GROUND-SOURCE HEAT PUMP SYSTEMS Economic performance …

GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University

Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli

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-15 -10 -5 0 5 10

CO

P*Tsource, [°C]

Tuser = 45°C

Qsource Quser

L

L

QCOP user=

sourceuser

user

TT

TCOP

−=*

Tout

Tin

Tsource

Tuser





SubsidenceThermal short-circuitGW regulation

(200 l/h;5 K) → 1 kWt

• 0.9 MW (H/C)• 1.0^6 m3/y• ∆T = 5°C• 48 l/s• 2+2 wells (80m)



20 m

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-15 -10 -5 0 5 10

CO

P*

Tsource, [°C]

Tuser = 45°C

• 1.2 MW (H/C)• 212 BHEs x 146 m/BHE (31 km)• 2.2 M€

70 €/m38 W/m

1.85 €/W



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Sem

i-osc

illaz

ion

e te

rmic

a, [°C

]

Profondità, [m]

T∞=15°

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-15 -10 -5 0 5 10

CO

P*

Tsource, [°C]

Tuser = 45°C



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T_SURFACE T_d T_Ferrara Flat_D_2Y



HGHE BHEEnergy performance � ☺

Soil use restriction � ☺

Maintenance ☺ �

GW contaminant risk ☺ �

Building cost ☺ �

Building equipment ☺ �

Building permission ☺ �

Design � ☺

Thermal drift ☺ �

20-25 m/kWt

H CdT 10 15Tmax - 35Tmin ≅0 -



ESI – 1st International Conference – Pernik, 9-10 June, 2011European University Polytechnical

Field monitoring of a HGHE flat panelBottarelli & Di Federico

piezometric well

V1

V2

V3V4

V5

V6

V7

V8

benchmark

L1 L2

piping

FP1

line data

valves

tank

pump

α βγ δFP2

M1M2

M3

tank

FP1FP2

Tank

α β δγFlat Panel

n.2Flat Panel

n.1Chiller

Electrical resistence

23 x 14 m2 of grasslandhydraulic closed looptwo flat-panelsmonitoring system



0,80

6,50

6,10

1,85

1,151,65

1,77

1234567

1234567

0,20 probe line H1 0,40 probe line H2

V7 V5

FP_1FP_2αβγδ

SIDE VIEW

TOP VIEW

probe line H2

probe line H1

sensor

0,90

FP1

FP2

αβ γ

δH1.1

H2.1H2.2

H2.3H2.4

H2.5 H2.6 H2.7/-1,15

H1.2H1.3

H1.4H1.5 H1.6 H1.7/-1,65

V1

.1/-

.2/-

.3/-

.4/-0,15

.5/-0,99

.6/-1,91

.7/-2.29

V2

.1/-

.2/-0,35

.3/-0,99

.4/-1,72

.5/-2,55

.6/-3,46

.7/-4,46

V3

.1/-

.2/-

.3/-

.4/-0,28

.5/-1,11

.6/-2,03

.7/-3,04

V5

.1/-

.2/-

.3/-

.4/-

.5/-0,42

.6/-1,34

.7/-2,32

V6

.1/-

.2/-

.3/-0,06

.4/-0,82

.5/-1,65

.6/-2,57

.7/-3,55

V8

.1/-

.2/-

.3/-

.4/-0,54

.5/-1,40

.6/-2,31

.7/-3,28

V7

.1/-

.2/-

.3/-0,21

.4/-0,96

.5/-1,79

.6/-2,71

.7/-3,68

V4

.1/-

.2/-

.3/-

.4/-

.5/-

.6/-

.7/-

0,54

3,11

1,41

1,93

3,15

0,89

0,91

FP_1FP_2

0,4

The monitoring system reaches 5 m deep in soil.The nearest sensor is 0.2 m far from GHX, the farthest one 3.2 m

H Horizontal probe indexV Vertical probe index2.1/-2.03 Sensor ID and its depth [m]FP_1 Flat Panel IDa,b,g,d GHX inlets/outlets

H2

FP1





This area was an island of the old river Po. The groundwater is 5 meters deep (dry conditions).

The soil is sandy silt /sandy clay, and its properties are following:

Department

City of Ferrara

Old River Po

But, the ground is very non homogeneous.In the first 2 meters, we found rubble, pottery and … bones.

