What people seem to forget is that thermodynamics is merciless and unforgiving. Petroleum is so much more efficient than any current alternative from a thermodynamic perspective, and so much cheaper, from an economic one, that it is difficult to imagine anything replacing it entirely or even principally.
One popular suggestion is Ethanol. It's made from plant matter (renewable), burning it produces few noxious emissions, and we can make it right here at home. But in terms of solving our energy needs (and yes, they're needs, unless you want to go back to the 1600s), it stands to create more problems than solve them.
Take these comments from The Belmont Club. It begins with this question from Mətušélaḥ:
Why live under the threat of nuclear armed Jihadis, when all it takes is about $20 USD to convert your current US motor vehicle to one that can use ethanol as its main fuel. Why?
To which Cedarford responds,
Well, for starters, conversion of all US cropland to ethanol production would give us 24% of the net oil substitute we need.
Second, ethanol, unsubsidized, costs about 6.50 to 7 bucks for the same energy as gasoline priced 2 bucks less taxes. Far more than the cheaper sugar cane cycle now created by chopping down unproductive, cheap virgin Brazilian rainforest.(Emphasis added.)
I don't know where Cedarford gets his numbers, so I can't substantiate them. Mətušélaḥ responds by quoting "A 2005 joint study by the US Dept of Energy and Dept of Agriculture [which] suggests that 30% of US oil imports can be replaced by bio-based ethanol by assuming relatively modest changes in agricultural and forestry practices."
Commenter Bart Hall, a farmer in Kansas, cites the work of David Pimentel a researcher at Cornell who argues that Ethanol isn't a viable alternative.
[G]rowing the corn, harvesting it, moving it, grinding and fermenting it, distilling it, and handling it requires about 130,000 btu per gallon. Ethanol provides 75,000 btu per gallon.
Some have criticised the work for using '80s era corn yields, but yield increases since that time depend on massive applications of fertiliser manufactured using natural gas.
I am a soil chemist and agronomist by training, and earn my living as a farmer. Corn as a crop is very hard on the land. Destruction of soil organic matter is a non-trivial contributor to greenhouse gases.
We need to be taking land out of corn production, not increasing the acreage so we can convert corn to ethanol at a net energy deficit of 50,000 per gallon.
Ethanol is a political boondoggle, pure and simple. That some folks have been suckered in to the naive hope it can provide an energy alternative ... simply adds another layer to that whole unfortunate mess.
If you include the fact that pure Ethanol is useless when the temperature drops below 20°F (adding petroleum lowers the temp, but not by much), you've pretty much closed the case.
If some people want to pour vegetable oil into their diesel engines so that they emit the aroma of french fries, that's fine with me. But to look to it as a Silver Bullet?
Sorry, folks, but the hard truth is that energy production and consumption always involves a trade-off, and never a good one. The only possible exception is geothermal, but that's only if you live in Iceland, and who wants to do that?