Title;; 1 Background Fuel cells are devices that convertthe chemical energy of hydrogen, methanol or other chemical compounds directly intoelectricity, without combustion or thermal cycles.
They are efficient, scalableand silent devices that can provide power to a wide variety of utilities, fromportable electronics to vehicles, to nation-wide electric grids. The high energy conversionefficiency of fuel cells, together with their high fuel flexibility, make theman ideal candidate for a better exploitation of fossil fuels and for anefficient conversion of renewable energy sources into electricity. Highly efficient energy devicessuch as the fuel cell systems that are in the development stages are at theforefront of sustainable energy development (Nigel et al,…..)Clean energy conversion systemscapable of operating efficiently on fossil fuels, as well as on renewableenergies represent the ideal final ring of a sustainable energy chain (Robert et al,…). Based on… there are types of fuelcells The Polymer Electrolyte Membrane(PEM) fuel cells (also known as Proton Exchange Membrane fuel cells) have highpower density, solid electrolyte, and long cell and stack life, as well as lowcorrosion. They have greater efficiency when compared to heat engines and theiruse in modular electricity generation and propulsion of electric vehicles ispromising. Using pure hydrogen as fuel can eliminate local emissions problemsin densely populated urban environments.
(Jay et al, 2005) 2 AimsAnd Objectives This research intends to advanceknowledge in control of fuel cells, focusing on high-temperature proton-exchange-membranefuel cells.Inthe relatively small body of available literature, there are some apparentlycontradictory statements: sometimes the slow dynamics of fuel cells is claimedto present a control problem, whereas in other articles fuel cells are claimed tobe easy to control and able to follow references that change very rapidly.These contradictions are mainly causedbydifferences in the sets of phenomena and dynamics that the authors decided toinvestigate, 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 muchmore important measure of performance.—-rephrase Properly state aim andobjectivesThe aim of this project is to consider the main phenomena influencingthe dynamics of fuel cells, to properly define the control problem and suggestpossible approaches and solutions to it.
This project seeks to integrate previous work from different areas withnew insights, in order to acquire a complete view of the control issues offuel-cell systems. Methodology This project will focus on aparticular type of fuel cell, a variation of proton-exchange-membrane fuelcells with a membrane of polybenzimidazole instead of the usual, commerciallyavailable Nafion .The advantages of this particular type of fuel cells forcontrol stem from their operation at temperatures higher than those typical ofNafion-based cells: these new cells do not have any water-management issues,can remove more heat with their exhaust gases, and have better tolerance topoisons such as carbon monoxide. The first part of this project willbe 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 afuel-cell stack’s overall behaviour. The dynamics of interest are thoseof chemical engineering (heat and mass balances), of electrochemistry(diffusion in electrodes, electrochemical catalysis) and of electricalEngineering (converters, invertersand electric motors).
The first part of the thesis will first present someexperimental results of importance for the electrochemical transient, and willthen develop the equations required to model the four dynamic modes chosen torepresent a fuel-cell system running on hydrogen and air at atmosphericpressure: cathodicOvervoltage, hydrogen pressure inthe anode, oxygen fraction in the cathode and stack temperature. The second part will explore someof 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 foreach dynamics to control. It is a major point of the project, however, that thetask of controlling a fuel cell is to be judged exclusively by its final result,which is power delivery:all other control loops, howeverindependent, will have to be designed bearing that goal in mind. The overvoltage, which correspondsnonlinearly to the rate of reaction, is controlled by operating a buck-boostDC/DC converter, which in turn is modelled and controlled with switching rules.Hydrogen pressure, being described by an unstable dynamic equation, requiresfeedback to be controlled. A controller with PI feedback and a feedforward parttoImprove performance is suggested.The oxygen fraction in the cathodic stream cannot be easily measured with asatisfactory bandwidth, but its dynamics is stable and disturbances can bemeasured quite precisely: it is therefore suggested to use a feedforwardcontroller.
Contrary to the most commonapproach for Nafion-based fuel cells, temperature is notcontrolled with a separate coolingloop: instead, the air flow is used to cool the fuel-cell stack. Thissignificantly simplifies the stack design, operation and production cost. Tocontrol temperature, it is suggested to use a P controller, possibly with afeedforward component. Simulations show that this approach to stack cooling isfeasible and poses no or few additional requirements on the air flow actuatorthat is necessary to control air composition in the cathode. Significance,Originality and /or Anticipated Impact of Work Electricity is the most convenienttype of energy that addresses man’s energy demand but its generation through thermodynamicprocedures is inefficient and transmission to the user results in furtherlosses. Due to that higher quantities of fuel are used, resulting in highercarbon dioxide emissions .
Arrangement of the transmission framework requires abroad foundation, and is costly. Fuel cells, when fully developedwill offer a more advantageous path of generating electricity, as the equipmentis not restricted by the Carnot effectiveness, with lower carbon dioxide emissionsand no other harmful emissions. Subsequently offering opportunity ofestablishment close to the client to suit their nearby necessities. It is intendedthat this research work will advance knowledge in the control of the fuel cellso that it can work at its optimum.