Optical Characterization of Liquid Crystal Switchable Mirror

Philippe LemarchandSchool of Physics

You Supervisors’ Names Here

Prof. Brian NortonDr. John Doran

15 February 2013

Project BackgroundConcentration of Solar Energy Using Switchable Mirrors.– What are Switchable Mirrors?Optical element that can be switched between a Transparent state and a Reflective state.

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Technology Advantages Inconvenients

Prismatic Cheap Use water – not practical

Gasochromic(RE;Mg-RE;Mg-TM)

Reflect wide SpectrumColour Neutral

Use hydrogen gas – not practical and potentially dangerousResearch Level – not commercially availableShort lifetime – 100s to 1000s of cycles

Electrochromic(RE;Mg-RE;Mg-TM)

Solid State (KOH electrolyte)Reflect wide SpectrumColour NeutralSwitch Electronically

Research Level – not commercially availableShort lifetime – 100s to 1000s of cycles

Chiral Liquid Crystal (CLC)

Solid StateReflect wide Spectrum(Custom designed: 400-3600nm)Bandwidth tailored from 50 to 1,000 nm Colour NeutralSwitch ElectronicallyCommercially available>10 years lifetimes (indoor)

New patented technology – presently expensive

Project BackgroundConcentration of Solar Energy Using Switchable Mirrors.– Why for Solar Concentration?

1) optically track the sun Remove need for mechanical parts and costs associated2) collect a wide proportion of the solar diffused component More light collected than

standard stationary concentrators3) optically regulate the solar heat flow Reduce the need for cooling and cost associated4) optically concentrate and transfer the reflected energy onto a photovoltaic (PV), thermal (T)

or PV/T absorber Reduce area of expensive absorber and cost associated

– How? choice of material, weather conditions and concentrator design.

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Example 1:Flat absorber with booster mirror

Linear Fresnel Mirror Configuration

Example 2:Inward Facing CPC

Example 3:Tubular Concentrator

Dynamic Optical Behaviour of LC Switchable MirrorConcentration of Solar Energy Using Switchable Mirrors.

Experiment on a Visible [400-700nm] Switchable Mirror (5x5cm)

– Transmission and Reflection bandwidth with light incidence angle.– Percentage Transmission and Reflection in the Clear and Reflective states with light

incidence angle– Switching Speed from Clear to Mirror and from Mirror to Clear states.

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CLC polymer(diacrylate+monoacrylate), nacrylate=1.49

Polyimide coating, np=1.90

ITO layer,np=1.89

Glass substrate,np=1.52

LC moleculeExample: 5CB (4-pentyl=4'-

cyanobiphenyl)n0=1.54, ne=1.71

Dn=0.18Rair-g

Rmol T

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2.5mm 2.5mm20 mm

square wave 0-~250V at 25Hz<10mA Average

Power consumption: 40 mW/cm2

𝜆𝑖=𝑛∗𝑃 𝑖

∆ 𝜆𝑖=Δ𝑛∗𝑃 𝑖

Mol. Reflection Central Wavelength:

Mol. Reflection Bandwidth:

Mirror Reflection Bandwidth:

∆ 𝝀=𝒏∗𝜟𝑷

Setups

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Lambda900 spectrophotometerMeasure spectral Transmission UV to NIR.Normal light incidence angle with mirror Clear and Reflective at Sample port and Integrating Sphere port.Scanning time of visible range: ~1minAt T=10% accuracy of ± 0.08%At T=35% accuracy of ± 0.05% Results used as a reference for comparison with the second setup.

Spectrophotometer with Angular ResolutionSwitchable Mirror Rotational Stage: Varies light incidence angle on mirror.Detector Rotational Stage: Rotate independently of ‘mirror stage’ collecting all light reflected by or transmitted through the mirror.Spectrometer A: Detector for spectral reflection and transmission.Spectrometer B: Detector for source fluctuation detectionAngular accuracy: 1 degreeScanning time of visible range: millisecondTransmission/Reflection accuracy unknown at this stage Using lambda900 results as reference

Results by Lambda900

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380-780nm

Global Sun Power: 549 W.m-2

Direct Sun Power:479 W.m-2

780-1350nm

Global Sun Power: 324 W.m-2

Direct Sun Power:305 W.m-2

Results by Lambda900

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In Clear State:- UV cut off (@T≤50%): 383nm- T ≥80% from 419nm

In Reflective State:- <330nm UV transmission <1%- NIR cut off (@T≥50%): 732nm- Light leakage ≤5% in the range

423nm-681nm

In Clear and reflective state:>75% transparent at wavelengths >800nm

Results by Lambda900

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In Clear State:- UV cut off (@T≤50%): 383nm- T ≥80% from 419nm

In Reflective State:- <330nm UV transmission <1%- NIR cut off (@T≥50%): 732nm- Light leakage ≤5% in the range

423nm-681nm

In Clear and reflective state:>75% transparent at wavelengths >800nm

Additional Information:- Transmission modulate by 80% between Clear and Reflective state in [434-685nm]

Absolute Transmission Dynamic between Clear and Reflective States.

