June 2012

Ben-Gurion University of the Negev Faculty of Engineering Sciences

Faculty Unit of Energy Engineering

By: Oleg Shkolnik

Supervisor: Prof. Daniel Feuermann

Photothermal coatings based on functional nanomaterials for the efficient conversion of solar

energy

Supported by: Ministry of Energy and Water Resources

Outline

Theoretical short introduction and motivation

Project description

Review of different types of coatings

Experimental setup and methodology

Current results and future experiments

משטרי מעבר חום -הקדמה

𝑞 בהולכה מעבר חום = −𝐴 ∙ 𝑘∂𝑇∂𝑥

𝑞 חום בהסעה מעבר = 𝐴 ∙ ℎ(𝑇𝑠 − 𝑇∞)

Thermal conductivity of various materials at 0 °C

Material k ( W/ m °C )

Copper 385

Aluminum 202

Carbon steel 1% C 43

Glass wool 0.0038

Air 0.024

מעבר חום בקרינה

𝜀 𝜆 = 𝛼 𝜆 according to Kirchhoff’s law

𝑞 = 𝜎 ∙ 𝐴1 ∙ 𝜀1(𝑇1 4 − 𝑇2

4 )

𝐸𝑏 = 𝐸𝜆𝑑𝜆 ≡ 𝜎 ∙ 𝑇4∞

0 Boltzmann law

𝛼 + 𝜌 + 𝜏 = 1 according to energy balance

Net heat transfer from surface 𝐴1

What is selective coating

A selective absorber coating or material is a thin layer (of the order of µm) on the surface, or a material which exhibits optical properties that drastically vary from one spectral region to another The solar spectrum and the re-radiated spectrum of the hot absorber are separated at relatively low temperature of the absorber. However, the higher the temperature of the surface the more the absorbed solar radiation and the emitted spectra overlap Selective coating was pioneered by Tabor in 1956 in Israel

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C=100 T=550 C

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C=25 T=800 C

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( ) (0.95) (0.07) (0.95) (0.07)cr cr

cr cr

cr sun sun bb bbq C I d C I d I d I d

Evaluation of critical wavelength and efficiency estimation

Selective coating the complete project This work is a part of a collaborative project of developing thin nanocomposite layers for high temperature (above 500 C) The samples are prepared at Hebrew University at chemical department by two groups of researchers (Mandler and Magasi et al.,) The Electro-Dynamic analysis of particles mixture is performed at TAU Electrical Engineering department by Steinberg et al. The optical properties measured at Sede Boqer and TAU ( Feuermann and Kribus et al.,)

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Goals of the project The global purpose of this research is to construct a spectrally selective

ceramic matrix with embedded nanoparticles for photo thermal conversion, stable at high temperature applications.

Each group has its specific requirements to meet.

Hebrew University group :

Solar absorption at 300-2500 nm range (AM 1.5),

above 95% of the total energy.

Low thermal emittance < 0.1

The coating will be designed for continuous

use at a temp >500 C (in air)

Optical properties of selective coatings

BGU : Detailed measurements of spectral and angular reflectance at ambient

and elevated temperatures

Measurements of solar absorptance at high solar concentration

TAU: Prof. Kribus Group

Measuring the of emissivity

• IR Camera 2–14 μm

• Pyroelectric sensor 0.3–16 μm

Prof. Steiberg group Theoretical calculations based on use

Maxwell equations in order to evaluate the α by knowing the dielectric constant of the absorbing particles

Basics assumptions: Homogeneous distribution particles should be one 0.1-0.2 of

the wavelength Spherical, ellipsoidal, or needle

like particles

Types of absorbers

1. Intrinsic absorbers- e.g., HfC(Hafnium Carbide ): two categories of materials fall into this group : transition metals and semiconductors. Unfortunately most selective metals exhibit the cutting edge, of good absorber to pore emitter too earlier at the solar spectrum and therefore not suitable for solar applications

Carl M. Lampert Solar Enegy materials 1( 1979)

2. Multilayer absorbers (e.g., Al203/Mo/ Al203):

Multilayer thin films are dielectric-metal combinations which act

like a selective filter for energy absorption Particular wavelengths of solar energy are absorbed by multiple

reflections in the dielectric-metal layers Such coatings are commonly produced by sputter deposition

technologies with AR layer on the top

N. Selvakumar, Harish C. Barshilia, Materials & Solar Cells (2011)

3. Optical trapping: it is possible to produce a surface with a such roughness that will absorb the visible part of the solar spectrum by pure geometrical means while being opaque to the infrared spectrum

4. Metal dielectric (cermet): highly absorbing metal dielectric composites consisting of nano or micro scale particles embedded in a dielectric or ceramic matrix

Structure of cermet: thin protective layer to prevent diffusion single layer with uniform particles distribution single layer with graded particles distribution double layer - according to fundamental modeling it has higher photo-thermal

conversion efficiency

Cermet types and structures

Offers high degree of flexibility , solar selectivity can be optimized by proper

choice of coating thickness Particle size- it is possible to shift the absorbing and scattering cutoffs to higher

wavelengths when the particle radius increases Possible to predict optical properties using effective medium theories for example

the optimum pore diameter dp=0.08 μm of nickel pigmented alumina coatings have been determined

Usage at broad range of temperatures In addition solar absorbance can be boosted with a proper choice of substrates and AR layer. After a long research and development in solar materials, the cermet remains one of the best and promising candidates for high temperature application

Advantages of cermets over other candidates for high temperature applications

International Energy Agency (IEA) performance criterion (PC) developed for a flat-plate collector selective absorber testing.

