Solid oxide fuel cell systems for Heavy Goods Vehicle propulsion

Taylor, Marcus (2025). Solid oxide fuel cell systems for Heavy Goods Vehicle propulsion. University of Birmingham. Ph.D.

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Abstract

The performance of solid oxide fuel cell (SOFC) systems is investigated in this thesis for the propulsion of heavy goods vehicles (HGVs) from the perspective of reducing fuel consumption and greenhouse gas emissions in this transport sector. Advantages of SOFC systems over equivalent low-temperature fuel cell systems include their higher electrical efficiencies and their high-temperature off-heat for thermal integration opportunities. Vehicles with SOFC systems can utilise existing liquefied natural gas (LNG) refuelling infrastructure, and are compatible with the phase-in of synthetic natural gas produced from renewable resources. LNG has the advantages over hydrogen of higher volumetric energy density and lower energy requirement for liquefaction.

This thesis presents a dynamic model of an SOFC system. The current density voltage characteristics of the SOFC stack are validated against experimental results, and steady-state system modelling results including anode off-gas recirculation are compared to modelling results in literature. The chemical reaction and thermal dynamics are studied in the transient response, and control strategies are developed to maintain the SOFC stack temperature, fuel utilisation, and steam-to-carbon ratio.

A novel aspect of this thesis is the investigation of the dynamic response of SOFC systems when subjected to standardised drive cycles for the propulsion of a 40 t HGV, and as part of a hybrid system coupled with a battery. During load changes the time derivative of the stack temperature can be maintained within acceptable limits, helping to maximise the lifetime of the SOFC system, but hybridisation of the SOFC system with a battery is required to handle more violent accelerations of the vehicle. The battery enables downsizing of the SOFC system power to 54 kW, and for energy recuperation through regenerative braking. The ageing of SOFC stacks reduces their power and efficiency over their lifetimes, and is highlighted as a challenge for their operation in HGV applications.

A heat loss model predicts the cool down of the SOFC systems when the vehicle is not in operation, and a detailed start-up strategy investigates thermal transients set up by the SOFC stack reactions. One challenge for the application of SOFC systems in HGVs is the 30 minutes required to start-up. This means early start-up of the SOFC system before vehicle operation is required, or a larger battery to compensate for the reduced electrical power output of SOFC system during that period. With the predictable usage cycles of commercial vehicles such as HGVs, the slow start-up times of SOFC systems may be more acceptable than for passenger cars.

Start-up also creates larger thermal transients in the SOFC stack than other operating modes. The stack is cooled initially by the endothermal methane steam reforming reaction, before heating from the electrochemical oxidation of hydrogen reaction as the current ramps up. Sufficient insulation of the hotbox components means that they cool by less than 100°C overnight, and therefore the system can be directly started the following morning. After weekend cool-down additional heating is required to bring the SOFC stack back up to its operating temperature, which is provided by electrical heating elements integrated into the hotbox components.

The dynamic modelling framework and transient results provide an insight into the potential and challenges of SOFC systems for HGV propulsion, which advances the progress towards the application of SOFCs systems in commercial vehicles. The framework can be used as a basis for more detailed component design or enhanced for greater insight into the thermal gradients during operation. With a generalised modelling approach, the framework can be adapted to capture future improvements in the performance of SOFC technology and their systems.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):