Improving the indoor air quality (IAQ) in naturally ventilated lecture hall with a single facade by solar chimneys

In this paper, the indoor air pollution was investigated inside an educational building which contains air pollutants with elevated concentrations. A field study was conducted in a naturally ventilated, single-faceted lecture’s hall to evaluate the indoor air quality (IAQ). Both air velocity and carbon dioxide (CO2) concentrations were measured at the respiratory area level to compare these values with ASHRAE standard (62.01-2019). The computational fluid dynamics (CFD) 3D model was utilized to predict the air velocity, and CO2 concentrations, and to validate the measured air concentrations. The measured results fairly agree with the numerical CFD data with a 6.2% difference between both values. This paper deals with experimental work to study the effect of the cross-section area, the number, and the height of the solar chimneys. The results showed that using solar chimneys improved the natural ventilation in the hall and minimized the CO2 concentrations. Additionally, using the chimney cross-section area of 0.25*0.25 m, 0.30*0.30 m, and 0.40*0.40 to 0.50*0.50 m can reduce the CO2 concentrations to (3%, 6.2%, 6.4%, and 6.7%, respectively). While using three chimneys instead of only one, the ventilation flow rate increased from 61 to 70.9%. The effect of the height of the chimney on the average of CO2 concentrations inside the hall was examined. The modeled height rates (1, 3, 5, and 7 m, respectively) were improved to 26%, 33.6%, and 48.7%, respectively.


Introduction
Indoor air pollution is one of the most important environmental threats to public health worldwide, given the increase in the number of indoor air quality diseases. Previous studies have found that the concentration of indoor pollutants is two to five times higher than in the outdoor environment, which in some cases, is reaching 100 times higher than the concentration of outdoor pollutants. Most people spend between 80 to 90% of their lives indoors; therefore the indoor air quality has a significant impact on public health [1][2][3].
The IAQ in classrooms should be considered carefully, especially in the third world countries, where most classes are ventilated naturally. In some classrooms, natural ventilation is a requirement for the design of a low-energy consuming building [4,5].
Cairo city, Egypt, has high concentrations of atmospheric pollutants including particulate matter (PM), carbon monoxide (CO), carbon dioxide (CO 2 ), and nitrogen oxides (NO x ) [6,7]. Transport and distribution of air pollutants generally depend on the ventilation system [6,7]. Some studies of naturally ventilated university classrooms have shown that the measured levels exceed the recommended value of 1000 ppm [8].
The main objective of ventilating indoor environments is to provide a comfortable and healthy environment for students [9]. The main goal herein is to provide fresh and pleasant indoor air without any negative impact on health taking into account energy efficiency and sustainability [9].
In comparison to K-12 schools, the research studies that focused on IAQ in higher education buildings were limited [10,11]. The effect of IAQ and ventilation rates on performance and health using psychological tests was investigated [12]. It is reported that carbon dioxide was the most influencing pollutant to be considered when calculating the outdoor air supply rates [9,12].
The use of solar chimneys improved the air movement in buildings with natural ventilation using the sun's renewable and clean energy [13]. Egypt has an average daily solar energy of 4.9 kWh/m 2 [13]. These climatic features encourage the application of solar chimneys to provide comfortable conditions in educational buildings [14].
Previous studies have examined the solar chimneys by their potential advantages in terms of energy requirements [15]. A steady-state mathematical model was developed based on the thermal network approach to predict natural induced ventilation using passive solar flues [16]. Some studies focused on the number and height of solar chimneys, air gap width (e.g., cross-sectional area), and direction of the chimney to natural ventilation in space [17]. It is reported that using 1, 2, or 3 solar chimneys can reduce temperature to 6%, 10%, and 12%, respectively. Also using 2 and 3 chimneys instead of 1 increased the ventilation flow rate to 13% and 33%, respectively [17]. The study found that the speed in a living area is achieved by using a solar chimney with a height of 1.85 m, a width of 2.65 m, an angle of inclination of 75 degrees, and an air gap of 0.28 m [18]. Results showed that the natural ventilation was improved by using multiple solar energy chimneys [18]. It is reported that the use of solar chimneys increased the air exchange in the classroom and reduced the levels of CO 2 concentration [17,19].
The application of computational fluid dynamics (CFD) has widely investigated the IAQ role in the classrooms [20,21]. Awbi and Gan [20] used CFD modeling to make predictions of air movement and thermal comfort in a mechanically well-ventilated office unit [20]. The CFD can be used to predict airflow and thermal comfort in offices with natural and mechanical ventilation with same accuracy [22]. The CFD was used to predict the airflow pattern in an air-conditioned seminar room [23]. Despite the abovementioned studies, there are little studies focused on the use of CFD simulation in the lecture's hall particularly with natural ventilation [24].
This paper aims to evaluate the naturally ventilated lecture's hall and verify if the standard IAQ values recommended by ASHRAE standards [25] are applied and satisfied. The novelty of this work is conducting IAQ in a large-size lecture's hall (dimensions: 12.55 m*19 m*4 m; full capacity and critical case: 300 students) located in a square with high traffic density in relation to small internal halls that used in other similar studies. The CFD model was used to simulate the IAQ standards in a lecture's hall at Faculty of Engineering at Cairo University, Egypt. The measured data were compared with the experimental modeled data and also to those of similar studies. In addition, the solar chimney was chosen to improve the rate of ventilation inside the lecture's hall to reduce its CO 2 concentrations until it reaches the acceptable concentration ratios.

