Advancements and Challenges in Direct Air Capture Technologies: Energy Intensity, Novel Methods, Economics, and Location Strategies

Abstract

Direct air capture (DAC) technology is increasingly recognized as a key tool in the pursuit of climate neutrality, enabling the removal of carbon dioxide directly from the atmosphere. Despite its potential, DAC remains in the early stages of development, with most installations limited to pilot or demonstration units. The main barriers to its widespread implementation include high energy demands and significant capture costs. This literature review addresses the most critical research directions related to the development of this technology, focusing on its challenges and prospects for deployment. Particular attention is given to studies aimed at developing new, cost-effective, and efficient sorbents that could significantly reduce the energy intensity and costs of the process. Alternative technologies, such as electrochemical and membrane-based processes, show promise but require further research to overcome limitations, such as sensitivity to oxygen presence or insufficient membrane selectivity. The economic feasibility of DAC remains uncertain, with current estimates subject to significant uncertainty. Governmental and regulatory support will be crucial for the technology’s success. Furthermore, the location of DAC installations should consider factors such as energy availability, options for carbon dioxide storage or utilization, and climatic conditions, which significantly affect process efficiency. This review highlights the necessity for continued research to overcome existing barriers and fully harness the potential of DAC technology.

1. Introduction

The increase in carbon dioxide concentration in Earth’s atmosphere is currently one of the most significant anthropogenic environmental challenges. According to the Global Carbon Project report [1], considered one of the most reliable sources of information on global greenhouse gas emissions, the CO₂ concentration in the atmosphere was approximately 417.2 ppm in 2022. In comparison, it was around 278 ppm before the industrial era. Carbon dioxide, as one of the primary greenhouse gases, significantly contributes to global temperature rise and intensifies ongoing climate change. To limit the anthropogenic impact of CO₂ on the climate, a number of international resolutions and agreements have been signed over the past few years. These commitments obligate signatories to reduce carbon dioxide emissions. Among the most notable documents is the Paris Agreement, which aims to limit global warming to well below 2 °C—preferably to 1.5 °C—compared to pre-industrial levels and achieve carbon neutrality. Achieving these goals is primarily envisioned through the reduction and control of greenhouse gas emissions, particularly anthropogenic CO₂. A critical tool for realizing the net-zero emissions (NZE) scenario is the widespread implementation of carbon sequestration technologies, such as carbon capture and storage (CCS), carbon capture and utilization (CCU), and direct air capture (DAC) [2,3,4,5,6,7]. DAC stands out among these technologies because its application is independent of emission sources. It could play a crucial role in reducing carbon dioxide emissions, especially in sectors like aviation, maritime transport, and heavy industry, where electrification is relatively challenging [3,8,9]. This technology has also garnered interest from some of the world’s largest companies, including Microsoft, Amazon, Aramco, and Siemens [10,11,12,13,14,15]. Microsoft and Amazon have committed to achieving net-zero carbon emissions by 2030 and 2040, respectively. To meet these goals, these companies are purchasing tokens representing the removal of specific volumes of carbon dioxide from the atmosphere while simultaneously investing in DAC-related companies.

1.1. Sorption Technologies in DAC Processes: L-DAC and S-DAC

To date, 27 DAC facilities have been launched worldwide, with an additional 130 planned units in various stages of development [2,16]. Among the numerous technologies under development, two main approaches to CO₂ capture using DAC can currently be distinguished. These technologies are based on liquid sorbents (L-DAC) and solid sorbents (S-DAC). A conceptual diagram of the currently applied DAC systems is presented in Figure 1 [17]. Essentially, both processes involve alternating sorption and regeneration of sorbents under different temperature conditions tailored to the specifics of the process. The captured carbon dioxide, released during the desorption process, is then stored in geological formations or oceans, or it can be utilized as a raw material in various industries.

Liquid DAC is one of the most promising and mature technologies currently applied in DAC processes. L-DAC systems function by passing air through aqueous chemical solutions, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or amine-based solutions, which absorb CO₂ [18]. On an industrial scale, alkaline solutions based on KOH or NaOH are predominantly used [19,20,21]. The process fundamentally relies on two interconnected chemical loops, as illustrated in Figure 2 [19,20,22]. In the first chemical loop, known as the absorption loop, a liquid solution absorbs atmospheric carbon dioxide in an air contactor. The second chemical loop, called the calcium loop, is responsible for the desorption process. The key chemical reactions occurring within the system, depending on the liquid sorbent used, are detailed in

Table 1. Reactions occurring in the different parts of the L-DAC system depending on the liquid sorbent used [19,20,22].

Section	Reaction	Enthalpy of Reaction
Air Contactor	$2\mathrm{K}\mathrm{O}{\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}+\mathrm{C}{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{K}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$ Or $2\mathrm{N}{\mathrm{a}\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}+\mathrm{C}{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{N}\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$	$\mathsf{\Delta }\mathrm{H}°=-95.8\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$ Or $\mathsf{\Delta }\mathrm{H}°=-109.4\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$
Pellet Reactor	${\mathrm{K}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_{2\left(\mathrm{s}\right)}\to 2\mathrm{K}\mathrm{O}{\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}+\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{s}\right)}$ Or ${\mathrm{N}\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_{2\left(\mathrm{s}\right)}\to 2\mathrm{N}{\mathrm{a}\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}+\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{s}\right)}$	$\mathsf{\Delta }\mathrm{H}°=-5.8\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$ Or $\mathsf{\Delta }\mathrm{H}°=-5.3\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$
Calciner	$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{s}\right)}\to \mathrm{C}\mathrm{a}{\mathrm{O}}_{\left(\mathrm{s}\right)}+\mathrm{C}{\mathrm{O}}_{2\left(\mathrm{g}\right)}$	$\mathsf{\Delta }\mathrm{H}°=+178.3\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$
Slaker	$\mathrm{C}\mathrm{a}{\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to \mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_{2\left(\mathrm{s}\right)}$	$\mathsf{\Delta }\mathrm{H}°=-63.9\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$

