International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 3, Issue 2, February 2014
ISSN 2319 - 4847
Simulation and Design of Multilayer Interference
Filters of Coloured Glazed Thermal Solar
Collectors for Different
Design Wavelengths
Zainab I. Al- Assadi 1 , Nadhim Khaleel Ibrahim 2 , Hazim G. Daway 3 ,
Abdul- Hussain Kh. Elttayef 4 and Amal M. Al- Hillou 5
1
Al- Mustansiriyah University / Baghdad/ Republic of Iraq
Ministry of Sciences and Technology / Baghdad / Republic of Iraq
3
Al- Mustansiriyah University / Baghdad/ Republic of Iraq
4
Ministry of Sciences and Technology / Baghdad / Republic of Iraq
5
Al- Mustansiriyah University / Baghdad/ Republic of Iraq
2
Abstract
This study presents simple idea to get the a desired colours hiding the black colour, and the visibility of tubes and corrugations of
the metal sheet (absorber) of the thermal solar collectors which is consider the main obstacle to facade integration buildings of
solar thermal collectors by designing a multilayer optical interference filter during theoretical simulation and by changing the
design wavelength (λ ) for three values (500,600,700) nm which will cause shifting toward long wavelengths and for even number
of layers from (2-40) layers by employing appropriate dielectric materials with high refractive index (H) such as SiO 2 and low
refractive index (L) such as MgF2 deposited on glass substrate for quarter wave thickness and for the optical model air//HL//glass
An aesthetic aspects play an important role in facades integration buildings of the thermal solar collectors , the principle of the
coloured appearance is based on interference in the thin-films coating on the reverse side of the cover glass .
The presence of multilayered optical interference filters on the collector glazing can produce a coloured reflection in the visible
region, the coloured appearance should not cause excessive energy losses , while transmitting the non-reflected radiation in the
near IR region entirely to the absorber sheet to increase the efficiency of the thermal solar collector.
The computer simulation exhibit the optical behavior of this filter such as maximum reflection peak (R max), visible reflectance
(Rvis.) , solar transmission (Tsol.) , solar reflectance (Rsol.) and merit factor (M) .
Keywords: Coloured glazing , Coloured façades , Façade integrated solar collectors, Multilayer thin films .
1. Introduction
The thermal solar collectors absorbs and converts the incoming solar radiation to heat, which is transported by a heat
carrier fluid to a buffer store. The efficiency at which the collector converts the incident radiation to thermal energy
output is determined by light absorption and heat loss processes. The collector efficiency decreases due to increased heat
loss as the temperature difference between the collector and its surroundings increases, up to a point where no further
useful energy can be withdrawn (stagnation) [1] .
Thermal solar collectors, typically equipped with black, optical selective absorber sheets, exhibit in general good energy
conversion efficiencies. However, the black colour, and sometimes the visibility of tubes and unwanted undulations of the
thin metal sheets, limit the architectural integration into buildings. A recent opinion poll showed that 85% of architects
would prefer other colours than black, even if a lower efficiency was the price to pay [2] .
One solution to this problem would be to colour the absorber sheets. In this case, the absorber surface combines the
functions of optical selectivity (high solar absorption/low thermal emission) and coloured reflection. Alternatively, we
propose to establish a coloured reflection not from the absorber but from the cover glass. This approach has the advantage
that the black, sometimes ugly absorber sheet is then hidden by the coloured reflection . In addition to that, the functions
of optical selectivity and coloured reflection are separated, giving more freedom to layer optimization. No energy should
be lost by absorption in the coating: all energy, which is not reflected, should be transmitted. Therefore, multilayer
interference stacks of transparent materials are ideally suited for this purpose [3],[4] .
Colour is a subjective, human, response to the spectral quality of light. The response varies with the individual observer.
In order to observe the colour there must be an acceptable level of reflected or transmitted light [5],[6] . The technology
producing the desired colour effect is based on thin films interference filters, by using successive layers . A large palette
of colours can be obtained by varying thickness and/or number of layers [7].
One recent idea is the use of coloured glazing of cover glass for thermal solar collectors and building faces by depositing a
multilayer thin film on the glass surface see fig.(1) , for this purpose the reflecting multilayer consisting of oxides
materials have to fulfill some requirements. Firstly, a large amount of power from solar radiation must be transmitted
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 3, Issue 2, February 2014
ISSN 2319 - 4847
through the coatings. Secondly, there is a need for zero or near zero absorption materials to avoid energy loss within the
coating. Another important factor is the stability of colours with respect to a varying angle of reflection. Lastly, another
critical factor is a narrow peak reflectivity in the visible range fixing the desired colour of the reflected light. The ideal
reflectivity of the glass-film system should be a narrow band of the visible light while transmitting the rest of the sunlight
towards the black body to minimize energy losses see fig.(2) [8] .
Fig. (1) Principle of a coloured glazing thermal solar collector [9].
To obtain coloured reflected light, the cover glass of the collector should be coated on one side or both by thin films. To
avoid any absorption, the thin films must be made by dielectric and transparent materials, such as SiO2 , MgF2 , Al2O3,
TiO2 or a mixture of these oxides [8] .
Fig. (2) : The idea: Reflect a narrow band in the visible range [10].
In this work, we study the simulation and design of multilayer optical interference filters based on Mgf2/SiO2 , dielectric
films deposited on glass substrate , the design is air//HL/Glass with a thickness quarterwave stacks and for three cases of
the design wavelength () (500,600,700) nm and for even number of layers .
2. Façade Integrated Solar Collectors
Solar energy is not only gained from the rooftop but also from the building's outside walls. If all building facades facing
east, south, and west were used to produce energy, then the building could even transform itself from an energy user to an
energy supplier (Energy-Gaining Building, Plus-Energy Building). Solar facades can be equipped with photovoltaic
modules, thermal collectors, air collectors, honeycomb- air collectors or transparent heat insulation. Even a hung-up glass
glazing with a large surface area, such as is sometimes used for climate control in office buildings, or a simple glass
covering effectively making balconies into sunrooms or winter gardens can be considered a solar facade. A uniform or
standard solar facade technology does not exist. Depending on the building and the desired results there are various
solutions when constructing a solar facade. Such things as heating, ventilation, and climate control of the building must
be taken into consideration. Architecturally innovative solar facades are found at newly constructed buildings as well as at
older buildings being renovated [11]. Fig. (3) show the use of thermal solar collector into facades buildings.
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Fig (3) : The use of thermal solar collector into facades buildings [12] .
3. Advantages of Façade Integrated Collectors
• cost savings as a result of joint use of building components
• replacement of the conventional façade
• suitable both for new buildings and for the renovation of old buildings [11] .
4. Theory of Thin Films
Thin films are fabricated by the deposition of individual atoms on a substrate. A thin film is defined as a low-dimensional
material created by condensing , one-by-one, atomic/molecular/ionic species of matter. The thickness is typically less than
several microns. Thin films differ from thick films. A thick film is defined as a low-dimensional material created by
thinning a three-dimensional material or assembling large clusters/aggregates/ grains of atomic/molecular/ionic species
[13].
In this context a thin film supports interference effects while a thick film does not. Thus the term thin implies that the
film has surfaces that are sufficiently flat and parallel that when illuminated by a plane harmonic wave the infinite
number of waves reflected back and forth between the two surfaces have a constant unambiguous phase relationship that
does not depend on their lateral position Fig. (4) show that [5].
Fig.(4) : A single thin film [5] .
Let another film be added to the single film so that the final interface is now denoted by c , as shown in figure (5). The
characteristic matrix of the film nearest the substrate is :
Fig. (5) : Notation for two films on a surface [5] .
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 Eb 
 
