A review of recent studies of both heat pipe and evaporative cooling in passive heat recovery

1 Introduction

Recently, scientific research has prominently addressed the challenge of recuperating lost energy, particularly within heating, ventilation, and air conditioning (HVAC) application systems. The substantial energy consumption associated with refrigeration and air conditioning has spurred a focused effort on developing energy recovery techniques. These techniques play a pivotal role in diminishing energy consumption rates while concurrently enhancing the efficiency and performance of refrigeration and air conditioning systems [1].

This article delves into the pivotal studies about energy recovery technologies. A comprehensive examination of two distinct types of evaporative cooling, namely direct and indirect, is undertaken. Additionally, this article conducts an in-depth analysis of the heat exchangers utilized in energy recovery technologies [2]. The objective is to present a thorough research survey that supports both experimental and theoretical studies on passive heat recovery. Specifically, this research focuses on the dual impact of evaporative cooling in HVAC applications.

2 Studies related to the double effect of evaporative cooling

Evaporative cooling, defined as the process of cooling air through the partial evaporation of water, involves the exchange of both thermal energy and mass between air and water [3]. This exchange leads to a decrease in air temperature and an increase in the relative humidity of the air [4]. By converting part of the sensible heat into latent heat, evaporative cooling enhances air quality and reduces energy consumption, offering a cost-effective solution [5]. The unique advantages of evaporative cooling have spurred extensive research into its improvement and application [6]. For instance, a study by Harby et al. utilized evaporative cooling to significantly decrease power consumption by 58% in the vapor compression cycle, consequently increasing the coefficient of performance (COP) by approximately 113% [7]. Furthermore, the implementation of evaporative cooling, as demonstrated in an experimental study by Yang et al., led to energy savings ranging from 2.4 to 13.5%, contributing to reduced compressor energy consumption and an enhanced performance factor in air conditioning systems [8]. Evaporative cooling systems employ various methods based on the heat and mass transfer mechanisms between water and air. These include direct evaporative cooling (DEC), indirect evaporative cooling (IEC), and hybrid systems combining both methods. In a notable experimental study conducted by Alhosainy and Aljubury, direct and indirect cooling with water containing geothermal energy was employed, resulting in a remarkable 26°C reduction in air temperature and an evaporative cooling effectiveness reaching 167% [6]. DEC technology serves as a traditional, straightforward, and cost-effective solution, resulting in a notable 70% reduction in energy consumption. Historically, this method involved the use of porous pottery filled with water and wet pads positioned in the supply air passageways. The direct contact between hot outside air and water induced a drop in air temperature, facilitated by the change in the air supply. During this process, a portion of sensible heat transforms into latent heat. However, a significant drawback lies in the substantial water consumption associated with this method [9].

DEC can be categorized as passive or active, depending on the requirement for electrical energy. Passive DEC operates without the need for electrical input, while active DEC necessitates electrical assistance [10]. Refer to Figure 1 for an illustration of the basic structure and main components of the DEC system. Additionally, Figure 2 depicts the fundamental workings and psychometrics chart of a direct evaporative cooler [11].

Figure 1

Basic structure and principal components of DEC system [11].

Figure 2

Working principle and psychometrics chart of a direct evaporative cooler [11].

IEC stands as a contemporary technique aimed at reducing air temperature by manipulating moisture content through the utilization of a heat exchanger. It can be categorized into Wet-bulb temperature IEC and Sub-wet-bulb temperature IEC based on the method of delivering air cooling [12,13].

Refer to Figure 3 for an illustration of the fundamental structure of the IEC system. Additionally, Figure 4 outlines the working principle and psychometric chart of wet-bulb IEC [14] (Figure 5).

Figure 3

Basic structure of IEC system [14].

Figure 4

Working principle and psychometric chart of wet-bulb IEC [14].

Figure 5

The evaporative cooling classification [10].

Researchers have exhibited significant interest in advancing both DEC and IEC system technologies. Consequently, a plethora of study publications have extensively documented experimental and numerical investigations on evaporative coolers [15].

Bishoyi and Sudhakar [16] compared different evaporative cooling pads, showing that the honeycomb-shaped cooling pad was better than the aspen-shaped one. The change in air temperature decreased by 8°C when the cooling pad was of the honeycomb type, while under the same conditions, the air temperature dropped by 5°C when the aspen wood cooling pad was used. While the materials for manufacturing evaporative cooling pads were tested by Khobragade and Kongre [17], different materials were tested according to air velocity parameters, water consumption rate, and evaporative cooling effectiveness. The experimental results showed that the cellulose material was the best among other materials, reaching the highest point of evaporative cooling by 93%. Unique category evaporative pad from inexpensive dense polyethylene mesh material constructed by Martínez et al. [18], which was experimented with verified sizes available, measuring 492 mm × 712 mm and having thicknesses of 80, 160, and 250 mm, respectively. The lowest pressure drop arrives at 250 mm thickness and front air velocity two m/s with evaporative effectiveness of 81%.

