Available via license: CC BY-NC-ND 4.0
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Procedia Engineering 121 ( 2015 ) 1788 – 1794
1877-7058
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015
doi: 10.1016/j.proeng.2015.09.158
ScienceDirect
Available online at www.sciencedirect.com
9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC) and the 3rd
International Conference on Building Energy and Environment (COBEE)
Thermal Process of Windows in Hot Summer and Cold Winter
Climate
Shunyao Lu
a
, Zhengrong Li
a,
*
, and Qun Zhao
b
a
School of Mechanical Engineering, Tongji University, Shanghai, China
b
College of Architectural and Urban Planning, Tongji University, Shanghai, China
Abstract
Heat gain through the windows is of great importance as it is the main part of cooling load in the hot summer and cold winter
climate. This paper attempts to find out the most efficient energy saving technology for windows by analysing the thermal
process of the south and west oriented windows in
Shanghai. By numerical calculation, the ratios of different compositions are
compared to find out which is the main source of heat gain of windows. The calculation result shows that solar radiation heat
gain is much bigger than the heat transmission caused by temperature differe
nce between indoor and outdoor air. Solar heat gain
for west windows is much higher than for south windows after 12
:00. For south windows, diffuse radiation and reflected
radiation from ground are the main source of solar heat gain. As for west windows, direct radiation heat gain takes up most of
solar heat gain after 12:00. Therefore different strategies should be taken according to different orientations.
© 2015 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of ISHVACCOBEE 2015.
Keywords: Thermal process, Solar radiation heat gain, Direct radiation, Energy saving
1. Introduction
Building envelope is of great significance in building e
nergy saving as its thermal properties influencing the
cooling or heating load greatly. Compared to wall, window is a weak component of building envelope and attracts
much attention.
* Corresponding author. Tel.: 86-021-65988869; fax: 86-021-65988869.
E-mail address: qdlsy611@163.com
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer
-review under responsibility of the organizing committee of ISHVAC-COBEE 2015
1789
Shunyao Lu et al. / Procedia Engineering 121 ( 2015 ) 1788 – 1794
Heat flow through windows contains two parts. One is caused by temperature difference between indoor and
outdoor air, and the other is solar heat gain because windows are transparent. There are already lots of researches
about these two parts effect on energy saving. Yang and Gao [1] studied the effect of different glass types on indoor
therm
al environment and building energy consumption. Stegou-Sagia et al. [2] figured out the impact of glazing on
energy c
onsumption. Bahaj et al. [3] assessed several emerging glazing technologies’ potential reductions in cooling
dem
and in hot arid climates. Serra et al. [4] made an experimental evaluation of a clim
ate façade’s energy efficiency.
Jin J. Y. [5] figured out that outside sunshade ca
n reduce 77.5% solar heat gain in a glass curtain wall building.
Tzempelikos and Athienitis [6] studied the impact of shading design and contr
ol in building cooling and lighting
demand. Palmero-Marrero and Oliveira [7] did similar research on louver shading devices. Those studies proved that
changing the glass properties and using shading devices are both efficient technologies to building energy saving.
But which method is more effective depends on the value of
heat flow of the two parts. And compared to solar
radiation, long-wave radiation from outdoor space is always ignored. T
herefore, this paper focuses on the thermal
process of windows in hot summer and cold winter climate and tries
to find a most proper and efficient method to
reduce building energy consumption.
2. Thermal Process Model
In order to
find out the each part’s proportion of heat gain through windows, it is necessary to establish windows’
thermal process model and perform theoretical calculations and numerical analysis.
2.1. Heat transmission caused by temperature difference between indoor and outdoor air
ݍ
௧
ൌ݇
ሺݐ
௨௧
െݐ
ሻ (1)
Where
୲
represents heat transmission caused by temperature difference of indoor and outdoor air,
is the heat
transfer coefficient,
୭୳୲
and
୧୬
are the temperatures of outdoor air and indoor air.
2.2. Solar heat gain
Solar radiation contains direct radiation, diffuse radiation from
sky and reflected radiation from ground.
2.2.1Direct radiation
ܫ
ௗ
ൌܫ
(2)
Where
ୢ୧୰
is the direct radiation that reaches the Earth’s surface,
is solar constant and
is atmospheric transparency.
Direct radiation for the vertical walls is calculated acc
ording the solar incident angle.
ܫ
ௗǡ௪
ൌܫ
ௗ
ή ܿݏ݅ (3)
Where
ୢ୧୰ǡ୵
is the direct radiation for a specific vertical wall while is the corresponding solar incident angle.
The angle is calculated by solar altitude and solar azimuth.
2.2.2 Diffuse radiation form sky
ܫ
ுǡௗ
ൌܥ
ଵ
ሺݏ݄݅݊ሻ
మ
(4)
1790 Shunyao Lu et al. / Procedia Engineering 121 ( 2015 ) 1788 – 1794
Where
ୌǡୢ୧
is the diffuse radiation from sky that received by a horizontal plane, is solar altitude and
ଵ
/
ଶ
is
empirical coefficient which depends on atmospheric transparency.
