Both absorption and Camping Fridge 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 itself.
The absorption cooling cycle can be described in three phases:
Camping Fridge A liquid refrigerant evaporates in a low partial pressure environment, thus extracting heat from its surroundings (e.g. the refrigerator’s compartment). Because of the low partial pressure, the temperature needed for evaporation is also low.
Absorption: The now gaseous refrigerant is absorbed by another liquid
Regeneration: The refrigerant-saturated liquid is heated, causing to evaporate out. The hot gaseous refrigerant passes through a heat exchanger, transferring its heat outside the system (such as to surrounding ambient-temperature air), and condenses. The condensed (liquid) refrigerant supplies the evaporation phase.
In comparison, a compressor refrigerator uses an electrically powered compressor to increase the pressure on the gaseous refrigerant. The resulting hot, high-pressure gas is condensed to a liquid form by cooling in a heat exchanger („condenser“) that is exposed to the external environment (usually air in the room). The condensed Camping Fridge now at a temperature near to that of the external environment, then passes through an orifice or a throttle valve into the evaporator section. The orifice or throttle valve creates a pressure drop between the high pressure condenser section and the low pressure evaporator section. The lower pressure in the evaporator section allows the liquid refrigerant to evaporate, which absorbs heat from the refrigerator food compartment. The now-vaporized refrigerant then goes back into the compressor to repeat the cycle.
Another difference between the two types is the refrigerant used. Compressor refrigerators typically use an HCFC or HFC, while absorption refrigerators typically use ammonia or water.
Simple salt and water system
A simple absorption refrigeration system common in large commercial plants uses a solution of lithium bromide and Lithium chloride 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 system drives the water off the lithium bromide solution with heat.
Water spray absorption system for Camping Fridge
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 cooled air with another spray providing the outlet of cool, dry air.
The salt is regenerated by heating it under low pressure, causing water to evaporate. The water evaporated salt solution is re-condensed, and rerouted back to the evaporative cooler.