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GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
5
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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
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
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)
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
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
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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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
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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T_SURFACE T_d T_Ferrara Flat_D_2Y
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
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 -
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
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
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
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
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
ESI – 1st International Conference – Pernik, 9-10 June, 2011European University Polytechnical
Field monitoring of a HGHE flat panelBottarelli & Di Federico
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.
0
5
10
15
20
25
30
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
5
10
15
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
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
-5
0
5
10
15
20
25
30
35
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
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
ESI – 1st International Conference – Pernik, 9-10 June, 2011European University Polytechnical
Field monitoring of a HGHE flat panelBottarelli & Di Federico
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.
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
Does GSHP pay off ?
Energy requirements
in heating
Building & operating cost
vs.
Climate zones
Energy labels
EMREnergy Mix Ratio
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
France
Germany
Italy
Netherlands
Spain
Sweden
UK
1
23
4
5
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
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
GROUND-SOURCE HEAT PUMP SYSTEMSDept. of Architecture, Ferrara University
Economic performance of ground source heat pump: does it pay off?Laura Gabrielli, Michele Bottarelli
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
-50
-40
-30
-20
-10
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10
20
30
40
50
60
70
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
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⋅−⋅
−⋅−+=
-600
-300
0
300
600
0
5
10
15
20
25
30
35
40
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
-50
-40
-30
-20
-10
0
10
20
30
40
50
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
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.
-20
-10
0
10
20
30
24
25
26
27
28
29
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
5
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
1
10
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
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