Current US electricity generation (4.2 x 1012 kWh, [46]) could produce roughly 24% of the Nation’s crude oil demands; the US installed base operating at 100% utiliza­tion (1.12 TW nameplate capacity, [15]) could produce roughly 55% of the Nation’s crude oil demands (Supp Calc 5). The economical production of fuel at even moder­ate scales would require significant renewable energy feedstocks: at least 1 GW of renewable electricity would be required to synthesize 10,000 BOE/day via an Electrofuels approach (Supp Calc 5). No electrochemical process has ever approached such scales and implementation will clearly require significant advances in engineering. Still, the potential advantages of an Electrofuels platform for the production of liquid fuels through processes that avoid the constraints of photosyn­thetic approaches is hard to overstate; replacement of 100% of the Nation’s gasoline demand using 20% efficient solar collectors could be achieved harvesting the solar resource of approximately 7,000 mi2 of nonarable land (Supp Calc 10), an area less than 5% of the acreage currently used for corn production in the USA [54].

4 Conclusions

The Electrofuels program offers an opportunity to transform the US energy infra­structure. By converting renewable electricity at times of low demand into fungible liquid fuels, engineered chemolithoautotrophic organisms could enable further inte­gration of wind and solar energy on the electricity grid while diminishing US depen­dence on foreign oil. The technical challenges remain significant, and the approach is still in its infancy. Those challenges notwithstanding the use of nonphotosynthetic autotrophic organisms offers a feedstock flexible and potentially high efficiency synthesis of liquid fuels directly from renewable energy resources, without compe­tition for arable land or scarce water resources. Continued development of the Electrofuels program offers a hedge against future resource constraints and the costly and unpredictable supply of foreign oil.

The Advanced Research Projects Agency (ARPA-E). ARPA-E was created by the US Congress in 2007 to enhance our energy and economic security, strengthening national security through the way we generate, store, and use energy. ARPA-E invests in and manages the development of transformational energy technologies that hold the potential to radically shift our Nation’s energy reality. Modeled after the Defense Advanced Research Projects Agency (DARPA), ARPA-E considers high-risk/high-impact routes to energy innovation. ARPA-E aims to promote the rapid development of technologies toward a point where private investment funds demonstration at scale and deployment.

Acknowledgments We thank Dr. Nicholas Cizek, Dr. Philippe Larochelle, Dr. Dawson Cagle, and Mr. Gregory Callman for their contributions to this chapter. We thank the 13 ARPA-E Electrofuels performer teams whose efforts make these goals possible.

Appendix A. Supplementary Calculations

A.1 Supplementary Calculation 1: Energy Captured by Corn-to-Ethanol

Critical values

Annual solar radiance: 4.1 kWh/m2/day (Des Moines, IA—[40]) Corn yield Midwest: 152.8 bushels/ac-yr [55]

Corn-to-ethanol yield: 2.8 gal ethanol/bushel corn [6, 10]

Annual solar energy

(4.1kWh/m2 /day) x (3.6 x 106 J/kWh) x (4,047m2 /ac) x (365days/yr) = 2.2 x 1013 J/ac — yr

Annual processed fuel energy

(152.8bushels corn/ac — yr) x (2.8gal EtOH/bushel corn) x(3.785L EtOH/gal EtOH) x (24x 106 J/L EtOH)

= 3.9x 1010 J/ac — yr

Fraction solar energy harness in ethanol fuel (3.9 x 1010 J/ac — yr) / (2.2 x 1013 J/ac — yr) = 0.18%

A.2 Supplementary Calculation 2: Energy Captured by Sugarcane-to-Ethanol

Critical values

Annual solar radiance: 5.6 kWh/m2/day (Brazil—[21, 23])

Brazil sugarcane yield: 33.4 tons harvested/ac-yr [55] Sugarcane-to-ethanol yield: 19.5 gal ethanol/ton sugarcane [53]

Annual solar energy

(5.6kWh/m2 /day) x (3.6 x 106 J/kWh) x (4,047m2 /ac) x (365days/yr) = 3.0 x 1013 J/ac — yr

