Plantpredict: Solar Performance Modeling Made Simple

1. PVSYST SA - Route du Bois-de-Bay 107 - 1242 Satigny - Suisse
www.pvsyst.com
Any reproduction or copy of the course support, even partial, is forbidden without a written authorization of the author.
PVsysts new framework to simulate bifacial systems
PVPMC Workshop
24-26.10.2016 Freiburg, Germany
André Mermoud, Bruno Wittmer
Bruno.Wittmer@pvsyst .com

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Overview
• Introduction
– Bifacial PV modules
– Modelling the backside irradiance
• PVsyst Approach do model bifacial installations
– Model describing shed installations
– Calculation of backside irradiation
– Some first qualitative results
• Summary and Outlook

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Approach for Bifacial Modules in PVsyst
Treatment of bifacial modules
• Front side irradiance is added to
backside irradiance x bifaciality factor (default is 0.8)
• From this Effective irradiance follows the IV-curve.
• Loss factor describing shadings of mounting structure
and junction boxes
• An additional mismatch factor is foreseen to account
for inhomogeneous rear side illumination
• This approach is an approximation
The main challenge is to calculate the additional
backside irradiance including its inhomogeneity
Front side irradiance
Back side irradiance
x
bifaciality factor
Effective irradiance
IV-Curve

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Standard Irradiance Calculation
• Direct
Subject to near shadings depending on sun position
• Diffuse
Subject to shading factor that is constant for a given plane orientation
Calculation of angles that shade the diffuse
• Albedo
Subject to shading factor that is constant for a given plane orientation
Calculation of azimuth angles that are blocked
Irradiance on PV modules has 3 components
Shadings of the direct irradiance Shadings of the diffuse irradiance
Azim 0° Azim -20° Azim -40°
He 10°
He 20°
. . .
...
Diffuse shading factor is an integral
over all hemisphere directions

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Albedo and Near Ground Scattering
• Albedo (of transposition model)
Reflections and scattering from ground that is far away.
Obstacles will shade all the albedo for a given azimuth.
• Near Ground Scattering (for bifacial simulations)
Light scattered back from ground that is close to the PV modules.
Subject to near shadings with solid angle calculation.
Albedo is not the same as Near Ground Scattering
Bifacial PV installations can not be described by a modified albedo only..
Albedo scattering
Albedo visible
Albedo shaded
Near ground scattering
Ground scattering is
only partially shaded

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Systems with Bifacial Modules in PVsyst
Basic approach for bifacial modelling
• Fraction of direct irradiance that reaches the
scattering ground (depends on sun position)
• Fraction of diffuse irradiance that reaches the
scattering ground (single factor)
• Factor describing the scattering off the ground
(Ground Albedo)
• Factor for backside acceptance of scattering
ground (form factor)
• (Constant loss factor describing shadings of
mounting structures, cabling and junction boxes)
• Only light scattered back from the ground contributes to backside illumination.
• Direct and sky diffuse irradiance contribute to ground illumination
• Sky diffuse is isotropic
• Only scattering is considered (no specular reflections)
• The diffuse reflection is isotropic (Lambertian Surface)
• Non-homogeneous illumination of backside is neglected at this stage
Assumptions for bifacial calculation
Sketch explaining bifacial model
Direct Irradiance
Diffuse Irradiance
Ground Scattering
Bifacial Module
Incoming light, and light scattered back to modules
depend on the position

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Bifacial modules in PV installations
• Vertical mounting
Diffuse and direct light can reach the PV
module on both sides.
Bifacial modules are used in different situations
Sketch showing vertical bifacial systems
In a first step, PVsyst will model bifacial systems only for shed geometries.
Sketch showing shed bifacial systems
• Sheds
Light can reach the ground between
the sheds and scatter back to the
module backside.

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Bifacial Modules in Sheds
PVsyst Model to determine bifacial parameters for regular shed configurations
Simplified 2D calculation
Rows without boundary effects (infinitely long)
Parameters:
• Tilt, Azimuth
• Width, Pitch
• Height above ground
• Ground Albedo
The factors for the bifacial calculation can be
determined by integrating over the distance between
rows
Direct and sky diffuse are only computed for front side.
Near ground scattering is only computed for backside.
Calculation proceeds in three steps
1. Ground Acceptance of
direct light
2. Ground acceptance of
diffuse light
3. Backside acceptance of
ground (form factor)

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Calculation of Direct Light on Ground
Ground Acceptance of direct light
1. Profile Angle and Limit Angle
determine the amount of directly
illuminated ground surface
2. Height over ground and profile angle
determine the position of the
illuminated strips.
Limit Angle
P
q alim
tan 𝛼𝑙𝑖𝑚 =
sin 𝜃
𝑃
𝑊
− cos 𝜃
aprof
Profile angle
Azimuth a
Profile angle aprof

