Development of Key Components for 5 kW Ammonia–Water Absorption Chiller with Air-Cooled Absorber and Condenser

Abstract

An absorption chiller is an alternative cooling system that operates using heat from renewable energy sources and employs environmentally friendly working fluids, such as ammonia–water or lithium bromide–water. Given Indonesia’s high solar energy potential, solar cooling systems using absorption chillers are particularly promising. Solar thermal energy has been demonstrated to effectively power absorption chiller systems through both simulations and experiments. In Indonesia, there is significant potential to utilize small-capacity solar absorption chillers for buildings, particularly those employing air-cooled condensers and absorbers, which can reduce operational and maintenance costs. This research aimed to design a prototype of a 5 kW solar-assisted ammonia–water absorption chiller system specifically for residential applications. The system will be air-cooled to minimize space requirements compared to traditional water-cooled systems. The study addressed the design and specifications of the system’s components, dimensional considerations, and an analysis of the impact of the measurement instrument on the research outcomes. The results provide precise dimensions and specifications for the system components, offering a reference for the development of more advanced systems in the future.

1. Introduction

In tropical country such as Indonesia, climate conditions are almost constant throughout the year and dominated by high temperatures and humidity [1]. As a result, Indonesia has a significant demand for cooling [2], particularly in its capital, Jakarta [3]. Approximately 31% of households in Jakarta have air conditioners, driven by average air temperatures ranging from 27.1 °C to 28.9 °C. However, conventional vapor compression cooling systems, which are widely used, have several notable drawbacks. The primary disadvantage of vapor compression systems is their high electricity consumption [4]. Buildings in Southeast Asia’s tropical regions allocate about 56% of their total energy consumption to air conditioning [5]. This substantial energy demand stems from the need to power the compressors within these systems. Reducing electricity consumption in cooling systems is crucial to minimize reliance on fossil fuels [6].

Another significant drawback of vapor compression systems is their use of environmentally harmful refrigerants [7]. Hydrofluorocarbon (HFC) refrigerants, commonly used today, have replaced the more ozone-depleting hydrochlorofluorocarbon (HCFC) and chlorofluorocarbon (CFC) refrigerants [8]. HFCs that are commonly used, such as R-134a, R-32, R-404A, R-407C, and R410A, are widely used for residential and commercial vapor compression air conditioning systems [9]. Although HFCs have a reduced ozone depletion potential with favorable thermodynamic properties and low toxicity, since they are potent greenhouse gases, they do still have a high global warming potential (GWP), which has led to efforts to phase them out in favor of lower GWP alternatives [10,11]. Therefore, exploring alternative refrigerants with minimal environmental impact, such as ammonia, is essential [12].

Absorption refrigeration systems were developed earlier than electrically powered vapor compression systems. The first practical absorption system was patented by Ferdinand Carré in 1859, using ammonia and water to produce cooling through heat rather than mechanical work. These systems were widely used in the 19th century, particularly in areas without electricity, as they could operate on various heat sources. In contrast, the vapor compression system, first demonstrated by Jacob Perkins in 1834, only became practical with the development of efficient electric motors in the late 19th century, particularly through Carl von Linde’s innovations [13,14]. While vapor compression systems eventually dominated due to their higher efficiency and reliability once electrically powered, an absorption chiller offers a promising alternative to conventional cooling systems by replacing the mechanical compressor with a thermal compressor, consisting of components like an absorber, generator, solution pump, and expansion valve [15,16]. Unlike mechanical compressors, thermal compressors can operate using various heat sources, including solar heat, waste heat, and biomass [17]. In Indonesia, solar heat is a viable energy source for absorption chillers due to the year-round availability of sunlight. Indonesia benefits from high solar irradiation [18], averaging between 4.6 kWh/m²/day and 7.2 kWh/m²/day [19], making solar energy an optimal choice for reducing fossil fuel consumption for cooling needs. Additionally, absorption chillers use environmentally friendly refrigerants, such as ammonia–water or water–LiBr [20].

The choice of ammonia–water over ionic liquids as the refrigerant–absorbent pair in this study was influenced by several practical, economic, and technical considerations. On the one hand, ammonia–water systems excel in applications requiring low or sub-zero temperatures [21]. Lithium bromide–water systems, on the other hand, are primarily limited to above-freezing temperatures, restricting their use in certain applications [22]. The assembly process of ammonia absorption chillers closely resembles the methods used for vapor compression chillers, both of which function under pressurized conditions. One of the alternatives other than the ammonia–water pair used in absorption chillers is ammonia–lithium nitrate, which also has a lower risk of crystallization compared to lithium bromide–water systems [23]. However, compared to ammonia–water, ammonia–lithium nitrate often requires more complex designs due to the higher operating pressures and the need to manage corrosion effectively, which can increase the overall system cost [16]. Other refrigerant–absorbent pairs that have been studied recently (such as ionic liquids and nano-refrigerants) are still in ongoing research and development. Thus, the maturity and reliability of ammonia–water systems make them a safer and more dependable choice for most applications [24,25].

