AR760 – The Optical Properties Of Solar Energy Material

The Optical Properties Of Solar Energy Material

The Optical Properties Of De AR760 of Leeuw Ltd – Reflex-Rol (UK)

Sample AR760

M.G. Hutchins

Solar Energy Materials Research Laboratory

Sonnergy Limited

Report Nº 09/312

December 2009

Contents

1. Introduction
2. De Leeuw Reflex-Rol (UK) Blind Sample
3. Experimental procedures
                3.1. Measurement of Spectral Transmittance and Reflectance
   • 3.2. Measurement of Infrared Spectral Transmittance and Reflectance
4. Calculation Methods
   • 4.1. Visible Transmittance and Reflectance
   • 4.2. Solar Transmittance and Reflectance
   • 4.3. Ultraviolet Transmittance
   • 4.4. Thermal Emittance
   • 4.5. Total Solar Energy Transmittance, Shading Coefficient and Shading Factor 
5. Results

1 – Introduction

The Solar Energy Materials Research Laboratory of Sonnergy Ltd undertakes ultraviolet, visible and infrared spectral optical properties measurements of materials for a wide range of industrial clients. The Laboratory is approved by the Ministry of Defence to perform measurements in compliance with Defence Standard DS0023/1 ‘NATO Infra Red Reflective (IRR) Green Colour for Painting Military Equipment’ (1), The laboratory operates in accordance with ISO 17025 for which compliance is being sought (2). Spectrophotometric instruments are serviced annually by  the respective manufacturers. All measurements are made in accordance with recognised international procedures and instruments are calibrated using traceable reference standards. The Laboratory participates regularly in proficiency tests and interlaboratory comparisons for the measurement of optical properties (3, 4, 5, 7). Sonnergy serves as the Chair of the European Union Cool Roofs Council Technical Committee (6) with responsibility for recommending measurement test procedures for product certification, is a full member of the International Commission on Glass Technical Committee 10 ‘Optical properties of glass and coated glass products’ (7) and provides the European representative for the peer review of optical properties spectral data for inclusion in the International Glazing Database (IGDB) (8) administered and maintained by the Lawrence Berkeley Laboratory, USA.

In this report measurements are presented of the total and diffuse near-normal hemispherical spectral reflectance and transmittance of the AR760 sample supplied by De Leeuw Ltd Reflex-Rol (UK) for the wavelength range 280 – 2500 nm. From these measurements integrated ultraviolet, visible and solar optical properties are calculated in accordance with accepted international standard procedures (9).

The total near-normal hemispherical spectral transmittance and spectral reflectance was measured for wavelength range 2.0 – 18.0μm using a Bruker IFS 66 Fourier transform spectrometer with a gold integrating sphere reflectance/transmittance accessory. Reflectance measurements were made for both sides of the sample. From these measurements the spectral absorptance was determined. The integrated emissivity was then calculated by weighting the spectral absorptance data with a 283K blackbody spectral distribution using the recommended procedure of EN 12898 (10,11).

The European Standards EN 14501 and EN 13363-1 (12, 13, 14) are used to calculate values of the total solar energy transmittance, gtotal, shading coefficient and shading factor, Fc, for complex glazing employing a fully closed internal blind in combination with the default glazing of each respective standard.

From the measured spectral optical properties data, an ASCII text file has been prepared which conforms to the format as required for the British Blinds and Shutters Association (BBSA) SHADE shading device optical properties database (15, 16).

2 – De Leeuw Reflex-Rol (UK) Blind Sample

The De Leeuw Reflex-Rol (UK) sample submitted for measurement is identified in Table 1.

Table 1. Identification of the De Leeuw Reflex-Rol (UK) blind sample.

3 – Experimental Procedures

        3.1 Measurement of Spectral Transmittance and Reflectance

Measurements of near-normal hemispherical spectral transmittance, τ(λ), and spectral reflectance, ρ(λ), were made using a Perkin Elmer Lambda 900 spectrophotometer using the PELA 150 integrating sphere accessory. Measurements were made over the spectral range 280 – 2500 nm (UV/Vis/NIR) to enable calculation of the integrated ultraviolet, visible and solar optical properties.

