Journal of Heat Island Institute International Vol. 9-2 (2014)

Academic Article

Effects of windows with heat ray retro-reflective film on
outdoor thermal environment and building cooling load
Shinji Yoshida*1

Saori Yumino *2 Taiki Uchida *3Akashi Mochida*4*1*2

Associate Professor, Faculty of Engineering, University of Fukui
JSPS research fellow, Dept. of Architecture and Building Science, Graduate School of Engineering, Tohoku University*3
Graduate Student, Dept. of Architecture and Building Science, Graduate School of Engineering, Tohoku University*4
Professor, Dept. of Architecture and Building Science, Graduate School of Engineering, Tohoku University
Corresponding author: Shinji YOSHIDA, y-shinji@u-fukui.ac.jp

ABSTRACTThis study aimed to compare the effects of a heat ray retro-reflective film and other countermeasure techniques for windows,
from the perspective of reducing the cooling load and mitigating the effects on thermal environment. In this study, we simulated theradiation field around an isolated building during the summer season using a method that considers the directional reflection. Fourdifferent windows installed on the building surface were compared: (1) single float glass with a heat ray retro-reflective film, (2)untreated single float glass, (3) single float glass with heat-shading film, and (4) low-e double glass. Through comparison, it wasclarified that the adoption of the heat ray retro-reflective film on the building surface improved the radiant environment in outdoor
space and reduced the cooling load during the summer season.
Key Words : Urban heat island, Retro-reflective film, Energy saving

1. Introduction

retro-reflective film and other countermeasure techniques forwindows on thermal environment and the cooling load

Recently, various countermeasures have been launched tosimultaneously.

combat urban warming. There are three perspectives ofcountermeasures of urban warming: mitigating global warming,
mitigating urban heat island, and adapting to urban warming(1)(2).
However, their distinction remains unclear among researchers.
For example, low-e double glazing and heat shading films haveSpecularSpecularreflection
reflection

become popular for reducing building cooling load. Thus,
adopting these countermeasures has a positive impact on

Retro-reflection
Retro-reflectionHeat	
  ray	  is	
  reflected	
  to	
  padestrianHeat	
  ray	  is	
  reflected	
  to	
  incoming	
  direction	  (1) previous heat shading films(2) retro-reflective film

mitigating global warming; however, it has a negative impact on
adapting to urban warming because reflected solar radiationfrom a building surface worsens outdoor thermal environment(Fig.1). Hence, in this example, simultaneous evaluation of theFig.1: Difference of a retro-reflective film and previous heatshading films

two perspectives (mitigating global warming and adapting tourban

warming)isimportantinselectingtheproper
2. Outline of radiant analysis considering directionalcountermeasures against urban warming.

reflection

In recent years, heat ray retro-reflective films for window
application have been attracting attention. It is expected that the
adoption of this film will have a positive impact through2.1 Definition of elevation and azimuth angles

reduction in the cooling load and mitigation of the effects on

Figure 2 illustrates a local coordinate system on a surface

thermal environment in outdoor spaces, as shown in Fig.1.

element i in order to define an elevation and an azimuth angle in

The aim of this study was to compare the effects of the heat ray

this study. The elevation angle θ is defined as the angle between- 67 -

thenormal

line

to

the

surfaceelement

i

and

the

Z:	
  Normal	
  line	  to	
  surface	
  i (θ = 0°)
Emitted	  heat	  ray(θ = θe ,	
  ϕ= ϕe )incident/reflective heat ray. Hence, θ = 0° is normal to i, whileθ = 90° is a tangent to i. The azimuth angle ϕ is defined as theY(ϕ= 90°,	
  θ = 90°)

angle between the X-axis in the local coordinate and theθeincident/reflective heat ray, with counter clockwise rotation
taken to be a positive angle. In the field of building
environmental engineering, the angle for which rotation isϕeclockwise is defined as the azimuth Az, as shown in Fig. 2.
Surface	  element	
  iHowever, in mathematical coordinates, the former angle (ϕ) ismore readily available. Hence, both definitions are used together

Az=	
  −ϕ

X	
  
(ϕ= 0°,	  θ = 90°)

Fig.2: Definition of an elevation and an azimuth angle in a local

in the present paper.

coordinate system on a surface element i.

