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

Determining Optical Efficiency 

At our outdoor Solar Concentrator Optical Efficiency Test Loop, 

we determine the optical efficiency of a parabolic trough by 

comparing the direct solar radiation falling on the trough 

aperture operating near ambient temperature to the thermal 

energy collected by the fluid flowing through the receiver tube. 

A parabolic trough, up to 16 meters long, is mounted on a 

two-axis tracker that points directly at the sun throughout 

the day. The trough’s receiver is connected via flexible lines 

to a closed-loop system that maintains the fluid at specified 

temperatures and cools the fluid after it passes through 

the receiver tube. A water-based coolant, at near-ambient 

temperature and low pressure, is pumped to the receiver. 

Because heat loss is extremely low, optical efficiency can be 

determined independent of thermal efficiency. Both are 

needed to determine the overall efficiency of a parabolic 

trough, but thermal efficiency is calculated separately (see 

Measuring Heat Loss).

Our researchers test commercial collectors and identify where 

performance gains can be made, allowing designers of new 

collectors to improve their products before production. Contact 

Chuck Kutscher, Chuck.Kutscher@nrel.gov, 303-384-7521.

Measuring Heat Loss

Our Receiver Test Laboratory measures heat loss in the 

receiver tubes of parabolic trough collectors. Heat loss results 

can be combined with collector optical efficiency tests (see 

Determining Optical Efficiency) to create an overall collector 

efficiency curve.

Testing takes place indoors under controlled conditions. A 

receiver is mounted on the Heat-Loss Test Stand, and electric 

heaters are placed inside the receiver to heat the absorber 

tube to desired test temperatures. Currently, testing occurs 

between 100° and 500°C and requires one to two weeks.

We work in partnership with private companies to test their 

latest receiver designs. Entering the results into the Solar 

Advisor Model, we can determine the effect that the reduced 

heat loss of a new receiver tube would have on a solar plant’s 

levelized cost of electricity. For example, annual, hourly 

simulations of one recently-tested design showed that its 

reduction in heat loss—compared to the previous mode—

would reduce the cost of electricity by half a cent per kilowatthour and would increase annual electricity production by 

5%. Contact Frank Burkholder, Frank.Burkholder@nrel.gov, 

303-384-7564.

Developing and Testing Concentrators

Researchers at our Concentrator Optical Measurements 

Laboratory develop and test concentrators for concentrating 

The National Renewable Energy Laboratory (NREL) tests the performance of concentrating solar power (CSP) system components— 

often while plants remain in operation—to determine durability, performance, and heat loss as well as to identify potential 

enhancements. Our work falls primarily within the following five capabilities.

In our Receiver Test 

Laboratory, a receiver 

for a solar collector is 

mounted on this HeatLoss Test Stand and 

tested for heat loss at 

temperatures between 

100° and 500°C. Results 

can show the impact 

of reduced heat loss on 

both the cost of electricity and amount of 

electricity produced by a 

solar plant.

A parabolic trough 

is mounted on a 

two-axis tracker 

at our Solar 

Concentrator 

Optical Efficiency 

Test Loop. As the 

trough tracks the 

sun, we can 

determine its 

optical efficiency.

PIX 17117; Credit, Frank Burkholder 

PIX 17116; Credit, Keith Gawlik

Collector/Receiver Characterization

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-550-48659 • August 2010

solar power systems. The Video Scanning Hartmann Optical 

Test (VSHOT) characterizes the optical performance of both 

point-focus and line-focus optical concentrators. 

VSHOT, which can be used both in laboratory and field 

settings, combines laser ray tracing with fast video imaging to 

mathematically describe the mirror concentrator surface and 

determine the deviation from the desired surface. It reports 

slope errors and provides plots of error location, magnitude, 

and direction. VSHOT testing of a new, commercial parabolic 

trough collector led to significant design improvements and 

helped garner an R&D 100 Award from R&D Magazine, which 

NREL shared with the manufacturer. 

We use the Receiver Infrared Imaging System (RIIS) in the 

field to locate and document unacceptable heat losses of 

receivers in operating parabolic trough plants. The RIIS 

consists of a vehicle equipped with an infrared camera 

and a Global Positioning Satellite (GPS) system. As the 

vehicle is driven down each row of a solar field, a computercontrolled data acquisition system uses the camera to identify, 

photograph, and process the receiver glass temperatures. 

One operator can determine the glass temperatures of 

about 6,000 receivers in a single day. Contact Tim Wendelin, 

Tim.Wendelin@nrel.gov, 303-384-7475.

Concentrating the Sun’s Power

At our Concentrated Solar Radiation Facility, the High-Flux 

Solar Furnace (HFSF) and the Ultra-Accelerated Weathering 

System (UAWS) concentrate the power of the sun to test the 

durability of materials and coatings, conduct high-temperature 

experiments, and demonstrate the potential of solar power in 

industrial processes.

In operation since 1990, our 10-kilowatt High-Flux Solar Furnace can quickly generate up to 1,800°C over an area of 

1 cm2

—3,000°C with a secondary concentrator—and can 

create heat equal to the intensity of more than 20,000 suns. 

Used in myriad research applications, it is ideal for hightemperature processes. The furnace is also a clean, lower-cost 

alternative to laser and electric furnaces used in industry. 

The UAWS, a winner of an R&D 100 Award, is an ultraviolet 

(UV) concentrator that can simulate 10 years of UV damage in 

just two months. It uses a parabolic dish with special mirror 

facets to concentrate and focus the sun’s UV rays to simulate 

aging on a wide variety of surfaces, from automotive paint 

and building materials to coatings on solar panels. Contact 

Judy Netter, Judy.Netter@nrel.gov, 303-384-6258 (HFSF); 

Matthew Gray, Matthew.Gray@nrel.gov, 303-384-3917 (UAWS).

Taking a Step Forward in Optical Characterization

Our prototype Distant Observer (DO) tool is a dramatic step 

forward in the optical characterization of CSP plants. When 

operational, the DO will be able to rapidly characterize a 

single module and, in the future, survey a large parabolic 

trough field in less than four hours, identifying collector and/

or receiver misalignments, mirror slope errors, and tracking 

errors while the collector field remains online. An improvement of 1% in optical efficiency is worth about $600,000 in 

annual revenue for a typical CSP plant. 

Among the important advantages of the DO are its speed and 

its ability to characterize collectors in operational orientations. 

Our technique uses an aerial platform (such as an aerostat or 

remote-controlled plane) deployed above a trough field. The 

aerial platform incorporates a high-resolution (16 megapixel) 

optical camera, which can be operated by just one person 

on the ground. Once problem areas are pinpointed, they can 

be revisited for more quantitative analyses. Contact Gary 

Jorgensen, Gary.Jorgensen@nrel.gov, 303-384-6113.

Thermal Systems Group: CSP Capabilities

The Video Scanning Hartmann Optical Test (VSHOT) is conducted on 

a parabolic trough concentrator at NREL’s Concentrator Optical 

Characterization Laboratory. 

PIX 17033; Credit, Allison Gray

At our High-Flux Solar 

Furnace, a heliostat 

redirects sunlight 

onto the primary 

concentrator, shown 

here, which focuses the 

light to a point inside 

our test facility, part of 

the Concentrated Solar 

Radiation User Facility. 

The furnace can 

generate temperatures 

that exceed the intensity 

of 20,000 suns.

PIX 06975; Credit, Warren Gretz