Hydrogen supply in proton exchange membrane fuel cell systems 2018-09-13 17:52:03

1.1 Thesis objective
The primary task of a hydrogen supply system in a PEMFC is to deliver hydrogen to a fuel cell. Without a hydrogen supply, the PEMFC cannot operate. The hydrogen supply system also has secondary tasks, such as preventing the buildup of impurities (contaminants and inert gases), removing liquid water, humidifying the hydrogen, and recirculating hydrogen. These tasks are important for efficient and long lasting PEMFC operation. The objective of this
thesis is to study devices and methods for hydrogen supply that could improve PEMFC system performance and durability.
Hydrogen purge is an efficient way of removing inert gases and other impurities  as well as liquid water from an anode. However, it has not been clear how
the anode inert gas content, system humidity level, and hydrogen purge parameters affect the PEMFC and fuel efficiencies. Methods for measuring the required quantities were developed, and the aforementioned questions were studied in publications [I] and [II].
Although hydrogen humidification adds system complexity and cost, it is often employed because it improves PEMFC efficiency. A bubble humidifier has a
simple design and working principle and is, thus, a good humidifier candidate for PEMFC systems. However, downsides such as hydrostatic pressure drop and
the lack of controllability are factors often considered to outweigh the benefits. In publication [III], a bubble humidifier was designed, characterized, and modelled,and improvements on its design and control were discussed.
Hydrogen recirculation is typically employed in PEMFC systems because it improves PEMFC performance. Mechanical pumps often employed for hydrogen
recirculation have been proven too unreliable. Ejectors, on the other hand, are seen as promising devices for this application. However, the limited working
range of ejectors and the challenges in ejector primary flow control are seen as factors limiting their usability. Publications [IV]-[VI] examined the possibility
of using the simplest thinkable approach to hydrogen recirculation – a single ejector with discrete primary flow control.
1.2 Thesis structure
The thesis first gives a brief introduction to PEMFC technology and PEMFC systems in Chapter 2. Chapter 3 provides an overview of hydrogen supply in
PEMFC systems and describes the three secondary tasks of the hydrogen supply system: hydrogen purge, hydrogen humidification, and hydrogen recirculation. Chapters 4-6 discuss the three secondary tasks of hydrogen supply in more detail and present the methods employed and results obtained in publications [I]-[VI]. Chapter 4 focuses on inert buildup and hydrogen purge, which was studied experimentally in publications [I] and [II]. Chapter 5 presents design, characterization, and modelling of a bubble humidifier for hydrogen humidification based on publication [III]. Chapter 6 presents methods and results of the work done in publications [IV]-[VI], which targeted hydrogen recirculation with a single, fixed geometry ejector and a discrete primary flow control. A summary of the results is given in Chapter 7. The publications that thesis is based on are attached at the end.