Thermal electric generators are solid state gadgets that are usually applied to convert heat flux directly into electrical energy through a phenomenon known as Seedbeck effect. Among their applications include: in geothermal, power station, vehicle exhaust recovery and woodstoves. However, their relatively low efficiency when compared with typical conversion systems restrict their applications in large scale. Regardless, promising applications of the thermal electric generators have been found to be in small-scale applications, like the “self-powered” wireless sensor networks. In recent times, these wireless sensor networks have found wide scale applications but their power supply issue inhibit their usage. Herein, a thermoelectric energy harvesting system designed to harvest tens of microwatts to several miliwatts from low-voltage thermoelectric generators, based on a two-stage boost system with self-startup ability, has been proposed.
Researchers from Xiamen University and The Chinese University of Hong Kong led by Professor Wei-Hsin Liao proposed a study for a low-voltage thermoelectric energy harvesting and management system for powering wireless sensor nodes. The researchers aimed at developing a high efficiency boost converter system with self-startup ability for low-voltage thermoelectric generators for evaluation and comparison. Their research work is now published in the peer-reviewed journal, Energy Conversion and Management.
To begin with, the researchers combined a high-efficiency two-stage energy harvesting scheme working under normal working mode and a self-startup scheme working during self-startup in the energy harvesting system. They then applied the DCM mode and maximum power point tracking algorithm technique in the low-voltage converter for high efficiency. The maximum power point tracking technique intelligently adjusts the on/off times of the switches according to the open-circuit voltage of the thermal electric generators. The expressions of the optimal switching on/off times of the first-stage converter were derived. The research team then applied the low-power designs in switching mechanisms between the two energy harvesting schemes so as to reduce the quiescent power dissipation. Eventually, low power designs were applied to the MCU so as to reduce its power consumption and enhance the whole system’s efficiency.
From the experiments undertaken, the authors of this paper were able to observe that the first stage converter could achieve a high efficiency ranging 72% to approximately 87% for the thermal electric generators with open circuit voltage range of 62–400 mV to 1.255 V. For the low voltage starter, the energy harvesting system was observed to self-start from a low input voltage, as low as 20mV. Experimental results showed that with a 6.8 ohm thermal electric generators and an input voltage of 62 mV, the self-startup scheme would take 196.05 s to switch to two-stage harvesting scheme.
The empirical results obtained in their study suggest that when the wireless sensor network node sends signals every three minutes, the lowest open-circuit voltage for the energy harvesting system and wireless sensor network node to be self-powered is 62 mV, which is much lower than for the other converter used in the test. All in all, the whole system’s efficiency is much higher than the BQ25504 chip converter.
Mingjie Guan, Kunpeng Wang, Dazheng Xu, Wei-Hsin Liao. Design and experimental investigation of a low-voltage thermoelectric energy harvesting system for wireless sensor nodes. Energy Conversion and Management, volume 138 (2017) pages 30–37.Go To Energy Conversion and Management