SOLAR SELECTIVE ABSORBER FUNCTIONALITY OF CARBON NANOPARTICLES EMBEDDED IN SiO 2, ZnO and NiO MATRICES

Transcription

1 SOLAR SELECTIVE ABSORBER FUNCTIONALITY OF CARBON NANOPARTICLES EMBEDDED IN SiO 2, ZnO and NiO MATRICES G. KATUMBA 1, G. MAKIWA 1, L OLUMEKOR 1 & A FORBES 2 1 Physics Department, University of Zimbabwe, Harare, Zimbabwe 2 CSIR-National Laser Centre, Pretoria 0001, South Africa SAIP 2006

2 Concept and motivation Power density (W/m 2 µm) Solar (AM1.5) Ideal selective absorber 300 o C Blackbody 200 o C 100 o C Ideal absorber reflectance Wavelength (µm) 0

3 Electromagnetic wave propagation Interaction of light with matter. Complex refractive index. E m = E 0 exp ( kωx / c) exp[ i( ωt nωx / c) ] Magnetic field is not altered in optical and IR. Logarithmic reduction in intensity for a film thickness d is ln (I/I 0 ) = -αd = -4πkd/λ.

4 Tailoring materials Factor kd/λ determines the efficiency of a solar absorber surface. Proper combination of k and d. Homogeneous films can only vary d. Composite films change k! Practically constant for most materials used. Can also tune n by changing porosity.

5 Tailoring materials Therefore can only tune d to place the crossover at an appropriate wavelength. Reflectance and efficiency of absorber surface depends on k and d. Near-normal reflectance of bulk material is: 1 N 2 R = 1 + N

6 Device structure Many designs are possible. A tandem device: a composite layer deposited on a metallic substrate. Composite layer Aluminium substrate

7 Effective medium approximations Bruggeman: Maxwell-Garnett: 2 = N ( ) = Br b Br b a Br a Br a f a f ( ) ( ) b a a b a b a a b a b MG f f =

8 Theoretical optimisation Get n & k data. EMAs Brugeman andmaxwell- Garnett. Variable fill factor, f. Vary fill factor, f. MULTILAYER PROGRAM. Calculate theoretical reflectance, R. Variable fill factor, f and thickness, t. Vary thickness, t. No OPTIMUM? (qualitative) No Yes Reflectance data treatment. Calculate theoretical solar absorptance, α and thermal emittance,. END

9 Theoretical reflectance Bruggeman Maxwell-Garnett t BR =0.20 um t BR =0.50 um 0.8 t MG =0.20 um t MG =0.50 um 0.6 t BR =1.00 um 0.6 t MG =1.00 um R Wavelength (µm) Wavelength (µm)

10 Experimental procedure Sol production Tube furnace

11 Experimental procedure Carbonization Filtration of nitrogen gas

12 Optical characterisation Varian Cary 500 UV-VIS-NIR Bomem DA8 NIR-IR

13 Non-optical characterisation SEM: Philips XL30 Surface mophology XRD: Philips PW1840 Diffractometer Cu K α, 35 kev, 30 ma Crystal structure

14 Experimental results: SiO 2 samples Spin-coating speed Carbon precursor g SUC 9 g SUC 11 g SUC 12 g SUC R rpm (9:6) 3000 rpm (9:6) 4000 rpm (9:6) 4000 rpm (9:12) 5000 rpm (9:12) 1 10 Wavelength (µm) R Wavelength (µm)

15 Experimental results: IR spectrum FTIR reflectance spectrum R 0.6 OH OH Shoulder Wavenumber (cm -1 ) Si - O - Si

16 Experimental results: defects Problem- cracked films Solutions to cracking MTES & Ac 2 O

17 Experimental results: defects

18 Experimental results: spectra 1 Addition of MTES Addition of Ac 2 O R R wt.% MTES 20 wt.% MTES 30 wt.% MTES 0 wt.% MTES 1 10 Wavelength (µm) wt.% A c O 2 15 wt.% A c O 2 20 wt.% A c O 2 15 wt.% A c O (9:11) 2 0 wt.% A c O W avelength (µm)

19 Experimental results: X-HRTEM c (a) TEOS only (b) TEOS + Ac2O (c) TEOS + MTES Uniform distribution Segregation a b 7 nm 7 nm

20 Experimental results: EELS EELS mapping Carbon K peak (a) TEOS only (b) TEOS + Ac 2 O (c) TEOS + MTES a 7 nm c b 7 nm

21 Experimental results: ZnO samples Reflectance ZnO (A) ZnO (B) ZnO (C) ZnO (D) ZnO (F) Wavelength (µm)

22 Experimental results: NiO samples 1 Reflectance NiO (A) NiO (B) NiO (C) NiO (D) NiO (E) NiO (F) NiO (G) NiO (H) Wavelength (µm)

23 XRD results: NiO and ZnO

24 Experimental results: Comparison NiO (E) ZnO (C) SO LKOTE SiO 2 Reflectance Wavelength (µm)

25 Experimental results: XRD NiO (B) ( ) NiO (A) ZnO (C) ZnO (B) ( ) ( ) ( ) ZnO (A) ( ) Al ( ) ( ) θ ( ο )

26 Conclusions a possibility to produce low cost selective solar absorbers with sol-gel technique. Addition of 15 wt.% Ac 2 O appeared to solve the problem of cracking in SiO 2 samples better than 20 wt.% MTES.

27 Conclusions New and interesting microstructure of the sol-gel derived samples have been revealed: A short chain-like structure of both a silica matrix and carbon nanoparticles is quite evident. Homogeneity of the coatings at nanoscale is very encouraging.

28 Conclusions Sample α α/ C-SiO C-ZnO C-NiO

29 Acknowledgement University of Zimbabwe. Uppsala University. NMMU. Defence Peace Safety & Security, CSIR. Rental pool. African Laser Centre.