Life Cycle Assessment of Direct Air Carbon Capture and Storage with Low-Carbon Energy Sources
Direct air carbon capture and storage (DACCS) is an emerging carbon dioxide removal technology, which has the potential to remove large amounts of CO 2 from the atmosphere. However, DACCS systems have hardly been evaluated regarding their environmental life-cycle performance. We present a comprehensive life cycle assessment (LCA) of different DACCS systems with low-carbon electricity and heat sources required for the CO 2 capture process -both stand-alone and grid-connected system configurations. The results demonstrate negative greenhouse gas (GHG) emissionsi.e. a net removal of CO 2 from the atmosphere -for all eight selected locations and five system layouts. Highest GHG removal potential is found for Norway with a gridcoupled system layout and waste heat usage with a GHG removal efficiency of almost 97%. Autonomous system layouts -entirely supplied by solar energy -prove to be a promising alternative at locations with high solar irradiation to avoid the consumption of fossil fuel based grid electricity and heat: Their GHG removal efficiency (79%-91%) is at a similar -or even higher -level as grid-connected DAC in countries with rather lowcarbon electricity supply and available waste heat, such as Switzerland with a GHG removal efficiency of 87%. The analysis of environmental burdens other than GHG emissions shows some trade-offs associated with CO 2 removal -especially land transformation for system layouts with PV electricity supply -and confirms the need for a comprehensive LCA approach. Further, the sensitivity analysis reveals the importance of selecting appropriate locations for grid-coupled all-electric system layouts, since the deployment of DACCS at geographic locations with CO 2 -intensive grid electricity mixes leads to net GHG emissions instead of GHG removal today. However, a prospective global analysis with the integration of two future energy scenarios shows net-negative GHG emissions for the all-electric system layouts on all continents in 2040. KeywordsLife cycle assessment (LCA), Direct air carbon capture and storage (DACCS), Carbon dioxide removal (CDR), Negative emission technologies (NETs).
Introduction<!>Methodology: Life Cycle Assessment<!>Goal and Scope<!>System Boundaries and Technical Description<!>Autonomous (HTHP + PV)<!>HTHP + Grid<!>Waste heat + Grid<!>Waste heat + PV + Battery<!>Reduced electricity consumption<!>Electrification: HTHP + Grid alternative<!>Future electrification: HTHP + Grid alternative<!>Assessment of Climate Change impact<!>Assessment of other environmental impact categories<!>Reduced electricity consumption<!>Future electrification: HTHP + Grid alternative<!>Discussion -limitations and future work<!>High Temperature Heat Pumps (HTHPs)<!>Assessment of fresnel heat plant in other locations<!>Competition between electricity sources<!>Battery storage in (semi)-arid countries<!>Autonomous system layouts<!>Location-specific limitations<!>Future electrification: REMIND regions<!>Comparison with literature on DACCS<!>Comparison with CDR technologies<!>Conclusion<!>Conflicts of interest
<p>Carbon dioxide removal (CDR) technologies, or negative emission technologies (NETs), are expected to play a crucial role in the decarbonization of the global energy system, with carbon removal potentials of more than 20 gigatonnes year -1 in 2050. 1,2 Prospective energy scenarios, generated by integrated assessment models (IAMs), demonstrate that the 1.5 • C target of the Paris Agreement is likely to be infeasible without the large-scale deployment of CDR technologies, and most IAM scenarios rely on the large-scale deployment of CDR technologies to have a chance of more than 50% to reach the 2.0 • C target. 3,4 Further, CDR technologies are required to a greater extent when climate mitigation is postponed, to compensate for an overshoot of GHG emissions. 5 A wide portfolio of CDR technologies has been proposed, such as the application of biochar, enhanced weathering (EW), ocean fertilisation (OF), bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS). 1,2,6 However, CDR technologies can have substantial environmental side-effects, such as impacts on land, water and/or soil. 4,6 For example, CDR technologies associated with biomass feedstock (e.g. BECCS and biochar) typically result in intensive land use, soil quality changes and water consumption, while potential unintended side-effects of other CDR technologies (e.g. OF) still need to be investigated. 4,6,7 DACCS systems could largely avoid impacts on the water and food security nexus and can be considered as technology ready for small-scale deployment. 4,6,8,9 DACCS systems aim to extract CO 2 from ambient air and permanently store the captured CO 2 in a geological storage medium. 6 Direct air capture (DAC) usually includes two steps, the adsorption (or absorption) step and the desorption (regeneration) step. During the former process, sorbents with strong absorption characteristics are used in a contacting area to bind CO 2 , which is challenging due the extreme dilute concentration of CO 2 in ambient air. 8 The regeneration process aims to regenerate the sorbents and to separate CO 2 . The latter process is energy-intensive due to the requirement of heat at relatively high temperature levels. 8 DACCS systems offer some flexibility compared to other CDR technologies, since they can remove CO 2 independently from point sources, both in time and space. 6,10 Consequently, optimal locations can be selected, considering CO 2 storage potential and local costs for energy supply. 11 DAC systems are usually based either on aqueous solutions with high temperature regeneration (HT DAC), or on solid sorbents with low temperature regeneration (LT DAC). 8 This distinction is established based on the temperature level and the sorbent used in the CO 2 capture process. 8 Few companies currently offer such DAC systems. Carbon Engineering (Canada) implemented HT DAC systems on the North-American market. 8,12 Climeworks (Switzerland) installed pilot LT DAC plants in Europe and realized the first DACCS project with negative CO 2 emissions in Hellisheiði (Iceland) in 2017. 8,13 Global Thermostat recently deployed pilot and demonstration plants based on LT DAC in the United States. 8 Other DACCS initiatives and pilot plants usually apply the LT DAC approach. 14 Generally, not all current DACCS demonstration or pilot plants are designed as DACCSi.e. with permanent storage of captured CO 2 -but rather connect to an industrial carbon utilisation process to reach carbon neutrality.</p><p>As emerging technology with a supposedly decisive role in future low-carbon energy systems, DACCS systems must be thoroughly evaluated regarding their environmental performance in a transparent and consistent way over their entire lifetime. 6 Life cycle assessment (LCA) is a suitable and flexible assessment tool to identify environmental hotspots (i.e. the main contributors to environmental burdens) and to evaluate the total life-cycle environmental performance of a product or service. 15,16 Only few DACCS LCA studies -with mostly limited scopes -have been conducted so far.</p><p>De Jonge et al. 17 assessed the life-cycle carbon removal efficiency of a HT DAC system and determined the main environmental contributors to overall LCA scores. The contribution analysis revealed a high environmental impact due to energy needed for the CO 2 capture process. A recent DACCS LCA study conducted by Deutz and Bardow 18 showed that two commercial LT DAC(CS) plants -in Hinwil (Switzerland) and Hellisheiði (Iceland)   produced and operated by Climeworks -achieved GHG removal efficiencies of 85% and 93%, respectively. The latter study also determined the environmental impacts of six different adsorbents and concluded that climate benefits are mainly influenced by energy sources used for CO 2 capture.