. Sorbent regeneration takes place through the reaction of Na₂CO₃ or K₂CO₃ with Ca(OH)₂. The regenerated sorbent, in the form of KOH or NaOH, is returned to the air contactor unit, while the resulting CaCO₃ is directed to the calciner, where it decomposes into CO₂ and CaO under high temperatures. The heat supplied to the calciner can be sourced from oxygen combustion of natural gas. In such cases, the system must also include an Air Separation Unit (ASU) and a gas turbine or a combined-cycle system. The CO₂ is removed from the system, while the CaO is sent to the slaker unit, where it reacts with water to convert back to Ca(OH)₂, completing the loop. This technology has been developed primarily by the Canadian company Carbon Engineering since 2009 [23]. Currently, based on this technology, the company 1PointFive is constructing the world’s largest CO₂ capture facility under the STRATOS project [24,25]. The installation is scheduled to achieve operational readiness by mid-2025 and is expected to capture 500,000 tons of CO₂ annually.

S-DAC systems utilize solid sorbent filters characterized by high specific surface area and porosity, which enables the adsorption of carbon dioxide on their surface. A key advantage of the S-DAC process is that adsorption typically occurs under ambient temperature and pressure conditions. Among the solid sorbents used or considered are amine-based sorbent materials, functionalized alkalis, zeolites, activated carbon, biochars, and lime [22,26,27,28,29,30,31,32]. The desorption process is typically carried out under high vacuum conditions with the application of low-temperature heat. Adsorption processes can be categorized based on the conditions under which desorption occurs. These include pressure swing adsorption (PSA or VSA when conducted under vacuum), temperature swing adsorption (TSA), or a combination of both techniques in a method called temperature vacuum swing adsorption (TVSA) [33,34,35]. Leading companies in the S-DAC sector include the American Global Thermostat (now acquired by Zero Carbon Systems), which uses amine-based sorbent materials, and the Swiss Climeworks, which uses alkali-functionalized adsorbents [32,36,37,38].

1.2. Energy Intensity of DAC Process

DAC systems are highly energy-intensive processes. This is due to the relatively low concentration of carbon dioxide in the atmospheric air compared to exhaust gases from, for example, power plants, as well as the need for sorbent regeneration. In modern combined-cycle gas-turbine power plants, CO₂ concentration in exhaust gases can be up to 5%, while in coal-fired plants, it ranges from 12–14%, compared to around 400 ppm in atmospheric air. The energy intensity of direct air capture systems based on sorption methods, depending on the type of sorbent used, is shown in Figure 3 [2]. Both solid and liquid sorbents require regeneration to release the bound carbon dioxide. The regeneration process, depending on the sorbent used, requires the supply of either low or high-temperature heat, which constitutes the primary energy expenditure in sorption-based DAC processes. According to Figure 1 and Figure 3, the regeneration process for solid sorbents occurs with the supply of low-temperature heat, typically in the range of 80–120 °C [2]. Therefore, this process can be powered by renewable energy sources such as solar or geothermal energy [39,40,41,42,43]. These systems, however, have much higher process energy intensity, reaching up to 9.5 GJ/t CO₂. In comparison, systems based on liquid solvents have a process energy intensity of up to 6.6 GJ/t CO₂. Regeneration of liquid solvents, however, requires high-temperature heat, typically in the range of 300–900 °C. As a result, the heat needed for the desorption process in liquid DAC systems is usually generated through the combustion of natural gas.

The aim of this article is to review the current and developing direct air capture technologies, with a focus on energy intensity, economics, and the impact of location on the efficiency of these technologies. The article seeks to identify the challenges and development directions that could contribute to the more efficient and cost-effective implementation of DAC in various contexts.

2. Research on New Sorbents

Reducing the energy intensity of the process is one of the main technological challenges facing the DAC concept. As a result, numerous studies in the scientific literature focus on the development of new, highly efficient liquid and solid sorbents, the use of which could reduce the energy consumption of the DAC process. An overview of selected studies on alternative sorbents is presented below, with a summary in

Table 2. A summary of the discussed sorbents, categorized by process type.

DAC Process Type	Sorbent	Reference
L-DAC	Diamines	[44]
L-DAC	Aqueous amino acid salts	[45]
L-DAC	Ionic liquids	[46]
S-DAC	Lime-based sorbents	[47]
S-DAC	TEPA-Al2O3	[48]
S-DAC	Amine-grafted SBA-15 by boron doping and acid treatment	[49]
S-DAC	Zeolites	[29,50]
S-DAC	KOH-activated olive pomace biochar	[27]
S-DAC	NaOH-activated biochar prepared from sewage sludge	[51]
S-DAC	Ionic liquid-modified carbon	[46]

.

