Why fuel cell cars don't work - part 3

Door mux op maandag 16 maart 2015 12:02 - Reacties (19)
Categorie: -, Views: 13.901

We are at part three of the fuel cell blog series, and it's high time we look at other things than hydrogen going into a PEM fuel cell. Other things like ethanol, methanol, split cycle vehicles, solid oxide fuel cells, using hydrogen in combustion engines and direct solar photolysis. And you'd be surprised in how many ways all these technologies have major to insurmountable problems. Probably the reason why we don't see them used at all.

This is a continuation of a blog series, here are Part 1 and Part 2 if you haven't read those yet! This series concludes in Part 4.

This is an extremely long, in-depth blog series, so I'll start by giving you a summary. This summary will exist at the top of every part of this series. If you're interested in the technical details, please do read on and make sure to come back for the next parts.

First of all, HFC cars are perceived to be a good bridge between fossil fuels and full electric because:
  • You can still fill up like you do with a gasoline or diesel powered car
  • The mileage you can get out of hydrogen is perceived to be more adequate than what you get from batteries
  • Hydrogen fuel cells are thought not to wear out as quickly as batteries (or conversely, batteries are thought to wear out very quickly)
  • Hydrogen as a fuel is perceived to be a relatively small infrastructural change from gasoline and diesel
  • Hydrogen is perceived as a cleaner solution than gasoline, diesel or natural gas
In reality,
  • You cannot fill up like you do with gasoline or diesel. It is actually pretty ridiculous how hard it is to fill up a HFC powered car
  • You won't even go 100 miles on current tech hydrogen tanks that are still safe to carry around in a car
  • Fuel cells wear out crazy fast and are hard to regenerate
  • Hydrogen as a fuel is incredibly hard to make and distribute with acceptably low losses
Additionally,
  • Hydrogen fuel cells have bad theoretical and practical efficiency
  • Hydrogen storage is inefficient, energetically, volumetrically and with respect to weight
  • HFCs require a shit ton of supporting systems, making them much more complicated and prone to failure than combustion or electric engines
  • There is no infrastructure for distributing or even making hydrogen in large quantities. There won't be for at least 20 or 30 years, even if we start building it like crazy today.
  • Hydrogen is actually pretty hard to make. It has a horrible well-to-wheel efficiency as a result.
  • Easy ways to get large quantities of hydrogen are not 'cleaner' than gasoline.
  • Efficient HFCs have very slow response times, meaning you again need additional systems to store energy for accelerating
  • Even though a HFC-powered car is essentially an electric car, you get none of the benefits like filling it up with your own power source, using it as a smart grid buffer, regenerating energy during braking, etc.
  • Battery electric cars will always be better in every way given the speed of technological developments past, present and future

Fuel cell types

In all of the previous posts, all I talked about was PEM or Proton Exchange Membrane fuel cells. But fuel cells, like everything in engineering, exist in many different variations. A fuel cell is defined very simply: an apparatus that converts chemical energy into electricity directly. So using a combustion engine to drive an electric generator doesn't qualify: the conversion must fundamentally be direct. No cheating!

For an extended overview of fuel cells, I highly recommend (again) Wikipedia's great article on fuel cells. I will only go over them very briefly.

Alkaline fuel cell
http://www.fuelcell.no/Space-Shuttle-fc.jpg
The AFC used in the NASA Apollo missions. Image from fuelcell.no

This is the oldest and most developed type of fuel cell; it has been used in multiple NASA missions (including the Apollo missions and ISS). It is also, by quite a margin, the most electrically efficient type of fuel cell (excluding external systems). In an AFC, pure hydrogen and oxygen are led into two chambers separated by a porous material that is drenched in an alkaline solution (most often potassium hydroxide, KOH). At the electrodes, the following half-reactions occur:

2 H2 + 4 OH- -> 4 H2O + 4 e-
O2 + 2 H2O + 4 e- -> 4 OH-

http://image.slidesharecdn.com/lmad5kpithsrvpckbtgc-signature-f555648a221aa96a0ffd63b23897d89f16b326e4a6ed30112ff6e13bc04a9104-poli-141222043350-conversion-gate02/95/renewable-energy-sources-27-638.jpg?cb=1419244609
AFC cell schematic (very simplified). Image originally from Wikipedia

This is different from any other hydrogen fuel cell in that the hydroxide ions (which cause alkalinity) are necessary for the reaction. This reaction produces water at the anode and consumes (half the) water at the cathode, so the most straightforward construction involves an electrolyte that can flow from anode to cathode as well as outwards (to dispose of the excess water). In older designs (and this is a great example of the age of these kinds of cells), asbestos was often used as an electrolyte matrix.

AFCs can be made (and work pretty well) without catalysts and with exceedingly cheap materials, while still attaining pretty good efficiency. This is the fundamental reason why AFCs are still being used (on quite a large scale) for certain applications. This is, however, where the good news ends.

