The first primitive refrigerators were holes dug in the ground, lined with straw, and sometimes packed with snow found in the mountains. the thermal energy of whatever you were trying to keep cool would decrease as the temperature decreases from the addition of the ice. The straw would also insulated the hole keeping it cool for longer. Meat was usually salted or smoked before being placed in the holes to keep the meat from spoiling faster. The first model of the the modern refrigerator for household use was created in the late 19th century but only became popularized for daily use in the early 20th century. The refrigerators from the 19th century and the refrigerators we use now have the same basic cooling technique called vapor compression. The system includes four main parts; the compressor, the condenser, the evaporator, and a throttling device. The system pumps a refrigerant throughout every compartment. The purpose of a refrigerator is to reduce the temperature of the food inside, so the system starts by running the refrigerant (low pressure vapour at this stage) through the pipes closest to the inside of the refrigerator (the evaporator). This absorbs the heat from the food and raises the temperature of the refrigerant. It then runs through the compressor to turn into a high pressure vapor and the temperature of the refrigerant increases. Then the refrigerant is passed through the condenser which is found on the outside of the refrigerator. The heat is transferred to the air and the refrigerant becomes a high pressure liquid at its natural temperature. Then it passes through the throttling device, its temperature drops, and its transformed into a low pressure vapor again. Then the whole cycle repeats. The only things that have changed in refrigeration technology is the materials used. Refrigerators used to be made out of wood and looked more like cabinets, while now they’re made out of metal and plastic. The type of refrigerants used also changed. The first few models used toxic chemicals like ammonia, methyl chloride, and sulfur dioxide. There were some unfortunate deaths due to refrigerant poisoning in the United States during the 1920s after the pipes leaked and the odorless and invisible gas was expelled into the air. Therefore the use of these refrigerants were band and a new refrigerant was introduced. Chlorofluorocarbons (CFCs) were introduced as refrigerants during the 1930s and had even more disastrous effects on humans then the first set of refrigerants. They are more commonly known as R-12 or Freon-12. Copious amounts of these CFCs were released into the atmosphere and consequently were the number one contributor to the depletion of our ozone. The hole in our ozone was first discovered in the 1970s and measures to remove these refrigerants from daily use was only put into motion in 1978. The refrigerant used since then, and still commonly used today is R404A but will soon be replaced due to its inefficiency, and it relatively large impact on the ozone. It could be replaced by a refrigerant known as R407F which is more efficient (cools faster) and is better for the environment. The efficiency of refrigerators currently on the market vary. You have the average refrigerator that consumes 1-2 kWh each day, then older models that can use up to twice that amount. The reason for this is because new models recognize when the fridge is at an optimal temperature, and only runs the cooling system when needed. Older models do not have this feature. Therefore the newer models use less power and create less waste, making them much more efficient. All of the refrigerators created in the past 20 years are much more efficient than those used in the 1970s. Back then refrigerators would use about 1800 kWh per year and now a days most refrigerators use well below 500 kWh per year. Making them more then 300% more efficient.