There are currently several issues with using hydrogen as a fuel in vehicles, but first I'd like to clarify that hydrogen is not
an energy source, but a means of storing
energy. Hydrogen as a fuel is really more akin to the battery in an electric vehicle than the gasoline in a conventional vehicle.
What are the advantages of using hydrogen in vehicles?
Hydrogen has a chemical energy per gram 2.6x that of gasoline. In a hydrogen-powered vehicle that energy is converted to electrical energy quite efficiently with a device called a hydrogen fuel cell. Then the electrical energy is used to drive an efficient electrical motor which makes the vehicle move forwards. This whole process is much more efficient than using gasoline and an internal combustion engine. The "tank to wheel efficiency" (what percentage of the fuel's energy can be converted into moving the vehicle) of a typical internal combustion vehicle is around 15%. Current hydrogen prototype vehicles can get anywhere from 30%-60% tank to wheel efficiency. Additionally, the hydrogen fuel cell process only creates one product, good old non-polluting water, while the internal combustion process creates greenhouse gases and other pollutants.
That sounds great for hydrogen as a fuel for vehicles, so why isn't it being used? There are several contributing factors.
Currently the world has a well-developed infrastructure for obtaining, transporting, and selling fossil fuels. There is effectively no infrastructure for hydrogen. There has to be a substantial investment in hydrogen infrastructure before it can be used viably as a fuel.
We get fossil fuels from deposits deep inside the earth, that's what makes it an energy source. Where would we get our hydrogen for all these vehicles? We have to create it from an existing energy source. There are two main options: create it through electrolysis (separating with an electrical current) of water
, or through a process called steam methane reforming
. Currently most hydrogen is produced through the latter method, by taking methane (a fossil fuel) and transforming it into hydrogen. I've always thought it interesting that most of what some tout as "the fuel of the future" is made from a fossil fuel in a process that releases carbon dioxide. Even if we made most of the hydrogen through electrolysis, that electricity would most likely come from a fossil fuel burning power plant. Both of these factors significantly affect the environmental advantages of using hydrogen as a fuel.
The main physical problem with hydrogen is its density. While its chemical energy content per gram is higher than gasoline, hydrogen exists as a gas unless it is compressed. In fact, even when you liquefy hydrogen, its density is still 10x less than that of gasoline. This makes it rather difficult to use liquid hydrogen an economical fuel, since its energy content per volume
is actually about 5x worse than gasoline. Also, it's rather tricky to store hydrogen as a liquid. If it is heated even slightly, it expands violently. The only way to ensure this doesn't cause problems is with a thick steel tank. Obviously this is a problem for vehicles since it adds considerable weight.
What about storing hydrogen as a gas instead? Since it won't expand as violently when stored as a gas this approach somewhat alleviates the potential problem of the hydrogen blowing up, but the density issue has only been made worse. Suppose you store the hydrogen at 66x atmospheric pressure, its density is now about half of what it is in a liquid form. The energy content per volume has now been halved, and it is now 10x
worse than that of gasoline. Since the hydrogen is still being stored under pressure, the tank weight has a considerable contribution as well.
The last issue is, in a word, economics. Currently all of the contributing factors make gasoline a cheaper fuel to use than hydrogen. Until the cost of using gasoline meets or exceeds that of hydrogen, the use of hydrogen in vehicles will be limited to fringe cases and prototype vehicles.
(published on 04/03/12)