- Email: [email protected]
dielectric coated energy-efficient glass windows for warm climates
Energy and Buildings 36 (2004) 891–898
Dielectric/Ag/dielectric coated energy-efficient glass windows for warm climates S.M.A. Durrani a,∗ , E.E. Khawaja a , A.M. Al-Shukri b , M.F. Al-Kuhaili b a
Center for Applied Physical Sciences, Research Institute, King Fahd University of Petroleum and Minerals, Box 1831, Dhahran 31261, Saudi Arabia b Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Accepted 15 February 2004
Abstract Energy-efficient glass windows for warm climates were designed and fabricated using a three-layer system of dielectric/metal/dielectric (D/M/D) on glass. Silver was used as a metal layer. The design parameters for optimum performance of D/M/D on glass-systems for dielectrics, having refractive indices in the range 1.6–2.4, were obtained by numerical calculations. Based on these parameters, D/M/D films on glass substrates were deposited using dielectrics such as TiO2 , WO3 , and ZnS. Upon testing these coated glass windows, it was concluded that the window with any of the three dielectrics performed well and the efficiencies of the windows with different dielectrics were nearly the same. © 2004 Elsevier B.V. All rights reserved. Keywords: Dielectrics; Energy-efficient glass windows; Coated windows
1. Introduction Over the past 40 years the introduction and development of an important new area of technology in the architectural field has occurred, namely, the use of thin film coatings to enhance the thermal performance of glass windows. Introductory installations of coated glass began in the 1960s with the development of large-scale coating facilities, and these were expanded considerably over the last four decades as the merits of coatings began to be appreciated [1]. Most of the applications have been in the commercial area. Now, in view of the growing energy crises, there is widespread interest world over for higher performance products, especially those that could contribute to energy savings [2–4]. The energy content of the solar spectrum is roughly split between the visible and near-infrared regions. A glass window to be energy-efficient in warm climates should have spectrally selective coatings such that it transmits nearly all the energy in the visible and reflects all the energy in the infrared. Thus an ideal energy-efficient window for a warm climate would have transmittance (T) and reflectance (R) to ∗
Corresponding author. E-mail address: [email protected] (S.M.A. Durrani). 0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.02.003
be given by: T = 1 and R = 0 for wavelength 400–700 nm (i.e. visible region), and, T = 0 and R = 1 for wavelength >700 nm (infrared, IR, region). Three-layer systems of dielectric/metal/dielectric (D/M/D) on glass substrates have been used for spectrally selective coatings for various purposes including the energy efficiency (e.g. Ref. [5–11]). By varying the material and thickness of the three layers, the optical properties of the D/M/D films can be tailored to suit different applications. The main objective of the present work was to develop a laboratory-scale version of thin film coated energy-efficient glass windows for applications in a warm climate. The three-layer system of D/M/D on glass was used. Numerical method was used for optimizing such a window to give high near-infrared reflection and low visual loss. Results are presented for windows with Ag film between TiO2 , WO3 , and ZnS layers.
2. Optimization of the optical performances of coated windows The D/M/D films on a glass substrate were used as a spectrally selective filter [5–11] that reflects infrared radiation (due to the properties of the metal layer) and transmits
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most of the visible spectrum. The highly reflective metal film, that otherwise transmits very little energy in the visible, was sandwiched between the two dielectric layers that act as anti-reflective coatings so as to enhance the energy transmitted in the visible region. Thus, the visible transmittance was chosen to be the parameter to be optimized for a given metal thickness, which in turn controls the IR reflectance. The optimum thickness of each dielectric layer was estimated. For the design of the D/M/D films, a computer program was written and implemented in order to perform calculations of the optical properties of multilayer films. For this purpose the characteristic matrix formulation, given by Heavens [12] was used. A systematic search was carried out in order to determine the effect of each layer on the optical properties of D/M/D type films. The film arrangement was taken to be:
air (n0 ,k0 )/D(n1 , k1 )/M(n2 ,k2 )/D(n3 ,k3 )/glass (n4 ,k4 ), where n’s and k’s are the refractive and absorption indices, respectively, of each of the layers and the media surrounding the three layers. The absorption of the solar radiation in air, dielectrics and glass was considered to be insignificant, that is k0 = k1 = k3 = k4 = 0. The two dielectric layers were taken to be of same material, that is n1 = n3 . The calculations (for D/M/D films on glass) were performed for normal incidence transmittance (T) and reflectance (R) using the equations given in Ref. [12]. It may be noted that the equation for T [12], gives the transmittance into the glass substrate. The transmittance measured in the air across the back-face of the substrate is about 4% less than that in the glass. The results of such calculations are discussed below. The T across the back-face of the substrate is discussed in the following.
0.68
0.554
n = 1.6
n = 1.8
f = 1.0
0.552
0.67
f = 0.9
f = 1.1
f = 0.9
0.550
Transmittance
f = 1.0
f = 1.1
0.66
(a) 0.548 52 0.78
54
56
58
60
(b) 0.65 42
62
0.85
n = 2.0
f = 1.0
0.77
0.84
0.76
0.83
46
50
54
n = 2.2
f = 1.0
0.82
0.75 f = 0.9
f = 1.1
f = 0.9
f = 1.1
0.81
0.74
(c) 0.73 35
58
40
45
(d) 50
0.80 30
0.90
35
40
45
f = 1.0
n = 2.4
f = 1.1 0.89
0.88 f = 0.9
(e) 0.87 28
30
32
34
36
38
40
Thickness, d 1 (nm) Fig. 1. Computer simulations for optimization of D/M/D layers on glass-system. The two D-layers in the system are of the same dielectric. Effects of thickness (d1 and d3 ) of dielectric layers on transmittance at wavelength of 550 nm for different values of f (=d3 /d1 ) are shown for dielectrics with refractive indices of (a) 1.6, (b) 1.8, (c) 2.0, (d) 2.2, and (e) 2.4.