Regarding the water consumption rate, an experimental comparison of a mathematical model modified by Kabeel and Bassuoni [19] investigated the effect of the concentration of salts in the water used in evaporative cooling on the water consumption rate. The study results showed an inverse relationship between the concentration of salts in water and the water consumption rate for evaporative cooling. As for the thickness of the evaporative cooling pad, Martínez et al. [20] investigated an evaporative cooling pad made of cellulose with different thicknesses (50, 100, and 150 mm) to reduce the electrical energy consumption of the split air conditioning unit. The experimental results showed that the split air conditioning system performed best under the influence of an evaporative cooling pad with a thickness of 100 mm, where the consumed energy decreased by 11%. DEC is affected by several factors, the most important of which are (the inlet air temperature of the cooling pad, the air mass flow rate, the relative humidity of the entering air, and the water flow rate); experimental studies conducted by Al-Badria and Al-Waaly [21] proved that the evaporative effectiveness is highly affected by the air flow rate through the cooling pad, and decreasing inlet air temperature contributes significantly to increasing the evaporative cooling effectiveness. Kovačević and Sourbron [22] developed a computational model for the system, which operates based on the interaction between evaporated water and air, and a metal pad was specifically engineered to enhance this interaction between the air and water. The study results show that high evaporative cooling arrives at a low air flow rate without affecting the inlet air humidity. Dhamneya et al. [23] A thermodynamic analysis was performed to improve the effectiveness of evaporative cooling in an aspen fiber cooling pad by exploring various geometric compositions of different sorts. The experimental findings indicated that the triangular layout of the evaporative cooling system achieved a saturation efficiency of 97%. Evaporative cooling has been utilized to enhance the efficiency of various applications, such as air conditioning and refrigeration units. Eidan et al. [24] conducted a practical study to improve the performance of a small air conditioner by DEC, in addition to verifying the efficacy of evaporative cooling for reducing energy consumption in cooling units. Additionally, He et al. [25] built a system using evaporative cooling for a humidification reduction system powered by renewable energy, with an established relative humidity of approximately 70%. Dwivedi and Shah [26] investigated the effect of evaporative cooling on the performance of air condition split unit 2,800-W cooling capacity with changing cooling pad thickness and material. The findings from the study reveal that employing a cellulose evaporative cooling pad with a layer thickness of 100 mm results in a 4.6% reduction in compressor usage of electricity and a 10.8% increase in cooling capacity. In a recent practical study by Ketwong et al. [27]. The cooling capacity and energy efficiency ratio can be increased by about 20 and 36%, respectively, by employing DEC technology to cool the condenser. DEC can increase the cooling capacity and reduce the compressor’s electrical energy consumption by 30 and 20%, respectively, when a heat exchanger heat transfer device is used with it, as stated in the experimental study by Eidan et al. [28]. The parameters studied in the comprehensive review study by Hammdi et al. [29] are the inlet air temperature, the relative humidity of the air, the air entry and exit speed, and the water temperature. Three airflow speeds were experimentally used; a commercial cooler and a honeycomb packing pad were used, as shown in Figure 6.

Figure 6

A schematic diagram for a honeycombed more excellent packing pad [29].

Sellami et al. [30] devised a computational framework to assess the efficiency of a direct evaporative cooler that utilizes porous ceramic plates as a wet medium. The mathematical equation was resolved using the finite volume approach. The findings suggest that the cooler can meet the cooling needs in the dry climate by achieving a temperature reduction of 15°C below the surrounding temperature. The effectiveness of evaporative cooling in dry, high-temperature weather has been investigated with different air flow rates and cooling pad thicknesses by Laknizi et al. [31]. The numerical study found that the cooling effectiveness is directly proportional to the airflow rate and thickness of the pad, where the air temperature difference was 17°C. In addition to the factors mentioned above affecting the performance of DEC, an important factor was studied by Mehrabi et al. [32] in an experimental study. The cooling pad angle at which it is placed was analyzed based on various factors that included (the difference in the flow rate of air and water and the temperature of both air and water), in addition to changing the cellulose pad angle of placing. This study aims to obtain the lowest temperature of the air leaving the cooling pad by changing the installation angle of the cooling pad within the evaporative cooling system. On the other hand, the study investigates the optimal angle at which the highest cooling capacity is achieved with the lowest water consumption. The research paper showed that changing the angle of the evaporative cooling pad increased evaporative cooling performance by 12% more than not changing the angle.

A new group of porous natural materials was tested as an evaporative cooling pad by Doğramacı and Aydın [33]. The materials included (eucalyptus fibres, ceramic tubes, yellow stone, dry papyrus basket, and cypriot marble) to verify their effect on DEC performance. The study results showed that eucalyptus fibers and ceramic pipes are the best materials for reducing air temperature and are affected by air velocity, where the rate of change in air temperature reached 6 and 7°C.

