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Keywords:

  • dish Stirling;
  • life cycle assessment;
  • meta-analysis;
  • parabolic trough;
  • power tower;
  • renewable energy

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

In reviewing life cycle assessment (LCA) literature of utility-scale concentrating solar power (CSP) systems, this analysis focuses on reducing variability and clarifying the central tendency of published estimates of life cycle greenhouse gas (GHG) emissions through a meta-analytical process called harmonization. From 125 references reviewed, 10 produced 36 independent GHG emissions estimates passing screens for quality and relevance: 19 for parabolic trough (trough) technology and 17 for power tower (tower) technology. The interquartile range (IQR) of published estimates for troughs and towers were 83 and 20 grams of carbon dioxide equivalent per kilowatt-hour (g CO2-eq/kWh),1 respectively; median estimates were 26 and 38 g CO2-eq/kWh for trough and tower, respectively.

Two levels of harmonization were applied. Light harmonization reduced variability in published estimates by using consistent values for key parameters pertaining to plant design and performance. The IQR and median were reduced by 87% and 17%, respectively, for troughs. For towers, the IQR and median decreased by 33% and 38%, respectively. Next, five trough LCAs reporting detailed life cycle inventories were identified. The variability and central tendency of their estimates are reduced by 91% and 81%, respectively, after light harmonization. By harmonizing these five estimates to consistent values for global warming intensities of materials and expanding system boundaries to consistently include electricity and auxiliary natural gas combustion, variability is reduced by an additional 32% while central tendency increases by 8%. These harmonized values provide useful starting points for policy makers in evaluating life cycle GHG emissions from CSP projects without the requirement to conduct a full LCA for each new project.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

Among renewable energy technologies, concentrating solar power (CSP) is expected to play an important role in diversifying the global electricity generation portfolio. Studies have projected 55 gigawatts (GW)2 of CSP could be deployed in the United States by 2050 (Blair et al. 2006) and could provide up to 11% of global electricity production in 2050 (IEA 2010). Three major CSP technologies are typically used in practice: parabolic trough (trough), power tower (tower), and parabolic dish (dish).

Life cycle assessment (LCA) is well recognized as a holistic and standard approach for quantifying environmental impacts of renewable energy technologies. LCAs account for the impacts resulting from all activities that transpire over the life of a power plant, including those that are upstream and downstream from the operational phase. After an exhaustive literature search carried out as a part of this analysis, several published LCAs were identified that estimate the life cycle GHG emissions of CSP technologies. As it exists today, significant variability can be found in the CSP LCA literature, caused by a range of factors, including the type of technology being investigated, scope of analysis, assumed performance characteristics, location, data source, and impact assessment methodology used.

Aims of the present meta-analysis include identifying, explaining, and, where possible, reducing variability in published estimates of life cycle GHG emissions for utility-scale CSP systems. Variability is reduced through a meta-analytical process defined here as “harmonization,” by which influential assumptions of the CSP LCA literature are identified, set to standardized values, and the life cycle GHG emissions recalculated. Furthermore, by clarifying the central tendency of estimates of life cycle GHG emissions through improving consistency and reducing variability, decision making and future analyses that rely on such estimates can be better informed. Although a similar analysis has been completed for fuels (Farrell et al. 2006), our work fills a gap in the literature by conducting the first meta-analysis of the CSP LCA literature. In addition to this article on CSP, five other articles produced under the same umbrella project (the LCA Harmonization Project3) led by the U.S. National Renewable Energy Laboratory (NREL) are to be published in this special issue: crystalline silicon photovoltaic (Hsu et al. 2012), thin film photovoltaic (Kim et al. 2012), coal (Whitaker et al., 2012), wind (Dolan and Heath 2012), and nuclear (Warner and Heath 2012). These and the other articles published in this special issue will help to push the boundaries of meta-analysis into LCA, which provides fertile ground for leveraged insights and future development.

The reader should keep in mind that 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 described herein. Appendix A of the supporting information document available on the Journal's Web site suggests a method by which the harmonized life cycle GHG emissions can be adjusted using different assumptions for environmental conditions and performance characteristics. Nevertheless, the most accurate approximation of life cycle GHG emissions associated with a specific CSP plant or design will always be obtained by conducting a full LCA using site-specific data.

Harmonization Method

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

Literature Collection/Screening Approach

An exhaustive search of the English-language literature yielded 125 references pertaining to the environmental impacts of CSP electricity generation. Multiple GHG emissions estimates from a single reference were possible if alternative CSP electricity generation scenarios or technologies were analyzed. Each estimate of life cycle GHG emissions was independently subjected to two rounds of review, consistent with the established screening methodology for the LCA Harmonization Project. Although a reference wasn't necessarily eliminated if only one of its estimates was screened out, most screening criteria applied to the reference as a whole, therefore the results of screening are reported at the level of the reference.

Primary Screening

The primary screen eliminated references based on several gross discriminators:

  • • 
    Conference papers less than or equal to five double-spaced pages
  • • 
    Trade journal articles less than or equal to three published pages
  • • 
    PowerPoint presentations, posters, and abstracts
  • • 
    Publication date prior to 1980
  • • 
    The technology did not produce electricity as a product (and if heat was coproduced, if results could not be determined for electricity alone)
  • • 
    Not a full LCA (less than two phases of the life cycle were evaluated)

The three major life cycle phases are defined below and depicted in figure 1. Transportation steps between each stage, where relevant, are included.

image

Figure 1. Activities included in the life cycle of a concentrating solar power plant, including those required to pass screens, those harmonized, and those unharmonized. The framing box defined by the dotted line shows the systems boundaries assumed in harmonization.

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  • • 
    Upstream processes: extraction of raw materials, materials manufacturing, component manufacturing, site improvements, and power plant assembly.
  • • 
    Operational processes: manufacture of replacement components and their transportation to the site, fuel consumption in cleaning/maintenance vehicles, on-site natural gas combustion, and electricity consumption from the regional power grid.
  • • 
    Downstream processes: plant disassembly and disposal or recycling of plant materials.

(See table S1 in the supporting information for life cycle GHG emissions reported in the literature disaggregated by life cycle stage.)

Primary screening eliminated 80 references from the initial 125 identified in the literature search.

