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Light-driven process turns greenhouse gases into valuable products

Much research has been done in order to reduce the use of fossil petroleum products as fuels. In that respect syngas (synthetic gas) seems as a great opportunity for sustainable energy developments. Syngas is the mixture composed of hydrogen (H2) and carbon monoxide (CO) as its main components. It represents an important chemical feedstock used widely for industrial processes for generating chemicals and fuels:

Global use of syngas in industrial processes.

Syngas can be produced from methane (CH4) in a reforming reaction with water (H2O), oxygen (O2) or carbon dioxide (CO2). The process called methane dry reforming (MDR) can be combined with carbon dioxide:

CH4 + CO2 → 2 H2 + 2 CO

It is an environmentally friendly path, turning two greenhouse gases into a valuable chemical feedstock.

However, the MDR is process requires chemical catalysts and high temperatures in the range between 700 − 1,000°C. Usually, it suffers from coke deposition and, in consequence, catalyst deactivation.

Some chemists have recently demonstrated that light, and not heat, might be a more effective solution for this energy-hungry reaction.

The photocatalytic solution

A team of researchers at the Rice University in Houston, Texas, together with colleagues from Princeton University and the University of California have developed superior light-stimulated catalysts that can efficiently power MDR reactions without any heat input. This work has been published in the prestigious journal Nature Energy.

They have reported a highly efficient and coke-resistant plasmonic photocatalyst containing precisely one ruthenium (Ru) atom for every 99 copper (Cu) atoms. The isolated single-atom of Ru obtained on Cu antenna nanoparticles provides high catalytic activity for the MDR reaction. On the other side, Cu antennas allow strong light adsorption and under illumination and deliver hot electrons to ruthenium atoms. The researchers suggested that both, hot-carrier generation and single-atom structure are essential for excellent catalytic performance in terms of efficiency and coking resistance.

The optimal Cu-Ru ratio have been investigated in synthesized series of CuxRuy catalysts with varying molar ratios of plasmonic metal (Cu) and catalytic metal (Ru), where x,y are atomic percentage of Cu and Ru. Overall, the Cu19.8Ru0.2 was the most promising composition in terms of selectivity, stability and activity. In comparison to pure Cu nanoparticles, the Cu19.8Ru0.2 mix exhibits increased photocatalytic reaction rates (approx. 5.5 times higher) and improved stability with its performance maintained over 20 h period. Calculations showed that isolated Ru-atoms on Cu lower the activation barrier for the methane dehydrogenation step in comparison to pure Cu without promoting undesired coke formation.

In addition, the research has been supported by different methods (CO-DRIFTS with DFT) in order to unravel and prove single-atom Ru structures on Cu nanoparticles occurring in Cu19.9Ru0.1 and Cu19.8Ru0.2 compositions.

The comparison between thermocatalytic and photocatalytic activity at the same surface for MDR has also been demonstrated. The thermocatalytic reaction rate at 726°C (approx. 60 µmol CH4 / g / s) was less than 25% of photocatalytic reaction rate under white-light illumination with no external heat (approx. 275 µmol CH4 / g / s). This enhancement in the activity is attributed to the hot-carrier generated mechanism which is predominant in the photocatalytic MDR. The role of the hot-carrier is an increase in C−H activation rates on Ru as well as improved H2 desorption.

The scientists also reported the catalyst achieving a turnover frequency of 34 mol H2  / mol Ru / s and photocatalytic stability of 50 h under focused white light illumination (19.2 W / cm2) with no external heat.

As the synthesized photocatalysts is primarily based on Cu which is an abundant element, this approach provides a promising, sustainable catalyst operating at low-temperatures for MDR. This allows cheaper syngas production at higher rates, bringing us closer to a clean burning carbon fuel.

(Photo: Wikipedia)

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Microbial Power-to-Gas in exhausted oilfields as bridge between renewable and fossil energy

An abandoned or unproductive oilfield can be reused for methane production from CO2 using renew­able electrical power. Exhausted oilfields can be reactors for the conversion of renewable energy to natural gas using microbes. To achieve this, an oilfield can be made electrically conductive and catalytically active to produce natural gas from re­newable power sources. The use of natural gas is superior to any battery because of the existing infra­structure, the use in combustion engines, the high energy density and because it can be recycled from CO2. Oil­fields are superior to any on-ground production because of the enormous storage capaci­ties. They are already well explored and these geological formations underwent environmental risk assessments. Lastly, the microbial power-to-gas technology is already available.

Process summary

Whole process (end-to-end via methane)

50% electrical efficiency

Energy density of methane

180 kWh kg1

Storage capacity per oilfield

3 GWh day1

Charge/Discharge cycles

Unlimited

Investment (electrodes, for high densities)

$51,000 MW1

Cost per kWh (>5,000 hours anode lifetime)

<$0.01 kWh1

Electrolyte

Seawater

The Problem

To address the problem of storing renewable energy, batteries have been proposed as a possible so­lution. Lithium ion batteries have a maximum energy storage capacity of 0.3 kWh kg−1. To date, this is consid­ered the best trade-off between cost and efficiency but these batteries are still too inefficient to replace gasoline, which has a capacity of about 13 kWh kg−1. This makes battery driven cars heavier than conventional cars. Lithium air batteries are considered a possible al­ternative because they can reach theoretical capacities of 12 kWh kg−1 but technical difficulties have prevented them from being used for transportation.

In contrast, methane has an energy density of 52 MJ kg−1 corresponding to 180 kWh kg−1 which is sec­ond only to hydrogen with 500 kWh kg−1, not counting in nuclear energy. This high energy den­sity of methane and other hydrocarbons along with their facile usage, is the reason why they are used in combustion and jet engines that drive nearly all transportation to date. While electrical cars seem to be a tempting green alternative, the fact that combustion engines and the fueling infrastructure are so wide-spread makes it difficult to switch.

In addition to the difficulty of changing habits, battery-driven electrical cars need other limited natural resources such as lithium. To equip all 94 million automobiles produced worldwide in 2017, 3 mega tons lithium carbonate would need to be mined annually⁠. This is nearly 10% of the entire recoverable lithium resources of 35 mega tons worldwide. Although lithium and other met­als can be recycled, it is clear that metal based batteries alone will not build the bridge between green en­ergy and tradi­tional ways of transportation due to the low energy densities of metals. And this does not even take into account other energy de­mands such as industrial nitrogen fixation, aviation or heating.

For Germany, with its high proportion of renewable energy, fuel for cars is not the only problem. As re­newable energy is generated in the north, but many energy consumers are in the south, the grid ca­pacity is frequently reached during peak production hours. A steadier energy output can only be ac­complished by decentralizing the production or by en­ergy storage. To decentralize production, home­owners were en­couraged to equip their property with solar panels or wind­mills. As tax incentives phase out, homeowners face the prob­lem of energy storage. The best product for this group of cus­tomers so far are again lithium ion batteries but investment costs of $0.10 kWh−1 are still unattractive espe­cially be­cause these products store the en­ergy as electricity which can only be used for a short time and is less efficient than natural gas when used for heating.

