evaluation of relevant reflector properties

Aránzazu Fernández‐Garcí[email protected]

Florian Sutter (DLR)

4th SFERA Summer SchoolDLR

Hornberg, 15th May 2013

Evaluation of relevant reflector properties

4th SFERA Summer SchoolHornberg, 15th May 2013

Contents

1. Introduction2. Solar reflectors 3. Reflectance: soiling and

aging4. Shape


Introduction

• Concentrating solar thermal systems

3


Introduction

• Classification

4

Concentrator: reflector with the proper shape


• Efficiency

Introduction

5

solar

lossthoverall P

PK ,

ρ

γτ

αPth,loss

Psolar


Macroscopic (concentrator shape)

γ

Microscopic (material scattering)

ρ

• Efficiency

Introduction

6

solar

lossthoverall P

PK ,


Introduction

• The reflector is the first key component in the energy conversion process of concentrating solar technologies

• Any solar radiation that is not reflected by the mirror in the direction of the receiver is lost to the system

• The feasibility of these technologies strongly depends on the material and manufacturing process used to achieve a suitable solar reflector‒ Appropriate optical properties: reflectance‒ Suitable concetrator geometry: shape ‒ Cost effective component

7

ργ

€


Contents


aging4. Shape


Solar reflectors

9

• Reflective metals used in solar reflectors


Solar reflectors

10

• Silvered thick‐glass reflectors

Low-iron glass (<0.015 %). 4 mm thickness

Reflective layer : Silver (0.7-1.2 g/m2) ReflectanceDurabilityShapeCost

Back layer : Copper (> 0.3 g/m2)

Paint layer (20-2.5% Pb). Pb free: 0.15 %

Paint layer (10-1% Pb). Pb free: 0.15 %


Solar reflectors

11



Solar reflectors

12



Solar reflectors

13

• Silvered thin‐glass reflectors

Low-iron glass (<0.015 %). < 1 mm thickness

Reflective layer : Silver (0.8-1.2 g/m2)

Back layer : Cooper

Paint layer (20-2.5% Pb). Pb free: 0.15 %

Paint layer (10-1% Pb). Pb free: 0.15 %

ReflectanceDurabilityCostShape (back)Cost (back)


Solar reflectors

14

• Silvered thin‐glass reflectors


Solar reflectors

15

• Laminated silvered glass reflectors

ReflectanceDurabilityShapeCost

Low-iron glass (<0.015 %). 1.6 mm thickness

Low-iron glass (<0.015 %). 2.3 mm thickness

Reflective layer : Silver

Adhesive layer: Polyvinyl Buytral (PVB)


Solar reflectors

16

• Laminated silvered glass reflectors


Solar reflectors

17

• Aluminum reflectors

Sol-gel SiO2

SiO2

TiO2

Polished Al substrate

Anodization Al2O3

PVD Al (pure)

CostShapeShape (back) ReflectanceDurability

< 5 μm


Solar reflectors

18

• Aluminum reflectors with metal structure


Solar reflectors

19

• Aluminum reflectors with composite material structure


Solar reflectors

20

• Silvered polymer films

(Kennedy, 2010)

< 5 μm

Anti-soiling Layer

Adhesion Promoting LayerPMMA superstrate

Substrate

Metal back layer: Cu

Reflective layer: Silver

Pressure Sensitive Adhesive (PSA)

CostShapeShape (back) ReflectanceDurability


Solar reflectors

21

• Silvered polymer films


Solar reflectors

22

Type of reflector Reflectance

Silvered Thin Glass 0.95Silvered Thick GlassLaminated silvered glass 0.93—0.94Silvered Polymer Film 0.90‐0.93Aluminum 0.83‐0.86

• Reflectance of different solar reflectors


Type of reflector Cost ($/m2)Silvered Thick Glass 43‐65Silvered Thin Glass 16‐43Silvered Polymer Film 20‐25Aluminum 20‐22

Solar reflectors

23

• Cost of different solar reflectors

(Kennedy and Terwilliger, 2005)


Contents


aging4. Shape


Reflectance

• To enhance the feasibility of CSP systems, quality and lifetime guarantees of the components must be increased. Those guarantees can only be given with the appropriate testing methods and measurement tools

• The proper optical parameter to evaluate the quality of reflectors is the solar‐weighted specular reflectance

25

Solar‐weighted reflectance:Whole solar spectrum

Specularity:Directed to the receiver


Reflectance

• Scheme of specular reflectance

),,( SWs

26


Reflectance

• Reflectance decrease mechanismsAbsorption Scattering/beam spread

• Both mechanisms are produced by these sources

27

Soiling deposition:cleaning

Aging due to environmental stress: durability


Reflectance: soiling/cleaning

28

• Reflectance decrease due to soiling deposition• Cleaning is one of the main of aspect of maintenance tasks• Cleaning strategy depends on the reflector and the location



29

• Cleaning methods typically used are mainly based on water ‒ Minimization of the water consumption by:

• Using some additives (mainly detergents)• Applying a brush, a foam, a tissue, etc.• Collect and reuse!!!!

‒ Optimization of the water treatment to reduce the cost‒ Combination of pressure and temperature of the water to

have a good compromise between efficiency and cost • Dry cleaning methods in some locations because in wet

ambients particles are strongly attached to the reflector surface



30

• Anti‐soiling coatings to reduce soiling rate‒ Easy‐to‐clean effect‒ Dust repellent properties



31

• Water based methods

(Abengoa)


Reflectance: aging

• Typical guaranties requested involve the goal of 10‐30 years of real time in outdoor exposure with low degradation

• The materials evolve quickly and their competition in the market is strong accelerated conditions are necessary in service lifetime prediction

• Prediction of outdoor lifetime based on accelerated aging is not an easy task because it depends on:– The failure mechanisms, which is specific for each type of reflector– The real outdoor conditions, which depends on the location

• Commercial reflectors change composition and structure32


Reflectance: aging

• Degradation mechanisms:

• Factors:

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Top coating: degradation and transmittance loss

Back coating: degradationReflective layer: corrosion

TemperatureHumidity

Radiation (UV)

Chemicals:‐ NaCl

‐ SO2, NOX‐ Particles

Abrasion:‐ Part + wind‐ Cleaning


Reflectance: aging

34


Contents


aging4. Shape


Shape

36

• Concentrator shape must be according to the design to focus the reflected radiation onto the receiver

Paraboloid Paraboloid/spherical/cylindrical

(large radius)

Parabola (cross section)


Shape

37

• Shape measurement techniques:– Deflectometry (distortion of reflected patterns)– Close‐range photogrammetry (3D point probing)– Flux density measurements (as indirect measurement)– V‐Shot (laser)– Distant observer (inverse optical path)

(Fernández‐Reche and Fernández‐García, 2009)(Ulmer et al., 2008)

(Lüpfert et al., 2007)


Shape

38

• Intercept factor is calculated by ray‐tracing, usingmeasured shape of the concentrator and considering:

• Results obtained are useful in:‒ Design process‒ Efficiency assessment‒ Quality control

‐ Sun shape‐ Reflector panel alignment

geometry‐ Receiver geometry‐ Receiver real position‐ Tracking accuracy‐ Other factors and loads


Thank you for your attention!!!!!

[email protected]

39

evaluation of relevant reflector properties

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