ASHRAE ST-16-018-2016 A Simulation-Based Study on Different Control Strategies for Variable-Speed Pumps in Distributed Ground-Source Heat Pump Systems.pdf

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1、 2016 ASHRAE 173ABSTRACTMost commercial ground-source heat pump (GSHP)systemsintheUnitedStatesareinadistributedconfiguration.These systems circulate water or an anti-freeze solutionthrough multiple heat pump units via a central pumpingsystem, which usually uses variable-speed pumps. Variable-speed p

2、umps have potential to significantly reduce pumpingenergyuse;however,theenergysavingsinrealitycouldbefarlower than its potential due to improper pumping systemdesignandcontrols.Inthispaper,asimplifiedhydronicpump-ing system was simulated with the dynamic Modelica modelsto evaluate three different pu

3、mping control strategies. Thepumping control strategies include two conventional controlstrategies: one strategy is to maintain a constant differentialpressure across either the supply and return mains and theotheristomaintainaconstantdifferentialpressureatthemosthydraulically remote heat pump. Ther

4、e is also an innovativecontrol strategy that adjusts system flow rate based on thedemand of each heat pump. The simulation results indicatethat a significant overflow occurs at part-load conditionswhen the variable-speed pump is controlled to maintain aconstant differential pressure across the suppl

5、y and returnmains of the piping system. On the other hand, an underflowoccursatpart-loadconditionswhenthevariable-speedpumpis controlled to maintain a constant differential pressureacross the furthest heat pump. The flow-demand-basedcontrolcanprovideneededflowratetoeachheatpumpatanygiven time and wi

6、th less pumping energy use than the twoconventional controls. Finally, a typical distributed GSHPsystem was studied to evaluate the energy saving potential ofapplying the flow-demand-based pumping control strategy.This case study shows that the annual pumping energyconsumption can be reduced by 64%

7、using the flow-demand-basedcontrolcomparedwithusingtheconventionalpressure-based control to maintain a constant differential pressureacross the supply and return mains.INTRODUCTIONHydronic pumping systems are commonly used in heat-ing, ventilation, and air-conditioning (HVAC) applications tocirculat

8、e water or other heat-carrier fluids through variousHVAC equipment such as chillers, boilers, heat pumps, cool-ing towers, and fan coils. The heart of a hydronic pipingsystem is the circulation pump.Pumping power in hydronic piping systems contributessignificantly (18% to 40%) to the total energy co

9、nsumption ofHVAC systems (Balta et al. 2010). Because pumps are acces-sory equipment to facilitate the space-conditioning operationof HVAC systems, their energy use should be reduced toimprove the operational efficiency of HVAC systems. Reduc-ing pumping energy use is even more desirable for ground-

10、source heat pump (GSHP) systems, which use water (or anti-freeze solution) tempered with various ground sources (e.g.,the ground or groundwater) as the heat sink and source. Watertempered with various ground sources as the heat sink andsourcewillnotonlyimprovesystemoperationalefficiencybutalso reduc

11、e the heat rejection loads to the ground source,which means smaller and cheaper ground-heat exchangers incooling-dominated applications. It is thus critical to optimizepumping design and control for GSHP systems in order tomaximize the system operational efficiency and minimize theinitial cost.Durin

12、g the past several decades, the costs of variable-frequency drives (VFDs) have come down significantly due toA Simulation-Based Studyon Different Control Strategies forVariable-Speed Pumps in DistributedGround-Source Heat Pump SystemsFuxin Niu, PhD Xiaobing Liu, PhD Zheng ONeill, PhD, PEStudent Memb

13、er ASHRAE Member ASHRAE Member ASHRAEFuxin Niu is a doctoral candidate and Zheng ONeill is an assistant professor in the Department of Mechanical Engineering, University ofAlabama, Tuscaloosa, AL. Xiaobing Liu is part of the research and development staff at the Building Technologies Research and In

14、tegrationCenter (BTRIC) of Oak Ridge National Laboratory, Oak Ridge, TN.ST-16-018Published in ASHRAE Transactions, Volume 122, Part 2 174 ASHRAE TransactionsadvancesinVFDtechnology.ItenableswidespreadapplicationsofvariablespeedpumpsinHVACsystems.AccordingtoANSI/ASHRAE/IES Standard 90.1-2013, Energy

15、Standard for Build-ings Except Low-Rise Residential Buildings, if the pump powerismorethan10hp(7.46kW),aVFDisrequired.However,fieldstudies of installed HVAC systems (Henderson et al. 2000;Kavanaugh and Kavanaugh 2012) indicated that most centralvariable-speed pumps did not operate at expected low sp

16、eedsduring part-load conditions, which reduces or even eliminatesthe energy savings benefits of variable-speed pumps. Recentcase studies of a few newly implemented GSHP systems (Liu,Malhotra, Walburger et al. 2015; Liu, Malhotra, Xiong et al.2015)indicatedthatthepumpingpowercontributed16%to45%in the

