Refrigerators may either be free-standing, or built into a kitchen. Refrigerators may also be combined with freezers, stacked with either the refrigerator or freezer above, below, or side by side. Other types include the French door style and the less expensive compact refrigerator, built to save space but with less storage space inside. Around 215 million refrigerators are sold each year worldwide, generating more than 125 billion U.S. dollars in annual market revenue.
By far the most common type of refrigerator is the compressor refrigerator. Also available are refrigerators that use oil and gas (natural gas or propane) as well as refrigerator models with dual gas/electricity units (typically found in RVs). Other types of refrigerators in use with specialized cooling mechanisms are the solar refrigerators and the thermal-mass refrigerators.
