1. Introduction
Absorption refrigeration systems are compression refrigeration systems that use thermal compressors. Such systems have the advantages of reduced electricity consumption, lower maintenance costs, and the elimination of CFC and HCFC use as refrigerants. They also allow cogeneration and solar energy as an input source to drive the system [
1,
2,
3,
4].
However, such equipment has the following disadvantages: (i) low COP (coefficient of performance) when compared to vapor compression systems, and (ii) absorption chillers are heavy equipment, and their capital cost is relatively high. Besides the critical points mentioned, the choice of working fluids (refrigerant and absorbent) is crucial for the efficiency and performance of absorption cooling systems, as many authors have demonstrated over the years, by conducting studies in different areas of the cooling thermal comfort or even for refrigeration purposes [
5,
6].
In commercial manufacturing products, two types of solution pairs are commonly used: lithium bromide/water (LiBr/H
2O) [
7,
8] and ammonia/water (NH
3/H
2O) [
9,
10,
11]. Usually, the pair LiBr/H
2O is used for thermal comfort systems and the pair NH
3/H
2O for refrigeration systems [
12]. As part of the analysis, those equipment have been used for different applications to achieve the energy demands of hospital sector [
13], hotel and business buildings [
5,
6,
14,
15], refinery industry [
16], and also for the configuration of new control strategies on absorption solar systems [
17].
In the last decades, new solution pairs have been presented and investigated by different authors [
18,
19,
20] to increase absorption chillers’ heat and mass transfer process. One of the cited pairs, ammonia/lithium nitrate (NH
3/LiNO
3), is very popular around the world due to its advantages, such as the elimination of the rectifier, high solubility in ammonia, and the absence of corrosion of the metal, among others [
19,
20,
21,
22]. Besides the advantages mentioned above, Heard et al. [
23] observed a higher tolerance to operation parameters other than the ideal condition, crystallization risk.
Several studies were conducted to evaluate the NH
3/LiNO
3 solution as an alternative work pair for the absorption systems. Some studies assessed this fluid by employing experimental apparatus [
24,
25], real prototypes [
26,
27], heat and mass transfer studies [
18], and different components and configurations [
28,
29]. These studies demonstrated the effective performance of these working fluids in absorption systems and showed their technology limitations.
The thermal energy storage by using absorption systems has been strongly analyzed considering cycles and thermodynamic systems, working fluids, and different system configurations to meet specific thermal demands. Recently, Mehari et al. [
30] published a review article on absorption refrigeration systems, aiming to discuss and present the cycles and configurations of thermal absorption energy storage, integrating of storage systems using conventional absorption refrigeration systems, and heat pumps.
To date, a few review papers on absorption chillers have been published [
30,
31,
32,
33,
34,
35]. However, they discuss solar energy as the input source of the systems [
36], the absorption refrigeration cycles’ evolution over the last decades [
37], or even the progress and new features in solar cooling applications [
35]. Regarding the critical description of the literature in the area of absorption refrigeration, Nikbakhti et al. [
38] reported in detail, several technologies implemented to improve the COP of absorption refrigeration systems. Promising alternatives were presented and discussed to increase the thermal performance of absorption refrigeration systems, aiming at optimizing the original design of the system. Another critical aspect verified was that the optimization of the absorption chillers’ operational conditions had improved its performance. The discussion on how mathematical or thermodynamic methods had been used to improve their performance is overviewed in Best and Rivera [
39]. Even with these studies, updating this subject with new technology and applications on the field is mandatory. It helps the scientific community understand and improve these systems to better technical performance to reach financial feasibility. In the literature, there is a lack of consistent studies about designing requirements for absorption chillers.
It is interesting to see that many authors have been discussing solar powered energy as an input source for absorption chillers [
32,
33,
36]. Others aim towards the type of heat exchanger technologies on those systems [
40] and the use of membrane contactor to enhance the absorption process [
31].
Figure 1 presents the central paper reviews about absorption chillers presented in the scientific community in the last 10 years.
Figure 1 shows no reviews about designing requirements for absorption chillers with the relevance of thermophysical properties in this process and the heat and mass transfer processes in the refrigeration absorption systems using ammonia/nitrate-lithium and water.
In this context, there is a need to present a more updated review to cover recent advances and suggested solutions to improve the use and operation of those absorption refrigeration systems using different working fluids. Hence, this review tries to fill the gap mentioned above by presenting the state-of-the-art of ammonia/absorbent based absorption refrigeration systems, considering the most relevant studies, describing the development of this equipment over the years.
