Absorption refrigerator
An absorption refrigerator is a cooling device that operates using thermal energy rather than mechanical compression, employing a cycle where a refrigerant vapor is absorbed into a liquid absorbent, separated by heat, and then re-evaporated to produce refrigeration effects.[1] This system typically utilizes working fluid pairs such as ammonia and water or water and lithium bromide, allowing it to run on low-grade heat sources like waste heat, solar energy, or natural gas, making it suitable for applications where electricity is scarce or expensive.[2] The fundamental principle of an absorption refrigerator involves four main processes: absorption, generation, condensation, and evaporation. In the absorber, the refrigerant vapor (e.g., ammonia) is absorbed into the liquid absorbent (e.g., water), releasing heat and forming a strong solution; this solution is then pumped to the generator, where heat input desorbs the refrigerant, creating a weak solution that returns to the absorber and pure refrigerant vapor that proceeds to the condenser for liquefaction. The liquid refrigerant expands through a valve to the evaporator, where it absorbs heat from the cooled space at low pressure, before returning as vapor to the absorber, completing the cycle without moving parts beyond a small pump.[1] Key advantages include silent operation, longevity due to minimal mechanical wear, and environmental benefits from using natural refrigerants, though it achieves a lower coefficient of performance (COP) of 0.6–1.2 compared to vapor-compression systems (typically 3–5).[2] Absorption refrigeration has a rich history dating back to the mid-19th century, with Edmond Carré inventing the first intermittent absorption machine in 1850 using water and sulfuric acid, followed by his brother Ferdinand Carré's development of a continuous ammonia-water system patented in 1860.[3] The technology gained prominence in the early 20th century through innovations like the Einstein-Szilard pump-less design patented in 1930, which influenced diffusion absorption refrigerators still used in recreational vehicles and portable units today.[4] Modern applications span large-scale chillers in industrial cogeneration plants, solar-powered systems for off-grid cooling, and hospitality sectors leveraging waste heat, with ongoing research focusing on improving efficiency through advanced working pairs and hybrid configurations.[1]Fundamentals
Basic Principle
An absorption refrigerator operates on a heat-driven thermodynamic cycle that achieves cooling through the absorption and desorption of a refrigerant in a liquid absorbent, without relying on mechanical compression. In this system, a refrigerant vapor, such as ammonia, is absorbed into a liquid absorbent, typically water, forming a solution; heat is then applied to separate the refrigerant, allowing it to circulate and produce cooling effects.[1] This process contrasts with conventional vapor-compression refrigerators, which use electrical energy to power a compressor, by instead utilizing thermal energy as the primary driver.[5] The cycle consists of four main processes. In the generation stage, heat supplied to the generator causes desorption, releasing refrigerant vapor from the absorbent solution. The vapor then travels to the condenser, where it cools and condenses into a liquid, rejecting heat to the surroundings. Next, in the evaporator, the liquid refrigerant expands and evaporates at low pressure, absorbing heat from the cooled space to provide refrigeration. Finally, the refrigerant vapor is reabsorbed into the weakened absorbent in the absorber, completing the cycle and preparing the solution for recirculation via a small pump.[6] This mechanism enables the use of low-grade heat sources, such as natural gas, solar thermal energy, or industrial waste heat, to drive the refrigeration process, making it suitable for applications where electricity is limited or costly.[1] The basic schematic includes a generator for desorption, an absorber for reabsorption, a condenser for liquefaction, an evaporator for cooling, and a solution pump to maintain circulation, all interconnected in a closed loop.[7]Key Components
The absorption refrigerator operates through a network of interconnected components that facilitate the separation, circulation, and recombination of refrigerant and absorbent in a closed-loop configuration. The primary elements include the generator, absorber, condenser, evaporator, solution pump, and heat exchangers, each performing a specific function to enable heat-driven cooling without mechanical compression.[8] The generator, often heated by an external source such as steam or waste heat, desorbs refrigerant vapor from the absorbent-refrigerant solution by elevating its temperature, typically to 55–225°C depending on the system design; the resulting weak solution (depleted of refrigerant) is then directed back to the absorber.[8] The absorber mixes the incoming refrigerant vapor with the weak solution under cooling conditions, forming a strong solution that releases heat to an external sink; this process relies on the affinity between the refrigerant and absorbent to achieve efficient absorption.