In the case of liquid sorbents, research focuses on, among other things, the search for new diamine, amine, or ionic liquid compounds. For instance, in [44], the authors conducted studies on the ability of five different diamine solutions to remove CO₂ from the air. The research was carried out using both aqueous and non-aqueous solvents, with a mixture of ethylene glycol and 1-propanol. The results of the 24-h absorption tests showed that diamine solutions in both types of solvents are capable of effectively removing CO₂ from the air. However, the removal efficiency is slightly higher in the case of aqueous solutions. On the other hand, organic solutions could lead to significant energy savings during sorbent regeneration in the desorption process. The authors emphasize the need for further research to evaluate the commercial potential of the tested sorbents. In the paper [45], the authors present experimental studies on the effectiveness of CO₂ removal from air using the absorption mechanism with amino acid salts in aqueous solutions. The studies also cover their regeneration in a reaction with meta-benzo-bis(iminoguanidine) (m-BBIG). This reaction, occurring at room temperature, forms hydrated carbonate salt crystals, which can subsequently release the bound CO₂ through low-temperature heating in the range of 60–120 °C. This process does not require heating the liquid sorbent, thus eliminating issues related to thermal degradation and oxidation at high temperatures. While the energy intensity of this process is higher compared to the chemical calcium loop process (8.2 GJ/t vs. 6.3 GJ/t), the regeneration process takes place at much lower temperatures. Ionic liquids (ILs) are also considered as sorbents [52,53,54]. This is due to their relatively high thermal stability, resistance to oxidative degradation, low viscosity, and high ability to tune the enthalpy of the reaction with CO₂ [53,55]. For example, in the study [54], the authors designed 26 different ionic liquids, selecting the most promising sorbents. Then, using Aspen Plus v12 software, they performed an exergy, energy, and efficiency analysis for various process variables. The process was based on the air contactor technology from Carbon Engineering. Among the examined ionic liquids, [P66614][Im] exhibited the highest exergy efficiency (5.44–16.73 MJ/kg) and energy intensity (15.15–35.42 MJ/kg). This process, in terms of exergy, is comparable to those based on amine sorption and shows better results than alkaline sorption. Preliminary economic analyses showed a potential capture cost of 200$/tCO₂.

The literature, however, seems to contain significantly more studies on the development or modification of various solid sorbents. The use of lime as a solid sorbent for CO₂ capture from the air is presented in [47]. The studies showed that lime-based sorbents can effectively adsorb CO₂ from the air. Granulated limestone exhibited the highest effective diffusivity due to its greatest porosity. The studies also identified the dominant influence of relative humidity and sorbent hydration on the reaction rate and conversion. In [48], the authors introduced an ultrasonic impregnation method to load tetraethylenepentamine (TEPA) onto aluminum oxide (Al₂O₃) as an adsorbent. Additionally, the authors applied gravitational adsorption technology to significantly enhance the system’s efficiency. The research showed fast and effective CO₂ capture from the air using the TEPA-Al₂O₃ adsorbent under high gravity conditions, as well as significant adsorbent stability. The developed adsorbent showed a CO₂ adsorption capacity of 48.5 mg/g, significantly surpassing traditional fixed-bed systems.

In [49], the authors significantly increased the CO₂ adsorption capacity at low concentrations by using citric acid on an SBA-15 adsorbent doped with boron, creating new silanol nests. The literature also highlights the potential of zeolites as promising adsorbent materials with a high potential for direct air capture of CO₂ [56,57]. Zeolites, as microporous materials, are characterized by low cost, non-toxicity, and high structural stability. While the first zeolite-based installation was commissioned in 2022 in Norway as part of the Removr project, research into the use of zeolites is continuing [2,58]. In [29], researchers using Na-X zeolites demonstrated the ability to capture CO₂ with high purity, above 99%, in a TVSA process. In [50], researchers examined the CO₂ separation capability of air using a synthesized zeolite containing iron (Fe@13X). The applied sorbent showed high stability and required 3.6 times less desorption energy compared to the iron-free 13X zeolite. An additional advantage of zeolites is their ability to enhance adsorption capacity through modification, e.g., with amines [59,60]. The literature also emphasizes the potential of biochar as an inexpensive and effective sorbent with low regeneration energy [61]. For example, in study [27], the authors investigated the possibility of CO₂ adsorption from the atmosphere using biochar made from olive pomace activated with KOH. The activation process aims to improve the functionality of the produced biochar by increasing its specific surface area and porosity. As a result, it leads to a significant improvement in adsorption capacity [27]. The authors obtained an adsorbent with higher adsorption capacity and speed compared to unactivated biochar. The produced adsorbent has a regeneration process occurring at 65 °C. In [51], the authors studied the adsorption potential of chemically activated biochar prepared from sewage sludge. They emphasize the crucial impact of pyrolysis temperature on the resulting morphological properties of the biochar, which in turn affect its adsorption capacity. The studies also showed that chemical activation with NaOH could improve its sorptive properties.

The previously mentioned ionic liquids are also used to modify solid sorbents. This concept, based on the modification of carbon sorbents, is presented in the study [46]. The studies showed that ionic liquids fill the micropores of carbon and form a thin layer on the surface of mesopores. This improves the availability of active sites for CO₂ binding. Essentially, IL provides active CO₂ binding sites by forming O-C bonds, while the carbon substrate supports IL by increasing its thermal stability and improving sorption kinetics. This is crucial for CO₂ capture from diffuse sources such as ambient air.

