Feasibility of a medium-depth U-tube ground-source heat pump system in a severe cold zone in China

ABSTRACTIn China, residential buildings have a high-volume ratio and density, which limit the amount of buried U-tube space available, thereby hindering the application of traditional ground-source heat pump systems (GSHPs). Due to the imbalance in heat transfer, the operating efficiency of GSHPs decreases annually in severe cold zones. To alleviate these issues, we assessed the feasibility of medium-depth U-tube GSHPs. An experimental platform in Fuxin City, China, was established, and a simulation model was designed with TRNSYS software. The area affected by the heating load was analyzed, and the proposed system was compared with solar-soil and medium-depth casing pipe GSHPs. The proposed system was found to be stable and efficient for long-term operations, delivering not only the lowest equivalent annual cost but also a 4.61% improvement in heating (compared to solar-soil GSHPs), an 81.54% improvement in cooling, and a 23.08% improvement in emission reductions (compared to medium-depth casing pipe GSHPs).The application of a special U-shaped pipe elbow and a dual-temperature switching valve allowed heat exchange conversion between the heating and cooling conditions. The results of the simulation model demonstrated that dual-temperature operation resulted in greater cooling performance and cheaper operating costs than did the single-temperature system.KEYWORDS: Medium-depth GSHPsU-tubegeothermal energydual-temperaturesevere cold zones Nomenclature COPr=Rated coefficient of performance of heat pump unitsEERr=Rated energy efficiency ratio of heat pump unitsTin,r=Rated water inlet temperature of heat pump unitsttop=Temperature on top of storagetsurface=Surface temperature of storage volumeΔt=Thermal gradient of storage volumeλ=Thermal conductivityN=NumberD=DistanceH=Depthϕ=DiameterCi=Investment costr=Discount ratet=Life cycleCm=Management costMEC=Major equipment costDC=Drilling costBC=Backfill costCBC=Comprehensive borehole costf=Unit feeV=VolumeEC=Excavation costh=Excavation height of V-shaped channelsn=Number of boreholes in a single rowAOAC=Additional occupied area costA=Land areaOC=Operation costP=Total energy consumptionε=Estimate indexQ˙=Rated capacityP˙=Rated powerV˙=Rated flowH˙=Rated headAc=Area of collectorsηc=Collector efficiencyηL=Rate of heat loss rateECI=Energy conservation indexERI=Emission reduction indexEI=Economy indexηe=Rate of energy conservationG=Annual cumulative reductionSubscripts=hp=heat pumps=storagef=fillp=pipeb=boreholel=layero=outeri=innerex=excavationt=transactiong=green landel=electricityH=heatingC=coolingsp=Single-speed pumpc=collectorh=heat storageO=outer pipeI=inner pipesys=systemCO2=carbon dioxide emissionSO2=sulfur dioxide emissiondust=dust emissionAcronyms=GSHPs=Ground source heat pump systemTRNSYS=Transient system simulation toolGDP=Gross domestic producttce=Ton of standard coal equivalenttCO2=Ton carbon dioxideHVAC=Heating, ventilation and air-conditioningTp=Temperature penaltyCOP=Coefficient of performanceEER=Energy efficiency ratioBHE=Borehole heat exchangerVGHE=vertical ground heat exchangerType557a=Mode of U-tube ground heat exchanger in TRNSYSRTWD160HE=Ground source heat pump units of TRANEType 225=Developed mode of heat pump in TRNSYSType 927=Mode of heat pump in TRNSYSType 742=Mode of variable pump in TRNSYSType 271=Developed mode of variable pump in TRNSYSType 114=Mode of single pump in TRNSYSType 557d=Mode of casing pipe ground heat exchanger in TRNSYSEAC=Equivalent annual costDST=duct Ground Heat StorageDisclosure statementNo potential conflict of interest was reported by the author(s).Additional informationFundingThe work was supported by the National Natural Science Foundation of China [Grant Number 52078097].Notes on contributorsTingting ZuoTingting Zuo is an engineer in Integrated Energy Management Department of China Construction Carbon Technology Co., LTD. She received her Bachelor's degree from Tiangong University and her Master's degree from Dalian University of Technology. Her research interests are renewable energy applications and building energy efficiency.Xiangli LiXiangli Li is an associate professor and doctoral supervisor of major Heating Ventilation and Air Conditioning in faculty of Infrastructure Engineering of Dalian University of Technology. He received his Ph.D. from Harbin Institute of Technology. His research interests include heat pump technology and building energy efficiency.Lifan WangLifan Wang is the chairman and founder of Fuxin Manulife New Energy Heating Co.. He and his company specialize in developing innovative technologies in construction technology. His research interest is in medium-depth geothermal energy apCang Tong is a researcher in Nanjing institute of future energy system, Sector heat exchange. He received his master's degree and PhD degree from Dalian University of Technology. His research interest is in thermal storage technology.Shiwei XueShiwei Xue is the chief executive officer of China Construction Carbon Technology Co.. He is responsible for the development of the company's integrated energy services and dual carbon research businesses. His research interest is in carbon reduction in the construction industry.Zhijie ZhangZhijie Zhang is the vice general manager of China Construction Carbon Technology Co.. He received his master's degree from Tongji University and is currently studying for his PhD at Tianjin University. His research interest is carbon reduction in the construction industry.