1
A Comparative Technoeconomic Analysis of Renewable
Generation of Methane Using Sunlight, Water and
Carbon Dioxide
: Supplementary Information
Alex J. Welch,
1
†
Ibadillah A. Digdaya,
1
†
Ron Kent,
2
Paul Ghougassian,
2
Harry A. Atwater
1
*
, and
Chengxiang Xiang
1
*
1
Liquid Sunlight Alliance
, Department of Applied Physics and Material Science, California Institute of
Technology, Pasadena, California 91125, United States
2
Southern California Gas
Company
555 W 5
th
St., Los Angeles, CA, 90013
*To whom correspondence should be addressed:
haa@caltech.edu
,
cxx@caltech.edu
†
Alex Welch and Ibadillah Digdaya
contributed equally to this work.
2
Analytical basis
Relevant process assumption and cost information for direct air capture (DAC),
1
low
-
temperature
electrolysis (LTE),
2,3
high
-
temperature electrolysis (HTE),
3,4
solar thermochemical hydrogen (STCH),
and thermochemical methanation
5
were solicited from available reports. Various sources were ado
pted to
develop process and cost estimates for ocean capture,
6
–
10
photoelectrochemical (PEC) hydrogen
generation,
11,12
g
rid electrolysis methanation,
3,4
PEC methanation,
11,12
and biochemical methanation.
13,14
The plant design capacity was assumed to be 240 ton/day for CO
2
capture, 45 to
n/day for H
2
generation,
81 ton/day for CH
4
production.
In some cases where cost information were already available in a certain plant capacity, the costs of similar
items of different sizes were approximated to our specific design capacity using exponen
tial scaling factor,
as follows:
퐶
D
=
퐶
B
(
푆
퐷
푆
퐵
)
푁
(2)
where
C
D
is the cost at design capacity,
C
B
is the cost at known baseline capacity,
S
D
is the design capacity,
S
B
is the known baseline capacity, and
N
is the scaling factor exponent that varies from 0.1 to unity. Unless
otherwise specified, an average value of 0.6 was as used for
N.
All equipment capital costs reported in the past were adjusted to 2019 dollars by multiplying the reported
base cost
from an earlier year by the ratio of a cost index (
I
) in 2019 to a base cost index (
I
base
) that
corresponds to the year of which the cost information was obtained:
10
퐶
2019
=
퐶
R
퐼
2019
퐼
R
(2)
where
C
2019
is the equipment purchase cost in 2019,
C
R
is the equipment purchase cost in reference year,
I
2019
is the Chemical Engineering Plant Cost Index (CEPCI) in 2019 and
I
R
is CEPCI in reference year.
The calculated results of the levelized product costs were
adjusted to 2020 dollars using inflation rate of
1.9%:
퐿퐶푃
2020
=
퐿퐶푃
2019
(
1
+
1
.
9%
)
2020
−
2019
(3)
where
LCP
2020
and
LCP
2019
are the levelized cost of product in 2020 dollars and in 2019 dollars,
respectively.
3
Table S1:
The default H2A hydrogen
production economic model and its financial values were applied
to all systems studied.
Financial Parameters
Assumptions
Length of Construction Period (years)
1
Year of analysis
2020
% of Capital Spent in 1st Year of Construction
100%
Start
-
up Time
(years)
1
Plant life (years)
40
Analysis period (years)
40
Depreciation Schedule Length (years)
20
Depreciation Type
MACRS
% Debt Financing
60%
% Equity Financing
40%
Interest rate on debt (%)
3.70%
Debt period (years)
Constant debt
% of Fixed
Operating Costs During Start
-
up (%)
75%
% of Revenues During Start
-
up (%)
50%
% of Variable Operating Costs During Start
-
up (%)
75%
Decommissioning costs (% of depreciable capital investment)
10%
Salvage value (% of total capital investment)
10%
Inflation rate (%)
1.9%
State Taxes (%)
6.0%
Federal Taxes (%)
21.0%
Total Tax Rate (%)
25.74%
Working capital (% of yearly change in operating costs)
15%
After
-
tax real IRR
6%
After
-
tax nominal IRR
8.01%
4
Table S2:
Assumptions for calculating the cost of direct air capture of CO
2
currently. For further
information see excel sheet titled “Direct Air Capture of CO2 (current)”.
