Recent Developments in Ground Source Heat Pump Research

RECENT
DEVELOPMENTS
IN GROUND
SOURCE HEAT
PUMP RESEARCH
Jeffrey D. Spitler
Oklahoma State University
Outline
• A brief history
• Recent research developments
• Foundation Heat Exchangers (Residential)
• Simple simulation tool (Both)
• GSHP vs. VRF - ASHRAE HQ Building (Commercial)
• Renewable?
William Thomson (Lord Kelvin)
• 1852: proposed a heat
pump for heating
buildings or, in tropical
climates, cooling them.
• 1853: provided
mathematical proof of
same, interacting with
the works of Joule,
Carnot, Mayer,
Rankine
Heinrich Zoelly
• A Mexican-Swiss turbine
engineer, born in Mexico
in 1862.
• Better known for
development of steam
turbine as an alternative to
steam engines for
locomotives.
• Issued Swiss patent
59350 in 1912 for a
ground source heat pump.
1940s
• Some pre-World War II
installations were done
in the U.S.; numbers
increased after the
war.
A start – a fizzle
• 1940s: A dozen research/monitoring projects reported in
the literature
• After the early 1950s reports of ground source heat pump
systems essentially vanished from the U.S. literature.
Why? Apparently,
• problems with drying around horizontal ground-loop heat
exchangers,
• leakage,
• and undersizing.
Take Two – 1970s
• In 1974, Oklahoma State University began a
research program in response to a request from
some local businesses.
• First GSHP by modifying an air-source unit.
• 1978 – first residential installations in Oklahoma
• Late 1970s – several US projects on solarassisted ground source heat pump systems
Early OSU Work
“The major problem is the complexity of the controls…”
From Bose, et al. (1979)
Challenges addressed 70s-90s
• Leakage: Fused plastic (HDPE) pipe
• Undersizing: Design tools
• Ground thermal properties: Thermal response
test
• Research-to-practice:
• Commercialization,
• IGSHPA,
• technology transfer
Mid-1990s - Present
• Transition from residential to commercial.
• Primary challenge remaining: economic.
• Addressed by:
• Hybrid systems
• Improved ground heat exchangers
• Identification of niche applications, e.g.:
• Schools
• Light retail
• Dissemination of best design practices
• Avoid over-pumping
• Avoid excess controls
• Improved design and simulation tools
Recent research developments –
a sample
• Foundation Heat Exchangers (Residential)
• Simple simulation tool (Residential and Commercial)
• GSHP vs. VRF - ASHRAE HQ Building (Commercial)
Foundation Heat Exchangers
Ground source heat pump (GSHP)
systems
• First cost the most
significant barrier.
• For typical US house,
extra cost for drilling
boreholes is $3000-$6000
(USD)
An alternative: Foundation Heat
Exchangers (FHX)
• Experimentally-proven
technology!
• For well-insulated houses
• For houses with excavated
basements (or drainage)
• Significant cost reduction
possible.
FHX
Experimental Houses
• Two low energy
houses have been
constructed with FHX
at Oak Ridge,
Tennessee, USA.
• Data collected over a
one year period has
been use to validate a
number of design tools
and simulation models
After earlier experimental success questions
• Proven in a temperate climate – where else might they
•
•
•
•
•
work?
Proven for highly-insulated houses – how good does the
insulation need to be?
How can we design such a system?
How big of a problem is short-circuiting?
How can we calculate energy consumption in EnergyPlus
in a reasonable amount of time?
Most of these questions can be at least partially answered
with an experimentally-validated simulation.
Simulation
• Which phenomena need be modeled?
• Conduction heat transfer
• Surface convection & radiation
• Evapotranspiration
• Freezing/thawing
• Moisture transport
• Snow
• Methodology?
• Speed?
• Accuracy?
Simulation Approaches
• Numerical Models
• 2d & 3d FVM using boundary-fitted coordinates
• 2d “coarse grid” finite volume method (FVM)
• 3d “dual coordinate system” FVM
• Response Factor Model – “Dynamic Thermal Networks”
• Analytical Model
Dual-coordinates FVM
• Combines nonuniform
coarse grid with radial
grid surrounding each
pipe.
• Final solution
implemented in
EnergyPlus
• 4000 rectangular cells;
360 radial cells
• Similar approach
developed by Piechowski
Heat pump Entering fluid temperature (C)
Experimental Validation
35
30
Experimental result
DCS-FV E+ model
25
HVACSIM+ model
2D/3D E+ Model
20
15
10
5
0
0
50
100
150
200
Days
250
300
350
FHX for well-insulated house
“Marginal”
may require
additional
horizontal
ground heat
exchanger
Simple simulation tool for vertical GHE
GHX Simulation
• Approaches
• Analytical
• Numerical
• Response factors (g-functions)
• Short time-step g-functions
• DST model
• But…
25
GSHP System Simulation
• To be useful, needs to be part
of a modeling tool, e.g.:
• eQuest
• EnergyPlus
• HVACSIM+
• TRNSYS
• Modelica
26
Problem
• What to do when heat pump or
system is “non-standard”?
