Nonstop, coast to coast by Lesley V. Kriewald
Skyrocketing prices at the pump are making us more interested than ever in vehicles that get good gas mileage. How about one that gets 90 miles a gallon and runs on garbage?
Imagine climbing into your car in California and driving to New York — without stopping once to fill the fuel tank.
Mark Ehsani, professor and holder of the Robert M. Kennedy ’26 Professorship in Electrical Engineering, has spent 15 years working on hybrid vehicles. Ehsani is now working to hybridize the superefficient StarRotor engine.
For engineers Mark Holtzapple and Mark Ehsani, it’s more than a fantasy trip. For them, that 90-miles-per-gallon car is the future, and they’re already partway down the highway. Their crop-to-wheel concept should provide both a remarkably efficient engine and a sustainable source of fuel, one that doesn’t depend on foreign oil producers.
The Crop-to-Wheel Initiative focuses both on new fuels and vehicle power train technologies. Holtzapple, a professor in the Artie McFerrin Department of Chemical Engineering, and electrical engineering professor Ehsani are developing technologies to transform crops into liquid fuels that can be burned in high-efficiency cars.
Garbage to gas
Fuels first. Holtzapple has developed the MixAlco process, so named because of the mixed alcohols that result. It converts biomass — trees, grass, manure, sewage sludge, garbage — into mixed alcohols for use as fuel. His research group operates a pilot plant on campus.
“We can use anything biodegradable,” Holtzapple says. “If you put it outside and it rots, we can use it.”
The process also can use high-productivity feedstocks, such as sweet sorghum and “energy” cane. Alcohol fuels produced from these crops are more productive in terms of net energy per acre than ethanol produced from corn. And water hyacinth, a weed that chokes waterways if left to grow uncontrolled, is even more productive, Holtzapple says.
“You’ve heard of alchemists trying to turn lead into gold,” Holtzapple says. “We turn manure into rubbing alcohol. We’re turning something useless into something useful.”
In the MixAlco process, the biomass feedstock is treated with lime and then fermented to form organic salts. Water is removed and then the mixture is heated to become ketones, such as acetone, a common ingredient in nail polish remover. Adding hydrogen to the ketones forms mixed alcohols, which can be used as biofuels.
“MixAlco is a robust process that uses naturally occurring organisms derived from soil,” Holtzapple says, “so no sterility is required in the process. In contrast, other researchers use genetically engineered organisms that require sterile — and expensive — equipment.”
In addition, biofuels are kind to the environment: Combustion of biofuels doesn’t contribute to global warming because no net carbon dioxide is released into the atmosphere. Carbon dioxide released from the combustion of biofuels is recycled through photosynthesis, unlike carbon dioxide released from the combustion of fossil fuels, which accumulates in the atmosphere.
Crop-to-wheel Initiative
Biomass feedstock — high-yield energy crops such as energy cane or sweet sorghum; agricultural residues such as corn stalks and wheat straw; manure and even municipal wastes such as refuse and sewage sludge — is processed in a biorefinery, which uses microorganisms derived from soil to convert the feedstock to organic acids. The acids are converted to ketones such as acetone and then transported to an oil refinery where they are hydrogenated to alcohols. Carbon dioxide can be injected into oil wells to enhance oil recovery. The mixed alcohols are combined with conventional gasoline at an oil refinery and then transported through existing pipelines and petroleum infrastructure to your local gas station. Finally the mixed alcohol–gasoline fuel is pumped into your high-efficiency or hybrid vehicles, while any carbon dioxide emitted from your tailpipe is consumed by the growing biomass, releasing no net carbon dioxide into the atmosphere and starting the whole cycle again.
How does ethanol stack up against mixed alcohols? |
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| Octane. Mixed alcohols and ethanol have a high octane rating, which is required to prevent internal combustion gasoline engines from knocking, which can cause damage. |  |
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| Low volatility. Volatile emissions from the fuel tank cause air pollution. Ethanol is very polar, which raises the fuel volatility. Mixed alcohols have a low volatility. | |
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| Pipeline shipping. Fuel components should be shipped through pipelines to lower costs, but ethanol is so polar that it absorbs water in the pipelines, which causes fuel problems. To prevent this, ethanol is shipped by train or truck to the terminal, where it is “splash” blended — an expensive proposition. Mixed alcohols can be shipped through the pipelines. | |
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| High energy content. The purpose of fuel is to store energy. Fuels with a high oxygen content, such as ethanol, have a low energy content, whereas fuels with a lower oxygen content, such as mixed alcohols, have a high energy content. | |
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| Heat of vaporization. Ethanol requires a lot of energy to vaporize, which can cause engine-starting problems. Mixed alcohols have a lower heat of vaporization. | |
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| Groundwater damage. Fuel is stored in underground tanks, which tend to leak. Mixed alcohols and ethanol do not damage groundwater. | |
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Goodbye, V-8?
The StarRotor engine uses the Brayton thermodynamic cycle used in jet engines. But unlike jet engines, which use spinning fan blades, the StarRotor engine uses gerorotors for the compressor and expander. The compressor raises the air pressure to about 6 atmospheres. This high-pressure air is preheated in a heat exchanger, while in the combustor, fuel is added to raise the air to the final temperature. Then, this hot, high-pressure air is sent to the expander where work is produced. Finally, the air is released at 1 atmosphere. The air is still hot, so it is sent to a heat exchanger where most of the remaining heat is captured and recycled within the engine. New fuels deserve a new engine, and Holtzapple has one: the StarRotor engine. It uses the Brayton cycle, the same thermodynamic cycle used in jet engines. Air is compressed, fuel is combusted and the hot high-pressure gas expands, thus doing work. Conventional jet engines use high-speed spinning fan blades to accomplish the compression and expansion, but the StarRotor engine uses lower-speed positive-displacement rotors, which are much more suitable for automotive applications, Holtzapple says.