The problem is: hydrogen in water (aqueous solution) does not often spontaneously dissociate to form loose protons and electrons. You wouldn't get large amounts of power from a fuel cell unless you make the fuel cell house-sized. This is where catalysts come in, and AFCs become a lot less interesting. I'll go into how catalysts work in the subchapter 'the platinum problem'. Suffice it to say, the only catalyst that works well in hydrogen and all other fuel cells at low temperature is platinum. You need quite a bit of platinum for a decently sized fuel cell; about 3g/kW for PEM and about double that for AFCs*. Now, this is a problem that all fuel cells have, so it's not unique.

What is unique to AFCs, is the problem they have with using normal air as an oxygen source. Air is composed of about 21% oxygen, 78% nitrogen, a little bit of water and trace amounts of other compounds. Unfortunately, because the electrolyte in AFCs is some alkali hydroxide (potassium, calcium, etc. hydroxide) it will react very favourably with CO2 in the air, producing potassium carbonate or calcium carbonate - chalk-like substances. These are not very soluble in water and will, over time, clog up the electrolyte. This reduces performance and puts a relatively short lifetime on AFCs.

The tendency of AFCs to irreversibly deteriorate over time unless you use either large/expensive scrubbers or a separate tank of pure oxygen makes AFCs pretty much uninteresting compared to more modern PEM fuel cells.

* Relatively recently, platinum-less catalysts for AFCs have been developed. Currently they are still more than an order of magnitude worse than platinum, but this is already much better than any other non-platinum catalyst developed so far. The stack cost and size is still very much unsuitable for vehicles, but it may be a solution for stationary AFCs.

Hydrogen PEM fuel cell
Proton Exchange Membrane (PEM) fuel cells use, like most types of fuel cells, the following half-reactions to get from hydrogen+oxygen to water+electric current:

H2 -> 2 H+ + 2 e-
1/2O2 + 2 H+ + 2 e- -> H2O

http://upload.wikimedia.org/wikipedia/commons/9/9b/PEM_fuel_cell.svg
PEM (and most other) fuel cell overview. Image courtesy of Wikipedia

The electrons are, again, siphoned off via the catalyst surfaces and substrate to power whatever is attached and the hydrogen ions (=protons) are moved through a very, very special material called a proton exchange membrane. This is a fairly revolutionary material: a polymer that effectively blocks everything except protons. It doesn't even conduct electrons. Well, it kind of does a little bit (as well as hydrogen and oxygen), but it works remarkably well nonetheless.

PEM fuel cells can be made extremely compact. Because the membrane material can be made almost arbitrarily thin and the catalyzing surfaces are very thin as well, the only limit on miniaturization of PEM fuel cells is water, reagent and heat management. This makes it theoretically possible to make PEM fuel cells that power cell phones.

http://static.tweakers.net/ext/f/KMrgTelQUY1n2SRM6MPj3o17/full.jpg
Just as an aside: how cool is this. We worked on the fuel cell race kart in the same hall where the Solar Boat, Nuna, Ecorunner, Formula Student and Wasub were being worked on. As well as a bunch of other smaller student projects like DARE, a record-setting rocket project I also contributed to

PEM fuel cells are low-temperature cells, meaning they operate typically below the boiling point of water at near-atmospheric pressure. Because there is effectively no electrolyte solution (the 'electrolyte' is the PEM itself), these fuel cells are fairly insensitive to poisoning by 'normal' trace gases in the atmosphere, so not much scrubbing is needed to purify the incoming oxygen from the air. Because they operate at almost-ambient conditions, start-up times are very reasonable (in fuel cell terms), to the point that it is practically feasible to put them into cars. They're not quite as instantaneous as batteries or gasoline powered cars, but they can be and they are really close already.

There are problems, though. Aside from the platinum problem, two issues that have not seen any improvement in the last 5 years or so are:
  • Gas and electron leakage over the membrane. Hydrogen is a tiny molecule that will eventually find its way through the membrane. But also oxygen leaks through. Because of the relatively large surface area of the membrane, although it is a fairly good insulator the membrane still leaks electricity significantly (up to about 10 percent of the losses in PEMFCs are due to this phenomenon).
  • Activation losses on the oxygen side. Even though platinum is a great catalyst for the hydrogen-side half-reaction, it is not super duper on the oxygen side. It is still the best we have, though. A large part of the losses in PEMFCs are due to the platinum catalyst on the oxygen side. Again; science would love to find a new, cheaper, better catalyst - but has failed to so far. The main issue here is the strong oxidative strength of oxygen, which rules out almost all non-precious metals.
Besides these issues, water and heat management are both significant issues that cause current-generation fuel cell stacks to be quite big. Most of the heat in a PEM fuel cell is produced at the catalyst on the oxygen-side, however in order to maintain good reaction slew rate (i.e. throttling performance) as well as good efficiency, the fuel cell may not heat up much and the temperature should be roughly equal over both reaction sites. This means that high power densities are most likely impossible, so you need a relatively large fuel cell. Water management has similar issues - with especially the membrane needing just the right amount of water in it to function well. So why didn't I put these issues in the nice list above? Well, even though these two issues are, at the moment, by far the biggest practical roadblocks for PEM fuel cell application, I don't view them as important to the discussion because the issues aren't fundamental to the issues with PEM fuel cells. These are, in my view, just engineering challenges that will be solved given enough time and money. The diffusivity and catalyst issues are much less likely to ever be solved.

