|Title:||Modelling and optimization of solid oxide cells for energy conversion and storage|
|Advisors:||Ni, Meng (BRE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Building and Real Estate|
|Pages:||xxiv, 109 pages : color illustrations|
|Abstract:||Solid oxide cell (SOC) is considered as a promising energy conversion device in the context of penetration of renewable and sustainable energy to the market. The intrinsic high temperature endows it various advantages, including high energy conversion efficiency, fuel flexibility, environmental friendly and the possibility of heat recovery in combined heat power systems. Besides, the reversible operating of SOC at solid oxide electrolyser cell (SOEC) mode additionally casts a light on the electricity storage realized by the power to gas process in SOEC, which is attractive in grid balancing and distributed energy system. The understanding of the highly coupled multiphysical processes inside SOC is necessary for researchers to optimize the microstructure of SOC anode-electrolyte-cathode assembly and the further integration with other power devices or chemical processes such as gas turbine, electricity engine, refrigeration cycles and hydrocarbon synthesis, etc. Modelling of SOC using computational fluid dynamics (CFD) theories offers an efficient option for researchers to understand and predict the coupled multiphysical processes with a good balance of the computation cost and labor cost, compared to the traditional experimental testing and other atomic level simulations such as molecular dynamics and ab initio method. Under the environment of COMSOL, a powerful multiphysics CFD simulation tool, the author developed comprehensive models of solid oxide fuel cell (SOFC) and SOEC at the single cell level, serving for the performance enhancement study and application investigations as abstracted below: In chapter 2, a numerical model is developed for a button SOFC, focusing on the effects of finger-like channels on the gas transport process in anode support. The performance of channelled button cell and un-channelled button cell are compared at different operating temperature and voltage with H₂ as the fuel. The H₂ transport is detailed discussed, such as the mole fraction in the anode support, the diffusion flux and the convective flux of H₂. Then, the model is further developed to study 2D-plannar cell fuelled with syngas. The mole fraction gradients of H₂, CO, CH₄, CO are all mitigated by the finger-like channels compared to un-channelled planar cell, providing a higher percentage of performance enhancement than that in button cell cases.|
In chapter 3, a one-step methanation tubular cell integrating high temperature SOEC section at 800°C and low temperature Fischer-Tropsch-like reactor (FT section) at 250°C is a novel design for energy conversion from power to fuel. In this simple and compact design, syngas (H₂/CO mixture) produced from co-electrolysis of H₂O/CO₂ inside SOEC section can undergo reversed methane reforming reaction at favorable low temperature in the FT section for CH₄ production. A 2D-axisymmetry model is developed to study the coupled transport and reactions in the methanation cell. The heat and mass transfer, electrochemical reactions and chemical reactions such as Waster Gas Shift Reaction (WGSR) and Methane Steam Reforming Reaction (MSR) are fully considered. Parametric simulations are conducted to investigate the effects of applied voltage, length of FT section, operating temperature and inlet gas composition on CH₄ generation. Optimal inlet gas composition is found both with (H₂:CO₂=3.566, 20 vol% H₂O) or without (CO₂:H₂O=0.3566) H₂ assisting. In chapter 4, the model developed in chapter 3 is further extended to investigate the pressure effect on the physical-chemical processes in SOEC-FT reactor It is predicted that the optimal operating pressure is around 3 bar, beyond which the CH₄ conversion ratio (2.5 times enhanced than 1 bar operating) cannot be further increased. It is also found that it is feasible to operate the pressurized SOEC at a lower temperature for CH₄ production with improved catalyst activity. Chapter 5 proposes a proof-of-concept of a novel solid oxide steam electrolyser with in-situ hydrogen storage capacity by integrating a magnesium hydride (MH) section with proton-conducting SOEC section. Dynamic simulation results show that it takes 1950 seconds to fully charge the MH section with a 56% H₂ storage efficiency without any flow recirculation, when the electrolyser is operated at 1.4 volts and 4 atm, yielding a current density of 4956.40 A/m². The evolution of temperature, H₂ partial pressure and reaction of Mg powder through the charging process are analyzed. It is found that the exothermic H₂ adsorption process of MH section can enhance the performance of the electrolysis process of SOEC section. The effects of operating parameters including operating pressure, electrolysis voltage, and cooling air temperature on the performance of the novel design are investigated by sensitivity studies. Results show that it is beneficial to operate the electrolyser at elevated pressure for shorter adsorption time and higher H₂ storage efficiency. Increasing the operating voltage can shorten the adsorption time, but lower H₂ storage efficiency. An optimal cooling air temperature is found at 521 K when the electrolyser is operated at 1.4 V and 4 atm.
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