Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 384

ing to that regulation, systems for (deep) geothermal energy utilisation below
400 m were subsidised by carrying the risk of the drilling process. As there had
not been any plants in operation at depths between around 200 and approximately
500 m for a long time, this was seen as the boundary. A value of 400 m as the ap-
proximate lower limit of shallow geothermal energy utilisation in the meantime
has been adopted by other normative guidelines as well (e.g. German VDI
guideline 4640). However, to set such an exact limit for the transition between
shallow heat utilisation and heat utilisation from the deep underground is prob-
lematic, as the continuous technical development enables e.g. geothermal energy
probes to reach increasingly deeper. Thus the limit between shallow and deep
geothermics is becoming increasingly fluid.
Ambient air and shallow geothermal energy can be harnessed by a number of
different technologies, methods and concepts. As the utilisable energy is normally
generated at a low temperature level (mainly below 20 °C), a device to increase
the temperature is generally required in order to enable the technical utilisation of
the heat (e.g. to heat a residential building). This means that a heat pump needs to
be built into the system. Alternatively, the subsoil temperature level can be in-
creased by storing additional heat (e. g. from solar energy using solar collectors or
excess heat from industrial processes). This option has hardly been put into prac-
tice so far. In order to harness ambient air and shallow geothermal energy, addi-
tional externally supplied energy is always required (e. g. electricity from the pub-
lic grid, natural gas or biogas, fuels).
Hence a system to supply useful or final energy through ambient air and
shallow geothermal energy utilisation generally consists of three system elements:
− Heat source system to enable the withdrawal of energy from ambient air and
near-surface ground.
− Heat pump or another technical system essential to increase the temperature
level and
− Heat sink; the system to feed or utilise the heat at a higher temperature level.
This increase is obtained by using a heat pump.
The principles and their technical realisation, on which the first two main system
elements are based, will be described in the following. Heat sink systems, how-
ever, are standard systems for heating and are thus not dealt with separately. Ad-
ditionally - after a description of the principles of the heat pump as the basis for
the utilisation of low-temperature heat - the different technical concepts of heat
source systems will be discussed initially. Afterwards, the technical principles of
the heat pump together with the resulting overall systems will be presented. This
leads to analysing these systems from an economic and environmental point of
view, also showing their potential and their forms of utilisation.