Natural refrigerant https://en.wikipedia.org/wiki/Natural_refrigerant :

> Natural refrigerants are considered substances that serve as refrigerants in refrigeration systems (including refrigerators, HVAC, and air conditioning). They are alternatives to synthetic refrigerants such as chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), and hydrofluorocarbon (HFC) based refrigerants. Unlike other refrigerants, natural refrigerants can be found in nature and are commercially available thanks to physical industrial processes like fractional distillation, chemical reactions such as Haber process and spin-off gases. The most prominent of these include various natural hydrocarbons, carbon dioxide, ammonia, and water.[1] Natural refrigerants are preferred actually in new equipment to their synthetic counterparts for their presumption of higher degrees of sustainability. With the current technologies available, almost 75 percent of the refrigeration and air conditioning sector has the potential to be converted to natural refrigerants

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Heat pump and refrigeration cycle >
Thermodynamic cycles: https://en.wikipedia.org/wiki/Heat_pump_and_refrigeration_cy... :

> According to the second law of thermodynamics, heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this.[3] An air conditioner requires work to cool a living space, moving heat from the interior being cooled (the heat source) to the outdoors (the heat sink). Similarly, a refrigerator moves heat from inside the cold icebox (the heat source) to the warmer room-temperature air of the kitchen (the heat sink). The operating principle of an ideal heat engine was described mathematically using the Carnot cycle by Sadi Carnot in 1824. An ideal refrigerator or heat pump can be thought of as an ideal heat engine that is operating in a reverse Carnot cycle.[4]

> Heat pump cycles and refrigeration cycles can be classified as vapor compression, vapor absorption, gas cycle, or Stirling cycle types.

Heat pump: https://en.wikipedia.org/wiki/Heat_pump :

> [...] When in heating mode, a refrigerant at the warmer temperature is compressed, becoming hot. Its thermal energy can be transferred to the cooler space. After being returned to the warmer space the refrigerant is decompressed — evaporated. It has delivered some of its thermal energy, so returns colder than the environment, and can again take up energy from the air or the ground in the warm space, and repeat the cycle.

> Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps. Large-scale heat pumps are also used in district heating systems.[2]

> The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is. When used for space heating, heat pumps are typically much more energy-efficient than electric resistance and other heaters. Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps can play a key role in climate change mitigation.[3][4] Consuming 1 kWh of electricity, they can transfer 3 to 6 kWh of thermal energy into a building.[5] The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions in mild climates.[6] Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, in 2021 they only met 10%.[7]