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

ed NREL logo, June 15, 2009

Blue

Ideally, we want reflector materials in a CSP plant to last 20 to 

30 years and cost less than $2.50 per square foot (or $25 per 

square meter) to manufacture. Highly specular mirrors should 

have better than 95% reflectance into a 4-milliradian full-cone 

angle and should resist soiling in all outdoor conditions. NREL 

focuses on achieving these goals by creating and applying 

testing procedures that accurately predict the performance 

and lifetime of materials. Some testing is relatively brief, 

lasting several weeks, whereas other processes may take 

several months or even years.

We evaluate the potential of reflector (mirror) and absorber 

(receiver) materials in the three areas described below, working 

with our industry partners to develop technologies that will 

move CSP toward economic parity with electricity generated 

by coal and natural gas. 

Evaluating Performance

NREL evaluates the performance of mirrors, solarselective coatings, glazings, lenses, and glass by measuring 

reflectance, transmittance, and absorptance. We also test 

for tensile strength, adhesion, permeability, and scratch 

resistance. In many cases, our partners submit samples for 

testing as a step in developing a new product. 

For mirrors, we measure both hemispherical and specular 

reflectance, using ultraviolet-visible-near infrared (UVVis-NIR) spectrophometers and specular reflectometers. In 

2010, NREL will add the capability to measure the specular 

reflectance of mirrors in a variety of cone angles across the 

entire solar spectrum. This added capability will enable us 

to rapidly measure the solar-weighted specular reflectance to 

determine how much light is being scattered from a mirror’s 

surface for all CSP technologies.

For solar-selective coatings, we measure the following 

parameters: solar absorptance (with a goal of greater than 

96%); thermal emittance (with a goal of less than 7%); and oxidation resistance at high temperatures (with a goal of 

650oC operating temperature). We use the UV-Vis-NIR 

spectrophotometer to measure solar absorptance and an 

infrared spectrophotometer to measure emittance between 

2.5 and 50 micrometers. Using hand-held instruments, our 

researchers can measure emittance at ambient temperature 

and determine emittance by fitting to the blackbody 

temperature of interest. 

NREL will also be adding the capability of measuring 

absorption and emittance of curved surfaces at operating 

temperatures. This will allow our partners to understand 

the optical properties of their receiver tubes at real-world 

operating temperatures. 

Determining Degradation Rate and Lifetime

We expose samples to a variety of natural outdoor and 

accelerated exposure tests to determine their rate of 

degradation and to estimate lifetime. Typically, we measure 

samples until they have degraded by 10% of their initial 

value. The samples are measured at regular intervals over 

a period of months and years, which helps to ensure that a 

new product meets desired optical requirements and avoids 

failure in the field.

NREL has developed a Service Lifetime Prediction 

methodology that allows us to predict the lifetime of a 

The National Renewable Energy Laboratory (NREL) can test and develop optical materials—mirrors and receiver tube coatings—

which are the major components of the solar field of concentrating solar power (CSP) systems. In evaluating the performance of 

these materials, we can help to improve the overall efficiency of CSP plants. Because the solar field accounts for nearly half the 

total plant construction cost, an increase in optical efficiency can be a significant step toward making CSP competitive with fossil 

fuels as a utility-scale energy source.

PIX 16817; NREL

Working at NREL, 

visiting scientist 

Florian Sutter of the 

German Aerospace 

Center (DLR) builds 

a space-resolved 

specular reflectometer, which can be 

used to measure 

the amount of light 

scattered from a 

mirror’s surface in a 

CSP plant. 

Advanced Reflector and Absorber Materials

Thermal Systems Group: CSP Capabilities
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401

303-275-3000 • www.nrel.gov 

Printed with a renewable-source ink on paper containing at least 

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-550-48662 • August 2010

sample with as little as 12 to 18 months of accelerated 

exposure testing. Our exposure chambers simulate a 

variety of conditions, including sunlight, various 

temperature and humidity levels, rain, freeze/thaw, 

hail, and salt spray. Chambers use xenon arc light that 

accelerates light exposure by a factor of 7.5. 

The testing chambers run 24 hours a day, seven days a week, 

replicating specified conditions on an ongoing basis. The 

newest weatherometers simulate the freeze/thaw cycle and 

rain with light exposure. Some testing may accelerate outdoor 

weathering by a factor of 30, helping to identify potential 

long-term problems. Scientists can then determine failure 

mechanisms and develop strategies to mitigate possible 

product failures. 

To ensure that accelerated exposure testing replicates the 

failure mechanisms observed, leading to the most accurate 

results possible, we test samples outdoors at three testing 

facilities that represent different environmental conditions: 

Atlas Weathering Service DSET Laboratories (Phoenix, 

Arizona); NREL (Golden, Colorado); and Atlas Weathering 

Service South Florida Test Service (Miami, Florida). 

Candidate samples are submitted to all three sites for 

exposure, and a standard series of accelerated tests is 

performed in parallel to outdoor testing. NREL can currently 

expose solar-selective coatings at high temperatures in an 

inert gas environment. In 2010, we will add the capability of 

exposing solar-selective materials at operating temperatures in 

a vacuum, enabling our partners to determine the reliability of 

their receiver tubes at real-world operating temperatures. 

Developing New Coatings

In NREL’s Thin Film Deposition Laboratory, we develop new 

mirror and solar-selective coatings and determine if they will 

provide improved optical performance and durability. We 

use a three-chamber system:

•	 First, we place a sample in a load-lock chamber, where it is

pumped down to a vacuum before it moves into the 

deposition chambers.

•	 Second, the sample is coated, using pulsed direct-current

sputtering in another chamber. For example, an 

aluminum target can be bombarded with heavy argon 

molecules, which sputter off aluminum and coat the 

substrate with the metal.

•	 Third, an electron beam in a final chamber evaporates

molecules, and an ion gun is used to compact them in 

the coating, thus creating a dense “hard coat.”

This process helps to develop new coatings that cost less than 

current ones and have better optical qualities and to produce 

mirrors and receivers that are easier to install and maintain. 

One new solar-selective coating developed in the laboratory 

was recently commercialized, based on its excellent properties of high absorption, low emittance, and resistance to 

oxidation. 

Contact Cheryl Kennedy, Cheryl.Kennedy@nrel.gov; 

303-384-6272 (office) or 303-384-6166 (lab).

Thermal Systems Group: CSP Capabilities

Researchers in NREL’s Thin Film Deposition Laboratory develop mirror and 

solar-selective coatings to improve the efficiency of CSP systems. Here, Senior 

Scientist Cheryl Kennedy is holding a sample in the evaporation chamber of 

the three-chamber deposition system. 

PIX 17034; Credit, Pat Corkery

 PIX 16581; Credit, Joe Poellot

NREL and Atlas 

Material Testing 

Technology researchers 

stand before the UltraAccelerated Weathering 

System, an ultraviolet 

solar concentrator that 

won an R&D 100 Award 

for innovation in 2009. 

The system replicates 

the effects of years of 

outdoor weathering 

in very abbreviated 

timeframes.