Amín Altamirano
email: amin.altamirano-cundapi@univ-smb.fr

Nolwenn Le Pierrès

Benoît Stutz

Benoit Stutz

Review of small-capacity single-stage continuous absorption systems operating on binary working fluids for cooling: Theoretical, experimental and commercial cycles

Keywords: Absorption chiller, Binary working fluid, Single-stage, New refrigerants, Thermodynamic comparison, COP

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Nomenclature and abbreviations

Energy demand is increasing at alarming rates. According to the International Energy Agency, the increase in the global energy demand between 2000 and 2040 is expected to be around 70%, and from the total energy consumed in 2040, 74% will still be from fossil fuels, whose combustion generates pollution and causes global warming [1]. Moreover, owing to human activity, there is an expected increase in the global average temperature in the range of 1.4-5.8°C for 2100 compared with 1990 [START_REF] Aliane | An illustrated review on solar absorption cooling experimental studies[END_REF], which will greatly impact the biggest single contributor to the world energy consumption and greenhouse emissions, the building sector [START_REF] Allouhi | Energy consumption and efficiency in buildings : current status and future trends[END_REF]. Today, in the United States, 40% of electricity during the summer is consumed by commercial buildings for air conditioning (AC) purposes [START_REF] Agrouaz | Case Studies in Thermal Engineering Energy and parametric analysis of solar absorption cooling systems in various Moroccan climates[END_REF]. New international agreements require the development of systems that contribute to the fight against climate change and global warming [START_REF] Calm | The next generation of refrigerants -Historical review, considerations, and outlook[END_REF].

Absorption systems replace the mechanical compression of standard refrigeration systems by thermochemical compression. These systems are still more expensive than common vapor compression systems. However, they will be of great interest in the near future because they are able to run from renewable energy and heat waste [6] [7]. Moreover, they are gaining acceptance and seem to be a true alternative for cooling residential, light commercial, and industrial applications [START_REF] Zhu | Second law-based thermodynamic analysis of ammonia/sodium thiocyanate absorption system[END_REF]. Their low operating cost has made them more attractive in recent years [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF]. These systems can use renewable energy (e.g., solar energy) as main the driving source, which could contribute significantly to the reduction of fossil fuel consumption and harmful emissions to the environment [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF]. The main advantage of absorption cooling systems over other heat-driven cooling technologies, such as adsorption, desiccant, or ejector systems, is their higher coefficient of performance (COP) [START_REF] Hassan | A review on solar cold production through absorption technology[END_REF] [START_REF] Best | A review of thermal cooling systems[END_REF].

Considering their thermodynamic cycle arrangement, absorption refrigeration systems can be divided into three categories: single-effect, half-effect, and multi-effect (e.g. double-effect or triple-effect) cycles [12] [13]. The single-effect and half-effect cycles require lower driving temperatures compared with the multieffect cycles [START_REF] Aliane | An illustrated review on solar absorption cooling experimental studies[END_REF]. From all the possible configurations, the single-stage is the most advantageous in terms of cost, low driving temperatures, and simple configuration [START_REF] Hassan | A review on solar cold production through absorption technology[END_REF]. At the same time, it is the most commercially available. Two absorption pairs are commonly encountered: H2O-LiBr (water-lithium bromide) and NH3-H2O (ammonia-water). A typical single-stage absorption system consists of an evaporator, a condenser, an absorber, a generator (sometimes also called a desorber), a solution pump, a solution heat exchanger (SHX), and two valves, as shown in Figure 1. The solution at the generator absorbs heat and desorbs refrigerant vapor. This vapor is condensed at the condenser before passing through a throttle valve and being vaporized again at a lower pressure and a lower temperature at the evaporator. The evaporation heat is taken from the cold source. The vapor produced by the evaporator is absorbed by the weak solution (in refrigerant) in the absorber. The resulting rich solution is pumped from the absorber to the generator, where the cycle starts again [START_REF] Best | A review of thermal cooling systems[END_REF]. The solution heat exchanger (SHX) is added between the absorber and the generator to increase the efficiency of the machine by recovering internal heat. When a volatile solution is used, such as ammonia/water, a vapor purification process has to be added after the generator to improve the quality of the refrigerant [START_REF] Ullah | A review of solar thermal refrigeration and cooling methods[END_REF] [START_REF] Bogart | Ammonia Absorption Refrigeration in Industrial Processes[END_REF]. A refrigerant heat exchanger (RHX) can also be added as an internal heat recovery method, but the effect of the SHX in the overall system performance is more important [START_REF] Herold | Absorbtion Chillers and Heat Pumps[END_REF] [START_REF] Shiran | Computerized Design and Economic Evaluation of an Aqua-Ammonia Solar Operated Absorption System[END_REF]. Moreover, a storage tank can be implemented to absorb the heat rate variation at the generator [START_REF] Aliane | An illustrated review on solar absorption cooling experimental studies[END_REF].

Figure 1: Schematic of a single-effect solar absorption chiller (adapted from [START_REF] Aliane | An illustrated review on solar absorption cooling experimental studies[END_REF]).

Absorption systems can be categorized according to different variables. For example, their firing method, driving source, the number of effects and stages, heat sink type, working fluid, application, capacity, or continuous/discontinuous operation. Usually, the reviews available in the literature focus on one of these characteristics or even multiple characteristics, providing information in general about absorption systems [START_REF] Sun | A review of working fluids of absorption cycles[END_REF] [19] [START_REF] Srikhirin | A review of absorption refrigeration tehnologies[END_REF], a specific application [START_REF] Rivera | A review of absorption heat transformers[END_REF], a specific driving source [START_REF] Aliane | An illustrated review on solar absorption cooling experimental studies[END_REF] [10] [START_REF] Fan | Review of solar sorption refrigeration technologies: Development and applications[END_REF] [23] [START_REF] Siddiqui | A review of solar powered absorption systems[END_REF] [25] [START_REF] Alobaid | A review of solar driven absorption cooling with photovoltaic thermal systems[END_REF] [27]

[28] [START_REF] Al-Alili | Review of solar thermal air conditioning technologies[END_REF], the application of these systems in specific regions of the world [START_REF] Jaruwongwittaya | A review: Renewable energy with absorption chillers in Thailand[END_REF] [31], as part of reviews including other thermally driven technologies [START_REF] Best | A review of thermal cooling systems[END_REF] [14] [32] [START_REF] Kim | Solar refrigeration options -a state-of-the-art review[END_REF], and about specific families of working pairs [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF] [34] [START_REF] Zheng | A review of imidazolium ionic liquids research and development towards working pair of absorption cycle[END_REF]. These variables make the task of comparing the different working pairs complicated, and very rarely are specific conclusions about their advantages and disadvantages. Moreover, it is not usually stated whether the tested systems are commercially available machines or experimental prototypes, which is an important factor for comparison.

As stated earlier, absorption systems will receive increasing attention over the coming years. More specific reviews are needed to make a more complete, in-depth, and comprehensive comparison between the systems and the cycle performance of the different working pairs. Therefore, the present review aims to provide an up-to-date global overview of the small-capacity single-stage continuous absorption systems operating on binary working fluids for cooling from the theoretical, experimental, and commercial points of view. The upper limit for the term "small-capacity", depending on the author, can go from 20 to 100 kW [19] [31] [36] [START_REF] Kühn | Thermally driven heat pumps for heating and cooling[END_REF]. For the sake of the present review, only systems with a capacity of 50 kW or less were investigated. Special focus is placed on comparing the cycle performance between the different families of refrigerants and working pairs as the choice of working fluid directly affects the performance of an absorption cycle [START_REF] Perez-Blanco | Absorption heat pump performance for different types of solutions[END_REF] [START_REF] Hassan | A review on solar cold production through absorption technology[END_REF]. The article provides useful information about the cycles performance and the challenges faced by researchers in the development of such systems. It is not in the scope of the review to compare the exchanger technologies, brands, or configurations. The main objective is to provide helpful information for a good understanding of the development of these systems over the past 40 years and of the main advantages and disadvantages of using different working pairs.

Comparative criteria

The performance of absorption systems depends on different factors and can be measured according to different criteria [START_REF] Pons | Performance comparison of six solar-powered air-conditioners operated in fi ve places[END_REF]. These factors depend on internal parameters such as the working pair, the heat exchanger technology, or the monitoring procedure; and external parameters, such as the specific climate conditions or the type of source (liquid, solid or gas), which directly impact the operating temperatures and thermal transfers in the system. Compared with conventional compression systems, which possess two temperature sources (the heat rejection temperature and the cooling temperature), absorption cooling systems have three temperature levels. In addition to the heat rejection and the cooling temperature, there is an additional high-temperature driving source. All three temperature sources are of great importance for the system's performance. In the case of air-cooled systems, the ambient temperature is fundamental (as it cannot be modified). While the performance of conventional compression AC is generally measured in terms of their electric coefficient of performance (COPelect, defined as the cold production divided by the electricity consumption), the performance of absorption cooling systems is usually measured by another coefficient of performance (COP) defined by Eq. 1, Some authors use the thermal COP (COPth), which neglects the effect of the pumping power, because in general, this parameter does not significantly affect the COP. However, some studies have shown that electricity consumption in absorption systems might be nonnegligible and therefore the COPelect has earned a relevant importance in some studies [40] [41].

= (1)
Apart from the COP, which can be optimized by means of the first law (conservation of energy), the second law (quality of energy and material) can be used to maximize the exergetic efficiency and minimize the entropy generation within the system [START_REF] Alefeld | Heat Conversion Systems[END_REF]. The exergetic efficiency represents the capacity of the system to avoid the destruction of available energy and is defined as the ratio between the recovered exergy at the evaporator and the provided exergy at the desorber [START_REF] Kilic | Second law-based thermodynamic analysis of water-lithium bromide absorption refrigeration system[END_REF]. However, for the sake of the present review, special focus is given to the COP as the main parameter for comparing the different systems. Another important parameter is the Carnot COP, which represents the maximum possible coefficient of performance of the absorption cooling system, a function of the system's operating temperatures. This is defined by Eq. 2. The COPcarnot implies a reversible process with isothermal heat transfers at the three temperature levels, which ideally requires infinite energy sources. Therefore, for an ideal system, the temperatures from the external heat transfer fluids should be taken into account at the inlet of the generator (Tg), at the outlet of the evaporator (Te), and at the inlet of the absorber and the condenser (Ta and Tc), or in the case of air-cooled systems, the ambient temperature (Tamb). In most experimental studies, the same heat sink source is used in parallel for the intermediate temperature, and in these cases, some authors use the term "intermediate temperature" or "heat sink temperature" (Ta=Tc=Tint=Tsink). The external temperatures data at the nominal conditions of absorption chillers is provided by the manufacturers in their catalogs. However, theoretical studies generally use the temperatures of the internal working pair or refrigerant at the exit of the components in order to neglect the irreversibilities of a real machine. On the other hand, prototypes in the literature provide information depending on the research objectives and the specific configuration and instrumentation implemented in the machine (internal or external temperatures located in different sections), making it difficult to standardize the data. Therefore, the authors carefully extracted this information with the best possible accuracy if the values were not directly given and when the provided information allowed to, adding some degree of inconsistency, to present the data contained in the tables and figures of the present review. Hence, these data may be interpreted with care and the reader must refer to the source if more detailed information is required.

= (2)
Absorption systems have been present for more than a century. Currently, different research teams are focusing on improving these systems to reduce their manufacturing costs, complexity, volume, maintenance, and environmental concerns. One example of this is the search for the perfect refrigerant, for which, besides good thermal properties, the concern of its environmental impact has increased in recent years [START_REF] Calm | The next generation of refrigerants -Historical review, considerations, and outlook[END_REF]. The choice of refrigerant is related to the specific requirements of the envisaged application and has a strong impact on the system's exchangers technology and configuration [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF]. Moreover, different from conventional vapor compression systems, the study of alternative refrigerants in absorption systems leads to another important field of study which is the investigation of the best adapted refrigerant-absorbent combination (working pair) [START_REF] Herold | Absorbtion Chillers and Heat Pumps[END_REF].

3 Ammonia-based working fluids

The general advantages of NH3 over H2O as the refrigerant are its low freezing point (-77°C) [START_REF] Wang | State-of-the-art review on crystallization control technologies for water/LiBr absorption heat pumps[END_REF] and the higher working pressures, which allow for negative cooling temperatures and eliminate the need for special and expensive equipment for vacuum conditions [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF]. The present chapter covers the theoretical, experimental, and commercial systems operating on NH3-based systems.

Conventional ammonia-based working pair: NH3-H2O

NH3-H2O is, together with H2O-LiBr, the most widely used working fluid in absorption systems [44][16].

It is commonly used for applications where negative refrigeration temperatures are required [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF]. Compared with H2O-LiBr, it allows for a large range of operation with no risk of crystallization. However, a vapor purification process is needed at the output of the generator to remove the remaining water that evaporates together with the refrigerant [9][45][20]. This process is composed of a distillation column with partial or total condensation, a rectifier (externally cooled device to condense water vapor), or a combination of both [START_REF] Bogart | Ammonia Absorption Refrigeration in Industrial Processes[END_REF]. The term "rectifier" is commonly used to refer to the vapor purification process in general. Indeed, a poor vapor purification would lead to a low system performance due to the presence of water in the evaporator [START_REF] Bogart | Pitfalls in ammonia absorption refrigeration[END_REF], reducing the COP by over 30% and requiring higher driving temperatures [START_REF] Dardouch | Study of an Absorption Refrigeration Machine Improved with Distillation Column[END_REF].

NH3-H2O: Theoretical studies

Some theoretical studies have focused on identifying the parameters that most impact the performance of absorption systems. Very high Tg negatively impact the COP, in which case an increase in the Tc improves it [START_REF] Kaushik | Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems[END_REF]. On the other hand, at low Tg, a decrease in the Ta and Tc improves the COP [START_REF] Best | Thermodynamic design data for absorption heat pump systems operating on ammonia-water-Part I. Cooling[END_REF] [50], with Ta having the highest impact, and thus Ta<Tc is more preferable [START_REF] Kaushik | Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems[END_REF].

Other studies have focused on analyzing the optimum operating conditions of these systems. In the case of negative refrigeration, Alvares and Trepp (1986) concluded that for a refrigeration temperature of -10°C, a single-stage system with a rectifier, SHX, and RHX could reach a maximum COP of 0.6021 at Tg=120°C, Pe=2.85 bar, and a Tint=25°C [START_REF] Alvares | Simulation of a solar driven aqua-ammonia absorption refrigeration system Part 1: mathematical description and system optimization[END_REF]. [START_REF] Clerx | Performance of an aqua-ammonia absorption solar refrigerator at subfreezing evaporator conditions[END_REF] performed a computer-aided thermodynamic analysis and concluded that the evaporator pressure must be between 0.21 and 0.41 MPa to maintain negative evaporator temperatures, while the generator pressure must be between 1.38 MPa (to force a Tint of over 37.8°C and use air-cooled condensers) and 2.04 MPa (for a lower maintenance and safety requirements), the Tg between 82.2°C (to maintain an acceptable COP over 0.2) and 204°C (due to the maximum pressure determined before and also to avoid excessive water evaporation), and the preheater, SHX, and rectifier must have at least an effectiveness of 70% to maintain a COP of no less than 0.5 for ice production [START_REF] Clerx | Performance of an aqua-ammonia absorption solar refrigerator at subfreezing evaporator conditions[END_REF].

