The case for estimating carbon return on investment for near-zero emissions energy systems: illustrations from CO2 sequestration and methodological recommendations

Abstract:

Modeling results have indicated that deployment of large-scale CO₂ capture and sequestration (CCS) platforms is likely to comply with a net-zero emissions target. Prior life-cycle assessment (LCA) studies of CCS indicate that there is an urgent need for more transparent accounting of carbon removal efficiency; with better focus on the CO₂ uptake as the primary objective for CCS technologies. We posit that uniform application of a novel ratio metric (of sequestered versus emitted carbon), carbon return on investment (CROI), will improve the usefulness of CCS LCAs in articulating their capacity to achieve true CO₂ sequestration. CROI estimation could be used complementary to the more commonly used energy return on investment (EROI)

In this presentation, we will showcase the utility of CROI estimation using two illustrative examples: covering both biological and geologic sequestration. For the first example, we performed an LCA of aquatic bioenergy systems with CCS. We found that considering idealized sequestration in this case delivered a high-CROI system that also delivers net energy production nominally. However, when the low carbon retention of the solid and liquid co-products is considered, the CROI declines to ~1, indicating emission of as much CO₂ as the intended sequestration. Thus, the estimated CROI value would be different if we assumed idealized “permanent” sequestration versus realistic leakages of the stored carbon.

The second example pertains to the tradeoff between CROI and energy investment in comparing CO₂ capture from fossil sources and ambient air. Our analysis illustrates the extent to which energy and CO₂ performance can be decoupled from one another, especially as the grid becomes increasingly decarbonized over time. Usage of CROI is helpful in demonstrating that energy metrics (e.g. CO₂ separation energy) alone are not sufficient to inform decision-making about CCS.

Our ongoing work aims at formalizing the time value of sequestration to consider if discounting CO₂ fluxes can deliver a refined CROI estimation. In addition to the role of sequestration permanency discussed above, this could improve representation of methane emissions that have time-varying contributions to global warming and ensure compatibility with CO₂ avoidance costs that are generally represented post-discounting.