The only emissions from the tailpipe are water vapor and heat, and a fuel cell has over double the efficiency of even a diesel engine. You can fill up your hydrogen tank in the same amount of time it takes to fill a gas tank. It’s the most abundant element on Earth. These are all true, yet hydrogen is still never going to be a good idea for powering cars.
The first thing you need to understand about hydrogen is, it isn’t a fuel. The gas, which is nearly non-existent in a pure state on Earth, is an energy carrier. You will always need to invest energy into getting hydrogen and you will never get that same amount back. That isn’t to say we don’t have a use for it, but in cars, it’s not an ideal fit.
Right now, almost all of the world’s hydrogen supply comes from natural gas. A process known as steam-methane reforming. High temperature steam and methane are combined, under pressure, with a catalyst, producing hydrogen. But, after all is said and done, you also get carbon dioxide and carbon monoxide. Those are two of the main reasons we don’t like fossil fuel.
As soon as you read temperature and pressure in the reforming process, your mind should say, “energy!” The process does require energy input and is reported to be over 90% efficient, in the most efficient production facilities. Over 90% sounds pretty good, but keep in mind, that is always a net energy loss, so it will always require energy from another source.
Hydrogen has an extremely high energy density per mass, upwards of 140 MJ per kilogram. For reference, gasoline is 45 MJ per Kilogram. The difference, 1 kilogram of gasoline occupies 0.001 cubic-meters(0.05 cubic-feet). At 1 bar of pressure, 1 kilogram of hydrogen would occupy nearly 13 cubic-meters(450 cubic-feet), or 3 Toyota Siennas with the 2nd and 3rd row seats removed, full of hydrogen. But, we can compress hydrogen, compressing and cooling requires more energy put into the process. So again, subtracting from the efficiency, although the exact amount varies by process, you end up with anywhere from 50% to 85% of your initial energy.
One of the things that’s supposed to make hydrogen more appealing than a battery electric vehicle(BEV) is the convenience. Consumers expect a switch from gasoline to whatever the next thing is, to be seamless and convenient. They expect to pull up to something resembling a gas station, fill up and leave within minutes, just like they do now. If you haven’t noticed, there aren’t that many hydrogen stations around. The infrastructure can be built. We could use pipelines from factories to distribution centers and trucks from distribution to filling stations. That would require more energy for pumping and cooling at every step. The other option is to produce the hydrogen at every filling station. That just requires building stations, powering them and stocking snack-shacks full of jerky and corn nuts.
If we would decide to “make” the hydrogen at the stations, a different process would be used. And, to be honest, this is where the process gets more appealing. Isolating the hydrogen locally gets rid of the transportation through pipelines and trucking waste. It also makes more sense to use a process called electrolysis. In this method, an electrical current is passed through water using electrodes. It splits the water molecule into oxygen and hydrogen for collection. There is no byproduct or waste. It is energy intensive and you will get roughly 70-80% of the energy value in hydrogen from the electricity put in, and you still need to compress it.
Finally, we get to the fuel cell itself. This is the part that is extremely clean and makes fuel cells look so attractive as a way to power vehicles. Similar to the electrolysis process of splitting a water molecule, two electrodes are used with the addition of an electrolyte membrane. Hydrogen is passed over the anode, the positive electrode, while oxygen is passed over the cathode, the negative electrode. The hydrogen is then pushed through the electrolyte, stripping off the electrons which are then run through a circuit, just like it would work in a battery, generating a DC electrical current. The hydrogen ions, oxygen and electrons returning from the circuit, all combine at the cathode, creating the water and heat that is the exhaust from the fuel cell. Again, if you see “heat” and immediately think “loss of energy,” Mrs. Cornwalski, your high school physics teacher is very proud.
Fuel cells, with current technology, are 50-60% efficient, according to the U.S. Department of Energy. There are several new technologies on the horizon that look promising to raise that number to almost 80% efficiency, but they are several years off.
If we look at total numbers here, hydrogen starts to look pretty grim. Let’s assume we are isolating at the point of distribution using electrolysis. We are down to 80% efficiency. Compressing and cooling the gas to 10,000 psi is at best, 80% efficient, so now we’re down to 64% of the energy we started with. If you have the best fuel cell, we can call that 60% efficient and even if we round up, we can very generously call the whole process from water to wheel, 40% efficient.
Your argument at this point should be: “But, we can do that entirely with clean, zero-emissions energy from wind, solar, hydro, nuke, over-caffeinated-chihuahuas-on-treadmills, whatever it’s all zero-emissions and doesn’t matter.” And I mostly agree with this. However, one of the biggest arguments against battery electric cars is, “where are we going to get all this electricity to charge them?”
Most all-electric BEVs are currently using lithium-ion(Li-ion) batteries. A Li-ion battery pack is in the neighborhood of 99% efficient. Electric motors are in the neighborhood of 80%-90% efficient. At worst, that means a BEV is going to use half the energy for the same number of miles as a hydrogen powered fuel cell electric car. If you’re concerned at all about where all this electricity is going to come from, that last sentence has to concern you.
It’s likely we will need to beef-up the electrical infrastructure before the mass market switches over to electric vehicles. Although, the ideal situation would be for a large number of EV owners to bolt solar panels to the roofs of their houses the same time they bring there shiny new cars home. In Europe, some car manufacturers even experimented with pilot programs of bundling home solar solutions in with a car purchase, that sounds like an ideal fix.
There are definitely downsides to lithium-ion batteries. They aren’t the cleanest or most ethical things to produce. However, we are on the verge of several new battery technologies that will likely make Li-ion look like lead-acid. It might be five, or so, years off, but how long do you suspect it would take to build a nationwide hydrogen infrastructure? We’re slowly getting a BEV charging infrastructure and that is far less complex.
Don’t get me wrong, I don’t hate the idea of hydrogen fuel cells. If you are getting the hydrogen using clean energy, it probably has a place in certain applications. It is very lightweight and energy dense in that sense. It certainly makes sense for spacecraft and possibly even aircraft, where the weight of a battery pack is far more detrimental to performance. With improved efficiency, it could be a good option for excess energy storage. I don’t think we should give up on pushing forward with research into fuel cells. But unless we somehow find a way to isolate, compress and distribute it without the big losses, it just doesn’t make sense to keep putting highly subsidized hydrogen into cars. Oh, did I forget to mention it costs the equivalent of over $5 per gallon of gas?