A photo showing the appearance of the fabricated module is shown in Fig. 3. The module was precisely crafted with a size of 5.9mm x 7.0mm x 2.6mm with no visible cracking, chipping, unpeeling between layers, or other defects, and had a relative density of at least 95%. A scanning electron microscope (SEM) image of the junctions of the thermoelectric materials in this module is shown in Fig. 4. In this figure, the area surrounded by dotted lines is the insulation layer. As shown in the figure, there is no noticeable elemental dispersion between the layers, and the junctions of the thermoelectric materials are also clear.
Fig. 3 Photograph of monolithic multilayer module
Fig. 4 SEM image of p/n junctions
The temperature dependence of a no-load voltage of the fabricated module is shown in Fig. 5. As shown in the figure, the no-load voltage increases linearly as the temperature difference increases, and a value of 0.6 V is obtained at ⊿T=100°C, and 2.0 V is obtained at 300°C. For the power generation characteristics when connected to a load, a maximum value of 187 mW (at 0.15 A) is obtained for a temperature difference ⊿T=360°C. The generated power per unit area calculated from this value and the module heat transfer area was 450 mW/cm2. To develop this thermoelectric module for energy harvesting applications, the power generation performance at small temperature differences must be evaluated. Therefore, the generated power at ⊿T=10°C (hot side: 30°C, cold side: 20°C) was evaluated, and the results of this evaluation are shown in Fig. 6. As shown in this figure, the voltage at no load is 52 mV, and the maximum output was 105 µW (at 4 mA).
Fig. 5 Temperature characteristics of no-load voltage and maximum output
Fig. 6 Power generation characteristics at temperature difference of 10°C
Devices that operate using energy harvesting technology generally have two modes of operation. In one mode, the generated power exceeds the power consumption of the load so that continuous operation of the device is possible. In the other mode, the generated power is less than the power consumption of the device, and so the generated power must be stored in a capacitor, and the power is supplied to the load after a certain amount of charge is built up. For this process, the capacity of the capacitor storing the charge must be selected to match the characteristics of the operating device. For this study, we fabricated an integrated wireless sensor device as shown in Fig. 7 for using a temperature sensor and communication module as the load.
This device used a heater of approximately 40°C as the heat source and had a structure that enabled heat that passed through the thermoelectric module to flow to the casing. In this design, in an external environment of 25°C, a temperature difference of 10°C was obtained in the monolithic multilayer module. Temperature measurements were conducted once about every 10 seconds with this configuration, and it was possible to send this information wirelessly. To enable operation under additional loads such as for operation under lower temperatures, shorter operation intervals, and for driving of multiple sensors, the output characteristics of the module will have to be improved, and advancements are also needed for improved conversion efficiency of the materials, reduced loss in the module, and other issues.
Fig. 7 Energy harvesting module (Thermoelectric power generator + temperature sensor + wireless module)
Original Paper: "Monolithic oxide-metal composite thermoelectric generators for energy harvesting"