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Door mux op donderdag 01 februari 2007 19:57 - Reacties (1)
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Why fuel cell cars don't work - part 1

This is an extremely late article. Hydrogen fuel cells (HFC) are and have been part of the yearly news cycle for more than 10 years now. Multiple car companies have attempted and only very recently has one sort of production car been announced. Meanwhile, electric cars have taken off like nobody's business, despite the big downsides the public (and car companies) think electric cars have compared to HFC cars. I have been involved in quite a few ways in the nuts and bolts of electric cars and fuel cells, so I pretend to know a thing or two about why this is.

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
  • 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
So now, on to the proper reasons and math. Even if you don't have much interest in electric or, well, any type of car, this is still good stuff. Today we will be looking at what a fuel cell car is and why people think they are cool.

Reasons why people like HFC cars

There are actually a bunch of reasons why people seem to historically like hydrogen fuel cell cars. There are a lot of differences pertaining to the age of the individuals asked, level of education and of course political leanings. But wait, first I'll talk a bit about myself.

Tinkering with the Forze II in Zaragoza

I've been involved in the first international hydrogen racing championship. Started in 2007, it was called Formula Zero with 'zero' pertaining to being zero-emission. I was in one of the teams, known as Formula Zero Team Delft ('Forze') and my main raison d'Ítre was designing and assembling electronics for the race kart. Yeah, they weren't actually full-fledged formula one cars; it was just small class racing karts. Top speed around 110-120km/h, 0-60 in about 3.5-4sec, nothing especially interesting about them from a racing perspective. But they were hydrogen powered, which was extremely cool back then and kind of still is - at least from a technical point of view. The Formula Zero championship eventually merged with Formula Student, so if you're interested in more info, take a look at their website. I am no longer involved with any of this - I've gone on to do my Master's thesis about power conversion in electric cars and I now have a couple of businesses that mostly do electronic system design for optimizing power conversion. In computers. Not quite the same anymore.

Nevertheless, I do have a lot of hands-on knowledge from my time at Formula Zero and I know what goes into building a hydrogen fuel cell powered car. I've done a lot of literature research and kept up with the technology. And: during all this time that hydrogen fuel cells have been in the news, I have never come across any kind of public article that properly explains WHY things are the way they are with these enigmatic machines. I've been meaning to write about it since at least 4 years, and thought I would have been beaten to the punch many times already. But no... so, here we go.

http://www.goodsense.nu/wp-content/uploads/2007/07/banner_h-racer_landscape.gif

Whoo! A hydrogen fuel cell car! (courtesy of goodsense.nu)

People don't like change
People don't like change. I don't actually believe that statement, but almost always the first reason that people give me when I ask them 'what do you like about the concept of a hydrogen fuel cell car?' is: well, I can just stop at a gas station and fill 'er up! No worries about having to wait for 6 hours for the stupid battery in an electric car to charge. People imagine it's basically the same as a gasoline-powered car. People imagine the same kind of filling stations with the big gas trucks distributing hydrogen to them.

http://www.hydrogencarsnow.com/images/AirProducts/hydrogen-filling-station.jpg

The next reason people give is range, or the distance you can drive on a single tank. They imagine for various reasons - hydrogen being very light is an often heard one - that you an fit a bunch of hydrogen in there and just go for 500 miles on a tank. I'm of course deliberately saying 'they imagine' and not using any definite statements, because I'll tell you later why this is not (entirely) true. But it's true in the eyes of a lot of people!

Of course the obvious reason people like it is because it is much like a normal car *and* it is clean. It solves the CO2 problem without needing to ride a bike everywhere. Hydrogen can just be made from water, and when it reacts in a fuel cell it becomes water again, right? Absolutely no carbon emissions. This is much the same argument that is given for nuclear power plants.

When presented with the ecologically responsible alternatives, electric cars are closest to home. But batteries are of course horrible. Batteries are no good. I mean, look at phone batteries. They die all the time.

