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Abstract

In recent years, many studies have been carried out on green roof systems because of the multiple benefits that they offer. The energy savings within the building is one of the benefits that have been studied more than the other benefits. The results obtained show that energy saving is achieved by decreasing cooling during summer and reducing heating during winter. This paper examines the current literature related to energy savings in buildings due to green roof systems, phenomenological models that describe the heat transfer process, and the experimental analysis of green roofs as well as highlights some of the factors with more impact. The results of this work show that it is necessary to analyze the heat transfer processes in green roofs at different length scales to evaluate the energy savings in buildings.
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 105
Building Energy Savings with a Green Roof
S. Quezada-García,1 R. Vázquez-Rodríguez,2 J. J. Ambriz-García,2 and
G. Espinosa-Paredes,2,*
1Department of Energy Systems, National Autonomous University of Mexico, Mexico
2Area of Energy Resources Engineering, Metropolitan Autonomous University -
Iztapalapa, Mexico
Received October 05, 2012; Accepted October 24, 2012
Abstract: In recent years, many studies have been carried out on green roof systems
because of the multiple benefi ts that they offer. The energy savings within
the building is one of the benefi ts that have been studied more than the
other benefi ts. The results obtained show that energy saving is achieved by
decreasing cooling during summer and reducing heating during winter.
This paper examines the current literature related to energy savings in
buildings due to green roof systems, phenomenological models that describe
the heat transfer process, and the experimental analysis of green roofs as
well as highlights some of the factors with more impact. The results of this
work show that it is necessary to analyze the heat transfer processes in green
roofs at different length scales to evaluate the energy savings in buildings.
Keywords: Energy saving, experimental analysis, green roof, heat transfer, phenomeno-
logical models
1 Introduction
In recent years, many studies have been carried out on green roofs systems (GRSs)
due to the multiple benefi ts that they offer, such as Increase in urban forest area [1],
air purifi cation [2,3], reducing runoff [1,4,5], reduction in the urban heat island effect
[6–10], life extension of the roof [11–13], and energy saving in the building [14–18].
To take advantage of the benefi ts of green roofs, the governments in some of
the highly urbanized societies such as Japan, Singapore, Germany, and Belgium
have offered incentives to encourage or even impose their use [10]. Mexico City
has laws that encourage the use of green roofs. The Green School Project, imple-
mented in South Korea, undertook the installation of GRSs in existing elementary
and middle school facilities [18].
*Corresponding author: gepe@xanum.uam.mx
DOI: 10.7569/JSEE.2012.629506
S. Quezada-García et al.: Building Energy Savings with a Green Roof
106 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
However, sometimes incentives alone are not suffi cient for project develop-
ment. The economic aspects need to be taken into account for decision making.
Therefore, some studies have assessed the fi nancial viability of GRSs and have
found that the net present value of GRSs is lower than that of conventional roofs
mainly because of energy savings [12,19].
To know the potential energy savings in a building, it is necessary to know the
heat transfer through the roof before and after installing the GRS. Therefore, it
is desirable to have mathematical models that accurately describe the heat fl ow
through the various biotic and abiotic layers that form the GRS to predict energy
exchange and determine the temperature inside the building.
This paper examines the current literature related to energy savings in build-
ings due to GRSs, phenomenological models that describe the heat transfer pro-
cess, and the experimental analysis of green roofs as well as highlights some of the
factors with more impact.
2 Building Energy Savings
The roof is a component of the building envelope that can provide advanced solu-
tions for energy savings. On average, the roof of a building receives twice as much
solar radiation than the walls [20]. GRSs can reduce the magnitude of heat fl ux
through one of the building envelopes as a result of the insulation provided by the
growing medium and provide additional benefi ts such as evapotranspiration that
can cool the roof under the sun [16]. Therefore, GRSs have the potential to reduce
energy consumption for heating and cooling [14,15].
Teemusk and Mander analyzed GRSs in an extreme climate such as in Estonia
and found that during summer the GRS with a substrate layer of 100 mm depth
signifi cantly reduced temperature fl uctuations compared to a conventional ceil-
ing [21]. During fall and spring, the substrate layer protects the roof from rapid
cooling and freezing, and also provides effective thermal insulation during
winter. Using dynamic simulations Zinzi and Agnoli analyzed GRSs and found
that they can improve the energy effi ciency of buildings. The authors concluded
that the water content in the roof signifi cantly impacts the energy savings in the
building; that is, the lack of water has a negative impact on reducing building
energy savings by cooling [17]. Hong et al. demonstrated, using numerical simu-
lations, the energy saving effect due to the introduction of GRSs in elementary
schools [18].
