Photovoltaic Glass Technologies Thin Glass for PV Modules: Mechanical Reliability Considerations Dr. James Webb Research Associate May 28, 2009
Agenda Introduction to thin glass Glass strength Reliability requirements Wind and snow load modeling Impact resistance testing Summary 2
Glasses come in many thicknesses; How thick is thin? Window & Architecture 4 mm 4 mm Automotive Windshield 3 mm Display Applications 2 mm Proposed range for thin-film PV panels 1 mm Glass thickness 3
Glass configuration for thin film PV applications Typical PV module cross section Cross section with thin glass 3.2 mm Soda-lime glass Thin film & encapsulant Soda-lime glass <1.5 mm Thin specialty glass Thin film & encapsulant Soda-lime glass 3.2 mm 4
Why thin specialty glass for PV? Enables higher conversion efficiency Higher transmission Lowers manufacturing and BOS costs Shorter heating times Shorter cooling times Reduced weight Lower transportation costs Lower installation labor costs Thin specialty glass adds value 5
Thin glass is stronger than most people think Glass strength characteristics Statistical Determined by surface imperfections from processing, handling, and use Fatigues over time when exposed to environmental and mechanical stresses Different glasses have different fatigue resistances Quantified by fatigue exponent (large exponent is better) High quality glass without imperfections can be 30X stronger than high strength steel 6
Effect of statistics and lifetime on glass strength Failure Probability log(strength) 99.5 95 80 60 40 20 10 5 2 1 25 50 75 100 125 S STRENGTH (MPa) design 1 n = S test t t ftest fdesign 1 n Allowable Stress = [ F af x F fp x F sd ] σ o F fp = [ ln { 1/ (R) } ] 1/m Probability Factor F af = [ A spec / A prod ] 1/m Scale Factor F sd = [ 1/ ( τ ) ] 1/n Fatigue Factor n = fatigue exponent τ = stress duration R = reliability A = area σ 0 = tested strength m = Weibull modulus log(time to failure) 7
Not all glasses are created equal; Glass composition affects long term strength Glass Fatigue Exponent Relative Strength (30 Year Life) Soda-lime (window glass) ~15 1.0 Corning PV glass ~20-23 1.4 1.6 TiO 2 doped Silica (mirrors) ~33 2.0 Fused Silica ~45 2.4 8
Thin glass must meet PV module qualification requirements 25 to 30 years - no glass breakage Wind, rain, hail, snow, blowing sand IEC 61646 Qualification Wind load test Uniform 2400 Pa pressure to both sides Total 6 hour duration Heavy snow load test (optional) Uniform 5400 Pa pressure Hail Impact choice of hail sizes and velocities 25.0 mm iceball at 23 m/s 9
We have modeled several mounting configurations 2 Edge Rails Framed Edges 2 Rails 3 Rails Simple constrained in Z 0.6 x H Rubber +Z PV Panel +Z (1/3)x H 10
Module mounting is more important than glass thickness 150 Maximum glass stress under IEC 61646 snow load test 1100 x 1300 mm module Maximum Glass Stress, MPa 125 100 75 50 25 0 2 Edge Rails Framed Edges 2 Rails 3 Rails 0 1 2 3 4 Glass thickness (mm) 11
Model predicts thin specialty glass to pass heavy snow load test Model results of IEC 61646 wind and snow load test 1100 x 1300 mm module 3 Rails Support Configuration Framed Edges 2 Rails 2 Edge Rails 0.7 1.1 1.3 2 3.2 Glass Thickness (mm) Pass Inconclusive, experimental validation needed 12
We also conducted multiple ice ball impact tests to simulate hail simply supported 600 mm 5 7 9 Sub-module 4 3 8 1 2 6 Middle support End support cantilevered 600 mm 8 9 10 3 7 5 2 Experiment details 95-500 X 600 sub-modules 25 mm diameter ice ball 23 m/s velocity 18 to 20 impacts each Thin glass 5 thicknesses 0.7 to 23.2 mm 400 grit bullnose edges Heat strengthened 3.2 mm soda-lime 0.76 mm PVB 0.7 3.2 mm thin specialty glass 6 4 1 13
Thin specialty glass passed the ice ball impact test Ice ball impact video All 95 sub-modules passed >1800 ice ball impacts 25 mm diameter ice ball 23 m/sec velocity Configuration Substrate Superstrate sub-module hail test matrix Simply supported Cantilevered Thickness (mm) 5 5 0.7 5 5 1.3 5 5 1.6 5 5 2.0 1 5 3.2 5 5 0.7 5 5 1.3 5 5 1.6 5 5 2.0 4 5 3.2 14
In summary, thin specialty glass is reliable for thin-film PV applications Glass strength is determined by surface imperfections and fatigue over time Not all glasses are created equal. Glass composition affects long term strength Thin specialty glass can withstand heavy snow loading Wind and snow load stresses depend primarily on mounting design Optimal and more cost efficient mounting configuration must be considered Thin glass passes ice ball impact testing Thin specialty glass is reliable and is recommended for thin-film PV applications 15
Acknowledgements David I. Wilcox Kevin L. Wasson Suresh T. Gulati 16