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Field Survey of Parabolic 

Trough Receiver Thermal 

Performance 

Preprint 

H. Price, R. Forristall, T. Wendelin, and 

A. Lewandowski 

National Renewable Energy Laboratory 

T. Moss 

Sandia National Laboratories

C. Gummo 

Apple Valley, California

To be presented at Solar 2006 Conference(ISEC’06) 

Denver, Colorado 

July 8–13, 2006 

Conference Paper 

NREL/CP-550-39459 

April 2006 
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Proceedings of ISEC2006 

ASME International Solar Energy Conference 

July 8-13, 2006, Denver, Colorado, USA 

ISEC2006-99167 

FIELD SURVEY OF PARABOLIC TROUGH RECEIVER THERMAL PERFORMANCE 

Henry Price, Russell Forristall, Timothy Wendelin, Allan Lewandowski 

National Renewable Energy Laboratory 

Golden, Colorado 

Timothy Moss 

Sandia National Laboratories 

Albuquerque, New Mexico 

Carin Gummo 

Apple Valley, California 

ABSTRACT 

This paper describes a technique that uses an infrared (IR) 

camera to evaluate the in-situ thermal performance of parabolic 

trough receivers at operating solar power plants. The paper 

includes results to show how the glass temperature measured 

with the IR camera correlates with modeled thermal losses 

from the receiver. Finally, the paper presents results of a field 

survey that used this technique to quickly sample a large 

number of receivers to develop a better understanding of how 

both original and replacement receivers are performing after up 

to 17 years of operational service. 

INTRODUCTION 

 During the period of 1984 to 1990, Luz International 

Limited built nine large solar power plants, 14 MW to 80 MW 

in size, in the California Mojave Desert [1]. These plants, 

referred to as Solar Electric Generating Systems or SEGS, have 

continued in daily operation and remain the largest solar power 

plants in the world today. During this period, these plants have 

demonstrated impressive operational availability and have 

sustained a high level of solar performance. These plants utilize 

a large field of parabolic trough solar collectors to heat a 

thermal fluid that is in turn used to generate steam to power a 

conventional steam Rankine cycle power plant. For this 

application, the parabolic trough solar field must operate at 

temperatures of 293oC to 391oC. As a result, the long-term 

performance of the linear parabolic trough receiver, a fourmeter long stainless steel pipe coated with a solar selective 

coating and enclosed by an evacuated glass envelope, becomes 

one of the keys to achieving good sustained performance. 

BACKGROUND 

The parabolic trough linear receiver, also called a heat 

collection element (HCE), is one of the primary reasons for the 

high efficiency of the Luz parabolic trough collector design. 

The HCE consists of a four meter long, 70-mm outside 

diameter (O.D.) stainless steel tube with a cermet solarselective absorber surface, surrounded by an antireflective

(AR) evacuated glass tube with a 115 mm O.D. (Figure 1). The 

HCE incorporates conventional glass-to-metal seals and metal 

bellows to achieve the necessary vacuum-tight enclosure and to 

accommodate for thermal expansion difference between the 

steel tubing and the glass envelope. The vacuum enclosure 

serves primarily to significantly reduce heat losses at high 

operating temperatures and to protect the solar-selective 

absorber surface from oxidation. The vacuum in the HCE, 

which must be at or below the Knudsen gas conduction range 

to mitigate convection losses within the annulus, is typically 

manufactured at about 1x10-4 torr (0.013 Pa). The multilayer 

cermet coating is sputtered onto the steel tube to result in 

excellent selective optical properties with high solar 

absorptance of direct beam solar radiation and a low thermal 

emittance at operating temperature to reduce thermal reradiation. The outer glass cylinder has an AR coating on both 

surfaces to reduce Fresnel reflective losses from the glass 

surfaces, thus maximizing the solar transmittance. Getters, 

which are metallic compounds designed to absorb gas 

molecules, are installed in the vacuum space to absorb 

hydrogen and other gases that permeate into the vacuum 

annulus over time. The receivers include an evaporable barium 

getter that is used to monitor the vacuum in the receiver. The 

barium getter will have a silver appearance when the receiver 

has good vacuum, but will turn white if the receiver loses 

vacuum and is exposed to air. Because of the higher operating 

temperatures at the latest plants, substantial thermal 

decomposition of the heat transfer fluid is expected, and as a 

result, hydrogen buildup in the vacuum becomes more of a 

 1
concern. In addition to getters, Luz developed a special 

hydrogen removal (HR) membrane made from a palladium 

alloy to help remove excess hydrogen from the vacuum annulus 

[2]. Unfortunately, a number of receiver reliability problems 

were experienced when receivers with the HRs were first used 

[3]. As a result, the HR was eliminated from the design on 

replacement receivers. 

