Journal of Power and Energy Engineering, 2014, 2, 113-119
Published Online April 2014 in SciRes.
How to cite this paper: Ahasan, T., Ahmed, S.F., Rasul, M.G., Khan, M.M.K. and Azad, A.K. (2014) Performance Evaluation of
Hybrid Green Roof System in a Subtropical Climate Using Fluent. Journal of Power and Energy Engineering, 2, 113-119.
Performance Evaluation of Hybrid Green
Roof System in a Subtropical Climate
Using Fluent
T. Ahasan, S. F. Ahmed*, M. G. Rasul, M. M. K. Khan, A. K. Azad
School of Engineering and Technology, Central Queensland University, Rockhampton Campus, Queensland,
Email: *
Received December 2013
Energy disaster is one of the major obstacles in the progress of human society. There are some
on-going researches to overcome this for a sustainable environment. Green roof system is one of
them which assist to reduce energy consumption of the buildings. The green roof system for a
building involves a green roof that is partially or completely covered with vegetation and plant
over a waterproofing membrane. Green roofs provide shade and remove heat from the air through
evapotranspiration, reducing temperatures of the roof surface and the surrounding air. This pa-
per reports the thermal performance of hybrid green roof system for a hot and humid subtropical
climatic zone in Queensland, Australia. A thermal model is developed for the green roof system
using ANSYS Fluent. Data were collected from two modelled rooms, one connected with green roof
system and other non-green roof system. The rooms were built from two shipping containers and
installed at Central Queensland University, Rockhampton, Australia. Impact of air temperature on
room cooling performance is assessed in this study. A temperature reduction of 0.95˚C was ob-
served in the room with green roof which will save energy cost in buildings. Only 1.7% variation in
temperature was found in numerical result in comparison with experimental result.
Thermal Performance; Green Roof; Subtropical Climate; ANSYS Fluent
1. Introduction
Total world energy consumption is rising with a significant amount each year. The total world energy consump-
tion refers to the total energy used by all of human society. Population and income growth are the key drivers
behind this growth of energy demand. By 2030 world population is projected to reach 8.3 billion, which indi-
cates an additional 1.3 billion people will need energy and world income in 2030 is expected to be roughly dou-
bled the 2011 level [1]. Total world energy use rises from 524 quadrillion British thermal units (Btu) in 2010 to
*Corresponding author.
T. Ahasan et al.
630 quadrillion Btu in 2020 and to 820 quadrillion Btu in 2040 [2]. This energy consumption will grow by 56%
between 2010 to 2040. Much of this energy consumption occurs in the countries outside the organization for
economic cooperation and development (OECD), known as non-OECD. Energy use in non-OECD countries in-
creases by 90% while in OECD countries it is 17%.
Four major sectors such as commercial, industrial, residential, and transportation are responsible for this en-
ergy consumption. Rapid growth in global demand for Australia’s energy resources has driven growth in domes-
tic production. However, the share of domestic consumption in Australian energy production has declined, from
an average of 49% in the 1980s to an average of 42% in the 1990s, and has continued to decline, to an average
of 34% over the past decade [3]. According to the Department of climate change and energy efficiency (Austra-
lia), the Australian residential sector energy consumption in 1990 was about 0.28 quadrillion Btu (electricity,
gas, LPG and wood) and that by 2008 had grown to about 0.38 quadrillion Btu and is projected to increase to
0.44 quadrillion Btu by 2020 under the current trends. This represents 56% increase in residential sector energy
consumption over the period 1990 to 2020.
There are many options to reduce the energy consumption. Green roof system is seen as a viable option to
save energy. Green roofs assist a building in different forms such as absorbing rainwater, providing insulation,
making a habitat for wildlife and helping a lower urban air temperature and mitigate the heat island effect. Green
roofs are a recognized part of modern building in Europe. Australian examples are less common but in 2007 a
national organization was formed to promote green roofs and Brisbane City Council included green roofs in its
proposed action plan for dealing with climate change [4]. Green roofs are well known for their wide range of
environmental, ecological, social and economic benefits [5,6]. In a green roof, plant absorbs large amount of so-
lar energy through biological functions and remaining solar radiation affects the internal temperature of the
building is much less than that of a bare roof. Only 13% of total solar radiation is transmitted into soil and other
27% reflected and 60% absorbed by the planted roofs [7]. Both plants and layers of green roof add insulation
value with existing building insulation. Del Barrio [8] has stated that green roof reduces the heat flux through
the roof which does not act as cooling device rather as insulation.
