Principles of Fuel Cells

Xianguo Li

• Publisher: CRC Press
• Year: 2005
• ISBN:
  9781135201791
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Energy

The book is engineering oriented and covers a large variety of topics ranging from fundamental principles to performance evaluation and applications. It is written systematically and completely on the subject with a summary of state-of-the-art fuel cell technology, filling the need for a timely resource.

This is a unique book serving academic researchers, engineers, as well as people working in the fuel cell industry. It is also of substantial interest to students, engineers, and scientists in mechanical engineering, chemistry and chemical engineering, electrochemistry, materials science and engineering, power generation and propulsion systems, and automobile engineering.

CHAPTER
1

INTRODUCTION

1.1 INTRODUCTION

Fuel cells are environmentally friendly devices for energy conversion and power generation, and are one of the most promising candidates as a zero-emission power source. Hence, they are often regarded as one of the advanced energy technologies of the future. In reality, fuel cells are one of the oldest energy conversion devices known to humankind, although their development and deployment for practical applications lag far behind other competitive technologies, mainly heat engines such as steam turbine and the internal combustion engine. Presently, heat engines employing combustion of fossil fuels have become an essential and integral part of modern civilization, being increasingly relied upon since the Industrial Revolution.

This almost exclusive reliance on combustion of fossil fuels has resulted in severe local air pollution due to the pollutant emissions, including SO_x, NO_x, CO, and particulates, which pose a severe threat to the health of millions of people living in many of the world’s urban areas. It continues to contribute significantly to the increase in atmospheric carbon dioxide concentrations, thus intensifying the prospect of global warming and threatening the very existence of our civilization and humankind on planet Earth. The specter of climate change is becoming so ominous that it is the substantial reduction of greenhouse gas emissions that is driving research and development into cleaner, more efficient energy technology and alternative energy sources and carriers (primary and secondary fuels). In addition to the health and environmental concerns, a steady depletion of the world’s limited fossil fuel reserves calls for new energy technology for energy conversion and power generation, which is more energy efficient than the conventional heat engine with minimal or no pollutant emissions and also compatible with renewable energy sources or carries for sustainable development. Fuel cells have been identified as one of the most promising and potential energy technologies which meet all of these requirements for energy security, economic growth, and environmental sustainability.

Therefore, this chapter discusses the motivation for the study, research, development, and deployment of fuel cells and then describes some basic concepts and ideas of fuel cells by raising a series of questions. Some of the fundamental questions are: What is a fuel cell? How does it work? Why is it regarded as an advanced energy technology of the future? By addressing these questions, we begin to appreciate the physical and electrochemical mechanisms that underlie fuel cell operation and the relevance and significance of fuel cells in our efforts to address industrial and environmental problems and develop an environmentally benign energy system sustainable in the future. We next illustrate a typical fuel cell system for power generation with various components described. Then we provide the classification and overview of fuel cells, present a brief account of historical development, and illustrate fuel cells as an emerging energy technology of the future. Finally, the scope and outline of the book is given along with the objective for the book.

1.2 MOTIVATION FOR FUEL CELLS

As discussed earlier, conventional power generation by using heat engines based on combustion of fossil fuels produces a significant amount of pollutant emissions that increasingly contribute to the degradation of the environment on which humans and other life forms dearly depend. A variety of alternative approaches has been proposed and implemented for power generation, such as hydroelectric power, wind, wave, solar, bioenergy, geothermal energy, and so on. These renewable energy sources can be used with relative ease to generate electricity for utility applications, but for example, they are subject to seasonal and irregular fluctuations in terms of the amount of energy available and are limited in the quantity of harvestable energy. Further, these renewable energy sources and associated power generation technologies remain virtually impossible for direct utilization in transportation applications, which presently account for a significant amount of pollutant emissions.

