Solar Optical and Infrared Radiative Properties of Transparent Polymer Films (original) (raw)

Abstract

Polymer films are used in various solar energy devices, such as transparent insulation materials, encapsulation of solar cells, windows, substrates for functional layers, etc. In many cases the radiative properties of the films in the solar and infrared range are of importance for the specific application. This paper describes averaged solar optical and infrared radiative properties for various transparent polymer films including different polymer types and film thicknesses. The measurements were performed from the near ultraviolet to the medium infrared, covering the range of interest for calculating solar and thermal radiative transfer through glazings (e.g., transparent insulation). Concerning the solar optical properties, the extinction is dominated by scattering. In general, semicrystalline polymer films show higher solar losses than amorphous polymer films. Furthermore, additives frequently used in commercial polymer films to improve e.g. manufacturing, tear resistance and weathering behaviour also affect the solar optical properties of polymer films. In the medium infrared range, extinction is caused by absorption due to intramolecular bond rotations and vibrations of molecular groups and segments of polymeric chains. For blackbody temperatures from 10 to 100 °C ether-, fluoride-, imide-, sulphone-and siloxane-groups are of major relevance.

Figures (9)

Table 1. Investigated polymer film types (polymer nomenclature; polymer morphology (amorphous, a, or semicrystalline, sc), index of refraction; maximum long term service temperature; film thickness; film production process (casting, c, or extrusion, e)

Table 1. Investigated polymer film types (polymer nomenclature; polymer morphology (amorphous, a, or semicrystalline, sc), index of refraction; maximum long term service temperature; film thickness; film production process (casting, c, or extrusion, e)

Fig. 1. Calculation procedure for the integral solar absorption coefficient, K°, and the solar scattering coefficient, oO. a a a a A a > a At first the collimated-collimated transmittance and reflectance values were calculated subtracting the collimated-diffuse data from the collimated-hemispherical data. The spectral averages (or integral data) were determined by weighting the measured spectral radiometric property such as transmittance or reflectance by the AM 1.5 Global Irradiance source function. Due to the fact that radiation outside a cone of approximately 5° belonged to the diffuse component and because of the small differences between diffuse reflectance and diffuse transmittance (the difference was in many cases less than 0.01), isotropic scattering was assumed and the transmittance and reflectance data were adapted appropriately. On the one hand, these experimental integral properties were input data for the optimization routine. On the other hand, the 4-flux model (a generalization of the Maheu model (Maheu et al., 1984)) was used to calculate theoretical transmittance and reflectance values as a function of the integral absorption and_ scattering coefficients. In order to apply the 4-flux model, a number of parameters appearing in the theory must be known. The forward scattering ratio for both, collimated and diffuse light, was kept constant at 0.5. The average pathlength parameter for diffuse flux was defined as 2.0. The 4-flux model calculations gave theoretical values for the following four parameters: collimated-collimated transmittance, T, collimated-diffuse transmittance, Tg, collimated-collimated reflectance, P.., and collimated-diffuse reflectance, Q.g. Subsequently, the error function, that is the sum of the quadratic differences between experimental and theoretical values, was minimized and the variables KS and O° were optimized numerically. As starting parameters the absorption coefficient was defined as 0, and the scattering coefficient was set equal or larger than the overall extinction coefficient calculated from the experimental T., value. At first the collimated-collimated transmittance and reflectance

Fig. 1. Calculation procedure for the integral solar absorption coefficient, K°, and the solar scattering coefficient, oO. a a a a A a > a At first the collimated-collimated transmittance and reflectance values were calculated subtracting the collimated-diffuse data from the collimated-hemispherical data. The spectral averages (or integral data) were determined by weighting the measured spectral radiometric property such as transmittance or reflectance by the AM 1.5 Global Irradiance source function. Due to the fact that radiation outside a cone of approximately 5° belonged to the diffuse component and because of the small differences between diffuse reflectance and diffuse transmittance (the difference was in many cases less than 0.01), isotropic scattering was assumed and the transmittance and reflectance data were adapted appropriately. On the one hand, these experimental integral properties were input data for the optimization routine. On the other hand, the 4-flux model (a generalization of the Maheu model (Maheu et al., 1984)) was used to calculate theoretical transmittance and reflectance values as a function of the integral absorption and_ scattering coefficients. In order to apply the 4-flux model, a number of parameters appearing in the theory must be known. The forward scattering ratio for both, collimated and diffuse light, was kept constant at 0.5. The average pathlength parameter for diffuse flux was defined as 2.0. The 4-flux model calculations gave theoretical values for the following four parameters: collimated-collimated transmittance, T, collimated-diffuse transmittance, Tg, collimated-collimated reflectance, P.., and collimated-diffuse reflectance, Q.g. Subsequently, the error function, that is the sum of the quadratic differences between experimental and theoretical values, was minimized and the variables KS and O° were optimized numerically. As starting parameters the absorption coefficient was defined as 0, and the scattering coefficient was set equal or larger than the overall extinction coefficient calculated from the experimental T., value. At first the collimated-collimated transmittance and reflectance