S.M.A. Durrani et al. / Energy and Buildings 36 (2004) 891–898
893
ness. The results of Fig. 1a-e are summarized in Fig. 2. Using the results of Fig. 1a-e, the maximum in transmittance was plotted as a function of the refractive index of the dielectric in Fig. 2a. It is clear from Fig. 2a that for high transmission in the visible region, the dielectric to be used needs to have high refractive index. Using the results of Fig. 1a-e, the thickness d1 of the dielectric layer that gave maximum transmittance was plotted as a function of the refractive index of the dielectric in Fig. 2b. Once the dielectric is selected, Fig. 2b may be used to identify the thickness of the two dielectric layers for the optimum performance of the D/M/D system. Further calculations show that in a D/M/D system, when the thickness of both dielectric layers are changed by 60%, the change in IR reflectance is a mere 2.5%. It can, therefore, be concluded that the thickness of the dielectric layers can be altered in order to optimize transmission in the visible without any undue changes in the IR reflectance. Based on these findings, the spectral responses of various D/M/D were calculated, as discussed below.
2.1. Effects of (a) thickness and (b) refractive index of dielectric layer on T The design wavelength used here is 550 nm. The optical constants of Ag at 550 nm are n2 = 0.05 and k2 = 3.55 [13]. The thickness of the metal layer d2 = 20 nm was used. It is well known that Ag films, thinner than 15 nm, tend to be inhomogeneous and granular. More realistic values for the silver layer thickness are considered to be those of 16–24 nm [11]. Such films are semi-transparent with significant reflectance in the infrared. In Fig. 1a, the transmittance is plotted as a function of thickness d1 of the top dielectric layer for different values of the ratio, f = d3 /d1 (where d3 is the thickness of the bottom dielectric layer). Fig. 1a is obtained for a dielectric with refractive index n1 (or n3 ) = 1.6. Similar curves for other dielectrics with n1 = 1.8, 2.0, 2.2, and 2.4 are shown in Fig. 1b-e, respectively. It is apparent from Fig. 1, that for maximum transmittance at the design wavelength f = 1. This means that the dielectric layers in the D/M/D system should have the same thick-
60
0.90
Dielectric Layers
0.85 55
(a)
0.80
Tmax
0.75 45
0.70
Thickness, d 1 (nm)
50
40
(b)
0.65
35 0.60
0.55 1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
30 2.4
Refractive Index (n1 or n2) Fig. 2. Results of Fig. 1 are summarized here. Dependences of (a) maximum value of the transmittance and (b) thickness of the dielectric layer(s) on the refractive index of a dielectric are shown.
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Table 1 Optical constants (refractive index n and absorption index k) of Ag films at different wavelengths (λ) from Ref. [13] λ (nm)
n
k
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
0.25 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.09 0.07 0.06 0.06 0.05 0.05 0.05 0.05 0.05
14.50 13.75 13.15 12.50 11.80 11.03 10.10 9.50 8.70 8.00 7.18 6.40 5.60 4.80 4.00 3.07 2.25
2.2. Optical constants of thin films
Table 3 Refractive index (n) at different wavelengths (λ) of WO3 films λ (nm)
n
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
1.94 1.94 1.95 1.95 1.96 1.96 1.97 1.97 1.98 1.99 1.99 2.00 2.00 2.01 2.02 2.04 2.05
TiO2 deposited on unheated substrates (1.9), Table 3 for WO3 (2.03), Table 4 for TiO2 deposited on heated substrates (2.23), and Table 5 for ZnS (2.36). The values of the refractive indices of the dielectrics at a wavelength of 550 nm are given in parentheses.
The optical constants (n and k) for Ag-films given in Table 1, were taken from Ref. [13]. Dielectric films, such as TiO2 , WO3 , and ZnS, are non-absorbing in the wavelength region from 400 nm to 2000 nm; the region of interest in the present work. Therefore, the absorption index k = 0, and we are left with refractive index alone. A method given in Ref. [14] for determining both the refractive index (as a function of wavelength) and the thickness of a transparent film on a transparent substrate (glass) from measurement of transmittance at normal incidence was used in the present work. Average values of the refractive index at different wavelengths for the films of different thickness are given in Table 2 for
Spectral dependence of reflectance and transmittance of D/M/D systems such as (a) TiO2 /Ag/TiO2 , (b) WO3 /Ag/WO3 , (c) ZnS/Ag/ZnS, and based on the data of Tables 1–5 (optical constants of individual material), were calculated for different thickness of the Ag layer. Thicknesses of the dielectric layers used were obtained from Fig. 2. The results are shown in Fig. 3. It is clear from
Table 2 Refractive index (n) at different wavelengths of TiO2 films (λ) deposited on unheated substrates
Table 4 Refractive index (n) at different wavelengths of TiO2 films (λ) deposited on heated substrates (at 300 ◦ C)
λ (nm)
n
λ (nm)
n
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
1.79 1.79 1.80 1.80 1.80 1.81 1.81 1.82 1.82 1.83 1.83 1.84 1.85 1.87 1.89 1.93 2.05
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
2.10 2.10 2.10 2.11 2.11 2.12 2.12 2.13 2.13 2.14 2.14 2.15 2.16 2.18 2.21 2.27 2.40
2.3. Effects of thickness of Ag layer on spectral response of R and T
S.M.A. Durrani et al. / Energy and Buildings 36 (2004) 891–898
895
1.0
e
e
R
R
T
T 0.8
a
a
0.6
TiO2/Ag/TiO2 0.4
Transmittance (T) and Reflectance (R)
WO3/Ag/WO3
(Unheated Substrate)
a
a 0.2
(a)
(b)
e
e 1.0
e
T
e
T
R
R
0.8
a
a 0.6
TiO2/Ag/TiO2
ZnS/Ag/ZnS
(Heated Substrate) 0.4
a
a
0.2
(c)
(d)
e
e 0.0 400
800
1200
1600
400
800
1200
1600
2000
Wavelength (nm) Fig. 3. Computer simulation of spectral transmittance and reflectance of D/M/D on glass for different thickness of M-layer and for different dielectric materials. Curves marked ‘a’ to ‘e’ correspond to Ag-layer thickness of 16, 18, 20, 22 and 24 nm, respectively. For these simulations, data on optical constants given in Tables 1–5 were used.