[8] In the condenser, the pure refrigerant vapor from the generator is cooled—often to 25–50°C—and liquefied, rejecting latent heat to the surroundings before flowing to the evaporator.[8] The evaporator allows the liquid refrigerant to expand and vaporize at low pressure (around 0–15°C), absorbing heat from the cooled space to produce the refrigeration effect, after which the vapor proceeds to the absorber.[8] The solution pump circulates the strong solution from the absorber to the generator, requiring minimal energy input compared to vapor compression systems.[8] Heat exchangers, such as solution heat exchangers, enhance overall efficiency by transferring heat between the strong and weak solutions, potentially increasing the coefficient of performance (COP) by 44–66% through thermal recovery.[8] Construction materials for these components are selected for compatibility with the working fluids and operational pressures. In ammonia-based systems, pressure vessels and piping are typically fabricated from steel, as ammonia is incompatible with copper or brass, ensuring durability and preventing corrosion when combined with corrosion inhibitors like sodium chromate.[2] For water-lithium bromide systems, corrosion-resistant alloys are employed to mitigate degradation from the hygroscopic absorbent.[8] These components integrate into a sealed, closed-loop system where the refrigerant and absorbent continuously cycle without external replenishment, maintaining system integrity. A still or rectifier is often incorporated at the generator outlet to purify the refrigerant vapor by separating impurities (e.g., water in ammonia systems), preventing contamination in downstream components and improving efficiency.[8] Safety features are essential, particularly in ammonia-based systems due to the refrigerant's toxicity and pressure risks. Pressure relief valves are standard on pressure vessels to automatically vent excess pressure, preventing ruptures and ensuring compliance with safety standards. Additional measures, such as excess flow valves and leak detection, may be integrated to contain potential releases.[9]History
Invention and Early Development
The absorption refrigerator was first invented by French engineer Edmond Carré in 1850, who developed an intermittent machine using water as the refrigerant and sulfuric acid as the absorbent.[10] This pioneering device marked the beginning of absorption-based cooling through a chemical process, though it required manual reheating for each cycle and was limited in practicality. Building on this, Edmond's brother Ferdinand Carré developed the first practical ammonia-water absorption system in 1858.[11] This system utilized ammonia as the refrigerant and water as the absorbent, enabling more efficient production of artificial cold. Ferdinand Carré refined the design in the late 1850s, achieving continuous operation that reduced manual intervention and improved reliability. He secured a French patent for the ammonia-water machine in 1859 and a U.S. patent in 1860 (No. 30,201), which described an apparatus capable of producing ice at rates up to 200 kg per hour in industrial scales.[12][13] Further enhancements in the 1860s included adaptations for greater efficiency, such as integrating agitators inspired by earlier vacuum systems to optimize fluid circulation, allowing for more automatic operation powered by external heat sources, though early versions still required some manual oversight.[12] The technology gained public attention through demonstrations, notably at the 1867 Paris Universal Exposition, where Ferdinand Carré's apparatus was exhibited for producing artificial cold and ice, highlighting its potential for commercial applications like food preservation during the American Civil War era. Despite these innovations, early absorption refrigerators faced substantial challenges, including low thermal efficiency—often requiring excessive heat input for modest cooling output—and persistent need for manual intervention in charging and regenerating the absorbent, which hindered scalability and led to limited adoption outside specialized industrial settings by the late 19th century.[14][12]Commercialization and Modern Advancements
In the 1910s and 1920s, Albert Einstein and Leó Szilárd collaborated on developing an innovative absorption refrigerator design that eliminated the need for mechanical pumps, addressing the noise issues associated with compressor-based systems of the era.[4] Their partnership began after Einstein, motivated by reports of fatal accidents from leaking mechanical refrigerators, sought a safer alternative; they filed a patent application in Germany on December 16, 1926, followed by filings in the UK and US, culminating in US Patent 1,781,541 granted in 1930.[15] This pump-free system relied on natural circulation driven by heat and gravity, using ammonia as the refrigerant, water as the absorbent, and butane as an auxiliary gas to enhance efficiency without moving parts.[16] Commercialization gained momentum in the late 1920s through Servel Inc., which licensed and produced gas-fired absorption refrigerators for household use, capitalizing on the design's quiet operation and fuel flexibility.