Secondary Screen

The secondary screen further narrowed the pool of references to undergo harmonization by assessing the quality of the studies. Specifically this screening step assessed

  • • 
    the quality of the LCA and GHG emissions accounting methods (for instance, adhering to the International Organization for Standardization [ISO] 14040 series of LCA standards (ISO 2006));
  • • 
    the completeness of reporting regarding the investigated technology, including adequate description of the inputs and methods such that the results could be traced and trusted. Studies were permitted to use either empirical or theoretical data (noted in table 1); and
  • • 
    the modern or future relevance of the technology. Existing and future technologies were included (noted in table 1).
Table 1.  Published values of light harmonization parameters and other important characteristics of studies that passed the first and second screens
Scenario Author Pub. year Tech. Cap. (MW) C.F. (%) S.F. (%) DNI (kWh/m2/yr) Life (years) Eff. (%) Temp. vint. Data type Study loc.
  1. Notes: Pub. year = year of publication for the given reference; Tech. = technology type. 1 = trough, 2 = tower, 3 = parabolic dish; Cap. = capacity; C.F. = capacity factor; S.F. = solar fraction; DNI = direct normal irradiance; Life = lifetime; Eff. = solar-to-electric efficiency; Temp. vint. = temporal vintage (C = existing technology case study, H = existing technology hypothetical study, F = future technology); Data type: E = primarily empirical data, T = primarily theoretical data; Study loc. = primary country or location for the study: EUR = Europe, NDL = NORDEL countries (Denmark, Finland, Sweden, Norway), other country codes are based on United Nations three-letter codes (United Nations 2010); NR = not reported, indicates no value reported for that parameter. MW = megawatt, kWh/m2/yr = kilowatts-hours per square meter per year.

 1 Becerra-Lopez and Golding200715037762,3733014HTUSA
 2 Burkhardt et al.20111103471002,7243016HTUSA
 3 Burkhardt et al.20111103491002,7243015HTUSA
 4 Burkhardt et al.20111103471002,7243016HTUSA
 5 Burkhardt et al.20111103471002,7243016HTUSA
 6 Burkhardt et al.20111103471002,7243016HTUSA
 7 DLR 2006110,000881002,8353012FTDZA
 8 DLR 2006110,000871002,8023012FTEGY
 9 DLR 2006110,000891002,8653012FTLBY
10 Lechon et al.200815044852,0162516HTESP
11 Martin 19971803575NR30NRCTUSA
12 Viebahn et al.200815044822,0003015CTESP
13 Viebahn et al.20081200731002,0003516FTESP
14 Viebahn et al.20081200731002,0003516FTESP
15 Viebahn et al.20081400731002,0004016FTESP
16 Viebahn et al.20081400731002,0004016FTESP
17 Viebahn et al.20081200731002,0003519FTESP
18 Viebahn et al.20081400731002,0004019FTESP
19 Weinrebe et al.199818036752,3003014CTUSA
20 Kreith et al.19902100401002,8483019CTUSA
21 Lechon et al.200821771851,9972517HTESP
22 Lenzen 19992100381002,5002516CTAUS
23 Lenzen 19992100381002,5002516CTAUS
24 Lenzen 1999230261002,5002515FTAUS
25 Lenzen 19992100351002,5002518FTAUS
26 Lenzen 1999230261002,5002515FTAUS
27 Lenzen 19992100351002,5002518FTAUS
28 Lenzen 1999230261002,5002515FTAUS
29 Lenzen 19992100351002,5002518FTAUS
30 Vant-Hull 19922100381001,9143020HTUSA
31 Viebahn et al.200821571822,0003016CTESP
32 Viebahn et al.20082180731002,0003518FTESP
33 Viebahn et al.20082180731002,0004018FTESP
34 Weinrebe et al.1998230301002,3003014CTN/A
35 Weinrebe et al.1998230361002,3003014CTN/A
36 Weinrebe et al.1998230361002,3003014CTN/A
37 Cavallaro and Ciraolo200631NR100NR3018CTITA
38 Cavallaro and Ciraolo200631NR100NR3018CTITA
39 Lenzen 19993100271002,5002530CTAUS
40 Lenzen 19993100271002,5002530CTAUS
41 Ordóñez et al.20093025100NR20NRCTESP
42 Wibberley 200131271002,3623018CTAUS

In addition, to avoid transcription error, only GHG emissions estimates that were reported numerically (and not just graphically) were included for harmonization. At a minimum, each study was required to provide enough detail such that results could be reported in grams of carbon dioxide equivalents per kilowatt-hour of electricity generated (g CO2-eq/kWh) and report the value for at least one of the “light harmonization” parameters, as defined in the following sections. Duplicate estimates from one study quoting another or from the same author group publishing the same estimate multiple times were not recorded.

The secondary screen eliminated 32 references, leaving a total of 13 references that provided 42 GHG emissions estimates (19 trough, 17 tower, and 6 dish). Because the pool of literature for dish CSP only provided six life cycle GHG emissions estimates from four references, the results of harmonization are not considered reliable indicators of variability or central tendency for this technology. In addition, the parabolic dish technology evaluated in five of the six estimates is not representative of the dish-Stirling configuration, which is the dish design closest to commercial deployment. Because of the very small statistical population and the irregular parabolic dish configuration analyzed in the literature, the discussion in this article will focus on the results obtained for the trough and tower technologies. Harmonization results for dish technology are reported numerically in table S2 and graphically in figure S1 in the supplementary information on the Web.

Additional Screening

Unique to this study under the LCA Harmonization Project, an additional screen was applied to the body of LCA literature passing the secondary screen. The purpose of this additional screen was to identify studies with exceptional documentation and, more importantly, inclusion of life cycle inventory (LCI) data so they may undergo an additional thorough and resource-intensive level of harmonization, analogous to what was done by Farrell and colleagues (2006). This final screen identified five studies to undergo this second level of harmonization.

Harmonization Approach

Sources of Variability in Life Cycle Assessment

Before harmonization could be carried out, the main areas of variability needed to be identified. Variability found in CSP LCAs can be attributed to a variety of factors, but it can generally be traced back to three of the four main stages of LCA methodology (as defined in ISO 2006):

  • 1
    Goal and scope of analysis: Specific aspects of the scope that may lead to variability between studies include the system boundaries, location, and depth of analysis (e.g., screening LCA, full LCA), or the assumed operational lifetime of the plant. Selection of the LCA method can also influence results, attributional and consequential being the two main divisions, and within the attributional category, process based and economic input-output based. Because of limitations in the LCA literature for CSP, only process-based LCAs are evaluated in this article.
  • 2
    Life cycle inventory: Details regarding the design, performance, embodied materials, and construction activities of the power plant are sources of variability. LCA practitioners frequently rely upon commercially available LCI databases to estimate the environmental impacts of common materials and processing activities, which can often represent a significant portion, if not all, of their LCA model inputs. Because there are several LCI databases available and several variations of a given process within a single database, many inconsistencies still remain between LCA studies that rely upon these common databases.
  • 3
    Impact assessment: The potential environmental impacts resulting from the power plant's life cycle are calculated using the data from the LCI. In some cases, a weighting factor may be applied based on the impact's perceived relative importance. Because the impact on climate change from the release of GHGs is of interest, the main factor that leads to variability between studies is the global warming potentials (GWPs) selected to convert the mass of major GHGs to CO2 equivalents.
Levels of Harmonization

For the LCA Harmonization Project as a whole, two levels of harmonization were devised. The first level harmonizes at a more gross level the entire set of literature estimates of life cycle GHG emissions passing the secondary screen. It does so by proportional adjustment of the estimate of life cycle GHG emissions to consistent values for several influential performance characteristics and, by addition or subtraction, to a common system boundary (at the level of major life cycle stage). For brevity, we refer to this first level of harmonization as “light” harmonization.