If you're interested about how this translates into a practical fuel cell, Here is the technical product specification of a fairly widely used fuel cell stack, which at the moment retails for about §40 000-§50 000 (with subsidies).

Phosphoric acid fuel cells
http://www.ika.rwth-aachen.de/r2h/images/thumb/a/a8/Fuel_cell_bus.JPG/800px-Fuel_cell_bus.JPG
This bus runs on a PAFC. Image courtesy of Institut fŁr KFZ Aachen.

Phosphoric acid fuel cells (PAFCs) are extremely similar to PEM fuel cells, but the proton exchange membrane is replaced by molten phosphoric acid (H3PO4), and the fuel cell works at slightly elevated temperatures of about 200C. PAFCs are advantageous as compared to PEM because they don't need to hydrate the electrolyte (meaning much longer electrolyte life) and the produced water is in the form of steam, which can be used in a small steam turbine or other cogeneration plant to recoup the thermal energy from the exhaust. Maybe even more interesting is that it can run natively on some simple hydrocarbon fuels (most notably methane and methanol), as the electrodes nor the electrolyte are poisoned by CO2. PAFCs, because they run at higher temperatures, also have an easier time getting rid of their heat so the power density (output power per kg) can be better than PEM fuel cells.

All this makes PAFCs more efficient and easier to implement than PEM fuel cells. So why aren't we using them as a prime candidate for cars? Well, the big problem is that they need to run at higher temperatures (=long startup time) and even though power density of the stack itself is very good, the supporting systems are so big that complete system power density (and total efficiency) is actually quite low. It's still compact enough to be used in things like buses and trucks, but most of the applications are in static electricity generation. After AFCs and PEMFCs, PAFCs are the most widely implemented fuel cells.

High-temperature (auto-reforming) fuel cells
Then there is a whole sub-class of very high temperature (typically 500-1000C) fuel cells, with the most well-known type being the Solid Oxide Fuel Cell (SOFC). To start off with the biggest reason why these will probably never be used in cars: the start-up times are in the order of hours, not minutes. A PAFC can start up in about 5-15 minutes, a decently sized SOFC reaches equilibrium after about an hour and will consume a very significant amount of energy during start-up.

http://4.bp.blogspot.com/-hGGOgpR8oFg/TdHB_-qi-9I/AAAAAAAAAjc/3qNB5aKib8E/s1600/Scheme+of+a+solid-oxide+fuel+cell.png
A schematic representation of a solid oxide fuel cell. Image originally from Wikipedia

If we just look at the technology: SOFCs and other high-temperature fuel cells are again similar to PEMFCs and PAFCs in that they have two electrodes with an electrolyte in the middle, but in this case both the electrodes and electrolytes are generally ceramic materials. Because they operate at much higher temperatures, just because of thermal energy there is enough activation energy in the fuel inlet to transform hydrocarbon fuels, through gas reformation, into hydrogen (given the appropriate catalysts are present). Combined with the even better immunity against electrode and electrolyte poisoning, this means that basically any kind of volatile hydrocarbon fuel can be fed in and will generate electricity remarkably efficiently.

SOFCs and other high-temperature fuel cells are increasingly being considered for applications like back-up power supplies because of their improved energy efficiency and higher reliability for intermittent operation as compared to diesel generators.

http://toc.vniitf.ru/images/images-f/image037.gif
One of a couple Siemens Westinghouse co-generating SOFCs. This particular one outputs 250kW, using the waste heat from the stack to power a turbine, which yields very impressive efficiency (about 50% total system efficiency, including reforming losses which is almost 10% more than a traditional coal-fired system). Note the immense size of the installation, even though its power output is only a couple times more that of a typical car engine

The platinum problem
So, I talked about this already in the AFC section, but this is a really big problem. People are worried that lithium will run out for battery electric vehicles - well, we know for a fact that platinum will run out at leastan order of magnitude sooner.

Catalysts are materials that have a certain shape which allows reagents - materials that want to react together to form some other compound - to do their thing very well. If you have reagents just floating around in a liquid, the chance is fairly small that they will bump into each other and perform the desired reaction. More importantly, in the case of fuel cells we don't just want them to perform a reaction: we want to siphon off those electrons that are involved in the reaction. If the electrons just start floating around in a liquid they will most likely find an undesired alternative way to react and reduce reaction efficiency.

