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Comparison of carbon
sequestration efﬁcacy between
artiﬁcial photosynthetic carbon
dioxide conversion and
Santiago Gonzalez Hernandez and Stafford W. Sheehan ,
Air Company, 407 Johnson Avenue, Brooklyn, NY 11206, USA
Address all correspondence to Stafford W. Sheehan at email@example.com
(Received 15 June 2020; accepted 11 August 2020)
A comparison between electrochemical carbon dioxide conversion and reforestation is presented. By comparing thermodynamic and
forestry data, recommendations for technology development can be made.
With the global average temperature steadily increasing due to anthropogenic emission of greenhouse gases into the atmosphere, there
has been increasing interest worldwide in new technologies for carbon capture, utilization, and storage (CCUS). This coincides with the
decrease in cost of deployment of intermittent renewable electricity sources, speciﬁcally solar energy, necessitating development of new
methods for energy storage. Carbon dioxide conversion technologies driven by photovoltaics aim to address both these needs. To ade-
quately contribute to greenhouse gas reduction, the carbon dioxide conversion technology deployed should have a substantially higher
rate of carbon dioxide removal than planting an equivalent-sized forest. Using consistent methodologies, we analyze the effectiveness
of model photovoltaic-driven carbon dioxide conversion technologies that produce liquid alcohols as compared to planting an equivalent
forest. This analysis serves to establish an energy use boundary forcarbon dioxide conversion technology, in order to be a viable alternative
as a net carbon negative technology.
Key words: carbon dioxide; electrochemical synthesis; ethanol; life cycle assessment; photovoltaic; sustainability
Technologies that actively increase the rate of terrestrial car-
bon ﬁxation by utilizing or storing carbon equivalents have been
identiﬁed as important within the portfolio of solutions that
humanity must develop to decrease atmospheric greenhouse
In the chemical industry alone, by
capturing and reusing carbon dioxide there is the potential to
reduce annual greenhouse gas emissions by up to 3.5 gigatons
carbon dioxide equivalent (GtCO
e) by 2030.
that global energy-related CO
emissions in 2019 were around
33 Gt total,
implementation of carbon capture, utilization,
and storage (CCUS) technologies throughout this one industry
could have a substantial impact toward reducing global carbon
emissions and limiting the increase in global average tempera-
ture to less than 2 °C. When combined with other industries
utilization technologies can have a considerable
impact, such as building materials and transportation fuels,
there is an opportunity for these technologies to substantially
contribute to humanity’s efforts to counteract anthropogenic
While there are several approaches for CO
among the most widely researched are artiﬁcial photosynthetic
or solar fuel technologies.
Technologies such as these, that fol-
low the principles of photosynthesis, are distinguished from
others in that they use only carbon dioxide, water, and solar
energy as reagents, and produce only reduced-carbon products
and oxygen gas on a system level. For the purposes of this study,
•Carbon dioxide conversion is an important tool to reduce the
concentration of greenhouse gases in the atmosphere.
•Reforestation sequesters 5–10 kgCO
/day per hectare of forest and
is another important strategy in our portfolio to mitigate climate
•A well-curated forest absorbs carbon dioxide at around 1% of the
rate of an electrochemical technology powered using an equivalent
area of solar panels.
MRS Energy and Sustainability
page 1 of 8
© The Authors, 2020, published on behalf of Materials
Research Society by Cambridge University Press
we also include wind energy as a potential energy source for arti-
ﬁcial photosynthetic technologies, as it is renewable and ulti-
mately driven by the sun.
Nature has demonstrated that this
mass balance for artiﬁcial photosynthetic technologies, requir-
ing only H
, and solar photons, is compatible with our
global climate and atmosphere, as it is the same approach that
has been taken by photosynthetic organisms for the last two bil-
lion years that terraformed Earth to enable life as we know it.
While nature originally pioneered photosynthesis, its efﬁ-
ciency in plants is low since evolution has prioritized resilience
and reproduction rather than thermodynamic efﬁciency.