Fig. 3 that as the Ag-layer thickness increases, an average transmitttance in the visible region (TVIS ) decreases while an average infrared reflectance (RIR ) increases. An ideal heat mirror is such that it has TVIS = 1.0 and RIR = 1.0 (see Section 1). This ideal behavior cannot be achieved in practice because, when the Ag film thickness is changed, one of the two TVIS and RIR , increases while the other decreases. Therefore, a compromise is needed. For optimum performance a figure of merit, Z, may be defined as Z=
TVIS dλ dλ
VIS
RIR dλ dλ
IR
where λ is the wavelength. For an ideal case, since TVIS = RIR = 1, the above equation gives Z = 1. However, for
the actual case, both TVIS and RIR are less than 1.0 (see Fig. 3). Therefore, Z is expected to be less than 1.0. Using the results of Fig. 3 in the above equation, the values of Z were calculated as a function of the thickness of the Ag-layer for the different D/M/D systems. The results are shown in Fig. 4. For the ZnS/Ag/ZnS and TiO2 /Ag/TiO2 (on heated substrate) systems, Z is maximum when the Ag-layer has a thickness in the vicinity of 18 nm. On the other hand, for the WO3 /Ag/WO3 and the TiO2 /Ag/TiO2 (on unheated substrate) systems, the maximum value of Z could be obtained for an Ag-layer of thickness that is smaller than 18 nm (Fig. 4). It is well known that Ag films thinner than 15 nm tend to be inhomogeneous and granular. More realistic values for the silver layer thickness are considered to be those of 16–24 nm [11]. Thus, Ag film thickness in the vicinity of
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Table 5 Refractive index (n) at different wavelengths (λ) of ZnS films Wavelength λ (nm)
Refractive index n
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
2.23 2.23 2.24 2.24 2.25 2.25 2.26 2.26 2.27 2.28 2.29 2.30 2.32 2.33 2.35 2.39 2.56
Table 6 Design parameters for optimum performance of D/M/D on glass-system for various dielectrics Dielectric
]a
[TiO2 WO3 [TiO2 ]b ZnS a b
0.70 ZnS/Ag/ZnS TiO2/Ag/TiO 2 Heated Substrate
0.65
Z-Factor
WO3/Ag/WO3
0.60 TiO2/Ag/TiO2 Unheated Substrate
0.55
17
18
19
20
21
22
Dielectric layer, d1 = d3
Metal layer, d2
46 43 38 34
18 18 18 18
Deposited on unheated substrate. Deposited on heated substrate.
3. Fabrication and results of D/M/D films on glass
18 nm seems to be a reasonable compromise. For this range of thickness, all the D/M/D systems discussed here, have values of Z that are greater than 0.6. Design parameters for optimum performance of D/M/D on glass-systems for various dielectrics are listed in Table 6. These parameters were used to fabricate the D/M/D films on glass substrates.
0.50 16
Thickness (nm)
23
24
Thickness of Ag Layer (nm) Fig. 4. Z-factor derived from the results of Fig. 3 as a function of the thickness of Ag-layer for the different dielectrics.