A developed mathematical model for plastic pad evaporative cooling that will assist a solar chimney under the influence of different atmospheric conditions will be employed, as proposed by Soto et al. [34]. The purpose of the study is to save the energy needed for evaporative cooling. The numerical results indicated that 0.8 watt-hours per cubic meter saved points. Additionally, DEC has the potential to contribute to energy savings. A modern design that performs two functions together: evaporative cooling and reducing the relative humidity of the air, based on a practical study conducted by Ahmadu et al. [35] and shown in Figure 7, where DEC consists of a pad made of loofah fibers mixed with charcoal and the moisture-reducing pad is made of activated carbon. The study’s results showed that in the presence of a humidity-reducing pad, the air’s relative humidity from evaporative cooling decreased from 84 to 49%, and the air temperature decreased by 10°C. In contrast, the average decrease in air temperature was 11°C without the humidity-reducing pad.

Figure 7

Illustrates the structure of a (a) DEC system. (b) DEC procedure on a psychrometric chart [35].

Chen et al. [36] investigated the air condensation conditions in IEC. They conducted simulations utilizing different situations and solved the effectiveness-number of transfer units (ε-NTU) equations. The study utilized the TRANSYS software, which yielded satisfactory results. It was confirmed that in hot and humid regions, the duration of operation for condensation with IEC of the air conditioning unit may be extended. IEC was employed to cool the air before it entered the vapor compression unit through a numerical study of the mathematical model prepared by Cui et al. [37] to take advantage of the exiting air under the influence of hot and humid weather conditions. The study verified the success of IEC by reducing the air temperature and relative humidity simultaneously before it enters the vapor compression system. The design of IEC combined with a heat exchanger was verified by Fakhrabadi and Kowsary [38]. The factors affecting the heat exchanger and evaporative cooling, including (the characteristics of the inlet air for both evaporative cooling and the heat exchanger, as well as the effectiveness of evaporative cooling) were studied. The heat exchanger transmits the evaporative cooling effect. The study results concluded that the cooling capacity and evaporative cooling effectiveness are affected by the inlet air characteristics, including (temperature and relative humidity of the air). The possibility of saving energy by employing IEC technology was studied under Indian summer conditions by Jain and Hindoliya [39]. Different and diverse climates were studied, including hot, dry, and humid climates. The ability of the evaporative cooler to provide human comfort conditions and its electrical energy consumption have been verified. The results showed that IEC consumes 55% less electrical energy than an air conditioner consumes under the same conditions. The factors affecting IEC were investigated in an experimental study conducted by De Antonellis et al. [40]. The temperature of the inlet air for direct and IEC was studied, in addition to the flow rate of both water and air. The results showed that the effectiveness of evaporative cooling, called wet bulb efficiency, is minimally affected by both the ambient temperature and the outlet air flow rate. The effect of the working situation and water flow rate on the IEC has been investigated by Liberati et al. [41]. The study developed a modified technique by spraying the water on a heat exchanger with evaporative cooling. The cooling effect will be increased at a reduced water flow rate; therefore, the energy will be saved in all working conditions. A modern technique of IEC, which involves the integration of heat exchanger tubes with porous ceramic tubes, was configured by Amir [42]. The proposed cooling mechanism is based on the regenerative IEC principle. The study found that IEC reduces water consumption while increasing cooling effectiveness. The energy consumed in IEC decreased due to the improvement in cooling effectiveness with the decrease in inlet air velocity and the increase in inlet air temperature. IEC performance is inversely proportional to the thickness of the ceramic pipes. IEC meets the comfort conditions in desert areas, provided that it considers the number of rows of heat pipes in the evaporative cooling system. IEC was employed to develop energy and mass heat exchange through a numerical comparison between the heat exchanger in the presence and absence of IEC by Wang et al. [43]. The numerical results validated with the experimental results showed that the heat exchanger with evaporative cooling is 20% better than without IEC at the same conditions and air mass flow rate. Evaporative cooling gives higher results than traditional systems used in energy conservation, as stated in the study conducted by Cichoń et al. [44] when they employed evaporative cooling technology to save energy under moderate conditions by cooling air conditioners. The mathematical model of the system was analyzed by the ε-NTU method. The variables affecting IEC were verified and studied by preparing a numerical model developed by Al-Abbasi and Al-Alawi [45], where the airspeed, duct height, and airflow ratio, in addition to the length of the air passage, were the height of the air passage were verified. On the effectiveness of evaporative cooling. Dry and humid climates were modeled in this study. The study concluded that evaporative cooling is more effective in wet than dry conditions and depends on water and airflow rates. The IEC system has been enhanced and modified by incorporating dehumidification processes. Three important modifications to traditional systems were included. The results indicate that increasing the air flow rate improves the evaporative cooling coefficient. The problems of sagging films can be overcome using a vertical heat exchanger. The updated model by Shahzad et al. [46] increased the effectiveness of evaporative cooling and reduced humidity. The performance of IEC was verified through a mathematical study by Rajski et al. [47]. The advantages of the thermosyphon heat exchanger (THE) were employed to verify the performance of IEC transmitted through the heat exchanger, as shown in Figure 8. The critical speed rate must be adhered to raise the performance factor of the cooling unit and reduce energy consumption and the number of heat exchanger rows. The maximum air velocity must not exceed 1.5 m/s to satisfy the desirable result. Figure 9 shows an illustration of the design of a cooler using IEC.