5. Heat and Mass Transfer Processes in Absorption Cooling System Ammonia/Absorbent
The intensification of mass and heat transfer processes in absorption systems represents the main goal of its optimal thermal behavior. That is why many authors have been investigating ways to enhance those processes [
85,
93,
97]. Some possible improvements are the use of different working fluids, water added on the absorbent, configuration of the absorber, and generator, as seen in
Figure 8 showing a flow chart on how and why the intensification on the heat and mass transfer has been so important to enhance the performance of the absorption refrigeration systems.
Lima et al. [
85] evaluated the influence of the ammonia mass fraction and the flows of the absorbent solution and refrigerant vapor to allow the optimization and simulation of absorption cooling systems. The absorption process is one of the limiting factors of the absorption cooling system’s performance. Using the same working fluids, Chandrasekaran et al. [
98] presented a characterization of a new microchannel shell-and-tube absorber for absorption refrigeration systems. The idea was to design an absorber that allows significant heat absorption capacities considering a wide range of fluid flow rates and ambient temperatures, using a 10.5 kW NH
3/H
2O absorption chiller. Numerical modeling was developed and validated with the experimental data collected, and found excellent fit between them (errors lower than 4%). As a main result from the study, it was the development of a sizing and simulation tool that allowed to optimize highly compact and efficient absorbers for absorption systems. Another important advance of this proposal was presented by Kini et al. [
99] where through the thermodynamic analysis they verified that the new absorber integrated to the absorption chiller led to excellent behavior proving the scalability of this technology to attend the thermal demand for a residential scale absorption heat pump.
Boiling and absorption processes are the key to sorption refrigeration systems, so their improvement must reach high levels to provide higher refrigeration capacity with less driving energy. In this context, Oronel et al. [
93] conducted an experimental study of the boiling and absorption process with the binary mixture (NH
3/LiNO
3) and ternary mixture (NH
3/LiNO
3 + H
2O) under the operating conditions of absorption refrigeration systems driven by low-temperature heat sources. They were performed using a conjugated flat plate heat exchanger formed by three channels, with the absorption occurring in the central channel. The results showed that the ternary mixture’s mass flow absorption was higher than that obtained with the binary mixture in the same operating conditions. The heat transfer coefficient from the ternary mixture’s solution was higher than that of the binary mixture. The ternary mixture’s low viscosity increased the heat transfer and mass of the absorber compared with the binary mixture.
Studying heat and mass transfer in absorption cooling systems, Aramis et al. [
18] performed an experimental analysis using the same working fluids as Oronel et al. [
93]. The test’s objective was to characterize the absorption and desorption process using flat plate heat exchangers, looking for a cost and size reduction of the absorption chiller. It was found that the ternary mixture has a higher affinity than the binary mixture, which increases the performance of the absorption and desorption processes.
Using the same working fluids, Taboas et al. [
100] presented an experimental study to estimate the heat transfer coefficient by flow boiling and the pressure drop by friction for a plate heat exchanger considering 20% weight of water in the absorbent. The results showed that the solution’s mass flow was the parameter that had the most significant influence on the flow boiling coefficient in the operating conditions applied, representing a characteristic of boiling by convection.
The increase in the flow’s boiling coefficient was significant in low heat flow values when the mass flow was increased for the binary mixture. However, with the ternary mixture, the improvement of the flow’s boiling coefficient, achieved by the increase in the solution’s mass flow, was analogous for all values considered for the heat flow.
Using the NH
3/LiNO
3 pair, Jiang et al. [
92] presented through experimental analysis the influence of parameters such as mass and heat flow, diameter, and titer of vapor on the coefficient of heat transfer by boiling in a horizontal tube. An increase in the heat transfer coefficient with the increase in mass flow and heat flow was observed, where the mass flow was more impactful with high heat flow. A new correlation (
Table 9) was also proposed to predict the influence of the analyzed parameters on the boiling heat transfer coefficient, obtaining better results than other works found in the literature.
Applying the falling film method in the heat and mass transfer process, the authors of [
101] presented a parametric analysis to determine the coefficient of performance, cooling capacity, and recirculation of the solution in different operating conditions in an absorber and generator cooling system. Cooling capacities of 4.5 kW, temperatures around 4 °C from the evaporator, COPs between 0.3 and 0.62 were found depending on the operating system.
