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Anabsorption refrigerator is arefrigerator that uses a heat source to provide the energy needed to drive the cooling process.Solar energy, burning a fossil fuel,waste heat from factories, anddistrict heating systems are examples of heat sources that can be used.
An absorption refrigerator uses two coolants, the first coolant performingevaporative cooling before being absorbed into the second coolant. Heat is then needed to reset the two coolants to their initial states.
Absorption refrigerators are commonly used inrecreational vehicles (RVs),campers, andcaravans because the heat required to power them can be provided by either a propane fuel burner, a low-voltage DC electric heater (from a battery or vehicle electrical system) or amains-powered electric heater. Absorption refrigerators can also be used toair-condition buildings using the waste heat from agas turbine orwater heater in the building. Using waste heat from a gas turbine makes the turbine very efficient because it first produceselectricity, then hot water, and finally, air-conditioning—trigeneration.
Unlike more commonvapor-compression refrigeration systems, an absorption refrigerator has no moving parts.
In the early years of the 20th century, the vapor absorption cycle using water-ammonia systems was popular and widely used, but after the development of thevapor compression cycle it lost much of its importance because of its lowcoefficient of performance (about one fifth of that of the vapor compression cycle).[citation needed] Absorption refrigerators are a popular alternative to regular compressor refrigerators where electricity is unreliable, costly, or unavailable, or where noise from the compressor is problematic; or where surplus heat is available.
In 1748 in Glasgow,William Cullen invented the basis for modern refrigeration, although he is not credited with a usable application. More on history of refrigeration can be found in the paragraphRefrigeration Research on pageRefrigeration.
Absorption refrigeration uses the same principle asadsorption refrigeration, which was invented byMichael Faraday in 1821. Instead of using a solid adsorber, an absorber in an absorption system absorbs the refrigerant vapour into a liquid.
Absorption cooling was invented by the French scientistFerdinand Carré in 1858.[1] The original design used water and sulphuric acid. The principle was enhanced with a three-fluid configuration in 1922 byBaltzar von Platen andCarl Munters, two students at theRoyal Institute of Technology inStockholm. This "Platen-Munters" design can operate without a pump.
Commercial production began in 1923 by the newly-formed companyAB Arctic, which was bought byElectrolux in 1925. In the 1960s, absorption refrigeration saw a renaissance due to the substantial demand for refrigerators forcaravans. AB Electrolux later established a subsidiary in the United States, named Dometic Sales Corporation. The company marketed refrigerators forrecreational vehicles (RVs) under theDometic brand. In 2001, Electrolux sold most of its leisure products line to the venture-capital companyEQT which createdDometic as a stand-alone company. Dometic still sold absorption fridges as of July 2025.[2]
In 1926,Albert Einstein and his former studentLeó Szilárd proposed an alternative design known as theEinstein refrigerator.[3]
At the 2007TED Conference,Adam Grosser presented his research of an "intermittent absorption" vaccine refrigeration unit for use in third world countries. The refrigerator is a small unit placed over a campfire, that can later be used to cool 15 litres (3.3 imp gal; 4.0 US gal) of water to just above freezing for 24 hours in a 30 °C (86 °F) environment.[4] The concept was similar to an early refrigeration device known as anIcyball.
Common absorption refrigerators use a refrigerant with a very lowboiling point (less than −18 °C (0 °F)) just likecompressor refrigerators. Compression refrigerators typically use anHCFC orHFC, while absorption refrigerators typically useammonia orwater and need at least a second fluid able to absorb the coolant, theabsorbent, respectively water (for ammonia) or brine (for water). Both types useevaporative cooling: when the refrigerant evaporates (boils), it takes some heat away with it, providing the cooling effect. The main difference between the two systems is the way the refrigerant is changed from a gas back into a liquid so that the cycle can repeat. An absorption refrigerator changes the gas back into a liquid using a method that needs only heat, and has no moving parts other than the fluids.
