Guest Post by Willis Eschenbach
As a result of my post on energy storage entitled Getting Energy From The Energy Store, a few people brought up the idea of replacing our current energy sources such as gas and oil with hydrogen (often written as H2, because two hydrogen atoms make up one hydrogen molecule).
You see this on the web, that we could power our civilization on hydrogen, convert all the trucks and buses to run on hydrogen, they call it shifting to a “hydrogen economy”… and why not? You can burn hydrogen just like natural gas, you can run an internal combustion engine on hydrogen, what’s not to like? Here’s a typical intro to an analysis:
A Comparison of Hydrogen and Propane Fuels.
Hydrogen and propane have long histories of being used as fuel. Both fuels can be used safely if their physical, chemical, and thermal properties are understood and if appropriate codes, standards, and guidelines are followed. Although the properties of hydrogen have been compared to those of propane and methane, these comparisons were made to facilitate appreciation of the physical and chemical differences and similarities among these fuels. It is not possible to rank these fuels according to safety because plausible accident scenarios can be formulated in which any one of the fuels can be considered the safest or the most hazardous.
So what’s wrong with this comparison, between hydrogen and other competitive fuel sources like propane and methane?
What’s wrong is that people misunderstand hydrogen. Unlike say propane or methane, hydrogen is not an energy source. There are no hydrogen mines. You can’t go out and drill somewhere into a deposit of pure hydrogen and bring it back home to burn.
And why can’t we mine or drill for hydrogen and bring it home and burn it to power our cars?
The reason we can’t mine and burn hydrogen is simple … it’s all been burnt already. The nerve of nature! I mean, people are always warning that we’ll burn up all the fossil fuels, and now we find out that nature has already gone rogue on us and burned up all the hydrogen …
Figure 1. Burnt hydrogen, showing the hydrogen and oxygen atoms.
Most of the burnt hydrogen we call “the ocean”. Another bunch of burned hydrogen we call ice and rain and rivers and lakes. But there’s no hydrogen that is available for drilling or mining—it’s all bound up in other compounds. And as a result, hydrogen is not a source of energy—it is merely a way to transport energy from Point A to Point B.
And this in turn means that the main competition to hydrogen, what we should be comparing it to, is not natural gas, nor propane as the quote above says, nor any other gas.
Instead, the main competition to hydrogen, the true comparison, is to electricity, which is our current means of transporting energy. Saying that we could “power our civilization on hydrogen” is as meaningless as saying we could “power our civilization on electricity” … neither one is a source of power, they’re just different ways to transport energy around the planet.
This is not to say that hydrogen is not useful, merely to clarify what it is useful for—transporting energy from one place to another. It is not a source of energy, it is a way to move energy. This distinction is very important because it lets us make the proper comparison, which is not comparing hydrogen to propane as they did above, but comparing hydrogen to its real competition—electricity.
Now, compared to electricity as a means of transporting energy, hydrogen suffers from a number of disadvantages.
The first disadvantage results from what I modestly call “Willis’s Rule Of Small Stuff”, which states:
It is far easier to move electrons than to move molecules.
This rule has ramifications in a number of fields, particularly the transportation of energy. For example, consider the difference between moving a large amount of energy on a constant basis over say a hundred miles (160 km) by the two competing transportation methods, electricity and hydrogen.
For electricity, you just have to move electrons. So you string a pair of copper wires up on poles from Point A to Point B, and … well … that’s about it. You hook one end to a generator of electricity, and a charge appears at the other end of the wires. There’s not much leakage, not many problems of any kind. The system is robust and relatively safe, and able to withstand storms and temperature extremes.
Now, consider moving the same amount of energy as hydrogen. For that, you have to move molecules. First off, you need a pipeline. Now, we’re all familiar with pipelines for moving energy. The trans-Alaska pipeline is a fine example. Pump oil in at one end, add some pumping stations along the route to keep it moving, and oil pours out the other end.
Hydrogen, though, is a very difficult beast to pump through a pipeline. To start with, hydrogen has very, very low energy density. So you have to pump a huge amount of it, about 4,000 times the volume of gasoline or oil for the same energy.
Next, hydrogen is incredibly sneaky. Do you know how a rubber balloon filled with helium gradually loses its helium over time? The helium is small enough to go through holes in the rubber balloon, tiny holes that are too small for air to pass through. Well, hydrogen is even worse. It can escape right around a piston in a pump, and run happily out through the pipe threads in any pipeline connectors. It requires special gas-tight connections from end to end of the delivery chain. You can’t just stick the hydrogen pump nozzle into your gas tank like you can with gasoline or diesel. So moving the molecules of hydrogen turns out to be a much, much harder problem than moving the electrons with electricity.
