Malaysia experiences rapid increase in energy consumption in the last decade
due to its high economic growth and increase in the standard living of household
(EIA, 2009). Energy is becoming more costly and the situation
is worsened by the global warming as a result of green house gas emission. A
more efficient energy usage and significant reduction in the released emission
is therefore required. Space cooling with the use of air conditioners is practiced
all year round in Malaysia and this accounts for 42% of total electricity energy
consumption for commercial buildings and 30% of residential buildings (Saidur
et al., 2009). Since the energy cost tends to increase further in
the future, reduction in the energy used for cooling in built environment is
a vital step to energy conservation in Malaysia. A study on the factors affecting
energy performance in buildings is necessary for a better understanding of the
design and operational strategies related to energy conservation; these are
possible with the use of energy simulation software.
At present, there is no legislation pertaining to building efficiency in Malaysia. Architectural designs of certain buildings seemed to portray more on the aesthetics values but at the same time failed to consider the climate situation in the country. The extensive use of glazing for buildings perimeter walls, which results in higher cooling load due to sun radiation, is an example of poor consideration in energy conservation since sun light is available all year round in Malaysia. In many situations, lack of attention has been given to the aspects of operation and maintenance of the building.
As for the building owners, little can be done on the aspect of design to conserve
energy when a building has already been constructed and handed over. On the
other hand, there are energy conservation opportunities in the operation and
maintenance aspects of the air-conditioning system. Overcooling, for instance,
was investigated in Hong Kong (Lam, 2000) in which it
was reported that there could be a 3% reduction in total building electricity
use for every °C rise in the indoor design temperature.
In the present study, the cooling energy resulted from the as-installed system
is studied and a few possible energy conservation mechanisms is studied for
centralized air-conditioning systems of glazed buildings in a University. The
study is conducted by using computer simulation software, Energy
Plus (2010). Possible weaknesses in the aspect of air-conditioning operation
are identified and potential solutions are simulated using the software.
DESCRIPTIONS OF BUILDINGS
The study involved 16 buildings that were constructed next to each other within an academic complex in a university campus, as illustrated in Fig. 1. Each of the buildings has four floors with the exception of Buildings 5 and 15. The buildings aspect ratio is 3.1. The major functions of the buildings are as offices, classrooms, laboratories and computer rooms. The total air-conditioned floor areas for each building are approximately 4833 m2. The building can be accessed 24 h a day by the staff, but the air-conditioning is only supplied 12 h per day, typically between 7 a.m. and 7 p.m. from Mondays to Fridays. On Saturday, a few areas are air-conditioned based on the requests by the occupants; this is normally for selected laboratories and lecturers offices.
The external walls of the buildings, including the doors, are nearly fully
glazed with aluminium frames. Most of the walls are constructed by a single
layer of tinted glass with an overall U-value of 1.43 W m-2 K. Selected
laboratories are constructed with double glass and the overall U-value of 0.72
W m-2 K. External walls that are not glazed and are composed of a
layer of Medium Density Fibre (MDF) board, which is sandwiched by Corian®
solid panels (DuPont, 2010). The total thickness of the
composite wall is 200 mm and its overall U-value is 0.72 W m-2 K.
The indoor walls are also made of glass of the same heat transfer properties.
The floor slab is 300 mm thick concrete. The ceiling finish is 13 mm gypsum
Each of the air ducts in the buildings is equipped with a Variable Air Volume
(VAV) system as means to vary and reduce the energy consumption based on the
varying load of the air-conditioned area. Descriptions of the VAV system can
be found in various references e.g., Pita (2001) The buildings
also have other energy saving mechanisms which were installed in the system
during initial construction such as the variable speed drive for the blowers
motor of the AHU, heat recovery wheels and motorized valve for modulation of
chilled water supply.
SIMULATION OF COOLING LOADS
In the present study the buildings cooling energy was simulated based
on a few operational conditions or patterns using EnergyPlus. Hourly and annually
cooling loads were calculated by EnergyPlus version 3.0 (EnergyPlus, 2010),
a building energy simulation software developed by the US Department of Energy,
based on its previous energy analysis software; i.e., BLAST (Building Loads
Analysis and Systems Thermodynamics) and DOE-2. The EnergyPlus software was
developed to model thermal loads, lighting, ventilating and other energy related
systems. The simulation program is based on the heat balance method which allows
for simultaneous calculation of radiant and convective effect at both interior
and exterior surface during each time step. With this method, all heat balances
on the outdoor and indoor surfaces and the transient heat conduction through
the building construction are taken into account (Eskin
and Turkmen, 2008).
The design data was based on that for Kuala Lumpur, which was available in
the EnergyPlus database. The location of study, near Tronoh was situated less
than 200 km to the North of Kuala Lumpur and had nearly the same ground elevation
(62 m average) and climatic conditions. The outdoor design condition corresponded
to dry bulb and wet bulb temperatures of 32.5 and 26.9°C, respectively.