Figure 1. Heat collection element (HCE) 

(Source: Flabeg Solar International)

Although Luz went out of business in 1991, two new 

companies are supplying parabolic trough receivers for 

replacements at existing plants and for new plants. Solel Solar 

Systems1

 of Israel took over the Luz receiver manufacturing 

facility and has continued to improve the receiver design and 

performance [3]. Schott Glass of Germany2

 has also recently 

developed a new receiver design that it sells commercially [1]. 

The thermal performance of parabolic trough receivers has 

been tested by a number of sources. Dudley et al [4] tested the 

original Luz cermet and black chrome receivers on an LS-2 

collector mounted on the rotating platform at the National Solar 

Thermal Test Facility in Albuquerque, New Mexico. The 

Sandia test results provide a performance baseline for the 

original Luz receivers in new condition. Solel developed a new 

parabolic trough receiver to improve reliability issues with the 

glass to metal seal and to improve the durability and properties 

of the selective coating. The relative thermal performance of 

these receivers was measured in a test loop of collectors at the 

SEGS VI and compared with original Luz receivers operating 

in a parallel loop of collectors [3]. The test was conducted at 

SEGS VI plant because it uses the optically superior LS-2 

collector and it operates up to 391oC. Although a significant 

increase in performance of 20 to 30 percent was observed with 

the new Solel receivers, it was not determined whether the 

increased performance was entirely a result of improvements in 

the new receiver, or caused, in part, by degradation of the 

original receivers. Sandia has subsequently tested the new 

receivers from both Solel and Schott on the rotating platform 

 1

 Solel Solar Systems, Ltd., Jeruslam, Israel, www.solel.com. 2

 Schott Glass, Mainz, Germany, www.schott.com/solar. 

[5]. These tests indicate that the new receivers do show 

significantly improved performance, however, not the 20 to 30 

percent seen in the loop tests at SEGS VI. In general, there has 

been extensive testing of new receiver performance, but there 

has been very little done to look at the thermal performance of 

receivers after they have been in service for many years. This 

paper looks at a new approach for evaluating the in-situ thermal 

losses of parabolic trough receivers in an operating solar plant. 

METHODOLOGY 

The modular nature of a parabolic trough power plant 

allows periodic maintenance on individual collectors while the 

rest of the plant continues to operate. A single collector can be 

defocused for mirror replacement, optical alignments, or 

electronic repairs, while hot fluid continues to pass through the 

receivers. The glass envelope temperature on receivers with 

good vacuum remain reasonably cool (approximately 70oC to 

100oC), even when heat transfer fluid at operating temperatures 

is flowing through them (on the non-tracking collector). When 

receivers lose vacuum, e.g. air gets into the annulus between 

the steel and glass tubes, the temperature of the glass increases 

significantly. The maintenance crews know to be careful 

around receivers that have lost vacuum because they represent 

a potential burn hazard if touched with bare skin. Receivers 

that have lost vacuum are generally easy for crews to identify 

because the barium detector getter turns white when exposed to 

air. Several years ago, the maintenance crews began noticing 

that the glass temperature was elevated on some receivers that 

otherwise still appeared to have good vacuum. Field 

inspections showed that some receivers that still appeared to 

have good vacuum actually had glass temperatures above those 

that had lost their vacuum. When a hole was drilled through 

the bellows of one of these receivers, air was observed to be 

sucked into the annulus. The glass temperature then cooled off 

to that of a receiver with air in the annulus. Based on this 

behavior, the hot receivers appeared to be showing signs of a 

build-up of hydrogen in the vacuum annulus. The hydrogen is 

assumed to come from decomposition of the heat transfer fluid 

(HTF) which is permeating through the stainless steel tube into 

the vacuum annulus. 

In 2005, FPL Energy, the operating company of the SEGS 

III-IX plants requested support from SunLab3

 to conduct a 

survey of the solar fields to try to assess the extent of receivers 

experiencing the hot glass behavior. 