Process Assumptions
Design capacity
244444.44
kg CO
2
/day
Average production
220000
kg CO
2
/day
Capacity factor
0.9
Baseline capacity
2684931.507
kg CO
2
/day
Scale ratio
0.09
Fan energy
61
kWh/t
-
CO
2
Fluid pumping energy
21
kWh/t
-
CO
2
Fluid pumping energy
27
kWh/t
-
CO
2
Power produced from slaking heat
-
77
kWh/t
-
CO
2
Energy consumption
32
kWh/t
-
CO
2
ASU power usage
238
kWh/t
-
CO
2
Compressor power usage
132
kWh/t
-
CO
2
Calciner
369
kWh/t
-
CO
2
Water consumption
4.7
t
-
water/t
-
CO
2
Water price
0.69307362
$/t
-
water
Industrial electricity
0.049
$/kWh
Natural gas price
3.5
$/GJ
Baseline system cost
Baseline
installed cost in
startup year dollars
Installation
cost factor
Combined
plant scaling
I
nstalled cost
Air contactor
$135,867,821
1.86
0.09
$22,984,912
Pellet reactor
$91,490,678
1.70
0.09
$14,167,891
Calciner
-
slaker
$52,110,425
1.77
0.15
$13,591,553
Air separation unit
$45,209,958
1.43
0.24
$15,339,052
CO
2
compressor
$20,463,455
1.16
0.20
$4,777,228
Steam turbine
$7,971,229
1.12
0.19
$1,667,188
Power plant
$38,904,359
1.07
0.30
$12,564,408
Fines filter
$20,939,349
1.76
0.24
$8,728,853
Other equipment
$115,285,393
1.06
0.24
$29,039,678
Buildings
$2,974,339
2.68
0.43
$3,445,575
Transformer
$22,129,085
1.06
0.33
$7,822,927
Total
$134,129,266
Capital costs
Depreciable capital costs
Value in startup year dollars
Direct capital cost
$134,129,266
Indirect capital cost
Site preparation (2% direct capital cost)
$2,682,585
Engineering and design (10% direct capital cost)
$13,412,927
Project contingency (15% direct capital cost)
$20,119,390
Upfront permitting cost (legal and contractors fees) (7.5% direct
capital cost)
$10,059,695
Total capital cost
$180,403,863
Fixed operating costs
Value in startup year
Labor cost ($50/FTE) ($/year) (11 FTE)
$1,282,289
G&A ($/year) (20% labor cost)
$256,458
Property taxes and insurance ($/year) (2% total capital cost)
$3,608,077
Production
maintenance and repairs ($/year) (2.9% direct capital
cost)
$3,889,749
Total fixed operating costs ($/year)
$9,036,573
Variable operating costs
Value in startup year
Energy utilities costs
$17,823,995
Non energy utilities
costs
$3,032,682
Total variable operating costs ($/year)
$20,856,677
Replacements
Value in startup year
Unplanned replacement capital cost (0.5% of total direct capital
costs/year)
$670,646
5
Table S3:
Assumptions for calculating the cost of direct air capture of CO
2
in the future. For further
information see excel sheet titled “Direct Air Capture of CO2 (future)”.
Process Assumptions
Design capacity
2684931.51
kg CO
2
/day
Average production
2550684.932
kg CO
2
/day
Capacity factor
0.95
Baseline capacity
2684931.507
kg CO
2
/day
Scale ratio
1
Scaling factor exponent
0.6
Electricity equivalent gas input
478.3333333
kWh/t
-
CO
2
Electricity input
77
kWh/t
-
CO
2
Water consumption
4.7
t
-
water/t
-
CO
2
Water price
0.69307362
$/t
-
water
Industrial
electricity
0.01
$/kWh
Natural gas price
3.5
$/GJ
Baseline system cost
Uninstalled cost in startup year
dollars
Installation
cost factor
Installed cost
Air contactor
$157,996,906
1.00
$157,996,906
Pellet reactor
$112,786,948
1.00
$112,786,948
Calciner
-
slaker
$75,667,193
1.00
$75,667,193
Air separation unit
$55,560,659
1.00
$55,560,659
CO
2
compressor
$18,440,904
1.00
$18,440,904
Steam turbine
$6,900,467
1.00
$6,900,467
Power plant
$31,765,944
1.00
$31,765,944
Fines filter
$29,505,446
1.00
$29,505,446
Other equipment
$91,609,652
1.00
$91,609,652
Buildings
$6,900,467
1.00
$6,900,467
Transformer
$19,868,587
1.00
$19,868,587
Total
$607,003,173
Capital costs
Depreciable capital costs
Value in startup year
dollars
Direct capital cost
$607,003,173
Indirect capital cost
Site preparation (2% direct capital cost)
$12,140,063
Engineering and design (10% direct capital cost)
$60,700,317
Project contingency (15% direct capital cost)
$91,050,476
Upfront permitting cost (legal and contractors fees) (7.5% direct capital
cost)
$45,525,238
Total capital cost
$816,419,268
Fixed operating costs
Value in startup year
dollars
Labor cost ($50/FTE) ($/year) (46.34
FTE)
$5,402,081
G&A ($/year) (20% labor cost)
$1,080,416
Property taxes and insurance ($/year) (2% total capital cost)
$16,328,385
Production maintenance and repairs ($/year) (2.9% direct capital cost)
$17,603,092
Total fixed operating
costs ($/year)
$40,413,974
Variable operating costs
Value in startup year
dollars
Energy utilities costs
$17,823,995
Non energy utilities costs
$3,032,682
Total variable operating costs ($/year)
$20,856,677
Replacements
Value in startup year
dollars
Unplanned replacement capital cost (0.5% of total direct capital
costs/year)
$670,646
6
Table S4:
Assumptions for calculating the cost of ocean CO
2
capture currently. For further information
see excel sheet titled “Ocean CO2 Capture (current)”.