In the subject literature, there are also numerous review papers on the sorbents used and developed for CO₂ capture. The authors in [62] presented the main types of sorbents intended for CO₂ capture used in DAC, along with their classification according to the sorption mechanism. In [18], the authors focused solely on the review of various liquid solvents, whereas in [63], the authors conducted a critical evaluation of amine-based solid adsorbents. In the review paper [64] on adsorption technology research from 2016 to 2021, the authors point out the lack of literature data regarding the properties of adsorbents needed for modeling and designing DAC processes.

3. Alternative Direct Air Capture Technologies

Despite extensive research on new sorbents aimed at improving CO₂ capture efficiency, challenges related to high energy consumption and process costs remain a significant issue. In response to these challenges, alternative technologies are being developed that could reduce energy consumption and lower operational costs. This chapter provides an overview of the developing alternative methods for atmospheric carbon dioxide capture, such as Electric-Swing Adsorption, electrochemical processes, and membrane separation.

3.1. Electric-Swing Adsorption

Among the studied technologies, the Electric-Swing Adsorption process stands out [2,65,66,67,68]. This technology is a modification of the TSA process through the direct regeneration of the sorbent by heating with electric current, utilizing the Joule effect [35,69]. This allows for a significant increase in the desorption process speed through enhanced heating efficiency. Consequently, this process requires materials with high adsorption capacities that also exhibit good electrical conductivity, such as activated carbon [66,69]. However, activated carbon has a lower CO₂ adsorption capacity compared to, for example, zeolites, which do not conduct electric current. Researchers in [69] noted this problem. In response to these issues, researchers synthesized an innovative hybrid of activated carbon (from phenolic resin) and NaUSY zeolite monolith. The newly developed adsorbent exhibited twice the adsorption capacity compared to the phenolic resin material. In [68], the authors presented the possibility of using electric current regeneration of the sorbent with a carbon monolith to remove CO₂ from spaces with increased concentrations. In the work [70], the authors presented the Electric-Swing Adsorption process for CO₂ capture from the atmosphere, using sorbent-coated carbon fibers. Experimental results showed that CO₂ desorption occurs very quickly, within less than 10 min, with 95% of the adsorbed CO₂ being recovered. The module surface temperature did not exceed 50 °C during the heating of the fibers to around 110 °C, indicating efficient heat management. The scaling of the technology to larger modules enabled their operation in windy conditions without fans. A preliminary techno-economic analysis showed a CO₂ capture cost of 160$ per ton under optimal operating conditions.

3.2. Electrochemical Methods

Considerable attention is also given to electrochemical methods of carbon dioxide capture. For instance, an electrochemical process for sorbent regeneration is proposed, allowing the elimination of steam use through the electrification of the desorption stage [71,72,73,74]. A diagram of the process is shown in Figure 4 [74]. In this process, the solution, after capturing carbon dioxide and containing an alkaline carbonate electrolyte, enters the electrolyzer. There, carbonate anions and metal cations separate and electrochemically move to the respective chambers: acidic and alkaline. In the acidic chamber, CO₂ is released due to proton-induced pH changes, while in the basic chamber, the capturing liquid is regenerated by the electrochemical production of hydroxide [73]. In the study [73], the authors evaluated the potential of an electrochemical system for the regeneration of alkaline CO₂ capture liquids (AE) for DAC processes. The system demonstrated stability over 100 h of operation without degradation of the electrocatalyst. However, limitations related to the presence of oxygen in the capture liquid were identified. The reduction of O₂ was limited to ~0.5% of the electrolyzer’s current density during the regeneration process. The authors also emphasized that laboratory-scale studies might not reflect advanced large-scale processes. In [75], the authors presented an electrochemical regeneration process of KOH combined with hydrogen co-generation, aiming to reduce process costs. The studies showed that achieving an energy consumption level of 250–500 kJ/mol-CO₂ is possible while simultaneously producing hydrogen. The electrochemical regeneration process of NaOH was presented in [76]. The authors conducted experimental studies on an electrochemical process for simultaneous solvent regeneration and CO₂ desorption in a continuous system with H₂ recycling. The process achieved an energy consumption of 374 kJ/mol CO₂, with CO₂ purity exceeding 95%. Theoretical calculations indicate the possibility of reducing energy consumption to 164 kJ/mol CO₂. The electrochemical cell with H₂ recycling allows for the simultaneous desorption of high-purity CO₂ and regeneration of up to 59% of the sorbent’s CO₂ capture capacity.

Part of the research on electrochemical DAC processes focuses on the use of bipolar membrane electrodialysis (BPMED). The advantage of this solution is the ability to create pH fluctuations through direct water dissociation with minimal voltage drop [77,78]. In [78], the application of bipolar membrane electrodialysis was evaluated in two different configurations: cation exchange membrane (CEM) and anion exchange membrane (AEM). The results showed that BPM-CEM achieves a minimal energy requirement of 24 MJ/kg CO₂, which is competitive with other technologies but higher than the calcium loop process. The AEM configuration requires improved selectivity to compete with BPM-CEM. Adding neutral salt to the solution can reduce energy requirements in BPM-CEM to 17 MJ/kg CO₂, and further membrane improvements could lower costs to below USD 250/ton CO₂, making this process a viable alternative to chemical regeneration technologies. BPMED-based processes, however, may exhibit higher energy consumption compared to other electrochemical methods. In [71], the authors conducted a direct comparison of DAC systems based on electrodialysis and electrolysis through numerical modeling. The results showed minimal energy requirements of 1255 kJ/mol CO₂ for electrolysis and 1451 kJ/mol CO₂ for electrodialysis. Additionally, a 20-fold higher capture rate was achieved for the electrolysis process. Under ideal process conditions, energy requirements could be reduced by approximately 90% to 162 and 154 kJ/mol CO₂, respectively. The authors noted that energy consumption by electrochemical cells constitutes over 96% of the total energy requirement.

