Review

# Net-zero emissions energy systems

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Science  29 Jun 2018:
Vol. 360, Issue 6396, eaas9793
DOI: 10.1126/science.aas9793

## Path to zero carbon emissions

Models show that to avert dangerous levels of climate change, global carbon dioxide emissions must fall to zero later this century. Most of these emissions arise from energy use. Davis et al. review what it would take to achieve decarbonization of the energy system. Some parts of the energy system are particularly difficult to decarbonize, including aviation, long-distance transport, steel and cement production, and provision of a reliable electricity supply. Current technologies and pathways show promise, but integration of now-discrete energy sectors and industrial processes is vital to achieve minimal emissions.

Science, this issue p. eaas9793

## Structured Abstract

### BACKGROUND

Net emissions of CO2 by human activities—including not only energy services and industrial production but also land use and agriculture—must approach zero in order to stabilize global mean temperature. Energy services such as light-duty transportation, heating, cooling, and lighting may be relatively straightforward to decarbonize by electrifying and generating electricity from variable renewable energy sources (such as wind and solar) and dispatchable (“on-demand”) nonrenewable sources (including nuclear energy and fossil fuels with carbon capture and storage). However, other energy services essential to modern civilization entail emissions that are likely to be more difficult to fully eliminate. These difficult-to-decarbonize energy services include aviation, long-distance transport, and shipping; production of carbon-intensive structural materials such as steel and cement; and provision of a reliable electricity supply that meets varying demand. Moreover, demand for such services and products is projected to increase substantially over this century. The long-lived infrastructure built today, for better or worse, will shape the future.

Here, we review the special challenges associated with an energy system that does not add any CO2 to the atmosphere (a net-zero emissions energy system). We discuss prominent technological opportunities and barriers for eliminating and/or managing emissions related to the difficult-to-decarbonize services; pitfalls in which near-term actions may make it more difficult or costly to achieve the net-zero emissions goal; and critical areas for research, development, demonstration, and deployment. It may take decades to research, develop, and deploy these new technologies.

A successful transition to a future net-zero emissions energy system is likely to depend on vast amounts of inexpensive, emissions-free electricity; mechanisms to quickly and cheaply balance large and uncertain time-varying differences between demand and electricity generation; electrified substitutes for most fuel-using devices; alternative materials and manufacturing processes for structural materials; and carbon-neutral fuels for the parts of the economy that are not easily electrified. Recycling and removal of carbon from the atmosphere (carbon management) is also likely to be an important activity of any net-zero emissions energy system. The specific technologies that will be favored in future marketplaces are largely uncertain, but only a finite number of technology choices exist today for each functional role. To take appropriate actions in the near term, it is imperative to clearly identify desired end points. To achieve a robust, reliable, and affordable net-zero emissions energy system later this century, efforts to research, develop, demonstrate, and deploy those candidate technologies must start now.

### OUTLOOK

Combinations of known technologies could eliminate emissions related to all essential energy services and processes, but substantial increases in costs are an immediate barrier to avoiding emissions in each category. In some cases, innovation and deployment can be expected to reduce costs and create new options. More rapid changes may depend on coordinating operations across energy and industry sectors, which could help boost utilization rates of capital-intensive assets, but this will require overcoming institutional and organizational challenges in order to create new markets and ensure cooperation among regulators and disparate, risk-averse businesses. Two parallel and broad streams of research and development could prove useful: research in technologies and approaches that can decarbonize provision of the most difficult-to-decarbonize energy services, and research in systems integration that would allow reliable and cost-effective provision of these services.

## Abstract

Some energy services and industrial processes—such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing—are particularly difficult to provide without adding carbon dioxide (CO2) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO2 to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

People do not want energy itself, but rather the services that energy provides and the products that rely on these services. Even with substantial improvements in efficiency, global demand for energy is projected to increase markedly over this century (1). Meanwhile, net emissions of carbon dioxide (CO2) from human activities—including not only energy and industrial production, but also land use and agriculture—must approach zero to stabilize global mean temperature (2, 3). Indeed, international climate targets, such as avoiding more than 2°C of mean warming, are likely to require an energy system with net-zero (or net-negative) emissions later this century (Fig. 1) (3).

Energy services such as light-duty transportation, heating, cooling, and lighting may be relatively straightforward to decarbonize by electrifying and generating electricity from variable renewable energy sources (such as wind and solar) and dispatchable (“on-demand”) nonrenewable sources (including nuclear energy and fossil fuels with carbon capture and storage). However, other energy services essential to modern civilization entail emissions that are likely to be more difficult to fully eliminate. These difficult-to-decarbonize energy services include aviation, long-distance transport, and shipping; production of carbon-intensive structural materials such as steel and cement; and provision of a reliable electricity supply that meets varying demand. To the extent that carbon remains involved in these services in the future, net-zero emissions will also entail active management of carbon.