 3

 SunLab is a partnership between Sandia National Laboratories and the 

National Renewable Energy Laboratory. The labs work together as one virtual 

laboratory to support DOE’s research and development efforts in Concentrating 

Solar Power. 

 2
Modeling of Receiver Thermal Losses

A model has been developed by Forristal [6] based on an 

approach developed by Mancini in Dudley et al [4], to predict 

the thermal performance of parabolic trough receivers. The 

model was developed to assess the trade-offs of different 

designs and properties of materials used in the receiver for a 

range of operating and ambient conditions. The model can also 

account for different levels of vacuum and gas compositions in 

the annulus. Figure 2 shows the modeled thermal losses of a 

new Solel Universal Vacuum Receiver (UVAC) based on 

properties reported in [3] with good vacuum at 1x10-4 torr, and 

receivers with different pressures of air, hydrogen and helium 

in the annulus space. The figure shows losses for a one-meter 

length of the receiver with a fluid temperature of 350oC. If the 

receiver loses vacuum and air enters the annulus the pressure in 

the annulus will be atmospheric (approximately 760 torr). If 

hydrogen permeates into the vacuum space through the steel 

tube from the HTF, and the getters or HR are not able to adsorb 

or remove it, then the hydrogen pressure can build up until it is 

at equilibrium with the partial pressure of hydrogen in the HTF. 

The rate at which hydrogen migrates depends on many factors 

including the temperature and properties of the steel tube, and 

the partial pressure of hydrogen in the HTF. Limited data 

currently exists on the partial pressure of hydrogen in the HTF, 

but we believe it is about 1 torr at the plants experiencing the 

hot glass problem. Figure 2 shows that a loss of vacuum in the 

receiver doubles thermal losses, and hydrogen in the annulus 

can cause thermal losses to be almost four times that of a 

receiver with good vacuum. 

Vacuum @

10-4 torr

Hydrogen 

@ 1 torr

Lost Vacuum

Air @ 760 torr

0

100

200

300

400

500

600

700

800

900

1000

0.0001 0.001 0.01 0.1 1 10 100 1000

Annulus Pressure (torr) 

Receiver Thermal Losses (W/m)

Air Hydrogen Helium

Helium

@ 0.004 torr

Assumes: LS-2 collector with new Solel UVAC cermet receiver, 

Therminol VP-1 heat transfer fluid @ 350oC, 140 gpm, DNI 950 

W/m2

, wind speed 1 m/s, 35oC ambient temperature . 

Figure 2. Receiver Thermal Losses 

Experience from evacuated glass tubular solar water 

heating collectors has shown that helium from the atmosphere 

can diffuse through the glass into the vacuum over time. 

Harding [7] found that the time it takes the helium pressure in 

the vacuum to come to equilibrium with the partial pressure of 

helium in the atmosphere (~0.004 torr) depends on the type and 

temperature of the glass. In the water heating case, borosilicate 

evacuated tubes at 100oC took 6 years and tubes at 400oC took 

only 3 months to reach equilibrium pressure. Because parabolic 

trough receivers use borosilicate glass, it is likely that helium 

permeates into the vacuum annulus over time. Figure 2 also 

shows the effect on thermal losses as helium from the 

atmosphere permeates into the vacuum space. The circle on the 

helium line represents the partial pressure of helium in the 

atmosphere. Helium in the vacuum annulus at equilibrium with 

helium in the atmosphere would increase thermal losses by 

about 20% over a receiver with vacuum at 10-4 torr. 

Figure 3 shows how receiver thermal losses vary as a 

function of HTF temperature and receiver annulus condition. It 

can be seen that helium causes a slight increase in thermal 

losses, but not nearly as significant as air or hydrogen in the 

annulus. The receiver model can also be used to determine the 

glass envelope temperature. Figure 4 shows the temperature of 

the glass envelope for the same cases shown in Figure 3. From 

Figure 4, it is clear that glass temperatures are elevated when 

receivers lose vacuum, or if hydrogen builds up in the annulus. 