To improve the thermal performance of heat and mass transfer using an adiabatic absorption process, Zacarías et al. [
29] conducted an experimental study of the ammonia vapor absorption process in the NH
3/LiNO
3 solution using a full cone nozzle and an upstream single pass subcooler to analyze the influence of the absorption rate, subcooling output, mass transfer coefficient, and the equilibrium ratio approach. The results recommended using full cone nozzles in spray absorbers at high mass transfer values due to the high availability, low cost, ease of construction, compacting, and large bore diameter. Equation (1) presented a Sherwood number correlation, expressed as:
It is important to note that the efficiency of absorption systems comes from the effectivity of the heat and mass transfer process through the absorbers, where the intensity of heat and mass transfer is high, and they could represent the element with a greater possibility of bottlenecking the energy process [
82,
90]. That is why heat exchangers have been identified as an energy barrier element of absorption chillers [
40], hence the importance of studying and understanding the operation of heat exchangers. A detailed review of different technologies of heat exchangers in absorption chillers was carried out where detailed works and experimental simulations studies of simple effect absorption systems that use binary solutions with LiBr/H
2O, NH
3/H
2O, and NH
3/LiNO
3 were presented. The aim, in detail, was on the use of heat exchangers with innovative technologies, conventional and special geometries, mechanical treatments to provide information on the use of heat exchanger technologies and their development in the last 40 years. Another critical aspect discussed was the performance in the heat and mass transfer process and its relevance in choosing the correct heat exchanger configuration, which not only takes the potential for heat and mass transfer performance but factors such as manufacturing costs, type of exchanger and heat source, working fluids, compactness, among others.
By improving the process of the heat and mass transfer in a plate heat exchanger (PHE)-type model NB51 absorber with NH
3/LiNO
3, Chan et al. [
102] conducted an experimental analysis of a bubble mode absorption using a special vapor distributor aiming to increase the mass transfer considering solar cooling operating conditions. A range of 35–50% of solution concentrations and 11.69–35.46 kg s
−1 mass flow rates of the dilute solution range were used. As main result from the study, a set of Nusselt number correlations, Equations (2) and (3), governing the ammonia vapor by the NH
3/LiNO
3 solution in the absorber was determined considering two ranges of Reynolds number.
An experimental analysis to verify the influence of operational conditions on the heat and mass coefficients in a heat and mass transfer process in a horizontal falling film tube absorber using NH
3/H
2O was carried out in Bohra et al. [
103]. The absorber was constructed with transparent housing to visualize flow and heat and mass transfer measurements at the local and component levels, as shown in
Figure 9, where a basic schematic drawing is seen [
103].
The results showed that most of the time, the absorber operates in a falling film mode. It was concluded that while the Nusselt number of the solution increased, Sherwood vapor and liquid numbers remained relatively unchanged by the Reynolds number
Using the falling film technique to analyze the heat and mass transfer process, Nagavarapu and Garimella [
104] presented a heat and mass transfer model for NH
3/H
2O absorption on a bank of microchannel tubes, analyzing three regions in the absorber: solution pool, droplets in evolution, and falling film. It was found that most of the absorption occurred in the region of the film, and 7% of the process occurred in the droplets. The heat transfer coefficients’ values varied between 1788 and 4179 W/m
2K, presented through Nusselt numbers. An empirical correlation of the Nusselt number of films was developed as presented in, Equation (4). This correlation can be used to integrate a hydrodynamic model and a heat and mass transfer model. A total of 135 of the185 data points were predicted within ± 25% accuracy.
where
,
and corresponds to Reynolds and Prandtl numbers of the film
, represents the Reynolds numbers of the vapor, respectively.
The applicability of the correlation presented in Equation (4) is limited to the range of parameters shown in
Table 10.
The unfavorable properties of fluid transport, over the years, have been highlighted as the main factors that prevent the application of ammonia and ionic liquids in absorption refrigeration systems. Despite this information, few studies link this problem to the heat and mass transfer of these ionic fluids. Hence, Wang et al. [
61] developed an experimental/numerical study to fill this gap on the process of heat and mass transfer of ionic fluids in absorption chillers using a corrugated plate heat exchanger. The experimental results are used as input for the numerical model developed to study heat and mass transfer performance during the absorption of NH
3 vapor in the NH
3/ILs pair. The heat transfer coefficient was 1.4 kW/m
2-K, and the proposed effective mass diffusivity was exponentially related to the ionic fluid viscosity with an exponent of −1.45 for the analyzed pair.