In 2014, difficult-to-eliminate emissions related to aviation, long-distance transportation, and shipping; structural materials; and highly reliable electricity totaled ~9.2 Gt CO2, or 27% of global CO2 emissions from all fossil fuel and industrial sources (Fig. 2). Yet despite their importance, detailed representation of these services in integrated assessment models remains challenging (46).

Here, we review the special challenges associated with an energy system that does not add any CO2 to the atmosphere (a net-zero emissions energy system). We discuss prominent technological opportunities and barriers for eliminating and/or managing emissions related to the difficult-to-decarbonize services; pitfalls in which near-term actions may make it more difficult or costly to achieve the net-zero emissions goal; and critical areas for research, development, demonstration, and deployment. Our scope is not comprehensive; we focus on what now seem the most promising technologies and pathways. Our assertions regarding feasibility throughout are not the result of formal, quantitative economic modeling; rather, they are based on comparison of current and projected costs, with stated assumptions about progress and policy.

A major conclusion is that it is vital to integrate currently discrete energy sectors and industrial processes. This integration may entail infrastructural and institutional transformations, as well as active management of carbon in the energy system.

## Aviation, long-distance transport, and shipping

In 2014, medium- and heavy-duty trucks with mean trip distances of >160 km (>100 miles) accounted for ~270 Mt CO2 emissions, or 0.8% of global CO2 emissions from fossil fuel combustion and industry sources [estimated by using (79)]. Similarly long trips in light-duty vehicles accounted for an additional 40 Mt CO2, and aviation and other shipping modes (such as trains and ships) emitted 830 and 1060 Mt CO2, respectively. Altogether, these sources were responsible for ~6% of global CO2 emissions (Fig. 2). Meanwhile, both global energy demand for transportation and the ratio of heavy- to light-duty vehicles is expected to increase (9).

Light-duty vehicles can be electrified or run on hydrogen without drastic changes in performance except for range and/or refueling time. By contrast, general-use air transportation and long-distance transportation, especially by trucks or ships, have additional constraints of revenue cargo space and payload capacity that mandate energy sources with high volumetric and gravimetric density (10). Closed-cycle electrochemical batteries must contain all of their reactants and products. Hence, fuels that are oxidized with ambient air and then vent their exhaust to the atmosphere have a substantial chemical advantage in gravimetric energy density.

Battery- and hydrogen-powered trucks are now used in short-distance trucking (11), but at equal range, heavy-duty trucks powered by current lithium-ion batteries and electric motors can carry ~40% less goods than can trucks powered by diesel-fueled, internal combustion engines. The same physical constraints of gravimetric and volumetric energy density likely preclude battery- or hydrogen-powered aircraft for long-distance cargo or passenger service (12). Autonomous trucks and distributed manufacturing may fundamentally alter the energy demands of the freight industry, but if available, energy-dense liquid fuels are likely to remain the preferred energy source for long-distance transportation services (13).

Options for such energy-dense liquid fuels include the hydrocarbons we now use, as well as hydrogen, ammonia, and alcohols and ethers. In each case, there are options for producing carbon-neutral or low-carbon fuels that could be integrated to a net-zero emissions energy system (Fig. 1), and each can also be interconverted through existing thermochemical processes (Table 1).

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.

View this table:

### Hydrogen and ammonia fuels

The low volumetric energy density of hydrogen favors transport and storage at low temperatures (–253°C for liquid hydrogen at atmospheric pressure) and/or high pressures (350 to 700 bar), thus requiring heavy and bulky storage containers (14). To contain the same total energy as a diesel fuel storage system, a liquid hydrogen storage system would weigh roughly six times more and be about eight times larger (Fig. 3A). However, hydrogen fuel cell or hybrid hydrogen-battery trucks can be more energy efficient than those with internal combustion diesel engines (15), requiring less onboard energy storage to achieve the same traveling range. Toyota has recently introduced a heavy-duty (36,000 kg), 500-kW fuel cell/battery hybrid truck designed to travel 200 miles on liquid hydrogen and stored electricity, and Nikola has announced a similar battery/fuel cell heavy-duty truck with a claimed range of 1300 to 1900 km, which is comparable with today’s long-haul diesel trucks (16). If hydrogen can be produced affordably without CO2 emissions, its use in the transport sector could ultimately be bolstered by the fuel’s importance in providing other energy services.

Ammonia is another technologically viable alternative fuel that contains no carbon and may be directly used in an engine or may be cracked to produce hydrogen. Its thermolysis must be carefully controlled so as to minimize production of highly oxidized products such as NOx (17). Furthermore, like hydrogen, ammonia’s gravimetric energy density is considerably lower than that of hydrocarbons such as diesel (Fig. 3A).