The effectiveness of solar-powered absorption chillers has been demonstrated through simulations and experiments. Geothermal energy and gas engines are less applicable for absorption chillers due to challenges in consistency and efficiency. Geothermal energy, while stable, often provides heat at lower temperatures than required for efficient absorption chiller operation; thus, using a solar thermal heat source is a favorable option for regions with hot climates [26]. Gas engines, though capable of high-temperature output, are generally less efficient due to complex installation, since they are more favorable for use in combination heat and power (CHP) systems and thus require extra pre-operational costs [27]. Solar energy, while variable, can be more sustainable and is often paired with thermal storage, making it a more viable option for smaller-scale absorption chillers. For instance, dynamic simulations by Thomas and Andre showed a 34.9% energy savings when using solar heat for absorption chillers compared to vapor compression systems [28]. The solar absorption chiller is widely used and has been tested by simulation and experimentally in hot weather countries such as Iran and Saudi Arabia [29,30]. Pedro et al. developed and validated a TRNSYS model for the solar air conditioning system installed at the Universidad Miguel Hernández de Elche in Spain. The model was tested under different climatic conditions, functioning both as an autonomous system and as a solar-assisted system with a backup heat source. The autonomous system was able to maintain the comfort temperature for 60.8% of the hours in Sevilla and 78.3% of those in Madrid during the study period. When integrated with a backup compression chiller, the absorption chiller met 52.8% of the total cooling demand in Bilbao and 75.3% of it in Madrid [31]. Andrea et al. developed a prototype plant in Italy using a hybrid storage configuration, which included a double-effect absorption chiller powered by a parabolic trough solar collector. This system demonstrated a better performance in terms of primary energy savings when compared to traditional HVAC systems and low-temperature solar cooling plants [32]. Jan Albers developed a novel control strategy for a solar-assisted absorption chiller that simultaneously manages both hot and cooling water temperatures to optimize the system. Installed in the building automation system at UBA Dessau, Germany, this approach led to significant improvements: the seasonal energy efficiency ratio (SEER) exceeded 0.75, the electrical efficiency increased by 35%, and the water consumption was reduced by 70% [33].

Additionally, the University of Indonesia implemented a similar system using a lithium bromide–water pair with a 239 kW cooling capacity, achieving an energy reduction of 11% to 48% compared to equivalent vapor compression systems [34]. However, the use of water-cooled systems in these implementations makes them unsuitable for residential applications due to their large component sizes. In contrast, the air-cooled system that consists of tubes and fins to facilitate the heat transfer process is more compact than the other one [35].

This case study aimed to address these challenges by developing a 5 kW air-cooled absorption chiller system using an ammonia–water refrigerant pair, specifically designed for residential applications. The ammonia–water pair is preferred for its lower maintenance requirements compared to the water–LiBr pair, which is prone to crystallization issues [36]. The outcome of this study includes detailed specifications and dimensions for the heat exchanger components of the absorption chiller, as well as an analysis of the measurement instrument uncertainties to ensure the system’s reliability for future experimental research. This innovative approach not only highlights the potential for energy-efficient cooling in tropical residential applications but also paves the way for the development of more advanced and sustainable cooling systems in the future.

2. Methods

2.1. System Description

This case study focused on the comprehensive design, construction, and testing of a solar-assisted, single-effect ammonia–water absorption chiller system. The process began with an extensive literature review, where key concepts of absorption chiller systems and heat exchanger calculations were explored to build a solid theoretical foundation. Absorption chillers use a heat source (such as waste heat, solar energy, natural gas, or steam) to drive the refrigeration cycle instead of a mechanical compressor [16,37]. The cycle relies on the absorption of a refrigerant by an absorbent material, and then heat is used to separate the refrigerant and absorbent to complete the cycle. The four main components of absorption chillers are as follows: the generator: the heat source drives the separation of the refrigerant from the absorbent; the condenser: the refrigerant vapor from the generator is condensed back into a liquid; the evaporator: the liquid refrigerant absorbs heat from the environment, producing cooling, and evaporates; and the absorber: the refrigerant vapor is absorbed back into the absorbent, releasing the refrigerant into the cycle again [16].

Integrating absorption chillers with variable solar energy sources, particularly at low capacities, presents unique challenges but can be effectively managed with thoughtful design and operational strategies. Absorption chillers have high thermal inertia, meaning that they take longer to reach their optimal operating conditions and respond to changes in heat input, while solar energy is intermittent and variable, which can lead to mismatches between the heat available and the heat required by the chiller. The solution to this matter is using a thermal energy storage system to buffer the variability in solar energy. Thermal storage (such as water tanks and phase-change materials) can store excess heat when solar radiation is high and release it when needed, helping to smooth out fluctuations and provide a steady heat source to the absorption chiller [38]. Another solution that can be implemented is installing a hybrid system that combines solar energy with a backup heat source (e.g., natural gas, electricity, or waste heat) to ensure a continuous and reliable heat supply to the chiller, even when solar input is low [39]. In addition, the use of advanced control algorithms can adjust the chiller’s operation based on the available solar energy and the thermal storage level, minimizing the impact of variability [40].