0

200

400

600

800

1000

1200

0 50 100 150 200 250 300 350 400

Heat Transfer Fluid - Ambient Air Temperature (C)

Receiver Thermal Losses (W/m)

Vac Air @ 760 torr

H2 @ 1 torr He @ 0.004 torr

Figure 3. Receiver Thermal Losses as a Function of Fluid 

Temperature and Annulus Condition 

 3
0

20

40

60

80

100

120

140

160

0 50 100 150 200 250 300 350 400

Heat Transfer Fluid - Ambient Air Temperature (C)

Glass Envelop - Ambient Temp. (C)

Vac Air H2 @ 1 torr He @ 0.004 torr

Figure 4. Receiver Glass Temperature as a Function of 

Fluid Temperature and Annulus Condition 

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Glass Envelop - Ambient Air Temperature (C)

Receiver Thermal Losses (W/m)

0 m/s 1 m/s 2 m/s 4 m/s 8 m/s

 Because the receiver glass envelope is essentially opaque 

to infrared (IR) energy, all the thermal losses from the receiver 

must pass, via conduction, through the glass envelope. As a 

result, the radiant and convective thermal losses from the 

receiver are directly related to the glass envelope temperature, 

and it does not matter what is happening inside the glass tube 

(for example, the type and quality of the selective coating, 

whether the vacuum is good or bad, or the HTF fluid 

temperature). The thermal losses can be correlated directly with 

the glass envelope temperature for known air temperature and 

wind speed. Figure 5 shows receiver thermal losses as a 

function of glass envelope temperature for known wind 

velocities and an air temperature of 35oC. Figure 5 becomes a 

very useful tool for evaluating the relative thermal performance 

of receivers in the solar field. 

One problem with Figure 5 is that the model assumes the 

wind direction is perpendicular to the receiver. In practice, the 

wind velocity near the receiver is not measured, and the 

influence of the wind direction being non-perpendicular to the 

receiver has not been determined. To complicate matters, at the 

existing plants the prevailing wind direction is from the west. 

As a result, the mirrors shelter the receiver from the wind in the 

morning, but the receivers are more exposed to the wind in the 

afternoon when the collectors face into the wind. It is clear 

from the figure that accurate estimates of the wind speed are 

needed to determine receiver thermal losses. 

Receiver Field Survey

The operators of the SEGS plants developed a procedure 

for measuring receiver glass temperature. The procedure 

required collectors to be defocused while measurements were 

taken on each receiver with a hand-held IR temperature 

measurement gun. The collector had to be defocused so that the 

crews were able to get close enough to take a measurement. 

The IR gun reads the temperature of a relatively small spot. 

Several readings were taken on each receiver to obtain a more 

representative average glass temperature. Although the 

operators have a database that is used to track failures and 

repairs in the solar field, the actual receiver type and year of 

manufacture is not tracked directly. Thus, a visual inspection of 

each receiver was required to identify this information. The 

approach allowed a 2-person crew to measure the glass 

temperature on about 200 receivers in a day. 

At FPL Energy’s request, NREL developed a new 

approach for measuring receiver temperatures using a solar 

blind IR camera. A ThermaCAMTM S40 IR camera and 

Researcher software (trademark of FLIR Systems) was used 

[8]. The camera operates in the far IR region (8-14 μm) and is 

not affected by incident solar radiation (wavelengths 0.3-2 μm) 

on the glass envelope. As a result, the glass temperature can be 

measured while the collectors are tracking the sun. 

Figure 6 shows an example of an IR image taken of a 

parabolic trough receiver. From the image, the receiver, 

receiver supports, mirrors, and sky are clearly identifiable. The 

color of the image represents the intensity of the IR signal and 

its temperature. 

Figure 5. Receiver Thermal Losses as a Function of Glass 

Temperature and Wind Speed 

Figure 6. IR Camera Image of Parabolic Trough Receiver 

 4
A custom Excel spreadsheet was developed that links 

directly to the ThermaCAM Researcher software to record IR 

images of the receiver, analyze the receiver image to determine 

its average temperature, and store the image on a laptop 

computer for any subsequent analysis that might be required. 

In addition, NREL developed a new data query methodology 

that used the FPL Energy field maintenance database to 

determine the type and year of manufacture of each receiver in 

the solar field. Detailed maps of receiver types and locations in 

the solar field were developed. During 2005, SunLab 

conducted several field surveys of receiver glass temperatures 

at six of the nine SEGS plant solar fields with the IR camera 

system. In total, the glass temperatures on over 12,000 

receivers were measured using the IR camera technique. When 

the equipment was debugged, it was possible for two people to 

image up to 5000 receivers in a single day. A 30-MWe plant has 

approximately 10,000 receivers. 