CFD modeling and case study
This study includes two models: numerical (assumed) and measured (experimental). The initial model of the case study was drawn within 3D AutoCAD (source: https://www.autodesk.com/products/autocad), and then, it was exported into the ANSYS CFD model [26]. The selected lecture's hall at the Faculty of Engineering, Cairo University, contains 110 students with dimensions of (12.55 m*19 m*4 m) with (X, Y, Z) Cartesian coordinates. The air ventilation has naturally inlet through single-facade windows. The lecture's hall has 12 sliding windows, each of which has a dimension of 0.675 m*1.45 m and the total ventilated area was calculated as 11.745 m 2 . The doors are open during the teaching period, where supplied air was measured from the inlet windows at an average velocity of 0.4 m/s. Figure 1 shows the specific features and components of the lecture's hall which is located in the third floor in the north direction (NNW, 15°).

Numerical meshing
In this study, meshes with various sizes were used and it was found that small and dense sizes delivered more accurate results ( Table 1). The mesh used in this study was very dense more relative to the contaminant sources and then gradually increased till it reached to the reasonable internal size of elements to the whole volume. To do this, a size function was generated with a starting size of 0.02 to 0.2 m of mesh (Table 1). Steady-state boundary conditions (input data) In the case study, the air inlet velocity was measured at the inlet flow window with an average value of 0.4 m/s and temperature of 27°C which was measured in October 2017 because it was the beginning of the semester with a relative humidity of 45%. The measured outdoor CO 2 was measured at 490 ppm. The occupants' bodies have a temperature of 37°C [27], and with no species diffusion. The faces of the occupants are treated as a species source due to the presence of the CO 2 in the expired air of the respiratory system of the occupants [28]. The volume of 0.5 L is considered as the volume of an average breath per occupant [29], and the volume of the gases in the dry expired air under standard conditions are 74.5% N 2 , 15.7% O 2 , 3.6% CO 2 , and 6.2% H 2 O. The mass flow of expired air from the occupants is calculated as 2*10 −4 kg/s per occupant based on 20 times per minute during normal activity [28].

Measurement of indoor parameters
The main indoor measured parameters are CO 2 concentration and inlet velocity. The measurement devices (EXTECH portable CO 2 meter and Anemometer) were used in this study to measure the CO 2 concentrations and wind velocity ( Fig. 2) (specifications as reported in Table 2). All measurements were taken to compare the output of the experimental work with the numerical predictions.

Velocity measurement
The measuring device used in this study was a hot-wire thermal anemometer (Fig. 2b). The device is based on relating the amount of heat removed by air stream passing on the sensor to the mean velocity of the air stream (Fig. 2b). The air velocity at the windows inlet (with A, B, and C sections) was measured at 7 points within the window area (Fig. 3).
The average air velocity of the measured 7 points for the case study was 0.4 m/s.

CO 2 measurements
The indoor CO 2 was released in the exhausted air from the students' breathing operation, given that the students are an important source of CO 2 inside the lecture's hall and gave high CO 2 levels. The outdoor CO 2 measured at 490 ppm due to the high traffic density in the university square in front of the lecture's hall. CO 2 is a colorless and odorless gas, with a density of 1.7878 kg/m 3 [28]. CO 2 was measured using EXTECH portable CO 2 meter, which can store the maximum, minimum, and average values of CO 2 (Fig. 2a).

Numerical modeling
The numerical three-dimensional modeling has been carried out under the assumption of steady-state conditions using ANSYS*2021 R1 Fluent commercial CFD software [26]. The numerical modeling included measurements of air inlet velocity and CO 2 at the windows inlet. In addition, CO 2 was set at the breathing zone level (1.20 m) at 21 points as shown in Fig. 4. Multiple field measurements were done in the lecture's hall with various students' density from 9:00 to 16:00 in 3 days (22-24 October 2017 (fall)) and the number of measured occupants were varied from 0 (empty hall), 50, 80, and 110. However, the study was focused only on the worst case scenario with the maximum capacity (300 occupants) which is delivered from the CFD model. Additionally, the measurements were carried out at different levels with the same direction in the hall to determine the effect of the University's Square and traffic on the air quality in the facility.

