[Lewis Thomson (OP)]

Eddy has sent The Naked Scientists a question all the way from the other side of the world.

"Here in Australia there is a great debate about the pros and cons of hydrogen as a fuel source (fuel cells etc) versus electricity as a fuel source (batteries etc). I'd love to hear a detailed explanation and critique of hydrogen as a fuel source.."

What do you think? Let's discuss the impact of hydrogen as a fuel source in the comments below.

[alancalverd]

Hydrogen has 3 times the energy density of diesel fuel (roughly 150: 50 MJ/kg) and is the ideal liquid fuel in theory. It is rapidly becoming a practicable fuel in small quantities (for road vehicles and aircraft) thanks to the development of large scale applications in rocketry.

The best lithium batteries can only manage 1 MJ/kg.

Thanks to Carnot, you can only turn about half the energy of a combustible  fuel into useful work, so the ratio of available energy per unit mass in, say, a car, is closer to 75:25:1, but you have to generate the electricity to charge your lithium battery and most of that still comes from a combustible fuel.

The best solution seems to be a hydrogen fuel cell driving electric motors, with an intermediate battery providing rapid acceleration and regenerative braking.   

The engineering problem with aircraft is that hydrogen storage in the wings is inefficient as a wing has a large surface/volume ratio (= significant  heat input)  so you have to rethink the configuration  with the fuel tanks in the fuselage and flat fuel cells in the wings (for dispersal of waste heat) feeding multiple electric motor/propellors. A hydrogen-fuelled jet engine may well be feasible but doesn't seem to have hit the headlines just yet.   

Domestic and industrial fuel was "town gas" (50% hydrogen) for about 100 years, so no problem using it for static power, which accounts for more than half of our energy consumption.

[Petrochemicals]

It's pretty good but exessivley volumous. Liquid fuels of some sort are the easiest to handle. At present methane butane etc has always been a great fuel because of its cleanliness and ease of mixing but storage has always been a problem.

The engineering problem with aircraft is that hydrogen storage in the wings is inefficient as a wing has a large surface/volume ratio (= significant  heat input)  so you have to rethink the configuration  with the fuel tanks in the fuselage and flat fuel cells in the wings (for dispersal of waste heat) feeding multiple electric motor/propellors. A hydrogen-fuelled jet engine may well be feasible but doesn't seem to have hit the headlines just yet.   

Nothing about heavy duty cylinders or cryogenic refrigeration and deadweight batteries is particularly aerospace. Petrol on the other hand sounds fantastic, your plane just burns that weight away as it progresses.

[wolfekeeper]

it's energetically and financially expensive to make, financially expensive to store, it burns reasonably efficiently, so it's got that going for it, which is nice. Or you can put it through expensive hydrogen fuel cells, which are very efficient,  but expensive. Did I mention that it's expensive?

Oh yeah, and it can explode, it's got the widest explosive and flammable range of any fuel.

What's not to love.

Oh yeah, then there's blue hydrogen. You take methane, chemically hack off the carbon and turn it into carbon dioxide, a greenhouse gas, and then sell the hydrogen and release the carbon dioxide into the atmosphere. The fossil fuel industry want you to use this. It's like methane, which at the moment has quadrupled in price, but even more expensive.

[alancalverd]

It's pretty good but exessivley volumous.

Thanks to its high energy density, the fuel tank volume for hydrogen is only about twice that for diesel or JETA1., and the weight is about a third. So we'll need to change the shape of aircraft a bit but they will be lighter at takeoff and therefore have a bigger payload.

Nothing about heavy duty cylinders or cryogenic refrigeration and deadweight batteries is particularly aerospace.

Apparently NASA and ROSCOSMOS don't know that. Amateurs. 

[alancalverd]

it's energetically and financially expensive to make,

Like electric cars, hydrogen vehicles will be powered by free wind-generated electricity. The difference is that

(a) the power/weight ratio of a hydrogen vehicle is vastly superior to a battery-driven one
(b) the on-board energy store is a lot simpler and cheaper
(c) refuelling rate is of the order of 150 MW compared with 40 MW for diesel and 0.5 MW for battery-electric, so total journey times will be shorter
(d) the interim demand can be met by modifying existing internal combustion engines without scrapping them

My only reservation is the longevity of fuel cells. There are plenty in current road vehicle use so we will have good data soon. Reliability certainly hasn't been a problem in space vehicles, but they aren't particularly cost-sensitive. That said, the entire Apollo moon program cost less than one third of the new London-Birmingham railway track, so cost is obviously not a major problem.

