borehole heat exchanger
TRANSCRIPT
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Design of BHE-Fieldsas a source of shallow geothermal energy
Marc Sauer
Dipl.-Geol. M. Sauer GbRZum Boden 6 35580 Wetzlar Germany
0049 6441 212910
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Picture by Thomas Blomberg, Blocon
BHE: Borehole Heat Exchanger
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VDI 4640
German
guidelineVDI 4640
For the design of smaller
BHE-fields (up to 30kW
heating load) the so-
called specific heat
extraction may be used.
Here the heat
conductivity and the full
load hours (heating) are
roughly taken into
account.
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VDI 4640: Example
Heating load: 10,0 kWFull load hours 1.800,0 h/aAnnual heating work: 18,0 MWh/a
Lithology: Limestone (normal underground)Specific heat extraction: 60,0 W/m
SPF heat pump: 4,0
Evaporator load: 7,5 kW
Needed BHE length: 125,0 m
( )1= SPFSPF
loadHeatingloadEvaporator
extractionheatSpecific
loadEvaporatorlengthBHE =
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Larger Systems (> 30 kW)
Very often the design of the BHE-fields for medium and large
facilities is also based on the specific heat extraction.
German
guideline
VDI 4640
An accurate dimensioning often results in completely different
BHE-field sizes.
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Larger Systems (> 30 kW)
soil heat conductivity
undisturbed underground temperatures
groundwater
building requirements (load, full load hours and work respectively)
kind of use (only heating, heating + cooling direct / with heat pump)
seasonal performance factor SPF (required temperatures)
length of BHE
borehole resistance (BHE type, borehole diameter, filling material)
spacing
BHE-field geometry
The guideline demands that the temperatures in the underground have to becalculated (simulated) over the planned life time (e.g. 25 years) of the system.For this task several computer programs exist (e.g. Earth Energy Designer EED)They take into account the following important influencing factors:
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Calculating Larger Systems (> 30 kW) with EED
The dimensioning of a BHE-field is an iterative process, which means the input
parameters will be changed until the fixed temperature limits are hardly maintained, i.e.:monthly middle temperatures in a heat transfer medium (base load): not below 0 C
minimum temperatures in a heat transfer medium (peak load): -5 C
Base load
Peak cool load
Peak heat load
Year 25
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Fluidtemperature[C]
5
4
3
2
1
0
-1
-2-3
-4
-5
Peak min
Peak max
Base min
Base max
Year
252015105
Annualm
in-maxfluidtemp.
[C] 10
8
6
4
2
0
-2
-4
Simulation with Earth Energy Designer
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Examples
Residental Area with 28 Houses
Heating: 6,5 kW
2.400 h/aCOP HP: 3,8
Underground: limestone
= 2,8 W/(mK)T0= 12,8C
As a single system:
Spec. extraction acc. VDI 4640= 60 W/m
Evap.Load: 4,8 kW
BHE-Length: 80 m
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ExamplesResidental Area with 28 Houses
Heating: 6,5 kW2.400 h/a
COP HP: 3,8
Underground: limestone
= 2,8 W/(mK)T0= 12,8C
After simulation
6,8 kW
16,3 MWh
6,4 kW
15,4 MWh
7,0 kW
16,8 MWh
6,4 kW
15,4 MWh
6,4 kW
15,4 MWh
6,4 kW
15,4 MWh
6,4 kW
15,4 MWh
6,4 kW
15,4 MWh
89,0 m109,0 m 85,0 m93,0 m98,5 m102,5 m102,5 m102,5 m
8m
+ 36% + 6%+28 %
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Examples
Year 1
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Fluidtemperature[C
]
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
Year 25
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
li
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-12
-13
-14
Year
25242322212019181716151413121110987654321
Annualmin-maxfluidte
mp.
[C]
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
Heating load (Building): 100,0 kWHeating load (Heatpump) 30,0 kWFull load hours 3.500 h/aAnnual heating work: 105 MWh/a
Lithology: Limestone (normal underground)Specific heat extraction: 60,0 W/m
SPF heat pump: 4,0Evaporator load: 22,5 kW
BHE length: 375 m (4 x 94m)
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Examples
9m
Year
25242322212019181716151413121110987654321
Annualmin-maxfluidtemp
.[C]
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
+ 100% Spec.Extraction: 30 W/m
Heating load (Building): 100,0 kW
Heating load (Heatpump) 30,0 kWFull load hours 3.500 h/aAnnual heating work: 105 MWh/a
Lithology: Limestone (normal underground)Thermal Conductivity: 2,8 W/(mK)Undisturbed Temp.: 12,8 C
SPF heat pump: 4,0
BHE length: 752 m (8 x 94m)
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Examples
eating load (Building): 100,0 kWeating load (Heatpump) 30,0 kWull load hours 3.500 h/annual heating work: 105 MWh/aPF 4,0
ooling load (Building): 100,0 kWooling load (Heatpump): 30 kWull load hours 1.700 h/a
nnual cooling work: 51 MWh/aPF (active cooling): 5,0
HE length: 564 m (6 x 94m)Year
25242322212019181716151413121110987654321
Annualmin-maxfluidtemp.
[C]
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
6m
Spec.Extraction: 40 W/m
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Calculating Larger Systems (> 30 kW)
Some pricipals:
Thermal conductivityA higher thermal conductivity always increases the efficency of theBHE. The heat is transported faster when conductivity is higher
Number of BHEWhen changing the number of BHE also the required depth changes.
But a 100m BHE can't be replaced by 2 x 50m.Factors:1. Increase of temperature with depth (3K/100m)2. A low number of BHE reduces the unfavourable effect of
mutual influence.3. With a greater number of BHE the accessible volume of
soil /rock can be increased.
4. Also the envelope area of the BHE-field is increasing
BHE spacingWith increase spacing between the BHEs the accessible volume of soilcan be increased and the mutual influence can be reduced.
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Calculating Larger Systems (> 30 kW)
Some pricipals:
Field geometryDepending on the field geometry a BHE has more or lessneighbours. With larger number of immediate neighboursthe mutual influence increases.
Thermal borehole resistance
The thermal borehole resistance describes the loss of temperatureduring heat transmission from rock to brine and vica versa.A low borehole resistance always has positive effect on theefficency of the BHE.
Storage effectAs a rule, the maximum efficiency can be achieved with BHE-plants
used for building heating and cooling.The discharged heat from the building cooling is (partly) stored in theunderground and is available for building heating in winter and vice versa.The consideration of the storage effects can cause a significant reductionin needed depth and/or number of BHE.
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Summary
W/m
50
For smaller single systemswithout special requirementsthe specific heat extractionvalues may be used for the
design of BHEs.
For medium sized (> 30 kW)and larger systems as well asfor systems with specialrequirements (e.g. high fullload hours, cooling) asimulation has to be made.
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W/m50
Thank you
for your attention