The funny thing is that basically all of these perceived advantages are... well, they are correct! Most of them at least - except for the cleanliness argument, although even that has some merit. The big problem here, though, is that technology isn't ready and probably more importantly: the environmental and financial cost of a switch to this kind of a situation is basically insurmountable.

The contrast here is that battery electric cars can be introduced en masse today, no problem. And they are much cleaner and better for the environment, both in the short and long run. The argument against hydrogen fuel cells is not that they can't be a good alternative to fossil fuel cars or that they 'just don't work', but that there are many technological and even fundamental physical problems (i.e. the laws of nature are against us) that need to be solved to get there. And you can prove that even if you get there, batteries will be better in every single way possible. This is the central theme of this blog series and what I will be trying to prove. So where to start?

Let's start with the fuel itself. What is hydrogen?

What is hydrogen and why is it so tantalizing?

http://cdn.zmescience.com/wp-content/uploads/2013/06/hydrogen.jpg

Hydrogen, the atom, is the smallest atom you can make in our universe. It's just one proton and one electron, nothing else. When you combine two hydrogen atoms together, you get H2 - hydrogen molecules, normally a gas at room temperature and pressure. Hydrogen is very light: a cubic meter of hydrogen only weighs 90 grams. As a comparison, air at sea level weighs about 1300 grams per cubic meter. So at sea level, hydrogen is 14 times lighter than air. So, like a bubble of air in water, in nature hydrogen gas will rise up into the stratosphere if you don't keep it in a container. Yes, this is a problem but no, we're not at the part of the blog where I talk shit about hydrogen.

If you take a hydrogen molecule and combine it with one oxygen atom (from the oxygen molecule O2), and you add a little bit of activation energy, the oxygen and hydrogen combine to form a water molecule. This is called combustion. Well, actually this is a reduction-oxidation, or redox reaction. Redox reactions happen because one part of the equation, the hydrogen, kind of has excess electrons while the other part is - again kind of - missing electrons. When combining, the electrons around the atoms merge together. The fact that there is electrons involved means that if you can somehow separate those two reaction compounds (reagents) and redirect those electrons, you can get a little bit of electric current from the reaction. This is what fuel cells do - with untold gazillions of these reactions happening per second to get enough power that we can actually use it for something.

http://www.olympusmicro.com/primer/java/fuelcell/fuelcelljavafigure1.jpg


Normally, if you want to perform a chemical reaction, you have to supply all the reagents. In this case: you'd need a canister of hydrogen and a canister of oxygen. But... oxygen is already really plentiful in the air we breathe. The big advantage of hydrogen in a fuel cell is the fact that it uses oxygen from the air and thus only needs to supply the hydrogen. This is, by the way, exactly the same with fossil fuels. They also need oxygen to combust inside the engine, but you can just pull that from the air. A regular car would need a ginormous oxygen tank if the atmosphere didn't exist. And so would humans. We breathe air just like our superior mechanical masters.

About that, hydrogen is really light. For the reaction from hydrogen+oxygen->water, you need about 8 times as much weight (well, mass, but we're not doing science here) in oxygen as you need hydrogen, so it's a giant weight saving you do if you just get oxygen from the air. And because the reaction product is just plain old water, you can expel that into the environment with zero danger. But there is another part to this equation. This all sounds great, but how much energy do you actually get from hydrogen? How much do you need to carry around?

To give you some perspective: a liter of gasoline contains about 46MJ of energy. A lithium ion battery contains only about 0.7MJ per kg, or about 60 times as little. A kilogram of hydrogen? A whopping 146MJ/kg, more than three times as much as gasoline. Crazy. This is an awesome fuel. Hydrogen gives you, by far, the most energy out of any chemical energy source known to man - if you don't have to carry any oxidizer. The only 'better' energy sources use fundamentally different types of energy, e.g. nuclear fuel. This has all to do with the nuclear forces vs. electromagnetic forces, but I won't go into that here.

How do electric and fuel cell cars work?

