This document summarizes three case studies on the vertical potential of bifacial photovoltaic modules: 1) Integrating bifacial modules into a double-skin facade, 2) Using a vertical facade demonstrator, and 3) Analyzing vertical ground-mounted bifacial systems. For the double-skin facade, parameters like coating reflectivity and distance between walls impacted electrical gains up to 17%. The vertical facade demonstrator characterized indoor performance of different module architectures. Ray-tracing then simulated outdoor performance, showing gains of 4-6% for textured glass. Vertical ground-mounted bifacial systems in desert regions can gain over 30% annually compared to monofacial systems due to high ground albedo

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VERTICAL POTENTIAL OF BIFACIAL MODULES
1
2nd workshop on BIFACIAL-PV
26/27 May 2014, Chambéry, France
Ya Brigitte ASSOA - Building energy lab (LCEB)
Bruno SORIA - PV-modules lab (LMPV)
Masakazu ITO - PV-systems lab (LSPV)
Eric GERRITSEN - PV-modules lab (LMPV)

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Vertical potential of bifacial modules
3 case studies :
Double-skin facade integration
BI2PV: bifacial building integrated-PV
Vertical facade demonstrator
methodology to evaluate the annual potential of innovative
bifacial module architectures
optical / electrical characterization & simulation
Vertical ground-mounted bifacial systems
global simulation of annual yield using direct and diffuse radiation
plus land reflected albedo

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3 case studies :
Double-skin facade integration - Brigitte Assoa
Vertical facade demonstrator
Vertical ground-mounted bifacial systems

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ventilation
IAQ/IEQ
air tightness
glazed surfaces
Coupling of PV, thermal &
visual comfort.
Passive heat gain
structures
thermal inertia
BIPV systems
facades & roofs
insulation layers
Thermal exchange
Water transfer
Life cycle analysis (LCA)
INES - Building Energy Lab
rolling shutters
control strategy
solar thermal systems
Hot water systems (CESI/ CESCAI)
Solar combined systems (SSC)
Interseasonal thermal storage
Solar cooling

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integration of bifacial PV-modules
into a double-skin facade
Objectives :
• impact of configuration parameters on thermal and electrical performance.
• impact of solar heated air in the double-skin on building heating demand
Parameters :
• distance between front- and backside of double-skin facade (10 to 80 cm)
• reflectivity (color/roughness) of the inner surfaces
south-oriented double-skin
facade of a PASSYS unit with
two bifacial PV modules
reflective surfaces inside
double-skin facade
test unit / reference unit
(reflective coating / plywood)

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Effect of coating reflectivity on relative electrical gain
(in Wh, relative to plywood reference) :
white 1: R = 85-90% gain = 8%
white 2: R = 70-80% gain = 6%
white 3: R = 50-80% gain = 3%
reference (plywood): R = 20-80% gain = 0
black: R = 5% loss = -9%
gain (bifacial – monofacial) = 17%
Effect of distance between inner and outer wall
on relative electrical gain (compared to plywood reference)
gain increases with distance …..
spectral reflectivity
white 1
white 2
white 3
white 1
white 2
white 3
The choice of reflector material and double-skin thickness depends
on customer requirements for either electrical ( white coating / large gap)
or thermal ( dark coating / narrow gap) energy mix optimisation

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3 case studies :
Double-skin facade integration
Vertical facade demonstrator – Bruno Soria
Vertical ground-mounted bifacial systems

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INDOOR CARACTERIZATION
OUTDOOR VALIDATION
RAY-TRACING SIMULATION: TracePro®
double mirror setup in PASAN solar simulator
vertical façade test bench
& additional configurations
‘ the sky is the limit ’
diffuse/direct
reduced scale vertical façade application
with 2x2 cell modules
BIF vertical
east-west
monofacial
latitude
angle

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1st step: INDOOR CARACTERIZATION
IV parameters for various architectures
resistive losses with
increasing irradiance
upto 2 suns
use half-cells
standard half-cells
gain of half-cells vs. standard :
linear gain with irradiation
due to lower resistive losses
textured glass
Isc gain with textured
glass vs. flat glass:
angular response for 2
orientations