2.2 Existing method for radiant computation

i intercepted by a surface element j [W/sr], Ei(j) is the radiation

Many researchers have developed and used radiant simulation to

per unit solid angle emitted from surface i to surface j [W/sr],

evaluate the effects of radiation on thermal environment inρki(j) is the fraction of the radiosity reaching surface j fromoutdoor space. In most of these methods, radiosity, or the total

surface k via surface i per unit solid angle [1/sr], κki is theradiation energy flux leaving a surface per unit area and unit

correction coefficient of the distribution of the reflected radiosity

time, is defined by Eq. (1):from surface k to surface i, and ρhemi(k,i) is the reflectivity
measurement value from surface k via surface i to the

𝑅! = 𝐸! +    𝜌!

!
!!! 𝐹!" 𝑅!

surroundings.

(1)

where Ri is the radiosity [W], ρi is the reflectance of the surface

In this study, we incorporate this method into the analysis of theelement i, Ei is the radiation emitted at the surface element i [W],spatial distributions of solar radiation. In this case, Ei(j) is the sum

and Fij is the form factor, i.e. the fraction of radiation leaving theof the reflective components of incident, direct, and diffusive

surface element i that is intercepted by a surface element j. In

solar radiation at surface i to j:

this method, each surface in the computational domain is

𝐸! ! = ρ !! ,!! ;  !,! 𝐸!" +assumed to be a perfectly diffusively reflecting (or Lambertian)!!"#!!! κ!" ρ!" ! 𝐴! 𝐹!" 𝐼!"(5)

surface. Hence, the radiosity of surface element i intercepted bywhere θS and ϕS are the elevation and the azimuth angle of thea surface element j per unit of solid angle Ri(j) is defined by the

sun’s rays to the plane, respectively, EDi is the direct solar
radiation gain to surface i [W], Nsky is the number of surface

following equation:

elements that comprise the sky area, Ai is the area of surface i,𝑅!(!) = 𝑅! 𝜋

and ISH is the incident sky solar radiation on a horizontal surface(2)

As shown in the Eqs. (2), most of the existing methods could not[W/m2].

evaluate the radiant field that is strongly affected by the
directional reflection, such as the radiant field around a window

2.4 Calculation of directional reflectivity per unit solid angle

with a heat ray retro-reflective film.

In the method described in sections 2.2 and 2.3, the distributionsof directional reflectivity per unit solid angle ρi,j(k) affect the

2.3 The equations of the radiant computation considering

calculation results considerably. Ichinose et al. (2005) set ρi,j(k)using the Anisotropic body of rotation of the Normal Distribution

directional reflective property
In this study, to consider the directional reflection, radiant heatexchanges between urban surfaces were calculated by a method

function (AND) model that was proposed by Makino et al. (1999)(5). In this study, we adopt the AND model for the calculation of

proposed by Yoshida et al. . This method is revised theρi,j(k) for the heat ray retro-reflective film applied to windows.

progressive radiosity method extended to the directional radiantIn the AND model, the total directional reflectivity per unit solid(3)

computation(4)

angle ρ(θi; θo; ϕo) is divided into a directional and a diffusive

for outdoor space.

component of reflectivity as shown in Eq. (6):The equations of the expanded radiosity method are as follows:
𝑅!(!) = 𝐸!(!) +κ!" = 𝜌!!"# !,!

!
!!! κ!" 𝐹!" ･ρ!" ! ･𝜋･𝑅! !
!
!!! 𝐹!" ･𝜋･ρ!" !

𝜌(!! ;!! ;!! ) = 𝜌!(! ) + 𝜌!(!! ;!! ;!! )

(3)

!

(6)

where ρD(θi) is the diffusive reflectivity per unit solid angle at the(4)

incident elevation angle θi [1/sr]. The symbol ρS(θi; θo; ϕo) is thewhere Ri(j) is the radiosity per unit solid angle of surface element

directional reflectivity per unit solid angle at the incident elevation- 68 -

	%
angle θi, the reflect elevation angle θo, and the reflect azimuth
angle ϕo [1/sr].
Then, ρS(θi; θo; ϕo) is calculated using Eqs. (7), (8), (9), (10),𝜌! !! ;!! ;!! 𝑐𝑜𝑠𝜃! =(7)(𝑝! + 𝑞 ! )(8)

