Comment by tomas789 | Hacker News Reader

Comment by tomas789 | Hacker News Reader

Solving hydrogen storage has great implications. Hydrogen has high gravimetric energy density (1 kWh worth of H2 is light) but very low volumetric density (1 kWh takes up a large volume). Easy solutions are compressed hydrogen (H2 is a small molecule and easily escapes even through a material, steel embrittlement and energy for compression are big issues), cryogenic cooling (energy intensive and cryocoolers have significant capex) and metal hydrids. To put it simply the crystalic structure of the metal acts as a high pressure storage tank. H2 is typically released by heating the material up.

One thing they didn’t mention is energy used for round trip per unit of H2 and cost of the material itself. Neither was discussed there. Many similar materials exist already.

Many CO2 neutral pathways with other molecules exist. Amonia, methanol, ethanol, DME, urea, formic accid and so on.

PS: If you’re into this stuff, keep an eye on the Fraunhoffer institude. They have many cool projects. (I am not affiliated with them in any way)

> One thing they didn’t mention is energy used for round trip per unit of H2 and cost of the material itself.

I also did not see anything about recycling the carrier materials which seems suspiciously odd in a context so closely related to environmentalism.

But the big picture news is that this is another datapoint that shows how, like you said, the solution space for what I like to call "bound hydrogen" is apparently not exhausted at all yet. In recent months I have seen (all on hn) power-to-ammonia, that Dutch group that proposes iron powder as a heat fuel of roughly coal performance that is recycled by reducing the returned iron oxide powder with hydrogen (and started to fuel a brewery as a pilot) and now this "powerpaste" which sound like straight out of the back to the future future. The claimed energy density is absolutely amazing! If the required infrastructure hardware is small enough it could be a gamechanger for BEV: size the battery to be sufficient for 80% of driving days and install the "1000 miles extra" block where it can be accessed for replacing, e.g. where an ICE car would have it's engine. Chances are the extra miles fuel won't be cheap, but you'll surely be able to buy a lot of RX refills for the battery cost saved if a strong majority majority of your driving can be done on battery only.

I always considered the term "hydrogen economy" silly because of how annoying H2 is to deal with at scale, but "bound hydrogen" can change that in so many ways. It's almost as if hindsight was trying to win a bet or something wrt how we look back at George W. Bush, first him appearing so unexpectedly presidential compared to a certain successor, then suddenly hydrogen economy ceases to be a joke. What's next, discovery of actual WMD so they could have well stayed honest had they just looked a little harder? Harris becoming president after "pretzel incident 2"? Bound hydrogen is the most exciting technology field since many years.

Bound hydrogen is what is in gasoline and any other hydrocarbons. So it is nothing new, but a method for storing energy already used for a few billions of years, since free dioxygen appeared on Earth.

The only chemical substances that can approach hydrocarbons in energy density must be composed of light elements, preferably able to lose many electrons by oxidation.

So aluminum hydride would be better from this point of view than the magnesium hydride and ammonia is a very good solution if it is desired to avoid making hydrocarbons from carbon dioxide.

Nevertheless, nothing practically usable beats hydrocarbons in energy density. However it is unknown yet whether another form of storage, e.g. ammonia or another hydride a.k.a. "bound hydrogen", would not be better for the efficiency of a complete cycle of storing the energy by chemical synthesis, then recovering it using a fuel cell or a thermal engine.

> "The claimed energy density is absolutely amazing!"

Specific energy of magnesium hydride is 7.7% of H2. Energy density by volume is 13.3 MJ/L, about 40% that of gasoline. The fuel cell should be more efficient than an internal combustion engine, however, so you probably make back the difference.

Well, I don't think hydrogen is the answer for all these reasons. It seems, that sodium (Na) can under certain conditions (very thin film, e.g. when applied to a surface with great speed of 10s of m/s) react directly with water and produce stable electric current. That is not so hard to do, e.g. hard disks, regular turbines etc. pretty much would provide such conditions :-) The patents for such use in Lockheed-Martin rockets expired long ago but it seems based on the details, that it worked reasonably well.

