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geotHeRmal collectoR
Geothermal collector
the ground as heat source in this
context is the upper layer of the
earth down to a depth of 2 m. Heat is
yielded via a heat exchanger that is
buried in an area, where no buildings
are located, near the building to
be heated. the heat relevant to the
extraction from the ground is stored
solar energy that is transferred to the
ground through direct irradiation,
air-borne heat transfers and
precipitation. this is also the energy
source for the rapid regeneration of
the supercooled ground at the end of
the heating season. the heat rising
from lower strata upwards amounts
to only 0.05 to 0.12 w/m² and can
therefore be ignored as heat source
for the upper strata. the available
heat and therefore the size of the
required area is largely dependent on
the thermo-physical properties of the
ground and the irradiation energy, i.e.
the climatic conditions. the thermal
properties, such as the volumetric
thermal capacity and thermal
conductivity are mainly dependent
on the consistency and condition of
the ground. the control variables that
are of particular relevance are the
proportion of water, the proportion of
mineral constituents, such as quartz
and feldspar as well as the proportion
and size of pores filled with air. to put
it simply, the storage characteristics
and thermal conductivity are higher
the more the ground is enriched with
water, the higher the proportion of
mineral constituents and the lower
the proportion of pores. the extraction
rate of the ground is subject to the
soil quality and lies between 10 and
40 w m² at a spacing between pipes
of 0.6 to 1.0 m and a depth of between
1.2 and 1.5 m. to be able to utilise
the ground as heat source, plastic
pipe loops (geothermal collectors)
are buried under ground. the heat
transfer medium circulates through
these pipes. the mixture transfers
the heat extracted from the ground
to the heat pump. the heat transfer
medium must provide adequate frost
protection. in addition, the medium
must not damage the groundwater in
case of a leak. the antifreeze based on
ethylene glycol offers these properties.
it was specifically developed for the
heat transfer and the frost/corrosion
protection in heat pump systems.
Extraction rate
(VDi 4640)
for dry, non-binding soil
qe = 10-15 w/m²
for moist, binding soil
qe = 15-20 w/m²
for very moist, binding soil
qe = 20-25 w/m²
for soil saturated with water
qe = 25-30 w/m²
for areas, where groundwater runs
qe = 30–40 w/m²
A surface area results, subject to
the heat demand of the house and
the consistency of the ground. the
required area of ground is determined
on the basis of the cooling load Q
K
of the heat pump. the cooling load
of the heat pump is the difference
between the heating output Q
HP
and
the power consumption P
HP
.
Q
K
= Q
wP
– P
wP
Example:
At a heat source temperature of 0 °C
and a heating flow temperature of
+35 °C, the wPF 10 delivers a heating
output of 9.9 kw and consumes
2.2 kw.
Q
K
= 9.9 kw – 2.2 kw
Q
K
= 7.7 kw
A specific extraction rate qe of
25 w m² results in the following area:
Area = Q
K
/ q
e
Area = 7700 w / 25 w/m²
Area = 308 m² ground
A pipe spacing of 0.6 m results in the
following pipe length:
308 m² / 0.6 m = 513 m pipe, which
equals five pipe circles of 100 m
length each.
Summary of Contents for Heat pumps
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