Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
MEASUREMENT, MODELING AND SIMULATION OF AN
EARTH-TO-AIR HEAT EXCHANGER IN MARBURG (GERMANY)
Rainer Wagner, Stefan Beisel, Astrid Spieler, Klaus Vajen
Philipps-Universität Marburg, Department of Physics, D-35032 Marburg, Germany, Phone: ++49/6421/282-4131, Fax: -6535
[email protected]
Andreas Gerber
FH Biberach, Department of Architecture, D-88400 Biberach, Germany
Abstract – Earth-to-air heat exchangers are simple systems to save energy in buildings which are equipped with an
active ventilation system: In addition to the conditioning and distributing of the air in the building, the fresh air is
sucked through (e.g.) pipes buried in the ground. In this way the fresh air is pre-cooled in summer and pre-heated
during the winter. The energy delivered by the system strongly depends on the different parameters of the earth-to-air
heat exchanger like length beneath foundations, length beneath undeveloped ground, depth, diameter, material, number
and distance of the pipes. In some cases especially the outlet temperature of the earth-to-air heat exchanger is of
interest to avoid the freezing problem of a heat recovery system. A detailed model, that makes a compromise between
calculation time and reached accuracy, was developed further to weigh up some of the different parameters. This model
was compared with measurements in a passive solar office building in Cölbe (near Marburg, Germany). Measurements,
the earth-to-air heat exchanger model and some of the simulation results are presented in this paper.
1.
INTRODUCTION
Detailed models are needed to investigate the different
parameters that influence the thermal output and the outlet
temperature of earth-to-air heat exchangers. When dealing with
cartesian coordinates (like Pfafferott, 1997) one might have the
problem of long calculation times. One possible solution is
discussed by Neugebauer (Neugebauer, 1998): The grid is
adjusted to the temperature field round the earth-to-air heat
exchanger: Cylindrical near the pipes and cartesian far away
from the pipes. Conduction is only allowed in radial direction.
Moreover the grid can be adjusted to the temperature
differences, hence only a few nodes/m³ are used far away from
the pipes whereas the pipes themself and the near region are
represented by a higher density of nodes.
To test this model with the earth-to-air heat exchanger of
the passive solar office building in Cölbe, some further
development was carried out by Beisel (Beisel, 1999).
In the first part of this work measurements from March
1999 to April 2000 regarding health and comfort in the
building, temperatures in the ground, possible freezing of the
heat recovery unit and cooling/heating output are presented.
Further the model development with special coordinates, a
comparison with measurements and some parametric studies
are discussed.
2.
THE INVESTIGATED SYSTEM
2.1. The concept of the ventilation system
The earth-to-air heat exchanger belongs to the solar assisted
passive office building which was build by the Wagner & Co
Company (Germany) (see e.g. Wagner, 1999 or Spieler, 2000).
The earth-to-air heat exchanger consists of 4 pipes of concrete
connected in parallel laying partly beneath the foundations
(length 32 m, diameter 0.5 m, distance 0.15 m, depth 1.5 m).
Corresponding to the hygienic need, the air exchange rate
typically amounts to 3400 m³/h ( ≈1.2 m/s in the pipes.),
which is equivalent to 0.4 /h related to the volume of the
building. Even though the air exchange rate is very small, the
ventilation system can be used for distributing the low heat
demand which is less than 15 kWh/m²a in the house. Fig. 1
shows the earth-to-air heat exchanger under construction.
Fig. 1: Earth-to-air heat exchanger of the solar assisted
passive office building during the construction.
The earth-to-air heat exchanger is part of an active
ventilation system which consists of an heat recovery system (4
cross-flow-heat-exchangers, dry efficiency of about 80%), one
central pre-heater (water-air heat exchanger, 42 kW) and 9
decentrally located heaters in the different zones of the
building (water-air heat exchangers, 2...6 kW).
The earth-to-air heat exchanger operates all year round
whereas the heat recovery system can partly or completely be
bypassed. Except for a small active cooling system for the
Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
server-computers in this office building, there is no further airconditioning. Fig. 2 shows a scheme of the active ventilation
system with the positions of the fans and the filter-bags.
Intake air
return air
max.
50°C
exit air
EU3
2
fresh air
EU3
4
EU5
1
3
max.
25°C
min.
