A scientific plan is presented that proposes the construction of carbon dioxide (CO2) deposition plants in the Antarctic for removing CO2 gas from Earth’s atmosphere. The Antarctic continent offers the best environment on Earth for CO2 deposition at 1 bar of pressure and temperatures closest to that required for terrestrial air CO2 “snow” deposition—133 K. This plan consists of several components, including 1) air chemistry and CO2 snow deposition, 2) the deposition plant and a closed-loop liquid nitrogen refrigeration cycle, 3) the mass storage landfill, 4) power plant requirements, 5) prevention of dry ice sublimation, and 6) disposal (or use) of thermal waste. Calculations demonstrate that this project is worthy of consideration, whereby 446 deposition plants supported by sixteen 1200-MW wind farms can remove 1 billion tons (1012 kg) of carbon (1 GtC) annually (a reduction of 0.5 ppmv), which can be stored in an equivalent “landfill” volume of 2 km × 2 km × 160 m (insulated to prevent dry ice sublimation). The individual deposition plant, with a 100 m × 100 m × 100 m refrigeration chamber, would produce approximately 0.4 m of CO2 snow per day. The solid CO2 would be excavated into a 380 m × 380 m × 10 m insulated landfill, which would allow 1 yr of storage amounting to 2.24 × 10−3 GtC. Demonstrated success of a prototype system in the Antarctic would be followed by a complete installation of all 446 plants for CO2 snow deposition and storage (amounting to 1 billion tons annually), with wind farms positioned in favorable coastal regions with katabatic wind currents.
Since the beginning of observations of atmospheric carbon dioxide (CO2) at the Mauna Loa Observatory in 1958, which established the famous Keeling curve for CO2 increases (see Bacastow et al. 1985), there has emerged an unprecedented scientific concern (and alarm) regarding climate change attributed to fossil fuel emissions by the industrialized world. Preindustrial CO2 values of 295 ppmv have risen to the current global average of 392 ppmv and the more recent trend of CO2 increase is illustrated in Fig. 1 at 1.88 ppmv yr−1. Trenberth (1981) has determined that 1 ppmv of CO2 increase amounts to 2.13 × 1012 kg, and therefore the 1.88 ppmv represents about 4 billion tons of carbon (4 GtC) added per year (based on the 2000–09 decadal average). Global surface observations of air temperature, coupled with climate model simulations, support the conclusion that CO2 is the principal greenhouse gas (GHG) responsible for much of the 1°C rise in temperature during the twentieth century [see numerous scientific references in Solomon et al. (2007)]. Further, climate model simulations through the twenty-first century (through different scenarios of continued CO2 increase) show additional global warming increases from 1° to 5°C, which would result in unprecedented modern-day geophysical and economic disasters.
To date, several efforts have been made to address this global emergency, from the Kyoto Protocol (1997) to the Copenhagen summit in December 2009 and from national and state and city planning for mitigation (e.g., due to sea level rise) to calls for CO2 sequestration. The social and economic impacts of global warming are too numerous to discuss in this paper, and the reader is referred to the various working group reports prepared by the Intergovernmental Panel on Climate Change. One particular effort that is relevant to this proposal is the Virgin Earth (VE) Challenge (see http://www.virgin.com/subsites/virginearth/), a $25 million prize initiated and financed by Sir Richard Branson. The requirement of this challenge is to remove 1 GtC from the atmosphere per year, for a 10-yr period. The concept presented in this paper was not submitted for competition, since the deadline for submission was 8 January 2010. Nonetheless, the VE prize is a simple illustration of the perceived importance of removing CO2 from the atmosphere (with $5 million awarded at the beginning and $20 million awarded at the end of a successful decade of removal). As of 2 November 2011 there were no winners; however, 11 leading organizations with promising ideas were announced at the Global Clean Energy Conference in Calgary, Alberta, Canada, to establish next steps for the Virgin Earth Challenge. It is noted that 1 GtC yr−1 is ~0.5 ppmv of CO2 removal and thus represents only a 25% decrease in the rate of increase.