In the case of positive cooling, [START_REF] Shiran | Computerized Design and Economic Evaluation of an Aqua-Ammonia Solar Operated Absorption System[END_REF] developed a program that evaluates the cooling and heating demands of buildings depending on the location and design the absorption cycle along with the solar collectors and auxiliaries [START_REF] Shiran | Computerized Design and Economic Evaluation of an Aqua-Ammonia Solar Operated Absorption System[END_REF]. Other studies centered on observing the behavior of the system in local geographic conditions [START_REF] Kouremenos | PERFORMANCE OF SOLAR NH3/H2O ABSORPTION CYCLES IN THE ATHENS AREA[END_REF] or in a specific working range [54] [55]. Special configurations have also been theoretically analyzed. [START_REF] Uppal | A Low-Cost Solar-Energy Stimulated Absorption Regrigerator for Vaccine Storage[END_REF] proposed a system with no moving parts for vaccine storage in remote locations [START_REF] Uppal | A Low-Cost Solar-Energy Stimulated Absorption Regrigerator for Vaccine Storage[END_REF], and [START_REF] Ahachad | Study of an improved NH3H2O solar absorption refrigerating machine in Rabat (Morocco)[END_REF] proposed to substitute the rectifier column by a bubble exchanger fed with the strong solution coming out of the absorber (see Figure 2a); the proposed system showed an improvement in the COP ranging from 15% to 35% [START_REF] Ahachad | Study of an improved NH3H2O solar absorption refrigerating machine in Rabat (Morocco)[END_REF].

Finally, optimization studies are also available in the literature. [START_REF] Bulgan | Optimization of the thermodynamic model of aqua-ammonia absorption refrigeration systems[END_REF] observed that at fixed Ta and Tc, for each ammonia concentration level in the refrigerant and varying the Tg, there is a local COPmax. A theoretical maximum COP of 0.888 could be achieved [START_REF] Bulgan | Optimization of the thermodynamic model of aqua-ammonia absorption refrigeration systems[END_REF]. Le [START_REF] Lostec | Finite time thermodynamics study and exergetic analysis of ammonia-water absorption systems[END_REF] presented an optimization study for a system under steady-state conditions. Three optimum COP values were identified: a COP of 0.56 with a minimum heat exchange surface, a COP of 0.62 with a minimum overall irreversibility, and a COP of 0.69 with a maximum exergetic efficiency. Finally, the destruction of exergy takes place mostly in the absorber and the desorber [START_REF] Lostec | Finite time thermodynamics study and exergetic analysis of ammonia-water absorption systems[END_REF].

NH3-H2O: Experimental prototypes development and experimentation

A summary of the small-capacity prototypes of NH3-H2O absorption cooling systems developed in laboratories and their operating conditions is presented in Table 1. The prototypes described in the literature have often been built for the use of renewable energy sources. A prototype for the use of geothermal energy was constructed at the Cerro Prieto Geothermal Field (Mexico) to cool a small storage unit at negative temperatures. The system provided evaporating temperatures of -10°C at generator temperatures of 125°C [START_REF] Delgado | An experimental study of heat driven absorption cooling systems[END_REF]. The use of solar energy for these systems is also of great interest. De [START_REF] Francisco | Development and testing of a prototype of low-power water-ammonia absorption equipment for solar energy applications[END_REF] simulated and constructed a 2-kW air-cooled (by natural convection) solar system (with SHX and rectifier). The experimental COPs were lower than 0.05, compared with 0.53 in the simulations. There were expected improvements in the solar collectors, new fans for forced convection, an ice storage tank, a new pump system, and a new automatic control process [START_REF] Francisco | Development and testing of a prototype of low-power water-ammonia absorption equipment for solar energy applications[END_REF]. [START_REF] Sözen | Development and testing of a prototype of absorption heat pump system operated by solar energy[END_REF] developed and tested a solar prototype in Ankara (Turkey) driven by a parabolic collector. The system reached cooling temperatures as low as 3°C, and the evaporator and absorber possessed the highest exergy losses [START_REF] Sözen | Development and testing of a prototype of absorption heat pump system operated by solar energy[END_REF]. [START_REF] Mendes | A rich solution spray as a refining method in a small capacity, single effect, solar assisted absorption machine with the pair NH3/H2O: Experimental results[END_REF] designed and constructed a solar system with SHX and RHX in which the rectification column was substituted by a refining spray of rich solution directly integrated in the desorber (see Figure 2b). The system showed an increase of 1% in ammonia content in the refrigerant after the refining process (before refining, the NH3 content was in the range of 95.5-97.6%) at nominal conditions of 4 kW of cooling with a COP of 0.54 [START_REF] Mendes | A rich solution spray as a refining method in a small capacity, single effect, solar assisted absorption machine with the pair NH3/H2O: Experimental results[END_REF]. [START_REF] Said | Analysis of a solar powered absorption system[END_REF] designed and simulated a solar system (based on a design presented by the University of Stuttgart [63] [64]) that recovered waste heat from the rectifier and contained a refrigerant storage unit. A total increase of 18% in the COP was observed compared with a conventional generator/rectifier configuration [START_REF] Said | Analysis of a solar powered absorption system[END_REF] and the system (installed in Saudi Arabia) delivered cooling capacities between 4 and 10 kW with COP values between 0.4 and 0.7 [START_REF] Said | Design, construction and operation of a solar powered ammonia-water absorption refrigeration system in Saudi Arabia[END_REF]. Another innovative proposal was made by [START_REF] Zotter | Experimental analysis of a novel concept of a "thermally driven" solution pump operating a small-capacity ammonia/water absorption heat pumping system[END_REF], who adapted a commercial PinkChiller PC19 with a thermally driven solution pump (instead of an electrically driven pump).

Thermally driven pumps can be a suitable option owing to the lower production cost, fewer leakage problems, and no electricity consumption; however the experimental system's COP was reduced by at least 0.1 (15%) compared with the system using an electrical pump [START_REF] Zotter | Experimental analysis of a novel concept of a "thermally driven" solution pump operating a small-capacity ammonia/water absorption heat pumping system[END_REF]. [START_REF] Boudéhenn | Development of a 5 kW cooling capacity ammonia-water absorption chiller for solar cooling applications[END_REF] developed and constructed a compact and low-cost 5-kW system at INES (French National Institute for Solar Energy). The system, which uses compact plate heat exchangers (PHEs) available on the market, showed good potential. Nonetheless, further optimization opportunities exist for the SHX and the absorber [START_REF] Boudéhenn | Development of a 5 kW cooling capacity ammonia-water absorption chiller for solar cooling applications[END_REF]. refining spray directly integrated in the desorber [START_REF] Mendes | A rich solution spray as a refining method in a small capacity, single effect, solar assisted absorption machine with the pair NH3/H2O: Experimental results[END_REF].

Other uses of NH3-H2O absorption chiller prototypes are miniaturized and mobile applications. Seeking to make use of the exhaust gases from trucks equipped with refrigeration systems for food transport, Koehler et al. simulated, constructed, and tested a system that delivered sufficient cooling capacities to be in compliance with the German Industrial Standard DIN 8959 [START_REF]Normblatt DIN 8959: Prüfung von wärmegedämmten Beförderungsmitteln für Lebensmittel[END_REF], but with low COP values due to the airtransfer medium in all the exchangers of the system [START_REF] Koehler | Absorption refrigeration system for mobile applications utilizing exhaust gases[END_REF]. Determan and Garimella (2012) presented the design, fabrication and experimental demonstration of a microscale monolithic absorption heat pump with a nominal cooling capacity of 300 W. All the exchangers were fabricated in a single manufacturing process, reducing working fluid inventory and leakage. The system delivered cooling capacities of 136-300 W and COPs in the range of 0.25-0.43. However, the desorber and rectifier had to be improved to increase the system's COP [START_REF] Determan | Design, fabrication, and experimental demonstration of a microscale monolithic modular absorption heat pump[END_REF].

NH3-H2O Commercial systems: Presentation and experimentation

The absorption systems market has historically witnessed a slow but constant evolution at varying paces depending on the geographical region since the situation changes in terms of economics, needs, and governmental regulation [START_REF] Herold | Absorbtion Chillers and Heat Pumps[END_REF]. A summary of the companies producing small-scale NH3-H2O absorption cooling systems (to the best of our knowledge) is presented in Table 2, and the experiments performed on these systems available in the literature are presented in Table 3 (only the ones with enough data to be represented in the table are shown). The Robur brand was first investigated by [START_REF] Lazzarin | Ammonia-water absorption machines for refrigeration: Theoretical and real performances[END_REF], who simulated the performance of a 17.5-kW air-cooled machine with a rectifier and RHX [START_REF] Lazzarin | Ammonia-water absorption machines for refrigeration: Theoretical and real performances[END_REF] by using a linear relationship between the inverse COP and a dimensionless temperature ratio (function of the inlet temperatures of the three temperature levels and the outlet temperature of the evaporator temperature) first proposed by Ng et al.

(see Eqs. 3 and 4) [START_REF] Ng | Theoretical and experimental analysis of an absorption chiller[END_REF]. In Eq. 4, & and ' are variables that depend on the operating temperatures and irreversible losses. The predicted results were close to the experimental ones, except for extreme temperature values (high Tint or low Te). Horuz and Callander modified a 10-kW Robur Servel ACD-3600 gas-fired system by replacing the air-cooled heat sink by a water-cooled one. The system performed slightly better (less than 10%) with the water-cooled heat exchanger [START_REF] Horuz | Experimental investigation of a vapor absorption refrigeration system[END_REF].

( * = ,+ ,+ (4)
Another tested brand was SolarNext and their models Chilli PSC10 (10 kW) and Chilli PSC12 (12 kW), which have shown good experimental agreement in some studies [86] [87]. Le Lostec et al. ( 2012) tested the PSC10 model and observed that when Te decreases, the COP decreases significantly, perhaps due to an evaporator overfeed. Cooling capacity oscillations due to refrigerant expansion control were also observed. Finally, the chiller's performance is also impacted by the generator, condenser, and absorber temperatures as well as the solution flow [START_REF] Lostec | Experimental study of an ammonia-water absorption chiller[END_REF].

NH3-LiNO3 and NaSCN

Some alternative absorbents to replace water have been proposed so as to avoid the need for a rectifier in ammonia-based systems and at the same time to improve other properties such as the solubility in ammonia, less corrosion of steel, and lower generator temperatures [START_REF] Zhu | Second law-based thermodynamic analysis of ammonia/sodium thiocyanate absorption system[END_REF][89] [START_REF] Mansoori | Thermodynamic basis for the choice of working fluids for solar absorption cooling systems[END_REF]. Ammonia/salt refrigeration systems seem to be the most plausible application in refrigeration units with small cooling capacity, thanks to their low driving temperature and good adaptability for high sink temperatures (Ta/Tc) [START_REF] Cai | Thermodynamic analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle driven by low grade energy[END_REF]. The most popular salts are NaSCN and LiNO3 [START_REF] Blytas | Concentrated Solutions of NaSCN in Liquid Ammonia. Solubility, Density, Vapor Pressure, Viscosity, Thermal Conductance, Heat of Solution and Heat Capacity[END_REF][92] [START_REF] Srikhirin | A review of absorption refrigeration tehnologies[END_REF]. According to Ferreira, the systems based on these working fluids both present COPs about 10% (absolute) higher than the ones with NH3-H2O [START_REF] Ferreira | Thermodynamic and physical property data equations for ammonia-lithium nitrate and ammonia-sodium thiocyanate solutions[END_REF].

NH3-LiNO3: Theoretical studies and experimental prototypes

LiNO3 has been more extensively studied than NaSCN [START_REF] Sc | Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents[END_REF], probably because it was first discovered [START_REF] Cai | Experimental evaluation on thermal performance of an air-cooled absorption refrigeration cycle with NH3-LiNO3 and NH3-NaSCN refrigerant solutions[END_REF] or because it possesses a wider range of operating conditions [START_REF] Davis | Vapor Pressure of Lithium Nitrate: Ammonia System[END_REF]. This salt was already mentioned as an absorbent for NH3 in the GB358844 (A) patent filled by Electrolux Ltd in 1930 [START_REF] Ltd | Improvements in or relating to absorption refrigerating apparatus[END_REF]. LiNO3 is slightly toxic, noncombustible, stable, and noncorrosive to stainless steel [START_REF] Heard | Carbon and stainless steel in solutions of lithium nitrate in ammonia at moderate temperatures[END_REF]. If appropriate security measurements are taken, the resulting system is considered safe [START_REF] Cai | Exergy analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle with NH3-NaSCN and NH3-LiNO3 refrigerant solutions[END_REF]. NH3-LiNO3 has presented some disadvantages during experiments such as low heat and mass transfer (caused by elevated viscosity) in the absorber and desorber, which results in lower experimental performance than expected [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF][100]. [START_REF] Best | Thermodynamic design data for absorption heat pump systems operating on ammonia-lithium nitrate-Part One[END_REF] presented a simulation for systems using NH3-LiNO3 as the working fluid and provided the thermodynamic design data for this kind of system. They obtained thermal COPs between 0.012 and 0.462 [START_REF] Best | Thermodynamic design data for absorption heat pump systems operating on ammonia-lithium nitrate-Part One[END_REF].

