Application of CFD to Reduce the Cooling Load of a Building: An Alternative Approach for Green Building Design

Article Info Abstract Article History Received Mar 01, 2022 Revised Mar 25, 2022 Accepted Mar 28, 2022 Approximately 23 % of total CO2 emissions are attributed to the construction industry each year; in 2009, the industry produced 5.7 billion tonnes of CO2. Green building design is concerned with constructing a building in such a way that its carbon footprint is reduced throughout its life span. Lecture rooms are key components of educational institutions because they are where students spend the majority of their time. This study aims to propose a methodology to convert the existing designs of building to green buildings by optimizing the airflow. The lecture room at UET was chosen for this investigation because the air velocity distribution inside the room is not uniform due to the current design, which influences the convective heat loss from the room by creating pressure and temperature gradient. SOLIDWORKS 2021 was used to create the model of a lecture theatre. Because the majority of the rooms are naturally ventilated, a fluid dynamics analysis was performed on ANSYS Fluent 19.1 to ensure proper air ventilation. Natural airflow was improved by the addition of an extra column of windows. The total electric energy load has been computed, and the quantity of solar panels necessary to meet the demand has been recommended. Keywords


Introduction
The effort is being made by the scientific community to transform present construction techniques into ones that are more environmentally friendly. Engineers are attempting to develop more diversified and advanced designs for increasing efficiency, decreasing loads, and harnessing renewable energy to conserve the environment around buildings while also making them more sustainable.
As a result of the dramatic environmental and ecological changes that have occurred, a strong emphasis is being placed on lowering carbon emissions. In 2009, the construction industry was accountable for 23 % of global carbon emissions, producing 5.7 billion tonnes of CO2, with 94 % of these emissions coming from indirect sources, such as the production of building materials and the provision of electricity for a building [1] be taken to cut CO2 emissions. Green buildings were emphasized as critical components to reduce carbon emissions [2]. Designs aiming towards lowering the carbon footprint of buildings refer to green building designs. A strong collaboration between builders, designers, and engineering professionals is necessary to reduce the carbon emissions from a building. For green buildings, it is preferable to use locally created materials and sustainable energy sources whenever possible.
The lecture halls of educational institutions are where students spend the vast majority of their time.
When it comes to maintaining a comfortable temperature in a room, the airflow circulation within the space becomes critically significant. Fresh air should be drawn into the room, and hot air should be expelled from it consistently through proper air ventilation. When dealing with a single room with a ceiling height of sixteen feet, it can be regulated with the aid of ceiling fans that are set in a regular pattern [3].
Mechanical or natural ventilation is used to ventilate lecture halls at Pakistani institutes depending on the climate. Most of the energy in Pakistan is derived from the burning of fossil fuels. HVAC systems utilize around two-thirds of the total energy consumed by cities each year [4]. It is critical to create a pleasant studying atmosphere inside the room by maintaining the room's ambient temperature.
To construct or improve existing rooms that have already been constructed, numerical simulations and experimental research are two methods that can be used. In recent years, computational fluid dynamics (CFD) techniques have shown to be an extremely useful tool, since they can avoid engineers from making costly mistakes during the testing process while also substantially reducing costs [5][6][7].
Multiple scholars had examined the performance of ventilation in an enclosed space using computational fluid dynamics (CFD). Mokhtarzadeh-Dehghan was the first to use computational fluid dynamics (CFD) to optimize airflow in a controlled volume in 1990 [8]. Then Ayad completed experiments on the distribution of airflow using computational fluid dynamics (CFD), as well as studied the effects of various air openings, such as windows and doors [9]. Analyzing the height of a naturally ventilated space with a single open side, Gan computed the height of the room using simple numerical analysis [10].
Convection is the primary mechanism through which heat is transferred out of a human body to the surrounding air. Because the temperature of fresh air is lower than that of the temperature of the body, it works as a constant sink, allowing heat to be transferred from the body to the sink through convection. The air that comes into contact with the human body will be heated and forced upward, allowing fresh air to replace it. It is possible to increase the efficiency of convective heat loss from a body by providing adequate ventilation in the surrounding area. If the ceiling fans and windows are not appropriately located in the room, the air will be distributed unevenly, resulting in the formation of a pressure gradient in the room. The pressure in the room will fluctuate as a result of the varying air velocities present in different regions of the space. Following Bernoulli's Equation, pressure will be high in regions where velocity is low and will be low where velocity is high.
where P denotes pressure, denotes density, v denotes velocity, and h denotes elevation at two separate points in a two-dimensional flow. friendly, this energy should be used in environmentally friendly constructions. For the duration of its operational life, it will emit zero carbon dioxide. Figure 1 shows the solar radiation per annum in Pakistan. To achieve the aim of converting a current building to a green building, all lighting, ceiling fans, and other electrical equipment in institutional buildings should be powered by solar energy. It takes 100 Watts to run seven ceiling fans and ten tube lights in the lecture hall at UET. An average of 1.1 kW is consumed by one room's total power consumption.

