Hydrogen Production ..Paul Martin ..chemical engineer.
Reference to Ballard and Plug ,,, Elon also ,, but I lost that part in the shuffle.
Hydrogen production is itself roughly 70% efficient- regrettably, that’s at best. A recent conversation I had with Hydrogenics, a major producer of both alkaline and PEM electrolyzers, puts the efficiency of their cheaper alkaline units at around 60%, and the efficiency of the PEM units at around 70%. From what I understand, that’s scraping pretty near the ~ 77% ultimate efficiency for electrolysis, in terms of LHV of H2 product out per unit electrical energy input. As I mentioned, that in and of itself is a loss- it’s acknowledging that you’ve put energy into the production of hydrogen that you will not get back because you’re not recovering the heat of condensation of the product water.
Note that most electrolysis vendors state their efficiencies in HHV terms, i.e. including the heat of condensation of the product water. On that basis. 70% LHV efficiency (the figure I'm using) is about 83% HHV efficiency. That's achievable in a PEM electrolyzer running at a low current density- you could likely produce significantly more hydrogen from that same unit by simply turning up the current and losing more energy due to reduced efficiency.
The trouble with electrolysis is that some of the energy obviously goes into making oxygen (though in Gibbs energy terms, you don't get any credit for that). That might be valuable itself and hence worth a credit if you’re doing it in large enough systems to make oxygen clean-up and compression for sale a reasonable thing to do, or if you’re using the H2 not as fuel but as a process feed and that process also needs oxygen. Regrettably, a vehicle refuelling station isn’t going to get any benefit from the product oxygen- it’s going to vent it.
So, let’s take 70% (of LHV) for the conversion of electricity, presumably renewable electricity, to energy in the form of the LHV of hydrogen. To be fair, we’ll have to throw in a 6% penalty for grid losses on the way from the power plant to the electrolyzer.
The figure of 70% of product LHV per unit feed LHV happens to match pretty closely to the best estimate for the best available technology for hydrogen production from natural gas, the large centralized steam methane reformer (SMR). An SMR takes advantage of huge scale to provide benefits from heat integration and thermal energy recovery, including burning the waste gases produced when purifying hydrogen to the extreme levels required for the long-term survival and efficiency of our “engine”, the PEM fuelcell. These devices are really sensitive to carbon monoxide (CO), which poisons the precious metal catalyst. Regrettably you get some CO any time you reform a hydrocarbon fuel to make hydrogen. Even worse, the catalyst itself can also make CO from CO2, so your hydrogen feed has to be purified to remove both CO and CO2 to ppm levels. In fact, even inerts like argon or nitrogen in the feed hydrogen have negative effects on the efficiency of the PEM fuelcell by requiring more anode tail gas venting, so in fact you need very pure hydrogen to feed a fuelcell- something you’ll rapidly find out if you get the purity spec requirements for a PEM fuelcell from someone like Ballard, Plug Power etc.
Regrettably, the efficiency of SMR drops like a rock the smaller you make the unit. Heat losses become more profound, which matter a lot in a high temperature process like SMR. And as the scale drops, so does the opportunity for beneficial heat integration etc. Everything just gets worse at smaller scale for this process, as anyone rapidly discovers when they try to design such a unit for a small application like, say, a chemical pilot plant, or a vehicle refuelling station…
Distribution from a natural gas well through a gas plant and pipeline to the SMR, and then distribution of hydrogen from a centralized large SMR to fuelling stations, is likely going to cost us a great deal more than 6% of the energy in the product hydrogen, but let’s be generous and keep that loss at 6% total just so we have less math to do (spoiler alert- it won’t matter in the end!). So whether we start with electricity or methane, we’re down to 0.7*0.94 or about 66% of the feed energy by the time we’ve made hydrogen- at best, without much room for improvement because we’re up against the thermodynamic limits already.
Note also that it is possible to achieve very high apparent efficiencies (even, somewhat strangely, higher than 100%) if you electrolyze steam at high temperature rather than starting with water (for instance by running a solid oxide high temperature fuelcell in reverse). However, those efficiencies are artifacts of the calculation- the energy used to evaporate water to make steam and then superheat steam to high temperature are not included in the calculation. Nobody uses steam electrolysis to make hydrogen unless they have a use for either hot hydrogen or hot oxygen or preferably both.
Hydrogen StorageNow we have to store the hydrogen, and the devil in that detail again arises from the molecule itself. Though its energy density per unit mass is quite impressive, hydrogen even as a cryogenic liquid (at 21 degrees above absolute zero...) is only 75 kg/m3…so the only currently practical means of storing hydrogen for small vehicle applications is as a high pressure gas. Any means used to increase the storage density or to reduce the storage pressure (things like metal hydrides, adsorbents, organic hydrogen carriers etc.) either significantly increases the mass of the tank, or increases the parasitic loss of hydrogen during storage, or requires energy to recover the hydrogen, or a combination of those things. So high pressure gaseous hydrogen it shall be, and I wouldn’t count on some magical breakthrough to change that- we’ve had plenty of time to consider the alternatives in the thirty years hydrogen has been a serious contender as a vehicle fuel.
While much ado is made about how dangerous hydrogen is, there will be no pictures of the Hindenburg in my paper! In fact we’ve been handling hydrogen quite safely in industrial settings for a long time- we know what it takes to keep it safe. The wide flammability range is offset by its low density and high diffusivity, making hydrogen explosions rather less likely in practice than in the imagination of people doing HAZOP reviews. With proper precautions during design and operation, high pressure hydrogen is quite safe- in industrial settings that is! I don’t want my neighbours to even think about making 6,000 or 9,000 psig hydrogen using their home solar panels…that gives me nightmares on many levels.
