WEBINAR: 1MW PEM Electrolysis September 2024 Kacso Hunor - Hydrogen Training Solutions, London
So, we’ll just jump right into it. I like to start with the basics. What is an electrolyzer? Well, an electrolyzer is an electrochemical device that takes water and electrical power and turns it into hydrogen and oxygen. It uses the electrical power to split the water into its constituents, which are, of course, hydrogen and oxygen.
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There are multiple different electrolyzer system types. Today, we’ll be talking about PEM only. We also have alkaline, which we won’t be talking about today, and, of course, there’s solid oxide. There’s also AEM for relatively smaller systems. But as far as large systems go, it’s going to be alkaline and PEM.
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Here are some of the advantages of PEM. It uses pure water, with no harsh chemicals. If you know anything about alkaline, it uses potassium hydroxide mixed into water at a concentration of about 25 to 30%. PEM just uses pure water. We have the ability to run PEM at a very high current density, which reduces the footprint. We’re also able to achieve higher voltage efficiency with PEM electrolyzers, which means we put in less electricity and get more hydrogen compared to the amount of electricity we put in.
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It’s also capable of holding pressure. The hydrogen side of a PEM electrolyzer—well, the industry standard is typically 30 bars. You don’t need a compressor to hold the pressure at 30 bars because the electrolyzer will generate the hydrogen at that pressure. You also have a turndown ratio, which means you’re able to turn it up and down very quickly. You’re able to increase and decrease the current density really quite fast. And, of course, you’ll be able to achieve very pure hydrogen—high hydrogen purity. For that, you will have to use a hydrogen purifier.
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I’d like to look at a typical containerized PEM electrolyzer system, and I’m just going to run you through it. We’ve got the airblast cooler on the top here, a refrigerant chiller over here, and a rectifier—transformer rectifier—over here. The water purification system is going to be inside this container. We also have the stack inside the container, along with hydrogen-water separation, oxygen-water separation, and the hydrogen purification system, all inside the container.
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This is a Nel electrolyzer. Let’s look at a different one. Of course, this is going to be somewhat different. We’re going to have an airblast cooler, a refrigerant chiller over here, and, of course, the airblast cooler over here, with the rectifier. The rectifier is going to be in one of these containers, likely the one on the right-hand side. The water purification system is inside the container, the stack is inside, hydrogen separation is inside, oxygen separation, and the hydrogen purification system, etc., are all inside the container.
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This is another PEM system, a containerized PEM system, but from a different manufacturer. This one is from Plug Power. What’s the difference? These systems are very similar. They are typically so similar that you’ll have the airblast cooler outside here, the refrigerator, the transformer, etc. These systems are relatively similar. They’re going to differ slightly. You’ll have different materials used and different designs. Some designs, for example, will allow higher water flow across the stack, etc. However, these systems are relatively similar.
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What I’m going to do in this webinar is talk about these systems—what we typically see in an electrolyzer system. Obviously, things will differ system by system, but I’ll go into what we see typically in these systems. What about a very large system, like a multiple megawatt-scale system? Again, we’re going to have an airblast cooler. I’ve got this little star here because I know people are going to say, “Yeah, you can’t see the airblast cooler here.” No, you can’t see it on this diagram. Also, the refrigeration area is outside the scope of this diagram, but we’ve got the rectifier and the transformers. You can see the rectifiers and transformers over here, the water purification system over here, and the stacks—all located here. We have the hydrogen purification and oxygen separation. That’s what these vessels are for. And we’ve got the hydrogen purification system, which is a temperature swing desiccant dryer located over here.
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Let’s look at a typical PFD (process flow diagram) of an electrolyzer. This is pretty typical for a 1-megawatt system. We’ve got the stack over here. Let’s start from the beginning. For a 1-megawatt system, this will vary, but typically, you will have around 325 liters an hour of water. We’re assuming this is potable water—tap water—flowing into the water purification system. We’ll come back to how the water purification system works shortly. There will be this water flow going into the water purification system. There are reverse osmosis membranes, and from those, we are going to have around 125 liters per hour drained. This will typically just go to drain. We will have around 200 liters of highly pure water, with very close to the lowest conductivity of water—around 0.08, 0.09, or 0.1 microsiemens per centimeter—going into the oxygen-water separation vessel.
