Predicting Yields for Bifacial PV Systems Dr. Christian Reise Fraunhofer Institute for Solar Energy Systems ISE christian.reise@ise.fraunhofer.de www.ise.fraunhofer.de
Bifacial PV Modules and Systems Bifacial gain Some bifacial gain values from literature (mainly experimental / test systems): 24% 15% 22% bsolar 2014 bsolar 2014 Sanyo 2009 22% 57% 20% ISC Konstanz 2014 EdF R&D 2014 ECN 2014 2
Predicting Module Yields Exemplary Results Some bifacial gain values from our consulting work: 5% 8% 13% 80% 80% 9% 17% 24% 3
AGENDA Bifacial Gain: Definitions Module Properties System Properties Exemplary Results 4
Bifacial Gain: Definitions monofacial bifacial 5
Bifacial Gain: Definitions monofacial bifacial 6
Bifacial Gain: Definitions monofacial bifacial BG OPT back side eff. BG MOD power limit. BG SYS 7
AGENDA Bifacial Gain: Definitions Module Properties System Properties Exemplary Results 8
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? 9
Module Properties Front 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 (G E ) method (increased irradiance on front side) 10
Module Properties Simultaneous measurements of both sides Photo: Eternal Sun 11
Module Properties Simultaneous measurements of both sides Photo: Fraunhofer ISE 12
Module Properties Standardization 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... 13
AGENDA Bifacial Gain: Definitions Module Properties System Properties Exemplary Results 14
System properties An example: Standard roof-top design Module properties define PV system yield 15
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 16
System properties What, if bifacial: Mounting Structure needs to be adapted Junction boxes and cables need new places Yield prediction needs optical modeling 17
System properties Validation of ray tracing procedure The prototype: Bifacial module test installation, monitored by Fraunhofer ISE in 2009 18
System properties Validation 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% 19
System properties Less complex yield estimations Up to now, I know: PVsyst SolarWorld web site 20
AGENDA Bifacial Gain: Definitions Module Properties System Properties Exemplary Results 5% 8% 80% 80% 9% 17% 21
Exemplary Results Ground mounted systems Some bifacial gain values from our consulting work: 5% 8% 13% 80% 80% 9% 17% 24% 22
Exemplary Results Ground 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% 23
Exemplary Results Flat 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% 24
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 25