Though controversial and in their early stages, negative emission technologies (NETs) such as carbon dioxide removal (CDR) have won recognition as potential responses to the Paris Agreement objectives to limit future global warming to well below 2 °C and preferably 1.5 °C. Direct air carbon capture and storage (DACCS) has become a focus of attention, but ongoing research lacks a composite overview of its potential, methods, and applications. Despite the growing interest in NETs, no comprehensive studies have been conducted in the southern hemisphere to provide a comparison of the available data on the cost and functionality of the two major DACCS variants. Using Australian energy costs as a basis for analysis, the research address the gap in this paper, which advances a detailed exposition of the two major DACCS variants and compares their approach, location, infrastructure requirements, capture, and capital costs, with a focus on the Australian context.

2. The Australian Context for DACCS

2.1. Policy Settings

Given the expanding nexus between commercial and policy initiatives, some countries are starting to integrate CCS technology as part of their NDCs. For example, the United Kingdom aims to sequester at least 5 Mt of CO₂ per year using engineered GHG gas removal (GGR) technologies, including DACCS, BECCS, biochar, and enhanced weathering. However, the cost of CCS technologies and the lack of infrastructure to transport and store CO₂ have hindered large-scale commercial deployment. In addition to delivering negative emissions, the United Kingdom’s policy efforts supporting GGR technologies currently focus on research and innovation, the designation of CCS clusters, and a GBP 1 billion carbon capture and storage infrastructure fund [47]^[1]. Similarly, Australia in October 2021 updated its NDC to commit to net-zero by 2050 through seven low-emission technologies within a long-term reduction plan and a Technology Investment Roadmap [48]^[2]. As part of this ensemble, CCS attracts a proposed investment of over AUD 300 million. Australia has a competitive advantage in implementation due to its sizeable geological storage basins and proximity to mining and other industries producing highly concentrated CO₂ emissions [48]^[2]. Although the Roadmap presented by the federal government mentions that CCS technologies will contribute significantly to the net-zero goals, no information emerges as to how the relevant modeling was undertaken. This much is certain: a projected increase in national fossil fuel exports by mid-century will make reliance on CCS or other offsets important to achieving the Paris targets. So, how economically viable could a technology such as DACCS be in the Australian context?