Averaged over an annual growth cycle, the solar-to-chemical
conversion efﬁciency of trees is approximately 1%.
photovoltaic (PV) modules, on the other hand, routinely reach
solar-to-electric conversion efﬁciencies as high as 20%.
explanation for this discrepancy is the complex kinetics of
removing four electrons from water to form oxygen, which
drives down the efﬁciency of solar-to-chemical conversion pro-
cesses. For photoelectrochemical (PEC) or photocatalytic
solar-to-fuel processes, which presents a more direct compari-
son with natural photosynthesis since water oxidation is per-
formed and products are generated at the site of solar energy
realistic efﬁciencies for deployed systems with
earth-abundant elements have been estimated at approximately
Due to the cost, efﬁciency, and product collection chal-
lenges presented by PEC technologies, the present study
focuses on integrating photovoltaic modules with electrically
The comparatively high efﬁciency of photovoltaics suggests
that electrically driven CO
conversion has substantial promise
to increase the rate of terrestrial carbon utilization and seques-
tration if we are able to convert the electric energy from photo-
voltaic modules into products at any reasonable system-level
efﬁciency. Methods of utilizing photovoltaic electricity as
means to diverge from non-renewable fossil fuel-based energy
include, most notably, water electrolysis driven by photovoltaic
modules (PV-electrolysis) which can be deployed using com-
mercially available, off-the-shelf components and require only
solar energy as an input to produce H
enable the production of H
at high thermal efﬁciencies.
These methods combined with CO
utilization at reasonable rates and system-level
However, before committing substantial effort to applied
research and deployment of a CO
conversion technology, it
is useful to conﬁrm that the technology can capture and seques-
at a faster rate than simply planting trees. By analyzing
data to answer questions like this one, we can help to set guide-
lines for artiﬁcial photosynthetic technology development so
that we do not ultimately produce technology that is less effec-
tive at sequestering CO
than this obvious and natural low-cost
Of the commercially relevant methods that use the principles
of photosynthesis to convert CO
to chemicals, production of
alcohols that are liquids at standard temperature and pressure
are among the most widely researched and implemented.
Among the most mature technologies in the world that use
O and CO
to produce an essential chemical is CO
nation to methanol powered by H
produced via electrolysis.
Several pilot and demonstration plants producing thousands
of tons per year are in operation.
Production of ethanol via
and renewable H
in a similar manner has also been dem-
onstrated on the pilot scale.
Direct electrosynthesis of ethanol
has been demonstrated since the 1980s
and is an
active area of research on the lab scale with several recent
advances to improve efﬁciency and rate of production.
reduction to produce methanol has also
seen substantial recent progress,
as has electrochemical CO
conversion to either methanol or ethanol.
Methods that use
syngas, which can be produced electrochemically in a single sys-
to produce methanol or ethanol have also been imple-
mented in the industry. Once fully developed, there are few
substantial technical challenges to power any of these technolo-
gies with solar photovoltaics or wind turbines, thus accomplish-
ing liquid alcohol production from CO
O, and solar energy.
In this study, we analyze liquid alcohol production from CO
to provide an understanding of the energy requirements and
removal rate at different efﬁciencies derived from thermo-
dynamic data along with published energy consumption data for
balance-of-plant systems. In parallel, we calculate the CO
removal rate of timberland reforestation at different tree-
densities per unit area using data from the United States
(USFS), which can be compared to similar
climates in Europe and Asia that share the same latitude and
biome. We compare the annual CO
removal rate between the
two at a benchmark CO
conversion system-level efﬁciency of
60% and one hectare of the solar capture area. Lastly, we assess
how varying the efﬁciency of the CO
conversion system com-
pares to reforestation at different tree densities, to understand
the minimum system-level efﬁciency required to provide a sub-
removal beneﬁt over simple reforestation.
Methods and model development
While artiﬁcial photosynthetic technologies can be charac-
terized by their system-level mass balance, there are several
components that make up a system that makes chemical prod-
ucts from H
, and renewable electricity with interdepen-
dent components. Figure 1 shows the general modeling
approach that we take for these systems, with the most substan-
tial energy costs being CO
capture, conversion, and product
separation. Several different methods for electrochemically
conversion can be applied to this model provided
the kWh/kg CO
, and percent conversion
are known, and the chemical energy for conversion comes
from an electrolysis reaction. For example, depending on the
technological risk tolerance for the deployment and product
concentration requirements, one can envision a single-step
approach, such as CO
electrolysis; a two-step approach, such
as electrochemical H
production combined with CO
nation; or a three-step approach, such as electrochemical H
2▪MRS ENERGY AND SUSTAINABILITY / / VO LU M E 7 // e3 2 / / www.mrs.org/energy-sustainability-journal
production combined with thermochemical reverse water-gas
shift to produce CO, combined with CO electrolysis, and several
Any of these combinations can be factored into a single over-
all system-level efﬁciency based on the total energy content per
mole (or higher heating value, HHV) of the product chemical.
