NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

ASME ES2009

San Francisco, CA

Garvin Heath

John Burkhardt

Craig Turchi

Terese Decker

Chuck Kutscher

July 20, 2009

NREL/PR-6A2-46875

LCA of Parabolic Trough CSP: 

Materials Inventory and Embodied GHG 

Emissions from Two-Tank Indirect and 

Thermocline Thermal Storage
National Renewable Energy Laboratory 2 Innovation for Our Energy Future

Issue

• Significant env. impacts from energy systems

• Renewable energy systems shown to be much 

better than conventional (fossil), but requires 

different approach to estimate

– Vast majority of env emissions from conventional 

systems in operation (fuel combustion)

– Majority of env emissions from RE in component 

manufacture

• Requires accounting for all life cycle stages to 

evenly compare

• CSP touted as an important RE technology

– Especially for ease of integration of energy storage to 

smooth solar resource fluctuations and extend into 

evening peak hours or longer

• Very few LCAs of CSP, especially of modern 

design and on US-based systems
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Goal: determine the life cycle 

environmental impacts (e.g., GHG 

emissions) and net energy balance of 

modern parabolic trough CSP in US

Today: comparison of materials 

inventory and embodied GHG 

emissions from two-tank and 

thermocline thermal energy storage 

sub-systems

- TES ~40% of CSP system embodied 

GHG emissions (Lechon et al., 2006)

NREL’s LCA of CSP
National Renewable Energy Laboratory 4 Innovation for Our Energy Future

Approach

Operation Resource 

Extraction

Component 

Manufacturing Construction Deconstruction

• Life cycle assessment

– Inventory level

– Conforms with ISO standards

• Significant industry input

• Focus on impacts embodied in component manufacture

– Construction, operation and decommissioning impacts generally 

found to be small for RE technologies, including CSP 

(Veibahn et al., 2008)
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• Two-tank

– Supporting spreadsheets to Kelly, 2006, 

Nexant Parabolic Trough Solar Power 

Plant Systems Analysis, Preferred Plant 

Size (NREL/SR-550-40162)

– Extensive conversations with Kelly

• Thermocline

– Uses two-tank as base, from which 

subtractions and substitutions were made 

based on expert judgment

• Materials life cycle GHG emissions

– EcoInvent v2 LCI database (1st choice)

• Mass-based

• www.ecoinvent.ch

– US Economic Input Output (EIO-LCA)

• Cost-based

• CMU: www.eiolca.net

• Global warming potentials

– 2007 IPCC

Data Sources
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System Definition and Boundary

• 50 MWe parabolic trough plant

• 6 hours thermal storage
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Systems – Indirect two-tank 

•Storage tanks 

– Hot and cold tanks (2)

– Immersion heaters

– Insulation

– Nitrate salt and therminol 

pipes, heat tracing pipes and 

insulation

•Thermal mass – molten salt

– 60% Potassium nitrate; 

– 40% Sodium nitrate

•Tank foundations

– Concrete and rebar

– Insulation

– Steel plate

– Thermal slab and rebar

•Oil-to-salt heat exchangers

– Tubes, shells, covers

– Insulation

– Heat tracing pipes

•Pumps

– Nitrate salt pumps for each tank

– Oil-to-salt HX

•Elevated platform

– Structural steel

– Structural concrete

– Rebar 

•Nitrogen ullage system

– Nitrogen

– Tank

– Compressor

– Pipes

– Insulation

– Heat tracing

– N2-to-air heat exchanger
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Systems – Thermocline

• Single tank

– Model as hot tank (slightly larger)

• Replace 70% of salt with silica sand

• Half of most materials

– Some retained in full (N2 system)

Source: Sandia National Laboratories test
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Materials and Proxies

• Nitrate salts (40% Potassium Nitrate, 60% Sodium Nitrate)

– Neither in LC databases

– Decided that both could be approximated by same material

– Most nitrate salt used in CSP application is mined

– Surrogate = KCl, a known mined product

• Steels

– Carbon steel (tanks, pipes, rebar) 

– Stainless steel (oil-to-salt HX and piping, immersion heaters)

• Concrete

• Insulations

– Calcium Silicate

• Also not in LC databases

• Closest analogue = sand-lime brick

– Closest based on peak process temperature and process steps

– Refractory bricks

– Mineral wool

– Foam glass

• Pumps

– Because highly manufactured, used economic value with EIO
National Renewable Energy Laboratory 10 Innovation for Our Energy Future

Results: Materials Inventory

MATERIAL MASS [KG] Thermocline 

Reduction (%)

TWO-TANK THERMOCLINE

Silica Sand - 8,950

Molten Salt* 25,600 7,680 70%

Concrete 5,140 3,360 35%

Carbon Steel 1,615 936 42%

Stainless Steel 417 182 56%

Calcium Silicate 134 67 50%

Foam Glass 91 44 52%

Mineral Wool 283 158 44%

Refractory Brick 667 432 35%

Pumps** - - 38%

* Molten salt = 40% Potasium Nitrate 60% Sodium Nitrate

** Based on reduction of GHG emissions associated with pumps, estimated from EIO-LCA.
National Renewable Energy Laboratory 11 Innovation for Our Energy Future

Results: GHG Emissions by Material

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

Two-Tank Thermocline

GHG emissions (MT CO2e)

Pumps**

Refractory Brick

Mineral Wool

Foam Glass

Calcium Silicate

Stainless Steel

Carbon Steel

Concrete

Molten Salt*

Silica Sand

17,100 MT CO2e

7700 MT CO2e

insulation

pumps

steels

thermal mass

concrete

% reduction in GHG emissions per material = % mass reduction
National Renewable Energy Laboratory 12 Innovation for Our Energy Future

Results: GHG Emissions by Component

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

Two-Tank Thermocline

GHG emissions (MTCO2e)

Elevated Platform

Pumps

Storage Tank(s) Foundation(s)

Storage Tank(s)

Oil-to-Salt HX

Thermal Mass

17,100 MT CO2e

7700 MT CO2e

50%

37%
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Results: Sensitivity Analyses

• Nitrate salts

– Nitrate salts for thermal storage could also come from synthetic 

pathway

• Haber process – uses considerable natural gas and releases N2O

– Synthetic nitrate salt proxy = KNO3 (fertlizer)

– Per unit mass, synthetic salt > 8x GWP vs. mined

– Impact on GHG emissions from TES:

• 2-tank = increase total emissions nearly 5x

• Thermocline = increase total emissions over 3x

• Calcium silicate

– So small a contribution that using different (higher GWP) proxy not 

matter
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Conclusions

• Thermocline system demonstrates greatly reduced

– Materials inventory

– GHG emissions (-55%)

– Cost 

• Relative results expected to be robust to

– Inclusion of construction, operation and decommissioning-related 

impacts

– Selection of material proxy for nitrate salts (though difference is 

enhanced)

• Further research needed

– Life cycle inventory of thermal storage nitrate salts (underway by 

manufacturer)

– Completion of LCA for full plant

– Comparison to previous work and other power systems
National Renewable Energy Laboratory 15 Innovation for Our Energy Future

Questions?

Andosol