| Author: | Liang, Zhaojian |
| Title: | Theoretical investigations of the transient characteristics of solid oxide electrolysis cells (SOECS) under unstable operational conditions |
| Advisors: | Li, Mengying (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Energy storage Renewable energy sources Solid oxide fuel cells Electrolytic cells -- Mathematical models Hydrogen Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxi, 95 pages : color illustrations |
| Language: | English |
| Abstract: | A promising solution to the storage of intermittent renewable energy is the integration of solid oxide electrolysis cells (SOEC) with solar/wind power. This trend requires comprehensive and quantitative investigations on the transient characteristics of SOEC, especially under varying power supply conditions. For this purpose, a high-resolution, 3-dimensional, transient numerical model, as well as an adaptive time-stepping strategy, is proposed in this study. This study analyzes the electrical, gaseous, and thermal responses of SOEC to voltage ramps with different ramp rates and ramp magnitudes. The results show that electrical undershoots or overshoots occur after fast voltage changes. This phenomenon reflects the discrepancies between the steady and transient current-voltage characteristics and may lead to unsteady hydrogen production rates in practice. The electrical undershoots or overshoots are caused by the different transfer rates in SOEC – electronic/ionic transfer rate is faster than the mass transfer rate and the mass transfer rate is faster than the heat transfer rate. Furthermore, electrical undershoots or overshoots can be divided into two parts. One part is related to mass-transfer lag, and the other part is related to heat-transfer lag. The former can be alleviated or eliminated simply by slowing the voltage ramp, while the latter needs a more effective control strategy, rather than simply adjusting the voltage ramps. Apart from the electrical conditions, cell structure also has significant impacts on the electrical responses, e.g., the rib and the length of channel are related to the non-uniform electrical responses in the functional layer. Upon expanding our study from a single solid oxide cell (SOC) to various types, we encounter the inherent challenge of diverse specifications and operating conditions that hinder the generalizability of design and control strategies. General formulas describing the relationship between SOC transients and multiple parameters remain elusive. Through comprehensive numerical analysis, we find that the thermal and gaseous response times of SOCs in rapid electrical variations are on the order of two characteristic times (τh and τm), respectively. The gaseous response time is approximately 1τm, and the thermal response time aligns roughly with 2τh. These characteristic times represent the overall heat and mass transfer rates within the cell, and their mathematical relationships with various SOC design and operating parameters are revealed. The validation of τh and τm is achieved through a comparison with an in-house experiment and data from the existing literature, achieving the same order of magnitude for a wide range of electrochemical cells, showcasing their potential use for characterizing transient behaviors in a wide range of electrochemical cells. Moreover, two examples are presented to demonstrate how these characteristic times can streamline SOC design and control without the need for complex numerical simulations, thus offering valuable insights and tools for enhancing the efficiency and durability of electrochemical cells. To facilitate the broader application of SOECs from individual cells to integrated systems, it is crucial to develop control strategies that mitigate temperature gradients and the rates of change in temperature within the cell. This is essential for maintaining the safety and longevity of SOECs, especially under the variable conditions imposed by fluctuations in solar power. Recognizing that the supply of the reactant influences the current, a novel control strategy is developed to modulate the internal heat source in the SOEC by adjusting the steam flow rate. The effectiveness of this strategy is assessed through numerical simulations conducted on a coupled photovoltaic (PV)-SOEC system using actual solar irradiance data, recorded at two-second intervals, to account for rapid changes in solar exposure. The results indicate that conventional control strategies, which increase airflow rates, are inadequate in effectively suppressing the rate of temperature variation in scenarios of drastic changes in solar power. In contrast, our proposed strategy demonstrates precise management of SOEC internal heat generation, thus reducing the temperature gradient and variation within the cell to less than 5 K cm−1 and 1 K min−1, respectively, and maintaining a high electricity-to-hydrogen conversion efficiency of 94.9%. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/13398