Thermal Performance and Energy Consumption of Air Conditioning Systems with Heat Exchangers for Water Heater Efficiency Improvement

Sommas Kaewluan,1 Nathawat Unsomsri1 and Songkran Wiriyasart2,*

1Biofuel and Bioenergy Technology Research and Development Laboratory Department of Mechanical Engineering, Faculty of Engineering, Srinakharinwirot University, 63 Rangsit-Nakhonnayok Road, Ongkharak, Nakhon-Nayok, 26120, Thailand

2Thermal Solution and Energy Technology Research and Development Laboratory, Department of Mechanical Engineering, Faculty of Engineering, Srinakharinwirot University, 63 Rangsit-Nakhonnayok Road, Ongkharak, Nakhon-Nayok, 26120, Thailand

*Email: songkranw@g.swu.ac.th (Songkran Wiriyasart)

Abstract                                                                                                             

 

This study aims to evaluate the impact of refrigerant heat removal via a double-pipe heat exchanger on the energy consumption of air conditioning systems. Experimental parametric studies were conducted, focusing on variations in water mass flow rate and inlet water temperature. The results demonstrate that incorporating heat exchangers not only enhances the thermal efficiency of water heaters but also lowers the refrigerant temperature before it enters the condenser. As a result, the COP of the systems showed a significant improvement, depending on the water mass flow rate. An increase in the water mass flow rate through the heat exchanger led to a higher heat transfer rate, rising from 0.7 kW to 1.58 kW. This corresponded to an approximate increase in COP from 22 % to 44 %, along with a decrease in power consumption. However, to maintain stable thermal performance, the temperature of the water entering the heat exchanger should be controlled. These findings offer valuable insights for future research on heat recovery applications, particularly in air conditioners, which are increasingly in demand for residential use due to their essential role in maintaining indoor comfort.

Keywords: Coefficient of performance (COP); Air conditioner; Waste heat recovery; Heat exchanger.

 

Table of Contents

Innovative Description: Heat recovered by the heat exchanger helps increase the coefficient of performance (COP) of both the air conditioner and the water heater.

 

 

 

1. Introduction

Energy plays a vital role in modern human life, and global energy demand is increasing rapidly. It is essential for industrial processes such as those in the steel, iron, food, and ceramic industries,[1] as well as for powering a wide range of electrical appliances, including televisions, electric stoves, water heaters, water pumps, lighting, refrigerators, and air conditioners. According to the first law of thermodynamics, also known as the law of conservation of energy, energy cannot be created or destroyed but can only be transformed from one form to another.[2] When energy is converted into heat and released into the environment, it contributes to global warming. Air conditioners are commonly used to provide thermal comfort in tropical regions and are also utilized in various industrial processes. The excess heat and refrigerants emitted by air conditioners significantly impact global warming.[3–5] Recovering waste heat from air conditioners presents a challenge, particularly as the quantity of waste heat correlates with building size and usage.[6] However, this excess heat can be recovered for practical applications such as clothes drying,[7] domestic water heating,[8–12] improving the thermal efficiency of water heaters,[13] and powering thermoelectric generators.[14–18] Heat recovery can increase water temperature by up to 30.3 % and improve system COP by 3 % to 13.66 %.[19,20] When integrated with heat recovery devices, systems can recover approximately 30 % of waste heat,[21] with a temperature differential of around 10°C suitable for electricity generation via TEGs.[22] Efficient heat recovery can be achieved using water as a working fluid in conjunction with heat exchangers.[16,17] Among these, heat pipe heat exchangers[16,23,24] and runaround coil heat exchangers[25,26] are highly recommended due to their excellent performance and high efficiency. Other advanced technologies such as thermosiphons, pulsating heat pipes, loop heat pipes, wire on a tube heat exchanger,[27] and phase change materials are also effective. These systems are favored for their high heat transfer performance enabled by phase change mechanisms, along with benefits such as simple design, low cost, and the absence of moving parts.[28–30] Overall, heat recovery tools offer versatile applications in waste heat management.[31-34] However, their selection should be guided by an understanding of their working principles, advantages and disadvantages, and performance-affecting parameters to ensure optimal efficiency.

