Although hybrids seem ultimately ideal to most people there is one major problem with putting dependence in hybrids is that they still use petroleum.
Yeah they do a remarkable job using less of it but that still means that they take gas and burn it up into our atmosphere where we can no longer harness power. What we need to learn now is how to run transportation with renewable sources of energy. Biofuels, hydrogen fuel cells, and pure electric cars are all aimed in the right direction however some attempts are ultimately not as significant.
Although E85 ethanol has got media attention it is not an effective alternative to gasoline simply because it does not make as much sense as other options. It is true that it is built up with only 15% gasoline and the other 85% all renewable ethanol fuel but there are shortcomings to this mix. A factor in this is price. In economy and marketing, price is everything and producing ethanol not only costs alot to make but it takes more energy to produce E85 than the amount of energy available for use when burned so it is a double loss situation. For emmisions sake however, this fuel burns cleaner than gasoline and is made from vegatation primarily.
Other options such as hydrogen fuel cells have a future but have some roadblocks along the way to overcome. We know how to apply the technology but we don't know how we will pay for it if fuel cells go mainstream.
Fuel cell demonstration models currently cost near a million dollars to make. It will also be extremely expensive to replace the gasoline infrasturcture in the U.S. with hydrogen. There will also be weather concerns with hydrogen fuel cell technology. Hydrogen is water and water freezes. Currently, there is no easy way to address the fuel cell vehicles endurance to harsh weather. It also takes electricity to charge the fuel cells. The bright side of using water however is that the only emissions of the car consist simply of water. While hydrogen fuel cell technology has potntial, it has to overcome a multitude of hardships before it can go mainstream.
Bio-fuels are a great example of truly renewable energies. Produced from bio-mass (derived from organic material or products) including agriculture products, forms of bio-fuel can be made such as bio-diesel. Bio-diesel is compatible and will be burned smoothly with any diesel engine. Many countries including the U.S. are already making extensive use of bio-fuels because of the practicality of it. Not only does it help prevent climate change but it also helps waste management and fuel security. Another form of bio-fuels are bio-alcohols including ethanol. This is showcased by Brazil and its use of bio-alchohols produced with sugercane.
There are also flaws to bio-fuels. These can include the fact that currently bio-fuels are not cost-effective to produce, mass bio-fuel use can rise food prices, and while bio-fuels themselves are easy on the environment, the productions that make the fertilizers and equipment that go into the roots of bio-fuel produce emmisions.
Pure electric cars (FEV's) are definately a consideration for our future as electric motors don't produce emmisions, have nearly 100% energy efficiency, and have the potential to run with combustion powered cars in terms of performance. However there are still hardships with electricity as with any energy source.
Compressed Air Vehicles have their advantages and disadvantages. They have a lot of potential of being implemented in our society and are not expensive to make. We already have the infrastructure in place for making large amounts of compressed air and air turbine technology has been used for over 50 years. Low tech materials are required and a compressed air system does not produce pollutants or emmisions since the product of the system is simply air (http://www.treehugger.com/files/2005/10/what_about_comp.php).
This seems like a practical option but the downsides are also numerous, several of them related to thermodynamics. First of all, the high amount of energy that is stored in the compressed air tanks could make car accidents much more harmful. The practicality of the air tanks are also of concern. It takes a significantly longer time to fill a tank with compressed air than gasoline and the cruise range of the vehicle is stronly dependent on the size of the tank (which has to be built to a grade where it won't shatter under pressure). Compressed air motors are also more inefficient than electric motors and the machines used to produce the compressed air create emmisions even though the specific compressed air vehicle does not.
Ultimately, there is no fuel quite like petroleum but in our energy dependency situation, we need to look towards other sources of energy such as fuel cells and electricity to pull us through the hardships that are to come.