Figure 8

Schematic depiction of the design of an indirect evaporative cooler employing a gravity-assisted heat pipe with a staggered array of tubes [48].

Figure 9

Indirect evaporative cooler schematic [49].

The effect of natural cooling pad type on the IEC effectiveness at a variable air mass flow rate has been studied. Pineapple paper fibers were tested by Sofia et al. [49]. It was the ideal type of cooling pad supported by a heat exchanger that could achieve an evaporative cooling efficiency of more than 80% and a temperature difference of about 10°C.

3 Studies related to heat pipe heat exchanger (HPHE)

The effectiveness of HPHE and its energy-saving potential were evaluated and theoretically examined by gathering and evaluating data from several cities in India [50]. HPHE offers a significant and immediate potential to decrease energy usage, as confirmed by urban research showing its economic advantages [51,52].

Multiple types of heat exchangers are accessible for energy recovery purposes [53,54]. Running a coil around is cost-effective, but it requires a recirculation pump and a fluid tank to operate the system. The plate-to-plate heat exchanger is efficient yet cumbersome, expensive, and very challenging to maintain. Condensate can accumulate on the plates, leading to mold formation. Wheel heat recovery is difficult to clean and prone to cross-contamination. In addition to these limitations, wheel heat recovery is not effective in removing film condensation. Figure 10 demonstrates the primary distinction among various heat exchanger types. HPHEs are the only heat exchange devices that do not have these frequent shortcomings found in other devices [55,56].

Figure 10

(a) Run around coil, (b) air-to-air heat exchanger, (c) rotary wheel heat exchanger, and (d) THE [98].

The thermal performance of heat exchangers has increased interest recently when the use of multiple types of heat exchangers increased in several fields. Recovering lost energy in air conditioning systems is achieved by employing the advantages of heat pipes. The following section will review previous studies dealing with various types of heat exchangers [57]. Waste heat recovery systems may be categorized into three main forms: run-around coils, wheel heat recovery, and different types of heat exchangers, such as wick heat pipes, wickless heat pipes, oscillation heat pipes, and plate-to-plate heat pipes. Figure 11 describes the psychometric analysis of an HVAC unit that includes a THE [58]. Currently, heat recovery technologies have the capability to reclaim around 60–95% of the energy that is lost, demonstrating great potential. The benefits of heat exchangers include stationary components, no need for an extra power source, excellent dependability, independent air duct, small size, and simple maintenance; however, they are limited to recovering sensible heat only [59]. Conversely, the fixed-plate has many benefits. These include being compact, having a reasonably high heat transfer coefficient, avoiding cross-contamination, being simple to maintain, being compatible with countercurrent flow, and recovering both sensible and latent heat. The advantages of the Rotary Wheel might be emphasized as follows: this equipment is characterized by its high efficiency and small size, and it has the ability to recover both sensible and latent heat as shown in Table 1.

Figure 11

The psychometric process of an HVAC system with THE [58].

The heat pipe consists of three main parts (the evaporator, the condenser, and the adiabatic insulated part). The evaporator absorbs the heat, which leads to the liquid boiling inside the evaporator, and the heat is expelled to the outside through the condenser part. The main difference between the types of heat exchangers is the method of transferring steam from the evaporator to the condenser and vice versa. The gravity assistant moves the condensed vapor from the condenser to the evaporator in the thermosiphon. Historically, the first heat pipe was manufactured in Los Alamos in 1966 by Grover [60]. A heat pipe is a passive device that efficiently transfers thermal energy. Heat pipes are used to efficiently move substantial amounts of thermal energy over a wide surface area while maintaining low-temperature differentials. These devices are distinguished by their simplicity and the ability to be operated easily without requiring external power [61].

The heat transmission efficiency of heat pipes is estimated by the process of boiling and condensation of the working fluid [62]. As a result, the effective thermal conductivity of heat pipes is 200–500 times higher than copper’s. Additionally, the effectiveness of heat pipes is influenced by several parameters, such as the heat pipe type, the percentage of the evaporator filled with a working fluid, the length of the evaporator and condenser, and the thickness of the pipe. Furthermore, the heat transmission capability of a heat pipe is contingent upon many physical phenomena, including the boiling limit, sonic limit, viscous limit (also known as the vapor pressure limit), and capillary limit [61].

Heat pipes can be used in several applications to improve heat transfer and raise the efficiency of various applications because heat pipes have many advantages and different sizes that can be used in multiple applications [63]. Heat pipes may be categorized into different types: conventional heat pipes (CHP), wickless heat pipes (sometimes referred to as gravity-assist heat pipes or two-phase closed thermosyphon), oscillating heat pipes as shown in Figure 12 [64], and evacuated tube heat pipe [65]. The efficiency of the wickless HPHE was verified by Mutalikdesai et al by manufacturing a heat exchanger filled with pure acetone with filling ratios ranging from 30 to 100%. This study investigated the effect of the inclination angle and the added heat to reach the highest performance of the heat pipe. the results show that a best inclination angle of 30° with a filling ratio of 60% [66].

Figure 12

(a) pulsating heat pipe, (b) wickless heat pipe (HPHE), and (c) CHP [64].

