Review

Net-zero emissions energy systems

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Science  29 Jun 2018:
Vol. 360, Issue 6396, eaas9793
DOI: 10.1126/science.aas9793
  • A shower of molten metal in a steel foundry.

    Industrial processes such as steelmaking will be particularly challenging to decarbonize. Meeting future demand for such difficult-to-decarbonize energy services and industrial products without adding CO2 to the atmosphere may depend on technological cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

  • Fig. 1 Schematic of an integrated system that can provide essential energy services without adding any CO2 to the atmosphere.

    (A to S) Colors indicate the dominant role of specific technologies and processes. Green, electricity generation and transmission; blue, hydrogen production and transport; purple, hydrocarbon production and transport; orange, ammonia production and transport; red, carbon management; and black, end uses of energy and materials.

  • Fig. 2 Difficult-to-eliminate emissions in current context.

    (A and B) Estimates of CO2 emissions related to different energy services, highlighting [for example, by longer pie pieces in (A)] those services that will be the most difficult to decarbonize, and the magnitude of 2014 emissions from those difficult-to-eliminate emissions. The shares and emissions shown here reflect a global energy system that still relies primarily on fossil fuels and that serves many developing regions. Both (A) the shares and (B) the level of emissions related to these difficult-to-decarbonize services are likely to increase in the future. Totals and sectoral breakdowns shown are based primarily on data from the International Energy Agency and EDGAR 4.3 databases (8, 38). The highlighted iron and steel and cement emissions are those related to the dominant industrial processes only; fossil-energy inputs to those sectors that are more easily decarbonized are included with direct emissions from other industries in the “Other industry” category. Residential and commercial emissions are those produced directly by businesses and households, and “Electricity,” “Combined heat & electricity,” and “Heat” represent emissions from the energy sector. Further details are provided in the supplementary materials.

  • Fig. 3 Comparisons of energy sources and technologies.

    A) The energy density of energy sources for transportation, including hydrocarbons (purple), ammonia (orange), hydrogen (blue), and current lithium ion batteries (green). (B) Relationships between fixed capital versus variable operating costs of new generation resources in the United States, with shaded ranges of regional and tax credit variation and contours of total levelized cost of electricity, assuming average capacity factors and equipment lifetimes. NG cc, natural gas combined cycle. (113). (C) The relationship of capital cost (electrolyzer cost) and electricity price on the cost of produced hydrogen (the simplest possible electricity-to-fuel conversion) assuming a 25-year lifetime, 80% capacity factor, 65% operating efficiency, 2-year construction time, and straight-line depreciation over 10 years with $0 salvage value (29). For comparison, hydrogen is currently produced by steam methane reformation at costs of ~$1.50/kg H2 (~$10/GJ; red line). (D) Comparison of the levelized costs of discharged electricity as a function of cycles per year, assuming constant power capacity, 20-year service life, and full discharge over 8 hours for daily cycling or 121 days for yearly cycling. Dashed lines for hydrogen and lithium-ion reflect aspirational targets. Further details are provided in the supplementary materials.

  • Table 1 Key energy carriers and the processes for interconversion.

    Processes listed in each cell convert the row energy carrier to the column energy carrier. Further details about costs and efficiencies of these interconversions are available in the supplementary materials.

    To
    FromeH2CxOyHzNH3
    e Electrolysis ($5 to 6/kg H2)Electrolysis + methanationElectrolysis + Haber-Bosch
    Electrolysis + Fischer-Tropsch
    H2Combustion Methanation ($0.07 to 0.57/m3 CH4)Haber-Bosch ($0.50 to 0.60/kg NH3)
    Oxidation via fuel cell Fischer-Tropsch ($4.40 to $15.00/gallon of gasoline-equivalent)
    CxOyHzCombustionSteam reforming ($1.29 to 1.50/kg H2) Steam reforming + Haber-Bosch
    Biomass gasification ($4.80 to 5.40/kg H2)
    NH3CombustionMetal catalysts (~$3/kg H2)Metal catalysts + methanation/Fischer-Tropsch
    Sodium amide

Supplementary Materials

  • Net-zero emissions energy systems

    Steven J. Davis, Nathan S. Lewis, Matthew Shaner, Sonia Aggarwal, Doug Arent, Inês L. Azevedo, Sally M. Benson, Thomas Bradley, Jack Brouwer, Yet-Ming Chiang, Christopher T. M. Clack, Armond Cohen, Stephen Doig, Jae Edmonds, Paul Fennell, Christopher B. Field, Bryan Hannegan, Bri-Mathias Hodge, Martin I. Hoffert, Eric Ingersoll, Paulina Jaramillo, Klaus S. Lackner, Katharine J. Mach, Michael Mastrandrea, Joan Ogden, Per F. Peterson, Daniel L. Sanchez, Daniel Sperling, Joseph Stagner, Jessika E. Trancik, Chi-Jen Yang, Ken Caldeira

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
    • References 

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