Measuring Thermophysical Properties

In our Thermal Storage Materials Laboratory, we use a variety 

of instruments to measure the thermophysical properties of 

heat transfer fluids and storage materials to predict both the 

behavior of the materials in a storage system and the overall 

performance of the system.

The facility functions as an analytical laboratory, supporting a 

wide variety of materials research for NREL and its university 

and industry partners.

We use a range of instruments—including a differential 

scanning calorimeter, thermal gravimetric analyzer, rheometer, 

high-precision balance, and oven—to measure the melting 

point, boiling point, heat capacity, density, viscosity, and phasetransition enthalpies for storage materials at temperatures from 

ambient to 600°C. For measuring thermal conductivity, we are 

designing and building an instrument based on the transient 

hot-wire method.

We measure the thermophysical properties of known and 

new heat transfer materials, as well as phase-change 

materials. By determining the thermal properties, we can 

identify those materials that will increase thermal storage 

capacity, thermodynamic efficiency, and dispatchability—as 

well as lower the levelized cost of energy—of a CSP plant.

Measuring Fluid Flow and Heat Transfer 

Our Thermal Storage Process and Components Laboratory 

is being established as a testing lab with the equipment 

and accessories needed to measure the fluid flow and heat 

transfer behavior/performance of process components 

of storage systems—complete with oil, steam, and salt 

heat transfer fluids. Such an evaluation of process unit 

operations is essential for determining the overall 

performance of a complete thermal storage system. We 

will use test results to evaluate potential storage systems 

before they are tested at full scale.

The facility will encompass roughly 1,500 square feet and 

will be equipped with about 100 kilowatts of electrical 

heating and chilling. It will include data acquisition and 

control instrumentation to monitor and control process 

components during testing.

In the testing process, data will be collected over a period of 

time sufficient to cover many temperature cycles of thermal 

charge and discharge. The data will include measures of 

maximum storage capacity, plus charge and discharge rates 

over time. By analyzing these data, our researchers will 

determine if the system is able to function consistently with 

high performance.

PIX 17115; Credit, Greg Glatzmaier

We evaluate the properties of fluids that transfer and store heat in concentrating solar power (CSP) plants to improve 

the thermal-to-electricity efficiency and lower the operational cost of the plants.

Traditionally, CSP plants have used synthetic oils as heat transfer fluids and molten salts for thermal energy storage. 

At the National Renewable Energy Laboratory (NREL), we are improving these materials as well as developing and 

characterizing advanced nanofluids and phase-change materials (PCMs) for thermal storage applications.

Our capabilities in thermal storage and advanced heat transfer fluids center primarily around the following three areas:

An NREL scientist in the new Thermal Storage Materials Laboratory uses a 

rheometer to measure fluid viscosity as part of his work to characterize new 

heat transfer fluids for CSP plants.

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Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

ed NREL logo, June 15, 2009

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Thermal Storage and Advanced Heat Transfer Fluids

Thermal Systems Group: CSP Capabilities
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401

303-275-3000 • www.nrel.gov 

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50% wastepaper, including 10% post consumer waste.

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

NREL/FS-550-48660 • August 2010

Simulating Flow of Thermal Energy and Fluid 

At NREL, we use thermal-storage heat-transfer and fluidflow modeling to simulate the flow of thermal energy and 

fluid over time in complex geometries such as tanks, piping, 

and packed beds. Over a relatively short period of time, the 

techniques can help to predict the performance of complex 

heat transfer and flow systems—without constructing a 

physical system.

Specially trained staff who understand the mathematical 

basis for modeling set up simulations using computer 

simulation software, such as FLUENT, and associated 

hardware. The software code solves a set of mathematical 

equations that represent energy and material balances.

For each simulation, the results are a set of temperatures, 

pressures, and fluid-flow velocity maps that specify the 

conditions for a particular geometry and period of 

simulation time. The information helps us to evaluate 

sensitivities that result from changes in storage system 

design and the resulting impact on CSP power plant 

performance and cost.

Contact Greg Glatzmaier, Greg.Glatzmaier@nrel.gov, 

303-384-7470.

Thermal Systems Group: CSP Capabilities

This graphic shows computer modeling results for a thermocline storage 

system, which stores thermal energy in a single vessel that contains a 

stationary filler material. The system has hot (top) and cold (bottom) regions, 

separated by a sharp temperature partition or “thermocline.” The system’s 

behavior is highly dependent on the properties of the filler material and the 

fluid that flows through it. Modeling these systems provides a wealth of 

information about their operation and performance without actually 

constructing a physical system.

The Thermal Storage Process and Components Laboratory will operate in a temporary location while its permanent home, illustrated here, is developed on the 

NREL campus. Initial testing in the lab may relate to storing the thermal energy content of steam and will include the use of phase-change materials.

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