The daily temperature range of Dry Bulb (DB) temperature was 8.2°C. As for
the indoor design condition, the dry bulb temperature was nominally set by the
building operator to be 24°C with a relative humidity of 50% and these were
within the range of values commonly practised in Malaysia (Leong,
2009). Controls of the indoor air temperature were done by thermostats.
Most of the buildings were used mainly as lecture rooms, laboratories and lecturers
offices. The official working hours is between 8.00 a.m. and 5.00 p.m. and most
AHUs are operated approximately between 7.30 a.m. and 5.30 p.m. In some
cases, subject to requests by the occupants, the AHUs were turned off
at 7.30 p.m., or later if requested by the occupants.
|| Aerial view of the academic complex
The lecturers offices, which were located on the highest floor, were
generally fully occupied during the office hours. However, a number of lecture
rooms and laboratories were not always occupied for various reasons.
ASSESSMENT OF THE INSTALLED LOAD
In analyzing the systems cooling load, it was necessary to compare the equipments rated cooling load to that calculated by the software. This would determine if there was significant overdesign or under design of equipment. Table 1 showed the rated equipment loads for AUs at the right wing of Level 3 in Buildings 1, 5, 17, 21, 22 and 23, as specified by the design engineers. Also shown in Table 1 are the refrigeration load calculated by the EnergyPlus software for comparison. Although the study was done only for selected buildings the results were expected to be able to represent the other buildings.
It is observed in Table 1 that the equipments load varied although, they are basically of the same geometry and construction. The calculated load for Buildings 21, 22 and 23 had high refrigeration loads of over 87 kW each as compared to only 50-55 kW in Buildings 1, 5 and 17. This was likely due to the variation in the orientation of the buildings with respect to the direction of sun.
From the results in Table 1 it is shown that the installed
AHUs were overdesigned by an average of 28%. It is obvious that the trend
of the designer was to overdesign by at least 20%. Nevertheless, in Building1
the overdesign factor was nearly 50%, which was considered as very high. In
practice, sizing of air-conditioning equipments is normally being overdesigned
by 10-15% (Yu and Chow, 2000) due to factors such as
future extensions, to overcome uncertainty in assumptions during sizing calculation
phase and also due to clients request. Although a system should be designed
to be flexible, overdesigning is not a good engineering practice. The penalty
of equipment over sizing is lower plant efficiency. If there is a possibility
of future extension or change of usage, the system should be designed so that
it will be easy and inexpensive to add equipment or change equipment.
EFFECT OF BUILDING ORIENTATIONS
The effect of building orientation with respect to North towards the cooling
energy requirement, such as those for Buildings 21, 22 and 23, were investigated.
The indicated bearing reflects the position of the building with respect to
the sun. EnergyPlus simulation was performed for all six buildings listed in
|| AHU loads of selected building
||Variation of simulated annual cooling energy for different
The simulation was made only for Level 3 (lecturers offices) of those
buildings due to similarity in the design and construction (e.g., wall material,
floor area, internal heat gain) except for building orientation.
It is shown in Fig. 2 that Building 23 (14°N) experienced the highest annual cooling energy at about 155MWh, while the lowest is Building 5 (90°N). It is also observed in Fig. 2 that the closer a building orientation to 0°N the higher would be the cooling energy, as that for Buildings 23, as well as for Buildings 21 and 22. When a building is aligned in parallel with the North direction (0°N) the highest solar radiation through the glazed walls is experienced. This is due to the fact that solar radiation intensity received by a surface is the highest at right angle when the glass area is perpendicular to solar radiation. On the other hand, for Building 5, which was aligned nearly parallel to the East or West, the glazed wall area received the minimum amount of solar radiation, which resulted in a relatively lower annual cooling energy. Thus, it can be concluded that building orientation with respect to the sun is one of the factors that result in the variation of annual cooling energy in each building.
EFFECT OF WALL SHADES
Since the study involved highly glazed buildings, a common approach of reducing
the heat gain through solar radiation would be by introducing internal shadings
either internally or externally in a building. The effectiveness of a shading
device is usually gauged by its shading coefficient or the ratio of the solar
energy transmitted through a window to the incident solar energy. In the present
study, blinds and internal shading devices were chosen for simulation as they
are commonly used for typical buildings in Malaysia. Three types of chosen blinds
were the High Reflectivity (HIREF), Medium Reflectivity (MIDREF) and Low Reflectivity
(LOWREF) slats blinds. As for the shading devices, the High Reflectivity Low
Transmittance (HRLT), Medium Reflectivity Low Transmittance (MRLT) and Low Reflectivity
Low Transmittance (LRLT) shades were considered. Energy Plus simulation was
done for Building 23 for all floors (Levels G, 1, 2 and 3).
The simulation results indicate that blinds and shades with high reflectivity material give the highest reduction of base case cooling load. Table 2 shows the percentage of reduction in the annual cooling energy for all types of blinds and shades. It is shown in Table 2 that the highest saving in the annual cooling energy can be achieved by installing the HRLT shades; i.e., reduction by 26.5% which accounts for 181,783 kW h. As for window blinds, a reduction of 14.2% in the annual cooling energy can be achieved by installing HIREF type of blinds and this is equivalent to 97, 203 kW h of cooling energy. Both types of shadings yield high energy savings due to their high reflectivity and low transmittance properties against the solar incident energy.
