Fuel cells are devices that convert
the chemical energy of hydrogen, methanol or other chemical compounds directly into
electricity, without combustion or thermal cycles. They are efficient, scalable
and silent devices that can provide power to a wide variety of utilities, from
portable electronics to vehicles, to nation-wide electric grids.
The high energy conversion
efficiency of fuel cells, together with their high fuel flexibility, make them
an ideal candidate for a better exploitation of fossil fuels and for an
efficient conversion of renewable energy sources into electricity.
Highly efficient energy devices
such as the fuel cell systems that are in the development stages are at the
forefront of sustainable energy development (Nigel et al,…..)Clean energy conversion systems
capable of operating efficiently on fossil fuels, as well as on renewable
energies represent the ideal final ring of a sustainable energy chain (Robert et al,…).
Based on… there are types of fuel
The Polymer Electrolyte Membrane
(PEM) fuel cells (also known as Proton Exchange Membrane fuel cells) have high
power density, solid electrolyte, and long cell and stack life, as well as low
corrosion. They have greater efficiency when compared to heat engines and their
use in modular electricity generation and propulsion of electric vehicles is
promising. Using pure hydrogen as fuel can eliminate local emissions problems
in densely populated urban environments. (Jay et al, 2005)
This research intends to advance
knowledge in control of fuel cells, focusing on high-temperature proton-exchange-membrane
the relatively small body of available literature, there are some apparently
contradictory statements: sometimes the slow dynamics of fuel cells is claimed
to present a control problem, whereas in other articles fuel cells are claimed to
be easy to control and able to follow references that change very rapidly.
These contradictions are mainly caused
differences in the sets of phenomena and dynamics that the authors decided to
investigate, and also by how they formulated the control problem. For instance,
there is little doubt that the temperature dynamics of a fuel cell can be slow,
but users are not concerned with the cell’s temperature: power output is a much
more important measure of performance.—-rephrase
Properly state aim and
The aim of this project is to consider the main phenomena influencing
the dynamics of fuel cells, to properly define the control problem and suggest
possible approaches and solutions to it.
This project seeks to integrate previous work from different areas with
new insights, in order to acquire a complete view of the control issues of
This project will focus on a
particular type of fuel cell, a variation of proton-exchange-membrane fuel
cells with a membrane of polybenzimidazole instead of the usual, commercially
available Nafion .The advantages of this particular type of fuel cells for
control stem from their operation at temperatures higher than those typical of
Nafion-based cells: these new cells do not have any water-management issues,
can remove more heat with their exhaust gases, and have better tolerance to
poisons such as carbon monoxide.
The first part of this project will
be concerned with defining and modelling the dynamic phenomena of interest.
Indeed, a common mistake is to assume that fuel cells have a single dynamics:
instead, many phenomena with radically different time scales concur to define a
fuel-cell stack’s overall behaviour.
The dynamics of interest are those
of chemical engineering (heat and mass balances), of electrochemistry
(diffusion in electrodes, electrochemical catalysis) and of electrical
Engineering (converters, inverters
and electric motors). The first part of the thesis will first present some
experimental results of importance for the electrochemical transient, and will
then develop the equations required to model the four dynamic modes chosen to
represent a fuel-cell system running on hydrogen and air at atmospheric
Overvoltage, hydrogen pressure in
the anode, oxygen fraction in the cathode and stack temperature.
The second part will explore some
of the possible approaches to control the power output from a fuel-cell stack.
An attempt will be made to produce a modularised set of controllers, one for
each dynamics to control. It is a major point of the project, however, that the
task of controlling a fuel cell is to be judged exclusively by its final result,
which is power delivery:
all other control loops, however
independent, will have to be designed bearing that goal in mind.
The overvoltage, which corresponds
nonlinearly to the rate of reaction, is controlled by operating a buck-boost
DC/DC converter, which in turn is modelled and controlled with switching rules.
Hydrogen pressure, being described by an unstable dynamic equation, requires
feedback to be controlled. A controller with PI feedback and a feedforward part
Improve performance is suggested.
The oxygen fraction in the cathodic stream cannot be easily measured with a
satisfactory bandwidth, but its dynamics is stable and disturbances can be
measured quite precisely: it is therefore suggested to use a feedforward
Contrary to the most common
approach for Nafion-based fuel cells, temperature is not
controlled with a separate cooling
loop: instead, the air flow is used to cool the fuel-cell stack. This
significantly simplifies the stack design, operation and production cost. To
control temperature, it is suggested to use a P controller, possibly with a
feedforward component. Simulations show that this approach to stack cooling is
feasible and poses no or few additional requirements on the air flow actuator
that is necessary to control air composition in the cathode.
Originality and /or Anticipated Impact of Work
Electricity is the most convenient
type of energy that addresses man’s energy demand but its generation through thermodynamic
procedures is inefficient and transmission to the user results in further
losses. Due to that higher quantities of fuel are used, resulting in higher
carbon dioxide emissions .Arrangement of the transmission framework requires a
broad foundation, and is costly.
Fuel cells, when fully developed
will offer a more advantageous path of generating electricity, as the equipment
is not restricted by the Carnot effectiveness, with lower carbon dioxide emissions
and no other harmful emissions. Subsequently offering opportunity of
establishment close to the client to suit their nearby necessities.
It is intended
that this research work will advance knowledge in the control of the fuel cell
so that it can work at its optimum.