Electrochemical processes can also be conducted based on absorber/desorber cycles using specific redox-active sorbent carriers [77]. Redox-active compounds such as quinones, phenazine, 4,4′-bipyridine, and thiolates coat the electrode surfaces and can serve as CO₂ carrier molecules [79,80,81,82,83,84]. This process is referred to as electro-swing adsorption (ESA). A diagram of the process is shown in Figure 5 [74]. In the first phase, electrodes coated with redox compounds are electrochemically reduced, leading to the adsorption of CO₂ from the feed gas stream. In the second phase, applying a positive charge causes electrochemical oxidation with the release of CO₂. However, most currently used redox-active compounds are highly sensitive to the high oxygen content in the feed gas [74,81,85].

Recently, some progress has been made to adapt this technology for DAC processes. For example, in [82], researchers successfully conducted a semi-conductive Faradaic ESA process using quinone-functionalized electrodes for CO₂ concentrations of 10% and in the range of 0.6–0.8%. The process exhibited high Faradaic efficiency (>90%) and cell durability. In a recent study [85], an electrochemical CO₂ capture and release system based on an aqueous electrolyte with a nitro-reductant redox system (NR/NRH₂) was presented. This system enables efficient CO₂ capture from both high-concentration gases (15%) and atmospheric air. The authors estimated the minimal energy requirements of the process to be about 35 kJe/mol CO₂ for 15% CO₂ and 65 kJe/mol CO₂ for air, making the technology energetically competitive compared to other capture methods. The studies showed that the NRH₂ solution exhibits a fast CO₂ absorption rate within the concentration range of 1% to 15%. A higher adsorption capacity was observed at higher gas concentrations. Additionally, the material remained stable in the presence of oxygen for at least a week, increasing its potential for practical applications. ESA technology is currently being developed by the American company Verdox [86].

3.3. Membrane Separation

A completely separate approach involves the use of membrane separation processes for direct air capture (m-DAC). Unlike the other technologies discussed, this process does not require an energy-intensive sorbent regeneration process or the use of chemicals. Membrane separation is a well-developed technology, widely proposed in the literature as an alternative to sorption processes for CO₂ capture from flue gases. It is characterized by significantly lower energy consumption [87,88,89,90]. A significant advantage of the m-DAC process is its low technological complexity. Essentially, besides the membrane module, this process only requires flow machines responsible for creating pressure differentials in the system, which are the main driving force of the process [88,91]. Membrane separation systems can operate under different strategies to ensure pressure differentials between the feed stream (feed gas stream) and the permeate stream (separated gas stream), as shown in Figure 6. The low partial pressure of carbon dioxide in atmospheric air and the lack of membranes with suitable separation properties have so far completely prevented the adaptation of this process within m-DAC [92,93]. Membranes with low CO₂ permeance require large pressure differences within the system. Alternatively, they must have a sufficiently large surface area to effectively capture the gas. This is because the required membrane surface area is inversely proportional to the permeance of the separated gas [8,94,95]. Therefore, it is proposed that membranes used in this process should achieve gas permeance above >10,000 GPU and CO₂/N₂ selectivity > 30 [92,95].

The dynamic development in research on developing highly selective and permeable membranes makes it possible to consider this process as a realistic alternative to sorption methods in the future. Among other developments, the announcement of a freestanding siloxane membrane with a record CO₂ permeance reaching almost 40,000 GPU [96] is noteworthy. In another study [96], the authors reported the development of a multilayer membrane with CO₂/N₂ selectivity = 70. Based on the obtained separation properties of the membranes, the authors presented a preliminary feasibility assessment of m-DAC based on the numerical simulation results in [95]. The authors managed to achieve a 30% carbon dioxide concentration in a 4-stage separation system with an energy consumption of 16 kWh/kgCO₂/day for a pressure ratio in the system ϕ = 20, using a vacuum pumping strategy. The authors also performed calculations for a membrane with a significantly lower permeance of 10,000 GPU, while significantly increasing the pressure ratio in the system to ϕ = 55. The results showed that with a much higher pressure ratio, a similar process energy consumption could be achieved for CO₂/N₂ selectivity = 30. Additionally, the 4-stage process enabled the production of a gas product with a 41.6% CO₂ content. The dominant influence of the pressure differential in the system for the vacuum pumping strategy is also noted by the authors in [8]. In their work, the authors conducted extensive parametric analyses using numerical modeling, particularly as a function of membrane permeance. The performance maps of the m-DAC system presented by the authors may serve as a useful optimization tool for future researchers. Additionally, the authors included the energy consumption of the pre-fan in their calculations, resulting in a significant energy consumption of 22.5 kWh/kgCO₂, which is considerably higher than the value for typical sorption systems. However, it was emphasized that flow machines were not optimized, and the system could be applied in hybrid arrangements. Another solution could be the use of membranes with high selectivity at the cost of reduced permeance, but this induces a significant increase in the required membrane module surface area [97,98]. In [98], the authors performed numerical simulations, among other things, based on the separation properties of commercially available modules. The results of the calculations showed that in the case of a single-stage membrane separation process based on currently available market technologies, the maximum CO₂ concentration in the permeate could be 2%, and the minimal process energy consumption is about 18,000 kWh/t CO₂. The energy demand can be minimized through vacuum pumping, but the authors mention difficulties in achieving a pressure ratio below 0.01 on an industrial scale using conventional vacuum pumps. When calculations were performed for highly selective materials with CO₂/N₂ > 100, the authors reported the possibility of achieving 12% CO₂ concentration after the first separation stage, and the minimal process energy consumption could be 3000 kWh/t CO₂. This highlights the importance of further developing highly efficient membranes to adapt this technology within the DAC system.