 Hb 

i sin  2   2 
 cos  2
i 2 sin 

2
cos 
 Ec 
  
  Hc 
2
(1)
Since the tangential components of E and H are continuous across a boundary, and since there is only a positive-going
wave in the substrate, this relationship connects the tangential components of E and H at the incident interface with the
tangential components of E and H which are transmitted through the final interface. The 2 × 2 matrix on the right-hand
side of equation (1) is known as the characteristic matrix of the thin film. We can apply equation (1) again to give the
parameters at interface a , i.e.
 Ea 
 
 Ha 

 cos 1
 i 1 sin 

1
i sin 1  1 
cos 
i sin  2   2 
 cos  2
  i 2 sin 
 
2
1
cos 
2
 Ec 
  
  Hc 
(2)
and the characteristic matrix of the assembly is :
B
 
 C 

 cos  1
 i  1 sin 

1
i sin  1  1 
cos 
i sin  2   2 
 cos  2
  i  sin 
  2
2
1
cos 
2
1
  
  3 
(3)
and where we have now used the suffix m to denote the substrate or emergent medium .
m = y Nm cos m
for s-polarisation (TE)
m = y Nm / cos m
for p-polarisation (TM)
If ϑ0 , the angle of incidence, is given, the values of ϑr can be found from Snell’s law, i.e.
N0 sin ϑ0 = Nr sin ϑr = Nm sin ϑm.
(4)
A useful property of the characteristic matrix of a thin film is that the determinant is unity. This means that the
determinant of the product of any number of these matrices is also unity [3], [14]-[16] .
i sin  r  r   1 
 B   q  cos 
     i sinr
C  r  1  r
r
cos 

