martes, junio 30, 2009

Is Farming for Electricity More Efficient?

New York Times, Green Inc. May 11, 2009, 7:20 am
James Estrin/The New York Times

Raising crops to produce electricity, which will in turn power cars, is more efficient, a new study says, than raising crops to create ethanol to use as fuel in cars.

According to a study by three California researchers, an acre planted with corn for ethanol will provide far fewer miles of transportation fuel as the same acre growing trees or switchgrass, which are then burned in power plants that provide the power to charge the batteries of electric cars.

In fact, even ethanol made from cellulose, a technology that does not now exist in commercial form, is not as efficient a use of biomass as burning it in a power plant would be, the researchers found.

In a paper published in the current issue of Science magazine, Chris Field, a professor of biology at Stanford and director of the Department of Global Ecology at the Carnegie Institution, Elliott Campbell of the University of California, Merced, and David Lobell of Stanford’s Program on Food Security and the Environment, write that the size of the advantage would depend on many factors.

These include the number of miles per gallon any particular vehicle will go on ethanol, and what a battery weighs per kilowatt-hour of energy stored. As batteries get lighter, for example, it takes less energy to move them.

But the researchers estimated that a small battery-powered S.U.V. would go nearly 14,000 miles on the highway on the energy from an acre of switchgrass burned to make electricity, compared to about 9,000 miles on ethanol.

One reason, the researchers say, is that the electric vehicle is inherently more efficient than an internal combustion burning ethanol — or anything else.

Greenhouse gas production would be lower, too, and could even be negative, the researchers said, if carbon-capture technology is used.

If one grows a tree or annual crop, for example, which pulls carbon dioxide out of the air, burns it in a power plant that captures and stores escaping CO2, and then replaces it with another crop, which pulls yet more carbon dioxide out of the air, the process becomes carbon-negative.

The “miles per acre” question and the amount of farmland diverted for use in producing transportation fuel is a sensitive political question, with American use of corn for ethanol blamed in part for last year’s run-up in global grain prices.

And the effect of ethanol on global warming is also under debate. Last week, the Environmental Protection Agency published a draft rule on renewable fuels that endorsed the concept that if more American corn is diverted for ethanol, then somewhere else in the world, someone is going to clear a patch of forest, or jungle or grassland, and plant food crops.

Clearing virgin land releases more carbon dioxide.

Advocates of biomass say that it can be sustainably withdrawn from land that is not cropland. But some researchers have already found that while this is true, almost anything grows better on cropland than it does in less favorable environments.

But ethanol has a leg up, because internal combustion engines and tanks for liquid fuel are far less expensive that batteries and motors, and the tanks and engines are already in common use. And consumers like vehicles that run on liquid fuel, because refueling takes minutes instead of hours, and they can run hundreds of miles between refuelings.

And there may be other environmental issues, the authors said, like water use by the power plant, or the toxicity of batteries.

No hay comentarios.: