Sustainable Hybrid Cooling: Integrating Indirect Evaporative and Split Air Conditioning for Improved Indoor Air Quality in Tropical Climates

Abstract

To address the limitations of conventional split air conditioners (SACs) that lack proper ventilation, resulting in indoor pollutant buildup and health risks, this study develops and evaluates the performance of a sustainable hybrid air conditioning system that integrates Indirect Evaporative Cooling (IEC) with SAC to enhance indoor air quality (IAQ), thermal comfort, and energy efficiency in tropical climates, compared with a standalone SAC system. The hybrid SAC + IEC system is designed to meet stringent comfort criteria while reducing indoor formaldehyde and carbon dioxide concentrations. Experiments were conducted in a controlled classroom environment using a cross-flow tubular heat exchanger with optimized nozzle configurations. Temperature, humidity, and pollutant levels were continuously monitored under varying tropical conditions. The IEC achieved an average cooling capacity of 1430 W, substantially exceeding the target of 566 W, and reduced the fresh air dry-bulb temperature by up to 8.79 °C, maintaining primary air near 25.2 °C, with energy efficiency ratios varying between 30% and 100%. The hybrid SAC + IEC system outperforms the standalone SAC system in maintaining acceptable formaldehyde and CO₂ levels while delivering comfortable thermal conditions within the indoor standards. These results demonstrate that the Hybrid SAC + IEC system optimizes energy efficiency and improves cooling performance and indoor air quality (IAQ) for tropical environments.

1. Introduction

In tropical regions, maintaining indoor air quality (IAQ) and thermal comfort is challenging due to consistently high temperatures and humidity levels [1]. These environmental conditions not only affect occupant comfort but also contribute to the deterioration of indoor environmental quality (IEQ) by promoting the accumulation of indoor pollutants such as carbon dioxide (CO₂) and formaldehyde, which originate from human respiration, furniture emissions, and other indoor sources [2,3,4,5]. The widespread use of split-type air conditioners (SACs) in offices, classrooms, residences, and industrial facilities often exacerbates these issues, as these systems are typically installed without adequate ventilation to minimize energy consumption and reduce cooled air loss. However, this lack of proper ventilation leads to poor air exchange rates, resulting in the continuous recirculation of stale indoor air and the accumulation of pollutants [6]. Insufficient ventilation rates (VRs) have been linked to increased risks of respiratory illnesses, Sick Building Syndrome (SBS), and cognitive impairment due to prolonged exposure to elevated CO₂ levels [2,3,7]. Studies indicate that inadequate ventilation increases the risk of respiratory infections and discomfort by 1.5 to 6 times, depending on pollutant exposure levels [7,8,9]. Given that people spend over 90% of their time indoors, ensuring effective ventilation and air exchange is critical for maintaining occupant well-being, productivity, and overall comfort [10,11]. While mechanical ventilation systems and conventional air conditioning are widely used in commercial and residential buildings, these methods often come at the cost of high energy consumption and environmental impact [12]. This underscores the urgent need for innovative and energy-efficient ventilation solutions that balance cooling performance, air quality improvement, and sustainability [1,12].

One promising approach to enhancing both IAQ and cooling efficiency while minimizing energy consumption is the integration of Indirect Evaporative Cooling (IEC) with SAC systems, forming a hybrid ventilation strategy. Indirect evaporative coolers (IECs) operate by using water evaporation to absorb heat, effectively cooling the air without increasing indoor humidity, making them highly suitable for tropical and humid climates [13]. Unlike direct evaporative cooling, which introduces moisture into the conditioned space, IEC technology ensures that the cooled air remains dry, preventing excess humidity buildup. Research has shown that IECs can achieve wet-bulb efficiencies of 50% to 90% [14,15], energy efficiency ratios (EERs) of up to 80% [15], and consume approximately 55% less energy than conventional air conditioning systems [16,17]. These low-energy cooling technologies are gaining interest due to their minimal environmental impact, adaptability across diverse climate conditions, and ability to supplement traditional air conditioning systems. However, standalone IECs may be less effective in extremely humid environments, where evaporative cooling efficiency is diminished. As a result, hybrid air conditioning systems that integrate IECs with SACs have emerged as a promising solution for improving energy efficiency, indoor comfort, and ventilation effectiveness in high-occupancy indoor spaces such as classrooms, offices, and industrial buildings [18,19,20]. Despite this potential, limited research has been conducted on optimizing IEC-SAC hybrid systems, particularly in tropical settings where humidity regulation is a critical factor. Additionally, research on performance improvements related to spray nozzle arrangements and the integration of ejector venturi scrubbers in IECs remains scarce. Ejector venturi scrubbers utilize preformed sprays to draw working air through a single nozzle, reducing fan usage, enhancing airflow efficiency, and lowering energy consumption. However, the energy-saving potential and practical applications of these devices in hybrid IEC-SAC systems require further exploration to determine their long-term feasibility and real-world benefits.