</p><p>To the best of our knowledge, no comprehensive LCA of DACCS has been published in the scientific literature. We acknowledge that the study of Deutz and Bardow 18 is comprehensive for the DAC system and the associated supply chains, but lacks a detailed assessment of the CO 2 storage stage, since their work only considered electricity demand for CO 2 injection. However, the CO 2 storage stage includes for example environmental impacts from infrastructure (e.g. pipelines, injection wells and the compression station), the drilling of injection wells and CO 2 leakage during the transportation of CO 2 . 19,20 Besides, the latter study excludes energy storage when intermittent (renewable) energy sources are integrated and claim that their energy system layouts can be installed at remote locations, while we believe that appropriate energy storage units need to be considered when (renewable) intermittent energy sources are used for the CO 2 capture process -which inevitably leads to additional environmental impacts. Further, the DACCS study of de Jonge et al. 17 only focused on the life cycle carbon efficiency, hence excluded other potentially important environmental impacts. In addition, the latter study reported limitations regarding the quality of their life cycle inventory (LCI) data for the DAC infrastructure.</p><p>Other available LCA studies mainly focus on DAC, thereby excluding the carbon storage stage required for permanent CO 2 removal from the atmosphere. In addition, these LCAs are simplified regarding LCA modelling choices, such as the exclusion of life-cycle phases and of environmental impact categories besides climate change. 6 Further, some studies do not consider a certain amount of CO 2 equivalents removed from the atmosphere as functional unit, which impedes the comparison of different CDR technologies. 6 Essentially, a comprehensive LCA on the entire DACCS supply chain, which assesses multiple environmental impact categories, uses an appropriate functional unit, is transparent in the methodology used, thoroughly assesses all life-cycle stages and examines a wide set of energy sources for CO 2 capture including autonomous system designs, is missing.</p><p>In this context, the contributions of this paper can be summarized as follows:</p><p>• We present a detailed and transparent LCA of a LT DACCS system -based on Clime-works' technology and verified with available data on LT DAC -with different electricity (i.e. grid and photovoltaics (PV) power) and heat sources (i.e. electricity, waste and solar heat) for CO 2 capture.</p><p>• Different innovative and autonomous system layouts are included, namely the integration of high temperature heat pumps (HTHPs), the integration of a fresnel solar heat plant at locations with high solar irradiation and system layouts with electricity and heat storage.</p><p>• We include several processes needed for CO 2 storage -parameterized on transportation distance, geographical storage location and storage depth: energy needs for compression, infrastructure requirements (e.g. pipelines and compression stations), drilling of boreholes, country-specific electricity for the injection of CO 2 , and CO 2 leakage resulting from the transportation in pipelines.</p><p>• We address land transformation in detail: while grid-connected DACCS systems could exhibit low land requirements compared to other CDR technologies, 6 direct solar electricity and heat supply for the DAC unit might result in substantial land transformation. 4,14 • A global analysis is included for all-electric grid connected DACCS systems.</p><p>• We integrate two future IAM scenarios [21][22][23] and modify the ecoinvent background LCA database to determine the future global carbon removal potential of grid connected DACCS systems.</p><p>The structure of the paper is as follows. Section 2 describes the methodology, where LCA modelling steps and different DACCS system layouts are discussed. Section 3 presents the LCA results, discussions and limitations. Finally, conclusions are drawn in Section 4.</p><!><p>LCA is a methodology which aims to quantify environmental impacts of a product or service over its entire life-cycle. 15,16 LCA is standardized by the international organization for standardisation (ISO), where ISO 14040 describes the general principles and framework of LCA 24 and ISO 14044 presents guidelines and general practices for LCA. 25</p><!><p>Our goal is to quantify the environmental impacts and to determine environmental hotspots (i.e. main drivers and processes determining the environmental performance) along the DACCS life-cycle considering low-carbon energy sources for CO 2 capture in different DACCS system layouts. Our work focuses on innovative low-carbon energy sources and electrification of DACCS systems for the following reasons. Previous analysis of DAC(CS) demonstrated that impacts on climate change mainly depend on the carbon-intensity of electricity and heat sources. 17,18,26 However, the overall environmental impact of the integration of low carbon energy sources -using a wide set of environmental impact categories -has not been well examined until now. Further, we expect that the penetration of low carbon energy sources and the electrification of energy systems will expand further in the future. 22 And finally, site dependent boundary conditions -such as the lack of waste heat, geological CO 2 storage sites or low-carbon electricity from the grid -might require novel system designs including heat and electricity storage allowing for autonomous DAC operation in proximity of geological CO 2 storage sites.</p><p>In our analysis, we consider the following countries, which exhibit different climates: Greece, Mexico, Jordan, Spain and Chile are included as (semi)-arid countries with high annual solar irradiation. Besides, cooler and temperate climate regions are covered with Norway, Iceland and Switzerland. We have selected these countries based on their geological storage potentials, 27 difference in grid electricity mix, 28 climate variations and data availability for the fresnel solar collector. A comprehensive argumentation for the selection of these countries is provided in the supplementary information (SI) Note S2.</p><p>Our functional unit is defined as 'Gross removal of 1 ton CO 2 from the atmosphere via the use of a DAC plant combined with geological CO 2 storage', with a reference flow of a DAC unit removing 100 kt CO 2 year -1 with varying system layouts and electricity and heat inputs as specified in the individual scenarios (see Section 2.2.1). Consequently, total GHG emissions produced from all upstream and downstream DACCS activities should be less than 1 ton CO 2 -eq. to result in net negative GHG emissions, i.e. net CO 2 removal from the atmosphere.</p><p>We identify no multi-functionality of our DACCS system, since the main purpose is to remove CO 2 from the atmosphere in a permanent way. Hence, allocation or system expansion is not required. 29 We use the ILCD 2.0 (2018) life cycle impact assessment (LCIA) method 30 to assess the environmental performance of the proposed DACCS system layouts. We adopt 16 midpoint categories from ILCD in the protection areas climate change, ecosystem quality, human health, and resources. Further, we add one additional impact category to capture water consumption with the Water Depletion impact category of the ReCiPe 2016 LCIA methodology (1.1 (20180117)). 31,32 Finally, we are interested in the total amount of land transformed associated with different DACCS configurations, since we include different PV-coupled DACCS configurations which could be land-intensive. 