The term air-conditioning refers to a refrigeration process that is applied to comfort cooling. Comfort cooling systems are intended to cool and maintain the temperature of buildings and structures during periods when the temperature within the structure is higher than desired. Although this typically occurs in the warmer summer months, there are applications and situations where comfort cooling may be required year-round. Air-conditioning and comfort-cooling processes are classified as high-temperature refrigeration systems, as they operate with evaporator saturation temperatures above freezing and also operate to maintain the conditioned space at temperatures above freezing. Air-conditioning systems are similar to other refrigeration systems in that they use four major system components (compressor, condenser, metering device, evaporator) and refrigerant to cool the space. In operation, the air-conditioning system absorbs heat from within the structure and deposits this heat to the outside. Some of this heat absorbed into the air-conditioning system likely leaked into the structure from outside, while some was likely generated inside the structure itself. People are sources of internally-generated structural heat, as are cooking equipment, computers, televisions, and other appliances.