Density 1,720 kg/m3 Porosity 0.36 Specific heat 1.35 kJ/kgK Thermal conductivity 1.4 W/mK

5 m deep

Daily temperature in Ferrara for air and groundwater

Ferrara is characterized by a continental climate.Hot summer (38°C) and cold winter (0°C) .

Relative humidity is frequently close to the saturation.

The shallow groundwater temperature is 15°C.

The heat transfer mode was carried out with different temperatures of the working fluid and several operations.

Unlike with the vertical systems, long-term subsurface thermal energy build-up or depletion wouldn’t be expecting by shallow GHE.

Period Mode Day[d]

Energy[kWh]

Time on[h]

Length[m]

Power[W/m]

2011, March → Sept. Heating 161 990 2907 4.2 61 / 812011, Nov.→ Dec. Free 42 28 351 6.0 5 / 132012, January Free 31 13 225 6.0 4 / 102012, Feb. → April Cooling 56 225 843 6.0 28 / 442012, July → Sept. HeatingP 68 264 585 6.0 27 / 752012, Nov. → Dec. CoolingP 48 117 364 6.0 17 / 542013, Jan. → Feb. CoolingP 41 101 352 6.0 17 / 48

Mode Unalteratedsoil temperature

(1.4 m deep)

Working fluid temperature

∆T

Heating 12÷22 35÷38 16÷23Cooling 10÷19 2÷8 8÷11

Free 12÷19 6÷12 6÷7

The maximum temperatures did not change in August 2011 and 2012, even if the heat transfer was different.

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35

a n f m a n f m a n f

Tem

pera

ture

, o C

V3.5H1.4H2.4

Still up to 40 cm far from the GHX, the pulsed operation mode is clear.

β γH2.3

H2.4

H1.4H1.5

V3

.1/-

.2/-

.3/-

.4/-0,28

.5/-1,11

.6/-2,03

.7/-3,04

0,4

0

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10

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20

25

30

35

a n f m a n f m a n f

Tem

pera

ture

, o C

V6.4V6.5V6.6

The temperatures did not change in November 2011 and 2012, even if a considerable heat transfer was carried out during the summer 2011.

The heat transfer achieved clearly 1.4 m far from the GHX.

H2.1H1.2

V6

.1/-

.2/-

.3/-0,06

.4/-0,82

.5/-1,65

.6/-2,57

.7/-3,55

0,54

1,41

FP_1



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f m a m g l a s o n d g f m

Tem

pera

ture

, o C

air V3.5 V3.6 V3.7air V3.5 V3.6 V3.7

20112012

Even if the system transferred a lot of heat in spring 2011, the maximum temperature were the same in both summers 2011 and 2012.

The temperatures in February were comparable, even if a cooling mode was operating in winter 2012.

V3

FP2FP1

β γH2.3

H2.4

H1.4H1.5

V3

.1/-

.2/-

.3/-

.4/-0,28

.5/-1,11

.6/-2,03

.7/-3,04

0,4

-3

-2

-1

00 5 10 15 20 25 30

m/oC0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30

Heating

Jan. Feb. Mar. Apr. May Jun July Aug. Sept. Oct. Nov. Dec.Cooling

Free

2010 O O O

2011 X X X X X X X X X X X X

2012 � � � � � � � � � � � �

2013 w w





The flat-panel shows high energy performance:− 45 W/m in cooling mode, with a thermal

average working difference of 10 K− 80 W/m in heating mode, with a thermal

average working difference of 15 K

Similar temperatures were naturally achieved after few time of inactivity.

So, the heat transfer over the soil surface deletes the thermal memory of the energy exploitation carried out by shallow GHXs.

Unlike with the vertical exchangers, its behaviour highlights that long-term subsurface thermal energy build-up or depletion wouldn’t be expecting by shallow GHXs.



Does GSHP pay off ?

Energy requirements

in heating

Building & operating cost

vs.

Climate zones

Energy labels

EMREnergy Mix Ratio



France

Germany

Italy

Netherlands

Spain

Sweden

UK

1

23

4

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0.25

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0.35

0.40

0.45

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0.55

€ 0.20 € 0.40 € 0.60 € 0.80 € 1.00 € 1.20 € 1.40

EM

R -

Ene

rgy

Mix

Ra

tio

Gas price for households, 2009 [€/Nm3]

Data from Europe's Energy Portal,

www.energy.eu

� Only heating mode� No reduction� GHE cost : 1 €/W� Cut-off: 30 y



For shallow GHEs, PCMs could represent a method:1. to restore the UTES benefit, according to the

seasonally regeneration2. to smooth the thermal wave produced by the HP

Two kinds of energy requirement: heating & coolingThen, two melting points.

