How Does a PEM Electrolyser Work?
Hello everyone, and welcome to this lesson entitled what is an electrolyzer and how does it work?
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My name is Hunor and I'm a hydrogen engineer. And this is lesson two of a series of lessons on hydrogen. And the next lessons are going to be about PEM cells and PEM stacks. And I'll also be talking about hydrogen safety in one of the next lessons. So let's jump into it.
Well, by the end of this lesson, you will be able to, number one, describe what is an electrolyzer. Number two, you'll be able to name two different electrolysis technologies. Number three, you'll be able to name three advantages of PEM over other electrolysis technologies. Number four, you'll be able to describe the balance of plant (BOP) system around PEM electrolysis.
So what is an electrolyzer? Well, an electrolyzer is a device that takes water and it takes electrical power. It uses the electrical power to split the water into its constituents, which are hydrogen and oxygen. So let's look at the most popular electrolyzer system types out there. We can talk about alkaline electrolysis and we can also talk about PEM electrolysis, which are the most common types of electrolysis methods out there today. When we talk about alkaline electrolysis, it has actually been invented over 200 years ago, which makes it a very mature technology today. And we can also talk about PEM electrolysis, which has been invented in the 1960s during the space race. And it is also a relatively mature technology. PEM electrolysis has a number of distinct advantages over alkaline electrolysis, which we'll talk about in just a second.
PEM stands for Proton Exchange Membrane, utilizes a membrane between the anode side and the cathode side of the cell. And what happens is the water flows into the anode side of the cell, and when the electrical power is applied, the water is split into hydrogen and oxygen. And the hydrogen atom consists of one proton and one electron. Well, the electron splits off the proton via electrolysis, and the electron goes through the electrical connections and it goes over to the other side of the membrane. So because we have the Proton Exchange Membrane the PEM membrane, will allow the proton to go through from one side of the membrane to the other side of the membrane. And the proton and the electron on the other side of the membrane on the cathode side will join up and will form a hydrogen atom. And two hydrogen atoms, as we know, is an H2 molecule.
So what are those distinct advantages of PEM? Well, PEM is considered to be a very advanced technology. And the advantages are that you're able to run a PEM electrolyzer at a high current density. What that essentially means is that you can have a smaller electrolyzer on a smaller footprint, and you're able to run that electrolyzer at a high current density, which means that you'll have a lot of hydrogen coming out of a small electrolyzer system.
The next advantage we can talk about is high voltage efficiency, which means that you're having to put less electrical power into the electrolyte to get essentially the same amount of hydrogen out of it. So it's more efficient. We can talk about the capability to hold pressure, which means that when you're generating the hydrogen, you can generate the hydrogen at pressure so you can integrate it with so many different things and it's more useful, more clients will want it.
Essentially what it means is you can generate the hydrogen at 5 bar, 10 bar, 20, 30, 50 bar if you have to. And in fact, it has been demonstrated that we can use PEM electrolysis to generate hydrogen at up to 350 bars. That's a very high pressure.
The other advantage we can talk about is turn down ratio, which means that, number one, you can turn on and off the electrolyzer in a very short period of time. And you can also manipulate the output of the electrolyzer very quickly. You can go from a low, let's say, 50% output to 100% output very quickly. We can also talk about the highly pure hydrogen.
Most PEM electrolysis systems are designed in a way that they have a hydrogen purifier on them, most of them, and they will give you very, very pure hydrogen that you can even use in fuel cells. Don't let this diagram confuse you, though. It can look quite daunting when you look at it for the first time. However, I will be dedicating my next video entirely to explaining how the PEM cell works and how a PEM stack will work as well. Okay, so let me explain the balance of plant surrounding a PEM electrolyzer if you'd like.
The more information on the balance of plant of a PEM electrolyzer, there is a great book called PEM Water Electrolysis. I'm holding volume two in my hand and the author is Dimitri Bessarabov and Pierre Millet. I'm hoping I'm pronouncing their names right, but this is a fantastic book on PEM water electrolysis and it talks about the balance of plant, it talks about the electrolyzer stack. So the jargon that I'm going to be using is going to actually line up with this book. When we talk about an electrolyzer balance of plant, we first have the water going from the tap into so this is tap water into a water purification system. From there the water is going to go into a tank. And this book that I just mentioned calls this the O2 two separation tank. And from this vessel we are pumping the water into the stack. I'm going to draw a picture of the stack. So I'm going to say this is a PEM stack and the water is running the air pump into the hemp stack. Then the water and the oxygen on the other side of the stack will go back into the oxygen water separation tank. And from there we have the O2. The oxygen is being vented off into the atmosphere. And the hydrogen go out from this side of the stack and it will go into another vessel.
This vessel is going to be called the hydrogen water separation vessel. So the hydrogen separation tank. From there, let's just say that this is the water level in here. The hydrogen will go into this vessel and the hydrogen will bubble up to the vessel and the water will stay on the bottom. Why do we have water in PEM electrolysis? Why don't we just have pure hydrogen? Well, it's just how PEM electrolysis works. In PEM electrolysis, you have water go over to the other side of the membrane and you'll have to later separate the water out from the hydrogen.
The water that you have is quite a lot of water and it's pretty pure water as well. It's relatively clean. So you are actually able to reuse that water instead of wasting this water. Most electrolyzer manufacturers will design a system where the water is able to be reused.
So you can put the water back into the oxygen water separation tank and you can reuse the water. So from here, the hydrogen on the top will go into a dryer unit. So that in a lot of electrolyzers, according to that book, for example, there is going to be a deoxidizer. And then from there you are going to have a vessel and another vessel. And these are going to be your dryers. Your dryers are going to remove all of the moisture from the hydrogen to make it extremely pure hydrogen. Now from there, your hydrogen is going to exit.
We must provide electrical power to the stack. This is going to be our positive and then this is going to be our negative. And this has to be DC power. It cannot be AC power because AC power won't work with electrolysis whatsoever. So I'm going to draw this box and I'm going to call this box the rectifier. And a rectifier is essentially an AC to DC converter. And upstream of that there's normally in most systems, there needs to be a transformer and then you're basically feeding that from the grid or whatever your electrical source is. Another thing I'd like to talk about is that during electrolysis, there's going to be quite a lot of waste heat. Where is that waste heat going to go to? Well, we need to cool down the water. So this is the water that we use for electrolysis, and we have the water coming out of the stack being really quite hot, and that's where we "lose" that energy. And then we could have either here or here, we could put in a cooling unit, which we would have to cool down the water and just keep the stack temperature reasonable.
Okay, so if there's any questions, let me know in the description. This is just in very simple terms, how a PEM electrolyzer works, how the balance of plant works around that. So again, you've got the water going in and you've got the electrical power going in. And then from there you'll have the water split into hydrogen oxygen. The oxygen will just leave normally to the atmosphere, and then the hydrogen will exit and will go to the client over there.
Okay, so let's finish up with a couple of questions here. Question number one. Name two different types of electrolysis systems, and I'll give you a couple of seconds to think about the answers. Okay? So we can talk about PEM, we can talk about alkaline, and if you looked at my previous slide, you will also remember solid oxide. But any of those two will do. Let's look at the other question. Name an advantage of PEM electrolysis and it could be any of the following answers. I'm going to give the answers in a second or two. All right, so high current density, high voltage efficiency, capable to hold pressure, turn down ratio, highly pure hydrogen. Next week's topic is going to be how a PEM electrolyzer stack works. Let me know in the description if you like this video. And have a great day. Bye.