NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Measuring the Optical 

Performance of Evacuated 

Receivers via an Outdoor 

Thermal Transient Test

Preprint

Charles Kutscher, Frank Burkholder, 

and Judy Netter

To be presented at SolarPACES 2011

Granada, Spain

September 20-23, 2011

Conference Paper

NREL/CP-5500-52466

August 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1 

MEASURING THE OPTICAL PERFORMANCE OF

EVACUATED RECEIVERS VIA AN 

OUTDOOR THERMAL TRANSIENT TEST

Charles Kutscher1, Frank Burkholder2

Abstract

, Judy Netter3

Evacuated receivers used with parabolic trough collectors can be characterized in terms of their heat loss and their 

optical efficiency. The latter is the fraction of sunlight striking the receiver tube that is absorbed. It is equal to the 

product of the transmittance of the glass cover (τ) and the absorptance of the absorber surface (α). This paper 

describes an outdoor transient test method for measuring the τα product. The receiver is filled with cold water and 

exposed to outdoor solar radiation. The slope of the temperature vs. time curve is taken symmetric about the point at 

which the average glass temperature is equal to the average absorber temperature (i.e., the point at which there is no 

heat loss or gain from the absorber tube), and this is used to determine the τα. This method has the advantage of 

using the actual solar spectrum and has an uncertainty of ±2%. Preliminary measurements of an actual receiver tube 

yield a τα that is reasonably close to the manufacturer’s specifications.

1. Introduction

Modern parabolic trough solar collectors operate with high fluid temperatures to provide the heat input to Rankine 

steam power cycles. High performance is achieved via the use of a receiver tube with a selective surface having a 

high absorptance for incoming short-wave solar radiation and a low emittance for outgoing long-wave infrared 

radiation, as well as an evacuated glass envelope around the receiver tube to essentially eliminate convective and 

conductive heat losses. NREL [1], DLR, and receiver manufacturers measure heat loss in the laboratory by 

supplying electric resistance heat and measuring the heat input at steady-state conditions. NREL measures the 

overall optical efficiency of a trough collector on an outdoor test stand [2]. The optical efficiency of an evacuated 

receiver is the product of the transmittance of the cylindrical glass envelope and the short-wave absorptivity of the 

receiver coating. Techniques are available for measuring these quantities separately for an unassembled receiver, but 

this is not easily done for the final assembled product. DLR measures the optical efficiency of an assembled and 

evacuated receiver using a steady-state method in an indoor solar simulator [3]. This paper describes a new method 

that determines overall receiver optical efficiency by exposing a fluid-filled, pre-cooled receiver to one sun outdoors 

and measuring the slope of the temperature curve at the point where the average absorber temperature is equal to the 

average glass envelope temperature (that is, the point at which there is no heat gain or loss from the absorber). The 

use of an aperture or masking device allows testing with either total solar irradiance or direct normal irradiance. This 

transient test method offers the potential advantages of simplicity, high accuracy, and the use of the actual solar 

spectrum.

2. Methodology

The optical efficiency of a receiver can be represented as τα, the product of the transmittance of the glass envelope, 

τ, and the short-wave solar absorptivity of the receiver coating, α. To conduct a transient heat transfer test on a 

receiver with the goal of measuring τα, the receiver tube is first filled with cold water at a temperature below that of 

the ambient air. The receiver is then exposed to one sun (either total solar irradiance or direct normal irradiance). By 

 

1 Principal Engineer, Ph.D., National Renewable Energy Laboratory, MS 5202, 1617 Cole Blvd., Golden, CO 80401 

USA (303) 384-7521 chuck.kutscher@nrel.gov 2 Engineer III, Ph.D., National Renewable Energy Laboratory, MS 5202, 1617 Cole Blvd., Golden, CO 80401 USA 3 Engineer III, National Renewable Energy Laboratory, MS 3313, 1617 Cole Blvd., Golden, CO 80401 USA
2 

orienting the receiver tube along a North-South line and at an angle to the horizontal corresponding to the zenith 

angle at solar noon, tests performed around noon result in the sun striking normal to the collector, although any 

cosine effect can be accounted for in the incident radiation calculation. The water is circulated during the test to 

maintain a uniform temperature. 

