GEOTHERMAL HEAT PUMP SYSTEMS: CLOSED-LOOP DESIGN CONSIDERATIONS

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GEOTHERMAL HEAT PUMP SYSTEMS CLOSED-LOOP DESIGN
CONSIDERATIONS

• Andrew Chiasson
• Geo-Heat Center, Oregon Institute of Technology

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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

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General Decision Tree
Unique Opportunity (gray water, etc.)
Evaluate resource obtain permits, agreements, etc.
Groundwater for open loop, existing well use or
need
Good disposal options
Aquifer test, groundwater chemistry
Hard rock, good quality groundwater
Evaluate standing column well
Enough land for horizontal loop, good soil for
excavation
Good conditions for pond loop, interested owner
Pond thermal evaluation
Test bores, Thermal conductivity test
Good conditions for vertical loop
Annual unbalanced loads, AND/OR thermal storage
opportunity
DESIGN DEVELOPMENT
Other HVAC System
Hybrid
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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

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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 gt higher vendor markups

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System ConstructionWhat 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! gt 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

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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

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System ConstructionVertical 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)

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System ConstructionVertical Loops
Mixing grout
Installing vertical loop
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System ConstructionVertical Loops
Drilling fluids flowing from hole as grout is
pumped in
Inserting u-tube tremie-pipe With geo-clips
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System ConstructionVertical 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
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System ConstructionHorizontal Loops
4 6 ft burial depth
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System ConstructionHorizontal Loops
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System ConstructionPond Loops
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System ConstructionPond Loops
Copper Pipe
Geo Lake Plate
HDPE Pipe
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System ConstructionFlushing/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

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System ConstructionBuilding Interior
(from Water Furnace)
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System ConstructionBuilding Interior
(from Water Furnace)
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System ConstructionBuilding Interior
(from Water Furnace)
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System ConstructionBuilding Interior Hydronic
Systems
Using water-to-water heat pumps for hot water
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System ConstructionBuilding 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
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System ConstructionBuilding Interior Outdoor
Air

• Several options
• Introducing too cold or too hot outdoor air
  directly to a heat pump decreases its capacity
  gt 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

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Loop DesignImportant ParametersVertical Closed
Loop
Undisturbed Earth Temperature
Average Thermal Conductivity
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Loop DesignImportant ParametersHorizontal
Closed Loop

• Various loop configurations gt Borehole
  resistance concept is replaced by trench
  resistance
• Trench depth dictates average earth temperature!
  gt Twinter, Tsummer

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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!! gt 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

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Loop DesignKnow 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

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Loop DesignOverview of Procedure
Building Loads (from loads calculation
software, residential may use spreadsheets)

• Ground-loop software that considers
• Peak hour
• Monthly run fraction
• Annual full load hours
• OR monthly loads

• Peak hour
• Design month run fraction (usually from degree
  days)

• 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

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Loop DesignDesign Software
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Loop DesignThermal Conductivity

• Thermal conductivity is generally dependent on
  density, moisture content, mineral content
• Soils
• Clays (15 moisture) 0.4 - 1.1 Btu/hr-ft-F
• Clays (5 moisture) 0.3 - 0.8
• Sands (15 moisture) 0.6 - 2.2
• Sands (5 moisture) 0.5 1.9
• Rocks
• Granite 1.3 2.1 Btu/hr-ft-F
• Basalt 1.2 1.4
• Limestone 1.4 2.2
• Sandstone 1.2 2.0
• Shale 0.8 1.4
• Grouts
• Standard bentonite 0.42
• Thermally-enhanced 0.85 1.40

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Loop DesignThermal Conductivity

• An in-situ thermal conductivity test (or thermal
  response test) is recommended on commercial jobs

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Loop DesignHybrid 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

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Loop DesignHybrid Systems
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Loop DesignHybrid Systems

• Need software for analysis gt current ASHRAE
  research project to study design and control

Example School in Northern U.S.
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Loop DesignLoop Lengths for Planning

• Generalized loop lengths for planning purposes
• NOT recommended for final designs gt use
  software

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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 gt check Reynolds Number
  (Re gt 2300)
• More turbulence means more convection heat
  transfer, but more pumping energy

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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 gt
  impacts pumping energy

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Pumping

• ASHRAE grading system
• A-Excellent 0.05 hp/ton
• B-Good 0.05-0.075 hp/ton
• C-Mediocre 0.075-0.1 hp/ton
• D-Poor 0.1-0.15 hp/ton
• In other words, pumping kW should be lt10 of
  total system demand
• Reduce friction losses by
• Reverse-return piping
• Parallel circuits
• Use larger-diameter pipe in deeper bores

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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

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Open Loop Option

• Advantages
• 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

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Summary

• Closed Loops vertical vs. horizontal vs. pond
• Vertical loops generally have highest first cost
• Consider practical considerations for loop
  installation gt hybrid systems, open loop option
• Think system interior HVAC components, outdoor
  air
• Efficiency and lower cost through design
• Final designs should use design software