GEOTHERMAL HEAT PUMP SYSTEMS: GEOTHERMAL HEAT PUMP SYSTEMS: CLOSED-LOOP DESIGN CLOSED-LOOP DESIGN

CONSIDERATIONSCONSIDERATIONS

Andrew ChiassonGeo-Heat Center, Oregon Institute of Technology

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

General Decision TreeUnique Opportunity(gray water, etc.)

Groundwater for open loop,existing well use or need

Hard rock,good quality groundwater

Enough land for horizontal loop, good soil for excavation

Good conditions for pond loop,interested owner

Good conditions for vertical loop

OtherHVAC System

Hybrid

Evaluate resourceobtain permits, agreements, etc.

Good disposal options

Aquifer test,groundwater chemistry

Evaluate standing column well

Pond thermalevaluation

Test bores,Thermal conductivity test

DESIGNDEVELOPMENT

YES

YES

YES

YES

YES

NO

NO

NO

NO

NO

YES

YES

Annual unbalanced loads,AND/OR thermal storage opportunity

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

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

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! => 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

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

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)

System ConstructionVertical Loops

Installing vertical loop

Mixing grout

System ConstructionVertical Loops

Inserting u-tube & tremie-pipeWith geo-clips

Drilling fluids flowing from hole as grout is pumped in

System ConstructionVertical Loops

150 – 300 ft typical depthReverse-return piping arrangement

1 bore per circuitu-tubes can range in diameter from ¾ to 1 ¼ inch(1-inch is most common)

System ConstructionHorizontal Loops

4 – 6 ft burial depth

System ConstructionHorizontal Loops

System ConstructionPond Loops

System ConstructionPond Loops

Geo Lake Plate

HDPE Pipe

Copper Pipe

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

System ConstructionBuilding Interior

(from Water Furnace)

System ConstructionBuilding Interior

(from Water Furnace)

System ConstructionBuilding Interior

(from Water Furnace)

System ConstructionBuilding Interior – Hydronic Systems

Using water-to-water heat pumps for hot water

System ConstructionBuilding Interior – Hydronic Systems

Using water-to-water heat pumps

Baseboards

Fan Coil Units

Max. output water temperatures are about 120oF (cast iron radiators generally designed for 160-180oF)

System ConstructionBuilding 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

Loop DesignImportant ParametersVertical Closed Loop

~~

~

Ceiling-Mounted Units

Console Units

Vertical Units

Mechanical Room

To/FromGround

Loop

AverageThermal

Conductivity

UndisturbedEarth

Temperature

Heat Gains and Losses

BoreholeThermal Resistance

or

BoreholeSpacing

SOLAR COLLECTOR ARRAYCOOLING TOWER

Loop DesignImportant Parameters

Horizontal Closed Loop

Various loop configurations => Borehole resistance concept is replaced by trench resistance

Trench depth dictates average earth temperature! => Twinter, Tsummer

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

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

Loop DesignOverview of Procedure

Building Loads(from loads calculation software,residential may use spreadsheets)

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

Ground thermal properties

Design lengths: IGSHPA method Proprietary software (usually employs IGHSPA method)

Ground-loop software that considers:

Peak hour Monthly run fraction Annual full load hoursOR monthly loads

Ground thermal properties

Design lengths (NO UNIFIED METHOD):

ASHRAE method Proprietary software

Residential Commercial

Loop DesignDesign Software

Software Product Vendor

CLGS Intl. Ground-Source Heat Pump Assoc., Stillwater, OK, USA

ECA Elite Software, Inc., Bryan, TX, USA Earth Energy Designer (EED) University of Lund, Sweden GEOCALC Ferris State University, Big Rapids, MI, USA GeoDesigner ClimateMaster, Oklahoma City, OK, USA GchpCalc Energy Information Services, Tuscaloosa, AL, USA GL-Source Kansas Electric Utility, Topeka, KS, USA GLHEPRO Intl. Ground-Source Heat Pump Assoc., Stillwater, OK,

USA Ground Loop Design GBT, Inc., Maple Plain, MN, USA GS2000 Buildings Group, Natural Resources Canada Lund Programs University of Lund, Sweden RIGHT-LOOP Wrightsoft, Lexington, MA, USA WFEA Water Furnace Intl., Fort Wayne, IN, USA

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

Loop DesignThermal Conductivity

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

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

Loop DesignHybrid Systems

HEATINGLOADS

COOLINGLOADS

Loop DesignHybrid Systems

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

30

35

40

45

50

55

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Time (yrs)

He

at

Pu

mp

Ent

eri

ng

Flu

id T

em

pe

ratu

re (o

F)

Critical Design Temperature

Example School in Northern U.S.

Loop DesignLoop Lengths for Planning

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

0

50

100

150

200

250

300

350

400

450

44-47 48-51 52-55 56-59 60-63 64-67 68-70

Avg. Ground Temperature (F)

Len

gth

of

Tre

nch

or

Bo

re p

er T

on

Slinky 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:

• Propylene glycol• Ethanol• Methanol• CPTherm (new product)

• Need to check viscosities at low temperatures => impacts pumping energy

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 <10% of total system demand

• Reduce friction losses by:• Reverse-return piping• Parallel circuits• Use larger-diameter pipe in deeper bores

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

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

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