So far, they’ve built the compressor half of the StarRotor engine. Recent measurements indicate that a complete StarRotor engine would be about 55 percent to 65 percent efficient, which is about three times more efficient than today’s reciprocating engines. Holtzapple says that building a complete engine will take about one year once the funding is raised.
The more fuel-efficient an engine is, the less fuel it needs, and the less energy cane or sweet sorghum must be grown to fuel the vehicle.
“The combination of high-efficiency engines and high-productivity crops greatly reduces the required land area to supply the nation’s motor fuels,” Holtzapple says. “This overcomes the most common objection to corn-derived ethanol: that there simply is not enough land to make a big impact on the nation’s fuel needs.”
Enter the hybrid
Conventional vehicles throw away energy every time they brake. Ehsani explains that capturing this energy can make vehicles more fuel efficient, hence, the hybrid.
Why Not Corn?
Ethanol produced from corn grain may be the most talked-about biofuel, but Holtzapple says it isn’t the only option, or the best. Alcohol fuels from high-productivity crops such as energy cane or sweet sorghum are far more productive than corn ethanol, so scientists can minimize land area required for growing feedstock by using these higher-productivity crops. Second, farmers can gross two to three times more per acre by growing these high-productivity energy crops instead of corn. Third, there is less environmental impact — water, fertilizer, pesticides, soil erosion and herbicides — when growing energy cane or sweet sorghum than when growing corn grain.
Hybrid cars have a fuel-powered engine plus an electric machine that can function as either a motor or a generator. In generator mode, a battery charges and slows the vehicle. In motor mode, the battery drains and speeds the vehicle. Ehsani says that this hybrid system captures energy normally lost in braking into the battery, which in turn increases fuel mileage, particularly in stop-and-go city driving.
Hybrid cars have another advantage: They can baby the engine. Any engine, including the StarRotor engine, operates most efficiently when run at a constant speed, but driving in traffic means the engine must speed up as you accelerate and slow down when you idle at a stoplight. Incorporating the StarRotor engine into Ehsani’s Electrically Peaking Hybrid (ELPH) Vehicle will produce the best efficiency, emissions, performance and cost.
“The StarRotor engine delivers average power,” Ehsani says, “but a battery provides peaking power that allows the vehicle to accelerate quickly. This compact and efficient traction system has a battery that never needs charging and minimizes fuel consumption.”
Holtzapple says, “We plan to get 90 miles per gallon in a conventional car equipped with a hybridized StarRotor engine. That means we can drive from Los Angeles to New York City on 31 gallons.”
Someday, there will be an economic end to the petroleum age, the researchers say, and our economy will suffer if we don’t prepare for it. “The previous energy crisis in the 1970s was just practice. This is the real one,” Holtzapple says.
Beyond all or none
Mark Holtzapple and Mark Ehsani have spent their careers preparing for an energy crisis that hasn’t yet happened. But they say it will happen, and their integrative, multidisciplinary Crop-to-Wheel Initiative may be the answer.
But biofuels won’t replace fossil fuels entirely — at least not soon. We have invested trillions of dollars in the fossil fuel infrastructure — drilling rigs, refineries, pipelines, trucks and the like — and it will take a long time to replace it. Instead, as biofuels become available, they can be mixed with gasoline, transported in existing pipelines and finally pumped into cars at gas stations. And as gas gets more expensive, the percentage of biofuels in the mix can be increased.
Ehsani and Holtzapple call this an enabling technology. “Put the whole picture together,” Holtzapple says. “We can convert fossil fuels such as coal or natural gas to hydrogen and carbon dioxide, which can be stored underground to address global warming.
“The hydrogen is chemically bound to a biomolecule, which can be safely burned without adding net carbon dioxide to the atmosphere. This approach allows us to embrace both fossil fuels and the so-called hydrogen economy, while still building sustainable energy systems.”
“There’s a path from where we are to where we’re going,” says Ehsani, who holds the Kennedy Professorship in Electrical Engineering.
And, Holtzapple adds, “It’s not ‘all or none.’ We can use individual pieces of the technology.”
Holtzapple and Ehsani say the whole crop-to-wheel concept can be refined and perfected by Texas A&M researchers. For instance, associate professor Othon Rediniotis in the Department of Aerospace Engineering is working to reduce aerodynamic drag on the car, while plant scientist Erik Mirkov at the Texas A&M Agricultural Research and Extension Center in Weslaco is working to make energy cane more cold tolerant so it can grow more widely.
“Our vision is to pull in as many faculty members as possible,” Ehsani says. “This is truly multidisciplinary, and Texas A&M is uniquely positioned to achieve this vision. It’s no accident that this integrated idea happened here. Where else do you have a world-class agricultural school side by side with a world-class engineering school?”
Energy and gas mileage may be the latest new things, but Holtzapple and Ehsani have been working on the problem for decades.
“We’ve dedicated our careers to a crisis that hasn’t happened yet,” Ehsani says. “Dr. Holtzapple was doing bio when bio wasn’t cool, and I was doing hybrids when hybrids weren’t cool.”
The crop-to-wheel idea is unique, Holtzapple says, because it’s integrative: It makes sense from beginning to end.
“We are the only group that we know of that is solving the problem in an integrated way, from the crop to the wheel.” 
Texas A&M Engineer Online
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[ Back to 2006 Issue ]Business
Energy
- Biomass and clean air
- Energy 101
- Keeping the lights on
- Nonstop coast to coast
- Nuclear by the numbers
- Petroleum under pressure
- Policy + technology = security
- Tapping the trash alternative
- To drill or not to drill
