Borehole Heat Exchanger

8/11/2019 Borehole Heat Exchanger

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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


Fluidtemperature[C

]

10

9

8

7

6

5

4

3

2

1

0

-1

-2

-3

-4

-5

-6

-7

Year 25


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

Documents

Borehole Heat Exchanger