How Does a Refrigerator Work? Moving Heat Uphill

Two pieces of physics do the work. The first is that evaporating a liquid absorbs a large quantity of heat without any change in temperature, the latent heat of vaporisation, and condensing it back releases the same amount; this is why sweat cools skin and why steam scalds worse than boiling water. The second is that the temperature at which a fluid boils depends on its pressure, so the same substance can be made to boil at minus twenty-five degrees in one part of a circuit and condense at forty-five degrees in another, purely by controlling pressure. Combine the two and you have a way of picking up heat at a low temperature and releasing it at a high one, which is exactly what is needed. The second law of thermodynamics is not violated, because work is put in by the compressor, and the total entropy of the system plus the kitchen still rises.

Almost every domestic refrigerator, freezer, air conditioner and heat pump is the same circuit with the same four parts:

• •The compressor, the humming unit at the back, which takes low-pressure gas and compresses it, raising both its pressure and its temperature above that of the room
• •The condenser, the black coil on the back or underneath, where that hot high-pressure gas gives up heat to the kitchen air and condenses into a liquid
• •The expansion device, a narrow capillary tube or a valve, which is the only deliberate restriction in the loop and across which the pressure drops sharply, causing part of the liquid to flash into vapour and the temperature to fall steeply
• •The evaporator, the coil inside the cabinet, where the cold low-pressure mixture boils as it absorbs heat from the food and the air, then returns to the compressor as gas
• •A thermostat switches the compressor on and off to hold the set temperature, which is why the noise is intermittent

The choice of refrigerant is the part of the technology with the most history. Early machines used ammonia, sulphur dioxide and methyl chloride, all of which are toxic and several of which are flammable, and a series of fatal domestic leaks in the 1920s prompted a search for something safe. Thomas Midgley's team produced chlorofluorocarbons, marketed as Freon, which were non-toxic, non-flammable, stable and ideal, and which turned out decades later to destroy stratospheric ozone, leading to the phase-out under the Montreal Protocol of 1987. Their replacements, hydrofluorocarbons, do not harm ozone and are potent greenhouse gases, thousands of times worse than carbon dioxide per kilogram, and are themselves being phased down under the Kigali Amendment of 2016. Modern domestic appliances in Europe mostly use isobutane, a hydrocarbon with negligible climate effect that is flammable, which is manageable in the very small quantities a fridge contains. Midgley, incidentally, also led the team that put lead in petrol, which gives him a strong claim to having had more adverse impact on the atmosphere than any other single organism.

Nothing in the circuit knows which side you care about. An air conditioner is the identical arrangement with the evaporator indoors and the condenser outside, and a heat pump is the same machine again, used with the intention of collecting heat from outdoor air or the ground and delivering it inside. That is why a heat pump can deliver three or four units of heat for each unit of electricity consumed, which looks like a violation of energy conservation and is not: the machine is not generating that heat but moving it, and the electricity pays only for the transport. Efficiency is measured as a coefficient of performance, the heat moved divided by the work done, and it falls as the temperature difference grows, which is why heat pumps become less efficient on very cold days and why a freezer costs more to run per litre than a refrigerator.

The compressor cycle dominates and is not the only option. Absorption refrigerators replace the compressor with a heat source, using ammonia and water and a chemical rather than mechanical pressure difference, which makes them silent and able to run on gas or waste heat; they are found in caravans, hotel minibars and off-grid installations, at the cost of lower efficiency. Thermoelectric coolers use the Peltier effect, in which current through a junction of two materials moves heat, and are small, silent, solid-state and inefficient, which suits wine coolers and cooling electronics. Evaporative coolers simply evaporate water into dry air, which works well in desert climates and not at all in humid ones, and is the oldest method, used with porous pots for thousands of years. Magnetic refrigeration, which exploits materials that warm when magnetised and cool when the field is removed, is a promising laboratory technology that has not yet reached the kitchen.

A refrigerator moves heat from a cold interior to a warmer room by circulating a fluid that boils inside the cabinet, absorbing latent heat, and condenses outside it, releasing that heat, with the boiling point controlled by pressure. The four components are a compressor, a condenser, an expansion restriction and an evaporator, and the same circuit reversed is an air conditioner or a heat pump, which delivers several units of heat per unit of electricity because it transports rather than generates. Refrigerants have moved from toxic gases to ozone-destroying CFCs to potent greenhouse gases and now to hydrocarbons.