𝑇𝐷𝐴𝑏𝑠=𝑇 𝐶𝑙𝑒𝑎𝑟−𝑇 𝑅𝑒𝑓𝑙𝑒𝑐 .

Results by Lambda900

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In Clear State:- UV cut off (@T≤50%): 383nm- T ≥80% from 419nm]

In Reflective State:- <330nm UV transmission <1%- NIR cut off (@T≥50%): 732nm- Light leakage ≤5% in the range

423nm-681nm

In Clear and reflective state:>75% transparent at wavelengths >800nm

Additional Information:- Transmission modulate by 80%

between Clear and Reflective state in [434-685nm]

-High reflective efficiency of the mirror [419nm-708nm]

-Reflection bandwidth 400-780nm

Relative Transmission Dynamic between Clear and Reflective States.

𝑇𝐷𝑟𝑒𝑙=1−𝑇 𝑅𝑒𝑓𝑙𝑒𝑐𝑡

𝑇 𝐶𝑙𝑒𝑎𝑟

Spectrometer with Angular ResolutionMethod to Measure Absolute Reflection and Transmission

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<B> +sB

<B> <A>

<A> +sA

Step 2: During Transmission and Reflection measurements with spectro A, measure the source fluctuation with spectro B and calculate what would have been the source spectrum recorded by spectro A.

𝐵𝑠𝑜𝑢𝑟𝑐𝑒

⟨𝐵 ⟩ = 𝑓 ∗𝐴𝑠𝑜𝑢𝑟𝑐𝑒

⟨ 𝐴 ⟩=*

BsourceAsource

Step 1: Without mirror, measure the fluctuation of the source over several minutesSince both spectrometers are looking to the fluctuation in intensity by the same source, to a percentage of source fluctuation detected by spectro B corresponds a percentage of fluctuation detected by spectro A by a factor ‘f’.

⟨𝐵 ⟩+𝜎𝐵

⟨𝐵 ⟩ = 𝑓 ∗⟨ 𝐴 ⟩+𝜎 𝐴

⟨ 𝐴 ⟩ )

Spectrometer with Angular ResolutionMethod to Measure Absolute Reflection and Transmission

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Useful Spectral Range of the setup: 380-795nmCorrection factor varying between 0.97 and 1

Spectrometer with Angular ResolutionComparison Method with Lambda900

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Results by Spectrophotometer with Angular ResolutionMirror Reflective – Reflection Measurement

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Increasing Incidence Angle Induces:1) Reflection spectrum shrinks towards blue

wavelength (Red light is lost and blue light is increasingly reflected)

2) Reflection bandwidth decreases3) Average reflection percentage decreases!Reflection varies within 380-780nm range!!Reflection >70% at Normal incidence within 420-700nm!

Mirror Reflective – Transmission measurement

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Increasing Incidence Angle Induces:1) Increased transmission of red

wavelengths and decrease of blue wavelengths

2) Transmitted wavelength plateau at 80% transmission

3) Average transmission increases

Mirror Clear – Reflection Measurement

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Uniform spectral reflectionReflection increases with angle solely due to glass reflection

Mirror Clear – Transmission Measurement

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Quasi uniform spectral transmissionTransmission decreases due to increased reflection by glass

Note: An optical coating could be applied to maintain transmission to its maximum…

…but reflection in the reflective state would also be decreased

Mirror Clear – Summary within [380-780nm]

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Transmission

Reflection

Abs./Diff.

Perfect Match between Lambda900 and Spectrophotometer experiments; T=84% at Normal incidence

-Transmission constant (84%) up to glass critical angle (41deg)-Transmission decrease follows glass transmission trend.

-Reflection constant (9%) up to glass critical angle.-Molecules are adding an extra 5-6% reflection compared to glass.

-Constant optical loss (7%) is believed to be mostly due to diffusion ; not absorption

Mirror Reflective – Summary within [380-780nm]

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- Reflection decreases from 69% to 46% between 0° and 62°- Transmission increases from 16,5% to 40% between 0° and 62°- Optical losses constant to 14.4%. Molecules in planar alignment add 9-10% loss.

!Mirror maintains its optimum performances within 12 degrees incidence angle!

