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Comment by jandrewrogers | Hacker News Reader

> Ammonia is a great not-on-demand consumer of electricty.

This does not follow. The cost efficiency of ammonia production is highly dependent on the process being continuous and steady state. Every analysis that says ammonia is cost effective as a fuel is based on an efficient continuous process as a cost assumption.

If you are constantly starting and stopping based on electricity availability then your ammonia just became much more expensive. In which case, it is probably no longer cost effective as a fuel. Mixing "best possible price" and "worst possible process" and pretending these represent the same instance of reality is misleading to say the least.

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> The cost efficiency of ammonia production is highly dependent on the process being continuous and steady state.

Hydrogen is the overwhelming energy input to ammonia production. Hydrogen is readily storable -- this is done even today, when the hydrogen comes from natural gas, to smooth things out to keep the ammonia plant running -- so intermittency of renewables will be almost entirely countered by doing the same thing and storing the green hydrogen.

What matters is cost of electrolysers, but they have been getting very cheap in China.

It is not just the cost of inputs.

Ammonia synthesis is a high-pressure high-temperature process. One of the reasons to use a continuous steady-state process is that cycling it up and down causes thermal and pressure fatigue in the reactor. The safe operating life of a reactor can be surprisingly short if it is not operated at a steady state. If you want this to scale, it needs to be low maintenance and have a long operating life.

You could in principle centralize ammonia production with sufficient reactant reserves to ensure continuous production from variable low-density energy sources like solar or wind. However, this would require hydrogen pipelines that largely don't exist and would take a long time to build. We can't repurpose existing natural gas infrastructure and similar because they weren't built with alloys resistant to hydrogen embrittlement. One of the big economic advantages of using methane for ammonia is that it takes advantage of the millions of kilometers of natural gas distribution pipeline that already exists.

I'm not averse to the idea but the enthusiastic proponents are pretending like the practical realities of industrial chemistry don't apply to them. We aren't going to get to a green future with rainbows and unicorns, we need to brutally realistic about the true requirements.

I wasn't talking about the cost of inputs, I was talking about their putative intermittency.

The argument that was being made seemed to be "renewables are intermittent, therefore ammonia synthesis based on renewable energy must be intermittent, or else use expensive storage". The counterargument is that hydrogen is the overwhelmingly most important input, and it is highly storable, so the intermittency of the inputs can be largely avoided at modest cost, allowing the ammonia plant to run 24/7.

You may not be aware, but we already have hydrogen pipelines coupled to ammonia plants. The US has ~1000 miles of hydrogen pipelines for this purpose. It's also not obvious to me why pipelines would necessarily be needed. After all, the ammonia plant could be built where the hydrogen is stored.

Can factories install local battery banks to cover a day’s utilization? Charge up the batteries on the cheap electricity during noon and run the plant off of those reserves. I assume other industries are already running these cost optimization analyses as the renewable electricity market continues to develop. There is a balancing point between the capex and opex, but unless it is insanely energy hungry (like aluminum), that seems possible.

> but unless it is insanely energy hungry (like aluminum)

Producing a ton of hydrogen by electrolysis requires ~3x the energy to produce a ton of aluminum. It is, in fact, "insanely energy hungry". This isn't necessarily a problem but it does create logistical challenges.

What comes back to the fact that you only need batteries for the ammonia production. Hydrogen production is a low-pressure process, and optionally even low-temperature.

It takes 11 MW of electricity to make 1 ton of ammonia, ammonia plants can make 1000 to 3000 tons a day. Providing battery storage for that production rate for 24 hours would probably cost more than the plant itself.

> It takes 11 MW of electricity to make 1 ton of ammonia

Do you mean MWh? Because the units given don't make sense.

But 11 MWh would be too low.

> You could in principle centralize ammonia production with sufficient reactant reserves to ensure continuous production from variable low-density energy sources like solar or wind.

I'm probably missing something here but why would you need to pipe hydrogen to the plant, rather than just generating it on site from power drawn from the grid?

