GEOTHERMAL HEAT PUMP SYSTEMS:
CLOSED-LOOP DESIGN CONSIDERATIONS
Andrew Chiasson
Geo-Heat Center, Oregon Institute of Technology
Outline
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Geothermal options - decision tree
System construction
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System design
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Ground heat exchanger materials and
layout
Inside the building
Geothermal loop design
Pumping
The open-loop option
General Decision Tree
Unique Opportunity
(gray water, etc.)
YES
NO
Groundwater for open loop,
existing well use or need
YES
Evaluate resource
obtain permits, agreements, etc.
Good disposal
options
Aquifer test,
groundwater chemistry
NO
Hard rock,
good quality groundwater
Evaluate
standing column well
YES
NO
Enough land for horizontal loop,
good soil for excavation
YES
NO
Good conditions for pond loop,
interested owner
NO
Good conditions for vertical loop
YES
Pond thermal
evaluation
YES
Test bores,
Thermal conductivity test
Annual unbalanced loads,
AND/OR thermal storage opportunity
YES
Other
HVAC System
Hybrid
DESIGN
DEVELOPMENT
GHP Pros/Cons
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Advantages
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Energy efficiency
Simplicity
Low maintenance
Water heating
No auxiliary heat (in most cases)
No outdoor equipment
Packaged equipment
Environmentally “green”
Lowers peak demand
Low life-cycle cost
Allows more architectural freedoms
Better zone comfort control
GHP Pros/Cons
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Disadvantages
• First (capital) cost
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However, incentives, energy-savings
mortgages or loop-leasing are some ways of
off-setting costs
Limited qualified designers
Geographically limited contractors
Supply/demand => higher vendor markups
System Construction
What does the Loop Do?
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The closed-loop is a heat exchanger, where fluid
flowing through the loop exchanges heat with the
earth
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The earth is a solid material! => thermal storage
effects
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Synonyms: Ground (or ground-loop heat
exchanger), earth energy exchanger, ground (or
earth) coupling, borehole field, loop field,
Geoexchange (GX)
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Design goal is to size the loop to provide fluid
temperatures to the heat pump(s) within the
design target range (usually 35oF – 90oF) to meet
thermal loads of the building
System Construction
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All underground piping is high-density
polyethylene (HDPE) with thermally-fused joints
(according to ASTM standards)
Field installation procedures have been
standardized by IGSHPA
DX systems:
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Copper refrigerant lines are direct buried
Standards and operating experiences do not exist to
the level of water-source heat pumps
System Construction
Vertical Loops
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Installed by standard drilling methods
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Auger: soils, relatively shallow holes
Mud-rotary: soft sediments and sedimentary rocks
Air-rotary: soft to hard relatively dry rocks
Air-hammer: hard rock
Cable-tool: hard rock, deep holes (slow drilling)
Sonic drilling: high drilling rates in most materials
Loop (or borehole heat exchanger) is rolled off a
reel into borehole
Borehole is grouted from the bottom to the top
with a “tremie pipe” to insure a good seal
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Standard bentonite grout
Thermally-enhanced grouts (bentonite/sand mixture)
System Construction
Vertical Loops
Mixing grout
Installing vertical loop
System Construction
Vertical Loops
Drilling fluids flowing from
hole as grout is pumped in
Inserting u-tube & tremie-pipe
With geo-clips
System Construction
Vertical Loops
1 bore per circuit
u-tubes can range in diameter from ¾ to 1 ¼ inch
(1-inch is most common)
150 – 300 ft typical depth
Reverse-return piping arrangement
System Construction
Horizontal Loops
4 – 6 ft burial depth
System Construction
Horizontal Loops
System Construction
Pond Loops
System Construction
Pond Loops
Copper Pipe
Geo Lake Plate
HDPE Pipe
System Construction
Flushing/Purging
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The loop must be designed so it can be
