UC Berkeley
UC Berkeley Previously Published Works
Title
Net-zero emissions energy systems.
Permalink
https://escholarship.org/uc/item/16109441
Journal
Science (New York, N.Y.), 360(6396)
ISSN
0036-8075
Authors
Davis, Steven J
Lewis, Nathan S
Shaner, Matthew
et al.
Publication Date
2018-06-01
DOI
10.1126/science.aas9793
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
REVIEW SUMMARY
◥
ENERGY
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*
BACKGROUND: Net emissions of CO
2
by
human activities—includ ing not only en-
ergy services and industrial production but
also land use and agriculture—must ap-
proach zero in order to stabilize global
mean temperature. Energy services such
as light-duty tr ansportat ion, heati ng, cooling,
and light ing may be relatively straight-
forward to decarb oniz e by ele c-
trifying 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). How-
ever , other energy services essent ial
to modern civilization entail emis-
sions that are likely to be more
difficult to fully eliminate. These
difficult-to-dec arbonize energy ser-
vices include aviation, long-distance
transport, and shipping; production
of carbon-intensive structural mate-
rials such as steel and cement; and
provision of a reliable electricity
supply that meets varying demand.
Moreover, demand for such ser-
vices and products is projected
to increase substantially over this
century. The long-lived infrastruc-
ture built today, for better or worse,
will shape the future.
Here, we review the special chal-
lenges associated with an energy
system that does not add any CO
2
to the atmosphere (a net-zero
emissions energy syst em ). W e
discuss prominent technolog-
ical 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 re-
search, development, demonstration, and de-
ployment. It may take decades to research,
develop, and deploy these new technologies.
ADVANCES: A successful transition to a
future net -zero emissions energ y system
is likely to depend on vast amounts of in-
expensive, emissions-free electricity; mecha-
nisms to quickly and cheaply balance large
and uncertain time-varying differences be-
tween demand and electricity generation;
electrified substitutes for most fuel-using
devices; alternative materials and manu-
facturing processes for structural materials;
and carbon-neutral fuels for the parts of the
economy that are not easily electrified. Re-
cycling and remova l of
carbon from the atmo-
sphere (carbon manage-
ment) is also likely to be
an important activity of
any net-ze ro emissio ns
energy system. The spe-
cific 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 a ctions in t he near t erm, it is
imperative to cle arly ident ify desir ed end
points. To achieve a robust, reliable, and af-
fordable net-zero emissions energy system
later this century, efforts to research, develop,
demonstrate, and deploy those candidate
technologies must star t now.
OUTLOOK: Combinations of known tech-
nologies could eliminate emissions related
to all essential energy services and pro-
cesses, b ut substanti al increases in costs
are an immediate barrier to a voiding emis-
sions in each category. In some cases, in-
novation a nd deployment can be expected
to reduce costs and create new options. More
rapid changes may depend on coordinat-
ing operations across energy and industr y
sectors, which could help boos t utilization
rates of capital-intensive assets, but this
will requ ire overcoming instituti onal and
organizational challenges in order to create
new markets and ensure cooperation among
regulators and disparate, risk-averse busi-
nesses. Two parallel and broad streams of
research and development could prove use-
ful: 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.
▪
RESEARCH
Davis et al., Science 360, 1419 (2018) 29 June 2018 1of1
The list of author affiliations is available in the full article online.
*Corresponding author. Email: sjdavis@uci.edu (S.J.D.);
nslewis@caltech.edu (N.S.L.); kcaldeira@carnegiescience.edu
(K.C.)
Cite this article as S. J. Davis et al., Science 360, eaas9793
(2018). DOI: 10.1126/science.aas9793
Ashowerofmoltenmetalinasteelfoundry.Industrial
processes such as steelmaking will be particularly
challenging to decarbonize. Meeting future demand for
such difficult-to-decarboniz e energy services and industrial
products without adding CO
2
to the atmosphere may depend
on technological cost reductions via resear ch and innovation,
as well as coordinated deployment and integration of
operatio ns across currently discr ete energy industries.
ON OUR WEBSITE
◥
Read the full article
at http://dx.doi.
org/10.1126/
science.aas9793
..................................................
TOMO RR OW’ SEARTH
Read more articles online
at scim.ag/TomorrowsEarth
on June 29, 2018 http://science.sciencemag.org/Downloaded from
REVIEW
◥
ENERGY
Net-zero emissions energy systems
Steven J. Davis
1,2
*, Nathan S. Lewis
3
*, Matthew Shaner
4
, Sonia Aggarwal
5
,
Doug Arent
6,7
, Inês L. Azevedo
8
, Sally M. Benson
9,10,11
, Thomas Bradley
12
,
Jack Brouwer
13,14
, Yet-Ming Chiang
15
, Christopher T. M. Clack
16
, Armond Cohen
17
,
Stephen Doig
18
, Jae Edmonds
19
, Paul Fennell
20,21
, Christopher B. Field
22
,
Bryan Hannegan
23
, Bri-Mathias Hodge
6,24,25
, Martin I. Hoffert
26
, Eric Ingersoll
27
,
Paulina Jaramillo
8
, Klaus S. Lackner
28
, Katharine J. Mach
29
, Michael Mastrandrea
4
,
Joan Ogden
30
, Per F. Peterson
31
, Daniel L. Sanchez
32
, Daniel Sperling
33
,
Joseph Stagner
34
, Jessika E. Trancik
35,36
, Chi-Jen Yang
37
, Ken Caldeira
32
*
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 (CO
2
) 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 CO
2
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.
