Adsorption is the process in which molecules from one phase (usually vapor) accumulate on the surface of a solid adsorbent. Conversely, absorption involves the transfer of a substance from one phase into the bulk of another. The key differences between these two mechanisms lie in the nature of the sorbent and the time required for the sorption cycle, which is generally longer for adsorption [7]. To evaluate the performance of a sorption machine, the solar COP ( COP Solar ) and the thermal COP ( COP Thermal ) must be evaluated. These two COPs are defined as follows:

Q ˙ cold : Cooling capacity (W); Q ˙ hot : Heat supplied to the generator (W); Q ˙ solar : Solar energy input (W); η solar collector : Efficiency of the solar collector.

3.1.1. Operating Principle

The ideal adsorption refrigeration cycle consists of four thermodynamic processes, commonly represented on a Clapeyron diagram (Figure 1). During heating (A-B-D), the adsorbent releases the refrigerant vapor, which condenses at point C. Upon cooling (D-F-A), adsorption resumes as the refrigerant vapor is reabsorbed, producing the cooling effect in the evaporator [8].

Figure 1.Clapeyron diagram of ideal adsorption cycle.

3.1.2. Critical Review and Comparative Analysis of Solar Adsorption Refrigeration

Adsorption cooling dates back to the 19th century, when Faraday first demonstrated vapor adsorption using solid sorbents. Early solid-vapor systems were technically simple but inefficient [9]. Recent developments have focused on improving performance through material optimization and system integration.

Early industrial development of adsorption refrigeration technology, notably introduced to the U.S. market by Nishiyodo Kuchouki Company Ltd. in 1986 [10], demonstrated the feasibility of adsorption systems for thermal refrigeration and related applications such as separation and purification processes [11]. However, subsequent research has highlighted that system performance remains highly sensitive to design choices and operating constraints.

From a working-pair perspective, activated carbon-methanol and silica gel-water dominate the literature, each offering distinct advantages and limitations. Mohand Berdjaa etal. [12], developed a solar-powered adsorption refrigeration system based on the activated carbon-methanol pair, explicitly targeting socio-economic constraints through low-cost components, minimal maintenance, and a valve-free configuration. Their semi-pilot prototype achieved a thermal COP of 0.49, which is relatively high for methanol-based systems operating with low-grade solar heat. This performance reflects a deliberate compromise: the system prioritizes technological simplicity and compatibility with low-temperature solar collectors over maximum thermodynamic efficiency. Nevertheless, the limited adsorption capacity and slower mass transfer kinetics of methanol on activated carbon inherently restrict further COP improvement.

In contrast, systems employing the silica gel-water pair generally report higher COP values due to superior adsorption affinity and faster kinetics. Ghilen Najeh etal. [13] achieved a COP of 0.62 and a cooling capacity of 5.64 kW using a two-bed silica gel-water system. The enhanced performance is not solely attributable to the working pair but also to the adoption of a multi-bed configuration and improved thermal management. Their validated numerical model further demonstrates that precise control of heat and mass transfer processes is essential to exploit the full potential of silica gel-water systems. However, these gains come at the expense of increased system complexity and a higher sensitivity to ambient temperature, particularly during heat rejection.

Experimental investigations by Himsar Ambarita and Hideki Kawai [14], further illustrate the influence of adsorbent composition on performance. By testing activated alumina, activated carbon, and mixed adsorbent beds under identical flat-plate solar collector conditions, they showed that pure activated carbon achieved the highest average COP (0.074), although absolute COP values remained low. These results emphasize that material selection alone is insufficient to achieve competitive performance without optimized heat transfer and cycle control.