4. Economic Aspects

Research on new sorbents and alternative CO₂ capture methods may, in the future, enable increased process efficiency; however, the DAC technology itself remains uncertain in terms of costs. As a result, it is difficult to definitively assess the viability of further development of this technology in terms of its cost-effectiveness. Despite the technology moving beyond the demonstration and pilot scales, the costs associated with the high energy consumption of the process remain a significant barrier. Cost estimates remain imprecise and show considerable variability, which complicates the long-term profitability assessment of this technology. Even before the deployment of the first DAC units, estimates of capture costs in installations in the literature ranged from 100 to 1000 USD per ton of CO₂ [99,100,101,102]. As noted in [102], a significant issue is the high discrepancy between the results presented in the scientific literature and the lower values declared by DAC startups. According to recent IEA indications from 2022, cost estimates for DAC remain uncertain and may range from 125 to 335 USD per ton of CO₂ for currently constructed large-scale plants [103]. The Intergovernmental Panel on Climate Change (IPCC) estimates the cost of CO₂ capture at 100–300$ per ton in its 2022 report [104]. The Boston Consulting Group, on the other hand, cites current costs in the range of USD 600–1000 per ton of CO₂, emphasizing the need to reduce costs below USD 200 per ton by 2050 for widespread adoption of DAC technology [105]. Future targets set by Frontier’s Advance Market Commitment (AMC) and the US Department of Energy aim to achieve capture costs of 100 USD per ton of CO₂ [106,107]. A significant reduction in costs can be achieved through appropriate political support in the form of capital grants or tax incentives. However, so far, only a few governments have committed to financial support for DAC. Start-ups primarily rely on private funding, mainly from the oil sector [105,108,109] Consequently, there is an urgent need to develop support instruments for this emerging technology as it scales beyond demonstration units. The literature also emphasizes the importance of reducing the cost of energy needed for regeneration, with geothermal energy appearing to be the least variable and cost-effective option [110].

While the studies cited earlier in this review include preliminary estimates of capture costs, these usually focus exclusively on energy costs related to the process’s energy demand. The literature still lacks reliable and comprehensive techno-economic assessments of direct air capture systems [108]. The first document with a commercial compilation of engineering costs based on energy and mass balance data from a pilot plant was presented in [19] for the chemical calcium loop technology developed by Carbon Engineering. The authors of the document indicate that the average cost of carbon dioxide capture, depending on financial assumptions, ranges from 94 to 232 USD/tCO₂. However, they point out the difficulties in estimating technology costs before its widespread implementation, which means their cost estimates are subject to significant uncertainty.

In [111], the authors conducted a comparative technical assessment of three technological approaches to the DAC process. The analyses involved two liquid DAC systems based on monoethanolamine (MEA) and an alkaline KOH solution, respectively. The third variant was the VTSA adsorption process for four different adsorbent materials. Thermodynamic calculations allowed the determination of carbon dioxide capture costs as a function of heat and electricity prices and installation costs related to the required air contactor capacity. The results showed that the amine system, due to its dominant heat energy demand, is highly sensitive to fluctuations in heat costs. In contrast, the alkaline solvent system, due to its lower capture process efficiency, is highly dependent on the cost of the air contactor. The lowest average process costs were obtained for the solid sorbent technology with a high mass transfer coefficient, which combines low energy demand with high productivity. This technology shows the greatest dependence on variable heat prices. With low mass transfer, costs become more similar to the MEA technology, although still more favorable. It was also highlighted that all three technologies could achieve a capture cost below 200 USD/tCO₂ under optimistic calculation assumptions.

An important issue is the estimate of future DAC costs, which should decrease as the technology scales up. In a paper [112], the authors analyzed the economic aspects of implementing high-temperature (HT DAC) and low-temperature DAC (LT DAC) from 2020 to 2050. The authors note that despite higher energy demand, low-temperature systems based on solid sorbents have a greater potential to reduce capture costs by 2050. This is due to the possibility of powering them using heat pumps and waste heat, while simultaneously not requiring process water. With a learning curve for capital expenditures set at 10%, the expected capex is 230 and 207 USD/t CO₂ by 2050 (assuming 1 EUR = 1.04 USD). The learning curve can play an important role in reducing capture costs. This process includes improvements in construction practices, process optimization, and cost reductions in materials and equipment due to increased production scale and experience gained by suppliers and operators [110]. In another study [107], researchers used a learning curve approach to estimate the future costs of carbon capture, transport, and storage (DACCCS) for three different capture technologies by 2050. The analysis covered a potassium hydroxide absorption system with calcium looping regeneration, temperature-swing adsorption, and the CaO ambient weathering DAC technology. For facilities with a cumulative capture capacity of 1 GtCO₂/year, the estimated costs ranged from 226–544$, 281–579$, and 230–835$ per ton of CO₂, respectively. Consequently, achieving a capture cost below 100$ per ton of CO₂ by 2050 appears unlikely.