Figure 1. Solar Radiation per annum in Pakistan
In this study, a CFD investigation of UET's lecture halls is conducted. The existing design of the lecture theatre will be improved to maintain a moderate and pleasant temperature by optimizing the natural airflow. This method can be used to change an existing building into a green and sustainable one.

Materials and Methods
This research was focused on a lecture hall on the first floor of the lecture theatre at the University of Engineering and Technology (UET) in Lahore. Figure 2 shows the actual lecture theatre.

Geometry of Design Using CAD
SOLIDWORKS was used to create the design of the lecture theatre. Equipment such as whiteboards, seats, and desks were not included in this edition. Windows' thicknesses were taken into account in the design process. The model is depicted in Figure 3 and Figure 4.

Boundary Conditions
The following assumptions were made during this study.
• The wind is blowing from the west side windows at a velocity of 1.34 meters per second.
• The temperature outside is 300 o C.
• The windows and entrance on the east side of the room serve as an outlet for the room.
• All of the windows and doors are open entirely.
• The openings for the air conditioning system are treated as walls, and no air can enter or depart via them at any time.
• The fluid is of inviscid nature

Contours of pressure
Because it is the center rows that are of concern, a distinct plane has been included. This plane is used to investigate the characteristics of the flow. In figure 9, the findings of pressure are produced on the addi-

Contours of Velocity
The results of the velocity experiment are displayed in a contour plot. It can be seen that the largest velocity occurs at the entrance region, which is comprised of windows, and that the magnitude of the velocity decreases gradually as the airflow flows away from the inlet. Zero-velocity zones are denoted by a dark blue region around the windows and along the walls. Because of the no-slip condition, a layer of zero velocity is generated near the walls, which implies that the flow stays to the walls and that the velocity progressively increases as the flow goes away from the walls. As there are no windows, air strikes the walls and shifts away from the wall, moving towards the roof or the floor. Due to the absence of windows to serve as exit regions, the airflow is disrupted, and ventilation is affected. As there is no flow in that region, the amount of heat lost through convection would decrease. Resultantly, the amount of cooling load and electricity required to cool the middle rows would increase. Figure 10 depicts the results of the velocity contours of the initial design.

Design Optimization
According to the CFD results of the current design, it is necessary to improve the flow in the central zone of the room. To do this, a minor modification to the design is recommended, which is the addition of windows of the same size and height on the east side. That would allow air to escape through the windows in the center rows.

Distribution of velocity inside the room
The findings of the velocity distribution in the suggested design are shown in figure 11. It can be seen that the velocity has been increased in the middle region of the lecture theatre due to the addition of windows. The air molecules do not bounce back off the walls, and the air that comes from the west side can now easily depart through the windows on the opposite side, resulting in an undisrupted airflow and better heat removal due to convection than in the previous case. Figure 11 shows the results of the velocity distribution for optimized design.

Contours of Pressure inside optimized design
The pressure contours of the suggested design were generated. The airflow is not disrupted, as can be seen on the left, and as a result, the regions of high pressure have been reduced on the left side. Though high-pressure areas remain toward the roof, the area where students will be seated has a more uniform distribution of pressure. Figure 12 shows the results of the pressure distribution for optimized design.

Contours of Velocity inside optimized design
The findings of the velocity contours of the suggested design show that the flow is uniform from the input to the output. It can be seen that there is no stagnation of air in the middle rows of the lecture theatre for the proposed design. Figure 13 shows the results of the velocity distribution for optimized design.

Results Comparison
To study the distribution of velocity from the west side to the east side, a line was added as a reference in the middle region. When there is no window present on the east wall, the air velocity decreases to zero.
However, when a column of windows was added, the air velocity doesn't become zero and shows a small rise in magnitude. In the initial design, the slope of the curve was on the downward side, whereas the proposed design has a curve that is flat in the room area and gradually increases as it nears the exit. Because of the improved airflow, greater heat loss can occur as a result of convection, which would result in a reduction in cooling loads. A reduction in cooling load means a reduction in power consumption, which is a step forward in the direction of green building design.

Single Panel Energy
On a bright sunny day, a single solar panel may generate 300 watts of power. The power output is estimated to be 335 Watts, taking into account weather changes and the efficiency loss.
In Pakistan, the annual sunlight hours range between 2,900 and 3,300 hours.

Consumption of Power
The lecture hall is equipped with seven fans and ten tube lights.

Consumption of single light= 40 Watts
For eight hours each day, the daily energy consumption is 8.8-kilowatt-hours (KWh).

Required Solar Panels
The required amount of energy = 8. So, from the above approximation, the required number would be 4.

Conclusions
In this study, an alternative approach to design the green building was presented. A 3D model of the existing lecture theatre was modeled using SOLIDWORKS and a CFD analysis was conducted to study the air distribution inside the room. It was observed that stagnation of air takes place on the door side of the lecture theatre because of the absence of windows. Due to stagnation of air, heat loss due to convection reduces in that region, and the cooling load required to remove the heat from this region increases. An additional column of windows was added in the second design and CFD analysis was conducted on it. The problem of stagnation of air was reduced by the addition of a new column. Hence, it can be concluded that the current buildings can also be converted into green buildings by optimizing the natural airflow and reducing the cooling load required.