-2°C
Fig. 2: Scheme of the active ventilation system
consisting of: Earth-to-air heat exchanger (1), heat
recovery system (2), one central pre-heater (3) and nine
heaters (4) (EU3, EU5: Filterbags).
The main part of the pressure drop of approximately 900 Pa
for air exchange rates of 1/h can be assigned to the air
distributing system with the filter bag (EU5), the heat recovery
system and the pre-heater and the heaters, respectively. This
(high) pressure drop yields to a measured power consumption
of 0.54 W/(m³/h) by the two fans (10.99 – 04.00). This is a
little higher than the proposed value for passive houses of
0.4 W/(m³/h) (Feist, 1998).
2.2. Measurement system
Validating and testing of earth-to-air heat exchanger models
requires detailed measurements. Therefore the temperatures
were measured in the undeveloped ground in different
distances to the pipes (0 m up to 6.5 m), in different depths (32
sensors), beneath the foundations in a depth of 1.5 m (13
sensors), on the outer surface of the pipes (5 sensors) and in
the air current (10 sensors) with Pt-100 sensors. Moreover the
humidities of the fresh air at inlet and outlet of the earth-to-air
heat exchanger and of the return air are monitored.
3.
MEASUREMENTS
3.1. Mould loads and radiative exposure
To make sure that there is no risk to health due to radiative
exposure (radon) coming from the concrete pipes or the
ground, the air quality was measured by the Institut für Umwelt
und Gesundheit (IUG, 2000)). They did not state any increased
values of Radon (220Rn, 222Rn).
Another risk to health might be an increased bacterium or
mould load. Flückiger (Flückiger, 1999) has already shown
that usually the number of colony forming units decreases due
to the filter bags. But in case of leakages in the pipes, they
suggested to control the air quality periodically. The Wartig
Chemieberatung GmbH (Wartig, 1999) has done
measurements of the mould load in one seminar room, that was
closed for 48 h and ventilated as usual in September 1999.
Although the measurements were done without germ
differentiation, one can say, that the number of colony forming
units was decreased by a factor of 40(!) compared to the
ambient air due to the filter bags.
3.2. Comfort: Temperatures and humidity
High ventilation rates in conjunction with a re-heating of
the air leads to an uncomfortable dry indoor climate. To solve
this problem without the use of a humidifier the air exchange
rate was adjusted to the hygienic need of only 0.4 /h.
In contrast to this, an earth-to-air heat exchanger cools down
the fresh air in summer and may lead to uncomfortably moist
indoor air or even to condensation in the pipes. To consider
this problem the humidity was measured at the inlet and outlet
of the earth-to-air heat exchanger. During the period from
March 1999 until October 1999 no condensation could be
measured. In less than 10% of the measured hourly mean
values the humidity rose above 80%.
To consider the comfort in the house, both the indoor air
temperature and the humidity are of interest. Fig. 3 shows
measured hourly mean values of the humidity versus the indoor
air temperature. In 84% of all cases the measured values are in
the comfortable range (during the winter still 58% of the time
the air quality is “comfortable”; all along the air quality was at
least “still comfortable”).
air humidity
in %
100
90
80
70
60
50
40
30
20
10
0
comfortable
84%
still comfortable
air temperature in °C
12
14
16
18
20
22
24
26
28
Fig. 3: Hourly mean values of the indoor humidity
versus indoor air temperature out of the heating period:
In 84% of all hourly mean values the measured values
of indoor air humidity and temperature are comfortable
(both measured in the return air; March 1999 - October
1999; Definitions from Leusden and Freymark cited in
RWE, 1996).
3.3. Temperatures in the ground
In Fig. 4 some measured temperatures are shown, from
which the influence of the earth-to-air heat exchanger on the
ground can be seen.
From Fig. 4 many aspects can be pointed out: (i) The
temperatures beneath the foundations are more damped than
the temperatures in the undeveloped ground (see bottom).
Beneath the foundations the standard deviations are typically
2 K smaller (compare top: T1 with T8 or T2, T3, T4 with T5, T6).
(ii) Whereas the temperatures near and far from the pipes are
different within a few days (see bottom: T1, T2, T3), the average
Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
undeveloped ground
foundations
11.1±5.2 11.2±5.4 10.7±5.1 11.6±3.7 11.9±3.2
T8 13.4±1.7
T2
T3
T4
T5 T6
N
T7 12.8±2.1
S
O
W
measured thermal output and the temperature difference
between inlet and outlet of the earth-to-air heat exchanger. It
can be seen that in more than 50% of the cases the hourly
mean values for output or temperature difference are less than
2 kW or 2 K, respectively. In only 8% of the time the average
output in one hour is higher than 8 kW.