As highlighted in the existing literature, recent studies have explored the recovery of waste heat from air conditioners for general applications. However, limited research has specifically examined the impact of heat recovery on enhancing the COP of air conditioning systems integrated with heat exchangers. Previous studies have already presented results on water heaters that utilize recovered heat emitted from the condenser of air conditioners. Therefore, this study investigates the effect of heat recovery using a double-pipe heat exchanger on air conditioner performance, building on prior experimental work.[13] The temperature distribution of the working fluids is analyzed, and the system's COP is calculated and compared with that of a conventional air conditioner.

 

2. Experimental setup

The experimental setup comprises five main components: an air conditioning unit, a heat exchanger unit, a water supply unit, a control unit, and a data acquisition unit, as illustrated in Fig. 1. The configuration and installation of the heat exchanger adhere to the specifications outlined in a previous study.[13] A double-tube heat exchanger is integrated into the air conditioning system between the compressor and the condenser, as shown in Fig. 2. The inner tube contains the refrigerant, while the outer tube carries water in a counter-current flow. Three thermocouple heads were installed near the compressor, the heat exchangers, and the condenser to measure variations in the refrigerant. Additionally, two thermocouple heads were placed at the inlet and outlet of the heat exchanger to monitor changes in the water. The water supply unit consisted of a water tank and a pump connected to the water line of the heat exchanger unit. The water pump was connected to the inlet of the heat exchanger, while the outlet was connected to the water heater. A thermocouple was installed between the heat exchanger and the water heater to measure the water temperature at the outlet before it entered the water heater. The evaporator was mounted on the wall at a height of 1.8 meters from the floor inside the test room, while the condenser and heat exchanger were placed on the opposite wall, spaced 45 cm apart to allow for air circulation. Three thermocouples were installed in front of the evaporator, as well as at the front and back of the condenser, to measure the air temperature during the experiment. These thermocouples were connected to a data acquisition system to provide real-time data, which was recorded by a computer using commercial software. The control unit uses a programmable logic controller (PLC) to manage pump and water operations through a human-machine interface (HMI).

 

Fig. 1: Photograph showing the interior of the experimental setup.

 

Fig. 2: Schematic diagram of the experiment.

 

3. Methodology

Air conditioners are extensively used to provide thermal comfort in residential and commercial environments. However, the heat they release into the surroundings contributes significantly to global warming. Waste heat recovery presents a viable solution to mitigate this environmental impact by redirecting excess thermal energy for practical use. Previous studies have demonstrated that capturing waste heat from split-type air conditioners can enhance the thermal efficiency of domestic water heating systems. In these configurations, a heat exchanger is installed between the compressor and condenser to recover heat from the refrigerant through circulating water. While the advantages for water heating are well established, the influence of such recovery on the overall performance of the air conditioning system itself remains less explored. In the experiment, the test room was built within a temperature-controlled environment. The air conditioning unit features a fixed-speed system with a cooling capacity of 9,000 BTU/hr, which supplies conditioned air to the test room. The evaporator unit was installed inside a test room measuring 3×2 ×2 m. To simulate internal heat gain, the heat source used in the experiment was based on the assumption that the test room may contain two people working, collectively emitting approximately 150 W of heat.[35] A 150 W LED lamp was used as the heat source in all test scenarios. The water heater and preheated water from the heat exchanger are neglected, as they are located outside the test room. The double-pipe heat exchanger was designed, built, and used in the experiment. The refrigerant flows through the inner tube, which has a total length of 2000 mm and an inner diameter of 9.625 mm. Water circulates through the outer tube, which is 1500 mm long and has the same inner diameter.[13] The water supply system includes a 0.325 m3 water tank and two circulation pumps. One pump circulates water through a compact chiller, rated at 0.75 kW, to maintain the water tank at the desired temperature. The second pump circulates water through the heat exchanger unit, which functions as a heat recovery device. Natural water was circulated through the heat exchanger and subsequently stored in a hot water storage tank. The mass flow rates of both hot and cold water were measured using rotameters. The power consumption of the air conditioning unit was monitored using a power meter. Temperature variations of the water and refrigerant, along with the air conditioner's power consumption, were continuously monitored throughout the experiment. Temperature data were collected using a data acquisition system consisting of a Wisco AI210 analog input module connected to a computer. Type T thermocouples were connected to the Wisco AI210 and used to measure temperature variations in both the refrigerant and the water.

