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 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Technical Report NREL/TP-1900-72633 October 2018

NREL Comparison Between Absolute Cavity Pyrgeometers and Pyrgeometers Traceable to World Infrared Standard Group and the InfraRed Integrating Sphere Ibrahim Reda,1 Mike Dooraghi,1 Afshin Andreas,1 Julian Gröbner,2 and Christian Thomann2 1 National Renewable Energy Laboratory 2 Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC)

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 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory 15013 Denver West Parkway Golden, CO 80401 303-275-3000 • www.nrel.gov

Technical Report NREL/TP-1900-72633 October 2018

NREL Comparison Between Absolute Cavity Pyrgeometers and Pyrgeometers Traceable to World Infrared Standard Group and the InfraRed Integrating Sphere Ibrahim Reda,1 Mike Dooraghi,1 Afshin Andreas,1 Julian Gröbner,2 and Christian Thomann2 1 National Renewable Energy Laboratory 2 Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC)

Suggested Citation Ibrahim, Mike Dooraghi, Afshin Andreas, Julian Gröbner, and Christian Thomann. 2018. Comparison Between Absolute Cavity Pyrgeometers and Pyrgeometers Traceable to World Infrared Standard Group and the InfraRed Integrating Sphere. Golden, CO: National Renewable Energy Laboratory. NREL/TP-1900-72633. https://www.nrel.gov/docs/fy19osti/72633.pdf.

NOTICE

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy’s, Solar Energy Technologies Program and the Atmospheric Radiation Measurement (ARM) Research Facility. The views expressed herein do not necessarily represent the views of the DOE or the U.S. Government.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

U.S. Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.

Cover Photos by Dennis Schroeder: (clockwise, left to right) NREL 51934, NREL 45897, NREL 42160, NREL 45891, NREL 48097, NREL 46526.

NREL prints on paper that contains recycled content.

iv This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Acknowledgments We sincerely appreciate the support of Solar Radiance Research Laboratory (SRRL) staff and National Renewable Energy Laboratory (NREL) management, the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy/Solar Energy Technologies Program, Environmental Research/Atmospheric Radiation Measurement Program, and NREL’s Environment, Safety, Health, & Quality center (ESH&Q).

v This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Table of Contents 1 Introduction ........................................................................................................................................... 1 2 Instrument List ...................................................................................................................................... 2 3 Measurement Equations ...................................................................................................................... 3 4 Results ................................................................................................................................................... 4

vi This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

List of Figures Figure 1. ACPs’ irradiance ........................................................................................................................... 4 Figure 2. Average ACPs’ irradiance minus the irradiance measured by each ACP ..................................... 5 Figure 3. Average ACPs’ irradiance vs irradiance measured by PIR and KZ .............................................. 6 Figure 4. Average of ACPs’ irradiance minus irradiance measured by PIR and KZ vs PWV ..................... 7

List of Tables Table 1. Average ACPs’ irradiance minus the irradiance measured by each ACP....................................... 5 Table 2. Average ACPs’ irradiance minus the irradiance measured by the PIR and KZ ............................. 7

1 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

1 Introduction The comparison of the absolute cavity pyrgeometers (ACPs) with a precision infrared radiometer (PIR) pyrgeometer traceable to the World Infrared Standard Group (WISG) and a Kipp & Zonen (KZ) pyrgeometer traceable to the InfraRed Integrating Sphere Radiometer (IRIS) was held during the National Renewable Energy Laboratory (NREL) Pyrheliometer Comparison (NPC-2018) from September 24 to October 5, 2018. Data from all instruments was collected during nighttime clear sky conditions only. The irradiance measured by the ACPs and the PIR pyrgeometer traceable to the WISG were collected every 10 seconds during the ACPs’ self-calibration and every 30 seconds during the measurement runs. The measurement runs lasted for two hours, while the calibration runs lasted for 6 minutes. The KZ pyrgeometer collected irradiance measurements every 30 seconds. During the comparison, the average (av) irradiance difference measured by three ACPs varied from 0.2 W/m2 to -0.5 W/m2, with a standard deviation (sd) from 0.5 W/m2 to 0.8 W/m2. The average irradiance difference measured by the three ACPs minus the irradiance measured by the PIR was 4.3 W/m2 and sd 4.2 W/m2, and the average irradiance difference measured by the KZ was 2.3 W/m2 and sd 3.5 W/m2.

2 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

2 Instrument List • Absolute Cavity Pyrgeometer (ACP): 57F3, 95F3, and 96F3. • PIR pyrgeometer: 31197F3 • KZ pyrgeometer: CGR4 110390

3 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

3 Measurement Equations

ACP

𝑊𝑊 =𝐾𝐾1 ∗ 𝑉𝑉𝑡𝑡𝑡𝑡 + (2−∈) ∗ 𝐾𝐾2 ∗ 𝑊𝑊𝑟𝑟 − (1+∈) ∗ 𝑊𝑊𝑐𝑐

𝜏𝜏

Where,

• W is the atmospheric longwave irradiance (W.m-2). • K1 is the reciprocal of the ACP’s responsivity (W.m-2.uV-1). • Vtp is the thermopile output voltage (uV). • ϵ is the gold emittance. • K2 is the emittance of the black receiver surface. • Wr is the receiver irradiance (W.m-2). • Wc is the concentrator irradiance (W.m-2). • τ is the ACP’s throughput.

