sustainability
Article
High-Reflectance Technology on Building Façades:
Installation Guidelines for Pedestrian Comfort
Hideki Takebayashi
Department of Architecture, Graduate school of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501,
Japan; thideki@kobe-u.ac.jp; Tel./Fax: +81-78-803-6062
Academic Editors: Constantinos Cartalis, Matheos Santamouris and Marc A. Rosen
Received: 15 June 2016; Accepted: 9 August 2016; Published: 10 August 2016
Abstract: The focus of this study is on the impact of solar radiation reflected from the building
façade to a pedestrian. The possibility of using high-reflectance technology on building façades was
evaluated by using a two-dimensional simple building façade model. The effectiveness of applying
retroreflective materials to building façades was also evaluated in regards to avoiding adverse effects
on pedestrians. The ratio of diffusely-reflected solar radiation to a pedestrian from a given floor is
proportional to the ratio of the angle of the reflective arc reaching a pedestrian from that floor to the
angle of the reflective arc from the entire building. Specular reflection of solar radiation from the
building façade is calculated by ray-tracing method corresponding to solar angle θ. In Japanese cities
that are located at mid-latitudes, applying high-reflectance technology to a building façade at the
fourth floor and above produces reflection of solar radiation that does not have adverse effects on
pedestrians. High-reflectance technology is applicable on building façades above the fourth floor
at any latitude, if we ignore a negative effect, since incident direct solar radiation to the building
façade around noon is small at low latitude. Retroreflective material was considered for use on
building façades below the third floor in order to avoid impacts on pedestrians from the reflection of
solar radiation.
Keywords: building façade; pedestrian comfort; high-reflectance technology
1. Introduction
As a measure to counter the urban heat island phenomenon, high-reflectance technology, such as
cool roofs and cool pavement, can be applied to places with large incident solar radiation [1–7].
Since the solar radiation incident on a building façade is small compared with that on roofs and
pavement [8], the priority for cooling measures on building façades is low. In addition, high-reflectance
technology applied to building façades reflects solar radiation downward as well as upward.
To address this problem, retroreflective materials have been studied [9–20]. Retroreflective
materials have already been used in various applications, such as on street signs [21,22]. Yuan et al. [9]
evaluated the durability of retroreflective materials in long-term outdoor exposure, and estimated their
retro-reflectance using a spectrophotometer and the heat balance calculated from outdoor temperatures.
Yuan et al. [10] also evaluated the total albedo and the cooling and heating loads for an urban canyon
that had retroreflective materials applied. Yuan et al. [11,12] evaluated the retro-reflectance of these
materials using an emitting-receiving optical fiber system. Rossi et al. [13] measured the angular
reflectance of retroreflective materials experimentally, and then compared their measurements with
the ones obtained through an equation that they devised for evaluating angular reflectance based on
diffuse reflection. Rossi et al. [14] made more measurements of the angular reflectance of several types
of retroreflective materials and evaluated the energy subsequently entrapped in an urban canyon at
various latitudes and days. Rossi et al. [15] further evaluated the urban heat island mitigation potential
of retroreflective pavement in urban canyons through an experiment in an existing small-scale urban
Sustainability 2016, 8, 785; doi:10.3390/su8080785
www.mdpi.com/journal/sustainability
evaluated the urban heat island mitigation potential of retroreflective pavement in urban canyons
through an experiment in an existing small-scale urban canyon. Measurement and calculation
methods for determining angular reflection from building materials have been studied by Ichinose
et al. [16], Nakaohkubo
et al. [17], Nishioka et al. [18,19], and Yoshida et al. [20].
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2016, 8, 785
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In the current study, the appropriate introduction of highly-reflective technology on building
façades is evaluated. Effects on the indoor environment have been previously evaluated, e.g., by
canyon.
andnot
calculation
methods
for determining
angular reflection
buildinga
Pisello etMeasurement
al. [23], and are
considered
here. Several
thermal comfort
indices forfrom
evaluating
materials
have
been
studied
by
Ichinose
et
al.
[16],
Nakaohkubo
et
al.
[17],
Nishioka
et
al.
pedestrian thermal comfort have been proposed such as PMV (predicted mean vote),[18,19],
SET*
and
Yoshida
et
al.
[20].
(standard new effective temperature), ET* (new effective temperature), PT (perceived temperature),
the current study,
the appropriate
introduction
of highly-reflective
technology
building
PET In
(physiologically
equivalent
temperature),
UTCI (universal
thermal climate
index),on
and
WBGT
façades
is
evaluated.
