A Modelling Study of SOFC - GT Systems: Optimization of Design Conditions and Development of Simulation Test Platforms

Abstract

Hybrid systems comprising solid oxide fuel cells and gas turbine engines are considered to be a promising energy conversion technology because of their ultra-high efficiency over different scales and low level of emissions. While thermodynamic performance has been widely concerned, mechanical issues are yet to be fully understood, especially how these two devices of very different nature can be coupled together physically. The purpose of this PhD project is to develop a simulation tool for conducting effective design optimization and off-design analysis of such hybrid systems and to acquire knowledge about mechanical equilibrium operation at a mechanism level. The novel simulation approach has successfully integrated turbomachinery matching expressions with thermodynamic and electrochemical descriptions to guarantee stable running states throughout the research. New energetic and operational insights are provided into three typical SOFC - GT systems: basic single-shaft layout, recuperative single-shaft layout and twin-shaft free turbine layout. Design analysis shows there are mutual restrictions between the design parameters of SOFC, recuperator (if included) and turbine(s); proper design values selection is important for system performance and operation. Observing mass flow and temperature increase along the stream under different values of SOFC design parameters, determination mechanism of mechanical equilibrium operation is proposed for the first time to explain the disruptive influence of SOFC on turbomachinery operation at the level of mathematical expressions. A full view of power output and system efficiency within the equilibrium running range under different SOFC fuel addition (reflected by air utilization) has been presented for basic and recuperative single-shaft layouts resting on ideal load assumption. For the proposed twin-shaft layout, the depiction of equilibrium running line is not relying on specific load assumptions; how the air utilization affects the operation of turbomachinery and the performance along the running lines, as well as the output under different power turbine speeds are studied respectively. On the basis of separate off-design analysis of the three systems, a comparative study is carried out in terms of output characteristics and operation strategies under both constant and variable speed loads. Results show that recuperative single-shaft layout is very suitable for electricity generation (constant speed load) because of high efficiency maintained when coping with load drops, even adopting an easier to implement operation strategy. For simultaneous supply of electric power and shaft work (variable speed load), twin-shaft free turbine layout is more suitable due to high flexibility in output adjustment of the two forms. These discussions indicate that SOFC - GT hybrid systems have the potential to be applied in various practical scenarios.

Date of Award	13 Jan 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Hector Iacovides (Main Supervisor) & Alistair Revell (Co Supervisor)