4,14 Therefore, we aggregate all life cycle inventory flows associated with land transformation (in m 2 ), since the considered environmental impact category 'land use' of the ILCD 2.0 LCIA method represents land use impacts in terms of points and as such does not lead to meaningful land transformation surface areas.</p><p>Results are shown for impacts on climate change in the main text, while the complete set of results is shown in SI Note S6.</p><p>An attributional LCA perspective with the ecoinvent database (v3.6, system model Allocation, cut-off by classification 33,34 ) as source of background inventories is applied. The open-source Python package Brightway2 is used to conduct our LCA. 35 Our LCIs and corresponding assumptions will be discussed in Section 2.2 and Section 2.2.1. In addition to the analysis for the specific countries listed above, a sensitivity analysis shows GHG emissions of an all-electric system layout for 144 countries on a world map, see Section 3.3.2.</p><!><p>We evaluate five different DAC configurations with specific supply of heat and electricity for CO 2 capture:</p><p>(1) Electricity from the grid, heat from a waste heat source.</p><p>(2) Electricity from the grid, heat from a HTHP operated with electricity from the grid.</p><p>(3) Electricity from a PV installation, heat from a fresnel solar-thermal heat collector.</p><p>(4) Electricity from a PV installation, heat from a HTHP operated with electricity from the PV installation.</p><p>(5) Electricity from a PV installation, heat from a waste heat source.</p><p>These configurations are detailed in subsequent sections. A simple representation of our system boundaries is shown in Fig. 1. The DACCS product system includes -depending on the specific system configuration -the production and transportation of system components, energy generators and storage units, such as the DAC plant, a fresnel heat plant, PV systems, heat storage tanks and batteries as well as transport and injection of CO 2 and business flights (the so-called "foreground processes"). Detailed assumptions regarding the infrastructure and dismantling of the DAC plant (based on Climeworks' technology), sorbent used for CO 2 capture, business trips as well geological storage of CO 2 are provided in the SI Note S5.</p><p>Energy requirements for CO 2 capture are estimated on 500 kWh t -1 CO2 for electricity (without electricity consumption for CO 2 compression) and 1500 kWh t -1 CO2 for heat (at around 100 • C). 18 Earlier DAC(CS) LCAs showed that energy consumption for the CO 2 capture can be perceived as the crucial process in terms of environmental impacts. 17,18,26 Therefore, we evaluate system layouts with different energy sources for the CO 2 capture process. We focus on solar energy, waste heat and all-electric system layouts for the following reasons. First, solar energy is one of the fastest growing renewable energy sources with large potential for further expansion and comparatively low costs and can be used for both heat and electricity production. 36 Second, waste heat -if available -can be considered as the optimal heat source due to its low cost and the fact that it comes (almost) burden-free in terms of environmental impacts. 8,34 Third, further low-carbon electrification of future energy systems can be expected. 22 Large-scale and economically attractive implementation of DACCS might, however, require remote installations close to proper sites for geological CO 2 storage. Therefore, we introduce two autonomous off-grid system layouts entirely based on solar energy. Besides, three grid-coupled alternatives are considered. DAC operators generally aim for renewablesbased energy sources and low-carbon DAC operation, so that exclusively low-carbon grid electricity is an option for them. An overview of the different system layouts (system components, capacities, lifetimes) is given in SI Note S3. The common system lifetime is indicated as 20 years -which is the lifetime of the DACCS unit. In case of a longer lifetime of a system component, we assign a proportional fraction of the inventory of the system component to the DACCS system by dividing the common system lifetime with the lifetime of the system component. An overview of the main parameters used in the DACCS system configurations in provided in Table 1. The Autonomous (Fresnel + PV) system layout is supposed to allow for an autonomous off-grid DAC system operation entirely based on solar energy. However, solar energy is intermittent, resulting in fluctuations in power and heat output of the PV and fresnel units. 37 These fluctuations are mitigated by two storage media: heat storage tanks and battery electricity storage. This system design enables an assessment based on the common functional unit with the same goal, i.e. to capture 100 kt CO 2 annually from the ambient air. Less CO 2 would be captured without a storage medium when the same DAC capacity is installed, since fluctuating electricity and heat supply would not allow for continuous DAC operation.</p><p>Solar heat can be generated with fresnel solar collectors when sufficient solar irradiation is present. 38 Steam temperatures up to 400 • C can be achieved with fresnel solar collectors, which makes fresnel solar heat an appropriate heat source for industrial applications as well as for DAC systems. 39 For the desorption of CO 2 heat at a temperature of 100 • C is required. 8 LCI of the fresnel solar collector has been generated in collaboration with Industrial Solar (Freiburg, Germany), based on an existing fresnel plant in Jordan. Detailed information and LCI of the fresnel system is provided in SI Note S4.</p><p>Site-specific annual solar irradiation is a key factor for the design and heat output of fresnel units and therefore, location specific plant designs are required. 38 We use data received from Industrial Solar for eight locations in five countries with a direct normal irradiance of more than 2000 kWh/m 2 year -1 , which were comprehensively modelled regarding their techno-economic performance. The sites considered are located in Chile (Antofagasta), Greece (Creta), Jordan (Amman), Mexico (San luis Potosí) and Spain (Tabernas).</p><p>Electricity is supplied by large scale ground mounted PV arrays. Therefore, countryspecific LCI datasets are generated and used to represent multi-Si PV modules (see SI Note S5). Further, we use a stationary battery system to store excess PV electricity during daytime in order to be consumed during night-time. We include a lithium Nickel-Manganese-Cobalt oxide (NMC) battery, representing mainstream technology for stationary electricity storage today. 40 We assume that the NMC battery is able to store 12 hours of the electricity load to provide sufficient electricity during night for CO 2 capture and compression, in line with the heat storage sizing and the aim to capture 100 kt CO 2 year -1 . The minimum battery storage capacity requirement (C bat,req ) is calculated to capture 100 kt CO 2 year -1 considering 12 hours of storage. We oversize the NMC battery to consider battery degradation. 41 Hence, the battery capacity (C bat ) is determined by considering the Depth of Discharge (DoD), the discharge efficiency (η dis ) and the percentage of the original storage capacity left, required at the end of its lifetime (EoL). We use a DoD of 93%, a discharge efficiency of 94.3% and an EoL capacity percentage of 80%. 37 Equation 1 is used to size the energy capacity of the NMC battery. 37</p><p>Besides, we assume a C-rate of 0.5 C (i.e. two hours for battery charge or discharge) as most appropriate, since the NMC battery is installed in (semi)-arid locations with high PV power peaks. Consequently, a high power capacity could be beneficial to charge during PV power peaks in order to avoid curtailment of PV electricity. Further, we consider additional electricity needed for system layouts with battery deployment to compensate for the Roundtrip Efficiency (RE) related losses of the battery. Hence, the required PV electricity is divided by the RE (i.e. 89%) of the NMC battery to function as a safety factor. 