[walnutclose]

Hydrogen has 3 times the energy density of diesel fuel (roughly 150: 50 MJ/kg) and is the ideal liquid fuel in theory.

Hydrogen has 3 times the energy per mass as diesel fuel, but if you're actually talking about density, even compressed to 10,000PSI, hydrogen has about 1 quarter the energy per volume.   So, to get comparable range to what cars and light trucks now get with gasoline or diesel, you'd need a 200 liter tank.   And that tank will have to be cylindrical with semispherical ends, or spherical over all.   Think 55 gallon oil drum sized.   No fitting this in a flat space under the trunk or seats.

It's also basically a bomb if the vehicle is involved in a serious accident.   That 200 liters of hydrogen really wants to occupy 140,000 liters of space.    Lose structural integrity of the tank, and it will first blow apart through explosive decompression of the fuel, and then, more than likely, explode into a fireball.

I'm not saying those challenges can't be solved, but saying hydrogen is better than batteries for vehicles because "energy density" is both misleading, and leaving out some real challenges.

"Here in Australia there is a great debate about the pros and cons of hydrogen as a fuel source (fuel cells etc) versus electricity as a fuel source (batteries etc). I'd love to hear a detailed explanation and critique of hydrogen as a fuel source.."

You don't want to think of hydrogen as an energy source.   There simply isn't any source of hydrogen in it's reduced form accessible to us.    Hydrogen is an energy transmission and storage medium.   It fits between the source of energy, and it's end-use.

There are basically two energy sources that can be used to generate hydrogen that matter to the discussion.   You can create hydrogen by breaking down methane, using more methane as the heat source.   But that does nothing to decarbonize our energy system, and just creates a harder to manage fuel (hydrogen) out of an easier to manage one (methane).   It's a dead end. 

Or, you can generate hydrogen through electrolysis, with the electrical energy coming from renewable sources like wind or solar, or from other carbon-free sources like nuclear.    This is what most advocates of a hydrogen economy are imagining - basically, you still build all the renewable or carbon neutral electric generation capability we're talking about for the renewable future, but instead of moving the energy around in electric fields (wires) and storing it in batteries or other reverse generation technologies, you move it around in the form of compressed hydrogen gas, and store it as compressed hydrogen gas or liquid hydrogen.   Big parts of the fossil fuel infrastructure industry will advocate for this approach.   It keeps them busy building and operating wells (hydrogen plants), pipelines, and tanks.   It keeps transportation tied to roadside refueling (gas stations, literally this time around).   It feels like v.next of things they understand.   But it has little advantage to the electric infrastructure industry.   They may get to sell lots of electricity at commodity wholesale rates to the gas/liquid fuel industry, but it doesn't electrify the economy.

It doesn't benefit consumers particularly either.   Hydrogen won't be cheaper than electricity, since for all practical purposes, it is electricity transformed into chemical energy, before being transformed into heat or kinetic energy.   It won't re-use much of the infrastructure they own, since hydrogen cannot be efficiently utilized by their natural gas plumbing.   They will lose the advantage of being decoupled from roadside refueling for most of their transportation energy needs (electric personal cars and trucks will be fueled largely in their own garages, after all).   They get to keep rapid, roadside refueling, but more than a few will notice that the cost of that is that they are riding with a bomb the size of a 55 gallon oil drum in their trunk.

Nations need to answer the hydrogen economy question from a systems point of view - is the storage and delivery technology you get with hydrogen enough better than what you can get with batteries, at all scales, to justify the additional complexity?   It could be.   One can imagine Australia generating hydrogen in the 350 days of sun a year outback, and piping it to the populated areas, upgrading their infrastructure, and making all that work.  Personally, I doubt it.  I think there are better utility-scale energy storage options that hydrogen, electricity is a better distribution method, and batteries are good enough - and easier to get to - for transportation.

(There is a third way: you could crack methane with heat from nuclear reactors, to make hydrogen.  But you'd still be freeing fossil carbon, although not necessarily into the atmosphere - it could be a solid phase carbon with enough chemical engineering magic.   But you'd have to overcome your country's nuclear jitters.)

[hamdani yusuf]

(a) the power/weight ratio of a hydrogen vehicle is vastly superior to a battery-driven one

Have you considered the weight and cost of the container?
also, structural battery is being developed, which will increase overall power/weight ratio of a battery vehicle.

