The biggest obstacle holding us back is us. None of these technologies will ever get off the ground without a large percentage of the population being behind it. Heck everyone's heard about the engine that runs off of water...
YouTube - Engine Runs on Water!
But for some crazy reason, hardly anybody seems interested in seeing it become mainstream. I know the water theory works because I saw it work in several vehicles a few years ago. I used to work with this guy who modified cars to run off of a water vapor/gasoline mix. He learned the technology from some guy he knew down in Georgia or something like that. It took him a while to figure out how to actually get it to work (and we all thought he was a lunatic when he was telling us about it), but before I left that job he had succesfully converted 4 coworkers vehicles, including his own Cadillac Northstar V8, over to this gas/water hybrid.
It involved a glass jar (like you use for canning) with copper wire wrapped tightly around something that he dropped down into the jar (this was the part of the design he kept trying to perfect. He tried several designs, and ended up using a really thin guage copper wire wrapped very tightly, overlapping a few times). Then he'd fill it half way full of water (can't remember if he added anything to the water or not), seal it up, and run a copper tube out of the top of the jar and into the fuel system. In the lid of the jar there were two bolts that stuck out of the top of the lid, then ran down into the water. Then he ran a positive wire to the first bolt, and a negative wire to the second bolt. I can't remember all the details, but he explained to me how somehow mixing the water, copper and electricity created a very combustible gas vapor that he then used to supplement the regular petroleum powering the engine, to greatly increase fuel efficiency. When I left he was still tweaking the design out quite a bit, trying to get the vehicles he'd converted to get better and better fuel efficiency.
The one that I remember the most was his good buddies big Dodge 3500 dually pickup truck that got around 12 - 16 mpg of gas on the highway. This guy was a farmer who drove 45 minutes to and from work everyday. They were good friends, but this farmer guy absolutely did not believe running an engine on water vapor was feasible. He kept telling his buddy that the whole project was like trying to find Big Foot... couldn't be done because it was all a fairytale! So he was dead set on proving it to his farmer friend.
Once he'd figured it out and hooked up his water contraption to the guys truck, the farmer dude was totally astonished when his average fuel economy shot all the way up past 75 miles per gallon... with WATER VAPOR! The part I had a hard time understanding, was that you didn't have to keep filling the jar up all the time. He said one jar of water half full would produce enough of this vapor to last an entire days worth of driving. Extremely efficient.
The system wasn't perfect of course. For starters he wouldn't perform the moddification to certain engines that used a lot of a specific metal (which I can't think of off hand). He said the likelyhood that the water vapor would cause rust damage to the interior of those certain engines was too high, and he didn't want to be responsible for that. Can't remember which cars they were.
Anyways, he was a really smart, and a good guy. But man he was paranoid as hell. Naturally everyone wanted him to modify their engines with this cheap gas saver once he proved it really worked. But he turned down just about everyone except a few of his closest friends because he said he wasn't trying to, "make a lot of noise and step on anyones toes..." the 'anyone' in his reference meaning those who make billions of dollars on oil sales who don't want to see a potentially huge decline in demand for it.
So one time when I was assisting him on the laminator, I asked why he didn't try and make money off of this design. Hell the way I saw it he was sitting on a gold mine unlike any gold mine anyone's ever dreamed of. So he told me that his guy from Georgia who'd given him the idea for it was trying to sell the technology he'd perfected to the government and the military since it cost next to nothing to completely convert any car, truck, SUV or anything else on the road into a water burning vehicle instead of a gas burning vehicle... no gas at all was required. And then one day the guy up a vanished, nobody's seen his since.
So he thinks either the government saw this invention as a clear and present danger... because it would cost them untold trillions of dollars in tax's that they make from the sale of gasoline. Or somehow the oil companies found out about his revolutionary idea, and decided he needed a permanent vacation.
I'm not making any comments on all that, all I know is that there are proven technologies available to us. But for now anyways, everyone seems pretty content with the way things are... giving all our hard earned cash away to the Saudi Arabians for their fossil fuels.