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Rahman, Ehsanur

University of British Columbia

Thermionic converters are promising candidates for static heat-to-electricity conversion due to their flexible form factor, low maintenance requirement, high power density, and potential for high efficiency. However, practical applications of thermionic converters have been hindered in the past by various engineering challenges such as the space charge effect and lack of materials with desirable properties. With the advent of microfabrication and nanotechnology, there has been renewed interest in thermionic conversion in the recent past. In particular, the micro-gap architecture has drawn significant attention as an elegant solution to mitigate the space charge effect. For example, micro-gap thermionic converters could enable chip-scale power generators. However, to make this a reality, apart from overcoming the engineering challenges, a thorough understanding of the devices' operation is necessary. The work presented in this thesis addresses this need by laying a foundation for multiphysics computational models for micro-gap thermionic converters both for single-stage devices and for various hybrid configurations including other mechanisms. For the single-stage thermionic converter, we develop self-consistent iterative models that consider the thermal and charge balance to accurately determine the electrode temperatures, space charge, thermal radiative coupling between the electrodes and its possible enhancement at small gaps due to the near-field effect, and the electrical and thermal losses in the lead resistance. This model shows how the micro-gap device performance and electrode temperatures are affected by the interelectrode gap size under the constraint of finite input power. Moreover, we reveal how the cathode material and its thermal coupling with the input energy source determine the nature of the device's response to light and its combined photo-thermionic behaviour. We also develop multiphysics models to investigate the prospects of hybrid thermionic-thermoelectric and thermionic-photovoltaic devices. We show that these different mechanisms can be operated in a complementary manner due to their different optimal temperature ranges of operation. Additionally, we show that, depending on the conversion mechanism of the second stage, such a hybrid device may or may not be more efficient than a single thermionic device. The above models represent a powerful approach to designing thermionic converters, developing new device concepts, and understanding their operation.

Text

2022-08-23

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/82488

Electrical and Computer Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/