2.2.3 Reflected radiation from ground
Considering ground as a scattering surface, solar radiation is reflecte
d from ground. It is considered that
horizontal plane can’t receive reflected radiati
on from ground, and for vertical surfaces, it can be calculated as the
following equation:
ܫ
ோ
ൌ
ଵ
ଶ
ߩ
ொ
ܫ
ௌு
(5)
Where
ୖ
is the radiation arriving at vertical surfaces which is reflected from ground,
ୗୌ
is globe solar radiation
for horizontal plane and ɏ
୕
is the average reflectivity of ground. As for urban, the approximate value of ɏ
୕
is 0.2.
A part of solar radiation is transmitted into the room because the glass is transparent. Besides that, some radiation
is absorbed by
windows, and then conducts convection heat transfer with both indoor and outdoor air and long-wave
heat tra
nsfer with indoor surfaces.
ݍ
௦
ൌ ܵܪܩܥ
ௗ
ܫ
ௗǡ௪
ܵܪܩܥ
ௗ
ሺܫ
ுǡௗ
ܫ
ோ
ሻ (6)
Where
ୱ
is solar heat gain,
ୢ୧୰
is the solar heat gain coefficient for direct radiation for windows and
ୢ୧
is for diffuse radiation.
2.3. Long-wave radiation from outside
Besides the above heat transfer method, long-wave radiation from outside is also a
part of heat gain through
windows which is ignored constantly. Long-wave radiation comes from atmosphere, ground
and ambient buildings.
Among them, long-wave from ambient buildings varies according to th
e relative positions and façade, so it’s not
discussed in this paper.
Radiation intensity for long-wave radiation from atmosphere and ground can be calculated according to Stefan-
Boltzmann’s Law:
ܫ
ൌܥ
ቀ
்
ೞ
ଵ
ቁ
ସ
߮
ሺሻ
Where
ୠ
ൌͷǤȀሺ
ଶ
ή
ସ
ሻ, it’s blackbody radiation constant, ɔ is view factor for vertical walls to sky or
ground,
ୱ
is equivalent temperature of sky or ground.
Glass is selective for radiation with different wave
length and it’s nontransparent for long-wave radiation. So
when long-wave radiation reaches window surface, it’s absorbed a
nd increases the window temperature. Also the
windows conduct long-wave heat transfer with atmosphere and ground. The heat gain of long-wave radiation
depe
nds on the temperature of glass, atmosphere and ground.
ݍ
ൌܥ
ሺܫ
ǡ
ܫ
ǡ
െܧ
௪ǡ
െܧ
௪ǡ
ሻ
ሺͺሻ
Where
୪ǡୟ
is long-wave radiation from atmosphere while
୪ǡ
is from ground,
୵ǡୟ
is the long-wave radiation from
windows to atmosphere and
୵ǡ
is to ground.
The glass temperature is always higher than temperature of
atmosphere and ground because the absorbed solar
radiation heats the glass. Long-wave radiation from atmosphere and ground is negative for non-shading windows.
Hence
there is no heat gain of long-wave radiation from outside so its specific value will not be calculated in this
pape
r.
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Shunyao Lu et al. / Procedia Engineering 121 ( 2015 ) 1788 – 1794
3. Results
July 21st is the longest day in hot summer and cold winter clim
ate and is summer solstice. Take a building in
Shanghai as an example and calculate the thermal process of windows in July 21.
3.1. Heat transmission caused by temperature difference between indoor and outdoor air
Whether the AC system is on or not, the temperature di
fference between indoor and outdoor air is not very
significant in July 21, so the heat transmission is around 5~20Ȁ
ଶ
(when heat transfer coefficient of windows is
2.4Ȁሺ
ଶ
ήԨ)).
3.2. Solar heat gain
Solar heat gain contains three parts: direct radiation, diffuse radiation and reflected radia
tion from ground. There
are all short-wave radiations and can get through the windows.
3.2.1 Solar heat gain for south windows
Figure 1(a) presents the value of each part of solar heat gain for south windows, the peak turns up at 12:00 and
direct radiati
on is bigger than the other two. Figure 1(b) shows the percentage of each part, the maximum of direct
radiation is 39% which happened at 12:00. Except 11:00 and 12:00, reflected radiation from ground accounts for the
biggest proportion during 8:00~15:00, diffuse radiation takes the lead after 15:00.
Fig. 1. (a) solar heat gain for south windows; (b) percentage for each part.
3.2.2 Solar heat gain for west windows
Figure 2(a)&(b) shows the details of west windows. There i
s no direct radiation heat gain for west windows until
12:00, diffuse radiation is steady around 30 W/m^2 while reflected radiation from
ground increases from 24 Ȁ
ଶ
to 42 Ȁ
ଶ
during 8:00~12:00. After 12:00, direct radiation has a sharp rise and takes up most of solar heat gain for
west windows.
1792 Shunyao Lu et al. / Procedia Engineering 121 ( 2015 ) 1788 – 1794
Fig. 2. (a) solar heat gain for west windows; (b) percentage for each part.
