2 Case study 1: Refrigerators and the ozone layer
2.1 Refrigeration and chlorofluorocarbons
A domestic refrigerator consists essentially of two elements. First, it has a well-insulated box that minimises the flow of heat energy from the warmer outside environment to the cold space inside. Second, it has a motor to circulate a cooling liquid or refrigerant which extracts heat from the cold space and carries it to the outside, where it is released, usually through a radiator at the back. Most refrigerators make use of the principle that when liquids vaporise – that is, change from liquid to gas – they take heat from their surroundings; when the gases condense back to liquid they return heat to the surroundings. As the fluid refrigerant is pumped around in closed circulation, the trick is to expand and vaporise it while it is inside the cold space, and to compress and condense it again once it has moved outside the cooled region.
Early refrigerators used a variety of potentially unpleasant and dangerous chemicals for refrigerants, for example, ammonia and sulphur dioxide. So there was a lot of interest from the industry when, in 1928, DuPont discovered an entirely new range of chemicals known as chlorofluorocarbons, or CFCs, which were neither toxic nor flammable, seemed practically non-reactive, and had suitable thermal properties. They were introduced as refrigerants in the 1930s, relatively soon after their discovery. Later, in the 1960s, CFCs found another use as blowing agents for foam insulation, to replace the less effective glass fibre insulation then used in refrigerator cabinets.
Alas, we now know that these chemicals have the potential to damage the ozone layer in the stratosphere. The ozone layer in the upper atmosphere, high above our weather systems, absorbs ultraviolet (UV) radiation from the sun and prevents it from reaching the surface of the planet, where it can be harmful to life. Sherwood Rowland was one of the two scientists awarded the Nobel Prize for the discovery, in the early 1970s, of the potential dangers of CFCs. One night, while he was working on the problem, his wife asked him how his work was going. Rowland replied despondently, ‘Very, very well. But it looks like the end of the world’ (Ochert, 1999).
This discovery in the 1970s gave a major jolt to the science establishment; after all what possible threat could non-toxic, non-flammable, non-reactive chemicals pose? But it is precisely because CFCs are so stable that they last a long time in the atmosphere and are eventually able to reach the stratosphere where they react with ultraviolet radiation to release chlorine atoms. In certain circumstances, for example, over the Antarctic during winter and spring, we now know that each chlorine atom in the wrong place has the potential to destroy up to 100,000 ozone molecules.