Effects
on
the
indoor
environment
have
been
previously
evaluated,
e.g.,
(wet bulb globe temperature) [24,25]. Generally, six thermal parameters (air temperature, humidity,
by
et al.
[23],radiant
and aretemperature,
not considered
here. Several
thermal rate)
comfort
fortoevaluating
air Pisello
velocity,
mean
col-value,
and metabolic
areindices
required
calculate
aindex
pedestrian
thermal
comfort
have
been
proposed
such
as
PMV
(predicted
mean
vote),
SET*
values. In this study, the focus is on the impact of solar radiation reflected from the
building
(standard
effective temperature),
(new
effective
PT (perceived
temperature),
façade to new
a pedestrian.
The objective ET*
of this
paper
is totemperature),
assess the impact
of the high-reflectance
PET
(physiologically
equivalent
temperature),
UTCI
(universal
thermal
climate
index),
andeffect
WBGT
technology introduced on building facades, by comparing with the current condition. The
on
(wet
bulb
globe
temperature)
[24,25].
Generally,
six
thermal
parameters
(air
temperature,
humidity,
the pedestrian through the longwave radiation heat exchange on a building façade with a low
air
velocity,
mean radiant
temperature, col-value,
and is
metabolic
rate)which
are required
to calculate
surface
temperature
that high-reflectance
technology
introduced,
is calculated
in the index
same
values.
In
this
study,
the
focus
is
on
the
impact
of
solar
radiation
reflected
from
the
building
façade
way as the diffuse reflection calculation, is not so large as compared with the effect by the reflected
to
a pedestrian.
The objective
this paper
to surface
assess the
impact of the
high-reflectance
technology
solar
radiation from
building of
façade,
since isthe
temperature
reduction
effect by introducing
introduced
on building
facades,
comparinginto
withconvection,
the current condition.
on the pedestrian
high-reflectance
technology
is by
distributed
radiation, The
andeffect
conduction
heat flux
through
the
longwave
radiation
heat
exchange
on
a
building
façade
with
a
low
surface
temperature
reductions. Therefore, I have focused only on the change of the reflected solar radiation
from the
that
high-reflectance
technology
is
introduced,
which
is
calculated
in
the
same
way
as
the
diffuse
current condition.
reflection calculation, is not so large as compared with the effect by the reflected solar radiation
from
building
façade,
since the
surface
temperature
2. Solar
Radiation
Reflected
from
Building
Façadesreduction effect by introducing high-reflectance
technology is distributed into convection, radiation, and conduction heat flux reductions. Therefore,
Figure 1 shows reflection from a building façade. A simple, two-dimensional building façade
I have focused only on the change of the reflected solar radiation from the current condition.
was used for all computations. The incident angle of the direct solar radiation is specified by the sun
angle
(a function
of Reflected
date, time,from
and latitude),
2.
Solar
Radiation
Building regardless
Façades of the orientation of the objective façade. The
diffuse reflection of solar radiation is taken as half upward and half downward from the horizontal.
Figure 1 shows reflection from a building façade. A simple, two-dimensional building façade
Reflectance values of 0.25 for conventional materials and 0.75 for high-reflectance materials were
was used for all computations. The incident angle of the direct solar radiation is specified by the
used in all computations.
sun angle (a function of date, time, and latitude), regardless of the orientation of the objective façade.
Since the diffuse reflection does not depend on the sun angle, the change of the sun angle may
The diffuse reflection of solar radiation is taken as half upward and half downward from the horizontal.
not be taken into account. Additionally, since the most significant impact on pedestrians near the
Reflectance values of 0.25 for conventional materials and 0.75 for high-reflectance materials were used
wall by the specular reflection occurs in the high sun angle condition, the influence by the specular
in all computations.
reflection should be considered in the high sun angle condition.
from aa building
building façade.
façade.
Figure 1. Diffuse and specular reflection from
In thethecase
of areflection
completely
surface,
as concrete,
paint,
or sun
wood,
used
Since
diffuse
does diffuse
not depend
on thesuch
sun angle,
the change
of the
angle
mayasnota
conventional
building Additionally,
façade, 12.5%since
of thethe
solar
radiation
is reflected
upward
and 12.5%
is reflected
be
taken into account.
most
significant
impact on
pedestrians
near the
wall by
to
a
pedestrian
area.
Approximately
75%
of
solar
radiation
is
absorbed
by
the
building
façade,
and is
the specular reflection occurs in the high sun angle condition, the influence by the specular reflection
redistributed
as
sensible
or
radiative
heat
flux
into
the
street
canyon
and
to
the
building
interior,
should be considered in the high sun angle condition.