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From the oxygen-water separation vessel, we are going to have around 1,200 liters per minute of water flow. This is approximate. Some electrolyzer systems will have 1,500 liters per minute, while others might only have 1,000 liters per minute. Typically, the higher the water flow rate per megawatt, the better. We’ll come back to that shortly. Let’s just assume 1,200 liters per minute of water flow. What we have to understand here is that most of this water flow will come right back into the oxygen-water separation vessel, and we will have this flow of water circulating. Most of the water that goes into the PEM electrolyzer stack will come out. By mass, when you’re running at 100% output on a 1-megawatt electrolyzer, about 99.8% of the water will come out as water, and only about 0.2% per mass will be used for the reaction at each cycle of the water going around.
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Most of the water is used to either wash away chemicals or cool down the PEM electrolyzer stack. The oxygen evolution reaction, and to some extent the hydrogen evolution reaction, requires quite a lot of energy. There’ll be quite a lot of inefficiencies, especially in the oxygen evolution reaction, more so than the hydrogen evolution reaction. Therefore, that will require cooling. When the water, along with some oxygen, comes out of the stack, it will pass through a plate heat exchanger, typically, and the water will get cooled down. Then it will go into the oxygen-water separation vessel, and the cycle will repeat over and over again.
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We have a power supply, and in this particular situation, it will be around 1 megawatt because it’s a 1-megawatt system. The power supply will include several things, typically a transformer and a rectifier. This is very much simplified. We need a rectifier because electrolysis only works with DC power, not AC power. The rectifier takes alternating current and converts it into direct current.
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In normal operation in a PEM electrolyzer, you are going to have water transport from the anode side, which is the water and oxygen side. The water flows through the anode side, and on the cathode side is the hydrogen. The green line here shows the flow of the hydrogen, but in normal operation, we are going to have water flowing over to the cathode side, and this is completely normal. This water flowing over to the cathode side will go into the hydrogen-water separation vessel, where the hydrogen will bubble up to the top, and the water will stay at the bottom. We’ll come back to this later. This water is still relatively good quality, which we can reuse with relatively little polishing required.
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The hydrogen flowing out of the hydrogen-water separation vessel will flow into the deoxidizer. Remember, this hydrogen is still saturated with moisture. So, we’re going to have a dryer system, typically a temperature swing desiccant dryer. From there, the hydrogen is going to be highly purified, which will then go to the client. The hydrogen coming out of the hydrogen-water separation vessel will have to go into a hydrogen vent, or we will need to put the hydrogen through some sort of process to rid the hydrogen bubbles out of this water because we cannot send any hydrogen back into the water purification system for safety reasons—obvious safety reasons.
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Okay, conscious of time, we’ve got to move on. We’ll look at the PEM electrolyzer cell. We’ve got the anode side on the left-hand side here and the cathode side on the right-hand side. Water goes into the anode side, and water and oxygen come out on the outlet of the anode side. What we have here is the hydrogen. Once we apply the DC power and reach the minimum voltage, called the threshold voltage, which is 1.48 volts, something very interesting will start to happen.
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Because hydrogen is so positive—excuse my terrible drawing here, but I’m having to do this with a mouse—this is a proton in the middle, and we’ve got the electron, the e-minus, up here. Once we split off the electron, the proton is going to be extremely tiny. Because it’s so tiny, it’s small enough to go through the PEM membrane. The proton is positively charged, the anode side is positively charged, and therefore, the anode side will repel the positively charged proton, and the cathode will attract it into the cathode chamber. Once the proton goes over to the cathode chamber, the electron goes through the electrical connections to the other side. The proton and electron will meet, forming a hydrogen atom almost immediately. Once we have two hydrogen atoms, that will form an H2 molecule.
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We don’t only have hydrogen going over to the cathode side. We also have water transport over to the other side. The way I imagine this is to think of the PEM membrane as a sponge. If you hydrate one side of the PEM membrane, the water will go over to the other side. This is completely normal because electrolysis will not happen unless the PEM membrane is hydrated. The PEM membrane has to be hydrated. Because of that, we have quite a lot of water making its way over to the hydrogen side, to the cathode side, and that is completely normal.
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There are two mechanisms for the water to go over. One is diffusion: if you have water on one side, you will have water on the other side because of diffusion from high concentration to low concentration. The other is electro-osmosis. Electro-osmosis happens whenever we are electrolyzing and is proportional to the current density. The higher the current density, the higher the rate of electro-osmosis. The more hydrogen we’re generating, the more water is being transported over to the cathode side via electro-osmosis.