### Biofuels

Conversion of biomass currently provides the most cost-effective pathway to nonfossil, carbon-containing liquid fuels. Liquid biofuels at present represent ~4.2 EJ of the roughly 100 EJ of energy consumed by the transport sector worldwide. Currently, the main liquid biofuels are ethanol from grain and sugar cane and biodiesel and renewable diesel from oil seeds and waste oils. They are associated with substantial challenges related to their life-cycle carbon emissions, cost, and scalability (18).

Photosynthesis converts <5% of incident radiation to chemical energy, and only a fraction of that chemical energy remains in biomass (19). Conversion of biomass to fuel also requires energy for processing and transportation. Land used to produce biofuels must have water, nutrient, soil, and climate characteristics suitable for agriculture, thus putting biofuels in competition with other land uses. This has implications for food security, sustainable rural economies, and the protection of nature and ecosystem services (20). Potential land-use competition is heightened by increasing interest in bioenergy with carbon capture and storage (BECCS) as a source of negative emissions (that is, carbon dioxide removal), which biofuels can provide (21).

Advanced biofuel efforts include processes that seek to overcome the recalcitrance of cellulose to allow use of different feedstocks (such as woody crops, agricultural residues, and wastes) in order to achieve large-scale production of liquid transportation fuels at costs roughly competitive with gasoline (for example, U.S. $19/GJ or U.S.$1.51/gallon of ethanol) (22). As technology matures and overall decarbonization efforts of the energy system proceed, biofuels may be able to largely avoid fossil fuel inputs such as those related to on-farm processes and transport, as well as emissions associated with induced land-use change (23, 24). The extent to which biomass will supply liquid fuels in a future net-zero emissions energy system thus depends on advances in conversion technology, competing demands for bioenergy and land, the feasibility of other sources of carbon-neutral fuels, and integration of biomass production with other objectives (25).

### Synthetic hydrocarbons

Liquid hydrocarbons can also be synthesized through industrial hydrogenation of feedstock carbon, such as the reaction of carbon monoxide and hydrogen by the Fischer-Tropsch process (26). If the carbon contained in the feedstock is taken from the atmosphere and no fossil energy is used for the production, processing, and transport of feedstocks and synthesized fuels, the resulting hydrocarbons would be carbon-neutral (Fig. 1). For example, emissions-free electricity could be used to produce dihydrogen (H2) by means of electrolysis of water, which would be reacted with CO2 removed from the atmosphere either through direct air capture or photosynthesis (which in the latter case could include CO2 captured from the exhaust of biomass or biogas combustion) (27, 28).

### Demand management

Technologies that allow electricity demand to be shifted in time (load-shifting or load-shaping) or curtailed to better correlate with supply would improve overall system reliability while reducing the need for underused, flexible back-up generators (90, 91). Smart charging of electric vehicles, shifted heating and cooling cycles, and scheduling of appliances could cost-effectively reduce peak loads in the United States by ~6% and thus avoid 77 GW of otherwise needed generating capacity (~7% of U.S. generating capacity in 2017) (92). Managing larger quantities of energy demand for longer times (for example, tens of petajoules over weeks) would involve idling large industrial uses of electricity—thus underutilizing other valuable capital—or effectively curtailing service. Exploring and developing new technologies that can manage weekly or seasonal gaps in electricity supply is an important area for further research (93).

### Outlook

Nonemitting electricity sources, energy-storage technologies, and demand management options that are now available and capable of accommodating large, multiday mismatches in electricity supply and demand are characterized by high capital costs compared with the current costs of some variable electricity sources or natural gas–fired generators. Achieving affordable, reliable, and net-zero emissions electricity systems may thus depend on substantially reducing such capital costs via continued innovation and deployment, emphasizing systems that can be operated to provide multiple energy services.

## Carbon management

Recycling and removal of carbon from the atmosphere (carbon management) is likely to be an important activity of any net-zero emissions energy system. For example, synthesized hydrocarbons that contain carbon captured from the atmosphere will not increase atmospheric CO2 when oxidized. Integrated assessment models also increasingly require negative emissions to limit the increase in global mean temperatures to 2°C (9497)—for example, via afforestation/reforestation, enhanced mineral weathering, bioenergy with CCS, or direct capture of CO2 from the air (20).

Capture and storage will be distinct carbon management services in a net-zero emissions energy system (for example, Fig. 1, E and J). Carbon captured from the ambient air could be used to synthesize carbon-neutral hydrocarbon fuels or sequestered to produce negative emissions. Carbon captured from combustion of biomass or synthesized hydrocarbons could be recycled to produce more fuels (98). Storage of captured CO2 (for example, underground) will be required to the extent that uses of fossil carbon persist and/or that negative emissions are needed (20).