In the case of a completely diffuse surface, such as concrete, paint, or wood, used as a conventional
building façade, 12.5% of the solar radiation is reflected upward and 12.5% is reflected to a pedestrian
area. Approximately 75% of solar radiation is absorbed by the building façade, and is redistributed as
Sustainability 2016, 8, 785
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sensible or radiative heat flux into the street canyon and to the building interior, which increases the
indoor cooling load. In total, 87.5% of solar radiation is converted into heat near the ground surface.
In the case of a high-reflectance, but completely-diffuse building façade, 37.5% of solar radiation is
reflected upward and 37.5% is reflected to a pedestrian area. Approximately 25% of the solar radiation
is absorbed by the building façade, and is redistributed as sensible or radiative heat flux into the street
canyon and to the building interior. In total, 62.5% of solar radiation is converted into heat near the
ground surface. Therefore, heat generation near the ground surface is reduced 25% when the solar
reflectance of a completely-diffuse building façade changes from 0.25 to 0.75.
In the case of a completely-specular surface, such as metal, glass, or tile, used as a conventional
building façade, 0% of solar radiation is reflected upward and 25% is reflected to a pedestrian area.
75% of solar radiation is absorbed by the building façade, and is redistributed as sensible or radiative
heat flux into the street canyon and to the building interior. In total, 100% of solar radiation is converted
into heat near the ground surface.
In the case of a completely-specular high-reflectance building façade, 0% of solar radiation is
reflected upward and 75% of solar radiation is reflected to a pedestrian area. The rest, i.e., 25% of solar
radiation is absorbed by the building façade, and is redistributed as sensible or radiative heat flux
into the street canyon and to the building interior. In total, 100% of solar radiation is converted into
heat near the ground surface. Since specular materials introduce such large amounts of heat near the
ground surface, cool façades are recommended to reduce heat island effects.
The solar radiation reflected to a pedestrian area is increased 25% in the diffuse surface case and
50% in the specular surface case when the solar reflectance of the building façade changes from 0.25 to
0.75. The impact of solar radiation reflected from the building façade to a pedestrian must, therefore,
be further evaluated.
3. Effects on Pedestrians
3.1. Diffuse Surfaces
The effect of diffuse reflection of solar radiation from the building façade to a pedestrian depends
on the distance from the objective building façade surface to the pedestrian. Pedestrians are modeled
as points for computational purposes. The ratio of diffusely-reflected solar radiation to a pedestrian
from a given floor is proportional to the ratio of the angle of the reflective arc reaching a pedestrian
from that floor to the angle of the reflective arc from the entire building (Figure 2) [26]. A Monte Carlo
calculation method to track a sufficient number of particles between the divided small components of
the façade, road, and other surfaces are used for diffuse reflection of solar radiation from the building
façade in previous studies [8,10,12,18]. In this study, since assuming a simple two-dimensional model
without trees or eaves, a fairly simple method is adopted. The angle from the objective building façade
of each floor to a pedestrian is calculated as:
H´B
B
` tan´1
A
A
(1)
pi ´ 1q H ´ B
iH ´ B
pi “ 2, 3, ¨ ¨ ¨q
´ tan´1
A
A
(2)
α1 “ tan´1
αi “ tan´1
where αi is the angle from the objective building façade on each floor (rad), i is floor number, A is the
distance from a pedestrian to the objective building façade (m), H is the height of each floor (m), and B
is the height of a pedestrian (m).
The effect on pedestrians from diffuse reflection of solar radiation from the building façade on
floor i can be evaluated by multiplying the solar reflectance value of the building façade on floor i by
the angle αi . The effect on pedestrians from high-reflectance technology with a solar reflectance of 0.35,
0.45, 0.55, 0.65, 0.75, or 0.85 that is applied to the first (ground), second, third, fourth, or fifth floor and
above, respectively, is calculated as a ratio to that of a conventional building façade. A floor height (H)
of 3.5 m is used in all cases.
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3.5
Time: t
3.5
3.5
Time: t
Time: t
Time: t
α5
Θt
Θt
3.5
3.5
α4
α5
α3
α4
α2
α3
B
B
α1
α2
α1
A
3.5
AｔａｎΘｔ
3.5
Θt
H=3.5
H=3.5
h
AｔａｎΘｔ
3.5
Θt
A
B
h
B
Figure 2.