The use of nanoparticles on the solution absorption fluid pair is also seen in Jiang et al. [
105] where a new type of absorber for an absorption refrigeration system that uses NH
3/H
2O as the working fluids to analyze the processes of heat and mass transfer with different concentrations of TiO
2 nanoparticles (0.1%, 0.3%, and 0.5%) was designed and tested. Unlike other studies, the absorber has been integrated into the real absorption system. The absorption performance was mainly determined by the strong solution’s concentration at different evaporation, activation, and inlet cooling water temperatures. It was found that the addition of TiO
2 nanoparticles has an essential effect on the absorption process as it allows the temperature of the strong solution to be lower and the concentration of the strong solution to be higher. The phenomena could be explained in
Figure 10 where the Prandtl number rises slowly with increasing temperature with the same quantity of nanoparticles. At the same temperature, and a higher concentration of nanoparticles, the Prandtl number is higher, too, since the nanofluid’s viscosity was increased, and the thermal conductivity growths slightly correspondingly. The figure also shows the schematic design used to model the mass and heat transfer [
103]. It was also found that the thermal conductivity of nanofluids dominates the absorption process
In the second part of this study, Jiang et al. [
106] proceeded on the effect of different amounts of TiO
2 nanoparticles, but now on the coefficient of performance of the absorption refrigeration system. In this new experiment, the evaporation, activation, and cooling temperature ranges were varied (−18–0 °C), (105–150 °C), and (22–33 °C), respectively. The addition of nanoparticles can lead to an increase of 27% of the heat and mass transfer, as shown in
Figure 11, where the increase of the TiO
2 nanoparticles increases the COP and the generation and evaporation temperatures. The COP improvement is strongly related to the number of nanoparticles dispersed in the fluid [
106]
It is evident from these studies that the addition of water as an absorbent in the mixture with NH
3 allowed a better performance in the absorption and desorption process [
18,
93,
100]. Better results were obtained when this process was performed in plate heat exchangers due to the size, ease of installation, cheaper cost, and ease of testing. Introducing a falling film method and a new component as a full cone nozzle [
29,
101] could increase heat transfer and mass thermal performance. Therefore, the thermal performance of absorption systems and the appropriate heat exchanger for the absorption chiller could lead to a better COP performance, as seen in Altamirano et al. [
40]. Another fact seen around the heat and mass transfer process literature is the positive effect of using ionic nanoparticles on the absorption system, increasing the COP of the chiller [
105,
106].
6. Chiller Activation by Absorption through Solar Energy
Due to the lower driving temperature, absorption chillers that make use of the absorbent fluid ammonia/absorbents have the possibility of activation by thermal energy obtained through solar collectors [
1]. This advantage has led several authors to study this application in generator activation. As shown in
Figure 12, it can be seen that the main reason for the use of solar energy as input to drive absorption refrigeration systems is to attend to the residential and industrial sector demand for cooling and refrigeration. The figure also shows how important it is to design small components to compound the absorption chiller, search for different working fluids to operate them, and how the conventional solar and concentrated collector systems impact the technical and financial behavior of those absorption cycles.
Rivera and Rivera [
78] performed a theoretical study of an intermittent cycle operating with NH
3/LiNO
3, using solar energy as the driving source. The authors used data from Texmico, Mexico, and obtained a compound parabolic concentrator efficiency throughout the year with values between 0.33 and 0.78, and the system can produce up to 12 kg of ice when the temperatures of the generator and condenser are 120 and 40 °C, respectively.
Through theoretical analysis, Vasilecu and Infante-Ferreira [
75] analyzed a double-effect cooling system with the NH
3/LiNO
3 work pair driven by solar energy for industrial use. The study was conducted in a dynamic regime considering the Mediterranean summer conditions and horizontal parabolic solar collectors. The authors could conclude that the solar collector’s thermal energy could supply more than 50% of the thermal energy required for the system to operate at maximum cooling load.
Using experimental analysis, the prototype operating with NH
3/LiNO
3 developed by Rivera et al. [
107] has an operating capacity of 8 kg of ice per day using exclusively solar energy. The authors could observe a positive correlation between the COP and the solar radiation. In the same context, Moreno-Quinanar et al. [
24] compared the absorption chiller’s performance with the work pairs NH
3/LiNO
3 and NH
3/(LiNO
3 + H
2O), and driven by solar energy. A better COP was obtained with NH
3/(LiNO
3 + H
2O), 25% higher than the other mixture.
Llamas-Guillén et al. [
108] presented the results for a prototype absorption refrigeration system using the NH
3/LiNO
3 mixture. In an environment with a temperature of 25–35 °C, the system reached a temperature below 10 °C in the evaporator and 110°C in the generator, which was only possible to achieve due to high efficiency evacuated tube collectors. A COP between 0.3 and 0.4 and a thermal load of 4.5 kW was obtained.
The heating and cooling systems powered by solar energy have been proving their efficiency and flexibility, aiming at environmental aspects and saving energy consumption, by using renewable energy that does not harm the environment and saving electricity thanks to a total or partial activation related to solar irradiation. Besides, they have technical and functional feasibility since it represents an alternative to replace conventional energy sources such as fossil fuels and electricity.
