1. GEOTHERMAL HEAT PUMP SYSTEMS: CLOSED-LOOP DESIGN CONSIDERATIONS
Andrew Chiasson
Geo-Heat Center, Oregon Institute of Technology

2. Outline Geothermal options - decision tree System construction
Ground heat exchanger materials and layout
Inside the building
System design
Geothermal loop design
Pumping
The open-loop option

3. General Decision Tree YES NO YES NO YES NO YES NO YES NO YES YES
Unique Opportunity
(gray water, etc.)
Evaluate resource
obtain permits, agreements, etc. NO
YES
Groundwater for open loop,
existing well use or need
Good disposal
options
Aquifer test,
groundwater chemistry NO
Hard rock,
good quality groundwater
YES
Evaluate
standing column well NO
YES
Enough land for horizontal loop,
good soil for excavation NO
YES
Good conditions for pond loop,
interested owner
Pond thermal
evaluation NO
YES
Test bores,
Thermal conductivity test
Good conditions for vertical loop
Annual unbalanced loads,
AND/OR thermal storage opportunity
YES
DESIGN
DEVELOPMENT
Other
HVAC System
Hybrid

4. GHP Pros/Cons Advantages 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

5. GHP Pros/Cons Disadvantages First (capital) cost
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

6. System Construction What does the Loop Do?
The closed-loop is a heat exchanger, where fluid flowing through the loop exchanges heat with the earth
The earth is a solid material! => thermal storage effects
Synonyms: Ground (or ground-loop heat exchanger), earth energy exchanger, ground (or earth) coupling, borehole field, loop field, Geoexchange (GX)
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

7. System Construction
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:
Copper refrigerant lines are direct buried
Standards and operating experiences do not exist to the level of water-source heat pumps

8. System Construction Vertical Loops
Installed by standard drilling methods
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
Standard bentonite grout
Thermally-enhanced grouts (bentonite/sand mixture)

9. System Construction Vertical Loops
Mixing grout
Installing vertical loop

10. System Construction Vertical Loops
Drilling fluids flowing from hole as grout is pumped in
Inserting u-tube & tremie-pipe
With geo-clips

11. 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

12. System Construction Horizontal Loops
4 – 6 ft burial depth

13. System Construction Horizontal Loops

14. System Construction Pond Loops

15. System Construction Pond Loops
Copper Pipe
Geo Lake Plate
HDPE Pipe

16. System Construction Flushing/Purging
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

17. System Construction Building Interior
(from Water Furnace)

18. System Construction Building Interior
(from Water Furnace)

19. System Construction Building Interior
(from Water Furnace)

20. System Construction Building Interior – Hydronic Systems
Using water-to-water heat pumps for hot water

21. 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 oF)
Baseboards
Fan Coil Units

22. System Construction Building Interior – Outdoor Air
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

23. Loop Design Important Parameters Vertical Closed Loop
SOLAR COLLECTOR ARRAY
Heat Gains and Losses
COOLING TOWER
BoreholeThermal Resistance or
Undisturbed
Earth
Temperature
Average
Thermal
Conductivity
Borehole
Spacing

24. Loop Design Important Parameters Horizontal Closed Loop
Various loop configurations => Borehole resistance concept is replaced by trench resistance
Trench depth dictates average earth temperature! => Twinter, Tsummer

25. Loop Design 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

26. Loop Design Know the Loads Profile of the Building
Zone loads determine the heat pump size (a zone is the area controlled by a thermostat)
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
Block loads depend on the building “diversity”
For example, residential buildings have no diversity, a school with wings may have a 50-60% diversity

27. Loop Design Overview of Procedure
Building Loads
(from loads calculation software,
residential may use spreadsheets)
Residential
Commercial
Peak hour
Design month run fraction (usually from degree days)
Ground-loop software that considers:
Peak hour
Monthly run fraction
Annual full load hours
OR monthly loads
Ground thermal properties
Ground thermal properties
Design lengths:
IGSHPA method
Proprietary software (usually employs IGHSPA method)
Design lengths (NO UNIFIED METHOD):
ASHRAE method
Proprietary software

28. Loop Design Design Software

29. Loop Design Thermal Conductivity
Thermal conductivity is generally dependent on density, moisture content, mineral content
Soils:
Clays (15% moisture) Btu/hr-ft-F
Clays (5% moisture)
Sands (15% moisture)
Sands (5% moisture) – 1.9
Rocks:
Granite – 2.1 Btu/hr-ft-F
Basalt – 1.4
Limestone – 2.2
Sandstone – 2.0
Shale – 1.4
Grouts:
Standard bentonite
Thermally-enhanced – 1.40

30. Loop Design Thermal Conductivity
An in-situ thermal conductivity test (or thermal response test) is recommended on commercial jobs

31. 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
Boiler
Solar collector array
Cooling tower
Pond or swimming pool
Snow melting system
Refrigeration load

32. Loop Design Hybrid Systems
HEATING
LOADS
COOLING

33. Loop Design Hybrid Systems
Need software for analysis => current ASHRAE research project to study design and control
Example School in Northern U.S.

34. Loop Design Loop Lengths for Planning
Generalized loop lengths for planning purposes
NOT recommended for final designs => use software

35. 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

36. 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:
Propylene glycol
Ethanol
Methanol
CPTherm (new product)
Need to check viscosities at low temperatures => impacts pumping energy

37. Pumping ASHRAE grading system:
A-Excellent 0.05 hp/ton
B-Good hp/ton
C-Mediocre hp/ton
D-Poor 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

38. Pumping Flow management
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

39. Open Loop Option Advantages : Disadvantages : Water quality dependent
Low cost, especially for large loads and residential applications that need a drinking water well
Water well drilling technology is well-established
Stable source temperature
Standing column well option in certain circumstances
Disadvantages :
Water quality dependent
Scaling
Corrosion
Iron bacteria, well fouling
Water disposal
Laws and regulations
Permits, water rights

40. 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