Development of indirect evaporative cooler based on a finned heat pipe with a natural-fiber cooling pad

Abstract

Evaporative cooler is one of the methods that have been used to keep buildings at a comfortable temperature since ancient times. This type of cooler is particularly suitable for hot and arid areas. This research examines the design and testing of an indirect evaporative cooler system utilizing heat pipes as heat exchangers and natural fibers as cooling pads. An experiment was performed on a laboratory scale with three different types of natural fiber cooling pad materials: pineapple leaf, ramie, and luffa fibers. The air temperatures to be conditioned was 40 °C, with three variations of the intake airflow velocities of 0.4, 1.1, and 1.8 m/s. According to the results of this experiment, an indirect evaporative cooler system utilizing pineapple leaf fiber as a cooling pad performs better than those employing luffa and ramie fibers as cooling pads. The maximum wet bulb effectiveness of the system using pineapple fiber was 85%, maximum dew point effectiveness was 65%, and maximum energy efficiency ratio was 52.5 Btu/W.As a passive heat exchanger, the performance of finned heat pipes is also investigated, with a maximum heat absorption of 527.6 W and a temperature reduction of 9.9 °C.

Indirect evaporative cooler; Pineapple leaf fiber; Cooling pads; Natural fiber; Finned heat pipe.

1. Introduction

Since ancient times, evaporative cooling has been used as one of the methods that could keep buildings at a comfortable temperature. This system has several advantages and disadvantages. Its advantages include easy to use, lower maintenance costs, better indoor air quality, less air pollution, and low energy use. The operation of evaporative coolers involves a conversion of sensible heat in the air into latent heat. When heat and mass are transferred from air to water, the driving forces behind the process are the difference in temperature of the steam and the partial pressure of the steam. When using water as the working fluid in an evaporative cooler, it is significantly more environmentally friendly than Chlorofluorocarbon (CFC) or Hydrochlorofluorocarbon (HCFC) which is used in vapor compression refrigeration. In addition, evaporative cooler uses significantly less energy than electric vapor compression refrigeration systems to move air and water [1, 2]. The primary drawback of an evaporative cooler is its dependence on outside air. The difference between the ambient temperatures of the dry bulb and the wet bulb fuels the evaporation process [3]. Even though evaporative cooler systems can be assembled in a variety of ways, they are essentially variations on the three primary types: indirect evaporative cooler, direct evaporative cooler, and semi-indirect evaporative cooler [4].

Evaporative cooler technology is primarily used in hot and dry climates. For example, India has a wide range of climate zones, including hot-dry, cold, warm-humid, temperate, and composite. Many areas experience either hot-dry, warm-humid conditions, and composite. In areas that have a warm-humid climate zone, direct evaporative cooler cannot be employed efficiently during the summer due to the high relative humidity (RH) of around 80%–90%. Because of the lower RH in the range of 30%–50% in this environment, direct evaporative cooler is more commonly used. Direct evaporative cooler enables simple temperature and humidity control [5].

In the meantime, research shows that an indirect evaporative cooler with low energy consumption and high efficiency for a variety of applications will be developed in the near future. In comparison to traditional refrigeration, indirect evaporative cooler technology is more environmentally friendly and has a far lower influence on global warming [6].

Several studies have revealed that evaporative cooler application is able to reduce energy consumption in hot and dry places worldwide. Alternatively, the type of evaporative cooler system used determines the building's ability to maintain comfortable conditions [1, 2, 4, 7, 8, 9, 10].

The majority of earlier work on evaporative cooler centered on the thermodynamics and performance optimization of several of its fundamental configurations, including tube- or plate-type direct and indirect evaporative cooler. Insufficient comprehensive research has been implemented on emerging evaporative cooler technologies such as heat pipe indirect evaporative cooler, dew point indirect evaporative cooler, and semi-indirect evaporative cooler. Additional empirical and theoretical studies should be conducted to better understand the transfer of heat and mass that happens in these new evaporative cooler systems [1, 11].