Following these aspects, the dimensions and layout of the heat exchanger components were determined, leading to the creation of an initial system design using Autodesk Inventor 2021 software. This design phase included precise specifications for each component to ensure a coherent and functional layout. Once the design was finalized, the manufacturing of the system was outsourced to a third-party company that collaborated closely with the author. This collaboration involved ensuring that the heat exchanger dimensions calculated by the manufacturer aligned with the theoretical calculations provided by the author, thus validating the design’s accuracy. Post-manufacturing, the system underwent rigorous pressure and connection testing to check for leaks and ensure structural integrity. Following successful testing, appropriate sensors and electrical equipment were selected and purchased. These sensors were then verified and installed, and a control panel was constructed for the system’s operation. Finally, the results and conclusions were meticulously documented.

The entire case study workflow, depicted in Figure 1, outlines the systematic process for the design and testing of an ammonia–water absorption chiller system. The process begins with a literature study, which informs the preparation of the system design. The design phase includes both heat exchanger dimension calculations and component layout design. For the heat exchanger dimension calculations, there is a comparison with the vendor calculations. If discrepancies arise, the design is revisited until the dimensions are matched. Once the design and dimensions are finalized, the manufacturing of the absorption chiller components proceeds. After manufacturing, the workflow transitions to testing the components, followed by the purchase of equipment and sensors necessary for the system. The next stage involves verifying the accuracy of the measuring instruments used in the system. The final phase includes the analysis of the results and drawing conclusions, leading to the completion of the research.

The components required for the absorption chiller system are categorized into two groups: those that are custom-manufactured and those that are purchased as off-the-shelf items. The custom-manufactured components are dimensioned based on the system requirements derived from steady-state simulations conducted in a prior investigation. These components include the absorber vessel, generator, evaporator, condenser storage, absorber storage, condenser heat exchanger, and absorber heat exchanger, as illustrated in Figure 2, along with the frame, skid, and piping. The absorption chiller operates on a closed-loop thermodynamic cycle involving the following steps [16]:

• Heating (generator): The heat source heats a mixture of refrigerant and absorbent in the generator, and this heat causes the refrigerant (ammonia) to vaporize and separate from the absorbent (water). Solar energy was utilized as a potential heat source in this study; thus, future work should investigate the integration of solar energy with the hot-water loop (in the generator) with detailed design and operational strategies;
• Condensation (condenser): the refrigerant vapor flows to the condenser, where it releases heat to the surrounding environment and condenses into a liquid;
• Expansion and evaporation (evaporator): The liquid refrigerant then passes through an expansion valve or device, which reduces its pressure. In the evaporator, the low-pressure refrigerant absorbs heat from the space being cooled, evaporating in the process;
• Absorption (absorber): The refrigerant vapor flows to the absorber, where it is absorbed back into the absorbent solution. This absorption process releases heat, which is usually rejected to the environment. In this study, the absorber was split into two sub-units: the absorber vessel, which functions as the mixing chamber for ammonia vapor from the evaporator and the weak ammonia–water mixture, and the absorber heat exchanger, which releases heat into the ambient air so that the condensation of the refrigerant can occur.

Figure 2. Schematic diagram and component capacity of the absorption chiller system.

Figure 2. Schematic diagram and component capacity of the absorption chiller system.

A solution heat exchanger (SHX) is commonly used in ammonia–water absorption chiller systems to improve the efficiency by transferring heat from the hot, strong solution leaving the generator to the cold, weak solution returning from the absorber [16]. However, in this case, the SHX was not implemented due to practical considerations, such as the cost, system complexity, and space constraints, as this was the first version of the system built for this research study.

The manufactured components must be able to meet the system requirements according to the results of the previous steady-state simulation study. The results of the steady-state simulation are depicted in Table 1. Based on the simulation results, the components need to meet the following system requirements:

• The component can withstand pressures up to 20 bar;
• The component can withstand temperatures up to 100 °C;
• The component is made of materials suitable for handling ammonia solutions.

Table 1. Steady-state simulation results of the studied ammonia–water absorption chiller.

State Point (Outlet of Each Component)	Temperature (°C)	Pressure (Bar)	Ammonia–Water Mass Fraction (%)	Mass Flow Rate (L/min)
Evaporator	6	5.3	99.6	0.6
Absorber	38	5.3	52.3	6
Generator	84.4	14.7	47.3	5.4
Condenser	38	14.7	99.6	0.6

Table 1. Steady-state simulation results of the studied ammonia–water absorption chiller.