A number of temperature sensors (e.g., thermocouples) are positioned at various locations to determine the average 

temperature of the mass as a function of time and assess its uniformity. The better the uniformity, the higher the 

confidence that the measured average temperature represents the bulk temperature. The value of τα can be 

experimentally determined as follows: 

τα = MC⋅

ΔT

Δt

⋅ 1

IA

where M = the mass of the receiver tube and its contents (kg) 

C = the average (mass-weighted) heat capacity of the receiver tube and its contents (J/kg-°C) 

ΔT = the change in mass temperature over a small time period centered around the point at which the 

average absorber temperature is equal to the average glass temperature (°C) 

Δt = the time period over which the slope is determined (sec) 

I = the solar radiation to which the receiver is exposed, either total or direct normal (W/m2

) 

A = the area of the receiver tube exposed to the radiation (m2

) 

The receiver glass temperature is higher than ambient temperature because it absorbs some sunlight. When the 

absorber temperature is at the average glass temperature, there is no heat loss or gain from the absorber tube and 

equation 1 holds. By determining the slope centered on the point at which the average absorber temperature equals 

the average glass temperature and inserting it into equation 1, the τα product can be determined. An example of a 

plot generated from a simple model is shown in Figure 1. Note that the absorber temperature leads the fluid 

temperature but that the slopes of the two curves are virtually identical. Note also that the plots are fairly linear, but 

are slightly concave downward due to the fact that the receiver loses more heat to the ambient as its temperature 

goes up. To obtain the best data, one wants a high value or IA (that is, high irradiance and a large aperture), so that 

as one deviates from the zero heat transfer point, the heat exchange term is small compared to the solar input term. 

This results in high linearity, and hence latitude in where the slope is measured (as there is some uncertainty in 

determining the average glass and absorber temperatures and hence the zero point location). One also desires that 

the radiation be steady on either side of the zero heat transfer point so that the slope measured over a finite time 

interval symmetric about the zero point will closely match that at the zero point. 

Figure 1. Example temperature plot for a transient experiment showing absorber and fluid temperatures vs. time. 

11 11.25 11.5 11.75 12 12.25 12.5 12.75 13

15

20

25

30

35

40

45

Time (h)

Temperature (°C)

Absorber temperature

Fluid temperature

Avg. glass temperature

τα = 1

hf = 100 W/m2

-°C

Fluid: water

ΔΤ

Δt

(1) 
3 

An uncertainty analysis indicates that the various parameters can be determined with the following uncertainties:

Table 1. Uncertainties in the parameters

Combining the various uncertainties (see Section 5) results in an overall uncertainty in τα of ±0.02. The error can be 

further reduced if a standard receiver is available.

3. Apparatus

The test stand (see Fig. 2) is designed to simultaneously measure the performance of two test receivers side-by-side 

with a standard reference receiver. The τα of each test receiver can then simply be determined by multiplying the τα

of the standard receiver by the ratio of the temperature vs. time slope for the test receiver to that of the standard 

receiver. This improves the overall accuracy (assuming τα for the reference receiver is known to high accuracy) 

because the irradiance (which has the largest uncertainty of the measurements) is the same for the reference and test 

receivers and so need not be measured. Where a standard receiver is not available, equation 1 is solved. Each 

receiver under test is surrounded by a shroud to prevent radiation reflected off the ground from striking the receiver 

and to allow only direct normal irradiance (DNI) to strike the absorber tube. Our current shroud uses a black channel 

in the front through with the DNI passes. Because some solar radiation can reflect off that channel and because the 

channel can get warm and emit infrared radiation, we intend in the future to replace the channel in the shroud with a 

simple slit.

Figure 2. a. Schematic of test rig showing 3 receivers. b. Photo of actual shrouded commercial receiver under test.

(Photo: C. Kutscher)

A pump is used to circulate water inside the absorber tube to achieve uniform temperature. The pump at the top of 

each receiver tube pumps water down a small feeder tube inside the steel absorber tube and the water returns in the 

annular region between the feeder tube and the absorber tube. Thermocouples are used to determine the 

temperatures of the water and absorber tube at various locations as shown in Figure 3.
4 

Figure 3. Locations of thermocouples inside receiver tube.

To obtain an accurate slope corresponding to the zero heat transfer point, it is important to determine the average 

glass and absorber temperatures. For the data taken to date, we measured the front of the glass and assumed that the 

back of the glass is at ambient temperature (due to the low circumferential heat conductance around the thin glass). 