(b) the on-board energy store is a lot simpler and cheaper

If you consider the necessary preventive countermeasures to the hazards, it would be a lot complicated and expensive.

refuelling rate is of the order of 150 MW compared with 40 MW for diesel and 0.5 MW for battery-electric, so total journey times will be shorter

It's possible to increase charging rate of battery electric vehicle. The technology hasn't met its theoretical limit yet.
In principle, the BEV can be charged wirelessly. So, it's possible to install wireless charger along a highway or toll road, which makes the battery increase its stored energy during the journey. That's impossible with hydrogen.

[alancalverd]

Hydrogen has 3 times the energy per mass as diesel fuel, but if you're actually talking about density, even compressed to 10,000PSI, hydrogen has about 1 quarter the energy per volume. 

Liquid hydrogen has about one tenth of the density of diesel, so you need about 3.5 times the fuel tank capacity for a given range. This requires some variation of vehicle design but is by no means as restrictive as a battery. On the other hand if folk are happy with the 200 mile range of a practical electric car, there's no significant redesign at all.

[alancalverd]

Have you considered the weight and cost of the container?

Every liquid-fuelled rocket designer has.

https://www.riversimple.com/service/#therasa  shows a hydrogen gas powered city car with 300 mile range and 5 minute refuelling.
 

[alancalverd]

It's possible to increase charging rate of battery electric vehicle.

Let's increase the charging  rate to 40 MW and assume that 1000 people in the UK want to recharge their cars at the same time. You need an entire new national grid and its generating capacity to meet the demand. Assuming, of course, that you have a radical new battery and radiator design that can dissipate 500 kW of heat, or not generate it in the first place, whilst charging.

[alancalverd]

So, it's possible to install wireless charger along a highway or toll road,

Aha! The electric railway! Brilliant idea. No need for personal ownership or driving test: just rent a seat for the trip you want to take.

[walnutclose]

Liquid hydrogen has about one tenth of the density of diesel, so you need about 3.5 times the fuel tank capacity for a given range.

Honestly, it never occurred to me that someone might imagine using liquid hydrogen for automobile fuel.  It's completely impractical.   Every liquid hydrogen storage facility must be continuously maintained at cryogenic (20^oK or thereabouts) temperatures.   That takes expensive equipment, and that equipment has to keep running all the time.   If it fails, you have to vent the hydrogen safely into the open atmosphere as it boils off, to avoid turning your garage into a bomb.   So every car or pickup truck fueled by liquid hydrogen: 1) would continuously use fuel in order to maintain it's cryogenic state; 2) would have to be parked in a location with facilities for venting hydrogen in the case of a failure.   Not practical at national scale.

[hamdani yusuf]

So, it's possible to install wireless charger along a highway or toll road,

Aha! The electric railway! Brilliant idea. No need for personal ownership or driving test: just rent a seat for the trip you want to take.

You seem to miss the wireless part. Also where your car can go in with low batt and go out fully charged.

[hamdani yusuf]

This requires some variation of vehicle design but is by no means as restrictive as a battery.

Are you referring to hydrogen intrrnal combustion engine, or hydrogen fuel cell?
How many hydrogen cars were delivered last year? How is it compared to battery cars?

[hamdani yusuf]

You need an entire new national grid and its generating capacity to meet the demand.

Not necessarily if the charging stations are equipped with their own batteries or super capacitors.

[Bored chemist]

Is hydrogen a better fuel source for the environment?
Only if we find a hydrogen mine.

[alancalverd]

You need an entire new national grid and its generating capacity to meet the demand.

Not necessarily if the charging stations are equipped with their own batteries or super capacitors.

And they are charged by what magic? A typical filling station on an A road may have 10  or more pumps, each delivering 40 MW at roughly 20% duty cycle (a minute to deliver 50 liters, 3 minutes to pay, 1 minute to bring up the next car) so you need 80 MW average and 400 MW peak input to meet normal demand.

[alancalverd]

You seem to miss the wireless part.

For a very good reason.You want to bury charging inductors all along every major road. Suppose each vehicle is consuming 30 kW and the inductors are one vehicle length apart. Then every inductor must be capable of supplying 30 kW to the receiver which is at least 15 cm away. Possibly just feasible. But not if the car is made of steel, which will look like a short-circuit. My cooker has 2 kW induction plates that heat a steel pan to cooking temperature in about 10 seconds. Not a pleasant experience for the passengers.

[alancalverd]

Is hydrogen a better fuel source for the environment?
Only if we find a hydrogen mine.

Gas companies owned several "hydrogen mines" in the past, but electrolysis of sea water is the only feasible source since Mrs Thatcher closed them. It's good enough for all the buses, council vehicles and airport ground power in Kirkwall.