If you want to save the planet, don't drive a flex-fuel vehicle, says Stanford's Mark Jacobson.

Jacobson, a professor of civil and environmental energy, has completed a study that ranks sources of fuel according to both their benefits and harmful effects on the planet. While wind came out as the most promising form of energy, nuclear, coal and ethanol ranked at the bottom.

Cellulosic ethanol – the fuel made from wood chips and forest waste that has drawn millions of dollars from green VCs like Vinod Khosla – is even worse than corn ethanol because it takes more land and is worse for wildlife, Jacobson claimed. Cellulosic ethanol may even emit more global warming pollutants than fossil fuels when everything – mining, carbon released in forest clearing, loss of forests, lower mileage – is calculated.

"The energy alternatives that are good are not the ones people have been talking about the most. And some options are downright awful," he said in a prepared statement. "Ethanol based biofuels will actually cause more harm to human health, wildlife, water supply and land use than  current fossil fuels."

The full study – sure to raise eyebrows and draw objections – will be debated at the American Geophysical Union next week in San Francisco. Jacobson will speak at the conference, one of the major annual highlights on the conference calendar for earth scientists, on Thursday and Friday next week.

Wind took the top spot as both a source of conventional electrical power and as a source of electricity for transportation for a number of reasons. For one, with wind, there is a 99 percent reduction in air pollutants. Second, it doesn't take that much land. The entire U.S. vehicle fleet could be operated on power produced by 3 square kilometers of land planted as wind farms, he claimed. Getting the same amount of energy from corn or cellulosic ethanol would take 30 times the amount of land.

Approximately 73,000 to 144,000 5-megawatt turbines could supply the power needed for U.S. transportation, he claimed. And, assuming that livestock won't be afraid of freak decapitations from loose turbine blades, the land between the turbines can be used for grazing.

Connecting all of these wind farms could also reduce the intermittent nature of wind power. The fact that the wind doesn't blow in a predictable manner has always been a huge problem and is one of the reasons ocean power, which can be predicted days and even decades in advance, has drawn investors.

Jacobson made two lists. First, he ranked technologies for their overall potential to generate electricity. From best to worst, the list goes as follows: 1. wind; 2. solar thermal (using mirrors to collect heat and power generators); 3. geothermal; 4. tidal; 5. solar photovoltaics (solar panels); 6. wave; 7. hydroelectric. Nuclear and "clean coal" with carbon sequestration tied for last place in eighth.

Then he ranked different sources of energy for transportation. The cleanest cars are potentially battery-powered electric cars charged by wind. No. 2 on the list were hydrogen fuel cell vehicles with hydrogen produced in plants powered by wind. The rest of the list goes: 3. battery powered vehicles charged by solar thermal; 4. geothermal-charged cars; 5. tidal-charged cars; 6. solar panel-charged cars; 7. wave-charged cars;  8.hydroelectric-charged cars; 9. A tie between cars charged with nuclear or coal. (Hydrogen fuel cells were only examined in how they would work if hydrogen production were powered by wind, so you can slot some extra fuel cell spots in there when thinking of other technologies.)

Corn-based ethanol ranked tenth, or worse than nuclear or clean coal, while cellulosic ethanol cars came in last at 11.

Ethanol and even fossil fuel cars, though, can drive far farther on a tank of fuel than batteries can carry an electric car. Charging a car battery can also take hours. Filling up at a station with gas or hydrogen takes a few minutes. As a result, fully electric cars face other challenges to market acceptance.

Jacobson examined both the potential for these different technologies to deliver power but also the impacts on global warming, human health, energy security, water supply, space requirements, wildlife, water pollution, reliability and sustainability.

Tags: biofuel, cellulosicethanol, coal, ethanol, markjacobson, nuclear, solar, stanford, vinodkhosla, wind