2. Angle
Angle from
from each
each floor
floor to
to aa Apedestrian
pedestrian (left),
(left), and
and the
the height
heightA at
at which
which specular
specular reflection
reflection does
does
Figure
notFigure
affect2.pedestrians
pedestrians
(right).
not
affect
(right).
Angle from each floor to a pedestrian (left), and the height at which specular reflection does
not affect pedestrians (right).
The ratio of the diffuse reflection of solar radiation from the building façade to pedestrians (A =
The ratio of the diffuse reflection of solar radiation from the building façade to pedestrians
2 m, B The
= 1.5 m) of
is shown
in Figure 3. The
floors
at andfrom
above
high-reflectance
technology
radiation
thewhich
building
façade to pedestrians
(A = is
(A = 2 m, B ratio
= 1.5 m)the
is diffuse
shown reflection
in Figureof
3. solar
The floors
at and above
which high-reflectance
technology
applied
indicated
on the
horizontal
axis.
Theat reflection
solarhigh-reflectance
radiation from a conventional
2 m, Bare
= are
1.5
m) is shown
3. The
floors
aboveof
which
is
is applied
indicated
on in
theFigure
horizontal
axis.
The and
reflection
of
solar radiation from technology
a conventional
building
is takenonasthe
0.25.
The effect
onThe
pedestrians
high-reflectance
technology
is larger
appliedfaçade
are indicated
horizontal
axis.
reflection by
of solar
radiation from
a conventional
building façade is taken as 0.25. The effect on pedestrians by high-reflectance technology is larger when
when
it is applied
totaken
lowerasfloors,
its solar
reflectanceby
is larger,
and whentechnology
the distance
building
façade is
0.25. when
The effect
on pedestrians
high-reflectance
is between
larger
it is applied to lower floors, when its solar reflectance is larger, and when the distance between the
it is applied
to lower floors,
when When
its solar
reflectance is larger,
and when
the distance
between
thewhen
building
and pedestrians
is smaller.
high-reflectance
technology
is applied
above
the third
building and pedestrians is smaller. When high-reflectance technology is applied above the third floor,
the building
and
is is
smaller.
When
high-reflectance
is applied
abovefaçade.
the thirdThe
floor,
the effect
onpedestrians
pedestrians
less than
120%
of that fromtechnology
a conventional
building
the effect on pedestrians is less than 120% of that from a conventional building façade. The introduction
floor,
the
effect
on
pedestrians
is
less
than
120%
of
that
from
a
conventional
building
façade.
Thethe
introduction of high-reflectance technology above the third floor is, therefore, acceptable from
of high-reflectance technology above the third floor is, therefore, acceptable from the viewpoint of
introduction
of
high-reflectance
technology
above
the
third
floor
is,
therefore,
acceptable
from
the
viewpoint of adverse effects on pedestrians.
adverse
effects
on
pedestrians.
viewpoint of adverse effects on pedestrians.
Figure3. 3.Diffuse
Diffuse reflectionof
of solar radiation
radiation from high-reflectance
paint onona abuilding
façade to ato a
Figure
Figure
3. Diffuse reflection
reflection of solar
solar radiationfrom
fromhigh-reflectance
high-reflectancepaint
paint on abuilding
buildingfaçade
façade to
pedestrian
as
a
ratio
to
that
of
conventional
paint
(A
=
2
m,
B
=
1.5
m,
solar
reflectance
forfor
pedestrian
as
a
ratio
to
that
of
conventional
paint
(A
=
2
m,
B
=
1.5
m,
solar
reflectance
a pedestrian as a ratio to that of conventional paint (A = 2 m, B = 1.5 m, solar reflectance for conventional
conventional
paint
is
0.25).
conventional
paint
is 0.25). paint is 0.25).
3.2.
SpecularSurfaces
Surfaces
3.2.
Specular
3.2. Specular Surfaces
The effect on a pedestrian by specular reflection of solar radiation from the building façade
The effect on a pedestrian by specular reflection of solar radiation from the building façade
The effect
onincident
a pedestrian
bydirect
specular
of solar
radiation
from
building
façade
depends
on the
angle of
solar reflection
radiation and
the distance
from
the the
objective
building
depends on the incident angle of direct solar radiation and the distance from the objective building
depends
onathe
incident(Figure
angle of
solar at
radiation
and
the distance
from
objective
façade to
pedestrian
2).direct
The height
which the
specular
reflection
of the
solar
radiationbuilding
from
façade to a pedestrian (Figure 2). The height at which the specular reflection of solar radiation from
the building façade does not affect pedestrians is calculated as:
the building façade does not affect pedestrians is calculated as:
affect pedestrians (m), A is the distance from the objective building façade to a pedestrian (m), θ is
solar angle (°), and B is the height of a pedestrian (m). Specular reflection of solar radiation from the
building façade is calculated by a ray-tracing method corresponding to the solar angle θ, as well as
Nishioka et al. [18] and Yuan et al. [10,12].