A lot of people talking about fuel cell and even electric cars either assume that whoever is reading their arguments knows how these things work, or they themselves actually never looked at how they work. This is a big, big problem because you really need to know what is going on to understand the (dis)advantages of either drive system, as well as the routes towards optimization. I'll give a brief overview of both technologies from a systems perspective.
Electric cars
Let's start with electric cars. An electric car system looks like this:

Battery electric vehicle system overview

The most important and by far biggest component in an electric car is the battery; electric cars require much less energy to work in the same way as a gasoline powered car, but batteries store so much less energy per kg that this technical advantage is completely lost. To get what is considered adequate range, you need hundreds of pounds of battery to get there. With the energy consumption of modern electric cars hovering between 90-160Wh/km, an adequate battery is around 50kWh of usable capacity (or about the equivalent of 4.5L or 1 gallon of gasoline). If you would make the battery pack exactly 50kWh with the highest gravimetric (energy/mass) density batteries around, such a battery would weigh 200kg (450lbs), but in reality in order to extend the life of the battery pack the batteries are slightly oversized. Also, as the really high density battery chemistries are prone to fire and explosions and all that, car manufacturers like to use slightly less energy dense but overall much safer types of batteries, e.g. LiFePO4. This means that these batteries are usually in the range of 275-350kg.

Besides the battery, the biggest component(s) by weight is/are the motor(s). Current generation electric cars use still fairly heavy central motors with an actual axle to the wheels. Upcoming generations will use in-wheel or near-wheel motors that have almost no drive train associated with them: no transmission, no gearing at all, no weight lost to things that aren't necessary. This is one of the reasons electric cars can actually become lighter than traditional gasoline-powered cars; the chassis can be reduced because there are almost no driving forces on it anymore. Note though: this is an advantage of both battery and fuel cell electric cars!

The other two major components in an electric car are the motor controller and some kind of cooling system. Motor controllers, as the name implies, regulate the power going into the motors. The cooling system is necessary to keep the motor and controller, but mostly also the battery at a reasonable temperature. Batteries don't particlularly like high temperatures and as efficient as lithium chemistry batteries are, they still generate some heat when you discharge them rapidly. As a rule of thumb, the cooling system needs to remove about 1/10th of the rated power of the car in heat at 50 degrees C. For some perspective: a gasoline powered car needs to be able to remove about 2x the rated engine power at 95C. That's 20x as much energy, but at a higher temperature which means it's about 3x as easy to do. Do the math and you get a cooling system that should be about 1/6th the size of that of a regular car.

Emile's Motor Controller - or EMC for short
This is an image of a 2x125A 150V motor controller I once made for Formula Zero, but never finished the firmware for. Still an awesome piece of high performance electronics

However, altogether the weight of the motors, cooling system and motor drive are nowhere near the size of the battery. In some current production hybrids, these non-battery components are actually pretty heavy and large (Toyota Prius II: altogether about 110kg), but this is a transient phenomenon. In the future this will all go down to a couple tens of kg.

There is a question mark in the block marked 'charger/dcdc'. As it stands, most cars incorporate their battery charger into the car, sometimes combined with a dc/dc controller that regulates the voltage coming from the battery into the motors. This is because every car is different, and you cannot really design one type of charger (at least not at the moment) that can charge any car optimally. So charging stations - e.g. the ones in parking lots you see often these days - are not much more than a three-phase outlet, and the actual charging algorithm necessary to properly charge the battery is entirely inclusive to the car.

This may very well change in the (near) future with direct charging, where the charging outlets actually have the conversion built-in and all the car does is identify itself and tell the charger 'hey, can you give me 400V 200A DC?'. This saves on a lot of cost mostly, as well as a little bit of weight.
Fuel cell cars
Something a lot of people only barely realize is that fuel cell cars are just electric cars, but with a fuel cell and hydrogen tank instead of a battery. Well, almost. Fuel cell powered cars look a bit like this:

[systeemoverview FCH]

There's the motor, controller and cooling system much like an electric car. But as fuel cells (at least portable ones) are appreciably less efficient than batteries, they need to get rid of a lot more heat they produce. As a rule of thumb, a vehicle fuel cell just on its own is about 40% efficient. By the way, if you're ever interested in researching these numbers: don't believe what manufacturers tell you without checking the type of efficiency they mean. There's a big difference between theoretical energy content (what I talked about before, about 150MJ/kg), higher heating value, lower heating value and even some other ways of calculating fuel cell efficiency. In general though, you need about as much cooling as the rated power of the vehicle at 60 degrees, in other words, about as much of a cooling system as a traditional gasoline powered car. In an ideal world, if the hydrogen can be used refrigerated, the theoretical maximum efficiency of fuel cells is about 85% and the theoretical maximum efficiency at room temperature is about 70%.