  m
r
r = y Nr cos r
for s-polarisation (TE)
r = y Nr / cos r
for p-polarisation (TM)
(5)
5. Optical Properties of Multilayer Films
As already mentioned, a large fraction of power from the solar radiation must be transmitted through the coatings. The
transparency of the film permits avoiding absorption energy losses. At the same time, the multilayer films must present a
narrow reflection band in the visible range fixing the colour of the reflected light. To estimate if a multilayer coated glass
sample is suitable to be used as a coloured solar collector glass, it is characterized by its solar transmission Tsol and its
solar reflectivity R sol, defined respectively by the following relations :
Tsol 
 T  I  d
 I  d
sol
(6)
sol
Rsol 
 R   I  d
 I  d
sol
(7)
sol
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T(λ) is the transmission of the film, R(λ) the total hemispherical reflectivity and Isol the intensity of the solar spectrum
AM1.5. The integration range is given by the limits of the solar spectrum. The visible reflectance Rvis is determined from
the photopic luminous efficiency function V(λ), the standard illumination D65(λ) and the total hemispherical reflectivity
R(λ):
Rvis 
 R   D  V  d
 D   V  d
65
(8)
65
The standard illuminant D65 closely resembles the relative spectral energy distribution of north-sky daylight and is
accordingly important for colour specification in northern Europe [8] .
Merit factor M defined as the ratio of the visible reflectance Rvis and the solar reflectivity Rsol . M is then large for a high
visible reflectance or low solar energy losses and consequently describes the energy efficiency of the visual perception
(“brightness per energy cost”) , the potential of colored thermal solar collectors can be expressed by a figure of merit M .
Following this definition, we obtain [9],[13] :
M 0  
Rvis 0  D65 0   V 0 
 I sol  d