The height at which the specular reflection of solar radiation from the building façade does not
Sustainability 2016, 8, 785
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affect a pedestrian (A = 2 m, B = 1.5 m) at 12:00 on August 1st in Osaka (34°40′ N; θ = 73.6°) was
calculated as 8.3 m from the ground, or above the fourth floor if H = 3.5 m. Since specular reflection is
likely to
at high(Figure
sun angles,
it height
is recommended
that
high-reflectance
technology
be used
on
façade
to occur
a pedestrian
2). The
at which the
specular
reflection of
solar radiation
from
building
façades
above
floor in thisisparticular
case
the
building
façade
doesthe
notfourth
affect pedestrians
calculated
as: (A = 2 m, B = 1.5 m, in Osaka) (Figure
4).
h “
Atanθ
` Bof the urban canyon which is dependent(3)
While Yuan et al. [10] have studied the
total
albedo
on
the urban canyon shape and the angle of incident solar radiation, the focus is on the impact of solar
where
h is reflected
the heightfrom
at which
specularfaçade
reflection
solar radiation
theHowever,
building the
façade
does not
radiation
the building
to a of
pedestrian
in thisfrom
study.
influence
by
affect
pedestrians
(m),
A
is
the
distance
from
the
objective
building
façade
to
a
pedestrian
θ is
the opposite side building should be considered. Since the distance from the opposite side (m),
building
solar
angle
(˝ ),influence
and B is the
height
of a pedestrian
(m).
Specular reflection
of solar
radiation
from
is large,
the
by the
diffuse
reflection is
sufficiently
small. Though
the
influence
by the
the
building
is calculated
a ray-tracing
method corresponding
the solar angle
θ, asnear
wellside
as
specular façade
reflection
cannot beby
ignored,
if the influence
by the speculartoreflection
from the
Nishioka
et
al.
[18]
and
Yuan
et
al.
[10,12].
building is avoided, pedestrians can walk on this side.
The
Theheight
heightat
atwhich
whichthe
thespecular
specular reflection
reflectionof
ofsolar
solarradiation
radiationfrom
fromthe
thebuilding
buildingfaçade
façadedoes
doesnot
not
˝ 401 N; θ = 73.6˝ ) was
affect
a
pedestrian
(A
=
2
m,
B
=
1.5
m)
at
12:00
on
August
1st
in
Osaka
(34
affect a pedestrian (A = 2 m, B = 1.5 m) on August 1st was calculated for every hour and every
calculated
as 8.3 m
the ground, or
above theisfourth
floor ifonHbuilding
= 3.5 m. façades
Since specular
reflection
latitude (Figure
5).from
High-reflectance
technology
applicable
above the
fourth
isfloor
likely
to
occur
at
high
sun
angles,
it
is
recommended
that
high-reflectance
technology
be
used
on
at any latitude, if we ignore a negative effect, since incident direct solar radiation to the
building
façades
above
the
fourth
floor
in
this
particular
case
(A
=
2
m,
B
=
1.5
m,
in
Osaka)
(Figure
4).
building façade around noon is small at low latitudes (Figure 6).
Diffuse ○
Specular ○
Apply Highreflectance paint
4F
Diffuse ○
Specular ×
Do not apply
High-reflectance
paint
3F
Diffuse △
Specular ×
○ × ×
B
Do not apply
High-reflectance
paint
2F
A
1F
Figure4.4.The
Theheight
heightat
atwhich
whichspecular
specularreflection
reflectionof
ofsolar
solarradiation
radiationfrom
fromthe
thebuilding
buildingfaçade
façadedoes
doesnot
not
Figure
affectpedestrians
pedestrians(A
(A== 22m,
m,BB== 1.5
1.5 m,
m, at
at 12:00
12:00 on
on 11 August
August in
in Osaka).
Osaka).
affect
While Yuan et al. [10] have studied the total albedo of the urban canyon which is dependent on
the urban canyon shape and the angle of incident solar radiation, the focus is on the impact of solar
radiation reflected from the building façade to a pedestrian in this study. However, the influence by
the opposite side building should be considered. Since the distance from the opposite side building is
large, the influence by the diffuse reflection is sufficiently small. Though the influence by the specular
reflection cannot be ignored, if the influence by the specular reflection from the near side building is
avoided, pedestrians can walk on this side.