37 The latter assumption can be perceived as a worst case scenario in order to be conservative, assuming that all produced PV electricity will go through the NMC battery to be used for electricity for the DAC plant.</p><!><p>The Autonomous (HTHP + PV) system layout is an all-electric off-grid system entirely supplied by PV electricity, including a HTHP to deliver high temperature heat for the CO 2 capture process. Hence, the difference with our previous system layouts is the replacement of solar heat with heat produced by a HTHP. Further, an NMC battery is used to store PV electricity during the night; the storage capacity of the battery is calculated using equation 1.</p><p>Note that the battery storage capacity is larger compared to Autonomous (Fresnel + PV), due to the larger electricity requirement for this all-electric system layout. We assume a coefficient of performance (CoP) of 2.9 for the HTHP, which is conservative and at the lower range of presented CoPs of HTHPs.</p><!><p>The all-electric HTHP + Grid system layout contains a HTHP connected to the electricity grid. The same assumptions for the HTHP are used as in the previous system layout. Other energy sources are not required, since grid electricity is available in all selected countries.</p><p>Consequently, the environmental impact of energy consumption (predominantly) depends on the national grid electricity mix and the performance of the HTHP. Country-specific LCI datasets of the ecoinvent database are used for grid electricity. 34 We conduct a sensitivity analysis for future electricity mixes, see Section 2.3.2.</p><!><p>The Waste heat + Grid system layout consumes (industrial) waste heat and is connected to the electricity grid. Note that waste heat comes (almost) burden-free in the Allocation, cut-off by classification system model of ecoinvent. 34 The Waste heat + Grid system layout is only applicable when waste heat at the correct temperature level is available for example generated from industrial processes. Therefore, a location specific assessment is required to identify the potential of waste heat. For simplicity, we decided to include all 8 countries with country-specific LCI from the ecoinvent database for waste heat and grid electricity. 34</p><!><p>The Waste heat + PV + Battery system layout consumes PV electricity and waste heat as energy sources for CO 2 capture. For waste heat, the same assumptions are used as in the previous system layout. Assumptions for the provision of PV electricity and battery storage are adopted from the Autonomous (Fresnel + PV) layout. Complete LCIs of all system layouts are provided in SI Note S5.</p><!><p>DACCS is an emerging technology and will profit from technological improvements. 8 This could result in a reduction of energy consumption during the CO 2 capture process. Current figures used for energy consumption (500 kWh t -1 CO2 electricity and 1500 kWh t -1 CO2 heat 18 ) are based on a very high CO 2 purity in the resulting CO 2 stream. 8 However, a lower CO 2 purity seems to be compatible with carbon capture and storage (contrary to carbon capture and utilization applications), which might result in lower energy requirements. Therefore, we examine the performance of the proposed system layouts with an electricity consumption of the CO 2 capture process reduced by 20%.</p><!><p>The all-electric HTHP + Grid system layout is further examined on a global scale to determine GHG emissions (i.e. Climate Change) of DACCS in 144 countries. We specifically focus on this all-electric system layout in our geographical sensitivity analysis, since energy system models predict an increase of low-carbon electrification in future energy systems. 22 Besides, this system layout is the simplest and (theoretically) could be implemented in all locations with a (low-carbon) grid connection providing sufficient electricity, as grid electricity is the only energy source needed. Climeworks has committed -and probably also other DAC suppliers -to only offer carbon dioxide removal services with grid-coupled DAC systems at geographical locations with low GHG intensive grid electricity.</p><p>For simplicity, we exclude environmental impacts for transportation and business trips in this sensitivity analysis, since GHG emissions from transportation processes are small according to our results. Further, we use average electricity supply (i.e. 'market group for electricity, ..' ) for countries which are modelled with multiple regional electricity datasets  in the ecoinvent database. In case there is no country-specific electricity dataset available, we use the market (group) for electricity for the geographical area in which this country is located as approximation. Further, we assume a generic transportation distance for CO 2 of 500 kilometers to the injection wells and a generic storage depth of 2000 meters.</p><!><p>Projections of IAMs show that electricity grid mixes will become less CO 2 -intensive, even in the most carbon intensive energy scenarios. 21,22 Therefore, we examine GHG emissions of the HTHP + Grid system layout based on future grid electricity mixes for 2040. To achieve this, we use these future grid mixes for electricity supply for the DAC unit by modifying the ecoinvent background database with the rmnd-lca Python package, 44 adapting future electricity mixes in our background database, based on the output figures of the REMIND model scenarios. 21 Note that this results in geographically aggregated future electricity datasets, since the REMIND model subdivides the world into only 11 regions.</p><p>Firstly, we use the SSP2-Base energy scenario to determine future GHG emissions of the HTHP + Grid system layout. The SSP2-Base scenario is a scenario with no additional climate policy. 21,44 Secondly, we include a more ambitious future climate policy with the SSP2-PkBudg1300 energy scenario, which corresponds to a maximum average temperature increase of 2 • C. 44 3 Results and discussion</p><!><p>The outcome of the assessment of the "Impacts on Climate Change" of DACCS technologies for the different system layouts and countries considered is illustrated in Fig. 2  (XURSH>(1762(@ :RUVWFDVHUHQHZDEOHV Figure 3: Effect of the GHG-intensity of electricity used -needed for capture and compression of CO 2 -for all alternatives on the overall GHG emissions of DACCS systems. We use the LCI of Norway as proxy for HTHP + electricity and Waste heat + electricity. Further, LCI of Spain is used for the following configurations: Waste heat + intermittent electricity + Battery, Autonomous (Fresnel + electricity) and Autonomous (HTHP + intermittent electricity). Note that the Autonomous (Fresnel + intermittent electricity) alternative can only be installed in (semi)-arid locations due to the requirement of high annual solar irradiation. Besides, the waste heat alternatives can only be installed with sufficient supply of waste heat. The DAC configurations operated with intermittent electricity are shown up to a GHG intensity of electricity supply of 0.15 kg CO 2 -eq./kWh, which represents a worst case for renewables. The countries and specific power generation technologies on the x-axis represent the GHG-intensities of national electricity grids and technologies according to ecoinvent, see SI Note S9. 34 Note that for all cases -considered in Fig. 2 -the climate change impact is smaller than the amount of CO 2 removed and stored, i.e. negative emissions are indeed generated in all cases. However, the range of carbon removal efficiencies is large, going from only 9% all the way to 97%. It is worth noting that carbon removal efficiency has a strong impact on unitary costs per net CO 2 removed. If we assumed that the two value chains leading to 9% and to 97% removal costed the same per ton CO 2 removal, say 100 monetary units, then the unitary cost per net CO 2 removed would be 1,111 and 103 monetary units, respectively. Besides, the carbon removal efficiency can be strongly dependent on the GHG footprint of electricity generation, as illustrated in Fig. 3. The main findings can be summarized as follows.