Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 398

cally to the wound axis) in a way that makes the windings overlap. Afterwards
the trench is filled up again. Such a collector can also be sunk vertically into a
small, slot-die trench.
− For the Svec-collector /9-7/ a plastic tube is wound up on a roll during produc-
tion. When sunk into a prepared trench, the tube can then be stretched and
fixed like a coil (vertically to the wound axis). Afterwards the trench is filled
up again.
With all these more compact ground-coupled collectors there is a danger that the
necessary heat recovery does not take place during the summer if they are only
used for heating purposes. The reason is that the boundary marking area of the
surrounding soil and the earth's surface is relatively small compared to the acces-
sible volume. Therefore such a set-up is more appropriate for energy storage, thus
compact ground-coupled collectors are most appropriate for heating and cooling
systems. For heat pumps exclusively used for heating purposes, plane ground-
coupled collectors are more suitable.
There are two ways of withdrawing heat from the ground and transferring heat
from the heat source to the heat pump.
− Heat withdrawal and transport can be achieved with an intermediate circuit us-
ing a heat carrier ("brine") that absorbs heat from the ground and discharges it
to the heat pump evaporator. In Germany, a mix of monopropyleneglycol and
water (partly also monoethyleneglycol) has been most effective, which is frost
proof at 25 % glycol up to a temperature of approximately -10 °C and at 38 %
of glycol up to a temperature of approximately -20 °C. Plastic tubes with an ex-
terior diameter of up to 40 mm are used for the ground-coupled heat exchanger.
These materials have a sufficient ageing and corrosion resistance and are elas-
tic and chemically stable at the given temperatures. The individual tube lines
are welded or screwed together.
− Heat withdrawal and transport can also be realised through the so-called "direct
evaporation". In that case the working medium of the heat pump circulates di-
rectly in the tubes of the ground-coupled heat exchanger. It evaporates there
and thus withdraws heat from the earth. The evaporator of the heat pump is
sunk into the ground. In general, copper tubes with a plastic coating to protect
them from corrosion are used. The advantage of this direct evaporation is in the
lower level of machinery input and the higher achievable SPF of the heat
pump. However, it requires a system design that is exactly adjusted in terms of
refrigeration. Furthermore, the filling volumes of the working medium are
much larger than for systems with an intermediate circuit.
Vertically installed ground-coupled heat exchangers. Vertical ground-coupled
heat exchanger for closed systems (so-called ground probes) require significantly
less space compared to the horizontal heat transfer media. They are normally used
in tight spaces or for the retrofit of heating systems, because only little garden
space is affected during the installation.