A summary of the small-capacity prototypes of NH3-LiNO3 absorption cooling systems developed in laboratories and their operating conditions is given in Table 4. To the best of our knowledge, the first experimental study on an NH3-LiNO3 system was reported in the PhD thesis of Infante [START_REF] Ferreira | VERTICAL TUBULAR ABSORBERS FOR AMMONIA-SALT ABSORPTION REFRIGERATION[END_REF], who constructed a single-stage chiller (with RHX and SHX) that provided between 0.8 and 3.69 kW of cooling capacity [START_REF] Ferreira | VERTICAL TUBULAR ABSORBERS FOR AMMONIA-SALT ABSORPTION REFRIGERATION[END_REF]. One decade later, [START_REF] Heard | An experimental comparison of an absorption refrigerator using ammonia/water and ammonia/lithium nitrate[END_REF], adapted the 10.5-kW NH3-H2O absorption chiller presented in [START_REF] Delgado | An experimental study of heat driven absorption cooling systems[END_REF] to use NH3-LiNO3 instead. The system provided cooling temperatures between -10 and 0°C with COPs between 0.3 and 0.6. Moreover, the cooling capacity was reduced by about half of the design value (with the NH3-H2O working pair) [103] [104]. Some years later, other experimental prototypes were presented. For example, the HVAC manufacturer CIAT and the Rovira i Virgili University conducted a project to develop a 10-kW system operating with the NH3-LiNO3 working pair. Two pre-industrial prototypes were developed, a water-cooled (of 12.9 kW) and an air-cooled system (of 9.3 kW), both using brazed plate heat exchangers (BPHEs) in all the thermal components [START_REF] Zamora | A new ammonia/lithium nitrate absorption chiller for solar cooling applications[END_REF]. At Tamb=41°C, the air-cooled chiller could still provide 64% of its nominal capacity (at Tamb=35°C) [START_REF] Zamora | Pre-industrial development and experimental characterization of new air-cooled and water-cooled ammonia/lithium nitrate absorption chillers[END_REF]. [START_REF] Zamora | Part-load characteristics of a new ammonia/lithium nitrate absorption chiller[END_REF] later determined the performance characteristics of the water-cooled prototype and concluded that at part-load operation, it is much more convenient to use an ON-OFF control than to change the hot water temperature; in this way, the behavior of the system was more agile and responsive [START_REF] Zamora | Part-load characteristics of a new ammonia/lithium nitrate absorption chiller[END_REF]. In a different context, Llamas-Guillén et al. (2014) changed the evaporator of an air-cooled 5-kW system (first presented in [START_REF] Llamas | Development of a small capacity ammonia-lithium nitrate absorption refrigeration system[END_REF]) to improve its performance and compactness [START_REF] Best | Two novel solar sorption refrigerators using salt-ammonia mixtures for air-conditioning and ice production. 30th ISES Bienn Sol World Congr[END_REF]. The first prototype, which included an air-cooled evaporator, did not yield useful cooling temperatures. During experiments, the new prototype produced evaporator temperatures below 10°C with COPs in the range of 0.3-0.4 and a maximum cooling capacity of 4.5 kW [START_REF] Llamas-Guillén | Experimental results of a direct air-cooled ammonia-lithium nitrate absorption refrigeration system[END_REF]. Hernández-Magallanes et al. ( 2014) presented the development and experimental assessment of a 3-kW absorption solar cooling system. The system delivered up to 2.7 kW of cooling and a minimum evaporator temperature of 1°C. The experimental COP ranged between 0.45 and 0.7 [START_REF] Hernández-Magallanes | Experimental assessment of an absorption cooling system operating with the ammonia/lithium nitrate mixture[END_REF]. Domínguez-Inzunza et al. (2016) reported on an experimental assessment of a system using a falling film shell-and-tube absorber and generator. The system achieved a cooling capacity of 4.5 kW at a generator temperature of 100°C. Evaporator temperatures as low as 4°C were obtained, and the COP ranged from 0.27 to 0.62 [START_REF] Domínguez-Inzunza | Experimental assessment of an absorption cooling system utilizing a falling film absorber and generator[END_REF]. Finally, Jiménez-García and Rivera designed and constructed a system using only PHE in the thermal components. The system achieved a cooling power of 3.1 kW, cooling temperatures as low as 6°C, and a COP or around 0.62 [START_REF] Jiménez-García | Parametric analysis on the performance of an experimental ammonia/lithium nitrate absorption cooling system[END_REF]. Cai et al. (2016) presented a system designed to operate with NH3-salt pairs. It was tested both with LiNO3 and NaSCN. These results are presented in Section 3.4 as a comparison between the two working pairs.

NH3-NaSCN: Theoretical studies and experimental prototypes

NaSCN was first considered as a possible absorbent for the NH3 refrigerant by Blytas and Daniels in 1962 [START_REF] Blytas | Concentrated Solutions of NaSCN in Liquid Ammonia. Solubility, Density, Vapor Pressure, Viscosity, Thermal Conductance, Heat of Solution and Heat Capacity[END_REF]. It is inexpensive, nonexplosive, chemically stable, and noncorrosive on iron, steel, and copper [START_REF] Tyagi | Ammonia-salts vapour absorption refrigeration systems[END_REF] [114]. NH3-NaSCN seems to perform better than the NH3-LiNO3 working pair, especially at high generator temperatures. Indeed, the COPs of this working fluid are about 10% higher than the ones of the NH3-H2O system working under the same conditions [START_REF] Zhu | Thermodynamic analysis of a novel thermal driven refrigeration system[END_REF]. However, the system is limited to working at evaporator temperatures over -10°C to avoid crystallization [116][117]. NaSCN is considered a moderately toxic noncombustible material. Its use in absorption systems is considered safe if the appropriate security measures are taken [START_REF] Cai | Exergy analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle with NH3-NaSCN and NH3-LiNO3 refrigerant solutions[END_REF]. The properties of this working pair have been studied and presented by different authors [START_REF] Sun | Comparison of the performances of NH3-H2O, NH3-LiNO3 and NH3-NaSCN absorption refrigeration systems[END_REF][89] [START_REF] Ferreira | Thermodynamic and physical property data equations for ammonia-lithium nitrate and ammonia-sodium thiocyanate solutions[END_REF]. [START_REF] Best | Thermodynamic design data for absorption heat pump systems operating on ammonia-sodium thiocyanate-I. Cooling[END_REF] presented thermodynamic design data for an absorption cooling system using NH3-NaSCN as a working fluid. They obtained a range of COPs between 0.123 and 0.555 in the simulated conditions [START_REF] Best | Thermodynamic design data for absorption heat pump systems operating on ammonia-sodium thiocyanate-I. Cooling[END_REF]. A summary of the small-capacity prototypes of NH3-NaSCN absorption cooling systems developed in laboratories and their operating conditions is presented in Table 5. Zhu et al. ( 2008) presented a theoretical and experimental study on the characteristics and performance of an NH3-NaSCN system with SHX [START_REF] Zhu | Performance investigation of a thermal-driven refrigeration system[END_REF]. This system can work with generator temperatures over 58°C (see Figure 3a). Moreover, it can operate at high condenser and absorber temperatures (around 23°C) with low evaporator temperatures and reasonable COPs (around 0.35, see Figure 3b). Their simulated versus experimental COP comparison is shown in Figure 3b; the discrepancies are mainly due to thermal losses and the mass transfer resistance of vapor ammonia in the absorber, not considered in the simulation. Zhu and Gu (2010) developed a second law-based thermodynamic analysis for the same system and concluded that the COP increases if Tg and Te increase, while it decreases if Ta and Tc increase. On the other hand, when the COP increases, the second law efficiency may decrease mainly because of the exergy loss in some components [START_REF] Zhu | Second law-based thermodynamic analysis of ammonia/sodium thiocyanate absorption system[END_REF]. Cai et al. later reported that the mean ionic activity coefficient of NaSCN in liquid ammonia had not been provided before and that the one used for the last analysis by Zhu and Gu seemed to be that of the LiBr in water, consequently, the entropy calculation results might be inaccurate [START_REF] Cai | Exergy analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle with NH3-NaSCN and NH3-LiNO3 refrigerant solutions[END_REF]. Therefore, they presented the thermophysical properties of NH3-LiNO3 and NH3-NaSCN based on a new regression of experimental data and studied their thermodynamic cycles (see Figure 4b) [START_REF] Cai | Thermodynamic analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle driven by low grade energy[END_REF].

NH3-Ionic Liquids: Theoretical studies

Ionic liquids (IL) are salts in liquid form at room temperature (with a melting point lower than 100°C) usually with organic cations and inorganic anions [START_REF] Seddon | Ionic liquids: A taste of the future[END_REF]. They have been proposed as new absorbents or additives to address the disadvantages presented by the conventional working pairs [START_REF] Cera-Manjarres | Vapour pressure measurements of ammonia/ionic liquids mixtures as suitable alternative working fluids for absorption refrigeration technology[END_REF]. Many of them present good physical and chemical properties such as chemical stability, thermal stability, high miscibility in most refrigerants, nonflammability, low corrosive character, nontoxicity, and a broad range of liquid temperatures [START_REF] Kim | Analysis of[END_REF] [122] [START_REF] Khamooshi | Overview of ionic liquids used as working fluids in absorption cycles[END_REF]. According to Shiqiang et al., the most favorable working pairs with ILs are the ones with higher absorption properties, otherwise, they yield to high circulation ratios and low COPs [START_REF] Liang | The Latent Application of Ionic Liquids in Absorption Refrigeration[END_REF]. Scammellls et al. [START_REF] Scammells | Ionic liquids: The neglected issues[END_REF] warned that research on the potential problems of ILs lagged behind the explosion of interest in their application. In recent years, the scientific community warned about the risk of their application when considering the entire life cycle of the systems [START_REF] Bubalo | A brief overview of the potential environmental hazards of ionic liquids[END_REF]. Some issues discussed were the presence of impurities generated in their preparation, the degradation of their properties over time, their toxicity in some cases (which could lead to severe health threats), and the need for recycling due to poor biodegradability [START_REF] Scammells | Ionic liquids: The neglected issues[END_REF] [126] [START_REF] Bubalo | Toxicity mechanisms of ionic liquids[END_REF].

In 2006, Yokozeki and Shifflet patented the application of ILs as working fluid in absorption refrigeration [START_REF] Shiflett | Absorption cycle using ionic liquids as working fluid[END_REF]. They later presented the solubilities of NH3 in different ILs and a simple model for COP comparison. Results showed the highest ever observed solubilities in ILs, comparable to the NH3-H2O case. However, lower COPs were obtained for the NH3-ILs working fluids [129] [130]. Kotenko et al.

(2011) compared various NH3-ILs using ASPEN Plus. NH3-[bmim][PF6] led to the highest COP values, even higher than the NH3-H2O system (around 8% better); however, the circulation rate (f) was ten times higher. Moreover, NH3-ILs showed a greater COP decrease compared with the NH3-H2O system when the difference between the heat rejection temperature and the evaporation temperature (temperature lift) increased [START_REF] Kotenko | Thermodynamic analysis of ammonia/ionic liquid absorption heat pumping processes[END_REF]. [START_REF] Bedia | Screening ionic liquids as suitable ammonia absorbents on the basis of thermodynamic and kinetic analysis[END_REF] screened ILs as NH3 absorbents on the basis of thermodynamic and kinetic analysis and proposed [choline][NTf2] as the most suitable solvent [START_REF] Bedia | Screening ionic liquids as suitable ammonia absorbents on the basis of thermodynamic and kinetic analysis[END_REF]. [START_REF] Ruiz | Evaluation of ionic liquids as absorbents for ammonia absorption refrigeration cycles using COSMO-based process simulations[END_REF] studied the thermodynamic performance of NH3-ILs using COSMO-based process simulations with Aspen Plus/Aspen HYSYS and concluded that this method effectively predicts the properties of solutions and gives reasonably good estimations of the cycle performance [START_REF] Ruiz | Evaluation of ionic liquids as absorbents for ammonia absorption refrigeration cycles using COSMO-based process simulations[END_REF]. Metal ions were proposed to increase the solubility of NH3 in common ILs [START_REF] Sel | NH3 gas sensing applications of metal organic frameworks[END_REF]. In this context, Chen and Bai (2016) proposed [emim]Cu2Cl5 as an NH3 absorbent. They experimentally measured vapor pressures and simulated the system's thermal performance. The [emim]Cu2Cl5-NH3 pair performed better than the NH3-H2O working pair, and even if it had lower performance than the LiBr-H2O system, it had a wider scope of operating temperature conditions [START_REF] Chen | Thermal performance of an absorption-refrigeration system with [emim]Cu2Cl5/NH3as working fluid[END_REF]. Cera-Manjarres et al. (2018) presented the measured vapor pressures of NH3 with four different ILs, concluding that the most suitable absorbent is [EtOHmim][BF4], as it has a high affinity to NH3 at T<60°C in the absorber. However, this affinity decreases with the operating temperature in the generator (80°C<T<100°C). More analyses need to be performed considering safety requirements, compatibility of materials, and all the range of possible operating conditions of the considered system [START_REF] Cera-Manjarres | Vapour pressure measurements of ammonia/ionic liquids mixtures as suitable alternative working fluids for absorption refrigeration technology[END_REF]. To the best of our knowledge, no experimental results on absorption chillers working on the NH3-ILs have been presented to date.

Comparisons of NH3-based absorption systems

As mentioned earlier, the NH3-H2O working pair is the most conventional working pair for ammoniabased systems. However, his drawbacks have led scientists to look for other options to replace it. The theoretical thermodynamic cycle comparison is the first step in the study of the viability of new working pairs. The present section describes theoretical comparative studies that have been presented in the search for these innovative working pairs. [START_REF] Sc | Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents[END_REF] reported a comparative evaluation of a system using NH3-LiNO3 and NH3-H2O as working fluids. For a single-stage chiller, the COP of NH3-LiNO3 is higher than the one of NH3-H2O, especially for higher generator temperatures (due to the heat loss in the partial rectification process of the NH3-H2O system). Considering that the need for a rectifier is also eliminated, this makes NH3-LiNO3 a very suitable option for compact refrigeration units [START_REF] Sc | Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents[END_REF]. Similar conclusions were obtained by Táboas et al. ( 2014), who compared the same working fluids for the use of waste heat provided by diesel engines of fishing ships [START_REF] Táboas | Analysis of ammonia/water and ammonia/salt mixture absorption cycles for refrigeration purposes in fishing ships[END_REF].

