Climate change and water scarcity are important issues for today's power sector. To inform capacity expansion decisions, hybrid life cycle assessment is used to evaluate a reference design of a parabolic trough concentrating solar power (CSP) facility located in Daggett, CA, along four sustainability metrics: life cycle (LC) greenhouse gas (GHG) emissions, water consumption, cumulative energy demand (CED), and energy payback time (EPBT). This wet-cooled, 103 MW plant utilizes mined nitrates salts in its two-tank, thermal energy storage (TES) system. Design alternatives of dry-cooling, a thermocline TES, and synthetically derived nitrate salt are evaluated. During its LC, the reference CSP plant is estimated to emit 26 g of CO(2eq) per kWh, consume 4.7 L/kWh of water, and demand 0.40 MJ(eq)/kWh of energy, resulting in an EPBT of approximately 1 year. The dry-cooled alternative is estimated to reduce LC water consumption by 77% but increase LC GHG emissions and CED by 8%. Synthetic nitrate salts may increase LC GHG emissions by 52% compared to mined. Switching from two-tank to thermocline TES configuration reduces LC GHG emissions, most significantly for plants using synthetically derived nitrate salts. CSP can significantly reduce GHG emissions compared to fossil-fueled generation; however, dry-cooling may be required in many locations to minimize water consumption.

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Keywords:dish Stirling life cycle assessment meta-analysis parabolic trough power tower renewable energy Supporting information is available on the JIE Web site SummaryIn 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 metaanalytical 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 CO 2 -eq/kWh), 1 respectively; median estimates were 26 and 38 g CO 2 -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.

A hybrid life cycle assessment (LCA) is used to evaluate four sustainability metrics over the life cycle of a power tower concentrating solar power (CSP) facility: greenhouse gas (GHG) emissions, water consumption, cumulative energy demand (CED), and energy payback time (EPBT). The reference design is for a dry-cooled, 106 MW(net) power tower facility located near Tucson, AZ that uses a mixture of mined nitrate salts as the heat transfer fluid and storage medium, a two-tank thermal energy storage system designed for six hours of full load-equivalent storage, and receives auxiliary power from the local electric grid. A thermocline-based storage system, synthetically derived salts, and natural gas auxiliary power are evaluated as design alternatives. Over its life cycle, the reference plant is estimated to have GHG emissions of 37 g CO2eq/kWh, consume 1.4 L/kWh of water and 0.49 MJ/kWh of energy, and have an EPBT of 15 months. Using synthetic salts is estimated to increase GHG emissions by 12%, CED by 7%, and water consumption by 4% compared to mined salts. Natural gas auxiliary power results in greater than 10% decreases in GHG emissions, water consumption, and CED. The thermocline design is most advantageous when coupled with the use of synthetic salts.

In the United States, concentrating solar power (CSP) is one of the most promising renewable energy (RE) technologies for reduction of electric sector greenhouse gas (GHG) emissions and for rapid capacity expansion. It is also one of the most price-competitive RE technologies, thanks in large measure to decades of field experience and consistent improvements in design. One of the key design features that makes CSP more attractive than many other RE technologies, like solar photovoltaics and wind, is the potential for including relatively low-cost and efficient thermal energy storage (TES), which can smooth the daily fluctuation of electricity production and extend its duration into the evening peak hours or longer. Because operational environmental burdens are typically small for RE technologies, life cycle assessment (LCA) is recognized as the most appropriate analytical approach for determining their environmental impacts of these technologies, including CSP. An LCA accounts for impacts from all stages in the development, operation, and decommissioning of a CSP plant, including such upstream stages as the extraction of raw materials used in system components, manufacturing of those components, and construction of the plant. The National Renewable Energy Laboratory (NREL) is undertaking an LCA of modern CSP plants, starting with those of parabolic trough design. Our LCA follows the guidelines described in the international standard series ISO 14040-44 [1]. To support this effort, we are comparing the life-cycle environmental impacts of two TES designs: two-tank, indirect molten salt and indirect thermocline. To put the environmental burden of the TES system in perspective, one recent LCA that considered a two-tank, indirect molten salt TES system on a parabolic trough CSP plant found that the TES component can account for approximately 40% of the plant’s non-operational GHG emissions [2]. As emissions associated with plant construction, operation and decommissioning are generally small for RE technologies, this analysis focuses on estimating the emissions embodied in the production of the materials used in the TES system. A CSP plant that utilizes an indirect, molten salt, TES system transfers heat from the solar field’s heat transfer fluid (HTF) to the binary molten salts of the TES system via several heat exchangers. The “cold tank” receives the heat from the solar field HTF and conveys it to the “hot tank” via another series of heat exchangers. The hot tank stores the thermal energy for power generation later in the day. A thermocline TES system is a potentially attractive alternative because it replaces the hot and cold tanks with a thermal gradient within a single tank that significantly reduces the quantity of materials required for the same amount of thermal storage. An additional advantage is that the thermocline design can replace much of the expensive molten salt with a low-cost quartzite rock or sand filler material. This LCA is based on a detailed cost specification for a 50 MWe CSP plant with six hours of molten salt thermal storage, which utilizes an indirect, two-tank configuration [3]. This cost specification, and subsequent conversations with the author, revealed enough information to estimate weights of materials (reinforcing steel, concrete, etc.) used in all components of the specified two-tank TES system. To estimate embodied GHG emissions per kilogram of each material, two life cycle inventory (LCI) databases were consulted: EcoInvent v2.0 [4], which requires materials mass data as input, and the US Economic Input-Output LCA database [5], which requires cost data as input. IPCC default global warming potentials (GWPs) give the greenhouse potential of each gas relative to that of carbon dioxide [6]. Where certain materials specified in Kelly [3] were not available in the LCI databases, the closest available proxy for those materials was selected based on such factors as peak process temperature, and similar input materials and process technology. The thermocline system was modeled using the two-tank system design as the foundation, from which materials were subtracted or substituted based on the differences and similarities of design [7]. Table 1 summarizes the results of our evaluation. Embodied emissions of GHGs from the materials used in the 6-hour, 50 MWe two-tank system are estimated to be 17,100 MTCO2e. Analogous emissions for the thermocline system are less than half of those for the two-tank: 7890 MTCO2e. The reduction of salt inventory associated with a thermocline design thus reduces both storage cost and life cycle greenhouse gas emissions. While construction-, operation- and decommissioning-related emissions are not included in this assessment, we do not expect any differences between the two system designs to significantly affect the relative results reported here. Sensitivity analysis on choices of proxy materials for the nitrate salts and calcium silicate insulation also do not significantly affect the relative results.