Annual processed fuel energy

(33.4tons sugarcane/ac — yr)x (19.5gal EtOH/ton sugarcane) x(3.785L EtOH/gal EtOH) x (24x 106 J/L EtOH)

= 5.9 x 1010 J/ac — yr

Fraction solar energy harnessed in ethanol fuel (5.9 x 1010 J/ac — yr) / (3.0 x 1013 J/ac — yr) = 0.20%

A.3 Supplementary Calculation 3: Minimum Water Used by Electrofuels

Critical values

4CO2 + 5H2O + 28e — ^ 1C4H10O + 6O2 (Electrofuels cell metabolism, 20e — to convert 2CO2 to 2AcCoA, 8e — to convert 2AcCoA to 1BuOH, assuming no diverted energy)

Minimum water use

(5mol H2O/1mol BuOH)x (18g H2O/1mol H2O) x (1mol BuOH/74g BuOH) x(1,000g BuOH/36.6 x 106 J) x (1L H2O/1,000g H2O) x (34.2 x 106 J/L gasoline)

= 1.1gal H2O/gal of gasoline equivalent

A.4 Supplementary Calculation 4: Solar Efficiency of Electrofuels

Critical values

4CO2 + 5H2O + 28e — ^ 1C4H10O + 6O2 (Electrofuels cell metabolism) Efficiency of solar panel: 20% [25]

Voltage of delivered electricity: 1.5 V [39]

Solar efficiency of electrofuels

(0.2J e- /J solar energy) x (1C(@1.5V) /1.5J e-) x (6.24 x 1018 e — /C) x(1BuOH/28 e-) x (1mol BuOH/6.02 x 1023BuOH) x(74g BuOH/mol BuOH) x (36.6 x 106 J/1,000g BuOH)

= 13.3%

= 29.1%

(with state of art 43.5% solar cell, [25])

A.5 Supplementary Calculation 5: Land Requirements for Fuels Production

Critical values

Electrofuels solar efficiency: 13.3% (Supp Calc 4)

Joule unlimited solar efficiency: 7.2% [49]

Annual solar radiance: 5.7 kWh/m2/day (El Paso, TX — [40])

Land use requirements

(5.7kWh/m2 /day) x (3.6 x 106 J/kWh) x (4,047m2 /ac) x(1BOE/6.12 x 109J) x (365days/yr)

= 4,950BOE/ac — yr(solar energy)

= 660BOE/ac — yr(Electrofuels@13.3% Efficiency-Wood-Ljundahl)

= 360BOE/ac — yr(Electrofuels@7.2% Efficiency-3 — hydroxypropionate bicycle or Calvin cycle, anaerobic)

= 360BOE/ac — yr(Joule Unlimited@7.2% Efficiency)

A.6 Supplementary Calculation 6: Electrofuels Production from Wind Energy

Critical values

Annual wind energy: 0.053 kWh/m2/day (North Dakota—[41])

4CO2 + 5H2O + 28e — ^ 1C4H10O + 6O2 (Electrofuels cell metabolism)

Electrical efficiency of electrofuels

(1C(@1.5V)/1.5Je-) x (6.24 x 1018 e- /C) x (1 BuOH/28e-)

x(1mol BuOH/6.02 x 1023 BuOH) x (74g BuOH/mol BuOH) x(36.6 x 106 J/1,000g BuOH)

= 66.8%

Land use requirements

(0.053kWh/m2 /day) x (3.6 x 106 J/kWh) x (4,047m2 /ac) x(1 BOE/6.12 x 109 J) x (365 days/yr)

= 46 BOE/ac — yr (Wind Energy Energy)

= 31 BOE/ac — yr (Electrofuels @ 66.8% Efficiency-Wood-Ljungdahl) = 17 BOE/ac — yr (Electrofuels @ 36.0%

Efficiency-3 — hydroxypropionate bicycle or Calvin cycle, anaerobic)