Sun (1998) compared the performance of the NH3-LiNO3 and NH3-NaSCN working pairs with the conventional NH3-H2O pair (see Figure 4a) and concluded that NH3-LiNO3 and NH3-NaSCN lead to higher COPs and do not need a rectifier. The NH3-NaSCN working pair seems to be slightly better than NH3-LiNO3 (especially at high Tg); however, it is limited to working at Te over -10°C to avoid crystallization risks [START_REF] Sun | Comparison of the performances of NH3-H2O, NH3-LiNO3 and NH3-NaSCN absorption refrigeration systems[END_REF]. Abtulateef et al. (2007,2008) conducted a similar study and obtained similar results and conclusions [117][136]. [START_REF] Farshi | First and second law analysis of ammonia/salt absorption refrigeration systems[END_REF] confirmed this and also reported that the NH3-NaSCN is limited in the range of Tg (due to crystallization) and presents a higher f [START_REF] Farshi | First and second law analysis of ammonia/salt absorption refrigeration systems[END_REF]. Cai et al. (2014) confirmed that the NH3-LiNO3 working pair has a wider range of operating generator temperatures and that NH3-NaSCN has higher COPs at higher Tg (see Figure 4b) [START_REF] Cai | Thermodynamic analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle driven by low grade energy[END_REF]. In a separate paper, they presented an exergy analysis of the same system, which contains an air-cooled non-adiabatic type absorber, taking into account the variation in solution concentration for the ammonia/salt solution exergy calculation. The proposed configuration was proved theoretically viable as it showed relatively high performance for practical applications [START_REF] Cai | Exergy analysis of a novel air-cooled non-adiabatic absorption refrigeration cycle with NH3-NaSCN and NH3-LiNO3 refrigerant solutions[END_REF]. Antonoupoulos and [START_REF] Antonopoulos | Performance of solar-driven ammonia-lithium nitrate and ammonia-sodium thiocyanate absorption systems operating as coolers or heat pumps in athens[END_REF] found that NH3-LiNO3 is more suitable than NH3-NaSCN for heating purposes (winter), as it provides a superior heat-gain factor and useful thermal power; while for cooling (summer), however, the choice depends on the specific working conditions, as NH3-LiNO3 offers a higher cooling power, while NH3-NaSCN provides a higher COP [START_REF] Antonopoulos | Performance of solar-driven ammonia-lithium nitrate and ammonia-sodium thiocyanate absorption systems operating as coolers or heat pumps in athens[END_REF][139], especially for air-cooled systems. Regarding experimental comparisons, as mentioned in Section 3.2.1, Cai et al. (2016) presented a system designed to operate with NH3-salt working pairs. Two salt absorbents were tested, LiNO3 and NaSCN, and two series of tests were run, one for ice production and another to test for variable working conditions. In the ice production test, the NH3-LiNO3 system reached the lowest evaporation temperature of -13.1°C at an average COP of 0.15. On the other hand, the NH3-NaSCN system reached -7.5°C at an average COP of 0.2. Concerning the variable working conditions test, the NH3-NaSCN system's COP ranged from 0.2 to 0.35, while for the NH3-LiNO3 system, the COP ranged from 0.15 to 0.29. The experimental COP of the NH3-LiNO3 system is on average 30-40% lower than that of the theoretical results, while for the NH3-NaSCN system the deviation is lower and even close to the theoretical results in some cases (see Figure 5). The NH3-NaSCN system needs higher generator temperatures than the NH3-LiNO3 system for the same evaporator temperatures, and the NH3-LiNO3 can withstand higher values of generator temperatures with no risk of crystallization. Hence, the NH3-LiNO3 system is more suitable for practical applications where a lower temperature is needed [START_REF] Cai | Experimental evaluation on thermal performance of an air-cooled absorption refrigeration cycle with NH3-LiNO3 and NH3-NaSCN refrigerant solutions[END_REF]. Finally, Chen et al. compared the IL [bmim]Zn2Cl5 with NaSCN as ammonia absorbent through a thermodynamic analysis [START_REF] Chen | Thermodynamic analysis of an absorption system using [bmim]Zn2Cl5/NH3as the working pair[END_REF]. The COP and exergetic efficiency of the NH3-[bmim]Zn2Cl5 system were slightly higher than for the NH3-NaSCN system at high Tg (higher than 73°C for the exergetic efficiency and higher than 80°C for the COP at Tint=25°C and Te=-10°C) and low Tc and Ta (lower than 28-30°C at Tg=90°C and Te=-10°C). In a separate paper, the same authors presented the development of ILs as working pairs and concluded that the NH3-[bmim]Zn2Cl5 pair was the most ideal, similar to the H2O-LiBr working pair, but with a wider range of operating conditions [START_REF] Liang | Ionic Liquids Facilitate the Development of Absorption Refrigeration[END_REF].

4 Water-based working fluids

Water is a natural refrigerant, abundant, low-cost, and harmless to the environment. Moreover, its good thermal properties such as high latent heat of evaporation make it a very good option as a refrigerant in absorption systems [START_REF] Banu | Review of water based vapour absorption cooling systems using thermodynamic analysis[END_REF]. In general, the performance of water-based absorption systems is higher than that of ammonia-based systems. However, water has some disadvantages, such as its restriction to positive cooling temperatures and its subatmospheric evaporating pressure, which leads to leak problems and expensive components for vacuum operation [START_REF] Wu | An overview of ammonia-based absorption chillers and heat pumps[END_REF] [18] [142] [START_REF] Muthu | Experimental studies on R134a-DMAC hot water based vapour absorption refrigeration systems[END_REF].

4.1 Conventional water-based working pair: H2O-LiBr H2O-LiBr [START_REF] De Vega | Performance of a LiBr-water absorption chiller operating with plate heat exchangers[END_REF] is preferred over NH3-H2O because of its higher performance [START_REF] Horuz | A comparison between ammonia-water and water-lithium bromide solutions in vapor absorption refrigeration systems[END_REF] and no need for a rectifier [START_REF] Sun | A review of working fluids of absorption cycles[END_REF]. However, besides the general drawbacks of water as refrigerant mentioned before, there is a risk of crystallization (especially for high Ta or low Te, which cause problems for air-cooled absorption) [START_REF] Izquierdo | Crystallization as a limit to develop solar aircooled LiBr-H2O absorption systems using low-grade heat[END_REF][43], a higher viscosity (which increases the pumping requirements and decreases the heat and mass transfer) [START_REF] Sun | A review of working fluids of absorption cycles[END_REF], and corrosion problems [START_REF] Brotzu | Pipeline Corrosion Failure in an Absorption Chiller[END_REF] with the H2O-LiBr solution.

H2O-LiBr: Theoretical studies

Several studies in the literature focused on the impact of the different components or conditions on the performance of the system [START_REF] Farnós | Dynamic modelling of an air-cooled LiBr-H2O absorption chiller based on heat and mass transfer empirical correlations[END_REF]. Eisa et al. (1986) simulated a system with no SHX and obtained a maximum COP of 0.857 [START_REF] Eisa | Thermodynamic design data for absorption heat pump systems operating on water-lithium bromide: Part I-Cooling[END_REF]. De Vega et al. ( 2006) used correlations in the literature to simulate a system operating with PHEs and obtained COP values as high as 0.8 for low ambient temperatures (around 20°C) [START_REF] De Vega | Performance of a LiBr-water absorption chiller operating with plate heat exchangers[END_REF]. For specific cases of electronics cooling, Kim et al. ( 2008) presented a miniature system with a dual-channel evaporator, a micro-channel desorber, and a hydrophobic membrane condenser/absorber. Experimental validation is still needed [START_REF] Kim | An absorption based miniature heat pump system for electronics cooling[END_REF]. Other studies aimed to compare the heat sink or heat sources. Kühn et al. (2011) studied the constraints and demands of different heat sources and heat sinks (boreholes, groundwater, air) [START_REF] Kühn | A 10 kW indirectly fired absorption heat pump: concepts for a reversible operation[END_REF] and concluded that every heat source and heat sink combination imposes different restrictions. More work has to be done regarding the seasonal performance and techno-economic studies for the different configurations [START_REF] Kühn | Thermally driven heat pumps for heating and cooling[END_REF]. [START_REF] Marcos | New method for COP optimization in water-and air-cooled single and double effect LiBr-water absorption machines[END_REF] presented a new method for COP optimization in water-and air-cooled machines taking into account the effect of Tc, the solution concentration, and defining the crystallization limits (see Figure 6) [START_REF] Marcos | New method for COP optimization in water-and air-cooled single and double effect LiBr-water absorption machines[END_REF]. The problem with air cooling in H2O-LiBr systems is that as the heat-rejection temperature is higher than for water-cooled systems and, consequently, the Tg needed to achieve the same Te is higher and the cycle comes closer to the crystallization limit [START_REF] Chen | Experimental and analytical study on an air-cooled single effect LiBr-H2O absorption chiller driven by evacuated glass tube solar collector for cooling application in residential buildings[END_REF].

H2O-LiBr: Experimental prototypes development and experimentation

A summary of the small-capacity prototypes of H2O-LiBr absorption cooling systems developed in laboratories and their typical operating conditions is presented in Table 6. Great attention has to be given to the design procedure of the system's components as the dimensioning of each one of them is of great importance [154] [155]. The University of Jordan developed a low-cost solar cooling unit with H2O-LiBr.

The first-generation unit (FGU) consisted in a 1.75-kW unit manufactured locally with a 3.6-m 2 flat-plate solar collector in parallel with a 0.15-m 2 elliptical concentrating collector, without an SHX or RHX. This system presented a temporal COPmax of 0.55 [START_REF] Hammad | Performance of a solar LiBr-water absorption refrigeration system[END_REF]. An optimization study was developed to determine the minimum costs and maximum benefit/cost ratio, and Hammad and Abuzahra (1995) concluded that an optimum design only existed for systems with a capacity of at least 12 kW [START_REF] Hammad | Optimization Study of a Solar refrigeration System Works by Li-Br -H2O Absorption Cycle[END_REF]. After this study, Hammad and Zurigat (1998) presented a 5.27-kW system that replaced the workshop-fabricated components by commercially available ones. The system, coupled to a 14-m 2 flat-plate solar collector, presented a maximum experimental COP of 0.85 [START_REF] Hammad | Performance of a second generation solar cooling unit[END_REF]. Air-cooled systems are an important challenge and subject of study in H2O-LiBr absorption chillers [START_REF] Kim | Air-cooled LiBr-water absorption chillers for solar air conditioning in extremely hot weathers[END_REF]. Some prototypes were developed and tested with success and far from the crystallization limits of the solution. Crystallization problems occurred owing to inadequate shutdowns of the machines [START_REF] Ohuchi | A Study on a Hot-Water Driven Air-Cooled Absorption Refrigerating Machine[END_REF] [161] [START_REF] Castro | Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O-LiBr absorption cooling machine[END_REF].

Drost and Friedrich (1997) studied miniature heat pumps that took advantage of the elevated heat and mass transfers attainable in microstructures (constrained micro-thin films and microchannels). A manportable cooling device was designed to provide 0.68 W with a COP of 0.68 and a weight of around 4-5 kg, while a space conditioning device was designed to provide 350 W with a COP of 0.71 [START_REF] Drost | Miniature heat pumps for portable and distributed space conditioning applications[END_REF]. To our knowledge, no experimental results were presented. Another example of micro-technology in absorption chillers is the one of [START_REF] Hu | Study of a micro absorption heat pump system[END_REF], who presented a micro-absorption cooling system with a target cooling capacity of 40 W. The experimental results were different from the design conditions owing to the presence of non-condensable gases, heat losses, and imperfect control of the flow rates [START_REF] Hu | Study of a micro absorption heat pump system[END_REF].

After years of investigation and collaboration between Phönix SonnenWärme AG, TU Berlin, ZAE Bayern, and the Institute of Rehabilitation and Modernization of Buildings (IEMB), Kühn et al. (2005) presented a compact 10-kW chiller for low driving temperatures called Suninverse. At nominal conditions, the system produced chilled water at 15°C with a COP of around 0.76 [START_REF] Kühn | Operational results of a 10 kW absorption chiller for low-grade driving heat[END_REF]. This system was also tested in heating operation mode [START_REF] Kühn | Operational results of a 10 kW absorption chiller in heat pump mode[END_REF] and was later commercialized under the brand SonnenKlima [START_REF] Kohlenbach | Solar cooling with absorption chillers: Control strategies and transient chiller performance[END_REF]. Kohlenbach and Ziegler published two papers describing a dynamic simulation model for transient absorption chiller performance [START_REF] Kohlenbach | A dynamic simulation model for transient absorption chiller performance. Part I: The model[END_REF], the numerical results, and sensitivity checks to prove the model's functionality [START_REF] Kohlenbach | A dynamic simulation model for transient absorption chiller performance. Part II: Numerical results and experimental verification[END_REF].

With the aim of developing a new generation of highly efficient absorption chillers, the TU Berlin, ZAE Bayern, and Vattenfall Europe developed a system with low heat losses, simple manufacturing, compact, and low weight. The 50-kW prototype, which can achieve COPs of around 0.8, was operated in two field tests supplying 44 kW of office cooling driven by district heating and 35 kW of data center cooling driven by solar energy. The new machine was expected to decrease the manufacturing costs by around 50-70% compared with the current technologies [START_REF] Kühn | Thermally driven heat pumps for heating and cooling[END_REF].

In recent years, there has been an increasing interest in adiabatic absorbers for absorption systems. Gutiérrez-Urueta et al. ( 2011) compared two adiabatic absorber configurations (droplets and liquid sheets). The liquid sheet configuration proved to be better in terms of performance and would lead to a significant reduction in the absorber's size (up to 50%) [START_REF] Gutiérrez-Urueta | Experimental performances of a LiBr-water absorption facility equipped with adiabatic absorber[END_REF]. One year later, they presented an extended characteristic equation of the machine that showed good agreement with experimental results [START_REF] Gutiérrez-Urueta | Extension of the characteristic equation to absorption chillers with adiabatic absorbers[END_REF]. González-Gil et al. ( 2011) developed a solar direct air-cooled chiller that uses an adiabatic flat-fan sheets absorber and can switch to a double-effect mode. The prototype worked efficiently at COP values of around 0.6 with a cooling power of between 2 and 3.8 kW (about 85% of the nominal capacity) [START_REF] González-Gil | Experimental evaluation of a direct aircooled lithium bromide-water absorption prototype for solar air conditioning[END_REF]. Lizarte et al. ( 2012) later tested the same system, obtaining an evaporator temperature of 14.3°C and a mean COP of 0.53 [START_REF] Lizarte | An innovative solar-driven directly air-cooled LiBr-H 2O absorption chiller prototype for residential use[END_REF]. Chen et al. ( 2017) presented an experimental and analytical study of a 6-kW solar air-cooled chiller for residential buildings. The absorber was of an adiabatic type with a finned tube pre-cooler. Experimental results showed that the system's COP is significantly affected by the temperature conditions, with its average value being 0.61 [START_REF] Chen | Experimental and analytical study on an air-cooled single effect LiBr-H2O absorption chiller driven by evacuated glass tube solar collector for cooling application in residential buildings[END_REF].

For heat transfer enhancement, Achuthan et al. ( 2011) added micro-nozzles to the evaporator and the absorber of a solar absorption system, obtaining an increase in the COP from 0.3 to 0.6 [START_REF] Achuthan | Experimental analysis on the performance and characteristics of compact solar refrigeration system[END_REF]. Finally, Zhai et al. ( 2012) constructed and tested an 8-kW chiller coupled to 96 m 2 of solar collector arrays. The system delivered 4.5 kW of cooling to a 50-m 2 test room with an average COP of 0.32 [175][176].