This study addresses research gaps by developing and evaluating a novel hybrid ventilation system that integrates a self-air-circulating Indirect Evaporative Cooler (IEC) with a cross-flow tubular heat exchanger and four ejector venturi scrubbers, coupled with a split air conditioner (SAC). The system is designed to deliver effective ventilation, maintain comfortable indoor temperatures, and achieve significant energy savings in tropical climates by balancing energy efficiency, ventilation performance, and pollutant control. Using a controlled comparative design in a tropical-like indoor environment, the study measures cooling capacity, energy efficiency, thermal comfort, indoor pollutant levels (CO₂ and formaldehyde), and overall energy consumption. Through a comprehensive analysis of these parameters, the research provides empirical evidence on the viability of hybrid IEC-SAC systems. The findings are expected to advance sustainable indoor climate control, inform building ventilation policies, and support the transition to energy-efficient, eco-friendly cooling solutions for tropical environments.

2. Materials and Methods

2.1. Design and Optimization of an IEC

The design and optimization of an Indirect Evaporative Cooler (IEC) for a controlled indoor environment begins with a thorough assessment of classroom characteristics—specifically a 4.25 m × 3.95 m × 2.72 m experimental room positioned to analyze interactions between indoor and outdoor air. To enhance cooling efficiency, key heat transfer parameters—such as heat gains from occupants and equipment, ventilation rates, and wall heat transfer coefficients—are carefully considered. The Log Mean Temperature Difference (LMTD) method is then employed to determine optimal heat exchanger dimensions, ensuring robust cooling performance. Engineered to manage a 566 W sensible heat load from ventilation and reduce primary air temperature from 35 °C to 27.5 °C using a water flow rate of 0.901 kg/h, this cross-flow tubular heat exchanger comprises 621 aluminum tubes, each 0.01285 m in diameter and 0.4 m in length, arranged in a 23 × 27 pattern for maximum heat dissipation. The system directs primary airflow over the tube exteriors, while secondary airflow travels through the tube interiors, thereby improving heat transfer efficiency. Additionally, ejector venturi scrubbers are integrated to minimize reliance on mechanical fans, enhancing energy efficiency while maintaining proper ventilation. The system’s key components and schematic diagram are shown in Figure 1.

To optimize the IEC’s cooling capacity, key experimental parameters (two nozzle types, three primary airflow rates, two secondary air inlet conditions, and two secondary air-driven modes) were systematically evaluated under realistic tropical conditions (

Table 1. Independent Variables, Levels, and Fixed Conditions for Optimizing IEC Performance.

Independent Variables	Levels	Fixed Conditions
Nozzle Types	- Type A (200 µm droplet size); - Type B (290 µm droplet size)	Fixed: low primary airflow rates; exhausted air condition; and Fan + Ejector venturi air mode
Primary Air Flow Rates	- Low (0.202 m3/s); - Medium (0.211 m3/s); - High (0.214 m3/s)	Fixed: nozzle type A; exhausted air condition; and Fan + Ejector Venturi air mode
Secondary Air Inlet Condition	- Outdoor air (Fresh air); - Exhausted air (Cooled air)	Fixed: nozzle type A; low primary airflow rates; and Fan + Ejector Venturi air mode
Secondary Air-Driven Mode	- Ejector Venturi (0.050 m3/s); - Fan + Ejector Venturi (0.075 m3/s)	Fixed: nozzle type A; low primary airflow rates; and exhausted air condition

). Each parameter set was tested under fixed conditions for one hour daily across five consecutive days, encompassing sunny, cloudy, and rainy weather. Temperature, relative humidity, and airflow rates were continuously recorded at each inlet and outlet every minute. This extended testing period provided a comprehensive evaluation of IEC functionality, confirming its adaptability to fluctuating environmental conditions while maintaining stable cooling efficiency.

2.2. Experimental Setup and Configurations

Experiments were conducted in a controlled classroom that accommodated 11 occupants, utilizing a conventional wall-mounted SAC (DAIKIN, Model FTKQ-UV2S, 12,300 BTU/h; Daikin Industries, Chonburi, Thailand), operating in cooling mode with a set point of 25 °C to evaluate various configurations [21]. As shown in

Table 2. Characteristics of Each System Type Related to Air Conditioning Unit, Ventilation Method, and Air Change Rate.