Thin films were prepared by physical vapor deposition (PVD) in a vacuum of 10−6 mbar, using a Leybold model L560 box coater pumped by a turbomolecular pump onto glass substrates. A tungsten boat was used for evaporation of Ag, while molybdenum boats were used for WO3 and ZnS. The TiO2 films were prepared by electron beam evaporation. Purities of the materials used for evaporation were better than 99.9%. In order to produce films with uniform thickness, the substrates were rotated while deposition took place. For monitoring the evaporation process of these films, a Leybold Inficon XTC quartz crystal monitor and evaporation controller were employed. Calibration of the quartz thickness monitor for individual material was achieved by measuring the actual thickness of single films by optical methods [14,15]. Normal incidence T and R from films was measured using JASCO V570 spectrophotometer. Various three-layered systems of dielectric/metal/dielectric on glass were fabricated using Ag as a metal layer while for a set of two dielectric layers either ZnS or WO3 or TiO2 was used. Designed parameters used were those given in Table 5. The three-layered system was fabricated in one go without breaking the vacuum, which required two different evaporation sources. Measured spectral reflectance and transmittance of D/M/D films on glass substrates are shown in Fig. 5 for ZnS/Ag/ZnS/glass, WO3 /Ag/WO3 /glass, and TiO2 /Ag/TiO2 / unheated-glass. For an ideal energy-efficient glass window the transmittance in the visible region should be close to one. It may be noted that for an uncoated glass window the transmittance is about 0.92. The systems such as ZnS/Ag/ZnS/glass, WO3 /Ag/WO3 /glass, and TiO2 /Ag/TiO2 /unheated-glass, are acceptable as these have average transmittance of about 0.7 in the visible region (Fig. 5). These three systems perform equally well as far as their transmittance in the visible region is concerned (Fig. 5). On comparing the measured reflectance and transmittance (Fig. 5) with the corresponding reflectance and transmittance obtained through computer simulations (Fig. 3) we find that for the different dielectrics the two are in good agreement (within 10%) for the entire spectral region that was covered. However, for the system of
S.M.A. Durrani et al. / Energy and Buildings 36 (2004) 891–898 1.0
897
48
(a) R
T
0.8
44
(a) uncoated glass
0.6
TiO2/Ag/TiO2
0.4
(unheated substrate)
40
Reflectance and Transmittance
o
Temperature ( C)
0.2
(b)
T
R
0.8
0.6
36
(b) coated glass
32
0.4
WO3/Ag/WO3
28
0.2
1.0
(c) 0.8
24
R
T
0.6
20
0
4
ZnS/Ag/ZnS
12
16
20
24
28
32
Time (min)
0.4
Fig. 6. Measured temperatures inside the two cubes as a function of time: (a) cube using uncoated glass and (b) cube using D/M/D coated glass.
0.2
0.0 400
8
800
1200
1600
2000
Wavelength (nm) Fig. 5. Measured reflectance and transmittance of D/M/D on glass-systems for the different dielectrics.
TiO2 /Ag/TiO2 /heated-glass the average transmittance in the visible region was smaller than 0.4, and thus it may not have been of real use. Inter diffusion of the two materials at the interface may have been the cause for such a low transmittance. Field tests of the coated glass were performed using a set-up that is described below. A cube (0.1 m × 0.1m) was constructed such that its walls were made from the coated glass plates, while wooden plates were used for the floor and the roof. Another cube of the same dimensions was constructed; however, in this case the coated glass plates were replaced with uncoated glass plates. Each wall of the two cubes was exposed to a 100 W incandescent lamp, and the temperatures inside both the cubes were measured as a function of time, using two thermometers. It may be mentioned that the spectrum of radiation emitted by a lamp somewhat resembles the solar radiation spectrum. In fact, there are special lamps that are commercially available and have radiation spectrum that matches the solar spectrum. However,
these lamps were not available locally. Therefore, in the present work ordinary filament lamps were used. The results are shown for a WO3 /Ag/WO3 /glass in Fig. 6. Inside temperature of the cube with coated windows was lower by about 8 ◦ C when compared with the inside temperature of the cube with uncoated windows (Fig. 6). The energy efficiency of the glass window is clearly reflected by the plots of Fig. 6. Similar results were obtained for ZnS/Ag/ZnS/glass and TiO2 /Ag/TiO2 /unheated-glass-systems and these were within 5% of those that are shown in Fig. 6.
4. Conclusions Energy-efficient glass windows for warm climates were designed and fabricated using a three-layer system of dielectric/metal/dielectric (D/M/D) on glass. Silver was used as a metal layer. Optimization of the optical performances of the three layers on glass through computer simulations was carried out. Based on some of the parameters obtained through the simulations, successful fabrication of the energy-efficient windows was achieved. Overall optical performances of the D/M/D systems with different dielectrics were in close agreement with those predicted by the simulations. On testing it was found that the D/M/D systems were
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highly energy-efficient. Temperature difference of about 8 ◦ C was observed inside the two cubes, one using coated and the other uncoated glass windows. Acknowledgements This work is part of an internal project # CAPS 1202, supported by the Research Institute of King Fahd University of Petroleum and Minerals. References [1] P.H. Berning, Appl. Opt. 22 (1983) 4127. [2] C.M. Lampert, Sol. Energy Mater. 11 (1984) 1.
[3] C.G. Granqvist, Thin Solid Films 193/194 (1990) 730. [4] R.B. Goldner, J. Vac. Sci. Technol. A 13 (1995) 1088. [5] C. John, C. Fan, F.J. Bachner, G.H. Foley, P.M. Zavracky, Appl. Phys. Lett. 25 (1974) 693. [6] J.A. Pracchia, J.M. Simon, Appl. Opt. 20 (1981) 251. [7] C.G. Granqvist, Appl. Opt. 20 (1981) 2606. [8] H. Köstlin, G. Frank, Thin Solid Films 89 (1982) 287. [9] C.C. Lee, S.H. Chen, C.C. Jaing, Appl. Opt. 35 (1996) 5698. [10] X. Zhang, S. Yu, M. Ma, Sol. Energy Mater. Sol. Cells 44 (1996) 279. [11] G. Leftheriotis, P. Yianoulis, D. Patrikios, Thin Solid Films 306 (1997) 92. [12] O. S. Heavens, Optical Properties of Thin Solid Films, Dover Publications, New York, 1991. [13] P.B. Johnson, R.W. Christy, Phys. Rev. B 6 (1972) 4370. [14] E.E. Khawaja, J. Phys. D: Appl. Phys. 9 (1976) 1939. [15] E.E. Khawaja, S.M.A. Durrani, A.M. Al-Shukri, Thin Solid Films 358 (2000) 166.