100%
T1 11.7±3.2
cdf
25.02.00
27.12.99
28.10.99
29.08.99
30.06.99
01.05.99
90%
25
temperature
difference
in K
80%
70%
60%
50%
40%
Ta
20
T1
T8
30%
20%
T7
15
output
in kW
10%
0%
-10 -8
-6
-4
-2
0
2
4
6
8
10
10
5
T6
T2, T3
0 T/°C
-5
Fig. 4: Daily mean values of the ambient temperature
and some temperatures in the ground: near (0.5 m) and
far away (2.0 m) from the earth-to-air heat exchanger,
in the undeveloped ground and beneath the foundations
(May 1999 – April 2000). All mentioned sensors are
installed in the same depth as the earth-to-air heat
exchanger. In the upper part the average temperatures
and the standard deviations of some sensors were given
(from daily mean values)
values over the year are quite the same (top: T1, T2, T3 and
T4). That means that there is no exhaustion over a year. This
bases on the fact, that the output for heating and cooling are
nearly the same. (iii) The average temperature beneath the
foundation without earth-to-air heat exchanger is about 2 K
higher than in the undeveloped ground even though the
contribution of the building will be small since the foundations
are insulated with 24 cm of foam glass against the ground. The
influence on the output will be studied later.
3.4. No freezing of the heat recovery unit
Especially for passive houses, where a heat recovery unit is
essential, earth-to-air heat exchanger can avoid a freezing of
the heat recovery unit. The minimum inlet temperature for a
heat recovery system with an efficiency of 80% can be
estimated to be –4 °C (Treturn = 21 °C, no vaporization enthalpy
considered). Up to now the measured outlet temperature of the
earth-to-air heat exchanger was at least -2 °C (hourly mean
values). However, the longest cold period was only 3 days at
-8 °C.
3.5. Cooling and heating power
Fig. 5 shows the cumulated distribution function of the
Fig. 5: Cumulated distribution function (cdf) of
measured hourly mean values for heating and cooling
gains and temperature difference, respectively (May
1999 – April 2000).
3.6. Output of the earth-to-air heat exchanger and
the heat recovery unit in 99/00
Whereas the heat recovery unit was partly or completely
bypassed, the earth-to-air heat exchanger operates without
bypass full year. Unfortunately this causes sometimes a cooling
output during a winter day or a heating output during a summer
day leading to a decrease of the useful energy output. The
heating-, cooling- and useful energy output are calculated for
different evaluation periods in Tab. 1: S-I: Summer, with
naturally driven night ventilation cooling of the building (June
to September), S-II: Summer without night ventilation (May,
June and September), TP: Transitional period (October,
November and March, April), W: Winter (November to
March).
evaluation
period
HRU
MWh
S-I
S-II
TP
W
sum:
Earth-to-air heat exchanger
heating
cooling
output
MWh
MWh
MWh
0
0
5.9
48.3
2.0
0.8
1.9
6.2
-3.7
-1.6
-0.3
-2.5
1.7
0.8
1.6
3.7
54.2
10.8
-8.0
7.9
Tab. 1: Measured output of the heat recovery unit
(HRU) and earth-to-air heat exchanger (April 1999
– March 2000) (Abbreviations see text). The useful
of the earth-to-air heat exchanger calculated
positive.
The limits for the evaluation periods just show the typical
operation conditions, however when calculating the outputs no
day will be skipped or calculated twice in different periods.
Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
The total useful energy delivered by the earth-to-air heat
exchanger in case of cooling and heating was measured to be
7.9 MWh/a (Tab. 1). The heat recovery unit and the earth-toair heat exchanger delivered together (54.2 + 6.2) MWh/a in
the periods TP and W. This is an important contribution to the
energy balance of the building, making possible the low
heating energy demand of only 23 MWh/a for the whole
building (corresponds to 12 kWh/m²a, see Spieler, 2000).
However the contribution of the earth-to-air heat exchanger is
very low due to the poor design.
4.
MODELING THE SOURROUNDINGS
4.1. Soil types
The temperature field in the ground depends on the soil
type and the moisture contained, respectively. In most cases
there is not any detailed information about soil characteristics
available and the moisture varies throughout the year.