2. The idea
The National Aeronautics and Space Administration’s (NASA) Mars Global Surveyor and Odyssey missions have revealed the presence of a CO2 ice cap on Mars’s South Pole (see Fig. 2), which is annually subjected to deposition and sublimation. The presence of this CO2 ice cap triggered the idea to consider the possibility of terrestrial air CO2 deposition at Earth’s South Pole, considering that this is the coldest location on Earth and the energy required to sequester CO2 from the atmosphere (and to maintain insulated storage) might be within the scope of reality. A depositional plant constructed on Antarctica could conceivably pull air into a refrigerated chamber, where sufficient cooling could result in CO2 “snow” deposition. To pursue this idea, it is first noted that N2, O2, and Ar all would remain in the gas phase as terrestrial air CO2 is brought down to its depositional temperature. Since the atmosphere is only 392 ppmv of CO2, the Clausius–Clapeyron equation, in conjunction with the CO2 vapor pressure curve, can be considered to calculate the atmosphere’s depositional temperature for CO2. Appendix A is presented to show that the relevant depositional temperature for terrestrial air CO2 snow is 133 K, an achievable chilled temperature for the deposition plant. Alternatively, one could consider placing the ambient air under 10 bars of pressure, and the depositional temperature would increase to 152 K. It is noteworthy that liquid N2 has a very high efficiency as a cooling agent at this depositional temperature (considering that pure N2 at 10 bars of pressure condenses at 105 K). A more reasonable target for deposition is the use of liquid N2 at T = 120 K (under a pressure of P = 29.61 bars), within a closed-loop vapor-compression refrigeration system. This will be discussed in a later section.
The coldest surface air temperature ever measured on Earth was at the Vostok Station in 1983 (see Fig. 3), a reading of T = −89.2°C (or 184 K), which is reasonably close to CO2 snow deposition temperature of 133 K (1 bar) or 152 K (10 bars). In fact, much of Antarctica has been getting colder (largely attributed to the O3 hole; see Thompson and Solomon 2002), although the Western Antarctic Ice Sheet is warming (also see Stieg et al. 2009; Franzke 2010). The mean annual temperature of the Antarctic interior is approximately = 226 K (−57°C), and this continent will continue to be the most favored location for implementing the proposed CO2 sequestration methodology. It is further noted that the vastness of the Antarctic interior with multiple international scientific participation (see Fig. 3) lends itself to the global theme of this paper. The Antarctic Treaty (see http://www.nsf.gov/od/opp/antarct/anttrty.jsp) provides a forum for international governance and scientific cooperation. As discussed later in this paper, the construction of CO2 snow deposition plants that are supported by wind farms offers the opportunity for unique international expertise to join forces to develop the CO2 sequestration facilities that can substantially curtail the effects of anthropogenic GHG warming.
4. Design of the CO2 sequestration facility
The components of the proposed Antarctic facility are illustrated in Fig. 4, which shows environmental air (A) entering the right side of the depositional chamber (B). Refrigeration is powered by wind farms that drive a closed-loop liquid N2 cooling facility (see Fig. 5). CO2 snow deposition, at rates of approximately 40 cm day−1 (falling to the bottom of a 100 m × 100 m × 100 m chamber; see appendix B), is excavated into the insulated dry ice landfill (D). Appendix C shows the appropriate calculations and design criteria that would remove 1 GtC per year, which could be accomplished by approximately sixteen 1200-MW wind farms. Most individual wind turbines in midwestern United States wind farms are 1–3 MW; however, more powerful turbines could be considered but are not necessary. An example of a wind farm that exists in Antarctica can be found at http://www.antarcticanz.govt.nz/scott-base/ross-island-wind-energy. Appendix D shows the calculations for meeting the energy requirements to support the depositional plants.
5. Engineering design and operations
The refrigeration cycle and energy requirements for CO2 snow deposition are illustrated in Fig. 5, based on a “closed-loop liquid-vapor cooling system.” Liquid nitrogen is the refrigerant of choice and is effective at the required depositional temperature for CO2 in terrestrial air. Engineering details regarding “compressor” size and “expansion valve” size are under consideration, as is the size of the “heat exchanger.” Multiple components of smaller size (e.g., the compressor) might reduce the energy requirements. Current plans for a 45-MW wind farm (fifteen 3-MW towers) will run one prototype deposition plant. The wind farm should be designed to expand to 1200-MW to supply energy to 28 deposition plants. The CO2 snow landfill for this prototype plant will be 380 m × 380 m × 10 m (for each year of CO2 snow deposition).