Hydrological performance of extensive green roofs was investigated in New York City [9]. Three full-scale
green roofs were monitored in the City, representing the three extensive green roof types. A mathematical model
was developed for evaluating cooling potential of green roof and solar thermal shading in buildings [10]. A con-
trol volume approach based on finite difference methods was used to analyse the components of green roof such
as green canopy, soil and support layer. Thermal properties and energy performance of the green roof was ana-
lysed by Niachou et al. [11]. A model of the energy balance of a vegetated rooftop was developed and integrated
into Energy Plus building energy simulation program [12]. This green roof module allows the energy modeller
to explore green roof design options including growing media thermal properties and depth, and vegetation
characteristics such as plant type, height and leaf area index. Although there are a few researches on green roof
system, but no credible researches on hybrid green roof system using air conditioning are practiced in Australia
as well as in the world. In these view, the main aim of this study is to investigate the thermal performance of
green roof system in combination with air conditioning.
2. Green Roof System
Green roof is a rooftop garden, park or meadow that carries a host of environmental and economic benefits. It
absorbs heat and acts as insulators for buildings, reducing energy needed to provide cooling. A green roof re-
places traditional roofing with a lightweight, living system of soil, compost, and plants. By using soil and plants
on rooftops in place of hard roofing materials, the homeowners can extend the life of the roof two or three times
more. Hybrid green roof system is a green roof system in combination with any other treatment such as air con-
ditioning. There are two types of green roof systems: extensive and intensive. Extensive green roof is more
common between the two categories which employs a shallow growing medium of five inches or less. The
plants chosen for extensive roofs usually include succulents and moss. The design of extensive green roofs is
geared towards low maintenance and limited irrigation. The extensive green roof system has been used in this
present study. At most, maintenance occurs one to two times a year with limited access to the roof [13]. Inten-
sive roofs use hardier plants that require a growing medium of at least 6 - 12 inches. Roofs of this system can
assist the multiple functions such as providing an outdoor garden space for food production. Intensive roofs in-
volve a greater load of more than 150 kg/m2 and have more than 200 mm of substrate and require maintenance
T. Ahasan et al.
in the form of weeding, fertilizing and watering [14]. Figure 1 shows the green roof cross section.
The cost of constructing a safe and effective green roof depends on the building and its existing roof structure.
An estimated cost of installing a green roof is $10 per square foot for simpler extensive roofing, and $25 per
square foot for intensive roofs (Environmental Protection Agency, USA). Annual maintenance costs for either
type of roof may range from $0.75 to $1.50 per square foot.
3. Experimental Set Up and Measurement
Two identical shipping containers with a dimension of 5.63 m × 2.14 m × 2.26 m were used in this study as
modelled rooms. They were installed in the sustainable precinct at Central Queensland University, Rockhamp-
ton, Australia for the experimental measurement. An experimental bed of green roof has been set over one ship-
ping container whereas other container has bare roof. An air conditioner was installed in both the containers to
increase the efficiency of the green roof system. Temperature of both the air conditioner was set at 24˚C to pro-
vide optimal comfort and energy savings. The experimental set up of the green roof system has been shown in
Figure 2.
For excess rain water runoff, the containers were set with a 3˚ pitch from North to South. Both the containers
have single sliding 5 mm dark grey float toughened glass door and window along with aluminium frame. Both
hand rail and stairs are built as per Australian Standard (AS 1657-1992) which ensures safety while working on
the rooftop. Figure 3 shows the schematic diagram of the experimental green roof setup.
Figure 1. Green roof cross section.
Figure 2. Experimental set up.
T. Ahasan et al.
Data Collection
Through a suite of experimental design, experimental tests and field investigation, the cooling performance was
assessed against air temperature. Air temperature was measured to investigate their impact on cooling perform-
ance of the system. Indoor average air temperature and indoor top room temperature were collected using the
measuring tools from both the containers. The data of the months of December, 2012 and January, 2013 were
collected for this study. Temperature profile of both green roof and non-green roof systems on one of the hottest
day (26th February 2013) have been shown in Figure 4.
