Thursday, February 9, 2017

Hydrogen versus gas in cars

For a long time people have talked about the idea of using hydrogen to fuel our transportation systems.  There are some obvious advantages to this.  When you burn hydrogen you don't release any CO2, the hydrogen combines with oxygen to form simple water.  Batteries like you'd find in an electric car also store and release energy without emitting CO2 but those don't hold as much energy as the equivalent weight of hydrogen.  Hydrogen theoretically stores 40,000 Wh/kg whereas even a good battery like the one used in a Tesla only stores  100 Wh/kg.  The theoretical values aren't the whole story since converting hydrogen to motive force is less efficient than converting charge in a battery.  And also you need tanks to store the hydrogen which I'll come back to later.  But those don't overcome the magnitude of the difference and long range hydrogen cars are feasible in a way that long range electric cars aren't.

On the other hand hydrogen has some big problems compared to batteries or other fuels.  At room temperature and pressure hydrogen only nets you 3 Wh/L compared to 9,500 Wh/L for gasoline or roughly 500 Wh/L for lithium ion batteries (I couldn't find Tesla's exact number).  To make hydrogen feasible you have to compress it a lot.  According to the ideal gas law you'd need to keep your hydrogen at 450 bars to get up to a good-for-long-range 1,250 Wh/L but Wikipedia tells me it's really 690 for some reason.  690 bars is a lot, requiring a very heavy pressure vessel if it's going to be robust to car crashes.  For a 10 L, .3 kg tank of hydrogen the back of my envelope tells me you'd need something like 100 kg of steel.  I have no idea if that's actually accurate but it suggests that hydrogen's range advantage over batteries isn't so very great.

Lets say you've got a bunch of the hydrogen you were going to put into your car.  There are other things you can do with it instead.  Something that's been used industrially for over a hundred years is the Sabatier reaction, turning hydrogen and carbon dioxide into methane.  Methane is also a gas, like hydrogen, but it's a far denser one cutting down on the pressure needed to get a tank of it into something that can fit into a car.   Some energy is lost in the conversion but only around 25%.  And that's just the 100 year old technology.  Turning hydrogen and carbon dioxide into methanol, ethanol, or conventional hydrocarbons is something that currently exists in pilot plants.  A development would be needed to make this practical to generate enough to replace all our current gasoline production but it would be far, far easier than converting all our cars, infrastructure, etc to run on hydrogen.

But of course this scheme is currently missing one of the chief advantages that both electric and hydrogen cars enjoy: the federal subsidy for zero-emission vehicles.  A vehicle running on synthetic gas might absorb a gram of CO2 from the air for every gram that comes out its tailpipe but there's still CO2 coming out of its tailpipe.  So legally it's not in the right category to benefit from existing government subsidies.  Maybe if this caught on the government might do something, who knows.  But really this sort of issue is why I'm much prefer a carbon tax or some other sort of universal and evenhanded program rather than our current mostly ad hoc system for trying to reduce carbon emissions.  Because really, nobody knows how many other situations like this there are where a fixation on details distracts us from the problem of reducing the amount of carbon dioxide in the air.

We're trying to make our SARS-2 tests better than we should

Ok, that's a somewhat provocative title but I think it's basically accurate.  During this pandemic the US in particular has had a pr...