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Molten Salt vs. Liquid Sodium 

Receiver Selection 

Using the Analytic Hierarchy Process 

Craig Turchi, PhD craig.turchi@nrel.gov

Thermal Energy Science & Technologies Group

National Renewable Energy Laboratory

Cara Libby, Electric Power Research Institute

Joe Coventry and John Pye, Australian National University
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Overview

• Gen3 Liquid Pathway project seeks to demonstrate potential of chloridebased molten salt for energy storage at > 700°C.

• Chloride salt’s high freeze point and poor thermal conductivity are 

challenges for use in a solar receiver.

• Project choose to evaluate liquid-metal sodium as an alternative receiver 

heat transfer fluid via a structured analytic hierarchy process.
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Analytic Hierarchy Process (AHP)

• Decision-making process developed in the 1990s to help work through complicated prioritization 

scenarios; widely used in the military, government, private sector, and academia. 

• Encourages decisions based on knowledge that supports the decision-making process, rather than 

intuition. 

• Simplifies the process by comparing two criteria at a time (i.e., pairwise comparisons) to 

determine which is more important with respect to the decision goal.

• Employs a multi-level (hierarchical) structure centered around an objective, weighted criteria, and 

alternatives. 

– For each criterion, the options are compared to one another in a series of pairwise comparisons. 

– With accurately weighted decision criteria in-place, the feasible alternatives can then be evaluated 

and scored against each criterion in a systematic fashion. 

– The result is a ranked order list of alternatives that summarizes the scoring team’s knowledge and 

wisdom. 

• AHP provides the ability to compare relative benefits and risks of alternatives instead of simply 

identifying a single top-performing option. 
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NREL’s Process

1. Define the decision team

2. Build the decision model

3. Prioritize the criteria

4. Score the alternatives

5. Analyze the results

6. Make the Decision
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Weighted Benefit Criteria

Maximize longterm reliability 

and availability 

Maximize 

efficiency and 

performance

32.0% 33.5%
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Weighted Risk Criteria

Minimize risk to 

people and the 

environment

50.7%
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Benefit Scoring 

(Higher Scores = Higher Benefit)

Accommodate 

different plant 

sizes and 

configurations

Maximize ease of 

operations and 

maintenance

Maximize 

efficiency and 

performance

Maximize longterm reliability and 

availability

Maximize 

stakeholder 

support

Minimize the time 

required to transition 

from the pilot phase 

demonstration to 

large-scale plants

Criteria Legend

Sodium Design

Salt Design
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Risk Scoring 

(Higher Scores = Higher Risk)

Minimize the risk 

of manufacturing 

issues

Minimize the risk 

of a schedule delay

Minimize the risk 

of design issues 

specific to the 

solar receiver

Minimize the risk of 

obtaining bank 

financing and 

insurance for a 

commercial plant

Minimize the risk of 

unplanned outages 

due to operational 

instability

Minimize the risk 

to people and the 

environment

Criteria Legend

Sodium Design

Salt Design
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LCOE Model Validation

• SolarTherm

checked against 

SAM for the saltreceiver case

• Agreement within 

2%

• SolarTherm then 

used for sodium vs. 

salt comparison
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Summary and Decision

• Sodium case has 

11% lower LCOE

• Benefit/Risk ratio:

 Sodium = 1.19

 Salt = 0.86

 Team selected the 

Sodium Receiver 

design
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Proposed

Integrated 

System Design

Sodium Receiver:

• Higher receiver efficiency

• Lesser freeze risk and 

simpler fluid handling

• Greater design flexibility

• Greater operating 

flexibility

Salt

Tanks
www.nrel.gov

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.

Thank you!

NREL/PR-5700-77819

This work was authored by the National Renewable Energy 

Laboratory, operated by Alliance for Sustainable Energy, LLC, for the 

U.S. Department of Energy (DOE) under Contract No. DE-AC36-

08GO28308. Funding provided by the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy Solar Energy 

Technologies Office. The views expressed in the article do not 

necessarily represent the views of the DOE or the U.S. Government. 

The U.S. Government retains and the publisher, by accepting the 

article for publication, acknowledges that the U.S. Government retains 

a nonexclusive, paid-up, irrevocable, worldwide license to publish or 

reproduce the published form of this work, or allow others to do so, for 

U.S. Government purposes.