Development of application technology and hybrid genera-tor to improve practicality of thermoelectric generator

Abstract

This dissertation focuses on the development of application technology to improve the practicality of thermoelectric power generator and the implementation of new concept hybrid devices to overcome limitation in power conversion efficiency. The purpose of this study is to verify the feasibility of thermoelectric generator in real industrial environment through the output power optimization process by implementing a self-powered wireless sensor node using thermoelectric generator. In addition, by hybridizing the thermoelectric generator with photovoltaic module, propose a breakthrough that can overcome limitation in power conversion efficiency problems that energy harvesters faces. As industrial environments expand and become more automated, wireless sensor networks are attracting attention as an essential technology for efficient operation and safety. A wireless sensor node (WSN), self-powered by an energy harvester, can significantly reduce maintenance costs as well as the manpower costs associated with the replacement of batteries. Among the many studies on energy harvesting technologies for self-powered WSNs, however, the harvest-ed power has been too low to be practically used in industrial environments. In this work, we demonstrate a self-powered WSN driven by a flexible thermoelectric generator (f-TEG) with a significantly improved degree of practicality. A study was conducted to optimize the performance of the f-TEG for a particular WSN application, and an f-TEG fabricated with an area of 140 × 113 mm$^2$ harvested 272 mW of energy from a heat pipe at a temperature of 70℃. We also tested a complete self-powered WSN system capable of the remote monitoring of the heat pipe temperature, ambient temperature, humidity, CO2 and volatile organic compound concentrations via LoRa communication. The fabricated self-powered WSN system can wirelessly transmit the data at distances as long as 500 m. The second study is to fabricate a high-performance photovoltaic-thermoelectric hybrid generator by hybridized a thermoelectric generator and a photovoltaic module for the advancement of new and renewable energy technology and improvement of commercial feasibility. First, a high-performance monolithic photovoltaic-thermoelectric hybrid generator with a light-to-heat conversion layer was implemented. The layer, a thin acrylic film dyed with black colored AZO dye, effectively absorbs photons in the near-infrared region. The AZO dye supports an exothermic reaction through non-radiative relaxation. The hybrid generator was fabricated through monolithic integration. A thermoelectric (TE) module was fabricated directly on the back surface of the photovoltaic (PV) module without using the ceramic substrate typically employed in conventional TE modules, so that thermal resistance between the PV module and TE module could be minimized. The 51.35 cm$^2$ fabricated hybrid generator showed a power conversion efficiency of about 22.5% with an open-circuit voltage of 0.94 V and a maximum power of 1.15 W under standard AM 1.5G illumination. The hybrid power generator with the photo-thermal conversion layer showed about 40% higher output power compared to the control PV only, about 16% higher than an acrylic control film, and about 7.4% higher than a common black dyed interface layer. Furthermore, a series/parallel reconfigurable hybrid generator was also successfully implemented to supplement the passive aspect of thermoelectric generator. A series/parallel reconfigurable thermoelectric generator composed of several unit devices actively reconfigures the electrical configuration according to the output of the photovoltaic module. This means that the maximum output can be generated under all solar irradiation environments, unlike the conventional hybrid generators which have an optimal driving point only under the AM 1.5G environment. As a result, the cumulative harvested energy during the day was 3.3 times higher than that of the conventional hybrid generator in a area of 100 cm$^2$ and 4.3 times higher in 14,640 cm$^2$. The second hybrid generator deals with the successful implementation of a new concept high-performance hybrid generator by material-level hybridization of a dye-sensitized solar cell and a thermoelectric generator. Unlike conventional devices, charge generation and transport occur within one element by combining an N-type dye-sensitized solar cell and a P-type thermoelectric element. It excites electrons through photon absorption from sunlight and controls the movement of holes by using thermal energy generated by photons in the near-infrared region. In addition, the P-type thermoelectric element was able to generate a synergistic effect to further increase the power conversion efficiency by supplying electrons to the electrolyte of the dye-sensitized solar cell to promote the reduction of the dye. As a result, the hybrid generator achieved 10.8% of power conversion efficiency under the AM 1.5G environment, which in-creased by 33% compared to the dye-sensitized solar cell. It has achieved the highest level among the existing tandem-type devices and material synthesis-type devices. The third hybrid generator deals with the successful implementation of a wearable organic solar cell-thermoelectric hybrid generator by hybridizing a flexible thermoelectric generator and a flexible organic solar cell. The fabricated hybrid generator is flexible therefor, it can be worn on the human body and it is suitable for outdoor activities for a long time because it uses sunlight during the day and body temperature at night. In addition, the open-circuit voltage loss, a chronic problem of organic solar cells, could be minimized through the hybridization with thermoelectric generator. It was confirmed that the fabricated organic solar cell-thermoelectric hybrid generator had a higher circuit open circuit voltage, carrier recombination lifetime, and carrier concentration under all solar irradiation environments compared to organic solar cells. The increase in the open-circuit voltage of the hybrid generator was larger than the arithmetic sum of the open-circuit voltages of the organic solar cell and the thermoelectric generator. This could be interpreted as minimizing voltage loss due to non-radiative recombination of organic solar cells. The fabricated wearable hybrid generator was stable even after repeated bending 1,000 times and was able to generate up to 34 mWh during 30 minutes of outdoor activity during the day and 5.9 mWh at night.