Refrigeration

Refrigeration is the artificial cooling of a space, substance, or system to lower and/or maintain its temperature below the ambient temperature. Early refrigeration used consumable coolants such as ice and dry ice, which required periodic replenishment. Modern refrigeration is a self-sustaining heat exchanger process that transfers thermal energy against a temperature gradient using a heat-transfer working fluid, or refrigerant. This fluid absorbs heat from a low-temperature medium and releases it to a higher-temperature medium, typically involving active phase change via a compressor and a radiator system. Energy transfer in refrigeration is traditionally driven by physical means, such as ice melting or an electromechanical machine, but can also be driven by heat pumps, magnetism, electricity, or laser cooling. Applications include household refrigerators, industrial freezers, cryogenics, and air conditioning. Heat pumps may use the heat output of the refrigeration process, such as for water heating, and can be designed to be reversible.

Refrigeration has had a large impact on human lifestyle, agriculture, industry, and settlement patterns. The idea of preserving food by cold storage dates to prehistory, but for millennia, societies were limited in their means. In most regions, surplus foods were preserved by curing or drying, while cold storage was generally unavailable except in arctic, subarctic, or alpine areas where natural ice was harvested from glaciers or frozen water bodies, or where natural coolness was used in caves, root cellars, and winter weather. The 19th-century ice trade enabled cold chains, and from the late 19th through mid-20th centuries, mechanical refrigeration was invented and expanded, leading to temperature-controlled rail cars, trucks, and household refrigerators. Refrigerated rail cars contributed to the settlement of areas not on main transport channels and spurred the growth of large cities in hot climates, such as Houston and Las Vegas. Today, most developed countries rely on refrigeration in food processing, distribution, and retail to maintain food safety. This improvement in preservation reduced food loss and spoilage, enabling a supply chain where large quantities of meat, seafood, and produce come from fewer, larger farms and fisheries, diversifying food products and impacting nutrition and culinary arts.