High temperature thermoelectric units using oxide materials

Human society has become much more prosperous by increasing its energy use. The combination of fossil energy and heat engines brought big changes to our life. It provides us fun to drive, warmth to escape from cold, brightness of illumination, many kinds of industrial products, etc. It is not too much to say that our society is controlled by energy. But these fuels should run dry in near future. Moreover, the carbon dioxide, occupying the largest amount of greenhouse gases is emitted from many places by combustion of the fossil energy. The total utilization efficiency of the primary energy is as low as 30% on the average with the remaining 70% exhausted to the air as the waste heat. It is clear that improving the total efficiency of energy conversion by capturing and using the waste heat could provide great impact to decrease in the energy consumption. However, since the waste heat is widely and thinly spread, no efficient conversion technologies from the waste heat to electricity have developed yet. Electricity is one of the highest quality and the most convenient energy forms in points of view of transportation, storage, and conversion. Thus, the direct conversion of waste heat to electricity is placed high expectations. Thermoelectric conversion using Seebeck effect is paid attention as the strongest candidate for the effective utilization of the diluted waste heat.
Oxide thermoelectric materials are considered to be promising ones because of their durability against high temperature in air, low cost for producing and toxicity etc. Thermoelectric modules using p-type Ca3Co4O9 (Co-349) and n-type CaMnO3 (Mn-113) have been produced using Ag paste to form junctions [1, 2]. The maximum power density against area of the substrate of the module reaches 4.3 kW/m2 at 973 K of the heat source temperature [3]. Cascade modules piled up the oxide and Bi2Te3 modules have been prepared. The maximum power density of 7.8 kW/m2 has been obtained at 1000 K of the heat source temperature.
Moreover, thermoelectric power generation units including the cascade modules have been demonstrated to recover waste heat from industrial furnaces and incinerators. The maximum out-put power was obtained over 700 W from the water-cooled thermoelectric unit. In order to improve the convenience of thermoelectric generation, water should be unwanted for cooling. After increase in the out-put power and durability of the thermoelectric modules using oxide materials, the air-cooled thermoelectric generation units have been developed.
The power factor is doubled by repetition of hot-forging technique for the Co-349 bulks. The out-put power of the thermoelectric module composed of Co-349 and Mn-113 devices is enhanced by 2 times. The maximum power density of the oxide module was increased to 7.2 kW/m2 against the total cross-sectional surface of the devices at 1073 K of the heat source temperature (TH) by water cooling at 293 K (Tc).
The durability against high temperature, heat cycling, and vibration of the oxide modules was investigated quantitatively. Long life time tests have been carried out for the oxide modules up to 1073 K of the heat source temperature by water circulation at 293 K under the air atmosphere. No degradations in both generated power are observed up to 1073 K of TH. The durability against heat cycling was investigated between 873 and 373 K of TH in air. The maximum out-put power is kept constant during more than 200 times of the heat cycling.
The air-cooled thermoelectric units have been developed using heat pipes for the cold side. The maximum out-put power reaches 2.2 W at 823 K of the heat source temperature. The power generation can be shown by lighting LED lamps, charging the smart phone and portable TV, and wireless transmission of data and moving images by the temperature sensor and web camera, respectively using the combustion of natural gas or firewood as the heat sources.
Reference
1. R. Funahashi, et al., Jpn. J. Appl. Phys., Vol. 39, L1127-L1129 (2000).
2. D. Flahaut, et al., J. Appl. Phys., Vol. 100, 084911 (2006).
3. R. Funahashi, et al. J. Mater. Res., Vol. 30, 2544-2557 (2015).