The second, more resource-intensive level of harmonization was reserved for those studies that completed the light harmonization process and passed the additional screen for exceptional documentation of LCI data. The goal of this second level of harmonization is to further reduce variability in the published estimates of life cycle GHG emissions by selecting consistent global warming intensity (GWI—defined in the following section) values for all items reported in the LCI of each of the five studies that passed the additional screening process. Furthermore, in order to identify a more realistic magnitude of life cycle GHG emissions that can be expected from CSP technology, the system boundaries of the five studies are expanded to include auxiliary natural gas combustion and electricity consumption, which are often excluded from CSP LCAs. Hereafter, we refer to this second level of harmonization as “full harmonization.”

Throughout the screening and harmonization process, estimates were not audited for accuracy and no exogenous assumptions were employed.

Harmonization Parameters
Light Harmonization Parameters

Several characteristics pertaining to scope and plant performance are reported in nearly all studies and can be extracted with minimal effort—these characteristics act as light harmonization parameters. Table 1 reports the published values of the parameters used for light harmonization and other important characteristics of each study. Each light harmonization parameter was set to a standard value and used to calculate an adjusted, harmonized life cycle GHG emissions estimate. If the value for a harmonization parameter was not reported, that harmonization step was not applied to that estimate. The description of each parameter and the value selected for light harmonization are listed below with additional details provided in Appendix B of the supporting information on the Web.

  • • 
    Solar fraction: The proportion of electricity produced only from solar energy. A CSP facility with a solar fraction of 1 (or 100%) is defined here as a “solar-only” operating plant. A facility with a solar fraction less than 1 is a “hybrid” operating plant that combusts natural gas (hereafter referred to as natural gas cofiring) to generate a portion of its electrical output. The harmonization value for the solar fraction was chosen to be 100% to better estimate the GHG emissions resulting from a solar-only CSP plant. Note that when harmonizing to a solar fraction of 1, not only are the absolute GHG emissions from these hybrid operating plants reduced by removing the emissions associated with natural gas cofiring, but the annual energy output also decreases by eliminating the portion of electricity produced by the natural gas.
  • • 
    Direct normal irradiance (DNI): The amount of solar energy per unit area incident upon the collector area of the solar field during 1 year. The harmonization value for DNI was chosen to be 2,400 kilowatt-hours per square meter per year (kWh/m2/yr). This value was not chosen to be reflective of any one location, but rather is representative of a high-quality solar resource that is incident upon thousands of square kilometers in several global locations (Trieb et al. 2009). Also, CSP developers typically require about 2,000 kWh/m2/yr to justify construction (IEA 2010)
  • • 
    Lifetime: The assumed life span of the power plant used for the LCA analysis. The harmonization value for lifetime was chosen to be 30 years. This 30-year lifetime duration is frequently assumed in CSP LCAs and in economic analyses of CSP plants (e.g., Turchi 2010).
  • • 
    Solar-to-electric efficiency: The percentage of solar energy converted to electricity at the CSP facility. The harmonization values for solar-to-electric efficiency are chosen to be 15% and 20% for trough and tower technologies, respectively. These solar-to-electric efficiencies are representative of current state-of-the-art designs for each technology (IEA 2010).
  • • 
    Global warming potentials (GWPs): A metric used to measure the radiative forcing of a given GHG over a 100-year time period relative to that of CO2 (GWPCO2= 1). The GWPs of two major GHGs, methane (CH4) and nitrous oxide (N2O), were harmonized by updating the GWP values to those reported in the latest Intergovernmental Panel on Climate Change (IPCC) assessment report (IPCC 2007).
  • • 
    Removal of auxiliary natural gas combustion and electricity consumption: In addition to its use in cofiring activities, natural gas is used during miscellaneous operation and maintenance (O&M) activities, such as heat transfer fluid (HTF) freeze protection activities and system start-up procedures. Electricity is drawn from the regional grid to satisfy the plant's parasitic load when the plant is not generating its own power (e.g., pumping HTF to the auxiliary boiler to prevent freezing at night and supplying heating and cooling to on-site buildings). A thorough engineering analysis must be conducted to obtain a reasonable estimate of the amount of auxiliary natural gas combusted and electricity consumed for a given CSP plant design. Although we cannot accurately calculate the GHG emissions contributions from these two parameters for each of the 36 estimates, we can, however, accurately remove their contribution from those few studies (Burkhardt et al. 2011; Lechon et al. 2008) that include their impacts so all studies share a common system boundary.

It is important to emphasize that to exclude the GHG contributions from auxiliary natural gas combustion and electricity consumption is to underestimate the real magnitude of life cycle GHG emissions to be expected from a CSP. To illustrate the potential change in magnitude in life cycle GHG emissions when auxiliary natural gas combustion and electricity consumption are accounted for, their contributions to life cycle GHG emissions are estimated for the five studies used in the full harmonization step (additional details will be provided in the following section). Furthermore, Appendix A of the supporting information on the Web contains details on how to estimate the GHG contribution from natural gas combustion and electricity consumption for a specific plant design, using the light harmonized results as a starting point.

Equation 1 displays how most of the above-listed parameters are used to estimate life cycle GHG emissions:

  • image(1)

where the numerator of equation (1) is the sum of the masses of the three major GHGs (CO2, CH4, N2O) emitted during the plant's life cycle, converted to CO2 equivalents using up-to-date GWPs. The denominator of equation (1) (E) is the life cycle electricity output from the plant and is a function of solar-to-electric efficiency (η), DNI, and lifetime (t).

In general, the published life cycle GHG emissions values are harmonized through two main operations:

  • 1
    multiplying the denominator of equation (1) by the ratio of the harmonized parameter value and the published parameter value, and
  • 2
    adding or subtracting GHG emissions from the numerator or electricity output from the denominator.

A more detailed discussion of the methodology used to calculate the harmonized life cycle GHG emissions can be found in Appendix B of the supporting information on the Web.

Full Harmonization

The embodied emissions of plant materials and other construction activities reported in the LCI are important variables that strongly influence the total life cycle GHG emissions of a CSP plant. Because there may be dozens of unique materials and processing activities included in the LCI of a study, each with its own GWI, even small inconsistencies in the GWI may lead to significant variability in the final life cycle GHG emissions value. To address this source of variability, we selected consistent values for the GWI of each material and construction activity in the five studies’ LCIs.

GWIs are defined as the mass of GHGs emitted from the production of common materials and from other activities (e.g., transportation) per functional unit (e.g., mass of material, unit distance transported). With the exception of the nitrate salt storage medium and the synthetic oil HTF, the GWI of all LCI entries were estimated using unit processes from the Ecoinvent v2.0 LCI database (Swiss Center for Life Cycle Inventories 2010). The Ecoinvent LCI database is commonly used in LCAs, containing up-to-date information that is thoroughly documented and peer reviewed. As for the nitrate salts and HTF, to improve the accuracy of their GWIs compared to the use of proxy materials in Ecoinvent, their GWIs were estimated using values reported by Burkhardt and colleagues (2011), which were obtained directly from manufacturers.

