Investigating a New Means for Carbon Remediation – Freezing CO2

The inability of global society to modify energy and transportation infrastructures to mitigate the release of CO2 and other greenhouse gas emissions has lead to a steady increase in atmospheric concentrations of these elements resulting in a steady increase in global surface and ocean temperatures. Unfortunately the existing temperature rise is not representative of the additional carbon emissions that have been released because temperature follows CO2 in two separate feedback systems, slow and fast. The temperature increases to date are representative of the fast feedback, more temperature increases will occur even if emissions are balanced again natural carbon sinks tomorrow due to the slow feedback element.1 Therefore, it is difficult to rationally argue that humans need only to mitigate carbon emissions to avoid significant detrimental long-term damage to the environment; humans must also devise a means to remove CO2 from the atmosphere. If one could be confident that the global community would cease excess carbon emissions in the very near-future then slower more natural means of carbon remediation, like planting trees and restoring peat/swamp lands would be appropriate, but existing human behavior indicates that such a condition is not forthcoming; therefore, faster means of atmospheric carbon remediation utilizing technology will be required.

Numerous methods to accomplish this accelerated remediation have been proposed ranging from enhancing mineral weathering and extracting CO2 from the atmosphere with sorbents/amines to oceanic iron fertilization and biochar expansion from feedstocks, all of which have been discussed at one point or another on this blog. A new option was recently proposed involving the extraction of CO2 by freezing it.2 Basically the idea is to install gigantic freezers on Antarctica to solidify CO2 separating it from the atmosphere and then storing that CO2 underground. The selection of Antarctica is practical for two obvious reasons: first the naturally cold temperatures reduce the energy demands associated with freezing the CO2 reducing costs, time and infrastructure and second there is little competitive interaction. Competitive interaction relates to the fact that very little can be done on Antarctica so building this freezer infrastructure will not deprive resources from other human endeavors (no food growth, no settlement, no mineral acquisition, no alternative power infrastructure for normal operation, etc.).

While the general physics and thermodynamics of this process do not raise any significant concerns, there are two major issues that need further evaluation. First, the lead author of this proposal, Professor Ernest Agee, believes that using wind power from a fleet of 16 1.2 GW wind farms will be sufficient to address the necessary energy requirements. The first concern with this plan is that an efficiency of 100% is assumed for these wind farms, which is not appropriate. While wind energy is ‘free’ and the intermittency is less of a problem due to separation from societal necessity, including this intermittency is still important to understand the time requirements associated with annual CO2 extracted. Included in this concern is the imbalance of the intermittency. If a wind farm produces an annual power output of 1.2 GW that output is rarely evenly spaced. Some days will produce 10 MW and other days will produce 0 MW, but not all of that excess MW may be utilized by the freezers. Therefore, without storage or some other means to address this overflow there will be additional non-use that must be considered beyond the normal capacity factor when calculating CO2 extraction.

The second concern with using wind power is that the temperature, which was a boon to reducing the energy requirement for solidifying CO2, but will more than likely also reduce the energy provided. There is a lack of extensive studies regarding how wind turbines are able to function in cold conditions and almost none regarding how they function in extremely cold conditions and the results from those that exist do not provide confidence in ensuring the theoretical outcomes.

Cold weather can influence wind power generation in one of three major ways: 1) the low temperatures affecting the physical properties of the materials; 2) ice accumulation on physical structures/surfaces themselves; 3) the presence of snow in the vicinity of the turbine.3 Of these three issues low temperatures is the largest concern. Both the steel and various composite materials used in turbine construction are negatively influenced by cold; steel becomes more brittle, which reduces their ability to absorb energy and resist deformations before failure.3,4 Composite materials suffer from unequal shrinkage creating greater levels of residual stress which leads to microcracking which results in faster deterioration of the material.4,5 Generators, yaw drive motors, transformers and other electrical devices can also respond negatively to the cold largely due to thermal shocks between down-time and operational-time.

Another problem is cold temperatures increasing viscosity for various lubricants and hydraulic fluids making these beneficial elements more detrimental. When oil is thick and unable to circulate effectively it dramatically increases the probability of damage to gears and other moving parts. Loss of lubrication can also maintain higher internal friction reducing power transmission levels for gearboxes and other elements. Finally small connectors and supports like seals and cushions can loose flexibility and result in more inefficiency.6,7 These failures also are problematic because the composite materials that would be useful for cold weather turbines, those with similar thermal coefficients, are not mass produced thus when developed would cost significantly more money because they would be more unique composites (if they exist at all) and they would not be mass-produced eliminating scale-up savings.3

Not surprisingly the biggest problem with addressing the problems caused by low temperatures result in sacrificed efficiency. The probability of thermal shocks can be reduced by placing heaters inside the nacelle to create lead-time for increasing temperatures during start-up.3 However, these heating elements must be powered, which requires batteries and/or storage systems. Lower viscosity lubricants can be used to aid cold starts and maintain some fluidity, but the drop in viscosity will reduce its protective ability at normal operating temperature increasing the probability of mechanical erosion and damage.3,6 Changes in drive-train activation could be applied avoid full torque until a safe temperature is reached, but such a start-up methodology is not currently applied in moderate temperature turbines and would also reduce electricity generation.3,4