flushed to remove debris and entrained
air upon commissioning or at any time
necessary
Install provisions (shut-off valves, hose
ports) on the supply and return runouts
Large systems use one or more vaults
Smaller systems can have valves on
headers in mechanical room
System Construction
Building Interior
(from Water Furnace)
System Construction
Building Interior
(from Water Furnace)
System Construction
Building Interior
(from Water Furnace)
System Construction
Building Interior – Hydronic Systems
Using water-to-water heat pumps for hot water
System Construction
Building Interior – Hydronic Systems
Using water-to-water heat pumps
Max. output water
temperatures are about
120oF (cast iron
radiators generally
designed for 160-180oF)
Baseboards
Fan Coil Units
System Construction
Building Interior – Outdoor Air
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Several options
Introducing too cold or too hot outdoor air
directly to a heat pump decreases it’s
capacity => but, increasing heat pump
capacity may result in too much air flow
In commercial buildings, some type of
heat recovery system is generally
recommended
Water-water heat pumps tied to the
ground loop can be used to pre-condition
outdoor air
Loop Design
Important Parameters
Vertical Closed Loop
SOLAR COLLECTOR ARRAY
COOLING TOWER
Heat Gains and Losses
Average
Thermal
Conductivity
BoreholeThermal Resistance
Borehole
Spacing
or
Undisturbed
Earth
Temperature
Loop Design
Important Parameters
Horizontal Closed Loop
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Various loop
configurations =>
Borehole resistance
concept is replaced
by trench resistance
Trench depth
dictates average
earth temperature!
=> Twinter, Tsummer
Loop Design
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RULES OF THUMB ARE NOT RECOMMENDED
FOR FINAL DESIGN
Why? The earth is a solid material, so effects of
run time are important in the design!! => Heat
pump run hours must be considered
Loop design for residential buildings is generally
handled differently than commercial buildings
Why? Internal gains in commercial buildings, load
diversity, etc. affect annual heat
rejection/extraction to the ground, so the building
life-cycle must be considered
Loop Design
Know the Loads Profile of the Building
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Zone loads determine the heat pump size (a zone
is the area controlled by a thermostat)
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In U.S. & Canada, accepted practice is to size heat pump
equipment based on the peak cooling load, and should
NOT be oversized; want to minimize on-off cycling,
maximize humidity control
If necessary, supplemental electric heat can make up the
difference
Block loads (greatest sum of hourly zone loads)
determine the loop size
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Block loads depend on the building “diversity”
For example, residential buildings have no diversity, a
school with wings may have a 50-60% diversity
Loop Design
Overview of Procedure
Building Loads
Residential
(from loads calculation software,
residential may use spreadsheets)
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Peak hour
 Design month run fraction
(usually from degree days)
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Ground thermal properties
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Ground-loop software that
considers:
 Peak hour
 Monthly run fraction
 Annual full load hours
OR monthly loads
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Design lengths:
 IGSHPA method
 Proprietary software
(usually employs IGHSPA
method)
Commercial
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Ground thermal properties
Design lengths (NO
UNIFIED METHOD):
 ASHRAE method
 Proprietary software
Loop Design
Design Software
Software Product
CLGS
ECA
Earth Energy Designer (EED)
GEOCALC
GeoDesigner
GchpCalc
GL-Source
GLHEPRO
Ground Loop Design
GS2000
Lund Programs
RIGHT-LOOP
WFEA
Vendor
Intl. Ground-Source Heat Pump Assoc., Stillwater, OK,
USA
Elite Software, Inc., Bryan, TX, USA
University of Lund, Sweden
Ferris State University, Big Rapids, MI, USA
ClimateMaster, Oklahoma City, OK, USA
Energy Information Services, Tuscaloosa, AL, USA
Kansas Electric Utility, Topeka, KS, USA
Intl. Ground-Source Heat Pump Assoc., Stillwater, OK,
USA
GBT, Inc., Maple Plain, MN, USA
Buildings Group, Natural Resources Canada
University of Lund, Sweden
Wrightsoft, Lexington, MA, USA