A heat pipe is a heat exchanger which has a high capacity for heat transfer. Moreover, it is a passive heat transfer device that requires no additional power from an external source [12]. Heat pipes are widely used as heat exchangers in HVAC systems, particularly for heat recovery in air conditioners. Wang et al. presented an air-conditioning system employing a heat pipe heat exchanger (HPHE) as secondary heat recovery [13]. Ragil et al. developed an HPHE for HVAC systems in airborne-infection isolation rooms for hospitals [14]. Hakim et al. has done a study on the use of a vertical designed U-shaped finned HPHE to reduce energy for cooling and reheating in HVAC systems [15].

In 2003, FJR Martinez et al. [16] began investigating the potential benefits of heat pipes combined with evaporative cooler. This study aims to develop a mixed-air energy recovery system to enhance indoor air quality. This study's key finding is that adding an air recovery system, which comprises of a couple of heat pipes and indirect evaporative cooler, to an air conditioning installation can recover the energy from the air flow, enhancing energy efficiency and lowering environmental impact.

Liu Y et al. has also experimented using heat pipes in conjunction with evaporative cooler. His research offers a hybrid cooling system that includes a dew point evaporative cooler and micro channel heat pipes, both of which result in substantial energy savings [17].

However, only a few researchers have attempted to use finned heat pipes for evaporative cooler. In 2004, Riffat et al. [18] were the first to use heat pipes in evaporative cooler. In subsequent years, several authors continued the investigation [19, 20, 21]. The study presented the indirect evaporative cooler principle, which employs porous ceramics as cooling sources and heat pipes as heat exchangers. Figure 1 shows a schematic of an indirect evaporative cooler.

Figure 1.

Indirect evaporative cooler schematic [18].

In the course of this research, the type of evaporative cooler developed was regenerative evaporative cooler, which was integrated with a heat pipe. The principle is, through a dry channel, hot and dry ambient air is circulated – sending its heat to the heat pipes, with a portion of the surface remaining cool and dry. The wet ceramic cuboids, on the other hand, are in contact with the other portion of the air that is routed via the wet channel, resulting in a cooling effect. The water inside the ceramic cuboid's walls will seep out via its micropores which forms a thin water layer in the surface. In the wet channel, the moving air surrounding the ceramic containers cools the water within the containers, which in turn cools the heat pipe condenser. This causes water to evaporate from the cuboid surface at all times. Figure 2(a) depicts the schematic of the heat pipe-based porous ceramic indirect evaporative cooling system, while Figures 2(b) and 2(c) depict the original photograph of the system and heat pipe 2(c).

Figure 2.

Schematic of the heat-pipe-based porous ceramic indirect evaporative cooler system (a) original photograph of the system (b) and heat pipe (c) [21].

The next study focused on the combination of heat pipes and evaporative coolers. In 2009, Bintang et al. [22] conducted research on the use of heat pipes in multistage direct–indirect evaporative coolers to determine the saturation efficiency, output air humidity, and water consumption in sump. In the first stage of this experiment, the heat pipe served as a pre-cooler, and in the third stage, it was utilized for indirect evaporative cooler. Figure 3(a) depicts the heat pipe module for pre-cooling, while Figure 3(b) depicts the heat pipe module for indirect evaporative cooler utilized in this experiment.

Figure 3.

(a) First heat pipe module and (b) second heat pipe module (b) [22].

According to research on the use of heat pipes in evaporative cooler systems, some unresolved challenges remain. This study investigated the design and testing of a regenerative evaporative cooler system based on heat pipes as heat exchangers and natural fibers as cooling pads. As a restriction, a heat pipe made of copper, using water as a working fluid, that is 10 mm in diameter and 400 mm in length is employed. As a heat exchanger, the heat pipe causes heat from the dry side to move to the wet side. The natural fibers that were used are pineapple leaf, ramie, and luffa cylindrica fibers, which functioned as parts that absorb water to extend the evaporation process. The test temperature is set to 40 °C, and the relative humidity is uncontrolled and dependent on the ambient temperature.