State Point (Outlet of Each Component)	Temperature (°C)	Pressure (Bar)	Ammonia–Water Mass Fraction (%)	Mass Flow Rate (L/min)
Evaporator	6	5.3	99.6	0.6
Absorber	38	5.3	52.3	6
Generator	84.4	14.7	47.3	5.4
Condenser	38	14.7	99.6	0.6

The coefficient of performance for a refrigeration system (COPR) obtained from the steady-state simulation is around 0.3, which is unusually low compared to the typical range of 0.6–0.8 commonly observed in such systems. This lower COPR can be traced to several factors, including the following:

• The evaporator temperature
  Operating at a lower evaporator temperature can reduce the overall system efficiency because the temperature difference between the evaporator and the surroundings increases, requiring more energy input to achieve the desired cooling effect. This results in a lower COPR as the system works harder to maintain cooling [16,41]. As stated by Brice L.L. et al., according to the simulation that they conducted on an ammonia–water absorption chiller, as the evaporator temperature decreased by 10 °C, the COPR decreased by 25% as well [42];

2.

The rectifier

The rectifier in an absorption system is responsible for separating the refrigerant from the absorbent. In this study, the system does not use a rectifier; thus, it directly impacts the COPR by increasing the energy required to achieve the same level of cooling [16];

3.

Solution heat exchanger

The solution heat exchanger (SHX) is critical in preheating the rich solution before it enters the generator. If the SHX is not used, as in this study, more energy is needed to heat the solution in the generator, leading to greater energy consumption without a corresponding increase in cooling output. This inefficiency further reduces the COPR [16].

In addition to the custom-manufactured components, the system also incorporates several off-the-shelf components. These purchased components consist of the solution pump, expansion valve, valves for controlling the piping route, and various instrumentation equipment. This strategic division ensures that the system is both accurately tailored to the specific requirements and efficiently assembled using readily available, high-quality parts.

2.2. Determination of Design and Dimensions of the Absorption Chiller System Components

This absorption chiller system employs two types of heat exchangers: shell-and-tube heat exchangers used in the generator and evaporator, and finned-tube heat exchangers used in the condenser and absorber. The shell-and-tube heat exchanger for the generator and evaporator was chosen due to several benefits, such as the high heat transfer efficiency as a result of the large surface area within a compact design, the versatility in design, since the number and size of tubes can be adjusted to optimize heat transfer based on the specific needs of the system, and its ability to handle high-pressure and -temperature differences, making it suitable for the demanding conditions found in absorption chillers [43,44]. The finned-tube heat exchanger for the condenser and absorber was selected because the fins on the tubes increase the effective surface area available for heat transfer, where the finned design also helps to improve the airflow around the tubes, which can enhance the cooling process by facilitating better heat removal from the refrigerant [43]. Finned-tube heat exchangers are designed to be more compact compared to non-finned types while still providing effective heat transfer [45].

To calculate the heat exchanger area ($A$) of the absorption chiller system components, the process involves understanding the heat transfer required to achieve phase change in the refrigerant, the temperature differences across each component, and the physical properties of the working fluids. $UA$ represents the thermal conductance of a heat exchanger, which is the product of the overall heat transfer coefficient ($U$) and the heat exchanger area ($A$). The following equations (Equations (1)–(4)) are the formulations used to calculate the $UA$ value for each main component, as referenced in Figure 2:

Absorber:

$${\dot{m}}_6{h}_6+{\dot{m}}_{13}{h}_{13}-{\dot{m}}_1{h}_1=U_a{A}_a{∆T}_{LMTD,a}$$

(1)

Generator:

$${\dot{m}}_2{h}_2-{\dot{m}}_5{h}_5-{\dot{m}}_7{h}_7=U_g{A}_g{∆T}_{LMTD,g}$$

(2)

Condenser:

$${\dot{m}}_7{h}_7-{\dot{m}}_{10}{h}_{10}=U_c{A}_c{∆T}_{LMTD,c}$$

(3)

Evaporator:

$${\dot{m}}_{12}{h}_{12}-{\dot{m}}_{13}{h}_{13}=U_e{A}_e{∆T}_{LMTD,e}$$

(4)

where $\dot{m}$ is the mass flow rate of the refrigerant/solution at the specific point of the flow, $h$ is the enthalpy of the refrigerant/solution, and ${∆T}_{LMTD}$ is the log mean temperature difference across a specific component.

