Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Methodology to Assess Potential Glint and Glare Hazards from Concentrating Solar Power Plants: Analytical Models and Experimental Validation

Clifford K. Ho, Cheryl M. Ghanbari, and Richard B. Diver

Concentrating Solar Technologies DepartmentConcentrating Solar Technologies Department Sandia National LaboratoriesSandia National Laboratories

Albuquerque, NM 87185Albuquerque, NM 87185 ckho@sandia.govckho@sandia.gov

ASME 2010 Energy Sustainability Conference

Phoenix, AZ, May 17 – 22, 2010

2

Overview

• Introduction

• Safety Metrics

• Glint and Glare Analysis

• Model Validation

• Summary

3

Introduction

• Glint and glare may cause unwanted visual impacts• Glint is momentary flash of light; glare is more continuous

source of excessive brightness• Visual impacts range from flash blindness to retinal burn

• Need quantified analysis of glint/glare to reduce uncertainties associated with visual impacts of CSP installations• Industry, military, government agencies (e.g., California

Energy Commission, Transportation Research Board)

4

Examples of Glint/Glare

Solar OneSolar One (10 (10 MWMWee power power tower, Daggett, tower, Daggett,

CA)CA)

Central Receiver Central Receiver Test FacilityTest Facility (SNL, NM)(SNL, NM)

Kramer JunctionKramer Junction (150 (150 MWMWee parabolic trough, Mojave parabolic trough, Mojave

Desert, CA)Desert, CA)

National Solar Thermal Test FacilityNational Solar Thermal Test Facility (SNL, NM)(SNL, NM)

5

Overview

• Introduction

• Safety Metrics

• Glint and Glare Analysis

• Model Validation

• Summary

6

Potential Impacts

(0.15 s exposure)

7

Retinal Irradiance

choroid

retina

lens conjunctiva

cornea

pupil iris

nodal point

f s

ds dr ω ω

• Need to calculate• Retinal irradiance

• Function of irradiance at the cornea• Subtended angle of glint/glare source

8

Overview

• Introduction

• Safety Metrics

• Glint and Glare Analysis

• Model Validation

• Summary

9

Modeling Approach

θ θ

Specular Reflection

Diffuse Reflection

• Analyze two different types of reflection

(polished surfaces; e.g., mirrors)

(rough surfaces; e.g., receivers)

10

Specular Reflections

• Point Focus and Line Focus Collectors

Dish Heliostat Parabolic Trough

11

Off-axis Dish

Specular Reflections

• Potential for glint and glare from collectors• Off-axis; misalignment; moving to or from stow• End-loss and spillage for troughs

Off-axis Trough End-Loss from Trough

12

Analysis Steps

• Calculate retinal irradiance using equations in paper for specular or diffuse reflections• Collector optical properties, DNI, pupil diameter

• Calculate subtended angle using equations in paper• For diffuse reflections, source is given by size of receiver

or reflecting source• For specular reflections, use equations

• Identify potential impact using plot of retinal irradiance vs. subtended source angle

13

Specular Irradiance

0.001

0.01

0.1

1

10

100

1000

1 10 100 1000 10000Distance (m)

Maximum for point-focus mirrors

point-focus (b=100m)

ρ = 0.92β = 9.4 mradD h = 6.86 mQ = 0.1 W/cm2

point-focus (b=10m)

line-focus (b=2m)

point-focus (b=1000m)

flat mirror

b=focal length

one sun

Flat Collector

Focused Collector

Irrad

ianc

e at

the

Cor

nea

(W/c

m2 )

14

Subtended Source Angle

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.00 10.00 100.00

Distance (m)

Sub

tend

ed S

ourc

e A

ngle

(rad

) ρ = 0.92β = 9.4 mradD h = 6.86 mfocal length = 10 m

oobbsseerrvveerr

ccoolllleeccttoorr

15

Comparison to Safety Metrics

ρ

= 0.92

β

= 9.4 mrad

Dh = 6.86 m

focal length = 10 m

50 m viewing distance

⇒⇒

Retinal irradiance Retinal irradiance = 7 W/cm= 7 W/cm22

⇒⇒

Subtended source Subtended source angle ~ 2 angle ~ 2 mradmrad

16

Overview

• Introduction

• Safety Metrics

• Glint and Glare Analysis

• Model Validation

• Summary

17

Specular Reflection Testing

18

Specular Reflection Validation

0.001

0.01

0.1

1

10

0 20 40 60 80 100 120Distance, south of dish (m)

Nor

mal

ized

Irra

dian

ce (S

uns)

MeasuredAnalytical SolutionRay Tracing (ASAP®)

7/1/09~9:30 AM (MDT)

Looking north

19

Diffuse Reflection Testing

20

Diffuse Reflection Validation

0.00%

0.25%

0.50%

0.75%

1.00%

1.25%

1.50%

0 50 100 150 200 250Distance, north of tower (m)

Irrad

ianc

e (%

of o

ne s

un)

Measured

Analytical

7/2/09~10:00 AM (MDT)

21

Overview

• Introduction

• Safety Metrics

• Glint and Glare Analysis

• Model Validation

• Summary

22

Summary

• Glint and Glare can cause unwanted visual impacts

• Analytical models have been developed to quantify glint and glare• Specular reflections

• Point-focus and line-focus• Diffuse reflections

• Safety metrics have been compiled• Plot of retinal irradiance vs. subtended source angle can be

used to assess potential impact of quantified glint/glare

• Models have been validated with test data

23

Ongoing Work

• Reduce uncertainties associated with glint/glare for permitting and certification of solar power systems• Transportation Research Board

• Synthesis Report on “Investigating Safety Impacts of Energy Technologies on Airports and Aviation”

• California Energy Commission• Military• Industry

• Develop web-based tool for glint/glare analysis