Because it is the only electrical infrastructure common to all entries in the Ecoinvent database, all materials that undergo harmonization are assumed to use the average U.S. electricity generation profile. Using engineering judgment, each material in the LCIs of the five studies was paired with an Ecoinvent process. The LCI of each study can be found in tables S3–S7 in the supporting information on the Web. The standardized GWI selected for each material can be found in table S8 in the supporting information.

To carry out the GWI harmonization step, the total mass of life cycle GHG emissions (i.e., g CO2-eq) is estimated as the product of the mass of each material, or the energy consumed during an activity (e.g., diesel fuel combusted by a building machine) reported in the LCIs of each of the five selected studies (tables S3–S7), and standardized GWIs (table S8), summed for all materials or activities. The total mass of life cycle GHG emissions is then divided by the respective life cycle electricity production for each of the five plants to obtain normalized life cycle GHG emissions (i.e., g CO2-eq/kWh). Using this approach, not only are GWIs of common materials made consistent, but they are also updated to the latest available information and thus reflective of a plant being constructed today.

Because full harmonization is labor intensive, it was decided to focus resources on just one CSP technology. The trough technology was selected because it had the most life cycle GHG emissions estimates that qualified for full harmonization. In addition, troughs account for the largest share of the current CSP market (IEA 2010). Table 2 lists the five trough LCAs selected for full harmonization and the LCI database used in the published work.

Table 2.  List of references used in full harmonization and their life cycle inventory (LCI) database
Scenario Author Pub. year LCI database
  1. Notes: Scenario corresponds to the scenario number shown in table 1.

  2. Pub. year = publication year.

 2 Burkhardt et al.2011Ecoinvent v2.01
10 Lechon et al.2008Ecoinvent v1.21
11 Martin 1997TEMIS2
12 Viebahn et al.2008Ecoinvent v1.31
19 Weinrebe et al.1998ETH Zurich3

As mentioned in the previous section, the GHG contributions from electricity and natural gas consumption are estimated in the full harmonization step. Auxiliary natural gas use for a modern trough plant is estimated to be 91,000 megajoules per megawatt per year (MJ/MW/yr) in an article by Burkhardt and colleagues (2011). Because it is the only estimate for auxiliary natural gas combustion, 91,000 MJ/MW/yr is used to harmonize the remaining four studies. Two values of electricity consumption in trough plants are reported in the literature: Burkhardt and colleagues (2011) reports 36 megawatt-hours per megawatt per year (MWh/MW/yr), while Lechón and colleagues (2008) report an electricity consumption rate of 327 MWh/MW/yr. The lead author of the latter study reports that rules of the Spanish feed-in tariff at the time of their study incentivized CSP plants to purchase all required power from the grid even when it could be self-produced, because power generated garnered the (higher) feed-in tariff price and was not required to be net of parasitic loads (Lechon 2011). The rules have since been changed to eliminate this arbitrage opportunity, and it is unlikely that any new CSP plant would operate under similar rules again. Therefore the estimate reported in the Burkhardt and colleagues (2011) article is used to harmonize the remaining four studies, as it appears more realistic of grid electricity consumption for trough CSP.

Statistical Assessment

Statistical assessments of the variability and central tendency of the published and harmonized datasets are used to characterize estimates of life cycle GHG emissions that passed the screens for each level of harmonization. For light harmonization, central tendency is reported using the median value of the datasets. Variability is described using the interquartile range (IQR) (75th percentile value minus the 25th percentile value). When analyzing larger datasets, IQR is a more robust measure of variability than the range because IQR is not influenced by extreme estimates. Because the dataset used in full harmonization is quite small (five data points), variability is better described by the range (maximum minus minimum), so that the contribution from each of the five estimates is captured. For each harmonization step, changes in central tendency and variability are compared with published estimates to describe the impact of the harmonization step. Decreases in measures of variability indicate effective harmonization in terms of a tightened IQR or range of life cycle GHG emissions from the evaluated technology.

Results and Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

Parabolic Trough

Summary of Published Results

Of the 13 references that passed the two-tiered literature screening, 7 provided 19 estimates of life cycle GHG emissions for various trough plant designs in five different countries (see table 1). Nine estimates were representative of futuristic scenarios, three were case studies, and the remaining seven evaluated hypothetical trough plants in the “present day.” The year of publication ranged from 1997 to 2011. The initial range of life cycle GHG emissions was 230 g CO2-eq/kWh. The IQR was 83 g CO2-eq/kWh with a median of 26 g CO2-eq/kWh. The initial variability in trough life cycle GHG emissions estimates was strongly influenced by those studies that evaluated hybrid operating plants.

Harmonized Results

Figures 2(a)–(h) display the impacts of light harmonization for trough technology starting with the published estimates in figure 2(a), reporting each light harmonization step applied independently (figures 2b–h), and concluding with cumulative harmonization by all parameters in figure 2(i). The original rank order for the published results is maintained throughout each frame. A summary of the results obtained from each harmonization step is discussed in the following sections. Table 3 summarizes changes in the measures of central tendency and variability from the application of each light harmonization step independently and then cumulatively for all. Numerical results for each evaluated life cycle GHG emissions estimate displayed in figure 2, and including the other two CSP technologies, are detailed in table S2 of the supporting information on the Web.

image

Figure 2. Rank order estimates of life cycle greenhouse gas emissions for trough concentrating solar power electricity generation technology. Panel (a) reports only published estimates. Panels (b)–(f) show the impacts of the independent application of light harmonization steps (open circles) relative to the published data (black filled circles): (b) harmonizes by solar fraction, (c) harmonizes by global warming potentials, (d) harmonizes by direct normal irradiance, (e) harmonizes by plant lifetime, (f) harmonizes by solar-to-electric efficiency, (g) harmonizes by removing auxiliary natural gas consumption, (h) harmonizes by removing auxiliary electricity consumption, and (i) harmonizes by all parameters.

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Table 3.  Changes to central tendency and variability from independent application of light harmonization steps and cumulatively by all light harmonization parameters
Technology [g CO2-eq/kWh]a Pub. S.F. GWP DNI Life Eff. Aux. gas Aux. elec. All light parameters
  1. Notes: aValues have been rounded to maintain a maximum of two significant digits.

  2. Pub = published value; S.F. = solar fraction; GWP = global warming potential; DNI = direct normal irradiance; Life = lifetime; Eff. = solar-to-electric efficiency; Aux. gas = auxiliary natural gas combustion removal; Aux. elec. = auxiliary electricity consumption removal; SD = standard deviation; IQR = interquartile range. “—” denotes that the harmonization parameter does not apply to that technology owing to a lack of reported parameter values. Counts of estimates and references refer to the number for which that harmonization step was applied.

