Overview
This section explains the functioning, purpose, and performance metrics of refrigerators and heat pumps, highlighting their similarities and how they are essentially heat engines operating in reverse.
Introduction to Refrigerators and Heat Pumps
- Refrigerators and heat pumps are heat engines running in reverse.
- A refrigerator removes heat from a designated cold space, e.g., food compartment or building interior.
- A heat pump dumps heat into a chosen hot space, such as a home for heating.
How Refrigerators and Heat Pumps Work
- The device absorbs heat ( Q_c ) from the cold reservoir at temperature ( T_c ).
- It discards heat ( Q_h ) to the hot reservoir at temperature ( T_h ).
- Work ( W ) is done on the system, typically by an electric motor.
- In refrigerators, coolant extracts heat at the evaporator, vaporizes, is compressed, then releases heat at the condenser.
Coefficient of Performance (COP)
- COP for a refrigerator (( K_R )) measures effectiveness as heat removed divided by work input: ( K_R = \frac{Q_c}{W} = \frac{Q_c}{Q_h - Q_c} ).
- COP for a heat pump (( K_P )) is heat delivered to hot space divided by work input: ( K_P = \frac{Q_h}{W} = \frac{Q_h}{Q_h - Q_c} ).
- Both formulas use energy conservation: ( W = Q_h - Q_c ).
Key Terms & Definitions
- Refrigerator — a device that removes heat from a cold reservoir and expels it to a hot reservoir using work input.
- Heat Pump — a device that delivers heat to a hot reservoir by absorbing heat from a cold reservoir using work input.
- Coefficient of Performance (COP) — a ratio indicating efficiency: for refrigerators, ( K_R = \frac{Q_c}{W} ); for heat pumps, ( K_P = \frac{Q_h}{W} ).
- Cold Reservoir (( T_c )) — the location where heat is absorbed (cooled area).
- Hot Reservoir (( T_h )) — the location where heat is dumped (warmed area).
- Work (( W )) — energy supplied to the system, typically by electricity.
Action Items / Next Steps
- Practice calculating COP for both refrigerators and heat pumps using given values for ( Q_c ), ( Q_h ), and ( W ).
- Review diagrams and processes for refrigerators and heat pumps in textbook figures (Figures 4.6 and 4.7).