The Performance Analysis And Heat Transfer Behavior Of Single U-bend Loop Ground Heat Exchanger Of The Direct Expansion Ground-source Heat Pump System

With the direction of sustainable development, the ground source heat pump (GSHP)has been attracting much interest in recent decades. The Direct Expansion GroundSource Heat Pump(DX-GSHP) has satisfactory system performance which will beused more widely in china，specially in the much larger villages and towns and Urban.And it will be benefit to the building energy saving in these areas. This paper tries tofind the coupled heat transfer in the single U-bend loop Ground heat exchanger andundersoil. An optimization design method for the single U-bend loop Ground heatexchanger is put forward. According to the actual operating conditions, the designoperating conditions is searched for the DX-GSHP. Research results provide thetheory basis of widespread use for the DX-GSHP.Physical and mathematical models of the coupled heat transfer in single U-bendloop Ground heat exchanger and undersoil were constructed firstly. After somereasonable assumption and simplicity, an analysis method has been got withseparation of variables. Based on the theory of cylindrical, a classical problem oftransient thermal transfer in single U-bend loop Ground heat exchanger and undersoilis analysed quantitatively. The function of the single “equivalent” pipe is obtained bythe use of MATLAB and an experiment is designed to validate the results.The effectsof some parameters on the thermal performance are discussed, such as backfilledmaterial, the drilling-hole diameter, ground thermal properties. By considering theflow and heat transfer performance of refrigerant and pressure loss control, a newmodular method is proposed for single U-bend loop Ground heat exchanger.Themathematical model of oil return for refrigerant is established and the minimalrefrigerant velocity in order to ensure oil return is derived.With regard to the systemperformance of DX-GSHP and the initial investment of single U-bend loop Groundheat exchanger, the optimum design program is developed. As the solution to the oilreturn and the quantity of refrigerant in system operation process, correspondingcontrol strategy is proposed, which got the certification of National Invention Patent.Depended on the help of the National Key Technologies R&D Program of Chinaduring the11th Five-year Plan Period (“Nos.2006BAJ04A13”)-The key technicalresearch of use low temperature heat in villages and small towns, the demonstrativeproject was designed and constructed with refrigerant R134a, which refrigerating capacity is5kW. This paper studies the the DX-GSHP in actual operation, and thetrained for operating data in diverse operation conditions were to be proved theDX-GSHP distinct capable advantages. Finally above-mentioned theories andfeasibility of the method have been proved. This work may reveal insights into thedevelopment of DX-GSHP systems in China.The design and construction of demonstrative project are analyzed. The experienceof design cycle and construction technique aiming at DX-GSHP are discussed.Analyzing the successful experience of this mode can provide a reference for us.Combining designed example, the paper describes design conditions under variousregion. By monitoring analysis, some suggestions are presented for the similarprogramming design of engineerings. Generally speaking, in cooling mode, thedesigned outlet-temperature of the single U-bend loop Ground heat exchanger shouldbe annual average air temperature adding another5~8oC. The condensing temperatureis annual average air temperature adding another11~13oC. The evaporatingtemperature is7.2oC, which is specified in air source heat pump Unit nominalrefrigeration capacity testing working condition of summer by Air-conditioning,Heating and Refrigeration Institute(AHRI).With the comparison of the DX-GSHP and the secondary loop ground-coupled heatpump(SL-GCHP)in actual operation, the condensing(evaporating) temperature ofDX-GSHP is lower (higher) than that of SL-GCHP. The average cooling performancecoefficients (PFsys) of the DX-GSHP system was obtained to be6.03and that of theSL-GCHP system was determined to be5.64, which means that DX-GSHP has a23.8%higher efficiency than SL-GCHP in cooling mode.The average input powervalues for the DX-GSHP system and the SL-GCHP system are found to be1.39and1.715kWh respectively.The compressor performs the continuous process that includesthe suction, oil return without any difficulties by the means of the more shallowburied Ground heat exchanger and controlling the minimal refrigerant velocity.Withthe economic comparison of the DX-GSHP and the SL-GCHP in actual operation bymeans of Annual Cost (AC) and Present value approach(PW).The DX-GSHP is betterthan the SL-GCHP. The DX-GSHP is substitute for the SL-GCHP in an suitableworking place.