𝑃𝐶 = −∆𝑎𝑠 + 0.25 ∙ ∆𝜀 ≤ 0.05 lifetime of at least 25 years and decrease in the annual solar fraction of 5% Service lifetime testing for this criterion is performed by exposing the absorber coating for 200 h at 250°C If the material survives , it is then exposed for 75 h at 300 °C hand roll followed by 600 h at 40 °C and 95 %relative humidity No similar criterion has been developed for testing the service lifetime of high temperature absorbers for CSP. Degradation of the high temperature absorbers usually causes increasing emittance ; in addition while the emittance of many materials after exposure to high temperature does not return to the original emittance measured ( e.g., paint), for some materials the emittance changes at high temperature and returns to the original value

Long term stability

Different matrices were investigated for CERMET layers, more exactly 2 categories

of matrices (silica and alumina). Both alumina and silica matrices are transparent to IR and are stable at high temperature.

Black coatings were prepared by dispersing black pigment (black 26) in the

different matrices, adding dispersant and additives. The coatings were dried at 750°C during 1 hour under air.

SEM of alumina film with Black 26

First set of experiments: Solar irradiation – one sun Ambient temperature Measurements of 4 samples

θs

θi

wet thickness 40 µm

wet thickness 6 µm

achieve 1000K

experimental setup

4 side faces

of furnace

back face of

furnace Front face Total loss

heat loss by convection 5.7 W 2 W 23.9 W 31.6 W

heat loss by radiation 6.6 W 2.4 W 13.6 W 22.6 W

Results for HU samples

θi

θs

Azimuthal effects inside the “specular region”

rotating the sample to observe any difference

in surface structure θi

θs

Φs The structure of the surface has obviously a strong effect

θi

θs

Φs

rotating the sample to observe any difference

in surface structure

Azimuthal effects outside the “specular region”

θi

θs

polar angle effect around specular direction

B-CZ-10.1A ferro 8µm

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polar angle θs(deg)

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θi=45° Φs=0°

here θi=θs

Specular effects in polar direction (θs) at two incidence angles

We simply average the directional reflectance over the wavelengths. This is of course not a correct measure but should give some qualitative indication of a relative ‘specularity’ of the different samples.

all the results on one graph

polar angle effect at θi=30° Φs=0°

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polar angle θs (deg)

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B-CZ-STS-10.2 4.7µm

B-CZ-STS-10.2A 6µm

B-CZ-10.2B 5.9µm

B-CZ-STS-9.2 2.6µm

B-CZ-10.1 4µm

B-CZ-10.1A 8µm

θi

θs

15°

45°

θs=60°

30°

MS 1.3 (two layers) at elevated temperature of 600K

we had to cut off at 1100nm because thermal radiation was too large at the longer wavelengths.

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relative intensity from MS13 sample in the dark compared to gray reference under illumination

we get 10 times the intensity from thermal radiation than from light reflected

MS 1.3 at 600°K

•attempt to increase signal to noise ratio by optics • better means of measuring temperature • measurement in the solar furnace (to obtain α directly) and will be closer to realistic conditions.

at high temperatures measurements even a very bright lamp is not sufficient while dealing with very absorbing samples

ultimately will be used in solar furnace

Sample inside solar furnace

The next day photos after heating to max temperature 430 °C

Experimental setup using a PCX Condenser Lens 125mm Diam x 500mm Focal Length

A) we have considered using a lens and the sun

Lens position

Problem: Stability of direct beam solar radiation. Ephemeral nature of solarradiation.

Region that the sensor ‘sees’

Light spot that sensor “sees”

Placing the sample at focal plane

Sample position

What did we achieve? much higher intensity light spot completely in the measured region –

experiments at wide angles Hopefully solved uniformity problem

sample holder

sample holder

Region that the sensor “sees”

Light spot that sensor “sees”

θs

θs

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ecta

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wavelength(nm)

polar angle effect θi=30° Φs=0° MS-CZ- 17C 8.7µm θs=-10°

θs=0°

θs=10°

θs=20°

θs=30°

θs=40°

θs=50°

θs=60°

θi

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polar angle θs (deg)

polar angle effect θi=30° Φs=0°-corrected by solar spectrum 17A÷17F 2.5÷ 15.7µm

17B - 2.5µm

17F - 4.8µm

17A - 5.16µm

17C - 8.7µm

17E - 9.1µm

17D - 15.7µm

sample reflectance in non-

specular region

reflectance in specular

region

weighted average

reflectance over both regions

absorbtance

α(θi=30°)

17A -5.16µm 0.050 0.198 0.053 0.947±0.004

17B-2.5µm 0.067 0.223 0.070 0.930±0.005

17C-8.7µm 0.050 0.161 0.053 0.947±0.004

17D-15.7µm 0.043 0.158 0.045 0.955±0.003

17E-9.1µm 0.043 0.170 0.046 0.954±0.003

17F-4.8µm 0.044 0.183 0.046 0.954±0.003

Acknowledgments

This research is supported by the Ministry of energy and water within the framework of the program of scholarships in the field of renewable energy.

I would like to acknowledge my supervisor Prof. Daniel

Feuermann for invaluable support, guidance and scientific assistance.

Thank you

for your attention