P
eople 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). Mean-
while, net emissions of carbon dioxide (CO
2
)from
human activities—including not only energy
and industrial production, but also land use and
agriculture—must approach zero to stabilize glo-
bal mean temperature (2, 3). Indeed, interna-
tional climate targets, such as avoiding m ore
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 transpor-
tation, heating, cooling, and lighting may be
relatively straightforward to decarbonize by
electrifying and generating electricity from var-
iable renewable energ y sources (such as wind
and solar) and dispatchable (“on-demand”)non-
renewable sources (including nucl ear energy
and fossil fuels with carbon capture and storage).
However, other energy services essential to mo-
dern 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 materi-
als such as steel and cement; and provision of
a reliable electricity supply that meets varying
demand. To the extent that carbon remains in-
volved 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 CO
2
, or 27% of global
CO
2
emissions from all fossil fuel and industrial
sources (Fig. 2). Ye t des pite their im portance,
detailed representation of these services in in-
tegrated assessment models remains challeng-
ing (4–6).
Here, we review the special challenges asso-
ciated with an energy system that does not add
any CO
2
to the atmosphere (a net-zero emissions
energy system). We discuss prominent techno-
logical opportunities and barriers for eliminat-
ing 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 econo-
mic modeling; rather , they are based on compar-
ison 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 infrastruc-
tural and institutional transformations , as well as
active management of carbon in the energy system.
Aviation, long-distance transport,
and shipping
In 2014, medium- and h eavy-duty trucks with
mean trip dist ances of >160 km (>100 miles)
accounted for ~270 Mt CO
2
emissions, or 0.8%
of global CO
2
emissions from fossil fuel com-
bustion and industry sources [estimated by
using (7–9)]. Similarly long trips in light-duty
vehicles accounted for an additional 40 Mt CO
2
,
and aviation and ot her shipping modes (such
as trains and ships) emitted 830 and 1060 Mt
CO
2
, respectively. Altogether, these sources were
responsible for ~6% of global CO
2
emissions
(Fig. 2). Meanwhile, both global energy demand
for t ransportation 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 perfor-
mance 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 highvolumetricandgrav-
imetric density (10). Closed-cycle electrochemical
batteries must contain all of their reactants and
products. H ence, fuels that are oxidized with
RESEARCH
Davis et al., Science 360, eaas9793 (2018) 29 June 2018 1of9
1
Department of Earth System Science, University of California, Irvine, Irvine, CA, USA.
2
Department of Civil and Environmental Engineering, University of California, Irvine, Irvine, CA, USA.
3
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA.
4
Near Zero, Carnegie Institution for Science, Stanford, CA, USA.
5
Energy Innovation, San
Francisco, CA, USA.
6
National Renewable Energy Laboratory, Golden, CO, USA.
7
Joint Institute for Strategic Energy Analysis, Golden, CO, USA.
8
Engineering and Public Policy, Carnegie Mellon
University, Pittsburgh, PA, USA.
9
Global Climate and Energy Project, Stanford University, Stanford, CA, USA.
10
Precourt Institute for Energy, Stanford University, Stanford, CA, USA.
11
Department
of Energy Resource Engineering, Stanford University, Stanford, CA, USA.
12
Department of Mechanical Engineering, Colorado State University, Fort Collins, CO, USA.
13
Department of Mechanical
and Aerospace Engineering, University of California, Irvine, Irvine, CA, USA.
14
Advanced Power and Energy Program, University of California, Irvine, CA, USA.
15
Department of Material Science and
Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.
16
Vibrant Clean Energy, Boulder, CO, USA.
17
Clean Air Task Force, Boston, MA, USA.
18
Rocky Mountain Institute,
Boulder, CO, USA.
19
Pacific National Northwestern Laboratory, College Park, MD, USA.
20
Department of Chemical Engineering, South Kensington Campus, Imperial College London, London, UK.
21
Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, CA, USA.
22
Woods Institute for the Environment, Stanford University, Stanford, CA, USA.
23
Holy Cross Energy, Glenwood Springs, CO,
USA.
24
Department of Electrical, Computer, and Energy Engineering, University of Colorado Boulder, Boulder, CO, USA.
25
Department of Chemical and Biological Engineering, Colorado School of
Mines, Golden, CO, USA.
26
Department of Physics, New York University, New York, NY, USA.
27
Lucid Strategy, Cambridge, MA, USA.
28
The Center for Negative Carbon Emissions, Arizona State
University, Tempe, AZ, USA.
29
Department of Earth System Science, Stanford University, Stanford, CA, USA.
30
Environmental Science and Policy, University of California, Davis, Davis, CA, USA.
31
Department of Nuclear Engineering, University of California, Berkeley, Berkeley, CA, USA.
