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Title: Structure and properties of one-dimensional fiber-based thermoelectric generators
Authors: Zhang, Lisha
Degree: Ph.D.
Issue Date: 2019
Abstract: To collect scavenged energy from objects with large and three dimensional surface like human body, it requires the energy harvesting devices to be flexible, deformable, stretchable and light weight. Traditional rigid thermoelectric (TE) generators cannot fulfill this purpose, although they can convert thermal energy into electrical energy directly without any moving parts or working fluids. In the past, very few research has been reported on flexible and conformal fiber-based TE generators (FTEGs). Therefore, this thesis focuses on one-dimensional (1D) FTEGs, that is, only fiber or yarn structures will be considered and they can be further fabricated into two-or three-dimensional devices. A systematical research is conducted on the geometric structure of 1D FTEGs and their thermoelectric performance. Based on a systematic literature review, the research gaps were identified. A theoretical model of 1D FTEGs was designed analytically and numerically simulated with COMSOL MultiphysicsR, a finite element software package. From the structure viewpoint, the 1D FTEGs were designed as a coaxial shell/core structure: TE layer and electrodes were the shell; fiber/filament was the core. Initially, the convergence of the numerical simulation was verified with appropriate discretization approach. The reliability of the numerical simulation was examined and confirmed through the comparison of simulation results with those from the previously published articles and the analytical solutions of the theoretical models. Then, based on the appropriate mesh configuration, the output power and energy conversion efficiency of 1D FTEGs with different geometric parameters were derived. Their influencing factors were studied in three cases of different conditions: (1) conduction heat transfer only; (2) conduction with thermal and electrical contact resistance (TCR and ECR); (3) conduction and radiation heat transfer. In the first two cases, the hot and cold side temperature were fixed as constants. In the third case, the cold side was a free end, whose temperature was determined as the result of radiative and conductive heat transfer. The geometric parameters included the radius of filament, the thickness and length of TE layer. The simulations showed that, among all these parameters, the thickness of TE layer was the primary factor, because it brought the highest variation in the maximum output power and energy conversion efficiency in several orders of magnitude. Besides, although the large TCR and ECR caused the deterioration of the device's performance, they can hardly lead to the decrement in orders of magnitude. Finally, the influence of the radiative heat transfer was rather complex, indicating the increment in filament radius resulted in the increasing efficiency first and then decreasing one. In order to verify the theoretical analysis and numerical simulation of 1D FTEGs, experimental investigations were carried out. 1D FTEG samples were fabricated with poly(ethylene terephthalate) (PET) filament and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT: PSS), one polymer thermoelectric (TE) material. The TE material was characterized by thermo-gravimetric analysis (TGA), crystalline structure with X-ray diffraction (XRD), Seebeck coefficient, thermal conductivity and electrical conductivity. The 1D FTEG samples were characterized with scanning electron microscope (SEM) and an optical microscope. The energy conversion performance of these samples were measured with a lab-made measurement system, which was composed with a heat source, real-time temperature measurement and output electrical potential measurement. Finally, the experimental results were compared with the numerical solutions. The experiment results showed good agreement with that from the model and simulation: the variation of TE layer thickness caused the huge variation (in serval orders of magnitude) of maximum output power.
The 1D FTEGs are commonly used in an array. The radiation influence in FTEG arrays would be much more prominent as the 1D FTEGs were closely packed. Therefore, an assembly unit of parallel 1D FTEGs was further considered by taking account of the surface-to-surface radiation and the air conduction simultaneously. This investigation has been divided into two parts: the first part was the exploration of the necessary and limitation of considering these progress of thermal transmission; the second part focused on the issue of the performance of the assembly unit of parallel 1D FTEGs under the condition of various emissivity and distance between 1D FTEGs. For the two parts of this investigations, the temperature at the hot side was fixed as a constant. But the cold side was set as a free end. In order to accomplish the exploration assignment, the theoretical models were design as the 1D FTEG being encircled with a thin wall. Under distinct thickness of TE layer conditions, the influences on output performance were demonstrated with various distances between the adjacent surfaces of 1D FTEGs and thin wall with a series of varied emissivity. Under this condition, the decrease of temperature at the cold side was caused by two factors: the major one was the increasing distance between the surfaces of 1D FTEG and thin wall; another was the enlarged emissivity. Whereas, for the second part of the investigation, in the assembly unit of 1D FTEG array, the decrement of temperature at free end was the result of the increasing emissivity as the primary reason and the increment of the distance as the second one. For all cases, if the temperature at the free end dropt down, the output power of devices raised up. In this study, the multi-physics models have been established and used to explain the relationships between the performance and geometric structure of 1D FTEGs, for the first time. The models of 1D FTEG arrays were designed, whose performance and the influential factors were also explored. A simple and feasible method has been developed to fabricate 1D FTEGs, which can facilitate the application of these devices. More 1D FTEGs and their arrays are expected to be fabricated and characterized in the future. Then, the experimental results will be compared to the simulation results of the developed models. This comparison may help to modify the current models if necessary. The study of 1D FTEGs also provides a solid foundation for the development of FTEGs in two- or three-dimensions. For the long term, this study could provide engineering guidance to the design and fabrication of FTEGs.
Subjects: Hong Kong Polytechnic University -- Dissertations
Thermoelectric generators
Thermoelectric generators -- Materials
Pages: xi, vii, 204 pages : color illustrations
Appears in Collections:Thesis

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