Several studies underline the critical role of solar heat supply and thermal management. Rifat Ara Rouf etal. [15] demonstrated that integrating thermal energy storage with silica gel-water adsorption chillers improves operational stability and overall efficiency compared to direct solar coupling, albeit at the cost of additional collector area. Similarly, Sourav Mitra etal. [16] showed through simulation that cycle time optimization is essential to balance specific cooling capacity and COP, particularly under varying heat source temperatures. Their results indicate that COP is relatively insensitive to heat source temperature, whereas cooling capacity strongly depends on cycle timing and thermal input quality. Climatic operating conditions also significantly affect system performance. Under Dhaka climatic conditions, Rifat Ara Rouf etal. [17] reported a COP of approximately 0.35 and a cooling capacity of 10 kW, highlighting the detrimental impact of high ambient temperatures on heat rejection processes. These findings confirm that adsorption systems must be specifically adapted to local climatic constraints to maintain acceptable performance.

Material enhancement and compact system design represent two additional pathways explored in the literature. Shahab Edin Hamrahi etal. [18] demonstrated that incorporating nano-activated carbon into activated carbon-methanol beds can improve adsorption capacity and enhance performance by up to 33%, partially mitigating intrinsic material limitations. Shu Xu [19] and Zhaohong He etal. [20] independently emphasized the importance of heat and mass recovery, compactness, and advanced valve control. Their results show that simple single-bed systems suffer from low COP due to poor thermal utilization, whereas multi-bed configurations and compact designs significantly improve specific cooling power and volumetric performance.

Advanced cycle configurations further expand the operating envelope of adsorption systems. Z. H. He etal. [21] demonstrated that two-stage adsorption chillers can operate with driving source temperatures as low as 45˚C, albeit with moderate COP values. Complementarily, K. M. Ariful Kabir etal. [22] showed that incorporating mass recovery into two-bed solar adsorption systems can enhance cooling capacity by approximately 9%, confirming that system-level optimization strategies are as critical as material selection.

Overall, the reviewed studies highlight a fundamental trade-off in solar adsorption refrigeration technology. Systems emphasizing low cost, simplicity, and low-temperature operation achieve moderate COP values but offer high socio-economic relevance, particularly for decentralized applications. Conversely, higher-efficiency systems rely on advanced materials, multi-bed configurations, heat and mass recovery, and higher-quality solar heat, resulting in increased complexity and cost. Consequently, the selection and design of solar adsorption refrigeration systems must be guided by application-specific requirements, climatic conditions, and economic constraints rather than COP alone.

3.1.3. Critical Synthesis of Solar Adsorption Cooling Technologies

Beyond individual experimental and numerical studies, the performance of solar adsorption cooling systems strongly depends on the choice of the working pair, the driving heat source temperature, and the solar collector technology. Activated carbon-methanol systems typically operate at relatively low generation temperatures (80˚C - 120˚C), making them compatible with flat-plate or evacuated tube collectors. However, their COP generally remains limited (0.1 - 0.4), mainly due to low adsorption capacity and poor heat transfer within the adsorbent bed.

In contrast, silica gel-water systems require higher regeneration temperatures (above 80˚C) but exhibit superior mass transfer characteristics and higher achievable COP values, reaching up to 0.62 in optimized two-bed configurations. The performance gap between studies such as Berdjaa etal. (COP ≈ 0.49) and Najeh etal. (COP ≈ 0.62) can be primarily attributed to differences in adsorbent thermal conductivity, bed configuration, and cycle time optimization.

Several studies highlight that system-level enhancements, rather than material selection alone, are decisive for performance improvement. The integration of heat and mass recovery processes, thermal energy storage, and nano-enhanced adsorbents has been shown to increase COP by 10% - 33%. Furthermore, the choice between direct solar coupling and stored solar heat significantly affects system stability and daily cooling output, especially under fluctuating tropical solar conditions.

Overall, adsorption cooling systems remain attractive for off-grid and low-temperature applications; however, their deployment in tropical climates requires careful matching between collector type, driving temperature, and cycle control strategy to overcome inherent limitations in specific cooling capacity.

