Geothermal Heat Pumps For School Applications

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Geothermal Heat Pumps For School Applications. John A. Shonder Oak Ridge National Laboratory. Rebuild America Geothermal Workshop March 5, 2002 Concord, NH. Mission of the U.S. Department of Energy's Federal Energy Management Program (FEMP). - PowerPoint PPT Presentation


Geothermal Heat PumpsFor School ApplicationsJohn A. ShonderOak Ridge National LaboratoryRebuild America Geothermal WorkshopMarch 5, 2002Concord, NHMission of the U.S. Department of Energy's Federal Energy Management Program (FEMP)Reduce the cost of government by advancing energy efficiency, water conservation, and the use of renewable energy sources, and by helping agencies manage their utility costs.FEMP relies on ORNL and other national labs to carry out its programs and provide expert assistance to federal agencies working to reduce their energy use and costs. Executive Order 13123Through life-cycle cost-effective measures, each agency shall reduce energy consumption per gross square foot of its facilities, excluding facilities covered in section 203 of this order, by 30 percent by 2005 and 35 percent by 2010 relative to 1985.GHP is one technology being used to meet these goalsORNL is the home of FEMPs GHP Core TeamMission: Provide support and technical assistance to Federal agencies developing and implementing GHP projectsAlso, to develop "tools of the trade" to make GHP technology just as easy for Federal facilities to procure and install as more conventional equipment. Presentation outlineWhy GHPs are popular for school applicationsBasics of GHP operation and performanceGHP system configurationsSite characterizationDesign of GHP systemsPresentation outlineConstruction costsMaintenance costsAdvantages and disadvantages of GHPsCase study of the Maxey Elementary School in Lincoln, NebraskaSome reasons why GHPs are popular for schoolsReduced energy use and energy costsOne of the most efficient HVAC technologies availableReduced maintenance costsModular, residential-sized unitsNo complicated controls requiredLower life cycle costSaves money for school districtSome other reasonsNo outdoor equipmentImproved aestheticsReduced vandalismQuiet operationReduced mechanical room spaceIndividual room controlLow source energy use and low air pollutant emissions green technologyAccording to EPA, in 2000, geothermal heat pumps were installed in 450 schools in 30 states (undoubtedly more by now) technology goes by many different namesGround source heat pumps (GSHP)Ground coupled heat pumps (GCHP)Geothermal heat pumps (GHP)GeoExchange (GX?)Earth energy systemsWhy GHPs are more efficient than other HVAC equipmentGHPs exchange heat with the earth, rather than with ambient airEarth provides a much better heat exchange mediumStable temperature year-roundGenerally cooler than ambient air when cooling is needed, and warmer than ambient air when heating is neededDeep earth (and groundwater) temperatures in the U.S.Capacity of a typical 4-ton geothermal heat pump vs. EWTHeating capacityCooling capacityEfficiency of a typical 4 ton GHP as a function of EWTBasics of GHP operationCompressorWater-to-refrigerant coilAir-to-refrigerant coilExpansion valveFanAir filterAir inlet (Return + OA)Conditioned air (Supply)Heat recovery coilReversing valveGHP System optionsOne heat pump on its own ground loopGround loops can be vertical or horizontalGenerally requires 1500-3000 ft2 land area per tonCommercial system: multiple GHPs on a common loopHDPE u-tubes in vertical bores. Common loop conditioned byvertical ground heat exchangerGenerally requires 250-300 ft2 of land area per tonProductionwell pumpRiver orother surface bodyOptionalinjectionwellPlate heat exchangerAACommon loop conditioned by groundwaterGenerally requires wells with flow of 2-3 gpm/tonCommon loop conditioned by surface water (closed loop)Typically 15 tons/acre (depth15-20 ft) or as high as 85 tons/acre for well stratified deep lakesCommon loop conditioned by surface water (open loop)AAPlate heat exchangerWater intakeWater dischargePumpMore suited to warm climates, or cooling-only applicationsSingle or common loop conditioned by standing column wellsubmersible pumpperforated intakedischargesleeveformationsoilAAoptional bleedOther methods of conditioning a single or common loopWastewater streamsCommunity loopPotable water supplies (where allowed)Hybrid systems:condition with 2 or more of abovecondition with 1 or more of above and outdoor airMany ways to configure indoor equipment as well Each room has its own air-to-water heat pumpOne heat pump serving several roomsWater-to-water heat pumps provide hot and/or chilled water to coils (2-pipe or 4-pipe systems) or console unitsOutdoor air pretreatment (see below)Site Characterization: evaluation of sites geology, hydrogeology, and other characteristicsPresence or absence of waterDetermines whether groundwater or surface water heat pumps can be usedDepth to waterAffects drilling costs for open loop GWHPWater (or soil/rock) temperatureAffects required flow rate (open loop GWHP) or heat exchanger length (closed loop systems)Site Characterization (continued)Depth to rockInfluences drilling cost, borefield layout (closed loop)Rock typeAffects thermal conductivityAffects drilling costsInfluences feasibility of certain system typesNature and thickness of unconsolidated materials overlying the rock.Affects drilling costsOther important site characteristicsLand area availablePlanned or current land use (e.g. parking lot, playground, tennis court, etc.)Location of underground utilitiesState regulatory climateCan impact every aspect of the designSources of information for site characterizationState water regulatory agencyState geology departmentWell completion reports from nearby water wellsGeologic and hydrologic mapsU.S. Geological SurveyLocal experienceOnce the