Therefore, although there are other more cost-effective ambient air capture techniques, which involve more natural processes (planting trees or synthesizing bio-char), these processes are significantly slower than technological methods. Speed is important, not just on a general level, because also a feedback level for during the process of removing the necessary CO2 the now more acidic ocean could lose a significant amount of its sink capacity has it begins out-gassing previously absorbed CO2 back into the atmosphere due to the change in the concentration gradient, thus the process must be fast enough to accommodate any further reduced sink capacity. Also the threat of some permafrost melt will also increase atmospheric CO2 concentrations which will need to be effectively managed beyond mitigation of directly derived human emissions.
Most economists are troubled by the calculated theoretical (no large scale system has been empirically tested yet) costs of technology driven air capture ($400-600 dollars per ton of CO2) and they should be, but as explained here the options available to the global community to address global warming consequences are quite small. Realistically due to the deficiencies of natural sinks there are only two choices: rapidly deploy both emission reduction programs and direct air capture technology or prepare to live in a much harsher environment which should reduce life expectancy. The reason for the parameters of the first choice is that natural sinks (land and ocean) will be unable to remove enough CO2 from the atmosphere, even in a scenario of rapid emission reduction because there is already too much CO2 and other greenhouse gases in the atmosphere, to avoid significant detrimental environmental effects.
However, these air capture unit must be efficient, otherwise they will lose their speed advantage over augmenting natural sources. Therefore, there are some important operational issues that must be addressed before deployment. The first major issue is water use. Regardless of the system, the chemical reaction utilized to absorb CO2 from the atmosphere requires the use of water. In most situations the water is supposed to act as a catalyst, but due to the open-air nature of the reaction system a significant percentage of the water (how much is heavily based on overall process design) is absorbed into the atmosphere becoming water vapor making water recovery more difficult.
Another important consideration is that most of the costs associated with air capture relate to gross costs per ton of CO2 captured because the estimates do not take into consideration what energy source is utilized to power the capture unit. A general background regarding where energy in most system designs is utilized can be found here. If a trace emission source is utilized (nuclear, geothermal, wind or solar) then the gross cost can be reasonably estimated as 90-99% efficient (thus the net cost will be 1.01-1.1 times more than the gross cost). However, if a fossil fuel source is utilized then the net cost will be higher than the gross cost (largely dependent on the exact fuel mix), but most of the time at least 1.3-1.5 times. Not only does the use of a fossil fuel energy source increase per unit costs and overall long-term costs, but it also reduces the overall speed of CO2 removal making technology air capture less attractive vs. natural sources. Therefore, it is important that all air capture units be powered by trace emission sources.
The final consideration is developing an endpoint for the captured CO2. A number of air capture developers have dreamt to using the captured CO2 as a marketed product either to enhance oil recovery, in a methane or hydrocarbon based fuel or in commercial industry (soda, etc.). Unfortunately the first two options add CO2 back to the atmosphere, just another means of disrupting the overall extraction efficiency of these units making them less desirable relative to expanding natural sources. The commercial option is incredibly insufficient at utilizing a vast majority of the CO2 that needs to be captured (gigatons of CO2). The best means to address this glut of CO2 appears to be sequestering it underground.
With all of these additional considerations to take into account it is not wise to simply build these air capture units at random. These units clearly need to be constructed in an orderly and cohesive manner perhaps even in a localized autonomous network. This network needs to contain a water source, a power source and a means of utilizing the captured CO2 in addition to having recycling pathways for all necessary materials used in the selected air capture reactions.
The most important element in such an air capture complex is selecting a power source. Recalling that volumetric speed is one of the principle elements behind the need for technology air capture the selected power source will need to be reliable and have as little downtime (intermittence) as possible. This requirement limits the viability of using wind or solar power as those energy methodologies cannot reliably power this proposed complex 24 hours 7 days a week.
Now one could argue that wind or solar would be appropriate with storage, but the general lack of storage options and empirical track record hurts the viability of this response. For example Solar Tres uses molten nitrate salt as a storage medium, but it is difficult to conclude that enough storage could be generated on a consistent basis to generate the 24-7 operational period. Remember energy can only stored if it is in excess, which will not be true most of the time if the solar panels are already providing power to various elements in the complex. Pumped hydro shares the same problem, as well as limiting the location for the complex because of its required topography. Also the addition of a storage medium would increase the cost associated with capture.
Without being able to rely on solar or wind power due to consistency concerns, viable energy options fall to nuclear power and geothermal. Nuclear power in a conventional plant is a little tricky because the provided power would be too much for the needs of the complex, thus if nuclear power was used in this way it would have to come from an outside plant, brought in by transmission lines. Another nuclear option may be to utilize a small modular unit design for a given complex.
Geothermal power is an attractive option when considering an Enhanced Geothermal Systems (EGS) model, but not a conventional model. A conventional geothermal model is similar to utilizing a pumped hydro storage system in that it limits where the complex can be constructed. An EGS model is also attractive as a secondary means to utilize some of the captured CO2 as a supercritical fluid in the system itself. So it appears that from the perspective of the complex itself the best power sources in decreasing order of effectiveness are EGS > Nuclear > Solar > Wind.
There are two main strategies for developing a water source: desalination or atmospheric capture of water vapor. The advantage of an air capture complex is that both of these strategies can be utilized because the air capture unit itself does not rely on natural wind patterns, but creates its own direct air flow to drive ambient air into the reaction section. The only boundary condition for the unit is tied to the power source utilized. This flexibility advantage is useful because the air capture complex requires a system to provide the initial water and would be heavily aided by a system which recycles water (reduces loss from air absorption).
Therefore, the complex could be constructed near a source of salt water, use desalination to provide in the initial water supply and utilize atmospheric water condensers to limit the amount of water required from desalination after the initial reactant amount. Note that tapping a continuous water source (such as desalination) may not be necessary as long as a sufficient amount of water is made available at the beginning of the process until the atmospheric collectors can successfully begin the recycling process, it just appears to be an easier overall strategy.
Desalination use has always involved two major concerns: the energy required for the process itself and determining an endpoint for the brine. The energy requirement is not a significant issue if using one of the two best-fit energy sources for the complex. Under normal circumstances the brine is a very significant issue that has environmental repercussions as returning it to the sea (standard practice) is through to have serious detrimental effects in the localized region where it is returned. Other than injecting it back into the source, effective ideas to address the leftover brine are far and few between.
Similar to the absorbed CO2, the total raw amount of salt from the brine, which would be generated from such a desalination project, is so large that using it in commercial endeavors does not appear viable. Some have proposed ammoniating the brine and using it to increase the volume of CO2 capture. The concern with that strategy is providing the necessary ammonium to react with the brine to create a consistent and worthwhile reactant volume. Another option that has been floated is incorporating the brine into a set of molten salts that would be used in either nuclear power reactors or batteries. However, the viability of such an idea is still questionable.
It is understandable that if the economic impact of developing such a complex was quantitatively calculated that it would be high; however, the nature of the complex is that all of these elements will be required in the future based on the current environmental-use path humans have embarked upon, thus the cost is not based on luxury, but necessity. The idea behind such a complex is actually to lower costs by tying many of the air capture units into the same required operational elements, thus making the technology air capture strategy more economical, saving money for investment in other environmentally necessary avenues like emission reduction. Overall while the manifestation of such a complex may not be exactly as described in this blog post, the reality is that such a complex will be needed in one form or another.
The figure below is a crude visual representation regarding what the complex may look like without mileage delineations between units as such an element needs to be modeled to maximize the efficiency of each given operational unit.
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