Dielectric/Ag/dielectric coated energy-efficient glass windows for warm climates S.M.A. Durrani a,∗ , E.E. Khawaja a , A.M. Al-Shukri b , M.F. Al-Kuhaili b a
Center for Applied Physical Sciences, Research Institute, King Fahd University of Petroleum and Minerals, Box 1831, Dhahran 31261, Saudi Arabia b Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Accepted 15 February 2004
Abstract Energy-efficient glass windows for warm climates were designed and fabricated using a three-layer system of dielectric/metal/dielectric (D/M/D) on glass. Silver was used as a metal layer. The design parameters for optimum performance of D/M/D on glass-systems for dielectrics, having refractive indices in the range 1.6–2.4, were obtained by numerical calculations. Based on these parameters, D/M/D films on glass substrates were deposited using dielectrics such as TiO2 , WO3 , and ZnS. Upon testing these coated glass windows, it was concluded that the window with any of the three dielectrics performed well and the efficiencies of the windows with different dielectrics were nearly the same. © 2004 Elsevier B.V. All rights reserved. Keywords: Dielectrics; Energy-efficient glass windows; Coated windows
1. Introduction Over the past 40 years the introduction and development of an important new area of technology in the architectural field has occurred, namely, the use of thin film coatings to enhance the thermal performance of glass windows. Introductory installations of coated glass began in the 1960s with the development of large-scale coating facilities, and these were expanded considerably over the last four decades as the merits of coatings began to be appreciated [1]. Most of the applications have been in the commercial area. Now, in view of the growing energy crises, there is widespread interest world over for higher performance products, especially those that could contribute to energy savings [2–4]. The energy content of the solar spectrum is roughly split between the visible and near-infrared regions. A glass window to be energy-efficient in warm climates should have spectrally selective coatings such that it transmits nearly all the energy in the visible and reflects all the energy in the infrared. Thus an ideal energy-efficient window for a warm climate would have transmittance (T) and reflectance (R) to ∗
Corresponding author. E-mail address: [email protected] (S.M.A. Durrani). 0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.02.003
be given by: T = 1 and R = 0 for wavelength 400–700 nm (i.e. visible region), and, T = 0 and R = 1 for wavelength >700 nm (infrared, IR, region). Three-layer systems of dielectric/metal/dielectric (D/M/D) on glass substrates have been used for spectrally selective coatings for various purposes including the energy efficiency (e.g. Ref. [5–11]). By varying the material and thickness of the three layers, the optical properties of the D/M/D films can be tailored to suit different applications. The main objective of the present work was to develop a laboratory-scale version of thin film coated energy-efficient glass windows for applications in a warm climate. The three-layer system of D/M/D on glass was used. Numerical method was used for optimizing such a window to give high near-infrared reflection and low visual loss. Results are presented for windows with Ag film between TiO2 , WO3 , and ZnS layers.
2. Optimization of the optical performances of coated windows The D/M/D films on a glass substrate were used as a spectrally selective filter [5–11] that reflects infrared radiation (due to the properties of the metal layer) and transmits
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S.M.A. Durrani et al. / Energy and Buildings 36 (2004) 891–898
most of the visible spectrum. The highly reflective metal film, that otherwise transmits very little energy in the visible, was sandwiched between the two dielectric layers that act as anti-reflective coatings so as to enhance the energy transmitted in the visible region. Thus, the visible transmittance was chosen to be the parameter to be optimized for a given metal thickness, which in turn controls the IR reflectance. The optimum thickness of each dielectric layer was estimated. For the design of the D/M/D films, a computer program was written and implemented in order to perform calculations of the optical properties of multilayer films. For this purpose the characteristic matrix formulation, given by Heavens [12] was used. A systematic search was carried out in order to determine the effect of each layer on the optical properties of D/M/D type films. The film arrangement was taken to be:
air (n0 ,k0 )/D(n1 , k1 )/M(n2 ,k2 )/D(n3 ,k3 )/glass (n4 ,k4 ), where n’s and k’s are the refractive and absorption indices, respectively, of each of the layers and the media surrounding the three layers. The absorption of the solar radiation in air, dielectrics and glass was considered to be insignificant, that is k0 = k1 = k3 = k4 = 0. The two dielectric layers were taken to be of same material, that is n1 = n3 . The calculations (for D/M/D films on glass) were performed for normal incidence transmittance (T) and reflectance (R) using the equations given in Ref. [12]. It may be noted that the equation for T [12], gives the transmittance into the glass substrate. The transmittance measured in the air across the back-face of the substrate is about 4% less than that in the glass. The results of such calculations are discussed below. The T across the back-face of the substrate is discussed in the following.
0.68
0.554
n = 1.6
n = 1.8
f = 1.0
0.552
0.67
f = 0.9
f = 1.1
f = 0.9
0.550
Transmittance
f = 1.0
f = 1.1
0.66
(a) 0.548 52 0.78
54
56
58
60
(b) 0.65 42
62
0.85
n = 2.0
f = 1.0
0.77
0.84
0.76
0.83
46
50
54
n = 2.2
f = 1.0
0.82
0.75 f = 0.9
f = 1.1
f = 0.9
f = 1.1
0.81
0.74
(c) 0.73 35
58
40
45
(d) 50
0.80 30
0.90
35
40
45
f = 1.0
n = 2.4
f = 1.1 0.89
0.88 f = 0.9
(e) 0.87 28
30
32
34
36
38
40
Thickness, d 1 (nm) Fig. 1. Computer simulations for optimization of D/M/D layers on glass-system. The two D-layers in the system are of the same dielectric. Effects of thickness (d1 and d3 ) of dielectric layers on transmittance at wavelength of 550 nm for different values of f (=d3 /d1 ) are shown for dielectrics with refractive indices of (a) 1.6, (b) 1.8, (c) 2.0, (d) 2.2, and (e) 2.4.