The intermittent nature of power generation from the renewable sources favors the use of energy storage for power provision. Although batteries, super-capacitors, and flywheels, can be used as energy storage devices, they have not been shown to be an ideal option, especially for transportation applications.^{1, 5} Ithas been proposed that the most promising option for environmentally benign operation is to use hydrogen as the energy carrier, produced from the renewable energy sources such as solar energy and hydroelectric power, and to adopt fuel cells as the clean and efficient means of energy conversion and power generation for mobile applications. Figure 1.1 shows the relative amount of carbon dioxide produced by various fuels for one unit of energy consumed. It is seen that hydrogen, as expected, does not produce carbon dioxide at all and that hydrogen is most efficiently used in fuel cells for power generation as opposed to combustion in heat engines. The beauty of the hydrogen option lies in the fact that its production from water produces only oxygen as a byproduct, and its recombination with oxygen from the air forms water once again when power is needed and produced. Figure 1.1 also shows that methane yields the lowest carbon dioxide production among the hydrocarbon family of fuels due to the highest hydrogen-to-carbon ratio of its molecular structure. It might also be pointed out that methyl alcohol or methanol produces slightly more carbon dioxide emissions than propane in this comparison, with their carbon dioxide emissions above only those from methane. Both methane and methanol can be directly or indirectly used in fuel cells for electricity generation and both can be manufactured from the fossil fuels or biomass resources. Renewable sources are the only possible and plausible path because fossil fuels have been dismissed as a viable option for a sustainable future. Therefore, a sustainable future energy system will include fuel cells, with hydrogen, methane and methanol derived from renewable sources, to produce electricity in areas where and in times when power is needed or electricity is in demand.

Figure 1.1 Comparison of the relative amount of carbon dioxide production from various fuels for one unit of energy. Note wood is commonly regarded as CO₂-neutral fuel for the entire cycle since CO₂ is absorbed during the wood growth stage.

Fuel cells can also be employed for power generation from fossil fuels and biomass-derived materials with reduced pollutant emissions due to higher practical efficiencies. The option of fuel cells with fossil fuels can be regarded as an interim method of introducing fuel cells into the market place without significant barriers arising from the lack of fuel distribution infrastructure needed for a future sustainable energy system. Biomass-derived fuels include methane from municipal solid wastes, sewage sludge, forestry residues, landfill sites, and oil-field flare gases and can be used efficiently by fuel cells for power generation, instead of their direct release into the atmosphere. Hundreds of thousands of landfill sites can be found in the world that could be economically exploited and the deployment of small unattended power plants of fuel cells running on these gases could make them highly profitable sources of energy. Biofuels from agricultural and animal waste are another attractive source of energy for fuel cell implementation. Biofuel options can be implemented in an overall carbon dioxide neutral process if appropriate methods are used.

Furthermore, a significant portion of the energy need is in the demand for electricity, which is, at present, almost exclusively produced by large power plants burning fossil fuels and then distributed through networks of high-voltage transmission wires over long distances. Even though these large power plants have optimal energy efficiencies because of their size, electric energy losses occur during long distance transmission, amounting to 7%–8% of electricity generated in Europe and as much as 10% in North America. As such, overall energy efficiency is reduced when accounting is done at the end-user site. In addition, these networks of transmission wires may not function properly all the time, especially when they are needed most, as evidenced by what happened during the ice storm in Quebec in January 1998. Logically, electricity generation should be decentralized and located at or near where electricity is needed. Because of their unique characteristics for electricity generation, fuel cells can be used for cogeneration, on-site, and distributed power generation without the need of complex, long-distance transmission networks. Hydrogen fuel cells thus can make the energy system more decentralized, and contribute to a more secure energy supply system.

On the other hand, the fossil fuel reserve is quite limited and is unevenly distributed throughout the world. The latter also leads to severe geopolitics and regional conflicts that threaten world peace. The limited fossil fuel reserve is being rapidly depleted and will not be able to support world energy demand in a “business as usual” manner for long. It has been estimated⁶ that natural gas and petroleum oils may last another half-century, while coals and nuclear energy may be sufficient for only another few hundred years. This limited supply and huge demand situation drives up the cost of fossil fuels rapidly, as it is being encountered in many parts of the world today. The end of the cheap oil era is rapidly approaching.⁷ In comparison, renewable energy sources such as solar, wind, geothermal, biomass, and tidal waves may be regarded as lasting forever. Therefore, future energy technology for power generation must be compatible with the renewable energy sources, and fuel cells meet this requirement nicely.

In summary, fuel cells are an energy conversion technology, with operation on hydrogen to produce electric power on demand. Therefore, they are not a primary energy source and they do not compete with electricity generation from the renewable energy sources. Rather they are complementary to each other and fuel cells are compatible with ...