Environmental Engineering Reference
In-Depth Information
Drive power
Compressor
from heat
source
to heating
system
Low
temperature
Low pressure
High pressure
High
temperature)
to heat
source
from heating
system
Vaporizer
Condenser
Expansion valve
Figure 1.19
Principle of a Compression Heat Pump
Heat pumps can technically utilize the low-temperature heat in the ground and
air as already mentioned in the section on geothermics. An exact division of such
low-temperature heat into solar energy and geothermic energy is very difficult.
Figure 1.19 shows the operating principle of compression heat pumps that
utilize low-temperature heat. A compressor, which is driven by an electric
motor, or a gas or petrol motor powered with external energy, compresses a
vaporous working medium. Pressurizing the working medium also results in
significant heating. A condenser removes the heat from the working medium
until it becomes a liquid. The heat at the higher temperature level is used for
room or domestic water heating. The pressurized working medium expands
back across an expansion valve and reaches the vaporizer. Heat from the low-
temperature heat source vaporizes the working medium again and the
compressor completes the cycle.
Depending on the working medium and pressure, a heat pump can even
provide useful heat at high temperatures from sources with temperatures below
0°C. Ambient air, groundwater or soil can be heat sources. The mechanical
energy
W
needed by the compressor is usually smaller than the useful heat
Q
out
provided. The ratio of both values is called coefficient of performance
(COP) as defined by:
•
Q
out
Q
out
ε
=
=
(1.8)
W
P
where
P
represents the electrical or mechanical power supplied to the
compressor. The ideal or Carnot coefficient of performance
ε
c
of the heat pump
is dependent on the temperature difference of the target temperature T
2
(the
high temperature to the heating system) and the lower final temperature T
1
:
T
2
ε
C
=
(1.9)
T
2
- T
1
From this equation is can be seen that low temperature differences and high
temperatures of the heat source are essential for high COP values. However,