It is necessary to determine the overall heat transfer coefficient ($U$), which is calculated by considering the convective heat transfer coefficient of the refrigerant fluid ($H_i$), the thermal conductivity of the tube material ($k$), and the convective heat transfer coefficient of the external fluid that acts as a cooler or heater ($H_o$), as shown in Equation (5), where $x$ is the tube wall thickness. The calculation for the convective heat transfer coefficient ($H$) can be performed using Equation (6). Once the overall heat transfer coefficient ($U$) value has been obtained, the heat exchanger area ($A$) can be determined:

$${\displaystyle \frac{1}{U}}={\displaystyle \frac{1}{H_i}}+{\displaystyle \frac{x}{k}}+{\displaystyle \frac{1}{H_o}}$$

(5)

$$H=Nu{\displaystyle \frac{K}{d}}$$

(6)

The Nusselt Number ($Nu$), calculated via the Dittus–Boelter equation, is the ratio of convective to conductive heat transfer in a fluid. It is calculated based on the Reynolds Number and Prandtl Number, which are derived from the type of fluid, temperature, and pressure. $K$ is the thermal conductivity for the fluid conditions inside the heat exchanger pipe, and $d$ is the tube diameter. The value of the Nusselt Number is obtained from Equations (7) and (8). These equations have been confirmed experimentally for the range of conditions: 0.6 ≲ $Pr$ ≲ 160; $Re$ ≳ 10,000; and the ratio between the tube length and diameter: ($L/d$) ≳ 10:

$$Nu=0.023{Re}^{0.8}{Pr}^{0.4} \left(\mathrm{heating}\right)$$

(7)

$$Nu=0.0296{Re}^{0.8}{Pr}^{0.3} \left(\mathrm{cooling}\right)$$

(8)

2.3. Testing Method for Components of Absorption Chiller System

The heat exchanger is designed to meet the calculated heat exchanger surface area dimensions. Once manufacturing is complete, it is imperative to test the system components before they are used in experiments. These tests include pressure leak testing, weld quality testing, and system operation testing.

Ensuring the absence of pressure leaks is crucial for maintaining the absorption chiller system’s performance, especially since ammonia, the working fluid, is toxic if inhaled. Pressure leak testing is conducted following the ASTM E1003 standard to ensure the system’s integrity and safety [46].

Each connection within the vessel of the absorption chiller system must be secure, as these connections are primarily weld joints. Weld quality testing is performed according to ASTM E165 standards using a Non-Destructive Test (NDT) method, specifically Visible Penetrant Examination Method A [47]. This method employs a visible penetrant, typically red, that highlights defects against a white developer background under normal lighting conditions. Although this inspection does not require UV light, it should still be carried out under adequately bright lighting to ensure the visibility of any flaws.

2.4. Instrumentation Equipment Verification

Every instrumentation tool used in the system must first be verified with a credible measuring instrument (sensor) to ensure that the values measured by the instrumentation tool correspond to the real conditions. Instrumentation tools are used to collect the necessary data for the observation process. The types of instrumentation tools used are as follows:

• Flow sensor: to measure the flow rate of the working fluid in the system channel;
• RTD PT100: to measure the temperature of the working fluid in the system channel;
• Pressure transmitter: to measure the pressure of the working fluid in the system channel.

Data acquisition for the flow sensor is performed using Arduino UNO (by Arduino, manufactured in Ivrea, Italy), for the RTD PT100 using the ADAM-4015 module (by Advantech Co., Ltd., manufactured in Taipei, Taiwan), and for the pressure transmitter using the ADAM-6017 module (by Advantech Co., Ltd., manufactured in Taipei, Taiwan). The signals from the ADAM DAQ are forwarded to the Advantech EKI-2528 Ethernet Switch (by Advantech Co., Ltd., manufactured in Taipei, Taiwan), which is connected to the computer using an Ethernet cable. The signals from the Arduino UNO are also connected to the computer using a USB cable. On the computer, the voltage signals from the sensors are read and converted into flow rate and temperature or pressure units using LabVIEW 2021 software.

2.4.1. Flow Sensor

Verification of the flow sensor is conducted by measuring the water flow rate constantly from the tap using both the flow sensor and the rotameter. The water flow rate from the tap is varied to range from 3 to 7.5 L per minute. The flow sensor sends pulses to the Arduino, and the number of pulses is stored in the computer. The number of pulses from the flow sensor is sent to the computer every 4 s and compared with the flow rate value read on the rotameter. The image during the flow sensor verification process and the scheme can be seen in Figure 3. By regressing the number of pulses against the flow rate value on the rotameter, the slope and intercept values are obtained.

2.4.2. RTD PT100

The verification of the RTD PT100 is carried out by measuring the temperature of the water inside the Circulating Thermal Bath (CTB), as shown in Figure 4. Verification is conducted for six RTD PT100 sensors in a single experiment due to limited space in the CTB. The temperature of the CTB is varied in the range of 10–90 °C. The RTD PT100 sends voltage signals to the computer. These voltage signals are converted into temperature values by the LabVIEW application. The temperature values of the RTD PT100 are recorded on the computer and then regressed with the temperature values read by the standard thermometer.

2.4.3. Pressure Transmitter

The verification of the pressure transmitter (PT), as shown in Figure 5, is conducted by measuring the pressure of the nitrogen gas inside a pressure vessel. Verification is performed for two pressure transmitters in a single experiment due to limited outlets on the pressure vessel, which only has two outlets. The nitrogen pressure is varied in the range of 5–19 bar. The pressure transmitter sends a current signal to the computer. The current value is recorded by the LabVIEW software and regressed with the pressure values read by the calibrated pressure gauge.