Parabolic troughMean672767646867676323
 SD801980757877807810
 Minimum121212101411121213
 25th percentile171617182018171715
 Median262226272828242122
 75th percentile1002810091100100998426
 Maximum2408624023024023024024055
 IQR831283748081826811
 Range2307423022023022023023042
 Change in mean0%−60%0%−4%1%−1%−1%−6%−65%
 Change in median0%−15%0%5%8%7%−6%−18%−17%
 Change in SD0%−77%0%−6%−2%−3%0%−3%−87%
 Change in IQR0%−85%0%−12%−3%−2%−1%−19%−87%
 Change in range0%−68%0%−2%−1%−4%0%0%−82%
 Count of estimates1951187185619
 Count of references751626127
          
Power towerMean48344644404522
 SD49184247454010
 Minimum111191110119
 25th percentile23232223162316
 Median38353931293823
 75th percentile43394638364329
 Maximum2008117019019015042
 IQR20162415192013
 Range1907017018018014034
 Change in mean0%−29%−5%−7%−16%−6%−53%
 Change in median0%−8%4%−17%−22%0%−38%
 Change in SD0%−65%−16%−4%−9%−19%−81%
 Change in IQR0%−18%18%−25%−3%0%−33%
 Change in range0%−63%−14%−6%−7%−26%−82%
 Count of estimates172171117-117
 Count of references6263616
          
Parabolic dishMean2727212620
 SD3131193220
 Minimum77755
 25th percentile1414141010
 Median1515141413
 75th percentile2021172420
 Maximum8989608960
 IQR6731411
 Range8282528454
 Change in mean0%1%−22%−2%−24%
 Change in median0%2%−9%−3%−12%
 Change in SD0%0%−38%2%−35%
 Change in IQR0%15%−51%121%73%
 Change in range0%0%−36%3%−34%
 Count of estimates63356
 Count of references4  2234
All three technologiesMean543054525250535023
 SD642064596261646011
 Minimum777775775
 25th percentile161516171615161614
 Median262426282828262321
 75th percentile423842453838424227
 Maximum2408924023024023024024060
 IQR262326282223262612
 Range2308223023023023023023054
 Change in mean0%−44%0%−4%−4%−6%0%−6%−58%
 Change in median0%−10%0%9%6%6%−2%−12%−18%
 Change in SD0%−69%0%−8%−2%−4%0%−7%−82%
 Change in IQR0%−11%0%10%−15%−12%0%0%−53%
 Change in range0%−65%0%−3%0%−3%0%0%−77%
 Count of estimates42713821405742
 Count of references1351103111213
Light Harmonization
Solar Fraction

Five of the 19 published trough GHG emissions estimates passing the literature screens represented hybrid operating plants. Solar fraction values for the five estimates ranged from 0.75 to 0.85. Harmonizing the solar fraction to a value of 1 significantly reduces the IQR of the trough life cycle GHG emissions estimates (−85%). The median value is also reduced to 22 g CO2-eq/kWh (−15%).

Global Warming Potentials

This harmonization step was intended to update the GWPs of GHGs to the values reported by the IPCC (2007) for those studies that separately reported mass emissions of CH4 and N2O. Twenty-four of the 42 GHG emissions estimates for all CSP technologies were published before the IPCC (2007) publication; however, only one study—a trough estimate (Martin 1997)—provided enough details to update the GWPs of CH4 and N2O. The overall change in variability and central tendency for the entire pool of trough estimates is negligible and resulted in only a 1% increase in life cycle GHG emissions for Martin (1997).

Direct Normal Irradiance

Eighteen of 19 trough estimates could be harmonized by DNI. The DNI values reported for the 18 estimates ranged from 2,000 to 2,865 kWh/m2/yr. Harmonizing DNI to a common value reduces the variability in trough GHG emissions estimates by 12% (IQR = 74 g CO2-eq/kWh). The central tendency of the trough life cycle GHG emissions estimates remains relatively constant after harmonization by DNI (the median increases by 5%).

Lifetime

A total of seven trough GHG emissions estimates are modified by the lifetime harmonization step. One estimate used a value of less than 30 years and six others assumed a value greater than 30 years under futuristic scenarios. Because the majority of the affected estimates have a shorter lifetime after harmonization, the median value of life cycle GHG emissions increases by 8%. As for variability, the IQR is reduced by 3% relative to published values.

Solar-to-Electric Efficiency

Using the efficiency value of 15% obtained from the IEA (2010) publication, the harmonization of solar-to-electric efficiency reduces the IQR of trough estimates by 2% and increases the median by 7%. Only 6 of 19 estimates assumed an efficiency less than the harmonized value; therefore the overall effect of this harmonization step is to reduce solar-to-electric efficiency for most of the estimates considered in harmonization, thus increasing normalized life cycle GHG emissions.

Auxiliary Natural Gas Combustion Removal

The study of Burkhardt and colleagues (2011) is the only one to consider auxiliary natural gas consumption. To maintain consistency with the remaining studies, auxiliary natural gas combustion is removed from the five estimates provided by Burkhardt and colleagues (2011). The removal of auxiliary natural gas combustion has only a small impact on variability (IQR decreases by 1%), while central tendency decreases by 6%. Recall, however, that the most accurate estimate of life cycle GHG emissions will include the contributions from auxiliary natural gas combustion. One can approximate the potential impacts on life cycle GHG emissions of accounting for auxiliary natural gas combustion using the approach outlined in Appendix A of the supporting information on the Web.

Auxiliary Electricity Consumption Removal

Only two studies consider GHG emissions from auxiliary electricity consumption (Burkhardt et al. 2011; Lechon et al. 2011). To maintain consistency with the bulk of the LCA literature, this factor was removed from those two studies’ estimates. Once removed, the IQR of trough life cycle GHG emissions estimates is reduced by 19% and the median value decreases by 18%. Like auxiliary natural gas consumption, the GHG emissions from electricity consumption are an integral part of the CSP life cycle and should be considered to ensure an accurate assessment. Using the method described in Appendix A of the supporting information on the Web, one can estimate the impact of auxiliary electricity consumption on the total life cycle GHG emissions of a CSP plant.

All Light Harmonization Parameters

The final harmonization step required the sequential application of the independent harmonization steps previously described. Harmonizing the published estimates sequentially by all harmonization parameters decreases the trough IQR by 87% (to 11 g CO2-eq/kWh) and decreases the median by 17% (to 22 g CO2-eq/kWh). The moderate decrease in the median value of life cycle GHG emissions is due to the removal of natural gas cofiring and electricity consumption.

Full Harmonization

Two of the five references (table 2) chosen for full harmonization (i.e., Burkhardt et al. 2011; Viebahn et al. 2008) evaluated more than one plant design in their analysis. Each scenario evaluated in these two studies relied upon their original LCI data, with only minor alterations made to the system boundaries and other scoping assumptions. If properly harmonized, each alternate scenario would return to the original value calculated for the base case plant design, thus resulting in a clustering effect that would dilute the contributions from the other three studies if all were considered collectively. For this reason, only the base case scenarios from Burkhardt and colleagues (2011) and Viebahn and colleagues (2008) (i.e., scenarios 2 and 12 in table 1, respectively) were considered in full harmonization along with the single scenarios evaluated in Lechon and colleagues (2008), Martin (1997), and Weinrebe and colleagues (1998). Table 4 displays the summary statistics for the five trough life cycle GHG emissions estimates that underwent full harmonization.