2.2. Economic Viability of DACCS

In order to answer the above question, a two-step analysis is needed. It is not merely ‘speculative’ but should be regarded as ‘indicative’ rather than ‘definitive’, given the absence of NET projects and limited financial information about them in Australia. Amplifying the foregoing technical summary, a comparison of capture and capital costs for DACCS absorption and adsorption is first needed. Second, the results of that exercise should be calibrated with contemporary energy costs in Australia. Discussion ideally begins with the comment that, though DACCS is a cutting-edge CCS alternative, it is not necessarily a high-technology operation in regard to either its material usage or plant componentry. The technical challenge is processing large volumes of air to extract small concentrations of CO₂. The system can create considerable pressure drops inside the contactor units, controlled by expensive capital equipment and energy inputs to keep the air moving in the plant. As is well known in other industries, ancillary system components suffer similar problems (e.g., the slaker, causticizer, and clarificatory). Thus, the operation moves beyond known technology regarding cooling towers; the integration of DACCS componentry comes with significant learning commitments, especially in endeavors to scale up the output of CO₂. Entries were calculated from electricity requirement data from the National Academies of Sciences, Engineering, and Medicine [12]^[3], using an American cost of 0.06 USD/KWh for electricity and 3.25 USD per GJ for natural gas required for heating. In Australia, the respective prices are much higher, reported (ex-Brisbane) by the Australian Energy Regulator [49]^[4] as being 0.167 USD/KWh for electricity and 9.42 USD/GJ for natural gas. Keeping other plant and institutional cost factors the same (i.e., ceteris paribus), an international comparison of DACCS electricity and heating costs is provided in Table 1. Applying Australian prices exclusive of capital costs, the total energy cost becomes 123 to 192 USD/tCO₂ for liquid absorption and 58 to 98 USD/tCO₂ for solid adsorption technology. The two DACCS production methods have specific footprints and energy consumption, implying different capital and operating investments. Superimposed upon this observation is uncertainty about the actual cost of implementation; divergent figures typically circulate within the private and academic sectors [51]^[5]. Keith et al. [3]^[6] published the first paper with a commercial engineering breakdown for a 1 MtCO₂/year DACCS plant, and the approach of his team has informed the foregoing tables and figures. Costs as low as 100 USD and as high as 600 USD per tonne of CO₂ captured have been reported [52]^[7]. Regarding energy, Australia is one of the top 10 global producers. It relies heavily on coal for electricity production [53]^[8]. Fossil fuels generate 71 percent, and renewables only 29 percent of the energy delivered [54]^[9]. Interstate variations also occur: in Queensland, for example, renewables comprise less than 20 percent of the energy mix. Being a large user of both water and energy, DACCS will clearly have to eschew expensive solutions and rely on cheap sources of each commodity [55,56]^[10][11]. As indicated by Temple [57]^[12], many countries rely on large amounts of carbon removal to achieve their net-zero plans, but no clear pathway to implement the technology has been presented by any one of them. Proof is indicated that the technology is easily scalable. Thus, carbon-free electricity is required for the technology to be feasible; otherwise, and futilely, the operation will release more CO₂ into the atmosphere than it captures [58]^[13]. A joint leadership effort from industrialized economies is needed, taking advantage of regional characteristics that can promote cooperation and the advancement of technology [59]^[14]. The Commonwealth Scientific and Industrial Research Organization (CSIRO), in its GenCost 2021–2022 Consultation Draft, presents the levelized cost of electricity (LCOE) from a range of generation techniques [60]^[15]. Wind and solar emerge as the cheapest sources of new supply. With planned integration, their costs are predicted to fall compared with those of new fossil fuel electricity plants, which should remain unchanged over the next three decades [61]^[16]. The total cost is for a 1 MtCO₂ plant. The black area shows the minimum and maximum values and was calculated from the best-case scenario ambient conditions that will allow the plant to work with less energy and heat requirements to the worst-case scenario ambient conditions that will cause the facility to require more electricity and heat [12,60]^[3][15]. The total cost is for a 1 MtCO₂ plant. The shaded area represents the minimum and maximum values, calculated from a best-case scenario of ambient conditions that allow the plant to work with less energy and heat to a worst-case scenario in which the ambient conditions will cause the facility to require more electricity and heat [12,60]^[3][15]. Apart from debate over a facilitating regulatory framework (i.e., involving a carbon price and a market for captured CO₂ credits), the issue in any DACCS initiative will inevitably be who is to pay for the establishment and running of plants and the implications if costs fall on different sectors of society [62]^[17]. Economics and equity come to the fore as governments acknowledge CDR technologies as integral to their commitment to Net-Zero (e.g., the United Kingdom [62]^[17]). Australia’s Technology Investment Roadmap within its GHG emissions policy incorporates a financial strategy to support selected processes such as DACCS [63]^[18]. The latter are mostly judged controversial by the public and contrary to the conventional (renewable energy) routes to decarbonization. There is the risk that, if measures fail to mitigate regressive impacts, costs could be greater for low- than high-income groups. After extensive modeling, Owen et al. [62]^[17] found that the former will be disproportionately affected unless measures to fund the technologies are weighted by a household’s contribution to the total country’s income tax payments. Sovacool et al. [15]^[19] list several approaches toward funding. The first would be to integrate DACCS into a carbon tax emission trading system, with credits awarded for negative emissions. A second would replace existing carbon pricing schemes with carbon removal obligations, and a third advocates placing the obligation for removing carbon back on the emitting industries. ‘State-of-the-art’ reports and ongoing research are crucial for Australia’s goal of achieving net-zero emissions by 2050 through CCS technologies. However, more information is needed on the real-life costs of deploying and operating DACCS facilities. Our findings show that, while the construction costs of absorption and adsorption plants are similar, a liquid sorbent plant requires more than double the energy for heating. This divergence is significant, as heat and water are costly resources. High energy prices in Australia increase CO₂ capture costs by up to 60% compared with the United States. The viability of building and operating a DACCS facility in either nation could expand considerably if renewable generation technologies become cheaper and energy is solely derived from these sources, as predicted by the CSIRO. Additionally, with competition in construction increasing, the capital costs of a plant will likely decrease over time. Furthermore, academics and policymakers need to be well-informed about DACCS and other NETs to enable a proper interdisciplinary appraisal and debate. The major research issues include the dimensions of the carbon market, regulatory and funding frameworks, and a downstream issue well beyond the scope of this article, namely the technology and commercial viability of geological storage of captured carbon.