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Comparison of carbon
sequestration efficacy between
artificial photosynthetic carbon
dioxide conversion and
timberland reforestation
Santiago Gonzalez Hernandez and Stafford W. Sheehan ,
Air Company, 407 Johnson Avenue, Brooklyn, NY 11206, USA
Address all correspondence to Stafford W. Sheehan at staff@aircompany.com
(Received 15 June 2020; accepted 11 August 2020)
ABSTRACT
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
Introduction
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
gas concentrations.
1
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
2
e) by 2030.
2
Considering
that global energy-related CO
2
emissions in 2019 were around
33 Gt total,
3
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
where CO
2
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
climate change.
4
While there are several approaches for CO
2
utilization,
among the most widely researched are artificial photosynthetic
or solar fuel technologies.
5
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,
DISCUSSION POINTS
•Carbon dioxide conversion is an important tool to reduce the
concentration of greenhouse gases in the atmosphere.
•Reforestation sequesters 5–10 kgCO
2
/day per hectare of forest and
is another important strategy in our portfolio to mitigate climate
change.
•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
doi:10.1557/mre.2020.32
ORIGINAL RESEARCH
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.
6
Nature has demonstrated that this
mass balance for artificial photosynthetic technologies, requir-
ing only H
2
O, CO
2
, 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.
7
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%.
8
Modern
photovoltaic (PV) modules, on the other hand, routinely reach
solar-to-electric conversion efficiencies as high as 20%.
9
One
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
capture,
10–14
realistic efficiencies for deployed systems with
earth-abundant elements have been estimated at approximately
5.4%.
15
Due to the cost, efficiency, and product collection chal-
lenges presented by PEC technologies, the present study
focuses on integrating photovoltaic modules with electrically
driven CO
2
conversion processes.
The comparatively high efficiency of photovoltaics suggests
that electrically driven CO
2
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
2
,
16
where electrolyzers
enable the production of H
2
at high thermal efficiencies.
17
These methods combined with CO
2
conversion technologies
enable CO
2
utilization at reasonable rates and system-level
efficiencies.
18
However, before committing substantial effort to applied
research and deployment of a CO
2
conversion technology, it
is useful to confirm that the technology can capture and seques-
ter CO
2
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
2
than this obvious and natural low-cost
alternative.
Of the commercially relevant methods that use the principles
of photosynthesis to convert CO
2
to chemicals, production of
alcohols that are liquids at standard temperature and pressure
are among the most widely researched and implemented.
19
Among the most mature technologies in the world that use
H
2
O and CO
2
to produce an essential chemical is CO
2
hydroge-
nation to methanol powered by H
2
produced via electrolysis.
20
Several pilot and demonstration plants producing thousands
of tons per year are in operation.
21
Production of ethanol via
CO
2
and renewable H
2
in a similar manner has also been dem-
onstrated on the pilot scale.
22
Direct electrosynthesis of ethanol
from CO
2
has been demonstrated since the 1980s
23
and is an
active area of research on the lab scale with several recent
advances to improve efficiency and rate of production.
24
Electrochemical CO
2
reduction to produce methanol has also
seen substantial recent progress,
25
as has electrochemical CO
conversion to either methanol or ethanol.
26
Methods that use
syngas, which can be produced electrochemically in a single sys-
tem,
27
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
2
,H
2
O, and solar energy.
In this study, we analyze liquid alcohol production from CO
2
to provide an understanding of the energy requirements and
CO
2
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
2
removal rate of timberland reforestation at different tree-
densities per unit area using data from the United States
Forestry Service
28
(USFS), which can be compared to similar
climates in Europe and Asia that share the same latitude and
biome. We compare the annual CO
2
removal rate between the
two at a benchmark CO
2
conversion system-level efficiency of
60% and one hectare of the solar capture area. Lastly, we assess
how varying the efficiency of the CO
2
conversion system com-
pares to reforestation at different tree densities, to understand
the minimum system-level efficiency required to provide a sub-
stantial CO
2
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
2
O, CO
2
, 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
2
capture, conversion, and product
separation. Several different methods for electrochemically
driven CO
2
conversion can be applied to this model provided
the kWh/kg CO
2
,kgH
2
O/kg CO
2
, 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
2
electrolysis; a two-step approach, such
as electrochemical H
2
production combined with CO
2
hydroge-
nation; or a three-step approach, such as electrochemical H
2
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
other combinations.