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ness. The results of Fig. 1a-e are summarized in Fig. 2. Using the results of Fig. 1a-e, the maximum in transmittance was plotted as a function of the refractive index of the dielectric in Fig. 2a. It is clear from Fig. 2a that for high transmission in the visible region, the dielectric to be used needs to have high refractive index. Using the results of Fig. 1a-e, the thickness d1 of the dielectric layer that gave maximum transmittance was plotted as a function of the refractive index of the dielectric in Fig. 2b. Once the dielectric is selected, Fig. 2b may be used to identify the thickness of the two dielectric layers for the optimum performance of the D/M/D system. Further calculations show that in a D/M/D system, when the thickness of both dielectric layers are changed by 60%, the change in IR reflectance is a mere 2.5%. It can, therefore, be concluded that the thickness of the dielectric layers can be altered in order to optimize transmission in the visible without any undue changes in the IR reflectance. Based on these findings, the spectral responses of various D/M/D were calculated, as discussed below.
2.1. Effects of (a) thickness and (b) refractive index of dielectric layer on T The design wavelength used here is 550 nm. The optical constants of Ag at 550 nm are n2 = 0.05 and k2 = 3.55 [13]. The thickness of the metal layer d2 = 20 nm was used. It is well known that Ag films, thinner than 15 nm, tend to be inhomogeneous and granular. More realistic values for the silver layer thickness are considered to be those of 16–24 nm [11]. Such films are semi-transparent with significant reflectance in the infrared. In Fig. 1a, the transmittance is plotted as a function of thickness d1 of the top dielectric layer for different values of the ratio, f = d3 /d1 (where d3 is the thickness of the bottom dielectric layer). Fig. 1a is obtained for a dielectric with refractive index n1 (or n3 ) = 1.6. Similar curves for other dielectrics with n1 = 1.8, 2.0, 2.2, and 2.4 are shown in Fig. 1b-e, respectively. It is apparent from Fig. 1, that for maximum transmittance at the design wavelength f = 1. This means that the dielectric layers in the D/M/D system should have the same thick-
60
0.90
Dielectric Layers
0.85 55
(a)
0.80
Tmax
0.75 45
0.70
Thickness, d 1 (nm)
50
40
(b)
0.65
35 0.60
0.55 1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
30 2.4
Refractive Index (n1 or n2) Fig. 2. Results of Fig. 1 are summarized here. Dependences of (a) maximum value of the transmittance and (b) thickness of the dielectric layer(s) on the refractive index of a dielectric are shown.
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Table 1 Optical constants (refractive index n and absorption index k) of Ag films at different wavelengths (λ) from Ref. [13] λ (nm)
n
k
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
0.25 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.09 0.07 0.06 0.06 0.05 0.05 0.05 0.05 0.05
14.50 13.75 13.15 12.50 11.80 11.03 10.10 9.50 8.70 8.00 7.18 6.40 5.60 4.80 4.00 3.07 2.25
2.2. Optical constants of thin films
Table 3 Refractive index (n) at different wavelengths (λ) of WO3 films λ (nm)
n
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
1.94 1.94 1.95 1.95 1.96 1.96 1.97 1.97 1.98 1.99 1.99 2.00 2.00 2.01 2.02 2.04 2.05
TiO2 deposited on unheated substrates (1.9), Table 3 for WO3 (2.03), Table 4 for TiO2 deposited on heated substrates (2.23), and Table 5 for ZnS (2.36). The values of the refractive indices of the dielectrics at a wavelength of 550 nm are given in parentheses.
The optical constants (n and k) for Ag-films given in Table 1, were taken from Ref. [13]. Dielectric films, such as TiO2 , WO3 , and ZnS, are non-absorbing in the wavelength region from 400 nm to 2000 nm; the region of interest in the present work. Therefore, the absorption index k = 0, and we are left with refractive index alone. A method given in Ref. [14] for determining both the refractive index (as a function of wavelength) and the thickness of a transparent film on a transparent substrate (glass) from measurement of transmittance at normal incidence was used in the present work. Average values of the refractive index at different wavelengths for the films of different thickness are given in Table 2 for
Spectral dependence of reflectance and transmittance of D/M/D systems such as (a) TiO2 /Ag/TiO2 , (b) WO3 /Ag/WO3 , (c) ZnS/Ag/ZnS, and based on the data of Tables 1–5 (optical constants of individual material), were calculated for different thickness of the Ag layer. Thicknesses of the dielectric layers used were obtained from Fig. 2. The results are shown in Fig. 3. It is clear from
Table 2 Refractive index (n) at different wavelengths of TiO2 films (λ) deposited on unheated substrates
Table 4 Refractive index (n) at different wavelengths of TiO2 films (λ) deposited on heated substrates (at 300 ◦ C)
λ (nm)
n
λ (nm)
n
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
1.79 1.79 1.80 1.80 1.80 1.81 1.81 1.82 1.82 1.83 1.83 1.84 1.85 1.87 1.89 1.93 2.05
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
2.10 2.10 2.10 2.11 2.11 2.12 2.12 2.13 2.13 2.14 2.14 2.15 2.16 2.18 2.21 2.27 2.40
2.3. Effects of thickness of Ag layer on spectral response of R and T
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895
1.0
e
e
R
R
T
T 0.8
a
a
0.6
TiO2/Ag/TiO2 0.4
Transmittance (T) and Reflectance (R)
WO3/Ag/WO3
(Unheated Substrate)
a
a 0.2
(a)
(b)
e
e 1.0
e
T
e
T
R
R
0.8
a
a 0.6
TiO2/Ag/TiO2
ZnS/Ag/ZnS
(Heated Substrate) 0.4
a
a
0.2
(c)
(d)
e
e 0.0 400
800
1200
1600
400
800
1200
1600
2000
Wavelength (nm) Fig. 3. Computer simulation of spectral transmittance and reflectance of D/M/D on glass for different thickness of M-layer and for different dielectric materials. Curves marked ‘a’ to ‘e’ correspond to Ag-layer thickness of 16, 18, 20, 22 and 24 nm, respectively. For these simulations, data on optical constants given in Tables 1–5 were used.