For example, the efﬁciency of methanol production from CO
O would consider the thermal efﬁciency of electrolysis
combined with hydrogenation for the conversion step, along
with any additional losses from CO
and product separation. If water puriﬁcation and electrolysis
result in a net 75% efﬁcient component,
hydrogenation, and product separation give a thermal efﬁciency
of 80% assuming efﬁcient heat reuse from the exothermic CO
hydrogenation reaction, the overall efﬁciency of the system
would be 60%. For the purposes of this study, we focus on eth-
anol and methanol production, which have HHV thermody-
namic characteristics (i.e., energy consumption at unity
efﬁciency) calculated below from CRC Handbook values
Beyond the thermodynamic conversion values, for the pur-
poses of this study and as a model cost of CO
assumed to be concentrated from ﬂue gas using off-the-shelf
amine scrubbing technology. The industry standard amine
scrubbing technology pumps ﬂue gas containing CO
an absorbing ﬂuid, typically monoethanolamine, which is
then fed into a stripping column to condense out water and puri-
Amine scrubbing is largely used in fossil fuel plants
where it is expected to continue to be the industry standard in
the near term, and adds a 0.37 MWh/tCO
The rate of energy generation per hectare of land from solar
photovoltaics using approximately 20% efﬁcient photovoltaic
modules was calculated using the literature values.
these energy generation per unit area rates and energy required
to capture and sequester each ton of CO
, the CO
rate per hectare of artiﬁcial photosynthetic technologies at
different MeOH and EtOH conversion efﬁciencies was
To analyze and compare to reforestation, USFS provides
comprehensive and centralized data on the state of forests and
timberland, which includes biomass volume and area calcula-
tions for each region of the United States. The biomass present
in a hectare of forest as measured by the tree density represents
sequestered over the lifetime of the trees measured. To
determine the overall CO
sequestration rate of a newly planted
forest, ﬁrst, the average density of biomass per hectare must be
calculated as a basis to determine the amount of solid carbon,
and thus CO
, sequestered per hectare. Biomass volume data,
which ranged from 1953 to 2017, was divided by the total forest
area in each US region (North, South, Rocky Mountain, and
Paciﬁc; Fig. 2)
for each year in which USFS performed a sur-
vey, with the results shown in Table 2.
Figure 1. General tech-agnostic approach for artiﬁcial photosynthetic carbon dioxide conversion to liquid or gaseous products driven by renewable electricity.
Table 1. Thermodynamic characteristics and minimum energy required to
to produce methanol (MeOH) and ethanol (EtOH).
Parameter MeOH EtOH
Energy content per mole (kJ/mol) 726.6 1367
Energy required to produce at thermodynamic
efﬁciency, volumetric (kWh/l)
Mass of product produced, per ton of CO
Energy required to sequester a ton of CO
thermodynamic efﬁciency (MWh/tCO
MRS ENERGY AND SUSTAINABILITY // V OL U ME 7 // e 32 / / www.mrs.org/energy-sustainability-journal ▪3
From these data, we can also see there is a change in biomass
volume in each forest biome on an annual basis, noting that
these numbers also vary based on forest management practices,
wildﬁres, and other natural events that are to be taken into con-
sideration when reforesting.
The volume per area ratios calculated in Table 2 serve as the
basis for developing the model for CO
via timberland reforestation. Forests can have different densi-
ties depending on management practices and tree species.
Since 1953, tree densities have changed due to improved forest
management practices; therefore, most natural forests in similar
biomes will have similar tree densities as 1953, from which the
low projections in our sequestration calculations were derived.
