In office, commercial and public buildings, a significant volume flow of air is steadily circulated for ventilation and cooling or heating purposes. A typical volumetric exchange rate for office buildings is 5−10 times per hour. Although the concentration of CO 2 in the air is low, the absolute amount of CO 2 in contact with the A/C device can be substantial.

As an example, one of the landmark buildings in Frankfurt am Main, Germany, its Fair Tower (see Fig. 4), offers 63,000 m2 of office space. Assuming 3 m ceiling height yields a total volume of ca. 200,000 m3. Based on the above-mentioned recommended ventilation rate for office buildings, this gives an estimated airflow of 1–2 M m3 h−1. With 400 ppm CO 2 in air33, this translates to 0.75–1.5 t CO 2 h−1 ready for capture. According to recently published data for capturing CO 2 from air, the energy demand per metric ton of captured CO 2 is about 1.43 MWh heat and 0.37 MWh electric power34. If combined with an air conditioning system, the demand for additional electric power would be substantially lower because only the additional pressure drop of the CO 2 absorber would have to be covered. Thus, mainly an additional heat input of 1.07–2.15 MW would be required if all the CO 2 in the contacted air would be recovered. The temperature level needed is 100 °C, which is compatible with solar heat, district heating or reaction heat from the following exothermic fuel synthesis step, e.g., Fischer-Tropsch synthesis.

Fig. 4 The Frankfurt Fair Tower. Copyright permission by Gerd Bezner (2018) Full size image

Pertinent are recent results by the Karlsruhe Institute of Technology (KIT), Climeworks AG, sunfire GmbH, and INERATEC GmbH from the ongoing flagship project “Power-to-X”35 which is part of the so-called “Kopernikus-Initiative” of the German Federal Ministry for Education and Research BMBF to support the Energy Transition. They report a containerized plant integrating CO 2 capture from thin air, high-temperature co-electrolysis of CO 2 together with steam to produce synthesis gas, and an ultra-compact two-stage fuel synthesis based on the low-temperature Fischer-Tropsch route and hydrocracking, would produce 344 kg of liquid hydrocarbon fuels and 24 kg of wax per metric ton of CO 2 captured. Carbon efficiency is high due to the recycling of the gaseous product fraction after the fuel synthesis back into the co-electrolysis. The recycle stream mainly contains unreacted CO 2 , CO and H 2 as well as some CH 4 and minor amounts of C 2 - to C 4 -hydrocarbons formed in the synthesis as side products. Due to the presence of some inert gas in the feed, a small part of the recycle stream must be discharged. However, even in the worst case studied, per metric ton of CO 2 captured, at most 80 kg of the recycle stream had to be discharged which corresponded to a loss of carbon of only 10.1%. Hence, the carbon efficiency of the integrated process ranges between 89.9% and close to 100%.

These numbers have been derived from the experimental performance of the individual units together with a process simulation for the integrated process within the detailed engineering of an experimental proof-of-concept plant with a design throughput of 1.25 kg h−1 of CO 2 . Moreover, an overall energy efficiency of 50–60% has been determined for the integrated process at a scale between 100 kW and 10 MW for the co-electrolysis unit. Key to the high overall energy efficiency is the utilization of the reaction heat of the Fischer-Tropsch synthesis for providing the steam for the co-electrolysis, which is enabled by the advanced reactor technology used. Based on these data, the amount of CO 2 potentially captured by the Frankfurt Fair Tower corresponds to a production rate of liquid hydrocarbon fuels of 250–500 kg h−1 or 2000–4000 metric tons per year, which needs 40−80 microstructured Fischer-Tropsch synthesis modules or 5−10 containers of INERATEC’s current design as well as the matching co-electrolysis units. Note that the same calculation assuming the whole available office area in Frankfurt am Main, Germany, which is 11.59 million square metres36, would be equipped with this technology results in a tentative potential production rate of 370,000–740,000 metric tons per year. Accordingly, for the five cities with the largest office space in Germany together one obtains 2.4–4.8 M metric tons per year.

Moreover, considering the 25,00036 grocery stores of just the three biggest players of the German food retailing industry (with an approx. average area of 1200 m2 36), by ventilation (heating and air conditioning) approx. 10,000 m3 h−1 37 and for the rooftop condensers of the cooling system for chillers and refrigerators approx. 40,000 m3 h−1 37,38, in total approx. 50,000 m3 h−1 of air circulates within each individual store. This corresponds to a CO 2 amount of 40 kg h−1 to be captured and converted into 14 kg h−1 of liquid hydrocarbon fuels, which can be dealt with in a containerized plant. Having a huge scaling effect provided by tens of thousands of stores, each equipped with a number of synthesis units and the matching co-electrolysis units within one or more compact containers, implies that altogether about 1000 metric tons of CO 2 per hour, corresponding to 350 metric tons CO 2 per year per store could be captured for processing into hydrocarbon fuels. Impressively, using on-site conversion this would allow provision of 3 M metric tons of hydrocarbon fuels per year, which is about 8% of Germany’s total consumption of diesel of 38.7 M metric tons or 30% of its total consumption of kerosene of 10 M metric tons39.