Natural gas is widely used as energy source today and the global energy infrastructure is designed for natural gas and other fossil fuels. Increasing demand and lim­ited resources for these fossil fuels were the main driv­ers of oil and gas prices during the last years, slowed by the recent economic crises and hydraulic fractur­ing (fracking). The high oil price attracted in­vestors to recover oil using techniques that be­come in­creasingly expensive and are environmental risks such as deep-sea drilling or tar sand extraction. Ironically, the high oil price made costly renew­able ener­gies an economically feasible alterna­tive and helped driving down their cost. Since habits are difficult to change and building an entirely new infrastructure only for renewable energies does not seem economically feasible today while CO2 drives global warming, a more realistic solution needs to be found.

Microbial Power-to-Gas could be a bridging technology that integrates renewable energy into the existing fossil fuel in­frastructure. It reaches break even in less than 2 years if certain preconditions are met. This is ac­complished by integrating methane produced from renewable energy into the current oil and gas pro­ducing infrastructure. The principal idea is to use carbon instead of metals as energy carrier because of its high energy density when bound to hydrogen. The benefits are:

  • High energy density of 180 kWh kg−1 methane
  • Low investments due to existing infrastructure (natural gas, oilfield equipment)
  • Carbon is not a limited resource
  • Low CO2 footprint due to CO2 recycling
  • Methane is a transportation fuel
  • Methane is the energy carrier for the Haber-Bosch process
  • Inexpensive catalysts further reduce initial investments
  • Low temperatures due to bio-catalysis
  • No toxic compounds used
  • No additional environmental burden because existing oilfields are reused

The solution

Methane can be synthesized by microbes or chemically. Naturally, methane is produced by anaerobic (oxygen-free) microbial biomass decomposition. The energy for biomass synthesis is provided by sun­light or chemical energy like hydrogen. In the case of methanogens (methane producing microbes), energy is harvested after CO2 and hydrogen were re­leased from biomass de­composition following a 1-to-4 stoichiometry:

CO2 + 4 H2 → CH4 + 2 H2O

Without microbes, methane is produced by the Nobel-prize winning Sabatier reaction and several attempts are currently underway to use it on industrial scale. It is necessary to split water into hydrogen and use this to re­duce CO2 in the gas phase. A major drawback of the Sabatier reaction is the need for high tempera­tures around 385ºC, and a nickel catalyst that becomes quickly spent. Methanogens use iron-nickel enzymes called hydro­genases to harvest energy from hydrogen, but do so at ambient tempera­tures.

To produce abiotic hydrogen, water is split using precious metal catalysts. Microbes split water using hydrogenases in reverse direction and the produced hydro­gen is oxidized by methanogens that grow in the electrolyte or on electrodes to pro­duce methane⁠. This reaction oc­curs at the correct 1-to-4 stoichiometry⁠ at potentials that are near to the theoretical hydro­gen production potential of −410 mV obtained from the Nernst equation in neutral aqueous solu­tions⁠. Methanogenic microorganisms are able to reduce the overpoten­tial.

Power-to-Gas concept for exhausted oilfields. Electrolysis catalyzes water splitting inside the oilfield producing methane gas and O2.

The future challenge will be to accelerate methane production rates as has been reported for a high tem­perature oilfield cul­tures⁠. Besides increasing the temperature, the most obvious solution is to use a higher reactive surface and bringing both electrodes closer together. Using carbon brushes that are poor hydrogen catalysts but provide a higher sur­face for microbial attachment is one possibility. Methane production correlates with microbial cell numbers in the reactors.

The number of methanogens in microbial electrolysis reactors correlates with the electrode surface.

To overcome the problem of expensive carbon (and also steel) brushes for large scale applications,exhausted gas and oilfields could be used. They provide a high surface area and are usually eco­nomic liabili­ties and not assets. Methano­gens inhabit oilfields where they carry out the final step in anaero­bic petroleum degradation⁠. Hence, oilfields can be seen as bioreactors at geological scale. Geological formations provide ideal con­ditions for produc­ing, storing and ex­tracting methane.

Open questions and potential solutions

Oilfield porespace volume

The Californian Summerland oilfield, for instance, has been abandoned and extensively studied in the past. It produced 27 billion barrels of oil and 2.8 billion m3 methane during its lifetime of 90 years. This maximum load of 3.5 billion m3 left the same volume of porespace filled with seawater behind. Only 2% of these pores are larger than 50 µm, which is necessary for microbial growth assuming dimensions of 1 x 2 µm of a methanogen cell. Experiments showed that the resulting 70 million m3 accessible porespace have a storage capacity of 35,000 TW.  That is a lot of methane assuming a solubility of 0.74 kg methane m−3 seawater at 500 m water depth⁠. All German off-shore windfarms together have a capacity of 7,000 MW. Obviously, the limiting factor is not the volumet­ric storage capacity of an oilfield.

Microbial methane production rates

But how fast can microbes produce methane in an hypothetical oilfield? Under optimal conditions, methanogens that grow on electrodes (typically the genus Methanobacterium or Methanobrevibacter) can produce methane at a rate of 100-200 nmol ml−1 day−1 (equals 2.24-4.48 ml l−1 day−1) depending on catalyst and potential. Using a produc­tion rate of 15 J ml−1 day−1 of methane (190 nmol ml−1 day−1), the en­tire microbially accessi­ble oilfield (2%) has a ca­pacity of 3.6 mil­lion MBtu per year. Mi­crobes would theoretical­ly consume 1 TWh per year for 3.6 mil­lion MBtu meth­ane pro­duction if there were no losses and elec­trical power is translate­d into methane 1-to-1. A power genera­tor of 121 MW would be suffi­cient to supply the entire oil­field at these rates. However, all Ger­man off-shore wind­farms produce 7,000 MW mean­ing that only 3% off-peak power can be cap­tured by our ex­ample oilfield. There­fore, the catalytic sur­face and activity must be in­creased to accel­erate methane conversion rates.

Since methanogens produce methane from hydrogen, not only the 2% porespace big enough for cells can be used resulting in an increased catalytic surface to nearly 60%. A hydrogen cata­lyst needs to be found that does not out-pace methanogen growth to keep the reservoir pH within the limits of 6-8 re­quired for methanogen growth. This hydrogen catalyst must be cheap and render an oilfield electrically conductive. A chemical formulation that mimics microbial hydrogen catalysis could be used. It has the potential to turn a non-conductive and non-catalytic oilfield into a con­ductive hydrogen catalyst sufficient to sustain methane produc­tion needed to store all of Germany’s electricity produced by off-shore wind­farms. This catalyst is solu­ble in water when inactive. To become active, it coats mineral surfaces by precipitation that can be triggered by indigenous microbes or by electrical polarization. The investment would be $2.3 mil­lion per MW storage capaci­ty ($16 bil­lion for the entire 7,000 MW). Due to microbial growth, the cat­alytic activity of the system improves dur­ing operation and there is no need for the second component if an immediate production is not crucial. The investments made on the cathode side would then be as low as $600 per MW ($4.2 million for 7,000 MW).