The schematic diagram for the CO2 snow deposition chamber is given in Fig. 6. This chamber consists of a 100 m × 100 m × 100 m cubical volume on four support pillars with reversible air intake and exhaust fans for the refrigeration process of the ambient air. The front and back sides of this chamber will have embedded coils of liquid nitrogen coolant. The “floor” of the depositional chamber will be allowed to open for excavation into an insulated CO2 landfill. The prototype system will process ambient air at a depositional rate of 0.4 m of snow per 24-h operational day. This amount of solid CO2 can be stored in an insulated CO2 snow landfill that is 380 m × 380 m × 10 m, which amounts to 2.24 × 10−3 GtC. The intake–exhaust fans will allow reversed airflow to permit the chamber to operate with the ambient wind direction (although typically there will be katabatic flow from the ice sheet to the coastal region). It is further noted that five insulated landfills (380 m × 380 m × 10 m for each) will be constructed in a semicircle in close proximity to each deposition plant to accommodate for 5 yr of CO2 sequestration (one landfill filled per year at each deposition plant and maintained at 195 K).
Figure 7 is an illustration of the landfills (per deposition plant), and they will be insulated with polyisocyanurate (effective down to 93 K). Snowcat-type excavators will operate in groups of five to compact the dry ice into the insulated landfills. A partial vacuum or even refrigeration could be some alternative considerations for maintaining solid CO2.
6. Summary and conclusions
A plausible scientific plan has been presented for removing annually 1 GtC from the atmosphere through refrigeration of terrestrial air and 4 Gt of CO2 snow deposition. The CO2 snow will be stored in insulated landfills onsite in the Antarctic, and the energy for deposition plant operations will be provided by wind farms that will be positioned appropriately for both logistics and katabatic wind currents. The basic scientific concepts presented here are viewed as plausible, while additional engineering details can be provided as the project goes forward. Consideration will also need to be given to other related topics, such as modeling of CO2 global diffusion to the Antarctic (once a CO2 “hole” is created by the deposition plants). It is also noted that diffusion of global CO2 to the Antarctic region should increase as the CO2 is depleted. Last, a global partnership is envisioned and required to solve the global problem, and the Antarctic is the perfect location.
Depositional Temperature for Terrestrial Air CO2
a. Clausius–Clapeyron equation
The Clausius–Clapeyron equation is given by
where L ≡ latent heat of phase change, T ≡ temperature, and ΔV ≡ specific volume. The equation of state can be given as
Here, R = R*/m ≡ universal gas constant.
b. Solution for CO2 deposition in Earth’s atmosphere ≡ Tdep (P = 1 bar)
First, R* = 8.314 j mol−1 K−1 and m = 44 g mol−1, and thus or Second, the enthalpy for sublimation of CO2 (gas–solid) is Third, from the Chemical Engineering Research Information Center (CHERIC), one can obtain the equilibrium curve for equilibrium temperatures of CO2 vapor over solid CO2 (see Fig. A1). From the equilibrium curve, one can choose T1 = 20°C = 293.15 K and P1 = 56.5 atm. Fourth, to solve for Tdep (≡T2), one knows the partial pressure of CO2 in the terrestrial atmosphere is P2 = 3.9 × 10−4 atm. Thus, Eq. (A3) becomes
Substituting into Eq. (4) for (P1, T1) and (P2, T2) one obtains
Fifth, for the mean Antarctic surface temperature, = 226 K, the CO2 snow deposition temperature becomes Tdep = 133 K. Please note that a difference in the enthalpy for sublimation from 293.15 K (vapor to solid) versus sublimation from 226 K accounts for a slightly lower value of the deposition temperature.
Deposition of Snow Layer on Floor of Chamber (Refrigerator)
To calculate the daily CO2 snow deposition depth, the following 12 steps are used:
Chamber volume is (100 m)3 = 10−3 km3 = 106 m3.
