(628h) A Rooftop Approach for Direct Air Capture of Carbon-Dioxide

The goal of max 1.5 °C of global warming by the end of the century cannot be reached without atmospheric CO₂ capture [1-3], and several approaches are being researched. Capacities for the following approaches until 2050 are (i) BECCS (0.5–5 GtCO₂/yr) (ii) afforestation and reforestation (0.5–3.6 GtCO₂/yr) (iii) DAC capture and storage – DACCS (0.5–5 GtCO₂/yr) (iv) enhanced weathering (2–4 GtCO₂/yr) (v) ocean fertilization (vi) biochar (0.5–2 GtCO₂/yr) (vii) and soil carbon sequestration (5 GtCO₂/yr) [3]. Each of these approaches has limitations. For example, reforestation, improved forest and crop management, and BECCS have land constraints, with a displacement of natural habitats and agriculture. Hence, DAC will be an essential component in the full range of Negative Emission Technologies (NETs), especially when addressing CO₂ emissions from sources that are not easy or cost effective to capture with BECCS. DAC has attributes of high value to the politics similar to that of emergency response [4]. It is inherently a cleanup approach. Such response benefits from (i) systems that are modular, scalable, and highly controllable by the society, government and industries/companies that invest (ii) CO₂ removals are verifiable (iii) deployment does not interfere with existing practices nor political/industrial interests. DAC does not have biophysical limits [5] unlike BECCS, nor does it require a large area as compared to reforestation. To capture 1 ton/yr CO₂, a unit for DAC requires a footprint of 0.4–24 m² (depending on the source of energy to provide regeneration heat), which can be compared to the forest footprint of about 900 m² to capture the same amount of CO₂. DAC can be considered an artificial and a quick method to remove CO₂ from atmosphere.

Among technologies for DAC of CO₂, wet scrubbing with aqueous alkali hydroxides [6] and vacuum temperature swing adsorption (VTSA) using supported sorbents [7] have gained attention. The wet-scrubbing technology requires a very high temperature (about 900 °C) in the regeneration of the solvent and also has challenges with the caustic recovery of alkali hydroxides [8]. The VTSA process with its aminated solids has an advantage because of lower temperatures of regeneration (80–130 °C) [9] required to regenerate the amines. The VTSA process also gives flexibility in using low-grade waste heat to regenerate the amines. In addition, the VTSA technology can be scaled from modular to large scale deployments.

In this study, we present system analysis for a DAC value chain with a modular rooftop approach. The DAC system is based upon a VTSA process interconnected to both the heat and electricity from renewable sources (mainly solar). Here, we consider DAC system to be installed on rooftops of household and commercial buildings supported by solar panels (thermal or PV) for both heat and electricity demands. The captured CO₂ is stored temporarily in small pressurized containers. The CO₂ is collected periodically from the buildings and sent to a main collecting terminal, from where it is transported for either utilization or permanent storage. This solution is similar to the smart-grid approach for renewable electricity. The advantages of such a value chain are (i) it optimizes the space and the costs for deploying DAC, since rooftops are generally freely available (ii) enables all the actors in the society to participate in CO₂ removal and thereby contribute to climate targets (iii) the energy penalty can be significantly reduced by capturing the CO₂ in the ventilation air (in addition to atmospheric air) which has higher concentration of CO₂ than in atmosphere. In this article, we present the potential of the envisioned modular rooftop DAC value chain in terms of (i) total solar energy required in top 10 countries with most CO₂ emissions (ii) total CO₂ removed in top 10 countries with highest solar energy potential. We also present the results in terms of cost per ton CO₂ captured, transported and utilized/stored.

Acknowledgements

This work is financially supported by the Swedish governmental initiative StandUp for Energy.

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