Water Furnace Intl., Fort Wayne, IN, USA
Loop Design
Thermal Conductivity
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Thermal conductivity is generally dependent on density, moisture
content, mineral content
Soils:
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0.4 - 1.1 Btu/hr-ft-F
0.3 - 0.8
0.6 - 2.2
0.5 – 1.9
Rocks:
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Clays (15% moisture)
Clays (5% moisture)
Sands (15% moisture)
Sands (5% moisture)
Granite
Basalt
Limestone
Sandstone
Shale
1.3 – 2.1 Btu/hr-ft-F
1.2 – 1.4
1.4 – 2.2
1.2 – 2.0
0.8 – 1.4
Grouts:
– Standard bentonite
– Thermally-enhanced
0.42
0.85 – 1.40
Loop Design
Thermal Conductivity
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An in-situ thermal conductivity test (or thermal response
test) is recommended on commercial jobs
Loop Design
Hybrid Systems
• Unbalanced loads over annual cycle
• A school in a cold climate with no summer
occupancy, or office/school in warm climate
• A supplemental piece of equipment (or another
process) handles some of the building space load
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Boiler
Solar collector array
Cooling tower
Pond or swimming pool
Snow melting system
Refrigeration load
Loop Design
Hybrid Systems
HEATING
LOADS
COOLING
LOADS
Loop Design
Hybrid Systems
Need software for analysis => current ASHRAE
research project to study design and control
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Example School in Northern U.S.
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Heat Pum p Ente ring Fluid Tempe ra ture ( F)
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45
40
35
Critical Design Temperature
30
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10 11 12 13 14 15 16 17 18 19 20
Time (yr s)
Loop Design
Loop Lengths for Planning
• Generalized loop lengths for planning purposes
• NOT recommended for final designs => use software
Length of Trench or Bore per Ton
450
400
350
300
250
200
150
100
50
0
44-47
48-51
52-55
56-59
60-63
64-67
68-70
Avg. Ground Temperature (F)
Slink y
6-Pipe
4-Pipe
2-Pipe
Vert. U-Tube
Pumping
• Flow requirement for heat pumps is 2 to 3
gpm/ton
• Flow requirement for 1-inch u-tubes is similar in
order to maintain turbulent flow
• Total loop flow rate should be based on BLOCK
LOADS, not total heat pump capacity
• Desire just enough flow to maintain turbulence,
especially at peak hours => check Reynolds
Number (Re > 2300)
• More turbulence means more convection heat
transfer, but more pumping energy
Pumping
• If freezing temperatures are expected from heat
pumps, loop should be freeze-protected
(temperature drop across heat pumps of 10oF
should be assumed)
• Use as little antifreeze as necessary!
• Types of antifreeze:
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Propylene glycol
Ethanol
Methanol
CPTherm (new product)
• Need to check viscosities at low temperatures
=> impacts pumping energy
Pumping
• ASHRAE grading system:
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A-Excellent
B-Good
C-Mediocre
D-Poor
0.05 hp/ton
0.05-0.075 hp/ton
0.075-0.1 hp/ton
0.1-0.15 hp/ton
• In other words, pumping kW should be <10% of
total system demand
• Reduce friction losses by:
• Reverse-return piping
• Parallel circuits
• Use larger-diameter pipe in deeper bores
Pumping
• Flow management
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Variable speed drives in central systems
Sub-central pumping
Individual flow centers if possible
Constant flow pumping NOT recommended
• De-centralized loop fields in buildings
with diverse floor plans
Open Loop Option
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Advantages
:
Low cost, especially
for large loads and
residential
applications that need
a drinking water well
Water well drilling
technology is wellestablished
Stable source
temperature
Standing column well
option in certain
circumstances
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Disadvantages :
Water quality dependent
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Scaling
Corrosion
Iron bacteria, well fouling
Water disposal
Laws and regulations
Permits, water rights
Summary
• Closed Loops: vertical vs. horizontal vs. pond
• Vertical loops generally have highest first cost
• Consider practical considerations for loop
installation => hybrid systems, open loop option
• Think system: interior HVAC components,
outdoor air
• Efficiency and lower cost through design
• Final designs should use design software