Table 4.  Changes to central tendency and variability from application of full harmonization to the five trough life cycle assessments passing the additional screening criteria
[g CO2-eq/kWh] Published Light harmonization Full harmonization
  1. Notes: SD = standard deviation; IQR = interquartile range; g CO2-eq/kWh = the units of all quantities reported in the table (grams of carbon dioxide equivalents per kilowatt-hour). The bold type distinguishes data from the five studies that underwent full harmonization from the statistical metrics reported below them.

Burkhardt et al.  26 21 34
Martin 166 13 23
Lechon et al. 185 22 20
Weinrebe et al. 241 21 25
Viebahn et al. 161 33 23
Mean1562225
SD 79 7 5
Minimum 261320
25th percentile1612123
Median1662123
75th percentile1852225
Maximum2413334
IQR 24 1 3
Range2152013
Change relative to published values for the five full harmonization studies:
Change in mean0%−86%−84%
Change in median0%−87%−86%
Change in SD0%−91%−93%
Change in IQR0%−96%−89%
Change in range0%−91%−94%
Count of estimates555
Count of references555

Each of the five estimates relied upon a different LCI database to estimate the embodied GHG emissions of the materials and processing activities used during the life cycle of their trough power plants (see table 2). The goals of this harmonization step are to ensure that consistent GWI values are used for each material or construction activity in all five studies and to update the older studies (i.e., Martin 1997; Weinrebe et al. 1998) that rely on outdated information regarding the impacts of materials. In addition, full harmonization is expected to provide a more realistic estimate of the magnitude of life cycle GHG emissions due to the inclusion of auxiliary natural gas combustion and electricity consumption, which were excluded in the light harmonization steps.

The points of comparison for this full harmonization step are the range and median of only the five trough life cycle GHG emission estimates before and after light harmonization. Before light harmonization, the range and median of life cycle GHG emissions of the five studies was 215 g CO2-eq/kWh and 166 g CO2-eq/kWh, respectively. After light harmonization the range and median of the five estimates were reduced by 91% (to 20 g CO2-eq/kWh) and 87% (to 21 g CO2-eq/kWh), respectively.

Recall, the GHG emissions from auxiliary natural gas combustion and electricity consumption are estimated using the values reported in the article by Burkhardt and colleagues (2011). After GHG emissions from natural gas combustion were added to the four estimates that did not consider this factor, the range decreased by 34% and the median increased by 8% relative to the light harmonized statistics for the five estimates. As for electricity consumption, the range is reduced by 52% and the median increased significantly (by 36%) after including these two factors in all studies.

The last step in the full harmonization process is to harmonize the GWI values for all embodied materials and construction/operational activities using the appropriate Ecoinvent processes (see tables S3–S8). After its application, the life cycle GHG emissions of Burkhardt and colleagues (2011) increase slightly (11%), while the remaining four estimates saw large reductions (all greater than 40%). Overall, GWI harmonization (considered independently) reduces the range by 18% and the median by 43% relative to the five light harmonized values.

When all full harmonization steps are applied sequentially (i.e., addition of auxiliary natural gas combustion and electricity consumption, and GWI harmonization), the range of the five light harmonized values decreases by 32% and the median increases by 8%. Compared to the published values of the five estimates, full harmonization reduces variability by 94% (from 215 to 13 g CO2-eq/kWh) and the central tendency by 86% (from 166 to 23 g CO2-eq/kWh). Figure 3 displays the distribution of published and harmonized life cycle GHG emission estimates for the trough and tower technologies.

image

Figure 3. Published and harmonized box plots for trough and tower concentrating solar power electricity generation technologies (“tech.”). The middle panel (separated by the dashed line) shows the published values of the five estimates that underwent full harmonization and corresponding changes in variability and central tendency after full harmonization. Note: # of Est.: number of estimates; # of Ref.: number of references.

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Power Tower
Summary of Published Results

Six references passing the two-tiered literature screen provided 17 estimates of life cycle GHG emissions for tower plants in three different countries (see table 1). Futuristic scenarios dominate the dataset with eight estimates, case studies represent seven additional estimates, and the remaining two estimates represent hypothetical, present-day tower plant designs. Published estimates of life cycle GHG emissions for tower CSP exhibited a tighter range and IQR (190 and 20 g CO2-eq/kWh, respectively) relative to the trough estimates, primarily because fewer estimates pertained to hybrid operating plants. The median value for the tower technology (38 g CO2-eq/kWh) was moderately higher than for the trough technology.

Harmonized Results

Plots for published estimates, and their response to each harmonization step, are shown in figure S2 in the supporting information on the Web for the tower technology (and figure S1 for dish technology). Refer to table 3 for changes in the measures of central tendency and variability from the application of each light harmonization step independently and then cumulatively. A summary of the results obtained from each harmonization step is discussed in the following sections.

Light Harmonization
Solar Fraction

Two of the 17 published tower GHG emissions estimates passing the literature screens represented hybrid operating plants. The two solar fraction values were 0.82 (Lechon et al. 2008) and 0.85 (Viebahn et al. 2008). Harmonizing the solar fraction to a value of 1 reduces the IQR and median by 18% and 8%, respectively.

Direct Normal Irradiance

All 17 life cycle GHG emissions estimates are harmonized by DNI. The DNI values ranged from 1,914 to 2,848 kWh/m2/yr. Harmonizing DNI to the common value of 2,400 kWh/m2/yr increases the variability in tower GHG emissions estimates by 18% (IQR = 24 g CO2-eq/kWh). Eleven of the 17 tower GHG emissions estimates have a published DNI value within 6% of the harmonized value, while five estimates are at least 22% lower. The effect of maintaining the majority of the tower GHG emissions estimates near their published DNI values while notably increasing a few others increases the overall variability. The central tendency of the tower life cycle GHG emissions estimates remains relatively constant after harmonization by DNI (the median increases by 4%).

Lifetime

Eleven tower GHG emissions estimates are modified by this harmonization step. Nine estimates use a value of less than 30 years and two others assume a value greater than 30 years under futuristic scenarios. After harmonization by lifetime, IQR is reduced by 25% and the median decreases by 17%.

Solar-to-Electric Efficiency

Using the efficiency value of 20% obtained from the IEA (2010), independent harmonization of solar-to-electric efficiency reduces the IQR of tower estimates by 3% while the median decreases by 22%. For the tower GHG emissions estimates, all published efficiencies were less than the harmonized value, so the effect of this harmonization step is to increase the solar-to-electric efficiency and decrease the normalized life cycle GHG emissions for all estimates (opposite of that seen in trough estimates).

