Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 395

type are embedded in solid concrete components and thus utilise the ambient heat
indirectly absorbed by the external concrete surfaces. A large amount of this am-
bient heat is solar radiation energy. Because of the large component masses, the
solid absorbers can store large amounts of heat and thus largely compensate for
the fluctuations in ambient air and solar radiation. Normally solid absorbers also
have some kind of functionality as a building component of the house. To give
some examples, estate walls, noise protection walls, external walls of buildings or
concrete garages can be built as solid absorbers.
9.2.2 Heat source systems for shallow geothermal energy utilisation
Heat sources using the near-surface ground generally utilise the heat stored in the
ground (i.e. in the soil or the rocks and their pore filling (mostly groundwater)).
They differ mainly in the form of heat withdrawal from the subsoil or heat release
into the subsoil. Two basic variations can be distinguished (Table 9.2).
− Closed systems. One or more heat exchangers are - either horizontally or verti-
cally - installed in the ground. A heat transfer medium (or heat carrier) (e.g.
water (with antifreeze compound in most cases, or refrigerant) flows through it
in a closed circuit. This process withdraws heat from the subsoil (i.e. from the
soil or rock matrix and the pore filling) (or charges it with heat for space cool-
ing in summer). Heat transfer between the heat carrier and the subsoil takes
place by heat conduction. The heat carrier is not in direct contact with the soil
or rock matrix and the pore filling. Therefore systems of that type can theoreti-
cally be used almost everywhere.
− Open systems. When utilising groundwater, the water is pumped directly
through wells from the layers with groundwater (aquifers). Thus the groundwa-
ter itself is a heat carrier. It is cooled down afterwards (or heated up for space
cooling in the summer) and transferred into the same aquifer via an injection
well. In the subsoil, a heat transfer takes place between groundwater and the
soil or rock matrix. Groundwater as the heat carrier is not circulated in a de-
fined circuit and that, furthermore, is in direct contact with the aquifer; there-
fore these systems are called open systems (Table 9.2). The prerequisite for
such systems is the existence of appropriate layers with groundwater in the
subsoil.
− Other systems. In addition, there are other variations that do not fit exactly into
the categories mentioned so far. These are systems that are not entirely sealed
off from the groundwater; systems utilising water from artificial hollow under-
ground spaces and a system for air preheating, where the heat carrier is sealed
off against the underground, but does not circulate, as new air is continuously
sucked in.
These different heat source systems for the utilisation of near-surface ground en-
ergy are described in the following.