Experimental and validation results
To examine the reliability and accuracy of the CFD model, the RNG k-ε turbulence model simulation results are compared with the experimental measurements of CO 2 (ppm). The computed contours of CO 2 concentrations expressed as Mole fraction, which was converted to ppm (PPM equals Mole *10 6 ) at the student's breathing level zone as shown in Fig. 5. It is noted that the CO 2 concentrations was high at B and C areas in the hall except near the inlet window, where the CO 2 values were reduced due to the fresh air inlet. The comparison between the simulated and the measured results are presented in Fig. 6. The CO 2 concentrations were plotted for the horizontal plane, specifically at the levels 1 to 7 for sections (A, B, C) values of the horizontal plane (Fig. 6).
It is noted that the measured and simulated values of CO 2 in the hall exceed the recommended values by the ASHRAE standard [25] and the Egyptian CODE for ventilation of 1000 ppm [29]. This means that natural ventilation in the lecture hall is not enough, and a ventilation system is needed to maintain the required CO 2 concentration levels. The CFD model was validated and the deviation between the numerical and  experimental results have an average of 6.2% (Fig. 6), which is in agreement with the average error of [30]. Obviously, the CO 2 concentrations in section A are low, which is referred to the reduction by the fresh air near the inlet window. While, in section B, the CO 2 concentrations were higher (Fig. 6), due to lack of air movement. In section C, the CO 2 values were significantly increased to 1600 ppm, which exceeds the ASHRAE standard [25]. This can be referred to the far location of the measured point and the lack of air movement inside the lecture's hall.
Accordingly, the conditions of single facade of the studied hall with number of students (110), at time (10 A.M.) and under natural ventilation are not sufficient and requests extra techniques for air movement to reach the acceptable levels of CO 2 concentration according to the ASHRAE standards [25].

Proposed natural ventilation retrofitting technique
Several techniques for retrofitting natural indoor ventilation have been proposed to improve the IAQ [5,7,13]. The solar chimney is a better ecofriendly alternative to improve the building ventilation and particularly for the single facade [31]. The proposed technique increases the ventilation performance of the indoor environment using passive solar air heating [17].
In this study, a mathematical model was developed for the solar chimney to determine the optimum design solutions that can achieve the standard ventilation rate [14]. Improvements in the performance of a solar chimney largely depend on the design variables [16]. The relevant design variables such as the engineering parameters controlling the shape of the solar chimney are reported in Table 3 and Fig. 7. The work is carried out on the entire student density (300 students), and the worst level (e.g., the third floor) in ventilation was chosen to be compared with other levels with halls of similar size and orientations, and it was treated as a general case that can be applied to in this study. In the recent decade, the designers should create ventilation techniques with environmental awareness. Therefore, the chimney is made of glass and the heat absorbent part is darkcolored, and the solar chimney was set to the south direction to increase the air velocity air from 12:00 to 16:00 where the sun rays are oriented southward.

Results and discussion
The  Fig. 7 and Table 3.

Case one: The squared cross-section
The effect of the cross-section is tested on the solar chimney squares (0.25*0.25, 0.30*0.30, 0.40*0.40, 0.50*0.50 m), which provides the highest performance and is shown in Fig. 8. The average indoor air velocity was increased to (38%, 57%, 61%, and 64%, respectively) causing a significant decrease in the CO 2 concentrations to (3%, 6.2%, 6.4%, and 6.7%, respectively) at the student level (Fig. 8). It is noted that the cross-section (0.50*0.50 m) has the highest improvement (6.7%) compared to the ASHRAE standard level (1,000 ppm), while, the rest 93.3 % has an unacceptable level in the lecture's hall (>1000 ppm) (Figs. 8 and 9). It was observed that the smaller cross-section square (0.25*0.25 m) has an average indoor air velocity (38%) which is improved to 64% at 0.50*0.50 m square within the lecture's hall compared to the ASHRAE standards (Figs. 10 and 11). These results are in agreement with those of [17,32].