2. Methodology

2.1. Cooling pad material

In addition to the heat pipe, which serves as a heat exchanger in regenerative evaporative cooler, the cooling pad material has an important role in improving the performance. According to Zhao et al. [23], the airflow conditions and thermal properties of a material affect the cooling performance of the system. For airflow conditions, increasing the difference in temperature of the dry and wet channels results in an increased heat and mass transfer of the system. The increase in airflow velocity in the two channels also contributes to the formation of this potential trend, and the water-retaining capability, thermal conductivity, and cooling pad thickness are factors that can affect the system’s performance.

Several studies on cooling pads in evaporative cooler have been implemented. An in-depth investigation of cellulose cooling pads was carried out by Ali et al. Computational and statistical software (SCST) was used to predict the behavior patterns of the performance features of the cellulose evaporative cooler bearing system [24]. In 2016, Hou et al. has done a study on cooling media using cellulose pads [25]. Maurya et al. studied the cooling properties of numerous cooling media in 2014, including cellulose, aspen, and coconut pads [26]. Jie et al. proposed a new hollow-fiber-integrated evaporative cooler system where the hollow fiber module served as both a humidifier as well as an evaporative cooler [27, 28].

Cooling pads, as materiakls used for heat and mass transfer materials, are classified into five types: metals, fibers, zeolites, ceramics, and carbon. All of these material types have their own optimal configuration for their use as a medium for heat or mass transfer in indirect evaporative cooling systems. Among the five types of materials, fibers are the least expensive material [23]. Fibers are materials with high porosity and low replacement costs. Although fibers have a significantly lower thermal conductivity than metals, they have the ability to transfer heat and mass. In indirect evaporative cooler, fibers are the most common porous materials used. Natural, polymer, and fabric fibers are currently the most commonly utilized fibers in indirect evaporative cooler [29].

Several other researchers have been interested in cooling pad materials created from natural fibers. Al-Sulaiman investigated cooling pads fabricated from natural fibers (luffa, jute, and palm) and compared them with commercial cooling pads [30]. Sonawan et al. [31] investigated a cooling pad material fabricated from banana midrib fiber. Dogramaci et al. [32, 33] conducted a study on a cooling pad material fabricated from eucalyptus fibers. Jain et al. [34] conducted laboratory testing on coconut and palash fibers as novel cooling pad materials.

Based on the results of the study done by James et al., the wet surface area in evaporative cooler is directly proportional to the porosity and absorption of the medium; the efficiency increases with porosity [35].

Generally, the cooling pad material significantly affects the evaporative cooler performance. Cooling pads that use natural fibers have advantages in terms of availability and practicability, which affect their cost. In this study, the natural fibers that were studied are pineapple leaf (Figure 4a), ramie (Figure 4b), and luffa cylindrica (Figure 4c). These fibers are highly available, particularly pineapple leaf fibers.

Figure 4.

Pineapple leaf (a), ramie (b), and luffa fibers (c).

The cooling pad materials fabricated from natural fibers, such as pineapple leaf, ramie, and luffa fibers, were characterized in this investigation. Figure 5 (a, b, c) show the internal microstructures of the fibers which was observed using scanning electron microscopy (SEM) at a magnification of 50 x, and Figure 5 (d, e, f) show those at a magnification of 700 x.

Figure 5.

SEM images of pineapple leaf, ramie, and luffa fibers at (a, b, c) 50 × magnification and (d, e, f) 700 × magnification.

Pineapple leaf and ramie fibers appear to have the same shape; both are fiber-shaped and more flexible compared to luffa fiber types. This shape enables pineapple leaf and ramie fibers to easily fit into the heat pipe’s condenser section. Luffa fibers are unique because each fiber forms a cavity that enables air to be trapped in the cavity but are less flexible to install in the condenser heat pipe.