The IR camera measures the emitted IR radiation from an 

object. Because the emitted radiation is a function of the 

surface temperature of the object, the temperature of the 

surface can be determined if the emittance of the surface is 

known. The signal measured by an IR camera must also be 

corrected for attenuation by the atmosphere, emission of the 

atmosphere, and reflected emission from ambient sources. The 

camera accounts for both attenuation and emission from the 

atmosphere based on an assumed ambient temperature, relative 

humidity, and distance to the object. Reflected IR energy can 

also be a concern when the measured object is close in 

temperature to its surroundings. In this case, the receiver glass 

temperature that we are attempting to measure is generally 

significantly higher than the ambient and surrounding 

temperatures. It is possible that the reflected energy from the 

receiver supports and bellows shielding could affect the 

measured glass temperatures near the ends of the receivers. 

The ThermaCAM Researcher Analysis Software was used 

to evaluate the IR images to determine the receiver glass 

temperatures. Figure 7 shows a typical example screen from the 

ThermaCAM software used to analyze the temperature of a 

receiver. Field measurements showed that the glass temperature 

varies significantly along both the length and cross-section of a 

single receiver. Therefore, the analysis software was used to 

evaluate ten temperature cross-sections on each receiver. The 

upper left corner of Figure 7 shows the ten cross-sections. The 

bottom half of the figure shows the temperature profile for each 

of the cross-sections. The temperature profiles assume the 

emittance of glass ( 0.85) for the entire image. 

For each cross-section, the maximum temperature is 

determined. This temperature is assumed to be the glass 

Max of 

Cross-section

Maximums

107oC

10 Cross-Sections

Sky Temperature

Mirror Glass Temperature

Temperature Profiles 

of Cross-Sections

Receiver Glass Temperature

Min of

Cross-section

Maximums

93oC

Top of Line

Average of

Cross-section

Maximums

98oC

Max – Min of

Cross-section

Maximum

14oC

Temperature (oC)

Bottom of Line

Max of 

Cross-section

Maximums

107oC

10 Cross-Sections

Sky Temperature

Mirror Glass Temperature

Temperature Profiles 

of Cross-Sections

Receiver Glass Temperature

Min of

Cross-section

Maximums

93oC

Top of Line

Average of

Cross-section

Maximums

98oC

Max – Min of

Cross-section

Maximum

14oC

Temperature (oC)

Bottom of Line

Figure 7. ThermaCAM Researcher Analysis Software 

 5
temperature of the receiver at this cross section. The maximum 

value for each of the ten cross-sections is used to calculate 

average, maximum, and minimum glass temperatures for each 

receiver. The example in Figure 7 shows a calculated average 

glass temperature of 98oC, maximum temperature of 107oC, 

and a minimum temperature of 93oC. It is also useful to 

evaluate the difference between the maximum and minimum 

temperature, in this case the difference is 14oC. 

To minimize the impact on the plant operations and 

performance, one of the goals of the new receiver assessment 

effort was to test the receivers while the collectors were 

tracking. It is important to note that the glass temperature will 

change depending on whether the collector is tracking the sun 

(i.e. concentrated light is focused on the glass and metal 

absorber) or not. This is because the absorber temperature is 

higher when light is focused on the receiver and also because 

the glass absorbs some of the light passing through it. To verify 

that reasonable results could be achieved with the collectors 

tracking, a number of receivers were tested twice. First while 

the collectors were tracking and again when they were not 

tracking. Figure 8 shows the glass envelope temperature for a 

new (~2 years old) Solel UVAC receiver when the HTF 

temperature is about 330oC. The HTF temperature is estimated 

based on the inlet and outlet HTF temperatures for the loop of 

collectors being tested. At design conditions, the HTF 

temperature increases about 0.5oC across each receiver. In this 

case, the glass envelope is about 68oC -71oC when the collector 

is tracking and 63oC -66oC when the collector is not tracking. 

The UVAC shows only a minor temperature variation along the 

length of the receiver, about 3oC. In addition, there was only a 

small increase of about 5oC in glass temperature between the 

non-tracking and tracking measurements. This result is 

consistent with what would be expected based on modeled 

results. 