system type is decided, further tests are often requiredVertical loop: Thermal Conductivity TestMeasures deep earth temperature as wellImportant for loop field designOpen loop: Well TestWater flowWater quality (including chemistry)Design of GHP Systems (inside the building)Outdoor air treatment is a primary aspect of designSchools have high ventilation air requirementsDuring peak heating conditions, unheated OA mixed with RA results in low (Several ways of treating outdoor airDedicated water-to-water heat pump supplying OA preheat coilSensible heat recovery unit to absorb heat from exhaust airConventional heat source (gas boiler, electric resistance, etc.)Each method has advantages and disadvantages in terms of first costs and operating costsMany aspects of GHP design are similar to the design of any conventional HVAC systemDetermine peak heating and peak cooling load for each zone of the buildingSelect heat pumps to meet design loads (remember that capacity depends on EWT)Heat pumps should be located with due consideration for serviceabilitySize ventilation system components: ductwork, fans, preheating coils, etc.Retrofits: Existing equipment influences cost-effectiveness of various GHP designsCentral/terminal systems (ductwork or no)Water loop for two- or four-pipe system? If so, potential for re-use as GHP water loopWhat space is available in zones for individual heat pumps?Above dropped ceiling horizontal unitsMechanical closets vertical unitsThrough-wall PTAC units & perimeter console unitsThree basic configurations of geothermal heat pumpsverticalhorizontalconsoleOutside the building: borefield sizing (for vertical bore systems)Decide on borehole and u-tube size, groutingDetermine ground properties from in situ testDesign borefield layout (rows columns, spacing)Determine whether antifreeze is requiredSelect design EWTEnter all information (including loads) into borefield design program iterate if necessaryDesign exterior headersA number of programs available to design ground heat exchangerDesign packages ORNL has testedGchpCalc ( ( ( (Caneta Research)Design programs have greatly improved since 1996Important considerations for design of groundwater heat pumpsGroundwater availability, quality and temperatureRequired flowWater disposal methodDesign of plate heat exchanger Other system types have their own idiosyncrasiesOpen and closed loop surface water heat pumpsStanding column wellsPumping/Piping Subsystem is another key design elementFollow Kavanaughs grading system for vertical bore systemsPump hp/100 tonsGrade5A -- Excellent5-7.5B -- Good7.5-10C -- Mediocre10-15D -- Poor>15F -- BadWays of minimizing pumping power (closed loop systems)Water flow rate GHPs should also be considered for domestic water heatingA heat pump is the most efficient technology for water heatingCan use dedicated water-to-water heat pump, or desuperheaters on selected heat pumps supplementing conventional DHWIn cooling-dominated applications, can reduce required bore length and system costsResources for GHP DesignersGround-Source Heat Pumps: Design of Geothermal Systems for Commercial and Institutional Buildings, Kavanaugh and Rafferty (ASHRAE, 1997)Commercial/Institutional Ground Source Heat Pump Engineering Manual. Caneta Research. (ASHRAE, 1995)Closed Loop/Ground Source Heat Pump Systems: Installation Guide (IGSHPA)Learning from Experiences with Commercial/Institutional Heat Pump Systems in Cold Climates, Caneta Research. (CADDET, 2000)Chapter 31: Geothermal Energy. 1999 HVAC Applications Handbook (ASHRAE)Problem: Lack of construction-cost-estimating guide for GHP familyRS-Means is the industry standardReferenced by HVAC cost-estimating softwareDoes not include some GHP family membersAvailable GHP cost data are largely anecdotal important factors are unknownType of estimate preliminary, per square foot, bidder price, etc.Scope whether costs for site work, design, electrical, demolition, controls, etc., are includedSolution: HVAC Construction- and Maintenance-Cost DatabaseConstruction Cost DatabaseConstruction Cost DatabaseSummary for Vertical Bore:Commercial New Construction$14.61 per square foot ( 6%)Commercial Retrofit$11.50 per square foot (wide variation due to small dataset)Large Residential Retrofits$3,312 per ton (wide variation due to small dataset)Groundwater heat pumps$3000-$5000 per ton for well water systems (open loop and standing column well)Optimizing the Cost/Benefit Ratio of GHP ProjectsIn vertical bore projects, minimize bore length by using industry-standard borefield design programPerform thermal conductivity testAvoid complex control systemsAvailable Maintenance Cost DataGHP/Conventional HVACSheet1SourceASHRAE HandbookCaneta/ASHRAEMEANS (Army CERL)ORNLDate Reported19861998mid-70's1999/2000HVAC System Types80's conventional, no GHPGHP70's conventionalGHP, ACC/GHWB, WCC/GHWB# of Bldgs34225task-based20Bldg TypesCommercialSchools, Commercial, ResidentialMilitarySchoolsAge of Bldgs or Systems2-25 years1-15 yearsn/a2-32 yearsAvg. Ann. Maint. Cost per Sq Ft32/ft29.3 - 10.9/ft2n/a9.27/ft2 (GHP), 8.75/ft2 (ACC/GHWB), 18.71 /ft2 (WCC/GHWB)Estimating energy use of GHP systems (as required by LEED, ASHRAE 90.1)DOE-2 (PowerDOE, etc.)Fairly good model, includes ground loopsTrane TraceCan be used, but no ground loop modelTRNSYSContains sophisticated ground loop model, but somewhat difficult to useTrane System AnalyzerIncludes GHP, but not an hourly programWeb sites with information on Google search on the term geothermal heat pump turns up other interesting linksAdvantages of GHPsHigh efficiencyLower energy consumptionLower energy costLow maintenance costLow life cycle costNo outdoor equipmentGreater occupant comfortDisadvantages of GHPFirst cost can be higher than for conventional systemsNot all system types feasible in all locationsLimited pool of designers


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