Refrigeration is the artificial cooling of a space, substance, or system to lower and/or maintain its temperature below the ambient temperature. Early refrigeration uses consumable coolants such as ice and dry ice, while modern refrigeration is a self-sustaining heat exchanger process by which thermal energy is transferred against the temperature gradient via the use of a heat-transfer working fluid, which absorbs heat from a low-temperature medium and releases it to another higher-temperature medium, typically involving active phase change via a compressor and aided by a radiator system. Energy transfer in refrigeration is traditionally driven by physical means, but it can also be driven by heat pump, magnetism, electricity, laser cooling, or other means. Refrigeration has many applications, including household refrigerators, industrial freezers, cryogenics, and cool store air conditioning. Heat pumps may make use of the heat output of the refrigeration process, and also may be designed to be reversible, but are otherwise similar to air conditioning units.