H2O-LiBr commercial systems: Presentation and experimentation

A summary of the commercial small-scale H2O-LiBr absorption cooling systems (to the best of our knowledge) is presented in Table 7, and the experiments performed on these systems are presented in Table 8 (only experiments with enough data to be represented are shown). Yazaki is historically the most widely studied brand in the branch of commercial absorption systems. The commercial H2O-LiBr chillers tested in the twentieth century were the discontinued Yazaki models WFC 400 (4.5 kW) and WFC 600 (7 kW). [START_REF] Yeung | Performance of a solar-powered air conditioning system in Hong Kong[END_REF] reported the performance study of a solar-powered WFC 400S system installed in Hong Kong. The integrated system showed an annual overall system efficiency of 7.8% [START_REF] Yeung | Performance of a solar-powered air conditioning system in Hong Kong[END_REF]. In the case of the WFC 600 system, Nakahara et al. (1975) presented the results of the machine coupled to solar collectors and observed that the optimal operating generator temperature was about 78°C [START_REF] Nakahara | Experimental study on house cooling and heating with solar energy using flat plate collector[END_REF]. [START_REF] Bong | Performance study of a solar-powered air-conditioning system[END_REF] also studied this system with solar collectors in Singapore. The system provided an average cooling capacity of 4 kW with a COP of 0.58 [START_REF] Bong | Performance study of a solar-powered air-conditioning system[END_REF]. [START_REF] Ng | Theoretical and experimental analysis of an absorption chiller[END_REF] later presented a theoretical and experimental analysis of the same chiller. The theoretical model proposed a new linear correlation between the inverse COP and the non-dimensional temperature ratio (presented in Section 3.1.3).

Experimental results in a wide range of operating conditions proved the theoretical model. The cooling capacities obtained from the test were lower than the ones in the catalog by 10-25% [START_REF] Ng | Theoretical and experimental analysis of an absorption chiller[END_REF]. Prasartkaew studied the possibility of integrating solar/biomass sources to drive an absorption chiller [START_REF] Prasartkaew | A low carbon cooling system using renewable energy resources and technologies[END_REF]. The experimental system used a renovated WFC 600 model (working fluid and control system) and operated at 75% of its nominal capacity reaching COPs of around 0.54 at steady state [START_REF] Prasartkaew | Experimental study on the performance of a solar-biomass hybrid airconditioning system[END_REF]. Moreover, the chiller needed to be operated at driving temperatures higher than 85°C to obtain high COP values [START_REF] Prasartkaew | Performance test of a small size LiBr-H2O absorption chiller[END_REF]. [196] [197] Another model that has been extensively studied is the WFC-SC 5 (17.5 kW) from Yazaki. Asdrubali and Grignaffini (2005) simulated and tested an experimental system with this model. The machine operated with acceptable efficiency for driving temperatures in the range of 65-70°C, showing the highest COP values at 70°C. However, the experimental performance results were lower than in the catalog because of the refrigerant overflow and low flow rate in the heat sink [START_REF] Asdrubali | Experimental evaluation of the performances of a H2O-LiBr absorption refrigerator under different service conditions[END_REF]. The same machine was modeled in TRNSYS and tested by [START_REF] Martínez | Design and test results of a low-capacity solar cooling system in Alicante (Spain)[END_REF] in Alicante (Spain) integrated with solar collectors, a cooling tower, hot-water storage tanks, and a cold-water storage tank. An average COP value of 0.691 was registered and a good accuracy was reported with the theoretical model [START_REF] Martínez | Design and test results of a low-capacity solar cooling system in Alicante (Spain)[END_REF]. For the cases in which the machine operates outside the designed temperature range, Martínez et al. ( 2016) developed a model in EES of a Yasaki WFC-SC5 chiller and later validated it. The simulated results show a deviation of 0.75 °C for the output generator temperature and of 0.35 °C for the output evaporator temperature [START_REF] Martínez | Development and experimental validation of a simulation model to reproduce the performance of a 17.6 kW LiBr-water absorption chiller[END_REF]. Rodríguez et al. constructed a test bench to characterize small-capacity thermal refrigeration systems and heat pumps [START_REF] Carabias | Desarrollo de un banco de ensayos multifuncional y de los procedimientos para caracterizar equipos térmicos de refrigeración y bombas de calor de pequeña potencia[END_REF] and tested a Yazaki WFC-SC 5. The experimental results showed good agreement with the catalog data.

Another model that has been extensively studied is the Yazaki WFC-SC 10 (35.2 kW) [START_REF] Beausoleil-Morrison | The experimental characterization of a lithium bromide-water absorption chiller and the development of a calibrated model[END_REF]. [START_REF] Haywood | Thermodynamic feasibility of harvesting data center waste heat to drive an absorption chiller[END_REF] used a simulator provided by Yazaki to study the possibility of using data center waste heat as the driving source for this chiller. The heat produced by the data center under standard conditions is enough to run the system [START_REF] Haywood | Thermodynamic feasibility of harvesting data center waste heat to drive an absorption chiller[END_REF]. The same model was studied for solar applications by [START_REF] Syed | A novel experimental investigation of a solar cooling system in Madrid[END_REF] in Madrid (Spain). It provided a maximum cooling capacity of 7.5 kW with a cut-off temperature of 65°C. At 81°C, the system provided 6.4 kW of cooling. The maximum measured COP was 0.6, and the maximum daily average COP was 0.42 [START_REF] Syed | A novel experimental investigation of a solar cooling system in Madrid[END_REF]. The same installation was tested by Rodríguez [START_REF] Hidalgo | Energy and carbon emission savings in Spanish housing air-conditioning using solar driven absorption system[END_REF]. In this case, the system provided a mean cooling power of 3 kW for a period of 6.5 h of complete solar autonomy on an average day. Further improvements were to be made in terms of the machine size and control strategy [START_REF] Hidalgo | Energy and carbon emission savings in Spanish housing air-conditioning using solar driven absorption system[END_REF]. [START_REF] Pongtornkulpanich | Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling system in Thailand[END_REF] presented their results with a solar-driven Yazaki WFC-SC 10 installed in Thailand. The solar collectors, which were designed for the nominal chiller's COP of 0.7, provided 81% of the total thermal energy required [START_REF] Pongtornkulpanich | Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling system in Thailand[END_REF]. [START_REF] Zambrano | Model development and validation of a solar cooling plant[END_REF] installed this chiller integrated with a solar collector and developed its dynamic model, which showed good agreement with experimental results [START_REF] Zambrano | Model development and validation of a solar cooling plant[END_REF]. Hamza et al. ( 2008) tested a solar-powered Yazaki WFC-SC 10 chiller combined with free cooling (recuperation of low external ambient temperatures at times of the day when they are low to assist AC) installed in Germany. On sunny days, the chiller operated with COPs in the range of 0.37-0.81 [START_REF] Ali | Performance assessment of an integrated free cooling and solar powered single-effect lithium bromide-water absorption chiller[END_REF]. Another big issue for absorption chillers is the overflow in the evaporator. Albers et al. ( 2010) presented a simulation model of the part load behavior of the WFC-SC 10 and its experimental validation. The model can predict the transition from partially dry to a refrigerant overflow in the evaporator [START_REF] Albers | Simulation and Experimental Analysis of a Solar Driven Absorption Chiller With Partially Wetted Evaporator[END_REF].

The second most studied brand in the literature is Rotartica and its 045v model (4.5 kW). [START_REF] Izquierdo | Air conditioning using an air-cooled single effect lithium bromide absorption chiller: Results of a trial conducted in Madrid in August 2005[END_REF] studied the performance of this chiller installed in Spain in 2005. The driving temperature varied in the range of 80-107°C and the average COP for the total period was 0.49 [START_REF] Izquierdo | Air conditioning using an air-cooled single effect lithium bromide absorption chiller: Results of a trial conducted in Madrid in August 2005[END_REF]. The same model was installed in the UK coupled with solar collectors and tested in 2007. Results indicated that the average chiller's COP was 0.58, and chilled water temperatures as low as 7.4°C were achieved [START_REF] Agyenim | Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store[END_REF]. Monné et al. ( 2011) developed a TRNSYS model and experimentally analyzed the performance of a solar-powered cooling installation in Zaragoza (Spain). In 2007, the average COP was 0.6 and the cooling power was between 4 and 5.6 kW. In 2008, the COP ranged from 0.46 to 0.56 and the cooling capacity from 3.6 to 5.3 kW. Both the simulated and experimental results showed a strong influence of the heat sink and driving temperature. For this reason, a geothermal heat sink was selected to integrate the system, and the first theoretical results showed a COP increase of up to 42% [START_REF] Monné | Monitoring and simulation of an existing solar powered absorption cooling system in Zaragoza (Spain)[END_REF]. [START_REF] Evola | Proposal and validation of a model for the dynamic simulation of a solar-assisted single-stage LiBr/water absorption chiller[END_REF] presented a model for the dynamic simulation of a solar-assisted Rotartica 045v chiller considering the non-steady behavior due to thermal and mass storage in the components. The simulated and experimental results had a good agreement for both a daily and seasonal basis [START_REF] Evola | Proposal and validation of a model for the dynamic simulation of a solar-assisted single-stage LiBr/water absorption chiller[END_REF].

Some other machines were also tested in a few studies.

H2O-LiI and LiCl

As mentioned earlier, LiBr is the most common absorbent in water-based absorption chillers. However, other salts have been studied as possible alternatives. H2O-LiI has been proposed as a viable working pair for absorption systems. Patil et al. presented the thermodynamic design data of this working fluid for cooling applications [START_REF] Patil | Thermodynamic design data for absorption heat pump systems operating on water-lithium iodide-Part I. Cooling*[END_REF], achieving a similar performance to the H2O-LiBr working pair. However, LiI has mostly been studied and used as one element in multi-component absorbents, and no other studies of this salt as single absorbent are available in the literature [START_REF] Sun | A review of working fluids of absorption cycles[END_REF].

Another proposed working pair is H2O-LiCl, which has various advantages over H2O-LiBr such as triplestate point (for non-continuous systems), long-term stability, and lower cost. Several studies have also proved that compared with a H2O-LiBr system, it presents higher COPs and exergy efficiency, mainly due to a lower circulation rate [START_REF] Grover | Thermodynamic design data for absorption heat pump systems operating on water-lithium chloride-Part One[END_REF][220] [START_REF] Patel | Exergy Based Analysis of LiCl-H2O Absorption Cooling System[END_REF]. However, the higher risk of crystallization for the H2O-LiCl system generates difficulties in its design and maintenance [221] [222]. Moreover, it seems that depending on the studied operating conditions, the performance of the H2O-LiCl pair might be lower than that of the conventional H2O-LiBr working pair [223] [224]. There are no experimental results with single-stage absorption systems for cooling using the H2O-LiCl working pair, probably because of the high risks of crystallization [221] [222].

H2O-Ionic Liquids: Theoretical studies and experimental prototypes

ILs have been proposed as H2O absorbents. Shifflet and Yokozeki patented the use of water and at least one IL as the working fluid for absorption systems [START_REF] Shiflett | Absorption cycle utilizing ionic liquid and water as working fluids[END_REF][225] and presented a comparison for the H2O-[emim][BF4] and the conventional NH3-H2O and H2O-LiBr working fluids, showing the proposed working pair a lower performance and much higher f. A summary of the small-capacity prototypes of H2O-IL absorption cooling systems developed in laboratories and their operating conditions is presented in Table 9.

Römich et al. (2011) performed a simulated comparison between H2O-[dema]

[CH3SO3] and H2O-LiBr as working fluids. The H2O-IL system yielded COP values around 6% close to but lower than the H2O-LiBr system [START_REF] Römich | Comparison between lithiumbromidewater and ionic liquid-water as working solution for absorption refrigeration cycles[END_REF]. However, during experiments (using the machine presented in [START_REF] Kühn | Operational results of a 10 kW absorption chiller for low-grade driving heat[END_REF]) the COP of the H2O-IL system was lower than simulated (see Figure 7). Considering the experimental results of the H2O-LiBr working fluid, both working fluids are comparable in terms of COP. In terms of experimental cooling capacity, the one with H2O-IL was much lower than with H2O-LiBr (1.3 kW vs. 10 kW) owing to the high viscosity and low wettability of the H2O-IL solution in a system designed to work with the H2O-LiBr working fluid [START_REF] Seiler | Sustainable cooling strategies using new chemical system solutions[END_REF].

Comparisons of the water-based working fluids

Saravanan and Maiya (1998) performed a thermodynamic analysis of systems running on four binary working fluids (H2O-LiBr, H2O-NaOH, H2O-LiI and H2O-LiCl) and other multi-component working fluids (see Figure 8). Among the binary working fluids, the H2O-LiCl is the best option from the point of view of cut-off temperature and circulation ratio [START_REF] Maiy | Thermodynamic comparison of water based working fluid combinations for a vapor absorption refrigeration system[END_REF]. Several studies have proved that the main advantages of H2O-LiCl over H2O-LiBr are a higher COP, higher exergy efficiency (and thus less solar collecting area), lower irreversibilities, and lower driving temperatures [START_REF] Gogoi | Exergy analysis of a H 2 O -LiCl absorption refrigeration system with operating temperatures estimated through inverse analysis[END_REF] [221] [START_REF] She | A novel low-grade heat-driven absorption refrigeration system with LiCl-H2O and LiBr-H2O working pairs[END_REF]. However, a higher crystallization risk exists compared with the H2O-LiBr working pair (see Figure 11) [START_REF] Bellos | Exergetic and energetic comparison of LiCl-H2O and LiBr-H2O working pairs in a solar absorption cooling system[END_REF], and this might be the main reason why there are no experimental prototypes running in continuous mode.

20

TU Berlin and Evonik Industries AG studied the potential of ILs in absorption chillers, and different ILs were tested in a glass shell prototype. Results are shown in Figure 9 for the initial machine and the machine with an improved condenser and generator (new C/G) at Tg=75°C, Tint=27°C, Te=18°C. H2O-IL systems reach COP values close to the H2O-LiBr system. However, their cooling capacity is always lower owing to a mass transfer inhibition probably due to a higher solution concentration [START_REF] Kühn | Thermally driven heat pumps for heating and cooling[END_REF]. According to Seiler et al. [START_REF] Seiler | Sustainable cooling strategies using new chemical system solutions[END_REF], COP simulations using plain energy balances and phase equilibria are not enough to evaluate new working fluids, and thus in the future, more research has to be done to underline the heat and mass transfer phenomena. The properties of working pairs with alcohols as the refrigerant (thermal stability and high output temperature) make them more interesting for heating or heat transformer applications [START_REF] Sun | A review of working fluids of absorption cycles[END_REF]. However, there have been some theoretical studies for cooling applications, for which, just as with water as refrigerant, alcohols would usually operate in vacuum conditions [START_REF] Chen | Thermodynamic performances of [mmim]DMP/methanol absorption refrigeration[END_REF]. Wang and Zheng (2008) studied a system using TFE (2,2,2-trifluoroethanol) as the refrigerant and three ILs as absorbents ([BMIM]Br, [BMIM]Br4, and [EMIM]Br4). The TFE-[BMIM]Br system presented the highest COP and lowest circulation ratio [START_REF] Wang | Performance analysis of absorption cooling cycle utilizing TFE-[BMIm] Br as working fluid[END_REF]. Liang et al. [START_REF] Liang | Absorption refrigeration cycle utilizing a new working pair of ionic liquid type[END_REF] presented a theoretical study of a system based on the [MMIM]DMP-CH3OH working fluid, concluding that this new proposed working pair shows a higher COP with a lower Tg and Tc than the conventional ones (NH3-H2O and H2O-LiBr). Moreover, it has a refrigerating capacity and circulation ratio that lie between them. Chen et al. (2012) studied a system using the [mmim]DMP-methanol working fluid and concluded that (1) f of the proposed working fluid is higher than that of the H2O-LiBr working pair, but still acceptable and tolerable, (2) the COP of the new working fluid lies between the two conventional ones (NH3-H2O and H2O-LiBr) under most operating conditions, and (3) the system working with the proposed working fluid operates with high COPs when the driving temperature is too high to drive the H2O-LiBr system [START_REF] Chen | Thermodynamic performances of [mmim]DMP/methanol absorption refrigeration[END_REF].