And what about miniaturized versions? Very small-scale residential CO 2 capture and conversion units, like for instance developed in the framework of the willpower-energy project funded by the EU Horizon 2020 Programme and guided by Gensoric GmbH40, could profitably be implemented in low-energy houses, which technically need ventilation to fulfil hygiene-standards (humidity and odours), because their highly insulated walls and windows don’t allow sufficient air exchange. Especially interesting is the implementation in neighbourhoods with a high share of low-energy houses like for instance the Vauban neighbourhood in Freiburg, Germany (see Fig. 5). For a 70 m2 flat (average for one flat in Vauban41) with one bathroom, one extra WC, one kitchen and a cellar room a minimum rated airflow of 140 m3 h−1 can be estimated following the national standard of DIN1946-642. With 5−6 flats per house (average in Vauban41) and 400 ppm CO 2 in ambient air33, one building could capture at most 0.5 kg of CO 2 per hour which, converted to hydrocarbons, would yield 4−5 kg fuel daily. The power required at the electrolysis unit in that scale is in the range of 4–5 kW. System integration, very compact design, low cost and fully automated operation as well as long-term stability and robustness would be imperative. An alternative to individual production units would be a crowd-owned larger production facility in the neighbourhood. All 354 buildings in Vauban41 together would for instance capture approx. 200 kg h−1 CO 2 , which converted in one to two containers of INERATEC’s current design translates to approx. 620 metric tons of hydrocarbon fuel per year. The 5500 inhabitants in the futuristic and ecological neighbourhood Vauban own 1146 cars41. Admittedly this is way less than the average in Germany, where more than 46 million cars43 are owned by a bit more than 82 million people32. But assuming a consumption of 6 L hydrocarbon fuel per 100 km, each of the cars in Vauban could drive 11,000 km per year with the 620 metric tons of fuel synthesized by the crowd’s own production facilities, which is close to the average mileage of a car in Germany (approx. 14,000 km per year44).

Fig. 5 Aerial view of Vauban neighbourhood in Freiburg, Germany. Copyright permission by Erich Meyer, Hasel|Stadt Freiburg (2012) Full size image

These three different examples illustrate the potential of A/C systems equipped with CO 2 capture to serve as a carbon source for production of a relevant amount of hydrocarbon fuels. At the same time, one can easily calculate that for substitution of the total current refinery output in Germany of 106 M metric tons39 based on air-captured CO 2 , about 455 × 1012 m3 of air would have to be contacted if all the CO 2 could be removed. Given the total surface area of Germany of ca. 358 × 109 m2 the latter translates into a height of 1300 m of air all over Germany that would have to be processed during the course of 1 year. This sounds very large, but per day it is approximately 3.5 m “only” which may appear less threatening. Yet as 100% removal is certainly unrealistic the actual values would be larger.

One may argue that biomass is a more appropriate source of carbon than thin air given the high dilution of the CO 2 in air. However, if the amount of carbon needed for 106 M metric tons of oil products had to be supplied by biomass grown within the German borders, the required volume of air per land area would not be lower. In that case, the growing plants would have to extract the CO 2 from the air and only a fraction of that CO 2 would be found in the final products.

On a side note, around 302 × 109 m3 of air per year are passed through the lungs of the whole German population for providing the oxygen to drive their metabolism (derived from 10 m3 per day and person). This is about three orders of magnitude less compared to the volume of air to be processed in order to provide the carbon to fuel their vehicles and produce the chemicals they use.

The renewable electrical energy needed to synthesize 106 M metric tons of oil products from CO 2 is huge. Assuming a conversion efficiency from electrical energy to fuel of 50%, it would be in the range of 2.800 TWh. Today, about 146 TWh of renewable energy45 is harvested per year from all the wind turbines and solar panels installed in Germany. There is certainly potential for increasing this value, but the amount of renewable electrical energy required is an enormous challenge. According to statistical data, 9.1% of all land area in Germany is covered by buildings46 and an annual average solar radiation of 1050 kWh a−1 m−2 is measured47. With a panel efficiency of 15% roughly half of the whole rooftop area would have to be covered by solar panels to generate 2800 TWh per year.

In summary, these numbers tell us that the overall consumption of oil products must be reduced to the minimum needed. Furthermore, air-captured CO 2 is a relevant carbon source, which should be considered. And finally, viable mechanisms have to be identified and implemented for quickly building up new infrastructure for decentralized conversion of renewable electrical energy into hydrocarbon fuels.