NREL developed and applied a systematic approach to review literature on life cycle assessments of concentrating solar power (CSP) systems.
NREL developed and applied a systematic approach to review literature on life cycle assessments of concentrating solar power (CSP) systems, identify primary sources of variability between these assessments, and, where possible, reduce variability in greenhouse gas (GHG) emissions estimates.
Figure 1 summarizes the life cycle stages (based on harmonized data) for utility-scale CSP systems.
The harmonization for CSP technologies was done by adjusting published greenhouse gas estimates based on consistent application of:
- Four CSP system performance parameters: Solar fraction, direct normal irradiance, solar-to-electric efficiency and operating lifetime
- System boundary, for trough systems only by addition of auxiliary consumption of natural gas and electricity processes in the ongoing operational phase
- Global warming potentials (GWP) (based on IPCC 2007)
The table below summarizes the range of published values, harmonized value, and the impact on central tendency and variability of GHG estimates for each harmonization parameter. The central tendency is characterized by the median value. Variability is characterized by the interquartile range (75th minus 25th percentile values) and total range.
Figure 2 compares the published and harmonized life cycle GHG emissions for CSP technologies. Harmonization reduced the variability of published trough technologies by 69% and by 26% for tower technologies. The harmonization parameters that are most effective in reducing variability in the published life cycle GHG emissions estimates are solar fraction for trough CSP and operating lifetime for tower CSP.
The harmonized median life cycle GHG emission estimates for tower and dish technologies are similar but somewhat lower than those for trough technologies: 46, 25, and 13 g CO2eq/kWh, respectively.
Impact of Harmonization on CSP GHG Emissions Estimates1
|Harmonization Parameter||Range of Values||Harmonized Value||Impact on GHG Emissions Estimates|
|Solar Fraction (%)||75-100%||100%||Reduced 10%||Reduced 11%||Reduced 65%|
Irradiance (DNI) (kWh/m2/yr)
|1,914-2,865||2,400||Increased 9%||Increased 10%||Reduced 3%|
|20-40||30||Increased 6%||Reduced 15%||No change|
|Increased 6%||Reduced 12%||Reduced 3%|
|System Boundary Adjustment||—||—||—|
|—Auxiliary Natural Gas
|Removed when included 2||Reduced 2%||No change||No change|
|Removed when included 2||Reduced 12%||No change||No change|
|25 g CO2 eq/g CH4;
298 g CO2 eq/g N2O
|No change||No change||No change|
|ALL PARAMETERS||Reduced 18%||Reduced 53%||Reduced 77%|
*IQR= interquartile range, which represents the spread of the middle 50% of estimates (75th percentile – 25th percentile)
** IPCC Values for 100-yr time horizon (2007)
- Results reported in this table represent the impact of harmonization on trough, tower and dish CSP technologies collectively. Results for each technology are separately reported in the journal article.
- A deeper level of harmonization performed only on the trough LCA literature harmonized to a system boundary which included accounting for auxiliary natural gas and electricity consumption and is a more accurate result. Removal of these two factors was the only option for consistent harmonization of all CSP technologies.
- Solar Fraction is the percentage of electricity produced only from solar energy
- Direct Normal Irradiance is the amount of solar energy per unit area incident upon the collector area of the solar field during one year
- Solar-to-Electric Efficiency is the percentage of solar energy converted to electricity at the CSP facility
NREL’s analysis shows that CSP technologies are similar to other renewables and nuclear energy and much lower than fossil fuel in total life cycle GHG emissions. By adjusting published estimates to consistent gross system boundaries and to consistent values for key input parameters, the harmonization process increased the precision of life cycle GHG emission estimates in the literature. For tower and dish CSP systems, the central tendency shifts down with harmonization. For trough CSP systems, the central tendency shifts up with harmonization. The life cycle GHG emissions of a specific power plant will depend on many factors and could legitimately differ from the generic estimates generated by the harmonization approach, but the harmonized results provide a useful approximation of life cycle GHG emissions for generic CSP facilities that could, for certain purposes, obviate the need to conduct a full life cycle assessment of a new project.