The high projections were taken using 2017 numbers, repre-
senting a forest curated to optimize tree density, and the average
values were taken from an average of all years in Table 2 to rep-
resent reforestation with moderate upkeep. The USFS data gives
only 2007 and 2017 live tree biomass data, therefore, to develop
a comprehensive model an average density was calculated for
each region utilizing the 2007 and 2017 biomass data, in dry
pounds per cubic feet. Utilizing the calculated average density
for each region, the volumes per area were divided by the tree
densities to get the cubic meters of tree volume per hectare.
The mass per area was multiplied by the mean tree carbon com-
position of about 0.5,
and multiplied by the carbon to CO
atomic mass ratio of 3.67 to attain the CO
sequestered in a for-
est planted for each region.
Results and discussion
The overall goal of both reforestation as outlined in this study
conversion is to reduce the concentration of CO
], which we attempt to keep in the range of
To minimize the effect of anthropogenic cli-
mate change, the increase in atmospheric CO
due to man-made CO
) can be mitigated
by equal and opposite decreases in atmospheric CO
tion from reforestation (−[CO
), air capture with CO
), or capture prior to release to the
atmosphere and dilution by recapturing ﬂue gas which is
more favored entropically and used in this study (−[CO
Fig. 3). We note that there are also other major carbon sinks,
such as oceans, but since the aim of our model is to compare
conversion technologies that produce liquid alcohols
with reforestation our model focuses on these methods.
is large and increasing at a faster rate than
any of the three carbon sinks we discuss here. It should, there-
fore, be noted that the best way to reduce CO
emissions is to
from being emitted in the ﬁrst place by reducing
the amount of fossil fuels burned, rather than trying to capture
it afterward. However, there will almost always be CO
sions that are challenging to avoid, for which we employ
Figure 2. Map of the United States showing the North (blue), South (orange),
Rocky Mountain (red), and Paciﬁc (green) regions as denoted by the USFS.
Regions were colored based on data from Ref. .
Table 2. The ratios for biomass volume over the area of the US forest
according to the year and the region.
Volume/Area (cubic meters/hectares)
North South Rocky Mountain Paciﬁc USA
2017 107.5 91.0 69.6 87.2 90.1
2007 100.9 86.0 72.9 84.5 86.7
1997 88.0 77.6 69.4 78.3 78.8
1987 80.5 73.2 62.6 78.6 74.7
1977 69.5 66.4 55.7 79.7 69.1
1953 45.1 43.3 52.9 86.9 58.1
Avg all yrs 81.9 72.9 63.9 82.5 76.2
Figure 3. Diagram of reforestation and CCUS as methods help reduce the
concentration of CO
in the atmosphere.
4▪MRS ENERGY AND SUSTAINABILITY / / VO LU M E 7 // e3 2 / / www.mrs.org/energy-sustainability-journal
Reforestation and CO
conversion technologies serve to
increase the rate at which CO
is removed from the atmosphere.
Quantifying trees’potential for CO
reforestation requires assumptions on their overall carbon
mass content ratio. The variance of carbon content in trees
has been widely reported in recent years, varying from 41%
up to 60% depending on the biome and tree species.
on these results and for the calculations in the present study,
a carbon composition ratio of 50% was used. To determine
the rate of CO
conversion by reforestation, we ﬁrst determine
the ratio of embodied CO
to forest hectare in each of the
biomes studied in the United States, including the high, aver-
age, and low reforestation sequestration potential for each
region, and for the US average (Table 3).
From these data, values for the CO
sequestration rates, in kg
per tree-day, were calculated for the average tree for each
US region. The USFS provides the age for trees in each region
of the US, which when utilized through a weighted average
can be used to calculate the average age and rate of growth for
current forests of each US region. The CO
from Table 3 are the CO
sequestration projections assuming
that the reforested available area reachesthe same level of matu-
rity as the current average age of forests for the corresponding
region. Meaning, for example, that for the average projection
of 151.4 tons CO
per hectare for the Northern region, it
would require that reforested area to reach the reported average
age of forests in that region. This also leaves room for further
sequestration in forest regions as they mature. Thus, to cal-
culate the CO
sequestration rates of kg CO
per tree-day, the
average forest age for each region, the CO
and the US national average ratio of trees per hectare (241
in forests were used to develop high, average,
and low CO
sequestration rates for the average tree of each
region (Table 4).
