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

Test & Evaluation

Matthew Muller

Sept 22-23, 2010

PV Performance 

Modeling Workshop

Albuquerque, NM

NREL/PR-520-49505

Measuring and Modeling Nominal Operating Cell 

Temperature (NOCT)
Outline

• Overview of NOCT (IEC 61215)

• Motivation for NOCT research

• Basic Heat transfer model in relation to NOCT

• Hypothesis and Testing at NREL

• Results for single module over 8 months

• Side by Side comparison of 3 modules with 

substantially different reported NOCT values

• Problems with the NOCT procedure

• Unanswered questions

• Conclusions and continuing work

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NOCT (800 W/m2,20C, 1m/s)

(IEC 61215 primary method)

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• Mount module on an open rack at a 45 degree tilt (Voc)

• Border test module with at least 0.6 m of black aluminum plating or similar modules

• At a 5 s intervals measure the following:

Irradiance, Ambient Temp, Cell Temp, Wind Speed, Wind Direction

• Record data before and after solar noon covering at least a 300 W/m2 range

• Reject the following data:

10 minute intervals after the irradiance varies more than 10% in10 minutes 

10 minutes after the wind speed > 4m/s 

Wind speeds outside 0.25-1.75 m/s

Ambient temperature outside 5-35C 

Wind direction within +/-20 ̊ of E or W

Irradiance<400 W/m2,

Data sets in which the ambient temperature varies more than 5C • For a single day with data meeting the above requirements, plot the modules temperature rise above ambient 

temperature as a function of irradiance

• Use linear regression to fit the data plotted. Use the regression equation to determine the module 

temperature rise above ambient at 800 W/m2.

• Add 20C to determine module temperature at reporting conditions.

• Apply correction factor based on data set average wind speed and ambient temp

• Complete the above procedure for 3 days and then average all three to report NOCT
Example of a Single Day NOCT calculation

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y = 0.0174x + 12.355

R2 = 0.7182

15

20

25

30

35

Tmodule

500 600 700 800 900 1000

-Tambient

Irradiance W/m2

Acceptable NOCT data

Average Ambient = 7.8C

Average Wind Speed = 1.08 m/s

Correction Factor = (-1 or 0)

NOCT1day=0.0174*800+12.355 + 20 - (1or 0)

Regression equation 

20C ambient reporting condition

NOCT1day= 46.3 or 45.3
Motivation for NOCT research 

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• Participate in NOCT round robin testing to understand seasonal and other variation 

in the NOCT procedure.

• The California energy commission reports NOCT values for rack-mounted standard 

silicon modules in a glass/Tedlar package that range from 41.6C to 52.3C (these are 

test results from 3rd party labs)

• A 10.7C variation coupled with a power coefficient of 0.5%/C suggests up to a 5% 

improvement in power output based on module selection.

• If modules are in open circuit and have the same basic package of materials, heat 

transfer theory indicates they should reach the same steady state temperature.

• If heat transfer theory is correct, this NOCT research has the potential drive changes 

in how NOCT is measured or replace measurements with an analytical approach to 

determining NOCT
Basic NOCT Heat Transfer Model

(steady state, no power produced)

Tm

Tsky

Tg

qRad,ground

qRad,sky

qSun

qconvection

qconvection

Tamb

Assumptions for a standard glass front, plastic back silicon PV module

αs=0.92 (module absorptivity), Gs= (global irradiance on module) εg=0.84 (glass emissivity)

σ = 5.67e-8 (Stefan-Boltzmann constant), εb=0.893 (back of module emissivity)

Tm, Tsky, Tamb, and Tg are module, sky, ambient, and ground temperatures respectively

Wavg = 5 minute average wind speed,

(1.2*Wavg+4.8) data fit for convection heat transfer, applies to Wavg ranging from 0-4 m/s
Heat Transfer Model Against NOCT data

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295

300

305

310

315

320

325

330

335

7:12 9:36 12:00 14:24 16:48

Tcell,Kelvin

Time

3/16/2010

Tmeasure Tpredicted

305

310

315

320

325

330

335

340

7:12 9:36 12:00 14:24 16:48

Tcell, Kelvin

Time

9/17/2010

Tmeasured Tpredicted

Model inputs

•Tsky (site downwelling IR)

•Tg (site upwelling IR)

•Tamb

•Wind Speed (5 min avg)

•POA irradiance

•Data gaps occur when NOCT 

conditions are NOT achieved 

•NOCT conditions are an 

approximation of steady state
Model Used to indicate NOCT variation

Modeled Cell Temperatures under possible NOCT measurement conditions

Tsky, C

Tground, 

C Tamb, C

Wind 

(m/s)

Cell Temp, 

C 

Tcell-Tamb, 

C Correction

NOCT, 

C

baseline -5 20 20 1 47 27 0 47

cold sky -43 20 20 1 42.5 22.5 0 42.5

hot sky 12 20 20 1 49.8 29.8 0 49.8

cold day -5 5 5 1 36.7 31.7 -1 50.7

hot day -5 45 35 1 59.7 24.7 2 46.7

low wind -5 20 20 0.25 49.1 29.1 -2 47.1

high wind -5 20 20 1.75 45.2 25.2 1 46.2

NREL 

winter -25 5 5 1 33.9 28.9 -1 47.9

NREL 

summer 10 45 35 1 61.9 26.9 2 48.9

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Hypothesis and Testing at NREL

