Gregoire's CleanTech Blog

An Alternative to your Alternator

New thermophotovoltaic materials could replace alternators in cars and save fuel.

A "thermophotovoltaic" (TPV) generator burns fuel to heat a material that emits light (labeled "radiator"). The light is then filtered and converted into electricity in photovoltaic (PV) cells. (Image courtesy of John Kassakian, MIT.)

Researchers at MIT are developing new technology for converting heat into light and then into electricity that could eventually save fuel in vehicles by replacing less-efficient alternators and allowing electrical systems to run without the engine idling.

The technology, called thermophotovoltaics, uses gasoline to heat a light-emitting material, in this case tungsten. A photovoltaic cell then converts the light into electricity. The idea has been around since the 1960s, says John Kassakian, MIT electrical engineering and computer science professor. But until now, the light emitters for the photovoltaics produced inefficient and very costly systems. Improvements in the materials used in these latest devices -- possible in part because researchers can modify the material structure at the nanoscale -- are now making much more efficient systems, Kassakian says.

According to Kassakian, the system could potentially be a more efficient way to power electrical systems in a vehicle than the current alternator-based one, which wastes energy in two stages: the internal combustion engine converts only about 30 percent of the energy in fuel into movement, and then the alternator is only 50 percent efficient in converting the mechanical energy into electricity. He says a small prototype thermophotovoltaics device that could confirm the system's improved efficiency might be ready in a year.

The researchers modified the surface structure of the light emitter, etching into it nano-sized pits to tune the wavelengths of light emitted to precisely those a photovoltaic cell can convert most efficiently into electricity. They further refined the device with the use of filters that allow the desired wavelengths of light to pass through to the photovoltaic cells, but reflect other wavelengths back to the light emitter. The reflected light carries energy that helps keep the emitter hot, reducing the amount of fuel needed.

In addition to replacing the alternator with a thermophotovoltaic module, says Kassakian, the technology could be part of an air-conditioning system for vehicles that doesn't require a compressor. Because this would significantly decrease the load on an engine, it could make it possible to turn off the engine when the vehicle stops in traffic and easily restart it. Today's hybrids use this technique to save gas, but require large batteries to provide electricity for the radio and lights, and to restart the engine, and they have to turn the engine back on when the battery charge runs out. In the new MIT system, these batteries wouldn't be necessary.

Indeed, if the new materials do make more efficient thermophotovoltaics, there will be no shortage of potential uses. Kassakian says a particularly good application, when combined with the a new air-conditioning system, could be for use in trucks with sleeper cabs to cool the cabs and provide electricity for televisions and other equipment. And, says Donald Chubb, a researcher at the NASA Glenn Research Center, "The military has had a lot of interest in it for portable power supplies in the field. Because there are no moving parts, there wouldn't be any noise, so you couldn't detect it."

It will be several years before the technology can used in vehicles, though. Challenges include scaling up the fabrication process, developing effective insulation and cooling systems to manage the heat generated, and ensuring that the system remains efficient on a larger scale. The MIT research is funded in part by Toyota, but the automaker has not committed to implementing the technology.

By Kevin Bullis

Source : Technology Review

Better than Hybrids : A proposed engine design approaches the efficiency of gas-electric hybrids, but could be far cheaper.

Consumers hoping to cut gasoline spending, with average gas prices nearing $3 a gallon, could opt for hybrids. But even with gas prices high, the added cost of hybrid cars can cancel money saved at the pump, suggesting the need for lower-cost alternatives.

A new type of ethanol-boosted, turbocharged gasoline engine could be the answer. The engine would be almost as efficient as gas-electric hybrids, but cost much less, according to its MIT inventors -- Leslie Bromberg and Daniel Cohn, plasma science and fusion center researchers, and John Heywood, professor of mechanical engineering.

The new engine would improve efficiency in two ways. The first is to decrease the size of the engine, which reduces friction, thus saving fuel at light engine loads, such as during city driving. When more power is needed, a turbocharger kicks in. It uses exhaust flow to compress air, making it possible to combust more air and fuel in a smaller space.

The second approach is to engineer the engine to have a higher compression ratio -- the ratio of the volume of air and fuel before and after it is compressed in an engine. A higher compression ratio "makes the engine more efficient, because you expand the burned gases more and extract more energy out of them," Heywood says.

Neither of these are new ideas. But in the past, such efforts have been limited by a phenomenon called knock: high compression ratios and extreme turbocharging cause gasoline to spontaneously combust when the engine is under heavy loads, such as during acceleration or at high speeds, potentially causing serious damage. The MIT researchers have found a way to prevent knock, allowing them to crank up the turbocharger and increase the compression ratio -- and thereby increase the power of an engine by 250 percent.

If this increase in power is taken advantage of to reduce the size of the engine -- which would go against long-time trends emphasizing performance over fuel economy -- it could save gas. "This allows very large pressure turbocharging, very large downsizing of the engine, and makes it possible to have a small engine with much higher efficiency," Cohn says.

The researchers solved the knocking problem by injecting into combustion chambers precisely controlled amounts of ethanol at moments when the engine is working hard enough to cause knock. Compared with gasoline, ethanol has higher octane, a rating of how much a fuel can be compressed before it combusts spontaneously, that is, before it causes knocking. The injected ethanol also cools the mixture, so it effectively increases the octane rating of the fuel mix to about 130 -- as good as high-performance racing fuels, Cohn says.

