[wolfekeeper]

Sure, but the safety works one way, the natural gas molecule is bigger than hydrogen, so replacing hydrogen with natural gas is easy you just change the jets, but the other way is hard because the hydrogen molecule is tiny and so leaks extremely easily, it's odorless, and is highly explosive, and burns with an invisible flame.

The easiest way to test for a hydrogen fire is to wave a broomstick  around in front of you. If it bursts into flames, there's a hydrogen fire, try not to walk into it because spoiler: that's bad.

[alancalverd]

Going back a few posts, I note that hydrogen has been declared  by an expert consensus uneconomic and unfeasible for domestic heating and urban transport. Precisely the uses for which it was put in every city 200 years ago, and currently in Orkney, respectively.

I caught a short TV clip a couple of days ago, discussing high speed trains. It seems that one very promising bit of British technology was abandoned after a brief demonstration of feasibility. The vehicle had an aircushion suspension  over an inverted T concrete track. I think it might be time to resuscitate the idea.

Now that we have to put concrete and steel barriers along the middle of motorways, why not raise the barrier height and run a concrete  monorail along the road, with an aircushioned or maglev hydrogen-powered train floating over it? Fixed distance between termini means that you can start with just one or two refueling points, say London and Birmingham, with minimal environmental impact  and speeds of 200+ mph. Hydrogen power offers a compromise between the fuel weight of a diesel generator and the cost and engineering limitations of overhead power lines.

At 200 mph, the train becomes time-competitive with air travel between all UK cities.

[walnutclose]

Going back a few posts, I note that hydrogen has been declared  by an expert consensus uneconomic and unfeasible for domestic heating and urban transport. Precisely the uses for which it was put in every city 200 years ago, and currently in Orkney, respectively.

The right economic comparison is between making heat by burning hydrogen generated by electrolysis using carbon-free electricity, and using the electricity directly to make heat.  From a systems cost point of view, hydrogen loses in this comparison.   First, electrolysis is at best about 75% efficient in capturing the energy content of electricity.   So you need a lot more green electricity sources to make up for that loss.   Second, you need an essentially duplicative energy distribution grid to get the hydrogen to where the heat is needed.   The cost of that network raises again the cost of hydrogen relative to electricity.  Finally, hydrogen is substantially less efficient as a source of heat in a home or building.  A hydrogen fueled furnace is at best about 95% efficient in delivering the energy content of its fuel as heat; in all but the coldest climates, an electrically driven air-source heat pump can easily deliver 300% annualized efficiency (and in climates where there is any demand for summer cooling, the equipment for heating and cooling are essentially the same, so there is additional advantage there).

Over all, hydrogen from electrolysis loses the economic equation horribly as a domestic heat source.

Fixed distance between termini means that you can start with just one or two refueling points, say London and Birmingham, with minimal environmental impact  and speeds of 200+ mph. Hydrogen power offers a compromise between the fuel weight of a diesel generator and the cost and engineering limitations of overhead power lines.

I agree on the value of high speed intercity trains.   Far superior to air travel for most, and maybe all travel on the island of Britain and in most of Europe.   When you figure in the ability of trains to move people city-center to city-center, I think the advantage is there for distances well over 200 miles.   I have no opinion on air-levitated trains, although the idea makes sense.   But trains, of all forms of transportation, most readily lend themselves to battery-electricity power.   Refueling is a matter of switching discharged battery trucks off the train, and full charged ones on, or for that matter, if you're building an air levitated system, building induction recharging coils into select track runs.   (In other words, I think the need for friction-contact overhead electrical lines is soon to pass).    The mass disadvantage of batteries for transport that you've assiduously plugged here simply are not present in "rail" based transport options.

[alancalverd]

The problem with nearly all green electricity is its unreliability. Given that the energy is free, it makes sense to find some means of storing it. Hence batteries (0.8 MJ/kg) or liquid fuel (43 MJ/kg) or hydrogen (120 MJ/kg).

It is true that a battery-powered train could be overall more energy-efficient than one powered by electrolytic hydrogen, but only before it moves. To deliver 8 MW for 3 hours (i.e to run a reasonable train from London to Aberdeen at 200 mph) a battery would have to weigh over 100 tonnes - about the weight of four carriages, not including the weight of the truck itself. Accelerating the battery truck to running speed will consume 30% of the train's power, so you will probably need to add another 20 tonnes or so. And you will need two battery trucks per train, one at each end of the track. This doesn't compare well with less than a tonne of hydrogen for the same trip. The only question is whether it should be oxidised in a fuel cell or a gas turbine to maximise power/weight ratio.