• Based on the basic heat transfer model, the following is hypothesized:

– 10C variation from NOCT procedure based on varying sky/ground/ambient temperature

– Modules with a similar package should have NOCTs within 2C of each other 

• Varying module absorptivity by 5%, heat transfer suggests a 1.5C cell temp change

• Varying glass emissivity by 5%, heat transfer suggests a 0.5 C cell temp change

• A testbed was established at NREL to measure NOCT following the IEC procedure 

for a single module across all 4 seasons

• Data has been gathered and analyzed from January 2010 to the present.

• In July 2010 two additional glass/silicon/plastic modules were mounted in a side by 

side configuration with the original module

• Previously reported NOCT values for the three modules are 42.4C, 47.9C, and 52.3C

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8 Month Results for Single Module

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NOCT TEST DATA -OPEN CIRCUIT, January -September 2010

Date 1/15 3/12 3/16 3/17 6/15 6/17 6/25 7/26 8/10 9/17

Time(start/end)

9:00-

15:35

9:18-

16:09

9:29-

16:09

9:11-

14:07

10:19-

16:04

9:09-

13:54

7:58-

12:36

11:06-

15:17

8:12-

16:03

11:08-

16:02

# data points 898 275 680 121 199 274 287 105 250 322

Ambient Temp 

C, average 7.8 11.8 14.9 16.9 24.4 26.3 31.1 33.6 27.9 26.1

Wind Speed 

m/s, average 1.08 1.07 1.04 1.17 1.01 0.94 0.96 1.02 1.16 1.15

Irradiance range 

W/m2

517-

1037

494-

1135

489-

1168

806-

1106

448-

1024

730-

1032

468-

1012

629-

966

508-

902

428-

1092

Irradiance 

W/m2, average 775 727 1009 933 840 876 646 707 646 760

NOCT correction 

factor, C -1 0 0 0 0 0 1 2 1 1

NOCT reported 45.3 46.9 46.6 45.3 45.9 45.9 45.4 49.1 49.8 47.9

Note see* see* see* see**

* Temperature measurements on the module backskin, assumed cell is 1.3 C hotter at NOCT

** This day only had 5 data points before solar noon
Comments on 8 month Results

• Lowest 3 day average is 45.33 C NOCT

• Highest 3 day average is 48.9 C NOCT

• This is within +/-4C uncertainty associated with the NOCT procedure

• Only 10 days over 8 months suggest procedure is too limited

• Several good days were unusable because the temperature increased more 

than 5C over the needed irradiance range

• 2 potential days were lost due to measurement problems

• 2 days had to be thrown out due to snow on the ground. Module 

temperature was elevated by 2 degrees due to irradiance to the back side

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NREL Test Results for 3 Modules Reported to Have 

NOCT Values of 42.4C, 47.9C, and 52.3C

IEC 61215 Side-by-Side Testing

7/26 8/10 9/17 3 day Avg

Module 1 NOCT 49.1 49.8 47.9 48.9 C

Module 2 NOCT 49.1 49.7 47.1 48.6 C

Module 3 NOCT 48.5 50.2 47 48.6 C

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Note that Module 1 is that reported on for 8 months
Problems with NOCT Procedure

•10C range of NOCT values for modules that show identical NOCT values in 

side-by-side testing

•Procedure reported to have +/-4 C uncertainty

•For many locations restricted conditions are difficult to achieve

(In Golden, CO 10 NOCT days over 8 months) 

•Neglects the heat transfer parameters of sky and ground temperature

•Does not represent temperature of modules under load

•Provides no information about how temperature varies with wind, irradiance, etc

•Test reports from more than one lab indicate procedure is not being followed in all

aspects. (An argument to simplify)
Unanswered Questions

• Will the NREL site eventually show the full 10C range of NOCT for the 3 

modules under test?

• How are Labs measuring cell temperature with glass on glass modules?

• How much would NOCT vary if the procedure is changed so that the 

module is under load?

• Changes are being suggested to the IEC procedure. What will the data 

gathered at NREL show if applied to this alternative procedure?

• Are labs accurately following the current NOCT procedure? Examination 

of test reports for 2 of the three modules showed deviations from 61215.

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Conclusions and Continuing Work

• The IEC 61215 Procedure does not guarantee repeatable results

• 8 months of NREL data result in NOCT values ranging from 45.3 to 48.9C

• A Steady state heat transfer model supports that a 10C NOCT variation 

can result from changing sky, ground, and ambient temperatures

• 3 modules with previously reported NOCT values of 42.4C, 47.9C, and 

52.3C show identical NOCT values in side-by-side testing

• Future work with include examination of suggested changes to the IEC 

procedure and gathering of data over continued months at NREL

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