Anodes

As the cathodic side of the reaction can be excluded as limiting factor, the anode needs to be de­signed. Several commercially available anodes such as mixed metal oxides (up to 750 A m−2) with platinum on carbon black or niobium anodes (Pt/C, 5-10 kA m−2) could be used. Anodes based on platinum are the most cost-efficient material available on the market. Invest­ments made for Pt/C (10%, 6 mg cm−2) anodes will amount to $50,000 per MW ($350 million for 7,000 MW). How­ever, the exact amount of Pt needed for the reaction still needs to be evaluated in an experiment be­cause the corrosion rate at 2 V cell voltage is unknown. An often cited value for the life­time of fuel cells is 5,000 hours and is used here to determine the costs per kWh. For 5,000 hours life­time, the costs per kWh will be at the targeted limit of $0.01 but may be well below that because Pt/C anodes can be re­cycled and the Pt load may be reduced to 3 mg cm−2 (5%). Alternatively, steel anodes (SS316, 2.5 kA m−2, $54,000 per MW) can be used but it is unclear when steel anodes fail to elec­trolyze. In conclusion, the anodic side is the cost-driving factor. Hope­fully, better water splitting anodes will lower these costs in future.

Cost estimation summary

Windfarms

Already in place

CO2 injection

Already occurred

Natural gas capturing equipment

Already in place

Microbial seed

Wastewater from oil rig

Cathode costs

$600 MW1

Anode costs

$50,000 MW1

Electrolyte (seawater)

Free

Total (>5,000 hours anode lifespan)

<$0.01 kWh1

Energy and conversion efficiencies

The whole cell voltage for microbial power-to-gas reactions varies from 0.6 to 2.0 V, depending on ca­thodic rates, anodic corrosion and the presence of a membrane. Higher voltages will accelerate an­ode corrosion, again, making anodes the limiting factor. As the voltage decreases, methane production rates become slower but also more efficient. The voltage also depends on the pH of the oil­field. An oil­field that underwent CO2 injection as enhanced recovery method will have a low pH, provid­ing better condi­tions for hydrogen production but not for microbial growth and must be neutralized using seawa­ter. As stated above, the oilfield, being the cathode, is not limiting the the sys­tem. The use of Pt/C an­odes eliminates the overpotential problem on the anode side. Hence, we can assume an ideal system that splits water at 1.23 V. However, the voltage is often 2 V due to anode and cathode overpotentials. Optimized cul­tures and cathodes produce about 190 nmol ml−1 day−1 methane which equals 0.15 J ml−1 day−1 using the energy of combustion of 0.8 MJ mol−1. The same electrolysis cell consumes 0.2 mW at a cell voltage of 2 V which equals 0.17 J ml−1 day−1 and the resulting energy effi­ciency is 91%. The anodes can be simple carbon brushes and the two cham­bers of the cell are separated by a Nafion™ membrane. The system can still be optimized by using Pt/C anodes and by avoiding mem­branes.

The overall electricity-methane-electricity efficiency also depends on the consumption side efficiency where methane is used in com­bustion engines and gas fired power plants. Such power plants fre­quently operate at efficiencies of 40- 60%. Assuming a reasonable power efficiency of 80% (see above), the overall electrical power recov­ery using gas fired power plants will be up to 50%. Besides the high efficiency of gas fired power plants, they are also easy to build and therefore contribute the a better power grid efficiency. Coal fired power plants can be upgraded to gas fired power plants.

Experimental approach

The conversion efficiencies of charge (Coulombs) transported across the circuit are usually between 70-100% in these systems depending on the electrode material⁠. Another efficien­cy limitation could arise from mass transport inhibition. Mass transport can be improved by pump­ing electrolyte adding more costs for pumping which still have to be de­termined. However, since most oil­fields undergo seawater injection for enhanced oil recovery the addi­tional cost may be negligi­ble. The total efficiency has yet to be determined in scale-up experiments and will depend on the fac­tors men­tioned above.

The reactor simulates oilfield conditions using sand as filling material under continuous flow of electrolyte.

Controlling the pH is crucial. Alkaline pHs significantly impede hydrogen pro­duction and therefore methanogenesis. This can be addressed by a software that monitors the pH and adjusts the po­tential accordingly. Addition of acids is not desired as this drives the costs. The software can also act as potentiostat that then fully controls the methane production process. To test the process under more realistic conditions, a drill core from an oilfield must be obtained.

Results show methane production in the simulation reactor. The appearance of methane in the anode compartment was a result of flow from the cathode to the anode, carrying produced methane with it.

Return of investment of the microbial power-to-gas process

The the microbial power-to-gas process in unproductive oilfields is economically superior to all other storage strategies because of the low start-up and operating costs. This is achieved because the ma­jor investments are the installation of oil- and gas production equipment and renewable power plants which are already in place as a precondition. These investments break even in a short amount of time.

But how can the microbial Power-to-Gas process accelerate the return of investment in renewable en­ergy? Only 8 out of 28 active off-shore windfarms reported their investment costs. These 8 produce roughly half the overall power of 1,600 MW corresponding to $7 billion. While the maximum production of an oilfield with unlimited supply of elec­tricity would yield hypothetical 3.6 million MBtu natural gas per year resulting a return of $13 million per year the real production is limited by off-peak power gener­ated by re­newable energy production. Assuming that the maximum annual methane production corresponds to 10% excess electrical power, $15 million per year can by gener­ated by selling 4.3 million MBtu meth­ane per year on the market. These are $15 million that are not lost during off-peak shutdowns. Clearly, this conservative estimate can help to compensate the invest­ment in renewable energy earlier. It also decreases the investment risk because the investment calcu­lations for new wind farms can be made on a more reliable basis.

In the example using all German windfarms (7,000 MW) this compensation roughly doubles. Using the $60 million generated by methane sales per year, the investment of $4 million for the cathodic cata­lyst and the $36 million for the Pt/C anodes are compensated for within less than a year. No other invest­ments are required because the target oilfield already produced oil and gas and all necessary installa­tion are in working condition. The target oilfield is swept using seawater as sec­ondary extraction method. Electrical installations are in place for cathodic protection of production equipment in order to prevent microbial corrosion, which, however, may need to be upgraded to pass the now higher power densities. Moreover, CO2 is used from CO2 injection as tertiary enhanced oil re­covery method. Only the pH may then need to be adjusted to sustain life by sweeping with seawater.

And this is not the end of oilfield storage capacity. In theory, an oilfield can store the entire amount of renewable energy produced in one year globally, allowing more than enough head room for future development and CO2 sequestration.

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Autonomous production of methane on Mars using microbial electrolysis and algal oxygen for a safe return to Earth

How do we shape human exploration on Mars to minimize what we must bring with us and to maximize the value and utility of what we bring, and augment it with what is already there?

To provide sufficient fuel for a safe return of the Mars crew, we can produce methane and oxygen on Mars which could be aided by microbes that are brought to Mars. We suggest lightweight perovskite solar panels that may be transported to Mars. In the optimistic scenario, about 18 months are required for the installation of surface solar power and fuel production for the failsafe return after which the crewed mission com­mences. The pessimistic scenario takes 4 years. To save oxygen, we also suggest Martian perchlorates as rocket fuel component. For later missions’ food supply, we suggest to use lichens as primary colonizers to produce organic rich soil.