Fig. 3. Collimated-hemispherical and collimated-diffuse solar transmittance (jp, Ta) spectra for 50 um thick PC and FEP films.

Fig. 3. Collimated-hemispherical and collimated-diffuse solar transmittance (jp, Ta) spectra for 50 um thick PC and FEP films.

Fig. 4. Solar scattering coefficient, O°, of transparent polymer films with service temperatures of approx. 100 °C as a function of film thickness, Z (single film interaction for PMP, CA, CTA; subsequent film interactions for PET, PMMA, PC).

Fig. 4. Solar scattering coefficient, O°, of transparent polymer films with service temperatures of approx. 100 °C as a function of film thickness, Z (single film interaction for PMP, CA, CTA; subsequent film interactions for PET, PMMA, PC).

Fig. 5. Solar scattering coefficient, O°, of transparent polymer films with service temperatures ranging from 150 to 200 °C as a function of film thickness, Z (single film interaction for ETFE, FEP; subsequent film interactions for PEI, PES, PSU).

Fig. 5. Solar scattering coefficient, O°, of transparent polymer films with service temperatures ranging from 150 to 200 °C as a function of film thickness, Z (single film interaction for ETFE, FEP; subsequent film interactions for PEI, PES, PSU).

REFERENCES Fig. 6. Collimated-collimated infrared transmittance, T.,, spectra for 50 um thick PC and FEP films. Fig. 7. Infrared absorption coefficient, Kip, of transparent polymer films with service temperatures of approx. 100 °C as a function of film thickness, Z.

REFERENCES Fig. 6. Collimated-collimated infrared transmittance, T.,, spectra for 50 um thick PC and FEP films. Fig. 7. Infrared absorption coefficient, Kip, of transparent polymer films with service temperatures of approx. 100 °C as a function of film thickness, Z.

the lowest and the modified cellulose based polymers (CA CTA) revealing the highest values for infrared absorption. Thi high absorption coefficients of CA and CTA are predominantly a result of the high density of C-O bonds in the molecula structure of these polymers. While the low temperatur polymer film types of PET, PMMA and PC also contain C-C groups, compared to the cellulose based films their density is significantly lower. Consequently, with regard to their infrarec absorption coefficients, PET, PMMA and PC films were foun to be positioned in the midrange between PMP and CA/CTA (see Fig. 7).

the lowest and the modified cellulose based polymers (CA CTA) revealing the highest values for infrared absorption. Thi high absorption coefficients of CA and CTA are predominantly a result of the high density of C-O bonds in the molecula structure of these polymers. While the low temperatur polymer film types of PET, PMMA and PC also contain C-C groups, compared to the cellulose based films their density is significantly lower. Consequently, with regard to their infrarec absorption coefficients, PET, PMMA and PC films were foun to be positioned in the midrange between PMP and CA/CTA (see Fig. 7).

Fig. 8. Infrared absorption coefficient, Kip, of transparent polymer films with service temperatures ranging from 150 to 200 °C as a function of film thickness, Z. freedom of mobility, all these polymer types contain a certain density of IR absorbing groups (fluoride, sulphone, ether and imide groups), thus yielding Kya values in the upper range of those covered by the low temperature films of Fig. 7.

Fig. 8. Infrared absorption coefficient, Kip, of transparent polymer films with service temperatures ranging from 150 to 200 °C as a function of film thickness, Z. freedom of mobility, all these polymer types contain a certain density of IR absorbing groups (fluoride, sulphone, ether and imide groups), thus yielding Kya values in the upper range of those covered by the low temperature films of Fig. 7.

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