combining a thermally supported ground source heat … 110.pdf · combining a thermally supported...
TRANSCRIPT
Combining a thermally supported ground source
heat pump with an ORC
ASME ORC 2013 | Rotterdam | October 8, 2013
Stefan Schimpf, Roland Span Chair of Thermodynamics
Faculty of Mechanical Engineering
2 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Motivation
Conventional solar systems - collector standstill whenever
the maximum temperature of the storage is reached
Approach:
• Combination of market proven technology
• Minimal additional investments
Goal:
• Selection of appropriate models
• Simulation and optimisation of the combined system
• Energetic and economic analysis
The resulting excess heat can be harnessed in an ORC!
3 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Combined solar system
Combined solar system
• Space heating
• Domestic hot water
Ground source heat pump
• Space heating
• Domestic hot water
ORC
• Scroll expander?
• pump
• valves
• advanced controls
Solar thermal loop
• Coiled tube
• Solar station
• Stratification device
4 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Overview of the System
Single family house
German low-energy standard
Floor space: 150 m²
Heating power: 5 kW
Borehole depth: 80 m
Evac. tube collector area: 10 m²
Storage tank volume: 900 l
5 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Software SH-PORT
Target group:
architects, heating installers
Inputs in Microsoft Excel, no
additional software required
Intuitive use, detailed
knowledge not necessary
Calculation by a dll written
in C++
State of the art modeling of
the components
Small number of predefined
systems
Fast calculation due to
multithreading
6 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Simulation results
Denver ORC conv.
ORC gain / kWh 112 -
Energy demand / kWh 1735 1849
savings (20 years) / € 333 -
• Electr. pricing Ankara: 14,7 ct/kWh
• Electr. pricing Denver: 8,9 ct/kWh
• Electr. pricing Bochum: 21,4 ct/kWh
• Inflation electricity: 5 %
Ankara ORC conv.
ORC gain / kWh 96 -
Energy demand / kWh 1444 1524
savings (20 years) / € 389 -
Bochum ORC conv.
ORC gain / kWh 37 -
Energy demand / kWh 2144 2164
savings (20 years) / € 142 -
The combined ORC-system is energetically superior
but under current conditions economically unfeasible!
7 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Modelling the ground heat exchanger
Soil λs, αs
Grout
λg, αg
U–tube
pipe 1
Fluid
Sp, Rf U–tube
pipe 2 Tf,
Tb
Tp, Rp
T0
Rb
ro
rb
Rs
injq
pq
bq
x
Eskilson (1987)
• Long-term response of a borehole
• Heat capacities inside the borehole are neglected,
model becomes valid after a few hours
• Representative for models in building energy analysis software
Relation between
heat extraction rate
and fluid temperature
8 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Analytical short timestep model
Soil λs, αs
Grout
λg, αg
Fluid
Sp, Rf
Tf,
T(r)
Tp
T0
re rb injq
pq
bq
Tb
2
0
·
f ,f 0nj0 i(1
· d)2
t t
t
t
G t t
T te
LqT
2
p
0
p
t
b s
1
1·
11
1
p
L
Ct R
KR
K K
Javed and Claesson (2011): Simplifies geometry but considers
all parts of the borehole and thermal resistances.
9 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Simulation results – Comparison of GHX-models
Ankara JC ESK
Energy demand / kWh (conv.) 1524 1500
ORC gain / kWh 96 115
Energy demand / kWh (ORC) 1444 1402
savings (20 years) / € 389 476
Denver JC ESK
Energy demand / kWh (conv.) 1849 1817
ORC gain / kWh 112 135
Energy demand / kWh (ORC) 1735 1682
savings (20 years) / € 333 397
+22%
+19%
Long-term response models in state of the art
building energy analysis software are not suited
for ORCs coupled with ground heat exchangers!
10 Schimpf | ASME ORC 2013| Rotterdam, October 8, 2013
Javed and Claesson (2011): analytical short-time step model
simplifies geometry
Conclusion
• ORC-system saves energy but its economic feasibility is
questionable
• Comparison of two ground heat exchanger models
Eskilson (1987): long-term response
neglects heat capacities of fluid and grout
representative for building energy software
• Long-term response model overestimates ORC-gain
• Simulation of small-scale ORCs with condensation in a
ground heat exchanger requires a short-time step model