4 Passive heat recovery with HPHE in HVAC systems

Heat pipes may be employed to save energy and recover lost energy. The primary purpose of heat pipes is to efficiently recover lost energy by ensuring that the two fluids flowing through the condenser and evaporator of the heat pipe remain distinct [3]. HVAC units require high electrical power at high air temperatures, reaching about 50°C [67]. At the same time, the air leaving the building is at the temperature of the air-conditioned room, about 25°C, representing wasted energy. Wasted energy can be utilized by using one of the types of heat exchangers, which are heat pipes (thermosyphons), to cool hot air before it enters the HVAC unit, which contributes to reducing the consumption of electrical energy spent on cooling units by utilizing the energy wasted. Energy conservation techniques can be improved by combining more than one method. Since the heat exchanger mainly depends on the difference between the temperatures of the air passing through the evaporator and the condenser of the heat exchanger, both direct and indirect types of evaporative cooling can be used: first, to cool the air passing through the condenser before it enters the heat exchanger and to increase the difference between the temperatures of the heat exchanger. Second, contribute to effectively reducing the temperature of hot air. An HPHE often has many heat pipes, each functioning independently. HPHE has exceptional heat transmission capabilities and operates without the need for power and without any mechanical components. The functionality may be achieved by altering the phase of the working fluid inside the pipe. This feature utilizes HPHEs to enhance the performance and decrease the energy consumption of HVAC systems in several ways [68]. The wickless HPHE has several benefits compared to traditional heat exchangers, including the absence of external power requirements, prevention of cross-contamination among air flows, simplified production, and simple maintenance [69]. The effect of heat pipe diameter and air mass flow rate on the effectiveness of the heat exchanger and energy recovery through it was studied by Abdelaziz et al. [67]. Thirty-two copper wickless heat pipes were used, and three different diameters (9.5, 12.7, 19.05 mm) were tested. The effect of fresh air temperature was studied by changing the temperature from 30 to 50°C in each experiment, as shown in Figure 13. This study found that adding a wickless heat exchanger could reduce the required energy by 30%. The cost of establishing the energy recovery system can be covered within 36 months, and its expected life is 240 months. The ability of heat exchangers to remove contaminated air, not transfer pollutants, and save energy was verified in a practical study on COVID-19 patient isolation rooms (AII) conducted by Sukarno et al. [70]. Heat pipes were added in the heating, ventilation, and air conditioning system (HVAC) to pre-cool the air before it reached the HVAC unit. Four heat pipes per row were added in different rows (3, 6, and 9) to study the row number’s effect on the heat exchanger’s performance. The practical study found that using heat exchangers to save energy meets the design standards of AII rooms and prevents the transfer of pollutants. The energy required to operate the HVAC units with the heat exchanger is 46% less than without it. The temperature of the air entering the cooling units with the heat exchanger is 9.4°C lower than without it. Ali and Sarsam [71] used a wickless HPHE to investigate the thermal performance of an air‐to‐air HPHE, which is charged by two refrigerants, R22 and R407c. The gravity assistant, HPHE, was made from copper with two rows and 11 pipes per row. The experimental test was conducted with different airflow rates of 0.14, 0.18, and 0.22 m³/h, with varying temperatures of air of the evaporator section (40, 44, and 50°C), with different filling ratios as 45, 70, and 100%. The maximum effectiveness was reached at a 100% filling ratio and Ce/Cc was 1.5. The experimental result showed that the evaporator inlet temperature and the heat transfer rate are directly proportional. Alshukri et al. [72] conduct a comprehensive experimental study was conducted to verify the effect of nanomaterial in addition to phase change materials (PCMs) on the performance of the evacuated tube heat pipe.

Figure 13

Experimental test rig of [67]. (a) Photograph of HPHX-Duct System. (b) Block and line diagram of HPHX-Duct System. (c) Cooling coil.

On the other hand, Yang et al. [73] used a pulsating heat pipe heat exchanger (PHP-HE) as a new type of heat pipe for saving waste energy from the air conditioning system. R134a is the working fluid charged the PHP-HE at a 50% filling ratio. Different parameters were tested, such as extra fresh air with varying air velocity and installation angles. The study result shows that the effectiveness of PHP-HE is directly proportional to the new air temperature; therefore, the energy-saving increases in the summer and hot seasons. The effect of gravity and different angles on the performance of the heat exchanger was verified by an experimental study conducted by Ramkumar et al. [74]. To select the best, the concentric heat exchanger is filled with different working fluids. The working fluids used in this study were water, methanol, and acetone. Experimental results showed that acetone was better than methanol and water, and the appropriate angle was 60°.

