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Fig. 5
Mollier diagram of ideal heat cycle (see sec-
tion 8.2)
The idealised version of the heat pump cycle in-
volves four steps: compression (1→2), liquefac-
tion (2→3), controlled expansion (3→4) and va-
porisation (4→1):
Compression:
The gaseous refrigerant is sucked in by the com-
pressor without changing the entropy (s
then compressed from pressure p
causes excess heat to be generated. The temper-
ature rises from T
to T
1
done per unit mass is Δw = h
Liquefaction:
The fluid cools sharply inside the condenser caus-
ing it to liquefy. The heat emitted by this process
(latent heat) heats up the surrounding reservoir to
temperature T
. The change in heat per unit mass
2
is Δq
– h
= h
.
2
2
3
Controlled expansion:
The condensed refrigerant reaches the expansion
valve where it is allowed to expand to a lower pres-
sure without any mechanical work being done. This
results in a drop in temperature since work needs to
be done against the force of attraction between re-
frigerant molecules (Joule-Thomson effect). En-
thalpy remains constant (h
Vaporisation:
In the evaporator, the refrigerant absorbs heat and
vaporises completely. This causes the surrounding
reservoir to cool to a temperature T
sorbed per unit mass is Δq
The vaporised refrigerant is sucked back in again
by the compressor to start the compression pro-
cess anew.
= s
). It is
1
2
to p
which
1
2
. The mechanical work
2
– h
.
2
1
= h
).
4
3
. The heat ab-
1
– h
= h
.
1
1
4
Note:
The expanded refrigerant evaporates and with-
draws heat from the left reservoir.
Under ideal conditions, the pipe system carries
pure gaseous refrigerant from the evaporator via
the sight glass to the compressor.
As the water temperature decreases, the heat ab-
sorption via the evaporator coil decreases.
Therefore as a result drops of refrigerant can be-
come visible in the left sight glass.
This has practically no influence on the function
of the heat pump, but should be reduced to a min-
imum by constantly stirring the water.
For the determination of the coefficient of perfor-
mance, a limited temperature window should be
used:
Start temperature approx. 20°C to 25°C, termina-
tion temperature in the left reservoir approx. 10°C
to 12°C.
8. Example experiments
8.1 Efficiency of the compressor
The efficiency of the compressor η
the ratio of the change in energy ΔQ
the warm water reservoir per time interval Δt, to
the power P supplied to the compressor to per-
form its work. It decreases as the temperature dif-
ference between the condenser and the evapo-
rator increases.
Q
c
m
2
η
co
P
t
P
c = specific heat capacity of water and
m = mass of water.
Determining the efficiency:
Connect the heat pump to the mains supply.
Fill up the water containers with 2000 ml wa-
ter and mount them into the retaining plates
(see point 6.1). For the following measure-
ment, keep at least 4 l of water at 20°C ready.
Allow the compressor to run for about 10
minutes before starting the experiment until it
reaches its operating temperature (the com-
pressor should not heat up during the meas-
urement).
Empty the water container and fill it with water
at a temperature of 20°C. Reset the energy
meter (point 9)
Switch on compressor and start timing (stop
watch, smartphone, etc.).
Stir the water in the containers thoroughly
throughout the experiment.
5
is given by
co
provided to
2
T
2
t
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