Hydrogen – the colorless, odorless, non-toxic fuel. It’s the simplest element, consisting of just a single electron orbiting around a single proton. It’s the lightest gas, lighter than helium, so light in fact that it will actually rise out of the Earth’s atmosphere and leak into space. Fuel cells use hydrogen by directly converting its energy into electricity, without burning it and without pollution. In this process, this simple element is converted into a compound we all know well and rely on, pure water.
Consider that for a minute. You can take a clean, non-toxic substance and convert it to pure water, and in the process get electricity. Or in reverse, you can take pure water, add electricity, and make pure hydrogen. You can do this at home – or, actually anywhere – and create a fuel that you can use where you don’t have electricity, like in your car. It seems too good to be true. Why are we not using it more?
There are two issues with hydrogen. The first is it has a profound image problem. In the public’s mind, hydrogen is associated with spectacular displays of “energy gone wild” such as in the Hindenberg disaster and with fusion bombs (commonly misnamed “hydrogen bombs” even though they do not contain hydrogen, only hydrogen isotopes). Another day I’ll address this in more detail along with other safety considerations on this fuel.
The second issue, and the one we’re going to look at today, is that hydrogen is very difficult to contain in small spaces. From a safety perspective this is a good thing, because once it reaches free air it immediately zooms upward as fast as possible, leaving no traces around to (literally) flare up into problems. But to store hydrogen to use later, such as in a car, this means it takes up a LOT of space. To put this into perspective, let’s say we wanted to store hydrogen in a basketball, which is normally inflated to 8 psi. And let’s say we want to store the amount of hydrogen that equals the same amount of energy in 1 gallon of gasoline. To do so, our hydrogen-filled basketball would need to be over 8 feet in diameter! This is clearly a problem for any kind of transportation application such as a car, truck, ship, or airplane.
Why is hydrogen so darn big the first place? At the atomic level, hydrogen the molecule is made up of two hydrogen atoms, which is why we write “H2” instead of just H, and it contains two protons and two electrons. The shape of the molecule is simple, just like a dumbbell where the ends are the protons. More complex molecules have different shapes, like the open-triangle of H2O, or the tetrahedral of methane (CH4). When you start adding different elements and shapes, the molecules can become “polarized” like a magnet, and also get tangled up easier. But hydrogen is so simple that it is not polarized and it cannot get tangled. In short, it does not like being next to its neighbors. And like kids at a 7th grade dance, it needs a lot of external help to overcome its repulsion.
Over the years, many creative ideas have been used to solve this problem. These include a good DJ, dim lighting, and… oh wait I was talking about hydrogen. To reduce the size of hydrogen, one way is to increase the pressure in our theoretical hydrogen basketball. For example, today’s hydrogen fuel cell cars store hydrogen in thick carbon fiber tanks at 10,000 psi. If we had a magic basketball that could hold 10,000 psi without bursting, it would end up being about 15 inches in diameter. Not bad! But this comes at a large cost because instead of a magic basketball we would need a highly-engineered thick carbon fiber tank (making our basketball more like 19 inches in diameter), and it also takes quite a bit of energy and machinery to get the hydrogen up to 10,000 psi.
Another solution is to turn hydrogen into a liquid. Chemistry magic happens when substances turn into liquids. The molecules lose so much energy during this process that they slow way down and end up getting much closer together. Going back to the previous example basketball, if we could fill the basketball with the same amount of hydrogen as before, only now it’s a liquid, it would be just over 12 inches in diameter, about half the volume as before, without any added pressure.
However, liquifying hydrogen is an even more intense process than pressurizing it to 10,000 psi. Hydrogen doesn’t become a liquid until it is cooled down to -423 °F, just 36 ° above absolute zero. And unlike propane, it will not automatically liquify when pressurized. Once it becomes a liquid, adding just a little heat will cause it to boil into a gas again, so the tank needs to be insulated with a vacuum and layers of so-called “super-insulation”. This is even more complex than a high-pressure carbon fiber tank, but with similar cost and volume penalty. Liquifying hydrogen requires special turbomachinery to get it so cold, and this uses a lot of energy: the energy required to liquefy hydrogen is 1.5 to 2-times that of compressing it to over 10,000 psi. When added to the energy to create the H2 in the first place, this means the total energy required to create LH2 is about 10% higher than that to create compressed H2. This higher energy input typically results in a corresponding 10% higher cost per kg of LH2.
Despite this energy penalty – and resulting cost increase – of producing LH2, it is still the fuel of choice for many applications. As illustrated above, hydrogen stored as a liquid in one tank would be about half the volume as that stored as a gas at 10,000 psi in another tank – including the volume of the tanks themselves. Weight is another issue. A tank of 10,000 psi compressed hydrogen weighs nearly 3-times more than a tank of the same amount of hydrogen in liquid form.