Thus, two PCMs are needed.

A numerical model has been implemented to analyze the benefit occurring by their application

A 2D numerical approach was carried out to assess the behaviour of a flat-panel with/without PCMs

2D transversal section 10x15 mPCM layer 30x170 cmN° elements 23.000Min element size 0.16 cm2

Max element size 1600 cm2

Model domain

COMSOL’s module:Heat Transfer in Solids, advanced

( )Tt

Tc eqeqeq ∇⋅∇=

∂∂ λρ

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

270 275 280 285 290 295 300

H [1

], D

[1/K

]

T [K]

D_waterD_PCMH_waterH_PCM

H&D functions

( ) Sii

n

iiG

n

ii Hrr λλ ⋅−⋅+⋅

− ∑∑==

1111

( )∑∑==

⋅−⋅+⋅

−n

i

SiiiG

n

ii THrr

11

)(11 ρρ

SOLID

LIQUID

( ) ( )( )∑=

⋅+⋅⋅n

ii

SLi

Lii TDhcTHr

11

( )( ) ( )( )∑∑==

⋅+⋅−⋅+⋅

−n

ii

SLi

SiiiG

n

ii TDhcTHrcr

11

11

( )∑=

⋅⋅n

ii

Lii THr

1

ρ

( )∑=

⋅⋅n

i

Liii THr

1

λ

Two functions (H&D) control the phase change in the model

H(T) controls the phase change D(T) modulates the latent heat

The latent heat was introduced as Equivalent Specific Heat

• Time varying heat flux at the GHE wall

• Time varying heat flux at the soil surface

• Constant temperature at the bottom• All other boundaries as adiabatic

Boundary conditions

0.5 100.1

T=15°C

q=

0 W

/m2

qGHE(t)

q=

0 W

/m2

qG(t)

0.2

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389 390 391 392 393 394 395 396

Heat flux, [W

/m2

]Tem

per

atur

e, [°C

]

Day of Year

Air temperatureSoil surface temperatureGHE heat fluxSoil surface heat flux

Heat fluxes

( ) ( ))()()( //on

chbbbon

airchon tTTcrtTT

V

SUtq −⋅⋅+−⋅⋅= ρ&

( ) ( ) ( )( )( )

bbb

off

cVr

ttUS

airoff

air etTTtTtT ρ⋅−⋅

−⋅−+=

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-300

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600

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300 330 360 390 420 450 480 510 540 570 600 630 660

Da

ily energ

y, [Wh

/day]

Tem

per

atu

re, [°

C]

Day of Year

Soil temperature

Daily soil heat flux

Temperatures without PCMsCASE G

Temperatures with PCMs, case PCMCASE PCM

Winter week

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417 418 419 420 421 422 423 424

Heat flu

x, [W/m

2]Tem

pera

ture

, [°C

]

Day of Year

G_GHE temperature

PCM_GHE temperature

GHE heat flux

Summer week

Numerical Analysis of a Novel Ground Heat Exchanger Coupled with Phase Change Materials Bottarelli et al.

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613 614 615 616 617 618 619 620

Heat flu

x, [W/m

2]Tem

pera

ture

, [°C

]

Day of Year

G_GHE temperature

PCM_GHE temperature

GHE heat flux

Solid phase

Benefits of Phase Change Material in conjunction with ground heat exchangers Bottarelli et al.

0%

20%

40%

60%

80%

100%

0

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10

15

20

25

300 330 360 390 420 450 480 510 540 570 600 630 660

So

lid phase in

trenchTem

per

atu

re, [

K]

Day of Year

Average trenchtemperature

Total solidphase

Water solidphase

Paraffin solidphase

Equivalent specific heat

0

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100

1.0

1.1

1.2

1.3

1.4

1.5

300 330 360 390 420 450 480 510 540 570 600 630 660

Equ

ivalent specific h

eat, [kJ/kg*K

]T

her

mal

co

ndu

ctiv

ity, [

W/m

K]

Day of year

PCM_k

PCM_Cp

Documents

GROUND-SOURCE HEAT PUMP SYSTEMS Economic performance …