On the theoretical side, Danielewicz et al. [53] have introduced a transient TRNSYS thermosyphon HPHE component. This component is constructed using the ɛ-NTU method and has been rigorously validated against experimental outcomes. The model demonstrates a commendable accuracy, predicting outlet temperatures and energy recovery within a 15% margin of error and maintaining an average discrepancy of 4.4% when compared to existing experimental results. This level of precision is deemed acceptable for engineering applications. Al Jubori and Jawad [75] advanced numerical model was built and solved to predict the process of condensation and boil inside the THE, in addition to the mass flow rate of air outside the boundaries of the exchanger and liquid inside the heat pipe. The ANSYS-Fluent program in two dimensions was used to simulate the numerical model of the thermosyphon and fill the gap in previous research that lacked a simulation process for the features of the heat exchanger. The effect of the working fluid ratio inside the thermosyphon evaporator and the heat absorbed by the evaporator, in addition to the inclination angle of the exchanger, was studied during this study. The results highlight that it is possible to reduce the thermal resistance of the thermosyphon through high heat absorption in the evaporator section. On the other hand, the results demonstrated that the increase in the heat addition to the evaporator section increases the effect of the filling ratio on the thermosyphon performance. The developed model can be used to predict the performance of the heat exchanger under other conditions and at different sizes.

Górecki et al. [76] used a long-finned heat pipe to investigate the heat recovery compared to the bare HP. They found that the heat recovery increased by 80% with a long-finned heat pipe. Jouhara et al. [77] investigated the performance of a multi-pass HPHE experimentally and theoretically. The experimental results have highlighted the strong correlation between heat exchanger performance and the Reynolds number. By increasing the number of passes from one to five, the effectiveness of the HPHE was improved by more than 25%. The study has demonstrated that increasing the number of keys increases the Reynolds number of the flow, leading to higher heat transfer coefficients and lower thermal forced convection resistances. The HPHE overall performance, as well as the outlet temperatures of the fluids, was predicted through two theoretical models based on the log mean temperature difference method and the ε-NTU method. Birajdar and Sewatkar [78] conducted an experimental study with different working fluids, such as ethanol, acetone, and methanol, with different adiabatic lengths, filling ratios, and heat loads. This study aims to select the best working fluid, adiabatic measurement, and filling ratio at which the wickless heat pipe performs best for given geometrical and working conditions. The filling ratio of each working fluid varies as 30, 40, 50, 60, and 70%. The adiabatic length is verified by 200, 500, and 800 mm at each test with different heat inputs at evaporators as 500, 1,000, 1,500, and 2,000 W. The results showed that the mass flow rate is inversely proportional to the adiabatic length; the mass flow rate decreases (from 0.06 to 0.03 kg/s) when the adiabatic length increases (from 200 to 800 mm), significantly, the mass flow rate is the maximum at a higher heat load. The best filling ratio of the loop thermosyphon system is 60%, with an optimum heat load range (1,500–3,000 W). The acetone working fluid has a low overall thermal resistance compared with other working fluids; this means the acetone has a more excellent HPHE thermal performance.

In the experimental study, Gopi Kannan and Kamatchi [79] used a wickless heat exchanger with PCM for cooling electronic modules. The different working fluids fill THE, such as n-hexane, benzene, and DI water. The modern point for this work is the PCM melting behavior through vaporized working fluids by the thermosyphon to cool the electronic modules. The experiment was tested with input heat varying from 70 to 110 W. These results showed that the working fluid n-hexane works better than other working fluids. The result was selected according to the high point of the superheated and low enthalpy of vaporization value. Wickless HPHE with PCM heat exchanger is highly reliable for all electronic modules that store sustainable energy to reduce power consumption. Arat et al. [80] they investigated the effect of the heat exchanger's angle and the filling ratio in an experimental study to determine the thermal performance of evacuated copper HP at different vacuum pressures. The outer diameter of the tubes filled with distilled water is 28 mm, the inner diameter is 26 mm, and their length is 1,500 mm. The practical results of this study showed that the wall-mean heat transfer coefficient (h) value is inversely proportional to the vacuumed copper pipe time. Jouhara et al. [81] conducted an experimental, theoretical, and numerical energy saving using a new HPHE containing 180 spiral-finned heat pipes arranged in 18 rows. The updated heat pipe was used to cool the exhaust stack section and enabled recovery of up to 100 kW at a steady state without cross-contamination or excess contamination. The pipe diameter used in the study was 38 mm, and the total length was 1.5 m. Fins are distributed on each heat pipe to enhance the heat transfer rate. Carbon steel is used to manufacture the heat pipes filled with water as the working fluid. The current heat pipe system consists of only a condenser and evaporator section, and the adiabatic section has disappeared. The results showed a decrease in the annual consumption of the station with the presence of the heat exchanger at a rate of 876 megawatts per hour compared to the yearly consumption without the heat exchanger. On the other hand, numerical results showed that it is possible to recover thermal energy amounting to about 101 kW per unit. Finally, the numerical results were compared with experimental data, and the error between the experiment under steady-state conditions and the calculations was about 1%.

