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BACKGROUND AND MOTIVATION
Energy is becoming an issue of serious concern in the world today. It is inevitable for human life and a secure supply of energy is required for sustainability of human societies [1]. The need to satisfy world energy demand, which actually determines the living standard of the populace, is increasing. This energy is utilised to generate the electricity we need for our homes, businesses, schools and factories. It energises our computers, lights, refrigerators, washing machines and air conditioners, to mention only a few. Also, the quantity of energy required in the industrial sector of the economy for its production activities is ever-increasing. This energy is mostly obtained from fossil fuel stock combustion processes and great deals of pollutant gases (CO2, NOX, etc.) are emitted to the atmosphere [2, 3]. Some of these gases, especially CO2, are a major contributor to global warming and its attendant consequences, such as rise in global average temperatures, rise in sea levels, flooding and deforestation. Therefore, the effects of global warming have become an issue of major concern to goverments, policy makers and environmentalists. Hence, in recent times, numerous researches and commissioned studies have focused on the development of carbon-free energy sources that are environment-friendly, sustainable and cheaply available so as to minimise the amount of pollutant gases emitted into the atmosphere as a result of energy consumption [4].
The available energy sources in the world today are divided into two groups: renewable and non-renewable sources. Renewable energies are those that come from natural resources and are replenished naturally. Non-renewable energies are those that are not replenished or only replenished very slowly. The available renewable energy systems range from solar power systems, wind power systems, geothermal power systems, fuel cells, etc. Renewable systems have different comparative advantages which usually determine their applications. Both renewable and non-renewable energy sources can be used to produce secondary energy sources, including electricity
and hydrogen. However, most of our energy sources today are from non-renewable sources, which include the fossil fuels, i.e. oil, natural gas and coal [3]. Renewable energy resources become an important option to fossil fuel as the negative environmental consequences of fossil fuel increases and its utility cost (electricity) climbs. The quality of renewable energy technologies, that makes it a viable substitute to fossil fuel, includes its modular nature, lower operating cost and its flexibility and
adaptability. These energy sources are considered by many as a direct replacement of existing fossil fuel technologies and this has made the evaluation of its benefit in terms of cost to be rated low when compared to traditional fossil technologies. The baseline is to view these renewable technologies as a complementary modular addition to existing energy systems with short lead-times [1]. This will adequately reduce the pressure on the national grids and ensure availability of energy to people in remote areas. Moreover, it will help reduce the amount of pollutant gases released
into the atmosphere as a result of fossil fuel usage.
The world energy consumption projection by 2030 is estimated at about 700 Quadrillion British thermal unit (BTU) [5]. This figure equates to two-thirds more energy than the present usage. Fossil fuels will remain the dominant sources of energy, accounting for more than 90% of the projected increase in demand [5].
Problems associated with energy supply and demand are much more than global warming threats, but environmental concerns such as ozone layer depletion, pollution, deforestation and radioactive emission are increasing today [1]. These environmental problems need to be addressed quickly if the world is to achieve a sustainable energy future. The drive today is to seek for sustainable development through the utilisation of energy sources that has little or no adverse impact on the environment [6, 7]. These energy sources (i.e. solar, wind, etc.) are easily replenished once consumed, as compared to finite fossil fuels (oil, coal and natural gas).
Hydrogen, a clean and renewable fuel source, is generally available in abundance and is a safe energy source [8, 9]. This fuel type can be generated from different kinds of sources, including most renewable sources and fossil fuels (natural gases and coal gasification). Figure 1.1 illustrates a typical comparison of utilising gasoline and hydrogen as fuel for transportation and mobile applications in the service sector [10].
The figure illustrates that hydrogen sources are diverse on the energy sector side and that the emission characteristics are quite limited on the service sector side, making hydrogen a key candidate for future energy currency.
Hydrogen has long been recognised as a potential fuel source for application in engines due to some unique and desirable properties [11]. These properties include its combustion in oxygen that produces only water as a waste, though, when combusted in air, could generate some oxides of nitrogen. Table 1.1 is a comparison of combustion properties of hydrogen with other fuels. The table shows the outstanding properties of hydrogen in terms of performance when compared with other
conventional fuels.