2.2. Experimental setup

In this study, an evaporative cooler system was developed in which the designed type of evaporative cooling can be categorized as indirect evaporative regenerative cooler. The cooling system has two channels: wet and dry. The dry channel is the primary channel, which is the channel in which air is to be conditioned, whereas the wet channel is the secondary channel, in which air is in contact with water through the cooling pad.

The air temperatures will be conditioned at 40 °C, in the RH range of 31%–43% and they are three variations of the intake airflow velocities: 0.4, 1.1, and 1.8 m/s. The test was performed on three different cooling pad materials: pineapple leaf, ramie, and luffa fibers. Furthermore, other tests were performed without a cooling pad for comparison.

Air was passed on the dry side (T_dcin) of the channel with dimensions of (150 × 180) mm². Air was passed through the heat pipe’s evaporator side. A fin was installed on this side to expand the heat transfer area. The temperature that passed through this section (T_dcout) decreased because the sensible heat on that side was absorbed by the heat pipe to evaporate the working fluid in the heat pipe. The steam then moved to the condenser in the wet channel (T_wcin) and released heat on that side. On the wet channel side, the condenser heat pipe section was covered by a cooling pad (pineapple leaf, ramie, and luffa fiber), and water was sprayed on that section for an evaporation process to occur; then, the air that passed through that section (T_wcout) was rejected. Furthermore, experiments were also performed on the system without a cooling pad covering the condenser in the wet channel.

Heat pipe module and the placement of the temperature sensor on the heat pipe can be seen in Figure 6 (a) and 6 (b). The heat pipe module arrangement was installed transversely between the dry and wet channels, where the dry channel was attached to the evaporator section (the part which contains the fins), and the condenser was attached on the wet side. Ten heat pipes with lengths of 400 mm were arranged in parallel (two rows and five columns). The condenser located in the wet channel was covered by a cooling pad created from natural fibers (pineapple leaf, ramie, and luffa cylindrica). For this investigation, the heat pipes were fabricated according to the design parameters. The heat pipe was created from copper with a sintered copper wick structure on its inner surface. Water was used as the working fluid with a filling ration of 50%. It was composed of three sections: a 180 mm evaporator section, 40 mm adiabatic section, and 180 mm condenser section.

Figure 6.

Heat pipe module (a) and placement of the temperature sensor on the heat pipe (b).

In this example, the conditioned air temperature was represented by the intake air temperature (T_{dc, in}). A temperature of 40 °C was set with the heater connected to the voltage regulator. The type-K thermocouple used in this investigation was coupled to an NI 9213 module with an accuracy of ±0.01 °C, and data was gathered via an NI cDAQ-9172 system. Thermocouple sensors were positioned on the channel's intake and outlet sides, the heat pipe (evaporator, condenser, and adiabatic section), the reservoir's water, and the ambient air. Meanwhile, the RH was measured using a THD series Autonic sensor, which was connected to an NI 9219 module placed at the entry and exit points of the two channels. The conditioned airflow velocity was adjusted at three airflow velocities: a maximum of 1.8 m/s, a medium of 1.1 m/s, and a minimum of 0.4 m/s. A hotwire anemometer (Lutron AM-4204) sensor was used at the center of the duct to detect the fresh and exhaust air entrance velocities with an accuracy of ±0.01 m/s. The complete experimental setup is shown in Figure 7.

Figure 7.

Experimental setup.

2.3. Balance of energy in the system

In the dry channel, the process of lowering the temperature accomplished through a combination of convection and conduction heat transfer, which incorporates the convection thermal resistance between the dry channel airflow and heat pipe wall, and the wet channel airflow and fiber surface, as well as the conduction thermal resistance of the heat pipe and heat pipe wall to the fiber. Figure 8 (a,b) illustrates model control and a simplified thermal resistance network for the system under an intake air temperature (T_dcin) of 40 °C at 0.4 m/s using a pineapple leaf fiber cooling pad.

Figure 8.