3. Results

3.1. Calculation of Heat Exchanger Dimensions

A shell-and-tube heat exchanger is utilized to evaporate the ammonia–water mixture inside the generator and evaporator, while the condenser and absorber use a finned-tube heat exchanger. Finned-tube heat exchangers are particularly effective in air-cooled systems because they significantly enhance heat transfer by maximizing the surface area available for heat exchange. This makes them well suited for applications like condensers and absorbers, where efficient heat dissipation to the air is crucial. Shell-and-tube heat exchangers are preferred in industrial settings due to their durability, ability to operate under high pressures, and capacity to achieve efficient thermal exchange within a compact design, making them versatile for a range of demanding applications. To determine the heat exchanger surface area ($A$), we first obtain the $UA$ value from steady-state simulations. The value of A is then calculated by dividing the UA value by the overall heat transfer coefficient ($U$), which is determined through heat transfer calculations. The calculated surface area ($A$) for each component is listed in

Table 2. Heat exchanger surface area calculation results.

Component	$\mathit{U}\mathit{A}$ (W/K)	$\mathit{U}$ (W/m2K)	$\mathit{A}$ (m2)
Evaporator Shell and Tube	1859	2752.046	0.675
Generator Shell and Tube	825.1	2979.367	0.277
Absorber Finned Tube	812.5	57.151	14.217
Condenser Finned Tube	1310	61.508	21.298

.

The calculation of the heat exchanger dimensions performed by the author was compared to the calculations conducted in the software as the validation by a third-party collaborator responsible for manufacturing the absorption chiller system. The software validation calculations were performed using Unilab Coils 8.0 software. This comparison ensures the accuracy and reliability of the design specifications. The comparative analysis of the heat exchanger dimension calculations, conducted by both the author and the software, is presented in

Table 3. Comparison of heat exchanger dimension calculation results.

Component	Heat Exchanger Dimension Calculation Results		Deviation
	Author(s)	Software Validation
Evaporator Shell and Tube	0.675	0.716	5.66%
Generator Shell and Tube	0.277	0.36	23.07%
Absorber Finned Tube	14.217	15.1	5.85%
Condenser Finned Tube	21.298	22.4	4.92%

. This table highlights any discrepancies and validates the consistency between the theoretical calculations and practical design considerations, thereby confirming the robustness of the heat exchanger design for the evaporator component.

From the comparison of the results, it was found that the heat exchanger dimensions calculated by the software are larger than those calculated by the author. The resulting deviation is also quite significant, with the highest deviation occurring in the shell-and-tube component of the generator at 23.07%. The dimensions calculated by the software are larger than those calculated theoretically because the software takes into account real-world factors that theoretical calculations might not fully address. These factors include manufacturing tolerances, pressure drops, safety margins, fouling factors, and design standards specific to commercial products. Additionally, the software might optimize the design for better performance or compliance with industry regulations, which could result in larger dimensions compared to purely theoretical calculations. However, the larger calculated results are considered safe because they compensate for heat losses that occur in real conditions. Over time, heat exchangers can accumulate fouling and scaling, which reduces their effective heat transfer area and performance; thus, designing with a larger area compensates for this reduction and ensures continued efficient operation [48]. A larger area also ensures that the heat exchanger can handle the variations in temperature in the real condition, which can affect heat transfer rates, without compromising performance [49]. Therefore, the heat exchanger calculations carried out by the third-party collaborator are valid, as their values align with the calculations based on heat transfer principles.

3.2. Component Construction

The construction of the absorption chiller components is depicted in the Detailed Engineering Design (DED). The creation of the DED was aided by a third-party collaborator, which also assisted in manufacturing the absorption chiller system. Initially, the author described the components according to the desired dimensions using Autodesk Inventor 2021 software. These drawings were then handed over to the third-party collaborator, where they were refined to meet industry standards using Microsoft Visio 2021 software. The assembly overview of the expected system to be achieved in this study is depicted in Figure 6. The detailed construction of each absorption chiller component will be discussed in the following subsections. The system consists of key components, including the shell-and-tube evaporator/generator and finned-tube condenser/absorber, which are central to the heat exchange process. However, other components, like vessels and the storage tank, also play crucial roles in the system’s overall functionality. These elements ensure proper fluid containment, flow management, and thermal regulation, which are essential for the system to operate according to its intended principles and desired performance.

3.2.1. Shell-and-Tube Heat Exchangers

The shell-and-tube heat exchanger (Figure 7) functions to provide heat to the fluid, facilitating its evaporation. This design was chosen for its ease of manufacturing and its ability to offer a large heat transfer area within compact dimensions. The heat exchanger is installed on both the evaporator and generator using flanges, allowing for easy removal during repairs or inspections.