[17] By 1927, Servel had partnered with Electrolux to manufacture these models in the US, achieving widespread adoption as they offered a reliable alternative to electric units during a period of limited electrification.[18] Popularity peaked in the 1930s, with Servel selling millions of units before the dominance of cheaper, more efficient electric compression refrigerators in the 1940s led to a decline in market share.[19] Following World War II, absorption refrigerators experienced a revival in the 1950s and 1960s, particularly for off-grid applications such as recreational vehicles (RVs) and boats, where access to electricity was unreliable.[20] Manufacturers like Electrolux and its Dometic brand developed compact, propane-powered models that operated without external power, enabling cooling in remote settings and boosting the RV industry's growth.[21] These units, often with capacities around 6-12 cubic feet, became standard in mobile applications by the mid-1960s due to their durability and multi-fuel compatibility.[22] Modern advancements since the 2000s have focused on enhancing efficiency and sustainability, including integration with solar thermal systems to utilize renewable heat sources for cooling.[23] Double-effect cycles, which employ two generators to achieve coefficients of performance (COP) up to 1.2-1.4—nearly double that of single-effect systems—have been refined for higher thermal efficiency, particularly in large-scale chillers.[24] Research into alternative absorbents, such as ionic liquids and biomass-derived solvents, aims to reduce toxicity and global warming potential compared to traditional ammonia-water or lithium bromide pairs, with studies demonstrating stable performance at lower environmental impact.[25] In the 2020s, developments in waste heat recovery have targeted data centers, where absorption systems capture low-grade exhaust heat (around 40-60°C) to drive cooling, achieving significant energy savings in hybrid setups and supporting sustainable operations in high-density facilities.[26]Thermodynamic Principles
Absorption Cycle
The absorption refrigeration cycle operates through four primary stages that facilitate cooling without mechanical compression, relying instead on thermal energy to drive the separation and recombination of a refrigerant-absorbent mixture. In the generation stage, heat is supplied to the generator, inducing an endothermic desorption process where the refrigerant vaporizes and separates from the absorbent solution, concentrating the absorbent. This vapor is then directed to the condenser. The absorption stage follows, where the refrigerant vapor, after cooling, is reabsorbed into the dilute absorbent solution in the absorber, an exothermic mixing process that releases heat to the surroundings. The condensation stage involves the refrigerant vapor releasing its latent heat to condense into a liquid at higher pressure. Finally, in the evaporation stage, the liquid refrigerant expands and evaporates at low pressure in the evaporator, absorbing latent heat from the cooled space to produce the refrigeration effect.[27][2] The performance of the cycle is quantified by the coefficient of performance (COP), defined as the ratio of the cooling provided to the total energy input: \text{COP} = \frac{Q_c}{Q_g + W_p} where Q_c is the heat absorbed in the evaporator (cooling capacity), Q_g is the heat supplied to the generator, and W_p is the work input from the solution pump, which is often negligible compared to Q_g due to the low pressure differentials. Typical COP values for single-effect systems range from 0.6 to 0.75, reflecting the cycle's reliance on heat rather than work.[28][2] Irreversibilities in the cycle arise from processes such as non-equilibrium heat and mass transfer, mixing, and temperature gradients, leading to entropy generation that can be analyzed through the entropy balance equation for each component: \Delta S = \sum \frac{Q}{T} + S_{\text{gen}}, where S_{\text{gen}} > 0 accounts for irreversibilities, and exergy losses are given by I = T_0 S_{\text{gen}} with T_0 as the ambient temperature. The highest entropy generation typically occurs in the generator and absorber due to the heat of mixing and separation.[29][27] The cycle's pressure-temperature relationships are depicted on a temperature-entropy (T-s) diagram, where isobars represent constant pressure lines for the refrigerant and solution paths. Standard multi-pressure operations (typically two levels) maintain low pressure in the evaporator and absorber to enable evaporation at low temperatures, while high pressure prevails in the generator and condenser to facilitate desorption and condensation at elevated temperatures; this setup follows isobaric processes on the T-s diagram, with entropy increasing during absorption and decreasing during generation. In contrast, single-pressure variants, such as diffusion absorption cycles, operate at uniform pressure using an auxiliary inert gas to aid refrigerant diffusion, simplifying the system but often at reduced efficiency, as shown by flatter isobars on the T-s plot.[27][30][2] Heat exchangers play a crucial role in enhancing cycle efficiency by recovering sensible heat between the hot concentrated solution leaving the generator and the cool dilute solution entering it, thereby minimizing exergy losses from temperature mismatches and reducing the required generator heat input. These exchangers, often solution-to-solution types, operate on principles of counterflow to approach ideal effectiveness, lowering overall irreversibilities by up to 20-30% in optimized systems.[29][28]Working Fluids and Their Properties