Portable fuel cells are, at the moment at least, almost all Proton Exchange Membrane or PEM fuel cells. This is the lightest type of fuel cell. As far as I know, the current concept fuel cell cars all use PEM fuel cells at about 60 degrees C and close to 1 bar operating pressure. Now, here's where all that other stuff in the diagram comes in.

A kilogram of hydrogen takes up a couple cubic meters. In order to take some reasonable amount of fuel with you, you need to compress it down a *lot*. A couple hundred atmospheres of pressure is what you need to take the equivalent of a full tank of gas with you. But the fuel cell won't accept this directly; you need to reduce the pressure. And you can't just do that willy nilly. As you relieve a gas of its pressure, it cools down. If you would use a reduction valve to go directly from 200 to 1 bar, it will freeze to close to absolute zero, become brittle and shatter. This reduction is usually done in steps; first from 200 to about 25 bar, then to 5-10 bars and then to the final pressure. Other thermal and gas flow effects necessitate some additional pipework and in-between stages.

PEM fuel cells use hydrogen and oxygen either directly from the air or in solution with a proton exchange membrane in the middle. Using a catalyst, the hydrogen and oxygen bond to form water and give off an electron to conveniently placed electrodes in the fuel cell. I won't bother illustrating this here; wikipedia can help you with that. This process generates lots of heat and it's actually so much heat that it is not enough just to 'cool' the fuel cell by piping off the excess water that has been produced. You absolutely need additional cooling. This is done in one of three ways: 1) by putting a closed loop water cooling system on the hydrogen side of the fuel cell, 2) doing the same on the water side and 3) by using a separate cooling system that uses heat exchangers to interface with the 'hot' parts of the fuel cell.

All three of them have problems. The simplest one is by putting the water cooling in the hydrogen side. Why is that the simplest? By exclusion:
- If you put a closed water circuit on the oxygen side, all the water that is produced ends up in this circuit as well, increasing pressure. So you need to get rid of the excess water with a valve.
- Also, on the oxygen side you're putting a lot of oxygen in water, which is corrosive to your cooling system.
- You need to make sure the water is super clean because it is very easy to 'poison' (clog up) the fuel cell catalysts
- If you put the cooling system completely separate from the reagent flows, you need quite large heat exchangers and a modified fuel cell stack (which increases lsoses) to adequately pull out all the heat. If you don't, you will get hot spots which severely reduce fuel cell life

On the hydrogen side, you don't need pressure regulators and you don't need to worry about water purity because once you put the cooling fluid in, it stays there unaltered. Also, hydrogen is not nearly as reactive as oxygen in solution. But... you do lose a *lot* of hydrogen. Only a small part of what you put into the water actually gets to the electrodes. The fraction of hydrogen that doesn't will cycle through the entire system and then maybe get a second chance, but in the process a certain amount of hydrogen will diffuse through the walls of the cooling system into the surrounding air. Also, because the partial pressure of hydrogen is lower than if you just directly force hydrogen onto the electrode, fuel cell efficiency is lower.

I've seen fuel cells with all three methods, and ones that combine everything: all reagents are put into solution and a heat exchanger extracts heat from the water into a separate system. Very small fuel cells are simpler still; they don't need nearly as much cooling and can just rely on the evacuated water to get rid of the heat.

I'm telling you all this because this is what the 'extra stuff' in the water cooling path represents. There is a lot to it! This is not a job for a standard radiator and water pump. And this is why the hydrogen, oxygen and cooling paths in fuel cells are a prime target for optimization.