat in gas, fluid, and solid phases. The most convenient form of thermoelectric power generation that employs temperature difference is to use high-temperature waste heat. In general, high-temperature waste heat at a certain energy level can be recovered by heat converter using boilers. Therefore, in thermoelectric generation research, the main focus was on the development of material that functioned below 700 K. However, since the exergyTerm 1 decreases with decreasing temperature of waste heat, the thermoelectric generation system to recover heat will become large in scale. Diversification of energy conversion with such systems as medium- to-small-scale cogeneration system and boiler using biomass being in progress, effective use of waste heat was difficult from efficiency and cost aspects since practical amount of waste heat could not be obtained using the existing heat recovery system. In other words, in using waste heat for small systems, high temperature waste heat was preferred. However, this might decrease the efficiency of the thermal system to which the thermoelectric generation would be installed. The Authors, therefore, considered recovering energy through thermoelectric conversion at higher temperatures than required by the thermal system, and then operating the thermal system afterwards. Thinking from a different angle, we proposed a topping heat recovery system in which waste heat from thermoelectric generation was used in the “main house” thermal system (Figure 2). This system would improve energy efficiency of the entire system by optimizing energy use in thermoelectric generation and thermal system. In the development of the topping system, the Authors looked at water heaters using natural gas.In home-use gas water heater, the combustion temperature of natural gas reaches 1473 K, while the hot water obtained is only about 323 K at the most. It is very wasteful when looking just at temperatures. Therefore, the Authors conducted joint research with Osaka Gas Co., Ltd. to create a cogeneration system where topping heat recovery and water heating can be done simultaneously through thermoelectric conversion in gas water heater. Moreover, we attempted generation of superheated steam as well as hot water in this cogeneration system.3 Necessity of thermoelectric power generation in gas appliancesGas appliances used in homes including water heater, cooking stove, and fan heater require electricity for ignition or control of the device. Users are faced with inconveniences such as requirement of a power outlet in addition to a gas valve, inability to heat the house or make hot water during power outage, and having to pay for electricity . If the gas appliance can generate its own power so self-sustained operation is possible without power supply from an outlet, the usability will increase dramatically. Also, household use of superheated steam is becoming common such as for cooking and sauna. With the development of small steam generator, electrical appliances are ahead, but considering energy efficiency and instantaneity of heating, steam can be generated in high volume and a short time if gas combustion is used. However, due to issues of heat deterioration in heat exchangers and incomplete combustion from a decrease of flame temperature (production of CO), small steam generators using natural gas combustion have not been widely used. In other words, the key to development of small steam generator using natural gas is the development of technologies that protect the surface of the heat converter and prevent the decrease of flame temperature. To solve the above problems, it is effective to coat with material with excellent heat durability such as oxides, just to a level that does not compromise heat exchange property. If thermoelectric conversion function can be added to this coating layer, both steam and electricity can be generated simultaneously by gas combustion, and a new cogeneration system that is extremely useful to the user can be developed.4 Technological issues for gas-hermoelectric cogeneration systemFigure 3 shows the technological issues for constructing small cogeneration system using natural gas. We started our investigation from “downstream.” To produce superheated steam and hot water by gas combustion, it is necessary to heat cool water by heat conversion. Therefore, we decided that the thermoelectric module should be in pipe form, and the temperature difference would be created by heating the exterior of the pipe and by running water inside the pipe to conduct simultaneous thermoelectric generation and heat exchange. In the main-stop type water heater for home used in this study, the heat converter was located 15~20 cm above the burner, and the space between was empty. To generate superheated steam, it was necessary to bring the pipe-type Fig. 2 Concept of bottoming and topping waste heat recovery.Energy sourceBottomingWaste heatEnergy sourceToppingWaste heatThermoelectric generation systemThermoelectric generation systemThermal system, heat exchange, fuel cell, etcThermal system, heat exchange, fuel cell, etc(14)−

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