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How crazy is this?: Direct air capture from buildings

So here is a cute little idea: Integrate direct air capture (DAC) into the HVAC system of a building, both to capture carbon and to improve the air quality of the building.

There are two reasons to think that this is clever. The first is that the HVAC system of a large building already contains a fan. Given that buildings HVAC systems experience about 1 Pa of pressure drop for every meter of ducting, adding a drop of 20 Pa or so by installing a sorbent in-line with the system would only negligibly restrict flow. So you more or less get your fans for free.

The second advantage that buildings have is that the CO2 level inside is always higher than ambient, at least when they are occupied, because of all of the people exhaling. An occupied conference room can easily get above 1000 ppm even when well-ventilated, and a packed, stale classroom can experience an increase in CO2 to above 3000 ppm. This means that less air must be filtered to capture an equivalent CO2 to outdoor conditions, and capture methods with imperfect selectivity for CO2 might fare better.

Of course, buildings lack the economies of scale that centralized capture facilities have. And they are usually not connected to CO2 pipelines for carbon disposal. But it seems worthwhile to check, and so inspired the recent paper "Optimizing chemisorption based direct air capture unit efficiency in HVAC systems: A study on the impact of DAC location and adsorption conditions as a response to the climate crisis and indoor air quality". The researchers looked at HVAC designs for a building in Qatar, where a cross-linked amine was used as sorbent. They found that with optimal positioning of the capture unit within the building, they could achieve a capture energy as low as 13 MJ/kg CO2.

This is actually pretty good! The canonical number from a liquid sorption process such as the one used by Carbon Engineering is in the range of 10 MJ/kg CO2, There are probably all sorts of other integration advantages that could be achieved, such as using passive solar in the regeneration process, or even recirculating the treated air back into the building, since much of the reason for exhausting conditioned air is to lower its CO2 levels. (You'd want to add a carbon filter in-line as well.) Of course the capture system is only really useful during the day, when the building is occupied. But in the summer in Qatar or Phoenix, it's not inconceivable that a combination of these improvements could make such a process economic.

All told, I would classify this idea as mostly but not entirely ridiculous. The amount of CO2 exhaled by people is too small to make a difference to the climate, and even a substantial office building would not be able to capture enough carbon dioxide to qualify for the minimum standard of 1,000 tons/yr set by the US 45Q credit.. But if carbon capture systems can be made economically enough, and small enough, thanks to work by companies such as Climeworks, it is not impossible to see such things deployed in buildings to make air handling more energy efficient. Technology sometimes benefits society in surprising ways.

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