Eidan et al. [82] investigated the innovative HPHE to improve the window-type air-conditioning system performance with different working fluids with two different filling ratios. HPHE is charged with three fluids, distilled water, acetone, and r-134a, with a filling percentage (FR = 50 and 100%) to obtain the best fluid with a filling ratio. The experimental result shows the energy recovery by working fluids at a 100% filling ratio (2.01, 2.195, and 1.33%) for each fluid, respectively, compared with the conventional AC system. HPHE improves the refrigeration effect in the air condition system by working fluids with a 100% filling ratio of about 3.5, 6.03, and 3.97% for each one, respectively. The HPHE, which charges with acetone and water, has the maximum effectiveness by range 48–75% compared with HPHE filling by R-134a. Temimy et al. [83] studied the effecting wickless HPHE performance by experimentally and numerically inserting the internal tube packing (tp) inside it. The copper was used to manufacture a thermosyphon heat pipe (THE) with a 17.4 mm inner diameter and 600 mm length. The vapor and condensate were transported by new TP. The heat pipe was improved, and the design was completed by TP, which was inserted inside HPHE. The practical result shows that the transient time of the thermosyphon heat pipe with tp reduced from 16 to 11 min, in addition to reducing the thermal resistance for the new composition depending on the heat source. The thermal resistance of the new composition wickless heat pipe with TP is independent of inclination angle and filling ratio. Abedalh et al. [84] used HPHE to save energy in HVAC systems applications while keeping a comfortable temperature level in the air-conditioner space with different air velocities with other HPHE rows and heat flux at each test. The thermosyphon HPHE was installed between two air ducts. The HPHE has a 73 cm length, 1.23 cm outer diameter, and 1 cm inner diameter, with four rows of HPHE for each row and ten tubes made from copper. The effect of air velocity on different air temperatures was studied at other heat fluxes. The heat pipe thermosyphon was charged with water at about 50% filling ratio. The practical results show that the HPHE effectiveness was increased when the air velocity decreased with increased heat flux. The maximum effectiveness value reached 2 m/s with a heat flux of 1,400 W, and it is value was 923.4 watts, while the best effectiveness value at the air temperature condition of the inlet evaporator, outlet temperature, and condenser temperature (33.6, 31.7, and 29.2°C) respectively. Ibnu Hakim et al. [85] used a finned vertical configuration U-shaped HPHE containing a wick structure to save an HVAC system’s cooling and reheating waste energy. The HPHE was charged by water with a 50% filling ratio. Presented the different factors of HVAC performance: U-shaped HPHE effectiveness, HVAC COP integrated with U-shaped HPHE, and dehumidification capability. The U-shaped HPHE was inserted at the inlet fresh airstream ducts wrapped around the cooling coil. The experimental test was conducted with different air velocities from 1.5 to 2.5 m/s and different new air temperatures from 30 to 45°C. The HPHE configurations were arranged in one and two rows with eight heat pipes per row. Compared to the conventional HVAC system, the precooling and reheating processes were affected by two-row U-shaped HPHE. The COP was enhanced with two-row U-shaped HPHE by 39.9% over the HVAC without HPHE. The result shows that the maximum energy recovery for precooling and reheating was 288.1 and 340.2 W, respectively, at 0.080 m³/s air volume. The relative humidity was reduced by 21.6%.

Sukarno et al. [86] investigated the effect of thermosyphon HPHE used in a staggered set with different heat pipe rows. The experimental test was carried out with different inlet air temperatures with variations in air velocity. The ε-NTU method was used in theoretical analysis to predict the outlet evaporator temperature, the HPHE energy saving, and the HPHE effectiveness. The thermosyphon THPHE, as shown in Figures 14 and 15, was contained in four heat pipes in three, six, and nine rows. THPHE was charged with water as a working fluid with a 50% filling ratio. The test result shows that the HPHE effectiveness was increased when the inlet air temperature rose, and the HPHE row increased. The HPHE effectiveness was reduced when the air-inlet velocity increased. The energy recovery is directly proportional to the number of rows, air-inlet temperature, and air velocity in the evaporator section. The ε-NTU method can succeed as a theoretical method in analyzing energy-saving systems that use HPHE HVAC systems. The HVAC efficiency system can be enhanced using thermosyphon heat pipes that use cold-air exhaust and reduce emissions.

Figure 14

Illustration of wickless heat exchanger section [86].

Figure 15

HVAC energy saving by HPHE in AII room [86].

Barrak et al. [87] enhanced the dehumidification capability of the cooling coil experimentally by studying the energy-saving parameter in an HVAC system by applying oscillating HPHE technical with different working flitches charged with methanol, binary, and water at a 50% filling ratio. HPHE enhanced the air conditioning with 25% for methanol, 21% for binary, and 17% for water. The highest energy recovery was reached at 1,932 W by methanol, 1,849 W by binary, and 1,645 W by water. The result shows that mixing the methanol with water can improve energy recovery by 14%. Xu et al. [88] utilized different nanoparticle mixing with the water with a different filling ratio and concluded that the 25% Al₂O₃ + 75% TiO₂–H₂O and TiO₂–H₂O were the best-working fluids. The HPHE, which is charged with nanoparticles, can be comforted with the working conditions. Meiss et al. [89] deduced a simple chart that can be used to predict the air change efficiency value in rooms by a numerical study in many cases. Gallego et al. [90] used nanofluids in experimental fluid with different concentrations with different thermosyphon factors: filling ratio, heat input, and limit condition of thermosyphon. They concluded that the thermosyphon charged with water could arrive at the production level set by nanofluids/water if the filling ratio increases above 60%. Eidan et al. [91] presented an experimental and numerical study of THEs in HVAC application to enhance energy saving and dehumidification. The THEs were used with a comprehensive different working fluid with a wide variation of outside air condition at each test. The results show that using THEs in HVAC successfully increased moisture removal ability.

