pvsysts new framework to simulate bifacial systems

PVSYST SA - Route du Bois-de-Bay 107 - 1242 Satigny - Suissewww.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 WittmerBruno.Wittmer@pvsyst .com

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

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 irradianceBack side irradiance

xbifaciality factor

Effective irradiance

IV-Curve

Standard Irradiance Calculation

• DirectSubject to near shadings depending on sun position

• DiffuseSubject to shading factor that is constant for a given plane orientationCalculation of angles that shade the diffuse

• AlbedoSubject to shading factor that is constant for a given plane orientationCalculation 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

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

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 modulesdepend on the position

Bifacial modules in PV installations

• Vertical mountingDiffuse 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

• ShedsLight can reach the ground between the sheds and scatter back to the module backside.

Bifacial Modules in Sheds

PVsyst Model to determine bifacial parameters for regular shed configurations

Simplified 2D calculationRows without boundary effects (infinitely long)Parameters:• Tilt, Azimuth• Width, Pitch• Height above ground• Ground AlbedoThe factors for the bifacial calculation can be determined by integrating over the distance between rowsDirect and sky diffuse are only computed for front side.Near ground scattering is only computed for backside.

Calculation proceeds in three steps1. Ground Acceptance of

direct light2. Ground acceptance of

diffuse light3. Backside acceptance of

ground (form factor)

Calculation of Direct Light on GroundGround 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

tan𝛼𝑙𝑖𝑚 =sin 𝜃

𝑃𝑊 − cos 𝜃

aprof

Profile angle

Azimuth a

Profile angle aprof

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

Calculation of Diffuse Irradiance on GroundGround 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

Calculation of Form FactorBackside 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.

Calculation of Total Irradiance on BacksidePutting 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

+

Irradianceon ground

Absolute irradiance Normalized to horizontal

Absolute irradiance Normalized to Front

For this location and geometry, the additional bifacial gain isobtained mainly in summer

Example shed installationBasic PV system with sheds:

90 kWp in 6 rows of 3 x 20 modules landscapeLocation Geneva, Switzerland: 46.3° N, 6.1° E25° Tilt, 6m Pitch, 3m WidthMounted 1m over groundPV surface: 600 m2

Ground surface: 33m x 33m = 1000 m2

3D shading scene

Definition of bifacial shed model

Simulation ResultsPVsyst Report showing results for bifacial modelling

Global incident on groundGround scatteringBackside form factorShadings on backsideBifaciality factorMismatch for back irradiance

Height over groundHeight 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

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 ContributionOn Rear Side

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

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

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

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 backStatistical 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

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

pvsysts new framework to simulate bifacial systems

Technology