With an understanding of model CO
sequestration rates for
reforestation per hectare and model CO
costs based on thermodynamic values, we use data from com-
mercially available photovoltaic modules to determine the
energy production per hectare for a deployed photovoltaic sys-
tem. Commercial solar cell modules produced using multicrys-
talline silicon ingots manufactured by directional solidiﬁcation
(DS-Si) have a solar-to-electricity conversion efﬁciency of
20.4% under AM1.5 light.
Our calculations assume a reference
solar irradiation of 4.0 kWh/m
/day over one hectare with
25% system losses, and our results are consistent with kWh
output values calculated by the NREL PVWatts calculator for
a deployment with 20.4% module efﬁciency in localities
and with array conﬁgurations (ﬁxed, single-axis tracking, or
dual-axis tracking) that result in an annual irradiance of
Figure 4 shows the comparison between three different sce-
narios of timberland reforestation; the US high, average, and
sequestration rates, compared to CO
tems at different overall efﬁciencies based on the HHV of meth-
anol and ethanol driven by solar photovoltaic panels under
/day, which is near the average irradiance at
many locations in the USA and Europe. Thermal efﬁciencies
only up to 80% are shown, with the understanding that CO
hydrogenation coupled with H
O electrolysis faces thermody-
namic energy losses in the production of intermediate H
while direct CO
electrolysis faces kinetic challenges (selectiv-
ity, overpotential) that contribute to lost energy. If we assume
an overall efﬁciency of 60% for a realistic CO
tem, and the rate of CO
sequestration by well-curated forests
is equivalent to a system with overall efﬁciency around 0.5%,
reforestation sequesters CO
at around 1% the rate of the artiﬁ-
cial photosynthetic system.
Table 4. The CO
sequestration rate through photosynthesis for the average
tree in each U.S. region assuming the national average number of trees per
hectare (241). Average values are weighted based on forest tree density and
age. U.S. averages are weighted based forest coverage of each region.
Region High Average Low
North 0.035 0.027 0.015
South 0.045 0.036 0.021
Rocky Mountain 0.011 0.010 0.009
Paciﬁc0.020 0.019 0.020
U.S. 0.028 0.024 0.018
Table 3. The calculated CO
sequestration potential per hectare for
reforestation in different US regions, based on existing forests in these
regions. Average values are weighted based on forest tree density and age.
U.S. averages are weighted based forest coverage of each region.
Sequestered in Forests
(metric tons CO
Region High Average Low
North 198.7 151.4 83.5
South 165.2 132.4 78.6
Rocky Mountain 94.9 87.1 72.2
Paciﬁc139.5 132.0 139.0
US 153.3 129.8 98.9
MRS ENERGY AND SUSTAINABILITY // V OL U ME 7 // e 32 / / www.mrs.org/energy-sustainability-journal ▪5
We can see from this analysis that artiﬁcial photosynthesis
has a much greater potential to increase the direct ﬂux of CO
out of the atmosphere and into terrestrial sinks. Furthermore,
the potential of CO
sequestration through reforestation has a
capped value, due to ﬁnite area available for reforestation, as
determined both by forest management practices and the efﬁ-
ciency of natural photosynthesis. The duration of time that
is sequestered can be long in the case of reforestation; how-
ever, granting reforestation an advantage in certain scenarios
that require permanent sequestration.
The duration of sequestration also depends on the life cycle
of the reduced-carbon product; since timberland can be har-
vested and liquid alcohols can be used to produce ﬁxed chemical
products, and both can be burned for energy. Further CO
duced from burning wood or alcohols for energy generation can
still be captured and recycled again to maintain a circular car-
bon cycle, minimizing CO
emissions. If used as fuel, re-capture
and reuse of the CO
produced from energy generation by either
of the pathways presented in this paper would be necessary for
long-term reduction of atmospheric CO
For either case, if products are made, is most advantageous if
they displace a legacy alternative that substantially burns fossil
fuels to produce and distribute. This is more realistic with
conversion technology, which allows for control over the
product made from CO
. As noted previously, the best way to
reduce the concentration of CO
in the atmosphere is to prevent
fossil fuels from being burned and their emission in the ﬁrst
place, which displacement via a CO
-derived product can
accomplish. One such example is consumer ethanol;
emit 0.8–2.3 kg CO
Production from CO
sumes approximately 1.5 kg CO
/L ethanol produced. Provided
a renewable source of electricity powers the transformation,
much of the carbon mitigation of producing spirits from CO
can come from displacement of the legacy product made by
burning fossil fuel. Distribution-related emissions can also be
lessened since deployment requires only CO
O, and renew-
able electricity which enables production closer to use.
