Comment by ncmncm | Hacker News Reader

Comment by ncmncm | Hacker News Reader

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Hydrogen can be produced electrically from water. A positively stupendous amount of electrolysis equipment will need to be built out, but there will be demand for as much as can be produced. Burning hydrogen produces mainly water. Burned in an engine, it also produces nitrous oxides, so probably some ammonia would be dissolved in to scavenge that.

Liquid hydrogen takes up a lot more room than kerosene. The insulated liquid-hydrogen tank is quite a lot bigger than the kerosene tank -- maybe it is a whole other car -- but so what? Room on a train is cheap. What matters is weight. The tank is light, and the weight of LH2 needed to go the same distance is much less. The difference is weight that doesn't itself need to be transported.

Furthermore, the LH2 can be produced and banked on-site at the railyard, and during peak-production hours, so cheaply.

So, a win all around. Not as big a win as LH2-powered aircraft, but better, and much easier to retrofit.

> Hydrogen can be produced electrically from water.

Which adds three energy transformation steps (Electricity for H2O to H2 + compression, H2 + 02 to water and heat, water and heat to mechanical movement) as opposed to electricity to motor.

Hydrogen is a meme.

Solar panels on the train car roofs would utterly fail to propel the train, even at noon. So, the energy has to be concentrated somehow. Batteries are heavy, and also involve a pair of chemical transformations. Hydrogen is the single most energy-dense (per unit mass) medium.

But hydrogen is not best compressed, but liquified. There might be some value in compressing it on the way to liquifaction. Liquifying hydrogen consumes some energy too.

Just because the tank is light, doesn't mean that a completed fuel carriage will be much lighter than any other car.

A tanker that holds 80,000 pounds of kerosine will hold about 8000 pounds of LH2 in the same dimensionality. 0.07 = density of LH2, Density of water 1.0, kerosine 0.7 +/- varies with grade. So it take 10 times the cubeage plus Dewar grade insulation. At a permeant depot they will have a super refrigerator to condense boil off into LH2. They are trying to get a viable clathrate hydrate to hold a higher density form of bound but not reacted Hydrogen

The measure of fuel you need to carry is not weight, but energy content. It takes a much smaller weight of LH2 to match the energy content of diesel. So, correspondingly less than 10x volume. It is this better "gravimetric energy density" that makes LH2 an attractive fuel in many cases. You end up needing only about 2-3x tankage space, including the insulation.

Better gravimetric energy density doesn't make so much difference for a train, so its other advantages -- production on-site, zero emissions, carbon-neutral -- are more important.

At the trainyard, the LH2 would be banked in an underground tank where insulation is easy. It is not necessary to refrigerate it after it is liquified. It will gradually boil off, so you just make more anytime the level goes down, whether from boil-off or pumping out to a locomotive tank.

There is truth in that energy density. but it is still cheaper energetically to chill boil off to liquid back to the tank that to make new hot hydrogen you then chill to liquid back to the tank. The end game of clathrate storage will solve many of these problems.

True, keeping the LH2 below its boiling point is, energetically, strictly cheaper than producing as much new hydrogen as would have boiled off, and then chilling that... But, it incurs the added complexity cost of building refrigeration plumbing into the tank, and maintaining that. That could mean the tank has to be above ground, for easy access (which might be needed anyway).

My expectation was that native electrical energy will be cheap enough that wasting some is better than complicating your system. Each tradeoff will be an engineering analysis. In this case it will all depend on how good your insulation is, which is of course better in an underground tank. Many old conventions will be overturned as the marginal cost of electrical power at peak times gets cheaper.

Both will be tried, so we will see.

Yes, flow batteries, gravity batteries and high water storage will all make their presence felt. Those ~~20-50 tonne bricks they haul and drop would be costly, but I expect them to make them as strong walled brick shapes, with a lower cost fill - be wise to add salt to limit freezing in cold weather.

The bricks, anyway, will not happen. It's the dumbest idea I have run across.

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