Environmental Engineering Reference
In-Depth Information
9.1 Principles
During isenthalpic throttling of a real gas, the temperature normally decreases if
no external heat is added. In reversal, during isenthalpic compression of a real gas
or a fluid, the temperature increases. This process, known as the Joule-Thomson-
Effect is based on inter-molecular interaction between the gas molecules. During
expansion work is required to counteract the molecular attraction. This work re-
duces the internal energy, which leads to a temperature decrease. Hence the Joule-
Thomson effect is a measurement for the deviation from real to ideal gases.
Without auxiliary devices, heat flow is only possible from a higher to a lower
temperature. In order to make ambient air heat or shallow geothermal energy uti-
lisable, the direction of the flow has to be reversed. Heat is absorbed at lower
temperatures (i.e. from the environment) and then released again at higher tem-
peratures (e.g. to a radiator, for domestic water heating). To enable such a "heat
pumping" process from lower to higher temperatures, the appropriate equipment
and additionally high-quality energy (e.g. electricity) is required.
A refrigerating agent (refrigerant) is circulated to keep the temperature almost
constant during heat absorption and release, mostly using evaporation and con-
densation (cold vapour process). On the low-pressure side heat is absorbed at a
constant low temperature by the evaporating refrigerant (i.e. energy from the am-
bient air and/or the near-surface ground). Heat flows from this cold heat source to
the refrigerant, which is even colder. Afterwards pressure is added to the evapo-
rated refrigerant by a compressor. This leads to an increase in temperature caused
by the Joule-Thomson effect and the almost isentropic and non-isenthalpic com-
pression. At this higher pressure and temperature level, heat can be discharged in
a condenser to the heat source that needs to be heated up. Again heat flows from a
high to a slightly lower temperature level. During the real heat pump process the
refrigerant is condensed in the condenser and sometimes supercooled slightly. Af-
terwards it is lead via a throttle where it is expanded isenthalpically and thereby
cooled by the Joule-Thomson effect. Then the evaporation process starts again.
Fig. 9.2 shows two forms of presenting the thermodynamic cycle for a com-
pression heat pump. The lg p-h diagram (pressure-enthalpy-diagram) on the right
is clearer, as both pressure levels are clearly visible here. The internal energies in
the individual system parts do not change assuming a stationary circular process.
Thus the heat flow Q . of each part of the circular process results from the mass
flow m . and the enthalpy difference ∆ h of that part of the circular process. Thus
the heat flows of the individual partial processes can be checked from the lg p-h -
diagram as sections on the x-axis (enthalpy) (Equation (9.1)).
&
&
&
Q
=
m
h
=
m
T
s
(9.1)
In the T-s -diagram (temperature-entropy-diagram) on the left of Fig. 9.2, it is
apparent that there is a significantly higher temperature at the compressor outlet
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