Refrigerators and Heat Pumps

Understand how refrigerators move heat from cold to hot regions using work input.

1. What Are Refrigerators and Heat Pumps?

Refrigerators and heat pumps are devices that move heat from a colder region to a hotter region. This goes against the natural direction of heat flow, so external work is required.

Both devices work on the same principle—their use is different. A refrigerator keeps the inside cold, while a heat pump warms a space.

2. Why External Work Is Needed

According to the Second Law of Thermodynamics, heat does not flow spontaneously from cold to hot. To make this happen, the device must do work.

This work is usually supplied by an electric motor or compressor.

2.1. Direction of Heat Flow

  • Natural: hot → cold
  • Refrigerator/heat pump: cold → hot (requires work)

3. Basic Working Principle

Both devices operate using a cyclic process. A working fluid (called the refrigerant) absorbs heat from the cold region and releases it to the hot region.

3.1. Main Steps in the Cycle

  • The refrigerant absorbs heat from the cold space at low temperature.
  • The compressor does work on the refrigerant, increasing its pressure and temperature.
  • The hot refrigerant releases heat to the surroundings.
  • The refrigerant returns to a low-pressure, low-temperature state and the cycle repeats.

4. Coefficient of Performance (COP)

The efficiency of refrigerators and heat pumps is measured using COP (Coefficient of Performance). It tells how effectively the device moves heat.

4.1. COP of a Refrigerator

For a refrigerator:

\( \text{COP}_R = \dfrac{Q_c}{W} \)

where:

  • \( Q_c \) = heat removed from the cold region
  • W = work input

4.2. COP of a Heat Pump

For a heat pump:

\( \text{COP}_{HP} = \dfrac{Q_h}{W} \)

where:

  • \( Q_h \) = heat delivered to the hot region
  • W = work input

4.3. Relation Between COP of Heat Pump and Refrigerator

\( \text{COP}_{HP} = \text{COP}_R + 1 \)

This comes from the energy balance \( Q_h = Q_c + W \).

5. Refrigerator: Working Idea

A refrigerator keeps the inside cold by removing heat from it. The removed heat is dumped into the room.

5.1. Everyday Insight

The back side of a refrigerator feels warm because that heat is released there.

6. Heat Pump: Working Idea

A heat pump warms a place by taking heat from the outside (even if cold) and delivering it indoors.

6.1. Everyday Insight

Air conditioners running in reverse act like heat pumps.

7. Similarities and Differences

Both devices use the same components and the same working cycle, but the objective is different.

7.1. Similarities

  • Both move heat from cold to hot regions.
  • Both require external work.
  • Both use a refrigerant and a compressor.

7.2. Differences

  • A refrigerator’s goal is to cool a small space.
  • A heat pump’s goal is to heat a larger space.
  • COP definitions differ because the useful heat flow differs.

8. Examples in Daily Life

  • Kitchen refrigerator
  • Deep freezers
  • Air conditioners
  • Room heaters that use heat pump technology
  • Industrial cooling systems