3.2.2. Finned-Tube Heat Exchangers

The finned-tube heat exchanger (Figure 8) functions to transfer heat from the working fluid to the environment. The fins on this heat exchanger are circular, chosen for their ease of manufacturing compared to plate-shaped fins. The dimensions of the tube and fins are based on the calculated area results. Additionally, the heat exchanger is equipped with a fan to enhance forced convection, thereby maximizing the heat transfer efficiency.

3.2.3. Generator and Evaporator Vessels

The generator and evaporator function to evaporate a mixture of ammonia–water by providing heat to the fluid. These components are equipped with shell-and-tube heat exchangers to facilitate this process. Their outlets are designed to efficiently release the vapor generated during evaporation. The vapor collects above the vessel dome and flows through the outlet, as illustrated in Figure 9.

To ensure that all the fluid entering the generator and evaporator comes into contact with the heat from the shell-and-tube heat exchanger, the inlet components are designed to spray the fluid directly towards the heat exchanger, as shown in Figure 10. The generator and evaporator are also equipped with flanges that provide a secure location for the shell-and-tube heat exchanger. Additionally, sight glasses made of acrylic are installed in the generator and evaporator to monitor the fluid levels inside these components.

3.2.4. Absorber Chamber

The absorber functions as the mixing chamber for ammonia vapor from the evaporator and the weak ammonia–water mixture. The inlet for the liquid ammonia–water mixture from the generator is designed as a sprayer (Figure 11) to facilitate the absorption of ammonia vapor from the evaporator. Additionally, the absorber is equipped with an acrylic sight glass to monitor the fluid level inside the component.

3.2.5. Storage Tank

The storage tank (Figure 12) serves as a reservoir for the fluid that has passed through the finned-tube heat exchanger. It also functions as a storage space for ammonia and water when the system is not in operation.

3.3. Determination of Expansion Valves

The expansion valve to be used in the system is a regulating valve type, allowing for the manual adjustment of the valve opening by Danfoss. A thermal expansion valve was avoided to prevent potential valve closure due to a decrease in the fluid temperature at the valve outlet. The selected pipe diameter is ½ inch, necessitating the use of a valve with a 15 mm diameter, as specified by Danfoss (hand-operated regulating-valve-type REG-SA and REG-SB) [50]. In addition to the valve diameter, the type of cone and the valve opening size needed to be determined based on the pressure drop and the mass flow rate through the valve. In this single-effect ammonia–water absorption chiller system, the pressure drop across the valve is 10 bar, and the mass flow rate is 162 kg/h. With these specifications, the appropriate type of cone and valve opening size by Danfoss could be identified, and, consequently, the selected valve type is REG-SA 15, with an opening set to approximately 50% to 60% [50].

3.4. Absorption Chiller Component Testing

3.4.1. Pressure Testing

The method for testing pressure leaks in the system is outlined in ASTM Standard 1003. This testing involves immersing the component in a tank filled with water and introducing pressurized N₂ gas into the component. The component is then left for 1 h. After this period, the author inspects for leaks by checking for the presence of air bubbles in the water tank. If the component is too large to be submerged, leak inspection can alternatively be performed by applying soapy water to the connections and observing for bubbles. If no bubbles are observed, the component is deemed leak-free.

An example of a pressure test for the heat exchanger component is shown in Figure 13. The test was conducted at a pressure of 24 bar, which was approximately 150% of the operational pressure. This pressure level was consistent with the recommendations specified in ASTM Standard 1003. After being pressurized for 1 h, no pressure drop was observed in the heat exchanger, indicating the absence of leaks.

3.4.2. Weld Joint Testing

In addition to conducting pressure testing, weld joint testing was also performed for every weld joint in the component. This testing followed ASTM E165 standards, specifically using Visible Penetrant Examination Method A, as illustrated in Figure 14. The process begins by thoroughly cleaning the weld joint with a cleaner. After cleaning, a red penetrant is sprayed onto the weld joint and left to dwell for approximately 5 min. The penetrant is then cleaned off with a cloth. Subsequently, a white developer is sprayed onto the weld joint. If a red color appears on the joint when the developer is applied, it indicates that the joint has defects and requires repair.

3.5. Control Panel Construction

For operating the absorption chiller, a control panel is needed as a location to distribute electrical energy from the power source at the Faculty of Engineering, Universitas Indonesia, to the components within the absorption chiller. The components include fans, pumps, and data acquisition devices (ADAM-4015, ADAM-6017, Advantech PSD-A60W24, and Advantech EKI-2528 by Advantech Co., Ltd., manufactured in Taipei, Taiwan).

To ensure the safety of the components and operators from hazardous electrical currents when operating the absorption chiller, several electrical components are required, such as the following:

• A Miniature Circuit Breaker (MCB) to interrupt the flow of the electricity in case of electrical current surges from the power source or components;
• Contactors as switches that deliver electricity from the power source to the components to be operated;
• A Push Button as a trigger for the contactors to deliver electricity to the components of the absorption chiller;
• Busbar Grounding to channel excess electrical currents back to the ground to prevent endangering the operator.