The huge advantage of airplanes is the absence of any infrastructure. As they say, a mile of road will take you nowhere: a mile of runway will take you anywhere. The hydrogen airlev train requires less infrastucture than a road but overhead wires are hugely expensive and require a lot of maintenance, and a track-powered maglev is even more complicated and capital-intensive. A battery maglev seems a bit like a low-flying elephant.

[walnutclose]

The problem with nearly all green electricity is its unreliability. Given that the energy is free, it makes sense to find some means of storing it.

Yes, that is true for wind and solar.   Not for geothermal or nuclear, both of which ought to be components of a carbon-neutral energy grid.   With a well-considered mix of low-variability sources like those, and high variability sources like wind and solar combined with short and mid-term storage, you get a resilient, stable energy system.

But yes, you need storage.   Hydrogen for storage is going to be a tough sell, though.   It has the advantage of density, but the disadvantage of horrible efficiency.   You lose 20% or more in making hydrogen from electricity, another 20% or so in reconverting to electricity, and a fair bit in managing it (if you liquify it to store it, you have another 30% loss; if you merely compress it, you've got at best another 10%).     And a lot of equipment, since each stage requires completely different technology.   I predict with high confidence that no one will build substantial electrical energy storage systems in the form of electricity to hydrogen to electricity full cycle.   But I would also say that it's a question that is easily left to the market to decide.    Grids will build hydrogen storage if it is cost effective for the volumes and cycles they need.

The hydrogen airlev train requires less infrastucture than a road but overhead wires are hugely expensive and require a lot of maintenance, and a track-powered maglev is even more complicated and capital-intensive. A battery maglev seems a bit like a low-flying elephant.

Not sure what you're on about here, since I said nothing at all about maglev.   I was making the point that if you've got an air-levitated train, then you can probably power it electrically with relatively modest batteries that are recharged in-motion through induction coils in the train bed.   An air-levitated train by design solves the problem of running the mass of the train along a fixed path with tightly controled, close approximation to the bed, so inductive power transfer to the train is a straightforward engineering challenge.

Accelerating the battery truck to running speed will consume 30% of the train's power

I think you're overly pessimistic here.   Much of the energy used to bring the train up to speed goes into the kinetic energy of the train itself, and that can be efficiently recaptured back into the battery through regenerative braking.

Overall, levitated high-speed trains are almost the perfect platform for pure electric operation: tightly controlled, predictable paths that enable efficient energy use, coupling and recapture.

[alancalverd]

Geothermal has very limited applicability. High temperature GT systems work in geologically active areas like Greenland but low temperature systems as in Marchwood, UK, have very low power output because they rely on thermal diffusion from the core, not frictional generation from tectonic plates.

Nuclear had its day when oil was cheap - it takes about 5 - 10 years to extract more energy from a nuke than you used to build it, so it made strategic sense  as a bulwark against inevitable oil price rises, but civil nukes are probably  uneconomic nowadays.

If you are going to recharge the airlev train by induction coils along the track, you might as well turn the track into a linear motor which gives you maglev and forward motion with no moving parts on the train. Huge infrastructure cost but very fast.

Adding 100 tonnes of batteries to the weight of the train means you need  bigger motors to drive it, adding more weight, hence more batteries....it's the rocket problem. It is solvable, but rocketeers long ago decided that liquid fuel was the only practical solution. Kinetic energy recovery is significant at low speeds, so it enhances the performance of a 70 mph stop-start car, but at 200 mph or more continuous travel you are more concerned with aerodynamic drag - airplane speed increases with the square root of power - so there's not a lot to be gained by regenerative braking at the end of a 300 mile journey.

Where the resource is free, we tend not to worry about efficiency as long as the waste is nonpolluting. We throw away 90% of wheat plants to eat a tiny bit of sunshine. The energy loss in electrolysis can be used as domestic or industrial heat, and we reduce helium loss problem in MRI machines with a recirculating pump - hydrogen is a lot easier to reliquefy. The gas distribution grid is well established in the UK at least, and can be upgraded with 18th century technology.

[hamdani yusuf]

Kinetic energy recovery is significant at low speeds, so it enhances the performance of a 70 mph stop-start car, but at 200 mph or more continuous travel you are more concerned with aerodynamic drag - airplane speed increases with the square root of power - so there's not a lot to be gained by regenerative braking at the end of a 300 mile journey.