The energy saving depends on many factors such as the composition of the
growing medium, depth, moisture content, type of plants, irrigation, and local cli-
mate [17,20–22]. Plants may reduce the amount of heat stored in a building through
an increase in the amount of radiation refl ected, the shade provided by the canopy,
and evapotranspiration. Plants with green leaves have better temperature reduc-
tion effects than plants with purple leaves [20]. It has been found that increasing
the depth of the substrate layer up to 20 cm results in an increase in the thermal
S. Quezada-García et al.: Building Energy Savings with a Green Roof
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 107
DOI: 10.7569/JSEE.2012.629506
transmittance coeffi cient (U), but for depths of the substrate layer greater than 20
cm the U value decreases [23]. Experimentally, it has been found that solar radia-
tion accounts for 99.1% of the total heat gain in green roofs. Evapotranspiration
from plants was of the total heat loss in green roof 58.4% is due to the evapo-
transpiration, net long-wave radiative exchange correspond to 30.9%, and the net
photosynthesis of plants is around 9.5% [24].
Increasing the depth of the growing medium and supplying water for irriga-
tion allow the use of plants with higher biomass and greater leaf area index, which
results in higher evapotranspiration rates [22].
GRSs are passive systems for energy saving and are based on four fundamental
mechanisms: (i) interception of solar radiation by the canopy of the vegetation; (ii)
thermal insulation provided by the vegetation and the substrate; (iii) evaporative
cooling that occurs by evapotranspiration from the plants and the substrate; and
(iv) effect of wind on the building [25].
3 Mathematical models
Some authors have calculated heat transfer through GRSs in terms of a global heat
transfer coeffi cient to evaluate the thermal performance of GRSs in real scale and
dynamic conditions [23]. These studies are generally conducted on green roofs
that are already installed and are valid only for the roof in question.
To predict energy savings, it is necessary to know the heat fl ux before and after
installing a GRS. Some authors have developed mathematical models for the anal-
ysis of energy fl ow through GRSs [26–30].
The energy balance in the energy exchanges between the plants–soil system and
the environment, which is illustrated in Figure 1, is given by equation (1) [24]. In
this equation, the thermal effects of plant metabolism such as respiration and tran-
spiration and the thermal effects of microorganisms in the soil are not considered;
similarly, conditions such as precipitation and dew are not included. Moreover, it
is assumed that the green roof is large enough to assume horizontal homogeneity
and apply one-dimensional (vertical) analysis.
Ambient air
qsr
qss
qtf
qlr
qep
qrp qsp
qcv qem qtp System boundary
Plants
Soil
Roof
Figure 1 Energy exchange between a green roof and its environment [24].
S. Quezada-García et al.: Building Energy Savings with a Green Roof
108 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
sr lr cv em tp ep sp ss tf ps rp
+ + + + + + + + + + = 0qqq q qq q qqq q , (1)
where sr
q is the heat gain from solar radiation, lr
q is the heat gain from long-wave
radiation, cv
q is the heat transferred by convection, em
q is the heat loss by emis-
sion, tp
q is the heat loss by transpiration, ep
q is the heat loss by evaporation, sp
q is
the heat storage by plants, ss
q is the heat storage by soil, tf
q is the heat transferred
into a room, ps
q is the solar energy converted by photosynthesis, and rp
q is the
heat generation by respiration.
Jim and Tsang presented a mathematical model by considering that conduction
is the main factor affecting soil temperature [31]. A typical heat conduction equa-
tion, equation (2), can be used to estimate the i-layer temperature:
r
⎛⎞
=⎜⎟
⎝⎠
∂∂
1
ii
i
ii
dT T
k
dt C z z , (2)
where i
T is the i-layer temperature and ri, i
C, and i
k are the mass density, spe-
cifi c heat, and thermal conductivity of the i-layer, respectively.