Sodium is very easy to handle. There is no need for large pressure or any bonding elements. The hydroxide can be turned into sodium again by well known processes that were used on industrial scale more than 100 years ago already. This process and the "burn" of sodium with water both release hydrogen as a by-product. This could solve the winter energy storage issue. Also you could quite likely safely transport sodium in former oil tankers or by pipe if heated to a bit more than 100 °C/ or in the form of concentrated hydroxide (the "waste")... so the infrastructure is mostly there. As you all well know, original sodium can be extracted from regular NaCl salt that we have plenty of in sea water and salt mines.

Best of all, sodium reacts so rapidly, it could under minor adjustments replace diesel in +- regular engines too. (That is also the contents of one of the patents.) That is of course not so efficient, but there is a large installed base.

> Sodium is very easy to handle.

It only blows up when it touches water.

From an engineering point of view, that might still count as easy. Especially compared to hydrogen.

You can carry gasoline in an open pail, spill it all over yourself, and even throw a lit cigarette right in the pail, all without harm, all while having very high energy density.

Sodium is not remotely in that class.

Doesn't most of what you said also apply to magnesium? Seems like we'll never run out (2.5% of earth's crust), there are well established methods to refine it and you just react the hydride with water to generate hydrogen.

Hehe never run out. Like we never ran out of IPv4 addresses or crude oil... I think any solution needs to be cyclic.

Looks like there is ~10^−8 earth masses of oil and coal [1]. I can't seem to find the mass of magneisum on earth but its 2.5% of the earth's crust, which is apparently <1% of the earth's volume, lets estimate 0.1% of the total mass to be conservative, not sure how reasonable this is but seems ok for this back of the envelope calculation [2]. This gives about ~10^-5 earth masses of magnesium, still orders of magnitude more than the oil and coal and this process won't consume it, its just for hydrogen storage/transport.

More numbers on the mass of the crust in [3].

[1] https://osf.io/dkmwy/download [2] https://en.wikipedia.org/wiki/Earth%27s_crust [3] ttps://ui.adsabs.harvard.edu/abs/2007AGUFM.V33A1161P/abstract

If we were to use sodium on such massive scale, I'm wondering what we would do with all the chlorine.

This magnesium hydride technology certainly does not solve the hydrogen storage problem in the sense of being better than using hydrocarbons for energy storage, as the living beings have been doing for billions of years.

For many applications, using magnesium hydride should be much better than using compressed or liquefied hydrogen, but it remains far worse than gasoline despite the misleading statement from the article "POWERPASTE offers a range comparable to – or even greater than – gasoline".

This statement is false. Magnesium has twice the atomic weight of carbon and the hydrogen from magnesium dihydride provides only 2 electrons per magnesium atom, instead of 6 electrons per carbon atom, as in gasoline.

Because of that, a fuel cell using hydrocarbons (those exist, but the current prototypes do not have an acceptable lifetime) would have a 6-times higher energy capacity per fuel weight.

While the energy per weight of magnesium dihydride is very poor, the energy per volume is more decent, because gasoline has low density.

Nevertheless MgH2 has only twice the density of gasoline, which means that the hydrogen content per volume is about the same as for gasoline. Because when used in a fuel cell gasoline would provide 3 times more current per volume, magnesium hydride remains uncompetitive regardless how the storage cost is computed.

Pure dihydrogen would provide a voltage around 20% higher than hydrocarbons in a fuel cell. That is much too little to make a difference compared to factors of 6 and 3 in current per weight and current per volume.

Instead of wasting time and money to search for impossible ways of storing hydrogen, the energy research should be directed to improving the (already existing) technologies for making hydrocarbons from carbon dioxide and for making hydrocarbon-using fuel cells with better characteristics.

Are you seriously suggesting we should waste our renewables and nuclear to capture CO2 from the atmosphere and then turn that stuff back into hydrocarbon fuels only to turn 80% of it to waste heat?