HFCs and HCFCs-based working fluids

Hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) are both banned under the 1987 Montreal Protocol owing to their ozone depleting potential. Hydrofluorocarbons (HFCs) raised as an alternative to those refrigerants as they are non-ozone depleting gases. These refrigerants have the advantages of immiscibility with water and aqueous streams for direct-contact heat transfer and a lowgrade driving temperature; moreover, they operate at similar pressure levels as ammonia. However, these refrigerants have a considerable lower enthalpy of vaporization compared with NH3 and H2O, and most of them possess an important global warming potential (GWP) [START_REF] Borde | Development of absorption refrigeration units for cold storage of agricultural products[END_REF]. For this reason, HFCs are also to be progressively phased down under the 2016 Kigali Amendment to the Montreal Protocol that will take effect between 2019 and 2028, depending on the country's level of development [START_REF] Iea | The Future of Cooling[END_REF].

The HCFC R22 together with the organic absorbent DMETEG have been extensively investigated as a working pair for absorption systems [START_REF] Jelinek | Density of organic binary mixtures from equilibrium measurements[END_REF]. [START_REF] Borde | Development of absorption refrigeration units for cold storage of agricultural products[END_REF] developed a model to test the R22 refrigerant with different organic absorbents such as the NMP, MCL, DMMP, DMETEG, and DMF. The most efficient working fluid in terms of COP and circulation ratio was the R22-DMF, and therefore, a prototype was constructed. Experimental results were close to the predicted performance [START_REF] Borde | Development of absorption refrigeration units for cold storage of agricultural products[END_REF]. [START_REF] Borde | Absorption system based on the refrigerant R134a[END_REF] investigated the possibility of using the HFC R134a refrigerant coupled with three organic absorbents (DMETEG, MCL, and DMEU). At Tg=100°C, R134a-DMETEG was the optimum working fluid with a similar COP to the others but a lower circulation ratio. Compared with R22 as refrigerant [START_REF] Borde | Development of absorption refrigeration units for cold storage of agricultural products[END_REF], R134a shows a reduction in COP and an increase in the circulation ratio [START_REF] Borde | Absorption system based on the refrigerant R134a[END_REF]. [START_REF] Borde | Working fluids for an absorption system based on R124 (2chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents[END_REF] studied the possibility of using the HCFC R124 coupled with DMAC, NMP, MCL, DMEU, and DMETEG [START_REF] Borde | Working fluids for an absorption system based on R124 (2chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents[END_REF]. In this case, the working fluid with the best performance was the R124-DMAC. Even if the NMP and DMEU options had a similar COP and circulation rate, they suffered from stability problems at high temperatures. Finally, when comparing the R124 [START_REF] Borde | Working fluids for an absorption system based on R124 (2chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents[END_REF] refrigerant with the R22 [START_REF] Borde | Development of absorption refrigeration units for cold storage of agricultural products[END_REF] and R134a [START_REF] Borde | Absorption system based on the refrigerant R134a[END_REF], it was found that R124 had lower COPs than R22, but higher than R134a, and in terms of circulation ratio, R124 had lower values than both R22 and R134a [START_REF] Borde | Working fluids for an absorption system based on R124 (2chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents[END_REF]. Levy et al. (2000) analyzed a system based on the HFCs R134a and R125 refrigerants and using DMETEG as absorbent. The maximum COP depended on the working pair, while the COPmax of R134a was slightly higher. However, the working fluid based on the R125 was preferred since the f and Tg were lower [START_REF] Levy | Single stage absorption system based on refrigerants R125 and R134a with DMETEG[END_REF].

ILs have also been studied as suitable absorbents for HFCs. Kim working pair, such as the high overall solubility and its much lower viscosity, resulted in higher system performance [START_REF] Kim | Analysis of[END_REF].

Other refrigerant bases

A summary of the small-capacity prototypes of absorption cooling systems with refrigerant bases other than H2O and NH3 developed in laboratories and their operating conditions is presented in Table 10. [START_REF] Pilatowsky | Thermodynamic analysis of monomethylamine-water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures[END_REF] studied the monomethylamine-water working pair operating at low driving temperatures (60-80°C). This working fluid requires a rectifier. However, it possesses interesting properties at moderate heat sink temperatures (25-35°C) and a Te between -10 and 10°C [START_REF] Pilatowsky | Thermodynamic analysis of monomethylamine-water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures[END_REF]. [START_REF] Romero | Monomethylamine-water vapour absorption refrigeration system[END_REF] compared the same system with the ammonia-water system, finding that the monomethylamine-water system possesses higher COP values in a short range of low Tg (between 63 and 80°C); however, the conventional working fluid keeps higher COPs in a wider range of temperatures for the rest of the Tg. Moreover, the proposed system operates at lower vapor pressures than the NH3-H2O cycle, which allows for smaller wall thickness in the system's components [START_REF] Romero | Monomethylamine-water vapour absorption refrigeration system[END_REF].

The acetone-zinc bromide (C3H6O-ZnBr2) working pair was studied and modeled for an absorption chiller by [START_REF] Karno | Thermodynamic analysis of an absorption refrigeration machine with new working fluid for solar applications[END_REF]. The model was validated with an experimental plant that was operated at low Tg (higher than 50°C) achieving a COP of around 0.4. However, more work needs to be done to study the influence of the operating temperatures and the efficiencies of heat exchangers [START_REF] Karno | Thermodynamic analysis of an absorption refrigeration machine with new working fluid for solar applications[END_REF].

CO2 is, together with ammonia and propane, an environmentally friendly substance studied for compression refrigeration. However, special attention should be paid to the operating pressures, since they are at higher levels than ammonia as the refrigerant, which creates new technological challenges. ILs have a high CO2 absorption capacity, which indicates the possibility of both substances being used as the working fluid in absorption systems [START_REF] Sen | Using carbon dioxide and ionic liquids for absorption refrigeration[END_REF], and this is why this working pair has been studied during the past decade. [START_REF] Cai | Dynamic modeling of an absorption refrigeration system using ionic liquids[END_REF]. The COP of the system yielded much lower values compared with the NH3-H2O working fluid; however, the authors questioned the accuracy of the ideal-gas heat capacity coefficients for the working pair. Martín and Dolores Bermejo (2010) discussed the possibility of using CO2 as the refrigerant with ILs as absorbents. The CO2-IL working pair yielded lower COP values (around 0.21 for the CO2-[bmpyrr][Tf2N] working pair) compared with the conventional NH3-H2O working fluid (around 0.55) owing to the higher f required for the CO2-IL system (2.4 times higher) [START_REF] Martín | Thermodynamic analysis of absorption refrigeration cycles using ionic liquid + supercritical CO2 pairs[END_REF]. 6 General comparisons between working pairs 6.1 Theoretical general comparisons in the literature Wilbur and Mancini (1976) compared the NH3-H2O and H2O-LiBr working fluids for a solar AC in the conditions of Atlanta (USA). The comparison also included the type of collectors, the heat rejection alternative, and the thermal storage type. The H2O-LiBr working fluid gave a significantly better performance, and the best configuration was a H2O-LiBr air-cooled system with chilled water storage and a conventional flat-plate collector [START_REF] Wilbur | A comparison of solar absorption air conditioning systems[END_REF]. For the same working fluids comparison, Alizadeth et al. (1979) confirmed the better performance and higher simplicity of the H2O-LiBr system; moreover, they reported that the most important variable to take into account is Tg [START_REF] Alizadeh | Design and optimisation of an absorption refrigeration system operated by solar energy[END_REF]. [START_REF] Mansoori | Thermodynamic basis for the choice of working fluids for solar absorption cooling systems[END_REF] also confirmed the advantages of H2O-LiBr, especially for hot climates, and mentioned that the NH3-H2O working pair is more versatile as it can work in a wider range of operating conditions, but with a lower COP. Finally, the NH3-NaSCN working pair was considered to lie between the two options [START_REF] Mansoori | Thermodynamic basis for the choice of working fluids for solar absorption cooling systems[END_REF]. The better efficiency of H2O-LiBr over NH3-H2O was also verified by Karakas et al. [START_REF] Karakas | Second-Law Analysis of Solar Absorption-Cooling Cycles using Lithium Bromide / Water and Ammonia / Water as Working Fluids Auxiliary source[END_REF] and Sun [START_REF] Sun | Thermodynamic design data and optimum design maps for absorption refrigeration systems[END_REF] (see Figure 10); however, this working fluid also entails more design difficulties because of its working pressures at subatmospheric levels, and, moreover, crystallization and positive cooling temperatures must be taken into account [START_REF] Horuz | A comparison between ammonia-water and water-lithium bromide solutions in vapor absorption refrigeration systems[END_REF]. Considering an application for storage rooms (frozen and chilled products that require storage temperatures between -35 and 6°C), Infante [START_REF] Ferreira | VERTICAL TUBULAR ABSORBERS FOR AMMONIA-SALT ABSORPTION REFRIGERATION[END_REF] compared 11 promising working pairs with 5 different refrigerants (H2O, NH3, CH3OH, NH2CH3, and CHClF2) and concluded that for the required cooling temperatures, the best adapted working pairs would be NH3-LiNO3 and NH3-NaSCN, being the latter the most efficient but also the most restricted due to crystallization risks [START_REF] Ferreira | VERTICAL TUBULAR ABSORBERS FOR AMMONIA-SALT ABSORPTION REFRIGERATION[END_REF]. [START_REF] Lecuona | Absorption solar cooling systems using optimal driving temperatures[END_REF] determined the optimum driving temperature and compared a system working with H2O-LiBr, another with NH3-LiNO3, and a third one with NH3-LiNO3 but with a low-pressure compressor booster [START_REF] Lecuona | Absorption solar cooling systems using optimal driving temperatures[END_REF]. The NH3-LiNO3 working pair offers a lower refrigeration production at positive temperatures compared with the H2O-LiBr system. However, the boosted cycle offers 15.6% more cold than the singleeffect H2O-LiBr cycle using a pressure ratio of 2. Absorption cooling systems working with the NH3-LiNO3 working fluid were found to be feasible and attractive. Flores were evaluated (900 ILs and eight refrigerants) and adequate refrigerant-absorbent pairs competitive with conventional systems were selected (see Figure 12). The optimum working fluid depends on the cooling temperature. Water showed excellent thermodynamic properties owing to the its high enthalpy of vaporization and good affinity to ILs; however, because of its limitation to working at positive cooling temperatures, methanol and ammonia are good options for cooling temperatures between -5 and 5°C [START_REF] Moreno | Absorption refrigeration cycles based on ionic liquids: Refrigerant/absorbent selection by thermodynamic and process analysis[END_REF]. The data from the present comparisons were obtained from the different tables in the previous sections about experiments conducted on in-lab-developed prototypes and on commercial chillers. Figure 14 shows the regrouping of the different working fluids (commercial systems and prototypes) and their experimental COPs or catalog COPs. As mentioned earlier, only two working fluids are commercially available (H2O-LiBr and NH3-H2O). It is clear from this figure that there is a different value range of COPs depending on the selected working fluid and that the three most experimentally studied working pairs in the literature are the H2O-LiBr, NH3-H2O, and the NH3-LiNO3 working pairs. However, in the case of the experimental prototypes, a big dispersion can be observed mainly because of three reasons: the physical differences of the systems (heat exchanger technologies, piping, and insulation), the different operating temperatures, and the inconsistency added by de variability of the operating conditions presented in the literature regarding experimental machines (as discussed in Section 2). Similar value ranges are observed for the two commercially available working pairs compared with the experimental prototypes; however, as the commercial machines have already gone through an optimization process before the market launch, there is considerably less data dispersion (0.2-0.25 compared with 0.53-0.69 of the experimental prototypes). It can also be deduced from the trend lines in Figure 15a and b that at the same source temperature conditions (COPcarnot), the H2O-LiBr working fluid has a higher performance than the NH3-H2O working pair, which is in good agreement with their differences in their theoretical performance. Finally, from Figure 15a, a better performance can be observed at low COPcarnot for the NH3-LiNO3 system compared with the conventional NH3-H2O system, which proves the theoretical comparisons presented earlier.

However, as the COPcarnot increases, no substantial increase in the experimental performance can be observed, and at COPcarnot>3, the NH3-H2O working pair performs even better than the NH3-LiNO3 working pair, which might be due to the reported problems of the NH3-LiNO3 working pair regarding its elevated viscosity [9][100]. For the rest of the experimental systems in Figure 15a, the R134a-IL and the C3H6O-ZnBr2 working pairs do not seem to have an especially advantageous performance, the performance of the NH3-NaSCN system seems to be comparable to those of the NH3-H2O and NH3-LiNO3 systems, and the H2O-IL working pair performance seems to be similar to that of the H2O-LiBr working pair. However, more experimental data are needed to confirm these findings. Finally, experimental studies on the commercial chillers are represented in the same Figure 15b. Only the experimental studies with enough information to represent them graphically are shown. As expected, owing to the different and sometimes uncontrollable conditions in experimental installations, and also to the fact that for research purposes, the commercial machines are usually tested at conditions out of the optimal design values, the performance of the installed systems is generally lower than the one provided by the manufacturer. This is even more accentuated for driving temperatures higher than those of the nominal conditions (higher COPcarnot than in the range of the trend lines). Indeed, as the generator temperature increases, the theoretical performance tends to increase; however, this generates exergy losses in the generator, condenser, and absorber as their average temperature increases, reducing the exergetic efficiency of the system. This and the dry zones generated at elevated Tg on the heat exchanger surface due to the elevated heating lead to an increase in the difference between the expected performance and the experimental performance of the systems, reducing the system's "ideality".