The absorption cooling cycle can be described in three phases:
The system thus silently provides for the mechanical circulation of the liquid without a usual pump. A third fluid, gaseous, is usually added to avoid pressure concerns when condensation occurs (see below).
In comparison, a compressor based heat pump works by pumping refrigerant gas from an evaporator to a condenser. This reduces the pressure and boiling temperature in the evaporator and increases the pressure and condensing temperature in the condenser. Energy from an electric motor or internal combustion engine is required to operate the compressor pump. Compressing the refrigerant uses this energy to do work on the gas, increasing its temperature. The warm, high pressure gas then enters the condenser where it undergoes a phase change to a liquid and releases heat to the condenser's surroundings. Warm liquid refrigerant moves from the high pressure condenser to the low pressure evaporator via an expansion valve, also known as a throttling valve or a Joule-Thomson valve. The expansion valve partially vaporizes the refrigerant cooling it via evaporative cooling, and the resulting vapor is cooled via expansive cooling. This is a combination of Joule-Thomson cooling and work done by the expanding gas, both at the expense of the internal energy of the gas. The cold, low pressure liquid refrigerant will now absorb heat from the evaporator's surroundings and vaporize. The resulting gas enters the compressor, and the cycle begins again.
A simple absorption refrigeration system common in large commercial plants uses a solution oflithium bromide orlithium chloride salt and water. Water under low pressure is evaporated from the coils that are to be chilled. The water is absorbed by a lithium bromide/water solution. The system drives the water out of the lithium bromide solution with heat.[5]
Another variant uses air, water, and a salt water solution. The intake of warm, moist air is passed through a sprayed solution of salt water. The spray lowers the humidity but does not significantly change the temperature. The less humid, warm air is then passed through anevaporative cooler, consisting of a spray of fresh water, which cools and re-humidifies the air. Humidity is removed from the cooled air with another spray of salt solution, providing the outlet of cool, dry air.
The salt solution is regenerated by heating it under low pressure, causing water to evaporate. The water evaporated from the salt solution is re-condensed, and rerouted back to the evaporative cooler.
A single-pressure absorption refrigerator takes advantage of the fact that a liquid's evaporation rate depends upon thepartial pressure of the vapor above the liquid and goes up with lower partial pressure. While having the same total pressure throughout the system, the refrigerator maintains a low partial pressure of the refrigerant (therefore high evaporation rate) in the part of the system that draws heat out of the low-temperature interior of the refrigerator, but maintains the refrigerant at high partial pressure (therefore low evaporation rate) in the part of the system that expels heat to the ambient-temperature air outside the refrigerator.
The refrigerator uses three substances:ammonia,hydrogen gas, andwater. The cycle is closed, with all hydrogen, water and ammonia collected and endlessly reused. The system is pressurized to raise the boiling point of ammonia higher than the temperature of the condenser coil (the coil which transfers heat to the air outside the refrigerator, by being hotter than the outside air.) This pressure is typically 14–16 standard atmospheres (1,400–1,600 kPa), putting thedew point of ammonia at about 35 °C (95 °F).
The cooling cycle starts with liquid ammonia at room temperature entering the evaporator. The volume of the evaporator is greater than the volume of the liquid, with the excess space occupied by a mixture of gaseous ammonia and hydrogen. The presence of hydrogen lowers thepartial pressure of the ammonia gas, thus lowering theevaporation point of the liquid below the temperature of the refrigerator's interior. Ammonia evaporates, taking a small amount of heat from the liquid and lowering the liquid's temperature. It continues to evaporate, while the largeenthalpy of vaporization (heat) flows from the warmer refrigerator interior to the cooler liquid ammonia and then to more ammonia gas.
In the next two steps, the ammonia gas is separated from the hydrogen so it can be reused.
The pure ammonia gas then enters the condenser. In thisheat exchanger, the hot ammonia gas transfers its heat to the outside air, which is below the boiling point of the full-pressure ammonia, and therefore condenses. The condensed (liquid) ammonia flows down to be mixed with the hydrogen gas released from the absorption step, repeating the cycle.