4. Discussion
4.1. Heat transmission caused by temperature difference between indoor and outdoor air
According to equation (1), heat transmission caused by temperat
ure difference of indoor and outdoor air depends
on the heat transfer coefficient and temperature difference betwee
n indoor and outdoor temperatures. Temperature
difference between indoor and outdoor air is small with the AC system off, if the AC system is on, temperature
difference is still not very significant in July 21, so the heat transmission is no more than 20 Ȁ
ଶ
. Even in hot
summer when temperature difference between indoor and outdoor air is up to 15ć, the heat transmission is still no
m
ore than 36 Ȁ
ଶ
. And high performance glass (smaller heat transfer coefficient) will make the value of heat
transmission even smaller. Heat transmission caused by temperature difference of indoor and outdoor air is much
sm
aller than solar heat gain.
4.2. Solar heat gain
Solar heat gain contains three parts: direct radiati
on, diffuse radiation and reflected radiation from ground. There
are all short-wave radiations and can get through the windows.
4.2.1 Comparison between south and west windows
As presented in Figure 3(a), compare the solar heat gain for south and west windows, we find they are same at
8:00, but quite differe
nt later. Solar heat gain for west windows increases significantly after 12:00 and achieves its
highest value 360Ȁ
ଶ
at 17:00. On the other hand, solar heat gain for south windows reaches peak at 12:00 with
122Ȁ
ଶ
and then declines gradually.
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Shunyao Lu et al. / Procedia Engineering 121 ( 2015 ) 1788 – 1794
Fig. 3. (a) solar heat gain for windows; (b) direct radiation heat gain for windows.
Diffuse radiation is non-directional, so diffuse radiation and reflected radiation from ground for south windows
are identical with west windows. The difference between solar heat gain for south and west windows is direct
radiation heat gain. Figure 3(b) shows the direct radiation heat gain for south and west
windows. It’s clear that south
windows only get direct radiation during 9:00~15:00 while west windows get direct radiation after 12:00 because of
the solar azimuth. For south windows, the highest value appears at 12:00 with 48Ȁ
ଶ
. And for west windows, it
reaches peak of 337Ȁ
ଶ
at 18:00. The intensity of direct radiation depends on normal direct ration and the solar
incident angle. Solar altitude changes a lot during 8:00~18:00, it arises in morning and goes down in afternoon, so
its peak
value (81ι) appears at 12:00. So the direct radiation heat gain for south window
s is not very big. On the
contrary, for south windows after 12:00, solar altitude’s decline leads to the dec
reasing of solar incident angle, so
the direct radiation heat gain goes up to 337Ȁ
ଶ
at 18:00.
4.2.2 Solar heat gain for south windows
As shown in figure 1(a), the three parts all reach t
heir peak value at 12:00.Direct radiation is not steady because
the solar incident angle (calculated by solar altitude and solar azimuth) changes with time. Direct radiation is non-
zero only during 9:00~15:00 because of solar azimuth. It reaches peak value 48Ȁ
ଶ
at 12:00 and is the bigger
than the other two. It is similar at 11:00 with value 45Ȁ
ଶ
. But during other time, it’s not the biggest and even
less than 20Ȁ
ଶ
at 9:00 and 14:00.Diffuse radiation is steady. It begins with 25Ȁ
ଶ
and reaches peak value
31Ȁ
ଶ
at 12:00, then decreases to 11Ȁ
ଶ
gradually. Reflected radiation from ground changes its value from
25Ȁ
ଶ
to 42Ȁ
ଶ
and decreases to 3Ȁ
ଶ
at 18:00.
Figure 1(b) shows the percentage of each part, the maximum of direct radiation is 39% which happened at 12:00,
and it takes t
he lead only during 11:00-12:00. Reflected radiation from ground accounts for the biggest proportion
during the most time, diffuse radiation takes the lead after 15:
00. So the direct radiation is not the main source of
solar heat gain for south windows.
4.2.3 Solar heat gain for west windows
Unlike direct radiation, diffuse radiation and reflected radia
tion from ground are not related with solar azimuth.
So diffuse radiation and reflected radiation from ground for west
windows are the same as for south windows. As
shown in figure 2(a), direct radiation is non-zero after 12:00 and increases from 25Ȁ
ଶ
to 337Ȁ
ଶ
(18:00)
rapidly. Figure 2(b) shows that reflected radiation from ground takes the lead during 8:00~12:00 and reaches 57% at
11:00. After 13:00, direct radiation tak
es the lead and its proportion changes from 61% to 96%.
1794 Shunyao Lu et al. / Procedia Engineering 121 ( 2015 ) 1788 – 1794
5. Conclusions
Compared to solar heat gain, heat transmission caused by
temperature difference of indoor and outdoor air is
small. Therefore most of the heat gain through windows is caused by solar radiation in hot summer and cold winter
climate and the most efficient method for building energy saving is using shading devices.
Solar heat gain for west windows is m
uch higher than that for south windows after 12:00 so west windows need
special attention.
For south windows, diffuse radiation a
nd reflected radiation from ground are the main source of solar heat gain.
As for west windows, direct radiation heat gain takes up most of solar heat gain after 12:00. Consequently, different
strategies should be taken according to different orientations.
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