Auxiliary Natural Gas Combustion and Electricity Consumption Removal

The tower estimates are unaffected by the removal of auxiliary natural gas combustion, as no studies consider its contribution to life cycle GHG emissions. As for auxiliary electricity consumption, one estimate (from Lechon and colleagues 2008) includes the GHG contributions from electricity use. Despite its significant contribution (48 g CO2-eq/kWh) to total life cycle GHG emissions in the article by Lechon and colleagues (2008), the removal of electricity consumption has a negligible impact on both central tendency and variability. As noted before, it is important to remember that a complete accounting of life cycle GHG emissions should consider any auxiliary natural gas and electricity consumption and that by using a common system boundary that excludes these factors, the results of light harmonization underestimate true GHG emissions from tower CSP.

All Light Harmonization Parameters

The final harmonization step required sequential application of the independent harmonization steps previously described. Harmonizing the published data cumulatively by all harmonization parameters decreases tower IQR by 33% (to 13 g CO2-eq/kWh) and median by 38% (to 23 g CO2-eq/kWh). An interesting result of this analysis is that, after light harmonization, life cycle GHG emissions from trough and tower technologies are very similar: the median value of trough life cycle GHG emissions was reduced from 26 to 22 g CO2-eq/kWh and the median value of tower life cycle GHG emissions was reduced from 38 to 23 g CO2-eq/kWh.

Limitations of the Analysis

Other Environmental Impacts

A general limitation of this analysis pertains to its scope: only GHG emissions are evaluated. In reality, GHG emissions are only one of many factors that must be assessed when determining the total environmental impacts of an electricity generation system. Other important impacts include water consumption, habitat destruction, and health effects resulting from air and water pollutant emissions.

Unharmonized Parameters

Another limitation is that not all parameters selected for light harmonization could be applied in all studies. Martin (1997) evaluates a plant design based on the 80 MW Luz solar energy generating systems (SEGS) VIII facility where the DNI and solar-to-electric efficiency were not reported and therefore could not be harmonized. Furthermore, 24 of the 42 GHG emissions estimates were published before the latest IPCC assessment report (2007), but only one estimate could be harmonized using the IPCC 2007 GWPs. Some of these unharmonized estimates report life cycle GHG emissions in terms of CO2 equivalent, which implies GHGs other than CO2 were considered. In other studies it is unclear whether or not CO2 was the only GHG evaluated.

The overall impact of these unharmonized parameters, however, is not expected to significantly impact the results of harmonization or change the conclusions of this analysis. For the first example, the impact of harmonization on the estimate of Martin (1997) would likely follow that from Weinrebe and colleagues (1998), who also evaluates an 80 MW SEGS plant and reports both DNI and solar-to-electric efficiency. After harmonization the published life cycle GHG emissions value of Weinrebe and colleagues (1998) is reduced by only 3% and 4% for the DNI and solar-to-electric harmonization steps, respectively, suggesting a similarly small change in results of Martin (1997).

Regarding the unharmonized GWPs, the contributions from CH4 and N2O to total life cycle GHG emissions from CSP plants are typically small relative to life cycle CO2 emissions (Burkhardt et al. 2011; Viebahn et al. 2008). In addition, the change in GWP values for CH4 and N2O are relatively small in magnitude across the IPCC reports. Based on these two factors, it is reasonable to assume the potential impact of leaving other GWPs unharmonized will be small.

Adjusting Life Cycle Greenhouse Gas Emission Estimates for Different Parameter Estimates

As demonstrated in this analysis, the overall environmental performance of a CSP plant is influenced by several key parameters. Because the values chosen for the harmonization parameters do not represent any one location or specific plant design, it is useful to understand how life cycle GHG emissions are affected by changing the environmental- and performance-related conditions. Appendix A of the supporting information on the Web outlines a relatively simple approach that policy makers and LCA practitioners can use to develop a first-order estimate of the life cycle GHG emissions of a CSP plant under a variety of conditions using the harmonized life cycle GHG emissions as a starting point. The reader should keep in mind the most accurate approximation of the life cycle GHG emissions associated with a specific CSP plant design will always be obtained by conducting a full LCA using site-specific data.

Light Harmonization Limitations
General Underestimation of Light Harmonization Results

Auxiliary natural gas combustion and electricity consumption are important and necessary parameters to consider when determining the life cycle GHG emissions of the CSP plant, but were excluded from the light harmonization results due to limitations in data availability. To illustrate the potential increase in GHG emissions that can result from their inclusion, the GHG contributions from electricity and auxiliary natural gas consumption are estimated for the five estimates used in full harmonization. The method by which the contributions were added is a rough approximation and therefore the results should be used with caution.

Adding the GHG contributions from auxiliary natural gas combustion during full harmonization increased the median value of the five estimates by 8% (21 to 23 g CO2-eq/kWh) compared to their light harmonization values. The GHG contributions from auxiliary electricity consumption had a much more noticeable effect on life cycle GHG emissions: the median value increased by 36% (29 g CO2-eq/kWh) compared to light harmonization results. It is reasonable to assume that a similar increase in GHG emissions would result if these two terms were included during light harmonization. Assuming the GHG emissions from light harmonization increase by the same magnitude (i.e., an 8% increase from auxiliary natural gas and a 36% increase from auxiliary electricity), the median value of tower GHG emissions for light harmonization would increase by 9 g CO2-eq/kWh (to 31 g CO2-eq/kWh). To more accurately estimate the GHG contributions from auxiliary natural gas and electricity consumption, it is recommended to use the method described in Appendix A of the supporting information on the Web.

Direct Normal Irradiance

The solar multiple of a CSP plant is the ratio between the amount of instantaneous thermal energy that can be provided by the solar field and the design-point thermal energy required to operate the power block at rated power. In most cases the solar multiple will be selected to ensure that the solar field is oversized to smooth out fluctuations in the solar resource. As a result, the annual output from CSP plants is designed to remain relatively constant despite small fluctuations in DNI because of the enlarged solar field. Consequently our approach to harmonizing by DNI (see Appendix A of the supporting information on the Web) should be considered a first approximation of the impact on life cycle GHG emissions because the effect on power output has not been modeled in detail for each study's plant design. Harmonizing small deviations in DNI will introduce less uncertainty than larger deviations, but a more specific estimate of the direction or magnitude of under- or overestimate in the estimate of true life cycle GHG emissions from harmonization is beyond the scope of this study.

Statistical Population

A pool of previously published papers is not an independent “population” in the statistical sense, and potential biases within the pool are possible. Of the original 125 references identified, 42 estimates were obtained, but from only 13 separate first authors. In particular, only seven lead authors contributed to the 19 trough life cycle GHG emissions estimates, 11 of which are coauthored by at least one of the other seven authors. As a result, estimates in the pool may tend to cluster as inherent author assumptions and biases are carried through serial publications and as multiple GHG emissions estimates from the same reference may share common author assumptions. Clustering may also occur as independent authors cite the same data sources. As the population of GHG emissions estimates does not constitute a true independent sample, the statistical measures reported in this study should be interpreted with caution and should be viewed only as indicative of the central tendency and variability for the given technology.