The height at which the specular reflection of solar radiation from the building façade does not
affect a pedestrian (A = 2 m, B = 1.5 m) on August 1st was calculated for every hour and every latitude
(Figure 5). High-reflectance technology is applicable on building façades above the fourth floor at
any latitude, if we ignore a negative effect, since incident direct solar radiation to the building façade
around noon is small at low latitudes (Figure 6).
Sustainability 2016, 8, 785
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20
(50 m)
1820
18
16
16
14
14
12
12
10
10
8
8
6
6
4
4
2
2
0
0
(50 m)
The height
at which
specularh=A
reflection
does
(m)
not affect
pedestrians
tan+B
not affect pedestrians h=A tan+B (m)
The height at which specular reflection does
Sustainability 2016, 8, 785
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applicable abobe 6th floor
applicable abobe 6th floor
applicable abobe 5th floor
applicable abobe 5th floor
Osaka at noon
Osaka at noon
applicable abobe 4th floor
applicable abobe 4th floor
applicable
applicable
abobe
3rd floor
abobe 3rd floor
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
00
10
10
20
20
30
30
40
40
(Latitude)
50 (Latitude)
50
Figure
5. 5.
The
height
solar radiation
radiationfrom
fromthe
thebuilding
building
façade
does
Figure
The
heightatatwhich
whichspecular
specular reflection
reflection of
of solar
façade
does
notnot
affect
pedestrians
at
every
hour
and
every
latitude
(A
=
2
m,
B
=
1.5
m,
on
1
August).
(A == 2 m, B = 1.5 m, on 1 August).
affect pedestriansatatevery
everyhour
hourand
andevery
every latitude
latitude (A
Incident direct solar radiation to facade
Incident direct
radiation
to facade
2)
IDcossolar
 (kW/m
IDcos (kW/m2)
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
Osaka at noon
Osaka at noon
0.1
0.1
0
0
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00(Latitude)
15:00 16:00 17:00 18:00
0
0
10
10
20
20
30
30
40
40
50
50 (Latitude)
Figure
6. 6.Incident
solarradiation
radiationtoto
building
façade
at every
and latitude
every latitude
Figure
Incident direct
direct solar
thethe
building
façade
at every
hour hour
and every
on 1
Figure
6. Incident direct solar radiation to the building façade at every hour and every latitude on 1
August.
on
1 August.
August.
EffectsofofUsing
UsingRetroreflective
Retroreflective Materials
Materials
4. 4.Effects
4. Effects of Using Retroreflective Materials
Adverseeffects
effectson
onaapedestrian
pedestrian from
from the
the reflection
Adverse
reflection of
of solar
solar radiation
radiationare
areananissue
issuewhen
when
high-reflectance
technology
is appliedfrom
to building
façades.ofRetroreflective
materials
have
been
Adverse
effects
on
a
pedestrian
the
reflection
solar
radiation
are
an
issuestudied
when
high-reflectance technology is applied to building façades. Retroreflective materials have been
studied
as
a
way
to
address
this
problem
[9–20].
The
shape
and
reflection
characteristics
of
high-reflectance
technology
is applied
building
façades.
Retroreflective
materials
have been
as
a way to address
this problem
[9–20].toThe
shape and
reflection
characteristics
of retroreflective
retroreflective
tile
and
the
effects
of
using
retroreflective
material
on
the
lower
floors
are
shown
in
studied
way to
this problemmaterial
[9–20]. on
Thethe
shape
and
reflection
characteristics
tile
and as
theaeffects
of address
using retroreflective
lower
floors
are shown
in Figure of
7.
Figure 7. A retroreflective
tile [27]ofthat
is already
commercially
available
inlower
Japanfloors
was used
in this in
retroreflective
tile
and
the
effects
using
retroreflective
material
on
the
are
shown
A retroreflective tile [27] that is already commercially available in Japan was used in this
study.
study.
a very small tile
fraction
of
solar
radiation
is reflected downward
from
the was
retroreflective
Figure
7. Only
A retroreflective
[27] radiation
that
is already
commercially
available
in
Japan
used
in this
Only
a very
small fraction of
solar
is reflected
downward
from the
retroreflective
material
material (Figure 7).
study.
Only
a
very
small
fraction
of
solar
radiation
is
reflected
downward
from
the
retroreflective
(Figure 7).
material (Figure 7).
