by Michael R. Gerhardt
Our climate is rapidly changing, and many countries are beginning to take action. In the United States, President Barack Obama has announced the Clean Power Plan to reduce greenhouse gas emissions from electric power generation, while Chinese president Xi Jinping has announced economic incentives to reduce emissions [1,2]. Even oil companies have publicly acknowledged the challenges we face and have voiced a desire to limit global warming . New technologies are being developed worldwide to cut down on greenhouse gas emissions. One of these up-and-coming energy technologies is the solid-oxide fuel cell, which promises more efficient use of abundant, inexpensive natural gas, permitting less overall carbon dioxide emissionsthan traditional combustion turbines.
Global energy use is changing our climate through emissions of greenhouse gases. Greenhouse gases cause the Earth’s temperature to rise by trapping heat from the Sun close to the Earth’s surface, much like a warm, cozy blanket. However, this greenhouse blanket has warmed the Earth enough to melt glaciers and swell the oceans, flooding low-lying coastal cities like Miami . In the United States, the two largest greenhouse gas emitters are power plants and vehicle (Figure 1) . Other greenhouse gas emissions come from industrial processes, such as steel production, and heating of households and commercial spaces. As a result of these activities, the atmospheric concentration of the well-known greenhouse gas carbon dioxide is higher now than it has been in the past 800,000 years, foreshadowing radical changes to our planet .
Interestingly, despite years of technological advancements, we have struggled to produce electricity without setting things on fire. Burning coal and natural gas, for example, accounts for two-thirds of electricity generation in the United States (Figure 2). Natural gas use presents a special challenge, because natural gas itself is a potent greenhouse gas, so it must be used very carefully and efficiently to prevent leaks . Replacing the traditional use of combustion turbines with new natural gas fuel cell technologywould result in less greenhouse gas emissions and more efficient electricity generation.
How is natural gas used to make electricity?
Natural gas power plants today make up nearly a third of electric plants in the United States. These power plants work by compressing and igniting natural gas as shown in Figure 3 . The ignition of the gas in the ignition chamber (1) begins a chemical reaction, converting natural gas and oxygen to carbon dioxide and water. This reaction releases enormous amounts of heat, warming the gas to 2000 °F (1100 °C), which causes the gas to expand rapidly. Ordinarily, this would be called an explosion, but since it happens in a device we built, we call it engineering. This gas then rushes past the blades of the turbine (2), causing the turbine to spin a generator to produce electricity (3).
Each of these steps, from the explosion to the spinning turbine to the electric generator, involves conversion of energy – from chemical, to mechanical, to electrical energy. Each of these conversions is not perfect. As a result, the most efficient turbine designs in the world are only 61% efficient, because of losses in all the aforementioned conversion processes, like friction in the turbine shaft, and heat lost to exhaust air . However, new fuel cell devices can convert chemical energy directly into electrical energy, skipping these steps. Current fuel cell designs are about 60% efficient, on par with many power plants, but also have the potential to climb to 80% or higher . A special type of fuel cell, the solid-oxide fuel cell (SOFC), may be able to raise efficiency to 85-90%. These fuel cells are small and quiet enough for household use, unlike combustion turbines. Furthermore, the waste heat generated from the cell can also be used to heat the home, which increases the energy efficiency .
What is a fuel cell, and how does it help?
Fuel cells are devices that take advantage of a chemical reaction between two input fuels, usually hydrogen and oxygen, to convert energy from the reaction directly into electricity, skipping all the inefficient conversion steps in a fuel combustion turbine. Figure 4 shows a schematic of such a device. Fuel cells use “reduction-oxidation” reactions, a class of chemical reactions in which electrons are transferred from one fuel, hydrogen (1), to the other, oxygen (2). In a fuel cell, hydrogen gas goes into one side and oxygen into the other. By keeping the hydrogen gas and oxygen separate with a specially designed separator (3), fuel cells can force this electron transfer reaction to occur through a wire (4). The flow of electrons down this wire is already electrical energy made directly from chemical energy without having to go through a mechanical energy step. However, hydrogen gas is not readily available naturally and must be made. Most hydrogen gas today is produced by reacting methane and pressurized steam at high temperatures to form hydrogen and carbon monoxide or carbon dioxide, which requires additional energy input and therefore lowers the efficiency of the fuel cell . Furthermore, hydrogen production must be kept separate from the fuel cell itself to prevent the high temperatures from melting the separator.
This is where the SOFC makes an improvement. The SOFC, uses a solid ceramic separator, which allows the cell to operate at high temperatures (700 °C or 1300 °F) at which ordinary fuel cell separators would melt . The higher temperature both accelerates the reaction between the fuels and allows the cell to produce hydrogen internally. In this way, solid oxide fuel cells can use a common fuel like natural gas, at 60% efficiency, comparable to combustion turbines .
Fuel cells have another energy efficiency trick up their sleeves. They can be designed smaller than turbines, and the waste heat from the fuel cell can be used to warm a building, in a process known as cogeneration. Other solid-oxide fuel cell designs incorporate an evaporative cooler, which can use the waste heat to provide cool air, like an air conditioner . When accounting for both the electricity and heat produced by such a device, the overall efficiency of the fuel cell jumps to 85-90% . Cogeneration at a household level with natural gas turbines is impossible, since the turbines are too large and too loud. A laboratory at the University of Maryland has recently formed a startup company, Redox Power Systems, to develop solid-oxide fuel cells capable of using natural gas, propane, or diesel fuel to cogenerate heat and electricity .
Carbon dioxide will still be produced during operation of a solid-oxide fuel cell. However, cogenerating electricity, heat, and cooling with these appliances reduces the total usage of fossil fuel, in turn reducing carbon dioxide emissions. In addition, the fuel cell will output an exhaust stream of carbon dioxide and water, from which the carbon dioxide can easily be captured and stored to prevent its release to the atmosphere. The ease of carbon capture and storage in fuel cells is a huge advantage over natural gas turbines, which exhaust carbon dioxide through a smokestack to the atmosphere.
So why don’t we all have one of these things yet?
As seen in an earlier Science in the News article, bringing clean energy technologies to market has proven very difficult . The main impediment to these fuel cells hitting the market today is production cost. Many of the components, like the ceramic separator, are still expensive to manufacture . Elevated operating temperature has its drawbacks, as the fuel cells take time to warm up before they can produce electricity. Plus, high temperatures require shielding and safety equipment to protect users, which also drives up cost . The Redox Cube is a step in the right direction; innovative separator materials have increased the cell power tenfold while reducing costs as much as 90% . The United States Advanced Research Projects Agency – Energy (ARPA-E) funds Redox Power and other advanced solid-oxide fuel cell technologies through its REBELS program (Reliable Electricity Based on Electrochemical Systems) to push fuel cell technology forward and drive down cost . These research programs and improvements in fuel cell technology bring us closer and closer to sustainable energy generation and use by enabling cogeneration of heat and electricity, squeezing as much energy from our fuel as we can. We may be able to shed this blanket someday after all.
Michael R. Gerhardt is a Ph.D. candidate in Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences.
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