Platinum has the unique property that it is an ideal 'place' for hydrogen molecules to sit down, split up and become H+ ions (which are bare protons). In the case of fuel cells, the other side of the reaction (the oxygen reaction) is also catalyzed particularly well by platinum, although other types of catalysts exist for this half-reaction. Catalysts are not active parts in the reaction; they are merely energetically favourable sites of reaction. An important property of catalysts is thus that they have very large surface areas.

Without platinum catalysts, the reaction rate in fuel cells would be many orders of magnitude slower. Depending on temperature and pressure, the reaction rate is at least 105 higher with a Pt catalyst.

So, how much catalyst do we need? Can we improve on this? Currently, platinum consumption is about 2-3g/kWe (i.e. output electrical kilowatts). This means that a typical electric car (35kWe) will need about 70-105g of platinum. Platinum prices have skyrocketed in recent years because of its popularity as an investment metal, and sits at around 37000 USD/kg. Just the raw platinum metal in a fuel cell (that is rather small, but sufficient for most purposes) costs $3000. But that's not all.

The most important aspect of a catalyst is its reaction surface area. To improve on this (and subsequently reduce required platinum consumption), most fuel cells today use tiny (2-5nm diameter) platinum nanoparticles seated on a carbon (graphite) substrate as catalyst. This is what allows for the relatively small amount of platinum catalyst in fuel cells. Not much possibility for improvement on this front, unfortunately.

Another, as of now, insurmountable problem is the actual production capacity of platinum in the world. Production at the moment is at an all-time high, but cannot keep up with demand. We're producing about 150 000 - 200 000 kg of platinum per year, and this is not expected to be able to go up. Half of this is already being used by, mostly, ammonia production and diesel exhaust scrubbers. The rest is split between investment and jewellery.

http://upload.wikimedia.org/wikipedia/commons/7/7d/Platinum_world_production.svg
World production of platinum 1990-2015. Image courtesy of Wikipedia

http://upload.wikimedia.org/wikipedia/commons/5/57/Pt_price_92_12.jpg
Platinum prices 1992-2012. Image courtesy of Wikipedia. 1 troy ounce == 30g

We're producing about 50 million cars per year in the world. If all these cars were to run on fuel cells, we would need 5 million kg of platinum per year. This is physically impossible. With the current state of technology, even producing 1 million fuel cell cars per year would be impossible. But wait, we can recycle the platinum from fuel cells, right?

Yeah, absolutely. With about 50-75% efficiency. The big problem here is that in order to use amounts of platinum that a mortal human can pay for in a fuel cell, we need to use either platinum nanoparticles or other microstructured forms of platinum to generate the surface area needed for the reaction to take place. As a fuel cell is used, some of these tiny particles erode from the electrodes and wash away, either through the exhaust or into the cooling system. The platinum particles that stay put are still extremely tiny and during the regeneration process, they cannot all be recovered. Of course, this can be improved upon and platinum recovery from fuel cells is hardly a big industry right now. But even in applications where platinum exists as a gauze or sponge - all in one place, easy to recover - the recovery rate is still usually less than 95% (higher for very large industrial gauzes). This means that we will still never be able to produce anywhere near the amount of cars we produce right now using fuel cells.

Even if platinum consumption can be reduced in the future - even if it could be reduced tenfold - the extremely limited supply and complicated substrate structures will still mean that electrodes in fuel cells will be a majority cost of your car. As it stands, the electrodes in production cars account for about $10 000 of the final bill of materials. If fuel cells are ever going to become a thing, the single biggest hurdle that needs to be overcome is not a reduction, but a complete elimination of precious metals as catalysts.

Using other fuels


Hydrocarbon fuels in PEMFCs using on-board reforming
In the above chapters we've fairly generally gone over different types of fuel cells and although no amount of blog posts can truly convey the technical details of this all, the general gist is that only PEM fuel cells are small, light and fast enough to be able to be used in smaller vehicles. We've also established in the previous parts of this blog series that hydrogen is a bitch. The logical step is to look for other types of fuels to use in fuel cells.

This is a great idea because - and for some reason I haven't said this already - there are more hydrogen atoms in a liter of uncompressed gasoline than there are in a liter of hydrogen gas compressed at 700 bars. Gasoline is an awesome storage medium for hydrogen. As are most other hydrocarbon fuels; simply the fact that the molecules are larger and packed together more tightly (because they can exists stably as a liquid at low pressures) counts for a lot of convenience.

