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Liquid CO2 storage vessels for a CO2 battery (ambient, ~70 bar): count, wall thickness, steel and cost per kWh

Carbon-steel bullets holding an Energy Dome-type CO2 battery's liquid CO2 at ambient temperature, 4 to 1,000 MWh. NIST densities, ASME UG-27 walls and the Seider vessel cost give vessel count, steel and installed EUR/kWh.

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Open Jack

Founder · OpenJack

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  • CurrencyEUR 2023
  • Battery limitinstalled
  • 67 documented assumptions
  • Updated2 Nov 2026

Mass and energy balance

What the asset draws and delivers for the case and load you choose.

Reference case

Full discharges a year you expect to sell. 365 is one cycle a day, the usual basis for daily-cycling long-duration storage. Fewer cycles spread the same vessel cost over fewer MWh.

Flows

  • In: Liquid CO2 from the charge compression train—

Asset

Liquid CO2 storage vessels for a CO2 battery (ambient temperature, ~70 bar)

  • Out: Liquid CO2 to the discharge expansion train—

Annual quantities at the current usage.

Electricity returned per full discharge of the CO2 battery. Sets the CO2 inventory the vessels must hold. 200 MWh is the Energy Dome standard plant (20 MW for 10 h, Sardinia and Latrobe Valley); the shared CO2 battery range runs from the 4 MWh Ottana demonstrator to 1,000 MWh.

CO2 inventory
— t
Liquid CO2 volume
— m3
Number of vessels
— vessels
Steel mass of the vessel bank
— t

CAPEX

Specific cost at this size, what backs it, and how it behaves with scale.

Specific CAPEX at this size

—

Reference size 200 MWh

Seider 4th ed. / IEEE Spectrum 2025

Model value: steel mass, Seider vessel price and installation factor. Customized: your own installed quote per kWh, used as is at every size; it is then no longer sourced.

Installed vessel CAPEX per kWh (applied) by energy capacity

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  1. Ex-factory
  2. Installed· this figure
  3. Fully financed
CAPEX at the battery limit—
Working capital, contingencyproject level

Included

  • Equipment
  • Installation, civil works and engineering

Not included

  • Working capital
  • Contingency

Added at project level

  • Working capital layer, from the project settings
  • Contingency, from the project settings

Project figures

Values passed to project mode when this asset is added to a project.

Concept sketch, not to scale · drag to rotate

Liquid CO2 from the charge compression train in
—
Liquid CO2 to the discharge expansion train outMain product
—
CAPEX to the battery limit
—
Fixed O&M
0 EUR/year
Variable O&M, excluding feedstocks
0 EUR/year
Asset lifetime
30 years

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Vessel bank cost per MWh discharged

A standalone indicator for a quick read. Project mode recomputes it for the whole system.

—

  • Vessel purchase part—
  • Installation part—

Linked to the balance above

Full discharges a year you expect to sell. 365 is one cycle a day, the usual basis for daily-cycling long-duration storage. Fewer cycles spread the same vessel cost over fewer MWh.

Hours of discharge at rated power. Sets the rated power and the CO2 flow leaving the vessels, not their size. A cycle also needs a charge: on the Energy Dome standard frame it takes about as long as the discharge, so cycles x (charge + discharge) must fit in a year.

Tonnes of CO2 cycled per MWh discharged. 10 t/MWh is printed for the 200 MWh Sardinia plant (2,000 t, IEEE Spectrum 2025). The Energy Dome 2022 deck implies about 11.5 t/MWh (66.7 kWh/m3 at 1.3 L/kg). The whole inventory is sized as liquid at full charge.

Absolute pressure in the vessels. Energy Dome's 2022 deck gives 70 bar at ambient temperature; IEEE Spectrum reports compression to about 55 bar. It must stay above the CO2 saturation pressure at the hottest ambient temperature, or the liquid cannot be held.

Hottest temperature the uninsulated vessels reach. It sets the liquid density (NIST: 860 kg/m3 at 10 °C, 710 at 25 °C, 592 at 30 °C) and the saturation pressure. CO2 has no liquid above 31 °C; at 70 bar storage the limit is about 28.6 °C.

Inside diameter of each horizontal bullet, 12 m between tangents with 2:1 ellipsoidal heads. 3 m reflects the 'school bus' sized vessels reported at Sardinia. A larger diameter means fewer vessels but thicker walls.

Real rate used to annualise the vessel CAPEX. Costs are in constant 2023 euros, so the rate must be real (after inflation).

Service life of the vessel bank. Energy Dome states a 30+ year plant life with no degradation; static carbon-steel pressure vessels have no wear part to replace.