Another important issue is the cost of the sorbents themselves. In [113], a universal cost analysis model for DAC technology was presented, based on a net present value maximization approach, allowing for the economic evaluation of the sorbents used. The authors point out that most currently proposed DAC sorbents require more detailed tests in real conditions, including long-term stability in loading and unloading cycles. The model indicates that the profitability of sorbents requires their durability at the level of tens or hundreds of thousands of cycles. Additionally, DAC process optimization may include shortening the cycle time and reducing the amount of CO₂ captured per cycle to lower operational costs. The importance of research on sorbent degradation and cycle times in real operational conditions, as well as the need for better estimation of investment and operational costs for different DAC configurations, was also highlighted.

5. Location of Installations

Although direct air capture processes are characterized by the ability to capture carbon dioxide independently of emitters, the literature emphasizes the importance of selecting optimal locations to increase capture efficiency and reduce costs. The choice of the optimal location should be tailored to the specifics of the given DAC technology, as different capture methods may require different locational conditions. Among the key factors influencing the choice of DAC installation locations are climatic conditions such as humidity and air temperature, access to energy sources, infrastructure enabling CO₂ transport, storage, or utilization, and local political conditions [41,114,115,116,117,118,119].

Studies pay particular attention to issues related to air temperature and humidity. These factors directly affect sorption capabilities and energy consumption. For example, in [116], the influence of climatic conditions, including temperature and relative humidity, on the efficiency and costs of CO₂ capture for the liquid solvent process developed by Carbon Engineering [19] was assessed. The process modeling was conducted using Aspen software, considering various climate scenarios, such as cold, dry, and warm, humid climates. The study showed that high capture rates are only possible in hot and humid climatic conditions. A capture rate of 75% is achievable only at temperatures above 17 °C and 90% humidity. Sensitivity analysis indicated that CO₂ capture efficiency is not very sensitive to climatic conditions, while the balanced CO₂ capture cost when using natural gas ranges from 240 to 409 USD/tCO₂ and is more sensitive to temperature than humidity. The authors define capture efficiency as the ratio of the difference between the captured CO₂ and the resulting greenhouse gas emissions (in CO₂ equivalent) to the captured CO₂.

In another study [115] on L-DAC, the authors found that when the system is powered by locally burned natural gas, the amount of sequestered CO₂ can be 30 to 50% greater than the amount of CO₂ removed from the atmosphere. This is due to directing exhaust gases from the gas turbine to the absorber, where about 90% of CO₂ is absorbed and mixed with air in the air contactor according to the process presented in [19]. The decision on the unit’s energy mode, such as grid energy, locally burned natural gas, or mixed mode, can be made without considering climatic conditions, as it depends solely on the greenhouse gas emission intensity related to electricity and natural gas production. It was also found that for the discussed L-DAC system, using the United States as an example, local weather conditions can lead to a threefold difference in system efficiency. Generally, similar to [116], regions with higher and less variable air temperature and humidity, such as southern states, are preferred locations.

The influence of climatic conditions when using solid sorbents was presented in [110]. The authors analyzed the impact of climatic conditions, including both relative humidity and air temperature, on the DAC process through multidimensional optimization of the TVSA process for the commercially available supported-amine sorbent, Lewatit^® VP OC 1065. The results indicate that climates with high average temperatures are the most attractive for DAC technology, while in cold and humid climates, capture costs are about 40% higher. Tropical and dry, hot climates, thanks to relatively stable weather conditions throughout the year, allow for easier process handling. In contrast, variable seasonal conditions in temperate and continental climates require dynamic process control to minimize operational costs throughout the year.

In contrast, researchers in [120] reached opposite conclusions. The researchers analyzed the impact of climate on the efficiency of the S-DAC process, identifying optimal regions for deploying these systems. The analyses were based on the same TVSA process and sorbent as in [118]. It was found that the most suitable regions for implementing DAC technology are cooler and drier areas, where the temperature does not drop below −15 °C for most of the year, assuming capital costs are not considered. However, such areas may not provide the lowest levelized cost of CO₂ capture (LCOD), as local WACC weighted average cost of capital) and energy costs significantly affect LCOD. The authors also note that about 25% of the Earth’s surface may not be suitable for DAC installations. In addition, the limitations associated with the lack of accurate isotherm models for various DAC sorbents, particularly those describing the coadsorption of CO₂ and H₂O under high relative humidity conditions, were also highlighted. The lack of mass transfer models describing the adsorption rates of CO₂ and H₂O, especially at very low temperatures, was also highlighted. In [119], the authors emphasize that locations with lower ambient temperatures should be preferred due to their lower environmental impact and lower energy consumption. Using mathematical modeling, the researchers analyzed the adsorption process in different countries worldwide, considering the average annual air temperatures in various locations. Three different Metal-Organic Frameworks (MOF)-based sorbents and two amine-functionalized sorbents were used. The results indicate that an increase in air temperature significantly reduces adsorption capacity in the case of metal-organic frameworks, more so than in the case of amine-functionalized sorbents. In contrast, energy consumption shows an opposite trend.