However, Jäger and Herz suggest a small number of different
soil-types, that are characterized as follows (Jäger, Reichert
and Herz, 1981):
soil type
sand, dry
sand, wet
loam, dry
loam, wet
λ
W/m²K
0.70
1.88
1.45
2.90
ρ
kg/m³
1500
1500
1800
1800
cp
J/kgK
922
1199
1339
1591
Whereas the influence of trickling water or geothermal
energy can probably be neglected (Shen, 1988; Sanner, 1992),
the solar radiation in Germany with approximately
αground⋅2.8 kWh/m²d ≈αground⋅116 W/m² and the long wave
radiation exchange between the ground and the sky have to be
considered. However, the temperature in the ground is
approximated quite well by using a formula that only depends
on monthly based average ambient temperatures and properties
of the ground, namely the heat transfer coefficient
αground-ambient, the thermal conductivity λ and the temperature
conductivity a. This formula is given by Grigull and Sandner
(Grigull and Sandner, 1990) as a solution of the heat
conduction equation in one dimension with the ambient
temperature approximated as cosine-periodic boundary
condition. The solution has the form of a dampened cosine
oscillation with the annual mean temperature as an offset and a
dephasing. The damping coefficient is found to be
(π/ (a⋅107 s))½ (for a detailed discussion see (Grigull and
Sandner, 1990 or Beisel, 1999). A comparison of measured and
calculated ground temperatures shows a good agreement (Fig.
7).
20
a
m²/s
50.7⋅10-6
104.5⋅10-6
60.2⋅10-6
101.3⋅10-6
18
16
14
8
6
4.2. Influences on the temperature field in the
ground
The temperature field in the ground is influenced by on
different quantities (see Fig. 6).
solar
radiation
rainfall
trickling water
conduction
ground water
geothermal energy
Fig. 6: Different environmental influences and heat
transfer mechanisms that influence the temperature
field in the ground.
beneath
foundation:
1,5 m
10
4
evaporation
Ta
12
Tab. 2: Reference soil-types suggested by Jäger
(Jäger, Reichert and Herz, 1981) derived from the
literature (thermal conductivity λ, density ρ, heat
capacity cp, temperature conductivity a).
radiation
T / °C
2
ground:
1,5 m
3,0 m
0
Dec Jan Feb Mar Apr May Jun Jul Aug
Fig. 7: Measured (lines; December 1998 to August
1999) and calculated (bars) temperatures in the ground
without the influence of an earth-to-air heat exchanger
(ambient, 1.5 m beneath foundation, 1.5 and 3.0 m in
the undeveloped ground). The calculations (bars) are
done with the formula of Grigull and Sandner (Grigull
and Sandner, 1999) for soil type “wet loam”.
Calculations of the damping coefficient for the different soil
types show skin depths of 2.25-3.25 m per year. An estimation
of the daily skin depth yields only 15 cm. When choosing the
simulation grid with the thermal lines, these temperature field
should be represented by the control volumes.
5.
MODELING THE EARTH-TO-AIR HEAT
EXCHANGER
5.1. Stationary temperature fields
In order to characterize the behaviour of the pipes in the
undeveloped ground and beneath the foundations the stationary
temperature fields a pre-study for the grid was made with the
Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
simulation program Therm (Therm, 1999). Fig. 8 shows both
cases for an extreme winter state with -10 °C inlet
temperature.
conditions for the simulation.
5.3. Coordinate system of the grid
The earth-to-air heat exchanger and the ground are
approximated with control volumes that allow heat flux in only
one dimension (cf. Fig. 10).
ρc pV
qin
qout
cylinder
Fig. 8: Steady state temperature field in the
undeveloped ground (top) and beneath the foundations
(bottom). Both simulations were done with the
simulation tool Therm (Therm, 1999) with an inlet air
temperature of -10 °C to give an idea of the
temperature field.
Roughly speaking, the isothermal lines are running
circularly close to the pipes and horizontally or vertically far
away from the pipes. The isothermal lines strike normally the
foundations. These observations can help to choose the
coordinate system of the grid.