Sustainability 2016, 8, 785
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4.1.
4.1. Ideal
IdealRetroreflective
Retroreflective Materials
Materials
4.1.
Ideal
Retroreflective
Materials
Based
Based on
on the
the study
studyresults
resultsin
in the
the previous
previous chapter,
chapter, we
we model
model using
using retroreflective
retroreflective materials
materials on
on
Based
on
the
study
results
in
the
previous
chapter,
we
model
using
retroreflective
materials
on
the
lower
levels
(third
floor
and
below)
(Figure
7).
Pedestrians
are
not
affected
by
retroreflective
the
lower
levels
(third
floor
and
below)
(Figure
7).
Pedestrians
are
not
affected
by
retroreflective
the lower levels (third floor and below) (Figure 7). Pedestrians are not affected by retroreflective
material
material that
that is
is applied
applied to
to the
the building
building façade
façadebelow
belowthe
thethird
third floor
floor so
so long
long as
as the
the solar
solar radiation
radiation is
is
material
that
is
applied
to
the
building
façade
below
the
third
floor
so
long
as
the
solar
radiation
is
reflected
completely
upward.
Only
the
diffuse
reflection
from
high-reflectance
technology
applied
reflected
completely
upward.
Only
the
diffuse
reflection
from
high-reflectance
technology
applied
to
reflected completely upward. Only the diffuse reflection from high-reflectance technology applied
to
the
building
façade
above
the
fourth
floor
would
affect
pedestrians.
The
amount
the
building
façade
above
of
solar
radiation
to
the
building
façade
abovethe
thefourth
fourthfloor
floorwould
wouldaffect
affectpedestrians.
pedestrians. The
The amount
amount of
of solar
solar radiation
radiation
reflected
to
a
pedestrian
from
an
ideal
retroreflective
material
is
calculated
as
a
ratio
to
that
of
aa
reflected to
to aa pedestrian
pedestrian from
froman
anideal
idealretroreflective
retroreflectivematerial
materialisiscalculated
calculatedasasa aratio
ratio
that
reflected
to to
that
of of
conventional
building
façade
with
solar
reflectance
of
0.25
(Figure
8).
A
reflectance
of
is
a conventional
building
façade
with
solar
reflectance
0.25
(Figure
A solar
reflectance
of
0.75
conventional
building
façade
with
solar
reflectance
of of
0.25
(Figure
8).8).
A solar
solar
reflectance
of 0.75
0.75
is
used
for
both
high-reflectance
materials
and
retroreflective
materials.
Results
were
calculated
for
is
used
for
both
high-reflectance
materials
and
retroreflective
materials.
Results
were
calculated
for
used for both high-reflectance materials and retroreflective materials. Results were calculated for
two
two cases.
cases. In
Inthe
thefirst
firstcase,
case,high-reflectance
high-reflectancetechnology
technologywas
wasapplied
appliedto
tothe
thefourth
fourthfloor
floorand
andabove,
above,
two
cases.
In
the
first
case,
high-reflectance
technology
was
applied
to
the
fourth
floor
and
above,
while
a
conventional
façade
was
used
on
the
third
floor
and
below.
The
calculated
value
for
this
while
a
conventional
façade
was
used
on
the
third
floor
and
below.
The
calculated
value
for
this
case
while a conventional façade was used on the third floor and below. The calculated value for this case
case
was
104.
In
the
second
case,
high-reflectance
technology
was
applied
to
the
fourth
floor
and
above
was 104.
104. In
In the
the second
second case,
case, high-reflectance
high-reflectance technology
technology was
was applied
applied to
to the
the fourth
fourth floor
floor and
and above
above
was
while
retroreflective
material
was
applied
to
the
third
floor
and
below.
The
calculated
value
for
while
retroreflective
material
was
applied
to
the
third
floor
and
below.
The
calculated
value
for
this
while retroreflective material was applied to the third floor and below. The calculated value for this
this
case
was
6.
case
was
6.
case was 6.
Figure
Figure 7.
7. Shape
Shapeand
andreflection
reflectioncharacteristics
characteristicsof
ofretroreflective
retroreflective tile
tile (left),
(left), and
and the
the effects
effects of
of using
using
Figure
7.
Shape
and
reflection
characteristics
of
retroreflective
tile
(left),
and
the
effects
of
using
retroreflective
material
on
the
lower
floor
(right).
retroreflective
material
on
the
lower
floor
(right).
retroreflective material on the lower floor (right).
Figure
Figure 8.