A detailed approach on the subject was reported in the review study carried out by Skeikhani et al. [
33], where the authors discussed the contributions directed to cooling systems powered by solar energy and integrated with other energy auxiliary devices. The explanation of technologies for capturing solar irradiation, such as flat plate collector, evacuated tube collector, composite parabolic collector, and trough parabolic collector, was conducted. Essential technical and financial parameters (COP, annual energy consumption, payback period, solar implementation systems) were discussed, emphasizing the quality of the activation source used and the configuration effect on the absorption chillers. It is clear that chillers with triple-effect absorption present a better COP but need a higher hot source temperature (around 200–250 °C). This leads to more efficient but, at the same time, more complex and expensive solar collectors, such as evacuated and parabolic collectors. In the case of a single-effect absorption chiller with smaller COP (approximately 0.7) a simpler and cheaper collector can be used (flat plate type).
Sharma et al. [
32] presented a review aiming at the importance of these variables (selection of solar collectors and thermal storage) in the performance of absorption chillers powered by solar energy in a critical survey of studies of cooling systems by solar absorption. Based on the information, it was considered that the correct choice of the type of solar collector directly influences the efficiency of the absorption cooling system. Solar energy is intermittent. It is a significant factor in storing this energy and guarantees that the demand for activation source will be attended in periods without solar radiation (night) and moments when the chiller’s operating conditions are excellent. These storage tanks act as buffer absorbers and significantly improve the COP of the chiller.
Regarding the financial viability aspects, other factors might affect the return period of the implementation of the absorption system, such as project, climatic conditions, region, and thermal load, which makes it necessary to subsidize the fixed cost of absorption systems powered by solar energy to encourage the use of this technology and, reduce the return period.
Using a sensitivity analysis as a tool to evaluate the performance of solar absorption chillers at different operational conditions, Luna et al. [
109] carried out an experimental analysis of a 5 kW absorption refrigeration system that uses NH
3/H
2O as the working fluids, where the solar drive system consists of a field of 15 parabolic collectors with a reflective surface made of aluminum, and the absorber tubes made of copper, representing a total area of 38.4 m
2. Useful heat of up to 6.5 kW, energy, and exergetic efficiency of up to 20% and 15%, respectively, through the field of the collectors and activation temperatures of up to 105 °C were found and a COP of 0.56 and exergetic efficiency of 0.13 were obtained, considering activation temperatures between 85 and 95 °C, condensation temperatures between 20 and 28 °C, and chilled water up to 6°C.
Through the application of the First and Second Laws of Thermodynamics, absorption chillers of the solar-type can be analyzed considering (i) the type of fluid as a function of the thermal properties for the performance of the system [
110], (ii) the type of solar collector and function improving irradiation absorption [
111], (iii) the use of nanofluids as a way to increase the efficiency of heat and mass transfer [
112], and (iv) the use of integrated polygeneration systems [
113].
A double-effect absorption cooling system that uses NH
3/H
2O as an air-cooled working fluid has been proposed in Du et al. [
9] for small applications powered by solar heating. Using a prototype of a rated capacity of 2 kW, it was built to verify the feasibility and performance of this configuration. The system provided a uniform and constant behavior during the tests. The COP stabilized in a range of 0.18–0.25 in thermal comfort conditions in the summer season. According to the promising results, this prototype presents the possibility of developing small systems with low-cost solar energy activation for residential applications.
Pandya et al. [
110] presented a thermo-economic comparison of the use of two working fluids (sodium ammonia-thiocyanate and ammonium-lithium nitrate) to evaluate the performance of the 15 kW solar absorption refrigeration systems. Different solar collectors were used, such as flat plate, evacuated tubes, flat plate with parabolic reflectors, and parabolic collectors, integrated with a thermal storage tank. The comparison of COP as a function of working fluids showed that the NH
3/LiNO
3, coupled to the evacuated tube collectors, was superior to the arrangement with coupled flat plate collectors. Absorption chillers using NH
3/LiNO
3, together with parabolic collectors, showed higher values of cost and thermal efficiency by 23% and 0.7%, if compared to the system’s values integrated to the evacuated tube collectors. Considering the two working fluids’ performances, it is observed that the NH
3/LiNO
3 pair was superior to the NH
3/NaSCN in all the analyzed solar collector configurations. Therefore, considering the thermodynamic and economic results, using the NH
3/LiNO
3 mixture in an absorption chiller activated with evacuated tube collectors was recommended.
Likewise, using the same working fluids as Pandya et al. [
110], but considering the use of nanofluids in the solar capture system, Mody et al. [
112] carried out an energy analysis on solar absorption chiller to evaluate this addition of nanoparticles in the performance of thermal parameters, such as heat transfer coefficient, thermal efficiency, and useful heat gain of the collector. A maximum increase of 122% in the heat transfer coefficient was determined with 2% nanoparticles concentration, the heat transfer coefficient with the use of NH
3/NaSCN as the fluid with the best performance 0.12% higher compared to the use of NH
3/LiNO
3 as fluid. However, in the case of the average COP of the chiller, the use of NH
3/LiNO
3 was 6% higher than the use of NH
3/NaSCN. Thus, the use of NH
3/LiNO
3 is recommended in absorption refrigeration systems coupled to plate collectors with the addition of nanofluid.