Model control (a) and simplified thermal resistance network (b) for the system at an intake air temperature (T_dcin) of 40 °C, airflow velocity of 0.4 m/s, and pineapple leaf fiber cooling pad.

The energy balance in the dry and wet channels at the control volume can be expressed as in Eqs. (1), (2), (3), (4), and (5). According to the control volume shown in Figure 8 (a,b), the sensible heat is only calculated on the dry channel side, where the evaporator heat pipe is located. This heat is absorbed by the evaporator and released on the condenser, which is in the wet channel.

$$Q_{dc}=\frac{T_{dc}-T_{e,hp}}{R_{dc}}$$ (1)

$$Q_{hp}=\frac{T_{e,hp}-T_{c,hp}}{R_{hp}}$$ (2)

$$Q_{wc}=\frac{T_{c,hp}-T_{wc}}{R_{cp}+R_{wc}}$$ (3)

As shown in Figure 8 (b), heat transfer occurs as a result of the observed temperature potential of the system. Heat transfer occurs in a single direction from the intake air temperature on the dry side of the channel to the temperature on the wet side of the channel. The heat transfer flow is mathematically defined by Eqs. (4) and (5):

$$\frac{T_{dc}-T_{e,hp}}{R_{dc}}=\frac{T_{e,hp}-T_{c,hp}}{R_{hp}}=\frac{T_{c,hp}-T_{wc}}{R_{cp}+R_{c,wc}}$$ (4)

$$Q_{dc}=Q_{hp}={-Q}_{wc}$$ (5)

The sensible heat amount in the dry channel (Q_dc) was calculated based on the balance of energy in the dry channel as shown in Eqs. (6) and (7):

$$Q_{sensibel}=Q_{dc}$$ (6)

$${\dot{m}}_{dc}{c}_{p,dc}\left(T_{dcin}-T_{dcout}\right)=Q_{dc}$$ (7)

Thus, the value of the existing thermal resistance of the system was calculated using Eqs. (1), (2), and (3). The overall heat transfer coefficient (U_T) is calculated using the three thermal resistances (R_T), mathematically written as Eqs. (8) and (9).

$$U_T=\frac{1}{R_t}$$ (8)

where,

$$R_t=R_{dc}+R_{hp}=R_{cp}+R_{wc}$$ (9)

The thermal resistance in the dry channel, which has fins, is represented by the temperature differences in the air in the dry channel and is denoted as R_dc.

The thermal resistance in the wet channel, which has a cooling pad and cooling water, is represented by the difference in air temperature in the wet channel.

3. Result and discussion

Three natural fiber-based cooling pads were used in the experiments to test the accomplishment of the indirect evaporative cooler system: pineapple leaf, ramie, and luffa fibers. When the values reported for all measuring instruments were in a steady state at all locations, data collection was performed. In addition, tests were conducted on systems without cooling pads as a reference.

Figure 9 shows the experimental results for the test at an intake air temperature of 40 °C and airflow velocity of 1.8 m/s. At 40 °C, the systems using pineapple leaf fiber (Figure 9a), ramie fibers (Figure 9b), luffa fiber (Figure 9c) and a system without a cooling pad (Figure 9d) were tested for comparison. Tests at 40 °C were also performed at airflow velocity variations of 0.4 and 1.1 m/s. According to the test results, the greatest temperature difference occurred when pineapple leaf fiber was used as the cooling pad material and when a cooling pad was not used. Although the system without a cooling pad had the same high temperature difference as the system with pineapple leaf fiber, the pineapple leaf fiber system had the advantage that the water sprayed on the wet side of the channel was focused on the cooling pad and minimal occurred splashes.

Figure 9.

Indirect evaporative cooler profile temperature at the intake air temperature of 40 °C and airflow velocity of 1.8 m/s, for pineapple leaf fiber (a), ramie fiber (b), luffa fiber (c) and system without a cooling pad (d).