The electrical components are assembled as shown in Figure 15. The installation of the electrical components on the panel also includes cable ducts to neatly arrange the cables and DIN Rails to support each component. After ensuring that each cable was connected perfectly without any electrical leakage, the author conducted a test on each contactor connection using a multimeter. In the multimeter, it was observed that each contactor delivered electrical current with an AC voltage of 220 V, in accordance with the voltage requirements of each component.

3.6. Measuring Device Verification Results

From the verification results of all sensors, regression equations were obtained, and the coefficient of determinations (R²) were calculated. The coefficient of determination determines the extent to which the independent variables in the regression can predict the values of the dependent variable [51,52]. If the R² value approaches 1, it indicates a higher ability of the independent variables to predict the values of the dependent variable, and vice versa. In this verification process, the independent variables were the values read by the sensors, and the dependent variables were the values read by the verification instruments. The R² value for each sensor is listed in

Table 4. R² value of each sensor’s calculated regression.

Sensor	R2	Sensor	R2
FM 1	0.997982911	RTD 15	0.999992314
FM 2	0.994972004	RTD 16	0.999997148
RTD 1	0.999904965	RTD 17	0.999996318
RTD 2	0.999998657	RTD 18	0.999996147
RTD 3	0.999997205	RTD 19	0.999983646
RTD 4	0.999994633	RTD 20	0.999962972
RTD 5	0.999992459	RTD 21	0.999996427
RTD 6	0.999998152	RTD 22	0.999994269
RTD 7	0.999990906	RTD 23	0.999988262
RTD 8	0.999991754	RTD 24	0.999995572
RTD 9	0.999988738	PT 1	0.999994229
RTD 10	0.999992051	PT 2	0.99999811
RTD 11	0.999996819	PT 3	0.99999855
RTD 12	0.999987421	PT 4	0.999997073
RTD 13	0.999995833	PT 5	0.999995329
RTD 14	0.999996889	PT 6	0.999997641

. The R² values from the regression results of all the sensor verifications are close to 1. Therefore, it can be concluded that the regression equations obtained from the verification process have the ability to predict values corresponding to the calibrated measurement instruments.

4. Conclusions

Absorption chillers, powered by solar energy and using environmentally friendly fluids like ammonia–water, are ideal for Indonesia’s high solar potential. This study focused on designing a 5 kW solar-assisted ammonia–water absorption chiller for residential use, utilizing air-cooled condensers and absorbers to reduce the space and operational and maintenance costs. The research includes the system design, component specifications, and an impact analysis of the measurement instruments. The outcomes provide precise dimensions and specifications, contributing to the development of advanced solar cooling systems in the future.

Based on the calculations and analysis conducted, it can be concluded that the suitable heat exchangers for this absorption chiller system are the finned-tube and shell-and-tube types. The finned-tube heat exchangers are used in the condenser and absorber components, with effective heat transfer areas of 13.976 m² and 22.618 m², respectively. The shell-and-tube heat exchangers are utilized in the evaporator and generator components, with effective heat transfer areas of 0.669 m² and 0.278 m², respectively.

The selection of heat exchangers is critical in absorption chiller systems, where efficiency and performance are heavily dependent on effective thermal management. Finned-tube heat exchangers are known for their ability to enhance heat transfer by increasing the surface area, making them ideal for air-cooled applications such as condensers and absorbers. Shell-and-tube heat exchangers, in contrast, are widely used in industrial applications due to their robustness, ability to handle high pressures, and efficient thermal exchange in compact spaces. In practical applications, these findings highlight the importance of precise heat exchanger selection to ensure the optimal performance of absorption chillers, particularly in systems where energy efficiency is paramount. The use of finned-tube exchangers in air-cooled components can be particularly beneficial in regions with fluctuating ambient temperatures, while shell-and-tube exchangers offer reliability in handling high thermal loads in the evaporator and generator.

From the insights gleaned from this case study, a series of developments can be performed to enhance the design and execution of future research activities. These developments include the following:

• Implementing an SHX unit, which could significantly enhance the system efficiency by recovering heat between the weak and strong solutions;
• Exploring advanced control strategies to optimize both the energy and exergy performances, potentially through real-time monitoring and adaptive algorithms;
• Investigating the use of alternative materials for heat exchangers and other components to reduce the weight and cost and improve the thermal performance;
• Integration with auxiliary energy sources (e.g., geothermal, gas engines) to provide continuous operation during low-solar-irradiance periods;
• Developing a sophisticated control panel for the dynamic regulation of the pump flow rates and fan speeds to further optimize the performance and energy consumption;
• Conducting a lifecycle analysis of the system to evaluate its overall environmental impact and further explore eco-friendly enhancements.