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Predicting Yields for Bifacial PV Systems

Dr. Christian Reise

Fraunhofer Institute for Solar Energy Systems ISE

christian.reise@ise.fraunhofer.dewww.ise.fraunhofer.de

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Bifacial PV Modules and SystemsBifacial gain

Some bifacial gain values from literature (mainly experimental / test systems):

24% 15% 22%

22% 57% 20%

bSolar 2014 Sanyo 2009bSolar 2014

ISC Konstanz 2014 EdF R&D 2014 ECN 2014

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Predicting Module YieldsExemplary Results

Some bifacial gain values from our consulting work:

5% 8% 13%

9% –17% 24%80% 80%

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AGENDA

Bifacial Gain: Definitions

Module Properties

System Properties

Exemplary Results

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Bifacial Gain: Definitions

monofacial bifacial

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Bifacial Gain: Definitions

monofacial bifacial

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Bifacial Gain: Definitions

BGOPT

BGMOD

BGSYS

monofacial bifacial

back side eff.

power limit.

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AGENDA

Bifacial Gain: Definitions

Module Properties

System Properties

Exemplary Results

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Module Properties

Main module characteristics:

STC power (of both sides)

Bifaciality (ratio of rear to front efficiency)

Temperature coefficient(s)

Dependency on irradiance level

Dependency on angle of incidence

MPP voltage and current

How to?

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Module PropertiesFront and back STC power

Two approaches for the precise measurement of bifacial modules:

Independent measurements of both sides(with cover on other side of module)

Simultaneous measurements of both sides(with two light sources or with a mirror setup)

Quick measurements within production lines:

Effective irradiance (GE) method(increased irradiance on front side)

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Module PropertiesSimultaneous measurements of both sides

Photo: Eternal Sun

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Module PropertiesSimultaneous measurements of both sides

Photo: Fraunhofer ISE

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Module PropertiesStandardization

Measurement of current-voltage characteristics – IEC 60904-1-2

Currently under discussion

Status is “Final Draft”, formal approval still pending

Marking and documentation requirements – EN 50380

Extension towards bifacial modules is essential ...

PV module performance testing and energy rating – IEC 61853-1 ... -4

Standard isn’t even finished for monofacial modules

Discussion on bifacal modules just started

PV module design qualification and type approval – IEC 61215

Junction box, hot spots, re-testing requirements ...

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AGENDA

Bifacial Gain: Definitions

Module Properties

System Properties

Exemplary Results

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System properties

An example:

Standard roof-top design

Module properties define PV system yield

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System properties

What, if bifacial:

Light may reach the back surface on several paths

Albedo (roof reflectivity) is of big importance

Row distance and height above roof have big influence

Shading of ground becomes important

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System properties

What, if bifacial:

Mounting Structure needs to be adapted

Junction boxes and cables need new places

Yield prediction needs optical modeling

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System propertiesValidation of ray tracing procedure

The prototype:

Bifacial module test installation, monitored by Fraunhofer ISE in 2009

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System propertiesValidation of ray tracing procedure

System model validation results (235 days in 2009):

height tilt angle albedo BG module

Measurement 0,2 m 15 0,64 21.9%

Model 0,2 m 15 0,64 21.1%

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System propertiesLess complex yield estimations

Up to now, I know:

PVsyst

SolarWorld web site

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AGENDA

Bifacial Gain: Definitions

Module Properties

System Properties

Exemplary Results

5% 8%

9% –17%80% 80%

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Exemplary ResultsGround mounted systems

Some bifacial gain values from our consulting work:

5% 8% 13%

9% –17% 24%80% 80%

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Exemplary ResultsGround mounted tracking systems

Example values for tracking systems in desert regions:

type height tilt angle albedo eff. GCR Region BG module

HSAT 2.2 m ±60 0,38 0,40 MENA 8.2%

HSAT 1.3 m ±55 0,38 0,29 MENA 11.0%

HSAT 2.8 m ±60 0,30 0,35 MENA 8.4%

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Exemplary ResultsFlat roof top systems

Example values for a rooftop system in Germany:

type height tilt angle albedo GCR Country BG module

rooftop 0.1 m 20 0,40 0,40 DE 6%

rooftop 0.3 m 20 0,40 0,40 DE 11%

rooftop 0.5 m 20 0,40 0,40 DE 14%

rooftop 0.3 m 20 0,20 0,40 DE 6%

rooftop 0.3 m 20 0,40 0,40 DE 11%

rooftop 0.3 m 20 0,60 0,40 DE 16%

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Summary

Bifacial modules show a big potential for increased yield and / or reduced electricity costs

Bifacial gain is not a module property

Each system layout needs an individual assessment

Small experimental or demonstration systems show bifacial gains of 15% to 25%

With larger commercial systems, realistic bifacial gains are expected in a range from 5% to 15%

Optimization of mounting geometry and mounting structure is essential in order to draw the full benefits from bifacial PV modules