Well, there's a little problem. PEM fuel cells only accept hydrogen and no hydrocarbon fuels (e.g. natural gas, gasoline, diesel, methanol). Fundamentally, this is because the fuel side and oxygen side are physically separated by the PEM which only conducts hydrogen cores (protons), whereas for a full hydrocarbon redox reaction you want the carbon from the fuel side to go over to the oxygen side and bond with the oxygen to form CO2. So in order to make PEM fuel cells work with other fuels, there is no other option than to bring your own hydrogen reforming plant into the car.

http://bioage.typepad.com/photos/uncategorized/renault_h21.png
Renault-Nuvera's on-board multi-fuel reformer concept from 2002-2006. This project was abandoned when Renault-Lexus went all-in on battery hybrids. Funny enough, just recently Toyota has partnered up with this exact same company.

Great. Like fuel cell cars weren't complex and inefficient enough already. This is obviously not an elegant solution, nor a cheap one. Hydrogen reforming is again something that depends quite a bit on (expensive) catalysts and needs a bunch of extra supporting systems. However, you do save on having to store hydrogen, which takes away a lot of problems. But... let's do some quick calculations. Let's take the LHV of a bunch of hydrocarbons, and the LHV of the recoverable hydrogen that can be extracted from them using gas reforming (all units per mole of original fuel):

FuelLHV [kJ]LHV of recovered H2 [kJ]Efficiency fuel->H2
Methane 802.34 488 60.8%
Methanol 638.55 488 76.4%
Ethanol 1329.8 732 55.0%
Gasoline (octane) 5074.9 2196 43.4%


Not that great. As it turns out, people have written papers on this. A lot of hydrocarbons contain very large amounts of hydrogen, but the actual stored chemical energy in the form of hydrogen you can extract from them is very limited. This is because a great deal of the chemical energy is in the form of C-H (and C-C) bonds, which is energy you lose when converting to pure hydrogen (and cannot recover using a fuel cell!). Especially if we look at the efficiency of using gasoline directly in a fuel cell car, we find something incredibly interesting: it's actually worse to use gasoline in a reformer and then generate electricity in a PEM fuel cell than it is to directly burn it in an internal combustion engine - technology that has been made very cheap and abundant already. ICE efficiency (especially more modern engines that use the Atkinson cycle) can get thermodynamic efficiencies in excess of 35%. If reforming is already just 43.4% efficient and requires a ~60% efficient hydrogen-to-electricity step after that, the fuel cell alternative is worse in every respect.

The table is quite limited, but there are two important trends to observe: 1) larger hydrocarbons are less useful for gas reforming, and 2) alcohols are better at hydrogen energy storage than other hydrocarbons. This is why methanol in particular has gotten quite some attention over the years for use in fuel cells. It has none of the storage problems that pure hydrogen has, it is fairly easy to produce from other chemical energy sources (like biomass) and it is pretty good at 'storing hydrogen' for hydrogen fuel cells. Mind you though, you still lose about a quarter of the energy stored in the methanol outright, and you need to get rid of this substantial amount of extra heat with a larger, heavier and more expensive cooling system. The fuel cell car only gets more complex. So why not do...

Direct hydrocarbon use in high-temperature fuel cells
Autoreforming fuel cell types, i.e. fuel cells that can reform hydrocarbon fuels in the stack itself instead of needing a separate reformer, relieve the need for additional complex subsystems. Very roughly, there are two types: proton exchange and cation exchange. Proton exchange autoreforming fuel cells only conduct hydrogen ions through the electrolyte, meaning that we end up with the same problem as before: a big proportion of energy in hydrocarbon fuels is 'wasted' because we cannot harvest the energy from the creation of CO2. However, specifically solid oxide fuel cells (SOFCs) transport the oxygen ions through the electrolyte, which means these fuel cells work very efficiently with hydrocarbon fuels. This is why SOFCs are such a popular research topic for static and e.g. shipbound applications; they can run directly on e.g. Diesel fuel with much greater efficiency than even very large, nearly perfect turbine and piston combustion engines. Unfortunately the exceedingly long start-up times, very high weight of the stack and high required temperatures fundamentally rule out SOFCs for vehicle use.

http://cdn01.androidauthority.net/wp-content/uploads/2015/01/kraftwerek-promo-image.jpg
The Kraftwerk portable SOFC. Courtesy of Kraftwerk

There are portable direct hydrocarbon fuel cells, e.g. direct methanol fuel cells. These have even found some moderate success in commercial applications, a recent example being the Kraftwerk mobile direct hydrocarbon fuel cell power supply. These suffer from quite low efficiency and power density, though. This product especially is marvellous... to demonstrate the poor efficiency and near-obsolescence of direct hydrocarbon fuel cells in portable applications. Let's do some calculations again!