Fig. 3 that as the Ag-layer thickness increases, an average transmitttance in the visible region (TVIS ) decreases while an average infrared reflectance (RIR ) increases. An ideal heat mirror is such that it has TVIS = 1.0 and RIR = 1.0 (see Section 1). This ideal behavior cannot be achieved in practice because, when the Ag film thickness is changed, one of the two TVIS and RIR , increases while the other decreases. Therefore, a compromise is needed. For optimum performance a figure of merit, Z, may be defined as Z=
TVIS dλ dλ
VIS
RIR dλ dλ
IR
where λ is the wavelength. For an ideal case, since TVIS = RIR = 1, the above equation gives Z = 1. However, for
the actual case, both TVIS and RIR are less than 1.0 (see Fig. 3). Therefore, Z is expected to be less than 1.0. Using the results of Fig. 3 in the above equation, the values of Z were calculated as a function of the thickness of the Ag-layer for the different D/M/D systems. The results are shown in Fig. 4. For the ZnS/Ag/ZnS and TiO2 /Ag/TiO2 (on heated substrate) systems, Z is maximum when the Ag-layer has a thickness in the vicinity of 18 nm. On the other hand, for the WO3 /Ag/WO3 and the TiO2 /Ag/TiO2 (on unheated substrate) systems, the maximum value of Z could be obtained for an Ag-layer of thickness that is smaller than 18 nm (Fig. 4). It is well known that Ag films thinner than 15 nm tend to be inhomogeneous and granular. More realistic values for the silver layer thickness are considered to be those of 16–24 nm [11]. Thus, Ag film thickness in the vicinity of
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Table 5 Refractive index (n) at different wavelengths (λ) of ZnS films Wavelength λ (nm)
Refractive index n
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
2.23 2.23 2.24 2.24 2.25 2.25 2.26 2.26 2.27 2.28 2.29 2.30 2.32 2.33 2.35 2.39 2.56
Table 6 Design parameters for optimum performance of D/M/D on glass-system for various dielectrics Dielectric
]a
[TiO2 WO3 [TiO2 ]b ZnS a b
0.70 ZnS/Ag/ZnS TiO2/Ag/TiO 2 Heated Substrate
0.65
Z-Factor
WO3/Ag/WO3
0.60 TiO2/Ag/TiO2 Unheated Substrate
0.55
17
18
19
20
21
22
Dielectric layer, d1 = d3
Metal layer, d2
46 43 38 34
18 18 18 18
Deposited on unheated substrate. Deposited on heated substrate.
3. Fabrication and results of D/M/D films on glass
18 nm seems to be a reasonable compromise. For this range of thickness, all the D/M/D systems discussed here, have values of Z that are greater than 0.6. Design parameters for optimum performance of D/M/D on glass-systems for various dielectrics are listed in Table 6. These parameters were used to fabricate the D/M/D films on glass substrates.
0.50 16
Thickness (nm)
23
24
Thickness of Ag Layer (nm) Fig. 4. Z-factor derived from the results of Fig. 3 as a function of the thickness of Ag-layer for the different dielectrics.
