8th PVPMC Workshop, May 9-10 2017

1. Progress Toward Efficient
Bifacial Rear Irradiance Models
Sara MacAlpine
Chris Deline, Bill Marion
8th PV Performance Modeling and Monitoring Workshop
May 10, 2017

2. 2
• Importance of rear
side irradiance for
bifacial modules
• Discussion of rear
side irradiance
models
• Validation of efficient
models under
development at NREL
• Model sensitivity and
key considerations
Outline

3. 3
Bifacial Energy Gain =
Module Bifaciality * Rear Side Irradiance Ratio
𝐵𝑖𝑓𝑎𝑐𝑖𝑎𝑙𝑖𝑡𝑦 =
𝑃 𝑚𝑝,𝑟𝑒𝑎𝑟
𝑃 𝑚𝑝,𝑓𝑟𝑜𝑛𝑡
(from single side flash data)
𝑅𝑒𝑎𝑟 𝑆𝑖𝑑𝑒 𝐼𝑟𝑟𝑎𝑑𝑖𝑎𝑛𝑐𝑒 𝑅𝑎𝑡𝑖𝑜
= 𝑓(𝑎𝑙𝑏𝑒𝑑𝑜, 𝑡𝑖𝑙𝑡, 𝑟𝑜𝑤 𝑠𝑝𝑎𝑐𝑖𝑛𝑔, ℎ𝑒𝑖𝑔ℎ𝑡, 𝑟𝑎𝑐𝑘𝑖𝑛𝑔, 𝑚𝑜𝑑𝑢𝑙𝑒 𝑡𝑟𝑎𝑛𝑠𝑝𝑎𝑟𝑒𝑛𝑐𝑦, 𝑐𝑙𝑖𝑚𝑎𝑡𝑒)
Bifacial Gain Calculation
Month
Performance%

4. 4
• PVSyst – 2D “unlimited sheds” model for back side
irradiance, module types have individual bifaciality
factors
• Solar World “Boost Calculator” – web interface for
user input, tailored to specific modules
• Various manufacturers of bifacial PV modules have in
house tools
PV modelers need an efficient model for rear side
irradiance that can be used for any bifacial module!
Industry Bifacial Models

5. 5
• NREL developing and validating two efficient rear
side irradiance models
• Ray Tracing model (Radiance)
o Fast run time *if using preprocessing*
o Suitable for standalone calculator
o User can create complex scenes if necessary
• 2D View Factor model
o Very fast run time and low memory requirement
o Suitable for SAM or standalone calculator
o Use for systems with regular row layouts
NREL Rear Side Irradiance Models

6. 6
Radiance GenCumulativeSky
Robinson, Stone “Irradiation modelling made simple: the cumulative sky approach” 2004
• Radiance simulations typically use Perez model to
generate hourly sky description
• Runtimes = hours for annual simulations
• GenCumulativeSky bins annual
irradiance into 145 sky patches
o Runtime reduced 1000x
completing annual simulation
in seconds.

7. 7
• GenCumulativeSky benchmarked in literature with
2.2% RMS error relative to Perez workflow
• Unclear what this means for rear side irradiance ratio
(validation underway)
Radiance GenCumulativeSky, cont.
Spatial resolution is reduced for single timepoint
Gendaylit GenCumulativeSky

8. 8
• Ground divided into n segments in row-to-row
direction and shading determined for each
• Irradiance on each ground segment found using view
of the sky (configuration factors)
• Rear side irradiance is sum of sky, ground reflected,
object reflected components
• Runtime 4 seconds for annual simulation
NREL View Factor Model

9. 9
Irradiance Validation: NREL Rooftop System
• Array of monofacial modules with
regular row spacing
• Irradiance measurements taken in
center of the upper array
• Challenges include racking, tilted roof,
nearby objects
• Front and rear side measurements
taken in plane of array using reference
cells
• Rooftop albedo measured, varies with
soiling

10. 10
Irradiance Validation: NREL Rooftop System, cont.
Measured Rear Side
Irradiance Ratio
17.4%
17.7% 16.4%
NREL VF NREL Raytrace
Predicted Rear Side
Irradiance Ratio

11. 11
Irradiance Validation: NREL Rooftop System, cont.
Measured Rear Side
Irradiance Ratio
14.2%
14.1% 14.7%
NREL VF NREL Raytrace
Predicted Rear Side
Irradiance Ratio

12. 12
Performance Validation: SolarWorld System
Solar World
Bifacial/Monofacial Test
Bed in VA
• Monitored for 7 months
• 10 degree tilt, 0.15m
height, 1.5m row spacing
• Grey/white roof 0.62
albedo
• Model uses TMY data
(source of uncertainty)
Measured Bifacial
Performance Gain
8.2%
8.1% 7.0% 7.9%
Solar
World
NREL
VF
NREL
Raytrace
Predicted Gains
Credit: http://www.solarworld-usa.com

13. 13
Rear Side Irradiance Sensitivity
Varying Parameters for NREL and SolarWorld Models
(“error” bars are just the range of irradiance on module rear sides)

14. 14
• Rear side irradiance nonuniform, especially for
modules close to high albedo ground cover.
Recommend middle points for resource estimation
• Albedo can vary seasonally and has large effect on
rear side irradiance (linear)
Key Considerations

15. 15
• Both Raytrace and View Factor models at NREL
undergoing continued validation
• View Factor model will be presented at PVSC:
"A Practical Irradiance Model for Bifacial PV Modules"
is in the oral session entitled "Modeling of System
Components" which will be on Wednesday, June 28 at
10:30 AM.
• View Factor model will ultimately be embedded in
NREL SAM tool
• Raytrace model will be deployed as standalone tool
Future Plans

16. Thank you!
sara.macalpine@nrel.gov
chris.deline@nrel.gov
bill.marion@nrel.gov