This page last updated on 01/26/2019.
Copyright © 2001-2019 by Russ Meyer
A Few Opening Remarks
The purpose of this article is to compare the efficiency of a hydrogen powered car to a conventional gasoline powered car. However, before getting started, I'd like to devote a few paragraphs to combating some hideous misconceptions promulgated by the technologically ignorant media.
I frequently hear politicians, homemakers, even fellow engineers extolling hydrogen power as the savior of the modern world. The truth is, there is no such thing as "hydrogen power." Hydrogen is an energy storage medium. It does not generate energy. The energy stored in hydrogen has to come from somewhere else. You cannot just go out and mine hydrogen, process it, and put it in your car. Hydrogen has to be produced somehow. For example, most people are familiar with the production of hydrogen by electrolysis of water. It's true that the hydrogen itself came from the water, but it took electrical energy to break the chemical bond it had with oxygen. A fraction of the electrical energy used to conduct the electrolysis has become locked as potential energy in the chemical configuration of gaseous hydrogen and oxygen. When you burn that hydrogen, you are recombining it with oxygen and extracting that fraction of stored energy. So, hydrogen is not a source of energy...it's just a way to store energy for a while; like a battery. The energy in hydrogen has to come from somewhere else.
Now that I've made that point so emphatically, let me take it right back. Actually, we can and do "mine" hydrogen. Know how? By mining coal, pumping oil, and piping natural gas. These are substances that contain stored solar energy in chemical configurations, namely hydrogenated carbon compounds, through the metabolic action of plants. In a sense, it's just like hydrogen that has been liberated from water via electrolysis. A fraction of the energy of the sun has become locked up as potential energy in the chemical configuration of hydrocarbons. These fossil fuels are just a storage medium for solar energy. When burned, they release years of absorbed solar radiation in just a few minutes.
The point of all this is that hydrogen does not address energy needs in any way at all. The only thing that can be said for hydrogen is that it provides a means to store and release energy that does not involve a carbon cycle. In particular it avoids environmental problems with carbon monoxide and carbon dioxide. Environmental concerns are the only possible justification for pursuing hydrogen vehicles. Even if we could wave a magic wand and change every vehicle on the planet to use hydrogen, it wouldn't address the energy production problem. How we choose to produce energy might still involve fossil fuels (like coal fired power plants) and the attendant carbon dioxide problem. At the moment, the only known way to escape fossil fuels with present technology is nuclear fission.
If you were going to embark today on a crusade to stop most carbon dioxide emissions and simultaneously ensure the energy independence of the United States, there would be but one practical approach:
Maybe that is exactly the groundwork the Bush administration is laying now.
Now then...back to the main topic; how does the efficiency of a hydrogen powered vehicle compare to a gasoline powered vehicle?
Hydrogen Verses Gasoline
Let's find out how much energy it takes to make a trip in a gasoline powered car verses a hydrogen powered one. First some assumptions; these assumptions basically fix the required amount of kinetic energy to make the trip:
To calculate the amount of fuel energy needed to move the cars that distance, we'll make a few more assumptions:
So, the fuel energy required by the gasoline car will be:
Expressed as an equation it would be:
The fuel energy required by the hydrogen powered car will be:
Expressed as an equation it would be:
Since the cars need the same amount of kinetic energy to make the trip, Ek is the same for both. We can use that to find the energy ratio between the two vehicles:
This means the gasoline powered vehicle will take about 30% more fuel energy to move the same distance as a hydrogen powered car. The hydrogen powered car converts the fuel energy into kinetic energy in two steps, but the combined efficiency of those two steps is a bit greater than that of a gasoline engine. The gasoline powered car converts fuel energy to kinetic energy in one step. Here's a comparative graphic. I tried to draw the width of the arrows to scale. They show proportionally where the energy is going.
Energy Flow of a Gasoline Powered Vehicle - One Step Conversion
Energy Flow of a Hydrogen Powered Vehicle - Two Step Conversion
Here's the Kicker
I frequently hear hard-over granola types saying hydrogen could be liberated from water using electrolysis. This electrolysis could be powered from wind or solar sources making hydrogen powered cars a clean transportation system of idyllic proportions. In fact, the article cited at the top of this page stated this very proposal. They seem rather taken aback at suggestions to generate hydrogen from fossil fuels or nuclear power. As described in the essay, "Storm Ciphers," the cost of producing energy using wind power is about 4 to 6 times that of fossil fuels or nuclear. Let's see what it would cost you to actually take a trip in a hydrogen powered vehicle using fuel created from wind power.
The cost to produce a given amount of energy using wind power is 4-6 times that of fossil fuels. Let's just split the difference and call it 5 times. We can express that with the following equation:
We also know from our earlier calculations that it takes about 1.3 times the amount of energy to go a given distance in a gasoline powered vehicle as compared to a hydrogen powered one. We'll have to buy 1.3 times as much gasoline to make the trip. So, the cost equation becomes:
In other words, it will cost you almost 4 times as much to make the trip in a hydrogen powered car using fuel created from wind power. Put another way, if it usually costs you $25 to fill up your gas tank, it will cost you almost $100 to fill up with hydrogen created from wind power. Yikes!
The Painful Conclusion
The real problem with hydrogen powered cars is not the energy efficiency of the vehicle. The hydrogen powered car is better in that regard. Rather, there are a number of other things keeping hydrogen powered cars off the road:
Many problems with hydrogen powered cars can be fixed by just using batteries instead of a hydrogen fuel cell. Batteries are 80-90% efficient at storing and releasing energy compared with 50% for a fuel cell. The only problem is that they weigh a lot and that detracts from the efficiency and utility of the vehicle. Still, maybe it would make more sense to put effort into improving battery technology. One objection to using batteries is that they take hours to charge. Maybe you could use interchangeable battery packs. You'd pull into a service station, swap out the whole battery pack, and get back on the road. A more difficult problem with batteries is their energy density. For the space they take up and how much they weigh, they don't provide much energy. To drive a mini-van 350 miles at 60 mph using present state-of-the-art battery technology, all the space behind the two front seats would have to be packed with batteries. What a bummer! Gasoline cars handle this task with aplomb because the energy density of the fuel is so much better.
Check out this article on hydrogen powered cars. It provides a good overview of the related technical and economic factors. Also, for an excellent overview of the issue, take a look at this article. Here is a fabulous, short video that does a thorough treatment of the efficiency issues for hydrogen powered cars.
Gee, I wonder why you don't see hydrogen fuel cell powered vehicles all over the road. Must be a conspiracy between Bush and his oilman cronies. Yeah right. Uhhh...maybe it has more to do with the fact that it doesn't make any economic sense at all...do ya think?