Reversed-Cycle Refrigerators and Heat-Pump Performance

All flashcards

How does a refrigerator or heat pump operate relative to a heat engine?

It runs a heat-engine cycle in reverse, using supplied work to transfer heat from a cold reservoir to a hot reservoir.

What do $Q_c$, $Q_h$, and $W$ represent in a reversed cycle?

$Q_c$ is heat absorbed from the cold reservoir, $Q_h$ is heat released to the hot reservoir, and $W$ is work supplied to the device.

What is the refrigerator coefficient of performance?

The heat removed from the cold reservoir per unit of work supplied: $K_R = Q_c/W$.

What is the heat-pump coefficient of performance?

The heat delivered to the hot reservoir per unit of work supplied: $K_P = Q_h/W$.

How are the refrigerator and heat-pump COPs related?

For the same cycle, $K_P = K_R + 1$.

Quiz questions

1. What distinguishes the purpose of a refrigerator from that of a heat pump?Answer: A refrigerator removes heat from a cold region; a heat pump delivers heat to a warm region.
2. In a reversed-cycle device, what does the work $W$ represent?Answer: Work input done on the working substance.
3. Which equation expresses energy conservation over one complete refrigerator or heat-pump cycle?Answer: $Q_h = Q_c + W$
4. A device removes $300\,\text{J}$ of heat from a cold region and receives $100\,\text{J}$ of work. How much heat does it release to the hot reservoir?Answer: $400\,\text{J}$
5. If a device has refrigerator COP $K_R = 3$, what is its heat-pump COP for the same cycle?Answer: $K_P = 4$