How Refrigerators Work: The Thermodynamics of Cooling
A refrigerator doesn't create cold — it moves heat from inside the fridge to the outside. This is thermodynamically "unnatural" (heat spontaneously flows from hot to cold, not the reverse), so it requires work — typically 100–200 W of electrical power. The same principle drives air conditioners, heat pumps, and industrial chillers.
1. The Second Law & Why Cooling Costs Work
The second law of thermodynamics states that heat spontaneously flows from hot to cold, never the reverse. A refrigerator moves heat from a cold space (5°C) to a warm space (room at 25°C) — against the natural direction. This requires external work input.
2. The Vapour-Compression Cycle
Nearly every domestic refrigerator, air conditioner, and heat pump uses the vapour-compression cycle. It exploits the fact that a fluid absorbs large amounts of heat when it evaporates (latent heat) and releases that heat when it condenses.
Compressor pressurises the low-pressure vapour → high-pressure hot vapour (60–70°C)
Hot vapour flows through condenser coils (back of fridge). Heat escapes to room air. Vapour condenses to high-pressure liquid.
Liquid passes through expansion valve (capillary tube). Pressure drops sharply → temperature drops to −20 to −30°C.
Cold liquid flows through evaporator coils (inside fridge). Absorbs heat from food. Liquid evaporates to low-pressure vapour. Returns to compressor.
3. The Four Components
- Compressor: The heart of the system. A hermetically sealed motor-pump unit in the bottom-rear of the fridge. Reciprocating piston or rotary scroll type. Consumes ~100–150 W (domestic). Inverter compressors (variable speed) are 20–40% more efficient than fixed-speed by running continuously at reduced power instead of cycling on/off.
- Condenser: Coils on the back or bottom of the fridge where hot refrigerant releases heat to room air. Some fridges use a fan for forced convection. Fins increase surface area. Condenser temperature: ~30–40°C above ambient in normal operation.
- Expansion device: In domestic fridges, a long thin capillary tube (inner diameter ~0.5 mm, length ~2 m). No moving parts. The pressure drop is caused by friction as refrigerant flows through the narrow tube. Commercial systems use thermostatic expansion valves (TXV) or electronic expansion valves (EEV) for better control.
- Evaporator: Coils inside the fridge (usually behind a panel in the freezer compartment). Cold refrigerant absorbs heat and evaporates. A fan circulates cold air throughout the fridge and freezer. Frost forms on the evaporator when moisture in air freezes — auto-defrost systems periodically heat the evaporator to melt frost.
4. COP & Carnot Limit
A COP of 3 means for every 1 W of electricity, 3 W of heat is removed from the fridge — and 4 W is dumped into the kitchen (Q_cold + W). Higher COP = better efficiency. The EU energy label rates fridges from A (best, COP ~4+) to G (worst).
5. Refrigerants: From CFCs to R-290
| Generation | Type | Example | ODP | GWP | Status |
|---|---|---|---|---|---|
| 1930s–1990s | CFC | R-12 (Freon) | 1.0 | 10,900 | Banned (Montreal 1987) |
| 1990s–2020s | HCFC | R-22 | 0.055 | 1,810 | Phased out (2020–2030) |
| 2000s–present | HFC | R-134a | 0 | 1,430 | Being phased down (Kigali 2016) |
| Modern | HFO | R-1234yf | 0 | 4 | Automotive AC replacement |
| Modern | Hydrocarbon | R-290 (propane) | 0 | 3 | Domestic fridges (EU standard) |
| Modern | Natural | R-744 (CO₂) | 0 | 1 | Commercial/heat pumps |
ODP = Ozone Depletion Potential (relative to R-11). GWP = Global Warming Potential over 100 years (relative to CO₂). The shift is toward natural refrigerants (propane, CO₂, ammonia) with near-zero GWP. R-290 (propane) is now used in ~50% of new European domestic refrigerators — flammable but safe in the small charges used (~57 g).
6. Heat Pumps: Refrigeration in Reverse
A heat pump is the same vapour-compression cycle, but the useful output is the heat rejected (Q_hot) rather than the cooling (Q_cold). It pumps heat from outside air (even at 0°C) into a building.
Air-source heat pumps work well down to about −15°C. Below that, COP drops significantly and supplementary heating may be needed. Ground-source heat pumps (using underground loops at ~10°C year-round) maintain higher COP but are more expensive to install.
7. Alternative Cooling Technologies
- Absorption refrigeration: Uses heat (gas flame, solar, waste heat) instead of a compressor. Ammonia-water or lithium bromide-water working pairs. No moving parts (except a solution pump). Used in gas-powered RV fridges and solar cooling. Lower COP (~0.5–1.0) but uses thermal energy instead of electricity.
- Thermoelectric (Peltier): Solid-state cooling using the Peltier effect — electric current through a junction of two different conductors pumps heat from one side to the other. No refrigerant, no moving parts. Very low COP (~0.5). Used in mini-fridges, CPU coolers, and wine cellars where silence matters more than efficiency.
- Magnetic refrigeration: Exploits the magnetocaloric effect — some materials heat up when magnetised and cool when demagnetised. Potentially 30–60% Carnot efficient (vs ~35% for vapour compression). No refrigerant gas. Still in R&D; prototypes by Cooltech, Haier.
- Evaporative cooling: Simple physics: water evaporation absorbs ~2,450 kJ/kg. Swamp coolers pass air through wet pads. Effective in dry climates (desert Southwest US); useless in humid climates where the air is already saturated.