Using the First and Second Laws of Thermodynamics as a technical evaluation tool, Khaliq et al. [
113] presented a study of a trigeneration system composed of a heliostat field (Duratherm600 oil), Rankine organic cycle, and a solar-powered absorption chiller (NH
3/LiNO
3) to produce cold demand at 0 °C. The study analyzed two hydrocarbons (isobutane and propane) as refrigerants in the Rankine organic cycle (ORC). The exergetic flow of the isobutane-operated trigeneration system was increased from 2562 to 4314 kW, while it increased from 1203 to 2028 kW when the system uses propane, considering that the normal direct irradiations were increased from 600 to 1000 W/m
2. The results showed that when the ORC uses isobutane as working fluids, 65% of the energy is transformed into useful output energy, and the remaining 35% is lost and exhausted to the environment. On the other hand, in exergetic terms, only 14% is transformed into useful exergy, 85% is destroyed due to irreversibility, and only 1% is transformed into exergetic losses.
Cerezo et al. [
114] developed a dynamic model by coupling two computational platforms, Equation Engineering Solver (EES) and TRaNsient System Simulation (TRNSYS), of a single-effect absorption chiller using five working fluids (NH
3/H
2O, H
2O/LiBr, NH
3/NaSCN, NH
3/LiNO
3, and H
2O/LiCl) driven by solar energy. The results showed that despite obtaining the best COP among all working fluids, due to the problem of crystallization of the solution, the H
2O/LiCl mixture obtained a maximum solar fraction of 0.67 and a minimum heating fraction of 0.33 with a maximum fraction of lost heat of 0.12. The NH
3/LiNO
3 and NH
3/H
2O mixtures obtained the most significant energy gain up to 6. Both got a maximum solar fraction of 0.91 and a minimum heating fraction of 0.09, using 89 and 100 m
2 of solar collector area.
Some disadvantages of solar absorption refrigeration systems are the complexity of these installations, the required area to capture useful energy for activation, installation control, the type of configuration if it is cold and heating, and, consequently, the high installation and operating costs. In this way, the search for more competitive technologies through hybrid configurations, and combined applications, can represent an effective alternative for the production of heat and air conditioning [
36].
The integration of the generator with a vapor separator from an absorption chiller that uses NH
3/LiNO
3 as the working fluid and considering a field of solar collectors reduces the monetary cost and makes the cooling and solar heating facilities more flexible was proposed by Lecuona-Neumann et al. [
111]. The flow established inside the linear receiver tube of the solar collector is driven by gravity and stratified in a counter-flow regime, and modeled in a one-dimensional way adapting convective boiling correlations and including modifications for the effects mixing. A low sensitivity was found to the chosen boiling heat transfer correlation in terms of heat and mass transfer. The theoretical and experimental results showed that the current use of the parabolic collector or Fresnel medium temperature solar collectors in the proposed flow layout was feasible since it allowed to produce vapor with efficiency similar to conventional type vapor generators, significantly if the subcooling length is minimized.
As it has been established and demonstrated, the use of solar energy as input for the cooling absorption systems has been a challenge due to the seasonality and periodicity of it, so the optimization and the uses an efficiency solar collector must be implemented to produce more activation heat, as the experimental studied conducted by Luna et al. [
109] showed, where a parabolic collector system was used to produce the heat to drive an ammonia/lithium nitrate absorption chiller with a 5 kW nominal capacity. The goal of them was to assess the operation of the chiller and its solar activation source considering several conditions on the weather of Cuernavaca, Mexico. Particularly comparison and evaluation of the solar absorption chiller, both of the systems (solar collector and absorption chiller) were considered coupled and uncoupled to see their effectiveness, using the first and the second Laws of Thermodynamic. It was determined that the integrated solar collector field system could generate up to 6.5 kW of heat, with up to 20% of thermal efficiency and exergy efficiencies up to 15%, at 105 °C temperature, which is good enough to drive a single effect absorption chiller. Regarding the absorption refrigeration systems, the cooling capacity produced could be up to almost 2 kW with almost 3.5 kW of input heat, with values COP up to 0.56. It was concluded, due of the results obtained, that the parabolic solar absorption cooling configuration system will be allowed to achieve air-conditioning demands with rational performance.
From the studies presented, it is evident that the use solar energy as driving input to activate the absorption refrigeration systems has been one of the main goals of investigations on absorption refrigeration systems, as confirmed by Sheikhani et al. [
33] and Sharma et al. [
32]. The motivation is that it allows reducing electrical consumption to activate cooling and air conditioning systems, using renewable energy as an input, and a decrease in the use of fossil resources [
109,
115]. However, this kind of resource is directly associated with specific regions and climatic conditions since solar energy can only be used directly during the day and depends on solar irradiation. It is, then, necessary to use storage systems, collection systems with better efficiency [
75,
78], and the use of considerable areas, depending on the cooling capacity and activation of the absorption systems [
114].