Recent studies [12-14] have shown the importance of hydrogen energy to sustainable development and in resolving the prevalent global environmental issues. The transition to hydrogen-based economy, where the main energy carrier is hydrogen and the main non-chemical energy form is electricity, is being made gradually and interest in this area is growing rapidly. However, generating electricity directly from hydrogen requires specific energy technologies such as the fuel cell. Fuel cell is a thermodynamic system that generates power by a direct conversion of the chemical energy in fuel into electrical power through electrochemical reaction [15].
CHAPTER 1: INTRODUCTION
1.1 BACKGROUND AND MOTIVATION
1.2 REVIEW OF RELATED LITERATURE
1.2.1 OPTIMAL OPERATING CONDITIONS FOR PEM FUEL CELLS
1.2.2 FUEL GAS CHANNEL OPTIMISATION FOR PEM FUEL CELLS
1.2.3 REACTANT GAS TRANSPORT IN PEM FUEL CELLS .
1.2.4 HEAT TRANSPORT AND COOLING IN PEM FUEL CELLS
1.3 JUSTIFICATION FOR THIS STUDY
1.4 RESEARCH OBJECTIVES .
1.5 ORGANISATION OF THE THESIS
CHAPTER 2: FUNDAMENTALS OF PEM FUEL CELL SYSTEMS
2.1 INTRODUCTION .
2.2 THE BASIC STRUCTURE OF A PROTON EXCHANGE MEMBRANE FUEL CELL
2.2.1 PROTON EXCHANGE MEMBRANE
2.2.2 CATALYST LAYERS
2.2.3 GAS DIFFUSION LAYERS
2.2.4 BIPOLAR PLATES
2.3 PEM FUEL CELL STACK DESIGN
2.3.1 HYDROGEN FUEL CELL SYSTEM COMPONENTS .
2.4 THEORIES OF TRANSPORT AND ELECTROCHEMICAL PROCESSES IN PEMFC .
2.4.1 CONSERVATION EQUATIONS .
2.4.2 NUMERICAL MODELS OF INDIVIDUAL PEM FUEL CELL COMPONENTS .
CONCLUSION69
CHAPTER 3: NUMERICAL MODELLING FRAMEWORK
3.1 INTRODUCTION
3.2 NUMERICAL METHOD
3.2.1 NUMERICAL MODELLING PROCEDURES
3.3 NUMERICAL OPTIMISATION
3.3.1 CONSTRAINED OPTIMISATION
3.3.2 THE DYNAMIC-Q METHOD
3.3.3 DYNAMIC-Q APPROACH: CONSTRUCTING SPHERICAL QUADRATIC SUBPROBLEMS
3.3.4 THE OBJECTIVE AND CONSTRAINT FUNCTIONS GRADIENT APPROXIMATION
3.3.5 ADVANTAGE OF DYNAMIC-Q ALGORITHM
CONCLUSION
CHAPTER 4: NUMERICAL OPTIMISATION OF OPERATING AND DESIGN PARAMETERS FOR A PEM FUEL CELL .
4.1 INTRODUCTION
4.2 MODEL DESCRIPTION
4.2.1 MODEL ASSUMPTIONS .
4.2.2 GOVERNING TRANSPORT EQUATIONS
4.2.3 CHANNEL CROSS-SECTION
4.2.4 FLUID FLOW THROUGH GAS DIFFUSION LAYER
4.2.5 BOUNDARY CONDITIONS
4.2.6 SOLUTION TECHNIQUE
4.2.7 MODEL VALIDATION
4.3 MODEL RESULTS AND DISCUSSION
4.3.1 PRESSURE DROP IN FLOW CHANNEL .
4.3.2 EFFECT OF PHYSICAL PARAMETERS ON PROTON EXCHANGE MEMBRANE FUEL CELL PERFORMANCE
4.3.3 EFFECT OF DESIGN PARAMETERS ON PROTON EXCHANGE MEMBRANE FUEL CELL PERFORMANCE
4.3.4 OPTIMAL CHANNEL GEOMETRY
CONCLUSION
CHAPTER 5: OPTIMISING REACTANT GAS TRANSPORT IN A PROTON EXCHANGE MEMBRANE FUEL CELL WITH A PIN FIN INSERT IN CHANNEL FLOW
CHAPTER 6: MODELLING AND OPTIMISATION OF COOLING CHANNEL GEOMETRIC CONFIGURATION FOR OPTIMAL THERMAL PERFORMANCE OF A PROTON EXCHANGE MEMBRANE FUEL CELL
CONCLUSION