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
2
and H
2
O would consider the thermal efficiency of electrolysis
combined with hydrogenation for the conversion step, along
with any additional losses from CO
2
capture, H
2
O purification,
and product separation. If water purification and electrolysis
result in a net 75% efficient component,
29
and CO
2
capture,
hydrogenation, and product separation give a thermal efficiency
of 80% assuming efficient heat reuse from the exothermic CO
2
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
(Table 1).
30
Beyond the thermodynamic conversion values, for the pur-
poses of this study and as a model cost of CO
2
capture, CO
2
is
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
2
through
an absorbing fluid, typically monoethanolamine, which is
then fed into a stripping column to condense out water and puri-
fied CO
2
.
31
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
2
capture cost.
31
The rate of energy generation per hectare of land from solar
photovoltaics using approximately 20% efficient photovoltaic
modules was calculated using the literature values.
9
With
these energy generation per unit area rates and energy required
to capture and sequester each ton of CO
2
, the CO
2
conversion
rate per hectare of artificial photosynthetic technologies at
different MeOH and EtOH conversion efficiencies was
determined.
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
the CO
2
sequestered over the lifetime of the trees measured. To
determine the overall CO
2
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
2
, 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)
28
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
sequester CO
2
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)
5.0 6.5
Mass of product produced, per ton of CO
2
sequestered (kg/tCO
2
)
728.2 522.7
Energy required to sequester a ton of CO
2
at
thermodynamic efficiency (MWh/tCO
2
)
4.6 4.3
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
2
sequestration potential
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,
32
and multiplied by the carbon to CO
2
atomic mass ratio of 3.67 to attain the CO
2
sequestered in a for-
est planted for each region.
Results and discussion
The overall goal of both reforestation as outlined in this study
and CO
2
conversion is to reduce the concentration of CO
2
in the
atmosphere, [CO
2
], which we attempt to keep in the range of
350–550 ppm.
33
To minimize the effect of anthropogenic cli-
mate change, the increase in atmospheric CO
2
concentration
due to man-made CO
2
emissions (+[CO
2
]
Em
) can be mitigated
by equal and opposite decreases in atmospheric CO
2
concentra-
tion from reforestation (−[CO
2
]
REF
), air capture with CO
2
con-
version (−[CO
2
]
DAC
), 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
2
]
FGC
;
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
CO
2
conversion technologies that produce liquid alcohols
with reforestation our model focuses on these methods.
Currently, [CO
2
]
Em
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
2
emissions is to
prevent CO
2
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
2
emis-
sions that are challenging to avoid, for which we employ
CCUS measures.
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. [28].
Table 2. The ratios for biomass volume over the area of the US forest
according to the year and the region.
Year
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
2
in the atmosphere.
4▪MRS ENERGY AND SUSTAINABILITY / / VO LU M E 7 // e3 2 / / www.mrs.org/energy-sustainability-journal
Reforestation and CO
2
conversion technologies serve to
increase the rate at which CO
2
is removed from the atmosphere.
Quantifying trees’potential for CO
2
sequestration through
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.