Procedure to produce energy on Mars

We suggest the use of in situ Martian resources for the production of methane as ascent and return propellant. Since oxygen generated during electrolysis is not sufficient for a safe return, we also suggest to use algae for the pro­duction of oxygen. Algal biomass will be used as soil for food production. Methane producing microbes will be contained in methanogenic electrolysis reactors (MER) while algae will grow in covered craters. The produc­tion of methane on Mars is to be car­ried out autonomously by robots and reactors that will land near the ice-rich polar regions to melt wa­ter as elec­trolyte for low temperature electrolysis. The Mars lander will autonomously construct facili­ties with the purpose of propellant production to fuel the Mars transfer vehicle which enables transport between Mars surface and Earth orbit. Only when enough propellant for a safe return to Earth has been produced, shall a manned Mars mission begin. Furthermore, meth­ane will be used as energy storage should there be failure of energy collectors such as solar panels. Goal is to erect a 3.5 MW solar power plant on Mars by the end of the first manned mission.

Schematic of the fuel production process. Red circles highlight the end products steel (top) and CH4/O2 (bottom). Top: 1, Lander on ice, 2, Nuclear fission reactor (NFR) with heat exchanger to melt ice, 3, mining unit, 4, iron/nickel ores, 5, induction steel foundry with energy supply from the NFR (2), 6, algae enrichment tank with water supply from the NFR (2), 7, crater algae reservoir producing O2 and biomass for 8, dehydrator separating water and 9, biomass pellets, 10, carbon steel for Bottom: 11, solar heat collector melting ice and producing water for 12, microbial electrolysis reactor (MER) producing methane and oxygen collected by 13, degasser, 14, solar panels mounted on carbon steel producing electricity for MER (11), 15, gas storage tank, 16, Mars orbit transfer vehicle fueled by CH4/O2

To sustain the life of 6 crew members a power production capacity of 170 kW (see “Surface habitat energy needs”) is required and has highest priority along with fuel production for a safe journey home. The procedure is sketched out in the figure above. It is a stepwise process during which most steps are interdependent and therefore self-accelerating as power production in­creases. To minimize risk of failure, we recommend at least four independent landers in the circum-polar region of Mars. The polar regions bear the highest surface water content which is crucial for manned explorer missions, methanogenic electrolysis as well as the production of oxygen and biomass. The landers will carry a small nu­clear fission device that will begin mining for iron and titanium ores⁠ to produce the steel required as structural support for solar panels. First, the lander begins mining for iron ores so that steel production can begin. Graphite or other reduced forms of carbon for steel production will be shipped from Earth because or­ganic matter production on Mars by algae is a slow process. Alternatively, lightweight modular carbon fiber elements to mount solar panels will be brought from Earth to Mars. Once the 170 kW solar plant is estab­lished, melting ice for the methanogenic elec­trolysis reactors (MER) begins. Combined power from the nuclear fission reac­tor (NFR) and the solar plant will be used. Only when the amount of methane for a failsafe return (Orion capsule size) has been produced, power will be diverted into three equal parts: continue to melt ice for algae, start or continue to mine ores for thermal collectors, continue to produce methane. After enough thermal collectors are produced to pace ice melting with algal growth, electrical ice melting will be shut off. Now, electrical energy is used for steel production to install more solar panels and meth­ane production until sufficient for a comfortable (more payload) return. By this time, the crew is arriving and decides what the highest priorities are. We recommend to focus on accelerating algal growth for oxygen and biomass production as complete independence from Earth requires the production of or­ganic carbon from CO2.

The composition of Martian soil as analyzed by NASA’s Curiosity and other rovers (Source: NASA 2012)

Energy need for the Mars Transfer Vehicle

Fuel produced on Mars will serve 3 purposes:

  • Failsafe: return of one Orion-sized vehicle
  • Desired: production of return fuel allowing for less stress (more comfort) during transit
  • Energy storage at night or power failure

Two options for Mars-Earth return seem possible. (1) Option one was suggested by NASA’s DRA 5.0 and involves one Orion-like vehicle of about 12 tons and a travel speed of 14 km/s⁠. This option requires only one vehicle but provides less comfort for the long journey home and hence, it exposes the crew to higher stress. However, it uses less fuel therefore enables a faster completion of the first Mars mis­sion. (2) We envision a second option that involves two vehicles, one Orion-like lift-off vehicle for transport of 6 crew members into a Mars orbit of 250 km and one transit vehicle to return to Earth. As option two is the preferred option, we recommend to use option one, the Orion-only scenario, as fail­safe scenario.

We assume the capacity of a manned Orion capsule with Dragon thrusters (Draco) as reference. The Orion-sized vehicle can carry 6 crew members and has a weight of 12 tons including fuel. For a Mars lift-off, a thrust-mass ratio of at least 5 N/kg will be needed resulting in 60 kN thrust or 150 Draco thrusters to lift 6 crew members. The travel time from Martian surface to a 250 km orbit would be 7 minutes at full thrust. About 600 m3 methane (at Earth atmospheric pressure) would be required for the lift-off. To produce this amount of methane at 210 kW (40 kW nuclear fission and 170 kW solar power, see “Surface habitat energy needs”), 3 years of dedicated fuel production are necessary. The proposed solar power installations of 1,400 m2 perovskite solar cells can produce ef­fectively 170 kW during a Mars day of 8h (that is, 20 kW/m2 can be captured by perovskite panels). Using the same Orion capsule also for the Mars-Earth transit, another 7 minutes or 17,000 km are required to reach the travel speed of 14 km/s and approximately the same for full stop. To produce enough fuel for the Mars-Earth transit, only 3 more days are required. Once enough fuel for the failsafe scenario has been produced, the crew will leave the lower Earth orbit heading for Mars.

For the comfort scenario, we assume the proposed 63-ton crewed payload option from the Mars DRA 5.0 used for Earth-Mars transit⁠. This seems reasonable as most equipment will be left behind on Mars and only the transit habitat, the Orion capsule (for emergencies), engines and fuel are needed. This scenario, however, requires considerably more fuel for which the 210 kW surface power generators must produce methane for at least 42 years. Since this is out of scope, we recommend an extension of the surface power production to 3,500 kW which would reduce the required production of methane to 30 months at 14 km/s travel speed or 12 months at 9 km/s. The reduced stress on the crew justifies the lower travel speed and the higher investment. How­ever, 6.3 tons (corresponding to 0.18 km2) of perovskite solar panels will be required to produce sufficient fuel. About 280 tons of steel are necessary as structural support for this sce­nario. Since 900 kWh/ton of steel will be con­sumed for electric smelting⁠, only about one year of steel production using a 40 kW NFR (or 2 months using the full combined 210 kW) justify the increased comfort. The shipment of carbon fiber elements form Earth may completely eliminate the need for steel production in this stage of the mission. The process will be self-accel­erating as power production increases during the solar panel as­sembly process.

To produce methane sufficient for one lift-off, a 200,000 liter MER is required using steel mesh/brush electrodes (anode and cathode) of 2,200 m2 pro­jected surface (see figure below). A modular redundancy system of smaller dimension will improve safety but requires more material. MERs have the theoretical capacity to produce sufficient ascent fuel for one Orion capsule within less than one or two days, if power sup­ply were sufficient. At maximum performance, this reactor would con­sume about 100 GWh or 220 kWh/mol methane. Microbes will facilitate electrolysis at low temperatures and these microbes will be carried by the Mars lander in small (100 mL) redundant batches. Since the limit for methane production is not the reactor capacity but the electrical power available, doubling the amount of solar panels will half the time required for methane production and so on. To store the so produced methane we recommend to first pressurize water to 200 bar before it is injected into the MER. To extract as from the electrolyte, a small pressure reduction is needed and the so obtained gas phase is then conducted into pressurized steel tanks for later use.