The temperature data were then used as parameters to calculate several factors affecting the indirect evaporative cooler system’s performance. The overall heat-transfer coefficient (U_T) was one of these variables.

The dry channel's U_T was calculated to determine the amount of heat transferred from the wet channel to the dry channel, or vice versa. Additionally, the coefficient value affected the quantity of heat that the heat pipe absorbs. Figure 10 shows the relationship between the airflow velocity and U_T for each cooling pad material and the system without a cooling pad at an intake air temperature of 40 °C and airflow velocity of 1.8 m/s. Henceforth, only the performance of the system at a 40 °C intake air temperature and RH ranging from 31%–43% is depicted. These conditions were considered to represent the hottest climates in several countries.

Figure 10.

Effect of airflow velocity on the overall heat transfer.

As shown in Figure 10, the U_T value for the system covered with natural fibers (pineapple leaf, ramie, and luffa) followed the same trend: the U_T value increased as the airflow velocity increased. Similarly, the systems without cooling pads exhibited the same pattern. The airflow velocity affected the increase and decrease in the convection heat transfer coefficient on the surface of the cooling pad or on the surface of the heat pipe wall. In convection heat transfer, a high air velocity increases the rate of heat transfer through the difference of temperature.

In the system with cooling pads of luffa and ramie fibers, the U_T value increased gradually, whereas for the system without cooling pads, the U_T value increased more steeply. This is because randomly arranged pineapple leaf fibers have more free space than luffa and ramie fibers, enabling the air conditioner to have a direct effect on the heat pipe walls and a lower resistance to resistance than luffa and ramie fibers. An indirect evaporative cooler system without a cooling pad has a lower thermal resistance value because the occurrence of heat transfer by convection between the air and the heat pipe wall in this system.

3.1. Indirect evaporative cooler system performance

Several variables that serve as indicators the performance of the system were observed in addition to the experimental results. Four variables were considered: cooling capacity, wet-bulb effectiveness, dew point effectiveness, and energy efficiency ratio [19, 20, 21].

The cooling capacity, as shown by Eq. (10), indicates how much heat can be absorbed by the heat pipe as a heat exchanger:

$$q_c={\dot{m}}_{dc}{c}_{p,dc}d{T}_{dc}$$ (10)

The airflow velocity and intake air temperature (ambient air) significantly affected the cooling capacity. The effect of the airflow velocity on the cooling capacity is depicted in Figure 11.

Figure 11.

Effect of airflow velocity on cooling capacity.

This indicated that the cooling capacity of each cooling pad material followed a similar pattern. The cooling capacity increased with the increase of intake air velocity, which was consistent with previous research findings [18].

The cooling capacity was affected by the airflow velocity and the difference in temperature between the intake and outlet air of the dry channel. The difference in temperature between the indirect evaporative cooling system with a pineapple leaf fiber cooling pad and the system without a cooling pad was greater than the temperature difference between the ramie and luffa fibers, indicating that the two had a greater cooling capacity.

Several studies [19, 36] have indicated that the evaporative cooler performance is comparable to the effectiveness of a wet bulb. The process of air-cooling using direct evaporative cooler is considered as a constant wet bulb process. The maximum allowable value for the temperature of the dry bulb in the air that is supplied (T_dcout) is then used as the intake air temperature of the wet-bulb (T_wb,dcin). This parameter provides an indication of the close connection between the dry-bulb temperature of the generated air and the temperature of the intake air temperature of the wet bulb. Therefore, the effectiveness of the wet bulb can be calculated as Eq. (11):

$${\epsilon }_{wb}=\frac{T_{dcin}-T_{dcout}}{T_{dcin}-T_{wb,dcin}}$$ (11)

In contrast, in indirect evaporative cooler, the air enters two streams. While passing through the heat exchange module, reasonable amounts of air are supplied (without added humidity). The intake air dew point is the limiting value of the air supply (T_dcout). Consequently, it is preferable to compare the performance based on the effectiveness of the dew point. The effectiveness of the dew point is calculated using Eq. (12):

$${\epsilon }_{dp}=\frac{T_{dcin}-T_{dcout}}{T_{dcin}-T_{dp,dcin}}$$ (12)

Figure 12 (a,b) depicts the effect of airflow velocity on the effectiveness of the wet bulb and dew point in the experiments.