Conclusions

In order to select a working fluid for an absorption system, different aspects have to be taken into account depending on the desired application and the most important objectives to be accomplished. Table 11 summarizes the advantages and disadvantages of the different refrigerants and working pairs presented in the previous sections. As stated earlier, the choice of the system must not only be oriented toward the thermodynamic cycle performance but also toward other factors that might impact the real application of the system, either technological (available properties, operating pressures and temperatures, safety precautions, corrosion avoidance, exergetic efficiencies, heat sink medium, driving source type, and system lifetime), economic (initial cost of the system and external components, cost of the working pair, operating costs, economic gains depending on the electricity price changes and inflation), or even social (global warming potential and renewable energy use). -Crystallization risks (higher than for H2O-LiBr).

Ionic Liquids + High solubility. + No crystallization risk. + Good performance.

-Depending on the chemical structure, long-term thermal and toxicity problems.

-High cost.

-Absence of properties documentation.

HFC

+ Moderate pressure ranges.

-Low enthalpy of evaporation.

-GWP.

-Will be banned in the future.

Organic absorbents + Low driving temperature. + Non-toxicity of fluids.

-Low performance.

-Absence of techno-economic studies.

Alcohol

+ High thermal stability.

-Vacuum operation.

Organic Solvents + Only convenient for high driving temperature applications.

-Low performance for cooling.

-In most of the cases possess the same toxicity as NH3.

-Absence of techno-economic studies.

In the case of the ammonia-based working pairs, the new absorbents LiNO3 and NaSCN seem to be good candidates to substitute H2O. However, experimental studies to explain and avoid the lower-thanexpected experimental performance on these systems and economic studies have to be performed before a commercial machine launch. In the case of the water-based systems, H2O-LiBr continues to be the most reliable and convenient working pair. The rest of the studied candidates up to date don't seem to present enough advantages to overcome their associated drawbacks. The H2O-LiCl pair has a higher theoretical performance; however, crystallization risks are increased for this working pair compared with the H2O-LiBr working pair, for which this problem is already important. Therefore, a commercial system based on the H2O-LiCl working pair for continuous operation seems difficult. There was an explosion of interest in ILs in recent years because of their excellent physical and thermal properties; however, more in-depth studies are necessary to assure the long-term effects of their use. Novel environmentally friendly ILs and deep eutectic solvents from natural sources such as organic acids or aminoacids, with similar properties to ILs and which are claimed to be non-toxic and biodegradable, could be the future direction in this research area [START_REF] Bubalo | A brief overview of the potential environmental hazards of ionic liquids[END_REF]. On the other hand, HFCs do not seem to be an option for the future as most of them have an elevated global warming potential and in any case, they will be banned under the Kigali

Figure 2 :

 2 Figure 2: Proposed innovations in the vapor purification process of NH3-H2O systems. (a) Bubble vapor purifier [57] and (b)refining spray directly integrated in the desorber[START_REF] Mendes | A rich solution spray as a refining method in a small capacity, single effect, solar assisted absorption machine with the pair NH3/H2O: Experimental results[END_REF].

  [START_REF] Garimella | Development and demonstration of a compact ammonia-water absorption heat pump prototype with microscale features for space-conditioning applications[END_REF] later escalated the system to a 3.5-kW prototype that delivered 3.2 kW of cooling with a COP of 0.53[START_REF] Garimella | Development and demonstration of a compact ammonia-water absorption heat pump prototype with microscale features for space-conditioning applications[END_REF], and Staedter et al. (2018) constructed a 7-kW system based on the same concept that delivered chilled water at 7.2°C with a COP of 0.51[START_REF] Staedter | Development of a micro-scale heat exchanger based, residential capacity ammonia-water absorption chiller[END_REF].[START_REF] Goyal | Experimental evaluation of a small-capacity, waste-heat driven ammonia-water absorption chiller[END_REF] constructed a waste-heat-driven and air-cooled chiller designed for a thermal COP of 0.55. The machine provided between 1.91 and 2.54 kW of cooling at ambient temperatures between 29.7 and 44.2°C. The absorber and desorber were the limiting components while the rest of the components performed near design.[74][75].

Figure 3 :

 3 Figure 3: Zhu and colleagues' (a) simulated COP against generator temperature (Ta=Tc=15°C, Te=-20°C) and (b) COP experimental vs. simulated (Ta=Tc=23°C, Tg=100°C) [119] for an NH3-NaSCN single-stage system.

Figure 4 :

 4 Figure 4: Comparison of COP against generator temperature (a) for the NH3-H2O, NH3-LiNO3, and NH3-NaSCN working pairs [116] and (b) for the NH3-LiNO3 and NH3-NaSCN working pairs at different evaporator temperatures (Ta=Tc=40°C) [91].

Figure 5 :

 5 Figure 5: Cai and colleagues' experimental and simulated COP comparison for the (a) NH3-LiNO3 system and the (b) NH3-NaSCN system [95].

Figure 6 :

 6 Figure 6: COP against generator equilibrium temperature (Tg=T8) for a (a) water-cooled (Te=5°C) and (b) air-cooled singleeffect absorption chiller [152].

Figure 7

 7 Figure 7 Römich and colleagues' COP against driving temperature for the H2O-[dema][CH3SO3] working fluid, simulated and experimental results (Tc=Ta=24°C, Te=11°C) [226]. Radspieler and Schweigler (2011) studied the H2O-[emim][(CH3)2SO4] working fluid with an experimental chiller Built at ZAE Bayern using an adiabatic desorber and absorber (spray chambers with external plate heat exchangers). COP values similar to the ones with the H2O-LiBr working fluid were obtained but with a much lower cooling capacity (about one-third at Tg=95°C, Tc=Ta=27°C, and Te=17°C). Moreover, the tested working pair showed a lack of chemical stability [37] [228].

Figure 8 :

 8 Figure 8: COP comparison against generator temperature for different binary [(1) H2O-NaOH, (2) H2O-LiI, (3) H2O-LiCl, (4) H2O-LiBr] and other multi-component working fluids (5 to 16) [229].

Figure 9 :

 9 Figure 9: Experimental results for different H2O-ILs and H2O-LiBr working pairs [37].

Figure 10 :

 10 Figure 10: Results for COP vs. Tg for the H2O-LiBr and NH3-H2O working fluids at Te= 2°C [255].

  Flores et al. (2014) studied six different working fluids (H2O/LiBr, NH3/H2O, NH3/NaSCN, NH3/LiNO3, H2O/LiCl and H2O/CaCl2) using a mathematical model developed in the EES software [258]. The NH3-H2O working fluid yielded the lowest COP values (around 0.53) due to the absence of a rectifier. The H2O-LiBr and H2O-LiCL working pairs obtained the highest COP values (around 0.81). The NH3-LiNO3 and NH3-NaSCN working pairs exhibited medium performance, the former having higher COP values than the latter. Cerezo et al.(2018) developed a dynamic simulation of a system comparing the NH3-H2O, H2O-LiBr, NH3-NaSCN, NH3-LiNO3, and H2O-LiCl working fluids using the EES and TRNSYS software tools[START_REF] Cerezo | Dynamic Simulation of an Absorption Cooling System with Different Working Mixtures[END_REF]. The H2O-LiCl working pair demonstrated the best performance; however, its operating temperatures are restricted owing to crystallization (see Figure11). For the case of adiabatic microchannel membrane-based absorbers, De Vega et al. compared theoretically the H2O-LiBr, H2O-LiCl, and NH3-LiNO3 working pairs and observed that the NH3-LiNO3 working pair would provide the highest cooling capacities with the lowest absorber volume ratio, followed by H2O-LiBr and H2O-LiCl at last, while the driving temperatures required are the lowest for H2O-LiCl, followed by NH3-LiNO3 and H2O-LiBr at last. Moreover, the H2O-LiBr working pair possesses the highest COPs[START_REF] De Vega | Modeling and performance analysis of an absorption chiller with a microchannel membrane-based absorber using LiBr-H2O, LiCl-H2O, and LiNO3-NH3[END_REF]. For the case of negative cooling, Crepinsek et al. (2009) compared the NH3-H2O, NH3-LiNO3, NH3-NaSCN, CH3NH2-H2O, R22-DMEU, R32-DMEU, R124-DMEU, R152a-DMEU, R125-DMEU, R134a-DMEU, TFE-TEGDME, CH3OH-TEGDME, and R134a-DMAC working pairs in terms of COP and f, and concluded that R124-DMEU, R125-DMEU, NH3-LiNO3, and NH3-NaSCN are possible alternatives [260].

Figure 11 :

 11 Figure 11: Comparison of different working fluids in terms of COP against Tg at Ta=Tc of (a) 30°C and (b) 44°C [259]. Kim et al. compared seven ILs ([emim][Tf2N], [emim][BF4], [bmim][BF4], [bmim][PF6], [hmim][Tf2N], [hmim][BF4], and [hmim][PF6]) as absorbents and HFCs (R125, R134a, R143a, R152a, R32), R114, R124, and H2O as refrigerants. R32 presented the lowest f and relatively high COP combined with the ILs with [PF6 -] and [BF4 -] anions. R124 would not be considered owing to its negative environmental impact, and H2O-[emim][BF4] presented the highest COP (0.91) [261] [262]. Kim and Gonzalez later presented an exergy analysis using different ILs and obtained and exergetic COP superior to the rest for the H2O-[emim][BF4] working pair [263]. Zheng et al. (2014) presented a review of ILs in working pairs and observed that the COP of H2O-ILs systems do not exceed the traditional H2O-LiBr working fluid, but H2O-[DMIM]DMP is close and overcomes crystallization and corrosion problems. Moreover, it broadens the operating range. For NH3-ILs systems, COPs are lower than for NH3-H2O systems, but no rectifier is needed. Finally, the TFE-ILs system COP is commonly between that of the H2O-LiBr and NH3-H2O systems (the TFE/[BMIM]Br system having the highest COP), but offers a wider operating range[START_REF] Zheng | A review of imidazolium ionic liquids research and development towards working pair of absorption cycle[END_REF]. Sujatha and Venkatarathnam (2018) studied the suitability of HFC and HFO as refrigerants for ILs and compared them using [hmim][TF2N] as the absorbent. HFC refrigerants proved to be better (a COP of 0.5 can be achieved using the R32 refrigerant). Hence, IL-HFC working pairs are suitable for these systems[START_REF] Sujatha | Comparison of performance of a vapor absorption refrigeration system operating with some hydrofluorocarbons and hydrofruoroolefins as refrigerants along with ionic liquid [hmim][TF2N] as the absorbent[END_REF].[START_REF] Moreno | Absorption refrigeration cycles based on ionic liquids: Refrigerant/absorbent selection by thermodynamic and process analysis[END_REF] presented a COSMO-based/Aspen HYSYS methodology to perform an extensive thermodynamic evaluation of ILs in absorption refrigeration cycles. In total, 7,200 systems were evaluated (900 ILs and eight refrigerants) and adequate refrigerant-absorbent pairs competitive with conventional systems were selected (see Figure12). The optimum working fluid depends on the cooling temperature. Water showed excellent thermodynamic properties owing to the its high enthalpy of vaporization and good affinity to ILs; however, because of its limitation to working at positive cooling temperatures, methanol and ammonia are good options for cooling temperatures between -5 and 5°C[START_REF] Moreno | Absorption refrigeration cycles based on ionic liquids: Refrigerant/absorbent selection by thermodynamic and process analysis[END_REF].

Figure 12 :

 12 Figure 12: COP vs. Tg for different refrigerant-IL systems [265].6.2 Discussion of the developed prototypes and commercial systemsFigure13shows the percentage of existing brands and experimentally developed prototypes during the past 35 years of small-capacity single-stage continuous absorption chillers divided by working fluid. It is clear that the dominant working fluids are H2O-LiBr and NH3-H2O, the former being the most predominant. Indeed, the manufacturers of H2O-LiBr chillers seem to possess the largest market share[START_REF] Labus | Review on absorption technology with emphasis on small capacity absorption machines[END_REF]. Moreover, Figure13bshows that the third most widely studied working pair is the NH3-LiNO3; most probably due to the advantages mentioned in the previous sections.

Figure 13

 13 Figure 13 Single-stage absorption chillers. (a) Percentage of available brands by working fluid and (b) percentage of the experimental prototypes developed during the past 35 years. Most of the comparative studies between absorption systems and working fluids are theoretical [253] [254] [90] [255] [259] [260] [261] [265]. At the same time, there is scarce information about the comparison of experimental systems owing to the different constraints in terms of technological design depending on the selected working fluid and the different experimental conditions [39]. In order to make a representative comparison between the different experimental systems found in the literature, their experimental COPs were compared directly with their maximum theoretical performance (COPcarnot), since this is a reference for comparison in function of the source temperatures at the conditions of the represented COPexp. An increase in the COPcarnot indicates an increase in the generator or evaporator temperatures or a decrease in the intermediate source temperature and vice versa. In real applications, the cooling temperature and intermediate temperature are regularly user-defined, which indicates that the

Figure 14 :

 14 Figure 14: Graphical representation of the laboratory-developed prototypes and commercial systems in separate colors by working pair.For a better clarification of the dispersed data, a graphical representation of the prototypes and commercial systems comparing the system's experimental or catalog COPs vs. their carnot COPs is presented in Figure15. A linear trend line of the same color for each working pair was traced to reduce the effects of the differences between the systems. These trend lines show how the experimental COPs of the different working pairs change as a function of the COPcarnot. One can see how the H2O-LiBr working fluid is restricted to working at low Tsink and Tg (low COPcarnot) to avoid crystallization, while the NH3-H2O and NH3-LiNO3 working pairs can operate at wider temperature ranges. Unfortunately, for the rest of the experimental working pairs, there are not enough experimental studies to allow for a comparison.

Figure 15 :

 15 Figure 15: Graphical representation of the systems grouped by working pair. (a) COPexp vs. COPcarnot of laboratory-developed prototypes and (b) COPcat/COPexp vs. COPcarnot for the commercial systems and the experimentations performed on them.

