These published results show that life cycle GHG emissions from technologies powered by renewable and nuclear resources are, in general, significantly less than from those powered by fossil fuel resources--typically by a factor of 10 or more. Only the very highest estimates for biopower overlap with the range of a fossil-fueled technology, and the central tendencies of all renewable technologies are between 400 and 1,000 g CO2eq/kWh lower than their fossil-fueled counterparts without carbon capture and sequestration (CCS). For fossil-fueled technologies, post-combustion CCS can bring total life cycle GHG emissions within the upper 25th percentile of estimates for nuclear and several renewable technologies. Biopower with CCS can display negative GHG emissions (without considering the impacts of land use change).
Harmonization of GHG Estimates
NREL adjusted the published GHG emission estimates from these select studies to a consistent set of methods and technology-specific assumptions using a new meta-analytical procedure called “harmonization,” which was developed by NREL researchers. Harmonization ensures that key drivers of variability are more consistent:
- System boundary assumptions and impact assessment method (e.g., global warming potentials of assessed GHG emissions)
- Technological performance factors such as thermal efficiency and capacity factor, primary energy resource characteristics such as solar resource and fuel heating value, and assumed system design characteristics and operating lifetime.
Figure 2 compares published (un-harmonized) results to harmonized life cycle GHG emissions for solar (crystalline silicon and thin film PV, as well as CSP), wind, nuclear, and coal technologies. (Results for natural gas-fired electricity generation technologies is in progress for publication. Check NREL's LCA Harmonization website for results once published.)
Figure 2. Comparison of published and harmonized estimates of life cycle GHG emissions for select electricity generation technologies on which methodological harmonization was performed. Source: Publications found on www.nrel.gov/harmonization
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