A.7 Supplementary Calculation 7: Electricity Requirements for Fuel Production

Critical values

4CO2 + 5H2O + 28e- ^ 1C4H10O + 6O2 (electrofuels cell metabolism)

Voltage of delivered electricity: 1.5 V [39]

US electricity nameplate capacity: 1.12 TW [15]

US electricity generation: 4.2 x 1015 Wh/yr [46]

US annual petroleum consumption: 7.0 x 109 barrels of petroleum [15]

Electricity required to produce 10,000 BOE/day

(10,000 BOE / day) x (6.12 x 109 J/BOE) x (1kg BuOH/36.6 x 106 J)

x(28mole — /0.074kg BuOH)x(6.02x 1023e-/mole-) x(1C/6.24 x 1018 e-) x (1.5J/1C(@1.5V)) x (1 day/86,400s)

= 1.06GW

Possible fuel production from current US electricity generation (4.2x 1015 Wh/yr) x (10,000 BOE/day/1.06x 109 W)x (1 day/24 h)

= 1.7 x 109 BOE/yr (24% U. S. annual consumption)

Possible Fuel Production from Current US Electricity Capacity ( @ 100% capacity factor)

(1.12 x 1012 W) x (10,000 BOE/day/1.06 x 109 W) x (365days/yr)

= 3.9x 109 BOE/yr (55% U. S. annual consumption)

A.8 Supplementary Calculation 8: Cost of Electrofuels from Electricity

Calculations for the cost of electricity, CO2 , labor, maintenance, taxes, materials, waste, water, and value of O2 coproduct and can be derived from base costs in Table 3, and Supplementary Calculations 1-7. The annual cost of capital is derived by numerically solving for a net present value (NPV) of zero. This can be done assuming: 3-year construction period with constant spending rate, working capital equal to 15% of the total capital investment, 20-year facility life, 5-year MARCS depreciation of capital, 10% interest rate, 2% inflation rate, 50% nameplate utiliza­tion in year 1, 75% in year 2, and 100% in years 3-20, and 10% down time in years 1-20.

A.9 Supplementary Calculation 9: Cost of Electrofuels from Natural Gas

Critical values

4CO2 +14 H2 + O2 ^ 1C4H10O + 9H2O (Electrofuels cell metabolism)

Cost of H2: $1/kg H2 (on-site production; [32])

Cost of CO2: $0/ton (coproduced with H2 effluent)

25% increase in the cost of capital No O2 coproduct

Balance of systems cost: $0.92/GGE

Cost of feedstock

(14mol H2 /mol BuOH) x (2g H2 /mol H2) x (1kg/1,000g) x($1/kg H2) x (1mol BuOH/74g BuOH) x (1,000g/kg) x(1kg BuOH/36.6x 106 J) x (34.2x 106 J/L) x (3.785L/gal)

= $1.34/GGE (H2 and CO2 combined cost contribution)

= $2.25/GGE (overall cost)

A.10 Supplementary Calculation 10: Land Use Requirements for Electrofuels

Critical values

US daily crude oil consumption: 19,148,000 barrels/day [15]

US daily gasoline consumption: 9,034,000 barrels/day [15]

Electrofuels solar efficiency: 13.3% (Supp Calc 4)

Annual solar radiance: 5.7 kWh/m2/day (El Paso, TX — [40])

Land use requirements

(19.148 x 106barrel crude oil/day) x (6.12 x 109 J/barrel crude oil) x(1kWh/3.6 x 106 J) x (1m2 /day/5.7kWh) x (1ac/4,047m2) x(1J solar/0.133J fuel)

= 10.6 x 106 acres = 16,600mi2

= 129mi x 129mi (for total U. S. crude oil production)

(9.034 x 106 barrel gasoline/day) x (42 gal/barrel) x (3.785L/gal) x(34.2 x 106 J/L) x (1kWh/3.6 x 106 J) x (1m2 /day/5.7kWh) x(1ac/4,047m2) x (1J solar/0.133J fuel)

= 4.4 x 106 acres = 6,950mi2 = 83mi x 83mi (for total U. S. gasoline production)