Evaluation of relevant reflector properties, Aránzazu Fernández

Transcripción

4th SFERA Summer School
DLR
Hornberg, 15th May 2013
Evaluation of relevant reflector properties
Aránzazu Fernández‐García
[email protected]
Florian Sutter (DLR)
Contents
1. Introduction
2. Solar reflectors 3. Reflectance: soiling and aging
4. Shape
Introduction
• Concentrating solar thermal systems 3
Introduction
• Classification
Concentrator: reflector with the proper shape
4
Introduction
• Efficiency
 overall         K   
Pth ,loss
Psolar
Psolar
ρ
γ
τ
α
Pth,loss
5
Introduction
• Efficiency
 overall         K   
Pth ,loss
Psolar
Microscopic (material scattering) ρ
Macroscopic (concentrator shape) γ
6
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 γ
‒ Cost effective component
€
ρ
7
Contents
1. Introduction
4. Shape
Solar reflectors
• Reflective metals used in solar reflectors
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Solar reflectors
• Silvered thick‐glass reflectors
Low-iron glass (<0.015 %). 4 mm thickness
Reflective layer : Silver (0.7-1.2 g/m2)
Back layer : Copper (> 0.3 g/m2)
Paint layer (20-2.5% Pb). Pb free: 0.15 %
Reflectance
Durability
Shape
Cost Paint layer (10-1% Pb). Pb free: 0.15 %
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Solar reflectors
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Solar reflectors
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Solar reflectors
• 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 %
Reflectance
Durability
Cost
Shape (back)
Cost (back)
13
Solar reflectors
• Silvered thin‐glass reflectors
14
Solar reflectors
• Laminated silvered glass reflectors
Low-iron glass (<0.015 %). 1.6 mm thickness
Reflective layer : Silver
Adhesive layer: Polyvinyl Buytral (PVB)
Reflectance
Durability
Shape
Cost
Low-iron glass (<0.015 %). 2.3 mm thickness
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Solar reflectors
• Laminated silvered glass reflectors
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Solar reflectors
Sol-gel SiO2
TiO2
SiO2
PVD Al (pure)
Anodization Al2O3
< 5 μm
• Aluminum reflectors
Cost
Shape
Shape (back) Reflectance
Durability
Polished Al substrate
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Solar reflectors
• Aluminum reflectors with metal structure
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Solar reflectors
• Aluminum reflectors with composite material structure
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Solar reflectors
Anti-soiling Layer
PMMA superstrate
Adhesion Promoting Layer
Reflective layer: Silver
Metal back layer: Cu
Pressure Sensitive Adhesive (PSA)
< 5 μm
• Silvered polymer films
Cost
Shape
Shape (back) Reflectance
Durability
Substrate
(Kennedy, 2010)
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Solar reflectors
• Silvered polymer films
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Solar reflectors
• Reflectance of different solar reflectors
Type of reflector
Reflectance
Silvered Thin Glass
0.95
Silvered Thick Glass
Laminated silvered glass 0.93—0.94
Silvered Polymer Film
0.90‐0.93
Aluminum
0.83‐0.86
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Solar reflectors
• Cost of different solar reflectors
Type of reflector
Silvered Thick Glass
Silvered Thin Glass
Silvered Polymer Film
Aluminum
Cost ($/m2)
43‐65
16‐43
20‐25
20‐22
(Kennedy and Terwilliger, 2005)
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Contents
1. Introduction
4. 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
Solar‐weighted reflectance:
Whole solar spectrum
Specularity:
Directed to the receiver
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Reflectance
• Scheme of specular reflectance
 s ( SW , ,  )
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Reflectance
• Reflectance decrease mechanisms
Absorption
Scattering/beam spread
• Both mechanisms are produced by these sources
Soiling deposition:
cleaning
Aging due to environmental stress: durability
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Reflectance: soiling/cleaning
• 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
28
• 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
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•
Anti‐soiling coatings to reduce soiling rate
‒ Easy‐to‐clean effect
‒ Dust repellent properties
30
• Water based methods
(Abengoa)
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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 structure
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Reflectance: aging
• Degradation mechanisms:
Top coating: degradation and transmittance loss
Reflective layer: corrosion
Back coating: degradation
• Factors:
Temperature
Humidity
Chemicals:
‐ NaCl
‐ SO2, NOX
‐ Particles
Radiation (UV)
Abrasion:
‐ Part + wind
‐ Cleaning
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Reflectance: aging
34
Contents
1. Introduction
4. Shape
Shape
• Concentrator shape must be according to the design to focus the reflected radiation onto the receiver
Parabola (cross section)
Paraboloid
Paraboloid/
spherical/
cylindrical
(large radius)
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Shape
• 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)
(Ulmer et al., 2008)
(Fernández‐Reche and Fernández‐García, 2009)
(Lüpfert et al., 2007)
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Shape
• Intercept factor is calculated by ray‐tracing, using
measured shape of the concentrator and considering:
‐ Sun shape
‐ Reflector panel alignment geometry
‐ Receiver geometry
‐ Receiver real position
‐ Tracking accuracy
‐ Other factors and loads
•
Results obtained are useful in:
‒
‒
‒
Design process
Efficiency assessment
Quality control
38
Thank you for your attention!!!!!
[email protected]
39

Evaluation of relevant reflector properties, Aránzazu Fernández

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