Seriously, man. There is no future where this is going to happen. We are not going to replace waste primary energy generated by fossil fuels by wasting our low carbon energy. We are going to reduce the amount of waste energy and thus lower the overall demand for primary energy. The energy density of gasoline is worthless if the efficiency is garbage.

A transition from ICEs to EVs works just fine without expanding power generation significantly precisely because we get rid of the inefficiency.

It wouldn’t be a permanent solution but there are trillions of dollars worth of infrastructure built around petroleum fuels. Moving personal vehicles and metro transit to EVs is straightforward, but it’s less so for freight/marine/aircraft and doesn’t account at all for industrial uses of petroleum fuels.

You are right that the main disadvantage of storing energy in hydrocarbons is the low efficiency of the complete cycle, which is indeed around 20% today.

It is likely that the efficiency can be improved a lot but it is improbable that the efficiency of the complete cycle could reach much above 50% any time soon.

So you are right, for the best efficiency rechargeable batteries are the best.

Nevertheless, there are many cases when a maximum autonomy time is more important than the efficiency, together with the possibility of storing the energy for an indefinite time without any losses (e.g. due to self-discharge). In those cases hydrocarbons are optimal.

So both technologies are necessary and each has uses for which it is the best.

For example, a healthy human can live about one month without eating, due to the stored hydrocarbons, i.e. fat.

No future robot using lithium batteries will be able to do that, while performing a similar activity level and having the size of a human.

> It is likely that the efficiency can be improved a lot

What makes you think that? I thought internal combustion engine efficiency is widely believed to have hit its reasonable limits?

Internal combustion, or any form of combustion, is only one way to get energy back out of a hydrocarbon.

And a hydrocarbon cycle doesn't necessarily have to be all that efficient to be practical, if the primary energy input is renewable, and the cycle is largely closed (IE, a plant somewhere consumes the same stuff the vehicle produces). The portability, safety, ease of handling, and rapid simple refill of hydrocarbons outweighs a lot of other factors at the point of use for some jobs, mainly vehicles.

Most other energy-consuming jobs can use a wide variety of other forms of energy, so this is mostly about vehicles, not energy usage in general. This is why your furnace doesn't burn gasoline.

Anyway what they're saying is that entire cycle can probably be improved a lot from what we can manage today.

We can surely continue discovering new and better ways to make a fuel cell that consumes a hydrocarbon.

And we are definitely still discovering a variety of different ways to synthesize hydrocarbons. Some have a lot of overhead like planting corn and eventually getting a small quantity of a low-density hydrocarbon (alcohol).

Some are more direct bulk inustrial processes that are fairly cyclic and lower overhead than farming.

We already have a variety of examples of both the consumer and generator parts of the cycle and we are definitely not done discovering all the possibilities.

TIL you can put gas in a fuel cell. Why don't we today? That seems like a handy way to run a hybrid

You don't need hydrogen, you need electrons. I prefer to store my electrons directly in atomic element 3, so I can fill it directly with electrons and get them back out, at much higher efficiencies than this process.

In all energy storage tech, you're trading off energy density (eg. energy per kilogram) against power rating (energy per second that you can transfer). Then you have trade-off considerations like safety, emissions (which have a value - see Tesla P/L), production cost, supply chain etc. For different use cases, different trade-offs may work out.

Lithium battery tech typically trades off power rating and energydensity (Earlier - how much acceleration and range, nowadays with better batteries - how fast you can fill up) for other benefits.

This tech prioritises safety and power rating at some production cost and supply chain.

Gasoline gives emissions and engine complexity vs other conveniences

Rockets prioritize both power and energy density over everything else and uses LH2 and LOX dealing with cryo complexity

Ion engines are another interesting technology. They are almost the opposite of rockets: very low power and very low force, but great energy density.

Keep in mind that both rockets and ion engines actually have fairly low realized energy density, because they can't just rely on atmospheric oxygen.

Unless you're thinking of a hybrid supercapacitor, you're still going to need a chemical reaction first.

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