In addition to climatic conditions, the local availability of the heat sources needed to power the regeneration process is also an important issue for the location of the plant. Such an analysis of the LT-DAC process is presented in the work [41]. The researchers considered four heat sources: geothermal energy, parabolic trough collectors (PTC), industrial waste heat (IWH), and high-temperature heat pumps (HTHP), located in Iceland, Spain, Germany, and Norway. The results show that Spain is particularly suitable for PTC, IWH, and HTHP systems, Iceland for geothermal energy, IWH, and HTHP, and Norway mainly for the HTHP system due to cheap and decarbonized electricity. For Germany, significant environmental and legal barriers were noted; however, the country has a high potential for using IWH due to its highly developed industry. The techno-economic assessment indicates significant variability in LCOD costs for different heat source systems, with the geothermal system showing the lowest costs at 182.65 USD/tCO₂ (assuming 1 EUR = 1.04 USD).

6. Perspective

The use of DAC technology seems essential for achieving climate neutrality. However, these installations are just moving beyond the demonstration phase, and their large-scale implementation faces significant barriers, such as high energy demand and capture costs.

The high energy intensity of the DAC process remains one of the main barriers, significantly affecting operational costs. Reducing the energy intensity of this process should be considered a key research priority necessary for its widespread implementation in the future. Among the available technologies, the L-DAC process, based on the calcium loop, seems to be the most advanced in terms of technological maturity [19]. The development of alternative capture technologies and advanced sorbents with improved adsorption properties shows great potential for reducing energy consumption. However, in most cases, these technologies have not yet reached the technological maturity required for the construction of demonstration or pilot units.

Future research and development efforts should focus on accelerating the maturation of technologies, supporting the transition from laboratory-scale to industrial-scale implementation. In the context of S-DAC processes, it is particularly important to develop sorbents with high adsorption capacity, low production costs, and long-term stability. With low-temperature regeneration of solid sorbents, there is an opportunity to utilize waste heat, which can help mitigate the issue of energy demand.

The development of electrochemical capture methods, membrane separation technologies, and electric swing adsorption may in the future enable full electrification of the DAC process. Such an approach opens the possibility of powering the installations solely with renewable energy sources, eliminating the need for natural gas. In the long term, in the context of the energy transition based on renewable energy sources, DAC units could serve as balancing systems for excess energy generation.

However, it should be noted that both the implementation of electrochemical methods and membrane separation face numerous technological barriers. Electrochemical methods are sensitive to the presence of oxygen, which constitutes a major obstacle to their further development. On the other hand, membrane separation processes are limited by the current separation capabilities of available membranes and most research is currently based solely on numerical modeling. Additionally, membranes, especially those made of polymers, are highly sensitive to humidity [121,122], an aspect that has so far been largely neglected. Therefore, future research should focus on experimentally confirming the effectiveness of these technologies and considering the impact of air humidity on their performance.

Current estimates of CO₂ capture costs are still subject to considerable uncertainty and range widely from 100 to 1000 USD per ton of CO₂, depending on the source [103,104,105]. Comprehensive techno-economic analyses conducted by independent government agencies and academic institutions are necessary to reduce this uncertainty and identify the most promising capture technologies. In the long term, it is anticipated that CO₂ capture costs will decrease significantly, similar to the trends observed in battery and photovoltaic technologies in recent years.

Legislative support mechanisms, such as grants and tax incentives, can play a key role in reducing costs and accelerating the implementation of DAC technologies. The profitability of DAC plants could be enhanced by using captured CO₂ as a raw material in various industries, which would make market availability a critical locational criterion.

7. Conclusions

This review paper addresses key issues discussed in the scientific literature aimed at overcoming the main barriers to the further development of DAC technology. The broad scope of the topics covered demonstrates the high research interest in this field and reflects the complexity of the challenges that lie ahead for the continued advancement of this technology. The main conclusions are as follows:

(a)

Research on developing inexpensive and highly efficient sorbents should continue to reduce regeneration energy demand and process costs. For liquid sorbents, significant issues remain, such as thermal degradation, oxidation, toxicity, and corrosivity. Solid sorbents, on the other hand, should be characterized by high durability and the ability to maintain their properties over many process cycles.

(b)

The development of alternative capture technologies could significantly increase the profitability of DAC technology in the future. Both electrochemical and membrane processes still require further research. Electrochemical processes are particularly sensitive to the presence of oxygen, while the efficiency of membrane-based DAC is limited by the current separation capabilities of available membranes. Among the available solutions, ESA technology, developed by the American company Verdox, seems the most promising.

(c)

The cost of CO₂ capture in DAC technology remains uncertain, with current estimates subject to significant uncertainty. Currently, DAC projects are primarily financed by the private sector; however, widespread implementation of this technology will require government support, including appropriate financial and regulatory instruments. The literature contains few techno-economic analyses based on actual data from pilot or demonstration units [98]. A significant problem is the considerable discrepancy between academic estimates and those declared by startups. Therefore, the need for more detailed and reliable economic analyses is emphasized, which will better assess the potential of DAC technology in the long term.

(d)

The greatest advantage of DAC systems is their independence from the location of CO₂ emission sources. Nevertheless, the location of installations is not entirely free from limitations. Key factors include energy availability and the possibility of storing or utilizing captured CO₂. Local climatic conditions also play a significant role. Air temperature and humidity can significantly impact the efficiency and energy consumption of the capture process, depending on the sorbent used. Therefore, the construction of installations should be preceded by a comprehensive location assessment, considering both the selection of appropriate capture technology and the suitable type of sorbent.