Advancements in Concentrating Solar Power (CSP) Reflectors and Solar-

Selective Coatings

C.E. Kennedy1National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, M/S 3321, Golden, CO

80401-3393, 303-384-6272, 303-384-6103 (fax), cheryl_kennedy@nrel.gov

E3 2007 University of Minnesota, MN

November 27, 2007

Concentrating Solar Power TechnologiesPower TowerParabolic Trough Dish-Stirling

CPV Heliostat CPV Winston Collector

Compact Linear Fresnel Reflector (CLFR)

Solar concentration allows tailored design

approaches

100kW LCPV Tracking

Goals for Improved Optical Materials

• >90% Specular reflectance into a 4-mrad cone angle – Unofficially 95%

• 10 - 30 year lifetime– Unofficially 30 y

• Manufacturing cost $10.76/m2 ($1/ft2)– 1992 Cost Goal

– Adjusted for inflation to $15.46/m2 ($1.44/ft2)

– Structural (self-supporting) mirror to $27/m2 ($2.50/ft2)

Technical Approach• Samples supplied by:

– Industry– Subcontracts– Developed in-house

• Optical Characterization:– Perkin-Elmer (PE) Lambda 9 & 900 UV-VIS-NIR

spectrophotometers (250-2500 nm) w/ integrating spheres

– PE IR 883 IR spectrophotometer (2.5-50 μm)– Devices & Services (D&S) Field Portable

Specular Reflectometer (7, 15, & 25-mrad cone angle at 660 nm)

• Outdoor (OET) & Accelerated Exposure Testing (AET):

– Atlas Ci65 & Ci5000 WeatherOmeters (WOM) (1X & 2X Xenon Arc/60ºC/60%RH)

– QPanel QUV (UVA 340@ 290- 340 nm/ 4 h UV at 40º / 4 h dark at 100%RH)

– 1.4 kW Solar Simulators (SS) (≈5X Xenon 300-500 nm. 1.4 kW-SS-4 quadrants 2 RH &T, light /dark)

– BlueM damp heat (85ºC/85%RH/dark)– 3 meterologically monitored sites at Golden,

Colorado (NREL), Miami, Florida (FLA), and Phoenix, Arizona (APS)

3

2

1

3

2

1

Reflective Layer (wet-silver)

Low-iron Slumped Glass (4- or 5-mm thick)

Acrylic (w/ high UV stability)

2nd coat Paint Layer (heavy Pb)(1% Pb)

1st coat Paint Layer (heavy Pb)(2.5% Pb)

Parabolic Trough Glass Mirror Architecture

Back Layer (Cu)

Three-coat paint system designed for outdoor applications

Mactac adhesive

Ceramic pad

Original vs. New Flabeg Mirror

85

90

95

100

0 10 20 30 40 50 60 70 80

Total UV Dose (100 x MJ/m2)

% H

emis

pher

ical

Ref

lect

ance

APS - OLDFLA - OLDNREL - OLDCi65 - OLD

Equivalent NREL Exposure Time (years)3 6 12 15 180 24219

Reflective Layer (wet-silver)

Low-iron Glass (3- or 4-mm thick flat)

2nd coat Paint Layer (lead-free <0.15% Pb EU)<1 ppm Pb US)

1st coat Paint Layer (lead-free <0.15% Pb EU)<1 ppm Pb US)

Alternate Thick Glass Mirror Architecture

Back Layer (Cu-less)

Adhesive (PS, spray)

Substrate (SS, Al)

Alternate Thick Glass Mirrors

65

70

75

80

85

90

95

100

0.0 3.3 6.7 10.0 13.3 16.7 20.0 23.3 26.6 30.0 33.3 36.6 40.0 43.3 46.6 50.0 53.3

Total UV Dose (100 x MJ/m2)

% H

emis

pher

ical

Ref

lect

ance

NREL - PilkingtonNREL - SpanishCi65 - PilkingtonCi65 - Spanish

Equivalent NREL Exposure Time (years)1 2 3 4 50 6 7 8 9 11 12 13 14 15 1610

Reflective Layer (wet-silver)

Low-iron Glass (~1 mm- thick)

Substrate (SS, Al)

Adhesive (PS, spray)

Paint Layer (Pb)(Pb-free)

Thin Glass Mirror Architecture

Back Layer (Cu)(Cu-less)

Thin glass mirrors are designed for indoor applications.