System Type	Description	Air Conditioning Unit	Ventilation Method	Air Change Rate
1. Hybrid SAC + IEC system	This system provides an innovative solution to enhance indoor air quality and ventilation by integrating diverse technologies for comprehensive air exchange and energy recovery.	SAC + IEC system	IEC system	5 AC/h
2. Standalone SAC system	This system represents the typical configuration in Thailand, where most buildings have air conditioning but lack adequate ventilation.	SAC system	None	0 AC/h

, two air-conditioning configurations were systematically tested: (1) a Hybrid SAC + IEC System, which integrates a self-air-circulating Indirect Evaporative Cooler (IEC) featuring a cross-flow tubular heat exchanger and four ejector venturi scrubbers (see Figure 2 and Figure 3) alongside a conventional SAC, and (2) a Standalone SAC System, a common split-type air conditioner with no additional ventilation. The air change rate for the hybrid SAC + IEC system was set at 5 AC/h in accordance with ASHRAE Standard 62.1-2022, ensuring adequate ventilation for a classroom with one teacher and ten students aged nine or older. Key parameters, such as the outdoor air rate per person ($R_p$) of 5 L/s, the area outdoor air rate ($R_a$) of 0.6 L/s·m², and a zone air distribution effectiveness ($E_z$) of 1.0, led to this 5 AC/h rate [22]. This ventilation rate also aligns with recommendations to reduce the spread of SARS-CoV-2 in compact indoor spaces, where 4 to 6 AC/h are advised [23].

To simulate indoor pollution, a 15 mm particleboard (1200 mm × 600 mm) with a formaldehyde emission rate of 0.938 mg/m²·h was placed on the teacher’s desk one day before data collection to ensure continuous off-gassing for 24 h, while CO₂ was introduced at an occupant breathing-zone height at 0.20592 m³/h from a reservoir to mimic exhalation by one teacher and ten students [24,25,26]. Experiments comparing the Hybrid SAC + IEC and Standalone SAC systems (see Table 2) were conducted as follows: On Day 1, pollutants were cleared by opening doors and windows and using fans. On Day 2, a formaldehyde source was introduced 24 h before data collection. On Day 3, data collection began at 8 AM with blinds closed to prevent overheating, and measurements started at 9 AM after conditions stabilized, with baseline data recorded before CO₂ introduction. Temperature, relative humidity (%RH), energy consumption, and CO₂ levels were measured over 2 h to simulate typical classroom activities, while formaldehyde was monitored in 24-hour experiments with air samples taken at intervals (0, 0.5, 1, 2.5, 5.5, 9.5, 15.5, and 24 h). Each cycle lasted 3 days, with the Standalone SAC system tested over three cycles. Pollutants were cleared for 1 day before testing the next system configuration. The Hybrid SAC + IEC system followed the same experimental cycle as the Standalone SAC system.

Additionally, the study compares energy consumption and indoor temperatures between the two configurations, detailing energy usage for both SAC and IEC components, and presenting total energy consumption alongside the final indoor temperature. This approach highlights differences in cooling performance and power usage, providing insights into each system’s efficiency and effectiveness.

2.3. Data Collection and Measurement Techniques

To ensure high precision and reliable data collection, this study employs a multi-method approach integrating sensor-based monitoring, air sampling, and computational analysis. This methodology allows for a comprehensive evaluation of cooling capacity, thermal comfort, indoor air quality (IAQ), energy consumption, and system optimization parameters, providing a robust assessment of the performance of hybrid ventilation and air conditioning systems.

2.3.1. The IEC Capacity Measurements

During the experiments, temperature and relative humidity were recorded using a 3 M Area Heat Stress Monitor (Model QUESTemp°34; range: −5 to 120 °C, 20 to 99% RH; accuracy: ±0.5 °C, ±5% RH; 3M Science Applied, St Paul, MN, USA) with data logged every minute. Sensors were installed at four locations: (1) the primary air inlet duct, (2) the primary air outlet duct, (3) the secondary air inlet duct, and (4) the classroom center at 1.2 m to represent occupied-zone conditions (see Figure 3). Air velocity, airflow rate, and pressure drop (ΔP) were measured using multi-function sensors from KIMO (Model AMI 300; range: 0.15 to 3 m/s, 0 to ±500 Pa; accuracy: ±0.03 m/s, ±0.2% Pa; Kimo Instruments, Montpon, France), while water flow rates were manually recorded with a calibrated flow meter. This setup ensured precise monitoring of the IEC’s cooling capacity.

