This document discusses using plane-of-array irradiance data in PV modeling and the System Advisor Model. It finds that using measured POA data directly provides comparable or better results than traditional global horizontal and direct normal irradiance data, with generally lower error. Direct use of POA data may allow irradiance measurement campaigns to balance cost and accuracy. Some models still require beam and diffuse components, and future work could explore differences between pyranometer types and selecting appropriate loss assumptions based on input data.

1. 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. 2
Measuring Irradiance Data is a
Balancing Act…
System Advisor Model
Cost Accuracy

3. 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. 4
Implementation in SAM
Also accessible through the SDK as part of pvsamv1
System Advisor Model

5. 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. 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. 7
Test Systems System Advisor Model

8. 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†
NormalizedRMSEComparedtoMeasuredPVProductionData
System
DNI&GHI HDKR
DNI&GHI Perez
POA as Reference Cell
POA as Pyranometer
*Measured GHI only
†Photodiode pyranometer
‡Single-axis tracking

9. 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†
AnnualError(SAM-Measured)/Measured
System
DNI&GHI HDKR
DNI&GHI Perez
POA as Reference Cell
POA as Pyranometer
*Measured GHI only
†Photodiode pyranometer
‡Single-axis tracking

10. 10
Annual Error- 3 Measured ComponentsSystem 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†
AnnualError(SAM-Measured)/Measured
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. 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. 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

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