Take on a podcast episode from Reviewer 2 does geoengineering, originally published Fri, 15 Ma. Listen: https://podcasters.spotify.com/pod/show/reviewer2geoengineering/episodes/Viridas-Pressurised-DAC--El-Sayed-e3jct2c

TL;DR

  • Viridas pitches “pressurized DAC”: compress incoming air to ~70 bar so CO2 partial pressure jumps from ~400 ppm to ~2.8%, shrinking contactors ~70x.
  • Sorbent is a cheap aqueous ammonia + alkali carbonate mix (carbamate formation + bicarbonate buffer), regenerated thermally at ~90–110°C. Sensible chemistry, nothing exotic.
  • Core thesis: solvent R&D is hitting thermodynamic limits; the real lever is CAPEX, attacked by retrofitting gas-turbine turbomachinery as compressor/expander.
  • Claimed round-trip efficiency target: ~97–98% (vs ~70% for current compressed-air energy storage). That’s the whole ballgame and it’s not demonstrated.
  • No technoeconomic assessment shared, no pilot, no funding disclosed, Gmail contact address. Treat as concept-stage.

The host of Reviewer 2 Does Geoengineering interviews Ahmed El-Sayed, co-founder of Viridas Technologies, about a pressurized DAC concept built around retrofitted gas-turbine machinery. The episode is recorded walking through a Cambridge park, which means a non-trivial fraction of the runtime is ice cream and hawthorn commentary — the substantive technical content is maybe 35 minutes.

The pitch in one line: stop optimizing sorbents, start compressing the feed air. At 70 bar, CO2 acts (in partial-pressure terms) like a 2.8% stream rather than 400 ppm, which collapses contactor volume, residence time, and per-cycle solvent loading. Viridas pairs this with a deliberately boring solvent — aqueous ammonia plus sodium/potassium carbonate, doing carbamate capture with a bicarbonate buffer — chosen for oxygen tolerance, gigaton-scale feedstock availability, and a modest ~90–110°C regen. El-Sayed frames it by analogy to reverse osmosis displacing multi-stage flash in desalination: the membrane wasn’t initially cheaper, but recovering pressure energy across the system eventually was.

The load-bearing claim is the turbomachinery round-trip efficiency. El-Sayed concedes that compressed-air energy storage today runs ~70% round-trip, and that pressurized DAC only pencils at “97 to 98% efficiency before you can start considering whether it’s comparable.” That is a very large gap to close, and it’s where the entire CAPEX-vs-fan-energy argument lives or dies. The host’s pushback is the right one: even granting the thermodynamics, you’re now coupled to a gas-turbine OEM supply chain that AI hyperscalers have already bid into the 2030s. El-Sayed’s answer — partner with Siemens et al. rather than build turbomachinery in-house — is correct but currently aspirational. No pilot data, no TEA, patent process cited as the reason numbers aren’t public.

For context on pressure-shifted DAC and adjacent compression-leveraging approaches, this sits in a small cluster with Mission Zero (electrochemical, ambient), RepAir, and the older Carbon Engineering liquid-solvent contactor lineage now inside Oxy. The “stop chasing sorbents, attack the balance of plant” argument echoes critiques from the Realmonte et al. modeling work and earlier APS 2011 DAC assessment that El-Sayed cites. The chemistry — aqueous ammonia plus carbonate buffer — is closer to chilled-ammonia post-combustion capture (GE/Alstom lineage) than to anything Climeworks-shaped, and inherits that family’s ammonia slip and water management questions, neither of which the episode addresses.

Worth an hour only if you specifically track pressurized or compression-based DAC architectures