Figure 12.

Effect of airflow velocity on the effectiveness of the (a) wet bulb and (b) dew point at air velocities of 0.4, 1.1, and 1.8 m/s with pineapple leaf, ramie, and luffa fibers as cooling pad materials.

Figure 12 depicts a clear trend comparing systems with and without fiber cooling pads. Furthermore, despite the same trend, the method utilizing pineapple leaf fiber has a better effectiveness value than other fibers. This can be attributed to randomly placed pineapple leaf fibers having more free space than luffa and rami fibers, resulting in a faster evaporation process in the wet channel.

Environmental conditions significantly affect the performance of evaporative cooling [2, 5, 8, 9, 10]; therefore, in addition to being influenced by the incoming air temperature, the effectiveness of the system was also influenced by the RH at the intake air temperature, and the test was performed in the RH range of 31%–43%.

The following psychrometric diagram (Figure 13) shows the processes occurring in the studied system. The graph uses an example of an intake air temperature of 40 °C, airflow velocity of 0.4 m/s, and a pineapple leaf cooling pad material. At some airflow velocity variations, the process trend shown in the psychrometric diagram was the same for both systems with other types of fiber cooling pads and systems without cooling pads.

Figure 13.

Psychometric diagram for pineapple leaf fiber at an intake air temperature (T_dcin) of 40 °C and velocity of 0.4 m/s.

3.2. Energy consumption

The amount of energy required to operate this indirect evaporative cooler system is very low; electrical power is used only to operate the fan and pump. The energy efficiency, or coefficient of performance (COP), of the system is the cooling capacity and the electrical power consumed ratio in units of W/W as written in Eq. (13). In an air-conditioning system, this energy efficiency value is occasionally referred to as the energy efficiency ratio (EER) in Btu/W; the EER is calculated by multiplying the COP by 3.413 as written in Eq. (14). The EER of an indirect evaporative cooler system is generally in the range of 30–80 [3,36].

$$COP=\frac{q_c}{P_{elek}}$$ (13)

EER = 3.413 COP (14)

The experimental results indicated that when pineapple leaf fiber is used as a cooling pad medium, the indirect evaporative cooler system has a maximum performance of 52.5 Btu/W. In terms of the group energy, the EER value ranks as C [5, 37]. Cooling energy rating based on EER can be seen in Table 1.

Table 1.

Energy ranking of coolers based on EER [5, 37]ss.

According to the bar chart in Figure 14, the EER value in the system with and without a cooling pad had the same tendency to increase with increasing airflow velocity. The EER value is directly proportional to the cooling capacity, and based on the experimental results, the largest cooling capacity value occurred in systems with pineapple leaf fiber cooling pad material and systems without cooling pads. Therefore, we can conclude that the EER values for both systems were higher than those with ramie and luffa fibers.

Figure 14.

Effect of airflow velocity on the effectiveness of the EER.

The complete results of the indirect evaporative cooler system performance with a finned heat pipe as the heat exchanger and natural fiber as the cooling pad material are listed in Table 2.

Table 2.

Performance result of the systems.

3.3. Uncertainty analysis

The accuracy of the measuring equipment is very important for determining the indirect evaporative cooler system performance based on finned heat pipes with natural fiber cooling pads. Each instrument has a sensor that is either very accurate or imprecise, and both these factors affect the accuracy of the calculations.

