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Life Cycle Assessment Harmonization Methodology

NREL conducted the Life Cycle Assessment Harmonization project in two phases. Initially, NREL evaluated estimates of life cycle greenhouse gas (GHG) emissions from electricity generation technologies through a comprehensive review and analysis of published literature. Then a meta-analysis procedure called harmonization was developed to adjust the published GHG emission estimates to a consistent set of methods and assumptions specific to the technology under investigation.

Systematic Literature Review

Starting in May 2009, NREL identified more than 2,100 published references as potentially being life cycle assessments (LCA) of electricity generation technologies. Each reference was then subjected to three rounds of screening by multiple experts to select references that met criteria for quality, relevance and transparency.

Approximately 25% of the total number of reviewed references passed our screening, with only about half of those (or approximately 14% of the total screened) providing estimates of life cycle GHG emissions.

Harmonization and Data Analysis

After completing the systematic review, NREL developed a meta-analytical procedure called "harmonization" to adjust the published GHG emission estimates to a consistent set of methods and assumptions, specific to the technology under investigation, in two main stages:

  1. First, system harmonization ensured studies used a consistent set of included processes (e.g., system boundary, set of evaluated GHGs) and metrics (e.g., global warming potentials).
  2. Then technical harmonization of key performance parameters (e.g., capacity factor) or primary energy resource characteristics (e.g., solar resources; fossil fuel heating value) ensured that the consistent values chosen to reflect a modern reference system (typically a modern facility operating in the United States) were completed.

The following table summarizes harmonization parameters by technology. As indicated, all technologies were harmonized to consistent system boundary conditions and to global warming potentials (GWP) as defined in IPCC (2007). The specific technical parameters harmonized vary by technology, although most technologies were harmonized to a consistent system operating lifetime.

Harmonization Parameters by Electricity Generation Technology

Harmonization Parameter1 Solar-Photovoltaics Concentrating Solar Power Wind Nuclear Technologies2 Coal
System Boundary3
Global Warming Potential (GWP)
Greenhouse Gases        
Harmonization Parameter1 Solar-Photovoltaics Concentrating Solar Power Wind Nuclear Technologies2 Coal
Operating Lifetime  
Capacity Factor      
Thermal Efficiency      
Fuel Heating Value        
Combustion CO2 Emission Factor        
Methane Leakage          
Solar-to-Electric Efficiency      
Solar Resource      
Performance Ratio        
Solar Fraction        
Global Warming Intensities of Materials        

1 All applicable harmonization parameters were considered in each study; those considered significant were selected for harmonization.
2 Other factors qualitatively evaluated for nuclear, including uranium ore grade, primary source energy mix and LCA method.
3 The specific system boundary varied by study.

By reducing the variability caused by inconsistent methods and assumptions, the resulting harmonized estimates show a technology's central tendency for life cycle GHG emissions and show how they can be used by certain analytical applications, policy, and investment decisions.

For more information on the harmonization process and results for each technology, visit:

For technology-specific results of NREL's review, analysis, and harmonization of published LCA estimates, visit:

For questions about this project, contact Garvin Heath via our Webmaster page.