3.2.1. Operating Principle

In absorption systems (Figure 2), the refrigerant vapor is absorbed into a liquid solution and then regenerated using heat. Depending on the regeneration process, systems can be classified as single-effect, half-effect, or double-effect [23]. Heat supplied to the generator separates the refrigerant (water or ammonia) from the absorbent. The vapor then condenses and evaporates in subsequent components, producing the cooling effect. This cycle uses a heat source (11) to produce cooling, unlike compression systems which require mechanical work. The provided heat in the generator separates the refrigerant vapor (1) from the solution. This vapor is then cooled and condensed into a liquid (2) in the condenser. After its pressure is reduced, the refrigerant enters the evaporator, where it absorbs heat from the environment (17) and evaporates, creating the cooling effect. The resulting low-pressure refrigerant vapor (4) is then reabsorbed by the weak solution (6) in the absorber, forming a new, rich solution (5). This solution is finally pumped (P) back to the generator to restart the cycle. This process is particularly efficient in applications where a low-cost or waste heat source is readily available.

Figure 2.Schematic diagram of the absorption system.

3.2.2. Critical Synthesis of Solar Adsorption Cooling Technology

Absorption chillers using solar thermal energy are excellent alternatives to mechanical refrigeration. Absorption cooling systems are mature technologies that have proven their ability to provide clean cooling using low-grade solar and waste heat [24]. Early thermodynamic and numerical investigations focused primarily on conventional working pairs and steady-state cycle behavior. Omar Ketfi etal. [24] performed a detailed thermodynamic analysis of a commercial 70 kW Yazaki single-effect LiBr-H₂O absorption machine, demonstrating that the COP increases with generator and evaporator temperatures while decreasing with condenser and absorber temperatures. Their reported maximum COP of 0.77 at a generator temperature of 92˚C closely matches manufacturer data, validating the reliability of classical LiBr-H₂O absorption models. Similar numerical conclusions were obtained by Sérgio Hanriot etal. [25] for NH₃-H₂O systems; however, their work clearly demonstrated that operating an absorption machine outside its design envelope-such as using highly fluctuating automotive exhaust heat-can lead to extremely poor performance, with COP values as low as 0.05.

Subsequent studies explored alternative working pairs and system configurations to reduce generator temperature requirements while maintaining acceptable performance. Ridha Ben Iffa etal. [26] investigated multiple absorption cycles using NH₃-H₂O, NH₃-NaSCN, NH₃-LiNO₃, and LiBr-H₂O. Their results showed that NH₃-LiNO₃ systems combined with a compressor-assisted configuration can achieve COP values up to 0.71 at generator temperatures as low as 60˚C, highlighting the potential of hybrid solutions. Comparable COP values were reported by Raluca Porumba etal. [27] for LiBr-H₂O systems through numerical simulation, reinforcing the maturity and efficiency of lithium bromide-water cycles under moderate thermal conditions.

The integration of solar energy as the primary heat source introduced new constraints related to heat source stability and temperature level. P. Soto etal. [28] demonstrated that a solar-driven NH₃-LiNO₃ absorption system could achieve evaporator temperatures as low as 7˚C with COP values ranging from 0.28 to 0.48, depending on solar availability. Optimization methodologies were further introduced by Virang H. Oza and Nilesh M. Bhatt [29], who applied Taguchi design and regression analysis to NH₃-H₂O systems, identifying an optimal COP of 0.65 and highlighting the strong sensitivity of system performance to generator temperature and flow ratios. Application-specific studies continued to expand the operational envelope of absorption systems. Manu Thimmaiah and T.K. Chandrashekar [30] showed that LiBr-H₂O absorption heat pumps used for electronic chip cooling can achieve COP values exceeding 0.84 under favorable thermal conditions. At the same time, hybrid configurations were explored by D. Lounissi etal. [31] who demonstrated that compression-absorption systems using novel working fluids can reach COP values comparable to conventional NH₃-H₂O systems while enhancing operational flexibility. Efforts to improve system robustness and reduce operating temperatures intensified in later studies. Osman Wageiallah Mohammed and Guo Yanling [32] showed that integrating internal heat exchangers into NH₃-H₂O absorption systems significantly lowers cut-in temperatures and improves both COP and exegetic efficiency. Diffusion absorption systems were also investigated during this period; F. Schmid etal. [33] and Osman Wageiallah Mohammed and Guo Yanling [34] reported COP values typically below 0.3, reflecting the intrinsic limitations of diffusion-based cycles despite their mechanical simplicity and suitability for small-scale solar applications. Comparative and parametric studies further clarified the trade-offs between different ammonia-based working pairs. Jasim M. Abdulateef etal. [35] demonstrated that NH₃-NaSCN systems outperform NH₃-H₂O cycles at generator temperatures above 80˚C, although crystallization risks limit their low-temperature operation. Practical demonstrations by Darwesh etal. [36] confirmed that solar absorption systems can be locally manufactured for agricultural applications, albeit with modest COP values under variable operating conditions.