The system would use relatively little ethanol, about

1 gallon

per

20 gallons

of gasoline, so Cohn estimates the separate ethanol tank would have to be refilled about as often as an oil change. Furthermore, since it would require relatively minor modifications to existing technologies, Cohn says the design could be in production vehicles as soon as 2011 -- with the help of a recent collaboration between their startup, Ethanol Boosting Systems (EBS), Cambridge, MA, and Ford Motor Company, Dearborn, MI.

The MIT researchers estimate their engine would add only $500-1000 to the cost of a vehicle, which includes the added costs of the high-end turbocharger, a direct-injection system, and a stronger, smaller engine. This modest premium compares favorably to that of hybrid cars. According to a review in Consumer Reports (April 2006), "none of the six hybrids we tested recovered its price premium in the first five years" -- even factoring in federal tax credits and gas prices that rise to $4 a gallon. In contrast, Cohn says, their engine would pay for itself in two to three years.

The new engine should be 30 percent more efficient than conventional engines, based on a computer model the researchers say accurately reproduces the behavior of internal-combustion gasoline engines. In comparison, a Toyota Prius gets about 30-35 percent better fuel economy than a comparable vehicle, according to tests by Consumer Reports. In the same review, the magazine showed a $5,700 price premium for the Toyota Prius over a conventional vehicle.

Rodney Tabaczynski, former director of powertrain research at Ford (who is not involved with EBS), says the ethanol "will definitely help the octane problem" and existing electronic controls and feedback systems should make the controlled injection feasible.

The challenges EBS is likely to encounter he says, have more to do with logistics -- two fuel tanks in a vehicle can be hard to implement, and there's the challenge of making sure ethanol is available at the corner gas station. Also, the engine will need a system that ensures it isn't damaged if the driver forgets to fill the ethanol tank.

Tabaczynski also cautions that real fuel savings will depend on an individual's driving habits. As with hybrids, cars with these engines will get their best mileage when driven in a city, not at 70-

75 miles

per hour on the highway with the throttle wide open.

By Kevin Bullis

Source : Technology Review

Clean Diesel from Coal : A novel catalytic method could let you fill up your tank with coal-derived diesel, cutting U.S. dependence on foreign oil.

As the cost of oil soars and worries over the U.S. dependence on foreign petroleum escalate, coal is becoming an increasingly attractive alternative as a feedstock to make a range of fuels. Now chemists have invented a new catalytic process that could increase the yield of a clean form of diesel made from coal.

The method, described in the current issue of the journal Science, uses a pair of catalysts to improve the yield of diesel fuel from Fischer-Tropsch (F-T) synthesis, a nearly century-old chemical technique for reacting carbon monoxide and hydrogen to make hydrocarbons. The mixture of gases is produced by heating coal. Although Germany used the process during World War II to convert coal to fuel for its military vehicles, F-T synthesis has generally been too expensive to compete with oil.

Part of the problem with the F-T process is that it produces a mixture of hydrocarbons -- many of which are not useful as fuel. But in the recent research, Alan Goldman, professor of chemistry and chemical biology at Rutgers University, and Maurice Brookhart, professor of chemistry at the University of North Carolina at Chapel Hill, use catalysts to convert these undesirable hydrocarbons into diesel. The catalysts work by rearranging the carbon atoms, transforming six-carbon atom hydrocarbons, for example, into two- and ten-carbon atom hydrocarbons. The ten-carbon version can power diesel engines. The first catalyst removes hydrogen atoms, which allows the second catalyst to rearrange the carbon atoms. Then the first catalyst restores the hydrogen, to form fuel.

Diesel fuel produced in this way has several potential advantages. Ordinary diesel contains molecules, called aromatics, that, when combusted, produce particulates, Goldman says. But the diesel formed by the new catalysts does not include aromatics, so it burns much cleaner, overcoming one of the major objections to diesel fuel. This could lead to more vehicles using diesel engines, which are about 30 percent more efficient than gasoline engines.

But the biggest advantage may be that the United States has huge amounts of coal: "We have as much energy in coal as the rest of the world has in oil. That's enough to last us the next hundred years or so," Goldman says. Thus, a more efficient, and so less expensive method of converting coal to diesel could significantly cut U.S. dependence on foreign oil, and do so for a long time.

"When I saw this I thought it was really a terrific contribution that could be very important," says Richard Schrock, professor of chemistry at MIT, who won the Nobel Prize in Chemistry in 2005, with two other scientists, for discovering the type of catalyst used in the second step. Combining two catalysts this way "is pretty rare," he says. "You can't just throw any two things together and expect to get the results you anticipated."

According to Robert Grubbs, professor of chemistry at Caltech, who shared the Nobel prize with Schrock, "The key is finding catalyst systems that are compatible, and will operate at the temperatures where you can do both processes together."

At this time, the new catalytic method is still a proof-of-concept, and not ready for commercial use. For example, the second catalyst tends to break down. But Schrock says this problem should be solvable: "It's theoretically possible that this could become practical. I e-mailed Alan Goldman and said, 'Look, we've got a lot of catalysts, and I can think of some things that might be thermally more stable.' So I'm going to send him some catalysts, and he's going to try them out."

It also might be possible to make catalysts that use products from the first reaction to regenerate themselves. "Then the catalyst wouldn't die, and you could in fact keep the reaction going," says Schrock.

By Kevin Bullis

Source : Technology Review