Thin Glass Corrosion

Thin Glass Mirror

0

20

40

60

80

100

250 500 750 1000 1250 1500 1750 2000 2250 2500

Wavelength (nm)

% R

efle

ctan

ce

0.0 MOBlueM 3.52 MOBlueM 7.21 MOCi65 3.16 MOCi65 6.15 MONREL 3.82 MONREL 9.57 MO

(Naug/Clearcoat/966

PVD Al Reflective Layer

Reflectivity-enhancing Oxide

Polished Aluminum Substrate

Protective Overcoat

Aluminized Reflector Architecture

Aluminized Reflectors

80

85

90

95

100

0 333 666 999 1332 1665 1998 2331Total UV Dose (MJ/m2)

% H

emis

pher

ical

Ref

lect

ance

OriginalImproved Miro2Improved Miro2 Set#2Miro/4270kk

NREL Exposure Time (y)

1 2 3 4 50 6 7

Aluminized Reflector Specularity

Alanod 4270/kk

FLA 11.8 m

APS 27.7 m

NREL 11 m

WOM 10.2 m0

20

40

60

80

100

0.0 3.3 6.7 10.0 13.3 16.7 20.0 23.3 26.6

Total UV Dose (100 x MJ/m2)

7-m

radi

an S

pecu

lar R

efle

ctan

ce a

t 660

nm

APSFLANRELCi65

Equivalent NREL Exposure Time (years)

1 2 3 4 50 6 7 8

Aluminized ReflectorSpecular Reflectance at 7- and 25-mradians at 660 nm of Alanod MiroSun mirrors after

accelerated exposure in Blue M (dark / 85oC / 85%RH), WOM (1 sun / 60oC / 60%RH) chambers, and outdoor exposure at NREL, APS, FLA, and Sandia

30

40

50

60

70

80

90

100

0 3 6 9 12 15 18 21 24

EXposure Time (Months)

% S

pecu

lar R

efle

ctan

ce

NREL - 25 mrNREL - 7 mrNREL - SWVAPS - 25 mrAPS - 7 mrAPS - SWVFLA - 25 mrFLA - 7 mrFLA - SWVWOM - 25 mrWOM - 7 mrWOM - SWVBlue M - 25 mrBlueM - 7 mrBlueM - SWVSandia -25mr

Silvered Polymer Reflector Architecture

UV-Screening Superstrate

Base Reflector

Bonding Layer

Flexible Polymer Substrate

Silvered Polymer

70

75

80

85

90

95

100

0 3.3 6.6 9.9 13.2 16.5 19.8 23.1 26.4

Total UV Dose (100 x MJ/m2)

% H

emis

pher

ical

Ref

lect

ance

UV-Screen/SS95-NRELReflecTech A-NRELReflecTech B-NRELUV-Screen/SS95-WOMReflecTech A-WOMReflecTech B-WOM

Equivalent NREL Exposure Time (years)

0 1 2 3 4 5 6 7 8

Advanced Selective Coating Goals• Receivers:

– 4 m long x 70 mm diameter– 64 MWe Nevada plant

• 820 collectors with 24 (96 m) receivers each

• 19,680 receivers• 82 km of receivers (50 mi)

– 3-4%/yr Failure Rate– ~$1000/tube

• To develop receiver coatings that have:– Good optical and thermal

performance: absorptance (α) ≥96%, & emittance (ε) ≤ 7% >450ºC

– High temperature stability in air at temperatures ≥ 550ºC

– Manufacturing processes with improved quality control

– Lower cost

200C (0.31 kW/m2)

300C (0.80 kW/m2)

400C (1.78 kW/m2)

500C (3.56 kW/m2)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

100 1000 10000 100000

Wavelength (nm)

Bla

ckbo

dy Ir

radi

ance

(W/m

2 -nm

)

0

20

40

60

80

100

% R

efle

ctan

ce ( ρ

)

Direct AM 1.5 (0.77 kW/m2)

Ideal Solar Selective

Conclusion• DOE, the WGA, state RPS mandates, and feed-in tariffs have

successfully jump-started growth in CSP technologies that would require 7 to 10 million square meters of reflector and more than 600,000 HCEs over the next 5 years.

• Commercial glass mirrors, Alanod, and ReflecTech may meet the 10-yr lifetime goals based on accelerated exposure testing. Predicting an outdoor lifetime based on accelerated exposure testing is risky because AET failure mechanisms must replicate those observed by OET.

• None of the solar reflectors available have been in test long enough to demonstrate the 10-year or more aggressive 30-year lifetime goal, outdoors in real-time

• Emittance excellent & absorptance of modeled coatings is very good but further improvements are expected. However, trade-off exists between emittance and absorptance. Key issue is making the coating and prototype development underway. Patent being pursued

AcknowledgmentsAlanod, Glaverbel, Naugatuck, ReflecTech, SAIC, and SES for providing

solar reflective samples.Schott and Solel for providing solar selective samples.AZ Technology and Surface Optics Corporation for high-temperature optical

measurements .Armstrong World Industries: Dr. J. S. RossNortheastern University: Dr. Jackie IsaacsPenn State University: Prof. Singh, Tom Medill, and Dale DonnerSAIC: Dr. Russell Smilgys and Steve WallaceStat-Ease: Wayne F. AdamsSwisher and Associates: Dick Swisher

NREL:Lynn Gedvilas, Gary Jorgensen, Mark Mehos, Judy Netter, Craig Perkins, Hank Price, Kent Terwilliger, and Student interns: Micah Davidson, Anthony Nelson, Michael Milbourne, and Christopher, and Andrea Warrick.

DOE supported this work under Contract No. DE-AC36-99GO10337.