The results of the calibration of the temperature sensor and data collection indicated that the errors associated with (T_dcin-T_dcout), (T_dcin-T_wb,dcin), and (T_dcin-T_dp,dcin) were ±0.13. The efficacy uncertainties (Sɛ_wb/ε_wb) and (Sɛ_dp/ε_dp) used in this study are expressed as Equation 15 and Equation 16.

$$\frac{{S\epsilon }_{wb}}{{\epsilon }_{wb}}=\sqrt{{\left(\frac{S\left(T_{dcin}-T_{dcout}\right)}{\left(T_{dcin}-T_{dcout}\right)}+\right)}^2+{\left(\frac{S\left(T_{dcin}-T_{wb,dcin}\right)}{\left(T_{dcin}-T_{wb,dcin,}\right)}+\right)}^2}$$ (15)

$$\frac{{S\epsilon }_{dp}}{{\epsilon }_{dp}}=\sqrt{{\left(\frac{S\left(T_{dcin}-T_{dcout}\right)}{\left(T_{dcin}-T_{dcout}\right)}+\right)}^2+{\left(\frac{S\left(T_{dcin}-T_{dp,dcin}\right)}{\left(T_{dcin}-T_{dp,dcin}\right)}+\right)}^2}$$ (16)

Eq. (10) can be used to calculate the cooling capacity (q_c) provided by the evaporative cooler. Eq. (17) can be used to predict the uncertainty cooling capacity (S_qc/q_c), assuming that the air density (ρ) and specific heat (Cp) are constant under the same channel area (A).

$$\frac{{Sq}_c}{q_c}=\sqrt{{\left(\frac{S_V}{V}\right)}^2+{\left(\frac{S_T}{T}\right)}^2}$$ (17)

Based on previous research [14, 19] and the calculations of Eqs. 15, 16, and 17, the maximum uncertainty value of the effectiveness of the dew point (Sɛ_dp/ε_dp) was 5.8%, the maximum wet-bulb effectiveness (Sɛ_wb/ε_wb) was 2.4%, and the maximum cooling capacity (S_qc/qc) was 3.7%.

Table 2 shows the results of the uncertainty analysis. Based on the table, the uncertainty values for all parameters are in the range of 5 percent. So it can be said that the overall test results from this study are acceptable.

The test results show that the highest temperature difference occurs when the system uses a pineapple leaf fiber cooling pad or does not use a cooling pad. Although the system without a cooling pad has the same high temperature difference as the system with pineapple leaf fiber, the system with pineapple leaf fiber has the advantage that the water sprayed on the wet side of the channel is focused on the cooling pad and minimal splashing occurs. The use of finned heat pipes as passive heat exchangers also shows good performance, especially at high intake air temperatures.

4. Conclusion

An indirect evaporative coolers utilizing heat pipe and several natural cooling pad was designed and tested to investigate its performance and its effectiveness in heat and mass transfer. It has demonstrated that the ambient temperature, the velocity of the air flow from the intake air, the material of the cooling pad, and the temperature of the water can affect the performance of the cooling system. It is found that the highest temperature difference occurs when the system uses a pineapple leaf fiber cooling pad and only heat pipe without a cooling pad. Although systems without cooling pads have the same high temperature difference as systems with pineapple fibers, systems with pineapple fibers have the advantage that water sprayed on the wet side of the drain is focused onto the cooling pads and splashing is minimal. Pineapple leaf fiber is the best fiber that can be proposed for a cooling pad, where it has the maximum wet bulb effectiveness is 85%, the maximum dew point effectiveness is 65%, the cooling capacity is 527.6 W, the temperature difference is 9.9 °C, and the maximum energy efficiency ratio is 52.5 Btu/W at low air intake velocity (0.4 m/s). In addition, indirect evaporative cooler based on finned heat pipes with pineapple fiber as a cooling pad can be applied as an alternative in air conditioning system.

Declarations

Author contribution statement

Evi Sofia: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Nandy Putra: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Engkos A Kosasih: Conceived and designed the experiments; Analyzed and interpreted the data.

Funding statement

Nandy Putra was supported by Direktorat Riset dan Pengembangan Universitas Indonesia.

Data availability statement

Data included in article/supp. material/referenced in article.

Declaration of interest’s statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.