In addition, Alizadeh and Ganji [92] carried out an experimental study with a wide range of different parameters such as (heat input, FR, coolant flow rate, the number of fins, and the initial pressure on the thermosyphon heat pipe) to reduce the wickless heat pipe thermal resistance by extending the fin on the top condenser section. They concluded that thermal resistance was decreased by increasing the fins number in addition to increasing thermosyphon efficiency. Ramadan et al. [93] presented a suggestion to use the hot air from air conditioning to save energy. They concluded that recovery of HVAC west energy by using it in air drying people’s clothes. The results were estimated by numerical study with agreement validation with experimental results. Ahmed et al. [94] compared three cases of HVAC to investigate the thermosyphon performance and its efficiency in saving energy and dehumidifying. The experimental and theoretical analysis was conducted with different dry air temperatures, velocities, and relative humidity. They concluded that using a thermosyphon heat pipe integrated with HVAC improves power consumption compared to other cases.

Suleiman et al. [95] carried out the impact of thermosyphon HPHE was used to decrease energy consumption by air conditioning at the same comfort level. The heat pipe was charged by nanoparticles of copper oxide with different concentrations with different outlet air conditioning. The result shows that HPHE effectiveness is directly proportional to nanoparticle concentration and fresh air temperature. Rajski et al. [47] studied numerically the performance enhancement of THEs supported by indirect evaporative coolers and developed a C.F.D. model to estimate the heat and mass analysis. The numerical model was validated with other experimental data from the literature study. They concluded that the thermosyphon supported by an indirect evaporative cooler can be successfully integrated with the HVAC application.

Furthermore, Jouhara and Meskimmon [58] investigated the water viability charged in Wraparound thermosyphon heat pipes in HVAC applications and compared it with different working fluids. Determined the thermosyphon effectiveness with a wide range of differences (tube orientation, size, and flow path) with varying numbers of pipe rows. The result shows that The efficacy of the Wraparound heat pipe charged by the water is better than the heat pipe set by R134a, which in water thermosyphon, the effectiveness was in the range of 16.9 and 16.4%, while for R134a is between 14.3 and 14%. Alammar et al. [96] carried out the geyser phenomenon in the thermosyphon HPHE and its impact on the performance of the heat exchanger through an experimental study of a wide range of different properties. They used water as a working fluid with diverse filling ratios, heat inputs, and inclination angles for the heat pipe. They concluded that the geyser phenomena related to serving ratio and thermosyphon orientation angle and at the maximum heat input the geyser reaches when the thermosyphon is 10° horizontal angle and it is sensations don’t get at the vertical direction. Sukarno et al. [97] built a correlation of heat pipe performance as an energy recovery device in air HVAC systems through an experimental and non-dimensional study using the Buckingham Pi theorem. The heat transfer properties of HPHEs were studied during the experiment. The hot air temperature of the evaporator section (T _e,in) varied between 30 and 45°C, the velocity of the hot air varied between 1.5 and 2.5 m/s, and the number of HPHE rows varied between 3, 6, and 9 rows. S _p number is the form of the new dimensionless number correlation. The findings supported that the number of HPHE rows (n), HPHE effectiveness, and Reynolds numbers impact the S _p numbers most (Re, D). The S _p equation can be used to estimate the thermal resistance of a single heat pipe, which is very useful in determining the performance of a heat pipe during the design and operation stages.

5 Conclusion

The thorough examination of existing literature and the comprehensive investigation of recent studies underscore a notable scholarly interest in advancing air pre-cooling techniques with the aim of reducing energy consumption in HVAC units. The pivotal role of heat exchangers in enhancing lost energy recovery techniques, especially in climates characterized by high temperatures and low humidity, is evident. A spectrum of technologies, including dual-effect evaporation, DEC, indirect evaporation, and the integration of HPHE technologies, has been explored. Extensive studies have scrutinized key parameters influencing the performance of these technologies, encompassing ambient temperature, air flow rate, choice of working fluids, the synergy of various technologies, the filling ratio of heat exchanger evaporators, and the optimization of the number of rows in heat exchangers. The dynamic landscape of energy recovery technologies is marked by ongoing development and modernization.

This exhaustive literature survey specifically targets the refinement of energy recovery techniques within the unique weather conditions of Iraq. The proposed approach involves the integration of the double effect of evaporative cooling with HPHE, strategically employed to curtail energy needs in HVAC applications by harnessing otherwise wasted energy. This study identified a gap in scientific funds to improve the performance of evaporative cooling used to recover lost energy by employing a heat exchanger to transmit its effect indirectly.

The extraction and synthesis of insights from prior literature not only provide robust support for the proposed methodology but also accentuate the imperative for an enhanced and integrated energy recovery system design. This synthesis not only contributes to the academic discourse but also positions the research within the broader context of sustainable and energy-efficient HVAC practices.