Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 386

than during the condensation that follows afterwards. One part of the heat sink
can be at a considerably higher temperature level than the condensation tempera-
ture. By exploiting condensation and evaporation, a large part of the heat can be
charged or discharged at a constant temperature, which is an advantage for most
heat sources and heat sinks. The heat capacities have to be read from the T-s -
diagram (temperature-entropy-diagram) as areas beneath the curve ( T ∆ s ) (Equa-
tion (9.1)).
Fig. 9.2 Compression heat pump process shown in a T - s -diagram (temperature-entropy
diagram) and an lg p - h -diagram (pressure-enthalpy diagram)
Heat pump principle. The heat pump is a "device which absorbs heat at a certain
specific temperature (cold side) and releases it again at a higher temperature level
(warm side) after adding drive work" /9-1/. Hence a heat pump can withdraw
thermal energy from a heat source at a low temperature level (e.g. ambient air).
The absorbed thermal energy including the drive work converted into heat can
then be supplied for utilisation as thermal energy at a higher temperature level.
Depending on the functionality of the heat pump, the heat pump can be charged
with the necessary drive energy in the form of mechanical energy or heat. Corre-
spondingly, according to the resulting drive principle compression and sorption
heat pumps are differentiated. In addition, sorption heat pumps are divided into
absorption and adsorption systems; so far the latter have hardly been significant
for the applications analysed here. The respective basic functionality principles -
with the exception of the adsorption system - will be presented in more detail in
the following. For compression heat pumps cold vapour processes will be as-
sumed.
Compression heat pumps. In compression heat pumps a steam cycle takes place in
a closed circuit that mainly consists of the four steps evaporation, compression,
condensation and expansion. These systems thus consist of
− an evaporator,
− a compressor with drive plus
− a liquefier (condenser) and
− an expansion valve (Fig. 9.3).