First up: the Hydrogen Fuel Cell. The next big thing in cars, we are told, is the hydrogen fuel cell. Claims of zero emissions and environmentally friendliness abound (see here or here). It's too bad then that hydrogen fuel cells can actually produce more greenhouse gases than our good old internal combustion engines. How can that be, you say? They couldn't be lying to us, could they? Well, in a sense, no, they aren't lying to you. The cars themselves would not produce any greenhouse gases. But whence comes this hydrogen to fill our cars?
Back to that question in a second; first, how does a fuel cell work? Basically, a hydrogen fuel cell works by combining hydrogen and oxygen to form water. Hydrogen enters from one side of the cell, and oxygen from the other. The hydrogen atom is split into a proton and an electron; the proton crosses a membrane toward the oxygen while the electron is forced to go around to meet the oxygen. That travel by the electron produces some electricity (electricity essentially is the flow of electrons) which can be used to power the car. The proton, electron, and oxygen then meet up to form water.
Ok, back to hydrogen. Hydrogen is not a normal part of the atmosphere: it is too light and gravitationally escapes from Earth. Oxygen is readily available and is produced by photosynthesis. Hydrogen, though, needs to be extracted from some hydrogen-containing source. There are two primary ways to acquire hydrogen for fuel cell use. The first, and worst, is through straightforward electrolysis. An electrical current is run through water to spit the molecules and produce hydrogen and oxygen. The hydrogen is then stored for use in the fuel cell. This is incredibly inefficient; almost 80% of the energy is lost, leaving us with an efficiency of 20%. Compare that to the processes of extracting and refining oil etc., which has an efficiency of 80%. The fuel cell vehicle itself is very efficient (almost 40%) at utilizing that energy once it gets to the car, but the process to get the hydrogen fuel is so costly that the overall efficiency of electric grid hydrogen fuel cells is actually less than your average combustion engine. Besides being so inefficient, a lot of the electricity needed to split the water comes from coal plants (around 50%). This leads us to an overall greenhouse gas emissions rating that is actually higher than normal cars (~440 grams per mile compared to ~380 grams per mile). Unbelievable.
Fortunately, there is a more efficient way to produce hydrogen. It's called steam reforming. The process involves using steam to break down natural gas into its components, which include hydrogen. The hydrogen can then be stored for use in a fuel cell. This process is way more efficient than grid electric (~60% efficient compared to ~20%). The high efficiency of the fuel cell combined with the good efficiency of the production method leads to the highest overall efficiency of the major engine types in automobiles, around 22%. This does not sound very impressive, but compared to normal gas engines (~16%) it's a good step up. Unfortunately, this method of obtaining hydrogen requires natural gas. This is, of course, a type of fossil fuel. One byproduct of the steam reform process is, you guessed it, carbon dioxide. At least this second method actually does produce less greenhouse gases (~140 grams per mile compared to the ~380 grams per mile) than gasoline engines. Still, we will eventually run out of natural gas, except for the relatively small amounts that are produced biologically (in swamps, etc.).
The larger message here is that despite the hype surrounding hydrogen fuel cells, they in fact do have emissions (indirectly, through their fuel sources) and in some cases are actually less energy efficient or produce more greenhouse gases than gasoline engines. And the more efficient method may only be sustainable on a small scale (e.g. using biological natural gas).
Next: biofuels (or maybe hybrids).