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.

Using POA Irradiance Data

in PV Modeling

5th PV Performance Modeling Workshop

Janine Freeman, David Freestate (EPRI),

William Hobbs (Southern Company), Cameron Riley (EPRI)

May 9, 2016

2

Measuring Irradiance Data is a

Balancing Act… System Advisor Model

Cost Accuracy

3

Why Use Plane of Array Data? System Advisor Model

Photo by Steve Wilcox, NREL 15552

• Cheaper and easier than measuring 2-3 components of irradiance

• Avoids potential errors introduced by transposition models

• Matched POA reference cell data has been shown to correlate better to system performance

4

Implementation in SAM

Also accessible through the SDK as part of pvsamv1

System Advisor Model

5

Adding Complexity…

• Some models still require beam and diffuse components of the POA:

o Shading of all types

o Concentrating PV (diffuse utilization factor)

o “Heat transfer” module temperature model

• “POA Pyranometer” option applies module cover effects to POA-beam

• GTI-DIRINT Model (Marion et al, Solar Energy vol. 122, December 2015)

System Advisor Model

6

What Introduces the Least Error? System Advisor Model

Measured GHI & DNI or

GHI & DHI

Measured DNI & DHI

Measured POA

(pyranometer)

Measured POA

(reference cell)

Measured GHI only

Calculate DHI or DNI

Separation model:

DHI and DNI

GTI-DIRINT model:

DHI and DNI

Transposition model

Transposition model

Transposition model

Module cover effects

Module cover effects

Module cover effects

Module cover effects

Module model

7

Test Systems System Advisor Model

8

RMSE Compared to Measured Data System Advisor Model

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

1.00E-04

EPRI1* EPRI2*‡ EPRI5*† EPRI3† EPRI4† SC1 SC2†‡ SC3†

No

rma

lize

d R

MS

E C

om

pa

red

to

Mea

su

red

PV

Pro

du

cti

on

Data

System

DNI&GHI HDKR

DNI&GHI Perez

POA as Reference Cell

POA as Pyranometer

*Measured GHI only

†Photodiode pyranometer

‡Single-axis tracking

9

Annual Error System Advisor Model

-15.0%

-10.0%

-5.0%

0.0%

5.0%

10.0%

15.0%

EPRI1* EPRI2*‡ EPRI5*† EPRI3† EPRI4† SC1 SC2†‡ SC3†

An

nu

al

Err

or

(SA

M-

Me

as

ure

d)

/ M

ea

su

red

System

DNI&GHI HDKR

DNI&GHI Perez

POA as Reference Cell

POA as Pyranometer

*Measured GHI only

†Photodiode pyranometer

‡Single-axis tracking

10

Annual Error- 3 Measured Components System Advisor Model

-12.0%

-10.0%

-8.0%

-6.0%

-4.0%

-2.0%

0.0%

2.0%

4.0%

6.0%

EPRI3† EPRI4† SC1 SC2†‡ SC3†

An

nu

al

Err

or

(SA

M-

Me

as

ure

d)

/ M

ea

su

red

System

DNI&DHI HDKR

DNI&DHI Perez

DNI&GHI HDKR

DNI&GHI Perez

GHI&DHI HDKR

GHI&DHI Perez

POA as Reference Cell

POA as Pyranometer

†Photodiode pyranometer

‡Single-axis tracking

11

Conclusions

• Initial comparison shows comparable results using POA as using traditional irradiance inputs

• As expected, RMSE is generally lower using POA options

• Direct use of POA data in PV modeling may allow irradiance measurement campaigns to achieve balance between cost and accuracy

System Advisor Model

12

Future Considerations

• Why does pyranometer option sometimes have higher RMSE for pyranometers than reference cell option?

• Is there a difference in how photodiode versus thermopile POA pyranometers should be treated?

• How do you select the “correct” loss assumptions for a system, and does it depend on your input data stream?

System Advisor Model

Thank you! Questions? Full report forthcoming at IEEE PVSC 2016.