1. Introduction
The demand for cooling energy in the form of chilled water is an important factor when considering energy management in many industrial plants. In most cases, the basic devices used for chilled water production are compression chillers with air-cooled condensers (monoblock-type) or water-cooled condensers (split-type). Using electricity to power the production of cooling energy limits core production, which requires electricity to power new processing lines. Replacing the above-mentioned systems with absorption units powered by hot water or steam, which are already available in many industrial plants, may therefore be an attractive alternative.
In order to select the variant of expansion or retrofitting of a chilled water production system, one needs to conduct an analysis taking into account the cooling needs, their variation over time, and the availability of energy carriers. The identified variants should be assessed according to multiple criteria, taking into account technological, energy-related, environmental, and economic aspects.
A review of absorption cooling technologies that may be applied in chilled water systems in industrial plants can be found in Nikbakhti et al. [
1]. The authors noted an increased interest in this field in connection with the energy crisis, rise in primary fuel prices, and the negative environmental impact of traditional compression chilling systems. They noted that despite its many advantages (e.g., the possibility of using low-potential heat as a power source, and using cooling mediums with minimal environmental impact), this technology is characterized by low energy efficiency. By analyzing research by many other authors, they pointed to options in terms of improving the energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER) by increasing heat recuperation, using new pairs of mediums, and modifying the operating parameters.
Many research papers on absorption technology are concerned with evaluations and possible improvements of energy efficiency.
Kaynakali et al. [
2] performed an energy and exergy analysis of a dual-stage absorption system powered by various energy sources. While studying the application of hot water, steam, and hot air as energy sources for a high-temperature boiler, they noticed that the internal exergy losses were lowest when the boiler was fueled by hot water and highest when it was fueled by hot air.
Saoud et al. [
3] conducted in situ research and numerical modeling of a single-stage absorption chiller. Those authors built a numerical model of the chiller based on the energy balance, which they validated against a commercially available, 106.1 kW water chiller. By analyzing the device’s EER, cooling efficiency, and temperature drop in the boiler, they concluded that the equipment operated correctly across a broad range of boiler-powering temperatures and enabled the efficient use of low-temperature sources of powering heat.
El-Shafie et al. [
4] conducted experimental research on a 1750 kW absorption chiller with a natural gas-fueled boiler. Two years of research allowed those authors to determine the energy-efficiency variability and internal exergy losses of the device based on operating time. They noticed a 22% drop in the EER and a 14% increase in exergy losses, which they attributed to the device becoming contaminated during use. Their final conclusion was that the equipment should undergo service checks more frequently.
Mróz (2006) [
5] conducted experimental research on a 500 kW single-stage absorption chiller fueled by steam. The research helped to evaluate the energy and economic efficiency of the studied system. It was discovered that the instantaneous cooling load of the device significantly influenced its EER.
The prospects for integrating absorption chilling systems as elements of a combined energy economy were described in [
6,
7,
8].
Trygg and Amiri [
6] noted that Sweden offers favorable conditions for implementing absorption technology using waste heat from electricity generation in heat and power plants. They calculated that applying tri-generation technology may result in an 80% national-scale reduction in carbon dioxide emissions and a 170% reduction in the costs of cooling energy generation as a result of the sale of electricity generated in this way.
A similar analysis of the Finnish energy market was conducted by Saastamoinen and Paiho [
7], who noted the attractiveness of using tri-generation systems.
Gąsiorowski and Mróz [
8] analyzed the energy and exergy efficiency of a low-power tri-generation system based on a gas-fueled microturbine and a single-stage absorption chiller. They demonstrated that in such systems, the key parameter impacting efficiency is the optimal use of waste heat, i.e., the heat of the exhaust gases generated in the gas microturbine.
From the point of view of synergy between absorption chillers and combined heat and power (CHP) systems, another important parameter is the required temperature of the heat vector supplied to the boiler. Gomari et al. [
9] studied the influence of supply temperature on the EER in single-, dual- and three-stage absorption systems. They found that the maximum EER value for a typical vaporization temperature (T
v = 4 °C) was 95 °C for single-stage systems (EER = 0.7), 190 °C for dual-stage systems (EER = 1.2), and 205 °C for three-stage stage systems (EER= 1.6). The rising required temperature of boiler power supply negatively impacted the operation of CHP production, resulting in a lower co-generation ratio.
M.M. Joybari and F. Haghighat [
10] conducted an analysis of systems with various heat exchanger designs, all sharing the same EER coefficient. Their study on the effects of the working fluid’s mass flow rate and the inlet temperatures of the cooling water, chilled water, and heating fluid from the heat source showed that a decrease in the working fluid’s mass flow rate resulted in reduced exergy loss for the system element in question. Additionally, they observed that the total system exergy rose when the heat source and inlet chilled water temperatures were lower and the inlet cooling water temperature was higher. The analysis found that the absorber and condenser experienced the most significant exergy losses. Consequently, the studies recommended modifying the cooling tower by decreasing the cooling water’s mass flow rate while increasing its temperature, ensuring that it can still satisfy cooling demand.