Due to these implications, several authors have directed efforts in the use of alternative mixtures to reduce the activation capacity, lower temperatures [
108], combining technologies as mechanical, ejector absorption cycles [
36], advanced technologies of heat exchanger [
111], the addition of substances [
112], among others, in order to activate absorption chillers exclusively/partially with solar energy [
113].
7. Ammonia/Absorbent Absorption Chiller Prototypes
In recent years, studies on the absorption cooling system using the binary mixture have taken a direction in designing and constructing prototypes [
27,
34,
116].
As mentioned, NH
3/H
2O is one of those usual working fluids of absorption chillers [
3,
9,
10,
117,
118]. That is why many authors are still researching to achieve better performances on different operation conditions. Hence, searching for better ways to minimize exergy destruction, Du et al. [
71] proposed a novel cycle considering the maximum internal heat recovery applying the pinch method technology. This application was able to verify that this proposed cycle’s performance was considerably improved, at least by 20%, compared with the traditional cycles, and even better at low evaporating temperature and when the highest generation temperature was considered. Du et al. [
70] also presented an analysis of the same system applying a graphical method to identify the characteristics of different cycles with different internal heat recovery strategies and find out the key points that significantly influence internal heat recovery.
After dealing with a single-effect, Du et al. [
72] continued to work using the pinch technology on a double-effect NH
3/H
2O ARS, aimed at the internal heat recovery of a mass-coupled considering freezing temperatures conditions to verify the reduction on the losses by irreversibility and to quantify the gain of uses of this kind of implementation on absorption chillers. The COP values from the derived refrigeration double-effect absorption chiller showed a significant increase, between 14% and 34%, under the tested conditions.
Currently, cooling demands are significant to the total energy consumption in buildings. Therefore, the focus on the design of more efficient and sustainable refrigeration systems is especially important, Neyer et al. [
116] using NH
3/H
2O as a working fluid, energetically and financially analyzed the influence of different heat rejection sources in a single-effect and half-effect ACH powered by solar or cogeneration energy. A functional chiller was developed and built based on flat plate heat exchangers. Models developed were used to simulate other operating conditions through the TRNSYS computational platform and evaluate the annual impact on the new single and half-effect absorption refrigeration prototypes. It was verified that the chiller presented a good and stable performance in different operating conditions, and the system was able to operate with a heat rejection temperature of up to 45 °C, which provides its use in hot and arid climates. Savings of primary nonrenewable energy of up to 70% were verified on this prototype powered by solar energy and cogeneration when compared to the conventional ones.
Considering the NH
3/LiNO
3 working fluids, Rivera et al. [
107] presented a performance analysis of the intermittent absorption cooling system. The developed prototype has a nominal capacity of 8 kg of ice per day and was based on the theoretical study developed by Rivera and Rivera [
78]. The measurements performed on the reported prototype evaporator presented temperatures as low as 11 °C obtained for several hours with solar coefficients of performance up to 0.08. It was verified that the coefficient of performance increased with the increase of solar radiation and the solution’s concentration, and there was no dependence on the coefficient of performance with the temperature of the cooling water. However, there were significant discrepancies between the numerical and experimental results of the CPC efficiency. This may be associated with inappropriate numerical correlations for the thermodynamic properties of NH
3/LiNO
3. Even with these errors, the proposed system could work exclusively with solar energy as the driving source and produces 8 kg of ice/day.
In Moreno-Quintanar et al. [
24], an experimental comparison of the solar-driven intermittent absorption cooling system developed by Rivera et al. [
107] considering the two mixtures NH
3/LiNO
3 and NH
3/(LiNO
3 + H
2O) was performed. The idea was to verify which mixtures present better performance. It was found that the evaporator temperature reaches 8 °C for 8 h driven exclusively by solar energy. The ternary mixtures’ system performance was better than the binary mixtures, obtaining a COP 25% higher. This increase in the COP may be related to the fact that the generator temperature was 5 °C lower than the binary mixture and the pressure reduces with the increase of water in the ternary mixture, reducing the pump’s consumption in comparison to the binary mixture.
Zacarías et al. [
119] performed an experimental analysis to evaluate ammonia and lithium nitrate solution’s adiabatic absorption. The authors used a flat plate absorber with a flat fan nozzle, and in the upstream, a single pass subcooler, obtaining a heat transfer coefficient twice as high as that obtained for the tubular vertical absorber in bubbles.