The second problem is with the accumulation of ice on various portions of the turbine most notably on the blades. There are two major forms of ice that can form on turbine blades: glaze and rime. Glaze forms through the typical process when liquid precipitation strikes a surface at the temperature below the freezing point. Basically glaze is similar to the ice that forms on objects and plants during ice storms, hard and transparent (you can clearly see the object encased). Rime forms when surfaces are below the freezing point and are exposed to clouds or fog with supercooled water droplets.5 Rime ice is white and opaque due to the presence of trapped air bubbles. The probability of rime ice formation increases with increasing elevation, thus rime ice formation could be a significant problem for wind turbines built in Antarctica. The potential problem of rime ice could be extended in determining whether or not it will be hard rime ice or soft rime ice. This is an important distinction due to large relative differences in density, which could adjust the strategy used to address its formation and accumulation.

Both forms of ice can form on the rotating and non-rotating surfaces of the turbine. The largest problems are associated with icing of the rotor due to operational interference of speed limiting devices like flaps and blade tips, increases in static load, changes in dynamic balance accelerating fatigue, reduction of energy capture due to changes in the aerodynamic profile of the rotor reducing lifespan and simple weight increases leading to tower collapse.3,5 Other problems stemming from ice accumulation are increases in static loading and wind loading because of increases in surface area (especially for rime ice).

A significant problem with both the ice and general cold temperatures is that due to the consistency of these temperatures and the location of Antarctica, maintenance of these turbines will be a complete nightmare. Icing on Antarctica is also concerning because it will significantly tax normal de-icing strategies. The most common method of de-icing is to apply an anti-adhesive coating, like Teflon, on the turbine blade. However, without frequent application this method will fail quickly on Antarctica. Active de-icing methods could be applied using thermal de-icing from heating units, but once again such a design requires energy and some of that energy may need to come from a non-wind source due to temperatures and wind intermittence. Some estimate that the total energy requirement for thermal active de-icing is between 6 – 12% of the output energy,7 but this estimate should be increased significantly due to the longer operational times that will be suspected for the original estimate only deal with temporary freezing conditions not consistent ones. Black coated blades will probably not work in Antarctica because there really is no opportunity to absorb heat over a long enough period of prep time, but they might as well be utilized due to no real detriment in their use.

Problems related to snow will be largely based on where the wind turbines are built. On the coast wind farms have a significant advantage in their ability to better harness the strong katabatic winds, which have reduced potency more inland. Note that due to its elevation moving inland will still produce higher wind profiles than a vast majority of locations on the other six continents. As a whole Antarctica is basically a frozen desert receiving less than 200 mm of precipitation on its coasts and less than 50 mm inland. Therefore, coast-based wind farms will have to deal with much higher rates of precipitation, usually in the form of snow, whereas inland the concern of snow is almost non-existent. The important aspect of this precipitation is its annual amount is generally divided into waves of large activity and almost no activity. This behavior could increase the probability of damage due to large volumes of precipitation limiting recovery time and overwhelming preventative measures. However, despite these possibilities overall snow is a limited concern regarding its potential negative impact on the efficiency of a wind farm.

The second important issue that must be addressed for this proposed method is how to manage the CO2 once it has been solidified. The overall idea desires to store the solidified CO2 (in snow form) in an insulated specialize ‘landfill’ of sorts with five being constructed for the initial design.2 The concern with this strategy relates back to the energy supply issue because in order to maintain the solidification of the CO2 this landfill will have to be cooled like a larger version of the freezers that created the solidification in the first place. As discussed above there are numerous reasons to anticipate wind power not being consistently supplied due to its natural intermittency as well as efficiency and maintenance problems brought on by weather conditions, thus storage in an extend landfill freezer may not be a sustainable idea because if the CO2 reverts to a liquid or gas the entire project fails.

Transferring the CO2 to pressure-sustaining containers for storage in effort to manage them after state changes from sold to liquid will probably not be economically effective because of the number of materials involved. There will be a storage ceiling regarding the concentration of CO2 that can be stored in a given pressure-sustaining container more than likely forcing the construction of numerous smaller volume containers, which will create logistical and space problems.

Another possibility for CO2 storage would be to inject it into the deep ocean where due to the low temperature/high pressure environment of the deep ocean the CO2 would react with water forming a solid hydrate. This CO2 hydrate is denser than seawater so it should remain at the bottom of the ocean sequestering it from the geochemical cycle.8 However, the process is exothermic, which will release heat and reject salt. These reaction bi-products could create localized instabilities on biological life in the environment. This effect should be limited because of eventual mixing due to ocean circulation, but there may be a scale problem if the CO2 is deposited in a single place over a consistent time period. There is some concern that this immediate release of CO2 before formation to hydrate could damage local biosphere largely due to low metabolic tolerance of CO2 and its resultant excess proton damage.9,10

There are two methods that could be applied for deep ocean storage. In the first method the logistics for a deep ocean storage methodology would require a long pipeline to transfer the CO2 to sufficient depth because if the CO2 is released at too shallow a depth the hydrate formation reaction is not fast enough to mitigate the density difference between the CO2 and the seawater, thus the CO2 would rise to the surface creating large inefficiencies in hydrate formation.8 The general floor depth for ensuring efficiency is about 2600 meters. Note that for this method the CO2 would have to move from a solid state to a liquid one, which is not an issue.