In an article by H. Ansarinasab and others [
11], a thermoeconomic analysis of a solar-powered absorption cooling system integrated with different types of collectors was presented. Their analysis revealed that the system performance factor increased with an increase in source temperature and evaporator temperature, while the exergy efficiency was proportional to the temperature of the water supplying the generator and inversely proportional to the temperatures of the evaporator, absorber, and condenser. It was pointed out that the most economical solution from the perspective of exergy efficiency was a solar absorption cooling system utilizing evacuated tube collectors (ETC), with an exergy efficiency of 0.66. However, when interpreting the system efficiency data, the use of parabolic trough collectors (PTCs) proved to be the most favorable solution.
Kerme et al. [
12] conducted a series of studies and analyses on an absorption cooling system using lithium bromide and water, powered by solar energy (flat plate collector). Aspects including the impact of different types of collectors on efficiency and heat gain, the inlet temperature to the generator, the efficiency of the heat exchanger, and the mass flow rate of the working fluid were assessed to determine their influence on system performance. The analysis indicated that using a collector with a selective coating increased the system’s efficiency and heat gain compared to using single- or double-glazed collectors. An increase in the heat exchanger efficiency significantly improved the overall system performance, while increasing the mass flow rate of the fluid reduced the chiller efficiency. The results showed that the main source of exergy losses in this system was the solar collector (which accounted for 84% of the total exergy losses), followed by the generator (8.3% of the total exergy losses). Increasing the inlet temperature to the generator led to a slight increase in cooling efficiency, but after stabilizing, it ultimately decreased. This also resulted in increased exergy losses in system components. Based on these findings, the authors noted that improving the heat exchanger efficiency reduced the exergy losses of the system and improved the cooling performance, while an increase in the mass flow rate led to a decrease in the efficiency of the system, contributing to higher exergy losses.
In his article, G. S. Dhindsa [
13] presented actions aimed at improving the performance of absorption cooling systems powered by solar energy. The author noted that the use of various types of waste energy led to a significant increase in the efficiency of the absorption cooling system. That article also highlighted the correlation between system performance and the increased latent heat of the working fluid. Attention was drawn to the beneficial effect of using iron oxide nanoparticles as a component of the refrigerant fluid. G.S. Dhindsa emphasized that under optimal operating temperatures for the generator, a constant pressure process demonstrated higher absorption cooling efficiency than a constant temperature process.
G. L. Szabó [
14] examined ways to optimize the operation of absorption chillers for cooling purposes, with the main requirement being the increase in exergy efficiency, which impacts both quantitative and qualitative indicators. The article stated that ways to improve system performance included changing the generator temperature, changing the condensation temperature, and consolidating changes in both the generator and condenser temperatures. For example, lowering the condenser temperature by 2.1 °C and increasing the generator temperature by 27.3 °C significantly increased the system performance indicators.
In an article by J. Asadi and others [
15], a thermoeconomic analysis of a solar-powered absorption cooling system integrated with different types of collectors was presented. The analysis found that the system performance factor increased with an increase in the source temperature and the evaporator temperature, while the exergy efficiency was proportional to the temperature of the water that supplied the generator and inversely proportional to the temperatures of the evaporator, absorber, and condenser. A solar absorption cooling system using evacuated tube collectors (ETC) was identified as the most economically favorable solution, with an exergy efficiency of 0.66. However, in terms of the efficiency of the solar-powered system, the use of parabolic trough collectors (PTC) turned out to be the most beneficial.
Banua and Sudharsanb [
16] conducted a thermodynamic analysis of absorption cooling systems. They demonstrated that choosing the right device parameters, such as the type of working fluid and the number of stages of the device, plays a crucial role in improving the efficiency of absorption cooling systems, which can be aided by the use of thermodynamic analyses (TDA). The article highlighted the optimal temperature ranges for generators based on the COP values of devices with a specific number of stages and configurations.
Rashidi and Yoo [
17] compared the KPCC system (using the Kalina cycle with an NH
3-H
2O absorption chiller) and the KLACC system (using the Kalina cycle with a LiBr-H
2O absorption chiller). The analysis showed that exergy losses in the KLACC system were 40% higher (mainly in the condenser and the preheater of the second separation tank). As a result, the efficiency of this system was close to the Kalina cycle’s efficiency, while the efficiency of the KPCC system was 6.8% higher. From an economic analysis perspective, the unit cost of energy production in the KPCC system was 20.5% lower than in the KLACC system. The analysis also emphasized that efforts to reduce exergy losses should focus on the absorber in KPCC systems and the second condenser in KLACC systems.
Alazwari et al. [
18] analyzed the operation of an absorption chiller in an air handling unit with heat recovery using phase change materials (PCM) on building partitions. The authors investigated the benefits of the improvements made to the basic configuration of the system. The use of PCM reduced the cooling energy demand by 6.22%, while heat recovery reduced the demand by 8.38%. The total reduction in energy consumption generated by these improvements was approximately 111 kWh/m
2. The study concluded that for cases with high cooling demand, it is worth analyzing the implementation of such improvements, especially when there is no possibility of using larger cooling-producing equipment.