𝑔 = (𝑘𝑝 + 𝑓 ! −    𝑘 ! 𝑞 ! )/𝑓
𝑝 =    sin 𝜃! cos 𝜑! − 𝜑! !"#𝑞 =    sin θ! sin 𝜑! − 𝜑! !"#𝑘 =    sin 𝜃!(!"#)

Absorptance[6]"

0.6"
0.4"

0"

(9)
(10)

−𝑘

Transmissivity[6]"
1"

0.2"
0"

0.5"
1"
Incident%angle%[rad](1) Case 1

(11)
(12)

(in the case that θo = θo(max) and ϕo = ϕo(max), f = 0; g = (areal value) ≠ 0),

1.5"

	
%

(single float glass with heat ray retro-reflective film, AND model)
Reﬂectance[2]"Transmissivity[2]"Absorptance[2]"1"0.8"where θo(max) and ϕo(max) are the elevation and the azimuth angle
for the maximum value of ρS(θi; θo; ϕo), respectively. The

rate%[']𝑓=

𝐴 ∙ 𝑒𝑥𝑝 − 𝑓 ! 2𝜎 ! cos  { 𝜋 2 (𝑓 𝑔)! }Retro"Reﬂectance"[6]"0.8"rate%['](11), and (12):Mirror"Reﬂectance"[6]"

0.6"
0.4"
0.2"

symbol σ is a representative value for the peak width of ρScosθo.

0"

The symbols A, ρD, ρS, σ, θo(max), and ϕo(max) are parameters that

0"

0.5"

[rad]

1"

1.5"

	
%
(2) Case 2 (single float glass, AND model)

should be set according to the characteristics of the material
surface.

Reﬂectance[2]"Transmissivity[2]"Absorptance[2]"1"3. Radiation analysis for an isolated building
rate%[']

0.8"

3.1 Study area

0.6"
0.4"

We investigated the effects of the heat ray retro-reflective film on

0.2"

the radiant thermal environment around a building and the building

0"0"cooling load during the summer season.

0.5"

[rad]

1"

1.5"

Figure 3 illustrates the computational domain in the present(3) Case 3

analysis. As a first step of the study, it was assumed that the(single float glass with heat shading film, AND model)
Reﬂectance[2]"Transmissivity[2]"Absorptance[2]"1"building is located in an area without any effects of complex
terrain or locations of other buildings. The window was installed
ratio was set to 80%. In this analysis, we evaluated differencesin the radiant environment due to changes in the window

0.8"
rate%[']

only on the western surface of the building, and the window

0"

Transmissivity[6]"1"summer day. The meteorological data measured at the Japan

1"

Absorptance[6]"0.8"rate%[']

Building	
  walls	
  is	
  assumed	
  to	
  be	
  concrete(albedo	
   0.2,emissivity	
  0.9)Wall	
  area	  ratio	
  of	
  surfaces	
  except	
  for	
  west	  side	  surface:1.0Wall	
  area	  ratio	  of	
  west	  side	  surface
:0.2
20	
  m

0.5"

[rad]

	
%1.5"
(4) Case 4 (low-e double glass, AND model)
Mirror"Reﬂectance"[6]"Retro"Reﬂectance"[6]"We investigated the thermal environment on a particularly hotTarget	  day	
  and	
  time:7/23	
  15:00

0.6"
0.4"
0.2"

East

40	
  m
North

Window	
  area	  rate	
  of	
  
west	
  side	  surface:0.8

0.4"

0"

3.2 Meteorological conditions

140	
  m

0.6"

0.2"

properties on this surface.

20	
  m

	
%

A

Ground	  is	
  covered	
  with	  asphalt(albedo	
  0.1,emissivity	
  0.9)
South

0"
0"

0.5"
1"
Incident%angle%[rad]

1.5"

5	
  m

West

(5) Case 1

140	
  m

(single float glass with heat ray retro-reflective film, measurement data)A	
  pedestrian	
  stands	
  5m	
  away	  from	  the	
  western	
  surface	
  (MRT	
  calculation)

Fig.4: Distributions of absorptance, reflectance, transmissivity

Fig.3: Computational domain in the present analysis

on each incident elevation angle to window- 69 -

Meteorological Agency in Tokyo were used in this study, andapproximately 46° and 83°, respectively. Under normal

the target date was set to July 23, 2010.