First of all, Kraftwerk demonstrates how much supporting structure you need to even make a low-power USB charger. The entire charger is 160g (200g filled up with LPG) and only produces 2W continuous (with a quoted 10W peak, which is most likely a 1-second rating as the electrolyte depletes, although this is conjecture as they give almost no specs). The total energy that can be generated from the 40g of fuel is about 56Wh. LPG has an energy density (LHV) of 46.28MJ/kg, or about 12.9kWh/kg. 40g of LPG then contains 514Wh of energy. Only 56Wh can be regenerated from that, giving an overall system efficiency of 11%. A lot of this energy loss is in reforming; LPG is usually a mix of propane and butane which have a thermodynamic reforming efficiency of 47.7% and 45.9% respectively. This leaves an actual fuel cell efficiency of about 23.5%. This is much, much worse than any combustion engine, but indicative of typical lower-temperature SOFCs.

The really interesting thing about Kraftwerk in particular is that they claim the exhaust is only water vapor and CO2. Typical SOFCs do not have 100% complete combustion and require CO scrubbing, as well as always leaving a small proportion of the fuel unburnt (which is exhausted as is). The description isn't quite clear on this, but the fact it is advertised as being usable indoors leaves me to think they have done something very innovative to optimize for clean exhaust gases. Either that, or the eventual product will actually say it cannot be used for prolonged periods of time in an enclosed space. The unit claims protection against overheating, which is consistent with the entire energy cycle being internal to the unit (i.e. it will produce 18W of heat when it's generating 2W of electricity), with the majority of the heat not being exhausted as gas. Also interesting to note is that the unit only has a 2-year specified working life 'during normal use'. This is also consistent with low-temperature autoreforming fuel cells, which tend to have a useful life of 1000-2000 hours.

http://www.ttcart.com/images/STX-6000II_5.jpg
This is actually better at a lower price. Image courtesy of STX

But for all this innovation, a 200-g lithium ion battery unit with solar panel stores almost the same amount of energy (40Wh), can literally be used everywhere without even needing to fill up with gas and has a much higher power density, being able to fast-charge phones and even tablets in much less time. And these units are commodity and cost about half of the Kickstarter price. Cutting-edge (truly innovative) fuel cell technology cannot even start to compete with commodity battery tech.

Using hydrogen in existing ICEs

A couple of people have asked why we don't use hydrogen directly in ICEs. I don't need to devote much text to this; the simple answer is that it's inefficient. But we can get into a little bit more detail.

If we just quickly gloss over all the practical problems with storing and injecting hydrogen into the cylinders of your existing car, let's just look at the raw theory. The idea behind burning hydrogen in your car is that hydrogen has a much higher combustion temperature than gasoline or diesel; whereas gasoline burns at about 550K, hydrogen burns at roughly 800K. Because an ICE is a heat engine, we can express the theoretical maximum efficiency of the engine as:

efficiency = 1 - TC/TH

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/imgheat/carnot2.gif
Carnot cycle efficiency. Image courtesy of hyperphysics

Where TC is the ambient temperature and TH is the combustion temperature (this is a gross simplification for practical engines, but bear with me). If we calculate this for gasoline, diesel and hydrogen, we get the respective efficiencies:

FuelCombustion temperatureCarnot efficiency
Gasoline550 K46.7%
Diesel673 K56.4%
Hydrogen800 K63.4%


Theoretically, burning hydrogen in an ICE is quite a bit more efficient than using gasoline or diesel. By the way, here's one of the biggest reasons why Diesel fuel is 'better' (in terms of efficiency) than regular gasoline. Actually, things get even better for hydrogen. One of the simplifications in this approach is that the actual combustion inside a cylinder doesn't happen simultaneously in the entire mixture. Instead, there is a flame front going through the mixture over a not-insignificant period of time. During this period of time, the piston actually moves which means that the combustion process doesn't happen under the most ideal circumstances everywhere in the cylinder. Diesel has a leg up on gasoline because it can achieve autoignition at multiple points in the cylinder instead of just having the single point of ignition (the spark plug). Hydrogen, though, has even more of a leg up in that its flame front speed is much, much higher than either hydrocarbon fuel. This improves considerably on the practical efficiency.

However, this is where the party stops. Because even though it's theoretically over 60% efficient, practical efficiency will most likely never surpass about 47% because of multiple technical issues (e.g. the actual practically usable heat engine cycles). Even the much more mature gasoline technology has not yet surpassed 38% efficiency in practical engines. And all of this while PEM fuel cells already, even in its relative infancy, achieve about 50-60% practical efficiency. Fuel cells are simply better than ICEs and will always be better because ICEs have this unbreakable efficiency limit. Not to mention the significant inefficiency of generating and storing hydrogen, which makes the comparison between gasoline/diesel and hydrogen in ICEs extremely unfavourable for hydrogen.

A commonly heard argument is that we already have ICE cars, and that we don't need to change much to regular cars to let them drive on hydrogen. Should I even type a rebuttal after 3 blogs full of the technical details here? No, it actually is quite complicated to adapt existing cars. This is not a sane argument.

