The Silver Bullet: Plug-in Biofuel Hybrids

By · February 02, 2009

The trash bin of car history is filled with failed attempts to produce an emissions-free petroleum-liberated automobile. Hydrogen, ethanol, compressed natural gas, battery-electric, and ultra-lightweight cars have all been trotted out as silver bullet solutions, only to fail the tests of practicality and/or market acceptance. Environmentalists and policy wonks fed up with the next technology-du-jour have swapped silver bullets for silver buckshot, often described as a “portfolio” approach to reducing tailpipe emissions and oil dependence.

Unfortunately, by giving up on an ultimate answer, we have become too timid to choose any winners and losers. That’s a shame, because it looks as if the silver bullet is right before us: the plug-in biofuel hybrid. Let’s parse these words and technologies.

Why Plug In?

Mini EV Plug

Electricity is a cost-effective fuel source for cars.

We should definitely be plugging in our cars—for one main reason: electric 'fuel' is cheap. At $2 a gallon, it costs about 13 cents per mile to gas up a conventional vehicle with a city EPA rating of 15 mpg. The same vehicle cost 26 cents per mile with $4 gas—which hopefully we won’t see again (but don’t count on it). Of course, Europeans are still paying more than $6 a gallon, most of it tax.

How much does it cost to run a 4,000-pound car on grid electricity? If you charge it overnight, about 2 cents per mile, and during a relatively expensive recharge during the day, about 6 cents per mile even with relatively expensive public re-charging.

There are obvious environmental benefits for running as many miles as possible from electrons, but the limited range of full battery-electric cars, and the cost of those batteries—considering limited battery durability—means that lifetime costs are prohibitively high. For example, if a battery lasts 50,000 miles—say 2,000 recharge cycles times 25 miles per day—and costs $5,000 to replace, it adds 10 cents per mile to the overall cost-per-mile of the vehicle. Until battery durability improves and costs fall much further, it will be difficult for most economically rational American car buyers to justify purchasing a battery-only vehicle. But that doesn’t mean throwing away the batteries. Time to hybridize.

Why Hybrids?

Subaru R1E

Small battery-only cars, like the Subaru R1E concept, are great for city speeds, but larger all-electric vehicles on the highway will require much heavier and more expensive batteries.

It’s a matter of physics. As a car speeds up, it requires more energy to cover each mile, and the main reason is air resistance. At city speeds, air resistance is almost irrelevant, so small battery-only cars and delivery vans will be able to provide acceptable range with small and relatively inexpensive batteries when running at city speeds. However, take a battery-only vehicle on a free-flowing freeway—if you can find one—and a tall, blunt, electric van or SUV might need twice as heavy and expensive a battery to cover the same distance.

The answer is to extend the range of an electric vehicle by adding a small engine running liquid fuel. That was the idea behind the original hybrid concept—from more than 100 years ago. Get the batteries to carry you for the first few miles of each trip, but once the batteries have been drained, fire up the engine to drive the wheels.

Dramatic reductions in battery cost, size and weight will be needed before an all-electric vehicle will become as economically viable as a plug-in hybrid with the same functionality. Don't hold your breath.

Why Biofuel?


Electricity can take us most of the way toward reduced oil consumption. Biofuels, like cellulosic ethanol from switchgrass, can get us the rest of the way.

The general enthusiasm for electric drives—especially from renewable grid energy—doesn’t mean throwing away liquid fuels entirely. As batteries get cheaper and more robust, the proportion of annual mileage powered by a hybrid's engine should shrink, but it is unlikely that 'engine-less' cars will gain more than a small percentage of light-duty vehicle sales for the coming decade. The latest forecast from the Boston Consulting Group suggests it will take until at least 2020 before all forms of plug-ins take more than 5 percent of the US market.

Here’s the good news. If sleek, energy-efficient, plug-in hybrids gain a foothold in the market, we could be much closer to energy independence for transportation. The rest of the distance can be made up with domestically produced advanced biofuels and our remaining petroleum resources.

The US has sufficient land already set aside from food production to be able to grow enough environmentally benign crops—such as indigenous switchgrass and miscanthus—to meet most of the remaining demand for liquid fuels in a national fleet of plug-in hybrids.

Biofuels Digest estimated that US venture capitalists invested $437 million in cellulosic ethanol firms during 2008. Verenium Corporation just announced it will start building its first commercial cellulosic ethanol plant in Florida later this year. The plant will be able to produce up to 36 million gallons a year, starting in 2011. So 'cellulosic' is no longer just a clever lab experiment but is set to take over from corn ethanol, releasing more corn for animal feed, etc. The large-scale production of biofuels does not necessarily reduce US food production. (Also, see "The World's Energy Problem and What We Can Do About It," a lecture by Steven Chu, US Secretary of Energy. Dr. Chu's lecture begins at the 9:00 mark in the video.)

In addition, the next generation of hybrid drives will be much more efficient allowing the use of smaller engines—that can take full advantage of the superior combustion efficiency of biofuels. As a result, biofuel will cost less per mile than gasoline. And smaller engines will be less expensive, partially offsetting the cost of adding hybrid functions.

Boldly Look Ahead

The combination of electric fuel and biofuel is a winner. So forget mpg. That's so 'last millennium!” From now on, it's going to be about total cost of ownership and cents per mile, and for most governments and all manufacturers, it will be net CO2 per mile. The plug-in biofuel hybrid is the silver bullet we shouldn’t be afraid to use.

This article was contributed by Chris Ellis, CEO, HyKinesys.

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