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Examples of Direct Light Ground Acceptance
Ground Acceptance of direct light
Ground irradiance over one day
Momentary Ground irradiance
Geneva, 21. June, 19:00h
Geneva, 21. June, 12:00h 21. June
Illuminated ground is composed of homogeneously illuminated strips

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Calculation of Diffuse Irradiance on Ground
Ground acceptance of diffuse light
- Diffuse irradiance from sky is isotropic.
- Ground acceptance of diffuse light is a
function of the position on the ground.
- Underneath the sheds the irradiance is
smaller.
- Inhomogeneity tends to level out with
increasing mounting height.
sheds at ground level 1m over ground 2m over ground

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Calculation of Form Factor
Backside acceptance of ground
(Form Factor)
Ground scattering is isotropic
(Lambertian Scattering)
sheds at ground level 1m over ground 2m over ground
- Ground scattering is isotropic
(Lambertian Scattering)
- Form Factor is a function of the position on
the ground.
- Underneath the sheds the form factor is
large.
- Inhomogeneity tends to level out with
increasing mounting height.

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Calculation of Total Irradiance on Backside
Putting it all together
Direct Diffuse
Irradiance on Ground is specific
for location and geometry.
In this case (Geneva):
- Almost no direct in winter
- Fraction of diffuse on ground
is constant over the year
Irradiance
on Backside:
Combine Ground
acceptance with
Form Factor
+
Irradiance
on ground
Absolute irradiance Normalized to horizontal
Absolute irradiance Normalized to Front
For this location and geometry,
the additional bifacial gain is
obtained mainly in summer

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Example shed installation
Basic PV system with sheds:
90 kWp in 6 rows of 3 x 20 modules landscape
Location Geneva, Switzerland: 46.3° N, 6.1° E
25° Tilt, 6m Pitch, 3m Width
Mounted 1m over ground
PV surface: 600 m2
Ground surface: 33m x 33m = 1000 m2
3D shading scene
Definition of bifacial shed model

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Simulation Results
PVsyst Report showing results for bifacial modelling
Global incident on ground
Ground scattering
Backside form factor
Shadings on backside
Bifaciality factor
Mismatch for back irradiance

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Height over ground
Height over ground
With higher mounting, the opposite behavior of
ground illumination and acceptance gets
attenuated.
For the shed model with no boundaries
(infinitely long sheds), the increase saturates
(ground will appear homogeneously illuminated)
sheds at ground level
1m over ground
2m over ground
Diffuse on Ground Form Factor Diffuse Contribution
to rear side

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Impact on best Tilt
Impact on best tilt
Best tilt does not change significantly for this specific
case.
Maximum in bifacial curve is slightly flatter.
Bifacial definitions:
- 2 m over ground
- 80% bifaciality factor
- GCR: 50%
- Ground albedo factor: 0.3
Bifacial Contribution
On Rear Side

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Impact of pitch on bifacial gain
Gain increase as function of pitch
Increase of pitch (row spacing) reduces
mutual shadings and thus increasing the
yield.
Ground in between rows also gets more
irradiance, leading to an increased yield
gain for bifacial systems.
4%
3%
Normalized to
monofacial, pitch 5m

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Best tilt as function of Pitch
Complete optimization has to consider Tilt together with Pitch
Optimization tool to study yield as function of tilt and pitch
Monofacial Bifacial

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Ground reflection (Albedo factor)
Bifacial Gain as function of Albedo Factor
For common Albedo factor ranges and the considered
90 kWp shed installation, the simulation predicts
4-10% bifacial gain.
Ground Type Albedo
factor
Worn Asphalt 0.12
Bare Soil 0.17
Green Grass 0.25
Desert sand 0.40
New concrete 0.55
Fresh snow 0.8-0.9

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Next Steps for Bifacial Framework
Bifacial treatment of any PVsyst 3D scene
Define a ground surface shape.
Compute Global irradiance on ground (direct and Diffuse).
Calculate view factors for all PV backsides.
Limitations:
- No specular reflections
- Only the ground surface scatters back
Statistical approach with a random distribution of ground points
Add further specific bifacial models
Model for vertically mounted modules
- Both module sides need to be treated equally
- Irradiance of Direct, Diffuse, Albedo and Ground Scattering computed for
both faces

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Summary and Outlook
– The challenge in bifacial calculations for PV systems is to determine the irradiance on the
module back side
– The module behavior under a given total effective irradiance is similar to a standard PV
module
– Simulation of Bifacial PV systems with shed (row) layout will be possible with PVsyst
V6.60
– Several approximations are made to handle the calculation
• Only light scattered off the ground reaches the module back side
• Ground reflection is diffuse and isotropic
• Shadings of the mounting structure on the backside are accounted with a constant derate factor
• An additional factor accounts for inhomogeneous illumination
– Main contributions of backside illumination are captured
– The approach will be generalized to allow the bifacial calculation for any 3D shading scene
– The final goal is to treat front and back illumination in the same way.
– Validation with measured data is still necessary