Visible Image of Receiver – Not Tracking 

Infrared Image – Not Tracking (Glass Temp. 63oC-66oC )

Infrared Image – Tracking (Glass Temp. 68oC-71oC) 

Figure 8 New Solel UVAC Receiver with Getters 

Figure 9 shows an example of one of the original Luz 

cermet receivers at SEGS VI that has been in service for 17 

years. The receiver visually appears to be in good condition, 

and the barium getter indicates good vacuum. The receiver is 

also operating with HTF at approximately 330oC. However, the 

glass temperature is significantly higher than the UVAC 

receiver. This is due in part to the higher emittance of the 

original Luz cermet coating relative to the new Solel UVAC 

selective coating. The temperature variation along the tube is 

also significantly greater, almost 130oC. On inspection, it can 

be seen that many of the older receivers have metallic dust 

from getter decomposition inside the glass tubes. When the 

collector is tracking, concentrated sunlight heats the dust, 

which in turn causes localized heating of the glass envelope. 

The hot spots in Figure 9 are of concern, but are not related to 

the hydrogen problem being investigated. Care must be taken 

to make sure the getter dust problem does not corrupt the 

search for receivers with hydrogen build-up. In general, 

receivers with the hydrogen buildup will have a higher 

minimum temperature and may or may not also suffer from the 

getter dust problem. It was decided to use the minimum crosssection temperatures to evaluate the condition of the receiver 

vacuum. 

Visible Image of Receiver – Not Tracking 

Infrared Image – Not Tracking (Glass Temp. 124oC-141oC )

Infrared Image – Tracking (Glass Temp. 138oC-267oC) 

Getter dust is causing hot spots on the glass

Figure 9. Original Luz Cermet Receiver with HR & Getters 

The glass temperatures measured with the IR camera were 

independently measured on several receivers with a stick on 

Type T thermocouples (accuracy is 1oC or 0.75% of reading, 

whichever is greater) attached to the glass. IR camera 

measurements were within 2oC to 6oC of the thermocouple 

temperature measurement, depending on whether the collectors 

were tracking or not. 

 6
RESULTS 

 A field survey was conducted with the IR camera of the 

southwestern quadrant of the SEGS VI solar field. There are 

three types of receivers installed at SEGS VI. The plant initially 

had Luz black chrome receivers on the cold half of the 

collector loops, and Luz cermet receivers on the hot side of the 

collector loops. The black chrome receivers used getters only. 

The cermet receivers had the HR membranes and some getters. 

The third type of receiver present is a replacement cermet 

receiver. These have getters but no HR membranes. Figure 10 

shows the glass envelope temperature (minus ambient air 

temperature) as a function of HTF temperature (minus ambient 

air temperature) for each receiver surveyed. Receivers that have 

lost vacuum, have coating failures, or have the glass envelope 

broken are not shown. 

0

50

100

150

200

240 260 280 300 320 340 360

Heat Transfer Fluid - Ambient Temperature (oC)

Glass Envelop - Ambient Temp. (oC)

Black Chrome w/ Getters

Cermet w/ Hydrogen Remover

Cermet w/ Getters

Figure 10. SEGS VI Receiver Survey Results 

The SEGS VI testing shows some interesting results. The 

replacement cermet receivers with just getters show distinct 

low- and high-temperature bands. The receivers in the lower 

band represent receivers performing as expected based on 

testing of new receivers at Sandia. The upper band represents 

receivers that show signs indicative of hydrogen build-up in the 

vacuum annulus. The solar field receiver database was used to 

identify the receivers in each band. The receivers in the lower 

band are typically the latest UVAC generation with up to four 

times the getter material, and with the getters mounted off the 

absorber to keep them cool and increase their hydrogen 

absorption capacity. It is interesting to note that the optical 

quality of the replacement receivers in both the upper and 

lower bands is typically excellent, and it is generally not 

possible to tell visually whether a receiver is hot. 

The cermet receivers with the hydrogen remover are 

generally performing as expected even after 17 years of 

service. The glass temperature is generally higher than the best 

new replacement tubes, because the selective coating is not 

quite as good. There is more scatter in the temperature of these 

tubes, probably because variations in how well the hydrogen 

palladium membrane is working and variations in the quality of 

the selective coating. The palladium membranes work best 

when hot, however the O&M company has shaded them to 

prevent failures of the glass envelope. Thus, the membranes 

may not be operating at their optimal temperature. 

