How NREL is improving GHG calculations for power plants

How NREL is improving GHG calculations for power plants

With clean energy and energy security important in the national dialogue, the financial, power generation, power purchase, and policy making communities are seeking more precise information on greenhouse gas (GHG) emissions from various sources of energy. These stakeholders need credible estimates of GHG emissions from power plants (both existing and planned) to develop and comply with environmental and energy requirements, and to help quantify the financial implications of a potential future carbon cost for specific electricity generation technologies.

The National Renewable Energy Laboratory’s (NREL) life-cycle assessment (LCA) harmonization project gives these stakeholders more precise estimates of GHG emissions for renewable and conventional electricity generation technologies. The research also clarifies inconsistent and conflicting estimates in the published literature, reducing uncertainty. Results can inform analysis and decision-making by policymakers, project developers, investors, manufacturers, and utilities or can be used to develop estimates for specific projects.

The results of NREL’s study are detailed in a special supplemental issue, Meta-analysis of Life Cycle Assessment, of the Journal of Industrial Ecology, a peer-reviewed international journal owned by the Yale University School of Forestry and Environmental Studies. Overall, the harmonization process increased the precision of life cycle GHG emission estimates in the literature while having little impact on the median--or central tendency--of these estimates. Results of this research reveal, for instance, that from cradle to grave; solar, wind, and nuclear energy emit similar levels of GHGs per unit of generated electricity, whereas life cycle GHG emissions from coal-fired electricity are about 20 times higher than for those technologies.

LCA Literature Review

NREL considered more than 2,100 published life cycle assessments of electricity generation technologies—renewable, fossil, and nuclear. Systematic review, comprising three rounds of screening by multiple independent experts, narrowed the field to select references that met strict criteria for quality, relevance, and transparency. Only 14% of the original pool of references passed this review process. Figure 1 compares the screened life cycle GHG estimates for hydropower, ocean, geothermal, biopower, solar (photovoltaic [PV] and concentrating solar power [CSP]), wind, nuclear, coal, and natural gas technologies.

Figure 1. Comparison of as-published lifecycle greenhouse gas emission estimates for electricity generation technologies. The impacts of the land use change are excluded from this analysis. Source: Figure 9.8 of Sathaye et al. (2011)

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

Like the published data, the harmonized results show that life cycle GHG emissions from solar (both PV and CSP), wind, and nuclear technologies are considerably lower and more consistent than emissions from technologies powered by combustion-based coal technologies. In addition, harmonization has little impact on the central tendency of the technologies evaluated. Harmonization does, however, reduce the variability of GHG emissions estimates to varying degrees—from about 25 percent to 70 percent for the technologies evaluated. This increased precision helps clarify the impacts of specific electricity generation choices, producing more robust and policy-relevant results.

To learn more about this work, see the special supplemental issue of the Journal of Industrial Ecology on Meta-Analysis of LCA, visit NREL's LCA Harmonization website, or custom visualize and download the numerical results and bibliographies on OpenEI.org.

[SIDEBAR] Background on Life Cycle Assessment of Energy Systems

LCA is a standardized technique that tracks all material, energy, and pollutant flows of a final product—from raw material extraction, manufacturing, transport, and construction to operation and end-of-life disposal. Life cycle assessment can help determine environmental burdens from "cradle to grave" and facilitate consistent comparisons of energy technologies. The generalized life cycle stages for an energy technology are shown in the figure below.

Figure 3. Life cycle stages for energy technologies. (Fuel cycle is only applicable to certain technologies such as fossil and nuclear, and some other stages are only applicable to certain technologies.) Source:  Figure 9.7, Sathaye et al., 2011

 

Hundreds of LCAs published on electricity generation technologies over the last 30 years show that life cycle GHG emissions from renewable electricity generation technologies are, in general, considerably less than from those from fossil fuel-based technologies. In addition, the proportion of GHG emissions from each life cycle stage differs by technology:

  • For fossil-fueled technologies, fuel combustion during operation of the generation facility emits the vast majority of GHGs.
  • For nuclear and renewable energy technologies, the majority of GHG emissions occur during manufacturing and construction.

Image of solar panels and cooling towers by Vaclav Voirab via Shutterstock