The second method could use a mini-refrigerator that would store the solidified CO2 maintaining its state while the CO2 was transferred to the appropriate depth before it was released. Upon release the CO2 will more than likely move from a solid state to a liquid state, but the added time as a solid will increase the efficiency of the hydrate reaction relative to the depth of the CO2. This method could create more radical shifts in localized environments because it would entail larger concentrations of CO2 over shorter time frames versus the first method, which releases smaller concentrations over longer more consistent time frames.

Overall the value of this idea is predicated on its economic viability relative to other carbon remediation techniques. This blog has discussed numerous times that there is an economic separation between remediation and mitigation techniques because both will be required to evade the more significant detrimental consequences of global warming. The chief advantage of the technique discussed above is that it does not require the use of water. Almost every other legitimate remediation technique requires significant amounts of water be it direct air capture, enhanced weathering, biochar synthesis (the process of growing the feedstock because relying on residue is not an effective strategy) to even growing trees. The chief disadvantage of this technique is supplying the power for the process.

Unfortunately as it stands currently wind power does not appear to be able to supply consistent power to this process due to the extreme cold temperatures reducing efficiencies and increasing probabilities for maintenance, low consistency and lack of cost-effective storage options (no pumped hydro). There has been some discussion regarding the creation of a very low temperature wind turbine, but this design is in the kW range not the MW range which is required for GW sized wind farms.11 Also there are some questions to whether this design could survive wind speeds above 60 m/s, which leads to questions for larger wind turbines as well. Storage is not a big specialized concern because it will be an issue for all non-biological (trees/biochar) carbon remediation methods.

The advantage of not having to use additional supplies of water makes this method quite attractive because water will have multiple elements vying for its dwindling supplies in the future. However, due to the remote location of Antarctica, powering this system is difficult. Solar will not be efficient due to long stretches of no sunlight, tidal power is too expensive and cannot produce nearly enough energy, geothermal is a non-starter for obvious reasons and using any type of significant carbon emission technology will defeat the purpose of this carbon remediation method. As outlined above there appear to be significant issues with using wind power, which makes its inclusion questionable. Therefore, powering this method may rely on the use of small modular nuclear breeder reactors. Note that consistent power is important for scale because unless this system is taking in a significant amount (at least 1 billion tons of CO2 per year) it loses significant economic value versus using those funds on other strategies like direct air capture or enhanced weathering. Overall if the power issue can be addressed this method is actually a welcome possibility to the range of options for carbon remediation and should be pursued barring any negative surprises.

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Citations:

1. Hansen, J, et Al. “Climate Change and Trace Gases.” Phil. Trans. R. Soc. A. 2007. 365:1925-1954.

2. Agee, E, Orton, A, and Rogers, J. “CO2 Snow Deposition in Antarctica to Curtail Anthropogenic Global Warming.” Journal of Applied Meteorology and Climatology. 2012. doi: http://dx.doi.org/10.1175/JAMC-D-12-0110.1

3. Lacroix, A and Manwell, J. “Wind Energy: Cold Weather Issues.” Renewable Energy Research Laboratory. 2000.

4. Dutta, P. and Hui, D. “Effects of Cold Regions Environment on Structural Composites.” Proceedings of the International Conference on Advanced Technology in Experimental Mechanics. Japan Society of Mechanical Engineers. 1997.

5. Peltola, E, et Al. Expert Group Study on Recommended Practices: 13. Wind Energy Projects in Cold Climates. International Energy Agency. 2011.

6. Brugada, R. “Performance of Thermoplastic Elastomers (TPEs) at Subzero
Temperatures.” Proceedings of the Subzero Engineering Conference. 1989. 67-73.

7. Jasinski, W, et Al. “Wind Turbine Performance Under Icing Conditions.” Journal of Solar Energy Engineering. 1998. 120:60-65.

8. Brewer, P, et Al. “Direct Experiments on the Ocean Disposal of Fossil Fuel CO2” Science. 1999. 284:943-945.

9. Seibel, B, and Walsh, P. “Potential Impacts of CO2 Injection on Deep-Sea Biota.” Science. 2001. 294:12-13.

10. Millero, F. “The Marine Inorganic Carbon Cycle.” Chem. Rev. 2007. 107:308-341.

11. Stander, J. “Selecting a Small Wind Turbine for the South African Antarctic Research Base SANAE IV.” Winter Wind Conference in Sweden. December 2008.