 

4. Procedures

During testing, the initial room temperature was maintained at 30 ± 1°C to initiate the air conditioner's operation. The temperature setting on the remote controller was set to 25 °C for all experiments. Water mass flow rates ranging from 2.5 to 12.5 liters per minute were tested under various inlet water temperatures, specifically between 20 °C and 40 °C. At the beginning, the data acquisition system and computer were operated to read and record temperature data for 30 seconds to establish the initial readings. After that, two pumps were operated to circulate water through the test system. The temperature readings were monitored to ensure there were no errors or abnormal operations. Then, the air conditioner was turned on to begin the testing. The water mass flow rates for the circulation loop and the water heater loop were manually controlled using ball valves. Digital rotameters with a range of 0–25 L/min were used to measure the current mass flow rates. Data were recorded at 3-second intervals over a total duration of 60 minutes for each test.

 

5. Data reduction

The rate of heat transfer of water is given using Eq. (1).[4]

(1)

where  is the temperature difference of water across the heat exchanger.

The mass flow rate of the water, as determined from the experiment, can be calculated using Eq. (2).[13]

(2)

where  is the volume flow rate and  is the density of the water.

The coefficient of performance of the air conditioner can be calculated using Eq. (3).[3]

(3)

The output capacity refers to the cooling capacity of the air conditioner, which is approximately 9,000 BTU/h. Since 1 BTU/h is equivalent to 0.293 watts, this corresponds to about 2,637 watts. The power input, meanwhile, is measured experimentally using a power meter.

 

6. Uncertainty analysis

The uncertainty of the measurement are given in Table 1.

Table 1: Uncertainty of measurements.

Instrument

Uncertainty

Rotameter, (L/m)

±0.2

Thermocouple type T (°C)

±0.1

Power meter (W)

±0.2

 

7. Results and discussion

This study investigates the effect of integrating a double-tube heat exchanger into an air conditioning system for waste heat recovery from the refrigerant. The aim is to enhance the thermal efficiency of water heaters and assess the impact on air conditioner performance. Fig. 3 presents the refrigerant temperature distribution with and without the heat exchanger. The data clearly indicate a significant reduction in refrigerant temperature when the double-tube heat exchanger is employed, as compared to a conventional system. The outlet refrigerant temperature displays a wave-like pattern, corresponding to the air conditioner's operational cycles. During compressor operation, the refrigerant temperature increases due to compression, while in the off cycles, it decreases owing to the absence of compression. In the initial phase of operation, approximately the first 800 to 1200 seconds, the temperatures of both refrigerant and water rise rapidly before stabilizing. Once the room reaches the setpoint temperature, the compressor shuts off, leading to a temporary temperature drop. It restarts when the room temperature exceeds the setpoint, repeating the cycle. The results consistently show lower refrigerant temperatures when the heat exchanger is used, suggesting that the refrigerant reaches a saturated liquid state before entering the condenser. The average maximum refrigerant temperatures with and without the heat exchanger are approximately 43 °C and 52 °C, respectively. Similarly, the average minimum temperatures are about 32 °C with the heat exchanger and 42 °C without it. The mean refrigerant temperature ranges from 33 °C to 39 °C with the heat exchanger, compared to 40 °C to 42 °C without. These findings clearly demonstrate that the heat exchanger significantly lowers the refrigerant temperature prior to condensation, thereby enhancing overall system efficiency.

 

Fig. 3: Refrigerant temperature with and without heat exchanger.