34
Based
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
2
conversion by reforestation, we first determine
the ratio of embodied CO
2
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
2
sequestration rates, in kg
CO
2
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
2
sequestration ratios
from Table 3 are the CO
2
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
2
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
CO
2
sequestration in forest regions as they mature. Thus, to cal-
culate the CO
2
sequestration rates of kg CO
2
per tree-day, the
average forest age for each region, the CO
2
sequestration ratios,
and the US national average ratio of trees per hectare (241
trees/hectare)
35
in forests were used to develop high, average,
and low CO
2
sequestration rates for the average tree of each
region (Table 4).
With an understanding of model CO
2
sequestration rates for
reforestation per hectare and model CO
2
conversion energy
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.
9
Our calculations assume a reference
solar irradiation of 4.0 kWh/m
2
/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
4.0 kWh/m
2
/day.
36
Figure 4 shows the comparison between three different sce-
narios of timberland reforestation; the US high, average, and
low CO
2
sequestration rates, compared to CO
2
-to-alcohols sys-
tems at different overall efficiencies based on the HHV of meth-
anol and ethanol driven by solar photovoltaic panels under
4.0 kWh/m
2
/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
2
hydrogenation coupled with H
2
O electrolysis faces thermody-
namic energy losses in the production of intermediate H
2
,
while direct CO
2
electrolysis faces kinetic challenges (selectiv-
ity, overpotential) that contribute to lost energy. If we assume
an overall efficiency of 60% for a realistic CO
2
conversion sys-
tem, and the rate of CO
2
sequestration by well-curated forests
is equivalent to a system with overall efficiency around 0.5%,
reforestation sequesters CO
2
at around 1% the rate of the artifi-
cial photosynthetic system.
Table 4. The CO
2
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.
CO
2
Sequestration Rate
(kg CO
2
/ tree-day)
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
2
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.
CO
2
Sequestered in Forests
(metric tons CO
2
/ hectare)
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
2
out of the atmosphere and into terrestrial sinks. Furthermore,
the potential of CO
2
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
CO
2
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
2
pro-
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
2
emissions. If used as fuel, re-capture
and reuse of the CO
2
produced from energy generation by either
of the pathways presented in this paper would be necessary for
long-term reduction of atmospheric CO
2
.
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
CO
2
conversion technology, which allows for control over the
product made from CO
2
. As noted previously, the best way to
reduce the concentration of CO
2
in the atmosphere is to prevent
fossil fuels from being burned and their emission in the first
place, which displacement via a CO
2
-derived product can
accomplish. One such example is consumer ethanol;
22
spirits
emit 0.8–2.3 kg CO
2
e/L produced.
37
Production from CO
2
con-
sumes approximately 1.5 kg CO
2
/L ethanol produced. Provided
a renewable source of electricity powers the transformation,
38
much of the carbon mitigation of producing spirits from CO
2
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
2
,H
2
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
2
conversion deployments are
made.
39,40
The calculated cost per ton of CO
2
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
41
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
42
per ton of CO
2
sequestered, while more extreme estimates report as high as
$1654 USD per ton of CO
2
sequestered.
42
Most recent analyses
report values of sequestration through reforestation as less than
or about $100 per ton of CO
2
.
43
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
of CO
2
is consumed per kg of H
2
reacted, one can calculate a
base sequestration cost (not inclusive of the cost of the CO
2
con-
version unit) of $832 per ton using the levelized cost of H
2
from
a grid-supplemented photovoltaic array coupled with electroly-
sis at $6.1 per kg.
44
Conclusions
From this analysis, we can see how much higher the rate of
direct CO
2
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
2
emissions reduction, (ii)
techno-economic analysis, (iii) upstream and downstream CO
2
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
quantify CO
2
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
2
sequestration.
45
Figure 4. Direct CO
2
conversion rates for reforestation with the US high,
average, and low CO
2
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
2
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
2
conversion
of an artificial photosynthetic technology to that of trees is a
useful exercise to predict direct CO
2
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
2
emissions mitigation, or the displace-
ment of the CO
2
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
2
removal between timberland reforestation and electrochemi-
cally driven alcohol production from CO
2
.
Acknowledgments
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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