You can see this arrow here that says “water.” This is optional. Most OEMs don’t do this, but there are one or two OEMs that run water into the cathode side. Yes, this is a thing. They do run water into the cathode side, not only for small electrolyzers but also for some large megawatt-scale or even gigawatt-scale electrolyzers. Some OEMs aspiring to build gigawatt-scale electrolyzers do this. Why? They’ve realized that cooling the anode side is great because of the oxygen evolution reaction, but there’s also some heating generated by the hydrogen evolution reaction. Therefore, some OEMs pump water to cool down the hydrogen side as well.
We’ve got to move on. The stack is over here. We’ve got the water going in, the water going out, the hydrogen going out, and the power supply unit connected. Next, we’ll look at water purification. In short, the water purification system includes a carbon and particulate filter, typically an element of water softening, and reverse osmosis. After the reverse osmosis, we’ll see the conductivity of the water drastically reduce, but not quite enough for PEM electrolysis.
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Depending on what you read, some people think PEM electrolyzers can get away with water conductivity of about 5 microsiemens per centimeter. This is wrong. The lower the conductivity, the better for the stack. Less than 1 microsiemen per centimeter would be fantastic; even 0.5 microsiemen per centimeter is completely achievable. If you were to run the stack at 5 microsiemens per centimeter, it would reach the end of life very shortly, trust me. To reduce the conductivity and increase the purity of the water, you need the mixed bed DI resin. Downstream of the mixed bed DI resin, you’re going to see water conductivity levels very close to the theoretical lowest conductivity of water, which is 0.056 microsiemens per centimeter. You won’t see it go that low, but you can achieve 0.08, 0.09, or 0.1 microsiemens per centimeter downstream of the DI bed resin.
There are two parts to the water purification system. Typically, you’ll have the water inlet, the activated carbon filter, the particulate filter, reverse osmosis, etc. The role of this water purification system is to get you from potable or tap water to water that you can use in your PEM electrolyzer. However, what you will see is that when water passes through the stack, something interesting happens. The water going into the stack will be highly pure, but coming out of the stack, the water conductivity will increase. You’ll essentially have muck in the water. One issue is fluoride release. The rate of fluoride release is actually quite high at low current density, contrary to common belief. In addition, especially in brand-new stacks, the water coming out will increase in conductivity.
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What do we do? There are two ways to fix this. Both ways include one part of the water purification system. Some systems send the water back with a repolishing pump to the water purification system. Approximately 1 to 10% of the water going through the main circulation pump would be circulated back into the water purification system to be repolished. This water won’t go through all the steps, only through the mixed bed DI resin or something else, which we’ll come to in a second.
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I can already see we’re going to go over time here because I’ve barely started, and we only have two minutes to go. We’re going to go over a little bit, sorry, guys. The other way to do this is instead of having a repolishing pump, there’s going to be a completely separate water polishing loop with a pump and a water repolishing system, and then the water will go back into the oxygen-water separation vessel. Water polishing can be done in two ways, and this is a relatively recent development in the PEM electrolyzer sphere. DI resin is what we’ve been using for a long time. You can regenerate DI resins, but that hasn’t been done much in PEM electrolysis. A lot of companies have looked at regenerating the resin catalyst, but it hasn’t really been done to this point.
What we have to do is, every so often—let’s say every four, six, or eight months—you would have to go on site and replace the DI resin inside these containers. The trouble with that is it increases maintenance and costs. What a lot of OEMs are doing now is providing an option for electrodeionization, which you see on the left-hand side here. Electrodeionization does not require any replacement of material or parts because it uses DC electricity to polish the water, saving costs. If you speak to five OEMs, three of them will provide you with an option for electrodeionization.
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Let’s look at oxygen and hydrogen separation quickly. The first thing to notice is the water going into the stack. I’m hoping this makes it quite clear, but 100% of the water going into the stack versus 99.8% of the water coming out means we have a reduction of 0.2% because that 0.2% has been turned into hydrogen and oxygen. While we have a slight reduction in the water coming out, we have a massive increase in the volume we have to transport out of the stack on the anode outlet. Out of the 0.2% water, the oxygen generated increases this volume by 38%. We have to transport out a lot more volume than we put in because we have a lot of oxygen in this stream leaving the stack.