Vessel bank cost per MWh discharged by full cycles per year

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Benchmarks

This model's figures against published references and reference cases.

CO2 inventory per MWh, t/MWh

10 IEEE Spectrum 2025

11.53 Energy Dome NETL 2022 (derived)

Liquid specific volume, L/kg

1.3 Energy Dome NETL 2022

Liquid volume per MWh, m3/MWh

14.99 Energy Dome NETL 2022 (derived)

Reference cases at this size and usage

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All results

Every model output at the current settings, grouped, with the sensitivity charts.

55 outputs · 6 groups · 13 also shown above, dimmed

Storage cost

5
Vessel bank cost per MWh discharged
ComputingEUR/MWh
Vessel purchase part
ComputingEUR/MWh
Installation part
ComputingEUR/MWh
Annualised vessel CAPEX
ComputingEUR/year
Capital recovery factor
Computing1/year

CAPEX

14
Installed vessel bank CAPEX
ComputingEUR
Installed vessel CAPEX per kWh (applied)
ComputingEUR/kWh
Installed vessel CAPEX per kWh, scale law
ComputingEUR/kWh
Installed vessel CAPEX per kWh (model)
ComputingEUR/kWh
Purchased vessel cost per kWh
ComputingEUR/kWh
CAPEX per kWh as built
ComputingEUR/kWh
Whole-vessel rounding
Computingratio
Purchased vessels
ComputingEUR
Installation
ComputingEUR
CAPEX per tonne of CO2 stored
ComputingEUR/t
Purchased cost of one vessel, 2013
ComputingUSD
Purchased cost of one vessel, 2023 euros
ComputingEUR
Purchased cost per kg of steel
ComputingEUR/kg
Share of Energy Dome's whole-plant price
Computing%

CO2 inventory and liquid

9
CO2 inventory
Computingt
CO2 inventory per MWh
Computingt/MWh
Liquid CO2 density at the hottest day
Computingkg/m3
Liquid specific volume
ComputingL/kg
CO2 saturation pressure at the hottest day
Computingbar
Pressure margin above saturation
Computingbar
Margin to the critical temperature
Computing°C
Liquid CO2 volume
Computingm3
Liquid volume per MWh
Computingm3/MWh

Vessels and steel

15
Number of vessels
Computingvessels
Vessels needed (continuous)
Computingvessels
Volume of one vessel
Computingm3
Required vessel volume
Computingm3
Vessel volume per MWh
Computingm3/MWh
Vessel volume per tonne of CO2
Computingm3/t
Vapour-space penalty
Computing%
Gauge pressure
Computingbar
Design pressure (gauge)
Computingbar
Allowable stress
ComputingMPa
Shell wall thickness
Computingmm
Steel mass of one vessel
Computingt
Vessel weight for the cost law
Computinglb
Steel mass of the vessel bank
Computingt
Steel per MWh
Computingt/MWh

Code and correlation checks

4
Thickness to radius ratio
Computingratio
Pressure to stress ratio
Computingratio
Thin-wall formula out of range
Computingflag
Cost law out of range
Computingflag

Duty and chaining

8
Rated discharge power
ComputingMW
Discharge load hours
Computingh/year
Charge to discharge time ratio
Computingratio
Charge load hours
Computingh/year
Discharge capacity factor
Computing%
Energy discharged a year
ComputingMWh/year
CO2 cycled through the vessels a year
Computingt/year
CO2 flow leaving the vessels in discharge
Computingt/h

Sensitivities

5 charts
Vessel cost per MWh vs full cycles per year

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Vessel count and CAPEX per kWh vs energy capacity

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Wall thickness and steel per MWh vs vessel diameter

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Vessel volume per MWh vs hottest ambient temperature

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Installed vessel CAPEX per kWh vs storage pressure (25 °C hottest day)

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Sources

67 documented assumptions · 11 sources

About

Calculator context

An Energy Dome CO2 battery keeps its charged state as liquid CO2 at ambient temperature, in carbon-steel pressure vessels at about 70 bar (Energy Dome, NETL 2022). This calculator sizes and prices that vessel bank. It takes the energy capacity (MWh, the key shared by the six CO2 battery calculators), the CO2 inventory per MWh, the storage pressure, the hottest ambient day and the vessel diameter. It returns the liquid volume, the number of vessels, their wall thickness and steel mass, the installed CAPEX per kWh, and the vessels' share of the plant LCOS.

Inventory. 10 t of CO2 per MWh discharged is the default: IEEE Spectrum (Dec 2025) prints 2,000 t for the 200 MWh Sardinia plant. Energy Dome's 2022 deck implies about 11.5 t/MWh (66.7 kWh/m3 at 1.3 L/kg). That is a derived cross-check, within 15 %. The slider runs from 9 to 12 t/MWh. The whole inventory is sized as liquid at full charge.