Total near-normal hemispherical spectral reflectance measurements were made with the sample mounted on the rear sample port of the 0.15 m diameter PELA 150 integrating sphere. The basic experimental configuration is shown in Fig.1. Calibration was made using 2 Labsphere Spectralon white reflectance standards (17). The measurement procedures were performed in accordance with EN 14500 and CIE 130 (18, 19).

For measurement of the total near-normal hemispherical spectral transmittance, τn-h(λ), the blind sample is located at the sample entrance port of the integrating sphere (Position A) and the rear sample mounting port (Position B) is covered with a white reflectance standard.

For measurement of the near-normal diffuse spectral transmittance, τn-dif(λ), the blind sample is located at the sample entrance port of the integrating sphere (Position A) and the rear sample mounting port (Position B) is left open (uncovered) to enable any direct component of the transmitted light to exit the sphere through this port.

For measurement of the total near-normal hemispherical spectral reflectance, ρn-h(λ) the blind sample is located at the rear sample mounting port (Position B) of the integrating sphere and the sample entrance port (Position A) is left open (uncovered).

For measurement of the near-normal diffuse spectral reflectance, ρn-dif(λ) the blind sample is located at the rear sample mounting port (Position B) of the integrating sphere and the integrating sphere specular reflectance exit port cover located at Position C is removed to allow the regularly reflected component to exit the integrating sphere.

Figure 1 – Experimental configuration for the measurement of spectral transmittance and reflectance (UV/Vis/NIR) using the PELA 150 integrating sphere reflectance accessory.
(D1: Photomultiplier detector; D2: PbS detector) 

3.2 Measurement of Infrared Spectral Transmittance and Reflectance

Measurements of total near-normal hemispherical spectral transmittance and  reflectance in the range 2.0 – 18.0 µm were made using a Bruker IFS 66 Fourier transform spectrometer using a 0.2 diameter diffuse gold coated integrating sphere reflectance attachment. A globar source and potassium bromide (KRr) beamsplitter combination were employed. The signal level inside the integrating sphere was detected using a wall mounted liquid nitrogen cooled mercury cadmium telluride (MCT) solid state detector with 3 x 3 mm² detector area.

For transmittance measurements the sample was mounted to cover the entry port of the integrating sphere and irradiated with a beam at normal incidence.

For reflectance measurements the sample was mounted on the rear sample port of the integrating sphere and irradiated with a beam at 10ºangle of incidence. Reflectance measurements were made for both sides of each sample. 

The system was calibrated using two diffuse gold reflectance standards (20) and a bare gold mirror calibrated to a traceable NPL gold mirror (21).

4 – Calculation Methods

4.1  – Visible Transmittance and Reflectance

The visible transmittance and reflectance of a sample is calculated using the relative spectral power distribution Dλ of illuminant D65 (22) multiplied by the spectral sensitivity of the human eye V(λ) and the spectral bandwidth Δλ.

Measurements are made of the spectral transmittance, τ(λ), and the visible transmittance, τν, is then calculated using a weighted ordinate method (9): according to EN 410 using the relationship:

Measurements are made of the spectral reflectance ρ(λ), and the visible reflectance, ρv is also calculated by weighted ordinates according to EN 410 using the relationship:

To evaluate these expressions the values of spectral transmittance and reflectance are taken at 10nm intervals from 380 – 780 nm and the values are normalised since ΣDλV(λ)Δλ = 1. The normalised fractional contributions of each interval to the total sum are tabulated in EN 410 (9).

4.2. Solar transmittance and Reflectance

The solar transmitttance, τs, is defined (23) as:

where Gλ is the spectral solar irradiation, τλ is the spectral transmittance and λ1 and λ2 respectively define the short and long wavelength limits of the solar spectral distribution.

The solar absorptance, ∝s, and solar reflectance, ρs, are similarly defined:

where ∝λ and ρλ are the spectral absorptance and spectral reflectance respectively.

It is normal only to measure ρλ and τλ and to deduce ∝λ from the conservation relationship ∝λ + ρλ + τλ = 1.

To evaluate the integrals the recommended procedure of EN 410 (9) is used and a weighted ordinate method is employed. Each of the integrals reduces to the form

where the family fi are the relative proportions of the total solar energy in each equal wavelength interval and their sum is normalised to unity.