An experimental MER needs to be constructed on Earth to prove this concept. Like the Mars reactor, this experimental MER will be a 5 x 5 m cylindrical reactor of ei­ther one or two chambers each. The advantage of the two-chamber system is the separation of oxygen and methane but it requires more water while the single chamber reactor is easier to build and holds less water but O2/CH4 separation is required after production. Unfortunately, the ratio of oxygen to methane is difficult to predict as it depends on the anodic pH. A mass ration greater than 2:1 is required. We therefore propose the use of algae as additional oxygen source (see “Photosynthesis crater to produce oxygen and biomass”). As electrodes brush or spiral steel mesh electrodes will be used. Steel mesh (40 x 40 mesh) produced on Mars will be used having a 1,100 m2 projected surface of each electrode.

A detailed description of the reactor can be found here.

Alternative oxidants in cold methane fuel cells or rocket fuel

It is anticipated that oxygen scarcity imposes severe limitations on any manned Mars mission. Oxygen is crucial as propellant and for any human presence. The use of methane for energy storage makes only sense if there is an adequate electron acceptor. While methane can be burnt in turbines at acceptable efficiencies for electricity production, it may also be used in fuel cells. However, no catalysts exist that oxidize methane on electrodes at room temperature or below. The only possible exception are anaerobic methane oxidizing consortia that naturally use biological electron transport chains. The use of biological electron transport chains opens the possibility to capture the energy stored in electrons during transport. Since this is electron acceptor independent, oxidized metal minerals, which are abundant on Mars, can be used as electron acceptors. The disadvantage of this methane fuel cells is that less energy will be captured compared with oxygen. Also, they only exist in theory.

(1) CH4 + 2 O2 → HCO3 + H+ + H2O ;∆G°’ = −830 kJ/molCH4

(2) CH4 + 4 Fe2O3 + 15 H+ → HCO3 + 8 Fe2+ + 9 H2O ;∆G°’ = −250 kJ/molCH4

The high acidity on Mars, however, is in favor of reaction, forming additional water from iron oxides and protons using the reductive power of methane. Soluble Fe2+ may be used for electric steel production as the reduction of Fe2+ to Fe0 requires a considerably lower redox potential and therefor lower energy.

Martian perchlorate salts may serve as oxidant in rocket fuel. Ammonium perchlorate and, on Mars, the more prevalent calcium perchlorate are explosive oxidizers. To transform calcium perchlorate into the ammonium salt, ammonium can be produced by a variety of microbial process such as nitrogen fixation (by way of the nitrogenase enzymes) and catabolic ammonification of amino acids or waste urea (by the urease enzyme). Should collection and compression of the photosynthetically derived O2 gas prove impractical for in rocket fuel, our solid oxidizer approach is also submitted. This dual oxidizer strategy will provide for far greater flexibility and more breathable oxygen. Mined perchlorate may also be used to disinfect water.

Photosynthesis crater to produce oxygen and biomass

Oxygenic biophotolysis of water using psychrophilic (cold-loving), dinitrogen fixing cyanobacteria, i.e. blue green algae, grown in covered craters is one proposed plausible means of generating the need for oxygen and biomass. The surplus of oxygen will be required as propellant and component of artificial air in the SHAB. While initially not crucial for a Mars mission, the production of organic matter is useful for more extended missions with larger teams and longer presence. Organic matter is essential for rich soil which, in turn, is pivotal for vegetable food production on Mars. Moreover, cyanobacteria and algae require little engi­neering and energy which makes them ideal for autonomous production of utility compounds such as organic matter and oxygen.

The amount of damaging cosmic rays and UV can be higher due to the lack of an ozone layer and protective magnetosphere. The amount of cosmic radiation (est. 0.076 Grays per year) is certainly within the tolerable range for many Earthly microbes as it is only around what the interior of the international space station is exposed to. UV light, with its shorter wavelength, can be readily blocked by a thin covering of Martian soil whereas longer wavelengths of photosynthetically active radiation can penetrate further. The microbes will be selectively enriched in their survival zones. Alternatively, a UV protective cover could be used over the crater. The lightweight but durable and robust crater coverings could take the form of an inflatable inverted dome anchored around the crater edge by cables and spikes. The clear upper canopy would admit sunlight but have coating to block harmful radiation while the curved lower surface could be reflective (to maximize photosynthesis) or black to absorb heat. Solar powered gas pumps could adjustably increase the internal gas pressure to accelerate carbon and nitrogen fixation rates and water accumulation from the trace water vapor available.

Conversion of limited amount of solar energy and frozen water plus copious CO2 into biologically generated oxygen plus organic matter will require a phototroph capable of survival at extremely low temperatures consistent with the Martian surface. We propose to identify terrestrial cyanobacteria capable of this by selectively enriching them from mixed biofilm consortia obtained from the Earth’s Arctic and Antarctic regions⁠. Samples obtained from rocky coastal brines will be subjected to intensive evaluation in selective enrichment freezers outfitted to replicate the polar Martian habitat. The finding that the lichen Pleopsidium chlorophanum (gold cobblestone lichen) can survive⁠, adapt and grow under Martian environmental conditions bodes well for this approach.

During the initial surface resource utilization phase (see figure above), the growth of algae is the most time consuming step and therefore the production of liquid water has highest priority. The use of craters will eliminate the need for containers for growing and there reduce the amount of material brought to Mars. Ideally, such craters are equatorial flat water ponds that ensure maximum sunlight capture and minimum water reheating. These preconditions do not align with the initial mission setup (landing near polar ice caps) but should be prepared during the first mission. That is, water pipes from the circum polar regions to the equatorial areas must be constructed. The pipes may need to be heated which requires additional energy or heating the melted water to high temperatures and pressures to prevent ice formation during transport.

Production of water as medium for methanogenic electrolysis and algae

The lack of liquid water is a major hindrance since active metabolism requires a fluid aqueous medium. In addition to the production of methane fuel, melting ice is the greatest challenge for the first manned Mars mission. Liquid water is essential for MERs and algae craters. Hence, all excess of heat or electrical power produced should be directed to melting of ice after methane fuel production is secured. The obtained CO2-rich brine is the electrolyte for MERs. The high acidity is not inhibitory for microbial growth as acidophilic methanogens⁠ and algae⁠ were reported from terrestrial environments. Fortunately, the low pH will reduce the electrical overpotential needed for hydrogen generation, which is the intermediate step during methanogenic electrolysis⁠. The low pH, on the other hand, inhibits oxygen formation which is why corrosion of steel anodes is anticipated to become a possible problem. Anode corrosion must be monitored and shall not exceed a certain, yet to be determined, threshold before spent anodes are recycled in steel foundries brought with the first mission.