For industrial CO2 capture, research and development are needed to reduce the capital costs and costs related to energy for gas separation and compression (99). Future constraints on land, water, and food resources may limit biologically mediated capture (20). The main challenges to direct air capture include costs to manufacture sorbents and structures, energize the process, and handle and transport the captured CO2 (100, 101). Despite multiple demonstrations at scale [~15 Mt CO2/year are now being injected underground (99)], financing carbon storage projects with high perceived risks and long-term liability for discharge remains a major challenge (102).

## Discussion

We have estimated that difficult-to-eliminate emissions related to aviation, long-distance transportation and shipping, structural materials, and highly reliable electricity represented more than a quarter of global fossil fuel and industry CO2 emissions in 2014 (Fig. 2). But economic and human development goals, trends in international trade and travel, the rapidly growing share of variable energy sources (103), and the large-scale electrification of other sectors all suggest that demand for the energy services and processes associated with difficult-to-eliminate emissions will increase substantially in the future. For example, in some of the Shared Socioeconomic Pathways that were recently developed by the climate change research community in order to frame analysis of future climate impacts, global final energy demand more than doubles by 2100 (104); hence, the magnitude of these difficult-to-eliminate emissions could in the future be comparable with the level of total current emissions.

Combinations of known technologies could eliminate emissions related to all essential energy services and processes (Fig. 1), but substantial increases in costs are an immediate barrier to avoiding emissions in each category. In some cases, innovation and deployment can be expected to reduce costs and create new options (32, 73, 105, 106). More rapid changes may depend on coordinating operations across energy and industry sectors, which could help boost utilization rates of capital-intensive assets. In practice, this would entail systematizing and explicitly valuing many of the interconnections depicted in Fig. 1, which would also mean overcoming institutional and organizational challenges in order to create new markets and ensure cooperation among regulators and disparate, risk-averse businesses. We thus suggest two parallel broad streams of R&D effort: (i) research in technologies and processes that can provide these difficult-to-decarbonize energy services, and (ii) research in systems integration that would allow for the provision of these services and products in a reliable and cost-effective way.

We have focused on provision of energy services without adding CO2 to the atmosphere. However, many of the challenges discussed here could be reduced by moderating demand, such as through substantial improvements in energy and materials efficiency. Particularly crucial are the rate and intensity of economic growth in developing countries and the degree to which such growth can avoid fossil-fuel energy while prioritizing human development, environmental protection, sustainability, and social equity (4, 107, 108). Furthermore, many energy services rely on long-lived infrastructure and systems so that current investment decisions may lock in patterns of energy supply and demand (and thereby the cost of emissions reductions) for half a century to come (109). The collective and reinforcing inertia of existing technologies, policies, institutions, and behavioral norms may actively inhibit innovation of emissions-free technologies (110). Emissions of CO2 and other radiatively active gases and aerosols (111), from land use and land-use change could also cause substantial warming (112).

## Conclusion

We have enumerated here energy services that must be served by any future net-zero emissions energy system and have explored the technological and economic constraints of each. A successful transition to a future net-zero emissions energy system is likely to depend on the availability of vast amounts of inexpensive, emissions-free electricity; mechanisms to quickly and cheaply balance large and uncertain time-varying differences between demand and electricity generation; electrified substitutes for most fuel-using devices; alternative materials and manufacturing processes including CCS for structural materials; and carbon-neutral fuels for the parts of the economy that are not easily electrified. The specific technologies that will be favored in future marketplaces are largely uncertain, but only a finite number of technology choices exist today for each functional role. To take appropriate actions in the near-term, it is imperative to clearly identify desired endpoints. If we want to achieve a robust, reliable, affordable, net-zero emissions energy system later this century, we must be researching, developing, demonstrating, and deploying those candidate technologies now.

## References and Notes

Acknowledgments: The authors extend a special acknowledgment to M.I.H. for inspiration on the 20th anniversary of publication of (1). The authors also thank M. Dyson, L. Fulton, L. Lynd, G. Janssens-Maenhout, M. McKinnon, J. Mueller, G. Pereira, M. Ziegler, and M. Wang for helpful input. This Review stems from an Aspen Global Change Institute meeting in July 2016 convened with support from NASA, the Heising-Simons Foundation, and the Fund for Innovative Climate and Energy Research. S.J.D. and J.B. also acknowledge support of the U.S. National Science Foundation (INFEWS grant EAR 1639318). D.A., B.H., and B-M.H. acknowledge Alliance for Sustainable Energy, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under contract DE-AC36-08GO28308. Funding was in part provided by the DOE Office of Energy Efficiency and Renewable Energy. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. government. The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. government purposes.

Correction (29 June 2018): Figure 2 has been revised; “Other industry” has been correctly labeled “14%”.

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