 

Fig. 4 illustrates the refrigerant temperature distribution with and without the use of a heat exchanger. In the system equipped with a heat exchanger, the inlet water temperatures were set at 20 °C, 30 °C, and 40 °C, while the water mass flow rate was maintained at a constant 12 L/min. The results indicate that the refrigerant temperature profile fluctuates in response to compressor cycling. In the conventional air conditioning system, the refrigerant consistently exhibits higher temperatures compared to the water-cooled refrigerant in the system utilizing the heat exchanger. The average refrigerant temperature entering the condenser in the conventional setup is approximately 49 °C. In contrast, the corresponding average temperatures for the systems incorporating the heat exchanger are approximately 33 °C, 27 °C, and 25 °C for inlet water temperatures of 20 °C, 30 °C, and 40 °C, respectively. These findings clearly demonstrate that the refrigerant temperature is significantly reduced after passing through the heat exchanger. This considerable temperature drop suggests that the refrigerant reaches a saturated liquid state at a lower temperature before entering the condenser, thereby enhancing the performance of both the condenser and evaporator units and potentially improving the overall efficiency of the air conditioning system.

 

Fig. 4: Temperature distribution of refrigerant with and without heat exchanger.

 

A group of blue and red linesAI-generated content may be incorrect.

Fig. 5: Heat transfer of refrigerants with different mass flow rates.

 

Fig. 5 illustrates the transient heat transfer rate of the heat exchanger at various water mass flow rates. The results show that the heat transfer rate fluctuates in response to the compressor’s operational cycles. Lower water mass flow rates consistently result in reduced heat transfer rates compared to higher flow rates. Peak heat transfer typically occurs at the onset of compressor operation and gradually decreases, stabilizing within the range of 0.2 kW to 1.5 kW, as depicted in Fig. 5(a) through 5(d). The average refrigerant heat transfer rate ranges between 0.2 kW and 0.6 kW. Notably, heat transfer persists even when the compressor is off, which is attributed to the residual heat retained within the air conditioning system.

 

A graph of the same type of graphAI-generated content may be incorrect.

Fig. 6: Heat transfer performance of water at different mass flow rates: (a) maximum heat transfer rate, (b) minimum heat transfer rate.

 

Fig. 6 presents the minimum and maximum heat transfer rates for systems with and without a heat exchanger under two conditions: uncontrolled inlet water temperature and a controlled inlet water temperature fixed at 25 °C. The results indicate that the system with a fixed inlet temperature of 25 °C achieves higher heat transfer rates compared to the system without temperature control. This finding suggests that heat transfer between the refrigerant and water is influenced by both the water mass flow rate and the inlet water temperature. At higher mass flow rates, the difference in heat transfer rates between the two conditions becomes negligible. A slight reduction in heat transfer rate is observed within the 7–12 L/min range under the fixed inlet temperature condition. Overall, the maximum heat transfer rates range from 0.7 kW to 1.58 kW across water mass flow rates from 2.5 to 12.5 L/min, with peak values occurring during active compressor operation. Notably, residual heat transfer persists even during compressor off-cycles due to thermal accumulation within the system. These findings confirm that the heat generated during compression is effectively removed by the heat exchanger, resulting in a substantial reduction in refrigerant temperature. Nevertheless, further research is required to fully assess the potential performance benefits this configuration may offer to air conditioning systems.

 

Fig. 7: Power consumption of the air conditioner under various water mass flow rates and inlet water temperature conditions.

 

 

Fig. 7 illustrates the energy consumption of air conditioners with and without a heat exchanger, under varying water mass flow rates and both controlled and uncontrolled inlet water temperatures. The conventional system (without a heat exchanger) exhibits the highest power consumption, peaking at approximately 2.32 kW. In contrast, the system equipped with a heat exchanger shows a reduction in power consumption ranging from 0.21 kW to 0.55 kW, depending on the water mass flow rate. Notably, the configuration with a heat exchanger and a fixed inlet water temperature of 25 °C demonstrates the lowest energy usage, consuming approximately 0.52 kW to 1.09 kW less than the conventional unit, and 0.31 kW to 0.53 kW less than the system with uncontrolled inlet water temperature. Although higher water mass flow rates improve heat transfer, they can also lead to increased energy consumption. This is due to the relatively small temperature differences between low and high flow rates, as noted in a previous study,[13] and the longer compressor operation periods required at higher flow rates, which offset the gains in thermal efficiency.