Thin films were prepared by physical vapor deposition (PVD) in a vacuum of 10−6 mbar, using a Leybold model L560 box coater pumped by a turbomolecular pump onto glass substrates. A tungsten boat was used for evaporation of Ag, while molybdenum boats were used for WO3 and ZnS. The TiO2 films were prepared by electron beam evaporation. Purities of the materials used for evaporation were better than 99.9%. In order to produce films with uniform thickness, the substrates were rotated while deposition took place. For monitoring the evaporation process of these films, a Leybold Inficon XTC quartz crystal monitor and evaporation controller were employed. Calibration of the quartz thickness monitor for individual material was achieved by measuring the actual thickness of single films by optical methods [14,15]. Normal incidence T and R from films was measured using JASCO V570 spectrophotometer. Various three-layered systems of dielectric/metal/dielectric on glass were fabricated using Ag as a metal layer while for a set of two dielectric layers either ZnS or WO3 or TiO2 was used. Designed parameters used were those given in Table 5. The three-layered system was fabricated in one go without breaking the vacuum, which required two different evaporation sources. Measured spectral reflectance and transmittance of D/M/D films on glass substrates are shown in Fig. 5 for ZnS/Ag/ZnS/glass, WO3 /Ag/WO3 /glass, and TiO2 /Ag/TiO2 / unheated-glass. For an ideal energy-efficient glass window the transmittance in the visible region should be close to one. It may be noted that for an uncoated glass window the transmittance is about 0.92. The systems such as ZnS/Ag/ZnS/glass, WO3 /Ag/WO3 /glass, and TiO2 /Ag/TiO2 /unheated-glass, are acceptable as these have average transmittance of about 0.7 in the visible region (Fig. 5). These three systems perform equally well as far as their transmittance in the visible region is concerned (Fig. 5). On comparing the measured reflectance and transmittance (Fig. 5) with the corresponding reflectance and transmittance obtained through computer simulations (Fig. 3) we find that for the different dielectrics the two are in good agreement (within 10%) for the entire spectral region that was covered. However, for the system of
S.M.A. Durrani et al. / Energy and Buildings 36 (2004) 891–898 1.0
897
48
(a) R
T
0.8
44
(a) uncoated glass
0.6
TiO2/Ag/TiO2
0.4
(unheated substrate)
40
Reflectance and Transmittance
o
Temperature ( C)
0.2
(b)
T
R
0.8
0.6
36
(b) coated glass
32
0.4
WO3/Ag/WO3
28
0.2
1.0
(c) 0.8
24
R
T
0.6
20
0
4
ZnS/Ag/ZnS
12
16
20
24
28
32
Time (min)
0.4
Fig. 6. Measured temperatures inside the two cubes as a function of time: (a) cube using uncoated glass and (b) cube using D/M/D coated glass.
0.2
0.0 400
8
800
1200
1600
2000
Wavelength (nm) Fig. 5. Measured reflectance and transmittance of D/M/D on glass-systems for the different dielectrics.
TiO2 /Ag/TiO2 /heated-glass the average transmittance in the visible region was smaller than 0.4, and thus it may not have been of real use. Inter diffusion of the two materials at the interface may have been the cause for such a low transmittance. Field tests of the coated glass were performed using a set-up that is described below. A cube (0.1 m × 0.1m) was constructed such that its walls were made from the coated glass plates, while wooden plates were used for the floor and the roof. Another cube of the same dimensions was constructed; however, in this case the coated glass plates were replaced with uncoated glass plates. Each wall of the two cubes was exposed to a 100 W incandescent lamp, and the temperatures inside both the cubes were measured as a function of time, using two thermometers. It may be mentioned that the spectrum of radiation emitted by a lamp somewhat resembles the solar radiation spectrum. In fact, there are special lamps that are commercially available and have radiation spectrum that matches the solar spectrum. However,
these lamps were not available locally. Therefore, in the present work ordinary filament lamps were used. The results are shown for a WO3 /Ag/WO3 /glass in Fig. 6. Inside temperature of the cube with coated windows was lower by about 8 ◦ C when compared with the inside temperature of the cube with uncoated windows (Fig. 6). The energy efficiency of the glass window is clearly reflected by the plots of Fig. 6. Similar results were obtained for ZnS/Ag/ZnS/glass and TiO2 /Ag/TiO2 /unheated-glass-systems and these were within 5% of those that are shown in Fig. 6.
4. Conclusions Energy-efficient glass windows for warm climates were designed and fabricated using a three-layer system of dielectric/metal/dielectric (D/M/D) on glass. Silver was used as a metal layer. Optimization of the optical performances of the three layers on glass through computer simulations was carried out. Based on some of the parameters obtained through the simulations, successful fabrication of the energy-efficient windows was achieved. Overall optical performances of the D/M/D systems with different dielectrics were in close agreement with those predicted by the simulations. On testing it was found that the D/M/D systems were
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highly energy-efficient. Temperature difference of about 8 ◦ C was observed inside the two cubes, one using coated and the other uncoated glass windows. Acknowledgements This work is part of an internal project # CAPS 1202, supported by the Research Institute of King Fahd University of Petroleum and Minerals. References [1] P.H. Berning, Appl. Opt. 22 (1983) 4127. [2] C.M. Lampert, Sol. Energy Mater. 11 (1984) 1.
[3] C.G. Granqvist, Thin Solid Films 193/194 (1990) 730. [4] R.B. Goldner, J. Vac. Sci. Technol. A 13 (1995) 1088. [5] C. John, C. Fan, F.J. Bachner, G.H. Foley, P.M. Zavracky, Appl. Phys. Lett. 25 (1974) 693. [6] J.A. Pracchia, J.M. Simon, Appl. Opt. 20 (1981) 251. [7] C.G. Granqvist, Appl. Opt. 20 (1981) 2606. [8] H. Köstlin, G. Frank, Thin Solid Films 89 (1982) 287. [9] C.C. Lee, S.H. Chen, C.C. Jaing, Appl. Opt. 35 (1996) 5698. [10] X. Zhang, S. Yu, M. Ma, Sol. Energy Mater. Sol. Cells 44 (1996) 279. [11] G. Leftheriotis, P. Yianoulis, D. Patrikios, Thin Solid Films 306 (1997) 92. [12] O. S. Heavens, Optical Properties of Thin Solid Films, Dover Publications, New York, 1991. [13] P.B. Johnson, R.W. Christy, Phys. Rev. B 6 (1972) 4370. [14] E.E. Khawaja, J. Phys. D: Appl. Phys. 9 (1976) 1939. [15] E.E. Khawaja, S.M.A. Durrani, A.M. Al-Shukri, Thin Solid Films 358 (2000) 166.