The black chrome receivers are generally performing 

worse than other receivers in the solar field. This is likely 

because of the higher thermal emittance of the black chrome 

selective coating and because the receivers use a relatively 

small amount of getters to handle hydrogen permeation. 

DISCUSSION 

The SunLab receiver model was used to evaluate the 

influence of receiver vacuum on receiver thermal losses and 

receiver glass temperature. The model showed that hydrogen 

build-up in the receiver annulus can result in substantial 

increase in receiver glass temperature. In addition, the model 

shows that receiver glass temperature provides a good 

indication of thermal losses from the receiver. Therefore, 

measurement of the envelope glass temperature provides a 

good measure of receiver performance. 

The IR camera methodology presented here provides a 

useful technique for rapid evaluation of parabolic trough 

receivers in an operating solar field. A large number of 

receivers can be evaluated in a short period with the IR camera. 

Wind velocity has a significant effect on glass temperature, so 

it is important to do the measurements when there is very little 

wind. 

The field-testing has helped identify that hydrogen buildup appears to be an issue in some receivers, but not others. The 

initial Luz cermet receivers with HR appear to be performing 

adequately without a noticeable buildup of hydrogen. However, 

a number of replacement receivers without the HR show signs 

of elevated glass temperature that is indicative of hydrogen 

build-up. Receiver vendors are addressing this issue with new 

designs that increase the hydrogen capacity of the getters. 

Schott Glass is also changing the composition of the steel tube 

and using additional hydrogen barrier coatings to reduce 

hydrogen permeation rates into the vacuum to increase the 

receiver lifetime. 

Additional receiver testing is being conducted in controlled 

laboratory experiments on a new receiver thermal loss test 

stand at NREL. The testing will further explore the relationship 

between thermal losses and receiver glass envelope temperature 

as a function of receiver type and gas composition in the 

annulus. A new non-destructive instrument has been developed 

[9] to identify gas composition and pressures in the receiver 

vacuum annulus. Preliminary test results on a number of 

receivers removed from the solar field show that hydrogen is 

present in receivers that exhibit elevated glass temperatures. 

 7
ACKNOWLEDGMENTS 

This work has been conducted with funding from the U.S. 

Department of Energy’s Solar Energy Technologies Program. 

We appreciate the technical and logistical support from FPL 

Energy the operator of the SEGS III-IX parabolic trough solar 

power plants. 

 

REFERENCES 

[1] Kearney, D., Price, H., "Advances in Parabolic Trough 

Solar Power Technology," Advances in Solar Energy. Vol 

16, Kreith, F., Goswami, D.Y. (Eds.), ASES, Boulder, 

Colorado, 2005. 

[2] Labaton, I., Y. Harats, “Hydrogen Pump,” United States 

Patent 4,886.048, Dec. 12, 1989. 

[3] Price, H., M. Hale, R. Mahoney, C. Gummo, R. Fimbres, 

R. Cipriani, “Development in High Temperature Parabolic 

Trough Receiver Technology,” Proceeding of ISEC: Solar 

2004, Portland, Oregon, July 11-14, 2004. 

[4] Dudley, V. G. Kolb, A. R. Mahoney, T Mancini, C 

Matthews, M. Sloan, D. Kearney, “Test Results SEGS LS2 Solar Collector,” SAND94-1884, December 1994. 

[5] Moss, T., D. Brosseau, “Final Test Results for the Schott 

HCE on a LS-2 Collector,” SAN2005-4034, Sandia 

National Laboratory, July 2005. 

[6] Forristall, R., “EES Heat Transfer Model for Solar 

Receiver Performance,” Proceedings of ISEC, Solar 2004, 

Portland, Oregon, July 11-14, 2004. 

[7] Harding, G. L., “Helium Permeation in All-Glass Tubular 

Evacuated Solar Energy Collectors,” Solar Energy 

Materials 5 (1981), pp. 141-147, North-Holland Publishing 

Company. 

[8] FLIR Systems, “ThermaCAMTM Researcher Professional 

Edition,” Version 2.8, web: www.flr.com. 

[9] Latent Structures, “Spectral Evaluation Experiments of 

HCEs,” NREL Subcontract ACO-5-55519-01, Boulder, 

CO, Feb-Mar 2006 

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Field Survey of Parabolic Trough Receiver Thermal Performance: 

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This paper describes a technique that uses an infrared camera to evaluate the in-situ thermal performance of 

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