Energy density mostly and being able to deliver that power where you need it. Aluminum plants are co-located with large-scale power plants, famously hydroelectric, for the same reason. Above certain power requirements, you essentially need the power generation to be onsite.

Hydrogen requires 3x the energy of aluminum per ton, so it is an even bigger problem for hydrogen. Unlike aluminum, it is feasible to have large numbers of small hydrogen production plants but then you need to transport all that hydrogen at an acceptable scale.

> Above certain power requirements, you essentially need the power generation to be onsite.

Is this due to transmission losses or just because you couldn't feasibly build enough capacity cables to transmit large amounts of power over long distances?

A single ton of hydrogen requires ~50 MWh of electricity. Small special-purpose ammonia plants, which are common for some industrial applications, typically require on the order of 50 tons of hydrogen per day. This would require ~2.5 GWh of electricity per day via electrolysis. To put that in context, that is in the same ballpark as the average output of the largest solar farms ever built in the US.

The largest ammonia plant in the US requires around 2,000 tons of hydrogen per day. That would require 100 GWh per day to produce by electrolysis, which would require the entire output of a large hydroelectric or nuclear power plant, much like large aluminum refineries. Otherwise, you need to move a lot of electricity or a lot of hydrogen to have good efficiency, and there is infrastructure for neither.

Converting natural gas into hydrogen is also energy intensive. One of the big advantages of natural gas is that your hydrogen source is also your energy source and there is vast infrastructure for moving natural gas around.

Building green hydrogen pipelines likely makes more sense than trying to backhaul electricity from diffuse sources. A single hydrogen pipeline can reify a lot of electricity production without the concomitant transmission and management infrastructure.

A ton of hydrogen seems to occupy a cube with a 23 meter side. Wonder if a bunch of those could be built to hold the excess gas for night time operation.

100GWh is not small, but it's not impossible. The largest solar farm in operation is 5GW, and that could get you theoretically halfway there operating 10 hours a day.

It feels like the challenges are a lot easier to solve than with fusion or nuclear.

It is doable in theory but would require the construction of large-scale supporting infrastructure that currently doesn't exist. I am not optimistic about our ability to undertake infrastructure projects of this magnitude without it taking several decades and incurring obscene cost overruns that make even the most pessimistic economic models look optimistic.

This will definitely be harder than nuclear. The expansive land use requirements means the legal battles pertaining to that would almost certainly span many decades. At least with nuclear there is a limited number of people that need to sign-off to have a viable project -- reforming that process probably would be simpler.

The issue doesn't appear to be storage, but transmission. Hydrogen can leak through metal and lead to it becoming brittle, so you can't use conventional natural gas pipelines to transport it.

Exactly, these tanks can be giant balloons right next to the solar panels all feeding the in-situ ammonia plant.

That was my thinking, but I think what he is saying is that power plants won't generate enough electricity to make building an in-situ ammonia plant economical. You need to network power plants together to operate a centralized ammonia plant 24/7, and the network to move this energy (whether in the form of hydrogen or electricity) doesn't currently exist.

The land area of an all-in-one plant is maybe the biggest unknown for me with respect to just getting ownership and permits and such. But it's fun to imagine just picking a giant plot somewhere in the desert and plopping down 20 GW of solar panels, enough hydrogen storage to keep the less energy intensive steps operating throughout the night, and presumably batteries for whatever still requires electric power while panels are offline. Water and air in, sweet sweet ammonia out. :-)

Cloudy weather would be an interesting problem I guess.

I can’t imagine it wouldn’t be stored either as liquid or at least pressurized.

Steam has something like 200x the volume of the water it’s boiled from.

One thing you can do there is have an onsite energy storage mechanism (battery, gravity, etc) and run the process 24/7, keeping the energy storage topped up whenever the cost of electricity falls below whatever threshold.

Worst comes to worse you run on grid for a few hours.

Liquid ammonia takes less energy and volume than liquifying hydrogen.

This has nothing to do with hydrogen as an input to the ammonia production process. This hydrogen is not liquefied, even if it is temporarily stored (as a compressed gas, for example underground in solution mined salt caverns.)

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