Multi-physical structural and geometric optimization of thermoelectric generator and its application with solar power and radiative cooling technology

Abstract

Thermoelectric technology has attracted tremendous interest in recent decades due to its special advantages: noticeably no emission, no noise, and no moving parts, making it a promising solution for waste heat recovery and renewable energy generation. Nonetheless, its widespread application is limited by its low efficiency compared to other existing energy conversion technologies. Therefore, there are still strong calls for further improving its output power and energy conversion efficiency from a large variety of practical engineering applications. This thesis addresses this challenge through systematic numerical and experimental studies into heat transfer mechanisms, distributed thermal and electric contact resistances (TCRs and ECRs), and the structural and geometric optimization of thermoelectric generators (TEGs) under both steady-state and transient conditions. The research also explored the application of TEG with solar power and radiative cooling technologies.
From the experimental perspective, detailed magnitudes of the distributed TCRs and ECRs at different interfaces in a TEG system and their specific influence on TEG performance are not fully revealed in the literature. To this end, the overall TCR and ECR, as well as their distributed values at different interfaces for the given interfacial materials (air, graphene sheet, and thermal grease) were obtained. The results suggested that the external TCR contributed more than 80% to the overall TCR in these three scenarios, which was far beyond its internal counterpart. These experimental studies were then incorporated with a series of numerical simulations to detail the impact of TCR locations and elaborate on how the distributed TCRs and ECRs affect TEG performance. Interestingly, it was found that as compared to the numerical analysis using the distributed TCRs across different interfaces, the conventional treatment of TCR as a lumped variable has led to a discrepancy of up to 16.9% in the TEG output power and 24.5% in efficiency (take the case at Th=523 K and Re=0.1 Ω as an example). These numerical findings indicate that not only the magnitudes of TCR but also its distributions do matter to TEG performance.
Based on these findings, this thesis developed different numerical models with the distributed TCRs and ECRs considered to further understand the mechanism within the TEG under different steady and transient conditions. It first revealed the magnitude of various types of heat, including conductive heat, Joule heat, Peltier heat, and Thomson heat. Then, structural and geometric optimization of TE legs under various steady-state and transient conditions were conducted. It showed that a trade-off is required between achieving the maximum output power and efficiency under the fixed temperature conditions. However, both targets can be satisfied simultaneously under fixed heat input conditions.
Besides, the numerical analyses under transient conditions further detailed the influence of key properties on TEG performance, such as the heat input waveforms, periods, duty cycles, and amplitudes, and compared their output power subject to three different types of waveforms (i.e., rectangular, sine, and triangular waveforms). It was found that the waveform characterized by “A1-0W-t1-t2” (standing for “a periodic heat input wave starting at a heating power of A1 and ending at 0W, with durations of t1 and t2, respectively.”, and is termed as “Pattern A”) could improve the average TEG output power, regardless of its specific waveform, with the rectangular waveform achieving the greatest improvement. On the other hand, all three waveforms with “ 0W-A1-t1-t2” (i.e., Pattern B) did degrade TEG performance, with the case using the rectangular waveforms suffering the most.
Then, nine different TE leg structures were numerically designed. Detailed geometric parameters of these TE legs were discussed under both the steady-state and transient thermal boundaries. It was found that under the given heat input conditions, the TE legs in the hollow cuboid, trapezoid j-U, j-L, and diamond shapes could significantly improve the output power, P, and output power per volume, PV, compared to cuboid TE legs used in conventional designs, regardless of the boundary conditions being steady-state or transient. Based on these findings, a novel TEG design using hollow trapezoid TE legs was proposed, and the numerical simulations show that in comparison to those conventional cuboid TE legs, this design can achieve up to 61% (average output power, Pave) and 193% (output power per volume, PVave) improvement under transient conditions. These results were also 16% and 51% higher than those using the best trapezoid j-U design.
Lastly, a 24-hour solar-powered radiative cooling TEG (SP-RC-TEG) system integrating the novel TEM with hollow trapezoidal TE legs was proposed. Through detailed numerical simulations, the performance of this system was examined with a large variety of thermal management schemes. It was found that TEG with appropriate thermal management can essentially improve the performance of SP-RC systems.

Date of Award	15 Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Yong (Sean) Shi (Supervisor), Yong Ren (Supervisor) & Yuying Yan (Supervisor)