

While most green attention has been focused on wind, solar and other renewable resources, a team at MIT has proposed an alternative power plant that would use natural gas, but wouldn't emit carbon dioxide.

Crucially, the new plants wouldn't burn natural gas, they'd feed it to solid oxide fuel cells, electrochemical devices that convert the energy stored in the gas into electricity through a chemical reaction that's more efficient than traditional combustion.

Theoretically, the plant would be able to turn heat into electricity with an efficiency of 74 percent, as compared with just 50 percent at the very best natural gas plants (.pdf). And what's left over isn't the mix of gases that traditionally goes up a power plant's smokestack, but relatively pure water and carbon dioxide.

"Because we're keeping the nitrogen out of there, it's very, very easy to take the CO2 out," said MIT engineer Tom Adams, co-author of a paper in the Journal of Power Sources on the new plant design.

Though some of the scientists who have been working on solid oxide fuel cells for a long time don't think the MIT model is realistic, it does showcase some of the advantages of solid oxide fuel cells that could make them a major part of the low-carbon energy future. Specifically, solid oxide fuel cells make capturing carbon dioxide emissions easier and less expensive compared to other ways of using fossil fuels.

"The basic point is that we're able to avoid the CO2-capture penalty," Adams said.

Adams and his co-author, MIT engineer Paul Barton, have built on a decade-long effort by the Department of Energy: The Solid Energy Conversion Alliance, a consortium of heavy-hitting fuel-cell scientists and companies like Siemens coordinated by the National Energy Technology Laboratory, has been working to develop solid oxide fuel cells for commercial use.

The group has been steadily progressing towards building fuel-cell power plants. Right now, solid oxide fuel cells like the ones described by Adams are nearing commercialization by Siemens, but at the kilowatt scale, not the megawatt scale. But Adams believes megawatt prototypes could be operational by 2012.

Fuel cells might not sound like the hottest field in energy, but that might be because you're thinking about the wrong kind of fuel cell.

"You say fuel cells and it's like the kiss of death," said Michael Tucker, a chemical engineer at Lawrence Berkeley National Laboratory who is researching new ways of making fuel cells. "But that's because [people] associate the fuel cell with two things: the hydrogen economy, which doesn't exist, and PEM fuel cells, the hydrogen kind."

PEM, or polymer electrolyte membrane, fuel cells can convert hydrogen into electricity with the help of a catalyst at fairly low temperatures. They were supposed to be used largely in transportation to power cars. Whatever their merits, they haven't had the impact that some analysts predicted years ago.

But solid oxide fuel cells are different. While they are conceptually less attractive because they run at high temperatures (more than 1,500 degrees Fahrenheit) and high pressures (10 times atmospheric pressure), they don't require the fragile membranes and expensive catalysts made from precious metals like PEM cells do.

Despite this savings, the cost of solid oxide fuel cells is still too high, which is a major deterrent to their adoption.

"There needs to be a benefit to overcome cost differential," said Tucker. "You need to offer something better, and cheaper."

Right now, projections by backers of solid oxide fuel cells show that if they were able to manufacture them in large numbers, they'd be commercially competitive, Tucker said. But it's hard to know if those projections are realistic. Despite all the technical advances and DOE-directed research, cheap fuel cells still aren't really on the market.

"There is a reason that you can't buy one," said Tucker. "No one wants one at the cost that they can manufacture it at."

He's working on a new way of making the fuel cells largely out of stainless steel instead of the ceramic commonly used. This could be radically cheaper than the current technology, which would make it competitive with standard power sources.

Natural gas has had an up-and-down career in the American energy supply. After massive growth throughout the 1950s, the mistaken perception that natural gas was fairly scarce stalled its adoption for electricity production. Now, new finds and extraction methods mean that natural gas is often viewed as a bridge fuel from the oil and coal of the past into some energy future based on renewable and/or nuclear power

Still, natural gas burned in a regular power plant produces between one-third and one-half the carbon dioxide emissions of a coal plant. And it's very difficult to separate out the carbon dioxide from the rest of the flue gases that come out of the combustion process. It may be cleaner, but it's certainly not as CO2-light as nuclear or solar power.

Solid oxide fuel cells are composed of a stack of three ceramic layers: the anode, an electrolyte and a cathode. Oxygen atoms pick up electrons in the cathode and travel through the electrolyte to the cathode, where the charged oxygen atoms are combined with hydrogen to produce electricity and water.

Higher-efficiency fuel cells would make using natural gas as clean as possible. But what form an eventual fuel-cell plant with carbon capture would take is not yet clear.

Scott Samuelsen, director of the National Fuel Cell Research Center at the University of California, Irvine, criticized the MIT paper for making assumptions that underestimated fuel cells while overstating the benefits of their theoretical power plant.

"The analysis that is conducted here is a bit naive," Samuelsen said.

One special property of solid oxide fuel cells is that they can use any fuel stuck into them, an ability known as internal reformation. But Adams' model does not incorporate this ability. Instead, it adds a step to transform natural gas into a different gas mixture, heavy in hydrogen and carbon monoxide before it goes into the fuel cell, without accounting for the energy needed in that process, Samuelsen said.

"The paper ignores the internal reformation capabilities," Samuelsen said. "It's like taking the heart out of the patient and describing how the patient behaves."

Adams countered that some data shows that putting natural gas directly into the fuel cells can lead to carbon deposit buildups that reduce efficiency and cause the cells to die earlier. Though he noted that many researchers were trying to solve exactly this problem, his team preferred to work around that problem in their model plant instead.

Tucker also took issue with some of the assumptions in the paper. He thought they were too optimistic about both the cost of solid oxide fuel cells and the price of carbon that could flow out of any climate legislation. But he said that researchers at his own lab and elsewhere were trying to come up with ways of making fuel cells cheaper.

Even if fuel-cell power plants that capture the carbon dioxide they produce began to spring up across the country in the coming years, they'll have to store all that CO2 somewhere. Carbon dioxide sequestration research continues, but serious doubts have been voiced by energy researchers, like Vaclav Smil of the University of Manitoba, about the volume of material that the carbon capture and sequestration industry would have to handle.

To sequester just 10 percent (.pdf) of the world's carbon dioxide emissions would require building an industry "that would have to force underground every year the volume of compressed gas larger than or (with higher compression) equal to the volume of crude oil extracted globally," Smil wrote in 2008 paper.

Some of that CO2 could be diverted to industrial processes that need it, like brewing or enhancing oil recovery in depleted fields. And fuel-cell power plants could fit elegantly into such a future system because, unlike traditional turbines, their efficiency is not dependent on their size. Fuel cells work well at whatever size fits the need for power on the site.

Images: 1) Power plant at Morro Bay, CA. whittiz/Flickr. 2) Siemens. 3) DOE.

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