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GEOTHERMAL HEAT PUMP SYSTEMS: CLOSED-LOOP DESIGN CONSIDERATIONS : 

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

Outline: 

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 Tree: 

General Decision Tree Unique 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 Other HVAC System Hybrid Evaluate resource obtain permits, agreements, etc. Good disposal options Aquifer test, groundwater chemistry Evaluate standing column well Pond thermal evaluation Test bores, Thermal conductivity test DESIGN DEVELOPMENT Annual unbalanced loads, AND/OR thermal storage opportunity

GHP Pros/Cons: 

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: 

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?: 

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

System Construction: 

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: 

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)

System ConstructionVertical Loops: 

System Construction Vertical Loops Installing vertical loop Mixing grout

System ConstructionVertical Loops: 

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

System ConstructionVertical Loops: 

System Construction Vertical Loops 150 – 300 ft typical depth Reverse-return piping arrangement 1 bore per circuit u-tubes can range in diameter from ¾ to 1 ¼ inch (1-inch is most common)

System ConstructionHorizontal Loops: 

System Construction Horizontal Loops 4 – 6 ft burial depth

System ConstructionHorizontal Loops: 

System Construction Horizontal Loops

System ConstructionPond Loops: 

System Construction Pond Loops

System ConstructionPond Loops: 

System Construction Pond Loops Geo Lake Plate HDPE Pipe Copper Pipe

System ConstructionFlushing/Purging: 

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

System ConstructionBuilding Interior: 

System Construction Building Interior (from Water Furnace)

System ConstructionBuilding Interior: 

System Construction Building Interior (from Water Furnace)

System ConstructionBuilding Interior: 

System Construction Building Interior (from Water Furnace)

System ConstructionBuilding Interior – Hydronic Systems: 

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

System ConstructionBuilding Interior – Hydronic Systems: 

System Construction Building 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: 

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

Loop DesignImportant ParametersVertical Closed Loop: 

Loop Design Important Parameters Vertical Closed Loop Average Thermal Conductivity Undisturbed Earth Temperature

Loop DesignImportant ParametersHorizontal Closed Loop: 

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

Loop Design: 

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: 

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

Loop DesignOverview of Procedure: 

Loop Design Overview 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 hours OR monthly loads Ground thermal properties Design lengths (NO UNIFIED METHOD): ASHRAE method Proprietary software

Loop DesignDesign Software: 

Loop Design Design Software

Loop DesignThermal Conductivity: 

Loop Design Thermal 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: 

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

Loop DesignHybrid Systems: 

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

Loop DesignHybrid Systems: 

Loop Design Hybrid Systems

Loop DesignHybrid Systems: 

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

Loop DesignLoop Lengths for Planning: 

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

Pumping : 

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 : 

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 : 

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 : 

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 : 

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 : 

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