The low temperatures on Mars that reach only 20°C in equatorial regions also impose a major hurdle on liquid water maintainance. That is, water may need to be heated by parabolic heat collectors to remain liquid. However, Fischer et al. recently found that “when the salts are in contact with water ice, liquid brine forms in minutes, indicating that aqueous solutions could form temporarily where salts and ice coexist on the Martian surface and in the shallow subsurface.” If our crater canopy is fitted with an internally reflective coating in the infrared spectrum small green houses can be created an the brine will remain liquid longer.

The MERs use methanogenic microorganisms for methane production which will be brought to Mars along with algae seeds by the landers. The methanogenic microbes are highly efficient in methane production, resulting in electricity capturing efficiencies close to 100%⁠. Precious metal catalysts are not required. In contrast, for effective oxygen production, platinum or palladium coating may be required on the anodic side of the MERs. Anodic algae appear to be an alternative but need to be further explored. Since the amount of platinum used is very low, it may be transported as salt to Mars and electroplated on steel electrodes once they are ready. Electroplating is an easy procedure so that a robot can accomplish this task within few minutes. However, platinum recycling requires 1-2 days of work of one crew member.

About 280 tons of steel for structural support of solar panels are required (see “Production of steel for structural support of Mars surface components”). The carbon content of steel should not exceed 2.1% to guarantee high stability and therefore we chose 1.5% carbon for Mars steel. That is, about 4 metric tons of carbon are required for steel produc­tion. This is the bottleneck of steel production. Assuming cold conditions on Mars comparable to the Antarctic, a good approximation for biomass concentration in brine is 5 mg/m3. At this concentration, nearly 1 billion m3 water need to be processed. While the existing amount of 821,000 km3 would be more than sufficient, it is impossible to melt this amount of ice within the mission’s timeframe using an NFR of 40 kW even if other power sources were counted in. Therefore, parabolic heat collec­tors could be shipped from Earth as well. At an energetic efficiency of parabolic heat collectors of 80%, 300 tons would be required to melt this amount water within 2 years. Using 10 tons of parabolic collectors, one can melt only 26,000 m3 for algae during 2 years. This is enough to produce 130 g algal carbon in little more than 2 years assuming a constant concentration of 5 mg/m3. It is more efficient to bring 4 tons of graphite to Mars for initial steel production or consider reflecting surfaces other than polished steel.

Alternative use of covered craters to accumulate water using native perchlorates

In light of the considerable difficulties associated with installing long water pipelines to fill craters with water, we outline an elegant alternative strategy for gradually capturing water from the atmosphere using native perchlorates in the Martian sediments.

Perchlorate salts have been detected in Martian sediments and craters such as the Dale Crate and at concentrations of 0.5-1% globally. Calcium perchlorate is an extremely hygroscopic component of the Martian soil that was recently discovered to cyclically draw H2O from the Martian atmosphere into the soil by night to form saline liquid brine⁠. By sealing the crater covers by day, when the water normally sublimates off, and then opening in-current or one way valves by night after photosynthetically formed O2 has been recovered, atmospheric water vapor can slowly be accumulated as brine liquid/ice inside of the crater at zero or minimal energetic cost.

Halophilic algae tolerate high salt concentrations and low temperatures. Due to the high concentration of CO2 in the native atmosphere the crater covering will amplify the warming effects of this greenhouse gas to prolong the duration of liquid state water needed for nitrogen and carbon fixation. By transporting more soil-derived perchlorates, possibly with crushed water ice deposits, into the covered crater, water can slowly be accumulated. Biologically formed nitrous oxide gas might further accentuate the internal greenhouse warming and thus biological rates of activity.

Perchlorates salts from inside the crater can be recovered from the liquid brine to gradually reduce the salinity of the water. This could be done using parabolic evaporation troughs periodically lifted above the briny surface. Since perchlorates are considered a human toxin, they can be removed by some microbes such as perchlorate reducing bacteria (PRBs) which use percholorates as alternative electron acceptors. Such PRBs could be introduced at a later stage to eventually render the water-filled covered craters non toxic to higher forms of life.

Soil conditioning through phototrophic primary productivity

Lichen and blue green algae have both been used as foods on Earth for hundreds of years. Spirulina is one example of a widely consumed cyanobacterium that uses sunlight to synthesize essential vitamins, antioxidants like beta-carotene and fatty acids from CO2. One major advantage of using a nitrogen fixing cyanobacterium is that they can use solar energy to convert atmospheric nitrogen gas directly into the essential amino acids that future manned missions will need to build and maintain muscle on the Red planet. This will reduce the amount of fuel spent on shuttling food supplies in. Surprisingly, some species of cyanobacteria contain 60% protein per dry gram which is more protein than beefsteak, without the high amount of deleterious cholesterol. Gaseous nitrogen makes up roughly 2.7% of Mars’ thin atmosphere and is available globally. Nitrogen gas is not the only bioavailable form of nitrogen needed to grow oxygenic phototrophs. Nitrates are an ideal fertilizer. The Curiosity rover identified bioavailable nitrates as a significant component of the sediment on Mars⁠. Trace elements are also present in rocks and soil but may require processing.

Soil conditioning of the Martian landscape initiated by this pioneer mission would be required for subsequent longer term human habitation and colonization. Lichen and cyanobacteria are common pioneer species on Earth that grow in the rocky wake of retreating glaciers. These phototrophs are known to accelerate rock weathering and to facilitate the release of essential minerals. Phosphorous, much like nitrogen, is an important macro-phytonutrient that is now known to be a significant component of the Martian surface. Indeed, some nitrogen fixing cyanobacteria can up-regulate their expression of phosphorous liberating phytase enzymes when exposed to phosphorus limitation⁠. Cyanobacteria also build and stabilize soils by reducing their susceptibility to wind erosion through formation of organic extracellular polysaccharides that help trap and retain moisture. Lichen can also release acids and metabolites that contribute to rock break down and soil formation. While lichens and cyanobacteria may adapt to higher UV light dosages on Mars, they can be protected initially by a thin cover as described in “Photosynthesis crater to produce oxygen and biomass”.

Production of steel for structural support of Mars surface components

Steel cannot be brought to Mars as there will be at least 2.2 tons of steel necessary for structural support for 1,400 m2 perovskite solar panels. While lightweight carbon fiber modules could be used as structural support, it is possible to produce steel in situ. Steel production on Mars seems an apparent alternative to transport of construction material given the abundance of iron, nickel and titanium on Mars. However, it also requires organic carbon which is to be produced by CO2-fixing algae that grow first in enrichment tanks (transparent plastic bags) and later in covered craters. After dehydrating the algae medium, recycling and reheating it, dry algae pel­lets will be used as supplement for steel production. The dehydration and reheating process require additional energy which can be provided as heat using parabolic collectors. Parabolic collectors are more efficient in terms of energy capturing and easier to construct as polished steel can be used as opposed to organic Pb/I composites in perovskite solar cells. Steel is then shaped and pol­ished to build parabolic thermal collectors to melt more ice and provide more energy until the NFR and solar panels can be fully replaced by parabolic collectors that can also produce electricity. The produc­tion of steel is limited by the amount of organic carbon available. Therefore, we recommend to ex­plore the possibility to use methane gas as reductant and carbon source for steel production. Methane gas production is faster and requires less water resources than algae.