4.3. Ultraviolet Transmittance

The ultraviolet transmittance, τuv, is calculated as (9)

where τλ is the spectral transmittance, Uλ is the relative distribution of the ultraviolet part of the global solar radiation and Δλ is the wavelength interval (5 nm).

4.4.  Thermal Emittance

The spectral emittance, ελ, is derived from the relationship (23_

ελ = 1 – ( ρλ + τλ )

For an opaque sample, where τλ = 0, this relationship reduces to ελ = 1 – ρλ. The spectral emittance, ελ, derived from spectral reflectance measurements is convoluted with the Planck blackbody spectral distribution, Ebλ, for a temperature of 283 K (4) and normalised to the ideal emitter (ε = 1) to give the total near-normal hemispherical thermal emittance εn.

The thermal emittance is thus expressed as

where λ1’ and λ2’ are the respective wavelength limits of the blackbody spectral distribution for the temperature of interest.

To evaluate this expression, the selected ordinate method prescribed in EN 12898 and EN 673 was used (10, 11).

4.5. Total Solar Energy Transmittance, Shading Coefficient and Shading Factor

Window and glazing thermal performance is described in relation to thermophysical properties denoting energy gains and losses. For the characterization of the energetical performance of a window the three main areas of interest are the determination of the heat transfer through the window, the solar gain through the window, and the light distribution behind the window. The quantitative properties are the overall heat loss coefficient (U-value), the total solar energy transmittance, which is termed the g value, and the visible light transmittance (τv).

The total solar energy transmittance, g, is the measure of the total energy passing through the glazing when exposed to solar radiation, It is the sum of the solar transmittance, τs, and the secondary internal heat transfer factor qi, i.e. g = τs + qi, the latter term arising from absorption of solar radiation in the glazing and subsequent re-radiation at thermal wavelengths to both the outside and the inside of the enclosure. The g-value is also called the Solar Heat Gain Coefficient (SHGC) and the Solar Factor.

The g value may be calculated for single or multiple glazing from the spectral transmittance and reflectance data and from knowledge of the heat resistances and surface heat transfer coefficients. A simplified method for the calculation of the g-value for gazing employing solar protection devices, such as blinds, is described in EN 13363-1 (13, 14). This method is also recommended when performing calculations in accordance with EN 14501 (12).

For the blind used internally, i.e. placed on the room side of the glazing, the total solar energy transmittance of the glazing-blind configuration, gtotal, is calculated from

gtotal = g ( 1 – g ρsb – ∝sb ( Λ / Λ2))

where:

g is the total solar energy transmittance of the glazing without the blind

ρsb is the solar reflectance of the blind facing the glazing

∝sb is the solar absorptance of the blind facing the glazing

Λ represents the effective heat transfer through the configuration defined as

Λ = 1 / ((1/U) + (1/Λ2))

where U is the heat loss coefficient of glazing without the blind and Λ2 assumes the value 18 W m¯² ºC¯¹.

The shading coefficient is derived by comparing the total solar energy transmittance of the glazing with a clear float glass having a total solar energy transmittance of 0.87. This corresponds to float glass of thickness 3-4 mm. The shading coefficient is the total solar energy transmittance, g, divided by 8.87.

The gtotal and SC values of the glazing/blind configuration are calculated for the blind in combination with default glazing cases. The two European standards EN 14501 (12) and EN 13363-1 (13) each identify 4 reference glazing.

The 4 reference glazing which represent the default cases defined in EN 14501 (12) together with their respective g and U values shown in Table 2.

The 4 reference glazing which represent the default cases defined in EN 1363-1 (13) together with their respective g and U values are shown in Table 3.

The Shading Factor, Fc, is defined (17) as the ratio of the total solar energy transmittance of the glazing-blind assembly, gtotal, to the total solar energy transmittance, g, of the glazing alone, i.e.

Fc is sometimes also termed z.

Note that for any given blind, the value of Fc is dependent upon the glazing with which the blind is combined, i.e. there is not a unique value of Fc for a given blind product.

Tabela 2. Values of the glazing thermal transmittance, U, and total solar energy transmittance, g, used to calculate the gtotal and shading coefficient values for the blind fabrics placed internally (taken from EN 14501 (12)).