EFFECT OF SPACE OVERCOOLING
Space overcooling is caused when the actual air temperature in the conditioned
space is lower than the designated set point value. In this study, the set point
temperature for the buildings was 24°C, which was within the indoor design
temperature recommended by ASHRAE Standard 55. Nevertheless, it was usual that
the control room staff received instant requests from some of the occupants
to reduce their room temperatures; mainly at the time when the sun was facing
their rooms. Even though the change in the temperature set point should be made
by the control room operator for only one room or zone, it was common that the
operator would reduce the set point temperature for the whole floor, usually
to 20°C. As a result, other occupants who were readily feeling comfortable
started to feel colder and uncomfortable. Through interviews with a number of
the occupants, it was discovered that most of them would rather remain silent
about the situation and thus the problem prolonged. This was evident when a
number of them started to wear additional clothing such as coats or sweaters
in offices. Similar problems were also reported in Hong Kong (Lam,
2000), where many commercial premises were being over-cooled by 2-4°C.
Shown in Table 3 is the annual simulated cooling energy for
a Building 23 as a result of different set point temperatures; i.e., 20, 22
and 24°C. The increases in cooling energy are calculated relative to the
energy required at a set point temperature of 24°C. It is shown in Table
3 that a reduction in the air temperature from 24 to 22°C and 20°C
will cause increases in the cooling energy by 36% and 68%, respectively. In
term of cost, these are equivalent to RM 146, 880 and RM 278, 920 per year,
respectively. The cost calculation is based on the standard commercial electric
tariff rate of RM 0.397 for every kW h (TNB, 2010). It
must be highlighted that in reality due to inefficiencies of motors and energy
conversion system, the actual costs are expected to be higher than those listed
in Table 3. This shows that overcooling of a space involves
huge amount of additional energy and cost and thus should be avoided in the
first place. Apart from the issue of operating cost, overcooling should also
be avoided in order to provide a room temperature that is within the level of
comfort of most occupants.
EFFECT OF OUTDOOR AIR INFILTRATION
Infiltration load is one of the elements of space cooling load. Normally, in
design phase, designers will incorporate infiltration load based on the level
of air tightness of a building. However, in actual daily usage, additional infiltration
load may be imposed to the system through door or windows openings.
|| Comparison of various AHU loads for a typical building
|| Effect of set point temperature on Building 23
||Variation of simulated monthly cooling energy for building
23 if the doors are left open with infiltration rate of 1.0 cfm ft-1
of door area
In analyzing the severity of the addition of cooling energy from such air infiltration,
a simulation of cooling energy was performed for Building 23 with the assumption
that each floor has one door opened during the day time (8 a.m. to 5 p.m.).
The value of infiltration was taken as one cubic foot per minute per square
foot of the door area (Pita, 2001).
From the simulation computed by EnergyPlus, annual cooling energy for Building 23 was increased from 685, 160 kW h to 1, 026, 043 kW h as a result of infiltration load from the door openings. The increase is about 49.7% which would cost an additional RM135, 330. Therefore, the results in Fig. 3 imply that the infiltration load from door or window openings can result in a significant increase in the cooling energy. Therefore, the building operator must ensure that doors are not left open for a long time to minimize infiltration load.
Simulation of the annual cooling energy for highly glazed academic buildings
was performed using Energy Plus. From the study, the followings can be concluded:
||The installed AHUs were found to be overdesigned by at least 20%.
It was found to be high in Building 1, which was 47%. Although overdesign
is practiced by design engineers, the normal range is within 10-15%. With
the equipment not operating close to its maximum capacity would lower the
plant efficiency. Although at the present stage, this cannot be corrected,
plans should be made in the future, such as allocation for greater occupancy
in order to optimize the system
||The building orientation with respect to the sun was one of the important
factors that results in the variation of annual cooling energy in each building.
Due to the large area of glazed walls, the effect of heat gain through solar
radiation was significant for the buildings
||Application of blinds or shadings for the glazed academic buildings could
yield high savings in the annual cooling energy due to their high reflectivity
and low transmittance properties against the solar incident energy.
||Overcooling of a space by reducing the set point temperature of the thermostat
in the academic building would involve significant increase in the cooling
energy and operational cost. In some situations, overcooling was found to
cause discomfort among a large number of occupants. A balance between comfort
and cost should be made by setting the most suitable set point temperature
||Infiltration of outdoor air through opened doors or windows could lead
to a significant increase in the cooling energy. Hence, the building operator
should ensure that doors are not constantly left open during regular buildings
The authors would like to express their kind appreciation to Ir. Mohd Fatimie Irzaq Khamis and his team members, as well as Universiti Teknologi PETRONAS for their technical assistance and support in this study.