 

Fig. 8: Comparison of the ideal and modified vapor compression refrigeration cycles, illustrating the impact of the integrated heat exchanger on refrigerant temperature and system performance.

 

The integration of a heat exchanger into the system alters the vapor compression refrigeration cycle by lowering the refrigerant temperature, as illustrated in Fig. 8. The red line represents the conventional vapor compression cycle, which consists of four primary processes: (1–2) isentropic compression, (2–3) heat rejection in the condenser resulting in a saturated liquid, (3–4) isenthalpic expansion, and (4–1) evaporation leading to a saturated vapor. In standard operation, the refrigerant temperature rises during compression and is subsequently reduced at constant pressure in the condenser, reaching a saturated liquid state. However, incorporating a heat exchanger between the compressor and condenser modifies process 2–3 by allowing partial heat rejection before the refrigerant enters the condenser. Consequently, the refrigerant temperature decreases from point 2 to 2′ within the heat exchanger, and then further drops from 2′ to 3′ in the condenser. This results in the formation of a saturated liquid at a lower temperature than that of a conventional system. The subcooled refrigerant then passes through the expansion valve and undergoes vaporization in the evaporator, absorbing heat more effectively. The increased temperature differential between the refrigerant and ambient air in the evaporator enhances heat absorption, thereby improving the system's overall thermal performance. This efficiency gain can reduce the compressor's duty cycle, ultimately lowering the system’s energy consumption.

 

Fig. 9: Coefficient of performance (COP) of the air conditioner under various water mass flow rates and inlet water temperature conditions.

 

Fig. 9 illustrates the COP of the air conditioner. As expected, the COP is inversely related to input power, lower power consumption results in a higher COP. The data show that the system integrated with a heat exchanger consistently achieves a higher COP compared to the conventional system, primarily due to reduced energy input. When the inlet water temperature is maintained at 25 °C, the air conditioner's COP increases significantly, ranging from 1.47 to 2.04, representing an improvement of approximately 25 % to 45 % over the conventional system. Even under uncontrolled inlet water temperature conditions, the COP shows a notable increase of approximately 18 % to 25 %.

 

8. Conclusion

This article investigates the effects of integrating a double-pipe heat exchanger on air conditioner performance, with particular emphasis on refrigerant behavior, energy consumption, and the system's COP. The main conclusions of the study are summarized below.

The refrigerant temperature before entering the condenser of the air conditioner integrated with the heat exchanger is approximately 7 °C to 11 °C lower than that of the conventional system.

The lower mass flow rate and lower temperature effectively contribute to the reduction of refrigerant temperatures.

The COP of the air conditioner with a heat exchanger is 25% to 45% higher than that of the conventional system when the inlet water temperature is controlled at 25 °C, and 18% to 25% higher when the inlet water temperature is not controlled.

Recovering waste heat from air conditioners for general applications is a crucial strategy for reducing environmental heat output. The results obtained take into account strategies for managing thermal emissions from air conditioners, which significantly contribute to global warming. However, the effect parameters should be considered to be consistence with the air conditioners to develop for commercial.

 

Acknowledgments

The authors would like to express their sincere appreciation to the Strategic Wisdom and Research Institute, Srinakharinwirot University (SWRI), and to the Faculty of Engineering, Srinakharinwirot University, for their financial support through Research Grant No. 191/2566.

 

Conflict of Interest

The authors declare that there is no conflict of interest.

 

Nomenclature

Q

Rate of heat transfer

W

mo

Mass flow rate

ms-1

T

Temperature

°C

Specific heat

kJkg-1 °C -1

COP

Coefficient of performance

 

Greek symbols

Density

[kgm-3]

Volume flow rate

[kgm-3]

Δ

Differential

 

Subscripts

c

Cooling

 

w

Water

 

r

Refrigerant

 

o

Outlet

 

i

Inlet

 

hex

Heat exchanger

 

 

Supporting Information

Not applicable.

 

CRediT Statement

Sommas Kaewluan: Conceptualization, Supervision, Writing-reviewing and editing. Nathawat Unsomsri: Data curation, Formal analysis, Investigation. Songkran Wiriyasart: Funding acquisition, Investigation, Writing-reviewing and editing.

 

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