4. Modelling Approaches
Green roof problem has been solved numerically by using the CFD code “Fluent in ANSYS 13.0, which uses
the finite volume method for discretization of the computational domain. Standard
turbulence model was
used to develop the green roof model. The model satisfies certain mathematical constraints on the Reynolds
stresses and consistent with the physics of turbulent flows. The modelling equations were described in ANSYS
Fluent theory guide [16].
4.1. Geometry of the Model
A 2D geometry for this model was created in DesignModeler which is shown in Figure 5. The geometry con-
sists of a room with dimension of 5.63 m × 2.14 m × 2.26 m.
4.2. Mesh Generation
A typical mesh is presented for green roof system in Figure 6. This mesh was generated using DesignModeller
Figure 3. Diagram of experimental green roof [15].
Figure 4. Temperature profile of both green roof and non-green roof systems
in summer (26th February 2013).
T. Ahasan et al.
Figure 5. Model geometry in DesignModeler.
Figure 6. A typical 2D mesh.
in ANSYS 13.0. A number of cells, namely 127,340, were created in the meshing to get an accurate result as a
large number of cells may result in long solver runs and a small number of cells may lead to inaccurate results.
Cell zone condition for the surface body was defined as fluid. All the zones used in the geometry were defined
such as inlet and room wall. No slip boundary conditions were applied on the pipe walls and room walls.
4.3. Solver Approach
A 2D pressure-based-segregated solver was used for the green roof modelling. The solver uses a solution algo-
rithm where the governing equations are solved sequentially. The pressure semi-implicit with splitting operators
(simple) was adopted for the numerical calculations. The simple algorithm uses a relationship between velocity
and pressure corrections to enforce mass conservation and to obtain the pressure field [17]. It uses a segregated
approach where the equations are solved in sequential steps letting to the iterative process the care of non-line-
arity as well as the coupling between the equations [18].
Spatial discretization of second-order upwind scheme was used for momentum, turbulent kinetic energy and
turbulent dissipation rate as the discretization of the viscous terms is always second-order accurate in Fluent.
Moreover, the second-order upwind differencing scheme was used to overcome numerical diffusion. The stan-
dard initialization in the entire domain used in this study allows setting the initial values for the flow variables
and initializing the solution using these values.
5. Results and Discussion
Experimental results were obtained for air temperature at different points in both the modelled rooms. The air
temperature was measured using HOBO Pendant Temp Logger. The average room temperatures of 299.54 K
(26.39˚C) and 300.95 K (27.8˚C) were measured in the modelled rooms of green roof and non-green roof re-
spectively. Performance of the green roof system was also calculated numerically at different iterations of 10,000
and 15,000. Temperature profiles have been shown at these iterations in Figure 7. The thermal variables for the
boundary and cell zone conditions were imposed on the boundaries of the model. Average maximum room air
T. Ahasan et al.
Figure 7. (a) Temperature profile at the iteration of 10,000; (b) Temperature
profile at the iteration of 15,000.
temperature of 299.23 K (26.08˚C) collected from the top of the green roof room was imposed at the top wall of
the room.
The numerical results for the maximum predicted room temperature of using green roof system was found to
be 300 K (26.85˚C) as shown in Figure 7(b). This predicted temperature is minimum 0.95˚C less than the
measured temperature of 300.95 K (27.8˚C). The results obtained in this study were validated at different points
by comparing the numerical data predicted by Fluent with the measured data collected from the modelled room
containers. The maximum room temperature of 300 K (26.85˚C) predicted by Fluent which is shown in Figure
7(b) was compared with the measured room temperature of 299.54 K (26.39˚C) of green roof system. The dif-
ference between the numerical data predicted by Fluent and the experimental data was found to be within 1.7%
of the measured maximum room temperature.
T. Ahasan et al.
6. Conclusion
Thermal performance of green roof system was measured in this study by developing a CFD model for a sub-
tropical climate in Queensland, Australia. Standard
turbulence model was used to develop the model.
The impact of air temperature was assessed on the room cooling performance using simulation. The simulation
result shows a minimum temperature reduction of approximately 0.95˚C in the room. The developed model was
validated at different points of the room with the measured data, and only 1.7% temperature variation was found
between numerical result predicted by Fluent and the experimental result. Further investigation is being under-
taken in this study.
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