Process Assumptions
Design capacity
246575.34
kg CO
2
/day
Average production rate
221917.8082
kg CO
2
/day
Capacity factor
0.9
Extraction
efficiency
0.9
Electrodialysis acidified stream target pH
0.40
Oceanwater target pH
4
Current density
100
mA/cm
2
Voltage
1.2
V
Electrodialysis energy at oceanwater target pH
0.97736927
kWh/kg CO
2
CO2 stripping energy
0.072994345
kWh/kg
CO
2
Intake energy (0.5 bar, 5 m intake)
0.206827333
kWh/kg CO
2
Pre
-
treatment energy (3 bar ultrafiltration, 7.6 bar nanofiltration, 0.5 bar
electrodialyzer)
0.04804125
kWh/kg CO
2
Pump energy (0.5 bar membrane contactor)
0.206827333
kWh/kg CO
2
Industrial electricity
0.049
$/kWh
Baseline system cost
U
ninstalled cost in startup year
dollars
Installation
cost factor
I
nstalled cost
Electrodialyzer
$15,133,812
1.20
$18,160,574
Pre
-
treatment
$6,657,344
1.20
$7,988,813
Gas stripping
$21,691,109
1.20
$26,029,331
Intake
$6,717,271
1.20
$8,060,725
Screening
$22,973,169
1.20
$27,567,802
Pumping
$3,747,976
1.20
$4,497,572
Total
$92,304,818
Capital costs
Depreciable capital costs
Value in startup year dollars
Direct
capital cost
$92,304,818
Indirect capital cost
Site preparation (2% direct capital cost)
$1,846,096
Engineering and design (10% direct capital cost)
$9,230,482
Project contingency (15% direct capital cost)
$13,845,723
Upfront permitting
cost (legal and contractors fees) (7.5% direct capital
cost)
$6,922,861
Total capital cost
$124,149,981
Fixed operating costs
Value in startup year dollars
Labor cost ($50/FTE) ($/year) (34.95 FTE)
$4,074,146
G&A ($/year) (20% labor
cost)
$814,829
Property taxes and insurance ($/year) (2% total capital cost)
$2,483,000
Production maintenance and repairs ($/year) (2.9% direct capital cost)
$2,769,145
Total fixed operating costs ($/year)
$10,141,120
Variable operating
costs
Value
in startup year
Energy utilities costs
$6,001,364
Non energy utilities costs
$0
Total variable operating costs ($/year)
$6,001,364
Replacements
Value in startup year dollars
Unplanned replacement capital cost
(0.5% of total direct capital
costs/year)
$461,524.09
Replacement costs (15% of depreciable capital cost/5year)
$18,622,497.08
Specified replacement cost (electrodialyzer/5year)
$18,160,574.35
Specified replacement cost (60% pre
-
treatment/5year)
$4,793,288.03
Specified replacement cost (gas stripping/10year)
$26,029,331.20
7
Table S5:
Assumptions for calculating the cost of ocean CO
2
capture in the future. For further information
see excel sheet titled “Ocean CO2 Capture (future)”.