The dominant form of heat dissipation in GRSs is evapotranspiration; therefore,
it is important to take into account the mass balance [15,16,24,26,27,32]. The mass
balance for the substrate layer is given by equation (3) [31]:
⎡⎤
=⎢⎥
∂∂
⎣⎦
gg
g
dW W
D
dt z z
, (3)
where g
W is the volumetric water content that the soil can hold and g
D is the dif-
fusion coeffi cient of water in soil.
A basic model was proposed for calculation of the convective heat transfer,
equation (4), which is a modifi ed version of Newton’s cooling law [33]. Plants
have a symmetric leaf structure that can be assumed to have equal two-sided heat
transfer rates for both forced and free convection conditions. Therefore, h values
can be doubled for a two-sided leaf:
()
=−2la
HhTT
, (4)
where a
T
is the air temperature, l
T is the leaf temperature, h is the convection
coeffi cient, and H is the sensible heat fl ux.
The mathematical models to describe the heat fl ow through green roofs have
been validated by some authors through experimentation.
4 Experiments in the literature
To validate the mathematical models, some authors have used simulation pro-
grams while others have resorted to experimentation to verify the accuracy of a
mathematical model [23,26,34]. Jim and Tsang validated their model with empiri-
cal results from three experimental plots [31]. Jim and He performed a similar
S. Quezada-García et al.: Building Energy Savings with a Green Roof
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 109
DOI: 10.7569/JSEE.2012.629506
experiment and measured variables such as solar radiation and microclimatic and
soil conditions; four solar radiation components were recorded: incoming and out-
going shortwave and incoming and outgoing longwave; soil moisture and tem-
perature; meteorological factors including the relative humidity of air, air tem-
perature, dew point temperature, wind speed, wind direction, and rainfall [35].
The quantitative evaluation of the heat transfer through a GRS requires precise
knowledge of the thermal properties of the growing medium. Previous experi-
ments have measured the thermal conductivity, the heat capacity, and the thermal
diffusivity of different soil samples [36,37].
5 Discussion
It has been shown that GRSs offer many benefi ts; however, using current math-
ematical models and experimentation we are still far from adequately describ-
ing the heat fl ux through these systems. The transfer of energy through GRSs
has been studied until now at the macroscopic level , that is, global energy bal-
ances where the length scale is of the order of magnitude of the green roof.
This type of approach has limitations as it does not go deep into the problem of
understanding the mechanisms of heat transfer in the energy transfer processes
in green roofs, especially near the boundaries between adjacent layers that con-
stitute the GRS.
Of the different layers that make up a GRS are porous media in which important
phenomena for the heat and mass transfer processes take place, but the porosity
is not taken into account in many existing mathematical models. Another limita-
tion of macroscopic description is that it is not possible to analyze the interaction
between the leaves of the plants and the environment.
A more complete study of heat transfer through GRSs can be done by using dif-
ferent length scales, that is, by studying the phenomenon at a large and small scale
(Figure 2) [38,39]. The small-scale approach has the advantage that all the factors
involved in the interfacial heat fl ux in the GRS are taken into account and the lay-
ers are considered to be multiphase systems.
Regarding experimental work, measurements to validate the mathematical
models and calculate the energy savings have been done to get experimental plots
for green roofs that are already installed, that is, these have been made in outdoor
experiments with multiple factors that affect the heat transfer. In some cases, the
models are only valid for the particular case of the study. Therefore, it is important
to carry out controlled experiments in a laboratory [40].
6 Conclusions
GRSs offer many benefi ts, one of which is the energy savings in buildings by
reducing the need for cooling in summer and heating in winter. However, many
of the studies that quantify the energy savings are only valid for the building in
question.
S. Quezada-García et al.: Building Energy Savings with a Green Roof
110 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
To predict the energy saving, it is necessary to know the heat transfer processes
in the roof. This requires mathematical models based on the physical processes
that describe the net heat fl ux both in conventional roofs and in green roofs. At
present, although mathematical models have been proposed to describe the heat
ow through GRSs, it is desirable to have more precise models that can be applied
in any situation (e.g., Figure 2). Solar radiation, type of plants, the composition and
depth of the growing medium, moisture content, leaf area index, irrigation, and
local climate are some of the factors involved in the heat fl ow that must be taken
into account in the mathematical models.
To describe the phenomenon in each stage, it is more convenient to carry out
controlled experiments. Therefore, to get a deeper understanding of the mecha-
nism of heat transfer processes in GRSs, it is necessary to develop more rigorous
experiments and models.
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