POSTED 8 FEBRUARY 2007
Renewable fuel: Help for global warming?
In terms of the greenhouse warming that's cooking the home planet, here is the renewable-fuel question that matters: How much greenhouse gas gets made by burning comparable amounts of various fuels? (Okay, it might be even smarter to measure passenger miles per ton of greenhouse gas produced, but given the furious pace of global warming, should the perfect be the enemy of the good?)
Photo: ©David Tenenbaum
When Alexander Farrell of the Energy and Resources Group at the University of California at Berkeley examined greenhouse-gas production from ethanol, he learned the importance of the process: "How do you make the ethanol? The number of greenhouse gases associated with ethanol varies a great deal, by a factor of two or three, depending on the feedstock and process used to convert feed into fuel."
In a 2001 study of U.S. ethanol production (see #2 in the bibliography), Farrell calculated that corn-derived motoring-moonshine "probably yielded a slight decrease in greenhouse gases, per unit of energy delivered to the car, in the neighborhood of 15 percent." He admits "there are many uncertainties associated" with this calculation, ranging from a 36 percent reduction to 29 percent increase.
Getting something for nothing, or getting nothing for something?
As the United States tries quelling its swelling thirst for fossil fuels, biofuels are also being hyped as a renewable way to move vehicles -- as fuel from the sun. But in this age of ballooning energy shortages, some skeptics wonder how much energy you get back from biofuels.
When you tap an energy source, you want more get than give. This, after all, is the payback from petroleum: You get 15 joules of gasoline energy for your gas tank if you give 1 joule of energy to drilling, refining and shipping the fossil fuel.
That's a 15 to 1 payback. Unfortunately the corn-ethanol payback is not so happy. Depending on which study you believe, motoring moonshine yields 0.75 to 1.5 times the energy you invest to make it.
Payback calculations count the energy flow in and out of the ethanol process. Growing plants use solar energy to make the sugars and carbohydrates that can be converted to ethanol. But this "biomass energy" is not free: to get it from corn, you need to plow, plant, fertilize, cultivate, harvest, dry and transport the crop. Then you need to ferment the kernels into ethanol, and distill (concentrate) the alcohol into fuel. Each step takes energy.
Improved from DOE.
What's in it for me?
To get gasoline from petroleum, you give energy exploring, drilling, transporting, storing and refining. You give the energy equivalent of 1 liter of gasoline, and you get 15 liters of gasoline. The 3.2 billion liters (20 million barrels) of crude oil guzzled each day in the United States proves that 15 to 1 is an acceptable payback.
Let's return to the net energy calculation. A 1991 study by David Pimentel of Cornell University (see #3 in the bibliography) found that corn ethanol would produce only about 0.75 times the input energy -- in other words, it was an energy sink, not a source.
Ethanol advocates have since contested Pimentel's results, claiming that he misunderestimated corn production per acre, for example. But even if we accept that corn ethanol has a positive payback (the highest estimates reach about 1.8), is the game worth the candle?
In 2006, Robert Costanza and colleagues calculated that a middle-range positive payback (give 1 liter to get 1.2 liters) would mean that a good corn crop would yield "a mere 60 gallons [of ethanol] per acre net yield, not even two fill-ups for an SUV. The entire state of Iowa, if planted in corn, would yield approximately five days of gasoline alternative" (see #4 in the bibliography).
Corn ethanol: Not all it's cracked up to be?
Costanza, who is now director of the Gund Institute for Environmental Economics at the University of Vermont, should know about net energy. In 1984, he first proposed to analyze biofuels with an approach he called Energy Return on Investment (EROI; see #5 in the bibliography).
The EROI method compares the output of an energy source to its fossil-fuel inputs. If you spend too much energy getting energy, you'll have less left over to haul your carcass to the mall, power your Wii, or heat your house.
The exact payback of a particular energy source is, Costanza, acknowledged on the phone, "an open question, but the numbers do not put these energy sources anywhere near what we been used to with oil or coal. That's the real concern. We are going to be putting lot a more energy back into just getting energy."
These slightly-better-than-breakeven numbers for corn ethanol are based on faulty analysis, says Farrell. Comparing energy "give" to energy "get" makes little sense it this case, because the point of converting corn into ethanol is to make high-value vehicle fuel.
Corn ethanol does not simply increase the energy output by a factor of 1.2 or 1.5. Instead, it makes motor fuel from solar energy (which makes sugar in the corn kernels), coal (source of electricity for the ethanol plant), natural gas (source of corn fertilizer), and diesel fuel (which runs tractors and trucks).
So instead of talking about net energy, Farrell says we should be discussing the benefits of energy conversion. That, he says, is how we evaluate comparable technologies. "Do we really pay attention to how much energy went into mining the coal or natural gas that makes electricity for our computers? No, you think the electricity is really valuable. The net energy is really lousy for electrical conversion [in a generating plant], but we don't care." Electric generating plants and ethanol plants both "take fuel of one quality in, and produce energy of a higher quality," he says.
Although the energy return for corn ethanol sounds straightforward (you just measure inputs and outputs), "There are a lot of uncertainties about how you measure the input and output, where you draw the system boundaries," says Cutler Cleveland, professor of geography and environment at Boston University. "Do you include the energy used to pump irrigation water? Some analysts don't use that. What about the energy consumed by laborers on the farm?" If half your corn crop is getting fermented into fuel, do you count the energy needed to make tractors and tractor factories?
One key "system-boundary issue" concerns the effect of growing so much corn on the soil. "If we are growing corn and depleting the soil, it's not sustainable," says Costanza. "The yield will go down unless we put organic matter back on the soil, and that has a real cost in energy or dollar terms." It may be difficult to pinpoint the dollar cost of soil degradation, but cost-benefit analysis requires it, Costanza says. "It's better to be approximately right than precisely wrong. If you leave [soil costs] out and assume they are zero, you are wrong."
And while it may be true, as Farrell says, that petroleum (mainly diesel fuel for tractors, harvesters and trucks) is only a minor input to corn ethanol, a vast amount of natural gas is used to make corn fertilizer, and natural gas is a tip-top fossil fuel. It has low carbon emissions, and it can even power cars without conversion to fertilizer to grow corn for ethanol.
A further issue concerns the dried-out junk left over after the yeast finish fermenting the corn mash. These "brewer's dried grains" can be added to cattle feed (up to about 20 percent of the feed), so it seems legitimate to add their energy value to the ethanol's fuel value, which raises the payback. But soaring ethanol production may flood the market for dried grains. As Cleveland observes, "You can't burn animal feed in your Toyota." Under those circumstances, is it still legit to count the energy value of the cow-food?
Confused yet? Here's another factor in the biofuel debate: All the energy in gasoline comes from fossil fuel, but corn ethanol contains some solar energy. "Corn ethanol still has better energy balance than petroleum," says Bob Wallace, of the National Bioenergy Center at the National Renewable Energy Laboratory. "Far less fossil fuel is used per gallon of ethanol produced, and it has much stronger environmental benefits per gallon, in terms of greenhouse gas reduction, carbon reductions."
Despite the unlimited level of squabbling on the subject of corn ethanol, here's the weird part: Few (if any) experts expect corn ethanol to "make" a whole lot of energy, or to significantly reduce greenhouse gases. "I don't think corn will ever displace gasoline," says Wallace. "It's not possible to grow enough corn to make a serious displacement of petroleum fuel. We used 120 billion gallons of gas in 2004; there is no way we could make that much corn ethanol."
In other words, no matter what you may have heard, corn ethanol will not stem global warming. Any chance we could make moonshine motorfuel from cellulose?