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The water comes out of the stack filled with oxygen. If this was a clear tube, around 38% of what you’d see in that tube would be oxygen bubbles. These oxygen bubbles and the water go into the oxygen-water separation vessel. The vessel will have water at a certain level, the water will go to the bottom, and the oxygen will exit at the top. There’s a little bit of hydrogen here as well, up to 1.8, 1.9, or maybe even 2% hydrogen in this vessel locally. The water will go down to the bottom, and any bubbles in the water will come up to the top. It normally takes about 15 to 30 seconds for the bubbles to rise and the water to settle, and the cycle repeats.
The pressures in the oxygen-water separation vessels vary. Some systems use atmospheric pressure, so the vessel is simply open to the atmosphere. Others increase the pressure slightly, up to about two or three bars. It’s quite low pressure; you don’t want to go very high. If the vessel isn’t open to the atmosphere, it prevents the ingress of impurities and provides slightly better recirculation with a slight positive pressure. One reason is that on the surface of the membrane, the bubbles will be smaller due to the slight increase in pressure, covering a larger area of the membrane surface.
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For hydrogen-water separation, it’s normal for the PEM membrane to allow water to cross over to the cathode side. The hydrogen-water separation vessel is at the running pressure of the hydrogen, typically 30, maybe 32, or even 35 bars in some systems, but 30 bars is the industry standard. This water is still pretty good and can be reused after running it through the water purification system to repolish it. Before that, we need to remove the hydrogen bubbles because this is at 30 bars. When you reduce the pressure to atmospheric or just above, a lot of hydrogen bubbles will be released and must be vented safely. There are complex systems to safely vent the hydrogen out of this recirculated water.
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The hydrogen going to the dryer is very close to 100% relative humidity, so there’s quite a lot of water vapor in this flow of hydrogen. Therefore, it goes to the dryer. We can see this on a larger scale with the dryers downstream, deoxidizers, etc.
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Lastly, we’ll look at the cooling systems. There are a couple of different cooling systems on these electrolyzers. One typically uses airblast coolers, and the other uses refrigerant coolers. The airblast cooler is used to cool down the process water used for the electrolysis process, which we put through the stack. We’ve got a plate heat exchanger and an airblast cooler. If you have a larger airblast cooler, your capital expenditure (CAPEX) will increase, but you’ll likely run the fans for a shorter period each day, reducing your operating expenditure (OPEX). Depending on the material used, if you’re at least two miles from the shore, you can use normal copper airblast coolers. If you’re very close to the shore, you’ll likely need a different material, like stainless steel, which is less efficient, requiring a larger airblast cooler, increasing both CAPEX and OPEX.
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In relatively large electrolyzer systems, especially in warm or hot climates, adiabatic coolers are used. An adiabatic cooler has a sprinkler system that precools the inlet air. For example, if the outside air temperature is 38°C, the wet bulb temperature might be about 23-24°C, so you can precool the air to approximately that temperature with the sprinkler system. We’ve got the airblast cooling plate heat exchanger for context and the cooling system for the refrigeration cooler, which is used for the hydrogen purification system.
There’s quite a lot of research going into reducing the size of refrigerant coolers or moving to airblast coolers because refrigerant coolers are very expensive in terms of both CAPEX and OPEX. We want to put as much electricity into the electrolyzer stack and not into the balance of plant.
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With that, we’ll jump right into the questions. The courses I do are the Principles of PEM Electrolyzer Operation training course, typically held in person in Kuala Lumpur, and another course on stack design, development, and operation for PEM electrolyzer stacks, also in Kuala Lumpur in April. An electrolyzer plant design course is another upcoming course we’re working on right now.
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Mai: Do we have any questions? Yes, we have a lot of questions here. The first one is: It is similar to alkaline electrolyzer?
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Hunor: Is the question, “Is PEM similar to alkaline electrolyzer?” Well, PEM is an electrolyzer that generates hydrogen, but that’s about it. In alkaline, you can only run at low current density, and you’ll struggle to ramp it up or down quickly, so you can’t really use it for grid balancing. Also, you’re using a chemical—potassium hydroxide mixed with water at approximately 25 to 30%. So, it’s not very similar.
