Optical Characterization of Liquid Crystal Switchable Mirror Philippe Lemarchand School of Physics You Supervisors’ Names Here Prof. Brian Norton Dr. John Doran 15 February 2013

Project Background Concentration of Solar Energy Using Switchable Mirrors. – What are Switchable Mirrors? Optical element that can be switched between a Transparent state and a Reflective state. 2 TechnologyAdvantagesInconvenients PrismaticCheapUse water – not practical Gasochromic (RE;Mg-RE;Mg-TM) Reflect wide Spectrum Colour Neutral Use hydrogen gas – not practical and potentially dangerous Research Level – not commercially available Short lifetime – 100s to 1000s of cycles Electrochromic (RE;Mg-RE;Mg-TM) Solid State (KOH electrolyte) Reflect wide Spectrum Colour Neutral Switch Electronically Research Level – not commercially available Short lifetime – 100s to 1000s of cycles Chiral Liquid Crystal (CLC) Solid State Reflect wide Spectrum (Custom designed: nm) Bandwidth tailored from 50 to 1,000 nm Colour Neutral Switch Electronically Commercially available >10 years lifetimes (indoor) New patented technology – presently expensive

Project Background Concentration of Solar Energy Using Switchable Mirrors. – Why for Solar Concentration? 1)optically track the sun  Remove need for mechanical parts and costs associated 2)collect a wide proportion of the solar diffused component  More light collected than standard stationary concentrators 3)optically regulate the solar heat flow  Reduce the need for cooling and cost associated 4)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. 3 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 Mirror Concentration of Solar Energy Using Switchable Mirrors. Experiment on a Visible [ nm] 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. 4 CLC polymer (diacrylate+monoacrylate), nacrylate=1.49 Polyimide coating, np=1.90 ITO layer, np=1.89 Glass substrate, np=1.52 LC molecule Example: 5CB (4-pentyl=4'- cyanobiphenyl) n0=1.54, ne=1.71  n=0.18 Rair-g RmolT 1 2.5mm 20  m square wave 0-~250V at 25Hz <10mA Average Power consumption: 40 mW/cm 2 Mol. Reflection Central Wavelength: Mol. Reflection Bandwidth: Mirror Reflection Bandwidth:

Setups 5 Lambda900 spectrophotometer Measure 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: ~1min At 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 Resolution Switchable 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 detection Angular accuracy: 1 degree Scanning time of visible range: millisecond Transmission/Reflection accuracy unknown at this stage  Using lambda900 results as reference

Results by Lambda nm Global Sun Power: 549 W.m-2 Direct Sun Power: 479 W.m nm Global Sun Power: 324 W.m-2 Direct Sun Power: 305 W.m-2

Results by Lambda900 7 In Clear State: -UV cut off 383nm -T ≥80% from 419nm In Reflective State: -<330nm UV transmission <1% - NIR cut off 732nm - Light leakage ≤5% in the range 423nm-681nm In Clear and reflective state: >75% transparent at wavelengths >800nm

Results by Lambda900 8 In Clear State: -UV cut off 383nm -T ≥80% from 419nm In Reflective State: -<330nm UV transmission <1% - NIR cut off 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 [ nm] Absolute Transmission Dynamic between Clear and Reflective States.

Results by Lambda900 9 In Clear State: -UV cut off 383nm -T ≥80% from 419nm] In Reflective State: -<330nm UV transmission <1% - NIR cut off 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 [ nm] -High reflective efficiency of the mirror [419nm-708nm] -Reflection bandwidth nm Relative Transmission Dynamic between Clear and Reflective States.

Spectrometer with Angular Resolution Method to Measure Absolute Reflection and Transmission 10 +  B +  A 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. B source A source Step 1: Without mirror, measure the fluctuation of the source over several minutes Since 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’.

Spectrometer with Angular Resolution Method to Measure Absolute Reflection and Transmission 11 Useful Spectral Range of the setup: nm Correction factor varying between 0.97 and 1

Spectrometer with Angular Resolution Comparison Method with Lambda900 12

Results by Spectrophotometer with Angular Resolution Mirror Reflective – Reflection Measurement 13 Increasing Incidence Angle Induces: 1)Reflection spectrum shrinks towards blue wavelength (Red light is lost and blue light is increasingly reflected) 2)Reflection bandwidth decreases 3)Average reflection percentage decreases !Reflection varies within nm range! !Reflection >70% at Normal incidence within nm!

Mirror Reflective – Transmission measurement 14 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 15 Uniform spectral reflection Reflection increases with angle solely due to glass reflection

Mirror Clear – Transmission Measurement 16 Quasi uniform spectral transmission Transmission 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 [ nm] 17 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 [ nm] 18 - 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 State 1 spectrum recorded every 30 secs for 1 hour. 19 Mirror switched to reflective state in <30secs within [ nm]

Switching Time – from Clear to Mirror State 20

Switching Time – from Clear to Mirror State 21 Switch from Clear to Mirror; Meas. In Reflection at 11deg; Time between spectra:30ms Mirror fully switched to reflective state in secs within [ nm]

Results by Spectrophotometer with Angular Resolution 22 Switch from Mirror to Clear; Meas. In Transmission at 0deg; Time between spectra:30ms Mirror fully switched to Clear state in <200ms within [ nm]

LC Switchable Mirror Solar Transmission and Reflection 23 TransmissionReflection % Solar Power (W.m-2) % Solar Power (W.m-2) Mirror State Reflective9%4369*-46%331*-220 Clear84%40216,5*-40%79*-191 Reflection bandwidth at normal incidence: nm Reflection bandwidth for R>70% at normal incidence: nm Spectral range with transmission and reflection fluctuation within 62degrees: nm Power consumption (clear state): ~400W.m-2 *Values constant within 12 degree light incidence angle. Spectral Range considered: nm Direct Solar Input power: 479W.m-2 Consideration : Powering a 1m 2 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: nm

LC Switchable Mirror Solar Transmission and Reflection 24 TransmissionReflection % Solar Power (W.m-2) % Solar Power (W.m-2) Mirror State Reflective9%7069*-46%541*-361 Clear84%65816,5*-40%129*-313 Reflection bandwidth at normal incidence: nm Reflection bandwidth for R>70% at normal incidence: nm Spectral range with transmission and reflection fluctuation within 62degrees: nm Power consumption (clear state): ~400W.m-2 * Values constant within 12 degree light incidence angle. Spectral Range considered: nm Direct Solar Input power: 784W.m-2 A 1m 2 mirror for 1m 2 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 25 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 nm range. – Optical simulation of solar concentrator designs using switchable mirrors

Results by Lambda Mirror Clear at Integrating Sphere Square wave voltage applied High voltage: Molecules forced to align 0V: 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 27

Mirror Reflective – Absorption/Diffusion within CLC layer 28 High Diffusion of blue/Green wavelengths

Mirror Clear – Absorption Measurement 29

Mirror Clear – Absorption/Diffusion within CLC layer 30 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 31 Diffusion has a linear/proportional function with molecules angle?!

Mirror Reflective – Summary (2) within [ nm] 32

Average Photon Energy (APE) – Expression of spectral shift and intensity variation 33 Reflective State Reflection Meas. Clear State Reflection Meas. Clear State Transmission Meas. Reflective State Transmission 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 APE REFERENCE

Results by Spectrophotometer with Angular Resolution 34