# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 6 Conclusions  
The stringent environmental agreements urge not only CO2 mitigation technologies but also the removal of CO2 from the atmosphere through NETs. One of these is DAC. In this process, CO2 is captured from the atmosphere using several systems, such as absorption, adsorption, ion-exchange resin, mineral carbonation, membrane, photocatalysis, cryogenic separation, electrochemical and electrodialysis approaches, and is put in storage or used in other chemical processes, closing the carbon cycle (as a human-controlled carbon cycle).  
In this work, an extensive review of the above-mentioned DAC processes reported in the literature in recent years has been presented. Each DAC system, at commercial and lab scale, has advantages and disadvantages that underline the need for an integrated portfolio of solutions to satisfy climate goals.  
Absorption, using an aqueous solution of strong base, and adsorption, mainly based on chemisorbents with amine-functionalized sorbents, are the most intensively investigated and mature systems used by the principal companies that have emerged in this field. Among these, only one--Climeworks--has the first commercial plant in operation.  
As an interesting feature of the present work, KPIs are proposed as a way to compare DAC technologies. Thermal and electrical energy consumption, OPEX and CAPEX (or general total costs), land use, water demand, TRL and environmental impact are the most important factors to consider. It is worth pointing out how costs and energy consumption are higher than those required by CO2 capture from flue gas, and the TRL is not at industrial scale for all technologies. Moreover, a wide range of cost and energy consumption values have been suggested in the literature due to the fact that it is a new emerging technology, and more efforts are needed to concretize all of the research presented in this work. An independent and accurate economic and energetic analysis is required for this new process and should be developed in future research, as has already been conducted by [PERSON] et al. [148].  
Overall, the objective of researchers and companies is to decrease costs and energy consumption while increasing the amount of captured CO2. To achieve this objective, some suggestions can be provided, such as to consider a trade-off between the energy required for regeneration and how tightly the sorbent/solvent binds CO2, in order to reduce the energy consumption. Moreover, it is possible to supply low-carbon heat energy and exploit renewable energy and waste heat, as well as reduce capital costs, through optimal design of air contactors. It is suggested that an optimal location be found which has low-carbon heat and power availability, favorable air conditions with low levels of moisture that can reduce the amount of captured CO2, favorable temperature to reduce the heat energy required for regeneration and favorable wind to reduce fan power consumption.  
In the near term, in order to reduce total costs, government support, e.g., through grants, tax credits and public procurement of CO2 removal, should be provided for this emerging technology. In the long term, actions related to CO2 pricing mechanisms and accounting frameworks that recognize and value the negative emissions associated with storing captured CO2 should be adopted.  
Future research should be focused on the development of an optimal sorbent with a low regeneration energy, high CO2 selectivity, fast reaction time, long stability and low degradation rates. For air contactors, attention should be put on increasing the contact area while minimizing pressure drop and the energy requirement for air movements. The systems should minimize sorbent/solvent losses and increase capture capacity.  
Conceptualisation, methodology, writing-review and editing: G.L., P.S.F. and N.S.; investigation and writing-original draft preparation: G.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the Hadley Trust Charity.  
Not applicable.