The second disadvantage of hydrogen as a means of transporting energy is the low energy density mentioned above. In general, the energy content of fuels varies with their density. So for example, diesel has more energy per liter than gasoline, which is lighter. And alcohol is even less dense, so it contains less energy per liter than either gas or diesel.
Now, consider hydrogen gas. Figure 2 shows a chart comparing various materials regarding energy density in two different measures—megajoules per liter (MJ/L), and megajoules per kilogram (MJ/kg).
Figure 2. Energy density of selected materials. Vertical scale is in megajoules per litre, and the horizontal scale is in megajoules per kilogram. Hydrogen is at the lower right. Click to embiggen. SOURCE
This leads to an oddity. Hydrogen gas has a huge amount of energy per kilogram … but almost no energy per liter. Gasoline holds about 35 megajoules per liter (MJ/L). But even compressed at 700 bar (about 10,000 psi) hydrogen has only about 5 megajoules per liter. That means that you have to move a lot of hydrogen, or pack a lot of it into a car or truck fuel tank, to have enough energy for practical purposes.
Now folks are always claiming that this problem will be solved by adsorbing the hydrogen onto the surface of an as-yet-unknown sponge-like substance from which it can be recovered as hydrogen gas by heating the substrate. But that can’t possibly be as energy-dense as liquid hydrogen, and liquid hydrogen has only a measly ten megajoules per liter. So adsorbing it will not solve the problem.
Nor will liquefying it, seeing as how you have to keep liquid hydrogen at about 240 degrees C below zero (-405°F) … not practical.
And that means that if someone wants to store much energy, say at a hydrogen fueling station, well, they’ll need a whole lot of high-pressure tanks with special fittings, and they’ll need to be about six times as large as the corresponding gasoline tanks to contain the same amount of energy. Or if they are storing the hydrogen adsorbed onto the surface of some as-yet-undiscovered material, they won’t need to be high pressure, but they’ll need to be even bigger.
The third disadvantage of hydrogen as a transportation medium is safety. Yes, electricity is dangerous, of course. But electricity isn’t flammable, and hydrogen is extremely flammable. Hydrogen has an unusual quality. Most fuels only burn when there is a certain ratio of fuel to oxygen. But hydrogen will burn whether it’s a little hydrogen mixed with a lot of air, or a lot of hydrogen mixed with a little air. Not only that, but the hydrogen flame is colorless, smokeless, and invisible in sunlight … a very bad safety combination.
The fourth disadvantage of hydrogen is something called “hydrogen embrittlement”. Here’s a crazy fact. Hydrogen molecules are so small that they can leak out through solid steel. It can either react with steel, or it can leak right through the steel. But that minuscule slow leakage is not the real problem. The real issue is that hydrogen penetrating into and through the metal lattice weakens the steel, fatigues the metal, and decreases fracture resistance. In the long run, it can embrittle solid steel to the point where it cracks all the way through after only a slight impact.
The final disadvantage of hydrogen as a transportation medium is that after using hydrogen to transport energy from A to B, it is hard to convert the hydrogen back into other useful forms of energy. For example, electricity can be used to drive a crankshaft, heat a cup of tea, shoot a railgun projectile at supersonic speeds, energize a magnet, wash my clothes, power a laser, propel a train via linear induction, light up a football stadium, split water into hydrogen and oxygen, charge my computer’s battery, or to drive a chemical reaction against an energy gradient.
Out of all of those uses, hydrogen can drive a crankshaft with very low efficiency compared to electricity, and it can heat a cup of tea … yes, you can use hydrogen in a fuel cell to convert it back to electricity, but that kinda defeats the purpose.
I mean, isn’t it rather goofy to use electricity to create hydrogen, transport the hydrogen, and then convert it back to electricity??? What’s wrong with this picture? Why not just use the electricity directly?
All of this taken together, of course, is the reason that our civilization did not adopt the use of hydrogen as an energy transportation medium, and we settled on electricity instead … because, well, it’s kind of a no-brainer. For just about any purpose you can name, including transporting energy around for powering cars and trucks, hydrogen is well down on my list of good candidates.
Now, if I can just find out who burned up all the hydrogen and didn’t save it for the grandkids like Death Train Jim Hansen advised us to do …