Effect Of Borehole Depth On Performance Of Ground-source Heat Pump And The Structure Optimization Analysis Of Buried Pipe

The building energy consumption accounts for about1/3in total energyconsumption, changing the development mode of building filed, combined with therenewable energy has become an inevitable trend in the development ofenergy-efficient buildings. Compared with other renewable energy sources, shallowgeothermal energy has the advantages of abundant resources, stable energy supply. Inaddition, the development technology of shallow geothermal is relatively mature andthe cost is relatively low, so the shallow geothermal energy should be vigorouslypopularized. However, in practical engineering applications of GSHP, the surface areaof single U-shaped ground heat exchanger (GHE) accounts for1/5to1/3in buildingair conditioning area. While the initial investment of GHE is very high, the drillingcost only make up about1/3of system initial investment. Therefore, how toeffectively utilize the surface area and strengthen heat transfer of GHE is one of thecore subject of development of GSHP.In this paper, hourly load of building was calculated based on weather data ofTianjin and the GSHP simulation model was established by TRNSYS. The number ofboreholes, performance coefficient of heat pump unit and system, energyconsumption and land occupation of GHE ware analyzed under different boreholedepth. The calculation method of the number of boreholes was proposed by iteration.When the borehole depth is higher than300m, the method of increasing boreholedepth is not an effective way to reduce the number of borehole. And the average soiltemperature increases firstly and then decreases with the increasing of borehole depthunder long-running, effective mitigation of increasing of average soil temperature canbe achieved by increasing the borehole depth. The winter system COP changing trenddecreases firstly and then increases with the increasing of borehole depth, and reachesup to maximum under the borehole depth of60m; When the borehole depth is60m,the system summer COP has a minimum value, while system summer COP has amaximum value under the borehole depth of200m. The total energy consumptionincreases with the increasing of borehole depth, and obtain the minimum value underborehole depth of100m, but the annual energy consumption reduce year by year. Atthe same time, initial investment of GHE sharply increases with the borehole depth, the initial investment in borehole depth of150m or200m is about1.5times of100m.And the land occupation of GHE reduces with the increasing the borehole depth, it isabout1/2of100m under the borehole depth of150or200m. Comprehensiveconsideration of energy consumption and initial investment, when the surface area islarge enough, the preferred borehole depth is100m, but the surface area is limited, theborehole depth can be extended to150m or200m. And the formulas are fitted aboutthe relationship of borehole depth, land occupation, system operating costs and initialinvestment of GHE.The comparative simulation and experimental studies of helical GHE werecarried. The results show that, when the inlet water temperature is higher, the heattransfer performance of helical pipe have been strengthened, the transferred amount ofheat per unit mass of water per unit time are higher than smooth round pipe, theaverage increasing rate of helical pipe is about13%under different flow rate, but as toheat exchange rate of ground heat exchanger, the helical pipe is not the dominant. Atthe low inlet water temperature, the heat exchange rate of helical ground heatexchanger is higer than smooth round ground heat exchanger, the extent ofstrengthened heat transfer increases with the decreasing of inlet water temperature. Sothe helical pipe is more suitable in winter condition. The amount of heat exchange ofintermittent operation mode is15.7%higher than continuous operation mode.Andanalysis the structure of helical pipe, and the optimization recommendations aregiven.