Conclusion

http://static.tweakers.net/ext/f/foxeorgfq2EotKaSGbbay6Rb/full.jpg

I think it's best to leave this side of the technical discussion on fuel cells here. I was planning on talking about direct solar photocatalytic conversion, Sabatier carbon capture and a few other things but the post is already long enough and these subjects - in my opinion - don't add too much of value to the discussion. But if you disagree, I encourage you to type up a comment. In general, comment away! I don't make any money off of these blogs, I do these just for the love of the subject. I love to hear your thoughts, comments, corrections!

Today we've looked at alternatives to hydrogen and alternatives to PEM fuel cells. Basic conclusion: they're generally not that great, to be honest. Actually, I'm wrong here. They are great. SOFCs and AFCs are excellent fuel cell types that, in my opinion, will certainly come to dominate the fuel cell market. But the fuel cell market won't dominate vehicles. This will be for stationary applications. Fuel cells are great when they're sitting still.

In the next and last post in this series, I will tell you all about the way I think the future of vehicles will be. I will tell you why battery electric vehicles are better than both combustion engine-driven cars as well as fuel cell vehicles. I will also give you some sound investment advice. Seeya!

Volgende: Why fuel cell cars don't work - part 4 03-'15 Why fuel cell cars don't work - part 4
Volgende: Mmmm, lekker eten! 03-'15 Mmmm, lekker eten!

Reacties


Door Tweakers user Ryceck, maandag 16 maart 2015 14:46

I actually learned something today that I had no idea of that it even existed :)

Thanks for the great post; I am a wiser man concerning the subject of Hydrogen Fuel Cells and why they aren't a good solution to an important problem

Door Tweakers user himlims_, maandag 16 maart 2015 17:36

projectje; enkele jaren in ontwikkeling, en wat ik begrijp, recentelijk live: AIRPod - auto op lucht

Door Tweakers user mux, maandag 16 maart 2015 19:31

himlims_ schreef op maandag 16 maart 2015 @ 17:36:
projectje; enkele jaren in ontwikkeling, en wat ik begrijp, recentelijk live: AIRPod - auto op lucht
Dit is in BraziliŽ ook al een keer op vrij grote schaal gedaan. Het probleem met auto's op perslucht is dat je maar relatief weinig energie kunt meenemen, en nog steeds vrij grote verliezen hebt bij zowel het 'maken' als 'gebruiken' van de perslucht. Het voornaamste voordeel hiervan is niet dat het een vervoersprobleem oplost, maar dat het extreem goedkoop en makkelijk is om een persluchtauto te maken. Je kunt op die manier hele nuttige korteafstands-auto's (heftrucs, industriele voertuigen, zelfs taxi's en minibusjes) maken voor een paarhonderd ŗ een paarduizend dollar. Extreem nuttig voor ontwikkelingsgebieden.

Door Tweakers user H!GHGuY, maandag 16 maart 2015 20:30

I agree that efficiencies are important, but I'd like to use a different efficiency metric:
ecological impact/energy output.

The point is that I care less about end-to-end efficiency when ecological impact is significantly lower. If E2E efficiency is reduced by a factor 5, while reducing ecological impact by a factor 50, I'll happily build more while still significantly lowering ecological impact.

That's why I see SOFC and PAFC is minor improvements over just burning carbon-based fuels, rather than solutions to their replacement by hydrogen-based fuel.

Of course, as you point out throughout the blog series: feasibility and cost are 2 other important factors, especially for wide-scale deployment like in cars.

I also tend to see electricity generation as a mostly solved problem due to the advent of mass-produced solar and wind technology. Both are ultimately sun-powered and I think that we'll see us harvest a lot more solar power in the future. Efficient and environmentally friendly storage, however, remains a bit of an issue, in my opinion...

Door Tweakers user mux, maandag 16 maart 2015 21:31

That is a great point of discussion to include in the final part of the blog series!

Door Tweakers user marcop23, maandag 16 maart 2015 21:59

I don't really know a good suggestion, but I just want to say I'm looking forward to the next blog! This is a very useful, and interesting blog series.

You said in a DM, some years ago:
Heh, bedankt. Ja, er komt zeker nog eens een blog over transportefficiŽntie.
- Maandag 26 september 2011 21:40

And, here it is!

[Reactie gewijzigd op maandag 16 maart 2015 22:00]


Door Tweakers user mux, dinsdag 17 maart 2015 09:27

Yeah... this has taken a heck of a long time to finish...

Door Tweakers user Sissors, dinsdag 17 maart 2015 09:59

While I largely agree with your conclusions, I am a bit surprised about the number of times you mentioned this point in your blogs:
•Efficient HFCs have very slow response times, meaning you again need additional systems to store energy for accelerating
While true, I don't really see any technical problems with integrating a battery for very short distances, I expect it to be significantly easier than current hybrid cars.