The trouble with high pressure hydrogen storage is that you have to compress the gas from a modest ~300 psig exiting an SMR, or perhaps from near atmospheric pressure exiting a PEM electrolyzer- a compression ratio ranging from ~20:1 to over 400:1. That takes thermodynamic work, which takes energy, typically electricity. And regrettably, the heat of compression, although available, needs to be rejected at a rather low temperature to protect the compressors’ components, and hence is rather difficult to use in any meaningful way. Even worse is the fact that you need a tank at, say, 6,000 psig pressure which can fall only to 5900 psig when filling the tank on the vehicle, so all the compression is done at the highest compression ratio- and the tanks themselves at the filling station need to be very large indeed.
When done on a massive scale with large compressor trains, high pressure hydrogen storage can be as good as 90% efficient in terms of LHV of H2 stored per unit electrical energy used to run the compressors, which is surprisingly good given all these considerations. (Note that the polytropic efficiency of the compressors themselves is a small fraction of that number- this is a very different measure of efficiency). Regrettably though, when you reduce the size of the compressors, the efficiency plummets. A single-vehicle multistage diaphragm compressor may be as little as 50% efficient on that basis or even less - this is something which, along with the unit capital cost, gets much worse as the scale decreases. That’s a shame, because distributing hydrogen over long distances is infeasible for exactly the same reasons it’s hard to store- the properties of the molecule. All the dreams about a “hydrogen economy” are predicated on small, distributed hydrogen generation systems so the thing we need to move around from place to place isn’t hydrogen, which leaves us in my view with only one realistic option: electrolysis.
OK, so we’re at 70% (H2 production) x 94% (grid/distribution loss) x 90% (high pressure storage) = 59% from energy source to tank, compared with 80% for gasoline. Clearly we’re not going to be feeding that hydrogen into a lossy ICE as a replacement for gasoline, especially if the source of the H2 is fossil- we'd be far better off feeding the ICE directly with whatever fossil we started with. And if we care about GHG emissions, we certainly can’t make that H2 from fossil sources- we’d be better off with the Prius for sure. Electrolysis from renewable electricity is our only hope.
The Proton Exchange Membrane (PEM) FuelcellSadly, we’re not done losing energy yet- next is the loss in the PEM fuelcell. Despite the fact that it is not a heat engine, it still has its own limiting thermodynamics. PEM fuelcells are achieving efficiencies of about 50-60%, and that is not far off the ultimate thermodynamic limit of about 83% for an ideal fuelcell.
princeton.edu
So let’s be generous and take 60% as the fuelcell efficiency- that will get us from the well or power plant all the way to the output of the fuelcell.
The FCEV From Energy Source to WheelsNow we have the electric drivetrain (inverter and motor) and its 90% efficiency- so “well to wheels”, or “power plant to wheels”, we’re now at 94%x70%x90%x60%x90% = 32%. I’ll remind you that on a well to wheels basis, the Prius achieved about 30% on gasoline- so we’re doing better than the Prius, and with no tailpipe emissions! And rapid refuelling. Hurray! Right? Right?...
I remind you that my home-made electric vehicle, the E-Fire, on the same basis, was achieving76.5%...and it had no tailpipe emissions either. And despite having a very small pack by OEM EV standards- only 18.5 kWh- it has an adequate range for my commute. We’re just crossing 12,000 nearly fossil-free miles driven so far, and I’ve never waited around for it to charge- I just plug it in once at night, and once in the morning at work. It doesn’t replace everything a gasoline car can do, doesn’t try to, and doesn’t HAVE to in order to serve a very valuable purpose- getting me to work and back with acceleration that makes my neck sore.
The FCEV Loses on Cycle Efficiency- But We're Not Done Losing Yet!So when we talk about the FCEV, if we’re honest, we’re talking about a technology which has a best case energy source to wheels efficiency of roughly 2.4x inferior to that of an existing alternative technology (the battery EV). What do we get in return for that huge efficiency hit? Faster refuelling, and possibly modestly greater range between refuellings- that’s it.
Seems like too much of a price to pay? Wait- we’re not done yet! We haven’t even started talking about cost…
Hydrogen is a very expensive fuel, regardless how you slice it.
The 2.4x inferior efficiency- again at best- means we’ll have to build out at least 2.4x as much renewable infrastructure as if we used it to recharge EVs. That alone should give hydrogen promoters significant food for thought.
Then there’s the hydrogen distribution infrastructure. You’re not going to be refuelling at home, folks, unless the local fire marshal falls asleep at the wheel of his diesel fire truck. So that means businesses are going to have to build out that infrastructure, and they’re going to want a return on that investment. They’re not going to MAKE that investment, because they know that a return is impossible.
And as if that weren’t enough, now let’s talk about what else is in a FCEV. There’s of course a hydrogen storage tank and a PEM fuelcell. Oh, and every other part of the battery EV, including the battery! The battery will be smaller- closer to that used in a Prius than to that used in a BEV, but it’s still needed to capture regenerative braking energy, to manage the power demand on the fuelcell stack to keep its costs down, and to manage the start-up and shutdown process of the fuelcell system. So the FCEV will be a hybrid.
Furthermore, we’ve had a long time to drive down the costs of the PEM fuelcell, and the costs are still very high. Though that would certainly drop further as a result of the “learning curve” with mass adoption and mass production, just as it continues to do for Li-ion batteries, there’s a nagging limiter to dropping that price too far: platinum group metals (PGMs) such as platinum and palladium that are used in the fuelcell as catalysts. Reduce the PGM content and the fuelcell becomes even more susceptible to hydrogen impurities, and the efficiency drops too I suspect. Replace the PGMs with cheaper metals like nickel and most of the benefits of the PGM fuelcell go away- you’ll need to operate it at higher temperature etc. |