Cooling capacity, calculated from experimental data, represents the enthalpy change in the primary air stream, as it traverses the dry channels of the IEC heat exchanger and is expressed as [27,28]:

$$Cooling capacity=Output cooling energy in BTU=Sensible heat$$

(1)

$$Sensible heat\left(h_s\right)=c_p\times {\rho }_a\times Q_{p,out}\times (T_{p,db,in}-T_{p,db,out})$$

(2)

The conversion from kW to BTU/h is given by:

$$P_{\left(\mathrm{B}\mathrm{T}\mathrm{U}/\mathrm{h}\right)}=3412.142\times P_{\left(\mathrm{k}\mathrm{W}\right)}$$

(3)

2.3.2. Thermal Comfort Measurements

Indoor thermal comfort was continuously monitored by measuring temperature, relative humidity, air velocity, and airflow rates. Temperature and humidity were measured using a 3 M Area Heat Stress Monitor (Model QUESTemp°34, range −5 to 120 °C, 20 to 99% RH; accuracy ±0.5 °C, ±5% RH) and a KIMO AMI 300 (range −20 to 80 °C, 5 to 95% RH; accuracy ±0.25 °C, ±5% RH), strategically placed at 1.2 m to simulate the breathing zone of seated occupants (see Figure 3). Air velocity and airflow rates were recorded using a KIMO hotwire anemometer (Model AMI 300, range 0.15 to 3 m/s; accuracy ±0.03 m/s; Kimo Instruments, Montpon, France), ensuring accurate assessment of ventilation performance. These measurements are essential for evaluating the effectiveness of various air conditioning configurations in maintaining optimal indoor conditions, particularly in tropical environments.

2.3.3. Indoor Air Quality (IAQ) Monitoring

To assess ventilation effectiveness and pollutant control, real-time carbon dioxide (CO₂) concentrations are monitored using NDIR sensors from KIMO (Model AMI 300, range 0 to 5000 ppm; accuracy ±3% ppm), providing continuous feedback on indoor air exchange rates. Formaldehyde (HCHO) concentrations are measured using two complementary methods: (1) electrochemical sensors from RAE (Model MultiRAE Lite, range 0 to 10 ppm; resolution 0.01 ppm; RAE Systems, San Jose, CA, USA) for real-time monitoring, and (2) the NIOSH 2016 Method (accuracy ±19%), which involves active air sampling with solid sorbent tubes (silica gel coated with 2,4-DNPH), followed by HPLC-UV analysis for highly accurate, laboratory-validated results. Additionally, ventilation rates (VRs) are estimated through CO₂ decay analysis, a technique that tracks CO₂ concentration fluctuations over time to assess air exchange effectiveness. This comprehensive IAQ assessment provides a detailed understanding of pollutant dynamics and evaluates each system’s ability to maintain safe and healthy indoor air quality standards.

2.3.4. Energy Consumption and Energy Performance Analysis

Energy consumption, in terms of electric power (W), energy (Wh), voltage (V), and current (A), was continuously monitored using a multi-function meter (Model D52-2066; 0 to 300 V, 0 to 100 A, 0 to 9999.9 W; accuracy ±1%; Wenzhou Taiye Electric, Zhejiang, China) and intermittently measured with a precision smart digital AC and DC clamp meter (Model PEAKMETER-PM2128; 0 to 1000 V, 0 to 1000 A; accuracy ±0.8% V, ±3% A; Guilin Huayi Peakmeter Technology, Guilin, China). Sensors were clamped onto the line wires supplying power to each system, and data were recorded every 5 min throughout the experiment. To evaluate system efficiency and energy performance, key metrics, including wet-bulb effectiveness (${\epsilon }_{wb}$), dry-bulb effectiveness (${\epsilon }_{db}$), Energy Efficiency Ratio (EER), and Coefficient of Performance (COP), were calculated. These indicators quantify cooling efficiency relative to power input and provide insight into potential energy savings and sustainability. The wet-bulb effectiveness is computed as follows [29]:

$${\epsilon }_{wb}={\displaystyle \frac{\left(T_{p,db,in}-T_{p,db,out}\right)}{\left(T_{p,db,in}-T_{s,wb,in}\right)}}$$

(4)

and the dry-bulb effectiveness as [30]

$${\epsilon }_{db}={\displaystyle \frac{\left(T_{p,db,in}-T_{p,db,out}\right)}{\left(T_{p,db,in}-T_{s,db,in}\right)}}$$

(5)

Power consumption, measured in watts (W) or kilowatts (kW), reflects the electrical energy used by the fan and pump of the IEC system. The EER is defined as the ratio of output cooling energy (in BTU) to electrical input energy (in Wh) [28], typically ranging from 30 to 80, while the COP is the ratio of cooling capacity (in J) to power consumption (in J/s). Converting EER to COP using the conversion factor (1 BTU/Wh = 0.293 COP) yields: COP = EER × 0.293. This analysis is crucial for determining the feasibility of integrating IEC with SAC in tropical climates.