So my question would be: Why would integrating a battery for a few hunderd meters, until the HFC is delivering the power, be an issue, but integrating a battery for a few hunderd kilometre not be an issue? It seems to me that the order of magnitude is so different, that adding some batteries in a fuel cell powered car should be an insignificant issue.

Door Tweakers user mux, dinsdag 17 maart 2015 10:15

Yes, using batteries for that kind of work wouldn't actually work. You would run that battery into the ground unless it is incredibly overdimensioned. Think about it: if it only has to work for a couple hundred meters, the charge/discharge rate has to be in the tens of C. No type of battery lasts more than a couple tens to a couple hundreds of charges at these kinds of rates.

So you need to overdimension it to the point where it charges and discharges roughly at 1-2C, which means you need in the order of 10kWh. And that's very expensive.

So the current status quo is using ultracapacitors or LiCaps (lithium ion capacitors, hybrid stuff). These still add a significant amount of volume and cost to the car, but at least they're relatively light and not that complex to integrate.

Door Tweakers user Sissors, dinsdag 17 maart 2015 10:21

Ah of course, stupid of me to forget that the relative discharge rate gets way too high.

Door Tweakers user matroosoft, dinsdag 17 maart 2015 10:41

Misschien suggestie: een keer een Nederlandse versie van deze blog posten? :)
De Engelse versie is voor mij redelijk te lezen, maar m'n Engels is nog niet zo heel goed. Daardoor is het best vermoeiend om zulke lange teksten in het Engels te lezen. Overigens wel een goede oefening. ;)

Door Tweakers user mux, dinsdag 17 maart 2015 10:44

matroosoft schreef op dinsdag 17 maart 2015 @ 10:41:
Misschien suggestie: een keer een Nederlandse versie van deze blog posten? :)
De Engelse versie is voor mij redelijk te lezen, maar m'n Engels is nog niet zo heel goed. Daardoor is het best vermoeiend om zulke lange teksten in het Engels te lezen. Overigens wel een goede oefening. ;)
De hele blogserie is bedoeld voor internationale lezers (en wordt ook vrij goed gelezen buiten NL; 50% van m'n clicks komen vanaf reddit/hackernews/etc.). Het is nogal veel werk om deze dingen in elkaar te zetten, dus ik beslis al vroeg tijdens het schrijven of ik het in het Engels of Nederlands doe. Dit keer dus EN.

Door Tweakers user matroosoft, dinsdag 17 maart 2015 10:51

Jammer, maar begrijp het wel. Het is in ieder geval een interessante serie!

Door burne, dinsdag 17 maart 2015 23:44

Don Quixote is er niets bij, mux vertelt het stelletje idioten wel even dat de aarde rond is. Rond, als een pannekoek!

Door Tweakers user mux, woensdag 18 maart 2015 09:41

burne schreef op dinsdag 17 maart 2015 @ 23:44:
Don Quixote is er niets bij, mux vertelt het stelletje idioten wel even dat de aarde rond is. Rond, als een pannekoek!
Wat ik in deze blogs vertel is geen nieuws (althans, geen nieuws voor mensen binnen dit veld), het wordt alleen niet zo goed gecommuniceerd buiten de wetenschappelijke wereld/wereld van engineering. Er wordt wel regelmatig gezegd 'brandstofcellen zijn dom' ('fool cells' ->Elon Musk), maar zelden wordt er een fatsoenlijke technische uitleg gegeven waarům dit is.

Door Tweakers user hans_debruin, woensdag 18 maart 2015 13:14

What a great blog!

Reading this I become more and more convinced that solar power is the future for many things like this. Also if you look in nature, everything is solar powered. The key is that things in nature require so little energy that solar power suffice. We need to develop things that require less and less power to operate.

Door Tweakers user Sissors, woensdag 18 maart 2015 19:52

I think many things in nature are way less power efficient than our machines. The key of nature is concentrating energy, where plants and herbivores perform the role of concentrating the solar energy. But there is no way for example a lion could obtain enough energy from the sun if he had chlorophyle (or just regular solar panels on his back ;) ).

Of course many things can be made more efficient, but at the same time there are also laws of nature that we cannot circumvent.

Door Maarten de Wit, donderdag 7 april 2016 11:19

I already had te idee that the FCEV for personel transport is not gonna be a serious thred to the FEV (see my blog on maartendewit66.wordpress.com), but after I red your 3-part blog, i'm totaly convinced. But how is it possible that Prof. Dr. Ad van Wijk (Professor in ‘Future Energy Systems’ at the Delft University of Technology) is convinced that in 10 years there will be more commercial FCEV's on the roads in Europe than FEV's?

Door Tweakers user mux, donderdag 7 april 2016 11:36

Well... you'll have to ask him for his motivations. There's not much to discuss without arguments :P

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