Atmospheric CO2 gaseous content in percent by weight is 0.046.
Density of terrestrial air at T = 226 K (the approximate mean temperature for Antarctica) is ρair = 1.534 kg m−3.
Density of CO2 in terrestrial air at 226 K is 1.534kg m−3 × 0.046 × 10−2 = 0.071 × 10−2 kg m−3.
Mass of CO2 in a 100 m × 100 m × 100 m chamber in Antarctica is 0.071 kg m−3 × 10−2 × 106 m3 = 7.1 × 102 kg.
One chamber flush per 10 s → 6 × 60 × 24 flushes day−1 → total flushes per day = 8640.
Total CO2 mass flushed per day is 7.1 × 102 kg × 8640 = 6.13 × 106 kg day−1.
Density of dry ice = 1.561 × 1012 kg km−3.
Surface area of chamber bottom = 104 m2.
CO2 snow depth is found from ; now 3.93 × 10−6 × 109 m3 ÷ 104 m2 = 0.393 m.
Bottom chamber cumulative depth per hour = 0.393 m ÷ 24 = 0.0164 m h−1.
CO2 snow depth per day = 0.393 m.
ΔCO2 Mass and Deposition Plant
a. ΔCO2 mass (4 Gt of CO2 = 1 GtC)
According to Trenberth (1981), the total mass of the atmosphere is 5.137 × 1018 kg, and 1 ppmv of CO2 is 2.13 × 1012 kg of carbon mass. On the basis of global atmospheric CO2 values from 2000 to 2009, the ΔCO2 = 1.88 ppmv yr−1. The carbon mass added to the atmosphere per year is 1.88 × 2.13 × 1012 kg = 4.004 × 1012 kg. The Virgin Earth Challenge of 1012 kg yr−1 of carbon = 1 ÷ 2.13 = 0.47 ppmv. To summarize, 4 GtC into the atmosphere annually increases the atmospheric content of CO2 by ~2 ppmv and the VE Challenge would reduce the annual increase by ~25% (~0.5 ppmv).
b. Deposition plant
The calculations for number of deposition plants involve five steps:
Volume = (100 m)3 = 106 m3 = 106 (10−3 km)3 = 106 (10−9) km3 = 10−3 km3.
Mass of CO2 in the depositional chamber is 7.1 × 102 kg.
360 chamber flushes per hour (for sidewall exhaust velocity = 10 m s−1) leads to a mass of CO2 processed in 1 h = 360 × 7.1 × 102 kg = 2.56 × 105 kg.
Depositional plant mass per year = 24 × 365 × (2.56 × 105 kg) = 8.76 × 103 × (2.56 × 105) kg = 2.24 × 109 kg = 2.24 × 10−3 Gt.
Number of plants for ΔCO2 = 4.004 × 1012 kg ÷ 2.24 × 109 kg = 1.787 × 103 = 1787; therefore, N = 1787 for 4 Gt and N = 446 for 1.0 Gt (Virgin Earth goal), where N is the number of 43-MW depositional plants.
Energy and Power Plant Requirements
The calculations for energy and power plant requirements involve six steps:
The CO2 deposition at 136.1 K is 617 J g−1.
The ΔCO2 (1.88 ppmv) is 1.88 × 2.13 × 1012 kg = 4.004 × 1012 kg.
Energy = (617 × 103 J kg−1) (4 × 1012 kg) = 2.47 × 103 × 1015 kg= 2.47 × 1018 J.
Time in seconds per year is 60 × 60 × 24 × 365 = 6 × 6 × 2.4 × 3.65 × 105 s = 315.36 × 105 s = 3.1536 × 107 s.
Power plant rating of 1200 MW = 1.2 × 109 J s−1 and 1 year = 3.1536 × 107 s → one plant for one year: (1.2 × 109) × (3.15 × 107) J = 3.78 × 1016 J.
Number of power plants N* needed = 2.47 × 1018 J/3.78 × 1016 J = 0.653 × 102 ≅ 65.3, so N* = 65 for 4 Gt and 16.2 for 1.0 Gt (Virgin Earth goal). Therefore, N* ≡ sixteen 1200-MW wind farms, with 28 deposition plants powered per wind farm.