Process
Assumptions
Design capacity
2739726.03
kg CO
2
/day
Average production rate
2602739.726
kg CO
2
/day
Capacity factor
0.95
Extraction efficiency
0.98
Electrodialysis acidified stream target pH
0.40
Oceanwater target pH
4
Current
density
1000
mA/cm
2
Voltage
1.6
V
Electrodialysis energy at oceanwater target pH
1.196778698
kWh/kg CO
2
CO2 stripping energy
0.072994345
kWh/kg CO
2
Intake energy (0.5 bar, 5 m intake)
0.189943469
kWh/kg CO
2
Pre
-
treatment energy (3 bar
ultrafiltration, 7.6 bar nanofiltration, 0.5 bar
electrodialyzer)
0.044119516
kWh/kg CO
2
Pump energy (0.5 bar membrane contactor)
0.189943469
kWh/kg CO
2
Industrial electricity
0.01
$/kWh
Baseline system cost
U
ninstalled cost in startup
year
dollars
Installation
cost factor
I
nstalled cost
Electrodialyzer
$7,721,333
1.00
$7,721,333
Pre
-
treatment
$25,033,436
1.00
$25,033,436
Gas stripping
$73,347,986
1.00
$73,347,986
Intake
$68,543,583
1.00
$68,543,583
Screening
$234,420,087
1.00
$234,420,087
Pumping
$38,244,657
1.00
$38,244,657
Total
$447,311,082
Capital costs
Depreciable capital costs
Value in startup year dollars
Direct capital cost
$447,311,082
Indirect capital cost
Site preparation (2% direct
capital cost)
$8,946,222
Engineering and design (10% direct capital cost)
$44,731,108
Project contingency (15% direct capital cost)
$67,096,662
Upfront permitting cost (legal and contractors fees) (7.5% direct capital
cost)
$33,548,331
Total
capital cost
$601,633,406
Fixed operating costs
Value in startup year dollars
Labor cost ($50/FTE) ($/year) (126.79 FTE)
$14,780,298
G&A ($/year) (20% labor cost)
$2,956,060
Property taxes and insurance ($/year) (2% total capital
cost)
$12,032,668
Production maintenance and repairs ($/year) (2.9% direct capital cost)
$13,419,332
Total fixed operating costs ($/year)
$43,188,359
Variable operating costs
Value in startup year
Energy utilities costs
$16,090,905
Non energy utilities costs
$0
Total variable operating costs ($/year)
$16,090,905
Replacements
Value in startup year dollars
Unplanned replacement capital cost (0.5% of total direct capital
costs/year)
$2,236,555
Replacement
costs (15% of depreciable capital cost/5year)
$60,163,341
Specified replacement cost (electrodialyzer/5year)
$7,721,333
Specified replacement cost (60% pre
-
treatment/5year)
$25,033,436
Specified replacement cost (gas stripping/10year)
$73,347,986
8
Table S6:
Assumptions for calculating the cost of membrane water capture. For further information see
excel sheet titled “Membrane Water Capture”.
Process Assumptions
Design capacity
365000
kg H
2
O/day
Average production rate
357700
kg H
2
O/day
Capacity factor
98%
System cost
Uninstalled cost in startup year
dollars
Installation
cost factor
Installed cost
Equipment cost
$52,581,424
1.00
$52,581,424
Total
$52,581,424
Capital costs
Value in
startup year dollars
Depreciable capital costs
Direct capital cost
$52,581,424
Indirect capital cost
Site preparation (2% direct capital cost)
$761,944
Engineering and design (10% direct capital cost)
$3,809,721
Project contingency
(15% direct capital cost)
$5,714,581
Upfront permitting cost (legal and contractors fees) (15% direct capital
cost)
$5,714,581
Total capital cost
$68,582,251
Fixed operating costs
Value in startup year dollars
Labor cost ($50/FTE)
($/year) (3 FTE)
$362,980
G&A ($/year) (20% labor cost)
$67,339
Property taxes and insurance ($/year) (2% total capital cost)
$1,371,645
Production maintenance and repairs ($/year) (3% direct capital cost)
$1,577,443
Total fixed operating costs
($/year)
$3,379,407
Variable operating costs
Value in startup year dollars
Total variable operating costs ($/year)
$321,067
Replacements
Value in startup year dollars
Unplanned replacement capital cost (1% of total
direct capital
costs/year)
$525,814
9
Table S7:
Assumptions for calculating the cost of condensation water capture. For further information
see excel sheet titled “Condensation Water Capture”.
Process Assumptions
Design capacity
365000
kg H
2
O/day
Average production rate
357700
kg H
2
O/day
Capacity factor
98%
System cost
Uninstalled cost in startup year
dollars
Installation
cost factor
Installed cost
OPUR structure
$19,993,088
1.00
$19,993,088
Pumps
$214,000
1.00
$214,000
Total
$1,273,210,134
Capital costs
Value in startup year dollars
Depreciable capital costs
Direct capital cost
$327,803,864
Indirect capital cost
Site preparation (2% direct capital cost)
$4,750,123
Engineering and design
(10% direct capital cost)
$23,750,615
Project contingency (15% direct capital cost)
$35,625,923
Upfront permitting cost (legal and contractors fees) (15% direct capital
cost)
$35,625,923
Total capital cost
$427,556,448
Fixed operating
costs
Value in startup year dollars
Labor cost ($50/FTE) ($/year) (3 FTE)
$362,980
G&A ($/year) (20% labor cost)
$67,339
Property taxes and insurance ($/year) (2% total capital cost)
$8,551,129
Production maintenance and repairs ($/year)
(3% direct capital cost)
$9,834,116
Total fixed operating costs ($/year)
$18,815,564
Variable operating costs
Value in startup year dollars
Total variable operating costs ($/year)
$0
Replacements
Value in startup year
dollars
Unplanned replacement capital cost (1% of total direct capital
costs/year)
$3,278,039