The average glass temperature is taken as the average of the front glass temperature and ambient. Because we have 

found that the τα result is more sensitive to the exact temperature at which that slope is centered than originally 

expected, we intend to improve the experimental apparatus by installing more glass temperature measurements to 

obtain a better average glass temperature. We also plan to improve our measurement of average absorber 

temperature.

4. Results and Discussion

The test apparatus is located at NREL’s mesa top test facility in Golden, Colorado. Because of unusually rainy 

weather in the summer of 2011, there were few long periods of steady DNI. However, test data taken on July 28 

with less-than-ideal solar radiation are reported here. A commercial evacuated receiver tube was exposed for 200 

minutes to an average of 928 W/m2 of direct normal insolation. The variation of the DNI can be seen in Figure 4.

This shows that there were some downward spikes in the radiation as clouds passed overhead.

Figure 4. DNI and average water during the test period. 

A plot of water temperature vs. time for the various temperature sensors throughout the water is shown in Fig. 5.

Three general observations can be made. First, there is very good temperature agreement at all of these locations. 

Second, and more importantly, the slopes of the temperature vs. time curve are in close agreement at all locations. 

The curves appear fairly linear, although the slope is still somewhat sensitive to the specific time where it is 

measured.

860

870

880

890

900

910

920

930

940

950

960

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200

Beam radiation (W/m2)

Average water temp. (°C)

time (min.)

Average Water Temp.

Beam radiation
5 

Figure 5. Time-temperature profiles at 12 different locations in the water.

In general, small variations in irradiance can put jiggles in the temperature curve, and a better slope value can be 

determined if we smooth the curve using a running average. Figure 6 shows plots of average water temperature vs. 

time for running averages ranging from 10 seconds to 15 minutes for data taken with an aperture mask. These curves 

all have roughly the same average slope. The longer running averages are shifted to the right, reflecting the fact that 

it takes some time to accumulate the running average. In choosing a slope we want to pick two points symmetrically 

on either side of the point at which the average absorber temperature equals the average glass temperature. If we use 

the top curve, the small jiggles in that curve mean that the slope is sensitive to the locations of the points chosen. On 

the other hand, if we use the much smoother lower curve, the slope is less sensitive to the selection of end points. 

Figure 6. Average water temperature vs. time curves for different running averages. 

To calculate τα from equation 1, one needs not only the slope of the temperature curve at the zero heat transfer point

(as well as the radiation and receiver aperture area through which that radiation passes) but also the MC product. In 

calculating the MC product, one must take into account all the thermal mass being heated. Table 1 shows the 

thermal capacitance values of the various components undergoing transient heating.

0

10

20

30

40

50

0 50 100 150 200

Water temperature (°C)

time (min.)

B-side

B-top

B-side

B-bottom

M-side

M-top

M-bottom

M-side

T-side

T-top

T-side

T-bottom

20

25

30

35

40

45

75 125 175

Average water temperature (°C)

time (min.)

10 sec.

5 min.

15 min.

Running 

average:
6 

Table 2. Thermal capacitance values.

This shows that the thermal capacitance is dominated by the recirculating water in the receiver tube, which 

represents 87.8% of the thermal capacitance. When this is combined with the thermal capacitance of the absorber 

pipe, these two components account for 98.1% of the total thermal capacitance. As the thermal capacitance (mass 

times specific heat) of these two quantities is well known, there is high confidence in the total thermal capacitance 

value. 

With the values of MC and the integrated value of the DNI during the relevant portion of the test, we can now pick 

the slope from 15-minute running average curve of temperature. With the average glass temperature estimated as 

32°C, the average water temperature reaches this value at minute 125. (For these initial measurements, we did not 

have an adequate measurement of absorber temperature and so used the water temperature, but this will be rectified 

in the future.) Table 2 shows calculations for τα using slopes obtained from six different time intervals of varying 

length, but all centered on the time of 125 minutes. Also shown is the standard deviation for each as well as twice 

the standard deviation, which would correspond to the 95% confidence value.

Table 3. Results for τα using test time intervals of different lengths.