Case two: The rectangular cross-section
The effect of different values of the rectangular cross-section (wall chimney) on the indoor air velocity distribution and the CO 2 concentrations on the students' breathing level is tested to reach the proper rectangular cross-section which provides the highest ventilation performance and the best air distribution inside a single-facade lecture's hall.
The effect of wall chimney on the indoor air velocity was also examined as shown in Fig. 15. It is noted that the proposed 0.25 * 13 m cross-section area improved the average indoor air velocity by 40% compared to other wall chimney cross-section areas (Fig.  15), and as a result, the CO 2 concentrations (ppm) was improved by only 5% at the student's breathing level inside the single-facade lecture's hall according to the ASHRAE standards (Fig. 13). Based on this, the average indoor air velocity was improved to (40%, 50%, 70%, and 73%, respectively) at the wall chimney dimensions (0.25 * 13, 0.30 * 13, 0.40 * 13, and 0.50 * 13 m, respectively) (Figs. 14 and 15).
Case three: The number of chimneys Figure 16 shows the effect of the number of chimneys on the average CO 2 concentrations (ppm) inside the lecture's hall, the more number of chimneys, the lecture's hall more ventilated. This means that the air is more mixed and better distributed inside the hall. The number of chimneys, which can be used, depends on the timing which requires reducing the average CO 2 concentrations inside the lecture's hall. Using one chimney along the wall has decreased the average CO 2 concentrations to 34%, and by using three chimneys the CO 2 reduction was up to 40% (Figs. 16 and 17). Note that the use of multiple solar chimneys can increase the ventilation flow rate inside the lecture's hall by using multiple stacks (Fig. 18). Accordingly, it was noticed that in one chimney, the ventilation flow rate reaches 61%, and increasing the number of chimneys to three can increase the ventilation flow rate to 70.9% (Fig. 19). These results are much higher than 13-33% of [17].
Case four: The chimney's height (1, 3, 5, and 7m). Figure 20 shows the height of the chimney and its effect on the average CO 2 concentrations for the solar chimneys. When the chimney's heights are more facing the lecture's hall surface, it can be a negative pressure zone toward the sun direction (to the south in this case). The more the chimney's height is, the more the chimney's temperature increases. This can be attributed to the thermal energy gained through the air as it passes through the chimney (Fig. 20). The effect of the chimney's height on the average CO 2 value inside the lecture hall was improved at the different chimney's heights (1, 3, 5, and 7 m, respectively) to 26%, 33.6%, 48.7%, and 49.7%, respectively (Fig. 21).
It is noted that the chimney's height of 5 m was the most proper because it is located in the air comfort area of the students and it should not exceed 15 CFM (0.9 m/s) per person, while the height 7 m was excluded because it exceeds the recommended ASH-RAE standards (Figs. 22 and 23). The average indoor air velocity was improved to (33%, 70.9%, 74.2%, and 89.6%, respectively) at the wall chimney heights (1, 3, 5, and 7 m, respectively) (Figs. 22 and 23). These results are in agreement with similar trends of [17]. The adopted methodology in this paper has its limitations such as the measured data were carried out in a single-façade, large-size hall, with natural ventilation, located in a square with high traffic density on October 2017 (fall season) which is the beginning of the first semester with low air masses entering the hall. However, other studies were focused on small classroom [30] and small residential rooms [31]. These limitations should be overcome in any future research studies. Additionally, the authors recommend conducting a future research on halls in the coastal areas, in variable seasons (summer, winter, and spring) and focusing on the effect of adequate ventilation systems on reducing other pollutants such as dust, fungi, bacteria, viruses, and VOCs through periodical measurements.

Conclusions
A field study in a naturally ventilated, single-faceted lecture's hall at the Faculty of Engineering, Cairo University, was investigated to evaluate the indoor air quality (IAQ). The air velocity and CO 2 concentrations were measured at the student's breathing level and the results were compared with the ASHRAE standards. Numerical 3D CFD modeling was applied on the measured data using ANSYS Fluent software. The CFD model was validated, and the deviation between the numerical and measured results has an average of 6.2%, which has a fair agreement with literature. The fresh air was insufficient and poorly distributed in the studied lecture's hall. Accordingly, the hall was polluted with CO 2 concentrations higher than the ASHRAE standard. In addition, the number of students was an effective parameter in this study up to the maximum capacity of 300 students that give higher CO 2 concentrations. On the other hand, the solar chimney worked to move the air inside the hall and significantly reduced the average CO 2 concentrations inside the hall through the proper cross-sectional area of 0.50*13 m. The multiple solar chimneys helped to distribute the air inside the hall in an acceptable manner and using three chimneys delivered the best performance. Similarly, the chimney height of 5 m on the air flow rate gave the best performance. It is noted that there are few studies investigating the IAQ role in the lecture's halls in the developing countries compared to those in the developed world, which can be referred to the limited resources and logistic issues.