2.4. Data Analysis and Statistical Methods

To assess the impact of key independent variables (including intake air temperature, relative humidity, airflow rate, and nozzle type) on IEC performance, a comprehensive experimental framework was developed. The study systematically varied these parameters to measure effects on wet-bulb effectiveness, dry-bulb effectiveness, energy efficiency ratio (EER), and coefficient of performance (COP). To ensure robust data interpretation, a combination of descriptive and inferential statistical analyses was employed. Descriptive statistics, such as mean, standard deviation, and range, were used to analyze temperature, humidity, pollutant concentrations, and energy usage, with bar and line graphs illustrating performance variations. Inferential statistics involved the Kolmogorov–Smirnov (KS) test to assess data normality. For non-normally distributed data, the Wilcoxon signed-rank test compared median indoor temperature, humidity, CO₂, and formaldehyde concentrations to their acceptable threshold values. The Mann–Whitney U test was employed to compare median values of temperature, humidity, energy consumption, and pollutant levels between the Hybrid SAC + IEC and Standalone SAC systems. Additionally, simple linear regression was applied to predict outlet primary air dry-bulb temperature and the temperature reduction in primary air in the IEC under varying inlet conditions.

2.5. Research Validity and Reliability

To ensure the accuracy, consistency, and reliability of findings, this study implements rigorous quality control measures. All sensors are calibrated before each experimental cycle to minimize measurement errors and enhance data precision. Air sampling strictly follows NIOSH and ASTM standards, ensuring high-precision pollutant detection and reliable IAQ assessments. To strengthen statistical validity, repeated trials are conducted, reducing anomalies and increasing the robustness of the results. Additionally, external validation is performed by comparing the findings with existing literature and previous studies, reinforcing the credibility and applicability of the research outcomes.

2.6. Ethical Considerations

This study adheres to ASHRAE 62.1 guidelines for ventilation and indoor air quality (IAQ), ensuring compliance with industry standards for safe and effective air management. No human subjects are directly exposed to pollutants during experiments, and all IAQ measurements are conducted in accordance with health and safety regulations, maintaining ethical integrity and prioritizing occupant well-being. Additionally, this study was approved by the Committee on Human Rights Related to Human Experimentation, Faculty of Public Health, Mahidol University, Bangkok (MUPH 2019-082).

3. Results

This study investigated sustainable hybrid ventilation systems combining Indirect Evaporative Cooling (IEC) and mechanical air conditioning across diverse conditions. Experiments measured cooling capacity, pollutant reduction, and energy usage, analyzing design elements like nozzles, airflow rates, and humidity control. Results revealed effective temperature and humidity regulation, notable energy savings, and enhanced indoor air quality. These findings underscore the promise of hybrid ventilation in tropical settings, supporting healthier, more sustainable indoor environments.

3.1. Cooling Capacity of the IEC

This study was conducted in a classroom under sunny, cloudy, and rainy conditions, simulating tropical operating environments for the IEC. The intake primary air dry-bulb temperature ranged from 27.81 °C to 36.49 °C, and the secondary air from 25.81 °C to 27.90 °C, with corresponding relative humidity values of 47.54% to 87.43% and 63.57% to 81.84%, respectively. The outlet primary air dry-bulb temperature ranged from 25.20 °C to 28.80 °C, with relative humidity between 63.57% and 81.84%. The IEC achieved an average cooling capacity of 1430 W (4881 BTU/h), substantially exceeding the 566 W design target, with a standard deviation of 410 W and a range of 401 to 2223 W, demonstrating its adaptability. Notably, the IEC reduced the fresh air dry-bulb temperature by up to 8.79 °C, maintaining a minimum product air temperature of 25.2 °C, with wet-bulb effectiveness ranging from 47% to 64%, and an energy efficiency ratio of 30 to 100%.

Figure 4 illustrates the IEC’s performance metrics under various configurations, showing its wet-bulb and dry-bulb effectiveness, COP, and EER across multiple tests. Results reveal that Nozzle Type A outperforms Type B, a secondary airflow rate of 0.075 m³/s is better than 0.050 m³/s, the exhausted air mode is superior to the outdoor air mode, and increasing primary airflow to 0.214 m³/s further boosts cooling capacity. These findings indicate that optimizing secondary airflow, using exhausted air, and employing finer droplets from Type A nozzles significantly enhance IEC efficiency and energy performance. Overall, the study demonstrates the importance of fine-tuning nozzle type, airflow rates, and ventilation mode to maximize IEC performance in tropical climates.