How do you get to that conclusion? Additional mass from battery doesn't significantly add air friction.

[alancalverd]

Drag force = ½ρC_DAV² where ρ is air density, C_D is the drag coefficient, A the effective area of the moving body and V its speed. Thus terminal speed ≈ k√(power) at subsonic speeds. Nothing to do with the mass of the battery, just the size and shape of the train.

Unlike cars, planes and trains can accelerate and decelerate fairly slowly  but spend most of the journey in a constant-speed cruise at about 75% of full power, so the kinetic energy available for regeneration is a small fraction of the energy expended in cruise. Regen braking is used in all modern trains because it's more efficient and reliable than friction brakes but it doesn't have much impact on the energy required for a long trip.

[wolfekeeper]

The problem with nearly all green electricity is its unreliability. Given that the energy is free, it makes sense to find some means of storing it.

Yes, that is true for wind and solar.   Not for geothermal or nuclear, both of which ought to be components of a carbon-neutral energy grid.   With a well-considered mix of low-variability sources like those, and high variability sources like wind and solar combined with short and mid-term storage, you get a resilient, stable energy system.

Nuclear and geothermal both only really work if propped up by renewables (especially hydroelectricity). If you haven't got a lot of hydroelectricity, you can never really have very high levels of nuclear power. It's not mostly a technical issue, it's primarily economic. (Although there are reasonably severe technical constraints on ramp times and power cycling in nuclear reactors).

Even if you have a lot of (non pumped) hydroelectricity, the key issue is seasonal variability of demand.

That's why France isn't 100% nuclear power. It only has nuclear baseload, and France is pretty much the global geographical best case for nuclear power. The UK SUCKS for nuclear because it has almost no non pumped hydroelectricity.

So the 'reliability' of nuclear doesn't do you a lot of good in the end. You still need backup from something else. Wind and solar just need pumped hydroelectric storage. It's quite a lot of storage, but it is as nothing compared to the amount of storage nuclear power would need.

[hamdani yusuf]

Drag force = ½ρCDAV2 where ρ is air density, CD is the drag coefficient, A the effective area of the moving body and V its speed. Thus terminal speed ≈ k√(power) at subsonic speeds. Nothing to do with the mass of the battery, just the size and shape of the train.

Then why did you say this?

It is true that a battery-powered train could be overall more energy-efficient than one powered by electrolytic hydrogen, but only before it moves. To deliver 8 MW for 3 hours (i.e to run a reasonable train from London to Aberdeen at 200 mph) a battery would have to weigh over 100 tonnes - about the weight of four carriages, not including the weight of the truck itself. Accelerating the battery truck to running speed will consume 30% of the train's power, so you will probably need to add another 20 tonnes or so. And you will need two battery trucks per train, one at each end of the track. This doesn't compare well with less than a tonne of hydrogen for the same trip. The only question is whether it should be oxidised in a fuel cell or a gas turbine to maximise power/weight ratio.

Adding 100 tonnes of batteries to the weight of the train means you need  bigger motors to drive it, adding more weight, hence more batteries....it's the rocket problem. It is solvable, but rocketeers long ago decided that liquid fuel was the only practical solution. Kinetic energy recovery is significant at low speeds, so it enhances the performance of a 70 mph stop-start car, but at 200 mph or more continuous travel you are more concerned with aerodynamic drag - airplane speed increases with the square root of power - so there's not a lot to be gained by regenerative braking at the end of a 300 mile journey.

Tesla Semi prototype has been demonstrated to work well. Several Semis connected head to tail would resemble a train. At least it can be used as proof of concept.

[alancalverd]

Then why did you say this?

I didn't say that battery mass affected air friction! I pointed out that, like a plane and unlike a car, most of the energy expended by a high speed train is lost in friction and drag during the long cruise phase and is therefore unrecoverable. A commuter train  is different: like a car, it starts and stops frequently and rarely exceeds 50 mph, so regeneration can recover a significant amount of kinetic energy that would otherwise be lost by friction braking. Regen is indeed used for high speed trains but primarily as a means of slowing the vehicle, not for overall energy efficiency.

Having estimated the battery mass required to move a 4- or 8-car train 300 miles at 200 mph, we can calculate the total mass of the train and the fraction of that mass attributable to the battery. This then determines the power required to accelerate the whole assembly to its cruising speed in a reasonable time (say 0.1g acceleration), which determines the size of the motors and cooling system. For comparison, a Shinkasen car weighs about 4.5 tonnes and delivers about 1 MW. A 100 tonne 8 MW battery is a burden, not an asset!