Liquid. The density is the NIST saturated-liquid value at the hottest ambient temperature, interpolated between NIST points at 10, 25 and 30 °C. The interpolation is a little low between the points, which is conservative. Liquid can only be held while the CO2 saturation pressure at that temperature stays below the storage pressure. The calculator refuses a design that breaks this rule, and any hottest day at or above the 31 °C critical point. At 70 bar the limit is about 28.6 °C. The vessels are filled to 95 % at most: the 49 CFR rule for pressure receptacles, transposed to stationary storage.

Vessels. Each vessel is a horizontal bullet, 12 m between tangents, with 2:1 ellipsoidal heads; about 3 m in diameter matches the 'few dozen' school-bus-sized vessels reported at Sardinia. The shell thickness follows ASME VIII Div. 1 UG-27. The design pressure is 10 % above gauge (Seider rule), the allowable stress is 15 ksi carbon steel by default or 20 ksi for SA-516 Gr 70, welds are fully radiographed, and 3.175 mm is added for corrosion. Steel mass uses the Seider weight formula. Per-MWh results use the continuous vessel count, so they do not jump with rounding. The vessel count, total steel and total CAPEX use whole vessels.

Cost. Each vessel is priced from its weight with the Seider et al. (4th ed.) horizontal-vessel correlation. That gives an f.o.b. price including nozzles, manholes and supports, escalated with CEPCI 2023 / 567 and converted at the ECB 2023 rate. It is then multiplied by the Towler & Sinnott fluids-processing installation factor of 3.2, which gives installed ISBL direct cost in constant 2023 euros. The vessel bank is a numbering-up of identical vessels, so no economy of scale is applied: the cost per kWh does not fall with plant size. The cost per MWh discharged annualises that CAPEX with a real capital recovery factor over the vessel service life (30 years by default, Energy Dome) and the cycles a year. It is the vessels' share of the plant LCOS only.

What to defend, and what not. The vessel-only CAPEX comes out high. On default settings it is a large share of the roughly EUR 220/kWh Energy Dome quotes for the whole 20 MW / 200 MWh plant (vendor claim, 2023 euros, the same year as this calculator). That share is shown as an output. Each modelling choice pushes the cost up: 15 ksi rather than 20 ksi steel, a generic fluids installation factor whose 0.8 piping term is large for simple storage bullets, and 70 bar where IEEE reports about 55 bar. At 55 bar the hottest day must stay below about 19 °C for liquid storage. Fixed O&M is not modelled, because no sourced figure was found. Engineering, offsites and the first CO2 fill are excluded.

Model

126 variables — inputs, calculations and outputs, with their dependencies.

126 variables shown of 126
VariableValueUnitDepends on
200MWh—
10h—
365cycles/year—
1——
0——
150EUR/kWh—
10t/MWh—
70bar—
25°C—
3m—
0.07ratio—
30years—
VariableFormulaUnitDepends on
if((<=)+(<=)+(<=)+(<=)+(<=)+(<=)+(<)+(>=)+((+(-)*(-)/(-))>)+(**(+(/(*)))>)+(<)+(<)+(<)+(>)+(<)+(>)+(<=)>0,1,0)bool
VariableFormulaUnitDepends on
/max(,)MW
t/MWh
*t
if(<=,+(-)*(-)/(-),+(-)*(-)/(-))kg/m3
/max(,)L/kg
+(-)*(-)/(-)bar
-bar
-°C
*/max(,)m3
/max(,)m3/MWh
(/-)*%
/max(*,)m3/t
*m3
/max(,)m3/MWh
*^2**(+*)m3
/max(,)vessels
ceil()vessels
-bar
max(+,*)bar
if(==,,)MPa
***/max(**-**,)+mm
(/)/max(/,)ratio
*/max(*,)ratio
if((>)+(>)>0,1,0)flag
*(+/)*(+*)*(/)*/t
*/lb
if((<)+(>)>0,1,0)flag
exp(+*ln()+*ln()^2)USD
*//EUR
/max(*,)EUR/kg
*t
*/max(,)t/MWh
*/max(*,)EUR/kWh
*EUR/kWh
clamp(*(/)^(-),,)EUR/kWh
if(>=,,)EUR/kWh
if(>=,,/max(,))ratio
***EUR
/max(*,)EUR/kWh
/EUR
-EUR
/max(,)EUR/t
/*%
*h/year
/(*)ratio
*h/year
/*%
*MWh/year
*t/year
/max(,)t/h
if(<,/,(*(+)^)/max((+)^-,))1/year
*EUR/year
*/max(,)EUR/MWh
*/max(,)EUR/MWh
+EUR/MWh

Assumptions

67 assumptions used in the calculations

  • Avoids a literal 0 in formulas.

