25 pvpmc workshop(u iowa)

May 15, 2017 8th PV Performance Modeling and Monitoring Workshop
Ray Tracing Models for Bifacial PV
Performance
Amir Asgharzadeh Shishavan, Fatima Toor
Electrical and Computer Engineering Department
University of Iowa
May 10, 2017

Agenda
 Analysis of the impact of installation parameters and system
size on bifacial gain and energy yield of PV systems
 RADIANCE PV modeling
 Combined effect of tilt angle, height, and albedo
 Effect of the system size
 Simulation of Sandia’s Fixed-Tilt String-Level Arrays
 Results and analysis
 Conclusions and next steps
2

RADIANCE PV Modeling
 We modeled the PV systems using RADIANCE, which is a simulation software to
compute the radiance profile of physical systems by ray-tracing methods
 This study approximates the Perez direct and diffuse model [1]
 In our model, we utilized the dimensions and electrical characteristics of Prism Solar’s
Bi60-368BSTC bifacial module (front and backside efficiencies of 17.4% and 15.6%,
respectively, which is equivalent to a bifaciality of ~90%)
 TMY3 weather (hourly) data for Albuquerque, NM was used to acquire global
horizontal irradiance (GHI), diffuse horizontal irradiance (DHI) and direct normal
irradiance (DNI)
 Azimuth and zenith angles (also hourly data) were derived from PV_LIB Toolbox
3
[1] R. Perez, R. Seals, and J. Michalsky, "All-weather model for sky luminance distribution - preliminary configuration and validation," Solar Energy, vol. 50, pp.
235-245, Mar 1993.

Single moduleA row consisting of
five modules
Five rows, each with five modules
4
 Three south-facing PV system configurations were considered

 Installation parameters
 h: Height (0.1m to 3m)
 θ: Tilt angle (10º to 90º)
 α: Albedo (21% , 43% , 81%)
 We ran hourly simulations sweeping parameters mentioned above around
three important dates of the year: summer solstice, winter solstice and fall
equinox
θ
h α
5

Agenda
6

Combined Effect of Tilt angle, Height and
Albedo – Energy yield
7
Summer Solstice

8
Fall Equinox

9
Winter Solstice

Albedo – Bifacial Gain
10
Summer Solstice
𝐵𝐺𝐸 ≡
𝐸 𝑏
𝐸 𝑚
− 1
(𝐸 𝑏 and 𝐸 𝑚 are energy yield for bifacial
and monofacial cases respectively)

11
Fall Equinox
𝐵𝐺𝐸 ≡
𝐸 𝑏
𝐸 𝑚
− 1

12
Winter Solstice
𝐵𝐺𝐸 ≡
𝐸 𝑏
𝐸 𝑚
− 1

Agenda
13

Single moduleA row consisting of
five modules
Five rows, each with five modules
14
 Three south-facing PV system configurations were considered

Effect of the System Size
 Energy yield with optimum tilt angle, height of 1.5 m and albedo of 21%
 Increase in the system size results in decrease in the performance
 Middle module in multi-row bifacial array generates about 7% less energy comparing
with single module on summer solstice, 6 % on fall equinox and 5 % on winter
solstice.
15

Agenda
16

Sandia’s Fixed-Tilt String-Level Arrays
 Four rows at 15˚, 25˚, 35˚, and 45˚ tilt
 Each row has two strings of 8 modules (one monofacial and one bifacial)
 Two types of bifacial modules are used :
 Prism Solar
 Sunpreme
17

RADIANCE Simulation
 Each row has 2 reference cells (top and bottom)
for backside and one reference cell for front side
 We simulated both front side and backside
irradiance using RADIANCE modeling
 Simulated data was compared to field
measurements for a partially cloudy day
(September 25, 2016)
18
Top reference cell
Bottom reference cell

Simulated Scene with RADIANCE
S15 S25
S35 S45
19

Simulated Scene with RADIANCE -top view
244 cm
191 cm
305 cm
S15
S25
S35
S45
20

RMSD and NRMSD
 Calculated RMSD (Root mean square deviation) and Normalized RMSD
(NRMSD) for backside simulations to compare the simulated data with
measured data
NRMSD =
𝑅𝑀𝑆𝐷
𝑥1,𝑚𝑎𝑥 − 𝑥1,𝑚𝑖𝑛
𝑥1 ∶ 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑛𝑐𝑒
𝑥2 ∶ 𝑠𝑖𝑚𝑢𝑙𝑎𝑡𝑒𝑑 𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑛𝑐𝑒
n : number of data
21

Front side Irradiance
Measured
Ref. cell simulation
 Simulated and measured data match very well (NRMSD~2.5%, RMSD~25 𝑊/𝑚2)
22

First row (Tilt = 15˚) – Backside
 Simulation data has NRMSD value of 4.6% and 11.6% for top and bottom ref. cells
respectively
 The highest non-uniformity among the rows
23
Measured-top
Measured-bottom

Second row (Tilt=25˚) - Backside
respectively
 RMSD (NRMSD) values for the second row is lower than for the first row, which
shows it has better match with the measured data
24
Measured-top
Measured-bottom

Third row (Tilt=35˚) - Backside
respectively
25
Measured-top
Measured-bottom

Fourth row (Tilt=45˚) - Backside
respectively
 The least non-uniformity among the rows
26
Measured-top
Measured-bottom

Agenda
27

Conclusions and Next Steps
 Installation parameters (tilt angle, height and albedo) impact the energy yield
and bifacial gain of bifacial PV systems in a complex manner
 System size impacts the performance of the system (Energy yield of a large
array with ground albedo of 21% can decrease up to 7% comparing to a
single module)
 Simulation of Sandia’s fixed-tilt string-level arrays shows a good match
between the simulated data and field measurements
 The next steps for the project include compare simulation and measurement
data for different sky and weather conditions and consider the impact of
shading on bifacial gain as a function of the time of the day
28

May 15, 2017 8th PV Performance Modeling and Monitoring Workshop 29
Thanks for your attention
Any Questions?

25 pvpmc workshop(u iowa)

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