Perfect Match between Lambda900 and Spectrophotometer experiments; T=16,5% at

Normal incidence

Transmission

Reflection

Abs./Diff.

Switching Time – from Clear to Mirror State1 spectrum recorded every 30 secs for 1 hour.

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Mirror switched to reflective state in <30secs within [430-680nm]

Switching Time – from Clear to Mirror State

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Switching Time – from Clear to Mirror State

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Switch from Clear to Mirror; Meas. In Reflection at 11deg; Time between spectra:30ms

Mirror fully switched to reflective state in 10-20 secs within [430-680nm]

Results by Spectrophotometer with Angular Resolution

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Switch from Mirror to Clear; Meas. In Transmission at 0deg; Time between spectra:30ms

Mirror fully switched to Clear state in <200ms within [380-780nm]

LC Switchable MirrorSolar Transmission and Reflection

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Transmission Reflection

% Solar Power(W.m-2) % Solar Power

(W.m-2)

Mirror StateReflective 9% 43 69*-46% 331*-220

Clear 84% 402 16,5*-40% 79*-191

Reflection bandwidth at normal incidence: 400-780nmReflection bandwidth for R>70% at normal incidence: 420-700nmSpectral range with transmission and reflection fluctuation within 62degrees: 380-780nmPower consumption (clear state): ~400W.m-2

*Values constant within 12 degree light incidence angle.

Spectral Range considered: 380-780nmDirect Solar Input power: 479W.m-2

Consideration: Powering a 1m2 switchable mirror consumes as much as the solar power received!Including absorption loss by solar absorber and power conversion losses -> Results in power deficit!

Solution to consider:- Improve the material conductivity- Adapt driver duty cycle and voltage for reduce power consumption and maintain transmission.- Apply optical coating for optimum transmission (best case scenario: gain 77 W.m-2)- The area of the switchable mirror in the design has to be several time smaller than the solar area of

collection- Use a mirror with widest transmission/reflection bandwidth possible: 380-1380nm

LC Switchable MirrorSolar Transmission and Reflection

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Transmission Reflection

% Solar Power(W.m-2) % Solar Power

(W.m-2)

Mirror StateReflective 9% 70 69*-46% 541*-361

Clear 84% 658 16,5*-40% 129*-313

Reflection bandwidth at normal incidence: 400-780nmReflection bandwidth for R>70% at normal incidence: 420-700nmSpectral range with transmission and reflection fluctuation within 62degrees: 380-780nmPower consumption (clear state): ~400W.m-2

* Values constant within 12 degree light incidence angle.

Spectral Range considered: 380-1380nmDirect Solar Input power: 784W.m-2

A 1m2 mirror for 1m2 collection are would provide 258W.2 gain if power consumption remains identical for a switchable mirror of 1000nm bandwidth

Power consumption of switchable mirror demands to be lowered for best results.

Conclusions

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• The visible range CLC switchable mirror has been optically characterized.• Data can be integrated into ray tracing software for designing solar

concentrators• Power consumption of mirror in the Clear state limits the area of switchable

mirror that can be used. Design is critical.• Further technical investigations shall focus towards:

– Power consumption– Optical performances of switchable mirror active in the 380-1380nm range.– Optical simulation of solar concentrator designs using switchable mirrors

Results by Lambda900

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Mirror Clear at Integrating SphereSquare wave voltage applied

High voltage: Molecules forced to align0V: Molecules quickly relaxing in a partially reflective state

Results: Noise during scanning Mirror Clear at Sample Port

Mirror Reflective at Sample Port

Mirror Clear at Sample Port

Mirror Reflective – Absorption measurement

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Mirror Reflective – Absorption/Diffusion within CLC layer

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High Diffusion of blue/Green wavelengths

Mirror Clear – Absorption Measurement

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Mirror Clear – Absorption/Diffusion within CLC layer

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Symmetrical diffusion in blue and red wavelengths.

Effect of the film structure rather than only the material??

Mirror Clear – Absorption/Diffusion induced by molecules rotation

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Diffusion has a linear/proportional function with molecules angle?!

Mirror Reflective – Summary (2) within [380-780nm]

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Average Photon Energy (APE) –Expression of spectral shift and intensity variation

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Reflective StateReflection Meas.

Clear StateReflection Meas.

Clear StateTransmission Meas.

Reflective StateTransmission Meas.

Clear State: the mirror acts as a simple glazing; the APE deviation from the source is quasi constant (-12nm); Colour neutral.

Reflective State:- In transmission: APE deviation varies between +55nm and +75nm; Colour variation from yellow to red- In reflection: APE deviation varies from -20nm to -68nm; Colour variation from neutral to white-blue

Source APEREFERENCE

Results by Spectrophotometer with Angular Resolution

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