The results indicate a value of 0.95 ±0.027. Whereas the result is not very sensitive to the length of the time interval, 

it is more sensitive to the location of the center of that interval. We can examine this by evaluating the impact of the 

glass temperature being 5°C lower or 5°C higher than assumed. If instead of 32°C, the average glass temperature 

were 27°C, the steeper slope at that part of the curve would yield a τα value of 1.004. If the average glass 

temperature were 37°C, a τα value of 0.894 would result. The manufacturer of the receiver tube quotes a 

transmittance value of 0.97 and an aborptance value of 0.96. The product of these is 0.931. While our measured 

value of 0.95 is within 0.02 of the manufacturer’s value, it is quite dependent on the assumed average glass 

temperature. It is important to have accurate measurements of both the glass and absorber temperatures to determine 

the zero heat transfer point. This is especially important if the radiation is not steady, as was the case with the data 

reported here.
7 

5. Experimental Uncertainty

The major sources of experimental uncertainty are in the measurement of the solar radiation and the temperature 

rise. The experiment is located immediately adjacent to NREL’s Solar Radiation Research Laboratory (SRRL). 

SRRL estimates a combined random and systematic error for these measurements of ±1.5%. As the direct normal 

irradiance is on the order of 1000 W/m2 and the diffuse radiation is less than 10 percent of that, the error in diffuse 

radiation is less than one-tenth that for the direct normal. The accuracy of the measurement of the temperature rise is 

very high because the same thermocouples are used at the two points in time. Also, the temperatures at the two 

points in time are very close, differing by only about 15 degrees Celsius, so one expects very little difference in 

thermocouple behavior. Also, we average the results from many different locations. Thus there should be virtually 

no systematic error in the temperature rise measurement. Random error is minimized by taking a large number of 

samples and is estimated at only 0.1C. When all of the errors are combined using a root-sum-square method, the 

total error in τα is estimated at ±0.015.

An alternative way to evaluate this is to apply all the various uncertainties and vary the values using a Monte Carlo 

approach. This was done using Matlab, and the results (based on the 32°C assumed glass temperature) are shown in 

Figure 7. Using the 2σ (95% confidence) result, this shows a value of 0.95±0.02, which is in close agreement with 

the other approach.

Figure 7. Monte Carlo uncertainty analysis, in which the various parameters in equation 1 are varied in Gaussian 

distributions according to their individual uncertainties and τα is calculated in each case.

It is important to note, however, that neither calculation properly takes into account the uncertainty in measuring the 

glass and absorber temperatures in order to accurately locate the zero heat transfer point, and this needs to be 

explored further.

6. Conclusions

As new low-cost receivers are developed, it is important to understand not just the heat loss performance but also the 

optical efficiency, i.e., the product of the glass transmittance, τ, and the absorber absorptance, α. The transient 

thermal test method described in this paper offers a way to measure the τα product of a receiver with potentially 

high accuracy. The temperature rise of the receiver during a transient test can be measured with higher accuracy than 

the outlet minus inlet temperature difference in a steady-state test. Also, outdoor solar radiation can be measured 

with high accuracy and an outdoor test provides results that correspond to the true wavelength spectrum that the 

receiver will operate under in actual practice.
8 

7. Future Work

Because results have shown that the slope calculation (and hence the value of τα) is sensitive to the location of the 

zero heat transfer point, especially when the irradiance is not steady, we intend to analyze this effect using a heat 

transfer model. We will also install additional instrumentation to obtain a better average glass temperature and 

absorber temperature to allow us to more accurately determine when these temperatures are equal. The mass of 

water was obtained from a volumetric measurement. We intend to explore more accurate mass measurements of the 

other components. We will improve the shroud to minimize any heat transfer of reflected radiation between the 

shroud and the receiver while maintaining a large value of IA, which minimizes the impact of any heat exchange.

We will evaluate the repeatability from additional tests and we also plan to test other tubes, in particular those that 

have been tested by other laboratories. 

Acknowledgments

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the 

National Renewable Energy Laboratory. We would like to thank Mike Martinez and Victor Castillo for their 

assistance in building the experimental apparatus.

References

[1] Burkholder, F., and Kutscher, C. “Heat Loss Testing of Schott’s 2008 PTR70 Parabolic Trough Receiver,” 

National Renewable Energy Laboratory Technical Report, NREL/TP-550-45633, 2009.

[2] Stynes, Kathleen, Keith Gawlik, Charles Kutscher, and Frank Burkholder, Optical Efficiency Measurements of 

the SkyTroughTM Solar Collector, National Renewable Energy Laboratory, April 2010.

[3] QUARZ Test and Qualification Center for Concentrating Solar Power Technologies, DLR.