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Comment by tomsto | Hacker News Reader

This is a really interesting idea and the unit economics are great. However when I researched this (or rather Gemini 2.5 Pro researched this) the problem appears that Haber-Bosch produces ammonia, whereas Birkeland-Eyde produces nitrate, which then needs to be converted in nitric acid. Getting from nitric acid to ammonium nitrate (which dominates the fertiliser market) needs ammonia from somewhere, which destroys the economics.

However calcium nitrate, potassium nitrate production via B-E looks really interesting.

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This paper provides an in-depth analysis of the feasabilty of this idea: https://pubs.rsc.org/en/content/articlelanding/2021/ee/d0ee0...

"Due to the emergence of low cost renewable electricity from solar and wind, there is renewed interest in decentralized opportunities for electricity-driven nitrogen fixation."

"This analysis shows that the energy consumption for NOX synthesis with plasma technology is almost competitive with the commercial process with its current best value of 2.4 MJ mol N−1, which is required to decrease further to about 0.7 MJ mol N−1 in order to become fully competitive"

Note that this measure of competitivity is based on energy, not cost. So the (intermittently) ultra-low cost of electrical energy generated by modern PV installations (where substantial overprovisioning is becoming normal) has not been taken into account.

An Agri-PV installation that produces all the fertilizer it needs from its own surplus electricity would be cool indeed.

Plants are happy with nitrate as the only nitrogen source.

Ammonium nitrate is preferred because it provides more nitrogen per weight, i.e. at similar transportation costs with the alternatives, and also because introducing a too great amount of metal cations together with the nitrate, e.g. potassium or calcium, can be detrimental for the soil and can make it too alkaline after the nitrate is consumed.

> needs ammonia from somewhere, which destroys the economics.

Urine collection.

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