Alkaline vs. SOEC: What Topsoe’s Green Ammonia Cost

 

Alkaline vs. SOEC: What Topsoe’s Green Ammonia Cost Model Actually Shows

Anyone trying to evaluate a green ammonia project proposal eventually runs into the same question: which electrolyzer technology, and which plant configuration, actually gets you closest to a competitive cost? A 2022 paper from Haldor Topsoe A/S, by Yawar Abbas Naqvi and Joachim Harteg Jacobsen, lays out a specific techno-economic answer, and it’s worth understanding the reasoning behind it rather than just the headline conclusion.

Why Ammonia as an Energy Carrier

Start with why ammonia is even in this conversation as an energy carrier rather than just a fertilizer feedstock. It contains no carbon, so it emits no CO2 when burned as a fuel. Its volumetric energy density beats hydrogen’s, and critically, it liquefies at –33°C — mild compared to the –162°C needed for natural gas or the –253°C needed for hydrogen itself. That means ammonia can move through existing tanker infrastructure without the expensive cryogenic insulation hydrogen transport requires. The catch is cost: green ammonia isn’t price-competitive with conventional grey ammonia yet, because of electricity cost and high upfront capital expenditure — though the paper expects that gap to narrow as electrolyzer and renewable costs keep falling.

Designing for Renewable Power Variability

There’s also a design problem specific to running an ammonia plant on renewable power: conventional plants are built around a steady pipeline gas feed, not the frequent starts, stops, and wide swings in output that wind and solar actually produce. Topsoe’s answer is a “dynamic” plant design that can run anywhere from 10% to 100% load without needing hydrogen storage as a buffer — it holds the ammonia-synthesis loop at constant pressure via gas recirculation and uses fast-reacting controls to absorb the swings. This isn’t just theoretical; it’s the basis for the REDDAP demonstration project (with Skovgard Invest and Vestas, funded by Danish EUDP) pairing 12 MW of wind and 50 MW of solar with a 10 MW electrolyzer and a 24 MTPD synthesis loop, targeted for Q2 2023 startup.

Comparing Electrolyzer Scenarios

Now the part that actually answers the electrolyzer question. The paper models three scenarios: oversized solar PV with hydrogen storage; a wind/solar hybrid without storage using an alkaline electrolyzer; and the same hybrid setup but swapping in a Solid Oxide Electrolysis Cell (SOEC) instead of alkaline. The two alkaline-based scenarios land at 9.7 MWh of total energy consumption per metric ton of ammonia produced. The SOEC scenario comes in at 7.7 MWh/MT — a meaningful improvement, and worth comparing against the 7.89-8.4 MWh/MT that today’s best natural-gas-based plants achieve. In other words, on pure energy efficiency, SOEC-based green ammonia is already competitive with conventional gas-based production, even though electricity cost still makes it more expensive on a total-cost basis today.

Why SOEC Wins on Cost

Why does SOEC win here specifically? Because electricity cost dominates the overall production cost equation, and SOEC uses 25% less electricity than alkaline for the same output — even though the two technologies land at roughly comparable investment cost. That’s the paper’s core recommendation: at scale, SOEC is the better bet, not because it’s cheaper to build, but because it’s cheaper to run. The paper even gives specific thresholds for when a large plant (4,000 MTPD or bigger) becomes price-competitive with conventional ammonia: electricity below 15 EUR/MWh with electrolyzer costs under 300 $/kW for alkaline, or below 20 EUR/MWh with costs under 350 $/kW for SOEC.

Ammonia as Hydrogen-Trade Vector

The bigger picture the paper draws is that ammonia isn’t just a decarbonization play for fertilizer — it’s a hydrogen-trade vector too, via cracking ammonia back into hydrogen and nitrogen at the destination, cited by the International Energy Agency as potentially the cheapest way for import-dependent markets like Japan and Korea to bring in hydrogen at scale.

 

SOURCE & ATTRIBUTION Source: Y. A. Naqvi, J. H. Jacobsen, “Production and Conversion of Green Ammonia for Current and Future Applications by Topsoe,” Haldor Topsoe A/S, Nitrogen + Syngas 2022 International Conference & Exhibition, Berlin, 28-30 March 2022.