3.2. Factors Affecting the Performance of IEC

3.2.1. Effect of Water Nozzles

The Indirect Evaporative Cooler’s (IEC) performance was meticulously examined under variations in water nozzle characteristics, revealing that nozzle type plays a crucial role in overall cooling effectiveness. Figure 4 shows the average values of wet-bulb effectiveness (${\epsilon }_{wb}$), dry-bulb effectiveness (${\epsilon }_{db}$), coefficient of performance (COP), and energy efficiency ratio (EER) for two nozzle types. The results indicate that Type-A nozzles, with smaller droplet diameters, significantly outperformed Type-B nozzles (p < 0.01)—highlighting that decreasing the mean droplet diameter has a greater impact on performance than simply increasing the water flow rate, as smaller droplets evaporate more quickly and enhance the cooling effect [31]. By optimizing nozzle configuration [32,33] and reducing mean droplet diameter [34], the IEC achieved higher wet-bulb and dry-bulb efficiencies, improved COP, and an increased EER, underscoring the critical influence of nozzle characteristics on the IEC’s overall effectiveness.

3.2.2. Air Outlet Conditions Under Varying Intake Conditions

Figure 5 shows that varying primary and secondary air conditions significantly affect the primary air outlet temperature. Simple linear regression analysis indicates that a 1 °C drop in the inlet primary air dry-bulb temperature ($T_{p,db,in}$) decreases the outlet primary air dry-bulb temperature ($T_{p,db,out}$) by 0.359 °C (R² = 0.8612, p < 0.01), a 1 °C decrease in the inlet secondary air dry-bulb temperature ($T_{s,db,in}$) reduces $T_{p,db,out}$ by 1.629 °C (R² = 0.9024, p < 0.01), and a 1 °C drop in the inlet primary air wet-bulb temperature ($T_{p,wb,in}$) lowers $T_{p,db,out}$ by 1.654 °C (R² = 0.5023, p < 0.01). Conversely, a 1 °C increase in the inlet secondary air wet-bulb temperature ($T_{s,wb,in}$) decreases $T_{p,db,out}$ by 1.735 °C (R² = 0.4118, p < 0.01). These findings underscore the intricate interplay between primary and secondary air streams and highlight the need for precise temperature management to optimize system efficiency.

3.2.3. Influence of Dry-Bulb and Wet-Bulb Temperature Differentials on Primary Air Cooling Performance for IEC

Figure 6a shows a strong relation between primary air temperature reduction (${∆T}_p$) and the difference between dry-bulb and wet-bulb inlet temperatures (${∆T}_{db-wb,in}$) (R² = 0.9511, p < 0.01). As this temperature differential increases, primary air cools more effectively. A similar trend is observed for secondary air (R² = 0.8426, p < 0.01). These findings underscore that optimizing the dry-bulb/wet-bulb temperature gap is essential for maximizing the efficiency of the Indirect Evaporative Cooler and maintaining comfortable indoor conditions.

3.2.4. Influence of Inlet Air Humidity on Primary Air Cooling Performance for IEC

Figure 6b shows a strong relation between inlet air humidity and the IEC’s temperature reduction. The simple linear regression for primary air humidity (R² = 0.9523, p < 0.01) indicates that higher humidity decreases cooling effectiveness, with a similar trend for secondary air humidity (R² = 0.8527, p < 0.01). These findings underscore the need to optimize both primary and secondary inlet air humidity to maximize IEC efficiency. Additionally, the study demonstrates the IEC’s remarkable energy-saving potential in tropical climates, attributed to its advanced sensible tubular cross-flow heat exchanger, which effectively handles large ventilation heat loads with minimal power usage. This cutting-edge system is poised to revolutionize air conditioning and ventilation in educational, residential, and commercial environments.

3.3. Effectiveness of Hybrid Air Conditioning and Ventilation Systems in Reducing Indoor Pollutant Concentrations

3.3.1. Effectiveness of Hybrid Air Conditioning and Ventilation Systems in Reducing Indoor Formaldehyde Concentration

This subsection evaluates the effectiveness of hybrid air conditioning and ventilation systems in reducing classroom formaldehyde levels, as confirmed by measured concentrations and statistical analysis. Advanced air quality monitors tracked formaldehyde before and after implementation, and both descriptive and inferential statistics were used to analyze the data. Figure 7a shows that the Hybrid SAC + IEC system reduced formaldehyde levels to 0.08 ppm within 29 min, while the Standalone SAC System remained at an average of 0.241 ppm (±0.0096 ppm) and exceeded acceptable levels of 0.08 ppm (p < 0.01) [21]. The Hybrid SAC + IEC system maintained an average indoor formaldehyde concentration of 0.043 ppm (±0.0123 ppm), well below the acceptable levels of 0.08 ppm (p < 0.01) [21], demonstrating its superior performance in enhancing classroom air quality.