While not the focus of the present study, we note that
techno-economics and cost of each method will play a central
role in determining how CO
conversion deployments are
The calculated cost per ton of CO
captured by refor-
estation vary widely in the literature due to the unique nature of
trees affected by variables such as tree species, soil quality, cli-
mate, labor, and other factors. To provide a brief overview, there
are reported costs (in United States Dollars, USD) that range
between $5 and $43
for the average national forest in the
United States. In other studies, reforestation costs ranged
between values as low as $0.272 and $2.180
per ton of CO
sequestered, while more extreme estimates report as high as
$1654 USD per ton of CO
Most recent analyses
report values of sequestration through reforestation as less than
or about $100 per ton of CO
While more efﬁcient, the cost
of solar photovoltaics and electrochemical systems is much
higher than the cost of reforestation. For example, using the sto-
ichiometric mass balance for ethanol production where 7.33 kg
is consumed per kg of H
reacted, one can calculate a
base sequestration cost (not inclusive of the cost of the CO
version unit) of $832 per ton using the levelized cost of H
a grid-supplemented photovoltaic array coupled with electroly-
sis at $6.1 per kg.
From this analysis, we can see how much higher the rate of
conversion using artiﬁcial photosynthetic technol-
ogy is than an equivalent forest, provided an overall system-level
thermal efﬁciency higher than around 0.5% is achieved. This
study can be expanded upon by analyzing several other factors
such as (i) indirect CO
emissions reduction, (ii)
techno-economic analysis, (iii) upstream and downstream CO
emissions, and (iv) life cycle analysis of energy generation and
equipment production, among other considerations. In our
future work, we plan to perform more in-depth analysis of
each contributing factor. For reforestation, this includes analy-
sis of forestry practices, population of individual tree species,
their carbon composition, and their growth rate. To adequately
beneﬁts and utility of reforestation in speciﬁc
regions, data on total forest biomass in the region must be gath-
ered to determine the quantity of available forest volume. This
ties into forest management practices, which greatly affects
reforestation sequestration potential after a forest has been
replanted and the Intergovernmental Panel on Climate
Change (IPCC) has identiﬁed as a strategy with one of the larg-
est potentials for CO
Figure 4. Direct CO
conversion rates for reforestation with the US high,
average, and low CO
sequestration projections (black, green, and gray dotted
lines, respectively) compared with artiﬁcial photosynthetic methanol (red) and
ethanol (blue) production at thermal efﬁciencies between 0.1% and 80% with
a 0.37 MWh/tCO
capture energy cost with both normalized to one hectare of
solar energy capture.
6▪MRS ENERGY AND SUSTAINABILITY / / VO LU M E 7 // e3 2 / / www.mrs.org/energy-sustainability-journal
While comparing the rate and efﬁciency of CO
of an artiﬁcial photosynthetic technology to that of trees is a
useful exercise to predict direct CO
sequestration efﬁcacy, we
note that there are several other factors that go into decisions
to research and deploy technologies. In today’s world, economic
factors are usually the deciding factor as to whether a technology
is deployed. Indirect CO
emissions mitigation, or the displace-
ment of the CO
emissions of production from a product that is
currently energy intense to make or derived from fossil fuels, are
also an important and complex issue. While natural and artiﬁ-
cial photosynthetic methods have been compared on more fun-
damental levels, we provide an example comparing direct CO
removal between timberland reforestation and electrochemi-
cally driven alcohol production from CO
The authors thank Prof. Matthew Hayek for insightful discus-
sions, Vi Dang for assistance in the design of Figure 3, and the
New York State Energy Research and Development Authority
(NYSERDA) for their Clean Energy Internship Program (S.G.H.).
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