Recently, Toyota announced production of the Mirai hydrogen fuel cell car to be sold in California. But what is a hydrogen fuel cell? Without going into all the details, the basic idea is that you can get an electrical current when you combine hydrogen with oxygen (you also get water).
But why would you make a hydrogen fuel cell vehicle? I think Elon Musk said it best: "I think that they're extremely silly". He's right. Hydrogen fuel cells work great. However, you have to have hydrogen first. You can't just go to the store and get hydrogen, you have to make it by breaking apart water. This takes a bunch of energy to accomplish. Oh, that's not the only place you can get hydrogen. You can also produce hydrogen from fossil fuels. Yes, getting hydrogen from fossil fuels sort of defeats the main reason to use hydrogen in the first place.
Ok, hydrogen fuel cells aren't always silly. If you need a reliable battery with a large fuel supply, hydrogen fuel cells are a good choice. For example - in space, hydrogen fuel cells are a perfect choice. If everyone on Earth used the fuel cell, we would have a problem making enough hydrogen (and storing it).
Are there other reasons to make hydrogen?
Ok, here comes the speculation about Toyota's FCV (fuel cell vehicle). What if Toyota is promoting the FCV because they have an abundance of hydrogen? If there is a bunch of hydrogen just sitting around doing nothing, then maybe the FCV is a good idea. But why hydrogen? This next brilliant step comes from my colleague Eric Booth. He suggests that perhaps Toyota is looking at making (or someone else making) deuterium with hydrogen as a by product.
What is deuterium and what would you use it for? Deuterium could be used in a nuclear fusion reactor. So maybe Toyota is counting on a nuclear fusion reactor in the near future. Of course there are many questions left to answer (to fully understand this speculation).
What is the difference between nuclear fusion and nuclear fission?
Let's start with nuclear fission. A fission reactor starts with a heavy element like uranium. If you shoot a neutron at a uranium atom, you can get the atom to break into two smaller pieces. Here is the magic part. The mass of the original uranium atom is larger than the combined mass of all the pieces it broke into. Mass (by itself) is not conserved.
How about an analogy. Supposed I have 10 dollars and I give it to some to make change. They hand back a 5 dollar bill, 4 one dollar bills 3 quarters, two dimes and 4 pennies. You might think - "hey! Where's my extra penny!" Yes, in the conversion you lost a penny. The same thing happens with uranium. You lose a little bit of mass when you break apart the atom - but it's not really lost. The tiny bit of mass was converted into energy. Let me write this as an equation.
Yes. This is the famous E = mc2 equation. In it, m is the mass of an object and c is the speed of light (2.99 x 102 m/s). Since the value of c is large, a small difference in mass can produce a lot of energy (that is the KE - the kinetic energy of the products). Technically, there could be electromagnetic radiation as energy produced too.
This is how a fission reaction works. You start with something like uranium and break it down. This produces extra energy which you then use to turn water into steam and drive a turbine to generate electricity. The biggest draw back is the left over "stuff" after the nuclear reaction. The products are radioactive and generally "not good" - so you have to store them somewhere. Say hello to nuclear waste.
Now what about fusion? Fusion is JUST like fission except that you gain energy by combining elements. However, you can't just take any elements and put them together to get energy - it only works for lower mass elements like hydrogen, helium and stuff like that. The big advantage to fusion is that you end up with not-so-bad stuff like helium. Everyone loves helium.
Why do you need deuterium for nuclear fusion?
Great. But what does this have to do with deuterium and hydrogen? Suppose I have two protons (a hydrogen atom is just a proton and an electron). If I put them near each other they repel since they are both positive. In fact, the closer they get the greater the repulsive force.
This repulsive force is the Coulomb force and it exerts forces on objects with electric charge (like the proton). There is another force - the Strong Nuclear Force. This is an attractive force between particles like protons and neutrons. If you could get the two protons close enough, the strong force would make a them combine into one atom. But you can't (well, not very easily). You can't get two protons close enough because the repulsive force is too great. This is where deuterium comes into the picture. What is deuterium? Well, it's just like hydrogen except that it has a proton and a neutron in the nucleus instead of just a proton.
Everyone knows that protons are red and neutrons are gray - right? If you add a neutron to the proton, you don't change the electric charge. However, you do increase the attractive force between two deuterium nuclei. The neutron does not experience the Coulomb force but it does have the attractive strong force. This difference makes nuclear fusion between two deuterium atoms possible. In the end you would get helium and energy. Boom.
That's why you need deuterium. But wait! If you take two deuterium and an oxygen, you get a molecule that is similar to H2O but is different. This is called heavy water. You can find heavy water mixed in with normal water - but here is the important part. Heavy water can be used in nuclear fission reactors in a way to produce weaponizable plutonium. So people like to keep track of the heavy water. This site has lots of useful (and easy to understand) explanations of nuclear weapons production.
How do you make deuterium?
The easiest way to make deuterium is by having a Big Bang. It's assumed most of the deuterium we see today was created during the Big Bang (which if you think about it, that's pretty awesome). Ok, if you don't have a Big Bang handy then the next best thing is to find the left over pieces from the first Big Bang. If you look at sea water, you will find a lot of H2O (as you would expect). However, you can also find some D2O (heavy water). About 1 out of every 5,000 H2O will actually be heavy water instead.
If you collect enough water, you can separate the heavy water from the water. Yes, this no simple task - but it can indeed be accomplished. Once you have heavy water, you can separate the oxygen and deuterium using electrolysis. The basic idea is to put two electrodes into the heavy water with an electric potential across it. By running a current through the liquid, you will break up the D2O into deuterium and oxygen.
There is another way to get deuterium. If you want, you could take water and use electrolysis to break it into hydrogen and oxygen. Some of that hydrogen would actually be deuterium. Once you have just hydrogen there are ways to just get the deuterium. In this last case you end up with a BUNCH of hydrogen left over.
What could you do with all this left over hydrogen? Oh, you could use it in a fuel cell vehicle. Now we are back to Toyota's FCV. Oh, don't forget that Lockheed Martin said that are working on a compact fusion reactor. Coincidence? Maybe.
Remember, this is just speculation.
Here is a quick summary.
- Toyota is making fuel cell vehicles which require hydrogen.
- I believe Elon Musk when he says that hydrogen fuel cells are silly.
- Lockheed Martin said that nuclear fusion is just around the corner.
- You probably need deuterium to make nuclear fusion work.
- If you make a bunch of deuterium, you probably get even more hydrogen as a by product. This leads to.... hydrogen fuel cells.
It all makes sense now. Well, even if it's not true it's still a great excuse to talk about fusion and fission.