PIR 𝑊𝑊 = 𝐾𝐾0 + 𝐾𝐾1 ∗ 𝑉𝑉𝑡𝑡𝑡𝑡 + 𝐾𝐾2 ∗ 𝑊𝑊𝑟𝑟 + 𝐾𝐾3 ∗ (𝑊𝑊𝑑𝑑 −𝑊𝑊𝑟𝑟)

Where,

• K0, K1 , K2 , and K3 are the calibration coefficients. • Wd is the dome irradiance, in W/m2.

KZ

𝑊𝑊 =𝑉𝑉𝑡𝑡𝑡𝑡𝐶𝐶 + 𝐾𝐾2 ∗ 𝑊𝑊𝑐𝑐

Where C and K2 are the calibration coefficients and Wc is the case irradiance.

4 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

4 Results Figure 1 shows the irradiance of ACPs 57F3, 95F3, and 96F3; and Figure 2 shows the difference between the three ACPs’ average irradiance and the irradiance measured by each ACP. Table 1 shows that the difference varied from 1.0 W/m2 to 1.7 W/m2, with a 95% confidence level.

Figure 3 shows the ACPs’ average irradiance vs that of the PIR and KZ, as well as the PIR output thermopile voltage (Vtp). The Vtp is correlated to the net longwave irradiance, i.e., Vtp approaches zero when the sky gets cloudier. As is illustrated in the figure, the average irradiance of the ACPs is larger than that of the pyrgeometers when Vtp is less than -300 µV (i.e., during clear sky conditions), and smaller than irradiance measured by the pyrgeometers when Vtp is more than -300 µV (i.e. thin clouds). This behavior might be correlated to the spectral response of the pyrgeometers and the estimated precipitable water vapor (PWV), yet more comparisons and data might help in resolving this issue.

Figure 4 shows the estimated PWV vs the difference between the ACPs’ average irradiance and the irradiance measured by the PIR pyrgeometer with traceability to WISG and the KZ pyrgeometer with traceability to the IRIS. As is shown in Table 2, the difference is 9.3 W/m2 for the PIR and 7.3 W/m2 for the KZ, with a 95% confidence level.

Figure 1. ACPs’ irradiance

260265270275280285290295300305310

19:5

721

:02

21:4

922

:43

23:3

00:

171:

121:

592:

463:

414:

285:

2318

:43

19:3

020

:17

18:5

719

:44

23:4

40:

311:

292:

163:

17

W/m

2

Mountain Standard Time (MST)

ACPs' Irradiance from September 24 to 28, 2018

Win 95F3 Win57F3 Win 96F3

5 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 2. Average ACPs’ irradiance minus the irradiance measured by each ACP

Table 1. Average ACPs’ irradiance minus the irradiance measured by each ACP

WavACPs - W(95F3) WavACPs - W(57F3) WavACPs - W(96F3)

av 0.3 0.2 -0.5

sd 0.6 0.5 0.8

U95 1.2 1.0 1.7

nrdg 1754 1754 1754

-3

-2

-1

0

1

2

3

20:3

121

:11

21:5

122

:32

23:1

923

:59

0:39

1:26

2:06

2:46

3:34

4:14

4:54

5:43

18:5

519

:35

20:1

50:

121:

031:

432:

233:

17

W/m

2

MST

ACP Average Irradiance Minus Each ACP Irradiance from September 24 to 28, 2018

Wav-W95F3 Wav-W57F3 Wav-W96F3

6 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 3. Average ACPs’ irradiance vs irradiance measured by PIR and KZ

-500-450-400-350-300-250-200-150-100-500

230.000235.000240.000245.000250.000255.000260.000265.000270.000275.000280.000285.000290.000295.000300.000305.000

20:3

1:31

21:2

3:31

22:1

5:31

23:1

5:16

0:07

:16

1:06

:50

1:58

:50

2:50

:50

3:50

:30

4:42

:30

5:43

:22

19:0

7:39

19:5

9:39

0:08

:37

1:11

:23

2:03

:23

2:55

:23

µV

W/m

2

MST

ACPs' Average Irradiance vs PIR and KZ Irradiance from September 24 to 28, 2018

Wav ACPs W(PIR) W(KZ) Vtp-PIR

7 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 4. Average of ACPs’ irradiance minus irradiance measured by PIR and KZ vs PWV

Table 2. Average ACPs’ irradiance minus the irradiance measured by the PIR and KZ

WavACPs - W(PIR) WavACPs - W(KZ)

av 4.3 2.3

sd 4.2 3.5

U95 9.3 7.3

nrdg 1754 1754

0

2

4

6

8

10

12

-8-6-4-202468

10

20:3

121

:20

22:0

923

:06

23:5

50:

451:

402:

293:

264:

155:

0418

:34

19:2

320

:12

0:18

1:18

2:07

3:10

mm

W/m

2

MST

ACPs' Average Irradiance Minus PIR and KZ Irradiance vs PWV from September 24 to 28, 2018

Wav-W(PIR) Wav-W(KZ) PWV (mm)