CALC-069

Marginal abatement cost of SBSP against gas, and why it is undefined against renewables

Feeds CLM-0315. Answers QST-004. Basis: DELIBERATELY MIXED - FY2022 USD (NASA LCOE), 2021 USD (EIA gas LCOE, 2027 vintage), NREL life-cycle medians as printed in SRC-012 Table 25 and EIA's CO2 factor for 2023 utility-scale gas generation (SRC-770). Not reconciled to one basis year. Illustrative order-of-magnitude only.

Why it was needed. CLM-0125 records that SBSP's carbon intensity is already competitive even where its cost is not. That is a favourable finding, and the obvious next question is whether it rescues the economic case when expressed as a cost per tonne avoided. It does not, and the reason is structural rather than numerical: the realistic substitute for SBSP is another low-carbon technology.

Inputs.

Unit conversion applied throughout: 1,000 gCO2eq/kWh = 1 tCO2/MWh.

Working, transcribed.

-- Against terrestrial renewables --
cost premium    = 610 USD/MWh - 20 to 50 USD/MWh
                = approximately 560 to 590 USD/MWh
emissions saved = 26-40 gCO2eq/kWh  against  13-43 gCO2eq/kWh
                = near zero, and of INDETERMINATE SIGN

MAC = (a large positive number) / (approximately zero)
    => not merely high. UNDEFINED.

-- Against natural gas --
cost premium    = 610.07 - 37.05               = 573.02 USD/MWh
emissions saved = 0.486 - 0.026 tCO2/MWh       =   0.460 tCO2/MWh   (life-cycle median)
MAC             = 573.02 / 0.460               = approximately 1,246 USD/tCO2

with EIA's combustion factor instead:
emissions saved = 0.435 - 0.026 tCO2/MWh       =   0.409 tCO2/MWh
MAC             = 573.02 / 0.409               = approximately 1,400 USD/tCO2

with the p.viii label as printed (1,001 for gas), the old figure:
MAC             = 573.02 / 0.975               = approximately 588 USD/tCO2

Result: approximately 1,250 to 1,400 USD per tonne of CO2 against gas, and undefined against renewables. Until the 2026-10-03 repair of CLM-0315 (VER-2026) this CALC gave 588, from the swapped p.viii label; that figure understated the cost.

State what 1,250 to 1,400 is a cost OF, because the number invites the wrong reading. It is the cost of the last tonne avoided by choosing SBSP over a combined-cycle gas plant, at NASA's own RD1 baseline price, with no firming credited to either side. It is not the cost of decarbonising a grid with SBSP, and it is not comparable with a carbon price. The claim's own limits field restricts it to "an order-of-magnitude illustration", which is the correct restriction.

Conclusion, and the undefined half is the stronger half. 1,250 to 1,400 USD/tCO2 is comparable with the most expensive abatement options available anywhere, such as direct air capture. But the renewables comparison is the one that matters, because that is the substitute an actual buyer faces, and there the emissions difference is not merely small - the two ranges overlap (26-40 against 13-43), so the sign of the saving is not established. A favourable carbon finding does not survive being expressed as cost per tonne avoided, because the denominator collapses.

Where this fails. - Three basis years mixed, deliberately and declaredly: FY2022 for NASA's LCOE, 2021 USD for EIA's gas LCOE at a 2027 vintage, NREL life-cycle medians and EIA's combustion factor for the emissions. Nothing here is basis-adjusted and the claim forbids quoting it as a precise figure. - The about 1,400 end sets EIA's COMBUSTION CO2 for gas against SBSP's LIFE-CYCLE CO2eq; the about 1,250 end is life-cycle on both sides. - verified: VER-2256 CONFIRMED 2026-10-09 (3d-04 independent re-derivation, with the EIA input held as SRC-770; ON-436). The working matches CLM-0315 as repaired and CONFIRMED by VER-2026 (2026-10-03). This CALC text had its own check on 2026-10-09 (3d-04, outputs/review/calc069-verify-3d04-2026-10-09.md, PARTIAL); with SRC-770 held, the 0.96 lb/kWh input is now confirmed for 2023 data (outputs/review/eia-faq74-oldnew-2026-10-09.md). - The gas comparison uses a life-cycle median across published studies, or EIA's US average combustion figure, rather than a marginal plant's, and the EIA LCOE is a 2027-vintage projection rather than an observed price.

A SEPARATE FINDING ON THE SOURCE ID, FIXED 2026-10-09. The claim's support field cited SRC-174's Table 4a as "PSRC-02 Table 4a" after the remap; it now reads "SRC-174 Table 4a".

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