</p><p>Energy supply -heat and electricity used for CO 2 capture -is the key factor driving GHG emissions of DACCS configurations. Waste heat is environmentally attractive as heat supplier when available, as waste heat comes (almost) burden-free in the Allocation, cut-off by classification system model. GHG emissions generated from energy consumption can be reduced by using renewable energy: in our analysis solar energy in countries with high annual solar irradiation.</p><p>The GHG-intensity of the grid electricity mix is a crucial factor for grid-coupled DACCS configurations and is very country-specific. Consequently, countries with low GHG intensive electricity mixes in combination with waste heat exhibit lowest GHG emissions, with GHG removal efficiencies of almost 97% and 94% for Norway and Iceland, respectively. On the contrary, high GHG emissions are generated -almost approaching net-positive GHG emissions in Greece with 0.91 ton CO 2 -eq. per ton of gross CO 2 removal -for grid-coupled DACCS configurations when the national electricity grid relies on a large share of fossil fuels. Fig. 3 shows that the break-even point -in terms of net-negative GHG emissions for the grid-coupled HTHP DACCS configuration (HTHP + Grid ) -is on ∼0.87 kg CO 2 -eq. kWh -1 electricity used for the capture and compression of CO 2 .</p><p>In absolute terms, minor GHG emissions are generated from the DAC construction (6 kg CO 2 -eq. per ton of gross CO 2 removal) and sorbent consumption (10 kg per ton of gross CO 2 removal). However, these components can be substantial in relative terms, if energy supply for CO 2 capture is clean in terms of GHG emissions. The relative Climate Change impact of transportation processes directly related to DAC (i.e. business and freight transport, for the collectors and process units) can be considered small, with contributions of less than 1%.</p><p>GHG emissions from the storage and transport of CO 2 are significant -and their relative contribution fluctuate between 11-52% -mainly driven by electricity requirements for the compression, recompression and injection of CO 2 as well as CO 2 leakage during long distance CO 2 transport, respectively. Note that we included the Medium scenario presented in Holloway et al. 20 for CO 2 leakage during transportation. The contribution of CO 2 leakage Figure 4: Spider graphs illustrating environmental trade-offs for a selection of DACCS configurations on nine environmental impact categories with a focus on Greece and all-electric DACCS configurations, please refer to the main body of the text for an explanation of this selection. Complete spider graphs considering all DACCS system configurations and environmental impact categories are provided in SI Note S6.</p><p>during the transportation of CO 2 in pipelines would increase by a factor of 10 if we used their Pessimistic scenario for CO 2 leakage, which emphasizes the need for a well-designed pipeline network for CO 2 transportation. DACCS-related GHG emissions in countries with a CO 2 -intensive grid electricity mix can be substantially reduced by shifting to renewable energy sources as energy suppliers and can be installed as (nearly) autonomous DACCS configurations. The GHG removal efficiencies of autonomous system layouts -entirely supplied by solar energy -are in between 79-91% and are therefore at a similar -or even higher -level as grid-connected DAC in countries with rather low-carbon electricity supply and available waste heat, such as Switzerland with a GHG removal efficiency of 87%.</p><p>Autonomous system layouts with a fresnel solar collector exhibit slightly higher GHG removal efficiencies (84-91% for Autonomous (HTHP + electricity)) in comparison with autonomous DACCS configurations entirely based on PV arrays in combination with a HTHP (79-89% for Autonomous (HTHP + electricity)), mainly due to the provision of lower carbon heat with a fresnel solar collector and the requirement of a smaller battery (∼125 MWh vs. ∼221 MWh). A bigger battery is required for autonomous configurations with a HTHP as electricity needs to be stored to provide sufficient electricity to the HTHP during night.</p><p>Further, the production of heat storage tanks (made of low-alloyed steel) causes lower GHG emissions compared to the production of bigger battery energy storage systems, which means lower GHG emissions for DACCS configurations with fresnel heat.</p><p>The production of electricity storage (NMC battery) mediums can -in relative termshave a significant impact on GHG emissions, from 10% to 24%, while that of the production of the heat storage (steel tanks) units is small with a relative contribution of less than 1%.</p><p>Consequently, GHG emissions from the infrastructure of autonomous systems are comparably higher (see the lower-left part of Fig. 3), due to the impact of the production of energy storage units, such as battery energy storage systems. Fig. 3 illustrates how alternatives with waste heat consumption, and the autonomous system layout coupled with fresnel heat (Autonomous (HTHP + electricity)), are less sensitive when using high GHG intensive electricity, due to less electricity consumption required for CO 2 capture. For example, the autonomous DACCS configuration with fresnel heat generates solar heat and therefore less electricity is required for CO 2 capture, although this alternative can only be installed at locations with sufficient annual solar irradiation.</p><!><p>Fig. 4 illustrates the overall environmental burdens and trade-offs for a selection of DACCS configurations and environmental impact categories. The spider graphs show the normalized scores (scores from 0 to 1, visualized with the colored line) of DACCS configurations on nine environmental impact categories, normalized to the system configuration with the highest environmental impact among all configurations (i.e. highest score = 1, presented in SI Note S6) on a specific environmental impact category. A greater colored area therefore demonstrates an overall higher environmental burden for a specific DACCS configuration.</p><p>Environmental impact categories -presented in Fig. 4 -are selected based on recommendations by Fazio et al. 30 We only show impact categories of recommendation level I and II in the main article and add land transformation, since these are important for CDR technologies in general. 6 Greece is included as a reference and worst-case scenario regarding GHG emissions. All-electric DACCS system configurationsi.e. HTHP + grid electricity -exhibit a large sensitivity regarding overall environmental impacts between countries.</p><p>We therefore include three other countries with cleaner electricity mixes in terms of GHG emissions -Switzerland, Norway and Iceland -since Climeworks has committed to only offer carbon dioxide removal services with grid-coupled DAC systems at geographical locations with low GHG intensive grid electricity. The complete set of spider graphs and results for all DACCS configurations and environmental impact categories is provided in SI Note S6.