The working fluids in absorption refrigerators consist of a refrigerant-absorbent pair, where the refrigerant is volatile and the absorbent has a strong affinity for it, enabling the cycle's absorption and desorption processes.[31] The most widely used pairs are ammonia-water and lithium bromide-water, selected for their thermodynamic compatibility with low-grade heat sources. Ammonia-water (NH₃-H₂O) operates at higher pressures, making it suitable for compact, small-scale units such as domestic or portable refrigerators. Ammonia serves as the refrigerant due to its low boiling point (-33°C at atmospheric pressure) and high latent heat of vaporization (approximately 1370 kJ/kg), while water acts as the absorbent with high solubility for ammonia across a wide temperature range (-80°C to 180°C).[31] The pair's vapor-liquid equilibrium follows solubility curves modeled by equations like those from Patek and Klomfar (1995), showing ammonia concentrations up to 40% by weight without phase separation.[31] However, the mixture requires rectification in the generator to remove water vapor, as residual water can freeze in the evaporator and reduce efficiency; this adds complexity but allows operation down to -50°C evaporation temperatures.[32] Regarding safety, ammonia is toxic (irritation threshold at 25 ppm) and flammable in air concentrations of 15-28%, necessitating robust containment, though its global warming potential (GWP) is 0 and ozone depletion potential (ODP) is 0. Corrosivity is moderate, primarily affecting copper but manageable with steel alloys.[33] The circulation ratio (mass flow of solution to refrigerant) is low, typically 2-10, minimizing pumping requirements while maintaining efficient heat transfer.[34] In contrast, lithium bromide-water (LiBr-H₂O) is favored for large-scale chillers operating under vacuum (evaporator pressures around 0.8-1 kPa), with water as the refrigerant and lithium bromide as the non-volatile absorbent. The solution's low vapor pressure, governed by Dühring's rule and models from Conde (2014), allows evaporation temperatures as low as 5°C without compression.[31] Solubility is high up to 70% LiBr by weight, but crystallization risks occur below 55% concentration at temperatures under 10°C, limiting use to chilled water applications above 5°C.[35] Toxicity is low, as both components are non-flammable and environmentally benign (GWP=0 for water), though LiBr's corrosivity to carbon steel and copper requires inhibitors like lithium chromate or molybdates.[31] Operating temperatures span 20-100°C, ideal for waste heat or solar-driven systems, with a high circulation ratio of 10-25, increasing solution pumping power but enabling effective absorption.[36][2] Alternative pairs include ammonia with salts like lithium nitrate (NH₃-LiNO₃) for reduced circulation ratios (around 8-10) and organic fluids such as methanol-LiBr for mid-temperature ranges, though these are less common due to stability issues.[31] Selection criteria prioritize the pair's operating temperature range to match heat source and sink (e.g., 80-100°C for single-effect cycles), circulation ratio to balance pumping versus absorption efficiency, and rectification needs to prevent evaporator fouling.[37] Environmental and safety factors, including toxicity and corrosivity, further guide choices, with non-toxic options preferred for indoor applications.[33] Emerging research as of 2025 explores ionic liquids (ILs) as absorbents, paired with ammonia or water, to address limitations of traditional fluids. Examples include [EMIM][OAc] with ammonia, offering tunable solubility via NRTL models, low vapor pressure, and operation at 25-60°C without crystallization.[31] ILs exhibit low toxicity, negligible flammability, and reduced corrosivity compared to LiBr, with environmental benefits from high thermal stability and recyclability, potentially improving COP by 20-30% in compact systems.[38] Zeolite-based solid absorbents, such as 13X or SAPO-34 with water, are under investigation for hybrid absorption-adsorption cycles, providing high uptake (up to 0.3 kg/kg) at low pressures but requiring further scaling for refrigeration efficiency.[39]| Property | Ammonia-Water | Lithium Bromide-Water | Ionic Liquids (e.g., NH₃-[EMIM][OAc]) |
|---|---|---|---|
| Operating Pressure | High (5-20 bar) | Vacuum (0.01-0.1 bar) | Moderate (1-10 bar) |
| Temperature Range | -80 to 180°C | 20 to 100°C | 25 to 60°C |
| Circulation Ratio | 2-10 | 10-25 | 10-15 |
| Toxicity | High (ammonia) | Low | Low |
| Corrosivity | Moderate | High | Low |
| GWP | 0 | 0 | 0 (negligible) |