    Market range Not applicable.

    0
    OpenJack DSL convention

  • Avoids a literal 1 in formulas.

    Market range Not applicable.

    1
    OpenJack DSL convention

  • Converts radius-based UG-27 to a diameter basis.

    Market range Exact.

    2
    Geometry

  • Cross-section of the shell.

    Market range Exact.

    0.25
    Geometry

  • Display in percent.

    Market range Exact.

    100%
    Unit conversion

  • Mass conversion.

    Market range Exact.

    1000kg/t
    Unit conversion

  • Volume conversion.

    Market range Exact.

    1000L/m3
    Unit conversion

  • Length conversion.

    Market range Exact.

    1000mm/m
    Unit conversion

  • Pressure conversion for UG-27.

    Market range Exact.

    0.1MPa/bar
    Unit conversion

  • The Seider cost correlation takes the vessel weight in lb.

    Market range Exact.

    0.453592kg/lb
    Unit conversion

  • Gauge pressure = absolute storage pressure minus one atmosphere.

    Market range Exact.

    1.01325bar
    Standard atmosphere

  • Lower node of the density interpolation.

    Market range Tabulated.

    10.153°C
    NIST WebBook

  • Liquid density at a cool site.

    Market range Saturated liquid only; compressed liquid above saturation is a few percent denser (not credited).

    859.99kg/m3
    NIST WebBook

  • Middle node of the interpolation.

    Market range Tabulated.

    25.038°C
    NIST WebBook

  • Default design density at a 25 °C maximum ambient.

    709.92kg/m3
    NIST WebBook

  • Lower node of the saturation-pressure line.

    Market range Tabulated.

    64.399bar
    NIST WebBook

  • Upper node of the interpolation.

    Market range Tabulated.

    30.022°C
    NIST WebBook

  • Liquid density 1 °C below the critical point.

    Market range Tabulated.

    592.32kg/m3
    NIST WebBook

  • Highest ambient temperature at which 70 bar still holds liquid.

    Market range Tabulated.

    28.611°C
    NIST WebBook

  • Upper node of the saturation-pressure line, extrapolated linearly below 25 °C (slightly low by under 1 bar at 20 °C).

    Market range Tabulated.

    69.886bar
    NIST WebBook

  • No liquid CO2 can exist above it: the calculator refuses a maximum ambient at or above it.

    Market range 30.98 °C (NIST).

    31°C
    Energy Dome NETL 2022

  • Leaves vapour space so thermal expansion of the liquid does not overfill the vessel.

    Market range No independent range cited.

    0.95ratio
    49 CFR 173.304b

  • Design pressure margin above the operating gauge pressure.

    Market range No independent range cited.

    1.1ratio
    BioSTEAM / Seider

  • Minimum design margin.

    Market range No independent range cited.

    2.068bar
    BioSTEAM / Seider

  • Reference case 1 allowable stress.

    103.4MPa
    BioSTEAM / Seider

  • Reference case 2 allowable stress.

    Market range No independent range cited.

    137.9MPa
    ASME II-D via secondary sources

  • Documents the source unit.

    Market range Not applicable.

    15000psi
    BioSTEAM / Seider

  • A 70 bar vessel of this thickness is fully radiographed.

    Market range 0.85 spot radiography, 0.70 none.

    1ratio
    ASME VIII Div. 1 UW-12 via CheCalc

  • Added to the pressure thickness.

    3.175mm
    BioSTEAM / CheCalc

  • Circumferential stress thickness.

    Market range Exact.

    1.2
    ASME UG-27

  • Flags a design outside the thin-wall formula.

    Market range Exact.

    0.5ratio
    ASME UG-27

  • Flags a design outside the thin-wall formula.

    Market range Exact.

    0.385ratio
    ASME UG-27

  • Shell and heads mass.

    7861kg/m3
    IDAES / Seider

  • Adds the head steel to the shell.

    Market range Exact within the method.

    0.8ratio
    IDAES / Seider

  • Unit vessel length.

    12m
    IEEE Spectrum 2025 (indicative)

  • Adds the head volume to the shell volume.

    Market range Exact.

    0.333333ratio
    Geometry

  • Purchased cost of one vessel, incl. nozzles, manholes and supports.

    Market range Valid 1,000-920,000 lb.

    5.6336
    Seider 4th ed.

  • Cost-weight elasticity.