</p><p>In general, lowest overall environmental burdens can be achieved with DACCS configurations with waste heat in combination with renewable electricity supply, for example in Norway and Iceland. Fundamental differences between environmental impact categories and countries are found for grid-coupled DACCS configurations in combination with a HTHP. This is illustrated in the second row of Fig. 4. Low overall environmental burdens can also be achieved with grid-coupled DACCS configurations -in combination with a HTHP (Autonomous (HTHP + electricity)) -when the national grid electricity mix is based on a large-share of renewables as for example in Norway and Iceland, which both rely on a high share of hydropower. Switzerland has slightly higher overall environmental burdens compared to Norway and Iceland, mainly due to a higher reliance on nuclear power and fossil fuels, respectively. On the contrary, substantially higher environmental burdens are generated from gridcoupled DACCS configurations when the national grid electricity mix is based on a large share of fossil fuels as for example in Greece (see Fig. 4). This generally results in high environmental burdens on almost all environmental impact categories.</p><p>Autonomous DACCS configurations generate low environmental burdens on almost all environmental impact categories, although several environmental trade-offs occur. The most important one concerns land use, which can be considered as the main "outlier" among all the impact categories: Autonomous DACCS layouts with solar energy supply perform in general worse than grid-connected systems on land transformation (see the first 2 spider graphs in the top row of Fig. 4). Fig. E -in SI Note S7 -presents a detailed contribution analysis on land transformation (in m 2 ), and reveals that autonomous energy systems exhibit large land transformations (up to 1.87 m 2 per ton of gross CO 2 removal), mainly due to large surface area requirements for ground mounted PV panels and the fresnel heat collector, while the DAC unit as such exhibits very low land transformation impacts (a life-cycle land transformation of 0.01 per ton of gross CO 2 removal). In addition, autonomous energy systems exhibit comparatively high environmental scores on the Minerals and Metals impact category (see SI Note S6), mainly resulting from material requirements for the production of PV arrays and the NMC battery. Note that this impact category is associated with substantial uncertainties and also methodological shortcomings, and is therefore assigned with recommendation level III. 30 These examples demonstrate the importance to assess and compare system layouts on a wide set of environmental impact categories to examine environmental trade-offs.    removal. Countries which fulfill this requirement are shaded in green and orange (i.e. orange meaning higher GHG emissions than green). Net positive GHG emissions are shown in dark red.</p><!><p>Significant variations between countries can (mainly) be explained by the difference of the CO 2 -intensity of national grid electricity mixes. For the HTHP + Grid system layout, the GHG break-even point is reached with a grid electricity mix GHG intensity of ∼0.87 kg CO 2 -eq. kWh -1 electricity (see also Fig. 3 for the break-even point) -which means that grid electricity mixes with a lower GHG intensity than the GHG break-even point exhibits GHG removal from the atmosphere. It turns out that most countries in Europe, North-America, South-America and central Africa already show large GHG removal potentials with DACCS HTHP + Grid system layouts today. However, few countries in these continents exhibit net-positive GHG emissions. In general, Australia and countries in Asia, Southern Africa and Northern Africa are nowadays not (or less) suitable to install DACCS HTHP + Grid systems.</p><!><p>Fig. 6 demonstrates the possible future impact (in the year 2040) regarding the Climate Change impact category for the HTHP + Grid system layout according to the SSP2-Base scenario of the REMIND model, 21,23 which extrapolates observed trends, but is not particularly ambitious in terms of climate policy. It turns out that the deployment of HTHP + Grid DACCS system layouts could result in net-negative GHG emissions in all world regions in 2040. North-America, South America, Europe, Australia and Africa exhibit a large net GHG removal potential, while India and Asia seem to be less suitable due to their still relatively high CO 2 -intensive grid electricity mix.</p><p>When implementing a more ambitious climate scenario (i.e. SSP2-PkBudg1300 ) DACCS in all regions exhibits GHG emissions of less than 230 kg CO 2 -eq. per ton of gross CO 2 removal in 2040. Hence, all regions would be green if the world map for this case were drawn using the same color codes as used for Figures 5 and 6. Therefore, effective climate policy would be very beneficial for all-electric DACCS system layouts connected to the grid. Note that in these two prospective evaluations we not only use future regional electricity mixes for electricity supply to the DACCS unit, but also populate the scenario-specific development of the electricity sectors throughout the entire background LCA database. This procedure results, for example, in a reduction of GHG emissions due to PV electricity, since the electricity used for PV wafer production exhibits a scenario-specific GHG intensity lower than today's.</p><!><p>Our analysis shows that within the LCA of DACCS systems, there are few elements and key factors, which determine the results: first of all, energy required for DAC operation and the sources of supply. While we consider data availability and quality regarding energy demand of and supply for, respectively, the CO 2 capture process -the most crucial factor by far driving LCA results -to be high, there are other issues, which call for further analysis in the future. More sophisticated modeling integrating operational experience of DAC units of different scale and at different locations to be gained in the future will allow for a more precise representation of the environmental burdens due to DACCS. We discuss the most important of these elements in the following subsections.</p><!><p>We included a HTHP in different DACCS system layouts. Currently, no complete LCA of HTHP has been conducted. We received information regarding potential future refrigerants used in HTHPs from MAN Energy Solutions (Zürich, Switzerland), but had to establish most of our LCI as an extrapolation of a low-temperature HP. Future analysis should consider the utilization of different types of refrigerants, since refrigerants usually exhibit large contributions to environmental burdens caused by heat pumps. 42,45</p><!><p>Only five potential locations for the fresnel solar collector were included in the analysis due to the site-specific performance of fresnel solar collectors and limitations on data for further locations. The fresnel performances at these locations in semi-arid climate regions were thoroughly modelled by Industrial Solar (Freiburg, Germany). Since the integration of fresnel solar heat systems turned out to be beneficial from the environmental perspective, especially for autonomous DACCS systems, site-specific assessments and modelling should be expanded to determine the environmental merits in other geographical locations.</p><!><p>Future energy systems tend to show higher reliance on electricity than current energy systems. Additional electricity will be required by the mobility and heat sector, possibly on top of increased power demand due to population growth and higher electrification rates.</p><p>Increasing the pressure on the power sector through the installation of grid-coupled DACCS systems competes with other demands of electricity. This was not a topic of our research, but has to be considered.</p><!><p>We proposed to install NMC batteries in (semi)-arid countries (i.e. Chile, Spain, Greece, Jordan and Mexico) with high ambient air temperatures. Temperature levels within these countries could easily reach more than 40 • C during warm periods. The acceptable temperature range for lithium-ion batteries is between -20 • C and 60 • C, with an optimal operation temperature between 15 • C and 35 • C. 46 Therefore, location-specific measures should be considered when outside temperatures approach critical temperature levels to avoid battery damage. These measures could result in additional energy and/or material requirements, i.e. higher environmental impacts.