circumstance, the study of temporal and seasonal change isrequired, because the radiant property of the window depends on

3.3 Computational cases

the incident angle of the sun ray. Hence we plan to study it in the

In this study, the following four computational cases werenext step of this study.

investigated. In Case 1, it was assumed that the single float glasswith a heat ray retro-reflective film was used for the western

3.4.1 Radiant properties of the western surface with window

window of the building, while the single float glass was used inTable 1 summarizes comparisons of radiant properties between

Case 2. The single float glass with heat shading film was used in

the western surfaces with windows in each case at the target

Case 3, and low-e double glass was evaluated in Case 4. In thetime of the present analysis. The values are estimated frompresent analysis, we model the radiant properties of theseradiant amounts of the calculation results described in thewindows using the AND model according with experimental

following sections. The value of the reflectance of mirror

results for each window. Figure 4 illustrates distributions ofcomponent in Case 1 is 7.5%, while the values in Cases 2, 3, andabsorptance, transmissivity, mirror and retro reflectance on the

4 are 13.0%, 26.0%, and 26.0%, respectively. In Case 1, theincident elevation angles of each case calculated by the AND

value of the reflectance of retro component that is reflected tomodel. In this figure, the experimenatl measurement data forthe sky is also 12.1%. Hence, we can see the window with heat

Case 1 is also included. We can see that there is a relatively wide

ray retro-reflective film contibute to apploximately 12% decrese

variation of values in Case 1. This is due to the fact that theTable 1: Comparison of solar radiant properties between western
surfaces with window in each case (weighted average ofthe radiant properties of window and wall at 15:00 on
July 23).
Case 1 Case 2 Case 3 Case 4

radiant property of the heat ray retro-reflective film is affectedby the incident azimuth as well as the incident elevation angle,
while the other windows are only affected by the incident
elevation angle.

Reflectance of mirror
3.4 Resultscomponent [%]In this section, we evaluate effects of installing the window with

Reflectancethe heat ray retro-reflective film using the results obtained atcomponent [%]15:00 on July 23, as a first step of this study. Solar position at

Absorptance [%]this time was nearly west, and solar altitude and azimuth wereTransmissivity [%]East&[W/m2]&
800&

East&

[W/m2]&
800&

700&
125&

600&

500&

400&

575&

300&
Solar&Radia*on&
West&(1) Case 1(Retro-reflective film)

300&

200&

Solar&Radia*on&

100&

West&

200&

13.0

26.0

26.0

51.3

21.8

40.8

40.8

32.5

66.0

34.6

34.6

12.1

[W/m2]&
800&[W/m2]#
80#
70#
60#
50#
40#
30#
20#
10#
0#
−10#
−20#
−30#
−40#
−50#
−60#
−70#
−80#

600&
500&

500&
550&
625&

575&

600&
Solar&Radia*on&
West&[W/m2]#
80#
70#
60#
50#
40#
30#
20#
10#
0#
−10#
−20#
−30#
−40#
−50#
−60#
−70#
−80#

550&

400&

625&

300&

575&

400&
300&

600&

200&

200&
Solar&Radia*on&
100&

West&

(4) Case 4(The low-e glass)

East&

−70&
−40&
−20&
Solar&radia)on&West&[W/m2]&
80&
70&
60&
50&
40&
30&
20&
10&
0&
−10&
−20&
−30&
−40&
−50&
−60&
−70&
−80&

(1) Case 1 － Case 2(2) Case 1 － Case 3(3) Case 1 － Case 4(Difference from the float glass)(Difference from the heat shading film)(Difference of the low-e glass)Fig.6: Distributions of the difference between Case 1 (heat ray retro – reflective film) and other cases (at 15:00 on July 23)- 70 -[W/m2]&
800&
700&

125&

600&

100&

−70#
−40#
−20#

East&

700&
125&

(2) Case 2(3) Case 3(The float glass)(The heat shading film)
Fig. 5: Distributions of absorbed solar radiation at 15:00 on July 23.