32
Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, USA.
33
Institute of
Transportation Studies, University of California, Davis, Davis, CA, USA.
34
Department of Sustainability and Energy Management, Stanford University, Stanford, CA, USA.
35
Institute for Data,
Systems, and Society, Massachusetts Institute of Technology, Cambridge, MA, USA.
36
Santa Fe Institute, Santa Fe, NM, USA.
37
Independent researcher.
*Corresponding authors: Email: sjdavis@uci.edu (S.J.D.); nslewis@caltech.edu (N.S.L.); kcaldeira@carnegiescience.edu (K.C.)
on June 29, 2018 http://science.sciencemag.org/Downloaded from
ambient air and then vent their exhaust to the
atmosphere have a substantial chemical advan-
tage in gravimetric energy density.
Battery- and hydrogen-powered trucks are now
used in short-distance trucking (11),butatequal
range, heavy-duty trucks powered by current
lithium-ion batteries and electric motors can car-
ry ~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). Auto-
nomous trucks and distributed manufacturing
may fundamentally alter the energy demands of
Davis et al., Science 360, eaas9793 (2018) 29 June 2018 2of9
Fig. 1. Schematic of an integra ted system that can provide
essential energy ser vices without adding any CO
2
to the atmo-
sphere. (A to S) Colors i ndicate the domi nant role of spe cific
technologies and processes. Green, electricity generation and trans-
mission; blue, hydrogen production and t ransport; pur ple,
hydrocarbon production and transport; orange, ammonia production
and transport; red, carbon management; and black, end uses of
energy and materials.
RESEARCH | REVIEW
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the freight industry, but if available, energy-dens e
liquid fuels are l ikely to remain the pre ferred
energy source for long-distance transportation
services (13).
Options for such energy-dense liquid fuels in-
clude 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 intercon-
verted through existing thermochemical p rocesses
(Table 1).
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 pres-
sure) and/ or high pressure s (350 to 700 bar),
thus requiring heavy and bulky storage contain-
ers (14). To contain the same total energy as a
diesel fuel storage system, a l iquid hydroge n
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 t rave ling 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 hy-
drogen can be produced affordably without CO
2
emissions, its use in the transport sector could
ultimately be bolstered by the fuel’simportance
in providing other energy services.
Ammonia is a nother 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
NO
x
(17). Furthermore, like hydrogen, ammo-
nia’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 re-
newable 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 ra-
diation to chemical energy, and only a fraction
of that chemical energy remains in biomass (19).
Conversion of biomass to fuel also requires en-
ergy for processing and transportation. Land
used to produce biofuels must have water, nu-
trient, soil, and climate characteristics suitable
for agriculture, thus putting biofuels in competi-
tion 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
captureandstorage(BECCS)asasourceofnega-
tive 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 trans-
portation fuels at costs roughly competitive with
gasoline(forexample,U.S.$19/GJorU.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 emis-
sions 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 produc-
tion with other objectives (25).
Synthetic hydrocarbons
Liquid hydrocarbons can also b e 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 en-
ergy 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 elec-
tricity could be used to produce dihydrogen (H
2
)
by means of electrolysis of water, which would
be reacted with CO
2
removed from the atmo-
sphere either through direct air capture or photo-
synthesis (which in the latter case could include
CO
2
captured from the exhaust of biomass or
biogas combustion) (27, 28).
At present, the cost of electrolysis is a major
barrier. This cost includes both the capital costs
of electrolyze rs and the cost of emissions-free
electricity; 60 to 70% of current electrolytic hy-
drogen cost is electricity (Fig. 3C) (28, 29). The
cheapest and most mature electrolysis technology
available today uses alkaline electrolytes [such as
potassium hydroxide (KOH) or sodium hydroxide
Davis et al., Science 360, eaas9793 (2018) 29 June 2018 3of9
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
From e
–
H
2
C
x
O
y
H
z
NH
3
............ ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ ............... .......
e
–
Electrolysis ($5 to 6/kg H
2
) Electrolysis + methanation Electrolysis + Haber-Bosch
............. ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ .
Electrolysis + Fischer-Tropsch
............ ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ ............... .......
H
2
Combustion Methanation
($0.07 to 0.57/m
3
CH
4
)
Haber-Bosch ($0.50 to
0.60/kg NH
3
)
............. ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ .
Oxidation via fuel cell Fischer-Tropsch ($4.40
to $15.00/gallon of
gasoline-equivalent)
............ ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ ............... .......
C
x
O
y
H
z
Combustion Steam reforming
($1.29 to 1.50/kg H
2
)
Steam reforming +
Haber-Bosch
............. ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ .
Biomass gasification
($4.80 to 5.40/kg H
2
)
............ ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ ............... .......
NH
3
Combustion Metal catalysts
(~$3/kg H
2
)
Metal catalysts + methanation/
Fischer-Tropsch
............. ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ .
Sodium amide
............ ................ ................ ................ ............... ................ ................ ................ ............. ................ ............... ................ ................ ................ ................ ............... ................ ................ ................ ................ ............... .......
RESEARCH | REVIEW
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