</p><!><p>Two autonomous system layouts -entirely based on solar energy supply -were included:</p><p>Autonomous (Fresnel + PV), and Autonomous (HTHP + PV). Twelve hours of energy storage capacity was assumed for storage mediums for these two system layouts. This led to a large storage capacity needed for both the battery system (∼125 MWh and ∼221 MWh, respectively) and the heat storage tanks.</p><p>Alternatively, a doubling of the DAC capacity (in order to comply with our functional unit) with discontinued operation over night could be installed to reduce the need for energy storage in the proposed two autonomous system layouts. However, we emphasize that such a system with doubled DAC capacity would still require energy storage for intermittent (renewable) electricity generation, which would result in additional environmental impacts from the production of storage mediums. Further, a doubling of the capacity of the DAC plant would incur large additional capital expenditures for the DAC plant, since capital expenditures of DAC systems are still high. 8 Therefore, we argue that such a DAC system is currently unrealistic from an economic and technological point of view -and further research is required for a complete assessment of the operation of such an autonomous DAC system. Further, more sophisticated research is needed to confirm the self-sufficiencye.g. the 12 hours of energy storage -of the proposed autonomous system layouts. For the Autonomous HTHP + PV, the environmental and economic performance can possibly be (slightly) improved with the installation of a heat storage medium to reduce the size of the battery. In this way, heat supply -needed for the CO 2 capture process -can be stored in a heat storage medium instead of electricity storage in a battery.</p><p>In addition, autonomous system designs with DAC units entirely powered by solar energy require site-specific assessments in terms of land use. The area covered by PV arrays required for electricity supply for a DAC unit removing 100 kt CO 2 year -1 in our most land-intensive case and location (i.e. Autonomous HTHP + PV in Mexico) amounts to a direct land transformation (for the DAC unit and PV panels considering their lifetimes) of almost 4.7 km 2 . Such a land surface area will not always be available. The scale up of solar PV-coupled DACCS to the gigatonne scale -for example removing 1 Gt CO 2 year -1 from the atmosphere with a GHG removal efficiency of 79% (representing worst case Autonomous HTHP + PV for Mexico) -would therefore require a direct land transformation area as large as ∼59,000 km 2 ; which is almost 1.5 times the total land surface area of Switzerland.</p><p>Land surface area requirements can be significantly reduced by for example using grid electricity sources, although this would result in indirect land use transformations mainly generated from electricity generators. Note that the land transformation for PV panels includes area requirements for the open ground power plant including the space in between the modules as well as some surrounding area, 34 and can therefore be considered as conservative.</p><p>Further, the land between and underneath the PV modules may be used for grazing of small livestock such as sheep or poultry, bee-keeping, or other innovative agricultural uses. Such land use options should already be considered during the design phase of the solar field.</p><!><p>Location-specific assessments are required to identify the availability of waste heat from industrial applications. For example, waste heat could be inaccessible in remote locations, which would limit the design options. Furthermore, decarbonization of the energy system with reduced combustion of fossil energy carriers might result in a reduction of waste heat sources in general, and/or at the required temperature level. However, grid-coupled and autonomous DACCS energy systems provide -in environmental terms -a good alternative for waste heat in these situations.</p><p>Another site-specific boundary condition of DACCS is the availability of geological storage sites. We therefore examined different transportation distances from CO 2 capture to the injection wells, although the transportation distance of CO 2 can be minimized -to avoid CO 2 leakages from the transportation of CO 2 -by installing the DAC facility close to geological storage sites. However, the large scale deployment of DACCS requires large geological storage potentials and therefore CO 2 transportation might be required. This could result in competition for geological storage sites between DACCS and for example carbon capture and storage from conventional power plants, which could limit the large-scale implementation of DACCS.</p><!><p>We used the outputs of the IAM REMIND 21 for quantification of future GHG-intensity of electricity supply and to modify our background LCA database to assess the future performance of all-electric DACCS systems. However, the geographical resolution of the REMIND model is limited to 11 world regions. Therefore, the future environmental potential of the electrified system layouts had to be aggregated to those regions, while our results for current systems showed that regional differences can be significant in terms of GHG emissions (see Fig. 5). Further geographical disaggregation would be beneficial for prospective LCA of DACCS, especially for all-electric system layouts. However, our findings demonstrate very well how effective climate policy could improve the environmental performance of all-electric DACCS systems. On the other hand, the decarbonization of the power supply system could lead to additional environmental impacts on other life-cycle environmental impact categories.</p><p>Luderer et al. 47 investigated benefits and side-effects of the decarbonization of power supply.</p><!><p>The single purpose of DACCS systems is to remove CO 2 from the atmosphere in a permanent way. Our analysis focused on LT DACCS systems and must not be considered as representative for HT DACCS systems, due to the need of different processes for CO 2 capture and their associated energy requirements. 8,48 One recent DACCS LCA study of Deutz and Bardow 18 examined the same LT DAC technology of Climeworks although focused on a smaller DAC plant (with a capture capacity of 4 kt CO 2 year -1 ), and therefore provided less detailed results for the larger DAC plant (with a capture capacity of 100 kt CO 2 year -1 ). Their GHG removal efficiencies are generally slightly higher compared to the reported GHG removal efficiencies in this work. For example, Deutz and Bardow 18 found a carbon removal efficiency of 88.8% for Switzerland -consuming the same amount of grid electricity and waste heat as energy sources for CO 2 capture as our analysis -for the 4 kt CO 2 year -1 DAC facility. The CO 2 removal efficiency can be expected to be higher for the 100 kt CO 2 year -1 DAC facility, due to reductions of sorbent consumption as well as the optimization of the DAC infrastructure (see Section 2.2 and SI Note S1). Reported GHG emissions of the carbon storage phase turned out to have a minor contribution with less than 2% on total GHG removal efficiencies for DACCS configurations with GHG removal efficiencies higher than 86%.