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Mai: The next question is: How do you avoid intermixing of gases in the electrolyzer itself if they don’t get separated for any reason?
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Hunor: We don’t mix gases. We separate water from the hydrogen and water from the oxygen. The electrolyzer stack is where the reaction happens. You have water on one side, the oxygen stays, and the hydrogen goes to the other side. There is no situation where the gases won’t separate. There could be a situation where, if there is damage to the stack, the cathode side is pressurized, and hydrogen could potentially go back into the anode side. Most systems have extensive monitoring of the oxygen-water separator to ensure the hydrogen level doesn’t go too high.
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Mai: What is the system efficiency of PEM for hydrogen production?
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Hunor: If you don’t include heat recovery, the voltage efficiency can be anywhere from 60% to 70%. 70% is quite high, and 60% is relatively low. To generate one kilogram of hydrogen, you typically use between 50 to 55 kilowatt hours of electricity. One kilogram of hydrogen contains 33 kilowatt hours of energy, so it’s around 60 to 66%, potentially even 70% efficiency on very high-efficiency electrolyzers. If you include heat recovery, you can achieve very high efficiencies, but few systems include heat recovery.
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Mai: What is the power consumption per kilogram of hydrogen in PEM?
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Hunor: 52 kilowatt hours per kilogram is quite typical, between 50 to 55, as I mentioned earlier.
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Mai: What is the largest type of PEM electrolyzer available on the market?
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Hunor: The largest PEM electrolyzer today is 24 megawatts, in Quebec, Canada. I’ve seen announcements of companies aspiring to build a 1-gigawatt electrolyzer, and some clients I work with are looking to build 3 gigawatts of electrolysis by 2030, which is a lot. But currently, the largest operational PEM electrolyzer is 24 megawatts.
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Mai: Is the PEM membrane hydrated with water or some electrolyte?
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Hunor: The PEM membrane is hydrated with pure water. There’s no electrolyte on the surface of the PEM membrane being circulated through it. PEM stands for proton exchange membrane, but it also stands for polymer electrolyte membrane, which means the membrane itself is the electrolyte. We don’t need to flush anything on the surface of the membrane that is an electrolyte; all we flush is very pure water.
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Mai: Where are the rare earth minerals used in PEM?
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Hunor: There are several locations where we can source iridium for the anode side and platinum for the cathode side. Iridium oxide and platinum—iridium isn’t even originally from Earth; it’s from stardust.
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Mai: What is the power consumption per kilogram of hydrogen?
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Hunor: Around 52 kilowatt hours, approximately, depending on the system, between 50 to 55.
Mai: What is the lifetime of a PEM electrolyzer compared to an alkaline electrolyzer?
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Hunor: Nobody knows. OEMs typically claim their systems will last for 80,000 hours. Show me one large-scale PEM electrolyzer in the megawatt scale that has provably been running without a stack failure or major balance-of-plant failure for 80,000 hours—you can’t. Alkaline electrolyzers have a bit more data, but there are also issues. Look at the world’s largest alkaline electrolyzer system in China by Sinopec—just Google that. I believe PEM electrolyzers will improve lifetime by implementing redundancy systems and improving reliability.
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Mai: What is the efficiency and purity of a PEM electrolyzer?
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Hunor: The voltage efficiency we’ve discussed. For purity, depending on your dryer, you can easily achieve grade hydrogen for fuel cells, down to one part per million (ppm) water vapor. For oxygen content, 5 ppm is what many uses require, but it’s difficult to reliably achieve. You might achieve it for most of a 24-hour day, but it can peak over that. 10 ppm of oxygen in the hydrogen is easier to achieve, along with 1 ppm moisture.
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Mai: How much demineralized water is needed to produce 1 kilogram of hydrogen?
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Hunor: About 9 liters of pure water will generate one kilogram of hydrogen.
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Mai: When will it be possible to construct 20-megawatt PEM electrolyzers?
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Hunor: It’s already happened. Look at the Eliquid site built by Cummins PEM Electrolyzer in Quebec, Canada—24 megawatts.
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Mai: Why is a heat exchanger provided at the outlet of the PEM, not the inlet?
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Hunor: One reason is you want to cool the outlet so you’re not running hot water and hydrogen through pipelines for very long. You want to put cooler water into the oxygen-water separation vessel. However, some OEMs do it at the inlet, so it varies.