But perhaps the most important distinction between compressed gas and liquid hydrogen storage is the ability to scale. It is more and more difficult to make larger compressed hydrogen tanks. The stresses from the high pressure require thicker and thicker walls. Eventually it becomes prohibitive. The largest commercially-available compressed hydrogen tanks hold less than 20 kg. This works fine for applications which don’t use a lot of hydrogen, such as forklifts and cars, but anything more than this requires multiple tanks piped together in an array. While an array of many tanks can function as well as a single tank, there is a significant increase in the overall volume of the array because of unused space between tanks. In one example of a commercially-available array of eight hydrogen tanks at 5,000 psi (a common storage pressure for heavier-duty applications), the array’s volume is nearly twice as big than if the gas could have been stored in a single tank.
LH2 tanks have the reverse trend. As an LH2 tanks gets larger, the insulating layer around the tank takes up a smaller and smaller portion of the overall volume. While inefficient at storing small amounts of hydrogen, this allows them to store massive quantities in a single tank, such as the 100 ton (100,000 kg) tank at Praxair’s LH2 production facility in Ontario, CA, or the 380 ton tank planned for NASA’s Kennedy Space Center. Coming back down to earth, LH2 storage tanks for industrial facilities are commonly in the 500 to 2,000 kg range.
The quantity that can be transported by truck is an important result of this scaling difference. While LH2 tanker trucks are commonly sized between 3,000 to 4,000 kg each, the largest truck trailer of compressed hydrogen (with multiple tanks in arrays) carries 880 kg, and most carry just 250 kg. That means that, at best, it takes more than three truckloads of compressed hydrogen to equal a single truckload of LH2, and at worst the ratio can be over 10:1. Every truck trip adds a significant amount of energy use to the overall energy requirement of the fuel. It has been shown that for a usage facility more than just 50 miles away from a production facility, the energy savings of trucking liquid hydrogen more than makes up for the energy penalty required to make it.
All in all, the choice between compressed hydrogen and liquid hydrogen should be made with careful consideration of the usage. For mobile applications this is, at the simplest level, a consideration both of the available volume and weight on-board, and of the consumption rate. At the ends of the spectrum this is a simple analysis. A forklift requires 2 kg to last an 8-hour shift, and at this size an LH2 tank would be too large because of the required insulation layers. Case closed, has to be compressed gas. At the other end, a cargo ship needs to store 60 tons of hydrogen to last the 12-day journey across the Atlantic ocean. The corresponding compressed gas array is much too large for this, so the choice must be liquid. Done. But in many cases this becomes a tradeoff analysis.
For example, let’s look at a 600-passenger tourist boat that uses about 200 kg per day. The boat has sufficient space for about 260 kg of compressed gas at 5,000 psi, or in the same space it can fit about 1,050 kg of LH2. Compressed gas will work, and we know that compressed gas is likely going to be about 10% cheaper than LH2, so do we go with that? Looking a little deeper, going with this option means that it must be refueled every day, but with 1,050 of LH2 on-board the refueling could be spaced out to every four or five days. Today’s delivery logistics are structured such that there are two components to the cost: (1) the cost of the hydrogen and (2) the cost of the truck. In this case, the four- or five-times higher delivery cost will certainly dwarf any cost savings of the hydrogen itself, resulting in a higher overall cost to the operator.
There are other, more subtle considerations as well. Where in the world does this need to be refueled? While hydrogen is made just about everywhere in the world, liquefaction facilities are relatively uncommon. Currently there is a robust, 300 ton/day LH2 production and distribution network in North America (built up primarily to support the US space program), but this is not the case around the world. The EU has a small amount (26 ton/day) as well as Japan (35 ton/day) but in the rest of the world the availability of LH2 is nearly non-existent.
Another consideration is safety. While both forms of the fuel have decades of experience developing and using safe handling practices, in some applications the high pressure of compressed gas is a concerning hazard, in others the cold temperatures of cryogenic liquids is a concern. Long term storage is another factor. Even with super-insulation, LH2 tanks absorb a small amount of heat that boils the hydrogen. Some of this can accumulate within the tank, but if more time passes and the tank is not used, it must be slowly released into the atmosphere. After a long time of non-use, all of the LH2 in the tank will boil away. For applications that are sitting unused for long periods of time, compressed gas may be the only choice.
The options for storing hydrogen are not limited to liquid or compressed gas, but these are by far the most common today. Other ways to store hydrogen include reversible (rechargeable) materials such as chemical hydrides, sorption materials, and metal hydrides. There are also substances that are made from hydrogen, such as ammonia and synthetic methane, where hydrogen is stored as a compound that is irreversibly broken apart later for use. I’ll address these in a subsequent column.
When it comes to selecting the method of hydrogen storage today between LH2 and compressed gas, a rule of thumb we’ve developed through experience over the years is that applications requiring less than 200 kg on-board are probably better suited for compressed gas, while those over 500 kg are more likely to work best with LH2, notwithstanding the other considerations discussed above. Between 200 kg to 500 kg the choice becomes more complicated but can become clear with a deeper understanding of the tradeoffs.
The fundamental chemistry of hydrogen helped us to understand why hydrogen is such a “volumetrically-challenged” fuel. Compression and liquefaction are the two primary ways to reduce hydrogen’s volume to manageable proportions. Each method has its pros and cons, and there is not one solution for everything. An understanding of the tradeoffs allows us to make the best decision and enables us to use this remarkable fuel across a wide variety of applications, from small forklifts up to the largest ships.