The classical Carnot heat engine
An absorption refrigerator is a refrigerator that uses a heat source (e.g., solar energy, a fossil-fueled flame, waste heat from factories, or district heating systems) which provides the energy needed to drive the cooling process.
Absorption refrigerators are often used for food storage in recreational vehicles. The principle can also be used to air-condition buildings using the waste heat from a gas turbine or water heater. This use is very efficient, since the gas turbine then produces electricity, hot water, and air-conditioning (called cogeneration/trigeneration).
The standard for the absorption refrigerator is given by the ANSI/AHRI standard 560-2000.
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used, but after the development of the vapor compression cycle it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where waste heat is available or where heat is derived from solar collectors. Absorption refrigerators are a popular alternative to regular compressor refrigerators where electricity is unreliable, costly, or unavailable, where noise from the compressor is problematic, or where surplus heat is available (e.g., from turbine exhausts or industrial processes, or from solar plants).
In 1922 Baltzar von Platen and Carl Munters, while they were still students at the Royal Institute of Technology in Stockholm, Sweden, enhanced the principle with a 3-fluid configuration. This "Platen-Munters" design can operate without a pump.
Commercial production began in 1923 by the newly formed company AB Arctic, which was bought by Electrolux in 1925. In the 1960s the absorption refrigeration saw a renaissance due to the substantial demand for refrigerators for caravans. AB Electrolux established a subsidiary in the U.S, named Dometic Sales Corporation. The company marketed refrigerators for RVs under the Dometic brand. In 2001 Electrolux sold most of its leisure products line to the venture-capital company EQT which created Dometic as a stand-alone company.
At the 2007 TED Conference, Adam Grosser presented his research of a new, very small, "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 liters of water to just above freezing for 24 hours in a 30 degree Celsius environment.
Both absorption and compressor refrigerators use a refrigerant with a very low boiling point (less than 0 °F (−18 °C)). In both types, when this 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 refrigerant itself.
The absorption cooling cycle can be described in three phases:
- Evaporation: A liquid refrigerant evaporates in a low partial pressure environment, thus extracting heat from its surroundings (e.g. the refrigerator's compartment). Due to the low pressure, the temperature needed for evaporation is also lower.
- Absorption: The now gaseous refrigerant is absorbed by another liquid (e.g. a salt solution), reducing its partial pressure in the evaporator and allowing more refrigerant to evaporate.
- Regeneration: The refrigerant-saturated liquid is heated, causing the refrigerant to evaporate out. This happens at a significantly higher pressure. The refrigerant is then condensed through a heat exchanger to replenish the supply of liquid refrigerant in the evaporator.
In comparison, a compressor refrigerator uses an electrically powered compressor to increase the pressure on the gas, and then condenses the hot high pressure gas back to a liquid by heat exchange with a coolant (usually air). Once the high pressure gas has cooled and condensed into a liquid, it passes through an orifice which creates a pressure drop, which causes the liquid to evaporate. The evaporation process absorbs heat, and the temperature of the refrigerant drops to its boiling point at the now low pressure.
Simple salt and water system
A simple absorption refrigeration system common in large commercial plants uses a solution of lithium bromide salt and water. Water under low pressure is evaporated from the coils that are being chilled. The water is absorbed by a lithium bromide/water solution. The water is driven off the lithium bromide solution using heat.
Water spray absorption refrigeration
Another variant, depicted to the right, 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 an evaporative 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.
Single pressure absorption refrigeration
A single-pressure absorption refrigerator uses three substances: ammonia, hydrogen gas, and water. The system is pressurized to the point where the ammonia is liquid (14-16atm). The cycle is closed, with all hydrogen, water and ammonia collected and endlessly reused.
The cooling cycle starts with liquefied ammonia entering the evaporator at room temperature. The evaporated ammonia is mixed with hydrogen. The partial pressure of the hydrogen gas is used to regulate the total pressure of the ammonia (liquid) and hydrogen (gas) solution, which in turn regulates the boiling point of the ammonia. As the ammonia boils in the evaporator, it requires energy to overcome the enthalpy of vaporization. This energy is drawn from the refrigerator's interior and provides the cooling required.
The next three steps exist to separate the gaseous ammonia and the hydrogen:
- The ammonia (gas) and hydrogen (gas) solution flows through a pipe from the evaporator into the absorber. In the absorber, the solution of gas flows into a solution of ammonia (liquid) and water (liquid). The ammonia dissolves in the water allowing the gaseous hydrogen to collect at the top of the absorber, while the ammonia (liquid) and water (liquid) solution remains at the bottom.
- The next step separates the ammonia and water. In the generator, heat is applied to the solution to distill the ammonia from the water. Some water vapor and bubbles remain mixed with the ammonia. This water is removed in the final separation step, by passing it through the separator, an uphill series of twisted pipes with minor obstacles to pop the bubbles, allowing the water vapor to condense and drain back to the generator.
- Finally, the ammonia gas enters the condenser. In this heat exchanger, the hot ammonia gas transfers its energy to the ambient air allowing it to condense. This provides liquid ammonia, which flows down to be mixed with hydrogen gas, allowing the cycle to repeat.
- PDF document for download at http://www.ahrinet.org/App_Content/ahri/files/standards%20pdfs/ANSI%20standards%20pdfs/ANSI%20ARI560-2000.pdf
- Eric Granryd & Björn Palm, Refrigerating engineering, Stockholm Royal Institute of Technology, 2005, see chap. 4-3
- "US Patent 1781541".
- Adam Grosser (Feb 2007). "Adam Grosser and his sustainable fridge, 552 word transcript and video 3mins34". TED. Archived from the original on 19 April 2010. Retrieved 2010-04-18.
- Sapali, S. N. "Lithium Bromide Absorption Refrigeration System". Textbook Of Refrigeration And Air-Conditioning. New Delhi: PHI learning. p. 258. ISBN 978-81-203-3360-4.
- Levy, A.; Kosloff, R. (2012). "Quantum Absorption Refrigerator". Phys. Rev. Lett. 108: 070604. arXiv:1109.0728. Bibcode:2012PhRvL.108g0604L. doi:10.1103/PhysRevLett.108.070604.
- Absorption Heat Pumps (EERE).
- Arizona Energy Explanation with diagrams
- Design Analysis of the Einstein Refrigeration Cycle, Andrew Delano (1998). Retrieved September 13, 2005.
- Air Conditioning Thermodynamics, published by the California EPA, Air Resources Board
- Lithium-Bromide / Water Cycle - Absorption Refrigeration for Campus Cooling at BYU.