Table 1 :

 1 In-lab developed prototypes of small-capacity NH3-H2O absorption cooling systems and operating conditions.

	_ (kW)	Heat source	Heat sink	R E C T	S H X	R H X	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	_ !" (kW)	COPexp	COPcarnot	Comments	REF
	10.5	Geothermal	Water	x	x		125	30	30	-10	-	0.4	1.57		[59]
	-	Exhaust gases	Air	x	x	x	180	30	30	0	7	0.255	3.01		[70]
	2	Solar	Air	x	x		109	40	40	20	-	0.05	2.65	Air-cooled by natural convection.	[60]
														Spray integrated in
	4	Solar	Air	x	x	x	110	32	32	11	-	0.54	2.75	desorber for vapor	[62]
														purification
	10	Solar	Air	x	x	x	102	27	27	15	10.7	0.74	4.8	Focus on generator	[63] [64]
	0.3	Electric heater	Water	x	x	x	115.4	30.2	30.2	7.1	0.24	0.34	2.66	Microscale monolithic AHP	[71]
	5	Electric heater	Water	x	x	x	80	27	27	18	-	0.65	4.86		[68]
	3.5	Combustion module	Water	x	x	x	180	38.5	38.5	9.5	3.2	0.53	3.04		[72]
	10	Solar	Air	x	x	x	114	23	23	-2	10.1	0.69	2.55		[66]
	2.71	Waste heat from diesel generator	Air	x	x	x	398.9	51.67	51.67	19.8	2.45	0.55	4.75	Nominal temperatures and COP values.	[74] [75]
	7	Combustion module	Air	x	x	x	183	39.4	39.4	7.9	6.9	0.51	2.81		[73]

Table 2 :

 2 NH3-H2O small-capacity commercial absorption chillers and their catalog data.

	Company	Model	_#$% (kW)	Heat source	Heat sink	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	COPcat	COPcarnot	Comments	REF
	AGO (GER)	ago congelo	50	-	Water	95	20	20	0	0.6	2.78		[76]
	SolarNext (GER)	Chillii PSC10 / PSC12	10 / 12	-	Water	75 85	24 24	24 24	15 6	0.65 0.62	4.69 2.64	Cooling ceiling Fan coils	[77] [78]
			12.3	-	Water	95	24	24	-3	0.47	1.93	Minus-cold
	Pink (AT)	PC19	18.6 19.5	--	Water Water	85 75	24 24	24 24	6 15	0.62 0.72	2.64 4.69	Fan coils Cooling ceiling	[79]
		GAHP-AR	16.9		Air	-	35	35	7	0.67	-	Gas
		GAHP-GS	7 / 12.1 / 16.4	Natural	Geothermal	-	40	40	7	0.67	-	Utilization
	ROBUR (IT)	GAHP-WS	10.6 / 16.6 / 18.7	Gas or	Water	-	30	30	7	0.72	-	Efficiency	[80]
		GA-ACF	13.3 17.1 / 17.7	GPL	Air Air	--	35 35	35 35	-5 7	0.52 0.7	--	(GUE) is used as COPcat.
	Tranter
	Solarice	XS30 / XS50	30 / 50	-	-	95	20	20	10	0.6	5.77		[81] [82]
	(GER)

Table 3 :

 3 Experimental studies on small-capacity commercial NH3-H2O absorption chillers reporting the operating conditions.

	Model	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	COPexp	COPcarnot	_ !" (kW)	REF
	SolarNext Chillii PSC10	75 86.3	22 22.3	22 22.3	14 15.4	0.63 0.62	5.46 7.45	6.6 10.5	[88] [86]

Table 4 :

 4 In-lab developed prototypes of small-capacity NH3-LiNO3 absorption cooling systems and operating conditions.

	_ (kW)	Heat source	Heat sink	S H X	R H X	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	_ !" (kW)	COPexp	COPcarnot	REF
	-	Electric heater	Water	x	x	90	20	20	-10	1.6	0.54	1.69	[102]
	10.5	Geothermal	Water	x		134.6	20	20	-7	5.24	0.4	2.77	[103] [104]
	7.82	Electric heater	Air	x		143	29	29	10	4.5	0.4	4.08	[109]
	10	Electric heater	Air	x	x	90	35	35	15	9.3	0.6	2.18	[41]
	10	Electric heater	Water	x	x	90	37.5	37.5	15	10.16	0.67	1.85	[41]
	3	Electric heater	Water	x		95	18	18	6.6	2.63	0.68	5.13	[110]
	4.5	Electric heater	Water	x		100	20	20	8.6	4.5	0.42	5.3	[111]
	-	Electric heater	Air	x		94	37.4	44.6	6	1.43	0.33	1.19	[95]
	-	Electric heater	Water	x		95	20	20	8	3.1	0.62	4.77	[112]

Table 5 :

 5

	(kW)	Heat source	Heat sink	S H X	R H X	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	_ !" (kW)	COPexp	COPcarnot	REF
	-	Electric heater	Water	x		100	23.5	23.5	5	0.82	0.481	3.08	[119]
	-	Electric heater	Air	x		104	33	43.8	-2.4	0.65	0.35	1.22	[95]

In-lab developed prototypes of small-capacity NH3-NaSCN absorption cooling systems and operating conditions.

_

Table 6 :

 6

	(kW)	Heat source	Heat sink	S H X	R H X	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	_ !" (kW)	COPexp	COPcarnot	Comments	REF
	1.76	Solar	Water			90	27.5	27.5	15	-	0.55	3.97		[156]
	7	Hot water	Air	x		160	35	35	6	5	0.8	2.78		[160]
	5.28	Solar	Water	x		71	29.5	32.5	16.2	6.2	0.69	2.43		[158]
	10	Electric heater	Water	x		75	27	27	15	10.2	0.76	3.31		[165] [166]
	2	Hot water	Air	x		95	35	35	12	2.6	0.62	2.02		[161]
	0.04	Electric heater	Water			100	45	30	15	-	0.47	1.80		[164]
													Micro nozzles in
	-	Solar	Water			85	28	28	15.5	0.28	0.6	3.68	evaporator and	[174]
													absorber
	3.5	Electric heater	Water	x		87	28	28	5	1.2	0.28	1.98	Adiabatic absorber	[170]
	4.5	Solar	Air			105	45	42.5	10	3.4	0.64	1.34	Adiabatic absorber	[172]
	8	Solar	Water	x		88	30	30	11	4.5	0.27	2.40		[175]
	-	Electric heater	Water	x		75	27	27	15	4	0.7	3.31		[37]
													Condenser and
	-	Electric heater	Water	x		75	27	27	15	4.5	0.71	3.31	generator improved	[37]
													from the last one
	50	Electric heater	Water			90	30	30	16	50	0.8	3.41		[37]
	6	Solar	Air	x		90	28	28	15	8.93	0.65	3.78	Adiabatic absorber	[153]

In-lab developed prototypes of small-capacity H2O-LiBr absorption cooling systems and operating conditions.

_

Table 7 :

 7 H2O-LiBr small-capacity commercial absorption chillers and their catalog data.

	Company	Model	_#$% (kW)	Heat sink	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	COPcat	COPcarnot	REF
		Wegracal SE 15 / SE 30 / SE 50	15	Water	90	30	30	11	0.71	2.47	[183]
	EAW GER)	(Discontinued)	30 / 50	Water	90	30	30	11	0.73	2.47	[184] [185]
		Wegracal Maral 1 / Maral 2	15 / 30	Water	86	27	27	9	0.75	2.58	[186] [187]
		RXZ-11	11	Water	90	30	30	10	0.65	2.34
	Jiangsu Huineng (CHIN)	RXZ-23	23	Water	90	30	30	10	0.68	2.34	[188]
		RXZ-35	35	Water	90	30	30	10	0.7	2.34
	Rotartica (SP)	045v (Discontinued)	4.5	Air	90	35	35	12	0.63	1.88	[189]
	Shandong Lucy New Energy Technology Co., Ltd (CHIN)	TX-11/TX-23/TX-35	11.5 / 23 / 35	Water	90	27	27	10	0.69	2.89	[190]
	PURIX (DEN)	A25	2.5	Air	80	35	35	13	0.81	1.66	[191]
		SHL003/SHL005/SHL008/SHL0	10.5 /
	Sakura (JP)	10	17.5 /	Water	88	31	31	8	0.7	1.93	[192]
		(Discontinnued)	28 / 35
	SonnenKlima (GER)	suninverse 10	10	Water	75	27	27	15	0.76	3.31	[193]
	Thermax (IN)	LT 0.5 / LT 1	17.5 / 35	Water	90	29	29	7	0.66	2.14	[194]
	Yazaki (JAP)	WFC-400 / WFC-600 (Discontinued) WFC-SC5/WFC-SC10	4.6 / 7 17.6 / 35.2	Water Water	75 88	29.5 31	29.5 31	9 7	0.6 0.7	1.80 1.84	[195]

Table 8 :

 8 Experimental studies on small-capacity commercial H2O-LiBr absorption chillers reporting the operating conditions.

	Model	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	COPexp	COPcarnot	_ !" (kW)	REF
	EAW Wegracal SE 30	67.5	25	25	10	0.67	2.36	20	[216]
		100	33	33	15	0.52	2.87	4.6	[210]
		77.1	24	24	10.1	0.66	3.09	2.82	[211]
	Rotartica 045v	91 92.5	27.7 31.2	27.7 31.2	11.5 11.5	0.57 0.51	3.05 2.42	5.7 4.4	[212]
		90	33	33	13	0.74	2.25	5.6	[213]
		78	16	16	7.5	0.75	5.83	5.75	[178]
		81	28	28	12	0.58	2.67	4	[179]
	Yazaki WFC-600	75	29.5	29.5	10.5	0.47	1.95	2.15	[84]
		86	32	32	14.5	0.54	2.47	5.25	[181]
		80	32	32	15	0.51	2.30	4.2	[182]
	Yazaki WFC-SC5	80.7	18	18	8.5	0.24	5.25	4.7	[198]
		90	28	28	16	0.66	4.11	29.5	[207]
	Yazaki WFC-SC10	85	29.5	29.5	12	0.72	2.53	25	[209]
		93	29	29	9.5	0.75	2.53	40.5	[202]

Table 9 :

 9 In-lab developed prototypes of small-capacity H2O-IL absorption cooling systems and operating conditions.

	_ (kW)	Heat source	Heat sink	S H X	R H X	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	_ !" (kW)	COPexp	COPcarnot	REF
	10	Electric heater	Water	x		79	24	24	11	1.3	0.75	3.41	[226]
	-	Electric heater	Water	x		75	27	27	15	2.7	0.73	3.31	[37]

  Shelide proposed the CO2-[bmim][PF6] working fluid for absorption refrigeration systems [248]. Shiflett and Yokozeki measured the solubilities and diffusivities of CO2 in [bmim][PF6] and [bmim][BF4] [249]. Cai and Paolucci (2007) developed a dynamic modeling of a system using the CO2-[bmim][PF6] working pair and studied the transient response, design, and operating parameters affecting its performance

Table 10 :

 10 In-lab developed prototypes of small-capacity absorption cooling systems with refrigerant bases other than H2O and NH3 and operating conditions.

	REF	ABS	_ (kW)	Heat source	Heat sink	S H X	R H X	Tg (°C)	Tc (°C)	Ta (°C)	Te (°C)	_ !" (kW)	COPexp	COPcarnot	REF
	R134a	IL	0.1	Electric heater	Water			70	41	41	28	0.03	0.3	1.96	[252]
	C3H6O	ZnBr2	10.6	Solar	Water	x	x	50.5	22	22	13	0.7	0.4	2.8	[246]

Table 11 :

 11 Comparison of the main binary working fluids studied for single-stage absorption cooling.

	REFRIGERANT	ADVANTAGES AND DRAWBACKS OF REFRIGERANT	ABSORBENT	ADVANTAGES OF THE WORKING PAIR	DRAWBACKS OF THE WORKING PAIR
				+ Well documented properties and	-Rectifier needed.
			H2O	materials compatibility.	-Lower COP than H2O-LiBr system.
				+ No crystallization risks.	-Alkaline and corrosive solution.
				+ Low driving temperature.
				+ Higher COP than NH3-H2O.
			LiNO3	+ No rectifier needed. + Low crystallization risk.	-Limited performance due to high viscosity.
	NH3	+ High enthalpy of evaporation. + Refrigeration under 0°C. + Moderate pressure ranges. -Toxicity of NH3. -Explosion hazard potential for confined spaces.	NaSCN	+ High solubility in NH3. + No corrosion of steel. + Low driving temperature (higher than for LiNO3). + Higher COP than NH3-H2O. + No rectifier needed. + High solubility in NH3.	-Risk of crystallization for evaporator temperatures below -10°C. -Limited performance due to high viscosity (but less than for LiNO3).
				+ No corrosion of steel.
					-Depending on the chemical structure,
				+ High solubility.	long-term thermal and toxicity
			Ionic Liquids	+ No rectification needed. + No crystallization risk.	problems. -High cost.
				+ Good performance.	-Absence of properties
					documentation.
				+ Well documented properties and
				materials compatibility.	-High viscosity.
			LiBr	+ High refrigerant vapor purity.	-Corrosion risks.
	H2O	+ High enthalpy of evaporation. + Abundantly available. + Low-cost. + Environmentally friendly. -Only positive refrigeration	LiCl	+ Higher COP compared to NH3-H2O systems. + Good stability. + Better cycle performance than H2O-LiBr.	-Crystallization risks.
		temperatures.
		-Vacuum operation.

& -./ = & + ' ( *

Acknowledgements

The authors would like to thank the Mexican sectorial fund "CONACYT-SENER-SUSTENTABILIDAD ENERGÉTICA" and the PACs-CAD project for their support to this research. "PACs-CAD" is a cross border project supported by Interreg France-Switzerland, a European territorial cooperation program. It has received 463 185 € through the European regional development fund (ERDF) and 150 000 CHF through the Swiss Federal Interreg funds.

Amendment to the Montreal Protocol [START_REF] Iea | The Future of Cooling[END_REF]. Finally, alcohols seem to be better adapted for heating and heat pump applications. None of these mentioned refrigerants have shown special advantages in terms of performance compared with the conventional H2O-LiBr and NH3-H2O working pairs. CO2 has also been proposed in recent years as the refrigerant for absorption cooling systems and may be an option in the future; however, new absorbents for this refrigerant have to be studied for better performance and special attention must be paid to its high operating pressures, which raise new challenges in terms of design.

The increasing energy demand, specifically in the building sector, and the search for low CO2 depletion systems will bring growing attention to absorption cooling systems in the following years. Therefore, their study must continue. However, there are some scientific challenges such as new and better collaborations between research groups, academia, and industry [START_REF] Seiler | Sustainable cooling strategies using new chemical system solutions[END_REF]. More information is needed about the physical interactions and surface properties of the new working pairs (e.g., surface wetting or droplet formation) that lead to more efficient systems. Studies need to be conducted in the search for new additives to induce Marangoni convection, better wettability properties, and less corrosion [START_REF] Seiler | Sustainable cooling strategies using new chemical system solutions[END_REF]. In this review, an up-to-date global overview of the single-stage continuous absorption cooling systems operating on binary working fluids was provided from a theoretical, experimental, and commercial point of view. A regrouping of the experimental prototypes, commercial systems, and their operating conditions was done regarding their performance and working pair. The authors aim to provide a useful tool to better understand the advantages and disadvantages of the studied working pairs, and, consequently, to help in making a more informed choice of working fluid depending on the specificities of the required project.