8. The
Theratio
ratioof
ofsolar
solarradiation
radiationreflected
reflectedto
toaaapedestrian
pedestrianfor
foran
anidealized
idealizedretroreflective
retroreflectivematerial
material
Figure
8.
The
ratio
of
solar
radiation
reflected
to
pedestrian
for
an
idealized
retroreflective
material
(left),
and
aa realistic
retroreflective
material (right).
(left),
and
realistic
retroreflective
(left), and a realistic retroreflective material (right).
4.2.
4.2. Actual
ActualRetroreflective
Retroreflective Materials
Materials
4.2.
Actual
Retroreflective
Materials
The
retroreflective
tile
used
study
consists
of
to
The retroreflective
forfor
thisthis
study
consists
of an of
upward
slope ofslope
45 degrees
horizontal
The
retroreflectivetile
tileused
used
for
this
study
consists
of an
an upward
upward
slope
of 45
45todegrees
degrees
to
horizontal
and
a
downward
slope
of
30
degrees
to
horizontal
(Figure
5).
Since
a
part
of
solar
and a downward
slope of 30slope
degrees
to horizontal
5). (Figure
Since a part
of solar
radiation
is
horizontal
and a downward
of 30
degrees to (Figure
horizontal
5). Since
a part
of solar
radiation
is
reflected
diffusely
from
each
slope
surface,
it
is
not
a
completely
retroreflective
material.
reflected
diffusely
from
each
slope
surface,
it
is
not
a
completely
retroreflective
material.
Even
so,
radiation is reflected diffusely from each slope surface, it is not a completely retroreflective material.
Even
so,
relatively
amount
of
radiation
reflected
According
to
a relatively
amountlarge
of solar
radiation
reflected
upward.is
measurements
provided
Even
so, aalarge
relatively
large
amount
ofis solar
solar
radiation
isAccording
reflectedtoupward.
upward.
According
to
˝
measurements
provided
by
Harima
et
al.,
when
incident
solar
radiation
is
60°
to
the
objective
by Harima et al.,provided
when incident
solar radiation
is 60 incident
to the objective
building façade,
measurements
by Harima
et al., when
solar radiation
is 60° almost
to thetwo-thirds
objective
building
building façade,
façade, almost
almost two-thirds
two-thirds of
of solar
solar radiation
radiation is
is reflected
reflected upward,
upward, and
and another
another one-third
one-third is
is
reflected
downward.
Therefore,
50%
of
solar
radiation
is
reflected
upward
and
25%
reflected downward. Therefore, 50% of solar radiation is reflected upward and 25% of
of solar
solar
Sustainability 2016, 8, 785
8 of 9
of solar radiation is reflected upward, and another one-third is reflected downward. Therefore, 50% of
solar radiation is reflected upward and 25% of solar radiation is reflected downward from an overall
solar reflectance of 0.75 for retroreflective tile. For comparison, when the overall solar reflection
of high-reflectance paint is 0.75, 37.5% of solar radiation is reflected both upward and downward.
When the overall solar reflectance of a conventional façade is 0.25, 12.5% of solar radiation is reflected
both upward and downward. Therefore, the effect on pedestrians from using retroreflective materials
is similar to the effect from using high-reflectance paint.
5. Conclusions
The possibility of using high-reflectance technology on building façades was evaluated by
using a two-dimensional simple building façade model. The effectiveness of applying retroreflective
materials to building façades was also evaluated in regards to avoiding adverse effects on pedestrians.
In Japanese cities that are located at mid-latitudes, applying high-reflectance technology to
a building façade at the fourth floor and above produces reflection of solar radiation that does
not have adverse effects on pedestrians. Urban heat island mitigation effects can also be expected.
Retroreflective material was considered for use on building façades below the third floor in order to
avoid impacts on pedestrians from the reflection of solar radiation. However, solar radiation reflected
from a realistic retroreflective material still affects pedestrians. It is necessary to further quantify the
impacts of solar radiation reflected from such materials on pedestrians. Retroreflective material is
most effective for building façades composed mostly of materials that have specular reflection, such as
glass, tile, and metal, for both urban heat island mitigation and the improvement of the pedestrian
thermal environment.
Acknowledgments: Many thanks to the retroreflective materials sub-working group members in Osaka HITEC
for their beneficial discussions. Measurement data of solar reflectance for retroreflective materials were provided
by Harima, Ohmura, and Nagahama of Dexerials Corporation.
Author Contributions: The author contributed to all in this study.
Conflicts of Interest: The author declares no conflict of interest.
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