3.3.2. Effectiveness of Hybrid Air Conditioning and Ventilation Systems in Reducing Indoor Carbon Dioxide Concentration

This subsection examines the impact of hybrid air conditioning and ventilation systems on indoor CO₂ levels using precise gas analyzer measurements from classrooms with and without these systems. Descriptive and inferential statistics show that CO₂ concentrations are significantly reduced, consistently remaining below 1000 ppm [21]. As shown in Figure 7b, the Hybrid SAC + IEC system maintains an average CO₂ concentration of 789 ppm (±155 ppm), well below 1000 ppm (p < 0.01), whereas the Standalone SAC system averages 1928 ppm (±860 ppm) and climbs to approximately 3200 ppm over 120 min (p < 0.01). Statistical tests confirm these differences are highly significant (p < 0.01), demonstrating that the hybrid approach is markedly more effective than traditional air conditioning in maintaining safe indoor CO₂ levels. These findings highlight the significant advancements offered by hybrid systems, which not only reduce but also stabilize indoor CO₂ concentrations, setting a new standard for healthy and efficient air quality management in educational settings.

3.4. Impact of Hybrid Air Conditioning and Ventilation Systems on Thermal Comfort

This section demonstrates how hybrid air conditioning and ventilation systems improve classroom thermal comfort, as confirmed by descriptive and inferential statistics. Figure 8 compares indoor temperature and relative humidity between the Hybrid SAC + IEC and Standalone SAC systems. The Hybrid SAC + IEC system maintained an average temperature of 25.25 °C (range: 25.0–25.6 °C) and a relative humidity of 61.38% (range: 60.9–61.8%), whereas the Standalone SAC system averaged 23.98 °C (range: 23.7–24.2 °C) and 54.45% relative humidity (range: 53.45–55.53%). Statistical analysis confirmed that the Hybrid SAC + IEC system produced significantly higher temperature and humidity levels than the Standalone SAC system (p < 0.01), although the hybrid system still maintained conditions within acceptable ranges (24–26 °C and 50–65% relative humidity, p < 0.01) [21]. These findings suggest that although the hybrid system is slightly warmer and more humid, it consistently maintains comfortable indoor conditions within acceptable values [21].

3.5. Energy Consumption of Hybrid Air Conditioning and Ventilation Systems

This study found that the average total energy consumption of the Hybrid SAC + IEC system is 1125 Watt/hour (1046 Watt/hour for SAC and 79 Watt/hour for IEC), maintaining indoor conditions of 25.25 °C and 61.38% RH, while the Standalone SAC system consumes 864 Watt/hour to sustain conditions of 23.98 °C and 54.45% RH. Figure 9 illustrates that the Hybrid SAC + IEC system’s energy consumption is significantly higher than that of the Standalone SAC system (p < 0.01), with the Standalone SAC remaining stable between 866 and 900 W/h and the Hybrid system ranging from 1049 to 1080 W/h. Despite slightly higher energy use, the Hybrid SAC + IEC system delivers greater cooling output, reduces energy loss through temperature recovery and venturi-driven airflow, and improves indoor air quality through enhanced ventilation.

4. Discussion

The findings underscore the remarkable cooling capacity and adaptability of the Indirect Evaporative Cooler (IEC), which surpassed its initial target of 566 W by attaining an average of 1430 W under diverse tropical conditions. Smaller droplet diameters emerged as a key factor for boosting evaporative efficiency, along with wet-bulb/dry-bulb performance, coefficient of performance (COP), and energy efficiency ratio (EER) [31,35]. Empirical data drawn from 660 measurements confirm that humidity, inlet air temperature, and airflow rates strongly influence both the IEC’s cooling capacity and its wet-bulb/dry-bulb effectiveness. Optimizing the temperature gap between dry-bulb and wet-bulb air streams proved crucial for improved heat transfer, while managing inlet humidity was shown to be particularly important, given its inverse relationship with the IEC performance [36,37]. The viability of the sensible cross-flow tubular heat exchanger—comprising 621 parallel aluminum tubes—was further demonstrated by its ability to stabilize indoor conditions in hot, humid environments [32,37,38]. Together, these results highlight the IEC’s adaptability, supporting broader sustainable ventilation goals that combine indirect evaporative cooling with mechanical air conditioning [36,39].