Tesla Semi prototype has been demonstrated to work well.

Hardly a surprise - people have been using battery-powered goods vehicles for at least 100 years. But there's a huge difference in speed and payload ratio between road and passenger rail transport. The high-speed train is competing with airplanes, not trucks.

[alancalverd]

So the 'reliability' of nuclear doesn't do you a lot of good in the end. You still need backup from something else. Wind and solar just need pumped hydroelectric storage. It's quite a lot of storage, but it is as nothing compared to the amount of storage nuclear power would need.

UK baseload is currently 20 GW and will probably double with the advent of electric road transport. At present we only have about 5 GW of nuclear availability, so it would make sense, at least in power terms, to build a lot more nukes. Indeed if we could settle on a single, known effective design, it might even make economic sense.

Daily demand fluctuation is another 0 - 20 GW on top of baseload. To meet this with renewables., assuming an all-nuclear baseload, we would have to install another 200 GW of generating capacity and 2,400 GWh of storage. That's a very big battery, but an entirely feasible hydrogen grid. Multiply by 3 if you don't build the nukes.

[Bored chemist]

If only we could take the battery off the train and connect it to the engine via the rails or wires or something...

[hamdani yusuf]

I pointed out that, like a plane and unlike a car, most of the energy expended by a high speed train is lost in friction and drag during the long cruise phase and is therefore unrecoverable.

Why cars are different? Which one has the bigger drag coefficient? SUV or bullet train?

[hamdani yusuf]

If only we could take the battery off the train and connect it to the engine via the rails or wires or something...

Of course we could. I took the train almost regularly a few years ago.

[hamdani yusuf]

A 100 tonne 8 MW battery is a burden, not an asset!

Hydrogen system is a bigger burden overall.

[alancalverd]

If only we could take the battery off the train and connect it to the engine via the rails or wires or something...

Which is why the capital and maintenance costs of high speed trains require public subsidy. HS2 costs £1,000,000 per meter to build and god knows how much to maintain the track and wiring.

Why cars are different? Which one has the bigger drag coefficient? SUV or bullet train?

Cars start and stop frequently and don't exceed 70 mph. HS trains run for hours at a constant 200 mph. Drag depends on the square of speed, so even if a car and a train were the same size and shape, the train would dissipate 8 times as much unrecoverable power when cruising.

Hydrogen system is a bigger burden overall.

My London-Aberdeen example would burn 1 tonne of hydrogen. Fuel cell delivers about 2 MW/tonne, so add one power car to a Shinkasen 8-car set and get rid of the overhead wires and all that outdoor maintenance in the middle of nowhere.

[Bored chemist]

Which is why the capital and maintenance costs of high speed trains require public subsidy. HS2 costs £1,000,000 per meter to build .

I doubt that the wires are a big part of that.
I think the cost is pretty clearly not related to the actual physical track, but is essentially politics.

[Petrochemicals]

If only we could take the battery off the train and connect it to the engine via the rails or wires or something...

Which is why the capital and maintenance costs of high speed trains require public subsidy. HS2 costs £1,000,000 per meter to build and god knows how much to maintain the track and wiring.

It does not cost £1m a metre, it is totaling £1m a metre due to many thechnical kickbacks in the same guise as the ppe covid contracts, exorbitant property purchaces and compensations and technical consultations that are exessive, overcharged and needless.

Cars start and stop frequently and don't exceed 70 mph. HS trains run for hours at a constant 200 mph. Drag depends on the square of speed, so even if a car and a train were the same size and shape, the train would dissipate 8 times as much unrecoverable power when cruising.

Cars will soon be computer driven meaning better flow and safety at higher speed. You will soom be able to get in a car and go to sleep, whilst the car does a steady 150mph to Scotland, rendering it faster than hs3 as that has a scheduled completion date to Glasgow of 2365ad.

[alancalverd]

However it happens, that is the cost to the UK taxpayer, even before subsidising the purchase and running of the trains.

Clearly the simpler the track system, the fewer opportunities there would be for corruption in the contracting, hence a concrete inverted T track with airlev may be the only acceptable technology, followed by a maglev track with aluminum panels. Using the central reservation of a motorway eliminates the corruption of land purchase and access wayleaves. Nobody cares about the disruption of motorway traffic nowadays.