Conference Paper

NREL/CP-6A2-45857

July 2009

Life Cycle Assessment of 

Thermal Energy Storage: 

Two-Tank Indirect and 

Thermocline

Garvin Heath, Craig Turchi, John Burkhardt and

Chuck Kutscher

National Renewable Energy Laboratory

Terese Decker

University of Colorado

Presented at the American Society of Mechanical Engineers (ASME) 

Third International Conference on Energy Sustainability

San Francisco, California

July 19-23, 2009
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1

LIFE CYCLE ASSESSMENT OF THERMAL ENERGY STORAGE: TWO-TANK INDIRECT 

AND THERMOCLINE

Garvin Heath

Strategic Energy Analysis Center, National Renewable Energy 

Laboratory, Golden, CO

Craig Turchi

Electricity, Resources and Building Systems Integration Center, 

National Renewable Energy Laboratory, Golden, CO

Terese Decker

Department of Mechanical Engineering, 

University of Colorado, Boulder, CO

John Burkhardt

Strategic Energy Analysis Center, National 

Renewable Energy Laboratory, Golden, 

CO

Chuck Kutscher

Electricity, Resources and Building 

Systems Integration Center, National 

Renewable Energy Laboratory, Golden, 

CO

EXTENDED ABSTRACT

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.
2

REFERENCES

[1] International Organization for Standardization (ISO). 2006. 

Environmental management – Life cycle assessment – Requirements 

and guidelines. ISO 14044:2006.Geneva, Switzerland. 46 pp. 

[2] Lechon Y, de la Rua C, Saez R, 2006. “Life Cycle Environmental 

Impacts of Electricity Production by Solar Thermal Technology in 

Spain.” Paper #B5-S5. 13th SolarPACES International Symposium, 

Seville, Spain, June 20-23, 2006.

[3] Kelly B. 2008. 50 MWe_6 hour_Material quantities for Garvin 

Heath.xls. Spreadsheet provided to Garvin Heath, National Renewable 

Energy Laboratory, on July 16, 2008, and subsequent personal 

communications on August 4 and 5, 2008.

[4] Swiss Center for Life Cycle Inventories, 2008. EcoInvent v2.0,

Duebendorf, Switzerland. Available at: http://www.ecoinvent.org/. 

Accessed July 2008 – February, 2009. 

[5] Carnegie Mellon University Green Design Institute. 2009.

Economic Input-Output Life Cycle Assessment (EIO-LCA), US 1997 

Industry Benchmark model [Internet], Available from:

http://www.eiolca.net. Accessed July, 2008 – February, 2009.

[6] Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. 

Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. 

Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007, Changes in 

Atmospheric Constituents and in Radiative Forcing. In: Climate 

Change 2007: The Physical Science Basis. Contribution of Working 

Group I to the Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, 

M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. 

Cambridge University Press, Cambridge, United Kingdom and New 

York, NY, USA.

[7] Turchi C and Kutscher C. 2008. Personal communication with 

Garvin Heath and Terese Decker. July-August, 2008.

TABLE 1: COMPARISON OF THE EMBODIED LIFE-CYCLE GREENHOUSE GAS EMISSIONS FROM THE MATERIALS USED IN 

TWO-TANK INDIRECT AND THERMOCLINE INDIRECT MOLTEN SALT THERMAL ENERGY STORAGE SYSTEMS DESIGNED TO 

SUPPLY 6 HOURS OF THERMAL STORAGE FOR 50 MW CSP PLANTS

MATERIAL MATERIAL MASS [kg] EMISSIONS (MTCO2e)

TWO-TANK THERMOCLINE TWO-TANK THERMOCLINE

Thermal Mass

Silica Sand - 17,900,000 - 376

Molten Salt 

(40% Potasium 

Nitrate 60% Sodium 

Nitrate)

25,600,000 7,680,000 8,470 2,540

Oil-to-Salt Heat 

Exchanger

Calcium Silicate 77,800 38,900 4.9 2.4

Stainless Steel 411,000 179,000 1850 927

Storage Tank(s)

Calcium Silicate 51,300 25,700 3.2 1.6

Carbon Steel 885,000 456,000 1,270 654

Mineral Wool 283,000 158,000 382 212

Stainless Steel 6,110 3,080 31.7 16.0

Storage Tank 

Foundations

Carbon Steel 258,000 134,000 371 192

Concrete 3,850,000 2,070,000 474 255

Foam Glass 90,700 44,000 105 51.0

Refractory Brick 667,000 432,000 1,540 996

Nitrogen Ullage 

System

Calcium Silicate 4,930 2,460 0.31 0.16

Carbon Steel 20,000 17,900 28.7 25.8

Nitrogen 429,000 28,100 184 12.1

Pumps - - - 1,930 1,190

Compressors - - - 6.6 6.6

Elevated Platform

Carbon Steel 194,000 194,000 278 279

Concrete 1,290,000 1,290,000 159 159

TOTAL: 17,100 7890

Notes: Three significant figures are used to convey the uncertainty in the input data and assumptions. Materials listed are those specified in Kelly 

[3]; proxy materials were used for LCA modeling in some cases. Materials and masses are not reported for pumps and compressors because the EIOLCA was used to estimate GHG emissions in those cases [5]. 
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Life Cycle Assessment of Thermal Energy Storage: Two-Tank 

Indirect and Thermocline

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DE-AC36-08-GO28308

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G. Heath, C. Turchi, J. Burkhardt, C. Kutscher, and T. Decker

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NREL/CP-6A2-45857

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PVB7.6301

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14. ABSTRACT (Maximum 200 Words)

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.

15. SUBJECT TERMS

life-cycle assessment; concentrating solar power; CSP; solar; RE technologies; thermal energy storage; greenhouse 

gas emissions; GHG; TES

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