• eQuest
• EnergyPlus
Wait or approximate
• HVACSIM+
• TRNSYS
Use existing components
or write new Fortran code.
• Modelica
Write new Modelica code
(Good Luck!)
27
Our Solution
• Problem complicated by
“simultaneity”
• Use successive substitution with
full-duration, separate,
simulations of
• GHX
• Heat pump(s) and supplementary
devices
28
Our Solution
• Hourly, multi-year simulation
• GHX simulation: standalone, pre-compiled exe.
(derived from HVACSIM+)
• Heat pump / auxiliary components: Excel/VBA
• G-functions from database or Javed and
Claesson (2011)
• Post-processing: Excel/VBA
• Converges “rapidly”: 4 or 5 iterations
ASHRAE Headquarters Building Study
ASHRAE Building
• 2008: Major renovation
• Three state-of-the-art systems:
• 2nd floor: Ground source heat pump (GSHP) system
• 1st floor: Variable refrigerant flow (VRF) system – a multiple-split air
source heat pump system
• Dedicated Outdoor Air System (DOAS) to provide fresh air
• 1600 data points are measured! Includes:
• Total power of each system
• Lighting power consumption
• Plug loads
• Objective: Compare performance of GSHP and VRF
systems
Analysis of loads on both systems
• Measured:
• Lighting
• Plug Loads
• DOAS
• Estimated:
• Envelope (Walls, windows, roof)
• Occupants
Net monthly building loads
1,2
GSHP
Monthly Net Loads, kWh/sq ft
1,0
0,8
0,6
0,4
0,2
0,0
-0,2
VRV
22
Average Power, W/m2
20
18
16
14
12
VRF
10
8
6
4
GSHP
2
0
-10
-5
0
5
10
15
20
Outside Air Temp, °C
25
30
35
40
36
VRF system power – contributions of
cooling/heating
22
20
Average VRF Power Use, W/m2
18
heating
cooling
16
14
12
10
8
6
4
2
0
-8
-2
3
8
13
18
Outside Air Temp, °C
23
28
33
38
37
GSHP system power – contributions of
cooling/heating
22
20
heating
Average GSHP Power Use, W/m2
18
cooling
unallocated
16
14
12
10
8
6
4
2
0
-8
-2
3
8
13
18
Outside Air Temp, °C
23
28
33
38
38
Ground Loop Supply Temp.
Ambient Dry Bulb Temp.
Ground Loop Supply Fluid Temp.
40
35
30
Temperature, °C
25
20
15
10
5
0
-5
-10
7/1/11
12/31/11
7/1/12
12/31/12
7/2/13
39
Conclusions – System performance
• 2nd floor (served by GSHP system) has (per unit floor
area):
• Higher cooling demand than 1st floor, but
• Lower heating demand
• Total cooling energy requirements >> total heating energy
requirements
• The GSHP system used less energy per unit floor area
than the VRF system while maintaining similar room
temperatures
• Up to 40% less energy in summer
• Up to 70% less energy in winter and shoulder seasons
40
Conclusions—Reasons for the Difference
• Ground loop supply temperature was more favorable than
the ambient air temperature for heat pump operation
• The control strategy of the VRF resulted in more
simultaneously heating and cooling than the GSHP,
especially in shoulder season
• VRF system “over-controlled” – leading to heating mode
operation even in summer.
• Defrosting operation of VRF in winter
Renewable Energy Perspectives
Perspectives
• What is the goal of renewable energy?
• Not (in my opinion) an end in itself
• Rather:
• to maximize human comfort and productivity
• while minimizing consumption of non-renewable energy,
• also minimizing adverse environmental effects,
• and do it cost-effectively.
Comparison
• Comparing GSHP system to an advanced air
source heat pump system (VRF), GSHP system:
• Reduces electrical energy required for cooling (~40•
•
•
•
70%)
Reduces electrical energy required for heating (~65%)
Delivers (mostly) renewable heat to the building
Gives the same human comfort and productivity
Has ~25% lower initial cost, including boreholes.
Conclusions
• Like any other cooling system, GSHP systems
require electricity.
• They use less energy, so for any mix of power
sources, they use less non-renewable energy.
• They can use even less non-renewable energy
as the power source mix becomes more
renewable.