Beyond thermal benefits, the hybrid system integrating SAC and IEC demonstrated a strong capacity to lower formaldehyde and CO₂ concentrations, thereby promoting healthier indoor environments—especially in classrooms with stringent air quality needs [40,41,42]. Specifically, formaldehyde levels dropped to 0.08 ppm in under 30 min, outperforming standalone air conditioning in pollutant control. These findings resonate with research from the National Institute of Standards and Technology (NIST) and studies by Hamdy M. and Mauro G.M. (2019) [41] and Jung C. and Mahmoud N.S.A. (2023) [40], which highlight the synergy between robust ventilation, low-emitting materials, and hybrid strategies in maintaining acceptable pollutant levels [31,35,43,44,45]. With the Hybrid SAC + IEC system sustaining an average temperature of 25.25 °C and relative humidity near 61%, it effectively balanced indoor comfort and energy consumption (1125 Watt/h versus 864 Watt/h for the Standalone SAC) while still achieving superior pollution control [35,38,46]. From an applied perspective, precise configuration—encompassing sensor-driven controls, careful nozzle selection, and humidity management—can substantially enhance the effectiveness of hybrid ventilation across various high-occupancy settings, including commercial and healthcare facilities.

Despite promising results, this study has certain limitations. Much of the evidence is drawn from controlled or short-term data, thus not accounting fully for real-world fluctuations in weather, occupant behavior, and building layouts. Moreover, focusing on specific pollutants like formaldehyde overlooks the broader spectrum of indoor contaminants, while cost-effectiveness and maintenance needs remain underexplored. Future research should, therefore, incorporate year-round, multi-site evaluations, including additional variables (e.g., airflow velocity, occupant density, building envelope properties), to gauge the feasibility and scalability of hybrid ventilation systems more comprehensively. Long-term monitoring is also necessary to confirm durability, maintenance requirements, and ongoing operational costs.

The implications of this study extend to building design, policy, and management in tropical regions. By integrating an efficient IEC that reduces indoor temperatures and stabilizes humidity levels, designers and facility managers can promote occupant comfort and well-being without excessively increasing energy consumption. Precisely tuning system parameters—such as spray nozzle type, airflow modes, and air change rates—enables robust thermal control and pollutant reduction, a valuable strategy for schools, offices, and other high-occupancy environments. Moreover, coupling indirect evaporative cooling with mechanical air conditioning broadens the system’s adaptive capacity, mitigating the effects of varying weather conditions and occupant loads while maintaining energy efficiency and thermal comfort. With the Hybrid SAC + IEC system rapidly lowering formaldehyde to 0.08 ppm and keeping CO₂ below 1000 ppm, its relevance for education and other sectors requiring stringent indoor air standards is evident. Wider adoption could support sustainability targets, improve occupant health, and reduce institutional energy footprints.

Finally, although the IEC showed strong versatility and performance across numerous tests, further studies are needed to validate these outcomes under different seasons and more complex building types. Scaling the system, accounting for diverse real-world pollutant sources, and assessing variations in occupant behavior will be essential for verifying reliability under everyday conditions. Investigations could also refine water flow rates, optimize airflow patterns, and incorporate advanced building controls to enhance efficiency. By effectively balancing temperature, humidity, and energy consumption while controlling pollutants, the Hybrid SAC + IEC system emerges as a promising solution for tropical environments. Continued research and development will help solidify its role in sustainable building design, contributing to healthier indoor spaces and more responsible resource use.

5. Conclusions

This study successfully developed and evaluated a new hybrid air conditioning and ventilation system integrating Split Air Conditioning (SAC) with Indirect Evaporative Cooling (IEC), significantly improving indoor air quality, thermal comfort, and energy efficiency in tropical climates. Quantitative results confirmed that the Hybrid SAC + IEC system significantly outperformed conventional SAC systems, reducing formaldehyde concentrations to an average of 0.043 ppm—below the 0.08 ppm threshold within 29 min—and keeping carbon dioxide levels at an average of 789 ppm, substantially below the acceptable value of 1000 ppm. The IEC achieved an average cooling capacity of 1430 W, significantly exceeding its design target of 566 W, while reducing fresh air temperature by up to 8.79 °C and producing air temperatures near 25.2 °C, with wet-bulb effectiveness of 47–64% and an Energy Efficiency Ratio (EER) of 30–100%. Although the hybrid SAC + IEC system consumed more energy than the standalone SAC (1125 W/h versus 864 W/h for the standalone SAC), it provided significantly enhanced indoor air quality and maintained thermal comfort within acceptable ranges (average of 25.25 °C and 61.38% RH), demonstrating a trade-off between energy use and environmental health performance in tropical classrooms. The novelty of this research lies in the integration of IEC with ejector venturi scrubbers and optimized airflow configurations, demonstrating a sustainable environmental control solution for high-occupancy tropical environments. Future research should explore long-term performance and scalability for further validation.