The Mars landers will also mine iron ores and silicates for the production of wires, solar panels and construction materials. Steel will be produced in an induction furnace using iron ores and graphite or organic biomass. Organic biomass from algae tanks will be used for steel production. This organic biomass will also be used for graphite production at a later stage of the mission. Alterna­tive furnace concepts are possible. For example, methane can be used as reductant. Another alterna­tive would be an electric arc furnace or sacrificial graphite electrodes. Graphite can be produced from organic carbon as follows

  1. Organic carbon from CO2 by cold adapted algae
  2. Organic carbon + 800ºC → C
  3. C + SiO2 + 1,400ºC → SiC
  4. SiC + 4,200ºC → graphite

The steel and graphite induction furnaces will be carried by the landers

Energy for initial steel production for the construction of the 170 kW solar plant (see “SHAB energy needs”) is produced by an NFR. A 40 kW reference NFR is recommended. Steel production from iron ore using electric smelting requires 900 kWh per ton of steel⁠. That is, to produce enough structural support for solar panels for 6 people, about 2,000 kWh are necessary or about 3 days of energy production at full performance. This is based on the assumption that steel of 2 mm thickness and 10% of the perovskite area of 1,400 m2 is sufficient. To produce enough structural support for 3.5 MW (0.18 km2) perovskite solar cells needed for the comfortable return option, 280 tons of steel are required. At 50 days of steel production using the entire 210 kW (40 kW NFR + 170 kW solar per­ovskite life support) are anticipated. To add 1.5% carbon, 4 tons of graphite are necessary which will be carried as cargo from Earth.

Steel is necessary for parabolic heat collectors on site as well. Parabolic heat collectors are required for melting ice for algae growth. The production of steel sufficient for parabolic collectors to melt 1 billion m3 of ice is approximately 600 tons, i.e. 9 additional tons of graphite need to be shipped. To make this amount of steel on Mars another 2 years will be necessary at least. This appears to be the best trade-off between cargo transport and waiting time for a Mars mission but is still an approximation. The algal side product oxygen, also justifies this approach. Since the launch of a manned mission is not dependent on algae production, it is not counted as wait time, which would add another 4 years prior to launch. This shall only demonstrate the feasibility of in situ steel production on Mars. As alternative, the use of methane produced on Mars as carbon and electron source for steel needs to be explored as this may eliminate the need for graphite transport or carbon fiber construction materials are transported to Mars.

Possible perovskite production and reuse of lead produced by the nuclear fission reactor

Lead remnants from the 235U nuclear fission reactor aboard the landers can be used as they are a side product of the radioactive decay of contaminating 238U. There are no confirmed higher concentrations of iodine on Mars and this element needs to be brought in the form of elemental iodine, KI or NaI with the lander to produce the PbI and methyl ammonium iodide. However, since iodine can be re­placed by the element chlorine for perovskite production, the initial amount iodine may not need to be replenished because chlorine is an abundant element on Mars.

Solvents required for perovskite cells can be produced in situ using methane gas and acetic acid (also a possible side product of MERs) as precursors as soon as they become available. To bridge the intermediate gap, solvents and organic reactants brought to Mars by the landers may be used. These materials are⁠:

  • N,N-dimethylformamide (solvent)
  • 2-propanol (solvent)
  • <ethylammonium iodide (reactant)
  • 2,2′,7,7′-tetrakis(N, N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD, reactant)

As synthesis of perovskite cells may still be to difficult on Mars, we recommend the transport of these components. As shown below (Surface habitat [SHAB] energy needs), only 9 kg perovskite solar panels will be required. As photo voltaic systems are constantly improved, one can expect better performance and lighter weights in future.

Devices included in this chart of the current state of the art have efficiencies that are confirmed by independent, recognized test labs (NREL, AIST, JRC-ESTI, and Fraunhofer-ISE) and are reported on a standardized basis (Source: NREL 2019)

Surface habitat (SHAB) energy needs

The average energy need per capita in the European Union was 150 GJ/year. Although this estimate is likely too high for a permanent colony on Mars we used this as reference herein. A perovskite solar panel operating at 12% efficiency⁠ can produce 14 MJ/day/m2 from Mars aphelion 8 hour solar radiation assuming 170 W/m2⁠. That is, 240 m2 methyl ammonium lead halide perovskite solar panels are required to sustain the presence of one person on Mars. This requires 350 g mesoporous TiO2, 370 g Au per capita. The light weight of about 720 g per capita makes a transport of these components from Earth to Mars feasible (8.5 kg total). Mounting the thin solar power collector on a stable steel surface can be achieved on Mars but only if steel is produced in situ. NASA’s Human Exploration of Mars Design Reference Architecture recommends a mission of 6 explorers. That is, at about 1,400 m2 perovskite solar panels or 170 kW capacity will be required only to sustain life of one manned explorer mission. Before installation of these panels, steel will have to be produced on which they can be mounted. For the envisioned larger energy needs, 3.5 MW are necessary and for this, about 6.3 tons of perovskite may be shipped to Mars.

Compounds produced on Mars (purpose in brackets)

  • Iron, Fe0 (steel)
  • Steel (construction, wires, electrodes)
  • Graphite (steel, electrodes)
  • Silicium dioxide, SiO2 (silicon carbide, graphite)
  • Silicon carbide, SiC (graphite)

Compounds brought to Mars (with optional later in situ production):

  • Graphite (for initial steel production)
  • Or carbon fiber elements (for construction without steel)
  • Platinum chloride (for electroplating anodes, alternative to steel)
  • Perovskite solar panels (3.3 tons)
  • Mesoporous titanium dioxide, TiO2 (perovskite solar cells, photon trap)
  • Gold (perovskite solar cells, conductor)
  • Lead iodide (perovskite solar cells)
  • N,N-dimethylformamide (perovskite solar cells, solvent)
  • Methylammonium iodide, CH3NH3I (perovskite solar cells, reactant)
  • 2-propanol (perovskite solar cells, solvent)
  • Hydroiodic acid (perovskite solar cells, reactant)
  • Spiro-MeOTAD (perovskite solar cells, reactant)

(Prof. John Pisciotta of the West Chester University contributed to this article. Image: NASA/Wikipedia)

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Framework for a global carbon budget

Over the past decade, numerous studies have shown that global warming is roughly proportional to the concentration of CO2 in our atmosphere. In this way one can estimate our remaining carbon budget. This is the total amount of man-made carbon dioxide that can still be released into the atmosphere before reaching a set global temperature limit. The nations of the world agreed on this limit in the 2015 Paris Agreement. It should not exceed 1.5°C, and in any case be well below 2.0°C. However, diverging estimates have been made for the remaining carbon budget, which has a negative impact on policy-making. Now, an international research group of renown climate experts has published a framework for the calculation of the global CO2 budget in Nature. The researchers suggest that the application of this framework should help to overcome the differences when estimating the carbon budget, which will help to reduce uncertainties in research and policy.