5.2. Boundary conditions
Since the earth-to-air heat exchanger buried in a depth of
1.5 m and the ground water level is approximately at 3 m
under the earth's surface, the ground water level can be
considered as an isothermal boundary. The lateral boundaries
of the region to be considered and the earth’s surface
temperature in the undeveloped ground were given by the
equation of Grigull and Sandner (Grigull and Sandner, 1990)
considering the skin depth. The foundations of the house can
be seen as an adiabatic boundary, since the house is insulated
against the ground with 24 cm of foam glass
(λ= 0.035 W/mK).
earth-to-air
heat exchanger
foundations (adiabatic)
earth's surface +
lateral boundaries
(Grigull et al)
ground water (isothermal)
Fig. 9: Scheme of the earth, the earth-to-air heat
exchanger and the foundations to show the boundary
coil
rung
∂T ∂(q out − qin )
=
∂t
∂t
adapter
ashlar
Fig. 10: Different control volumes (ashlars, coil rungs,
adapters, cylinder) for the ground and the pipes. The
material properties can be set individually. Heat flux is
allowed in only one dimension.
The whole system is composed with these control volumes
in the following manner: Several ashlars, coil rungs and so on
are assembled to form “chains”. These chains can be easily
connected to the pipes (consisting of four thermal nodes) and
the air in the pipes via standardized interfaces, forming one
“slice”. This slice is a cross section of the system, representing
a length of approximately 2.7 m for this earth-to-air heat
exchanger in flow direction. Twelve slices together make up
the whole system with approximately 2500 thermal nodes.
The modeling is done with the simulation tool Smile
(Smile, 1997), since it supports aggregation and inheritance of
classes, which is very useful in this case. The simulation time
was about 2 h for one year simulation (Smile-version 1.0pre19,
max. time step 4 h, PII-333 MHz).
6.
COMPARISON BETWEEN MEASUREMENTS AND
SIMULATION RESULTS
6.1. Comparison
When the work on the simulation was finished the
measurements were not completed. Hence, comparison was
possible only for short periods, for example: “cooling” (one
week in July 99), “heating” (one week in February 99) and
“both heating and cooling” (the whole of May 99). The
simulation was done for the period December 1998 until July
1999 with the measured boundary conditions (cf. 5.2).
The comparison shows temperature differences of only
0.2 K up to 0.7 K. This leads to an error of typically 10% for
the delivered energy in the investigated time periods (Beisel,
1999).
Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
90
measurement
simulation
difference
kWh/d
75
kWh/d
20
60
45
10
30
15
0
0
-15
-30
-45
-10
-60
-75
day in May 1999
-90
-20
01
06
11
16
21
26
31
Fig. 11: Comparison between measured output and
simulations with soil type “wet loam” for May 1999.
The simulation seems to overestimate the cooling
output in some cases. However, the typical simulation
error is about 10% on a weekly basis.
Now a detailed model is available to study one year
performance of the earth-to-air heat exchanger and the
influence of different parameters like installation depth and
pipe length to the output respectively.
6.2. One-year simulation with TRY
Fig. 12 shows the simulated daily and weekly mean output
for the test reference year TRY04 (Blümel, 1986). The earthto-air heat exchanger runs continuously with a flow rate of
3000 m³/h. Even during the summertime a heating output of up
to 80 kWh/d can be observed and during the wintertime a
cooling output of up to –40 kWh/d occurs. This problem might
be reduced by installing a bypass. On the other hand the
continuous operation will prevent the earth from exhaustion.
400
300
200
100
0
-100
kWh / week
-200
-300
27.12
28.10
29.08
30.06
01.05
02.03
-400
01.01
kWh / day
100
80
60
40
20
0
-20
-40
-60
-80
-100
Fig. 12: Simulated daily and weekly output of the
earth-to-air heat exchanger (soil type “wet loam”,
TRY04 (Blümel, 1986)). The daily output (dots)
sometimes changes between cooling and heating daily.
The simulated hourly mean values of the outlet temperature
never fall below -6 °C, and in only 6 h/a they are below -5 °C .
Hence, freezing of the heat recover unit (Tmin > -4 °C) will not
occur in practice if the weather is comparable to the German
test reference year TRY04 (Blümel, 1986).
6.3. Installation depth and pipe length
The influence of different parameters on the thermal output
of the earth-to-air heat exchanger were studied by Beisel
(Beisel, 1999). Tab. 3 shows the results for the parameters
“pipe length in the undeveloped ground”, “pipe length beneath
the foundations” and “installation depth”. Without a coupled
dynamical simulation of the building, the cooling output cannot
be studied, because the room temperatures can vary in a certain
region without influencing the comfort in the building.