17、 total power consumption of the GSHP systems. Theconventional control strategy for the variable-speed pump,which adjusts the pump speed to maintain a fixed differentialpressure (DP) between the main supply and return of the pipingsystem, contributed to excessive pumping during part-loadconditions. T

18、his DP setpoint is usually arbitrarily determinedand is often much higher than needed (Henderson et al. 2000;MooreandFisher2003;SuandYu2013).DynamicallyresettingDP is recommended in the 2015 ASHRAE HandbookHVACApplications, but no specific recommendations are provided.Afewcontrolstrategiesforresetti

19、ng DPsetpointhavebeenstudied previously. Wang and Burnett (2001) presented a strat-egy to reset DP setpoint based on the estimated derivative of thepoweruseofthecoolingsystemwithrespecttothechangeofDP.AcasestudyindicatedthatthisDPsetpointresetcontrolresultedina10%reductioninthepoweruseofthecoolingsy

20、stem.Mooreand Fisher (2003) discussed a control strategy that can dynami-cally optimize the DP based on the position of the control valvesin the piping system. The control was to keep at least one valvealmost completely open at all times. This control strategy wasimplementedina900,000ft2(83,612m2)bu

21、ilding,andthefieldtest results showed that a 44% reduction of pumping powerconsumptionwasachievedfromusingthenewDPsetpointresetcontrol. While pumping power was always lower with the DPreset control compared with conventional controls, largerpercentage savings were achieved at lower load conditions.

22、Maand Wang (2009) investigated a similar control strategy to opti-mize the DP setpoint. This strategy used the maximal openingsignal among all water control valves and the number of valveswith this maximal opening signal to determine an optimal DPsetpoint. The DP setpoint determined by this strategy

23、 was justbig enough for the most heavily loaded subcircuits in the pipingsystem, and, thus, one of the control valves is kept almost fullyopen. A simulation-based study performed by the authors indi-cated that pumping energy use could be reduced by 12% to 32%with the proposed optimal control strateg

24、y, compared withconventional controls.Mescher(2009)introducedaone-pipeloopdesign.Inthisdesign,alltheheatpumpswereconnectedinserieswithaone-pipe water loop. Each individual heat pump had a dedicatedcirculator to extract water from and reject it back to the waterloop after exchanging heat in the heat

25、pump. A pair of centralpumps circulated water through the one-pipe loop and theground-heat exchanger. The operation of the central pumpswas controlled based on the loop temperature (e.g., runningone or more of the central pumps when the returning watertemperature from the one-pipe loop is out of a d

26、esired range).Most GSHP systems in the United States are in a distrib-uted configuration. With the distributed configuration, eachzoneofthebuildingisconditionedwithanindividualheatpumpunit, and the multiple heat pumps are connected in parallelthrough a common water loop. Traditionally, a two-pipe wa

27、terloop is used with a central pumping station, as shown inFigure 1. Each heat pump can be turned on and off inde-pendentlytosatisfythefluctuatingheatingorcoolingloadofthezone it serves. A two-way solenoid valve is usually installed ateach heat pump and this valve will be closed to block the waterfl

28、ow when the heat pump is not in operation and the valve willbe open when the pump is in operation. Variable-speed pumpsarecommonlyusedindistributedGSHPsystemsandcontrolledto maintain a fixed DP either between the main supply andreturn pipes of the water loop or at the critical subcircuit.Given the o

29、n/off operation of the two-way valves and thefixed flow-rate requirement at each heat pump, the DP resetcontrol strategy described above can be simplified for thedistributed GSHP systems. The circulation pump can becontrolled to deliver just the needed flow rate, which can bedetermined by the number

30、 of the heat pumps that are in oper-ation (which can be determined through the existing buildingenergy management system or with additional temperaturesensors at each heat pump) and the design flow rate of theseheat pumps. The feedback used for this control could be themeasured total system flow rat

31、e or the temperature differenceacross each GSHP unit. This innovative strategy is referred toas flow-demand-based control in this paper.Figure 1 Typical configuration of a distributed GSHP systemwith a pressure-controlled central variable-speedpump.Published in ASHRAE Transactions, Volume 122, Part

32、2 ASHRAE Transactions 175In this paper, three different pumping control strategiesare studied to evaluate their impacts on the pumping perfor-mance and associated energy use. The studied control strate-gies include the following: (1) conventional pressure-basedcontrol to maintain a fixed DP between

33、the main supply andreturn pipes, (2) conventional pressure-based control to main-tainafixedDPatthehydraulicallyfurthestloop,and(3)flow-demand-based control. Finally, a simulation-based case studyfor applying these three control strategies to a typical distrib-uted GSHP system is presented.PUMPING SY

34、STEM CONTROL STRATEGIESA simplified computer model for the hydronic pipingsystem of a typical distributed GSHP system was developedusing the Modelica model (Modelica 2015) on the Dymolaplatform (Dassault Systems 2015). The component models ofhydronic piping systems were adapted from Lawrence Berke-l