Since the fifth report of the Intergovernmental Panel on Climate Change (IPCC), the concept of a carbon budget has become more important as an instrument for guiding climate policy. Over the past decade, a series of studies  has clarified why the increase in the global average temperature is roughly proportional to the total amount of CO2 emissions caused by human activity since the Industrial Revolution. In the framework, the research group cites numerous published documents that provide evidence for the linearity of this correlation. This literature has allowed scientists to define the linear relationship between warming and CO2 emissions as a transient climate response to cumulative CO2 emissions (TCRE). The linearity is an appealing concept because of the complexity of the Earth’s response to our CO2 emissions. Additional processes that affect future warming have been included in recent models, among them, for example, the thawing of the Arctic permafrost. These additional processes increase the uncertainty of current climate  models. In addition, global warming is not just caused by CO2 emissions. Other greenhouse gases, such as methane, fluorinated gases or nitrous oxide, as well as aerosols and their precursors affect global temperatures. This further complicates the relationship between future CO2.

In the case of global warming caused by CO2, every tonne contributes to warming, whether that ton is emitted in future, now or in the last century. This means that global CO2 emissions must be reduced to zero, and then remain zero. This also means that the more we emit in the next years, the faster we have to reduce our emissions later. At zero emissions, warming would stabilize, but not disappear. It may also reverse. An overdraft of the carbon budget would have to be compensated by removing the CO2 later. One way of removing CO2 from the atmosphere would be a technology called direct air capture, which we reported earlier. Ultimately, this will probably be the only way left, as carbon neutral renewable energy source sources only make up 5% of our energy mix. Establishing a global carbon budget will further highlights the urgency of our clean energy transition. Unfortunately, there is a large divergence when it comes the amount of the CO2 remaining in our carbon budget. In their framework, the researchers cite numerous studies on carbon budgets to maintain our 1.5°C target. Starting 2018, these range from 0 tonnes of CO2 to 1,000 gigatons. For the 2.0°C target, our carbon budget ranges from around 700 gigatons to nearly 2,000 gigatons of remaining CO2 emissions. The aim of the researchers is to limit this uncertainty by establishing a budget framework. The central element is the equation for calculating the remaining carbon budget:

Blim = (TlimThistTnonCO2Thist) / TCRE − EEsfb

The budget of the remaining CO2 emissions (Blim) for the specific temperature limit (Tlim) is a function of five terms that represent aspects of the geophysical and human-environment systems: the historical man-made warming (Thist), the non-CO2 contribution to the future temperature increase (TnonCO2), the zero emission commitment (TZEC), the TCRE, and an adaptation for sources from possible unrepresented Earth system feedback (EEsfb).

 

Term Key choices or uncertainties Type Level of understanding
Temperature limit Tlim Choice of temperature metrics that allow global warming, the choice of pre-industrial reference and consistency with global climate targets Choice Medium to high
Historical man-made warming Thist Incomplete data and methods for estimating the man-made component; see also Tlim Choice and uncertainty Medium to high
Non-CO2 contribution to future global warming TnonCO2 The level of non-CO2 contributions coinciding with global net zero CO2 emissions; depends on policy choices, but also on the uncertainty of their implementation Choice and uncertainty Medium
Non-CO2 contribution to future global warming TnonCO2 Climate reaction to non-CO2 forcers, such as aerosols and methane Uncertainty Low to medium
Zero-emissions commitment TZEC The extent of the decadal zero emission commitment and near-zero annual carbon emissions Uncertainty Low
Transient climate response to cumulative emissions of CO2 TCRE TCRE uncertainty, linearity and cumulative CO2 emissions that affect temperature metrics of the TCRE estimate Uncertainty Low to medium
Transient climate response to cumulative emissions of CO2 TCRE Uncertainty of the TCRE linearity, value and distribution beyond peak heating which is affected by cumulative CO2 emissions reduction
Uncertainty Low
Unrepresented Earth system feedback mechanisms EEsfb Impact of permafrost thawing and duration as well as methane release from wetlands on geomodels and feedback Uncertainty Very low

In the CO2 budget, the unrepresented Earth system feedback (EEsfb) is arguably the greatest uncertainty. These feedback processes are typically associated with the thawing of permafrost and the associated long-term release of CO2 and CH4. However, other sources of feedback have been identified as well. This include, for example, the variations of CO2 uptake by the vegetation and the associated nitrogen availability. Further feedback processes involve changes in surface albedo, cloud cover, or fire conditions.

It remains a challenge to adequately characterize the uncertainties surrounding the estimates of our carbon budget. In some cases, the reason of these uncertainties is inaccurate knowledge of the underlying processes or inaccurate measurements. In other cases the terminology is used inconsistently. For better comparability and flexibility, the researchers propose to routinely measure global surface air temperature values. This method gives robust data for models and model runs over selected time periods. More detailed comparisons between published estimates of the carbon budget are currently difficult because the original data used for publication often are missing. The researchers therefore propose to provide these in the future along with publications.

Breaking down the carbon budget into its individual factors makes it possible to identify a number of promising pathways for future research. One area of ​​research that might advance this field is to look more closely at the TCRE. Future research is expected to narrow down the range of TCRE uncertainties. Another promising area of ​​research is the study of the correlation between individual factors and their associated uncertainties, for example, between uncertainties in Thist and TnonCO2. This could be achieved by developing methods that allow a more reliable estimate of historical human-induced warming. It is also clear that less complex climate models are useful to further reduce the uncertainties of climate models, and hence the carbon budget. Currently, each factor of the framework presented by yhr researchers has its own uncertainties, and there is no method to formally combine them.

At Frontis Energy, too, we think that progress in these areas would improve our understanding of the estimates of our carbon budget. A systematic understanding of the carbon budget and is crucial for effectively addressing global warming challenges.

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Cheap, high-octane biofuel discovered

Researchers from the National Renewable Energy Laboratory (NREL) have developed a cheap method for producing high-octane gasoline from methanol. They recently published their method in the journal Nature Catalysis. Methanol can be synthesized from CO2 via various routes, as we reported last year. Biomass, such as wood, is one possibility.

The production of biofuels from wood, however, is too expensive to compete with fossil fuels. To find a solution to this problem, the researchers combined their basic research with an economic analysis. The researchers initially aimed at the most expensive part of the process. Thereafter, the researchers found methods to reduce these costs with methanol as an intermediate.

So far, the cost of converting methanol to gasoline or diesel was about $1 per gallon. The researchers have now reached a price of about $0.70 per gallon.

The catalytic conversion of methanol into gasoline is an important research area in the field of CO2 recovery. The traditional method is based on multi-stage processes and high temperatures. It is expensive, producing low quality fuel in small quantities. Thus, it is not competitive with petroleum-based fuels.

Hydrogen deficiency was the initially problem the researcher had to overcome. Hydrogen is the key energy containing element in hydrocarbons. The researchers hypothesized that using the transition metal copper would solve this problem, which it did. They estimated that the copper-infused catalyst resulted in 38% more yield at lower cost.

By facilitating the reintegration of C4 byproducts during the homologation of dimethyl ether, the copper zeolite catalyst enabled this 38% increase in product yield and a 35% reduction in conversion cost compared to conventional zeolite catalysts. Alternatively, C4 by-products were passed to a synthetic kerosene meeting five specifications for a typical jet fuel. Then, the fuel synthesis costs increased slightly. Even though the cost savings are minimal, the resulting product has a higher value.

Apart from the costs, the new process offers users further competitive advantages. For example, companies can compete with ethanol producers for credits for renewable fuels (if the carbon used comes from biogas or household waste). The process is also compatible with existing methanol plants that use natural gas or solid waste to produce syngas.