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Mai: What is the control philosophy for pressure of electrolysis?
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Hunor: I’m not sure if I understand this question, Rahul Saini. I apologize. Could you rephrase it?
Mai: How is hydrogen at pressure generated in PEM?
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Hunor: On the cathode side, hydrogen atoms appear, and as more hydrogen molecules form, there’s a back-pressure regulator or pressure control valve downstream controlling the pressure on the cathode side. The higher the pressure, the higher the voltage at the same current density. If you’re running at 10 bars, your voltage will be lower than at 30 bars for the same amount of hydrogen generated. That’s where the energy goes to generate the pressure.
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Mai: How does a developer decide the operating pressure of PEM? What is the design basis for operating pressure?
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Hunor: The operating pressure is typically 30 bars, an industry standard. In the future, to reduce wear and tear on the stack, customers like those producing ammonia-based fertilizers or refineries, which don’t need high-pressure hydrogen, may reduce the pressure to increase stack lifetime and efficiency. It’s difficult, but most systems are at 30 bars for now.
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Mai: What is the operating temperature of PEM?
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Hunor: The operating temperature varies system by system, typically around 60°C. Some OEMs use 50°C, others 65°C or 67°C. The higher the temperature, the more risk of membrane degradation due to hotspots, a main degradation mechanism. Higher temperatures increase fluoride release and efficiency but risk degradation. Lower temperatures make the electrolyzer less efficient. Running at 90°C would be very efficient but would cause rapid degradation.
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Mai: What is the life of the membrane in PEM?
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Hunor: OEMs claim 80,000 hours, but some say much less than 30,000 hours, and others are even more pessimistic. We don’t really know because OEMs won’t advertise short lifetimes.
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Mai: I understand the oxygen and hydrogen separation, but how do you separate the oxygen water generated on the anode?
Hunor: You let the oxygen bubble up to the top. The oxygen is gaseous, not liquid, and it will go up to the O2 vent, while the water stays at the bottom.
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Mai: What is the cost per kilogram for PEM?
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Hunor: I’m not sure if I understand the question, Ravikant Sanki. Do you mean the cost per kilogram of hydrogen or the cost per kilowatt for PEM? Could you rephrase that?
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Mai: What is the cost of 1 megawatt of PEM versus alkaline electrolyzer?
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Hunor: Prices have varied massively. In the past 12 months, there’s been a significant increase in prices for both PEM and alkaline electrolyzers, especially in the West. For PEM, one megawatt is around a billion pounds or a million to 1.2 million US dollars, though some OEMs have increased prices by up to 50-55%, with most seeing about a 30% increase.
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Mai: How do you decide the operating pressure and temperature of an electrolyzer?
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Hunor: The OEM decides based on the components and materials used in the stack. If components can’t handle over 60°C, you either change them or adjust the temperature. The operating pressure is typically 30 bars, but you can promise clients a lower pressure to increase lifetime.
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Mai: What is the maximum conductivity of water for operating PEM?
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Hunor: You can run a PEM electrolyzer on tap water quality, but it won’t last long—you might as well throw it in the bin. For a long-lasting electrolyzer, the water conductivity must be around 0.5 microsiemens per centimeter at all times.
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Mai: What is the current density of PEM?
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Hunor: Typically, the maximum current density is around 2.5 to 4 amps per square centimeter, with the minimum between 0.5 and 1 amp per square centimeter. Some manufacturers, like Electric Hydrogen, are increasing it to 6 amps per square centimeter, or even 11 amps per square centimeter. However, high current density increases stack degradation unless mitigated by measures like increasing water flow, reducing water conductivity, or cooling the cathode.
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Mai: Is only 0.5% of the total water input converted to hydrogen in PEM?
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Hunor: No, for every minute you’re pumping, say, 1,200 liters of water, about 0.2% of that water is used up per cycle when running at 100% output.
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Mai: How much actual power is required to run the PEM?
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Hunor: An average is about 52 kilowatt hours per kilogram of hydrogen, between 50 and 55 kilowatt hours.
About 10% of this is for the balance of plant, which you want to keep as low as possible because it doesn’t generate hydrogen.
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I hope that answers everybody’s questions. Feel free to find me on LinkedIn and DM me if you have any further questions. Thank you so much, everybody, and I hope you’ve enjoyed the webinar. All the best. Take care.