0#
120#

East&

500&
550&

400&

575&

retro

700&
125&

600&

550&

of

7.5

100&

Table 2: Comparison of the MRT values and the amounts of solar
radiation transmitted into the building.
MRT
Heat load by solar
Building coolingin the absorbed solar radiation on the window.
3.4.2 Distributions of absorbed solar radiation
Figure 5 shows the distributions of absorbed solar radiation at the
ground surface in each case. In Cases 3 and 4, the peak value ofsolar radiation was observed around the western surface of thebuilding, while a peak was not observed in Case 1. For Case 1,
these differences resulted from the reduction of the specularatradiation transmitted

load during the

15:00

into building during theperiod fromon July

period from 15:00 to

15:00 to 16:00 on

23

16:00 on July 23

July 23 [MJ/h].

[MJ/h].
Case 1

64.4

520

1570

Case 2

64.7

1054

2059

Figure 6 shows the distributions of the difference between Case 1

Case 3

67.9

646

1686

and the other cases. Focusing on the difference between each case,

Case 4

68.7

551

1602

reflection component of solar radiation by adoption of the heat ray
retro-reflective film.

the absorbed solar radiation around the western surface of thebuilding in Cases 3 and 4 was larger by up to 70 W/m2 than thatfor Case 1. In Case 2, the absorbed solar radiation around thewestern surface of the building was slightly larger than that for
Case 1.

load during the period from 15:00 to 16:00 on July 23. The
amount of the transmitted solar radiation in Case 1 isapproximately 520 MJ/h, while those in Cases 3 and 4
approximately 640 MJ/h and 550 MJ/h, respectively. The
building cooling load in Case 1 is also approximately 1570 MJ/h,while those in Cases 3 and 4 approximately 1680 MJ/h and 1600

3.4.3 Evaluation of results from the perspectives of outdoor

MJ/h, respectively. From these results, it has been found that

thermal environment and building cooling load
Table 2 summarizes the value of Mean Radiant Temperature(MRT) for an entire body at approximately 1 m height at point A
in Fig. 3 in each case. The result of MRT was related to outdoor
thermal environment for pedestrians. These values were
calculated using the method for analyzing inhomogeneousradiant environments proposed by Yoshida et al. (2014) (3). In
this analysis, it is assumed that a pedestrian stands 5 m awayfrom the center of the western surface of the building, as shown

installing the window with a heat ray retro-reflective film
mitigates both the trasmitted solar radiaton and the building
cooling load, compared with the window with heat shading film
and the low-e double glass.
4. Conclusions(1) The radiant environment around a building with four
different glazing types on the exterior window surface wasin Fig. 3. The pedestrian also faces to the southern direction. The

simulated using a method that considers the directionalheight of the pedestrian is assumed to be 1.736 m. We can see
the MRT in Case 1 was lower by up to 4.3°C than the MRT in
Cases 3 and 4. It seems that the differences of the MRT between

reflection.
(2) The results indicated that the MRT around the single float
glass window with the heat ray retro-reflective film wasCase 1 and Cases 3 and 4 are not so large. It has been thought

lower by up to 4.3°C than that around the low-e double glassthat this result has a connection with the computational domain

window. The amount of solar radiation transmitted into theof the present analysis. As mentioned above, in the present

building with the single float glass window with the heat ray

analysis, it is assumed that a building stands in the domain
where no effects of complex terrain and other building locations
are included. Hence, the results of the distributions of absorbed

retro-reflective film was lower than that of Case 2.
(3) As a future goal of this study, the thermal environment of areal town block based on the application of the heat ray

solar radiation and ground surface temperature are simple toretro-reflective film will be simulated, and the effects of theunderstand. However, in the area where the solar radiation that isfilm will be evaluated.

reflected from the windows is large, the amount of direct solar
radiation is also large due to the simple computational domain.
Hence, the influence of the direct solar radiation is dominant inmost of the study area. At the next step of this study, we plan tochange the study area to the building block, because the result ismore complex than that of the present analysis. We expect that
we can find the area where the influence of the reflected solar

Acknowledgements
This work was supported by JSPS Grant-in-Aid for Scientific
Research (B) (Grant Number 26289200) and Grant-in-Aid for
JSPS Fellows.

radiation is more dominant than that of the direct solar radiation.
Table 2 also summarises the amounts of the heat load by solar
radiation transmitted into the building, and the building cooling- 71 -

Table of symbols
A

Maximum

value

of

the

directionalreflective component

1/sr

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ISH
Nsky
Ri

radiation per unit solid angle emitted fromsurface i to surface j
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Number of surface elements that comprisethe sky area
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- 72 -