Recommendations for Future Work
Completion of Additional Dish Life Cycle Assessments

Among the 125 original references identified in our exhaustive literature search, only four references provided six life cycle GHG emission estimates for dish/engine systems. With a pool of only six data points, robust conclusions cannot be drawn from the statistical assessments applied during the harmonization process for this CSP technology. In addition, only one of the six estimates explicitly states that the system under evaluation is the dish-Stirling engine configuration, which is the technology closest to commercial deployment. Therefore future work that attempts to harmonize the life cycle GHG emissions of dish systems would greatly benefit from the completion of additional high-quality LCAs that evaluate dish/Stirling systems.

Full Harmonization of Life Cycle Greenhouse Gas Emissions from Tower Facilities

While fewer life cycle GHG emissions estimates are available for the tower technology than the trough technology, 4 of the 17 tower GHG emissions estimates were identified as potential full harmonization candidates in the preliminary stages of the CSP harmonization project. Future work may consider applying the full harmonization methodology to the pool of tower GHG emissions estimates, including addition of auxiliary natural gas and electricity consumption estimates.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

Of 125 references identified in an exhaustive literature search, 13 passed two levels of quality screening to provide a total of 42 life cycle GHG emissions estimates for three CSP technologies. Nineteen GHG emissions estimates were provided for trough technology and 17 for tower technology, while the remaining 6 pertained to parabolic dish technology. Given the small pool of estimates for dish CSP, trough and tower systems were the focus of this article.

The IQR of the published life cycle GHG emissions estimates was 83 and 20 g CO2-eq/kWh for trough and tower, respectively. The median of the published estimates was 26 and 38 g CO2-eq/kWh for trough and tower, respectively. The first level of harmonization (light harmonization) reduced the variability of published trough GHG emissions estimates by 87% (IQR = 11 g CO2-eq/kWh) and by 33% (IQR = 13 g CO2-eq/kWh) for tower. Interestingly, the median values of trough and tower life cycle GHG emissions are reduced to similar values: 22 and 23 g CO2-eq/kWh for trough and tower, respectively. Light harmonization, however, underestimates true life cycle GHG emissions because it excludes auxiliary natural gas and electricity consumption.

The light harmonization parameter that is most effective in reducing variability in the published life cycle GHG emissions estimates for trough CSP is the solar fraction. When applied independently, the IQR decreases by 85% after solar fraction harmonization. Two factors contributed to the magnitude of the decrease: high life cycle GHG emissions from natural gas cofiring compared to solar-only CSP, and the relatively large fraction of the trough literature evaluating hybrid plants. As for the published life cycle GHG emissions estimates for towers, the largest reduction in IQR results from the lifetime harmonization step (−25%). This reduction is realized because 9 of the 17 life cycle GHG emissions estimates for towers assume a lifetime of less than 30 years, while only two estimates assume a lifetime greater than 30 years.

The light harmonization parameter that has the most significant impact on central tendency for tower estimates is solar-to-electric efficiency (decreasing median by 22%), as all the published efficiencies were less than the harmonized value. As for trough systems, the median value decreases 18% upon removal of electricity consumption, which illustrates its importance to the overall life cycle.

A second, more resource-intensive level of harmonization (full harmonization) was applied to five life cycle GHG emissions estimates of troughs, which provided sufficient documentation to carry out additional analysis. By harmonizing the values of GWIs for each material and construction activity provided in the LCI of each study, the median value of the five life cycle GHG emissions was reduced by an additional 43% (12 g CO2-eq/kWh) and the range by 18% (16 g CO2-eq/kWh) compared to the results of light harmonization. Adding auxiliary natural gas and electricity consumption effectively reduces the range of the five light harmonized estimates by 34% and 52%, respectively. The median value of the five light harmonized estimates increases by 8% and 36% for auxiliary natural gas combustion and electricity consumption, respectively.

Published estimates of life cycle GHG emissions from CSP passing screens ranged to nearly 250 g CO2-eq/kWh, leading to confusion over CSP's GHG emissions profile and relative benefits compared to fossil-fueled generation technologies. By adjusting published estimates to consistent gross system boundaries and to consistent values for key input parameters, the meta-analytical process called harmonization clarifies the existing literature in ways useful for decision makers and analysts. 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 results presented in this article provide a useful first approximation of life cycle GHG emissions for generic CSP facilities that could, for certain purposes, obviate the need to conduct a full LCA with each new project. To reflect different conditions and performance parameters than selected in this article and to add the GHG contribution from auxiliary natural gas and electricity consumption, future analysts can make reasonable adjustments to the median values of the light harmonized results using the approach outlined in the supporting information on the Web.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

The authors wish to acknowledge funding from the U.S. Department of Energy (U.S. DOE), Office of Energy Efficiency and Renewable Energy. Many National Renewable Energy Laboratory (NREL) and U.S. DOE staff members helped guide this project, most importantly Margaret Mann (NREL), and also Austin Brown (formerly at U.S. DOE, now at NREL), Ookie Ma (DOE), and Gian Porro (NREL). Additional contributors to the LCA Harmonization Project include Pamala Sawyer, Stacey Dolan, Patrick, O’Donoughue, and Ethan Warner, all of NREL, and Vasilis Fthenakis and Hyung-Chul Kim of Brookhaven National Laboratory.

Notes
  • 1

    One kilowatt-hour (kWh) ≈ 3.6 × 106 joules (J, SI) ≈ 3.412 × 103 British thermal units (BTU). Carbon dioxide equivalent (CO2-eq) is a measure for describing the climate-forcing strength of a quantity of greenhouse gases using the functionally equivalent amount of carbon dioxide as the reference.

  • 2

    One megawatt (MW) = 106 watts (W, SI) = 1 megajoule/second (MJ/s) ≈ 56.91 × 103British thermal units (BTU)/minute.

  • 3

    Additional data and results of the project are available at http://openei.org/apps/LCA.

References

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  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information
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About the Authors

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

John J. Burkhardt III was a research analyst and Elliot Cohen was a research participant at the National Renewable Energy Laboratory (NREL), Golden, CO, USA, when performing this research. John Burkhardt is now a private consultant working out of Boulder, CO, USA. Garvin Heath is a senior scientist at NREL, Golden, CO, USA.

Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Harmonization Method
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgements
  8. References
  9. About the Authors
  10. Supporting Information

Supporting Information S1: This supporting information provides two appendices and a series of tables and figures. Appendix A presents a methodology for estimating life cycle greenhouse gas (GHG) emissions from concentrating solar power (CSP) plants with varying environmental and performance characteristics. Appendix B describes the methodology used to select the standardized values of harmonization parameters.

Table S1 shows the published life cycle GHG emissions by life cycle phase. Table S2 shows the changes to published life cycle GHGs from application of harmonization steps. Tables S3–S7 show the life cycle inventory values from selected studies with published global warming indices (GWIs) and the selected GWI for each material or process. Table S8 lists the life cycle inventory (LCI) processes and their respective GWIs. Figures S1 and S2 depict the rank order estimates of life cycle GHG emissions for dish and tower CSP electricity generation technologies, respectively.

FilenameFormatSizeDescription
JIEC_474_sm_SuppMatS1.pdf1024KSupporting info item

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