Page 140 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

The first and second laws apply equally well to cycles working in the reverse direction to

those of heat engine. In general, heat only flows from a high-temperature source to a

low-temperature sink. However, a reversed heat engine can be utilized to pump the heat

from a low-temperature region to a high-temperature region. The reversed heat engine
is called heat pump.

In the case of a reversed cycle, the net work done on the system is equal to the net heat

rejected by the system. Such cycles occur in heat pumps and refrigerators. The
equivalent diagram of the heat pump (or refrigerator) is shown in Fig. 4.1-2.

High-temperature

HOT RESERVOIR

Q1
WORK INPUT
HEAT
W

PUMP

Q1 = W + Q2
Q2

Low-temperature

COLD RESERVOIR

Figure 4.1.2- Reverse heat engine

In the heat pump (or refrigerator) cycle, an amount of heat, Q2, is supplied from the cold
reservoir, and an amount of heat, Q1, is rejected to the hot reservoir.

Q1 = W + Q2 (4.4)

In the second law, we can say that work input is essential for heat to be transferred from
the cold to the hot reservoir,

i.e. W > 0

The first law sets no limit on the percentage of heat supplied, which can be converted

into work. Nor does it indicate whether the energy conversion process is physically
131 | P a g e