System-level modifications and solar collector integration were addressed by M. Benramdane etal. [37] and Jasim Abdulateef etal. [38], who showed that multi-generator and evacuated-tube-based NH₃-H₂O systems can operate efficiently at generator temperatures around 70˚C, achieving COP values close to 0.6. Experimental investigations of diffusion absorption systems by R. Mbarek and W. Mbarek [39] Abir Hmida etal. [40], Mohamed Izzedine Serge Adjibade etal. [41] and Asmaa A. Harraz etal. [42] consistently reported COP values between 0.24 and 0.8, depending on system scale, working fluids, and heat input quality. Climatic effects and seasonal performance became a major focus in more recent studies. Y. Agrouaz etal. [43] demonstrated significant COP variations across different Moroccan climates, while Dione F. etal. [44] showed that high-temperature solar collectors such as linear Fresnel reflectors can reliably supply NH₃-H₂O absorption machines during peak solar hours. Abu Shaban et al. [45] confirmed that both NH₃-H₂O and LiBr-H₂O systems exhibit comparable COP values within the generator temperature range of 90˚C - 120˚C, reinforcing the thermodynamic consistency of absorption models. Seasonal analyses by Elbir [46] further highlighted the influence of local climate on COP stability.

Finally, high-performance systems operating under optimized thermal and climatic conditions were reported by Chikh etal. [47] and Al-Harbi etal. [48] who demonstrated that advanced NH₃-H₂O absorption systems coupled with hybrid solar technologies can achieve COP values exceeding 1.0. These results confirm that absorption refrigeration performance can be significantly enhanced when system design, heat source quality, and operating conditions are coherently optimized.

Overall, the chronological evolution of the literature clearly indicates that absorption refrigeration technology has progressed from basic thermodynamic validation toward climate-adapted, high-performance solar systems. While LiBr-H₂O cycles remain the most efficient option for air-conditioning, NH₃-H₂O and alternative ammonia-based systems offer greater flexibility for refrigeration applications in tropical and arid regions. Consequently, future developments should prioritize integrated system design and solar heat management rather than isolated improvements in individual components.

3.2.3. Comparative and Critical Analysis of Solar Absorption Systems

The reviewed absorption refrigeration studies clearly demonstrate that system performance is predominantly governed by the thermodynamic properties of the working pair and the achievable generator temperature provided by the solar heat source. LiBr-H₂O systems consistently exhibit higher COP values (0.7 - 0.8) under moderate generator temperatures (70˚C - 95˚C), which makes them well suited for flat-plate and evacuated tube collectors. However, their operation is restricted to evaporation temperatures above 0˚C - 2˚C, limiting their application to air conditioning and chilled water production.

Ammonia-water absorption systems, while offering lower average COP values (0.4 - 0.6), present a wider operational range, including sub-zero evaporation temperatures. This makes them more suitable for refrigeration and cold storage in tropical and arid regions. The variability in reported COP values among NH₃-H₂O systems is primarily linked to generator temperature level, heat exchanger effectiveness, and solution heat recovery.

Studies employing advanced solar collectors, such as evacuated tubes or concentrating collectors like Linear Fresnel Reflector, report significantly improved system performance by enabling higher and more stable generator temperatures. In tropical climates, where high solar irradiance coincides with high ambient temperatures, absorption system efficiency becomes particularly sensitive to condenser and absorber heat rejection, emphasizing the importance of system integration and thermal management.

Consequently, absorption refrigeration systems powered by solar energy represent a mature and scalable solution, provided that the working pair, collector technology, and climatic constraints are jointly considered during system design.