Table 3.  Values of the glazing thermal  transmittance, U, and total solar energy transmittance, g, used to calculate the gtotal and shading coefficient values for the blind fabrics place internally (taken from EN-13363-1 (13)).

For the blind used externally, i.e. placed on the outside of the glazing, the total solar energy transmittance of the glazing-blind configuration, gtotal, is calculated from

gtotal = τsb g + ∝sb (Λ / Λ2) + τsb (1 – g) (Λ / Λ1)

where:

g is the total solar energy transmittance of the glazing without the blind

τsb is the solar transmittance of the blind

∝sb is the solar absorptance of the blind

Λ represents the effective heat transfer through the configuration defined as

Λ = 1 / ((1/U) + (1/Λ1) + (1/Λ2))

where:

U is the heat coefficient of the glazing without the blind,

Λ1 = W m¯² ºC¯¹ and Λ2 = 18 W m¯² ºC¯¹

5. Results

The two sides of the sample are designated as Ace A and Face B. Reflectance measurements are made for each face of the sample.

The measured UV/Vis/NIR (300 -2500 nm) total near-normal hemispherical and near-normal-diffuse spectral transmittance of De Leeuw Reflex-Rol (UK) sample AR760 are show in Figure 2.

The UV/Vis/NIR (300 – 2500 nm) total near-normal hemispherical and near-normal diffuse spectral reflectance of the De Leeuw Reflex-Rol (UK) AR760 sample are shown in Figure 3.

From these data, and using the expressions and methods described in Section 4, the respective total and diffuse visible transmittance from the measured total transmittance.

The integrated total near-normal hemispherical, near-normal diffuse and normal-direct solar and visible reflectance and transmittance of the De Leeuw Reflex-Rol (UK) AR760 sample are shown in Table 5.

The measured infrared total near-normal hemispherical spectral transmittance and spectral reflectance of the De Leeuw-Rol (UK) AR760 sample in the range 2.0 – 18.0 μm are shown in Figure 4.

The emissivity values derived from the infrared measurements of reflectance and transmittance, for each face of the sample are shown in Table 6.

The estimated uncertainty of all ultraviolet, visible and solar values is ± 0.02.

The estimated uncertainty of all emissivity values is ± 0.04.

Total solar energy transmittance, gtotal, shading coefficient, SC, and shading factor, Fc, values were calculated for the AR760 sample in combination with the reference glazing of the two European standards EN 13363-1 (7) and EN 14501 (17), in all cases with the blind placed on the inside of the respective glazing, using the simplified methods described in EN 13363-1. The results for the complex glazing using the EN 14501 reference glazing are presented in Table 7 and for EN 13363-1 reference glazing in Table 8 respectively.

Table 4. Integrated total near-normal hemispherical solar, visible and ultraviolet optical properties of the De Leeuw Reflex-Rol (UK) aviation foil AR760 sample. (Solar and visible reflectance values for Side A and Side B).

Table 5. Integrated total near-normal hemispherical, near-normal diffuse and normal-direct solar and visible reflectance and transmittance of the De Leeuw Reflex-Rol (UK) AR760 sample.

Table 6. Integrated total near-normal hemispherical emissivity, infrared reflectance and infrared transmittance of the De Leeuw Reflex-Rol (UK) AR760 sample.

Table 7. Calculated total solar energy transmittance, gtotal, shading coefficient, (SC), and shading factor, Fc, values of the De Leeuw Reflex-Rol (UK) AR760 sample used as internal shading in combination with the four standard glazing of EN 14501 (12). 

Table 8. Calculated total solar energy transmittance, gtotal, shading coefficient, (SC) and shading factor, Fc, values of the De Leeuw Reflex-Rol (UK) AR760 sample used as internal shading in combination with the four standard glazing of EN 13363-1 (13).

Wavelength (nm)

Figure 2. Total near-normal hemispherical and near-normal-diffuse spectral transmittance of De Leeuw Reflex-Rol (UK) sample AR760.

Wavelength (nm)

Figure 3. Total near-normal hemispherical and near-normal diffuse spectral reflectance of De Leeuw Reflex-Rol (UK) sample AR760 (Face A and Face B).

Wavelength (μm)

Figure 4. Total near-normal hemispherical infrared spectral  transmittance and reflectance of De Leeuw Reflex-Rol (UK) sample AR760 (Face A and Face B).