In [
19], the economic and environmental costs of using of a device based on solar energy-powered absorption cooling was analyzed and compared to those of a standard inverter device with a heat pump. Those others found that absorption cooling devices used in Australia consume at least 50% less energy than inverter air conditioners, resulting in half the CO
2 emissions. Additionally, absorption chillers consume approximately 75% less peak electrical power (kWp), which is particularly important given the increasing performance issues affecting the power grid. However, due to the significantly higher capital cost of solar-powered absorption chillers, with a long payback time exceeding 20 years, the use of such systems, despite their numerous benefits, is difficult to justify.
Kheiri et al. [
20] analyzed a gas turbine-based trigeneration system (GTBS) utilizing waste gasification fuel, which produces electricity and heat that may be used by absorption chillers to produce cooling. It was found that during the winter period, the energy utilization factor of such a device was 47.62% and its exergy efficiency was 20.42%. An environmental analysis showed that the use of the GTBS system reduced potential CO
2 emissions of 9233 tons annually. Further parametric studies conducted by those authors indicated that increasing the gasifier temperature and the inlet gas temperature to the gas turbine increased the energy utilization factor of the device while reducing its exergy efficiency and overall system operating costs.
Nondy and Gogoi [
21] compared the optimal performance of two gas turbine-based CCHP trigeneration systems: the first, consisting of a steam turbine, a Rankine regenerative cycle with heat recovery, two stages of absorption cooling, and a water heater, and the second, in which the Rankine regenerative cycle was replaced by a steam turbine condensation cycle. Based on a parametric analysis, the optimization of these systems resulted in a slight improvement in energy and exergy efficiency, while the device costs were reduced by 9% and 5.3%, respectively. With such a configuration, the payback time was found to be 10.83 years for the first system and 13.27 years for the second. Furthermore, it was found that under optimal operating conditions, the overall energy production and efficiency of the systems were nearly identical, but the overall cost of the first system was significantly lower than that of the second. In addition, the first system exhibited the lowest specific CO
2 emissions, i.e., 91.75 kg/MWh.
In [
22], a new CCHP system powered by biogas was proposed, based on secondary exhaust gas injection (due to high exergy losses in systems based on chemical recovery). Huang and other authors noted that the new system would feature an increased methane conversion factor. In the new system, electricity production increased by 55.52 kW (corresponding to 6.2%) and cooling increased by 542.49 kW (corresponding to 101.49%). The analysis revealed that the exergy efficiency of the system increased by 8.31%. The study indicated that an excessive inlet temperature of the gas to the turbine was unfavorable for exergy efficiency from the perspective of the whole system.
Mirzaee and other authors [
23] analyzed a co-generation system modeled in the EES software (Aspen Plus), consisting of a gas turbine, absorption chiller, boiler, and heat exchanger. The system was studied under nine characteristic scenarios and the results were presented and compared in terms of energy efficiency, energy consumption, fuel consumption ratio, and CO
2 production. The optimal solution for combined electricity and cooling production was found in Scenario 5 (two absorption chillers installed in series, i.e., Stages I and II), with a fuel consumption ratio of 45,325.5 kJ/kg. For the combined production of electricity and heat, Scenario 7 was the most beneficial (with a fuel consumption ratio of 39,541.9 kJ/kg), where all heat could be recovered. That article also noted that the highest CO
2 production (88.18 kg/s) was observed in the devices presented in Scenarios 1 and 6, which were found to be insufficient and inefficient. On the basis of the conducted studies, it was concluded that for combined heat and electricity production, reducing the pressure in the heat recovery system of the gas turbine led to a decrease in the output temperature from the heat exchanger, significantly affecting the fuel consumption ratio.
Modi et al. studied the energy efficiency of a single-stage absorption chiller with variable parameters in terms of boiler power supply and absorber cooling [
24]. They showed that the decreasing absorption temperature increased the device’s energy efficiency, i.e., the possibility of cooling the absorber to 30 °C allowed for the maximization of the EER (0.77) when the boiler’s supply temperature was lowered to 82 °C. Similar research results were obtained by Karamangil et al. [
25], Sun [
26], and Aphornratana and Sriveerakul [
27].
A literature survey revealed that there is limited interest in the application of such solutions in industrial plants. This is partly due to a lack of knowledge among industry energy managers concerning the availability of absorption cooling technologies. New approaches to the evaluation of industrial plants based on the Environmental-Social-Governance (ESG) model require a broader look at existing, technically acceptable solutions which could help to reduce energy consumption and the environmental footprint of industry.
Despite relatively low EERs and SEERs, absorption chillers used in synergy with combined heat and power sources—i.e., tri-generation systems—may be attractive solutions in terms of energy usage (primary energy) and environmental sustainability (CO2 emissions) in comparison to traditional compressor systems powered by electricity.
Here, we present an energy and environmental analysis of implementing the dual-stage absorption chillers supplied from an existing gas-fueled co-generation plant at a selected pharmaceutical plant. It is proposed that such an action may lead to the reduction of primary energy consumption and carbon dioxide emissions.