    Market range Exact within the method.

    0.4599
    Seider 4th ed.

  • Curvature of the cost-weight law.

    Market range Exact within the method.

    0.00582
    Seider 4th ed.

  • Flags extrapolation.

    Market range Exact.

    1000lb
    Seider 4th ed.

  • Flags extrapolation.

    Market range Exact.

    920000lb
    Seider 4th ed.

  • Escalation base.

    Market range Exact.

    567
    Seider 4th ed.

  • Escalates the 2013 correlation to 2023.

    Market range Exact.

    797.9
    CEPCI 2023

  • Converts 2023 USD to 2023 EUR.

    Market range Exact.

    1.0813USD/EUR
    ECB

  • Turns the f.o.b. vessel price into installed ISBL direct cost (erection, piping, instruments, electrical, civil, structures, lagging). Engineering, contingency and offsites are not included.

    Market range No independent range cited.

    3.2ratio
    Towler & Sinnott 2022

  • Documents the largest term of the installation factor.

    Market range No independent range cited.

    0.8ratio
    Towler & Sinnott 2022

  • Shows how the vessel bank compares with the whole-plant price: the model's vessel-only figure is already about 70 % of it, which says the conservative choices (S 15 ksi, generic factor 3.2, 70 bar) are high.

    Market range No independent range cited.

    220EUR/kWh
    Energy-Storage.news 2023

  • Reference size of the scale law.

    200MWh
    IEEE Spectrum 2025

  • Numbering-up of identical vessels: the specific CAPEX per kWh does not change with plant size; whole-vessel rounding is shown separately.

    Market range 1.0 for modular numbering-up; 0.6-0.7 only for a single larger vessel, which is outside this design.

    1
    Seider 4th ed. / IEEE Spectrum 2025

  • Prevents a degenerate value outside the domain.

    Market range Not applicable.

    10EUR/kWh
    OpenJack DSL convention

  • Prevents a degenerate value outside the domain.

    Market range Not applicable.

    2000EUR/kWh
    OpenJack DSL convention

  • Reference case 1: Seider/BioSTEAM carbon steel, S = 15 ksi.

    Market range Not applicable.

    1
    OpenJack DSL convention

  • Reference case 2: SA-516 Gr 70 at 20 ksi.

    Market range Not applicable.

    2
    OpenJack DSL convention

  • Charge power of the standard frame, used only to check that charge + discharge fit in the year.

    Market range No independent range cited.

    24MW
    Energy Dome TMCES 2024

  • Discharge power of the standard frame.

    Market range No independent range cited.

    18MW
    Energy Dome TMCES 2024

  • Charge energy = discharge / RTE; with 24/18 MW the charge then lasts 1.0 x the discharge.

    0.75ratio
    Energy Dome 2024

  • Default of the CO2 inventory slider (9-12 t/MWh).

    10t/MWh
    IEEE Spectrum 2025

  • Benchmark for the CO2 inventory per MWh.

    Market range Single plant.

    10t/MWh
    IEEE Spectrum 2025

  • Second benchmark for the CO2 inventory per MWh.

    Market range Not applicable.

    11.53t/MWh
    Energy Dome NETL 2022 (derived)

  • Inventory of the reference plant; the model returns 2,000 t at 200 MWh and 10 t/MWh by construction (not a validation).

    Market range Single plant.

    2000t
    IEEE Spectrum 2025

  • Benchmark for the liquid specific volume.

    Market range Not applicable.

    1.3L/kg
    Energy Dome NETL 2022

  • Basis of the 15.0 m3/MWh liquid-volume benchmark.

    Market range Not applicable.

    66.7kWh/m3
    Energy Dome NETL 2022

  • Benchmark for the liquid volume per MWh.

    Market range Not applicable.

    14.99m3/MWh
    Energy Dome NETL 2022 (derived)

  • Default of the storage pressure slider; a chain contract variable.

    70bar
    Energy Dome NETL 2022

  • Default of the diameter slider.

    3m
    IEEE Spectrum 2025 (indicative)

  • Scope: the liquid CO2 vessels only. The dome, compressor train, expander train and thermal storage are separate calculators. Chaining rule: the CO2 inlet comes from the charge compression train (upstream) and the CO2 outlet feeds the discharge expansion train (downstream) with the same yearly tonnage; link energy_capacity_mwh and co2_inventory_t one to one, never with a partial coefficient.

    Market range Not applicable.

    —
    OpenJack chain contract

  • Fixed O&M (inspection, recertification) is not modelled and exported as zero; add a project-level line if needed.

    Market range No independent range cited.

    —
    Research gap