</p><p>On the contrary, our results show a lower GHG removal efficiency of 86.8% for the waste heat and grid electricity configuration (Waste heat + Grid ) in Switzerland; (even) for a DAC plant with a capacity of 100 kt CO 2 year -1 . First, this can be partly explained by the application of different background LCA data; ecoinvent 3.6 in our analysis vs. both Gabi and ecoinvent 3.5 in Ref. 18 Second, our analysis includes a more thorough (and conservative) analysis regarding the transportation and storage of CO 2 , while Ref. 18 expected sufficient CO 2 storage sites close to the DAC facility as well as the utilization of existent infrastructure for CO 2 storage; they therefore excluded CO 2 transportation and infrastructure requirements for geological storage sites. However, the availability of geological storage sites for DACCS can be limited due to geographical availability as well as the competition with conventional CCS and other CDR technologies, such as BECCS (see Section 3.4.6). For Switzerland, CO 2 leakage during transportation (1500 km) reduced the GHG removal efficiency already by more than 2%, while the compression, recompression and injection of CO 2 as well as infrastructure requirements further reduced the GHG removal efficiency to a total of 6% in our analysis.</p><p>In addition, the work of Deutz and Bardow 18 reported a generic land use requirement of 4,450 km 2 (3,683 DAC plants with a capture capacity of 100 kt CO 2 year -1 , with 1.21 km 2 per DAC plant) for PV-coupled DACCS to capture 1% of global CO 2 emissions in year 2019 (0.368 Gt CO 2 18 ). Our main environmental trade-off was found for land transformation; manifested by the big range of both direct and life-cycle land transformations between DACCS configurations and countries. The worst case land transformation scenario in our analysis demonstrated much higher land use requirements for autonomous PV-coupled DACCS systems; almost 4.7 km 2 per DAC facility (with a capacity of 100 kt CO 2 year -1 ), mainly due to lower annual solar irradiation for this specific location (and for the reason given in Section 3.4.5).</p><p>Since our analysis and this comparison show that specific DACCS system configurations at different locations exhibit very different environmental performances and that transport and storage of CO 2 can cause substantial burdens, we argue to perform site-specific assessments of DACCS configurations during the design phase, including a thorough analysis of the transportation and storage of CO 2 as well as a location-specific assessment of land use requirements.</p><!><p>Comparison of DAC(CS) LCA results with other CDR technologies or CO 2 sources for carbon capture and utilization should consider the variability of the life-cycle GHG removal efficiency, as demonstrated by our analysis. This is, however, not always the case and the presentation of the GHG removal performance of DAC(CS) as one single number can be misleading (e.g. by Müller et al. 29 for DAC).</p><p>Further, DACCS systems should be compared with other CDR technologies on the same functional unit -to evaluate benefits and potential trade-offs of various CDR technology options. 6 We propose to compare CDR technologies per unit of CO 2 -removal from the atmosphere. Unfortunately, we are currently not able to present such a comparison between CDR technologies due to the immature research state of CDR technologies in general. 6</p><!><p>Our paper aimed to determine the life-cycle environmental performance of low temperature DACCS systems with low carbon energy sources for CO 2 capture. The results were presented per ton of gross CO 2 removal with a DAC plant of an annual capture capacity of 100 kt CO 2 year -1 based on Climeworks' technology. We included different energy sources -solar heat, waste heat and electricity by means of a HTHP -to deliver heat to the DAC plant. In addition, different electricity sources -PV and grid electricity -were considered to supply electricity to the CO 2 capture process. The results were presented for different system layouts for eight geographic locations with specific assumptions regarding CO 2 transport and storage after capture.</p><p>The results demonstrated that energy supply -heat and electricity used for CO 2 capture -is the key factor driving GHG emissions of DACCS configurations. All eight selected countries exhibited net-negative GHG emissions on all proposed system layouts, meaning that GHG emissions associated with energy supply and material demand for DACCS are below the amount of gross CO 2 removal, equivalent to a net-removal of CO 2 from the atmosphere.</p><p>However, the level of "net-negativity" showed substantial variation between DACCS layouts and countries of application: while in our best case almost 97% of the captured CO 2 is permanently removed from the atmosphere, our worst case resulted in a life-cycle GHG removal rate of less than 9%. The best climate change-related performances were achieved by system layouts using waste heat -and in countries with low CO 2 -intensities of the national electricity grid. The CO 2 -intensity of the national grid electricity mix turned out to be the crucial factor for grid-coupled system layouts.</p><p>Autonomous DACCS layouts -which consume solar energy -are from an environmental perspective (except for land use considerations) promising alternatives in regions where the grid electricity mix relies on a high share of fossil fuels and at remote locations without grid access (potentially close to CO 2 storage sites). Therefore, we recommend solar-based autonomous DACCS systems for countries with (semi)-arid climates, which have a CO 2 -intensive grid electricity mix and the required land area available. All-electric DACCS system layouts are recommended when the national grid electricity mix relies on low-carbon electricity sources. Further contributions to life-cycle GHG emissions of DACCS -associated to for example DAC infrastructure, CO 2 transport and storage, and energy storage unitscan be substantial in relative terms, if energy supply for CO 2 capture is clean in terms of GHG emissions, but absolute GHG emissions due to these contributions are rather small.</p><p>The assessment of a wide variety of environmental impact categories -in addition to impacts on climate change -showed different trade-offs on a few environmental impact categories, especially regarding land transformation associated with PV-coupled DACCS configurations.</p><p>This observation confirms the importance of a comprehensive LCA approach not only focusing on climate change.</p><p>The sensitivity analysis demonstrated the large variation of GHG emissions between countries for all-electric DACCS systems. Hence, selecting inappropriate locations in countries with CO 2 -intensive grid electricity mixes could lead to net GHG emissions instead of GHG removal. Consequently, the operation of all-electric DACCS systems with fossil-fuel based grid electricity mixes should be avoided, and we recommend to assess the suitability of DACCS systems based on site-specific conditions, such as the availability of (renewable) energy sources as well as land, waste heat and the potential of carbon storage sites. Further, our prospective analysis demonstrated that more ambitious climate policy will have beneficial effects on GHG emissions for all-electric DACCS systems.</p><p>Finally, we demonstrated significant carbon removal efficiencies of DACCS, although with a large dependency on the energy sources used for CO 2 capture. We therefore argue to utilize low-carbon energy sources for CO 2 capture and to compare alternative DACCS configurations with a comprehensive and transparent LCA approach. Such an approach enables policy-makers to select the most environmentally friendly DACCS configuration under given local boundary conditions. Besides environmental aspects, effective policy is required to facilitate the development of the DAC industry to improve the economic viability of DACCS.</p><p>With sufficient geological storage potentials and an economically viable DAC system, we foresee DACCS as a key component in the pathway towards net-zero GHG emissions.</p><!><p>There are no conflicts to declare.</p>