However, during the heating period the outlet temperature of
the earth-to-air heat exchanger will be less than the set point
temperature of the intake air for all the time. The heating
output was calculated for the heating period in a passive house,
that runs from December, 1st until February, 28th.
depth
pipe length:
heating cooling
m
udg + bf = “total“
MWh
MWh
m
output
MWh
1.5
2.0
3.0
5.0
32.0
32.0
32.0
32.0
+
+
+
+
0.0
0.0
0.0
0.0
=
=
=
=
32
32
32
32
2.9
2.9
3.5
4.1
-1.0
-1.0
-0.8
-0.7
1.8
2.0
2.7
3.5
1.5
1.5
1.5
1.5
1.5
1.5
18.7
26.7
39.7
66.7
32.0
0.0
+
+
+
+
+
+
13.3
13.3
13.3
13.3
0.0
32.0
=
=
=
=
=
=
32
40
53
80
32
32
3.8
4.3
5.1
6.3
2.9
5.1
-0.6
-0.7
-0.8
-0.9
-1.0
-0.4
3.2
3.6
4.3
5.5
1.8
4.7
Tab. 3: Heating-, cooling- and useful output of the
earth-to-air heat exchanger for different installation
depths and pipe lengths in the undeveloped ground
(udg), beneath the foundations (bf). The built earth-toair heat exchanger was printed in bold characters
(simulations with soil type “wet loam”, and TRY04
(Blümel, 1986) for the heating period December to
March).
Increasing the installation depth leads to an increasing
useful output. This effect can be explained when calculating
the temperatures in the ground without heat exchanger with the
formula of Grigull (Grigull and Sandner, 1990): In a depth of
1.5 m the temperatures reach 4...7 °C, whereas in a depth of
5 m 8...10 °C can be calculated for the winter time December
to February. This is also the reason why the cooling output
increases when lengthening the pipes in a depth of 1.5 m in the
undeveloped ground.
The measurements of the temperature beneath the
foundation show more than 12 °C in the winter time. Hence the
simulation shows that an earth-to-air heat exchanger running
completely beneath the foundations yields a higher output than
an earth-to-air heat exchanger of the same length in a depth of
5 m in the undeveloped ground.
Rainer Wagner, Stefan Beisel, Astrid Spieler, Andreas Gerber, Klaus Vajen: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in
Marburg (Germany), 4. ISES Europe Solar Congress, Kopenhagen, Dänemark, 2000.
7.
CONCLUSIONS
The modeling of the earth-to-air heat exchanger was done in
special coordinates: Cylindrical close to and cartesian far away
from the pipes. Simulated and measured thermal output of the
earth-to-air heat exchanger showed differences of only 10% on
a weekly basis. Simulations show that the heating output
would have decreased by 15% if this earth-to-air heat
exchanger had been installed completely in the undeveloped
ground, whereas the heating output would have increased by
22% if it would have been installed completely beneath the
foundations.
The simulations with standard weather conditions
measurements have shown, that the earth-to-air heat exchanger
in Cölbe can avoid a freezing of the heat recovery system. In
only 6 h/a the simulated outlet temperature falls below –5 °C.
The mould loads decreased by a factor of 40 due to the filter
bags and no increased radon exposure was found out. In 58%
of the year the air quality of the building was “comfortable”
but never uncomfortable, as far as humidity and indoor air
temperature are concerned.
The measured/simulated output was up to 100 kWh/d for
cooling or heating. This led to an useful energy output of about
7.9 MWh/a in the year 1999/2000 in the passive solar office
building. As far as heating is concerned the output of the heat
recovery unit and the earth-to-air heat exchanger was
(54.2 + 6.2) MWh/a - a worth mentioning share leading to the
(very low) heating energy demand of only 23 MWh/a for the
whole building. The low contribution of the earth-to-air heat
exchanger is due to the poor design (e.g. installation depth,
distance between the pipes).
ACKNOWLEDGEMENT
The co-operation with Ulrich Rustige, Klaus Schweitzer,
Karsten Tent (Wagner & Co, Cölbe), Wolfgang Feist, Jürgen
Schnieders (Passivhaus-Institut, Darmstadt) and Christoph
Nytsch, Tobias Schrag (Institut für Energietechnik der
Technischen Universität Berlin) is gratefully acknowledged.
The project is supported by Bundesministerium für
Wirtschaft und Technologie (contract No. 0335006L).
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