In Zacarías et al. [
120], a study of an absorber using NH
3/LiNO
3 solution was performed. The injection of the absorbent solution was performed through a mist injector. The adiabatic equilibrium factor was 3.7% higher, and the mass transfer coefficient was half of the value obtained in Zacarías et al. [
119], respectively. In both studies, correlations for the equilibrium factor and Sherwood’s number were presented.
Testing a prototype ARS, Hernández-Magallanes et al. [
25] analyzed a single-effect system with 3 kW nominal cooling capacity operating with NH
3/LiNO
3 as a working fluid designed and built for food conservation and air conditioning purposes. The developed prototype is presented in
Figure 13.
The generator and absorber are heat exchangers with internal coils and the condenser, evaporator, and solution HEX are compact plate heat exchangers. It has been reported that the system produces up to 3 kW of cooling capacity with a hot water temperature of 95 °C and can reach evaporator temperatures around 1 °C. Additionally, the COP can range from 0.45 to 0.70. It was found that the system can work with a hot water temperature of 80 °C, which is adequate for the use of solar energy as a driving source.
There were significant discrepancies between the numerical and experimental results that the authors attributed the components’ inefficiencies and the heat losses to the environment but did not mention the uses of the correlations, found from the literature, to determinate thermodynamic properties. The previous studies found in the literature [
19,
20,
21,
73,
74], confirmed the imprecision on thermodynamic properties’ use could bring inefficiencies of the ARS.
As mentioned before, there are no manufacturers that produce absorption chillers with these working fluids (binary or ternary solution), but there is specific research studying this type of absorption chiller. Zamora et al. [
26] developed two preindustrial absorption chillers, a water-cooled and an air-cooled one, both using welded plate heat exchangers.
Figure 14 shows the absorption chiller prototype’s schematic and the test bench installed at the Rovira and Virgili University in Spain, where the water circuits are presented and the part-load circuit configuration, adapted from Zamora et al. [
26,
27].
A new rotary pump replaced the circulation pump with lower energy consumption. The water-cooled prototype produced almost 13 kW of cooling capacity and an electric COP
elec of 19 when operating at 15, 90, and 35 °C of chilled, hot, and cooling water, respectively. In the air-cooled prototype, a cooling capacity of 9 kW and an electric COP
elec of 6.5 to 15, 90, and 35 °C of chilled, hot, and air temperatures, respectively.
Table 11 shows the coefficients of performance, thermally and electrically, achieved by this absorption chiller prototype considering the driving temperature and two evaporation temperatures and its operating condition.
The COP values, from a thermal point of view, are those expected for single-effect ARS reaching values of 0.5–0.6. However, what is essential to highlight were the COPs electrically achieved which represent significant values (varying from 19 to 27), making it an excellent alternative to ARS prototype. This system meets energetic demands for thermal comfort processes and even lower temperature systems, such as data centers.
Figure 15 shows the two absorption chiller prototypes’ global electricity consumption distribution presented in Zamora et al. [
26,
27].
It is possible to see in
Figure 15 that there is still a potential space for improving the performance of the prototype chillers due to the high electricity consumption of the cooling water pump. Hence, characterizing in partial load mode, Zamora et al. [
27] conducted an experimental test. The electrical performance coefficient’s partial load curve was obtained by adjusting the experimental data to the curve shape proposed in the standard prEN-14825:2011 for air–water chillers. These prototypes were described by the characteristic equation of the experimental data collected. The uses of welded plate heat exchanger have transitory response times similar to those of vapor compression machines, and the results in partial load operation achieve higher electric COP, where it was better to use an ON-OFF control than to modify the hot water temperature. These prototypes were intended to produce a commercial absorption cooling chiller for residential use, driven exclusively by solar sources. This was a project in cooperation with the Engineering Department of Rovira and Virgili University and CIATESA Corporation.
There is still no mass marketing of absorption refrigeration systems that use ammonia with different water absorbers. However, several studies have shown the technical and financial viability of some prototypes that allow the use of LiNO
3, LiNO
3 + H
2O, and others, as seen in Zamora et al. [
26], through the use of new configurations of absorbers [
29,
77,
119,
120], as the main component in the process of heat and mass transfer in absorption refrigeration systems [
93,
103,
104]. An important fact found in this critical survey of the state-of-the-art of refrigeration systems by absorption of ammonia/absorbents is the use of hot sources at low temperatures, aiming at the use of solar energy to activate the systems [
107] for different applications, such as food preservation [
25], ice production [
24], and air-conditioning [
25].
Therefore, the present work shows the importance of these applied materials in the pursuit of bringing to the market other sorbents that can help to reduce the size of the absorption refrigeration equipment [
27], as well as in energy flexibility with the complete introduction of solar energy as an activating source of absorption chillers [
25].