35、ey National Laboratorys Modelcia Building Library V1.6(Wetter et al. 2014). Parameters of the simulated hydronicsystem are listed in Tables 1 and 2. The performance of thethree aforementioned pumping control strategies is investi-gated using this model. The variable-speed pump is modeledusing a user

36、-provided pump performance curve, which isderived from three operation points of a pump when it runs atfull speed. The pump model calculates the pump head, effi-ciency, and power use for other speeds based on the perfor-mance curve and the affinity law (Wetter 2009). The pumpingsystems with two conv

37、entional control strategies are modeledas shown in Figure 2(a) (fixed DP at the furthest loop) andFigure 2(b) (fixed DP between supply and return). Threehydronic loops are included in these models: two loops with atwo-wayvalveoneachandabypassloopwithoutanytwo-wayvalve. As typically used in real pump

38、ing systems, the bypassloopallowscontinuouspumpingwhenotherloopsareblockedoff by the two-way valves. In these simulation models, theoperation of the two-way valves is controlled independentlyby two schedules. Figure 2(c) shows the model for the flow-demand-based control, which does not need the bypa

39、ss loop.As used in real pumping systems, an expansion tank assemblyis included in each of the models to provide a reference pres-sure.Allthehydronicloopsinthesimulatedpipingsystemarebalanced so that each loop has its design (nominal) flow rateat the full-load condition (i.e., when the two-way valves

40、 inLoops 1 and 2 are all open).To evaluate the pumping system performance underdifferent part-load conditions, four scenarios of operation aresimulated by applying different on/off schedules for the two-wayvalvesinLoops1and2,respectively.AsshowninFigure3,bothvalvesarekeptopenduringthewholeday(100%of

41、thetime period) in Scenario A, and the time percentages when thetwo valves are open in Scenarios B, C, and D are 75%, 50%,and 25%, respectively.CONTROL STRATEGIESEVALUATION AND DISCUSSIONSThesimulationpredictedwaterflowrateineachloop,therequired pump head, and the associated power use resultingfrom

42、each of the three control strategies under the four-partload conditions. Figure 4 presents the volume flow rate inLoop 1 for all four scenarios. In Scenario A, all three controlTable 1. Operating Points of the PumpUsed in the ModelVolume Flow Rate,gpm (m3/s)Pressure Head,ft (Pa)0 (0) 30 (90,000)21 (

43、0.0013) 20 (60,000)35 (0.0022) 0 (0)Table 2. Flow Rate and Pressure Drop of Each Component at Full-Load ConditionNameVolume Flow Rate,gpm (m3/s)Pressure Drop,ft (Pa)NoteRes_Main_1 21 (0.00133) 3.3 (10,000) Main pipe resistance 1Res_Main_2 21 (0.00133) 3.3 (10,000) Main pipe resistance 2Res_Sub_Main_

44、 1 19 (0.0012) 6.7 (20,000) Submain pipe resistance 1Res_Sub_Main_ 2 9.5 (0.0006) 6.7 (20,000) Submain pipe resistance 2Res SC _1 9.5 (0.0006) 6.7 (20,000) Subcircuit 1 resistanceRes SC _2 9.5 (0.0006) 6.7 (20,000) Subcircuit 2 resistanceRes SC _3 2.1 (0.0013) 26.8 (80,000) Subcircuit 3 resistanceVa

45、lve_1 9.5 (0.0006) 6.7 (20,000) Valve 1Valve_2 9.5 (0.0006) 12.9 (40,000) Valve 2Published in ASHRAE Transactions, Volume 122, Part 2 176 ASHRAE TransactionsFigure 2 (a) Pumping system with fixed DP at the hydraulically furthest loop, (b) pumping system with fixed DP between themain supply and retur

46、n, and (c) pumping system with flow-demand-based control.Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 177strategies are able to maintain the design flow rate, 9.5 gpm(0.0006 m3/s), in Loop 1 when the hydronic system is stabi-lized. However, the simulation results for Scen

47、arios B, C, andD indicate that maintaining a fixed DP at the main supply andreturn results in overflow in Loop 1 (i.e., 17% higher than thedesign flow rate) when the valve in Loop 2 is closed. In thesecases,thepressuredropatthesubmainpipesbetweenLoops 2and3(Res_Sub_Main_1)decreasesduetothereducedsys

48、temflow rate. However, because the DP between the main supplyand return is maintained at the same level as at the full-flowcondition, more than the designed water flow (i.e., overflow)is pumped through Loop 1.Figure 5 shows the predicted volume flow rate in Loop2 for the four scenarios. It can be seen that, when a fixed DPis maintained between the main supply and return, excessiveflow occurs in Loop 2 when Valve 1 is closed but Valve 2 isopen. On the other hand, when the fixed DP is maintained atthefurthestloop(i.e.,Loop1),theflowrateinLoop2is18%lower than the design flow