Rsol 0 
I sol 0 
 D65    V  d
(9)
It is independent of the intensity of the reflection. The integrals just correspond to a normalization, the dependence on the
wavelength λ0 is simple [3],[4],[8],[13][17].
6. Structure Study and Computer Simulation
In this study the optical model air//HL//Glass was used , in this model all individual layers are of optical film thicknesses
n.t =λ/4 , where (λ) is called the design wavelength .
Usually layers of a low refractive index material (L) alternate with a high refractive index material (H) resulting in stack
of the LHLHLH… and with even number of layers from 2 to 40 layers , we take the design wavelength (λ) for three cases
the first one at (500) nm , the second at (600) nm and the third at (700) nm , we use the dielectric materials silicon
dioxide (Sio2) with a high refractive index (1.47) and magnesium fluoride (Mgf2) with a low refractive index (1.38)
deposited on glass substrate with (1.52) refractive index .
The difference in refractive indices between the high index and the low index material governs the peak height [13] .
The larger the difference in the refractive indices , the larger is the spectral region of high reflection . We are interested in
the opposite , a narrow reflection peak , which can in principle be created by employing a large number of layers (40
layers) , we chose the refractive indices to be very close to each other (but not identical) because the reflection at each
interface is weak now, by choosing a low refractive index material such as Mgf2 the level of background oscillation in the
reflectance spectra can be lowered thus gaining colour saturation and some percent in solar transmission [3],[16], fig.(6)
below show the reflectance spectrum curve for 2,4 and 6 layers at 500 nm design wavelengths and fig.(7) from 2-40 layers
for this case .
Fig.(6): Reflectance spectrum curve computed for a quarterwave stacks
consisting of 2,4 & 6 layers at λ = 500 nm .
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Fig.(7): Reflectance spectrum curve computed for a quarterwave stacks
consisting of 40 layers at λ = 500 nm .
The same behavior occur for the reflectance spectrum curve at (600 and 700 ) nm design wavelength , see fig.(8) and
fig.(9) .
Fig.(8):Reflectance spectrum curve computed for a quarterwave stacks
consisting of 40 layers at λ = 600 nm .
Fig.(9): Reflectance spectrum curve computed for a quarterwave stacks
consisting of 40 layers at λ = 700 nm .
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7. Result and Discussion
In this study simulation results show that increase the number of layers for even number from (2-40) layers will produce
increasing in peak high Rmax. of reflectance for three cases at (500, 600 &700) nm of design wavelength (λ) and it is
observe that identification in Rmax for the three cases see fig.(10) .
Fig. (10): increase the peck value of reflectance versus increasing the number of layers
for three values of design wavelength λ (500,600,700) nm .
Changing design wavelength will produce curve shifting toward long wavelengths this give us the possibility to chose the
colour coating that we need . When we use (500) nm design wavelength the coating will exhibit green colour reflectance ,
when we use (600) nm design wavelength the coating will exhibit orange colour reflectance in the same time using (700)
nm design wavelength the coating will exhibit red colour reflectance see fig. (11,12,13) . Increasing the number of layers
will decrease the width of the curves and become very narrow when the coating contain 40 layers see fig. (6,7,8,9) this
confirm the idea of reflect a narrow band in the visible range [10] , see fig.(4) .
Fig.(11): Reflectance spectrum curve for 2 layers at three cases
of design wavelength (λ) (500,600,&700) nm .
Fig.(12): Reflectance spectrum curve for (10) layers at three cases
of design wavelength (λ) (500,600,&700) nm .
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Fig.(13): Reflectance spectrum curve for (40) layers at three cases
of design wavelength (λ) (500,600,&700) nm .
The coloured reflection will add esthetic value to the thermal solar collectors while the near infrared region still antireflection region this mean the solar transmittance (Tsol.) value become high and its values varies from (95.7 - 97.8)% ,
see fig.(14) and the solar reflectance (Rsol.) very few and varies from (2-4.3)% see fig. (15) , consequence the efficiency of
the thermal solar collector will increase .
Fig (14): Solar transmittance (Tsol.) values versus number of layers .
Fig (15): Solar reflectance (Rsol.) values versus number of layers .
In the same time when we use matlab program and extracting the visible reflectance Rvis. depending on the equation (8) ,
we find for (500,600) nm design wavelengths increasing Rvis. with increasing number of layers while at (700) nm design
wavelength Rvis. increase slightly with increasing number of layers see fig.(16) .
A visible reflectance of 12% , which is already considerable for a colour (since100% corresponds to white) [13] .
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Fig (16): increasing visible reflectance values versus increasing number of layers .
The general potential of coloured thermal solar collectors is promising, and can be expressed by a figure of merit M this
number describes the energy efficiency of the visual perception (‘‘brightness per energy cost’’) [13]. Fig.(17) show the
varying merit factor with increasing number of layers for three cases (500,600 and 700) nm design wavelengths , the
figure show for two cases of design wave length (500,600) nm merit factor will exhibit progressive vibration with
increasing the number of layers while at (700) nm merit factor vibrate without increasing with increase the number of
layers .The efficiency of coloration at 600 nm design wavelength is better than using (500 , 700) nm design wavelength
this mean that at (600) nm design wavelength Rvis. values is higher than Rvis. values at (500 and 700) nm design
wavelengths , increasing Rvis. and opposite decrease Rsol. will increase merit factor M , see eq. (9) .
Fig.(17) : Merit factor versus increasing number of layers .
8. Conclusions
Multilayer optical interference filter work as anti reflection coating in the near IR region to increase the efficiency of the
thermal solar collector and as (green , orange , red) coloured reflection coating in the visible region to gain esthetic
aspect for the thermal solar collector which is used as building facades has been obtained by a theoretical simulation
made by using matlab program we designed it for this purpose , the structure of optical model is air//HL//Glass , for
quarterwave stacks and for even number of layers from (2-40) layer .
The behavior of the designed multilayer is analyzed by the computer simulation yielding the maximum peak of reflection
(Rmax), visible reflectance (Rvis.) , solar transmission (Tsol.) , solar reflectance (Rsol.) and merit factor (M) .
The proposed coloured glazed solar collectors will be ideally suited for architectural integration into buildings, e.g. as
solar active glass facades.
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Proceedings of the Industry Workshop of the IEA Solar and Cooling Programme, Task 26 in Delft, The Netherlands ,
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[3] A. Schüler , C. Roecker, J. Boudaden , P. Oelhafen , J.-L. Scartezzini ,"Coating for coloured glazed thermal solar
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AUTHOR
Zainab I. Al- Assadi received the B.S. and M.S. degrees in Physical Science / Laser and Molecule from AlMustansiriyah University in 2005 and 2008 , respectively. She now PhD. Student .
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