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2nd step: RAY-TRACING SIMULATION
15-22 % bifacial gain
for optimized distance
standard architecture
linear power gain from
indoor characterization
(‘quantity’)
+
irradiance non-uniformity
on front and backside
(‘quality’)
half-cell architecture
non-uniform distribution on back
beneficial impact of half-cell
architecture ?
angular response from
indoor characterization
+
angular distribution
on front and backside
textured glass architecture
example : 20/02/13 at 12h
FRONT BACK
angular distribution of diffuse
irradiance beneficial
impact of textured glass ?
BACKFRONT

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3rd step: OUTDOOR VALIDATION
on several configurations and different seasons
model validated by
outdoor measurements
standard
architecture
gain (half/std) with increasing
irradiance. Additional gain for
non-uniform irradiance
half-cells
large gain for direct flux in
morning & evening.
Low gain for diffuse flux
throughout the day
overall gain: 4-6%
textured glass

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3 case studies :
Double-skin facade integration - Brigitte Assoa
Vertical facade demonstrator
Vertical ground mounted bifacial systems - Masakazu Ito
visiting scientist Tokyo Institute of Technology
now at Waseda University (Japan)

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Irradiation data:
NASA/SSE
Albedo data:
ISLSCP II MODIS
(collection 4)
Example on next slide
GREEN: vertical bifacial oriented East/West performs at least 5% better (in annual kWh) than
monofacial oriented South at latitude tilt angle. YELLOW: at least 5% worse. -5%< GREY <+5%
starting points :
• Bifacial double-glass modules resistant to harsh desert conditions (like N-Africa)
• Vertical installation may reduce the effects of soiling ~ 20% performance gain
But ….is vertical installation compatible with the lower geographical lattitudes of these regions ?
vertical ground mounted bifacial systems
Yes ! when oriented East/West and by exploiting the high local albedo

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example :
worldmap albedo in May

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close-up of ‘BIFACIAL BELT’
green on map below
• gain upto 30% in the bifacial belt for vertical bifacial E/W orientation
• this adds up with the ~20% expected gain from reduced soiling on vertical modules

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gain bifacial @ vertical compared to monofacial @ lattitude angle
GREEN > +5% gain -5% < GREY < +5% gain YELLOW > 5% loss
bifacial back-to-front ratio (BTFR)
critical for vertical bifacial performance
BTFR=100% BTFR=90%
BTFR=80% BTFR=55%

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conclusions & perspectives :
Integrated in double-skin facades bifacial modules can offer a 15-20% performance
gain over monofacial ones
A half-cell module architecture can offer a gain from 3 to 6% in bifacial modules
and allows to reduce sensitivity to non-unform backside irradiance
Linearly textured glass on vertical modules can offer a performance gain of 4 to 6%
Vertical installation in desert areas can take a double-benefit from ground
albedo and reduced soiling losses.
Bifacial building integrated PV (‘BI2PV’) remains to be explored and exploited for its
combination of thermal and electrical energy, daylighting and sunshading
Opto-geometrical modelling by Ray-Tracing useful to optimise the optical
environment of modules for bifacial and BIPV applications.

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vertical..…
WHY ?
30°
northsouth

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EVOLUTION !
north
east
west
south

20. Commissariat à l’énergie atomique et aux énergies alternatives
INES RDI | Savoie Technolac – BP332 – 50 avenue du Lac Léman
73377 Le Bourget-du-Lac
T. +33 (0)4 7979 2804
Email : eric.gerritsen@cea.fr
public establishment with industriel et commercial caracter| RCS Paris B 775 685 019
thank you for your attention !
Division of Technological Research
Solar Technologies Department
Laboarorory for PhotoVoltaic Modules
acknowledgements :
• Paul Lefillastre & the INES module team
• Yannick Veschetti
• Yves Delesse †
• Luc Bernhard (AGC Glass Europe)